freespace, optoelectronic circuits

A multichannel optical oscilloscope for sampling broadband, freespace, optoelectronic circuits Rick L. Morrison, Steve G. Johnson*. Anthony L. Lentine, and Wayne H. Knox** AT&T Bell Laboratories, 2000N. Naperville Rd., Naperville, IL 60566 *Massachusetts Institute of Technology, Cambridge, MA 02139 **AT&T Bell Laboratories, 101 Crawfords Corner Rd., Holmdel, NJ 07733 Fundamental Operation The advent of large-scale, free-space, optoelecironic interconnections, as demonstrated in recent system prototypes'4, requires new sampling methods to reveal diagnostic information. Several factors contribute to the difficulty of probing optical communications channels without disrupting their operation. High-speed electronic connections to the chip periphery are not available in sufficient number and would contribute an undesirable thermal load. Electronic and optical56 physical contact probes would obscure many of the optical channels that are relayed to a common surface of the chip in current systems. Optical sampling provides the better method although many standard techniques are either too time consuming or complex to implement. We will describe a tool we developed that delivers diagnostic information on a large number of high-speed, optical data channels simultaneously and operates analogously to the conventional sampling electronic oscilloscope. The optical oscilloscope is constructed using CCD cameras and video capture boards that are controlled by a software application resident in a personal computer. Sampling is based on a stroboscopic method of using short pulsed laser probe beam synchronized to a data stream to illuminate optical modulators within the opto-elecironic circuit. We have demonstrated and will discuss the tool's capability of simultaneously monitoring arrays of broadband optoelectronic devices operating at speeds from several hundred Megabit/s to a few Gigabit/s. In current free-space photonic systems, data is transmitted optically between electronic processor cells by modulating the intensity of light beams. Arrays of light beams, externally generated by laser diodes and diffractive components, are focused by lenses onto small reflective windows underlying multiple-quantumwell (MQW) material. The optical absorption of the MQW5 is electronically governed by attached processing circuitry. In this manner, the absorption of the MQW encodes data onto each optical channel. An optical infrastructure then routes the reflected modulated channels to the subsequent chip or fiber. An optoelectronic chip may embody thousands of optical channels each operating at speeds of hundreds of Mbits/sec, thus posing a serious challenge in collecting diagnostic information. In present-day investigations, it is typically necessary to simultaneously monitor a large number of parallel channels to determine the optimal operation parameters. Rather than design a complex array of high-speed photodetectors that must be accurately aligned to a remote image of the modulator array, it is far simpler to sample the modulators' states using a repetitive, short duration light pulse and collect the image with an inexpensive CCD video camera. Thus in the same manner that a stroboscopic light source apparently freezes or slows the motion of rotating fan blades, the pulsed illuminator highlights the evolution of a periodic data stream for a large set of modulators. 158 ISPIE Vol. 2692 O-8194-2066-2/96/$6.OO SYNCHRONIZATION CONTROL A schematic of the multichannel, optical oscilloscope is shown in figure 1. The three primary functions of the oscilloscope hardware modules are: • to generate pulses that create short duration readout light beam arrays for probing the modulator absorplion and to synchronize these pulses with a periodic data stream (synchronization control module) • to sample the readout light from several optical data channels in parallel and focus the individual channels separately onto a photosensor array, typically a CCD video camera (optical probe unit) • to digitize and analyze the video signal and display the sampled waveforms in a format similar to that of an oscilloscope (analysis and display processor) Currently, the separate modules have been only loosely integrated since the system to be investigated influences the design of the probe and synchronization units. The synchronization control unit is typically custom designed to match the system. For example, generation of the short-duration readout light pulses can be performed by connecting new signal lines to the existing readout lasers in some systems, while in other cases a pulsed, broad-area illuminator can be integrated with the optical probe assembly. Since the readout pulse usually occurs repeatedly during the time sampling window of the photosensor, it must occur at the same point in the data stream throughout that window. In our demonstrations, we have maintained synchronization by using coupled data and pulse generators referenced to a common clock, but differing by about 1Hz at the bit frequency so that the pulse slowly scans the data pattern. SPIE Vol. 2692 / 159 HIGH SPEED ELECTRONICS WITH INTEGRATED MODULATOR ARRAY Figure 1. Schematic of test photonic system and optical oscilloscope modules.