FR2483608A1 - Demultiplexer for light signals with different wavelengths - has photodetectors on upper and lower side of transparent support plate, which selectively absorb desired wavelength - Google Patents

Abstract

The device comprises two photodetectors (5,6) e.g. photodiodes, each absorbing selectively one of the two wavelengths. Two photodetectors (5,6) are superimposed and fixed, e.g. glued, on the top face (7) and bottom face (8) of a plate (9) which forms a support and which is transparent to the light, e.g. glass. The outer face (11,12) only of each photodiode (5,6) is covered by a metallisation, in the form of a ring. The second metallisation of each photodiode is a thin layer (14) on each face (7,8) of the support (9), using an electrically conductive transparent material, e.g. tin oxide. The two photodiodes (5,6) give on their terminals (15,16) an electrical signal whose amplitude is proportional to the optical power of each wavelength. To demultiplex, more wavelengths one can add other photodiodes.

Description

 The present invention relates generally to transmission systems and relates more particularly to a device for separating two light signals of different wavelengths transmitted over the same transmission channel, such as for example an optical fiber.

 To demultiplex two different wavelengths simultaneously, for a transmission system on optical fiber, it is known to use complex optical devices comprising for example dielectric thin film filters, diffraction gratings or dispersive prisms. These devices are, however, bulky and fragile.

 Instead of such optical devices, it is known to use a photodiode whose structure makes it possible to detect and demultiplex simultaneously two wavelengths, as described in the article published in the journal "Applìed Physics Letters" volume 34, n0, March 15, 1979, page 401, entitled "Dual-wavelength demultiplexing InGaAsP photodiode".

According to this article, the photodiode consists of a substrate on which a succession of layers has been grown by epitaxy. Two of these layers are doped so as to form two photodiodes. In addition, these two layers have different crystal structures, so that they selectively absorb the wavelengths of two light signals transmitted over an optical fiber.

 However, a photodetector of the type described above has drawbacks. Indeed, from the technological point of view, it is very difficult to achieve perfectly homogeneous crystal growth of the different layers. Furthermore, perfect separation of the spectral responses of the two layers forming photodiodes is very difficult to obtain. In addition, the values of the detected wavelengths, 1.08pm and 1.17pu, do not constitute the wavelength values usually used in a transmission system over optical fibers and, because of their relatively close values, these wavelengths are difficult to control.

On the other hand, the crosstalk between the responses of the two layers forming photodiodes is high.

 The object of the present invention is to remedy these drawbacks by proposing a device for separating at least two light signals of different wavelengths transmitted over an optical fiber, which is of simple structure and is inexpensive. .

This device therefore lends itself very well to mass production, and adapts perfectly to microelectronics.

 More specifically, the separation device according to the invention mainly comprises at least two superimposed photodetectors and fixed respectively on the upper and lower faces of at least one plate forming a support made of material transparent to light, the photodetectors selectively absorbing the lengths of wave.

 The invention also relates to a receiver of a transmission system comprising such a device for separating two different wavelengths.

 Other characteristics and advantages of the invention will appear better in the detailed description which follows and refers to the single appended drawing given solely by way of example and which is a longitudinal section of the separation device according to the invention.

 According to an exemplary embodiment, and with reference to the accompanying drawing, a single-mode or multi-mode fiber is shown comprising a core 1 and a sheath 2, the end 3 of the fiber being cut at a bevel (360).

 In order to increase the capacity of the transmission over optical fiber, it is customary to use several sources, such as for example lasers which simultaneously emit light signals of wavelengths, that is to say of distinct colors. These light signals are transmitted in the optical fiber, as shown in 4, and undergo a total reflection on the end 3 of the fiber. These reflected light signals are then separated by the device according to the invention, which will now be described.

 As can be clearly seen in the figure, this separation device comprises two photodetectors 5 and 6, such as for example photodiodes, intended to selectively absorb the two wavelengths usually used in transmission over optical fiber, equal to 0.85m and 1.3pu.

Advantageously, the two photodiodes 5 and 6 are chosen from those existing on the market, therefore with known absorption properties. Thus, for example, the photodiode 5 is made of silicon which is known to be absorbent at 0.85 pm and transparent at 1.3 pm; and the photodiode 6 is made up of bodies belonging to groups III and V of the periodic classification of the elements of Mendeleeff, commonly known as the photodiode with the compounds III - V. Thus, the silicon photodiode 5 is placed above the photodiode 6 with compounds III - V, and the fiber is positioned in such a way that the light reflected by the end 3 of the fiber passes through the two photodiodes; each photodiode will then extract a preferred wavelength;
More specifically, the two superimposed photodiodes 5 and 6 are fixed, by gluing, respectively to the upper 7 and lower 8 faces of a support plate 9 made of a material transparent to light, such as for example glass.

 As can be clearly seen in the figure, only the upper 11 and lower 12 faces of the respective photodiodes 5 and 6, that is to say their respective faces furthest from the glass plate 9, are covered with a metallization in the shape of a ring. The second metallization of each photodiode 5 and 6 is provided by a thin layer 14 uniformly deposited on the faces 7 and 8 of the glass plate 9, and made of an electrically conductive material and transparent to light, such as by example of tin oxide. The connections made on the metallizations 11 and 14 of the photodiode 5 are shown at 15 and at 16 the connections made on the metallizations 12 and 14 of the photodiode 6.

 When the two light signals of wavelength 0.85pm and 1.3pm are reflected by the end 3 of the fiber, the silicon photodiode 5 absorbs the wavelength 0.85pm and lets through, like a filter, the light signal of wavelength 1.3pu. The latter passes through the tin oxide layer 14 and the glass plate 9, and the photodiode 6 with compounds III - V absorbs the wavelength 1.3 pu. In the case where parasitic reflections appear at the interface between the photodiodes and the glass plate, it suffices to cover these photodiodes with a thin layer of anti-reflective material.

 Consequently, electrical signals are detected at terminals 15 and 16, the amplitude of which is proportional to the optical power emitted by the two sources. These electrical signals are then processed by an appropriate electronic circuit.

 Of course, the device which has just been described for demultiplexing two wavelengths, could just as easily be designed to demultiplex a third wavelength equal for example to 1.06pu, without departing from the scope of the invention. In this case, it would suffice to mount on a glass plate a third photodiode, of the III-V compound type, doped in such a way that it is absorbent at 1.06 μm.

 A device for demultiplexing at least two wavelengths has therefore been produced according to the invention, which is particularly simple, very effective, and advantageous from the point of view of manufacturing cost.

Claims (7)

 1 - Device for separating at least two light signals of different wavelengths transmitted on the same transmission channel, characterized in that it comprises at least two photodetectors 5, 6) superimposed and fixed respectively on the upper faces ( 7) and lower (8) of at least one support plate (9) made of material transparent to light, the photodetectors selectively absorbing the wavelengths.  2 - Device according to claim 1, characterized in that the photodetectors (5, 6) are photodiodes of which only the respective faces (11, 12) furthest from the plate (9) are metallized.  3 - Device according to one of claims 1 and 2, characterized in that the upper (7) and lower (8) faces of the plate are uniformly covered with at least one thin layer (14) conductive of electricity and transparent to light  4 - Device according to claim 3, characterized in that the plate (9) is glass etzla layer (14) conductive in tin oxide.  5 - Device according to one of the preceding claims, characterized in that the photodetectors (5, 6) are fixed to the plate (9) by bonding.  6 - Device according to one of claims 2 to 5, characterized in that one of the photodiodes (5) is silicon and the other photodiode (6) with compounds III - V.  i  7 - Receiver of a transmission system, characterized in that it comprises a separation device according to any one of the preceding claims.