Optical-Electrical-Optical (OEO) wavelength translation (WT) based on small-form-factor pluggable (SFP) transceivers is meeting with increased interest in metro optical-network applications. It provides a flexible and low-cost solution when it interfaces with legacy optical equipment that does not support International Telecommunications Union (ITU) wavelengths. WT improves the performance of optical systems by changing the operating wavelength of the incoming optical signal to a wavelength that enables longer reach through lower attenuation or lower dispersion penalty.
Besides being protocol independent, WT presents an opportunity to deploy an optical source with a narrower linewidth, extending the optical link through better dispersion management. This is especially useful for higher-speed systems, such as OC-48 running on 1300- nm systems, requiring link extension. Data-com signals at 850 nm running on multimode fiber can profit also from WT that allows the system to run on single- mode fiber at 1550 nm, eliminating multimode dispersion.
The primary component that has made WT possible is the SFP transceiver consisting of a transmitter, receiver, and microcontroller. The transmitter module consists of a laser and laser-drive circuitry that may include a thermoelectric cooler (TEC). The laser could be: an uncooled, single-mode, distributed-feedback (DFB) laser with its center wavelength anywhere in the range of 1480 nm to 1580 nm (typically at 1550 nm); a coarse wavelength division multiplexing (CWDM), ITUgrid wavelength, cooled DFB laser at a wavelength whose peak is centered at 100-GHz spacing on the ITU-grid; a 1310-nm multimode Fabry-Perot (FP) laser; or a multimode 850-nm vertical cavity surface emitting laser (VCSEL).
The receiver is a PIN- or APD-based module, depending on the link budget, with a trans-impedance amplifier (TIA) and limiting post amplifier. The attainable link reach after WT is a function of both transmitter and receiver and could be short (up to 10 km), intermediate (up to 60 km), long (up to 100 km), or extended-reach (up to 160 km).
Today, most SFP transceivers comply with a multi-source agreement (MSA) signed by many vendors to guarantee compliance and interchangeability of modules. These transceivers offer hot-swappable modules that utilize single 3.3-V power sources to minimize power consumption.
The SFP transceiver receives a non-return- to-zero (NRZ)-encoded optical signal and converts it into a low-noise CML (current mode logic) or LVPECL (low-voltage positive referenced emitter-coupled logic) compatible electrical signal. The transmitter is compatible with both CML or LVPECL input data levels. SFP transceivers are incorporated with a digital diagnostic feature to report transmitter and receiver status. They can be used for data rates from 50 MHz to 2.7 GHz and support SONET, Gigabit Ethernet, and Fiber Channel in addition to datacom products. A datacom SFP transceiver has to satisfy IEEE standard (Gigabit Ethernet 802.3) or American National Standards Institute’s (ANSI) Fiber Channel, FC-PI specifications. SONET protocol SFP transceivers must satisfy Telcordia qualification requirements.
Figure 1 shows a WT block diagram used to convert an 850-nm TDM signal to a 1550-nm signal for transmission to a remote site. The 850-nm signal could be a 1 Gb/s data signal on 200 MHz-km, multimode fiber that needs to be sent to another network site kilometers away. The conversion to 850 nm can be to a TDM signal at 1550 nm, a CWDM wavelength or a DWDM (dense wavelength-division multiplexing) wavelength on the ITUgrid. The choice of system is a matter of distance between sites, bit rate, and budgetary consideration.
Wavelength translation and regeneration find applications in link extensions to overcome loss and dispersion limitations in network upgrades. For example, a 1310-nm legacy network can be converted to a DWDM network that would improve link capacity and network management without resorting to the expense of electrical multiplexing and the transmission of higher bit-rate signals. Link capacity is increased by converting many 1310-nm signals in different fibers to a corresponding number of 1550-nm DWDM channels, each operating at the original signal bitrate, that could be multiplexed optically into a single fiber and managed using current technologies such as reconfigurable add/drop multiplexers (ROADM).
An example of the type of practical problems that can be addressed with wavelength translation is shown in Figure 2. In this case, the number of bits transmitted between the networks is the same. The lower bit-rate signals are multiplexed optically and then transmitted using optoelectronics designed to handle the original bitrates. Alternatively, these lower-bit-rate signals can be multiplexed electrically to a higher rate so that higher bit-rate optoelectronics must be used. With the signals translated to 1550 nm using a proper optical source (e.g., a narrow-linewidth laser), the stage is set for the use of optical amplification, dispersion compensation, and other signal-massaging techniques required to transmit over longer distances.
This article was written by Ahmad K. Atieh, Ph.D., senior optical engineer, and Paul Vella, Ph.D., vice president of optical engineering, for BTI Photonic Systems. For more information on wavelength translation using SFP transceivers, contact the authors at