ADC Uses Optical Sampling and Clocking
John Baliga, Associate Editor -- Semiconductor International, 2/1/2001
Leading-edge ICs are using clock frequencies of a gigahertz or more. The practical matter of examining the signals generated by those ICs requires sampling rates of 10 GHz or more. Also required is an analog-to-digital converter (ADC) that is clocked at the sampling rate. Researchers at HRL Laboratories (Malibu, Calif.) have developed a system that uses optical sampling and clocking and an InP heterojunction transistor (HBT) optoelectronic integrated circuit (OEIC).1 One key to the operation of this system is a high-quality source of laser pulses. The system uses a laser mode-locked at 10 GHz, generating 3 psec pulses with a timing jitter of 16 fsec and an amplitude jitter of 0.058%. This allows a sampling accuracy up to 11 bits, though the IC was designed to produce 4-bit data. The pulse train was fed into a lithium niobate electro-optic modulator, where it is modulated by the analog signal (Fig. 1). The modulated pulse train feeds into the InP-HBT quantizer IC. The input train goes to photodetectors on the chip, which are connected to differential integrate-and-dump amplifier circuits, which convert the pulses into an electronic signal for the converter. The converter is a resistive ladder with comparitors and other formatting converters.
The clock signal for the integrate-and-dump amplifiers and the converters comes from the same reference used to control the sampling laser pulse train. It is converted from an optical to an electrical clock signal on-chip.
The OEIC is made using the InAlAs/GaInAs/InP material system (Fig. 2). The InAlAs and GaInAs layers are grown epitaxially on an InP substrate, and vertical devices are defined by mesa etching. Optoelectronic devices made with this system are well suited for the common fiber communication wavelengths in the 1.3 — 1.6 µm range, and many types of high-speed electronic devices can be made in it as well.
1. The system used for sampling at 10 GHz includes a laser, electro-optic modulator and InP-HBT quantizer with optical clocking. (Source: HRL Laboratories) |
For this OEIC, the same epitaxial stack is used for both the detectors and HBTs.2 The detectors are made using the base-collector junction of an HBT structure as a pin diode. After the HBT structures are etched, the "emitter" of each detector is metallized, shorting the emitter to the base, and a hole is etched to define the aperture. An antireflective layer is added to enhance detector responsivity.
Many other types of electronic devices can be made in this material system, such as high-electron mobility transistors (HEMTs), or planar metal-semiconductor-metal (MSM) detectors. One advantage of the approach used here is that one epitaxial stack can be used for both detectors and transistors. Some other approaches require a custom stack for each type of device, sometimes requiring selective growth in the middle of device processing
2. The epitaxial structure built on the InP substrate allows the monolithic integration of photodiodes and HBTs. (Source: HRL Laboratories) |
The HBTs used to realize all the circuitry are designed to have a cutoff frequency of about 150 GHz. The HBTs do not limit the performance of the device, regardless of any trade-offs made to allow the use of one stack.
REFERENCES
1. T. Broekaert, W. Ng, J. Jensen, D. Yap, R. Walden, "InP-HBT Optoelectronic Integrated Circuits for Photonic Analog-to-Digital Conversion," Procedings of the 22nd IEEE GaAs IC Symposium, Nov. 2000.
2. R. H. Walden, "A Review of Recent Progress in InP-Based Optoelectronic Integrated Circuit Receiver Front-Ends," International Journal of High-Speed Electronics and Systems, Vol. 9, p. 631-642, 1998.
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