Tunable VCSELs: Enabling Wavelength-on-demand In Metro Networks (Cover Story - VCSELs)
Tunable VCSELs offer a reliable, cost-effective way to dynamically provision bandwidth in metro networking systems, says Bandwidth9's Connie Chang-Hasnain.
An unquenchable thirst exists for bandwidth. Nowhere is this more evident than in the explosive growth of Internet traffic. This demand is driving the rapid development of DWDM networks: these systems now regularly handle more closely packed channels operating at speeds of up to 10 Gbit/s, with maximum channel counts reaching 200 per fiber. Currently the market for optical components for DWDM systems stands at around $89 billion, and is expected to approach $25 billion by 2004. The metro environment While a great deal of innovation and development has been focused on the deployment of long-haul optical systems, there also exists a growing demand for DWDM systems optimized for metro environments. Traffic patterns in metropolitan areas can be distinguished from long-haul data flow patterns largely by the uneven flow of data. The patterns are also differentiated by a need to accommodate multiple protocols and different quality-of-service requirements. These pre-requisites lead to a need for different hardware solutions. In short, metro DWDM systems should be smart and flexible. Metro DWDM systems that are based on fixed wavelength add/drop nodes work well when a limited number of wavelengths and system reconfigurations are required. However, as the pattern of traffic and customer requirements change, costly and time-consuming system reconfigurations are needed. In addition, to limit downtime caused by system failures, carriers use spare components to establish infrastructure redundancy. In the case of laser sources, this requires a redundant line card with fixed wavelength lasers and filters for each working wavelength, which represents a large capital investment. Additional costs associated with this redundancy include those associated with increased use of space and higher power consumption. The tunable solution The capital expenditure for fixed sources can be greatly reduced by using broadly tunable sources, which allow a single replacement or spare line card to be substituted for a wide range of wavelengths. Further cost reductions are possible if system reconfigurations can be implemented remotely and quickly. Another cost-related issue facing providers of metro infrastructure relates to the fact that many customers employ systems operating at slightly different wavelengths. This requires locking equipment that allows the various systems to match wavelengths and "talk" to one another. The availability of tunable wavelength sources would eliminate such mismatch issues. Remote access For DWDM systems, while inventory reduction and lower system costs represent very considerable gains, an equally important benefit is afforded by tunable sources that offer remote access. The tunable laser can be reconfigured remotely, providing wavelengths on demand, and is able to dynamically provision, activate, deactivate and redirect data flow. In essence, this can be viewed as a physically connected fiber ring, over which there exists a connective mesh network that can be changed as needed. Even greater flexibility is possible if, in addition to tunable sources, tunable filters are included in the network. In this instance there is no need to pre-allot certain channels to particular fibers: any fiber can carry any wavelength. This offers a major advantage. Typically it takes several weeks to remove or add a subscriber to the network, or to modify a subscriber s fiber requirements when the system is based on fixed wavelengths and filters. This is because components must be physically replaced. In the case of tunable sources and filters, these changes can be done remotely and in less than one millisecond. Types of tunable laser There are three basic ways to tune lasers: these involve the use of temperature, current or a mechanical method to change the wavelength. Temperature tuning is typically used in distributed feedback (DFB) lasers. These lasers are edge-emitting devices that are employed as fixed wavelength transmission sources that can also be tuned to different wavelengths by altering the temperature. With this approach, the disadvantage is that a tuning range of only about 5 nm is possible, and a settling time of several seconds is needed once the temperature has altered the wavelength. Current tuning can be implemented in a multi-section distributed Bragg reflector (DBR) laser. In this kind of laser, an integrated Bragg grating reflector performs the wavelength selection. Under different levels of current injection, the refractive index of the DBR is changed, which then modifies the output wavelength. For this kind of laser, injecting multiple levels of current into different sections of the device allows a wide tuning range. One of the limitations of this approach relates to the control of tuning. This is complex and often does not produce a continuous change in wavelength when one of the injection currents is altered. It is therefore difficult to use a simple universal wavelength locker with such a device. Mechanical tuning is the most recent approach. This method employs a VCSEL in combination with a micro-electromechanical system (MEMS) tuning device. Unlike edge-emitting lasers, the VCSEL reflects light between two mir-rors in a vertical cavity, emitting light vertically from the surface of the laser. With a MEMS structure it is possible to take advantage of this vertical cavity geometry, and the laser s upper mirror can be repositioned and adjusted using a cantilever (see ). The VCSEL possesses another major advantage compared to both DBR and DFB lasers; at around 3 m, its optical cavity is two to three orders of magnitude smaller. As a result, the VCSEL is intrinsically a single wavelength device. Therefore, adjustments to the cavity spacing cause continuous changes to the wavelength. VCSEL fabrication The key to implementing a mechanically tunable laser for DWDM applications is the ability to first fabricate a high-quality VCSEL operating in the 1.51.6 m wavelength range. This band targets telecommunication frequencies and includes the important 15301610 nm range, which caters to erbium-doped fiber amplifiers with high gain and wide optical spectra. However, due to the small variation in the refractive index afforded by lat-tice-matched InAlGaAs/InAlAs superlattices, the quality of the DBRs as reflectors is poor. Other techniques have been employed to alleviate this problem, including wafer bonding of materials with a higher refractive index. But while high-quality semiconductor and oxide-based DBRs can be bonded above the active layer of the VCSEL structure to improve the laser s operating characteristics, this is a challenging and costly fabrication process. In Bandwidth9 s devices, the active region is typically comprised of strained InGaAs QWs, and the bottom DBR reflectors fabricated from lattice-matched InAlGaAs/InAlAs superlattices. The whole structure is based on a single epitaxial growth on an InP substrate. Metamorphic mirrors Bandwidth9 has improved the performance of these devices with a new proprietary design that includes a metamorphic (or non-lattice matched) top DBR mirror comprised of an AlGaAs/ GaAs superlattice. A high-quality DBR can be fabricated with a relatively small number of AlGaAs/GaAs superlattice periods by exploiting the large change in refractive index of AlGaAs as the aluminum content is increased. From a fabrication standpoint, the entire VCSEL structure - including the substrate contact layer, the upper tuning contact, the active layer, and the top and bottom DBRs - can be produced in a single MBE growth sequence. The use of the high-quality metamorphic DBR above the active region greatly improves the laser performance: an output power of 0.45 mW with a threshold voltage of 1.7 V can be obtained for a 9 m aperture device. In addition, the current and optical confinement can be enhanced by sidewall oxidation. In terms of the other key parameters of the device, a single transverse mode of operation is achieved, with a side-mode suppression ratio of 45 dB. The polarization mode suppression is 45 dB, and the operating parameters are maintained for temperatures up to 55C. Cantilever tuning Bandwidth9 s tunable VCSEL is designed with the DBR divided into three sections, with an air gap of about 1 m located between the upper and lower region of the top DBR. The tuning contact is a very basic MEMS structure manufactured from a simple GaAs/AlGaAs cantilever beam (). Applying an electrostatic force - which is obtained by applying a voltage to the tuning contact - can deflect this beam. This deflection changes the VCSEL cavity length, which in turn changes the wavelength of the device, allowing continuous tuning as a function of the applied voltage. The tuning spectra of a C-band VCSEL controlled by a single tuning voltage is shown in . The tunable VCSEL can be directly modulated with high-speed data streams by varying the current through the QW region via the laser drive contact, in common with all directly modulated diode lasers. The direct modulation performed at two different lasing wavelengths in the L-band is shown in , where fully open eye diagrams attest to data rates of 2.5 Gbit/s. As important as the ability to tune is the speed of tuning. The shorter the length of the cantilever, the greater its resonance frequency, and therefore the faster the cantilever can respond to a change in voltage. Research into differing cantilever lengths has shown that the frequency response of even the longest cantilevers (100 m), which also provide the largest tuning range, feature a resonant frequency of around 100 kHz. This allows a VCSEL to tune to a new wavelength in around 10 s. The shortest cantilever (25 m) is able to tune to a new wavelength in just 1 s. Another inherent advantage springs from the circular nature of beam, which is controlled by the oxide aperture. This property considerably eases coupling of the VCSEL to a fiber, and Bandwidth9 s device features a coupling efficiency of over 90% (). Staying in tune It is also important that once the desired wavelength has been selected, it can be quickly locked into position. Bandwidth9 has developed a locker for the tunable VCSEL that is able to lock to within a 2.5 GHz bandwidth in less than 200 s. This is achieved using a special filtering process that rapidly damps transient ringing (). This technology has been integrated into Bandwidth9 s MetroFlex tunable optical transmitter, which offers continuous tuning and locking in less than 100 s (). Measuring 3 5 0.5 inches, MetroFlex is SONET and ITU-T compliant up to OC-48 and STM-16, and features the full C- and L-band tuning range. Future applications In the near future, other applications could benefit from tunable VCSELs. An example is optical cross-connects (OXCs). Whether electrical or all-optical switch fabrics, OXCs are emerging as a way of handling high-density optical terminations typically found at the interconnection points between long-haul and metro DWDM systems. OXCs are used to regulate and re-route traffic across optical pathways. They also provide protection and restoration of the signal. These functions can be distributed throughout the metro DWDM transport network if tunable sources are used to control the selection of wavelength interchanging. Wavelength tuning capabilities will also increase the degree of connectivity and number of logical connections, and will support a variety of protection and restoration methods on physical ring deployments. A combination of these currently distinct network elements could ease the migration to efficient implementation of metro WDM systems.