PHOTONIC FRONTIERS: FREQUENCY-SHIFTED DIODE LASERS: Shifting semiconductor laser wavelengths poses challenges

FIGURE 1. Corning's green laser module for a pico-projector is 4 mm thick and shown sitting on a smart phone for scale.

Nonlinear optics offer invaluable ways to fill gaps in the laser spectrum, from simple harmonic generation to more complex optical parametric oscillators (OPOs). Frequency doubling of diode-pumped neodymium (Nd) lasers has made green laser pointers cheap and compact, but why can't developers drop the diode pumping and directly double semiconductor laser output to produce hard-to-find wavelengths?

It's been done for green light and is already reaching the market in pico-projectors from companies like MicroVision (Redmond, WA). But it's not easy. Nonlinear wavelength conversion requires not only raw power but also high beam quality and narrow-line emission. It's tough to combine all those features in a semiconductor laser. Yet progress is being made. The first products are on the market, and developers are reporting more encouraging results, including new laser designs, diode pumping of OPOs, and both harmonic generation and difference-frequency generation with quantum cascade lasers.

Quests for doubled diodes

Serious work on direct doubling of diodes started in the early 1990s, when diodes had reached high powers and the diode laser spectrum stopped in the red. Doubling the output of near-infrared diodes promised inexpensive sources for the short end of the visible spectrum. It also offered directly modulatable short-wavelength lasers for applications such as laser displays.

Coherent Inc. (Santa Clara, CA) succeeded in developing a product called the D3 (for direct-doubled-diode) laser, which frequency doubled the roughly 100 mW of an 860 nm diode to produce about 10 mW of blue light at 430 nm.¹ It required a distributed Bragg reflector laser for narrow-line output, and the diode output had to be mode-matched and phaselocked into the external harmonic generator. It was a first, but it found few applications and eventually faded away—no doubt partly because of Shuji Nakamura's remarkable success in developing blue indium-gallium nitride diode (InGaN) lasers at the Nichia Chemical Corp. (Tokushima, Japan). Coherent eventually developed optically pumped surface-emitting semiconductor lasers, which are frequency doubled to emit at visible wavelengths but behave more like solid-state lasers than diodes.

The success of blue diode lasers left a gap in the green center of the visible spectrum, which emerged as a problem a few years later as the consumer electronics industry looked for new technology for projection television. Laser back-projection promised a better color gamut than flat-panel displays, if a suitable laser source were available at about 530 nm. Doubled Nd might have seemed a logical choice, but it couldn't be directly modulated at the required speed, so developers turned to doubling 1060 nm diodes or other lasers to produce 530 nm green beams. Many of those projects wound down as rear-projection television faded from the consumer market, but a few shifted toward compact pico-projectors for mobile devices, where cost points are lower than for televisions, says John Nightingale, an optical consultant in Portola Valley, CA.

Corning Inc. (Corning, NY) is already carving a niche out of the young pico-projector market. Last year it introduced a commercial version and is supplying lasers to MicroVision for its Showwx projector for iPods and laptops. Corning's green laser doubles the 1060 nm output of a distributed Bragg reflector (DBR) laser emitting a single-spatial mode and single frequency. The laser includes three sections: one a DBR grating, a second for phase adjustment, and a third for gain. Corning initially reported generating up to 104.6 mW at the 530 nm second harmonic by coupling the infrared DBR output into a periodically poled lithium-niobate second-harmonic generator.² Measurements showed that the green light could be modulated at rates above 50 MHz as required for projectors, and later laboratory versions reached green output of 184 mW.³

Corning's first commercial model, introduced last year, emits 60 mW (see Fig. 1). In May 2010 the company introduced a prototype 80 mW version that it says has wall-plug efficiency of 8% and can be modulated at speeds to 150 MHz as needed for extended graphics resolution.

Tapered amplifier lasers

Another approach to generating the high-quality, high-power beam needed for efficient harmonic generation is combining a single-mode ridge waveguide DBR diode laser with a tapered amplifier stage (see Fig. 2). Götz Erbert's group at the Ferdinand Braun Institute for High-Frequency Technology (Berlin, Germany) is in the midst of a five-year project to develop compact second harmonic sources with output to a few watts in the visible for a range of applications, from cinema-scale projectors to precision spectroscopy. The group has generated fundamental output at 980 nm with 0.012 nm linewidth, power to 12 W, and a nearly diffraction limited beam with vertical divergence less than 15°.⁴ Single-pass second-harmonic generation in periodically poled lithium niobate generated more than 1 W at 488 nm. The group also is exploring nonlinear techniques for generating light from the ultraviolet to the infrared, and together with Sina Riecke of PicoQuant GmbH (Berlin, Germany) has produced 30 ps pulses at 531 nm and megahertz repetition rates.⁵

FIGURE 2. The tapered amplifier developed at the Braun Institute includes a 2 mm length of 4 µm ridge waveguide, with a 1 mm DBR at the back and a 1 mm gain section. The remaining 4 mm amplifier stage is tapered at 6° [4].

The Braun Institute group also is working with the University of Potsdam on a coupled ring resonator for harmonic generation (see Fig. 3). The main ring optically locks a tapered amplifier laser with the ring resonance, and couples the fundamental output into a smaller ring that contains a periodically poled lithium niobate harmonic generator. Recent experiments generated 310 mW of 488 nm output with 50 MHz linewidth at 18% optical conversion efficiency.⁶

FIGURE 3. Coupled ring resonators for diode-laser harmonic generation developed by Potsdam-Braun Institute Team feature a tapered amplifier laser (TA) in the top ring, along with a holographic diffraction grating (G), an optical diode, a half-wave plate (HWP), a polarizing beamsplitter (PBS), a beamsplitter (BS), and several lenses. A periodically poled lithium niobate harmonic generator (PPLN) is shared by the upper and lower ring [6].

A joint project with Paul Michael Petersen's group at the Technical University of Denmark (Roskilde, Denmark) yielded fundamental output of 1.38 W from a tunable diode between 659 and 675 nm with linewidth of 0.07 nm.⁷ The output is the highest from a tunable diode in this range, and could be doubled to the 335 nm range, shorter than current ultraviolet diode lasers.

Combining the high-quality near-infrared lasers with nonlinear optics also can generate longer infrared wavelengths where good sources are not available, says Erbert. Together with the University of Twente (Enschede, the Netherlands) his group used 8.05 W at 1062 nm from a monolithic diode amplifier to pump a singly resonant optical parametric oscillator of periodically poled lithium niobate. Tuning ranges were 1541 to 1600 nm for the signal wave and 3154 to 3415 nm for the idler. The idler power exceeding 1.1 W at 3373 nm, the highest yet from a diode-pumped OPO, and its 44% optical-to-optical conversion efficiency made overall electrical to optical efficiency 14.9%, about seven times higher than from pumping an OPO with a diode-pumped laser.⁸

Shifting quantum cascade laser wavelengths

Nonlinear wavelength shifting is also a hot topic for quantum cascade lasers, where the major goals are second-harmonic conversion, and difference-frequency mixing to generate terahertz frequencies.

The main interest in second harmonic generation is to reach C-H, O-H, and N-H stretching bands of hydrocarbons near 2.5–3.5 µm, which would open important new applications, says theorist Alexey Belyanin of Texas A&M University (College Station, TX). Problems with heterostructure growth and current injection have hampered development of quantum cascade lasers with direct output at such short wavelengths. The first observations of harmonic generation in quantum cascade lasers were made several years ago by a collaboration involving Belyanin and Frederico Capasso and Claire Gmachl, then both at Bell Labs (Murray Hill, NJ), but the power was limited to tens of nanowatts.⁹ By 2004, they had raised second harmonic power to milliwatt levels at 4.45 µm.¹⁰ The harmonic generation occurs within the quantum cascade structure itself. "Since we basically do 'quantum engineering' of the optical nonlinearity, we can control it at our will," says Belyanin. That increases nonlinearity far above that of other materials, and aids phase matching by shifting electron resonances.¹¹ He recently proposed a way to generate second harmonics as short as 1.5–2.5 µm and is working with experimentalists to demonstrate the idea.¹²

The appeal of difference-frequency mixing for generating terahertz radiation is the ability to operate at room temperature, which for most applications is preferable to the cryogenic cooling required for direct terahertz emission from quantum cascade lasers. The tradeoff, says Belyanin, is that "there's no free lunch because you lose a lot of power."

Difference-frequency generation requires producing a quantum cascade laser that operates at two separate wavelengths, which mix in the laser cavity to generate the difference frequency. "The terahertz output is modest, but it is there," says Mikhail Belkin of the University of Texas at Austin (Austin, TX), who mixed 7.6 and 8.7 µm wavelengths to generate the 60 µm difference frequency while working with Capasso at Harvard.¹³ "Theoretically we could get milliwatts, but experimentally we only get about 1 µW at room temperature," adds Belkin, who is continuing this work in Austin. The good news is that he sees plenty of opportunities to raise the power further. Cryogenically cooled terahertz quantum cascade lasers can emit up to 150 mW but only at very low temperatures, declining at higher temperatures and to zero above 190 K.

Competition and outlook

Recent progress on green diode lasers shows clear competition. At Photonics West, Nakamura, now at the University of California at Santa Barbara and Kaai (Santa Barbara, CA), announced a 523 nm green diode, and Osram Opto Semiconductors (Regensberg, Germany) reported 50 mW output in the laboratory from a 515 nm InGaN laser.¹⁴ Yet Osram isn't giving up on doubling optically pumped surface-emitting semiconductor lasers to produce green light, and Corning is delivering products that are being built into pico-projectors. Green diodes will have to catch up in power and reliability, and for all their appeal, there is no guarantee they can beat doubled diodes—especially at the watt level. Other nonlinear wavelength shifting is on the cutting edge of progress. Results are encouraging, but we will have to wait for the final results.

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http://www.repairfaq.org/sam/laserdio.htm#diocod
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