Here for the first time the technique of the resonant modes coupling is used for spectrally-selective fundamental mode suppression. In our work we considered the fiber design consisting of the low-index core surrounding by the appropriately chosen high-index absorbing rods. Mode suppression in this case happens due to the resonant core mode deformation owing to mode-anticrossing effect and its partial absorption into the rods. According to our calculations it was established that stop-band of the core fundamental mode can be easily adjusted for different practical aims by fiber bending. Furthermore, in the present work we implemented and studied passive fiber with three high-index absorbing rods incorporated into fiber cladding. The Sm was chosen as an absorbing element of the high-index rods.
Highly doped with ytterbium (up to 20 wt% of Yb2O3) Er-Yb aliminosilicate glass core fibers have been fabricated and thoroughly investigated. It has been discovered that in a particular condition the lasing properties of both Yb and Er ions can be nearly completely suppressed. Due to a very high Yb ions absorption in the 900-1000 nm range (~ 3000 dB/m at 915 nm) all together with low background losses the developed glass matrix is very promising for such applications as pump isolation in fiber lasers as well as spectral and mode filtering using highly absorbing glass dopants.
In this paper, we demonstrate the spectrally selective fundamental core mode suppression in single-mode fiber by mode anti-crossing technique. A unique feature of the method proposed in the current work is that the fundamental core mode can have rectangular-like excess loss spectrum with bandwidth controllable by fiber bending. This property made the proposed fiber design to be promising for suppression of lasing at unwanted wavelengths in different fiber lasers and amplifiers.
In this paper, we have developed Yb-doped fiber suitable for creation of all-fiber seed laser schemes operating near 977 nm. The fiber was based on a ring-doping design (cladding was partially doped with Yb-ions), which allowed us to fabricate a relatively small core and provide mode field diameter (MFD) of the active fiber comparable with standard fibers (to achieve small splicing losses with commercially available optical fibers) and, simultaneously, increase absorption from the cladding to keep a reasonably high lasing efficiency. So MFDx of the fiber was 12 μm, MFDy was 14 μm. Outer silica cladding of the active fiber was decreased to diameter of 80 μm and a special pump and signal combiner was used to inject pump and signal into the active fiber. Based on the developed Yb-doped fiber an all-fiber polarization maintaining mode-locked laser with central wavelength around 977 nm was demonstrated for the first time. SESAM was used as a saturable absorber. The laser was self-starting for pump powers above 4.6 W, with the output power of 3 mW. The autocorrelation was the best fitted with sech2 profile and pulse duration was estimated to be as long as 9.5 ps. The fundamental cavity frequency corresponded to the pulse repetition rate of 33.532 MHz. Signal-to-noise ratio measured in the radio frequency range was more than 50 dB, the line width was below 1 kHz, which indicate ultimate stability of the fabricated mode-lock laser.
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.
We developed a highly efficient double-clad Yb-doped polarization-maintaining fiber to be implemented for small-signal amplification near 0.976 μm. The fiber was designed to have a relatively small mode field diameter compatible with standard step-index single-mode optical fibers. Another feature of the fiber was a small threshold for 0.976 μm signal amplification, which was achieved by a creation of a thin inner cladding (80 μm diameter). The unique design of the fiber allowed us to construct successfully an all-fiber picoseconds mode-locked laser at 0.98 μm for the first time to the best of our knowledge.
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.
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