WDM systems could use tunable lasers around 1.55 µm, and fiber-to-the-home systems are likely to need tunable lasers around 1310 nm. Researchers Ajay Nahata and colleagues at NEC (Princeton, NJ, and Tsukuba, Japan) and the S. I. Vavilov State Optical Institute (St. Petersburg, Russia) have developed a tunable laser that uses a distributed Bragg reflector in a structure that is simpler than previous tunable lasers that used DBRs.
The laser uses a dynamic holographic grating mirror to set the wavelength. The grating is formed in the volume of a gallium-doped CdF2 crystal, a material that contains bistable defects known as "DX centers." Crystals containing these defects respond to optical excitation by releasing electrons from the deep DX trap states, resulting in a change in refractive index.
A tunable external-cavity laser, can be formed if the back mirror is a Bragg grating holographically written into the CaF2:Ga crystal. The grating is formed by the interference of two 488-nm beams. The angle of the mirror determines the period of the gratings.
An argon ion laser, operating at 488 nm is used to write the refractive index grating into the crystal using two-beam interference (the same way many holograms are made). The beam's power density is 24 mW/cm2 at most. By altering the angle at which the beams intersect, users can alter the grating period.
At room temperature, the grating fades within seconds of the argon laser beam being turned off. Therefore, in operation, the grating-writing laser needs to be on. "At room temperature, the gratings take several seconds to tune," Nahata said. "Basically, the old grating has to fade before the new grating can work properly." While the gratings would fade more quickly at higher temperatures, the writing power required would change as well.
The system uses a 1290 nm laser diode with an antireflection-coated back facet. Light output from the facet is collimated and sent into the 11 x 12 x 7-nm crystal. The grating in the crystal acts as a mirror, reflecting the light at a specific wavelength depending on the period of the mirror.
The external-cavity laser is tuned from about 1260 to 1285 nm and maintains a sidemode rejection ratio of more than 40 dB across the entire range. In principle, the technique should allow tuning at 1550 nm in the CaF2:Ga crystal.
- Ajay Nahata et al., IEEE Phot.Tech. Lett. 12 (11) 1525 (November 2000).
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.