Optoelectronic Integrated Circuits (OEICs)
This is a continuation from the previous tutorial - Photonic Integrated Circuits (PIC).
The first OEIC was reported in the mid 1970s by Yariv and coworkers. It was a simple device - A Gunn diode integrated with a laser. Since then, various types of integrated laser-driver and photodiode-amplifier with single and multichannel capability have been reported. So far, most of the OEIC developments have been towards lightwave transmitters and receivers.
1. Receiver OEIC
The receiver OEIC generally consists of PIN photodiode integrated monolithically with an amplifier. The amplifiers using FET- and HBT-based technologies have been fabricated. The first receiver OEIC was an InGaAs PIN photodiode integrated with a single InP-based FET.
The schematic of a receiver OEIC which consists of an InGaAs PIN photodiode and an HBT-based amplifier is shown in Figure 12-11. The entire layer structure is grown by the MOVPE growth technique. The detected photocurrent from the PIN photodiode is amplified by transistors T1 and T2. Transistor T3 acts as an output buffer stage.
Arrays of photodiodes and integrated amplifiers for use in optical interconnection have also been reported. The amplifiers used here are FET-based. Figure 12-12 shows the photograph of a PIN-MODFET (modulation-doped FET) array fabricated with a center-to-center spacing of 250 μm.
The fabrication involves two epitaxial growths by MBE. First the layers for the InGaAs PIN photodiode are grown by MBE. Then channels are etched on the wafer using a SiO2 mask and wet chemical etching. The layers for the MODFET are then grown in the channels by MBE with the SiO2 mask in place. No growth occurs on the top of the SiO2.
The performance of both the MODFET and PIN photodiodes on the regrown structure is comparable to that of single devices. A measure of the performance of a receiver is the receiver sensitivity. Hence it is important to compare the sensitivity of integrated PIN-amplifier-based receivers to those of their hybrid counterparts.
The measured data as a function of bit rate are shown in Figure 12-13. Note that the hybrid-technology-based receivers have 3-4-dB better sensitivity than the integrated receivers.
2. Transmitter OEIC
Several types of transmitter OEICs, both using GaAs lasers and InGaAsP lasers, have been reported. GaAs OEICs generally use metal-semiconductor field-effect transistors (MESFETs) in their driver circuits. InGaAsP OEICs use HBTs in their driver circuits.
The circuit diagram and the structure of an integrated GaAs transmitter OEIC is shown in Figure 12-14. The GaAs laser is a single quantum-well laser. The circuit has a waveguide-type photodiode for monitoring the performance of the laser. The transistors T1 and T2 are arranged as a differential pair for better stability. The circuit shown in Figure 12-14 operated at 1 Gb/s.
An InGaAsP-laser-based transmitter OEIC has also been fabricated. This device structure is shown in Figure 12-15. It utilizes HBTs for driving the laser and transistors are arranged as a differential pair. The epitaxial layers were grown by MOVPE on an Si-InP substrate. The circuit operated at 5 Gb/s.
3. Regenerator OEIC
Integration of a light source, a photodiode and an electronic circuit will, in principle, lead to an optical regenerator. Up to now, such monolithic regenerators have behaved as simple optical amplifiers. An example of such a device built on an Si-GaAs substrate is shown in Figure 12-16.
It has 3 FETs and a laser. One of the FETs also serves as a photodiode for the input light. An overall power gain of 10 dB has been achieved for this OEIC. Similar regenerator circuits using InP material systems have also been reported.
4. Logic OEIC
OEICs which perform some optical logic functions have been fabricated. Many of these OEIC developments are aimed for applications in computing and switching systems where parallel two-dimensional interconnection technology is important. The surface-emitting laser or LED is the source of choice for these two-dimensional interconnection systems.
An example of an optical bistable switch using a surface-emitting laser is shown in Figure 12-17. The switching operation is performed by a p-n-p-n photothyristor monolithically integrated with the surface-emitting laser. The device has a high contrast (~ 30 dB) between its on and off states.
The next tutorial discusses about Lead-Salt Semiconductor Lasers.