Incoherently pumped high-power linearly-polarized single-mode random fiber laser: experimental investigations and theoretical prospects

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.

Cascaded Raman fiber lasers with ultrahigh spectral purity

Random distributed feedback (RDFB) cascaded Raman fiber lasers (CRFLs) are simple, wavelength agile, and enable high-power fiber lasers outside emission bandwidths of rare-earth doped fiber lasers. However, the spectral purity, defined as the percentage of total output power in the desired Stokes wavelength band, and relative intensity noise (RIN) of these systems are limited due to the intensity noise of the pump source used for Raman conversion. RIN gets amplified and transferred to Raman Stokes orders which causes incomplete Raman conversion and hence limits the spectral purity. Here, we demonstrate a low-intensity noise (<-100 dBc/Hz from 9 kHz to 10 GHz) CRFL with a record spectral purity of ∼99%, tunable over six Stokes orders, using a very low-intensity noise, narrow linewidth Yb-fiber amplifier as a pump source.

Supercontinuum generated in an all-polarization-maintaining random fiber laser structure

We demonstrated a linearly-polarized supercontinuum (SC) directly generated in an all-polarization-maintaining random fiber laser (RFL) structure. Owing to the comparatively high Raman gain of the polarization-maintaining germanium doped fiber (GDF), the spectrum of the output SC shows an enhanced bandwidth and improved spectral flatness compared to the unpolarized counterpart. The output SC has an average output power of 4.43 W with a spectrum covering from 600 nm to 1900nm. The polarization extinction ratio (PER) is measured to be greater than 18 dB from 800 nm to 1700nm at the highest output power level. To the best of our knowledge, this is the first demonstration of a linearly-polarized SC generated directly from a RFL. This work is meaningful to help further expand the bandwidth of SC generated from a RFL and provides a simple and cost-effective method of generating linearly-polarized SC for practical applications.

Tailoring the efficiency and spectrum of a green random laser generated by frequency doubling of random fiber lasers

Frequency doubling of random fiber lasers could provide an effective way to realize visible random lasing with the spectrum filled with random frequencies. In this paper, we make a comprehensive study on the efficiency and spectral manipulation of a green random laser generated by frequency doubling of an ytterbium-doped random fiber laser (YRFL). To tailor the efficiency of green random lasing generation, the ytterbium-doped random fiber lasing is filtered at different spectral positions, and then amplified to watt-level to serve as the fundamental laser source for frequency doubling in a periodically poled lithium niobate (PPLN) crystal. We found that by selecting different spectral components of ytterbium-doped random fiber lasing, the temporal intensity fluctuations of the filtered radiations vary dramatically, which plays an important role in enhancing the efficiency of frequency doubling. By fixing the filtering radiation wavelength at 1064.5 nm and tuning the central wavelength of YRFL, we experimentally demonstrate that, compared to the filtered radiation in the center of the spectrum, the efficiency of frequency doubling can be nearly doubled by utilizing the filtered ytterbium-doped random fiber lasing in the wings of the spectrum. As a result, the conversion efficiency of the generated green random laser at 532.25 nm can be more than 11% when the input power of the polarized 1064.5 nm fundamental light is 2.85W. For spectral manipulation, we realize a spectral tunable green random laser in the range of 529.9 nm to 537.3 nm with >100 mW output power for the first time by tuning the wavelength of YRFL and the temperature of PPLN simultaneously. The system can be naturally modified to simultaneously realize the efficiency enhancement and wavelength tuning, thus providing a new route to generate high efficiency and tunable visible random laser via frequency doubling that are potentially useful for imaging, sensing and visible light communication applications.

Tunable narrowband cascaded random Raman fiber laser

Random Raman fiber lasers (RRFLs) with half-opened cavity have been used as a new platform for designing high performance, wavelength-agile laser sources in the infrared region due to their intrinsic modeless property and structural simplicity. To provide the point feedbacks for cascaded random Raman lasing at different wavelengths, wavelength-insensitive broadband reflectors are commonly used in cascaded RRFLs, resulting in the rather broad high-order random Raman lasing with several nanometers of typical spectral width. Here, we experimentally demonstrate a tunable narrowband cascaded RRFL with an air-spaced etalon assisted point reflector. To realize narrowband, single- or dual-wavelength emission for each order of random lasing, the etalon is specially designed to have broad operation wavelength range, narrowband transmission lines and large free spectral range (FSR) associated with the Raman frequency shift. As a result, 1st- to 3rd-order random Raman lasing with single-wavelength emission in 1.1-1.27 μm region are generated in a 15 km single mode fiber (SMF) with -3 dB bandwidths below 0.4 nm, which are approximately four times less than those of cascaded RRFL without etalon. The maximum output power of the 3rd-order random Raman lasing is 615 mW, with 10% of optical conversion efficiency. Moreover, a tunable cascaded RRFL is performed by tuning the wavelength of pump laser or tilting the etalon. Dual-wavelength emission for each order of random lasing can also be realized at specific pump wavelengths. We also verified, by employing shorter fiber (10 km), more than 1.5 W output power of high-order RRFL can be achieved with -3 dB bandwidths less than 0.6 nm. To the best of our knowledge, this is the first demonstration of tunable sub-1 nm narrowband cascaded RRFL with single- or dual-wavelength emission for each order of random lasing.

Advances in Random Fiber Lasers and Their Sensing Application

Compared with conventional laser, random laser (RL) has no resonant cavity, reducing the requirement of cavity design. In recent years, the random fiber laser (RFL), a novel kind of RL, has made great progress in theories and experiments. The RFL has a simpler structure, a more flexible design, and higher reliability. It has valuable applications for earth sciences, biological life sciences, and national defense security, due to these unique properties. This paper reviews the development of RFLs in the last decade, including their configurations based on various optical fibers and their output properties, especially the method of control. Moreover, we also introduce their applications in the optical fiber sensing system, which is a very important and practical orientation to study. Finally, this paper presents the prospects of RFLs.

Cascaded telecom fiber enabled high-order random fiber laser beyond zero-dispersion wavelength

Four-wave mixing induced spectral broadening near the zero-dispersion wavelength (ZDW) of the fiber is a bottleneck factor that limits the further wavelength extending in cascaded random fiber lasers (RFLs). In this Letter, we successfully suppress the spectral broadening near the ZDW of the fiber in the cascaded RFL by simply combining two kinds of commercial telecom fibers with different ZDWs, G655C fiber with ZDW around 1.52 µm and G652D fiber with ZDW around 1.31 µm. As a result, an 8th order Stokes light component at 1721 nm with a maximum output power of 2.1 W and a spectral purity of 96.94% is realized in this telecom-fiber-based cascaded RFL. This work provides a reference of nonlinear effect management in fiber lasers as well as affords a cost-effective way with great potential of realizing high-power widely tunable fiber lasers.

Broadband pumping enabled flat-amplitude multi-wavelength random Raman fiber laser

A flat-amplitude multi-wavelength random Raman fiber laser with broad spectral coverage and a high optical signal-to-noise ratio (OSNR) is challenging and of great interest. In this Letter, we theoretically and experimentally proved that broadband pumping can help realize a broader, flat-amplitude multi-wavelength random Raman fiber laser. The influence of pump bandwidth, tunability of the spectral envelope, and channel spacing are investigated. As a result, with a 40 nm pump bandwidth, a spectral coverage of 1116-1125 nm with 19 laser lines and 31 dB OSNR is achieved, and the standard deviation in the peak intensities of the central nine lines is ${\sim}{1}.{1}\;{\rm dBm}$∼1.1dBm. This technique can also be applied to the multi-wavelength Raman (or random Raman) fiber lasers at other wavelengths and provide a reference for multi-wavelength applications in sensing, communication, and optical component testing.

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.

Temporal ghost imaging with random fiber lasers

Ghost imaging in the time domain has opened up new possibilities to retrieve ultrafast waveforms. A pre-requisite to ghost imaging in the time domain is a light source with random temporal intensity fluctuations that are fully uncorrelated over the duration of the temporal waveform being imaged. Here, we show that random fiber lasers are excellent candidates for ghost imaging in the time domain. We study the temporal correlations of the intensity fluctuations of a random fiber laser in different operating regimes and compare its performance in temporal ghost imaging configurations with that of a conventional multi-mode cavity-based fiber laser. Our results demonstrate that random fiber lasers can achieve superior performance for ghost imaging as compared to cavity-based fiber lasers where strong correlations at the cavity round-trip time can yield artefacts for waveforms of long duration.

2 kW high-efficiency Raman fiber amplifier based on passive fiber with dynamic analysis on beam cleanup and fluctuation

In this paper, we study the power scaling in high power continuous-wave Raman fiber amplifier employing graded-index passive fiber. The maximum output power reaches 2.087 kW at 1130 nm with an optical conversion efficiency of 90.1% (the output signal power versus the depleted pump power). To the best of our knowledge, this is the highest power in the fields of Raman fiber lasers based merely on Stokes radiation. The beam quality parameter M² improves from 15 to 8.9 during the power boosting process, then beam spot distortion appears at high power level. This is the first observation and analysis on erratic dynamic properties of the transverse modes in high power Raman fiber amplifier.

All-fiberized cascaded random Raman fiber laser with high spectral purity based on filtering feedback

Cascaded random Raman fiber lasers (CRRFLs) with simple configuration and high spectral purity have become a great candidate for power scaling over the 1.1 µm-2 µm spectral band. Recently, CRRFLs with high spectral purity over 90% have been proposed by applying a highly temporal-stable pump source or a free-space short-pass filter, at the cost of increased system complexity. In this work, pumped directly by a Yb-doped fiber oscillator at 1080 nm, an all-fiberized and simplified CRRFL with a short-pass optical filter based on bending fiber and a thin-film wavelength division multiplexer is demonstrated. The transmission loss of the filter for 5th Stokes order at 1440 nm is up to 70 dB. Spectral purity over 92% for all the first four Stokes orders is achieved. The highest output power is 15 W for the 4th Stokes order at 1341 nm.

Tunable random Raman fiber laser at 1.7 µm region with high spectral purity

We demonstrate a tunable, high order cascaded random Raman fiber laser (RRFL) with high purity at 1.7 µm band by using a high power amplified spontaneous emission source (ASE) with both wavelength and linewidth tunability as pump source. The influence of the spectral bandwidth of the ASE source on the spectral purity of the output at 1.7 µm band is investigated. By adjusting the spectral bandwidth of the ASE source to the optimized 20 nm, output power >14 W with spectral purity up to 98.29% at 1715 nm is achieved. As far as we know, this is the highest spectral purity ever reported for a RRFL at 1.7 µm region. Furthermore, by adjusting the central wavelength of ASE source, the output of the RRFL can be tuned from 1695 to 1725 nm with >10 W output power. What's more, the spectral purity is above 92% over a tuning range from 1705 to 1725 nm.

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.

More than 200 W random Raman fiber laser with ultra-short cavity length based on phosphosilicate fiber

An over 200 W high-power first-order random Raman fiber laser (RRFL) at 1238 nm is demonstrated. The laser is based on a half-open cavity with a piece of 30 m phosphosilicate fiber. This RRFL is pumped by a conventional 1064 nm Yb-doped fiber laser. After suppressing the silica Raman component, a maximum output power of 206.7 W is obtained with a full width half-maximum linewidth of 7.1 nm at a pump power of 346.3 W, corresponding to an optical-to-optical efficiency of 59.7%. To the best of our knowledge, this is the highest reported output power of RRFL on the basis of phosphosilicate fiber with the shortest cavity length.

High-power low spatial coherence random fiber laser

A high-power multi-transverse modes random fiber laser (RFL) is investigated by combining a master oscillator power-amplifier (MOPA) configuration with a segment of extra-large mode area step-index multimode fiber (MMF). Spatial coherence of the high-power multi-transverse modes RFL has been analyzed, which shows that speckle contrast is reduced dramatically with the output power increasing. In this way, considerably low speckle contrast of ~0.01 is achieved under high laser power of ~56 W, which are the records for multi-transverse modes RFLs in both spatial coherence and output power. This work paves a way to develop high-power RFLs with very low spatial coherence for wide-range speckle-free imaging and free-space communication applications.

Power scalability of linearly polarized random fiber laser through polarization-rotation-based Raman gain manipulation

Random fiber laser based on Raman gain and random distributed feedback has drawn great attention in recent years. One of the most widely-studied fields is to improve the optical efficiency and the output power. However, the power scaling of a random fiber laser is instinctively restricted by the high order Stokes generation. In this manuscript, we propose a simple yet effective method, which employs a homemade all-fiber Lyot filter to manipulate the polarization dependent Raman gain, thus increasing the threshold of the 2nd-order Stokes wave and enhancing the maximum output power of the linearly polarized random fiber laser. Through reliable theoretical analysis, we optimize the design of the wavelength dependent Lyot filter. Moreover, the performance of the filter and the power scaling capability of the linearly polarized random fiber laser are investigated in detail. A proof-of-principle experiment is carried out by inserting the homemade Lyot filter into a half-opened random fiber laser. The experimental results indicate that the 2nd-order Stokes wave can be effectively suppressed, and the maximum output power of the 1st-order Stokes wave is significantly increased with a range of ~50% (from 43.6 to 63.2 W).

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 491 W, 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.

High order cascaded Raman random fiber laser with high spectral purity

An up to 8th order cascaded Raman random fiber laser with high spectral purity is achieved with the pumping of a narrow linewidth amplified spontaneous emission source. The spectral purity is over 90% for all the 8 Stokes orders. The highest output power is 6.9 W at 1691.6 nm with an optical conversion efficiency of 21% from 1062.0 nm. As a comparison, with conventional FBG-based fiber oscillator as pump source, only 47% spectral purity is achieved at 8th order. The temporal stability of the pump laser is proved to play a key role, because the time fluctuation of pump laser is transferred directly to Raman outputs and results in power distribution among different Stokes orders.

Flexible spectral manipulation property of a high power linearly polarized random fiber laser

Fiber lasers with flexible spectral manipulation property could provide a flexible tool for scenes where the temporal coherence property accounts, for example, coherent sensing/communication and nonlinear frequency conversion. Due to the good laser performance and relative simplicity of implementation, random fiber lasers (RFLs) based on random distributed feedback and Raman gain have earned more and more attention in the past few years, and a variety of RFLs with substantially different spectral properties have been developed. In this presentation, we demonstrate a high power linearly polarized RFL with flexible spectral manipulation property, in which the central wavelength and the linewidth of the spectrum can be tuned independently through a bandwidth-adjustable tunable optical filter (BA-TOF). The central wavelength of the RFL can be continuously tuned from 1095 to 1115 nm, while the full width at half-maximum (FWHM) linewidth has a maximal tuning range from ~0.6 to more than 2 nm. Moreover, the output power of 1102.5-1112.5 nm reaches ~23 W with polarization extinction ratio (PER) value > 20 dB. To the best of our knowledge, this is the first demonstration of a powerful linearly polarized RFL with both wavelength and linewidth tunability.

More than 400 W random fiber laser with excellent beam quality

We report on the demonstration of a high power, high beam quality random fiber laser. It is a half-open cavity based on 130-m-long large mode area step-index fiber with a core diameter of 20 μm and a numerical aperture of 0.08. The random laser cavity, whose central wavelength is 1120 nm, is pumped by a 1070 nm Yb-doped fiber oscillator. The maximum output power is 418 W with respect to the injected pump power of 588 W, corresponding to the optical-to-optical efficiency of 71%. The output power, spectral, and temporal characteristics are carefully presented and discussed, showing that such a cavity can export high power random laser stably and effectively. The far field beam quality is also measured, and the profile is nearly a Gaussian shape. This work provides an impactful method to advance the output power of random fiber lasers.