FR2718588A1 - Device for equalizing, as a function of the wavelength, a gain applied to an optical signal. - Google Patents

Abstract

Device for equalization, as a function of the wavelength, of a gain applied to an optical signal, characterized in that it comprises so-called routing means (C), for routing an optical signal on the one hand received on a so-called input channel of these switching means, and consisting, or not, of the optical signal whose gain is to be equalized, depending on the place of application of said gain with respect to this device, to a so-called channel balancing including wavelength-selective reflection means (R1 ... Rn, R'1 ... R'n) making it possible to compensate for differences in gain as a function of the wavelength, and to direct on the other hand, the optical signal coming from this balancing channel to a so-called output channel constituting the output of this device.

Description

 Equalization device. depending on the wavelength.

gain applied to an optical signal
The present invention relates to an equalization device, as a function of the wavelength, of a gain applied to an optical signal.

 The term gain equalization as a function of the wavelength is understood to mean compensation for differences in gain as a function of the wavelength, these differences possibly being those appearing during the processing of an optical signal by an optical component (particularly a optical amplifier) amplifying differently the different wavelengths present in the signal applied to it, or those appearing after wavelength multiplexing of different optical signals from different optical components (particularly lasers) each providing a gain at a wavelength determined, but with a gain difference between these components.

 The present invention is particularly applicable to an optical transmission link on which is used, in order to increase the transmission capacity, wavelength division multiplexing, and which also includes optical amplifiers as repeaters arranged at intervals regular on this link.

 In such an application, a problem arises in the absence of equalization of the gain of these optical amplifiers.

 Indeed, if the link is dimensioned so that the gain at the wavelength, denoted Xo, for which the gain characteristic of these amplifiers passes through a maximum, exactly compensates for the attenuation between two amplifiers so arranged consecutive on the link, the gain at a wavelength, denoted X1, different from Xo, will not completely compensate for this attenuation, and, after a certain number of amplifiers crossed, the signal level & this length d X1 wave may no longer be sufficient to ensure correct reception.

 It is therefore necessary to ensure equalization of the gain of each of these amplifiers.

 It is known from patent application WO 93/07 664 to achieve such gain equalization by aligning this gain with its lowest values by using an optical filter receiving the signal from an optical amplifier whose gain is & equalize and retransmit it with a wavelength attenuation.

 Such a way of doing things has in particular a drawback, which is the difficulty of producing, with very high precision, an optical filter whose characteristic makes it possible to compensate very exactly that of the optical amplifier whose gain is & equalize.

 The present invention relates to a gain equalization device based on a different principle, and in particular making it possible to avoid this drawback.

 The present invention thus relates to a device for equalization, as a function of the wavelength, of a gain applied to an optical signal, this device being essentially characterized in that it comprises means, called switching , to direct on the one hand an optical signal received on a so-called input channel of these switching means and constituted, or not, by the optical signal whose gain is & equalize, according to the place of application of said gain by with respect to this device, towards a so-called balancing path including selective wavelength reflection means making it possible to compensate for differences in gain as a function of the wavelength, and to route the optical signal from the other hand from this balancing path to a so-called output path constituting the output of this device.

 The device according to the invention also makes it possible to achieve gain equalization by aligning this gain with values greater than its lowest values, which does not make it possible to obtain the solution mentioned above under the prior art , and which allows, in the aforementioned application & an optical link to repeaters with optical amplifiers, greater spacings between repeaters on the link.

Other objects and characteristics of the present invention will appear on reading the following description of exemplary embodiments of the present invention, made in relation to the attached drawings in which:
FIG. 1 illustrates a first embodiment of a device according to the present invention,
FIG. 2 is a diagram intended to illustrate the principle on which this first exemplary embodiment is based,
- Figure 3 illustrates a second embodiment of a device according to the present invention.

The device illustrated in FIG. 1 is & by way of example intended to compensate for the differences in gain appearing during the processing of an optical signal, denoted s, in which n wavelengths are present, denoted X1 to
Xn, by an optical amplifier.

 This device comprises so-called switching means, or circulator, denoted C, intended to direct on the one hand an optical signal received on a so-called input channel of these switching means, connected to an access marked 1 of these means switch, to a so-called balancing channel connected to an access marked 2 of these switching means, and to direct the signal from this balancing channel to a so-called output channel, connected to a access identified 3 of these referral means.

 The optical signal to be amplified s is in this case received on said input channel, the balancing channel in this case includes the optical amplifier, denoted A, and the amplified optical signal, with an equalized gain, denoted S , is in this case obtained on said exit channel.

 The optical amplifier A then being of the so-called double pass type, the gain equalization is done by alignment not with the lowest gain values of this amplifier, (considering here as gain that due to a single pass) but on values at least equal to twice (in the case of gain expressed in dB) these lowest values, (the case of gain values greater than twice these lowest values being explained later).

 In the exemplary embodiment illustrated in FIG. 1, the balancing path comprises an amplifying optical fiber called balancing fiber, and a set of reflective optical filters, denoted R1 to Rn, each selective for one of the wavelengths X1 & Xn, and located, on this balancing amplifier optical fiber, & different locations making it possible to compensate for the gain differences of amplifier A & these different wavelengths.

 By way of example, the optical amplifier A is an amplifier & fiber, having a certain length, corresponding to the gain due to a single pass, denoted 1, in which case the balancing amplifier fiber may or may not be included in this length 1, depending on whether the gain equalization is done by aligning values equal to twice (in the case of gain expressed in dB) the lower gain values (single-pass) of amplifier A, or on values greater than twice these lower values.

 A reflective filter denoted Rn + l, selective for a wavelength Xn + l, may furthermore, as illustrated in dotted lines in FIG. 1, be provided on said balancing channel to reflect an optical signal called a pump, denoted Sp, & the wavelength Xn + l, applied & the amplifier A and multiplexed with the optical signal to be amplified s via a multiplexer, noted N.

 FIG. 2 illustrates the shape of the gain characteristic G, due to a single pass, of an amplifying fiber as a function of the length L of this amplifying fiber, for n wavelengths hl to Xn. The term amplifying fiber is, in the embodiment illustrated in FIG. I, used to denote either all or part of the fiber forming the optical amplifier, or all or part of the assembly comprising the fiber forming the amplifier optical and the balancing amplifier fiber, depending on whether the gain equalization is done by aligning on values equal to double (in the case of gain expressed in dB) the lower gain values (single pass) of amplifier A, or on values greater than twice these lower values.

 It can be seen in this figure that there exist, for these n wavelengths, n lengths of amplifying fiber, denoted respectively L1 to Ln, for which the gain G has the same value, denoted Go.

We therefore deduce the different locations at which the different reflective filters R1 to
Rn so that the gain of amplifier A is equalized & the value 2 Go (in the case of a gain Go expressed in dB) corresponding to a double pass.

In the case where the balancing amplifier optical fiber is included in the fiber optic amplifier A, we thus have:
1-li = Li with i between 1 and n, and li designating the location of the reflective filter Ri on the fiber forming the optical amplifier, this location being measured relative to that of the ends of this fiber which is not connected to access 2 of the switching means.

In the case where the balancing amplifier fiber is not included in the amplifier & fiber, we have:
l + li = Li with i between 1 and n, and li designating the location of the reflective filter Ri on the balancing amplifier fiber, this location being measured relative to that of the ends of this fiber which is connected to the end, not connected to access 2 of the switching means, of the fiber forming the amplifier A.

 It will be noted that so far, no account has been taken of any gain differences (in this case negative gain, or attenuation) as a function of the wavelength, originating within the means. 2, but that If such differences exist, they will of course have to be taken into account to determine the locations of the various reflective filters Ri. This will be taken into account simply by no longer considering lengths Li corresponding to the same gain value 2 GB (in the case of a gain Go expressed in dB), but to distinct gain values, verifying an inverse law of that, & compensate, of these referral means.

 It should also be noted that the number of accesses of the switching means could be chosen to be greater than three; for example, in the case where it would be chosen equal to four, the optical amplifier A and the balancing amplifier fiber could, in the case where the latter is not included in the amplifier, either be arranged, as in Figure 1, on the same track, but on separate tracks.

The various reflective optical filters can for example be constituted by Bragg gratings inscribed in the amplifying balancing fiber. This inscription can for example be based on the photorefractive effect, and be carried out by transverse holographic printing around a wavelength of 240 nm, as described for example in the document
Electronics Letters, May 24, 1990, vol 26, no 11, pp 730-731: wAll-Fiber Narrowband Reflection Gratings At 1500 nm "R.

 KASHYAP, J.R. MARMITAGE, R. WYATT, S.T. DAVEY, D.L. WILLIAMS.

 The device illustrated in FIG. 3 is also, by way of example, intended to compensate for the differences in gain appearing during the processing of an optical signal, also denoted s, in which n wavelengths are present, denoted hl to Xn, by an optical amplifier.

 This device comprises switching means similar to those of FIG. 1, and, in this exemplary embodiment, the balancing track connected to the access 2 of these switching means does not comprise any amplifier fiber. balancing, but only includes n reflective optical filters, denoted R'1 to R'n, close, or spatially confused, each selective for one of the wavelengths Xi & Xn, and each providing a different attenuation & this length d wave, in order to compensate for the differences in gain of the optical amplifier as a function of the wavelength.

 The optical amplifier, also noted A, and symbolized by way of example by an optical fiber amplifier, may or may not be disposed on the balancing channel, the example of FIG. 3 corresponding more particularly to the case where it is arranged on this balancing channel, in which case, this amplifier being of the so-called double pass type, the gain equalization is done by alignment, not on the lowest gain values of this amplifier (also considering here as gain that due to a single pass), but on values equal to double (in terms of gain expressed in dB) of these.

 Note however that in the exemplary embodiment illustrated in FIG. 3, it is not possible to perform an equalization by alignment on values greater than twice (in the case of gain expressed in dB) the lower gain values of the amplifier.

 As in the case of FIG. 1, a reflective filter, also noted Ru + 1, selective for a wavelength kn + l, can also be provided on the balancing channel to reflect a pump signal sp, å the wavelength Xn + l, applied to amplifier A and multiplexed with the optical signal to be amplified s via a multiplexer M.

 The various reflective filters can for example be produced according to the technique recalled above for the case of FIG. 1, and the different attenuation values obtained by the filters R'1 & R'n can for example be obtained by providing lengths from different networks.

 It will be noted that the precision with which the equalization is carried out no longer depends on the precision with which optical filters having a determined transfer function can be produced, but only either with the location of these filters or with their behavior a a determined wavelength, such precision can be obtained in a much simpler way than in the solution mentioned above under the prior art.

 It will be noted that in the exemplary embodiment illustrated in FIG. 3, the n reflective filters R'1 to R'n could however be replaced by a single reflective filter of adequate spectral response, that is to say taken such that the differences in gain of the optical amplifier at the different wavelengths are compensated by different attenuations of this reflective filter at these wavelengths.

 It will also be noted that the exemplary embodiments of FIGS. 1 and 3 could be combined, each of them making it possible to partially compensate for the differences in gain of the optical amplifier as a function of the wavelength.

 It will also be noted that although the embodiments described in the foregoing correspond more particularly to the gain equalization of an optical amplifier, the present invention is not limited to such an application and can also be used in particular to equalize gains in optical signals from lasers operating at various wavelengths.

Claims (6)

1 / Equalization device, as a function of the wavelength, of the gain applied to an optical signal, characterized in that it comprises so-called switching means (C), for switching on the one hand an optical signal received on a so-called input channel of these switching means, and constituted, or not, by the optical signal whose gain is to be equalized, according to the place of application of said gain with respect to this device, to a so-called channel balancing including selective wavelength reflection means (Ri .. Rn, R'l ... R'n) making it possible to compensate for differences in gain as a function of the wavelength, and to direct d on the other hand the optical signal from this balancing path to a so-called output path constituting the output of this device. 2 / Device according to claim 1, characterized in that said balancing path comprises an amplifying optical fiber called balancing and in that said wavelength selective reflection means comprise a set of reflective filters (Ri. Rn), each selective for one of the wavelengths likely to be in the signal they receive, and located on this balancing amplifier optical fiber, & different locations making it possible to compensate for differences in gain & these different wavelengths. 3 / Device according to claim 1, characterized in that said wavelength selective reflection means comprise a set of reflective filters (R'1 ... R'n), each selective for one of the likely wavelengths to be in the signal they receive and each providing a determined attenuation so as to compensate for differences in gain & these different wavelengths. 4 / Device according to one of claims 1 to 3, characterized in that said gain differences depending on the wavelength are those arising in an optical amplifier (A). 5 / Device according to claim 4, characterized in that said optical amplifier is located on said balancing channel. 6 / Device according to claims 2 and 5, characterized in that said optical amplifier (A) is a fiber optical amplifier and in that said balancing amplifier optical fiber is included in this optical amplifier.