High Power Gain-Switched Diode Laser Master Oscillator and Amplifier
- M. Poelker
- Appl. Phys. Lett. 67 (1995) 2762.
A high power cw modelocked Ti-sapphire laser has been constructed to drive the Jefferson Lab polarized photoinjector and provide > 500 mW average power with 50 ps pulsewidths at 499 MHz or 1497 MHz pulse repetition rates. This laser allows efficient, high current synchronous photoinjection for extended periods of time before intrusive steps must be taken to restore the quantum efficiency of the strained layer GaAs photocathode. The use of this laser has greatly enhanced the maximum high polarization beam current capability and operating lifetime of the Jefferson Lab photoinjector compared with previous performance using diode laser systems. A novel modelocking technique provides a simple means to phase-lock the optical pulse train of the laser to the accelerator and allows for operation at higher pulse repetition rates to ~ 3 GHz without modification of the laser cavity. The laser design and characteristics are described below. 1 SYNCHRONOUS PHOTOINJECTION Picosecond pulse lasers with repetition rates synchronized to the accelerator cavity rf frequency are used to drive the polarized photoinjector at Jefferson Lab. With such lasers, electrons are extracted from the photocathode only during the portion of the rf cycle when they can be accelerated in the machine. Few electrons are dumped at the injector chopper as would be the case for dc laser illumination. This efficient use of the extracted electron beam helps to preserve the operating lifetime of the photocathode. Synchronous photoinjection also provides a means to generate high bunch charge without the need for subharmonic bunching and chopper cavities, as demonstrated at the photoinjector at the Jefferson Lab Free Electron Laser. The largest obstacle to synchronous photoinjection has been the lack of commercially available high power picosecond-pulse laser sources with GHz repetition rates. Diode laser systems  are extremely reliable and provide high repetition rates but average output power from these systems is low, typically < 100 mW. Low output power combined with inherently low quantum efficiency from strained layer GaAs photocathodes limits the maximum beam current with high polarization to approximately 70 μA. Moreover, a key component of these systems (diode optical amplifier, SDL Inc. Model 8630-E) is no longer manufactured and an alternate vendor has not been identified. 2 JLAB MODELOCKING TECHNIQUE At Jefferson Lab, we have built an actively modelocked Ti-sapphire laser that emits 50 ps pulses and provides > 500 mW output power with ~ 30 nm wavelength tunability centered at 850 nm. Modelocked operation is obtained by introducing light from an injection-locked gain-switched diode laser into the Ti-sapphire laser cavity. The diode laser light introduces gain-modulation within the Ti-sapphire laser cavity, which initiates and maintains modelocked operation when the repetition rate of the gain-switched diode laser is set to the axial mode spacing of the Ti-sapphire laser. Moreover, the pulse repetition rate of the Ti-sapphire laser can be varied by setting the repetition rate of the gain-switched diode laser to multiples of the axial mode spacing, without changing the Ti-sapphire laser cavity length. Repetition rates from 250 MHz to 3 GHz have been obtained (in 250 MHz intervals) from one laser having a constant cavity length of ~ 60 cm. The Ti-sapphire laser optical pulse train follows that of the gain-switched diode laser which is easily be locked to the accelerator. 2.1 Description of the Laser A schematic of the laser is shown in Figure 1. The Tisapphire crystal (20 mm x 6 mm dia., 0.03% dopant) is mounted in a water-cooled, temperature-controlled copper block, midway between two curved mirrors (radius of curvature, 10 cm). The double-fold geometry provides correction for astigmatism introduced by the Brewster cut Ti-sapphire crystal faces . The ~ 60 cm long cavity length is very manageable; it provides a compact design but still allows the use of relatively inexpensive commercial optic mounts. A prism located near the flat high reflector mirror provides wavelength selection. Figure 1. Schematic of the modelocked Ti-sapphire laser. L, lens; M, laser mirror; OC, output coupler; FR, Faraday crystal and halfwave plate; DL, gain-switched diode laser. _______________