Joshua Bradford

Highly polarized all-fiber thulium laser with femtosecond-laser-written fiber Bragg gratings.

We demonstrate and characterize a highly linearly polarized (18.8 dB) narrow spectral emission (<80 pm) from an all-fiber Tm laser utilizing femtosecond-laser-written fiber Bragg gratings. Thermally-dependent anisotropic birefringence is observed in the FBG transmission, the effects of which enable both the generation and elimination of highly linearly polarized output. To our knowledge, this is the first detailed study of such thermal anisotropic birefringence in femtosecond-written FBGs.

Thulium fiber laser and application development

Within the past 10 years, thulium (Tm)-doped fiber lasers have emerged as a flexible platform offering high average power as well as high peak power. Many of the benefits and limitations of Tm:fiber lasers are similar to those for ytterbium (Yb)-doped fiber lasers, however the ~2 µm emission wavelength posses unique challenges in terms of laser development as well as several benefits for applications. In this presentation, we will review the progress of laser development in CW, nanosecond, picosecond, and femtosecond regimes. As a review of our efforts in the development of power amplifiers, we will compare large mode area (LMA) stepindex and photonic crystal fiber (PCF) architectures. In our research, we have found Tm-doped step index LMA fibers to offer relatively high efficiency and average powers at the expense of fundamental mode quality. By comparison, Tm-doped PCFs provide the largest mode area and quasi diffraction-limited beam quality however they are approximately half as efficient as step-index fibers. In terms of defense related applications, the most prominent use of Tm:fiber lasers is to pump nonlinear conversion to the mid-IR such as supercontinuum generation and optical parametric oscillators/amplifiers (OPO/A). We have recently demonstrated Tm:fiber pumped OPOs which generate ~28 kW peak power in the mid-IR. In addition, we will show that Tm:fiber lasers also offer interesting capabilities in the processing of semiconductors.

Mode-selective amplification in a large mode area Yb-doped fiber using a photonic lantern

We demonstrate selective spatial mode amplification in a few mode, double-clad Yb-doped large mode area (LMA) fiber, utilizing an all-fiber photonic lantern. Amplification to multi-watt output power is achieved while preserving high spatial mode selectivity. We observe gain values of over 12 dB for all modes: LP01, LP11a, and LP11b, when amplified individually. Additionally, we investigate the simultaneous amplification of LP01+LP11a and LP11a+LP11b, and the resultant mode competition. The proposed architecture allows for the reconfigurable excitation of spatial modes in the LMA fiber amplifiers, and represents a promising method that could enable dynamic spatial mode control in high power fiber lasers.

Nonlinear processes associated with the amplification of MHz-linewidth laser pulses in single-mode Tm:fiber

This work studies the accumulated nonlinearities when amplifying a narrow linewidth 2053 nm seed in a single mode Tm:fiber amplifier. A <2 MHz linewidth CW diode seed is externally modulated using a fiberized acousto-optic modulator. This enables independent control of repetition rate and pulse duration (>30 ns). The pulses are subsequently amplified and the repetition rate is further reduced using a second acousto-optic modulator. It is well known that spectral degradation occurs in such fibers for peak powers over 100s of watts due to self-phase modulation, four-wave mixing, and stimulated Raman scattering. In addition to enabling a thorough test bed to study such spectral broadening, this system will also enable the investigation of stimulated Brillouin scattering thresholds in the same system. This detailed study of the nonlinearities encountered in 2 μm fiber amplifiers is important in a range of applications from telecommunications to the amplification of ultrashort laser pulses.

Rapid thermo-optical quality assessment of laser gain media

We describe a technique for the quick and simple assessment of doped optical materials for use as laser gain media. To demonstrate this technique, referred to as Rapid Thermo-Optical Assessment (RTOA), we analyze a set of ceramic and crystalline Yb3+:YAG samples. RTOA is based on Shack-Hartmann wavefront sensing and thermal lensing to evaluate the media’s thermal response, giving a relative overall quality assessment of the material. The technique is also broadly applicable to optical media considered for high power or thermal loading conditions, and useful for the refinement of fabrication methods.

A VBG-stabilized narrow linewidth, spectrally tunable, Yb:YAG thin-disk laser

We present an Yb: YAG thin-disk oscillator providing a tunable linewidth of 50-pm via VBG-based feedback. Output powers of up to 2 W have been recorded while maintaining an excellent spatial profile.Volume Bragg gratings

Progress on high-power Yb, Tm and Raman fiber lasers

To advance the science of high power fiber lasers, in-house drawn specialty optical fibers are investigated. Ongoing research involves the fabrication and testing of Yb- and Tm-doped fibers at 1μm and 2μm. Using specialized fiber and pump mixing geometries, dopant profiles and system configurations, the performance of our in-house drawn active fibers has been examined. Results on a highly multi-mode, high average power pulsed Raman fiber amplifier pumped by a thin disc laser are presented. The Raman fiber is a large mode-area graded index fiber, also drawn in house. Finally, the development of capabilities for kilometer range propagation experiments of kW-level CW and TW-level pulsed lasers at the TISTEF laser range is reported.

Influence of Temperature on Nanosecond Pulse Amplification in Thulium Doped Fiber Lasers

Thulium silica doped fiber (TDF) lasers are becoming important laser sources in both research and applications in industry. A key element of all high-power lasers is thermal management and its impact on laser performance. This is particularly important in TDF lasers, which utilize an unusual cross-relation pumping scheme, and are optically less efficient than other types of fiber lasers. The present work describes an experimental investigation of thermal management in a high power, high repetition-rate, pulsed Thulium (Tm) fiber laser. A tunable nanosecond TDF laser system across the 1838 nm – 1948 nm wavelength range, has been built to propagate 2μm signal seed pulses into a TDF amplifier, comprising a polarized large mode area (PLMA) thulium fiber (TDF) with a 793nm laser diode pump source. The PLMA TDF amplifier is thermally managed by a separately controlled cooling system with a temperature varied from 12°C to 36°C. The maximum output energy (~400 μJ), of the system is achieved at 12°C at 1947 nm wavelength with ~32 W of absorbed pump power at 20 kHz with a pulse duration of ~ 74 ns.

Demonstration of passively cooled high-power Yb fiber amplifier

This work investigates the feasibility of passive cooling in high-power Yb amplifiers. Experimentally, an all-glass airclad step-index (ACSI) amplifier is diode-pumped with 400W and provides 200W power levels. With only natural convection to extract heat, core temperatures are estimated near 130°C with no degradation of performance relative to cooled architectures. Further, advanced analysis techniques allow for core temperature determination using thermal interferometry without the need for complicated stabilization or calibration.

High average power pulsed multi-mode Raman fiber laser in graded index fiber

Raman fiber lasers have seen increased interest recently, due to their ability to access difficult wavelength ranges without the use of specially doped materials and to avoid some of the obstacles of very high power rare-earth doped fiber lasers, including modal instability and photodarkening. Though most modern works in Raman fiber lasers are based on fiber laser or direct diode pumping, solid state lasers have been developed with extremely high average powers and are readily available commercially. This work explores a very short fiber length high average power multi-mode Raman laser system. The custom 200um graded index fiber is pumped by 30ns pulses with average powers up to 750W and pulse energies up to 7.5mJ at 1030nm, by a solid state commercial laser system. Pump-only and seeded configurations are examined. In the seeded case, higher order mode activation is demonstrated by detuning the single mode seed to preferentially feed energy to the less confined modes. 5 orders of Stokes are demonstrated, ranging from 1078nm to 1350 nm. Beam enhancement is observed by qualitative measurement of minimum beam waist, and average powers up to 70W are achieved at an energy of 1.4mJ.