Phase conjugation of depolarized light with a loop PCM

Abstract

Fax: (310) 317-5679The loop phase conjugate mirror (PCM) has attracted much of attention recently for application to high power solid state lasers, because of its ability to work with a unique combination of laser radiation parameters, such as high aberrations, Q-switched through CW pulse formats, low peak and high average powers, broad spectral bandwidths [ 1,2]. Another challenge for any kind of PCM in general is to accommodate depolarized laser beams, especially, that the commonly used technique, first shown for SBS [3], and based on the transformation of depolarized light before entering a PCM into linear polarized component, is very difficult to apply for a loop type of PCM.In [4] we suggested an idea how to modify the basic loop PCM optical scheme in order to work with depolarized radiation. In this paper we present experimental results showing near 100% polarization conjugation fidelity, in combination with compensation of phase distortions and high reflectivity.Basically, loop PCM is a laser that is seeded and triggered by an external signal beam, which we want to be phase conjugated. The optical scheme is shown in Fig.1. The ring resonator of this laser contains a nonlinear medium, which acts as one of the resonator mirrors after the signal beam writes a hologram (or grating) in it. The signal beam E, enters the loop, passes through the nonlinear medium, reflects from mirrors, and then as beam E, it intersects with E, inside the nonlinear medium. The interference pattern of these two beams records a hologram via a nonlinear mechanism. When the hologram reflectivity, combined with the gain of laser amplifier fulfills a threshold condition, spontaneous emission builds up, giving rise of a strong return E,, which, under certain conditions, may be close to a phase conjugate replica of the signal beam E,. A 'leaky" optical diode (which usually consists of a Faraday rotator, polarizers, and other optical elements) inside the resonator serves to prevent saturation of the amplifier by the input signal E,, and to support only unidirectional generation of the return beam E,, providing controllable attenuation for the input signal and insignificant losses for the generated return E,.For an arbitrary polarized signal beam, we actually have two loop resonators operating in parallel: one for the S and another for the P polarization components, which are wraped up together in one optical scheme. To accommodate the loop scheme with this situation, we first modified the optical diode to allow it to work with two independent S and P polarization components of the input and return beams. In addition, we placed a half-wave plate inside the loop resonator to serve as a polarization mixing element. This Diode \3 Figure 1 0-7803-4950-4/98/$10.00 1998 IEEE 288

2-kW Average Power CW Phase-Conjugate Solid-State Laser

“…Second, an LPCM offers broad wavelength coverage, and finally, it is compatible with lasers producing short coherence lengths (∼1 cm or less). Several low-power experimental LPCM demonstrations have been reported [10], [11]; these demonstrations are consistent with the advantages asserted earlier. However, to our knowledge, there have been no prior reports of LPCMs that might be used in high-power CW phase-conjugate master oscillator power amplifier (PC-MOPA) laser systems.…”

“…The amplifier transverse dimensions correspond to an aspect ratio of 18:1, which approximately matches the far-field divergence (10 • × 0.6 • ) of typical fast-axis-collimated laser diode arrays. This allows coupling the output from a 2-D laser diode array into the slab with a single lens.…”

“…The basic operation of an LPCM has been described earlier [10], [11], and a detailed description of the present LPCM is also available [13]. The key components of the LPCM are the NC, the LPCM amplifier A4, and the isolator (see Fig.…”

“…In view of these limitations to the applicability of SBS phase conjugation, we began developing an alternative NOPC architecture approximately 10 years ago. This alternative architecture is called a loop phase-conjugate mirror (LPCM), and this name is derived from the fact that the nonlinear medium is incorporated into a loop resonator [10]. Our LPCM exploits a thermal nonlinearity, whereby four-wave mixing (FWM) occurs in an absorbing medium.…”