Fiber Sagnac Loop Research Articles

High-Speed FBG Interrogation With Electro-Optically Tunable Sagnac Loops

We report high-speed Fiber Bragg Grating (FBG) interrogation based on fiber interferometry. An electro-optical modulator, used as a tunable retarder, is implemented in a fiber Sagnac loop, enabling fast tuning of the interference fringes. Applying an active modulation scheme, the wavelength of the FBG can be retrieved without any requirement of the spectral position of the FBG with respect to the fringes, with no need of tunable sources or spectral analysis. We report experimental results of wavelength measurements with a bandwidth of 100 kHz (only limited by our data acquisition rate) and a dynamic wavelength resolution of σ = 3.7 fm/√Hz (corresponding to a strain resolution of 3.1 nϵ/√Hz). We also show that the system can track a continuous tunable laser sweep of 40 nm along the C-band. The technique is polarization-independent and allows wavelength division multiplexing schemes using only one interferometer for all the sensing points. The system is fully based on low-cost off-the-shelf fiber-optical components, and the free-spectral-range of the interferometer can be easily modified by just changing the length of the polarization-maintaining fiber segment.

Switchable multi-wavelength linearly-polarized lasing oscillations in a figure eight EDF laser based on spatial-mode beating by means of weakly-coupled FMF

A switchable multi-wavelength figure eight Erbium-doped fiber (EDF) laser with linearly-polarized (LP) beam generation in the C-band region is proposed and experimentally demonstrated. Spatial-mode beating occurs owing to effect of multi-mode interference in the weakly-coupled few-mode fiber (FMF). A bandpass filter of multiple specific wavelengths is created by incorporating a weakly-coupled FMF with a Sagnac fiber loop. The LP modes are generated by cascaded mode-selective couplers (MSCs), which are designed and fabricated by tapering the proposed FMF with single-mode fiber (SMF). Combination of the cascaded MSCs and a simple N × 1 optical switch is employed to switch the LP modes. In experiments, the proposed fiber laser delivers versatile lasing outputs in states of single-, dual-, triple-wavelength oscillations for LP01, LP11 and LP21 modes. We investigate the stabilities for LP01, LP11, LP21 modes in 24-min. Moreover, the mode intensity profiles, optical signal-noise ratios (OSNRs) of lasing output and slope efficiencies are also measured. The proposed fiber laser has promising applications in WDM-MDM optical fiber communication systems, optical fiber sensor systems, optical instruments and spectroscopy.

Polypyrrole-Coated Polarization Maintaining Fiber-Based Vernier Effect for Isopropanol Measurement

A fiber optic isopropanol sensor based on polypyrrole-coated polarization maintaining fiber (PMF) is demonstrated. The sensor configuration is based on the combination of the Vernier effect of a single fiber Sagnac loop and the isopropanol sensitive material of polypyrrole, which swells in corresponding to the change of isopropanol concentration. And this swell effect can induce the strain effect on the fiber Sagnac loop. In this paper, the sensitivity of the proposed sensor exhibits ∼4.2 times (239 pm/ppm) higher than the conventional Sagnac interferometer (56.89 pm/ppm) from the range of 0 to 42 ppm with a satisfactory stability and linearity.

High-Sensitivity Temperature Sensor Based on Polarization Maintaining Fiber Sagnac Loop

A high-sensitivity all-fiber temperature sensor based on a Sagnac interferometer is demonstrated by splicing a section of polarization maintaining fiber (PMF) between two sections of standard single mode fibers (SMFs). In this sensor, the SMF-PMF-SMF structure in the Sagnac loop is bent into a circle to enhance the sensitivity. The length and curvature of the PMF in the loop are investigated and can be optimized to further increase the temperature sensitivity of the sensor. Results show that the radius of the circle has an important effect upon temperature sensitivity due to the bend-induced birefringence variation of the PMF. The SMF-PMF-SMF structure bent into a circle with a radius of 30 mm exhibits a high-sensitivity temperature of 1.73 nm/°C. The sensor is provided with the advantages of easy fabrication, low-insertion loss, and high sensitivity, which may find potential applications in the field of high precision temperature measurement.

Passively Q-switched wavelength-tunable 1-μm fiber lasers with tapered-fiber-based black phosphorus saturable absorbers

Abstract In this paper, we demonstrated passively Q-switched wavelength-tunable 1-μm fiber lasers utilizing few-layer black phosphorus saturable absorbers. The few-layer BP was deposited onto the tapered fibers by an optically driven process. The wavelength tunability was achieved with a fiber Sagnac loop comprised of a piece of polarization maintaining fiber and a polarization controller. Stable Q-switching laser operations were observed at wavelengths ranging from 1040.5 to 1044.6 nm at threshold pump power of 220 mW. Maximal pulse energy of 141.27 nJ at a repetition rate of 63 kHz was recorded under pump power of 445 mW.

High power high signal-to-noise ratio multiwavelength Raman fiber laser based on random distributed feedback

A multiwavelength Raman fiber laser based on random distributed feedback and a Sagnac fiber loop filter (SFLF) that can deliver 2.33 W output power is demonstrated. The effect of polarization state on the signal-to-noise ratio (SNR) of multiwavelength emission is investigated by the insertion and removal of a polarization beam splitter (PBS) in the cavity. With the help of a PBS and a polarization controller (PC), stable six-wavelength emission with SNR > 21 dB is achieved. To the best of our knowledge, this is the highest output power in a multiwavelength Raman fiber laser ever reported.

Cascaded fiber Sagnac loop-based microwave photonic multiband bandpass filter with a selectable passband frequency

A passband frequency-electable microwave photonic multiband bandpass filter based on a cascaded fiber Sagnac loop is presented and experimentally demonstrated. A broadband light is sliced by the cascaded fiber Sagnac loop, which serves as a spectrum slicer. After broadband light passes through the spectrum slicer, the sliced broadband light will have the varied periodical spectral characteristics that will cause a transmitted spectral distribution with uniform or multiple periods, then will be modulated by an optical phase modulator and delayed by a dispersive medium. Therefore, a frequency-band-selectable microwave photonic multiband bandpass filter with a suppressed baseband is achieved that can be switched between the single-passband, dual-passband, and triple-passband state. The presented filter exhibits a maximal out-of-band rejection ratio of about 30 dB.

Wavelength-Spacing Switchable Dual-Wavelength Single Longitudinal Mode Thulium-Doped Fiber Laser at 1.9 μm

We propose and demonstrate a spacing switchable dual-wavelength single longitudinal mode thulium-doped fiber laser at 1.9 μm. The fiber laser employs a ring cavity and a segment of 4-m thulium-doped fiber pumped by a 1563 nm laser. A fiber Sagnac loop is used as a comb filter to generate a stable dual-wavelength laser. A multimode interference filter consisting of a single-mode–multimode–single-mode fiber and a rotary fiber squeezer is used to adjust the wavelength spacing. When pump power is fixed at 260 mW, a stable dual-wavelength laser at 1.9 μm is obtained by adjusting the polarization controller in the Sagnac filter, and the wavelength spacing can be tuned from 6.7 to 26.7 nm by adjusting the rotary fiber squeezer in the multimode interference filter. Each wavelength of the dual-wavelength laser operates at a single longitudinal mode state, and the maximum linewidth is 7.5 MHz analyzed by a Fabry–Perot scanning interferometer.

Wavelength-tunable dissipative pulses from Yb-doped fiber laser with Sagnac filter

We experimentally demonstrate a wavelength-tunable passively mode-locked Yb-doped all-normal-dispersion fiber laser by exploiting a high-birefringence Sagnac fiber loop as a flexible filter. The wavelength of pulses is controlled by properly rotating the polarization controllers in a Sagnac loop. The spectrum of output pulses could be shifted by up to 15 nm at a repetition rate of 18 MHz. Output power of more than 43 mW at pulse durations below 11.4 ps is achieved at the central wavelength of 1035 nm. The pulse temporal frequency and output power change slightly over the tunable range.

Fiber-based frequency-degenerate polarization entangled photon pair sources for information encoding.

In this paper, a generation scheme of telecom band frequency-degenerate polarization entangled photon pairs is proposed and demonstrated experimentally. It is based on the vector spontaneous four wave mixing (SFWM) process in a Sagnac fiber loop, in which two frequency-degenerate and polarization orthogonal biphoton states have equal probabilities for generating along the clockwise and counter-clockwise directions. The quantum interference between them at the 50:50 fiber coupler of the fiber loop separates the two frequency-degenerate photons in a pair, leading to the generation of polarization entanglement. The raw fringe visibilities of the two-photon interferences under two non-orthogonal polarization bases are 91% and 90%, respectively. Information can be encoded on the generated photon pairs using the polarization entangled Bell states, which requires the frequency-degenerate property. It is demonstrated by a simplified Bell state measurement with a fringe visibility of 83%.

Discrete Frequency Entangled Photon Pair Generation Based on Silicon Micro-ring Cavities

In this paper, we propose and demonstrate a scheme to generate discrete frequency entangled photon pairs based on a silicon micro-ring resonator. The resonator is placed in a Sagnac fiber loop. Stimulated by two pump lights at two different resonance wavelengths of the resonator, photon pairs at another two resonance wavelengths are generated along two opposite directions in the fiber loop, by the nondegenerate spontaneous four wave mixing in the resonator. Their states are superposed and interfered at the output ports of the fiber loop to generate frequency entangled photon pairs. On the other hand, since the pump lights come from two continuous wave lasers, energy-time entanglement is an intrinsic property of the generated photon pairs. The entanglements on frequency and energy-time are demonstrated experimentally by the experiments of spatial quantum beating and Franson-type interference, respectively, showing that the silicon micro-ring resonators are ideal candidates to realize complex photonic quant...

Pressure sensor based on the Sagnac effect and fiber loop ringdown spectroscopy

A fiber-optic sensing scheme of measuring pressure is described here. The high reflective mirror in a laser cavity is replaced by a Sagnac loop in this scheme. The method combining the Sagnac effect with fiber loop ringdown technology fully embodies the advantages of both. The working principle is discussed in detail, and the whole sensing performance is demonstrated by applying sensing force to the sensor area. The sensing force can be obtained by measuring the ringdown time. The pressure measurement range of this device is 40 to 350 N. The detection sensitivity 0.4 ns/N can be realized in this system. The experimental curve reveals a close relationship between the sensing strain and the ringdown time.

A Dual-Parameter Sensor Using a Long-Period Grating Concatenated With Polarization Maintaining Fiber in Sagnac Loop

In this paper, a novel dual-parameter measurement scheme using a long-period grating (LPG) concatenated with polarization maintaining fiber (PMF) in a Sagnac loop was proposed and experimentally demonstrated to measure temperature and refractive index (RI) simultaneously. There are two types of dips in the output spectrum of the proposed sensor, which are formed by the LPG and Sagnac loop. Compared with the traditional Sagnac loop sensors using PMF only, the birefringence induced by the LPG changes the phase difference and makes the dips of the Sagnac loop sensitive to surrounding RI. Due to different interference mechanisms, the dual-parameter measurement is realized by Sagnac loop and LPG with different sensitivities to temperature and RI. Experiments show that in the output spectrum of the sensor, the dip of the LPG has the sensitivities of 0.201 nm/°C and -8.36 nm/RIU, and the dip of the Sagnac loop has the sensitivities of -1.06 nm/°C and -23.068 nm/RIU. The proposed sensor has a simple structure and great potential in biological, chemical, and ocean fields.

Magnetic field fiber sensor based on the magneto-birefringence effect of magnetic fluid

In this study, the magneto-birefringence effect of magnetic fluid (MF) is adopted to form an innovative fiber optic magnetic field sensor. The sensitive section is fabricated via a D-shaped microstructure inscribed in a high-birefringence fiber Sagnac loop with a femtosecond laser. The D-shaped microstructure facilitates good combination of the optical-fiber Sagnac interferometer with the magneto-birefringence effect of MF without suffering from absorption loss and manual alignment. Experimental results show the good performance of the magnetic field fiver sensor, particularly its high stable extinction ratio. Preliminary results are provided, and the magnetic field sensitivity of 0.0823nm/mT can be further improved by increasing the depth and length of the D-shaped microstructure.

Experimental research on generating and splitting degenerate correlated photon pairs in Sagnac fiber loop

Degenerate correlated photon pairs (DCPPs) have been widely used in quantum information science,especially in the areas of quantum computation,quantum state control and precision measurement,which are typically generated in a (2) nonlinear crystal through the spontaneous parametric down-conversion.However,such a source is not compatible with optical fiber as large coupling losses occur when the pairs are launched into it,which restricts its direct application to quantum information processing system.More recently,DCPP generation from spontaneous four-wave mixing in (3) optical fiber has aroused strong interest,due to its advantages of compatibility with existing fiber networks and free of alignment.The process of generating DCPP in fiber can be described as follows:two pump photons at different frequencies p1 and p2 scatter through the (3) nonlinearity to create a pair of identical photons at the mean frequency c,such that p1+p2=2c.Because the collinear tensor component xxxx(3) in a Kerr nonlinear medium is 3 times as large as the tensor component xyxy(3),the co-polarized four-wave mixing is preferred,which means the two pump photons and new-born twin photons are both co-polarized.Therefore,it is very challenging to deterministically separate the fiber-based DCPP,since the twin photons share the same properties in all degrees of freedom:frequency,polarization and spatial.Sagnac fiber loop (SFL),composed of a piece of nonlinear fiber and 50/50 coupler,is presented as the splitter for DCPP based on the reversed Hong-Ou-Mandel quantum interference of counter-propagating DCPPs.The SFL can be configured as a total reflector,total transmitter or equally transmissive and reflective state,which sets the differential phases of counter-propagating DCPPs meeting at 50/50 coupler to be ,0 and -,respectively.In order to satisfy the differential phase requirement for completely splitting the DCPP,the SFL is always set to be equally transmissive and reflective state,however,the polarization-mode matching of counter-propagating DCPPs is not easily achieved due to the disturbance of fiber birefringence.According to the Jones matrix derivation of DCPP propagating in the SFL,the polarization mode of counter-propagating DCPPs when interference at 50/50 coupler is automatically matched,if the SFL is set as a total reflector or total transmitter.In experimental scheme,utilizing the SFL as a total reflector,the 1.1 nm bandwidth and 1544.53 nm central wavelength DCPPs are generated by two pulsed light beams pumping the 300 m dispersion-shifted fiber in the SFL.Using the two pieces of single mode fiber connecting the 300 m dispersion-shifted fiber and 50/50 coupler,whose length difference is fixed at 3.3 m,the differential phase of counter-propagating DCPPs highly dependent on the dispersion properties of single mode fiber is managed at 2 for fully distributing DCPPs into which degrades the fidelity of DCPP source.The measured ratio of coincidence to accidental-coincidence of DCPPs from one port is approximately 1.8:1,which indicates that the coincidence counts mainly originate from accidental coincidence counts and extra coincidence counts from photon bunching and there are not any DCPPs outputting from one port.Meanwhile,the ratio of best measured coincidence to accidental-coincidence of DCPPs from two ports reaches 47:1,when the average power of two pumps is fixed at 0.026 mW.The experimental results demonstrate that the high purity and fully spatial separation DCPPs are successfully prepared in optical fibers,which is a very useful tool for realizing various quantum information systems.How the spatial state of outputting DCPPs depends on the length difference between single-mode fiber and detuning wavelength is also discussed in detail.

Seawater Temperature Measurement Based on a High-Birefringence Elliptic Fiber Sagnac Loop

A new method for seawater temperature sensing based on a high-birefringence elliptic fiber (HBEF) Sagnac loop is proposed in this letter due to the linear relationship between the change of temperature and the shift of interference peak. The transmission spectra of HBEF Sagnac loop at different seawater temperatures are studied theoretically and experimentally, which show good agreement with each other. The results show that sensing sensitivity increases with the increasing HBEF length and the decreasing wavelength, which is just opposite to the dependence of free spectrum range on the fiber length and the probing wavelength. Thus, there may exist an optimal fiber length and probing wavelength for the highest sensitivity and the largest detection range. By optimizing the parameters of probing system, the sensitivity of −472 pm/°C is obtained for temperature sensing, and the detection range from 10 °C to 30 °C is kept with the wavelength of 1310 nm (communication window) and the fiber length of 98.9 cm. Seawater temperature sensors demonstrated here show the advantages of small size, low cost, easy fabrication, high sensitivity, and easy integration with fiber systems.

Large-region tunable optical bistability in saturable absorber-based single-frequency Brillouin fiber lasers

The optical bistability in saturable absorber-based single-frequency Brillouin fiber laser (BFL) has been experimentally investigated. A Sagnac fiber loop with unpumped erbium-doped fiber (EDF) is embedded in the BFL cavity as a saturable absorber (SA). This component serves as both an auto-tracking narrowband dynamic grating to guarantee single-longitudinal-mode (SLM) operation, and an intensity-dependent loss switch leading to the bistable behavior. The feature of the intensity-dependent loss is well verified and characterized. A bistable region is found as large as 418.8 mW in the laser with the Brillouin gain media of 50 m single mode fiber. Furthermore, the flexible tunability of bistable region can be implemented by designing the unpumped EDF length and the Brillouin gain media with a proper fiber length or fiber type.

Multiwavelength thulium-doped silica fiber laser incorporating an all-fiber phase modulator

A novel multiwavelength thulium-doped silica fiber laser incorporating an all-fiber phase modulator is first presented in 2μm band. In the laser cavity, a phase modulator with a modulation frequency is efficient and beneficial to overcome the mode competitions. A polarization maintaining fiber Sagnac loop mirror is used as the filter. Large number of lasing wavelengths can easily and conveniently obtained at a proper modulation frequency under room temperature. The fiber laser can achieve ten output channels within a 15dB bandwidth from 1982nm to 1998nm.

Multiwavelength tunable ring fiber laser operating at 1.9 μm

ABSTRACTA multiwavelength tunable fiber laser operating at 1.9 µm is demonstrated. A 4 m long Tm‐doped single‐mode fiber is pumped by a 1.57 µm fiber laser. A Sagnac fiber loop is used in the fiber laser cavity as a tunable filter, and the fiber loop consists of a 3‐dB coupler, a polarization controller (PC), and a segment of polarization‐maintaining fiber. Adjusting the PC state and pump power, multiwavelength laser with wavelength spacing of 2.5 nm 1895–1915 nm is observed. The number of output wavelength can be tuned precisely from one to four by adjusting the PC state. The 3‐dB linewidth and side‐mode suppression ratio are 0.045 nm and 45 dB, respectively. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:401–403, 2015

Temperature-Insensitive Hydrogen Sensor With Polarization-Maintaining Photonic Crystal Fiber-Based Sagnac Interferometer

We demonstrated a temperature-insensitive hydrogen ( $H_{2}$ ) sensor based on polarization-maintaining photonic crystal fiber (PM-PCF). The sensing element is a section of PM-PCF coated with Pd/Ag composite film and is incorporated into a single mode fiber Sagnac loop interferometer. The birefringence of PM-PCF is modulated by $H_{2}$ absorption that induces deformation of the Pd/Ag composite film, resulting in shift of the interference spectrum at the output of the Sagnac interferometer. A magnetron facing target sputtering technique with special Pd/Ag rectangular target structures is developed to deposit Pd/Ag composite film on the cylindrical surface of the PM-PCF and the atomic ratio of Pd/Ag composite film is controlled by setting appropriate volume ratio in rectangular target. An empirical linear relationship between atomic ratio and volume ratio was obtained and the $H_{2}$ sensing capacity with different atomic ratio of Pd/Ag composite films was investigated experimentally. The results showed that increasing the proportion of Pd in the composite film improves the $H_{2}$ detection sensitivity. The sensitivity can also be improved by using as longer length of Pd/Ag coated PM-PCF. With ∼100 mm long coated PM-PCF as the sensing element, we obtained ∼1.310 nm shift in the interference spectrum for $H_{2}$ concentration from 0 to 1%. The sensitivity coefficient is ∼131 pm% in the range of 1% to 4% $H_{2}$ concentration. The sensor has low temperature dependence and good repeatability.