Abstract: Apparatus and method of monitoring fluid presence, wherein electomagnetic radiation sources (1, 2) provide respective reference and sample beams (12, 22), the wavelength of the radiation preferably being matched with that of the absorption spectrum of a fluid to be monitored. A first reference cell (13), containing a reference sample of the fluid to be monitored, has the reference beam (12) passed through it, whilst a second, sample cell (33) containing any working sample of the fluid to be monitored, has both the reference and sample beams (12, 22) passed through it. Subsequently, the reference and sample beams are detected independently of each other to provide respective signals representative thereof, which signals are then compared to determine the presence, and preferably also the concentration, of any sample fluid in the second cell.

Abstract: A combined optical power meter and receiver is provided in which the optical power meter retains high sensitivity and the optical receiver retains high bandwidth. A single photodiode provides current for both the optical power meter and the optical receiver. A first operational amplifier is configured as an integrating transimpedance amplifier having a current input which is referenced to a reference voltage and receives current from one port of the photodiode. A second operational amplifier is configured as a transimpedance amplifier having a current input which is referenced to ground and receives current from the other port of the photodiode. In this manner, a bias voltage is developed across the photodiode to decrease the capacitance and increase the operational speed of the receiver. A second material photodiode is provided which is shielded from the input light source is coupled to the current input of the first transimpedance amplifier.