Interference Demodulation Research Articles

On chip broadband and high extinction ratios Mach-Zehnder interferometer for DPSK signal demodulation

Differential phase shift keying (DPSK) signals are widely employed in optical communication systems due to the high sensitivity and simple self-delay interference demodulation scheme. In this paper, we demonstrate a broadband on-chip Mach-Zehnder interferometer (MZI) for multiple channel DPSK signal demodulation. The chip is constructed using a single-mode high-index doped silica glass waveguide with fiber-to-fiber insert loss less than 3 dB and small second-order dispersion. The MZI interference fringe extinction ratio is over 25 dB in the range of C- and L-bands. The MZI is employed to demodulate four parallel DPSK signals simultaneously with similar performance, indicating that the broad bandwidth MZI is suitable for a receiver of massive parallel optical communications.

Silicon-Cantilever-Enhanced Single-Fiber Photoacoustic Acetylene Gas Sensor.

A single-fiber photoacoustic (PA) sensor with a silicon cantilever beam for trace acetylene (C2H2) gas analysis was proposed. The miniature gas sensor mainly consisted of a microcantilever and a non-resonant PA cell for the real-time detection of acetylene gas. The gas diffused into the photoacoustic cell through the silicon cantilever beam gap. The volume of the PA cell in the sensor was about 14 μL. By using a 1 × 2 fiber optical coupler, a 1532.8 nm distributed feedback (DFB) laser and a white light interference demodulation module were connected to the single-fiber photoacoustic sensor. A silicon cantilever was utilized to improve the performance when detecting the PA signal. To eliminate the interference of the laser-reflected light, a part of the Fabry-Perot (F-P) interference spectrum was used for phase demodulation to achieve the highly sensitive detection of acetylene gas. The minimum detection limit (MDL) achieved was 0.2 ppm with 100 s averaging time. In addition, the calculated normalized noise equivalent absorption (NNEA) coefficient was 4.4 × 10-9 W·cm-1·Hz-1/2. The single-fiber photoacoustic sensor designed has great application prospects in the early warning of transformer faults.

Fiber-integrated cantilever-based nanomechanical biosensors as a tool for rapid antibiotic susceptibility testing.

There is an urgent need for developing rapid and affordable antibiotic susceptibility testing (AST) technologies to inhibit the overuse of antibiotics. In this study, a novel microcantilever nanomechanical biosensor based on Fabry-Pérot interference demodulation was developed for AST. To construct the biosensor, a cantilever was integrated with the single mode fiber in order to form the Fabry-Pérot interferometer (FPI). After the attachment of bacteria on the cantilever, the fluctuations of cantilever caused by the bacterial movements were detected by monitoring the changes of resonance wavelength in the interference spectrum. We applied this methodology to Escherichia coli and Staphylococcus aureus, showing the amplitude of cantilever's fluctuations was positively related on the quantity of bacteria immobilized on the cantilever and associated with the bacterial metabolism. The response of bacteria to antibiotics was dependent on the types of bacteria, the types and concentrations of antibiotics. Moreover, the minimum inhibitory and bactericidal concentrations for Escherichia coli were obtained within 30 minutes, demonstrating the capacity of this method for rapid AST. Benefiting from the simplicity and portability of the optical fiber FPI-based nanomotion detection device, the developed nanomechanical biosensor in this study provides a promising technique for AST and a more rapid alternative for clinical laboratories.

Laser ranging system based on double intensity modulation

Long-range, high-precision, and high-refresh rate absolute distance measurement based on double intensity modulation is proposed and experimentally demonstrated. In this scheme, a Mach-Zehnder modulator is utilized to perform bidirectional modulation by a reversible optical path. In the Mach-Zehnder modulator, interference demodulation is completed by bidirectional modulated light with time difference. By adjusting the driving frequency of the modulator, the curve of light intensity versus driving frequency is achieved. Consequently, the distance to be measured can be obtained by extracted the frequency interval between two adjacent light intensity minimum points. In the traditional double polarization modulation ranging, the optical carrier is polarized by a polarizing beam splitter (PBS) before phase modulator. Moreover, a quarter wave or Faraday rotating mirror need to be placed to adjust the polarization in front of the target object. Therefore, the polarization state is an indispensable factor in the traditional double polarization modulation ranging. Due to the advantage of the intensity modulation, absolute distance measurement is achieved without additional polarization control, greatly simplifying the system. Theoretical analysis of the system is developed, which is then demonstrated by experiments. In the experiments, we achieved the following results. Firstly, the relationship between the intensity of the output light of the system and the modulation frequency is theoretically analyzed, which proved to be a cosine form. Secondly, swing method is introduced to realize high-speed absolute distance measurement during the analytical distance algorithm, and we achieved a refresh rate of 2 kHz in the experiments. Thirdly, the relationship between measurement ranging precision and frequency stability is analyzed. When the modulation frequency is 8.9 GHz, the experimentally measured frequency stability is on the order of 10<sup>–7</sup>. And when the distance to be measured is 2.73 m, the standard deviation of ranging reaches 1 μm, which represents the precision of the system. That is, the relative measurement precision is also on the order of 10<sup>–7</sup>, which is consistent with theoretical analysis. Finally, when the distance to be measured increases from 1.57 m to 100.83 m, the measurement precision increases from 1 μm to 30 μm. It is worth mentioning that the relative measurement precision of the system is always stable in the order of 10<sup>–7</sup>. Our scheme has some significant advantages, such as long-range, high-precision, high-refresh rate, and a simple and compact configuration. Moreover, our method can be used in important applications such as precision instruments, metrology, and aerospace.

In-Fiber Integrated Quasi-Distributed Temperature Sensor Array With High Spatial Resolution for Silicon Nitride Igniter

As a key component in the engine system, silicon nitride igniter’s stable and reliable operation is a necessary condition for the proper functioning of the engine system. However, an online high temperature sensor of silicon nitride igniter inside the engine is absent. This paper presents and demonstrates a novel in-fiber integrated high temperature sensor array with high spatial resolution for online temperature field measurement on a silicon nitride igniter. The sensor array with a series of weak reflectors is written by a femtosecond laser in the core of single-mode fiber. The reflectivity of each reflector can be controlled from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−8</sup> by adjusting the written power of femtosecond laser. Two adjacent weak reflectors form a compact fiber sensor. The size of a single sensor can be as small as 3–5 mm, so it is able to measure the temperature fluctuation in a space down to several millimeters. The five-sensor-array, with the help of a white light interference demodulation system, can measure high temperature up to 1000°C. Due to the advantages of high reliability, compact construction and simple fabrication, this high temperature sensor array can be widely applied in high temperature sensing applications with space constrains.

White light interference demodulation of optical fiber Fabry-Perot micro-pressure sensors based on the Karhunen-Loeve transform and singular value decomposition.

In this paper, the Karhunen-Loeve transform (KLT) and wavelength domain interferometric spectral singular value decomposition (SVD) are used for the first time to demodulate the pressure of an optical fiber Fabry-Perot (F-P) micro-pressure sensor, and the feasibility of the proposed method is demonstrated experimentally. The eigenvalue decomposition of the dominant frequency part of the beam-domain interferometric spectrum after the fast Fourier transform (FFT) is performed using KLT, and the singular value decomposition of the wavelength domain interferometric spectrum is additionally performed using SVD. Both methods use high-order eigenvalues as a new metric and then derive the relation between the new metric and the reference pressure. The two demodulation methods are experimentally compared, and we used an optical fiber F-P pressure sensor with unknown structure and material for pressure measurements. Even though the interferometric spectral signal is acquired using a coarse spectrometer (2.5 nm wavelength resolution), one can still achieve high demodulation accuracy with both algorithms. However, the SVD demodulation accuracy decreases significantly after reducing the spectral data points in the wavelength domain from 1566 to 783. KLT still has high demodulation accuracy and linearity after spectral data points are reduced from 1024 to 256 in the wavenumber domain. The satisfactory linearity of the measured pressure versus reference pressure and low reading errors validate the feasibility of the proposed demodulation algorithm.

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.

Fluid velocity monitoring in oil well using fiber laser vibration sensing

A novel method of flow measurement in oil well based on turbulence vibration principle and fiber laser vibration sensor is proposed. The constitutive equations of flow velocity and pipe wall vibration were established, and a kind of optical fiber laser vibration sensor based on hinge structure was designed and installed on the outer wall of oil pipe to measure the fluid flow velocity inside oil pipe. When the fluid flows through the oil pipe, the dynamic pressure will cause the vibration of the pipe wall, resulting in the wavelength change of the fiber laser vibration sensor. The optical fiber interference demodulation technology is used to demodulate the wavelength change to obtain the vibration intensity. The sensitivity of the fiber laser vibration sensor is tested to be 25 pm/g. The field test in oil well showed that the relationship between the vibration intensity of the sensor and the flow rate is quadratic curve, and the minimum flow rate is measured to be 0.041 m/s, which indicates that the system can realize the real-time online measurement of the flow.

基于独热码有限状态机的斐索干涉解调相位补偿方法

基于独热码有限状态机的斐索干涉解调相位补偿方法

In-fiber integrated high sensitivity temperature sensor based on long Fabry-Perot resonator.

This paper presents and demonstrates a novel in-fiber integrated high sensitivity temperature sensor based on long Fabry-Perot (FP) resonator. In a quartz capillary, the FP resonator was composed of two single mode fibers (SMFs) whose end faces were coated by gold films. The temperature can be obtained by measuring the length variation of the FP cavity. A white light interference demodulation system was used to measure the length variation of the FP cavity. By the multiple reflections of light in the FP cavity, we achieved the sensitivity multiplication of the sensor. The proposed sensor measured the temperature up to 350°C for 2 hours, and the sensitivity of the sensor is six times that of the traditional interference temperature sensor. Due to the advantages of low cost, high sensitivity and simple fabrication, this temperature sensor can be widely used in high temperature applications.

Nondestructive and in situ determination of graphene layers using optical fiber Fabry–Perot interference

Thickness measurement plays an important role for characterizing optomechanical behaviors of graphene. From the view of graphene-based Fabry–Perot (F–P) sensors, a simple, nondestructive and in situ method of determining the thickness of nanothick graphene membranes was demonstrated by using optical fiber F–P interference. Few-layer/multilayer graphene sheets were suspendedly adhered onto the endface of a ferrule with a 125 µm inner diameter by van der Waals interactions to construct micro F–P cavities. Along with the Fresnel’s law and complex index of refraction of the membrane working as a light reflector of an F–P interferometer, the optical reflectivity of graphene was modeled to investigate the effects of light wavelength and temperature. Then the average thickness of graphene membranes were extracted by F–P interference demodulation, and yielded a very strong cross-correlation coefficient of 99.95% with the experimental results observed by Raman spectrum and atomic force microscope. The method could be further extended for determining the number of layers of other 2D materials.

Investigation of spectral shifts in a white light interferometer with a single interference peak

When the optical path difference between the two arms of an interferometer is so small that only one peak exists within the measurable interference spectrum, traditional white light interference demodulation techniques based on two adjacent interference peaks are not a suitable choice. We report an experimental approach to measure the peak wavelength shifts with only a single interference peak in a fiber white light interferometer. The temperature measurement experiments prove a linear relationship between the peak wavelength and the temperature. The temperature resolution is 0.1°C in theory.

Wideband optical phase modulator half-wave voltage measurement technique based on phase modulation optical link

We measured the half-wave voltage Vπ of LiNbO3 phase modulators in the broadband frequency range by analyzing the gain of phase modulation interference demodulation optical link. This is a new high practical value measurement method for half-wave voltage of LiNbO3 phase modulators in wideband frequency range, and can accurately predict the nonlinear frequency characteristics of phase modulation optical link.

Fiber-Optic Sensing of Coal Mine Gas Based on Fabry - Perot Interferometer

In this paper, According to mine gas accident frequent and the shortages of traditional gas monitoring system, puts forward some eigenvalue fabry-perot interference (EFPI) technology, its spectrum domain to study the mechanism of white light interference demodulation . establishing the wavelength fiber methane sensor system, realized the sensing information high resolution, wide measuring range and do not suffer light wave, the spectrum domain influence of disturbance synthesis technology. Experimental results show that have an accurate measurement results, it can be applied in underground bad environment method of measuring gas concentration, the research may realize all fiber optical signal measurement, underground only, no electrical signals, strong anti-jamming capability, suitable for inflammable, explosive and other environmental.

Development of a parallel demodulation system used for extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

A parallel demodulation system for extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors is presented, which is based on a Michelson interferometer and combines the methods of low-coherence interference and a Fourier-transform spectrum. The parallel demodulation theory is modeled with Fourier-transform spectrum technology, and a signal separation method with an EFPI and FBG is proposed. The design of an optical path difference scanning and sampling method without a reference light is described. Experiments show that the parallel demodulation system has good spectrum demodulation and low-coherence interference demodulation performance. It can realize simultaneous strain and temperature measurements while keeping the whole system configuration less complex.

Modulation‐frequency dependence of the nonlinear sensitivity of a Brüel &amp; Kjær 8103 hydrophone in parametric demodulation of amplitude‐modulated ultrasound

The interference demodulation by the nonlinearity of hydrophones cannot be ignored in parametric demodulation of amplitude‐modulated (AM) ultrasound in the near‐field media, where the intensity of the primary incident waves on hydrophones is very high. The interference demodulation, which is caused by the second‐order sensitivity of a hydrophone, N(f), is proportional to the square of the incident pressure p2 [Moffett and Blue, Naval Underwater System Center, Tech. Memo. 801150 (1980); Humphrey and Hsu, Proceeding of Specialists Conference on Underwater Acoustic Applications (1980)]. It is found in the measurement of parametric demodulation of AM ultrasound by a Bruel & Kjaer 8103 hydrophone that the N(f) is not only the function of the carrier frequency f, but also the function of the modulation frequency F. At a fixed carrier frequency f0=2.3 MHz, the interference demodulation can be observed at the modulation frequencies below 6 kHz, and becomes stronger toward low modulation frequencies at a distance o...