An optical system having a first order spectral range that is usable in an optical spectrum analyzer receives an broadband optical test signal and a optical calibration signal and couples the optical signals via two optically isolated paths to separate optical detectors. First and second pairs of optical fibers, with each pair having an input fiber and an output fiber, are positioned in a focal plane of a collimating optic that has an optical axis. The fiber pairs are symmetrically positioned on either side of the optical axis with the input fibers positioned on one side of the optical axis and the output fibers positioned on the opposite side of the optical axis. The input fibers receive the optical test signal and the optical calibration signal. The output optical fibers are coupled to first and second optical detectors. An optical calibration source generates second order or greater spectral lines that fall within the first order spectral range of the optical system. A diffraction grating receives the optical test signal and the optical calibration signal from the collimating optic and separates the first order spectral components of the broadband optical test signal and passes the second order or greater spectral lines of the optical calibration signal. The first optical detector that is responsive to the first order spectral components of the optical test signal receives the optical test signal from the collimating optic and converts the optical test signal to an electrical signal. A second optical detector that is responsive to the second order or greater spectral lines of the optical calibration signal concurrently receives the optical calibration signal from the collimating optic and converts the calibrations signal to an electrical signal.
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