Intracavity Second Harmonic Generation Research Articles

Compact efficient high-power continuous-wave Nd:YVO4/KGW/LBO Raman lasers for selectable wavelengths within 559-603 nm

Compact efficient high-power continuous-wave Nd:YVO4 Raman lasers for selectable wavelengths within 559-603 nm are achieved by using KGW crystal for intracavity stimulated Raman scattering (SRS) and lithium triborate (LBO) crystal for intracavity sum frequency generation (SFG) and second harmonic generation (SHG). The LBO crystal with the cut angle in the XY plane for the type-I phase matching is used to perform intracavity SHG or SFG. Experimental results reveal that the participated Stokes lines include the internal vibration mode at 901 cm-1, the external vibrational mode at 209 cm-1, and the combination mode of the 901 cm-1 and 209 cm-1 Raman shifts. By tuning LBO temperature for attaining the maximum output power, the output spectrum reveals the triple peaks of 588.7, 595.7, and 603.1 nm with the intensity ratio of 10:4:1. Under this circumstance, the output power can reach the highest value of 10.8 W at a pump power of 40 W. Furthermore, the output spectrum can be simply concentrated on the single peaks among 588.7 (orange), 565.7 (yellow), and 559.1 nm (lime) by tuning LBO temperature to fulfill the selection of the critical phase matching. The output powers at a pump power of 40 W can be up to 8.0, 6.1, and 9.8 W for the single-peak emission at 588.7, 565.7, and 559.1 nm, respectively. Finally, a dual-peak emission of 565.7 and 572.3 nm with total output power of 5.2 W can be generated by tuning LBO temperature to match the SFG for 572.3 nm.

Highly efficient diode-pumped passively Q-switched Nd:YVO4/KGW Raman lasers at yellow and orange wavelengths.

Efficient diode-pumped passively Q-switched Nd:YVO4 yellow and orange lasers are developed with the pulse pumping scheme and the intracavity stimulated Raman scattering (SRS) and second harmonic generation (SHG). A Np-cut KGW is exploited in the SRS process to generate the yellow 579 nm laser or the orange 589 nm laser in a selectable way. The high efficiency is achieved by designing a compact resonator to include a coupled cavity for intracavity SRS and SHG and to provide a focused beam waist on the saturable absorber for reaching an excellent passive Q-switching. The output pulse energy and peak power can reach 0.08 mJ and 50 kW for the orange laser at 589 nm. On the other hand, the output pulse energy and peak power can be up to 0.10 mJ and 80 kW for the yellow laser at 579 nm.

Intracavity second-harmonic generation tunable dual-frequency passively Q-switched Nd:YAG laser with polarization splitting

An intracavity, second-harmonic generation, tunable, dual-frequency, passively Q-switched Nd:YAG laser based on a T-resonator configuration with polarization splitting is proposed, whose frequency difference could be doubled in comparison with fundamental lasing. The Nd:YAG, Cr4 + : YAG, and potassium titanyl phosphate (KTP) crystals were set at the shared arm, which could considerably reduce thermal fluctuation and pulse timing jitter between dual-frequency lasers. One birefringent filter consisting of a polarized beam splitter and a half-wave plate (HWP) is placed in each divided arm to select their single longitudinal mode. As a result, the p-polarized and s-polarized passively Q-switched components of 532 nm are simultaneously operated, whose power easily reaches to same and frequency is beneficial to tune theoretically throughout the whole gain bandwidth. The main characteristics of the power, longitudinal mode selection, and the pulse have been tested experimentally. Moreover, the frequency difference of the dual-frequency laser at 532 nm has been widely tuned from 9.6 to 117 GHz, by slightly adjusting the tilt angles of the HWPs. We offer a simple and widely tunable source with potential for portable frequency reference applications in absolute-distance interferometry, terahertz-wave generation, and other fields.

Wavelength-versatile deep-red laser source by intracavity frequency converted Raman laser.

We demonstrate an efficient wavelength-selectable output in the attractive deep-red spectral region from an intracavity frequency converted Nd:YLF/KGW Raman laser. Driven by an acousto-optic Q-switched 1314 nm Nd:YLF laser, two first-Stokes waves at 1461 and 1490 nm were generated owing to the bi-axial properties of KGW crystal. By incorporating intracavity sum-frequency generation and second-harmonic generation with an angle-tuned bismuth borate (BIBO) crystal, four discrete deep-red laser emission lines were yielded at the wavelengths of 692, 698, 731, and 745 nm. Under the incident pump power of 50 W and the repetition rate of 4 kHz, the maximum average output powers of 2.4, 2.7, 3.3, and 3.6 W were attained with the pulse durations of 3.4, 3.2, 4.3, and 3.7 ns, respectively, corresponding to the peak powers up to 177, 209, 190, and 245 kW. The results indicate that the Nd:YLF/KGW Raman laser combined with an angle-adjusted BIBO crystal provides a reliable and convenient approach to achieve the selectable multi-wavelength deep-red laser with short pulse duration and high peak power.

Mobile Optical Communications Using Second Harmonic of Intra-Cavity Laser

Optical wireless communication (OWC) meets the demands of the future six-generation mobile network (6G) as it operates at several hundreds of Terahertz and has the potential to enable data rate in the order of Tbps. However, most beam-steering OWC technologies require high-accuracy positioning and high-speed control. Resonant beam communication (RBCom), as one kind of non-positioning OWC technologies, has been proposed for high-rate mobile communications. The mobility of RBCom relies on its self-alignment characteristic where no positioning is required. In a previous study, an external-cavity second-harmonic-generation (SHG) RBCom system has been proposed for eliminating the echo interference inside the resonator. However, its energy conversion efficiency and complexity are of concern. In this paper, we propose an intra-cavity SHG RBCom system to simplify the system design and improve the energy conversion efficiency. We elaborate the system structure and establish an analytical model. Numerical results show that the energy consumption of the proposed intra-cavity design is reduced to reach the same level of channel capacity at the receiver compared with the external-cavity one.

Yb-fiber-pumped high-average-power picosecond optical parametric oscillator tunable across 1.3−1.5 μm

We report the generation of high-repetition-rate picosecond pulses in the 1.3-1.5µm spectral range by internal second harmonic generation (SHG) of an idler-resonant optical parametric oscillator (OPO) based on MgO-doped periodically-poled LiNbO3 (MgO:PPLN), synchronously pumped by ∼20 ps pulses at 80 MHz using an Yb-fiber laser at 1.064 µm. By taking advantage of the high spatial quality of the resonant idler beam in the 2503-3030 nm wavelength range and using a second MgO:PPLN crystal with fanout grating structure for intracavity SHG, we have achieved spectral coverage across 1272-1515 nm with up to 1.23 W average power. The second harmonic output exhibits a power stability of 3% rms over 1 hour in pulses of 8.3 ps with Gaussian beam profile. The described approach overcomes the spectral limitation of 1.064 µm-pumped OPOs based on MgO:PPLN and other oxide-based nonlinear crystals, where signal generation below ∼1.45 µm is precluded by multi-phonon absorption of idler radiation above ∼4µm.

Simulations and Experiments Toward Continuous Wave 167 nm Laser Generation for ARPES With High Energy Resolution

Continuous wave (CW) laser at a vacuum ultraviolet (VUV) range with the narrow-linewidth is an ideal optical source in angle-resolved photoemission spectroscopy (ARPES) for the research of superconductors with a narrow band gap. In this study, we present an eighth-harmonic-generation (EHG) laser scheme for CW laser generation at the VUV range, in particular at 167.75 nm, based on the cascaded power enhancement cavities. An intracavity second-harmonic generation (ICSHG) 671 nm laser with the narrow-linewidth and active frequency stabilization is built as the first stage, delivering the 2.55 W output power. A resonant cavity for fourth-harmonic-generation (FHG) constitutes the second stage, which generates the 335.5 nm laser with the output power of up to 1.25 W. The third stage is designed for the EHG of 167.75 nm based on the KBBF crystal. To realize the efficient CW laser generation at 167.75 nm, a theoretical analysis concerning the enhancement factor and the conversion efficiency of the KBBF-based EHG is carried out. The results show that it is possible for mW-level 167.75 nm generation if the transmittance of the KBBF prism-coupled device is increased to 97%. A 59 W circular intracavity power is observed in the 335.5 nm enhanced cavity experiments, corresponding to the peak power density of up to 20.86 MW/cm2. This work paves a solid way for CW VUV laser generation with the narrow-linewidth, which would be an ideal tool for the extremely high resolution ARPES.

A systematic study of PPLN length dependence in intracavity SHG

In this paper, a systematic study of the relationship between nonlinear crystal length and intracavity second-harmonic generation (SHG) using MgO-doped periodically-poled lithium niobate (MgO:PPLN) is presented. The experimental results demonstrate a relationship between the maximum SHG power generated and the full-width at half maximum (FWHM) of the crystal’s temperature tuning curve to the length of the nonlinear optical crystal. It was shown that maximum SHG power increases rapidly with the increase of MgO:PPLN length, reaching a saturation length (~ 2 mm), which is much shorter than that predicted by the single-pass SHG theory. This saturation length of the MgO:PPLN crystal is almost independent on 808 nm pump power for typical powers used in continuous wave intracavity SHG lasers. In addition to this saturation effect, a broadening effect was also observed, the FWHM of the temperature tuning curve was shown to have a larger FWHM than that predicted by the single-pass SHG theory for MgO:PPLN shorter than the saturation length. This work has the benefit of allowing engineers to optimize nonlinear crystal length when developing intracavity SHG based diode-pumped solid state (DPSS) lasers.

Study of fundamental wave depletion in intracavity second harmonic generation.

In this paper, a novel approach to modelling intracavity second harmonic generation (SHG) in periodically poled MgO-doped lithium niobate (PPLN) is presented and verified against experimental results. This approach involves combining the coupled nonlinear wave equations with a rate equation model for a diode-pumped solid-state Nd:YVO4 laser, taking into account both the depletion of the fundamental wave due to the energy conversion from the fundamental wave to the SHG wave and the reduction of the fundamental wave within a laser cavity due to the loss as a result of the SHG nonlinear process. It was shown that the theoretical simulation matched the experimental results well, while also providing physical insight into the importance of the fundamental wave depletion in the intracavity SHG nonlinear processes. The resulting model is computationally simple and has the potential to generalize to the other nonlinear processes such as three-wave mixing and optical parametric oscillation.

Investigation on Intracavity SHG With Controllable Coherence in a Degenerate Laser

We investigate intracavity second harmonic generation (SHG) in a degenerate laser for generating green laser with controllable degree of coherence. It is shown that spatial coherence of the SHG emission can be controlled by varying the diameter of the limiting aperture in the degenerate laser resonator. The pumping power also affects the number of the oscillating transverse modes, so that influences the spatial coherence of the output laser. The laser of low coherence for speckle reduction has been demonstrated. It is shown that the laser of low spatial coherence may find applications in imaging, and laser display etc.

Intracavity second-harmonic pulse generation at 261 and 320 nm with a Pr3+:YLF laser Q-switched by a Co2+:MgAl2O4 spinel saturable absorber.

We demonstrated the power scaling of a passively Q-switched Pr3+:LiYF4 (YLF) laser at 523, 607, and 640 nm with a Co2+:MgAl2O4 (MALO) saturable absorber and analyzed the experimental results with a numerical simulation based on rate equations. A maximum pulse energy of 33.5 µJ was obtained with a pulse width of 30.9 ns and a repetition rate of 64.0 kHz at 640 nm. We demonstrated ultraviolet pulse generation at 261 and 320 nm by intracavity frequency doubling and obtained 63.3-ns pulses with a pulse energy of 7.0 µJ and a repetition rate of 64.6 kHz at 320 nm as well as 356-ns 261-nm pulses with a pulse energy of 0.2 µJ at a repetition rate of 82.0 kHz. To the best of our knowledge, a 261-nm pulse is the shortest wavelength laser directly generated from the intracavity second-harmonic generation of passively Q-switched lasers.

Intensity noise suppression of a high-power single-frequency CW laser by controlling the stimulated emission rate.

The intensity noise of a high-power single-frequency continuous-wave (CW) laser is very harmful for applications in precise measurements and quantum communication. By simply lengthening the length of a laser resonator to decrease the stimulated emission rate of the laser, the coupling strength of all noise sources in the resonant laser field will be reduced, and thus the intensity noise of the output laser will be prominently suppressed. Based on theoretical analyses of the laser noise spectra, we experimentally implement a low-noise high-power single-frequency laser with a 1050 mm long resonator. With the assistance of an intracavity imaging system and nonlinear second-harmonic generation, the amplitude of the resonant relaxation oscillation peak and the shot-noise level (SNL) cutoff frequency are successfully reduced to 8.6 dB/Hz above SNL and 1.0 MHz, respectively, under the output power of 16 W. The work provides an effective way to develop a high-quality laser with high output power and low intensity noise.

Compact self-illuminated image upconversion system based on intracavity second-harmonic generation.

We present an image upconversion system based on intracavity Type II second-harmonic generation to create an image in the visible spectrum of a target illuminated by an infrared laser. The system has the novelty of being self-illuminated. It uses some fractional leaking power of the infrared laser to illuminate a target located in the object focal plane of the system, and to couple back a created infrared target image to an intracavity nonlinear crystal, where it mixes with the cavity laser beam to obtain a second-harmonic image, visible with a silicon CCD camera. For a proof of concept, we have built a system based on a diode-pumped Nd3+:YVO4 continuous-wave (cw) laser and an intracavity KTP crystal to upconvert 1342nm target images to 671nm. The upconverted cw power allowed us to capture real-time video in a standard nonintensified CCD camera, with 2.5W of a diode pump.

Single-frequency linear orthogonally polarized modes resulting in unidirectional traveling wave cavity green laser

A narrow linewidth (∼ 8 MHz , instrument limited) CW green laser at 532 nm is demonstrated in linear orthogonally polarized modes resulting in an unidirectional travelling wave cavity using intracavity second-harmonic generation technique with 250-mW output power.

50-kHz, 50-ns UV pulse generation by diode-pumped frequency doubling Pr3+:YLF Q-switch laser with a Cr4+:YAG saturable absorber.

We demonstrate intracavity second-harmonic generation at 320 nm of a diode-pumped praseodymium-doped YLF laser Q-switched by a Cr4+:YAG crystal. By employing two 3.5-W high-power blue InGaN diode lasers as the pump source, we obtained 50-ns Q-switched pulses with a pulse energy of 1.54 μJ at a repetition rate of 50 kHz. A rate equation analysis shows good agreement with the experimental results.

Diode-Pumped Yb:S-FAP Thin-Disk Laser Operating at 985 nm and the Second-Harmonic Generation at 492.5 nm

We report a diode-pumped continuous-wave (CW) thin-disk Yb3+-doped Sr5(PO4)3F (Yb:S-FAP) laser operating at 985 nm. We achieve a power of 4.34 W at 980 nm in the CW operation regime with a fiber-coupled laser diode emitting 17.2 W at 914 nm. Furthermore, we demonstrate intracavity second-harmonic generation in the continuous-wave mode with a power of 893 mW at 492.5 nm using a BiB3O6 (BiBO) nonlinear crystal. The fluctuation of the blue output power was better than 3.57%. The M2 factors are about 1.15 and 1.18 in the X and Y directions, respectively.

Frequency-comb formation in doubly resonant second-harmonic generation

We theoretically study the generation of optical frequency combs and corresponding pulse trains in doubly resonant intracavity second-harmonic generation (SHG). We find that, despite the large temporal walk-off characteristic of realistic cavity systems, the nonlinear dynamics can be accurately and efficiently modelled using a pair of coupled mean-field equations. Through rigorous stability analysis of the system's steady-state continuous wave solutions, we demonstrate that walk-off can give rise to a new, previously unexplored regime of temporal modulation instability (MI). Numerical simulations performed in this regime reveal rich dynamical behaviours, including the emergence of temporal patterns that correspond to coherent optical frequency combs. We also demonstrate that the two coupled equations that govern the doubly resonant cavity behaviour can, under typical conditions, be reduced to a single mean-field equation akin to that describing the dynamics of singly resonant cavity SHG [F. Leo et al., Phys. Rev. Lett. 116, 033901 (2016)]. This reduced approach allows us to derive a simple expression for the MI gain, thus permitting to acquire significant insight into the underlying physics. We anticipate that our work will have wide impact on the study of frequency combs in emerging doubly resonant cavity SHG platforms, including quadratically nonlinear microresonators.

Optimization of nonlinear crystal location for intracavity second-harmonic generation

We discuss optimization of locations for the nonlinear crystal in intracavity generation of second-harmonic radiation and show that optimizing the distance between the nonlinear crystal and the two-wave end mirror in the laser cavity may support nearly a 100% increase in the maximum emission power of the second harmonic. The optimum distance depends on the type of end mirror used. We describe a variety of effects that mask the advantages conferred by the optimum location of the nonlinear crystal when performing experiments on intracavity generation of strong second-harmonic laser radiation. We describe various approaches for experimental determination of the optimal location of the nonlinear crystal in the cavity.

Highly efficient 400 W near-fundamental-mode green thin-disk laser.

We report on the efficient generation of continuous-wave, high-brightness green laser radiation. Green lasers are particularly interesting for reliable and reproducible deep-penetration welding of copper or for pumping Ti:Sa oscillators. By intracavity second-harmonic generation in a thin-disk laser resonator designed for fundamental-mode operation, an output power of up to 403W is demonstrated at a wavelength of 515nm with almost diffraction-limited beam quality. The unprecedented optical efficiency of 40.7% of green output power with respect to the pump power of the thin-disk laser is enabled by the intracavity use of a highly efficient grating waveguide mirror, which combines the functions of wavelength stabilization and spectral narrowing, as well as polarization selection in a single element.

Tunable femtosecond near-infrared source based on a Yb:LYSO-laser-pumped optical parametric oscillator**Project supported by the National Key Basic Research Program of China (Grant No. 2013CB922402), the National Key Scientific Instruments Development Program of China (Grant No. 2012YQ120047), the National Natural Science Foundation of China (Grant Nos. 61205130 and 11174361), and the Key Deployment Project of Chinese Academy of Sciences (Grant No.

We demonstrate a widely tunable near-infrared source from 767 nm to 874 nm generated by the intracavity second harmonic generation (SHG) in an optical parametric oscillator pumped by a Yb:LYSO solid-state laser. The home-made Yb:LYSO oscillator centered at 1035 nm delivers an average power of 2 W and a pulse duration as short as 351 fs. Two MgO doped periodically poled lithium niobates (MgO:PPLN) with grating periods of 28.5–31.5 μm in steps of 0.5 μm and 19.5–21.3 μm in steps of 0.2 μm are used for the OPO and intracavity SHG, respectively. The maximum average output power of 180 mW at 798 nm was obtained and the output pulses have pulse duration of 313 fs at 792 nm if a sech2-pulse shape was assumed. In addition, tunable signal femtosecond pulses from 1428 nm to 1763 nm are also realized with the maximum average power of 355 mW at 1628 nm.