Fiber Sagnac Loop Research Articles

Separation of twin beams generated in a Sagnac fiber loop

We theoretically demonstrate the propagating performance of twin beams generated in a Sagnac fiber loop by developing a practical model of optical parametric loop mirror consisting of an optical coupler with adjustable splitting ratio, a nonlinear fiber, a dispersion element, and two tunable attenuators placed at different positions in the loop. Two situations of frequency degenerate and frequency non-degenerate four-waving mixing are considered and various of factors including dispersion conditions, internal losses in the loop, and splitting ratios of the coupler are investigated in detail. When the splitting ratio of the coupler is 1:1 and no loss is in the loop, both the frequency degenerate and non-degenerate twin beams can propagate from different ports completely by introducing proper dispersion, and switching between phase-sensitive and phase-insensitive amplification can be realized by changing the dispersion within the loop. In general, the propagation of signal and idler beams at the two output ports of the fiber loop are influenced by both the splitting ratio of the coupler and the internal loss within the loop. Adjusting the internal loss can partially compensate for the asymmetry of the splitting ratio of the coupler, facilitating spatial separation of the signal and idler beams. Our research is useful for the generation of twin beams using Sagnac fiber loop and the realization of switchable phase-sensitive amplifiers.

Sagnac interferometer embedded with fiber Bragg grating for relative humidity and temperature sensing

In this paper, we first propose and demonstrate an ultra-compact fiber sensor consisting of fiber Bragg grating (FBG) and Sagnac loop interferometer with a specific taper-based coupling region. It is utilized for the relative humidity and temperature monitoring at the same time. The taper-based coupling region in the Sagnac loop can enhance the interaction between high-order propagating modes and the surrounding perturbations, therefore improving the fiber-optic sensitivity. The Sagnac interferometer demonstrates the humidity sensitivity of 89.4 p.m./%RH and the temperature sensitivity of −8.7 p.m./oC. FBG, the structure embedded in the Sagnac loop, shows the temperature sensitivity of 11.6 p.m./oC, while along with no feedback on the ambient relative humidity. It could be used as the reference component to demodulate the humidity information from the total reflection spectrum, finally realizing the simultaneous response of temperature and relative humidity.S

PANDA Fiber Sagnac Interferometer for Temperature Sensor Application

Fiber Sagnac loop mirror has great potential for use in various application such as multi-wavelength laser source, strain and temperature sensors. This paper demonstrates an optical fiber temperature sensor based on two lengths of PANDA polarization maintaining fibers (PMFs) in a fiber Sagnac interferometer. The temperature sensing is based on the change of spectral spacing of the generated optical comb with the temperature difference between two PMFs. The performance of the system was characterized numerically and experimentally by measuring the spectral spacing when raising the temperature of one of the PMFs. A high temperature sensitivity up to 0.36 nm/°C is obtained, with detuning range of 43 nm. This makes it a good candidate for various in-field sensing application.

Sagnac interferometer based optical coherence tomography

We present in this paper an all-fiber Optical Coherence Tomography OCT system in the time domain by employing a Sagnac fiber loop. The optical fiber version of TD-OCT uses directional couplers to construct a Michelson interferometer instead of a beam splitter which is used in the bulk optics version. Instead of implementing the optical delay by an external movable mirror, we present a new configuration that employs a Sagnac interferometer. Avoiding the use of an external movable mirror, eliminates the problems of alignment to couple back the optical signal into the fiber after being delayed by the external mirror. Moreover, this all-fiber system has the advantage of being compact and having large values of optical path difference.

A Compact Optical Fiber Sagnac Loop Inserting With A Nematic Liquid Crystal Film for Highly Sensitive Temperature Sensing

Since the high temperature response characteristics of nematic liquid crystal (NLC) molecules, a compact optical fiber Sagnac loop inserted a segment of NLC film for temperature sensing was proposed and experimentally demonstrated. The NLC film was formed by filling into the micron-scale gap between the two single-mode fibers inserted into a ferrule matching sleeve. After two single-mode patch cables were rotated, the NLC film molecules were oriented due to exogenous effects. Experimental results showed that the temperature sensing performance of the designed sensor was improved by decreasing the thickness of NLC film in Sagnac loop. The thickness of NLC film in the ferrule matching sleeve was measured accurately by Fabry-Perot interference. The wide temperature measurement range of 2.2 ∼ 50.2°C and a high temperature sensitivity of -9.34 nm/°C were achieved at the NLC film thickness of 31.30 μm. Owing to the compact structure, large measurement range, and high sensitivity, the designed sensor shows a broad application prospect in high spatial resolution and high sensitivity temperature sensing.

Adaptive Fiber Ring Laser Based on Tapered Polarization Maintaining Fiber in Sagnac Loop for Temperature and Salinity Sensing

An optical fiber ring laser (FRL) cavity-based sensitive temperature and salinity sensor is proposed and experimentally demonstrated. The sensor consists of a Sagnac loop with a waist of 15 µm and a total length of 30 cm made of tapered polarization-maintaining fiber (PMF). Sagnac loop dual parameter sensing was theoretically modeled and presented. The salinity sensitivity of 0.173 nm/‰ was made possible by the efficient interaction between the tapered PMF cladding mode and the external refractive index. In addition, temperature sensitivity of 0.306 nm/°C was achieved through ultrahigh birefringence of PMF. Apart from that, the previous sensing system used a broadband light source (BBS) as the input light, resulting in a wide bandwidth and a poor signal-to-noise ratio (SNR). The Sagnac loop integrated into the FRL system can achieve a high SNR of approximately 50 dB and a narrow bandwidth of 0.15 nm while serving as the filter and sensor head. Additionally, the developed sensor has the advantages of simple design, low cost, and easy fabrication. It can also extend sensing distance indefinitely within a given range, which is anticipated to have positive effects on the testing of marine environments in laboratories.

High-Sensitivity Transverse-Load and Axial-Strain Sensor Based on Air-Bubble Fabry–Pérot Cavity and Fiber Sagnac Loop Cascaded

We have designed a highly sensitive optical fiber transverse load and axial strain sensor based on the cascaded Fabry–Perot (F–P) cavity and fiber Sagnac loop. F–P cavity is composed of an air-bubble formed by a Section of quartz capillary discharge. Fiber Sagnac loop is made by winding a Section of panda fiber (PF) through a 3 dB coupler. When the free spectral ranges (FSRs) of two interferometers are similar, they are cascaded to form the optical Vernier effect, which amplifies the sensitivity of the sensor. The experimental results show that the sensitivities of transverse load and axial strain of the sensor are up to 19.1 nm/N and 28.9 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , respectively. The sensitivities of a single F–P cavity for measuring the transverse load and the axial strain are 1.5 nm/N and 2.4 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , respectively. It can be seen that by using the optical Vernier effect, the sensitivity of the transverse load and axial strain of the sensor is improved by 12.7 and 12 times, respectively. The measurement matrix is constructed by using the sensitivities of a single F–P cavity and the cascaded structure to realize the simultaneous measurement of the transverse load and axial strain and eliminate the cross-sensitivity between them. The designed sensor has a certain attraction in small load and small strain monitoring.

Detecting correlated errors in twin-field quantum key distribution

We experimentally demonstrate that we can detect correlated errors in a twin-field quantum key distribution (TFQKD) system by using a technique that is related to self-consistent tomography. We implement a TFQKD system based on a fiber-Sagnac loop, in which Alice and Bob encode information in the phase of weak coherent states that propagate in opposite directions around the loop. These states interfere as they exit the loop and are detected by a third party, Charlie, who reports the results of their measurements to Alice and Bob. We find that it is possible for Alice and Bob to detect correlated state-preparation and measurement errors while trusting only their own individual states, and without trusting Charlie’s measurements.

Highly sensitive temperature sensing based on a birefringent fiber Sagnac loop

The usage of a 1 m Polarization Maintaining Fiber (PMF) as a passive sensing element for the experimental demonstration of a highly sensitive all-fiber temperature sensor based on a Sagnac interferometer configuration is presented here. Using a simple interrogation method tracing the wavelength shift of the obtained interference peaks, we were able to detect variations in a water bath temperature with a highly linear response with coefficient of determination (R2) of 0.9999 and a sensitivity of −1.59 nm/°C for the 18–42°C temperature range. This result demonstrates the feasibility of using this all-fiber device as a temperature sensor for many applications that demands very precise and reliable measurements.

All-Fiber High Efficiency Wavelength-Tunable Mode-Locked Cylindrical Vector Beam Laser

A wavelength-tunable mode-locked cylindrical vector beam (CVB) fiber laser with a dumbbell-shaped structure was proposed and demonstrated experimentally. A homemade broadband long-period fiber grating (LPFG) and a high-birefringence Sagnac fiber loop work as mode converter and comb filter respectively. The mode-locking mechanism is based on the nonlinear polarization rotation (NPR) effect. The central wavelength can be tuned from 1023.257 nm to 1046.123 nm with mode-locked operating state. The laser has a high slope efficiency of 15.75% with the mode-locking threshold value of 232.5 mW at the central wavelength of 1041.361 nm. The mode-locked CVB pulses have a repetition rate of 7.06 MHz with the duration of 107 picoseconds. Radially and azimuthally polarized beams can be obtained by eliminating the degeneracy of LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode, with the purity of 94.86%. This CVB laser with controllable operating wavelength may have potential applications in mode division multiplexed (MDM) systems, optical manipulation, and electron acceleration.

High-Performance Hyperentanglement Generation and Manipulation Based on Lithium Niobate Waveguides

Here, we demonstrate the generation and manipulation of hyperentanglement in polarization and time-energy degrees of freedom. The hyperentangled photon pairs are generated by cascaded second-harmonic generation and spontaneous parametric down-conversion processes in a high-efficiency periodically poled lithium niobate (PPLN) waveguide in a fiber Sagnac loop. The states achieve the maximum value of Clauser-Horne-Shimony-Holt Bell inequality of $S=2.76\ifmmode\pm\else\textpm\fi{}0.03$ with two-photon interference visibility of higher than 99% for polarization entanglement and the Franson-type two-photon interference visibility of 98.5% for time-energy entanglement. In addition, the polarization maximally entangled states can be fast switched to each other by using a PPLN on insulator chip through the electro-optic effect. After the entanglement manipulation, the two-photon interference visibility for polarization entanglement is still higher than 97% while keeping the quantum characteristics of time-energy degrees of freedom maintained. Our system paves a way toward reducing the complexity of quantum systems and has potential applications in many quantum-information tasks like fast superdense coding and teleportation.

A Multi-Wavelength Thulium-Doped Fiber Laser Using a Photonic Crystal Fiber-Based Sagnac Loop

A multi-wavelength thulium-doped fiber laser incorporating a photonic crystal fiber (PCF) Sagnac loop was presented and experimentally demonstrated. In the laser cavity, the Sagnac loop mirror used a 0.5 m PCF as a comb filter. A nonlinear polarization rotation (NPR)-based 600 m single-mode fiber (SMF) was used to suppress the mode competition. By adjusting the polarization controllers (PCs), up to nine stable wavelength outputs near the 2 μm spectrum were obtained at room temperature with an optical signal-to-noise ratio (OSNR) of up to 30 dB. The channel spacing of this fiber laser is 4.88 nm, and the 10 dB bandwidth encompasses 1950.39 to 1989.43 nm.

Sagnac mirror loop with two polarization maintaining fibers for twist measurement by tracking adjacent dips with tunable measurement range

The transmission characteristics of Sagnac fiber loop interferometer which consists two sections of polarization-maintaining fiber (PMF) spliced at different offset angles were analyzed. Based on the simulation results, an application for twist measurement where PMFs were spliced at different offset angles was designed. When the lengths of two PMFs are equal, a clear spectrum can be obtained for twist or torsion sensing. A phenomenon that the dips would split or merge with twist angle alteration can be used for sensing. Measurement range and sensitivity can be adjusted by changing the PMFs offset angle. The sensitivity was improved by calculating the distance between two adjacent dips, being 0.73 nm/° or 8.37 nm/(rad/m) when the offset angle was 75°.

A Sagnac Interferometer-Based Twist Angle Sensor Drawing on an Eccentric Dual-Core Fiber

A fiber twist angle sensor has been theoretically proposed and experimentally verified by employing a fiber Sagnac loop interferometer based on a self-made eccentric dual-core fiber. The asymmetry of the dual-core fiber structure made the deformation of the fiber more obvious under the action of twist and enabled the designed sensor to identify the direction of twist angle. A clockwise (CW) and counterclockwise (CCW) rotation of a 5-cm-long eccentric dual-core fiber in the Sagnac loop generated a phase difference, which induced a red or blue shift of the interference spectrum, and the purpose of twist angle sensing was achieved by monitoring the spectral shift. The size of the twisted object should be larger than 5 cm, so that the sensor can be placed on the surface of the object to measure the twist deformation. In the measuring range from &#x2212;90&#x00B0; to 90&#x00B0;, the twist angle-based sensitivity and twist rate-based sensitivity of the designed sensor were, respectively, investigated in CW and CCW directions: the former was 45.2 pm/&#x00B0; in CCW direction (&#x2212;90&#x00B0; to 0&#x00B0;) and 12 pm/&#x00B0; in CW direction (0&#x00B0;&#x2013;90&#x00B0;), and the latter was 129.49 pm/(rad/m) in CCW direction (&#x2013;31.4 to 0 rad/m) and 34.38 pm/(rad/m) in CW direction (0&#x2013;31.4 rad/m). The satisfactory sensitivity, stability, and repeatability make it a competitive alternative in twist angle sensing region, which can be widely used in engineering, biomedical, and other twist angle sensitive fields.

Cascaded fiber non-adiabatic taper and Sagnac loop for refractive index sensing

We propose and experimentally demonstrate a refractive index sensor by cascading a fiber non-adiabatic taper and a fiber Sagnac loop. The modal interference of the non-adiabatic taper produces a regular interference spectrum, which shifts with the refractive index change of the external medium. The interference spectrum of the Sagnac loop can be tuned by changing the polarization state and length of polarization maintaining fiber (PMF). Cascading the two interferometers with different free spectral ranges (FSRs) will generate the superimposed envelope. The cascade structure will change the sensing capability of the single interferometer. Experiment results show that the refractive index sensitivity of our proposed sensor can be adjusted from 921.43 nm/RIU (single tapered fiber) to 787.24 nm/RIU (cascaded configuration). The low cost and easy fabrication of the proposed method highlight a beneficial choice for changing the refractive index sensitivity. © 2021 Elsevier Science. All rights reserved.

Visible-Wavelength-Tunable, Vortex-Beam Fiber Laser Based on a Long-Period Fiber Grating

In this letter, we report the first demonstration of a visible-wavelength-tunable, vortex-beam fiber laser based on a Sagnac-loop fiber filter and efficient long-period fiber grating. A 30 cm Pr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -codoped ZBLAN fiber pumped by a 445 nm laser diode provides ~ 635 nm strong optical gain. A homemade dielectric mirror on the fiber end-facet and a Sagnac-loop fiber filter are used to build a wavelength-tunable fiber laser with a wavelength range of 633.89 to 636.02 nm. A self- fabricated long-period fiber grating acts as a mode converter in the cavity, and one-order vortex-beam including cylindrical vector beams (CVBs) and orbital angular momentum beams (OAMs) with a high mode purity are successfully obtained. Furthermore, we also measured the speckle contrast of the visible-wavelength fiber laser, and the speckle contrast is reduced by 17.13% through CVBs. This work could offer an effective approach to develop compact, wavelength-tunable vortex fiber lasers in the visible spectrum for applications to high-resolution imaging, laser display, visible light communications, and scientific research.

Research on Smart Mattress Based on Fiber Unbalanced Sagnac Loop

Because the contact vital sign detection system is monitored through skin contact, it affects the user’s comfort and causes inconvenience. And most of the non-contact equipment is not high precision, or the cost is too high. To solve the above problems, this article is based on the physiological activity of slight vibration in multimode optical fiber micro bending deformation which result in the change of light intensity principle, considering the structure of the optical fiber Sagnac loop sensor has simple structure, low cost, but the signal to noise ratio is poor and is not sensitive to low frequency signal, this paper designed a based on intelligent mattress of non-equilibrium sagnac loop structure. The mattress can obtain the heart impact map signal in real time by changing the small light intensity, and obtain the heart rate, respiration rate and other information through the algorithm processing the signal. Mattress in the design of the sensing optical fiber module will be connected to two different types of optical fiber of Unbalanced Sagnac loop structure, solve the balance Sagnac loop structure of optical fiber access signal noise is bigger, poor sensitivity were weakness, go through the serpentine line way of embedded mattress, and using the mattress cover material for optical fiber micro bending cycle critical mechanical adjustment cycle, the mattress is of high sensibility. Cardiac and respiratory signals obtained in the laboratory were compared with those obtained from the balanced Sagnac loop, and through the fast Fourier transform (FFT) in frequency domain signal is analyzed, the vital signs of signal more clearly this mattress stable and less noise, change the tester for many times after tests, get the signal of high accuracy, the error is very small. The experiment shows that the mattress can realize the non-sensory vital signs signal detection, convenient for users to monitor their physical conditions in real time in daily work and life, and can timely notify the family members and medical staff in emergency, to avoid the occurrence of accidents.

Simultaneous measurement of temperature and acoustic impedance based on forward Brillouin scattering in LEAF.

The detection of the surrounding environment is one of the most fundamental challenges in optical fiber sensors research. In this Letter, we report two new, to the best of our knowledge, phenomena in large effective area fiber (LEAF), which are the linear dependence of the radial acoustic mode induced forward Brillouin scattering (FBS) spectral linewidth on temperature and the linear dependence of the radial acoustic mode induced FBS spectral frequency shift on acoustic impedance. By utilizing these linear relationships, we present a novel temperature and acoustic impedance simultaneous measurement method based on FBS in LEAF. With a fiber Sagnac loop structure, the obtained measurement uncertainties of temperature and acoustic impedance surrounding LEAF are 0.1°C and 0.006kg/(s⋅mm2), respectively. The corresponding measurement uncertainties in the form of relative percentage values are 0.5% and 0.4%, respectively.

Switchable multi-wavelength erbium-doped random distributed feedback fiber laser incorporating two-stage Sagnac filter

A low-threshold and simple structure switchable multi-wavelength fiber laser based on random distributed feedback (RDFB) is presented. The experiment setup incorporates a two-stage Sagnac fiber loop mirror, an 8 m long erbium-doped fiber (EDF), and a 25 km long single-mode fiber (SMF). The SMF provides RDFB via Rayleigh scattering. Due to the implementation of the half-open cavity structure and the gain generated by the EDF, the laser threshold can be significantly suppressed to 60 mW, and the slope efficiency of output lasing power against pump power is ∼0.9%. By adjusting the polarization controllers of the proposed two-stage Sagnac loop mirror, single-, double-, triple-, and quadruple-wavelength lasing channels in the range of 1527–1534 nm are achieved. In the case of quadruple-wavelength channel lasing, the optical signal noise ratio is up to 33 dB under the 180 mW pump power. The proposed random fiber laser can be applied in distributed sensing systems and biomedical imaging.

Observation of binary phase states of time-multiplexed degenerate optical parametric oscillator pulses generated using a nonlinear fiber Sagnac loop.

We generated time-multiplexed degenerate optical parametric oscillator (DOPO) pulses using a nonlinear fiber Sagnac loop as a phase-sensitive amplifier (PSA), where the pump and amplified light in pump-signal-idler degenerate four-wave mixing can be spatially separated. By placing the PSA in a fiber cavity, we successfully generated more than 5000 time-multiplexed DOPO pulses. We confirmed the bifurcation of pulse phases to 0 or π relative to the pump phase, which makes them useful for representing Ising spins in an Ising model solver based on coherent optical oscillator networks. We also confirmed inherent randomness of the DOPO phases using the National Institute of Standards and Technology random number test.