In the previous works the spin Hall effect (SHE) of light was usually detected by the quantum weak measurement technique in which the complete information of the field was lost. In this work, we investigate the spatial evolution of the whole field in the SHE of light on reflection. First, we establish a model to describe the polarization state of reflected light and disclose its relationship with the SHE. Then, it is found that the reflected polarization generally becomes a vector field. The SHE due to the polarization gradient is manifested as a spin-dependent splitting. Further, it is found that both the incident angle and the incident polarization can affect the polarization of reflected light noticeably. Finally, the evolution of the energy flow is analyzed to disclose the underlying physical mechanism.
Here, we review our recent works about the realization of the spin-dependent manipulation of structured light by tailoring the polarization. We develop a theory for spin-dependent manipulation based on the polarization design. As the exemplary demonstration, our experimental results show that such a theory can attain the spin-dependent splitting, and spindependent diffraction. Furthermore, we demonstrate this theory can be also applied to the spindependent focusing. The scheme provides a possible route for the manipulation of spin states of photons, and enables spin-photonics applications.
Two-dimensional (2D) atomic crystals have extraordinary electronic and photonic properties and hold great promise in the applications of photonic and optoelectronics. Here, we review some of our works about the spin-orbit interaction of light on the surface of 2D atomic crystals. First, we propose a general model to describe the spin-orbit interaction of light of the 2D free standing atomic crystal, and find that it is not necessary to involve the effective refractive index to describe the spin-orbit interaction. By developing the quantum weak measurements, we detect the spin-orbit interaction of light in 2D atomic crystals, which can act as a simple method for defining the layer numbers of graphene. Moreover, we find the transverse spin-dependent splitting in the photonic spin Hall effect exhibits a quantized behavior. Furthermore, the spin-orbit interaction of light for the case of air-topological insulator interface can be routed by adjusting the strength of the axion coupling. These basic finding may enhance the comprehension of the spin-orbit interaction, and find the important application in optoelectronic.
Development of spin-photonic devices requires the integration of abundant functions and the miniaturization of the elements. Pancharatnam-Berry phase elements have fulfilled these requirements and can be attained by using dielectric metasurfaces with subwavelength nanostructures. Here, we review some of our works on Pancharatnam- Berry phase elements and make an introduction of some integrated spin-photonic devices. We propose to integrate Pancharatnam-Berry phase lens into dynamical phase lens, which can be conveniently used to modulate spin states of photons. By integrating a Pancharatnam-Berry phase lens into a conventional plano-concave lens, we can obtain spin-filtering of photons. Moreover, we demonstrate that the generation of complex wavefronts characterized with different spin states can be implemented by the Pancharatnam-Berry phase lens. Further, based on the spin-dependent property of Pancharatnam-Berry phase element, we realize the three-dimensional photonic spin Hall effect with lateral and longitudinal spin-dependent splitting simultaneously. We foresee that this optical integration concept of designing Pancharatnam-Berry phase elements, which circumvents the limitations of bulky optical components in conventional integrated optics, will significantly impact multipurpose optical elements, particularly spin-based photonics devices.
Dielectric metasurfaces with spatially varying birefringence and high transmission efficiency can exhibit exceptional abilities for controlling the photonic spin states. We present here some of our works on spin photonics and spin-photonic devices with metasurfaces. We develop a hybrid-order Poincaré sphere to describe the evolution of spin states of wave propagation in the metasurface. Both the Berry curvature and the Pancharatnam-Berry phase on the hybrid-order Poincaré sphere are demonstrated to be proportional to the variation of total angular momentum. Based on the spin-dependent property of Pancharatnam-Berry phase, we find that the photonic spin Hall effect can be observed when breaking the rotational symmetry of metasurfaces. Moreover, we show that the dielectric metasurfaces can provide great flexibility in the design of novel spin-photonic devices such as spin filter and spin-dependent beam splitter.
We report the demonstration of intrinsic spin Hall effect (SHE) of cylindrical vector beam. Employing a fan-shaped aperture to block part of the vector beam, the intrinsic vortex phases are no longer continuous in the azimuthal direction, and results in the spin accumulation at the opposite edges of the light beam. Due to the inherent nature of the phase and independency of light-matter interaction, the observed SHE is intrinsic. Modulating the topological charge of the vector beam, the spin-dependent splitting can be enhanced and the direction of spin accumulation is switchable.
Observation of photonic spin Hall effect (SHE) manifested by spin-dependent splitting of light in a dielectric-based birefringent metasurface is reported experimentally. By designing the metasurface with homogeneous phase retardation but space-variant optical axis directions, we govern the photonic SHE via space-variant Pancharatnam-Berry phase originated from the local polarization manipulation of the metasurface, essentially, the spin-orbit interaction between the light and the metasurface. Modulating the polarization distribution of the incident light and/or the structure geometry of the metasurface, the photonic SHE could be tunable. This type of metasurface offers an effective way to manipulate the spin-polarized photons and a route for spin-controlled nanophotonic applications.
The photonic spin Hall effect (SHE) is generally believed to be a result of an effective spin-orbit coupling, which
describes the mutual influence of the spin (polarization) and the trajectory of the light beam. The photonic SHE
holds great potential for precision metrology owing to the fact that the spin-dependent splitting in photonic SHE
are sensitive to the physical parameter variations of different systems. Remarkably, using the weak measurements,
this tiny spin-dependent shifts can be detected with the desirable accuracy so that the corresponding physical
parameters can be determined. Here, we will review some of our works on using photonic SHE for precision
metrology, such as measuring the thickness of nanometal film, identifying the graphene layers, detecting the
strength of axion coupling in topological insulators, and determining the magneto-optical constant of magnetic
film.
Graphene, due to its linear energy band structure and the Pauli blocking principle, exhibits broadband optical saturable
absorption. We experimentally demonstrated the saturable absorption in graphene with different layers at 800 nm. By
performing the balanced twin-detector method, we are able to characterize the nonlinear optical property of the
as-fabricated different layer graphene samples. Under strong illumination, we find that absorption of graphene decreases
with the increase of incident power, and by fitting the experiment data with theoretical model, the saturable intensity and
the normalized modulation depth is measured to be 7.911 GW/cm2 and 10.6% in 3 layers sample and 2.77 GW/cm2 and
73.6% in monolayer sample, respectively. The experimental result shows that graphene may be a promising saturable
absorber, with the potential laser photonics applications, such as laser mode locker or Q-switcher, at 800 nm band.
We present a theoretical and experimental investigation of the spin Hall e®ect (SHE) of light in graphene. When
a light beam impinges onto graphene-prism interface near Brewster angle, an enhanced and switchable spin-
dependent splitting can be detected via the signal enhancement technique known from weak measurements. Our
preliminary experimental results show that the SHE of light can become an advantageous metrology tool for
characterizing the refractive index of graphene. In addition, the SHE of light may have a potential of probing
the spatial accumulations of spin electrons in the graphene, which builds a bridge between electronic SHE and
photonic SHE.
We propose a simple method to fabricate graphene saturable absorber (GSA) through drop casting the graphene dispersion prepared by liquid-phase exfoliation of graphite onto quartz plate and realize the passively mode-locked of an Erbium-doped fiber laser by this saturable absorber (SA). In contrast with previous GSAs, due to the incorporation of additional free space alignment from the SA component, we are able to deliberately control the laser radiation spot size and location on GSA, leading to flexibility in mode-locked performance. Finally, we can obtain stable soliton pulse emission with central wavelength at 1564.42 nm, 3 dB bandwidth of 1.02 nm, and pulse duration of 2.89 ps, and also note that soliton parameters (pulse energy, central wavelength, and pulse duration) sensitively depend on the position of GSA, which could be traced back to nonuniformity of saturable absorption from the nonhomogeneous morphology of the as-prepared graphene sample.
KEYWORDS: Modulation, Pulsed laser operation, High power lasers, Beam splitters, Laser systems engineering, CCD cameras, Mirrors, Solid state lasers, National Ignition Facility, Fluctuations and noise
It is well known that small-scale self-focusing is one of the major factors to restrict the capability and efficiency of
high-power solid-state laser driver. B-integral is used to measure the severity extent of small-scale self-focusing and the
typical value of B-integral is less than 1.8 in narrow band high power laser by NIF. We experimentally obtained the
relationship of B-integral with spatial contrast for broadband pulse in case of active modulation and random noise
modulation, and found that the growth of contrast is still very slow after B-integral up to 1.8 for 12 nm ultrashort pulse
laser. So we can consider that the broadband laser is able to unloosen B-Integral criterion and then improve the system
loading.
Pulse compression of initial negative frequency chirp pulses in silicon photonic nanowire has been analyzed numerically.
The effects of the initial negative frequency chirp and core diameter variation on pulse compression have been
numerically investigated. By analyzing the interaction of SPM, positive group-velocity dispersion and the initial negative
frequency chirp, we find that the compression factor increases but the quality factor and the optimum fiber length
decrease with the increasing initial negative chirp when the core diameter is specified. A compression factor of 8.02 can
be achieved by a pulse with initial negative frequency chirp of -20 and initial FWHM duration of 30 fs in a 4.35 μm long
silicon nanowire.
We have investigated the dispersion and the nonlinearity properties of the subwavelength-diameter optical fiber (SDOF)
with high-index-contrast dielectric thin films. The dispersion length and the nonlinearity length have been calculated to
discuss the propagation properties of pulse in SDOF. The waveguide dispersion and nonlinearity analysis is used to
guide the waveguide dimension design for possible applications. The high nonlinearity of SDOF makes it especially
suitable for nonlinearity devices.
We extend the Green's function integral method to investigate the propagation of electromagnetic waves
through an anisotropic dielectric-magnetic slab. From a microscopic perspective, we analyze the interaction
of wave with the slab and derive the propagation characteristics by self-consistent analyses. Applying the
results, we find an alternative explanation to the general mechanism for the photon tunneling. The results
are confirmed by numerical simulations and disclose the underlying physics of wave propagation through
slab. The method extended is applicable to other problems of propagation in dielectric-magnetic materials,
including metamaterials.
We have investigated the effective electromagnetic parameters of a two-dimensional photonic crystal even though the
wavelength is on the order of its lattice constant. For the photonic crystal within the first band gap, negative effective
permittivity or negative effective permeability has been found. Utilizing the Finite-difference time-domain method, a flat
slab imaging for TE waves in the near field has been demonstrated for the photonic crystal with effective negative
permittivity which is similar to silver superlens for TM waves. Based on these results, we can conclude that photonic
crystals in a certain frequency region can indeed mimic not only double-negative but also single-negative metamaterials.
Absorption or loss is inevitable for the metal-based metamaterials (MMs) due to the intrinsic loss of the metal, and
constitutes a major hurdle to the practical realization of most applications such as a sub-wavelength lens. Thus, to reduce
the losses becomes one of the major challenges in the MM field. However, the inevitable loss can also be harnessed to
take a positive role in the applications of MMs such as stealth technology or other types of cloaking devices. In this
presentation, after a brief review of the advances in MMs-based absorbers, we present several schemes to fulfill the
desired electromagnetic absorption properties, both linear and nonlinear. For linear absorption, we have experimentally
demonstrated that the absorption performance of an ordinary microwave absorbing material can be evidently improved
by using the electric resonance resulting from an array of subwavelength metallic circuit elements. For nonlinear
absorption, we show theoretically that the active linear magnetic permeability induces a nonlinear absorption, similar to
the two-photon absorption (TPA), of electric field in a lossy MM with a Kerr-type nonlinear polarization.
In this paper, we in detail investigate the dynamic behavior of pulse splitting in metamaterials (MMs) with a Kerr
nonlinear polarization, focusing on the anomalous propagation properties associating with the unique and engineerable
electromagnetic properties of such materials. The numerical results show that, the pulse symmetric splitting in MMs will
occur for the case of defocusing nonlinearity with anomalous dispersion due to the negative refractive index; while it
will appear for the case of focusing nonlinearity with normal dispersion in ordinary materials. Moreover, our further
analysis shows that, like the case in ordinary materials, the inclusion of self-steepening (SS) effect with positive value
gives rise to the asymmetry between the leading and trailing pulses, but the relative magnitudes of the two peaks are
reversed, namely, the leading pulse is higher than the trailing pulse. However, the negative SS does opposition.
It is well known that the omnidirectional photonic bandgap (zero-n bandgap) can be realized in the one-dimensional
photonic crystals containing metamaterials. However, these omnidirectional photonic bandgaps are not tunable. In this
manuscript, we show that the electrically tunable omnidirectional photonic bandgap can be obtained in one-dimensional
photonic crystal with third-order nonlinear composite materials and linear metamaterials. It is demonstrated this photonic
crystal possesses electrically tunable photonic band structures based on the fact that the effective refractive index of
nonlinear composite material has electric-field dependence due to the existence of third-order nonlinear responses.
Moreover, we also showed that the photonic bandgap of this photonic crystal can be tuned by controlling the filling
fraction f of the composite.
Through using the standard split-step Fourier method, it is found that the transverse modulation instability (MI)can
develop when beams copropagate in the positive- and the negative-index region of the metamaterials (MMs) respectively
and it is equivalent with the temporal MI in the case of two pulses copropagate in the anomalous and normal dispersion
regions of the optical fibers respectively, which is meaning that bright and dark electromagnetic spatial solitons may
generate simultaneously. Furthermore, it is confirmed that the bright and dark electromagnetic spatial solitons may even
generate simultaneously when beams copropagate in MM, which is a new way to generate spatial soliton pair for there is
only leading to the generation of bright or dark spatial solitons in conventional material when two beams copropagate.
KEYWORDS: Orthogonal frequency division multiplexing, Single mode fibers, Modulation, Radio optics, Receivers, Fiber Bragg gratings, Extremely high frequency, Dispersion, Optical amplifiers, Signal detection
We have experimentally investigated a full-duplex radio-over-fiber system transmitting 2.5-Gbit/s orthogonal frequency-division multiplexing (OFDM) signals with a 40-GHz optical millimeter wave as downlink. Meanwhile the central can be reused wavelength as the uplink connection for transmitting 2.5-Gbit/s on-off keying (OOK) signals. By experimentally comparing the transmission performance of OFDM downstream and OOK upstream signals, it can be seen that the power penalty for the downstream signals is about 1 dB at a bit error rate of 10−3 after transmission over a 50-km standard mode fiber. However, the power penalty for the upstream signals is less than 0.5 dB at a bit error rate of 10−9 after transmission over the same distance.
It has been investigated of nonlinear propagation of femosecond pulse and supercontinuum generation (SCG) in three
photonic crystal fibers (PCFs) with different dispersion profile in the 1550nm window by numerical stimulation. The
influence of higher-order effects on supercontinuum, including higher-order dispersion (HOD), self-steepening (SS), and
stimulated Raman scattering (SRS), are discussed in detail as well. The results of numerical simulations show that group
velocity dispersion and self-phase modulation take main effect in the initial stage. In the PCF with anomalous dispersion,
SRS plays main role when the propagation distance increases, which induces a red-shift of the central wavelength,
suggesting the appearance of soliton self-frequency shift. In the PCF with near-zero anomalous dispersion, HOD plays
main role. This results in the fission of higher-order solitons and remarkable broadening of the pulse spectrum. In the
PCF with normal dispersion, higher-order effects have almost no effect on the pulse. The Gaussian pulse wave broadens
to rectangular symmetrically, and the pulse spectrum broadens symmetrically, too. However, the broadening is smaller
than the former two cases.
KEYWORDS: Modulators, Radio optics, Signal generators, Eye, Channel projecting optics, Receivers, Radio over Fiber, Modulation, Oscillators, Optical engineering
We propose and demonstrate a novel radio-over-fiber (ROF) architecture by using only one phase modulator and one optical filter to generate an optical millimeter (mm) signal and realize wavelength reuse for upstream data connection. In the central office, the rf signals are generated by an electrical mixer that mixes the local oscillator (LO) and downstream data, and a phase modulator is used to generate an optical millimeter-wave signal. In the base station, an optical interleaver with two output ports is used to separate the optical carrier and the first-order sidebands. The separated first-order sidebands are used to generate optical millimeter-wave signals with a double LO frequency, while the separated optical carrier is reused for an uplink connection. There is no additional laser source for the upstream data generation in the base station and no dc-bias controller for phase modulator. By this method, we simplify the configuration of the radio-over-fiber system and reduce the cost of the system.
Metamaterials (MMs) are artificial structures, which can be engineered to satisfy the prescribed requirements. The most
important difference between an ordinary medium and a MM is that the former has a constant permeability, while the latter
has a dispersive and controllable permeability. MMs can extend the electromagnetic properties of conventional materials,
and the study of the nonlinear propagation of ultrashort pulses in MMs could lead to completely new electronic and optical
devices. In this paper, the research advances on the propagation of electromagnetic pulses in MMs with third-order nonlinear
response are briefly described. Special effort is focused on the typical nonlinear optical phenomena such as modulation
instability, bright and dark solitons.
The phenomenon of hot image in high power lasers is closely related to the spatial uniformity of laser beams and the
running safety of laser systems. Here we study the formation of the second-order hot image of phase defect in the
high-power broadband laser system, special attention is payed to the role of bandwidth of incident beam in the formation
of the second-order hot image. It is shown that, as the laser bandwidth increases by either increasing the positive
temporal chirp or reducing the pulse duration the intensity of the second-order hot image decreases. However, as the
amount of negative chirp increases, the intensity of the second-order hot image increases first but decreases after
reaching a maximum value. The distance between the second-order hot image and the nonlinear medium is found to be
about half that between the phase defect and the nonlinear medium. The influence of phase shift caused by the phase
defect and nonlinear effect on the formation of the second-order hot image is also numerically studied.
Two kinds of novel spatial filters constructed by metamaterials and their possible applications in high-power laser
systems have been investigated. The first one can be constructed by forming a compensating bilayer of indefinite
metamaterials. It is shown that the cutoff wave vector of the low-pass spatial filter can be adjusted desirably. The second
kind of low-pass spatial filter is based on the controllable dispersion characteristics of photonic crystals. With proper
design, the higher spatial frequency components, which are incident to the filter with angles exceed a critical value, are
reflected totally because no Bloch waves of the photonic crystals can be excited. However, the lower spatial frequency
components are coupled to the self-collimating modes and permeate with high transmission. The applications of the two
novel kinds of metamaterials-based low-pass spatial filter in high-power lasers are discussed.
Based on the angular spectrum theory of light propagation and the mean-field approximation, an expression for intensity
of hot image of the intense laser beam through a thick Kerr medium with gain and loss is obtained, beyond the
thin-medium approximation. Thereby the dependence of intensity of hot image on the thickness and gain/loss of medium
and the property of obscuration are identified analytically and numerically. It is shown that, for a given obscuration, the
intensity of hot image decreases as the medium thickness increases for definite B integral and increases monotonously
with the medium thickness for definite input power of laser beam. For definite output power of laser beam and gain/loss
of medium, the intensity of hot image firstly increases and subsequently decreases as the medium thickness increases.
Furthermore, it is shown that hot image from obscuration of phase modulation is more intense than that from obscuration
of pure amplitude modulation, even in the presence of medium gain and loss. As the size of obscuration increases, the
intensity of hot image first increases gradually, after reaching a maximum value, it decreases rapidly to a minimum
value.
We propose a novel scheme for simultaneously realizing optical
mm-wave generation and remote photonic down-conversion
by using optical phase modulator along with sidebands separation technique. In the central office, optical
double sideband (DSB) signal is generated by using an optical phase modulator. One sideband of the generated optical
DSB signal is filtered out to carry downstream data signal. In the base station, the sideband carried with downstream data
beats with part of the optical carrier, thereby generates mm-wave signal. Another sideband along with part of the optical
carrier is injected into the second phase modulator to implement photonic frequency down-conversion for uplink signal
transmission. We also theoretically show the principle of photonic frequency down-conversion based on the optical
phase modulator. By using the scheme, no additional optical local oscillator signal or RF signal is needed because the
full advantage of sidebands of the optical DSB signal has been taken.
Solitons have been found in many physical branches. Previous investigations on solitons mainly focused on various
nonlinear dispersive media in nature. It is well known that in the nonlinear dispersive medium, soliton formation is due
to the exact balance between the nonlinearity and the group velocity dispersion (GVD). When the signs of nonlinearity
and GVD are the same, that is, the positive (focusing) nonlinearity vs. positive (normal) GVD, or the negative
(defocusing) nonlinearity vs. negative (anomalous) GVD, the interaction between nonlinearity and GVD may lead to the
generation of dark solitons. In this paper, we study the formation and propagation of dark electromagnetic solitons in the
metamaterials, artificial structures that display properties beyond those available in naturally occurring materials. We use
an extended Tanh-function expansion method to solve the nonlinear equation for ultrashort electromagnetic pulse
propagation in metamaterial with a nonlinear polarization, and get dark solitary solutions under various conditions. It is
found that, due to the role of the second-order nonlinear dispersion resulted from the dispersive magnetic permeability,
dark solitons can be formed in the absence of linear dispersion, or even in the case of anomalous linear GVD for a
focusing nonlinearity, challenging the traditional conditions for dark soliton formation.
Optical label switching (OLS) has been regarded as an efficient technique to route and forward IP packets transparently
in the optical layer. In label switching system, orthogonal modulation has shown some advantages, such as its high
spectral efficiency. One example is the combination of amplitude-shift-keying (ASK) label and differential phase-shift-keying
(DPSK) payload. However, differential quadrature phase-shift-keying (DQPSK) payload has not been used in
optical label switching system yet. In this letter, we propose and demonstrate a novel optical label switching scheme by
the combination modified return zero (MD-RZ) and DQPSK for the first time. The transmitter is set up by cascading two
phase modulators (PM) and one dual-arm LiNbO3 modulator (LN-MOD). The signal is first phase modulated by two
PMs. The MD-RZ label is then impressed by the subsequent LN-MOD. Duty cycle and extinction ratio of the MD-RZ
labels are tunable. The DQPSK payload can be used in a 20Gbit/s optical label switching system, with 2-bit per symbol
modulation/demodulation. Within the network, the label information is read, processed and updated, a process known as
label switching, until the burst reaches the desired egress edge node. The use of balanced detection is expected to
improve performance for the payload. The transmission performance of 20Gbit/s DQPSK payload with 2.5Gb/s MD-RZ-ASK
label is also analysed. MD-RZ labeling of a DQPSK payload has been shown to be a promising candidate for the
implementation of OLS system.
Metamaterials (MMs) are artificial structures that display properties beyond those available in naturally occurring materials.
The most notable MM is the negative-index material with simultaneously negative electric and magnetic responses. Recently,
MMs exhibiting negative electric and magnetic properties in the infrared and optical frequencies have been realized, which
will affect substantially the conventional optics and their applications in devices of compact cavities, adaptive lenses, tunable
mirrors, isolators, converters, and so on. Moreover, MMs with nonlinear electric and/or magnetic response have also been
demonstrated. This will further stimulate intense investigation in MM-based nonlinear optics due to the richness and the
potential applications of the nonlinear property of MMs. In this paper, we report on our recent progress on investigation of
some typical third-order nonlinear optical phenomena in MMs, such as modulation instability, soliton propagation, self-focusing,
and so on. The controllability and the novel properties of the nonlinear phenomena in MMs are demonstrated.
We propose a novel scheme to generate duobinary RZ and dark RZ signals. It needs only one differential Mach-Zehnder
modulator (DMZM), which can adjust the operating point of the modulator to generate duobinary RZ and dark RZ
signals easily. It is more cost-effective compared to conventional schemes due to its simple configuration.
KEYWORDS: Modulators, Signal detection, Double sideband modulation, Modulation, Radio optics, Signal generators, Eye, Phase modulation, Radio over Fiber, Receivers
In this paper, a scheme of optical millimeter (mm)-wave generation by using only one phase modulator has been
investigated experimentally. To generate a mixing signal, the baseband signal is mixed with the radio frequency (RF)
signal using an electric mixer. And the double-sideband (DSB) signal generated from phase modulator driven by the
mixing signal is transmitted to base station via a single mode fiber. The central carrier and the first-order sideband are
separated by an optical interleaver at base station. The two peak modes of the first-order sideband will beat to generate
an mm-wave with a double repetitive frequency of the RF signal when they are detected by an optical receiver. The
transmission performance of the optical mm-wave is analyzed. According to our research, the different delay of two first
sidebands due to the fiber dispersion will lead to power loss of the mm-wave. And the duty cycle of the code is reduced.
It limits the largest transmission distance of the mm-wave. In the experiment, the 20GHz RF signal generates 40GHz
mm-wave and 2.5Gbps NRZ is carried by the mm-wave. For downlink connection, the power penalty is 0.2 dB after
transmission over 20-km.
We have both numerically and experimentally studied femtosecond chirped optical pulse undergoing self-focusing and splitting into multiple filamentation passing through carbon disulfide (CS2). From the experimental results, we obtain that broad bandwidth laser pass through a nonlinear medium (CS2) will occur modulation increase at the particular spatial position, and it shows good agreement with the simulation results. When the width of pulse increases (equal to the value of chirp add), modulation increase of chirped pulse beam will delay. After the modulation increase reaches to a certain value, it will stop increasing and appear new modulation frequency which bring optical beam to split into filamentation.
We experimentally and theoretically investigate the propagation of intense femtosecond pulse in a nonlinear medium, in
which the dynamic process of small-scale self-focusing of intense femtosecond pulse is experimentally investigated. By
using tabletop femtosecond laser, the pulse which is spatially modulated by the cross silk diffraction passes through the
nonlinear medium (carbon bisulfide, CS2). As the input power increases, the small-scale self-focusing of the pulse is
obtained. By analyzing the contrast gradient of the beam, we can obtain the dynamic process of small-scale self-focusing.
Finally, we find that the numerical simulations are in good agreement with experimental results.
Compared to the non-magnetic ordinary dielectrics, the
negative-index metamaterials have not only a dispersive electric
permittivity but also a dispersive magnetic permeability. The purpose of this paper is to identify the role of dispersive
magnetic permeability in nonlinear propagation of ultrashort electromagnetic pulses in metamaterials. Firstly, we derived
a generalized system of coupled three-dimensional nonlinear Schroedinger equations suitable for few-cycle pulse
propagation in the metamaterial with both nonlinear electric polarization and nonlinear magnetization, which clearly
demonstrates the role of dispersive permeability in nonlinear pulse propagation: In the linear propagation aspect, its
contribution is buried in the ordinary dispersive terms; while in the nonlinear propagation aspect, the dispersive
permeability manifests itself as a nonlinear polarization dispersion, although it is a linear parameter. Secondly, by
exemplificatively using the coupled nonlinear Schroedinger equations in the Drude dispersive model, we quantitatively
discussed the influence of dispersive permeability on pulse propagation in metamaterials.
We propose a novel scheme to generate return-to-zero differential phase-shift keyed (RZ-DPSK) and carrier-suppressed
RZ-DPSK (CSRZ-DPSK) formats based on the differential Mach-Zehnder modulator (DMZM), which can adjust the
operating point of the modulator and control chirp conditions easily. It is more cost-effective due to its simple
configuration.
We study the propagation property of soliton pulses in negative-index metamaterials with a nonlinear polarization, and
especially analyze the influence of the controllable self-steepening effect, which is resulted from the dispersive magnetic
permeability in negative-index metamaterials, on soliton formation and propagation. The results indicate that the
negative self-steepening effect also leads to the asymmetry of soliton pulse, the center shift and the decay of higher-order
soliton. In addition, the controllable self-steepening effect can be used to counteract the shift of soliton pulse resulted
from the third order dispersion effect to make the soliton pulse propagation without center shift to some extent.
Propagation of optical beam in optical lattices is studied. The conditions for spatial solitons formation are obtained by
using variational approach. A proposal for loss compensation for maintaining the spatial soliton propagation in loss
medium by varying the longitudinal depth of the optical lattice is demonstrated both analytically and numerically.
We report experimental results of wavelength dynamics in a soliton fiber ring laser passively mode locked by using the
nonlinear polarization rotation technique. We find central wavelength of pulses can be tuned by adjusting the
polarization controllers due to the fiber birefringence dependence on the light wavelength. The spectrum of pulses
generated from that of continuous waves (CW) is also studied. We find the wavelength shifted between the CW and the
mode-locked state exhibits hysteresis effect. The mechanism is investigated and it is found the wavelength tuning
hysteresis is the combined effects of fiber birefringence and cavity propagation property.
Based on the Maxwell-Bloch formalism, operation of a passively mode-locked fiber laser is numerically investigated. It
is found that even with the effects of coherence stable solitary waves can still be obtained in the laser due to the cavity
pulse peak clamping effect. And the discrepancies between the results obtained by the coupled GLEs and the Maxwell-
Bloch formalism will increase as the linear phase delay bias increases.
We proposed and demonstrated a novel full-duplex radio-over-fiber system using an optical phase modulator and optical
interleaver to generate DWDM optical millimetre wave for carrying downstream data and wavelength reuse for uplink
connection. Since the optical phase modulator we used has not a dc-bias controller, and the optical interleaver is not as
sensitive to temperature as a fiber Bragg grating, the stability of the generated DWDM optical millimeter wave is largely
improved, and the optical power is effectively utilized because the remaining optical carriers with high power have been
reused. Moreover, the limitation of chromatic dispersion is greatly reduced due to avoiding the generation of higher order
sidebands via driving the phase modulator with optimized RF signal. So this system shows cost-efficient configuration
and good performance over long-distance delivery.
KEYWORDS: Modulation, Signal detection, Radio optics, Modulators, Optical filters, Radio over Fiber, Signal generators, Extremely high frequency, Eye, Continuous wave operation
In this paper, a novel scheme to generate millimeter-wave with wavelength reuse by using one modulator has been
proposed. In such a system, a continuous wave lightwave is intensity-modulated via a dual-arm LiNbO3 Mach-Zehnder
interferometer modulator with DC biased at υ π and driven by two complementary modulation signals, which are
composed of a lower frequency RF cosine signal mixed with baseband signals by an electric mixer and a higher
frequency RF cosine signal. The optical carrier is suppressed and the generated optical signal mainly consists of four
sidebands, in which only two sidebands carry baseband signals. At the base station, an optical interleaver is used to filter
out the proposed optical mm-waves from the sidebands and another IL is used to filter out the wavelength reused for
uplink connection from the rest sidebands. The transmission performance of system is analyzed theoretically and
numerically.
Effect of the initial chirp on picosecond pulse breakup in the presence of noise is analyzed through numerically solving
the modified nonlinear Schrödinger equation, using the standard split-step Fourier method. It is found that, for shorter
pulses, the pulse breakup is caused by pulse collapse resulted from high-order soliton compression, even in the presence
of noise. For longer pulse, pulse breakup is triggered by nonlinear amplification of noise caused by modulation
instability. The effect of initial chirp on pulse breakup is closely related to the mechanism of pulse breakup, and the
process of short pulse breakup depends strongly on the strength of negative initial chirp.
KEYWORDS: Modulation, Eye, Modulators, Radio optics, Extremely high frequency, Signal detection, Optical filters, Single mode fibers, Signal generators, Linear filtering
We proposed and experimentally demonstrated a novel method of optical mm-wave generation using only one Mach-
Zehnder modulator (MZM) and a direct-modulator based on the method of optical carrier suppression. Using this
proposed method, the 2.5Gb/s data channel was successfully transmitted over 20 km for downstream with less than 1-dB
power penalty. Therefore the system configure of mm-wave generation can be further simplified and more cost effective.
The transmission performance of system was analyzed theoretically and numerically. According to our research, the
mm-wave generated by this scheme can be immune fiber dispersion, and the configuration of the base station can be
simplified and the cost is thus reduced.
A configuration of transmitter for superimposing DPSK signals on dark RZ signals with tunable duty
cycle was proposed. The transmitter comprises two optical phase modulators and a delayed line Mach-Zehnder
interferometer. The simulation results show that the spectral efficiency is doubled by using this scheme.
In this paper, we investigate the solitary wave propagation through the nonlinear periodic structure that consists of
alternating layers of both positive and negative Kerr nonlinear coefficients along the propagation direction, in which the
pulse dynamics is governed by the nonlinear coupled mode (NLCM) equations. Using the multiple scale analysis, the
NLCM equations are reduced into the perturbed nonlinear Schroedinger (PNLS) type equation, which incorporates both
the higher order dispersive effects and self-steepening effect. From the PNLS equation, dark solitary solutions have
been constructed by an extended Tanh-function expansion method. The effects of the physical parameters for nonlinear
periodic structure on soliton propagation are discussed.
The guided optical modes in the asymmetric slab waveguide with a core of the normal dielectric surrounded by two single-negative (SNG) materials are investigated. The condition for occurring surface waves in the SNG material is analyzed. It is found that the epsilon-negative (ENG) waveguide supports both oscillating and surface guided modes, which is a new feature that the conventional waveguide does not possess. The oscillating guided modes of TM polariton are absent of fundamental mode, while the TE polariton can support the fundamental mode. For higher frequency, larger slab thickness and constitutive parameters, the ENG waveguide accommodates more oscillating guided modes. Furthermore, it is found that the ENG waveguide only supports TM surface guided modes, while the mu-negative (MNG) waveguide only supports TE surface guided modes. The existence of various stable solutions to the surface guided modes depends on the combination of the different constitutive parameters and the structures of waveguide. Finally, the transverse profiles of the surface guided modes in three different regions are obtained.
We investigate the Goos-Hanchen (GH) shifts from an asymmetric configuration with single-negative materials by means of the stationary phase theory. The transmission and reflection coefficients for both TE- and TM-polarized incident beams are obtained using the transfer matrix method. A large GH shift was observed in the asymmetric configuration with single-negative materials when the surface polariton is properly excited for the TM polarization. The GH shift of the reflected beam is not equal to that of the transmitted beam. Furthermore, it is found that there is an optimum thickness and an optimum incident angle for the maximum GH shift time. The GH shift of the reflected beam can be detectable due to its large value and high reflectivity.
We have demonstrated an all optical switching by use of pulse trapping in photonic crystal fiber (PCF). A train of four pulses with temporal separation of 1 ps is used as the signal pulses, in which only one pulse is trapped by the soliton pulse in the PCF. The wavelength of the trapped pulse is blue shifted, and thus the trapped pulse can be picked off easily by use of a wavelength filter such as a fiber Bragg grating. In addition, The characteristics of the ultrafast all optical switching are analyzed numerically. The trapping efficiency decreases as the temporal separation increases. The low trapping efficiency impairs the performance of optical switching.
In the paper, we have numerically studied how the initial conditions influence the mode-locked soliton formation in the passively mode-locked fiber laser by using the nonlinear polarization rotation technique. We find that once the laser gain is fixed, a soliton with fixed peak power and pulse width will be formed, which is independent of the initial seed pulse conditions. Further numerical simulations have shown that both the peak power and the pulse width of the mode-locked soliton are varied with the linear cavity delay bias setting. We identified that the larger the linear cavity phase setting, the higher the soliton peak and the narrower the soliton pulse achievable in certain range, and adjustable pulse width passively mode-locked fiber laser can be formed by turning the linear cavity delay bias.
The transmission of a one-dimensional left-handed photonic crystal consisting of alternating slabs of two materials with
positive and negative refractive index respectively is investigated with the help of transfer matrix method (TTM). The
properties of defect modes of the left-handed photonic crystal were disclosed. Both the number and the location of defect modes strongly depend on the property and composition of defects. The defect modes resulted from negative
defect often appears in the zero average refractive index (ZARI) gap, while those resulted from positive defect usually
appears in the Bragg gap. The defect modes, no matter in the Bragg gap or in the ZARI gap, exhibit degeneracy and
split as the distance between the two defects decreases. Due to the coupling of the two defects, the split of defect mode
does not appear when two different kinds of defect are introduced.
Modulation instability in a fiber soliton ring laser is studied by a linear stability analysis of the underlying nonlinear
Schroedinger equation. Based on the modulation instability theory and the multisection fiber theory, we achieve the gain
flattening in the fiber soliton ring laser. We find that the perturbation gain can be improved when the erbium doped fiber
is in the normal dispersion region. When the erbium doped fiber is in the anomalous dispersion region, the perturbation
gain spectrum will evolve into a supercontinuum which is similar to a parabola as the small-signal gain of the erbium
doped fiber increases.
The paper investigate the effect and control of loss for the beam propagation in Kerr-type nonlinear optical lattices, and
the loss compensation by the longitudinal depth of the modulation analytically and numerically. It is shown that media
loss reduce the peak power of lattice soliton, decreasing the nonlinear effects, disturbing the balance between the
diffraction and nonlinear effects. As a result, lattice soliton can't propagate steadily and optical beam dispersed finally. It
is possible that proper changing the longitudinal lattice depth can complete suppression of the loss's influence and the
beam can be regenerated successfully. There are analogy characteristic in the action of periodic lattice's potential of
optical and nonlinear. The periodic potential offers the better method to control the lattice soliton formation and
propagation.
The effect of initial frequency chirp is theoretically investigated in photonic crystal fibers with two zero-dispersion wavelengths. Based on numerical simulations, it is shown that there exits an optimal propagation distance whether input pulse is chirped or not, where spectrums have maximal bandwidth. Furthermore, contrary to the complicated spectrum structure generated by negative-chirped pulse, positive linear chirp enhances supercontinuum generation and spectrum is much more regular. The efficiency of four-wave mixing is also improved because of initial positive chirps, and for enough large chirp values the incident pulse energy in the anomalous dispersion region is almost thoroughly transferred to the visible and near-infrared regions outside two zero-dispersion wavelengths.
A theoretical investigation of the pulse trapping in a birefringent photonic crystal fiber is presented. The strict coupled
nonlinear Schroedinger equations are solved numerically using a standard split-step Fourier algorithm. Different
phenomena of pulse trapping are observed depending on whether the two pulses are polarized along the same
polarization axes. The signal pulse suffers cross phase modulation (XPM) from the pump pulse and it is trapped to
copropagate with the pump pulse along the fiber. Much larger wavelength shift of the trapped signal pulse can be
obtained by increasing the power of pump pulse. Different initial temporal separation between the pump pulse and the
signal pulse leads to different effect in the pulse trapping.
The characteristics of femtosecond pulse trapping in photonic crystal fibers are investigated. Numerical simulations show that the efficiency of pulse trapping can be improved by adjusting the parameters of pump pulse. For a given pulse width, the trapping efficiency is fluctuant as the temporal delay increases and has a maximal value. The blue-shift of signal pulse and the red-shift of pump pulse, however, have no matter with temporal delay. The maximal trapping efficiency and the corresponding temporal delay both decrease as the pulse width increases.
We adopt the worst channel equalization (WCE) scheme to compensate polarization mode dispersion (PMD) in wavelength-division-multiplexing (WDM) systems. The degree of polarization (DOP) of single state of polarization (SOP) optical signal is used as the feedback signal of the PMD compensators, and the particle swarm optimization (PSO) algorithm is used as the searching algorithm. Simulation results show that the DOP of RZ format of the worst-performance channels after compensation is greatly improved, and the PSO algorithm is successfully experienced into adaptive multi-stage PMD compensators in a 40-Gb/s optical WDM communication system. The WCE scheme is an effective way for PMD compensation in WDM system.
We propose a polarization mode dispersion (PMD) compensation scheme for wavelength-division-multiplexing (WDM) system, in which two WDM channels share one PMD compensator at the receiver site. The effect of different modulation formats on multi-stage PMD compensators is studied and compared by numerical simulations in 40-Gb/s WDM optical fiber communication system. The degree of polarization (DOP) of single state of polarization (SOP) optical signal is used as the feedback signal in PMD compensators. The particle swarm optimization (PSO) algorithm is used as the searching algorithm in WDM systems. The compensated DOP values of return-to-zero (RZ) format and nonreturn-to-zero (NRZ) format by multi-stage PMD compensators have been increased distinctly compared to the corresponding cases without compensation. It is shown that the PSO algorithm is implemented successfully in adaptive multi-stage PMD compensation in a 40-Gb/s optical WDM system. The compensated eye diagrams for the two channels by multi-stage PMD compensators indicate that the three-stage PMD compensator which eliminates the influence of second-order PMD completely in WDM system takes the best efficiency in the multi-stage PMD compensators for RZ format and NRZ format.
We present a theoretical investigation of the nonlinear propagation of an optical pulse in a birefringent photonic crystal fiber (PCF). The strict coupled nonlinear Schroednger equations are solved numerically using a standard split-step Fourier algorithm. The phenomenon of pulse trapping is observed for different polarized angles except 0 degree and 90 degree, when the central wavelength of the input optical pulse is located in the anomalous dispersion region. With a single pulse which inclines from one axis launched into a birefringent PCF, the input pulse is split into two orthogonally components (signal and pump component) between the two orthogonally axes. The signal pulse suffers cross phase modulation by the pump (Raman shifted soliton) pulse and it is trapped and copropagates with the Raman soliton pulse along the fiber. A minimum trapping efficiency is obtained when the polarized angle is at 45o. For two complementary polarized angles, higher trapping efficiency can be obtained for smaller angle. As the input power of pulse is increased, the red-shift of the Raman soliton is considerably enhanced, leading to further red-shift of the trapped pulse to satisfy the condition of group velocity matching.
We propose an adaptive polarization-mode dispersion (PMD) compensation system in which the minor axis of the degree of polarization (DOP) ellipsoid in receiver is used as feedback signal, and the length of the DOP ellipsoid is optimized through the particle swarm optimization (PSO) searching algorithm to compensate PMD. The operation of adaptive equalization is successfully demonstrated by 40 Gb/s transmission system numerically. Furthermore, we show that using DOP ellipsoid as feedback signal and PSO as searching algorithm, the first and higher order PMD can be compensated effectively.
We study the transmission of a one-dimensional ternary left-handed photonic crystal which is consisting of three alternating slabs in the form of {ABC}, where A and C represent two kinds of positive- refractive-index materials, and B represents the negative-refractive-index material. Firstly, we obtain the dispersion equation based on the boundary conditions and Bloch theorem. By construing the obtained dispersion relation theoretically we demonstrated that a zero averaged refractive index (ZARI) gap which is around the frequency where the average refractive index vanishes appears. Secondly, we investigate the dependence of the transmission on the thickness of slabs by changing the thickness of slab A, B and C respectively. We find that the transmission of such a structure can be made tunable. This novel property may be very useful in designing tunable filters because the position of band gaps in such a structure can be chosen expediently. Furthermore, we study the property of defect mode of such a structure, and find that the defect mode doesn't always appear simultaneously in both Bragg and ZARI gaps of the transmission spectrum. It can be selected to appear either in the Bragg gap or in the ZARI gap.
We study the performance of all-optical switching in long period fiber grating based on the coupled nonlinear Schrodinger equations. The switching rate of long period fiber grating in the on-resonance and off-resonance case is obtained, respectively. It is found that in both cases, the on-off ratio of all-optical switching in long period fiber grating can be improved remarkably, and the phenomenon of pulse breakup can be avoided effectively, by using the higher order
super-Gaussian pulses.
We present a theoretical investigation of the ultrafast all optical switching based on pulse trapping in photonic crystal fibers. We numerically solve the coupled nonlinear Schrodinger equations for two independent ultrashort pulses propagation in photonic crystal fiber using a standard split-step Fourier algorithm to analyze the phenomenon of pulse trapping across the zero-dispersion wavelength. It is shown that one pulse (for example the second one) of the signal pulse train propagating in the normal dispersion region can be trapped by an ultrashort soliton pulse propagating in the anomalous dispersion region. The soliton and trapped pulse co-propagate along the fiber. The soliton pulse is red-shifted due to soliton self-frequency shift and the trapped pulse is blue-shifted to satisfy the group velocity matching. Only the second pulse among the signal pulse train is trapped by the soliton pulse and the optical spectrum of the trapped pulse is distinctly blue-shift through the cross phase modulation and is separated from the untrapped ones, thus it can be picked out easily by use of a wavelength filter such as a fiber Bragg grating. The ultrafast all optical switching with a 1THz repetition frequency is confirmed directly. As the input peak power of the pump pulse increases, the red-shift of the soliton is considerably enhanced with the simultaneous further blue-shift of the trapped pulse to satisfy the condition of group velocity matching.
We study modulation instability (MI) in the fiber Bragg grating with nonlinearity management based on the coupled-mode theory. The role of both average Kerr nonlinearity and variance of Kerr nonlinearity between the layers of fiber grating in MI is identified. It is found that the variance of Kerr nonlinearity affect MI gain spectrum remarkably in both anomalous dispersion and normal dispersion regimes. In the anomalous dispersion regime, when the variance of Kerr nonlinearity is much smaller than the average Kerr nonlinearity, the MI gain spectrum is similar to that without the variance of Kerr nonlinearity, but the range of wave number for MI to occur is narrowed, and the amplitude of gain decreased. When the variance of Kerr nonlinearity is enhanced to be equivalent to the average Kerr nonlinearity, the role of variance of Kerr nonlinearity in MI becomes important: At low intensity, the range of wave number for MI to occur shrinks notably, and the gain gets only a single peak compared with the original one which has two symmetrical side-bands. At high intensity, there appear three MI ranges. In the normal dispersion regime, near the lower edge of photonic band gap, the amplitude of MI gain is slowed down due to the influence of variance of Kerr nonlinearity, and only two small symmetrical MI range appear, in sharp contrast to the original case without the influence of variance of Kerr nonlinearity, in which MI occurs for all wave numbers. Whereas in the case that far away from the edge of photonic band gap, we find that the range of wave number for MI to occur and the amplitude of MI gain increase as the value of variance of Kerr nonlinearity increases.
The nonlinear Schrodinger equation with variable coefficients is analyzed by means of projection matrix method. An exact analytical solution is obtained, which clearly shows how the variable fiber dispersion, nonlinear, and loss coefficients affect the propagation of ultrashort optical pulses. The obtained solution is used to analyze the propagation properties of ultrashort pulses in dispersion-decreasing fibers. It is found that the ultrashort pulse can realize stable soliton transmission if the fiber dispersions have some certain profiles related to the fiber loss and nonlinear properties. A small variation in the dispersion has a similar perturbative effect to an amplification or loss. The exponentially dispersion-decreasing fiber is studied exemplificatively to demonstrate the obtained results.
We investigate modulation instability (MI) in the distributed fiber amplifier based on a modified Ginzburg-Landau equation. The role of gain dispersion and stimulated Raman scattering (SRS) in MI is identified. It is found that, due to SRS, the MI gain spectrum consists of two parts: the conventional MI gain spectrum and the Raman gain spectrum. Gain dispersion exerts little influence on the conventional MI spectrum, yet it deforms the Raman gain spectrum seriously, mainly by reducing its growth rate. Moreover, as the signal power increases, the bandwidths of the conventional MI and the Raman gain spectrum are simultaneously extended, with the latter spectrum being extended more quickly.
Supercontinuum generation is usually the synergy of some fundamental sophisticated nonlinear processes. Although it is well known that self-phase modulation, stimulated Raman scattering and four-wave mixing etc., are the dominant contributors, the comparative importance of these effects is still blurry to some extent. Based on the extended nonlinear Schrödinger equation describing pulse propagation in the microstructured fiber, we have identified the role of higher-order effects in supercontinuum generation. It is shown that in the initial formation of the supercontinuum self-phase modulation plays a main role while all the higher-order effects play a minor role. After a certain propagation distance the spectral width saturates, suggesting that the supercontinuum in microstructured fiber has a maximum spectral width. During the saturation stage, the higher-order effects exert more important influence on supercontinuum, mainly changing its shape. The third-order dispersion leads to spectral asymmetry, while SRS leads to a red shift of the supercontinuum spectrum. Furthermore, based on our theoretical results, we have discussed the way to controlling supercontinuum generation by using the nonlinear processes of microstructured fiber.
We propose a new method for ultrashort light pulse generation. This method is based on the spatiotemporal instability in dispersive Kerr medium. By formulating a linear instability analysis based on the (3+1)-dimensional nonlinear Schrodinger equation including arbitrary higher-order dispersions, we have disclosed the generic role played by arbitrary higher-order dispersions in spatiotemporal instability. We find that the fourth-order dispersion leads to the appearance of new instability regions which can extend far away from the original instability region. In addition, due to the fourth-order dispersion, temporal instability can develop for any spatial frequencies, and by increasing the spatial frequency, the corresponding temporal instability frequency can be easily increased. Numerical simulations show that the new instability regions can be used to generate a sequence of ultrashort light bullets. The repetition-rate of the generated bullets can be made quite large by increasing the corresponding spatial modulation frequency locating in the new instability region resulted from the fourth-order dispersion.
In this paper a method for obtaining a high-power ultrashort pulse
with ultrabroad bandwidth has been investigated. A theoretical
investigation of the propagation and supercontinuum generation of
the highly chirped pulse having been stretched and amplified are
presented by numerical simulations in microstructured optical
fiber. Comparing with the case of a chirp-free pulse, a chirped
pulse with the same peak power and initial bandwidth can produce a
broader supercontinuum spectrum with higher average power. After a
felicitous compression, a high power ultrashort pulse that has an
ultrabroad bandwidth much larger than that of the pump pulse can
be produced.
In this paper we find the spectral bandwidth is more important than the duration of pulse for analyzing the evolution of ultrashort laser pulses. A general novel (3+1) dimensional propagation equation first-order in the propagation coordinate is derived, which is independent of the envelope and carrier frequency. This equation provide an accurate description of the evolution of the ultrashort pulsed beam through the dispersive nonlinear medium, with the numerical value of the bandwidth being bigger than the carrier frequency and considering the true frequency. A family of exact solutions of the novel linear (3+1) dimensional equation in the dispersive medium has been derived, which represents the pulse with ultrawide bandwidth evolving due to gain (losses), dispersion, and diffraction. The use of the novel dispersive and loss coefficients is analyzed.
We report on the theoretical investigation of the amplification of highly chirped pulse with ultrabroad bandwidth. Based on the analysis of the properties of the pulse with a very big chirp, we build a theoretical model for the amplification of highly chirped pulse. The model includes the effects of homogeneous and inhomogeneous broadening. Based on our model, the difference
between the inhomogeneously and homogeneously broadening amplification is identified by numerical simulations. We believe that the obtained model can be used to evaluate the laser-performance, and optimize the laser design.
We use the standard linear stability analysis to study the role of stimulated Raman scattering in spatiotemporal instability in dispersive Kerr medium based on an extended (3+1)-dimensional nonlinear Schrodinger equation. We show that the spatiotemporal instability gain spectrum consists of two parts: the conventional spatiotemporal instability gain spectrum and the Raman gain spectrum. Stimulated Raman scattering doesn't affect the conventional spatiotemporal instability gain spectrum; yet it provides additional sidebands which have almost the same gain spectrum in all combinations of the signs of group velocity dispersion and nonlinear refractive index. It is interesting that spatiotemporal instability gain can appear for any spatial frequencies in the presence of stimulated Raman scattering, in sharp contrast to conventional spatiotemporal instability whose gain is located in a limited spatial frequency range.
We apply the variational approach to solve the nonparaxial nonlinear Schrodinger equation to disclose the nonparaxial propagation properties of a Gaussian beam. A system of differential equations for the evolution of the parameters of Gaussian beam is obtained. The obtained analytical results clearly show the nonparaxial propagation process of periodic focusing-defocusing. This process is significantly influenced by the initial power and chirp of the beam. A positive chirp retards the first self-focus, while a negative chirp brings forward the first self-focus. Both positive and negative chirps increase the subsequent focusing-defocusing cycles.
We study the phenomena of self-focusing and self-defocusing of an optical beam in a nonlinear negative-refractive-index material (NRM). It is shown that an optical beam propagating in NRM experiences self-focusing and self-defocusing for negative and positive nonlinear refractive index respectively. The critical power for self-focusing is the same as that in a conventional nonlinear positive-refractive-index material. Further, we show by numerical simulations that the nonlinear NRM can be used to compensate for self-focusing in the propagation of high power lasers in conventional nonlinear positive-refractive-index medium. The modulation generated by self-focusing in nonlinear positive-refractive-index medium can be reduced drastically by inserting a piece of nonlinear NRM.
KEYWORDS: Modulation, Laser beam propagation, Near field diffraction, High power lasers, Free space, Wave propagation, Pulsed laser operation, Diffraction, Laser systems engineering, Light wave propagation
We report on a numerical investigation of propagation property of high-power broadband laser beam in free space. We use Hankel transformation to solve Helmholtz equation numerically in the frequency domain. The modulation property of the broadband laser beam propagating in the Fresnel diffraction region is disclosed. It is found that the number of diffractive modulation rings equals to half the Fresnel number corresponding to the central wavelength of the broadband laser beam. The intensity modulation contrast of the broadband laser beam is smaller than that of monochromatic laser beam, suggesting that the broadband laser beam can suppress the harmful effect of near-field diffraction to some extent. Furthermore, it is demonstrated that the uniformity of near-field diffraction profile is determined by Fresnel number and bandwidth of the broadband beam.
We report on a theoretical analysis of propagation of monochromatic and broadband high-power laser beam through spatial filter. The law of formation of low-frequency modulation rings due to diffraction is obtained. It is found that the filling factor or the intensity modulation contrast of the laser beam is sensitive to times of diffraction limit of the filter’s pinhole. The uniformity of beam can be significantly improved by adjusting times of diffraction limit. For the same filter parameters, the broadband beam has a more uniform diffraction profile than the monochromatic beam, suggesting that the laser beam with a proper amount of bandwidth can suppress the harmful effect of diffraction to some extent.
We report on a theoretical analysis of a channel dielectric waveguide consisting of periodically interlaced negative refractive index medium and positive index medium. The dispersion relation for TM modes is obtained. The properties of guided wave modes have been numerically disclosed. It is shown that the waveguide can exhibit negative group velocity dispersion as well as extraordinarily large group velocity dispersion. The Poynting vector within the waveguide can change both sign and magnitude. The distribution of energy flux density of different sections relates to the frequency and the width of the channel waveguide.
The generic features of modulation instability (MI) in optical fibers are disclosed by application of an extended nonlinear Schroedinger equation. The role of arbitrary higher-order dispersions, stimulated Raman scattering (SRS) and self-steepening (SS) in MI is identified. It is shown that all odd-order dispersions contribute nothing to MI, whereas all even-order dispersions not only affect the conventional instability regions but may also lead to the appearance of new
MI regions. In the presence of SRS, the MI gain spectrum in optical fibers consists of two parts: the conventional MI gain spectrum and the Raman gain spectrum. In the case of normal dispersion, MI occurs due to SRS. In the case of anomalous dispersion, as the initial power increases, the SRS gain spectrum is gradually screened from the conventional MI gain spectrum. Self-steepening exerts little influence on MI in both normal and anomalous dispersion regimes.
Numerical simulation confirms the obtained analytical results.
Influence of space-time focusing on spatiotemporal instability in dispersive self-focusing media is investigated on the basis of a modified nonlinear Schrodinger equation (NLSE) beyond the slowly varying envelope approximation. It is found that, for both normal and anomalous dispersions, space-time focusing may lead to the appearance of new instability regions. Space-time focusing influences the original spectra obtained using the standard NLSE mainly by shrinking the instability regions, especially in the case of normal dispersion.
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