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.
The operation of fiber laser systems is not limited by average power, but by nonlinear optical phenomena such as optically induced damage, Stimulated Raman Scattering (SRS), and Self-Focusing. To overcome these limitations, fiber designs capable of delivering larger mode field areas than available in standard telecom fibers are needed. Increasing the mode field, and hence the non-linear thresholds, while maintaining good beam quality is of critical importance. Currently the limits of Er:Yb co-doped fiber technology capable of delivering good beam quality at high powers corresponds to fiber designs with core diameters around 18-μm and 0.17 NA.
In terms of amplifier design, maximum system flexibility is achieved by using multistage amplifiers with a telecom- like semiconductor laser as the seed source. Low-power pre-amplifier stages are made from telecom components keeping the costs low. The final stage in the chain is a double-clad fiber, cladding pumped by a fiber coupled diode bar as shown schematically in Figure 1.
Results from this system are presented in Figure 2, where the high slope efficiency for the final amplifier stage (30%) and lack of amplified spontaneous emission (ASE) are indicated. In the Er:Yb fiber system, high slope efficiency is achieved through a complicated balance of rare earth doping levels and the host glass composition while the low level of ASE indicates the high saturation level for the amplifier despite the multimode nature of the fiber.
The pulse duration is 3 ns at repetition rates 100 KPPs and the 10 W average power of Figure 2. This corresponds to a peak power of 30 kW from the final amplifier stage of the PM 18/250. The measured beam quality from the final amplifier stage is shown in Figure 2b, and to the best of our knowledge this represents the largest peak power reported in the 1550-nm eye-safe region with a single transverse mode operation resulting in an M2 ∼ 1.15.
The systems current 18-μm core fiber has a relatively high NA of 0.17, so the core supports more than five modes at 1550 nm. However, despite the multimode nature of the fiber several methods for achieving good beam quality have been developed and are readily applied to this class of “few-moded” fiber. The high NA is primarily a side effect of maintain good energy transfer characteristics between Yb and Er ions, as shown in Figure 3.
Maintaining high slope efficiency is critical to the efficient power conversion in the power amplifier stage, but the need to increase mode field diameter for further scaling of the peak powers (and still deliver good beam quality from such large fiber cores) becomes very difficult if the NA is maintained at this value. For comparison, 30-μm core LMA Yb-doped fibers are available with 0.06 NA and are capable of delivering good beam quality in many applications. The challenge for a new class of Er:Yb co-doped fibers is to increase mode field diameter in the complex co-doped system to the same 25-30-μm regime, while maintaining good efficiency and near diffraction limited beam quality.
This is now the focus of research to scale future systems to deliver higher peak powers at 1550 nm, as can be achieved with the current technology optimized for emission around 1060 nm. Research on a new generation of large mode area Er:Yb fibers specifically optimized for the eyesafe application discussed above, is being funded through ARFL-LADREA contract #FA9451-05-D-0218, with a goal to produce efficient ErYb-doped polarization maintaining LMA fibers by early 2006.