Data transmission is increasingly the bottleneck of modern communications with exponential growth projected well into the 21st century. Already, copper lines and coaxial cables are being phased out and replaced by optical fibers due to their high loss and low bit rate. A combination of the state-of-the-art semiconductor laser and optical fiber offers gigabit/s transmission data rate that is sufficient for most cases, however long term problem persisted. By combining the low loss optical fiber with erbium doped fiber amplifier, it is possible to have a few tens of terabit/s data transmission through an optical fiber. However, this enormous bandwidth has yet been taken advantage of. Specifically, Wavelength Division Multiplexing (WDM) has not been implemented.
Implementation of WDM using fixed wavelength laser arrays have temperature-control, system reliability, component aging and manufacturability problems. Because of these shortcomings, wavelength tunable lasers are indispensable elements of such an array since the lasing wavelength can be set, maintained or changed to any wavelength within their tuning range. Due to their long cavities, edge emitting lasers have small mode spacing which limits the continuous tuning range. Tuning beyond this causes the laser to mode hop. On the other hand, vertical cavity lasers have short cavities (wide mode spacings) which enable one to achieve continuous wavelength tuning without mode-hopping. Wavelength tuning range can then be made as wide as the gain spectrum of the laser. Therefore, the challenge is to develop wavelength tunable vertical cavity devices to satisfy the unique requirements of WDM architectures.
Our approach in realizing these devices originated from the idea patented by Dr. Bardia Pezeshki and Prof. James S. Harris in 1992. Instead of using conventional VCSEL top mirrors, we fabricated our top mirror as part of a deformable membrane that is suspended above the semiconductor cavity by an airgap. The airgap thickness can be modulated electrostatically by applying a bias between the membrane and the cavity. The "new" cavity is then made up of the semiconductor cavity plus the airgap. Modulation of the airgap thickness results in an additional phase shift that effectively modulate the resonance frequency of the whole structure resulting in a lasing wavelength shift that is "proportional" to the change in the airgap thickness.
Using this simple approach, we have been able to successfully fabricate, in chronological order, tunable filters with 30 nm tuning, tunable light emitting diode with 39 nm tuning, and tunable VCSELs with 19.1 nm continuous tuning range near 960 nm. Assuming a 2 nm channel separation, this tunable VCSEL represents a tenfold increase in data transmission rate. Wayne Martin has started some preliminary design work on WDM systems using Dragone multiplexer employing these tunable lasers. We believe the combination of tunable laser array with Dragone multiplexer will produce an enabling technology for low-cost, ultra-high bandwidth networks.
A schematic diagram of a tunable vertical cavity surface emitting laser is shown in the figure above. The bottom mirror and cavity are similar to those of conventional VCSELs. The top mirror is a deformable membrane containing quarter lambda GaAs, trilayer SiO2/Si3N4/SiO2 phase-matched to gold, and 150 nm gold. The membrane is attached to four rigid posts by four flexible beams suspended above the semiconductor cavity. Electrostatic force applied between the membrane electrode and the top p-layer of the cavity reduces the air gap spacing, changing the effective Fabry-Perot cavity length which changes the lasing wavelength. The laser is pumped by injecting current through the four top metal p-contact fingers and from the substrate. The lasing wavelength is controlled by applying bias to the membrane. Figure 2 shows the room temperature cw lasing spectrum at different membrane tuning voltages. A continuous wavelength tuning range of 19.1 nm is achieved by applying 17.8 V bias. The threshold current at 0 V membrane bias is 0.34 mA and the differential quantum efficiency is 6.5%. We are currently investigating higher reflectivity membranes that should enable us to lower threshold current even further in addition to improving the differential quantum efficiency and a wavelength tuning range of greater than 25 nm is expected.
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