Laser diodes are enabling sophisticated applications, as the legacy advantages of these lasers pair with emerging benefits.
JEFFREY UNGAR, QPC LASERS
More than 30 years ago, acclaimed physicist Edward Teller said, “No one should use a laser unless it’s a diode laser.” Although those of us engaged in the manufacture of laser diodes will perhaps be forgiven for our uncritical endorsement of Teller’s statement, ample scientific justification supports such a perspective.
Xenon (Xe) magnetic resonance imaging is an essential solution for specialized bioimaging, especially of the lungs. This method necessitates the hyperpolarization of Xe, which is achieved via laser diode-drive spin exchange optical pumping. Courtesy of Polarean Imaging Plc.
To start, consider the physical advantages that laser diodes offer users: For a given output power, laser diodes far
surpass all alternatives in their compactness, low mass, ruggedness, and simplicity. Regarding their performance capabilities, laser diodes convert electricity directly to photons, with efficiencies that approach 80% — a value that is far beyond their nearest competitors. Laser diodes also benefit from maintenance-free lifetimes ranging from tens of
thousands to millions of hours; operate
at wavelengths from the ultraviolet to
the infrared; are electrically or thermally tunable over hundreds of gigahertz;
and can be amplitude- and frequency-modulated. And with appropriate designs, these sources provide single-frequency outputs that can be coupled to single-mode fibers.
Additionally, laser diode chips are mass manufactured at the wafer level using techniques borrowed and adapted from the silicon industry — at costs per watt lower than other laser types.
Together, these remarkable capabilities are fueling revolutions in fiber optic communications, remote sensing, machine vision, and optical data storage.
Versatile, but not omnipotent
Why, indeed, would anybody use any other kind of laser?
The answer to this question lies in the specifics of a given application. In, for example, use cases that require specific wavelengths, semiconductor alloys with the necessary and/or desired properties may not be available. Other cases may require short high-power Q-switched pulses, which cannot be created by laser diodes.
But most frequently, the use of a laser diode is precluded for one of two reasons: either because a narrow emission spectrum, accurately centered at a specific wavelength, is needed, or because a high optical quality (nearly diffraction-limited) high-power output beam is required.
Narrow, tightly specified emission spectra present a challenge to laser diodes because the semiconductors from which they are constructed have broad, compositionally and temperature-dependent gain spectra. These useful properties allow laser diodes to be manufactured and tuned to emit at any desired point within a continuous band of wavelengths. At the same time, this also makes it difficult to impose tight tolerances on emission wavelength and linewidth.
If required by the application, tighter spectral control can be obtained by introducing volume Bragg gratings (VBGs) into the external optics to “lock” the diode spectrum to the grating. Yet this is a critical and delicate process that adds complexity and cost, and which becomes increasingly difficult to implement when large numbers of diodes must be locked to a common wavelength. Ensuring long-term stability of the spectrum is also challenging because minor shifts in alignment between the VBG and the beam that occur over time cause the output wavelength to drift. For these reasons, external VBG stabilization has seen only limited deployment.
Obtaining high beam quality, high-power beams from laser diodes also presents a challenge. The electrically pumped optical waveguide at the core of a laser diode governs the quality of the generated beam. If the waveguide is narrow and supports only a single lateral mode, the output beam will be essentially diffraction limited. If, on the other hand, the waveguide is large enough to support multiple modes, beam quality will worsen significantly because the output beam becomes a composite of many differently shaped beams, each of which carries a fraction of the total power.
Single-mode waveguide laser diodes with waveguides a few microns wide can generate powers of several hundred milliwatts, but higher-power laser diodes require larger waveguides that support tens or even hundreds of modes. Beams generated by these multimode laser diodes cannot be tightly focused or accurately collimated.
High-power laser diodes can be used for the direct welding of nonmetallic
materials, such as polymers, where wave-
length flexibility is a more important
application parameter than beam quality.
Laser diodes are also used for metal processing operations, such as heat treating and powder cladding, which do not require focused beams. But because
of insufficient beam quality, laser diodes are not well suited for the direct performance of metal processing operations, such as cutting and welding, and they
are relegated to supporting roles, such as optical pumping fiber lasers.
Enabling innovation
So much for the bad news.
The good news is that laser diode technology has been making steady progress toward eroding remaining performance gaps that separate these sources from fiber, solid-state, and gas lasers. Some of these gains are the result of steady but evolutionary improvement to the designs and epitaxial fabrication processes used to manufacture laser diodes. Others are the result of the incorporation of additional features to the architectures of these sources. For example, developments in epitaxial technology that enable the monolithic incorporation of diffractive spectral stabilization gratings into internals of the diode itself have considerably improved spectral performance (Figure 1). This Brightlock family of internal grating laser diodes accurately fixes the center wavelength and reduces linewidth by as much as an order of magnitude without external optical elements increasing cost or causing frequency drift.
Figure 1. A cross-sectional electron micrograph showing two “teeth” of an internal grating incorporated into a 976-nm Brightlock family laser diode. Courtesy of QPC Lasers.
In addition, adopting so-called master oscillator power amplifier (MOPA) laser diode designs can enable order-of-magnitude improvements in both beam quality and linewidth. As shown in Figure 2, these designs circumvent the multimode problem that plagues conventional laser diodes by combining a single-mode, single-frequency distributed feedback seed laser with a high-power tapered semiconductor amplifier monolithically. Laser diodes based on this principle have been developed and introduced into the Brightlase family at many infrared wavelengths, with single-mode power outputs as high as tens of watts per emitter.
Figure 2. A schematic of a high beam quality, single-frequency master oscillator power amplifier (MOPA) Brightlase laser diode. DFB: distributed feedback. Courtesy of QPC Lasers.
Emerging applications
A recently developed medical imaging technique provides an example of a technology that is enabled by newly developed laser diode capabilities. Detailed imaging of lung function is fundamental to the detection and treatment of respiratory disease, but even after decades of development, standard imaging techniques provide only limited resolution, which supplies insufficiently detailed information. Specialized imaging techniques require the patient to inhale radioactive agents, which limits their use to situations in which the medical benefits outweigh the risks of the procedure.
The technique of xenon (Xe) magnetic resonance offers a critically needed solution. This method pairs specially tuned MRI scanners with a contrast agent consisting of nonradioactive Xe-129 atoms, whose nuclear spins are strongly hyperpolarized along an ambient magnetic field. The preparation of the hyperpolarized Xe uses a process called spin exchange optical pumping, in which circularly polarized photons transfer their angular momentum to the valence electrons of rubidium (Rb) atoms and by subsequent collisions to Xe-129 nuclei. The implementation of this process requires a laser source to provide hundreds of watts precisely matched to the 794.8-nm Rb D1 absorption line with linewidths of a fraction of a nanometer.
Internal grating laser diodes have made generating the required photons a matter of routine. Figure 3 shows an FDA-
approved system for preparing hyperpolarized Xe-129 installed in a clinical
setting. The HPX Xenon Hyperpolarization System, manufactured by Polarean Imaging Plc, incorporates a standard Brightlock laser pump and enables lung imaging with high levels of detail and contrast.
Figure 3. The Polarean HPX Hyperpolarization System, using 694.8-nm Brightlock pump laser diodes to prepare hyperpolarized xenon (Xe) for medical imaging. Courtesy of Polarean Imaging Plc.
Beyond diagnostics, direct laser therapy represents another class of biomedical applications that relies on advancements in laser diode spectral stabilization. One representative example is the treatment of acne vulgaris, which in the U.S. alone is responsible for billions of dollars in costs to patients, and for which the baseline treatment is based on drugs, such as isotretinoin, that have serious side effects, including strong teratogenicity.
Academic studies dating back more than ten years showed that the overactive sebaceous glands that cause acne could be selectively destroyed without drugs and without damaging other tissue by narrowband laser illumination at 1726 nm. But at the time that many of these studies were published, clinically suitable sources of laser radiation were unavailable.
High-power narrowband 1726-nm sources using Brightlock laser diodes have made this laser treatment the reality. During the last two years, laser diode-based treatments have been FDA approved for all classes of acne, with
clinical trials suggesting that the treatment is both effective and long-lasting, and thousands of lasers have been
deployed in treatment centers.
Advanced laser diode designs are also making their way into emerging markets, many of which are already sizable and growing. One such market is the generation of electrical power by wind, which currently generates >10% of domestically produced electricity. Maximizing the power generated by the wind turbine and preventing damage to its blades requires dynamical optimization of airfoil pitch and direction to accommodate changes in incident airflow. Traditional anemometers — sensor devices that measure wind speed and direction — are typically located on the turbine nacelle, and only measure wind speed and direction after the gust has already reached the turbine. They cannot provide a proactive warning
of incoming airflow and offer limited utility in preventing turbine damage and optimizing power generation.
In contrast, Doppler lidar provides the required upstream wind measurement needed to optimize performance, and increases in power generated up to 5% have been reported. The lidars used in this method require watt-scale diffraction-limited beams with sub-megahertz spectral lines, preferably with emission at eye-safe wavelengths. This was once the domain of fiber lasers, but research conducted at the Technical University of Denmark and commercialized at Windar Photonics has led to the commercial deployment of wind sensors based on MOPA laser diodes that are much cheaper and lighter than fiber laser-based systems (Figure 4).
Figure 4. Windar Photonics’ WindEye laser diode-based anemometer used to gauge wind power and speed, deployed on a wind turbine.
Courtesy of Windar Photonics.
Environmentally beneficial applications of these new laser diode technologies are not confined to clean power generation; the narrow linewidth single-frequency, high-optical-quality beams are also helping to mitigate climate change by stemming the release of greenhouse gases. One major source of anthropogenic
atmospheric methane is leakage from natural gas production fields and pipelines. Remote detection of leaks that exploit methane’s strong absorption lines near 1650 nm is possible, but low-cost, compact sources with the required power, beam quality, and linewidth were commercially unavailable until recently. MOPA diodes have been transformational, and narrowband diodes providing
hundreds of milliwatts at methane absorption lines are now commercially available.
With increased regularity, laser diodes are filling roles previously reserved for other types of lasers, and this trend will continue with increasing awareness of the capabilities enabled by new diode technologies. Laser diodes will never entirely replace other laser technologies, but where they find application, they will bring benefits in the form of significantly lower costs, reduced size and weight,
reduced power consumption, and
improved robustness.
Meet the author
Jeffrey Ungar is the founder of QPC Lasers and was formerly director of advanced R&D at Ortel, which was later acquired by Lucent Technologies. He has been involved in the development of high-power laser diodes for 40 years; email: [email protected].