WO2011068460A1

WO2011068460A1 - Optical coupler

Info

Publication number: WO2011068460A1
Application number: PCT/SE2010/051327
Authority: WO
Inventors: Christofer Silfvenius
Original assignee: Ekklippan Ab
Priority date: 2009-12-04
Filing date: 2010-12-01
Publication date: 2011-06-09
Also published as: SE0950938A1

Abstract

An optical coupler (β), which coupler (6) is connected to a first (2), a second (4) and a third (5) optical waveguide, wherein the coupler (6) is arranged to couple light within a first wavelength interval from the first (2) to the third (5) waveguide, and also light within a second wavelength interval, different from the first wavelength interval, from the second (4) to the first (2) waveguide, wherein a predetermined bias voltage is applied to the coupler (6), and wherein the coupler (6) is manufactured from a semiconductive material, which in an unpumped state is absorbing for both the first and the second wavelength intervals, but which may be pumped electrically so that the material is inverted to decreased absorption, transparency or amplification for both the respective wavelength intervals. The invention is characterized in that the optical amplification spectrum for the material of the coupler (6) is selected so that the optical amplification is different at the first and the second wavelength interval, respectively, given the predetermined bias voltage and in absence of optical pumping.

Description

Optical coupler

The present invention refers to an optical coupler for use in an optical system for data and telecommunication or analyti- cal applications, as well as an optical communication system.

In for example optical data communication, integrated chips are often used as optics, in contrast to so called discrete bulk optic and hybrid solutions, which may both be used for emitting light from narrowband light sources, such as lasers or broadband light sources such as for example light diodes^', and for detecting laser light or light from other light sources, as for instance from light diodes. Other applications are also possible, such as for example measuring of luminescent light from biological samples, measuring of re^¬ fractive index in liquids or the like.

In particular, integrated chips comprising a laser for emit^¬ ting light and a light detector for detecting light are used. Consequently, the laser emits laser light that is modulated with the information to be sent, and the light detector, for example a photo diode, detects received laser light.

Such integrated chips may in general comprise one or more components, such as for example lasers, photo diodes, diplex- ers, or matrices of such components. Each component may in turn be built up by part components, such as for instance in the case where a diplexer comprises a number of waveguides. Swedish patent no. 0501217-4 describes such a chip which is monolithically integrated. In contrast to the conventional integrating method, which is based on the so called butt- joint coupling, the patent mentioned above describes a mono- lithic integration without splices, wherein the chip has a basic structure, which is the same from a carrier and up over the entire surface of the chip, without any splices in the waveguide. The chip comprises a number of waveguides and components, all of which are formed through etching of the basic structure. Such a chip offers low manufacturing cost combined with a high yield.

A problem during transmitting and receiving optical signals is the risk of optical overhearing between different components. This is particularly true in the case where a laser and a light detector are arranged in the same integrated chip. During digital or analogous fiber optic data communication by means of light signals, it is common that a centrally located communication station (central office) interacts with a local communication station (network terminal) , so that a light source, typically a laser, in the central office emits on a certain light wavelength and a light detector in the network terminal receives light of the same wavelength, at the same time as the network terminal emits and the central office receives light over a certain other wavelength, respectively. In other words, the emitting wavelength of the central office is the receiving wavelength of the network terminal and vice versa. If lasers and light detectors are both arranged in one and the same integrated chip, problems with overhearing may arise. This is particularly true in monolithically integrated chips. Since both lasers are arranged to emit light for de- tection in the other terminal, via a fiber optic network, high emitting powers are required. Also, the received light is weak in both terminals. Even if only a very small share of the emitted laser light reaches the light detector in the same integrated chip, for example by .undesired reflections in the facet of the chip or from other surfaces in the communicating link, disturbances result with respect to the interpretation of the information in the incoming signal.

The present invention solves the problems described above.

Consequently, the invention refers to an optical coupler, which is connected to a first, a second, as well as a third optical waveguide, wherein the coupler is arranged to couple light over a first wavelength interval from the first to the third waveguide, as well as light over a second wavelength interval, which is separated from the first wavelength interval, from the second to the first waveguide, wherein a prede- termined bias voltage is applied to the coupler, and wherein the coupler is manufactured from a semiconductive material, which in an unpumped state is absorbing for both the first and the second wavelength intervals but which may be pumped electrically so that the material is inverted to decreased absorption, transparency or amplification for both the respective wavelength intervals, and is characterized in that the optical amplification spectrum for the material of the coupler is selected so that the optical amplification is different at the first and at the second wavelength interval, respectively, given the predetermined bias voltage and in absence of optical pumping.

The invention will now be described in more detail, with reference to the exemplifying embodiments of the invention and, the accompanying drawings, wherein:

Figure 1 is a perspective view illustrating a" monolithically integrated chip; Figure 2 is a top view illustrating the chip of figure 1;

Figure 3 is a graph illustrating an amplification spectrum for an optical coupler according to the present invention; Figure 4 is a top view illustrating a first optical communi- cation system in accordance with the present invention; and Figure 5 is a top view illustrating a second optical communication system in accordance with the present invention.

Figure 1 illustrates schematically a monolithically inte- grated chip 1, prior to metallization, with a coupling waveguide 2, an optical coupler 6 and two output waveguides 4, 5. Such a chip is known from the Swedish patent No. 0501217-4.

According to the invention, the chip 1 is intended for data- or telecommunication or optical analysis of at least one, two or more wavelengths.

The chip 1 comprises a waveguide 2 with a first port 3 into or out from which light is intended to be conveyed. The wave- guide 2 is expanded, in an expanded part 6 in the form of an optical coupler, from the first port 3 in a direction towards a second waveguide 4 and a third waveguide 5. The various part components of the illustrated component are integrated monolithically and without splices. In other words, the chip has a basic structure which is identical from a carrier and upwards over the entire surface of the chip, and the waveguides 2, 4 and 5 are formed at the upper surface of the chip by way of the basic structure having been etched down so that protruding waveguides are formed.

Figure 2 shows an integrated chip similar to the one in figure 1. Corresponding parts have the same reference numbers as in figure 1. In the second waveguide 4, a laser 41 is arranged to emit light A, B. The laser 41 may be produced from the same monolithic basic structure as the rest of the chip, which is preferred since this results in lower manufacturing costs. In this case, a grating structure has been produced in the second waveguide 4, which grating structure defines the laser cavity. The laser 41 can be a Distributed FeedBack laser . (DFB laser), a Distributed Bragg Reflector laser (DBR laser), or any other known type of laser, which, for cost reasons, pre^¬ ferably includes a grating which is produced from the mono^¬ lithic basic structure. As an example, Fabryt-Perot (FP) lasers are also possible. The emitted light A is typically intended to be received by a receiver in the form of a light detector in an external unit, such as an external communication terminal.

Over the laser 41, and also over other components to which a bias voltage is to be applied, such as waveguides 2, 4, 5 and the expanded part 6, there are arranged separate or common electrical contacts (not shown) .

In the third waveguide 5, there is a light detector 51 arranged to detect incident light D. The light detector 51 may be produced from the same monolithic basic structure as the laser 41, which is preferred since this leads to low manufacturing costs, but not necessary. It may, furthermore, be any suitable known light detector, such as a conventional photo diode. The light D typically incides from an external emit- ting unit comprising a laser emitting the light D.

As has been mentioned above, according to the present invention the expanded part 6 consists of an optical coupler 6. The coupler 6 is wavelength selective, i.e. arranged, based on the wavelength of the light D incident through the first waveguide 2, to guide the light D towards either the second 4 or the third 5 waveguide, and also to guide light A of a .

5 certain wavelength from the second waveguide 4, and with a certain other wavelength from the. third waveguide 5, respec^¬ tively, towards the first waveguide 2.

According to a preferred embodiment, the expanded part 6

10 consists of an optical Wavelength Division Multiplexing (WDM) coupler of multimode type. Other types of couplers are also useful, for example a power coupler or an envanescent coupler .

15 According to the exemplified embodiment shown in figure 2, the laser 41 is arranged to emit light A within a certain wavelength interval, and the light detector 51 is arranged to detect light D within a certain other wavelength interval, wherein said wavelength intervals are different. Herein, that

20 the wavelength intervals are different should be interpreted to mean that they are essentially not overlapping and that the power maximums with respect to the wavelength for emitted or received light, respectively, are at least separated.

25 The coupler 6 is accordingly arranged to convey light A,

emitted from the laser 41, to and out through the first waveguide 2, and also light D, within another wavelength interval and incident from the first waveguide 2, to the third waveguide 5 and on to the light detector 51.

-30

In other words, the chip 1 is arranged, by means of the laser 41, to be able to emit optical signals within a first wavelength interval and at the same time to be able to receive optical signals, by means of the. light detector 51, within a second wavelength interval, said second wavelength interval being different from the first wavelength interval. However, due to for example undesired facet reflections of the laser light A, for example at the port 3, reflected light

C of the same wavelength as the light A also incides towards the third waveguide 5. Since the light A, emitted from the laser 41, is much strong^¬ er than the light D inciding into the expanded part 6 through the first waveguide 2, the undesirably reflected laser light C may in many cases severely disturb the incident light D towards the third waveguide 5. This is often true even though only a small share of the light A is reflected, that only a small share C of the reflected light, due to absorption in the coupler 6, reaches the third waveguide 5, and even though the wavelength selectiveness of the coupler in general couples the reflected light from the laser back towards^' the laser.

According to the invention, the coupler 6 is manufactured from a semiconductive material, which in an unpumped state, i.e. in absence of electrical and optical pumping, is absorb- ing for both the inciding D and the emitted A wavelength interval, but which may be pumped electrically and possibly also optically so that the material is inverted to reduced absorption, transparency or amplification for both wavelength intervals individually. Electrical pumping occurs in a way which is conventional as such, by means of such electrical contacts as described above. During operation, a predetermined bias voltage is thus applied to the coupler 6, through contacts that have been at^¬ tached against the coupler 6, to achieve desired amplification features in the material of the coupler 6.

A typical, conventional coupler of WDM type, manufactured from an optically passive material, can couple about 99% of inciding light of a wavelength to an outgoing port and the residual 1% to another outgoing port. If the WDM coupler is produced in an optically passive material, the relation 99:1 can be assumed to be relatively constant for all power levels of the incoming light.

According to a preferred embodiment, the coupler 6 is however manufactured from an optically active material. The term "optically active material" as used herein refers to a material that may be pumped electrically and possibly also optically. Moreover, the optical material according to the invention is of such type that its amplification spectrum is not the same for all wavelengths. Accordingly, there is an additional variable: the coupler 6 may be absorbing, transparent or amplifying, and the state may also be wavelength dependent. Optically active materials are conventional as such, and it falls well within the knowledge of the person skilled in the art to design a semiconductive material with optically active features.

This means that a WDM coupler 6 in accordance with the present invention may be designed so that it at the same time has different amplifications for two different wavelengths, for example so that it amplifies a certain first wavelength while it absorbs a certain second wavelength. The amplification of the material may thus in principal be designed in a way so that the coupler 6 at a suitable bias voltage is slightly absorbing for the wavelength of the laser 41 but slightly amplifying for the wavelength of the incom- ing signal. A reflex in the facet of the laser emitted wavelength will thereby be attenuated before it reaches the light detector 51. The incoming wavelength, on the other hand, will not be attenuated but may, depending on the design of the material of the coupler and the added bias voltage, even be amplified.

The coupler 6 according to the present invention will, in other words, have better performance than a conventional coupler .

According to a preferred embodiment, a bias voltage is in addition applied to one or more of the waveguides 2, 4, 5 to,in this way, select the degree of inversion or amplification of the material. Thi-s will achieve lower optical losses in the chip 1. Moreover, in the case where the different components of the chip are monolithically integrated in the same material system, the bias voltage applied to individual components may be controlled in order to attain a desired functionality locally.

Correspondingly, a bias voltage applied to the coupler 6 may be selected so that an undesired reflection C from the laser 41 to the third waveguide 5, which reflection typically has a power which is several magnitudes smaller than the emitted laser light A, can be completely absorbed in the material in the coupler 6 due to the fact that the reflected light C is not capable of inverting the material to transparency by way of optical pumping. If the incoming wavelength D is considered in parallel to internally reflected light C, the material amplification in the coupler 6 for the wavelength of the incoming signal D should be such that the incoming signal D reaches the photo detector, at the selected bias voltage. Due to the WDM func^¬ tion, light C, D travel with different wavelengths, with slightly different multimode patterns, through the coupler 6, between the first waveguide 2 and the third wave guide 5. The difference in multimode patterns is, however, often far too small to prevent the one wavelength from inverting the material to transparency so that the second wavelength can find a way through without being absorbed and vice versa. In conventional optical couplers, manufactured from passive materials, the optical damping is typically essentially the same for different wavelengths. In an integrated chip for simultaneous transmission and receiving within different wavelengths, it is possible to solve the problem identified above by absorption of undesired laser light reflections by means of a wavelength specific optical filter along the third waveguide 5, between the coupler 6 and the light detector 51. If such a filter is made opaque for the emitted wavelengths A, only incident light D will reach the light detector 51.

Such a filter will, on the other hand, make the production of such a chip more expensive, which is undesired.

Moreover, in the case where the coupler is manufactured from an active material, it is desired to apply a bias voltage to the coupler 6, which causes the material to be slightly absorbing for the emitted laser light A, in order to minimize the noise in the chip 1, and also, if the amplification is positive, reduce the risk for noise and unintentional lasing or that interference patterns are created in another cavity than the laser cavity. However, the circumstance that also incident light D has to be able to be conveyed through the coupler 6 without being absorbed too much limits the freedom of choice for the bias voltage applied to the coupler 6, which is a problem.

To solve these problems, according to the invention, the material for the optical amplification spectrum of the coupler 6 is therefore selected so that the optical amplifica^¬ tion, at a predetermined bias voltage and in absence of optical pumping, is different for, on the one hand, light within the wavelength interval that the light detector 51 is ar- ranged to receive and, on the other hand, light within the wavelength interval that the laser 41 is arranged to emit.

Such an amplification spectrum results in higher degree of freedom to select the bias voltage applied to the coupler 6, while retaining the function of the coupler 6.

The amplification spectrum for the coupler 6 is, according to an especially preferred embodiment, designed so that stronger amplification is achieved for the wavelength interval within which the light detector 51 is arranged to receive light as compared to the wavelength interval within which the laser 41 is arranged to emit light, at a certain predetermined bias voltage applied to the coupler 6 and in absence of optical pumping .

Such an amplification spectrum is illustrated graphically in figure 3, showing the optical amplification per unit length (y-axis) as a function of the wavelength of the light con- veyed through the material (x-axis) . -The different amplification curves, shown in figure 3, correspond to amplification spectrums for different values of the predetermined bias voltage applied to the coupler 6.

A material showing the illustrated amplification spectrum of figure 3 is suitable for use in an optical coupler 6 for a transmitter wavelength of about 1310 nm and a receiving wave^¬ length of about 1550 nm. The electrical bias voltage may in this case be balanced against possible optical pumping from light A, D conveyed through the material, so that the amplification in the material becomes positive for light at 1550 nm before it becomes positive for light at 1310 nm, resulting in that the incoming light signal D can be conveyed to the light detector 51 at the same time as too strong amplification of the emitted laser light A can be avoided. A proportionately low bias voltge may therefore be applied to the coupler 6, which is desirable since it lowers the noise in consequence of amplification of spontaneous emission (ASE) .

It should also be realized that other wavelengths, such as for example 1490 nm, may be used for transmission and/or reception. Moreover, it is possible that one or more wavelengths are used for transmission at the same time as one or more wavelengths are used for reception, for example by means of several cascade connected couplers of the type described herein. It is preferred that a shorter wavelength, such as 1310 nm, is used for transmission and a longer wavelength, such as 1550, for reception.

The predetermined bias voltage is selected, according to a preferred embodiment, so that the material of the coupler 6, in absence of optical pumping, is slightly damping for the wavelength interval within which the -laser 41 emits. The predetermined bias is selected, according to an especially preferred embodiment, so that the material in the coupler 6 in absence of optical pumping, is absorbing for both the emitting and the receiving wavelength interval, and so that the light within the receiving wavelength interval has a power large enough to invert the material in the coupler 6 to transparency for the receiving wavelength interval during passage of the light D.

This results in the advantage that the signal to the photo detector 51 may pass through without problem, possibly even under certain amplification, at the same time as the laser light A is absorbed somewhat in the coupler 6, which prevents undesired reflexes C to disturb the activity of the photo detector 51.

The amplification spectrum for the semiconductive material in the coupler 6 may be designed in different ways by adjusting the design of its material.

The material system in a monolithically integrated chip according to the present invention may, as can be seen in the Swedish patent no. 0501217-4, be designed with various mate- rials. However, the material structure generally constists of a number of layers with different semi conductive materials, mainly consisting of As, P, Ga, In and/or Al . The material can consist of bulk, multiple quantum wells (MQW) or a combination of quantum dots (QD) .

The material of the coupler 6 comprises, according to a first preferred embodiment, an array of quantum wells formed in a way which is conventional as such, by adjusting the choice of material in different groups of 'layers on different depths in the material, and so that the combined spectrum from the active material obtains an amplification spectrum which is different for at least two of the different used emitting and receiving wavelengths. This embodiment allows a simple and therefore cheap production in the case where higher amplification is desired for instance for the wavelength 1550 nm compared to the wavelength 1310 nm. Typical choices of ma^¬ terial are InGaAsP or AlInGaAs alloys for bulk, well and barriers .

According to a second preferred embodiment, the material from which the coupler 6 is built comprises a number of so called quantum dots, designed for optical amplification at the first wavelength interval, and a number of other quantum dots, designed for amplification at the second wavelength interval. The array of quantum dots in the material is designed so that the amplification is stronger for one of the wavelength intervals than the other. This design admits that the amplifi- cation spectrum for the material in the coupler 6 can be designed more freely, for example in that the amplification can be stronger for the wavelength 1550 nm than for 1310 nm for a certain bias voltage applied to the coupler 6. Typical material choices for quantum dots of 1310 nm are combinations of the alloys InAs,. InGaAs and GaAs, and for 1550 nm In- As/InP.

Figures 4 and 5, which are similar to each other and which also share reference numbers for corresponding parts, illu- strate the use of optical couplers in accordance with the invention in two different preferred systems for optical communication comprising a locally arranged communication station Si and a centrally arranged communication station S₂. The communication system may be -composed of a so called P2P (Point-To-Point) FTTH link, where both stations can constitute the local and the central sides, respectively. The local station Si may in such a system emit Bi on 1310 nm and receive Di on 1550 nm, while the central station S2 emits B₂ on 1550 nm and receives D₂ on 1310 nm. Other combinations of wavelengths and wavelength intervals are also possible, as long as one of the stations emits on the receiving wave- length (s) of the other station and vice versa. Both stations Si, S₂ are arranged for simultaneously emitting and receiving the optical signals, which for example are composed of data traffic over Internet, for digital TV or similar. According to this embodiment, the local station Si comprises a coupler 61 with an amplification spectrum that for a predetermined bias voltage applied to the coupler 61 results in a stronger amplification at 1550 nm than at 1310 nm, i.e.

stronger amplification for the receiving wavelength of the local station Si than for its emitting wavelength. This results in the advantages, described above, of low noise in combination with good light detection. This solution is espe^¬ cially preferred with regard to the fact that the local stations in this type of communication system tend to be numer- ous, and that such a design admits a relatively cheap manufacturing of the optical chip arranged in the station Si.

The central station S₂, on the other hand, cannot use the same type of amplification spectrum in its corresponding coupler 6₂, since its emitting and receiving wavelengths are inversed in relation to the local station Si. Instead of designing the material in the coupler 6₂ with an amplification spectrum that results in stronger amplification at 1310 nm than at 1550 nm at a certain 'bias -voltage, it is possible to design the material of the coupler 6₂ with an amplifica^¬ tion spectrum which leads to the amplification at both these wavelengths being essentially the same given the predetermined bias over the coupler 6₂.

To avoid disturbing internal reflections C₂ in the central station S₂, in this case an optical filter 52₂ is arranged in its third waveguide 5₂ between the coupler 6₂ and the light detector 51₂, which filter 52₂ is arranged to be opaque to light C₂ within the wavelength interval conveyed through the coupler 6₂ from the second 4₂ to the first 2₂ waveguide, i.e. for the light A₂ that gives rise to harmful internal reflections C₂. To the contrary, the filter 52₂ is of course transparent to the incident light D₂.

The filter 53₂ may in certain applications be supplemented by or replaced by a choice of material design in the coupler 6₂ that admits some absorption for both 1310 nm and 1550 nm given the predetermined bias voltage.

Moreover, it is preferred that an optical amplifier (not shown) is arranged between a possible filter 52₂ and the light detector 51₂, to secure enough power in the received light reaching the detector 51₂.

The communication system illustrated in figure 5 also comprises a local Si and a central S₂ station. In contrast to the communication system illustrated in figure 4, the material of the coupler 6₂ in the central station S₂ is designed so that its amplification spectrum admits that the amplification, given the predetermined bias voltage, is stronger at 1310 nm than at 1550 nm. This results in that the corresponding ad- vantages achieved for the local .station Si also can be achieved for the central station S₂, why the need of a filter 52₂, being present in the system of figure 4, is not present in the system illustrated in figure 5. This results in cheap^¬ er production of the coupler 6₂ of the central station S₂.

It is realized that the stated wavelengths in all the described embodiments above are illustrative by nature, and that the idea of the invention can be applied for other pairs of emitting and receiving wavelengths, where one wavelength is longer or shorter than the other.

Preferred embodiments have been described above. However, it is obvious for a person skilled in the art that many modifications can be made to the described embodiments. The invention shall thus not be limited to the described embodiments, but may be varied within the scope of the enclosed claims.

Claims

1. An optical coupler (6), which coupler (6) is connected to a first (2), a second (4) and a third (5) optical waveguide, wherein the coupler (6) is arranged to couple light over a first wavelength interval from the first (2) to the third (5) waveguide, and also light over a second wavelength interval, different from the first wavelength interval, from the second (4) to the first (2) waveguide, wherein a predetermined bias voltage is applied to the coupler (6), and wherein the coupler (6) is manufactured from a semiconductive material, which in an unpumped state is absorbing for both the first and the second wavelength interval, but which may be pumped electrically so that the material is inverted to decreased absorp- tion, transparency or amplification for both the respective wavelength intervals, c h a r a c t e r i s e d in that the optical amplification spectrum for the material of the coupler (6) is selected so that the optical amplification is different at the first and the second wavelength interval, respectively, given the predetermined bias voltage and in absence of optical pumping.

2. A coupler (6) according to claim 1, c h a r a c t e r i s e d in that the coupler (6) is wavelength selective.

3. A coupler (6) according to claim 2, c h a r a c t e r i s e d in that the coupler (6) is a WDM coupler.

4. A coupler (6) according to any one of the preceding claims, c h a r a c t e r i s e d in that the second wave^¬ guide (4) comprises a laser (41), arranged to emit light within the second wavelength interval, and in that the third waveguide (5) comprises a light -detector (51) arranged to receive light within the first wavelength interval.

5. A coupler (6) according to any one of the preceding claims, c h a r a c t e r i s e d in that the coupler (6) constitutes a part of an integrated chip (1) for data- or telecommunication or optical analysis for two or more wavelengths, in that the chip (1) can be used for emitting light and for detecting light, in that the chip (1) further com- prises the first (2), second (4) and third (5) waveguides, in that the coupler (6) consists of an expanded part of the first waveguide (2) from the first waveguide (2) in a direction towards the second (4) and the third (5) waveguides, in that the chip (1) has a basic structure which is identical, from a carrier and upwards over the entire surface of the chip, in that said waveguides (2; 4; 5) are formed at the upper surface of the chip (1) by way of the basic structure being etched down so that protruding waveguides are formed, and in that the chip (1) comprises monolithically integrated compo- nents.

6. A coupler (6) according to any one of the preceding claims, c h a r a c t e r i s e d in that the amplification spectrum for the coupler (6) is such that a stronger amplifi- cation is achieved for the first wavelength interval than for the second wavelength interval at the predetermined bias voltage applied to the coupler (6) and in absence of optical pumping .

7. A coupler (6) according to claim 6, c h a r a c t e r i s e d in that the material of the coupler (6) is built up from a number of layers of semiconductive materials, wherein different groups of layers constitute optical quantum wells, and in that the quantum wells are designed so that the ampli^¬ fication is stronger at the first wavelength interval than at the second wavelength interval.

8. A coupler (6) according to claim 6, c h a r a c t e r i s e d in that the coupler (6) comprises a number of quantum dots, which are designed for optical amplification at the first wavelength interval as well as at the second wavelength interval, and in that the quantum dots are designed so that the amplification is stronger at the first wavelength interval than at the second wavelength interval.

9. A coupler (6) according to any one of claims 6-8, c h a r a c t e r i s e d in that the predetermined bias voltage is selected so that the material in the coupler (6) , in absence of optical pumping, is slightly absorbing at the second wavelength interval.

10. A coupler (6) according to claim 9, c h a r a c t e r i - s e d in that the predetermined bias voltage is selected so that the material in the coupler (6), in absence of optical pumping, is absorbing both at the first and the second wavelength interval, and in that the light within the first wavelength interval inverts the material in the coupler (6) to transparency.

11. A coupler (6) according to any one of the preceding claims, c h a r a c t e r i s e d in that the first wavelength interval comprises the wavelength 1550 nm and that the second wavelength interval comprises the wavelength 1310 nm.

12. An optical communication system comprising a first (Si) and a second (S₂) station, wherein both stations (Si;S₂) are arranged for simultaneous emission ai¾d reception of optical signals, wherein emitting and receiving is performed over different wavelength intervals so that the emitting wavelength of the first station (Si) is the receiving wavelength of the second station (S₂) and vice versa, c h a r a c t e r i s e d in that the first station (Si) comprises a coupler (6i) according to claim 9 or 10, in that the second station (S₂) comprises a coupler (6₂) according to claim 4, but where the amplification spectrum for the material in the coupler (6₂) in the second station (S₂) is selected so that the amplification, given the predetermined bias voltage applied to said coupler (6₂), is the same at the first and second wavelength interval, and in that an optical filter (52₂) is arranged in the second station (S₂) between its coup- ler (6₂) and light detector (51₂), which filter (52₂) is arranged to be opaque to light (A ;C₂) within the wavelength interval conveyed through the coupler (6₂) in the second station (S₂) from its laser (41₂) to its first (2₂) waveguide.

13. An optical communication system comprising a first (Si) and a second (S₂) station, wherein both stations (Si;S₂) are arranged for simultaneous emission and reception of optical signals, wherein emitting and receiving is performed over different wavelength intervals so that the emitting wave- length of the first station (Si) is the receiving wavelength of the second station (S₂) and vice versa, c h a r a c e r i s e d in that both the first (Si) and the second (S₂) station comprise couplers (6i;6₂) according to claim 9 or 10.

14. An optical communication system according to claim 12 or 13, c h a r a c t e r i s e d in that one of the wavelength intervals comprises the wavelength 1550 nm and the other wavelength interval comprises the wavelength 1310 nm.