PM-clad fibers enable scaling with single transverse mode beam quality.
Recent advances in fiber technology have enabled a dramatic increase in the power delivered from fiber based amplifiers and lasers. This is particularly true at the 1060-nm wavelength where ytterbium (Yb)-doped fibers operate, with state-of-the-art peak powers now approaching 1 MW (1 mJ and 1 ns) delivered with single-mode, near diffraction limited beam quality. However, progress on scaling fiber-based eye-safe devices, such as those operating around the 1550-nm wavelength regime, based on Er:Yb (erbium:ytterbium) co-doped fibers has been much slower. The progress at this wavelength range hampered by the lack of suitable large mode area (LMA) fibers.
Until suitable LMA fibers are available, scaling methods are being pursued using current fiber technology. The highest peak powers yet achieved in a fiber device operating within the eye-safe wavelength range have been delivered by a polarization maintaining (PM) double clad fiber with singlemode, near diffraction limited beam quality (M2 ∼ 1.15). This method achieved 33-kW peak power (100-μJ pulses and 3 ns). The PM fiber operated for several hours at these peak irradiances, corresponding to 20 GW/cm2 and fluence levels of 65 J/cm2 with no sign of optical damage. The latter result is comparable to the single pulse optical damage threshold measured in fused silica.
Fiber devices are interesting for both military and civilian markets, particularly LIDAR (LIght Detection And Ranging) systems involving human presence and eye-safe requirements demanding operation at wavelengths longer than 1400 nm. Radiation above 1400 nm essentially exposes only the cornea to the damaging effects of radiation; these wavelengths constitute the eye-safe operating wavelengths. For pulsed operation, the permissible fluence exposure levels to radiation in this wavelength range are four orders of magnitude higher than at 1060 nm.
LIDAR applications for high-power 1550-nm fiber lasers and amplifiers systems include: obstacle avoidance laser radar systems for unmanned vehicles; free space optical communications architectures (e.g., ground-to-air, air-to-air, and inter-satellite); coherent laser radar for wind metrology and vibrometry; pump sources for nonlinear frequency down-conversion for counter-measures; and bio-chemical detection. Although most of these examples are primarily relevant to the military domain, deployment of civilian/commercial LIDAR systems is increasing.
In addition to the eye-safe operating wavelength and excellent beam quality (M2 ∼1.5), advantages of fiber-based sources for these applications are:
- broad wavelength tenability
- robustness with lower overall system weight
- high wall-plug efficiency (10-15% vs. 1-5% for solid state sources)
- high repetition rates with widely adjustable pulse parameters
- linear polarized output beam capability (improves system detection efficiency)
Good beam quality for efficient power delivery to the target, and relatively short pulses (1 ns or less) for good spatial resolution are also common requirements for modern LIDAR systems. Thus, an efficient fiber source operating at repetition rates of 10-100 KPPs (ranges of 1.5 -15 km) is necessary for these systems.