Singly-resonant Optical Parametric Oscillator Research Articles

High power, frequency agile comb spectroscopy in the mid-infrared enabled by a continuous-wave optical parametric oscillator.

While mid-infrared optical frequency combs have been widely utilized in areas such as trace gas sensing, chemical kinetics, and combustion science, their relatively low power has limited sensitivities and led to correspondingly long acquisition times. We have developed a new approach in which an electro-optic frequency comb is utilized to pump a continuous-wave singly-resonant optical parametric oscillator in order to spectrally translate the comb into the mid-infrared (2.19 µm and 4.00 µm) with watt-level powers. Through the use of electro-optic combs produced via chirped waveforms we have produced mid-infrared combs containing up to 2400 comb teeth. We show that an ultraflat comb can be generated on the non-resonant idler and we use these combs to perform high resolution spectroscopy on methane. In addition, we describe the underlying theory of this method and demonstrate that phase matching should allow for combs as broad as several THz to be spectrally translated to the mid-infrared. The high power and mutual coherence as well as the relatively low complexity of this approach should allow for broad application in areas such as chemical dynamics, quantum information, and photochemistry.

Auto-pump-depleted stable single-longitudinal-mode 1.5 μm source with the assistance of SHG.

To realize a stable single-longitudinal-mode (SLM) 1550-nm light source for the generation of non-classical states, a ring auto-pump-depleted singly resonant optical parametric oscillator (SRO) with the assistance of second-harmonic-wave generation (SHG) is designed and built in this Letter. A magnesium oxide doped periodically polarized lithium niobate (MgO:PPLN) crystal and a lithium triborate (LBO) crystal are employed as the optical parametric downconversion (OPDC) and SHG crystals, respectively. Especially, the introduced SHG can firstly increase the loss difference between the lasing and non-lasing modes so that the dual-mode or multi-mode coupling in the achieved SRO can be effectively eliminated and the stable SLM operation is achieved. At the same time, the SHG will automatically adjust the output coupling efficiency of SRO, so as to achieve efficient conversion efficiency and auto-pump depletion of SRO. In addition, due to the SHG, it is easy to achieve the low-intensity noise multi-wavelength output for the stable SLM SRO. As a result, the output powers of the SLM 1550 nm and 775 nm are up to 4.05 W and 3.25 W, respectively, and the total optical conversion of the built SRO can achieve 45.58%. The presented method paves a way to develop a compact stable SLM multi-wavelength SRO, and the obtained SRO is further beneficial to develop compact continuous-variable non-classical light fields.

Theoretical Analysis of Continuous-Wave Mid-Infrared Optical Vortex Source Generated by Singly Resonant Optical Parametric Oscillator

Due to the important application in the study of vibrational circular dichroism and helical dichroism of chiral molecules, the tunable vortex beam at mid-infrared region has attracted increasing attention. Based on orbital angular momentum (OAM) conservation in nonlinear interactions, the vortex pumped singly resonant optical parametric oscillator (SRO) is recognized as a versatile source of coherent vortex radiation providing high power and broad wavelength coverage from a single device. However, the low parametric gain and high oscillation threshold under continuous wave (cw) pumping has so far been the most challenging factor in generating cw tunable vortex beams. To predict the output characteristic of vortex pumped SRO, a theoretical model describing the vortex pumped SRO is needed. In this study, the theoretical model describing the vortex pumped SRO is set up under collimated Gaussian beam approximation. Output characteristics of different SROs are simulated numerically. By proper selection of pump scheme (such as double-pass pumping scheme), the vortex pumped mid-infrared SRO can oscillate at a relatively low pump power. By controlling the gain (mode overlap ratio between the pump and resonant wave in the nonlinear crystal) and loss (employing a spot-defect mirror with different defect size as the output coupler) of the resonant signal mode in the SRO, the OAM of the pump beam can be directionally transferred to a specific down converted beam. The transfer mechanism of the OAM among the pump light and the down-converted beams and factors affecting the transfer are studied. Our study provides the guidelines for the design and optimization of vortex pumped SRO under cw operation.

Green laser pumped congruently-grown LiTaO[formula omitted] based singly-resonant optical parametric oscillator

We report a continuous wave (cw) singly-resonant, single-frequency optical parametric oscillator (SRO) using 50 mm long congruently-grown 8-mol% MgO doped periodically poled LiTaO3 (MgO:cPPLT). The SRO is pumped by a high-power green laser derived from a near-infrared (NIR) fiber laser source. The SRO is tunable in visible band from 740–755 nm (signal) and mid-infrared (MIR) band from 1810–1930 nm (idler). The MgO:cPPLT crystal is characterized by a strong optical absorption at the pump wavelength and relatively small thermal conductivity which leads to substantial rise in crystal temperature which is spatially confined in the region where the pump is present. This local rise in temperature facilitates longitudinal dephasing by virtue of a large thermo-optic coefficient for MgO:cPPLT crystal and its impact manifests through an early saturation effect in conversion efficiency. The impact of thermo-optic manifestations could be observed in the measurement of spatial beam qualities of the modes.

Stimulated polariton scattering in an Yb laser pumped lithium niobate based continuous-wave singly resonant optical parametric oscillator

The effective refractive index of a dielectric medium modifies in the infrared spectral band due to a significant contribution from the vibrational modes by virtue of the presence of heavy ions. The coupling between electromagnetic (EM) modes and transverse optical (TO) vibrational modes is the underlying principle behind such a modification. This interaction manifests through the generation of polariton modes that can be observed in the Raman spectrum. However, they do not coincide with the peaks associated with the Raman scattering process. In the present work, we show that the polariton modes can be excited in a lithium niobate based continuous-wave singly resonant optical parametric oscillator (SRO) pumped by a single-frequency, near-infrared Yb-doped fiber laser. The stimulated polariton scattering process is triggered by a strong interaction between the mid-infrared resonant SRO signal beam and TO phonon modes of a lithium niobate crystal, which leads to generation of Stokes mode(s). Such mode(s) can be observed as side peak(s) in the SRO signal spectrum. The generated low energy polariton mode exhibits a dominant EM character, and this provides a plausible route to generate narrow linewidth far-infrared (FIR) or terahertz radiation by utilizing the high intra-cavity signal power in the SRO. In addition, the off-axis FIR generation process assists in reducing the thermal load on the lithium niobate crystal at high pump powers.

Experimental Study of Near-Infrared to Mid-Infrared Laser Output Based on Single Resonant Optical Parametric Oscillator

Experimental Study of Near-Infrared to Mid-Infrared Laser Output Based on Single Resonant Optical Parametric Oscillator

Development of single-resonant optical parametric oscillator with tunable output from 410 nm to 630 nm

A single-resonant low-threshold type-I β-Ba2BO4 (BBO) optical parametric oscillator (OPO) with tunable output from 410 nm to 630 nm at 5 kHz repetition rate is reported. By taking the noncollinear phase matching method, low-threshold OPO operation could be obtained compared with the configuration of collinear phase matching, and the maximum optical–optical conversion efficiency of 11.8% was achieved at 500 nm wavelength when 0.4 mJ pump pulse energy was applied. When the noncollinearity angle was preset at 1.6°, 4.8°, and 6.3°, a continuously tuning output with a total spectral range of 220 nm was successfully obtained by adjusting the phase matching angle of the BBO crystal.

Widely Tunable Narrow-Linewidth Intracavity Optical Parametric Oscillators Based on an Yb: KYW Laser

We report the generation of widely-tunable, narrow-linewidth midinfrared laser emission from an Yb: KYW-based intracavity singly-resonant optical parametric oscillator (ICSRO). The wavelength-tuning scheme is based on exploiting the wide emission-bandwidth of Yb: KYW crystals. Birefringent filters were used in the ICSRO cavity for linewidth narrowing and wavelength tuning. The benefit of this tuning scheme is that it does not rely on the temperature-tuning mechanism or the usage of customized fanout PPLN crystals. By using 6 different poling-periods of a 25 mm-long PPLN crystal, and synchronizing the tuning of the pump and signal waves, we obtained midinfrared idler light with a tuning range over 2600–4030 nm, having an estimated linewidth of less than 0.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> .

Tunable injection-seeded fan-out-PPLN optical parametric oscillator for high-sensitivity gas detection

We demonstrate the possibility to enhance spectroscopic and lidar capabilities of mid-infrared pulsed optical parametric oscillators (OPOs) based on fan-out periodically-poled lithium niobate (fan-out PPLN) by combining their own wavelength tunability with tunable continuous-wave injection seeding. To that effect, we have developed an original test-bed singly-resonant OPO configuration in which continuous wavelength tuning via intracavity translation of the fan-out PPLN is assisted by simultaneous tuning of a narrow-linewidth fiber-coupled seed diode laser. It is shown that such an approach provides strong (at least by one order of magnitude) spectral narrowing, wavelength stabilization and beam-quality improvement for the signal and idler radiations (tunable around 1.54 μm and 3.33 μm, respectively). The achieved spectral narrowing does not noticeably affect the OPO output pulse energy, but allows fine wavelength tuning and significantly increases the power spectral density which can be available at an analyzed gas absorption line. Thus, it leads to a noticeable increase in sensitivity of the photoacoustic gas sensing carried out with the developed OPO. We have achieved an almost double (∼80%) increase in the photoacoustic response of a methane detector to the reference gas mixture with low concentration methane. The application of the method can be simply extended to many gas absorptions lines within the range 3.1–3.4 μm, due to the potentially wide tunability of the developed OPO. The demonstrated OPO features simplicity, reliability and energy efficiency. It can advance various LIDAR techniques due to the unique combination of spectral and energy characteristics.

Thermo-Optic Effects in Congruent-LiTaO 3 Based Continuous-Wave Optical Parametric Oscillator

We present an experimental study on single-frequency, singly-resonant optical parametric oscillator (SRO) comprised of 50 mm long congruently-grown, 8-mol%-doped, periodically-poled LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (MgO:cPPLT) crystal. The SRO is pumped by a single-frequency, fiber laser pumped green laser. Our observations reveal that the SRO output (idler) scales linearly with the pump power ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ) near the SRO threshold ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p(th)</sub> ). However, the SRO idler power ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> ) exhibits strong saturation at pump power ≈ 1.5P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p(th)</sub> which eventually switches-off the SRO operation on further pump-power enhancement. By coarsely tuning the MgO:cPPLT crystal temperature from 50 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C to 170 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C, the SRO idler wavelength is varied from 1810-1920 nm wavelength band. A maximum idler power P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> =583 mW was obtained when the input pump power was ≈ 3.1 W. The idler power saturation is accompanied by discernible distortion in idler beam profile. An early onset of saturation in MgO:cPPLT is essentially a consequence of strong linear absorption exhibited by MgO:cPPLT at pump wavelength, its high thermo-optic coefficient as well as low thermal conductivity. The results are theoretically analyzed by solving the heat conduction equation which provides a deeper insight about the impact of pump-induced thermo-optic effects on a three-wave interaction process.

1.8-μm MgO: PPLN optical parametric oscillator pumped by Nd: YVO4/YVO4 2nd-Stokes Raman laser

An efficient MgO: PPLN singly resonant optical parametric oscillator (SROPO) pumped by a Nd: YVO4/YVO4 2nd-Stokes Raman laser at 1313 nm was demonstrated. The maximum conversion efficiency of 54.8% was obtained at the pump power of 1.16 W, which was higher than that of the OPO pumped by 1st-Stokes Raman laser at 1176 nm, due to the better beam quality, narrower line width and pulse width of the 2nd-Stokes Raman laser. The maximum output power of 1.57 W at 1855.5 nm with the pulse repetition frequency (PRF) of 40 kHz was achieved under the pump power of 4 W. The corresponding conversion efficiency was 39.7%, with the pulse width of 2.53 ns, linewidth of 0.45 nm, and M2 of 1.87. Meanwhile, with the crystal period ranging from 27.91 to 29.98 μm, the tuning range of signal wave was 1778.6–1861.8 nm.

Parametric oscillation of acoustical phonon mode in magnetized doped III–V semiconductors

Using the hydrodynamic model of semiconductor plasmas and coupled mode theory of interacting waves, we develop a theoretical formulation followed by numerical analysis to study parametric oscillation of acoustical phonon mode in magnetized doped III–V semiconductors. The origin of nonlinear interaction is assumed to lie in effective second-order optical susceptibility ( $$\chi^{(2)}$$ ) arising due to nonlinear induced current density and acousto-optic polarization of the semiconductor medium. Expressions are obtained for threshold pump intensity ( $$I_{{0,{\text{th}}}}$$ ) and operational characteristics such as conversion efficiency ( $$\eta_{k}$$ ) and single pass signal power gain ( $$g_{{{\text{sp}}}}$$ ) of the proposed single resonant optical parametric oscillator. Numerical analysis is made for a representative n-InSb crystal irradiated by 10.6 µm pulsed CO2 laser. The effects of some important parameters including external magnetic field ( $$B_{0}$$ ), mirror reflectivity ( $$R$$ ), crystal length ( $$L$$ ), etc. on $$I_{{0,{\text{th}}}}$$ , $$\eta_{k}$$ and $$g_{{{\text{sp}}}}$$ of the single resonant optical parametric oscillator are analyzed in detail. The analysis establishes the technological potentiality of magnetized doped III–V semiconductors as the hosts for fabrication of highly efficient and widely tunable single-resonant optical parametric oscillators.

High-resolution, broadly-tunable mid-IR spectroscopy using a continuous wave optical parametric oscillator.

We report on the design and automation of a mid-infrared, continuous wave, singly-resonant optical parametric oscillator. Hands-free controls and the implementation of a tuning algorithm allowed for hundreds of nanometers of continuous, effective-mode-hop-free tuning over the range of 2190-4000 nm. To demonstrate the applicability of this light source and algorithm to mid-IR spectroscopy, we performed a sample spectroscopy measurement in a C2H2 gas cell and compared the experimentally-measured absorption spectrum to HITRAN 2016 simulations. We found excellent agreement with simulation in both peak heights and peak centers; we also report a reduced uncertainty in peak centers compared to simulation.

A Wavelength Tunable CW Orange-red Laser Source Based on Magnesium Oxide-doped Periodically-poled LiNbO3 in an Intracavity Sum-frequency Generation

A watt-level continuous wave (CW) orange-red laser with wavelength tunable properties is experimentally implemented based on sum-frequency generation (SFG) in a composite cavity. The cavity is composed of an 880 nm laser diode (LD) side-pumped Nd: GdVO4 p-polarized 1062.9 nm cavity and a single-resonant optical parametric oscillator (SRO) of signal light using a magnesium oxide-doped periodically-poled LiNbO3 (MgO: PPLN) crystal with a poling period of 29.1 μm. In the overlap region of the two cavities, orange-red laser is generated by using a type-II critical phase-matched potassium titanyl phosphate crystal (KTP) crystal. In the temperature tuning range of the MgO: PPLN crystal (from 30 °C to 200 °C), the CW orange-red laser beams are generated in a tunable waveband from 612.24 nm to 620.72 nm (Δλ = 8.48 nm), corresponding to the mid-infrared idler light is also obtained with tunable wavelength from 4027.5 nm to 3708.2 nm (Δλ = 319.3 nm). At the lowest tuning temperature of 30 °C, the maximum CW output power of the orange-red laser at 612.24 nm and the idler light at 4027.5 nm are obtained, which are 1.145 W and 2.83 W, respectively.

High-Power High-Repetition-Rate Tunable Yellow Light Generation by an Intracavity-Frequency-Doubled Singly Resonant Optical Parametric Oscillator

An efficient intracavity-frequency-doubled singly resonant optical parametric oscillator (SR-OPO), pumped by a high-maturity 532 nm laser, tunable in the yellow wavelength region is demonstrated. An elaborate V-shaped idler cavity design is employed for a potassium titanyl phosphate (KTP) OPO nonlinear crystal to achieve a 5.06 W idler output at 1151.2 nm at a 10 kHz pulse repetition frequency. Using a second KTP crystal to intracavity frequency double the circulating idler wave, a 2.37 W yellow output at 575.6 nm is generated. High optical conversion efficiency (25.4% and 11.2%), 0.5 nm spectral linewidth, and 32 ns pulse duration of the 575.6 nm yellow output are achieved.

Two-Color Quantum Correlation between Down-Conversion Beams at 1.5 and 3.3 μm from a Singly Resonant Optical Parametric Oscillator

We demonstrated a two-color quantum correlation between the down-conversion beams with a telecommunication wavelength at 1.5 μm and mid-infrared wavelength at 3.3 μm generated by a singly resonant optical parametric oscillator (SRO) operated above the pump threshold with a magnesium-oxide doped periodically-poled lithium niobate crystal in the cavity. A maximum of 1.8 dB noise reduction of the intensity difference of the twin beams was measured at the analysis frequency of 5 MHz. Based on a theoretical model for the quantum correlation between the twin beams given by a semi-classical approach, the influence of the analysis frequency and pump parameter on the quantum correlation between the twin beams was discussed theoretically and experimentally. The quantum correlation between the twin beams was degraded at the analysis frequencies above 5 MHz due to the limitation of the bandwidth of SRO cavity and was degraded at the analysis frequencies below 5 MHz due to the excess intensity noise of the pump. The two-color quantum correlated twin beams at 1.5 and 3.3 μm have potential applications in high-precision measurements beyond the shot noise level.

Design of optical parametric cavity for broadband squeezed light field

The field of squeezed state is an important quantum resource in the study of quantum optics. In the application of quantum information, the spectrum bandwidth of the squeezed light field is an important index to limit the information transmission capacity. Currently, the optical parametric oscillator (OPO) is one of the most efficient ways to generate high squeezed non-classical optical fields. In this paper, the degenerate singly-resonant and doubly-resonant OPO structures are introduced. Both OPOs are composed of concave mirrors and periodically poled potassium titanyl phosphate crystals (PPKTP). The length of PPKTP crystal is 10 mm. The curvature radius of the curved surface is 12 mm, and it has high reflectivity at 1550 nm and 775 nm. The plane surface is coated with anti-reflection coating. The air gap length is 21 mm. The concave mirror is an output coupling mirror, and its radius of curvature is 25 mm. In the singly-resonant OPO, only the signal light resonates in the cavity, and the pump light passes through the nonlinear crystal twice and then outputs out of the cavity. The reflectivity of OPO output coupling mirror to the wavelength of 1550 nm is 88%. The linewidth of the corresponding fundamental frequency wave is 77.4 MHz. For doubly-resonant OPO, both the signal light and the pump light resonate simultaneously in the cavity. The reflectivity of OPO output coupling mirror to 1550 nm and 775 nm is 85% and 97.5%, respectively. The linewidth of the corresponding fundamental frequency wave and harmonic is 97.1 MHz and 15.6 MHz, respectively. Then the threshold of OPO is calculated. The threshold pump power of OPO increases with signal light transmittance increasing, but the threshold value of doubly-resonant OPO is obviously smaller than that of singly-resonant OPO. After that, the variation of the squeezing bandwidth of the squeezed light field generated by OPO with the transmittance of the signal is analyzed. Finally, we complete the design of quantum squeezer with low threshold (18 mW), broadband (84.2 MHz) and high stability (the standard deviation of locking baseline is 0.32 MHz) experimentally. The results show that compared with the singly-resonant optical parametric oscillator, the doubly-resonant cavity has the characteristics of low threshold and high stability, which is more suitable for the preparation and practical application of broadband squeezed light field.

Green-pumped optical parametric oscillator based on fan-out grating periodically-poled MgO-doped congruent LiTaO3.

We report a green-pumped optical parametric oscillator (OPO) based on periodically poled MgO-doped congruent lithium tantalate (MgO:cPPLT). Pumped at 532nm by a frequency-doubled Q-switched Nd:YAG laser, and using a fan-out grating structure, the singly-resonant OPO provides continuous tuning across 689-1025nm in the signal and 1106-2336nm in the idler at room temperature by simple mechanical translation of the crystal. The tuning range can be further extended to 677nm and 2479nm in the signal and idler, respectively, by temperature tuning the crystal to 200°C. With a 29-mm-long crystal, the OPO generates 131mW of average idler power at 1476.5nm for an input pump power of 1.8W at 25kHz repetition rate, with a slope efficiency of 11.3%. Bulk damage in the MgO:cPPLT crystal has been observed for pump powers above ∼1.8 W, and at pump powers beyond ∼1.4 W under long-term operation. The passive power stability of the generated idler is 3.9% over 30min, in a Gaussian spatial profile.

Mid-infrared dual-comb spectroscopy with absolute frequency calibration using a passive optical reference.

We demonstrate an absolute-frequency-calibrated mid-infrared dual-comb spectrometer by using a reference absorption cell. The source is based on a singly-resonant OPO containing two MgO:PPLN crystals in a common ring cavity, synchronously pumped by two mode-locked Yb-fiber lasers. The repetition-rate of the two pumps are stabilized while their offset frequencies and the OPO cavity length are not actively controlled. The reference spectrum is used to correct the frequency fluctuations in the sample spectrum providing a high-quality averaged spectrum with spectral resolution of 6 GHz and calibration precision of 120 MHz, without adding any complexity to the experimental setup or signal processing.

Tunable visible frequency combs from a Yb-fiber-laser-pumped optical parametric oscillator

We present a 100 MHz repetition rate Yb-fiber-laser-pumped synchronously pumped optical parametric oscillator (SPOPO) delivering tunable frequency combs covering almost the entire visible spectral range. By intracavity doubling both the signal and idler combs and using collinear residual pump light, nearly continuous tuning over the range of 420-700 nm is achieved with only small gaps near the OPO degeneracy condition. Output powers range from 10 mW to 200 mW, depending on wavelength, with pulse durations below 150 fs without external compression. Frequency locking of all three collinearly outcoupled combs (pump, doubled signal, and doubled idler) to a femtosecond enhancement cavity facilitates direct comparison of their optical phase noise and phase modulation transfer functions. In the singly-resonant OPO, optical phase modulation of the pump comb is transferred nearly completely to the non-resonant idler comb. This results in a resonant signal comb with reduced optical phase noise and also enables high-bandwidth modulation on the idler comb via phase modulation of the pump.