Source stabilization for high quality time-domain diffuse optical tomography

We report a new close-loop feedback control method to keep a Mach-Zehnder electro-optic modulator (MZ-EOM) biased at the quadrature point and simultaneously correct the bias drift caused by the temperature changes as well as the inherent photorefractive effect. The modulator is a key part of our high speed time-domain diffuse optical tomography system. It modulates the dual-wavelength near-infrared light with the high speed pseudorandom bit sequence (PRBS) signal for the temporal point spread function (TPSF) measurements. Our method applies a periodical low frequency square wave with 50% duty cycle as the pilot tone upon the MZ-EOM together with the PRBS and sweep the bias voltage of the MZ-EOM in a self-adaptive step. A constant fraction of the modulated output power is measured by a photodiode via a tap coupler. After demodulation, the modulation depth versus the bias voltage can be measured from which the peak value corresponding to the quadrature point can be located quickly by curve fitting. Our stabilization technique is simple, fast and cost effective and is effective to correct the bias drift caused by the photorefractive and the change of ambient conditions. The experiment results show the TPSFs measurements can be stabilized to within ±2% in an hour duration, which helps improved the image quality.