A 1D IR lock-in focal plane array (FPA) for extremely weak signal imaging has been demonstrated. The experimental system consists of an object with modulated image signal, a high speed InGaAs linear photodetector array as receiver, a CMOS lock-in linear array read-out circuit, and a focal plane array test system. The system can detect extremely weak signals immersed in strong background. Preliminary test shows that under room temperature each of the pixels in the 1D lock-in FPA can read out modulated signal 5 orders smaller than the background. The InGaAs detector array response is
from 0.8 μm to 1.6 μm (peak at 1.2 μm). The lock-in array read-out circuit uses a correlated multi-cycle integrator, which can operate in several modes such as gated integration, and phase-sensitive integration with background subtraction. The 1D lock-in FPA works as a pixel to pixel lock-in amplifier, wherein very small signals may be extracted from a much strong background if the frequency of the illuminating source (usually IR light sources) is known. Simulation results are also reported. Experimental results based on an IR illuminating source are demonstrated.
A gated multi-cycle integrator (GMCI) is presented to recover weak repetitive image signal from strong background. The GMCI could operate in several modes such as Capacitive- transimpedance amplifier (CTIA), gated integration (GI) and background-cancellation integration (BCI). When GMCI operates at BCI mode, the storage well of a pixel is mainly used for signal integration even there exists strong background or large dark current. Thus the signal-to-noise ratio (SNR), dynamic range and the sensitivity of detection are greatly improved. In addition, the transmission windows of BCI peak at odd harmonics of the modulation frequency. Therefore the detector's 1/f and other low frequency noises can be attenuated. A switched capacitor integrator (SCI) was designed to carry out the performance of GMCI. The switch induced fixed pattern can be obliterated by taking the differentia of two multi-cycle integrated signals with 180° phase difference. Preliminary chip test shows that the GMCI can read out modulated signal that is five orders less than the background.
A new readout method of coal pane array - correlated readout is presented for the first time. The interface circuit of this method is modeled as to charge and discharge the integration capacitor according to a control signal. Combined with an optical modulator, this readout method will be able to increase image's dynamic range and signal-to- noise ratio more effectively for strong background application. It will not only increase the output voltage swing but also attenuate noises that have lower frequency than the modulation frequency. Several interface circuits are designed and studied. SPICE simulation results show that correlated capacitive transimpedance amplifier may become a very promising correlated readout circuit. In order to study this new method experimentally by visible imager, a 16 by 16 silicon CMOS FPA demo chip has been designed, along with a test system. The application of this method on solar megnetrograph measurement is discussed.