FR2535555A1 - Device for long-range transmission by optical fibre. - Google Patents

Abstract

Device for long-range transmission by optical fibre. The various wavelengths leaving a transmission fibre 1 are directed by a monochromator 2, 3, 4 onto fibre segments 5 of different lengths in order to compensate for the differences in propagation time of these various wavelengths in the transmission fibre. Application to information transmission.

Description

Fiber optic long distance transmission device
The present invention relates to the transmission of information over a long distance by optical fiber. A device for producing such a transmission conventionally comprises the following elements - a generator delivering a multichromatic light which can be modulated by information to be transmitted, - a transmission optical fiber made of silica, receiving this light and transmitting it, - and a detector receiving the light at the output of this fiber to provide a signal representative of the information which modulated it.

 Transmission by means of such a device poses two fundamental problems which do not affect its principle - but which so affect its cost price that the practical possibility of making numerous connections depends essentially on the solutions provided to these problems. . A first problem is that of the attenuation of light during its propagation in its fiber. It is in principle solved by using a modular light generator capable of injecting as large a power as possible into the fiber, a detector as sensitive as possible, and amplifier repeaters sufficiently close to each other along the transmission fiber taking into account the generators and detectors available and the attenuation coefficient of the fiber. But we quickly meet technical and economic limitations with regard to the useful power of the generator and the sensitivity of the detector. As for the number of repeaters, it obviously increases the cost of the device. This is why çon has always sought to reduce the coefficient d attenuation of the fiber which obviously depends on the nature of the fiber and the light used. It has in particular been proposed for this purpose to use pure or doped silica fibers, and infrared light with a wavelength close to 1550 nm. It is desirable that these fibers be unimodal to obtain a useful spectral band large enough to ensure a high flow of information.

 The device according to the present invention uses these well known arrangements.

 Unfortunately, these do not solve a second fundamental problem which is that of chromatic dispersion. The light of the generator has in fact a spectral bandwidth which is by no means negligible, and the speed of group propagation in the fiber varies considerably depending on the spectral component considered of this light.

As a result, the width of the usefully transmitted band is the smaller the longer the length of the transmission fiber, which in practice limits the transmission distance. Two solutions have been proposed to increase this transmission distance while maintaining a suitable transmitted bandwidth.

A first known solution consists in reducing the spectral width of the light injected into the fiber. This solution is notably exposed in the article by K. UTAKA, K. Kobayashi, Y. Suematsu, "Rapid Modulation characteristics of 1.5-1.6 micron distributed Bragg reflector integrated twin-guide lasers", in Digest cf Technical Papers,
Boarding school. Conf. integrated opt. and opt. comm., San Francisco,
April 1981, Optical Society of America, Washington, DC, Tu D5 paper.

 Given the need to maintain a large injected power, this solution leads to a too high cost price of the generator at the same time as a very delicate manufacturing.

 A second known solution consists in acting on the law of variation of the optical index in the transmission fiber as a function of the distance from the axis thereof. A suitable choice of this law makes it possible to obtain that the group speed in the fiber has a minimum for the average wavelength used, and therefore that this speed varies very little in the frequency band used.

This solution is notably exposed in the article by M. Monerie "Propagation in doubly clad single mode fibersn (IEEE Journal of Quantum
Electronics, Vol QE 18, pp 535-542, April 1982).

 In the case of the average wavelength specified above and making it possible to obtain a very low attenuation, this second solution leads to very severe requirements as to the manufacture of the transmission fiber, and to a high cost price. of it.

 The object of the present invention is to enable transmission of information by optical fiber over a long distance and with a large bandwidth for a reduced cost price.

 The transmission device used according to the present invention is characterized in that it comprises - a monochromator receiving the light at the output of the optical transmission fiber, and separating it into a set of juxtaposed monochromatic beams of different wavelengths , - and a corresponding set of sections of flexible optical fibers whose inputs are juxtaposed to each receive one of these beams, and whose outputs converge on said detector, these sections having different lengths capable of compensating for time differences of group propagation of lights of different wavelengths in the transmission fiber.

 It makes it possible to obtain compensation for the differences in propagation time which is certainly partial, but sufficient in practice, with a cost price lower than that which would have resulted from the use of previously known solutions.

 Using the single diagrammatic figure attached, a mode of implementation of the invention will be described below, without implied limitation. It should be understood that the elements described and shown may, without departing from the scope of the invention, be replaced by other elements ensuring the same technical functions.

 This figure represents a view of a long distance transmission device by optical fiber according to the invention.

 This device comprises an olassic light generator 6- at a moderate cost price. This generator emits an infrared light beam which extends between 1548 and 1552 nm in wavelength and which is amplitude modulated by input information - applied to a terminal 7. It is equipped with an injection system 8 which injects light into a single mode jump fiber of index index 1 of a known type. Celletci has a very low attenuation coefficient at this wavelength, and it has a moderate cost price per kilometer, but it has a significant chromatic dispersion coefficient in the vicinity of this wavelength.

 We then meet the following elements - A collimating lens 2 (lens or mirror) making it possible to make the beam carrying the fiber 1 "parallel" 2 the end of the fiber 1 being in the object focal plane.

- A dispersive element in wavelength 3, such as a glass prism, or a network (engraved or holographic).

- And a focusing optic 4 (lens or mirror), focusing the beams of different wavelengths provided by the element 3.

Each wavelength leaving the fiber 1 therefore provides an image point in the image focal plane of the optics 4. Two wavelengths separated by an interval VL provide two points separated by an interval
VX in the focal plane, transverse to the optical axis of optics 4 and parallel to the dispersion plane.

 In fact, all of the elements 2, 3 and 4 constitute a conventional monochromator, and their characteristics can be calculated as for any monochromator, by a person skilled in the art. In particular, the digital aperture of the optic 2 must be greater than that of the beam leaving the fiber 1 so as not to lose too much energy.

A set of sections of optical fibers 5 is arranged so that at an input end, arranged along the "image line" of the end of the fiber 1, the fibers are arranged in a linear network, with tangent hearts, possibly deformed to avoid any loss of light.

 This can be achieved with glass-plastic or glass-glass fibers after abrasion or dissolution of the sheath. At the other end, the fiber sections are tightened in a network and converge on a photodetector 9 which, via a circuit 10, provides output information on a terminal 11. The length of each fiber section is adjusted so that the differences in propagation time (approximately 5 ns / meter) between sections of different lengths exactly compensate for the differences in propagation time applied by fiber 1 between the wavelengths corresponding to the various sections.

 More specifically, the transmission fiber 1 may overall have a chromatic dispersion of 1 ns / nm (typical case of single-mode fibers, with a dispersion of 15 ps / nm x km at 1550 nm in wavelength and 70 km in fiber length at most). Of course, for each practical embodiment, the exact value of dispersion must be measured on the installed line and the lengths of the sections of the assembly 5 adjusted exactly.

 It is desired that the overall dispersion of the system never exceeds 0.6 ns for a light of wavelengths between 1548 and 1552 nm.

 We will use for the set 2, 3, 4 an "olassic" arrangement of monochromator providing for example a VL / VX dispersion - 5 nm / mm.

 The compensator assembly then comprises 19 sections of optical fibers 5, the input axes of which are regularly spaced 0.055 mm apart, and the row of fibers is positioned so as to cover the specified wavelength range.

 By regularly going from the section of fiber which receives the wavelength having undergone the shortest transmission delay towards that which receives the wavelength having undergone the longest transmission delay, each section is regularly shortened by about 5cm relative to to the previous one (for a group index of 1.5), or a regular decrease of 025 ns of propagation delay every 0.5 nm of wavelength, exactly compensating for the dispersion of the line. As can be seen, the compensation being done in stages, there remains an overall dispersion of 0.5 ns, less than that desired of 0.6 ns.

 Finally, while without the device described here, the system would have seen its transmission capacity very limited (a source covering the spectrum 1548 nm - 1552 nm would have provided a broadening of transmitted pulses by 4 n9), the device described here considerably increases the transmission capacity (by a factor of 8 in the example taken).

Claims (3)

1 / Device for long-distance transmission by optical fiber comprising - a generator (6) emitting around 1550 nm of wavelength, a multichromatic light which can be modulated by information to be transmitted, - an optical transmission fiber (1) made of silica, receiving this light and transmitting it in a unique mode with low attenuation, - and a detector (9) receiving the light at the output of this fiber to provide a signal representative of the information which modulated it, - this device being characterized by the fact that it comprises: - a monochromator (2, 3, 4) receiving the light at the output of this transmission fiber (1), and separating it into a set of monochromatic beams juxtaposed with wavelengths - and a corresponding set of sections of flexible optical fibers (5) whose inputs are juxtaposed to each receive one of these beams, and whose outputs converge on said detector (9), these sections present t different lengths capable of compensating for the differences in the group propagation times of lights of different wavelengths in the transmission fiber. 2 / Device according to claim 1, characterized in that the inputs of said sections of optical fibers (5) are aligned while their outputs are arranged in a compact hexagonal network. 3 / Device according to claim 2, characterized in that the sheath of each of said sections of optical fibers (5) is at least partially eliminated at the two ends of this section.