1. Field of the Invention
The present invention relates to an injection-locked laser, an interferometer, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
FIG. 7 is a view showing the schematic arrangement of a conventional injection-locked laser (see J. Rahn, “Feedback stabilization of an injection-seeded Nd:YAG laser”, App. Opt., 24, 940 (1985)). The injection-locked laser shown in FIG. 7 adopts an injection locking method of minimizing the buildup time.
A pulse oscillator O for generating pulse light is, generally, of a ring type to avoid any influence of spectral hole burning. A PZT mount 4 mounts an output coupler of the pulse oscillator O. A PZT controller (PZT amplifier) 5 accurately drives the PZT mount 4. A laser gain medium 3 can employ, for example, a Ti:sapphire crystal. An excitation light source 2 made of, for example, Nd:YAG can be used to excite the crystal by irradiating the crystal with a light beam so that the crystal absorbs the light beam.
A seed laser 1 is an injection light source for injection locking and uses a single longitudinal mode light source having a sufficiently narrow full width at half maximum. Seed laser light output from the seed laser 1 is injected into the pulse oscillator O so that it matches the transverse mode of the pulse oscillator O. The seed laser 1 can use, for example, an external oscillator type semiconductor laser.
The injection locking means locking the wavelength of narrow-band laser light injected into the oscillator with the optical path length of the oscillator. Photons of the injected narrow-band laser light play the part of evoking stimulated emission for initial pulse oscillation. This facilitates pulse oscillation while concentrating excitation energy in a narrow band.
When the optical path length of the pulse oscillator O is an integer multiple of the oscillation wavelength of the seed laser 1, the efficiency of injection locking is highest and the buildup time is shortest. Under other conditions, the buildup time is long because the oscillator generates a loss with respect to the seed laser 1.
The buildup time means the time from pump laser emission until pulse light oscillation. The above-described principle is used for oscillator control based on the buildup time.
To detect the buildup time, an excitation light source photodetector 32 and pulse light photodetector 33 are inserted near the oscillator. The outputs from the photodetectors 32 and 33 are sent to a control circuit 34. The control circuit 34 calculates the buildup time based on the signals output from the two photodetectors 32 and 33, generates an error signal based on a change in buildup time, and executes PID filtering for feeding back the error signal.
The filtered signal is sent to the PZT controller 5. The PZT controller 5 drives the PZT mount 4 based on this signal so as to control injection locking.
Unfortunately, the conventional control method using the buildup time may generate a control error when a factor (e.g., pump laser intensity jitter or pointing jitter) other than the oscillator length changes the buildup time. Still worse, noise is likely to mix in a processing circuit for calculating the buildup time based on the output from the laser. This makes it difficult to generate an error signal with high SN. This produces a locking control error, resulting in laser characteristic deterioration such as intensity or wavelength jitter.