Thermal effects in singly resonant continuous-wave optical parametric oscillators

Stability and tuning characteristics of continuous-wave optical parametric oscillators (CW OPOs) are affected by various thermal effects arising from optical absorption in nonlinear crystals. In this paper, we present an experimental study of such effects in a singly resonant CW OPO. The OPO operates in the 3-μm mid-infrared region and it is based on a MgO-doped periodically poled lithium niobate crystal. We focus our study on two thermally induced phenomena that have been recently reported to exist in singly resonant CW OPOs: optical bi-stability and thermal self-locking. Thermal self-locking effect, which is known to alter the stability and tuning properties of doubly and triply resonant CW OPOs, is shown to be also of importance in singly resonant OPOs. We report the stability and tuning characteristics of a thermally loaded OPO and discuss a simple temperature-tuning method that can be used to scan the OPO idler frequency continuously over several THz.     

Active mode locking of continuous-wave doubly and singly resonant optical parametric oscillators

We report on what we believe to be the first active mode locking of near-degenerate, doubly and singly resonant cw-pumped optical parametric oscillators (OPOs). We show experimentally that a steady-state regime of short pulses is reached in a few tens of microseconds under cw pumping. The oscillation buildup dynamics of both OPOs is also explored, evidencing an unusual transient behavior in the mode-locking process.     

Intensity noise of pump-enhanced continuous-wave optical parametric oscillators

4 (RTA), which provides a threshold pump power of 16 mW. The maximum signal wave power is 23 mW at a wavelength of 1243 nm. The spectral bandwidth of the signal wave is less than 750 kHz. Received: 3 February 1998     

Diffractive effects in singly resonant continuous-wave parametric oscillators

I /k_P=0.33and0.50. It is found that the ratio of pump depletion to maximal depletion as a function of the ratio of pump power to threshold power agrees with the plane-wave prediction to within 5%, for a wide range of focusing conditions. The observed trends are explained as resulting from intensity- and phase-dependent mechanisms. Received: 19 January 1998/Revised version: 13 March 1998     

Dye laser pumped, continuous-wave KTP optical parametric oscillators

4 (KTP) optical parametric oscillators (OPOs) with pump and idler resonant cavities. With a linear two-mirror cavity the pump power at threshold was 70 mW. The single-frequency signal and idler output wavelengths were tuned in the range of 1025 to 1040 nm and 1250 to 1380 nm by tuning the dye laser in the range of 565 to 588 nm. With a dual three-mirror cavity the threshold was 135 mW. Pumped by 500 mW of 578 nm radiation the 1040 nm single-frequency signal wave output power was 84 mW. Power and frequency stable operation with a spectral bandwidth of less than 9 MHz was obtained by piezo-electrically locking the length of the pump resonant cavity to the dye laser wavelength. Similar performance was achieved by placing the idler resonant OPO inside the resonator of the dye laser. With this system power stable and single-frequency operation was achieved with a spectral bandwidth of less than 11 MHz for the idler wave. Received: 3 February 1998/Revised version: 9 March 1998     

Long-term frequency stability and linewidth properties of continuous-wave pump-resonant optical parametric oscillators

We present a frequency stability and linewidth analysis of two different setups of continuous-wave pump and signal-resonant optical parametric oscillators (pump resonant, singly resonant OPO, PR-SRO). Both designs, a common-cavity and a dual-cavity version, utilize a frequency-stable and narrow-linewidth pump laser and are stabilized without using an external reference. A long-term frequency stability better than ±30 MHz is reached over more than 30 minutes for both designs. The frequency jitter on a one-second time-scale is 56 kHz for the common-cavity PR-SRO and about 10 MHz for the dual-cavity PR-SRO. The short-term linewidths were measured using an external high-finesse cavity and are (9±2)kHz and (6±1)kHz in 20 s, respectively. To our knowledge, these are the lowest values demonstrated so far for a widely continuously wavelength-tunable all-solid-state laser source. PACS 42.65.Yj; 42.72.Ai; 42.62.Fi     

Femtosecond optical parametric oscillators

Development of femtosecond optical parametric oscillators (OPO) began a decade ago and has followed the technological progress in lasers and nonlinear crystals. There exist now such OPOs operating in the near or mid-infrared as well as in the visible. In this paper, we give an overview of the existing performances and of the prospects of this very active field.     

Nanosecond optical parametric oscillators

The main characteristics of nanosecond OPOs are presented: nonlinear crystals, tunability ranges, output energies, phase-matching conditions. A semianalytical model and a completely numerical model are described. They point out the importance of the complex intensity distribution that appears in the transverse planes of the output pulses. Techniques to improve the spectral and spatial quality of the output beams such as intracavity Fabry–Pérot filters or unstable cavity are detailed. Finally a modelisation and experimental results about intracavity OPO are presented.     

Classical and quantum effects in spatially modulated optical parametric oscillators

We study the effects of transverse spatial modulations in a multimode degenerate optical parametric oscillator. Intracavity photonic crystals allow us to tune the instability threshold and improve entanglement above threshold. Here we compare such results with the case in which the modulation is in the injected field profile.     

Management of thermal effects in high-repetition-rate pulsed optical parametric oscillators

We report on the investigation of thermal effects in high-repetition-rate pulsed optical parametric oscillators emitting in the mid-IR. We find that the thermal load induced by the nonresonant idler absorption plays a critical role in the emergence of thermally induced bistability. We then demonstrate a significant improvement of the conversion efficiency (more than 30%) when a proper axial temperature gradient is applied to the nonlinear crystal by use of a two-zone temperature-controlled oven.     

Fiber-feedback continuous-wave and synchronously-pumped singly-resonant ring optical parametric oscillators using reverse-proton-exchanged periodically-poled lithium niobate waveguides

We describe guided-wave singly-resonant reverse-proton-exchanged (RPE) periodically-poled lithium niobate (PPLN) waveguide ring optical parametric oscillators (OPOs) in which the feedback was provided via a single-mode fiber pigtailed to the waveguide. Wavelength selection was achieved by means of a fiber Bragg grating inside the feedback loop. The SRO threshold for the synchronously-pumped OPO was below 1 mW of average coupled pump power.     

Cross-phase-modulation-induced spectral effects in high-efficiency picosecond fiber optical parametric oscillators

We explore the spectral effects due to cross-phase modulation and walk-off in picosecond fiber optical parametric oscillators. The output spectrum exhibits pump-power-dependent broadening, which can be quite asymmetric associated with a redshift or a blueshift depending on pump synchronization. By slightly increasing the cavity length, one obtains a blueshifted spectrum and a conversion efficiency as high as 15%.     

Synchronously pumped optical parametric oscillators

Recent work on the development of high-repetition-rate, synchronously pumped picosecond and femtosecond Optical Parametric Oscillators (OPOs) is reviewed. With KTP, BBO or LBO crystals and solid-state pumps such as cw mode-locked Ti:Sapphire, Nd:YAG or Nd:YLF lasers, the singly resonant OPOs or their nonlinear optical accessories yield pulses as short as 40 fs, average powers up to hundreds of milliwatts, and tunability from 200 nm to > 10 µm.     

Fiber-laser-based green-pumped continuous-wave singly-resonant optical parametric oscillator

Stable, high-power, second-harmonic-generation (SHG) of a compact CW Ytterbium (Yb) fiber laser at 1064 nm into the green and its use as a pump source for CW singly-resonant optical parametric oscillator (SRO) is demonstrated. Using a simple single-pass SHG configuration in MgO:sPPLT, as much as 9.6 W of single-frequency green radiation at 532 nm is generated from 30 W of fundamental power at a conversion efficiency of 32.7% in a Gaussian spatial profile with a beam quality factor of M ² < 1.3. Thermal effects have been investigated at different fundamental power levels and various thermal management schemes are employed to maximize the second-harmonic power. The green source is successfully deployed to pump a CW SRO tunable over 855–1408 nm, generating up to 2.1 W of idler at 1168 nm. The peak-to-peak idler power stability is better than 10.7% over 40 min, with beam quality factor M ² < 1.26 for the idler and M ² < 1.52 for the signal.     

Fiber-laser-pumped continuous-wave singly resonant optical parametric oscillator

We report on what is to our knowledge the first continuous-wave (cw) optical parametric oscillator (OPO) that is pumped by a tunable fiber laser. The OPO is singly resonant for the signal wave and consists of a 40-mm-long periodically poled LiNbO(3) crystal in a four-mirror ring cavity. At a pump power of 8.3 W provided by the wavelength-tunable Yb-doped fiber laser, the singly resonant OPO generates 1.9 W of 3200-nm cw idler radiation. The singly resonant OPO was tuned from 1515 to 1633 nm (signal) and from 3057 to 3574 nm (idler) by means of the crystal temperature and poling period. We obtained a wide idler tuning range, from 2980 to 3700 mn, by tuning the wavelength of the fiber laser from 1032 to 1095 nm.     

Low-pump-threshold continuous-wave singly resonant optical parametric oscillator

We describe a compact all-solid-state continuous-wave singly resonant optical parametric oscillator (SRO) with a minimal pump-power requirement. The SRO is based on periodically poled LiNbO(3) as the nonlinear material and is pumped by a 1-W diode-pumped Nd:YVO(4) minilaser at 1.064 microm . By exploiting the intracavity pumping technique in a 50-mm crystal, we have achieved SRO operation threshold at a diode pump power of only 310 mW.At 1 W of input diode power, the SRO delivers 70 mW of output power in the nonresonant idler at 3.66 microm , at a photon conversion efficiency of 55%. Multiparameter tuning of the SRO yields a signal wavelength range from 1.45 to 1.60 microm and an idler wavelength range from 3.16 to 4.02 microm in the mid infrared. The device is characterized by robust turnkey operation and long-term amplitude-stable performance.     

Secondary transverse instabilities in optical parametric oscillators

We investigate the effect of coupling between diffraction and walk-off on secondary instabilities in nondegenerate optical parametric oscillators. We show that traveling waves that propagate in the walk-off direction, which are generated at the onset of absolute instability, experience Eckhaus and zigzag phase instabilities. Each of these secondary instabilities splits into absolute and convective instabilities that modify the Eckhaus and zigzag instability boundaries. As a consequence, the stability domain of modulated traveling waves is enlarged and may coexist with uniform steady states. The predictions are consistent with the numerical solutions of the optical parametric oscillator model.     

Nonadiabatic pattern formation in optical parametric oscillators

A nonadiabatic mechanism for pattern formation in parametrically forced systems subjected to a slow periodic modulation of the excitation frequency is proposed and discussed in detail for the case of an optical parametric oscillator. It is demonstrated that nonautonomous dynamics may induce nonadiabatic off-axis emission of down-converted photons even when the signal field is blueshifted from the nearby cavity resonance.     

Specific architectures for optical parametric oscillators

Optical parametric oscillators are coherent light sources showing properties that are not encountered with usual laser sources. This article deals with optical architectures that take advantage of the specific properties of the three-wave mixing parametric interaction. We show that optical cavities can be specifically designed to increase either the optical conversion of the parametric process or to reduce dramatically the emitted line width. To cite this article: A. Berrou et al., C. R. Physique 8 (2007).