The tunable semiconductor lasers that have recently been commercialized may be vital parts of future optical packet-switching fabrics. To be useful for this application, however, they must also be able to switch channels precisely. Chun-Kit Chan, Karl Sherman, and Martin Zirngibl at Bell Laboratories, Lucent Technologies (Holmdel, NJ) have incorporated these lasers into a 100-channel transmitter that can switch channels in less than 100 ns with a wavelength accuracy to within +/-0.005 nm.1
Commercially available tunable laser diodes have tuning ranges of 40 to 60 nm, a side-mode suppression ratio greater than 30 dB, and greater than 0-dB peak output power. Using a grating-assisted codirectional-coupler with sampled-grating-reflector (GCSR) laser, the researchers built an optical packet transmitter that can switch between 100 channels (spaced 0.4 nm apart).
A GCSR laser consists of one gain section and three current-tuning sections. The gain section is provided with more than enough current to maintain lasing, while the coupler, reflector, and phase sections are supplied with current that alters their refractive indices in the laser cavity, thus changing the lasing wavelength. This specific laser had a wavelength range from 1523.77 to 1567.77 nm.
To control the four laser sections quickly, the group built a high-speed driver board (see figure). A temperature control circuit stabilized the laser temperature at 25°C, while a current source circuit was used to inject a constant current of 97 mA into the gain region. The researchers calculated the current needed to each of the other sections in order to access each wavelength. These values were stored as lookup tables in three high-speed programmable logic devices (one each for the coupler, reflector, and phase sections). These digital values are converted to analog in fast 10-bit A/D converters that could supply step changes as small as 0.02 mA.
The output optical power, over all channels, was at least -5 dBm. The side-mode suppression ratio was 30 dB.
For more information contact Chun-Kit Chan at email@example.com.
- C.-K. Chan, K. L. Sherman, and M. Zirngibl, IEEE Photon. Tech. Lett. 13, 729 (July 2001).