- MEMS Enable Tunable Lasers for Telecom
Dynamically reconfiguring next-generation fiber optic telecom systems will require wavelength-tunable lasers that can provide a signal into any wavelength division multiplexed channel and that can switch among channels on the fly. Researchers around the world are investigating a rich variety of schemes to make stable, efficient and inexpensive tunable lasers, including designs that feature cantilevered microelectromechanical systems (MEMS) elements. Applying electrostatic force to the cantilever alters the thickness of the air gap between the cantilevered distributed Bragg reflector and the body of the laser, thus changing the resonator's frequency and tuning the output wavelength (Figure 1).
Figure 1. By varying the electrostatic force between the cantilevered structure and the body of the laser, the resonator's frequency and, hence, the output wavelength, can be tuned.
Whereas these lasers have performed well in the 1-µm spectral region, difficult thermal problems have prevented the efficient implementation of the technology at the 1.5-µm region of interest to telecom. Recently, a group of researchers at Bandwidth9 in Fremont, Calif., demonstrated several techniques that significantly alleviate these thermal issues. They lowered the electrical resistance of the semiconductor device by incorporating a tunnel junction into the laser's structure and increased the thermal conductivity of the laser by changing its physical composition.
Figure 2. The middle distributed Bragg reflector includes a tunnel junction, which reduces resistive heating in the laser.
The bottom, middle and top distributed Bragg reflectors act together with the air gap to set the boundary conditions that determine the laser's output wavelength (Figure 2). Electrons flow from the substrate to the contacts atop the middle distributed Bragg reflector, funneled into a narrow stream by an aperture in the insulating oxide layer. The tunnel junction, which allows for low-resistance current injection, lies just above the oxide layer. Although tunnel junctions have previously been fabricated into semiconductor lasers, this is the first time one has been incorporated into a tunable vertical-cavity surface-emitting laser (VCSEL).
The researchers successfully tuned the laser across 10 nm in the C-band, from 1542 to 1552 nm, while the output power varied from 0.28 to 0.80 mW in a single transverse mode. The variation resulted from changes in the effective output coupling as the air gap changed, and it could have been minimized by optimizing the output coupling.
They directly modulated the newly designed VCSEL at 2.5 GHz and transmitted its output over 100 km of fiber, demonstrating the first successful transmission over a telecom link using this type of laser. From measurements of the relative intensity noise and dispersion, the engineers concluded that the laser has a maximum speed greater than 5 GHz over 10 standard 100-GHz channels.
The bottom distributed Bragg reflector in the device is based on alternating layers of high-index InAlGaAs and low-index InAlAs, but the engineers have shown that a distributed Bragg reflector in which InP replaces InAlAs has more than three times the thermal conductivity. With this motivation, they designed and constructed the first electrically pumped VCSEL with an InP/InAlGaAs bottom distributed Bragg reflector (Figure 3). They included a tunnel junction in the laser's middle distributed Bragg reflector as well.
Figure 3. The InP/InAlGaAs lower distributed Bragg reflector in this laser has much better heat conductivity than a standard InAlAs/InAlGaAs distributed Bragg reflector.
When oscillating in a single transverse mode, this device was tuned across 14 nm in the C-band, from 1552 to 1566 nm, while its output power varied from 0.37 to almost 0.90 mW -- an increase in both spectral range and power over the laser in Figure 1. Again, optimizing the output coupling could have minimized the variation. In multimode operation, the laser was tuned over a slightly wider spectral range of 17 nm, and an output power as high as 1.5 mW was obtained.
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