We report our recent progress of solid-state lasers, the titanium-doped sapphire laser and the praseodymium-doped LiYF4 (Pr3+:LiYF4) laser, directly pumped by InGaN diode lasers of green and blue. While the titanium-doped sapphire laser can be pumped by both blue and green diode lasers because of its broad absorption spectrum, we find 450-nm diode laser pumping inefficient for its power scaling owing to the onset of an additional absorption loss induced by the blue pump laser. The degradation of the performance of a Ti:sapphire laser induced by a 450-nm diode pumping is experimentally revealed. Furthermore, we demonstrate a passively mode-locked Ti:sapphire laser pumped by two green diode lasers of 1-W, and 74 fs pulses at a repetition rate of 120 MHz with an averaged output power of 45 mW are obtained. The Pr3+:LiYF4 laser, one of the most promising candidates as a visible laser, has become possible to be pumped directly by blue diode lasers of 440-nm wavelength. We demonstrate the power scaling at 523 and 640 nm oscillation, a passive Q-switching at 640 nm with a Cr4+:YAG saturable absorber by utilizing polarization-combined blue diode lasers as the pump source.
Talbot cavity is passive method to synchronize the phase of array lasers. Because the Talbot cavity does not need any electrical feedback systems, we believe that Talbot cavity is the most suitable technique to combine a considerable number of laser array into a compact system. A well-known drawback of the Talbot cavity is that it can produce out-phased array and their far-field image has 2-peak profile. To solve this drawback, we developed a frequency doubled laser array based on intra-Talbot-cavity second harmonic generation. Basic concept is second harmonic generation of the out-phased array generated from the Talbot cavity. Because the second harmonic wave is generated proportionally to the square of the fundamental wave, out-phase flips to in-phase. Our Talbot cavity is composed of a pumping 808-nm laser diode array with 15 emitters, an Nd:YVO4 planar waveguide, a PPLN planar waveguide, an f =10 cylindrical lens, and an output coupler (high reflection for 1064 nm and high transition to 532 nm). The pump laser beams are directly launched into the Nd:YVO4. The fundamental wave (1064 nm) oscillates between the Nd:YVO4 and the output coupler and generates second harmonic wave (532 nm) at the PPLN placed next to the Nd:YVO4. The round-trip optical path of the cavity length is set to 1/2 Talbot length so that Talbot cavity forms for the fundamental wave. As a result, we obtained 1-peak far-field image of second harmonic wave from the intra-Talbot-cavity second harmonic generation.
Simultaneous spatial and temporal focusing (SSTF) multiphoton microscopy offers us widefield imaging with sectioning ability. As extending the idea to 2D SSTF, people can utilize a 2D spectral disperser. In this study, we use a 2D spectral disperser via a virtually-imaged phased-array (VIPA) and a diffraction grating to fulfill the back aperture of objective lens with a spectrum matrix. This offers us an axial resolution enhanced by a factor of ~1.7 compared with conventional SSTF microscopy. Furthermore, the small free spectral range (FSR) of VIPA will reduce the temporal self-imaging effect around out-of-focus region and thus will reduce the out-of-focus multiphoton excited fluorescence (MPEF) signal of 2D SSTF microscopy. We experimentally show that inside a sample with dense MPEF, the contrast of the sectioning image is increased in our 2D SSTF microscope compared with SSTF microscope. In our microscope, we use a 1 kHz chirped amplification laser, a piezo stage and a sCMOS camera integrated with 2D SSTF to realize high speed volume imaging at a speed of 50 volumes per second as well as improved sectioning ability. Volume imaging of Brownian motions of fluorescent beads as small as 1μm has been demonstrated. Not only the lateral motion but also the axial motion could be traced.
Spectroscopic optical coherence tomography (OCT) is accomplished by analyzing fringe-resolved coherent functions that are obtained by Fourier transform of spectral interferogram. By use of this analysis, spectral absorption and dispersion properties are obtainable. We also constructed a simple model and discussed the accuracy of this technique.
The timing jitter measurement scheme for low repetition rate pulse train is proposed. Measuring a spectrum of upconverted light generated from frequency mixing with a transform limited reference pulse and a linear-chirped amplified pulse, a relative time lag between two pulses can be obtained. The measurable range and resolution with this method are discussed.
Brighter far-field pattern of a phase-locked Nd:YAG microchip laser array was obtained by the Talbot effect in a three-mirror cavity. The laterally coupled Nd:YAG microchip laser array produced a pair of spots with sharper peaks with an angular separation of (lambda) /d in its far-field, where d is the distance between the adjacent sources, indicating an out-of-phase spatial mode coupling was improved. The far- field spot size is reduced by a factor of 9.3 compared with that obtained by incoherent pile of the individual microchip laser outputs. We demonstrated that the far-field pattern is improved by enhancing the self-imaging with side mirrors which make the array source disguise an infinity. A mode selecting slit placed right after the crystal is also effective to stabilize the out-of-phase mode.
A novel pump-noise-suppressed semiconductor laser scheme is proposed. In this scheme, electrons injected into a stable intersubband state dressed by strongly resonant optical couplings stimulate amplitude-squeezed light with extremely low-threshold. Because of its high conversion efficiency and short stimulated emission lifetime at low injection current regime, this scheme can generate weak broad-band amplitude- squeezed light. We numerically estimate the characteristics of gain coefficient and output noise spectrum of this scheme.
Numerical studies of the dynamic transfer of one chaotic oscillation appeared in a laser diode, which is subject to weak optical feedback, to a variety of periodic oscillations by modulating the injection current of the laser diode, are described. We evaluate also the response of periodic attractors in the stabilized oscillations by calculating statistic distributions of the transient time among those attractors. The average transient time is approximately 21 ns, which is longer than the full round time in the controlled periodic attractors.
We propose a new spatial-phase-code division multiple access system based on Fourier-holography switching. The proposed scheme was experimentally verified with double-multiplexed hologram.
A Nd:YAG microchip laser array is developed and the spatial phase correlation among each the microchip laser is attained utilizing the Talbot effect in a three-mirror cavity configuration. The laterally coupled Nd:YAG microchip laser array produces a pair of spots with angular separation of (lambda) /d in its far field, where d is the distance between the adjacent sources, corresponding to an out-of-phase spatial mode coupling. The far field spot size is reduced by a factor of 4.7 compared with that obtained by incoherent adding of the individual microchip lasers. Although an external mirror positioned at the 1/4 Talbot distance from the microchip lasers source is very effective to introduce the coherence among the laser chips, the present long Talbot distance limits the output laser power due to the extremely high diffraction loss in the auxiliary cavity. Smaller beam sizes for each microchip lasers will decrease the filling factor and improve the efficiency in the Talbot cavity.
The Kerr-lens mode-locked (KLM) laser with a novel five- mirror cavity is theoretically examined by applying the ABCD-matrix formalism for a Gaussian beam. Since the optimum configuration of the five-mirror cavity is obtainable at the middle of confined cavity condition, one can easily achieve the KLM alignment and stable mode-locking operation. Influence of the self-focus appearing in the five-mirror cavity configuration upon a semiconductor saturable-absorber mirror is also analyzed.
A novel, wide-range, high-speed, and tunable wavelength conversion scheme, 'a fiber Raman converter,' is proposed, in which an externally injected high power pump laser and the associated Stokes laser are used to assist the Raman conversion process of signal light coded with optical information. In order to get a large frequency difference between two carrier frequencies, this fiber Raman process is cascaded twice. However, since the common external pump laser can be used in two cascaded Raman processes as long as phase- matching conditions are attained, the entire configuration is still simple. We numerically demonstrate that wide-range wavelength conversion from 1.31 micrometer to 1.55 micrometer for optical fiber communication is feasible at up to 5 Gbit/s.
We propose and analyze a novel low-threshold amplification scheme by bichromatically coupling three upper states including a metastable state. We numerically show that it is possible to amplify VUV emission in Kr gases without population inversion at the probe transition. It is shown that the origin of low-threshold gain is the population trapping of upper states dressed by two intense coupling lasers, and there is a possibility of forming large population inversion in optically dressed systems.
A novel scheme to generate a subnanosecond pulse in VUV spectral region is proposed, by applying the dual-wavelength pumping method in conjunction with the polarization switching technique to the stimulated rotational Raman scattering process. The optimum conditions for the short pulse generation are presented through the numerical calculations. The proposed process is applicable to a seed pulse generation for subnanosecond pulse amplification by the pump laser in the VUV region.
The effectiveness of low-intensity light for clinical therapies is well demonstrated, however, the principles of these therapies are not well understood. Our aim of this study is the investigation of optical properties of mitochondria because it seems to be the most important cellular organella in terms of the light-cell interaction. We measured the optical absorption characteristics of an isolated mitochondrial suspension in the range of 300 to 900 nm by means of the transmission methods with an integral sphere. Mitochondrial specific absorption coefficients (ml/g/cm-1) were carried out by the transmission spectrum. The specific absorption coefficient at the maximum absorption of cytochrome c(gamma ) peaked about 560 ml/g/cm-1 at 418 nm. Many papers described that the effect of the low intensity light to the biological tissue would be drastically changed by the irradiation conditions. We could calculate a deposit energy of irradiated light in a mitochondrion from our results. We think our results may be useful to describe the quantitative explanation in energy balance for the interaction between the light and mitochondria.
Output characteristics of a transverse discharge pumped atomic Ne laser (585.3 nm) using H2 as a penning partner has been studied. The laser energy is decreased with increasing the operating pressures. Lasing is terminated at 250 Torr. Peak intensity of the laser is saturated at high excitation rates (1 MW/cm3(DOT)atm). Mixing He as a buffer gas with Ne/H2 mixtures leads to depletion of the laser energy.
We demonstrated a high energy delivery of a pulsed wavelength-selected HF chemical laser by fluoride glass fibers (core/cladding equals 450/500 micrometers ). The optical energy of 19 mJ in a 540 ns pulse was successfully delivered with a peak intensity of 22 MW/cm2 at the exit core surface. We have also theoretically investigated the operational characteristics of the pulsed chain first vibrational overtone HF chemical laser using a computer code. The model used can describe simultaneously both the fundamental and overtone oscillations. The higher overtone output energy of 2.73 J/l can be obtained with a gas mixture consisting of F2/H2/He equals 10/4/786 (Torr) by successfully suppressing the ASE.
A new method of anti-Stokes Raman conversion termed dual-wavelength pumped Raman- resonant four-wave mixing, where a relatively intense secondary pump is applied in the phase- matched direction to enhance anti-Stokes conversion of a main pump, is proposed. Conditions for optimizing the conversion efficiency are also investigated. A remarkable feature of this method is that the optimum conversion efficiency is independent of the initial intensity of the main pump. It has been shown that nearly 60% of anti-Stokes conversion efficiency is theoretically possible in the vacuum ultraviolet region.
Experimental results of spectroscopic measurement of spontaneous emission from argon gas excited electrically in a nozzle discharge followed by supersonic expansion are described. A strong line emission at 152.6 nm originating from Si(ll) 4s-3p is observed together with relatively weak emission line around 126 nm. A theoretical model that takes account of collisional gas kinetics, discharge processes, and empirical treatment of the free jet expansion is developed to examine the capability of the discharge excited gas as a laser medium for Ar2 excimer at 126 nm wavelength.
Energy extraction in XeCl amplifier is theoretically studied for 0.16- and 3-ps
initial laser pulse durations. Since an amplification gain is defined by the degree
of matching between a gain spectrum and a propagating laser pulse spectrum, when a
laser pulse is amplified by a gain medium with a multilined and inhomogeneously
broadened spectrum, the saturation behavior of the amplified energy depends on
characteristics of change in the laser spectrum during the amplification. Therefore,
a saturation energy cannot be simply defined as a unique value for certain initial
laser pulsewidth, and depends on amplification history of the pulse, amplified laser
energy level, and initial carrier freqency. When a laser pulse is amplified to
extremely high energy level ( > 5 mJ/cm2 ), the laser pulse spectrum, which is filtered
by the gain spectrum in small-signal amplification regime, is nonlinearly distorted
and tends to shift its peak to the red spectrum side.
KEYWORDS: Chemical lasers, Reflectivity, Hydrogen fluoride lasers, High power lasers, Energy efficiency, Mirrors, Laser energy, Laser optics, Computer simulations, Resonators
We have theoretically investigated the output performance of the pulsed discharge initiated H2/SF2 nonchain and H2/F2 chain first vibrational overtone HF chemical lasers.
By exciting 10-atm He/F2 gas mixtures by electric discharge at 66MW/cm3, a specific output energy of 2.9J/1 (83mJ) has been obtained in the F2 laser (157.6nm). To extend an available laser wavelength region with high output laser energies in the vacuum ultraviolet, frequency conversion of the laser radiation by stimulated Raman scattering in H2 has been demonstrated for the first time. The laser energy focused into a Raman cell was about lOmJ (22ns FWHM), which corresponds to the focused intensity of 141MW/cm . At H2 pressure of 5atm, 1st and 2nd Stokes lines (168.6nm, 181.4nm) and 1st anti-Stokes line (147.9nm) were observed. The energy conversion efficiency was ~10% for the 1st Stokes line, ~ 2% for the 2nd Stokes line, and ~ 0.8% for the 1st anti-Stokes line at a l0atm H2.
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