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Edward Flagg
Department of Physics and Astronomy

Stabilized Fabry-Perot Interferometer

The emission spectra of quantum dots (QDs) have structure too fine to be discerned by typical spectrometers. For example, the emission from a neutral QD is a pair of lines separated by 2-10 GHz, while the resolution of a good quality spectrometer is about 20 GHz. To measure the fine structure of the emission spectra, we combine the spectrometer with a scanning Fabry-Perot interferometer (FPI), which acts a narrow-band, tunable filter. The transmission wavelength can be changed by changing the length of the FPI with a piezoelectric actuator.

Schematic of Fabry-Perot interferometer.

Such devices have been constructed before, but they can have poor stability due to several factors, including piezo creep and temperature fluctuations. We have used several different versions of indirect feedback to stabilize the FPI (strain gauges, air pressure control), but they were not sufficiently stable. Direct optical feedback would be best, but is complicated by the need to separate the reference laser from the QD fluorescence and by the expense of the necessary tunable laser.

Example of an optical reference scheme.

Instead, we mechanically connect the FPI mirrors to another interferometer that is itself stabilized using a reference laser and a feedback loop. The second cavity will be a shearing interferometer, because the entire interferogram can be recorded at once rather than requiring one of the system parameters to be tuned over time (e.g. laser wavelength, or cavity length). The laser is a frequency-stabilized helium-neon laser (HeNe), which has a fixed frequency and can be purchased relatively inexpensively. The interferogram is measured by a photodiode array using an Arduino microcontroller, which extracts the relative displacement of the two mirrors using Fourier techniques to determine the phase of the interference fringes.

Schematic diagram of shearing interferometer and interference pattern

The phase of the interference fringes shifts when the separation between the mirrors changes. Thus, the phase can be used as a feedback signal to stabilize the mirror separation. We can achieve a long-term stability of about 1 nm rms deviation of the separation.

Related Publications

  1. KC RB, Lander GR, Nichols C, Lee J, Flagg EB. A tunable Fabry-Perot cavity stabilized via a mechanically connected shearing interferometer. Photonic and Phononic Properties of Engineered Nanostructures XII, 12010:49–54 (2022).