The spectral region between 1600 and 1800 nm is still a band where optical amplifiers struggle to achieve satisfactory gain and output power levels. The output power typically does not exceed few tens of milliwatts, what severely limits some applications. In this paper, the bismuth-doped fiber amplifier (BDFA) operating beyond 1600 nm is presented. We demonstrate the in-house developed BDFA capable of providing output powers that exceed 200 mW for wavelengths near 1700 nm. The performance of the amplifier is discussed and various properties of the device are presented such as gain characteristics and noise figure.
The stimulated Brillouin scattering (SBS) is the main power limiting factor in the high-power narrow-line fiber laser circuits. A good way to increase the SBS power-threshold is to reduce its coherent gain, which is directly connected with the width of the Brillouin gain spectrum (BGS). The Brillouin gain peak and the phonons life-time are inversely proportional to the full-width-at-half-maximum of the BGS. The fine structure of the BGS and its ranges are sufficiently affected by parameters of the acoustic waveguide of the fiber. We propose a novel approach for increasing the BGS width and lowering its maximum (related directly to the SBS threshold) for a given optical refractive index profile. The aim of the approach is to maximize both the number and spectral spread of guided acoustic modes, as well as equate the acousto-optic interaction coefficients (acousto-optical overlap integrals) for the maximum possible number of these modes. This is due to the fact that an increase in the number of acoustic modes effectively contributing to the BGS, while preserving the distribution of the optical mode intensity, causes the scattered Stokes optical power to be redistributed accordingly between the corresponding number of Brillouin spectral lines, providing proportional damping of the Brillouin gain. Such an acoustically multimode SBS suppression can be achieved by tailoring a proper radial acoustic refractive index profile which can be fabricated by co-doping of silica with phosphorous oxide and fluorine.
A bismuth-doped fiber amplifier (BDFA) operating between 1650 nm and 1700 nm will be presented. This wavelength region is particularly interesting due to potential application is laser-based methane detection. However, typical output power from laser diodes operating in this spectral region is only between 5 and 15 mW which may limit sensitivity and/or detection range in some spectroscopic systems. Application of fiber amplifiers could help to overcome these limitations. BDFA presented in this paper provides output powers up to 80 mW at 1651 nm and 100 mW at 1687 nm. We analyze the noise at the output of the amplifier and demonstrate its application to photothermal spectroscopy of methane near 1651 nm.
In this paper, we performed numerical and experimental study of the stability of bismuth-doped high-GeO2 glass core fiber used as an active medium in lasers operating in the wavelength region 1600 - 1800 nm. Mainly, we focus on the investigation of the joint effects of temperature and pumping radiation on the spectroscopic and laser characteristics of the fibers. Temporal evolution of the degradation of bismuth-related active centers (BACs) under pumping at 1550 nm, as well as the annealing of the fibers at temperature ranging from 300 to 550 °C was experimentally revealed and studied. A model describing the photochemical processes of the transformation of the BACs at different ambient conditions was proposed and used to make a long-term prediction of the dynamics of the process. The ability to simulate the long-term behavior of the medium might be instrumental since direct measurements are time consuming and therefore impractical. In addition, we performed numerical simulation to find out how the effect of photoinduced degradation of BACs affects the performance of a laser based on this type of fibers.
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.
Simple method to increase stimulated Brillouin scattering (SBS) threshold in MCVD fiber based on design with few concentric layers having different compound has been proposed. Two sets of fibers with core consisting of three layers with different alumina and germania concentrations have been fabricated. First set of fibers was designed for Raman amplifiers and had a relatively small mode field area of 23-28 μm2. The second set of fibers was designed for high peak power pulse delivery and had mode area of 225-325 μm2. SBS suppression (as compared to the Ge-doped fibers) was estimated from SBS gain spectra and direct observation of SBS threshold to be more than 6 dB and 3.3 dB for the first and the second set of fibers.
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