Frequency-doubling Crystal Research Articles

Impact of the phase-mismatch in the SHG crystal and consequential self-action of the fundamental wave by cascaded second-order effects on the THG efficiency of a Q-switched 1342 nm Nd:YVO₄ laser.

We report on the influence of self-focusing and self-defocusing in the phase-mismatched frequency doubling crystal on the third harmonic generation (THG) efficiency in a two crystal frequency tripling scheme. By detuning the temperature of the doubling crystal, the impact of a phase-mismatch in second harmonic generation (SHG) on the subsequent sum frequency mixing process was investigated. It was found that adjusting the temperature not only affected the power ratio of the second harmonic to the fundamental but also the beam diameter of the fundamental beam in the THG crystal, which was caused by self-focusing and self-defocusing of the fundamental beam, respectively. This self-action was induced by a cascaded χ(2) : χ(2) process in the phase-mismatched SHG crystal. Self-defocusing was observable for positive detuning and self-focusing for negative detuning of the phase-matching temperature. Hence, the THG efficiency was not symmetric with respect to the point of optimum phase-matching. Optimum THG was obtained for positive detuning and the resulting self-defocusing in combination with the focusing lens in front of the THG stage was also beneficial for the beam quality of the third harmonic.

Investigation of bulk and waveguide frequency doubling crystals for interrogation of a two-photon transition in rubidium

The 5S1/2 (F=3)–5D5/2 (F=5) two-photon transition of rubidium (85) at 778nm is an excellent candidate for a high-quality portable frequency standard at the telecommunication region. The interrogation of the two-photon transition requires efficient second harmonic conversion of a laser at 1556nm. In this work, a comparison is made between the conversion efficiencies of bulk and waveguide periodically polled lithium niobate (PPLN) crystals. Both crystals are doped with 5% from MgO and used to generate frequency doubled light at 778nm from a fundamental light source operating at 1556nm. In order to obtain the optimum operating conditions, the frequency doubled power is measured as a function of laser wavelength and crystal temperature. The conversion efficiency of the waveguide PPLN is found to be around two orders of magnitudes larger than that of the bulk PPLN crystal for input powers less than 150mW. At the end, the generated second harmonic light from both crystals are used to interrogate the two-photon transition in a natural rubidium gas cell. A comparison is made between the fluorescence spectrums resulting from the two-photon transition using second harmonic generated powers from both crystals.

Super-narrow frequency conversion

The spectral width of a nonlinear converter is usually thought to be inversely proportional to the length of the nonlinear crystal. We present a method to overcome this limitation using the concept of super-oscillations, thus creating an arbitrarily narrow converter. A “super-narrow” frequency doubler was fabricated by appropriate modulation of its quadratic nonlinear coefficient, showing spectral and thermal response that are narrower by 39% and 69% compared to the side lobes and main lobe of the sinc function response of a standard frequency doubling crystal with the same length. This is accompanied by corresponding reduction of the efficiency to 14% and 0.79% with respect to those of the first side lobe and the main lobe. We propose more advanced modulation patterns, and discuss implications such as nonlinear filtering with higher resolution than the standard crystal.

Study on temperature adaptability extension of KTP frequency-doubling device

For the most commonly used KTP frequency-doubling crystal, its temperature adaptability range should be effectively extended. For this purpose, a compromise design is given by compreflensively considering both its effective nonlinear coefficient and the half-width of temperature range. The design method of KTP frequency-doubling device with a wide temperature range is analyzed in detail; furthermore, the curves of effective nonlinear coefficients, acceptance angles, and walk-off angles as a function of phase-matching angle are plotted via computer simulation. According to the results of theoretical study, a device used in the temperature range from -20 ℃ to 50 ℃ is designed and validated experimentally by the KTP external cavity frequency-doubling laser. Experimental results indicate that a peak conversion efficiency of 22.7% at 15 ℃ with a 70 ℃ temperature halfwidth is achieved by using the designed device. Compared with the commonly designed KTP frequency-doubling device, the temperature adaptability range increases notably although its frequency conversion efficiency decreases a little. Additionally, the effective nonlinear coefficient is still bigger than that of the commonly used crystals such as LBO and BBO when temperature halfwidth increases to 70 ℃. The above method would have the potential for extending the temperature adaptability range of other frequency-doubling devices.

High Electrical-to-Green Efficiency 123 W Average-Power Quasicontinuous-Wave Laser at 532 nm in Compact Design

We present an efficient high-power quasicontinuous-wave (QCW) all-solid-state laser at 532 nm with a simple and compact symmetrical resonator. We design a stable green laser at 532 nm with an average power of 123 W and a repetition rate of 10 kHz by combining a single side-pumped Nd:YAG laser module with a type II LBO crystal for intracavity frequency doubling. The corresponding electrical-to-green conversion efficiency is 12.5%. To the best of our knowledge, this is the highest electrical-to-green efficiency in 532 nm lasers with above 100 W output power ever reported. The pulse width is 62 ns at 123 W, so the peak power is calculated up to 198.4 kW. Such a compact and efficient resonator is promising for integration into laser-application systems.

C-cut Nd-doped vanadate crystal self-Raman laser with narrow Q-switched envelope and high mode-locked repetition rate

In this paper, a passively Q-switched and mode-locked c-cut Nd-doped vanadate crystal self-Raman laser at 1.17 μm is firstly demonstrated by using Cr4+:YAG. Two crystals of Nd3+:YVO4 and Nd3+:GdVO4 are adopted to generate laser, respectively. With the incident pump power of 13 W, the average output powers of 678 mW and 852 mW at 1.17 μm are obtained with the durations of Q-switched envelope of 1.8 ns and 2 ns, respectively. The mode-locked repetition rates are as high as 2.3 Hz and 2.2 GHz, respectively. As far as we know, the Q-switched envelope is the narrowest and the mode-locked repetition rate is the highest at present in this field. In addition, yellow laser output is also achieved by using the LiB3O5 frequency doubling crystal.

Structure, thermal and optical properties of Ca9Y0·5La0·5(VO4)7 – potential nonlinear optical material

The structure of a new nonlinear optical crystal, Ca9Y0·5La0·5(VO4)7, was solved, for first time to our knowledge. It belongs to trigonal system with space group R3c, its unit-cell parameters are: a = 10·8717(2) Å, c = 38·0753(14) Å, V = 3897·34(18) Å3, Z = 6, dc = 6·3279 g cm−3. The thermal expansion, the special heat capacity and the thermal conductivity of the crystal were investigated in detail. The crystal’s relative intensity for second harmonic generation is about 1·8 times as large as that of KH2PO4. Its transmittance exceeds 70% in the range of 400–2700 nm. From the measured principal refractive indices, the Sellmeier equations were derived. The results show that Ca9Y0·5La0·5(VO4)7 is expected to be a promising new frequency-doubling laser crystal.

Investigation of optical inhomogeneity of MgO:PPLN crystals for frequency doubling of 1560 nm laser

The asymmetrical phase-matching temperature tuning curves of periodically-poled 5%-mol MgO-doped lithium niobate (MgO:PPLN) bulk crystals for frequency doubling of 1560nm telecom laser have been observed experimentally and analyzed theoretically. We find that the asymmetrical phenomena are resulted from the variation in the extraordinary refractive index by the doping concentration inhomogeneity of the crystals. The critical parameters of phase-matching temperature mismatching γ(γ=δTpm/ΔT, in which δTpm is the change of the phase-matching temperature due to the doping concentration inhomogeneity, and ΔT is the temperature acceptance bandwidth) and the thermal refractive index ∂(Δn)/∂T are investigated. Our analysis method is not only a reasonable explanation to the experiment, but also can be a simple evaluation way of optical quality of the given MgO:PPLN bulk crystals. Moreover, the MgO:PPLN bulk crystal is used in single-pass configuration for frequency doubling a 4.4W Erbium-doped fiber amplifier boosted 1560nm telecom laser, and 300mW of tunable 780nm laser is obtained, corresponding to a conversion efficiency of 6.9%. This source provides a simple and compact laser system for rubidium atom cooling and manipulation experiments.

Intracavity-frequency-doubled Q-switched and mode-locked Nd:Lu0.15Y0.85VO4/KTP green laser with AO modulator and central SESAM

By using mixed crystal Nd:Lu0.15Y0.85VO4 as laser medium, KTP as frequency-doubling crystal, a diode-pumped intracavity-frequency-doubled Q-switched and mode-locked (QML) Nd:Lu0.15Y0.85VO4/KTP green laser with acousto-optic (AO) modulator and central semiconductor saturable absorption mirror (C-SESAM) is realized by using a V-type cavity. The QML laser characteristics such as the pulse width, single-pulse energy, have been measured for different modulation frequencies of the AO modulator. In comparison with the singly passively QML green laser with central SESAM, the doubly QML green laser can generate more stable and shorter pulses with higher peak power. Based on the coupled rate equations for a diode-pumped doubly QML green laser with AO and C-SESAM, in which the saturated absorption mechanism of C-SESAM is considered, the recurrence relation of the relative amplitude of the mode-locking pulses is given and the related numerical simulations are in good agreement with the experimental results.

678 nm RTP electro-optical Q-switched ceramic laser

The RTP electro-optical Q-switched ceramic laser at the wavelength of 678nm with narrow pulse width is studied. We used the laser diode arrays side-pumped Nd:YAG ceramic crystal with 1.1at% Nd doping and dimensions of Φ3mm×50mm, designed folding cavity parameters, and discussed the variation of the beam radius in the ceramic crystal and frequency doubling crystal with the thermal focal length of ceramic crystal or KTP crystal. By using double RTP crystals as electro-optic Q-switch and KTP crystal type II phase matching for intracavity frequency-doubling, a narrow pulse width electro-optical Q-switched Nd:YAG ceramic laser was obtained. The output energy of 0.9mJ and the pulse width of 41.6ns at 678nm are obtained at the repetition rate of 1000Hz and pumped power of 144W. The results formed the basis for the further development of the high power and high efficiency ceramic red laser.

A simplified 461-nm laser system using blue laser diodes and a hollow cathode lamp for laser cooling of Sr

We develop a simplified light source at 461 nm for laser cooling of Sr without frequency-doubling crystals but with blue laser diodes. An anti-reflection coated blue laser diode in an external cavity (Littrow) configuration provides an output power of 40 mW at 461 nm. Another blue laser diode is used to amplify the laser power up to 110 mW by injection locking. For frequency stabilization, we demonstrate modulation-free polarization spectroscopy of Sr in a hollow cathode lamp. The simplification of the laser system achieved in this work is of great importance for the construction of transportable optical lattice clocks.

Carrier-envelope phase stabilized high temporal contrast femtosecond laser source at 1053 nm

We demonstrate a carrier-envelope phase (CEP) stabilized high temporal contrast generator at 1053 nm for the first time. The device relies on a collinear optical parametric amplifier (OPA) followed by a frequency-doubling crystal. It is driven by a femtosecond laser source centered at 800 nm and finally generates CEP passively stabilized pulses with an energy of 130 μJ and a FWHM for the spectrum of 40 nm with 46 fs pulse duration at 1053 nm. The temporal contrast reaches >1011 at a few picoseconds before the main pulse.

Picosecond pulse of variable duration by phase matching frequency doubling crystal

Report on a semiconductor saturable absorber mirror (SESAM) passively mode-locked picosecond Nd:YVO4 laser, with adjustable durations between 8.7 and 70 ps. The pulse duration was adjustable by varying the length of type II phase matching KTiOPO4 (KTP) crystal. Type II phase matched second harmonic interaction in combination with the anisotropic emission characteristic of Nd:YVO4 introduced additional bandwidth limiting through birefringent filtering.

Broadly wavelength- and pulse width-tunable high-repetition rate light pulses from soliton self-frequency shifting photonic crystal fiber integrated with a frequency doubling crystal

Soliton self-frequency shift (SSFS) in a photonic crystal fiber (PCF) pumped by a long-cavity mode-locked Cr:forsterite laser is integrated with second harmonic generation (SHG) in a nonlinear crystal to generate ultrashort light pulses tunable within the range of wavelengths from 680 to 1800 nm at a repetition rate of 20 MHz. The pulse width of the second harmonic output is tuned from 70 to 600 fs by varying the thickness of the nonlinear crystal, beam-focusing geometry, and the wavelength of the soliton PCF output. Wavelength-tunable pulses generated through a combination of SSFS and SHG are ideally suited for coherent Raman microspectroscopy at high repetition rates, as verified by experiments on synthetic diamond and polystyrene films.

Compact Q-switched and mode-locked Nd3+:YVO4/Cr4+:YAG self-Raman laser

Compact passively Q-switched and mode-locked Nd(3+):YVO(4)/Cr(4+):YAG self-Raman laser with high mode-locked repetition rate was demonstrated. The picosecond first Stokes at 1176 nm was successfully obtained. At the maximum incident pump power, the average output power was 95 mW with the mode-locked pulse repetition rate of 940 MHz. Yellow laser output of 588.2 nm was also realized by the use of LiB(3)O(5) frequency doubling crystal.

An intracavity frequency doubled H_2 Raman laser scheme for generating narrow-linewidth yellow radiation

We propose a cavity-based combined Raman and second harmonic generation scheme for generating hundreds of milliwatts of continuous-wave yellow radiation with a frequency linewidth suitable for spectroscopic applications. We suggest using H2 gas as the Raman medium in an external Raman ring resonator with an intracavity frequency doubling crystal. Using gas rather than a crystal allows for single-mode, narrow-linewidth operation suitable for many spectroscopic applications. In a specific numerical example, we predict the generation of more than 200 mW of narrow-linewidth 583 nm light from an input of 1 W of 785 nm light, which could be obtained from a low cost tapered amplifier diode system. Finally, we suggest methods for improving the laser performance.

Passively Q-switched a-cut Nd:GdVO4 self-Raman laser with Cr:YAG

A passively Q-switched a-cut Nd:GdVO4 self-Raman solid-state laser with Cr:YAG saturable absorber was firstly demonstrated. The first Stokes at 1173nm was successfully obtained. At the maximum incident pump power, the pulse width was about 1.8ns and the repetition rate was 27.5kHz. 586.5nm yellow laser output was also realized by use of an LBO frequency doubling crystal.

Q-switching and mode-locking in a diode-pumped frequency-doubled Nd : YAG laser

A new method for obtaining Q-switching simultaneously with mode-locking using one travelling-wave acousto-optic modulator in a frequency-doubled Nd : YAG laser cavity is described. Further shortening of output laser pulses (from 40 to 3.25 ps) is achieved by forming a Kerr lens in the frequency-doubling crystal. At an average power of ∼ 2 W and a Q-switching rate of 2 kHz, the peak power of the stably operating reached ∼ 50 MW.

High-temporal-quality injector generated by optical parametric amplification with hollow-core-fiber compression

We demonstrate a 100 μJ, <10 fs, high-temporal-contrast injector. The experimental setup is based on an optical parametric amplification system followed by a standard hollow-core-fiber compressor and a frequency-doubling crystal. Pulses near 800 nm with FWHMs of about 160 nm are generated. More than 11 orders of magnitude of the contrast ratio, which is limited by the measurement device, are obtained over a large temporal range extending from <1 ps ahead of the main pulse. The broad spectral bandwidth and the high contrast of this device make it a good front-end injector for a few-cycle, high-power laser system.

Laser-diode end-pumped intracavity frequency doubling semiconductor saturable absorber mirror passively mode-locked picosecond lasers

A diode pumped Nd:YVO 4 semiconductor saturable absorber mirror (SESAM) passive mode-locked intracavity frequency doubled laser was studied. A type II phase matching KTiOPO 4(KTP) crystal and a type I phase matching LiB 3O 5 (LBO) frequency doubling crystal were respectively inserted in the cavity. With a pump power of 4.5 W, a 90 mW output of frequency doubled beam and 29.7 ps pulse duration were achieved with frequency doubled by KTP. With the same pump power a 140 mW output of frequency doubled beam and 3.67 ps pulse duration were achieved with LBO.