External Fabry-Perot Interferometer Research Articles

Three-dimensional sound source localization system based on fiber optic sensor array with an adaptive algorithm

Sound Source Localization (SSL) based on microphone arrays nowadays is widely used in the fields of machinery rattle localization, weapon navigation, and robot hearing. However, there is also a need for SSL in some special environments, such as long-distance gas pipelines where there is a risk of leakage. In these environments, arrays built with ordinary microphones can be a safety hazard, which can be compensated by the External Fabry-Perot interferometer (EFPI) fiber-optic sensor's advantages of being uncharged, inexpensive, and highly sensitive. To expand the application areas of microphone array-based SSL techniques, this paper using four EFPI fiber-optic sensors to constructs a 1 m × 1 m × 1 m sensor array, which is used for real-time localization of sound sources based on three-wavelength adaptive intensity demodulation methods and Time Difference Of Arrival (TDOA) localization techniques. An improved method is proposed for the three-wavelength adaptive intensity demodulation method to avoid the occurrence of signal flips. Finally, formulas are provided for this sensor array to quickly calculate the sound source position based on the arrival delay. Several experiments show that the system can achieve sound source localization over a wide range with an average error of less than 3 cm.

Application of end-face fiber optic sensor for thermooptical research

The principles of operation and implementation features of the developed fiber-optic device for pulsed thermoreflectometry based on an external Fabry-Perot interferometer are discussed. The possibility of studying local thermal processes in the near-surface layers of samples by changing the thermal reflection of probing radiation during pulsed laser heating is shown. Comparison of experimental data with modeling calculations showed the predominant influence of the thermal deformation mechanism in the case of metal samples.

Diaphragm-free gas pressure sensor based on all-sapphire fiber Fabry-Perot interferometers.

An all-sapphire fiber external Fabry-Perot interferometer (EFPI) sensor for measuring gas pressure is proposed and investigated. The sensor head is manufactured from a sapphire fiber ferrule and sapphire tube, and the same material can ensure the stability of the sensor structure at a high temperature. The refractive index of the gas is linearly related to the gas pressure. Therefore, the gas pressure can be measured by studying the optical cavity length of the EFPI. A multi-stage coupled multimode fiber is used to pick up the interference signal of the fiber EFPI. The pressure response of the sensor at different temperatures was measured in the experiment. The experimental results show that the sapphire fiber EFPI can measure 0-5 MPa gas pressure in the environment of 17-1400°C. The sensitivity of the sensor decreases with the increasing temperature, and the maximum sensitivity is 1.1673 µm/MPa (20°C). The sensor is compact and suitable for gas pressure measurement at a high temperature.

Signal Pick-Up Technique for Sapphire Fiber External Fabry-Perot Interferometer Using Tapered Multimode Fibers

A tapered multimode fiber (MMF) to sufficiently and stably pick up the interference signal of a sapphire fiber external Fabry-Perot interferometer (EFPI) is proposed and investigated. The tapered MMF was fabricated following the adiabaticity criteria for the local tapering angle such that the excited fundamental mode in MMF barely couples to the higher-order modes, ensuring a high-visibility interference output from the sapphire fiber EFPI. Tapered MMF also guarantees a robust fusion splice between the leading single-mode fiber and the sapphire fiber. Experimental results show that using tapered MMF the excitation coefficient of fundamental mode can be improved by more than 3.5 times comparing with conventional technique using non-tapered MMF, and the fringe contrast of the interference spectrum can increase by 2.3 times. The response of the collected interference spectra from the sapphire fiber EFPI using tapered MMF was tested when ambient pressure and temperature was changed from 0.1 MPa to 6.1 MPa and 17 °C to 302°C respectively. The results validate that tapered MMF can provide a high stability against environmental pressure and temperature variations.

Higher-order mode suppression technique for multimode sapphire fiber external Fabry-Perot interferometers.

A signal picked-up technique to improve the demodulation stability and accuracy of sapphire fiber external Fabry-Perot interferometer is proposed and demonstrated. Through fusion splicing four pieces of multimode fiber in sequence with different core diameters, the in-step change of the core diameter is found to introduce a sufficient fliting effect on the transmitted higher-order guided modes in the sapphire fiber and further reduce their influence on the fundamental mode interference demodulation. Experimental results show that the proposed multi-stage coupling technique can suppress by five-fold the additional phase imposed on the fundamental mode demodulation when compared with the conventional single-stage coupling approach in which single-mode fiber is spliced with only one piece of multimode fiber. The standard deviation of the demodulated optical phase and cavity length can also be reduced by more than two times. The proposed technique provides a simple yet sufficient solution for the long-standing difficulty of multimode sapphire fiber Fabry-Perot interferometer demodulation.

Single-Wavelength Passive Phase-Shifted Demodulation Technique With the Dual-Cavity and EFA for the Interrogation of EFPI Diaphragm-Based Fiber Sensor

A single-wavelength passive phase-shifted demodulation technique for the interrogation of a diaphragm-based fiber sensor is proposed. It is based on the configuration of the external Fabry–Perot interferometer (EFPI) with dual-cavity (DC) and the ellipse-fitting algorithm (EFA). The technique can address two main concerns: the orthogonality of the DC-EFPI must be matched, the direct current values and fringe visibility of the DC–EFPI must be excluded or normalized. Experiment results show that the proposed technique demonstrates high accuracy amplitude measurement of reflective surface vibration with a high dynamic range over 135 dB and good effectiveness with various initial phase differences apart from two areas around <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</i> rad. It is a successful solution for interrogating of diaphragm-based fiber sensors.

Time-resolved measurements of light–current characteristic and mode competition in pulsed THz quantum cascade laser

Quantum cascade lasers (QCL) are widely adopted as prominent and easy-to-use solid-state sources of terahertz radiation. Yet some applications require generation and detection of very sharp and narrow terahertz-range pulses with a specific spectral composition. We have studied time-resolved light-current (L–I) characteristics of multimode THz QCL operated with a fast ramp of the injection current. Detection of THz pulses was carried out using an NbN superconducting hot-electron bolometer with the time constant of the order of 1 ns while the laser bias current was swept during a single driving pulse. A nonmonotonic behavior of the L–I characteristic with several visually separated subpeaks was found. This behavior is associated with the mode competition in THz QCL cavity, which we confirm by L–I measurements with use of an external Fabry–Perot interferometer for a discrete mode selection. We also have demonstrated the possibility to control the L–I shape with suppression of one of the subpeaks by simply adjusting the off-axis parabolic mirror for optimal optical alignment for one of the laser modes. The developed technique paves the way for rapid characterization of pulsed THz QCLs for further studies of the possibilities of using this approach in remote sensing.

Fabry–Perot Interferometer-Based Absolute Pressure Sensor With Stainless Steel Diaphragm

In this paper, we discuss the design, fabrication, and testing of an external Fabry–Perot interferometer (EFPI)-based absolute type fiber optic pressure sensor of range 0–50 mbar. The FPI has sapphire as the first reflecting surface and stainless steel (SS) sheet (SS316L of thickness 0.18 mm) as the second one. In order to achieve high specular reflectivity, the reflecting surface of sapphire is coated with a broad band thin film coating, while the SS316L sheet is surface finished with the Chemo Mechanical Magneto Rheological finishing process. The parallelism between two reflecting surfaces of FPI and their individual flatness is around $\lambda $ /15, which was achieved by precision machining and assembly with stretched diaphragm. The SS sheet also works as the deflecting diaphragm of the pressure sensor and causes change in the FPI gap with applied pressure. FPI gap is calculated from the reflected spectrum by an improvised yet simple scheme. In this paper, FPI is an enclosed cell with vacuum inside, which acts as the reference pressure of the sensor. Applied pressure of 0–50 mbar changed the FPI gap by around $17~\mu \text{m}$ . This sensor was tested with white light interferometric technique, and the best finesse obtained was 5.5. The proposed device is an optical analogous of a capacitance-based pressure sensor. To the best of our knowledge, FPI-based absolute pressure sensors of sub-atmospheric range with a metal diaphragm have not been reported anywhere.

Widely tunable laser frequency offset lock with 30 GHz range and 5 THz offset

We demonstrate a simple and versatile method to greatly extend the tuning range of optical frequency shifting devices, such as acousto-optic modulators (AOMs). We use this method to stabilize the frequency of a tunable narrow-band continuous-wave (CW) laser to a transmission maximum of an external Fabry-Perot interferometer (FPI) with a tunable frequency offset. This is achieved through a servo loop which contains an in-loop AOM for simple radiofrequency (RF) tuning of the optical frequency over the full 30 GHz mode-hop-free tuning range of the CW laser. By stabilizing the length of the FPI to a stabilized helium-neon (HeNe) laser (at 5 THz offset from the tunable laser) we simultaneously transfer the ~ 1 MHz absolute frequency stability of the HeNe laser to the entire 30 GHz range of the tunable laser. Thus, our method allows simple, wide-range, fast and reproducible optical frequency tuning and absolute optical frequency measurements through RF electronics, which is here demonstrated by repeatedly recording a 27-GHz-wide molecular iodine spectrum at scan rates up to 500 MHz/s. General technical aspects that determine the performance of the method are discussed in detail.

Noninvasive optical control of complex semiconductor laser dynamics

Time-delayed feedback control (TDFC) is transferred to the optical domain and applied to complex multisection semiconductor lasers as used in optical communication. Pyragas-type control is provided by purely optical feedback from an external Fabry-Perot interferometer. This all-optical setup needs no time-consuming signal processing and, thus, has practically no speed limit. Proof-of-principle experiments demonstrate noninvasive stabilization of unstable steady states, chaos control, and suppression of noise-induced oscillations on picosecond time scales. A Lang-Kobayashi type model with optical TDFC is investigated taking into account the dynamic details of the device as well as all-optical time-delayed feedback. The parameter regimes that allow for stabilization of stationary emission are mapped out in good agreement with the experiments. Including noise, an analytical expression for the power spectral density is derived, which is confirmed by the experimental all-optical suppression of noise-induced relaxation oscillations.

Relative line intensity measurement in absorption spectra using a tunable diode laser at 1.6 μm: application to the determination of 13CO2/12CO2 isotope ratio

The measurement of relative intensities in CO2 combination bands spectrum is performed using wavelength modulation spectroscopy (WMS) and a DFB (distributed feedback) diode laser operating at 1.6 μm. The diode laser is stabilized with an external Fabry–Perot interferometer and absorption spectroscopy is performed in a multipass gas cell. A spectrum containing spectral lines of both 13CO2 and 12CO2 isotopic species is recorded. The variation of laser power during frequency scanning and the line shape are taken into account to accurately extract line intensities from experimental data. The isotopic concentration ratio is deduced from the intensity ratio. Both ratios are measured with an accuracy of about 0.5% in pure CO2.

Coherent anti-Stokes Raman spectroscopy study of collisional broadening in the O2–H2O Q branch

The fundamental isotropic Raman Q branch of oxygen perturbed by collisions with water vapor has been studied at pressures up to 1.5 atm and for temperatures between 446 and 990 K. The spectra have been recorded by using coherent anti-Stokes Raman spectroscopy (CARS) which has been preferred to stimulated Raman spectroscopy (SRS) in order to obtain more signal and higher sensitivity as the mixture has a small percentage of oxygen. The high resolution CARS spectrometer uses a seeded Nd:YAG laser actively stabilized on an external Fabry–Perot interferometer to prevent any frequency drift during the course of the experiment. The line broadening coefficients have been determined for several rotational quantum numbers (up to N=31 at 990 K). The effect of the splitting into triplets at lower pressure and the effect of interferences between neighboring lines at higher pressure have been taken into account. The influence of Dicke narrowing has also been considered and special care has been taken to avoid Stark broadening. The line broadening coefficients have been calculated according to a semiclassical model. The rotational quantum number and temperature dependencies of the experimental line broadening coefficients have also been studied with another approach based on fitting and scaling laws. Among several laws, the modified exponential energy gap law (MEG), the statistical power-exponential gap law (SPEG), and the energy corrected sudden law with basis rate constants taken as a hybrid exponential-power law (ECS-EP) have given good results. We have used the fitting and scaling laws to extrapolate in temperature the linewidths at 2000 K.

Stabilization of the emission frequency of a cw dye laser using an external Fabry–Perot interferometer

The results are given of an experimental study of the stabilization of the emission frequency of a cw dye laser with the aid of the pass band of an external thermally stabilized Fabry–Perot interferometer. Two acoustooptic modulators were used to form a dispersive profile of the error signal and this profile had a linear region much wider than the slope of the pass band of the interferometer, which made it possible to achieve stable operation of the laser with a line width not exceeding 100 kHz and continuous tuning in the range ~ 3 GHz throughout the range of stimulated emission of the R101 dye (620–670 nm).

A continuously tunable GaAs diode laser with an external resonator

Design and characteristics of an “external cavity single mode GaAs diode laser” are described which allow continuous wavelength tuning over 50 GHz without mode hops. Tuning is achieved by synchronous tilting of both a thin solid etalon and a Brewster plate inside the laser resonator. Stabilizing the laser frequency to an external Fabry-Perot interferometer yields a frequency stability of ≦1 MHz at output powers up to 20 m W.

Waveguide CO2/OsO4laser with a frequency stability of 10−13

A high-pressure waveguide CO2 laser was stabilized using the resonance frequency of saturated absorption due to the A22P(46) transition in the ν3 band of 192OsO4 (28.46 THz). The laser had a preliminary frequency stabilization system based on a transmission resonance of an external Fabry–Perot interferometer, narrowing the emission spectrum to 1 kHz. An external absorption cell was used: it contained OsO4 vapor at a reduced pressure. The frequency instability was measured by comparison with the frequency of a low-pressure CO2/OsO4 laser stabilized using a resonance located at the center of the CO2 P(14) line. The relative instability of the frequencies of the lasers was 10−13(τ−1+2τ0) for values of τ from 1 msec to 400 sec. The long-term stability (flicker limit) of ~10−13 was probably governed by pulling effects arising, in particular, from parasitic reflections in the optical system.

Use of an external Fabry-Perot interferometer with a saturated absorbent for obtaining narrow resonances in laser radiation intensity

Use of an external Fabry-Perot interferometer with a saturated absorbent for obtaining narrow resonances in laser radiation intensity

Isotope Shift In Neutral Oxygen*†

An O18-enriched oxygen sample (donated by A. O. Nier) has allowed examination of isotope structure in 20 atomic lines between 2000 and 11 000A. Strong sharp lines were obtained in a liquid nitrogen-cooled electrodeless quartz discharge tube containing up to 0.5 mm of oxygen in 5 mm of helium excited by a 10 mc oscillator. The discharge was photographed through an external Fabry-Perot interferometer in series with a large prism spectrograph. The shifts found ranged from zero to as high as 0.5 cm−1 for λ4233 (4p3P–3d′3P) and λ2884 (3p3P–3d′3P). Enough data was available to allow a study of the internal consistency of the measured shifts as a test of the mass-effect theory. With the measured shifts in two lines, two theoretical parameters for the specific effect were empirically determined and used to predict shifts in several other lines. It is concluded that the mass-effect theory is generally applicable to oxygen and that the abnormally large shifts found for λ4233 and λ2884 are the result of interactions affecting the common upper level. A comparison is made with earlier nitrogen data. Use is made of the formal relationship of specific effect parameters to certain radiation theory integrals in calculating a transition probability and an f value for the λ1306 resonance triplet. The reasonable values obtained are a further indication of the validity of the theory.