Single-frequency DBR lasing by integrating FBGs into germanium-free photosensitive highly Yb3+-doped silica fibers.

Monolithic distributed Bragg reflector (DBR) cavity which directly integrates fiber Bragg gratings (FBGs) into the photosensitive RE-doped fibers is a promising configuration in constructing compact and efficient single frequency fiber lasers (SFFLs). Yet, the doping level of rare-earth (RE) ions has generally to be sacrificed in the classical Ge-photosensitized RE-doped silica fibers because of the dramatic refractive index increase caused by the introduction of Ge. Here, we demonstrate an approach to realize the trade-off between photosensitivity and RE doping concentration. We validate that the addition of a small amount of cerium (0.37wt.%) instead of Ge could photosensitize Yb3+-doped silica fiber (YDF), while maintaining fiber numerical aperture (NA) at 0.12 under a high 2.5-wt.% Yb doping level. Based on the short monolithic DBR cavity constructed by this germanium-free photosensitive highly YDF, a 1064 nm fiber laser with a 48.6% slope efficiency and an over 200 mW power on two orthogonally polarized modes could be realized. Further stable and linear-polarized 1064 nm SFFL is also demonstrated in a designed monolithic polarization maintaining cavity with an output power of 119 mW and an efficiency of 26.4%. Our results provide an alternative way to develop photosensitive highly RE-doped fibers towards monolithic laser cavity application.

Er-doped silicate fiber amplifiers in the L-band with flat gain.

We demonstrated an efficient way to enhance and flatten the emission cross sections of Er3+ ions at the L-band in the silicate fiber amplifier by increasing Mg2+ (up to 22.5 mol%) with high field strength. High values of Er3+ concentration, lifetime, and L-band emission cross section were achieved in our silicate fibers. Particularly, the flatness at the L-band was achieved to be 0.8 dB, and a high gain coefficient at 1625 nm (4.7 dB/m) was demonstrated by pumping meter-scale Er-silicate fibers. The as-prepared Er-silicate fibers are attractive for the L-band fiber amplifier.

High ytterbium concentration Yb/Al/P/Ce co-doped silica fiber for 1-µm ultra-short cavity fiber laser application.

We demonstrate a high ytterbium concentration Yb/Al/P/Ce co-doped silica fiber by conventional modified chemical vapor deposition (MCVD) technology and solution doping process. The fiber has a Yb concentration of about 2.5 wt%, and the corresponding core absorption coefficient is measured to be ∼1400 dB/m at 976 nm. The gain coefficient was measured to be approximately 1.0 dB/cm. It is found that the Yb/Al/P/Ce co-doped silica shows a lower photodarkening-induced equilibrium loss of 52 dB/m at 633 nm than the Yb/Al/P co-doped silica fiber of 117 dB/m. Using the heavily Yb3+-doped silica fiber, a compact and robust ultrashort cavity single-frequency fiber laser was achieved with a maximum output power of 75 mW and a linewidth of 14 kHz. Furthermore, a compact passively mode-locked fiber laser (MLFL) with a repetition rate of 1.23 GHz was also proposed using our developed Yb-doped fiber. The laser properties of the proposed lasers were systematically investigated, demonstrating the superior performance of this fiber in terms of photodarkening resistance and ultrashort-cavity laser application. Furthermore, utilizing an all-fiber structure based on silica-based fiber offers the significant advantage of high stability and reliability.

Sub-kHz linewidth 1.6-µm single-frequency fiber laser based on a heavily erbium-doped silica fiber.

We present a single-frequency erbium-doped fiber laser operated at 1608.8 nm using a homemade, heavily erbium-doped silica fiber as gain medium. The laser configuration is based on a ring cavity, which is combined with a fiber saturable absorber to achieve single-frequency operation. The measured laser linewidth is less than 447 Hz and the optical signal-to-noise ratio exceeds 70 dB. The laser exhibits an excellent stability, without any instance of mode-hopping during 1-hour observing. The fluctuations in both wavelength and power were measured to be 0.002 nm and less than 0.09 dB in a 45-minutes period. The laser produces over 14 mW of output power with a slope efficiency of 5.3%, which, to the best of our knowledge, is currently the highest power directly obtained from a single-frequency cavity based on an erbium-doped silica fiber above 1.6 µm.

Brillouin gain spectrum characterization in an acoustic anti-guided delivery fiber for high power narrow linewidth laser.

The Brillouin gain spectrum (BGS) provides key information for stimulated Brillouin scattering (SBS), such as the Brillouin frequency shift (BFS), Brillouin spontaneous linewidth, and Brillouin gain coefficient. In this study, we theoretically investigate the field distributions and BGS characterization of Ge-doped, Al-doped, and Al/Ge co-doped fibers. Additionally, we analyzed and compared the relationship between the BGS and acoustic refractive index. In particular, we demonstrate the crucial role played by acoustic modes in anti-waveguide structures. The simulation results show that the Brillouin gain coefficient decreases with a decreasing acoustic index in the fiber core region. Furthermore, we experimentally measure the SBS threshold and BGS of the Al/Ge co-doped fiber to examine the validity of the numerical model. The simulated and experimental results are consistent.

Extending laser wavelengths to 1630†nm in centimeter-scale Er-phosphate fiber.

The spectral bandwidth of Er-doped fibers limits their lasing wavelength at longer wave band. Here, to the best of our knowledge, we report a broad emission band (1420‒1680 nm) of Er3+ and demonstrate for the first time an Er-phosphate fiber, which supports laser oscillation at the extended wavelengths of 1627 nm and 1630 nm, with the output powers and slope efficiencies of 44 mW/12.5% and 16.5 mW/5.6%, respectively, pumped at 1480 nm. To the best of our knowledge, these are the highest output powers and slope efficiencies at 1627 nm and 1630 nm from an Er3+-doped all-fiber configuration.

Temperature Dependence of Absorption and Energy Transfer Efficiency of Er3+/Yb3+/P5+ Co-Doped Silica Fiber Core Glasses.

A high phosphorus Er3+/Yb3+ co-doped silica (EYPS) fiber core glass was prepared using the sol-gel method combined with high-temperature sintering. The absorption spectra, emission spectra, and fluorescence decay curves were measured and compared in temperatures ranging from 300 to 480 K. Compared to 915 and 97x nm, the absorption cross-section at ~940 nm (~0.173 pm2) demonstrates a weaker temperature dependence. Hence, the 940 nm pump mechanism is favorable for achieving a high-power laser output at 1.5 µm. Additionally, the double-exponential fluorescence decay of Yb3+ ions and the emission intensity ratio of I1018nm/I1534nm were measured to evaluate the energy transfer efficiency from Yb3+ ions to Er3+ ions. Through the external heating and active quantum defect heating methods, the emission intensity ratios of I1018nm/I1534nm increase by 30.6% and 709.1%, respectively, from ~300 to ~480 K. The results indicate that the temperature rises significantly reduce the efficiency of the energy transfer from the Yb3+ to the Er3+ ions.

Improved radiation resistance of an Er-doped silica fiber by a preform pretreatment method.

We report a novel pretreatment method to improve the radiation resistance of Er-doped fiber (EDF). The processing object of this method is EDF preform, and the pretreatment processing involves three steps: deuterium loading, pre-irradiation, and thermal annealing. The effects of pretreatment conditions on the optical loss, gain performance, and radiation resistance of EDF were systematically studied. The relevant mechanisms were revealed using Fourier transform infrared (FTIR), radiation-induced absorption (RIA), and electron paramagnetic resonance (EPR) spectroscopies. The results show that the pretreatment can not only greatly reduce the hydroxyl content of the EDF core, but it can also effectively improve the radiation resistance of EDF. The online test results show that the gain of the commercial, pristine, and pretreated EDFs were reduced by 19.0, 4.2, and 1.3 dB, respectively, corresponding to a decrease of 68.1, 16.2, and 4.7% after 98 krad X-rays irradiation. The high vacuum experiments show that the pretreatment method can maintain long-term stable high radiation resistance. This work provides a reference for the development of high-performance radiation resistant EDFs for use in the lower, middle, and geosynchronous earth orbit.