Tunable spatiotemporal mode-locked fiber laser at 1.55 µm.

We report the spatiotemporal mode-locked multimode fiber laser operating at 1.55 µm based on semiconductor saturable absorber mirrors with the mode-locking threshold as low as 104 mW. Benefiting from the multimode interference filtering effect introduced in the laser cavity not only the central wavelength can be continuously tuned from 1557 nm to 1567 nm, but also the number of the output pulses can be adjusted from 1 to 4 by simply adjusting the polarization controllers. This work provides a new platform for exploring the dynamic characteristics of spatiotemporal mode-locked pulses at negative dispersion regime. Moreover, this kind of tunable laser has potential applications in fields of all-optical signal processing, fiber sensing and information coding.

Spectral filtering in single-mode fibers using resonant coupling with absorbing rods.

We investigate (both theoretically and experimentally) a method for fundamental mode spectral filtering in single-mode optical fibers using the resonant mode coupling effect. We demonstrate the possibility of controlling the spectral bandwidth of the fundamental mode suppression band through appropriate choice of fiber parameters and fiber bending. The developed technique can be very useful for the design of fiber-based spectral filters (i.e., active fibers with suppression of laser emission at undesirable wavelengths, suppression of stimulated Raman scattering).

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.

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.

High-order mode suppression in double-clad optical fibers by adding absorbing inclusions.

We proposed and experimentally demonstrated a technique for the suppression of unwanted modes in double-clad fibers with a high core-to-clad diameter ratio by introducing high-index absorbing inclusions into the first cladding of the fibers. These inclusions disturb the shape of undesirable modes, and a noticeable part of the power becomes localized inside the inclusion, resulting in an increase in the propagation loss of these modes. Two fiber designs were studied and realized: one with cylindrical symmetry and an absorbing high-index ring as the inclusion and another with high-index absorbing rods inserted around the fiber core. In both cases, the possibility of achieving perfect single-mode propagation was demonstrated both theoretically and experimentally.

Effect of the AlPO4 join on the pump-to-signal conversion efficiency in heavily Er-doped fibers.

Heavily Er-doped fibers (EDFs) based on P(2)O(5)-Al(2)O(3)-SiO(2) (PAS) ternary glass have been studied. A unique feature of this glass is the formation of a AlPO(4) join having a structure similar to that of SiO(2) glass and a refractive index below it. It is found that the Er(3+) absorption and emission spectra in the PAS EDFs are defined by the dopant (Al(2)O(3) or P(2)O(5)) present in excess and are close to those of the corresponding binary glass (Al(2)O(3)-SiO(2) or P(2)O(5)-SiO(2)). The presence of the AlPO(4) join results in the enhancement of the pump-to-signal conversion efficiency in the PAS EDFs as compared with the EDFs based on the P(2)O(5)-SiO(2) and Al(2)O(3)-SiO(2) (with 1.5 mol. %Al(2)O(3) and less) binary glasses. The PAS host glass is advantageous in the case of large-mode-area active fibers.

High-average-power second-harmonic generation from periodically poled silica fibers.

The generation of 236 mW of second-harmonic power in a 32-cm-long periodically poled silica fiber, corresponding to an average conversion efficiency of 15.2+/-0.5%, is reported. This represents the highest normalized second-harmonic conversion and the highest average second-harmonic power ever reported for a periodically poled silica fiber, to our knowledge. The enhancement is attributed to an improved design of the specialty twin-hole fiber and the extension of the nonlinear interaction length.

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.