Abstract:
A device for expanding the wavelength band of an optical component is disclosed. The device has a beam splitter for splitting a wavelength division multiplexed beam into two beams at a desired separation angle. An optical grating separates the two beams into spectral components for each beam. The spectral components are focused on a receiving surface. The separation angle between the two beams expands the wavelength band of the WDM signal.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Patent Application Serial No. 60/340,221, filed on Dec. 13, 2001, entitled METHODS AND TECHNIQUES FOR ACHIEVING FLATTENED AND BROADENED PASS BAND SPECTRUM FOR FREE-SPACE GRATING-BASED DENSE WAVELENGTH DIVISION MULTIPLERS/DEMULTIPLEXERS, the contents of which are incorporated herein be reference.  
         [0002]    Furthermore, this patent application relates to U.S. Provisional Patent Application Serial No. 60/301,958, filed on Jun. 28, 2001, entitled METHODS AND DESIGNS FOR ACHIEVING WIDE WAVELENGTH PASS BAND IN OPTICAL COMMUNICATION DEVICES, U.S. patent application Ser. No. 10/185,586, filed on Jun. 28, 2002, entitled METHODS AND DESIGNS FOR ACHIEVING WIDE WAVELENGTH PASS BAND IN OPTICAL COMMUNICATION DEVICES, U.S. Provisional Patent Application Serial No. 60/338,858, filed on Dec. 7, 2001, entitled, PASS BAND FLATTENING AND BROADENING METHODS AND TECHNIQUES FOR FREE-SPACE GRATING-BASED DENSE WAVELENGTH DIVISION MULTIPLEXING DEVICES, U.S. patent application Ser. No. ______, filed on Dec. 6, 2002, entitled METHOD AND SYSTEM FOR PASS BAND FLATTENING AND BROADENING OF TRANSMISSION SPECTRA USING GRATING BASED OPTICAL DEVICES, the contents of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention relates generally to methods and devices of flattening and broadening the pass band spectra of optical elements and devices that require flat and wide wavelength pass band, and, in particular, to methods and devices of flattening and broadening the pass band spectrum for grating-based optical components used for transmitting and receiving laser light through a single-mode optical fibers of multi-channel optical communications networks.  
           [0005]    2. Status of the Prior Art  
           [0006]    Fiber optic networks are becoming increasingly popular and important for high-speed and large capacity data transmission. The networks are continuously growing due to the explosive expansion of telecommunications and computer communications, especially in the area of the Internet. This has created a dramatic increase in the volume of worldwide data traffic and has placed an increased demand for communication networks to provide increased bandwidth. To meet this demand, fiber-optic (light wave) communication systems have been developed to harness the enormous usable bandwidth (tens of tera-Hertz) of a single optical fiber transmission link. Because it is impossible to exploit all of the bandwidth of an optical fiber using a single high-capacity channel, wavelength division-multiplexing (WDM) fiber-optic systems have been developed to provide high-capacity transmission of multi-carrier signals over a single optical fiber thereby channelizing the bandwidth of the fiber.  
           [0007]    In accordance with WDM technology, a plurality of superimposed concurrent signals are transmitted on a single fiber whereby each signal has a different wavelength. WDM technology takes advantage of the relative ease of signal manipulation in the wavelength, or optical frequency domain, as opposed to the time domain. In WDM networks, optical transmitters and receivers are tuned to transmit and receive on a specific wavelength such that many signals operating at distinct wavelengths share the single fiber.  
           [0008]    Wavelength multiplexing devices are commonly used in fiber-optic communications system to generate a single multi-carrier communication signal stream in response to a plurality of concurrent signals each having different wavelengths and received from associated sources or channels for transmission on the single fiber. At the receiving end, wavelength division demultiplexing devices are commonly used to separate the composite wavelength signal into the several original signals each having a different wavelength.  
           [0009]    Some of the most important components in the wavelength division system are demultiplexers, multiplexers, optical/add/drop multiplexers, and wavelength selective switches. It is advantageous to have wide wavelength pass bands for these components without degrading the signal performance and increasing the insertion loss. Although the operating wavelength for each of the transmitter lasers is tuned to the ITU grid wavelengths as close as possible when it was manufactured, there is always some offset to the ITU wavelength grid. Accordingly, the wider the pass window (i.e., pass band), the more tolerant is the laser offset specification such that it is easier to adjust the system. Also, there is always some drift, both in terms of the laser center wavelengths and the center wavelength of the pass band itself, such that the wider pass band allows the system to be more tolerant so that the center wavelength can ‘walk out’ the ‘passing window’ of the demultiplexer. Furthermore, the wider the pass band, the flatter the pass window will be. Therefore when many components are cascaded in series, the total pass band shape will not deteriorate quickly and the signal can travel farther without re-conditioning.  
           [0010]    In free-space grating-based devices, such as multiplexers, demultiplexers, optical add/drop multiplexers, wavelength-selective switches, etc. . . . , either transmissive or reflective diffraction gratings are employed as spectral dispersion elements. The position of a given spectral component is a function of the diffraction angle. For example, in a narrow spectral range (C-band, L-band, or C+L-band), the geometrical separation between two neighboring ITU channels is approximately equal. Single-mode fiber arrays are used to couple the diffracted light field and transmit individual channel signals. The spectral response near the pass band portion outputted from a given single-mode fiber is substantially Gaussian with a “narrow” bandwidth. Such a spectral shape is not desirable and it would be better to broaden and flatten the Gaussian pass band spectra.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore an object of the present invention to provide optical methods to design and manufacture optical components with desired pass band spectral response.  
           [0012]    Another object of the present invention is to provide optical methods and techniques that can broaden the pass band shapes of optical components.  
           [0013]    Further another object of the present invention is to provide optical methods and techniques that can flatten the pass band shapes of optical components.  
           [0014]    Yet another object of the present invention is to provide a design process that reduces channel cross-talk while producing wide pass band optical components.  
           [0015]    Yet still another object of the present invention is to provide methods of manufacturing wide pass band optical components with large volume capacity.  
           [0016]    A further object of the present invention is to provide a design process that has far fewer process steps, thus far fewer numbers of equipment to manufacture.  
           [0017]    Another important object of the present invention is to provide free-space DWDM devices that are easy to manufacture in large quantity using components that are easy to make.  
           [0018]    Briefly, presently preferred embodiments of the present invention provide methods and processes for producing wide pass band optical components for fiber-optic networks, including methods and processes for making the bulk (free-space) grating-related optical components within or based on glass materials, and manipulating light beam distributions, in terms of both spatial and spectral distributions. The components include an input means comprising beam separation and collimating; a grating means built into one means for diffracting light; means for combining grating with beam shaping means made of optical materials; beam shaping means on micro optical array components; means for shaping the beam through optical components with phase structures.  
           [0019]    Important advantages of the methods and processes of the present invention is that it is able to provide manufacturing methods to manufacture DWDM optical components with wide pass band and, more importantly, at highly repeatable manufacturing process, and at lower cost.  
           [0020]    The forgoing and other objects, features, and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments, which make reference to the several figures of the drawing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein:  
         [0022]    [0022]FIG. 1( a ) is a diagram showing a prior art free-space grating-based demultiplexer;  
         [0023]    [0023]FIG. 1( b ) is a graph showing a typical Gaussian transmission spectrum for the demultiplexer shown in FIG. 1( a );  
         [0024]    FIGS.  2 ( a ) and  2 ( b ) are graphs schematically showing the coupling between a uniform diffraction field and a single-mode fiber;  
         [0025]    FIGS.  3 ( a ) and  3 ( b ) are graphs showing the concept of using a wider wavelength pass band for a given channel;  
         [0026]    [0026]FIG. 4( a ) is a diagram showing a microlens array with a cylindrical lenses;  
         [0027]    [0027]FIG. 4( b ) is a graph of the transmission spectrum for the microlens array of FIG. 4( a ).  
         [0028]    FIGS.  5 ( a )- 5 ( c ) illustrate a process of generating a wide wavelength pass band by combining two closely-spaced sub-spectra;  
         [0029]    [0029]FIG. 6 is an example illustrating broadened and flattened pass band profiles by using two narrow sub-spectra;  
         [0030]    [0030]FIG. 7 is a diagram illustrating a process of generating a wide wavelength pass band by using two incident beams according to the present invention;  
         [0031]    FIGS.  8 ( a )- 8 ( d ) illustrate devices for generating the two incident beams of FIG. 7;  
         [0032]    FIGS.  9 ( a ) and  9 ( b ) illustrate two devices for preparing the two incident beams of FIG. 7;  
         [0033]    [0033]FIG. 10 is a schematic diagram illustrating a grating with a thin glass wedge for the device in FIG. 7; and  
         [0034]    [0034]FIG. 11 is a schematic diagram illustrating a system to use with the grating shown in FIG. 10. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the present invention only, and not for purposes of limiting the same, FIG. 1( a ) illustrates a prior art grating-based demultiplexing device  1000  to separate the wavelengths of an optical beam from an input optical fiber. When the device  1000  operates as a demultiplexer, an input fiber  110  transmits a composite signal containing a plurality of wavelengths from a WDM network. A divergent beam  120  from the input fiber  110  is collimated by a lens  130 . A parallel beam  140  from the lens  130  is incident upon a diffraction grating  150 . The diffraction grating  150  can be either a transmission or reflection grating and is typically a free-space grating. A dispersed beam  160  is generated from the parallel beam  140  by the diffraction grating  150 . The dispersed beam  160  is focused by a focusing lens  170  onto a fiber array  180  having a series of single-mode fibers  190 . The fiber array  180  can be made by stacking one or more rows of substantially closely spaced, end-flushed and AR (anti-reflection) coated optical fibers  190  in well-aligned silicon V-grooves. It will be recognized by those of ordinary skill in the art that the device  1000  can also operate as a wavelength multiplexer. In that case, a series of channels are inputted from the fibers  190 , then assembled, or multiplexed, and outputted from the fiber  110 .  
         [0036]    The diffraction grating  150  may be a volume phase grating having two parts, a diffractive element and a substrate. Such volume phase gratings are further described in U.S. Pat. Nos. 6,108,471 and 6,275,630 B1, the contents of which are incorporated herein by reference. The substrate is preferably made from a low scattering glass material. All the surfaces are preferably coated with an anti-reflection coating to enhance the passage of radiation. The transmission diffractive element is made by a holographic technique utilizing photosensitive media having a sufficient thickness. Preferably a volume hologram is used for the diffractive element so that diffractive efficiency is high and the operating wavelength range is broad. The photosensitive media are preferably materials that are able to achieve high spatial resolution in order to generate high groove density and thus high spectral resolution for WDM applications. The photosensitive media are preferably materials that have low light scattering, low optical noise and are capable of transmitting the wavelengths of interest to fiber optic networks. An example of such a photosensitive media is dichromated gelatin (DCG).  
         [0037]    [0037]FIG. 1( b ) is a graph showing a transmitted signal spectrum received by the fiber array  190 . The spectrum peaks  192  have the shape of a Gaussian pass band profile on a top portion of the spectrum  194  and a pass band region  196 . This phenomenon not only happens in the transmission grating-based demultiplexer device  1000  shown in FIG. 1( a ), but also in reflection grating-based demultiplexer devices when a single-mode fiber is used to receive the demultiplexed signals.  
         [0038]    The Gaussian pass band profile is not desirable in many optical communications systems. However, it is generated when the diffracted field is coupled with the single-mode fibers because the fundamental mode (i.e., the first mode) in the fiber is approximately Gaussian. The cross integral between the fundamental Gaussian field and another uniform diffracted field, even another Gaussian field, is still Gaussian-like. FIG. 2( a ) illustrates the concept of coupling between a uniform incident field  200  and a single-mode fiber  210  that leads to a Gaussian spectral response such as the spectrum  230  shown in FIG. 2( b ). Physically, the coupling efficiencies for the different spectral components that are located at varying spatial positions are different. More specifically, the spectral components at the center of the fiber have the largest coupling efficiency. By contrast, the spectral components corresponding to beams at the edges of the fiber are less effectively coupled into fiber and thus weaker in output. Accordingly, the Gaussian spectral response  230  is generated.  
         [0039]    However, optical communications systems prefer to have a wavelength demultiplexer with a wide pass band and a flat-top profile and not the spectrum profile  230  shown in FIG. 2( b ). Although the operating wavelength for each of the transmitter lasers is tuned as close as possible to the ITU grid wavelengths when it was manufactured, there is always some offset to the ITU wavelength grid. Thus the wider the pass window, the more tolerant the laser offset specification can be and thus the easier for the system to be operated. Also, there is always some wavelength drift, both in terms of the laser center wavelength and the center wavelength of the pass band itself. A wider pass band allows the system to tolerate larger drifts so that the center wavelength is able to ‘walk off’ the ‘pass window’ of the demultiplexer.  
         [0040]    Referring to FIG. 3( a ), two respective transmission spectra  300  and  310  are shown. The spectrum  300  has a relatively narrow pass band  340  compared to the transmission spectrum  310  which has relatively wider pass band  330 . The insertion loss  320  is the vertical height measured from the peak point downward, typically to the 0.5 dB, or 1 dB point. The pass bandwidths  330 ,  340  are measured in terms of wavelength for the two spectra respectively. As seen in FIG. 3( a ), the pass bandwidth  330  is larger than  340  for the same insertion loss  320  because the two spectra are different in their shapes. Thus spectrum  310  has wider pass band  330 , so that the shape of spectrum  310  is more desirable than that of spectrum  300 . However, there is a trade-off between the broadening pass band and increasing channel isolation. For the Gaussian spectral response, the wider the pass band, the lower the isolation between adjacent channels.  
         [0041]    [0041]FIG. 3( b ) shows two preferred transmission spectra  350  and  360  that have the same pass bandwidth  370  at the same insertion loss  380 . The spectrum  350  is the same as spectrum  310  in FIG. 3( a ) such that pass bandwidths  370  and  330  are equal. The spectrum  360  differs from spectrum  350  by having a substantially flat-top spectral response, and having a narrow spectral width at a low power level (e.g., 40 dB). Accordingly, lower cross-talk between adjacent channels will be exhibited. The flat-top spectral response of spectrum  360  is more desirable than spectrum  350  when optical signals are transmitted through several spectral components in a WDM networks. The cumulative pass bandwidth of each channel does not become much narrower than that of a single-stage Mux/Demux device with the spectrum  360 . The flat-top spectral response spectrum  360  with a wide pass band and a high isolation level is preferred more.  
         [0042]    As previously mentioned for FIG. 2( a ), a uniform incident field  200  is coupled to a single-mode fiber  210  and leads to a Gaussian spectral response such as the spectrum  230  (FIG. 2( b )). A microlens array may be used to couple the diffracted wavelength components into the fiber array (and the single mode fibers thereof) in order to increase coupling efficiency. The microlenses of the array not only increase the coupling efficiency but may also be able to widen the pass band of the transmission spectrum for a receiving fiber. FIG. 4( a ) illustrates a process of obtaining a widened wavelength pass band with a microlens array  430 . A microlens  420  of the microlens array  430  collects a band of incident field components from incident field  410  ranging in wavelength from λ to λ+Δλ. The microlens  420  focuses and centers the incident field  410  on the center of a respective optical fiber  440 . Referring to FIG. 4( b ), a widened spectrum  450  with a broad Gaussian profile is generated with the microlens array  430 . For comparison, a spectrum  460  is generated without the use of the microlens array  430 . The microlens array  430  is normally placed in front of the receiving fibers  440 , as is commonly known. The microlenses  420  are made by photolithographic techniques and are commonly spherical lenses. When used in multiplexer/demultiplexer devices, cylindrical lenses are preferred because the lens in the perpendicular dimension has a radius of infinity. Alternatively, the surface of a microlenses  420  can be non-spherical, or even arbitrary so that the field components within the pass band wavelength range can be equally coupled into the fiber.  
         [0043]    Although the use of the microlens array  430  can broaden the pass bandwidth to finite extent, in practice, it is difficult to obtain a flat-top pass band and reduce channel cross-talk. The size of the microlenses  420  are quite small and further diffraction will result such that the focusing area will be a finite-size spot rather than a point. Accordingly, a better approach to achieve a flat-top pass band and reduce channel cross-talk is to use a double-spot principle to modify the fields received by a single-mode fiber so that the desired shape of transmission spectrum can be produced.  
         [0044]    Referring to FIG. 5, the fundamental concept and process of broadening and flattening the pass band in multiplexer/demultiplexer devices, independent of dispersion elements, is shown. FIG. 5( a ) illustrates that a flat-top pass band spectrum  520  with a narrow spectral skirt is achieved by combining two sub-spectra  510  and  515  together. Both sub-spectra  510  and  515  are Gaussian and have a substantially narrow pass bandwidth. Because both sub-spectra  510  and  515  have a steep spectral response, the combined spectrum  520  has a substantially narrow bandwidth at a low power level (skirt) so that the signal cross-talk between adjacent channels is reduced. In order to generate the flattened pass band requires that two similar sub-spectra be separated in wavelength by a proper amount. A dispersion element and associated optical system are needed to generate the two sets of sub-spectra with a proper wavelength shift.  
         [0045]    Referring to FIGS.  5 ( b ) and  5 ( c ), the typical optical paths and field distributions for the two angular dispersed beams  530  at the same wavelength are shown. The angular dispersed beams  530 , generated by a diffraction element before the focusing lens  540 , contain the two sets of spectra  510 ,  515  slightly shifted in angle (and thus in wavelength by a corresponding small amount Δλ). The two sets of spectra  510 ,  515  for beams  530  at a desired wavelength (e.g., about 1530.33 nm) are focused to generate beams  550  and  555  with a small separating angle. Accordingly, two spots  570  and  580  are generated on a receiving plane  560 . The angular distance between beams  550  and  555  corresponds to a wavelength separation of Δλ. Consequently, the two wavelength components with a wavelength difference Δλ will overlap at the same receiving point on the receiving plane  560  because the same wavelength components coming from the different spectra separate in space. The two overlapped spectra will give rise to the flattened pass band spectrum profile  520  shown in FIG. 5( a ). The dispersion elements may be a diffraction grating or dispersion prism.  
         [0046]    A numerical simulation is illustrated in FIG. 6 as an example. Specifically, a demultiplexer, as shown in FIG. 5( b ), with 100 GHz channel spacing is simulated. A broadened pass band spectrum with a substantial flat-top profile is generated from the combination from the two narrow sub-spectra. The resulting pass bandwidth at the 0.5 dB down power point is 0.31 nm. Each sub-spectrum has a pass bandwidth of 0.112 nm at the 0.5 dB down point and a spectral separation of 0.249 nm between sub-spectrum is required. The channel isolation is increased significantly with the isolation between adjacent channels as high as 45.8 dB. For a corresponding Gaussian response with a pass bandwidth of 0.31 nm, the isolation between adjacent channels is only 8.6 dB. The lower cross-talk is achieved as the pass bandwidth is reduced. Accordingly, the isolation level can be increased while the pass band spectrum becomes flat and wide.  
         [0047]    Referring to FIG. 7, a process of generating a wide wavelength pass band with high channel isolation by using a double input configuration on a transmission grating according to the present invention is shown. The optical system  7000  has of a pair of input optical fibers  710  and  715  positioned with a preferred angle β therebetween, a collimating lens  730 , a transmission volume phase grating  700 , a beam focusing unit  760 , and a receiving fiber array  780 . The input fibers  710 ,  715 , and lens  730  form an input unit. The input fibers  710  and  715 , transmit two identical signal beams generated from a beam division element, as will be further explained below. Two divergent beams of light  720  and  725  from respective input fibers  710 ,  715  are incident upon the collimating lens  730 . Accordingly, two beams  740  and  745  are generated by the collimating lens  730  with the angle β therebetween and are incident on a front surface of the grating  700 . Because the two groups of incident beams are separated by a finite angle β, their respective diffraction directions for a given wavelength will be slightly different. The two groups of diffracted beams  750 ,  755  from the grating  700  with respective diffraction angles are collected by the focusing lens  760  and focused onto the receiving plane  780  that has microlenses and fiber arrays, as previously described. For a given wavelength (i.e., about λ=1530.33 nm), two spots  790  and  795  are formed with a spatial separation D for each desired wavelength separated by the diffraction grating  700 . Accordingly, two groups of diffraction spectra corresponding to the two incident angles are shifted in space. The spatial shift for a given wavelength must be consistent with the desired spectral separation required on the plane  780  in order to generate the two appropriate narrow sub-spectra used in FIG. 5( a ). The angular distance between diffracted beams  750 ,  755  corresponds to the wavelength separation of Δλ. Consequently, the two wavelength components with a wavelength difference αλ will overlap at the same receiving point. The two overlapped spectra will give rise to the flattened pass band spectrum profile  520  shown in FIG. 5( a ).  
         [0048]    There are various ways to generate the two incident beams  720  and  725  shown in FIG. 7. Referring to FIG. 8( a ), a 50/50 beamsplitter  810  can be used to separate an incident beam  800  into two equal-intensity beams  820  and  825  with a separation angle β. A lens element (not shown) is also needed in conjunction with the beamsplitter  810  so that two collimated beams are produced. Alternatively, two symmetrical prisms  840  and  845 , as shown in FIG. 8( b ), can produce two equal-intensity beams  850 ,  855  with a preferred separation angle β. The uniform incident beam  830  is transformed into the two beams  850  and  855  by the prisms  840  and  845 . Because the prisms are thin and the spectral range of the incident signal is relatively narrow, prism dispersion is negligible. The beams  850  and  855  are incident upon the front surface  860  of a transmission grating.  
         [0049]    The two incident beams can also been obtained by using a 3 dB wide-band fused fiber coupler  875  as shown in FIG. 8( c ). An incoming optical signal is transmitted through a single-mode optical fiber  870  and outputted through two single-mode fibers  880  with equal intensities. Furthermore, the angle between the two incident beams can be manipulated with a dual fiber capillary followed by a GRIN lens. Referring to FIG. 8( d ), two input fibers  890  are positioned in a dual fiber capillary  895  with a GRIN lens  898 . The angle between the two emerging beams is controlled by the pitch of the GRIN lens  898 .  
         [0050]    Additionally, it is possible to use a fused fiber to fabricate a Y-shaped 3 dB coupler in order to generate the two incident beams. Referring to FIG. 9( a ), a schematic diagram for a coupler  920  is shown. The coupler  920  has in input fiber  910  which branches into a Y junction with a separation angle β. Output beams  930  and  940  are generated with the angular separation β. Furthermore, planar waveguide  950  can be used to fabricate a Y-junction. In such an example, planar input waveguide  960  branches into two waveguides  970  and  980  at a separation angle β.  
         [0051]    Referring to FIG. 10, another approach to create two spatially-shifted sub-spectra for the purpose of generating a wide wavelength pass band with high isolation in a grating-based demultiplexer device is shown. The input unit is the same as shown and described for FIG. 1. However, a thin glass wedge is placed after the transmission grating  150 . Specifically, the grating  150  shown in FIG. 1 is divided into two equal sections  1020  and  1025  as shown in FIG. 10( a ). A thin glass wedge  1030  is attached to section  1025  as shown in FIGS.  10 ( b ) and ( c ). The wedge  1030  can be cemented onto the grating or separated from the grating in order to provide freedom for adjustment.  
         [0052]    [0052]FIG. 11 shows the process of generating wide wavelength pass band spectrum by using a glass wedge  1130  with a grating  1125 . The glass wedge  1130  and grating  1125  are the same wedge  1030  and grating  150  previously described. Specifically, and incident field  1110  of an optical beam is diffracted by the grating  1120  into spectral components. The beam from the grating  1120  that is transmitted through the glass wedge  1130  will deviate from its original propagation direction. After a focusing lens unit  1140 , two groups of focused beams  1150  and  1160  with the same wavelength λ are formed. The focused beams  1150  and  1160  appear as two spots  1170  and  1190  on a receiving plane  1180  with a small spatial separation in the vertical direction determined by the shape of the glass wedge  1130 . The spatial separation corresponds to a desired wavelength difference Δλ determined from the glass wedge  1130 . Accordingly, two sub-spectra are generated and focused on the center of one particular receiving fiber at the location of the spots  1170  and  1190 . The two wavelength components with a wavelength difference Δλ will overlap at that same receiving point. The two overlapped spectra will give rise to the desired flattened pass band spectrum profile  520  shown in FIG. 5( a ).  
         [0053]    Additional modifications and improvements of the present invention may also be apparent to those of ordinary skill in the art. Thus, the particular combination of parts described and illustrated herein is intended to represent only certain embodiments of the present invention, and is not intended to serve as limitations of alternative devices within the spirit and scope of the invention.