Widely tunable continuous-wave mid-infrared radiation (5.5-11 microm) by difference-frequency generation in LiInS2 crystal

The first demonstration, to the best of our knowledge, of continuous-wave (cw) difference-frequency generation (DFG) in LiInS2 crystal is reported. Wide spectral coverage (5.5-11.3 microm) has been obtained with angle and wavelength tuning for type II (eoe) critically phase-matched parametric interaction. The phase-matching conditions in cw DFG have been investigated, which allowed us to improve the Sellmeier parameters by use of a two-pole dispersion equation. An effective nonlinear coefficient deff = 6.9 +/- 0.8 pm/V has been determined at approximately 7 microm relative to the well-known nonlinear coefficient d36 of AgGaS2, which yields a power-conversion efficiency of approximately 12.4 microW/(W2 cm). We evaluated the high-resolution spectral characteristics of the DFG source by recording C2H2 and SO2 spectra.     

Mid-infrared (5-12-microm) and limited (5.5-8.5-microm) single-knob tuning generated by difference-frequency mixing in single-crystal AgGaS2

We describe tunable 5-12-mum mid-infrared generation in single-crystal silver gallium sulfide (AgGaS(2)), from nonlinear optical difference-frequency generation. Signal and idler waves obtained from a Nd:YAG laser-pumped LiNbO(3) optical parametric oscillator (OPO) were mixed in AgGaS(2) crystal to yield difference-frequency waves. For the efficient generation of difference frequency, an unstable resonator was employed as the OPO to reduce output beam divergence. A maximum difference-frequency power of 95 muJ/pulse was obtained near 7.5 mum for a 1-cm-long AgGaS(2) crystal. Spectral noncritical phase matching within a specific tuning range was also investigated that permitted limited single-knob tuning (5.5-8.5 mum) of the difference-frequency generator.     

Phase-matching and femtosecond difference-frequency generation in the quaternary semiconductor AgGaGe5Se12

We present data on the linear (transmission, index of refraction) and nonlinear (second-order susceptibility) optical properties of the quaternary semiconductor AgGaGe5Se12 with orthorhombic symmetry--a solid solution in the AgxGaxGe1-xSe2 system with x = 0.17. The nonlinear coefficients are estimated from phase-matched second-harmonic generation near 3 microm. After numerical analysis of the phase-matching configurations for three-wave nonlinear interactions, the first experimental results on difference-frequency mixing, producing tunable (4-7.5-microm) femtosecond pulses at a 1-kHz repetition rate, are described. The pulses of only five optical cycles (FWHM = 84 fs) are generated near 5 microm with energy of 0.5 microJ. Because of its higher damage threshold, larger birefringence and bandgap, and greater variety of phase-matching schemes, AgGaGe5Se12 could become an alternative to AgGaS2 and AgGaSe2, more widely used in high-power and specific applications.     

Widely tunable rapid-scanning mid-infrared laser spectrometer for industrial gas process stream analysis

A mid-infrared spectrometer with a tuning range of >400 cm(-1) in the C-H stretching region of the spectrum has been designed and constructed. The spectrometer is based on the difference-frequency generation of two tunable diode lasers in periodically poled lithium niobate waveguides. Tuning is achieved by varying a single parameter, the wavelength of one of the near-infrared input lasers. The instrument can be tuned over the entire tuning range in less than 1 s. By taking advantage of the wide tuning range, the instrument has been used to analyze a mixture of methane, ethylene, and propylene. Each of these major components was measured with an accuracy of better than 2% (where the error is defined as a percentage of the measured value) in a single 30 s long scan. When optimized, the spectrometer has the potential to meet both the performance requirements and the practical requirements for real-time process control in petrochemical manufacturing. The general principles for the design of mid-infrared spectrometers with wide tuning ranges are explained, including the use of variable waveguide fabrication recipes to create broad phase-matching resonances (which lead to broad tuning) in the desired location.     

Room-temperature mid-infrared laser sensor for trace gas detection

Design and operation of a compact, portable, room-temperature mid-infrared gas sensor is reported. The sensor is based on continuous-wave difference-frequency generation (DFG) in bulk periodically poled lithium niobate at 4.6 mum, pumped by a solitary GaAlAs diode laser at 865 nm and a diode-pumped monolithic ring Nd:YAG laser at 1064.5 nm. The instrument was used for detection of CO in air at atmospheric pressure with 1 ppb precision (parts in 10(9), by mole fraction) and 0.6% accuracy for a signal averaging time of 10 s. It employed a compact multipass absorption cell with a 18-m path length and a thermoelectrically cooled HgCdTe detector. Precision was limited by residual interference fringes arising from scattering in the multipass cell. This is the first demonstration of a portable high-precision gas sensor based on diode-pumped DFG at room temperature. The use of an external-cavity diode laser can provide a tuning range of 700 cm(-1) and allow the detection of several trace gases, including N(2) O, CO(2), SO(2), H(2) CO, and CH(4).     

Poling-induced birefringence phase matching for 1.5-μm-band wavelength conversion in a polymeric rib waveguide

A birefringence phase-matching (BPM) scheme for second harmonic generation, difference frequency generation (DFG), and their cascading is developed in a polymer rib waveguide. Poling-induced birefringence (n_TM–n_TE) of PMMA:DANS side-chain polymer films showed quadratic increase upon applied DC poling fields, and was controlled for satisfying the BPM. The three conversion processes using the nonlinear coefficient of the d₁₅ were simultaneously phase matched at the poling field around 150 V/μm when the fundamental guiding modes of a pump (DFG: 0.775 μm, cascading: 1.55 μm) and a signal (∼1.55 μm) waves were used. In addition, a numerical simulation of the DFG including the waveguide losses showed high conversion efficiencies around 0 dB on the conditions: the pump power of 100 mW, the waveguide length around 2 cm, and the waveguide losses of 2 dB/cm, demonstrating that the poling-induced BPM is an effective phase-matching method in the wavelength conversion processes.     

Thermo-optic dispersion formula for AgGaS2

AgGaS(2) has been found to be 90 degrees phase matchable at 192 degrees C for sum-frequency mixing between the output of the KTP parametric oscillator at 3.2627 mum and its pump source at 1.0642 mum. The thermo-optic dispersion formula that can be used for good reproduction of the temperature-dependent phase-matching conditions thus far reported in the literature as well as our new data for second-harmonic generation of a CO(2) laser and its harmonics at 1.7652-5.2955 mum is presented.     

High-resolution tunable mid-infrared spectrometer based on difference-frequency generation in AgGaS2

We have built a high-resolution and high-signal-to-noise ratio spectrometer for line shape studies of greenhouse gases in the mid infrared. The infrared radiation is generated in a AgGaS2 nonlinear crystal by the well-known difference-frequency method. The choice of crystal is explained, and a brief literature review is presented. With two tunable dye lasers and a type I, 90 degrees phase-matching geometry, the infrared is continuously tunable from 7 to 9 microm when Rhodamine 6G and Sulforhodamine 640 dyes are used. The total infrared power exceeds 30 nW and is limited by both the damage threshold and thermal loading of the crystal. Phase-sensitive detection allows us to reach signal-to-noise ratios in excess of 3500:1 while maintaining an instrumental linewidth of 1.5 MHz. However, we show that the spectrometer may be used to measure the positions of spectral lines within +/-400 kHz.     

Localized direct boundary-domain integro-differential formulations for scalar nonlinear boundary-value problems with variable coefficients

Mixed boundary-value Problems (BVPs) for a second-order quasi-linear elliptic partial differential equation with variable coefficients dependent on the unknown solution and its gradient are considered. Localized parametrices of auxiliary linear partial differential equations along with different combinations of the Green identities for the original and auxiliary equations are used to reduce the BVPs to direct or two-operator direct quasi-linear localized boundary-domain integro-differential equations (LBDIDEs). Different parametrix localizations are discussed, and the corresponding nonlinear LBDIDEs are presented. Mesh-based and mesh-less algorithms for the LBDIDE discretization are described that reduce the LBDIDEs to sparse systems of quasi-linear algebraic equations.     

Compact, broadly tunable, mid-IR source for the spectroscopic investigation of molecular reference lines in the 27- to 33-THz range

We report on the improvement of a tunable, high resolution, diode laser-based, difference-frequency spectrometer using an AgGaS(2) nonlinear crystal. We use a type-II cut crystal as a part of the improvement compared with a type-I cut, which was used in our preliminary setup. The two tunable laser-diodes are operating near lambda(3)=778 nm (pump) and lambda(2)=842 nm (signal) with a sub-100-kHz linewidth. The high resolution spectrometer is being developed as an alternative to CO(2) laser spectrometers in the 9- to Il-mum range. Using a dual-arm cavity to enhance the two radiation powers, and with 35 mW in front of the 778-nm arm and 100 mW in front of the 842 nm arm, about 70 nW of the tunable 10-mum radiation are generated. This power level is enough to investigate the linear absorption spectroscopy of SF(6). Doppler-limited spectra over 2 GHz, are recorded, showing the wide continuous tunability of the spectrometer.     

Temperature-dependent Sellmeier equations of nonlinear optical crystal Na3La9O3(BO3)8

The principal refractive indices of Na₃La₉O₃(BO₃)₈ (NLBO) crystal in the wavelength range of 0.363–2.325 μm were accurately measured by using the minimum deviation method within the temperature range from 23.5 °C to 160 °C. We derived the expressions of thermal refractive index coefficient as a function of wavelength that could be used to calculate the principal refractive indices at different wavelengths. The temperature-dependent Sellmeier equations were also derived and used to calculate the phase-matching (PM) angles for a frequency conversion device based on NLBO crystal at different temperatures. We found that the thermal refractive index coefficients of NLBO crystal changed from positive to negative values with the increase of wavelength. In addition, the phase matching conditions for third harmonic generation (THG) at different temperatures were also investigated.     

Theoretical analysis of phase-matched second-harmonic generation and optical parametric oscillation in birefringent semiconductor waveguides

We analyze the phase-matching conditions for second-harmonic generation (SHG) and optical parametric oscillation (OPO) in birefringent nonlinear semiconductor waveguides and apply these results to the model system of ZnGeP(2) on a GaP substrate. The analyses and numerical results show that phase matching can be achieved for OPO and SHG for reasonable guide thicknesses throughout much of the infrared, indicating significant potential applications for nonlinear birefringent waveguides. For the fundamental mode of a relatively thick guide the region of phase matching and the phase-matching angles are similar to those in bulk material. However, the waveguide has the added flexibility that phase-matched coupling can occur between the various modes of the guide. For example, the phase-matching region for SHG can be considerably extended by coupling the pump into the guide in the fundamental, m =, mode and phase matching to the m = mode of the second harmonic. Significantly, the results indicate, among other things, that ZnGeP(2) waveguides with harmonic output in the m = mode can be used for efficient SHG from input radiation in the 9.6-10.6-mum region where bulk efficiencies in this wavelength range are too small to be useful.     

非线性声波方程的几种解法对比

在医学诊疗领域及微、介观损伤的无损检测行业中,经常需要对介质的材料非线性系数进行表征,以得到局部区域更加精细的力学性能变化.文章在简述各向同性固体和理想流体介质中的非线性声波方程的基础上,证实了它们具有相同的形式,这表明它们的解也应具有相同的形式和性质.介绍了求解非线性声波方程的五种方法,包括有限差分、有限元、摄动法、...     

Multiple-wavelength quasi-phase-matching for efficient idler generation in MgO:LiNbO3 based nanosecond optical parametric oscillator

We present numerical results for optimization of the overall idler conversion efficiency of a nanosecond optical parametric oscillator (OPO), wherein the signal generated in the OPO process is also used as the pump for a difference frequency generation (DFG) process in a quasi-periodic MgO:LiNbO(3) crystal. The phase-matching conditions are considered such that the generated idler frequencies in both the processes (i.e., OPO and DFG) coincide. Optimization for the idler generation has been performed with respect to the different parameters, such as input pump power, pump pulse duration, and the output coupler reflectivity, for quasi-phase-matched interaction in MgO:LiNbO(3). Wavelength of the pump, signal, and idler waves considered in the optimization are 1.064 μm, 1.456 μm, and 3.95 μm, respectively. A maximum overall idler generation efficiency of ≈33% could be obtained in the simultaneous OPO+DFG process for a pump pulse duration of 72 ns and output coupler reflectivity (R(s)) of 90%, whereas for the stand-alone OPO process, the maximum idler generation efficiency was found to be ≈15%. The optimization has been illustrated for an average pump power of 8 W at a pulse repetition frequency (PRF) of 10 kHz. This approach of simultaneous OPO+DFG process can be employed to significantly enhance the idler generation efficiency of nanosecond OPOs.     

Phase-matched UV generation at 0.1774 μm in KB(5)O(8) ·4H(2)O

Sixth-harmonic generation of the Nd:YAG laser frequency at 1.0642 μm has been achieved in KB(5) O(8) ˙ 4H(2) O by type-1 mixing the fundamental and fifth harmonic at room temperature. The phase-matching angles were measured to be (θ = 90°, φ = 68.5 ± 0.5°) and (θ = 80.0 ± 0.5 °, φ = 90°) in the x - y and y -z planes, respectively. Improved Sellmeier equations of this crystal are presented.     

New and fast calculation for incoherent multiple scattering

To model bidirectional measurements, a numerical method for computing the incoherent light scattered by a diffusing medium is presented. The results are expressed as a function of the incident and the observer angles (theta(i), phi(i)) and (theta(f), phi(f)), in contrast to the N-flux method, which gives no information about the azimuthal-distribution. To solve the multiple-scattering equations, an auxiliary function, expanded on the spherical harmonics, is introduced in the diffusion equation. A set of integral equations on the coefficients are obtained that are well suited for the numerical resolution. The boundary conditions are included in the linear operator of the integral equation, so that each boundary condition is associated with a specific equation. As an illustration, the method is applied to numerical simulations of maps of the light scattered by a thick refractive diffusing layer of refractive index n = 1.5, for two directive phase functions and for several incident collimated-beam angles.     

Widely Tunable Continuous-Wave Mid-Infrared Laser Source Based on Difference-Frequency Generation in AgGaS(2)

We demonstrate difference-frequency generation in the 6.8-12.5-mum range by mixing two high-power single-frequency laser diodes in a type II AgGaS(2) crystal. This compact all-solid-state scheme provides maximum output powers that exceed 1 muW and permits continuous adjustment-free scans larger than 2 cm(-1) across the entire tuning range.     

In situ and on-line monitoring of CO in an industrial glass furnace by mid-infrared difference-frequency generation laser spectroscopy

A compact mid-infrared (MIR) laser spectrometer based on difference-frequency generation (DFG) is applied as a portable and sensitive gas sensor for industrial process control and pollutant monitoring. We demonstrate the performance of such a MIR DFG gas sensor by recording the absorption spectra of the carbon monoxide (CO) P(28) absorption line in the atmosphere of a gas-fired glass melting furnace. For a gas temperature of approximately 1100 degrees C, the CO concentration in the recuperator channel is measured to be 400 parts per million.     

Generation of high-repetition-rate femtosecond pulses from 8 to 18 microm

We generated subpicosecond pulses from 8 to 18 mum by difference-frequency mixing in a 1-mm-thick AgGaSe(2) crystal, the 130- and 180-fs output pulses (1.45 < lambda < 1.85 mum) from an 84-MHz-repetition-rate optical parametric oscillator. Numerical simulations show that intrapulse and interpulse group velocity dispersion determine minimum pulse duration above and below 15 mum, respectively. By cross correlation (upconversion) of 10.5-mum pulses with 90-fs, 810-nm pulses in AgGaS(2), the pulse length was measured to be 310 fs in good agreement with simulations.     

Temperature phase-matching properties for harmonic generation in GdCa4O(BO3)3 and Gd(x)Y(1-x)Ca4O(BO3)3

GdCa4O(BO3)3 has been found to have phase-matching points where the temperature variations of the phase-matching angles become zero for type-1 sum-frequency generation in the zx plane. We also found that the temperature sensitivities of the phase-matching conditions in the zx plane are different along the phi = 0 degrees and phi = 180 degrees directions in this material. In addition, the thermo-optic dispersion formula of this material that reproduces the temperature phase-matching properties of GdCa4O(BO3)3 and Gd(x)Y(1-x)Ca4O(BO3)3 is presented.