Monolithic Fiber Laser Research Articles

1.5 kW ytterbium-doped single-transverse-mode, linearly polarized monolithic fiber master oscillator power amplifier.

A linearly polarized monolithic fiber laser based on a master oscillator power amplifier structure with a master oscillator and a one-stage power amplifier is reported. We design a homemade oscillator based on the theory that, in the coiled gain fiber, the higher modes and the polarized mode of the fundamental mode along the fast axis are suppressed effectively because of their obviously higher bend loss than that of the polarized mode of the fundamental mode along the slow axis. The oscillator operates at 1080 nm, launching a 30 W seed laser with a high polarization extinction ratio of 19 dB into the power amplifier via a mode field adapter. The power amplifier utilizes Yb-doped polarization-maintaining fiber of 20/400 μm, which produces nearly diffraction-limited output power of about 1.5 kW with an optical-optical efficiency of 81.5% and a polarization extinction ratio of 13.8 dB. Both the M(x)² factor and the M(y)² factor of the collimated beam are measured to be about 1.2. The spectral width of the output power is broadened approximately linearly, and the full width at half maximum of the spectrum at the maximum output power is about 5.8 nm. It is known as the highest linearly polarized output power to the best of our knowledge.

Monolithic Fiber Lasers Combining Active PCF With Bragg Gratings in Conventional Single-Mode Fibers

Novel monolithic fiber laser architectures utilizing large mode area (LMA) photonic crystal fiber (PCF) and fiber Bragg gratings (FBG) in conventional single-mode fibers (SMF) are presented. The main challenge is to address high cavity losses arising from the intrinsic 18-fold mode-field mismatch between the SMF and the active LMA PCF. Employing an all-fiber, robust and reproducible mode-field matching approach based on graded-index multimode fibers, we numerically and experimentally demonstrate that the SMF-to-LMA PCF coupling can be more than three-fold improved. This MFA approach is further implemented in monolithic fiber laser cavities combining FBGs in SMF and active LMA PCF. We demonstrate that cavity losses can be significantly mitigated when using appropriate MFAs resulting in a substantial increase of the laser output performances.

700-kW-Peak-Power Monolithic Nanosecond Pulsed Fiber Laser

We report a high-pulse energy, high-peak power, and monolithic nanosecond pulsed fiber laser source at ~1064 nm in master oscillator-power amplifier configuration. The seed source is a directly modulated laser diode, producing nanosecond pulses as short as ~3 ns with ~1-W peak power and a tunable repetition rate. Three core-pumped fiber amplifier stages and two double-cladding fiber amplifier stages were built to boost the peak power and the pulse energy of the ~3-ns seed pulses to ~697 kW and ~2.3 mJ, respectively. The all-fiber construction of the whole laser system enables compact size, maintenance-free, and robust operation.

Distributed Scheme Versus MOFPA Array on the Power Scaling of a Monolithic Fiber Laser

A couple of amplifying arrays for the power scaling of the fiber lasers under diverse pumping modes are studied. The multifiber series distributed side-pumping arrangement and the cascade amplifier chains in the form of the end-pumped master oscillator fiber power amplifier (MOFPA) scheme are investigated accordingly. Those are numerically modeled based on the propagation rate equations associated with stimulated Brillouin scattering (SBS) as well as the consideration of coupling and splicing losses for large mode area fibers. The continuous wave double-clad Yb:silica fiber laser amplifier is analyzed in various amplifying stages for several cavity lengths at forward pumping regime. Despite the distributed mode exhibits a desirable beam quality, however, it suffers a low SBS threshold. On the other hand, high output power is achieved in the MOFPA configuration far from the onset of SBS.

300-W-average-power monolithic actively Q-switched fiber laser at 1064 nm

We demonstrate a high power monolithic nanosecond pulsed fiber laser source at 1064 nm. This laser was configured as a master oscillator-power amplifier (MOPA) seeded by an acousto-optic Q-switched fiber laser with varied pulse duration and repetition rate. Over 300 W average power is achieved for ~ 475 ns pulses at 100 kHz repetition rate with an optical to optical efficiency of 75.4%. The M2 at both directions is <1.6 measured at the highest average power.

Monolithic Tm-Doped Fiber Laser at 1951 nm With Deep-UV Femtosecond-Induced FBG Pair

We report on a monolithic Tm-doped fiber laser emitting in the region <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim}{\rm 2}~\mu{\rm m}$</tex> </formula> . A pair of fiber Bragg gratings (FBGs) was written into the Tm-doped fiber using a two-beam interferometry and a deep ultraviolet (DUV) femtosecond laser source at 266 nm. Such a monolithic setup allows a simple setup of the laser made of a single fiber only. This configuration also offers greater reliability because fusion splices inside the resonator cavity could eventually generate failure in the laser cavity. To the best of our knowledge, this is the first rare-earth-doped fiber laser with an FBG pair written by DUV femtosecond laser radiation. Laser characteristics are presented as well as compared with the fiber laser with external FBGs conventionally spliced to the Tm-doped fiber. Dependence of the laser wavelength on the pump power is investigated in detail.

UV-induced photodarkening and photobleaching in UV-femtosecond-pulse-written fibre Bragg gratings

Photodarkening and photobleaching effects in the case of UV femtosecond-pulse exposure of optical fibres are investigated. We evaluate the existence of loss equilibrium states in Yb-doped and Yb-free fibres. Supposing that parasitic VIS to NIR losses induced by fibre Bragg grating (FBG) inscription can also be addressed by a photobleaching treatment, we find grating absorption remarkably reduced by a post-exposure treatment. We also present photobleaching applied directly after FBG inscription with UV femtosecond pulses to improve the wavelength and power stability of a monolithic FBG-based fibre laser as a noteworthy alternative to subsequent thermal treatment.

Towards the development of fiber lasers for the 2 to 4 μm spectral region

A growing number of applications are calling for compact laser sources operating in the mid-infrared spectral region. A review of our recent work on monolithic fiber lasers (FL) based either on the use of rare-earth fluoride fibers or on Raman gain in both fluoride and chalcoge- nide glass fibers is presented. Accordingly, an erbium-doped double clad fluoride glass all-FL operating in the vicinity of 3 μm is shown. In addition, we present recent results on the first demonstrations of both fluoride and chalcogenide Raman fiber lasers operating at 2.23 and 3.34 μm, respec- tively. It is shown that based on this approach, monolithic FLs could be

550-mW Output Power From a Narrow Linewidth All-Phosphate Fiber Laser

We present a compact monolithic all-phosphate glass fiber laser with up to 550 mW of output power operating on a single longitudinal mode. We measured a linewidth of less than 60 kHz and relaxation oscillation peak amplitudes below -100 dB/Hz without active RIN-supression. The laser cavity has been formed by inscribing fiber Bragg gratings (FBG) directly into heavily Er3+ Yb3+ doped phosphate glass fiber using femtosecond laser pulses and a phasemask. The compact form factor and higher output power combined with the low noise and narrow line width characteristic make this laser an ideal candidate for ranging, interferometry and sensing applications.

Splice-Free Fiber Laser Array

We propose and investigate a novel splice-free fiber laser array architecture that uses a double-core fiber where one of the cores is active and the other is passive. Fiber tapers were implemented to transmit the light at the pump and signal wavelengths between cores. Experimental results on coupling between cores are presented for two different fibers. A two-element monolithic fiber laser array is demonstrated.

Growth characterization of fiber Bragg gratings inscribed in different rare-earth-doped fibers by UV and VIS femtosecond laser pulses

Besides well-known applications of fiber Bragg gratings (FBGs) in sensing and telecommunication, current research focuses also on an integration of gratings in actively doped fibers in order to realize high-power monolithic fiber lasers. The inscription of Bragg gratings directly into the active laser material has been achieved successfully, but the influence of the different dopants in rare-earth-doped fibers on the inscription process has usually failed to be considered. In this paper we report on a systematic investigation of Bragg grating growth behavior during inscription with UV and VIS fs pulses in various actively doped fibers. Furthermore we present an analysis of the initial grating growth in active fibers with different rare-earth dopant concentration under different femtosecond laser exposure conditions. A positive influence of cerium doping on grating growth and photosensitivity has been found. Ytterbium doping has been found to decelerate grating growth, meaning decreased photosensitivity to femtosecond pulses. The specific doping type seems to play an important role in the fabrication of high-reflective Bragg gratings in rare-earth-doped fibers.

A linearly polarised, pulsed Ho-doped fiber laser

We report the highest power operation of a monolithic polarised gain-switched holmium fiber laser. In this paper we describe the pulsed operation of two monolithic holmium fiber lasers, which were gain-switched by a pulsed thulium fiber laser. The first holmium laser produced pulses with a minimum pulse duration of 85 ns, maximum pulse energy of 16 µJ and repetition rate of 600 kHz. This laser was linearly polarised with an extinction ratio of >18.5 dB under all operating conditions and achieved average powers of up to 5.1 W at 2.104 µm. The second laser demonstrated shorter pulses with a minimum duration of 38 ns at a maximum repetition rate of 300 kHz. This laser produced up to 1.5 W of average power at 2.112 µm.

High peak-power single-frequency pulses using multiple stage, large core phosphate fibers and preshaped pulses

We have developed a monolithic high power pulsed fiber laser in a master oscillator power amplifier (MOPA) configuration, which is capable of reaching 0.38 mJ pulse energy and 128 kW peak power for 3 ns pulses at ~1550 nm while maintaining transform-limited linewidth. The fiber laser pulse seed was achieved by directly modulating a CW single-frequency fiber laser using an electro-optic modulator. We used an arbitrary waveform generator to preshape the fiber laser pulses before amplification to avoid pulse steepening and dynamic gain saturation. Single-mode, polarization maintaining highly Er/Yb codoped large core phosphate fibers were used in the power amplifier stages to scale the transform-limited fiber laser pulses, avoiding any nonlinearities.

Fused angle-polished multi-points side-pumping coupler for monolithic fiber lasers and amplifiers

We have developed theoretical model to investigate characters of fused angle-polished side-pumping coupler for monolithic fiber lasers and amplifiers. In the simulation the interplay of different couplers is considered. Five pumping fibers with different polished angles are employed in the experiment. The results showed that, the backward coupling efficiency, forward and backward pumping leakage are lower than 4%, 8% and 10% respectively, while the forward coupling efficiency is about 100%, with the polished angle suitable for commercial fibers. The experimental results agree well with the simulations. Furthermore, we fabricate a two-points co-propagating fused angle-polished side-pumping coupler, with the coupling efficiency of 94.1% and 87.9%. These couplers are quite suitable for multi-points side-pumping high power monolithic fiber lasers and amplifiers.

220 μJ monolithic single-frequency Q-switched fiber laser at 2 μmby using highly Tm-doped germanate fibers

We report a unique all fiber-based single-frequency Q-switched laser in a monolithic master oscillator power amplifier configuration at ~1920 nm by using highly Tm-doped germanate fibers for the first time. The actively Q-switched fiber laser seed was achieved by using a piezo to press the fiber in the fiber Bragg grating cavity and modulate the fiber birefringence, enabling Q-switching with pulse width and repetition rate tunability. A single-mode polarization maintaining large core 25 μm highly Tm-doped germanate fiber was used in the power amplifier stage. For 80 ns pulses with 20 kHz repetition rate, we achieved 220 μJ pulse energy, which corresponds to a peak power of 2.75 kW with transform-limited linewidth.

Towards a monolithic fiber laser with deep UV femtosecond-induced fiber Bragg gratings

Fiber Bragg grating inscription with a deep ultraviolet (DUV) femtosecond laser and two-beam interferometry allows grating inscription in germanium-free fibers without classical UV photosensitivity. Therefore such a writing technology is a promising solution for the fabrication of fiber Bragg gratings in the active fiber material to realize a fully integrated all-fiber solution. In this paper we report on the use of the DUV femtosecond laser fiber Bragg grating technology to build a grating-stabilized fiber laser with high spectral purity. Output powers up to 158 W at 1083 nm and a slope efficiency of 76% have been demonstrated.

1.1-kW Ytterbium Monolithic Fiber Laser With Assembled End-Pump Scheme to Couple High Brightness Single Emitters

We demonstrate a continuous-wave all-fiber laser with master oscillator and two-stage power amplifier configuration. Diode pumping with single-emitter technology is used with the advantage of low cost and outstanding heat management. An assembled end-pumped scheme is employed to couple high brightness pump power into a gain fiber. The laser system generates 1.1-kW output power at 1085 nm, with the total optical-optical efficiency of about 66.5%, corresponding to 1655 W of applied pump power. The variable absorption of gain fiber is analyzed considering the launching spectra characteristics of single emitters, which is significant for the construction of the pump system.

Influence of fusion splice on high power ytterbium-doped fiber laser with master oscillator multi-stage power amplifiers structure

We have developed models to investigate the power properties of monolithic fiber laser with master oscillator multi-stage power amplifiers configuration. In the simulation the effect of optical characteristics of fiber fusion splice was considered, which was introduced by connecting fiber amplifiers. The theoretical modeling was established based on the classic theory of rate equations and the computational method was described in detail. On the basis of three high-power fiber lasers with different structures, we analyzed the splice loss and return loss, and how they influence the distribution of laser power in the system. The results showed that, on the present splicing level, both the enhancement of the pump capacity of fiber laser and the reduction of the quantity of power amplifier modules were effective strategies to accomplish fiber laser with tens of thousands of watts output.

Tunable terahertz-wave generation from DAST crystal pumped by a monolithic dual-wavelength fiber laser

For developing a continuous-wave (CW) tunable Terahertz-wave (THz-wave) source using difference-frequency generation (DFG) in highly nonlinear optical crystals, we proposed and demonstrated a dual-wavelength fiber ring laser system operating around 1060 nm based on wideband chirped fiber Bragg gratings (CFBGs) and semiconductor optical amplifier (SOA). Thermo-induced phase shift along the CFBG produces a very sharp transmission spike therefore two lasing wavelengths with single longitudinal mode operation are oscillating simultaneously within the fiber ring cavity. Due to the inhomogeneous gain broadening property of SOA, the wavelength spacing of our dual-wavelength fiber laser can be continuously adjusted from 0.3 to 9.5 nm. By using this single emitter dual-wavelength fiber laser to pump an organic nonlinear DAST crystal, type-0 collinear phase matching of DFG process can be fulfilled and monochromatic THz wave ranging from 0.5 to 2 THz has been successfully generated.

基于有源对准技术熔接双包层特种光纤

The fusion splicing of double-clad (DC) specialty fibers based on active alignment is crucial to the investigation of high-power monolithic fiber lasers. Given the wave-guiding characteristic of DC fiber, a light stripper is introduced in an active alignment experiment. We propose a novel method for stripping light that is convenient, highly efficient, and low cost. This method is also effective for low-numerical-aperture beams that escape from the fiber core. A splice loss as low as 0.05 dB is achieved.