Tunable Distributed Bragg Reflector Lasers Introduced at OFC
Two vendors introduced similar, widely tunable, monolithic lasers at the Optical Fiber Conference (OFC) held in Los Angeles from Feb. 23 to 27, and the companies' scientists presented papers describing the lasers at the conference's technical sessions. The two semiconductor lasers are similar in that both have a gain section, two distributed Bragg reflector grating sections and a phase section (Figure 1). A current through the gain section creates the population inversion necessary for laser operation, and the laser's wavelength is tuned by adjusting the three currents running separately through the two grating sections and the phase section.
Figure 1. This simplified, conceptual drawing shows the gain section and the three frequency-tuning sections of the new lasers introduced at OFC. The current through each of the four sections is controlled independently.
If the laser is to reach threshold, the reflectivities of both gratings must at least partially overlap each other and the frequency of a longitudinal resonator mode, as illustrated by the vertical red line in Figure 2. Because the refractive index of the grating section depends on the current passing through it, the gratings' reflectivities can be adjusted by changing their currents. Similarly, the optical length of the resonator, and hence the position of the longitudinal modes, can be adjusted by altering the current through the phase section. Once all three peaks have been aligned, the laser can be tuned to a specific frequency by adjusting the three currents appropriately. (Figures 1 and 2 should be considered for conceptual purposes only; the lasers introduced at OFC are significantly more sophisticated.)
Figure 2. The position of the longitudinal modes depends on the current through the phase section in Figure 1, and the positions of the gratings' reflectivities depend on the current through the respective grating section. The laser will lase only if all three peaks have sufficient overlap to provide net round-trip gain. (The perfect overlap indicated by the red line here may not be necessary for net round-trip gain.) The modes and reflectivities are plotted as a function of frequency.
Tuning with gratings
One of the new lasers, introduced by Bookham Technology plc of Milton, UK, incorporates a rear grating comb reflector and a linearly chirped front grating with multiple electrical contacts. The reflectivities of the two gratings are shown in Figure 3. The front grating (black line) is relatively flat when no current passes through it, but a current passing through appropriate electrodes causes the perturbation at ~1.53 µm. The double-headed arrow represents the frequency range over which the front grating provides sufficient reflectivity for the laser to reach threshold. However, the rear grating (red line) provides feedback at only one frequency within this range, so the device lases at the frequency indicated by the yellow triangle.
Figure 3. When a current is applied to the appropriate electrodes of the front grating, its reflectivity increases over a narrow range, indicated by the double-headed arrow.
To tune the laser, the current to the front grating is held at a fixed value, while the current to the rear grating is adjusted so that a single reflectivity peak sweeps across the region defined by the blue arrow. For fine frequency adjustment and for adjustment for any lasing drift over laser lifetime, the current into the phase section is altered. To tune the laser beyond the region defined by the double-headed arrow, different currents are applied to the front grating so that lasing occurs on another peak of the rear grating. In this fashion, the laser can be continuously tuned across the C-band.
The Bookham laser includes a semiconductor optical amplifier that is monolithically integrated into the laser chip. This amplifier boosts the device's output to a level competitive with conventional distributed feedback lasers, and its current can be adjusted to maintain a constant output level as the laser is tuned across the C-band.
In experiments, these chips were assembled into 26-pin butterfly packages and calibrated for 80 channels spaced by 50 GHz across the C-band. As the laser was tuned across the C-band, it maintained a minimum output of 40 mW and a side-mode-suppression ratio of better than 46 dB.
Agility Communications Inc. of Santa Barbara, Calif., also introduced a widely tunable distributed Bragg reflector laser transmitter, but its engineers took a different approach to tuning. The reflectors at either end of the resonator are sampled gratings whose reflectivities consist of a series of spikes (Figure 4). The spacing between the spikes of the two gratings is slightly different, so by adjusting the current through the gratings, one set of spikes is used like a vernier against the other, and lasing occurs only at the frequency that provides feedback from both front and rear gratings. By adjusting the two grating currents appropriately, the laser can be tuned across the C-band and into the L-band.
Figure 4. The reflectivities from the two gratings are adjusted like a vernier, so they match at only one frequency.
As with the Bookham laser, Agility's device includes a monolithically integrated semiconductor optical amplifier that boosts the CW output into the 20-mW range, and it may be adjusted to maintain a constant output as the laser is tuned. But unlike the Bookham laser, Agility's chip includes a monolithically integrated Mach-Zehnder modulator.
Electroabsorption modulators are more commonly integrated into monolithic transmitter chips, but Mach-Zehnder modulators have lower insertion loss while providing better, frequency-independent control of extinction ratio and chirp. The Agility modulator consists of two multimode interference sections with curved waveguides and 400-µm-long lumped electrodes. With a drive of less than 3 V, the modulator is capable of a DC extinction ratio of better than 20 dB, and at 10 Gb/s, it maintains a radio-frequency extinction ratio of 12 dB across the C-band.
For testing, the laser package was assembled into a cooled butterfly module and connected to 100 km of standard single-mode fiber. The Agility engineers observed error-free transmission with less than 2 dB of dispersion penalty across the C-band. They believe that this is the first demonstration of such error-free transmission from a widely tunable, monolithically integrated laser/modulator transmitter.
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