GB2220079A

GB2220079A - Optically controlled optical power divider

Info

Publication number: GB2220079A
Application number: GB8814659A
Authority: GB
Inventors: Terry Bricheno; Terry Victor Clapp
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-06-21
Filing date: 1988-06-21
Publication date: 1989-12-28
Anticipated expiration: 2008-06-21
Also published as: GB8814659D0; GB2220079B

Abstract

An optically controllable variable ratio optical power divider consists of a saturable absorber clad 2 X 2 fused optical fibre tapered coupler. The length of coupler's optical interaction drawn-down region is equal to approximately 12 beat lengths of the zero and first order modes in this region.

Description

OPTICALLY CONTROLLED OPTICAL POWER DiVIDER This invention relates to optically controllable variable ratio optical power dividers.

Light guided by the core/cladding interface of an optical fibre includes an evanescent component of the optical field in the region of the cladding immediately surrounding the core. In a conventional single mode fibre this evanescent field is confined substantially completely to only a small proportion of the total thickness of the cladding. This means that light propagation is substantially unaffected by the optical properties of the environment surrounding the optical fibre.

In the manufacture of a fused fibre tapered coupler two or more optical fibres are drawn down in side-by-side contact with each other over an intermediate portion of their length to produce a region of reduced cross-sectional area. The reduction in core diameter that is provided in this drawn-down region produces a spreading of the evanescent field associated with light guided by the core/cladding interface, and in a fused fibre tapered coupler the draw-down is made sufficient to produce an optical interaction region in which the evanescent fields of its constituent fibres overlap to an appreciable extent and thereby afford mutual optical coupling between those fibres.One ancillary result of this mutual coupling is that the evanescent fields may penetrate to an appreciable extent beyond the confines of the optical cladding material, and hence propagation characteristics can be effected by changes in the optical properties of the medium in which the optical interaction region of the coupler is immersed.

An n x n fused fibre tapered coupler constructed from n lengths of single mode fibre may be considered as a device having n input ports coupled to n output ports via the intermediate optical interaction region of the coupler. Operation of this coupler may be analysed by considering the optical interaction region as an optical waveguide capable of supporting n modes which are excited with a certain amplitude and phase relationship by the launch of light into any one of the input ports. The division of power between the output ports is then determined by the geometry of the power distribution at the far end of the optical interaction region.Differences in the proportion constants of the different modes propagating in the optical interaction region make the geometry of the power distribution change progressively faster with increasing distance down the length of the optical interaction region.

Hence, with a long optical interaction region, even quite a small change in the refractive index of the material surrounding the region, producing a small change in the propagation constants of all the modes, can produce a significant change in the geometry of power distribution at the far end.

According to the present invention an optically controllable variable ratio optical power divider is provided which power divider comprises a fused fibre tapered coupler whose reduced diameter optical interaction region is embedded in an optically non-linear material exhibiting an optical intensity dependent refractive index. A preferred class of such materials is the class saturable absorbers.

Saturable absorption is a resonant phenomenon observable at the band edge of semiconductors, and in dyes and organometallic complexes such as those used in dye lasers. It is phenomenologically a decrease in absorption with time as an intense beam of light pumps the electronic states at the top Of the valence band (in the band model of semiconductors) or the lowest, conveniently, accessible transition in molecular systems. This gives, via the Kramers-Kronig causality relationship, a change in refractive index of the material.

There follows a description of an embodiment of optically controllable variable ratio optical power divider constructed to investigate and demonstrate the principles of operation of the invention. The description refers to the accompanying drawings in which: Figures 1 to 3 depict successive stages in the manufacture of the power divider, and Figure 4 is a schematic diagram of an experimental arrangement for demonstrating the operation of the power divider.

Referring to Figure 1, a first piece 1 of acrylate coated single mode silica optical fibre has a short length of its acrylate coating softened with dichloromethane to enable it to be peeled off to expose a length of bare fibre 2, which is then acid cleaned. A second piece 3 of identical fibre is treated in the same way to expose a similar length of bare fibre 4. In this specific example the bare fibre is 125um in diameter, has a core diameter of 9um and an equivalent step index of n = 0.0033. The two fibres are then assembled for making a directional coupler. For this purpose the fibres need to be in glass-to-glass contact with each other along the region that is to be drawn-down to provide the mutual optical coupling.This region of contact is conveniently provided by stranding the fibres as illustrated in Figure 1, preferably taking care that the stranding introduces substantially no twist of either fibre about its own axis. Preferably the stranded fibres are clamped between independently driven carriages 5 and 6 for the making of the fused directional coupler by the progressive stretching method substantially as described in Patent Specification No.

2150703B.

According to this method the fibres are heated in a flame 7 of a burner 8 to produce local softening of the glass while the fibres are being moved in a longitudinal direction through the flame. To this end the leading carriage, carriage 5 for a rightward traverse, carriage 6 for a leftward traverse, is traversed at a slightly faster rate than the trailing carriage. This stretching process is repeated a number of times until the requisite amount of coupling has been produced. Typically, though not essentially, successive traverses are performed in opposite directions through the flame.

In a typical 3dB coupler the stretching process is halted when the strength of coupling between the two fibres first reaches a point providing 3dB coupling.

The drawn down optical interaction region 9 (Figure 2) supports both the zero order mode and a first order mode, and at this point the configuration is such as to provide a quarter beat-length of optical interaction region for these modes.

For a conventional 3dB coupler the draw-down factor for the coupling region is typically relatively small so that the first order mode shall not be too close to cut-off. Several disadvantages would follow from operating close to cut-off. First the beat length would be inconveniently short and hence there would be difficulties in attaining adequate precision in the length of the optical interaction region. Second the spread of the evanescent fields would be unnecessarily

large, penetrating to an unnecessary extent beyond the glass and thereby making the coupler unduly sensitive to adventitious refractive index changes of the medium surrounding the optical interaction region.

Additionally the optical interaction region would be unnecessarily fragile and prone to problems of bending losses.

In the present instance of the variable power ratio coupler the draw-down factor is significantly greater. In this instance it is necessary to have the evanescent fields penetrating significantly into the medium surrounding the glass of the optical interaction region 9 because it is by virtue of the penetration that the coupler is rendered capable of having its power division optically controllable. For this reason the variable ratio optical power divider now being specifically described was drawn down at a temperature to provide the optical interaction region 9 with a cross-section measuring approximately 5.5um by 3.5um, and to an extent to provide a beat length of approximately lmm for 1.3um wavelength light. The optical interaction region was made approximately 12 beat lengths long.

Once the stretching has been completed, and before the assembly is removed from carriages 5 and 6, the fibres are mounted on a substrate. Conveniently this may take the form of an open channel 13 (Figure 2) made of fused silica in which the fibres are secured by the application of blobs 14 and 15 of resin applied where the bare fibres emerge from their plastics protective coatings. In this way the portion of the fibres between the two resin attachment points 14 and 15 is held in a substantially strain-free fashion.

To make the coupler optically controllable, its optical interaction region needs to be embedded in an optically active medium, in this particular instance a saturable absorber. Normally this saturable absorber would be dispersed in a solid medium, but in this instance, since the device is an experimental model, the dye is dispersed in a liquid solvent which can be flushed out and replaced with different mixtures based on different saturable absorber materials and/or solvents. When a solid medium is-being employed in which the saturable absorber is dispersed, it may for instance take the form of a resin which acts as a solvent for the saturable absorber and can be applied in liquid form and then cured to a solid. Alternatively the saturable absorber may be dispersed in a glass.In this instance the glass may be applied as a frit, or gel, which is subsequently fired to form a glass sheath for the optical interaction region.

In the particular case of the experimental model coupler, the assembly is arranged for immersing in liquid by inserting it in a tube 30 (Figure 3) whose ends, after insertion of channel substrate 13, are plugged with fillets (not shown) of a suitable silicone, or other, elastomer. Communicating with the interior of the tube are two pipes 31 by way of which the interior may be filled and vented.

For use at a wavelength in the region of 1.3um a suitable saturable absorber is the laser dye bis(diethylaminodithiobenzil)Ni. The absorption band of this material can be employed to provide an operating condition in which the refractive index decreases as absorption saturates. A suitable concentration of this dye for use in the coupler is one to cause a modulation of the refractive index in the range of order 10 4 to 10 3 at 1.3um. The dye is dispersed in a suitable solvent, which in this instance can be conveniently provided by a mixture of 75% carbontetrachloride and 25% methylene dichloride. The precise proportions of, and species composing, the mixture are varied to produce the requisite refractive index which is typically about 3 x 10 3 beneath that of the optical cores of the fibres from which the coupler is constituted.

This particular dye has an excited state lifetime of about 5ns. By varying the nature of the absorber, for instance by changing to another dye or by changing to another type of saturable absorber such as a fine dispersion of semiconductor particles, it is possible to modify the non-linearity and thereby tailor the coupler to have a particular response.

Figure 4 depicts an experimental asrangement for demonstrating the operation of coupler by directing pulses of laser light of different amplitude through the coupler and observing how the division of power between its two output ports varies as a function of pulse amplitude. The laser light pulses are derived from a 50m peak power 1.3um emission wavelength GaInAsP laser 40. Light from this laser is launched into one input port of the optically controllable variable ratio optical power tapered coupler indicated at 41 via an optical fibre variable attenuator 42.This attenuator 42 operates by varying the small distance separating the ends of two aligned fibres, and by this means the amount of optical power launched into the coupler 41 can be controlled without altering the laser drive conditions, and hence the power can be adjusted substantially independently of spectral content. Light from the two output ports of the tapered coupler 41 is directed down fibres 43 and 44 to the two input ports of a conventional 3dB optical fibre fused tapered coupler 45. Fibre 43 incorporates a delay line 46 so that upon arrival at the 3dB coupler 45 the light pulses emanating from the two output ports of the variable power ratio tapered coupler are spaced in time.One of the outputs from the 3dB tapered coupler 45 is fed direct to a detector 47 to provide a measure of power launched into the variable power ratio tapered coupler 41, while the other output from the 3dB tapered coupler 45 is fed via monochromator 48 to a separate detector 49. A monochromator is required because the variable power ratio tapered coupler 41 is wavelength sensitive with a periodicity expressed in terms of wavelength equivalent term that is comparable with that of the spectral width of the laser source. Accordingly if the monochromator is set to a wavelength within the spectral emission of the laser for which the coupler 41 acts as a 3dB coupler to low intensity light, then the ouput from detector 49 takes the form of two pulses 50 of variable relative amplitude displaced in time by the differential time delay introduced by the optical fibre delay time 46.If now the optical power is increased by adjustment of the attenuator 42 to provide less attenuation, a measure of this power increase being provided by the output of detector 47, saturation effects in the dye of the coupler 41 assume progressively increasing significance and disturb the operation of the coupler away from its starting condition with the result that one of the pulses is increased at the expense of the other, either as depicted at 51 or as depicted at 52. With the arrangement substantially as described above a modulation depth of 30% has been observed.

The experimental arrangement just described above with reference to Figure 4 makes use of only one input of the optically controllable variable ratio optical power tapered coupler 41 because the signal launched into that input is itself used to produce the saturation effects. An alternative way of employing the device involves controlling the passage of a signal through the coupler which has been launched into its first input port by means of a control signal applied to the second input port. With this arrangement the two signals would not normally be of exactly the same frequency.

Claims

CLAIMS:

1. An optically controllable variable ratio optical power divider which power divider comprises a fused fibre tapered coupler whose reduced diameter optical interaction region is embedded in an optically non-linear material exhibiting an optical intensity dependent refractive index.

2. An optical power divider as claimed in claim 1 wherein said material exhibiting an opticaL intensity dependent refractive index is a saturable absorber.

3. An optical power divider as claimed in claim 1 or 2 wherein said fused fibre tapered coupler is a 2 x 2 coupler.

4. An optically controllable variable ratio optical power divider substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.