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Laser breakthrough could soon enable 1-TB optical storage

Photonics Spectra
Oct 2010
Lynn Savage, lynn.savage@photonics.com

SENDAI, Japan – Numerous research labs are looking for ways to increase the amount of information one can store on a CD-ROM or related optical disc, but the equipment needed to push past the boundaries of Blu-ray technology is exceedingly large and expensive. Recently, investigators affiliated with Tohoku University and with Sony Corp. in Atsugi have made a huge leap toward a laser that is portable yet potent enough to drive optical storage up to 1 TB.


Researchers at Tohoku University and at Sony Corp. have developed a semiconductor blue-violet laser that emits 100-W, 3-ps pulses at 1 GHz, perhaps ushering in a new generation of large-capacity optical storage devices. Images courtesy of Sony Corp.


The next iterations of large-capacity optical storage devices – as well as nano-scale photolithography systems, novel noninvasive biological imagers and other useful tools on the horizon – require cumbersome Ti:sapphire lasers to operate in the labs where they are being developed. To get them out of the laboratory and into consumers’ hands, the lasers powering such devices must shrink while maintaining the ability to generate picosecond-long high-energy pulses at high repetition rates.

Hiroyuki Yokoyama of Tohoku University and his colleagues there and at Sony have been studying the behavior of GaInN laser diodes that are placed under intense electronic excitement. In the July 12, 2010, issue of Applied Physics Letters, Yokoyama’s team reported that it achieved better than 100 W of output power from a GaInN laser diode operating at 405 nm. The experimental device provides peak power of 103 to 119 W at 1050-mA electrical input, doing so with 3-ps pulses at a rate of 1 GHz. The university reported that the peak power achieved is 100 times higher than that supplied by other lasers in the blue-violet emission range.


Schematic depiction of the new semiconductor blue-violet laser.


The researchers achieved the large power and short pulses without extraneous pulse compression techniques. Instead, they developed a master oscillator power amplifier (MOPA), using a single-stage GaInN semiconductor optical amplifier (SOA) to increase the power of a mode-locked laser diode. The MOPA comprises a 600-μm-long bisectional laser diode, an optical filter designed to pass 404-nm light and an external mirror with 5 percent reflectivity. The researchers passively mode-locked the MOPA to generate clean pulses without noticeable subpulse components.

“Tohoku University is responsible for the device physics controlling nonlinear-optic effects and optical noise problems, etcetera, while Sony has the excellent GaInN laser diode device technologies,” Yokoyama said.

Sony tested the laser for possible use in large-capacity optical storage, finding good results when creating 300-nm-diameter void marks at a 3-µm pitch inside a plastic substrate.

Besides optical storage, the semiconductor laser system may find utility in two-photon absorption experiments such as those used in biological imaging.

“The present stage is still a kind of basic research to confirm the novel device operation function,” Yokoyama said. “Therefore, low-cost production technologies and reliability evaluation for semiconductor laser devices would be important toward commercialization.”


GLOSSARY
amplifier
A device that enlarges and strengthens a signal's output without significantly distorting its original waveshape. There are amplifiers for acoustical, optical and electronic signals.
optical disc
A rigid medium, generally a polycarbonate substrate coated with a reflective aluminum layer, that stores information (such as audio, video or data) as digital bits in the form of variations in the reflective surface, and that modulates a laser beam to decode the stored data. Optical disc formats include read-only (ROM) and write-once/read-many (WORM); erasable discs include phase-change, magneto-optical and dye-polymer media.
optical filter
A device with characteristics of selective transmittance, capable of passing a certain part of the electromagnetic spectrum while being opaque to the other portions. Means of producing filters varies considerably. Color filters (for the visual) usually consist of glass, gelatin or plastic containing dyes or pigments. Band pass filters are found in signal processing and are commonly fabricated via electronics and complex circuitry and are designed to pass and reject parts of the spectrum based...
reflectivity
The ratio of the intensity of the total radiation reflected from a surface to the total incident on that surface.
semiconductor laser
A semiconductor material which is designed and grown for the efficient production of short wavelength stimulated emission through high gain as well as low internal losses. Materials with band gap energies which emit radiation efficiently within the desired wavelength region are used. Diodes may emit vertically in relation to the laser material junction in a VCSEL configuration or horizontally in an edge emitting configuration. Synonymous with laser diode.  Sometimes used...
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