FR2755246A1 - Optical switch for Telecommunications - Google Patents

Abstract

The Optical switch has a fibre optic cable (10) forming a right angle bend, and secured at both ends (20,21). There is an electromagnet (30) which moves a section (32) with an electrical command and changes the radius of curvature of the cable. The curvature change allows the optical light to pass, whereas with a right angle bend the optical rays are reflected.

Description

OPTICAL SWITCH
The present invention relates to an optical switch intended for use in various applications related to the fields of optical telecommunications and optoelectronics. Such a device makes it possible in particular to establish or interrupt the propagation of a light wave.

 Many types of optical components have already been developed, notably in the field of optical telecommunications. These components are divided into two families. The first family concerns semiconductor amplifier optical gates, couplers, semiconductor-based switches such as GaAs or based on lithium niobate and acousto-optical switches which are now widely known. All of these devices have very short switching times.

However, they are expensive and do not simultaneously present all of the required characteristics.

 The second family, which constitutes the state of the art closest to the present invention, relates to micro-switches of the electromechanical type with slow application. -The principle of these microswitches consists in establishing or interrupting the coupling between several fibers by different means. A first operating method is based on the alignment and misalignment of two optical fibers using a stepping motor capable of gripping one fiber and moving it relative to the other by performing micro-mechanical movements. The other existing microswitches are based on other operating methods that are just as complex to implement. They use either an acousto-optic deflector, a rotating mirror or a prism.

 The operation of all these existing microswitches requires very high precision, in particular for producing micro-alignments. This requirement for precision makes their manufacturing complex and their reliability is reduced. Their manufacturing cost is also very high.

 The present invention overcomes all these drawbacks since it provides an optical switch comprising on the one hand at least one curved optical fiber, the ends of which are kept fixed, and on the other hand an electromechanical control means intended to actuate a portion median of this optical fiber so as to vary its radius of curvature.

 The invention further relates to the use of such a switch for the production of a coupler.

 The invention also relates to the use of such a switch for the production of a switch.

 The invention also relates to the use of several switches for producing a router. A router is defined as a device ensuring both the function of a switch and that of a coupler.

 The invention also relates to the use of a plurality of switches for producing a distributor.

 The switch according to the invention is manufactured according to a simple technique and therefore has a low manufacturing cost. It is also very reliable and a low voltage is enough to activate it.

Other features and advantages of the invention will appear on reading the description given by way of illustrative and non-limiting example with regard to the appended figures which represent:
FIG. 1, a block diagram of a switch according to the invention,
FIG. 2, a block diagram of a router produced from switches according to the invention,
FIG. 3, a block diagram of a coupler produced from a switch according to the invention,
FIG. 4, a block diagram of a switch produced from a switch according to the invention,
FIG. 5, a block diagram of a distributor made using a plurality of switches according to the invention,
- Figure 6, a block diagram of another distributor made using a plurality of switches according to the invention.

 Figure 1 illustrates a block diagram of a switch according to the invention. This switch is in the form of a box 50 containing an optical fiber designated by the reference 10, the ends of which are kept fixed by means of clamping elements 20, 21 for example, attached to the box 50. Preferably, the optical fiber 10 is curved.

 Connectors, for example, can then be assembled to the fixing means 20, 21 so as to connect a first end of the fiber 10 to an optical device or to another optical fiber intended to guide an optical input signal and the other end of this fiber 10 to another optical device or to another fiber intended to guide an optical output signal.

 According to an alternative embodiment, it is possible to envisage that the optical switch according to the invention comprises more than one optical fiber.

On the other hand, an electromechanical control means 30 makes it possible to move a substantially middle portion of the optical fiber 10 so as to vary its radius of curvature. The electromechanical control means 30 shown in FIG. 1 is preferably constituted by an electromagnet. It is placed perpendicular to the axis of the optical fiber 10. The electromagnet 30 comprises a winding 31, capable of producing a magnetic field when it is traversed by an electric current, and a part 32 capable of moving under the effect of the magnetic field created. The part 32 can thus take several positions. In Figure 1, two positions referenced
A and B are shown. The middle portion of optical fiber 10 is integral with this part 32. It therefore moves during successive changes in the position of the part 32.

 The operating principle of the optical switch shown in FIG. 1 is based on the fact that below a certain critical radius of curvature Rc, the wave propagating in an optical fiber is no longer guided.

 Thus, for a radius of curvature Ron greater than the critical radius of curvature Rc, an optical fiber is capable of guiding an optical signal propagating in its core. On the other hand, for a radius of curvature Roff smaller than the critical radius of curvature Rc, the angle of reflection of an optical signal propagating in the core of the fiber is not constant from one reflection to another and becomes too weak for the reflections to be total. In this case, the light wave then enters the sheath of the fiber. This loss of signal only becomes significant for a value of radius of curvature less than the critical value Rc. In addition, the attenuation of the signal increases exponentially with the decrease in the radius of curvature.

 The critical value of the radius Rc strongly depends on the core diameter and the numerical aperture of the fiber. However, for a multimode optical fiber having a core diameter of between 80 and 120 μm and a digital aperture of the order of 0.2, it is generally less than 15 mm. On the other hand, for a single-mode fiber having a core diameter of between 10 and 25 ssm and a numerical opening of the order of 0.1, this critical value of the radius Rc is generally less than 12 mm.

 In the example shown in Figure 1, when the part 32 of the electromagnet 30 is in a first position, designated by the reference A, the optical fiber 10 has a radius of curvature Roff less than the critical radius of curvature Rc. The optical fiber 10 can then no longer guide the propagation of a light wave and the attenuation of the signal is such that it is considered that the propagation is interrupted.

However, when the part 32 moves back into position
B, shown in dotted lines in FIG. 1, the middle portion of the optical fiber 10, integral with this part 32, also moves. This displacement causes a variation in the radius of curvature of the optical fiber which becomes greater than the critical radius of curvature Rc and the optical fiber consequently allows the propagation of a light wave.

The voltage curve shown in the diagram in FIG. 1 shows that at each position of the part 32, that is to say at each value of radius of curvature Ron,
Roff of the fiber, corresponds to an operating voltage VA, VB. In an exemplary embodiment, it is possible to ensure that the position B of the part 32, corresponding to the radius of curvature Ron allowing a light wave to propagate in the core of the fiber 10, is obtained when no voltage is applied to the electromagnet 30.

The other position A, corresponding to the radius of curvature
Roff allowing the interruption of light propagation, is then obtained by applying a voltage
VA on the electromagnet. Advantageously, a low operating voltage VA is sufficient to actuate the part 32. This voltage VA is indeed of the order of a few volts.

 Figure 1 shows only a block diagram of the device according to the invention mounted as a switch.

Its operation depends not only on the type of electromagnet used, but also on the mechanical part intended to move the middle portion of the fiber 10. One can indeed envisage the displacement of this middle portion of fiber by means of a system more sophisticated mechanics, such as a lever arm.

 Even more advantageously, it is possible to use a bistable electromagnet. In this case, a single operating voltage makes it possible to activate the part 32. This voltage is applied to the electromagnet 30 only when the position of the part 32 changes, in the form of a short pulse. duration. The position changes are in fact activated either by the rising edge or by the falling edge of the pulse, depending on the electromagnet used. This type of bistable electromagnet is very interesting since the voltage consumption is very low since it takes place only at changes in the positions of the part 32, that is to say at changes in the value of the radius of curvature of the optical fiber 10.

 Thanks to this optical switch, the attenuation of a wave propagating in the optical fiber 10 can reach several tens of decibels (dB) depending on the value of the radius of curvature Roff.

The value of the radius of curvature R, ff, making it possible to interrupt the propagation of an optical signal, is however subject to a compromise. Indeed, to preserve the life of the optical fiber 10 as long as possible, it is necessary that the stresses, due to the additional mechanical stresses applying to the surface of the fiber when the latter is bent, be the most weak possible. In practice, therefore, the value of the radius of curvature Roff must be much greater than the value
Limit of elastic limit and breaking of the fiber.

 This Rlim value strongly depends on the nature of the sheath surrounding the core of the fiber, but it is generally less than 3 mm. Furthermore, the value of the critical radius of curvature Rc of a multimode optical fiber is generally between 10 and 15 mm.

Consequently, the value of the radius of curvature Roff of the fiber, when the latter is of the multimode type, must be between 3 and 10 mm, preferably between 7 and 10 mm, to cause an interruption in the propagation of a light signal.

 On the other hand, the value of the critical radius of curvature Rc of a single-mode optical fiber is generally between 7 and 12 mm. Consequently, the value of the radius of curvature Roff of the fiber, when the latter is of the single-mode type, must be between 3 and 7 mm, preferably between 4.5 and 7 mm to cause an interruption in the propagation of a light wave.

 In addition, when the optical fiber 10 guides the propagation of a light wave, the value of its radius of curvature R on must be greater than the critical value Rc. It is therefore preferably greater than or equal to 15 mm when it is of the multimode type, and greater than or equal to 12 mm when it is of the single-mode type.

 The use of an electromagnet to move the optical fiber 10 is only one embodiment, other types of electromechanical control means can be used. Thus in an alternative embodiment, one could consider using a piezoelectric element.

 Advantageously, the electromechanical control means is controlled by a microcomputer using suitable software.

 According to another alternative embodiment, it is possible to envisage the production of a switch comprising several optical fibers, offset with respect to each other, and as many electromechanical control means to impose on them each a particular radius of curvature.

 The switch according to the invention is manufactured according to a simple and inexpensive technique. Its operation does not require great precision and its reliability is high. Furthermore, a low voltage is enough to activate it. Finally, it has no sensitivity to polarization or intrinsic optical losses.

 The switch according to the invention can also be applied to the manufacture of other optical components such as routers, switches, couplers or distributors for example.

 FIG. 2 represents an embodiment of a router comprising switches according to the invention.

This router has an input E1 and two outputs S1 and Two switches, shown diagrammatically by I1 and 12 in FIG. 2, are necessary for its realization. These two switches I1 and I2 are placed on each branch of a fiber Y junction 40. When a light wave propagates from the input E1 to the output S1, for example, the radius of curvature of the fiber of the switch I1 is greater than the critical radius Rc.

On the other hand, when the switch I1 is open, that is to say when no optical signal can propagate from the input E1 towards the output S1, then the radius of curvature is less than the critical radius Rc.

The switch I2 works in the same way to establish or stop the propagation of a light signal between the input E1 and the other output
The device shown in FIG. 2 not only makes it possible to connect the input E1 to the output S1 or to the output S2 in the manner of a switch, but also to connect the input E1 simultaneously to the outputs S1 and S2 in the manner a coupler. Consequently, thanks to two switches I1, I2 according to the invention, it is possible to make a router, in a simple manner, capable of ensuring either the function of a switch or that of a coupler.

 Furthermore, such a device, made from switches, does not exhibit any loss of alignment like conventional switches. The only existing optical losses are due to the Y junction at fiber 40 and are of the order of 3dB.

 FIG. 3 illustrates a block diagram of another alternative embodiment of a coupler with an input E1 and two outputs S1, S2, from a switch according to the invention. In this case, the switch according to the invention comprises two optical fibers 10 and 11 fixed on each branch of a fiber Y junction 40. The electromechanical control means 30 is placed between the two fibers 10, 11 and makes it possible to move simultaneously the middle portion of each fiber 10, 11 so as to vary their radius of curvature.

Thus, when the part 32 is in a first position denoted A in FIG. 3, the fibers 10 and 11 have a radius of curvature Roff1, Roff2 less than the critical radius of curvature Rc so as to interrupt the propagation of a signal between the entrance
E1 and outputs S1 and S2. On the other hand, when the part 32 is in position B, the fibers 10 and 11 have a radius of curvature Ronl, Ron2 greater than the critical radius of curvature Rc so as to guide the propagation of a light wave between the input E1 and outputs S1 and S2. In this case, the part 32 of the electromechanical control means moves in a back and forth movement and the device of FIG. 3 operates as a coupler since it makes it possible to interrupt or establish the propagation of a wave of the input E to the two outputs S1 and S2.

 FIG. 4 illustrates a block diagram of another alternative embodiment of a switch with an input E1 and two outputs S1, S2, from a switch according to the invention. In this case too, the switch according to the invention comprises two optical fibers 10 and 11 fixed to each branch of a Y junction with fiber 40. They are arranged symmetrically on either side of the electromechanical control means 30. The electromechanical control means 30 therefore makes it possible to simultaneously move the middle portion of each fiber 10, 11 so as to vary their radius of curvature.

Thus, when the part 32 is in a first position denoted A in FIG. 4, the fiber 10 has a radius of curvature Roffl less than the critical radius of curvature Rc so as to interrupt the propagation of a signal between the input E1 and the output while the other fiber 11 has a radius of curvature Ron2 greater than the critical radius of curvature
Rc so as to guide the propagation of a light wave between the input E1 and the output
Conversely, when the part 32 is in a second position, the radius of curvature of the fiber 10 will be Ron1 greater than the critical radius of curvature while that of the fiber 11 will be Roff2 less than the critical radius of curvature, so as to guide the propagation of a wave from input E1 to output and to interrupt it between E1 and
The device of FIG. 4 therefore operates as a switch since it makes it possible to guide the propagation of a light wave either from the input E1 towards the output S1, or from the input E1 towards the output
One can also provide, in an alternative embodiment, that the part 32 can take a third position, called the rest position, for which the two optical fibers 10 and 11 would pass. In this case, the switch is transformed into a coupler. Consequently, when the part 32 can take this third position, the device of FIG. 4 is transformed into a router capable of ensuring either the function of a switch or that of a coupler.

 The devices as illustrated in FIGS. 2 to 4 can then be applied to the manufacture of an optical distributor with two inputs E1, E2, and two outputs S1, S2 in tree structure, the schematic diagram of which is illustrated in the figure 5.

 In this case, the optical losses on each of the optical paths are very close to 6 dB and are only due to the two Y junctions 41 (and 42) in series. There are however other losses which are due to the connections between the various elements of the distributor, but these are negligible compared to the losses due to the Y junctions.

 The connections between the different elements of the distributor are made either by means of connectors or by means of splices. Splices appear to be the preferred solution because their cost is low, their reliability is high and their residual losses are less than a tenth of a dB. Furthermore, the sensitivity to polarization of such a distributor, on each of the optical paths, is less than 0.5 dB. In addition, its bandwidth is greater than a hundred nanometers and centered around 1.55 Hm. Finally, the switching time of such a distributor is at least of the order of a millisecond.

An optical distributor with four inputs E1, E2, E3,
E4 and four outputs S1, S2, S3, S4, manufactured according to the same principle and the same architecture as the previous one, is shown diagrammatically in FIG. 6. Such a distributor comprises four shots their C1 to C4 at one input and four outputs, four couplers C5 to C8 with four inputs and one output and sixteen switches with one optical fiber. In addition, 40 connection means, such as connectors or splices for example, make it possible to connect the various elements of the distributor. The theoretical optical losses on each of the optical paths are 12 dB. They can advantageously be compensated for by the addition, on each of the four outputs S1 to S4, of a conventional optical amplifier (A01 to A04). The gain of each amplifier must then be around fifteen dB to obtain an optical distributor with 4 inputs and 4 outputs without losses. In this case, 4 additional connection means allow the optical amplifiers to be assembled on the outputs.

 It is also conceivable to produce a distributor with eight inputs and eight outputs comprising eight couplers with one input and eight outputs, eight couplers with eight inputs and one output and 64 switches with an optical fiber. Furthermore, to compensate for the optical losses due to the couplers, provision is made to place an optical amplifier on each of the eight outputs. In this case, the optical losses on each of the paths being of the order of 18 dB, the gain of each amplifier must be set at around twenty dB to obtain a lossless distributor.

 More generally, an optical distributor with n inputs and n outputs in a tree structure comprises 2n couplers, of which n couplers with one input and n outputs and n couplers with n inputs and one output, n2 optical switches when these comprise only '' a single fiber and (2n + 2n2) connection means allowing the different elements of the distributor to be connected together, n being a multiple of two. Furthermore, an optical amplifier can be placed on each of the n outputs of the distributor so as to compensate for the optical losses in dB which are equal, on each of the optical paths, to 10 * log (1 / n). In this case, it is also necessary to add n additional connection means.

 Of course, the examples of optical devices which have just been described are not exhaustive. The applications that optical switches produced according to the present invention may have are very numerous indeed.

Claims (14)

 1. Optical switch comprising on the one hand at least one curved optical fiber (10), the ends of which are kept fixed, and on the other hand an electromechanical control means (30) intended to actuate a middle portion of this optical fiber ( 10) so as to vary its radius of curvature.  2. Switch according to claim 1, characterized in that the electromechanical control means (30) is placed perpendicular to the axis of the optical fiber (10) and in that it is constituted by an electromagnet comprising a winding (31), capable of producing a magnetic field when an electric current flows through it, and a part (32), integral with the middle portion of the optical fiber (10), able to move under the effect of the field magnetic created.  3. Switch according to claim 2, characterized in that the electromagnet is bistable.  4. Switch according to one of claims 1 to 3, characterized in that the ends of the optical fiber (10) are held fixed by clamping means (20, 21).  5. Switch according to one of claims 1 to 4, characterized in that the optical fiber (10) is of the multimode type and in that, to guide the propagation of a light wave, the value of the radius of curvature R on of the optical fiber (10) is greater than or equal to 15 mm.  6. Switch according to one of claims 1 to 4, characterized in that the optical fiber is of the multimode type and in that, to interrupt the propagation of a light wave, the value of the radius of curvature Roff of the optical fiber (10) is between 3 and 10 mm, preferably between 7 and 10 mm.  7. Switch according to one of claims 1 to 4, characterized in that the optical fiber (10) is of the single-mode type and in that, to guide the propagation of a light wave, the value of the radius of curvature R on of the optical fiber (10) is greater than or equal to 12 mm.  8. Switch according to one of claims 1 to 4, characterized in that the optical fiber is of the single-mode type and in that, to interrupt the propagation of a light wave, the value of the radius of curvature Roff of the optical fiber (10) is between 3 and 7 mm, preferably between 4.5 and 7 mm  9. Switch according to one of claims 1 to 8 applied to the production of a coupler with an input (E1) and two outputs (S1, S2) 'characterized in that it comprises two optical fibers (10, 11) fixed on each branch of a fiber Y junction (40) and in that the electromechanical control means (30) is placed between the two fibers (10,11) and simultaneously displaces the middle portion of the two optical fibers (10, 11) so as to vary their radius of curvature and to connect the input (E1) simultaneously to the two outputs (S1, S2).  10. Switch according to one of claims 1 to 8 applied to the production of a switch with one input (E1) and two outputs (S1, S2), characterized in that it comprises two optical fibers (10, 11) fixed to each branch of a fiber Y junction (40) and arranged symmetrically on either side of the electromechanical control means (30) and in that the electromechanical control means (30) simultaneously displaces the middle portion of the fibers optics (10, 11) so as to vary their radius of curvature and to connect the input (E1) to one or the other of the two outputs (S1, S2).  11. Switch applied to the production of a switch according to claim 10, characterized in that the part (32) of the electromechanical control means (30) can also take another position for which the two fibers (10, 11) are turned on, so as to turn it into a blow.  12. Router with one input (E1) and two outputs (S1, S2) comprising several switches (I1, I2) according to one of claims 1 to 8, characterized in that the two switches are placed on each of the branches of a fiber Y junction (40) so as to  - connect the input (E1) to one or other of the two outputs (S1, S2) to ensure the function of a switch, or  - connect the input (E1) simultaneously to the two outputs (S1, S2) to perform the function of a blow.  13. Distributor with n inputs and n outputs, produced from a plurality of switches according to claims 1 to 8, characterized in that it comprises n couplers with one input and n outputs, n couplers with n inputs and one output, n2 switches and (2n + n2) connection means making it possible to connect the different elements of the distributor to each other.  14. Distributor according to claim 13, characterized in that it further comprises n optical amplifiers capable of compensating for optical losses on each output and n additional connection means.