Patent Application
20080219619
1. Field of the Invention
Embodiments of the present invention relate generally to optical communication systems and, more particularly, to a fiber lens assembly and an optical device having a fiber lens assembly.
2. Description of the Related Art
In optical fiber-based communications systems, optic fibers are coupled to various signal processing devices for switching, attenuation, and broadcasting functions. These devices include wavelength selective switches (WSSs), optical add-drop multiplexers (OADMs), dynamic gain equalizers (DGEs) and wavelength selective routers, among others.
An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, where the core makes up the light-transmitting portion of the fiber and is generally on the order of about 10 micrometers in diameter. Relative to other optical components of a WSS, OADM, or DGE, which are generally free-space optical components, the fiber core is orders of magnitude smaller. Because the fiber core has such a small diameter, precise placement of the fiber in relation to the other optical elements of a device is necessary for the fiber to be optically coupled thereto. For example, if the optical fiber for an output port is displaced by only a few micrometers from the optical axis of an outgoing light beam, a substantial portion of the outgoing light beam will not be incident on the core of the fiber, resulting in significant signal loss. In fact, if the displacement between the fiber and the optical axis of the outgoing light beam is greater the radius of the fiber core, none of outgoing signal will reach the output fiber. Therefore, even a very small positional displacement of an optical fiber, e.g., five micrometers, can produce serious signal loss. Misalignment between the optical axis of an outgoing light beam and a fiber may occur when the fiber is initially coupled to an optical device, or as a result of drift. Drift is generally caused by the small relative motion between the components of an optical switching device that occurs due to thermal expansion and/or contraction of the optical switching device during operation.
Another source of signal loss in optical switching devices is the angular displacement between the ideal alignment of a fiber coupled to an optical switching device and the actual alignment of the fiber. As with positional displacement, angular displacement may be introduced during installation of the fiber and/or via thermal expansion/contraction of the optical switching device during operation. Collimated light beams, which are generally used in WSSs, OADMs, and other optical switching devices, are particularly sensitive to angular misalignment issues.
Fiber lens assembly 100B is substantially identical to fiber lens assembly 110A, except that fiber lens assembly 100B produces a collimated light beam 110B, which is centered on optical axis 103B. Optical axis 103B has an angular displacement θ from ideal optical axis 103, where angular displacement θ may be caused by imprecise installation of fiber lens assembly 100B, or by mechanical drift, thermal expansion, etc. As shown in
Angular misalignment and/or drift of optical elements can also produce high losses at an output port of an optical switching device.
Accordingly, there is a need in the art for a robust means of coupling an optical fiber to signal processing devices used in communications networks that is less sensitive to positional and angular alignment over prior art means.
Embodiments of the present invention generally provide a fiber lens assembly configured to optically couple an optical fiber to a wavelength division multiplexing (WDM) signal processing device having free-space optical elements.
In a first embodiment, a fiber lens assembly comprises an optical fiber for receiving a WDM signal and a diverging lens proximate the optical fiber, for optically coupling the optical fiber with another optical element, the diverging lens having the same optical axis as the optical fiber, a diameter that is greater than a diameter of the optical fiber, and a focal length that is no more than two orders of magnitude greater than the diameter of the optical fiber. The fiber lens assembly may be implemented in an input port for a WDM signal processing device, and the diameter of the diverging lens may be about 2 to 6 times greater than the diameter of the optical fiber.
In another embodiment, an optical device for processing a WDM signal comprises a light dispersing element for separating the WDM signal into its wavelength components and combining wavelength components of a WDM signal into the WDM signal, and a fiber lens assembly with an optical fiber and a diverging lens proximate the optical fiber, wherein the diverging lens is configured and positioned to optically couple the optical fiber and the light dispersing element.
In another embodiment, a wavelength selective switch comprises input and output ports, each including a fiber lens assembly with an optical fiber and a diverging lens proximate the optical fiber, a light dispersing element optically coupled to the input and output ports, and an optical switch. The optical switch receives wavelength components of an input light beam from the light dispersing element, alters the optical paths of the wavelength components of the input light beam, and directs the wavelength components of the input light beam to the light dispersing element after their optical paths have been altered.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
For clarity, identical reference numerals have been used, where applicable, to designate identical elements that are common between figures. It is contemplated that features of one embodiment may be incorporated in other embodiments without further recitation.
Embodiments of the invention contemplate a fiber lens assembly configured to optically couple an optical fiber to a signal processing device having free-space optical elements. Sensitivity of the fiber lens assembly to angular misalignment and positional displacement is reduced by optically coupling the fiber to the signal processing device using a diverging lens rather than a collimating lens, and by optimizing the focal length of the fiber lens assembly.
In operation, fiber lens assembly 200 optically couples core 201 with one or more free-space optical elements of a signal processing device contained in an optical communications network, such as a WSS, OADM, or DGE. Fiber lens assembly 200 may serve as an input port or an output port for such a device. As an input port, fiber lens assembly 200 converts the optical signal contained in core 201 into a free-space divergent light beam for subsequent processing, and as an output port, fiber lens assembly 200 directs a free-space convergent light beam into core 201.
If angular misalignment 0224 is present between fiber lens assembly 200 and ideal optical axis 223, then fiber lens assembly 200 directs a divergent light beam 224 toward image plane 222, where divergent beam 224 has angular displacement θ224 from divergent light beam 220, as shown. In this case, divergent light beam 224 intersects with image plane 222 at a region 225 that is offset from ideal target region 221 by distance 227. Region 225 does not entirely coincide with ideal target region 221, therefore some signal loss occurs. However, since divergent light beam 224 is not a collimated light beam, signal loss is not a strong function of angular displacement. For example, even if angular displacement θ224 is equal to divergence angle θ220, a significant portion of divergent light beam 224 will be coincident with ideal target region 221. In addition, because divergent beam 224 is not a collimated beam, signal loss stemming from angular misalignment of fiber lens assembly 200 does not increase when distance 226 between fiber lens assembly 200 and image plane 222 increases.
In addition, because diameter D2 of lens element 203 is significantly larger than diameter D1 of core 201, fiber lens assembly 200 is also relatively insensitive to positional displacement. For example, if convergent light beam 230 is vertically displaced 10 micrometers relative to ideal optical axis 231, the majority of the beam will still be inserted into core 201. In contrast, for a prior art fiber lens assembly, in which the diameter of the lens element is approximately the same as that of the fiber core, a 10 micrometer displacement of convergent light beam 230 will result in 100% insertion loss, since the beam will completely miss the lens element.
Because WSS 300 is configured to perform 1×4 routing of WDM wavelength channels, optical input/output assembly 360 optically couples a single WDM optical input signal from a communications network to WSS 300 and four optical output signals from WSS 300 back to the communications network. Optical input/output assembly 360 includes multiple fiber lens assemblies to perform the optical coupling between WSS 300 and the communications network. Optical input/output assembly 360 includes one input port 301 and four output ports 302A-D, where input port 301 and output ports 302A-D are fiber lens assemblies substantially similar to fiber lens assembly 200, described above in conjunction with
Base unit 310 is made up of a plurality of free-space optical elements, including a folding mirror 313, beam steering unit 314, cylindrical lenses 315 and 316, a light dispersing element 317, a folding mirror 322, cylindrical lens 321, a focusing lens 323, and a beam polarization unit 331. Light dispersing element 317 may be a diffraction grating or an arrayed waveguide grating. Base unit 310 optically couples optical input/output assembly 360 with switching optics assembly 330. In directing inbound light beams from input port 301 to switching optics assembly 330, base unit 310 shapes, separates, and focuses inbound light beams onto beam polarization unit 331. In directing outbound light beams back to optical input/output assembly 360, base unit 310 recombines, shapes, and directs outbound light beams to one or more of optical output ports 302A-D contained in optical input/output assembly 360.
Switching optics assembly 330 performs switching, attenuation, or other desired functions on an optical input signal, and includes an LC-based beam polarization unit 331, collimating lenses 332, 333, a beam steering unit 334, collimating lenses 335, 336, and an LC-based beam displacer assembly 337. Beam steering unit 334 may be a birefringent crystal or a Wollaston prism. The elements of switching optics assembly 330 are optically linked to enable the optical routing of a WDM optical input signal entering optical input port 301 to any one of the optical output ports 302A-D. The optical routing is performed by conditioning (via LC polarization) and vertically displacing inbound beams to produce outbound beams having a desired vertical position and polarization state. Switching optics assembly 330 selectively determines the vertical displacement of the outbound beams to correspond to the vertical position of the desired output port, i.e., optical output port 302A, 302B, 302C, or 302D, hence performing a 1×4 optical switching operation.
Because WSS 300 is optically coupled to a communications network with one or more fiber lens assemblies according to embodiments of the invention, signal loss associated with such unwanted positional displacement is significantly reduced or eliminated. In addition, by incorporating multiple fiber lens assemblies into a single structure, i.e., optical input/output assembly 360, the effect of positional errors associated with installation of a fiber lens assembly onto WSS 300 can be minimized. Namely, vertical irregularities on planar surface 390 may produce unwanted positional displacement between an input port and an output port. Although planar surface 390 acts as a datum plane, and is therefore assumed to be very flat, it is known in the art that any substrate cannot be perfectly so. Therefore, by incorporating input port 301 and output ports 302A-D into a single assembly, positional errors caused by vertical irregularities on planar surface 390 are minimized. For example, if input port 301 and output port 302A are each mounted in different locations on substrate 303, one may be positioned on a relative high spot on planar surface 390 and the other on a relative low spot on planar surface 390, thereby compounding the variation in flatness of planar surface 390. In so doing, significant vertical positional error may be produced, which may adversely affect the alignment of input port 301 and output port 302A to base unit 310 and to each other.
In a similar fashion, other WDM signal processing devices, such as optical add-drop multiplexers or dynamic gain equalizers, may also benefit when optically coupled to a communications network via one or more fiber lens assemblies, according to embodiments of the invention. In so doing, a WDM signal processing device is less sensitive to precise alignment and positioning of optical input and output ports to the optical axis of the device.
In one embodiment, optical input/output assembly 360 is coupled to a polarization management optics assembly 400, as illustrated schematically in
Polarization management optics assembly 400 includes a first birefringent beam displacer 401, a second birefringent beam displacer 402, a full-sized half-wave plate 403, and a half-sized half-wave plate 404, arranged as shown in
In operation, polarization management optics assembly 400 receives optical input signal 410 from optical input/output assembly 360 on surface 405 of first birefringent beam displacer 401. From surface 405, optical input signal 410 is split into a first component 410A and a second component 410B, which are initially s- and p-polarized, respectively, as indicated. First birefringent beam displacer 401 is oriented with respect to optical input signal 410 to direct s-polarized light, e.g., first component 410A, along an unaltered path and p-polarized light, e.g., second component 410B, along an angularly displaced path. Upon exiting first birefringent beam displacer 401, first component 410A and second component 410B return to parallel paths. Full-sized half-wave plate 403 the rotates the polarization state of first component 410A and second component 410B, so that first component 410A is p-polarized prior to entering second birefringent beam displacer 402 and second component 410B is s-polarized prior to entering second birefringent beam displacer 402. Thus, first component 410A follows an angularly displaced path through second birefringent beam displacer 402 and second component 410B passes directly through second birefringent beam displacer 402 without undergoing angular displacement. Upon exiting second birefringent beam displacer 402, first component 410A and second component 410B once again return to parallel paths. First component 410A then passes through half-sized half-wave plate 404 and is changed from p-polarized to s-polarized light. In this way, optical input signal 410, which has an arbitrary polarization state, is converted into two closely space, parallel light beams, i.e., first component 410A and second component 410B, having a single, known polarization state, i.e., s-polarized. Because the two parallel beams are closely spaced, the various active and passive optical elements of WSS 300 may treat these two beams as essentially one beam throughout WSS 300.
It is contemplated that optical input/output assembly 360 may only include second birefringent beam displacer 402 and half-sized half-wave plate 404 to convert optical input signal 410 into two closely spaced, parallel light beams. However, in such a configuration, the optical paths of first component 410A and second component 410B have different lengths, and may cause one or more of the following: polarization mode dispersion (PMD), polarization dependent losses (PDL), insertion loss due to optical aberration, and a decrease in the bandpass of the LC-based optical switching components of WSS 300 located “downstream” of polarization management optics assembly 400. Thus, full-sized half-wave plate 403 and first birefringent beam displacer 401 are included in polarization management optics assembly 400 so that the optical paths of first component 410A and second component 410B have substantially identical lengths.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/893,872, filed Mar. 8, 2007, entitled “Wavelength Selective Liquid Crystal Switch,” the entire contents of which are incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 60893872 | Mar 2007 | US |
