Relative intensity noise controller with maximum gain at...

Details Relative intensity noise controller with maximum gain at... Relative intensity noise controller with maximum gain at...

2002-01-08

2004-07-20

Turner, Samuel A. (Department: 2877)

Optics: measuring and testing

By light interference

Rotation rate

Reexamination Certificate

active

06765678

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to fiber optic gyroscopes, and methods of making and operating fiber optic gyroscopes.
2. Description of the Related Art
Fiber-optic gyroscopes are included in a powerful class of sensors which bring to measurement systems many of the advantages that optical-fiber technology has brought to communications systems. For example, the very high bandwidth of optical fibers used in fiber optic gyroscopes allows the fiber optic gyroscope to convey a large amount of measurement information through a single fiber. In addition, because optical fiber is a dielectric, it is not subject to interference from electromagnetic waves that might be present in the sensing environment. Furthermore, fiber-optic gyroscopes typically can function under adverse conditions of temperature and pressure, and toxic or corrosive atmospheres that generally erode metals at a rapid rate.
The problem inherent in many conventional fiber optic gyroscopes, however, is that they can be sensitive to excess noise disturbances at low rotation rates. For example, the well known Raleigh scattering (i.e., scattering of light due to inhomogeneities in material density smaller than a wavelength in size), polarization noise (i.e., polarization fluctuations observed via voltage fluctuations), and zero rotation drift due to the Kerr effect (i.e., the development of birefringence when an isotropic transparent substance is placed in an electrical field) are typical problems which often reduce the accuracy of the optical gyroscope output by introducing errors in rotation rate sensing.
To minimize the errors in rotation rate sensing resulting from excess noise disturbances, fiber optic gyroscope system designers typically use broadband optical sources in gyroscope system construction. More particularly, fiber optic gyroscope designers typically use broadband optical sources with a stable spectra, such as, for example, super luminescent diodes (SLDs) or super luminescent fiber sources (SLSs), etc. The downside to using these sources, however, is that due to their finite bandwidth, these broadband sources introduce an additional excess noise term into the gyro output. This, in turn, causes a reduction in performance and satisfaction of the fiber optic gyroscope systems. It is, therefore, desirable to eliminate the excess noise component introduced by the broadband source in the gyro output to achieve optimum gyroscope performance.
Unfortunately, where SLDs are implemented in fiber optic gyroscopes, the fiber optic gyroscope generally suffers from a high wavelength sensitivity to temperature, inefficient coupling to single-mode fibers, and a lack of immunity to optical feedback. Consequently, in recent years, fiber optic gyroscope designers have focused less on fiber optic gyroscope systems using SLDs and more on super luminescent fiber sources (SLSs), which do not typically exhibit most of the problems inherent in the SLDs.
For example, where SLSs are used the fiber optic gyroscope will have a more stable response over temperature ranges inside the SLS spectrum. That is, the temperature stability of the SLS spectrum (in particular, its center wavelength) is far superior to that of the SLDs, whose emission wavelength typically varies by about 0.05 nm/deg C. Furthermore, by designing with SLSs, the fiber optic gyroscope designer is capable of generating more power in an SLS than is available in an SLD. For example, in a typical SLD, the available power is approximately 30 mW, of which probably no more than a few milliwatts can be coupled to a single mode fiber. On the other hand, where a typical SLS is used, the fiber optic gyroscope designer is capable of generating approximately 40 mW to 200 mW of power. Additionally, in a practical system, unwanted spurious reflections from the source/system interface can greatly reduce the power which can be coupled to the system fiber. These reflections can be minimized in the SLS fiber device by splicing the source and system fibers with a fused glass-to-glass splice, which typically can not be realized with SLDs. Finally, the high conversion efficiency of the SLS fiber source and its broad character pump band make SLSs a beneficial choice over SLDs for many compact, laser-diode-pumped configurations.
Super luminescent fiber sources (SLSs) typically consist of a single-mode fiber with a core doped with an ionictrivalent rare earth element, such as model HG980 from Lucent Technologies in Chesterfield, Mo., a pump laser such as FLD148G3NL-S from Fujitsu of Japan, and a wavelength division multiplexer (WDM) such as model WS1415-LW from JDS Uniphase in Bloomfield, Conn. SLS's are well known in the art, and have been advantageously used to provide broadband (e.g., on the order of 10-30 nanometers) laser-like (highly directional) light beams for multiple applications, particularly in the communications field. For a description of an exemplary super luminescent fiber source, see “Amplification of Spontaneous Emission in Erbium-Doped Single-Mode Fibers,” by Emmanuel Desuvrie and J. R. Simpson, published by IEEE, in “Journal of Lightwave Technology,” Vol. 7, No. 5, May 1989, incorporated herein by reference.
As noted, an SLS typically includes a length of single-mode fiber, with a core doped with an ionic trivalent rare earth element. For example, neodymium (Nd3+) and erbium (Er3+) are rare earth elements that may be used to dope the core of a single-mode fiber so that the core acts as a laser medium. During operation, the fiber receives a pump input signal at one end, which is provided by a pump laser. The pump input signal is typically a laser signal having a specific wavelength &lgr;p. The ions within the fiber core absorb the input laser radiation at wavelength &lgr;p so that electrons in the outer shells of these ions are excited to a higher energy state of the ions. When a sufficient pump power is input into the end of the fiber, a population inversion is created (i.e., more electrons within the ions are in the excited state than are in the ground state), and a significant amount of fluorescence builds up along the fiber in both directions. As is well known, the fluorescence (i.e., the emission of photons at a different wavelength &lgr;s) is due to the spontaneous return of electrons from the excited state to the ground state so that a photon at a wavelength &lgr;s is emitted during the transition from the excited state to the ground state. The light which is emitted at the wavelength &lgr;s from the fiber is highly directional light, as in conventional laser light. One main characteristic of this emission which makes it different from that of a traditional laser (i.e., one which incorporates an optical resonator), however, is that the spectral content of the light emitted from the super luminescent fiber sources is generally very broad (between 1 and 30 nanometers). Thus, the optical signal output by the fiber will typically be at wavelength &lgr;p +/− about 15 nanometers.
The construction and operation of conventional fiber optic gyroscopes is well known, and as such, will not be discussed in detail. A typical discussion of fiber optic gyroscopes may be found in U.S. Pat. No. 5,465,149 issued Nov. 7, 1995 to Strandjord, et al., and incorporated by reference herein. For illustrative purposes,
FIG. 1
illustrates an exemplary fiber optic gyroscope system
100
, which may be found in the prior art. In general, the optical portion of the system
100
contains several features along the optical paths to assure that this system is reciprocal. That is, when considering the system, substantially identical optical paths occur for each of the opposite direction propagating electromagnetic waves except for the specific introductions of non-reciprocal phase difference shifts, as is described below. In general, the features along the optical paths include a fiber optic light source
112
, a fiber (tap) coupler
116
, a multifunctional processing chip (e.g., integrated optics chip)
120
, and a fi

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