Infrared Spectral Irradiance and Radiance Responsivity Calibrations
using Uniform Sources (IR-SIRCUS)

The objective of the IR-SIRCUS facility is to measure the spectral irradiance or radiance responsivity of infrared detector systems with reduced uncertainty. The method uses laser-illuminated integrating spheres to fill the entrance optics of the radiometer under test with uniform, tunable, monochromatic radiance and irradiance as measured by reference radiometers. The measurements of IR-SIRCUS facility will be linked through a short calibration chain to the Primary Optical Watt Radiometer (POWR).

Figure 1. Block diagram of the IR-SIRCUS facility, consisting of a tunable laser, an intensity stabilizer, chopper, speckle removal system, and an integrating sphere. Detector under test (or a reference detector) is placed at some distance from the integrating sphere opening.

IR-SIRCUS uses unique tunable lasers to illuminate a variety of integrating sphere sources. The purged and light-tight ambient background IR-SIRCUS facility covers the spectral range 0.7 µm to 5.3 µm with continuously-tunable intensity-stabilized lasers, and longer wavelengths out to 11 µm with discrete-tunable lasers. These include a lithiuim borate (LBO) optical parametric oscillator (OPO) for the 0.7 µm to 1.8 µm range, a (CW and mode-locked) periodically-poled lithium niobate (PPLN) OPO for the 1.4 µm to 5.3 µm range, CO (5 µm to 7 µm), and CO₂ (9 µm to 11 µm). The 12 W mode locked pump laser for the OPOs operates with a 7 ps pulse width at 220 MHz, and results in average power levels of about 1 Watt for both signal and idler. This pulse width is in the quasi-CW limit in that the peak-power per pulse is generally low enough so as not to drive the detectors non-linear. Development of continuously tunable lasers at wavelengths longer than 5.3 µm is also being performed at IR-SIRCUS, and a monochromator is being installed to fill in the spectral region out to 18 µm until such lasers become available.

Figure 2. A figure showing the wavelength coverage and available power of several tunable lasers used for the IR-SIRCUS. Shown in the figure include LBO/OPO, PPLN/OPO, CO, CO2 lasers, and Quantum cascade lasers (under development).

Spatially-uniform integrating spheres were developed for both point and extended source applications. IR diffuse materials have been introduced, tested, and applied for integrating-sphere-used sources and detectors to achieve higher spatial uniformity as compared to available methods. A liquid-helium-cooled Electrical Substitution Bolometer (ESB) is characterized and calibrated against irradiance mode Si trap radiometers and is used as the reference detector for the IR spectral range. InGaAs and InSb working standards are used for cases where adequate uncertainty can be achieved more conveniently.

The uncertainty for irradiance responsivity measurements that have been done to date is currently at about 3 % (k=2), and current developments are directed at reducing this to a general goal of 0.2 % (k=1) Also, the IR-SIRCUS is an optimum monochromatic uniform source facility that can provide detector-based calibration support for digital imaging systems such as detector arrays, infrared cameras, IR focal plane array radiometers, and spectrometers.

	Optical Power Primary Standard
POWR Facility Radiant Power Mode (collimated laser beam) VIS-NIR Wavelengths		0.01 %
	Si Trap
UV-VIS-NIR SIRCUS Facility Irradiance Mode VIS-NIR Wavelengths		0.1 %
	IR SIRCUS Reference Radiometer
IR SIRCUS Facility Irradiance Mode IR Wavelengths		TBD %
	Customer Radiometer to be Calibrated
Customer's Facility Irradiance Radiance Mode IR Wavelengths

Figure 3. A diagram showing the calibration chain of IR-SIRCUS, starting from Optical Power Primary Standard to Customer Radiometer to be Calibrated.

The increased accuracy transfer and working standard radiometers, calibrated on the IR-SIRCUS facility, will increase the accuracy of radiometric measurements in the infrared wavelength range. The IR-SIRCUS will provide an absolute calibration for InGaAs detectors, Extended InGaAs detectors, InSb detectors, pyroelectric detectors and HgCdTe detectors with high accuracy. The IR-SIRCUS facility is the basis to spread the new high-accuracy detector-based calibrations for military, academia, and industry in the infrared.

Development of a monochromatic, uniform source facility for calibration of radiance and irradiance detectors from 0.2 µm to 12 µm,
K.R. Lykke, P.-S. Shaw, L.M. Hanssen, and G.P. Eppeldauer,
Metrologia 35, 479-484 (1998).

An electrically substituted bolometer as a transfer-standard detector,
Rice J.P.
Metrologia 37, 433-436 (2000).