Polarization-maintaining Fiber Bragg Grating Research Articles

Switchable Dual-Wavelength Thulium-Doped Fiber Laser Based on Polarization-Maintaining Fiber Bragg Grating and Compound Cavity Filter

This paper presents experimental evidence regarding a novel switchable dual-wavelength thulium-doped fiber laser (TDFL). Wavelength switching is achieved by combining a polarization-maintaining fiber Bragg grating (PM-FBG) with a polarization controller (PC). The three-coupler double-ring compound cavity (TC-DRC) structure, acting as a mode-selection filter, is designed to select a single longitudinal mode (SLM) from the dense longitudinal modes. This paper introduces the design and fabrication method of the TC-DRC filter and analyzes, in detail, the mechanism for SLM selection. The experimental results demonstrate that the designed filter exhibits excellent performance. By adjusting the PC, the TDFL achieves stable SLM operation at the wavelengths of 1940.54 nm and 1941.06 nm, respectively. The optical signal-to-noise ratio (OSNR) is superior to 65 dB. When the TDFL is tested at room temperature, there is no significant wavelength drift, and power fluctuations are less than 1.5 dB. The operation of the SLM is verified through the self-heterodyne method, and the laser maintains stable SLM states for both wavelengths after continuous operation for an hour. Furthermore, based on the phase noise demodulation method, the linewidths of both wavelengths are measured to be less than 10 kHz at the integration time of 0.001 s.

Switchable and tunable orthogonal single-polarization dual wavelength SLM fiber laser based on parity-time symmetric system and PM-FBG

A dual-wavelength narrow linewidth orthogonal single polarization fiber laser based on a parity-time (PT) symmetric system is proposed and demonstrated, which achieves the function of tunable and switchable wavelength, and single longitudinal mode (SLM) operation. The PT symmetric system composes of a single physical loop, which includes two polarization controllers (PCs) and a polarization beam splitter (PBS). By modulating PCs to match gain, loss and coupling coefficient, the switchable wavelength and stable SLM laser output have been achieved. Through stretching the polarization maintaining fiber Bragg grating (PM-FBG), the wavelength adjustment range from 1549 nm to 1551 nm is obtained. In the process of tuning, the full width at half maxima (FWHM) linewidths and polarization states at each wavelength have been measured and keep basically stable. The measured FWHM linewidths are less than 13.66 kHz and 11.89 kHz. Further, the stable orthogonal single polarization state has been realized. The degree of polarization (DOP) at each wavelength exceeds 97% and basically remains unchanged during 20 min.

Polarimetric parity-time symmetry fiber laser with switchable and tunable orthogonal single-polarization narrow-linewidth output

Parity-time (PT) symmetry has garnered significant attention in the field of photonics owing to its remarkable properties. High-performance lasers, which require simultaneous gain and loss, serve as an ideal platform for harnessing the exceptional properties of PT symmetry. In this study, we present and experimentally verify a polarimetric PT-symmetry fiber laser that uses a polarization-maintaining fiber Bragg grating (PM-FBG). This design allows for tunable, switchable dual-wavelength operation with narrow linewidth and orthogonal single-polarization characteristics. By converting two equal-length yet distinct polarization states into two spatial subspaces sharing a common wavelength space, we can achieve PT symmetry. Regulating the polarization controllers (PCs) enables interconversion between PT-symmetry phases. During the PT-symmetric phase (or PT exact phase), a dual-wavelength laser is realized with wavelength fluctuations below 0.098 nm and power fluctuations below 2.175 dB. At this stage, light propagates through both ordinary and extraordinary wavelength-spaces. During the PT-broken phase, one waveguide has a net gain and the other incurs a net loss, leading to a single longitudinal mode (SLM) laser output. The wavelength and power fluctuations of left-wavelength λ1 and right-wavelength λ2 remain below 0.002 nm and 0.770 dB over a 20-minute period, during single-wavelength operation. By adjusting the axial strain on the PM-FBG, the laser output can be tuned within the wavelength range of 1549.754 nm to 1551.690 nm, while maintaining a linewidth variation of less than 0.130 kHz. As the wavelength is adjusted, the azimuth sum of λ1 and λ2 consistently remains close to 90 degrees, accompanied by degrees of polarizations exceeding 99%.

Femtosecond laser-inscribed polarization-maintaining fiber Bragg grating using line-by-line technique for generating wavelength-switchable erbium-doped fiber lasers

In this study, a polarization-maintaining fiber Bragg grating (PM-FBG) was inscribed by a femtosecond laser using a line-by-line method through fiber coating and used as a wavelength selector for a wavelength-switchable erbium-doped fiber laser (EDFL). The PM-FBG period was 1.075 μm, and the grating length was 3200 μm. For the proposed EDFL, the lasing threshold was 64 mW, and switchable lasing emissions with stable single and dual wavelengths were realized by adjusting a polarization controller. For the experiments, different single-wavelength switchable lasers with scope values of 1563.34–1564.02 nm were generated, six dual-wavelength lasers were obtained, and a triple-wavelength laser output was realized. The power fluctuations of the single- and dual-wavelength lasers were lower than 0.331 and 0.682 dB, respectively, over a 25 min monitoring time.

Tunable and switchable dual-wavelength SLM narrow-linewidth fiber laser with cascaded passive double-ring cavity narrowband filters

In this article, we have proposed and demonstrated a tunable, switchable dual-wavelength linear cavity fiber laser, which has a passive dual ring cavity structures (DRCs) and polarization-maintaining fiber Bragg grating (PM-FBG), and can be used for the operation of single-longitudinal mode(SLM), narrow-linewidth, and orthogonal single polarization. The passive DRCs, consisted of four 2 × 2 optical couplers (OCs), work as the narrow-band filter and mirror. The tunable and switchable wavelength of the fiber laser is realized by stretching PM-FBG and adjusting the polarization controller (PC). The optical signal-to-noise ratio (OSNR) at the dual-wavelength operating state can be more than 45 dB in the tunable range. While the OSNR of fiber laser at the each-wavelength operating state can reach 50 dB with the maximum wavelength and power fluctuation of 0.01 nm and 1.2 dB, during the monitoring time of 40 min. In addition, the measured laser linewidths are 375 Hz and 420 Hz, respectively. Meanwhile, the states of polarization are orthogonal single polarization, and the degree of polarization (DOP) is close to 100 %.

Temperature-independent multi-parameter sensor based on polarization maintaining fiber Bragg grating

Dynamic multi-parameter detection is of great significance in predicting fatigue damage to structures such as tunnels, bridges, and pipelines. Developing a high-sensitivity, environmentally friendly, low-cost, and easy-to-operate multi-parameter dynamic detection technology has always been the goal of the industry. The polarization-maintaining fiber Bragg grating (PM-FBG) has a special grating structure composed of fiber Bragg grating (FBG) directly written into high birefringence and polarization-maintaining fiber, and it supports two distinct polarization eigenmodes with two effective refractive indices. The PM-FBG couples the light beams polarized along the two principal axes corresponding to slow axis and fast axis at two different Bragg wavelengths. The two peaks of PM-FBG have different responses to external changes, which may be used to solve the cross-sensitivity problem of FBG sensor and realize the simultaneous multi-parameter measurement of the temperature, longitudinal strain, transverse strain, or twist. In order to solve the problems of complex structure and principle and high production cost of FBG-based multi-parameter sensors, a novel multi-parameter fiber-optic sensor with high sensitivity and temperature independence is designed based on PM-FBG in this work. The PM-FBG sensor proposed can simultaneously measure the changes of displacement and twist in two vertical directions at a certain point and has the function of temperature self-compensation. The external structure of the sensor is fabricated by using three-dimensional printing technology through the fused deposition method and the raw material for creating different components through using polylactic acid. Experimental results show that the fast axis and slow axis of the sensor have different temperature responses, with linear sensitivities of 11.4 pm/℃ and 10.6 pm/℃, respectively, and the temperature compensation coefficient and average torsional sensitivity of the PM-FBG sensor are 0.8 pm/℃ and 0.20 dB/(°), respectively. The fast axis and slow axis of the PM-FBG sensor have the same response to displacement, with a sensitivity of 31.5 pm/mm and an adjustable range of 0–20 mm. The sensitivity to displacement, torsion, and temperature sensitivities of the sensor are all superior over those of commercial FBG sensors. By changing the temperature field around the sensor, its displacement- and torsion-sensing performances are not affected, thereby realizing the temperature self-compensation. Consequently, the proposed sensor has potential applications in the multi-parameter dynamic detection due to its simple structure, high sensitivity, good mechanical strength, and low cost.

Single-frequency linearly polarized Q-switched fiber laser based on Nb2GeTe4 saturable absorber

We report a single-frequency linearly polarized Q-switched fiber laser based on an Nb2GeTe4 saturable absorber (SA). The Nb2GeTe4 SA triggers passive Q-switching of the laser, and an un-pumped Yb-doped fiber together with a 0.08-nm-bandwidth polarization-maintaining fiber Bragg grating (FBG) acts as an ultra-narrow bandwidth filter to realize single-longitudinal-mode (SLM) oscillation. The devices used in the laser are all kept polarized, so as to ensure linearly polarized laser output. Stable SLM linearly polarized Q-switching operation at 1064.6 nm is successfully achieved, producing a laser with a shortest pulse width of 1.36 μs, a linewidth of 28.4 MHz, a repetition rate of 28.3 kHz–95.9 kHz, and a polarization extinction ratio of about 30 dB. It is believed that the single-frequency linearly polarized pulsed fiber laser studied in this paper has great application value in gravitational wave detection, beam combining, nonlinear frequency conversion, and other fields.

Highly stable and wavelength-switchable single-longitudinal-mode Tm3+-doped fiber laser with a specially-designed tri-coupler dual-ring filter

A single-longitudinal-mode (SLM) Tm3+-doped fiber laser (TDFL) with a narrow linewidth, less than 8 kHz, is proposed and demonstrated. A superimposed polarization-maintaining fiber Bragg grating (SI-PM-FBG), a specially-designed tri-coupler dual-ring (TCDR) cavity filter, and a section of unpumped Tm3+-doped fiber are combined to realize output lasing with SLM operation for each wavelength channel in 2 µm band. The design, fabrication, and characterization of the TCDR filter are demonstrated in detail. The proposed four-wavelength-switchable TDFL can be switched among fifteen lasing states, including four single-wavelength, six dual-wavelength, four triple-wavelength, and one quadruple-wavelength operations by adjusting the drop-in polarization controller. The experimental results show that the four SLM single-wavelength lasings are all with a relative intensity noise below − 113.06 dB/Hz at frequencies above 2 MHz, and the wavelength and output power fluctuation below 0.02 nm and 0.40 dB whihin 60 min, respectively. Four lasing wavelengths at 2048.28, 2048.71, 2049.05, and 2049.52 nm exhibit good wavelength and power stability and can maintain SLM operation.

An actively Q-switched multi-wavelength ytterbium fiber laser with cascaded polarization-maintaining Bragg gratings

In this paper, we investigate the multi-wavelength lasing characteristics of an actively Q-switched ytterbium-doped fiber laser with two cascaded polarization maintaining fiber Bragg gratings (PM-FBGs). By properly adjusting the polarization states in the cavity, a total of 15 wavelength combinations, including single-, dual-, tri-, and quad-wavelength emission states, have been achieved. Stable Q-switched pulses with repetition rates ranging from 10 to 28 kHz are obtained. The influence of repetition rate and pump power upon the pulse width are also investigated. The high spectral stability in lasing operation indicates effectiveness of the proposed cascaded PM-FBGs in actively Q-switched multi-wavelength fiber laser configurations.

Optical Phase/Frequency Demodulation Using Polarization-Maintaining Fiber Bragg Gratings

Conventional techniques for demodulating phase/frequency-modulated (PM/FM) optical signals either require a sophisticated frequency-stable laser or employ a discriminator with poor characteristics. Here, we present a simple, compact, and robust technique featuring high linearity over a wide bandwidth and low background noise. Our technique exploits the reflection characteristics of fiber Bragg gratings written in polarization-maintaining fibers to create a frequency discriminator, which is able to convert PM/FM signals into intensity-modulated (IM) signals. A simple theoretical analysis is presented to highlight the advantages of the scheme. Experimentally, the transfer characteristics of the demodulator are characterized, yielding a linear FM-to-IM conversion slope of 1.238 mV/Hz in a 0.2-nm (∼25 GHz) bandwidth. Highly linear operation up to an FM-modulation index of 1.2 has been achieved. Background-free operation is shown by demonstrating negligible carrier leakage and a 10-dB IM rejection. Finally, high fidelity in demodulating arbitrary PM/FM optical signals is demonstrated with a correlation coefficient of 0.99 and a confidence of almost 100%. This simple yet efficient technique can potentially find applications in optical communications, microwave photonics and radar/lidar technologies.

Simultaneous Strain and Temperature Discrimination in 18650 Li-ion Batteries Using Polarization-Maintaining Fiber Bragg Gratings

In this work, a fiber Bragg grating (FBG) sensor network inscribed in a polarization-maintaining (PM) fiber is proposed to proceed with a multipoint simultaneous temperature and strain discrimination in different locations (positive and negative terminals, and middle) on a cylindrical Li-ion battery. The birefringence property of the PM fibers, together with FBG sensors, allowed such an application using only one fiber line fixed to the edges of the battery. The battery was subjected to two different charge/discharge cycles, one with nominal charging and discharging conditions (1.00 C and 1.13 C, respectively) and another with abusive conditions (1.88 C for charge and 2.39 C for discharge). The PM-FBG sensors registered maximum temperature and strain variations at the end of the abusive discharge process of the battery; the positive terminal achieved a 28.7 ± 0.3 °C temperature variation, while the center achieved 221 ± 10 με strain variation. The results indicate a different strain variation behavior in the middle location when compared to the negative and positive terminals, as well as a higher temperature variation in both terminals when compared to the middle location. The use of PM-FBG sensors successfully demonstrates their feasibility in locally tracking and discriminating strain and temperature shifts in a battery surface. To our knowledge, this is the first study using the application of PM-FBG sensors to monitor and discriminate critical safety parameters in Li-ion batteries.

Multi-wavelength thulium-doped fiber laser at 2.05 μm incorporating a superimposed polarization-maintaining fiber Bragg grating

A switchable multi-wavelength thulium-doped fiber laser with a high optical signal-to-noise ratio (OSNR) and stability is proposed and experimentally demonstrated at 2.05 μm. For the first time, a superimposed polarization-maintaining fiber Bragg grating (SI-PM-FBG) was utilized as a multi-channel narrow bandwidth filter to select the lasing wavelengths in the laser cavity in the 2-μm band. It also introduced polarization hole burning (PHB) to inhibit the strong wavelength competition induced by homogeneous gain broadening in the thulium-doped fiber. By adjusting the two polarization controllers (PCs), four stable switchable single-wavelength operations were obtained. The OSNR reached 72 dB at a pump power of 2.89 W, and the maximum wavelength and power fluctuations were 0.01 nm and 0.162 dB, respectively. Four combinations of dual-wavelength operations were obtained by adjusting the PCs. The polarization states between each dual-wavelength lasing were orthogonal. The stability of the dual-wavelength operation was investigated in detail. The proposed laser also has the capability for three- or four-wavelength operation. The excellent performance enables this fiber laser to be used in atmospheric sensing and free-space optical communication.

Switchable multi-wavelength mode-locked linear-cavity fiber laser of cylindrical vector beam

We have proposed and demonstrated a switchable multi-wavelength passively mode-locked linear-cavity fiber laser to generate cylindrical vector beam (CVB) pulses by using a semiconductor saturable absorption mirror (SESAM). A home-made broadband long-period fiber grating (LPFG) and two cascaded polarization-maintaining fiber Bragg gratings (PMFBGs) are used as mode converter and wavelength selectors, respectively. This fiber laser can be switched among four different single-wavelength and two different dual-wavelength lasing oscillations with CVB output. The laser has a mode-locking threshold value of 33.55 mW and delivers CVB pulses with duration of hundreds of picoseconds. The purity of the CVB is measured to be 94.1%. This laser can have some important applications in mode division multiplexed (MDM) systems and wavelength division multiplexed (WDM) systems in the future.

Switchable and tunable dual-wavelength passively mode-locked fiber laser

We demonstrate a switchable and tunable dual-wavelength passively mode-locked fiber laser in a compact linear cavity. Two cascaded polarization-maintaining fiber Bragg gratings (PM-FBGs) are used as wavelength-selective components, and a semiconductor saturable absorber mirror (SESAM) is used as a mode-locked component. Four kinds of switchable single-wavelength and two kinds of switchable dual-wavelength mode-locked operations are achieved in 1.0-μm band under lower pump power condition. Through temperature control, we obtain the tunable single-wavelength and dual-wavelength mode-locked operations, and the tunable wavelength range is about 0.5 nm when increase the temperature from 25 °C to 80 °C.

Switchable single- and dual-wavelength thulium/holmium co-doped fiber laser with single-longitudinal mode output

A single-longitudinal-mode ring thulium/holmium co-doped fiber laser (THDFL) possessing the switchable single- and dual-wavelength outputs is proposed with two subring cavities and two polarization-maintaining fiber Bragg gratings. The switching operation is realized by tuning the pump power and polarization controller together. When the THDFL working at single-wavelength output state, the optical signal-to-noise ratio (OSNR) is ∼51 dB, and the maximum power and wavelength fluctuation within 1 h are 0.804 dB and 0.04 nm, respectively. Using the phase noise analysis-based linewidth measurement system, the linewidth is measured to be 0.16 MHz with a measuring time of 0.002 s. When the THDFL working at dual-wavelength output state, the OSNR is ∼55 dB, and the maximum power and wavelength fluctuation within 1 h are 0.802 dB and 0.05 nm, respectively. The proposed THDFL may find wide applications in microwave photonics generation and optical fiber laser sensor.

Transverse strain measurements in polymer-embedded, polarization-maintaining Fiber Bragg Gratings

Fiber Bragg gratings (FBGs) have numerous advantages over traditional foil strain gages, most notably in that they can be embedded in host materials and used to evaluate local deformation. Among the wide variety of grating architectures that have been studied, FBGs inscribed in polarization-maintaining (PM) or other birefringent fibers provide a unique, quantifiable response to strain transverse to the fiber axis. In this work, we show that a PM FBG can accurately predict the asymmetry in transverse strain of a host material in embedded configurations. We do this by diametrically loading an epoxy cylinder that has a uniform PM FBG embedded in the center. We use elasticity and finite element modeling (FEM) to correlate the mechanical response of an embedded fiber with that of the host. An analytical solution of the nominal relationship between Bragg wavelength and strain is included for this loading condition.

Temperature-sensing characteristics of polarization-maintaining fiber Bragg grating inscribed directly by 800-nm femtosecond laser pulses

Abstract A temperature-sensing system based on a polarization-maintaining fiber Bragg grating (PM-FBG) with dual-wavelength peaks was proposed and demonstrated experimentally. First-order bow-tie- and panda-type PM-FBGs with 3-mm gratings were inscribed by a 800-nm femtosecond laser by the point-by-point method, in which the laser was focused onto the fiber core through the fiber coating and cladding. The FBG arrays were directly fabricated by the femtosecond laser writing technique. The PM-FBGs with dual-wavelength reflection-spectrum peaks were scanned by a cost-effective interrogator system using modulated-grating Y-branch (MG-Y) tunable laser to sample the wavelength center position. In the temperature-sensing experiment, the sensitivity of the bow-tie-type PM-FBG exceeded 14 pm/°C within the 30–90 ℃ range. The proposed PM-FBGs and its interrogation system can be widely used in the field of fiber sensing and fiber laser.

Ultrafine Frequency Linearly Tunable Single-Frequency Fiber Laser Based on Intracavity Active Tuning

An ultrafine frequency linearly tunable single-frequency fiber laser based on the intracavity active tuning is demonstrated. Single-frequency operation can be achieved by an ultrashort cavity and ultranarrow polarization-maintaining fiber Bragg grating. A sub-nanometer-accuracy piezoelectric ceramic actuator with closed-loop mode has been employed to realize the cavity-length active tuning scheme, which can achieve a linear tuning range of >1 GHz with a 3.2-MHz/nm tuning resolution. The periodic tuning repeatability and repeat accuracy show that the scheme can realize linear frequency tuning with great tuning repeatability and high tuning precision.

Switchable and tunable multi-wavelength emissions in pulsed ytterbium fiber lasers with black phosphorus saturable absorbers and polarization-maintaining fiber Bragg gratings

In this paper, we present a systematic study of ytterbium-doped pulsed fiber lasers, both Q-switched and mode-locked, that emitted in switchable and tunable multi-wavelength states. The pulsed lasers were initiated by black phosphorus saturable absorbers (BP-SAs) prepared by an optically driven process. One relatively thicker BP-SA with a modulation depth of 16.8% and a saturable intensity of 13.09 MW/cm2 was used for Q switching, and one thinner BP-SA with a modulation depth of 6.7% and a saturable intensity of 7.82 MW/cm2 was used for mode locking. The switchable and tunable wavelength-emitting status was achieved by a Bragg grating written in polarization maintaining fibers (PM-FBG). The fabricated fiber Bragg grating had two reflection peaks. By adjusting an in-cavity polarization controller, the wavelength-emitting status of the pulsed lasers was flexibly switched in three discrete states: individual wavelength of 1063.8 and 1064.1 nm, respectively; and simultaneously emitting both wavelengths with tuning capability. Stable Q-switched and mode-locked laser pulses were observed with all three wavelength-emitting states. Furthermore, by applying stresses at the fast or slow axis of the utilized PM-FBG, the wavelength spacing could be tuned from 0.02 to 0.52 nm in the dual-wavelength emission state.

Monitoring the failure forms of a composite laminate system by using panda polarization maintaining fiber Bragg gratings.

In this paper, the property of panda polarization maintaining fiber Bragg gratings (PPM-FBGs) embedded in a composite laminate system (CLS) under a transversal force from 1 × 105 Pa to 5 × 105 Pa is explored. Both the wavelengths shift and the rotation angle of the principal axes of the PPM-FBGs are surveyed theoretically. We investigate the corresponding relation between the direction of external force and the rotation angle of principal axes of the PPM-FBGs and prove that the magnitude and direction of the strain distribution in the CLS can be monitored simultaneously, which can realize the detection of interlaminar damage of the CLS. We find that the strain, which corresponds to the shift of wavelengths of the PPM-FBGs, has a different angular dependence, and therefore the potential failure forms of the CLS can be differentiated.