Modulators provide key to wdm
Designing a wdm network is hard enough without facing a choice of modulator technologies. Here`s a look at the pros and cons of some of the more common options.
ian croston, roger harley
Integrated optical components LTD.
The telecommunications industry`s insatiable demand for bandwidth, combined with the fiber-supply industry being "caught short," has been the impetus to develop and deploy wavelength-division multiplexing (wdm) systems. Original equipment manufacturers are pulling out all stops to bring new wdm systems to the market or expanding production to meet the needs of the information superhighway.
Designing high-speed fiber-optic telecommunications links is difficult enough without this kind of commercial pressure being brought to bear. It is only fair that component suppliers play their part by making life easier for the design engineer. To see how this might be done, let us consider the current state of OC-48 (2.5-Gbit/sec) and OC-192 (10-Gbit/sec) transmitter technology in commercially available wdm systems.
One of the most important decisions facing the wdm transmitter designer is the choice of modulator. There are four technologies to choose from, including
distributed-feedback (dfb) multiquantum-well lasers,
electroabsorption modulators,
III-V Mach-Zehnder modulators,
lithium-niobate Mach-Zehnder modulators.
dfb lasers employ direct modulation and are used in short-haul time-division multiplexed (tdm) systems up to 2.5 Gbits/sec. Formatted in butterfly packages with a built-in Peltier cooler and isolator, these lasers achieve adequate performance for tdm fiber-optic links up to approximately 120 km using singlemode fiber, if low-chirp devices are selected.
Because the modulating signal is applied directly to the laser light source, chirp is inevitable. The transmitter therefore generates a distribution of wavelengths. Each wavelength travels at a different velocity due to the dispersion in the singlemode fiber, and the transmitted signal is distorted as the link length increases.
While undesirable in tdm systems, this distortion can be disastrous in wdm systems, where 32 channels with 0.8-nm channel spacing must be guaranteed for the 20-year lifetime of the system. Reliability is of utmost importance to the telecommunications designer. Because chirp is inherent in direct modulated lasers, it is common practice to fabricate large quantities and select for the low-chirp devices, which command a premium.
In the optical spectrum, dfbs are narrowband devices with wavelengths that can be tuned by temperature up to ۪.5 nm using the integrated Peltier cooler. It is therefore necessary to fabricate a separate design for each of the 32 International Telecommunication Union-specified channels or select for the required wavelength, with yield becoming the dominant factor of the device availability. Direct modulation is understandably not the technology of choice for wdm systems.
Electroabsorption modulators
Electroabsorption modulators, which have been under development for years, are more complex than multiquantum-well dfbs and are a logical step to extending the use of the technology. A typical design incorporates a dfb laser producing continuous-wave (CW) light. The laser is collocated on a single epitaxially grown chip with an electroabsorptive element to which the modulated signal is applied. Spatially separating the laser source from the modulating element reduces but does not eliminate the chirp.
Both the laser source and the electroabsorptive element must be fabricated for each specific wavelength. Also, low-chirp devices must be selected from the wafer, with the associated yield, making high-volume manufacture difficult.
These devices, being relatively new, have yet to prove their reliability through extensive use. However, proponents of the technology claim a 20-year life expectancy for such modulators (see Lightwave, February 1997, page 56).
Chirp can be varied in electroabsorptive devices by the application of DC voltage, which is essential for long-haul fiber-optic links. However, application of this bias voltage is detrimental to both extinction ratio and output power, more than offsetting the 2.5V drive-voltage advantage these devices offer. It has been reported that, by selection of low-chirp devices and suitable control electronics, links up to 600 km have been achieved with the use of erbium-doped fiber amplifiers (edfas). Vendors of this technology must be concerned with selecting both for chirp and wavelength for each of the 40 channels over the range 1530 to 1564 nm with 0.8-nm spacing.
III-V Mach-Zehnder modulators
III-V Mach-Zehnder modulators are the least available technology deployed in wdm systems. Using gallium arsenide or similar materials, these devices employ a Mach-Zehnder interferometer and use the electro-optic properties of the semiconductor. Because external modulation is employed, low-chirp devices can be fabricated to achieve link lengths greater than 600 km, when devices are driven differentially with typically up to 3V in each arm.
However, with insertion losses between 10 and 12 dB, and a large differential between waveguide-mode size and fiber-mode size, coupling to high-power CW dfbs (greater than 40 mW) is essential to ensure suitable output power from the device. Such high powers and a somewhat cumbersome driver circuit have made this the least favored technology.
Lithium-niobate Mach-Zehnder modulators
The most commonly deployed devices in wdm systems are lithium-niobate modulators. A simple two-stage photolithography process is used to define a Mach-Zehnder structure, which is fabricated from an X-cut crystal for optimum temperature stability. First, titanium is deposited on the wafer surface and diffused at greater than 1000°C into the lithium niobate to form waveguides. Later, gold electrodes are deposited on a silicon dioxide buffer to form a coplanar microwave structure for high efficiency. Incorporated in a butterfly package and included as part of a wdm transmitter design, the device is capable of being integrated into singlemode fiber-optic links of up to 1000 km with the use of edfas.
Modulators operating at 2.5 Gbits/sec have a chirp of zero--a goal achieved by design, not by chance of fabrication. Extinction ratios of greater than 16 dB are typically achieved at 2.5 Gbits/sec from drive voltages of 4V, giving the designer a comfortable margin over the OC-48 specification. CW light is provided by a low-cost dfb laser connected to the modulator via a fusion splice or aligned fc/pc connector; the former offers optimum performance. Lithium-niobate modulators operating at 2.5 Gbits/sec have proven reliable, with more than 10,000 devices successfully deployed in the field--some for more than three-and-a-half years. Within the last year, multiple sourcing has been made possible, marking the formation of an industry standard.
The ability to upgrade such capability has been demonstrated by the successful deployment of 10-Gbit/sec wdm systems using lithium niobate as the core transmitter technology. Devices operating at 10 Gbits/sec are now commercially available with a footprint compatible with 2.5-Gbit/sec devices.
Although the lithium-niobate wdm systems offer good performance, reliability, low dispersion penalty, and high extinction ratios, there is still room for improvement (see figure). Currently, it is necessary to purchase dfb CW lasers with polarization-maintaining fiber (pmf) pigtails. While commercially available in high volume and at low cost, the wdm system would be better without the complexity and cost associated with fusion-splicing pmf pigtails between the laser and the modulator. Fiber management is still a concern for the production engineer.
Future developments
The future of wdm component supply can be summarized in one word: "integration." The leading component suppliers are actively engaged in subsystem-level integration of key components: lasers with modulators, modulators with drivers, and integration of transmitters, edfas, and wdms.
The most obvious method of combining laser and external modulator is collocating a CW laser chip on a carrier with a lithium-niobate modulator in the same package. An alternative approach--wdm laser modulators--will see deployment this year (see photo). Due for beta site testing in July, this design combines the field-proven reliability and performance of the lithium-niobate modulator with CW dfb laser while removing the complexity of pmf management. The performance of this device will be identical to that achieved by its discrete-solution predecessor, shown in the figure. The laser and the modulator use existing packaging technology and are connected via a conventional fc/pc interface, minimizing qualification requirements.
This approach offers significant advantages to the wdm system design engineer besides the elimination of fiber management. The device has a similar height and width as an fc/pc connector and is 4.75 inches long. When 32-channel systems are implemented, the small size of this design is likely to become a key advantage. The output connection is made via a conventional fc/pc singlemode connector, so no pigtail needs to be accommodated on the board.
Using this approach, CW lasers may be sourced from a range of vendors and to a range of specifications. Up to 40 itu channels are currently available in volume. Because the modulator is insensitive to wavelength over the entire edfa 1550-nm window, the selection of channel may be made at the final stages of printed circuit board population by simply plugging in the appropriate-wavelength laser.
"Time to market" is the prime concern of the telecommunications companies. Therefore, selecting the best component possible is vital to the success of wdm systems. Since low chirp is achieved by design rather than by selection, lithium-niobate modulators offer both the availability and performance that end-users desire. u
Ian Croston is principal member of technical staff, responsible for 2.5-Gbit/sec modulator designs, and Roger Harley is head of sales and marketing at Integrated Optical Components Ltd., a division of ioc (International plc), Witham, Essex, UK.