The development of optical bistable devices, such as self-electro-optic-effect devices (SEEDs), led to a flurry of activity in optical logic during the early 1980s. This work continues, but its practicality has been questioned: optoelectronic devices have traditionally offered very little computing functionality to warrant their high power consumption. To combat this problem, researchers at Kyoto University (Japan) have been developing integrated components with more complex operational capabili
Researchers integrate multistable devices
The development of optical bistable devices, such as self-electro-optic-effect devices (SEEDs), led to a flurry of activity in optical logic during the early 1980s. This work continues, but its practicality has been questioned: optoelectronic devices have traditionally offered very little computing functionality to warrant their high power consumption. To combat this problem, researchers at Kyoto University (Japan) have been developing integrated components with more complex operational capabilities. Recently, for instance, they demonstrated tri-, tetra- and pentastable devices that can be set and reset optically. Though they have not yet used these components in a system, the researchers claim that both the devices and the philosophy that went into making them could have a significant impact on optical processing.
The multistable devices work through the coupling of smaller components that are integrated together. For a tristable device (see Fig. 1), the Kyoto team used four heterojunction phototransistors (HPT-A, -B, -C, and -D) and a laser diode (LD). In this system, the placement of each component relative to the others is as critical as the wiring for more conventional circuits.1 Light falling on HPT-A is used to set the device (see Fig. 2). The photocurrent that flows when HPT-A is illuminated also flows through the laser diode. Because the laser is located directly underneath HPT-A, spontaneous emission caused by the photocurrent becomes optical feedback: the light goes into HPT-A, which creates more photocurrent, which causes more spontaneous emission.
If the light falling on HPT-A is strong enough (20 µW), then it turns the device into the first on-state. The optical feedback is now strong enough for self-oscillation to occur even when the light source is taken away, so the system is latched. Because the device is symmetrical, the spontaneous emission reaching HPT-A also reaches and causes optical feedback in HPT-B. If the incoming set light reaches 40 µW, then HPT-B is also turned on and self-oscillates with the laser diode even though it has never been illuminated from outside the device.
To reset the device, HPT-C is illuminated. The HPT-C and HPT-D sites are located slightly away from the LD stripe, so almost no coupling occurs between them. Current flowing through HPT-C simply causes a voltage drop across the laser. If this drop is big enough, it will prevent self-oscillation at HPT-B and eventually at HPT-A.
For tetra- and pentastable devices, the Kyoto researchers use a six-HPT, two-LD system with more complex light/electronic couplings.2 An AB pair that feeds back to one laser diode is coupled with an EF pair that feeds back to another. C and D HPTs are use for resetting.
These optical multistable devices are fabricated using a conventional InGaAsP/InP semiconductor structure. According to researcher Susumu Noda, even relatively simple components can be used to create useful optoelectronic components. By integrating them carefully, he says, you get "not only the combined characteristics of the constituent devices, but also new functions due to the mutual interaction between them." This leads to the more general philosophy of Noda`s group, led by Akio Sasaki--the higher the degree of integration, the higher the functionality of the device.
Using this approach, the group has developed many other optoelectronic components, including switches, amplifiers, and multistable flip-flops. Work continues to reduce the optical power needed to activate the devices and to incorporate surface-emitting, rather than edge-emitting, laser diodes into the design.
1. S. Noda et al., IEEE J. Quantum Electron. 31(8), 1465 (Aug. 1995).
2. V Ahmadi et al., Solid-State Electronics 38(3), 551 (1995).
SUNNY BAINS is a technical journalist based in Edinburgh, UK.
Polarized-gas MRI shows lungs in high resolution
A new magnetic-resonance-imaging (MRI) method that uses polarized helium is giving clinicians and biologists a better way to view tissues that are difficult or impossible to image with the conventional proton-based MRI technique. Researchers recently acquired the first high-resolution images of a living human lung with the spin-polarized gas approach (see photo). The method was developed by scientists from the Princeton University physics department (Princeton, NJ) and the Duke University Center for In Vivo Microscopy (Durham, NC), led by William Happer of Princeton.
Magnetic-resonance imaging, developed in the 1980s, relies on magnetization of hydrogen nuclei (protons) to image living tissue. Magnetic-resonance techniques detect the magnetization, which is analyzed to create an image of the tissue. While conventional MRI has been a valuable tool for studying tissues that contain significant amounts of water and therefore hydrogen nuclei, tissues that contain little water, such as the lungs, are difficult to image effectively. Other diagnostic methods such as x-rays or ventilation scans with radioactive isotopes (SPECT) use sufficient ionizing radiation for the dosage to be a consideration when they are used. X-ray techniques do not image the airways directly, and the spatial resolution of SPECT is only about 10 mm.
The new MRI method uses spin-polarized helium gas rather than protons as the detectable material. The patient inhales the hyperpolarized gas to fill the lungs, which are imaged with a conventional MRI detector assembly. In a normal state, the magnetic moments associated with spin have almost no net polarization, so the diagnostic gas must be optically pumped to induce spin alignment.
A single-neutron isotope of helium (3He), derived from the decay of tritium, is placed in a gas cell with rubidium. Heating the mixture to about 180°C vaporizes the rubidium, and a 120-W, continuous-wave diode-laser array from Opto Power Corp. (Tucson, AZ) pumps the gas mixture with circularly polarized light at 795 nm, preferentially aligning the electron spin of the rubidium atoms. This spin polarization is collisionally transferred to 3He, after which the rubidium atoms are free to be optically polarized. After a sufficient amount of hyperpolarized 3He is generated, the gases are cooled to room temperature, and rubidium condenses out of the mixture.
The compact, portable diode-laser array requires less than 300 W of input power, for a net conversion efficiency of about 35%. But there are trade-offs. The linewidth of the diode-laser array is 2 nm, exceeding the absorption line of rubidium. Although the rubidium vapor is optically dense and the high partial pressure of helium in the gas cell broadens the rubidium line, only about two-thirds of the available laser power can be absorbed. The high conversion efficiency, low cost, and compactness of the system outweigh this loss, however.
Says Hunter Middleton of the Princeton group, "The use of these diode lasers makes this technique economically feasible. Every two months or so we increase our laser power by a factor of two, and the costs seem to stay about the same." Citing system portability, simplified power requirements, and ease of operation, Middleton adds, "Without diode lasers this would never have become a useful medical technique. There`s no way this could ever have been done in a hospital environment."
The quantity of gas produced is primarily driven by optical power; with an earlier 20-W array, less than half a liter of hyperpolarized 3He was generated in 10 hours of pumping. This was sufficient for early tests on guinea pigs, but couldn`t efficiently provide the 1 to 4 l of gas necessary for human clinical tests. The 120-W array can polarize about one liter of gas in four hours, and the Princeton group will soon acquire a 250-W array for the next-generation system.
In late September, the group acquired the first hyperpolarized-helium MRI image of the lung of a human subject. They obtained data slices approximately 2 cm thick, with two-dimensional in-plane spatial resolution of about 1.25 ¥ 2.5 mm. In comparison, radioactive xenon SPECT scans only achieve resolutions on the order of 10 mm. "That`s why MRI is such a wonderful technique," says Middleton. "It offers high resolution while using materials that are nontoxic, nonradioactive, and noninvasive."
Fast-gated cameras capture plasma plumes
At The Queen`s University of Belfast (Northern Ireland), work on pulsed-laser deposition (PLD) of superconducting films, magnetic films, and soft x-ray mirrors is proceeding in parallel with more fundamental PLD plume studies (see Laser Focus World, Oct. 1994, p. 103). Queen`s researchers are using a range of diagnostic techniques based on fast-gated two-dimensional charge-coupled-device (CCD) detectors. Plume images are recorded using an Oriel Instruments (Belfast, N. Ireland) Instaspec V fast-gated intensified CCD (ICCD).
A plume is formed when 1-4 J/cm2 of energy from a 248-nm KrF excimer laser with 20-ns pulse duration hits the target. The spectrally resolved plume emissions were imaged 2 µs after ablation of a superconductive yttrium barium copper oxide (YBCO) target in 180-mTorr ambient oxygen (see photo). The compact air-cooled ICCD was operated at -25°C with a gate-on time of 20 ns. Images were recorded as single laser events: the spectrally integrated emission of the plume (top), the 603-nm chemiluminescent emission of YO from the reaction of atomic yttrium with ambient oxygen at the plume edges (middle), and the 440-nm Y+ ion emission (bottom), which is greatest at the high-temperature expanding plume front. The plumes extend about 2 cm from the target surface.
Other spectroscopic techniques used by the researchers include laser-absorption spectroscopy and laser-induced fluorescence (LIF). To obtain images with these methods, the ablated material is backlit by the expanded beam of a short-pulse dye laser with a narrow bandwidth--tuned to an absorption resonance of the plasma species. Recording the two-dimensional profile of the transmitted beam provides information on the total number of absorber particles along the line of sight.
Simultaneous imaging of the resulting LIF at right angles to the probe dye-laser beam determines the exact location of these absorbers along the line of sight. Such orthogonal two-dimensional imaging makes it possible to determine the three-dimensional distribution of absorbing species within the expanding plume at the instant of the probe laser pulse. Spectroscopy data are complemented by data from other ICCD-based diagnostics such as time-resolved Mach-Zehnder interferometry to measure electron density.
Capturing spatially resolved two-dimensional images of such transient systems requires appropriate fast-gated ICCD devices. Queen`s physicists Tom Morrow and Ciaran Lewis, in collaboration with Oriel Instruments research physicist Raied Al-Wazzan, are developing new techniques and diagnostic devices for convenient one-, two-, and three-dimensional mapping within the rapidly expanding plume.
Their current work, supported by the UK Engineering and Physical Sciences Research Council, aims to provide quantitative data to validate and further develop computer simulation codes of laser-induced plasma plumes. Long-term objectives are to provide practical and cost-effective monitoring of critical plasma parameters and, ultimately, provide feedback control during pulsed-laser deposition.
Compact visible laser sources on upswing
Highlighting the 1995 Annual Meeting of the Optical Society of America (OSA, Washington, DC) in Portland, OR, was a panel discussion entitled "Advances in compact and efficient visible sources," cosponsored by the OSA`s Lasers Technical Group and Nonlinear Optics Technology Group. James Kafka of Spectra-Physics Lasers (SPL, Mountain View, CA) and Martin Fejer of Stanford University (Palo Alto, CA) jointly chaired the standing-room-only session, in which panelists described current obstacles and challenges and predicted developments over the next five years.
Kafka opened the discussion with a review of predictions regarding compact visible lasers. He noted that in 1994 solid-state lasers had been expected to displace much of conventional gas laser technology in the visible laser market (output power <1 W), but, in fact, according to Laser Focus World estimates, growth in solid-state visible devices was flat in 1994, with the conventional technologies still thriving in total revenues and especially in unit sales.
Several panelists then pointed out that, in general, solid-state alternatives to low-power visible gas lasers are currently more expensive and must, therefore, offer more than just a one-for-one replacement technology. Benefits of such solid-state systems include compactness, efficiency, minimal heat dissipation, and vibration. Visible solid-state lasers have been most successful in applications that demand these features.
Shigeo Kubota of Sony Corp. (Tokyo, Japan) presented results of harmonic generation with both CW and Q-switched Nd:YAG lasers. He expressed concern over degradation problems associated with nonlinear materials for short-wavelength UV generation and stated his goal of producing 1 W of 213-nm output at 10% conversion efficiency. I myself reviewed high-power (>1 W), high-repetition rate (>10 kHz) harmonic generation of Q-switched Nd-doped lasers, in particular for OEM applications, and described recent results obtained at SPL with diode-pumped Nd:YVO4. For green-light generation with such devices, power increases will depend primarily on efficient scaling of the diode-pumped lasers. I expect that 20-25-W, Q-switched IR sources (and, therefore, ~10 W of green) should be available within five years at prices that could be supported by OEM users.
Prospects for diode-pumped lasers
Suzanne Lau of Uniphase (San Jose, CA) reviewed all commercial green solid-state lasers and described the twin challenges of providing good performance at a good price and of helping customers to climb the learning curve associated with a switch to solid-state diode-pumped lasers. In five years she expects to see smaller, cheaper, higher-power devices and shorter wavelengths.
Bill Risk of IBM Almaden (San Jose, CA) noted that the Coherent (Santa Clara, CA) 10-mW, 430-nm D3 laser is the only solid-state blue "catalog item" currently available based on frequency doubling. He described a possible window of opportunity for such sources that is likely to remain open until blue diode lasers are widely available. He also pointed out several technical barriers to these devices, including complexity, cost, and manufacturability of bulk and quasi-phase-matched (QPM) materials and related coatings.
Upconversion fiber lasers are the "crazy uncles" of the field, stated Tim Gosnell of Los Alamos National Laboratories (Los Alamos, NM), who reviewed wavelengths and output powers that have been demonstrated recently, including powers up to 300 mW and wavelengths from 381 to 635 nm. Gosnell highlighted the potential simplicity of such systems, but also pointed out that the cost of development is particularly high because the most promising results have been obtained with sophisticated ZBLAN fibers.
Reinhart Engelmann of the Oregon Graduate Institute (Beaverton, OR) surveyed recent progress in blue-green diode lasers. While the panel generally agreed that, eventually, "this would be the way to do it," Engelmann presented a plot of demonstrated lifetime versus calendar time that satirically suggested a maximum achievable diode lifetime of only about one hour! He then explained that certain specifics related to improved crystal growth would be among the keys to development of reliable blue-green diode lasers and compared the relevant properties of II-VI and III-V materials.
Panelists also presented their "wish lists" for the researchers in the audience, including requests for a good manufacturing process for QPM materials, new nonlinear materials that would allow still-shorter wavelengths into the UV, tabulations of oscillator strengths and cross sections for upconversion materials, better understanding of problems in the growth of BBO, the availability of bulk or QPM material that would allow for the generation of >1 W at wavelengths below 266 nm with industrial lifetimes, and, finally, means to greatly reduce the defect formation that limits the lifetime of blue-green diode lasers.
William L. Nighan Jr.
WILLIAM L. NIGHAN JR. is manager of the Advanced Product Technology Group at Spectra-Physics Lasers, POB 7013, Mountain View, CA 94039-7013.