GB2189902A - Single mode fibre adjustable attenuator - Google Patents

Abstract

An optical fibre adjustable attenuator in which the ends of a pair of single mode optical fibres (1 Fig. 1), 9 are held by clamps secured by brackets (35, 36 Fig. 3) to a sliding platform 27. Each fibre end rests in and is held in alignment by a groove formed by two needles 3, 4 or 7, 8. Optical coupling between the two fibres may be provided by a short loop of fibre (6) which is similarly held by clamps 11, 12 with its ends aligned in the two grooves, but in this instance the clamps do not move with the platform, and hence movement of the platform alters the spacing and hence the optical coupling between the opposed fibre ends. The fibre ends, the needles, the sliding platform and the clamps are all located within a refractive index matching liquid filled containment vessel formed by a base plate (30, Fig. 4) and a cover (46, Fig. 4). <IMAGE>

Description

SPECIFICATION Single mode fibre adjustable attenuator This invention relates to adjustable attenuators for single mode optical fibre systems. Variable attenuators have been constructed for multimode optical fibre systems which rely upon placing a graduated density neutral filter between the two halves of an expanded beam type connector, or rely upon adjusting the angular misalignment of the two halves of such a connector. A disadvantage of using either of these types of approach in the construction of a variable attenuator for a single mode optical fibre system is that they are liable to be too polarisation state sensitive for a number of applications. A design is required which provides an attenuation that is substantially independent of the state of polarisation of the light transmitted through the attenuator.

One design of adjustable attenuator which is substantially independent of state of polarisation is described in Patent Specification No.

2114769A, to which attention is directed. In that design the ends of two fibres are held in alignment by a Vee-groove formed in a glass block located in a vessel containing indexmatching liquid mounted on a slide mechanism. The fibres are held in position by clamps located beyond the two ends of the vessel, and hence the fibres have to penetrate glands located in the end walls of the vessel which prevent leakage of the liquid from the vessel. Operation of the slide mechanism causes one of the fibres to slide axially through its gland. This design suffers from a number of drawbacks at least some of which the present invention is concerned to ameliorate. These disadvantages include problems associated with the glands, and a problem of restricted dynamic range of attenuation provided by the device.In particular it has been found particularly difficult to achieve an adequately leakproof gland capable of sustaining the requisite translational movement of the fibre without introducing excessive hysteresis/backlash that makes the separation of the fibres at any given setting of the slide mechanism noticeably different according to whether the setting was arrived at by movement of the slide mechanism in one direction or by movement in the other.

Another limitation of that design is that the single gap between the two fibre ends typically affords a range of only about 15 to 20 dB over which attenuation increases monotonically with increasing separation of the fibre ends.

According to the present invention there is provided an adjustable attenuator in which the attenuation is adjustable by altering the spacing between the opposed ends of two single mode optical fibres held in spaced relationship by a pair of clamps located near said opposed ends of the two fibres and positioned so that the fibres are deflected by a pair of shoulders of an alignment member so as to constrain an end portion of both fibres to lie on a common axis, wherein a slide mechanism is provided by which one member of the pair of clamps is movable relative to the other in a direction substantially aligned with the direction of said common axis, and wherein the fibre ends and the alignment memer are located in a containment vessel for a refractive index matching liquid together with at least the one of the pair of clamps which moves relative to the vessel upon operation of the slide mechanism.

The adjustable spacing gap may be arranged to be optically in series with one or more other gaps that are mechanically ganged so that a given movement provides a correspondingly large change in attenuation. One effect of such ganging is to increase the dynamic range of the attenuator, and another is to increase the attenuation range over which the attenuation various substantially linearly with movement.

There follows a description of an adjustable attenuator embodying the invention in a preferred form. This attenuator, which incorporates two optical fibre adjustable gaps mechanically in parallel but optically in series, will be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating the arrangement of gaps between fibre ends in the attenuator, Figure 2 depicts in greater detail how the fibre ends are mounted, Figure 3 depicts the translation stage mounting assembly of the attenuator, and Figure 4 depicts the assembly in its index matching liquid containment vessel.

Referring to Fig. 1, the principle of operation of the attenuator is that light is launched into one end of an input single mode optical fibre 1 to propagate in the direction indicated by arrow 2. The far end of this fibre is constrained by means not shown in Fig. 1 to lie in an alignment guide, which in this instance is formed by a pair of steel needles 3 and 4 secured by fillets 5 of adhesive in spaced apart parallel relationship. The light emerging from fibre 1 crosses a small gap before being launched into one end of a short loop 6 of similar optical fibre. This end of the fibre loop 6 is similarly constrained to lie in the alignment guide formed by the two needles 3 and 4. The spacing between the fibre ends determines the efficiency with which light propagating in the fundamental mode of fibre 1 is launched into fibre 6 to propagate in its fundamental mode.At the other end of the fibre loop 6 there is a second alignment guide, identical with the first, that is formed by needles 7 and 8. Light emerging from fibre 6 is launched across a second gap and into an optical fibre 9 that constitutes the optical out put of the attenuator. The optical attenuation of the attenuator is varied by movement of the fibre loop 6 relative to the input and output fibres 1 and 9 so as to increase or diminish the spacing between their opposed ends held in alignment with each other in the two alignment guides.

The four fibre ends that are aligned by the alignment guides are held by two pairs of clamps that are mirror images of each other.

One such pair, the pair that holds the two ends of fibre 6, is depicted in Fig. 2. The loop of fibre 6 is a piece of fibre that has been encased in a plastics protective coating 10 that has been stripped off to expose the bare fibre at the ends of the loop where the fibre protrudes from the clamps. Each clamp has a lower part 11 secured with clamping bolts (not shown) to an upper part 12 so as to clamp the fibre between them by its plastics protective coating 10 in Vee-grooves 13 formed in their mating surfaces. The two clamps securing the fibre loop 6 are secured to the two slotted uprights 14 of a common mounting bracket 15 by means of knuried nuts 16. Coarse adustment of the profile of bend in the fibre between the clamp and the fibre end positioned on the alignment needles is provided by adjustment of the height and angle of the clamp before tightening the knurled nuts 16.Subsequent fine adjustment of this profile to maximise the coupling between the spaced ends of the fibres aligned by the alignment needles is provided by a rounded finger 17 bearing down on the bare fibre. This finger is mounted on a lever 18 that is itself mounted on a pivot 19 secured to the upper part of the clamp. The lever is biassed against an adjustment screw 20 by a leaf spring 21 engaging a notch 22 formed in the upper surface of the lever.

The two pairs of alignment needles 3, 4 and 7 and 8, are mounted on substrates 23 secured by screws 24 to a plinth 25. Slots 26 in each substrate permit lateral adjustment to locate the two needles symmetrically about the plane of the two fibres they are required to align. The plinth 25 is an integral part of a platform 27 to which the clamps (not shown) that hold the input and output fibres 1 and 9 are indirectly fixed. This platform 27 is movable relative to the mounting bracket 15 along an axis aligned with the axes of the alignment needles so that, by means of this movement, the magnitude of the gap separating the fibre ends located by the alignment needles may be altered.

The platform 27, which is shown in greater detail in Fig. 3, is mounted on a translation stage 28 which is itself mounted on a saddle bracket 29 secured to a baseplate 30 (not shown in Fig. 3). The translation stage is driven by rotation of a shaft 31. The central portion of translation stage is fixed to the saddle bracket 29 so that rotation of the drive shaft 31 produces translational movement of the outer frame of the translation stage in the direction of arrow 32. The platform 27, with integral plinth 25, is rigidly fixed to this outer frame. The platform 27 is provided with a recessed well 33, and a through hole 34, for freely accommodating a pillar (not shown) by which the mounting bracket 15 is rigidly secured through the fixed central portion of the translation stage to the saddle bracket 29.

The clamps for holding the ends of the input and output fibres 1 and 9 are respectively secured to the slotted uprights of brackets 35 and 36.

The position of bracket 35 on platform 27 is arranged to be capable of adjustment in the longitudinal direction of the movement of the translation stage so as to allow the gap between the ends of fibres 1 and 6 to be made equal to that between the ends of fibres 9 and 6. To this end the bracket 35 is trapped between a side wall 37 of the platform 27 and a guide 38 secured to the platform 27 by screws 39 extending through slots 40. The bracket 35 is similarly secured to the platform 27 by a screw 41 extending through a slot 42. Bracket 35 is biassed against the end of an adjustment screw 43 by means of a spring wire 44 that has one end located in an aperture in the side wall of the bracket and its other end secured in a mounting block rigidly attached to platform 27.

The mounting bracket for the clamps holding the two ends of the fibre loop 5 is also secured to its pillar (not shown) by means of screws (not shown) threading transversely extending slots (not shown) formed in the base portion of the bracket 15. This allows the slotted upright on the input side of this bracket to be aligned with the slotted upright of bracket 35 once this bracket is in position against the side wall 37. Similarly, bracket 36 is secured by means of two further screws (not shown) threading transversely extending slots (not shown) formed in the base portion of the bracket. This allows the slotted upright of this bracket 36 to be aligned with the slotted upright on the output side of bracket 15 once bracket 15 has been aligned with bracket 35.

Part of the base plate 30 to which the saddle bracket 29 is attached is depicted in Fig.

4. Clamped to this base plate by means of straps and bolts (not shown) is a cover 46. A gasket (not shown) is trapped between the lower lip of this cover and the base plate to provide an oil-tight seal. The translation stage drive shaft 31 protrudes through a rectangular aperture 47 in the front wall of the cover, and two further apertures accommodate oil-tight glands through 48 which are threaded the input and output optical fibres respectively provided with plastics protective coatings 49 and 50. It is to be noted that, since the clamps that are secured to the brackets 35 and 36, that move relative to the cover 46 when the translation stage is operated, are located within the volume enclosed by the cover, the glands 48 do not have to be capable of accommodating any leak-tight sliding movement through them of the fibres that thread them.

The rectangular aperture 47 is sealed off by clamping together, with an intervening gasket (not shown), two apertures plates (not shown), one on the inside of the front wall and the other on the outside. The inner plate is provided with a oil-tight rotating gland for the translation stage drive shaft, and the outboard end of this shaft is coupled to a stepping motor (not shown). The drive shaft is formd in two parts linked by a bellows mechanism 51 to allow for any slight misalignment between the axis of the motor and the axis of the translation stage.

Within the cover, the plastics coated input fibre extends from its gland 48 nearly to the further wall before being brought forward in a loop (not shown) past the lower edge of the saddle bracket 29, where it is additionally secured, and onward and upward in a further loop to enter the clamp secured to bracket 35. The output fibre 9 traces a similar path on the other side of the translation stage between its gland 48 and the clamp secured to bracket 36.

Once the assembly has been completed, the volume enclosed by the cover 46 and base plate 30 is fiiled to the brim with an index matching liquid such as liquid paraffin. The volume may also enclose some form of compliant member, such as a sponge, which will change its volume in a manner to prevent excessive pressure changes otherwise occasioned in thermal cycling due to the effects of differential thermal expansion. Filling may be conveniently be achieved via the aperture 47 in the cover after having stood the assembly on end.

In this attenuator much or most of the fundamental mode attenuation that occurs at each gap is the result of light being launched into the downstream fibre in cladding modes rather than in the fundamental core mode.

These cladding modes are generally more heavily attenuated than the fundamental core mode, and hence in a long length of fibre it is often not necessary to take any specific steps to suppress cladding mode propagation. This attenuator does not however incorporate a great length of fibre, hence it is generally desirable, in order to achieve adequate attenuation of cladding modes, to incorporate some form of cladding modes stripper at least in the output fibre. In the present instance steps have been taken to ensure that cladding modes are stripped not only from the output fibre 9, but also from the fibre loop 6 and from the input fibre 1.

Included in the input and output fibres are specific structures for cladding modes stripping, each of which is constituted by a stretch of fibre from which the plastic protective coating has been stripped and replaced with a coating of light absorbent lacquer. Mechanical protection for this stretch of fibre is provided by encasing it in a metal tube 52 secured to flanges 53 themselves secured either to the outside sides of the cover (as shown in Fig.

5), or to the sides of the base plate 30. Conveniently glands 54 of similar construction to glands 48 may be used both for sealing the ends of the tubes 52 and securing them to the flanges.

Any change in bend profile between the ends of fibre 6 is liable to change the partition between the various cladding modes of any optical power propagating in this fibre in cladding modes. It is therefore preferred for cladding mode power to be stripped from this loop so that such partition effects cannot effect the amount of power launched into the core mode of fibre 9. Since the loop 6 is to be totally immersed in index matching liquid it is in principle possible to arrange for this cladding mode power to be stripped merely by stripping away a portion of the plastics coating so as to expose a region of bare fibre. However, since this leaves the fragile fibre entirely unprotected in this region, it is preferred instead to choose for the fibre of loop 6 one that has been provided with a plastics coating that will itself function as a cladding modes stripper.

The choice of a pair of spaced apart needles 3, 4, or 7 and 8, for a fibre alignment, instead of the Vee-groove fibre alignment member of the attenuator described in Patent Specification No. 21 14769A to which previous reference has been made, is preferred because it has been found to provide better consistency of operation. It is suspected that this arises because small particles or bubbles are liable to get trapped under the fibres located in a Vee-groove and lift them out of alignment, whereas the spaced needles provide a greater space beneath the fibres into which small particles of dirt may drop so as not to interfere with fibre alignment, and along which bubbles may more readily spread and be dispersed so that they similarly do not interfere with fibre alignment.The lower coefficient of friction between the glass of the fibres and the steel of the alignment needles, as compared with the coefficient of friction between glass fibres sliding in a glass Veegroove, is also an advantage in minimising any problems of hysteresis/backlash in operation of the attenuator. For convenience of illustration the Figures have not shown that it is generally preferred to arrange for the fingers 17 mounted on the ends of the fine adjustment levers 18 to overlap the ends of the alignment needles 3, 4, 7 and 8, and for the diameter of the fibres 1, 6 and 9 in relation to the diameter of the needles and their spacing to be such that the fibres can be fully accommodated beneath the level of the tops of the needles. This preferred arrangement is chosen in order to reduce the risk of the fingers 17 breaking the fibres under conditions of substantial mechanical shock.

The particular stepper motor to which the translation stage drive shaft is connected is one that steps in units of 1.8 , and, with the particular fibres and translation stage of the attenuator over a range of 40 dB. The motor is operated under the control of a microprocessor (not shown), and, to enable it to interrogate the motor shaft to sense each turn in order to ensure that the motor does not attempt to turn the shaft beyond its permitted limits at the two ends of the four turns, a disc (not shown) is mounted on the shaft with a small hole to act as a shutter in an optical switch assembly (not shown). Operation of this switch provides single turn sensing.

Counting of the individual turns is effected by means of a pin mounted on the disc, this pin co-operating with a count wheel and cam assembly (not shown) operating a microswitch (not shown). The microprocessor includes an EPROM for storage of the measured values of attenuation at each one of the 800 shaft settings in order to enable the processor to select the appropriate shaft setting in response to the input of a specific required value of attenuation.

Claims (10)

1. An adjustable attenuator in which the attenuation is adjustable by altering the spacing between the opposed ends of two single mode optical fibres held in spaced relationship by a pair of clamps located near said opposed ends of the two fibres and positioned so that the fibres are deflected by a pair of shoulders of an alignment member so as to constrain an end portion of both fibres to lie on a common axis, wherein a slide mechanism is provided by which one memer of the pair of clamps is movable relative to the other in a direction substantially aligned with the direction of said common axis, and wherein the fibre ends and the alignment member are located in a containment vessel for a refractive index matching liquid together with at least the one of the pair of clamps which moves relative to the vessel upon operation of the slide mechanism.

2. An adjustable attenuator as claimed in claim 1, wherein both members of the pair of clamps are located in the containment vessel.

3. An adjustable attenuator as claimed in claim 1 or 2, wherein the two shoulders of the alignment member are formed by the curved surfaces of two cylinders extending in parallel spaced relationship.

4. An adjustable attenuator as claimed in claim 3, wherein the size and spacing of the two cylinders in such that where the two fibres are in contact with two shoulders they do not stand proud of the cylinders.

5. An attenuator as claimed in any preceding claim wherein each clamp is provided with means for coarse adjustment of the profile of the fibre where that fibre is deflected by its associated alignment member, and wherein said means for coarse adjustment is backed by further means for fine adjustment of this profile.

6. An adjustable attenuator as claimed in any preceding claim, wherein one member of a second pair of clamps is movable relative to the other by the slide mechanism as to vary the spacing between a second pair of opposed ends of optical fibres, which second pair of opposed ends is optically in series with the first.

7. An adjustable attenuator substantially as hereinbefore described with reference to the accompanying drawings.

8. An adjustable attenuator as claimed in any preceding claim, which attenuator is mechanically driven by a stepping motor under the control of an electronic microprocessor.

9. An adjustable attenuator as claimed in claim 8, wherein the control of the stepping motor is substantially as hereinbefore described.

10. An adjustable attenuator in which the attenuation is adjustable by simultaneously altering the gaps between the opposed ends of separate pairs of optical fibres, said gaps being optically in series.