More than 400 W random fiber laser with excellent beam quality

20 watt-level single transverse mode narrow linewidth and tunable random fiber laser at 1.5 µm band

High power 1.5 µm band fiber lasers are of great importance for many practical applications. Generally, the technical targets including high average output power, narrow linewidth, temporally suppressed intensity dynamics, high spectral purity, single transverse mode lasing, and excellent robustness are the major concerns when constructing a high-performance laser source. Here, we demonstrate the highest output power of a wavelength tunable 1.5 µm band random fiber laser based on the active fiber gain mechanism to the best of our knowledge. A master oscillator power-amplifier (MOPA) configuration is employed to greatly boost the output power to 20 watt-level with a single transverse mode lasing and the same linewidth as the seed, benefiting from the spectral broadening free feature when employing the random fiber laser as the seed. This work not only enriches the progress of random fiber laser, but also provides an attractive alternative in realizing high performance lasing light source at 1.5 µm band.

Kilowatt random Raman fiber laser with full-open cavity

Random Raman fiber laser (RRFL) has been widely studied in high-power laser generation due to its special lasing characteristics. However, all previous high-power results are based on the half-open cavity. In this letter, we demonstrate an applicable high-power RRFL with the simplest structure, that is, a full-open cavity. The lasing dynamic and output characteristics are theoretically and experimentally studied. Laser source with multi-longitudinal modes can result in the random laser output from one side even in the full-open cavity. The ratio of the backward output power is mainly determined by the reflectivity of fiber ends. The experimental results show that such a simple structure can easily generate kilowatts of random laser power and is a promising setup to achieve higher output power, which is also an important platform to study the laser dynamic in high-power full-open cavity without any point-action or regular distributed reflectors.

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.

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.

High efficiency Brillouin random fiber laser with replica symmetry breaking enabled by random fiber grating

We report a high efficiency Brillouin random fiber laser (BRFL) enabled by a random fiber grating (RFG) with demonstration of replica symmetry breaking (RSB). The RFG was characterized by optical coherence tomography (OCT) method, which measured the spatially resolved reflectivity of RFG by a tunable delay line. Multiple narrow linewidth peaks appeared in reflection spectrum of RFG, created by frozen scattering centers acting as narrow linewidth filters to select random modes in random fiber lasers based on Brillouin gain. With the scattering from RFG as disordered feedback, a BRFL with slope efficiency of 29.3% and lasing threshold of 10.2 mW was demonstrated with 1 kHz linewidth. Intensity dynamics show that RFG can reduce the noise of BRFL with a symmetric phase portrait, indicating the increased mean path length and coherence time of the Stokes photons. The probability distribution of the Parisi overlap parameter of intensity fluctuation spectra from trace to trace reveal a photonic spin-glass phase with RSB in the RFG enabled BRFL, providing a photonic platform to study the photon glassy behavior of random fiber lasers.

Brightness enhancement in random Raman fiber laser based on a graded-index fiber with high-power multimode pumping

A brightness-enhanced random Raman fiber laser (RRFL) with maximum power of 306 W at 1120 nm is demonstrated. A half-open cavity is built based on a graded-index (GRIN) passive fiber and single high-reflective fiber Bragg grating written in it directly. Due to the beam cleanup effect in the GRIN fiber enhanced in the half-open RRFL cavity, the output beam quality factor M² is improved from 9.15 (pump) to 1.76-2.35 (Stokes) depending on power, while the pump-Stokes brightness enhancement (BE) factor increases proportionally to output power reaching 6.1 at maximum. To the best of our knowledge, this is the highest power GRIN RRFL with BE.

Mode locking of a coherent random fiber laser with selectable repetition rates

Controlling emission of light in random structures/disordered systems, e.g., implementing mode-locked pulses in a laser system with a random structures/disordered systems, is a complex task. Usually, the generation of laser pulse by mode locking needs a stable fixed-length cavity that determines a specific repetition rate of the mode-locked pulses. Here, mode-locking laser pulses with selectable repetition rates are achieved in a typical one-dimensional disordered laser by passive mode locking. The laser includes disordered reflectors to provide multiple resonant modes associated with different cavity length. The regular pulses with adjustable repetition rates can be generated and selected by a nonlinear polarization rotator and a semiconductor saturable absorber mirror. The proposed work utilizing the advantages of multiple resonances in random lasers could pave a new way for regulating emission of light in the random structures/disordered system. And it displays an effective and realistic technical route to study ultrafast pulses generation and optical soliton dynamics in random structures/disordered systems.

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.

Three-dimensional beam profiling used to characterize dental light-curing units

Two-dimensional (2D) beam profiling is an emerging technique used to characterize the beam homogeneity in dental light-curing units (LCUs). This project developed a method to combine multiple 2D beam profiles that had been measured along the beam path to create a 3D profile of the beam. This allowed a quantitative investigation of beam divergence and homogeneity at different distances from the source. To illustrate the use of this technique, four representative dental LCUs were measured. In addition, the selected dental LCUs demonstrated the effects of LCU design, particularly that of fiber optic light guides, on beam quality. The results show the value of a program that can recombine multiple beam profile images made at different distances from the source to create a 3D beam profile of a light beam.

915 nm all-fiber laser based on novel Nd-doped high alumina and yttria glass @ silica glass hybrid fiber for the pure blue fiber laser

The fiber laser in the range of 900-1000 nm is essential to generate the blue fiber laser through frequency doubling for the laser display, laser underwater communications, and laser lighting. Yet, the well-developed three-level Yb-doped fiber laser can only realize the blue-green fiber laser at around 490 nm, which is far from the pure blue area (450 nm). To further achieve the pure blue fiber laser, the Nd-doped fiber has emerged as a proper choice to realize a shorter wavelength laser (<920  nm) through the F_3/24→I_9/24 transition of Nd³⁺. Here, based on the facile "melt-in-tube" (MIT) method, a novel Nd-doped high alumina and yttria glass @ silica glass hybrid fiber was successfully prepared using the Nd:YAG crystal as the precursor core. The crystal core converts to the amorphous glass state after the drawing process, as evidenced by Raman spectra. The gain coefficient at 915 nm of the hybrid fiber reaches 0.4 dB/cm. Further, the laser oscillation at 915 nm with over 50 dB signal-to-noise ratio was realized by a short 3.5 cm gain fiber. Our results indicate that MIT is a feasible strategy to produce novel fiber for generating fiber laser at special wavelengths.

Stable and low-threshold random fiber laser via Anderson localization

We report a stable and low-threshold Er-doped random fiber laser (RFL) based on a femtosecond-laser-inscribed random-distributed-grating array (RDGA) as the random feedback. The RDGA had a reflectivity of 93.5%, and its properties were numerically analyzed based on the transfer matrix method. The threshold of the laser was significantly reduced to 5.7 mW, and the linewidth was ~0.4 pm near the threshold as the Anderson localization effect existing in the RDGA significantly improved the laser quality factor (4 × 10⁶). In addition, we propose a method to select RFL lasing modes by stretching a fiber grating filter used in the cavity with different axial strains. The center wavelength hardly drifted and the maximum jitter value of the peak power was less than 0.12 dB over 1 hour for the selected three lasing modes, which indicated that our laser operation was quite stable.

Influences of position of ytterbium-doped fiber and ASE pump on spectral properties of random fiber laser

The influences of the position of the ytterbium-doped fiber and the parasitic lasing in the amplified spontaneous emission (ASE) pump source on the spectral properties of the random fiber laser are analyzed and discussed in this paper. The experimental results show that putting ytterbium-doped fiber in the random fiber laser's cavity and using an ASE pump source with parasitic lasing are beneficial for the generation of high-order Stokes. A near-infrared supercontinuum with 20 dB bandwidth of more than 500 nm can be generated directly from a random fiber laser, which proved that a random laser fiber cannot only works as a traditional random fiber laser, but also can be a novel, simple, low-cost, low-coherence and robust near-infrared supercontinuum generation method.

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.

Random Raman fiber laser based on a twin-core fiber with FBGs inscribed by femtosecond radiation

Narrowband Raman lasing in a polarization-maintaining two-core fiber (TCF) is demonstrated. Femtosecond point-by-point inscription of fiber Bragg gratings (FBGs) in individual cores produces a half-open cavity with random distributed feedback. The laser linewidth in the cavity with a single FBG inscribed in one core of the TCF reduced by ∼2  times with respect to the cavity with a fiber loop mirror. It is shown that the inscription of two FBGs in different cores leads to the formation of a Michelson-type interferometer, leading to the modulation of generation spectra near threshold. This technique offers new possibilities for spectral filtering or multi-wavelength generation.

2nd-order random lasing in a multimode diode-pumped graded-index fiber

Raman lasing in a graded-index fiber (GIF) attracts now great deal of attention due to the opportunity to convert high-power multimode laser diode radiation into the Stokes wave with beam quality improvement based on the Raman clean-up effect. Here we report on the cascaded Raman generation of the 2nd Stokes order in the 1.1-km long GIF with 100-μm core directly pumped by 915-nm diodes. In the studied all-fiber scheme, the 1st Stokes order is generated at 950-954 nm in a linear cavity formed at GIF ends by two fiber Bragg gratings (FBGs) securing beam quality improvement from M2 ≈ 30 to M2 ≈ 2.3 due to special transverse structure of FBGs. The 2nd Stokes wave is generated either in linear (two FBGs) or half-open (one FBG) cavity with random distributed feedback via Rayleigh backscattering. Their comparison shows that the random lasing provides better beam quality and higher slope efficiency. Nearly diffraction limited beam (M2 ≈ 1.6) with power up to 27 W at maximum gain (996 nm), and 17 W at the detuned wavelength of 978 nm has been obtained, thus demonstrating that the 2nd-order random lasing in diode-pumped GIF with FBGs provides high-efficiency high-quality beam generation in a broad wavelength range within the Raman gain spectral profile.

Decoherence of fiber supercontinuum light source for speckle-free imaging

Speckle-free imaging is attractive in laser-illuminated imaging systems. The evolutionary process of supercontinuum decoherence in extra-large mode area step-index multimode fiber is analyzed to provide high-quality broadband light source for speckle-free imaging. It is found that spectral bandwidth, number of spatial transverse modes, and decoherence among different modes all greatly contribute to speckle reduction. The combination of supercontinuum and extra-large mode area step-index multimode fiber can considerably increase the efficiency of decoherence process for speckle-free imaging. This work may enrich the research of speckle-free imaging and also provide guidance on speckle-free imaging using fiber-optics based light source.

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.