Abstract
Balanced photodetection is commonly used to detect a relative power change between two beams with equal power and from the same source. The advantage of balanced photodetection is removing the common mode noise in the photonic signals by subtraction. Therefore, the small differential signal could be sensed. The performance of balanced photodetection for measuring small effects has been proven to be very sensitive. However to balance photocurrents is critical and the noise subtraction is not always perfect. In 1990, Hobbs devised an auto-balanced circuitry to automatically balance the two photocurrents. Hobbs had investigated the noise cancellation capability, and found that the noise floor is close to the shot noise limit. This capability had also been found to depend on the photocurrent ratio of the two beams. The dissertation reports our studies for the auto-balanced photodetection. We measured the air Faraday effect and air Cotton-Mouton effect with the technology, and studied the measurement sensitivity. The air Faraday effect was induced by an axial magnetic induction with frequency of 10 kHz and an amplitude of 1.3 mT, which was produced by a solenoid. The effect was measured with an auto-balanced photoreceiver. The signal was found, and the angular sensitivity is 2.99×10^-8 rad Hz^-1/2, which is about 2.7 times the shot noise limit of the testing beam from diode laser. In measurements, in additional to noise level, we should also consider the signal size. Therefore, we used the signal-to-noise ratio in the same air Faraday rotation measurement as the index to optimize the power split ratio to the best sensitivity. We found the best sensitivity appears at the photocurrent ratio of 1. Particularly, in this experiment, we used He-Ne laser and achieved the best sensitivity of 3.02×10^-8 rad Hz^-1/2, which is about 1.3 times the shot noise limit. We also studied the performance of cavity enhancement. In this experiment, an optical cavity enclosing the solenoid was built to examine the enhancement effect. At resonance with this configuration, the Faraday rotation was amplified and the sensitivity also improved to 7.54 × 10^-10 rad Hz^-1/2, which agrees well with Jones matrix analysis. This value is only 3 times the best sensitivity of air Faraday rotation measurement, 2.93×10^-10 rad Hz^-1/2, obtained by Jacob et al. using a cavity with more than 45 times larger finesse. The air Cotton-Mouton effect is smaller than Faraday effect, and was induced by a permanent magnet with transverse magnetic field in our experiment. This transverse magnetic field is larger than 0.8 Tesla within 3 cm. The magnetic field was modulated by rotating the magnet in low frequency. Then, two schemes are used in the experiment. First, we modulated the polarization of the testing beam with electro-optical modulator, and measured the effect with an auto-balanced photoreceiver. The important finding is that the EOM produced noise to limit the sensitivity. Second, we measured the effect directly with log output of the auto-balanced photoreceiver. We proceeded many tests and verified there were some spurious signals with the measurement. Owing to the two issues, we did not find the air Cotton-Mouton signal.