4.3 Heimann (PDF, 1.4 MB)

Optical Gyroscope for Navigation Grade
Inertial Measurement Units for
Microsatellites at Low Earth Orbit
Marcus Heimann1, Norbert Arndt-Staufenbiel1, Karsten Hanbuch3, Benny
Hille3, Stephan Stoltz3, Michael Scheiding3, Henning Schröder2,
Klaus-Dieter Lang1
1Techical
University of Berlin, 13355 Berlin, Germany
2Fraunhofer
3Astro-
Institute for Reliability and Microintegration, 13355 Berlin, Germany
und Feinwerktechnik Adlershof GmbH, 12489 Berlin, Germany
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Outline

Application for Fiber Optic Gyroscopes

Sagnac Effect and Gyroscope classification

Interferometric Fiber Optic Gyroscopes

Optical components for Fiber Optic Gyroscopes

All-Polarization-Maintaining Fiber Optic Gyroscope

Summary / Outlook
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Applications of Gyroscopes

Gyroscopes for aerospace navigation
Microsatellite TET-1
(Source: Astrofein)
Picosatellite BeeSAT
(Source: TU Berlin)
Aircraft
Telecommunications satellite
(Source: EADS)
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Sagnac Effect
Static Condition:
Start
End
0,5
Output Signal
CW
Amplitude
Amplitude
0,5
Amplitude
Input Signals
time
-0,5
1
0,5
resulting
interference signal
time
CCW
-0,5
time
-1
-0,5
Rotation Condition:
Start
Input Signals
End
0,5
CW
Amplitude
Amplitude
Amplitude
Output Signal
0,5
Ω
time
-0,5
1
0,9
resulting
interference signal
0,5
time
CCW
∆Φ
-0,9
-1
time
-0,5
 Rotation results in a phase shift between counter
propagation waves
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4𝜋 ∙ 𝐿 ∙ 𝑎
∆Φ =
⋅Ω
𝑐∙𝜆
Gyroscope classification
Classification according to bias drift:
 Rate grade:
10 – 10000 deg/h
 Tactical grade:
0,01 – 10 deg/h
 Navigation grade: <0,01 deg/h
Requirements / Constraints for Gyros in Microsatellites at low Earth orbit:
S pecifications of IFOG
Measurement range
±10 °/s
Angular random walk (ARW)
<0.005 °/√h per channel
Bias drift
<0.01 °/h per channel
Scale factor
<10 ppm per axis
S pecifications es pecially for Micros atellites
Dimension of 3-axis Gyro
700 cm³
Total mass
1 kg
Electrical power consumption
8W
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Setup of Interferometric Fiber Optic Gyroscopes (IFOG)
Standard Configuration of an IFOG:
source
modulator
2x1
coupler
1x2
coupler
fiber-coil
detector
modulator
Requirements / Constraints for Gyros in Microsatellites at low Earth orbit:
S pecifications of IFOG
Measurement range
±10 °/s
Negative Effects:
Angular random walk (ARW)
<0.005 °/√h per channel
Bias drift
<0.01 °/h per channel
Kerr Effect, Faraday Effect,
Shupe Effect, …
Scale factor
<10 ppm per axis
S pecifications es pecially for Micros atellites
Env ironm ental conditions
Dimension of 3-axis Gyro
700 cm³
Temperature Range
-40 °C … +80 °C
Total mass
1 kg
Radiation dose
50 krad
Electrical power consumption
8W
lifetime
7 years
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Light Source: Super Luminescence Diode (SLD)
Properties of SLDs:

Small coherence length (avoid noise effects due to optical nonlinearities)

Broadband spectrum compared to a laser (around 50 nm)

Relative high optical output power (up to 40 mW)

Show wavelength and power drift over temperature

High degree of polarisation (DOP 95%)

Inherent reinforcement of SLD due to back
reflection from optical path
Disadvantages
to be solved
Alternative EDFS (Erbium-Doped Fiber Source):

Higher stability of wavelength and optical output power

Less compact package compared to SLDs

Disadvantage under radiation due to their dopants (degradation over time)
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SLD: Wavelength drift and power drift over temperature
Peak Wavelength in nm
Peak wavelength vs. temperature:
 A peak wavelength drift of 0.4316 nm/°C
is measured in a temperature range
between 10 °C to 35 °C
Temperature in °C
at 30 °C
at 25 °C
at 20 °C
Relative Intensity in %
wavelength and power drift
due to temperature:
 Both effects occur at
the same time
DL-CS5403A from Denslight was characterized
Wavelength in nm
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SLD: Compensation of Wavelength drift and power drift
over temperature
Monitoring of optical output power:

a monitor photodiode is required to measure the output power of the SLD continuously

the photo current is used to adjust the current controller of the SLD

such configurations are available in DIL or Butterfly Packages
Monitoring of wavelength drift:

Fiber Bragg Gratings (FBG) in combination with additional monitor photodiode are used

a differential measurement of the intensity is performed by using two FBGs with two PDs
arranged around the center wavelength of the SLD
@ 30 °C
FBG 1
FBG 2
FBG 1
Relative Intensity in %
Relative Intensity in %
@ 20 °C
Wavelength in nm
Wavelength in nm
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FBG 2
9
SLD: Degree of Polarization and Back reflection
Reduction of the Degree of Polarization:

SLD have a DOP of around 95 %, if SM-Fiber is used in the gyroscope system it will lead to
optical power drifts and instability of the system

Fiber Depolarizer can be used to reduce the DOP on a SLD down to 2 % (length 2100 mm)
Reduction of Back reflection:

Inherent reinforcement of SLD due to back reflection from signals of the optical path

Are significantly reduced by using fiber coupled optical isolators
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Fiber-Coil with Quadrupole winding
Winding scheme of
quadrupole fiber-coil
Quadrupole winded
Fiber-coil on
aluminium spool
with potting
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Fiber-Coil: thermally induced rotation rate errors
Problem:

High thermal gradients affect accuracy and stability of the fiber-coil
(Reason: the split optical waves propagate in both directions of the fiber, each wave experiences an
alternating refractive index. Their difference in propagation time results in a phase shift which can’t be
separated from the phase shift due to the Sagnac Effect)
Solution:
 the propagation of thermal gradients through the fiber-coil must be simulated and estimated
with live measurable data to eliminate measurement instability
Approach by Friedemann Mohr:

He established calculation of the measured rotation due to thermal gradients Ω𝐸
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Fiber-Coil: Verification of the model
Reconstructed fiber-coil
from experiment
(entire coil)
Reconstructed fiber-coil
from experiment
(one slide of the coil)
quadrupole winding
cylindric winding
time in s
rotation rate error in °/h
The approach and measurement results based on Friedemann Mohr‘s Journal article "Thermo
optically induced Bias Drift“ 1996
rotation rate error in °/h

Simulations and
Measurements show a
significant reduction of
rotation rate errors when
using quadrupole winding
(already known from
literature)

Simulation model fit to
measurements of F. Mohr
quadrupole winding
time in s
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
13
Fiber-Coil: prediction and regression of thermal gradients
Live temperature measurement regression model :
T1
T4
T2
rotation rate error in °/h
winded
fiber-coil
support
spole
time in s
T3
section of wound fiber-coil
with temperature sensors
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rotation rate error of the fiber-coil:
simulated (blue),
multiple linear regression (red)
and difference (green)
Extended configuration of an IFOG with “All-PM-Fiber”
Isolator to avoid back reflections
FBG with monitor
photodiode
1x2
coupler
source
isolator
1x2
coupler
FBG with monitor
photodiode
MIOC
modulator
1x2
coupler
2x1
coupler
fiber-coil
detector
modulator
Prediction and Regression of thermal induced
rotation rate errors
Monitoring of wavelength and power drift
with FBG and PDs
T1
winded
fiber-coil
support
spole
T4
T2
T3
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Setup of current „All-PM-Fiber“ Gyroscope

All optical components of the system are connected with polarization maintaining fiber

SLD: temperature and output power controlled, Degree of Polarization (DoP) > 93 %

Phase modulator: waveguides in LiNbO3 chip are linear polarizing

Fiber coil: fabricated with polarization maintaining optical bow tie fiber in quadrupole
winding

Minimal disturbing influences from external sources (thermal, mechanical) can effect the
system
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Aufbau des finalen Demonstrators
Y-coupler
Isolator
SLD
100 mm
photodiode
140 mm
fiber coil
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Summary / Outlook

Optical components for system stabilization
 Fiber depolarizer
 Wavelength monitoring
 FEM Simulation of quadrupole winded fiber coil
=> stabilization with temperature sensors for mathematical regression
model for compensation of thermo-mechanical impact

„All-PM-Fiber“ achieved following system parameters:
Param eter
target v alue
achiev ed v alue
Measurement range
±10 °/s
±12 °/s
Angular random walk (ARW)
<0.005 °/√h per channel
<0.001 °/√h per channel
Bias drift
<0.01 °/h per channel
=0.01 °/h per channel
 Implementation of several mechanism for system stabilization of “All-PMFiber” Gyroscope
 low-cost „All-Singlemode-Fiber“ Gyroscope
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Thank you for your attention
Project:
Gustav-Meyer-Allee 25
integrated optical gyroscope
13355 Berlin
Founded by:
Germany
Contact:
Marcus Heimann
Phone +49 (0)30 464 03-743
[email protected]
Cooperation of:
Stephan Stoltz
Fraunhofer IZM and Astro- und
Feinwerktechnik Adlershof GmbH
Phone +49 (0)30 6392-1066
© Fraunhofer IZM, Marcus Heimann
[email protected]
System Integration & Interconnection Technologies
[email protected]