Search

A tapered Er-doped fiber amplifier for high peak power pulses amplification has been developed and tested. The core diameter changed from 15.8 µm (mode field diameter (MFD) 14.5 µm) to 93 µm (MFD 40 µm) along 3.7 m maintaining single-mode performance at 1555 nm (according to the S2-method, the part of the power of high-order modes does not exceed 1.5%). The amplification of 0.9 ns pulses with spectral width below 0.04 nm up to a peak power above 200 kW (limited by self-phase modulation) with a slope pump-to-signal conversion efficiency of 15.6% was demonstrated.

In the paper we discuss current state of the art in the field of tapered fiber development. The best results in term of high peak and high average power achieved with this type of fibers are presented together with requirements to the tapered fiber amplifier design. The report is mainly focused on tapered fiber amplifiers operated near 1 μm (Yb-doped tapered fibers), but also extension of this technique to 1.55 μm spectral range is discussed.

In this paper we systematically study a limitation for maximum of Yb dopant concentration in silica based fibers. Two the most popular glass matrixes (F-Al2O3-SiO2 and Al2O3-P2O5-SiO2) were thoroughly investigated in this respect. A possibility to introduce ultra-high doping level of Yb2O3 (in excess of 2.5 mol%) with a relatively low optical losses in fibers was demonstrated. At the same time it was investigated that at ultra-high Yb concentration in the fiber core even with maintaining very low background losses (both initial and induced by photodarkening) such a fiber can nearly completely lose its active properties. Optimal glass matrixes and optimal concentration of Yb dopant in the glass core, which allow keep up lasing properties of the developed fibers high, were studied. An ultra-short length fiber amplifier (~3.5 cm) based on the developed Al2O3-P2O5-SiO2 glass core fiber doped with 1.2 mol% of Yb2O3 (Yb ions absorption was about 1000 dB/m at 920 nm) was created.

In this paper, we demonstrate possibility of simultaneous achievement of high peak and high average power in picosecond pulses using a monolithic amplifier based on a long Yb-doped tapered fiber. Due to a very high pump absorption (~ 25 dB/m at 976 nm) in the realized 2.4 m long tapered fiber most of the pump is absorbed near the thick tapered fiber end and a very small fraction of pump power reaches thin fiber end. As a result, signal passes through the thin part of the tapered fiber without an amplification and exhibits fast growth only near the output tapered fiber end, where a mode field diameter is large (35 μm at 1064 nm for 46 μm output core diameter), so that pulses can be amplified to a high peak power. Moreover, only a negligible fraction of pump radiation leaks at the conic part of the tapered fiber, because its most part was absorbed in the thick tapered fiber part. Thus a safe operation without polymer burning at a leakage point is possible up to a very high pump power. The developed tapered fiber was used in a final amplification stage of the all-fiber pulsed laser system, which allowed us to amplify 8.3 ps pulses with repetition rate of 18.4 MHz and central wavelength of 1064 nm to 150 W of average power and 0.92 MW peak power. The average power level was limited only by available pump power (230 W): no signs of transverse mode instability effects were observed.

Ultrashort pulse fiber laser systems with both high average and peak power are promising tools for a wide range of applications. Usually pulsed fiber laser systems exhibit either high average power [1] or high peak power [2]. Only utilization of rod-photonics-crystal-fiber technology [3] allows one to achieve both high average (130 W) and peak (870 kW) power with a price of non-monolithic design (mainly bulk elements are used in [3], which lead to the laser's high cost and lose of reliability). Moreover not perfectly single-mode output (M2 = 1.30) results in limitation of maximum average power caused by mode instability. Recently sub-MW peak power in monolithic all-fiber laser scheme was demonstrated in [4] using tapered fiber approach. Scalability of average power in tapered fiber to 28 W at peak power of 292 kW was demonstrated in [5]. In the current communication we demonstrated further progress in average and peak power scaling using this approach.

Single-mode Er-Yb fibers based on phosphorosilicate glass matrix highly doped with fluorine have been fabricated using modified all-gas phase MCVD technology. Fibers have core doped by 6.5 mol.% of P2O5, 0.9 wt.% of F and different concentration of Er and Yb. The core NA was about 0.07-0.08 relative to the pure silica level. Slope efficiency of more than 19% was achieved using amplifier scheme with co-propagating pump at 976 nm and signal at 1555 nm (input signal power was about 0.6W). Slope efficiency in the laser configuration has reached 34% relative to the input pump power.

Highly Er/Yb co-doped phosphosilicate fibers with low optical losses were fabricated by the MCVD method, utilizing an all-gas-phase deposition technique developed “in house.” Important characteristics such as photodarkening and photosensitivity of these fibers were thoroughly investigated. An almost negligible impact of photodarkening-induced losses (no more than 1 dB/m) was discovered in the near-infrared spectral region. The photosensitivity of hydrogen-loaded fiber samples was studied using excimer laser irradiation at a wavelength of 193 nm. A photoinduced refractive index value of 1 × 10 − 3 at the total UV irradiation exposure dose of 550 J / c m 2 was achieved. A single-frequency laser with a Fabry–Perot short cavity was designed by inscribing highly reflective ( R > 99.9 % ) and partially reflective ( R = 94.3 % ) fiber Bragg gratings directly in the Er/Yb co-doped phosphosilicate fiber core. The laser showed stable CW output emitting with a narrow peak centered at 1551.5 nm in the absence of any signs of Y b 3 + parasite lasing in a wavelength range from 1000 to 1100 nm.

A novel tapered Er3+-doped fiber design for high peak power amplification has been developed and tested. The core diameter was changing along 2.5 meters from 22.5 μm (single-mode operation) to 86 μm. Amplification of 2 ns pulses has resulted in peak power of 105 kW (0.25 mJ) with nearly diffraction-limited beam quality (M2

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.

In this work we demonstrate possibility of scaling peak power to MW level (before compression) using end-pumped monolithic amplifier based on a novel type of large mode area fibers, Yb-doped tapered fiber.

Single-mode Er–Yb fibers with the core based on a phosphorosilicate glass matrix (up to 6.5 mol.% P2O5) highly doped with fluorine (up to 0.9 wt.%) were fabricated using an all-gas-phase modified chemical vapor deposition method. The core numerical aperture was in the range of 0.07–0.08 relative to the pure silica, which allowed us to increase the single-mode core diameter up to 20 μm. The slope efficiency in lasers based on the fabricated fibers reached 34% relative to the launched pump power.

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.

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).

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.