In optical fiber applications, it is often necessary to couple light from one fiber into another. This might be done at a switching device where multiple fibers are brought together or by adding and/or dropping wavelengths in a dense wavelength division multiplexing (DWDM) application. A known way to do this is by directly butting the fibers together. The fibers can also be joined by electrical fusion, where an electric arc is used to heat the ends of the two fibers as they are brought into contact. The electric 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. Here, a physical connector holds the fiber and lenses in appropriate positions relative to each other.
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, commonly 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.
Additionally, processing of the optical signal between fibers is most easily performed if the optical beam from the fiber has also been collimated. FIG. 1 shows an example of a pair of conventional collimating lenses 16 and 18 being used to couple light from a source fiber 10 into a receiving fiber 20. It is known in the art that gradient index (GRIN) lenses are commonly used for this application. GRIN 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 lower 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 working 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, the optical modulator 17 shown in FIG. 2. The optical system shown in FIG. 2, is known as a transmissive system because the beam transmits through the optical components.
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. Referring to FIG. 2, a second collimating lens 18 produces a focused beam that is directed towards receiving fiber 20. 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 or increase the amount of required amplification.
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 use a method to separate out signals of different wavelengths present in the optical fiber, as shown in FIG. 3. A source fiber 22 is located near the back focal plane of a collimating lens 16. Light from the source fiber is collimated by lens 16 and directed at an optical filter 24. 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 24 is coupled into a receiving fiber 28. If filter 24 is aligned correctly, light reflected from the filter will be directed onto the end of a second receiving fiber 26. Note that fibers 22, 26, and 28 are located off the optical axis of the system. The optical system comprising source fiber 22, collimating lens 16, optical filter 24, and receiving fiber 26 is known as a reflective system, whereas the optical system comprising source filter 22, collimating lens 16, collimating lens 18 and receiving fiber 28 is known as a transmissive 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 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. For a single element optical system, the aperture stop should be located at or near the front or back focal plane of the lens. An optical system may be telecentric at different portions of the optical system. If the chief ray were parallel to the optical axis in object space, one would consider the system to be telecentric in object space. If the chief ray were 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 40 and a stop 42 wherein the system is telecentric in object space. FIG. 5 shows a similar system of a lens 50 and a stop 52 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 of the second collimating lens, 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, or the insertion loss will be increased. 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 location and type of the optical elements, and the location of the aperture stop affect the conditions of telecentricity.
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 receivers in combination with the optical elements. More specifically, if a beam is propagating in the system and it is undesirable to introduce any aperture that would limit the optical beam in any way, then the location of the stop is usually described by the location of where the chief ray crosses the optical axis of the system. The chief ray is defined to be the ray in the center of the beam distribution that is emitted from the source, and hence is not determined by physical apertures in the optical system.
It is known in the art that gradient index (GRIN) lenses can be used to collimate light from optical fibers. Nippon Sheet Glass, Somerset, N.J., makes such lenses. FIG. 6 shows a transmissive optical system using two GRIN collimator lenses. A Gaussian beam emanates from the source fiber 10 and is collimated by GRIN lens 16. The collimated beam 62 is then focused by GRIN lens 18 into receiving fiber 20. The paraxial front focal plane of a lens is located one effective focal length (EFL) from the second principle plane of that lens. The front focal plane 60 of the GRIN lens 16 is located in very close proximity to the front face 64 of that lens. This is because the second principle plane 66 is located inside the GRIN lens. For a reflective system (see FIG. 3) the optical filter 24 should be positioned at the front focal plane 60 of the input collimator lens to achieve maximum coupling efficiency, or minimum insertion loss. The close proximity of the optical filter 24 to the front face 64 of the GRIN lens may be advantageous to assembling a reflective photonic device, such as a DWDM demultiplexer, because the optical filter can be cemented directly to the front face 64 of GRIN lens 16 without incurring excessive insertion loss.
In order to have high coupling efficiency, the focusing lens must not introduce significant aberrations into the beam. For a gradient index lens, the shape of the refractive index profile must be tailored exactly to produce minimal aberrations. The control of the refractive index profile is difficult, since the shape of the profile is controlled only by diffusion of the dopant into the glass. It is a further disadvantage of gradient index lenses that one of the dopants commonly used in the diffusion is thallium. For example, the use of thallium in gradient index lenses is described in U.S. Pat. Nos. 3,941,474 and 4,246,474. Thallium is a toxic metal (even more toxic than lead).
In addition to gradient index glass lenses, previous attempts have used refractive lenses to couple light between fibers, as described in U.S. Pat. No. 4,421,383. However, U.S. Pat. No. 4,421,383 does not disclose the use of aspheric surfaces to improve optical performance, nor is the use of a symmetric bi-aspheric collimator lens discussed. FIG. 7 shows a transmissive optical system using two plano-convex refractive collimator lenses as described in U.S. Pat. No. 6,438,290. A Gaussian beam emanates from the source fiber 10 and is collimated by plano-convex lens 72. The collimated beam 62 is then focused by a second plano-convex lens 74 into receiving fiber 20.
For a plano-convex collimator lens, the front focal plane 82 is also located one effective focal length (EFL) from the second principle plane 80. Because all the optical power is located at surface 78, the second principle plane 80 is located approximately at surface 78. As a result, the front focal plane 82 is located approximately one effective focal length in front of the refractive optical surface 78. FIG. 8 shows a reflective system using a pair of plano-convex collimator lenses. The optical filter 24 must be positioned at the front focal plane 82 of the input collimator lens 72 to achieve maximum reflected light into receiving fiber 26. The relatively large distance from the front refractive surface 78 to the optical filter 24 may be perceived as a disadvantage in a reflective system because of positional changes in the optical filter (or mirror) during temperature changes. Reflected coupling efficiency is defined as the fraction of light that is coupled into the receiving fiber 26, assuming the optical filter (or mirror) 24 is perfect reflector. Reflected insertion loss quantifies the amount of light that is lost in a reflected optical fiber component.
Light that is reflected back into the source fiber from Fresnel reflections is known as return loss or back-reflection. Very small amounts of back-reflected light can cause serious performance degradation in the laser-diode source. To reduce this effect, it is well known in the art to polish an inclined facet on both the fiber and the collimator lens, as well as applying a high-efficiency antireflection coating to the fiber facet and lens surfaces. FIG. 9(a) shows a source fiber 100 with a polished inclined facet 102 and a GRIN lens 104 with a similar inclined facet 106. FIG. 9(b) shows a similar configuration for a plano-convex collimator lens 110 with an inclined facet 108. It is well known that an 8 degree inclined facet on the source fiber and collimator lens will produce acceptably small amounts of back-reflection.
The optimum design of a collimator lens is determined primarily by the index of refraction of the lens. The shape of the collimating lens and the ratio of the each radii of curvature is typically chosen to minimize 3rd order spherical aberrations. For index of refractions less than approximately 1.68, the optimum optical design is a bi-convex lens as shown in FIG. 10(a). The lens 122 focuses a collimated beam 120 down to a focal plane 124. If the index of refraction is approximately 1.68, then a plano-convex lens shape 126 is optimum (FIG. 10(b)). Finally, for index of refraction greater than approximately 1.68, a meniscus lens shape 128 is desired (FIG. 10(c)).
Additional optical wavefront performance can be achieved by using one or two aspheric optical surface. It is time consuming and costly to fabricate aspheric optical surfaces using traditional grinding and polishing. For high volume applications, molding of aspheric surfaces in glass or plastic is desirable. Several companies, for example, Lightpath Technologies and Hoya, manufacture a wide range of glass-molded, bi-aspheric collimator lenses. In each case, the shape of the lens and the ratio of the radii of curvature are typically chosen to maximize optical wavefront performance. Normally a symmetric bi-convex shape would not be chosen to minimize the 3rd order spherical aberrations of a collimator lens.
Eastman Kodak Co. commercially sells two glass-molded, symmetric bi-aspheric lens for use in collimating light from laser diodes that each contain a cover glass. The A-414 lens has a focal length of 3.30 mm, whereas the A-439 has a focal length of 0.71 mm. In both cases, the lenses were not designed to be telecentric.
Additionally, U.S. Pat. No. 5,301,249 describes the use of mirrored systems to couple light from a laser diode into a fiber. However, this patent does not quantitatively describe expected single-mode coupling efficiencies, nor does it describe off-axis performance of the system. As such, there is a need for a lens that can be used in optical fiber collimator assemblies that improve the packaging of these assemblies.