High-birefringence Fiber Bragg Grating Research Articles

1.94 & 2.04 µm dual-band four-wavelength-switchable single-frequency narrow linewidth fiber laser using a novel fs-laser direct-written polarization-dependent four-channel filter

We present a dual-band four-wavelength-switchable single-frequency thulium-holmium co-doped fiber laser (THDFL) utilizing a novel band-switchable polarization-dependent four-channel filter (BS-PDFCF). The filter incorporates two high-birefringence fiber Bragg gratings (HB-FBGs) and an electrical-controlled optical switch (EC-OS). Specifically designed for operation in the 1.94 μm and 2.04 μm bands, two HB-FBGs are fabricated using femtosecond (fs) laser direct-writing technology in two segments of polarization-maintaining fibers via a point-by-point method. Each HB-FBG features two narrow-band reflection channels with perpendicular polarization directions. The EC-OS switches the operation band of the BS-PDFCF. A 1.2 m long dynamic ultra-narrow-band filter is employed in the THDFL to achieve single-longitudinal-mode oscillation. The THDFL achieves exceptional output performance, including four single-wavelength and two dual-wavelength operations, spectral wavelength and power fluctuations of ≤0.012 nm and ≤0.386 dB, respectively, OSNRs of >76 dB, RINs of ≤−142.48 dB/Hz@≥5 MHz, linewidths of ≤3.59 kHz@1.94 μm band and ≤5.46 kHz@2.04 μm band, and output power fluctuations of ≤0.109 dB in a measurement time span of 30 min.

Eight-Wavelength-Switchable Narrow Linewidth Erbium-Doped Fiber Laser Based on Cascaded Superimposed High-Birefringence Fiber Bragg Grating

A narrow-linewidth eight-wavelength-switchable erbium-doped fiber laser is proposed, and its performance is demonstrated. A cascaded superimposed high-birefringence fiber Bragg grating is used to determine the lasing wavelengths. The combination of a Fabry–Pérot filter and a single-coupler ring is adopted to achieve the single-longitudinal-mode (SLM) oscillation. By introducing the enhanced polarization-hole-burning effect to suppress the gain competition between different wavelength lasers, the stable lasing output is guaranteed. When the pump power is 200 mW, by adjusting the polarization controller to balance the gain and loss in the laser cavity, 24 switchable lasing modes are achieved, including 8 single-wavelength operations and 16 dual-wavelength operations with orthogonal polarization states. For single-wavelength operations, every laser is in the SLM lasing state, with a high stabilized optical spectrum, a linewidth of approximately 1 kHz, an optical signal-to-noise ratio (OSNR) as high as 73 dB, a relative intensity noise of less than −150 dB/Hz, and very good polarization characteristics. For dual-wavelength operations, the lasers also have a stable spectrum and an OSNR as high as 65 dB. The proposed fiber laser has a wide range of applications, including long-haul coherence optical communication, optical fiber sensing, and dense wavelength-division-multiplexing.

Discriminative Monitoring of Seamless Rail Force by a High-Birefringence Effect-Based Fiber Optic Sensing Method

Nowadays, China has developed over 38,000-km high-speed railways (HSR). As one of the key elements in the HSR system, continuous welded rail (CWR) enables higher transportation efficiency, while demands for more robust inspection and maintenance techniques. The discrimination between longitudinal temperature force and longitudinal additional force is one of the main challenges in the rail health status evaluations. In this paper, the high-birefringence fiber Bragg grating (HB-FBG) is applied to reach the rail force discriminative measurement. By inscribing the Bragg grating on a high-birefringence fiber, two kinds of sensing modes can be generated and adopted to realize the discriminative measurement of longitudinal temperature force and longitudinal additional force. In the verification experiments, the longitudinal temperature and additional force are accurately measured with the error less than 1 kN. The accurate-measurement and easy-manufacture of the HB-fiber and HB-FBG make it possible to connect massive sensors in series to reach long-range monitoring of the rails on the land or cross-sea bridges.

Four-wavelength-switchable SLM fiber laser with sub-kHz linewidth using superimposed high-birefringence FBG and dual-coupler ring based compound-cavity filter.

We propose and demonstrate a four-wavelength-switchable erbium-doped fiber laser (4WS-EDFL) with a four-channel superimposed high-birefringence fiber Bragg grating (SI-HBFBG) and a dual-coupler ring based compound-cavity (DCR-CC) filter. Both for the first time, a SI-HBFBG as a four-channel reflective filter is used in a multi-wavelength switchable fiber laser to define wavelength channels and a DCR-CC filter is used to select a single mode from dense longitudinal-modes in a fiber laser. We present in detail how to design, fabricate, and characterize the DCR-CC filter with both theoretical analysis and experimental results, which we believe is the first systematic approach for making a compound-cavity based filter used for selecting single-longitudinal mode (SLM) in a fiber laser. The enhanced polarization hole burning effect in a 2.9 m long erbium-doped fiber, coiled inside a three-loop polarization controller, and the polarization-mismatch-induced losses are introduced into the laser cavity to achieve wavelength-switching operations. We show that the 4WS-EDFL can be switched among fifteen lasing states, including four single-wavelength operations, six dual-wavelength operations, four three-wavelength operations and one four-wavelength operation, all with high stability. For demonstration, in switchable single-wavelength operations, the four SLM lasing outputs measured are all with an optical signal to noise ratio of >80 dB, a linewidth of <700 Hz, a relative intensity noise of ≤-156.7 dB/Hz at frequencies over 3 MHz, an output power fluctuation of ≤0.555 dB and excellent polarization characteristics.

Tunable Dual-Wavelength Thulium-Doped Fiber Laser by Employing a HB-FBG

A novel tunable dual-wavelength thulium-doped fiber laser is demonstrated experimentally. The wavelength-tuning by employing a high birefringence fiber Bragg grating is proposed in 2-μm band lasing for the first time. By adjusting the polarization controller, stable dual-wavelength operation at the wavelengths of 1941.40 and 1942.21 nm is obtained. The optical signal-to-noise ratio is better than 48 dB. A 15.5% slope efficiency is achieved using a 90% output coupling ratio for laser extracting. The wavelength-tuning range in dual-wavelength operation is as wide as 6.93 nm.

194 μm switchable dual-wavelength Tm3+ fiber laser employing high-birefringence fiber Bragg grating

A 1.94 μm switchable dual-wavelength Tm(3+) fiber laser employing two high-birefringence fiber Bragg gratings (HB-FBGs) is demonstrated. The polarization hole burning effect enhanced by the HB-FBG is first observed and adopted to guarantee stable dual-wavelength operation in the spectral region near 2 μm at room temperature. The wavelength spacing between the dual lasing wavelengths is 0.81 nm. The polarization states of the dual-output lasers are orthogonal. By adjusting a polarization controller, a single-wavelength mode operating at one of the dual wavelengths can be selected. The side-mode-suppression ratio of each laser can be greater than 60 dB. The power fluctuation measurement at both operating wavelengths shows that this Tm(3+) fiber laser has good stability.

Novel in-line fiber polarization beam splitter using high-birefringence fiber Bragg grating

This study proposes a novel fiber-type polarization beam splitter (PBS) based on a high-birefringence fiber Bragg grating (Hi-Bi FBG). The reflective and transmitted modes of the Hi-Bi FBG are used to separate the p- and s-waves of an optical signal. The experimental results show that a 36dB extinction ratio can be obtained in the reflective mode. However, in the transmitted mode, the extinction ratio is just 4.78dB since the reflectivity of the Hi-Bi FBG is only 34.75%. It is shown analytically that the extinction ratio in the transmitted mode can be improved to 20dB by specifying a Hi-Bi FBG with a reflectivity of 99%. Finally, it is shown that the Bragg wavelength of the in-line fiber PBS can be tuned electrically to comply with the working wavelength.

Self-compensated high-pressure sensor with a high-birefringence fiber Bragg grating and a bulk modulus

High-pressure measurement is made by demodulating Bragg grating peak splitting caused by transverse stress differences in the core of a high-birefringence (HB) single-mode fiber, which is bonded on the exterior surface of a free active element bulk modulus. The two orthogonally polarized signals reflected from the HB fiber Bragg grating (FBG) can be independently detected with a simple FBG interrogation system, which is based on the light intensity measurement method. In addition, the cross-sensitivity of the FBG sensor can be self-compensated by this method. The measurement principle is introduced, and preliminary experimental results indicate that the measurement sensitivity is estimated to be ~12 pm/N in a linear measurement range from 0 to 120 N.

Simultaneous four-wavelength lasing oscillations in an erbium-doped fiber laser with two high birefringence fiber Bragg gratings

A novel multiwavelength erbium-doped fiber laser configuration is proposed and demonstrated. The laser can produce simultaneous four-wavelength lasing oscillations with a minimum wavelength spacing of only 0.36 nm in C-band via using two fiber Bragg gratings written in high birefringence fiber, while ensuring fairly stable room-temperature operation. The laser can also achieve switching modes among four wavelengths by simple adjustment of two polarization controllers in the cavities. Theconfiguration is based on the polarization hole burning and overlapping cavities principle. The laser has the advantages of simple all-fiber configuration, low cost, high stability and operating at room temperature.

Polarization synthesized optical transversal filter employing high birefringence fiber gratings

Polarization synthesizing is introduced into a Bragg grating based optical fiber transversal filter to realize incoherent operation. High birefringence fiber Bragg gratings are used to produce an optical transversal filter with tunable microwave frequency response and large free spectral range immune to optical coherent noise.