The History of the Blue Laser

Initially the development of a blue or blue-violet laser was seen as an impossible task because of the difficulty of crystallizing the compound semiconductor needed to produce this light. Furthermore, a semiconductor capable of emitting blue or blue-violet light would need to be manufactured from elements at the high end of the periodic table. This was a major challenge. Elements at the high end of the periodic table are difficult to crystallize because of their strong bonding characteristics.

Sony's development efforts started with the raw materials. It began to explore the characteristics of the various elements, using a trial-and-error approach. In July 1993, it created the world's first ZnSe semiconductor laser capable of room-temperature continuous-wave operation, an achievement that was previously regarded as impossible. In February 1996, Sony succeeded in maintaining continuous oscillation of a ZnSe semiconductor for 100 hours. These successes brought the commercial development of a blue semiconductor laser within reach. Subsequently it was decided to use GaN, which produces a more stable blue-violet light. Sony had succeeded in developing a blue-violet semiconductor laser suitable for use in consumer electronic products, and in 2003 it launched the world's first BD recorder, the BDZ-S77.

Figure 2: Output Power-Current and Voltage-Current Properties of a 405nm AlGaInN Laser (a) and a 780nm AlGaAs Laser (b) During Room-temperature Continuous-wave Operation

There are differences in the voltage-current properties of a BD laser and CD-R laser. Because a BD laser has a shorter wavelength, the energy per photon is greater. This means that the drive voltage and power consumption are higher than for a CD-R laser.

Creating a Triple-Wavelength Laser

By using a triple-wavelength laser, it is possible to simplify the optical pickup in the optical systems. The advantages include a reduction in the number of parts required, and in the number of assembly and manufacturing processes involved. However, the lasers used in CDs, DVDs, and BDs are all made from different materials, and the development of a simple three-in-one device was thought to be impossible.

The impossible was made possible through technology used in PlayStation 2. Sony had already succeeded in developing a hybrid CD/DVD pickup for PlayStation 2 and was able to apply this technology to the creation of a triple-wavelength pickup. It did this by fabricating a substrate with a BD GaN laser chip and mounting a hybrid CD/DVD chip on top. The excellent thermal conductivity of the BD GaN substrate ensures efficient removal of heat generated by the hybrid CD/DVD chip.

PLAYSTATION 3 was designed to accommodate three totally different optical disc standards—CD, DVD and BD—in a single unit. As a game console that also functions as a next-generation HD player, PLAYSTATION 3 has taken the market by storm.

Figure 3: The Structure of the Triple Wavelength Laser

The Future of the Technology

The evolution of optical discs as media for the storage of video has advanced from the CD to the DVD, and now from the DVD to the BD. The development of semiconductor lasers has been a major factor in this evolution.

With the emergence of the blue-violet laser, the development of lasers with shorter wavelengths appears to have reached a plateau. A semiconductor laser with a wavelength shorter than the blue-violet laser would produce ultraviolet light, and in addition to the development of the semiconductor laser itself, it would also be necessary to develop optical components and recording media capable of withstanding exposure to ultraviolet light. Future development efforts are likely to focus on the development of blue-violet semiconductor lasers with improved output power, and on the creation of media with faster recording speeds and more layers.