Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-06-22
2002-08-20
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S031000
Reexamination Certificate
active
06438290
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods of and apparatus for connecting optical fibers. In particular, it relates to lenses to couple light from one fiber into another, and to the design of such lenses so as to permit improved packaging of optical fibers, lenses, and related components.
BACKGROUND OF THE INVENTION
In optical fiber applications, it is often necessary to couple light from one fiber into another. This might be done at a switching facility where multiple fibers are brought together. A known way to do this is by directly butting the fibers together. The fibers can also be joined by electrical fusion. In this method, an electric arc is used to heat the end of the two fibers as they are brought into contact. The arc melts the fibers, causing them to join in a permanent and mechanically stable joint. It is also possible to use lenses to couple the light from one fiber into another, as described in U.S. Pat. No. 4,421,383, which describes lenses and a physical connector that holds the fiber and lenses in appropriate positions.
In many applications, it is desirable to perform processing or manipulation of the light after the light exits the source fiber and before it enters the receiving fiber. Examples of this processing include attenuation and filtering. In optical communication systems that utilize multiple wavelengths on one fiber, referred to as wavelength division multiplexing, an erbium-doped fiber amplifier is used to optically amplify the optical signal in the fiber over a broad wavelength range. Since each wavelength in a wavelength division multiplexed system comes from a different source, the signal power at each wavelength may need to be adjusted for optimum operation of the optical amplifier. The adjustment of the signal power requires variable optical attenuation of the optical signal, and this attenuation is often most easily performed on an expanded beam.
Processing of the optical signal between fibers is most easily performed if the optical beam from the fiber has been expanded and collimated.
FIG. 1
shows an example of a pair of conventional collimating lenses
10
and
12
being used to couple light from a source fiber
14
into a receiving fiber
16
. It is known in the art that gradient index lenses are commonly used for this application. Gradient index lenses are made by diffusing a dopant into a cylindrical glass body. The dopant produces a radial gradient in the refractive index of the lens. If the refractive index is less towards the periphery of the lens, then the lens will focus light from a distant source. The shape of the refractive index profile controls the imaging properties of the lens. After diffusion, the lenses are cut to a specific length and the ends are polished. When the light is collimated between the lenses, the beam stays nearly the same size over an appreciable distance “D” (typically 10's of millimeters). Since the beam is nearly the same size in this space, it is easier to put additional optical components that either attenuate or filter the beam, such as for example optical modulator “M” shown in FIG.
2
.
In systems involving the processing of optical signals, it is desirable to maintain as much signal power as possible when coupling the optical signal from one fiber to another. For the case of single mode optical fiber, the coupling efficiency can be computed by analytical methods. (See R. E. Wagner and J. Tomlinson, “Coupling efficiency of optics in single-mode fiber components,” Applied Optics, vol. 21, No. 15, 1982, pg 2671). For the case of coupling light from one fiber to the other, the lenses must be of a specific optical function in order to produce high coupling efficiency. Second collimating lens
12
produces a focused beam that is directed towards receiving fiber
16
. The percentage of light coupled into the receiving fiber will be reduced by any aberrations in the focused beam. Loss of optical power in a fiber system is highly undesirable, as it can limit the amount of information that can be transferred over a communication channel.
Recently, more optical fiber based communication systems utilize multiple wavelengths at one time in order to increase the quantity of information carried. The general concept of using multiple wavelengths is referred to as wavelength division multiplexing. Wavelength division multiplexing systems require a method to separate out signals of different wavelengths present in the optical fiber. This can be done by a method as shown in
FIG. 3. A
source fiber
18
is located near the front focal plane of a collimating lens
20
. Light from the source fiber is collimated by collimating lens
20
and directed at an optical filter
22
. A coating of the optical filter is constructed to reflect all light except that light in a very narrow wavelength band centered around a desired wavelength. Light that passes through filter
22
is coupled into a receiving fiber
24
. If filter
22
is aligned correctly, light reflected from the filter will be directed onto the end of a second receiving fiber
26
. Note that fibers
18
,
24
, and
26
are located off the optical axis of the system.
To achieve high coupling efficiency of a beam into an optical fiber, it is not sufficient that the beam be focused onto the fiber with a low amount of aberration. More specifically, the focused beam must match the fundamental mode of the fiber. This requires that the beam be of the same amplitude and phase of the fiber mode. To match the phase distribution of the fiber, the beam should enter the fiber along the optical axis of the fiber, or additional loss will result. If we assume that the end face of the fiber is perpendicular to the optical axis of the fiber, then the beam must be perpendicular to the fiber for the highest coupling efficiency. For a normal imaging system, the condition of a beam being parallel to an axis of the system is referred to as telecentricity. More specifically, telecentricity in a normal imaging system requires that the chief ray, which is the ray traveling through the center of the stop, be parallel to the optical axis at some point in the system. An optical system may be telecentric at different portions of the optical system. If the chief ray is parallel to the optical axis in object space, one would consider the system to be telecentric in object space. If the chief ray is parallel to the optical axis in image space, one would consider the system to be telecentric in image space. For example,
FIG. 4
shows a simplified system of a lens
28
and a stop
30
wherein the system is telecentric in object space.
FIG. 5
shows a similar system of a lens
28
′ and a stop
30
′ that is telecentric in image space.
Due to the nature of the fiber source, the beam coming from an optical fiber would normally be considered to be telecentric in object space, as that beam emerges from the fiber parallel to the optical axis. It is a desirable feature of the optical system for coupling fibers that the light is also telecentric in image space, in order to achieve the highest coupling efficiency of light into the receiving fiber, which is located in image space. If the light enters the optical fiber at a substantial angle to the axis of the optical fiber, then the coupling efficiency of the beam into the fiber will be significantly reduced. Although it may be possible to tilt fibers from the optical axis in order to reduce the effective angle between a beam and the optical axis of the fiber, tilting fibers can greatly increase the time and cost of assembling the final optical system. The conditions of telecentricity are affected by the location and type of the optical elements, and the location of the stop.
For systems used to couple light from one fiber to another, it is not desirable to have any apertures that limit the beam and thereby reduce optical power. Hence there is often no defined aperture or stop limiting the beam. When there is no physical aperture limiting the beam, telecentricity is determined by the characteristics of the source and receiver
Bietry Joseph R.
Bowen John P.
Ludington Paul D.
Eastman Kodak Company
Healy Brian
Sales Milton S.
Wood Kevin S
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