Bend-insensitive bismuth-doped P2O5-SiO2 glass core fiber for a compact O-band amplifier.

For the first time, we report on the fabrication of a bend-insensitive single-mode bismuth (Bi)-doped $ {{\rm P}_2}{{\rm O}_5} {-} {{\rm SiO}_2} $P2O5-SiO2 fiber having a depressed cladding design and study its gain characteristics at a spectral region of 1.3-1.4 µm. It was shown that the obtained Bi-doped fiber can efficiently operate in the spectral band even at a bend radius of 1.5 cm. In addition, it was shown that this type of fiber has a smaller mode-field diameter in comparison with a step-index single-mode Bi-doped $ {{\rm P}_2}{{\rm O}_5} {-} {{\rm SiO}_2} $P2O5-SiO2 fiber with $ \Delta {n} \approx 0.006 $Δn≈0.006 that resulted in a decrease of saturation power and, as a consequence, in a reduction of the total pump power required to a high-level-gain operation. The laser and gain experiments show the possibility of the construction of a compact high-performance optical amplifier for O-band based on the depressed-cladding Bi-doped fiber.

Photostability of laser-active centers in bismuth-doped GeO2-SiO2 glass fibers under pumping at 1550 nm.

We report experimental measurements and numerical calculations regarding the photostability of laser-active centers associated with bismuth (BACs) in Bi-doped GeO2-SiO2 glass fibers under pumping at 1550 nm at different temperatures. It was discovered that BACs are unstable under 1550-nm pumping when the temperature is elevated to hundreds of degrees centigrade. A simple numerical model was proposed to account for the discovered instability which turned out to be in good agreement with the experimental data.

Bismuth-doped fiber laser at 1.32 µm mode-locked by single-walled carbon nanotubes.

Bismuth-doped fiber is a promising active media for pulsed lasers operating in various spectral regions. In this paper, we report on a picosecond mode-locked laser at a wavelength of 1.32 µm, based on a phosphosilicate fiber doped with bismuth. Stable self-starting generation of dissipative solitons, using single-walled carbon nanotubes (SWCNT) as a saturable absorber, was achieved. Evolution of the pulsed regime, depending on pump power, and stability of the pulsing were investigated.

Formation of laser-active centers in bismuth-doped high-germania silica fibers by thermal treatment.

The effect of thermal annealing on the luminescent and laser properties of high-germania-core silicate fibers doped with bismuth was investigated. We studied the behavior of optical absorption assigned to the bismuth-related active centers associated with germanium as well as the behavior of unsaturable absorption in annealed fibers with respect to the Bi content. The dependence of the increment of the active center content on the Bi concentration in the annealed fibers was obtained. We achieved laser oscillations near a wavelength of 1700 nm with a slope efficiency of 18% using a 8.5 m long Bi-doped fiber. The comparison of the output parameters of the laser based on an annealed Bi-doped fiber with the ones of a pristine Bi-doped fiber laser is given. The performance of the obtained bismuth-doped fiber lasers was modeled using the propagation and rate equations of a homogeneous quasi-two-level laser medium. Theoretical results are compared with experimental ones.

NALM-based bismuth-doped fiber laser at 1.7 µm.

We demonstrate, to the best of our knowledge, the first bismuth-doped fiber laser operating at 1.7 µm mode-locked by means of Kerr nonlinearity. The laser setup has a figure-of-eight all-fiber design with a nonlinear amplifying loop mirror (NALM) and yields 17 ps pulses with a 3.57 MHz repetition rate and the energy 84 pJ. Using the master oscillator power amplifier scheme with a bismuth fiber amplifier, the output pulse energy of 5.7 nJ was achieved. Further pulse compression in the fiber compressor shortened pulses to 630 fs. The operation of the master oscillator was modeled using the nonlinear Schrödinger equation. Calculated data are in good agreement with experimental results.

Infiltrated bunch of solitons in Bi-doped frequency-shifted feedback fibre laser operated at 1450 nm.

Mode-locked fibre laser as a dissipative system is characterized by rich forms of soliton interaction, which take place via internal energy exchange through noisy background in the presence of dispersion and nonlinearity. The result of soliton interaction was either stationary-localized or chaotically-oscillated soliton complexes, which have been shown before as stand-alone in the cavity. Here we report on a new form of solitons complex observed in Bi-doped mode-locked fibre laser operated at 1450 nm. The solitons are arranged in two different group types contemporizing in the cavity: one pulse group propagates as bound solitons with fixed phase relation and interpulse position eventuated in 30 dB spectrum modulation depth; while the other pulses form a bunch with continuously and chaotically moving solitons. The article describes both experimental and theoretical considerations of this effect.

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.

Photobleaching effect in bismuth-doped germanosilicate fibers.

Photoinduced reduction of absorption (photobleaching) in bismuth-doped germanosilicate fibers irradiated with 532-nm laser has been observed for the first time. It was demonstrated that bismuth-related active centers having the absorption bands at wavelengths of 1400 and 1700 nm degrade under photoexcitation at 532 nm. The photobleaching process rate was estimated using conventional stretched exponential technique. It was found that the photobleaching rate in bismuth-doped germanosilicate fibers does not depend on type of bismuth-related active center. The possible underlying mechanism of photobleaching process in bismuth-doped fibers is discussed.

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.

All-bismuth fiber system for femtosecond pulse generation, compression, and energy scaling.

We demonstrate a 1.44-µm bismuth-doped master oscillator-power amplifier (MOPA) system for generating femtosecond pulses. The cavity of master oscillator comprises dispersion-compensating fiber for detuning the total dispersion to the normal regime and a carbon nanotube saturable absorber for triggering the mode-locked operation. The described multifunction bismuth fiber amplifier performs energy scaling, large spectral broadening, and pulse compression. The results show that the large chirp superimposed on the pulses in the bismuth oscillator with normal dispersion can be effectively suppressed in a subsequent bismuth power amplifier with anomalous dispersion and high nonlinearity, resulting in high-quality pulses with record duration of 240 fs. An all-fiber design provides a practical solution that avoids the need for supplementary pulse stretching and compression.

Anti-Stokes luminescence in bismuth-doped silica and germania-based fibers.

Luminescence excitation spectra of active centers in bismuth-doped vitreous SiO(2) and vitreous GeO(2) optical fibers under the two-step excitation have been obtained for the first time. The results revealed only one bismuth-related IR active center formed in each of these fibers. The observed IR luminescence bands at 1430 nm (1650 nm) and 830 nm (950 nm), yellow-orange (red) band at 580 nm (655 nm), violet (blue) band at 420 nm (480 nm) belong to this bismuth-related active center in the vitreous SiO(2) (vitreous GeO(2)), correspondingly.

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.

Experimental demonstration of spectral broadening in an all-silica Bragg fiber.

We present the first report on experimental observation of nonlinear spectral broadening in an all-solid photonic band gap Bragg fiber of relatively large mode area approximately 62 microm(2). The theoretically designed Bragg fiber for this specific application was fabricated by the well known MCVD technique. Nonlinear spectral broadening was observed by launching high power femtosecond pulses of 1067 nm pump wavelength. These first results indicate that fabrication of such Bragg fibers, once perfected, should potentially serve as an alternative route for realization of supercontinuum light.

Luminescence and optical gain in Pb-doped silica-based optical fibers.

IR luminescence and optical gain in a Pb-doped fiber have been observed for the first time. Absorption, luminescence and pump on/pump off optical gain spectra, as well as luminescence decay time, have been measured in these fibers. Comparison of optical active center characteristics in Pb-doped and Bi-doped fibers of the same composition indicates an essential difference of optical active centers in these two types of fibers.

Polarization-maintaining photonic bandgap Bragg fiber.

The possibility of fabricating a polarization-maintaining Bragg fiber has been studied. It is shown that violation of the cylindrical symmetry of a Bragg mirror in most cases results in a sharp increase in optical loss, which is caused by resonance transmission through the Bragg mirror at wavelengths near the cutoffs of the modes of the high-index rings with a nonzero azimuthal index. It is shown that placing stress-applied parts or air holes inside the Bragg fiber core allows one to avoid this effect. A polarization-maintaining Bragg fiber with perfect light confinement in the core is demonstrated for the first time to our knowledge.

Bi-doped fiber lasers and amplifiers for a spectral region of 1300-1470 nm.

Bismuth-doped fiber lasers operating in the range 1300-1470 nm have been demonstrated for the first time, to our knowledge. It has been shown that Bi-doped alumina-free phosphogermanosilicate fibers reveal optical gain in a wavelength range of 1240-1485 nm with pumping at 1205, 1230, or 808 nm.

High-power photonic-bandgap fiber laser.

An original architecture of an active fiber allowing a nearly diffraction-limited beam to be produced is demonstrated. The active medium is a double-clad large-mode-area photonic-bandgap fiber consisting of a 10,000 ppm by weight Yb(3+)-doped core surrounded by an alternation of high- and low-index layers constituting a cylindrical photonic crystal. The periodic cladding allows the robust propagation of a approximately 200 microm(2) fundamental mode and efficiently discriminates against the high-order modes. The M(2) parameter was measured to be 1.17. A high-power cw laser was built exhibiting 80% slope efficiency above threshold. The robust propagation allows the fiber to be tightly bent. Weak incidence on the slope efficiency was observed with wounding radii as small as 6 cm.