David C. Brown and Sten Tornegård, !%Snake Creek Lasers%!
The fundamental metrics used to determine effectiveness for military applications are size, weight, efficiency, reliability, performance and cost. In the commercial world, size and weight are less important but nevertheless desirable to decrease cost of ownership. Modern solid-state lasers with CW average powers exceeding 100 kW have been demonstrated, but large performance gaps exist at any power level, particularly with regard to wall-plug efficiency and beam quality.
Complexity is an additional issue that is often ignored. The “best solution” laser system for a particular application is the one that offers the least system complexity while providing just the performance needed to accomplish the task at hand. This general principle leads to the conclusion that laser systems that minimize the number of optical components, pump diodes and support equipment – and provide the best possible beam quality – are the most desirable.
Beam quality and efficiency
If we consider cases where the effects of a laser beam are determined primarily by the intensity achieved on target, it is not difficult to show that if the laser output beam is N times diffraction-limited, the intensity on target in the far field is reduced by N2. Thus, to achieve the desired intensity on target, one must increase the average power by N2. As an example, if the laser output is two times diffraction-limited, the laser average power must be increased to four times that needed to produce a diffraction-limited spot on target.
While for low-average-power lasers this is not very important, for high-average-power lasers operating at, say, the 25- to 100-kW power level, the increase in average power is significant and costly. In effect, if one defines the laser system to include the laser output beam all the way to the target, the system efficiency has been reduced by N2. If one includes atmospheric attenuation and aberrations, the system efficiency is degraded even further.
Laser system size and weight are approximately proportional. The size of a laser system is roughly inversely proportional to the system efficiency. Modern diode-pumped fiber lasers routinely achieve a true wall-plug efficiency of greater than 30 percent, while diode-pumped bulk solid-state lasers produce somewhat lower. The Northrop Grumman and Textron 100-kW laser systems, for example, are estimated to have wall-plug efficiencies of less than 25 and 15 percent, respectively. Both lasers have been reported to operate at less than two times diffraction-limited but require wavefront correction methods to do so.
Enter cryogenic lasers
A quiet revolution in solid-state laser technology has been taking place for the past decade, in which cryogenically cooled solid-state lasers have made important strides in addressing the beam quality and efficiency shortfalls. Why would someone want to operate a laser with liquid nitrogen (LN2) or other cryogen cooling? For starters, cryogenic cooling of Yb:YAG leads to optimum thermal, thermo-optic and laser kinetics performance by using a laser material that is simple, reasonably well understood, and free from upconversion, concentration quenching and other efficiency-robbing effects.
Cryogenic lasers are not difficult to build and operate, and LN2 is ubiquitous and inexpensive. Their beam quality is better than that of room-temperature devices, and they do not require wavefront correction methods. While cryogenic lasers require a source of cryogen and initially may be applicable only to ground-based or naval platforms, the technology is so compelling that the advantages far outweigh the resulting slight increase in system complexity.
Researchers at Snake Creek Lasers recently reported a high-efficiency Yb:YAG cryogenic oscillator, shown in Figure 1, that achieved a heat-fraction-limited slope efficiency of 91.9 percent, a maximum optical-optical efficiency of 86 percent and a 100 percent photon slope efficiency. All efficiencies are new world records.
Figure 1. Shown is an Yb:YAG cryogenic oscillator (D.C. Brown, T. Bruno and V. Vitali, Optics Express, Aug. 2, 2010).
The laser was pumped by a CW 946-nm Nd:YAG laser, producing a diffraction-limited output beam with an M2 of 1.0 to 1.1. High efficiencies were achieved via a resonator that produced a high ratio of internal resonator intensity to saturation intensity with a low saturation intensity at 77 K, a near-unity overlap efficiency of the pump and resonator beams, a near-unity extraction efficiency and the absence of thermal effects. In addition, a diffraction-limited pump beam was used, and the laser material chosen had a low heat fraction and negligible parasitic effects. The slope efficiency of 91.9 percent is the best one can achieve with Yb:YAG.
Other laser materials with lower heat fractions than Yb:YAG may produce even higher slope efficiencies in the future. This laser demonstration has provided a roadmap for producing solid-state lasers with high wall-plug efficiency, which in the future may be achieved by replacing the 946-nm pump source with high-brightness pump diodes and by increasing the pump absorption. In Figure 2, we show the most important efficiency steps in a Yb:YAG laser, as well as the cumulative laser wall-plug efficiency that can be achieved using the most realistic best values for each step.
Figure 2. This bar graph represents step and cumulative efficiencies for cryogenic Yb:YAG lasers.
Recently, a diode efficiency of 83.5 percent was demonstrated at 975 nm by employing advanced diodes operated at cryogenic rather than room temperature. If we assume that 80 percent efficiency can also be demonstrated at 940 nm, the result is a system with an overall wall-plug efficiency in excess of 68 percent. Using room-temperature diodes with an efficiency of 60 percent still results in a wall-plug efficiency of greater than 51 percent.
Even taking into account the ancillary equipment needed to operate any laser, we conclude that advanced all-cryogenically cooled solid-state lasers can be demonstrated with roughly 1.5 times the wall plug efficiency of fiber lasers operating today. We believe that this result will further hasten the development of cryogenic solid-state laser technology. Clearly, the implications for addressing the efficiency, size and weight performance gaps in current laser technology are compelling, and the benefits accrue across the board for both military and commercial applications.
Figure 3. This Yb:YAG cryogenic oscillator-amplifier system produces 963 W of average output power with a beam quality of M2 <1.3.
High-average-power CW and ultrafast results
Snake Creek recently produced two high-average-power performance milestones for Yb:YAG cryogenic lasers, the first operating in CW mode and the second producing a 50-MHz train of 12.4-ps pulses. In Figure 3, a CW oscillator using seven Yb:YAG disks, each pumped with two 30-W, 940-nm fiber-coupled diode sources, is used to drive a power amplifier with eight Yb:YAG disks, each pumped with two 100-W, 940-nm fiber-coupled diodes. Each Yb:YAG disk is pumped through dichroic windows that are totally reflecting at the lasing wavelength of 1029 nm and highly transmissive at the pump wavelength of 940 nm. The laser beam zigzags through each disk, mimicking a laser slab and providing a compact design. A flowing liquid nitrogen system is used to cool all the Yb:YAG crystals.
In Figure 4, a mode-locked ytterbium fiber laser producing a 50-MHz train of 12.4-ps pulses is directly amplified, first in the seven-disk amplifier, which is double-passed, and secondly in a single pass of the eight-disk amplifier. At the maximum average power of 758 W, energy per pulse was 15.2 μJ, and the peak power was greater than 1.23 MW. The performance of this high-average-power ultrafast laser was achieved without the use of chirped-pulse amplification, considerably simplifying the design and reducing the complexity.
Figure 4. The Yb:YAG ultrafast cryogenic laser depicted produces 758-W average output power and beam quality of M2 <1.3.
As a preliminary test of the capabilities of this high-average-power ultrafast system, we have, in collaboration with Jefferson National Laboratory, conducted a preliminary frequency-doubling experiment using a lithium triborate crystal, producing 60 W of 515-nm (green) average power with the fundamental 1029-nm output idling at 160 W. In the near future, much higher green average power is expected.
The future of cryogenic lasers
In the coming years, we may expect to see a dramatic increase in the average power of cryogenic lasers while maintaining outstanding beam quality, operation using many different laser materials and transitions, and ultimately very high wall-plug efficiency. It is now clear that high-average-power CW, Q-switched and ultrafast operation can be obtained using this technology.
While the need for a cryogen is often mentioned as an obstacle to the widespread use of this technology, the overwhelming advantages offered by cryogenic lasers substantially negate this assertion. In those situations where liquid nitrogen is not readily available, such as on aircraft platforms, it is likely that alternative low-temperature cooling methods may be used, such as low-temperature air at high altitudes, for example.
Cryogenic laser applications
It is clear that the major advantages offered by kilowatt-class cryogenic lasers, high wall-plug efficiency and excellent beam quality, which are critically important for military applications, are also applicable to micromachining applications, which currently are limited by room-temperature-cooled ultrafast lasers with an average power of approximately 50 W. With cryogenic lasers having average power a factor of 10 higher in the fundamental 1029-nm and second-harmonic (515 nm), third-harmonic (343 nm) and fourth-harmonic-generation (257 nm) wavelengths, laser processing speeds an order of magnitude faster are suddenly feasible for the photovoltaic, flat panel display, semiconductor and printed circuit board industries. The same argument is valid for the nuclear industry, which has been searching for an effective laser source to safely decontaminate surfaces from radioactive isotopes.
With Gaussian-shaped spatial beams, cryogenically cooled lasers are ideal pump sources for optical parametric amplifiers as well as photocathode illuminators for high-power free-electron lasers and other accelerator applications requiring well-timed picosecond electron injectors. It also is conceivable that Snake Creek’s cryogenic laser technology can be employed for ion-stripping of hydrogen beams because the technology is scalable to the 100-kW level and beyond.
The company is seeking collaborations with academic, government and business entities to further cryogenic laser technology development and to quickly bring it to market.
The work described in this article was supported by the US Army Research Laboratory.
Meet the authors
David C. Brown is president and chief technical officer of Snake Creek Lasers in Hallstead, Pa.; e-mail: firstname.lastname@example.org. Sten Tornegård is marketing manager at Snake Creek Lasers; e-mail: email@example.com.