This paper presents a new approach based on current mode circuits for fast and accurate optical level monitoring with wide dynamic range of a gigabit burst-mode laser driver chip. Our proposed solution overcomes the drawbacks that voltage mode implementations show at higher bit rates or in other technologies. The main speed-limiting factor of the level monitoring circuitry is the parasitic capacitance of the back facet monitor photodiode. We propose the use of an active-input current mirror to reduce the impact of this parasitic capacitance. The mirror produces two copies of the photo current, one to be used for the "0" level measurement and another for the "1" level measurement. The mirrored currents are compared to two reference currents by two current comparators. Every reference current needs only one calibration at room temperature. A pattern detection block scans the incoming data for patterns of sufficiently long consecutive 0's or 1's. At the end of such a pattern a valid measurement is present at the output of one of the current comparators. Based on these measurements the digital Automatic Power Control (APC) will adjust the bias (IBIAS) and modulation current (IMOD) setting of the laser driver. Tests show that the chip can stabilize and track the launched optical power with a tolerance of less than 1 dB. In these tests the pattern detection was programmed to sample the current comparators after 5 bytes (32 ns at 1.25 Gbps) of consecutive 1's and 0's. Automatic power control on such short strings of data has not been demonstrated before. Although this laser transmitter was developed for FSAN GPON applications at a speed of 1.25 Gbps upstream, the design concept is generic and can be applied for developing a wide range of burst mode laser transmitters. This chip was developed in a 0.35 µm SiGe BiCMOS process.

Recessed-gate DMTs (doped-channel hetero-MISFETs) with i-AlGaAs/n-GaAs structure and pseudomorphic i-AlGaAs/n-InGaAs/i-GaAs structure have been developed. Broad plateaus in gm and fT provide evidence that the DMTs make the devices suitable for high-speed large-signal operation. GaAs DMTs with 0.35 µm-length have gate turn on voltage of 0.7 V, maximum transconductance of 320 mS/mm and fT of 41 GHz. Pseudomorphic DMTs have gate turn on voltage of 0.9 V, maximum transconductance of 320 mS/mm, fT of 42 GHz and have the enhanced advantages of high current drivability and large gate swing. Further more, with the use of the recessed-gate DMTs, a high-speed laser driver IC for multi-Gb/s optical communication systems are demonstrated. This laser driver IC operates at 10 Gb/s with rise and fall times as fast as 40 psec, and it can drive up to 60 mA into a 25 Ω load.

We have successfully developed an optical receiver and a laser driver circuit which were implemented with 0.35 µm GaAs JFETs (junction Field Effect Transistors). The 0.35 µm GaAs. JFET had the typical transconductance of 480 mS/mm with small drain conductance. An interdigit MSM (Metal Semiconductor Metal) -type photodetector and the JFETs were monolithically integrated on a GaAs substrate for the optical receiver. The fabricated optical receiver demonstrated Gb/s operation with a very low power consumption of 8.2 mW. The laser driver circuit operated at up to 4.0 Gb/s.