We have demonstrated wideband tunable optical parametric oscillator (OPO) based on BaGa4Se7 crystal. A pump doublepass, short cavity, singly resonant OPO configuration was used to improve the optical conversion efficiency and reduce the OPO threshold. Using a commercial nanosecond laser with a repetition rate of 10 Hz as the pump source, the tuning range of BaGa4Se7-OPO achieved 3.10-4.84 and 8.24-13.34 µm in mid-wave and long-wave infrared band respectively. The maximum single pulse energy of reached 8.55 mJ at 3.88 µm and 0.855 mJ at 10.8 μm. To achieve high repetition rate operating, an electro-optic Q-switched laser with repetition rate of 1 kHz was built. The high repetition rate BaGa4Se7- OPO have a maximum output power of 564 mW at 3.87 μm and a tuning range of 2.99-5.04 µm.
A mid-infrared BaGa4Se7 (BGSe) optical parametric oscillator (OPO) pumped by a 1064 nm laser with a repetition frequency of 1 kHz was demonstrated. The maximum output power at the wavelength of 6.48 μm was 40.2 mW with a pulse width of 6.45 ns, corresponding to the peak power of 6.23 kW. The characteristics of MIR tuning for BGSe-OPO were investigated using temperature and angle tuning methods. A tunable output in the 6.48-6.89 μm range was obtained. The high-repetition-rate MIR lasers within the 6-7 µm range provide a kind of new source for a variety of practical applications.
A novel signal denoising framework (EEMD-VMD-IMWOA) for Rayleigh lidar is proposed to better suppress noise in an atmospheric lidar echo signal and improve retrieval accuracy. The ensemble empirical mode decomposition (EEMD) is used to retain the intrinsic mode functions (IMFs) of signal as the low-frequency effective component. Based on the denoising ability of variational mode decomposition (VMD) under high noise signal, the IMFs with noise is further denoised by VMD to obtain high-frequency effective component, wherein the improved whale optimization algorithm (IMWOA) is used to get the optimal decomposition layer K and the quadratic penalty α of VMD. Then, the low-frequency and high-frequency effective components are reconstructed to gain denoised signal. The simulation results show that the denoising effect of EEMD-VMD-IMWOA is superior to Wavelet threshold, EEMD and VMD, especially the far-field noise interference can be suppressed. Under the condition that the temperature retrieval error is less than ± 10 K, when the integration time is only 600s, the effective retrieval altitude can reach 59.6km, which is 17.3% higher than that without denoising. Finally, the retrieval accuracy of the measured lidar signal is significantly improved by EEMD-VMD-IMWOA.
In order to obtain a passively Q-switched sub-nanosecond microchip laser with a low pulse jitter of less than 10 ns, a scheme of injection-seeding stable nanosecond laser pulses was designed. The pulse timing jitter of the passively Q-switched laser was improved from μs-level to ns-level with the seeding pulse energy of around 70 μJ. Based on experimental measurements, the dynamic process of pulse locking by varying the seeding pulse energy was discussed. The locking threshold affected by the peak pump power and time delay (ΔtQ) between the initial passively Q-switched laser and seeding pulses was also analyzed.
To enhance the correlation in the orthogonal directions, a polarization self-modulation scheme with an intra-cavity quarter wave plate in a coaxial pumping orthogonally polarized laser was proposed. This quasi-isotropic cavity was compared with the traditional scheme in terms of the differences in the oscillation between dual components and the intra-cavity eigenstate distribution was obtained. Both theoretical and experimental results indicated that modes were effectively locked in TE and TM directions and dual-eigenstates output was achieved, which provided a half-free-spectrum-range frequency difference in ±45° directions. Q-switching and dual-wavelength-operation did not affect the polarization self-modulation process.
The Laser Diode (LD) end-pumped solid-state lasers with excellent output characteristics are extensively used in many fields. The thermal effects of the laser crystal are one of the key factors preventing the laser from achieving high-quality output. In this paper, the thermal effects of a laser at 1 kHz repetition rate were explored by developing a three-dimensional thermal model of an end-Quasi-Continuous Waves (QCW) LD pumped Nd: YAG crystal. Considering the radial and axial heat conduction of the laser crystal, the transient temperature field within the crystal was numerically calculated using a seven-point Finite Difference Method (FDM). The transient thermal effects of the composite crystal were compared with those of the non-composite crystal. The effects of different parameters on the transient thermal effects of the composite crystal were discussed in detail. This simulation work is believed to guide the design of thermally stable cavities for lasers operating at muti-kHz to attain favorable laser characteristics.
Depth image reconstruction has been of interest in single photon LiDAR. The difficulty of high-accuracy depth image reconstruction, for example, the low-reflection objects are ignored sometimes, results from the signal intensity in the reconstructing, which is heavily affected by the target characteristics. The confidence interval width is utilized to guide the recovery of depth images for achieving high-accuracy depth imaging under the non-negligible differences in target characteristics. This work proposes a confidence interval (CI)-guided depth imaging method, which evaluates the uncertainty of depth estimation with the 95% confidence interval of normal distribution. Three necessary steps exist in process with this CI-guided depth imaging method. Firstly, the noise responses are eliminated using the local gating method. Then, the depth image is reconstructed by a pixel-wise maximum likelihood estimator, and the CI guidance image is calculated from the 95% confidence interval of the mean normal distribution. Finally, the adaptive thresholding segmentation algorithm based on the CI guidance image is adopted to achieve the reconstruction of depth images. The CI-guided depth imaging method presents a unique perspective between signal and noise. This paves the way to improve the depth image recovery accuracy with the state-of-the-art photon-efficient imaging algorithm.
KEYWORDS: Air temperature, LIDAR, Atmospheric sensing, Monte Carlo methods, Measurement uncertainty, Temperature metrology, Statistical analysis, Atmospheric modeling
The measurement uncertainty is an important parameter to evaluate the reliability of the Rayleigh lidar in detecting atmospheric temperature. This presentation aims to study the atmospheric temperature measurement uncertainty of a floating platform-mounted Rayleigh lidar. A model was established for altitude correction considering the platform attitude, and the temperature uncertainty originating from the fluctuation in rolling and pitching angles was evaluated using the Monte Carlo method (MCM). The results show that the atmospheric temperature uncertainty due to platform fluctuation is confined to 10-2 K when the detection altitude is up to 65 km.
We report a compact and highly stable 1064-nm electro-optic Q-switched laser operating at the repetition rate of 1 kHz. A composite Nd:YAG crystal was used as the gain media and the cavity length was 105 mm. Under the average pump power of 11 W, the output power achieved 2.404 W with the pulse width of 4.558 ns, corresponding to the maximum peak power of 0.527 MW and the optical-to-optical conversion efficiency of 21.85%. The slope efficiency reached 42.69%. The beam quality in the horizontal (Mx2) and vertical (My2) directions were 1.81 and 1.58, respectively. The pulse timing jitter was less than 1 ns, and the average power fluctuation measured within 30 min was 0.83% (RMS). It is believed that such a compact and highly stable pulsed laser with high repetition rate, high peak power, and good beam quality has great potential in the fields of lidar, etc.
A theoretical model was proposed to simulate the broadband second harmonic generation (SHG) based on random quasiphase matching (RQPM) by Fourier transform mothed. A broadband SHG experiment system was built which could obtain the distribution of the SHG signal over a whole ZnSe sample. Both the simulated and experimental results demonstrated that the main feature of RQPM is the linear dependency of the SHG intensity with sample thickness.
Based on the rate equation of passively Q-switching, the effects of pump rate on the pulse timing jitter was simulated. The evolution of pulse jitter versus initial transmittance of the saturated absorber and pump power were experimentally investigated using different Nd:YAG/Cr:YAG bonded crystals. By adopting reasonable parameters, it was proved that the pulse jitter of passively Q-switching could be controlled within hundreds of nanoseconds. If an actively Q-switched laser was used as the seed laser for a passively Q-switched microchip laser, the pulse jitter could be reduced down to ~5 ns, and the output characteristics of the passively Q-switched laser with seed injection were discussed.
Efficient orthogonally polarized lasers (OPLs) with power balance is of great significance in many fields. A gain-selfbalanced coaxial-end-pumped orthogonally polarized laser is proposed in this presentation. Using the orthogonal Nd:YVO4 crystal arrangement and a quarter wave plate, different waves were amplified by both crystals and the OPL could operate under the optimized condition. Compared with traditional methods, the beam quality and the coherence of the OPL were greatly improved and the coherence could also be actively switched by pump conditions. Theoretical explanations and discussions were given from the view of thermal effects and laser resonators. It is believed the gain-self-balanced coaxialend-pumped OPL has broad application prospects in precision measurement and other fields.
A passively Q-switched dual-wavelength laser with pulsed LD coaxial end-pumped configuration was demonstrated. A theoretical model was built to simulate the dynamic process of the pulsed LD coaxial end-pumped dual-wavelength laser. Experimental verifications were carried out based on Nd:YAG/Nd:YAP crystals. When the reflectivity of the output mirror and the initial transmission of the saturable absorber were both 50%, the maximum output single-pulse energy of pulsed-pumped passively Q-switched dual-wavelength laser was 304 μJ, which was obviously enhanced compared with that in the CW pumping.
The atmospheric temperature measurement uncertainty was evaluated by the Guide to the Expression of Uncertainty in Measurement (GUM) method. The lidar measurement model was introduced considering the design of actual lidar instruments and standard retrieval method. The detection noise and the auxiliary temperature uncertainty were considered as two main uncertainty sources. Based on the simulation data of Rayleigh scattering lidar operating at 532 nm with 2- hour integration period, it was calculated that two main uncertainty sources resulted temperature standard uncertainties of around 2 K and 5 K at 60 km, respectively, and the combined standard uncertainty was 6 K.
A novel method to modulate the phase-matching (PM) condition based on the linear electro-optic (EO) effect in cubic nonlinear crystals was proposed to enhance the efficiency and broaden the PM bandwidth of terahertz generation. Taking ZnTe and CdTe crystals as examples, monochromatic terahertz waves can be difference-frequency generated (DFG) and agilely tuned under perfect PM condition over a range of 2.25 THz and 1.84 THz, respectively, which also corresponds to large allowable wavelength and divergence angle of the pump beam. Simultaneous wideband terahertz generation via optical rectification (OR) modulated by the EO effect was also investigated. It introduces an extra degree of freedom to fulfill PM condition of different excitation wavelengths and polarization states by OR, where the polarization selectivity can be optimized by controlling the applied voltage.
Theoretical models for the backscattering intensities of Lambertian tilted plates, spheres and cones in monostatic radar situation were proposed, and their one-dimensional (1D) range profiles were simulated in the terahertz range. Then the 1D range profiles of picosecond and nanosecond pulse incidence are compared, which reveal that more details of object shapes can be obtained with the ultrashort terahertz pulse. The influences of target size, posture, pulse width and waveform were also investigated, respectively.
A high-efficiency, high-peak-power, widely tunable optical parametric generator (OPG) based on a MgO-doped periodically poled lithium niobate (PPMgLN) crystal is reported. Pumped by a microchip passively Q-switched laser (duration: 330 ps, repetition rate:1 kHz) with the power output of 880 mW, the OPG could be continuously tuned from 1399 nm to 4443 nm by changing the grating period and working temperature. The OPG generated an output power of 591 mW for the signal (1758 nm) and the idler (2695 nm) waves, achieving the internal conversion efficiency of 67.16%, slope efficiency of 89.6% and peak power above 1 MW at 1758 nm. The evolution of linewidth of the signal wave during wavelength tuning were also studied and the theoretical models were proposed. The linewidth was narrowed from 100 GHz to GHz level using a continuous-wave (CW) tunable seeder. The linewidth reached 1.72 GHz at 1626 nm, close to the Fourier transform limit of the sub-nanosecond signal wave.
The Fourier transform treatment on random quasi-phase matching (RQPM) problems in nonlinear polycrystalline materials is proposed to simplify the simulation process. The spatial frequency spectrum information of the polycrystalline material is obtained directly by Fourier transform analysis in the space domain, which is closely related to wave number and coherence length. Using this method to simulate the second harmonic generation (SHG), the results are consistent with the previous studies, which verifies the feasibility of this method.
A widely tunable eye-safe noncollinear phase-matched (PM) KTP optical parametric oscillator (OPO) with fixed output direction was proposed. Based on a novel confocal optics system, the input pump beam from a pulsed 1064 nm Nd:YAG laser could be deflected into the OPO with a tunable and agile noncollinear angle while maintaining the resonator unaffected. As a result, stable OPO operation with a wide tuning range of 124 nm was achieved easily with a beam scanner. The pump threshold, output energy, linewidth and temporal pulse shapes during wavelength tuning were also measured and discussed.
High energy and widely tunable terahertz (THz) generation was demonstrated theoretically based on a semiconductor material 4H-SiC via difference frequency generation (DFG) process. Compared with the conventional THz nonlinear optical (NLO) crystals, 4H-SiC has the main advantages of extremely high optical damage threshold and wide optical transparent range, which implies the potential THz generation with high output energy and broadband tunability. Based on the basic NLO theories, the phase-matching (PM) characteristics, effective nonlinear coefficients, walk-off angles, and PM tolerance of DFG in 4H-SiC were calculated in the 2–15 THz range with different pumping wavelength. The output characteristics of THz generation were simulated in relation with the optical interaction length and the intensities of dual-wavelength pump beams via large-signal analysis among three coupled wave equations, which reveal that efficient and high energy THz generation based on 4H-SiC crystal could be achieved with appropriate crystal length and intensity ratio of dual-wavelength intense pumps, despite of a relatively low nonlinearity of the material.
A continuous-wave (CW) dual-wavelength laser with coaxial diode end-pumping configuration is demonstrated. A theoretical model was built to simulate the CW output power process of the dual-wavelength laser generation. The experiment was performed with Nd:YVO4/Nd:YAP composite laser crystals. The continuous-wave output power reached 5.28 W under the maximum LD pump power of 15 W, corresponding to optical-optical conversion efficiency of 35.2%. The power ratio between 1064 nm and 1080 nm could be tuned by varying the pump wavelength to balance the gains in two laser crystals.
Considering the impact of the pump focal position on the output characteristics in dual-wavelength lasers with coaxially arranged dual crystals, we study on the thermal effects based on two coaxial Nd:YVO4 crystals with variable pump focal position and incident pump power theoretically and experimentally. The output characteristics and thermal focal lengths were discussed at different cavity lengths using the resonator theories and compared with the monolithic crystal. The performance of the scheme with two discrete crystals when the focal position locates deep at different cavity lengths was also researched and the advantages over single crystal were verified.
We demonstrate the theory and experiments of a power-ratio tunable dual-wavelength laser with coaxial diode end-pumping configuration by varying the pump wavelength, which is realized by controlling the working temperature of the pump laser diode (LD). Composite laser gain media containing an Nd : YVO4 crystal and an Nd : GdVO4 crystal was used for example. The dynamics of the dual-wavelength laser generation based on practical input operating parameters were simulated, and in the experiment, a total power of 3.72 W was obtained under the maximum LD pump power of 8 W, corresponding to the optical–optical conversion efficiency of 46.5%. The characteristics of the power-ratio tuning and output power agreed well with the theoretical predictions.
We developed and verified a metrology and calibration equipment based on LabVIEW and USB-bus technology for measuring key parameters of medical laser therapy apparatus. In this paper, aiming at Q-switched Nd:YAG pulsed laser therapy apparatus whose safety and reliability issues are prominent, according to measurement requirements of key parameters during treatment, we designed a portable, high-precision, user-friendly and all-in-one measuring equipment. The designed equipment can carry out the measurement of pulsed laser key parameters including wavelength, pulse width, repetition frequency, pulsed energy or power, spot size of treatment area and beam divergence angle. For aiming beam, which is continuous wave laser, wavelength and power can be measured. In addition, the quantity values of the measuring equipment we designed were traceable to national standards of measurement effectively, which includes three items: measuring range of pulse width is 1 ns~100 ns and maximum permissible error (MPE) does not exceed ±10 %; measuring range of pulsed energy is 1 mJ~2 J, class of accuracy is up to 5, surface uniformity better than ±3 % and zero drift better than ±2 %; measuring range of treatment-area spot size is 2 mm~8 mm and MPE does not exceed ±10 %.
The dusty plasma sheath formed during the reentry process of hypersonic vehicle will interrupt the propagation of the communication signals, which is called the blackout problem. One of the most effective solutions to the blackout problem is to detect with terahertz wave, which refers to the electromagnetic wave with a frequency range from 0.1 to 10THz. Recently, the propagation characteristics of terahertz wave in dusty plasma have been studied based on the analyzed of the dielectric properties. Unfortunately, the influence of scattering caused by ablation particles is neglected in these studies, which can be ignored no more, especially for high terahertz wave. In this work, the propagation characteristics of dusty plasmas are analyzed considering both intrinsic absorption and scattering. Firstly, the electron densities and collision frequencies have been calculated based on the simulation of flow field around a return capsule model, and then the dielectric properties are analyzed. Secondly, the propagation characteristics are calculated and the results show that with the increase of detection frequency, the transmittance of THz wave in dusty plasma increases due to the decrease of absorption. But for higher frequency, the stronger scattering leads to the decrease of transmittance. Moreover, the influences of the flight speed of the vehicle, the diameter and density of ablation particle are also discussed. This research provides a basis for the selection of the best frequency band for the detection of return capsule.
A passively Q-switched dual-wavelength laser with coaxial diode end-pumping configuration is demonstrated. A theoretical model was built to simulate the dynamic process of dual-wavelength laser pulse generation. The experiment was performed with Nd:YVO4/Nd:GdVO4 composite laser crystals and a Cr4+:YAG absorber. The continuous-wave and Q-switched output power reached 3.48 W and 607 mW, respectively, under the maximum LD pump power of 8.0 W, corresponding to optical-to-optical conversion efficiencies of 43.5% and 7.6%. The power ratio between 1064.4 nm and 1063.5 nm could be tuned by varying the pump wavelength to balance the gains in two laser crystals.
A segmented RCS data measurement and processing method was proposed. Attenuation elements were introduced to improve the measurable signal range of the measurement system, and the accuracy was improved by segmented calibration of data. Based on this method, the warhead model was measured in a large dynamic range up to 63 dB. A compact-field radar cross section (RCS) measurement system applicable in the high-frequency terahertz range was built based on a seed-injected terahertz parametric generator (ips-TPG). The reliability of the system was verified by taking smooth stainless-steel spheres as the standard calibration objects and the RCS measurement of common target was performed at a high frequency point at 5 THz. The error between the measured and theoretical results was less than 4 dB.
A novel noncollinear phase-matching (PM) scheme by introducing a small tunable angle between two pump beams, was proposed to notably enhance the effective nonlinear coefficient (deff) in difference frequency generating (DFG) tunable terahertz waves in the ZnGeP2 crystal. Compared with the collinear PM condition, the noncollinear geometry transfers the PM angles to be around θ = 90° or θ = 30° for type-II (o→e→o) or type-I (o→e→e) PM to maintain large values of deff in the entire output frequency band, in which tunable bands of 1.90–4.5 THz or 0.47–4.30THz can be achieved, respectively, leading to a high conversion efficiency improved by tens of times. Based on the theory of noncollinear PM, the angletuning characteristics were studied and the crystal design was provided for efficient outcoupling. Rigorous theoretical models were built for both types under small-signal approximation to show the affecting factors of noncollinear PM and reveal its superiority compared with collinear PM. The idea presented in this paper not only provides a good solution for efficient terahertz generation in ZnGeP2, but it is also applicable in various optical frequency converters in different nonlinear materials.
In this paper, we demonstrated a passive Q-switched 2μm dual-wavelength intra-cavity PPLN optical parameter oscillator (OPO) with the advantages of compactness, large tuning range, and narrow linewidth. By using c-cut Nd:YLF crystal as the laser gain medium, the output power of the fundamental laser at 1053nm can reach up to 0.916W, corresponding to the incident pump power of 9.79W. When the temperature of PPLN crystal was varied between 40°C and 160°C, the orthogonal polarized dual-wavelength lasers can be tuned from 2026.4nm to 2193.9nm. As to the a-cut situation, the output power at the fundamental wavelength of 1047nm is about 1.192W. In the same temperature range, two orthogonal polarized wavelengths can be tuned from 1991.8nm to 2206.8nm. Such type-Ⅱ OPOs have potential abilities to work for sensing, spectroscopy, imaging, nonlinear frequency conversion and so on.
A Monte-Carlo simulation was performed on the efficiency of random quasi-phase matching (RQPM) nonlinear optical frequency conversion in polycrystalline materials to show the detailed influence of the statistical grain morphology properties. The simulation took second harmonic generation (SHG) for example and considered the parameters of beam size, beam distribution, fundamental wavelength variation, polycrystalline average grain size (mean), standard deviation (Std), etc. The results and conclusions could enrich the theory of RQPM and provide guidance for polycrystalline material processing and sample selection for specific nonlinear-optical experiments.
A multi-wavelength green laser is presented based on a coaxial diode-end-pumping configuration by intracavity frequency doubling with a nonlinear crystal. The composite gain media (Nd:YVO4 and Nd:GdVO4) are placed coaxially and share one pump diode around 808 nm to generate two competition-free fundamental laser at 1064.4 nm and 1063.2 nm. The nonlinear crystal (KTP or LBO) are satisfactory for second-harmonic generation (SHG) and sum-frequency generation (SFG) with different fundamental wavelengths. Stable multi-watt green lasers at 532.2 nm, 531.6 nm and 531.9 nm are simultaneously obtained. Through gain controlling by tuning the pump focusing depth and pump absorption, the power ratio for those wavelengths and pulse interval can be manipulated actively. A rate-equation model is proposed and the experimental results coincide with the simulations. By replacing the gain media (Nd:YAG, Nd:GSGG, Nd:YAP, etc), various green lasers with multiple and selectable wavelengths are possible, which have great potential in practical applications.
A gain-boosted terahertz-wave parametric generator (TPG) in high frequency tuning range based on MgO-doped nearstoichiometric LiNbO3 (MgO:SLN) crystal has been demonstrated with 1064 nm nanosecond pulsed laser pumping. The pulse-seed is provided by nanosecond singly resonant near-degenerated KTP optical parametric oscillator with the wavelength range of 1068.08 nm to 1084.76 nm. The terahertz tuning range of 0.97 THz to 4.07 THz was achieved. The maximum THz wave output signal was 4285mV at 1.82 THz under the pump energy of 180 mJ and pulse-seed energy of 20.2 mJ. During the frequency range of 1.25 THz to 3.43 THz, the THz output energies were larger than 2000mV. Compared with the maximum THz output energy, the THz energy attenuation factors of 0.55 dB, 1.71 dB and 3.31 dB were realized in pulse-seeded TPG at 2.5 THz, 3.0THz and 3.5THz, respectively. The significantly increasing of THz gain in high frequency range (<2.5 THz) was achieved.
Theoretical simulations were carried out to evaluate the properties of terahertz (THz) generation in β-BaTeMo2O9 (βBTM) crystal by stimulated polariton scattering (SPS) process. The effects of different polariton modes on THz generation were analyzed, from which we determined the optimal crystal design and polarizations of the coupled waves. The dispersion and absorption characteristics of these vibration modes were also given based on the first-principle calculation and correlation Raman spectrum. Finally, the angle phase matching property and THz-wave gain were calculated. Simulation results showed that β-BTM is a kind of potential material for high-power tunable THz generation.
A compact and flexible dual-wavelength eye-safe intracavity optical parametric oscillator (IOPO) configuration driven by a coaxially end pumped laser was proposed. Two fundamental waves were provided by a coaxially end pumped Qswitched dual-wavelength laser with combined two laser crystals, and the OPO cavity was placed inside the laser cavity for efficient conversion. Theoretical simulations showed that the power ratio for each signal wave, as well as the time interval between two pulses at different wavelengths, were both tunable by tuning the pump focusing depth or pump wavelength. Experimental results were performed with combined laser crystals (Nd:YAG and a-cut Nd:YLF) and a nonlinear crystal (KTA), demonstrating coincident conclusions. The maximum OPO output power was 724 mW (388 mW at 1506 nm and 336 mW at 1535 nm) with the LD pump power of 10 W at 6 kHz, corresponding to the opticaloptical conversion efficiency of 7.24%. As there was no gain competition between two fundamental waves, stable signal output could be obtained. Moreover, various wavelength pairs can be generated by using different laser crystal combinations. It is believed that this is a promising method for simultaneously generating dual-wavelength eye-safe lasers pulses.
We demonstrate an optical parametric oscillator (OPO) based on random phase matching in a polycrystalline χ(2) material, ZnSe. The subharmonic OPO utilized a 1.5-mm-long polished ZnSe ceramic sample placed at the Brewster's angle and was synchronously pumped by a Kerr-lens mode-locked Cr:ZnS laser with a central wavelength of 2.35 μm, a pulse duration of 62 fs, and a repetition frequency of 79 MHz. The OPO had a 90-mW pump threshold, and produced an ultrabroadband spectrum spanning 3-7.5 μm. The observed pump depletion was as high as 79%. The key to success in achieving the OPO action was choosing the average grain size of the ZnSe ceramic to be close to the coherence length (~ 100 μm) for our 3-wave interaction. This is the first OPO that uses random polycrystalline material with quadratic nonlinearity and the first OPO based on ZnSe. Very likely, random phase matching in ZnSe and similar random polycrystalline materials (ZnS, CdS, CdSe, GaP) represents a viable route for generating few-cycle pulses and multi-octave frequency combs, thanks to a very broadband nonlinear response.
A compact and flexible dual-wavelength laser with combined two laser crystals (a-cut and c-cut Nd:YLF) as the gain media under coaxially laser-diode (LD) end-pumping configuration was demonstrated and μW-level THz wave was generated based on difference frequency generation (DFG) in a GaSe crystal. The dynamics of coaxial pumping dualwavelength laser was theoretically investigated, showing that the power ratio and pulse interval for both wavelengths could be tuned by balancing the gains at both wavelengths via tuning pump focal position. Synchronized orthogonal 1047/1053 nm laser pulses were obtained and optimal power ratio was realized with the total output power of 2.92W at 5 kHz pumped by 10-W LD power. With an 8-mm-long GaSe crystal, 0.93 μW THz wave at 1.64 THz (182 μm) was generated. Such coaxially LD end-pumped lasers can be extended to various combinations of neodymium doped laser media to produce different THz wavelengths for costless and portable applications.
We demonstrated a data-processing method based on multiple beam interference and Fresnel equations that simultaneously gave the refraction index and absorption coefficient from the raw data of terahertz (THz) time-domain spectroscopy (TDS), which laid the foundation for obtaining the dielectric parameters. This method was independent of phase processing, and complete material information was reserved without having to cut the time-domain signal. The optical coefficients including refractive indices and absorption coefficients of white polyethylene and quartz samples at different thicknesses were obtained. The applicability and accuracy of this method were discussed and verified by comparison with the traditional data-processing method of THz TDS.
Degenerate (subharmonic) optical parametric oscillators (OPO) show great promise for the generation of broadband mid-infrared (MIR) frequency combs. Their main features are low pump threshold, dramatic extension of the spectrum of the pump laser, and phase locking to the pump frequency comb. Here we report on obtaining instantaneous spectrum ranging from 2.85 to 8.40 μm at -40 dB level from a subharmonic OPO pumped by an ultrafast Cr2+:ZnS laser. Our experimental setup includes a free running Kerr lens mode locked 2.35 μm Cr2+:ZnS laser, with 62-fs time-bandwidth limited pulse duration, 630-mW average power, and 79 MHz repetition rate that synchronously pumps a ring-cavity orientation-patterned (OP-GaAs) based OPO. A 0.5-mm-long OP-GaAs crystal has a quasi-phase-matching (QPM) period of 88 μm and is designed to provide a broadband parametric gain at OPO degeneracy. A 0.3-mm-thick ZnSe wedge inside the cavity was used to minimize group velocity dispersion. Spectral span of 1.56 octaves in the MIR that we achieved can be further improved by fabricating an in-coupling dielectric mirror with (i) broader reflectivity range and (ii) with compensation of the residual group velocity dispersion. The broad spectrum achieved, 2.85 - 8.40 μm (2320 cm-1 wide instantaneous span), overlaps with a plethora of fundamental molecular IR resonances and can be used for frequency comb spectroscopic detection applied to such fields as remote sensing, study of fast combustion dynamics and medical diagnostics, to name a few.
A compact optical terahertz (THz) source was demonstrated based on an efficient high-repetition-rate doubly resonant optical parametric oscillator (OPO) around 2 μm with two type-II phase-matched KTP crystals in the walk-off compensated configuration. The KTP OPO was intracavity pumped by an acousto-optical (AO) Q-switched Nd:YVO4 laser and emitted two tunable wavelengths near degeneracy. The tuning range extended continuously from 2.068 μm to 2.191 μm with a maximum output power of 3.29 W at 24 kHz, corresponding to an optical-optical conversion efficiency (from 808 nm to 2 μm) of 20.69%. The stable pulsed dual-wavelength operation provided an ideal pump source for generating terahertz wave of micro-watt level by the difference frequency generation (DFG) method. A 7.84-mm-long periodically inverted quasi-phase-matched (QPM) GaAs crystal with 6 periods was used to generate a terahertz wave, the maximum voltage of 180 mV at 1.244 THz was acquired by a 4.2-K Si bolometer, corresponding to average output power of 0.6 μW and DFG conversion efficiency of 4.32×10-7. The acceptance bandwidth was found to be larger than 0.35 THz (FWHM). As to the 15-mm-long GaSe crystal used in the type-II collinear DFG, a tunable THz source ranging from 0.503 THz to 3.63 THz with the maximum output voltage of 268 mV at 1.65 THz had been achieved, and the corresponding average output power and DFG conversion efficiency were 0.9 μW and 5.86×10-7 respectively. This provides a potential practical palm-top tunable THz sources for portable applications.
We present a data processing method based on multiple beam interference and Fresnel's formula that extract simultaneously the refraction index and the extinction coefficient from terahertz time domain spectra, and the dielectric coefficient can also be calculated. Typical THz-TDS system working in transmission mode was utilized for direct measurement of the transmission spectra with a frequency accuracy of 7.6 GHz and range from 0.3 THz to 4 THz at room temperature. This method is verified with a double-faced polished 350-μm 100-cut GaAs wafer, and the reasonable average relative error for refractive index in the whole range is less than 0.83% comparing with conventional method, which provides a new approach to process the transmission spectra with oscillations.
High-repetition-rate, monochromatic and tunable terahertz (THz) source is demonstrated. We use an orthogonally polarized dual-wavelength intracavity OPO to complete the type-II phase-matched collinear difference-frequency generation in GaSe. A high average-power 2 μm laser with 12 W output power and good beam quality based on an intracavity KTP OPO is experimentally designed. The KTP OPO is intracavity pumped by an acousto-optical Q-switched side-pumped Nd:YAG with the repetition rate of 10 kHz. Two identical KTP crystals were 7 × 8 × 15 mm3 in size, cut at θ = 51.2°, φ = 0°, which were tuned in the x-z plane to achieve type-II phase-matching. The KTP OPO consists of two identical KTP crystals to reduce the walk-off effect and improve the beam overlap area of the output signal and idler waves. The pulse-width of the 2-μm KTP OPO laser is about 11 ns with the linewidth about 0.8 nm. The focused OPO beam is injected into the uncoated GaSe with the length of 8 mm, and the generated THz wave is detected with a 4.2-K Si-bolometer after focusing with a polyethylene lens. The tunable and coherent radiation from 0.2 to 3 THz has been achieved based on the type-II phase-matching DFG when the two pump waves are in the range of 2.1064 - 2.1272 μm and 2.1516 - 2.1304 μm while symmetrically tuning the phase-matching angle of the KTPs. The maximum output THz average power can reach μW-level around 1.48 THz.
Cascaded optical parametric oscillations generating a tunable terahertz (THz) wave are analyzed to solve the problem of low quantum conversion efficiency in a THz-wave parametric oscillator. The forward and backward optical parametric processes are theoretically analyzed based on periodically poled lithium niobate (PPLN) as an example. Tuning characteristics of the THz wave that relate to the parameters of the pump wavelength, the grating period of PPLN, and the working temperature are numerically simulated. The gain and absorption characteristics of the generating THz wave are deduced in the situation of quasiphase-matching configuration at different working temperatures.
We report a widely tunable terahertz source by using QPM-GaAs crystals pumped by a near-degenerate dual-wavelength KTP OPO around 2.127μm, based on difference frequency generation (DFG). The tunable THz radiation from 0.06 THz to 3.34 THz has been achieved in QPM-GaAs crystal with coherence length of 650 μm. The maximum output THz energy is 45 nJ with the peak power of 10 W at 1.68 THz, corresponding to the energy conversion efficiency of 5×10-6 and the photon conversion efficiency of about 0.08%.
An asymmetric planar terahertz (THz) metamaterial (MM) is designed to be composed of two different single split-ring resonators (SRR) and its character is simulated in the THz region. Gallium arsenide (GaAs) is inserted between the gap of two separated single SRR, and through modulation of its conductivity (σ GaAs ) we achieved the switching of three different resonance modes and researched the influence of σ GaAs on transmission of MM. The resonant structure of MM can be switched from the two fundamental LC-modes to the new LC-mode and dipole resonance mode through the coupling LC-mode with the increasing σ GaAs . Such dynamical control of MM resonances provides an efficient way to manipulate electromagnetic wave to push mode-switching of resonance and could be implemented in terahertz devices to achieve additional functionalities.
Al0.5Ga0.5As/GaAs/Al0.2Ga0.8As multiple asymmetric quantum well (AQW) have been investigated based on optical difference frequency in the 9 to 11 μm region. Under an intense resonant excitation from a dual-wavelength CO2 laser, the saturation intensity of intersubband absorption for pump waves is estimated to be 0.3 MW/cm2. As the well width variation, the position of absorption peak for pump waves and the absorption of terahertz (THz) wave by DFG show concomitant changes. For an AQW of 7 nm deep well-width and 27 nm total well-width, the maximum of absorption for the THz wave is 6.01×105 m−1 when the two pump wavelengths are 9.69 and 10.64 μm, respectively. These manipulative transitions in AQW can be applied to tunable optical semiconductor devices and implemented in THz wave devices to achieve additional functionalities.
A high-powered pulsed terahertz (THz)-wave has been parametrically generated via a surface-emitted THz-wave parametric oscillator (TPO). The effective parametric gain length under the noncollinear phase matching condition was calculated for optimization of the parameters of the TPO. A large volume crystal of MgO:LiNbO3 was used as the gain medium. THz-wave radiation covering a frequency range from 0.87 to 2.73 THz was obtained. The average power of the THz-wave was 9.12 μW at 1.75 THz when the pump energy was 94 mJ, corresponding to an energy conversion efficiency of about 9.7×10−6 and a photon conversion efficiency of about 0.156%. The THz-wave power in our experiments is high enough for practical applications to spectrum analysis and imaging.
A high stability high average-power green laser was reported with composite ceramic Nd:YAG as gain material and
KTP crystal as frequency doubler. Average output power of 165 W is obtained at a repetition rate of 25kHz with a diodeto-
green optical conversion of 14.68% and measured pulse width of 162 ns. For the average output power of about 160
W, the power fluctuation is less than 0.6%. The experimental results show that the green laser system using this novel
ceramic Nd:YAG crystal offers better laser performance and output stability
With the development of terahertz (THz) technology, an efficient propagation waveguide is essential for the construction
of compact THz devices. Hollow core photonic crystal fiber with a large air core at the center and a cladding formed by a
periodic arrangement of polymer tubes has been demonstrated in this paper. The guidance mechanism is based on
anti-resonant reflection from struts of solid material in the cladding. Since most electromagnetic field is dominated in the
air core, hollow core fibers have obvious advantages in lower absorption. The propagation characteristics of the fiber,
such as the mode field distribution and the loss coefficient are numerically investigated through the finite element
method. The result shows that an effective way to reduce the absorption is to enlarge the central air core and reduce the
overlap between the field and material.
High-power nanosecond pulsed THz-wave radiation was achieved via a surface-emitted THz-wave parametric oscillator
(TPO). The effective parametric gain length under the condition of noncollinear phase matching was calculated to
optimize the parameters of the TPO. Only one MgO:LiNbO3 crystal with large volume was used as gain medium.
THz-wave radiation from 0.8 to 2.9 THz was obtained. The maximum THz-wave output was 289.9 nJ/pulse at 1.94 THz
when pump power density was 211 MW/cm2, corresponding to the energy conversion efficiency of 3.43×10-6 and the
photon conversion efficiency of about 0.05%. The far-field divergence angle of THz-wave radiation was 0.0204 rad at
vertical direction and 0.0068 rad at horizontal direction.
We have achieved a Terahertz (THz) DFG system based on a walk-off compensated intracavity pumped dualwavelength
KTP OPO employing two identical KTP crystals. The KTP OPO is doubly resonant and works near the
degenerate point at 2.128μm, which doubles the quantum efficiency compared with DFG using pump pulses around 1μm.
This THz source is simple and compact, about 10×10×40cm2 in size. Besides lower threshold and better stability, the
walk-off compensated KTP OPO greatly improves the pump beam quality and enhances the DFG conversion efficiency.
With an 8-mm-long GaSe crystal, the generated THz tuning range is from 0.186THz to 3.7THz with the maximum
output voltage of 489V on the bolometer at 1.68THz. An average enhancement of 76.7% for the THz energies is realized
using the walk-off compensated KTP OPO than a common one. The conversion efficiency can be improved with a
longer and better GaSe crystal.
We report the generation of 1178nm based on cascaded Raman scattering in KTA crystal intracavity pumped by a AO
Q-switched Nd:YAG laser. The output power at 1178nm is around 80mW when the diode pump power was 7.6W at
808nm. At the same time, the low-order Stokes waves at 1091nm, 1120nm, 1146nm and visible yellow laser at 573nm
(the second harmonic wave of 1146nm) are also detected. The total Stokes output power was 240mW and the yellow
laser was 115mW. The power at 1178nm can be increased with output mirrors that are more suitable. The spectra of the
generated wavelengths were experimentally analyzed and they accords well with theoretical results.
Based on the theoretical analysis on the phase-matching relations, effective nonlinear coefficients, walk-off and
acceptance angles, the generation of tunable and coherent nanosecond mid-infrared radiation covering the 8-12μm range
is realized by use of difference frequency generation (DFG) in a GaSe and a ZnGeP2 (ZGP) crystal. Using an 8-mm-long
GaSe crystal, we achieve the mid-infrared generation that is continuously tunable from 8.28μm to 18.365μm. The
maximum pulse energy is 31μJ at 8.76μm, corresponding to the conversion efficiency of 0.9% and the maximum midinfrared
peak power of about 7kW. In the case of using an 8-mm-long ZGP crystal, the tuning range is from 7.2μm to
12.2μm. The maximum pulse energy is about 10μJ at 9.22μm, corresponding to the conversion efficiency of 0.45% and
the peak power of 2.2kW.
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