Fiber optical rotation sensor with extended dynamic range

In a fiber optic gyro having a loop of optical fiber for guiding a pair of counter-propagating light waves and a device for combining said waves to form an output light signal, an apparatus for sensing rotation-induced phase shifts between said waves, comprising:

• a signal generator for providing first and second harmonic driving signals at first and second harmonic frequencies, respectively;
• a first phase modulator driven by said first harmonic driving signal for phase modulating the counter-propagating light waves in said loop at said first harmonic frequency;
• a second phase modulator driven by said second harmonic driving signal for phase modulating the counter-propagating light signals in said loop at said second harmonic frequency;
• means for detecting the first harmonic component of said output light signal;
• means, responsive to said detecting means, for controlling the amplitude of said second harmonic driving signal to reduce the amplitude of said first harmonic component in the output light signal substantially to zero;
• and
• means for converting said second harmonic signal in accordance with a non-linear transfer function to indicate the rotation rate of said loop.

An apparatus for sensing rotation rate comprising:

• a loop of a fiber optic material for guiding counter-propagating light waves;
• a first phase modulator for phase modulating said waves at a first frequency;
• and
• means for combining said waves to form an optical output signal;
• means for generating a feedback error signal proportional to the amplitude of the Fourier component in the optical output signal at said first frequency;
• a second phase modulator for phase modulating said counter-propagating waves at a second frequency in response to a driving signal at said second frequency;
• feedback means for adjusting the amplitude and phase of said driving signal at said second frequency so as to minimize the amplitude of said feedback error signal;
• and
• output means for converting, by means of a non-linear transfer function, the characteristics of said driving signal at said second frequency to data corresponding to the rotation rate of said loop.

In a Sagnac rotation sensor having counterpropagating lightwaves phase modulated at a first frequency which are combined to form an optical output signal having a component at said first frequency, a method of determining a rotation rate comprising:

• sensing the the component of the optical output signal at said first frequency;
• phase modulating said counter-propagating light waves at a second frequency and at an amplitude and phase relative to said first frequency so as to cancel said component at said first frequency;
• and
• converting, through a non-linear transfer function, the characteristics of said phase modulation at said second frequency to the corresponding rotation rate causing said component at said first frequency.

A method for sensing the rotation rate of a gyroscope having a loop of fiber optic material, the method comprising the steps of:

• introducing a light wave into said loop;
• sensing the light wave in the loop to produce a first signal responsive to rotation of said loop;
• producing a second signal related to the rotation rate of the loop, said second signal responsive to the first signal and tending to null said first signal;
• and
• superimposing a non-linear transfer function on the second signal to translate said signal to a rotation signal proportional to the rotation rate of the loop.