Raman Fiber Laser Research Articles

Wavelength tunable Raman fiber laser based on Raman gain spectrum control

Wavelength tunable Raman fiber laser based on Raman gain spectrum control

Polarization Dynamics of Narrow Spectral Modes of a Random Distributed Feedback Raman Fiber Laser

Polarization Dynamics of Narrow Spectral Modes of a Random Distributed Feedback Raman Fiber Laser

Broadband tunable Raman fiber laser with monochromatic pump.

Raman fiber laser (RFL) has been widely adopted in astronomy, optical sensing, imaging, and communication due to its unique advantages of flexible wavelength and broadband gain spectrum. Conventional RFLs are generally based on silica fiber. Here, we demonstrate that the phosphosilicate fiber has a broader Raman gain spectrum as compared to the common silica fiber, making it a better choice for broadband Raman conversion. By using the phosphosilicate fiber as gain medium, we propose and build a tunable RFL, and compare its operation bandwidth with a silica fiber-based RFL. The silica fiber-based RFL can operate within the Raman shift range of 4.9 THz (9.8-14.7 THz), whereas in the phosphosilicate fiber-based RFL, efficient lasing is achieved over the Raman shift range of 13.7 THz (3.5-17.2 THz). The operation bandwidths of the two RFLs are also calculated theoretically. The simulation results agree well with experimental data, where the operation bandwidth of the phosphosilicate fiber-based RFL is more than twice of that of the silica fiber-based RFL. This work reveals the phosphosilicate fiber's unique advantage in broadband Raman conversion, which has great potential in increasing the reach and capacity of optical communication systems.

High Energy Singular Beams Generation From a Dissipative Soliton Resonance Raman Fiber Laser

We demonstrated the high energy 1.65 μm singular beam generation from a dissipative soliton resonance (DSR) all-fiber Raman laser experimentally. By utilizing advantages of the high nonlinearity and home-made mode-selective coupler (MSC), different DSR pulsed singular beams, including cylindrical vector beam (CVB) and vortex beams (VB), have been generated experimentally. The maximum pulse energy can be enhanced up to 698.19 nJ with high signal to noise ratio of 80 dB by optimizing the intra-cavity nonlinearity. Our research may provide a new method to generate high energy pulsed singular beams with a flexible operation wavelength.

Modeless Raman fiber laser

The discrete multi-longitudinal mode structure and temporal periodic fluctuation are the intrinsic features of conventional lasers. However, longitudinal mode spacing limits the maximum resolution in high-resolution laser sensing systems. In addition, temporal periodic fluctuation reduces the security of secure communication and deteriorates the randomness in fast physical random bit generation. Therefore, it still remains an open challenge to realize a laser source without discrete longitudinal mode and temporal periodic fluctuation. Here, a modeless Raman fiber laser (RFL) with high efficiency was demonstrated. The output cavity mirror adopts a fiber Bragg grating with ultralow reflectance of −27dB. Due to the modulation instability, the discrete multi-longitudinal modes in the RFL gradually broaden with the increase of intracavity Stokes wave power and eventually fully overlap. At high power levels, the RFL no longer has discrete longitudinal modes like a conventional laser but generates a quasi-continuous spectrum. Benefiting from the modeless nature, the RFL can achieve extremely low relative intensity noise, favorable temporal stability, and rather low coherence. Modeless RFLs are expected to be ushered as ideal light sources into secure communication, optical sensing, and optical imaging.

Spectral characteristic of multi-wavelength random fiber laser using a microfiber knot resonator

We demonstrate a U-band multi-wavelength random Raman fiber laser (RRFL) based on a microfiber knot resonator (MKR). The RRFL has a forward-pump half-open cavity, wherein a 10-km single mode fiber provides both Rayleigh backscattering feedback and Raman gain. A MKR with a 0.18 nm free spectral range is used as the broadband comb filter. Up to 40 and 38 wavelength channels within 3 dB bandwidth were achieved from the intracavity and the end of the RRFL, respectively. The laser showed a good stability with maximum 0.38 and 0.1 dB peak power fluctuation within an hour at the two outputs, respectively. The spectral evolution with two envelopes was observed, and the impact of the MKR was discussed. The MKR is a small-size all-fiber and wavelength-insensitive broadband filter, which suits well with the broadband operation of the RRFL. The proposed RRFL has a simple structure and good potential tunability and provides guidance for flexible multi-wavelength lasers in the U-band and other wavebands, which have great potential in applications.

Laser operation at 1.2 µm in Ho3+-doped ZBYA glass fibers.

Powerful 1.2-µm laser operation was produced in Ho3+-doped single-cladding, in-house fabricated ZrF4-BaF2-YF3-AlF3 (ZBYA) glass fibers. The fibers were fabricated based on ZBYA glass with a composition of ZrF4-BaF2-YF3-AlF3. Pumped by an 1150-nm Raman fiber laser, the maximum combined laser output power emitted from both sides of a 0.5-mol% Ho3+-doped ZBYA fiber was 6.7 W, with a slope efficiency of 40.5%. We also observed lasing at 2.9 µm with an output power of 350 mW, which was ascribed to the transition of Ho3+:5I6 → 5I7. The effect of rare earth (RE) doping concentration and the length of the gain fiber were also investigated to determine their effect on laser performance at 1.2 µm and 2.9 µm.

Maximum Pump Power Coupled in Raman Resonator for Maximum Power Delivered at 1115 and 1175 nm

In this report, we present our analysis of the relationship between critical power and stimulated Raman scattering in Raman fiber lasers. Through our research, we have established a connection between the R.G. Smith constant at critical power and the necessary pump power required to reach the maximum power delivered by the first Stokes just prior to the generation of the second Stokes. In our experiments, two setups were successful in reaching the second Stokes generation, one utilizing a glass–air interface as the output coupler without HR mirrors and the other using HR-FBGs for both Stokes in conjunction with a glass–air interface. We found that the 1 Km 1060-XP fiber has an R.G. Smith constant of ~4.94 at critical power, which when multiplied by 2 gives ~9.88, a value close to the R.G. Smith constant (9.75) for maximum Stokes corresponding to a pump power of 5.5 W, with an approximation of ~98.6%. Our results demonstrate the importance of knowing the R.G. Smith constant at critical power in estimating the necessary pump power to achieve maximum power delivery in any Stokes component.

Widely tunable continuous-wave visible and mid-infrared light generation based on a dual-wavelength switchable and tunable random Raman fiber laser

Nonlinear frequency conversion of wavelength agile and high-power random fiber lasers can provide a promising way to generate continuous-wave (CW) visible and mid-infrared (MIR) light with unique properties such as the continuous modeless spectrum, low temporal/spatial coherence, and high temporal stability. Here, we report a dual-wavelength switchable and tunable random Raman fiber laser (RRFL) based on a phosphosilicate fiber that has two Raman gain peaks for the first time and demonstrate its superior capability to generate widely tunable CW visible and mid-infrared light via nonlinear frequency conversions. By using the combination of a tunable pump and two tunable gratings in Littrow configuration that can provide separated point feedback for the two Stokes wavelengths corresponding to silica- and phosphorus-related Raman peaks, the spectrum of an RRFL can be flexibly manipulated for the aim of nonlinear frequency conversions, including single-wavelength tunable emission at the 1.1 μm or 1.2 μm band for second-harmonic generation (SHG), dual-wavelength simultaneously tunable emission at the 1.1 μm and 1.2 μm bands for the sum-frequency generation (SFG), and dual-wavelength separation tunable emission for difference-frequency generation (DFG). As a result, with the combination of SHG and SFG in a periodically poled lithium niobate crystal array, we experimentally demonstrate the broadest tuning range (560–630 nm) of visible light generated from an RRFL, to the best of our knowledge. The tunable MIR light in the range of 10.7–12.3 μm is also demonstrated through DFG of an RRFL operating in separation tunable dual-wavelength emission mode in a BaGa4Se7 (BGSe) crystal, which is the first realization of >10 μm CW DFG in the BGSe crystal. We believe the developed dual-wavelength switchable and tunable RRFL can provide a new compact, robust, and cost-effective platform to realize broadly tunable light in both the visible and MIR regions, which can also find potential applications in imaging, sensing, and temporal ghost imaging in various spectral bands.

Cladding-pumped Raman laser with M2 of 1.3 and 400 µJ first Stokes energy

Cladding-pumped Raman lasers have often been cited for their potential for brightness enhancement, but so far have never achieved high power and beam quality simultaneously. By utilizing a fiber geometry with a larger cladding to core ratio than current high power Raman fiber lasers for brightness enhancement (BE-RFLs), a noise-seeded Raman fiber laser pumped by a 100 ns pulsed laser with record beam quality is achieved with M2=1.3 and 0.4 mJ first Stokes output, with an instantaneous brightness enhancement of 60. The results here support the hypothesis that the limiting factor to high beam quality in BE-RFLs is Raman conversion in the pump cladding, rather than higher order core modes as in most conventional fiber lasers.

Amplification of random lasing enables a 10-kW-level high-spectral-purity Yb-Raman fiber laser.

By amplifying the cascaded random Raman fiber laser (RRFL) oscillator and ytterbium fiber laser oscillator, we present the first, to the best of our knowledge, demonstration of a 10-kW-level high-spectral-purity all-fiber ytterbium-Raman fiber amplifier (Yb-RFA). With a carefully designed backward-pumped RRFL oscillator structure, the parasitic oscillation between the cascaded seeds is avoided. Leveraging the RRFL with full-open-cavity as the Raman seed, the Yb-RFA realizes 10.7-kW Raman lasing at 1125 nm, which is beyond the operating wavelengths of all the reflection components used in the system. The spectral purity of the Raman lasing reaches 94.7% and the 3-dB bandwidth is 3.9 nm. This work paves a way to combine the temporal stability of the RRFL seed and the power scaling of Yb-RFA, enabling the wavelength extension of high-power fiber lasers with high spectral purity.

Mode-modulation-induced high power dual-wavelength generation in a random distributed feedback Raman fiber laser.

An all-fiberized random distributed feedback Raman fiber laser (RRFL) with mode-modulation-induced wavelength manipulation and dual-wavelength generation has been demonstrated, where an electrically controlled intra-cavity acoustically-induced fiber grating (AIFG) is employed to adjust the input modal content at the signal wavelength. The wavelength agility of both the Raman effect and the Rayleigh backscattering in RRFL benefits on broadband laser output in case of broadband pumping. The feedback modal content at different wavelengths can be adjusted by AIFG, and then the output spectral manipulation can be ultimately manifested through the mode competition in RRFL. Under the efficient mode modulation, the output spectrum can be continuously tuned from 1124.3 nm to 1133.8 nm with single wavelength, while ulteriorly the dual-wavelength spectrum can be formed at 1124.1 nm and 1134.7 nm with a signal-noise-ratio of 45 dB. Throughout, the power is beyond 47 W with good stability and repeatability. To the best of our knowledge, this is the first dual-wavelength fiber laser based on mode modulation and the highest output power ever reported for an all-fiberized continuous wave dual-wavelength fiber laser.

45 W single frequency Er:Yb co-doped fiber amplifier at 1530 nm

A high-power continuous-wave single-frequency 1530 nm Er:Yb co-doped fiber amplifier is demonstrated with polarization-maintaining fiber as the gain medium. To achieve the greater than 50% population inversion required for efficient 1530 nm amplification, core pumping with a 1480 nm Raman fiber laser is utilized. The advantage of the 1480 nm core-pumping over the usual 9xx nm diode cladding pumping is proved by numerical simulation. The maximum single-frequency 1530 nm laser of 45.5 W with an amplified spontaneous emission suppression ratio of 46 dB is obtained at an incident 1480 nm laser of 60.7 W, corresponding to an optical efficiency of 74%. To date, this is the highest reported single-frequency 1530 nm laser, which will be used to pump an acetylene-filled hollow-core fiber laser.

1.1–1.6 μm Multi-Wavelength Random Raman Fiber Laser

Multi-wavelength fiber lasers have attracted great attention due to their application value in many fields. In this work, we demonstrated a seven-wavelength random Raman fiber laser in the range of 1.1–1.6 μm. A piece of 1-km-long high Raman gain optical fiber is utilized as the gain medium. The 1st-order to 7th-order Stokes waves are located, respectively, at 1133 nm, 1194 nm, 1260 nm, 1332 nm, 1414 nm, 1504 nm, and 1606 nm. In the 3-dB bandwidth of optical spectra of 1st-order and 2nd-order Stokes waves, four peaks with an average spacing of 1 nm and 20 peaks with an average spacing of 0.45 nm respectively, are recorded. Pumped by a 1080 nm/12.5 W/220 ns laser, the maximum output power can reach 4.16 W, corresponding to the optical-to-optical conversion efficiency of ~30.7%.

Modelling light wave propagation and interaction and dynamical regimes analysis using the Lyapunov exponents method in ring fibre cavities

The ‘Cabaret’ finite differences method allows the performance of fast and conclusive modelling of the microcavity temporal dynamics with group velocity dispersion, cross- and self- phase modulation included. The proposed scheme and model allow us to study the dynamics of a cavity with two counter-propagating pulses, second order dispersion, modulation instability, Rayleigh scattering and other effects along the cavity roundtrips number up to 106…7. The possibility of Lyapunov exponents mapping for the Raman and SBS fibre lasers and cavities is discussed.

364 km ultra-long Raman fiber laser based on high-order pumping and ultra-low-loss fiber

Raman fiber laser (RFL) based on distributed Raman gain along optical fiber is an ideal platform for realizing ultra-long-cavity laser, which is of great interest in fundamental laser science and multidisciplinary fundamental studies such as complex systems exhibiting turbulence-like characteristics. RFL based on standard single-mode fiber sets the record of the longest laser cavity reaching 270 km with a clearly resolvable mode structure. Here, we explore the effect of pumping scheme and key fiber parameters to further extend the ultimate length of RFL. We theoretically analyze and experimentally demonstrate that by adopting higher-order pumping and ultra-low-loss fiber (ULLF) with low transmission loss and low Rayleigh backscattering coefficient, the laser cavity length of RFL with resolvable mode structure can be significantly increased. As a result, a 364 km-long bi-directional pumped 6th-order ultra-long Raman fiber laser (URFL) based on ULLF is demonstrated with resolvable longitudinal mode, an extension of 35 % over the prior 270 km record. This work paves a way to lengthen cavity length of URFL and provides a new platform for fundamental laser physics study and the ultra-long fiber laser applications in communication and sensing.

High-spectral-purity random Raman fiber laser oscillator with full-open cavity counter-pumped by random lasing

Raman fiber lasers with high spectral purity and simple structure are essential for the high-power fiber lasers beyond the conventional emission band of active dopants. In this paper, we demonstrate a compact random Raman fiber laser (RRFL) oscillator with spectral purity of over 99%. The RRFL is generated in a full-open cavity which is counter-pumped by a random ytterbium-doped fiber laser with half-open cavity. They share the same piece of passive fiber to form the cavity, a fiber Bragg grating separates the signal light and pump light for the output. This configuration promises the compactness and high spectral purity of the RRFL. Leveraging the spatiotemporal gain-modulation, the RRFL can provide both continuous-wave output and stable pulsed output through adjusting the pump power. The output laser of continuous-wave operation and pulsing operation exhibits spectral purity of 99.4% and 99.5% around 1134 nm, respectively. This work offers a simple and robust solution for high-spectral-purity Raman fiber lasers of both continuous-wave operation and pulsed operation of hundred-watt level.

1.3/1.4 µm dual-wave band dissipative soliton resonance in a passively mode-locked Bi-doped phosphosilicate fiber laser.

We report the 1.3/1.4 µm dual-wave band dissipative soliton resonance (DSR) in a passively mode-locked bismuth-doped phosphosilicate fiber (Bi-PSF) laser. The low-water-peak Bi-PSF with two bismuth active centers associated with silicon and phosphorus supports the O+E-band gain. Using a 1239 nm home-made Raman fiber laser as pump source and nonlinear amplifying loop mirror for initiating mode-locking, stable DSR operation at 1343 and 1406 nm is achieved with the spectral bandwidth of 12 and 16 nm. The pulse duration with the pump power increases from 62 to 270 ps with a repetition frequency of 4.069 MHz. The average power is 11.05 mW corresponding to the maximum energy of 2.7 nJ. This is, to the best of our knowledge, the first demonstration of a mode-locked fiber laser in the ∼1.38 µm water absorption band and the O+E dual-wave band operation for applications in all-spectral-band communications, bio-medical imaging, and terahertz difference frequency generation.

Progress on mid-infrared glass optical fiber materials and Raman laser source (invited)

Progress on mid-infrared glass optical fiber materials and Raman laser source (invited)

Hundred-watt level high-order mode all-fiberized random distributed feedback Raman fiber laser with high mode purity

An all-fiberized random distributed feedback Raman fiber laser (RRFL) with LP11 mode output at 1134 nm has been demonstrated experimentally, where an intracavity acoustically induced fiber grating is employed for modal switching. The maximum output power of LP11 mode is 93.8 W with the modal purity of 82%, calculated by numerical mode decomposition technology based on stochastic parallel-gradient descent algorithm. To our best knowledge, this is the highest output power with high purity of LP11 mode generated from the RRFL. This work may pave a path towards advanced fiber lasers with special temporal and spatial characteristics for applications.