Photodetector Array Power Dissipation
For quantum-noise limited heterodyne detection we need a certain minimum optical local-oscillator power at each photodiode. Since we would like a 10 GHz bandwidth from each pixel, RC time-constant considerations dictate that each photodetector load must be of low impedance. There are two basic ways to minimize the amount of local-oscillator power required. Use transimpedance front-end amplifiers with a PIN photodetector array, so that while relatively high impedance loads may be employed, the transimpedance amplifier configuration produces small RC time-constants, or we can use Avalanche photodetectors (APDs). It might be possible to construct an array based on APDs at modest gain with 50-ohm loads, to avoid using trans- impedance amplifiers, and/or to reduce the local-oscillator laser power requirements, but there would be problems with gain stability, uniformity of gain, and tracking of gain across the array.
For the purposes of this analysis we will consider only PIN photodetector arrays with transimpedance amplifiers. At this time, transimpedance amplifiers are not available for this bandwidth, though hopefully they will be in a few years. Signetics has a 280 MHz 7-kohm transimpedance amplifier called the NE5210, but its bandwidth is too small for our purposes. However, state-of-the-art transimpedance amplifiers with 1 GHz bandwidths are becoming available.
A bandwidth at least as large as 10 GHz is preferred for the Multi-Channel Spectrum Analyzer (MCSA) so that the optical spectrum can be sampled more efficiently (rapidly) in 10 GHz chunks. Such an MCSA is substantially wider in bandwidth than proposed for the Microwave Observing Project (MOP), which is typically 300 MHz, with about 10 million 30 Hz top-level bins per polarization (left and right circular). However, because the bin bandwidth I have proposed for the optical search is about 100 kHz, the number of bins per polarization for a MCSA with an instantaneous bandwidth of 10 GHz, is only 100,000 (not accounting for any frequency overlap required). For a 128 X 128 array there are 16,384 pixels. If we require 128 MCSAs to sample an entire row of pixels at a time, the number of bins is slightly larger than proposed for MOP. Even if we are quite extravagant and stipulate one 10 GHz MCSA for each pixel, the number of top-level bins required is only 164 times that required for a two polarization-state MOP. On-going developments in MCSA technology should make this easier and less expensive to achieve in the future.
As previously mentioned, unlike with (incoherent) photon-counting arrays, (coherent) heterodyning arrays require a sufficient amount of local- oscillator (L.O.) laser power at each pixel to ensure shot noise limited detection. Ideally, we would like to use a simple 50-ohm load, but this would require a very high local oscillator power level to maintain the quantum noise limited sensitivity. At visible wavelengths, about 20 mW per pixel would be required to be quantum noise limited, so that a 128 X 128 array (16,384 pixels) would have to dissipate 328 W! This doesn't even include the electrical power dissipated in each photodetector, which is several times larger. Clearly, if we are going to illuminate all the pixels simultaneously with the L.O., this is not going to be possible with 50-ohm loads. If each load could be increased to 1-kohm, then the magnitude of the power dissipation problem could be substantially reduced.
Assuming that it is possible to produce a 10 GHz bandwidth optical front-end with an effective load or transimpedance of 1-kohm, a local-oscillator power of about 1 mW per pixel will be required for quantum noise limited detection. A transimpedance amplifier with a gain of about 20 dB would be suitable, but it is presently a technological challenge to produce a suitable very compact super-wideband amplifier which would be stable. Phase-gain margin considerations alone dictate that the amplifier must be physically very small, in order to prevent negative feedback becoming positive feedback at very high frequencies. If we assume a unity gain bandwidth of 30 GHz, the wavelength in air is only 1 cm.
For a L.O. power of 1 mW on each pixel (photodiode), the typical photocurrent produced by the L.O. laser in the red part of the spectrum will be about 0.45 mA per pixel. If the reverse bias applied to the photodiodes is 5 volts, then the electrical power dissipated per photodetector is 2.25 mW, and the total power dissipated is 3.25 mW. This is not a problem for a single photodiode, but in an array of 16,384 photodiodes, the total power would be 53 W! It is fortunate that we don't require a 2048 X 2048 array! This is another occasion when the expression "holy smokes" is again applicable.
If the bandwidth required was only 1 GHz, and if it was possible to produce a 10-kohm transimpedance, then the L.O. power could be reduced to 0.1 mW, with a reduction in photodiode current to 0.045 mA. The total dissipated power would then fall to 0.325 mW and the array power would be 5.3 W! This is still somewhat high, and would require a substantial amount of cooling. Note that this magnitude of continuous wave (C.W.) power level is available from dye, gas and Nd:YAG lasers.
There are a number of ways to get over the array heat dissipation problem:
1. Reduce the number of pixels that have to be simultaneously illuminated by the local-oscillator laser, by using a line-focused local-oscillator beam and a linear array, and scanning the image over the array with a mirror. Total power dissipated in the linear array for a 10 GHz system would be about 0.42 W.
2. Use a line-focused local-oscillator beam which scans across the two dimensional array in synchronization with the field scan rate. Total power dissipated in the two-dimensional array for a 10 GHz system would also be about 0.42 W.
3. Use a focused local-oscillator beam, and steer it so that it addresses each pixel sequentially in synchronization with the two-dimensional array output sampling, assuming sequential pixel sampling. Acousto- optic deflectors might be used for this purpose, though there are some problems here caused by the inherent varying frequency-shift of the scanned L.O. beam. Total power dissipated in the array for a 10 GHz system would be about 3.25 mW. We would probably have to use a bank of electronic mixers to remove the differential frequency offsets produced by the acousto-optic modulator(s). An added advantage of this approach is that a very low-power L.O. laser may be employed.
It might be possible to multiplex each element or row of elements with a single transimpedance amplifier. Recent years have seen major developments in MIMIC (Microwave/Millimeter-wave Monolithic Integrated Circuit) technology based on GaAs devices, so much may be possible here. What ever is done, we are basically talking about a state-of-the-art custom PIN photodetector array that has an integral transimpedance amplifier for each element or one that is shared between many pixels. We should look very closely at developments occurring in the fiber-optics industry, particularly relating to coherent fiber-optic communications and parallel signal processing arrays. I even wonder if it might be possible to tie all the photodiodes together to one transimpedance amplifier via extremely low capacitance optical switches (a bit like photodiodes), and actually sequentially switch each pixel into the on-state with a scanned L.O. beam? It would appear that an Optical SETI program will require the services of a photodetector manufacturer for developing this specialized product.
For the purposes of future discussions, we shall assume that the array, and mirror scanning system if employed, is equivalent to a 128 X 128 pixel device.
Coming Attractions: RADOBS.23 THE OPTICAL SEARCH STRATEGY RADOBS.24 TARGETED SKY SURVEY RADOBS.25 COPYRIGHT NOTICE RADOBS.26 LIST OF MAIN OPTICAL SETI FILES UPLOADED TO RADOBS BULLETIN BOARD
Last week, I happened to spot a book in the Bexley library by Walter Sullivan entitled "We are not alone - the search for intelligent life on other worlds". Chapter 15 in this book has the most detailed account of Optical SETI, i.e., using laser communications, that I have seen to date in a general interest publication, save for science fiction stories. The explanation for this is probably due to the book's publication date. It was published in 1964, only a few years after the laser was invented, and only a few years after the start of the microwave SETI era. At that time, there was more interest in new SETI ideas and the ideas of Schwartz and Townes, but eventually the microwave approach won out.
January 27, 1991 RADOBS.22 BBOARD No. 335
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