GaAs ICs Optimize Optical Data Systems

By Jack Browne, Microwaves & RF, June 1999

Ideal for Gigabit Ethernet and fibre Channel systems, these wide-bandwidth circuits offer good sensitivity and a reduction in cost compared to discrete solutions.

Fiber-optic communications offer many advantages over conventional copper-wired or even wireless systems. Despite the occasional high cost of installation, service providers trust fiber-optic links due to their lack of electromagnetic (EM) leakage, their immunity to surrounding EM fields, and wide, unrestricted bandwidths. In order to enhance the growth of fiber-optic-systems use, the engineers at ANADIGICS, Inc. (Warren, NJ) have combined two commonly used components - the transimpedance amplifier (TIA) and the photodetector - into a single package. These GaAs optoelectronic integrated circuits (OEICs), which target Gigabit Ethernet systems at 1.25 Gb/s and Fibre Channel applications at 1.064 Gb/s, also benefit from innovative microwave-rate power-supply circuitry.

The integrated photodetector/TIA products are the model AMT128502T46F/L, with an 850-nm metal-semiconductor-metal (MSM) photodiode detector and wideband TIA, and the model AMT121302T46F/ L, with a 1300-nm PIN-diode detector and TIA. The "F" and "L" letter designations refer to package options: the devices can be supplied in either a TO-46 ultraflat window package or a ball-lens package.

Each device has the company's patent-pending on-chip negative-voltage generator circuitry to bias the photodetector. In contrast to conventional switched-mode voltage generators that operate around the 200-kHz range, these circuits operate in the microwave region up to 10 GHz. This enables the shrinking of the circuit elements, allowing them to be implemented monolithically. In the MSM-TIA and PIN-TIA products, the on-chip negative-voltage generators run at approximately 6.5 GHz and are totally transparent to the operation of the device. The negative voltages are used to reverse bias the photodetector strongly and, hence, achieve low capacitance as well as excellent responsivity.

The MSM product (AMT128502T46F/L) has an integrated photodetector that is responsive in the 850-nm wavelength. This is ideal for the Gigabit Ethernet Short Reach (1000-SX) standard which specifies data transmission in multimode fiber at 1.25 Gb/s up to a distance of 550 meters. The photo detector is 100 mm square and the responsivity of the detector/TIA combination has a minimum value of 1000 V/W. The device provides a minimum bandwidth of 1.5 GHz.

High Sensitivity

The AMT128502T46F/L achieves an optical sensitivity of at least -20 dBm and typically -22 dBm, measured for a bit-error rate (BER) of 10-10 with a pseudorandom bit sequence of 27- 1 at 1.25 Gb/s. The device is rated for a maximum supply voltage of +7 VDC and a maximum input power of +5 dBm. It has an overload level (above which BER degrades) of 0 dBm. The photo-detector/ TIA operates on supply voltages from +4.5 to +5.5 VDC with maximum 50-mA current. The operating temperature range is 0 to +70°C.

Model AMT12130-2T46F/L employs a higher-wavelength (1300-nm) PIN photodetector with the same basic TIA as the model AMT128502T46F/L. It exhibits a minimum wave-length of 1270 nm and maximum wavelength of 1355 nm. The higher wavelength is better suited in single-mode fiber-optic systems, compared to the multimode 850-nm wavelength of the AMT128502T46F/L. In a single-mode fiber, optical energy is limited (by the small core size) to traveling across only one path. In a multimode fiber, light is able to travel through a number of paths, since the core diameter is larger.

The AMT121402T46F/L boasts a minimum bandwidth of 1 GHz and maximum bandwidth of 1.5 GHz, with a low cutoff frequency (where the amplitude response drops by more than 3 dB) of typically 800kHz. It achieves somewhat greater responsivity than its lower-wavelength counterpart, with small-signal differential responsivity (at 50 MHz) of at least 2500 V/W (and typically 3300 V/W) for the T46F package and at least 2000 V/W for the T46L package. Similar to the AMT-128502T46, it has differential output.

The AMT121302T46F/L features an optical sensitivity of at least -26 dBm (and typically -28 dBm) for the T46F (ultraflat) package and at least -25 dBm for the T46L (ball-lens) package. Optical overload occurs at 0-dBm input levels, using a BER of 10-10 with a 27- 1 PRBS at 1.25 Gb/s as test conditions.

Time-Domain Results

To maintain data integrity and simplify recovery of clock signals in a typical fiber-optic receiver, the AMT121302T46F/L has been designed for good time-domain performance. It achieves a 20-to-80- percent rise/fall time of typically 160 ps (and at least 260 ps), measured with -3-dBm optical input power and a differential output load of 100 ohms. Root-mean-square (RMS) jitter is typically only 15 ps, and no greater than 30 ps. Duty-cycle distortion is typically only 3 percent, and no worse than 6 percent. Total peak-to-peak jitter is less than 150 ps, and typically only 90 ps, ensuring that clock and data timing are preserved over long optical links. Similar to the 850-nm device, the AMT121302T46F/L operates with a single supply voltage of +4.5 to +5.5 VDC. It draws no more than 50-mA current and typically consumes only 35 mA.

With these components, the company has built upon its experience in high-speed GaAs TIAs, integrating additional components required for a fiber-optic receiver. The TIA, which functions essentially as a current-to-voltage converter, delivers high gain along with its conversion function through an open-loop architecture.

The bandwidth of the transimpedance amplifier circuit is essentially determined by the parasitic capacitance of the photodiode and the feed-back resistor. Through its monolithic implementation, the MSM-TIA controls this parasitic capacitance to achieve a wide band-width with minimal noise and wide dynamic range.

Optimized Integration

By integrating the photodetector and TIA, within a single package, an obvious savings in cost and circuit-board space has been achieved compared to separate, packaged components. More important, the commonly packaged photodetector and TIA have been closely matched for optimum performance, saving time in manufacturing and testing by facilitating optical alignment.

"Having a photodetector integrated directly onto the IC ultimately provides a lower-cost approach than discrete PIN diodes and preamplifiers," notes Dan Trepanier, ANADIGICS's Sales Manager for Fiber Optic Products. "This integration results in a higher performance product due to its optimization of the match between the TIA and the photodetector," he adds.

Evaluation boards are available for both devices. The evaluation boards mount one of the OEICs with sup- porting circuit elements and signal lines, and provide input/output (I/O) connectors to simplify evaluation and testing. Both of the units are rated for a maximum supply voltage of +7 VDC and maximum input power of +5 dBm. The availability of these pack-aged photodetector/TIAs should speed the development of new systems and the enhancement of existing systems for wider-bandwidth, higher- data-rate performance as required by many emerging datacom and telecom applications.

Microwaves & RF, June 1999