Distributed Feedback Raman Fiber Laser Research Articles

Comparative Study of Ultra-Narrow-Mode Generation in Random Fiber Lasers Based on Different Fiber Types

We studied the properties of ultra-narrow spectral modes, appearing in random distributed feedback Raman fiber lasers, for different fibers building up a laser cavity. Fibers with different nonlinear coefficients and dispersion were employed to obtain the generation. Ultra-narrow modes were observed in all fibers except those with the smallest dispersion. We measured the mode parameters, such as the average lifetime, as well as the maximum averaged output power that can support the ultra-narrow generation. The comparison revealed that the modes were more pronounced in high-dispersion fibers. Based on this comparative study, we conclude with the importance of the nonlinearity-dispersion interplay for regime stability.

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

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.

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.

Effect of Linewidth on the Relative Intensity Noise in Random Distributed Feedback Raman Fiber Lasers.

We experimentally explore the relation between spectral linewidth and RIN transfer in half-open cavity random distributed feedback Raman lasers, demonstrating for the first time the possibility of adjusting the pump-to-signal RIN transfer intensity and cut-off frequency by using spectral filtering in the reflector section. We apply this approach to a 50-km laser system, operating in the C-Band, reliant on a standard single-mode fiber. We obtained a minimum bandwidth of 13 pm, which translates into a visible RIN cut-off at 800 MHz.

Short single-frequency self-pulsing Brillouin-Raman distributed feedback fiber laser.

We demonstrate that the stimulated Brillouin scattering of a 250 mm long distributed feedback Raman fiber laser can self-pulse with repetition rates up to 7 MHz, pulse widths of 25 ns, and peak powers of 1.2 W. While both CW and pulsed lasing are produced from a bespoke grating at 1119 nm this laser design could be constructed at almost any wavelength, as the Raman and Brillouin gain regions are relative to the pump wavelength. The laser has a low lasing threshold for a Raman laser of 0.55 W, a peak slope efficiency of 14 %, and a maximum average output of 0.25 W. An investigation of beating between pure Raman and Raman-pumped Brillouin lasing shows that the outputs of the two processes are highly correlated and thus the Brillouin lasing is essentially single-frequency when CW and near transform limited for pulsed operation. A phenomenological model of the Raman-Brillouin interaction shows that the pulsing behaviour of such a cavity is expected and produces very similar pulsing to that the seen in experimental results.

Modal Dynamics in Kilowatt Cladding-Pumped Random Distributed Feedback Raman Fiber Laser With Brightness Enhancement

Random distributed-feedback Raman fiber lasers (RRFLs) are promising contenders in the applications of remote sensing, biotechnology, imaging, thanks to the unique properties on low coherence and wavelength agility. Despite recent advances on RRFL, the lack of brightness enhancement (BE) releasing high requirement on pump brightness remains a significant technological challenge for further power scaling. To this extent, we propose in this paper the first-ever cladding-pumped (CP) RRFL with BE to our best knowledge. The triple-clad purely passive fiber with designed cladding-to-core area ratio is utilized to suppress the generation of higher-order Stokes light. Pumped by two combined Yb-doped fiber lasers at 1080 nm, signal laser with maximum output power of 1 kW at 1132 nm is achieved with an optical-to-optical conversion efficiency of 92% (ratio between signal and absorbed pump power, which is 74% when vs. launched power). Moreover, the modal dynamics indicates the strong mode coupling induced by the complex interaction among the distributed gain, feedback, and other nonlinear effects, leading to the modal instability in the CP RRFL. Even so, this demonstration paves the way for powerful RRFLs beyond kilowatt with BE, which is of interest for diversified potential applications as diverse as industry and security.

Temporally stable fiber amplifier pumped random distributed feedback Raman fiber laser with record output power.

In this Letter, we propose a scheme to use a temporally stable pump source in a high-power random distributed feedback Raman fiber laser (RRFL) with a half-open cavity. Different from conventional pump manners, the pump source is based on an Yb-doped fiber amplifier, seeded by a temporally stable phase-modulated single-frequency fiber laser for suppressing the spectral broadening and second-order Raman Stokes generation in the output laser. Using a piece of 50-m-long 20/400 µm passive fiber, the maximum output power of 1570 W was obtained with a pump power of 2025 W. The conversion efficiency with respect to the pump power was 77.5%. To the best of our knowledge, this is the highest output power ever reported in a RRFL to date. This work could provide a novel method for power scaling of RRFLs.

Self-gain-modulation random distributed feedback Raman fiber laser with switchable repetition rate.

We experimentally demonstrate a pulsed operation in a random fiber laser operation via self-gain-switching. The pulses have low timing jitter and high average output power. We show that repetition rate switches abruptly while varying the pump power, and introduce a simple formula for oscillation frequencies.

Gratings with longitudinal variations in coupling coefficients: super-efficiency and unidirectionality in distributed feedback Raman fiber lasers

We propose a technique to design highly-efficient and -unidirectional DFB Raman fiber lasers based on the engineering of the grating’s coupling coefficient including a π-phase shift position in the fiber. For this purpose, first the ideal intra-cavity signal powers for different pump power levels are determined for given fiber lengths. Then, the sum and difference between the counter-propagating wave intensities at each small segment within fiber lengths are calculated resulting in determining the ideal grating’s coupling functions for co- and contra-directional-pumping. The steady-state behavior of the laser using realistic parameters is finally simulated for modified coupling functions considering the Kerr nonlinearity. For a 10 W co-directional-pumped, ∼1 m long single-mode super-efficient DFB, a ∼50% increase in the laser efficiency, more than 44 dB reduction in contra-directional lasing power, ∼15 times drop in the peak power of intra-cavity signal and ∼38% decrease in the unused-pump power are found, compared to those in a standard DFB with the same coupling-length product. Although an enhanced nonlinear refractive index due to thermal gradient reduces the output power of such lasers, it is shown that the super-efficient laser presents a better performance than the standard one, under such conditions.

Dual-wavelength random distributed feedback fiber laser with wavelength, linewidth, and power ratio tunability.

Owing to the special power distribution property, a random distributed feedback Raman fiber laser can achieve a high power spectrally flexible output with a low power spectrally tuning device. Here, an all-fiberized linearly polarized dual-wavelength random distributed feedback Raman laser with wavelength, linewidth, and power ratio tunability is demonstrated. By adopting two watt-level bandwidth adjustable optical filters, a spectrum-manipulable dual-wavelength output with nearly a 10 W output power is achieved. The wavelength separation can be tuned from 2.5 to 13 nm, and the 3 dB linewidth of the output can be doubled by increasing the bandwidth of the optical filter. The power ratio of each laser line can be tuned from 0 to nearly 100% with the help of two variable optical attenuators. A maximum output power of 9.46 W is realized, with a polarization extinction ratio up to 20.5 dB. The proposed dual-wavelength fiber laser can be employed as a pump source in frequency tunable, bandwidth adjustable terahertz microwave generation, and mid-infrared optical parametric oscillators.

Phosphosilicate fiber-based dual-wavelength random fiber laser with flexible power proportion and high spectral purity.

Phosphosilicate fiber has the inherent advantage of generating dual-wavelength output owing to the two Raman gain peaks at the frequency shifts of ∼13.2 THz (silica-related) and 39.9 THz (phosphorus-related), respectively. The frequency shift of 39.9 THz is often adopted to obtain long wavelength laser, while the control of Stokes light at 13.2 THz has attracted much attention currently. In this paper, a dual-wavelength random distributed feedback Raman fiber laser (RDFL) with over 100 nm wavelength interval and continuously tunable power proportion was presented based on phosphosilicate fiber for the first time. Through using the filtered amplified spontaneous emission (ASE) source as the pump source, the spectral purity of the Stokes light could be as high as 99.8%. By tuning two manual variable optical attenuators (VOAs), the power proportion of the silica-related Stokes light could range from ∼0% to 99.0%, and the maximum value is limited by the generation of second order Stokes light. Although the power handling capability of the VOA is merely 2 W, over 23 W total output power of the Stokes light was obtained thanks to the particular power distribution property of RDFL. This experiment demonstrates the potential to achieve a flexible high-power and high-spectral purity dual-wavelength RDFL output.

Spatial location of correlations in a random distributed feedback Raman fiber laser.

Nonlinear interactions between different components of multiwavelength radiation are one of the main processes shaping properties of quasi-CW fiber lasers. In random fiber lasers, nonlinear influence may be more complicated, as there are no distinct longitudinal modes in radiation because of the random nature of the feedback. In this Letter, we experimentally characterize internal correlations in the radiation of a multiwavelength random distributed feedback fiber laser. An analysis of Pearson correlation functions allows us to spatially locate the area over the fiber laser length in which correlations are more likely to occur. This, in turn, leads us to the conclusion about the main mechanism of spectral correlations-the relative intensity noise transfer from the pump wave.

Efficiency increase of distributed feedback Raman fiber lasers by dynamic control of the phase shift.

π-phase-shifted distributed feedback, ultralong fiber Bragg gratings (FBGs) with Raman gain have been shown to be excellent ultranarrow single-frequency lasers that can be operated at any wavelength. However, these lasers have shown unusually low slope efficiency (1%-10%), while theoretical simulations predict a much higher (30%-60%) number. We believe this poor performance is due to a thermally induced phase shift inside the FBG due to absorption of the high intensity of the signal oscillating in the cavity. To compensate for this, a thermally controlled dynamic phase shift is proposed to increase efficiency after lasing first occurs. We show here an increase in the slope efficiency of a factor of 4 and an increase in the total output efficiency by a factor of 6.5 with 6W of pump power by reducing the phase shift once the laser begins oscillating.

Tapered-fiber-enabled high-power, high-spectral-purity random fiber lasing.

Random distributed feedback Raman fiber laser is a new kind of light source that can be applied to generate a high-power laser. In this Letter, we report on a high-power, high-spectral-purity random Raman fiber laser based on tapered fiber, in which the four-wave mixing (FWM) effect has been sufficiently suppressed. By choosing an appropriate tapered fiber length, we achieve a maximum random laser output of 491W, and the spectral purity can reach to as high as 94%. We carefully compare the influence of different tapered fiber lengths and splicing patterns on the FWM effect by the cutting-back method and lateral-offset splicing. The results show that the transverse modes dispersion is responsible for the appearance of FWM by compensating the phase mismatch. It is believed that a kilowatt-level random laser can be obtained by further optimizing the parameters of tapered fiber.

Engineered <inline-formula> <tex-math notation="LaTeX">$\pi$ </tex-math> </inline-formula>-Phase-Shifted Fiber Bragg Gratings for Efficient Distributed Feedback Raman Fiber Lasers

Distributed feedback Raman fiber lasers with $\pi $ -phase-shifted uniform gratings are modeled and simulated in the steady state, to optimize their performance. Using the parameters of realistic devices, it is found that the position change of the $\pi $ -phase-shift in a constant-strength uniform grating has a significant impact on the laser performance including right and left-hand-side output power and emission frequency, and linewidth. Also, it is shown that the optimum phase-shift position to maximize the laser uni-directionality is dependent on pump power and fiber loss value. A new design approach based on an engineered $\pi $ -phase-shifted step-like-strength uniform grating is presented demonstrating that even for a high-loss fiber (0.1 dB/m), the laser power and linewidth can be efficiently increased and decreased by ~25% and ~15%, respectively, for the same total device length and pumping condition, just by the proper choices of the phase-shift position and two coupling coefficients.

Single-frequency low-threshold linearly polarized DFB Raman fiber lasers.

Distributed feedback (DFB) fiber lasers have been demonstrated to be excellent narrow-linewidth (kilohertz range) single-frequency laser sources. However, this type of laser is normally limited to rare-earth-doped fibers. Raman gain offers an alternative, operating at any arbitrary wavelength. We demonstrate here linearly polarized, single-frequency, DFB fiber Bragg grating Raman lasers with, to the best of our knowledge, the lowest pump threshold of 350 mW, an output power of up to 50 mW at 1120 nm, an 8.5% slope efficiency, and 300 mW of output power at 1178 nm. In the high-power regime, stimulated Brillouin scattering plays an important role in the laser dynamics. We also report the characterization of the power profile inside the fiber along its axis through a novel side-scatter technique.

Linearly-polarized random distributed feedback Raman fiber laser with record power

We demonstrate a linearly polarized random distributed feedback Raman fiber laser with record 86.3 W output power with a piece of 157 m long polarization maintaining (PM) fiber. The maximum optical efficiency of first-order-Stocks light at 1120 nm reaches ~74% while pumping at 1070 nm. The full width at half maximum (FWHM) at maximum output power is ~2 nm and the polarization extinction ratio (PER) is measured to be as high as 22 dB. To the best of our knowledge, this is the highest output power for linearly-polarized random fiber laser.

Nearly-octave wavelength tuning of a continuous wave fiber laser

The wavelength tunability of conventional fiber lasers are limited by the bandwidth of gain spectrum and the tunability of feedback mechanism. Here a fiber laser which is continuously tunable from 1 to 1.9 μm is reported. It is a random distributed feedback Raman fiber laser, pumped by a tunable Yb doped fiber laser. The ultra-wide wavelength tunability is enabled by the unique property of random distributed feedback Raman fiber laser that both stimulated Raman scattering gain and Rayleigh scattering feedback are available at any wavelength. The dispersion property of the gain fiber is used to control the spectral purity of the laser output.

Hundred-watt-level high power random distributed feedback Raman fiber laser at 1150 nm and its application in mid-infrared laser generation.

Two kinds of hundred-watt-level random distributed feedback Raman fiber have been demonstrated. The optical efficiency can reach to as high as 84.8%. The reported power and efficiency of the random laser is the highest one as we know. We have also demonstrated that the developed random laser can be further used to pump a Ho-doped fiber laser for mid-infrared laser generation. Finally, 23 W 2050 nm laser is achieved. The presented laser can obtain high power output efficiently and conveniently and opens a new direction for high power laser sources at designed wavelength.