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03 Jun 2009
The advent of cheap and efficient green lasers is eliminating one of the last remaining hurdles to the commercial success of laser displays and laser television. As Breck Hitz explains, laser vendors are rolling out a raft of innovative products to drive market growth.
Laser displays can deliver more than twice the colour gamut of LCD screens or plasma displays, an advantage that's reinforced by a thin, lightweight package that offers greater energy efficiency versus the competing technologies. What's more, unlike LCD or plasma products, laser displays do not degrade over their lifetime, which is expected to exceed 50,000 hours.
Despite these performance benefits, laser television to date has registered only limited commercial success. Until the first major electronics player, Mitsubishi of Japan, entered the market, no manufacturer had recorded significant sales. Mitsubishi's 65 inch model (which retails for around $7000) was introduced with considerable fanfare in October 2008, though the company has been less than forthcoming with sales figures. Earlier this year, production was also halted temporarily for "tooling problems" unrelated to the lasers' performance, according to Mitsubishi's David Alhart.
One of the biggest challenges to the commercial success of laser television is designing a suitable green laser. A television display requires three colours – red, green and blue (hence the term RGB display) – and the optical approach favoured by most designers is to employ direct-diode lasers for the red and blue light. There are no green diode lasers, however, which means an alternative approach is necessary: frequency-doubling of an infrared laser.
Think green
At Mitsubishi, engineers used slightly modified edge-emitting laser diodes intended for DVD reading and writing for the red (640 nm) and blue (447 nm) sources. For the green (532 nm) laser, they used an unusual planar-waveguide configuration for both the fundamental and second-harmonic wavelengths (figure 1). Both the Nd:YVO4 and LiNbO3 waveguides were several microns high and several millimetres wide. The Nd:YVO4 section was 1.5 mm long, and the LiNbO3 section was 4 mm long.
The 1.06 µm resonator was formed by mirrors on the right face of the LiNbO3 and the left face of the Nd:YVO4, so the resonator was 5.5 mm long and the second harmonic was generated inside the 1.06 µm resonator. A mirror on the left face of the LiNbO3 reflected the second harmonic that was generated travelling right-to-left and combined it with the second harmonic generated left-to-right.
The Mitsubishi engineers pumped the tiny Nd:YVO4 laser with a linear laser-diode array (a bar) with 15 individual ~100 µm-wide emitters separated by ~200 µm. This pumping geometry created 15 different lasing channels in the Nd:YVO4 and LiNbO3 waveguides. Each channel was guided by the waveguide in the vertical direction and focused by thermal lensing in the horizontal direction. The engineers induced the thermal lensing by designing the heat removal so that a parabolic temperature gradient existed in the transverse direction across each of the 15 channels. Spatial interference effects, in the far zone where the 15 green beams overlapped, were not critical because the beam was ultimately expanded to cover the entire television screen.
The diode array produced 27 W of 808 nm output, from which the doubled vanadate laser generated 11.4 W of green power for a 42% optical efficiency (from 808–1064 nm to 532 nm). The overall electrical (wall-plug) efficiency was 21%. The Mitsubishi engineers modulated the laser at 1.68 kHz and observed essentially a constant green peak-power output, independent of duty cycle ranging from 30 to 100%.
Different strokes
A different approach to green-light generation – actually two different approaches – has been pursued by QPC Lasers of Sylmar, CA. For the first technique, engineers designed a semiconductor master-oscillator power-amplifier. The oscillator was a distributed-feedback laser, coupled into a conical amplifier. A cylindrical lens focused the elliptical beam that emerged from the amplifier into a periodically poled LiNbO3 crystal. The laser generated 1 W of green light, in a beam with M2 of less than 2.
The second approach, according to Laurent Vaisse, a QPC engineer, involves a more conventional diode-pumped solid-state laser. Vaisse and his colleagues pumped the Nd:YVO4 laser with an 808 nm semiconductor laser that was stabilized with a planar grating in its epitaxial layer. The grating kept the pump laser locked onto the Nd:YVO4 absorption peak, boosting overall efficiency.
The YVO4 laser was frequency-doubled with an intracavity lithium-triborate (LBO) crystal, placed in a leg of the folded resonator so that the intracavity green powers generated in both directions were combined and emerged in a single beam from the folding mirror. The laser generated 6.5 W of green power, with an M2 of less than 2, stable to within 0.1 W over a base-plate temperature range from 10 to 30 °C.
Remember NECSELs?
In 1998, MIT scientist Aram Mooradian founded Novalux to commercialize an entirely new kind of semiconductor laser. The Novalux extended-cavity, surface-emitting laser (or NECSEL) was unique in that an etalon replaced the mirror that normally serves as a laser's output coupler. This gave the laser promising capabilities for mode control and intracavity nonlinear optics. And, unlike the elliptical beam produced by edge-emitting semiconductor lasers, the round beam from a NECSEL needed no complex cylindrical optics to focus it to low divergence.
Although the original plan was to make lasers for the telecommunications industry, Novalux shifted gear in 2003 (after the telecoms bust) and focused on lasers for displays. The company demonstrated a multiwatt green laser that was considered by many to be the leading candidate for televisions and other displays. But after absorbing over $250 m (€178.4 m) in investments in the decade after its formation, the company was purchased for $7 m in 2008 by Arasor International, an Australian vendor that's developing optical sources for televisions and other displays.
Arasor has continued to develop NECSELs for commercial applications, including displays, according to Arasor engineer Greg Niven. The company sells a three-colour engine for displays, consisting of 3 W green and blue NECSELs and a 4 W red direct-diode laser. Both the NECSELs are doubled with an intracavity crystal of periodically poled LiNbO3. Nondisclosure agreements prevented Niven from discussing the company's customers or details about the applications.
It's also worth noting that Mooradian had founded an earlier company, Micracor, in 1992. That company, which developed some of the earliest commercial surface-emitting semiconductor lasers, was sold to Coherent in 1996. Coherent's intention was to develop the lasers for displays, and at one point the company built a demonstration laser TV based on Micracor technology. But although the surface-emitting semiconductor laser has been a successful product at Coherent, its success stems from applications other than television displays (e.g. medical diagnostics and spectroscopy). What's more, the company is no longer pursuing the display application, according to Matthias Schulze, Coherent's director of marketing, OEM components and instrumentation.
On the fly
Frequency-doubled rare-earth lasers, like the Mitsubishi and QPC devices, are intended for display projection systems, in which three images – one red, one green and one blue – are projected simultaneously onto a screen, where they overlap to create a full-colour display.
An alternative is the flying-spot display, in which the beams from the three lasers are combined into a single beam that scans the screen. As the beam scans across the screen, the three lasers are modulated to produce the desired colour at each pixel. To obtain suitable resolution, the laser must be modulated with nanosecond switching speeds. However, because the spontaneous lifetimes of rare-earth lasers are typically of the order of hundreds of microseconds, only frequency-doubled diode lasers are suitable for flying-spot systems.
OSRAM Opto Semiconductors, Germany, is using precisely this approach to develop a green laser for a flying-spot system intended for portable displays in laptop computers and digital cameras. These tiny display devices would project an image onto any nearby surface. OSRAM has designed and built an optically pumped, surface-emitting semiconductor laser that is frequency doubled with an intracavity crystal of periodically poled MgO/LiNbO3. Like the QPC rare-earth laser, OSRAM's laser uses a folded resonator so that the green light generated in both directions is combined into a single beam that exits through the folding mirror.
The tiny OSRAM package occupies only 0.36 cm3 (compared with 5 cm3 for the Mitsubishi laser, for example). The laser generates 50 mW of green light in a single longitudinal and single transverse mode, from less than 1 W of electrical input. Other significant features of the design include modulation at tens of megahertz (simply by modulating the electrical input to the pump laser), passive cooling and green output that is constant despite variation of the duty cycle from 10 to 90%.
Meanwhile, scientists at Corning in the US have also investigated green lasers for flying-spot microdisplays. The Corning G-1000 laser is a distributed-Bragg-reflector (DBR) semiconductor laser, the infrared output of which is frequency-doubled in a periodically poled LiNbO3 waveguide.
Unlike the OSRAM laser, the Corning laser's doubling crystal is outside the resonator, and the infrared beam is coupled into the LiNbO3 waveguide with a pair of lenses. The waveguide is only several square microns in cross-section, and thermal drifts can easily misalign the doubling crystal from the incoming beam.
To compensate, Corning engineers employed a commercial adaptive optical element, the Konica Smooth Impact Drive Mechanism, to adjust the lenses between the laser and the doubling crystal. With this modification, the laser's green output remains between 84 and 90 mW as its temperature varies over a 30 °C range. Package size is just 0.7 cm3.
In summary, while all of these techniques for generating green laser light show promise, it is far from clear which of them will ultimately be best for televisions and other displays. The Mitsubishi approach seems to be leading the pack right now, but that lead could dissipate very quickly in the coming months. What is clear, however, is that laser displays are coming on strong and are set to become a major growth market sooner rather than later.
Back to basics on SHG
Second-harmonic generation (SHG) is a classical nonlinear light-matter interaction that converts part of the light in an intense laser beam to the second harmonic. If the second harmonic generated at every point in the nonlinear material is in phase with that from other points, an intense second-harmonic beam can emerge from the material parallel with (and sometimes colinear with) the fundamental beam.
An important issue is the phase relationship of the second-harmonic waves generated at different points along the fundamental beam. Normal material dispersion will cause the fundamental and second harmonic to slip out of phase with each other as they propagate, such that the second harmonic generated at one point will be out of phase with that from another point, and the total signal is zero.
• Birefringent phase-matching compensates for dispersion by orienting one wavelength in the ordinary polarization, and the other in the extraordinary. (So-called type-II birefringent phase-matching is slightly different.) Thus, the two wavelengths experience the same refractive index, and the second harmonic generated at one point along the beam is in phase with that generated at any other point.
• Quasi phase-matching does not depend on polarization or birefringence. Instead, a nonlinear material is periodically poled so the sign of its susceptibility switches every few tens of microns. As the fundamental and second harmonic propagate, they slowly slip out of phase and the rate of SHG slows. After a few tens of microns, they are 180° out of phase, and the second harmonic would diminish if they propagated farther. But at that point, the susceptibility switches and the fundamental and second harmonic are again in phase.
Quasi phase-matching can often achieve higher conversion efficiency than birefringent phase-matching. A big advantage of quasi phase-matching is that, because the crystal doesn't have to be oriented to take advantage of its birefringence, it can be oriented to utilize the crystal's greatest nonlinear coefficient. In LiNbO3, for example, quasi phase-matching can use a nonlinear coefficient seven times greater than the one available for birefringent phase-matching. Conversion efficiency scales as the square of this coefficient, so quasi phase-matching begins with a factor of 50 advantage over birefringent phase-matching.
• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.
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