System-on-chip technology adds options for laser driver control

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Don Alfano

In-system programmable, mixed-signal, system-on-chip technology is an alternative that enables space-efficient, smart regulators that implement control algorithms and readily accept change.

The venerable laser diode is the heart of broadband fiberoptic transmitter systems. While fiberoptic systems rely on laser stability, unregulated laser-diode characteristics can wander because of manufacturing tolerances, temperature variations, and/or parametric changes with age. Therefore, fiberoptic communication systems require dedicated circuitry to regulate key laser-diode parameters and maintain stable operation.

While laser control circuits are similar in their use of classic, closed-loop techniques, control-circuit implementations are as unique as snowflakes. Circuit designers frequently gravitate toward a discrete circuit strategy using multiple analog integrated circuits and passive components. While this strategy can result in a functional solution, it consumes excessive board space, increases system test complexity, and makes circuit revisions costly and time-consuming.

The latest trend in laser control-circuit implementation capitalizes on recent advances in mixed-signal system-on-chip (SoC) technology. These devices combine high-throughput processors, precision analog converters, and in-system programmable FLASH memory, enabling laser regulators that are small, highly versatile, economical, and "smart."

DISCRETE LASER REGULATORS
A 3641-type laser transmitter module, for example, is stabilized by feedback control loops that regulate laser temperature and optical output power. Each functional block, within both the temperature and the optical-power regulators, consists of multiple analog integrated circuits and passive components (see Fig. 1).

The optical-power regulator varies laser bias current to maintain constant optical output power. To achieve this, a photosensitive monitor diode within the laser module generates a current proportional to the optical power coupled into the fiber. This current is converted to a voltage and applied to the control amplifier.

The control amplifier is an integrating controller; its output voltage function is the time integral of the difference of the two input voltages. That is, the output voltage changes at a rate proportional to the difference of the input voltages, compared to a maximum rate set by the passive-component values in the circuit (see Fig. 2).

The integrating attribute of the control amplifier provides high gain to achieve tight loop regulation, and damping to prevent the loop from oscillating. The control amplifier output connects to a laser bias-current generator so that bias current decreases as measured optical output power increases. Control loop operation is straightforward: the control amplifier subtracts the monitor-diode power-output signal from the calibrated set-point (constant) voltage, and gently adjusts the bias-current generator until measured power equals the set-point value.

A decrease in output power causes a decrease in the monitor-diode output signal, lowering voltage on the control amplifier's negative input. This action causes the control-amplifier output voltage (and therefore the bias current and optical output power) to increase until the voltages on both inputs of the control amplifier are again equal.

The remaining circuitry in the optical-power regulator consists of an over-current protection safeguard and a laser bias monitor circuit for external circuitry, such as an analog-to-digital converter and processor.

The temperature regulator is another closed-loop controller similar in operation to the optical-power regulator circuit except temperature is the controlled variable. The temperature regulator circuit varies the current through the thermoelectric cooler (TEC) to keep laser temperature constant. The TEC can heat or cool the laser depending on the direction of the current passed through it—positive current lowers laser temperature; negative current increases laser temperature.

A thermally variable resistor (thermistor) within the laser package senses laser temperature, generating a voltage proportional to temperature when biased. The control amplifier compares this temperature signal to a set-point, and modulates the TEC interface circuit that ultimately adjusts the magnitude and direction of the current through the TEC. The interface circuit typically has a built-in current limiter to ensure TEC control current remains within safe limits. The control loop reaches a stable control point when measured temperature equals that of the set-point temperature.

SYSTEM-ON-CHIP REGULATION
A laser regulator can be implemented with an in-system programmable, mixed-signal SoC (C8051F006) and external interface circuitry (see Fig. 3). The SoC is a single-chip containing a high-speed 8051 CPU, 12-bit analog converters, and 32 Kbyte of in-system programmable FLASH memory.

Unlike the hardware implementation using discrete analog components discussed initially, the SoC-based design digitizes analog input variables, operates on this data with user-designed software control algorithms, and converts the required control variables back to analog signals. Unlike the discrete solution, this approach offers flexibility, high functional integration, and smaller installed size. In addition, on-board processing capability provides the means to create more sophisticated control and support algorithms without affecting hardware for a truly scalable solution.

This circuit implementation consists of the SoC and simple circuits that provide an interface to the laser transmitter module. The temperature and optical power analog signals are digitized by an 8-channel, 12-bit analog-to-digital converter that is on the SoC. The monitor diode connects via a current-to-voltage converter while the thermistor connects directly. Control interface to the laser module consists of two simple current generators driven by the digital-to-analog converters (DAC) on the SoC.

The SoC controls a second laser by adding two sample/hold amplifiers per channel (see Fig. 3, bottom left). In this scheme, user software "steers" the appropriate analog level from the DAC to each sample/hold amplifier using its I/O port lines. A dual DAC, controlled by one of the communications ports on the SoC, may also be used in place of the sample/hold amplifiers. In either case, laser-control-circuitry overhead is shared across two lasers for even lower cost and board-area per channel.

SOFTWARE-DRIVEN DESIGN
Perhaps the greatest attribute of the SoC approach is that system functionality is designed using software, allowing a user to create custom hardware solutions quickly by writing application software. Low-cost development tools allow a user to easily develop, download, and debug programs with the SoC installed in the target system.

Software routines for implementing temperature and optical-power regulators may be written in 8051 assembler, C-language, or any other language offering a compiler targeted at the 8051 instruction set. The software algorithm consists of a data-acquisition portion and a control portion (see Fig. 4). The data-acquisition software is straightforward: thermistor and monitor-diode signals are converted to digital words by the 12-bit ADCs and stored in memory. The control-portion software mimics the subtract-and-integrate behavior of the hardware control amplifier in the discrete analog circuits discussed earlier.

The SoC architecture uses a time base generated by one of the on-board timers to set the integration speed of the control algorithm (control output). This timer periodically calculates new values for each control output. The control output value is then incrementally increased or decreased (or neither if the calculated difference is zero), and written to the digital-to-analog converter.

The SoC functionality can be changed or updated via software download to the FLASH memory, simplifying system maintenance and reducing design risk. In addition to in-system programming capability, FLASH memory provides local data storage and update on demand. With the information storage directly fixed to the transmitter circuit assembly, critical information, such as service history, serial number, production date, maintenance history, and laser-diode parameter values at final factory test, is available instantly.

The integrated data-processing capability and serial connectivity of SoC enable other important benefits, including automated self-test. On-board serial-communication capability allows the SoC to exchange information with the main system processor, enabling more advanced diagnostics, system-configuration capability, and maintenance functions.

Don Alfano is vice president of marketing for Cygnal Integrated Products, 4301 Westbank Dr., Suite B-100, Austin, TX 78746. He can be reached at dalfano@cygnal.com.

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