2.1-

Basic principle

The FOG, like the RLG, is a rotation sensor based on the Sagnac effect [4]. Basically, this effect produces, in a ring interferometer, a phase difference ΔΦ_R proportional to the dot product of the rotation rate vector Ω by the area vector A enclosed by the closed optical path, usually written in the case of a fibre optic coil:

where L is the total coil length, D the mean coil diameter, c the light velocity in a vacuum and Ω_// the rotation rate component parallel to the coil axis.

To understand simply Sagnac effect, it is possible to consider the case of an “ideal” circular path in a vacuum. Light entering the interferometer is divided into two counter-propagating waves which return perfectly in phase after having travelled along the same path in opposite directions. Now, when the interferometer is rotating, an observer at rest in the inertial frame of reference sees the light travelling with the same vacuum velocity c in opposite directions, but during the transit time through the loop, the beamsplitter has moved. Therefore our observer sees that the co-rotating wave has to propagate over more than one complete turn while the counter-rotating wave has to propagate over less than one complete turn. This difference of transit time may be measured by interferometric means.

Sagnac effect in matter is more delicate to explain, but it may be shown that it is a pure temporal delay, completely independent of the indices of refraction or of the guidance conditions and that it keeps the same value as in a vacuum. In a FOG, the light propagates in a fibre optic coil and the sensitivity is enhanced by the use of a multi-turn coil, just as the flux of the magnetic field is enhanced in a multi-turn inductance coil.

To give an order of magnitude, a phase difference of 10^-7 to 10^-8 radian should be measurable through time averaging, while the absolute phase accumulated by the wave along 100 m to 1 km is 10⁹ to 10¹⁰ radians. Consequently, the sensitivity limit is 16 to 18 orders of magnitude below the total propagation path. Such a performance could look unrealistic when the temperature dependence is already on the order of 10^-5/K but the fundamental principle of reciprocity of light propagation in a linear medium is the key solution. The non-reciprocal Sagnac effect is a very small first order effect which should be buried in the changes of the zero order, the absolute phase accumulated in the propagation, but single-mode guided propagation and reciprocal configuration provide perfect “common-mode rejection” of this absolute phase.

2.2-

Optimal architecture and signal processing technique

After more than twenty years of research and development, the FOG technology has reached its maturity and the optimal configuration providing good performances is commonly recognized [5].

The optical architecture (Figure 1) requires:

Figure 1:

optimal architecture of a FOG

The signal processing technique used is as follows:

2.3-

Erbium-doped-fibre source

In order to be able to obtain both high performances and a correct behaviour under radiations, the source of a space FOG should have the following characteristics:

Deriving from the erbium-doped-fibre amplifier (EDFA) technology in the 1550 nm window, which is revolutionizing optical fibre communication, the erbium-doped-fibre source technology is perfectly suitable for this application. It is based on superluminescence, with a doped fibre excited by a pump laser diode. This superluminescence is also called ASE for Amplified (by stimulated emission) Spontaneous Emission. In addition to a broad spectrum and a high power which improves the signal over noise ratio, it brings an unpolarized emission which is useful to reduce polarization non-reciprocities and allows one to use ordinary (non-polarization maintaining) couplers.

We thus have developed such a source which we now use on all our medium to high performance FOG (Figure 2) [6]. Additionally, this source is wavelength and power stabilized and does not require any Peltier element, which improves the reliability and the overall power consumption.

Figure 2:

erbium-doped-fibre source

Under radiations, the main concerns are that first erbium is not an ideal dopant (problem of codopants) and second that the pumping wavelength being necessarily shorter than the emission one, its radiation induced attenuation (see for instance [7]) is higher, which decreases the conversion efficiency thus the emitted power. Hopefully, two effects counterbalance the radiation induced attenuation: the relaxation and the photobleaching [8]. Those effects are inverse chemical reactions to the one inducing attenuation, the photobleaching requiring additionally a photon, which implies it will occur only with incident light.

As a matter of fact, the development of our source lead to the choice of an erbium-doped-fibre using co-dopants allowing, for low dose rates typically observed in space, a real-time compensation of the induced attenuation by those two effects. The source has been tested up to 150 krad and we demonstrated its ability to function very correctly over its expected lifetime. Largely under-used at the beginning of its life, the source does not show any decrease in emitted power after a typical exposure to 50 krad.

Finally, it is interesting to note that even if in the past these sources might have been considered as a luxurious solution, today, with the very broad development of EDFA technology for communications, they are both cost effective and very reliable. This cost effectiveness is particularly advantageous with multi-axis operation since their high power may be used in a source sharing configuration with no penalty on noise.

3.1-

Creation of Ixsea

Continuously increasing since 1987, the Photonetics activity dedicated to the development of the FOG technology went through many steps. From the first laboratory prototypes demonstrating the validity of the technology, it has reached full production of medium and high performance FOG as well as attitude and navigation systems, the demonstration of very high performance and the development of space FOG. In September, the whole activity was transferred from the Navigation Division of Photonetics to a new independent company created for this purpose: Ixsea. Completely dedicated to the FOG technology, Ixsea will continue to develop its activity in its different fields of interest as a major European supplier of demanding inertial systems.

3.2-

FOG products: medium to very high performance range

The proposed range of standard FOG products concerns the medium performance (0.05 °/h class) with the FOG 90 and the high performance (0.01 °/h class) with the FOG 120, both declined in single or three-axes configuration. The performances are presented below (Table 1).

Table 1:

FOG 90 and FOG 120 performances

They both benefit from the developments achieved since 1993 concerning the hardening against typical military environment (temperature, vibrations, shocks), the reduction of the residual Shupe effect and the erbium-doped-fibre source.

More recently, the SOFIA (Stratospheric Observatory For Infrared Astronomy) project allowed us to demonstrate very high performance. This project, driven by the NASA, aims at the direct use of a telescope aboard a Boeing 747 for stratospheric infrared astronomy. MAN Technologies (Germany), in charge of the telescope development, including pointing and stabilization, chose us to develop the necessary gyroscopes. The requirements were ones of a very high performance FOG, more particularly concerning the random walk. We thus first developed a FOG 180 prototype which fulfilled perfectly the different specifications [9], then three additional identical units to be integrated in the system developed by MAN Technologies. The performances are presented below (Table 2).

Table 2:

FOG 180 performances

The FOG technology presents an intrinsically good bias stability at relatively stabilized temperature (without high temperature transients). As an example, we present below the typical result of an Allan variance analysis performed over a FOG 180 bias measurement at ambient temperature (Figure 3).

Figure 3:

Allan variance analysis of a FOG 180

3.3-

Attitude and navigation systems

A step beyond the “standard” FOG products, we have developed Octans, a strapdown AHRS (Attitude and Heading Reference System) based on a FOG 90 triad. Dedicated to the industrial, scientific and military marine markets, it is available in different models depending on the application: surface (IP 65 or IP 67) or subsea (1000 m, 3000 m or 6000 m). Octans provides, without any external reference, heading with a dynamic accuracy of ± 0.2 ° secant latitude and attitude (roll and pitch) with an accuracy of 0.01 °. Some models are presented below (Picture 1).

Picture 1:

Octans models

Much recently, we developed an evolution of Octans based on a FOG 120 triad, φ-INS, to propose a new strapdown Inertial Navigation System (INS) with a heading accuracy of 1 arc minute.

4.1-

Feasibility study

Beginning of 1996, a CNES contract allowed us to demonstrate in a paper study the feasibility of a FOG for space use. However, this study highlighted the different specific technical issues that would have to be solved: behaviour under radiations and vacuum, improvement of the gluing process reliability and development of a space electronics.

4.2-

Hardening of the optical head

End of 1995, an European invitation to tender from ESA/ESTEC was proposed for the development of a 0.01 °/h class FOG for space use and we were selected among major European manufacturers.

In the 1996-1998 period, we thus developed and manufactured two space FOG demonstrators deriving from the FOG 120 which were tested by the French LRBA. Those demonstrators were composed of an optical head (optic and optoelectronic part) designed for space use, including a purposely developed erbium-doped-fibre source (see § 2.3), and a standard ground electronics.

Beyond the successful demonstration of the specified FOG performances, the behaviour of the optical head was validated for use in typical space environment: vibrations, shocks and radiations (up to 50 krad).

The behaviour under thermal vacuum remained however to be verified.

4.3-

Improvement of the gluing process

In parallel, we benefited from a CNES contract in the 1997-1998 period to improve our gluing process. The results obtained, concerning essentially the behaviour against aging, temperature, humidity, vibrations and shocks, were considered very satisfactory and allowed us to reach a very good reliability level for this critical part of the FOG manufacturing process.

4.4-

Development of a complete one-axis qualification model

At the beginning of 1999, following the first ESA contract, CNES and ESA decided to join their efforts in two contracts running in parallel to continue the support of the space FOG development.

The CNES contract, which ended at the end of 1999, concentrated first on the yet to be verified behaviour under vacuum. It allowed us to successfully clarify this question with the verification of the very good behaviour of the optic and optoelectronic packaged components in vacuum, the study of the outgassing of organic materials used in the optical head and the test in thermal vacuum of one of the two ESA demonstrators. In a second part, we manufactured two new 0.01 °/h class optical heads deriving from the demonstrators ones and benefiting from the recent developments.

On the other hand, the ESA contract, still under progress, concerned first the development of a complete space electronics. This phase now completed, it was then originally planned to manufacture two complete one-axis FOG using the CNES optical heads: a qualification model and a flight model. The first unit, presented below (Picture 2), has been manufactured and entered in qualification in October.

Picture 2:

0.01 °/h class one-axis qualification model

4.5-

Development of a three-axis housing for the flight model

In the meantime, at the beginning of 2000, it was decided to go a step further with the flight model, which would benefit from the development of a three-axis housing under a new CNES contract.

This flight model will be delivered beginning of next year to be used in MAGI, equipped with the remaining real axis and two dummy axes due to a limitation on the available power supply. This CNES attitude measurement experiment should itself fly in 2002 onboard the French-Brazilian Microsatellite (FBM) [10] dedicated partly to technology evaluation. The purpose of the MAGI experiment is to evaluate the behaviour of a complete gyroscopic function during 13 months in orbit (circular LEO at 700 km) and MAGI will measure the angular movements of the satellite around one reference axis. The obtained results will then be compared with the attitude information provided by star sensors on several 90 minutes orbits, which will allow to check the short and medium term bias stability of the FOG in space.

The principal characteristics and specified performances of a complete 0.01 °/h class three-axis space FOG are presented below (respectively Table 3 and Table 4).

Table 3:

0.01 °/h class three-axis space FOG characteristics

Table 4:

0.01 °/h class three-axis space FOG specified performances

OTR: Over the Temperature Range (0/+50 °C full performance).

A first three-axis qualification housing having been successfully qualified in vibrations and shocks in September, the manufacturing of the flight model housing began in mid October, for a planned integration in December.