AU629650B2

AU629650B2 - Optical fibre termination

Info

Publication number: AU629650B2
Application number: AU57723/90A
Authority: AU
Inventors: Manfred Kaiser
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1989-06-30
Filing date: 1990-06-21
Publication date: 1992-10-08
Anticipated expiration: 2010-06-21
Also published as: DE3921440A1; CH680953A5; AU5772390A

Description

BY Signature of Applicant B. O'Connor To: The Commissioner of Patents ~9-e I ~rrrrrrP~lg 6296,)o 00 3 0 o COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 a no o -re COMIPLETE SPECIFICATION FOR THE INVENTION E TITLED "OPTICAL FIBRE TERMINATION" The following statement is a full description of this invention, including the best method of performing it known to us:i This invention relates to low-reflection termination of a single-mode glass fibre for use in fibre-optic transmission and measurement systems.

Such fibre end terminations are used for laser coupling for measurement tasks such as, for example, the determ,nation of the return loss of an optical connector or the noise of a semiconductor laser. In such situations, the power reflection from an unterminated fibre t is very disturbing.

A fibre coupling is known in which the signals to be transmitted are emitted by a laser which, depending on the application, is sensitive to light reflected from the coupled fibre. With direct modulation and high bit rates, this can cause considerable disturbance to the modulation characteristics. In the case of coherent light transmission, the laser may produce mode or fre- S quency jumps, or the line width of the laser may change.

S,,In order to avoid this an arrangement consisting of a monomode fibre and a graded index fibre can be used, in which one end of the monomode fibre is first butted up to one end of the graded index fibre. Subsequently the fibres are so adjusted that the core of the monomode fibre is at a part of the graded index fibre core for which the refractive indices of the two fibres agree as ao closely as possible. Finally the aligned fibres are fixed in this position so that through the combination of monomode and graded index fibres, the losses are small for light waves from the small-diameter core of the monomode fibre to the large-diameter core of the graded index fibre, but large in the opposite direction.

Another known method for protecting the laser against disturbing effects uses an optical isolator. Such an isolator is, however, expensive to produce.

Furthermore, it causes additional losses. Further known procedures for suppressing reflections are t'\e application of dielectric anti-reflection coatings to the fibre end, a'id the provision of a sloping face on the end.

Fibre enda with sloping faces are known, for example, from the optical fibre connectors of the Radiall company.

2

L

3 When a fibre is cut at right angles to its axis there is a sudden refractive index change from n= 1.5 (glass) to n 1 (air). This produces a reflection factor of approximately If no preventive measures are taken, this fraction of the power flows back to the laser and causes noise, or a deterioration of the signal-to-noise ratio in bidirectional communication systems, due to crosstalk.

An object of the present invention is to achieve a low-reflection termination with small insertion loss for a single mode glass fibre while retaining the forward transmission properties; it should be of simple construction and have low manufacturing cost.

According to the invention there is provided a low-reflection termination for a single-mode glass fibre for use in fibre-optic transmission and mcasurem nt systems, wherein the fibre end is tapered to form a tapered portion teminating in a point and at least said tapered portion of the fibre is fixed within a glass capillary by means of a transparent adhesive whose refractive index is greater than that of the glass capillary.

The invention will now be described with reference to the example shown in the drawing. The drawing shows a partly sectiocd side view of the end of a single-mode glass fibre with a low-reflection termination.

In the drawing, the single-mode fibre is designated I. This is a bare glass fibre, 20 free of its primary coating, with a diameter, for example, of 125 urn. The fibre end has a taper 2 formed on it and is fixed with transparent adhesive 3 into a glass capillary tube 4. The capillary tube is sufficiently long so th-'t at the rear it encloses a section of the cylindrical part of the bare single-mode glass fibre 1. The glass capillary tube 4 may have, for example, an inner diameter of approximately 130 um and an outer diameter of 500 um. However, exact dimensions are not important here o" as it is only necessary to ensure that the adhesive 3 has a boundary so that it can act as light guiding core. Thi, is achieved in the example considered here, which operates in the medium temperature range, by the use of an adhesive 3 with refractive index of 1.48, equal to, or closely approximating, that of the taper 2 of the B,

I

single-mode glass fibre 1; the a. sive fills the cavity of the glass capillary tube completely, the latter having a refractive index of 1.44. If slightly increased losses can be neglected, it is possible in many cases to use an internally mirrored metal capillary tube, instead of the glass capillary tube 4.

The taper 2 is approximately 1 to 2 mm long and its point lies at least mmn before the perpendicular end surface 5 of the coupling waveguide 6 which forms the low-reflection termination of the single-mode glass fibre. The angle of the sloping surface of taper 2 to the fibre axis is not critical here. Its i.0 only importance lies in th fact that an extremely small angle would increase the loss. A minimum dimension of at least 10 mm between the point of the taper and the free end of the coupling waveguide ensures that exit surface is Ifully illuminated.

To produce the taper 2, the glass fibre 1 is heated in a suitable gas I flame to the softening temperature and then drawn out so that its crosssection is continuously decreased. This steady decrease in area causes the fibre to lose its light guiding properties. The radiation energy that was originally guided in the fundamental mode of the fibre is thus separated into a number of modes which would form a spatial distribution determined by the taper function, if the taper was in air. In the example considered here, howo ever, the radiation does not emerge into air but into the optical waveguide o\ oo formed by the adhesive 3 glass capillary tube 4. The coupling waveguide 6 thereforc on the one hand has a low insertion loss (typically less than dB), and or the other hand has a high reflection or return loss, typically greater than 50 dB (and with a small insertion loss and linear radiation flow). The degree of suppression of the reflections is determined by the structural parameters of the two coupled waveguides and number of modes excited, or capable of being excited.

Since reflections appearing at the input end of the single-mode glass fibre 1 are undesirable for all fibre optic transmission systems which use a semiconductor laser as the transmitter, the glass fibr' termination described here is particularly advantageous for S transmission systems which may have short fibre lengths, eg. in the subscriber area; S transmission systems with severe requirements for laser stability, eg.

analogue transmission or coherent reception; S- fibre sensor techniques, eg. fibre gyroscopes Furthermore, the arrangement described here has operational and cost advantages for measuring or regulation tasks, because the fourth coupler port (which is normally terminated) can be used to measure the power actually flowing to the transmission line, without causing disturbing reflections. With analogue transmission systems, for example, the transmitter characteristic can be made linear despite the disturbing effects of the fibre coupling.

Claims

1. A low-reflection termination for a single-mode glass fibre for use in fibre-optic transmission and measurement systems, wherein the fibrc end is tapered to form a tapered portion teminating in a point and at least said Lapcred portion of the fibre is fixed withlin a glass capillary by means of a transparent adhesive whose refractive index is greater than that of the glass capillary.

2. A low-refcction termination as claimed in claim I, wherein the refractive index of the adhesive is substantially cqual to that of the single-mode glass fibre.

3. A low-refcction termination as claimed in claim I or 2, wherein the refractive index of the glass capillary is less than that of the singlc-mniode glass fibre.

4. A low-reflection termination as claimed in claim I, wherein instead of theic glass capillary, an internally reficcting capillary tube of metal is provided. A low-refl'ction termination substantially herein described with reference to the figure of the accompanying drawing. DATED TH-IIS THIRTIET-I DAY OF JULY 1992 00 2 ALCATEL N.V. :i 02 0 00 0 00t So 00 11i