WO1994015243A1

WO1994015243A1 - An optical isolator

Info

Publication number: WO1994015243A1
Application number: PCT/AU1993/000663
Authority: WO
Inventors: Robert Alan Pattie
Original assignee: Telstra Corporation Limited
Priority date: 1992-12-22
Filing date: 1993-12-16
Publication date: 1994-07-07
Also published as: US5912766A; US5768015A

Abstract

An optical isolator (2) comprising two polariser means (10, 12), two input/output ports (4, 6) formed respectively on the polariser means (10, 12), and optical rotator means (14, 18) disposed between the polariser means (10, 12), the optical rotator means (14, 18) including Faraday rotator means (14) and being selectively configured so the isolator (2) performs one of a plurality of isolator functions.

Description

AN OPTICAL ISOLATOR

The present invention relates to an optical isolator, and in particular, an isolator which is wavelength selective.

Optical isolators are used in optical communications systems to restrict, or isolate, the direction of travel of optical signals to one direction over a broad wavelength band. It has also recently been discovered that there is application for isolators which are wavelength selective and restrict a first wavelength band to one direction of travel, and a second wavelength band to the opposite direction, as discussed in International Patent Application No. PCT/AU93/00258 for a bidirectional isolator.

The present invention, more specifically, relates to using the wavelength dispersion characteristics of optical rotator materials, such as Faraday rotator materials, to provide isolator functions, and, in some instances, enhance performance.

Faraday rotators rotate polarised signals of a wavelength band λ by a selected number of degrees in a set clockwise or anticlockwise direction, regardless of whether the signals travel through the rotator in a forward or reverse direction. Faraday rotators are normally constructed from YIG, which is Yttrium based, or BIG, which is based on Bismuth substituted YIG. Optical rotators can also be provided by multi-order half-wave plates, which can be cut from quartz and used to rotate optical signals of wavelength λ by a predetermined number of degrees. Unlike Faraday rotators, the half-wave plates will rotate a signal of wavelength λ in one direction as it passes therethrough in a forward direction but will perform a reciprocal rotation, by rotating the signal in the opposite direction, when it passes through the wave plate in the reverse direction. Reciprocal optical rotators can also be fabricated from optically active material which continuously rotates polarised light along its length in a linear polarisation state. Half-wave plates rotate linearly polarised light by changing it to an elliptical polarisation state and then back to a linear polarisation state. Half-wave plates normally can achieve a desired rotation in a much shorter length of material than optically active material, which can also be cut from quartz.

The term wavelength dispersion is used herein to describe a characteristic of a device, which processes differently or has a different effect on signals of different wavelengths. Similarly, the term polarisation dispersion is used to refer to a characteristic of a device which processes differently or has a different effect on signals of different polarisations.

Previously the dispersion characteristic of a Faraday material in a cascaded isolator assembly has only been used to achieve isolation of a broader signal wavelength band, as discussed in Kazuo Shiraishi and Shorjiro Kawakami, "Cascaded optical isolator configuration having high-isolation characteristics over a wide temperature and wavelength range" Optics Letters, Volume 12, No. 7, July 1987, pages 462 to 464.

In accordance with the present invention there is provided an optical isolator comprising two polariser means, two input/output ports formed respectively on said polariser means, and optical rotator means disposed between said polariser means, said optical rotator means including Faraday rotator means and being selectively configured so that the isolator performs one of a plurality of isolator functions.

Advantageously the wavelength dispersion characteristics of said optical rotator means may determine said one of said isolator functions for at least two wavelength bands.

Preferred embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a side view of a first preferred embodiment of an isolator;

Figure 2 is polarisation diagrams for the isolator of Figure 1 for light of wavelength λ-_>; Figure 3 is polarisation diagrams for the isolator of Figure 1 for light wavelength λ,;

Figure 4 is a side view of a second preferred embodiment of an isolator;

Figure 5 is polarisation diagrams for the isolator of Figure 4 for light wavelength λ^;

Figure 6 is polarisation diagrams for the isolator of Figure 4 for light wavelength λ,;

Figure 7 is a side view of a third preferred embodiment of an isolator;

Figure 8 is polarisation diagrams for normal operation of the isolator of Figure Figure 9 is polarisation diagrams for the isolator of Figure 7 when errors oc in a second Faraday rotator,

Figure 10 is polarisation diagrams for the isolator of Figure 7 when errors oc in a first Faraday rotator,

Figure 11 is a side view of a fourth preferred embodiment of an isolator; Figure 12 is polarisation diagrams for normal operation of the isolator

Figure 11;

Figure 13 is polarisation diagrams for the isolator of Figure 11 when errors occ in a second Faraday rotator,

Figure 14 is polarisation diagrams for the isolator of Figure 11 when errors occ in a first Faraday rotator;

Figure 15 is a side view of fifth preferred embodiment of an isolator;

Figure 16 is polarisation diagrams for normal operation of the isolator Figure 15 for light of wavelength λjj

Figure 17 is polarisation diagrams for normal operation of the isolator Figure 15 for light of wavelength λj,;

Figure 18 is polarisation diagrams for the isolator of Figure 15 for light wavelength λ-j when errors occur in a first Faraday/optical rotator,

Figure 19 is polarisation diagrams for the isolator of Figure 15 for light wavelength λ- when errors occur in a first Faraday/optical rotator, Figure 20 is polarisation diagrams for the isolator of Figure 15 for light wavelength λ-, when errors occur in a second Faraday/optical rotator, and

Figure 21 is polarisation diagrams for the isolator of Figure 15 for light wavelength λ_j when errors occur in a second Faraday/optical rotator.

A first optical isolator 2, as shown in Figure 1, includes first and second inp ports 4 and 6 formed at the junction of respective graded refractive index (GRIN) lens 8 and spatial walk-off polarisers (SWP) 10 and 12. The GRIN lenses 8 are used connect the ends of fibres to the isolator 2 and direct incoming and outgoing optic signals between the isolator 2 and the optical fibres with minimum insertion loss. T isolator 2 also includes a Faraday rotator 14 and a reciprocal optical rotator 18 dispose between the SWPs 10 and 12, such that all of the components form an in line seri assembly. The first SWP 10 separates signals received on the first port 4 into verticall polarised and horizontally polarised components, and walks the vertical compone upwards, and performs the reciprocal operation on components travelling in the revers direction, i.e., from the second port 6 to the first port 4. The second SWP 12 also wal vertically polarised components upwards with respect to received horizontally polarise components, when the components travel in the forward direction from the first port to the second port 6. Again, the second SWP 12 performs the reciprocal operation f components travelling the reverse direction, by walking vertically polarised component down with respect to the received horizontal components. The SWPs may be bot formed from calcite. The assembly of the isolator 2 can be considered to have an upp and a lower section, where the first port 4 is aligned with the lower section and th second port 6 is aligned with the upper section.

The Faraday rotator 14 and the optical rotator 18 are configured so as to provid one of a plurality of isolator functions for the isolator 2 for two or more wavelengt bands. For example, if λ_j and λ^ denote first and second wavelength bands, the function may comprise:

(a) Isolate signals of λj and λ- for one direction. First wavelength λ_j may b 1300 nm and the second, λj may be 1500 nm. This function woul provide isolation for separate wavelength bands. (b) Isolate signals of λj for one direction and allow signals of , to trave through the isolator in both directions, i.e., allow the isolator 2 to b transparent to λj. For example, either 1300 or 1550 nm could be isolate and the other wavelength used for optical system communications. Al 1550 nm could be isolated and the assembly transparent to 1480 nm reduce undesirable noise in doped fibre devices, such as rare ea amplifiers. The uninhibited wavelength λj could also be used to perfo OTDR analysis on the system, as discussed in International Pat

Application No. PCT/AU93/00258.

(c) Signals of λ^ are isolated for one direction and signals of λ_j may partially isolated. This is relatively easy to achieve for most wavelen combinations of interest without producing a device which is over sensitive to temperature variations. This function may be used for t same applications as discussed for (b), except λ_j cannot be us effectively for OTDR analysis.

(d) Isolate signals of ^ in one direction and isolate signals of λ, in t opposite direction. This allows the isolator 2 to function as a bidirection isolator.

The different combinations of Faraday and optical rotators 14 and 18 which c be selected to provide the functions (a) to (d) are shown in Table 1 below, which discussed hereinafter.

TABLE 1

10

15

20

The functions (a) to (d) are listed in the left hand column, and the type of Farad rotator 14 required is marked with an *. The types of Faraday rotators are characterise by firstly the planes of polarisation which signals of the two wavelengths appear in aft rotation by the Faraday rotator 14, i.e., the same plane, orthogonal to one another, signals of λ-_> leading or lagging λj by 45°. The notation +45° refers to clockwise rotatio and -45° to anticlockwise rotation. Secondly, the type is characterised by whether t signals of λ_j or λ-, are rotated by an odd or even number of 45° rotations. The effectiv rotation required to be achieved by the optical rotator, for each wavelength λj and λ_j, t ensure the Faraday rotator 14 and reciprocal optical rotator 18 combination achieves th desired function for each case, is listed in the three right hand columns of Table 1.

The length of the Faraday rotator 14, which governs the length of the lig transmission path therethrough, is selected so as to provide the rotator with a wavelengt dispersion characteristic which gives rise to the desired polarisation component rotatio ±m 180°, where m is a non-negative integer. Similarly, the optical path length of th reciprocal optical rotator 18 is selected to provide a wavelength dispersion characteristi which achieves the desired effective rotation _tm 180°. The reciprocal optical rotator ma comprise half-wave plate or optically active material. The thinnest Faraday rotator 1 which provides an effective rotator combination is selected as the temperature sensitivit of the Faraday rotator 14 increases with increasing thickness.

The function (c) is a special case in that for the partially isolated wavelength it is only desired to ensure that signals of λ- travel in one direction, whereas in th reverse direction it is unimportant whether the λj signals are inhibited or allowed to pas through the isolator 2. The band λ-, may be used, for example, to pass pump signal through the isolator 2 to an optical amplifier. Normally pump signals are introduce between the isolator associated with an amplifier, and enabling the signals to be passe through one of those isolators instead significantly improves the noise characteristic o the optical system including the amplifier and isolators. The # in the column for λ_j o the optical rotator 18 and in the Faraday rotator column indicates that Faraday and optic rotators are selected to achieve the same total effective rotation as that for λ_j in th forward direction. Yet, the total effective rotation produced by the combination in th reverse direction for λj is of no concern.

If the reciprocal optical rotator is required to perform the same effective rotati according to Table 1, for λ_j and j then it can be omitted from isolator 2, which requi the SWP 12 to then be reoriented to take the omission into account. The Faraday rota must in this case rotate the polarisation components to be parallel and perpendicular the SWP walk-off direction.

The isolator 2 in Figure 1 may be configured to perform any one of the functio (a) to (d) but is described hereinafter as configured to perform function (b). T polarisation diagrams of Figures 2 and 3 show the polarisation components and t positions of the components at each of the interfaces of the parts 10 to 12 of the isolat 2, when viewed from the first port 4 of the isolator 2. The polarisation diagrams Figures 5, 6, 8, 9, 10, 12, 13, 14 and 16 to 21 adopt the same convention, and o representations for the interfaces between the Faraday rotators 14 and optical rotators 1

With reference to the forward direction diagram 20 of Figure 2, the verti component of light incident on the first port 4 is walked up to the upper section of t Faraday rotator 14 by the first SWP 10, whereas the horizontal component is allowed pass unchanged. Both components are rotated 90°, i.e., ± 90° ± ml80°, by t Faraday/optical rotator combination 14, 18, and then the lower vertically polaris component is walked up by the second SWP 12 to be incident on the second port 6 wi the horizontally polarised component. In the reverse direction diagram 22, light incide on the second port 2 follows the same path to the rotator combination 14, 18, but in t direction the components effectively undergo no rotation, i.e., ±m 180°. The upp horizontally polarised component is allowed to pass unchanged by the first SWP 10 as to be dispersed above the first port 4, and the lower vertically polarised component walked downwards out of the isolator 2 before reaching the first port 4. For signals wavelength λ,, with reference to the forward direction diagram 24 of Figure 3, t components of the signals incident on the first port 4 follow the same path as those λ-, as shown in diagram 20. However in the reverse direction, as shown in diagram 2 the signals of λj are rotated by 90° by the rotator combination 14, 18 so a vertical polarised component appears at the upper section of the first SWP 10 and can be walk down to a horizontal component so that both components are incident at the first port

With the ports 4 and 6 displaced vertically by the spatial walk-off distance illustrated in Figure 1, both polarised components travel the same distance through t isolator 2, which gives minimum polarisation dispersion. However the spatial walk- distance of the SWPs has a slight wavelength dependency, so as the components are o walked in one direction, the isolator 2 is more sensitive to the wavelength dispersion the SWPs 10 and 12. The insertion loss due to the wavelength dependence of the SW 10 and 12 can be minimised by aligning the GRIN lenses as shown in Figure 4 for second isolator 30. This however is at the cost of minimising polarisation dispersion the polarised components of signals will now travel different distances between the fi port 4 and the second port 6. The second isolator 30 can also be used to perform all the functions (a) to (d).

Any one of the isolator assemblies discussed herein may be configured with t first and second ports 4 and 6 aligned or not aligned, and the same isolator function performed provided the last SWP 12 and GRIN lens 8 or the last SWP 12, GRIN le 8 and last half-wave plate for isolators having multiple half-wave plates, are rotated 180°. This may, however, require some alteration of the effective rotation performed rotators in the isolator. A polariser can be included at the face of a Faraday rotator whi is the closest to an SWP 10 or 12 to enhance the performance of each isolator 2.

With reference to the polarisation diagram 32 of Figure 5, signals of waveleng λa incident on the first port 4 of the second isolator 30 are split into a vertically polaris component which is walked up to the Faraday rotator 14 and a horizontally polaris component is unchanged by the first SWP 10. The rotator combination 14, 18 is no configured to effect no rotation on the components in the forward direction but effe the 90° rotation on the components in the reverse direction, as the second SWP 12 h been rotated by 180° with the GRIN lens 8 of the second port 6. The SWP 12 therefo walks the vertically polarised component down to the horizontally polarised compone so that both components are incident on the second port 6. Light incident on the seco port 6, with reference to the diagram 34, travels the same path until the components a rotated 90° by the rotator combination 14, 18. A horizontally polarised component then passed by the first SWP 10 unchanged to be dispersed above the first port 4, and t vertical component received by the SWP 10 is walked out of the isolator 2 befo reaching the first port 4. For signals of wavelength λj, as shown in the polarisati diagrams 36 and 38 of Figure 6 for the forward and reverse directions respectivel signals incident on the first port 4 follow the same path as that shown in the diagram of Figure 5. However, in the reverse direction the rotator combination does not effe a rotation on the incident components so a vertical component is incident on the fir SWP 10 in the upper section, and a horizontal component in the lower section. Therefo the first SWP 10 walks the vertical component down to the horizontal component so th both are incident on the first port 4.

The first and second isolators 2 and 30 are sensitive to errors in the rotators whic may be induced by temperature or wavelength fluctuations. The third and fourth isolato

50 and 80 of Figures 7 and 11 are able to perform functions (a) and (d) and are le sensitive to Faraday errors. Erroneous components included by Faraday errors should n appear at the ports 4 and 6.

The third isolator 50 includes a rotator combination comprising two Farada rotators 14 with a polariser 54 disposed between them. The rotators 14 both perform a effective rotation of 45°. A first half-wave plate 52 is disposed between the rotat combination and the first SWP 10, and a second half-wave plate 56 is disposed betwee the rotated combination and the second SWP 12. The first and second ports 4 and 6 ar not aligned, and the half-wave plates 52 and 56 are aligned with the first port in th lower section of the isolator 50. The half-wave plates each perform an effective 90 rotation of all signals. The rotator combination 14,54 rotates horizontal and vertic components received into the same polarisation plane. The polariser 54 only allows lig polarised in the 45°-225° plane to pass therethrough. The isolator 50 is configured t perform function (a) with 0° effective optical rotator rotation, i.e., no optical rotators ar required. For designs which include a plurality of Faraday rotators or optical rotator as in the isolator 50 of Figure 7, each must meet the requirements of Table 1. Wit reference to the forward direction polarisation diagram 58 of Figure 8, light incident o the first port 4 is split into horizontal and vertical components by the first SWP 10, wit the horizontal component being incident on the first half-wave plate 52, and the vertica component being walked up to the transmissive medium, e.g., 0° half-wave plate immediately above the half-wave plate 52. The horizontal component is rotated 90° b the half-wave plate 52 so both components are polarised in the same plane when inciden on the first rotator 14a. The components are rotated into the 45°-225° plane and ar allowed to pass by the polariser 54 to the second rotator 14b, which rotates th components into the horizontal plane. The second half-wave plate 56 rotates the lowe component 90° into the vertical plane, and this vertical component is then walked up t the horizontal component by the second SWP 12, so both components are incident on th second port 6. Light incident on the second port 6 returns via the same path, as show in the reverse direction polarisation diagram 60, until the two horizontally polarise components are incident on the second rotator 14b. The two components are rotated b the second rotator 14b into the 135°-315° diagonal plane, which is orthogonal to th plane of the polariser 54. The components are therefore inhibited by the polariser 54 an no light appears in the first rotator 14a or at the first port 4.

If an error is induced in the second Faraday rotator 14b such that some effectiv rotation of the components is performed in the reverse direction, i.e., anticlockwise, the in the forward direction, as shown in the polarisation diagram 62 of Figure 9, the secon

Faraday rotator 14 outputs two components polarised in the vertical plane, instead of th horizontal plane. The lower component is rotated by the second half-wave plate 56 int the horizontal plane and is allowed to pass unchanged by the second SWP 12 so as to b dispersed below the second port 6. The vertical component passed above the half-wav plate 56 is walked up by the second SWP 12 out of the isolator 50 before reaching th second port 6. In the reverse direction, as shown in the reverse direction diagram 64, the second rotator 14b will output components polarised in the 45°-225° plane which ar allowed to pass to the first rotator 14a by the polariser 54. The components are rotate into the horizontal plane by the first rotator 14a, and the lower component is rotated 90° into the vertical plane by the first half-wave plate 52. The upper horizontally polarised component passes above the half-wave plate 52 through the first SWP 10 unchanged so as to be dispersed above the first port 4. The vertical component emitted by t half-wave plate 52 is walked out of the isolator 50 below the first port by the fir SWP 10.

If a similar error occurs in the first Faraday rotator 14a, for the forward directio as shown in the polarisation diagram 66 of Figure 10, the Faraday rotator 14a produc components polarised in the 135°-225° plane which are immediately inhibited fro proceeding further by the polariser 54. The reverse direction polarisation diagram 68 Figure 10 is the same as the reverse direction diagram 60 of Figure 8 for norm operation.

The fourth isolator 80 of Figure 11 is the same as the third isolator 50, except t first and second rotators 14a and 14b perform their effective 45° rotation in t anticlockwise direction, instead of the clockwise direction, so the polariser 54 polaris light in the 135°-315° plane, instead of the 45°-135° plane. Outside of the rotat combination 14,54, light travels precisely the same path through the fourth isolator 80 for the third isolator 50, as shown in the forward and reverse polarisation diagrams 8 and 84 of Figure 12 for normal operation of the isolator 80, the forward and rever direction polarisation diagrams 86 and 88 of Figure 13 where the second rotator 14 induces errors and the forward and reverse direction polarisation diagrams 90 and 92 Figure 14 where the first Faraday rotator 14a introduces errors.

The third and fourth isolators 50 and 80 illustrate that the same function can performed provided the Faraday rotator combination provides effective rotation which an odd multiple of 45°.

A fifth isolator 100 has the same structure as the third and fourth isolators 50 an 80, except the Faraday rotators 14 are replaced by two Faraday/optical rotators 102 an 104, which are Faraday rotator 14 and reciprocal optical rotator 18 combinations. Th structure of the fifth isolator 100 can be used to realise all of the four functions (a) to (d The structure can also invoke rotations which are odd integer multiples of 45° for bot λj and λj, where the polarised components appear in orthogonal planes for function (a and appear in the same plane for function (d), as indicated in Table 1. This cannot achieved using the structure of the third and fourth isolators. The structure is also n sensitive to rotator errors, as errors induced by the rotators 102 and 104 are not passe to the ports 4 or 6.

Considering the fifth isolator when configured to perform function (b), light of th second wavelength λ-_> incident on the first port 4 is split into a vertical component whic is walked up by the first SWP 10, and a horizontal component which is passed to the fir half-wave plate 52, as shown in the forward direction diagram 106 of Figure 16. Th horizontal component is rotated into the vertical plane and the two vertically polarise components are rotated by 45° in a clockwise direction by the first rotator 102. Th polariser 54 passes the components to the second rotator 104 which rotates them agai by 45° in the clockwise direction. The lower component is rotated into the vertical plan by the second half-wave plate 56 and is walked up to the other horizontal component b the second SWP 12 so as to be incident on the second port 6. Light incident on th second port 6 returns via the same path, as shown in the reverse direction diagram 10 of Figure 16, until the second rotator 104 rotates the components clockwise by 45° so a to be polarised in the 135°-225° diagonal plane. The polariser 54 then prevents th components from proceeding further to the first port 4.

Light of the wavelength λ-, incident on the first port 4 follows the same path a light of the wavelength λς to the second port 6, as shown in the forward directio polarisation diagram 110 of Figure 17. In the reverse direction, for light incident on th second port 6, as shown in the polarisation diagram 112, the horizontal component received by the second rotator 104 are rotated in an anticlockwise direction by 45 instead of the clockwise direction for wavelength λj. This occurs because for λ_j th optical rotator of the rotator 104 produces no effective rotation but for λ_j the optic rotator of the rotator 104 produces a 45° effective rotation, according to Table 1. Th 45°-225° polarised components are therefore allowed to pass by the polariser 54 and ar rotated into the vertical plane by the second rotator 102. The lower vertically polarise component is rotated into the horizontal plane by the first half-wave plate 52 and the tw components are then combined by the first SWP 10 so as to be incident on the firs port 4.

If the first Faraday/optical rotator 102 induces errors by producing an effective 45 rotation in the anticlockwise direction, for signals of λ-, incident on the first port 4, th components are prohibited from proceeding to the second rotator 104 and the second por 6 by the polariser 54, as shown in the forward direction diagram 114 of Figure 18. Fo the reverse direction, as shown in diagram 116, signals incident on the second port travel the same path as that shown in Figure 16. For signals of λj incident on the firs port 4, the path is the same as that for diagram 114 of Figure 18, as shown in the forward direction diagram 118 of Figure 19. In the reverse direction, as shown in diagram 12 of Figure 19, the components polarised in the 45°-225° plane which are incident on the first rotator 102 are rotated into the horizontal plane, instead of the vertical plane, and the lower component is then rotated by the half-wave plate 52 into the vertical plane. The lower vertical component is walked out of the isolator 100 before reaching the first port 4 and the upper horizontally polarised component passes above the half-wave plate 52 through the first SWP 10 and is dispersed above the first port 4.

If, however, errors are induced by the second Faraday/optical rotator 104, wherein the rotator 104 produces an effective 45° in a direction opposite to that shown in the diagrams 106 to 112 of Figures 16 and 17, then for light incident on the first port 4, as shown in the forward direction diagram 122 of Figure 20, the second rotator 104 rotates the components received into the vertical plane. The lower vertical plane is rotated in the horizontal plane by the second half-wave plate 56 and passes unchanged through thfc second SWP 12 so as to be dispersed below the second port 6. The upper vertical component passes above the second half-wave plate 56 and is walked out of the isolator 100 by the second SWP 12 before reaching the second port 6. For light incident on the second port 6, as shown in the reverse direction diagram 124 the second rotator 104 outputs components polarised in the 45°-225° diagonal plane, which are allowed to pass by the polariser 54 to the first rotator 102. The first rotator 102 rotates the components into the horizontal plane and the upper component passes directly to the first SWP 10 and is allowed to pass therethrough unchanged so as to be dispersed above the first port 4. The lower component is rotated into the vertical plane by the first half-wave plate 52 and is walked out of the isolator 100 by the first SWP 10. The signals of wavelength λ,, t path is the same in the forward direction as for signals of λ^ as shown in the polarisati diagrams 126 of Figure 21 and 122 of Figure 20. In the reverse direction, as shown diagram 128 of Figure 21, the second rotator 104 outputs components polarised in t 135°-315° .diagonal plane which are prohibited from proceeding to the first rotator 1 and the first port 4 by the polariser 54.

The rotator elements of all of the isolators 2, 30, 50, 80 and 100 can be select so that the various isolator functions can be performed for more than two wavelength For example, wavelengths λ,, λ-> and λ₄ may be isolated, and the isolator could be ma transparent to wavelengths λg and λj. The Faraday elements of all the isolators 2, 30, 5 80 and 100 may be a single Faraday element or a cascade of Faraday elements to obtai their required thickness or rotation characteristic.

Claims

CLAIMS:

1. An optical isolator comprising two polariser means, two input/output ports forme respectively on said polariser means, and optical rotator means disposed between sai polariser means, said optical rotator means including Faraday rotator means and bein selectively configured so the isolator performs one of a plurality of isolator functions.

2. An optical isolator as claimed in claim 1, wherein the wavelength dispersio characteristics of said optical rotator means determines said one of said isolator function for at least two wavelength bands.

3. An optical isolator as claimed in claim 2, wherein said functions include isolat one of said wavelength bands and allows the other one of said wavelength bands to pas between said ports in forward and reverse directions.

4. An optical isolator as claimed in claim 2, wherein said functions include isolatin light in both of said wavelength bands.

5. An optical isolator as claimed in claim 2, wherein said functions include isolating light in one of said wavelength bands and partially isolating light in the other one of said wavelength bands.

6. An optical isolator as claimed in claim 2, wherein said functions include isolating light in one of said wavelength bands in a forward direction and isolating light in the other one of said wavelength bands in a reverse direction.

7. An optical isolator as claimed in claim 1, wherein said optical rotator means further includes reciprocal rotator means.

8. An optical isolator as claimed in claim 1, which is substantially tolerant to errors induced by said Faraday rotator means. 9. An optical isolator as claimed in claim 1 or 7, wherein said optical rotator mea further includes segmented reciprocal rotator means having at least two light receptiv sections, one of said sections including a transmissive medium and the other of sai sections including optical rotating material.

10. An optical isolator as claimed in claim 9, including two of said optical rotat means abutting said polariser means, respectively, and having further polariser mean disposed therebetween.

11. An optical isolator as claimed in claim 10, wherein said segmented reciproc rotator means abut the polariser means on which said ports are formed.

12. An optical isolator as claimed in claim 1, wherein the optical path length of sai optical rotators means is selected to determine said one of said isolator functions.

13. An optical isolator as claimed in claim 1, wherein said polariser means include a spatial walk-off polariser.

AMENDED CLAIMS

[received by the International Bureau on 10 June 1994 ( 10.06.94 ) ; orig inal claims 2 and 12 cancel led ; original claims 1 , 3-9 amended ; new cl aims 14-20 added ; other claims unchanged (3 pages ) ]

1. (Amended) An optical isolator comprising two polariser means, two input/output ports formed respectively on said polariser means, and optical rotator means disposed between said polariser means, said optical rotator means including Faraday rotator means and having wavelength dispersion characteristics selected such that respective ones of a plurality of functions are performed on light of at least two different wavelength bands propagating, in use, through said isolator.

2. (Deleted)

3. (Amended) An optical isolator as claimed in claim 1, wherein said functions include isolating light in one of said wavelength bands and allowing light in the other one of said wavelength bands to pass between said ports in forward and reverse directions.

4. (Amended) An optical isolator as claimed in claim 1, wherein said functions include isolating light in both of said wavelength bands.

5. (Amended) An optical isolator as claimed in claim 1, wherein said functions include isolating light in one of said wavelength bands and partially isolating light in the other one of said wavelength bands.

6. (Amended) An optical isolator as claimed in claim 1, wherein said functions include isolating light in one of said wavelength bands in a forward direction and isolating light in the other one of said wavelength bands in a reverse direction.

7. (Amended) An optical isolator as claimed in claim 1, 14 or 15, wherein said optical rotator means further includes reciprocal rotator means.

8. (Amended) An optical isolator as claimed in claim 1, 14 or 15, which is substantially tolerant to errors induced by said Faraday rotator means.

9. (Amended) An optical isolator as claimed in claim 1, 14 or 15, wherein said optical rotator means further includes segmented reciprocal rotator means having at least two light receptive sections, one of said sections including a transmissive medium and the other of said sections including optical rotating material.

10. An optical isolator as claimed in claim 9, including two of said optical rotator means abutting said polariser means, respectively, and having further polariser means disposed therebetween.

11. An optical isolator as claimed in claim 10, wherein said segmented reciprocal rotator means abut the polariser means on which said ports are formed.

12. (Deleted)

13. (Amended) An optical isolator as claimed in claim 1, 14 or 15, wherein said polariser means includes a spatial walk-off polariser.

14. (New) An optical isolator comprising two polariser means, two input/output ports formed respectively on said polariser means, and optical rotator means disposed between said polariser means, said optical rotator means including Faraday rotator means and having wavelength dispersion characteristics such that said isolator performs different respective functions on light of at least two different wavelength bands propagating, in use, through said isolator.

15. (New) An optical isolator comprising two polariser means, two input/output ports formed respectively on said polariser means, and optical rotator means disposed between said polariser means, said optical rotator means including Faraday rotator means and having wavelength dispersion characteristics such that said isolator performs respective functions on light of at least two different and separate wavelength bands propagating, in use, through said isolator.

16. (New) An optical isolator as claimed in claim 14, wherein light in one of said wavelength bands is isolated and light in the other one of said wavelength bands is passed between said ports in forward and reverse directions.

17. (New) An optical isolator as claimed in claim 15, wherein light in both of said wavelength bands is isolated.

18. (New) An optical isolator as claimed in claim 14, wherein light in one of said wavelength bands is isolated and light in the other one of said wavelength bands is partially isolated.

19. (New) An optical isolator as claimed in claim 14, wherein light in one of said wavelength bands is isolated in a forward direction and light in the other one of said wavelength bands is isolated in a reverse direction.

20. (New) An optical isolator as claimed in claim 1, 14 or 15, wherein the optical path length of said optical rotator means is selected to provide said wavelength dispersion characteristics.