A Fully Automated Diode Laser Based Difference Frequency Spectrometer Tunable from 12-16 μm

A very convenient way to obtain widely tunable source of coherent radiation in the infrared region is through nonlinear frequency mixing processes like second harmonic generation (SHG), difference-frequency mixing (DFM) or optical parametric oscillation (OPO). Using commonly available Nd:YAG laser and its harmonic pumped dye laser radiation as parent beams, we have been able to generate coherent tunable infrared radiation (IR) in 2–16 μm region using different nonlinear crystals by DFM and OPO. We have also generated such IR source in the 4–5 μm region through SHG of CO2 laser in different infrared crystals. In the process we have characterized a large number of nonlinear crystals like different borate group of crystals, KTP, KTA, LiIO3, MgO:LiNbO3, GaSe, AgGaSe2, ZnGeP2, AgGa1−xInxSe2, HgGa2S4 etc. To improve the conversion efficiencies of such frequency conversion processes, we have developed some novel schemes, like multipass configuration (MC) and positive optical feedback (POF). The significance of the obtained results lies in the fact that to get the same conversion in SHG or DFM, one now requires fundamental input radiation with much lower intensity.

Recent years have witnessed significant advancements in hollow-core fiber gas lasers (HCFGLs), driven by developments in hollow-core fiber (HCF) design and fabrication. These novel lasers are characterized by hollow-core structure, providing an ultralong and tiny interaction region for light and gases, thereby enabling light amplification with high efficiency, high beam quality, and tunability. HCFGLs has achieved broadband emission spanning from ultraviolet to mid-infrared (MIR) region, with a record output wavelength exceeding 4.8 μm in silica-based fiber lasers. Output power has surpassed 100 W in the near-infrared and reached an impressive 21.8 W in the MIR region. With the continuing reduction of attenuation in HCFs, particularly in the MIR regime, HCFGLs holds promises for potential applications in trace gas detection, space communication, polymer processing, medical treatment, national defense, etc. In this review, we focus on the basic principles and research progress of HCFGLs. The paradigm shifts history of HCFs are reviewed first. The development history and representative works of HCFGL based on the population inversion and stimulated Raman scattering are then introduced in detail. Finally, the future trends toward power enhancement, spectral expansion, and practical applications are also outlined. We hope this review will provide valuable insights for HCFGL researchers and interested readers, while also offering potential pathways to achieve otherwise challenging laser wavelength outputs and further power scaling.

Compact CH 4 sensor based on difference frequency mixing of diode lasers in quasi-phasematched LiNbO 3

Thanks to a propagation direction at 50.3° from the c axis in a LiYF4:Nd crystal, we have equalized the emission cross-sections at 1047 and 1053 nm, respectively, for extraordinary and ordinary waves. The double refraction is used to balance the "o" and "e" pumping. Actively Q-switching the laser cavity leads to simultaneous pulses with duration less than 10 ns. Frequency conversion to THz radiation was performed through difference frequency mixing in two nonlinear crystals: GaSe and OH1. In the first case, we obtained 132 nW and in the second case we obtained 51 nW THz power at 6.67 kHz repetition rate.

We show by experiment and mathematical model that angular and frequency acceptance bandwidths for frequency mixing in a nonlinear crystal can often be improved by segmenting the crystal and reversing the spatial or temporal walk-off in alternating segments. We analyze nonlinear mixing primarily in real space, (x, t), rather than Fourier space, (k, ω), and show that acceptance bands for sum- and difference-frequency mixing can be increased by up to a factor equal to the number of crystal segments. We consider both high- and low-efficiency mixing as well as parametric gain, and show that in many cases of practical interest the increased bandwidth substantially improves conversion efficiency. We also attempt to clarify the role of acceptance bandwidths in frequency mixing.

Highly efficient terahertz (THz) wave sources based on the difference frequency generation (DFG) process in nonlinear optical crystals plays an important role for the applications of THz waves. In order to find more novel nonlinear crystals, here, we theoretically investigate the generation of THz waves using the isomorphs of periodically poled KTiOPO4 (PPKTP), including periodically poled RTP, KTA, RTA and CTA. By solving the cascaded difference frequency coupled wave equations, it is found that the intensities of the THz wave generated from the cascaded difference frequency processes are improved by 5.27, 2.87, 2.82, 3.03, and 2.76 times from the non-cascaded cases for KTP, RTP, KTA, RTA and CTA, respectively. The effects of the crystal absorption, the phase mismatch, and the pump intensity are also analyzed in detail. This study might help to provide a stronger THz radiation source based on nonlinear crystals.

The application of nonlinear crystals which enable the difference frequency generation (DFG) of two input lasers is apromising tool for light sources in the mid-infrared spectral region beside the well known lead salt diode laser andsideband CO- and CO-lasers.We present a high resolution spectrometer based on the DFG in the nonlinear ciystal AgGaS2 applying an AIGaASindex-guided single mode diode laser and a ring dye laser as pump sources. With an input power of only 5 mW (diodelaser) and 150 mW (dye laser) by applying non-critical type I 900

Several important applications in optical communications require high power fiber coupled laser sources operating at 1.3 /spl mu/m, 1.48 /spl mu/m and 1.55 /spl mu/m. Semiconductor diode lasers for these wavelengths can be fabricated using the InP/lnGaAsP material system. High power operation requires careful selection of the active region, p and n doping and the lateral waveguide structure. Additionally, distributed feedback (DFB) lasers require appropriate choice of grating feedback. High power, narrow linewidth and low noise single mode diode laser sources operating at 1.3 /spl mu/m and 1.55 /spl mu/m wavelengths have applications in cable television transmission systems, microwave photonics, interferometric sensors and as an alternative to diode-pumped solid state lasers. High output power is desirable to overcome losses in external modulators and for large scale distribution systems. It is particularly important for 1.3 /spl mu/m applications as reliable and efficient fiber amplifiers are not commercially available at this wavelength. 1.3 /spl mu/m DFB lasers with 440 mW chip power, over 150 mW fiber coupled power and 100 kHz linewidth have been demonstrated.

A proposed sub-femtosecond pulse synthesizer using separate phase-locked laser oscillators

We present the numerical study of terahertz generation via different frequency mixing in two-wavelength vertical external cavity surface emitting laser. Nonlinear crystal is placed inside the resonator to increase terahertz radiation power. The dynamical model is based on modified Lang-Kobayashi equations. Numerical simulation through varying round trip time in the external cavity and feedback rate is presented.

Semiconductor strained layers

We report the characterization of a 10-Hz pulsed, narrow-band source that is coincident with a fundamental ν(3) rovibrational absorption of methane at 3.43 µm. To generate this midinfrared wavelength, an injection-seeded 1.06-µm Nd:YAG laser is difference frequency mixed with first Stokes light generated in a high-pressure methane cell (1.06 ? 1.54 µm) to result in light at a wavelength of 3.43 µm, that is, the ν(1) Raman active frequency of methane (~2916.2 cm(-1)). With a modest-energy Nd:YAG laser (200 mJ), a few millijoules of this midinfrared energy can be generated with a pulse width of ~7 ns (FWHM). The methane ν(1) frequency can be pressure tuned over 8-32 atm (corresponding to ~13 GHz) and scanned across part of the ν(3)P(10) rovibrational level of methane, resulting in a peak measured methane absorption coefficient of 4.2 cm(-1) atm(-1).

Two flashlamp pumped dye lasers of very high spectral quality are mixed in a LiIO3 crystal to generate a tunable infrared beam through the difference frequency mixing. Thanks to the injection-locking process of the pulsed dye lasers leading to a linewidth of 6 MHz with peak powers of about 10 kW, we obtain an IR beam tunable from 3.5 to 5.9 μm with a peak power of 40 W and a linewidth of 9 MHz. As an application we present a spectrum of N2O obtained by differential absorption near 1880 cm−1. The lines of thisQ-branch are Doppler limited at the working pressure (102 Pa).

We have demonstrated a novel terahertz source based on dual-wavelength amplification in polarization maintaining Yb- doped fiber and frequency mixing in a zinc germanium phosphide (ZGP) crystal. The system consists of two orthogonally polarized signals, whose difference frequency is in the terahertz region, amplified in a single all-fiber amplifier chain and mixed in a ZGP crystal to generate high-peak-power terahertz radiation. Currently, 2 mW of average terahertz power (20 W peak, 20 nJ/pulse) has been produced at a repetition rate of 100 kHz and pulsewidths of 1 ns with a conversion efficiency of 0.137%. We have also developed a terahertz mixing model, which coincides well with our experimental data.

We analyze the dynamics of sum-frequency generation (SFG) and difference-frequency generation (DFG) for broadband input fields. We demonstrate that amplitude modulation of the input fields significantly affects conversion efficiency of SFG and DFG, but frequency modulation does not affect conversion efficiency. Analytic results for SFG and DFG output-field intensities are thereby available for arbitrary fields. Self- and cross-phase modulation can affect the dynamics in the limit of very high field strengths, but these effects are of higher nonlinearity and therefore usually negligible. The quantum efficiency of DFG is shown to be generally higher than SFG, but the energy efficiency is much lower if the difference frequency is significantly less than the input frequencies. Optimal SFG quantum conversion efficiency for such systems approaches that of second-harmonic generation only if the quantum fluence (total number of photons) in the two input beams are equal. Optimal DFG quantum conversion efficiency occurs when the number of photons of frequency ${\mathrm{\ensuremath{\omega}}}_{2}$ is small yet sufficient for significant stimulation. A recently developed method for obtaining efficient SFG for multimode input fields is theoretically analyzed. The method involves using an arrangement with two or more nonlinear mixing crystals with a time-delay line situated between the crystals that delays one of the fundamental fields relative to the other. The efficiency in the second crystal depends on the cross-correlation function of the two fundamental fields upon leaving the first nonlinear crystal. The time-delay method is not effective for multimode DFG.