JPS5929222A - Optical isolator - Google Patents

Abstract

PURPOSE:To obtain a practicable optical isolator, by arraying two kinds of Faraday elements which are opposite in rotatory polarization direction, are equal in the absolute value and temp. coefft. of optical rotatory power, have the Verdet's constant equal in both code and absolute value and have an equal length in the optical axis direction in the positions where the optical axes align to each other. CONSTITUTION:A Faraday element 5 is manufactured by pasting bismuth silicon oxide (BSO) or bismuth germanium oxide (BGO) 6 having left-hand optical rotatory power and BSO or BGO having right-hand optical rotary power of the same length in the positions where the optical axes are aligned to each other. A magnetic field H is applied in the optical axis direction of the element 5. The light from a light source 1 passes a polarizer 2 and is made into linearly polarized light which passes the element 5 and is negated of the rotation by the optical rotary power. Only the rotating angle by the Faraday effect is detected. Since the change of the optical rotary power with temp. is negated, the practicable optical isolator having excellent temp. stability is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical isolator using Faraday effect.

Optical isolator is the general name for an optical device that transmits light from one side to the other, but not in the parallel direction.
The Faraday effect is a phenomenon in which when light propagates through an optical material in which a magnetic field exists in the optical axis direction, the plane of polarization rotates in proportion to the strength of the magnetic field. Further, an element made of an optical element having such properties is called a Faraday element.

First, the Faraday effect will be explained with reference to FIG. Z
When a magnetic field of strength H exists in the axial direction, when polarized light parallel to the Z-axis passes through a Faraday element A of length t, the polarization direction is rotated by θ.

The rotation angle θ of the polarization direction is the length t of the Faraday element A,
Since it is proportional to the magnetic field H, it can be written as θ2Ve l )] (11. This proportionality constant Ve is called the Verdet constant. This light, which has passed through a Faraday element A of length t to which a magnetic field]
The strength of the magnetic field H and the length of the Faraday element 2 so that the angle is 5°
1, and the polarization plane of analyzer B is 45° with polarizer '.
When the light source 1 is rotated at an angle of (the direction of rotation is the same as θ), the light from the light source 1 reaches the photodetector 4 without being attenuated. On the other hand, the light reflected from the entrance surface of a certain optical system (for example, an optical fiber) passes through the analyzer in the opposite direction, and then undergoes the Faraday effect again, resulting in θ=4
Since the plane of polarization is rotated by 5 degrees, the plane of polarization is rotated by 90 degrees when compared to the original light exiting the polarizer 2, so it is completely stopped by the polarizer 2.

In other words, an optical device, an optical isolator, is constructed that transmits light from one direction to the other but not in the opposite direction.

Materials for the Faraday element constituting such an optical isolator include lead glass, glass containing a metal element such as Tb, and yttrium iron garnet (Y3Fe).
50.2 (hereinafter abbreviated as YIG), but each has drawbacks and has not been put into practical use. That is, although lead glass exhibits diamagnetic properties and is excellent in temperature stability, it has a small Faraday effect (O, D 93 min in 10 C-cm at 6 nm light).

On the other hand, glass containing metallic elements such as Tb has a Faraday effect that is approximately twice that of lead glass (approximately 0.2
4m1n/1Je-on), but it has the drawbacks of exhibiting paramagnetism and poor temperature stability.

In contrast, recently developed bismuth silicon oxite (BI 128 + 020, hereinafter referred to as BS○
), bismuth germanium oxide (Bi
□2GeO□ol (hereinafter abbreviated as BGO) has a Faraday effect twice that of lead glass (approximately 0.2 in 63 nm light).
rni n/○e ”, 77+) and exhibit diamagnetic properties, and the Faraday effect itself has good temperature stability, but it also has optical rotation power (the property of rotating the plane of polarized light even in the absence of a magnetic field). and its optical rotation power changes depending on the temperature (22 deg/1nrn for 635 nm light)
, it cannot be used as an optical isolator unless a means is provided to cancel the temperature dependence of the optical rotation power.Furthermore, the Faraday effect (15 min10 at a wavelength of 1.15 pm) has a high
e-1,m), but this material also has a strong optical rotation power (approximately 1
['30'/mouth] m ), and its optical rotation power shows temperature dependence, so it cannot be used as an optical inolator unless a countermeasure for the temperature dependence of the optical rotation power is elucidated.

As mentioned above, in terms of the thickness of the Faraday effect, BSO and BGOlYIG are suitable materials for optical isolators.
However, these materials all have the common drawback of having optical rotation power that varies as a function of temperature.

The object of the present invention is to utilize the advantages of these materials and eliminate their disadvantages to provide a practical optical isolator.

The present inventors have grown a large number of BSO and BGO single crystals (while growing these single crystals, dextrorotating BSO and BG
It was discovered that O, and BSO and BGO, which have levorotatory properties, exist. Here, a crystal whose plane of polarization rotates clockwise in the direction of propagation of light is called a dextrorotating crystal, and a crystal whose plane of polarization rotates counterclockwise is called a left-rotating crystal.

The present inventors actually grew bilaterally optically rotating BSO or BGO and measured their properties such as their Verdet constant, optically rotating power, and their temperature dependence.

The results are as follows.

(1) The Verdet constant is B501BGO with bilateral optical rotation.
were the same, and there was no temperature dependence of the Verdet constant.

In other words, if the magnetic field is the same, the direction of rotation of the plane of polarization is the same regardless of the direction of optical rotation.

(2) For BSO and BGO with left-handed optical rotation, if the optical power of right-handed rotation is expressed as positive and the optical power of left-handed rotation is expressed as negative, the absolute values of the optical powers are equal and the signs are opposite. Furthermore, although the optical rotation power has a temperature dependence, it was found that the above relationship holds true at any temperature.

Therefore, if the length of the crystal is e, the optical power per unit length is ρ0 (at the reference temperature), and the temperature coefficient of the optical power is k, then the rotation angle ψ of the polarization direction of the light propagating through the crystal is (1) ψB−p0e(
'l+ ΔT) + VeHe (2) (11) ψL−−ρ0e (1+ ΔT) for left-rotating BSO, BGO
+■eHe It is expressed by the formula (3). The present inventor believes that the relationships represented by equations (2) and (3) are B S
', 0. They discovered that this holds true for BGO. Note that ΔT is the amount of change in temperature from the reference temperature. In addition, the value of the temperature coefficient of optical rotation power is determined by measurement as -
I learned that 3X10=/℃.

Bilateral optical rotation BSO or BGO causes the rotation of the polarization direction as shown in equations (2) and (3), respectively, so
The influence of optical rotation power and its temperature coefficient can be eliminated by the following configuration.

A dextrorotating BSO or BGO element of the same length,
Left-optical rotation B80. It is clear that when the BGO elements of either force \ are combined, the rotation angle ψt of the polarization plane (polarization direction) as a whole becomes ψ - ψR + ψL −2VeHe (4).

In other words, J3 with the same length e but opposite direction of optical rotation
By combining SO or BGO, the optical rotation power and its temperature dependence can be completely canceled.

The present invention was made based on such discoveries and ideas.

FIG. 6 is a schematic diagram of an optical system of an optical isolator according to an embodiment of the present invention.

The Faraday element 5 is a left-handed rotatory BSO or BSO of the same length.
GO6 and dextrorotating BSO or B Go 7 are bonded together so that their optical axes coincide. It is assumed that the magnetic field IL exists in the optical axis direction of the Faraday element 5.

The light emitted from the light source 1 passes through a polarizer 2 and becomes linearly polarized light. The linearly polarized light enters the Faraday element 5. Light propagates inside the Faraday element while rotating its plane of polarization.

Part of this rotation angle is due to optical rotation power.

Therefore, the plane of polarization rotates counterclockwise in the left-handed optically rotating BSO or BGO6, and clockwise in the right-handed rotating BSO or BGO7. The rotation due to optical rotation has the same absolute value and opposite sign, so its value is large, but it is completely canceled out as it passes through the left and right optical rotatory particles, and in the end, only the rotation angle due to the Faraday effect is Therefore, there is no deterioration in isolation due to the temperature dependence of optical rotation power, which occurs in optical isolators made of one type of J, 3 SO or BGO.

In this way, by canceling out the large temperature change in the optical rotation power observed in BSO and BGO, it is possible to provide an optical isolator with excellent temperature stability.

In this example, B SO or BGO of the same length was used, one having left-handed rotation and the other having right-handed rotation. However, as already mentioned, the absolute value of the optical rotation power of BSO and 13Go,
Since the and Verdet constants are equal, one can be set to BSO and the other to 13 GO.

Alternatively, a large number of B501 or 13 GO- with left or right rotation may be laminated (in this case, the sum of the lengths of 1380 or BGO with left rotation and 13 SO with right rotation) Or B
It is easily understood that it is good if the sum of the lengths of GO is equal.

In addition, the elements do not necessarily have to be in close contact with each other (they may be arranged spaced apart from each other with their optical axes aligned in the direction in which the light travels).

The rotation angle θ of the plane of polarization due to the Faraday effect is proportional to the length of the Faraday element 5 and the magnetic field H, but if it is difficult to increase the length of the element or it is desired to set θ = 45° with a lower magnetic field, the Faraday element Mirrors 8 are provided on both sides,
This allows the light to be reflected and travel back and forth through the Faraday element a predetermined number of times.

When the Faraday material of length L (=2 e ) is reciprocated N times, the rotation angle of the plane of polarization 96N is ON = 2
V e HNL (5), so
The strength of the magnetic field can be 1/N compared to that shown in FIG.

Examples are shown below.

63371J++ Length 33 as an optical isolator for light
, using a combination of left-handed optical power B80 or BGO between 75 and right-handed optical power BSO or BGO, 2.00
A magnetic field of 0.00 was applied.

When the temperature was varied from -10℃ to 60℃ and the optical isolation was measured, the fluctuation due to temperature change was at the center value of 3.
It was within the range of ±2 dB with respect to 0 dB. In other words, it was confirmed that the optical isolator of the present invention has almost no temperature dependence and has extremely high performance.

For comparison, we made optical isolators of the same length using 'B80 or BGO with only dextrorotation power or only left-handed rotation power and conducted similar tests, but at temperatures between 110°C and 60°C.
When varying over the range, the isolation is 30 dB
±10 (IB) fluctuation, making it impractical in places with severe temperature changes.

The above results apply without exception when there is a pair of materials with opposite directions of optical rotation but the same absolute value of optical power, the same temperature coefficient, and the same Verdet constant, especially the large Faraday effect. When applied to YIG, which has optical rotation ability, the effects of the present invention are significant.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the Faraday effect, FIG. 2 is a diagram showing a configuration of an optical system of an optical isolator according to a conventional example, and FIG. 6 is a schematic diagram of an optical system of an optical isolator according to an embodiment of the present invention. FIG. 4 is a sectional view of the optical system of an optical isolator according to another embodiment. 1...Light source 2...Polarizer 6...Analyzer 4 Photodetector 5...
...Faraday element 6...Levorotatory BSO or BGO7...
- Right-handed optical rotation BSO or BGO8...Reflecting mirror A...Optical material H...-Magnetic field
θ・・・Rotation angle of polarization direction Patent applicant Sumitomo Electric Industries, Ltd.

Claims (1)

[Claims] (1) Although the directions of optical rotation are opposite, the absolute value of the optical rotation power and its temperature coefficient are the same, and the Verdet constants have the same sign and absolute value, and there are two types with the same length in the optical axis direction. An optical isolator formed by arranging Faraday elements made of a Faraday material or C) so that their optical axes coincide. (2) The two types of Faraday materials are bismuth silicon oxide (B112Sio2
D) Mata is bismuth germanium oxide (Bi, 2
2. The optical isolator according to claim 1, wherein the optical isolator is made of either bismuth silicon oxide or bismuth germanium oxide having left-handed optical rotation ability. t31 The Faraday material of the above 2 fi class is yttrium iron garnet (Y3Fe) which has dextrorotating ability.
5O12) and yttrium iron garnet having left-handed optical rotation. (4) A pair of reflecting means is provided before and after a Faraday element made of the two types of Faraday materials arranged in alignment in the optical axis direction, and the light is made of the two types of Faraday materials at least once. (5) The optical isolator according to claim 1 is characterized in that the optical isolator moves back and forth within a Faraday element. 2. The optical isolator according to claim 1, wherein the optical isolator comprises a Faraday element, and the total thickness of each type of Faraday element in the optical axis direction is equal.