Fig. 2-4 Schematic diagram of laser amplification of the outward and inward paths through a laser amplifier unit and a phase conjugation mirror The laser amplifier unit consists of flash lamps and a laser crystal where laser light is intensified. Laser light deformations are corrected by reflection in the phase conjugation mirror. The right hand photograph shows an external view of the phase conjugation mirror, which is filled with a fluorinated liquid compound in a 30 cm long stainless steel tube. Glass windows for incident and reflected laser light are installed on both ends of the tube.

Fig. 2-5 High-power laser system with phase conjugation mirror Laser light is generated first as a pilot beam in the laser oscillator and then amplified in both directions by the preamplifier. Following that, the laser beam is split into two beams. Each beam is amplified by using two amplifier systems with the phase conjugation mirror as shown in Fig. 2-4. Consequently, a high output power can be obtained. Bright light originates from the flash lamp (Xe lamp), which excites the laser crystal. The laser light has an infrared wavelength of 1064 nm and thus is invisible.

Thomson scattering of a laser beam is utilized for the measurement of electron temperature and density profiles of magnetically confined plasmas. Since the scattered light is quite weak with this method, it is necessary to develop high-power laser systems to improve the measurement accuracy. When a laser light passes through the laser crystal and is excited by the flash lamp in the amplifier unit, the laser light is intensified, and the output energy increases (Fig. 2-4). However, the crystal is deformed by thermal expansion due to the heat from the flash lamp. This deformation significantly disturbs the wavefront of the laser light. The disturbance is enhanced further by the amplification, which damages the crystal or the optical components. This has been a serious problem for the development of high-power laser systems. To solve this problem, a magic mirror, also referred to as a "phase conjugation mirror," has been developed by utilizing nonlinear optical interaction between laser beam and material. If a deformed glass is inserted between a normal mirror and a target, the reflected target image is also deformed. Nevertheless, it is possible to remove the deformation of the reflected target image using the magic mirror. Namely, if the reflected light from the mirror faithfully returns along its original path through the deformed glass, it is possible to recreate the undistorted image. Consequently, the deformation received in the outward direction is cancelled out in the inward direction. By applying this property to a laser amplifier unit, it is easy to obtain high power for the laser system since the nonuniformity of laser light in the crystal deformed by thermal expansion is compensated completely and with uniformity (Fig. 2-4). Development of a phase conjugation mirror to date has used a fluorinated liquid compound as a medium. Mirror reflectivity exceeding 95% in a stable condition was achieved at incident powers of 100 W levels (beam energy of 2 J at a 50 Hz repetition rate). Moreover, a further incremental increase in output power was attempted by incorporating a phase conjugation mirror with the existing laser diagnostic system to realize additional improvement in measurement accuracy. As a result, an output power of 350 W (pulse output power of 7 J at a 50 Hz repetition rate) was obtained without an additional amplifier unit. This value is eight times larger than that of conventional laser system without phase conjugation mirrors (original output power ~45 W). This system as shown in Fig. 2-5 has the best worldwide performance of flash-lamp excitation laser systems. This development brings an improvement in measurement accuracy and will promote research and industrial applications. This study was carried out in collaboration with the Institute of Laser Engineering in Osaka University.

Reference H. Yoshida et al., YAG Laser Performance Improved by Stimulated Brillouin Scattering Phase Conjugation Mirror in Thomson Scattering Diagnostics at JT-60, Jpn. J. Appl. Phys., 42, 439 (2003).

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