FR2670023A1 - Device for combining or splitting electromagnetic waves of mutually perpendicular polarisations, amplifying device and coherent detection device - Google Patents

Abstract

A filter (34) possessing a reflection coefficient substantially equal to 0 for a first direction of linear polarisation, and substantially equal to 1 for a second direction of linear polarisation, perpendicular to the first, is arranged at the ends of three waveguides (28, 30, 32), the first (28) and second (30) being for maintaining polarisation, for mutually perpendicular polarisations, and the third (32) allowing propagation of an electromagnetic wave independently of its polarisation. The invention also relates to an optical polariser drawn into a fibre, an amplifying device and a coherent detection device, each comprising a wave-combining or wave-splitting device of the aforementioned type. Application to optical telecommunications.

Description

DEVICE FOR COMBINING OR SEPARATING WAVES
ELECTROMAGNETICS OF PERPENDICULAR POLARIZATIONS
BETWEEN THEM, AMPLIFIER AND DEVICE
OF COHERENT DETECTION COMPRISING SUCH A DEVICE
COMBINATION OR SEPARATION SYSTEM.

Description
The present invention relates to a device for combining or separating electromagnetic waves of perpendicular polarizations between them, an amplifier device and a coherent detection device comprising such a combination or separation device. It applies to the field of guided optics, in particular for telecommunications.

 To combine or separate electromagnetic waves with perpendicular polarizations between them, a first device is known, shown diagrammatically in FIG. 1.

 This device comprises a first optical fiber 10 which allows the propagation without loss of an electromagnetic wave for a determined direction of polarization; in the example shown, the polarization is parallel to the plane of the sheet supporting the figure (p polarization).

 A second optical fiber 12 allows propagation without loss of an electromagnetic wave for a polarization direction perpendicular to the preferred direction of the first fiber 10; in the example shown, the polarization is perpendicular to the plane of the sheet supporting the figure (polarization s).

 The waves propagating in each fiber 10, 12 are divergent at the output. The lenses 14, 16 make it possible to make the light beams coming from the fibers 10, 12 parallel. The ends of the optical fibers 10, 12 are placed 900 from one another.

 A Glan prism 18 makes it possible to combine the beams made parallel by the lenses 14, 16 by reflecting the polarization beam p and by transmitting the polarization beam s.

A lens 20 makes it possible to focus the combined beam for its introduction into an optical fiber 22.

 The preceding description is given for the combination of two beams polarized perpendicularly to each other, but it is understood that it can be used in the opposite direction, for the separation of beams polarized perpendicularly to each other propagating in the optical fiber 22.

 This device is massive; the guidance of electromagnetic waves is broken at the output of the optical fibers, and lenses are necessary to make the diverging beams parallel. The device comprises a large number of elements which must be aligned with care and precision, during a difficult and often long operation.

 The faces of all the elements (ends of the optical fibers, lenses, Glan prisms) must undergo an anti-reflective treatment to avoid losses by reflection. This treatment considerably increases the cost of the device.

 These drawbacks make the use of this device inconvenient.

 Another device is known for coupling or separating arrays of light polarized perpendicularly to one another.

 This device consists of an assembly of two optical fibers connected to each other at X. The optical fibers forming the branches of the X are polarization conserved. The fibers undergo lateral polishing, the polishing plane having a well-defined orientation with respect to the axes of propagation of the fiber. The component is obtained by assembling the two resulting parts.

 The two output fibers allow the propagation of electromagnetic waves whatever the polarization.

 This polishing which must be precise, makes the production of the tong device and expensive.

 The object of the present invention is to provide a device for treating electromagnetic waves with polarizations perpendicular to each other, for coupling or separating them, and which is simple to implement and inexpensive.

More specifically, the present invention relates to a device for combining or separating electromagnetic waves of perpendicular polarizations between them, comprising
a first guide for the spread of a first
electromagnetic wave of rectilinear polarization
determined,
a second guide for the spread of a second
electromagnetic wave of rectilinear polarization
perpendicular to the polarization of the first
electromagnetic wave,
a third guide for the propagation of a wave
electromagnetic regardless of direction
of its polarization,
a filter with a reflection coefficient
substantially equal to 0 for a first direction
of determined and substantially rectilinear polarization
equal to 1 for a second direction of polarization
straight perpendicular to the first direction
polarization for a given wavelength and
a given orientation with respect to a wave
incident, this filter being arranged at one end
from each of the guides.

 The guides can be optical fibers or guides integrated in a substrate. The device of the invention does not require delicate implementation, the adjustments between the different guides being simpler than in the devices known hitherto.

 When transmitting an optical signal over a long path through an optical guide, it is sometimes necessary to amplify this signal to compensate for the losses occurring during propagation. Amplifying fibers are then used placed on the optical path. These amplifying fibers, like any amplifier, require a supply of energy which they transmit to the signal. The energy is supplied by an optical pumping of a material doping the fiber, thanks to the introduction into the amplifying fiber of a light beam of high energy and of wavelength less than that of the optical signal.

 The use of a powerful light source, such as a diode, poses problems of reliability and stability. This is why we combine several light beams from several stable and reliable sources, but of lower power. The device for combining light beams with perpendicular polarizations between them makes it possible to simply supply energy for the optical pumping of an amplifying waveguide.

The present invention therefore relates to an amplifier device comprising
a device for combining waves
electromagnetic with perpendicular polarizations
between them as described above,
a means for multiplexing a wave from the third
guide and an optical signal to be amplified,
an amplifier optical guide connected at the output
means for multiplexing,
a first source capable of delivering a wave
electromagnetic at one end of the first guide,
this wave being polarized rectilinearly so
to be able to spread in the first guide and
having a wavelength suitable for optical pumping
amplifier guide,
a second source capable of delivering a wave
electromagnetic at one end of the second guide,
this wave being linearly polarized so
to be able to spread in the second guide, and
having a wavelength suitable for optical pumping
of the amplifier guide.

 The invention also relates to a coherent detection device.

 We know that to improve the signal / noise ratio of a detection, we can correlate the signal, information carrier, with a reference signal from a local oscillator. For this, it is necessary that the reference signal has the same wavelength, the same phase and the same polarization as the information support signal.

However, during its propagation, the polarization of the latter fluctuates and deforms. For example, a rectilinear polarization becomes elliptical, and the beats obtained during correlation are of low amplitude. As will be seen, a device for separating waves of polarizations perpendicular to each other as presented above allows the coherent detection of a signal whose polarization has been deformed during its transmission.

 In addition, redundant information can be obtained which makes it possible to ensure the reliability of the detection.

The invention therefore relates to a coherent detection device comprising:
a device for the separation of waves
electromagnetic with perpendicular polarizations
between them as described above,
at least one local oscillator capable of delivering a
wavelength, phase electromagnetic wave
and polarization identical to the wavelength,
the phase and polarization of one of the waves which can
spread in one of the first and second guides,
for each Local oscillator, a connected coupler
by a first entry to that of the first and second
guides considered and by a second entry to a guide
for the transmission of the wave delivered by said
Local oscillator, this transmission does not introduce
no significant deformation of the polarization,
for each coupler, a connected output guide
at the output of the coupler considered,
for each outlet guide, a detector capable of
the detection of the beats between the waves
propagating in said outlet guide.

The features and benefits of
The invention will appear better after the description which follows given by way of explanation and in no way limiting. This description refers to the attached drawings, in which:
- Figure 1, already described and relating to the prior art, schematically represents a device for combining or separating electromagnetic waves of polarizations perpendicular to each other;
- Figure 2 schematically shows a device according to the invention for combining or separating electromagnetic waves of polarizations perpendicular to each other;
- Figure 3 shows schematically the variations of the reflection coefficient as a function of the wavelength of a filter used in the invention;
- Figure 4 schematically shows an alternative embodiment of a device according to the invention;
- Figure 5 schematically shows an amplifier device according to the invention;
- Figure 6 schematically shows a coherent detection device according to the invention.

 FIG. 2 schematically represents a top view of a device, in accordance with the invention, for combining or separating electromagnetic waves with polarizations perpendicular to each other.

 In this embodiment, a support 24, for example made of elastomer, has a T-shaped groove 26. Three optical fibers 28, 30, 32 (only their ends are shown) are arranged in this groove 26. These optical fibers 28 , 30, 32 are devoid of protective sheath.

 The first optical fiber 28 is of the "polarization preservation" type for a polarization directed parallel to the plane of the sheet supporting the figure (p polarization). The second optical fiber 30 is also of the "polarization conservation type" 1, for a polarization directed perpendicular to the plane of the sheet supporting the figure (polarization s).

 The third optical fiber 32 can be a standard single-mode fiber: it allows the propagation of a light wave regardless of the direction of its polarization.

 In the embodiment shown, a filter 34 is deposited on the end of the third optical fiber 32 which is bevelled at 450; The angle of incidence made by a light beam x leaving the fiber 32 with the filter 34 is therefore equal to 450.

 The filter 34 disposed at the end of the third fiber 32 is placed at the interconnection of the branches of the groove 26 so that it is also close to the ends of the first and second fibers 28, 30. In addition, the end of the first fiber 28 is bevelled at 450 so as to adapt to the end of the third fiber 32.

 The filter 34 is produced by depositing superimposed thin layers according to any known technique. It has a reflection coefficient R which depends on the polarization for a given wavelength and orientation with respect to an incident wave. As we have seen, the incidence of the beams entering or leaving any of the fibers 28, 30, 32 relative to the filter 34 is 450 in this example.

 The optical fibers 28, 30, 32 being contiguous or almost contiguous, no adaptation optics are necessary.

 FIG. 3 schematically represents the variations in the reflection coefficient R of the filter 34 as a function of the wavelength Xo. We see that for a wavelength Xo equal for example to 1.48 micron (and for an incidence equal to 450) the variations of the reflection coefficient of the filter 34 are different for a polarization s and a polarization p. If these variations have a form of walking in the two cases, R suddenly passes from approximately 0 to approximately 1 for a value close to but less than Xo in the case of an polarization s whereas R passes suddenly from approximately 0 to approximately 1 for a value close to but greater than X0 in the case of a polarization p. Ultimately, for a wavelength equal to X0, R is substantially equal to 1 for a light beam having a polarization s while it is substantially equal at 0 for a light beam having a polarization p.

 Again with reference to FIG. 1, it is understood that an electromagnetic wave of polarization p propagating in the first optical fiber 28 is transmitted entirely by the filter 34 and enters the optical fiber 32 directly without the guidance being interrupted.

 Similarly, an electromagnetic wave of polarization s propagating in the second optical fiber 30 is fully reflected by the filter 34 (after passing through a section of the third optical fiber 32) without the guidance being interrupted.

 The description which is given above relates to the combination of two waves of polarizations s and p, but it is easily understood that the device can be used in the opposite direction for the separation into two waves of a wave propagating in the fiber 32 and having a polarization component s and a polarization component p.

The fibers 28, 30, 32 once positioned in the groove 26 are held by gluing. For this, they are embedded in a fluid adhesive sensitive to UV radiation and plated in position under the effect of a glass plate 36 placed on the support 24. The assembly is then subjected to radiation
UV curing glue.

 In an alternative embodiment shown diagrammatically in FIG. 4, the device is integrated on a substrate 38, for example made of glass.

Waveguides 40, 42, 44 are integrated into the substrate 38, by diffusion of titanium for example.

As before, the first guide 40 allows the only propagation of a polarization wave p while the second guide 42 allows the only propagation of a polarization wave s. The guide 44 is suitable for the propagation of a wave whatever its polarization.

 The waveguides 40, 42, 44 are also arranged in a T shape, the guide 44 is in the extension of the guide 40 and the guide 44 is perpendicular to the other two, its end opening out at the ends of the guides 40 and 44 .

 A box-shaped space separates the ends of the guides 40 and 44. This box contains a filter 46 with characteristics identical to those described with reference to FIG. 3. The filter 46 is produced for example by depositing an even number of alternating layers on a glass substrate; the deposit formula is of the type: glass H B H B ... B glass where H represents a layer of high index material and B a layer of low index material.

For an application at 1.45 gm at an incidence of 450, an alternating stack of zinc sulfide and cryolite of 14 layers will be produced, the mechanical thicknesses of which are as follows: o
1603 A for zinc sulfide
o
2788 A for cryolite
An embodiment of this type of filter is described in the journal Journal of OPTICS of 1986 (volume 17, n01, pages 43 to 52).

 We see in Figure 4 that the box is tilted so that the filter makes an angle of 45C with each waveguide.

 FIG. 5 schematically represents an amplifier device according to the invention and using a device for combining two electromagnetic waves with polarizations perpendicular to each other of the type of that shown in FIG. 2; the references of FIG. 2 are therefore repeated.

 A first light source 48, for example a laser diode, of the type sold by the company OKI under the reference OL352A-20, delivers an electromagnetic wave of polarization p at the input of the first optical fiber 28.

 A second light source 50, for example a laser diode, of the same type as above, delivers an electromagnetic wave of polarization s at the input of the second optical fiber 30.

 The wavelength of these light beams can be for example equal to 1.48 microns and the average power of each beam is approximately equal to ten mW.

 Thanks to the transmission and the respective reflection of the one and the other beams by the filter 34, a combination of the two beams propagates in the third optical fiber 32 which is coupled at output to a first input of an optical multiplexer 52 for example of the type sold by ATI under the reference MUXOPTIC.

 The line fiber 54 which brings the signal to be amplified is connected to a second input of the optical multiplexer 52. The signal has a wavelength equal for example to 1.55 microns.

 An amplifying fiber 56 is connected at the output of the optical multiplexer 52. The light beams delivered by the fiber 32 are used for the optical pumping of the amplifying fiber 56 so that the signal is amplified during its propagation in the amplifying fiber.

 FIG. 6 schematically represents a coherent detection device in accordance with the invention using a device for separating components of polarizations perpendicular to each other of the type of that shown in FIG. 2; the references of FIG. 2 are therefore repeated.

 During a propagation in a standard optical fiber, the polarization of a signal fluctuates around the preferred direction constituted by the initial polarization. This deformation transforms for example a rectilinear polarization into an elliptical polarization.

 In the device shown in FIG. 6, the fiber 32 constitutes the line fiber in which the signal propagates. When the filter 34 is crossed, the polarization component p is transmitted in the optical fiber 28 while the polarization component s is reflected in the fiber 30. These components are delivered to the coupler inputs, respectively referenced 58p and 58s, of the type of those sold by the company PHOTONETICS under the reference PHCP.

 Each coupler has a second input to which an optical fiber 60s, 60p is connected, short enough so that the polarization of a wave does not change during propagation.

 Two local oscillators 62s, 62p deliver reference light signals on the inputs of the fibers 60s and 60p. These reference signals have a determined polarization, p for the signal coming from the oscillator 62p, s for that coming from the oscillator 62s, a wavelength and a phase identical to those of the signal to be detected.

 We thus obtain in the output fibers 64s, 64p connected to the respective outputs of the couplers 58s, 58p, beats between the component of the polarization signal s or p with the reference signal of the same polarization. These beats are detected by detectors 66s and 66p disposed respectively at the outlet of the fibers 64s, 64p.

 The electrical signals delivered by the detectors are then processed by processing means not shown. The detection carried out by the detectors 66p and 66s is redundant and makes it possible to ensure the reliability of the detection.

 The invention is in no way limited to the embodiments more specifically described and shown; on the contrary, it admits all variants. In particular, for the device for combining or separating electromagnetic waves with perpendicular polarizations between them, the angles between the different input and output guides can be different from 0 or 90 ". Similarly, the orientation of the filter can be different from that shown in example and the angle of incidence of electromagnetic waves can be different from 450.

 Another application of the invention is the production of a fiber optic polarizer. In fact, in FIG. 2, the fiber 32, filter 34 and fiber 28 assembly constitutes an optical polarizer. Whatever the direction of polarization of the incident wave which is present at the input of the fiber 32, only one direction of polarization is transmitted on the fiber 28; a linear polarized wave is thus obtained at the output.

Claims (8)

Claims  1. Device for combining or separating electromagnetic waves of perpendicular polarizations between them, characterized in that it comprises - a first guide (28, 40) for the propagation of a  first electromagnetic polarization wave  determined rectilinear, - a second guide (30, 42) for the propagation of a  second electromagnetic polarization wave  straight perpendicular to the polarization of  The first electromagnetic wave, - a third guide (32, 44) for propagation  of an electromagnetic wave regardless of the  direction of its polarization, - a filter (34, 46) having a coefficient of  reflection substantially equal to 0 for a first  determined direction of polarization  and substantially equal to 1 for a second direction  of rectilinear polarization perpendicular to the  first direction of polarization for a length  given wave and given orientation relative  to an incident wave, this filter (34, 46) being arranged  at one end of each of the guides.  2. Device according to claim 1, characterized in that the first, second and third guides (28, 30, 32) are optical fibers  3. Device according to claim 2, characterized in that the filter (34) consists of optical layers deposited on one end of one of the optical fibers (28, 30, 32).  4. Device according to claim 2, characterized in that it constitutes a fiber optic polarizer.  5. Device according to claim 3, characterized in that the ends of the fibers are held by gluing in a groove (26) made for this purpose in a support (24).  6. Device according to claim 1, characterized in that it is integrated.  7. Amplifier device, characterized in that it comprises  a device according to one of the claims  1 to 6,  means (52) for multiplexing a wave from the  third guide (32) and an optical signal to be amplified,  an amplifier optical guide (56) connected at output  means (52) for multiplexing,  a first source (48) capable of delivering a wave  electromagnetic at one end of the first guide  (28), this wave being polarized rectilinearly  so as to be able to spread in the first guide  (28) and having a wavelength suitable for pumping  amplifier guide optics (56),  a second source (50) capable of delivering a wave  electromagnetic at one end of the second guide  (30), this wave being polarized rectilinearly  so as to be able to spread in the second guide  (30), and having a wavelength suitable for pumping  amplifier guide optics (56).  8. Consistent detection device, characterized in that it comprises:  a device for the separation of waves  electromagnetic with perpendicular polarizations  between them conforms to any of the  claims 1 to 6,  at least one local oscillator (62s, 62p) capable of  deliver a wavelength electromagnetic wave,  of phase and polarization identical to the length  wave, the phase and polarization of one of the waves that can propagate in one of the first and second guides (28, 30), for each local oscillator (62s, 62p), a coupler (58s, 58p) connected by a first input to that of the first and second guides (28, 30) considered and by a second input to a guide (60s, 60p) for the transmission of the wave delivered by said local oscillator (62s, 62p), this transmission not introducing any significant deformation of the polarization, for each coupler (58s, 58p), an output guide (64s, 64p) connected at the output of the coupler (58s, 58p) considered, for each output guide (64s, 64p), a detector (66s, 66p) capable of detecting the beats between the waves propagating in said output guide (64s, 64p) considered.