The main requirements for an optical detector are low noise and high sensitivity. The old method used was only to cool the photomultiplier tube. This new method of compact cooled- PMT cools only the photocathode. The main noise of the photomultiplier tube is generated from the photocathode. By utilizing this new method of cooling the photocathode, the noise from the photomultiplier tube can be reduced significantly. A small thermo-cooler attached to the photocathode can cool the photomultiplier tube directly. As a result, within five minutes, the photocathode reaches a lower temperature, for example, of 5 degrees C in an ambient temperature of 20 degrees C. The performance of the photomultiplier tube is one tenth of the dark current of ambient temperature. Also, the dark count decreases by a factor of one tenth of the ambient temperature. Equivalent noise inputs wer 2.3 X 10-17 (at 600 nm).