Stabilization of Lasers
As lasers exhibit various kinds of laser noise, which can be detrimental in applications, it is sometimes necessary to use techniques for suppressing noise and stabilizing certain laser parameters. There are active and passive stabilization schemes, as discussed in the following.
See also the article on synchronization of lasers, which treats both timing and phase synchronization.
Active Laser Stabilization
Active stabilization schemes usually involve some kind of electronic feedback (or sometimes feedforward) system, where fluctuations of some parameters are converted to an electronic signal, which is then used to act on the laser in some way.
- The output power of a laser may be stabilized with a scheme as shown in Figure 1. The laser power is monitored with a photodiode and corrected e.g. via control of the pump power or the losses in or outside the laser resonator. In this way, both spiking after turn-on and the intensity noise under steady-state conditions can be reduced. Note that it is also possible to reduce intensity noise by acting on the output beam instead of the laser itself; see the article on noise eaters.
- The optical frequency of a single-frequency laser, or the frequency of one line of the frequency comb from a mode-locked laser, can be stabilized via resonator length control. The feedback signal can be obtained e.g. by recording a beat note with a second laser, by measuring the transmission or reflection of a very stable reference cavity or an interferometer, or by measuring the transmission of a gas cell (e.g. an iodine cell), possibly using Doppler-free laser absorption spectroscopy. A frequently used technique for generating an error signal with a reference cavity is the Pound–Drever–Hall method [3, 4], using a weak phase modulation of the light which is sent to the reference cavity. A scheme not requiring such modulation is the Hänsch–Couillaud method .
- The stabilization of the carrier–envelope offset phase or frequency of a mode-locked laser (CEO stabilization) can be based on, e.g., a phase measurement with an f−2f interferometer and feedback via some wedge or tilted mirror in the laser resonator. This kind of stabilization is important for frequency metrology.
- The timing of the pulses (→ timing jitter) from a mode-locked laser can be monitored by comparing the phases of a photodiode signal and of an electronic reference oscillator, and stabilized e.g. via cavity length control.
- Stabilization of the pointing direction of the output beam is possible via a beam position measurement (e.g. with a four-quadrant photodiode) and correction via piezo-controlled resonator mirrors.
The stability which is achieved with such active systems is determined by factors such as photodetection noise, the bandwidth of control elements, the design of the feedback electronics, and the stability of the reference standards (e.g. optical reference cavities).
Passive Laser Stabilization
Passive schemes do not involve electronics and are based on purely optical effects. Examples are:
- The frequency of a laser can be stabilized via optical feedback from a stable reference cavity. (This may also be considered as using an extended laser resonator, being a kind of composite cavity.)
- Synchronization of two mode-locked lasers is possible via cross-phase modulation in a Kerr medium, in which the intracavity pulses of both lasers meet.
The optical frequency of a laser may also be stabilized by injection locking, i.e., injecting a beam with a highly stable optical frequency from another laser.
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See also: laser noise, intensity noise, spiking, phase noise, linewidth, noise eaters, lasers, injection locking, carrier–envelope offset, frequency combs, frequency metrology, synchronization of lasers
and other articles in the categories laser devices and laser physics, fluctuations and noise, methods