WO2005006044A1

WO2005006044A1 - Optical multiplexer/demultiplexer and channel equaliser

Info

Publication number: WO2005006044A1
Application number: PCT/NO2004/000209
Authority: WO
Inventors: Ragnar Fagerberg; Steinar NEEGÅRD; Kjetil Johannessen; Lars Johnsen
Original assignee: Sinvent As
Priority date: 2003-07-09
Filing date: 2004-07-07
Publication date: 2005-01-20
Also published as: NO20033140D0; NO20033140L

Abstract

This invention relates to a multiplexing device for separating or combining optical signals between at least one common optical waveguide and a number of second optical waveguides, as well as a related equaliser means. The optical signal comprises a number of signals having different wavelengths each constituting an optical channel, the multiplexing device comprising a first diffractive optical element, said diffractive element being wavelength dependent focal points so as to focus light between the common fiber and a chosen number of directions. The device also comprises a set of mirrors positioned in said directions, said mirrors being adapted to transmit light between the first diffractive element and (a set of) (the) second diffractive element(s), said second diffractive element(s) being adapted to focus light to or from the second optical waveguides.

Description

OPTICAL MULTIPLEXER/DEMULTIPLEXER AND CHANNEL EQUALISER

This invention relates to a multiplexing device for separating or combining optical signals modulated at different carrier wavelengths between a common optical waveguide and a number of dedicated optical waveguides. In existing telecommunication systems at the present each channel may typically transmit 2.5 - 40Gb/s over one single optical fiber. This is further increased by using several separate optical channels in the same fiber by wavelength division multiplexing (WDM). In "Dense WDM" (DWDM) these channels are closely positioned in wavelength. The total capacity of the fiber may then be in the range of several Tb/s. In "Coarse WDM" (CWDM) the channel separation is larger and there are generally fewer channels on a single fiber. Since the channels are close in DWDM stable and expensive sources are required. CWDM opens for cheaper solutions, as the demand for lasers and other optical devices with extreme stability is relaxed. DWDM is especially suitable for long distance communication. Both DWDM and CWDM is increasingly used within smaller geographical areas, e.g. in so-called Metropolitan networks. Centrally positioned in WDM systems are components for combining light from several sources in one optical waveguide (multiplexing - MUX) and distributing different channels to individual outputs (demultiplexing - DEMUX). Such functions are implemented everywhere where sources (electrical to optical signal conversion), detectors (optical to elektrical conversion) and switches (electrooptic or all-optic) are present. hi WDM systems there is also a need to control or equalise the optical power over the wavelength range. In order to obtain this the composite WDM signal must be split into its respective channels (demultiplexed) or into a set of different wavelength segments. The different parts of the wavelength range may then be individually attenuated before being recombined to a complete and balanced WDM signal. These functions are typically positioned in the transmission system in or behind MUX devices, optical amplifiers and switches. Another use of an equaliser is in long chains of amplified sections. The attenuation is typically performed in a modulator which primarily is wavelength independent. Several such modulators are available based on different technologies, e.g., free-space diffraction based devices where the diffractive effect is based on a local and controlled height modulation of a reflective surface. Relevant products may be obtained through assembling several single modulators (or a pixelated modulator array) with wavelength sensitive components to a controllable multi-channel attenuator working on the entire WDM band. hi addition routing/switching of optical signals is an important function and there is a trend towards the implementation of all-optical solutions without electrooptic/optoelectric signal conversion. These functions also require the separation and combination of wavelength channels. Different component technologies are available for realising the wavelength dependant characteristics described above. In the most demanding solutions (DWDM with many channels) planar waveguide technology is a potential solution which however is not necessarily suitable in combination with free-space based modulators. Furthermore, planar waveguide devices remain expensive. Other solutions based on conventional diffraction gratings are also expensive, partly due to the requirement to align many optical components: gratings, lenses and possibly one or more attenuation devices. Recently a method for replicating polymer optical elements has been developed. These printed optical elements may have different designs, and may be configured as plane versions of conventional lenses, they can be provided with grating characteristics which through diffraction spread light with different wavelengths in chosen directions, and they may be designed to split each wavelength spatially. All these functions may be obtained within the same area. Further it is possible to realise different functions in different areas of the component. The complex design is transferred to a master component using electron beam lithography. This master may be replicated in polymer materials in large numbers and at low costs. This results in cheaper components and the current application opens for a potentially lower assembly cost than for other grating based solutions for telecommunication use due to a requirement a smaller number of elements, hi this invention these printed components are used in fiberoptic telecommunication systems. Diffractive optical elements (DOE) of this type are known, e.g. from international patent application No PCT/NO01/00476 where a DOE is used in spectroscopy. The DOE in the current application is provided with focussing abilities and for separating different wavelengths, but is used in a completely different context. In US patent application US 2002/0154855 and US patent 5,107,359

DOEs are used for wavelength-division multiplexing /demultiplexing purposes. The problem with these solutions is to obtain sufficient channel separation while coupling entire channels into the output fibers. US 2002/0154855 describes a complicated solution comprising diffraction gratings and lenses in combination to constitute a multiplexer/demultiplexer. This represents an expensive solution having the disadvantages mentioned above. In US 5,107,359, figure 1, the output fibers are positioned in focus of the DOE. As single mode fibers have small core diameters relative to the fiber diameter only two discrete wavelengths may be probed. This gives good channel separation but the transmission is low if the wavelength deviates from the ideal values, in which case the light is lead into the cladding of the fiber. Also, as the angular spread from the diffractive element is small the distance from the DOE to the fibers must be large if the wavelength resolution is to be good. Thus it is an object of this invention to provide an inexpensive optical multiplexer/demultiplexer without the requirement for delicate alignment of many optical elements, while maintaining the required resolution. The present concept is based on the combination of technologies in which components may be printed in more or less the same way as compact disks are produced. This and the other objects with the invention is obtained with an optical device as stated above being characterised as described in the accompanying independent claims. The device according to the invention thus provides a solution which may be inexpensive and compact, while maintaining the required channel separation. The device according to the invention also provides a multiplexer equaliser means for adjusting the relative intensity of the optical channels. The invention will be described below with reference to the accompanying drawings, which illustrates the invention by way of example.

Figures 1-8 illustrates the theory of the diffractive optical elements used according to the preferred embodiment of the invention. Figure 9a,b illustrates a multiplexer/demultiplexer according to the invention.

Figure 10 illustrates an embodiment of the invention comprising signal equalising means.

Figure 11 illustrates an equaliser according to the invention.

Figure 12 illustrates an embodiment of the equaliser in figure 11 having four channels.

Figure 13 illustrates a set of mirrors positioned according to the focal points of the first DOE element according to the invention. Figure 1 illustrates the principle of a reflective/phase only DOE with an orientation in relation to a reference plane P, a polychromatic source S and a detector D. The DOE is designed in such a way that for a given position θ_n of the diffractive element, the wavelength component λ„ of the source S is imaged on the detector D. Figure 1 illustrates the principles in case of two wavelengths λj of ₂-

It is necessary for the DOE design to consider the DOE plane as the reference plane (instead of the plane P) that is to say that the DOE keeps still while the source S and the detector D rotates over the scanning axis as shown by figure 2. This geometry is strictly identical to figure 1. Consider first the wavefront of the signal from a source s„ (abbreviation of S(Θ,J) whose location is defined by the vector r"_ource \x_Sn , y_Sn , z_Sn ) and its image d„

(abbreviation ofD(θ_n)) whose location is defined by the vector r_eteclor[x_d ,y_d ,z_d J. i.k„.r. S_n(r) is the spherical wavefront coming from the source: S„ (r) = A_sn ^■ e with k_n = 2π/λ_n and r_Sn = V + ²„ + . A is the wavefront amplitude at the DOE.

• D_n(r) is the spherical wavefront focusing on the detector: D_n{r) = A_d .e-^{i k}^

A_d is the wavefront amplitude at the DOE.

KB. S„(r) wavefront intensity is I_s =

= S„ (r)- S_n (r)^* = A_s ² . Likewise 1 - A ²

Regarding the DOE reflectance consider a reflective profile/TJc.j;,) as shown by figure 3. An incoming wave reflecting on this profile experiences a phase

delay Aφ = -2.f{x, y) , assuming that the phase delay caused by reflection on the n

DOE is a pure geometric addition to the optical path length.

The DOE reflectance function is given by: t(x, y) = A₀ (x, y) ■ e^lΛφ = A, (x, y) ■ e^-²-^⁾ with .f(x,y) the DOE profile function . λ_n is the wavelength of the incoming wave S„.

. Ao(x,y), the amplitude over the DOE.

Given Fourier optics, | Outcoming waves = S„(r) . t (x, y)

The calculations of the DOE profile f(x,y) may be described giving the right filter function (or reflectance) t(x,y) respectively for the single and two- wavelengths cases. In the single wavelength case the source sj emitting at is imaged on dj. Given equations 4 and 5: D_x = S - t(x,y) A₀ - e -i-h-2f{x,. y)

Resolving this equation results

Figure 4 illustrates a first possible source and detector location relative to the scanning axis of a DOE, wherein the source and detector is position on the Y-axis passing in the DOE plane through the optical centre of the DOE (see figure 6) and perpendicular to the rotation axis (that exist for the simplified situation). Figure 5 shows the phase of the reflectance function t(x,y) before it is chopped off to get the DOE profile (equivalent to a phase delay in the range [0, 2π]). Figure 6 shows the DOE profile, which in this case is the central part of a Fresnel lens. The central part of a Fresnel lens is, however, known to be wavelength- independent so that any wavelength coming from the source would focus at the same location on the detector. By shifting the source and the detector outside the Y-axis as illustrated in figure 7, the DOE profile becomes an off-axis Fresnel lens as shown by figure 8. This profile is now wavelength-dependent: a wavefront (wavelength λj) coming from the source is imaged on the detector whereas the other wavelengths focus at different locations. For a source containing a WDM signal, the WDM channels will be imaged along a line, resolved according to their wavelengths. Figures 9a and 9b show a basic DEMUX concept (alternatively MUX when reversing the light paths, as all the elements are bidirectional). Figure 9a shows the light path via a diffractive and focussing section of a DOE to a series of mirror elements, and figure 9b the light path from the same mirror elements to the output fibers via distinct sections of a DOE primarily performing a focussing action. The angle in and out of the DOE is arbitrary chosen in the figure for illustrative purposes. For packaging reasons with a demand for small physical size, it may for example be advantageous to have the series of mirror elements close to the in and output fibers. In a combination of the DEMUX concept with a free-space modulator, the detailed geometry will depend on the modulator type. WDM multiplexed light is for example transmitted through a single fiber in a fiber array and illuminates a distinct area of a diffractive optical element (DOE). The DOE shown (figure 9a) has an integrated grating and a focussing mirror function, which provides a wavelength dependent focussing where the different wavelengths are focussed at different positions along a line. Along this line a number of mirror elements are positioned. The length of each mirror element corresponds to the width of each WDM channel or alternatively a larger wavelength segment. The series of mirror elements directs, through designed mirror angles, each WDM channel or alternatively a larger wavelength segment, back to separate areas on the DOE element. In these areas directional mirror/lens functions are provided for focussing the light toward the different output fibers in the fiber array. These areas may also be designed with diffractive functionality that compensates for, e.g., the angular dispersion defined by the first DOE device so that channel is coupled to the output fiber with high efficiency and with further wavelength selectivity. Such a double pass through a dispersive element will reduce channel cross-talk that is an important parameter in WDM systems. Multiple passes between a mirror element and DOE being almost parallel may also be provided in another embodiment of the invention. Figure 9a and 9b shows a 1x4 demultiplexer as an example, but this channel count both in the input and output sides of the multiplexer maybe increased, also by defining different DOE channel areas in two dimensions on the element. Also, separate DOE elements may in some cases be used, wherein one is adapted to transmit light between the first waveguide and the set of mirrors, and one or more other DOE elements is adapted to transmit light between a set of mirrors and the output fibers. This is, however, a more complex and less advantageous solution. In the DOE elements a function may also be provided wherein the same wavelength is split into several directions, and thus onto a number of different output fibers, thus providing a channel splitter functionality, which e.g. may be used for monitoring means for controlling variations in the optical power of the channels. It is also possible to make a DOE which transmits a number of wavelengths in the same direction and thus into the same optical fiber. According to an alternative embodiment of the invention as illustrated in figure 10 the multiplexer/demultiplexer may also be provided with attenuation or equalising means for controlling the optical power in each channel. The module (which for simplicity is shown folded into the plane) is based on a multichannel variable optical attenuator (VOA) of a per se known type which will not be described in any detail here. Reference is, however, made to Asif Godil, "Diffractive MEMS technology offers a new platform for optical networks", Laser Focus World, May 2002, and to US 5,311,360 (Bloom et al) which described possible optical attenuators. The VOA is preferably positioned close to the fiber ends in such a fashion that one VOA channel corresponds to the beam from the first DOE element onto the mirrors, while each of the remaining VOA channels corresponds to a distinct beam from the mirrors onto the remaining DOE areas. Using such an arrangement it is possible to combine effects from decentering of the light beams and deviations from the fibers' numeric apertures to obtain the required damping. This is what a diffractive modulator does. They can be placed in the optical path without further arrangements. Of course it is also possible to use absorptive modulator or any other intensity-modulating element that can be fitted into the optical path. This possibility is especially advantageous when the budget for optical loss is not too critical, e.g. in metropolitan networks. In WDM optical communication systems with optical amplifiers the gain will vary substantially over the wavelength spectrum so that the different channels will be subject to different amplification levels. This may cause problems because of nonlinear effects in the fiber. Therefore there is a need for an equalisation so that the power is made sufficiently comparable for all channels. This function may be called a gain or channel equaliser, depending on whether the attenuation is performed on individual channels or on wavelength segments containing more than one channel. In order to realise this in a system with at least one fiber in and another fiber out, the DEMUX or MUX configurations may be used in combination with a VOA as described above. With two separate passes of the beams onto VOA channels and appropriate polarization controlling elements in the path, the controlled attenuation may be implemented while the polarisation dependency typically shown by some VOA devices is minimized. A modified equaliser may alternatively be used as a channel blocker, so as to block a selected number of optical channels. Another application is optical equaliser in a long distance WDM system where there are multiple optical amplifiers in a chain without any branching in-between. For this application a device can be made by making a device as described above but folding back from a DEMUX through the attenuators to a MUX. Such a device can be made folding multiple passes between a DOE plane and a mirror plane. A modulator array can be located as described above or more conveniently located in the middle of the device instead of a fiber array for the output of the DEMUX and input for the MUX. For monitoring and controlling the attenuation in the modulator a beam splitter may be provided in the light path for one or more wavelength, the beam splitter being adapted to direct part of the signal at each channel toward a detector. Thus the modulator is capable of adjusting the intensity in each channel according to the measured intensity in that channel, possibly also according to the intensity in the other channels so as to equalise the signals in said channels. The beam splitter may be comprised by the diffractive elements 2, as suggested in figures 11 and 12. The diffractive element will then be made to have more than one focal point for a given wavelength or range of wavelengths. Generally in multiple passes light will bounce back and forth between two parallel planes of the element. In most situations one plane holds the DOEs and the other plane holds mirror elements. However, other combinations are foreseeable like the above described on fiber equaliser where one of the reflector planes can be exchanged with a modulator plane. Other optical elements can also be inserted in the optical path changing the general pattern. Also, more complex systems featuring more than one input and output fibers may be contemplated. This may provide a possibility for combining some of the channels in two or more fibers into a common fiber and equalising the signal strengths in these channels. Figures 11 and 12 illustrates an equaliser function/modulator combined with both a DEMUX and a MUX in a double pass configuration to allow for a single fiber input and output device. The equaliser in this preferred embodiment comprises input and output fibers la, lb, e.g. for adjusting the relative intensity at different wavelength channels in the fiber. The concept illustrated in figure 11 is constituted by two DEMUX, MUX devices as referred to in relation to figure 10, with the additional monitoring detectors 10 and with a common modulator 7. Also, the illustrated concept shows only an equaliser having single fiber in and out. More complicated systems having several input and output fibers, and featuring multiplexing capabilities between the fibers, are also possible within the scope of this invention. Light emitted from the input fiber la is separated according to wavelength by the diffractive optical element 2a and directed toward a mirror array 3 a directing the separated wavelengths back to the same or different parts of the diffractive optical element 2a for being directed toward an optical modulator 7 as described above which directs parts of the light to a second diffractive optical element 2b which in the same manner, but in opposite sequence directs the light having been attenuated by the modulator 7 through a mirror array 3b and the diffractive optical element 2b again to an output fiber lb. hi figures 11 and 12 the signals at each wavelength are also split by the diffractive optical element so as to direct part of the light at each wavelength on both sides of the modulator 7 to a monitoring detector 10a,10b. This way control may be obtained for individual real time adjusting of the intensity at each wavelength at the modulator array 7. Figure 11 describes the principle of the equaliser according to the invention wherein the number of optical wavelength channels and waveguides may be chosen depending on the specific use. Figure 12 describes a specific four channel equaliser in which the DOEs are divided into an array of individual elements 2a,2b each handling a chosen wavelength and additional DOE elements 2c,2d for dividing or combining the optical signal to or from the optical waveguides la, lb. Thus, in figure 12 the signal from the input fiber la is separated by the first DOE 2c into four different wavelengths, each wavelength signal being directed toward a monitoring detector in a detector array 10a and through a mirror array 3 a to a DOE array 2a, which focuses the light toward a chosen element in the modulator array 7. Based on the information provided through the detectors 10a each wavelength signal is attenuated according to chosen parameters and directed to the second array of DOEs 2b. The attenuated wavelength signals are then directed through the mirror array 3b to the signal combining DOE 2d which focuses the light toward the output optical waveguide lb as well as to an array of monitoring sensors 10b for additional control of the signal attenuation. The mirror device is primarily based on fixed mirrors, although controllable mirrors may be possible. Both MEMS based mirror structures, grinded and polished cylindrical mirrors or other solutions may be contemplated within the scope of this invention. The preferred embodiment of this invention however, incorporates mirrors being produced by techniques corresponding to the ones used for producing the DOE (master/moulding), hi one embodiment of the invention it is therefore possible to make the DOE and the mirrors in one moulding. If controllable mirrors are used they may function as wavelength independent modulators through controlling the overlap between the beam and the succeeding DOE. Given the mirror action is orthogonal to the spectral spread of the focus line the spectral sensitivity is minimal. As an example a 16 channel CWDM system may be considered according to the ITU G.694.2 recommendation. Each channel carries 2.5Gb/s and there are 20nm between the channel centers. The carrier wavelength of a single channel can vary within the CWDM channel with time due to e.g., temperature fluctuations. If this 16-channel signal is coupled from the optical fiber onto a DOE element, which simultaneously performs a diffractive and focussing action, a focus plane will be defined at which the pattern will occur if all channels are positioned at the individual channel centers. The focus plane is of interest because here, maximum separation between the chamiels is reached. It is anticipated that a dispersion of at least lOμm/nm will be attainable in the focus plane with a practical DOE. The channel spots in a CWDM system will thus be spaced by 200μm. i this ideal situation, the individual channels could be picked up by placing a fiber array in the focus plane. With a real CWDM signal, the position of each point in the focus plane will be spatially shifted along the focus line according to the deviation of the individual channel carrier from the center channel wavelength. Since the actual position of each carrier may vary from one nominally identical laser source to another, and indeed over time for the same laser source, a DEMUX device must be able to collect all light that fall within a wavelength channel region. Again, the width of each channel is 200μm (still assuming a dispersion of lOμm nm and 20nm channel width). Standard single-mode fibers have a core of lOμm. Only light falling onto the core within the field of view will be effectively coupled into the fiber. In this case only about 5% or less of the channel will thus be transmitted into the fiber. Alternatively one could move the fiber out of the focus plane to allow for coupling over a broader wavelength range. This would however, reduce the maximum coupling efficiency and cross-talk between the channels would be introduced. Because of these factors it is of interest to add a channel separation to the angular dispersion defined by the first DOE element. The proposed way of obtaining an additional channel separation is to place a surface structured mirror at or close to the focus plane. As shown in Figure 13, each mirror surface overlaps with the allowed position of one channel. Each mirror has a distinct surface normal, which may be rotated about two axes - not only one as shown in Figure 13. The distinct surface normals and the abrupt transitions between the mirror elements ensure that the reflected and diverging light of neighboring channels is further separated. The arrows in Figure 13 indicate rays of the different channels incident on and reflecting off the mirror surface. At this scale all channels are not quite (due to the angular dispersion of the first DOE), but almost parallel onto the mirror surface. The mirror can be designed so as to take such deviations into account and may in principle, direct the channels individually to DOE segments positioned almost anywhere. The output DOE elements that each correspond to a distinct mirror surface must be designed so that the light coming from that mirror surface is focussed on a distinct fiber end, independent of the actual carrier wavelength (and thus the actual spot position on the mirror). The structured mirror will also introduce some diffractive effects. These will be minor in the example shown, where the channel separation and thus the mirror size is large as compared to the spot size, h a DWDM system, however, the mirror elements will have to be much smaller, say 10-20μm. In this case the diffraction off the mirror discontinuities will be more pronounced. The flexibility this invention introduces is to separate the angular separation of channels and the angular separation within the channel. The dead-band in- between the mirror elements are effective in introducing the desired stop band in- between the transmission channels. The flexibility in choosing stop band to transmission band ratio is also larger than in earlier designs. The DOE elements can also be used as described in No PCT/NO01/00476 to produce multiple replicas of the spectrum or focal point for a single wavelength. This possibility can be utilised splitting off a small part of the light into a separate set of mirrors or directly to a detector array for monitoring purposes. Alternatively, controllable splitting ratios of more equal proportion can be incorporated into the device introducing also a splitter function to the device. An application of this can be a WDM system where a limited number of users share a dedicated wavelength channel. A definite advantage with the present invention is that the DOE can be configured in such a way that this splitting ratio is virtually unchanged by small changes in wavelength or beam position. While many other approaches splitting off fractions of the light becomes very sensitive to movements of the beam and/or wavelength in the case or a partial intersection of the beam. Other options with a full field beam splitter introduce additional losses and elongated path length. Variations over the solutions described above are of course possible within the scope of this invention, which is defined by the accompanying claims. For example, the set of mirrors may comprise a two dimensional matrix of mirrors, so that in stead of a line of focal points the DOE focuses toward a matrix of mirrors.

Claims

C l a i m s

1. Multiplexing device for separating or combining optical signals between at least one common optical waveguide and a number of second optical waveguides, wherein the optical signal comprises a number of signals having different wavelengths each constituting an optical channel, the multiplexing device comprising a first diffractive optical element, said diffractive element being wavelength dependent focal points so as to focus light between the common fiber and a chosen number of directions, the device also comprising a set of mirrors positioned in said directions, said mirrors being adapted to transmit light between the first diffractive element and (a set of) (the) second diffractive element(s), said second diffractive element(s) being adapted to focus light to or from the second optical waveguides.

2. Device according to claim 1 , wherein the first and second set of diffractive element are constituted by the same diffractive element, said diffractive element comprising a number of areas with chosen focussing and dispersive characteristics.

3. Device according to claim 1, wherein said at least one common optical waveguide constitutes one of said second waveguides for guiding one of said multiplexed signals.

4. Device according to claim 1, wherein said set of mirrors is adapted to transmit light between said first diffractive element and chosen areas on said second diffractive element.

5. Device according to claim 4, wherein each mirror in said set of mirrors is controlled individually for directing light toward chosen areas of the diffractive element.

6. Device according to claim 1 , comprising a modulator positioned in at least one of the light paths for the different wavelengths, the said modulator being adapted to reduce the intensity of light passing through the modulator

7. Device according to claim 1, comprising a modulator positioned in at least one of the light paths where the wavelengths are multiplexed, said light path including at least two signal wavelengths, the said modulator thus being adapted to reduce the intensity of light passing through the modulator At all these multiplexed wavelengths.

8. Device according to claim 6 and 7, wherein the modulator is positioned in a number of said light paths, being adapted to reduce the intensity selectively in a chosen subset of said light paths.

9. Device according to claim 1 where parts of the light is split off by the

DOE either directly or through a separate mirror array to perform a splitting or channel monitoring function.

10. Device according to claim 1 where the wavelength dependent focal points are not aligned on a single line but distributed in a two dimensional pattern to allow for more channels within a given area and angular deviation from the original optical axis thus allowing for a higher channel count.

11. Optical equaliser for adjusting the relative effect of light at different wavelengths in a composite optical signal in at least one optical waveguide, wherein the equaliser comprises a first set of diffractive optical elements, each diffractive element having wavelength dependent focal points so as to focus light from one direction to at least one direction corresponding to different light paths, the equaliser also comprising a set of mirrors positioned in said light paths, said mirrors being adapted to transmit light between the first set of diffractive elements and the second set of diffractive element, said second diffractive elements being adapted to focus light to or from at least one second optical waveguides, and a modulator positioned in at least one of the light paths for the different wavelengths, the said modulator being adapted to reduce the intensity of light passing through the modulator, the modulator is positioned in a number of said light paths, being adapted to reduce the intensity selectively in a chosen subset of said light paths.

12. Equaliser according to claim 11, comprising beam splitter means for dividing light in at least one of said light paths into at least two separate secondary light paths, said beams splitter e.g. being the diffractive optical element or a mirror.

13. Equaliser according to claims 11 or 12, comprising monitoring means for monitoring the equaliser, said monitoring means being positioned in at least one of said light paths for monitoring the light in this light path.

14. Equaliser according to claim 11, wherein the equalising function/modulator is combined with both a DEMUX and a MUX in a double pass configuration to allow for a device with at least one one input and output fiber transferring signals at one or more wavelengths.

15. Equaliser according to claim 12 and 14 comprising at least one input and output fibers transferring signals at one or more wavelengths with monitored both input and output.