JPH04116616A - Optical isolator device - Google Patents

Abstract

PURPOSE:To make the length of an expensive polarization separating element short and to obtain a compact and inexpensive optical fiber type optical isolator without depending on polarization by inserting the polarization separating element between an optical fiber and a lens. CONSTITUTION:This device is constituted by arranging the optical fiber for incidence f1, the 1st polarization separating element 31, the 1st lens for collimating L1, a Faraday rotating element R1, the 2nd lens for focusing L2, the 2nd polarization separating elements B2 and B3, and the optical fiber for receiving light f2 in this order. The light which passes through the 1st element B1 from the optical fiber f1 and is emitted is converted into an almost collimated beam by the 1st lens L1 and further the collimated beam which passes through the element R1 is converted focusing beam by the 2nd lens L2 so as to be coupled on the fiber f2 through the elements B2 and B3. Thus, thin polarization separating elements B1 - B3 are applied and the optical isolator without depending on the polarization, whose cost is made low and which is miniaturized, is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to an optical fiber type optical isolator device with no polarization dependence used in optical communications etc. "Prior Art" FIG. An example of the configuration of a conventional polarization-independent one-optical fiber type router is shown below. 1° l, "2 is the tip fiber Ll, L2 is the collimating or focusing lens, R1
is a Faraday rotation element in which a saturation magnetic field is applied to a 710 crystal, B1. B2 and B3 are polarization separation elements composed of parallel flat plates of l-axis birefringent crystal.

The operation of this optical isolator will be explained using FIGS. 4(a) and 4(b).

First, the forward direction in which light passes will be explained using (a). The light from the optical fiber fl is converted into a collimated beam by the lens Ll, and the polarization separation element B1 separates the polarization components X, (ordinary light that travels in a straight line), Y (the extraordinary light that travels in a straight line), which are orthogonal to the polarization direction, respectively. (with parallel moving component)
The beam is separated into two parallel collimated beams.

In this example, polarization separation element B! It is assumed that the optical axis of is within the plane of the paper in the figure. At this time, only the component (Yl) parallel to the plane of the paper becomes extraordinary light and its optical axis is moved parallel within the plane of the paper. Note that the separation width d is proportional to the product of the length of B1 and the birefringent fold (in other words, d = k BL). Next, the polarized wave component x1 and the polarized wave component Y1 pass through the Faraday element R1, and are rotated by 45° in the polarization direction (clockwise in the figure).
After that, it enters the [stock] wave separation elements B 2 and B 3 on the multiplexing side. Several examples have been proposed for the combination of polarization separation elements B2 and B3, and one example will be shown here.

Polarization separation element B2. The thickness of B3 is 1 of the polarization separation element B1.
/, ff. First, the plane including the optical axis of the polarization separation element B2 is aligned with the polarization direction of the component Y1, which is a rotated 45° polarization. That is, the optical axis of the polarization separation element B2 is rotated by 45° clockwise around the optical axis along which light travels relative to that of the polarization separation element Bl. As a result, Yl also becomes extraordinary light for polarization separation element B2, and after passing through this polarization separation element B2, the short side of the right isosceles triangle (the long side is separated by polarization separation element Bl) as shown in the figure. It moves by I/VT of the previous separation width d along the direction of movement) to reach the position of the perpendicular vertex. On the other hand, for polarization separation element B3, rotate the plane of the optical axis by ±90 degrees with respect to polarization separation element B2, and align it with the polarization direction of component X, which has gone straight through polarization separation element B2. Since it becomes ×1 or extraordinary light, after polarization separation element B3 is directed, only Xl moves as shown in the figure and matches the position of component Y. That is, polarization separation element B 2 ,
By combining B 3 , the two polarized components X , , Y , completely match in position and direction and form one collimated beam. This beam is coupled to an optical fiber r2 by a condensing lens L2. Based on the above operating principle, it can be seen that there is no polarization dependence at all in the forward direction, and there is no fundamental transmission loss.

Next, the characteristics in the reverse direction will be explained in (b). The light returning through the optical fiber r2 is collimated by the lens L2 and passes through the polarization separation elements B 3 and B 2 .

As can be understood from the above description, the passed light is separated into polarization components Xx and Yt having exactly the same polarization direction and optical axis position as the forward direction components X, , Y. Next, X t
, Yt passes through the Faraday element R1, due to the characteristic of Faraday rotation that the rotation direction does not depend on the incident direction of light, the polarization direction is further rotated by 45 degrees clockwise and becomes orthogonal to the polarization direction of XYl, respectively. . That is, at the time when the backward component enters the polarization separation element Bl, the extraordinary light is incident at the x1 position (straight forward position) of the ordinary light component in the forward direction, and the position Y (movement) of the extraordinary light component is (where it is), ordinary light will be incident. Therefore, X, , Y.

When the light passes through the polarization separation element B1, the light returns to the position shifted up and down by d from the optical axis in the forward direction, as shown in the figure. A collimated beam with an axis deviation from the optical axis is focused by a lens 1 to a single point corresponding to the core of the optical fiber fl, but if the focal length of the lens is f and the amount of axis deviation is d, then the angle θ = ld. A shift occurs, and if θ is sufficiently large, there will be a large loss due to coupling with the optical fiber. That is,
It is possible to construct an optical fiber type optical isolator that has low loss in the forward direction and large loss in the reverse direction and has no polarization dependence.

As another configuration example, a polarization separation element Bl of the same thickness,
It is preferable to use a 1/2 wavelength plate or an optical rotator capable of rotating the polarization direction by ±45° as B3 and the polarization separation element B2.

In this case, polarization separation element B1. It differs from the above example in that it includes the optical axis of B3 and the optical axis (they are arranged coincident with each other or rotated by 180°).

"Problem to be solved by the invention" Here, we will explain the conditions and problems for obtaining sufficient isolation with the configuration of the optical isolator described in F.
Excluding the performance of 1, the axis deviation amount d determined by the length of the polarization separation element Bl and the radius of the collimated beam We
It depends on the ratio. 60d coupling of return light to fiber
In order to keep the value below B, the value of d/Wc should theoretically be about 4 or more. Normally, the beam radius Wc of a collimated beam with a small spread due to diffraction is required to be 250 μm or more, so the separation width must be d≧1 mm. By the way, even if rutile or calcite crystals, which have large birefringence and can obtain a large d, are used, d is 0.1 L (L=length of the crystal). From this, it can be seen that the conventional configuration shown in FIG. 3 requires a plurality of expensive rutile or calcite crystals with a length longer than TelCl as a polarization separation element.

From the above, the conventional tip isolator requires an expensive and large birefringent crystal, which makes the optical isolator extremely expensive, and the polarization separation element B1. B2. B
The disadvantage is that it is difficult to downsize because the total length of 3 is required to be 2.4 cm or more.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical isolator that is free from polarization dependence and allows the application of a thin polarization separation element, which can be reduced in price and size.

"Means for Solving the Problem" In order to achieve the above object, the invention of claim 1 provides an input optical fiber and a flat first optical fiber that shifts orthogonal polarized waves in parallel.
a polarization separation element, a first lens for collimating, a Faraday rotation element that rotates the polarization direction by 45 degrees, a second lens for focusing, and a flat plate-shaped device that combines two polarized waves shifted in parallel. A second polarized wave splitting element and a light receiving optical fiber are arranged in this order, and the light is emitted from the F input destination fiber through the first polarized wave splitting element. The first lens converts the collimated beam into a generally collimated beam, and the second lens converts the collimated beam that has passed through the Faraday rotation element into a focused beam, which is then sent to a light-receiving fiber via a second polarization separation element. This is the one I made to combine it with .

Further, the invention of claim 2 provides, as the second polarization separation element, two separation elements each having a deviation amount of the optical axis due to the polarization equal to I/ff of the deviation amount of the first polarization separation element. or a wavelength plate or optical rotator that rotates the polarization direction by 45 degrees, and the amount of deviation of the optical axis due to polarization is equal to that of the first polarization separation element. It is constructed by combining a separation element.

Further, the invention according to claim 3 provides the first isolator according to claim 1 or 2, in which the input optical fiber is arranged at approximately the third point! The distance between the principal surfaces of the first lens and the second lens is approximately equal to the sum of their focal lengths, and the projection q= approximately the second fiber
C is placed at the position of the image space point plane of the lens.
).

``Function J'' The first isolator of the present invention has a structure in which a first polarization separation element is inserted between the first fiber and the lens.The beam emitted from the optical fiber is first polarized by the first polarization. The ordinary light component and the extraordinary light component are separated into two parallel beams by the wave separation element, collimated by the first lens, and the positional deviation between the polarized waves is converted into an angular deviation, and then the Faraday rotation element R1 The polarization direction of each component is rotated by 45 degrees, and the collimated beam is further converted by the second lens into a beam that is focused on the receiving optical fiber, but at this time, the angular deviation between the two polarized waves becomes a positional deviation again. After conversion, these focused beams are subjected to a positional shift by a second polarization separation element to become a single beam with no angle or positional shift, and therefore both polarized waves are coupled to a receiving optical fiber.

In addition, as a second polarization separation element, two separation elements are arranged so that the deviation of the optical axis due to polarization is 1/n of the deviation amount of the first polarization separation element, and the deviation direction of each is orthogonal. It can be constructed with a combination of a wavelength plate (d), which rotates the polarization direction by 45 degrees, and a first polarization splitter, which rotates the polarization direction by 45 degrees. By combining the element with an equal separation element, the two polarized components completely match in both position and direction, forming one collimated beam, and the beam is coupled to the light-receiving optical fiber by the second lens.

Further, the input optical fiber is arranged approximately at the position of the object space focal plane of the first lens, and the distance between the principal surfaces of the first lens and the second lens is arranged at a position approximately equal to the sum of the focal lengths of both lenses. By arranging the output optical fiber approximately at the image space focal plane of the second lens, the two polarized waves are generated when the two lenses undergo position-angle transformation and angle-position transformation. This eliminates the angular deviation at the position of the optical fiber.

[Example 1] The present invention will be described in detail below. FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a diagram for explaining its operation.

As shown in FIG. 1, the optical isolator of this embodiment consists of an input optical fiber "I", a flat plate-shaped first polarization needle "M" which shifts orthogonal polarized waves in parallel, and a collimating optical fiber "I". A first lens L!, a Faraday rotation element R1 that rotates the polarization direction by 45 degrees, and a second lens L2 for focusing.
A second polarization separation element having a flat plate shape that combines two polarized waves shifted in parallel with each other and a light receiving optical fiber r2 are arranged in that order. The second polarization separation element includes two separation elements B in which the amount of deviation of the optical axis due to polarization is l/JT of the amount of deviation of the first polarization separation element Bl.
2 and B3 are combined so that their respective deviation directions are perpendicular to each other, but the wavelength plate or optical rotator that rotates the polarization direction by 45 degrees and the optical axis deviation amount due to polarization are the first. The polarization separation element Bl may be combined with an equal separation element.

Figure 2 (a) and (b) show the operation of the isolator with the above configuration.
This is explained by:

First, the forward direction operation will be explained using (a). Using the same operating principle as the conventional method explained in Fig. 3, the beam emitted from the end fiber is separated by the polarization splitter Bl into a beam in which the ordinary light component X1 and the extraordinary light component Y1 are shifted in parallel by d. (However, since the separation width is small and the light emitted from the optical fiber is greatly spread by diffraction (this state is not shown), even if it passes through the polarization separation element B1, the X and Y The components are not spatially separated beams.Therefore, the beam of light collimated by lens Ll appears as almost one beam.However, the positional deviation d between orthogonal polarized waves is equal to the positional deviation d after collimation. The polarization direction of each of the X3 and Y components is rotated by 45 degrees by the Faraday rotation element R1.Furthermore, the collimation is performed by the lens L2. The beam is converted into a beam that is focused on the destination fiber (not shown). At this time, the angular deviation θ between both polarizations is converted into m and positional deviation d. These focused beams are separated by polarization separation elements B2 and B3. The beam undergoes the same positional shift as in the conventional case, and becomes a single beam with no angle or positional shift. Therefore, both polarized waves are coupled to the optical fiber "2." Here, the focal lengths of lenses L1 and L2 are M
r1. r! Equally 1. By setting it as , it is possible to achieve low-loss coupling between the input side and output side optical fibers having the same mode field diameter.

In the above explanation of the operation, the occurrence of angular deviation at the position of the optical fiber, which occurs when two polarized waves undergo position-angle conversion and angle-position conversion at the lens Ll and the lens L2, has been ignored. However, this can be completely reduced to zero by adopting a confocal system in which the distance between the lenses Ll and L2 is equal to the sum of the two focal lengths.

Figure 2 is a special case.

Here, the influence of deviation from the above confocal system will be considered. The deviation from the confocal system of the interval is Δ, and the lens L1. L2
When the focal length of is f, the angular shift Δθ between the two polarized waves at the point where they are incident on the optical fiber is given by the following equation.

ΔOγ (d/l') (Δ/") ・
(]) Then, [1 is the polarization separation element B]. As a result, the angular deviation Δθ can be increased in proportion to Δ, or the increase in coupling loss can be reduced by decreasing d. For example, the coupling efficiency η (Δθ) for Δθ is given by the following equation.

η(Δθ) = exp knee (π・ω・Δθ/λ)
″)= eXpp−(π+ (IJ ・d ・Δ/r2
λ)2)・・・(2) Here, ω is the mode field radius (−5
μm). Then, the focal length of the lens is f=2+n
When n+, separation width d=50 μm, and wavelength λ−1, 55 μm, the coupling efficiency η becomes a function of Δ and is expressed by the following equation.

η (Δ) = exp((Q, 127XΔ (I!m)
)')... (3) In this case, even if Δ=2mm or 1mm, the loss is small at 0.279dB and 0.07dB, respectively, and the loss due to angular error due to deviation from the common point system can be sufficiently reduced.

Therefore, conventionally, before the beam emitted from the optical fiber is shifted and collimated, a polarization separation element is used to add a positional shift.
It was believed that because the two collimated beams are not parallel to each other, there would be a difference in the coupling efficiency between the two polarized waves, but by adopting a confocal system, this difference can be eliminated. It can also be seen that by reducing the separation width d, the deviation from the confocal position can be made relatively large.

Next, the operation characteristics in the reverse direction will be explained in (b). In this case as well, the operation of the polarization separation element is the same as in the conventional system shown in FIG. However, since the polarization separation element is inserted between the lens and the end fiber, the manner in which the two polarized waves in opposite directions are focused onto the optical fiber is different. That is, the Xt and Yv polarized waves in the opposite directions are transmitted through the lens L2 after receiving the positional shift d.
When collimated by , it is converted into an angular deviation θ, and the lens L! When it is focused at , it is returned to the positional shift d and enters the polarization separation element Bl. At this time, the relationship between the ordinary light and the extraordinary light is reversed, so at the position where the light is focused on the optical fiber, X t and Y t are softened by d in the vertical direction from the optical axis in the forward direction. Also, if it is close to a multi-1G focal system, the angle between Ct and Yz will not occur.

On the other hand, in the conventional case, there is no positional shift at the point where the light enters the optical fiber, and isolation is obtained by using only the angular shift.

Next, the necessary positional deviation ld will be mentioned.

Letting the mode field premature birth of the optical fiber be ω, the coupling efficiency η with respect to the positional deviation Id is given by the following equation.

y7 (d) = exp(-(d/ω)”3
- (4) From this, if the ratio of the positional deviation amount d and the mode field radius ω is d/ω≧4, theoretically 6
Isolation of 9dB or more can be obtained. Also, even if we assume that this ratio is 5 or more considering the safety factor, ω or 5μ
Since the polarization separation width d is as small as m, an extremely small value of 25 μm is sufficient for the separation width d of the polarization separation element.

As a result, the thickness of the separation element can be made extremely thin, approximately 250 μl. In contrast, in the past, a prism was inserted into a collimated beam with a large ω, which essentially required a beam with a large separation width.

"Effects of the Invention" As explained in detail above, according to the present invention, the polarization separation element is inserted between the tip fiber and the lens, so the length of the expensive polarization separation element can be significantly reduced. Can be made small,
Therefore, it is possible to realize a polarization-independent optical fiber optical isolator that is sufficiently compact and significantly cheaper than the conventional one, which is an excellent effect.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention, and FIG. 2 (A). (B) is a diagram for explaining its operation, FIG. 3 is a block diagram of a conventional optical isolator, and FIGS. 4 (A) and (B) are diagrams for explaining its operation.

Claims (3)

[Claims] (1) An input optical fiber, a flat plate-shaped first polarization separation element that shifts orthogonal polarized waves in parallel, and a first collimating optical fiber.
a Faraday rotation element that rotates the direction of polarization by 45 degrees, a second lens for focusing, a second polarization separation element in the form of a plate that combines two polarized waves shifted in parallel, and a second polarization separation element for light reception. and the optical fibers are arranged in this order, and the light emitted from the input optical fiber after passing through the first polarization separation element is roughly converted into a collimated beam by the first lens, and the light is converted into a collimated beam by the above Faraday rotation. An optical isolator device characterized in that the collimated beam that has passed through the element is converted into a focused beam by the second lens, which is passed through a second polarization separation element and coupled to a light-receiving fiber. (2) As the second polarization separation element, two separation elements whose deviation directions of the optical axes due to polarization are 1/√2 of the deviation of the first polarization separation element are orthogonal to each other. or a combination of a wavelength plate or optical rotator that rotates the polarization direction by 45 degrees and a separation element whose optical axis shift amount due to polarization is equal to that of the first polarization separation element. The optical isolator device according to claim 1. (3) Arrange the input optical fiber at approximately the position of the object space focal plane of the first lens, and set the distance between the principal surfaces of the first lens and the second lens to approximately the sum of their focal lengths. place,
Claim 1 or 2, wherein the output optical fiber is disposed approximately at the focal plane of the image space of the second lens.
The optical isolator device described in .