Article | April 19, 1999

Correlated Photon System Measures Radiance Directly, Part I

An experimental technique uses visible detectors to measure absolute infrared spectral irradiance without the use of calibrated standards.

By: Alan Migdall, National Institute of Standards and Technology

Using correlated photons, an experimental system at the National Institute of Standards and Technology (NIST) performs absolute infrared spectral radiometric measurements without requiring any externally calibrated radiometric standard. The technique is the only method that measures radiance directly, instead of using radiant power and aperture geometry measurements to deduce radiance indirectly. This technique has an additional unusual characteristic; it allows absolute radiometric measurements of infrared radiation to be made using high-quality visible detectors. Comparisons with conventional methods show agreement to within ~3%, which we believe to be the highest accuracy test of this technique.1

Optical parametric downconversion
The method uses correlated photons produced via optical parametric downconversion. 2-4 In optical parametric downconversion, which takes place in a nonlinear medium, photons from a pump laser beam essentially decay into pairs of photons under the restrictions of energy and momentum conservation (otherwise known as phasematching). 5 The photons are born two at a time into single entangled quantum states, so the detection of existence, wavelength, direction, and polarization of one photon tells of the existence and properties of the other.

For a practical system, a laser pumps a nonlinear crystal set up to produce correlated infrared (IR)/visible pairs of photons (see figure 1). The output of the IR source to be measured is imaged into the crystal such that it overlaps the region pumped by the laser, and overlaps the output direction of a portion of the down-converted light. The beam under test must overlap the downconverted output spectrally, as well as spatially and directionally. This IR input to the crystal enhances the "decay" of photons from the pump beam into downconverted photons along that overlap direction, but because these output photons must be produced in pairs, an increase is also seen along the correlated direction.

By analogy with an atomic system, this mechanism can be thought of as a "stimulated decay" of pump photons into correlated pairs, whereas the correlated photons produced with only the pump laser for input are the result of "spontaneous decay". It turns out that this spontaneous decay can be thought of as being produced by an omnipresent spectral radiance background of 1 photon/mode, which can be written as Rvac o=hc2/l5 (which has the more familiar units of spectral radiance, W/m3sr).2,3

The absolute IR spectral radiance of the unknown source is thus just the ratio of the increase in the visible channel signal observed with and without the addition of the IR. This ratio is the radiance in the fundamental units of photons/mode. Visible light and a visible detector can therefore be used to "see" the infrared beam. And because radiance is given by a ratio, no calibration is needed for the detector; it need only be linear.

To turn this system into a true metrological technique, it is necessary to consider several points. First, the radiance measured is that which is added to the crystal region pumped by the laser, so any input losses must be accounted for. Second, the size of that pumped region is essentially the spatial resolution of the measurement, so it is desirable to uniformly bathe that region with the IR field to be measured to avoid unwanted averaging. Similarly, it is necessary to angularly overfill the sensitive region. This angular extent is set by the phase matching conditions and the bandwidth of the measurement.

In part II of this article, Migdall will discuss the design and performance of a practical system based on an argon ion laser and a lithium iodide crystal.

1. A. Migdall, et. al., "Measuring Absolute infrared Spectral Radiance with Correlated Visible Photons: Technique Verification and Measurement Uncertainty," Appl. Opts. 37, 3455-3463, (1998).
2. Klyshko D. N., "Utilization of Vacuum Fluctuations as an Optical Brightness Standard," Sov. J. Quant. Elect. 7, 591-594, (1977).
3. Klyshko D. N., Photons And Nonlinear Optics, New York, Gordon and Breach,1988 325p.
4. G. Kh. Kitaeva et. al., "Measurement of Brightness of Light Fluxes Using Vacuum Fluctuations As a Reference," Sov. Phys. Dokl. 24, 564-566 (1979).

About the author…
A. Migdall is with the Optical Technology Division, 221/B208, National Institute of Standards and Technology, Gaithersburg, MD 20899.