WO1999006864A1 - Grating assisted coupler devices - Google Patents

Abstract

An add/drop filter (10) for optical wave energy incorporates a Bragg grating (27) in a very narrow waist region (12) defined by merged lengths of elongated optical fibers (30, 31, 34, 35). Light is propagated into the waist region via adiabatically tapered fibers (20, 21, 24, 25) and is tranformed 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 (12) is such that the reflected drop wavelength is polarization independent, without lossy peaks in the wavelength band of interest. Back reflections are shifted out of the wavelength band of interest. High strength gratings are written by photosensitizing the waist region fibers. Narrow spectral bandwidth gratings are apodized by both a.c. and d.c. variations in writing beams. A coupler device employing these is precisely arranged in a support structure which provides temperature compensation.

Description

GRATING ASSISTED COUPLER DEVICES

Background of the Invention

This invention relates to optical wave propagation systems and devices

utilizing electro-opucal devices, and more particularly to grating assisted devices for filtering, coupling and other funcϋons.

This application is a co muation-m-part of U.S. application entitled "Wavelength Selective Optical Couplers", filed August 29, 1995 , U.S. seπal no

08/703,357; is a continuation-in-part of U.S. application entitled "Wavelength Opucal Devices", filed October 27, 1995, U.S. seπal no. 08/738,068; and claims the benefit of

U.S. provisional applicauon No. 60/055,157 entitled "Fabrication of Add/Drop Filters for Wavelength Division Multiplexing", filed on August 4, 1997, the disclosures of which are hereby incorporated herein by reference.

CommunicaUon systems now increasingly employ opucal waveguides

(opucal fibers) which, because of their high speed, low attenuation and wide bandwidth characteπsucs, 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 muluple traffic can be carried on a single installed line. This application requires the use of multiplexers and demul plexers 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 mulu-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 energy of 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, having 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;

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.

Figure 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 LPj i mode with nominal propagation

vector βi 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 LPoi mode with propagation vector β₂, 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 βj and β₂ satisfy the Bragg law for reflection from an index grating of period Λ_g

at a particular wavelength, say λ,:

2π

|A ^{( )}| ⁺|A^{( )}| = Λ „

then the optical energy at λ, 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 λj, so that:

2π friλ_ji + l&iλ_j)

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 λi 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| ^} _I (λ₂ )| = k_g and 2JyS₂ ( L₃)| = k_g. To ensure that λ₂ and λ₃ are

outside the wavelength operating range of interest, the difference between βi and β₂ 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 the waveguide generally increases for smaller

diameters (smaller V's). This difference is maximized for small coupler waists, where

βi and β₂ correspond substantially to the LP₀₁ and LPπ modes of an air-glass optical

waveguide. The LPoi mode is a common representation of the HE_u ^e , HE," modes, and the LPn mode is a common representation of the HE₂₁ , HE₁°_X , EH , , and EH^

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 LP₀₁ into LPoi

modes and the LPn into LPn modes. The other consideration in selecting angle is to

maximize the mode conversion efficiency of the LP1₀ into LP_π and LPn into LPoi 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 λ₀ of the mode, the core

index n_core and the cladding index n_cι_afj: 2πTtaa n^{2 ~}λ^{~ ~} co —n 2 re clad ^■

For an air-glass waveguide n_core = 1.45 and n_cι_ad = 1.0. For an elliptical

cross section waveguide, the first or lowest order mode is nominally LP^ and the

second mode is nominally LPi ] . Typically, higher order modes exist within the

coupler waist, as the total number of modes supported by such a waveguide is

N ~ V² 12 , 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 ( |βι| - |β₂| ) / ( |βj| + |β₂| ) = 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. Figure 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/β₀ =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 LPoi (linearly polarized) mode is equivalent to a

linear combination of the even and odd HEn modes, and the LPn mode is equivalent

to a linear combination of the even and odd EHoi and HE₂ι 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'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 LPoi and LPn

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 (< 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 LPoi and

LPi i 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 LPoi and LPn 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. 4, 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., lβLPoι,_xl +

lβLP_π,χl = IβLPoi._yl + lβLP_{l lιy}l). Note that the left and right sides of this equation are

equal, even though individually lβLP₀ι_,xl is not equal to lβLP₀₁,_yl and IβLPn is not

equal to lβLPn_,yl. 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 LPn mode does not vanish for a circular waveguide

(that is, lβLPn_ιXl ≠ IβLPπ.yl for a circular waveguide), while the polarization

dependence of the LP₀ι modes does vanish (lβLPoι,_xl = IβLPoi.yi).

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? 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 « 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

CO₂ 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 CO₂ 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 (cos²(z)), sinc²(z), and

Gaussian (exp-z^σ²) 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 >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 cos² 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:

An(z) = Δn_n

The intensity of the a.c. beam is:

I(.z) = I₀ (sin k z + 1) cos² Ttz I L

and the intensity of the d.c. beam is:

/(z) = /„ sin² ττz / L .

Important advantages of the invention also reside in the features included in the example of Figs. 7-10, 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:

dε Λ_f dn_eff dh_e

JT n eff JT JT

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. 14, 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. 7), 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 US 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.

Claims

WE CLAIM:

1. A multi-mode light propagation element comprising a light confining region

defined by inherently optically independent merged optical fibers configured to

propagate different light modes independently except at a particular wavelength for

which at least one forward propagating optical mode is capable of being coupled to at

least one backward propagating optical mode.

2. An element as set forth in claim 1 above, wherein the optical fibers are of dissimilar cross-section in the region longitudinally adjacent to and on both sides of

the merged region.

3. An element as set forth in claim 1 above, wherein the optical fibers are merged along a length and the forward and backward propagating light modes are orthogonal.

4. An element as set forth in claim 3 above, wherein the shape of the merged

region is modified such that mode birefringence of each of the LPoi and LPn modes

are equal and opposite and the drop wavelength is polarization insensitive.

5. An element as set forth in claim 3 above, wherein the element is twisted

angularly about its propagation direction to an extent by which the difference in

propagation constant for orthogonal polarization components of the optical modes is substantially diminished and the particular wavelength of element is substantially

independent of polarization.

6. An element as set forth in claim 1 above, wherein the merged region has

diameter and index of refraction such that energy is propagated along the environment

surrounding the fiber body as well as by the fiber body.

7. An element as set forth in claim 1 above, wherein the fibers are stretched core- cladding optical fibers in which the original core is vestigial and the original cladding

comprises the substantial majority of the fiber.

8. An element as set forth in claim 7 above, wherein the fiber diameter is less than about 10 microns and selected to minimize the number of higher order modes which contribute to loss within the wavelength operating range of interest.

9. An element as set forth in claim 8, above, wherein the merged fiber region is of

generally dumbbell-ellipsoidal cross-section with a minor to major axis ratio of

approximately 0.8.

10. An element as set forth in claim 7 above, wherein the original cladding of at

least one of the fibers includes a diffraction grating.

11. An element as set forth in claim 10 above, wherein the element includes

tapered optical coupling branches transitioning adiabatically from merged region to optical fibers and diffraction grating reflecting propagating energy at a selected

wavelength between optical fibers.

12. An element as set forth in claim 11 above, wherein the element includes a

waist region in which the fibers are fused together longitudinally and the grating

planes are tilted at an acute angle relative to a perpendicular to the direction of

propagation.

13. An element as set forth in claim 10 above, wherein, in the waist region, variations in propagation constant along the length of diffraction grating are locally compensated.

14. An element as set forth in claim 13 above, wherein the compensation is induced by a laser.

15. An element as set forth in claim 10 above, wherein the diffraction grating is

apodized along its length.

16. An element as set forth in claim 15 above, wherein the apodization function

comprises variations in the index of refraction about a median line and includes both

a.c. and d.c. components.

17. An element according to claim 15 in which the envelope function of the a.c.

component substantially varies according to Δn cos²(π/L z) (sink_gz + 1 ) and the d.c. component substantially varies according to -Δn cos²(π /L z), where

L is the length of the grating and z = 0 is the longitudinal center of the grating.

18. An element as set forth in claim 1 above, wherein the adjacent fibers are fused

together to form a coupler element along the length and the length includes a

diffraction grating.

19. An element as set forth in claim 10 above, wherein the diffraction grating

reflects both a drop wavelength and a backreflection wavelength and wherein the

element includes means for increasing the separation between the drop wavelengths.

20. An element as set forth in claim 19 above, wherein the fibers in the region of the diffraction grating are of cross-sectional dimension sufficiently small so that the

backreflection wavelength is shifted outside the operating wavelength range of the element.

21. An element as set forth in claim 10 above, including in addition a support

body comprising at least one rigid unit coupled to engage opposite longitudinal ends

of the light propagating element of a selected spacing, the rigid unit including a

temperature compensating device for adjusting the spacing between the ends of the

element throughout a chosen temperature range.

22. An element as set forth in claim 21 above, wherein the support body is

configured to provide optical access to the grating region of the fibers.

23. An element as set forth in claim 21 above, including in addition a housing

means encompassing the support body and the optical fibers and including hermetic

seals isolating the housing interior from the environment, and wherein the housing

interior confines a protective gas.

24. A coupler element as set forth in claim 18 comprising:

an elongated hollow housing; an optical waveguide coupler element extending along and within the housing, the coupler element including a central, narrow waist extending along the housing,

and two pairs of terminal optical fibers extending out of the opposite end of the

housing;

a support element for the coupler element having at least one region of engagement to the housing, the support element being attached to the coupler element

^" at opposite ends of the coupler element and including elements supporting the coupler

waist which vary in length and vary the coupler waist tension over a temperature range

and spatially separated to allow optical access to the waist region; and

a hermetic seal means for closing the ends of the cylinders.

O 25. A coupler as set forth in claim 24 above, wherein the support element

comprises a prepackage having a pair of substantially temperature invariant rods

extending along the cylinder axis and a pair of spaced apart end hub elements

positioned along the rods with a predetermined spacing; and wherein the prepackage further comprises a third rod of different thermal

coefficient of expansion than the pair of rods and differentially coupled to the pair of

rods to compensate for variations in temperature.

^■ζ

26. A coupler as set forth in claim 25 above, wherein the prepackage includes a

pair of interior hub elements each engaging a different region of the third rod

mechanism to a different one of the pair of rods at a selected spacing, and one of the

interior hub elements for adjusting the temperature range in which compensation is afforded.

! θ

27. A coupler as set forth in claim 26 above, wherein the coupler comprises

metalized layers on the coupler element soldered to the end hubs at the opposite ends

of the waist region, and wherein the interior of the housing confines a protective atmosphere.

\S

28. A coupler as set forth in claim 27 above, wherein the waist region of the

coupler element has a diameter of less than 10 microns, the housing is a cylinder

having a diameter of less than about 5 mm and wherein the coupler includes means

adjacent to the ends of the housing for limiting bending of the extending ends of the

LO fibers.

29. A coupler as set forth in claim 28 above, wherein the coupler element

includes a diffraction grating in the narrow waist and the end hubs support the coupler

element under tension.

30. A coupler as set forth in claim 29 above, wherein the tension is adequate to

maintain the elongation of the coupler waist from 0.1 to 0.5%.

ST 31. A coupler as set forth in claim 30 above, wherein the pair of rods are of invar,

and the third rod is of stainless steel, and wherein the adjustable mechanism comprises

a fifth hub movable along the invar rods adjacent to one of the interior hubs and a

screw element threaded through the fifth hub and the adjacent interior hub.

I O 32. A coupler as set forth in claim 31 above, wherein the waist region is in the

range of about 2-3 cm long, and each pair of terminal optical fibers is in the range of

about 2-3 cm long, and wherein the waist region comprises fused fibers of hybrid dumbbell-elliptical cross-section of the order of 3-6 microns in cross-sectional dimension, one fiber being appropriately 15-25% smaller than the other.

[≤

33. A coupler as set forth in claim 32 above, wherein the hubs have circular

perimeters sitting within the cylinder, the invar rods extending through the hubs lie in a common plane, the two interior hubs have longitudinal peripheral grooves in

alignment, and the stainless steel rod mates in and is secured into the peripheral

QO grooves.

34. A coupler element as set forth in claim 18 including a Bragg grating for

providing a selected add/drop wavelength output in response to an input signal within

a wavelength band of interest comprising: an optical fiber coupler having a merged waist region of two fibers and

including a Bragg grating which extends along a longitudinal axis for the fiber

coupler, the fiber coupler including a pair of diverging arms extending from each end

of the waist region substantially along the longitudinal axis; a coupler carrier element supporting and fixedly joined to the diverging arms

at the spaced apart ends to establish tension on the waist region; the coupler carrier

element including a mechanism for stabilizing the tension on the waist region through

a selected temperature range to equalize the periodicity of the Bragg grating; and

a housing hermetically sealed about the coupler carrier element and the optical fiber coupler.

35. A coupler as in claim 34 above, wherein the coupler operates in a wavelength

band of interest from 1530 nm to 1565 nm and in a temperature range from 25 °C to

85 °C, and wherein the carrier element includes elongated elements having at least two ^" different thermal coefficients of expansion disposed to compensate for thermal

variation of the optical fiber coupler by adjusting the tension thereon, including an

adjustment device for setting the temperature range to be compensated.

36. A coupler element in accordance with claim 18 having two longitudinally

fused fibers, characterized by central waist region less than 10 microns in cross-

sectional dimension, comprising a photosensitive body longitudinally having only

vestigial cores, the photosensitive body including a wavelength selective Bragg

grating, wherein the photosensitive body has a cross-sectional geometry such that the

coupler is insensitive to the polarization of light propagated therein.

37. A waist region device as set forth in claim 36 above, wherein the central waist

region has an elliptical cross-section with minor to major axis ratios of about 0.8 and

the grating is tilted at an angle of about 3° to 5° relative to the longitudinal direction. s

38. A waist region device as set forth in claim 37 above, wherein the waist region

includes d.c. variations in the local index of refraction to compensate for dimensional

variations in the cross-section to reduce the spectral band of the wavelength selected

by the Bragg grating.

ισ

39. A waist region device as set forth in claim 38 above, wherein the cross-

sectional dimensions of the fibers are sufficiently small they are unsupportive of lossy

cladding modes and spread the wavelength separation between the wavelength

selected by the Bragg grating and the wavelength of back reflections from the grating.

\S

40. The method of making a coupler element in accordance with claim 18

including the steps of :

suspending the coupler device between two spaced apart points to a

suspension device;

J - writing a grating in a region of the coupler device between the two points;

tensioning the coupler to a level for which the drop wavelength of the grating

is responsive to the tension;

compensating the tensioning for temperature variations within a range, and

adjusting the temperature compensation range to a selected temperature range.

41. The method as set forth in claim 40 above, wherein the coupler device is

suspended between the two points and the grating is written in a central waist region

of the coupler device.

42. The method as set forth in claim 41 above, wherein the two points are

attached to suspension device by using a metallic solder to join the optical coupler to a

metallic substrate.

43. The method as set forth in claim 42 above, wherein the solder is selected from

a class of metals and metal alloys which maximize dimensional stability to precisely preserve the drop wavelength of the device during the operational lifetime of coupler device.

44. The method as set forth in claim 42 above, wherein the metallic solder

comprises substantially an Indium alloy.

43. The method as set forth in claim 42 above, including the step of coating at

least a portion of the optical coupler by evaporating a metallic coating on the glass

coupler material to affect adhesion to metallic solder.

44. The method as set forth in claim 40 above, further including the steps of

elongating a length of optical fibers to form a narrow cross-section waist with

adjoining tapered sections within coupler device prior to writing.

45. The method as set forth in claim 44 above, further including the step of

forming a merged waist region in the optical fibers.

46. The method as set forth in claim 45 above, including the further step of prestretching one of the fibers so that it is smaller than the other in the waist region.

47. The method as set forth in claim 46 above, wherein one fiber is 10-25%

smaller than the other in the waist region.

48. The method as set forth in claim 40 above, further including the step of in-

diffusing a photosensitizing material into the coupler during the writing step.

49. The method as set forth in claim 48 above, including the step of including a dopant in the coupler to enhance photosensitization properties, and wherein the in-

diffusion material is from the class comprising hydrogen and deuterium.

50. The method as set forth in claim 44 above, further including the step of

controlling the cross-sectional shape of the waist region during elongation.

51. The method as set forth in claim 50 above, wherein the waist region shape is

controlled to provide polarization insensitivity in the drop wavelength of the device.

52. The method as set forth in claim 51 above, wherein the coupler is imparted

with the cross-sectional shape of a hybrid dumbbell-ellipsoid in the waist region.

53. The method as set forth in claim 52 above, wherein the hybrid dumbbell- ellipsoid cross sectional shape has a minor to major ratio of about 0.8.

54. The method as set forth in claim 44 above, further including the steps of

measuring variations in the cross-sectional dimensions of the coupler waist and writing index of refraction variations in the waist region to compensate.

55. The method as set forth in claim 54 above, wherein the index of refraction

variations are written as d.c. variations in the background index of refraction of the optical waveguides.

56. The method as set forth in claim 44 above, wherein the elongated length of

optical waveguide is diminished in cross-sectional dimension to a size reducing the number of lossy modes and shifting backreflection wavelengths from the drop wavelength of the grating.

57. The method as set forth in claim 40 above, wherein the step of temperature

compensating by tensioning the coupler comprises using differential thermal

expansion of selected lengths of different materials to reduce the tension with

increases in temperature.

58. The method as set forth in claim 57 above, including the step of suspending

coupler with the central region unobstructed, whereby the grating may be written in

the central region.

59. The method as set forth in claim 58 above, further including the step of hermetically enclosing the optical coupler and suspension.

60. The method as set forth in claim 59 above, further including the step of

providing a protective atmosphere within the hermetic enclosure.