A Low-Drift Laser-Driven FOG Suitable for Trans-Pacific Inertial Navigation

Fiber optic gyroscope interrogated with three multiplexed broadened semiconductor lasers

“…[5] These metrics were shown experimentally to meet the FAA requirements for inertial navigation of an aircraft, with a positional error under 10 nautical miles after a 10-h flight. [6] In spite of this success, in earlier measurements the drift of this FOG was larger by a factor of ~3 than that measured in the same FOG probed with an SFS instead of a broadened laser. Experiments show that this drift is limited by one or more of the aforementioned nonreciprocal effects.…”

“…The ~1558-nm semiconductor lasers were selected to have similar center wavelengths, which could be tuned over a small range by adjusting their temperature. The laser outputs were combined in a 4x4 fiber coupler and sent into an EOM driven by the Gaussian noise source described in [6]. The broadened laser signal exiting the EOM was sent into an optical gate to (1) eliminate the optical spikes in the FOG output that result from the use of square-wave modulation to bias the FOG's interferometer [2], and (2) suppress the Kerr effect [1].…”

“…In each case, all laser powers exiting the EOM were adjusted to be the same, and the laser wavelengths were adjusted to be spaced by 1 nm by adjusting the temperature of N -1 lasers. Figure 2 shows that each laser spectrum has been broadened to a 3-dB bandwidth of ~0.2 nm (~25 GHz), while the noise voltage bandwidth was only ~10 GHz (see discussion of noise source in [6]). It is well known that in a FOG probed with coherent light, the two signals counter-propagating in the coil experience a differential phase shift due to the Kerr effect when they carry different powers.…”

“…To this end, the laser has been broadened by modulating its phase externally with either sine waves, [3] a pseudo-random bit sequence, [4] or broadband noise. [5,6,7] This last approach has been the most successful so far. The laser light is sent through an electro-optic phase modulator driven by broadband noise generated by amplifying the thermal noise of a resistor with a chain of high-speed amplifiers.…”

“…[7] These values are sufficient to eliminate most of the nonreciprocal noise and drift, but the residual Kerr effect still needs to be further suppressed by modulating the input laser power with a 50% duty cycle. [1,6] These combined improvements led recently to the demonstration of a 3-km laser-driven FOG with a noise as low as 900 µdeg/√h, a drift of 12.6 mdeg/h, [6] and a mean-wavelength stability of less than 1 part per million. [5] These metrics were shown experimentally to meet the FAA requirements for inertial navigation of an aircraft, with a positional error under 10 nautical miles after a 10-h flight.…”