Cladding Pumping Scheme Research Articles

High power tunable Raman fiber laser at 1.2 μm waveband

Development of a high power fiber laser at special waveband, which is difficult to achieve by conventional rare-earth-doped fibers, is a significant challenge. One of the most common methods for achieving lasing at special wavelength is Raman conversion. Phosphorus-doped fiber (PDF), due to the phosphorus-related large frequency shift Raman peak at 40 THz, is a great choice for large frequency shift Raman conversion. Here, by adopting 150 m large mode area triple-clad PDF as Raman gain medium, and a novel wavelength-selective feedback mechanism to suppress the silica-related Raman emission, we build a high power cladding-pumped Raman fiber laser at 1.2 μm waveband. A Raman signal with power up to 735.8 W at 1252.7 nm is obtained. To the best of our knowledge, this is the highest output power ever reported for fiber lasers at 1.2 μm waveband. Moreover, by tuning the wavelength of the pump source, a tunable Raman output of more than 450 W over a wavelength range of 1240.6–1252.7 nm is demonstrated. This work proves PDF’s advantage in high power large frequency shift Raman conversion with a cladding pump scheme, thus providing a good solution for a high power laser source at special waveband.Graphical

Cladding pumped bismuth-doped fiber amplifiers operating in O-, E-, and S-telecom bands.

Bismuth-doped fibers (BDFs) are considered nowadays as an essential part of the development of novel optical amplifiers, which can provide a significant upgrade to existing fiber optic telecommunication systems, securing multiband data transmission. In this paper, a series of BDF amplifiers (BDFAs) for O-, E-, and S-telecom bands based on a cladding pumping scheme using low-cost multimode semiconductor laser diodes at a wavelength of 0.7-0.8 µm were demonstrated for, it is understood, the first time. The developed BDFAs are characterized by a high peak gain of >25-30 dB in the corresponding telecom bands and a relatively low noise figure of 5-6 dB. Comparative analysis shows that most of the parameters of cladding pumped BDFAs are close to those of the best core pumped ones. This research opens up new opportunities for utilizing Bi-doped fibers as a key element of cost-effective and ready-to-work BDFAs for various practical applications.

Bending Induced Temperature Increases in Double-Clad Fibers for High-Power Fiber Lasers

Rare-earth-doped optical fibers are one of the most promising solid-state lasers. In these fiber lasers, a cladding-pumping scheme using large-mode-area double-clad fibers (DCFs) is utilized to increase the overall conversion efficiency of pumping light and to overcome the restriction owing to the onset of stimulated Raman scattering. On the other hand, it is extremely challenging to increase the fiber core size while retaining the excellent beam quality because fibers with large core size allow propagation of several higher-order modes (HOMs), except for the fundamental mode (FM). In order to suppress HOMs, DCFs are bent with a relatively small bend radius. Failures at bends in an optical fiber are caused by light leaking from the core when the fiber is accidentally bent tightly with a high power input. For the DCFs with core radii of 10 and 20 mm, the relationship between the bending induced temperature increases and the bend losses in the bent DCFs was investigated theoretically by the explicit finite-difference method using the thermochemical SiO_x production model. The temperature at the boundary between the inner and outer cladding layers increased with increasing optical power <I>P</I> at 1.080 mm and was a large value higher than the softening temperature <I>T</I>_s of the silica glass when <I>P</I> = 5 kW was entered to the bent DCF with a bend radius <I>R</I> of 150 mm and a core radius of 10 mm. On the other hand, the temperature at the boundary was a small value lower than the <I>T</I>_s when <I>P</I> = 10 kW was entered to the bent DCF with a large <I>R</I> of 245-275 mm. Furthermore, it was found that the initiation of the fiber fuse was fairly easy under a certain conditions where the temperature at the boundary was higher than the <I>T</I>_s.

Fiber Fuse Simulation in Double-Clad Fibers for High-Power Fiber Lasers

Rare-earth-doped optical fibers are one of the most promising solid-state lasers. In these fiber lasers, a cladding-pumping scheme using double-clad fibers is utilized to increase the overall conversion efficiency of pumping light. To maintain acceptable beam quality, the low-numerical aperture large-mode-area fibers is effective for the double-clad fibers because the effects of stimulated Raman scattering can be reduced via the corresponding reduction in the power density in the large fiber core. For the large-mode-area double-clad fibers, fiber fuse propagation was investigated theoretically by the explicit finite-difference method using the thermochemical SiO_x production model. In the calculation, we assumed the fiber to be in an atmosphere and that part (40 μm in length) of the core was heated to a temperature of 2,923 K. The threshold power for the double-clad fiber with the core radius of 10 μm was 1.6 W at 1.080 μm and it was close to the experimental value. The power dependence of the velocity of fiber fuse propagation was calculated for the double-clad fibers with the core radius of 10 and 15 μm. The calculated velocities were in fair agreement with the experimental values observed in the input power range from 1 kW to 3.5 kW at 1.080 μm.

A semi-analytic method for FM-EDFA intensity model

Based on the intensity model of few-mode erbium-doped fiber amplifiers (FM-EDFAs), we put forward a semi-analytic method for efficiently calculating the signal mode gains by solving the analytical expression for the average population inversion over each fiber segment. Our studies show that, the semi-analytic method can offer a desirable calculation accuracy, very close to that of the full approach method. According to the length dependency of the overlap integral between the normalized transverse distribution of population inversion and the signal modal field, a large iterative step size is verified to be feasible for the semi-analytic method, which is helpful for greatly reducing the iterative times. We also take the example of designing FM-EDFA’s overall gain using the semi-analytic method. The semi-analytic method is not only suitable for the FM-EDFAs with complex erbium-ion doping profile, the cladding pumping scheme and the cascaded FM-EDFAs, but also helpful for greatly improving the optimization efficiency of FM-EDFAs in combination with relevant optimization algorithms.

High-power cladding pumped Raman fiber amplifier with a record beam quality.

In this Letter, a high-power, high-brightness all-fiberized Raman amplifier based on a cladding-pumping scheme is presented for the first time, to the best of our knowledge. The triple-clad passive fiber is employed as Raman gain fiber in the laser system. The maximum output power is 762.6 W emitting at 1130 nm. To the best of our knowledge, this is the highest power in the fields of cladding-pumped Raman amplifiers. Through a cladding-pumping process, the beam quality parameter ${{\rm M}^2}$M2 improves from 6.12 of seed laser to 2.24 at maximum output power of 762.6 W, while the best ${{\rm M}^2}$M2 is 1.9 at 267.2 W. It is also the best beam quality of Raman laser with brightness enhancement in any kind of configuration (graded-index fiber or multi-clad fiber, laser or amplifier, all-fiber or free-space configuration) with power of over 100 W.

Improved Optical Amplification Efficiency by Using Turbo Cladding Pumping Scheme for Multicore Fiber Optical Networks

Improved Optical Amplification Efficiency by Using Turbo Cladding Pumping Scheme for Multicore Fiber Optical Networks

Theoretical Characterization of the Ultra-Broadband Gain Spectra at $\sim$1600–2100 nm From Thulium-Doped Fiber Amplifiers

In this paper, we present the theoretical modeling and characterization of the gain spectra at $\sim$ 1600–2100 nm from thulium-doped fiber amplifiers (TDFAs) for the first time. Our theoretical model was validated through the comparison with the experiments on the widely tunable TDFAs and thulium-doped fiber lasers using in-band pumping ( $^3H_6\rightarrow ^3F_4$ ) and 79 $\times$ -nm pumping ( $^3H_6\rightarrow ^3H_4$ ) schemes. Different emission and absorption cross sections of the commercial thulium-doped fibers were employed for comparison and comprehensive demonstration. We then calculated the performance of the in-band-pumped TDFAs under different spectral cross section profiles and amplifier configurations. It shows, without the restriction of the loss induced by the fiber-optic components, that high optical gains ( $>20$ dB in small signal regime and $>3$ dB in saturation regime) can be achieved over an ultrabroad spectrum ranging from $\sim$ 1580 to $\sim$ 2080 nm. We also demonstrate, although it is rarely used in the experiments of the broadband TDFAs, that the 79 $\times$ -nm cladding pumping scheme can effectively avoid the strong reabsorption at short wavelengths when long gain fibers are used.

Developing high-power hybrid resonant gain-switched thulium fiber lasers.

In this paper, we propose hybrid-pumped resonant gain-switched thulium fiber lasers to realize high-average-power and high-pulse-energy 2-μm laser emissions. Based on numerical simulation, laser dynamics (pulse peak power, pulse energy, pulse duration, etc.) of this kind of laser system are investigated in detail. By taking advantages of the 793 nm continuous wave pump and the 1900 nm pulsed pump, performance of the laser emission can be significantly improved, with the highest average power of 28 W, peak power of 3.5 kW, pulse energy of 281 μJ, and narrowest pulse duration of 92 ns, all of which can be further optimized through designing the cavity parameters and the pumping circumstance. Compared with the pump pulses, two times improvement in pulse energy and average power has been achieved. This hybrid resonant gain-switched system has an all-fiber configuration and high efficiency (low heat load), and can be steadily extended into the cladding pump scheme, thus paving a new way to realize high power (>100 W average power) and high pulse energy (>1 mJ) 2 μm thulium fiber lasers.

Raman fiber laser-pumped high-power, efficient Ho-doped fiber laser

We demonstrate a high-power and efficient Ho-doped fiber laser (HDFL) employing a high-power 1150 nm Raman fiber laser (RFL) as the pump source. The 1150 nm RFL provided 110 W pump power, and the output power of the HDFL reached 42 W with a slope efficiency of 0.37. The lasing wavelength ranges from 2046.8 to 2049.5 nm, with an optical signal-to-noise ratio of more than 30 dB. This is, to the best of our knowledge, the first demonstration of a high-power HDFL near 2 μm pumped by a 1150 nm RFL and the highest output power achieved in this pump band. Higher output power can be realized by launching more pump power into the HDFL and employing a cladding pump scheme.

Wavelength-tunable, picosecond fiber master-oscillator power amplifier source based on an erbium-doped large-core fiber

We present a direct amplification of wavelength-tunable picosecond optical pulses operating in the spectral range of 1535–1559nm using an erbium (Er)-doped large-core fiber (EDLCF) in a master-oscillator power amplifier (MOPA) configuration. The EDLCF has a core of 40μm in diameter and is dual-end-pumped by a counter-propagating 976-nm diode laser in the cladding-pumping scheme together with a co-propagating 1480-nm laser in the core-pumping scheme. The MOPA system can generate the peak power and pulse energy of 12.6kW and 107nJ, respectively, while keeping an excellent pulse shape and spatial beam quality.

High-energy, in-band pumped erbium doped fiber amplifiers

We have demonstrated and compared high-energy, in-band pumped erbium doped fiber amplifiers operating at 1562.5 nm under both a core pumping scheme (CRS) and a cladding pumping scheme (CLS). The CRS/CLS sources generated smooth, single-peak pulses with maximum pulse energies of ~1.53/1.50 mJ, and corresponding pulse widths of ~176/182 ns respectively, with an M2 of ~1.6 in both cases. However, the conversion efficiency for the CLS was >1.5 times higher than the equivalent CRS variant operating at the same pulse energy due to the lower pump intensity in the CLS that mitigates the detrimental effects of ion concentration quenching. With a longer fiber length in a CLS implementation a pulse energy of ~2.6 mJ is demonstrated with a corresponding M2 of ~4.2. Using numerical simulations we explain that the saturation of pulse energy observed in our experiments is due to saturation of the pump absorption.

Comparison of amplification in large area fibers using cladding-pump and fundamental-mode core-pump schemes

We compare the amplification of various modes in large mode area fibers when the pump is coupled into the fundamental core mode versus the standard cladding-pump scheme. Our simulations show that pumping in the LP(01) core mode results in differential gain for the fundamental signal mode which suppresses the higher order modes and amplified spontaneous emission compared to the cladding pump scheme. This differential gain effect is predicted to increase with core size and may provide a path to scale fiber mode area to several thousand square micron.

Numerical modelling and experimental investigation of double-cladding erbium-ytterbium-doped fibre amplifiers

A comprehensive model for characterising double-cladding erbium–ytterbium-doped fibre amplifiers (EYDFAs) is proposed and experimentally verified for the first time to our knowledge. The model is based on rate and propagation equations in a homogeneous two-level active medium and takes into account the modified pump absorption arising from the double-cladding structure and the presence of ytterbium ions. The influence of the cladding pumping scheme on the multimode pump absorption, radial population inversion and the amplified spontaneous emission are also modelled. The numerical mode is capable to predict amplifier gain, output spectra, gain saturation and amplifier noise. The model has been fully validated in a large number of operating conditions and a fairly good agreement is obtained between the experimental data and the numerical simulations over a wide range of input parameters.