The ability to achieve high quality periodic poling in lithium niobate (LN) has allowed quasi-phase-matching to be used for second-order nonlinear optics, leading to experimental demonstration of efficient optical frequency generation throughout its wide transparency range (0.35-4.5 microns). Applications of congruent lithium niobate involving visible or ultraviolet wavelengths are limited to low power or high temperature operation due to the effects of photorefractive damage (PRD) and green-induced infrared absorption (GRIIRA). The standard methods of suppressing PRD include doping with 5 mol-% MgO or ZnO and varying crystal stoichiometry. More recent methods employ a combination of lower doping level and near-stoichiometric composition. We use vapor transport equilibration (VTE) and significantly lower MgO doping (<0.5% in the melt) to obtain near-stoichiometric PRD-resistant crystals with improved parameters for periodic poling compared to the commercially available 5% MgO-doped congruent crystals. An efficient process for periodic poling at room temperature using baked photoresist as a patterned dielectric on one crystal surface with LiCl-solution electrodes was developed for periods as short as 8.3 microns for 0.5% and 7 microns for 0.3% MgO-doped VTE:LN. The quality of periodic poling improves as the MgO concentration is lowered. Stable second harmonic generation of 1.3-W continuous-wave 532-nm radiation was observed near room temperature (43 degrees Celsius, as determined by the phase matching condition) with no sign of degradation in a 1.5-cm long crystal of 0.3-% MgO-doped VTE:LN periodically poled with a period of 7.06 microns.

Crystal growth of two new quaternary crystals, AgGaGeS₄ and AgGaGe₅Se₁₂ , was performed in order to evaluate the usefulness of these materials for nonlinear optical frequency conversion of one-micron solid state lasers into the mid-infrared spectral region beyond 4 microns. Each compound was synthesized by vapor transport in sealed ampoules from high purity elemental starting materials, and crystals were grown by the gradient freeze technique in horizontal transparent furnaces. AgGaGe₅Se₁₂ exhibited incongruent melting behavior, and small optical samples extracted from an as-grown polycrystalline boule had high scattering losses. AgGaGeS₄ crystal growth proved to be far more favorable, resulting in a crack-free single crystal measuring 19mm in diameter and >80mm in length with as-grown 2.05-μm absorption losses < 0.05 cm^-1. The measured laser damage threshold of an uncoated AgGaGeS₄ crystal at 2.05μm was 1.1 J/cm², and room-temperature measurements of thermal diffusivity, heat capacity, and thermal conductivity yielded values of 0.224 mm²/s, 0.448 J/g/K, and 0.399 W/mK respectively for the sulfide. The growth and properties of AgGaGeS₄, despite being a quaternary compound with complex phase equilibria, appear indeed to be promising for shifting Nd-laser output directly into the mid-IR.

The interaction of damage initiating defect precursors in KDP and DKDP crystals with laser pulses is investigated as a function of laser parameters to obtain experimental results that contain information about the type and nature of the defects. Specifically, the focus is to understand a) the interaction of the precursors with sub-damage laser pulses leading to improvement to the damage performance (laser conditioning) and b) the synergetic effects during multi-wavelength irradiation. Our results expose complex behaviors of the defect precursors associated with damage initiation and conditioning at different wavelengths that provide a major step towards revealing the underlying physics.

Spatial self-phase modulation was observed when a CW laser beam propagated along a cell containing castor oil. The minimum power needed to excite this effect decreases when the sample length is increased, as well as when the laser wavelength approaches to the absorption band of the medium. The same phenomenon was also observed when a laser beam interacts with a colloidal solution of gold nanoparticles in castor oil. For this system the self-phase modulation minimum power decreased dramatically, which indicates that the nonlinear refractive index for this system is enhanced due to the gold nanoparticles. Moreover, for laser wavelength near to the plasmon resonance of the gold nanoparticles, this enhancement factor is even higher. Although the large value of those media nonlinearity, its temporal response is slow. This fact suggests that this phenomenon is due to thermal effects mainly.

The three-dimensional measurement of the global water vapor distribution in the atmosphere considerably improves the reliability of the weather forecast and climate modeling. A spaceborne Differential Absorption Lidar (DIAL) is able to per-form this task by use of suitable absorption lines of the broad absorption spectrum of water vapor. Because no interference with the absorption of other molecules exists, the range of 935/936 nm, 942/943 nm are the most preferred wavelength ranges for a water vapor DIAL. The challenge is to develop a dedicated efficient high power laser source emitting at these wavelengths. The comparison between frequency converters based on stimulated Raman scattering (SRS) and Ti:Sapphire and the directly generated Mixed Garnet laser shows the favorable properties of each concept and helps to evaluate the most suitable concept. Development of Raman frequency converters for high pulse energies concentrates on linear resonator de-signs and seeding using the Raman material as a direct amplifier based on Raman four-wave-mixing. In addition a seeded and frequency stabilized pulsed Ti:Sapphire laser system with output pulses up to 22 mJ injection-seeded at the water vapor absorption line at 935.684 nm with a spectral purity up to 99.9 % has been developed. Direct generation of the wavelengths 935/936 nm and 942/943 nm required for water vapor detection is possible with diode-pumped, Nd-doped YGG- and GSAG-crystals. First experiments resulted in pulse energies of 18 mJ in Q-switched and 86 mJ in free-running operation at 942 nm wavelength.

Generating the difference frequency of a frequency-doubled, widely tunable Ti:Al₂O₃ laser and a Nd:YAG laser provides tunable laser radiation in the visible spectrum range. The generated wavelength region closes the spectral gap between the fundamental and the second harmonic of the Ti:Sapphire laser. A prototype has being developed with a fully automated wavelength tuning, i.e. the wavelength tuning of the Ti:Sapphire laser, the angel tuning of the nonlinear crystals and the tuning of the temporal delay between the Ti:Sapphire and the Nd:YAG laser operate self-controlled. Design, theoretical modeling and experimental characterization of the system are closely discussed. At a repetition rate of one kilohertz, the frequency-doubled Ti:Sapphire laser provides pulses of approximately 20 ns, a spectral line width of 20 GHz, a nearly diffraction limited beam quality and pulse energies of up to 850 μJ. The tuning range reaches from 340 nm to 510 nm. For the three wave interaction process in a 8 mm long BBO crystal the Ti:Sapphire pulses (pump wave) are mixed with 3.5 mJ pulses of a Nd:YAG laser (signal wave). The generated idler wave has pulse energies of up to 280 μJ and pulse durations of approximately 10 ns in the spectral range between 510 nm and 680 nm. This yields to a conversion efficiency of about 33% and a quantum conversion efficiency of more than 50%. To our knowledge, this clearly exceeds the values that has been obtained with comparable setups so far. Further increase of the efficiency is currently under investigation.

Ultra broad band (from 1.5 to 37 THz) THz-wave generation using difference frequency generation (DFG) in an organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal was demonstrated. A DAST crystal is one of the promising materials for efficient and high power THz-wave generation, because of its very high nonlinearity and low refractive index dispersion between the near infrared region and the THz-wave region. We can use DAST's highest nonlinear component of, d11 (about 230pm/V), to generate THz-wave with by means of DFG, because the co-linear phase matching condition of the Type0 configuration is satisfied. We constructed a dual-wavelength optical parametric oscillator (OPO) with two KTP crystals pumped by frequency doubled Nd:YAG laser. Each KTP crystal was set on a Galvano scanner and the angle of each crystal was controlled independently. The OPO has a tunable range from 1300 - 1900 nm, results in an ultra broad tunable range of the THz-wave. We successfully generated ultra broad tunable THz-wave just using only one DAST crystal without any change of the experimental setup, except the computer controlled Galvano scanner angle change. The Hhighest THz-wave energy of 10 nJ was obtained at around 26 THz region under 2 mJ of pumping energy. And also, the THz-wave source can access an arbitral THz frequency at for every pulse (50 Hz at present). The Galvano scanner has a 1 kHz of responsibility response and we would obtain 1 msec of frequency access speed.

We have numerically modeled an efficient method of doubling the 1064 nm wavelength of a Q-switched Nd:YAG laser using a lambda-doubling nanosecond optical parametric oscillator (LDOPO). The LDOPO cavity is based on the four-mirror nonplanar RISTRA geometry, denoting rotated-image singly-resonant twisted rectangle, and contains a single type-II KTP crystal. By using the polarization-rotating properties of this cavity, and modifying its geometry to incorporate polarization-selective mirrors with angles of incidence near Brewster's angle, this design obtains stable, singly-resonant oscillation at degeneracy. If the pump laser is injection-seeded, and the LDOPO contains an intra-cavity etalon for single-longitudinal-mode oscillation, the phase of the wavelength-doubled 2128 nm light remains locked to the phase of the pump, independent of cavity length, so active frequency stabilization is not required. Numerical analysis indicates that a pulse-injection-seeded LDOPO can obtain 1064 nm to 2128 nm conversion efficiency exceeding 61%. However, analysis of a complete system incorporating a primary low-energy LDOPO that pulse-injection-seeds a secondary higher-energy LDOPO indicates total 1064 nm to 2128 nm efficiency of approximately 57%. A 2128 nm lambda-doubling system having conversion efficiency > 50% may offer a cost-effective alternative to conventional two micron laser sources such as Tm:Ho:YAG.

A high pulse energy widely tunable UV and IR laser is demonstrated for combustion imaging, LIDAR, etc. The laser output has superior beam quality because of rotated KTP OPO cavity, and it is highly efficient thanks to intracavity doubling and mixing.

Optical second harmonic generation in dense wavelength division multiplexing were used to monitor the performance of each channel in all optical networks. A poled Lithium-Niobate (PP-LNO3) device is used to convert C and L- Band optical WDM channels into half wavelength channels where silicon arrayed detectors and CMOS electronics were used to perform advanced digital signal processing to predict optical channel presence, channel power, signal to noise ratio and the quality (Q) factor parameter. With further processing, the bit error rate per channel can be estimated from the Q factor. The technique is realized on a system-on-a-chip CMOS technology which can potentially improve all optical networking architectures.

The nonlinear optical loop mirror (NOLM) is used in application like optical switching and demultiplexing, all-optical active mode locking, passive mode locking, pedestal suppression, pulse shaping, etc. This device offers a versatile way to obtain a nonlinear transmission behavior through the nonlinear differential phase shift between the two interfering beams due to the self-phase modulation. Recently we discussed a NOLM device using a symmetrical coupler, highly twisted fiber, and a quarter-wave (QW) retarder introducing the polarization asymmetry in the loop. We have shown high contrast operation, flexibility of characteristics, and stability in time. In this report we analyze theoretically and study experimentally the transmission behavior for different input polarization considering as well different output polarizations. We propose a simple description of the NOLM transmission for right- and left-hand circular output polarization at different output polarization states. The nonlinear characteristics depend on the QW retarder plate angle, but also on the polarization state at the NOLM input. Experiments were carry out with the NOLM consisted of a 500-m length SMF-28 fiber with twist rate of 7 turns/meter. Experiments show a good agreement with our theoretical approach. Appropriate choice of the input and output polarizations allows very high contrast, at least higher than 5000. Our measurements were restricted by sensitivity of our detection system to measure low power pulses at conditions when theoretically transmission must be equal zero. Experiments show flexibility and stability of characteristics. Adjustment of the QW retarder and input polarization allows tuning the critical power over a wide range.

The output of the high-repetition-rate all-solid-state picosecond oscillator is used for direct pumping of optical parametric amplifier seeded with the white-light continuum generated in optical fiber to generate tunable mid-IR radiation.

The generation of high energy femtosecond pulses in Optical Parametric Amplifier (OPA) pumped by fiber laser at a repetition rate of 1MHz is reported. Highly nonlinear fibers are used to create an intrinsically synchronized signal for the parametric amplifier. Seeding the OPA by a supercontinuum generated in a photonic crystal fiber, large tunability extending from 700 nm to 1500 nm of femtosecond pulses is demonstrated, with pulse energies as high as 1.2 μJ. Generating the seed using only SPM in a standard fiber, broadband amplification over more than 85 nm and subsequent compression down to 46 fs in a prism sequence are achieved. Pulse peak powers pulses above 10 MW together with 0.5 W of average power is achieved. This system appears to be very interesting due to scalability of pulse energy and average power of both involved concepts: fiber laser and parametric amplifier.

It is theoretically well known that the measurement of the second-harmonic power generated by a quasi-phasematched (QPM) grating as a function of the frequency detuning parameter yields the Fourier transform (FT) magnitude of the complex nonlinear coefficient profile along the QPM device. This measurement can be achieved by tuning either the wavelength of the fundamental laser beam or the temperature of the QPM grating. However, without the FT phase, the magnitude of the FT cannot be unambiguously inverted to uniquely recover the nonlinear coefficient profile of the QPM grating. In this work, we demonstrate that this ambiguity can be completely lifted by placing a stronger and thinner nonlinear sample against the input (or output) of the QPM device of interest and measuring the detuning curve of this composite assembly. The crux of this method is that by construction, the nonlinear profile of this assembly has a sharp peak due to the thinner sample, followed by the weaker, broader profile of the QPM grating, which essentially constitutes a minimum-phase function. As such, its FT phase can be simply and exactly calculated from its measured FT magnitude, for example by applying to the FT amplitude the logarithmic Hilbert transform or an error-reduction algorithm. The nonlinear coefficient profile of the QPM device can thus be fully recovered by processing the measured tuning curve with a fast and simple iterative error-reduction algorithm. In this paper, we demonstrate with numerical simulations that this powerful new technique can accurately recover the period, envelope, and chirp parameters of any QPM grating.

We have demonstrated all-epitaxially fabricated orientation-patterned AlGaAs waveguides with reduced waveguide core corrugation for the quasi-phase-matched second harmonic generation (SHG) pumped at 1.55 μm. The attenuation coefficient is measured to be ~4.5 dB/m at 1.55 μm, and ~9.7 dB/cm at 780 nm. The conversion efficiency at continuous wave operation is 43%W^-1 with an 8-mm long waveguide.

Efficient difference frequency generation (DFG) is obtained in the 2-μm region by using a direct-bonded periodically-poled LiNbO₃ (PPLN) ridge waveguide. The direct-bonding technique can utilize the bulk LN characteristics, which provide certain advantages including no additional absorption, precise device design and reproducible device fabrication. We achieved a conversion efficiency of 100%/W in the fabricated waveguide with a 0.94-μm pump laser diode (LD) and a 1.5-μm band tunable signal LD source. We also achieved a wide tunable range of over 0.1 μm in a 50-mm-long waveguide with a single-pitch PPLN at a constant temperature. This is because the DFG bandwidth is decided by the phase mismatch Δk. Generally, Δk=0 is only obtained at a certain wavelength, however, the Sellmeier equation shows that Δk~0 is easy to realize in the 2-μm region when the pump is set at 0.90-0.96 μm. Subtle Δk changes around 0 realized group velocity matching and a broadband output could be obtained. Compact and broadband tunable light sources are expected to be used for trace gas sensing in the near to medium infrared regions. This report also describes the bundle observation of carbon monoxide isotopomer absorption lines. The DFG output bandwidth is sufficient to observe ¹²CO and ¹³CO simultaneously. The absorption lines of the P and R branches for each gas are clearly observed between 2.30 and 2.45 μm. DFG in the 2-μm region using direct-bonded PPLN ridge waveguides is a promising approach for opening up new broadband applications.

Four-wave mixing (FWM) is an important mechanism for frequency conversion and fast optical switching in all-optical communication networks. In this paper, we demonstrate that microring resonator filters can generate significant conversion efficiency with sub-millimeter physical size. Microresonator based filters not only have high finesse, which can enhance nonlinear efficiency, but also have periodic passbands, which satisfy the phase matching condition of FWM automatically. Non-degenerate FWM in direct-coupled ring resonator (DCRR) filters and ring-based Mach-Zehnder interferometers (RMZI) is studied in this paper. Both structures provide greater than 10% conversion efficiency (defined by the converted intensity divided by the initial signal intensity) with low values of n₂*I_p, where n₂ is the nonlinear refractive index and I_p is the initial pump intensity; the DCRR allows greater frequency conversion bandwidth within the filter passband. We also compare the efficiency of the ring-based filters and a straight waveguide with the same effective length. We thus demonstrate that ring-based filters are effective devices for on-chip wavelength conversion for WDM network.

We have carried out a feasibility study of the excitation of π and 2π Raman solitons in gas filled hollow-core photonic crystal fibers. Using typical fiber parameters we present estimates for the soliton durations and powers and investigate the role played by first and second order fiber dispersions.

Fiber optical parametric amplifiers (OPAs) and Raman amplifiers (RAs) are both based on the third-order nonlinear susceptibility of glass fibers. Recently, there have been some efforts to combine these two nonlinear phenomena in order to extend the amplification and wavelength conversion windows to the S-band, or to lower the required parametric pump power in order to achieve the same signal gain. We propose a new technique utilizing these two amplication principles in a single piece of highly-nonlinear dispersion-shifted fiber (HNL-DSF). We call this Raman-mediated fiber OPA (RM-OPA), which is different from the previous Raman-assisted OPA (RA-OPA) work. The previously investigated RA-OPA required an extra Raman pump with power around 1 W, on top of the erbium-doped fiber amplifier (EDFA) required to amplify the OPA pump. Therefore, Raman amplification was simply employed as a power booster for the OPA pump, i.e. RA "assists" OPA. On the other hand, our new approach does not require an extra EDFA as pre-amplifier, and the single piece of HNL-DSF provides both RA and OPA effects. In other words, there is essentially no parametric amplification without the presence of the Raman pump. While combining RA and OPA have been investigated both analytically and experimentally before, it involved only introducing some extra Raman terms in the nonlinear Schrödinger equations (NLSE) additively, which is sufficient for the case when one pump is used to amplify the signal (and idler) through both RA and OPA. However, the situation is significantly different here, where the parametric pump is amplified by the RA, while the OPA gain varies along the gain medium as the parametric pump power itself is a function of the distance along the HNL-DSF. In this paper, we will present an approximate analytical model for this RM-OPA, in a co-propagating configuration. Through certain simplifying assumptions, we produce closed-form equations allowing intuitive insights into the RM-OPA operation, given the coupling amongst the parametric pump, Raman pump, signal and idler. These equations provide a framework for optimizing such kind of fiber amplifiers. Important RM-OPA design guidelines are also discussed.

The 1620-1700 nm region of the optical spectrum is important as it contains numerous molecular resonance lines of chemical species. We have investigated, theoretically and experimentally, the possibility of designing an efficient tunable wavelength converter (WC) based on a fiber OPA, to generate high-power in that range, by mixing radiation generated by C- and L-band fiber amplifiers. We have theoretically investigated the possibility of obtaining strong pump depletion in a one-pump fiber OPA, and of maintaining high conversion efficiency as the signal is tuned over a wide range. We have shown analytically that strong pump depletion can be obtained over a broad tuning range, when the signal input power is about one half of the pump input power. In experiments with a 40-m long highly-nonlinear fiber (HNLF) we have generated 900 mW of CW output power at 1665 nm, when pumping with 3W at 1612nm and 0.82W at 1562nm. The optical conversion efficiency was 23%, and the linewidth was less than 0.1 nm. To our knowledge this is the highest CW output power reported to date for a fiber OPA WC. We have also obtained similar output characteristics at 1684 nm, demonstrating the tunability of the device, which can in principle be tuned over the 1662-1697 nm region by tuning the signal wavelength over the C-band (1535 to 1565 nm). We anticipate that the output power can be scaled to higher powers.

We investigate supercontinuum generation with a femtosecond dual-pumping scheme. A 10 MHz oscillator delivering 300 femtosecond pulses at 1028 nm is frequency doubled and both the fundamental and second harmonic are coupled into a micro structured fiber. When the two pulses are temporally overlapped in the fiber a broad supercontinuum appears. By tuning the temporal delay between the two pulses, different regions of the spectrum can be enhanced which allows either improved flatness of the spectrum or selective amplification of regions of interest. The interesting result is shown to arise from cross phase modulation imposed on the visible pulse by fundamental solitons. Results from similar experiments with picosecond and nanosecond dual-wavelength pumping show the same qualitative behaviour and demonstrate that the governing mechanism in all cases is soliton fission and subsequent cross phase modulation of the co-propagating visible pulse.

We present an ultra-wideband supercontinuum source using a dispersion-shifted fiber and an amplified diode-laser pulse source. A gain-switched DFB laser operating at 1550-nm wavelength, which provides 30-ps pulses, was used for generating the seeding pulses. And serially cascaded low-cost EDFAs were employed to boost the peak power of the pulses to more than 1 kW. Single-mode supercontinuum spanning nearly the full near-IR band was obtained by passing the amplified pulses through a dispersion-shifted fiber. By investigating the characteristics of the generated supercontinuum pulses, the walk-off between the spectral components was found to limit the effective interaction length of the spectrum-broadening effects. In order to expand further the spectral range of the output, we have examined the time-gating ASE suppression scheme and use of a high-power EDFA. And the resulted outputs have reached wavelengths of 0.8 and 0.9 μm, respectively at the short-wavelength edges. Only the blue-shifted part that can be obtained using a short-wavelength-pass filter can exhibit 3-dB bandwidth more than 500 nm in the vicinity of 1.2 μm. The supercontinuum generation scheme provides a compact and reliable way to generate ultra-wideband flat spectrum that can be useful for high-resolution OCT.

Sellmeier equations of the mixture crystal AgGa(S_1-xSe_x)₂ for various concentration (x=0.3, 0.55, 0.75) are presented along with thermo-optic dispersion formulas at x=0.55, these equations were used to predict the 90^o phase-matching wavelength for Nd:YAG pumped OPO near 5.75μm for medical applications.

Advances in the development of nonlinear interference and holographic systems based on four-wave mixing point to great potentialities of these systems for real-time processing and correction of the light fields, formation of the desired space-time structures of laser radiation, image transfer, realization of logic and mathematical operations, creation of bistable devices and adaptive optics elements. This work presents the results of theoretical and experimental studies into the processes of light field transformations upon frequency-nondegenerate four-wave mixing in nonlinear Fabry-Perot interferometer. The principal aim is to develop a theory of intracavity four-wave mixing in complex molecular media in conditions of exhibited internal (scattering from dynamic gratings) and external (resonator) feedback, to determine a mechanism of light field transformations at the dynamic holograms and by nonlinear interferometers, to work out and introduce into practice novel nonlinear-optical methods for the control over characteristics of light beams. For theoretical description of typical experimental situations we used the round-trip model of interferometer adapted for the geometry of four-wave mixing, which can be realized in the scheme of symmetrical oblique incidence of reference and signal beams to the front and back mirrors of cavity. The conditions of magnification of dynamic gratings efficiency due to contribution from multiple interference of reading light beams have been studied experimentally and by means of theoretical modeling.

This paper studies the non-linear tolerance of several modulation formats in a four-span WDM system (8x10Gb/s) using low chromatic dispersion fiber. The narrow spacing between the channels (50 GHz) makes FWM to be the most detrimental effect experienced in each span made of G.655 fiber (80 km, D=2ps/nm.km) compensated by 0.8 km of DCF. A particular attention to the Q factor computed in the simulations enables a fair comparison between IMDD (Intensity Modulated Direct Detection) and phase modulated formats. It is shown that the various amplitude modulation alternatives result in more or less the same performance. Phase modulation schemes drastically increase the system performance leading to an increase of the Q-factor by almost 3dB.

Inspired by previous theoretical work, experiments on diffraction of 10 femtosecond ultrashort pulses passing through a single slit have been performed. Fringes are dramatically reduced or even eliminated in the diffraction of 10 fs ultrashort pulsed laser in the near field compared with that of the continuous-wave laser. This can be explained in the frequency domain as a result of the broadband spectrum contained in ultrashort pulses. Simulations are performed for Fresnel diffraction for both 10 fs ultrashort pulsed and continuous-wave lasers and the results agree with the experimental observation. The results of this work have important implication in biomedical imaging and remote imaging applications to name only a few.