Optical Manufacturing: Material improvements bring laser glasses into mainstream applications
TODD D. JAEGER
Laser glass (LG) consists of traditional amorphous glasses doped with rare-earth ions and is available in a variety of SCHOTT formulations, such as LG680, LG750, LG760, and LG940 (see Figs. 1 and 2). The formulations vary in their base glass material—often phosphate or silicate glass—as well as in their active dopant ions and ion levels. Many of the same characteristics that make laser glass the foundation of high-powered laser systems also make it the ideal material for everyday use: uniformity of material, peak power output, scalability, and cost-effectiveness.
Laser glass characteristics
High-homogeneity, meter-class slabs of neodymium-doped (Nd-doped) phosphate laser glasses are currently used in the National Ignition Facility (NIF) and Laser Megajoule programs and all commercially available laser glass types can be manufactured for large-aperture laser systems that use either slabs or rods for generating high-peak-power laser pulses. Examples of successful projects based on such large-aperture laser glass components include the Extreme Light Infrastructure (ELI), Apollon, Shenguang, GIST, Beamlet, Mégajoule, VULCAN, and PHELIX.
As an amorphous solid, glass can be manufactured to very high standards for purity and uniformity with minimal size and format limitations, with stock sizes up to 800 × 400 × 60 mm readily available. Even though crystalline materials might support higher repetition rates and greater average power, they can be prohibitively expensive to grow to large aperture and still would not have as large of a capacity for energy storage as laser glass possesses.
Research on Nd-doped phosphate and other types of rare-earth-element-doped glasses is pushing the boundaries of peak power and wavelength. How to generate petawatt pulses was unknown throughout the last decade, but state-of-the-art developments are now considering solutions for exawatt pulse lasers. For these applications, broad emission bands are favored, as optical parametric chirped-pulse amplification (OPCPA) is much easier with these kinds of emission bands.
Flexible prototyping and manufacturing processes make it possible to produce cost-effective custom laser glass and components for specific applications built on glass-based lasers. For crystals, in contrast, it is very hard to have a uniform concentration profile of doping ions and to reach high doping levels. For example, it is difficult to achieve doping levels of > 15 × 1020 Yb3+ ions/cm3 for crystalline materials.
Historically, one of the disadvantages of glass has been its low thermal conductivity as compared to rare-earth-doped crystals, but tremendous strides have been made in developing glasses with low refractive index changes as a function of temperature (low dn/dT) to avoid thermal lensing, as well as better overall thermomechanical figures of merit. With APG-1 glass, for example, dnrel/dT(20-40) =1.2 × 10-6/K can be reached.
By using new glass matrices and combinations of active dopants in cost-effective small-batch melts, the range of potential applications for laser glass has expanded greatly in recent years. It's not just for exawatt-class lasers anymore; in fact, laser glass is moving beyond large-scale applications and into everyday uses.
Beyond high-power laser research
Because of its broader emission bandwidth (typically more than 20 nm) and efficient energy storage, laser glass-specifically types such as LG680 and the LG7xx family-support petawatt-peak-power laser pulses (see Fig. 3). Because the manufacturing process is completely scalable and the resulting glass can be finished using traditional optical processing, it is an ideal material for use in extremely large systems such as NIF, Laser Mégajoule, and the ELI pillars, as previously mentioned. These are the highest-power systems that have ever been deployed, with the next generation being on the order of 10 to 20 PW.
With that much power, it becomes possible to efficiently generate neutrons, protons, and x-rays. The mechanics involve plasma generation and free-electron acceleration (wakefield), as described, for example, in the Czech ELI whitebook (see http://bit.ly/1cLZqjp).
The end goal for these systems is not just fundamental research. For example, fine streams of subatomic particles can focus on cancer tumors, minimizing radiation damage; beams of neutrons or x-rays can scan vehicles to detect nuclear devices and other harmful items by analyzing the interaction of subatomic particles and the materials onboard; and high-powered laser beams can impinge on a tritium target in both direct drive and inertial fusion, generating so much pressure that the target collapses and the hydrogen fuses into helium and releases massive amounts of energy.
But in addition to these obvious high-energy applications, laser glasses are now instrumental for a number of mainstream applications in materials processing, medicine, and research and development—to name a few.
Laser shock peening
Imagine a blacksmith heating up steel for a sword, hammering it, cooling it in water, then repeating the process. This method, called peening, adds stress to the steel, removes the weak spots, and strengthens the end product. Laser shock peening is a similar process, hitting the metal with a high-powered neodymium laser while cooling it with a fluid, such as water.
This process imparts stress and strength into the material, preventing fatigue that can cause premature failure. The result is parts that perform reliably for extended amounts of time, saving money and providing a safer work environment. For example, the wings of Boeing 747 airplanes are formed using a laser shock-peening process.
The wings begin as sheet metal and are subsequently formed when a pulsed glass laser is rastered across the surface of the metal in a specific pattern that adds stress to one side of the material and causes it to curve. The process is also applied to specific spots on jet-engine turbines, preventing failure at high-stress points and adding thousands of hours to the lifetime of such engines and generators.
Cosmetic medicine
The speed, power, and variety of wavelengths available have brought laser systems into medical offices for both cosmetic and surgical procedures. What once required in-patient surgery, high costs, extended recovery times, and risk of infection can now be done cost-effectively at the doctor's office, with a shorter, at-home recovery.
The wavelength of laser light is an important factor in determining its efficacy for different aesthetic applications. For example, a 1 μm laser system can burn the skin, causing scarring and extended recovery time. It can also harm the retina of the human eye.
Many medical device manufacturers are now offering an eye-safe 1.5 μm glass laser built with an erbium ytterbium-doped glass. This LG940 eye-safe laser glass is an erbium-ytterbium-chromium-cerium-doped phosphate-based laser glass used in flashlamp-pumped and diode-pumped solid-state (DPSS) laser systems. Phosphate glasses generally offer higher solubility of rare earth dopants, thus the amount of active ions can be significantly increased. In addition, the longer wavelength allows for greater depth of penetration and a more controlled, ablative process for skin rejuvenation. In addition, this is also safer for the human eye, as it would only harm the cornea, which could be replaced in surgery should it be necessary.
The 1.5 μm laser is finding particular success in fractional skin ablation and resurfacing for the removal of varicose veins and acne scars. Instead of focusing a laser on a single focal point, the beam (typically with a pulse duration of 10–20 ms and a pulse energy of 5–100 mJ) passes through a micro lens array that spreads the laser across several points. This beam array is then rastered over the skin surface, manipulating the skin based on the absorption of specific light wavelengths: 1540 nm and, according to the application, supported by 2940 nm or 532 nm. This causes ablation of the top layer of skin, and as the skin re-heals, patients are left with a smooth surface without lesions or discolorations.
Contrast this with competitive techniques such as dermabrasion and shorter-wavelength lasers that are more painful and increase the risks of infection. Using a 1.5 μm laser promotes faster healing, reduces infection risks, and is safer for the doctors, nurses, and patients.
A related application is laser tattoo removal. Because tattoos come in a variety of different colors, they require different wavelengths of light to break up the nanoscale metal particles present in different ink colors. Right now, the laser systems used for tattoo removal use multiple gain materials built into a single laser system. The system might be called an all-in-one laser, but it really only has wavelengths at 532, 755, and 1064 nm.
A new broadband laser glass (BLG) from SCHOTT is tunable across 80 nm of bandwidth around a 1 μm wavelength. A traditional yttrium aluminum garnet (YAG) system made from crystalline materials might offer 5–10 nm of bandwidth at best in the fundamental mode and in each of its harmonics, but light from this new phosphate-based laser glass offers 80 nm in the infrared that can then be frequency doubled to give off a broadband green light and tripled to transmit near-ultraviolet light.
If BLG is pumped with an alexandrite laser, the residual pump beam can be combined with the fundamental and harmonic beams to provide one system with coverage over multiple wavelengths. So, the laser system can be customized to the color of the tattoo, providing a more utilitarian tool, and (hopefully) a quicker, less-painful tattoo removal experience.
Ophthalmology
Another application for BLG is radial keratotomy (RK), a medical procedure used to correct nearsightedness that requires an extremely short laser pulse. Physical laws state that to generate very short pulses of light, you need to have substantial bandwidth. The two are inversely proportional to each other, so the more bandwidth you have, the shorter the pulse.
Thanks to BLG, manufacturers can create a simpler laser architecture, compared to much more complex systems requiring one laser system to pump another system, through direct-diode or flashlamp pumping of the BLG gain material. Pulses in the 150–100 fs range are possible—much shorter than the time regime for thermal energy transfer, allowing the laser to be used in RK to cut into the cornea without imparting collateral tissue damage.
Laser rangefinding
In the past, laser rangefinders (light detection and ranging [lidar] systems) or time-of-flight systems used either a diode or a YAG crystal laser system, each of which has some disadvantages. Diode systems tend to have more divergence and a poorer beam profile that can affect the accuracy of the measurements. And YAG-based systems operate in a wavelength that can damage the eye, making it dangerous to use in open environments. Additionally, active cooling is required for crystals to mitigate temperature-related beam instabilities, increasing the overall size and weight of a laser system.
For these reasons, there is a move toward eye-safe laser glass as the gain material for laser range finders (see Fig. 4). Erbium ytterbium phosphate glass lasing at 1.5 μm, for example, cannot pass through the lens of the eye and damage the retina. In former times, Nd:YAG lasers were often used—operating at a wavelength of 1064 nm that negatively affects the retina. Light at 1540 nm, in contrast, cannot enter the human eye because of its absorption characteristics, therefore making it retina- or eye-safe.
These eye-safe laser glass systems are a safer alternative that provide superior performance in terms of efficiency, beam quality, and accuracy-even up to several miles, making them perfect for road construction, architecture, astronomy for lunar ranging, atmospheric sensing, and for 2D and 3D mapping applications such as cave mapping and mapping of historical sites and buildings.
Rangefinders using Er-Yb-doped phosphate glasses have demonstrated stable performance from -30° to 80°C without active temperature stabilization mechanisms added to the cavity designs. With designs based on passive Q-switching, these systems can be ultracompact—often near the size of a U.S. quarter. Micron-sized chips of precision-finished glasses are typically used with rare-earth impurity profiles optimized to a particular laser design.
Laser glass is certainly pushing the boundaries of what can be done with extremely large laser systems, and it is also seeing increased use in more everyday applications, largely because it offers distinct advantages over crystal media such as easier production processes, higher doping levels, and higher levels of homogeneity.
Todd D. Jaeger, Ph.D., is Advanced Development Sales Manager for laser and optical components at SCHOTT North America, 555 Taxter Road, Elmsford, NY 10523; e-mail: [email protected]; www.us.schott.com.