FR2544883A1 - OPTICAL MULTIPLEXER / DEMULTIPLEXER - Google Patents

Abstract

A WAVELENGTH EMULTIPLEXER MULTIPLEXER INCLUDES A REFLECTION-TYPE DIFFRACTION NETWORK 34, A LENS 32 AND A LINEAR NETWORK OF SINGLE-MODE OPTICAL FIBERS 31. SINGLE-MODES 31 AND LENS 32. THIS CONFIGURATION ALLOWS BETTER USE OF THE AVAILABLE BANDWIDTH THAN ELSEWHERE POSSIBLE WITH SINGLE-MODE FIBERS, BECAUSE OF THEIR LOW RELATIONSHIP BETWEEN CORE DIAMETER AND SHEATH. APPLICATION TO OPTICAL TELECOMMUNICATIONS.

Description

the present invention relates to multiplexers

and optical demultiplexers.

  In conjunction with the extension of the

  wavelengths with low optical fibers, we studied

  techniques for using this increased bandwidth by simultaneously transmitting

  different wavelengths on each fiber

  of this type uses devices with ana-

  such as diffraction gratings (see for example:

  "Optical Devices for Wavelength Multiplexing and Demultiple-

  xing "by WJ Tomlinson, see also" High-capacity Wavelength Demultiplexer with a Diameter GRIN Rod Lens "by BD Metcalf et al, published in the March 1, 1982 issue of Applied Optics, Vol 21, No. 5, pages 794- 796, and "20-Channel Micro-Optic Grating Demultiplexer for 1 1-16 pm Band Using a Small Focusing Parameter Graded-Rod Lens Index" by M Seki et al, published in the March 18, 1982 issue.

  from Electronics Letters, Vol 18, No. 6, pages 257-258).

  Such devices typically include

  A network of fibers, a lens and a diffraction grating. When used as a demultiplexer, a set of signals at different wavelengths enter the device through an input fiber, are collimated by the lens and are directed to the diffraction grating

  o they are dispersed according to the wavelength Cha-

  each of the diffracted beams is then focused on a

  different fiber among the remaining fibers in this way.

  re, the signals are separated in space for later independent processing.

  the device in a reverse manner, it is possible to multiplex

  the fibers present in each of the fibers, for the transmission

simultaneous on a common fiber.

  Such devices are well suited to demulti

  plexing multimode and singlemode signals However, as Tomlinson says, they do not perform very well

  when used to multiplex single signals

  The problem is that in a single-mode fiber,

  the diameter of the core is small compared to the outside diameter

  It is therefore not possible to group the channels tightly, which leads to a misuse of

the bandwidth available.

  The invention provides a multiplexer or multiplier

  optical plexer comprising: a linear array of monomode input / output optical fibers, a diffraction grating

  to selectively transmit wave energy op-

  between one of the fibers and the other fibers, a

  designed to focus the transmitted energy, and a network

  convergent waveguide, in the form of an optically

  integrated tick, interposed between the fibers and the lens.

  The invention will be better understood on reading the

  description that will follow of embodiments and by

  Referring to the accompanying drawings in which: Figure 1 shows a known multiplexer / demultiplexer with diffraction grating of the reflection type; Figure 2, incorporated for an explanatory purpose,

  shows the response characteristic of the multiplexer / demulti

  plexeur of Figure 1; Figure 3 shows a multiplexer / demultiplexer according to the invention; and Figures 4 and 5 show parts of modes

  modified embodiments of the invention.

  Referring drawings, we see in Figure 1,

  a known multiplexer / demultiplexer 10, operating by way of

  diffraction grating-type wavelength partition of the reflection type For the purpose of illustration and explanation, the device is represented in the form of a demultiplexer comprising a common input multimode optical fiber, 9, and a linear array of multimode optical output fibers 11-1, 11-2 11-6 Signals at different wavelengths 11 '2' '* -6' provided by the fiber 9 are separated in space at the means of a diffraction grating 13 which is a grating grating of the reflection type A lens

  12, interposed between the fiber network and the network of dif-

  fraction, serves to focus the different

  optical cords. In operation, wave energy at the wavelengths A 1, 2 As emitted by the fiber 9 is focused on the diffraction grating 13 from which

  it is reflected selectively Figure 2 shows the dis-

  resultant intensity tribution, as a function of distance

  D along the fiber network By carrying out the measurement at

  firing from a certain arbitrary reference point, 0, we find that the first maximum of intensity at the wavelength 71

  appears at a distance Di along the axis D In a similar way

  wavelength maxima 72 '43 e 6

  appear at distances d 2, d 3 d 6 We can therefore

  to parry in space the various components of the incident signal, each of them corresponding to a separate signal channel, by placing a fiber at the focal point corresponding to each

  diffracted signals, as shown in Figure 1.

  The diffraction bucket 13 is advantageously designed so that the distance D between intensity maxima is equal to the outer diameter of the fibers. This leads to the best use of the available optical bandwidth. The bandwidth of a channel is a function of the diameter of the heart,

  c For multimode fibers, for which the ratio between

  the diameter of the core and the diameter of the sheath is

  viron 0.5, we thus achieve a good use of the available bandwidth. On the contrary, the ratio heart /

  sheath for singlemode fibers is much lower than

  meters of heart and sheath characteristics are respecti-

  The effectiveness of use is reduced from 50% to about 6%. What is needed is a means of increasing the density of implantation of the channels. arrives by interposing a

  network of waveguides converge between the fibers and the

  Reflecting diffraction bucket, as shown in Figure 3.

  More precise; ent, the multiplexer / demultiplexer comprises a network 31 of input / output fiber sections 31-1, 31-2 31-n; a network of waveguides converge in integrated optics, 30; a lens 32; and a sawtooth diffraction grating 34 Each fiber section is advantageously terminated by a suitable connector (not shown), to make the connection to the fibers of the system. In this embodiment, the lens 32 is a graduated lens. 1/4 pitch index, which can be more conveniently coupled to the waveguide array than a discrete lens An optical wedge 33 is incorporated to provide more effective coupling between

  the lens 32 and the diffraction grating 34.

  As stated above, a tight implantation

  signal paths is impossible when using

  Typical monomode fibers, because of the low core / sheath ratio The use of special monomode fibers, with very thin sheaths, and a core / sheath ratio increased by

  corresponding way, would present difficulties in

  The use of an integrated waveguide network avoids these two difficulties. As shown, each of the fibers 31-1, 31-2, 31-n is connected to one end of one of the waveguides 30-1, 30-2 30-n The waveguide network converges so

  that at the level of the lens the distance between the guides -

  waveform is much smaller than the fiber cladding diameter

  classical monomode Crosstalk will ultimately limit

  siting of the waveguides The crosstalk is

  low for spacings on the order of twice the size corresponding to the mode, and can be further reduced, if necessary, by forming grooves in the waveguide substrate between adjacent waveguides, such as FIG. 4 shows in this figure the end of the waveguide network which is adjacent to the lens. As an example, five waveguides 41, 42, 43, 44 and 45 are shown. In a suitable substrate 46 to more effectively isolate the different channels, grooves 51, 52 and 53 are formed in the substrate 46 in the region between adjacent waveguides.

  to maintain greater isolation by giving

  the waveguide propagation constants adja-

  cents. The multiplexer described above can be integrated

  on a common substrate It has been demonstrated that

  to use focusing and diffraction techniques

  for thin film optical waveguides using glass substrates The use of an electro-optically active substrate, such as a Li Nb O 3 substrate, would also allow for the integration of other circuit functions on the same substrate As an example, FIG. 5 shows a further modification of the waveguide array in which modulators 61-1, 61-2, 61-3 and 61-4 have been placed along the waveguides

  60-1, 60-2, 60-3 and 60-4 respectively. In this embodiment

  Waved wave signals at wavelengths -1 'X 2' \ 3 and 4 are coupled to the waveguide array. The output signal in waveguide 65 consists of

  modulated signals and multiplexed in wavelength.

  It goes without saying that many other modifications can be made to the device described and shown,

  without departing from the scope of the invention.

REMEïi DIM CAIOPS

  1 Optical multiplexer or demultiplexer

  a linear array (31) of input / output optical fibers, a diffraction grating (34) for selectively transmitting optical wave energy between

  fibers and other fibers, and a lens (32) for

  to focus the transmitted energy, characterized in that

  the fibers (31) are monomode fibers and that a

  convergent waveguide bucket (30), in integrated optics,

  is interposed between the fibers and the lens.

  2 Multiplexer or demultiplexer according to the

  1, characterized in that the waveguide grating

  (30) comprises a set of waveguiding strips (41-

  ) embedded in a substrate (46) having a refractive index

  lower fraction; and that the distance between

  Adjacent waveguide decreases from a maximum

  at one end of the waveguiding network, adjacent to

  (31), up to a minimum of one second ex-

  tremity of the waveguide network, adjacent to the lens

(32).

  3 Multiplexer or demultiplexer according to the claim

  2, characterized by grooves (50 53) which extend

  tooth inside the substrate (46), in the regions of the

  second end, between adjacent waveguiding strips

centes (41 45).

  4 Multiplexer or demultiplexer according to any

  of claims 2 or 3, characterized in that the

  substrate (46) consists of an electro-optical material.

  Multiplexer or demultiplexer according to the

  4, characterized by means (62) for modulating

  dulate an optical signal, incorporated along

  waveguide of the waveguide network.

  6 Multiplexer or demultiplexer according to any one

  one of claims 1 to 5, characterized in that

  constants of propagation of adjacent waveguides in the

  network of waveguides are unequal.