In-line Fabry-Perot interferometric temperature sensor with digital signal processing

A review of our recent work on ultrahigh resolution optical fiber sensors in the quasi-static region is presented, and their applications in crustal deformation measurement are introduced. Geophysical research such as studies on earthquake and volcano requires monitoring the earth’s crustal deformation continuously with a strain resolution on the order of nano-strains (nɛ) in static to low frequency region. Optical fiber sensors are very attractive due to their unique advantages such as low cost, small size, and easy deployment. However, the resolution of conventional optical fiber strain sensors is far from satisfactory in the quasi-static domain. In this paper, several types of recently developed fiber-optic sensors with ultrahigh resolution in the quasi-static domain are introduced, including a fiber Bragg grating (FBG) sensor interrogated with a narrow linewidth tunable laser, an FBG based fiber Fabry-Perot interferometer (FFPI) sensor by using a phase modulation technique, and an FFPI sensor with a sideband interrogation technique. Quantificational analyses and field experimental results demonstrated that the FBG sensor can provide nano-order strain resolution. The sub-nano strain resolution was also achieved by the FFPI sensors in laboratory. Above achievements provide the basis to develop powerful fiber-optic tools for geophysical research on crustal deformation monitoring.

A novel DSP scheme for eliminating the interaction between dispersion and transient frequency offset in optical burst-mode coherent systems

Biomedical researchers and clinicians have long sought the development of small, low cost and disposable sensors for in-vivo measurements of pressure and temperature. These sensors typically measure static or quasi-static pressure and temperature and are required for direct and accurate measurements over a wide range of operational, enviromental and (especially) handling conditions.

A novel monitoring system for a fiber Fabry-Perot interferometer (FFPI) temperature sensor has yielded a resolution of 0.013 degrees C (0.0025 fringe). Light from a broadband source passes through a scanned Michelson interferometer and is reflected from a FFPI to produce a fringe pattern, the temporal position of which is proportional to a change in the optical length of the fiber interferometer. A second Michelson interferometer with a distributed-feedback laser source is used to correct for variations in the translation rate of the motor-driven scanning mirror. Coherence multiplexing of three such sensors has also been demonstrated.

We have proposed and demonstrated a novel elastic optical fiber Fabry-Perot interferometer (FPI)-based ac magnetic field sensor with high magnetic sensitivity and correction of temperature crosstalk. The elastic FPI is formed by soft splicing using low Young's modulus polydimethylsiloxane material at the joint point of a hollow-core fiber and single-mode fibers and is fixed on a common supermendur rod, a kind of magnetostrictive material. AC magnetic field-induced length change of the magnetostrictive rod results in the obvious change of the FPI cavity length and finally is transduced to the intensity variation of the sensor signal, which makes the sensor highly sensitive to ac magnetic field. Taking advantage of the elastic FPI structure, the magnetic sensitivity (in terms of strain) is improved to 48 ppm/mT (75.01 pm/mT in terms of wavelength), compared with the low intrinsic sensitivity of the conventional magnetostrictive rod (usually below several ppm/mT and 0.39 ppm/mT in our case). Moreover, the impact of temperature cross-sensitivity is removed by retrieving the wavelength shift of the FPI interference spectrum based on the calibrated signal voltage versus wavelength shift. All the ac magnetic fields of different frequencies and intensities have been successfully detected and measured by our FPI sensor without the impact of temperature crosstalk. We believe such a simple, compact, and cost-effective sensor would be a promising candidate for accurate monitoring of ac magnetic field.

A highly sensitive demodulation method for the low-finesse Fabry-Perot interferometer (FPI) sensor is proposed and experimentally demonstrated. To generate the Vernier effect, the sampling interval of a tunable laser source is set close to the free spectral range of the FPI sensor. In order to improve the demodulation precision, the spline fitting process was applied to the sampling data. The dip wavelength of the fitted curve was chosen to measure the spectral shift. In the strain test experiment, the sensitivity of 0.01836 nm/μϵ was obtained. A controllable sensitivity is achieved by changing the sampling interval. It is convenient to balance the sensitivity and the measuring time. The method can be used to enhance the sensitivity of the preexistent low-finesse FPI sensors by changing an optical source in the sensing system. There is no need to make any changes to the FPI sensors and sensing nodes. Furthermore, the method can also be applied for other two-beam interferometry sensors.

The surge in interest regarding the next generation of optical fiber transmission has stimulated the development of digital signal processing (DSP) schemes that are highly cost-effective with both high performance and low complexity. As benchmarks for nonlinear compensation methods, however, traditional DSP designed with block-by-block modules for linear compensations, could exhibit residual linear effects after compensation, limiting the nonlinear compensation performance. Here we propose a high-efficient design thought for DSP based on the learnable perspectivity, called learnable DSP (LDSP). LDSP reuses the traditional DSP modules, regarding the whole DSP as a deep learning framework and optimizing the DSP parameters adaptively based on backpropagation algorithm from a global scale. This method not only establishes new standards in linear DSP performance but also serves as a critical benchmark for nonlinear DSP designs. In comparison to traditional DSP with hyperparameter optimization, a notable enhancement of approximately 1.21 dB in the Q factor for 400 Gb/s signal after 1600 km fiber transmission is experimentally demonstrated by combining LDSP and perturbation-based nonlinear compensation algorithm. Benefiting from the learnable model, LDSP can learn the best configuration adaptively with low complexity, reducing dependence on initial parameters. The proposed approach implements a symbol-rate DSP with a small bit error rate (BER) cost in exchange for a 48% complexity reduction compared to the conventional 2 samples/symbol processing. We believe that LDSP represents a new and highly efficient paradigm for DSP design, which is poised to attract considerable attention across various domains of optical communications.

The optical fiber Fabry-Perot interferometer (FPI) has been widely investigated as a potential temperature sensor. To function as a temperature sensor, the cavity of the FPI is typically constructed from either silica fibers or polymers. The silica cavity FPIs can function at temperatures exceeding 1 000 °C. However, their temperature sensitivity is constrained by their relatively low thermal optical coefficients and thermal expansion of silica materials. Although the polymer cavity FPI exhibits high temperature sensitivity, its cavity is susceptible to deterioration in high-temperature environments. Here, to overcome this challenge and achieve high-sensitivity temperature sensing in a high-temperature environment, we propose a new type of temperature FPI sensor by inserting and sealing a section of Cr20Ni80 metal microwire inside a section of a silica hollow core fiber (HCF) spliced to a standard single-mode fiber (SMF). The FPIs exhibit a high degree of temperature sensitivity due to the high thermal expansion of the Cr20Ni80 metal microwire. Since the Cr20Ni80 metal has a high melting temperature of 1 400 °C, such FPIs can function in high-temperature environments. Moreover, the temperature sensitivity of this FPI can be modified without affecting its reflection spectrum by changing the length of the metallic microwire within the hollow core fiber. The experimental results indicate that the proposed FPIs exhibit temperature sensitivity of greater than −0.35 nm/°C within the temperature range of 50 °C to 440 °C. Our proposed metal microwire-based FPIs are economical, robust, simple to fabricate, and capable of functioning in high-temperature environments, rendering them appealing options for practical applications.

In this research program, several optical instruments for high temperature measurement based on single crystal sapphire material are introduced and tested for real-time, reliable, long-term monitoring of temperatures for coal gasifier. These are sapphire fiber extrinsic Fabry-Perot interferometric (EFPI) sensor; intensity-measurement based polarimetric sapphire sensor and broadband polarimetric differential interferometric (BPDI) sapphire sensor. Based on current evaluation and analysis of the experimental results, the broadband polarimetric differential interferometric (BPDI) sensor system was chosen for further prototype instrumentation development. This approach is based on the self-calibrating measurement of optical path differences (OPD) in a single-crystal sapphire disk, which is a function of both the temperature dependent birefringence and the temperature dependent dimensional changes. The BPDI sensor system extracts absolute temperature information by absolute measurement of phase delays. By encoding temperature information in optical spectrum instead of optical intensity, this sensor guarantees its relative immunity to optical source power fluctuations and fiber losses, thus providing a high degree of long-term measurement stability which is highly desired in industrial applications. The entire prototype for BPDI system including the single crystal sapphire probe, zirconia prism, alumina extension tube, optical components and signal processing hardware and software have shown excellent performance in the laboratory experiments shown in this report.

This article reviews author’s research work on fiber-optic sensors over the last twenty years. It includes two aspects: low-coherence interferometric sensors (LCI) and fiber Bragg grating (FBG) sensors. For LCI sensors, author’s work mainly focuses on the interrogation and multiplexing methods for Fizeau and Fabry-Perot interferometric sensors at the University of Kent at Canterbury (UKC), UK, and study on novel Fabry-Perot interferometric sensors and their multiplexing methods at Chongqing University (CQU) and University of Electronic Science & Technology of China (UESTC), China, respectively. For FBG sensors, a number of multiplexing schemes are proposed and demonstrated at UKC, and then novel methods for realization of multi-parameter measurement and long-distance measurement based on the FBG sensor and its combination with other optical fiber sensors are also reported at CQU & UESTC. Thus, author’s study on these two topics can be divided into two periods, at UKC and at CQU & UESTC, China. This review is presented in such a time sequence.

A PDMS-optical fiber Fabry-Pérot interferometric (FPI) temperature sensor based on Vernier effect is proposed. The sensing probe is formed by cascading two single-mode fibers (SMFs) with a PDMS cavity, while the reference probe consists of two SMFs spliced to a hollow-core fiber (HCF). The two cavities have lengths of 146.8 and 218.4 μm, respectively. The structure is compact and easy to be fabricated. The air-filled cavity provides a highly stable reference interference signal, while the PDMS cavity, which is sensitive to temperature, undergoes significant cavity-length variation under temperature changes. Due to the slight mismatch between the free spectral ranges (FSRs) of the two cavities, a distinct Vernier envelope is generated in the reflection spectrum, effectively amplifying the temperature-induced spectral shift and thus greatly improving the sensing sensitivity. Experimental results show that the parallel FPI structure achieves a temperature sensitivity of approximately 21.54 nm/℃ in the range of 25-30 ℃, with a linearity better than 99.975%, which is significantly higher than that of conventional PDMS-based FPI sensors. The sensor also exhibits good temperature reversibility, demonstrating great potential for applications in industrial, medical, environmental, and aerospace fields.

Novel digital signal processing and synchronization schemes for high-speed digital receivers

A novel noise suppression method is proposed for the pressure surveillance of heavy oil thermal recovery well using white light extrinsic Fabry-Perot interferometric (EFPI) fiber-optic sensor. The moving grey model GM(1, 1) algorithm is applied to suppress the non-stationary noise and disturbance in downhole environment. Both theoretical analysis and field test results show that there exists a threshold effect for the moving window length selection, the optimized value of window length occurs at the threshold point which best removes the high frequency disturbance as well as retaining the fast change of actual pressure signal. Field test results show that the proposed method can increase the signal to noise ratio (SNR) of pressure profile with 10.4dB, and then can effectively suppress the large deviation in the forecasting result of daily oil production by combining temperature information.

A novel digital signal processing (DSP) scheme for special‐shaped circular (7,1) modulated signals based on quasi blind carrier phase estimation (QBCPE) is proposed. The results show that applying 0.33‰ overhead QBCPE provides similar performance as 3% training sequence data‐aided equalization of least mean square (LMS) algorithm.

A digital signal processing (DSP) scheme based on Volterra equalizer (VE) combined with adaptive noise-whitening post-filter and maximum likelihood sequence detection (MLSD) is proposed to mitigate nonlinear impairments in vertical-cavity surface-emitting lasers (VCSEL) multimode fiber (MMF) system. Successfully transmission of 108 Gb/s, 100 Gb/s and 60 Gb/s 4-ary pulse amplitude modulation (PAM4) signal over 5 m, 160 m and 460 m OM3-MMF is demonstrated below the 7% overhead hard-decision forward error correction (HD-FEC) bit error rate (BER) threshold by using a 20-GHz class VCSEL at 850 nm. Linear pre-equalization is applied to mitigate severe bandwidth limitation of the system. Our experimental results show that the scheme can well mitigate modulation nonlinearity induced by VCSEL and fiber nonlinearity induced by MMF. The BER decreases about two order of magnitude compared to linear equalizer after 100 m OM3-MMF transmission for 100 Gb/s PAM4 signal.