Pound Drever Hall Frequency Stabilized Laser for Parametric Oscillators

with Surendra P. Singh

Diode lasers have typical free running frequency bandwidths of a few GHz. Several schemes are used for narrowing their bandwidth. In one scheme the laser is locked to the center of an atomic line. This, however, is not convenient because the laser can only be tuned over the width of a line. Furthermore, a suitable atomic line may not available to satisfy the phase matching requirements for parametric oscillations. The most versatile frequency locking scheme is the Pound-Drever-Hall stabilizer. In this scheme, the light from the laser at frequency , typically 10¹⁴ - 10¹⁵ Hz, is frequency modulated at frequency , usually 10⁷ Hz. The light output from the modulator contains, in addition to the carrier frequency , sidebands at frequencies . The amplitudes of the sidebands are equal in magnitude but opposite in sign. This light is allowed to fall on a stable reference cavity tuned close to the carrier frequency . If the sideband frequencies lie outside the passband of the cavity, they are reflected back from the cavity. The reflected light is detected by a square law detector (a photodiode or a photomultiplier). In general, the output current from the photodetector contains a component at the modulation frequency . When the cavity resonance frequency coincides exactly with the carrier frequency, the sidebands are reflected with equal but opposite amplitudes. As a result the detector output has no modulation at the frequency . If the laser frequency drifts from the reference cavity resonance frequency, the two sidebands are reflected with different amplitudes. The output from the photodiode therefore contains a modulation signal at frequency . The amplitude of this signal is proportional to the difference in the cavity resonance frequency and the carrier frequency. This signal is filtered and mixed with a strong local oscillator signal at frequency . The output of the mixer is proportional to the amplitude of the modulation signal. This output from the mixer is our error signal. A correction proportional to the error signal is applied either to the laser to bring the lasers in resonance with the reference cavity or to the reference cavity to bring it in resonance with the laser frequency.

For diode lasers this scheme simplifies considerably. Unlike the dye or the Ti-sapphire lasers, where an external electrooptic frequency modulator is used, a laser diode can be frequency modulated by modulating its current. In our setup we do not need an extra reference cavity. The parametric oscillator cavity itself can be used as the reference cavity. Diode lasers stabilized in this fashion will be used for constructing optical parametric oscillators and studying their unique quantum mechanical noise properties. The REU student on this project will be expected to work on the locking scheme outlined here. The student will gain valuable experience with the state of the art in laser frequency stabilization schemes. The same scheme is applicable to other tunable lasers such as the dye or the Ti-sapphire lasers.