Tunable Fabry-Perot Filter Research Articles

Polarization-sensitive femtosecond mid-infrared spectrometer with a tunable Fabry–Perot filter and chirped-pulse upconversion

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular structure and vibrational dynamics. By using a polarized narrowband pump pulse in which center frequency is rapidly scanned with an electrically tunable Fabry-Perot filter, we have developed a femtosecond MIR spectrometer that is more sensitive to molecular structure. The pump energy and polarization dependences of transient MIR absorption spectra measured with the spectroscopic system revealed the appearance of a dark (infrared-inactive) mode due to a slight decrease in the molecular symmetry of a metal carbonyl complex, Mn2(CO)10, in solution. In addition, the rotational relaxation time of Mn2(CO)10 in solution was determined as approximately 20ps.

SOFIA-HIRMES: Looking Forward to the HIgh-Resolution Mid-infrarEd Spectrometer

The HIgh-Resolution Mid-infrarEd Spectrometer (HIRMES) is the 3rd Generation Instrument for the Stratospheric Observatory For Infrared Astronomy (SOFIA), currently in development at the NASA Goddard Space Flight Center (GSFC), and due for commissioning in 2019. By combining direct-detection Transition Edge Sensor (TES) bolometer arrays, grating-dispersive spectroscopy, and a host of Fabry-Perot tunable filters, HIRMES will provide the ability for high resolution ([Formula: see text]), mid-resolution ([Formula: see text]), and low-resolution ([Formula: see text]) slit-spectroscopy, and 2D Spectral Imaging ([Formula: see text] at selected wavelengths) over the 25–122[Formula: see text][Formula: see text]m mid to far infrared waveband. The driving science application is the evolution of proto-planetary systems via measurements of water-vapor, water-ice, deuterated hydrogen (HD), and neutral oxygen lines. However, HIRMES has been designed to be as flexible as possible to cover a wide range of science cases that fall within its phase-space, all whilst reaching sensitivities and observing powers not yet seen thus far on SOFIA, providing unique observing capabilities which will remain unmatched for decades.

Liquid crystal-based tunable photodetector operating in the telecom C-band

Liquid crystal (LC) microcells monolithically integrated on the surface of InGaAs based photodiodes (PDs) are demonstrated. These LC microcells acting as tunable Fabry-Perot filters exhibit a wavelength tunability of more than 100 nm around 1550 nm with less than 10V applied voltage. Using a tunable laser operating in the S and C bands, photocurrent measurements are performed. On a 70 nm tuning range covered with a driving voltage lower than 7V, the average sensitivity for the PD is 0.4 A/W and the spectral linewidth of the LC filter remains constant, showing a FWHM of 1.5 nm. Finally, the emission spectrum from an Er-doped fiber is acquired by using this tunable PD as a micro-spectrometer.

High Accurate and Stable Demodulation for 3-D Encoded Optical Fiber Sensing Network

An improved demodulation platform for 3-D encoded optical fiber sensing network is proposed with high accuracy and stability. The 3-D encoded optical fiber sensing network, consists of quantities of ultra-weak twin-grating-based microstructures encoded in wavelength, frequency, and time domains, requires high-precise wavelength interrogation due to the complexity of its backscattering spectrum. However, the tunable Fabry–Perot filter (TFF)-based system with high spectrum resolution suffers from nonlinear scanning effect and temperature sensitivity of TFF, which seriously affects the measurement reliability. A wavelength calibration unit composed of multiple fiber Bragg gratings (FBGs), as well as the third-order polynomial fitting, is adopted to establish the accurate wavelength–voltage relationship of TFF, so as to auto-compensate the real-time demodulation deviation. The prototype system is experimentally demonstrated with the wavelength drift of only 6 pm under the temperature change of about 10 °C, as well as the long-term measurement accuracy less than 3 pm. This improved demodulation scheme is universal for all FBGs sensing networks as well.

High-Speed and High-Resolution Demodulation System for the Hybrid WDM/FDM Based Fiber Microstructure Sensing Network

A hybrid wavelength-division multiplexing/frequency-division multiplexing (WDM/FDM) based optical fiber microstructure sensing network with large capacity, high resolution, and high-speed response is proposed and demonstrated. The sensing network is composed of a demodulation system and a fiber microstructure sensor array distributed along a single fiber. Parallel processing technology is utilized for investigating the envelope peak and the fringe peak of the spectrum simultaneously, which can be used to distinguish large-scale temperature shifts quickly. In combination with a high-speed tunable Fabry-Perot filter, the sensor array can be quickly demodulated with the speed up to 500 Hz as well as the wavelength resolution less than 3 pm. In a proof-of-concept experiment, an array composed of 16 sensors through four WDM and four FDM is employed for temperature measurement, demonstrating a temperature resolution less than ±0.3 °C, and a vibration experiment verified 500 Hz demodulation speed capacity. Compared with traditional demodulation schemes, the proposed demodulation system is suitable for large capacity networks and dynamic sensing applications.

A tunable Fabry–Perot filter ([formula omitted]/18) based on all-dielectric metamaterials

A tunable Fabry–Perot filter composed of two separated all-dielectric metamaterials is proposed and numerically investigated. Different from metallic metamaterials reflectors, the all-dielectric metamaterials are constructed by high-permittivity TiO2 cylinder arrays and exhibit high reflection in a broadband of 2.49–3.08 THz. The high reflection is attributed to the first and second Mie resonances, by which the all-dielectric metamaterials can serve as reflectors in the Fabry–Perot filter. Both the results from phase analysis method and CST simulations reveal that the resonant frequency of the as-proposed filter appears at 2.78 THz, responding to a cavity with λ/18 wavelength thickness. Particularly, the resonant frequency can be adjusted by changing the cavity thickness. This work provides a feasible approach to design low-loss terahertz filters with a thin air cavity.

Dynamic fiber Bragg grating strain sensor interrogation with real-time measurement

We demonstrate a 1550 nm band resonance Fourier-domain mode-locked (FDML) fiber laser with fiber Bragg grating (FBG) array. Using the FDML fiber laser, we successfully demonstrate real-time monitoring of dynamic FBG strain sensor interrogation for structural health monitoring. The resonance FDML fiber laser consists of six multiplexed FBGs, which are arranged in series with delay fiber lengths. It is operated by driving the fiber Fabry-Perot tunable filter (FFP-TF) with a sinusoidal waveform at a frequency corresponding to the round-trip time of the laser cavity. Each FBG forms a laser cavity independently in the FDML fiber laser because the light travels different length for each FBG. The very closely positioned two FBGs in a pair are operated simultaneously with a frequency in the FDML fiber laser. The spatial positions of the sensing pair can be distinguished from the variation of the applied frequency to the FFP-TF. One of the FBGs in the pair is used as a reference signal and the other one is fixed on the piezoelectric transducer stack to apply the dynamic strain. We successfully achieve real-time measurement of the abrupt change of the frequencies applied to the FBG without any signal processing delay. The real-time monitoring system is displayed simultaneously on the monitor for the variation of the two peaks, the modulation interval of the two peaks, and their fast Fourier transform spectrum. The frequency resolution of the dynamic variation could reach up to 0.5 Hz for 2 s integration time. It depends on the integration time to measure the dynamic variation. We believe that the real-time monitoring system will have a potential application for structural health monitoring.

Single-longitudinal-mode broadband tunable random laser.

In this work, we demonstrate a broadband tunable single-longitudinal-mode (SLM) random laser based on Rayleigh backscattering in a standard single-mode fiber. The wide tuning range of this SLM fiber laser over 1500-1570nm is demonstrated with a linewidth of 4.5-30kHz. The tuning is achieved using a tunable bandpass Fabry-Perot filter, and a semiconductor optical amplifier is used as the wide-bandwidth gain medium. The laser is able to operate in the S + C + L band.

A stepped FM/CW lidar system using a dual parallel Mach–Zehnder modulator

A stepped frequency-modulated continuous-wave lidar system using a dual parallel Mach–Zehnder modulator with heterodyne detection is demonstrated. The lidar transmitter utilizes a modulation loop circuit composed of an electro-optic dual parallel Mach–Zehnder modulator and a fiber amplifier, as well as a tunable Fabry–Perot filter to generate a bandwidth-enhanced stepped frequency-modulated signal. In addition, a centroid algorithm is used in the receiver to process the signal with both high precision and high accuracy. The simulation results demonstrated that the ranging precision of the proposed lidar was 2.09 cm and the ranging accuracy was 0.16 cm. A validation experiment verified that the data obtained in the simulation had a modulation bandwidth of 4 GHz using a 200-MHz signal source. The improvements resulted in a frequency-modulated continuous-wave lidar system with high precision and accuracy by operating a modulated signal with a wider modulation bandwidth using signal sources with low bandwidth in a low-cost and compact structure.

Continuously tunable wideband semiconductor fiber-ring laser

We demonstrate a wideband tunable semiconductor fiber-ring laser that can be continuously tuned from 1498 nm to 1623 nm. The proposed laser uses a semiconductor optical amplifier (SOA) as a gain medium and a fiber Fabry–Perot tunable filter as a selective wavelength filter. The optimized drive current of the SOA and the output coupling ratio are obtained by experimental research. This laser has a simple configuration, low threshold, flat laser output power and high optical signal-to-noise ratio.

Design and Experimental Study of Tunable Fiber Ring Laser of C+L Band

A tunable fiber ring laser based on L band EDFA (erbium-doped fiber amplifier) was proposed. It has high signal-to-noise ratio and wide tuning range. The influence factor of the tuning range and the time delay of relaxation oscillation were studied. And the FFP-TF(Fabry-Perot tunable filter) was used as a wavelength selector to produce tunable laser. Experiments show that by using appropriate time-delay and auto-adjusting the intra-cavity loss, the signal-to-noise ratio can be increased and the tunable range can be broadened to C+L band.

Simultaneous wavelength and orbital angular momentum demultiplexing using tunable MEMS-based Fabry-Perot filter.

In this paper, we experimentally demonstrate simultaneous wavelength and orbital angular momentum (OAM) multiplexing/demultiplexing of 10 Gbit/s data streams using a new on-chip micro-component-tunable MEMS-based Fabry-Perot filter integrated with a spiral phase plate. In the experiment, two wavelengths, each of them carrying two channels with zero and nonzero OAMs, form four independent information channels. In case of spacing between wavelength channels of 0.8 nm and intensity modulation, power penalties relative to the transmission of one channel do not exceed 1.45, 0.79 and 0.46 dB at the hard-decision forward-error correction (HD-FEC) bit-error-rate (BER) limit 3.8 × 10-3 when multiplexing a Gaussian beam and OAM beams of azimuthal orders 1, 2 and 3 respectively. In case of phase modulation, power penalties do not exceed 1.77, 0.54 and 0.79 dB respectively. At the 0.4 nm wavelength grid, maximum power penalties at the HD-FEC BER threshold relative to the 0.8 nm wavelength spacing read 0.83, 0.84 and 1.15 dB when multiplexing a Gaussian beam and OAM beams of 1st, 2nd and 3rd orders respectively. The novelty and impact of the proposed filter design is in providing practical, integrable, cheap, and reliable transformation of OAM states simultaneously with the selection of a particular wavelength in wavelength division multiplexing (WDM). The proposed on-chip device can be useful in future high-capacity optical communications with spatial- and wavelength-division multiplexing, especially for short-range communication links and optical interconnects.

Optimization of excitation of fiber Fabry-Perot tunable filters used in swept lasers using a phase-correction method.

In this paper we investigate a phase-correction method for compensation of the nonlinearity of conventional wavelength-swept laser sources based on a fiber Fabry-Perot tunable filter as a wavelength selective element. A triangular waveform signal is commonly used to drive the filter. We, however, extract the zero crossings from the interferograms and modify the shape of the triangular signal accordingly. This algorithm was tested for different values of the optical path length difference in the interferometer setup. Significant compensation for the nonlinearity of the filter was obtained.

Extended bandwidth wavelength swept laser source for high resolution optical frequency domain imaging.

Improving the axial resolution by providing wider bandwidth wavelength swept lasers remains an important issue for optical frequency domain imaging (OFDI). Here, we demonstrate a wide tuning range, all-fiber wavelength swept laser at a center wavelength of 1250 nm by combining two ring cavities that share a single Fabry-Perot tunable filter. The two cavities contain semiconductor optical amplifiers with central wavelengths of 1190 nm and 1292 nm, respectively. To avoid disturbing interference effects in the overlapping spectral region, we modulated the amplifiers in order to obtain consecutive wavelength sweeps in the two spectral regions. The two sweeps were fused together in post-processing to achieve a total scanning range of 223 nm, corresponding to 3.3 µm axial resolution in air. We confirm improved image quality and reduced speckle size in tomograms of swine esophagus ex vivo, and human skin and nailbed in vivo.

Large-Area MEMS Tunable Fabry–Perot Filters for Multi/Hyperspectral Infrared Imaging

This paper reports on a MEMS tunable Fabry–Perot filter technology capable of achieving nanometer-scale optical flatness across a large mirror area of up to square centimeters without any extraneous stress management techniques. The device employs a single-layer tensile silicon or germanium membrane for the suspended top mirror. Optical characterization of the fabricated single-membrane-based tunable filters for the SWIR, MWIR, and LWIR is presented. The fabricated 1000- $\mu$ m dimension Si-membrane-based SWIR and MWIR filters are demonstrated with a wavelength tuning range of 1.77–2.42 and 4.1–4.9 $\mu$ m, respectively, while the fabricated 200- $\mu$ m-dimension Ge-membrane-based LWIR filter is demonstrated with a wavelength tuning range of 8.5–11.46 $\mu$ m. All these filters are shown to achieve transmission characteristics that exceed the optical requirements for multispectral imaging applications. A large-area 1-cm dimension Si membrane-based SWIR tunable Fabry–Perot filter for multispectral imaging is demonstrated as a proof-of-concept, showing an excellent surface flatness in the order of 25 nm and an excellent optical uniformity with transmission peak wavelength variability less than 3% across the entire 1-cm dimension optical imaging area. In addition, the optical transmission behavior of the Fabry–Perot filters based on three-layer Si or Ge-based air-spaced DBRs for SWIR, MWIR, and LWIR is modeled, demonstrating that these filters can achieve a fine spectral resolution of several tens of nanometers suitable for hyperspectral imaging applications.

Deviation calibration method for fiber Bragg grating demodulation system based on tunable Fabry-Perot filter drived by triangular wave

Deviation calibration method for fiber Bragg grating demodulation system based on tunable Fabry-Perot filter drived by triangular wave

A Temperature-Independent Interrogation Technique for FBG Sensors Using Monolithic Multilayer Piezoelectric Actuators

In this paper, a temperature-independent technique for measurements of ac signals, such as voltage and electrical current, with fiber Bragg grating (FBG) sensors is presented. This technique is based on a twin-grating scheme, in which the measured quantity is obtained from the optical power of convolution between a sensing FBG and a tunable FBG. The proposed interrogator employs a low-voltage monolithic multilayer piezoelectric actuator (MMPA) to sweep the tunable FBG over the wavelength range of the sensing FBG, in order to obtain the convolution function accurately. A dedicated microcontrolled optoelectronic system with a firmware-based tracking routine is responsible for keeping the tunable FBG following the sensing FBG over temperature-induced wavelength shifts with subpicometric resolution, while the dc level of convolution is controlled precisely at the highest sensitivity point of operation. The MMPA employed allows the tracking of the tunable FBG after the sensing FBG over a maximum temperature difference that ranges from −29 °C to +90 °C. The proposed interrogator presents lower cost compared with the systems that employ frequency-scanning devices, such as high-speed Fabry-Perot tunable filters.

Widely tunable/wavelength-swept SLM fiber laser with ultra-narrow linewidth and ultra-high OSNR

We propose and demonstrate a novel single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) capable of operating at fixed-wavelength lasing mode with a tunable range more than 54 nm, an ultra-narrow linewidth of 473 Hz and an ultra-high optical signal-to-noise ratio (OSNR) more than 72 dB, or operating at wavelength-swept mode with tunable sweep rate of 10—200 Hz and a sweep range more than 50 nm. The excellent features mainly benefit from a triple-ring subring cavity constructed by three optical couplers nested one another and a fiber Fabry-Perot tunable filter which can be driven by a constant voltage or a periodic sweep voltage for fixed or wavelength- swept operation, respectively. The proposed EDFL has potential applications in high-resolution spectroscopy and fiber optic sensing.

Low cost, high performance white-light fiber-optic hydrophone system with a trackable working point.

A working-point trackable fiber-optic hydrophone with high acoustic resolution is proposed and experimentally demonstrated. The sensor is based on a polydimethylsiloxane (PDMS) cavity molded at the end of a single-mode fiber, acting as a low-finesse Fabry-Perot (FP) interferometer. The working point tracking is achieved by using a low cost white-light interferometric system with a simple tunable FP filter. By real-time adjusting the optical path difference of the FP filter, the sensor working point can be kept at its highest sensitivity point. This helps address the sensor working point drift due to hydrostatic pressure, water absorption, and/or temperature changes. It is demonstrated that the sensor system has a high resolution with a minimum detectable acoustic pressure of 148 Pa and superior stability compared to a system using a tunable laser.

Tunable Multiwavelength Fiber Laser Source for Continuous True-Time-Delay Beamforming

We present a novel approach of a true-time-delay (TTD) laser source for continuous phased-array beamforming employing a semiconductor optical amplifier (SOA) and a tunable Fabry–Perot filter (FPF). The wavelength tuning is achieved by tuning the cavity length of the filter, to which an SOA that generates the $C$ -band optical signal is applied. A five-channel TTD system based on a linear chirped fiber grating (LCFG) has been constructed and experimented using the proposed laser source. The signal-to-noise ratio of the obtained multiwavelength signal can be more than 35 dB. Thanks to the high position resolution of the translation stage, the time delay responses can be continuously controlled by changing the cavity length of the FPF.