Sagnac Ring Research Articles

Long-range temperature sensing based on forward Brillouin scattering in highly nonlinear fiber

We propose and experimentally implement a long-range forward Brillouin scattering (FBS) temperature sensor, in which a 25-m highly nonlinear fiber (HNLF) is included in a Sagnac ring and is employed as the main sensing unit, and a 40-km standard single-mode fiber (SMF) is used as the transmission channel to connect the main sensing unit to the data processing center. The approach for quickly acquiring an optimal R0, m mode spectrum is presented. After taking into account the signal-to-noise ratio, sensitivity, and Lorentzian spectral shape, the R0, 17 mode was selected to perform temperature sensing. The frequency shift of the long-range and local FBS sensing demonstrates a linear correlation with the temperature change from 30 ℃ to 60 ℃, with respective frequency shift-temperature coefficients of 73.1 kHz/℃ and 73.4 kHz/℃. Impact of the length of the SMF as the main sensing unit and the SMF as the transmission channel on the FBS gain spectrum has also been discussed, respectively. The proposed long-range FBS scheme is expected to find applications in the investigation of submarine oil and gas pipelines, as well as temperature gradients in underwater and mountainous environments.

Broadband tunable single-frequency Yb-doped fiber laser with dual ring subcavity and Sagnac ring filter

We demonstrate a broadband tunable ultra-narrow linewidth single-frequency (SF) Yb-doped fiber laser (YDFL). A dual ring subcavity (DRS) as a filtering component to eliminate the dense longitudinal mode inherent in YDFL and theoretically analyzed its filtering characteristics. By inserting a 0.8 m unpumped polarization-maintaining Yb-doped fiber in the Sagnac ring, a dynamic narrowband grating (DNG) based on the saturable absorption (SA) effect is constructed to ensure SF laser output over the entire wavelength tuning range. We achieve a wide wavelength tunable SF laser output in the range of ∼74 nm, covering 1021.06 nm to 1094.98 nm. The laser has an optical signal-to-noise ratio (OSNR) of more than 62.5 dB, a slope efficiency of 3.57 %, and an ultra-narrow linewidth of 622 Hz over the entire tunable range. In addition, the stability of the wavelength and power fluctuations below 0.018 nm and 0.67 dB, respectively. This tunable SF fiber laser has a compact structure, lower cost, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on.

Research on lateral pressure sensing based on Sagnac ring cascaded FBG

In the field of engineering, the deformation and lateral stress in structural health detection have attracted extensive attention to obtain the structural health information of building objects in time. Based on the birefringence effect of polarization maintaining fiber (PMF), a transverse pressure sensor based on Sagnac cascade fiber bragg grating (FBG) was proposed and prepared. The performance of the sensor is analyzed by the combination of the finite element method and experiment. The simulation results show that the cross section of PMF is deformed under the pressure along the fast axis and the slow axis and then modulates the refractive index. The experimental results show that by adding FBG structure, the lateral pressure sensitivity of the sensor is 408.7 pm/N, and the linear fit is 98.3%. The lateral pressure sensitivity of the structure without FBG is 281.7 pm/N, and the linear fit is 99.3%. That is to say, the FBG structure increases the sensitivity of the sensor by 45.1%. The experimental results are in full agreement with the theoretical derivation. The optical fiber lateral pressure sensor proposed in this paper has the characteristics of simple fabrication, low cost, and high repeatability. It plays an important role in the field of structural stress monitoring.

Tunable sensitivity microfiber Sagnac loop based on a virtual reference arm for microdisplacement detection

A virtual reference arm-based microfiber Sagnac ring (MFSL) microdisplacement sensor with tunable sensitivity is proposed and demonstrated in this study. We abandon the traditional physical reference arm and build a virtual reference arm to overcome the disadvantage of the physical reference arm being susceptible to environmental influences and to achieve tunable sensitivity. A MFSL with a diameter of 4 μm was fabricated via the winding and tapering process, and a microdisplacement sensitivity of −0.34 nm/μm was obtained in the range of 0–5 μm. Experimental results show that the microdisplacement sensitivity can reach −1.40 nm/μm after cascading the simulated reference arm, which is 4.12 times that of single MFSL. In addition, by changing the parameters of the virtual reference arm without remaking the reference arm, we obtained sensors with sensitivities of −1.74 and −2.30 nm/μm. The sensor has advantages of tunable sensitivity and low cost and has potential applications in microdisplacement measurement.

High sensitivity ethanol vapor sensor based on a nematic liquid crystal film embedded optical fiber Sagnac interferometer

Interferometric optical fiber sensors have become the preferred choice for ethanol vapor detection because of their high sensitivity and figure of merit. However, the response time of interferometric fiber optic ethanol vapor sensors is very long. To address this issue, we experimentally investigated an ethanol vapor sensor based on a nematic liquid crystal (NLC) film embedded optical fiber Sagnac interferometer. The high birefringent NLC film, which worked as the sensing media of ethanol vapor for its absorption of ethanol vapor, was penetrated into the Sagnac ring to generate the spectral interference. The results showed that the measurement sensitivity of ethanol gas concentration reached 2.22 pm ppm−1. The detection range was about 1210 ppm–10 000 ppm. Most importantly, the response time of the proposed sensor is only 15 s. The designed sensor, which showed the advantages of fast response, high sensitivity, and stability, could be a competitive candidate for ethanol vapor sensing.

Micro Displacement Sensor With Cascaded Micro Fiber Sagnac Ring Based on Vernier Effect

A cascaded micro fiber Sagnac ring (MFSR) micro displacement sensor based on the Vernier effect is proposed and studied in this article. A single-mode optical fiber is wound into a ring with a diameter of about 10 cm, an MFSR is obtained by fused tapering in the winding area, and another MFSR with a slightly different coupling length is fabricated by the same process. The transmission spectrum of each Sagnac ring is similar to the Vernier scale. The sensitivity of the sensor can be improved through the Vernier effect after the two rings are cascaded with single-mode fiber. The mode of MFSR is analyzed by using finite element analysis software. Its transmission spectrum is calculated through numerical simulation, and its micro displacement sensing characteristics are verified by experiments. Experimental results show that the micro displacement sensitivity of the sensor is as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${-{1.76} \,\,\text {nm}/\mu \text {m}}$ </tex-math></inline-formula> in the range of 0– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$35~\mu \text{m}$ </tex-math></inline-formula> , and its resolution is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${{0.057} ~\mu \text {m}}$ </tex-math></inline-formula> . Compared with a single MFSR, the sensitivity is improved by 7.65 times, which is consistent with the simulation. The cascaded MFSR micro displacement sensor has the advantages of low transmission loss and high measurement sensitivity and has important application prospects in industrial production and micro displacement detection.

Fiber Temperature Sensor Based on Vernier Effect and Optical Time Stretching Method.

A novel method for ultra-sensitive and ultra-fast temperature sensing has been successfully implemented by cascading Saganc rings to generate the Vernier effect and doing the same dispersive fibers to achieve the optical time-stretching effect. This is different from the traditional point fiber sensor demodulated by optical spectrum analyzer (OSA) whose demodulation speed is usually at the second level. The designed system maps the wavelength domain to the time domain through the dispersive fiber, which can realize the ultra-fast temperature monitoring at the nanosecond level. The cascaded Sagnac ring is composed of polarization maintaining fiber (PMF) which is significantly affected by the thermal-optical coefficient. When the temperature changes, the variation is as high as -6.228 nm/°C, which is 8.5 times higher than the sensitivity based on the single Sagnac ring system. Furthermore, through the optical time stretching scheme, the corresponding response sensitivity is increased from 0.997 ns/°C to 7.333 ns/°C, and the magnification is increased 7.4 times with a response speed of 50 MHz.

Tunable and switchable erbium-doped fiber laser based on the parallel filter of the FBGs embedded in Sagnac ring and the in-line MZI

A tunable and switchable erbium-doped fiber laser (EDFL) based on the parallel filter is proposed and experimentally verified. The parallel filter is composed of two branches. One branch is double fiber Bragg gratings (FBGs) embedded in Sagnac ring filter, which can realize the selection of two wavelengths; the other branch is an in-line Mach Zehnder interferometer (MZI) filter, which is composed of a 10 cm polarization-maintaining fiber (PMF) fused in the middle of single mode fibers (SMFs) with spheres at both ends, which can realize the selection of five wavelengths. When the parallel filter is inserted into the fiber laser cavity and the polarization controllers (PCs) are adjusted, the laser excitation from single-wavelength to quadruple-wavelength can be realized, including 7 different single-wavelength laser states, 16 different dual-wavelength laser states, 11 different triple-wavelength laser states and 2 different quadruple-wavelength laser states. In addition, the intervals of the dual-wavelength laser states are relatively regular, and the dual-wavelength laser intervals are obtained every ∼ 2 nm from 7 nm to 18 nm. And the optical signal-to-noise ratios (OSNRs) are more than 43 dB, and the highest OSNR is 60 dB. Finally, by changing the temperature applied to the FBG, tunability of dual-wavelength intervals can be achieved with a range of 1.1 nm. Due to the variety of lasing laser states of the fiber laser, easy switching, and regular intervals between the dual-wavelength laser states, the fiber laser proposed can be better applied to wavelength division multiplexing (WDM)system.

Sagnac ring and photonic crystal fiber structure refractive index sensor with high birefringence and low temperature sensitivity

In this paper, a novel refractive index sensor based on photonic crystal fiber and Sagnac ring is studied. The sensor adopts Mach-Zehnder interference principle. The production and experimental steps are as follows: The first step is to fuse the single-mode fiber with the photonic crystal fiber to form a basic sensing unit. The second part uses the coupling birefringence effect of the tapered coupler to fold and fuse the single-mode fiber together to form the Sagnac interferometer. Through this structure, the sensor has the characteristics similar to the polished photonic crystal fiber sensor, while the fabrication complexity is greatly reduced. The refractive index sensing capability and temperature stability of the two structures are analyzed experimentally. Simulation results show that the structure has high birefringence effect. Experimental results show that the proposed photonic crystal fiber combined with Sagnac ring sensor has good sensing performance in the refractive index range of 1.3355–1.3560. Compared with the sensor structure without Sagnac ring, the performance is greatly improved, the maximum sensitivity is up to 234 nm/RIU, and it has good temperature stability. The sensor has the advantages of miniaturization, high integration and high sensitivity, and can be used in industry, chemical detection, agriculture and other fields.

Fiber-optic temperature sensor based on fabry-perot laser diode strong feedback

A novel, simple, and low-cost optical fiber temperature detection system is proposed and demonstrated in this paper, which uses the strong feedback of the Fabry-Perot Laser Diode (FP-LD) at 1550 nm to demodulate the temperature in real time in the resonator cavity. The experimental system consists of a commercial telecommunication laser source (FP-LD) and a fiber-optic Sagnac ring. In the sensing test, the change in temperature has a perfectly linear relationship with the shift of beat frequency signals (BFS). Measuring signals of different frequency will have different sensing sensitivities. System has an external photodetector (PD) in the first experiment. The temperature monitoring sensitivities is estimated to be 14.87 kHz/°C@2.49 GHz. In the second experimental system, there is no external PD, and the built-in monitor PD of FP-LD is used for photoelectric conversion. The sensitivity of the temperature sensor is 8.14 kHz/°C@1.382 GHz. In this paper, a fiber sensing system with the lowest cost and simplest structure is proposed, which consists of only a laser diode and a Sagnac ring.

Simple continuous-variable quantum key distribution scheme using a Sagnac-based Gaussian modulator.

Continuous-variable quantum key distribution (CV-QKD) is a protocol that uses quantum mechanics to ensure that the distribution of an encryption key is secure even in the presence of eavesdroppers. The wide application of CV-QKD requires low cost, system simplicity, and system stability. However, owing to the particularity of Gaussian modulation in CV-QKD, an amplitude modulator (AM) and a bias controller are required, making the system structure complex and unstable. In this Letter, we achieve two-dimensional Gaussian modulation with only one phase modulator (PM) and a Sagnac ring structure, which significantly reduces the complexity of the system. We test the Gaussian modulation stability for 10 h, and the result shows that the expected secure key rate can be maintained at 80 kbit/s under a transmission distance of 50 km. This scheme opens up new, to the best of our knowledge, possibilities for a new generation of highly stable and simple CV-QKD systems.

Silicon-on-Insulator Optical Buffer Based on Magneto-Optical 1 × 3 Micro-Rings Array Coupled Sagnac Ring

Optical buffer is a key technology to control optical routing and solve channel competition, which directly determines the performance of information processing and storage. In this study, a switchable optical buffer using the nonreciprocal silicon-on-insulator (SOI) magneto-optical micro-ring (MOMR) array coupled with Sagnac ring was introduced, which can exceed the time-bandwidth limitation. The transmission equations and propagation characteristics of optical signal in 1 × 3 micro-rings and Sagnac ring coupled 1 × 3 micro-rings based on two kinds of phase-change materials were studied. The group time delay, effective buffer time and readout operation in the buffer were also investigated.

Tunable and switchable multi-wavelength random distributed feedback fiber laser based on SMF and a cascaded Sagnac ring filter.

A tunable and switchable multi-wavelength random distributed feedback fiber laser based on cascaded Sagnac loops is proposed and experimentally demonstrated. The random distribution feedback of the laser is provided by the Rayleigh scattering generated by the single-mode fiber (SMF). The cascaded Sagnac loops act as a filter and a reflector in the half-open cavity laser. The single-, dual-, three-, and four-wavelength channels can be realized by adjusting the angle of the polarization controller at the pump power of 300 mW. In the single-, dual-, and three-wavelength channels, the wavelength spacing can be maintained, and the laser wavelength position can be changed at the same time. The maximum wavelength tuning ranges of single-, dual-, and three-wavelength outputs are about 4.5 nm, 2.6 nm, and 1 nm, respectively. The proposed multi-wavelength random fiber laser has the advantages of simple structure and low threshold, and it has good application prospects in remote sensing and imaging systems.

MI-SI Based Distributed Optical Fiber Sensor for No-Blind Zone Location and Pattern Recognition

We designed a distributed fiber optic sensor based on a hybrid Michelson-Sagnac interferometer to overcome the defect that the conventional hybrid structure cannot locate when the disturbance position is close to the midpoint of the Sagnac ring. The sensor has excellent and flat frequency response for no-blind zone location. With the demodulation algorithm based on 3 × 3 optical coupler and simple mathematical operations, we can get two phase signals with time difference for location and pattern recognition. The phase signals were converted to two-dimensional images by Gramian Angular Field. Tagging images to create a dataset and feeding dataset into a convolutional neural network to achieve pattern recognition. After experiments, the frequency response capability of the structure was verified, and it was able to achieve location within 0-20 km with location errors of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 42 m. In a six-category pattern recognition, the testing set accuracy was 97.83%.

Narrow linewidth semiconductor laser with a multi-period-delayed feedback photonic circuit.

A multi-period-delayed feedback (MPDF) photonic circuit constructed by a Sagnac ring and two coupled rings was designed. By coupling a distributed feedback (DFB) laser diode (LD) with the MPDF, a narrow linewidth semiconductor laser was demonstrated. The linewidth of the DFB-LD with MPDF was narrowed to be around 2 kHz, which is reduced by three orders of magnitude, and the linewidth reduction capability could be maintained when the wavelength of the DFB-LD was tuned in a range wider than 3 nm. The laser frequency stability can also be improved using the proposed technique, and the frequency fluctuation was reduced for nearly 8 times in comparison with the DFB-LD.

Demodulation of Fiber Bragg Grating Accelerometer Using In-Line Sagnac Interferometers

We proposed a fiber Bragg grating (FBG) accelerometer demodulation method based an in-line Sagnac interferometric structure. In this method, an electro-optical phase modulator (EOM) is used as a phase-generating carrier (PGC) modulation module. The principle of in-line Sagnac optical path interference based on EOM is theoretically analyzed by Jones matrix, and the modulation mechanism of EOM is described. The experimental results of an FBG accelerometer based on flexible hinge show that the system background noise is <inline-formula> <tex-math notation="LaTeX">$1 \times {10^{ - 5}}\,\,{{\mathrm {rad}}} \cdot {{\mathrm {Hz}}^{-1 /2}}$ </tex-math></inline-formula>. The EOM based modulation scheme solves the problem of distortion of demodulation response due to the unstable modulation performance of the conventional piezoelectric transducer, which also provides a fast response to the interferometric optical path changes. The sensitivity of the in-line Sagnac based demodulation scheme is doubled compared to the Sagnac ring structure. The proposed scheme provides an integrated phase modulator for demodulation system for FBG sensors.

Simple Multiuser Twin-Field Quantum Key Distribution Network

Twin-field quantum key distribution (TFQKD) systems have shown great promise for implementing practical long-distance secure quantum communication due to its measurement-device-independent nature and its ability to offer fundamentally superior rate-loss scaling than point-to-point QKD systems. A surge of research and development effort in the last two years has produced many variants of protocols and experimental demonstrations. In terms of hardware topology, TFQKD systems interfering quantum signals from two remotely phase-locked laser sources are in essence giant Mach-Zehnder interferometers (MZIs) requiring active phase stabilization. Such configurations are inherently unsuitable for a TFQKD network, where more than one user-pair share the common quantum measurement station, because it is practically extremely difficult, if not impossible, to stabilize MZIs of largely disparate path lengths, a situation that is inevitable in a multi-user-pair TFQKD network. On the other hand, Sagnac interferometer based TFQKD systems exploiting the inherent phase stability of the Sagnac ring can implement asymmetric TFQKD, and are therefore eminently suitable for implementing a TFQKD network. In this work, we experimentally demonstrate a proof-of-principle multi-user-pair Sagnac TFQKD network where three user pairs sharing the same measurement station can perform pair-wise TFQKD through time multiplexing, with channel losses up to 58 dB, and channel loss asymmetry up to 15 dB. In some cases, the secure key rates still beat the rate-loss bounds for point-to-point repeaterless QKD systems, even in this network configuration. It is to our knowledge the first multi-user-pair TFQKD network demonstration, an important step in advancing quantum communication network technologies.

Sagnac Ring Humidity Sensor With a Melting Cone Based on Graphene Properties

This study proposes a new type of fused cone Sagnac ring humidity sensor based on graphene properties. First, the single-mode optical fiber (SMF) is fused with the photonic crystal fiber (PCF) to form SMF-PCF-SMF interference. Then, the SMFs at both ends of the PCF are folded in half to form a ring. The ring is placed on the fiber pulling cone translation platform for pulling cone coupling. A Sagnac interferometer that connects the PCF is formed by applying the birefringence effect of melting cone coupling. The prepared interferometer is placed on a special optical platform, and graphene is coated on the fused cone area and the PCF. The excellent properties of graphene are utilized to measure humidity. Experimental results show that sensitivity to humidity is within the range of 36.0% relative humidity (RH)-75.3% RH, reaching 340.13pm/% RH. The sensor proposed in this study exhibits the characteristics of a simple structure, easy manufacturing, and low cost. It can be used as reference for the design and development of new sensors.

Tapered two-mode fiber based Sagnac loop temperature sensor with improved interference spectrum extinction ratio

A temperature sensor constructed with tapered two-mode fiber (TTMF) based Sagnac loop is proposed, which features improved interference spectrum extinction ratio (ER). Both theory and experiment indicate that the TTMF-based Sagnac ring structure can significantly improve the ER of the interference spectrum generated by the TTMF without changing its free spectral range (FSR). The experimental results show that the temperature sensor with increased ER can obtain a sensitivity of 295 pm/℃ in the temperature measurement range of 20–60 ℃. The high ER may play an important role in enhancing the measurement accuracy and resolution of the sensing device.

Intensity Demodulation Technology of FBG Time Division Multiplexing Based on Sagnac Ring

Intensity Demodulation Technology of FBG Time Division Multiplexing Based on Sagnac Ring