Modelocking a tunable dual-wavelength fiber laser

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By Yvonne Carts-Powell

Researchers from McGill University (Montreal, Canada) and Aston University (Birmingham, England) recently demonstrated an actively modelocked, dual-wavelength fiber laser with very narrow wavelength spacing of 0.3 nm and stable, room-temperature operation.1 Dominik Pudo and Lawrence R. Chen at McGill University, as well as Domenico Giannone, Lin Zhang and Ian Bennion at Aston University showed that they could independently tune the power and wavelength of each output from their laser without affecting overall output stability. The operation occurs at a modulator drive frequency of 1035.38 MHz (corresponding pulsewidths of 115 and 130 ps).

Such compact lasers could be used for high-capacity systems that combine WDM with time-division multiplexing (TDM) access techniques. Other applications of the device could include use in characterizing photonic components.

Creating an erbium-doped fiber laser that emits at multiple closely spaced wavelengths is difficult because of homogeneous broadening in the fiber at room temperature. "Our solution for this particular problem was to use a separate gain medium for each wavelength," says Pudo (see figure). Each gain section uses a fiber Bragg grating (FBG) for wavelength selection. The FBGs are each 30 mm long with a 3-dB bandwidth of about 0.2 nm, and both have their reflection peak at roughly 1543 nm. One FBG is mounted on a translation stage so that the reflection wavelength can be tuned by applying strain. The pump powers are 35 and 60 mW.

This configuration creates two different cavity lengths for each wavelength, each with a different fundamental frequency. Pudo explains, "In order to be able to modelock both wavelengths simultaneously, we used a common modulator for both wavelengths, which had to operate at a multiple of both fundamental frequencies." A 10-Gbit/s electrooptic modulator driven by an RF synthesizer acted as the modelocking element. When the laser emits at 1543 and 1543.8 nm and the modulator is driven at 1035.38 MHz, the laser is modelocked at the 246th and 142nd harmonics for 1543 and 1543.8 nm, respectively. The time-bandwidth products for these wavelengths are 0.88 and 0.92, respectively, indicating that the pulses are chirped, mostly due to cavity dispersion.

By applying strain to the second FBG, the wavelength can be tuned while the laser is still modelocked. Stable modelocking was obtained over the entire wavelength range without modifying the RF frequency, and to wavelength spacings as narrow as 0.3-nm.

The researchers suggested several methods of optimizing the system to reduce loss and noise. The researchers are considering work on other tunable multi-wavelength configurations that may require fewer components.

For more information contact: Dominik Pudo at Dominik.Pudo@mail.mcgill.ca or Lawrence Chen at chen@photonics.ece.mcgill.ca.

REFERENCE
1. D. Pudo et al., IEEE Photon. Tech. Lett. 14(2), 143 (February 2002).

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