Konstantin K. Bobkov

MW peak power diffraction limited monolithic Yb-doped tapered fiber amplifier

Chirped pulse monolithic fiber amplifier based on a newly developed tapered polarization maintaining Yb-doped fiber has been developed and optimized. A novel amplification regime in a relatively long (220 cm) tapered fiber of improved design, which has been theoretically predicted, allowed us to achieve an ultimate high peak power. In this regime, the signal propagates most of the fiber without amplification and growths very rapidly only in the last 80 cm of the tapered fiber, which has a mode field area of approximately 1000 μm2 near the output. We have demonstrated amplification of 20 ps chirped pulses centered at 1056-nm with spectral width of 20 nm to 0.7 MW peak power directly from the tapered fiber amplifier. The pulses had a diffraction limited quality (M2 ~ 1.124) and could be compressed down to 350 fs with 50% efficiency. In addition, amplification of narrow-band 9 ps pulses centered at 1064 nm to a peak power of 1.8 MW directly from the tapered fiber amplifier was demonstrated.

Monolithic high peak-power coherent Doppler lidar system

In this work we present a monolithic lidar system, based on a newly-developed double-clad large mode area (LMA) polarization-maintaining Er-doped fiber and specially designed LMA passive components. Optimization of the fiber designs resulted in as high as 100 W of SBS limited peak power. The amplifier and its passive components (circulator and collimator) were integrated in an existing lidar system. The enhanced lidar system provides three times increase of scanning range compared to one based on standard telecom-grade amplifiers.

Monolithic sub-MW peak power tapered ytterbium-doped fiber amplifier

ABSTRACT In this study, we present a novel monolithic ytterbium-doped fiber amplifier with more than 0.5 MW peak power output power. The amplifier is based on a 2.1 m long tapered fiber with core/cladding diameters changing from 10/80 µm (at the signal input end) to 50/430 µm (signal output, pump input). The fiber has all-glass polarization-maintaining design, that make possible utilization of conventional FC adapter and standard angle polishing to 7° for thick end. Pump absorption was measured to be 8 dB at 915 nm for the whole fiber length. Despite a very large mode area (~30 µm) the tapered fiber demonstrates low bend sensitivity (it is possible to coil tapered fiber with 9 cm radius) and a diffraction limited beam quality. In the amplifier the pump power was coupled through the thick end by means of collimating and focusing lenses. Dichroic mirror employed to separate output signal and counter-propagating pump power. We obtained 3.5 W of average output power for a 5 mW seed signal (coupled by usual fusion splicing through a thin end) corresponding to a 28 dB gain. The amplified pulses have duration of about 5 ps and energy of about 3.3 µJ that corresponding to over 0.5 MW peak power. The spectral width was 28 nm operating with center wavelength of 1057 nm.To the best of our knowledge it is the highest peak power obtained directly from the all-fiber amplifier.. Keywords: Yb-doped tapered fiber, all-fiber megawatt system, low bend sensitive tapered fiber, ultrashort pulses amplification 1. INTRODUCTION The very rapid power scaling of both pulsed and cw Yb-doped fiber lasers dictates stringent requirements on the utilized fibers, mainly in terms of nonlinear effects threshold. Conventional single-mode Yb-doped fibers exhibit a relatively low threshold for nonlinear effects since the light is confined in a small core (6-10 µm in diameter) and propagates along long distances (above 10 m in some cases). The increasing of nonlinearity threshold is the urgent problem in the case of pulsed fiber systems, which have been demonstrated to reach power levels of several hundreds of kW and even approach MW power level. The most simple and obvious way to solve this problem is the enlargement of core diameter resulting in the enlarged mode filed diameter of the fundamental mode. In the same time, this approach has so me drawbacks: the fiber becomes a multimode, leading to degradation of output beam quality, and extremely bend-sensitive, resulting in requirement to keep the fiber straight, i.e. the system becomes cumbersome. The main goal of this work was to develop a novel approach based on utilizing of so-called tapered fibers that allows one to obtain large mode field areas and maintain an acceptable bend-loss sensitivity at the same time. The fiber parameters optimization and its using in the final amplification stage of picosecond chirped-pulse amplification system have been demonstrated. The obtained output peak power was record for all-fiber amplifying systems and approached by power level of microstructured fibers based systems.

5.5 W monolitic single-mode fiber laser and amplifier operating near 976 nm

An all-glass Yb-doped single-mode fiber laser and amplifier with output power of more than 5 W near 977 nm have been realized. Both laser and amplifier were based on a specially developed photodarking-free Yb-doped fibers with increased core-to-clad diameter ratio (up to 0.31).

The first experimental observation of long-term mode degradation in high peak power Yb-doped amplifiers

Experimental observation of a novel effect of long-term mode shape degradation in a high peak power ytterbium-doped pulsed fiber lasers based on large mode area step-index fibers has been reported for the first time. It is shown, that the degradation is caused by power coupling from LP01 to LP11 mode, occurring on a long period refractive index grating (LPG) formed in the active fiber core. The photo-darkening process was found to play a significant role in the formation of a LPG.

Monolithic diffraction-limited 976-nm laser based on saddle-shaped photo darkening-free Yb-doped fiber

Monolithic 976 nm laser design based on a newly developed saddle-shaped Yb-doped fiber has been proposed. The fiber has central single-mode part with core diameter of about 12 μm and ultra-thin square-shaped clad with side of about 42x42 μm. At the both ends of the saddle-shaped fiber the core and the clad sizes were adiabatically increased up to 20/(70x70) μm and the fiber could be spliced with standard (80..125 μm clad) passive fibers using commercially available equipment. Single-mode laser at 976 nm based on the developed fiber has been fabricated and photodarkening-free operation with output power of 10.6 W, which is the record high for all-fiber laser schemes, has been demonstrated.

1.5 MW peak power diffraction limited monolithic Yb-doped tapered fiber amplifier

High peak power lasers are of great importance for a wide number of applications like material processing, remote sensing and etc. The most impressive, and still record, result has been demonstrated with using of photonics crystal fibers (PCFs), where amplification to more than 1 MW peak power directly from amplifier was demonstrated [1, 2]. However such fiber are difficult in production, handling and requires use of bulk optics, that negates the benefits of truly all-fiber systems. For this reason, the development of alternative LMA design is still very actual task. One of the promising one is the tapered step-index fiber [3, 4]. Such fibers have robust single-mode (SM) input end part (to couple signal) and a very big core and outer diameters output end part. The output signal end is formally highly multi-mode, but slowly increasing diameter along the tapered fiber, ensuring of adiabatic enlargement of fundamental mode and suppress excitation of high order modes. A larger first cladding diameter at the output tapered fiber end makes possible counter-pumping using a low-brightness cheap pump diodes. Recently we have demonstrated chirped pulse amplification system, with final cascade based on Yb-doped PM fiber, generating diffraction limited (M2<1.2) 5-ps pulses at 1057-nm with 0.5 MW of peak power and 7-ps pulses at 1035-nm with 100 kW of peak power, which were compressed down to 130-fs with 60% efficiency [4].

Factors reducing the efficiency of ytterbium fibre lasers and amplifiers operating near 0.98 μm

We have studied the factors that reduce the efficiency of cladding-pumped ytterbium fibre lasers and amplifiers operating in the spectral range around a wavelength of 0.98 μm. It has been shown that the core of the active fibre being not single-mode leads to accelerated development of amplified spontaneous luminescence around λ = 1.03 μm, which reduces the highest possible amplifier efficiency. We have examined the effect of spontaneous luminescence propagating in the first reflective cladding. It has been shown theoretically that the pump-to-signal conversion efficiency is highest when the ratio of the core and cladding diameters in a multimode active fibre lies in the range 0.7 – 1. We have fabricated and characterised an optical fibre with a core-to-cladding diameter ratio of 0.76 (core and cladding diameters of 95 and 125 μm, respectively), which allowed a pump conversion efficiency of 66 % to be reached, a record level for cladding-pumped ytterbium fibre laser systems emitting near 0.98 μm.

Sub-MW peak power diffraction-limited chirped-pulse monolithic Yb-doped tapered fiber amplifier

We demonstrate a novel amplification regime in a counter-pumped, relatively long (2 meters), large mode area, highly Yb-doped and polarization-maintaining tapered fiber, which offers a high peak power directly from the amplifier. The main feature of this regime is that the amplifying signal propagates through a thin part of the tapered fiber without amplification and experiences an extremely high gain in the thick part of the tapered fiber, where most of the pump power is absorbed. In this regime, we have demonstrated 8 ps pulse amplification to a peak power of up to 0.76 MW, which is limited by appearance of stimulated Raman scattering. In the same regime, 28 ps chirped pulses are amplified to a peak power of 0.35 MW directly from the amplifier and then compressed with 70% efficiency to 315 ± 10 fs, corresponding to an estimated peak power of 22 MW.

Long-term mode degradation in ytterbium-doped pulsed fiber lasers

A novel effect of long-term mode degradation in a low-average-power high-peak-power ytterbium-doped pulsed fiber lasers based on large mode area step-index fibers has been reported for the first time.