AU605029B2

AU605029B2 - Magneto-optic device

Info

Publication number: AU605029B2
Application number: AU31645/89A
Authority: AU
Inventors: Xianda Dai; Yasuo Ito
Original assignee: Australian Telecommunications Corp
Current assignee: Telstra Corp Ltd
Priority date: 1988-03-25
Filing date: 1989-03-23
Publication date: 1991-01-03
Anticipated expiration: 2009-03-23
Also published as: AU3164589A

Description

Iwateal Uiis uaieu Gns ay OL Vt;L.it.L~ ij, 1W 7.

(a member of the firm of DAVIES COLLISON for and on behalf of the Applicant).

To: THE COMMISSIONER OF PATENTS Davies Collison, Melbourne and Canberra,

A

COMMONWEALTH OF AUSTRALIA PATENT ACT 1952 COMPLETE SPECIFICATION6 0 029

(ORIGINAL)

FOR OFFICE USE CLASS INT. CLASS Application Number: Lodged: Complete Specification Lodged: Accepted: Published: I *I a '5* ~b t 9* Sr f r' Priority: Related Art-: Tis document Contains the.

amendments made under Section 49 and is correct for printing.

CLCMUIPC)re A-h b I TELECOMMUNICATIONS 4QM-I-SS-I-QN NAME OF APPLICANT:

AUSTRALIAN

ADDRESS OF APPLICANT: 199 William Melbourne

VICTORIA

Street, 3000

AUSTRALIA

9 NAME(S) OF INVENTOR(S) YASUO ITO XIANDA DAI ADDRESS FOR SERVICE: DAVIES COLLISON, Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: MAGNETO-OPTIC

DEVICE

The following statement is a full description of this invention, including the best method of performing it known to us -f 2 t 4 cr MAGNETO-OPTIC

DEVICE

This invention relates to a magneto-optic device.

One form of magneto-optic device is known as an optical isolator. Such a device is intended to permit passage of light in one direction therethrough whilst substantially attenuating light directed in c the opposite direction through the device. Such C t devices find numerous applications. For example, laser sources used in telecommunications equipment may be required to direct light to enter the end of an optical fibre which fibre end may present substantial reflectance, tending to redirect light back towards the laser, which redirected light may interfere with or damage the laser. In such a case, an optical isolator positioned between the laser and the fibre end can be arranged whereby to permit the l l :t 1 1 1 1 1 1 1 1 i d relatively free passage of light from the laser to the fibre, as is required, whilst providing substantial attenuation of light when directed back towards the isolator from the fibre end. Similar uses occur when beams from lasers are required to be focussed or defocussed onto crystals, such as in optical imaging. These applications are characterized in that some means must be provided for focussing or defocussing the light beam travelling through the isolator in order that the beam may exhibit desired focussed or defocussed qualities at the output side. Generally, this has required that, in addition to the isolator, some mechanical focussing or defocussing mechanism be provided, such ,t 15 as an optical lens system which is capable of being mechanically moved in order to provide the desired focussing characteristics. Whilst such devices C f operate satisfactorily enough, this is at the expense of some inconvenience, particularly where large t c 20 numbers of optical devices are to be assembled t, together.

An object of the invention is to provide a f t magneto-optic device, such as an optical isolator, 2 5 which is capable of being focussed or defocussed by 25 non-mechanical means.

I In one aspect, the invention provides a magneto optic device of the kind having a first polarizer, having a preferred plane of polarization effective to substantially attenuate light passing therethrough, being light having a plane of polarization substantially differing from a preferred plane of polarization characteristic of the first 1 I I -i-r~BparrrrrC- l--rrr~ 4 polarizer, an optical rotator positioned for rotating light of said preferred plane emerging from said first polarizer through a predetermined angle, and a second polarizer effective to substantially attenuate light transmission therethrough when the plane of polarization thereof is substantially different from a preferred plane of polarization characteristic of the second polarizer, the second polarizer being arranged so that the plane for preferred light transmission therethrough corresponds to the l polarization plane of light having passed through the ;first polarizer and then through the rotator to be incident on the second polarizer, whereby such light passing through the device in a forward direction through the first polarizer, rotator and second polarizer, if reflected back through the device with the same polarization, is substantially attenuated in passage through the device by virtue of rotation of the plane of polarization thereof, once having passed back through the second polarizer, to exhibit a r polarization on incidence on the first polarizer which substantially differs from the preferred plane of polarization for transmission therethrough, characterised in that the rotator is capable of effecting focussing of light passing therethrough in the forward direction and to vary the focus point of such forwardly passing light, relative to the rotator, pursuant to variation in the power of such light.

Conveniently, the rotator includes a light transmissive element which is effective to effect both the rotation of the plane of polarization of light passing therethrough and to effect the required S- I ~s* focusing. Suitable materials may comprise materials having the general composition Mnl-x Cdx Te or Hgl-x Mn x Te or Hgl-.-y Mnx Cdy Te, (l>x>0) These materials are generally characterized in that the refractive index thereof varies pursuant to the energy of incident light thereon. Generally, these materials are crystalline, and are effective to exhibit the required property of rotating the plane S* of polarisation of polarised light passing eoo# 1 i0 therethrough under the condition that they are subjected to a magnetic field, most usually a magnetic field aligned in the direction of light

I

.o passage therethrough. This phenomenon is known as "Faraday rotation".

15 The invention is further described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagram of a prior art optical isolator, 20 Figure 2 illustrates, diagrammatically, an optical isolator constructed in accordance with the invention, Figure 3 is a graph of focal length versus input beam power, Figure 4 is a diagram of an optical system including an optical isolator constructed in accordance with the invention, Figure 5 is a graph of absorbance versus wave-length for certain crystalline materials useful 6 in the invention, Figure 6 is a graph of Faraday rotation against wave-length for materials useful in the invention, Figures 7a and 7b are diagrams illustrating the manner of operation of an isolator/limiter constructed in accordance with the invention, and Figure 8 is a diagram of energy gap 10 against manganese content for certain crystalline materials useful in accordance with the invention.

Referring firstly to Figure 1, there is shown a laser 10 effective to generate a coherent light beam 12 for incidence on a suitable sample, such as the end face 14a of an optical fibre 14 as shown. An optical isolator 16 is positioned between *I*l the laser 10 and the end face 14a and, as shown, comprises, in series arrangement, a first polarizer 18, a rotator 20, a second polarizer 22 and a 20 focussing lens system 25. Polarizers 18 and 22 are formed of conventional material, each adapted to preferentially pass therethrough light having a polarization plane which is arranged at a particular a physical orientation relative thereto. The rotator 20 is formed of a conventional crystalline material of Faraday-rotation type. The latter material, when subjected to a magnetic field (provided in the described arrangement for example by a cylindrical permanent magnet, not shown, with its axis aligned in the direction of travel of light beam 12), causes rotation of the plane of polarization of light travelling therethrough. The rotator 20 is arranged, in this instance, to rotate the polarization plane of S- -1-4 *r j

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C c C L 41C ~4 I t" C C Ct

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rC C~ LC CC light passing therethrough through 450 whilst the two polarizers 18 and 22 are so arranged that light, having once passed through the polarizer 18 and having the polarization plane thereof rotated by 450 through passage through the rotator 20 will have the plane of polarization thereof aligned with the preferred polarization plane for traverse through the polarizer 22. Thus, the polarizers 18, 22 will be arranged so that the preferred polarization planes 10 for light passing therethrough are at 450 to each other.

The lens system 25 is arranged to suitably focus the light beam from the laser 12 after passage through the isolator 16 on the end face 14a of the fibre 14. Generally speaking, the attenuation of light passing through the isolator 16 in the "forward" direction described, that is to say from the laser 10 to the fibre 14, can be made to be relatively slight. On the other hand, it will be observed that reflected light transmitted back from the end face 14a of the fibre 14, if polarized in the same fashion as the incident light thereon, will thus be able to travel back through the lens system and through the polarizer 22 to the rotator However, the light now traverses through the rotator in the "reverse" direction, ie in the direction opposite to the direction in which light from the laser 10 and polarizer 18 originally traversed the rotator. The rotator will thus be effective to 30 rotate the plane of polarization of this returning light by 450, but in the opposite directional sense to the direction of rotation of the Hforwardly" directed light. The reversely traversing light

I

1 1.

through the rotator 20 will have its plane of polarization rotated by 450 further relative to the orientation plane of polarization of the initially reflected light. The end result of this is that if, as shown in the diagram, the light passing through the polarizer 18 in the forward direction is counted as having a polarization of 00, the forwardly passing light through the rotator 20 and polariser 22 will have a polarization of, say, plus 450 which polarization prevails also for the reversely t travelling light passing through the polarizer 22.

However the relative polarisation of the reversely .Z travelling light will be increased to 90° after passage through the rotator 20. Thus, the reversely 15 travelling light falling on the polarizer 18 will be orientated at an angle of 900 relative to the plane of polarization of the forward light passing therethrough. Generally, a polarizer will exhibit a maximum attenuation of light therethrough when the plane of polarization of the incident light is rotated by 900 relative to the preferred plane of transmission therethrough so that the incident reversely travelling light at the polarizer 18 will be severely attenuated in passage therethrough.

25 Thus, practically speaking, a very severe attenuation of the reflected light can be achieved so that relatively little light can travel back from the end face 14a of the fibre 14 to reach laser 10. Thus, for example light reflected back from end face 14a through the isolator 16 will be so attenuated.

As mentioned, the prior arrangements such as shown in Figure 1 exhibit the disadvantage that, for proper operation, the lens system 25 must be ~cc~ -r C~"Idil;i i i ii 9 mechanically manipulated to bring the light beam passing through the isolator 20 to a focus at the end 14a.

Turning now to Figure 2, an isolator 24 constructed in accordance with the invention is arranged in a device similar to that shown in Figure i. In Figure 2 like components are referenced by like reference numerals to the reference numerals employed in Figure i. Thus, in this case, the laser is shown again as generating a light beam 12 which passes through the isolator 24 to the end face 14a of an optical fibre 14. Here, the isolator 24 again includes first and second polarizers 18 and 22 arranged so that the preferred planes for light transmission therethrough are at 450 one relative to the other. Between these, however, there is positioned, to form part of the isolator 24, a rotator 26 different to the rotator 20 previously described. Furthermore, the isolator 24 does not include the lens system 25 previously described.

44 tt r 4 4: 4 7 4 g ST4 r* 4 44 44 4 44 4 4 7 4.

The rotator 26 is formed from material capable of focussing the beam 12 on the end face 14a without the presence of the lens system 25 and is capable of varying the effective focal length of the isolator (that is to say the distance to the point at which light is brought to a focus having passed in a forward direction through the isolator).

It has been found that suitable rotators 26 may be formed from crystalline material of various compositions such as Hgl x Mn x Te or Cdl x Mn x Te. In any event, the material is selected so I_ I I~C 10 a C

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Stt 4 S 4. 4 that it exhibits the following two properties: a. The phenomenon of Faraday rotation. That is to say the property that polarized light passing therethrough has the plane of rotation thereof rotated through a substantial angle pursuant to application of an axial magnetic field applied thereto, and b. The ability to bring the beam light passing thereto a focus such that the focal length of the optical system provided by the isolator varies in accordance with the power of the incident light beam thereon.

Generally speaking, the materials in question will exhibit the property of variable focussing as above described pursuant to changes in the refractive index thereof in accordance with the power of the beam incident thereon.

Figure 3 shows a typical relationship between beam input power and focal length for a suitable material. Particularly, as the input beam power is increased, the focal length decreases.

By this arrangement, the operation of the isolator 24 is generally similar to the manner of operation of the isolator 16. The light beam having passed through the isolator may be brought as desired to a focus such as at the end face of the fibre 14 by the simple expedient of varying the intensity of light from the laser 10. Thus, the intensity of the beam 12 may be modulated by appropriately modulating

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i,: -1 "r 2/8 3i6~cs~sq llr~_ll-~i~-i^I the electrical power input to the laser 10, or by any other conventional means.

Figure 4 shows a variant arrangement.

Here, like reference numerals denote like parts in both Figure 3 and Figure 4. The laser 10 is shown again as generating a light beam 12 which passes through the isolator 24 to be brought to focus at the end face 14a of the fibre 14. Here, however, a partially light transmissive mirror 30 is arranged in the path of light from laser 10 to the isolator 24 and positioned at an angle of, say, 450 thereto whereby, in addition to light from the laser 10 which passes through the mirror 30 to the isolator 24, a light beam 35 from a further laser 32, incident on the mirror 30, can be reflected thereby to travel in alignment with the beam 12 through the isolator 24 to the fibre 14. In this arrangement, the light from the two lasers 10 and 32 is arranged to be of different wavelengths. Insofar as the laser 10 is S 20 concerned, the wavelength is chosen to be one at which considerable absorption occurs in the material forming the rotator 26. On the other hand, the wavelength of light from the laser 32 is chosen to correspond to one where the material forming the rotator 26 is relatively transparent.

p i Figure 5 shows a typical graph of J lightwave length against absorbance for material of the form of Cdl x Mn x Te. The absorption falls, over an initial range, from a high value, with increasing wavelength, until a relatively low value is reached, the absorbance remaining at this low value for a certain increase in wavelength. On still Ji lI; 12 further increase in wavelength, the absorbance again increases. The wavelengths need also to be chosen so that under a certain fixed magnetic field applied in the rotator 26, the polarization of the light from the laser 10 is rotated through 1350 in passing through the rotator 26 whereas the light from the laser 32 is rotated only by 45°, or in any event such that a phase difference substantially different from 0° and 1800 exists. In that regard, the materials S 10 employed for the rotator 26 typically exhibit Str considerable variation in the extent of Faraday t rotation which occurs, with a fixed magnetic field, s under variations in the wavelength of incident

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light. Figure 6 shows a typical variation where the 15 extent of Faraday rotation which occurs decreases with increasing wavelength. Figure 6 shows the selection of two wavelengths corresponding to the Srotations of 135° and 450 respectively Similarly, t i, r Figure 5 shows the selection of these two wavelengths as lying, for the wavelength X1 in a region where there is a considerable absorbance by the rotator 26, and for the wavelength X2 in a region where there is very little absorbance.

Generally speaking, the laser 10 should be chosen to have a relatively greater signal strerith than that of the laser 32, and the latter beam should be as weak as possible so as not to contribute to determining the focal length of the lens constituted by rotator 26.

By the arrangement of figure 4, then, the laser 10 can be adjusted to primarily influence the location of the point of focussing of light from the V, 4/8 device 24, whereas the beam from the laser 32 can be suitably modulated to provide signal information for transmission through the isolator 24 and to the fibre 14. The effects, insofar as the relative polarizations introduced by the polarizers 18 and 22 and the rotation caused by the rotator 26 are concerned, are shown in figure 4, both for the beam from laser 10 and that from laser 32. In the ideal case, the beam from laser 10, having been polarized i 10 by transmission through the first polarizer 18, is •r rotated through 1350 by the rotator 26, as well as being at least partially absorbed by the rotator 26, (as a result of the wavelength dependant absorption characteristics of the rotator as explained above 4' 15 with reference to figure The remaining portion of this 1350 polarized beam is then totally blocked by the 450 polarizer 22.

In the case where the beam from laser having been polarized by transmission through first polarizer 18, is rotated through an angle other than 1350°, complete absorption by polarizer 22 in the forward direction will not occur, and a portion of the beam passing therethrough may be reflected back through the Aevice at the end face 14a. When this occurs the reflected beam, being polarized in the direction of polarization of the polarizer 22, will x¢ pass through polarizer 22 uneffected, but wi'l be further attenuated, due to its wavelength, as well as being rotated, by the rotator 26. Furthermore the small portion of the reflected beam which remains after passage through the rotator 26 is further attenuated by action of polarizer 18. Thus the final portion of back-reflected beam originating from laser -i a i

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4 t 4 r t 4 14 will be of such minute power as to cause no or insubstantial interference to the emissions of laser Similarly, the light from the laser 32 is polarized by the polarizer 18 and rotated through 450 at the rotator 26 to then pass through the polarizer 22. Returned light of this wavelength is again rotated through a further 45° by the rotator 26 so that it will not pass through the polarizer 18.

Thus, the return light beams for both light wavelengths, as presented to the polarizer 18, are similarly affected by incidence on the polarizer 18, in the sense that both are rotated at an angle of ±900 to the preferred polarization plane, to cause maximum attenuation.

The arrangement in Figure 4 can thus satisfactorily prevent return light from both lasers and 32 from reaching the lasers whilst, at the same time, permitting focusing of the light from the laser 32 by largely independent operation, that is by variation of the intensity of the beam from laser The isolator 24 may also be arranged in the configuration shown in Figures 7a and 7b. Here like reference numerals denote like components in both Figure 7 and in Figure 2. In this arrangement, light from the isolator 24 is focussed at a predetermined location away from a sample 60 which it is desired to illuminate by light from the laser The focal point is arrnged, such as at the point 62 shown, whereby a certain area of the sample 60 is illuminated by non-focussed light from the beam 12.

t 4 1: t 41 4r 4 St 4 4t iip q 4-4 r X_~LI- 1- '-3 I i i 4 i ii The arrangement is such that this focal point 62 is established at a relatively low power of incident light from the laser 10. Under this circumstance, then, as the power increases from the laser 10 the focal point 62 will move somewhat closer to the isolator 24 with the result that the beam will become increasingly defocused as it emerges from the isolator 24. The result of this is that the area of the beam presented at the location of incidence on 1 0 the sample 60 will become greater so that the incident power will be lessened, or at least will not increase at a rate proportionate to increase in power of the beam 12 as it emerges from the laser r Thus, in instances where the sample 60 is susceptible to damage by subjecting it to too great an intensity of beam, an automatic power limiting effect is obtained.

Figure 8 shows a graph of manganese concentration in certain compounds useful in the invention, plotted against the energy gap (Eg), wavelength being inversely proportional to energy 'gap. By way of example, the optical system ofl figure 4 may be realised utilizing components as follows: Laser 10 A "Coherent 700" dye laser with 7210 cavity dumper, using Rhodamine 570 dye and operating at a wavelength tunable between 575 nm and 620 nm.

Laser 32 A "Quentron Optics" He-Ne laser of operating wavelength 632-8 nm.

j I w Polarisers 18, 22 "ORIEL" calcite polarizers Rotator 26 Formed from Cdlx Mn x Te with x=0.6.

The magnet field being created by permanent magnets.

With this arrangement, a rotation of the polarization of light from the lasers 10 and 32, of 1350 and respectively, can be achieved by use of a magnetic field of about 3570 gauss, using a rotator 26 of 3 mm thickness, and with laser 10 tuned to 586 nm.

The described construction has been advanced merely by way of explanation and many modifications and variations may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.

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Claims

1. A magneto optic device of the kind having a first polarizer, having a preferred plane of polarization effective to substantially attenuate Spassing therethrough, being light having a dlane of polarization substantially differing from a preferred plane of polarization characteristic of the first polarizer, an optical rotator positioned for t rotating light of said preferred plane emerging from 1 said first polarizer through a predetermined angle, n and a second polarizer effective to substantially attenuate light transmission therethrough when the plane of polarization thereof is substantially different from a preferred plane of polarization characteristic of the second polarizer, the second .t polarizer being arranged so that the plane for preferred light transmission therethcough corresponds to the polarization plane of light having passed through the first polarizer and then through the Srotator to be incident on the second polarizer, whereby such light passing through the device in a forward direction through the first polarizer, rotator and second polarizer, if reflected back through the device with the same polarization, is substantially attenuated in passage through the device by virtue of rotation of the plane of polarization thereof, once having passed back through the second polarizer, to exhibit a polarization on incidence on the first polarizer which substantially differs from the preferred plane of polarization for transmission therethrough, characterised in that the rotator is capable of effecting focussing of light passing therethrough in the forward direction and to lj I accordance with the invention, Figure 5 is a graph of absorbance versus wave-length for certain crystalline materials useful 18 vary the focus point of such forwardly passing light, relative to the rotator, pursuant to variation in the power of such light.

2. A magneto optic device as claimed in claim 1 wherein the rotator includes a light transmissive element which is effective to effect both the rotation of the plane of polarization of light passing therethrough and to effect said focussing.

3. A magneto optic device as claimed in claim 2 wherein said light transmissive element is of a kind in use causing rotation of the plane of polarization of polarized light passing therethrough r when subjected to a magnetic field, the rotator further including means in use generating said magnetic field.

4. A magneto optic device as claimed in claim rotct Ov is -fiwteCdk +rrc:,Yv, Wcv' 3 wherein saidAlkTer=t is characterised in that the S* refractive index thereof varies pursuant to the energy of incident light thereon. o' A magneto optic device as claimed in claim I4 wherein said material has the general composition Mnl-x Cdx Te or Hgl- x Mnx Te or Hgl-x-y Mn x Cdy Te, (l>x>O)

6. A light transmission system including a laser arranged to direct light to the end face of an optical fibre for propogation of the light lengthwise within the fibre, and including a magneto optic device as claimed in any preceding claim positioned between the laser and said fibre end to attenuate back reflected light from the end face of fibre in U LU 1 1 N T,. t 1 1 1 1 1 O* 1 the direction back to the laser, and means for modulating the power of said light whereby to enable control of the point of focus of the light passing from the laser through the rotator to the end face, for focussing the light on the end face.

7. A light transmission system as claimed in claim 6 wherein said laser comprises a first of two lasers, a second of said lasers being arranged for directing light through the magneto optic device together with light from the first laser, one said laser in use producing light having a wavelength such that variations in the power of that light effect corresponding significant variations in the location of said focus and the wavelength of the light in use produced by the other said laser being selected such that variations in the power of that light have lesser influence on the location of said focus.

8. An optical transmission system as claimed 4in claim 7 wherein said wavelengths are chosen whereby said rotator in use produces rotations of substantially 180* of relative phase displacement as between light from each respective laser, on passage through the rotator after passage through said first polarizer.

9. A light transmission system as claimed in any one of claims 6 to 8 wherein said light from said one laser is, in use of the system, stronger than that from the other laser. A light transmission system including a laser arranged to direct light to an object for illumination thereof and including a magneto optic device as claimed in any one of claims 1 to positioned between the laser and the object to attenuate back reflected light from the object tending to travel back to the laser.

11. A light transmission system as claimed in claim 10 wherein said object is positioned relative to said light emerging from the magneto optic device r whereby the incident light on the object is defocussed and whereby, on increase of power of light from the laser from a reference level, the light is B" further defocussed on the object.

12. A magneto optic device substantially as hereinbefore described with reference to Figures 2 to 8 of the accompanying drawings. e 13. A light transmission system substantially as hereinbefore described with reference to Figures 2 S" to 7b of the accompanying drawings.

14. ,4 csubstantially as hereinbefore described with J to reference to Figures58 of the accompanying drawing. Dated this 23rd day of March 1989 CeoPoa- dTo' AUSTRALIAN TELECOMMUNICATIONS GOMMSSO-IN- i By its Patent Attorneys DAVIES COLLISON .r r J 4 IA