Single-frequency Optical Parametric Oscillator Research Articles

Watt-level single-frequency optical parametric oscillator around 2 µm.

A single-frequency optical parametric oscillator (OPO) operating in the wavelength range around 2 µm is reported. The OPO comprises a periodically poled LiNbO3 (PPLN) crystal for quasi-phase matching and a four-mirror ring-cavity resonant to the signal. The OPO signal and idler are tuned from 1.89 to 2.05 µm and from 2.21 to 2.42 µm, respectively, by changing the crystal temperature. Output powers in excess of 2 W are demonstrated at a pump power of 9 W for both the signal and idler over all of the respective tuning ranges. A detailed characterization of the intensity and frequency noise is presented. The OPO exhibits an integrated intensity noise below 0.1% and a linewidth of ∼58 kHz for an observation time of 1 ms. The reported performance makes this source an ideal candidate for the generation of squeezed light around 2 µm via cascaded second-harmonic generation (SHG) and parametric downconversion (PDC).

Single-frequency optical parametric oscillator intracavity-pumped by a visible VECSEL for low-noise down-conversion to 1.55 µm.

We report, to the best of our knowledge, the first optical parametric oscillator (OPO) pumped by a visible AlGaInP-based vertical-external-cavity surface-emitting laser (VECSEL). Tunable emission over 1155-1300 nm in the signal and 1474-1718 nm in the idler are observed by temperature adjustment of a 40 mm-long 5%-MgO:PPLN crystal intracavity-pumped at 690 nm. When optimized for low oscillation threshold, and by implementing resonant idler output-coupling (TOC = 1.7%), extracted output powers of 26.2 mW (signal) and 5.6 mW (idler; one-way) are measured, corresponding to a total down-conversion efficiency and extraction efficiency of 70.2% and 43%, respectively. Further, a total down-conversion efficiency of 72.1% is achieved in the absence of idler output-coupling. Of particular interest for high-precision applications, including quantum optics experiments and squeezed light generation, high stability and single-frequency operation are also demonstrated. We measure RMS stabilities of 0.4%, 1.8% and 2.3% for the VECSEL fundamental, signal and idler, with (resolution-limited) frequency linewidths of 2.5 MHz (VECSEL) and 7.5 MHz (signal and idler).

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.

Long-wave infrared multi-wavelength optical source for standoff detection of chemical warfare agents.

We have designed and built a wavelength-tunable optical source for standoff detection of gaseous chemicals by differential absorption spectrometry in the long-wave infrared. It is based on a nanosecond 2 µm single-frequency optical parametric oscillator, whose idler wave is amplified in large aperture Rb:PPKTP crystals. The signal and idler waves are mixed in ZnGeP2 crystals to produce single-frequency tunable radiation in the 7.5-10.5 µm range. The source was integrated into a direct detection lidar to measure sarin and sulfur mustard inside a closed chamber, in an integrated path configuration with a noncooperative target.

Numerical analysis for single-frequency nanosecond optical parametric oscillators and optical parametric amplifiers

We numerically analyze single-frequency nanosecond optical parametric oscillators and amplifiers. It shows that a single-frequency optical parametric oscillator requires a single-frequency pump wave and a seed source for injection locking. The power of the injection seed should be increased dramatically to maintain single-frequency output as pump energy increasing. The results numerically verify that high energy and single-frequency output waves can hardly be reached simultaneously by optical parametric oscillators. Multistage optical parametric amplifiers are helpful to meet the shortcoming of optical parametric oscillators. During the amplification, the pump energy should match the signal energy in optical parametric amplifiers to obtain high conversion efficiency.

Microsecond fiber laser pumped, single-frequency optical parametric oscillator for trace gas detection

We report on the first microsecond doubly resonant optical parametric oscillator (OPO). It is based on a nested cavity OPO architecture allowing single longitudinal mode operation and low oscillation threshold (few microjoule). The combination with a master oscillator-power amplifier fiber pump laser provides a versatile optical source widely tunable in the 3.3-3.5 μm range with an adjustable pulse repetition rate (from 40 to 100 kHz), high duty cycle (~10(-2)) and mean power (up to 25 mW in the idler beam). The potential for trace gas sensing applications is demonstrated through photoacoustic detection of atmospheric methane.

High-power, continuous-wave, optical parametric oscillator pumped by an optically pumped semiconductor laser at 532 nm

We report a high-power, cw, single-frequency optical parametric oscillator pumped external to an optically pumped semiconductor laser (OPSL). The singly resonant oscillator (SRO), based on a 30 mm crystal of MgO:sPPLT, is pumped by a 6 W, cw OPSL at 532 nm. By deploying signal outcoupling and loose focusing in the presence of low pump power, we achieve significant improvements in SRO performance across the tuning range with regard to signal power, extraction efficiency, and pump depletion by reducing thermal effects. We generate >1.78 W of idler and >0.9 W of signal over 856-1404 nm while maintaining a total output power of >2 W over 93% of the tuning range at an extraction efficiency of 48.7% and pump depletion of 84%. The signal power remains within 0.85-1.05 W across the entire tuning range and has a TEM(00) spatial profile with M(2)<1.28 and circularity of >97%. Measurements of signal spectrum confirm single-frequency performance with an instantaneous linewidth of approximately 15 MHz and frequency stability better than 60 MHz over 60 s.

Combined wide pump tuning and high power of a continuous-wave, singly resonant optical parametric oscillator

A new singly resonant, single-frequency optical parametric oscillator (OPO) has been developed for the 2.6–4.7 μm infrared wavelength region, using a high power (>20 W), widely tunable (1024–1034 nm) Yb:YAG pump source. With the OPO frequency stabilized with an intracavity etalon, the OPO achieved an idler output power of 3 W at 2.954 μm. Tuning of the idler frequency was achieved by longitudinal mode-hop tuning of the pump source (FSR 100 MHz). In this way an idler frequency scan of 100–150 GHz could be obtained, after which the signal frequency hops ahead over the FSR of the intracavity etalon of the OPO (207 GHz). Due to un-optimized mirror coatings for the OPO cavity and PPLN crystal, the frequency stability was limited to 90 MHz over 1 s, with an unaffected long-term frequency stability of 250 MHz over 200 seconds.

Continuous-wave operation of a single-frequency optical parametric oscillator at 4–5 μm based on periodically poled LiNbO_3

We present a cw, Nd:YAG-pumped singly resonant single-frequency narrow-linewidth high-power optical parametric oscillator with idler tuning from 3.7 to 4.7 microm. In this spectral range the absorption of the idler wave in the LiNbO3 crystal is significant, causing the oscillation threshold to increase with a subsequent decrease in output power from 1.2 W at 3.9 microm to 120 mW at 4.7 microm. The optical parametric oscillator's cavity was stabilized and mode-hop tuned with a rotatable solid etalon but with a subsequent reduction in idler power of as much as 50%. We demonstrated the usefulness for spectroscopy by recording the photoacoustic spectrum of a strong CO2 absorption, using a 24-GHz continuous idler scan.

Coherent laser radar at 3.6 microm.

Coherent laser radar systems in the mid-IR wavelength region can have advantages in low-altitude environment because they are less sensitive to scattering, turbulence, and humidity, which can affect shorter- or longer-wavelength system. We describe a coherent laser radar at 3.6 microm based on a single-frequency optical parametric oscillator and demonstrate the system over short ranges outdoors. The system was used to make micro-Doppler measurements from idling trucks that were processed to give surface vibration spectra.

Single-frequency mid-infrared optical parametric oscillator source for coherent laser radar

We report on the design and characterization of a highly coherent mid-IR source at 3.57mum based on a single-frequency optical parametric oscillator. Detailed frequency and amplitude noise spectra have been measured. The rms intensity noise from 1.2 to 1000 Hz was 0.03%, and a rms frequency drift of 8 kHz in 1 ms was observed. We have also demonstrated the utility of this source for coherent laser radar applications by measuring micro-Doppler spectra from vibrating targets.

Photoacoustic trace-gas detection using a cw single-frequency parametric oscillator

H6) at 3.34 μm using a widely tunable cw single-frequency optical parametric oscillator. The high frequency and power stability and the continuous tunability of the parametric oscillator make it ideally suited for this application. Detection sensitivities of 0.5 ppb for ethane are obtained, which is comparable to the best results previously obtained with intracavity detection using line-tunable CO overtone lasers. The flexibility and compact size of cw single-frequency parametric oscillators can lead to portable photoacoustic trace-gas detection systems for environmental monitoring and process control.

Regenerative amplification of a single-frequency optical parametric oscillator

High-resolution high-energy near-diffraction-limited pulses are obtained by regenerative amplification in Ti:sapphire of a pulsed single-frequency optical parametric oscillator. The technique combines the virtues of tunable solid-state lasers and low-jitter parametric oscillators to yield ~2-ns pulses that can be rapidly tuned through the entire gain range of the Ti:sapphire crystal. The measured peak power is 20 MW at 850 nm.