METHODS FOR SUPPRESSING AMPLITUDE NOISE AND DRIFT

Two primary methods are used to stabilize amplitude fluctua- tions in commercial lasers: automatic current control (ACC), also known as current regulation, and automatic power control (APC), also known as light regulation. In ACC, the current driving the pumping process passes through a stable sensing resistor, as shown in figure 36.13, and the voltage across this resistor is monitored. If the current through the resistor increases, the voltage drop across the resistor increases proportionately. Sensing circuitry compares this voltage to a reference and generates an error signal that causes the power supply to reduce the output current appropriately. If the current decreases, the inverse process occurs. ACC is an effective way to reduce noise generated by the power supply, including line rip- ple and fluctuations.

With APC, instead of monitoring the voltage across a sens- ing resistor, a small portion of the output power in the beam is diverted to a photodetector, as shown in figure 36.14, and the voltage generated by the detector circuitry is compared to a refer- ence. As output power fluctuates, the sensing circuitry generates an error signal that is used to make the appropriate corrections to maintain constant output.

Automatic current control effectively reduces amplitude fluc- tuations caused by the driving electronics, but it has no effect on amplitude fluctuations caused by vibration or misalignment. Auto- matic power control can effectively reduce power fluctuations from all sources. Neither of these control mechanisms has a large impact on frequency stability.

Not all continuous-wave lasers are amenable to APC as described above. For the technique to be effective, there must be a monoto- nic relationship between output power and a controllable para- meter (typically current or voltage). For example, throughout the typical operating range of a gas-ion laser, an increase in current will increase the output power and vice versa. This is not the case for some lasers. The output of a helium neon laser is very insensi- tive to discharge current, and an increase in current may increase or decrease laser output. In a helium cadmium laser, where elec- trophoresis determines the density and uniformity of cadmium ions throughout the discharge, a slight change in discharge current in either direction can effectively kill lasing action.

If traditional means of APC are not suitable, the same result can be obtained by placing an acousto-optic modulator inside the laser cavity and using the error signal to control the amount of cir- culating power ejected from the cavity.

 One consideration that is often overlooked in an APC system is the geometry of the light pickoff mechanism itself. One’s first instinct is to insert the pickoff optic into the main beam at a 45-degree angle, so that the reference beam exits at a 90-degree angle. How- ever, as shown in figure 36.15, for uncoated glass, there is almost a 10-percent difference in reflectivity for s and p polarization.

In a randomly polarized laser, the ratio of the s and p compo- nents is not necessarily stable, and using a 90-degree reference beam can actually increase amplitude fluctuations. This is of much less concern in a laser with a high degree of linear polarization (e.g., 500:1 or better), but even then there is a slight presence of the orthogonal polarization. Good practice dictates that the pickoff ele- ment be inserted at an angle of 25 degrees or less.