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Karthik Srinivasan
Advisor: Afshin Daryoush, Ph.D.
Abstract:
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 µm) 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/µPa) is found to be 6-8 dB lower compared to a conventional piezo-electric hydrophone (-266 to -268 dB re 1 V/µPa). An increase in the optical power in an attempt to increase the signal level leads to a corresponding increase in the noise floor of a RIN dominated system; i.e., an increase of 2 dB in noise power for every dB increase in optical power and no improvement in signal to noise ratio (SNR).
A balanced detection scheme has been shown to be useful in canceling out common mode relative intensity noise (RIN) and hence making the system shot noise dominated, where SNR increases in 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. Detailed analysis of SNR has indicated that a noise cancellation of around 15 dB along with a 3 dB signal improvement is possible in the ideal balanced detection for a RIN dominated case. 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 practical implementation of noise cancellation is demonstrated using three different approaches in out-of-phase combination of the received signals: a broadband balun with excellent amplitude (>0.1 dB) and phase imbalance (<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 is realized and a noise cancellation of 11 dB is 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 6 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 is achieved by adjusting the optical power with attenuator to balance two arms of the optical receivers. Phase imbalance is corrected for by using control of laser wavelength since fine phase adjustment can be measured for phase errors associated with even a millimeter optical fiber length differences.
Monday, December 17th, 2007 at 10 a.m.
Bossone 302
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