These short-wavelength laser diodes are branching out from the optical storage market into digital, metrology and life sciences applications.
Dr. Gregory Flinn, Toptica Photonics AG
The recent availability of GaN-based laser diodes, nurtured by a strong and flourishing LED market, has opened up a host of new applications. The optical storage market, essentially the raison d’être for this technology, is leading the way with a new generation of writers/players that build upon established optical storage technology. The two contenders for a new storage format, the recently standardized Blu-ray Disc,1 proposed by the 10-strong, similarly named consortium, and AOD,2 proposed by Toshiba and NEC, both utilize 405-nm laser diodes but differ in the extent of their departure from the optics configuration for DVD. The expected storage capacities on a CD-size disc are approximately 25 and 15 GB, respectively.
A comparison of CD, DVD and Blu-ray Disc technologies helps illustrate the tight focusing and rapid intensity modulation available from violet diode lasers, properties usable in other photonic applications. Courtesy of Philips Research.
Nichia Corp.’s much-publicized techno-legal wrangles with other laser diode manufacturers have receded significantly, and in the meantime, the technology has progressed. On the one hand, to satisfy the requirements for double- and quadruple-speed drives, laser diode manufacturers project pulsed or even CW output powers up to 100 mW at 405 nm for 2004 or 2005, while on the other hand, sample laser diodes emitting around 375 nm (UV) and 440 and 470 nm (both blue) have been issued to technology leaders.3
Importantly, several other technology fields have been able to make use of these short-wavelength semiconductor lasers, in some instances well before Blu-ray Disc had been submitted for approval as a standard. One can conveniently separate these fields into digital, metrology and life sciences applications.
Digital applications
The print industry has enthusiastically adopted violet diode laser technology for direct computer-to-plate exposure systems, where the short-wavelength light and sophisticated driver technology enable a more rapid and accurate plate exposure. Although a move to even shorter wavelengths remains on the agenda (i.e., moving away from visible sources and the associated plate-handling difficulties), the higher powers available at 405 nm firmly anchor the current strategy in the violet for many plate and plate-setter manufacturers.
Violet diode lasers are already being used for optical storage technology. In general, a shorter wavelength than the read wavelength is used for the production of accurately sized pits on the disc master, utilizing exposure of a photoresist to generate a perfect master copy. Thus, we see the use of 532 nm for CD mastering, with the pressed copies being read at 780 nm. DVDs, although read with 650-nm light, are often mastered using a krypton-ion laser at 413 nm, but 405-nm violet laser diodes have been used and now dominate in this application.
Microlithography, or pattern writing on photoresists, is also starting to make use of these new laser sources. Clearly defined, submicron features have been made on standard chrome and iron-oxide masks, as well as by direct writing on final substrates (Si, GaAs, glass, etc.).4 Although not currently suitable for replacing HeCd lasers in the 325-nm, higher-resolution systems, 405- or 440-nm diode lasers offer performance enhancements for traditional 442-nm pattern generators. While 440 nm allows the direct replacement of HeCd lasers without introducing optics compatibility issues, 405 nm may nonetheless win because of better power and reliability. On the other hand, 375-nm diode lasers offer better compatibility with existing resists while retaining the use of standard optics materials.
All these technologies lie close to the core application; namely, optical storage. For this application, the laser must provide excellent wavefront and focusability, coupled with a rapid, high-contrast off/on modulation. A sophisticated laser diode driver and thermal management are required to guarantee pulse height/shape performance for asynchronous modulation ranging dynamically from DC up to several hundred megahertz.
he rapid modulation also permits the use of complex gray-scaling techniques: AM/FM screening in computer-to-plate processing gives improved visual results on the printed page, while gray-level patterning in microlithography allows the creation of diffractive optics.
Metrology
The production testing of optical components for DVD pickups is made using standard interferometry tools — i.e., with a stabilized HeNe laser at 633 nm — despite the moderate discrepancy with the read wavelength for the drives. Not so for violet-based optical storage; the highly wavelength-specific lens design has brought about the need for interferometric testing of optics at the very same wavelength: 405 nm.
Thus, the optics testing arena, in anticipation of the commercial availability of violet optical storage, requires the violet equivalent of the stabilized HeNe. At present, only non-OEM and bulky scientific lasers are on the market, offering little in the way of user comfort. The requirement is for a compact, actively wavelength-stabilized and OEM-compatible violet source capable of securing and signalizing stable single-mode operation (commensurate with high contrast in the interferogram) and thereby streamlining and guaranteeing the whole testing process.
Biophotonics
In the many varied applications of biophotonics — microscopy, fluorescence tagging and cytometry, to name but a few — one can also see applications for violet lasers. Principal needs are for very high focusability and compactness; for example, for single-mode fiber coupling in application areas such as confocal microscopy, where the coupling stability into a sub-3-μm fiber core represents a significant leap from that at telecom wavelengths.
The short wavelength of violet light offers enhanced spatial resolution in such applications as fluorescence tagging for total internal reflection fluorescence (an offshoot of confocal microscopy), while fluorescence lifetime measurements specify a requirement for rapid, subnanosecond-pulse operation. Development of the UV wavelengths for nanosystem fluorescence applications is receiving significant funding.
Compared with optical drive production, all of the technology applications mentioned are essentially low-volume applications. Nonetheless, their total market opportunity is some 10,000 laser systems per annum in relatively price-insensitive applications, with the result that manufacturers of lasers across the range are starting to address these markets.
For the end-user of violet wavelengths, critical concerns about lifetime expectancy or long-term availability of laser sources are a thing of the past, with improved lifetime/power performance at other wavelengths expected for the near future. In general, this technology offers several advantages through aspects such as reduced thermal issues, better overall efficiency, short warm-up times and compactness.
The appearance on the market of commercially available laser diodes from multiple manufacturers will signify an end to the exceptionally high unit price. Coupled with general optics support for this industry, we should then witness the enabling of further applications.
References
1. www.blu-ray.com.
2. www.blu-ray.com/faq/and www.labs.nec.co.jp/eng/topics/data/r020829.
3. www.nichia.co.jp/product/laser-main.html.
4. C. Arnone and G. Flinn (August 2003). Blue diode lasers mature for mask writing. EUROPEAN SEMICONDUCTOR, pp. 13-15.
Meet the author
Gregory Flinn is responsible for business development for R&D, OEM and print at Toptica Photonics AG in Martinsried, Germany.