The frequency output of a single-longitudinal-mode laser is sta-
bilized by precisely controlling the laser cavity length. This can be
accomplished passively by building an athermalized resonator
structure and carefully controlling the laser environment to elimi-
nate expansion, contraction, and vibration, or actively by using a
mechanism to determine the frequency (either relatively or
absolutely) and quickly adjusting the laser cavity length to main-
tain the frequency within the desired parameters.
A typical stabilization scheme is shown in figure 36.11. A por- tion of the laser output beam is directed into a low-finesse Fabry-Perot etalon and tuned to the side of the transmission band. The throughput is compared to a reference beam, as shown in the figure. If the laser frequency increases, the ratio of attenuated power to reference power increases. If the laser frequency decreases, the ratio decreases. In other words, the etalon is used to create a fre- quency discriminant that converts changes in frequency to changes in power. By “locking” the discriminant ratio at a specific value (e.g., 50 percent) and providing negative feedback to the device used to control cavity length, output frequency can be controlled. If the frequency increases from the preset value, the length of the laser cavity is increased to drive the frequency back to the set point. If the frequency decreases, the cavity length is decreased. The response time of the control electronics is determined by the char- acteristics of the laser system being stabilized.
Commercially available systems can stabilize frequency output
to 1 MHz or less. Laboratory systems that stabilize the frequency
to a few kilohertz have been developed.
A typical stabilization scheme is shown in figure 36.11. A por- tion of the laser output beam is directed into a low-finesse Fabry-Perot etalon and tuned to the side of the transmission band. The throughput is compared to a reference beam, as shown in the figure. If the laser frequency increases, the ratio of attenuated power to reference power increases. If the laser frequency decreases, the ratio decreases. In other words, the etalon is used to create a fre- quency discriminant that converts changes in frequency to changes in power. By “locking” the discriminant ratio at a specific value (e.g., 50 percent) and providing negative feedback to the device used to control cavity length, output frequency can be controlled. If the frequency increases from the preset value, the length of the laser cavity is increased to drive the frequency back to the set point. If the frequency decreases, the cavity length is decreased. The response time of the control electronics is determined by the char- acteristics of the laser system being stabilized.
Other techniques can be used to provide a discriminant. One
common method used to provide an ultrastable, long-term refer-
ence is to replace the etalon with an absorption cell and stabilize the
system to the saturated center of an appropriate transition. Another
method, shown in figure 36.12, is used with commercial helium
neon lasers. It takes advantage of the fact that, for an internal mir-
ror tube, the adjacent modes are orthogonally polarized. The cav-
ity length is designed so that two modes can oscillate under the
gain curve. The two modes are separated outside the laser by a
polarization-sensitive beamsplitter. Stabilizing the relative ampli-
tude of the two beams stabilizes the frequency of both beams.
The cavity length changes needed to stabilize the laser cavity are very small. In principle, the maximum adjustment needed is that required to sweep the frequency through one free spectral range of the laser cavity (the cavity mode spacing). For the helium neon laser cavity described earlier, the required change is only 320 nm, well within the capability of piezoelectric actuators.
The cavity length changes needed to stabilize the laser cavity are very small. In principle, the maximum adjustment needed is that required to sweep the frequency through one free spectral range of the laser cavity (the cavity mode spacing). For the helium neon laser cavity described earlier, the required change is only 320 nm, well within the capability of piezoelectric actuators.
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