Pound–Drever–Hall technique

The Pound - Drever - Hall method (PDH ) method is a technique to stabilize the frequency of lasers. It is named after Robert Pound, Ronald Drever and John Lewis Hall.

Principle

To maintain the laser frequency stability, a reference frequency is required, to which the laser is actively stabilized in a feedback loop. In PDH technology provides an optical resonator, this reference dar. whose resonant frequency depends only on the length thereof. Is this stable, the resonator an absolute reference dar.

Is the reflectance of an optical resonator measured, always show minima if and only if the laser frequency is a multiple of the resonant frequency. The idea of ​​the PDH method is now, the laser frequency to stabilize as a resonance by a variation in the frequency is detected by measuring the reflection and balanced over an active control of the laser.

The problem here is that the resonance is symmetrical in the frequency, ie the detection of a frequency deviation does not contain any information about their direction. The phase profile of the light field, however, is asymmetrical about the resonance frequency, that contains information about the direction. The trick now is to modulate the light field sidebands. If these are far outside the resonance, the phase of the sidebands is frequency independent and serves as a reference for the phase change of nahresonanten carrier. The phase is then measured by a homodyne detection and is used as a feedback signal for the control loop.

Operation

The error signal

Light to strong isere ligand laser is phase modulated, and the reflection by the resonator is measured using a fast photo detector. The frequency of this modulation is set by a local oscillator. The signal of the detector is electronically mixed with the local oscillator signal and filtered with a low pass. The resultant signal ( the error signal ) is essentially the derivative of the transfer function of the resonator is, and outputs the deviation of the actual frequency of the laser to the resonant frequency of the optical resonator to.

Feedback

The type of feedback to the laser depends on the type of laser used. With commercially available diode lasers, it is possible to set the frequency over the current flowing in the diode and the diffraction grating ( laser in the Littrow configuration ). The generated error signal at the PDH method is used to regulate the flow and the position of the grid via the control electronics of the laser.