Liquid crystal based polarized low coherence interferometer for optical demodulation in sensors.

This paper presents basic designs and operation concepts of a novel micro-electro-mechanical-systems (MEMS) filter integrating a tunable Fabry-Perot (FP) cavity and a grating. It is based on the filtering effect of the tunable FP cavity and the spatial separation function of the grating for the different wavelengths. The combination of two devices could widen the spectral range of the traditional FP filter without affecting its spectral linewidth. A MEMS filter configuration based on the silicon-glass bonded structure has been designed. The FP cavity length depends on the thickness of the silicon wafer itself to realise the long cavity length. The T-S cantilevers could decrease the micromirror deformation, and reduce the driving voltage appropriately. Calculational results indicate that narrow linewidth and wide spectral range could be obtained simultaneously at the long cavity length.

We investigate the behavior of cholesteric liquid crystals (CLC's) inside a Fabry-Perot (FP) cavity. Although FP cavities filled with various liquid crystals have been extensively studied, to our knowledge the behavior of CLC-based FP cavities has not been reported. In CLC the twisted structure can be changed because the pitch is a function of temperature. In a parallel-rubbed CLC FP cavity the balance between strong surface anchoring and elastic energy yield a steplike resonance spectrum. This corresponds to the quantized effective pitch that the system assumes when both surface alignments are fixed. Experiment results for parallel-rubbed samples are presented and explained theoretically by use of Jones matrix calculations.

In this article, we have proposed and experimentally demonstrated an optical fiber refractive index (RI) sensing interrogation system realized by extrinsic Fabry–Perot interferometer (EFPI) structure-based microwave photonic filter (MPF) technology. EFPI is considered as a sensing element, as well as a spectrum slicer of the MPF. The shift of the central frequency of the MPF’s passband caused by the change of the RI in the Fabry–Perot (FP) cavity can be monitored to demodulate the RI variation. In our experiment, four EFPI sensors with different FP cavity lengths have been applied for the RI sensing test. The experimental results show that the sensitivity can be tailored by adjusting the length of the FP cavity, and the longer the length of the FP cavity, the higher the sensitivity can be achieved. When the length of the FP cavity is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$960 ~\mu \text{m}$ </tex-math></inline-formula> , a high sensitivity of 1.2 GHz/RIU has been obtained. The proposed sensing system can transfer the changes of the interference pattern in the optical domain to the central frequency shift of the MPF’s passband in the electronic domain and thus avoid the limitation of measurement range induced by the periodic characteristics of the FP interference spectrum. The proposed sensing system also has the advantages of high sensitivity, the feasibility of multiplexing, temperature insensitivity, good repeatability and stability, simple fabrication, low cost, and so on.

A novel tunable laser which can achieve mode-hop-free single-electrode tuning based on a ring coupled cavity is proposed. The device comprises of a passive ring resonator coupled to two optical waveguides, two far ends of which are terminated by partially reflecting mirrors to constitute a Fabry-Perot (FP) cavity. The waveguide in the ring resonator is divided into a tuning segment and a bias segment. The FP cavity contains an active gain waveguide and passive waveguide segments. A tuning electrode covers the tuning segment inside the ring resonator and the passive waveguide segments in the FP cavity. Mode-hop-free operation with a single-electrode tuning is achieved by properly designing the tuning electrode lengths within the FP cavity and within the ring resonator. Numerical analysis shows that a large mode-hop-free tuning range (>6 nm) with a very high single-mode selectivity can be achieved with an injection current less than 100 mA.

Rydberg atom-based microwave (MW) atomic receiver is regarded as a promising atomic receiver to surpass the sensitivity of conventional receivers. This article proposes a tunable Fabry-Perot (FP) cavity that locally enhances the MW electric (E) field to improve measurement sensitivities. This FP cavity includes a 0.9947 reflection coefficient periodic metal slit structure and a reflective plate. In our experiment, 21-dB signal-to-noise ratio (SNR) enhancement is achieved by utilizing the FP cavity. Ultimately, the atomic receiver loaded with a FP cavity provides a sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ \mathbf {5 \mathbf {nv/cm/\sqrt {Hz}}}$ </tex-math></inline-formula>@8.57 GHz that is the most advanced sensitivity known based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {^{133}\textrm {Cs}}$ </tex-math></inline-formula> resonant transition <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {|44D_{5/2}\rangle \rightarrow |45P_{3/2}\rangle }$ </tex-math></inline-formula>. Since the FP cavity is different from the traditional closed cavity and split-ring resonance mechanism, a large enhancement tunable frequency bandwidth of 4.8–12.2 GHz can be achieved by adjusting the plate spacing. In addition, the FP cavity solves the direct coupling between the electromagnetic wave and the resonator in the X-band, which provides mobility and flexibility. We take all the vapor cell wall’s refractive index and metal reflections into account as the resonance-based system is more sensitive to errors.

In this paper, we investigate the effect of non-uniformities (enlargement of current passage, non-equal surface current densities, etc.) in axial as well as transverse directions of a porous silicon Fabry-Perot (FP) cavity as well as loss nature of bulk silicon on spectral properties of this cavity, even that cavity is created with an anisotropic etching process. Without correct and comprehensive characterization of such cavities by incorporating these non-uniformities and inherent lossy nature of a cavity, detection and identification of biological and chemical molecules by that cavity may yield unpredictable and misleading results. From our simulations, we note the following two key points. First, effects of the refractive index and the thickness of microcavity region of a lossless or lossy FP cavity on resonance wavelength is more prevailing than those of first and last layers. Second, the effect of some small loss inside the FP cavity is not detectable by the measurement of resonance wavelength whereas the same influence is noticeable by the measurement of reflectivity. We carried out some measurements from two different regions on the fabricated cavities to validate our simulation results. From a practical point of view in correct detection and/or identification of lossy biological or chemical vapor by FP cavities, we conclude that not only the measurement of resonance wavelength as well as its shift but also the reflectivity value at the resonance wavelength or some specific wavelengths should be utilized.

Ultra-stable lasers (USLs) based on Fabry-Perot (FP) cavities frequency stabilization take TEM00 mode as the reference frequency. When the laser does not match the FP cavity perfectly, high-order modes will be excited, affecting the performance of USLs. In this paper, influence of different laser alignment conditions on resonant modes of a FP cavity was numerically analyzed using Optical Simulation Containing Ansys Results (OSCAR) code. The relationship between energies of resonant modes and different laser incident conditions was revealed. The results can be a guide for aligning lasers and FP cavities.

Erbium fiber lasers emitting in the 1.55um wavelength region have been under intensive research for applications in telecommunications, spectroscopy and sensors. To ensure single frequency operation, fiber lasers constructed with either Fabry-Perot cavities or ring cavities usually require wavelength filtering components such as external gratings [1], in-fiber gratings [2] and Fabry-Perot filters [3]. Wavelength tuning can subsequently be achieved by filter adjustments. Recently a microchip erbium:ytterbium (Er:Yb) phosphate-glass laser has achieved single-mode operation in a 200um Fabry-Perot cavity without additional filtering [4]. Here we report a compact tunable single-mode fiber laser using a novel combination of an Er:Yb phosphate-glass fiber (developed under contract to Amoco Technology Company) within a fiber Fabry-Perot (FFP) cavity [5]. Attractive features of Er: Yb phosphate glass include efficient pump absorption at 980nm by the Yb ions and a large stimulated-emission cross-section for the Er. FFP technology allows fabrication of very short cavities ( ~ 8um) and continuous wavelength tunability with low loss. Here we demonstrate the shortest Er: Yb phosphate-glass FFP laser ever reported, having a 100um cavity length and a continuous wavelength tuning range over 4.52nm, limited only by the sharp gain peak.

A novel all-silica fiber optic Fabry-Perot (FP) pressure sensor with pressure leading-in tube based on microbubble structure is developed and experimentally demonstrated. The FP cavity is formed by fixing the end face of the single-mode fiber (SMF) parallel to the outer surface of the microbubble, in which the microbubble with a diameter of about 318 μm is constructed at the end of silica hollow tube. When external pressure is transmitted on the inner surface of the microbubble by the pressure leading-in tube, the FP cavity length changes with the diameter of microbubble. Experimental results show that such a sensor has a linear sensitivity of approximately 4.84 nm/MPa at room temperature over the pressure range of 1.1 MPa; the sensor has a very low temperature coefficient of approximately 2 pm/°C from room temperature to 600°C. The sensor has advantages of extremely low temperature coefficient, compact structure, and small size, which has potential applications for measuring pressure in high-temperature environment.

Excitonic optical bistability with nanosecond switching time in ZnSe-ZnS multiple quantum wells (MQWs) with a Fabry-Perot (FP) cavity is investigated at room temperature. The research result indicates that the threshold and contrast ratio for the optical bistability in ZnSe-ZnS MQWs with a FP cavity are about 0.2 MW/cm2 and 6:1, respectively. On the basis of the room temperature excitonic absorption spectra obtained here, we attribute the major nonlinear mechanism for the optical bistability to the excitonic saturating absorption due to phase space filling of excitonic states and excitonic band broadening.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe a stable lightwave synthesized frequency sweeper (LSFS) using a Fabry-Perot (FP) cavity composed of two Faraday rotator mirrors (FRMs). In the former half of this paper, we numerically simulate the behavior of amplified spontaneous emission (ASE) noise accumulations, and show that angular deviations of the FRMs can cause instability in the frequency sweep span. We also show that the span is stabilized by placing a polarizer in one of the FRMs because the FP cavity then works as a single-polarization cavity with a small transmittance fluctuation. Experimental results confirming the stability of this LSFS are provided in the latter half of this paper.

We have reported two different aspects of this work already [1,2]. Nevertheless as the point of view is unconventional we review it here strictly within the context of the theory of optical bistability and optical multistability. We are concerned to connect the envelope Maxwel1-Bloch equations with optical bistability (multistability) in a Fabry-Perot (FP) cavity in a rigorous and potentially quantitative way. One problem in this connection is an adequate statement about standing waves. We present methods which derive the standing wave equations of motion completely as a part of a comprehensive non-linear refractive index theory of multistabi1ity inside the FP cavity. The theory is a c-number one — but a comparable quantum theory seems possible. A key feature of the argument is generalisation of the famous 'optical extinction theorem' [1,2,3] to this non-linear regime. In practice it means we do not invoke any boundary conditions at the surfaces of the FP cavity — only conditions at infinity — and this offers advantages for the quantitative description as we show.

The direct patterning of colors using the bombardment of a focused beam of electrons onto a thin-film stack consisting of poly(methyl methacrylate) coated with a thin nickel film is demonstrated. This direct electron-beam color printing approach creates variations in the height of a Fabry-Perot (FP) cavity, resulting directly in a color print without the need for prepatterned substrates, distinct from some direct laser writing methods. Notably, the resolution of the color prints is defined by the electron beam. Height measurements with ∼5 nm accuracy through color image analysis of an electron-beam-patterned FP cavity were carried out. This technique also introduces a reflectance-based measurement of the point exposure function of a focused electron beam, aiding in rapid proximity effect corrections. In addition, the grayscale lithographic nature of this process was used to produce blazed gratings and could enable the fabrication of other 2.5D nanostructures with precise height control.

Adaptive infrared radiation modulator using reversible metal electrodeposition with silicon carbide-based fabry-perot cavity

Spectroscopy is one of the most powerful technical tools that are widely used in the study of natural sciences and the development of high technologies. To improve the performance of a traditional dispersive spectrometer, there is a competition between its spectral resolution and the geometric volume; a higher resolution is usually accompanied by a higher cost as well. However, a growing body of evidence suggests an urgent demand for miniaturized spectrometers with high spectral resolution and low costs. In this paper, we propose and study super-resolution spectroscopy via spectrum slicing by use of a Fabry–Perot (FP) cavity combined with a traditional spectrometer. The cavity functions as a spectral filter to slice into discrete pieces the continuous spectrum of the input light. For each given cavity length, the cavity outputs a set of sliced spectral pieces that are then analyzed and recorded by the subsequent spectrometer. By scanning the cavity length, one will have multiple sets of sliced spectral pieces that are then finally fused to recover a resolution-enhanced copy of the input spectrum. A theoretical analysis for the super-resolution spectroscopy is provided to prove the feasibility of the proposal, as further confirmed by MATLAB numerical simulation. The studied spectral analysis technique will benefit the research fields in need of high resolution spectrometers with moderate device volumes and low costs.