How Fiber Optic Gyros Work And Other Mysteries · F = Fiber (optical fiber) O = Optic (light based)...

�� 1998 All rights reserved

And Other Mysteries

Copyright Fibersense Technology Corporation 1998, all rights reserved

How Fiber Optic Gyros Work


What is a FOG

FOGs are: Angular rate sensors.– Rugged, solid state gyros.–They use interference of light to measure rotation.

–

Data output is usually an analog or digital signal.

–

FOGFog is visible moisture in the atmosphere. It may also be considered as a ground level cloud. Fog is perceived optically.

F = Fiber (optical fiber)O = Optic (light based)G = Gyro (short for gyroscope - rotation measurement)

How Fiber Optic Gyros Work

What is a FOG


An optical fiber can conduct light in two directions without interaction.

Fiber Coil

LightSource

Detector

Coupler Modulator & Splitter

Five Optical Components:

You may notice that a pipe will not simul- taneously carry water in two directions.

(3)

(2)(4)

(5)

(1)

(1) Light source drives the FOG with constant light.

(2) Modulator & splitter sends light into the fiber coil in counter rotating directions.

(3) The fiber coil is sensitive to rotation and is the heart of the FOG.

(4) Coupler separates returning light to the detector.

(5) Detector - Gyro output.

FOGs are Simple


Waves & InterferenceLight consists of waves.(Each color is a different wave length. Long waves, shown in black are used in FOGs and are invisible to the eye.)

•

Light waves can add together(constructive interference).

•

Light waves can subtract(destructive interference).

•

(1)

(2)

(1)+(2)=(3)

(1)

(2)(1)-(2)=(3)

This works with water which is a good way to demonstrate waves. Drop some pebbles in still water - Have Fun

(3)

(3)

FOGs are Simple

Waves & Interference


When a FOG coil is stationary the light traveling in both the clockwise and counterclockwise directions through the fiber coil travel the same distance, thus the emerging waves are in phase at the detector.

FOG at Rest

Light Source

Detectorcw

ccw

cwccwcw

ccw

cw = Light that transits the spool in a clockwise directionccw = Light that transits the spool in a counterclockwise direction

Ideal gyro without loss.

Transit time for the light around

the coil is typically 0.5 to 5 micro

seconds depending upon coil length.


How a Phase Shift DevelopsLight travels at a constant velocity. (That Einstien relativity stuff.)

The fiber coil is rotating in space.

Light travelling the opposite direction to the coil rotation does not have to travel as far in space before it exits the coil.

The rotation rate and wave length shown are greatly exaggerated. Normal rotation rates result in a fraction of a degree of phase shift between the counter rotating light beams.

Coil rotation

FOG at Rest

How a Phase Shift Develops


Phase Difference

Light Source

Detector

cwccw

cw

ccw

cw

ccw

When a FOG rotates, light traveling through the fiber coil is affected. The effect of rotation is to cause the light travelling in one direction to travel a different distance than light travelling in the opposite direction. During the time it takes for the light to transit the coil, the exit point of the coil will have moved. The two light waves will then have different phases at the detector, changing the amplitude detected.

Rotation

Extra distance traveled by the light traveling the coil cw.

Clockwise Rotation

Position when light leaves the light source

Position when light reaches the detector

Note: Since the speed of light is constant it is distance travelled in space that counts.


Detector Signal

ccwcw

ccwcw

ccwcw

ccwcw

Detector Output (1)

(2)(2A)

(3)

Increasing Angular Rate

(1) FOG at RestCW and CCW light adds.

(2) & (2A) FOG RotatingFOG is rotating sufficiently fast for CW and CCW light to cancel (180 deg phase shift).

(3) FOG RotatingFOG is rotating sufficiently for partial light cancellation.

Clockwise Rotation

Detector Signal


Phase Modulation

Detector OutputChange


Typical operating angular rate range

Phase Modulation

Detector OutputChange


(a) (b)

A simple FOG would provide a usable output only when the input angular rate is very large. For most applications high sensitivity is required at low angular rates.

By introducing phase modulation between the CW and CCW light the detector sensitivity can be greatly increased. In operation modulation is applied to move between operating point (a) and (b) at a high frequency.


Basic Design ConsiderationsFiber Coil Length

Longer coils increase the light transit time, increasing the period over which the phase difference develops.

–Longer coils increase sensitivity and signal to noise ratio.–

Fiber Coil Diameter Larger coil diameter results in greater velocity at the coil periphery increasing the phase difference between the CW and CCW beams for a given angular rate.

–

Larger coil diameter increases sensitivity and signal to noise ratio.–Wavelength

A shorter wave length increases FOG sensitivity.–Optical Fiber operating at long wave lengths have better radiation resistance for space applications.

–Practical Limitations

Larger coils are more prone to thermal and vibration perturbations.–For most applications FOG coils contain 100 to 1,000 meters of Fiber on spools ranging from 20 to 100 mm in diameter.

–Optical components are produced for 850, 1330 & 1550 nm wavelengths. 1330 and 1550 are preferred for space & satellite FOGs.

–

It is the details of the design and manufacture developed over many years that differentiate between an excellent and low accuracy FOGs.

Phase Modulation

Basic Design Considerations

How Fiber Optic Gyros Work And Other Mysteries · F = Fiber (optical fiber) O = Optic (light based)...

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