Diode-pumped lasers begin to fulfill promise

Diode-pumped lasers begin to fulfill promise

Bright pump sources, high-gain laser media, and crystals with large nonlinear coefficients are yielding a new class of visible lasers.

Larry Marshall

The invention of the Ti:sapphire laser in 1982 gave solid-state laser researchers a new tool for evaluating laser materials for diode pumping.1 Scientists demonstrated near-quantum-limited conversion of the Ti:sapphire pump laser light into infrared laser output, giving high hopes for diode pumping. The actual diode-pumped laser performance fell far below these expectations. But after nearly ten years of advances, diode lasers are finally starting to emulate those early Ti:sapphire sources.

Beam-shaping techniques developed by researchers at Southampton University (Southampton, England), the Fraunhofer Institute (Darmstadt, Germany), and SDL (San Jose, CA) have reformatted the output of linear diode arrays into circular apertures with an order-of-magnitude increase in brightness. When bright pump sources are combined with high-gain laser media and crystals with large nonlinear coefficients, a new class of visible lasers emerges offering simplicity, power, and efficiency.

High-brightness pumping

Most of the drive behind improved-brightness diodes is to replace solid-state continuous-wave (CW) lasers with semiconductor lasers. In industrial applications such as color-change marking, soldering, and wire stripping, single-mode (TEM00) output is unnecessary. The relatively poor divergence characteristics of multimode lamp-pumped Nd:YAG lasers or diffraction-limited carbon dioxide (CO2) lasers are far outweighed by their low cost compared to diode-pumped lasers. Ultimately, direct diodes can compete with the older technologies in these markets.

One of the biggest beneficiaries of the new high-brightness diode technology is the fiber laser. Researchers at Polaroid Corp. (Cambridge, MA) demonstrated 5-W TEM00 output from cladding-pumped fiber lasers in 1989, but were unsuccessful in power scaling to higher levels due to the limited brightness of diode pump sources. In these fiber lasers, diode laser light is focused into a 200-µm cladding that "traps" the pump light, allowing it to make many passes through the 1-µm fiber core that forms the lasing medium.

The high-brightness diodes allow 16 W of diode pump radiation to be coupled into the cladding and have enabled 50% optical efficiency in neodymium (Nd)-doped fibers, and 80% in ytterbium (Yb)-doped fibers, for TEM00 laser output. Such high optical efficiencies significantly improve the dollar-per-watt cost of diffraction-limited CW diode-pumped lasers.

CW lasers

The successes with fiber lasers have prompted scientists to take another look at conventional lasing media using the new high-brightness pump diodes. For instance, in CW-diode-pumped lasers, gain is always limited. To maximize gain, the CW diode light must be tightly focused. In typical end-pumped lasers, the diode light is of poor optical quality and can only maintain a tight focus over a length of a millimeter or so. This limited gain length is usually overcome with highly doped laser crystals that minimize absorption depth. Such designs form the basis of microchip lasers. These pump schemes are, however, plagued with thermal lensing and stress-fracture problems. The development of neodymium-doped yttrium vanadate (Nd:YVO4) offered researchers some relief from these issues; Nd:YVO4 has five times greater absorption than Nd:YAG (at nominal 1% Nd doping) and offers three times higher gain, which enables larger pump spots to be used. High-brightness diodes have extended the performance attributes of Nd:YVO4 compared to traditional laser materials such as Nd:YAG by use of "high-brightness pumping" (see Fig. 1).

High-brightness pumping is based on collimating the pump light over the length of the laser crystal, rather than having it be tightly focused. The collimation of pump light allows crystal doping levels to be reduced by extending crystal length. This extended absorption length offers a greater surface area to reject waste heat and lends the end-pumped laser the thermal benefits of side-pumping, with reduced thermal lensing and birefringence. There is an overall reduction in the aspheric component of thermal lensing, allowing smaller pump spots to be used but without the usual corresponding increases in pump-induced aberration. This reduced pump spot size enhances mode matching for TEM00 output.

We have demonstrated 8.4 W of TEM00 output from a 1-µm laser pumped by an Opto Power (Tucson, AZ) "Beamshaper" producing 16 W in a 200-µm pump beam waist that was collimated over a 5-mm length of 1% doped Nd:YVO4 crystal with diffusion-bonded end caps. These results represent 52% optical-to-optical efficiency, just as is observed in Nd-doped fiber lasers. Because the pump light is collimated, changing absorption depth does not vary pump diameter, so variations in diode wavelength do not affect mode or output power. These lasers are temperature insensitive over a wide range, which is an attractive characteristic for military applications.

Q-switched sources

While Nd:YVO4 has solved many of the problems associated with traditional end-pumped laser schemes, it cannot store large amounts of pump energy for Q-switched operation. Most industrial and nonlinear optical applications are driven by pulse energy (intensity). Conventional materials like Nd:YAG and Nd:YLF offer two to four times greater pulse energy, but these materials have lower gain than Nd:YVO4 and this, combined with their longer fluorescence lifetimes, causes relatively long pulse duration in Q-switched operation. Pulse lengths with Nd:YVO4 are two to three times shorter than with traditional laser media, which offsets its lower pulse energy. As a result, Nd:YVO4 is still the material of choice even for low pulse-repetition frequency (PRF) applications, because its "peak power" is greater than that obtained from conventional hosts (see Fig. 2). Clearly, if the short-pulse performance could be obtained from YAG or YLF lasers, they would offer higher power and pulse energy than YVO4.

The high gain offered by high-brightness pumping shortens output pulse duration even in materials with long storage lifetimes. As a result, the conventional materials can now offer short-pulse performance at high PRF as well as high pulse energy at low PRF. This dual ability offers a fundamental improvement in the nonlinear conversion ability of diode-pumped lasers by increasing their optical brightness.

Nonlinear conversion efficiency

The conversion efficiency of any nonlinear optical process is fundamentally limited by the brightness of the pump laser and the nonlinear coefficient of the nonlinear crystal. Eimerl has shown that the maximum nonlinear conversion efficiency depends solely on brightness.2

Most nonlinear materials have acceptance-angle restraints that limit how tightly pump radiation can be focused for nonlinear conversion. As peak pump power is reduced, the focus must be tighter to maintain high conversion efficiency (see Fig. 3). The efficiency for a joule-level quasi-CW pumped laser reaches 80%, while that of a CW-pumped Q-switched device with 10 W TEM00 output reaches only 55% and a true-CW 10-W source reaches only 40%.

High-brightness pumping allows shorter pulse duration and higher pulse energy from CW-diode pumped lasers, which increases the maximum nonlinear conversion efficiency for most processes. We have compared the external doubling efficiency for constant spot size using a conventionally end-pumped Nd:YAG laser and a high-brightness-pumped Nd:YAG laser. The doubling efficiency increased from 40% to 60%, enabling 6 W of green output from high-brightness pumping.

CW nonlinear optics

The high gain enabled by high-brightness pumping dramatically increases intracavity flux, which maximizes nonlinear drive. Noncritical phase-matching eliminates walkoff and allows further enhancement of nonlinear drive through tight focusing. Using type-I lithium triborate (LiB3O5; LBO) heated to 148°C, one can noncritically phase match the frequency-doubling process for a 1064-nm pump. High-brightness pumping, together with intracavity doubling, enables 35% conversion of diode light to green. This lowers cost and improves electrical efficiency of diode-pumped visible lasers to the point where multiple-watt devices can run directly from batteries (see Fig. 4).

Although LBO can be noncritically phase-matched, it still has a relatively low nonlinear coefficient compared to potassium titanyl phosphate (KTP). Researchers at Crystal Associates (East Hanover, NJ) have doped KTP with sodium to change the refractive indices and allow "custom" noncritical phase-matching. Nd:YVO4 produces strong lasing transitions at 1064 and 1342 nm. Using noncritically phase-matched KTP as an intracavity sum mixer we have produced 0.5 W of yellow output at 593 nm.

In an intracavity configuration, the intrinsic high gain exhibited by Nd:YVO4 offsets the losses introduced by additional intracavity elements. Even the inherent low gain of the third-harmonic conversion process is overcome by the high laser gain. We have produced several watts of Q-switched ultraviolet (UV) output using third-harmonic generation based on intracavity doubling of 1064 to 532 nm in KTP and subsequent sum mixing to 355 nm in LBO. By using noncritically phase-matched KTP (sodium-doped), we were able to demonstrate modest UV conversion for CW operation (200 mW).

Quasi-phase-matching

Just as improvements in laser brightness have increased nonlinear conversion, development of quasi-phase-matching has reduced the sensitivity of nonlinear crystals to focusing, allowing even greater increases in efficiency. The nonlinear coefficient of periodically poled lithium niobate (PPLN) is five times greater than that of KTP for doubling. This dramatic increase in nonlinear drive allows efficient single-pass CW frequency doubling external to the laser cavity. External doubling with noncritically phase-matched KTP achieves only 8% single-pass doubling efficiency, compared with 40% for PPLN.3,4 This efficiency is comparable to the efficiency of intracavity doubling without the complexities associated with the "green problem" (see Laser Focus World, June 1996, p. 143).

Just as Nd:YVO4 was heralded as the "ultimate" laser material, but has been discovered to be limited in its ability to deliver pulse energy; PPLN is the "ideal" nonlinear crystal, but is limited in its ability to handle pulse energy without damage. The combination of high PRF, Nd:YVO4 lasers, and PPLN makes an ideal source of efficient low-pulse-energy laser output. With advances in periodic poling of KTP and its isomorphs KTA (potassium titanyl arsenate), and RTA (rubidium titanyl arsenate)--materials that are capable of handling much-higher pulse energies than PPLN--increases in laser brightness have become even more important.

CW tunable sources

Researchers at Wright Patterson Air Force Base (Dayton, OH) have succeeded in operating an efficient extracavity CW optical parametric oscillator (OPO). The external-ring OPO was pumped by a 10-W CW diode-pumped laser to generate 1 W of CW tunable output around 4 µm. Using the same laser resonator, configured for intracavity OPO operation, Gonzalez and coworkers at the University of Dayton have also generated tunable mid-infrared (IR) output at the watt level.

The nonlinear drive of PPLN is so high that it is difficult to stop cascaded nonlinear processes. Just as in stimulated Raman scattering, in which the pump is shifted to a first-Stokes that in turn pumps a second-Stokes, the signal and idler outputs of the PPLN OPO can pump further OPO interactions, generating a multiplicity of spectral outputs.5 A PPLN OPO pumped at 1 µm produces outputs from the UV to the mid-IR (see photo on p. 63). The ability of PPLN to support these cascaded nonlinear processes with high efficiency has spawned some interesting results. Walt Bosenberg at Lightwave Electronics (Mountain View, CA) has generated 2.5 W of CW output at 629 nm from a 1-µm-pumped OPO/sum-mixer.6 The nonlinear element was PPLN fabricated with two quasi-phase-matched grating periods in series. The first grating was an OPO converting 1-µm pump light to a 1.54-µm signal, while the second grating sum-mixed the 1-µm pump with the 1.54-µm signal output from the PPLN OPO. This is a wonderful example of the unique abilities of PPLN to overcome the limitations of convertible nonlinear materials.

In an early attempt to realize tunable UV output from an all-solid-state laser, we developed a cascaded system with sum-frequency-mixing processes between the 1-µm pump laser, the third harmonic, and a parallel OPO stage. We recently revisited this system using a 10-W TEM00 laser to pump a PPLN OPO that allows temperature tuning of the 1.5-µm OPO signal. This tunable output was mixed with 355 nm in an LBO sum-mixer near noncritical phase-matching to generate 0.4 W of tunable UV output around 289 nm--a wavelength that is useful for detection of biological agents by absorption detection of amino acid complexes. The benefit of this approach is that sum-mixing requires equal numbers of photons rather than equal energies, which means that only a small amount of energy need be diverted into the OPO output. Researchers at Aculight Corp. (Bothell, WA) have sum-mixed the fifth harmonic at 213 nm with the 2.1-µm output of a PPLN OPO, using CLBO (cesium lithium borate, a new type of lithium triborate) with better absorption in deep UV light) to produce tunable UV output around 193 nm. In work at LightAge (Somerset, NJ), an alexandrite oscillator operating between 720 and 800 nm, was quadrupled into the 193-nm region.

The last two years have seen dramatic changes in the state of the art of diode-pumped lasers. Improvements in diode brightness have enabled CW diode-pumped lasers with quantum-limited performance through high-brightness pumping.

Periodically poled lithium niobate offers such strong nonlinear drive that it can be used to directly double CW laser output without complex intracavity or external resonant doubling schemes. This high gain also enables CW-pumped OPOs offering tunable near-IR output. The improved brightness of pump lasers gives a corresponding increase in the brightness of OPO sources and enables frequency mixing of OPO outputs generating tunable laser radiation upconverted to visible and UV. Most of these ideas were tried in the 1970s, but were limited by poor crystal quality, low-gain lasers, and inability to noncritically phase-match high-nonlinear-drive materials. As a result of these advances, all-solid-state lasers are now able to offer output across the spectrum. o

REFERENCES

1. P. F. Moulton, Optics News 8, 9 (1982).

2. D. Eimerl, IEEE J. Quantum Electron. QE-23(5), 575 (1987).

3. L. Marshall, "Frequency conversion using CW sources," Proc. Photonics West, SPIE 3263 (1998).

4. G. D. Miller et al., Proc. CLEO, paper CTUB2 (San Francisco, CA; 1997).

5. L. Marshall and J. A. Piper, IEEE J. Quantum Electron. 29(2), 515 (Feb. 1993).

6. W. Bosenberg, OSA Advanced Solid State Lasers, paper AMA-7 (Coeur d`Alene, ID; 1998).

Periodically poled lithium niobate supports cascaded nonlinear conversion processes, generating a multiplicity of wavelengths from the UV to mid-IR.

FIGURE 1. High-brightness pumping of the gain medium is based on collimating the pump laser light over the length of the laser crystal. Crystal doping levels can be reduced by extending the crystal length.

FIGURE 2. Plotting average power versus pulse repetition frequency (PRF) for Q-switched Nd:YAG, Nd:YLF, and Nd:YVO4 shows the rolloff in average power as the PRF is decreased.

FIGURE 3. Comparison of doubling efficiencies for 10-W CW diode-pumped laser (DPL), 0.5-mJ, Q-switched CW-pumped DPL, and 1-J quasi-CW pumped DPL shows the calculated maximum doubling efficiency for lithium triborate (LBO) as a function of Eimerl`s brightness parameter.

FIGURE 4. Diode-pumped visible-output lasers have reached a level of efficiency that allows a portable, battery-powered, passively cooled system to produce 3 W of CW green output.

FIGURE 5. Intracavity sum-mixing generates several watts of Q-switched output and hundreds of milliwatts of CW output at 355 nm.

LARRY MARSHALL is president of Light Solutions Corp., 1212 Terra Bella Ave., Mountain View, CA 94043.