Optical spectral analysis using moving space-charge field effects in photoconductive semiconductors

Abstract

The interference of two mutually coherent optical fields with unequal center frequencies generates moving space-charge electric fields inside photoconductive semiconductors that contain deep level traps. The motion of the internal space-charge field drifts free charge carriers out of the photoconductive sample and results in measurable photocurrents that consist of ac and dc components even in the absence of external bias electric fields. The dc photocurrent can be expressed as a convolution integral between the baseband power spectrum of the signal optical field and the characteristic impulse response of the photoconductive crystal, provided that the local oscillator has negligible optical linewidth. We have demonstrated experimentally a new kind of optical spectrum analyzer with a resolution bandwidth smaller than 15 kHz by sweeping the local-oscillator center frequency over the spectral region of interest and by subsequent deconvolution of the recorded photocurrent with the characteristic response of the crystal. The scanning range of the analyzer is limited only by the tuning range of the local oscillator.