Jason D. Tafoya

High-power “Watt-level” CW operation of diode-pumped 2.7 μm fiber lasers using efficient cross-relaxation and energy transfer mechanisms

We report the demonstration of high power (660 mW) CW operation of a diode-pumped mid-IR Er fiber laser. This was achieved by using efficient depopulation of the lower laser level via enhanced cross-relaxation between Er ions and energy transfer to Pr ions (at doping densities much higher than those used previously in Er:ZBLAN), along with optimal pumping of such lasers via custom-designed double-clad fluoride fibers.

A compact diode-pumped passively Q-switched mid-IR fiber laser

A compact cladding-pumped Er:ZBLAN mid-infrared pulsed fiber laser using a liquefying gallium mirror as a saturable absorber is demonstrated for the first time. Preliminary results give pulse widths of 7 µs and energies of 38 nJ.

A monolithic pump signal multiplexer for air-clad photonic crystal fiber amplifiers

We report on the performance of a monolithic 6+1X1 fiber pump signal multiplexer for use in fiber amplifiers. The key component of this coupler design is an etched taper that transforms the low-numerical aperture large diameter pump radiation into a high numerical aperture small diamter format for injection into the pump cladding of an air-clad fiber while maintaining a constant refractive index profile in the core for efficient signal coupling. This taper was then fused onto the 6+1 fiber bundle at the large end and to the air-clad large mode area polarization maintaining photonic crystal fiber at the small end. We employed 6 pump delivery fibers in a 200/220/0.22 core/clad/NA format and a 25/250 polarization maintaining step index signal delivery fiber for the bundle. The large end of the taper had a cladding diameter of 650 μm while the small end had a cladding diameter of 300 μm to match the pump cladding diameter of the PCF which was 314 μm. The core within the taper had a constant diameter of 40 μm and NA of 0.07 achieved through a step index profile. The mode field diameter of the PCF was 54 μm. Signal coupling efficiency at 1550 nm was measured to be 90% with a polarization extinction ratio > 20dB while pump coupling efficiency was measured to be 87% at 1532nm. The low pump coupling efficiency was found to be due to pump delivery fibers that had a numerical aperture of 0.24, higher than the specification of 0.22. A simple calculation shows that with 0.22 NA pump fibers, the pump coupling efficiency would increase to 94%.

Advanced components for multi-kW fiber lasers

We report on the development and performance of a key components that enable the construction of multi-kW fiber amplifiers for government and industrial applications that are both reliable and highly affordable. The usefulness of these components span the range from single frequency near diffraction limited kW class fiber lasers to multimode wide linewidth fiber lasers for welding and cutting applications. The key components for these amplifiers are a novel multi fiber-coupled laser diode stack and a monolithic 6+1x1 large fiber pump/signal multiplexer. The precisely aligned 2-D laser diode emitter array found in laser diode stacks is utilized by way of a simple in-line imaging process with no mirror reflections to process a 2-D array of 380-450 elements into 3 400/440μm 0.22NA pump delivery fibers. The fiber combiner is an etched air taper design that transforms low numerical aperture (NA), large diameter pump radiation into a high NA, small diameter format for pump injection into an air-clad large mode area PCF, while maintaining a constant core size through the taper for efficient signal coupling and throughput. The fiber combiner has 6 400/440/0.22 core/clad/NA pump delivery fibers and a 20/440 PM step-index signal delivery fiber on the input side and a 40/525 PM undoped PCF on the output side. The etched air taper transforms the six 400/440 μm 0.22 NA pump fibers to the 500 μm 0.55 NA core of the PCF fiber with a measured pump combining efficiency of 92% with zero brightness drop. The combiner also operates as a stepwise mode converter via a 30 μm intermediate core region in the combiner between the 20 μm core of the input fiber and the 40 μm fiber core of the PCF with a measured signal efficiency of 90% while maintaining polarization with a measured PER of 20 dB. We report the signal coupling efficiency and power handling capability as well.

KW monolithic PCF fiber amplifers for narrow linewidth and single mode operation

We report on the progress towards the development and performance of Photonic Crystal Fiber (PCF) based multi-kW fiber amplifiers that are both narrow linewidth and have near diffraction limited beam quality for government and industrial applications that are both reliable and highly affordable. These amplifiers based on the foundation of Photonic Crystal Fibers along with a novel multi fiber-coupled laser diode stack and a monolithic 6+ l×l large fiber pump/signal multiplexer. The pixilated nature of the PCF fabrication process allows for the inclusion of precise index of refraction control so designs to control SBS, prevent modal instabilities and integrate large quantities of pump light can be realized, The precisely aligned 2-D laser diode emitter array found in laser diode stacks is utilized by way of a simple in-line imaging process with no mirror reflections to process a 2-D array of 380-450 elements into 3 400/440μm 0.22NA pump delivery fibers. The fiber combiner is an etched air taper design that transforms low numerical aperture (NA), large diameter pump radiation into a high NA, small diameter format for pump injection into an air-clad large mode area PCF, while maintaining a constant core size through the taper for efficient signal coupling and throughput. A completely monolithic amplifier utilizing these components has been demonstrated at 967W of output power at 1085nm.

Compact infrared laser for stapedotomy

A laser has been developed with the aim of being a microsurgical tool for ear surgery. Its emission at 2.7 - 2.8 μm is readily absorbed by water in tissue. This makes it ideal for ablation of the stapes, while minimizing transmission to the inner ear. Slices of porcine otic capsule bone to represent the stapes were ablated with the laser. The mechanical stress imparted to the stapes during ablation was measured using a piezoelectric film. The Er:YAG laser has similar optical properties, but this laser offers the possibility of a more compact surgical tool.

High-power monolithic fiber amplifiers based on advanced photonic crystal fiber designs

We report on the development and performance of a fully monolithic PCF amplifier that has achieved over 400 W with near diffraction limited beam quality with an approximately 1GHz phase modulated input. The key components for these amplifiers are an advanced PCF fiber design that combines segmented acoustically tailored (SAT) fiber that is gain tailored, a novel multi fiber-coupled laser diode stack and a monolithic 6+1x1 large fiber pump/signal multiplexer. The precisely aligned 2-D laser diode emitter array found in laser diode stacks is utilized by way of a simple in-line imaging process with no mirror reflections to process a 2-D array of 380-450 elements into 3 400/440μm 0.22NA pump delivery fibers. The fiber combiner is an etched air taper design that transforms low numerical aperture (NA), large diameter pump radiation into a high NA, small diameter format for pump injection into an air-clad large mode area PCF, while maintaining a constant core size through the taper for efficient signal coupling and throughput. The fiber combiner has 6 400/440/0.22 core/clad/NA pump delivery fibers and a 25/440 PM step-index signal delivery fiber on the input side and a 40/525 PM undoped PCF on the output side. The etched air taper transforms the six 400/440 μm 0.22 NA pump fibers to the 525 μm 0.55 NA core of the PCF fiber with a measured pump combining efficiency of over 95% with a low brightness drop. The combiner also operates as a stepwise mode converter via a 30 μm intermediate core region in the combiner between the 20 μm core of the input fiber and the 40 μm fiber core of the PCF with a measured signal efficiency of 60% to 70% while maintaining polarization with a measured PER of 20 dB. These devices were integrated in to a monolithic fiber amplifier with high efficiency and near diffraction limited beam quality.

Monolithic eye-safer photonic crystal fiber lasers and amplifiers

We report on the performance of monolithic, polarization maintaining, Er-doped photonic crystal fibers (PCF) and amplifiers operating in the eye-safer wavelength regime from 1.55-1.6 um. As part of this effort, we have developed novel 6x1+1 pump/signal combiners for air-clad photonic crystal fibers with six 0.22 NA, 200/220 um pump input fibers and a 25/250 PM signal fiber that allow efficient pump and signal coupling to the air-clad Er-doped PCF. These etched air taper combiners have been demonstrated at the kilowatt level under 976 nm pumping and perform an efficient brightness transformation from 0.19 NA, 1532 nm fiber coupled diode pumps into the 0.6 NA air-clad Er-doped PCF with a measured pump throughput efficiency of 88-92% and a signal throughput of 65-80% with a PER of <18 dB. These novel combiners have been efficiently spliced to 40 um core, 200 um pump cladding Er-doped PCFs providing high efficiency resonantly pumped, monolithic, eye-safer PCF fiber lasers and amplifiers. Using grating stabilized 1532 nm pump diodes, our current experiments have demonstrated single transverse mode operation of both monolithic eyesafer PCF lasers and amplifiers at the multi-Watt level with slope efficiencies of over 55%.

Efficient and compact high-power mid-IR (~3 μm) lasers for surgical applications

3 μm Er:ZBLAN fiber lasers have many advantages for surgical applications compared to other lasers. Some key advantages include variable pulse width (CW to ~100 μs), high repetition rate (>1 kHz) and relatively high and easily adjustable average power (0 to >10 watts) in a compact air-cooled package. In addition, the simplicity of the laser increases reliability and decreases cost. In this paper, we describe some preliminary experiments with the use of such an Er:ZBLAN surgical fiber laser for the ablation of porcine otic capsule bone tissue; in particular, we describe ablation characteristics as a function of pulse energy and pulse duration.

Photonic crystal fiber pump combiner for high-peak power all-fiber thulium lasers

We report on the performance of a prototype pump combiner for use with thulium-doped photonic crystal fiber (PCF). This platform is attractive for “all-fiber” high energy and high peak power laser sources at 2 μm. We will report on the performance of this integrated amplifier in comparison to free space amplification in Tm:PCF. In particular, we carefully look for spectral/temporal modulation resulting from multimode interference between fundamental and higher order transverse modes in the amplifier to evaluate this for ultrashort chirped pulse amplification. The slope efficiency for the all-fiber amplifier is 22.1 %, indicating the need for further improvement. However, an M2 < 1.07 demonstrates excellent beam quality, as well as amplified polarization extinction ratios of ~25 dB.