GB2313675A

GB2313675A - Optical isolator

Info

Publication number: GB2313675A
Application number: GB9710814A
Authority: GB
Inventors: Ki-Seuk Seung; Jae-Chun Yun; Je-Se Yeon; Young-Jin Song
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1996-05-27
Filing date: 1997-05-23
Publication date: 1997-12-03
Anticipated expiration: 2017-05-23
Also published as: JPH1054957A; GB9710814D0; DE19721959A1; GB2313675B

Description

2313675 OPTICAL ISOLATOR The present invention relates to an optical

apparatus using an optical fibre, and more particularly, to an isolator installed between two optical fibres which allows a ray of light travelling in a forward direction to pass through it while blocking a ray of light travelling in a reverse direction.

Known optical isolators use a polarization mode of light to block a ray io of light from passing in a reverse direction.

Referring to Figure 1, a conventional optical isolator using the polarization mode is located along an optical path between a first optical fibre 18 and a second optical fibre 19, and allows a ray of light in a forward direction from the first optical fibre 18 to the second optical fibre 19 to pass 13 through the isolator while blocking a ray of light travelling in the reverse direction.

The optical isolator includes first and second glass ferrules 11 and 17 into which the ends of the first and second optical fibres 18 and 19 are inserted. Light emitted from the first optical fibre 18 becomes a parallel beam as it passes through a first GRIN (graded index) lens 12. The parallel beam travels toward the second optical fibre 19 via a first polarizer 13 comprising a wedge shaped birefringent crystal, a Faraday rotator 14, a second polarizer 1-5 and a second GRIN lens 16.

As shown in Figure 2A, as it passes through the first polarizer 13, a forward directional ray 20 splits into two rays of light, an ordinary ray 21 and an extraordinary ray 22, due to the birefringence of the first polarizer 13. The ordinary ray 21 is refracted according to a normal refractive index n, of the first polarizer 13 and is polarized in a direction parallel to the crystal's optical axis (not shown). The extraordinary ray 22 is refracted according to an extraordinary refractive index n. of the first polarizer 13 and is polarized in a direction perpendicular to the crystal's optical axis.

The polarization directions of each of the ordinary ray 21 and the extraordinary ray 22 passing through the first polarizer 13 are rotated at 450 by the Faraday rotator 14. Next, the ordinary ray 21 and the extraordinary ray 22 are refracted and become parallel beams as they pass through the second polarizer 15. The second polarizer 15 has a wedge shaped birefringent crystal like the first polarizer 13, and the crystal's optical axis (not shown) is oriented at 450 with respect to the optical axis of the first polarizer 13 in the direction in which the ray is rotated by the Faraday rotator 14. Accordingly, the ordinary ray 21 and the extraordinary ray 22 passing through the first polarizer 13 continue as an ordinary ray 21' and an extraordinary ray 221 in io the second polarizer 15, respectively.

An ordinary ray 21" and an extraordinary ray W' output from the second polarizer 15 have the same output angle 0 with respect to an output surface of the second polarizer 15. That is, the ordinary ray 21" and the extraordinary ray 2T' are parallel to each other and separated by a predetermined width S. The rays 21" and W' output from the second polarizer 15 are combined at the second GRIN lens 16 (see Figure 1) and converge into the end of the second optical fibre 19.

Referring to Figure 2B, a reverse directional ray 23 heading towards the first optical fibre 18 from the second optical fibre 19 is split into two rays of light, an ordinary ray 24 and an extraordinary ray 25, due to birefringence of the second polarizer 15. The ordinary ray 24 is refracted according to an ordinary refractive index no of the second polarizer 15, and the extraordinary ray 25 is refracted according to an extraordinary refractive index n, of the second polarizer 15.

The reverse directional ordinary ray 24 and extraordinary ray 25 are rotated 450 in the reverse direction with respect to the rotation direction of the forward directional ray 21 (see Figure 2A) as they pass through the Faraday rotator 14. Thus, the ordinary ray 24 passing through the Faraday rotator 14 has a polarization direction that is perpendicular to the first polarizer crystal's optical axis and changes into an extraordinary ray 24'.

Also, the extraordinary ray 25 passing through the Faraday rotator 14 has a polarization direction that is parallel to the optical axis and changes into an ordinary ray 25. Due to the changes to the two rays, the refractive angles of each of the two rays 2C and 2Y' passing through the first polarizer 13 become different from each other by 0:FAO so that the two rays do not become parallel to each other. The two rays 2C and 2Y1 are condensed by 3 the first GRIN lens 12 (see Figure 1) and the focal point thereof is not positioned at an input end of the first optical fibre 18. Therefore, the reverse directional ray 23 is blocked.

In a conventional optical isolator having such a structure, the ordinary ray 21" (see Figure 2A) and the extraordinary ray 22" split as they pass io through the first and second polarizers 13 and 15 and travel parallel to each other toward the second optical fibre 19. Here, an optical path difference between the ordinary ray W' and the extraordinary ray 2T' is defined as work-off. Thus, when the two rays 21" and 221' are combined and condensed by the second GRIN lens 16 (see Figure 1), a time delay is generated between the two rays 21" and 2T' due to the work-off so that polarization mode dispersion is generated.

To address this problem, an optical isolator has been disclosed in U.S. patent No. 5,557,692 entitled "Optical Isolator with Low Polarization Mode Dispersion". The disclosed optical isolator reduces work-off by further including a birefringent plate along the optical path of the optical isolator shown in Figures 1, 2A and 2B. However, this increases the number of parts required for the optical isolator.

The present invention seeks to provide an improved optical isolator without the need for additional optical components. According to an aspect of the present invention, there is provided an optical isolator to be disposed along an optical path between first and second waveguides, comprising: a first polariser configured to split a substantially parallel light beam, emitted from the first waveguide and travelling in a forward direction along the optical path, into an ordinary ray and an extraordinary ray; a second polariser configured to convert the ordinary ray into an extraordinary ray and the extraordinary ray into an ordinary ray; and a Faraday rotator disposed in the optical path between the first and second polarisers.

In an optical isolator according to the invention, the thickness of the polarisers may be arranged so as to compensate for the difference between the optical path lengths of the ordinary and extraordinary rays.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a diagram schematically illustrating a conventional optical isolator; Figure 2A is a diagram showing the path of a forward directional ray io which passes sequentially through the first polarizer, the Faraday rotator and the second polarizer shown in Figure 1; Figure 2B is a diagram showing the path of a reverse directional ray which passes sequentially through the second polarizer, the Faraday rotator and the first polarizer shown in Figure 1; Figure 3 is a view showing the optical configuration of an optical isolator according to.a first embodiment of the present invention; Figure 4 is a perspective view of the first polarizer, the Faraday rotator and the second polarizer according to the first embodiment of the present invention; Figure 5 is a diagram showing the path of a forward directional ray which passes sequentially through the first polarizer, the Faraday rotator and the second polarizer shown in Figure 4; Figure 6 is a diagram showing the path of a reverse directional ray which passes sequentially through the second polarizer, the Faraday rotator and the first polarizer shown in Figure 4; Figure 7 is a view illustrating the optical configuration of an optical isolator according to a second embodiment of the present invention; Figure 8 is a view illustrating the optical configuration of an optical isolator according to a third embodiment of the present invention; and Figure 9 is a view illustrating the optical configuration of an optical isolator according to.a fourth embodiment of the present invention.

Referring to Figure 3, an optical isolator according to a first embodiment of the present invention is located along an optical path between a first optical fibre 30 and a second optical fibre 40, and comprises a first GRIbJ lens 110, a first polarizer 120, a Faraday rotator 130, a second polarizer 140 and a second GRIN lens 150, sequentially disposed from the first optical fibre 30 toward the second optical fibre 40. It is preferable that each end of the first and second optical fibres 30 and 40 are fixed and aligned by first and second glass ferrules 35 and 45. The first GRIN lens 110 converts a divergent ray emitted from the first optical fibre 30 into a parallel beam. The first io polarizer 120 has a birefringent crystal wedge shape and splits a forward directional incident ray 50 into first and second rays 51 and 52. The first ray 51 is an ordinary ray which is refracted according to a normal refractive index no of the first polarizer 120. The second ray 52 is an extraordinary ray which is refracted according to an extraordinary refractive index n. of the first polarizer 120.

The crystal optical axis 1201 of the first polarizer 120 is at an angle of 22.51 with respect to the Y-axis on the Y-Z plane, as shown in Figure 4. The first ray 51 is polarized in the direction indicated by D., parallel to the crystal optical axis 120', whereas the second ray 52 is polarized in a direction indicated by D., perpendicular to the crystal optical axis 120'.

The second polarizer 140 is of a birefringent crystal wedge shape like the first polarizer 120, and its crystal optical axis 140' is at an angle of -22.511 with respect to the y-axis on the Y-Z plane. That is, the crystal optical axis 140' of the second polarizer 140 is angled at 45 with respect to the crystal 2.s optical axis 120' of the first polarizer 120 in the opposite direction to a polarization rotation direction of the Faraday rotator 130.

The Faraday rotator 130 rotates the incident rays 51 and 52 by 45.

The polarization direction D. of the first ray 51 passing through the first polarizer 120 is rotated by the Faraday rotator 130 by 450 and thus changes into a polarization direction indicated by D.'. The changed polarization direction Do' of the first ray 51 is perpendicular to the crystal optical axis 140' of the second polarizer 140 so that the first ray 51 becomes an extraordinary ray as it passes through the second poladzer 140. Also, the polarization direction D, of the second ray 52 after passing through the first polarizer 120 is rotated 450 by the Faraday rotator 130 and thus changes into a polarization direction indicated by D,'. The changed polarization direction D.' of the second ray 52 is parallel to the crystal optical axis 140' of the second polarizer 140 so that the second ray 52 becomes an ordinary ray as it passes through the second polarizer 140.

The shapes of the first and second polarizers 120 and 140 are symmetrical with respect to the Faraday rotator 130, and the respective first io and second polarizers 120 and 140 have a wedge shape in which the bottom surfaces are wider than the top surfaces.

Figure 5 shows a path of the forward directional ray 50 passing through the first and second polarizers 120 and 140 and the Faraday rotator 130. When the ray 50 travelling parallel to the X-axis is input to the first polarizer 120, the angle of incidence Oi of the ray 50 is the same as an inclination angle 0, of a first surface 121 of the first polarizer 120.

The ray 50 passing through the first surface 121 is split into a first ray 51 and a second ray 52 due to the birefringence of the first polarizer 120. The first ray 51 is an ordinary ray refracted according to the normal refractive index n. of the first polarizer 120, and the second ray 52 is an extraordinary ray refracted according to the extraordinary refractive index n, of the first polarizer 120.

The exit angles 01 and 01' of the first ray 51 and the second ray 52 with respect to the first surface 121 are as follows according to Snell's law.

61 sin-' (r sin 0) (1) no sin-' (r sin 0). (2) n.

-7where ni, is the refractive index of air.

Thus, the incident angles of the first and second rays 51 and 52 with respect to a second surface 122 of the first polarizer 120 are 01-01 and 01-011, respectively. The exit angles02and 02' of the first and second rays 51 and 52 5 at the second surface 122 are as follows.

02 = sin-' [ no sin (Q] (3) nair 02' = sin-' [ ne sin 01 Ifl (4) Also, the exit angles 03 and 03' of the first ray 51 and the second ray 52 with respect to a third surface 131 are as follows.

03 = sin-' (-a-r sin 02) (5) nf 03' = sin-' (r sin 621) (6) nf wherein the refractive index rif of the Faraday rotator 130 is the same for both rays 51 and 52.

Also, the exit angles 04and 04' of the first ray 51 and the second ray 52 15 at a fourth surface 132 are as follows.

04 = sin-' (---1sin (7) nai, = sin-' (---1sin 0.1) (8) nair Here, it is noted that the exit angle 0, in Equation (7) is the same as the exit angle 02 in Equation (5), and the exit angle 0,' in Equation (8) is the same as the exit angle 02' in Equation (6).

The exit angles 0, and 0,' of the first ray 51 and the second ray 52 at a 5 fifth surface 141 are as follows.

= sin-' ( sin (9) n.

es, = sin-' ( n a'. '- sin 041 (10) n.

Here, the first ray 51 is an extraordinary ray of which a refraction angle is determined according to the directions of the crystal optical axes 1201 io and 140', the rotation direction of the Faraday rotator 130 and the extraordinary refractive index n. of the second polarizer 140. Meanwhile, the second ray 52 is an ordinary ray of which a refraction angle is determined according to the directions of the crystal optical axes 120' and 140', the rotation direction of the Faraday rotator 130 and the ordinary refractive index n,, of the second polarizer 140.

The incident angles of the first and second rays 51 and 52 at a sixth -9surface 142 are 05 + 02 and 05' + 152, respectively. Here, 02 is an angle of inclination at the sixth surface 142 which is the same as the inclination angle 01 at the first surface 121.

The exit angles 06 and 06' of the first and second rays 51 and 52 at the sixth surface 142 are as follows.

06 = sin-' sin (05 + n.i.- 061 = sin-' n 0 sin (0,,' + 02)l (12) Here, with ni, = 1, 0; - 4o,, 01 02 - C, n. = 2.45 and n. - 2.709 applied to the above equations, the exit angles 0, and 0,' of the first and second rays 51 lo and 52 at the sixth surface 142 are - 16.8481' and - 16.842% respectively. It is noted that the first and second rays 51 and 52 passing through the second polarizer 140 are nearly parallel to each other.

Also, displacements h and h' of the first and second rays 51 and 52 along the Z-axis are as follows.

h=t,tanO, +t2tane2+ttano3+t4tan84+ttanO,, (13) hl=t,tane'+t2tane'+t,tan()3'+t4tane'+t,,tane' (14) 1 2 4 5 Here, t, represents the thickness of the first polarizer 120 on the X- - 10axis; t, represents the gap between the first polarizer 120 and the Faraday rotator 130; t3 represents the thickness of the Faraday rotator 130; t, represents the gap between the Faraday rotator 130 and the second polarizer 140; and t, and ts' are thicknesses of the second polarizer 140 at each place 5 where the first and second rays 51 and 52 are emitted. That is, t,-T,-(H+tltanOl+t2tanO2+ t3tan.03 + t4tanO4)tanO2 t5f = T5 - (H + tItan01' + t2tan02' + t3tan03' + t4tanO,')tanO2 Here, T, represents the maximum thickness of the second polarizer 140 and H represents the height from the bottom surface of the first polarizer 140 to an incident point of the ray 50. t, is represented by T,-Htanoo, and TI represents the maximum thickness of the first polaxizer 120.

The path lengths 1 and 1' of the first ray 51 and the second ray 52 between the first surface 121 and the second surface 142 can be represented as follows.

no ti - + nair t2 + nf t3 + nair t4 + n.ts cos (4b, -, 61) cos 02 cos 03 cos 04 cos 05 (15) 1 n. t, -- + n t2 + nf h + n.,,. t4 + n. ts 85f cos 41 - 011 COS 02' cos 031 COS 04' cos (16) Assuming that values L and M which are determined according to tI, tb t3 and t4 are as follows:

0 + nair t2 + nf + nair - t4 cos (I$1 - 01) COS02 cos 04 M = n. t, - + n,,,, k + nf t3 + t4 co's 01) cos 0,2 -is -0 13 1 COS()4 lo Equations 15 and 16 are represented as follows.

1 L + n. t5 (17) cos EN 1 M + %_ t ' 5 o (18) cos 5 - 12The difference in the path length between the first ray 51 and the second ray 52, i.e., Al=14', can be represented as follows from Equations 17 and 18.

W=(L-M) +n tt _n ts (19) e 0 cos 65 cos E) 5 Here, AI is the value for determining a polarization mode dispersion and the value becomes preferably the minimum value which satisfies the following equation.

When L -Mk:O, n. t5 -no tL -< 0 (20) COS05 COSEN, When L-M-<O, n. ts - no > 0 (21) cos 05 cos 65, That is, the polarization mode dispersion of the optical isolator can be io compensated for by adjusting the length of a ray path by controlling the thickness of the second polarizer 140 which is a factor for determining t, and tst.

In the meantime, referring to Figure 6, when a ray 60 emitted from the second optical fibre 40 travels towards the first optical fibre 30, the first ray 61 which is an ordinary ray split in the second polarizer 140 becomes an ordinary ray after passing through the Faraday rotator 130 and the first polarizer 120 and the second ray 62 which is an extraordinary ray split in the second polarizer 140 becomes an extraordinary ray after passing through the 13 Faraday rotator 130 and the first polarizer 120, which is determined according to a polarization rotation direction of the Faraday rotator 130 and the crystal optical axis of each of the first and second polarizers 120 and 140. The exit angles of the first and second rays 61 and 62 heading for the first optical fibre 30 are different from each other, as will now be described in detail.

The ray 60 incident upon the sixth surface 142 is split into first and second rays 61 and 62 proceeding in different paths from each other due to the birefringent characteristic of the second polarizer 140. The first ray 61 is io an ordinary ray which is refracted according to the normal refractive index n. of the second polarizer 140 and the second ray 62 is an extraordinary ray refracted according to the extraordinary refractive index n. of the second polarizer 140.

When an incident angle of the ray 60 is vi, an exit angle (p, of the first ray 61 and the exit angle o21 of the second ray 62 at the sixth surface 142 are as follows according to Snell's law.

sin-' (a sin T,) (22) no (p,' sin-' ( nai, sin T,) (23) n.

Accordingly, the incident angles of the first and second rays 61 and 62 at the fifth surface 141 are 02-ol and,02-011, respectively. The exit angles of 2othe f2 and p2' of the first and second rays 61 and 62 at the fifth surface 141 are as follows.

(P2 sin-' 1 sin(02 - (P1)l (24) T2' sin-' sin(02 -01')] (25) nair The exit angles p, and o3' of the first and second rays 61 and 62 at the fourth surface 132 are as follows.

n % = sin-' sin (Q (26) nf n, (27) = sin-' aT sin (P2) nf Also, the exit angles p, and p4' of the first and second rays 61 and 62 at the third surface 131 are as follows.

T4 = sin-' (---1sin (P3) (28) nai, 94' = sin-' (-f- sin T3) (29) nair Here, it is noted that the exit angle p, is the same as the exit angle 'P2, - 15from Equation 26, and also the exit angle (p,' is the same as the exit angle p,,, from Equation 27.

Also, the exit angles os and o,' of the first and second rays 61 and 62 at the second surface 122 are as follows.

sin-' (lair sin T4) (30) no T51 sin-' (" sin P4') (31) n.

The incident angles of the first and second rays 61 and 62 at the first surface 121 are ps+O, and os' +01, respectively. Here, 401 is the inclination angle of the first surface 121, which is the same as the inclination angle 02 of the sixth surface 142..

The exit angles 06 and (P61 of the first and second rays 61 and 62 at the first surface are as follows.

sin-' sin (o,, + obl)] (32) nair sin-' n f sin(T,,' (33) nai, 16- To simplify Equation 32 with Equations 22, 24 and 30, o, is represented as follows.

% = sin-' [-o- sin 42 +(bl - T1)l (34) nair To simplify Equation 33 with Equations 23, 25 and 31, iP6' 'S represented as follows.

sin-' [ ne sin((b2 +01 -(P,l (35) nair Here, when (pi 16.84% 01=02= 4% n. = 2.45, n. = 2.709 and ni, = 1, the exit angles 'P6 and 06' of the first and second rays 61 and 62 at the first surface 121 are -2.964 and -5.047% respectively. Accordingly, it is noted that the first and second rays 61 and 62 passing through the second polaizer 140 are lo not parallel to each other. Consequently, the ray in a reverse direction is blocked.

An optical isolator according to a second embodiment of the present invention will be described with reference to Figure 7. Here, the same reference numerals indicate the same elements performing the same function.

It is a characteristic feature of the second embodiment of the present invention to have a first GRIN lens 110' installed at the end portion of the first optical fibre 30, and combined, by a first holder 38, with a first glass ferrule 35' for aligning the first optical fibre 30. Also, the incident surface of the first polarizer 120, i.e., the exit surface 112 of the first GRIN lens 110' -17facing the first surface 121, is angled to be parallel to the first surface 121. From the above structure, a ray from the first optical fibre 30 can be refracted to a desired position due to the difference between the refractive indices of the first GRIN lens 110' and air so that the first optical fibre 30 can be aligned parallel to the X-axis.

In the same manner, a second GRIN lens 150', installed at the end portion of the second optical fibre 40, is combined with a second glass ferrule 45 by a second holder 48 for aligning the second optical fibre 40. Also, the incident surface 151 of the second GRIN lens 150' is angled to be parallel to io the exit surface of the second polarizer 140, i.e., the sixth surface 142 so that the second optical fibre 40 can be aligned to the X-axis, as described earlier.

As described above, the optical arrangement of the first and second optical fibres 30 and 40 can be facilitated by improving the structure of the first and second GRIN lenses.

13 Referring to Figure 8, an optical isolator according to a third embodiment of the present invention will now be described in detail. Here, the same reference numerals represent the same elements performing the same function. The optical isolator according to the present embodiment further includes a prism 160 which is disposed along an optical path between the first GRIN lens 110 and the first polarizer 120 and facilitates the optical arrangement of the first optical fibre 30.

An incident surface 161 of the prism 160 facing the first GRIN lens is perpendicular to the X-axis, i.e., an incident ray axis, and an exit surface 162 is angled to be p arallel to the first surface 121, i.e., the incident surface of - 18 the first polarizer 120. Thus, the direction of the ray 50 emitted from the first optical fibre 30 can be changed by refracting the ray 50 so that an optical arrangement of the first optical fibre 30 is facilitated. Here, the incident surface 161 of the first prism 160 is not restricted to being perpendicular to the X-axis. The incident surface 161 can be formed to be angled within a critical refractive angle with respect to the incident ray 50.

Referring to Figure 9, an optical isolator according to a fourth embodiment of the present invention further comprises a prism 170 for facilitating the optical arrangement of the second optical fibre 40, which is io installed along an optical path between the second polarizer 140 and the second GRIN lens 150.

It is preferable that an incident surface 171 of the prism 170 is angled parallel to the sixth surface 142, i.e., the exit surface of the second polarizer 140. Thus, the optical arrangement of the second optical fibre 40 is facilitated since the direction of the ray passing through the second polarizer 140 can be changed by refracting the ray.

An optical isolator (not shown) according to another embodiment of the present invention can have both the prisms 160 and 170 shown in FIGS. 8 and 9.

As described above, by rotating the optical axis of the second polarizer in a direction opposite to the polarization rotation direction of the Faraday rotator 130 with respect to the optical axis of the first polarizer 120 of Figure 3, an ordinary ray passing through the first polarizer 120 changes into an extraordinary ray with respect to the forward directional ray 50, and vice - 19versa. Accordingly, the first ray 51 and the second ray 52 are emitted parallel to each other without additional optical parts, and also, work-off which causes polarization mode dispersion can be reduced by adjusting the difference in an optical path between the first ray 51 and the second ray 52.

Claims

-20Claims

1. An optical isolator to be disposed along an optical path between first and second waveguides, comprising:

a first polariser configured to split a substantially parallel light beam, emitted from the first waveguide and travelling in a forward direction along the optical path, into an ordinary ray and an extraordinary ray; a second polariser configured to convert the ordinary ray into an extraordinary ray and the extraordinary ray into an ordinary ray; and lo a Faraday rotator disposed in the optical path between the first and second polarisers.

2. An optical isolator according to claim 1, wherein the thickness of the polarisers is arranged so as to compensate for the difference between the optical path lengths of the ordinary and extraordinary rays.

3. An optical isolator to be disposed in an optical path between first and second waveguides, comprising: a first polariser for splitting a substantially parallel input beam travelling from the first waveguide in a forward direction along the optical path, into laterally dispersed rays with different polarisation directions, a Faraday rotator which rotates the polarisation directions of said rays, a second polariser which receives the rays from the Faraday rotator and provides laterally dispersed output rays which travel substantially parallel along the optical path, with different polarisations, for passage to the second -21waveguide, the first and second polarisers having polarising planes so configured that a parallel light beam entering the second polariser to travel in a reverse direction along the optical path leaves the first polariser in a divergent path, and the optical path lengths presented by the polarisers for the 3 rays with different polarisations travelling in the forward direction, are configured so as to minimise polarisation mode dispersion for the rays.

4. An optical isolator disposed along an optical path between a first optical fibre and a second optical fibre to transmit a ray emitted from said io first optical fibre to said second optical fibre and block a ray emitted from said second optical fibre according to a polarization mode of an incident ray, said optical isolator comprises: a first GRIN (graded index) lens for converting the ray emitted from said first optical fibre into a parallel beam; a first polarizer, arranged along said optical path, for allowing said parallel beam to he birefringent into a first ray which is an ordinary ray and a second ray which is an extraordinary ray depending on a crystal's optical axis thereof and to pass therethrough; a Faraday rotator, arranged along said optical path, for allowing each of said first and second rays passing through said first polarizer to rotate in one direction and pass therethrough; a second polarizer, arranged along said optical path and having a crystal's optical axis angled 45 in a direction reverse to a polarization rotation direction of said Faraday rotator, for converting said first ray passing -22through said Faraday rotator into an extraordinary ray and said second ray into an ordinary ray and emitting the respective converted rays; and a second GRIN lens, arranged along said optical path between said second polarizer and said second optical fibre, for converging said first and said second rays passing through said second polarizer into an end portion of said second optical fibre.

5. An optical isolator according to claim 4, wherein assuming values L and M which are determined according to a thickness t, of said first polarizer, io a gap t, between said first polarizer and said Faraday rotator, a thickness t3 of said Faraday rotator, and a gap t, between said Faraday rotator and said second polarizer, L = no ti - + nair t2 + nf t3 + nair t4 cos ((bl - 81) cos 02 cos 03 COS 84 and ti t2 t3 t4 M = ne.0. (4bl - + n + nf - + nair' - el,) air COS EY COS 83' COS ()4' when path lengths 1 and 1' of said first and said second rays between said first and said second polarizers, respectively, are represented as follows:

1 = L + n. t5 and 1' = M + no t5 cos 05 cos 051 and when the difference Al=141 of the path length between said first and said -23 second rays is represented as follows:

Al (L M) + n. ts ts cos 05 cos 6) when L M ?: 0, n ts ts- 0 0 1 cos E) 5 cos 05 and when t5 t5 L - M -< 0, n. WS 05 no > 0 cos 05 wherein n, and n are an ordinary refractive index and an extraordinary io index of said first and said second polarizers, respectively; n., represents the refractive index of air; nf represents the refractive index of said Faraday rotator; t, represents the thickness of said first polarizer at a place in which a ray is incident upon it; t2. represents the distance between said first polarizer and said Faraday rotator; t3 is the thickness of said Faraday rotator; t, 13 represents the distance between said Faraday rotator and said second polarizer; ts and ts' respectively represent thicknesses of said second polarizer at points at which said first ray and said second ray are emitted; 01 and 01' respectively represent exit angles of said first and said second rays at an incident surface of said first polarizer; 0, and 02f respectively represent exit angles of said first and said second rays at an exit surface of said first polarizer; 03 and 03' respectively represent exit angles of said first and said second rays at an incident surface of said Faraday rotator; 0, and 0,' respectively represent exit angles of said first -24and said second rays at an exit surface of said Faraday rotator; and 0, and 0,' respectively represent exit angles of said first and said second rays at an incident surface of said second polarizer.

6. An optical isolator according to claim 4 or 5, wherein said first polarizer and said second polarizer each have a birefringent crystal wedge shape having a bottom surface larger than a top surface and disposed symmetrical to each other with respect to said Faraday rotator.

io

7. An optical isolator according to any one of claims 4 to 6, wherein a ray exit surface of said first GRIN lens facing a ray incident surface of said first polarizer is parallel to a ray incident surface of said first polarizer.

8. An optical isolator according to any one of claims 4 to 7, further comprising: a first glass ferrule for aligning an end portion of said first optical fibre; and a first holder for combining said first GRIN lens with said first glass ferrule.

9. An optical isolator according to any one of claims 4 to 8, wherein a ray incident surface of said second GRIN lens facing a ray exit surface of said second polarizer is parallel to a ray exit surface of said second polarizer.

-2510. An optical isolator according to any one of claims 4 to 9, further comprising: a second glass ferrule for aligning an end portion of said second optical fibre; and a second holder for combining said second GRIN lens with said second glass ferrule.

11. An optical isolator according to any one of claims 4 to 10, further comprising:

a prism, arranged along an optical path between said first polarizer and said first GR1N lens, for converting a proceeding path of a ray by refracting a ray passing therethrough.

12. An optical isolator according to claim 11, wherein a ray exit surface of said prism facing a ray incident surface of said first polarizer is parallel to a ray incident surface of said first polarizer.

13. An optical isolator according to any one of claims 4 to 10, further comprising:

a prism, arranged along an optical path between said second polarizer and said second GRIN lens, for converting a proceeding.path of a ray by refracting a ray passing therethrough.

14. An optical isolator according to claim 13, wherein a ray incident -26surface of said prism facing a ray exit surface of said second polarizer is parallel to a ray exit surface of said second polarizer.

15. An optical isolator as hereinbefore described with reference to the 5 accompanying drawings.