Broadband mid-IR fiber supercontinuum source for hyperspectral image projection

Abstract

Testing, calibration, and validation of imaging sensors and imaging systems require bright sources of emission to simulate scene artifacts. Ideally, such sources are broadband or tunable over a wide wavelength region to provide spectral content to the scenes and/or to target the spectral response of the system. In the mid-infrared (IR), projection systems for sensor testing typically use globars, blackbodies, resistor arrays, optical parametric oscillators (OPO), or quantum cascade laser sources. Globars and resistor arrays are not bright enough to simulate high temperature artifacts which may be necessary for testing of some sensors and systems. OPO sources aand quantum cascade laser sources do not have the spectral coverage. To meet testing requirements for IR sensor systems, we have developed a broadband fiber based supercontinuum mid-IR source for projection systems..The National Institute of Standards and Technology (NIST), Gaithersburg, MD, is developing a Hyperspectral Image Projector (HIP) for the testing, calibration, and system-level validation of multispectral and hyperspectral imaging sensors [1,2]. The system is capable of projecting scenes with specific spectral content at video frame rates. A schematic of the HIP system is shown in Fig. 1 below. The system is based upon two modified-commercial digital micromirror devices (DMDs) which are used in series to project the spectral content (DMD1) and the spatial content (DMD2) of the scene. For the HIP system, Naval Research Lab is developing an IR fiber supercontinuum source [3]. Requirements for illumination of the HIP system are spectral power density of -20 dBm/nm or higher, spectral flatness of 10 dB within the band of interest (3 µm to 5 µm), and continuous wave (CW) or high repetition rate pulse trains. The repetition rate must be ~1 MHz or greater so that it far exceeds the temporal response of the system under test. The output diameter of the fiber can be no greater than 20 µm in diameter to maintain the spectral resolution of the system.The supercontinuum source is composed of a 2 meter length of single-mode 10 µm core /140 µm clad As 2 S 3 fiber pumped with an amplified and Raman shifted 500 kHz, 80 ps Er mode-locked fiber laser. As the pump and generated supercontinuum fall within the normal dispersion regime of the fiber, the mechanism of supercontinuum is primarily cascaded Raman and self phase modulation. The use of a broadband pump with several cascaded 93TuB3.3 (Contributed Oral) 14:30 -14:45 U.S. Government work not protected by U.S. copyrightRaman shifts convoluted with the Raman gain bandwidth of the fiber and spectral broadening due to self phase modulation results in a broadband flat continuous spectrum from 1.5 µm to 5 µm as shown in Fig. 2. Currently, spectral flatness is 10 dB from 1.9 to 4.4 µm and 20 dB from 1.64 to 4.78 µm. Figure 2.Mid-IR supercontinuum from 2 meter length As 2 S 3 fiber. The black temperature lines indicate equivalent blackbody temperature of a projected scene from the HIP system using the source.In the HIP...