Light is especially useful for interconnection of data-carrying signals for two basic rea-sons. First, in the absence of nonlinearities, light obeys the principle of superposition,whereas electrons interact with one another. Second, light propagates at a very high fre-quency, which allows high-speed temporal encoding of data-carrying signals. We presentan approach that takes maximal advantage of these two facts.
The number of channels that can be interconnected by light entering a free-space op-tical system is limited by the volume of phase space that is allowed into and out of theoptical system. This volume is given by dΣ = 2(∆ν/c)(νo/c)2∆θ 2
F , where ∆ν is the rangeof temporal frequencies allowed into the optical system, νo is the mean optical frequency, cis the speed of light in the medium, and ∆θF is the field of view of the optical system.1
An additional factor of 2 is present to account for the two polarizations of light. Thecorresponding maximum number of independent channels of data-carrying information isdΣ/[(δν/c)(νo/c)2∆θ 2], where δν is the channel spacing in the frequency domain and ∆θis the angular spacing of the fiber sources. A reasonable value for this angular spacing is theangular resolution of the optical system, multiplied by a factor of ∼8 to ensure negligiblecross talk between neighboring channels. The system presented below allows us to achievethis upper bound of interconnectivity in reasonable way.
The approach combines imaging, and in particular telecentric 4- f imaging, with wave-length division multiplexing (WDM). The basic approach is shown in Fig. 1.2 The approachshows a bank for one polarization and for the simple case in which different fibers are usedfor reception and transmission of the data-carrying light. A more detailed description of thefull utilization of polarization and bidirectional fiber operation is discussed in Appendix A.
The array of fiber outputs are imaged onto a detector array (or the inputs to a micromirrorarray); then the signals are switched and retransmitted or redirected in a digital sense tothe appropriate output fiber, through the same imaging optics, for each wavelength, andindependently. Clearly, if two-dimensional arrays are used, as well as multiple colors, allbasic degrees of freedom of light are utilized and switched independently, and the remain-ing challenge is to pack the range of wavelengths and the field of view most efficiently withinformation to achieve best throughput.
Fig. 1. Basic approach. Light enters and exits from an array of fibers (a). After entering,the light from all fibers is collimated by a single lens (b) then optionally passed through apolarizing beam splitter (c) where the collimated, wavelength- and angle-multiplexed sig-nals are directed to a linear array of thin-film filters (e). For each wavelength of interest, theentire array of fibers are filtered by one thin-film filter (f), and signals of the correspondingwavelength are directed to a focusing lens (h), which images the fiber array onto a detectorarray (i), or the inputs to a micromirror array (not shown). The spatially distinct signalsare then detected, processed (j) and reemitted (k) (or redirected in the case of micromirrorarrays) and then imaged back through the system to the fiber bundle (a), with the sameoptics.
This approach differs from past approaches that involve WDM and spatial multiplex-ing. One recent past publication describes the use of microlens arrays to relay the lightfrom input multimode fibers to output multimode fibers,3 whereas in the approach herein,single lenses are used to perform most of the image relays, and WDM is included, allow-ing much greater interconnectivity. In addition, the image quality derived herein shouldbe sufficient to support the use of single-mode fibers, on the basis of detailed ray traces.Another recent publication shows the use of spatial multiplexing combined with WDM,but no mention is made of imaging and its associated single-hop interconnectivity, and thatsystem requires that each pixel consist of an array of multiple-wavelength VCSELs.4 Theshared optical function technology (SOFT) approach presented elsewhere differs from thatexhibited herein in that two-dimensional arrays of fibers can be efficiently demultiplexedin what follows, whereas previous research has anticipated only one-dimensional arrays offibers.5
Figure 1 shows the use of thin-film filters for wavelength separation. Such filters arenot the only components that can be used for free-space wavelength separation—gratingsare also a compatible technology for bulk free-space demultiplexing in the context ofinterconnection.6 On the other hand, the limited angular dispersion of differing wavelengthsachievable with gratings does not readily support the use of two-dimensional arrays offibers, rather only one-dimensional arrays. Hence thin-film filters offer significantly greaterscalability. Although gratings are less expensive, it was found that the optical system us-ing thin-film filters offers significantly lower cost of alignment and packaging. Because ofthese considerations, the use of gratings is not discussed further in this paper.
Optical interconnection can be done with arrays of steering mirrors.7,8 This approachcan be used to interconnect many optical channels, as many as 1000 to 1000.9 However,
when more than 1000 channels need to be interconnected, cross talk arising from diffrac-tive beam divergence becomes a basic issue.10−12 This issue can be partially circumventedby use of optical channeling.13 The approach proposed below is an optical architecture thatallows scaling far beyond a 1000 input × 1000 output switch and is strictly nonblockingwhen color switching is not required. The architecture also has the advantage that it is mod-ular so that it can be incrementally expanded and maintained, and different wavelengths canflexibly and independently be added to the wavelength channel plan.
2. Basic Concept and Extensions
The approach combines imaging with WDM as shown in Fig. 1. Optical fibers (a) emitlight to and receive light from the optical system. A collimating lens (b) collimates thelight emitted from all fibers within the field of view so that the light beams from the fibersare nearly flat wave fronts as they propagate toward a polarizing beam splitter (c). The po-larizing beam splitter (c) passes one of the polarizations (d) toward a bank of filters (e). Inthis embodiment the data-carrying light of each wavelength band with center wavelengthλi is reflected when it encounters the corresponding narrowband reflective filter (f) and thelight is then directed toward the corresponding switch module (g). In its simplest imple-mentation, the switch module comprises a focusing lens (h), a detection or input means(i), a processing means (j), and a retransmission or output means (k). In this embodimentthe received light is focused by focusing lens (h) to be detected at detectors (i) located atthe images of the input fibers. The electronic signal is directed by means of electronic pro-cessing (j), which in its simplest form is simply an electronic cross-point switch.14 Suchcommercially available cross-point switches have several benefits: They can be reconfig-ured in a few nanoseconds and can multicast signals. In addition, some models can alsoperform retiming. The switched signals are then sent to emitters (k) that are located at theimage points of the fibers. The emitters regenerate, reshape, and transmit the outgoing lightback through the optical system, retracing the path of the incoming light except for the tiltof wave as it propagates toward the fiber array. The light will then arrive at the fiber withthe proper position, size, and angular divergence to ensure effective coupling into the fiber,provided that the imaging is performed accurately over the entire field of view containingthe fiber array and that the system is properly aligned.
The unidirectional transmission of data in the fiber array is shown in Fig. 1. This ap-proach is compatible with networks deployed today. Unidirectional fiber operation is easilyperformed in this context by use of one subset of the fibers in the array for reception and asecond disjoint set for transmission. This clearly simplifies the design of the switch mod-ule, because the detectors and emitters can now be located in a common plane as shown inFig. 1, and no polarizing elements are needed in the system, as described in Appendix A.This unidirectional design allows much easier alignment, and is the preferred embodiment.This design still allows independent or redundant switching of two different polarizations,if an initial polarizing beam splitter is retained along with a corresponding second bank ofnarrow-bandpass filters and switch modules. At first glance, the device shown in Fig. 1 isjust a bulk wavelength multiplexer–demultiplexer (mux–demux), but further investigationshows that it offers interconnectivity as well, as seen in Fig. 2. In the schematic exam-ple shown, there are 3 fibers and 4 colors, leading to 3 muxes, 3 demuxes, and 24 fiberpatches with 2 connectors each. This is equivalent to a small version of the device shownon the right-hand side. The subject device is not only equivalent to the muxes and demuxes(b1) and (b2) but also includes the functionality of the fiber patches and connectors (c1)and (c2). The cost and complexity advantage becomes clear if 250 fibers and 10 colors arepresent, in which case 500 muxes and demuxes are used along with 5000 fiber patches. Thekey observation is that each module can independently switch between channels, becauseof the intervening free space, thus eliminating the gross complexity of intervening fiber
connectors. The cost, scalability, and manufacturability implications for switching systemsare the topic of this paper.
The above simple description overlooks several important optical issues that are suc-cessfully addressed in the prototype. First, commercially available, narrow-bandpass filtersare designed to operate at near-normal incidence and to pass the wavelength band insteadof reflect it. To accommodate these realities, a zigzag design was developed.15 As shownin Fig. 3, the zigzag angle θ was ∼2.5 deg, with a ±1-deg field of view. These shallowangles combined with ∼1-cm apertures imply significant path length between filters. Thispath length is typically ∼60 cm between filters. In turn, this path length, in combinationwith the ±1-deg field of view, implies significant beam walk for light from some of thefibers. This beam walk would result in unacceptably large apertures with the simple imag-ing shown in Fig. 1. To mitigate this problem, 4- f imaging was used between each channeland was implemented in the straightforward way shown in the Fig. 3. The figure shows twoplane waves (c0) indicated by straight lines that are imaged onto plane waves (c1) at filter1 and again as plane waves (c2) at filter 2, and so on. The focal lengths are indicated as fin the figure, with steering mirrors for alignment after the entrance lens (b) and along thebottom of the figure.
Fig. 2. Interconnectivity enabled by the approach. The figure on the right has the samefunctionality as the diagram on the left, wherein light of as many as four wavelengthsenters via three fibers (a), is switched between fibers independently for each wavelength,and is then transmitted to three output fibers (e).
A second important optical issue that was addressed was the angular sensitivity of thethin-film filters. The line center of the thin-film filters is known to shift with angle of in-cidence. This shifting is addressed in three ways. First, the 4- f imaging described in theprevious paragraph ensures that plane waves are delivered to the filters, rather than diverg-ing or converging waves that have significant angular width. Second, the field of view ofthe optical system was restricted to ±1 deg. This angle clearly can restrict the number offiber sources that can enter the optical system. As implied in Section 1, the number of fibersthat can be accommodated is [∆θF/(8λ/D)]2, where ∆θF is the field of view, λ is the meanwavelength, and D is the clear aperture. For example, with λ = 1.55 µm, D = 0.6 cm, and∆θF = 2 deg (34 mrad) full field of view, one obtains 256 fibers in a square field of view.So even with a restricted field of view, many fibers can be accommodated by the use ofa relatively small aperture D. Tens of thousands of fibers can be supported with a clearaperture that is approximately 3.6 cm or more in width.
On the other hand, even the ±1-deg field of view makes implementation of 100-GHzfilters difficult within a single bank. The third part of the solution to the problem of filterangular sensitivity is to use 200-GHz filters within a single bank, and then to use separate,additional banks with interleaved wavelengths to go to finer channel spacing. One mightwonder what an interleaver would look like in the context of this system. A bulk free-space
interleaver is shown schematically in Fig. 4 and is simply a modified Mach–Zehnder inter-ferometer. This design consists of reflective elements (r1)–(r4) and transmissive elements(t1)–(t4) that form a design that compensates for dispersion and transverse offsets. Thisdesign and refinements to compensate for a broad field of view have been analyzed and arebelieved to be a simple, effective, and robust bulk interleaver that is compatible with therest of the system and that should support operation to channel spacings as fine as 25 GHz.
A further challenge of the optical design is that of precise imaging over an appreciablefield of view, and additionally, precise imaging in the presence of highly divergent fiberoutputs. Fibers must be imaged accurately to less than a few micrometers of distortion overthe ±1-deg field of view. This challenge is also met, moreover, with low-cost optics andlenslet arrays. Results of a ray trace for the lab prototype are shown below in the Sections3 and 4. The resulting throughput into a 9-µm-diameter, single-mode fiber is estimatedwith ZEMAX power-in-bucket calculations and ranges from 75% to 80%. These ray tracesalso indicate that low cross talk is also expected for this design, approximately −25 dB,with 250-µm pitch between fibers and/or detectors. The 250-µm pitch also supports lowelectrical cross talk between detectors at the data rates of interest.
Fig. 3. Imaging and angular sensitivity of filters. Light enters and exits from fibers in bun-dle (a), is collimated (b), and then this plane is successively imaged with 4- f telecentricimaging onto bandpass filters (c1), (c2), and so on. Light that passes through the bandpassfilters is then focused down to an image of the fiber array (a).
The imaging conditions also now imply a more stringent requirement of throughput forthe various elements. For a nominal bank of 10 filters, light to the final filter encounters 22lenses, 11 mirrors, and 9 filters. A simple calculation shows that if the throughput is 99%for each of these optics, then an additional 73% loss (1.4 dB) is encountered at the finalchannel. The 99% throughput can be achieved with commercially available components,and the associated penalty does not seem excessive compared with other systems.
The alignment of the system is an important issue but one that is relatively straightfor-ward to address. The system can be completely aligned in four basic steps. First, a referencefiber near the center of the fiber array is lit, and the light is directed down the center of theoptical path by adjustment of an input lenslet array. Then it is transmitted to the final chan-nel by adjustment of an input steering mirror and subsequent steering mirrors shown on thebottom of the device in Fig. 3, or on the side of the device when in the orientation shownin Fig. 8, below. These adjustments are done with a screwdriver and take a few minuteseach. This will deliver the light to the center of the focusing lens at the final channel. Thenthe switch module is reoriented so that the light hits the detector. This final fine alignmentis done either with a screwdriver or is automated. The switch module consists of a lens
Fig. 4. Interleaver schematic. Light enters from one or more fibers (a); is collimated (b);then split by beam splitter (t1) and sent along upper path (r1), (t3), (t4), (r2), and alonglower path (r5), (r6), (r4), (t2). Transmissive elements (t3) and (t4) are included on theupper path to match the dispersion and path offset introduced by (t1) and (t2) along thelower path.
and a lenslet array and may include detector and emitters. The alignment of the lens to thelenslet array with proper design can rely on mechanical tolerances. However, the internallenslet arrays are manufactured together with detectors and emitters, and the whole lensletarray is adjusted in three translation axes separately to ensure that the detector positionsand emitter positions and projection angles are within tolerances. This adjustment can bedone easily and independently of the adjustments of the main unit with the proper test setupfor monolithic arrays. For multiple subarrays of detectors or emitters, current semiconduc-tor processing techniques support the needed accuracy.16,17 The subassembly consisting ofthe lenslets, and detectors and emitters (or alternative switching means), is then placed intothe switch module as a unit with the aforementioned mechanical tolerances (fraction of thewidth of a lenslet). The alignment for the switch modules is then repeated for each switchelement and is relatively quick and easy for manual alignment, and it can be automated aswell. Of course, the wavelengths of the light used for alignment must be chosen so that thelight is transmitted or reflected appropriately at the narrow-bandpass filters. Note that eachalignment can actually align hundreds or even thousands of channels at once, and so thealignment and integration cost per channel tends to zero as more fibers are used at the inputarray.
It should also be emphasized that the basic architecture can support several differenttypes of switches in the switch modules, as shown in Fig. 5. The concept described usesarrays of detectors and emitters, which would be bump bonded onto arrays of amplifiers,which would in turn connect to a crosspoint switch. However, emitter array technology isjust beginning to mature, and it is still difficult to find arrays of emitters, e.g., VCSELs,that are narrowband and have a uniform center wavelength in an array. One alternativeis a micro-electro-mechanical mirrors (MEMs) array as shown in Fig. 5 on the left-handside. In this embodiment, signals enter and exit in a one-dimensional array (a) and areswitched by a two-dimensional MEMs array (b) and exit in a spatially disjoint region (c).This implementation is transparent to data formats and uses fibers unidirectionally. Anotheralternative, shown on the right-hand side of Fig. 5, uses modulator arrays. In this approach,an emitter array is replaced with an array of modulators in conjunction with one or morecommercially available high-power diode lasers that supply light to them. In the figure,light is received by a detector array (d) after passing through a polarizing beam splitter(e) as described in Appendix A, and the detected electronic signals are switched by use ofa processor (f). The processor modulates output channels that are illuminated by a singlehigh-power, polarized laser (g) that is collimated with a lens (h), and is separately focused
and recollimated by lenslet arrays (i1), (i2) through the modulator array (j) elements beforeexiting the system. This approach requires a bit more alignment but is feasible, relativelylow in cost, and meets requirements. The modulator option along with the emitter option in-volve conversion of light to an electronic signal and back to optical signals and are referredto as opaque, or OEO (electronic–electrical–electronic), approaches.
The MEMs approach is more expensive at present, but MEMs arrays are commer-cially available with reasonable array sizes, and these arrays support transparent operation,which may have benefits in some cases. Furthermore, the scaling limitations associatedwith monolithic MEMs components are broken by use of the modular approach of thisarchitecture.10 The MEMs approach does have one significant potential benefit—it mayhave relatively low thermal dissipation. For example, we estimate that with correct choiceof components, OEO switching can be performed with emitters or modulators at ∼1/2 Wper 3-Gbit/s channel. When there are 250 channels per color, the implied power level perswitch module is ∼125 W, which is not inconsequential, considering that there may be 80modules (wavelengths) in a rack.
A fourth option supported by this architecture is to feed the signals at the switch mod-ule directly into fiber arrays as shown in Fig. 6. This approach essentially treats the ar-chitecture as a WDM mux–demux unit but with interconnectivity beyond that of a simplemux–demux. This option allows distributed switching as well as switching under differentprotocols, such as Internet Protocol (IP), asynchronous transfer mode (ATM), synchronousoptical network (SONET), and Ethernet. Thus more-sophisticated processing, includingrouting, can be performed for each data channel. It also allows a finer granularity of incre-mental modularity, since processors in this case can be added one fiber at a time.
In summary, the basic approach images an array of fiber outputs onto a detector array(or the inputs to a MEMs array), then switches the signals and retransmits or redirects theoutgoing light in a digital sense to the appropriate output fiber, through the same imag-ing optics, for each wavelength, and independently. Clearly, if two-dimensional arrays andmultiple wavelengths are used, as well as polarization as described in Appendix A, all ba-sic degrees of freedom of light are utilized and switched independently. The remainingchallenge is to pack information into the range of wavelengths and the field of view mostefficiently to achieve best throughput or some other objective. Many options are available,with trade-offs in terms of cost, capability, density, and thermal dissipation. All are strictlynonblocking if wavelength switching is not required. One big advantage of this systemthat is not immediately obvious is that alignment is very cheap, perhaps hundreds of timescheaper than other lightwave systems, because hundreds or even thousands of channelscan be aligned at once and with macroscopic alignment fixtures that are relatively easy tomanipulate.
The architecture also supports a next-generation parallel-router architecture whereinsignals can be routed to any destination fiber independently and in parallel for each WDMwavelength.18 The optical approach presented herein overcomes the interconnectivity issueassociated with prior router architectures.
The OEO version can support packet switching in one of several ways. This is becausethe electronic switches can switch their state in a few nanoseconds, during which relativelylittle packet information need be retained. The forwarding decision can be made on thebasis of information contained in a packet, though this approach usually requires inten-sive electronics for identifying the destination address and determining the correspondingswitch state. Alternatively, packet-forwarding information can be provided on a separate,supervisorial channel that can operate at a much lower bandwidth than the data-carryingchannel. Both these approaches can be implemented within this architecture, but the latteris certainly easier, as it would likely be for other switch architectures as well. One can alsoobserve that larger packets would further decrease processing requirements in any case.
Fig. 5. Schematic alternative switch modules. In the alternative to the left, incoming light(a) is imaged onto the entrance plane of a MEMs device, collimated by a microlens array(not shown), switched by pop-up mirrors (b), and then diverged by another microlens arrayat (c) whereupon this exit plane is imaged back to the fiber array. In the alternative to theright, incoming light is detected and modulated. The incoming light is first directed to de-tectors (d) possibly by use of a polarizing beam splitter (e), is electronically processed andswitched by use of a processor (f), is reemitted from a source (g) of the proper wavelength,focused by use of microlens array (i1) through a modulator array (j), and then refocused asneeded by another microlens array (i2). This second focal plane is imaged back to the fiberarray.
The above Figs. 1–3, 5, and 6 show devices that are capable of strictly nonblockingswitching, if color changing is not a requirement. However in many cases, color changingis useful, and in some cases it is required. There are two ways to perform color changingwithin the context of this device. The simplest, lowest-cost means is the use of an opti-cal foreplane as shown in Fig. 7. The foreplane is built into the switching architecture infront of the switching modules, hence the name. To switch light with the foreplane, light istransmitted from a receiving switch module (a) at the receiving wavelength, to one of thefiber ports (b). This port, denoted a utility port, is occupied by a fiber that leads to a colortransponder (c). At the transponder, the light is detected and then retransmitted at the de-sired wavelength by use of a fixed or tunable laser. The wavelength-converted signal goesback to the corresponding switch module (d). The light is received, switched, and sent out(e) at the desired transmitting wavelength in accord with the straightforward proceduresdiscussed above for a switch module. This foreplane approach allows easy upgrades forlimited wavelength switching, merely by addition of transponders into spare fiber ports.Two key observations can be made about the foreplane. First, the process involves at leastone extra OEO operation for the transponder. A second extra OEO operation is used ifthe switch modules use OEO processing. Hence as few as one and as many as three OEOsteps are needed with the foreplane, and the latter number of OEO steps can introduce sig-nificant power- and thermal-dissipation penalties. Second, if a significant fraction of thechannels require wavelength switching, the foreplane approach is overwhelmed. For ex-ample, assume that 250 fiber ports and 10 colors are used, and that 10% of the incomingsignals need wavelength switching. But 10% of the 250× 10 channels is 250 channels,which means that all 250 fiber ports would need to be utility ports if the foreplane wereimplemented to satisfy the requirement. Clearly, the foreplane approach is not a completesolution to the problem of wavelength switching. A more complete solution involves theuse of a backplane. We have developed a high-throughput optical backplane that uses one
Fig. 6. Exemplary distributed system. In this system, light from fiber array (a) is distributedto separate processing units according to wavelength band. These units, shown in the upperright part of the figure, denoted IP, ATM, SONET, and ETHERNET, receive and processthe signals and retransmit the signals to possibly different fibers (though within the sameband of wavelengths) within fiber array (a).
extra OEO conversion and that supports the scalability of the basic device. However, adiscussion of this backplane is beyond the scope of this paper and is therefore omitted.
Fig. 7. Optical foreplane.
A large-scale system has been designed by application of the subject architecture. Thesystem uses 1000 wavelengths packed into a band of 200 nm of wavelength variation, cor-responding to 25-GHz frequency spacing. An array of 100×100 fibers in 2.5 cm × 2.5 cmarray is used, corresponding to 250-µm spatial spacing. The clear aperture of the system’soptics is 3.6 cm. The system uses banks, each containing ten switch modules (wavelengths),as building blocks in accord with the prototype design. One hundred such banks are used,and wavelengths are distributed to the banks, first with a coarse-wavelength (20-nm filterspacing) version of the bulk mux–demux. Then for each 20-nm band a 25-GHz interleaveris followed by two 50-GHz and four 100-GHz interleavers, in order to use 200-GHz fil-ter spacings within each bank. Hence 70 interleavers are used. The total throughput ofsuch a system is 1000 wavelengths × 10,000 fibers × 10 Gbit/s per channel, or 100 Pbit/s(petabit/s). Of course, this number should be divided by 2 for actual throughput, but if thebidirectional approach is used as described in Appendix A, this number should be multi-plied by an additional factor of 2, resulting in the original 100 Pbit/s. Yet another factorof 2 can be obtained if polarization is also fully utilized, as also described in Appendix A.Each bank would be approximately 9 cm wide × 143 cm tall × 136 cm wide, with the light
entering the switch modules from beneath as in Fig. 3. The system will occupy a volumeof approximately 1.5 m high × 1.6 m deep × 9 m wide, accounting for the coarse mux–demux as well as the interleavers. The volume would probably be broken into two or moreunits of lesser width so that it would fit into a room of modest size. The total volume andenvelope is comparable with large-scale routers and switches that have been produced anddisplayed at trade shows in the past year.
Several observations are worthy of note for this large-scale system. First, it is clear thatsuch a system can be deployed one wavelength channel at a time, with banks added asneeded. Second, such a system is relatively compact, given the throughput. On the otherhand, it is large for an optical system, and there is the issue of stability in a thermally andmechanically varying environment (though it is believed that the design is relatively robustfor modest environmental variations). Also the 100-Pbit/s system would require that eachswitch module internally switch 10,000 channels, which is a beyond the current state of theart of electronic switches for bit rates in excess of 1 Gbit/s (though not for 50-Mbit/s chan-nels). Finally, power and thermal loading for switching at high rates would be astronomicalif 10 million OEO channels were switched, assuming 1/2 W per channel. A distributedsystem or a MEMs system could mitigate some of the latter challenges. So some practicalissues remain for the ultimate high-throughput device, but most of these challenges wouldbe confronted by other systems as well and are perhaps more easily mitigated with thisarchitecture. In summary, the basic issue of scalability has been overcome with this de-sign, including throughput and associated cost scaling; the latter actually diminishes on aper-channel basis for this architecture as it is incrementally filled.
3. Lab Prototype Description
A prototype bank using ten switch modules was built and is shown in Fig. 8. The unit asshown is approximately 32 cm high × 30 cm wide × 5 cm deep. The switch modules whenattached add ∼11 cm in width when the unit is oriented as shown. The preferred orienta-tion is to lay the unit flat, forming a 2U “pizza box.” When combined with other banks,the preferred orientation is one in which the light enters the switch modules vertically, asshown schematically in Fig. 3. The ten switch modules can accommodate up to ten differ-ent wavelength bands, with each band spaced by 200 GHz or more. This unit is designed toaccommodate up to 16 × 16 fiber arrays, or approximately 250 fibers. Hence the unit cansupport up to 10 ×250 = 2500 total (input plus output) WDM channels. The overall unit,as shown, is approximately the size of a laptop computer. The unit is constructed of Invarfor most of the pieces to ensure mechanical and thermal stability. The components weremanufactured to the tolerances required by the optical design.
One of the channels was attached to a state-of-the-art 68-input, 68-output electronicswitch (a larger 144-input, 144-output switch is now available), each channel capable ofoperating at 3.125 Gbit/s.14 If the above bank were fully loaded and using state-of-the-artswitches, it could support ∼3.9 Tbit/s of input and an equal amount of output.
The banks were also designed to be housed in a rack-mountable box that can hold up toeight banks, corresponding to 8×10 wavelength bands. Such a rack-mountable unit (frameonly) was built as well and could support ∼31 Tbit/s of switching capacity that is strictlynonblocking if intercolor switching is not required.
As mentioned above, the system requires very precise imaging, and additionally, preciseimaging in the presence of highly divergent fiber outputs. Fibers must be imaged accuratelyto less than a few micrometers of distortion over the ±1-deg field of view. This challengeis met, moreover, with low-cost optics and lenslet arrays. Figure 9 shows the results of aray trace performed in ZEMAX for several field positions, for the tenth (final) channel inthe bank. The figure shows spot diagrams at the switch module image plane. The circlesshow the Airy disk, which is equal to 15 µm in this case. The Airy disk is computed with
2.44 λF/D, where λ is the wavelength, 1550.9 nm in this case; F is the focal length ofthe lenslet, 1 mm; and D is the diameter of the ray bundle at the lenslet, 250 µm. Thespot diagrams show spots at 0, 6.3, 12.6, and 18.8 mrad of field angle. The spots show raybundles of rms radius ranging from 2 to 3 µm and a geometric (outer) radius of 4 to 5 µm.The image displacements are all less than 1 µm, indicating essentially zero field distortion.The resulting throughput into a 9-µm-diameter fiber is estimated with ZEMAX power-in-bucket calculations and ranges from 75% to 80%. The corresponding Strehls range from81% to 95%. These ray traces also indicate that low cross talk is also expected for thisdesign, with 250-µm pitch between fibers and/or detectors. The system is designed for crosstalk not to exceed −25 dB. These results hold for ray traces performed for all ten channelsand for the expected range of wavelengths coming into the unit. Minor adjustments to theswitch module are beneficial for extreme wavelengths.
Light was directed into the system with a 2×12 fiber bundle made by USCONEC, Incorpo-rated. The light passed through a lenslet, then to the entrance lens that collimates the light.The light passed through the system and was measured at the first through tenth channels.The system was aligned, and then purposefully not realigned for 3 months, during whichtime data were taken. During the 3-month period, the unit was bumped both purposefullyand accidentally, and the usual temperature swings in an office environment were encoun-tered.
The spatial profile of the light was measured both prior to passing through the lensletand after final focusing by the lenslet. The spatial profile was measured with a scanningprofiler made by PHOTON, Incorporated. Results are discussed for the first and the tenthmodules only.
4.A. First Channel (closest to input), Prelenslet
Most of the focal spots had a diameter that was close to the expected value. The mean spotexp(−2) intensity diameter was 127± 25 µm in x and 136± 19 µm in y. The theoretical,diffraction-limited exp(−2) intensity diameter is 132 µm for a Gaussian input beam of 2-mm diameter at the entrance lens and ∼88 µm for a Gaussian input beam of 3-mm diameter.A 4-mm diameter at the entrance lens results in a 66-µm diameter. The 2-mm diametersare most pertinent, on the basis of observations of the light at the entrance lens, madequalitatively with an IR-sensitive card sold by Edmund Scientific, Incorporated.
An example of the data taken at the lenslet with the beam scanner is shown in Fig. 10.The data were taken after adjustment for best focus of one of the fibers. However, somevariability of the distance from the fiber outputs to the lenslets was observed, apparentlyresulting from the imperfect parallelism between the plane of the fiber outputs and the planeof the lenslets. The departure from parallelism was measured at approximately 2.0± 0.5deg, and over the 1.5-mm half-field width this could lead to ∼±0.05 mm of variation ofseparation between fiber outputs and lenslets. A simple ray-matrix calculation was per-formed for a lenslet with a 1-mm focal length, a laser beam with a diameter of 150 µm atthe lenslet, and a distance of 134 mm from the lenslet to the input collimating lens. As thefiber–lenslet separation is varied, the resulting diameter of the Gaussian ranges from 1.6 to2.9 mm at the input collimating lens, which matches well with the observed variability ofthe diameter at that location.
This variation δ z of ±0.05 mm in separation between fiber outputs and lenslets in turnimplies a spot diameter at the output lenslet that varies by ∼15 µm. This is based on twofacts. First, the system approximately images the input lenslet array onto the output lensletarray. Second, the variation in spot diameter at the input lenslet array is given by ±δ z×2×NA, where z is the distance from the fiber to the input lenslets and NA is the divergenceangle of the light exiting the fiber. Using δ z = ±0.05 mm, and NA = 0.14, one finds avariation of ±14 µm. This explains a significant fraction of the observed variability in spotsize just prior to the lenslet array in the switch module. In other words, the variability in themeasured diameter at the prelenslet focus is consistent with the range of diameters observedat the entrance lens and with the known tolerances on the input lenslet array.
The results are much as expected for a handbuilt prototype version and support require-ments on cross talk and throughput.
4.B. First Channel (closest to input), Postlenslet
A sample of the data is shown in Fig. 11. The measured Gaussian spot width was 56.6 ±15 µm for x and 69.9 ± 11 µm for y, after averaging over all channels that were observed.Profiles were observed for all 24 input channels (though only 22 channels were observed
Fig. 10. Sample lab data at first channel, prelenslet.
for the prelenslet measurements). The camera plane is 1.1 mm behind the camera face, andthe point of best focus is ∼0.95 mm from the lenslets, implying that the best focus is inac-cessible to the camera. In addition, some variability of the distance from the camera planeto the lenslets was observed, approximately 1.3 ± 0.1 mm, resulting from the imperfectparallelism of the plane of the camera and the plane of the lenslets.
This observed variability results in some variability of the Gaussian spot diameter. Theexpected exp(−2) intensity focal spot diameter consists of the diffraction-limited spot (14µm for a Gaussian beam), and the increase due the camera separation from best focus{= [(1.3±0.1)−0.95] mm separation)/0.95 mm focal length)× 130 µm initial diameter}yields approximately 60 ± 15 µm. This estimate of 60 ± 15 µm is in good agreement withthe measured value quoted in the previous paragraph.
Of the observed spots, four had extraneous peaks spaced 250 µm from the main peakthat were greater than −20 dB in signal. The commercial profiling system could measureonly ∼−24 dB of signal in a resolution element of 10 µm. These types of degradationsamount to cross talk and so are serious degradations. Inspection of the optical system sug-gests that these failures are the result of two effects, both of which are readily mitigated:The lenslet array had some imperfections, and the alignment for this effort was done witha side fiber instead of a center fiber. Both these effects can be eliminated with better fabri-cation practices.
One additional effect should also be noted in the measurements. The camera resolutionis 10 µm, so the smallest measured spots will have as much as 10 µm of additional spread.For spot sizes that were observed herein, this extra spread should not be significant.
As discussed, all the degradations mentioned above are believed to be associated with
Fig. 11. Sample lab data at first channel, postlenslet.
the lenslets and can be eliminated by better optomechanical design of the coupling of thelenslets to the fibers or detectors/emitters. The results support very well a wavelength de-mux function for the unit, wherein the light is detected by detectors with diameter equal to50 µm. The results also support demux into multimode fibers that have a 50-µm diameter.Finally, the results are consistent with demux into single-mode fibers, but the concentrationof light to the required 9-µm diameter was not shown conclusively because of the size andresolution of the beam profiler.
4.C. Channel 10 (farthest from input), Prelenslet
In this case the mean spot exp(−2) intensity width was 616 ± 133 µm in x and 540 ± 80 µmin y. An example of a measured profile is shown in Fig. 12. Several types of degradationswere observed. First, only 20 of the 24 fiber channels made it to the profiler. The losseswere found to occur between the fourth and the fifth channel, and indeed resulted fromimperfect alignment.
It is believed that the extended beam diameters are due to the aberrations present inthe filters. On the basis of Zygo measurements of the filters, we determined that ∼0.14waves of spherical aberration (peak–valley) were present over the clear aperture, as well as∼0.88 waves of focus, at a wavelength of 1550 nm. These aberrations are due to coatingstresses and are very repeatable from one filter to the next. A numerical propagation wasperformed to determine whether these aberrations could explain the observed beam spreadat the various channels. Indeed, the effect of these aberrations using numerical wave-opticspropagation after bounces off of 9 filters showed 649 µm of the beam spread for a 2-mmbeam at the entrance to the switch module, in reasonable agreement with measured valuesof 616 ± 133 µm in x and 540 ± 80 µm in y mentioned above. In addition, the numerical
wave-optics calculation showed a beam spread of 129.8 µm for the first channel, also ingood agreement with the measured values of 127 ± 25 µm in x and 136 ± 19 µm in ymentioned above for channel 1, prelenslet. It should be mentioned that spot diameter in allcases was computed from beam intensity profiles with the clip-level technique, with a cliplevel of exp(−2) relative to the peak intensity.
Double peaks were the worst observed degradation of the focal spot. These doublespots can perhaps best be explained by interference from vignetting. Interference fringeswere present as evidence. Vignetting may occur because some of the adjustment mirrorswere aligned in a crude way, by use of a fiber on the side instead of in the middle.
The fact that most of the fiber inputs could be seen at the output was very encouraging.The issues associated with the tenth channel, i.e., excess beam spread, double peaks, andmissing peaks, are believed to arise primarily from filter aberrations and will be addressedmore thoroughly in subsequent research.
Fig. 12. Sample lab data at tenth channel, prelenslet.
A massively scalable, low-cost, compact technique, believed to be novel, has been de-scribed and shown to be experimentally feasible. The experiment validated almost all thekey optical issues that we can identify:
• Basic optical concept of imaging fiber arrays through the zigzag with thin-film filters
• Mechanical design and tolerances of components and integrated device
• Optical design and tolerances
• Long-term stability of mechanical design
• Ease of alignment of mechanical design
• Vibration insensitivity of mechanical and optical design
• Reflectivity and transmission of narrowband filters
• Spectral bandpass of narrowband filters
• Wave-front quality of narrowband filters in transmission
• Wave-front quality of off-the-shelf lenses and mirrors
• Adequacy of picomotors for initial alignment and alignment maintenance
• Basic feasibility and/or adequacy of the use of off-the-shelf fiber arrays
• Steering mirror approach, and acceptability of near focus on steering mirrors
• Overall low-cost approach
There were some issues that were not addressed. One issue that we feel was not vali-dated was the filter wave-front quality in reflection. We plan to address this issue through avariety of means, including the use of thicker filters and lower-stress coatings. Two other is-sues that were not fully validated were the packaging of electronics in the switch modules,and ruggedization of the overall unit.
The key OEO issue is thermal dissipation. This is manageable for smaller systems buta challenge for the larger systems and could be overcome by MEMs or by a distributedarchitecture. Thermal stability is also a challenge for the larger systems.
This architecture is intrinsically low in cost for two key reasons. First, the underlyingcomponents can utilize wafer-based semiconductor technologies that have enjoyed tremen-dous economies of scale for decades and will likely continue to do so in the near future.Cost per channel is approximately US$30 or less for essentially all components and isalready made in array formats that this architecture can best utilize. Second, a single align-ment will align hundreds or even thousands of individual channels at once. Hence both thecomponent cost and integration costs promise to be at least 1 and in some cases 2 ordersof magnitude cheaper than what exist today. These low estimated costs for switches mightaffect the relative usage of switches versus routers in networks of the future.
This architecture is readily scalable to a system with 10 million optical channels, or 100Pbit/s assuming 10 Gbit/s per channel with components that can be made today. The archi-tecture supports distributed systems by injection of light into fibers at the demux side asshown in Fig. 6. The architecture also supports a next-generation parallel-router architec-ture, for which signals can be routed to any destination fiber independently and in parallelfor each WDM wavelength.18
This massive scalability is achievable in a modular way, allowing a relatively smallsystem to grow to a much larger system as WDM wavelengths are added incrementally.This is in contrast to a monolithic MEMs device, in which the whole M ×M unit mustbe purchased at once, and if demand requires, must be replaced for a yet larger N ×Nunit. Reliability is also an important consequence of the modularity of this architecture—individual wavelength modules may be replaced instead of the whole switch unit.
This architecture can attain the maximal interconnectivity theoretically possible withinexpensive hardware means, especially when color switching is not required. The keyissues in further improving interconnectivity are (1) improving the allowed field of view,∆θF , which is currently limited by properties of gratings and filters; (2) full utilizationof the optical spectrum; and (3) efficient utilization of each spectral channel of spectralwidth δν and each spatial channel of angular width ∆θ . Bidirectional operation, as wellas utilization of both polarizations, is also possible with this system but is relatively morecostly and also is not compatible with most networks’ operation today.
Appendix A: Optical Design That Supports Maximal Interconnectivity
The design shown in Fig. 1 sends and receives signals on different sets of fibers. Hencethe design is unidirectional and does not utilize or manipulate polarization. In some cases,bidirectional transmission through fibers is useful, and certainly bidirectional transmissioncan double the amount of information passing through a given fiber. Similarly, in somecases, fiber can support independent transmission of information in both polarizations, andso using both states of polarization can again double the amount of information passingthrough a fiber. From discussions in Section 1, we see how utilization of these degrees offreedom allows attainment of the maximal interconnectivity supportable by a free-space op-tical device. The device shown in Fig. 1 can be modified to support both these throughput-improving variations. The modified device is shown in Fig. 13. In this case an additionalFaraday rotator (d) and half-wave plate (e) are used just to the right of the polarizing beamsplitter (c), and additional polarizing beam splitters (f) are used within each switch module.
Incoming light from the fibers (a) pass a polarization state, denoted y, and reflect asecond polarization state x to independent or redundant processing. The Faraday rotatoris a −45-deg rotator that rotates the state of polarization to the state (x + y)/21/2. Thesubsequent half-wave plate, oriented with fast axis along x, rotates the polarization state to(x − y)/21/2. The light then proceeds to a switch module where it encounters a polarizingbeam splitter that is oriented so that it reflects light of this polarization onto a detector array.
When the switched light is emitted back toward the fiber array, it passes through thepolarizing beam splitter (f) in the switch module and therefore emerges with polarization(x + y)/21/2. When the light passes through the half-wave plate (e), it has polarization state(x − y)/21/2, and the Faraday rotator applies its −45-deg polarization rotation so that thelight leaving it has a polarization state of −y. Light of this state passes through the originalpolarizing beam splitter (c) and arrives at the fiber array in the same polarization as did theincoming light. Note also that the outgoing light may go into any fiber, because the emittersin the switch module may be located at the same image points as the detectors, as a result ofthe beam-splitter configuration. This feature permits bidirectional operation and switching.
It should be noted that Faraday rotators and wave plates have fields of view. With properdesign, however, this field of view can be significantly larger than the ±1-deg field of viewallowed by the filters; hence these devices do not impose field-of-view constraints.
Variants of the above polarization control are available to accommodate polarization-sensitive filters. Other variants can accommodate transparent switching means such asMEMs.
In summary, additional means are described to switch both states of polarization in-dependently or redundantly. This approach manifestly allows bidirectional transmission of
Fig. 13. Approach for maximum interconnectivity afforded by light. Light enters and ex-its from an array of fibers (a) as in the basic approach. After entering, the light from allfibers are collimated by a single lens (b), then passed through a polarizing beam split-ter (c), a Faraday rotator (d), and half-wave plate (e). The collimated, wavelength- andangle-multiplexed signals are directed to a linear array of thin-film filters and distributedby wavelength as in the basic approach. The light then passes through a polarizing beamsplitter (f) as it is imaged onto a detector array. The detected signals are processed, reemit-ted, and pass back through the polarizing beam splitter (f) as it is imaged back through thesystem to the fiber bundle (a), with the same optics.
data to and from adjacent optical fibers, thanks to the imaging properties of the systemand aperture sharing associated with the polarizing beam splitters. These means allow themaximum interconnectivity permitted in a WDM optical free-space switch as defined inthe Section 1.
Acknowledgments
We thank the Woodside Fund of Redwood Shores, California, and in particular, RobertLarson for financial support. Technical advice from Alexander Sawchuk, Olav Solgaard,Jonathan Heritage, and Michael Haney are also gratefully acknowledged. Discussions withAdam Reif are much appreciated. Portions of this paper are protected for commercial ap-plications by U.S. patents 6,313,936, 6,335,782, and other patents pending.
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