AU620904B2 - Optical bandpass filter - Google Patents

Description

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COM VONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED :0 00 a "OPTICAL BANDPASS FILTER"

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The following statement is a full description of' this invention, including the best method of performing it known to us:- This invention relates to fibre-optic bandpass filters of the type in which a fibre having a light-conducting core is concatenated with at least one section of a dual-core fibre.

Such filters are known. (see "Fibre-optic bandpass filter consisting of concatenated dual-core fibres", K. Okamoto and J. Noda, Optical Fibre Conference, 1986, p.62). The known filters comprise concatenated dual-core fibre sections which have one uninterrupted core and a plurality of "dummy" core sections extending parallel to the core. Such filters are difficult to construct and, moreover, display a relatively high insertion loss.

16 It is therefore an object of the invention to provide an inproved S. SC o bandpass filter of the aforementioned type which is simpler to construct, has improved selectivity, and has reduced insertion loss.

*In such a simple and highly selective bandpass filter with reduced in- *:00 sertion loss, the bandpass characteristics depend only on the length of the 000.

dual-core fibre section. The width of the bandpass is substantially proportional to a 1/ length of the dual-core fibre section and can be prede- 0:°o termined accordingly. Such a fibre-optic bandpass filter is particularly 0* 0. suitable for use with optical Multiplex-circuits for the selection of wavelengths in telecommunications systems using optical waveguides.

With the known fibre-optic bandpass filters (Okamaoto/Noda) one employs, for the production of filters, many dual-core fibre sections connected in tandem. Each of these fibre sections must be linked to others, which, apart from the cost, is invariably associated with insertion losses.

The number of dual-core fibre sections should therefore be kept as small as possible. This condition is fulfilled by the present invention. Apart from that, the cores indicate an equal diameter and an equal law of refraction, i.e. there are further bandpass regions apart from the desired pass wavelength, for which the filter is permeable.

In order that the invention can be readily carried into effect, embodiments thereof will now be described in relation to the drawings, in which: i Figure. 1 is a schematic representation of a first embodiment of the present invention.

Figure 2 shows a graph of refractive indices -versus- wavelength for respective cores of a dual core fibre section.

Figure 3 is a schematic representation of a second embodiment of the present invention.

Figure 4 shows a graph of attenuation -versus- wavelength of the filter shown in Figure 3.

The bandpass filter according to Figure 1 comprises a glass fibre 1 with a core 2, a dual-core fibre section 3 having two cores 4 and 5, and a glass fibre 6 having a core 7. Both cores 4 and 5 of the dual-core fibre section 3 indicate various effective refractive indices ncff, ncj for core 4 and nc_ for core 5. They change in accordance with the wavelength as illustrated in Figure 2 and are equal with the wavelength 1. n is the refractive index of the core cladding.

Such dependency of the core refractive indices on the wavelength can be achieved in a known manner by a judicious selection of the materials.

0 The minor diameters are variable. The selection of the diameter in connection with the material results in the desired development according to Figure 2. In the embodiment according to Figures 1 and 2, core 5 has the smallest diameter, but the higher refractive index.

A light vibration with wavelength Xi, which satisfies the aforesaid prerequisite, assumes the development as illustrated in Figure 1, i.e. it will, after transmission from core 2 to core 4, be deflected in a sinusoidal-shaped manner into core 4, so that after a length of 11 3 (11/2) etc. develops in core 5; the sinusoidal wave broadening itself coaxially therein. The light vibration then enters into core 7 of glass fibre 6 which has only one core. For all wavelengths which deviate from the transmission of luminous efficiency from core 4 to core 5 of the dual-core fibre section 3 is incomplete. The light ray remains therefore largely in core 4 and does not travel further from there. As shown in Figure 1, core 4 abuts the core cladding material of glass fibre 6, but not the core directly. In this way an extraordinarily good selectivity arises with low insertion loss in the field of band frequency, i.e. bf the wavelength to be transferred, whereas the light ray in Figure 1 which does not follow the development of a wavelength unequal to is attenuated.

The minor diameters together with the core refractive Index determine the steepnesses of the curves shown in Figure 2. These refractive indices determine the propagation speed of the wave carried by the respective core.

A complete coupling of core 4 to core 5 can however only take effect if the actual refractive indices of both cores Li and 5 are the same, otherwise the coupling is incomplete. Should the diameters and refractive indices of both cores be equal, a complete coupling can arise with other wavelengths S and the desired filter efficiency would thus be lost. A typical diameter would for instance be 9,km, i.e. 7. and refractive index differences n4) /nm The curve as in Figure 4 indicates the bandpass charac- *eg: teristics of the bandpass filter. In the emb~odiment shown in Figure 3, there are two identical dual-core fibre sections 10 and 11 in an abutting arrangement with one section turned through 1800 to the other. Duial-core fibre section 10 includes core 14 and 15, and dual-core fibre section 11 includes core 14' and 15'. Core 15 of dual-core fibre section 10 is joined at its end to core 15' of section 11, which in respect of material and diameter is identical. In this way only a small loss of light rays occurs in the transmission from dual-core fibre section 10 to dual-core fibre section 11. At the other end of section 10, core 14 abuts core 2 of fibre 1, each having the same diameter. Similarly core 14' abuts core 6, each having the same diameter.

Thus, every core abuts a core of equal material and equal corethickness. In this way the insertion attenuation at X4 is extremely low.

At the same time, the adjacent channel suppression is doubled. The identi-

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cal dual-core fibre sections are equal in length, produce the same spot diameter and are arranged concentrically in the middle of the respective dual-core fibre sections. The torsion angle between the dual-core fibre sections may even be less than 1800. What is important, however, is that light transmission between cores 14 and 14' is sufficiently interrupted.

The closure attenuation of the filter is approximately twice as high as shown in Figure 4.

The invention envisages a filter arrangement comprising a planar arrangement of the waveguides, that is, to quote an example, through appropriate allocation of Lithium niobat, whereby the arrangement of the planar waveguides has to correspond with the configuration of the cores as represented in the aforementioned embodiments.

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Claims (6)

1. A fibre-optic bandpass filter wherein an optical fibre having a light-conducting core is concatenated with at least one section of a dual-core fibre, wherein the effec- tive refractive indices of the two cores of said at least one dual-core fibre section are chosen to be the same at the wavelength to be transmitted and different otherwise, and wherein the length of said at least one dual-core fibre section is equal to m. 11/2, where 1/2 is the length at which a light-wave of wavelength 2l couples com- pletely from one core to another core, and m is an odd number, and wherein on one end of said at least one dual-core fibre section one of the cores thereof butts directly against the core of the optical fibre, while on the other end on said dual-core fibre section the other core butts directly against the core of an optical fibre.

2. A fibre-optic bandpass filter as claimed in claim 1, wherein two or more dual-core fibre sections are joined, with each dual-core fibre section having a core equal in diameter to a core of the adjoining dual-core fibre section and wherein the 15 dual-core fibre sections are joined in such a manner that cores of the same diameter butt directly against each other.

3. A fibre-optic bandpass filter as claimed in claim 2, wherein the diameter and the refractive index of one of the cores of a dual-core fibre section are equal to the diameter and the refractive index of the core of the directly abutting optical fibre. 20

4. A fibre-optic bandpass filter as claimed in claim 3, wherein two abutting dual-core fibre sections of identical structure are rotated by a given amount with re- spect to each other.

An optical bandpass filter wherein on a substrate, a planar optical waveguide is concatenated with at least one section comprising two parallel optical waveguides, wherein the effective refractive indices of the cores of the two parallel optical waveguides of said at least one two-waveguide section are chosen to be the same at the wavelength to be transmitted and different otherwise, that the length of said at least one two-waveguide section is equal to mn 1,/2 where 1/2 is the length at which a light wave of wavelength l1 couples completely from one optical waveguide to the other, and m is an odd number, and that on one end of said at least one two- waveguide section, one of the parallel waveguides, and on the other end, the other parallel waveguide butt against the or an adjoining planar waveguide. 7

6. A fibre-optic bancipass filter substantially as herein described with reference to Figs. 1 to 4 of the accompanying drawings. DATED THIS ELEVENTH DAY OF NOVEMBER 1991 ALCATEL N.V. *e Goo* see** 250