source: © 2012 Optics express
Advances in fabrication of high-finesse optical resonators hold promise for the development of miniaturized, ultra-sensitive, wide-band optical sensors, based on resonance-shift detection. Many potential applications are foreseen for such sensors, among them highly sensitive detection in ultrasound and optoacoustic imaging. Traditionally, sensor interrogation is performed by tuning a narrow linewidth laser to the resonance wavelength. Despite the ubiquity of this method, its use has been mostly limited to lab conditions due to its vulnerability to environmental factors and the difficulty of multiplexing – a key factor in imaging applications. In this paper, we develop a new optical-resonator interrogation scheme based on wideband pulse interferometry, potentially capable of achieving high stability against environmental conditions without compromising sensitivity. Additionally, the method can enable multiplexing several sensors. The unique properties of the pulse-interferometry interrogation approach are studied theoretically and experimentally. Methods for noise reduction in the proposed scheme are presented and experimentally demonstrated, while the overall performance is validated for broadband optical detection of ultrasonic fields. The achieved sensitivity is equivalent to the theoretical limit of a 6 MHz narrow-line width laser, which is 40 times higher than what can be usually achieved by incoherent interferometry for the same optical resonator.  [Read more…]
Fig. 4 (a) The schematic of the system used to evaluate the effect of ASE on the noise in the detection scheme. The visibility and noise level were measured for OPDs varying from 0 to 15 mm (b) The noise at the differential amplifier for wideband pulsed (blue square markers) and CW (red circle markers) sources as function of OPD obtained when the source is filtered to a bandwidth of 0.3 nm. The noise at OPD = 0 was mostly a result of electronic noise and was similar for both optical sources. (c) the noise data of Fig. 4(b) scaled to the same level for better visualization displayed with the measured fringe visibility (dashed curve). The similar dependency of noise for both cases indicates that the ASE noise is dominant in the pulse interferometry scheme. (d) The system used to test the effect of ASE rejection on noise reduction in the pulse interferometry setup. The saturable absorber (SA) added to the system had a transmission approximately 2.8 higher for the pulses compared to CW. (e) The noise recorded with the SA (solid-blue curve) and with an attenuator replacing the SA to ensure the same signal level (dashed-red curve). A reduction of 2.3 in the noise was observed, in correspondence with the SA rejection ratio.