Single-pass Second-harmonic Generation Research Articles

20 Watt single-frequency 509 nm laser by single-pass second harmonic generation in an LBO crystal

High power 509 nm continuous-wave (CW) lasers have important applications in science and communication. Here we demonstrate a robust high-power single-frequency 509 nm laser system based on nonlinear phase demodulation technique and single-pass second harmonic generation (SHG) configuration. In experiments, the single-frequency fundamental wave at 1018 nm was linewidth-broadened by an electro-optical modulator and then amplified to 207 W in a ytterbium-doped fiber amplifier. In subsequent single-pass SHG stage, over 20 W CW single-frequency 509 nm laser was generated in a LiB3O5 crystal with a SHG efficiency of 9.7%. To the best of our knowledge, this is the highest reported power for CW single-frequency 509 nm laser, which could be used for advanced underwater optical communication and preparation of cesium Rydberg state.

Narrow Linewidth 510 nm Laser via Single-Pass Frequency-Tripling by Waveguide PPLNs

A single-frequency narrow linewidth green laser at 510 nm is a vital component for the study of Cesium Rydberg atoms. Here, we demonstrate a 510 nm laser based on single-pass second-harmonic generation (SHG) and sum-frequency generation (SFG) via waveguide Periodically Poled Lithium Niobate (PPLN) seeded with a common C-band laser (1530 nm). The final linewidth measured using the delayed self-heterodyne method reaches a narrow linewidth of 4.8 kHz. And, the optical-to-optical conversion efficiency is up to 13.1% and reaches an output power up to 200 mW.

Double-pass second-harmonic generation of picosecond pulses with custom-poled KTP crystal

We present the double-pass second-harmonic generation (SHG) of picosecond pulses with a custom-poled potassium titanyl phosphate (KTP) nonlinear crystal. The average output power of 466 mW at central wavelength of 515.7 nm is obtained with the input of 1.2 W fundamental laser pulses. Compared to the highest conversion efficiency of 29.1% in the single-pass SHG, the conversion efficiency in the double-pass SHG is increased to 38.8%. Moreover, the average RMS stability of 0.67% in 2 hours and high beam quality (M2 < 1.10) of the second-harmonic pulses is observed. The presented results provide an efficient method to enhance the conversion efficiency of SHG for picosecond pulses.

Second-Harmonic Generation of the Vortex Beams with Integer and Fractional Topological Charges

The single-pass second-harmonic generation (SHG) of a vortex beam under low fundamental wave depletion is systematically studied. Vortex modes at 1064 nm with integer topological charges from ±1 to ±9 and fractional ones at ±0.75 are generated by modulating the fundamental Gaussian beam with different spiral phase plates. The frequency doubling of these fundamental vortex modes is realized via single-pass SHG through the KTP. A detailed theoretical model is set up in the single-pass SHG of the vortex beams. Theoretical analysis indicates that the higher the order of the vortex beams, the lower the SHG efficiency, when the beam waists and fundamental power are given. The experimentally measured SHG output characteristics verify those obtained via theoretical analysis. Conservation of the orbital angular momentum during the SHG process is also verified, regardless of the fractional or integer vortex beams. SH LG0,2l vortex beams with high mode purity are obtained. The beam waists of fundamental/SH in KTP measured using a 4f system demonstrate that the Rayleigh ranges of the fundamental wave and SH wave are the same. The paper comprehensively presents some basic laws in the single-pass SHG of a vortex beam. In addition, it also indicates that SHG is an effective method to improve the mode purity of vortex beam.

Second and cascaded harmonic generation of pulsed laser in a lithium niobate on insulator ridge waveguide.

Nonlinear crystalline ridge waveguides, e.g., lithium niobate-on-insulator ridge waveguides, feature high index contrast and strong optical confinement, thus dramatically enhancing nonlinear interaction and facilitating various nonlinear effects. Here, we experimentally demonstrate efficient second-harmonic generation (SHG) and cascaded fourth-harmonic generation (FHG) in a periodically poled lithium niobate (PPLN) ridge waveguide pumped with pulsed laser at the quasi-phase matching (QPM) wavelength, as well as simultaneous SHG and cascaded third-harmonic generation (THG) waves when pumped at the non-QPM wavelength. Furthermore, the ridge waveguide achieves an efficient single-pass SHG conversion efficiency of picosecond pulsed laser at ∼62%. These results may be beneficial for on-chip nonlinear frequency conversion.

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.

A high-power and low-noise 532-nm continuous-wave laser for quantum gas microscopy.

Low-noise, high-power 532-nm lasers are of great interest in many scientific research studies, such as gravitational wave detection and ultracold atom experiments. In particular, in the experiments of quantum gas microscopy, a large power of laser is necessary during the imaging process, while low noise is important for preventing the atoms from being heated up. In this work, we report on the generation of such a 532-nm continuous-wave laser by coherently combining two laser beams produced by single-pass second-harmonic generation. The power of the combined laser is up to 17W. With the help of intensity stabilization, we are able to suppress the relative intensity noise to below -120 dBc/Hz. The generated laser satisfies the experimental requirements for integrating optical superlattices with a quantum gas microscope.

321 W high-efficiency continuous-wave green laser produced by single-pass frequency doubling of narrow-linewidth fiber laser.

In this study, a high-power continuous-wave green laser for copper processing is investigated. The laser is produced by single-pass second-harmonic generation (SHG) of a narrow-linewidth fiber laser. Based on Sellmeier equations, the factors (e.g., fundamental wave linewidth, temperature, and angle) that decide LBO noncritical phase matching are theoretically analyzed for optimizing the SHG setup. A narrow-linewidth polarization-maintained fiber laser is used as the fundamental laser with a linewidth of 20 GHz and polarization extinction ratio of greater than 15 dB. Type I noncritical phase-matching LBO is used as the nonlinear crystal in the SHG. A green laser (up to 321 W) is obtained from a 784 W fundamental laser with a conversion efficiency of 40.9%. The green laser is near-diffraction-limited and has a $ M^{2} $ factor of 1.07.

Nonlinear frequency conversion of 3D optical bottle beams generated using a single axicon

We demonstrate a novel experimental scheme to generate and study the nonlinear frequency conversion of a three-dimensional (3D) optical Bessel bottle beam (BBB). Using a single axicon and standard optical components and controlling the spot size and divergence of the input Gaussian beam to the axicon, we have generated stable micron-size, high-power optical BBB with tunable spatial characteristics. The BBB has a series of low-intensity regions surrounded by high intensity with diameters of ∼ 30 µ m and 17 µm, respectively, at a variable period of 2.3 to 6.4 mm along with the beam propagation. Using the single-pass second harmonic generation (SHG) of femtosecond BBB at 1064 nm in a bismuth triborate nonlinear crystal, we have generated BBB at 532 nm with output power as high as 75 mW and single-pass SHG efficiency of 1.9%. We also observed the self-healing of the BBB at both pump and SHG wavelengths. It is interesting to note that the pump beam truncation shows self-healing in the SHG beam. Such observation proves the direct transfer of the pump’s spatial characteristics to the SHG beam in the nonlinear process, potentially useful for imaging even in the turbid medium in biology. This generic scheme can be used at different wavelengths and timescales (continuous-wave to ultrafast).

Robust 17 W single-pass second-harmonic-generation at 532 nm and relative-intensity-noise investigation.

We demonstrate a 17 W single-frequency, low-intensity-noise green source at 532 nm, by single-pass second-harmonic generation of a 50 W continuous-wave fiber laser in a 30 mm MgO-doped periodically-poled stoichiometric lithium tantalate crystal. The maximum conversion efficiency is about 37%. A nearly Gaussian beam (M2<1.15 at 15 W) and low wavefront distortion are obtained. The system shows stable behavior over 100 h of uninterrupted operation. The evolution of the relative-intensity-noise transfer from the fundamental to the second harmonic is theoretically and experimentally investigated with high resolution.

Efficient watt-level 0.775 μm SHG and 3.3 μm DFG in annealed and reverse proton exchanged diced PPLN ridge waveguides

We report on the fabrication of ridge waveguides formed in congruent periodically poled lithium niobate (PPLN). The waveguides were fabricated by periodic poling, proton exchange, subsequent annealing and reverse proton exchange followed by diamond blade dicing. Up to 1 W of single-pass second-harmonic generation at 775 nm has been realized in 50 mm long ridge waveguides with internal conversion efficiency of 70%. Furthermore, difference frequency generation at 3.3 μm of two pump waves at 1.05 μm and 1.55 μm has been obtained in similarly fabricated PPLN ridge waveguides with an appropriate poling period.

Optimizing the efficiency of a periodically poled LNOI waveguide using in situ monitoring of the ferroelectric domains

Lithium niobate on insulator (LNOI) is a unique platform for integrated photonic applications and especially for high-efficiency nonlinear frequency converters because of the strong optical field confinement. In this work, we fabricated a 6-mm-long periodically poled LNOI ridge waveguide with an optimized duty cycle (50:50) using an active domain structure monitoring method. The performance of the single-pass second-harmonic generation and difference-frequency generation in the nanophotonic waveguide was characterized, and the normalized conversion efficiencies were ∼80% of the theoretical values. These high-quality frequency conversion devices can pave the way for the application of LNOI in nonlinear integrated photonics.

Compact, low-threshold squeezed light source.

Strongly squeezed light finds many important applications within the fields of quantum metrology, quantum communication and quantum computation. However, due to the bulkiness and complexity of most squeezed light sources of today, they are still not a standard tool in quantum optics labs. We have taken the first steps in realizing a compact, high-performance 1550 nm squeezing source based on commercially available fiber components combined with a free-space double-resonant parametric down-conversion source. The whole setup, including single-pass second-harmonic generation in a waveguide, fits on a 30 cm×45 cm breadboard and produces 9.3 dB of squeezing at a 5 MHz sideband-frequency. The setup is currently limited by phase noise, but further optimization and development should allow for a 19" sized turn-key squeezing source capable of delivering more than 10 dB of squeezing.

Frequency-conversion of vector vortex beams with space-variant polarization in single-pass geometry

Nonlinear frequency conversion processes depend on the polarization state of the interacting beams. On the other hand, vector vortex beams have space-variant polarization in the transverse beam plane. In light of these two points, is it possible to do nonlinear frequency conversion of the vector vortex beam in single-pass geometry and retain the characteristics of the beam? To address this question, here, we report an experimental scheme for single-pass second harmonic generation (SHG) of vector vortex beams. Using an ultrafast Ti:Sapphire laser of pulse width ∼17 fs and a set of spiral phase plates in a polarization based Mach–Zehnder interferometer (MZI), we have generated vector vortex beams of order as high as lp = 12 at an average power of 860 mW. Using two contiguous bismuth borate crystals with the optic axis orthogonal to each other, we have frequency-doubled the near-IR vector vortex beam into visible vector vortex beam with order as high as lsh = 24. The maximum output power of the vector vortex beam of order, lsh = 2, is measured be as high as 20.5 mW at a single-pass SHG efficiency of 2.4%. Controlling the delay in MZI, we have preserved the vector vortex nature of both the pump and frequency-doubled beams. Measurement on the mode purity confirms the generation of high quality vector vortex beams at pump and SHG wavelengths. This generic scheme can be used to generate vector vortex beams across the electromagnetic spectrum in all time scales, continuous-wave to ultrafast.

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.

2 W single-frequency, low-noise 509 nm laser via single-pass frequency doubling of an ECDL-seeded Yb fiber amplifier

A single-frequency low-noise green laser at 509nm is developed for the study of cesium Rydberg atoms. The laser is generated by single-pass second-harmonic generation of a Yb fiber amplifier seeded with an external cavity diode laser (ECDL) at 1018nm in a periodically poled MgO-doped lithium-niobate crystal. An up to 2.03W, 509nm laser is obtained from a 10.04W incident 1018nm laser with a conversion efficiency of 20.2%. The linewidth of the 509nm laser is estimated to be 40kHz according to the measured linewidth of 20kHz of the 1018nm fundamental laser. The relative intensity noise is 0.038% rms integrated from 10Hz to 10MHz.

On-axis intensity modulation-free, segmented, zero-order Bessel beams with tunable ranges

We propose and experimentally demonstrate a novel experimental scheme to generate high on-axis peak intensity, segmented, smooth, zero-order quasi-Bessel beams with tunable ranges. Illuminating the axicon with hollow Gaussian beams (HGBs) of different orders, we have generated Bessel beams of varying ranges at different positions away from the axicon. The presence of a dark core at the center of the HGBs removes the effect of imperfection in the axicon tip. As a result, the entire power of the input beam is transformed into a zero-order Bessel beam without any on-axis intensity modulation. We observe the decrease in range and increase in on-axis peak intensity of the zero-order Bessel beam with the order of HGBs. Controlling the superposition of the HGBs of different orders to the axicon, we have demonstrated the increase in the range of the Bessel beam. The current technique can also produce Bessel beams of different intensity distributions, including single-peak or multiple-peak Bessel beams. Using single-pass second-harmonic generation in nonlinear crystals of different lengths, we have further verified the increase of on-axis peak intensity of the Bessel beam with the order of the HGBs and the increase in the range of the Bessel beam due to the superposed HGBs.

Anisotropy of nonlinear optical properties in monoclinic SmxY1-xCa4O(BO3)3 crystals

The angular noncritical phase matching (A-NCPM) second-harmonic-generation (SHG) properties of SmxY1-xCOB (x = 0, 0.44, 0.55 and 1) nonlinear optical (NLO) crystals were investigated. The type-I A-NCPM SHG wavelengths along the Y and Z axes were found to be tunable in the ranges of 725–900 nm and 826–1116 nm, respectively, by adjusting the ratio of the rare-earth elements Sm and Y in SmxY1-xCOB crystals. Taking advantages of the fundamental laser pulses generated from a mode-locked Ti:Sapphire laser and a long Y-cut Sm0.44Y0.55COB crystal sample, the single-pass SHG average output power of ∼436 mW at 397 nm was obtained with a conversion efficiency of 41.5%. In addition, the SHG phase-matching (PM) curve and the effective NLO coefficients (deff) of SmCOB crystal for 1064 nm laser were studied, where the optimal type-I SHG PM direction in principal planes was determined to exist in the XY principal plane i.e. (90°, 52.4°), rather than the ZX plane i.e. (θº, 180°), which was different with those of TmCOB (32.5°, 180°), YCOB (30.7°, 180°), TbCOB (22.6°, 180°), GdCOB (19.7°, 180°) and LaCOB (15.1°, 180°) crystals in the same crystal family.

High efficiency single-pass SHG of low power CWML and QML laser in an MgO:PPLN

We present the second harmonic generation (SHG) of a low power and of both Q-switched mode-locked (QML) and CW (CWML) mode-locked picosecond (ps) laser at 1064 nm based on a single-pass frequency-doubling configuration of an end-pumped passively mode-locked Nd:YVO4 laser in a 3-mm periodically poled MgO-doped lithium-niobate (MgO:PPLN). The maximum SHG conversion efficiency is 27.4% when the laser is operated in the QML mode at an averaged pumping power of 895 mW and the conversion efficiency is reduced to 24.1% in the CWML mode with a maximum averaged pumping power of 1.37 W. The corresponding pulse width of the SHG output is estimated to be 15.5 ps. The repetition rates of the laser are 137 kHz for QML and 78.7 MHz for CWML. The theoretical efficiencies of SHG are calculated in both QML and CWML and the results agree very well with the experiments especially in the QML. This work provides a useful method for efficient frequency conversion for low power mode-locked lasers.

Blue and Orange Two-Color CW Laser Based on Single-Pass Second-Harmonic and Sum-Frequency Generation in MgO:PPLN

We demonstrate a compact blue and orange-two color continuous wave laser source emitting at 487 nm and from 597.4 to 600.3 nm, respectively. The temperature tunable coherent orange radiation is achieved by frequency mixing 974 nm laser diode (LD) and a C-band amplified spontaneous emission laser source while the temperature insensitive blue radiation is generated by second-order quasi-phase-matching frequency doubling of 974 nm LD. We implement the simultaneous nonlinear processes in a single magnesium oxide doped periodically poled lithium niobate bulk crystal without the need of an aperiodic design.