Application Description

Optical phase-locked loops (OPLLs) synchronize the relative phases of two (laser) light fields. As a result, the two fields have an adjustable frequency difference while their phase relation remains constant. Examples of popular applications of OPLLs are:

Coherent Raman transitions

Two atomic or molecular energy levels are connected through a third (virtual) energy level by two coherent light fields with a defined frequency difference. To perform well-defined coherent population transfers by means of Rabi oscillations, it is important to stabilize the relative phase of the involved lasers over the course of each experiment.

Coherence cloning, laser transfer lock (laser stabilization)

Optical phase-locked loops make it possible to transfer the coherence characteristics of one laser, e.g. its frequency or phase stability properties, to another laser provided that the bandwidth of the servo loop is high enough that it can handle the noise present in the receiving laser. In fact, multiple low-coherence slave lasers can be stabilized with a highly coherent master laser.

Frequency combs

The repetition rate as well as the carrier envelope offset (CEO) frequency need to be well-defined to use a frequency comb as an "optical ruler". The repetition rate can be directly inferred from the light and controlled by adjusting the laser cavity length. For the CEO, a so-called f−2f interferometer typically generates a beat note between the higher-frequency end of the comb spectrum and the frequency-doubled lower-frequency end (if the optical spectrum covers a frequency octave). Feedback is provided to the pump power to keep the CEO to a defined setpoint.

Coherent power combination

Synchronizing multiple lasers using an OPLL allows for the coherent combination of light waves to produce constructive and destructive interferences for phased-array optics, LIDAR and optical beam steering, among others.

The relative optical phase between the two light fields is typically detected by overlapping the fields on a beam splitter or combiner, creating a beat note at the frequency difference of the two lasers on a photodetector. From there, an electrical phase-locked loop references the beat note to a radio-frequency oscillator with high stability. The feedback signal is then fed to a frequency- or phase-shifting element within the setup. The latter can be an element inside one of the lasers or an external element such as an acousto-optical modulator.