Optical phase modulating scheme of a MIOC of an interferometer type fiber optic gyroscope

Abstract

An optical phase modulating method of a multi-integrated optical circuit of an interferometric fiberoptic gyroscope with a control feedback control with a digital modulation and demodulations approach includes the step of modulating at eight modulation levels. Each modulation levels has a duration of one-quarter of a light propagation time in a fiberoptic coil.

Claims

1. An optical phase modulating method of a multi-integrated optical circuit (MIOC) of an interferometric fiberoptic gyroscope (I-FOG) with a control loop feedback control with a digital modulation and demodulation approach, the optical phase modulating method comprising: modulating at eight modulation levels wherein each modulating level has a duration of one-quarter of a light propagation time τ in a fiberoptic coil.

2. The optical phase modulating method of claim 1, further comprising: correcting a ΔΦ.sub.R phase shift produced by a Sagnac effect in a sensor of the interferometric fiberoptic gyroscope after an ε.sub.R error measure with a period equal to the light propagation time τ.

3. The optical phase of claim 1, further comprising: correcting the MIOC modulation channel gain in a sensor of the I-FOG after an ε.sub.G error measure equal to the light propagation time τ.

4. The optical phase of claim 1, wherein a control frequency of the fiberoptic coil of a given length is increased by a factor of two as a consequence of the modulation level being one-quarter of the light propagation time τ.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) FIG. 1 illustrates the basic and high level scheme of an I-FOG system in an “all-digital” control configuration.

(2) FIGS. 2a-2c illustrate respectively a raised cosine pattern of interference obtained on the PinFET depending on the ΔΦ phase shift the selected working point depending on the biasing modulation given by the p.sub.0 and p.sub.1 levels, and the signal obtained at the PinFET over time.

(3) FIGS. 3a-3c illustrate respectively the interference pattern obtained at the PinFET depending on the ΔΦ phase shift, the selected working point depending on the biasing modulation, and the signal obtained on the PinFET depending on time.

(4) FIG. 4 illustrates what happens in a stationary condition when the modulation channel gain is not correct.

(5) FIGS. 5a, 5b and 5c illustrate a modulation scheme at 8 levels when the sensor is at rest and when it is subjected to rotation at constant angular rate ω, which generates a ΔΦ.sub.R phase shift by the Sagnac effect.

(6) FIG. 6 illustrates the effects of a gain error on the analogue modulation channel in stationary conditions when using the 8-level modulation scheme.

DETAILED DESCRIPTION OF THE INVENTION

(7) According to a preferred, but not limited embodiment, this invention relates a modulation technique of a MIOC of an interferometer fiber optic gyroscope, I-FOG, with closed-loop feedback control with application of a digital mod/demod approach. Said technique for modulation of the MIOC is an 8-level scheme and the duration of each level of modulation is τ\4.

(8) The technique for modulation of I-FOG sensors that is object of this patent application makes it possible to obtain higher dynamic performances with the same fiber length, doubling the frequency of demodulation of the signal rad at the interferometer and reducing control latency from 2τ to τ. This allows, with respect to the all-digital control techniques known in the state of the art, the dynamic response of the sensors to be improved, doubling the intrinsic bandwidth and reaching higher levels of accuracy and linearity with the same quantity of fiber.

(9) As can be seen in FIG. 5, illustrating the 8-level modulation scheme which drive the MIOC appropriately when the sensor is at rest and when it is subjected to rotation with constant angular rate ω, which generates a ΔΦ.sub.R phase shift by the Sagnac effect, a periodical phase pattern can be identified, through the modulation channel and control logic, given by Φ.sub.0=π−α, Φ.sub.1=π+α, Φ.sub.2=−π+α, Φ.sub.3=−π−α, Φ.sub.4=−π+α, Φ.sub.5=−π−α, Φ.sub.6=π−α e Φ.sub.7=π+α (FIG. 5b) which will correspond, at the interferometer to the values p.sub.0, p.sub.1, p.sub.2, p.sub.3, p.sub.4, p.sub.5, p.sub.6, p.sub.7. The duration of each of these values will be τ/4 (FIG. 5c) and, once again, the duration of the entire control cycle of the I-FOG will be 2τ. However, it will be noted how the following equivalences apply:

(10) 0$\begin{array}{cc}{\Phi }_4={\Phi }_2\stackrel{\mathrm{implies}}{.fwdarw.}{p}_4=p_2{\Phi }_5={\Phi }_3\stackrel{\mathrm{implies}}{.fwdarw.}{p}_5=p_3{\Phi }_6={\Phi }_0\stackrel{\mathrm{implies}}{.fwdarw.}{p}_6=p_0{\Phi }_7={\Phi }_1\stackrel{\mathrm{implies}}{.fwdarw.}{p}_7=p_1& \left(10\right)\end{array}$

(11) To modify the biasing modulation pattern so as to cancel the effect of the ΔΦ.sub.R phase shift it will be necessary to add the biasing signal provided to the MIOC by a serrodyne ramp with a step having ΔV.sub.RET amplitude of duration τ and proportional to the separation of the levels p measured at the PinFET. But, while with the techniques described in [1] and [2] it was necessary to wait for a time 2τ to have a set of updated samples so as to determine the ΔV.sub.RET value, which has been said to be, in first approximation, proportional to the integral of the instant ε.sub.R phase error, making it necessary to maintain the correction value unchanged for the next 2 steps of the serrodyne ramp, in the new proposed modulation scheme, as demonstrated by formulas (7) and (9), equivalences (10) and FIG. 5c, at each τ a complete and updated set of values p.sub.0, p.sub.1, p.sub.2, p.sub.3 will be available to determine:
€.sub.R=(p.sub.0+p.sub.3)−(p.sub.1+p.sub.2)=(p.sub.6+p.sub.5)−(p.sub.7+p.sub.4)  (11)
and
ΔV.sub.RET∝Σ∈.sub.R  (12)
The value of the serrodyne ramp step (and angular rate data) can therefore be updated every τ, thus doubling the tracking speed of the feedback control of the phase shift produced by the Sagnac effect and, consequently, the bandwidth of the I-FOG sensor.

(12) With the ε.sub.R error measurement there is the possibility of correcting the ΔΦ.sub.R phase shift produced by the Sagnac effect in the I-FOG sensor with a period τ, that is, for the time necessary for propagation of the light in the sensitive element of the sensor.

(13) The benefits of this innovation can be imagined by thinking of a situation where the rotation rate of the sensor is not constant, but varies very quickly and, for example, between two successive time intervals τ the rotational speed ω has undergone a large variation (not a difficult thing in the presence of high entity frequency vibrations).

(14) By using the prior techniques, by upgrading ε.sub.R only every 2τ, the phase variations produced by the Sagnac effect will be tracked with the same ΔV.sub.RET correction value for two consecutive times, producing a Φ.sub.RET phase that would no longer meet the Φ.sub.RET=−ΔΦ.sub.R equality, hence committing an error that, in highly performing sensors—that can require a quantity of fiber greater than 2,000 m and consequently a propagation time τ in the coil not negligible in presence of high dynamics—can become significant and also lead to instability of the feedback control. In fact, it must be reminded that the interference pattern of the light rays after the propagation in fiber is a raised cosine and, when moving away from the selected working point, the non-linear nature of that signal can be rapidly manifested. This phenomenon may even be exacerbated by an incorrect tracking by the feedback control.

(15) Thanks to an increase in the feedback dynamics of a factor of 2, the 8-level modulation technique greatly mitigates this phenomenon by reaching, for example, for a 2,000 m of fiber sensor, the same control speed and bandwidth that was, with the techniques illustrated in [1] and [2], for a sensor of 1,000 m, and to obtain, at the same fiber length, a reduction in the vibration correction effects of the sensors output, in the absence of any mechanical damping system, equal to at least one order of magnitude.

(16) By observing FIG. 6, showing the effects of an analogue modulation channel gain error under stationary conditions when using the modulation technique, object of this patent application, it can be understood how also the correction speed of the gain error can be doubled, being able to evaluate ε.sub.G at every τ through the formula:
ε.sub.G=(p.sub.0+p.sub.2)−(p.sub.1+p.sub.3)=(p.sub.6+p.sub.4)−(p.sub.7+p.sub.5)  (13)
and considering the equalities expressed in (10).

(17) But in this case the benefits are less apparent since the MIOC half-wave voltage variations, object of the feedback effected on the modulating channel gain, are related to generally slow phenomena, such as temperature variations. With the error measurement ε.sub.G it is possible to correct the MIOC modulation channel gain, in an I-FOG sensor, with periodicity equal to τ, that is, at the light propagation time in the sensing element of the sensor.

(18) The 8-level modulation technique allows obtaining a more accurate and linear dynamic response to the I-FOG sensors when measuring rotating rate profiles characterized by high variations (strong angular accelerations), such as it happens, for example, in the presence of vibrations. This allows mitigating the distortion effects that may occur due to the nonlinear nature of the signal measured at the interferometer, given by a raised cosine, and the latency of the feedback control.

(19) Through modulation technique, object of this patent application, the vibration rectification phenomenon of the I-FOG sensors can be reduced in absence of any mechanical damping system by a factor greater than one order of magnitude.

(20) The materials and dimensions of the invention as described above, illustrated in the accompanying drawings and as claimed below, may be any according to the requirements. In addition, all details are replaceable with other technically equivalent, without departing from the protective scope of this patent application.