Experimental setup. The beampath is represented as lines with arrows, the frequency stabilization system lies in the upper right corner of the figure, and the measurement signal generation in the lower left corner. The blocks on each cavity arm represent magnetic and/or electric fields zones that can be inserted in order to measure magnetoelectro-optic effects in gases. An optical isolator prevents feedback noise; the laser beam frequency is then frequency-shifted with an AOM in a cat’s eye retroreflector. A rEOM provides the phase modulation at frequency for the Pound–Drever–Hall frequency stabilization. The servo actuators are the laser TEC, the laser piezoelectric transducer (PZT), and the AOM. The light polarization is controlled all along the beampath by halfwave and quarterwave retardation plates and by polarizers (P). Light is injected into the cavity both in the cw and ccw directions; the and (respectively, and ) photodiodes monitor the reflected (respectively, transmitted) power in both directions.
Typical finesse measurement. After the laser is rapidly switched off, the intracavity power decays exponentially with a time constant proportional to the finesse .
Spectrum analysis of the reflected signal before demodulation. In our present experimental conditions, it represents fairly well the frequency detuning noise. The frequency resolution is limited by our spectrum analyzer 1 Hz resolution bandwidth. The shot-noise limit represented as a dashed line on the figure is evaluated from the average power impinging on the photodiode . It is situated about 15 dB under the noise spectral density in the region of interest. The bounce at resonance frequency is clearly visible. The two values of used in our experiment are chosen so that the noise spectral density is at its lowest.
Sensitivity range of our present apparatus. A laser frequency modulation of amplitude at frequency is created by a sinusoidal voltage applied to an EOM placed on the cw beampath. Since this modulation is corrected by the frequency servo-loop, it generates an out-of-phase mismatch of the laser frequency to the cavity resonance on the ccw beam. This mismatch thus appears as a spectral component at on the ccw error signal. Its amplitude , measured with a lock-in amplifier, is proportional to the frequency excursion . The linearity between and is excellent over more than 6 orders of magnitude, and the sensitivity is , achieved with a measurement time of 1000 s.
Review of recent experimental results concerning small birefringence measurements in gases. The reported is the noise equivalent birefringence at the detection frequency, while corresponds to the smallest measured birefringence value inferred from the published data.
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