Chapter 27 Laser Stabilization

For laser applications in which measurement precision is a key feature, frequency-stabilized lasers are preferred, if not essential. This observation was true in the gas laser days when the 10−6 fractional Doppler width set the uncertainty scale. Now we have diode-pumped solid state lasers with fractional tuning range approaching 10−2 or more, and laser diode systems with several percent tuning. Such tuning is useful to find the exact frequency for our locking resonance, but then stabilization will be essential. Locking to cavities and atomic references can provide excellent stability, even using a widely tunable laser source. Indeed, laser frequency stability between independent systems has been demonstrated at 5 × 10−14 in 1 s averaging time, and more than a decade better at 300 s. This incredible performance enhancement is possible because of a feedback from measurement of the laser’s frequency error from our setpoint, this signal being fed into a filter/amplifier system and finally to an actuator on the laser itself which changes its frequency in response. While such feedback in response to performance may be the most important principle in evolution, in machines and lasers feedback enables the design of lighter, less costly systems. The accuracy is obtained, not by great bulk and stiffness, but rather by error correction, comparing the actual output against the ideal. This continuous correction will also detect and suppress the system’s nonlinearity and noise. The performance limitation ultimately is set by imprecision of the measurement, but there is a lot of care required to get into that domain: we must have a very powerful correction effort to completely hide the original sins. This chapter is our attempt to lead the worker newly interested in frequency control of lasers on a guided tour of stabilized lasers, ideally providing enough insight for recruiting yet another colleague into this wonderful arena. As nonlinear optics becomes just part of our everyday tools, the buildup cavities that enhance the nonlinear couplings are taking on a more critical role: This is the reason that we focus on the taming of PZT-based systems. We then cover locking with other transducers, and present some details about their construction and use. We consider the frequency discriminator, which is a key element for these control systems. The chapter concludes with description of the design and performance of several full practical systems.