The requirements for today's high-speed fiber-optic interconnect include low cost, high performance, and flexibility in packaging designs. While each of these areas presents a formidable challenge to traditional semiconductor lasers, uncooled DFB technology's maturity level now meets all these challenges.
Shing-Shwang Yao, Ph.D., and Daniel Chen
Electronic Device Group
Mitsubishi Electric & Electronics USA, Inc.
One of many significant developments in high-speed fiber-optic interconnect is the transponder, which combines the physical-layer transceiver and optics. Transponders have gained increasing recognition and usage in optical networking in the metro-access, long-haul, and Ethernet frameworks because of their simple interface, space savings, and quick time-to-market for networking equipment suppliers.
Transponders that transmit at 10 Gbits/sec (10G) must satisfy industry standards developed by multiple organizations, such as ITU-T G702/707/783/825, IEEE802.3ae, Telcordia GR253/1377, OIF SFI-4/SPI-4, and ANSI T1.105/105.2/105.6/119/514. This involves subjecting the transponder to many different tests for optical power, jitter, eye masks, power penalty, and sensitivity at different fiber reaches to ensure its interoperability with different manufacturers' transponders.
For several years, transponders have used cooled 10G direct modulated (DM) distributed feedback (DFB) lasers to achieve short-reach fiber-optic communication. However, the thermal-electric cooler used with these devices consumes more power and space inside the laser module, so transponder designers have looked for an alternative method. One alternative is the uncooled 10G DM-DFB laser because it eliminates both the space and extra power requirements.
Uncooled 10G DM-DFB lasers have not enjoyed popularity because of their poor jitter and eye-mask performance, which is caused by increasing rise and fall times at high operating temperatures. However, innovative design improvements have enabled 1.3-mm wavelength, uncooled 10G DFB optical transmitters to emerge as a viable solution.
Use in SONET applications
The 10G transponder comprises a physical-layer transceiver and optics. In SONET applications, the physical-layer transceiver portion consists of an electrical multiplexer (mux), demultiplexer (demux), and timing generator, while the optics portion consists of an optical transmitter and receiver.
The transponder, as used in SONET, is designed to communicate with a system framer through an electrical interface and another transponder through fiber. On the framer side, 16 parallel electrical data/clocks and a reference clock are provided to the transponder's mux. In return, a transponder feeds back to the external framer the recovered parallel electrical data/clock from the demux. On the fiber side, an optical transceiver generates and receives 10G serial data to and from another transponder's transceiver.
Many options inside a transponder are available, such as jitter filter, line timing (recovered clock from optical receiver), I²C bus, monitors, alarms, and mirror clocks on the transmitter/receiver side.
The optics portion in a transponder includes an optical transceiver. The transmitter portion of the transceiver combines the laser module, laser driver, automatic power control (APC), and bias/modulation control. The optical receiver portion has a photodiode (PD) module, preamplifier (or transimpedance amplifier, TIA), post-amplifier (or limiting amplifier, LA), and clock and data recovery (CDR) circuitry. A well-designed optical transmitter relies on the laser driver and pays special attention to the output current, rise/fall times, crossing point, impedance matching between driver and laser, and a stable voltage supply to the driver.
In addition to the driver, a qualified laser should run hot without compromising output power, data rate, extinction ratio, and spectral width. A larger gain bandwidth (S21 parameter) and resonance frequency help to remove some background noise. Designers can improve eye-pattern observations such as jitter, overshoots, undershoots, double traces, and mask hits by fine tuning the driver's output current, S11 and S22 parameters (input and output reflection coefficients) of the laser module, driver output resistance, laser serial resistance and parasite capacitance.
Figure 1 shows a good wide-band eye-pattern waveform from a 1.3-µm wavelength, uncooled 10G DM-DFB transmitter at an elevated temperature with no SONET mask violation observed.
An optical transmitter cannot accomplish signal communication without a highly graded receiver. A well-designed receiver is capable of detecting a signal at a high data rate with good sensitivity/overload with a small power penalty over the fiber reach. Figure 2 shows data that illustrates measured sensitivity with bit-error rate (BER) and received optical power. With the performance illustrated, transponders can achieve short-fiber-reach communication for SONET applications using an uncooled 10G optical transmitter.
Uncooled versus cooled
Cooled and uncooled DFB lasers each have advantages and disadvantages: cooled laser modules offer better rise/fall times at an elevated temperature, while uncooled laser modules consume less power and have a smaller package size.
As shown in Figure 2, 1.3-µm wavelength DFB laser diodes can operate at high temperatures and function without failure in a heated transponder. Further improvement at even warmer temperatures would require reducing the rise/fall times, which designers can achieve by decreasing the laser diode's intrinsic RC constant (related to resonance frequency) or by decreasing the driver's rising/relaxation times.
An uncooled DFB laser also enables single-wavelength systems to be extended to coarse WDM (CWDM) systems. CWDM applications can afford to use uncooled lasers because CWDM offers a liberal spectral width of 20 nm. This makes CWDM systems easier to design and therefore achieve a faster time-to-market. Using uncooled lasers also means a cooler is unnecessary, which saves space and cost.
In addition to SONET applications, uncooled DFB lasers have also been successfully implemented into Ethernet transceivers. The most common transceiver package for 10-Gbit/sec Ethernet (10GbE) applications is called Xenpak (although other, smaller form factors have been announced).
Xenpak is designed to communicate with media access control (MAC) through 10-Gbit/sec Attachment Unit Interface (XAUI) and with another Xenpak through fiber. On the MAC side, four parallel electrical data inputs are provided to Xenpak. In return, Xenpak feeds recovered parallel electrical data back to an external MAC. Xenpak also generates reference-clock data internally.
The physical-layer transceiver for Ethernet applications is integrated together with 10G serializer/deserializer (SERDES), 64B66B coder/decoder (codec), 8B10B codec, 3.125-Gbit/sec SERDES, CDR, and phase lock loop (PLL) functions. The optics portion is an optical transmitter (uncooled DM-DFB laser module, driver, and APC) and optical receiver (PD module, TIA, LA, and CDR). The requirements on Ethernet optics such as eye mask and extinction ratio are more relaxed and easier to achieve than those on SONET devices.
In conclusion, recent technological advancements have made uncooled laser diodes a viable, cost-effective, and space-saving alternative to cooled lasers for short- to intermediate-reach 10G SONET, CWDM, and Ethernet applications.
Dr. Shing-Shwang Yao is senior optoelectronic engineering manager and Daniel Chen is assistant vice president, Communication Products Division, in the Electronic Device Group of Mitsubishi Electric & Electronics USA, Inc. (Sunnyvale, CA).