K. Urbanek

High-Power

We report on the development of novel high-power light sources utilizing a Yb3+-doped phosphate fiber as the gain element. This host presents several key benefits over silica, particularly much higher Yb2 O3 concentrations (up to 26 wt%), a 50% weaker stimulated Brillouin scattering (SBS) gain cross section, and the absence of observable photodarkening even at high population inversion. These properties result in a greatly increased SBS threshold compared to silica fibers, and therefore, potentially much higher output powers out of either a multimode large mode area or a single-mode fiber, which means in the latter case a higher beam quality. To quantify these predictions, we show through numerical simulations that double-clad phosphate fibers should produce as much as ~ 700 W of single-frequency output power in a step index, single-mode core. As a step in this direction, we report a short phosphate fiber amplifier doped with 12 wt% Yb2 O3 that emits 16 W of single-frequency single-mode output. We also describe a single-mode phosphate fiber laser with a maximum output power of 57 W. The laser slope efficiency is currently limited by the fairly high fiber loss ( ~ 3 dB/m). Measurements indicate that 77% of this loss originates from impurity absorption, and the rest from scattering.

\hbox{Yb}^{{\bm 3}{\bm +}}

Near-stoichiometric lithium tantalate (SLT) crystals were produced from congruent lithium tantalate by vapor transport equilibration, and several important optical and ferroelectric properties were measured. The effect of vapor transport conditions and surface preparation on reproducible ferroelectric engineering of SLT has been studied. Control of these effects along with dramatic decreases in the sensitivity to photorefractive damage and 532 nm absorption has allowed near-room-temperature generation of 10 W of continuous wave 532 nm radiation by second harmonic generation from 29 W of 1064 nm radiation in a 4 cm long device.

-Doped Phosphate Fiber Amplifier

We demonstrated a stable degenerate synchronously pumped femtosecond optical parametric oscillator (SPOPO) as a divide-by-2 subharmonic generator. The SPOPO exhibited passive all-optical self-phase-locking between the pump and signal/idler and thus required no external electronic feedback to produce the phase-locked subharmonic. We employed a type I phase-matched, 1-mm-long, periodically poled MgO:LiNbO3 crystal as the nonlinear gain element and an 80 MHz mode-locked Ti:sapphire laser with 180 fs pulses tuned at 775 nm as the pump. The SPOPO generated transform-limited 70 fs phase-locked output pulses centered at 1550 nm. The self-phase-locking operation was confirmed by separate beat-note measurement techniques with respect to the pump laser and with respect to an external cw laser.

Optical properties and ferroelectric engineering of vapor-transport-equilibrated, near-stoichiometric lithium tantalate for frequency conversion

We report on a system that produces 19 W of diffraction-limited radiation at 532 nm through single-pass frequency doubling of the output of a 1064 nm Yb3+-doped fiber MOPA in a periodically poled near-stoichiometric lithium tantalate (PPSLT) crystal. The output of the system was stable at the 19 W level for over one hour with no signs of photorefraction. The green power is believed to be limited by infrared-induced thermal dephasing in the PPSLT crystal.

Self-phase-locked degenerate femtosecond optical parametric oscillator

We report the first mode-locked, thulium-doped fiber figure-eight laser. The mode-locked oscillator produces 1.5-ps pulses with 63 pJ of pulse energy at a 10.4-MHz repetition rate with a 3-nm bandwidth at a center wavelength of 2034 nm. After amplification, the pulses are compressed to 370 fs with ~ 50 nJ of pulse energy. The oscillator can also operate in a square pulse regime, yielding stable pulses from ~ 100 ps to 20 ns long with ~ 100 nJ per pulse after amplification.

Room-Temperature Stable Generation of 19 Watts of Single-Frequency 532-nm Radiation in a Periodically Poled Lithium Tantalate Crystal

We demonstrate that silicate bonding an optical flat to the output facet of an active fiber device can increase the reliability of high-peak power systems and subsantially reduce the effective feedback at the termination of a double-clad fiber. We determine the bonding parameters and conditions that maximize the optical damage threshold of the bond and minimize the Fresnel reflection from the bond. At 1-mum wavelength, damage thresholds greater than 70 J/cm(2) are demonstrated for 25-ns pulses. We also measured Fresnel reflections less than -63 dB off the bond. Finally, we determined that the strength of the bond is sufficient for most operating environments.

Amplified 2-

A high-power linearly polarized Yb-doped silica fiber master oscillator power amplifier at 1150 nm is reported. It produced 3.35 W cw and 2.33 W of average power in 1 micros pulses at a 100 kHz repetition rate, both with 8 pm linewidth. This is the first report, to the best of our knowledge, of a high-power Yb-doped fiber amplifier at a wavelength longer than 1135 nm. The pulsed output was frequency doubled in a bulk periodically poled near-stoichiometric LiTaO(3) chip to generate 976 mW of average power at 575 nm with an overall system optical-to-optical efficiency of 9.8% with respect to launched pump power.

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We report the first measurement of the stimulated Brillouin scattering (SBS) gain coefficient of a phosphate fiber. Using single-pass amplification of spontaneous Brillouin scattering noise, we measured an SBS gain coefficient of 2.1 × 10-11 m/W in a 124.5-cm single-mode Yb3+-doped phosphate fiber. This is a factor of two less than the SBS gain coefficient of silica fiber, which puts phosphate glass fiber at an advantage for high-power applications.

Thulium-Doped All-Fiber Mode-Locked Figure-Eight Laser

Significance Microscopy and neurophysiology experiments in live animals commonly involve complex surgical preparations, which are often time-consuming, demand considerable manual dexterity, and can sharply limit experimental throughput. Here we present a spatially precise laser microsurgical technique using a pulsed UV laser. Our approach reduces surgical time by up to two orders of magnitude while substantially improving reproducibility. Using the fruit fly as a model, we show that laser microsurgery leaves complex behaviors intact and allows us to visualize brain activity in live flies for up to 18 h, more than four times longer than reported previously using hand dissection. We also demonstrate laser microsurgery on nematodes, ants, and the mouse cranium, illustrating broad potential utility for both optical and electrophysiological studies. Intravital microscopy is a key means of monitoring cellular function in live organisms, but surgical preparation of a live animal for microscopy often is time-consuming, requires considerable skill, and limits experimental throughput. Here we introduce a spatially precise (<1-µm edge precision), high-speed (<1 s), largely automated, and economical protocol for microsurgical preparation of live animals for optical imaging. Using a 193-nm pulsed excimer laser and the fruit fly as a model, we created observation windows (12- to 350-µm diameters) in the exoskeleton. Through these windows we used two-photon microscopy to image odor-evoked Ca2+ signaling in projection neuron dendrites of the antennal lobe and Kenyon cells of the mushroom body. The impact of a laser-cut window on fly health appears to be substantially less than that of conventional manual dissection, for our imaging durations of up to 18 h were ∼5–20 times longer than prior in vivo microscopy studies of hand-dissected flies. This improvement will facilitate studies of numerous questions in neuroscience, such as those regarding neuronal plasticity or learning and memory. As a control, we used phototaxis as an exemplary complex behavior in flies and found that laser microsurgery is sufficiently gentle to leave it intact. To demonstrate that our techniques are applicable to other species, we created microsurgical openings in nematodes, ants, and the mouse cranium. In conjunction with emerging robotic methods for handling and mounting flies or other small organisms, our rapid, precisely controllable, and highly repeatable microsurgical techniques should enable automated, high-throughput preparation of live animals for optical experimentation.

Investigation of the suitability of silicate bonding for facet termination in active fiber devices

We present measurements of the power noise due to optical amplification in a Nd:YAG free-space traveling-wave amplifier as the amplifier transitions from the linear regime into the heavily saturated regime. The quantum noise behavior is demonstrated by saturating the gain of a 100-W class zigzag slab amplifier with a high-power beam and measuring the power noise detected by a single-spatial-mode probe beam traversing the same optical path through the amplifier.