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Volume 36, Number 5, September-October, 2002
by Mark Malaeb (mark.malaeb@analog.com) The recent lifting of government regulations on the communications industry has fueled an explosion of new ideas and inventions, especially in the optical space. Many players, from startups to Fortune 500 companies, participated in the 1998 to 2000 growth period in an attempt to implement these ideas. Today, although the industry is going through a painful period and many of the startups are gone, the need for implementing these ideas and inventions is still alive and well. Optical fiber is the transport medium that has emerged to accommodate the expected long-term growth. In order to make the most of the wide bandwidth, which is the principal advantage of fiber transport, a method called wave-division multiplexing (WDM)—and later, dense WDM (DWDM)—was implemented. This method of transmission allows the transport of multiple wavelengths (one wavelength—or color—per laser) through a single fiber, but imposes stringent requirements on lasers. One major requirement is that the laser temperature be held constant so that its wavelength will not drift and interfere with other lasers. This usually involves a thermoelectric-cooler (TEC) controller (addressed later in this article). Controlling the laser power level and its modulation mechanism over time and temperature is another system requirement. This job is handled by the laser-diode driver (LDD). For long-haul applications, optical amplifiers are needed for signal reconditioning and retransmitting. Erbium-doped fiber amplifiers (EDFAs) and Raman amplifier types predominate. They recondition the signal without having to convert it from optical to electrical, and then back to optical—which was the widely used method (and only option) in the past. The amplification is now done solely in the optical domain. However, to control their parameters, optical amplifiers, need high-end A/D and D/A converters (ADCs and DACs), logarithmic amplifiers (log amps), and transimpedance amplifiers (TIAs), plus a controller. Optical Communications
Systems
The ADuC832 (Figure 2) includes: an 8-channel, 12-bit, 5-μs self-calibrating ADC; a 2-channel, 12-bit DAC with rail-to-rail outputs; an industry-standard 8052 microcontroller; 62 Kbytes of Flash program-RAM, and a host of peripheral functions. All of these features, plus a temperature monitor, programmable PLL clock, voltage reference, synchronous and asynchronous serial ports—and more—are integrated into a space-saving 56-lead chip-scale package (CSP), allowing the entire control system to fit within the housing of an optical module. We describe here the possible practical implementation of the various portions of an optical communication system, as shown in Figure 1, including a thermoelectric-cooler controller, laser-diode drivers (LDDs), photodetector-diode biasing, received optical signal-strength indicator (RSSI), and temperature sensors. Complete, fully tested software modules are available for each of these applications. These modules are written and commented especially for analog/optical designers who are not software savvy and never want to be. This will help cut design time and overall time to market for both experienced and novice programmers.
TEC Controller
One of the 12-bit DACs, connected to the TEMPSET pin, sets the target
temperature. The voltage corresponds to a specific target temperature
(typically 25 mV/°C for most laser diode applications). The TEC, through
its PID loop, maintains the target temperature to within Laser Diode Driver
The MicroConverter is used for both control and monitoring. Figure 4
shows that the extinction ratio (at the ERSET pin) and the average power
(at the PSET pin) are set using an ADN2850 dual 10-bit digital pot. This
pot is controlled through the serial peripheral interface (SPI)
port. The monitor photodiode currents, IMPDMON and IMPDMON2, flow to
ground through APD Monitoring and
Biasing The gain can be held constant by increasing the APD bias voltage as the temperature increases, as specified by the APD manufacturer. This change is typically expressed in %/°C, and ranges from 0.15%/°C to 0.30%/°C. The ADP3031 switching regulator can provide output voltages of up to 12 volts. Several ADP3031 boost stages can be cascaded to achieve the desired final voltage.
A DAC is used in the range of 0-to-2 V to vary the voltage across the diode with temperature. The actual voltage across the diode can be monitored with the A/D, thus providing complete closed-loop control. With the diode gain maintained at its target, the received optical signal strength can then be accurately monitored with a transimpedance amplifier (TIA) or a log amp plus another channel of the ADC. Calibration coefficients can be conveniently stored in Flash data memory, enabling adjustments to be made as needed. The 16-Bit
Interface
The ADC runs at a maximum update rate of one conversion per 5 μs. At this rate, the microcontroller has 5 μs to read the ADC result and store it in memory for further processing. It has to be done within this time interval or the next sample will be lost—especially time-consuming when using an interrupt routine for the ADC. Thus, in applications where the MicroConverter cannot service a very fast interrupt rate, DMA mode should be used. In DMA mode, ADC results are written directly to external memory. Like any standard 8051-compatible controller, this 16-bit interface can be used to exchange data with systems running a 16-bit processor. The dual port memory helps prevent bus altercation and contention—and provides a somewhat independent interface system. Because "real estate" (circuit-board area) is critical in most optical modules, integration of ADCs, DACs, Flash memory, and an 8052 MCU in a 56-lead CSP package provides the designer with a compact and powerful solution for optical communication systems. |
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