Diode-pumped, solid-state lasers offer several advantages over gas and lamp-pumped solid-state lasers in terms of size, ruggedness, reliability and cost in a variety of significant applications.
Neodymium-doped crystals and glasses such as Nd:YAG (neodymium:yttrium aluminum garnet) have long been used as laser materials. Optically pumped, they produce an output wavelength close to 1 µm, and the excited state lifetime of neodymium allows both CW and pulsed (Q-switched) operation.
The original pump sources were powerful flashlamps and arc lamps whose output is focused into a cylindrical laser crystal rod using elliptical reflectors to form a gain module. This module is then mounted in a laser cavity typically many inches in length. The cavity is defined by the usual high reflector and partial reflector, or output coupler.
There are several limitations to this approach. First, pumping is inefficient, partly because lamps are inefficient at converting electricity into pump light and produce a lot of unwanted heat. But even more critical, the lamps produce broadband emission throughout the visible and infrared. As a result, most of the light is not absorbed by the laser crystal and ultimately serves only to generate more heat in the pump module. This heat must be removed by water cooling of the laser head. The lamps also require a multikilowatt power supply.
For many industrial applications though, the biggest drawback is the short lifetime of CW arc lamps, which must be changed every 200 to 600 hours. When the lamps are replaced, the cavity optics usually require slight realignment in order to maintain a good output mode from the laser. This frequent routine maintenance actually conceals another limitation — their optical alignment tends to drift over time and would require periodic realignment, irrespective of any lamp change. Diode pumping eliminates these drawbacks.
The principles of diode pumping are simple. The Nd-doped laser crystal(s) has an intense, sharp absorption peak at 808 nm, a wavelength that is readily accessible to InGaAs semiconductor laser diodes (Figure 1). Laser diodes convert much of their electrical input into laser light, which is then very efficiently absorbed by the Nd-doped crystal. The end result is a wall-plug efficiency many times greater than lamp-pumped lasers.
Figure 1. A flashlamp emits over a wide spectral range (b), but laser crystals such as Nd:YAG absorb light only in narrow wavelength bands (a). Diode laser pumping is efficient because the diode laser emits in only one of these bands (b).
There are several other major advantages to this approach beyond electrical efficiency. These lasers generate relatively little heat and therefore do not require a high volume of cooling water like their lamp-pumped counterparts. Also, the diodes operate from a low-voltage power supply, which is compatible with a single phase (110/220 V) line.
Furthermore, because of the compact size of the semiconductor diodes, the size of the laser head can be greatly reduced.
For OEMs and industrial end users, the long lifetime of the diodes is another advantage because it minimizes maintenance downtime. In fact, the diodes used in diode-pumped lasers are capable of lifetimes of more than 10,000 hours.
Laser geometryThere are two basic, complementary methods for introducing diode laser pump light into a laser crystal — end and side pumping. In general, end-pumped lasers deliver high-quality output beams with state-of-the-art performance and stability at powers up to tens of watts, while side-pumped lasers sacrifice beam quality to offer raw power as high as several kilowatts.
In side pumping, laser bars or stacks are arranged cylindrically around the laser crystal (Figure 2). The output of each bar is focused by use of a cylindrical lens and/or a lens array. A large volume of the crystal is thus flooded with pump light, leading to high power and multimode output (M
2 values greater than 100). Each of these pumped crystals is mounted as a self-contained module. A high-power laser will contain multiple modules in series, each module serving to amplify the output of the previous module.
Figure 2. Side pumping enables a large number of pump bars (or stacks) to be arranged around a single laser rod.
Typically, side-pumped lasers have been derived from earlier, lamp-pumped designs. Nonetheless, they offer a significant reliability advantage over traditional lamp-pumped lasers and successfully compete with both lamp-pumped and CO
2 lasers in heavy materials processing applications such as welding and metal cutting.
With side-pumped lasers, a major development emphasis continues to be increased output power, with new records set seemingly several times a year. The goal of side pumping is simply to couple as much power as possible efficiently into the laser; in contrast, the aim of end pumping is to couple as much of the diode output as possible into the TEM
00 mode volume of the crystal. This not only produces a lower M
2 output, but also leads to the most efficient harmonic conversion, providing access to green and UV wavelengths.
One effective approach for end pumping is fiber coupling, as used in the FC
bar (fiber-coupled laser diode bar) technology developed by Spectra-Physics, where each diode laser facet is coupled into an individual fiber optic. The fibers are then circularly bundled such that the highly asymmetric diode bar output is converted to a high brightness spot suitable for efficient end pumping of the laser crystal (Figure 3). Also, since the FC
bar module(s) is mounted in the power supply and fiber connected to the laser head, it can be simply replaced in the field without any optical realignment.
Figure 3. End pumping allows the mode volume of the diode laser to be matched to the TEM00 mode volume of the laser cavity.
This architecture can produce a high-quality (M
2 <1.2) beam from a compact, rugged, hands-off package, with excellent maintenance lifetimes. Just as important, the flexibility diode-pumped technology delivers a wide range of output powers, with a choice of CW, Q-switched and even mode-locked outputs.
As a result, end-pumped solid-state lasers have replaced argon-ion and excimer lasers in many high-precision industrial applications, dominating areas such as PWB processing, stereolithography (rapid prototyping), inspection, graphics, micromachining and hard disc texturing. In addition, their low cost of ownership and operational simplicity are enabling applications that were uneconomical or impractical for earlier lasers.
Figure 4. In commercial diode-pumped lasers, the laser crystal can be shaped as a rod, thin disc, or slab.
Since the introduction of diode pumping, a number of different laser crystal geometries have been investigated, with varying degrees of commercial success. The most important geometries are cylindrical rods, slabs and thin discs (Figure 4). Slab- and rod-shaped laser crystals can be designed to be end pumped or side pumped, depending on the power/mode requirements, while disc-shaped crystals can only be end pumped. At this time, rod-shaped crystals dominate the low-power/high mode quality applications, whereas slabs and rods are commonly used in high-power lasers.
YAG and YVO4The most common Nd-doped material used in lamp-pumped lasers is Nd:YAG, which offers relatively simple-to-grow, large, defect-free crystals and is optically and mechanically robust. Another material Nd:YVO
4 (neodymium:yttrium orthovanadate) offers higher gain than Nd:YAG. However, it was long neglected as a commercial material because the crystals were difficult to grow (i.e., pieces long enough to make laser rods for lamp-pumped systems were not available). With the advent of end-pumped configurations, the use of much smaller laser crystals is making Nd:YVO
4 more attractive.
In quantitative terms, Nd:YVO
4 has a gain about 5.5 times greater than Nd:YAG. One implication is that this allows very short pulse (<10 ns) Q-switched output with superior pulse stability at high repetition rates (Table 1). In addition, Nd:YVO
4 is strongly birefringent and is naturally polarized, unlike Nd:YAG. The output of Nd:YVO
4 lasers is thus naturally polarized, eliminating the need for an intracavity polarizer.
With end-pumped lasers, Nd:YVO
4 is usually the preferred material for fast pulsed (>10 kHz) and CW operation. In fact, the maturation of this high-gain material proved critical in boosting the power of these lasers to market-enabling levels. On the other hand, Nd:YAG is still commonly used in many models operating at lower repetition rates. In the case of side pumping, however, a large crystal is still required, and these lasers therefore use Nd:YAG exclusively.
High-power pump diodesCommercial and industrial diode-pumped lasers are pumped by high-power diode laser bars or stacks. The diode bar is a monolithic chip that consists of a linear array of between 20 and 70 laser diode edge emitters (Figure 5). Output powers for this type of linear array continue to increase, but at this time the industry standard bar has a length of 1 cm and CW output powers of 40 W or more.
Figure 5. A diode bar is a monolithic linear array containing a large number of individual emitters. Bars can be combined to produce a two-dimensional stack, with very high output power.
Individual bars can be combined into stacks to produce higher total output power. This is usually done by stacking the arrays vertically, so that the rows of emitters are very close together (<2 mm). In principal, there is almost no limitation on how many bars can be combined in this manner. Typical commercial products offer CW output powers as high as 1 kW.
The completely sealed laserA major reason end-pumped lasers with rod-shaped crystals are preferred in many low-to medium-power applications is that they can offer zero-maintenance operation. If the diode bar is mounted in the power supply and fiber coupled into the head, even this field-replacement can be carried out without opening the laser cavity. Operating diode bars well below their maximum rated power (called derating) can extend the lifetimes of these devices to well beyond 30,000 hours.
This has allowed a new approach to laser design, the truly sealed resonator, setting new standards in terms of reliability, stability, beam quality, compact packaging and simplicity of operation. For industrial applications, these advantages translate into low cost of ownership and high process yields.
Figure 6. One method of achieving long-term stability of the laser cavity is mounting all the optics on a rigid I-beam platform.
Specifically, the traditional limitations of alignment drift and optical surface contamination are completely eliminated by use of a sealed cavity. There are several successful ways to do this. At Spectra-Physics, we use a monolithic approach, rigidly mounting all the optics along the cross-member of a closed box I-bar structure (Figure 6). This I-bar arrangement delivers excellent torsional rigidity and stability. Also, even if the cavity temperature changes, the I-bar structure expands uniformly, ensuring that the optics stay perfectly aligned along the same axis. In addition, the optical mounts are all metal, with no use of epoxies or other outgassing components. Since there is no need to adjust or clean the cavity optics, the head is assembled and tested in a cleanroom and then sealed at the factory, eliminating optical surface contamination as a failure mechanism. Just as important, the use of small, nonadjustable mounts and remote fiber-coupled diode bars allows for an extremely compact laser head.
Green and UVThe near-infrared (1.06 mm) output is useful for applications such as resistor trimming and surface marking of metals, but many laser applications require visible or ultraviolet wavelengths. Fortunately, the TEM
00 output beam produced by end-pumped lasers can be efficiently frequency doubled (to 532 nm), tripled (to 355 nm) and even quadrupled (to 266 nm) using nonlinear crystals such as LBO (lithium triborate) and BBO (barium betaborate).
With the current power level of diode-pumped CW lasers (>20 W at 1.06 mm), the nonlinear crystals must be placed inside the laser cavity in order to obtain useful second harmonic power levels. Beginning with Spectra-Physics’ Millennia in 1996, CW 532 nm lasers are now well-established with output powers up to 10 W.
Another route to higher green and UV power is provided by mode locking a CW laser. The latest SBR (saturable Bragg reflector) mirror technology enables simple mode locking that is robust enough for demanding industrial applications. The high peak power of mode-locked lasers, such as the Nd:YVO
4 based Spectra-Physics Vanguard, allows very efficient frequency doubling and tripling, providing multiple watts of power at 355 nm. Moreover, the high repetition rate (80 MHz) means that these quasi-CW lasers can replace bulky ion lasers in many CW ultraviolet applications.
With Q-switched pulsed lasers, the peak power is more than sufficient to permit extracavity doubling and tripling. Recently, this has been used to increase the available wavelengths from these lasers significantly. Specifically, the stable bore-sighting possible with a sealed monolithic laser head has allowed the development of simple "bolt-on" frequency doubling and tripling modules. This enables an end user to switch between using near-IR, visible and UV laser light, within a few minutes, with no optical alignment whatsoever. Before, an end user wishing to employ these different wavelengths would have to use three separate lasers or complicated laser realignment.
Recent developmentsWhen it comes to expanding applications for Q-switched diode-pumped lasers, there are currently two areas of laser development of particular note. These involve increasing the pulse repetition rate and pushing the output further into the UV. Increasing the pulse repetition rate is important in industrial applications since it leads directly to an increase in process throughput. Deep-UV output at 266 nm enables processing of difficult materials that exhibit poor absorption at 355 nm.
Pulse repetition rate is often the process-limiting factor in high-throughput applications such as scribing and marking. Here, the high scanning speed of the galvanometers used to sweep the laser beam cannot be fully exploited; the scan speed must be reduced to avoid producing a dotted cut or groove due to the individual laser pulses. Laser manufacturers have now responded to this limitation by developing Q-switched lasers capable of much higher repetition rates.
Nd:YVO
4 is usually the material of choice for higher repetition rate lasers, but typical end-pumped designs deliver peak performance at a maximum repetition rate of only 40 to 50 kHz. Pushing these lasers to higher repetition rates often results in lower energy per pulse, lower overall power and a significant increase in pulse-to-pulse noise.
The best route to higher pulse repetition rates in the near-IR is to use a low-loss laser cavity and an output coupler which is designed to ensure that the laser light is reflected many times inside the cavity before being emitted as laser light. An example of a laser that relies on this approach is the V-Xtreme from Spectra-Physics. This near-IR laser delivers over 8 W of average power at 400 kHz, where it has a pulsewidth of 110 ns; the laser can even be operated reliably at 500 kHz. Just as important, the pulse-to-pulse noise at 400 kHz is below 4 percent, comparable to that from conventional laser designs at 40 to 50 kHz.
Figure 7. The V-Xtreme laser enables pulse rates as high as 500 kHz. Because it uses a v-folded cavity, the compact laser head is less than 200 mm in length.
As already noted, one of the advantages of diode-pumped lasers is the compact size of the laser head. This advantage is maximized in the V-Xtreme laser by using a v-folded cavity so that the laser head is less than 200 mm in length.
One of the main applications for this type of laser is in disk texturing; the creation of a region of bumps on a hard disk where the read-write head can be safely parked without risk of damage due to stiction. In practice, each laser pulse is used to form a sombrero-shaped bump on the disk surface. The economics of this application dictate the need for a high-repetition-rate laser. However, it is vitally important to minimize bump-to-bump variations by use of a laser with a high degree of pulse-to-pulse stability. Otherwise the disk head could be subject to accelerated wear or even damage.
Another application is high-throughput marking where the laser is used to create an identifying mark such as an alphanumeric string. Here, the laser process does not enable operation of the final part nor add any significant value. The speed of this process must therefore be maximized to avoid adding undue cost to the parts. To this end, the laser incorporates a high-speed analog control port to enable fast on/off switching when moving the laser beam between individual marks.
Another important development has been the advent of fourth-harmonic modules — prealigned modules that bolt on to the front of several Q-switched laser models and deliver deep-UV output at 266 nm. There are a number of important and emerging industrial applications for pulsed deep-UV lasers including glass marking, FBG writing, wire marking and drilling exotic materials. However, 266 nm traditionally has been a difficult wavelength for solid-state pulsed lasers for two reasons. First, the nonlinear crystals used to quadruple the laser output frequency were easily damaged by any hot spots in the laser beam. In addition, the crystals had a short shelf life and operational life due to moisture absorption. The stable high-quality output of the latest diode-pumped lasers has now eliminated the issue of hot spots. Moreover, by using the latest CLBO (cesium lithium borate) crystals and enclosing them in factory-sealed modules, the crystal lifetime issues have also been successfully addressed. As a result, these new modules finally provide the OEM and/or end user with turnkey access to the deep-UV from an all-solid-state laser source.
In conclusion, diode-pumping has revolutionized the design of solid-state lasers. In particular, end-pumped Q-switched lasers now offer a unique combination of advantages, including low power consumption, no requirement for external cooling water, a compact overall package, excellent mode quality, high pulse-to-pulse stability, true hands-free operation, long lifetimes and very high reliability. And by tailoring the performance of these lasers to the specific needs of new applications, laser manufacturers have ensured a healthy market for these products in a variety of niche applications.