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  • Double-Ring Laser Increases Tunability

Photonics Spectra
Aug 2002
Gary Boas

Wavelength-tunable semiconductor lasers incorporating sampled-grating distributed Bragg reflector or superstructure-grating distributed Bragg reflector designs are boosting the capacity and flexibility of optical networks. Researchers in California now have proposed a novel tunable laser that promises even better characteristics for telecommunications applications.

A new design for a microring semiconductor laser promises improved tunability and overall performance for telecommunications applications. The passively coupled, double-ring structure couples only radiation at the resonance wavelength into the gain region. Electro-optically changing the relative radii or refractive indices of the rings would control the emission wavelength. Courtesy of Bin Liu.

The semiconductor laser would feature four main elements: a gain region for light amplification, two passive microring resonators for mode selection and wavelength tuning, passive waveguides to connect the resonators with the gain region, and absorption regions for extinguishing radiation outside the resonator's transmission bandwidth.

But the primary innovations are the use of passive resonators instead of the active ones found in conventional ring lasers, and the addition of the second ring.

The concept of the ring laser has been around for more than 20 years, said Bin Liu of Calient Networks in Goleta, Calif., who proposed the device with Ali Shakouri of the University of California in Santa Cruz and with John E. Bowers of the University of California in Santa Barbara. The conventional, active-ring laser, however, is limited in linewidth and wavelength tunability. Passive rings should reduce the linewidth by three orders of magnitude and the frequency chirp by one order of magnitude.

The improved tuning range is a result of the slightly different radii or effective indexes of the rings, which lead to different peak spacings in the transmission peak combs associated with the rings. The researchers propose that they could fine-tune the wavelength by aligning the peaks in the two sets of combs with the adjustment of index in either or both of the ring resonators.

"We slightly adjust the index in one ring to match one of the reflective peaks," Liu explained. "Then we can enhance the wavelength tuning via the Vernier effect. The large tuning effect means we only need a very small index change to cover the ultrawide wavelength range. This makes high-speed wavelength tuning possible by using the fast electro-optic effect."

A limitation of the double-ring resonator-coupled laser is the ~10-dB/cm optical loss in the microrings. The researchers point out, however, that the loss is acceptable and that the design nevertheless should improve wavelength tuning by approximately 50, with reduced linewidth and frequency chirp. With no loss in the rings, the tuning enhancement would be more than 1000.

Although only a theoretical analysis of the laser has been published, work is under way to demonstrate a prototype device. And Liu is optimistic about seeing the devices deployed. "We need to do some more work, but there is no big challenge, technically or theoretically. A couple of years is a reasonable time to commercialize the device."

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