Compact and efficient O-band bismuth-doped phosphosilicate fiber amplifier for fiber-optic communications.

During last decades there has been considerable interest in developing a fiber amplifier for the 1.3-[Formula: see text]m spectral region that is comparable in performance to the Er-doped fiber amplifier operating near 1.55 [Formula: see text]m. It is due to the fact that most of the existing fiber-optic communication systems that dominate terrestrial networks could be used for the data transmission in O-band (1260-1360 nm), where dispersion compensation is not required, providing a low-cost increase of the capacity. In this regard, significant efforts of the research laboratories were initially directed towards the study of the praseodymium-doped fluoride fiber amplifier having high gain and output powers at the desired wavelengths. However, despite the fact that this type of amplifiers had rapidly appeared as a commercial amplifier prototype it did not receive widespread demand in the telecom industry because of its low efficiency. It stimulated the search of novel optical materials for this purpose. About 10 years ago, a new type of bismuth-doped active fibers was developed, which turned out to be a promising medium for amplification at 1.3 [Formula: see text]m. Here, we report on the development of a compact and efficient 20-dB (achieved for signal powers between [Formula: see text] and [Formula: see text] dBm) bismuth-doped fiber amplifier for a wavelength region of 1300-1350 nm in the forward, backward and bi-directional configurations, which can be pumped by a commercially available laser diode at 1230 nm with an output power of 250 mW. The compactness of the tested amplifier was provided by using a depressed cladding active fiber with low bending loss, which was coiled on a reel with a radius of 1.5 cm. We studied the gain and noise figure characteristics at different pump and signal powers. A record gain coefficient of 0.18 dB/mW (at the pump-to-signal power conversion efficiency of above 27[Formula: see text]) has been achieved.

Narrowband random lasing in a Bismuth-doped active fiber.

Random fiber lasers operating via the Rayleigh scattering (RS) feedback attract now a great deal of attention as they generate a high-quality unidirectional laser beam with the efficiency and performance comparable and even exceeding those of fiber lasers with conventional cavities. Similar to other random lasers, both amplification and random scattering are distributed here along the laser medium being usually represented by a kilometers-long passive fiber with Raman gain. However, it is hardly possible to utilize normal gain in conventional active fibers as they are usually short and RS is negligible. Here we report on the first demonstration of the RS-based random lasing in an active fiber. This became possible due to the implementation of a new Bi-doped fiber with an increased amplification length and RS coefficient. The realized Bi-fiber random laser generates in a specific spectral region (1.42 µm) exhibiting unique features, in particular, a much narrower linewidth than that in conventional cavity of the same length, in agreement with the developed theory. Lasers of this type have a great potential for applications as Bi-doped fibers with different host compositions enable laser operation in an extremely broad range of wavelengths, 1.15-1.78 µm.

A 23-dB bismuth-doped optical fiber amplifier for a 1700-nm band.

It is now almost twenty-five years since the first Erbium-Doped Fiber Amplifier (EDFA) was demonstrated. Currently, the EDFA is one of the most important elements widely used in different kinds of fiber-optic communication systems. However, driven by a constantly increasing demand, the network traffic, growing exponentially over decades, will lead to the overload of these systems ("capacity crunch") because the operation of the EDFA is limited to a spectral region of 1530-1610 nm. It will require a search for new technologies and, in this respect, the development of optical amplifiers for new spectral regions can be a promising approach. Most of fiber-optic amplifiers are created using rare-earth-doped materials. As a result, wide bands in shorter (1150-1530 nm) and longer wavelength (1600-1750 nm) regions with respect to the gain band of Er-doped fibers are still uncovered. Here we report on the development of a novel fiber amplifier operating in a spectral region of 1640-1770 nm pumped by commercially available laser diodes at 1550 nm. This amplifier was realized using bismuth-doped high-germania silicate fibers fabricated by MCVD technique.

Single-frequency Bismuth-doped fiber laser with quasi-continuous self-sweeping.

Generation of regular pulses of linearly polarized radiation with periodic self-induced laser line sweeping by ~10 nm near central wavelength of ~1460 nm has been demonstrated for the first time in an all-fiber Bismuth laser without any tuning element. It has been shown that the radiation of each pulse is single-frequency, and the pulse-to-pulse frequency shift is as low as 1 MHz corresponding to one intermode interval in 100-m long laser cavity. The measured intra-pulse frequency chirp is below 1 MHz while the pulses are long (~10 µs) and overlapping. Thus the sweeping is nearly continuous in frequency and time domains.

Superfluorescent 1.44 µm bismuth-doped fiber source.

The first bismuth-doped superfluorescent fiber source (SFS) operating at 1.44 µm was developed. An SFS maximum output power in the double-pass backward configuration reached 82 mW with a fairly high efficiency of 31% at a pump wavelength of 1310 nm. The output spectrum is close to Gaussian shape with FWHM of ~25 nm.

Optical gain and laser generation in bismuth-doped silica fibers free of other dopants.

Luminescence emission and excitation spectra of bismuth-doped silica optical fibers free of other dopants have been obtained to construct an emission-excitation map in a wide wavelength range of 400-1600 nm. The main low-lying energy levels of the bismuth active centers in such fibers have been determined. For the first time (to our knowledge), optical gain and lasing have been obtained in such fibers. A gain of 8 dB has been achieved with a pump power of 340 mW, and a cw fiber laser emitting at 1460 nm with an output power of 40 mW and an efficiency of ≈3% has been created.