Abstract:
A multi-channel optical switching system particularly usable as a programmable optical add/drop multiplexer (POADM) in a multi-wavelength communication system. The switching system uses a thin film optical demux/mux that separates a multi-channel optical signal into a plurality of optical channels, and combines a plurality of optical channels into a multi-channel optical signal. The system also uses a plurality of optical ports optically connected to the thin film optical demux/mux and a selecting device to select which optical channel is directed to which of the optical ports.

Description:
CROSS-REFERENCE TO APPLICATION  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 09/955,741 filed Sep. 18, 2001, the content of which is expressly incorporated herein by reference, which claims the benefit of U.S. Provisional Application No. 60/280,660, filed Mar. 30, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to the field of optical communications, and more particularly, to a programmable optical add/drop system for use in optical multiplexing.  
         BACKGROUND OF THE INVENTION  
         [0003]    For several decades, fiber optics have been used for communication. Specifically, fiber optics are used for data transmission and other telecommunication applications. Despite the enormous information carrying capacity of fiber, as compared to conventional copper cable, the high cost of installing fiber optics presents a barrier to full implementation of fiber optics, particular as the “last mile”, from the central office to residences and businesses.  
           [0004]    One method of increasing carrying capacity without incurring additional installation costs has been to multiplex multiple signals onto a single fiber using various methods, such as time division multiplexing, where two or more different signals are carried over the same fiber, each sharing a portion of time. Another, more preferred multiplexing method is wavelength division multiplexing (WDM), where two or more different wavelengths of light are simultaneously carried over a common fiber.  
           [0005]    Until recently, typical fibers used for communications applications had preferred wavelength bands centered at 850 nm, 1300 nm, and 1550 nm, wherein each band typically had a useful bandwidth of approximately 10 to 40 nm depending on the application. Transmission within these bands was preferred by systems designers because of low optical attenuation. Recent advances in fiber design now provides fiber that have low attenuation over a very broad transmission range, from 1300-1620 nm.  
           [0006]    Wavelength division multiplexing can separate a fiber&#39;s bandwidth into multiple channels. Dividing bandwidth into multiple discreet channels, such as 4, 8, 16, 40, or even as many as 160 channels, through a technique referred to as dense channel wavelength division multiplexing (DWDM), is a relatively lower cost method of substantially increasing telecommunication capacity, using existing fiber optic transmission lines. Techniques and devices are required, however, for multiplexing the different discreet carrier wavelengths. That is, the individual optical signals must be combined onto a common fiber-optic line or other optical waveguide and then later separated again into the individual signals or channels at the opposite end or other point along the fiber-optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength sub-ranges) from a broad spectral source is of growing importance to the fiber-optic telecommunications field and other fields employing optical instruments.  
           [0007]    Optical multiplexers are known for use in spectroscopic analysis equipment and for the combination or separation of optical signals in wavelength division multiplexed fiber-optic telecommunications systems. Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tunable filters.  
           [0008]    Approaches for selectively removing or tapping a channel, i.e., selective wavelengths, from a main trunk line carrying multiple channels, i.e., carrying optical signals on a plurality of wavelengths or wavelength sub-ranges, is suggested, for example, in U.S. Pat. No. 4,768,849 to Hicks, Jr. Hicks, shows filter taps, as well as the use of gangs of individual filter taps, each employing high performance, multi-cavity dielectric pass-band filters and lenses for sequentially removing a series of wavelength sub-ranges or channels from a main trunk line. The filter tap of Hicks, returns a multi-channel signal to the main trunk line as it passes the desired channel to a branch line. One known demux is disclosed in Pan et al., U.S. Pat. No. 5,652,814, FIG. 25. In Pan et al., the WDM input signal is cascaded through individual filter assemblies, consisting of a fiber collamator, thin film filter, and a fiber focusing lens. Each filter is set for a given wavelength. However, aligning the fibers for each wavelength is costly and errors in the alignment contribute significantly to the system losses. Further, FIG. 13 of Pan et al. teaches the use of a dual fiber collimator, thin film filter, and a dual fiber focusing lens to selectively DROP and ADD a single wavelength or range of wavelengths. As discussed above, aligning the collimators is expensive. Other optical multiplexing devices eliminate many of the fiber to lens alignments.  
           [0009]    In U.S. Pat. No. 4,244,045 to Nosu et al, for multiplexing or demultiplexing a multi-channel optical signal. A row of individual optical filters are glued side-by-side onto the surface of an optical substrate, and a second row is similarly glued to the opposite surface of the substrate. Each individual filter transmits a different channel, that is, a preselected wavelength(s), and reflects other wavelengths. A multi-channel optical beam from a trunk line enters the optical substrate at an angle and passes through the substrate from filter to filter in a zig-zag fashion. Each filter transmits its preselected wavelength(s) and reflects the remainder of the beam on to the next filter. Each filter element is sandwiched between glass plates, and a prism element is positioned between each filter sandwich and a corresponding collimator positioned behind the filter sandwich to receive the transmitted wavelength(s). Nosu et al teaches the use of refractive index matching. The lenses, filters, optical substrate, etc. all have the same refractive index and are in surface-to-surface contact with one another, such that the light beam does not pass through air. This approach by Nosu et al involves the use of prisms as an optical bridge between the filter element and the collimators at each channel outlet. This elaborate design approach adds considerable cost and assembly complexity to multiplexing devices of the type shown in Nosu et al. The approach of Scobey, et. al, in U.S. Pat. No. 5,859,717, is similar to Nosu et al., except the zig-zag pattern is through air, not glass. A single spacer block with a hole is used to mount the individual filter. The block is dense and stable with low sensitivity to changes in the temperature and ambient humidity. Xu, in U.S. Pat. No. 6,118,912, also uses a zig-zag pattern between filters in air, but Xu tilts the individual filters to adjust the center bandpass of each of the wavelengths. Thin film multiplexing devices are economical for low channel count systems and have a desirable flat-topped pass bands. Those skilled in the art will recognize that these multiplexing devices can also be employed in reverse to multiplex optical signals from individual channels onto a multi-channel optical signal.  
           [0010]    Polarization dependent loss (PDL) is also a problem in WDM system because the polarization of the light drifts as it propagates through the fiber and furthermore this drift changes over time. Thus, if there is PDL in any component, the drifting polarization will change the signal level, which may degraded the system operation.  
           [0011]    Other multiplexer devices may be employed to add or drop channels in WDM systems. These systems are commonly known as optical add/drop multiplexers, or OADM. Another OADM, disclosed by Mizrahi U.S. Pat. No. 6,185,023, employs a fiber Bragg grating to a demux and mux signals in a WDM system. This method requires optical circulators and multiple components.  
           [0012]    However, the multi channel OADM designs discussed above are not programmable by the end user. That is, each multiplexers is designed and manufactured to mux (add) specific channels by the factory; or when used in reverse each multiplexers is also designed and manufactured to demux (drop) specific channels by the factory. This limitation mandates that the optical system&#39;s parameters be fixed before installation. Changes are not possible without replacing the fixed optical multiplexers with different designed multiplexers. This is expensive.  
           [0013]    One known programmable OADM is discussed in Boisset et al, International Publication No. WO01/13151. In Boisset et al., the desired add/drop channel is programmed by translating a segmented filter. To achieve this translation however, a large mechanical mechanism is employed. A further limitation to Boisset et al. is that only a single channel may be added or dropped per device. Designers may employ multiple devices, deployed in series, and programed as necessary to add/drop the correct channel, however this approach requires multiple devices and has multiple points of failure. Furthermore, the size of such a device would be overly large and therefore not practical for many applications where space is limited.  
           [0014]    Two other programmable OADMs are disclosed by Tomlinson, U.S. Pat. No. 5,960,133, and Aksyuk, et al, U.S. Pat. No. 6,204,946, both use bulk optics and gratings to demultiplex and multiplex WDM input and output signal. The grating-based systems are large, inefficient and tend to have sharply peaked pass bands, rather then the desired flat-topped pass bands.  
           [0015]    It is an object of the present invention to provide improved optical multiplexing devices which reduce or wholly overcome some or all of the aforesaid difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable and experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.  
         SUMMARY OF THE INVENTION  
         [0016]    In accordance with a first aspect of the invention, a programmable optical add/drop multiplexing device, programmed to add and/or drop one or more optical channels from/to a multi-channel light source, comprises four lenses, a first and second thin film demux/muxes, each in combination with a lens array, and programmable mirrors for directing light channels. The multi-channel light enters the device by way of a lens, and is directed through the first thin film demux/mux, where selected channels are filtered and directed through the first lens array and focused onto to the programmable mirrors. Depending upon the programmed state of the mirrors, the selected channels are either directed back through the first lens array and first thin film demux/mux and one of the lenses so as to enter a drop channel, or not reflected and allowed to pass through the second lens array and second thin film demux/mux and a lens so as to exit the device through a pass channel. In the instance where the programmed state of the mirrors directs one or more channels back through the first lens array and first thin film demux/mux and a lens so as to enter a drop channel, one or more add channels may enter the device by way of a lens, and is directed through the second thin film demux/mux and through the second lens array and focused onto to the programmable mirrors and is directed back through the second lens array and second thin film demux/mux and a lens so as to exit the device through the pass channel.  
           [0017]    To reduce polarization dependent loss (PDL) in the system a quarter-wave plate (QWP) may also be employed between the first thin film demux/muxes and the first lens array and the second thin film demux/muxes and the second lens array.  
           [0018]    In accordance with a second aspect of the invention, a programmable optical add/drop multiplexing device, programmed to add and/or drop one or more optical channels from/to a multi-channel light source, comprises a first and a second lens, a first and a second optical circulator, and a thin film demux/mux in combination with a lens array and programmable mirrors for directing light channels. The multi-channel light enters the device by way of the first optical circulator and the first lens, and is directed through the thin film demux/mux, where selected channels are filtered and directed to the lens array and focused onto the programmable mirrors. Depending on the programmed state of the mirrors, the selected channels are either directed back through the lens array and thin film demux/mux and the second lens and second optical circulator so as to enter a drop channel, or directed back through the lens array and thin film demux/mux and the first lens and the first optical circulator so as to exit the device through a pass channel. In the instance where the programmed state of the mirrors directs one or more channels back through the lens array and thin film demux/mux and the second lens and second optical circulator so as to enter a drop channel, one or more add channels may enter the device through the second optical circulator and second lens and be directed through the thin film demux/mux, where selected channels are filtered and directed to the lens array and focused onto the programmable mirrors, then back through the lens array and thin film demux/mux and the first circulator and first lens so as to exit the device through the pass channel.  
           [0019]    To reduce polarization dependent loss (PDL) in the system a quarter-wave plate (QWP) may also be employed between the thin film demux/mux and the lens array. The largest source of PDL is the thin film filters. The reflectance for light polarized perpendicular and parallel to the plane of incidence (contains the incident, reflected, and transmitted rays) differ. A QWP is located such that it is substantially normal to the propagating light beam and the retardance axis is at 45° to the light that was polarized parallel and perpendicular to the plane of incidence throughout the thin film demux/mux. If light leaving the thin film array is polarized parallel to the plane of incidence, then the QWP converts the light to a right circular polarization state. As it propagates through the lens array and is still substantially right circularly polarized when it is incident on the programmable mirrors. Reflection from the programmable mirrors changes the handiness of the light, so light is substantially left circularly polarized as it enters the QWP the second time. Passage through the QWP converts the light back to a linearly polarized state, but it&#39;s departing polarization state is orthogonal to the input state. Likewise, if light leaving the thin film array was polarized perpendicular to the plane of incidence, it leaves parallel. Thus, during one pass through the filter the light is polarized parallel and on the next is polarized perpendicular leaving a substantially zero PDL for the system.  
           [0020]    In accordance with a third aspect of the invention, a programmable optical add/drop multiplexing device, programmed to add and/or drop one or more optical channels from/to a multi-channel light source, comprises four lenses, and a thin film demux/mux in combination with a lens array and programmable mirrors for directing optical channels. The multi-channel collimated light enters the device by way of a collimator, and is directed through the thin film demux/mux, where selected channels are filtered and directed to the lens array and focused onto the programmable mirrors. Depending upon the programmed state of the mirrors, the selected channels are either directed back through the lens array and the thin film demux/mux and through a lens so as to enter a drop channel, or through a lens to exit the device through a pass channel. In the instance where the programmed state of the mirrors directs one or more channels back through the lens array and into thin film demux/mux and a lens so as to enter a drop channel, one or more channels may enter the device through a lens from one or more an add channels, and be directed through the thin film demux/mux, where selected channels are filtered and directed to the lens array and focused onto the programmable mirrors, and then back through the thin film demux/mux and a lens so as to exit the device through the pass channel.  
           [0021]    To reduce polarization dependent loss (PDL) in the system a quarter-wave plate (QWP) may also be employed between the thin film demux/mux and the lens array.  
           [0022]    In accordance with a fourth aspect of the invention, a programmable optical add/drop multiplexing device, programmed to add and/or drop four optical channels from/to a multi-channel collimated light source, comprises two or more lenses, and a thin film demux/mux comprised of at least two sides that demux and mux light sources, in combination with a first and second lens array and programmable mirrors for directing optical channels. The multi-channel collimated light enters the device by way of a lens, and is directed through the thin film demux/mux, where selected channels are filtered and directed to the first and second lens arrays and focused onto the programmable mirrors. Depending upon the programmed state of the mirrors, the selected channels are either directed back through the lens array and the thin film demux/mux and through a lens so as to enter a drop channel, or through a lens to exit the device through a pass channel. In the instance where the programmed state of the mirrors directs one or more channels back through the lens array and into thin film demux/mux and a lens so as to enter a drop channel, one or more channels may enter the device through a lens from one or more an add channels, and be directed through the thin film demux/mux, where selected channels are filtered and directed to the lens array and focused onto the programmable mirrors, and then back through the thin film demux/mux and a lens so as to exit the device through the pass channel.  
           [0023]    To reduce polarization dependent loss (PDL) in the system a quarter-wave plate (QWP) may also be employed between the thin film demux/mux and the lens array. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a perspective view of a first embodiment of a programmable optical add/drop multiplexer detailing the various channel paths through the device.  
         [0025]    [0025]FIG. 2 is a schematic plan view illustration of the embodiment of FIG. 1, detailing the various channel paths through the device.  
         [0026]    [0026]FIG. 3 is a perspective view of a second embodiment of a programmable optical add/drop multiplexer detailing the various channel paths through the device.  
         [0027]    [0027]FIG. 4 is a schematic side view of a MEMS mirror in IN/PASS and DROP/ADD modes, employed in the embodiment of FIG. 3.  
         [0028]    [0028]FIG. 5 is a perspective view of a third embodiment of a programmable optical add/drop multiplexer detailing the various channel paths through the device.  
         [0029]    [0029]FIGS. 6A and 6B are schematic illustrations of the relationship between the mirror angle and the beams in the Optical Add/Drop Multiplexer as employed in one embodiment of the invention.  
         [0030]    [0030]FIG. 7 is a perspective view of a fourth embodiment of a programmable optical add/drop multiplexer detailing the various channel paths through the device.  
         [0031]    [0031]FIG. 8 is a perspective view of one embodiment of a programmable optical add/drop multiplexer and optional mirrors.  
         [0032]    [0032]FIG. 9 is a perspective view of one embodiment of a programmable optical add/drop multiplexer and an optional detection array.  
         [0033]    [0033]FIG. 10 is a perspective view of one embodiment of a programmable optical add/drop multiplexer and an optional optical attenuator.  
         [0034]    [0034]FIG. 11 is a schematic illustration of the movement of optical attenuators employed in one embodiment of the invention.  
         [0035]    [0035]FIG. 12 is a perspective view of one embodiment of a programmable optical add/drop multiplexer and optional prisms. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    The programmable optical add/drop multiplexer of the invention has numerous applications, including for use in fiber optic telecommunications systems. For purposes of illustration, the preferred embodiments described below in detail multiplexing and demultiplexing, and adding and dropping channels, in wavelength division multiplexing and demultiplexing for a multi-channel fiber optic telecommunication systems. Exemplary references to an optical channel, or simply to a channel, should be understood to mean an optical signal with a centered wavelength and an upper and lower wavelength. Channel spacing is measured from the center of the first channel to the center of an adjacent channel.  
         [0037]    A four channel programmable optical add/drop multiplexer, employing one embodiment of the invention, is detailed in FIG. 1. It is of note that while only four channels are used in this example, a substantially larger number of channels/ports may be employed. The programmable optical add/drop multiplexer allows for demultiplexing and multiplexing four separate optical channels onto or off of a multi-channel light signal. The optical add/drop multiplexer of FIG. 1 may be dynamically programmed to demultiplex and multiplex any combination of channels onto or off the multi-channel light signal.  
         [0038]    A first embodiment of the programmable optical add/drop multiplexing device of FIG. 1 comprises a first set of two lenses, IN collimator  103  and DROP lens  121 , a first thin film demux/mux  105  and a first lens array  107 , a second set of two lenses  123  and  117 , a second thin film demux/mux  115 , and a second lens array  113 . All of these component are precisely aligned with each other, and mounted together so as to accommodate the entrance and exit of optical signals. The device of FIG. 1 may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like.  
         [0039]    In FIG. 1 a multi-channel light signal enters the device through the IN collimator  103 , and is directed through first thin film demux/mux  105 . The collimated light signal is directed inside the first thin film demux/mux  105  so as to enable each channel to separately exit the first thin film demux/mux  105  and be focused by first lens  107  onto a mirror array  109 . Gaussian beam waists are located mid-way through the thin film demux/mux and at the center of each of the mirrors in the mirror array to minimize insertion losses.  
         [0040]    The mirror array  109  contains a double-sided channel mirror for one or more of the channels; the first channel exits the first thin film demux/mux  105  and is focused onto channel mirror  111 , the second channel is focused onto channel mirror  119 , the third channel is focused onto channel mirror  125 , and the fourth channel is focused onto channel mirror  127 . For each channel, the lens array element relays the beam waist formed by the collimators in the middle of the demux/mux onto the channel mirror. Since the distance between the collimator waist and the lens array elements differs for each channel depending on the zig-zag path through the demux/mux, each lens array element optimally has a different radius of curvature and conic constant. To center the waists on each of the channel mirrors different wedges for each of the two sub-apertures used by the IN, DROP, ADD, and PASS beams are employed and different wedges for each channel are also employed. The radius, conic, and wedges can be easily designed using commercially available lens design software. This process is discussed in further detail below. It will be understood that devices may be employed where the mirror array  109  is constructed without channel mirrors for one or more particular channels.  
         [0041]    It will be understood by those familiar with the art that the centered wavelength of each channel need not be demuxed or muxed beginning with the highest centered or lowest centered, nor need the channels be arranged in order of their wavelength. In fact, it will be further understood by those familiar with the art that wavelengths may be demuxed or muxed in any order.  
         [0042]    Channel mirror  119  is engaged to reflect its corresponding channel back through the first lens array  107  and first thin film demux/mux  105  and through the DROP lens  121  so as to enter a DROP channel. In this regard, any channel mirror may be programmed to demultiplex its corresponding channel from the multi-channel light signal so as to allow the corresponding channel to exit the device through the DROP channel.  
         [0043]    On the other hand, channel mirror  111  is programmed not to DROP its corresponding channel, instead allowing the channel to pass through to the second lens array  113  and the second thin film demux/mux  115  and be multiplexed onto the multi-channel light signal, exiting the device through the PASS lens  117 .  
         [0044]    In the instance where one or more of the channel mirrors is engaged and DROPs its corresponding channel, one or more channels may enter the device through the ADD collimator  123  and be directed inside the second thin film demux/mux  115  so as to enable each channel to separately exit the second thin film demux/mux  115  and be focused by a second lens  113  onto the corresponding engaged channel mirror located on the mirror array  109 . The corresponding engaged channel mirror will reflect the ADDed channel back through the second lens array  113  and the second thin film demux/mux  115 , multiplexing the channel onto the multi-channel light signal exiting the device through the PASS channel.  
         [0045]    In this embodiment, the mirror array  109  is constructed using Micro Electrical Mechanical Systems (MEMS) pop-up mirrors that fully retract, or fully extend, into position. Programming of the channel mirrors is achieved by applying an electrical voltage to mechanical actuators. A larger mirror may be employed by design to control more then one channel. Of course, other types of mirror actuators could be used.  
         [0046]    By engaging the channel mirrors, one or more separate optical channels may be dynamically routed onto or off of a multi-channel light signal. Further, by engaging the channel mirrors as a function of time and in synchronous conjunction with other system components, time-division multiplexing of optical signals may be achieved.  
         [0047]    The first thin film demux/mux  105  and second thin film demux/mux  115  are typically identical to each other and sized according to the number of channels supported by the device.  
         [0048]    A first quarter-wave plate (QWP)  108  and a second QWP  112  reduce polarization dependent loss (PDL) in the system. QWP  108  and QWP  112  are located such that they are substantially normal to the propagating light beam and the retardance axis is at 45° to the light that was polarized parallel and perpendicular to the plane of incidence throughout the demux.  
         [0049]    In the event the channel mirrors  111 ,  119 ,  125 , and  127  are not engaged light passes thru the second lens array  113  and the second quarter-wave plate (QWP)  112 . The second quarter-wave plate (QWP)  112  axis is at 90 degrees to the axis of the first quarter-wave plate (QWP)  108 . As the light propagates through the lens array  107  and enters the second QWP  112  it is still substantially right circularly polarized. Passage through the second QWP  112  converts the light back to a linearly polarized state, but it&#39;s departing polarization state is orthogonal to the input state. Thus, during one pass through the system the light is parrallel and on the next is perpendicular leaving a substantially zero PDL for the system.  
         [0050]    Turning next to FIG. 2, the path of a four channel multi-channel collimated light signal  213  is more clearly illustrated. More specifically, of the four separate channels, the first and fourth channels are PASSED through the device, the second channel is DROPPED from the device, and the third is ADDED. Each of the four channel&#39;s path&#39;s are described below.  
         [0051]    The first channel is PASSED through the device as follows. The multi-channel light signal  213  enters the device through the IN collimator  103  and is directed into a first surface  201  of the first thin film demux/mux  105  through an entry port  202  and to a thin film filter  203  located on the second surface  210  of the first thin film demux/mux  105 . The entry port  202  is transparent to all wavelengths contained within the multi-channel collimated light signal  213  and allows all channels to enter the thin film demux/mux  105 . Thin film filter  203  is transparent to a first sub-range of wavelengths  211  that corresponds with the first channel included in the multi-channel collimated light signal  213 . Hereafter this first sub-range of wavelengths is referred to as the first channel  211 . Specifically, channel  211  passes through thin film filter  203 , demultiplexing the channel from the multi-channel collimated light signal  213 , whereas other channels contained within the multi-channel collimated light signal are reflected back into the first thin film demux/mux  105 . The first channel  211  is directed through the first lens array  109  where it is focused onto the channel mirror  111  that corresponds with this channel.  
         [0052]    Channel mirror  111  is not engaged. Accordingly, the first channel  211  is directed through the second lens array  113  and focused onto the second thin film demux/mux  115 . The first channel enters the second surface  218  of the second thin film demux/mux  115  through a thin film filter  219  and is multiplexed onto the multi-channel collimated light signal and exits the device through the PASS lens of FIG. 1, 117 (not shown on FIG. 2). Thin film filter  219  is transparent to the first sub-range of wavelengths  211  that corresponds with the first channel included in the multi-channel collimated light signal  213 .  
         [0053]    The fourth channel is PASSED through the devices as follows. The multi-channel light signal  213  enters the device through the IN collimator  103  and is directed into the first surface  201  of the first thin film demux/mux  105  to the thin film filter  203  located on the second surface  210  of the first thin film demux/mux  105 . Thin film filter  203  is transparent to a first sub-range of wavelengths  211  that corresponds with the first channel, all other channels contained within the multi-channel collimated light signal are reflected back into the first thin film demux/mux  105 . The multi-channel collimated light signal  213 , now comprised of channels two through four, reflects off the first surface  201  and encounters thin film  205 , which filters out channel two. The multi-channel collimated light signal  213 , now comprised of channels three and four, reflects off the first surface  201  and encounters thin film  207 , which filters out channel three. Lastly, the multi-channel collimated light signal  213 , now comprised only of channel four, reflects off the first surface  201  and encounters thin film  209 , which allows channel four to pass through to be focused by lens array  107  onto the mirror array  109 . Because channel mirror  127  is not engaged, channel four is directed through the second lens array  113  and focused onto the second thin film demux/mux  115 . Channel four enters the second surface  218  of the second thin film demux/mux  115  through a thin film filter  225  and is multiplexed onto the multi-channel collimated light signal. Thin film filter  225  is optically similar to thin film filter  209  in that it allows channel four to pass, and reflects all other channels.  
         [0054]    The multi-channel collimated light signal is then reflected by the first surface  217  of the thin film demux/mux  115  to thin film filter  223 . Thin film filter  223  allows channel three to pass, multiplexing channel three onto the multi-channel collimated light signal. The multi-channel collimated light signal, now channels three and four, is reflected by the first surface  217  to thin film filter  221 , multiplexing channel, then again by the first surface  217  to thin film  221 , picking up channel one. Lastly, the multi-channel collimated light signal, now channels four-one, exits the device through the PASS lens FIG. 1, 117 (not shown on FIG. 2).  
         [0055]    The second channel  227  is DROPPED from the device as follows: The multi-channel light signal  213  enters the device through the IN collimator  103  and is directed into the first surface  201  of the first thin film demux/mux  105  to a thin film filter  203  located on the second surface  210  of the first thin film demux/mux  105 . Thin film filter  203  is transparent to a first sub-range of wavelengths  211  that corresponds with the first channel  211  included in the multi-channel collimated light signal  213 . The multi-channel collimated light signal  213  is reflected off the second surface  210  and again of the first surface  201  to a thin film filter  205  located on the second surface  210 . Thin film filter  205  is transparent to a second sub-range of wavelengths  227  that corresponds with the second channel  227  included in the multi-channel collimated light signal  213 . Specifically, channel  227  passes through thin film filter  205 , demultiplexing the channel from the multi-channel collimated light signal  213 . The second channel  227  is directed through the first lens array  109  where it is focused onto the channel mirror  119  that corresponds with this channel.  
         [0056]    Because channel mirror  119  is engaged, the second channel  227  is reflected off the channel mirror back through the first lens array  107  and focused onto the first thin film demux/mux  105 . The second channel  227  enters the second surface  210  of the first thin film demux/mux  105  through thin film filter  205 , and is reflected by first surface  201  and second surface  210  to the DROP lens FIG. 1, 121 (not shown on FIG. 2), so as to exit the device through the DROP channel.  
         [0057]    A new channel three  215  may be ADDED as follows: In the instance where channel mirror  125  is engaged and DROPs channel three contained within multi-channel collimated light signal  213 , a new channel three  215  may be ADDED to the multi-channel collimated light signal exiting the devices FIG. 1, 117 (not shown). A new channel three  215  is directed into the first surface  217  of the second thin film demux/mux  115  by collimator  123 . The new channel three  215  is reflected off the second surface  218 , because thin film filter  219  is only transparent to channel one, back to the first surface  217 . The new channel three  215  is again reflected off the first surface  217  and then reflected off the second surface  218 , because thin film filter  221  is only transparent to channel two. The new channel three  215  is again reflected off the first surface  217  and then passes through thin film filter  223 , which is transparent to channel three, exiting the second thin film demux/mux  115 . The new channel three  215  is directed through the second lens array  113  and focused onto channel mirror  125 , which is engaged and reflects the new channel three  215  back through the second lens array  113  and into the second thin film demux/mux  115 . Thin film filter  223  allows channel three to pass, multiplexing channel three onto the multi-channel collimated light signal. The multi-channel collimated light signal, now channels three and four, is reflected by the first surface  217  to thin film filter  221 , multiplexing channel, then again by the first surface  217  to thin film  221 , picking up channel one. Lastly, the multi-channel collimated light signal, now channels four-one, exits the device through the PASS lens FIG. 1, 117 (not shown on FIG. 2).  
         [0058]    A programming table for the four channel programmed optical add/drop multiplexer functioning as described in the first embodiment is provide in table 1.  
                       TABLE I                       CHANNEL   MODE   MIRROR STATE                   One   PASS   Disengaged       Two   DROP   Engaged       Three   ADD   Engaged       Four   PASS   Disengaged                  
 
         [0059]    A second embodiment of the invention is detailed in FIG. 3, again by way of a four channel programmable optical add/drop multiplexer. The optical add/drop multiplexer of FIG. 3 may be dynamically programmed to demultiplex and multiplex any combination of channels onto or off the multi-channel collimated light signal. Optical add/drop multiplexer of FIG. 3 employs optical circulators as a bi-directional optical path to achieve various objects of the invention. An optical prescription for an eight channel programmable optical add/drop multiplexer with an air-space thin film filter optical demux and a silica lens array for the second embodiment is provided in Table 2.  
               TABLE 2                           This embodiment is intended to operate at wavelengths near 1550       nm with input fibers that support a single mode that is       10.4 microns in diameter and with a MEMS mirror that tilts       +/− 0.8 deg.       COLLIMATOR       Material: SF57       Radius: −2.957       Conic: −0.651       Thickness: 1.000                  
 
         [0060]    The collimator array has an aspheric surface described by the following equation  
           z=curv*y {circumflex over ( )}2/(1+ sqrt (1−(1+ k ) curv {circumflex over ( )}2 y {circumflex over ( )}2)  
         [0061]    wherin x and y are measured from the center of each lenslet, curv=1/radius, and k=conic.  
         [0062]    Demux/Mux Block  
         [0063]    Material: AIR  
         [0064]    Size  
         [0065]    Block: 2.551×15.317×12.000  
         [0066]    Filter: 2.551×1.051  
         [0067]    Tilt: 2.800 deg  
         [0068]    Lens Array  
         [0069]    Material: SILICA  
         [0070]    Lenslet aperture size: 3.000×1.498  
         [0071]    Array size: 3.000×11.986  
         [0072]    Substrate thickness: 2.000  
                                                                                   Right Hand Side       Pyramid (mrad) Maximum            Channel   Radius   Conic   X-tilt   Y-tilt   Lenslet Sag                    1   12.933   −0.552   4.887   0.000   0.116       2   12.796   −0.558   4.267   0.000   0.116       3   12.571   −0.567   3.214   0.000   0.117       4   12.270   −0.578   1.751   0.000   0.117       5   11.942   −0.589   0.068   0.000   0.118       6   11.645   −0.598   −1.537   0.000   0.119       7   11.418   −0.605   −2.821   0.000   0.119       8   11.268   −0.610   −3.693   0.000   0.119                  
 
         [0073]    The lenslets array has an aspheric surface described by the following equation  
           z=curv*y {circumflex over ( )}2/(1+ sqrt (1−(1+ k ) curv {circumflex over ( )}2 y {circumflex over ( )}2)+ xt*abs ( x )+ yt*abs ( y )  
         [0074]    wherin x and y are measured from the center of each lenslet, curv=1/radius,k=conic, and xt=x-tilt and yt=y-tilt of the pyramid term.  
         [0075]    All linear dimensions are in millimeters.  
         [0076]    The programmable optical add/drop multiplexing device of FIG. 3 comprises a first and second circulator  301  and  335 , a first and a second lens  307  and  325 , a thin film demux  309 , a first lens array  311 , and a mirror array  313 . All of these components are precisely aligned with each other, and mounted together so as to accommodate the entrance and exit of optical signals. Often, the device of FIG. 3 may be mounted within an enclosure optimize for optical transmission, including a gas-filled inclosure, or the like.  
         [0077]    In FIG. 3 a multi-channel light signal enters the device through the IN port  303  on the first circulator  301  and is passed via the Bi-directional port  305  of circulator  301  to the first collimator  307 , and directed into the thin film demux/mux  309  so as to enable each channel to separately exit the thin film demux/mux  309  and be focused by the lens array  311  onto the mirror array  313 . Gaussian beam waists are located mid-way through the thin film. Thin film filter demux/mux  309  contains four thin films located on the surface of the thin film demux/mux  309 , each filter is transparent to a sub-range of wavelengths contained in the multi-channel collimated light signal.  
         [0078]    The mirror array  313  contains four one-sided see-saw channel mirrors, one for each of the four channels of the device; the first channel exits the thin film demux/mux  309  and is focused onto channel mirror  315 , the second channel is focused onto channel mirror  323 , the third channel is focused onto channel mirror  325 , and the fourth channel is focused onto channel mirror  327 . For each channel, the lens array element relays the beam waist formed by the collimators in the middle of the demux/mux onto the channel mirror. Since the distance between the collimator waist and the lens array elements differs for each channel depending on the zig-zag path through the demux/mux, each lens array element optimally has a different radius of curvature and conic constant. To center the waists on each of the channel mirrors different wedges for each of the two sub-apertures used by the IN, DROP, ADD, and PASS beams are employed and different wedges for each channel are also employed. The radius, conic, and wedges can be easily designed using commercially available lens design software.  
         [0079]    It will be understood by those familiar with the art that the centered wavelength of each channel need not be demuxed or muxed beginning with the highest centered or lowest centered, nor need the channels be arranged in order of their wavelength. In fact, it will be further understood by those familiar with the art that wavelengths may be demuxed or muxed in any order.  
         [0080]    As in the previous example which detailed a four channel device, here the first and fourth channels are PASSED through the device, the second channel is DROPPED from the device, and the third channel is ADDED. Each of the four channel&#39;s path&#39;s are detailed below.  
         [0081]    Channel one  319  is PASSED through the devices as follows. The multi-channel light signal enters the device through the IN port  303  of the first circulator  301  and first collimator  307  and is directed into the thin film demux/mux  309 . Channel one  319  passes through the thin film filter corresponding to its wavelength, demultiplexing the channel from the multi-channel collimated light signal and then directed through the lens array  311  and focused onto channel mirror  315 .  
         [0082]    Channel mirror  315  is in PASS mode, accordingly, channel one  319  is reflected off the channel mirror back through the lens array  311  and focused onto the thin film demux/mux  309 . Channel one  319  enters the thin film demux/mux  309  and is muxed onto the multi-channel collimated light signal. Next, the multi-channel collimated light signal enters the first collimator  307  and the bi-directional port  305  of the first circulator  301  and exits the device through the PASS port  317  of the first circulator  301 .  
         [0083]    Channel four  320  is PASSED in identical fashion to channel one  319 , except that the channel four  320  passes through the thin film filter corresponding to its wavelength and is reflected back by channel mirror  327 .  
         [0084]    Channel two  321  is DROPPED from the device as follows: The multi-channel light signal enters the device through the IN port  303  of the first circulator  301  and first collimator  307  and is directed into the thin film demux/mux  309 . Channel two  321  passes through the thin film filter corresponding to its wavelength, demultiplexing the channel from the multi-channel collimated light signal and then is directed through the lens array  311  where it is focused onto channel mirror  323 .  
         [0085]    Channel mirror  323  is programmed to DROP/ADD the channel, and accordingly, channel two  321  is reflected off channel mirror  323  back through the lens array  311  and focused onto the thin film demux/mux  309 , then enters the second lens  323  and bi-directional port  329  of the second circulator  335  and exits the device through the DROP port  331  of the second circulator  335 .  
         [0086]    A new channel three  337  may be ADDED as follows: In the instance where channel mirror  325  is engaged and DROPs the channel three contained within the multi-channel collimated light signal, a new channel three  337  may be ADDED to the multi-channel light signal exiting the device. New channel three  337  enters the device through the ADD port  333  of the second circulator  335  and second collimator  325  and is directed into the thin film demux/mux  309 . The new channel three  337  passes through the thin film filter corresponding to its wavelength and is directed through the lens array  311  where it is focused onto channel mirror  325 .  
         [0087]    Channel mirror  325  is in DROP/ADD mode, and accordingly, channel three  337  is reflected off the channel mirror back through the lens array  311  and focused onto the thin film demux/mux  309 . Channel three  325  enters the thin film demux/mux  309  and is muxed into the multi-channel collimated light signal. Next, the multi-channel collimated light signal enters the first collimator  307  and the bi-directional port  305  of the first circulator  301  and exits the device through the PASS port  317  of the first circulator  305 .  
         [0088]    The quarter-wave plate (QWP)  310  reduces polarization dependent loss (PDL) in the system. QWP  310  is located such that it is substantially normal to the propagating light beam and the retardance axis is at 45° to the light that was polarized parallel and perpendicular to the plane of incidence throughout the demux.  
         [0089]    More specific details of the see-saw mirrors used in the FIG. 3 are provided in FIG. 4. The mirror&#39;s state detailed in  401  is as employed for IN/PASS mode. The mirror&#39;s state detailed in  403  is as employed for DROP/ADD mode. By reversing the voltage and ground applied the mirror changes its position, directing the optical signals as detailed above.  
         [0090]    A third embodiment of the invention is detailed in FIG. 5, again by way of a four channel programmable optical add/drop multiplexer. The optical add/drop multiplexer of FIG. 5 may be dynamically programmed to demultiplex and multiplex any combination of channels onto or off the multi-channel collimated light signal.  
         [0091]    The programmable optical add/drop multiplexing device of FIG. 5 comprises an IN collimator  503 , a PASS lens  505 , a DROP lens  507 , and an ADD collimator  509 , a thin film demux/mux  511 , a lens array  513 , a first, second, third and fourth channel mirror  515 ,  517   519  and  521 . All of these component are precisely aligned with each other, and mounted together so as to accommodate the entrance and exit of optical signals. The device of FIG. 5 may probably be mounted within an enclosure optimize for optical transmission, including a gas-filled enclosure, or the like.  
         [0092]    In FIG. 5 a multi-channel light signal enters the device through the first collimator  503  and directed into the thin film demux/mux  511 . The collimated light signal is directed inside the thin film demux/mux  511  so as to enable each channel to separately exit the thin film demux/mux  511  and be focused by the lens array  513  onto one of four see-saw mirrors  515 ,  517 ,  519 ,  521  corresponding to one of the four channels of the device; the first channel exits the thin film demux/mux  511  and is focused onto channel mirror  515 , the second channel is focused onto channel mirror  517 , the third channel is focused onto channel mirror  519 , and the fourth channel is focused onto channel mirror  521 . Gaussian beam waists are located mid-way through the thin film demux/mux and at the center of each of the mirrors to minimize insertion losses. For each channel, the lens array element relays the beam waist formed by the collimators in the middle of the demux/mux onto the channel mirror. Since the distance between the collimator waist and the lens array elements differs for each channel depending on the zig-zag path through the demux/mux, each area lens optimally has a different radius of curvature and conic constant. To center the waists on each of the channel mirrors different wedges for each of the four sub-apertures used by the IN, DROP, ADD, and PASS beams are employed and different wedges for each channel are also employed. The radius, conic, and wedges can be easily designed using commercially available lens design software.  
         [0093]    It will be understood by those familiar with the art that the centered wavelength of each channel need not be demuxed or muxed beginning with the highest centered or lowest centered, nor need the channels be arranged in order of their wavelength. In fact, it will be further understood by those familiar with the art that wavelengths may be demuxed or muxed in any order.  
         [0094]    As in the previous example, here the first and fourth channels are PASSED through the device, the second channel is DROPPED from the device, and the third is ADDED. Each of the four channel&#39;s path&#39;s are detailed below.  
         [0095]    Channel one  525  is PASSED through the devices as follows. The multi-channel light signal  501  enters the device through the IN collimator  503  and is directed into the thin film demux/mux  511 . Channel one  525  passes through the thin film filter corresponding to its wavelength, demultiplexing the channel from the multi-channel collimated light signal  501  and is directed through the lens array  513  where it is focused onto channel mirror  515 .  
         [0096]    Channel mirror  515  is in PASS mode, accordingly, channel one  525  is reflected off the channel mirror back through the lens array  513  and focused onto the thin film demux/mux  511 . Channel one  525  enters the thin film demux/mux  511  and is muxed onto the multi-channel collimated light signal. Next, the multi-channel collimated light signal enters the PASS lens  505  and exits the device through the PASS port.  
         [0097]    Channel four  531  is PASSED in identical fashion to channel one  525 , except that the channel four  531  passes through the thin film filter corresponding to its wavelength and is reflected back by channel mirror  521 .  
         [0098]    Channel two  527  is DROPPED from the device as follows: The multi-channel light signal  501  enters the device through the first collimator  503  and is directed into the thin film demux/mux  511 . Channel two  527  passes through the thin film filter corresponding to its wavelength, demultiplexing the channel from the multi-channel collimated light signal and is directed through the lens array  513  where it is focused onto channel mirror  517 .  
         [0099]    Channel mirror  517  is programmed to DROP/ADD the channel, and accordingly, channel two  527  is reflected off channel mirror  517  back through the lens array  513  and focused onto the thin film demux/mux  511 , then enters the DROP lens  507  exits the device through the DROP port.  
         [0100]    A new channel three  529  may be ADDED as follows: In the instance where channel mirror  519  is engaged and DROPs channel three contained within the multi-channel collimated light signal  501 , a new channel three  529  may be ADDED to the multi-channel light signal exiting the device. A new channel three  529   337  enters the device through the ADD collimator  509  and is directed into the thin film demux/mux  511 . The new channel three  529  passes through the thin film filter corresponding to its wavelength and is directed through the lens array  513  where it is focused onto channel mirror  519 .  
         [0101]    Channel mirror  519  is in DROP/ADD mode, and accordingly, channel three  529  is reflected off the channel mirror back through the lens array  513  and focused onto the thin film demux/mux  511 . Channel three  529  enters the thin film demux/mux  511  and is muxed onto the multi-channel collimated light signal, and exits the device through the PASS collimator  505 .  
         [0102]    The quarter-wave plate (QWP)  512  reduces for polarization dependent loss (PDL) in the system. QWP  512  is located such that it is substantially normal to the propagating light beam and the retardance axis is at 45° to the light that was polarized parallel and perpendicular to the plane of incidence throughout the demux.  
         [0103]    More specific details of the see-saw mirrors used in embodiment shown in FIG. 5 are provided in FIG. 6A and FIG. 6B. These two figures illustrate the angular arrangements of the beam incident on or reflected from on of the mirrors in the mirror array. To be precise, each beam  525 ,  527 ,  529 , and  531  represents an angular range of a conically shaped beam. FIG. 6A shows the angular relationship between the beam and the a normal  605  (represented by a cross) of the tilting mirror  515  when in IN/PASS mode. When in IN/PASS mode the mirror normal  605  is between IN port and the PASS port so as to reflect beam  525  from the IN port to the PASS port  505 .  
         [0104]    [0104]FIG. 6B shows the angular relationship between the beam and the a normal  607  (represented by a cross) of the tilting mirror  517  when in DROP/ADD mode. When in DROP/ADD mode the mirror normal  607  is between IN and DROP, and is also between PASS and ADD. When in DROP/ADD mode, the mirror normal  607  is between the IN and the DROP so as to reflect beam  527  from the IN port to the DROP port.  
         [0105]    A fourth embodiment of the invention is detailed in FIG. 7, again by way of a four channel programmable optical add/drop multiplexer. The optical add/drop multiplexer of FIG. 7 may be dynamically programmed to demultiplex and multiplex any combination of channels onto or off the multi-channel collimated light signal. An optical prescription for an sixteen channel programmable optical add/drop multiplexer with a silica lens array for the fourth embodiment is provided in Table 3.  
               TABLE 3                           This embodiment is intended to operate at wavelengths near 1550       nm with a MEMS mirror that tilts +/− 0.7 deg.       COLLIMATOR       Material: SF57       Radius: −1.929       Conic: −0.667       Thickness: 1.000                  
 
         [0106]    The collimator array has an aspheric surface described by the following equation  
           z= ( curv ) y {circumflex over ( )}2/(1+ sqrt (1−(1+ k ) curv {circumflex over ( )}2 y {circumflex over ( )}2)  
         [0107]    wherin x and y are measured from the center of each lenslet, curv=1/radius, and k=conic.  
         [0108]    Demux/Mux Block  
         [0109]    Material: BK7  
         [0110]    Size  
         [0111]    Block: 1.800×7.608×14.400  
         [0112]    Filter: 1.800×1.800  
         [0113]    Tilt: 10.000 deg  
         [0114]    Lens Array  
         [0115]    Material: SILICA  
         [0116]    Lenslet aperture size: 1.800×1.773  
         [0117]    Array size: 1.800×14.181  
         [0118]    Substrate thickness: 2.000  
                                                                                                                                                     Right Hand Side                Pyramid (rad)            Channel   Radius   Conic   X-tilt   Y-tilt                    1   9.127   −0.381   5.697e−003   5.610e−003       2   8.998   −0.403   4.989e−003   4.914e−003       3   8.828   −0.432   4.030e−003   3.970e−003       4   8.628   −0.467   2.844e−003   2.802e−003       5   8.411   −0.505   1.501e−003   1.480e−003       6   8.198   −0.542   1.107e−004   1.078e−004       7   8.004   −0.574   −1.218e−003   −1.201e−003       8   7.841   −0.603   −2.390e−003   −2.355e−003                    Left Hand Side                Pyramid (rad)            Channel   Radius   Conic   X-tilt   Y-tilt                    1   9.065   −0.391   5.363e−003   5.282e−003       2   8.915   −0.417   4.526e−003   4.457e−003       3   8.728   −0.449   3.444e−003   3.392e−003       4   8.517   −0.487   2.165e−003   2.134e−003       5   8.300   −0.525   7.816e−004   7.710e−004       6   8.095   −0.559   −5.894e−004   −5.818e−004       7   7.916   −0.589   −1.846e−003   −1.820e−003       8   7.770   −0.614   −2.913e−003   −2.870e−003                  
 
         [0119]    An optical prescription for an sixteen channel programmable optical add/drop multiplexer with a silica lens array for the fourth embodiment is provided in Table 4.  
               TABLE 4                           This embodiment is intended to operate at wavelengths near 1550       nm with a MEMS mirror that tilts +/− 0.7 deg.       COLLIMATOR       Material: SF57       Radius: −1.929       Conic: −0.667       Thickness: 1.000                  
 
         [0120]    The collimator array has an aspheric surface described by the following equation  
           z =( curv ) y {circumflex over ( )}2/(1+ sqrt (1−(1+ k ) curv {circumflex over ( )}2 y {circumflex over ( )}2)  
         [0121]    wherin x and y are measured from the center of each lenslet, curv=1/radius, and k=conic.  
         [0122]    Demux/Mux Block  
         [0123]    Material: BK7  
         [0124]    Size  
         [0125]    Block: 1.800×7.608×14.400  
         [0126]    Filter: 1.800×1.800  
         [0127]    Tilt: 10.000 deg  
         [0128]    Lens Array  
         [0129]    Material: SILICON  
         [0130]    Lenslet aperture size: 1.800×1.773  
         [0131]    Array size: 1.800×14.181  
         [0132]    Substrate thickness: 2.000  
                                                                                                                                                     Right Hand Side                Pyramid (rad)            Channel   Radius   Conic   X-tilt   Y-tilt                    1   50.874   −7.351   1.020e−003   1.005e−003       2   50.156   −7.081   8.937e−004   8.800e−004       3   49.216   −6.731   7.223e−004   7.111e−004       4   48.101   −6.307   5.102e−004   5.021e−004       5   46.897   −5.865   2.700e−004   2.654e−004       6   45.709   −5.453   2.062e−005   1.951e−005       7   44.631   −5.108   −2.173e−004   −2.151e−004       8   43.722   −4.809   −4.269e−004   −4.218e−004                    Left Hand Side                Pyramid (rad)            Channel   Radius   Conic   X-tilt   Y-tilt                    1   50.533   −7.225   9.606e−004   9.459e−004       2   49.697   −6.916   8.108e−004   7.984e−004       3   48.658   −6.519   6.174e−004   6.078e−004       4   47.484   −6.051   3.888e−004   3.824e−004       5   46.276   −5.647   1.412e−004   1.384e−004       6   45.135   −5.266   −1.047e−004   −1.041e−004       7   44.139   −4.928   −3.297e−004   −3.259e−004       8   43.328   −4.736   −5.205e−004   −5.142e−004                  
 
         [0133]    The lenslets array has an aspheric surface described by the following equation  
           z =( curv ) y {circumflex over ( )}2/(1+ sqrt (1−(1+ k ) curv {circumflex over ( )}2 y {circumflex over ( )}2)+ xtabs ( x )+ ytabs ( y )  
         [0134]    wherin x and y are measured from the center of each lenslet, curv=1/radius, k=conic, and xt=x-tilt and yt=y-tilt of the pyramid term.  
         [0135]    All linear dimensions are in millimeters.  
         [0136]    The programmable optical add/drop multiplexing device of FIG. 7 comprises an IN collimator  703 , a PASS lens  705 , a DROP lens  707 , and an ADD collimator  709 , a thin film demux/mux  711 , a first lens array  713 , a second lens array  716 , and a first, second, third and fourth channel mirror  715 ,  717   719  and  721 . All of these component are precisely aligned with each other, and mounted together so as to accommodate the entrance and exit of optical signals. The device of FIG. 7 may probably be mounted within an enclosure optimize for optical transmission, including a gas-filled enclosure, or the like.  
         [0137]    In FIG. 7 a multi-channel light signal enters the device through the first collimator  703  and directed into the thin film demux/mux  711 . The collimated light signal is directed inside the thin film demux/mux  711  so as to enable each channel to separately exit the thin film demux/mux  711  and be focused by either the first lens array  713  onto see-saw mirrors  715  or  717  or by the second lens array  714  onto see-saw mirrors  719  or  721  corresponding to one of the four channels of the device; the channel with the highest centered wavelength exits the thin film demux/mux  711  and is focused onto channel mirror  715 , the next highest centered wavelength is focused onto channel mirror  717 , the next highest centered wavelength is focused onto channel mirror  719 , and the next highest centered wavelength is focused onto channel mirror  721 . Gaussian beam waists are located mid-way through the thin film demux/mux and at the center of each of the mirrors to minimize insertion losses. For each channel, the lens array element relays the beam waist formed by the collimators in the middle of the demux/mux onto the channel mirror. Since the distance between the collimator waists and the lens array elements differ for each channel depending on the zig-zag path through the demux/mux, each lens element optimally has a different radius of curvature and conic constant. To center the waists on each of the channel mirrors different wedges for each of the four sub-apertures used by the IN, DROP, ADD, and PASS beams are employed and different wedges for each channel are also employed. The radius, conic, and wedges can be easily designed using commercially available lens design software.  
         [0138]    It will be understood by those familiar with the art that wavelengths need not be demuxed or muxed beginning with the highest centered and moving toward the lowest centered. In deed, it will be further understood by those familiar with the art that wavelengths may be demuxed or muxed in any order by simply adjusting the properties of the thin film demux/mux  711 .  
         [0139]    As in the previous example, here the first and fourth channels are PASSED through the device, the second channel is DROPPED from the device, and the third is ADDED. Each of the four channels&#39; paths is detailed below.  
         [0140]    Channel one  725  is PASSED through the devices as follows. The multi-channel light signal  701  enters the device through the IN collimator  703  and is directed into the thin film demux/mux  711 . Channel one  725  passes through the thin film filter corresponding to its wavelength, demultiplexing the channel from the multi-channel collimated light signal  701  and is directed through the first lens array  713  where it is focused onto channel mirror  715 .  
         [0141]    Channel mirror  715  is in PASS mode, accordingly, channel one  725  is reflected off the channel mirror back through the first lens array  713  and focused onto the thin film demux/mux  711 . Channel one  725  enters the thin film demux/mux  711  and is muxed onto the multi-channel collimated light signal. Next, the multi-channel collimated light signal enters the PASS  705  and exits the device through the PASS port.  
         [0142]    Channel four  731  is PASSED in identical fashion to channel one  725 , except that the channel four  731  passes through the thin film filter corresponding to its wavelength, demultiplexing the channel from the multi-channel collimated light signal  701  and is directed through the second lens array  714  where it is focused onto the second channel mirror  721 .  
         [0143]    Channel two  727  is DROPPED from the device as follows: The multi-channel light signal  701  enters the device through the first collimator  703  and is directed into the thin film demux/mux  711 . Channel two  729  passes through the thin film filter corresponding to its wavelength, demultiplexing the channel from the multi-channel collimated light signal and is directed through the second lens array  714  where it is focused onto channel mirror  717 .  
         [0144]    Channel mirror  717  is programmed to DROP/ADD the channel, and accordingly, channel two  727  is reflected off channel mirror  717  back through the second lens array  714  and focused onto the thin film demux/mux  711 , then enters the DROP lens  707  exits the device through the DROP port.  
         [0145]    A new channel three  729  may be ADDED as follows: In the instance where channel mirror  719  is engaged and DROPs channel three contained within the multi-channel collimated light signal  701 , a new channel three  729  may be ADDED to the multi-channel collimated light signal exiting the device. A new channel three  729   337  enters the device through the ADD collimator  709  and is directed into the thin film demux/mux  711 . The new channel three  729  passes through the thin film filter corresponding to its wavelength and is directed through the first lens array  713  where it is focused onto channel mirror  719 .  
         [0146]    Channel mirror  719  is in DROP/ADD mode, and accordingly, channel three  729  is reflected off the channel mirror back through the first lens array  713  and focused onto the thin film demux/mux  711 . Channel three  729  enters the thin film demux/mux  711  and is muxed onto the multi-channel collimated light signal, and exits the device through the PASS lens  705 . A first quarter-wave plate (QWP)  712  and a second quarter-wave plate (QWP)  716  reduce polarization dependent loss (PDL) in the system.  
         [0147]    In FIG. 8 a reflective mirror  803  is employed to directly reflect a channel back into thin film demux/mux  801  for the embodiments of the invention that use circulators. The mirror  805  may also be used to reflect multiple channels back into thin film demux/mux  801  by mounting the mirror  803  at a location in the optical path corresponding with a port on thin film demux/mux  801  that is transparent to multiple channels. Mirror  805  has a reflective surface that is angled in such as way as to reflect the channels into the PASS port of the system. Alternatively, a roof mirror or prism may be used to reflect the channels into the DROP port of the system. Those skilled in the art will recognize that a reflective prism may be employed instead of a reflective mirror to achieve the same result. A prism may also be directly affixed to the thin film demux/mux  801 . The reflective roof prism or roof mirror may be used to connect any pair of signals from the IN, ADD, DROP, PASS channels in the embodiments of this invention which use one demux and no circulators.  
         [0148]    A fifth embodiment of the invention is detailed in FIG. 9, again by way of a four channel programmable optical add/drop multiplexer. The optical add/drop multiplexer of FIG. 9 may be dynamically programmed to demultiplex and multiplex any combination of channels onto or off the multi-channel collimated light signal, as well as to provide power and signal monitoring features for each of the IN, DROP, ADD, and PASS ports.  
         [0149]    A small portion of the light split from at least one of the IN, DROP, ADD, and PASS ports using a fiber optic splitter  911  and the small portion is send to a second collimator array which contains one lens for each the IN, DROP, ADD, PASS port&#39;s that is being monitored, to the thin film demux/mux  903 , to a second lens array  915 , and finally to a detector array  917 . Those skilled in the art will recognize that this embodiment of the invention may be practiced with any of the embodiments of the invention described herein. The second lens array  915  contains a lens for each channel of the IN, DROP, ADD, and PASS ports that is being monitored, directing the light from each channel to be directed to an individual detector located on the detector array  917 .  
         [0150]    A sixth embodiment of the invention is detailed in FIG. 10, again by way of a four channel programmable optical add/drop multiplexer. Those skilled in the art will recognize that this embodiment of the invention may be practiced with any of the embodiments of the invention described herein. The optical add/drop multiplexer  1001  of FIG. 10 enables adjustable power attenuation of each of the channels by employing an array of MEMS shutters used as optical attenuators. It is also possible to place attenuators in the IN, ADD and DROP channels. Those skilled in the art will appreciate that other means of attenuating the beam are possible, including grating light valves, x liquid crystals, and interference modulators. MEMS shutters  1003 ,  1005 ,  1007 , and  1009  partially obscure each of the PASS beams. A closed-loop feedback circuit each of the PASS channel may also be provided to enable automatic power control. FIG. 11 depicts the placement and movement of the MEMS shutters, when employed as optical attenuators.  
         [0151]    A seventh embodiment of the invention is detailed in FIG. 12, again by way of a four channel programmable optical add/drop multiplexer. Those skilled in the art will recognize that this embodiment of the invention may be practiced with any of the embodiments of the invention described herein. The optical add/drop multiplexer enables the ADD channels to be introduced and the DROP channels to be removing to separate optical ports  1205 . Each port contains a single wavelength, a single collimator, and optionally reflecting means such as a prism  1203  or mirror (not shown) is used to redirect the channels. Light may be introduced or removed from the single wavelength ADD and DROP ports via an optical fiber. Alternatively, the ADD ports may directly collimate a laser transmitter and the DROP ports may be focused onto a receiving detector. This embodiment may be practiced on one or more channels, and in combination with other embodiments, of a multi-channel programmable optical add/drop multiplexer. When used with the second embodiment of this invention, a circulator is needed of each of the separate ADD/DROP channels. When the sixth and seventh embodiment of this invention is practiced, it is desirable to place the attenuators between the separate ADD/DROP ports and the lenses. It is most preferred for the attenuators to be close to the ADD/DROP ports where the beams are small, which allows for smaller and faster attenuators.