FIBER LASERS
Fiber sources reach
multimegawatt peak
power in ns pulses

Microchip SSF laser

PCF

FABIO DI TEODORO AND CHRISTOPHER D. BROOKS

BPF

Axial
channels

BPF

Endcap

RCore

are-earth-doped fiber lasers and amplifiers have emerged as practical and compact optical sources in many areas of interest. An example is the generation of multikilowatt continuous-wave (CW) power with good beam quality for laser welding and cutting. ¹ Under pulsed operation, fiber-based emitters are attractive as sources of highly repetitive ( multikilohertz pulse repetition frequency) pulses with durations of a nanosecond or shorter with high pulse energy, ^{peak power, and average power. Fiber-} based emitters offer a number of attrac- ^{Advances in large-core} tive features over traditional solid-state lasers. Their distributed thermal load, resulting from large surface/active-volume ratios, favors painless waste-heat removal from the gain medium. Moreover, the in the peak power and emitted beam quality is independent of power and resistant to thermomechanical disturbances, thanks to single-transverse-mode waveguiding. These benefits, combined with high optical efficiency (better _{than 60% to 70% in ytterbium [Yb]-doped retaining excellent beam} fibers), compact and modular form factor, quality and efficiency. and support for alignment-free and mono-

Output

FIGURE 1. A multistage MOPA contains Yb-doped PCF amplifiers (left inset shows schematic cross section). All fiber amplifiers are backward-pumped at 976 nm by injecting pump light into their output ends (SSF is solid-silica fiber; BPF is bandpass filter). The rodlike PCF has a large glass outer shell (right inset, top), a PCF pump waveguide and central core region (right inset, center), and a PCF air cladding (right inset, bottom), namely a ring of thin silica bridges connecting the pump waveguide to the outer shell and providing a high numerical aperture for acceptance of pump light. A beam-expanding endcap (lower-left inset) is created by thermally sealing the axial channels shut near the PCF output end.

lithic packaging, make pulsed-fiber sources des-

photonic-crystal fibers have ^{tined for widespread use}
in military, scientific,
led to important increases ^{medical, and industrial}
applications.

FABIO DI TEODORO is a senior scientist and CHRISTOPHER D. BROOKS is a development engineer at Aculight, 11805 North Creek Parkway South, Bothell, WA 98011; e-mail: Fabio.Di Teodoro@aculight. com; www.aculight.com.

Effects of high peak power

The generation of high peak powers in fibers has been limited by the onset of optical non-linearities, which can cause spectral degrada-

tion, power fluctuations, and feedback effects. Another problem has been the build up of amplified spontaneous emission (ASE) between pulses, which is a consequence of the high gain available in the fiber core and contin-

energy obtainable from a
fiber-based source, while