Configurable optical add/drop device

An add/drop device is described that allows channels from a multi-channel optical path to be dropped to a device and a new or modified channel to be added to the multi-channel optical path. The device thereby has access to a channel from the multi-channel optical path without having access to all of the channels. In one embodiment, the add/drop device of the invention includes one or more intermediate ports and a switch. The intermediate ports communicate the channels not dropped by the add/drop device and the switch selectively optically couples the dropped channel either to the device or to be added back into the multi-channel path. The switch also selectively optically couples the new or modified channel to be added to the multi-channel path. The one or more intermediate ports allow multiple add/drop devices to be optically coupled together to provide a configurable add/drop mechanism. The configurable add/drop mechanism allows channels from the multi-channel optical path to be provided to devices without the need of physically adding or removing an add/drop device from the multi-channel optical path.

FIELD OF THE INVENTION
 The invention relates to optical devices. More particularly, the invention
 relates to add/drop devices for optical communications networks.
 BACKGROUND OF THE INVENTION
 Fiber optic networks have the ability to communicate multiple channels of
 information on a single fiber. The ability to communicate multiple
 channels with a single fiber increases the bandwidth of networks and other
 devices including fiber optic networks as compared to communication
 channels that are limited to a single channel. Because each fiber can
 carry multiple channels, routing of channels is more complex than if each
 fiber carries a single channel.
 One routing scheme is to route each channel to each device and allow the
 devices to access the appropriate information. FIG. 1 is a block diagram
 of a simple network with each device having access to each channel of
 information. Network 100 includes devices 110, 130 and 160 that are
 connected by fiber optic communications paths. Paths 120 and 125
 communicate information between device 110 and device 130. Similarly,
 paths 140 and 145 communicate information between device 130 and 160, and
 paths 150 and 155 communicate information between device 110 and device
 160.
 However, for networks having many devices, an interconnection between each
 device can be prohibitively expensive, or even physically impossible. In
 order to provide interconnection of many devices to a fiber optic network,
 add/drop devices have been developed.
 FIG. 2 illustrates an add/drop device. Communications path 200 is a
 multi-channel fiber optic path that is optically coupled to drop filter
 210. Drop filter 210 filters a channel by passing the channel to
 communications path 220 and reflecting the remaining channels to
 communications path 230. The channels passed to communications path 220
 are delivered to device 240 that operates on the received channel.
 Device 240 generates information that is communicated via path 250 to add
 filter 260. Add filter 260 reflects the channels of path 230 and adds the
 channel of path 250 to provide a combination of channels to path 270. In
 this manner device 240 is allowed to access to data on a channel without
 requiring access to all available channels.
 However, the configuration of FIG. 2 is static and must be determined at
 the time of network configuration. Adding and dropping additional channels
 requires physical addition of additional add and drop filters as well as
 splicing into multi-channel paths 200 and 270. What is needed is an
 improved add/drop device.
 SUMMARY OF THE INVENTION
 An optical add/drop device is described. The add/drop device has an input
 port to receive multiple channels of information. A drop filter is
 optically coupled to the input port. The drop filter passes a channel or a
 group of channels and reflects the remaining channels to a first
 intermediate port. An add filter is optically coupled to a second
 intermediate port. The add filter combines the added channel or group of
 channels and reflects the remaining channels to an output port. A switch
 is optically coupled to the drop filter, to the add filter, to an add port
 and to a drop port. The switch selectively optically couples the drop
 filter to the drop port and the add port to the add filter when in a first
 state. The switch selectively optically couples the drop filter to the add
 filter and the add port to the drop port when in a second state. The basic
 add/drop function can be realized by optically linking the first and
 second intermediate ports. In one embodiment, the switch includes a
 diffraction prism to selectively optically couple the ports. In an
 alternative embodiment, the switch includes a mirror to selectively
 optically couple the ports of the add/drop device.
 In one embodiment, multiple add/drop devices are interconnected to allow
 multiple channels or groups of channels to be dropped and added. The input
 ports, output ports, first intermediate ports, and second intermediate
 ports of the multiple add/drop devices are interconnected to allow
 dropping and adding of multiple channels.

DETAILED DESCRIPTION
 An optical add/drop device is described. In the following description, for
 purposes of explanation, numerous specific details are set forth in order
 to provide a thorough understanding of the invention. It will be apparent,
 however, to one skilled in the art that the invention can be practiced
 without these specific details. In other instances, structures and devices
 are shown in block diagram form in order to avoid obscuring the invention.
 Reference in the specification to "one embodiment" or "an embodiment" means
 that a particular feature, structure, or characteristic described in
 connection with the embodiment is included in at least one embodiment of
 the invention. The appearances of the phrase "in one embodiment" in
 various places in the specification are not necessarily all referring to
 the same embodiment.
 The invention allows channels from a multi-channel optical path to be
 dropped to a device and a new or modified channel to be added to the
 multi-channel optical path. The device thereby has access to a channel
 from the multi-channel optical path without having access to all of the
 channels. In one embodiment, the add/drop device of the invention includes
 one or more intermediate ports and a switch. The intermediate ports
 communicate the channels not dropped by the add/drop device and the switch
 selectively optically couples the dropped channel either to the device or
 to be added back into the multi-channel path. The switch also selectively
 optically couples the new or modified channel to be added to the
 multi-channel path.
 The one or more intermediate ports allow multiple add/drop devices to be
 optically coupled together to provide a configurable add/drop mechanism.
 The configurable add/drop mechanism allows channels from the multi-channel
 optical path to be provided to devices without the need of physically
 adding or removing an add/drop device from the multi-channel optical path.
 FIG. 3 is one embodiment of an add/drop device according to one embodiment
 of the invention. Input port 300 is configured to receive a multiple
 channel optical communications device, for example, collimator optically
 coupled to a fiber that communicates multiplexed information corresponding
 to multiple channels.
 Input port 300 is optically coupled to drop filter 310. Drop filter 310 is
 configured to pass a channel received from input port 300. The remaining
 channels are reflected to intermediate port 330, which is also optically
 coupled to drop filter 310. The channel passed (or dropped) by drop filter
 310 is input to a first input port of switch 360. In one embodiment,
 switch 360 is a 2.times.2 optical switch. Various embodiments for switch
 360 are described in greater detail below. Other switching configurations
 can also be used.
 Add port 320 provides an input signal to a second port of switch 360. Add
 port 320 is optically coupled to a device (not shown in FIG. 3) that can
 receive data from and provide data to switch 360. Drop port 370 is a first
 output port of switch 360 and is optically coupled to the device. The
 second output port of switch 360 is optically coupled to add filter 380.
 In one embodiment add filter 380 provides the same filtering functionality
 as drop filter 310. Thus, add filter 380 passes the channel provided by
 switch 360 and reflects the channels provided from intermediate port 340.
 In this manner, add filter 380 adds a channel corresponding to the dropped
 channel to the multiple channels received by intermediate port 340. Output
 port 390 receives both the channels from intermediate port 340 that are
 reflected by add filter 380 and the channel passed by add port 320 that
 adds the channel to the multiple channels from intermediate port 340.
 The following example assumes five input channels; however, any number of
 channels can be supported. Input port 300 receives five channels (channels
 1, 2, 3, 4 and 5), one of which (channel 3) is to be communicated to a
 device optically coupled to add port 320 and drop port 370. Drop filter
 310 passes channel 3 to switch 360 and reflects channels 1, 2, 4 and 5 to
 intermediate port 330. Drop filter 310 can be configured in any manner
 known in the art. As described in greater detail below, intermediate port
 330 and intermediate port 340 can be used to interconnect multiple
 add/drop devices together. If a single add/drop device is used,
 intermediate port 330 is optically coupled to intermediate port 340 (not
 shown in FIG. 3).
 Switch 360 steers channel 3 to either drop port 370 or to add filter 380.
 In one embodiment, if switch 360 is in the cross state channel 3 is looped
 to add filter 380 and added to the channels from intermediate port 340. If
 switch 360 is in the bar state, channel 3 is communicated to drop port
 370. A device, for example, a computer system, is optically coupled to
 drop port 370 to receive channel 3. The device provides a new channel 3 to
 add port 320.
 Add port 320 is optically coupled to add filter 380 when switch 360 is in
 the bar state. Add filter 380 passes the new channel 3 to output port 390.
 The channels provided by intermediate port 340 are reflected by add filter
 380 to output port 390. When switch 360 is in the cross state, the new
 channel 3 from add port 320 is optically coupled to drop port 370. Switch
 360 can also be configured to communicate channel 3 to the device in the
 cross state and to add channel 3 back in the bar state.
 Thus, the add/drop device of FIG. 3 receives channels 1, 2, 3, 4 and 5 at
 input port 300. If configured in a first state the add/drop device
 receives channels 1, 2, 3, 4 and 5 and outputs channels 1, 2, 3', 4 and 5,
 where 3' is a new channel generated by a device optically coupled to the
 add/drop device of FIG. 3. If configured in a second state, the add/drop
 device receives channels 1, 2, 3, 4 and 5, and outputs channels 1, 2, 3, 4
 and 5.
 FIG. 4 is one embodiment of multiple interconnected add/drop devices
 according to one embodiment of the invention. Because each add/drop device
 of FIG. 4 includes two intermediate ports, multiple add/drop devices can
 be connected to a fiber optic line to allow reconfiguration of individual
 channel access by configuring switches rather than physically inserting or
 removing an add/drop device.
 For example, if an optical line communicates N channels, N add/drop devices
 can be built into a fiber optic network and the switches of the respective
 add/drop device can be set at cross or bar depending on whether access to
 the corresponding channel is desired. Thus, granting or denying access to
 channels is simplified as compared to inserting or removing an add/drop
 device to change access to a channel. Of course, more or fewer than N
 add/drop devices can be used to provide access to channels communicated by
 the optical line.
 In one embodiment, each add/drop device (420, 421 and 422) operates in the
 manner described above with respect to the add/drop device of FIG. 3.
 Interconnection of multiple add/drop devices as shown in FIG. 4 provides a
 configurable add/drop mechanism that allows modification of access to one
 or more channels of a multi-channel optical line without physical
 insertion or removal of hardware. Alternative embodiments of add/drop
 devices are described in greater detail below and can also be used to
 provide a configurable add/drop mechanism.
 Input port 400 of add/drop device 420 is optically coupled to receive a
 multi-channel optical communications device, for example, an optical
 fiber. Drop filter 410 passes a channel and reflects the remaining
 channels to intermediate port 430. As described in greater detail below,
 drop filter 410 can comprise multiple filters. Switch 460 causes the
 channel to be passed to Device 1 or to be passed to add filter 480. Add
 filter 480 receives either the channel dropped by drop filter 410 or a
 channel generated by Device 1. The channel received by add filter 480 is
 added to the channel(s) received by intermediate port 440.
 Output port 490 of add/drop device 420 is optically coupled to intermediate
 port 441 of add/drop device 421. Intermediate port 430 of add/drop device
 420 is optically coupled to input port 401 of add/drop device 421. Drop
 filter 411 operates to drop a channel to switch 461 and reflect the
 remaining channels to intermediate port 431. Switch 461 operates in a
 similar manner to switch 460. Add filter 481 adds the channel received to
 generate an output to output port 491.
 Multiple add/drop devices are interconnected in a similar manner up to
 add/drop device 422, which is optically coupled to Device N. Any number of
 add/drop devices can be optically coupled together. Input port 402 and
 intermediate port 442 are optically coupled to an intermediate port and
 output port, respectively, of another add/drop device (not shown in FIG.
 4). Drop filter 412 operates to drop a channel to switch 462 that either
 optically couples the output of drop filter 412 to Device N or to add
 filter 482. Add filter 482 adds the channel received to the channels
 received via intermediate port 442 to generate an output at output port
 492.
 In one embodiment, intermediate port 432 of add/drop device 422 is
 optically coupled to intermediate port 440 of add/drop device 420. The
 coupling can be either direct or through other devices, for example,
 dispersion management, cleanup filters or other devices.
 FIG. 5 is an add/drop device having a prism switch according to the one
 embodiment of the invention built with discrete optical components. Input
 port 500 and intermediate port 530 are optically coupled to drop filter
 510. As described in greater detail above, drop filter 510 drops a channel
 from multiple channels received via input port 500.
 Filter 515 provides further filtering of the signal passed by drop filter
 510. In one embodiment filter 515 has the same filtering properties as
 drop filter 510. For example, if drop filter 510 is used to drop channel 3
 of five incoming channels, in general, channel 3 is passed and the
 remaining channels are reflected. However, because physical filters are
 not ideal, some light representing the remaining channels may also be
 passed by drop filter 510. To minimize the unwanted light (or increase
 isolation to other channels), filter 515 is provided to double the
 isolation provided. For example, if 2% unwanted light passes drop filter
 510, 0.04% passes both drop filter 510 and filter 515. While filter 515 is
 not necessary to practice the invention, addition of filter 515 can
 provide better performance than an embodiment with a single drop filter.
 In one embodiment, collimators 550 optically couple drop filter 510, add
 port 520, drop port 570 and add filter 580 to diffraction prism 560. In
 alternative embodiments, diffraction prism 560 is replaced with other
 components, for example, a mirror. Add filter 580 operates to add a
 channel to the channels of intermediate port 540. The resulting channels
 are output to output port 590.
 The embodiment of FIG. 5 illustrates an add/drop device implemented with
 discrete optical components; however, six collimators are used that
 operate to interconnect the components of FIG. 5. The number of components
 through which an optical signal passes can be reduced, and performance of
 the add/drop device thereby improved, by integrating the components of the
 add/drop device and removing collimators 550.
 FIG. 6 is one embodiment of an integrated add/drop device having a prism
 switch according to one embodiment of the invention. The add/drop device
 illustrated in FIG. 6 has fewer collimators than the add/drop device of
 FIG. 5. In one embodiment, collimators 652 and 656 are dual-port
 collimators. Further, collimators 650, 652 and 656 include filters.
 Alternative embodiments having triple-port and quad-port collimators are
 described in greater detail below.
 Collimator 652 provides input port 600, intermediate port 630 and a filter.
 The embodiment illustrated by FIG. 6 includes two filters to provide drop
 filter 610; however, a single filter in collimator 652 or collimator 650
 can also be used. Collimator 650 provides drop port 670 and a second
 filter for drop filters 610. Collimator 654 provides add port 620.
 Collimator 656 provides intermediate port 640, output port 690 and add
 filter 680.
 Collimators 652 and 654 provide input paths to diffraction prism 560.
 Collimators 650 and 656 provide output paths from diffraction prism 560.
 When diffraction prism 560 is physically located between collimators 650,
 652, 654 and 656, input port 600 is optically coupled to output port 690
 and add port 620 is optically coupled to drop port 670. When diffraction
 prism 560 is not physically located between collimators 650, 652, 654 and
 656, input port 600 is optically coupled to drop port 670 and add port 620
 is optically coupled to output port 690.
 In one embodiment, diffraction prism 560 is physically moved by a solenoid
 or by an electric motor. In an alternative embodiment, collimators 650,
 652, 654 and 656 can be configured such that diffraction prism 560 is
 physically placed between collimators 650, 652, 654 and 656, input port
 600 is optically coupled to drop port 670 and add port 620 is optically
 coupled to output port 690. Similarly, when diffraction prism is not
 located between collimators 650, 652, 654 and 656, input port 600 is
 optically coupled to output port 690 and add port 620 is optically coupled
 to drop port 670.
 FIG. 7 is an add/drop device having a mirror switch according to the one
 embodiment of the invention built with discrete optical components. The
 add/drop device of FIG. 7 is illustrated with four dual-port collimators;
 however, other types of collimators can also be used.
 Input port 700 is optically coupled to drop filter 710. Drop filter drops
 one channel that is received via input port 700 and reflects the remaining
 channels to intermediate port 730. As described in greater detail above,
 drop filter 710 can include multiple filters. The channel dropped by drop
 filter 710 is provided to switch 760.
 In one embodiment, switch 760 includes mirror 763 and solenoid 765 that
 moves mirror 763 to selectively optically couple drop filter 710 and add
 port 720 to drop port 770 and add filter 780. When mirror 763 is
 physically placed between the collimators of switch 760, the channel
 dropped by drop filter 710 is reflected by mirror 763 to drop port 770.
 The channel that is provided to add port 720 is reflected by mirror 763 to
 add filter 780. Add filter 780 adds the channel received to the channels
 received by intermediate port 740 to provide an output signal to output
 port 790.
 When mirror 763 is not physically located between the collimators of switch
 760, drop filter 710 is optically coupled to add filter 780. Similarly,
 add port 720 is optically coupled to drop port 770. Thus, when mirror 763
 is not placed between the collimators of switch 760, the channel dropped
 by drop filter 710 is added back by add filter 780 and the device
 optically coupled to add port 720 and drop port 770 does not have access
 to any of the channels received at input port 700.
 FIG. 8 is an integrated add/drop device having a mirror switch according to
 one embodiment of the invention. The integrated add/drop device of FIG. 8
 includes fewer collimators than the add/drop device of FIG. 7. In one
 embodiment, the triple fiber collimators of FIG. 8 are configured as
 described in greater detail below with respect to FIGS. 10a and 10b.
 In one embodiment, triple fiber collimator 850 provides input port 800,
 intermediate port 830 and drop port 870. Triple fiber collimator 850 also
 includes drop filter 810. Alternative configurations can also be used. In
 one embodiment, triple fiber collimator 855 provides add port 820,
 intermediate port 840 and output port 890. Triple fiber collimator 855
 also includes add filter 880.
 In one embodiment, solenoid 865 moves mirror 863 such that mirror 863
 reflects signals or allows signals to pass between triple fiber
 collimators 850 and 855. Drop filter 810 drops a channel received via
 input port 800 and reflects the remaining channels to intermediate port
 830. The channel that is passed by drop filter 810 is reflected to drop
 port 870 if mirror 863 is between collimators 850 and 855.
 If mirror 863 is between collimators 850 and 855, the channel provided by
 add port 820 is reflected by mirror 863 to add filter 880 and is added to
 the channels provided by intermediate port 840 and output to output port
 890. If mirror is not between collimators 850 and 855, the channel dropped
 by drop filter 810 is passed to output port 890 and added to channels
 received via intermediate port 840 by add filter 880 and output to output
 port 890.
 FIG. 9 illustrates the basic optical principles of the integrated add/drop
 device of FIG. 8. For the example of FIG. 9, the input collimator that
 includes input port 900, intermediate port 910 and drop port 970 is
 configured as described below with respect to FIG. 10a. Similarly, the
 output collimator that includes add port 920, intermediate port 940 and
 output port 990 is configured as described below with respect to FIG. 10b.
 Other collimator configurations can also be used; however, corresponding
 modifications of the optical operation of FIG. 9 result.
 Lens 950 focuses light passing between input port 900, intermediate port
 910 and drop port 970 and drop filter 910. Similarly, lens 955 focuses
 light passing between add port 920, intermediate port 940 and output port
 990 and add filter 980. Mirror 963 is a double sided mirror and is movable
 to allow light to pass or to reflect.
 In one embodiment, the distance between the input ports and the distance
 between the output ports, the distance between the lenses and the ports as
 well as the lenses and the filters, and the angle of the filters are
 configured as described below. Other configurations can also be
 implemented.
 In embodiments of FIGS. 10a and 10b, r is the fiber radius, f is the lens
 focus distance, .alpha. is the input angle, .beta. is the filter
 reflection angle, and .theta. is the mirror reflection angle at the
 filter. FIG. 10a is an input triple fiber collimator according to one
 embodiment of the invention. FIG. 10b is an output triple fiber collimator
 according to one embodiment of the invention. In one embodiment,
 .alpha.=.beta.=.theta..congruent.1.4r/f. Because the filter spectrum
 shifts with the light incident angle, when light passes through a filter
 twice, the incident angles should be equal to each other.
 In the foregoing specification, the invention has been described with
 reference to specific embodiments thereof. It will, however, be evident
 that various modifications and changes can be made thereto without
 departing from the broader spirit and scope of the invention. The
 specification and drawings are, accordingly, to be regarded in an
 illustrative rather than a restrictive sense.