A high-sensitivity ultrasonic sensing system is proposed and demonstrated. In this system, a phase-shifted fiber Bragg grating (PS-FBG) is used as a sensor to achieve broadband and highly sensitive detection. The PS-FBG modulates the output of a tunable laser to detect the ultrasonic strain directly. Balanced photo-detector (BPD) is used for eliminating the DC component and further amplifying the AC component in the detected signal. Another major function of the BPD is to reject laser intensity noise. As a result, the minimum detectable strain is limited by the BPD’s noise and laser frequency noise. The sensitivity of the system is 9 ne/Hz1/2. Because of its high sensitivity, this system has the potential to be used in acousto-ultrasonic testing without amplifying the input signal and in practical acoustic emission detection.

High-sensitivity ultrasonic phase-shifted fiber Bragg grating balanced sensing system

Techniques for enhancing the absorption of photons in semiconductors with the use of microstructures are described. The microstructures, such as pillars and/or holes, effectively increase the effective absorption length resulting in a greater absorption of the photons. Using microstructures for absorption enhancement for silicon photodiodes and silicon avalanche photodiodes can result in bandwidths in excess of 10 Gb/s at photons with wavelengths of 850 nm, and with quantum efficiencies of approximately 90% or more.

Microstructure enhanced absorption photosensitive devices

High dynamic range analog photonic links : Design and implementation

We report on the first demonstration of velocity-matched distributed photodetectors and balanced photodetectors with p-i-n photodetectors. Record-high linear dc photocurrent of 45 mA has been achieved without suffering from thermal damage, thanks to the superior power handling capability of p-i-n photodiodes. A novel fiber alignment technique has been developed to achieve high linear photocurrent. More than 37 dB of common-mode-rejection ratio and 45-dB suppression of laser relative intensity noise over a broad frequency range have been achieved using the distributed balanced photodetectors in an RF fiber-optic link. The frequency response is flat from 1 to 35 GHz.

/pdf/velocity-matched-distributed-photodetectors-and-balanced-38fpadr9jl.pdf

Velocity-matched distributed photodetectors and balanced photodetectors with p-i-n photodiodes

A periodic parallel-fed traveling-wave photodetector (TWPD) with monolithically integrated power splitter is presented. The device exhibits an impedance match to the 50-Omega environment as well as a velocity match between the optical and electrical signals and shows a 3-dB bandwidth up to 85 GHz. The electrical output power reaches +10 dBm at 10 GHz. The device is based on evanescently coupled low-capacitance waveguide-integrated p-i-n photodiodes (PDs) with optimized optical matching layer which reveal a bandwidth of 145 GHz when employed as a stand-alone photodetector

High-Speed Miniaturized Photodiode and Parallel-Fed Traveling-Wave Photodetectors Based on InP

A novel velocity-matched distributed balanced photodetector with a 50-/spl Omega/ coplanar waveguide output transmission line has been experimentally demonstrated in the InP/InGaAs material system. Distributed absorption and velocity matching are employed to increase the saturation photocurrent. A common-mode rejection ratio greater than 27 dB has been achieved. The radio-frequency (RF) link experiment conducted at 4.16 GHz shows that the relative intensity noise of the laser has been suppressed by more than 24 dB and shot-noise limited performance has been achieved. Significant improvement of signal-to-noise ratio has been observed over a wide range of frequencies and phase mismatch of input RF signals.

/pdf/distributed-balanced-photodetectors-for-broad-band-noise-7hdl8uslk4.pdf

Distributed balanced photodetectors for broad-band noise suppression

We report on the microwave performance of a novel velocity-matched distributed balanced photodetector A 30-dB rejection of the common mode signals and a 17-dB suppression of the laser relative intensity noise have been achieved

Monolithic integration of distributed balanced photodetectors for high performance RF photonic links

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Distributed balanced photodetectors for broad-band noise suppression

Monolithic integration of distributed balanced photodetectors for high performance RF photonic links