Fabry-Perot Cavity Length Research Articles

Design and analysis of high-spectral resolution lidar discriminator

An accurate aerosol optical property can be obtained by a high spectral resolution lidar (HSRL) technique, which employs a narrow spectral filter to suppress Mie scattering in the lidar return signal. The ability for filter to suppress Rayleigh scattering is critical for the HSRL. In the HSRL system, Rayleigh scattering signal is obtained and aerosol scattering is suppressed at least by a factor of 10-5 through using the narrow filter. Usually, an atomic absorption filter can reach this level. While, the gaseous absorption lines do not exist at many convenient laser wavelengths, thus restricting the development of multi-wavelength HSRL instrument. A new and practical filtering method is proposed to realize the precise detection of atmospheric optical parameters by using the reflection field of Fabry-Perot (FP) interferometer. An optical splitting system with high spectral resolution is designed and its spectral characteristics are analyzed. Based on the characteristic of hyper-spectral lidar detection signal, the variations of spectral separation ratio and Rayleigh signal transmittance with reflectivity and cavity length are discussed. Spectral separation ratio is the transmittance ratio of aerosol scattering signal to molecular scattering signal through the spectral filter. With the increases of FP cavity length and surface reflectivity, the spectral separation ratio decreases and the Rayleigh signal transmission increases. The high spectral separation ratio and Rayleigh signal transmittance can be achieved by the reflection field of FP interferometer when the FP cavity length and reflectivity parameter can be chosen reasonably. We design an FP interferometer with a cavity length of 36 mm and reflectivity of 0.4. Its spectral separation ratio is affected by the echo divergence and incidence angle. The spectral separation ratio keeps unchanged when the beam divergence angle is within 3 mrad and the incident angle of the beam is within 0.5 mrad. In addition, a simulation analysis model is established based on the error propagation. An observed actual Mie-scattering profile is used for analyzing the errors. Moreover, the influences of the divergence angle and the incident angle of the echo beam on detection results are also discussed. The results show that the proposed FP interferometer can achieve fine spectral separation of Mie and Rayleigh scattering signal, and the error of detection result is not sensitive to laser divergence angle. Fine aerosol optical parameters can be achieved when the divergence and incidence angles are controlled within 10 mrad and 1.5 mrad, respectively.

In-fiber rectangular air fabry-perot strain sensor based on high-precision fiber cutting platform

An in-fiber rectangular air Fabry-Perot (FP) strain sensor based on a high-precision fiber cutting platform (HFCP) is proposed. The HFCP consisting of a CCD notation system, a micro-displacement platform, and an optical fiber cleaver can be used to precisely control the length of FP cavity. The microcavity of FP (even only tens of microns) with smooth reflective surface can be realized easily by using this system. The FP structures with different cavity lengths have been fabricated in this paper. Simulation and experimental results prove that the shorter length the cavity has, the higher strain sensitivity and the larger free spectral range (FSR) the sensor obtains. The strain sensitivity and FSR of in-fiber rectangular air FP sensor with a cavity length of 35µm can be up to 2.23pm/με and 28.5nm respectively. Moreover, the proposed FP strain sensor has a negligible temperature sensitivity in the range of 25–75°C. It is anticipated that such easy making, compact and low-cost fiber-optic strain sensors could find important applications in practice.

Theoretical Investigation of Ultraprecise Position Determination by Spatially Modulated Low-Coherence Interference Fringe in Optical Fiber Sensing

A position determination method for low-coherence interference fringe in optical fiber sensing is proposed. This method can be used to measure displacement or length. An amplitude ratio curve is created from spatially modulated interference fringes. There is a one-to-one correspondence relationship between the peaks of the amplitude ratio curve and the extremes of the low-coherence interference fringe. The peak positions of the amplitude ratio curve only rely on the Fabry-Perot (F-P) cavity length. In addition, the peaks of the amplitude ratio curve are very sharp, which can be easily identified. In the calculation of the amplitude ratio curve, the low-coherence interference fringe is spatially modulated by Pockels effects. The impacts of the modulated voltage on the sensing precision are analytically and numerically discussed. Compared with the envelop detection method, the position determination precision of low-coherence interference fringe is enhanced about 30 times by creating an amplitude ratio curve. This method can be widely employed in the sensing of all kinds of parameters that can be converted to displacement or length.

Nine-point phase demodulation for interferometric measurement

Pure high frequency periodic triangle phase shift is introduced into optical interference, calculates nine data points in each period and obtains discrete phase values. This paper proposes a novel phase demodulation technology analyzing phase information with a symmetric linear method for depressing sampling error. Traditional low finesse Fabry–Perot interferometer produces typical cosine interference signal, performance of which is greatly improved by proposed technology. In experimental configuration we situate an electro-optics crystal to induce phase shift. The variation of Fabry-Perot cavity length is measured with a resolution much higher than the fringe counting or frequency modulation techniques. Simulation and experimentation both demonstrate feasibility and robustness of proposed algorithm. Reconstructed deviation and noise sources are discussed in detail. Various vibration formats are measured as deeper verification.

Fiber Fabry-Perot interferometer with controllable temperature sensitivity.

We proposed a fiber taper based on the Fabry-Perot (FP) interferometer structure with controllable temperature sensitivity. The FP interferometer is formed by inserting a segment of tapered fiber tip into the capillary and subsequently splicing the other end of the capillary to a single-mode fiber (SMF), the tapered fiber endface, and the spliced face form the FP cavity. Through controlling the inserted tapered fiber length, a series of FP interferometers were made. Because the inserted taper tip has the degree of freedom along the fiber axial, when the FP interferometer is subjected to temperature variation, the thermal expansion of the fiber taper tip will resist the FP cavity length change caused by the evolution of capillary length, and we can control the temperature sensitivity by adjusting the inserted taper length. In this structure, the equivalent thermal expansion coefficient of the FP interferometer can be defined; it was used to evaluate the temperature sensitivity of the FP interferometer, which provides an effective method to eliminate the temperature effect and to enhance other measurement accuracy. We fabricated the FP interferometers and calibrated their temperature characters by measuring the wavelength shift of the resonance dips in the reflection spectrum. In a temperature range of 50°C to 150°C, the corresponding temperature sensitivities can be controlled between 0 and 1.97 pm/°C when the inserted taper is between 75 and 160 μm. Because of its controllable temperature sensitivity, ease of fabrication, and low cost, this FP interferometer can meet different temperature sensitivity requirements in various application areas, especially in the fields which need temperature insensitivity.

Interference characteristics analysis of optical fiber Fabry-Perot cavity with graphene diaphragm

With regard to the design of optical fiber Fabry-Perot cavity using graphene as sensitive diaphragm,the deflection change under uniformly distributed loads in graphene film was analyzed by finite element method based on the large deflection elastic theory of circular film. The pressure-sensing mathematical model of optical fiber Fabry-Perot cavity with graphene diaphragm was established based on the working principle of Fabry-Perot interferometer. The effects of graphene film layer and incident light angle on the film reflectivity were obtained according to the refractive index characteristics of the film. Then the interference spectra change,caused by both the cavity length losses and the film deflection deformations under the pressure loads,were analyzed. The simulation results show that adding the film layer can increase the film reflectivity and further improve the optical interference performance; however,a decreasing effect on the deflection deformation is caused by the film layer with the increase of pressure load. Thus,an 8 layer graphene film can achieve a reflectivity of 0. 715% and an approximate theoretical sensitivity of 10 nm / k Pa for a 40 μm Fabry-Perot cavity length with a membrane diameter of 25 μm. It provides a theoretical basis for the design and fabrication of high-sensitivity fiber-tip pressure sensor with multiple-layer graphene diaphragm.

A high-finesse fiber optic Fabry–Perot interferometer based magnetic-field sensor

A high-finesse extrinsic Fabry–Perot interferometric sensor for the measurement of weak dc magnetic fields is demonstrated. The Fabry–Perot cavity is formed by aligning the fiber end-face and the TbDyFe rod end-face, and each end-face is coated by a mirror with a micro-lens. The length of the TbDyFe rod is changed by the variation of an applied dc magnetic field, leading a change of the Fabry–Perot cavity length. By interrogating the white-light interferometric spectrum, the wavelength of the resonant peak is tracked and the length of the Fabry–Perot cavity is obtained. The sensor exhibits a high sensitivity of 1510nm/mT with a magnetic resolution of 25nT.

Simultaneous measurement of pressure and temperature by employing Fabry-Perot interferometer based on pendant polymer droplet.

We investigated a novel and ultracompact polymer-capped Fabry-Perot interferometer, which is based on a polymer capped on the endface of a single mode fiber (SMF). The proposed Fabry-Perot interferometer has advantages of easy fabrication, low cost, and high sensitivity. The variation of the Fabry-Perot cavity length can be easily controlled by using the motors of a normal arc fusion splicer. Moreover, the enhanced mechanical strength of the Fabry-Perot interferometer makes it suitable for high sensitivity pressure and temperature sensing in harsh environments. The proposed interferometer exhibits a wavelength shift of the interference fringes that corresponds to a temperature sensitivity of 249 pm/°C and a pressure sensitivity of 1130 pm/MPa, respectively, around the wavelength of 1560 nm.

Design of MEMS-based micro-filter integrating grating and tunable Fabry-Perot cavity

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.

Microcontroller-based locking in optics experiments.

Optics experiments critically require the stable and accurate locking of relative phases between light beams or the stabilization of Fabry-Perot cavity lengths. Here, we present a simple and inexpensive technique based on a stand-alone microcontroller unit to perform such tasks. Easily programmed in C language, this reconfigurable digital locking system also enables automatic relocking and sequential functioning. Different algorithms are detailed and applied to fringe locking and to low- and high-finesse optical cavity stabilization, without the need of external modulations or error signals. This technique can readily replace a number of analog locking systems advantageously in a variety of optical experiments.

Hollow fiber-based Fabry-Perot cavity for liquid surface tension measurement.

A Fabry-Perot (FP) cavity, formed by a single-mode fiber (SMF) fusion spliced with a section of hollow fiber, was proposed and experimentally used as a sensor for liquid surface tension measurement. Due to the FP cavity length changing with different liquids, the surface tension can be measured by monitoring the interference wavelength spacing. In our experiment, five kinds of liquids were measured, and the relationship between wavelength spacing and liquid surface tension was obtained. The experimental results show that the achieved sensitivity to liquid surface tension is 30.96nm/(N/m), and the maximum error is 5.43%.

The Characteristics of Optical Pumped GaN-Based Vertical Cavity Surface Emitting Laser Structures

Time-resolved photoluminescence spectra of vertical cavity surface emitting laser (VCSEL) structures under different excitation intensity are investigated. The effect of the distributed Bragg reflectors (DBR) on the laser emission and the mechanism of multi-longitudinal-mode are analysed. A broad peak around 2.80eV is observed from the structure without DBR cavity when pumped under low excitation intensity. At higher excitation density up to 21.4kW/cm2, a lasing peak appears at 2.86eV, and exhibits a rapid growth and red shift with the increase of the excitation density. The decay time of the peak is about 66ps. When the spectrum is measured after the deposition of DBR on the top side of the grown nitride structure, the number of peaks increases to 7, and the free spectral range 0.21×1014 Hz is close to the value that calculated by the Fabry-Perot cavity length. After depositing the second DBR, multi PL peaks around 2.87eV are observed. The relationship of laser emission intensity and angle of the polarizer shows nearly a cosine square variation. However, the polarization characteristics of the four main peaks are different, which indicates that these peaks occurr by the oscillation of different optical cavities.

DETERMINATION OF CONCENTRATION AND TEMPERATURE BY A FABRY-PEROT CAVITY FORMED BY TWO FIBER BRAGG GRATINGS

A sensing system based on a Fabry-Perot cavity formed by two fiber Bragg gratings for simultaneous measurements of temperature and liquid concentration is reported. Detailed analysis and discussion of the spectral transmission characteristics of the device are provided based on Coupled-Mode Theory. Through numerical simulation of spectral transmission, reflection spectra are shown to be influenced by the grating and Fabry-Perot cavity lengths, and sensing properties of temperature and liquid concentration are studied. A fast Fourier transform based demodulation method that simultaneously demodulates temperature and liquid concentration signals of the device was studied. The use of the fast Fourier transform provides absolute measurements with high sensitivity. The temperature sensitivity was 0.0104 nm/°C, the resolution was 1°C, the error of repeatability was 4%, the hysteresis was less than 0.010 nm, and the sensitivity of the measurement on liquid reflection was 0.01%.

Fiber-Optic Temperature Sensor Using a Fabry–Pérot Cavity Filled With Gas of Variable Pressure

We report a high-temperature fiber-optic sensor based on measuring the spectral fringes of a Fabry-Perot (FP) cavity on a microstructure fiber (MF) when the gas pressure in the cavity is varied through the holes in the MF. Theoretical analysis shows that the absolute temperature can be deduced from the slope of the spectral shift versus pressure curve, which requires no calibration and is insensitive to the FP cavity length variations. For demonstration, we fabricated a miniature sensor whose FP cavity is formed by sandwiching a fuse-silica tube between a side-hole MF and a solid-core fiber. Using the holes in the MF as gas channels, the pressure in the FP cavity is controlled. The sensor was tested for operation above 1000°C. Strain-insensitive temperature measurement was demonstrated at ambient temperature for a strain range up to 3600 μe.

Deep subwavelength Fabry-Perot resonances

Confinement of light by subwavelength objects facilitates the realization of compact photonic devices and the enhancement of light-matter interactions. The Fabry-Perot (FP) cavity provides an efficient tool for confining light. However, the conventional FP cavity length is usually comparable to or larger than the light wavelength, making them inconvenient for many applications. By manipulating the reflection phase at the cavity boundaries, the FP cavity length could be made much smaller than the wavelength. In this review, we consider the subwavelength FP resonance in a plasmonic system composed of a slit grating backed with a ground plane, covering the spectral range from microwave to THz and infrared regime. For very narrow slit width and spacer thickness, a typical zero-order and deep subwavelength FP resonance in the metallic slits can be strongly induced. Moreover, due to the subwavelength FP resonance, greatly enhanced electromagnetic pressure can also be induced in the system. The sign and magnitude of the electromagnetic pressure are dominated by the field penetration effect in the metal as well as the field enhancement in the FP cavities. The effect promises a variety of potential applications, such as detecting tiny motions and driving the mechanical oscillations.

Fiber-Optic Extrinsic Fabry-Perot Interferometer Sensors with Multi-Wavelength Intensity Demodulation

In this paper, a novel method used for Fabry-Perot cavity length's demodulation in high-speed and large scope measurement was proposed. Principle of the method is based on uniqueness of intensity of multiple wavelengths in the scope of Fabry-Perot vibration. This technique offers flexibility of selecting incident wave length and enhances the scope of demodulation of cavity length, compared with that popular triple wave length demodulation. In experiment, multi-wavelength demodulation is demonstrated for measurement of cavity length varying from 110um to 115um and the strain resolution was higher than 0.1um.

Temperature characteristics of a core-mode interferometer based on a dual-core photonic crystal fiber

A core-mode Fabry–Perot (FP) interferometer is constructed by using a dual-core photonic crystal fiber (DCPCF). The FP cavity is formed by a single piece of DCPCF, which can also serve as a direct sensing probe without any additional components. We theoretically and experimentally studied its temperature responses in the range of 40–480 °C. The temperature sensitivity is 13 pm/°C which matches the theoretical results. Since the temperature sensitivity of the proposed sensor is independent on cavity length, precise control of the length of FP cavity or photonic crystal fiber is not required. The sensor size can be as short as 100–200 μm, and its fabrication only involves splicing and cleaving, which make the sensor production very cost-effective. The proposed FP interferometric sensor based on a DCPCF can find applications in high-temperature measurement especially those that need accurate point measurement with high sensitivity.

MEMS Fabry-Perot sensor interrogated by optical system-on-a-chip for simultaneous pressure and temperature sensing

We present a micro-electro-mechanical systems (MEMS) based Fabry-Perot (FP) sensor along with an optical system-on-a-chip (SOC) interrogator for simultaneous pressure and temperature sensing. The sensor employs a simple structure with an air-backed silicon membrane cross-axially bonded to a 45° polished optical fiber. This structure renders two cascaded FP cavities, enabling simultaneous pressure and temperature sensing in close proximity along the optical axis. The optical SOC consists of a broadband source, a MEMS FP tunable filter, a photodetector, and the supporting circuitry, serving as a miniature spectrometer for retrieving the two FP cavity lengths. Within the measured pressure and temperature ranges, experimental results demonstrate that the sensor exhibits a good linear response to external pressure and temperature changes.

Temperature-compensated f iber optic Fabry-Perot accelerometer based on the feedback control of the Fabry-Perot cavity length

A temperature-compensated fiber optic Fabry-Perot accelerometer (FOFPA) formed by symmetrically bonding an all-silica in-line fiber Fabry-Perot etalon (ILFFPE) and a piezoelectric ceramic unimorph actuator (PCUA) to two surfaces of a silica cantilever is reported. The all-silica ILFFPE with feedback-controlled cavity length by the PCUA simultaneously senses acceleration and temperature. The results indicate that the fabricated FOFPA system simultaneously senses acceleration and temperature with active temperature compensation. The nonlinearity of the output voltage to acceleration is less than 0.65%. The nonlinearity of the control voltage to temperature is 1.75%. Furthermore, the maximum deviation of the sensitivity with temperature compensation at a temperature range from 25 to 50 oC is 0.025 V/g.

Enhanced electromagnetic pressure in a sandwiched reflection grating

We suggest that strongly enhanced electromagnetic pressure can be induced at the boundary of a sandwiched reflection grating due to a special kind of deep subwavelength Fabry-Perot (FP)-like resonance. A theoretical formula will be derived and employed to study this effect at both the infrared and microwave regimes. The magnitude or sign of the electromagnetic force can be tuned by adjusting the system parameters, which in turn adjust the balance between the electric and magnetic energies stored in the system. In the infrared regime, a strong attractive pressure was found, which is controllable by the FP cavity length. In the microwave band, a repulsive pressure can be realized in most situations. What is rather amazing is that a strong attractive electromagnetic pressure, which has not been found previously, can also be demonstrated in some geometries in the microwave regime.