[Show abstract][Hide abstract] ABSTRACT: This work reports on an optical hydrophone that is insensitive to hydrostatic pressure, yet capable of measuring acoustic pressures as low as the background noise in the ocean in a frequency range of 1 Hz to 100 kHz. The miniature hydrophone consists of a Fabry-Perot interferometer made of a photonic-crystal reflector interrogated with a single-mode fiber and is compatible with existing fiber-optic technologies. Three sensors with different acoustic power ranges placed within a sub-wavelength sized hydrophone head allow a high dynamic range in the excess of 160 dB with a low harmonic distortion of better than -30 dB. A method for suppressing cross-coupling between sensors in the same hydrophone head is also proposed. A prototype was fabricated, assembled, and tested. The sensitivity was measured from 100 Hz to 100 kHz, demonstrating a sound-pressure-equivalent noise spectral density down to 12 μPa/Hz(1/2), a flatband wider than 10 kHz, and very low distortion.
The Journal of the Acoustical Society of America 04/2011; 129(4):1837-50. DOI:10.1121/1.3543949 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An all-silica design and silicate bonding are used to demonstrate a Fabry-Perot acoustic fiber sensor for large-scale array applications with a high sensitivity, high thermal stability, and excellent reproducibility in displacement sensitivity (±0.6 dB).
Optical MEMS and Nanophotonics (OMN), 2011 International Conference on; 01/2011
[Show abstract][Hide abstract] ABSTRACT: A new design of miniature fiber acoustic sensors with high sensitivity and greatly improved thermal stability and reproducibility of the operating wavelength is presented. It consists of a high-finesse Fabry-Perot (FP) made of a photonic crystal (PC) reflector fabricated on a compliant Si membrane and placed in close proximity to the reflective end of a fiber. An incident acoustic wave deflects the membrane, which shifts the FP reflection spectrum. This shift is detected as a change in the power reflected by the FP at a fixed wavelength. The reported improvements over an earlier design include (1) the use of photolithography to fabricate the PC diaphragm (higher accuracy and yield), (2) making the sensor chip out of silica instead of Si (higher thermal stability and reproducibility of the FP spectrum), and (3) using silicate bonding to assemble the sensor (higher thermal stability). An experimental fiber acoustic sensor with a finesse of ~6 is shown to measure pressures in air with a resolution from 180 to 27 μPa/VHz from ~700 Hz to ~8.6 kHz, and as low as 5.6 μPa/VHz at 12.5 kHz. This last value is ~4 times better than previously reported. The measured thermal stability is ~70 times better than that of the earlier Si-based design.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that the reflection spectrum of a fiber Fabry–PÉrot formed by a single-mode fiber and an external mirror does not exhibit the standard Lorentzian profile of a conventional bulk-optic Fabry-PÉrot, but is instead highly asymmetric. Measurements indicate that the sign of this asymmetry is different for dielectric and metal mirrors. We show that this asymmetry is due to one of two mechanisms, namely the beam diffraction in the cavity and the complex phase upon reflection from a metal layer. We present an analytical approach to accurately model these spectra, and provide simple analytical formulas, useful in the design and optimization of fiber Fabry-PÉrot-based sensors. Specifically, we present expressions for the maximum finesse and the condition to obtain full contrast in a fiber Fabry-PÉrot. These results are widely applicable, in particular to both low- and high-finesse interferometers.
Journal of Lightwave Technology 01/2010; DOI:10.1109/JLT.2009.2032135 · 2.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In the above titled paper (ibid., vol. 27, no. 24, pp. 5648-5656, Dec. 09), there was an error with some embedded fonts. This has been corrected and can be accessed on Xplore.
[Show abstract][Hide abstract] ABSTRACT: Use of Photonic Crystals (PCs) to control electromagnetic fields
enables-unprecedented scaling of low-loss, free-space optical devices. In this paper, we
describe the operation, design, and fabrication of PC mirrors, and demonstrate their
applications in microoptical-systems.
Applications of Lasers for Sensing and Free Space Communications; 01/2010
[Show abstract][Hide abstract] ABSTRACT: We report a miniature mechanical gyroscope that utilizes optical means to detect rotation-induced displacements in a mechanical structure. It utilizes the Foucault pendulum principle used in some existing MEMS gyroscopes: a rotating reference frame induces a Coriolis force that oscillates the structure about an axis orthogonal to the driving-mode axis. The main difference with similar MEMS gyroscopes is that this rotation-induced oscillation is sensed using a pair of high-finesse fiber Fabry-Perot displacement sensors instead of a capacitive device. The drive axis is also driven by radiation pressure inside a set of auxiliary fiber Fabry-Perot cavities, making this device immune to electromagnetic interference. Calculations predict that a rotation sensitivity on the order of 1°/h/Hz1/2 is achievable. We show that this structure solves several problems associated with MEMS gyroscopes utilizing electrostatic sensing methods.
Proceedings of SPIE - The International Society for Optical Engineering 10/2009; DOI:10.1117/12.835141 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper reports on a highly sensitive fiber-optical acoustic sensor. The miniature acoustic sensor consists of a Fabry-Perot interferometer made of a photonic-crystal reflector embedded on a compliant silicon diaphragm placed at the tip of a single-mode fiber. A high-reflectivity multilayer dielectric mirror is deposited on the tip of the single-mode fiber. When combined with the high reflection from the photonic-crystal mirror, this second mirror forms a Fabry-Perot with a relatively high finesse (~15), giving rise to a high pressure sensitivity over the 10-50 kHz range. A minimum detectable pressure of 30 muPa/Hz<sup>12</sup> at 15 kHz is reported.
[Show abstract][Hide abstract] ABSTRACT: We present an analysis of the phase and amplitude responses of guided resonances in a photonic crystal slab. Through this analysis, we obtain the general rules and conditions under which a photonic crystal slab can be employed as a general elliptical polarization beam splitter, separating an incoming beam equally into its two orthogonal constituents, so that half the power is reflected in one polarization state, and half the power is transmitted in the other state. We show that at normal incidence a photonic crystal slab acts as a dual quarter-wave retarder in which the fast and slow axes are switched for reflection and transmission. We also analyze the case where such a structure operates at oblique incidences. As a result we show that the effective dielectric constant of the photonic crystal slab imposes the Brewster angle as a boundary, separating two ranges of angles with different mechanisms of polarization beam splitting. We show that the diattenuation can be tuned from zero to one to make the structure a circular or linear polarization beam splitter. We verify our analytical analysis through finite-difference time-domain simulations and experimental measurements at infrared wavelengths.
Journal of the Optical Society of America A 12/2008; 25(11):2680-92. DOI:10.1364/JOSAA.25.002680 · 1.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We show that modes in a photonic crystal slab that are uncoupled to outside radiation in a symmetric structure can be excited by breaking the mirror symmetry through introducing a protrusion on the side of the photonic crystal holes. We show that coupling to these resonances can be controlled by the strength of this asymmetry, and that it is also possible to choose among modes to couple to, through the shape of the asymmetry introduced. We provide simple theoretical arguments that explain the effect, and present eigenmode simulations and time-domain simulations. We confirm this predicted behavior with measurements on a photonic crystal with a broken mirror symmetry that exhibits an additional sharp resonant feature with a linewidth of 0.5 nm, in agreement with both calculated and simulated predictions.
[Show abstract][Hide abstract] ABSTRACT: We report a miniature fiber hydrophone that consists of a Fabry-Perot interferometer made of a photonic-crystal reflector embedded on a compliant silicon diaphragm placed at the tip of a single-mode fiber. A model was developed to show that after proper optimization to ocean acoustics, this sensor has a minimum detectable pressure that follows the minimum ambient noise of the ocean (reaching a minimum of ~10 µPa/Hz 1/2 at ~30 kHz) in the bandwidth of 1 Hz–100 kHz. By placing several such sensors with different acoustic power ranges within a single hydrophone head, the hydrophone is able of exhibiting a dynamic range in the excess of 200 dB. A prototype was fabricated, assembled, and tested that confirmed this high sensitivity and bandwidth.
Proceedings of SPIE - The International Society for Optical Engineering 06/2008; DOI:10.1117/12.785919 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Photonic-crystal slabs are two-dimensional photonic crystals etched into a dielectric layer such as silicon. Standard micro fabrication techniques can be employed to manufacture these structures, which makes it feasible to produce them in large areas, usually an important criterion for practical applications. An appealing feature of these structures is that they can be employed as free-space optical devices such as broadband reflectors. The small thickness of the slab (usually in the vicinity of half a micron) also makes it deflectable. These combined optical and mechanical properties make it possible to employ photonic-crystal slabs in a range of practical applications, including displacement sensors, which in turn can be used for example to detect acoustic waves. An additional benefit of employing a photonic-crystal slab is that it is possible to tailor its optical and mechanical properties by adjusting the geometrical parameters of the structure such as hole radius or shape, pitch, and the slab thickness. By altering the hole radius and pitch, it is possible to make broadband reflectors or sharp transmission filters out of these structures. Adjusting the thickness also affects its deformability, making it possible to make broadband mirrors compliant to acoustic waves. Altering the hole shape, for example by introducing an asymmetry, extends the functionalities of photonic-crystal slabs even further. Breaking the symmetry by introducing asymmetric holes enables polarization-sensitive devices such as retarders, polarization beam splitters, and photonic crystals with additional non-degenerate resonances useful for increased sensitivity in sensors. All these practical advantages of photonic-crystal slabs makes them suitable as key components in micromachined sensor applications. We report one such example of an application of photonic-crystal slabs in the form of a micromachined acoustic sensor. It consists of a Fabry-Perot interferometer made of a photonic-crystal reflector embedded in a compliant silicon diaphragm placed at the tip of a single-mode fiber. Measurements in air indicate that this sensor has a relatively uniform frequency response up to at least 50 kHz, which is at least one order of magnitude higher than existing all-fiber acoustic sensors. This sensor was also shown to be able to detect pressures as low as 18 muPa/Hz 1/2. This limit is four orders of magnitude lower than in similar types of acoustic fiber sensors that are based on a deflectable diaphragm at the fiber end. This significant improvement is to a large extent due to the higher reflectivity of the reflectors, which is itself due to the use of a photonic crystal. Through a modification in the design, such a sensor can also be used in water. In addition to the high compliance of the diaphragm, the advantage for using the photonic-crystal slab is that the holes provide a venting channel for pressure equalization. As a result, the hydrophone can be employed in deep-sea applications without suffering from the high static pressure. Measurements in water over the range of 10 kHz-50 kHz show that this hydrophone has a minimum detectable pressure of only 10 muPa/Hz1/2, close to the ambient thermal-noise level. A model was developed to show that after optimization to ocean acoustics, the sensor has a theoretical minimum detectable pressure that follows the minimum ambient noise spectrum of the ocean in the bandwidth of 1 Hz-100 kHz. This makes this sensor extremely broadband compared to commercial fiber hydrophones, which are bulky and poorly responsive to frequencies above a few hundred Hz, since they require a long length of fiber. By placing several such sensors with different acoustic power ranges within a single sensor chip, this hydrophone is capable of exhibiting a dynamic range in the excess of 200 dB (1010).
[Show abstract][Hide abstract] ABSTRACT: We demonstrate an acoustic sensor based on a Fabry–Perot interferometer formed by a single-mode fibre and an external silicon photonic-crystal mirror. Measurements in air indicate that this sensor has a relatively uniform frequency response up to at least 50 kHz, and detects pressures as low as 18 μPa Hz−1/2. This limit is four orders of magnitude lower than in similar types of acoustic fibre sensors. The sensor response agrees with mechanical calculations to within a few dB. We predict that this sensor has the potential to detect pressures as low as 600 nPa Hz−1/2, corresponding to the ambient thermal noise level in air at room temperature.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate theoretically and experimentally that a photonic crystal slab with form birefringence can act as a dual quarter-wave retarder based polarizing beamsplitter, which separates an incoming wave into two orthogonal polarizations.
[Show abstract][Hide abstract] ABSTRACT: Air-bridged photonic crystal slabs are demonstrated at visible and near-infrared wavelengths. We present experimental and theoretical studies of the transmission spectra of air-bridged photonic crystal slabs with free space illumination using collimated and focused beams. We show that the resonances of an air-bridged photonic crystal slab exhibit considerable differences in their sensitivity to the angle of illumination. We show that the variation in the quality factor of a resonance with the incident angle of illumination is determined by the symmetry of its field distribution. Last, we present experimental and theoretical results on the transmission spectra of air-bridged photonic crystal slabs with focused illumination, important due to the finite angular spread of real optical beams.
[Show abstract][Hide abstract] ABSTRACT: Non-degenerate resonances in a photonic crystal slab that are uncoupled in a symmetric structure are excited in a mirror symmetry lacking structure. The coupling to these resonances is controlled by the degree of asymmetry.
Lasers and Electro-Optics, 2005. (CLEO). Conference on; 06/2005
[Show abstract][Hide abstract] ABSTRACT: We characterize the transmission spectra of out-of-plane, normal-incidence light of two-dimensional silicon photonic crystal slabs and observe excellent agreement between the measured data and finite-difference time-domain simulations over the 1050-1600-nm wavelength range. Crystals that are 340 nm thick and have holes of 330-nm radius on a square lattice of 998-nm pitch show 20-dB extinction in transmission from 1220 to 1255 nm. Increasing the hole radius to 450 nm broadens the extinction band further, and we obtain >85% extinction from 1310 to 1550 nm. Discrepancies between simulation and measurement are ascribed to disorder in the photonic lattice, which is measured through image processing on high-resolution scanning electron micrographs. Analysis of crystal imperfections indicates that they tend to average out narrowband spectral features, while having relatively small effects on broadband features.
[Show abstract][Hide abstract] ABSTRACT: Photonic crystal slabs are experimentally demonstrated at visible wavelengths. These consist of freestanding silicon nitride membranes, patterned with square lattices of holes. Measured transmission spectra are in good agreement with finite difference time domain calculations.
[Show abstract][Hide abstract] ABSTRACT: It was recently demonstrated that a photonic crystal slab can function as a mirror for externally incident light along a normal direction with near-complete reflectivity over a broad wavelength range. We analyze the angular and polarization properties of such photonic crystal slab mirror, and show such reflectivity occurs over a sizable angular range for both polarizations. We also show that such mirror can be designed to reflect one polarization completely, while allowing 100% transmission for the other polarization, thus behaving as a polarization splitter with a complete contrast. The theoretical analysis is validated by comparing with experimental measurements.