Applications still abound at CLEO 97

FIGURE 1. Bell Labs researcher will present results on unipolar interminiband quantum cascade lasers (paper CThSI). Lasing at infrared wavelengths occurs between the first excited-state miniband and ground state of a superlattice. The emission wavelength is determined by the energy gap (minigap) between the two minibands and can be tailored by design over a large region of the infrared spectrum.

Applications of lasers and electro-optics technology remain a prominent focus of this year`s Conference on Lasers and Electro-Optics (CLEO `97), which will be held May 18-23, 1997, concurrently with the Quantum Electronics and Laser Science conference (QELS `97) in Baltimore, MD. At last year`s conference, the Optical Society of America (OSA; Washington, DC) organizers of CLEO/QELS `97, launched its laser and electro-optics applications program (LEAP), which will continue this year with sessions that feature optical technology in surgery, intellectual property and technology licensing, and product design and development processes. "People come for the technical presentations and to find out the latest and greatest research results," says CLEO General Co-Chair George Valley of Hughes Research Laboratories (Malibu, CA). "They wind up learning about applications they never dreamed of" in the exhibit hall and the LEAP program.

With the intent of broadening the appeal of the conference, the OSA is also launching a new program this year, the executive career advancement program (ECAP). "We`re enhancing the technical, application, and career program for attendees," says CLEO General Co-Chair Dennis Killinger from the University of South Florida (Tampa, FL). The program`s four workshops cover meeting people, taking technology to market, winning government contracts, and managing intellectual property. In addition to LEAP and ECAP, four special evening sessions cover delivering wideband data to the home, laser skin resurfacing, laser material processing, and evidence for life on Mars. These sessions are designed to tie in with LEAP in focusing on applications, explains Killinger.

At the plenary session on Monday afternoon Stephen Benton of the MIT Media Lab (Boston, MA) will discusses real-time three-dimensional video in his talk entitled "Bits in space: Speeding up holographic video." New results will be presented by Peter Stockman of the Space Telescope Institute (Baltimore, MD) in his presentation, "The Hubble Space Telescope: New and Improved," and David Shafer from MIT`s Lincoln Laboratory (Lexington, MA) will discuss optical semiconductor lithography.

The CLEO/QELS technical program includes more than 1490 papers in 20 topical areas—an increase of 14% compared to last year, according to the OSA. The program merges the most advanced scientific results with applications, says CLEO Program Co-Chair Connie Chang-Hasnain of Stanford University (Palo Alto, CA). Applications areas highlighted include printing, storage, display, and medicine.

Semiconductor lasers

The number of papers on semiconductor lasers has increased almost 30% compared to last year, and they address several research "firsts" (see also "Keeping up with III-V semiconductors," p. 114). In an invited paper, researchers from the Reed Center for Photonics at the University of Wisconsin-Madison (Madison, WI) working with Coherent Laser Group (Torrance, CA) report obtaining 5-W continuous wave (CW) output at 810 nm from an aluminum-free active region diode laser (paper CMA3). This result represents the highest CW output power from an aluminum-free 810-nm-emitting device. The groups also report obtaining catastrophic optical mirror damage--the first time for such a device--at an internal power density of 9.1 MW/cm², which is more than twice that of typical aluminum gallium arsenide (AlGaAs) devices used for producing 810-nm output. According to the researchers, this means the aluminum-free devices have the potential to be reliable at powers at least twice those of the AlGaAs-active counterparts with similar stripe geometry.

The possible superiority--in terms of catastrophic optical damage and long-term reliability--of aluminum-free diode-laser bar arrays makes them of interest for pumping Nd:YAG lasers at 808 nm. F. Daiminger and associates from Jenoptik Laserdiode GmbH (Jena, Germany), report 100-W output power diode bars, operating CW with wall-plug efficiency of 50%. The results compare to the best performance of aluminum-containing material (paper CFF1).

In another invited paper (CThS1), F. Capasso and colleagues from Lucent Technologies, Bell Labs (Murray Hill, NJ) describe the first unipolar interminiband laser—that is, a quantum cascade laser in which the transition occurs between energy minibands instead of between discrete levels (see Fig. 1). The device emits at 7.7 µm with a peak output power of 800 mW at 80 K. Also from Bell Labs, in paper CThS3, J. Faist and coworkers report on a new unipolar intersubband laser that uses photon-assisted tunneling and is electric-field-tunable between 6.6 and 6.2 µm. The laser operates at up to 80 K with a peak output power of 40 mW at 60 K. Such devices have potential as sources for spectroscopy.

A strain-compensated indium gallium phosphide/aluminum gallium indium phosphide (InGaP/AlGaInP) laser structure has produced a record room-temperature output power of 72 mW, according to researchers at Sanyo`s Microelectronics Research Center (Osaka, Japan). In invited paper CThF1 they describe a 630-nm-band device with a stable fundamental mode output capable of emitting 30 mW at temperatures up to 75°C. These lasers are potentially attractive sources for high-density rewritable optical-disk systems and high-speed laser printing.

Solid-state lasers

New materials and designs for increased pumping efficiency with consequent higher output powers are just two of the themes running through many of this year`s papers in the solid-state laser category. German researchers describe a "thin disk" design for a solid-state device that is intended to allow the laser to operate with high optical efficiency and good beam quality simultaneously.

In paper CFE1, A. Giesen from the Universität Stuttgart reports obtaining an output power of 224 W from a Yb:YAG disk laser pumped with 525 W of power output from 19 fiber-coupled InGaAs diode lasers at 940 nm. Instead of the conventional rod or slab of solid-state material, the laser medium is a thin disk mounted with its high-reflector-coated side on a cooling finger and pumped in a quasi-longitudinal manner with the fiber-coupled diode lasers. The end of the fiber bundle is imaged by four spherical mirrors and one flat mirror onto the laser crystal, resulting in an eightfold pass of the pump light through the crystal.

Also looking for improved pumping efficiency of solid-state lasers, researchers at Showa Optronics Ltd. (Kanagawa, Japan) have developed a stacked-glass-plate beamshaper for diode lasers. Described in paper CFO1 by T. Izawa and colleagues, the device improves the horizontal M2 value of a diode-laser bar output beam. The beamshaper consists of two sets of ten stacked, square-fused-silica plates. Each plate is 2.3 mm thick and is stacked with a twist offset and an air gap of 30 µm. The researchers describe operation of the device in an intracavity-doubled end-pumped Nd:YVO₄ laser. With a single 20-W diode-laser bar pump source delivering 14.7-W pump power to the vanadate crystal, they obtained 3.5 W of TEM00 output at 532 nm. This is reportedly the highest single-mode green output power obtained from an intracavity-doubled vanadate laser pumped by a single non-fiber-coupled laser diode bar.

Industrial applications of solid-state lasers often require high-brightness sources. At the Technische Universität Berlin in Germany, researchers have obtained very high average output power with near-diffraction-limited beam quality from an all-solid-state master oscillator power amplifier (MOPA) Nd:YALO laser and newly developed SBS (stimulated Brillouin scattering) fiber phase conjugators. The system produced average output powers of more than 70 W at kilohertz repetition rates and high beam quality.

Optoelectronic devices

The increasing importance of optoelectronics is reflected in the number of papers on the subject at this year`s conference, ranging from blue-emitting lasers to fiberoptics. Papers submitted on ultrafast optics, optoelectronics, and applications increased from 76 in 1996 to 97 this year.

Blue-emitting sources are required for color displays, printing, and data recording. A device generating 230 mW at 1123 nm from a thulium-doped ZBLAN upconversion fiber laser will be reported on by R. Paschotta and associates from the Optoelectronics Research Centre (Southampton, England) in paper CTuG3. The previous maximum output using a similar method was 106 mW. The researchers tested a more-powerful pump laser—a diode-pumped Nd:YAG laser—that produced a TEM₀₀ output of 1.6 W at 1123 nm for 7 W of pump power. This output was coupled into a single-mode core of ZBLAN fiber with a launch efficiency of about 50%. Although at higher power levels Paschotta saw a rapid degradation in performance, the fiber was entirely restored during a period of lasing at low output power.

Keith Beckley and Peter Herman from the Ontario Laser and Lightwave Research Center (Toronto, Ontario, Canada) extended telecommunications fiber photosensitivity to a record short wavelength of 157 nm using a fluorine laser. In addition, they successfully wrote a 0.75-cm-long Bragg grating in the core of the 7% doped fiber using a magnesium fluoride (MgF2) phase mask custom-made for the 157-nm radiation. Their results are discussed in paper CTuN3.

For the first time, a microactuated scanning microlens with hybrid integrated vertical-cavity surface-emitting lasers (VCSELs) was demonstrated using free-space micro-optical bench technology (see Fig. 2). In paper CME2, L. Fan and colleagues from the University of California (Los Angeles, CA) will describe a novel microlens that was raised to a 400-µm height above the substrate and integrated with two-axis microactuators for continuous two-dimensional scanning with scanning range of ۲ and a scan resolution of 0.002.

FIGURE 2. The microlens can directly collimate, or focus, light emitted from the VCSEL below it. The lens can be made into two-dimensional arrays and is suitable for monolithic use, if fabricated on III-V substrates, or hybrid integration with VCSEL arrays.

Ultrafast studies

A special joint session will be held on ultrafast lasers on Tuesday (JTuA). The session will begin with a tutorial on the physics of ultrafast lasers by Henry C. Kapteyn from the University of Michigan (Ann Arbor, MI).

One of the challenges for ultrafast laser techniques is the possibility of tracking and controlling the electronic wave packet dynamics in atoms and molecules. In an invited paper, JTuA3, Ch. Spielmann and associates from the Technische University of Vienna (Austria) discuss how high-energy 20-fs input pulses at 800 nm were spectrally broadened in a noble-gas-filled hollow fiber and then compressed in a broadband high-throughput dispersive system to 4.5 fs with an output energy up to 70 µJ (see p. 127). According to the researchers, these are the shortest pulses generated to date and contain less than two oscillations of optical radiation. The hollow-fiber technique combines the advantages of a guiding element with a large-diameter single-mode fiber and a fast nonlinear medium with a high threshold for multiphoton ionization.

The development of self-modelocked Ti:sapphire lasers generating pulses of less than 10-fs duration has revolutionized ultrafast spectroscopy. In invited paper CMC1, A. Baltuska and associates from the University of Groningen (The Netherlands) describe generating 5-fs pulses by compression of the white-light continuum with a 1-MHz repetition rate laser for ultrafast spectroscopy. The 5-fs pulses were obtained by injecting 13-fs pulses from a cavity-dumped self-modelocked Ti:sapphire laser into a single-mode polarization-preserving fiber. The researchers say that even shorter pulses may be feasible with custom-designed chirped mirrors.

In another invited paper (CMI3), researchers I. D. Jung and colleagues from the Ultrafast Laser Physics Laboratory at the Swiss Federal Institute of Technology (Zurich, Switzerland), describe self-starting 6.5-fs pulses from a Kerr-lens modelocking Ti:sapphire laser using prism pairs in combination with chirped mirrors (see Fig. 3). The mirrors balance the third-order dispersion of the prism pair. Self-starting is obtained by using broadband semiconductor saturable absorbers.

FIGURE 3. Self-starting Ti:sapphire laser produces 6.5-fs pulses. The laser cavity operates with a chirped or double-chirped mirror and a broadband semiconductor saturable absorber (SESAM) as a starting mechanism.

Medical applications of ultrafast lasers are also well represented, with a 35% increase in paper submission from 1996. High-speed, high-resolution optical coherence tomography using femtosecond lasers is discussed in an invited paper (CTuS1) by J.G. Fujimoto and colleagues from the Massachusetts Institute of Technology (Cambridge, MA).

Laser potpourri

In paper CTuR2 experimental and theoretical investigations on ultrashort pulse ablation of metals with 780-nm Ti:sapphire laser output are discussed by C. Momma and colleagues from the Laser Zentrum Hannover e.V. (Hannover, Germany). In another paper from Germany, CTuK2, researchers K. Dickmann and colleagues from FH Munster (Steinfurt, Germany) present their findings for nanoscale material processing by combination of laser and scanning probe technology. An intensity enhancement of up to 106 is possible in the near field of an illuminated scanning probe tip. The enhanced intensity was used for nanoscale material processing in terms of ablation and deposition.

Many applications will benefit from an optical holographic memory that can store and retrieve information in a format suitable for direct interface and transmission through an optical fiber network. Spatial-spectral holographic routing and processing devices are described in invited paper CWE1 by W. R. Babbit and associates from the University of Washington (Seattle, WA). The application of these devices is discussed, as well as an assessment of their expected performance.

K. Oba and colleagues from University of California at San Diego (San Diego, CA) have demonstrated nonvolatile storage of femtosecond pulses in photorefractive lithium niobate crystals by recording and readout of spectral holograms at wavelengths of 460 and 920 nm, respectively. The recorded spectral-domain holograms were read out for more than 24 h without degradation, indicating that the photorefractive hologram recorded at 460 nm is insensitive to the read-out wavelength of 920 nm. This work is discussed in paper CWE2.