Noise cancelled optical receivers in fiber optic hydrophone up to 100MHz?
Authors:   , Srinivasan Karthik , Daryoush Afshin S. 
Published: July 2021

The shift to high frequency ultrasound has necessitated the usage of hydrophones with a high bandwidth up to 100MHz. The fiber optic hydrophone is considered as an attractive low cost solution by taking advantage of the broadband associated with small (<10[mu]m) sensitive core diameter of the single mode optical fibers resulting in a reduced spatial averaging corrections required for the conventional hydrophones. However, the sensitivity of the fiber optic hydrophone (-274 dB re 1 V/[mu]Pa) is found to be 6-8 dB lower compared to a conventional piezo-electric hydrophone (-266 to -268 dB re 1 V/[mu]Pa). An increase in sensitivity can be achieved by increasing the optical power level. However, the increase in signal level is accompanied with a proportional increase in noise power in a RIN noise dominated system (i.e., an increase in 2 dB in the signal and the noise level is observed for every dB increase in optical power). The goal of this work is the implementation of balanced detection which is proven to be useful in canceling out common mode relative intensity noise (RIN) and hence making the system shot noise dominated, ensuring a SNR increase in 1 dB for every dB increase in optical power. This increase in SNR with increasing optical power provides an opportunity to improve the sensitivity of the fiber optic hydrophone without sacrificing the minimum detectable pressure. This work also presents a detailed analysis of SNR indicating that a noise cancellation of around 15 dB along with a 3 dB signal improvement is possible in the ideal balanced detection. The analysis has also considered the dependence of the achieved signal to noise ratio (SNR) due to imperfections, such as reduced coherency between two arms of balanced optical receivers, and amplitude/phase imbalance of optical receivers. The primary goal of this thesis is achieved by demonstrating noise cancellation using three different approaches for out-of-phase combination of the received signals: a broadband balun with excellent amplitude (better than 0.1 dB) and phase imbalance (better than 1 degree), a differential amplifier with a gain of 25 dB over a 100 MHz bandwidth, and finally a commercially available optical receiver implemented with matching anti-parallel photodiodes. Characterization of the realized circuits using balun and the differential amplifier indicates a Common Mode Rejection Ratio (CMRR) of at least 45 dB and 16 dB respectively. The CMRR of the anti-parallel photodiodes is specified to be at least 35 dB by manufacturer. Optical hydrophone using single mode optical fiber has been realized and a noise cancellation of 11 dB was achieved. The noise cancellation was found to be highest (12 dB) in the case of the balun as opposed to 9 dB in the case of the anti-parallel photodiodes and 8 dB in the case of the differential amplifier. This is justified by variation in CMRR for the different receivers. A 10 dB improvement in the SNR for a balanced optical receiver over a single ended receiver is achieved in the fiber optic hydrophone system. Amplitude balance has been achieved by adjusting the optical power with attenuator to balance two arms of the optical receivers. Phase balance is achieved by varying the bias current of the laser.