FR2776152A1 - Remote gain controlled optical fiber transmission system - Google Patents

Abstract

The system has an amplifier (6i), and a distance regulatable filter (9,10,11), allowing compensation of the multiplexing wavelength due to the variations produced by the aging of the transmission system. The optical communications line filters can be remotely adjusted to compensate for differential aging across the muptiplexed wavelengths, which occurs with time

Description

REMOTE EXCURSION CONTROL

  OF GAIN IN A FIBER TRANSMISSION SYSTEM

  OPTICAL AND WAVELENGTH MULTIPLEXED

  The present invention relates to a fiber optic transmission system with wavelength multiplexing. It also relates to a method of adapting a

  such a system, making it possible to compensate for the effects of aging.

  The invention relates to optical amplifiers, and more particularly to optical amplifiers intended to be used for optical fiber transmission systems with wavelength multiplexing. Such amplifiers are used to compensate for loss of line at regular intervals. These amplifiers are associated with filters so as to present a gain as flat as possible over the entire wavelength range used in the transmission system. The flatness of the gain, or even the limitation of the gain excursion is obtained during the installation of

  the line by an adequate optimization of the amplifiers and associated filters.

  One of the problems encountered for such optical fiber transmission systems is the aging of the components. The attenuation of the line fibers or amplifiers increases so as to decrease the power applied to the input of each amplifier. This decrease causes an increase in the gain of the amplifier, and the shorter wavelengths are then more amplified. This shift of the gain peak towards the low wavelengths causes a variation of the gain over the wavelength range of the multiplex; the gain variation as a function of the wavelength thus looks like a line of decreasing slope. This variation in gain over time causes an amplification imbalance between the different channels of the multiplex in wavelengths. This problem is mentioned in O. Gautheron et al., Robustness of WDM submarine transmission to degraded conditions, OFC'97 Technical Digest, p. 61. This presentation proposes a solution consisting in readjusting the power pre-emphasis in the transmission terminal, so as to compensate for the shift in gain towards short wavelengths; in other words, it is proposed to increase the transmission power of the longer wavelength channels, compared to the shorter wavelength channels, so as to compensate for the relative increase in gain. for wavelength channels

  weaker; this allows you to adjust the signal to noise ratio for the different channels.

  This solution leads to new problems. The relative power of the different channels cannot be increased indefinitely: the readjustment of the pre-emphasis is limited; in fact, the signal to noise ratio at the output of the link is a decreasing function of the pre-emphasis; a pre-emphasis of 7 dB thus induces a loss of the order of 0.5 dB in the signal to noise ratio at the link output, the loss being of the order of 1.2 dB for a pre-emphasis of 10 dB; in practice, the pre-emphasis is therefore most often limited to 7 dB. Increasing the pre-emphasis is also all the more difficult as the channels are numerous and the

long links.

  Limiting the pre-emphasis at the end of the life of a link to 7 dB leads to specifications of the amplifiers and associated filters which make their manufacture

difficult and expensive.

  The object of the invention is therefore to provide a solution to this new problem.

  It proposes a method which makes it easier to compensate for the aging of the transmission system, while maintaining a limited pre-emphasis. It even ensures at the end of link life a correct performance in terms of signal ratio

on noise.

  The French application filed by the applicant under the number 9800120 on 08.01.98 and having as its title Optical amplifier with optimal gain excursion for different input powers (internal reference F 100360) proposes using tunable bell filters to adjust the gain gain of amplifiers in a wavelength-division multiplex fiber-optic transmission system, in order to adapt the gain deviation of each amplifier when installing the

  link, depending on the losses of the different sections of line fiber.

  More specifically, the invention provides a fiber optic transmission system with wavelength multiplexing, comprising at least one amplifier and

  characterized by at least one remotely adjustable filter.

  Advantageously, the filter is adjustable so as to compensate in the wavelength range of the multiplex for the variations caused by the aging of the

transmission system.

  In one embodiment, the filter is adjustable so as to present in the wavelength range of the multiplex a transfer function substantially

  horizontal or a decreasing transfer function.

  In this case, the system preferably comprises at least one compensating filter

  the gain form of the amplifiers.

  This filter can include two complementary filters, one of which can be tuned remotely, for example two complementary filters are formed by two networks of

  Identical bragg used respectively in transmission and reflection.

  In one embodiment, the filter comprises means for switching between at least two separate filters and the switching means are controllable

remotely.

  In another embodiment, the remotely adjustable filter (s) compensate for the gain form of the amplifier (s). In this case, the filter is advantageously an inverted bell filter, for example a photorefractive filter, coupled to a temperature component that can be adjusted remotely, such as a Pelletier effect element. In one embodiment, the remotely adjustable filter includes a tunable wavelength filter. The invention also provides a method for adapting a fiber optic transmission system with wavelength multiplexing, comprising at least one amplifier and at least one filter that can be adjusted remotely, comprising the remote adjustment of said one or more filters to compensate for the gain excursion on the range

  of the multiplex caused by the aging of the system.

  Other characteristics and advantages of the invention will appear on reading the

  description which follows of embodiments of the invention, given by way of example and

  with reference to the accompanying drawings which show: - Figure 1 a schematic representation of a transmission system according to the invention; - Figure 2 a representation of an embodiment of a remotely adjustable filter usable in the assembly of Figure 1; - Figure 3 a representation of another embodiment of a remotely adjustable filter usable in the assembly of Figure 1; - Figure 4 a representation of yet another embodiment of a filter adjustable to

  distance usable in the assembly of figure 1.

  - Figure 5 a schematic representation of another embodiment of a transmission system according to the invention; - Figure 6 a representation of the transfer function of a remotely adjustable filter

  usable in the assembly of figure 1.

  The invention proposes, to compensate for the aging of a fiber optic transmission system with wavelength multiplexing, to have in the system one or more filters adjustable remotely. This or these filters can make it possible at any time to compensate, from one end of the system, for the offset of the gain peak of the amplifiers, and thus provide an alternative or cumulative solution to the variations in pre-emphasis on transmission. In other words, the adjustable filters allow to correct the variations of the gain excursion in the system of

  transmission caused by aging.

  In a first embodiment, described with reference to Figures I to 4, the transmission system has amplifiers associated with filters making it possible to compensate for the gain form of the amplifiers. The invention then proposes to further provide one or more filters making it possible to compensate for the variations in gain of the amplifiers caused by aging. In a second embodiment, described with reference to FIGS. 5 and 6, the transmission system does not have a filter associated with each amplifier. The filter or filters of the invention then have the function of compensating for the gain form of the amplifiers, and in addition of

  compensate for variations in amplifier gains caused by aging.

  Figure 1 shows a schematic representation of a transmission system according to an amplification device according to the invention; there is shown in the figure a transmitter Tx 1, a receiver Rx 2 connected to a fiber section 3, and n sections 4i, i = 1 to n. The transmitter 1 transmits to the first section i = 1 the signals of the different channels of the multiplex. The last section i = n is connected to fiber 3 which transmits the

pulses at the receiver 2.

  Each section 4i comprises a length of fiber 5i, the output of which is connected to an amplifier 6i; the output of the amplifier is connected to a filter 7i. The output of this one is connected to the fiber 5i + 1 of the following section, or for the section n, to the fiber 3. These various elements of the transmission system are given only by way of example, and the positions as well as the relative number of filters and amplifiers can

  vary, as will be apparent to those skilled in the art.

  According to the invention, the transmission system further comprises at least one remotely controllable filter, which makes it possible to compensate for the effects of aging on the gain excursion in the transmission system. In the example of FIG. 3, there are provided three remotely controllable filters 9, 10 and 11, for example regularly spaced along the transmission system. The remote control of the filters makes it possible at any time to compensate for the variations in the gain excursion due to the aging of the transmission system. According to the invention, the losses introduced by these filters are advantageously taken into account in the definition of the system so as to ensure a constant output power. One can for example shorten the range of the sections in which the filter (s) are registered, so as to take into account

  counts the losses caused by the filters.

  As an example, typical values of the effects of aging are given, for a transoceanic transmission system of 7 Mm, intended to last 25 years, and operating in wavelength multiplex over a range of 7 nm, with a distance of 60 between amplifiers. Losses due to aging are estimated at / 1000 dB / km after 25 years. It should be added 3 dB per 1000 km, corresponding to the addition of fiber necessary for a deep repair, supposed to occur every 1000 km. We estimate at 5 dB the power loss at output for I repeater on 20, and at 15 dB on each side of the link the losses for repairs in shallow waters. The average loss due to aging is then 12/1000 dB / km, which produces for each amplifier a displacement of about 0.3 nm towards the low wavelengths of the gain peak and therefore an increase in the gain of the amplifier of the order of 0.8 dB; this increase induces a variation in gain excursion over the width of the multiplex, which

  is 15 dB for the entire link.

  Such a variation in the gain excursion can be corrected using filters that can be adjusted remotely, without involving any intervention on the link. By having three filters each capable of inducing a variation in the gain excursion of 5 dB, in the opposite direction, it is possible to correct almost all of the effects due to aging, as the aging progresses. This means that the filters that can be adjusted remotely are likely to induce an increase over the wavelength range of the multiplex.

  relative gain for the highest wavelength channels.

  The invention can be implemented in the following manner. When installing the transmission system, the remotely controllable filter or filters are introduced onto the line fiber. The presence of these filters is taken into account in the definition of the line. As the system ages, the effects of aging on the gain excursion can be corrected by remotely adjusting the filters. We thus manage to preserve the performance of the transmission system, without having to introduce significant pre-emphasis. The filters are adjusted remotely,

  without requiring intervention on the transmission system.

  In the example of FIG. 1, the filters that can be adjusted remotely are distinct from the filters used for adapting the amplifiers during the design of the transmission system. It is clear that one can also replace one or more of the filters associated with. amplifiers by filters adjustable remotely to compensate for variations in gain of the amplifier (s) over time, as explained

  for example with reference to Figures 5 and 6.

  It is possible for the implementation of the invention to use various types of filters that can be adjusted remotely. It is thus possible to use filters whose transfer function is effectively adjustable remotely, to adapt to the necessary compensations. It is also possible to use a plurality of filters connected to a switch making it possible to select a filter from the plurality. You can still use filters with

  a wavelength tunable remotely.

  Figure 2 shows a representation of an embodiment of a remotely adjustable filter usable in the assembly of Figure 1. The filter of Figure 2 is formed of two filters 33 and 34; one has an inverted bell-shaped transfer function, and the other has a bell-shaped transfer function. The filter having a bell-shaped transfer function - the filter 34 in the example of FIG. 2, is a fixed filter. The filter having a transfer function in the form of an inverted bell - the filter 33 in the example of FIG. 2, is a filter tunable at a distance, which makes it possible to compensate for the variations in gain of the amplifiers due to aging. The filter 33 is remotely tunable so that

  allow any variations in amplifiers to be compensated at any time.

  In the example of FIG. 2, the filters 33 and 34 have substantially complementary transfer functions; thus, when the filters are tuned, the filter of Figure 2 has a substantially planar transfer function. On the other hand, when the central wavelength of the tunable filter 33 is changed remotely, the transfer function of the filter in FIG. 2 has a substantially straight line, decreasing for tuning from the central wavelength to short wavelengths. This compensates for variations in the gain of the amplifiers

caused by aging.

  The filter 33 of Figure 2 is for example a photorefractive filter, mounted on an element whose temperature is remotely controllable, such as a Pelletier effect component or a heating resistor. Such a photorefractive filter, such as that sold by the company Bragg Photonic under the reference Long period grating has a central wavelength which can vary from 50 μm / C, which ensures a variation of 0.5 nm for a variation of temperatures of 10 C. Such temperature variations can be obtained remotely using a Pelletier effect component or

a heating element.

  FIG. 3 is a representation of another embodiment of a remotely adjustable filter usable in the assembly of FIG. 1. The filter of FIG. 3 is a particular embodiment of the filter of FIG. 2, in which the two complementary filters are formed by two identical Bragg gratings used respectively in transmission and in reflection. The filter of Figure 3 includes a

  circulator with three inputs. The first input 35 of the circulator receives the signal to be filtered.

  On the second entrance 36 is mounted a Bragg network 37; on the third entrance 38

  another identical Bragg 39 network is mounted, the output provides the filtered signal.

  The network 39 used in transmission is remotely tunable.

  The operation of the filter in FIG. 3 is as follows. The signal to be filtered is received, and transmitted through the circulator to the first Bragg network 37, where it is reflected. The reflected signal passes through the circulator and then the second Bragg network 38, in transmission. When the two networks are tuned, by an appropriate setting of the network used in remote adjustable transmission, the transfer function of the filter is flat. However, when the two networks are not tuned, the filter

  may have a decreasing transfer function.

  We thus manage to insert into the telecommunications system, during its installation, a filter which has a substantially planar transfer function. If the system ages, one of the filters can be tuned, so as to compensate for the shift towards

  the left of the gain peak, as explained in the previous examples.

  Figure 4 shows a representation of yet another embodiment of a remotely adjustable filter usable in the assembly of Figure 1. The filter of Figure I comprises a remotely operable switch 20, which is connected to at least two separate filters 21 and 22; the outputs of the two filters are connected to a coupler 24. The separate filters 21 and 22 have, in the wavelength range of the multiplex, different transfer functions; switching from one of the filters to the other allows

  compensate for variations in gain excursion due to aging.

  The operation of the device of Figure 4 is as follows. When installing the transmission system, the coupler 20 is controlled so as to pass the signal through one of the filters, for example the filter 21; this filter is specified according to the initial characteristics of the transmission system. When the system ages and its characteristics change, it is possible to remotely control the switching of the switch so as to pass the signal through another filter, here filter 22. The characteristics of filter 22 are determined so as to ensure the compensation for gain variations over the range of the multiplex. Advantageously, in this embodiment, the average losses induced by the two filters are

  identical or close, so as not to vary the output power of the link.

  The embodiment of FIG. 4 allows compensation for variations to be implemented by a discrete number of filters; compared to the embodiment of Figures 2 or 3, the embodiment of Figure 4 allows a more limited number of settings, but makes possible a simpler adaptation of each of the filters. Figure 5 shows a schematic representation of another embodiment of a transmission system according to the invention. The system of FIG. 5 comprises a transmitter Tx 25, a receiver Rx 26 connected to a fiber section 27, and n sections 28i, i = 1 to n. The transmitter 25 transmits to the first section i = I the signals of the different channels of the multiplex. The last section i = n is connected to fiber 27 which

  transmits the pulses to the receiver 26.

  Each section 28i comprises a length of fiber 29i, the output of which is connected to an amplifier 30i; the output of the amplifier is connected to the fiber 29i + 1 of the following section, or for the section n, to the fiber 27. In this embodiment, the transmission system further comprises at least one filter that can be controlled remotely, which compensates for the effects of aging on the gain excursion in the transmission system, and which at the same time compensates for the appearance of the amplifiers. In the example in the figure, there are provided m filters 3 3 i which can be controlled remotely, for example regularly spaced along the transmission system,

  with a filter about every E (n / m).

  When the link is installed, the presence of the filters makes it possible to compensate for the variations in gain in the amplifiers, so as to equalize the gain; during the aging of the system, the filters also make it possible to compensate for the displacement of the gain peak of the amplifiers, to the left. Remote filter control allows you to compensate for variations in gain excursion due to aging

of the transmission system.

  Figure 6 is a graphical representation of the transfer function of a remote tunable filter, which can be used in the assembly of Figure 5. The filter has an inverted bell shape, and a tunable center frequency. The transfer function of the filter has a left flank 15 of negative slope, a central part 16 of substantially zero slope, and a right flank 17 of positive slope. The filter is chosen so as to present in the wavelength range of the multiplex an inverted form of the gain form of the amplifiers; the filter tuning range is sufficient to follow the offset of the gain peak of the amplifiers caused by aging: a possible tuning value of 0.3 nm is sufficient in practice. This ensures the correction of the gain excursion caused by the aging of the amplifiers over the range of the multiplex. In the example of FIG. 6, the central wavelength of the non-tuned filter is 1558 nm, and the tuning range is

  0.3 nm towards short wavelengths.

  The adjustable distance filters therefore make it possible at any time to compensate for the gain form of the amplifiers; as the filters are adjustable remotely, we can follow the gain peak, i. e. move the filters to short wavelengths to

  avoid gain variations caused by aging amplifiers.

  The filter of FIG. 6 can be made like the filter 33 of FIG. 2.

  We thus arrive, in the configurations of FIGS. I or 4, with or without filters associated with each amplifier, to ensure equalization of the gain of the amplifiers, and to compensate for the gain variations caused by the aging of the amplifiers. Of course, the present invention is not limited to the examples and embodiments described and shown, but it is susceptible of numerous variants accessible to those skilled in the art. The form of the transfer function of the filter is not limited to that shown in FIG. 6. The embodiments of FIGS. 1 to 4 can also be combined. The means for remotely controlling the filter are also not limited. like the Pelletier effect component. You can also use other types of filters that can be adjusted remotely, such as Fabry filters.

Perot or Mach Zender.

Claims (8)

  1.- A fiber optic transmission system with wavelength multiplexing, comprising at least one amplifier (6i), characterized by at least one filter (9, 10, 11) remotely adjustable. 2.- System according to claim 1, characterized in that said filter is adjustable so as to compensate in the wavelength range of the multiplex variations   caused by the aging of the transmission system.   3.- System according to claim 1 or 2, characterized in that said filter is adjustable so as to present in the wavelength range of the multiplex a substantially horizontal transfer function or a decreasing transfer function. 4.- System according to claim 3, characterized in that it further comprises   at least one filter (7i) compensating for the shape of the gain of the amplifiers.   5.- System according to claim 3 or 4, characterized in that the filter comprises two complementary filters (33, 34; 37, 39) one of which is remotely tunable. 6.- System according to claim 5, characterized in that the two complementary filters are formed of two identical Bragg gratings used   in transmission and reflection respectively.   7.- System according to one of claims 1 to 6, characterized in that the filter   includes switching means (20, 24) between at least two separate filters   (21, 22), and in that the switching means are remotely controllable.   8.- System according to claim 1 or 2, characterized in that the filter or filters   remotely adjustable (31 i) compensate for the gain form of the amplifier (s) (31 i).   9.- System according to claim 8, characterized in that said filter is a filter in inverted bell.   10.- System according to claim 8 or 9, characterized in that the filter is a photorefractive filter, coupled to a temperature component adjustable remotely, such as a Pelletier effect element.   1 l. System according to one of claims 1 to 10, characterized in that the filter   remotely adjustable includes a tunable wavelength filter.   12.- Method for adapting a fiber optic transmission system with wavelength multiplexing, comprising at least one amplifier (6i) and at least one filter (9, 10, 1 1) remotely adjustable, comprising remote adjustment of the said filter (s) so as to compensate for the gain excursion over the range of the multiplex   caused by the aging of the system.