Contributed editorial appearing in
Scientific Computing & Instrumentation 18:7, June 2001, pg. 22.
As a photon jockey, I get excited (‘pun intended) when learning about the hybridization of electronics and photonics. I once worked on a development team set to the task of deploying three-dimensional Planar Doppler Velocimetry (PDV) in wind tunnel measurements. PDV is useful for visualizing the flow velocity (speed and direction) of air around a model. This optical diagnostic technique measures the Doppler shift of light scattered from tracer particles entrained in the flow using a planar laser sheet and two-dimensional charge-coupled device (CCD) detectors. PDV is identical in theory to radio frequency Doppler-shift radar; however, the tracer particles must be extremely small in order to minimize momentum and thereby maximize their proclivity to follow the airflow. These miniscule tracer particles necessitate the use of short wavelength, high frequency electromagnetic radiation to achieve the requisite scattering efficiency. Also, since the CCD detectors collect still images, the Doppler shift of the high-speed particles must be measured quickly to avoid blurry images through the use of short, picosecond-duration laser pulses.The PDV system was based on the 532-nm frequency-doubled output of an injection seeded, Q-switched Nd:YAG laser having a signal frequency of 564 THz; a value that far exceeds the ca. 1-GHz upper frequency response limit of analog-to-digital converters (ADCs) currently available. In fact, the integrating CCD detectors used for data acquisition are continuous-wave (CW) devices capable of resolving spatial intensity but not temporal frequency. The enabling system component was a device used to modulate the high-frequency scattered laser light down to zero-Hz while transferring the frequency information into intensity modulation. This miraculous device is better known as a simple optical absorption filter. Iodine vapor was sealed in a high-pressure, temperature controlled, optically transparent cell and placed in front of the collection lens of the CCD detector. The nominally sharp absorption lines of iodine vapor are frequency broadened into bands through collisions with high-pressure inert gas introduced into the cell and the 532-nm source frequency of the Nd:YAG laser is precisely adjusted to lie at the half- height point of the absorption band.
If the source frequency is located on the “blue” or higher frequency shoulder of the absorption band, then blue-Doppler shifted light is less attenuated as it moves out of the absorption band while red-Doppler shifted light is more attenuated. The CCD detector then collects this intensity-encoded image. However, the position, number, size, and scattering efficiency of the entrained particles is not known or controlled during the experiment and therefore must be measured at the same instant the filtered image is recorded. This intensity reference image is obtained by placing a 50:50 beam splitting cube up stream of the iodine filter and imaging the split beam onto a second CCD detector. The reference and filtered images are then ratioed to extract the Doppler shift information, which is then converted to velocity.
This “sample vs. reference” approach is the foundation of analog-to-digital conversion. Successive approximation, flash, and delta-sigma ADCs all utilize comparison circuitry for the examination of differences between the sample signal and an internal reference. In the case of the PDV system, the external signal passes directly through the reference path of the split circuit and through the sample path only after interacting with a known, well-behaved modulator (the absorption filter). The “split the signal into two paths, filter one of the paths, and analyze the recombined signal” doctrine is exemplified in the Mach-Zehnder (M-Z) interferometer. This optoelectronic integrated circuit (OEIC) device is used extensively in fiber optic switching and wavelength division multiplexing (WDM) networking systems.
In late 1997, the Defense Advanced Research Projects Agency (DARPA) issued a Broad Agency Announcement for the Photonic Analog-to-Digital Converter Technology (PACT) program. Its goals are the development of ADCs capable of 12-to-14 bit resolution at conversion rates in the range of 1 to 100 Gsamples/s through the utilization of advanced photonic components. Among the critical technologies required are a precise high-speed clock and a high-resolution sample quantizer. These new optical ADCs are based on the M-Z interferometer layout and utilize mode-locked semiconductor lasers to produce short optical pulses having less than 10 fs jitter between pulses. In contrast to focusing an unknown optical signal into an M-Z interferometer consisting of a known modulation filter, as in the PDV system, the modulation filter of the PACT ADC is connected to the unknown input signal. Using an electro-optic material whose index of refraction is modulated by the input signal, the optical clock pulses are introduced into the M-Z interferometer and their precisely known frequency is modulated by the input signal and this change is subsequently measured. As all of the components are semiconductors, ultimately, the PACT ADC can conceivably exist as a single integrated circuit. The initial phase of the PACT program will be completed shortly, so be prepared to wear sunglasses in the lab.
