Research includes lasers, display devices and detector technologies.
Eric W. Van Stryland, University of Central Florida
The Center for Research and Education in Optics and Lasers (CREOL) has been in existence since 1986 at the University of Central Florida (UCF) in Orlando and recently morphed into the first US college devoted solely to optics — The College of Optics and Photonics. But we still refer to ourselves as CREOL.
Research is diverse here, focusing on topics from laser development (both large and small) to real-time information display devices, nonlinear optics, new detector technologies, and laser cutting and welding. The recent addition of the Florida Photonics Center of Excellence and the Townes Laser Institute, funded by initiatives of former Gov. Jeb E. Bush, are pushing research into biophotonics, lasers in medicine (associated with the new UCF College of Medicine), fiber and ceramic lasers, nanophotonics and imaging.
Dozens of research projects are under way by approximately 160 PhD candidates at CREOL.
Laser architectures
One area of interest is high-energy laser development, a rapidly emerging field of laser physics and engineering that includes CW devices with average power exceeding kilowatt levels. Portable and low-divergence systems (near the diffraction limit) should revolutionize industrial (welding, cutting, drilling) and military applications. Traditionally, gas lasers have been considered for such applications; however, recent developments in solid-state and fiber lasers are very promising.
A new approach to the development of high-energy lasers takes advantage of volume Bragg gratings produced in the labs of professor Leonid B. Glebov. Originating under the auspices of DARPA’s architecture for diode high-energy laser systems (ADHELS) program, the technique is based upon the creation of volume holographic elements (Bragg gratings) recorded in a photothermo-refractive glass, which provides high efficiency and robustness. The glass enables fabrication of phase volume diffractive gratings with absolute diffraction efficiency exceeding 95 percent, thermal stability to 400 °C and a laser damage threshold of 40 J/cm2 for 8-ns pulses. Tolerance to CW laser radiation in the near-infrared region is at least up to several tens of kilowatts per centimeter squared without measurable temperature shifts.
Such Bragg gratings provide narrow spectral selectivity down to 50 pm and narrow angular selectivity down to 100 μrad. These elements have low losses and can be used for high-power laser systems in intracavity applications and in passive elements for laser beam control.
Volume diffractive gratings are wave-vector filters with narrow spectral and angular selectivity. This feature enables a new view on semiconductor lasers, which are efficient sources of light but which previously were not considered for high-brightness systems because of their high divergence, and broadband and often unstable emission spectra.
Glebov’s group recently demonstrated that the emission spectra of high-power laser diodes can be dramatically narrowed and stabilized, and that they can be converted into a single transverse mode with diffraction-limited divergence using photothermo-refractive gratings in an external resonator. Moreover, it is possible to provide phase locking of laser diodes by resonant coupling with photothermo-refractive Bragg gratings. This design could enable coherent arrays of semiconductor lasers with power scalable to hundreds of watts.
In another method, laser beams at slightly shifted frequencies can be combined by a stack of photothermo-refractive gratings (spectral beam combining with low divergence), which also allows heat to be removed more easily.
Semiconductor diode lasers
Stabilized mode-locked semiconductor diode lasers also are under investigation at CREOL. They are being developed by professor Peter J. Delfyett’s group for applications in optical networking areas, and for coherent signal processing — all-optical analog-to-digital converters and digital-to-analog converters, for example. The concept is to utilize the three salient characteristics of a mode-locked laser: the temporally short optical pulse duration, the broad and discrete nature of the spectral bandwidth, and the coherent nature of the spectral comb that is generated.
Much work has been done recently using the technique of carrier-envelope-offset stabilization to realize a stable frequency comb from mode-locked solid-state and fiber lasers. The stabilization technique is difficult for diode lasers because of the large spectral bandwidth required.
Delfyett’s group used a modified Pound-Drever-Hall stabilization method with an optical etalon inserted in the cavity of an active harmonically mode-locked semiconductor diode ring cavity. The salient feature of realizing a stabilized comb in this configuration is that the free spectral range of the etalon and the active modulation frequency are chosen to be identical (and define the repetition rate), while the intracavity Pound-Drever-Hall control loop matches the fiber cavity length to a multiple of the etalon length (Figure 1).
Figure 1. Stabilized optical frequency comb mode-locked diode laser (left) technology is being developed for optical networking and coherent signal processing. The intracavity etalon and intracavity Pound-Drever-Hall control loop are pictured at right.
In this way, the investigators achieved a frequency-locked comb with ~200 combs, each with a linewidth of less than 10 kHz, on a frequency grid of 10 GHz. The measured stability of the entire comb set is limited to the stability of the reference laser used in the measurement and is less than 400 kHz, with a residual timing jitter of <4 fs (1 Hz to 10 MHz). The use of high-power slab-coupled optical waveguide amplifiers as the gain element in time-domain-based low-noise-cavity configurations have generated optical pulse trains at 10 GHz, with a residual timing jitter of 850 as (1 Hz to 10 MHz).
In related research, quantum dot laser diodes have shown great promise in replacing quantum well diode lasers for many applications, just as quantum wells replaced PIN diode lasers. For example, professor Dennis Deppe’s group is working with the DARPA SHEDS (super high efficiency diode sources) program to break the 80 percent wall-plug efficiency barrier by using molecular beam epitaxial growth technology of III-V semiconductors.
A new optical component
Reflectarrays are well-known in the radio-frequency portion of the spectrum, where a quasiperiodic array of metallic structures is placed at a particular distance above a ground plane. When electromagnetic radiation is incident on such structures, the reflected phase depends on the local dimension of the metallic structures. In this way, a flat surface can impress an arbitrary wavefront distribution on the reflected radiation. Recent research by professor Glenn D. Boreman’s team has demonstrated the feasibility of this methodology in the infrared, using electron-beam lithography for fabrication.
Figure 2. Researchers are designing infrared reflectarrays. An interferogram of the IR reflectarray is shown on the left, a visible image is shown in the middle, and on the right is a scanning electron microscope image of the 3.24-μm patch array.
The researchers use arrays of square-patch metallic elements in three sizes and measure the phase of the reflected wavefront using an interferometer at 10.6 μm (Figure 2). Infrared reflectarrays are desirable for many of the same reasons they are popular for use at lower frequencies: reduced cost, weight and volume for focusing surfaces. They facilitate direct stacking of multiple planar elements (filters and polarizers) on themselves for additional weight and volume reductions. Reflectarrays will provide additional degrees of freedom not previously available in conventional polished and diffractive IR optical surfaces for correction of monochromatic and chromatic aberrations.
Future vision
Imagine eyewear that doubles as a computer screen. In the near future, for example, consumers likely will be able to superimpose Internet content directly into a live environment. This wireless interactive technology is already a reality in professor Jannick P. Rolland’s labs (Figure 3).
Figure 3. Transparent head-worn displays could change how information is displayed in both military and consumer markets. Prototypes are pictured here.
This type of eyewear involves head-worn projection technology as well as eyeglass displays. The team that developed the first head-worn projection display with 6-g optics per eye is now building what could become an embedded technology in everyday life, creating an effective interface between people and this inspiring new technology. In the lab, the user is central to the design process. The team is gathering sensory input (such as visual perception) from the eyewear users to achieve ergonomically and perceptually viable high-performance systems.
Two applications are at the forefront to be the first to adopt this technology. One is to support the next generation of soldiers by enhancing information available to them in a combat zone. Another is surging consumer interest in cell-phone-based video content, which is prompting the rapid adoption of a small, comfortable and low-cost wearable display.
In the eyeglass project, a small and lightweight wearable computer monitor is being created. This combines research in optics and optical design with industrial and mechanical design to appeal to those with an active lifestyle. Designing off-axis combiners, free-form optics, and plastic and diffractive optical lenses to yield extremely compact optical designs is critical to success.
Perhaps most exciting is how many applications could benefit from wearable display technology. It could be used for augmented reality, simulation and training, field-dependent tasks and, of course, entertainment. Unlike the precursor “look at” display technology, where a pair of glasses is perched on the user’s nose and does not allow the person to see the outside world, these new displays are ergonomically integrated and provide users full access to their surroundings.
Professor Shin-Tson Wu’s students, meanwhile, are working on LCD technology that will drive the miniature displays needed for Rolland’s head-mounted displays.
Extreme lithography
Lastly, extreme-ultraviolet lithography is set to supersede conventional deep-UV lithography sometime after 2010, according to the International Sematech road map for chip technology. This next generation of lithography utilizes 13.5 nm of light (with wavelengths so short that ordinary refractive optics do not suffice) and could produce smaller and faster microchips. Professor Martin C. Richardson and his team are developing an extreme-UV source.
The illumination and imaging optics will be fabricated from precision reflective optics, often astigmatic and coated with highly reflective multilayer mirrors, so-called Bragg reflectors.
The traditional dividing line between the source and the stepper is called the intermediate focus (IF), with different vendors making each. ASML Netherlands BV and Japan-based Canon Inc. and Nikon Corp. are the principal stepper manufacturers, all of which are committed to the transition to extreme-UV. Two other companies — Cymer Inc. of San Diego, Calif., and Gigaphoton Inc., a Japanese consortium — now supply the 193-nm Ar:F excimer lasers required for current UV lithography. Two competing technologies vie to supply the extreme-UV light source for the first extreme-UV stepper (at least three have been ordered for delivery in 2009). They both use plasma sources of tin ions — a technique introduced by Richardson’s team several years ago. The most mature source, the discharge pulsed plasma, uses several designs of electrically driven dense plasma discharges, but these may not be able to supply the power required at the intermediate focus.
The other approach is a laser plasma source based on a multikilohertz-repetition-rate laser producing dense plasmas from small microdroplets of tin, a concept also introduced by Richardson’s team. Both Gigaphoton and Cymer are pursuing this approach, using solid tin droplets irradiated by pulsed CO2 lasers. Richardson’s team is collaborating with the British laser company Powerlase Ltd. in using pulsed diode-pumped solid-state lasers in combination with a special University of Central Florida target design.
All three groups (Gigaphoton, Cymer and Powerlase) have achieved the same power levels (~10 W at IF, though more than 100 W is ultimately required) but it would appear that the plasma debris from Richardson’s source is much less than that for other sources. Plasma debris leads to erosion of the extreme-UV optics, which must be in direct line of sight of the optics and is, therefore, a serious factor. Richardson’s team has patented two types of plasma shields to inhibit this erosion. Together with its Powerlase partners, it is now preparing to demonstrate the extreme-UV powers required for the first extreme-UV steppers.
In 2006, companies provided more than $5 million in research support, giving students valuable experience working with industrial clients. This mix of government and industrial research projects (~190 current projects) is essential for the modern educational experience.
Meet the author
Eric W. Van Stryland is dean of The College of Optics and Photonics at the University of Central Florida in Orlando; e-mail: [email protected].