Narrow-linewidth Fiber Research Articles

4.8-μm CO-filled hollow-core silica fiber light source

Mid-infrared (MIR) fiber lasers are important for a wide range of applications in sensing, spectroscopy, imaging, defense, and security. Some progress has been made in the research of MIR fiber lasers based on soft glass fibers, however, the emission range of rare-earth ions and the robustness of the host materials are still a major challenge for MIR fiber lasers. The large number of gases provide a variety of optical transitions in the MIR band. When combined with recent advances in low-loss hollow-core fiber (HCF), there is a great opportunity for gas-filled fiber lasers to further extend the radiation to the MIR region. Here, a 4.8-μm CO-filled silica-based HCF laser is reported for the first time. This is enabled by an in-house manufactured broadband low-loss HCF with a measured loss of 1.81 dB/m at 4.8 μm. A maximum MIR output power of 46 mW and a tuning range of 180 nm (from 4644 to 4824 nm) are obtained by using an advanced 2.33-μm narrow-linewidth fiber laser. This demonstration represents the longest-wavelength silica-based fiber laser to date, while the absorption loss of bulk silica at 4824 nm is up to 13, 000 dB/m. Further wavelength expansion could be achieved by changing the pump absorption line and optimizing the laser structure.

Switchable Dual-Wavelength Fiber Laser with Narrow-Linewidth Output Based on Parity-Time Symmetry System and the Cascaded FBG

In this paper, a dual-wavelength narrow-linewidth fiber laser based on parity-time (PT) symmetry theory is proposed and experimentally demonstrated. The PT-symmetric filter system consists of two optical couplers (OCs), four polarization controllers (PCs), a polarization beam splitter (PBS), and cascaded fiber Bragg gratings (FBGs), enabling stable switchable dual-wavelength output and single longitudinal-mode (SLM) operation. The realization of single-frequency oscillation requires precise tuning of the PCs to match gain, loss, and coupling coefficients to ensure that the PT-broken phase occurs. During single-wavelength operation at 1548.71 nm (λ1) over a 60-min period, power and wavelength fluctuations were observed to be 0.94 dB and 0.01 nm, respectively, while for the other wavelength at 1550.91 nm (λ2), fluctuations were measured at 0.76 dB and 0.01 nm. The linewidths of each wavelength were 1.01 kHz and 0.89 kHz, with a relative intensity noise (RIN) lower than −117 dB/Hz. Under dual-wavelength operation, the maximum wavelength fluctuations for λ1 and λ2 were 0.03 nm and 0.01 nm, respectively, with maximum power fluctuations of 3.23 dB and 2.38 dB. The SLM laser source is suitable for applications in long-distance fiber-optic sensing and coherent LiDAR detection.

Suppression of intensity noise and phase noise for thulium-holmium co-doped fiber laser by self-injection locking

The present study proposes and demonstrates a narrow-linewidth thulium-holmium co-doped fiber laser based on self-injection locking. The laser with single-longitudinal-mode operation is realized using a fiber Bragg grating as a wavelength-selection component and a dual-ring compound cavity as a mode-selection filter. The linewidth was compressed by increasing the photon lifetime by utilizing delay fibers of different lengths in the feedback cavity. The results show that the optical signal-to-noise ratio of the output laser is 57.2 dB, and the relative intensity noise is -130.43 dB/Hz with self-injection locking. The intrinsic linewidth is compressed from 39.45 kHz to 3.21 kHz, a reduction of nearly an order of magnitude. The linewidth compression mechanism solves the problems associated with deep linewidth compression in fiber lasers with long ring cavities and fosters the development of practical and reliable all-fiber structures. The laser source is characterized by low cost, high coherence, and low noise, which are highly desirable features in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing.

Tunable and switchable dual-wavelength SLM narrow-linewidth fiber laser with Lyot filter cascaded by double-ring cavity

An erbium-doped fiber (EDF) laser with single longitudinal mode (SLM), narrow linewidth, and switchable dual-wavelength based on a Lyot filter cascaded dual-ring cavity structure (DRCs) has been designed and constructed. In the experimental system, an Employed passive DRCs enhances the longitudinal mode spacing, while the Lyot filter effectively mitigates mode competition within the laser cavity. The composite filter structure facilitates stable dual-wavelength output while ensuring a consistent SLM output. Then, experimental results reveal optical signal-to-noise ratios (OSNR) with 62 dB for single-wavelength and 55 dB for dual-wavelength. Additionally, the power and wavelength fluctuations for both single-wavelength and dual-wavelength remained stable throughout monitoring periods of 20 min and 1 h, respectively. Meanwhile, a maximum tuning range of 2 nm is achieved by stretching a polarization-maintaining fiber Bragg grating (PM-FBG) using a fiber displacement stage. The linewidths of each wavelength are 2.8 kHz and 2.6 kHz, respectively. During the process of wavelength tuning, the fluctuations of linewidth do not exceed 0.3 kHz and 0.45 kHz, respectively. Simultaneously, the polarization states of each wavelength are measured, which reveals orthogonal single polarization. The relative intensity noise (RIN) is measured below −133 dB/Hz at frequencies above 1 MHz.

2 μm Low-noise Narrow-linewidth Single-frequency Fiber Laser Based on Self-injection Locking Technique

Abstract Based on the self-injection locking technique, we demonstrate a low-noise and narrow linewidth of 2 μm single-frequency distributed Bragg reflection fiber laser with an optical self-injection feedback loop (OSIFL). The OSIFL consists of a piece of delay fiber and two polarization-maintaining optical couplers, which can effectively narrow down the linewidth and suppress the relative intensity noise (RIN) of the laser. It was observed that the laser linewidth was narrowed down from the initial 6.5 kHz to 1.6 kHz while the laser relaxation oscillation peak was reduced from -100.3 dB/Hz to -113.5 dB/Hz. Experimental results demonstrate that by extending the photon lifetime we can effectively narrow down the linewidth and significantly reduce the RIN of the laser. This research is of great significance for promoting the development and application of 2 μm single-frequency lasers.

5 kW power-level 1050 nm narrow-linewidth fiber amplifier enabled by biconical-tapered active fiber.

Effective wavelength extension is vital in the applications of high-power narrow-linewidth fiber lasers. In this work, we demonstrate a 5-kW power-level narrow-linewidth fiber amplifier at 1050 nm utilizing a homemade biconical-tapered Yb-doped fiber (BT-YDF). Up to ∼4.96 kW fiber laser is achieved with a 3 dB linewidth of ∼0.54 nm and a beam quality factor of Mx 2 = 1.46, My 2 = 1.6. The experimental comparisons reveal that BT-YDF has the advantages of improving a stimulated Raman scattering threshold and balancing transverse mode instability suppression in the fiber amplifier. This work could provide a good reference for extending the operating wavelength of high-power fiber amplifiers.

Narrow linewidth fiber laser using compound-ring cavity filter and self-injection locked mechanism

Narrow linewidth erbium-doped fiber lasers (EDFLs) hold in significant potential optical fiber sensing, wavelength multiplexing transmission, and optical fiber communication. However, many EDFLs produced outputs with linewidth in several kHz level. Therefore, a high optical signal-to-noise ratio (OSNR), narrow linewidth single-longitudinal-mode (SLM) EDFL was proposed and investigated in this work. The EDFL utilized a combination of compound-ring cavity filter and a self-injection locked mechanism to generate a stable SLM output with an OSNR greater than 72 dB. The maximum wavelength drift and maximum peak power fluctuation over 60 min were maintained at low values of 0.0041 nm and 0.16 dB. Moreover, a narrow linewidth of 1.82 kHz at a measuring time of 0.002 s was realized. Furthermore, the research also investigated output power variations with different pump powers, relaxation oscillation, and the dependence of output central wavelength on the axial strain.

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.

Evaluation of the impact of weak end feedback on the SBS threshold in high-power narrow-linewidth fiber amplifiers

In this work, we quantitatively investigate the SBS threshold in high-power narrow-linewidth fiber amplifiers seeded with phase-modulated single-frequency lasers in presence of weak end feedback. The impacts of the end feedback and spectral linewidths on the SBS threshold are demonstrated in detail through comparative experiments and numerical simulations, respectively. In the experiment, we have pointed out a practical method to estimate the end reflectivity in high-power fiber amplifiers. Based on this estimation, the SBS threshold characters of the high-power narrow-linewidth fiber amplifiers with different end reflectivity and seed linewidths are investigated. By reducing the end reflectivity, a 2.85 times SBS threshold enhancement has been achieved at the most susceptible linewidth (16.8 GHz). Furthermore, we propose a spectral evolution model to investigate the SBS threshold in high-power narrow-linewidth fiber amplifiers, which is even capable for calculating SBS thresholds of the systems with tens of GHz linewidth while weak end reflection is considered. The simulation results demonstrate that end reflection will obviously affect the SBS threshold when the linewidth of the seed laser is broadened beyond 5 GHz, especially for the spectral linewidth of seed lasers nearing the Brillouin frequency shift. Besides, when the end reflectivity is set to be stronger than -65 dB, the SBS threshold performs a tendency to decline and then rise with the growth of seed linewidth. The experiment and simulation results provide a new optimization sight for the SBS effect suppression in high-power narrow-linewidth fiber amplifiers.

High-efficiency single-frequency DBR fiber laser at 1091 nm utilizing a Yb:YAG crystal-derived silica fiber

A single-frequency distributed Bragg Reflector (DBR) fiber laser operating at 1091 nm was demonstrated by using a Yb:YAG crystal-derived silica fiber (YDSF). The YDSF was prepared via the molten core (MC) method, with a Yb2O3 doping concentration of 5.60 wt.% in the core, resulting in a gain coefficient of 1.45 dB/cm at 1091 nm. Employing 0.8 cm of the YDSF, we attained a single-frequency laser with a maximum output power of 145 mW and a slope efficiency of 31.8%. The laser exhibited an optical signal-to-noise ratio (OSNR) exceeding 71 dB, a linewidth of ∼34 kHz, and a stabilized relative intensity noise (RIN) at -132 dB/Hz for frequencies over 4.5 MHz. The fiber laser could serve as an outstanding seed source for high-power, narrow-linewidth fiber amplifiers operating at 1091 nm.

Modelling and characterization of time–frequency evolution dominated by stimulated Brillouin scattering in narrow-linewidth fiber oscillators

This paper mainly introduces several ways to suppress stimulated Brillouin scattering (SBS) in a narrow-linewidth fiber oscillator and explains how the methods take effect. The experiments are designed to investigate the factors influencing SBS generation in oscillators. Experimental results demonstrate that higher output power, a high reflector (HR) with a narrower linewidth as well as injection of feedback all could help reduce SBS. A theoretical model of SBS-dominated time–frequency evolution in the oscillator is also constructed. Simulations reveal that SBS initially oscillates in the cavity and propagates forward outside the oscillator if the cavity gain is low. However, as the cavity gain increases sufficiently, the SBS would be eliminated through the selection of the output coupler (OC) FBG. Additionally, incorporating external feedback with a wider linewidth and steadier temporal characteristic as a boundary condition of simulation would result in stable ns-scale temporal sequence of the oscillator laser at low power. In fact, not only the stable state of the oscillator with feedback is helpful for SBS suppression but the chaotic state of the oscillator at low power provides a compact and simple structure for multi-order SBS generation which could be useful in multi-parameter sensing, tunable microwave generation and supercontinuum producing.

2 W, 1.5 µm single-mode fiber methane Raman laser pumped by a Yb-doped fiber amplifier.

We report here, to the best of our knowledge, the first 1.5 µm methane-filled fiber Raman laser pumped by a fiber laser. Based on the narrow-linewidth pulsed Yb-doped fiber laser pump source and a 15 m hollow-core fiber filled with 2.5 bar methane, the maximum power of 2.06 W Stokes wave at 1543 nm is obtained. The output laser has a narrow linewidth of 2.3 GHz, and the pulse repetition frequency can be adjusted flexibly. The output shows excellent near-diffraction-limited beam quality with a M2 factor of ∼1.09. This work proves the advantage of the fiber laser pump source with modest peak power and flexible temporal characteristics in 1.5 µm fiber gas Raman laser emission, providing good guidance for generating pulsed fiber source with narrow linewidth and high beam quality.

Tunable narrow linewidth fiber laser based on parity-time symmetric double rings and variable optical attenuator

A tunable narrow linewidth fiber ring laser based on parity-time (PT) symmetric double rings and variable optical attenuator (VOA) is demonstrated. The laser is composed of a fiber loop mirror (FLM), which creates two coupled rings: the reflective ring and the transmissive ring. The loss of the transmissive ring can be controlled by tuning the VOA, which balances with the gain of the reflective ring. When the PT symmetry is broken, a single-longitudinal-mode (SLM) laser is realized. The laser can be tuned by an optical tunable filter (OTF), and the tuning range is 38 nm, spanning from 1525 nm to 1563 nm. In the whole tuning process, the linewidth of the laser is at the sub-kHz level, and the optical signal to noise ratios (OSNR) is over 53 dB. The fluctuation of wavelength and power are less than 0.02 nm and 0.52 dB in an hour, respectively.

Theoretical study of filtered and amplified PRBS phase modulation for SBS suppression in a 2.5 kW, 10 GHz monolithic fiber amplifier

We report a theoretical and experimental study on stimulated Brillouin scattering (SBS) suppression in a monolithic fiber amplifier with filtered and amplified pseudo-random binary sequence (PRBS) phase modulation. Theoretically, we use a time-dependent three-wave coupled nonlinear system considering both active fiber and passive fiber to describe the acoustic phonon, laser, and Stokes characteristics in a fiber amplifier. The SBS threshold power after filtered PRBS phase modulation is numerically evaluated to obtain the optimal parameters, and the time-averaged distributions of the counter-pump power, laser power, and Stokes power at different positions along the fiber length of the fiber system are simulated. Also, we established a four-stage fiber amplifier system to verify our theory. The configuration of the fiber amplifier system includes a filtered and amplified PRBS phase-modulated single-frequency fiber laser, a three-stage pre-amplifier, and a counter-pumping main stage, subsequently. 2.5 kW output power with an FWHM linewidth of 9.63 GHz is accomplished by a domestic ytterbium-doped double-clad fiber with core/cladding diameters of 20.2/400 µm. The reflectivity of the main stage is 0.049‰ at the maximum output power, which indicates the proposed architecture is under the SBS threshold. The experiments verify the accuracy of the theoretical model, which provides a reliable reference for evaluating the SBS suppression capability of the high-power narrow-linewidth fiber amplifier phase modulated by the filtered and amplified PRBS signal.

Stabilized Brillouin laser with sub-Hz fundamental linewidth aided by frequency shifted optical injection locking

Stable laser emission with ultra-narrow linewidth plays an important role in making fundamental scientific breakthroughs. Here, we propose and demonstrate a new technique for the generation of an ultra-narrow linewidth and highly stable laser based on stimulated Brillouin scattering in combination with a frequency-shifted optical injection locking mechanism. The laser performance is characterized via a delayed self-heterodyne interference system, where the white frequency noise floor is ∼20 mHz2/Hz, corresponding to a fundamental linewidth of about 63 mHz. The maximum deviation in the output power is less than 1.5% over more than 10 min. The operation of the laser can be stabilized without the need for active optoelectronic feedback. The scheme presented in this work enables narrow linewidth and stable single-frequency fiber lasers in a robust and efficient way, which has shown promising potential for many applications.

Single-Frequency Ring Fiber Laser with Random Distributed Feedback Provided by Artificial Rayleigh Scattering

Femtosecond (fs) laser inscription technology allows for the production of in-fiber disordered structures with an enhanced level of Rayleigh backscattering with relatively few induced losses. These properties enable the application of these structures as reflectors in fiber lasers. In this study, a narrow-linewidth erbium fiber laser with random distributed feedback provided by a fs-induced random structure in a ring cavity configuration was developed. A single-frequency regime was observed over the entire lasing power range. At a maximum output power of 7.8 mW, the linewidth did not exceed 0.75 kHz.

Brightness enhancement on a narrow-linewidth fiber Bragg grating-based master oscillator power amplification fiber laser

Abstract In this paper, we have experimentally demonstrated a high-power and high-brightness narrow-linewidth fiber amplifier seeded by an optimized fiber oscillator. In order to improve the temporal stability, the fiber oscillator consists of a composite fiber Bragg grating-based cavity with an external feedback structure. By optimizing the forward and backward pumping ratio, the nonlinear effects and stimulated Raman scattering-induced mode distortion of the fiber amplifier are suppressed comprehensively, accompanied with the simultaneous improvement of beam quality and output power. The laser brightness is enhanced further by raising the threshold of transverse mode instability by approximately 1.0 kW by coiling the gain fiber with a novel curvature shape. Finally, a 6 kW narrow-linewidth laser is achieved with beam quality (M2) of approximately 1.4. The laser brightness doubled compared to the results before optimization. To the best of our knowledge, it is the highest brightness narrow-linewidth fiber laser based on a one-stage master oscillator power amplification structure.

Key technology analysis and research progress of high-power narrow linewidth fiber laser based on the multi-longitudinal-mode oscillator seed source

Key technology analysis and research progress of high-power narrow linewidth fiber laser based on the multi-longitudinal-mode oscillator seed source

A 301 W narrow-linewidth in-band pumped Er:Yb co-doped fiber amplifier at 1585 nm and related modeling for dynamics study and optimization

Abstract To overcome Yb lasing, a kilowatt-level 1535 nm fiber laser is utilized to in-band pump an Er:Yb co-doped fiber (EYDF) amplifier. The output power of a 301 W narrow-linewidth EYDF amplifier operating at 1585 nm, with 3 dB bandwidth of 150 pm and ${M}^2$ < 1.4, is experimentally demonstrated. To the best of our knowledge, it is the highest output power achieved in L-band narrow-linewidth fiber amplifiers with good beam quality. Theoretically, a new ion transition behavior among energy levels for in-band pumping EYDF is uncovered, and a spatial-mode-resolved nonlinearity-assisted theoretical model is developed to understand its internal dynamics. Numerical simulations reveal that the reduction in slope efficiency is significantly related to excited-state absorption (ESA). ESA has a nonlinear hindering effect on power scaling. It can drastically lower the pump absorption and slope efficiency with increasing pump power for in-band pumped EYDF amplifiers. Meanwhile, optimized approaches are proposed to improve its power to the kilowatt level via in-band pumping.

5.58 kW narrow-linewidth fiber laser based on house-made confined-doped fiber

5.58 kW narrow-linewidth fiber laser based on house-made confined-doped fiber