Integrated Tunable Transmitters for Wdm Networks

The integration of widely-tunable sampled-grating lasers with semiconductor optical amplifiers and either electroabsorption or Mach-Zehender modulators has been successful in creating high-performance optical transmitters. Details of this performance as well as recent advances in this technology will be reviewed in this paper. Introduction The desire to reduce operational costs with a universal WDM source that can access any wavelength channel across the C-or L-bands of a DWDM optical fiber system has provided a growing market for widely tunable lasers. The further desire to have analogous universal WDM transmitters, which integrate such sources with modulators, to enable low cost, low power dissipation, and small form-factor has also provided a market impetus to develop a viable photonic integration platform to provide such products. Most recently, many customers are expressing a preference to purchase at the transponder level to avoid contact with the optical components entirely. Here especially, a universal, programmable, full-band product is very desirable, and here especially, having small-form-factor, low-power, full-band optical components is very important to enable these characteristics in the universal DWDM transponder. In addition to the inventory issues, such a product will also enable the long anticipated ability to remotely dynamically provision wavelengths in the network. Past attempts to develop viable integration technologies have largely failed to produce the required performance metrics to compete with the conventional discrete approaches. For past DWDM systems, dozens of different DFB laser codes had to be available, and more importantly, each transmit/receive line card was specific to a particular wavelength, and dozens of these had to be inventoried for rapid replacement of failed parts. This inventory issue appears to have been one of the key issues that led to the virtual collapse of the industry over the past two years. Photonic Integration platform Work at UCSB and Agility Communications has aimed to address these issues by developing a lowcost ‘platform technology’ that is capable of providing a wide variety of PICs without changing the basic manufacturing process. This limits the required capital investment and enables higher volume by sharing the technology across a number of components. Figure 1 shows a photograph of a 2” InP wafer with arrays of seven-section photonic IC transmitters, each consisting of a full-band-tunable sampled-grating DBR (SGDBR) laser integrated with a monitoring detector, optical amplifier, and modulator. The SEM inset shows one of these mounted on a carrier ready to be inserted into a package. It is important to note that the wafer layer structure and processing procedure used is identical to that developed for the SGDBR laser alone. This same structure and processing procedure is also used in the more complex laser PICs to be discussed below. Note also a key advantage of photonic integration--only one optical coupling to fiber is required, as would be necessary for a simple DFB laser alone. Figure 1. Photo of wafer and SEM of mounted singlechip transmitter. Single-chip transmitter Figure 2 shows a schematic of the InP-based transmitter chip[1]. A common quaternary waveguide extends throughout the entire device and quantum well gain layers are included at the laser gain and SOA sections. The modulator bias is varied across the 40 nm tuning range to enable efficient modulation across this entire range[2]. Improvements in chirp Light Out Front Mirror Gain Phase Rear Mirror SG-DBR Laser Amplifier EA Modulator MQW active regions Q waveguide The device illustrated in Fig. 2 and characterized in Fig. 3 provides good results at 2.5 Gb/s for distances up to 350 km. However, for longer distances and/or higher bit rates, some sort of chirp control is necessary. Two approaches have been explored. The first is a ‘tandem EA’ modulator as shown in Fig. 5. Figure 2. Single-chip widely-tunable transmitter schematic showing a SGDBR laser integrated with an SOA and EAM. Light Out Front Mirror Gain Phase Rear Mirror Amplifier MQW active regions Q waveguide SG-DBR Laser Phase Modulator Amplitude Modulator DATA DATA Figure 3 shows the bit-error rate after transmission through 350 km of standard single-mode fiber for two different wavelengths. The data is applied directly to the EAM of the chip. The average modulated output power is about 3dBm in this case. Error-free operation was observed. Figure 5. SGDBR integrated with a two-section tandem EAM including inverted data applied to a phase modulator to compensate chirp. Amplitude modulator reverse biased for absorption; no bias applied to phase mod. -3