Pseudo-random-bit-sequence phase modulation for reduced errors in a fiber optic gyroscope.
Low noise and drift in a laser-driven fiber optic gyroscope (FOG) are demonstrated by interrogating the sensor with a low-coherence laser. The laser coherence was reduced by broadening its optical spectrum using an external electro-optic phase modulator driven by either a sinusoidal or a pseudo-random bit sequence (PRBS) waveform. The noise reduction measured in a FOG driven by a modulated laser agrees with the calculations based on the broadened laser spectrum. Using PRBS modulation, the linewidth of a laser was broadened from 10 MHz to more than 10 GHz, leading to a measured FOG noise of only 0.00073  deg/√h and a drift of 0.023  deg/h. To the best of our knowledge, these are the lowest noise and drift reported in a laser-driven FOG, and this noise is below the requirement for the inertial navigation of aircraft.
Spontaneous Vacuum Fluctuations and the Fundamental Sensitivity Limit of Wavelength-Multiplexed Fiber Bragg Grating Sensor Array Readouts
Multiplexed wavelength shift detectors are increasingly used to read out arrays of fiber Bragg grating (FBG) sensors that are illuminated by amplified spontaneous emission sources. The fundamental sensitivity limit of these systems is due to excess noise from the light source. This limit is evident when optical intensity is high enough to overcome the electronic noise of the measurement system. Here, we derive a formula for the fundamental sensitivity limit and show it is only a function of sensor optical bandwidth. The formula is corroborated by experiment by measuring the wavelength noise of 12 FBG sensors as a function of optical intensity. First, the expected noise vs. intensity is calculated from the formula with known sensor optical bandwidths and shown to agree to the measured results without fit. Then, the formula is fit to the experimental data, with optical bandwidth as a parameter, to extract the optical bandwidth of each FBG from the noise saturation behavior with respect to optical intensity.
Quantum noise in superluminescent diodes
Intensity noise in a superluminescent diode (SLD) has been studied over the frequency range from 100 Hz to 2 MHz. The "1/f" noise which dominates at low frequencies ( 500 kHz). A more extensive investigation has been carried out in this higher frequency regime, where the intensity noise is assumed to result from quantum fluctuation effects. For a given SLD driving current, the excess noise power is found to be a linear function of photodetector current to the maximum observed level of 12 dB. These results agree well with the behavior predicted by a quantum amplifier model for the SLD.