Tag: 2019

© 2019 Optics Express, OSA publishing.

Abstract
In optical detection of ultrasound, resonators with high Q-factors are often used to maximize sensitivity. However, in order to perform parallel interrogation, conventional interferometric techniques require an overlap between the spectra of all the resonators, which is difficult to achieve with high Q-factor resonators. In this paper, a new method is developed for parallel interrogation of optical resonators with non-overlapping spectra. The method is based on a phase-modulation scheme for pulse interferometry (PM-PI) and requires only a single photodetector and sampling channel per ultrasound detector. Using PM-PI, parallel ultrasound detection is demonstrated with four high Q-factor resonators.

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Fig. A schematic drawing of the PM-PI system used in this work to interrogate 4 resonators, implemented with π-phase shifted fiber Bragg gratings (π-FBGs). A wideband pulse laser with band-pass filters (BPFs) and an erbium-doped fiber amplifier (EDFA) create a source with a high spectral power density and sufficient bandwidth to cover the spectra of all the resonators. The modulation unit is an unbalanced Mach-Zehnder interferometer (MZI), composed of optical fiber couplers (FC) and a phase modulator (PM). The input phase signal to the PM, shown in the top-right plot, alternates between two values with a difference of 𝜋/2. For each phase value, the pulses interfere differently at the output of each resonator depending on the phase difference in the MZI for the specific resonance wavelength of that resonator. The bottom-right plot, shows a typical voltage signal measured for one of the resonators, which alternates between two states that correspond to the two phase values. As the bottom-right plot shows, in the current implementation, the duration of each phase value delivered to the PM corresponded to 5 laser pulses. We note that the limited bandwidth of our measurement did not allow full separation between the pulses in the bottom-right plot.

Yoav Hazan and Amir Rosenthal. 
Optics Express Vol. 27, Issue 20, pp. 28844-28854 (2019) https://doi.org/10.1364/OE.27.028844

 

© 2019 IEEE Photonics Journal

Impact Statement:
This paper gives a solution to one of the fundamental limitations of silicon-photonics based ultrasound detectors: the low photo-elastic response of silicon and silica. By using a BCB over-cladding, 5-fold increase in the acoustic sensitivity is achieved. We additionally provide a detailed analysis of the sensing mechanism, quantifying the different effects that contribute to the enhanced sensitivity.
Abstract:
Ultrasound detection via silicon waveguides relies on the ability of acoustic waves to modulate the effective refractive index of the guided modes. However, the low photo-elastic response of silicon and silica limits the sensitivity of conventional silicon-on-insulator sensors, in which the silicon core is surrounded by a silica cladding. In this paper, we demonstrate that the sensitivity of silicon waveguides to ultrasound may be significantly enhanced by replacing the silica over-cladding with bisbenzocyclobutene (BCB)-a transparent polymer with a high photo-elastic coefficient. In our experimental study, the response to ultrasound, in terms of the induced modulation in the effective refractive index, achieved for a BCB-coated silicon waveguide with TM polarization was comparable to values previously reported for polymer waveguides and an order of magnitude higher than the response achieved by an optical fiber. In addition, in our study, the susceptibility of the sensors to surface acoustic waves and reverberations was reduced for both TE and TM modes when the BCB over-cladding was used.

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Fig. – The measurement setup. For each of the polarizations, a Mach-Zehnder interferometer was constructed, where in each of the interferometer arms a chip with a different over-cladding material (BCB or silica) was connected. An ultrasound transducer was used to generate acoustic waves that impinged on only one of the chips, which were separated by more than 10 cm.

Resmi Ravi Kumar, Evgeny Hahamovich, Shai Tsesses, Yoav Hazan, Assaf Grinberg, Amir Rosenthal. IEEE Photonics Journal ( Volume: 11 , Issue: 3 , June 2019 )

© 2019 Optical Society of America

The detection of ultrasound via optical resonators is conventionally performed by tuning a continuous-wave (CW) laser to the linear slope of the resonance and monitoring the intensity modulation at the resonator output. While intensity monitoring offers the advantage of simplicity, its sensitivity is often limited by the frequency noise of the CW laser. In this work, we develop an alternative CW technique that can significantly reduce measurement noise by monitoring variations in the phase, rather than intensity, at the resonator output. In our current implementation, which is based on a balanced Mach–Zehnder interferometer for phase detection, we demonstrate a 24-fold increase in the signal-to-noise ratio of the detected ultrasound signal over the conventional, intensity-monitoring approach.[Read more…]

Fig. – Signals detected using the IM and PM interrogation methods with an optical power of 0.1 mW. In the PM measurement, the OPD was set to zero to maximize the SNR.

Lucas Riobó, Yoav Hazan, Francisco Veiras, María Garea, Patricio Sorichetti, and Amir Rosenthal.  Optics Letters Vol. 44, Issue 11, pp. 2677-2680 (2019) •https://doi.org/10.1364/OL.44.002677
© 2019 Optical Society of America