Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
  • G01C19/58—Turn-sensitive devices without moving masses
  • G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
  • G01C19/72—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers

Definitions

• the present invention relates to a fiber optical gyro of the kind described in the introductory part of the patent claim 1.
• Fiber optical gyros have during later years been subject to intensive research and development. Several laboratory prototypes have shown very high sensitivites. Nevertheless, the commersial introdukton of fiber optical gyros has not yet been successful. Among several reasons, the most crucial is that if they are to be highly sensitive, they also require high quality components and advanced procedures of manufacturing. This inevitably renders high costs.
• phase shift is propotional to the number of fiber loops (N) in the coil, to the, by the coil enclosed area (A) and to the rotation rate of the coil ( ⁇ ). Further, the phase shift is inversely proportional to the wave decorations ( ⁇ o).
• the inter ⁇ ference signal (I d ) is proportional to the light power (I 0 ) and the factor ( 1 +cos ⁇ s ) . The signal is thus not a linear funktion of ⁇ s and this limits the dynamics.
• the detected signal is not only a function of ⁇ s but also depends upon the light power I 0 . It is therefore impossible to distinguish between a real change in rotation and a fluctuation in lightpower. This can be circumvented if the system is made phase reading. This principle is known previously, but the present invention gives possibilities to make this in a new way.
• a fundamental problem with all fiber optical gyros is the backward radiarion which orginates from Raylength-scat- tering. If the scattered light is coherent with the signal carrier they will interfere and thereby give rise to false signals. The amount of scattered light interfering with the measurement signal is proportional to the coherence length of the light source. The most common way to reduce this form of noise is therefore to use super luminous diodes with a broad spectrum and small coherence length instead of common laser diodes. This, however, introduces other dis ⁇ advantages.
• Imperfections in the fiber give rise to coupling between the two orthogonally polarized modes in a single mode fiber. As these two modes have slightly different refractive index the mode coupling inside the fiber loop gives rise to irreciprocal phase shifts which can not be separated from the Sagnic-shift.
• polarizers have a limi ⁇ ted extinction ratio and to reach high sensitives it is in addition necessary to use a polrization maintaining fiber in the loop. This type of fiber is essentially more expen ⁇ sive than conventional single mode fiber.
• polarization selective modulation can be used (Swedish patent application nr 8900729-8) which lessens the demand for polarization maintaining fibres.
• the object of the present invention is therefore to pro ⁇ vide a fiber otical gyro using a laser diode as light source, having high sensitivy, high immunity against fluctuations in the intensity of the light source, high immunity to re ⁇ flections and Rayleight-backscattering and in principle with a linear relation between ouput signal and rotation rate.
• the present invention solves these problems in a way that is stated in the charaterizing part of the enclosed claim 1 whereby light from a light sourse, preferably from a laser diode or possibly from a super luminous diode, is devided and fed into both directions of a fiber-optic coil.
• the counter propagating beams are then combined where they were divided and their phase difference is interfero- metrically detected. Said phase difference between the two beams, The Sagnac-shift, appears when said coil is rotated.
• the light, before being fed into the fiber coil is in a time multiplexed manner divided into two channels.
• the multiplexing is achived with an opti ⁇ cal switch, wich alternatingly guides the light from the source into the two channels.
• Each channel, together with the light source, fiber coil and detector form a complete fiber optical gyro, but where the channels either are modu ⁇ lated differently or are designed differently.
• the applied phase modulation is such that the modulation together with the time multiplexing, i.e. the switching, gives a serrodyne modulation scheme.
• a serro- dyne modulation scheme offers the possibility to, in a linear way, directly measure said phase shift.
• Said modu ⁇ lation is preferably performed by using triangular waves and with a period wich corresponds to the double roundtrip time for the light passing the coil or odd multipples of this.
• the modulation of the two channels should be in coun ⁇ ter phase.
• the modulation dephs has to, in the case of triangular waves, be an integer multiple of ⁇ r/2.
• polarization selec ⁇ tive modulation can be used in the two-channel gyro to further reduce the above-mentioned polarization none- reciprocities. This is accomplished when the selective modu ⁇ lation is applied on both sides of the fiber coil, that is on both sides of the respective channels of the Y-branch, either alone or in combination with polarizers introduced in each channel. The signal is then detected in at least the eighth harmonic of the modulation frequency.
• figure 1 shows a conventional one-channel fiber optical gyro
• figure 2 shows how a complete two channel system according to the invention can be provided
• figure 3 shows howe the resulting phase modulation according to the invention in channel 1 respective 2 can be represented
• figure 4 finally shows a variant of the present invention where polarization selective modulation is used.
• Figure 1 shows a conventional one-channel fiber optical gyro, which also forms the basic design of a typical channel according to the invention in the two-channel gyro.
• the light source 1 is, preferably, a conventional laser diode. It is together with a detector 2 connected to a fiber coupler 4 integrated optical directional coupler or corresponding.
• a beamsplitter here as an integrated optical Y-branch 5, connected via a polarizer 6.
• the integrated optical Y-branch 5 is connec ⁇ ted to two phase modulators 7 - one on each arm - which in turn are connected to each end of the the fiber coil 3.
• FIG 2 a complete two channel system is shown with a light source 1 and a detector 2 connected to the first an second port of an optical switch, an integrated directional coupler or corresponding 14.
• the switch 14 replaces the fiber coupler 4 in the conventional fiber optical gyro and is necessary to preform the time multiplexing between the two channels.
• the third and fourth ports are then, possibly via two polarizers, connected to the both channels inte ⁇ grated optical Y-branches 15 of which outputs, one in each Y-branch, is coupled to a phase modulator 17.
• fiber couplers 18 or other beamsplitters which divides the light to res ⁇ pective channel from the optical fiber coil 3 is needed.
• the funktion is as follows: If the optical switch 14 changes between "bar-state” and “cross-state” with a period corresponding to twice the cycle time of the light through the fiber-loop, or odd multiples of this, a time multiplexed detection scheme is achived.
• the switch 14 When the switch 14 is in "bar-state” the light from light source only couples to channel 1.
• the beamsplitter 18 half of the light power When it reaches the beamsplitter 18 half of the light power will get lost while the other half proceeds through the fiber 3 like in a common fiber optical gyro. When it later comes back to the beam-splitters 18 half the power is coupled into channel 2 (the lower in the figure) while the rest remains in channel 1.
• phase modulators 17 e.g. triangle waves is applied (sinus waves are also possible, yet with less efficiency).
• the period will correspond to the double rota ⁇ tion period in the fiber coil or odd multiples of this and the modulations, the both channels between themselves will be in counter phase. This can also be achived in a way that the coupling is in phase but that the fiber coil in one of the channels is turned half a revolution in comparison to the other.
• the modulation depth is determed by the wave form, but will, in the case of triangle waves be an integer multiple of ⁇ r/2.
• This time multiplexed two-channel system together with the triangular modulation according to the present inven ⁇ tion makes it possible to achieve a true serrodyne modu ⁇ lation, being the phase difference between the beams carry ⁇ ing the Sagnac-shift after demultiplexing in addition is saw-tooth modulated.
• a saw-tooth modulation is, if it has proper modulation depth, equivalent with a frequency shift corresponding to the frequency of the saw-tooth modulation.
• Our signal from the detector will then have the form ( ⁇ m t+ ⁇ s ) where ⁇ m is the modulation frequency. From the expression it is clear that the modulation system has transformed the Sagnac-shift from the optical frequency domain and down to the frequency of the modulation which, makes it possible to directly measure the phase shift with a phase meter.

Landscapes

• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Gyroscopes (AREA)
• Optics & Photonics (AREA)
• Electromagnetism (AREA)
• Power Engineering (AREA)
• General Physics & Mathematics (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=20375372

Family Applications (1)

Country Status (3)

Cited By (1)

Families Citing this family (10)

Family Cites Families (2)

• 1989
  • 1989-03-16 SE SE8900942A patent/SE8900942D0/xx unknown
• 1990
  • 1990-03-16 WO PCT/SE1990/000172 patent/WO1990010843A1/en not_active Ceased
  • 1990-03-16 EP EP19900905717 patent/EP0434775A1/de not_active Withdrawn

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events