Continuous-wave Optical Parametric Oscillator Research Articles

Mid-infrared Doppler-free saturation absorption spectroscopy of the Q branch of CH4ν3 = 1 band using a rapid-scanning continuous-wave optical parametric oscillator.

We have developed a mid-infrared Doppler-free saturation absorption spectroscopy apparatus that employs a commercial continuous-wave optical parametric oscillator (CW OPO), complemented by a home-built automation and wavelength scanning system. Here, we report a comprehensive spectral scan of the Q branch transitions of the ν3 = 1 band of methane (CH4) with an average linewidth (FWHM) of 4.5 MHz. The absolute frequency calibration was achieved using previously reported transition frequencies determined using optical frequency combs, while a Fabry-Perot etalon was used for the relative frequency calibration. We report 15 transitions with improved accuracies of 1.13 MHz (3.76 × 10-5 cm-1).

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.

Widely tunable room-temperature green-pumped continuous-wave optical parametric oscillator from the red to mid-infrared

We report, to the best of our knowledge, the first realization of a room-temperature green-pumped continuous-wave (cw) optical parametric oscillator (OPO), providing wide and rapid tunability directly from the red to mid-infrared. The OPO, based on fanout MgO:sPPLT crystal and pumped at 532 nm, can provide a total signal and idler wavelength tuning range over 2554 nm, from 638 to 3192 nm. By deploying finite output coupling of the resonant wave at room temperature, also for the first time, to the best of our knowledge, we achieve enhanced output power and extraction efficiency, with the OPO delivering 4.6 W of total output power with pump depletion of 75% and total external efficiency as high as 50%. We believe 638 nm is the shortest signal wavelength generated from a cw green-pumped OPO in any fanout-grating nonlinear material to date. The passive power and frequency stability, linewidth, spatial beam profile, and beam pointing stability of the output-coupled signal, which are crucial for many applications, have been studied.

A Passively Wavelength-Stabilized Mid-Infrared Optical Parametric Oscillator

High-power and widely tunable continuous-wave optical parametric oscillators (cw OPOs), as mature radiation sources in the mid-infrared range, are limited by their frequency fluctuation. We built a cw OPO working at the near- and mid-infrared ranges and discuss the extent to which its frequency stability is affected by the mechanical temperature and the air pressure of the cavity. In addition, we artificially provoked mode hops by changing the above two factors to analyze the thermal-induced refractive index variation of the crystal. An inequality between the spacing of the hops and the free spectrum range (FSR) was observed. The wavelength stability of the cw OPO was guaranteed passively, and the longest mode-hopping-free time exceeded 13 h through sealing the cavity and thermal control. Further analysis of factors affecting frequency drift is critically essential for the reliability of cw OPO, particularly in environments outside the laboratory, and may provide a new method for high-precision tuning wavelengths.

Breakthrough instruments and products: Laser light tunable across the visible up to mid-infrared: Novel turnkey cw OPO with efficiency-optimized design.

Offering unique wavelength versatility, continuous-wave optical parametric oscillators (cw OPOs) are appealing sources of widely tunable laser light. Due to technical challenges, however, practical tunable cw OPO devices have so far been mostly limited either to near-infrared wavelength coverage or to maximum optical output powers in the 100 mW range-or both. We present a novel cw OPO designachieving output powers at the Watt-level throughout the very wide 500-765nm gap-free tuning range in the visible and coverage of up to 3540nm in the infrared spectral range. The system layout and its wavelength tuning options are tailored, in particular, to fundamental experiments in quantum and nanophotonics research as well as to applied holography.

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.

Application of a pulsed OPO seeded by a CW OPO to investigation of linewidth effects on TALIF excitation efficiency

In this article we present a widely tunable laser architecture for high-resolution nonlinear spectroscopy which incorporates a continuous wave optical parametric oscillator (OPO) for injection-seeding of a singly resonant actively-locked β-Ba(BO2)2 nanosecond OPO. Such an instrumental configuration is designed to provide near-transform-limited pulses over 450–650 nm signal, and 225–325 nm after signal doubling. Novel features that enable frequency agility and ramp-lock injection-seeding over a wide frequency range are described. System performance was demonstrated through investigations of two-photon laser induced fluorescence (TALIF) and the influence of excitation linewidth on the signal strength of the 5p6S0 - 6p[3/2]2 transition of atomic xenon at 252.49 nm. Measurements conducted at xenon partial pressures of 1×10-1-4×10-4 Torr, with and without active cavity locking and injection seeding, resulted in a 17× increased TALIF fluorescence yield during injection-seeded operation. The results are found to be consistent with modeling, taking into account the effects of lineshape, energy, and beam spatial cross-section. Modeling was informed by direct single-shot measurements of the signal beam lineshape in seeded and free-running operation using a virtually imaged phased array (VIPA) spectrometer.

Highly-Efficient Pulsed Mid-Infrared Generation Based on Intracavity Difference Frequency Mixing

We demonstrated highly-efficient pulsed mid-infrared generation based on intracavity difference frequency mixing inside a continuous-wave (CW) PPLN optical parametric oscillator (OPO). The near-infrared pulsed fiber source was located at 1120 nm with variable pulse width and repetition rate. The pump source of OPO was a CW linearly-polarized high-power 1060 nm fiber laser. The 1120 nm and 1060 nm pump beams were simultaneously incident into the OPO cavity. The 1060 nm pump beam built parametric oscillation firstly and generated high-power resonant signal beam inside the cavity. By controlling the PPLN temperature properly, the phase-matched difference frequency generation (DFG) occurred between the signal beam and 1120 nm pulsed pump beam in the PPLN crystal. Finally, the pulsed 1120 nm pump beam was successfully transformed to 3593 nm mid-infrared radiation at every power level and temporal characteristic. Both the slope efficiency and pump-to-idler conversion efficiency of the intracavity DFG process reached over 20%. In addition, the pulse shape of newly generated 3593 nm idler beam was basically the same as 1120 nm pump beam. The experimental results exhibited huge potential in frequency down conversion of low-power laser sources with special temporal characteristics.

Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe.

We report the first measurement of sub-Doppler molecular response using a frequency comb by employing the comb as a probe in optical-optical double-resonance spectroscopy. We use a 3.3 μm continuous wave pump and a 1.67 μm comb probe to detect sub-Doppler transitions to the 2ν_{3} and 3ν_{3} bands of methane with ∼1.7 MHz center frequency accuracy. These measurements provide the first verification of the accuracy of theoretical predictions from highly vibrationally excited states, needed to model the high-temperature spectra of exoplanets. Transition frequencies to the 3ν_{3} band show good agreement with the TheoReTS line list.

Measurement and assignment of double-resonance transitions to the 8900–9100- cm−1 levels of methane

Optical-optical double-resonance spectroscopy with a continuous wave pump and frequency comb probe allows measurement of sub-Doppler transitions to highly excited molecular states over a wide spectral range with high frequency accuracy. We report on assessment and characterization of sub-Doppler double-resonance transitions in methane measured using a 3.3-$\ensuremath{\mu}\mathrm{m}$ continuous wave optical parametric oscillator as a pump and a 1.67-$\ensuremath{\mu}\mathrm{m}$ frequency comb as a probe. The comb spectra were recorded using a Fourier transform spectrometer with comb-mode-limited resolution. With the pump tuned to nine different transitions in the ${\ensuremath{\nu}}_{3}$ fundamental band, we detected 36 ladder-type transitions to the $3{\ensuremath{\nu}}_{3}$ overtone band region, and 18 V-type transitions to the $2{\ensuremath{\nu}}_{3}$ overtone band. We describe in detail the experimental approach and the pump stabilization scheme, which currently limits the frequency accuracy of the measurement. We present the data analysis procedure used to extract the frequencies and intensities of the probe transitions for parallel and perpendicular relative pump-probe polarization. We compare the center frequencies and relative intensities of the ladder-type transitions to theoretical predictions from the TheoReTS and ExoMol line lists, demonstrating good agreement with TheoReTS.

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.

Green-pumped continuous-wave parametric oscillator based on fanout-grating MgO:PPLN.

We report the first green-pumped continuous-wave (cw) optical parametric oscillator (OPO) based on MgO:PPLN in a fanout grating design. Pumped by a single-frequency cw laser at 532 nm, the OPO provides tunable radiation across 813-1032 nm in the signal and 1098-1539 nm in the idler by simple mechanical translation at a fixed temperature of 55°C. By deploying a 25-mm-long crystal to minimize thermal effects and using output coupling for the signal wave, we generate a total output power of up to 714 mW at 30% extraction efficiency in excellent Gaussian beam quality with M2<1.1 and high output stability. Simultaneous measurements of signal and idler power result in passive stability of 2.8% and 1.8% rms, respectively, over 1 h. Strong thermal effects contribute to the high stability and excellent beam quality, while linear and green-induced infrared absorption limit the power scaling capabilities of the OPO. The output signal is single-mode with an instantaneous linewidth of ∼3MHz and frequency stability of ∼84MHz over 72 s.

Experimental study of the effect of phase mismatch on a CW-pumped cascaded quadratic nonlinear frequency comb

Optical frequency comb generation by continuous-wave-pumped cascaded quadratic nonlinearities (CQN) appears a promising alternative to well-established modelocked lasers, especially if access to the mid-infrared spectral region is needed. We report an experimental investigation of spectral properties of a CQN frequency comb, which is based on second-harmonic generation (SHG) and is pumped internally by a continuous-wave optical parametric oscillator. Our study focuses on SHG phase mismatch, which has drawn little attention in the previously reported CQN frequency comb research. The main observation of our study is that it is possible to improve spectral purity of the CQN frequency comb by adjusting the phase mismatch. In addition to the CQN process that generates a frequency comb in the near-infrared region, our experimental setup involves several other nonlinear processes. These processes lead to a composite comb, which extends from visible to mid-infrared.

All-fiber CW optical parametric oscillator tuned from 1642.5 to 1655.4 nm by a low-loss SMS filter

A continuous-wave tunable all-fiber optical parametric oscillator (FOPO) was demonstrated with a single-mode–multimode–single-mode (SMS) device used as wavelength selection filter. A highly nonlinear fiber (HNLF) was used to provide nonlinear gain, and two cascaded SMS fiber structures, having low insertion losses and containing multimode fibers of different lengths and core diameters, were used for effective wavelength selection. The maximum FOPO signal power was 0.5 W at 1642.5 nm, with 2 W of pump light incident on the HNLF end face. The power stability of the FOPO signal output was better than 0.2 dB over 30 min. A wavelength tuning range from 1642.5 to 1655.4 nm was realized by stretching a multimode fiber in the SMS device.

High-resolution analysis of the ν3 band of radiocarbon methane 14CH4

We present the first high-resolution measurements and rotational analysis of the fundamental asymmetric stretching vibrational band ν3(F2) of radiocarbon methane. A spectrometer consisting of a mid-infrared continuous-wave optical parametric oscillator and a cantilever-enhanced photoacoustic detector was employed to determine the wavenumbers of 43 rovibrational lines. A spectroscopic model could reproduce all the observed transition wavenumbers within the accuracy of the experiment. Our work contributes to the development of radiocarbon methane detectors based on laser spectroscopy.

Seeded optical parametric oscillator light source for precision spectroscopy.

Precision spectroscopy of fundamental bands of molecules in the mid-infrared (MIR) region is of great interest in applications of trace detection and testing fundamental physics, where high-power and narrow-linewidth MIR lasers are needed. By using a frequency-stabilized near-infrared laser as a seed of the signal light of a continuous-wave optical parametric oscillator, we established a broadly tunable MIR light source that has an output power of several hundred milliwatts and a linewidth of a few tens of kilohertz. The MIR laser frequency drift was reduced to below 1 kHz by using an optical frequency comb to stabilize the frequency of the 1064 nm pumping laser. The performance of the light source was investigated and tested by measuring the saturated absorption spectroscopy of a few molecular transitions at 3.3 µm.

Tunable ultraviolet vortex source based on a continuous-wave optical parametric oscillator

We report a continuous-wave (cw) optical parametric oscillator (OPO) generating optical vortices tunable in the ultraviolet (UV). Based on MgO:sPPLT as the nonlinear crystal, the singly resonant OPO is pumped by a cw vortex beam in the green, and deploying intracavity sum-frequency generation (SFG) between the undepleted pump and the Gaussian resonant signal in the crystal of BiB3O6, it can generate optical vortices of order, luv=1 and 2, tunable across 332-344nm in the UV with a maximum power of 12mW. Due to conservation of orbital angular momentum in the parametric process, the OPO also produces a non-resonant idler output beam in a vortex spatial profile of order li=1 and 2, identical to the pump vortex, with the signal beam in Gaussian distribution. The idler vortex is tunable across 1172-1338nm with maximum output power of 1.3W.

Widely tunable room-temperature continuous-wave optical parametric oscillator based on periodically-poled KTiOPO4.

We report the first realization of widely tunable continuous-wave (cw) optical parametric oscillator (OPO) based on periodically-poled KTiOPO4 (PPKTP) at room temperature. By exploiting fan-out grating design in a 30-mm PPKTP crystal, and configured in an output-coupled singly-resonant oscillator (OC-SRO) configuration pumped at 532 nm in the green, the OPO provides finely tunable radiation across 741-922 nm in the signal and 1258-1884 nm in the idler, at a fixed temperature of 25 °C. The use of output coupling for the signal wave enables enhancement of OPO extraction efficiency to 30%, providing a maximum total output power of 1.65 W (450 mW of signal at 901 nm and 1.2 W of idler at 1299 nm) for 5.5 W of pump power. The output idler exhibits passive power stability better than 3.2% rms over >2 mins, and the extracted signal exhibits frequency stability of 194 MHz over more than 35 seconds, in excellent beam quality. The OPO performance in pure SRO configuration has also been investigated.

Tunable, dual-wavelength, single-crystal, continuous-wave optical parametric oscillator∗∗This work was supported by Open Research Fund of State Key Laboratory of Pulsed Power Laser Technology, Electronic Countermeasure Institute, National University of Defense Technology (SKL2017KF04).

We demonstrated tunable, dual-wavelength (DW), continuous-wave mid-infrared generation in single-PPLN-based optical parametric oscillator (OPO) which was pumped by a DW Raman fiber laser located at 1060 and 1110 nm. Based on a singly resonant oscillator design, this device permits access to arbitrary signal and idler wavelength combinations within the parametric gain bandwidth and reflectivity of the OPO cavity mirrors. By tuning PPLN temperature, two pairs of signal and idler beams were obtained with arbitrary tuning across 1607–1626 nm and 3081–3583 nm. The frequency separation in the resonant signal waves was down to 5.7 THz. Moreover, near wavelength-matched condition, energy coupling between the resonant signal waves, resulted in reduced operation threshold and increased output power. A total DW output power (>6.8 W) is obtained. The experiment results exhibited great potential in mid-infrared generation using low-power pump beam with special characteristics.

Diode-laser-pumped continuous-wave optical parametric oscillator with a large mid-infrared tuning range

We report a singly resonant continuous-wave optical parametric oscillator (SRO), which is pumped by a simple semiconductor laser system and operates in the mid-infrared region (2.9 to 3.6μm). The pump laser system allows for a simple and cost-efficient SRO design with a small footprint. Tuning of the SROs mid-infrared output wavelength by more than 200 nm is demonstrated by scanning the pump laser wavelength through only 1.25 nm, without any need to adjust the SRO settings. The large tuning range is obtained by choosing a pump wavelength (780 nm) that corresponds to a turning point of the group velocity mismatch between the phase-matched signal and idler waves of the SRO.