Constant temperature operation of fiber-optic hot-wire anemometers

We demonstrate the constant temperature (CT) operation of a fiber-optic anemometer based on a laser-heated silicon Fabry-Perot interferometer (FPI), where the temperature of the FPI is kept constant by adjusting the heating laser power through a feedback control loop and the output signal is the heating laser power. We show that the CT operation can dramatically improve the frequency response over the commonly used constant power (CP) operation, where the laser heating power is kept constant and the output signal is the temperature of the FPI. For demonstration, we used a 100-μm-diameter, 200-μm-thick silicon FPI attached to the tip of a single-mode fiber as the anemometer. The FPI was heated by a 980-nm diode laser, and the temperature was measured using a 1550-nm diode laser. The effect of flow changes was simulated by exposing the silicon FPI to radiation from an external intensity-modulated laser beam. We show that the 10%-90% rise time of the step response in air was reduced from 625 ms for CP operation to 1.8 ms for CT operation, and the 3-dB bandwidth was increased from 0.5 Hz for CP operation to 2 kHz for CT operation. The response of the anemometer also shows good linearity to the radiation power.