Radha K. Pattnaik

High power, high efficiency diode pumped Raman fiber laser

We demonstrate a high power high efficiency Raman fiber laser pumped directly by a laser diode module at 976 nm. 80 Watts of CW power were obtained at a wavelength of 1020 nm with an optical-to-optical efficiency of 53%. When working quasi-CW, at a duty cycle of 30%, 85 W of peak power was produced with an efficiency of 60%. A commercial graded-index (GRIN) core fiber acts as the Raman fiber in a power oscillator configuration, which includes spectral selection to prevent generation of the 2nd Stokes. In addition, significant brightness enhancement of the pump beam is attained due to the Raman gain distribution profile in the GRIN fiber. To the best of our knowledge, this is the highest power Raman fiber laser directly pumped by laser diodes, which also exhibits a record efficiency for such a laser. In addition, it is the highest power Raman fiber laser (regardless of pumping source) demonstrated based on a GRIN fiber.

Nanoparticle doping for improved Er-doped fiber lasers

A nanoparticle (NP) doping technique was used for making erbium-doped fibers (EDFs) for high energy lasers. The nanoparticles were doped into the silica soot of preforms, which were drawn into fibers. The Er luminescence lifetimes of the NP-doped cores are longer than those of corresponding solution-doped silica, and substantially less Al is incorporated into the NP-doped cores. Optical-to-optical slope efficiencies of greater than 71% have been measured. Initial investigations of stimulated Brillouin scattering (SBS) have indicated that SBS suppression is achieved by NP doping, where we observed a low intrinsic Brillouin gain coefficient, of ~1× 10-11 m/W and the Brillouin bandwidth was increased by 2.5x compared to fused silica.

Erbium nanoparticle doped fibers for efficient, resonantly-pumped Er-doped fiber lasers

Nanoparticle (NP) doping is a new technique for making erbium-doped fibers (EDFs); the Er ions are surrounded by a cage of aluminum and oxygen ions, substantially reducing Er3+ ion-ion energy exchange and its deleterious effects on laser performance. Er-Al-doped NPs have been synthesized and doped in-situ into the silica soot of the preform core. We report the first known measurements of NP-doped EDFs in a resonantly-core pumped master oscillator-power amplifier (MOPA) configuration; the optical-to-optical slope efficiency was 80.4%, which we believe is a record for this type of fiber.

Power scaling of the in-band diode-pumped Er-nanoparticle-doped fiber laser (Conference Presentation)

Successful power scaling of the Er-doped fiber laser heavily depends on Er dopant concentration. In order to scale the power up to a kW class, core absorption should be well over the limits defined by current commercial doping techniques. Recently developed nanoparticle (NP) doping technique of fabrication of erbium-doped fibers allows the desired dopant concentration increases while mitigating both Er upconversion and clustering effects. Here we present the latest resonantly-pumped Er fiber laser power scaling results enabled by further development of the NP doping technique of Er-doped fiber fabrication. Using resonantly cladding-pumped (at 1530 nm) large mode area 20/125 µm fiber with the Er-NP-doped core we achieved pump-limited CW power of over 30 W at ~1605 nm with the slope efficiency versus absorbed pump power of ~63%. These are, to the best of our knowledge, the highest power and efficiency demonstrated so far for from the Er-NP-doped fiber. Further considerations on fiber design optimization are presented as well.

Yb-Er co-doping for resonantly pumped fiber lasers at eye-safer wavelengths (Conference Presentation)

Erbium doped fiber lasers are attractive candidates for high energy lasers (HELs) used in directed energy applications because they operate at wavelengths that are both safer to the eye and in a high atmospheric transmission window. A significant issue for erbium doped fibers is detrimental clustering effects such as upconversion and quenching. We have investigated the use of ytterbium-erbium co-doping in both solution and nanoparticle form, where Yb ions are used to help disperse and separate the Er ions in order to avoid these effects. Both solution doping and nanoparticle doping were investigated and optimal concentrations for both were determined. By optimizing variables such as Yb/Er ratio and Al precursor concentrations during synthesis we have been able to increase erbium concentration levels in Silica fiber cores while minimizing detrimental clustering effects. In-band pumping of Er ions at 1475 nm in a single-mode master oscillator power amplifier set-up was used to investigate lasing efficiency, and therefore the Yb ions do not absorb and are optically passive. This ensures that the fibers are operating in the eye-safer regime. We have achieved optical to optical slope efficiencies of 50% for Yb-Er NP and solution doping in a single mode fiber with Er concentrations that are much greater than are achievable with conventional solution doping. Results indicate improved potential for power scaling.

Clad-pumped Er-nanoparticle-doped fiber laser (Conference Presentation)

Erbium-doped fiber lasers are attractive for directed energy weapons applications because they operate in a wavelength region that is both eye-safer and a window of high atmospheric transmission. For these applications a clad-pumped design is desirable, but the Er absorption must be high because of the areal dilution of the doped core vs. the pump cladding. High Er concentrations typically lead to Er ion clustering, resulting in quenching and upconversion. Nanoparticle (NP) doping of the core overcomes these problems by physically surrounding the Er ions with a cage of Al and O in the NP, which keeps them separated to minimize excited state energy transfer. A significant issue is obtaining high Er concentrations without the NP agglomeration that degrades the optical properties of the fiber core. We have developed the process for synthesizing stable Er-NP suspension which have been used to fabricate EDFs with Er concentrations >90 dB/m at 1532 nm. Matched clad fibers have been evaluated in a core-pumped MOPA with pump and signal wavelengths of 1475 and 1560 nm, respectively, and efficiencies of ~72% with respect to absorbed pump have been obtained. We have fabricated both NP- and solution-doped double clad fibers, which have been measured in a clad-pumped laser testbed using a 1532 nm pump. The 1595 nm laser efficiency of the NP-doped fiber was 47.7%, which is high enough for what is believed to be the first laser experiment with the cladding pumped, NP-doped fiber. Further improvements are likely with a shaped cladding and new low-index polymer coatings with lower absorption in the 1500 – 1600 nm range.

Erbium nanoparticle doping for high power fiber lasers

A nanoparticle (NP) doping technique was used for making erbium-doped fibers (EDFs). The nanoparticles were doped into the silica soot of preforms which were drawn into fibers. Results have indicated optical-to-optical slope efficiencies of 80.4%.