Pound-Drever-Hall method

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

principle

Principle scheme

To keep the laser frequency stable, a reference frequency is required to which the laser is actively stabilized in a control loop . In PDH technology, an optical resonator is this reference. Its resonance frequency depends only on its length. If this is stable, the resonator represents an absolute reference.

If the reflection of an optical resonator is measured, minima always appear precisely when the laser frequency is a multiple of the resonance frequency. The idea behind the PDH method is to stabilize the laser frequency to such a resonance by detecting a fluctuation in the frequency by measuring the reflection and compensating for it by actively regulating the laser.

The problem here is that the resonance is symmetrical in frequency; H. the detection of a frequency deviation does not contain any information about its direction. The phase course of the light field, on the other hand, is asymmetrical around the resonance frequency, i.e. it contains information about the direction. The trick now is to modulate sidebands onto the light field . If these lie far outside the resonance, the phase of the sidebands is frequency-independent and serves as a reference for the phase change of the near-resonant carrier. The phase is now measured by means of a homodyne detection and serves as a feedback signal for the control loop.

functionality

The error signal

Light from the laser to be stabilized is phase-modulated and the reflection from the resonator is measured using a fast photodetector . The frequency of the modulation is specified by a local oscillator . The signal from the detector is electronically mixed with the local oscillator signal and filtered with a low pass . The resulting signal (the error signal ) essentially represents the derivative of the transfer function of the resonator and indicates the deviation of the actual laser frequency from the resonance frequency of the optical resonator.

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 via the current flowing in the diode and via the diffraction grating (for lasers in the Littrow arrangement). The error signal generated in the PDH process is used to regulate the current and the position of the grating via the control electronics of the laser.

literature

  • Eric D. Black: An introduction to Pound-Drever-Hall laser frequency stabilization . In: American Journal of Physics . tape 69 , no. 1 , p. 79-87 , doi : 10.1119 / 1.1286663 ( online [PDF]).
  • RWP Drever, GM Ford, J. Hough, IM Kerr, AJ Munley, JR Pugh, NA Robertson, H. Ward: A gravity wave detector using optical cavity sensing, in: Schmutzer (Ed.), Proc. 9th Int. Conf. Gen.Rel. Grav., Jena 1980, pp. 265-267
  • RWP Drever, JL Hall, FV Kowalsky, J. Hough, GM Ford, AJ Munley, H. Ward: Laser phase and frequency stabilization using an optical resonator, Appl. Phys. B, Vol. 31, 1983, pp. 97-105
  • RV Pound: Electronic frequency stabilization of microwave oscillation, Rev. Sci. Instr., Vol. 17, 1946, pp. 490-505
  • RL Barger, MS Sorem, JL Hall: Frequency stabilization of a cw dye laser, Appl. Phys. Lett., Vol. 22, 1973, pp. 573-575