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Stabilisation of the frequency of a diode laser with an external resonator to an atomic absorption line

In this illustrative application we explore the process of stabilising the frequency of a tunable diode laser using an external resonator, specifically designed to interact with an atomic absorption line.

In this application the frequency of a tunable laser is stabilised with the help of a reference cell or a gas reference cell. Suitable lasers can include tunable diode lasers, Ti:Sa or dye lasers.

The aim is to set the laser frequency to a value at which the sample has maximum absorption (or minimum absorption).

Laselock application

This application requires the following components:

• 1x digital LaseLock with HV option
• 1x laser with tuneable frequency, here via piezo-actuator (e.g. TOPTICA DL100 diode laser)
• 1x spectroscopic absorption cell*
• 1x beam splitter
• 2x photodetectors

Principle of operation

Two different methods can be applied:

• Side-of-fringe stabilisation
• Top-of-fringe stabilisation (to maximum or minimum, ‘lock-in’-technique)

Graph showing the LaseLock side of fringe

Side-of-fringe stabilisation

This method is used when a direct discriminator signal can be derived from the measurement signal. In other words the slope of the peak signal is used to convert frequency fluctuations of the laser into amplitude fluctuations, which can be detected and subsequently stabilised.

Graph showing the LaseLock top off fringe

Top-of-fringe stabilisation

This method uses a modulation technique and phase-synchronous detection.

For this the laser frequency (or a different physical measure like the resonator length) is modulated. A detector signal is multiplied by the modulation signal and then the product signal is averaged by a low-pass filter. The resulting ‘lock-in’-signal represents the derivative of the signal with respect to the laser frequency (or the respective varied physical measure).

This signal can be used directly for physical examinations. As in most cases it contains less disturbing signal parts (noise, offsets) than the directly measured signal.

The zero-crossing of the derivative represents a maximum (or minimum) of the detected signal structure. For stabilisation of a laser or resonator towards such an extremum the ‘lock-in’ signal is processed by a regulator. This generates a suitable control signal that is fed back (either directly, or for piezo actuators via a high-voltage amplifier) to the frequency-determining element of the laser (or resonator). In this way, the control loop is closed and the laser (or resonator) is locked actively to the maximum (or minimum).

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