A device for optical parametric amplification utilizing four mirrors oriented in a nonplanar configuration where the optical plane formed by two of the mirrors is orthogonal to the optical plane formed by the other two mirrors and with the ratio of lengths of the laser beam paths approximately constant regardless of the scale of the device. With a cavity length of less than approximately 110 mm, a conversion efficiency of greater than 45% can be achieved.

BACKGROUND OF THE INVENTION

The invention relates to an optical parametric oscillator and more particularly to an image-rotating, 4-mirror ring optical parametric oscillator.

Optical parametric amplification (OPA) is a nonlinear optical process whereby light at one wavelength, the pump wavelength, is used to generate light at two other (longer) wavelengths in a nonlinear optical material with a nonvanishing second order nonlinear susceptibility. Optical gain is established at two wavelengths, conventionally referred to as the signal and idler wavelengths. The sum of the energies of a signal photon and an idler photon are equal to the energy of a pump photon. There is no fundamental physical distinction between the idler wave and the signal wave. An optical parametric oscillator (OPO) is a resonant optical cavity containing a nonlinear material which provides OPA when pumped by a beam of laser radiation at a pump frequency from a pump source.

The content and orientation of the crystal and the design of the resonant cavity determines the signal and idler frequencies. The gain within the nonlinear medium combined with feedback within the resonant cavity permits oscillation, a process similar to build-up in a laser cavity. The cavity can either be singly resonant in which end mirrors reflect only the signal frequency or doubly resonant in which end mirrors reflect both signal and idler frequencies. End mirrors of the OPO are often transparent to the pump frequency, although they reflect the pump in some designs. OPOs with singly resonant cavities are typically more stable in their output than OPOs with doubly resonant cavities.

A schematic diagram of a prior art OPO appears in FIG. 1 (e.g., see Alford et al., U.S. Pat. No. 6,147,793, issued on Nov. 14, 2000). The pump 10 provides a source of intense coherent radiation in the form of the pump wave 14 . A suitable nonlinear optical material 13 is placed in the optical cavity formed by mirrors 11 and 12 . Mirror 11 is essentially transparent to pump wave 14 , thereby providing a pump source to nonlinear optical material 13 . Mirror 12 is partially transparent to the signal wave 16 , which along with the idler wave 15 is generated by nonlinear interaction of pump wave 14 with nonlinear optical material 13 . For simplicity, FIG. 1 shows all three waves propagating along a phasematch or quasi-phasematch direction within nonlinear optical material 13 , a situation known as collinear phase matching. More generally, collinearity of the three waves is not required for OPO function.

An average photon from signal wave 16 makes multiple passes through nonlinear optical medium 13 before escaping from the optical cavity through mirror 12 . Such apparatus can provide reasonably efficient (10-40%) conversion of pump photons into signal photons. Like excited optical laser media, OPA involves optical gain and amplification of light. In laser media, however, there is no fundamental relationship between the coherence or lack thereof of the excitation energy and the laser radiation. In contrast, in OPA the pump source must be coherent light, and the output energy is coherently coupled and phase-locked to the laser pump.

To obtain a useful device, it is necessary to be able to choose a specific signal wavelength. This is made possible within the nonlinear material itself, as useful gain appears only when the pump wave, the signal wave, and the idler wave can propagate and stay in phase with each other. This phase matching condition is difficult to establish. Optical materials generally exhibit a property called dispersion, in which the refractive index varies with wavelength. Normally, shorter wavelength light propagates more slowly than do longer wavelengths. Consequently, as waves with different frequencies propagate, they rapidly move in and out of phase with each other. The resulting interference prevents the signal wave from experiencing significant optical gain. The most common ways of phase matching are to take advantage of birefrigence often present in nonlinear crystals or to quasi-phase match by periodically changing the orientation of the nonlinear crystal to periodically rephase the pump, signal, and idler waves.

Because of constraints imposed by crystal nonlinearities and damage thresholds, scaling a pulsed OPO from low to high energy implies increasing the beam diameters while keeping the fluences, crystal lengths, and cavity length relatively unchanged. The result is a high-Fresnel-number (N F , where N F d 2 / L; d is the beam diameter, is the frequency and L is the cavity length) cavity that can support many transverse modes, often resulting in poor beam quality.

Beams from OPO's with small Fresnel numbers are often nearly diffraction limited because diffraction couples all transverse regions of the beams. However, as the beam diameters are increased to large Fresnel numbers, different portions of the beams uncouple and develop more or less independently of one another in cavities with flat mirrors. This allows uncorrelated phase and amplitude variations across the beam profile, resulting in poor beam quality. To improve the beam quality, all regions of the signal and idler beams must communicate in a way that establishes a more uniform phase and amplitude across the beams. One way to do this is to use a confocal unstable resonator (Clark et al., U.S. Pat. No. 5,390,211, issued on Feb. 14, 1995). Light originally oscillating near the cavity axis gradually spreads over the entire beam diameter by diffraction and cavity magnification. Light is also continuously lost from the edges of the gain region for the same reasons, so after a few round trips of the cavity all the resonated light has a common ancestry and, for proper cavity alignment, a common phase.

Anstett et al. (G. Anstett, G. Goritz, D. Kabs, R. Urschel, R. Wallenstein, and A. Borsutzky, 2001, Appl. Phys. B., DOI 10.1007) describe a method for reducing beam divergence using collinear type-II phase matching and back reflection of the pump beam. Alford et al. (U.S. Pat. No. 6,147,793, issued on Nov. 14, 2000) also describe a class of optical parametric oscillators that introduce means for reducing signal losses due to backconversion of signal photons in the nonlinear optical medium. Elimination of backconversion results in improved beam quality compared with an OPO in which backconversion is present.

Another way to communicate phase across the beam is by spatial walk off between the signal and idler beams, combined with image rotations (Smith, A. and Bowers, M., presented at University of Kaiserslautern, Kaiserslautern, Germany, May 5, 2000; incorporated herein by reference). Walk off, which describes the angle difference between the signal and idler Poynting vectors in the crystal (nonlinear medium), tends to smooth the phase of the signal beam over regions that interact with a particular portion of the idler beam. For a single pass through the crystal, this is a stripe of length equal to the walk off displacement within the crystal. Over successive passes of an OPO cavity, the stripe lengthens by this amount on each pass. This leads to a set of stripes of uniform phase oriented parallel to the walk off direction but with an independent phase for each stripe.

DETAILED DESCRIPTION OF THE INVENTION

An optical parametric oscillator (OPO) is an optical device pumped by a coherent light source, such as a laser, that provides optical parametric amplificaton by generating waves (radiation) in a nonlinear medium within a resonant optical cavity. The present invention provides optical parametric amplification using a 4-mirror, nonplanar ring design to produce 90 image rotation on each round trip of the formed cavity, maintaining polarization of the resonating light. The formed cavity (or OPO) advantageously has a shorter length than most previously designed ring OPOs.

FIG. 2 illustrates one embodiment of the 4-mirror, nonplanar ring system of the present invention. The system has 4 mirrors, designated as M 1 , M 2 , M 3 and M 4 . In general, the pump laser beam enters through a partially transmissive mirror (for example, M 1 ) and exits through another partially transmissive mirror (for example, M 2 ) although the other mirrors could also be used to admit and emit the pump light. Mirrors M 3 and M 4 are identical in order to maintain polarization with only one wave plate (WP 1 ), a half-wave plate, situated between mirrors M 2 and M 3 , although the wave plate could also be situated between mirrors M 1 and M 4 . This half-wave plate is included to maintain linear polarization at the crystal. These mirrors reflect the signal wave and could also reflect the idler and pump, although this is usually undesirable because it makes signal/idler wavelength selection difficult. Situated between mirrors M 1 and M 2 is at least one nonlinear optical medium C 1 (generally a crystal).

For the embodiment illustrated in FIG. 2 , a laser beam that traverses between mirrors M 1 and M 2 is defined to follow a laser beam path designated as L 1 , the beam path between mirrors M 2 and M 3 is designated as L 2 , the beam path between mirrors M 3 and M 4 is designated L 3 , and the beam path between mirrors M 4 and M 1 is designated as L 4 . For the purposes of the discussion herein, the plane formed by beam paths L 1 and L 2 is defined as optical plane A and the plane formed by beam paths L 1 and L 4 is defined as optical plane B. The optical plane defined by beam paths L 1 and L 2 is approximately orthogonal to the optical plane formed by L 4 and L 1 . By approximately orthogonal, it is defined as 90 10 . The ratio of L 2 (approximately equal to L 4 ) to L 1 (approximately L 3 ) is approximately 0.707 (where approximately is defined to be within 10%). These two geometric constraints determine the cavity geometry within a length scale factor that can be varied to accommodate different beam and crystal sizes. The design of the present invention produces an image-rotating cavity that is relatively insensitive to misalignment. Slight tilt (approximately 10 mrad) of the mirrors does not misalign the optical cavity. This makes possible the achievement of a resonant monolithic cavity with no adjustments on the mirrors. Additionally, the cavity is insensitive to mechanical vibrations and temperature changes compared with non-rotating cavities.

In another embodiment, multiple crystals are situated between M 1 and M 2 to walkoff compensate for broad tunability. Two identical crystals C 1 and C 2 can be used, situated side-by-side, as shown in FIG. 3 . For a crystal size of approximately 10 mm 10 mm 15 mm, a cavity length of only approximately 105 mm was achieved. The distance for L 1 was 31 mm. The distance for L 2 was 22 mm. The distance for L 3 was 31 mm. The distance for L 4 was 22 mm. The crystals were 15-mm long KTP crystals. The mirrors were all 33 angle of incidence. The beam had pump, signal and idler wavelengths of 532 nm, 800 nm, and 1588 nm, respectively, with a 1/e 2 pump beam diameter of 2.1 mm. A conversion efficiency of approximately 45% was achieved. In the operation of the configuration of the present invention to produce an optical parametric oscillator, an optical pump (such as a pump laser, as a nanosecond pulsed laser, and more particularly, a Q-switched solid state laser) generates a pump beam B (wave) with a suitable intensity at a pump frequency greater than a desired signal frequency. The pump beam B passes through a first mirror (M 1 ) that directs the beam through a nonlinear optical medium (in this case, crystals C 1 and C 2 ) subsequently is directed by a second mirror M 2 through a wave plate WP 1 to a third mirror M 3 that directs the beam back to mirror M 4 . All mirrors are at least partially reflective to the signal beam to allow multiple passes in the optical cavity. The waveplate aligns the beam polarization to avoid depolarization.

In another embodiment, one or two crystals can also be situated between M 3 and M 4 to increase gain. Such a system would require an additional half-wave plate to be situated in L 4 . Both half-wave plates are rotated to approximately 22.5 instead of 45 to maintain linear polarizations for both crystals.

The necessary image rotation angle of the nonplanar ring cavity for the system configuration of the present invention was determined by first developing the mathematical formalism for mirror reflections and then applying it to a nonplanar ring. The rotation angle was calculated from the area bounded by great arcs connecting propagation directions on a unit sphere. For the configuration depicted in FIG. 3 , various configurations of crystals and beam entry and exit points are possible for the resulting OPO. The simplest is with the nonlinear crystal or crystals situated between mirrors M 1 and M 2 , with the beam entering through M 1 and ultimately exiting through M 2 . Mirror M 2 is the signal output coupler, and the other three mirrors are high reflectors for the signal and low reflectors for the idler. The nonlinear crystals are oriented so the eigen polarizations of the crystal and the cavity lie in the horizontal and vertical planes. Starting at the crystal and following a horizontally polarized signal wave around the cavity, the half-wave plate in L 2 changes the polarization to vertical. Mirrors M 1 , M 3 , and M 4 rotate the polarization in the same fashion as they rotate the image because of the cancellation of phase shifts between M 2 and M 3 ; hence, the polarization is rotated back to horizontal after mirror M 1 . After one trip around the cavity, the horizontally polarized wave returns as a horizontally polarized wave.