Using optical-parametric-downconversion techniques to produce correlated photon pairs, physicists at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) have developed a method to measure absolute infrared (IR) spectral radiance to an initial accuracy of better than 3%. This is the only method directly sensitive to radiance, in contrast to conventional techniques, which use radiant power and aperture geometry to evaluate spectral radiance indirectly. The two-photon method also allows measurement of IR sources using high-performance visible radiometric detectors.
In two-photon downconversion, a nonlinear crystal pumped by a laser beam converts each photon of the pump-laser beam into a pair of lower-frequency photons (the signal and idler beams). The downconverted photons are created simultaneously, so the detection of one photon implies the existence of the other. The photon pairs obey energy- and momentum-conservation laws—energy and momentum information for one photon can be used to determine the energy and momentum of the other. The frequencies of the signal- and idler-beam photons sum to the frequency of the pump-beam photons (wp = ws + wi). A variety of nonlinear crystals can be used to create this effect. By adjusting the crystal optical axis tilt and choosing a particular output direction, the desired measurement wavelength can be selected from a wide spectral range (see Fig. 1).
Spectral-radiance measurements
Two-photon generation can be thought of as a spontaneous decay process, with a single pump photon spontaneously creating the correlated photon pair. If an additional beam with the same frequency and direction as one of the output beams is trained on the optically pumped nonlinear crystal, the beam will enhance the incidence of downconversion, essentially causing stimulated decay of pump photons into correlated photon pairs. The ratio of the output for the stimulated versus spontaneous output can be used to determine the spectral radiance of the stimulation source. This ratio is essentially the radiance of the source in units of photons/mode, which may be thought of as natural absolute units of radiance.
In the NIST system, a 300-mW beam from an argon-ion laser emitting at 457.9 nm pumps a 15
× 15
× 9.7-mm lithium iodate (LiIO
3) crystal (see Fig. 2). Alan Migdall and collaborators configured the system to emit visible and IR correlated photon pairs (0.5288 µm/3.415 µm, 0.5065 µm/4.772 µm) with the visible beam exiting the crystal at approximately 4 from the system optical axis and the IR beam exiting at 25 to 45 from the axis.