Response characteristics of Yb-doped fiber lasers to gamma-ray radiation

Summary form only given. Reliable, rugged and optically efficient fiber combiners are required for monolithic single frequency amplifier systems [1]. The most common fiber combiner type, a tapered fused fiber bundle (TFB), combines highly efficiently pump light up to several hundred watts, but with the drawback of an interrupted signal core, since this approach is based on the fiber end face pumping scheme [2]. Hence, the beam quality and the signal transmission can be influenced using the counter-propagation pumping (CPP) scheme. We developed an all-fiber side-pumping approach based on a coreless tapered intermediate fiber (see. Fig. 1) [3]. The signal feedthrough of this combiner offers the possibility to pass a high power signal in forward and, particularly, in reverse direction. Moreover, several pump feeding fibers can be laterally fused around a passive or active double-clad fiber (target fiber) with pump combining efficiencies of about 90%. We will present details and further progress of the optical fiber combiner design with the focus on the impact of the fiber combiner on the single-frequency amplifier performance in the case of CPP. Fig. 2 shows the all-fiber CPP Ytterbium fiber amplifier setup seeded by a preamplified nonplanar ring-oscillator operating at 1064 nm (spectral linewidth 1 kHz). The 4+1x1 pump combiner, based on the setup depicted in Fig. 1, consists of a centered double clad fiber (target fiber) with a cladding diameter of 250 μm (NA 0.46) and 4 pump feeding fibers with a core diameter of 105μm (NA 0.22). Each pump feeding fiber was axially fusion spliced to a coreless intermediate fiber with a cladding diameter of 125 μm. The 2.75 m long active fiber (Nufern, LMA-YDF-25/250-VIII) as well as the passive target fiber (Nufern, LMA-GDF-25/250) had a core diameter of 25 μm (NA 0.06). A single-frequency output power of 300 W was achieved with CPP at a wavelength of 976 nm [4]. That means, the pump combiner had to handle the pump power of 440 W and the high power signal propagating in reverse direction through the fiber component. The pump diodes were sufficiently isolated against the amplified signal light with about 30 dB. The measured signal insertion loss was less than 3 %. At an output power of 285 W the onset of stimulated Brillouin scattering (SBS) was observed. A further fiber integration step was realized by direct lateral fusion of four pump feeding fibers (with intermediate fibers) to a 3 m long polarization maintaining active fiber (Nufern, PLMA-YDF-25/250-VIII). Direct coupling of the pump light into the active fiber avoids the fusion splice at the high power output-side of the active fiber as well as the additional passive target fiber, which can affect the SBS threshold. In our first results with this setup we obtained an output power of 120 W without any indication of SBS. For both CPP amplifier configurations a beam quality factor M2 of about 1.2 and a stable polarized output beam was determined. In summary, the presented results show the progress in the development of highly integrated CPP single frequency amplifier systems.

Fiber lasers with fiber Bragg grating (FBG) mirrors inscribed directly into the active fiber, hereinafter referred to as monolithic fiber lasers, are of great interest thanks to simpler resonator design and thanks to elimination of fiber splices between the active fiber and passive fibers with FBGs that could lead to breakdown. In order to build such monolithic fiber laser configuration, different methods for FBG inscription were reported in literature, including phase mask inscription using UV light or fs lasers in deep UV light, visible and IR spectrum, and by point-by-point technique with IR fs radiation, for details see [1, 2] and references herein. We report to our knowledge first application of monolithic fiber laser with FBGs inscribed by so-called plane-by-plane method using femtosecond laser pulses operating in visible region [3, 4]. We used erbium and ytterbium co-doped double-clad fibre fabricated in-house. The fibre was drawn from preforms manufactured by modified chemical vapour deposition method and solution-doping of erbium and ytterbium ions with phosphorous oxide. Since the inner cladding cross-section of the fibre had a circular shape, a tailored coiling had to be applied in order to improve pump absorption [5, 6]. Indeed, the peak absorption at 976 nm increased from 3.5 dB of the standard coil to 17 dB for a tailored coil of 5 m long fibre sample. The high reflectivity mirror reflects more than 99 % and the reflectance of the low-reflective FBGs varied from 10 % to 50 % in the various samples. The laser characteristics were measured using 976 nm pump laser diode with the pump wavelength stabilized by volume Bragg grating. We report 30 % efficiency of the experimental fibre laser setup. The relatively low laser efficiency can be attributed to non-optimal absorption due to circular cross-section of the inner cladding and to non-optimized erbium-to-ytterbium concentration

In radiation environments, the radiation induced attenuation (RIA) of the active optical fiber can lead to a significant decline in the performance of fiber laser system. An effective way to solve this problem is to bleach the active fiber using pumps at certain wavelengths, namely photo-bleaching. Experiments have shown that output power of irradiated Yb-doped fiber laser experiences remarkable recovery under 976-nm pump. However, under 976-nm pump, signals at both 976 nm and 1070 nm co-exist in Yb-doped fiber. Moreover, it is difficult to distinguish which wavelength is responsible for the photo-bleaching process. Herein, a one-hundred-watt level Yb-doped fiber laser is irradiated with gamma-ray radiation. In the radiation process, a significant output decline from 129 W at 0 Gy to 81 W at 100 Gy is observed. Then, self-bleaching test is conducted with 976-nm pump. After 2-h bleaching, the output power is restored to 111 W, corresponding to a recovery ratio of about 37.0%. To verify the specific wavelength responsible for the performance recovery, photo-bleaching characteristics of Yb-doped fiber lasers are investigated under different pump wavelengths including 915, 976, 1070 and 1550 nm. Experiments show that laser signal at 1 μm waveband is the primary cause for the bleaching of Yb-doped fibers, while, the pump at 915, 976 and 1550 nm can hardly bleach the irradiated Yb-doped fiber. The RIA recovery curves of Yb-doped fibers are measured under different 1070-nm bleaching powers. And, related evolution parameters are obtained through curve fitting. With these parameters, the RIA evolution of the Yb-doped fiber and the corresponding output power evolution of the Yb-doped fiber laser in the radiation and bleaching process are simulated. Comparisons show that the numerical results are consistent with the measurements qualitatively, demonstrating the reliability of the model. This work has guiding significance for predicting the performance of fiber laser systems in radiation and bleaching environments.

Radiation hardening should be considered for various active fiber systems operated in adverse environments to reduce their sensitivity to complex ionizing radiations. And, architecture optimization through numerical simulation provides an efficient choice. Here, introducing radiation effects into the conventional fiber laser model, a radiation model concerning the design optimization of low- and moderate-power Yb-doped fiber lasers is developed. And, experiments at different radiation levels up to 750 Gy are carried out for validation, demonstrating the ability of this model to correctly simulate the performance of the Yb-doped fiber laser in harsh environments. Then, with this model, impacts of active fiber length, pump scheme, and pump allocation on the output characteristics of Yb-doped fiber lasers are analyzed numerically. And, optimization of Yb-doped fiber lasers are conducted through architecture design. Simulations show that a proper design with relatively short active fiber and dynamic pump allocation can remarkably improve the radiation tolerance of Yb-doped fiber lasers.

Effect of steady-state conditions on self-pulsing characteristics of Yb-doped cw fiber lasers

Fiber lasers have been actively developed in the last decade demonstrating unique efficiency and performance. It is known that the laser generation is possible not only in active fibers, but also in passive fibers owing to the effect of stimulated Raman scattering (SRS). In the SRS process, pump radiation induces amplification of the Stokes-shifted scattered light (by ~13 THz and ~40 THz for Si0 2 /Ge02 and P 2O5 , respectively), see [1] for a review. The Raman gain spectrum in silica-based fibers is rather broad (>10 THz). Therefore, Raman fiber lasers (RFLs) are treated as perspective laser sources in spectral bands, which are not available from rare-earth (RE) doped fiber lasers [2]. The RFL cavity is usually formed by a set of fiber Bragg gratings (FBGs) which reflect Stokes waves of the first and higher orders. As a result, a cascaded generation with stepwise tuning in a broad spectral range is possible, which may be combined with continuous tuning within the Raman gain spectrum of individual Stokes components. However, a relatively low Raman gain requires rather long (0.1-1 km) passive fibers with high-power pumping into the single-mode fiber core, e.g. by 1-μm single-mode Yb-doped fiber lasers (YDFLs) securing cascaded Raman generation in 1-1.5 μm range [1].

We present an efficient monolithic fiber laser incorporating fiber Bragg grating (FBG) mirrors inscribed directly into the active fiber using a femtosecond laser. The FBGs are inscribed in an erbium/ytterbium co-doped fiber using the direct-write, plane-by-plane femtosecond laser inscription method, which allows for complete spatial control of the refractive index change in the fiber core. This approach offers a significant advantage, as we eliminate the fiber splices typically required between the active and passive fibers that would, otherwise, incorporate the FBG. The gratings were characterized in transmission and reflection, and the performance of the fiber laser was recorded when the fiber was coiled into a circular and kidney shape. The kidney shape of the fiber coil improved the laser slope efficiency, as the inner cladding of the active fiber had a circular shape. Our results show that the control of the grating inscription parameters using the plane-by-plane inscription method can result in an efficient monolithic fiber laser.

In this work we present our result on investigation of pump power combiner fabrication process in configuration (N+1)x1. Our fabrication method is based on tapering of fiber bundle consisting of 6 multimode fibers and one signal fiber. We present results on fabrication power combiners based on single mode fibers and large mode area fibers for the signal. Full Text: PDF References D. Noordegraaf et al., "All-fiber 7x1 signal combiner for incoherent laser beam combining", Proc. SPIE 7914 79142 (2011) CrossRef E. Snitzer, H. Po, F. Hakimi, R. Tumminelli, B. C. McCollum,"Double Clad Offset Core Nd Fiber Laser", Opt. Fiber Sensor Topical Meeting., New Orleans, LA, 1988. CrossRef Zervas, M.N., Codemard, C.A., "High Power Fiber Lasers: A Review," Selected Topics in Quantum Electronics, IEEE Journal of, vol.20, no.5, pp.219,241, Sept.-Oct. 2014 CrossRef F. Hakimi, H. Hakimi, "New side coupling method for double-clad fiber amplifiers", Lasers and Electro-Optics, 2001. CLEO '01. Technical Digest. Summaries of papers presented at the Conference on, pp.116, 11-11 May 2001. CrossRef J.P. Koplow, S.W. Moore, D.A.V. Kliner, "A new method for side pumping of double-clad fiber sources", IEEE Journal of Quantum Electronics, 39, 4, Apr 2003. CrossRef Ch. Li, D. Shen, J. Song, N. S. Kim, K. Ueda, "Absorption characteristics of high-power disc fiber laser by side pumping of laser diode", Lasers and Electro-Optics. CLEO/Pacific Rim '99, 3 (1999). CrossRef J. Swiderski, A. Zając, M. Skorczakowski, "Pulsed ytterbium-doped large mode area double-clad fiber amplifier in MOFPA configuration", Opto-Electronics Review, 15, 2, (June 2007). CrossRef Y. Jeong, et al., "40-Gb/s Widely Tunable Transceivers", IEEE Journal of Selected Topics In Quantum Electronics, 13, 3 (May/June 2007). CrossRef W. Shi, Q. Fang, X. Zhu, R. A. Norwood, N. Peyghambarian, "Fiber lasers and their applications", Applied Optics 53, 28 (2014). CrossRef D. J.Richardson, J. Nilsson, and W. A. Clarkson, "High power fiber lasers: current status and future perspectives", J. Opt. Soc. Am. B 27, 11 (2010). CrossRef F. Gonthier et al., "High-Power all-fiber components: The missing link for high power fiber lasers", Fiber Lasers: Technology, Systems and Applications, 266-276, (2004). CrossRef A. Braglia, A. Califano, Y. Liu and G. Perrone, "Architectures and components for high power CW fiber lasers", International Journal of Modern Physics B, 28, 12 (2014). CrossRef G. Sobon, P. Kaczmarek, D. Sliwinska, J. Sotor, K. Abramski, "High-Power Fiber-Based Femtosecond CPA System at 1560 nm", IEEE Journal of Selected Topics in Quantum Electronics, 20, 5 (2014). CrossRef D. J. DiGiovanni, and A. J. Stentz, "Tapered fiber bundles for coupling light into and out of cladding-pumped fiber devices", U.S. Patent 5,864,644 A, Jan. 26, (1999). DirectLink A. Braglia, A. Califano, Y. Liu and G. Perrone, "Architectures and components for high power CW fiber lasers", International Journal of Modern Physics B, 28, 12 (2014). CrossRef J. K. Kim et al., "Monolithic all-glass pump combiner scheme for high-power fiber laser systems", Optics Express, 18, 12, (24 May 2010). CrossRef B. Sévigny, P. Poirier, M. Faucher, "Pump combiner loss as a function of input numerical aperture power distribution", Proc. SPIE 7195, 719523 (2009) CrossRef A. Kosterin, V. Temyanko, M. Fallahi,M. Mansuripur, "Tapered fiber bundles for high power applications", Technical Digest. OFC/NFOEC 2 (2005) CrossRef P. Koška, Y. Baravets,P. Peterka, J. Bohata, M. Písařík, "Mode-field adapter for tapered-fiber-bundle signal and pump combiners", Applied Optics, 54, 4 (1 February 2015) CrossRef R. Wiley, B. Clark, "High-power fused assemblies enabled by advances in fiber-processing technologies", Proc. SPIE 7914, 79140F (2011). CrossRef

In order to improve the output power and processing quality in industrial applications, it is important to optimize the output characteristics of the high-power fiber lasers. The slope efficiency, backward leaking power and stimulated Raman scattering are key issues in high-power fiber laser design. The parameters matching the fiber gratings, which are the critical components, have a direct influence on the whole fiber laser system. In this paper, the parameters matching the fiber gratings in fiber lasers are investigated. Firstly, the origin of slope efficiency, backward leaking power and stimulated Raman scattering are analyzed in theory. Then the influences on output characteristics of fiber lasers comprised of the output coupler gratings, which have different bandwidths and reflectivities, are experimentally studied. Finally, the optimized parameters and matching principle of fiber gratings in high-power fiber laser are obtained , thus providing an alternative method to improve the output characteristics of high-power continuous wave fiber laser.

We report an experimental study of the effect of fiber length and laser linewidth on self-pulsing dynamics and output stabilization of a single-mode Yb-doped double-clad CW fiber laser. It is found that initiation of self-pulsing under low-level pumping conditions is due to relaxation oscillations and saturable absorption in the weakly pumped region of the doped fiber, irrespective of the fiber length and the laser linewidth. However, with an increase in pump power, depending on fiber length and laser linewidth, the pulses initiated due to relaxation oscillation get amplified, and result in short-duration giant pulses due to either stimulated Brillouin scattering (SBS) or stimulated Raman scattering (SRS). In the case of fiber lasers that employ a broadband mirror and wherein the fiber length is sufficient to reach the SRS threshold, the giant self-pulses are generated by SRS, whereas in the case of fiber lasers using a fiber Bragg grating, characterized by narrowband reflection and with sufficient fiber length to reach the SBS threshold, the giant self-pulses are generated by SBS. Output stabilization and, hence, elimination of self-pulsations can be achieved either by suppressing the relaxation oscillations with the addition of an appropriate length of a passive fiber to sufficiently increase the cavity photon lifetime, or by increasing the pump power to achieve gain uniformity along the doped fiber such that relaxation oscillations and reabsorption effects are suppressed.

A monolithic fiber laser operating in the short wavelength infrared that is suitable for CO2 gas sensing applications is proposed and presented. The current study reports a laser design based on the direct inscription of a monolithic Fabry–Perot (FP) cavity in a thulium-doped optical fiber using the femtosecond laser (FsL) plane-by-plane inscription method to produce the cavity mirrors. The FP cavity was inscribed directly into the active fiber using two wavelength-identical fiber Bragg gratings (FBGs), one with high and one with low reflectivity. Initially the effective length of the fiber was defined using a single high reflectivity FBG and subsequently a very weak FBG was inscribed at the other end of the fiber in order to demonstrate a fully monolithic fiber laser. All fiber lasers were designed for continuous wave operation at 1950 nm and characterized with respect to the power output, slope efficiency, stability, and effective resonator length. The performance of the presented monolithic laser cavities was evaluated using the same active fiber as a reference fiber spliced to FBGs inscribed in passive fiber; an improvement exceeding 12% slope efficiency is reported for the presented monolithic laser.

Absorption coefficient is a very useful feature for active fiber. In fiber laser system, the length of active fiber is chosen according to absorption coefficient. And the length of fiber can directly influence the feature of fiber laser. Therefore, how to obtain an accurate absorption coefficient is very important. Because fiber exists re-emission in typical absorption band pumped by power. It is difficult to accurately measure absorption coefficient. The absorption coefficients of Yb-doped double cladding fiber at 975 nm measured by several methods were compared. In conclusion, for the fibers with same length pumped by white light, the absorption coefficient is the highest when cutback only once. Meanwhile, when fibers with different length were measured by the same method, the absorption coefficient is inversely proportional to optical fiber length.

We report on the mode field adaption of an active thulium-doped fiber by using the thermally-diffused expanded-core technique. The fiber core diffusion is analyzed by splice transmission measurements and visually from side view images. The obtained heating parameters are used to build a thulium-doped fiber laser emitting at 2036nm that is core-pumped by an erbium:ytterbium fiber laser. By allowing the fiber cores to diffuse, the mode fields of the active and passive fibers are adapted for both the signal and pump wavelength. The adaptation of the mode fields increases the slope efficiency from 66.1% to 75.0%. The obtained slope efficiency is close to the stoke efficiency of 77.0%. By comparing the results with a fiber laser simulation, the slope efficiency of 75.0% is verified to be the maximum slope efficiency taking the active fiber length into account.

The results of experimental and theoretical studies of the fiber laser based on active Yb3+ fibers are presented. It is shown that the design parameters, such as the length of the fiber resonator and the reflection coefficient of the Bragg gratings, in a continuous laser can be predetermined by simulation. An analytical solution for the output power of the laser is obtained. The modeling results are compared with experimental data. The dependences of the output power on the active fiber length and the reflection coefficient of the output Bragg grating are modeled, from which the parameters of the laser with the highest emission power are determined. For a given concentration of ytterbium ions, the length of the active fiber and the value of the reflection coefficient of the Bragg grating at the output of the resonator, which provide the maximum output power of the laser radiation, have been obtained. The comparison of the results of laser power modeling with experiments has shown that the discrepancy does not exceed 10% and the modeling can be used to select the laser parameters. The developed model is universal and suitable for describing the radiation of fiber lasers with resonators on Bragg gratings and active singlemode fibers doped with other rare earth elements.

The recently observed laser wavelength self-sweeping is known for its large sweeping range up to tens of nanometers, single- or few-longitudinal mode operation and self-pulsing output [1-5]. The self-sweeping can be explained by spatial hole burning along the active fiber. The spatial hole burning cause also weak refractive index modulation [6-7]. Self-swept fiber lasers recently attained attention thanks to its practical applications in laser spectroscopy and fiber sensor systems as well as revealing physical insight view into important laser physics phenomena, e.g., the origin of self-Q-switching regime and longitudinal mode-instabilities in fiber lasers. We present evaluation of the reflectivity of superimposed transient fiber Bragg gratings (FBG) using a numerical model of fiber laser with refractive index gratings build-up. In contrast to single refractive index grating demonstrated earlier [6-7] we present more realistic case of several superimposed gratings with damped modulation depth. We show that the 100 % reflectivity and thus sudden enhancement of the cavity Q-factor can be easily achieved despite small modulation depth of the gratings. The differences of reflectivities of the single FBG and of the multiple superimposed damped FBGs are discussed. This work was supported by the Czech Science Foundation, project No. 16-13306S. [1] A. Lobach, S. I. Kablukov, E. V. Podivilov, and S. A. Babin, “Broad-range self-sweeping of a narrow-line self-pulsing Yb-doped fiber laser,” Opt. Express 19, 17632-17640 (2011). [2] V. Kir’yanov and N. Il’ichev, “Self-induced laser line sweeping in an ytterbium fiber laser with non-resonant Fabry-Perot cavity,” Laser Phys. Lett. 8, 305-312 (2011). [3] P. Peterka, P. Navratil, J. Maria, B. Dussardier, R. Slavik, P. Honzatko, and V. Kubecek, “Self-induced laser line sweeping in double-clad Yb-doped fiber-ring lasers,” Laser Phys. Lett. 9, 445-450 (2012). [4] P. Peterka, J. Maria, B. Dussardier, R. Slavik, P. Honzatko, and V. Kubecek, “Long-period fiber grating as wavelength selective element in double-clad Yb-doped fiber-ring lasers,” Laser Phys. Lett. 6, 732-736 (2009). [5] P. Peterka, P. Navratil, B. Dussardier, R. Slavik, P. Honzatko, and V. Kubecek, “Self-induced laser line sweeping and self-pulsing in double-clad fiber lasers in Fabry-Perot and unidirectional ring cavities,” In Proc. SPIE 8433, 843309 (2012). [6] P. Peterka, P. Honzatko, P. Koska, F. Todorov, J. Aubrecht, O. Podrazký, and I. Kasik, Reflectivity of transient Bragg reflection gratings in fiber laser with laser-wavelength self-sweeping, Opt. Express 22:30024-30031, 2014. [7] P. Peterka, P. Honzatko, P. Koska, F. Todorov, J. Aubrecht, O. Podrazký, and I. Kasik, Reflectivity of transient Bragg reflection gratings in fiber laser with laser-wavelength self-sweeping: erratum, Opt. Express 24, 16222-16223 (2016)