We have built and tested a highly efficient source of pulsed 320 nm light based on intra-cavity sum-frequency-generation in a self-injection-seeded image-rotating nanosecond optical parametric oscillator. The four-mirror nonplanar ring optical cavity uses the RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle. The cavity contains a type-II xz-cut KTP crystal pumped by the 532 nm second harmonic of Nd:YAG to generate an 803~nm signal and 1576 nm idler, and a type-II BBO crystal to sum-frequency mix the 532 nm pump and cavity-resonant 803 nm signal to generate 320 nm light. The cavity is configured so pump light passes first through the BBO crystal and then through the KTP crystal with the 320 nm light exiting through the output coupler following the BBO sum-frequency crystal. The cavity output coupler is designed to be a high reflector at 532 nm, have high transmission at 320 nm, and reflect approximately 85% at 803 nm. With this configuration we've obtained 1064 nm to 320 nm optical-to-optical conversion efficiency of 24% and generated single-frequency λ = 320 nm pulses with energies up to 140 mJ.

We demonstrate the use of spectral angular dispersion in quasi-phase-matched second-harmonic generation (SHG) of 138 fs pulse at 1550 nm in a 1-cm-long crystal of periodically-poled lithium niobate (collinear acceptance bandwidth 13 times narrower than the first-harmonic bandwidth) with minimal spectral narrowing. In addition, we discuss the unique potential of quasi-phase-matched nonlinear materials in noncollinear SHG, and compare this technique to other methods for broadband frequency conversion.

We propose a method to compensate for group-velocity mismatch (GVM) effects that limit the efficiency-bandwidth product in optical frequency (OF) mixers. Integrated wavelength-dependent delay lines are introduced in a waveguide containing a series of QPM gratings. Appropriate choice of the time delays can compensate for GVM. We have demonstrated a two-stage quasi-group-velocity matching (QGVM) device in a periodically-poled lithium niobate waveguide. Two approximately 150-fs-long pulses generated 6 ps apart by SHG in two QPM gratings were re-synchronized by the fixed delay line, and their relative phase fine controlled by temperature tuning. The technique, which can be iterated to more than two segments, enables OF mixers of higher efficiency-bandwidth product than would be possible in a single grating short enough to evade GVM effects.

Pulsed optical parametric oscillators (OPO) are powerful sources of broadly tunable coherent radiation that covers the wide spectral range from the near ultraviolet (UV) to the infrared. Here, we investigate an optimum reflectivity of the output coupler for the signal wave in order to achieve the efficient ultraviolet LiB₃O₅ (LBO) optical parametric oscillator pumped by the fourth harmonics of a Q-switched Nd:YAG laser with 25-ns pulse duration and repetition rate of 10 Hz. Our LBO crystal of a rectangular shape has an aperture of 6 x 6 mm² with an interaction length of 10 mm, and it is cut for the non-critical phase matching of the type-II. The simple singly-resonant oscillator (SRO) configuration with two flat mirrors, M1 and M2, is used. The LBO crystal is mounted in the resonator in such a way that the wavelength of the signal wave is fixed at 313.5 nm. The input coupler M1 with high transmittance for the pump wave (T > 95%) is highly reflective (R > 99%) for the signal wave. In order to study the characteristics of signal output energy under the various conditions of output coupler M2, three kinds of M2 with different reflectivities for the signal wave are used. It is found that the maximum conversion efficiency as high as ~20% is achieved by using the dichroic output coupler of reflectivities for the signal wave of 65% and pump wave of 99%, respectively. This value is the highest efficiency in UV SRO among those ever reported to our knowledge.

Techniques are presented to model optical nonlinear frequency conversion of highly distorted beams with M-squared values as high as 30. Random superpositions of Gaussian-Hermite modes are used to create the field distributions of the incident beams. Split-step Fourier transform techniques are used for the calculation of nonlinear conversion.

Periodically poled, nearly-stoichiometric lithium tantalate has used to generate visible radiation (by second-harmonic generation using Nd:YAG laser) and mid-infrared radiation (by difference-frequency generation using a Nd:YAG laser and a tunable telecommunications-band laser). Phase-matching conditions have been measured for both interactions at temperatures between 25 degrees Centigrade and 131 degrees centigrade. The absolute conversion efficiency for SHG has been measured and used to derive an effective nonlinear optical coefficient for this process in the periodically poled material. These results can be used to guide the design of laser systems based on nonlinear optical frequency conversion in periodically poled nearly-stoichiometric lithium tantalate.

A continuous and tunable operation from 1.6 to 4.5 THz has been demonstrated using a difference frequency generation (DFG) scheme by using the nonlinear optical crystal 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) as the DFG crystal. The scheme was based on DFG between two seeded optical parametric generators. The THz bandwidth was 2.4 GHz. A high-resolution transmission spectrum of water vapor in air was used to demonstrate the utility of this source over the above THz spectral range.

We present experimental measurements and a corresponding theoretical analysis of a novel NOLM device made using a symmetrical (50/50) coupler, highly twisted fiber, and a quarter-wave (QW) retarder plate in the loop. The physical mechanism for the nonlinear properties is the polarization rotation of the counter-propagating optical fields. We also experimentally demonstrate that the nonlinear polarization rotation analysis is correct by controlling the transmission behavior as the QW retarder plate is rotated. We propose a simple description of the NOLM behavior, showing that nonlinear switching is obtained through the polarization asymmetry generated by the QW retarder plate. The proposed NOLM design is very attractive for applications like pedestal suppression and amplitude regularization of optical signals, since it operates stably without day-to-day drifting. We experimentally demonstrate the efficiency of the NOLM for high-order amplitude regularization of an optical pulse train subject to amplitude modulation, as an overall suppression of about 20 dB of the modulation was obtained over all frequencies. An environmentally stable NOLM will enable a wide range of applications, such as, optical switching and demultiplexing, all-optical active and passive mode-locking, and pedestal suppression.

This paper reports on a trace gas monitor using a dual wavelength coherent light source. The laser spectrometer is based on difference frequency generation (DFG) techniques in a periodically poled lithium niobate (PPLN) crystal, naturally having two laser sources (i.e., signal and pump laser sources) in it. A newly proposed idea is to use residual signal DFB-LD power to tune to NH₃ absorption lines around at 1.536 μm when the DFG output wavelengths are accessed at 3.46 μm. In the experiments, the signal DFB-LD was carefully tune the wavelength guided by HITRAN database in order to select appropriate mid-IR absorption lines of NO₂, while the DFB-LD can detect simultaneously NH₃ absorption lines. As results, both gaseous species were detected successfully with the same multi-pass optical cell in one DFG spectrometer scheme. Spectroscopic data both in near-IR and in mid-IR region, simulating flue gas condition are discussed in detail.

We report on the stabilization of a semiconductor laser’s frequency, using spectra-controlled etalon. As the spectra of an etalon are controlled by one of the Rb absorption lines, they provide highly stable reference frequencies in a broad frequency range. When we adapted the PEAK method to the etalon’s spectra and used a Doppler-free absorption line of Rb atoms as the control signal for the newest model of our system, relative optical frequency stability of 2.91x10^-11≤σ(2,τ)≤3.72x10^-10 was achieved in averaging time for 0.04s≤τ≤100s.

Currently in the initial stages of development, the endeavor aims to use satellite-to-satellite tracking laser interferometer-based optical technique, to document fluctuations in earth’s gravitational field indicating other critical changes in the environment. This system must be able to measure infinitesimal changes in the relative velocity of the two satellites, using a laser light source, which oscillates at frequency stability better than 10^-13 in the square root of the Allan variance. We have stabilized the laser’s oscillation frequency using the Faraday effect of Rb absorption lines. This method modulates the reference frequency of the stabilization system by modulating the magnetic field applied to the Rb absorption cell, instead of the oscillation frequency of the laser diode. Furthermore, we have adapted the “double optical feedback” to the laser diode for narrowing its oscillation spectrum and improving its frequency stability. In recent years, a “femtosecond optical comb generator” has been developed as a new reference frequency source for absolute frequency measurement. This optical comb generator is controlled by the microwave frequency standards systems and provides stability of 4x10^-13 at an averaging time of 1s and at the order of 10^-15 at 1000s averaging time. We have measured the frequency stability of our system using the optical comb. We obtained the best spectrum narrowing effect using two gratings as external reflectors in the double optical feedback setup. The obtained results were 6.269x10^-11 ≤ σ ≤ 1.516x10^-10 (24.11kHz ≤ f ≤ 58.31kHz) from 1s to 39s in the averaging time.

We report a study on the thermo optical properties of some nonlinear materials using the z scan technique. Both open aperture and closed aperture z scan transmittance signals were recorded to study the refractive and absorptive nonlinearities. The samples chosen were metal phthalocyanines viz CoPc, NiPc, ZnPc and CuPc. Dimethyl Formamide (DMF) and Dimethyl Sulphoxide (DMSO) were used as solvents. The laser source was the Q switched Nd: YAG with 10 Hz repetition rate and a pulse width of 7 ns. The Q switched envelope of the pulses give negative nonlinearity in all these samples which can be due to either thermal lens effect or electronic transitions among triplet states. Usually the thermal lens effect is neglected in z scan signals recorded using high repetition rate pulses. In our work, we have also carried out dual beam thermal lens measurements inside the sample, using a low power He-Ne as probe. We used the theory of the thermal lens formalism and also that of the Kerr type nonlinearities to interpret the obtained data. These studies yield the nonlinear optical constants and also the thermo optic constants of the samples under investigation.

We use a combination of vapor transport equilibration and moderate MgO doping (≤1%) to explore near-stoichiometric damage resistant lithium niobate crystals with improved properties for periodic poling and annealed-proton-exchange waveguide fabrication compared to the commercially available 5-mol% MgO-doped crystals. High damage resistance, measured by the saturated space-charge field generated in the crystal by 514 nm radiation, was obtained for all MgO doping concentrations (0.3, 0.5 and 1%) with appropriate equilibration. Green-induced infrared absorption was also measured in the 0.3-% doped crystal and was below the detection limit. Dispersion in the region 460-1550 nm was measured. Periodic poling was performed using LiCl solution electrodes. Poling quality improves with lowering MgO concentration. Waveguides for frequency doubling of 1550 nm were fabricated in the 1% doped crystal with losses as low as 0.4 dB/cm and normalized efficiency of ~10%/Wcm².

In contrast with conventional polarization maintaining fibers, a simple Ge-doped single-mode fiber is used to generate tunable femtosecond soliton pulses. Soliton self-wavelength-shift up to 200 nm is achieved in a 17 m long fiber. The generated “monocolored” soliton pulses have quasi-ideal sech spectral shapes. A high contrast optical switching scheme is proposed as an example of potential application of the soliton self-frequency shift.

We have presented a novel method of effective anti-Stokes SRS-generation in one-dimension photonic crystals by realization of SRS quasi-phase matching conditions. By numerical simulations we have studied crystals with different contrast in layers refraction indices. It was shown that the efficiency of anti-Stokes SRS generation may reach up to 30%.

The potentialities of diffraction methods for wavefront transformations may be considerably widened due to the use of nonlinear recording of dynamic holograms enabling multiwave mixing in media with the fifth and higher order nonlinearities. There is a great variety of such media including the resonant ones for which the presence of higher order nonlinearities is conditioned by the absorption saturation effect and transitions between different excited states of the molecules. However, in the majority of previous studies of multiwave mixing the resonant medium approximation has been used disregarding the induced anisotropy effect. This work presents a theoretical model and experimental studies of the energy efficiency of multiwave mixing in complex molecular media exhibiting higher order nonlinearities, in two cases: when the nonlinear cavity is introduced in the Fabry-Perot interferometer (IFP), and without it. Owing to the use of different combinations of mutually aligning polarizations for interacting waves, one is enabled to determine the contribution into the interaction efficiency by various dynamic gratings and nonlinearity mechanisms. The role of polarization gratings resultant from spatial modulation of the light field polarization state at the orthogonal polarization of the hologram recording waves has been established. Comparisons between the contributions of the above gratings and “normal” gratings recorded by identically polarized light beams into the process of multiwave mixing have demonstrated that the relation of these contributions is dependent on the intensity of interacting waves, and also it has been found that polarization of the diffracted wave is dependent on the diffraction order.

In this work we report the creation of color centers in LiF and YLF crystals by high intensity, ultrashort laser pulses. We used pure and Tm³⁺ and Oxygen doped samples, all irradiated with a Ti:Sapphire CPA laser system and also with electron beam, at room temperature. In both kinds of irradiations the production of photochromic damages and color centers that have absorption bands in UV and visible range was observed. A comparison between the two kinds of irradiation was done and the involved processes are described in this paper. F₂⁺ stable centers were produced by the ultrashort laser pulses irradiation in contrast to the well-known, short lived centers produced by electron beams, and a mechanism was proposed to explain the observed stability.

The damage performance of potassium dihydrogen phosphate (KDP, and its deuterated analog DKDP) crystals under multi-wavelength, simultaneous exposure to the harmonics of a nanosecond Nd:YAG laser system is of particular interest because it approximates the conditions taking place during frequency conversion. In this work, damage initiation under simultaneous exposure to two pulses at different wavelengths is investigated as a function of fluence in both KDP and DKDP. We have developed a novel damage testing instrumentation which allows us to measure the damage pinpoint density as a function of laser parameters, including wavelength, fluence, and pulse-length. This new method enables us to carefully quantify the damage effects of both, single-wavelength and dual-wavelength pulsed irradiation. In the latter case, we measure the laser-induced damage behavior when the fluence at one wavelength is varied while the fluence of the second wavelength is kept constant. Our results suggest that the behavior of laser-induced damage initiation under simultaneous multi-wavelength irradiation is complex and crystal dependent.

Non-ablative mechanism of femtosecond laser drilling of bulk dielectrics is proposed which is assuming concentrating and condensation of laser-generated vacancies into a void in a center of a laser waist at presence of picosecond photo-excited dense electron-hole plasma. Picosecond diffusive transport of neutral and/or charged vacancies and interstitials in the laser waist is driven by an inhomogeneous transient stress induced in the region via interactions of the dense electron-hole plasma and point defects with center-zone acoustic phonons. Simultaneously, inhomogeneous bandgap renormalization due to a coherent interaction of the plasma with center-zone optical phonons may contribute to the diffusive transport of charged point defects. This mechanism allows estimates of electron-hole plasma densities required for the "mild" micro-void fabrication regime, kinetics of void growth and their characteristic sizes as well as characteristic dimensions of surrounding laser-affected zones, consistent with known experimental results.

Laser-induced breakdown spectroscopy (LIBS hereafter) has emerged as a powerful diagnostic method in many application areas. LIBS operates by a focused laser beam inducing multiple ionization and dissociation of the target molecules. The high energy state at the focal point of the laser beam produces dissociated, excited elements which radiate characteristic emission bands while returning to ground states. These emission wavelengths and intensities can be used to infer the elemental composition of the sample. In air at atmospheric pressure, laser energy density of 10⁹ W/cm² is typically required to produce laser-induced breakdown, clearly visible as "sparks." There have been numerous fundamental and application studies of LIBS, particularly in recent years due to the increased interest in developing diagnostic methods for complex, toxic chemicals under arbitrary conditions. In this work, we present some results on in-situ LIBS measurements in flame environment. In particular, probably the most important utility of LIBS in flames is for measurements of temperature as will be validated against alternate methods.

Laser emission analysis is an efficient method used for determination of quantitative and qualitative composition of chemical elements in materials. For the analyser designs a proper choice of a laser type, stabilisation of its pulse parameters and performance of a high-resolution spectrometer of sufficient responsivity are crucial problems. The correlation-based method of laser induced breakdown spectrometry for soil contamination was presented. A system of laser induced breakdown spectroanalyser and the investigation of a portable laser emission spectro-analyser for analysis of chemical composition of soil samples are described. The results and examples of its applications are given.