AU623467B2

AU623467B2 - Optical transmission system

Info

Publication number: AU623467B2
Application number: AU47244/89A
Authority: AU
Inventors: Peter Dr. Kuppers
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1989-01-07
Filing date: 1989-12-21
Publication date: 1992-05-14
Anticipated expiration: 2009-12-21
Also published as: EP0378087B1; ES2063169T3; DE3900362A1; DE59006860D1; EP0378087A2; EP0378087A3; AU4724489A

Description

COMMONWEAIJPH OF AUSTRALIA PATENTS ACT 1952-1969 COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED "OPTICAL TRANSMISSION SYSTEM" The following statement is a full description of this invention, including the best method of performing it known to us:- 0~00 0 0000 *0 0 0 0 0 I~ 00 00 0 4 040 i 1 I~ This invention relates to an optical broadband communication system, particularly for the subscriber area, comprising a bidirectionally operable optical waveguide in which only the fundamental mode LP01 and the next higher-order mode LP11 can propagate in a wavelength range, eg., the 800-nm wavelength range, which is clearly below the cutoff wavelength of the optical waveguide, eg., 1280 nm, at least two optical transmitters in the form of semiconductor lasers and at least two receivers in the form of photodiodes whose operating wavelengths lie within said wavelength range in which the optical waveguide is to be operated, with the optical transmitters and the optical receivers coupled to the optical waveguide in such a way that two optical signals are transmitted bidirectionally in different modes LP01, LP11.

In a known optical broadband communication system, bidirectional transmission is achieved by providing somewhere in the optical waveguide, a mode 0 coupler which has the property of changing, in both directions, a signal 00,, from the fundamental mode LP01 to the next higher-order mode LP11. While 0 0 o no both optical transmitters transmit the signal in the fundamental mode LP01, 0 0 0 o a due to the insertion of a mode filter both optical receivers only receive S the next higher-order mode LP11, since the mode filter only couples this next higher-order mode LP11 to the optical receiver from the optical waveguide. This bidirectional communication system has led to a cost reduction of such communication systems, since the transmitters and receivers respectively can be identical and a standard 1300 nm optical waveguide can be used. Nevertheless, even this optical broadband communication system is not optimised for costs, so that it does not have acceptable costs for subscriber use.

The aim of the present invention is to achieve an optical broadband communication system, particularly for subscriber use, which is simpler and less costly to produce.

_C According to the invention there is provided a broadband optical cormunication system of the above mentioned kind, wherein one of the optical transmitters couples LP01 mode signals into the optical waveguide, that said LP01 mode signals are coupled from the optical waveguide to one of the optical receivers by radiation, and that the other optical transmitter and the i other optical receiver face a transmitter-waveguide-coupling device and a waveguide-receiver-coupling device, respectively, which are contained in the optical waveguide and serve to couple LP11 mode signals from the other optical transmitter into the optical waveguide and from the optical waveguide to the other optical receiver, respectively.

The system according to the invention can use optical transmitters in the form of semiconductor lasers without selection, that is multimode lasers, which can be produced cheaply in quantity. The input and output cou- Spling arrangements according to the invention are both simple and cost effective.

According to a preferred implementation of the invention, an optical 0 4 S component is used which is at present the simplest and cheapest to produce, S and which can be matched easily to the structure of the optical waveguide and the wavelength of the optical signal.

In order to improve the signal-to-noise ratio of such a system between the one optical transmitter and the waveguide-receiver-coupling device and between the one optical receiver and the transmitter-waveguide-coupling device, the optical waveguide contains a modal filter which passes only LP01 mode signals.

Near-end crosstalk is avoided modulating on the optical carrier a radio-frequency carrier modulated by an analog or digital technique. Different radio-frequency bands may be used for analog bidirectional transmission over forward and backward channels. Furthermore, the separation of the different frequency bands can be achieved with mass-production filters which are therefore cheap and readily available.

.Y.

i i The invention will now be described in more detail, using the drawings in which: Figure 1 shows a bidirectional optical communication system in accordance with a preferred embodiment of the invention; Figure 2A,2B show the input (output) coupling of the fundamental mode into (out of) the optical waveguide.

The preferred embodiment of an optical broadband communication system 11 shown in Figure 1 which is especially advantageous for subscriber use, has as the transmission medium an optical waveguide 12, which is in the form of a so-called 1300 nm Single-mode optical fibre. Each terminal 13, 14 (eg.

subscriber and/or exchange) has a transmitter 16, 18 in the form of a longitudinal multimode semiconductor laser or a CD-laser, and an optical receiver 17, 19 in the form of a semiconductor photodiode or PIN-diode. The identical optical transmitters 16, 18 and the identical optical receivers 17, 19 0 have an operating wavelength in the region of 800 nm. This is clearly below the cut-off wavelength of eg. 1280 nm for a standard single mode optical °o fibre 12 intended for transmission in the range 1300 nm to 1600 nm.

o The optical transmitter 16 of a first terminal 13 is connected optically via a coupling fibre 21 to an outward-coupling component 22 in the form of a bend, which in turn is connected to one end of the single-mnode optical fibre 12. The other end of this, or another, part of the single-mode fibre 12 is connected to an inward-coupling component in the form a bend 23 optically connected to a coupling fibre 24, which faces the optical receiver 19 of a second terminal 14. The dots shown in Figure 1 represent spliced connections between the individual optical components.

The optical transmitter 18 of the second terminal 14 is so arranged that its optical signal output side faces towards the inward-coupling bend 23. Correspondingly, the optical receiver 17 of terminal 13 is so arranged that its optical signal input side faces the outward-coupling bend 22.

4

A

2 As shown in Figure 2A, the coupling fibre 21 of terminal 13 is so made, and coupled in such a way to the optical transmitter 16 (multimode GD-laser), so that the coupling fibre 21 can only transmit the LP01 mode of the optical signal. To achieve this, the coupling fibre 21 is narrowed down at the end near the optical transmitter 16 and coated with an absorbing layer 16. After a certain length, a transition is provided from this narrowed region 27 to the normal 1300 nm region 28 of the coupling fibre 21.

The coupling-out of the LP01 mode signal from coupling fibre 2)4 to the optical receiver 19 at the second terminal 1~4, occurs through simple radiation, as shown on Figure 213.

The outward-coupling component 22 and the inward-coupling component 23 are bends with a specific radius. A function of the structure of the optical fibre 12 and the wavelength of the transmitted signal, this radius is essentially calculated in such a way that signals in the LP11 mode are coupled in or out by means of radiation. This characteristic of such bends is Sdescribed in ELECTRONIC LETTERS 21 (1985) p.1042. This means that of all ~the signals produced in the multimode optical transmitter 18 in the second terminal 1)4, only the LP11 mode signals are accepted by, that Is coupled into, the inward-coupling bend 23. In the same way as the fundamental mode LP01 signals, signals with the next higher mode LP11 are transmitted along the optical fibre 12 in the opposite direction. Because of the design of the outward-coupling bend 22, as mentioned above, these higher-order signals are exclusively coupled out and radiated to optical receiver 17 of the first terminal 13.

In order to improve the signal-to-noise ratio, a mode filter transmitting only the LP01 mode (not shown in the drawings) can be inserted before the optical transmitter 16 between the outward-coupling component 22 and the coupling fibre 21) and before the optical receiver 19 between the inward-coupling component 23 and the coupling fibre The two multimode CD-lasers 16, 18 are to a certain degree insensitive to signals reacting on their outputs. Therefore, low-level LP01 mode signals coupled out of the inward-coupling component 23 have no effect on the optical transmitter 18; neither do the low-level LP11 mode signals coupled from optical transmitter 16 to the optical fibre 12, which are also applied to the optical transmitter 18. Corresponding considerations apply to the optical transmitter 16.

In the examp~le described, the forward and backward channels of the analog or digital optical communication system occupy different frequency bands. For example, with an analog system the forward channel. may use a carrier of' 38.7 MHz, and the backward channel a carrier of 225 kHz,. Similarly, with the design example described here, the forward channel may use a carrier frequency of about 1 GHz and the backward channel 3 GHz. both being digitally modulated. The modulation frequency must be well below the carrier frequency.

The two terminals 13, 1~4 are provided with electronic circuits 31, 32 Swhich for the example above would contain filters for 38.7 MHz and 225 kHz, Sor for 1 GHz and 3 GHz, so that interaction between signials at different o 'carrier frequencies is minimised. Several carriers in each direction can S also be used, instead of just one carrier. In that case, tunable filters would be provided. This would have the advantage of giving multi-channel cormnication.

o 00

Claims

1. A broadband optical communication system, comprising a bidirectionally op- erable optical waveguide in which only the fundamental mode LP01 and the next higher-order mode LP 11 can propagate in a wavelength range which is clearly below the cutoff wavelength of the optical waveguide, at least two optical transmitters in the form of semiconductor lasers and at least two receivers in the form of photodiodes whose operating wavelengths lie within said wavelength range which is below, the cut-off wavelength, with the optical transmitters and the optical receivers coupled to the optical waveguide in such a way that two optical signals are transmitted bidirectionally in different modes LP01, LP 1, wherein one of the optical transmitters couples LPOI mode signals into the optical waveguide, and that one optical receiver receives this signal from the end of the optical waveguide, and that the other optical transmitter and the other optical receiver each face bends which are inserted into the optical waveguide and serve to couple LPI 1 mode signals from the other optical 15 transmitter into the optical waveguide and from the optical waveguide to the other o optical receiver, respectively.

2. A system as claimed in claim 1, wherein each of the bends have a radius so chosen, preferably depending on the structure of the optical waveguide and the wavelength of the optical signal, that only LPI mode signals are coupled in or out, 20 respectively, while LPO1 mode signals pass through said bends.

3. A system as claimed in claim I or 2, wherein between said one optical trans- mitter and the bend which serves to couple in LPI mode signals between said one optical receiver and the bend which serves to couple out LP11 mode signals, a modal filter which passes only LP01 mode signals.

4. A system as claimed in any one of claims I to 3, wherein a radio-frequency carrier modulated by an analog or digital technique is modulated upon the optical carrier.

A system as claimed in claim 4, wherein different frequency bands are used for respective direction of the bidirectional transmission signal, said frequency bands be- ing in the radio-frequency range.

6. A broadband optical communication system, substantially as herein described with reference to Figures 1-2 of the accompanying drawings. DATED THIS TWENTY-FIFTH DAY OF FEBRUARY 1992 IA ALCATEL N.V. I "n kc/o