Alireza Khorsandi

Thermal Effects in High-Power Continuous-Wave Single-Pass Second Harmonic Generation

We present a theoretical model which describes the effects of thermal load distribution on single-pass second harmonic generation (SP-SHG) of high-power continuous-wave (cw) radiation in MgO:sPPLT nonlinear crystal to provide green output at 532 nm. Numerical simulations are performed based on real practical values and actual operating conditions associated with a recent SP-SHG experiment, generating 10 W of cw green radiation using a Yb-fiber laser. The model is used for four oven configurations to simulate the implications of thermal effects on SH power. The observed asymmetric feature of the phase-matching curves, particularly at higher fundamental powers up to 50 W, are characterized and explained by considering the generation of heat due to crystal absorption. The concept of optical path difference (OPD) is introduced to study the formation of thermal lens and its effects on the displacement of focal point inside the thermally loaded crystal. We further study the dependence of the SH power on the different oven schemes by increasing the input fundamental power up to 50 W. It is found that a top-sinked oven design is the optimum configuration for achieving maximum SHG efficiency without saturation. Comparison of the simulation results with experimental data confirms the validity of the theoretical model.

Focusing Optimization for High-Power Continuous-Wave Second-Harmonic Generation in the Presence of Thermal Effects

We study thermal effects in high-power single-pass second-harmonic generation (SP-SHG) of continuous-wave (cw) green radiation in MgO:sPPLT and describe optimization of critical factors including the fundamental beam waist, position, and focusing parameter, for attainment of maximum SP-SHG efficiency. By developing a numerical model combining split-step Fourier and finite-element methods, we compute the relevant parameters for optimization of SP-SHG of a cw Yb-fiber laser at 1064 nm in a 30-mm-long crystal. Real experimental parameters and boundary conditions are used to simulate nonuniform heat distribution in the crystal under high-power operation and study its effects on SP-SHG optimization. The results indicate that for input powers exceeding a certain limit, the Boyd and Kleinman (BK) criteria for optimum focusing are no longer valid. By increasing the fundamental power to 50 W, we find that the optimum focusing parameter decreases by a factor of 4, from ξ =2.8 to 0.7. We also find that the maximum SP-SHG output power is always obtained for fundamental waist at the center of the crystal, and we present a general strategy for the choice of ξ at different fundamental powers. Using a top-sinked oven design, the BK focusing condition can be established even at high fundamental powers.

Power instability of singly resonant optical parametric oscillators: Theory and experiment

We present a theoretical model on the effects of mechanical perturbations on the output power instability of singly-resonant optical parametric oscillators (SR-OPOs). Numerical simulations are performed based on real experimental parameters associated with a SR-OPO designed in our laboratory, which uses periodically-poled LiNbO3 (PPLN) as the nonlinear crystal, where the results of the theoretical model are compared with the measurements. The out-coupled power instability is simulated for a wide range of input pump powers the SR-OPO oscillation threshold. From the results, maximum instability is found to occur at an input pump power of ~1.5 times above the OPO threshold. It is also shown theoretically that the idler instability is susceptible to variations in the cavity length caused by vibrations, with longer cavities capable of generating more stable output power. The validity of the theoretical model is verified experimentally by using a mechanical vibrator in order to vary the SR-OPO resonator length over one cavity mode spacing. It is found that at 1.62 times threshold, the out-coupled idler suffers maximum instability. The results of experimental measurements confirm good agreement with the theoretical model. An intracavity etalon is finally used to improve the idler output power by a factor of ~2.2 at an input pump power of 1.79 times oscillation threshold.

Active Control of Thermal Dephasing Effect in High Power Continuous Wave Single-Pass Second Harmonic Generation

In this paper, we present a theoretical model that describes the performance of a specific oven configuration used to actively control the effect of thermal dephasing on the output characteristics of a high-power single-pass second-harmonic generation (SHG) in a MgO:sPPLT nonlinear crystal. The provided model is based on using step and slope oven configurations and making a quantitative comparison with the recent results obtained by using open-top oven scheme. It is found that in the slope oven scheme the SHG power is enhanced by 190% at a fundamental power of 50 W, indicating the significance of the represented model. Eventually, the merit of the proposed slope oven indicated through saturation control in the generation of high-power SH radiation.

Multisegment oven scheme for Gouy phase shift compensation in second-harmonic generation.

We present a theoretical model that describes a multisegment oven configuration to compensate for the Gouy phase shift in the tight focusing regime. The model is provided to obtain maximum efficiency of the second-harmonic wave. Numerical simulations are performed based on the experimental setup employing an MgO:sPPLT nonlinear crystal. It is shown by the simulation that the three-segment oven is potentially capable of reaching the highest efficiency by canceling the effect of Gouy phase when the temperature of each segment is so optimized that the phase-matching condition is re-established. It is found that by using a multisegment oven scheme with optimized temperature, the highest attainable second-harmonic generation (SHG) efficiency exceeds about 4.4% at a confocal parameter of 3.32. Moreover, it is shown that for long crystals with large confocal parameter, the only way to attain the maximum efficiency is by using multisegment ovens. The results indicate that when the number of segments is changed from one to nine, the confocal parameter can be varied from 3.90 to 5.85. In the candidate three-segment oven, by applying active control on the temperature of each segment, for a certain combination of segment lengths, ∼4% improvement in the SHG efficiency is achieved. This results in a relatively large increase in the acceptance temperature bandwidth of the crystal of up to 2 times, reflecting a comparatively large enhancement in the second-harmonic power stability and efficiency.

Application of a characterized difference-frequency laser source to carbon monoxide trace detection

A tunable continuous wave (cw) mid-infrared (MIR) laser based on difference-frequency generation (DFG) in a 1.5-cm long AgGaS2 nonlinear crystal for trace gas detection is reported. Two visible and near-infrared diode lasers were used as pump and signal sources. The MIR-DFG laser was tunable in a wavelength range of 4.75 μm-4.88 μm. The phase-matching (PM) condition was non-critically achieved by adjusting the temperature of the crystal for fixed pairs of input pump and signal wavelengths. The required PM temperatures of the generated MIR-DFG wavelengths have been calculated by using three sets of recent Sellmeier equations and the temperature-dispersion equations of AgGaS2 given by Willer U, et al. (Willer U, Blanke T and Schade W 2001 Appl. Opt. 40 5439). Then the calculated PM temperatures are compared with the experimental values. The performance of the MIR-DFG laser is shown by the trace detection of the P(16) carbon monoxide (12C16O) absorption line in a laboratory-fabricated absorption cell. The enhanced sensitivity of about 0.6×10−4 was obtained through the long path absorption provided by consecutive reflections between coated cylindrical mirrors of a constructed cell.

Intracavity conf iguration for tuning and obtaining simultaneous dual-wavelength operation from CW Nd:YAG laser

In this letter, a thin slab of glass is used as Fabry–Perot etalon inside a cavity of a continuous wave (CW) Nd:YAG laser to change the behavior of its output longitudinal modes. The slab etalon is used as tuning element when it turns around the laser cavity axis. Two simultaneous longitudinal modes with a relatively wide tuning range from 5.83 MHz to 0.02 THz are obtained when the Nd:YAG laser is operated at moderate output power of about 120 mW. The mode structure of this configuration is modeled and simulated. Computer-generated diagrams are also presented schematically and compared with the experimental results.

Engineered aperiodically poled nonlinear crystal for high-power second-harmonic generation

We demonstrate the use of an engineered aperiodically poled (APP) crystal grating to generate a high-efficiency second-harmonic radiation of about 66% at 50 W of fundamental power at room temperature. By embedding several phase compensators along the propagation direction, it is shown that such structure is capable of compensating thermal de-phasing and Gouy phase shift without the need for a controllable oven for the crystal. Compared to a temperature-optimized conventional PP crystal, an APP device can produce second-harmonic generation light by a factor of about two times. Moreover, a multipath structure of an APP is theoretically investigated to decrease the sensitivity of the device to the pumping power variation. Eventually, the acceptance bandwidth of an APP scheme is found to be enhanced by 3.3°C and 0.5°C compared with temperature-optimized PP crystal.

Thermal dephasing compensation in high-power and high-repetition-rate second-harmonic generation using spillover loss

In this paper, the effect of thermal dephasing on the efficiency of a high-power and high-repetition-rate single-pass second-harmonic generation (SHG) in the nanosecond regime is studied. We found that inside a thermal loaded crystal, in addition to the optimization of the focusing parameter and oven temperature, spillover loss can also play a controlling role in retaining the SHG efficiency. It is shown that with a proper choice of the focusing parameter and oven temperature, at about 20 W of averaged fundamental power, at least 60% SHG efficiency is achievable. Moreover, exceeding a certain limit of input power up to 80 W, inducing a spillover loss of about 10%, will help to maintain the SHG efficiency at a maximum value of about 50%.

Thermal dephasing in an enhanced cavity based high-power second harmonic generation

We report on the performance of an enhanced-cavity (EC) designed for obtaining high-power and efficiency second-harmonic generation (SHG). This is performed by numerical simulation of SHG coupled equations in the presence of a thermal dephasing effect that is effectively intensified through embedding the periodically poled LiNbO3 crystal by an oven-surrounded scheme. It is found that by setting the PPLN temperature at an optimum value, adjusting the mirror reflectively, and pumping power at certain values, gaining SHG efficiency of more than 90% is possible. We further realized that by an ECSHG device SHG efficiency can be improved by about 15%-50%. Moreover, compared to a single-pass SHG scheme, the EC-based SHG device is shown to be a very promising candidate to reduce and suppress the effect of thermal dephasing on the stability and efficiency of SHG radiation.