Abstract:
An add/drop filter for optical wave energy incorporates a Bragg grating in a very narrow waist region defined by merged lengths of elongated optical fibers. Light is propagated into the waist region via adiabatically tapered fibers and is transformed from two longitudinally adjacent fibers into two orthogonal modes within the air-glass waveguide of the waist and reflected off the grating from one fiber into the other. The geometry of the waist region is such that the reflected drop wavelength is polarization independent, without lossy peaks in the wavelength band of interest. Additionally, back reflection are shifted out of the wavelength band of interest. High strength gratings are written by photosensitizing the waist region fibers by constantly in-diffusing pressurized hydrogen or deuterium. For narrow spectral bandwidth gratings, dimensional variations must be minimized or compensated, and the grating is apodized by both a.c. and d.c. variations in writing beams at a net constant power. A coupler device employing these precisely is arranged in a support structure which facilitates writing, testing and adjustment of gratings, and also provides temperature compensation and wavelength adjustment.

Description:
This application is a division of application Ser. No. 09/128,477, filed Aug. 4, 1998, and entitled “Grating Assisted Coupler Devices” now U.S. Pat. No. 6,236,782, which is a Continuation-in-part of application Ser. No. 08/738,068, filed Oct. 25, 1995, entitled “Wavelength Optical Devices”, now U.S. Pat. No. 5,875,272; which is a continuation-in-part of application Ser. No. 08/703,357 filed Aug. 26, 1996, now U.S. Pat. No. 5,805,751, and claims the priority of U.S. Provisional Application No. 60/002,916 filed Aug. 29, 1995 and Provisional application No. 60/005,915 filed Oct. 27, 1995. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to optical wave propagation systems and devices utilizing electro-optical devices, and more particularly to grating assisted devices for filtering, coupling and other functions. 
     Communication systems now increasingly employ optical waveguides (optical fibers) which, because of their high speed, low attenuation and wide bandwidth characteristics, can be used for carrying data, video and voice signals concurrently. An important extension of these communication systems is the use of wavelength division multiplexing, by which a given wavelength band is segmented into separate wavelengths so that multiple traffic can be carried on a single installed line. This application requires the use of multiplexers and demultiplexers which are capable of dividing the band into given multiples (such as 4, 8, or 16 different wavelengths) which are separate but closely spaced. Adding individual wavelengths to a wideband signal, and extracting a given wavelength from a multi-wavelength signal require wavelength selective couplers, and this has led to the development of a number of add/drop filters, the common terminology now used for devices of this type. 
     Since wavelength selectivity is inherent in a Bragg grating, workers in the art have devised a number of grating-assisted devices for adding or extracting a given wavelength with respect to a multi-wavelength signal. Typical optical fibers propagate waves by the use of the light confining and guiding properties of a central core and a surrounding cladding of a lower index of refraction. The wave energy is principally propagated in the core, and a number of add/drop filters or couplers have been developed using Bragg gratings in the core region of one of a pair of parallel, closely adjacent or touching fibers. The coupling region is commonly termed “evanescent” in that a signal propagated along one fiber couples over into the other, as an inherent function of the design. The wavelength selectivity is established by the embedded grating which provides forward or backward transmission of the selected wavelength, depending upon chosen grating characteristics. For modern communication systems, however, this approach has a number of functional and operative limitations, pertaining to such factors as spectral selectivity, signal to noise ratio, grating strength, temperature instability and polarization sensitivity. 
     The applications referenced above are based upon a novel theoretical concept and practical implementation. A narrow waist region of two fused dissimilar fibers is defined between pairs of tapered coupling sections at each end. At the waist, the merged fibers are formed by elongation of an optical fiber precursor of generally conventional size and are so diametrically small that the central core effectively vanishes. The wave energy is transferred through the merged fiber region in two spatially overlapping, orthogonal modes. Since the propagating energize the modes overlap, the coupling is potentially non-evanescent in the presence of a coupling mechanism such as a diffraction grating. For example, a reflective grating written in the waist region redirects only a selected wavelength of an input signal at the input port to the drop port, while all other wavelengths propagate through the waist section without reflection to the throughput port. This reflection grating couples light between two optical modes in a non-evanescent manner. Numerous advantages derive from this concept and configuration, but the realization of its full potential is dependent upon other developmental factors. 
     For example, modern applications require that any add/drop filter based upon this concept be very efficient at routing channels, have a strong grating which can be selectively and precisely placed at or adjusted to a specific wavelength and yet have a limited bandwidth, be temperature insensitive, compact, low cost, and not subject to spurious reflections or noise in the chosen wavelength band. Achieving high drop efficiency and low polarization dependence are particularly important. The problems of achieving these operative properties while at the same time providing a repeatably producible unit of very small size and high sensitivity have required much further innovation. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, the optical properties and performance of a grating assisted asymmetric fused coupler are highly dependent on the physical characteristics of the coupler waist. Polarization insensitivity of the drop wavelength can be achieved, for example, by controlling the shape during elongation or by applying a permanent twist to the coupler waist after the grating exposure. Furthermore, the small diameter waist renders the coupler sensitive to diameter non-uniformities but it is shown that these dimensional variations can be compensated by laser trimming or by impressing a compensated index of refraction grating. Further, the strength of the grating can be dramatically increased by in-diffusing a photosensitizing gas during the grating writing process. For improved spectral characteristics the grating is apodized and unchirped by being written with concurrent grating modulated (a.c.) and uniform (d.c.) intensity UV beams. Size and other characteristics of the waist region are selected such that the drop wavelength of the coupler is adequately separated from the backreflection wavelength and the latter wavelength lies outside the frequency band of interest. 
     A small coupler having these properties and wavelength adjustability as well is enclosed within a prepackage structure which enables optical access to the coupler waist for grating writing. An elongated structure consisting of materials having different thermal coefficients of expansion is arranged to the temperature dependence of the drop wavelength. Moreover. the structure provides fine tuning so that the drop wavelength is precisely adjusted and subsequently maintained throughout the desired operating temperature range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention arises by reference to the following description, taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a simplified and idealized view of the principal parts, namely the tapered coupling branches and the waist region, of a coupler in accordance with the invention, useful in describing the optical waveguide modes and characteristics; 
     FIG. 2 is an enlarged cross sectional view of the asymmetric waist section of the coupler of FIG. 1, with the extent of the optical wave energy propagating along the waveguide denoted by dotted lines; 
     FIG. 3 is a pair of graphs (A) and (B) illustrating the relationship between normalized propagation constants and V number for waveguide configurations employed in these devices, useful in understanding how lossy cladding modes are eliminated, how an adequate separation between drop wavelength and back-reflection wavelength is achieved, and how diameter uniformity tolerances relate to coupler diameter; 
     FIG. 4 is a simplified and idealized view of a coupler twisted at the waist region to impart polarization independence; 
     FIG. 5 is a graphical representation of the drop channel spectral characteristics; namely, the drop reflectivity versus wavelength, depicting the effect of twist on the polarization splitting characteristics of the drop channel; 
     FIG. 6 is an illustrative graph of (A) local Bragg wavelength variation along a typical non-uniform diameter coupler waist before correction and (B) local Bragg wavelength after correction; 
     FIG. 7 is an illustrative graph (not to scale) of UV induced index of refraction variations in a coupler corresponding to an apodized grating, such as a cosine-squared or Gaussian apodization function; 
     FIG. 8 is a break-away perspective view of an exemplary coupler in accordance with the invention having a cylindrical housing and an interior optical fiber support structure or prepackage; 
     FIG. 9 is a side sectional view of the coupler of FIG. 8; 
     FIG. 10 is a fragmentary side sectional view of a fine tuning mechanism for compensation of wavelength within the coupler of FIGS. 8 and 9; and 
     FIG. 11 is a fragmentary side sectional view of an end portion of the coupler, showing the manner in which the coupler housing is hermetically sealed and the exterior fibers are protected. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An optical fiber wavelength router in accordance with the invention is exemplified by a wavelength selective filter, here of the type usually referred to as an add/drop filter. Such a device, in which multiple channels at different wavelengths are applied, redirects in a low loss, highly efficient manner the selected wavelength channels into a first optical fiber while transferring the remainder of the channels to a second optical fiber. While the concepts employed may be used for other applications, such as switches, multiple channel routers, and crossconnects, the add/drop filter is perhaps of greatest immediate benefit for multiplexers and demultiplexers in wavelength division multiplex (WDM) systems. 
     FIG. 1 illustrates the physical structure of this device. The fused coupler consists of a first fiber  31 ,  35  and a second fiber  30 ,  34  dissimilar in the vicinity of the coupling region  12  wherein an index of refraction grating  27  has been impressed. The two fibers may be made dissimilar by locally pretapering one of them by 20% in the vicinity of the fused region. Light launched into the single mode core of upper fiber  31  evolves adiabatically into an LP 11  mode with nominal propagation vector β 1  in the waist, and adiabatically evolves back into the single mode core of the output fiber  35 . Light launched into the single mode core of the lower fiber  30  evolves adiabatically into the LP 01  mode with propagation vector β 2 , and adiabatically evolves back into the single mode core of the output fiber  34 . If an index of refraction grating  27  is impressed in the coupler waist  12 , and if the wavelength is chosen such that β 1  and β 2  satisfy the Bragg law for reflection from an index grating of period Λ g  at a particular wavelength, say λ i :                     β   1          (     λ   i     )            +            β   2          (     λ   i     )              =       2                 π       Λ   g         ,                          
     then the optical energy at λ i  in the single mode core of the first fiber  31  is reflected non-evanescently by the grating into the single mode core of the second fiber  30 . The spectral response and efficiency of this reflective and mode-converting coupling process is dictated by the non-evanescent coupling strength of the optical modes with the grating. If the wavelength of the input mode is detuned, say to λ j , so that:                   β   1          (     λ   j     )            +            β   2          (     λ   j     )              ≠       2                 π       Λ   g                              
     then the Bragg law is no longer satisfied and the input mode in the first fiber  31  travels through the coupler waist  12  and reappears as the output mode of the first fiber  35 , with minimal leakage into the second fiber  34 . For these wavelengths the coupler is transparent; that is, no coupling occurs, and the two fused fibers remain optically independent. Therefore, only a particular wavelength λ i  is coupled out of the first fiber  31 ,  35  as determined by the grating period in the coupling region  12 . 
     In addition to backwards coupling of light into the adjacent fiber, the grating typically reflects some light back into the original fibers at different wavelengths given by 2|β 1 (λ 2 )|=k g  and 2|β 2 (λ 3 )=k g . To ensure that λ 2  and λ 3  are outside the wavelength operating range of interest, the difference between β 1  and β 2  is made sufficiently large. The difference increases as the waveguides become more strongly merged or as the fused coupler waist decreases in dimension. This general trend is depicted in FIG. 3, whereby the vertical axis separation between adjacent characteristic curves for eigenmodes of waveguide generally increases for smaller diameters (smaller V&#39;s). This difference is maximized for small coupler waists, where β 1  and β 2  correspond substantially to the LP 01  and LP 11  modes of an air-glass optical waveguide. The LP 01  mode is a common representation of the HE 11,hu e , HE 11   o  modes, and the LP 11  mode is a common representation of the HE 21   e , HE 21   o , EH 01   e , and EH 01   o  modes, illustrated in FIG.  3 . It is common in the art to speak in terms of these LP modes in waveguide structures such as coupler waists that do not exhibit circular symmetry. 
     Furthermore, the tilt angle of the transversely asymmetric grating can be selected to reduce the coupling strength for backreflection of the LP 01  into LP 01  modes and the LP 11  into LP 11  modes. The other consideration in selecting angle is to maximize the mode conversion efficiency of the LP 10  into LP 11  and LP 11  into LP 01  modes. The typical angles to minimize backreflection coupling are in the range of 3 to 5 degrees and the angle increases as the coupler waist diameter decreases. This angle is slightly different than the angle to maximize the drop efficiency. 
     To form this fiber optic coupler, two locally dissimilar fibers are fused to a narrow waist typically 1 to 50 microns in diameter, forming a waveguide in the fused region which supports at least two supermodes or eigenmodes of the composite waveguide. The number of supermodes supported by this composite waveguide structure is determined by the index profile and dimensions of the structure. When this waveguide structure is significantly reduced in diameter, the waveguiding characteristics resemble that of an air-glass waveguide. The mode propagation behavior of this simplified step index waveguide is then partially described by a parameter defined as the V number, which decreases as the radius a of the waveguide core is decreased, and depends on the optical wavelength λ o  of the mode, the core index n core  and the cladding index n clad :        V   =         2                 π                 a       λ   o                  n   core   2     -     n   clad   2         .                              
     For an air-glass waveguide n core =1.45 and n clad =1.0. For an elliptical cross section waveguide, the first or lowest order mode is nominally LP 01  and the second mode is nominally LP 11 . Typically, higher order modes exist within the coupler waist, as the total number of modes supported by such a waveguide is N≈V 2 /2, which is 8-9 for a 4 micron diameter waist at 1550 nm. However, the two lowest order modes are principally important in the add/drop operation. In general, a lossy peak appears for each higher order mode greater than two. Because the two waveguides are sufficiently dissimilar and the tapered transition region is sufficiently long, the input optical modes traveling along the single mode cores of the original fibers adiabatically evolve into the supermodes of the coupling region. Upon exiting the coupling region, the supermodes will evolve adiabatically back into the original optical modes as the waveguide splits into the two original fibers. Thus, the optical energy passes from the input to the output without being disturbed. A typical fiber asymmetry of (|β 1 |−|β 2 |)/(|β 1 |+|β 2 |)=0.005 and a taper angle of 0.01 radians results in less than 1% in undesired leakage of optical energy from one fiber to the other. To achieve the asymmetry, a pair of identical fibers can be made dissimilar by stretching (adiabatically pretapering) one fiber in a central region. The two fibers are then merged or joined into one waveguide in the coupling region, yet the two fibers behave as if they were optically independent. A grating is next impressed in the coupling region to redirect light at a particular wavelength from one fiber to another. For example, a 125 micron diameter fiber is pretapered by 25%, then elongated and fused to another 125 micron diameter fiber to form a 4.5 micron diameter, 2 cm long waist region with taper lengths of 2 cm. The resulting taper angle is sufficient to produce a low loss, adiabatic taper. For a UV impressed grating period of 0.540 micron, the wavelength of the drop channel of representative devices is in the 1550 nm range. 
     A suitable starting fiber from which such a coupler may be fabricated is characterized in part by a photosensitive cladding which may be manufactured using known fabrication procedures by doping the cladding region at least partially with a photosensitive species while maintaining the waveguiding properties (i.e., the N.A.) of a standard single mode core fiber. The goal of the deposition processes for use in the present invention is to dope a significant volume fraction of the cladding. The farther the dopant (e.g., Ge) extends out along the radius of the fiber, the more photosensitive the resulting coupler waist will be after the fusion and elongation stages. It is also important that the fiber be doped in a manner that minimizes thermal stress and material property mismatch within the doped cladding. 
     WDM systems enable multiple wideband signals to be transmitted on a single optical fiber, provided that individual wavelengths can be precisely centered at given values and have narrow bandwidths with high signal-to-noise ratios. These properties must be provided by the add/drop filters, and the concept as disclosed and claimed in the above mentioned applications have special advantages in these respects. However, the technical requirements are so critical, as is described hereafter, that production of units in quantity at low cost without the need for instrumentation, testing and burning-in at each stage, presents formidable challenges. 
     As described in the previous applications and seen in FIG. 1, the add/drop filter, also referred to as a coupler  10 , has a narrow waist  12  formed by elongation from optical fiber precursors. The waist  12 , which is in the range of 2-3 cm long, comprises a pair of locally dissimilar, longitudinally merged fibers  14 ,  15  forming a merged region typically less than 10 microns in cross sectional dimension. Specifically in this example, the waist region  12  is a hybrid dumbbell-ellipsoid in cross-section, here having a major dimension (A) of 10 microns or less and with a minor (B) dimension that provides a 0.82 ratio between the axes. The hybrid dumbbell-ellipsoid (FIG. 2) is a shape having characteristics resembling a cross between a dumbbell shape and an ellipsoid. This shape also has a transverse asymmetry best characterized as a “peanut” or “pear” shape. The asymmetry is the result of the initial pretaper. The smaller fiber  15  in the waist  12  is prestretched before elongation and merging so that it is about 20% smaller (in this example), although the relative size can vary within a range of 10-30% or more. Where the facing sides of the fibers  14 ,  15  are fused and merged they introduce irregularity into the ellipsoid and retain the asymmetry of the original fibers. The waist region  12  is preferably elongated without twist to prevent the loss of the reference plane defining the centers of the original cores, now only vestigial in character. Maintaining this reference plane in the prepackage before exposure is essential to producing the proper grating tilt asymmetry. 
     At each end of the waist  12  the fibers extend outwardly in a divergent taper 2-3 cm long along separate tapered coupling branches  20 ,  21  and  24 ,  25 . This taper is adiabatic and transitions from the small diameter waist region  12  to the much larger single mode optical fibers (not shown) which have diameters of the order of 90-125 microns. These fibers have metallized outer surfaces (not shown) suitable for soldering the coupler to the prepackage and precisely and stably maintaining coupler tension after final packaging. Within the waist region  12 , a Bragg grating  27  is recorded that is of selected periodicity suitable for the chosen drop wavelength, and the grating planes are tilted (typically 3°-5°) with respect to the larger of the transverse dimensions of the waist  12 . A multi-wavelength input propagating along one branch, e.g., the first tapered coupling branch  21  into the waist region  12  is selectively filtered by the Bragg grating  27 , which couples only the drop wavelength into the second tapered coupling branch  20  and the other fiber  30 . 
     In accordance with the invention, the modal relationships, dimensions and properties of the coupler are selected and modified such that a number of advantageous properties are concurrently achieved. Referring now to FIG. 2, the reduced diameter waist sections  14 ,  15 , derived from precursor fibers are doped to be photosensitive (8 mol % germanium is suitable) in the original cladding region surrounding the small higher index of refraction core and have only minute vestigial cores after elongation. Energy is thus confined and propagated in what may be called an air-glass waveguide, the term “air” here meaning the surrounding environment about the physical fiber, whether air or some other medium. Some characteristics of such an air-glass waveguide include a large numerical aperture and multimode waveguiding properties. The radial extent of the field outside the fiber is represented by the dashed line  17 . 
     Within the air-glass waveguiding region of the waist (FIG.  2 ), the orthogonal optical modes completely occupy and overlap the internal volume of the adjacent fiber  14  or  15 , regardless of whether the light originated in fiber  31  or  30 . Because of this complete mode overlap, when a grating is impressed within the waist region, the coupling is “non-evanescent”, since the modes completely overlap with the grating. Note that the optical mode originally associated with a particular fiber is not localized within that original fiber region in the coupler waist. The modes in the waist are no longer waveguiding in their original fiber material alone. 
     The air-glass waveguiding property of the coupler waist leads to unique optical characteristics. First, all lossy cladding modes are eliminated. Unlike the precursor optical fiber, whose cladding-air interface also serves as a waveguide, the coupler waist has a new uniform cladding (air) that does not support secondary guiding. The waist supports multiple optical modes, but their number decreases as the diameter decreases. However, a very small waist diameter reduces the number of higher order modes that degrade the short wavelength transmission of this device. These modes are guided in the waist region, yet they escape from the fiber in the adiabatic transition regions of the taper sections and contribute only to background loss at particular wavelengths. In addition, by proper tilt asymmetry of the grating, the coupling strength to these higher order modes can be dramatically reduced or suppressed entirely. 
     These characteristics become clear upon analyzing the mode diagrams of elliptical cylinders representative of coupler waists, depicted in FIG.  3 . Each curve represents one particular mode supported by the waveguide. FIG. 3 [taken from Lewis, J. E. and G. Deshpande, “Modes on elliptical cross-section dielectric tube waveguides”, Microwaves, Optics, and Acoustics, Vol. 3, 1979, pp. 147-155] illustrates the normalized propagation constants for modes of a coupler waist with an ellipticity of 0.9 (i.e. greater than the present coupler example of 0.82). The top figure (A) illustrates the odd modes, and the bottom figure (B) illustrates the even modes. The horizontal axis corresponds to the V number of the waveguide, and the vertical axis corresponds to β/β o =n eff , equivalent to the modal index of refraction of the individual optical modes of the waveguide. 
     The waveguide characteristics may be expressed in terms of different mode expressions. For example, the LP 01  (linearly polarized) mode is equivalent to a linear combination of the even and odd HE 11  modes, and the LP 11  mode is equivalent to a linear combination of the even and odd EH 01  and HE 21  modes. LP mode descriptions assist in the analysis of polarization behavior. The mode evolution properties of elliptical waveguides are more amenable to an LP mode description. 
     The slope of these characteristic curves is a measure of the effective mode index sensitivity to diameter variations. The greater the sensitivity, the greater the chirping or broadening of the Bragg grating due to a given magnitude of diameter non-uniformity. For smaller diameter couplers (smaller V&#39;s) the slope increases and the diameter sensitivity increases. That is, smaller diameter couplers have more challenging diameter uniformity requirements to achieve a narrow spectral bandwidth grating. In addition, the separation between effective index for the LP 01  and LP 11  modes increases, corresponding to a larger wavelength separation between the drop and backreflection wavelengths (which can be important, as noted below). The separation between these modes and all the additional higher order modes also increases, ensuring that the lossy peaks associated with coupling to higher order modes are pushed out of the spectral region of interest (e.g., the 1530-1560 erbium doped fiber amplifier (EDFA) window). 
     Unlike fiber gratings, there are no lossy cladding modes which contribute to losses in grating assisted mode couplers, because the actual cladding material of the coupler (typically air) does not have a secondary waveguide structure which supports additional optical modes. Only the doped silica coupler waist supports optical propagation. 
     The grating assisted mode coupler reflects light at a particular wavelength from one fiber back into the same fiber (the backreflection), and reflects light at a different wavelength from one fiber into the other (the add/drop). The add/drop response leads to the desired wavelength routing of light from one fiber to another, while the backreflection response is usually undesirable. Therefore, the wavelength at which the backreflection occurs should lie outside the operating wavelength region of the add/drop filter. For example, for dense WDM applications in the 1530 to 1565 wavelength range, the backreflection wavelength should be either below 1530 nm or above 1565 nm, or lie at a wavelength between the active wavelength channels. The backreflection/drop wavelength splitting should be 18 nm or more. 
     This wavelength splitting is readily achieved by making the waist of the add/drop coupler sufficiently narrow (&lt;7 microns) such that the wavelength of the backreflection is far from the drop wavelength. By fabricating fused couplers with small waists, the difference between the modal propagation constants of the LP 01  and LP 11  modes increases. Therefore, the wavelength splitting of the drop and backreflection also increases. This wavelength splitting is in excess of 15 nm for an elliptical cross section waist with a major axis of approximately 3.5 microns using a specialty doped starting fiber. Further reduction in the coupler waist diameter leads to a further increase in wavelength splitting. The exact relationship between waist diameter and wavelength splitting depends strongly on the physical shape and the exact index of refraction profile of the coupler waist. A general rule would be to make the waist smaller than 5 microns. However, the required uniformity of the coupler waist diameter becomes increasingly stringent as the waist diameter decreases; therefore, the waist diameter is usually selected to be that diameter which gives a backreflection/drop wavelength splitting slightly larger than 15 nm. A given add/drop filter has a backreflection peak on either the short (for pretapered fiber input) or long (for non-pretapered fiber input) wavelength side of the drop peak. 
     For many telecom applications of add/drops, such as multiplexers/demultiplexers, this drop/backreflection wavelength splitting requirement is substantially relaxed to a splitting on the order of a WDM channel spacing (0.8 nm or 1.6 nm, for example). Therefore, larger splittings are not be necessary if the wavelengths are demultiplexed from the fiber in a sequential manner (shorter wavelengths to longer wavelengths, for example). Even though the longer wavelength devices have short wavelength backreflections, those channels at these wavelengths are already extracted from the fiber by the previous add/drops. Thus for these units, this waist diameter may be larger, reducing the diameter uniformity tolerance of the coupler. 
     For most telecom applications the polarization properties of the coupler are important. Optical fields are vectorial in nature; that is, they have direction. This direction is quantified by the state of polarization of the optical field. The polarization of an optical signal may be linear, circular, elliptical, or unpolarized. Two linearly polarized optical signals are othogonally polarized if the electric field vectors lie perpendicular to one another. For example, the LP 01  and LP 11  modes can be substantially polarized along the x and y directions, where x and y are the major and minor axes of an ellipse. 
     The grating assisted mode coupler can readily exhibit a polarization dependence of the wavelength of light coupled from the input fiber to the drop. This polarization dependence is due to the form birefringence of the coupler waist in the region of the Bragg grating. In general, the modal propagation constants β within the waists of fused couplers, for light in the two orthogonal polarization states, are not equal. Referring to FIG. 5, it can be seen, by referring to the two dotted line peaks, that a wavelength separation exists between the two orthogonally polarized modes under these conditions. However, for certain cross sectional shapes and index of refraction profiles of the waist, the polarization dependence vanishes (i.e. |βLP 01,x |+|βLP 11,x |=|βLP 01,y |+|βLP 11,y |). Note that the left and right sides of this equation are equal, even though individually |βLP 01,x | is not equal to |βLP 01,y | and |βLP 11,x | is not equal to |βLP 11,y |. In fact, counter-intuitively, the polarization dependence of the drop wavelength of a coupler waist of circular cross section does not vanish, because the polarization degeneracy of the LP 11  mode does not vanish for a circular waveguide (that is |βLP 11,x |≠|βLP 11,y | for a circular waveguide), while the polarization dependence of the LP 01  modes does vanish (|βLP 01,x |=|βLP 01,y |. 
     One waist cross sectional shape for which the polarization splitting does vanish at the drop channel is the hybrid dumbbell-ellipsoid with a ratio of minor to major axes of about 0.8. Alternate descriptors include “pear” or “peanut” shaped. Such a waist cross section is achieved when elongating a fused coupler under tension by heating it with a highly controlled and repeatable heat source that is varied in temperature and exposure time to achieve the desired cross section until the monitored polarization characteristics disappear. Examples of suitable heat sources are well known in the art and include CO 2  lasers, gas flames and resistive heaters. Alternate waist cross sections have also been designed to eliminate polarization dependence but the hybrid dumbbell-ellipsoid is more readily fabricated. It has been demonstrated that polarization dependence can be reduced to &lt;&lt;0.1 nm by manufacturing the coupler so that its waist has a precise amount of shape asymmetry, as with the preferred elliptical shape. The present add/drop filter has been fabricated in a manner that ensures that the polarization splitting of the add/drop wavelength is less than 0.05 nm. The operation of such a device is then essentially polarization insensitive for gratings of FWHM bandwidth a few times the polarization splitting, or about 0.2 nm. The optical transmission spectra are then independent of the polarization of the input signal. Under such conditions the orthogonally polarized modes merge into the single drop wavelength, as shown in FIG.  5 . 
     Alternately, if any polarization dependence of the fused coupler remains, it can be dramatically reduced by twisting the fused coupler waist in the region containing the grating, as shown in FIG.  4 . As another alternative, for suitable UV grating writing conditions (e.g., polarization and intensity), polarization dependence can be trimmed out by the UV exposure. The UV exposure process produces material birefringence within the glass that can compensate for the form birefringence of the coupler waist. 
     The response of a reflection grating in a coupler waist is often undesirably “chirped” or spectrally distorted if the diameter of the waist is non-uniform, because the local propagation constants and drop wavelengths vary with changing diameter. Therefore, a grating of constant period within a non-uniform waist will have a broader spectral width than a grating of constant period within a uniform waist. A grating with a 1 Angstrom spectral width requires that the variation in diameter be less than 0.01 microns for a 5 micron cross sectional coupler waist over that region of the waist containing the grating. Similarly, the shape of the coupler should be constant over this region containing the grating to prevent additional chirp and polarization dependence. A highly uniform heat source such as a reciprocating CO 2  laser or flame can be applied to give highly uniform coupler waists. Alternately, grating chirp due to small variations in the diameter of the coupler waist on the grating response can be substantially reduced or effectively eliminated, in accordance with the invention, by local CO 2  laser heating to correct diameter variations, by locally varying the grating period impressed in the coupler waist to maintain a constant Bragg wavelength, or by laser trimming of the background d.c. index of refraction along the grating. These techniques effectively reduce spatial variations in the local Bragg wavelength of the grating along the coupler waist, as illustrated in FIG.  6 . This confronts one of the key issues in the manufacture of narrow bandwidth add/drop devices, such as those required for 100 and 50 GHz WDM systems based on grating assisted mode couplers. To determine the non-uniformities of diameter in a manner that can be scaled up to a manufacturable process, they can be directly measured by scanning electron microscopy, atomic force microscopy, by reconstructing the index profile from the complex reflectivity profile, by measuring the local amount of UV induced fluorescence, by examining the position and wavelength dependence of reflectivity, or by analyzing the spectral and spatial characteristics of light scattered transversely off the coupler waist. 
     In addition to producing uniform gratings within fused couplers, precisely apodized gratings are necessary to reduce grating sidelobes and eliminate adjacent channel crosstalk. Apodized gratings are key to meeting the performance requirements of WDM systems. Apodization is understood to have been achieved by several methods, including variable speed scanning, dithering of the phase mask and apodized phase masks. 
     An apodized grating can be written by spatially varying the modulation amplitude or a.c. component of the index of refraction in the longitudinal direction along the grating. At the same time, however, the d.c. or background index of refraction must remain extremely uniform (variations less than 0.0001) to prevent undesirable chirp or broadening of the grating. Raised cosine (cos 2 (z)), sinc 2 (z), and Gaussian (exp-z 2 /2σ 2 ) apodization functions are all effective in reducing the grating sidelobes to below −30 dB. 
     An apodized grating exhibits a longitudinally varying index of refraction modulation amplitude as well as a uniform period pattern along the waveguide. That is, the grating is gradually (over a large number of grating periods &gt;1000) turned on and then off along the light propagation direction. This smoothly varying window function reduces the spectral ringing or sidebands resulting from gratings with a sharp, rectangle window function. In general, the frequency spectrum of the filter is the Fourier transform of the spatial window function of the filter. A superior method to achieve apodization is to use a scanning exposure, in which the contrast of the optical interference pattern is varied as the grating is recorded while the total incident intensity is contrast. To achieve this, the waist region is simultaneously exposed with a d.c. beam counter-propagating with the modulated a.c. beam while the interference pattern is imprinted. The sum of the intensities of the interference pattern and the uniform beam are kept constant, eliminating undesirable chirp arising from variations in the background index of refraction. As seen in FIG. 7, the index of refraction function for the cos 2  apodized grating is thus an apodized periodic wave varying in bipolar fashion about a center line over a grating, length L and is given by:          Δ                   n        (   z   )         =     Δ                       n   o          (   z   )            [     1   +     sin                   k   g        z                   cos   2            π                 z     L         ]                                
     The intensity of the a.c. beam is: 
     
       
           I ( z )= I   o ( sin k   g   z+ 1) cos   2    πz/L   
       
     
     and the intensity of the d.c. beam is: 
       I ( z )= I   o    sin   2    πz/L.   
     Important advantages of the invention also reside in the features included in the example of FIGS. 8-11, to which reference is now made. The add/drop coupler device  50  comprises a cylindrical housing  52  of stainless steel tubing that has a 0.270″OD and a length of 3.67″ and what may be termed a “prepackage” or support structure  54  internal to the housing  52 . The prepackage structure  54  is inside the housing  52  after assembly but used as a preliminary retainer to hold in the optical fiber coupler  53  precisely during processing steps in which the grating is written and adjustments are made. The prepackage structure  54  extends longitudinally along and within the housing  52 , and centrally supports and retains the optical fiber coupler  53 , in position along the approximate central axis. 
     In an initial assembly the opposite ends of the optical fiber coupler  50  are fixed to spaced apart brass end hubs  58 ,  59  on a pair of parallel invar rods  62 , 63  that extend along the majority of the inside length of the housing  52 . For solderability and freedom from contamination, the hubs  58 ,  59  and rods  62 ,  63  are nickel plated, preferably by an electroless process, as are the other elements within the housing  52 . 
     The prepackage structure is completed by interposition between the end hubs  58 ,  59  of a pair of spaced apart base hubs  66 ,  67 , one of which is proximate to the end hub  59  and is soldered or welded to the first invar rod  62 . The other base hub  66 , on the second rod  63 , is adjacent a reference hub  69  on the first end hub  58  side, also on the second rod  63 . 
     In the prepackage assembly and adjustment phase, there are two subassemblies of rods and hubs, longitudinally slideable relative to each other. One subassembly comprises the first invar rod  62 , the first end hub  58  and the second base hub  67 , each hub being fixed in position on the rod  62 . The other assembly comprises the second invar rod  63 , the reference hub  69 , the first base hub  66  and the second end hub  59 . Engaged in this way, the whole assembly may be mounted in a fitted jig or tray (not shown) with the waist region of the optical coupler  53  being open, for writing of a Bragg grating, to an optical system mounted on the side. 
     The optical fibers in the coupler  53  diverge from the waist region at each end to where they are fusion spliced to metallized optical fibers of standard dimension. The entire optical coupler  53  extends along the approximate central axis of the housing  52 , passing through radial slots  70  provided in each of the hubs  58 ,  59 ,  66 ,  67  and  69 . When the prepackage structure  54  is held rigidly in place in its positioning tray, the coupler  53  can then be soldered at its end regions to the central regions of the spaced apart end hubs  58 ,  59 . The waist region is thus stably configured for photosensitization and grating writing steps. 
     When a grating is written in the waist region with a selected periodicity its drop wavelength must be adjusted to sub-nanometer precision and this wavelength should be essentially constant over the required operating temperature range, normally −35° C. to 85° C. Although invar has a very low temperature coefficient, it alone cannot meet the athermal. The prepackage requirements are that the separation between the end hubs  58 ,  59  decreases as temperature is increased. This decreases the tension and resulting strain ε within the coupler waist with increasing temperature T is a manner that satisfies the following equation:            ∂   ɛ       ∂   T       =         -       Λ   g       n   eff                ∂     n   eff         ∂   T         -       ∂     Λ   g         ∂   T                                
     For Ge-doped silica glasses, the primary contribution to the temperature dependence arises from the first term on the right of the above equation; that is, the temperature dependence of the effective index of refraction n eff . The second term on the right, the thermal expansion contribution to the change in grating period, is typically an order of magnitude smaller than the first term. 
     Note that the base hubs  66 ,  67  include aligned longitudinal grooves  72 ,  73  respectively, in their peripheries, and that an adjustment screw  75  extends through the reference hub  69  and the first base hub  66 . Consequently, after first adjusting the end hubs  58 ,  59  to tune the grating, the wavelength is locked in by soldering a stainless steel rod to grooves  72 ,  73  in the base hubs  66 ,  67 . This inner structure within the prepackage establishes an interior length which has a different thermal coefficient of expansion than the invar rods  62 ,  63  which are seated at each end but not otherwise spatially defined except through the interior stainless steel connection. Each base hub  66 ,  67  is coupled to a different invar rod  62  or  63  respectively but has a different spacing along that rod from the end hub  58  or  59  which determines the grating periodicity. 
     With the stainless steel rod  80  in place, the unit can be inserted into a temperature controlled chamber and cycled through the required temperature range while reading the drop wavelength with optical spectrum analyzer instrumentation. To adjust the drop wavelength so that it is the same at 25° C. as 85° C., the adjustment screw  75  is threaded inwardly or outwardly relative to the reference hub  69 . As best seen in the fragmentary view of FIG. 10, the screw has a first short thread  76  mating in the reference hub  69 , and a terminal second thread  77  mating in the first base hub  66 . By turning the screw  75  at the screw head  78 , the screw engagement point with reference hub  69  can be shifted longitudinally, increasing or decreasing the length of one invar segment and increasing or decreasing the length of the stainless steel segment. 
     When the prepackage structure  54  including the optical coupler  53  is adjusted, it is removed from the holder or tray and inserted into the cylindrical housing  52 . The prepackage  54  is fixed in position relative to the housing  52  simply by crimping the housing  52  (see FIG.  11 ), onto the second end hub  59 . End caps  82 ,  83  with central bores  83  providing openings for the fibers are engaged into the housing  52  open ends, and soldered or welded into place. The outwardly extending fibers at the exit points are soldered to the end cages  82 ,  83  to produce hermetic seals. Preferably, for longer life, the housing  52  is filled with an inert gas before the housing  52  is sealed. The outwardly extending fibers are protected against kinking and strain by shrink fit tubes  87 ,  88  of a suitable length. 
     To reduce the propagation of cracks within the coupler waist and failure of the coupler after packaging, the coupler should be hermetically packaged. The presence of water within the package can lead to coupler failure. This problem is exacerbated when the coupler is packaged under tension, which is necessary to provide a temperature insensitive mount, for example. Preferably, the coupler is packaged either in argon, helium, nitrogen gas or a mixture thereof, or in vacuum. 
     A device similarly packaged can be rendered tunable by the addition of a active tuning mechanism. We have disclosed in U.S. Ser. No. 08/703,357 the design of a low loss, all-fiber optical switch using two add/drop filters whose add and drop ports are joined to one another. If the drop wavelength of the two filters coincide, light from a first fiber will be routed to a second fiber. If the drop wavelength of one of the filters is de-tuned so that the two drop peaks no longer coincide, then the light will return the first fiber. Thus, a light signal can be switched from one fiber to another. One mechanism to instantaneously de-tune an add/drop filter is to launch a high intensity switching beam down the coupler waist. If the beam is of sufficiently high intensity, the index of refraction of the waist can change almost instantaneously (on the order of fsec) through the Kerr effect. By modulated the switching beam, light can be rapidly switched from one fiber to another. This technique does not required precise control of the intensity of the switching beam. It only requires that the intensity be sufficient to de-tune the two filters. Since silica glass used in the manufacture of optical fibers does not display large optical nonlinearities, relatively high optical intensities are required. However, other optical materials, such as specially doped silica, electrically poled silica, crystalline or polymer materials are promising candidates for this application. Another mechanism to switch an add/drop filter is to surround the coupler waist by a gas, liquid, or other substance whose optical properties can be changed by the application of an electrical, magnetic, or optical control signal. One example is a liquid crystal, whose index of refraction varies upon application of an electric field. 
     Although there have been described above various forms and modifications it will be appreciated that the invention encompasses all variations and expedients within the scope of the appended claims.