JPS635318A - Cascade connection type isolator - Google Patents

Abstract

PURPOSE:To obtain an optical isolator having a high loss in a backward direction even if there is a temp. fluctuation by shifting the Faraday rotating angles of Faraday rotating elements to the angles above and below 45 deg.. CONSTITUTION:Polarizes 1-3 are so installed as to exist in the positions twisted by 45 deg. each; the plane of polarization of the polarizer 2 is twisted 45 deg. from the plane of polarization of the polarizer 1 and the plane of polarization of the polarizer 3 is twisted 90 deg. therefrom. The polarizers are prepd. by adequately cutting crystals such as calcite, rock crystal, KDP, and ADP. The 1st and 2nd Faraday rotating elements 4, 5 sandwiched by the polarizers 1-3 have the Faraday rotating angles theta1, theta2 which are not at 45 deg. at a standard wavelength lambda0 and standard temp. T0 and are shifted to the angles above and below 45 deg.. The excellent optical isolator characteristic is thereby obtd. even if there are fluctuations in the temp. and wavelength.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to an optical isolator whose isolation characteristics hardly deteriorate even when there is a change in temperature or a change in wavelength of a light source.

An optical isolake is an optical component that allows light to pass in one direction and not in the opposite direction, thereby preventing returned light from returning to the light source.

When a semiconductor laser is used as a light source and light is transmitted using an optical fiber, the light may return to the laser due to reflection at the end face of the optical fiber or back scattering within the fiber. If the returned light enters the semiconductor laser, oscillation becomes unstable, which is not desirable.

Therefore, an optical isolator is inserted between the laser and the optical fiber. If you cut the return light with an optical isolator,
The oscillation of the semiconductor laser is not disturbed.

(a) Conventional optical isolator structure 7, 4, a polarizer, a Faraday rotation element, an analyzer, a magnet, etc. A Faraday rotation element rotates the plane of polarization of linearly polarized light. When light propagates through a magnetically saturated Faraday rotation element, the Faraday rotation angle per unit length is called the Faraday rotation coefficient.

Many materials with Faraday rotation function are known, but
There are few ferromagnetic materials that can be put to practical use. Examples include I-IJium iron garnet, gadolinium iron garnet, and bismuth-substituted gadolinium iron garnet (CBIC).

Although it is difficult to make a single crystal of CBIG, it is relatively easy to make a small crystal. The Faraday rotation coefficient is also large.

The directions of the polarization planes of the polarizer and analyzer are twisted by 45 degrees.

The Faraday rotation element rotates the plane of polarization of light by 45 degrees.

The rotation angle is determined by the magnitude of the magnetic field, the length of the Faraday rotator,
Due to Faraday rotation coefficient.

The Faraday rotation coefficient changes depending on the wavelength of light and temperature. How does it change? That depends on the material.

The length of the element and the dimensions and strength of the magnet are set so that the Faraday rotation angle is 45° at a certain wavelength and temperature. The permanent magnet is designed to generate a magnetic field greater than the saturation magnetization of the Faraday rotation element.

Even if the temperature change of the magnet is small, the Faraday rotation coefficient of the Faraday rotation element varies depending on the temperature and wavelength. Even if the Faraday rotation coefficient itself fluctuates only slightly, the rotation angle deviates from 45°, resulting in a significant deterioration of the isolation characteristics.

Problems with the conventional technology Here, the term isolation characteristic will be explained. Since it is an optical isolator, 100% of the light is directed in the forward direction.
Ideally, it would be transmitted and 09 (transmitted) in the reverse direction.This is of course impossible, but optical isolators are required to have high reverse direction loss and forward loss close to 0. Ru.

Therefore, isolation characteristics are defined as (1) high reverse loss, and (11) forward loss close to O. ill depending on the purpose,
Emphasis is placed on either fll+.

Using an optical isolator using a Faraday rotation element made of a trinium iron net single crystal with bismuth substitution, we investigated how the forward and reverse losses change with changes in temperature and wavelength.

FIG. 3 shows changes in temperature.

The horizontal axis is temperature, and the vertical axis is forward and reverse loss.

The reverse direction loss is scaled on the left vertical axis, and the loss change is shown as a solid line. The forward loss is scaled on the right vertical axis, and the loss change is indicated by a broken line.

The reverse loss is designed to be maximum at 25℃,
There is a loss of 100 (dB) or more. However, when the temperature deviates from 25°C, the loss suddenly decreases. 4 at 20℃ and 30℃
It decreases to about Q d B.

The forward loss is set to the minimum QdB at 25°C. If the forward direction also deviates from 25° C., the loss changes.

As described above, when the temperature deviates from 25° C., the reverse loss in particular decreases rapidly, and the isolator characteristics deteriorate.

This is because it was designed to obtain the best isolator characteristics at 25°C. Although it is possible to set the optimum temperature at any number of times, it is not possible to cover a wide temperature range no matter what temperature is set.

FIG. 4 shows changes in reverse loss and forward loss with respect to wavelength changes. The horizontal axis is the wavelength of the light source. λ=1.5
The reverse direction loss is maximized for 7 μm. For this wavelength, the reverse direction loss is 100 dB or more. However, if you deviate a little from this, the reverse loss drops sharply.

For example, for λ=1.56 μm and 1.58 μm, the loss is reduced to about 4Q d B.

Even if the forward loss is set to 0 at λ=1.57 pm, it increases slightly as the wavelength changes.

(Problem to be solved by the invention) Such fluctuations in reverse loss with respect to temperature will be more or less small no matter what kind of Faraday rotation element is used.Assuming that the amount of temperature change is ±25℃ , for a temperature change of ±25° C., the change in the Faraday rotation coefficient is, for example, as follows.

(a) Cydz Bi Fes Ot2λ=0.
78μm Δθ=-3%725℃ ibl Cyd2,6Bi0.4Fe5012Δθ=
-4.2%/25℃ (cl Gd2.6Bio, 4Fes 012Δθ=
-6%/25°C. Here, Δθ is the amount of change in the Faraday rotation coefficient.

As described above, the GBtG Faraday rotation element has a temperature dependence of approximately 14% to 725°C.

As described above, since the Faraday rotation coefficient has a strong temperature dependence, the isolator characteristics deteriorate when the temperature deviates from the set temperature (for example, 25° C.).

All that is required is to maintain a constant temperature of the light source, isolator, and entrance part of the optical fiber, but this has the disadvantage that the peripheral circuitry becomes complicated and difficult to use. Changes in ambient temperature are often unavoidable.

Next, the deterioration of isolator characteristics due to wavelength fluctuation will be explained.

A semiconductor laser is used as a light source, but the oscillation wavelength of the laser diode itself varies.

For example, in the case of a laser diode whose standard oscillation wavelength is 1.30 μm, there are laser diodes with an oscillation wavelength in the range of 1.27 μm to 1.33 μm. There is a variation in the oscillation wavelength of about ±0.03 μm.

The oscillation wavelength also changes depending on the temperature. A temperature change of ±25°C changes the oscillation wavelength by about +0.01 μm.

The wavelength changes by about 0.04 μm due to variations in the oscillation wavelength between elements and fluctuations in the oscillation wavelength due to temperature fluctuations. Fourth
In the example shown in the figure, if the wavelength is shifted from the center wavelength by this amount, the reverse loss will be reduced to about 27 dB.

E) Purpose The first object of the present invention is to provide an optical isolator whose isolator characteristics do not deteriorate even when there are temperature fluctuations.

A second object of the present invention is to provide an optical isolator whose isolator characteristics do not deteriorate even if there are variations or fluctuations in the oscillation wavelength of a light source.

It is a third object of the present invention to provide an optical isolator that is small in size and easy to use.

As already mentioned, the isolator characteristics have two meanings. One is that the reverse direction loss is large. Another reason is that the forward loss is small.

It is difficult to reconcile the two conditions. Unless one is chosen, the problems of temperature dependence and wavelength dependence will not be solved.

Since it is an optical isolake, the most important thing is to block the return light. For this reason, it is first desired that the reverse direction loss be large.

Furthermore, it can be seen from FIGS. 3 and 4 that the dependence on temperature and wavelength is particularly large in the reverse direction loss.

Furthermore, the forward loss must be kept sufficiently high. It is sufficiently small that even if several optical isolators are stacked on top of each other,
It is possible that the forward loss is still small.゛Furthermore, the forward loss may be affected by small temperature changes.

For these reasons, of the two conditions required for an optical isolator, we choose the condition that the reverse direction loss is large.

When the set wavelength is χ0 and the temperature is TO, conventional optical isolators are designed so that the Faraday rotation angle is 45° at this time. Therefore, the reverse loss was the highest at this time.

In the present invention, there are two wavelengths, λ, that sandwich the set wavelength and temperature.
1. λ2. Consider two temperatures Tl and T2.

Then, one set λ1. An optical isolator L1 with maximum reverse loss at Tt is manufactured.

Another set λ2. Also at T2, the optical isolator L2 with the maximum reverse direction loss is manufactured.

Then, the Ll and B2 optical isolators are stacked to form an integrated optical isolator.

A1 × λ × A2 (1) It is. The distances between S1 and λ and between λ and S2 are appropriately determined.

Temperature T1. Also for T2, Tl (T (T2 (2) or T2 < T < TI (
3).

Optical isolator L1. as a function of wavelength and temperature. B
Bl (λ, T), B2 (λ.

T) (in dB), the loss of the stack of these two optical isolators is the product of both, but in dB
Since the unit is , it is given by the sum of both.

Let B(λ, T) be the reverse loss of the integrated optical isolator, B(λ, T)=Bl(λ,”r)−1−B+(λ,T)
(4). Ut (λ, T) is λ1. It reaches its maximum at T1. B2(A,T) is A2. It reaches its maximum at T2.

Then, both Bl (λ, T) and B2 (λ, T) are T1 × T × T2, A1 × λ × A2,
It will have a tail following both peaks. The sum of the two tails becomes the value of B(A,T). λ1. In the part close to T1, the contribution of Bl (λ, T) is dominant, and λ
2. In the portion close to T2, the contribution by B2 (λ, T) is dominant.

Both contributions are complementary, λ1. Tl~λ2. It works to increase the reverse loss during T2.

Even at Ao and To located between these, a considerably large reverse direction loss can be obtained.

In this way, the present invention uses two temperatures TI and T2 that sandwich the reference temperature and wavelength, and two wavelengths λ1. An optical isolator for λ2 is made and these are connected in cascade.

T1. The selection of T2 may be determined by considering the width of temperature change. The standard temperature is 25℃ and the minimum temperature is 0℃
, if it is used in an environment where the maximum temperature is 50℃,
Tl may be set near 0°C and T2 may be set near 50°C.

However, Bl (λ, T), B2 (λ, T) are Tl
Since there are tails outside of -, -T2, λ1 and λ2, T, , and T2 may be included in the lowest and highest temperatures.

Furthermore, correlations must be taken into account when choosing wavelength and temperature.

In the present invention, the number of optical isolators is not limited to two, but three or four optical isolators may be connected in cascade. In this case, there is a drawback that the forward loss decreases significantly, but there is no problem as long as the power of the light source is strong.

If four optical isolators are connected in cascade, (λ2
．． It is possible to connect four optical isolators such that TI) and (λ2.T2).

Consider how the Faraday rotation angle θ changes with respect to fluctuations in the temperature T1 wavelength λ. If, T
If θ has a positive or negative (same sign) differential coefficient with respect to lλ, then λ1. It is assumed that T1 is smaller than A2°T2.

In other words, if , then A1 × λ × A2 (6) Naturally (definition formula (1)), and regarding the temperature, (2)
The inequality % expression % ( is adopted.

If the Faraday rotation angle is 45° or less for the temperature and wavelength set (λ1.TI), then the other set (λ2.TI) is less than 45°.
T2), the Faraday rotation angle must be 45'' or more.

Since the Faraday rotation angle θ is a continuous function of λ and T,
It is preferable that θ monotonically increases or decreases while λ changes from S1 to S2. The same holds true for temperature T.

If the condition (5) exists and (6) is chosen as (force), the combination of wavelength and temperature that gives the minimum and maximum Faraday rotation angle θ is (λ1.Tl) or (Iz
, T2).

That is, λ1. If T1 gives the minimum rotation angle θmin, then λ2. T2 gives the maximum rotation angle θmax. The reverse is also possible.

Therefore, λl × λ × λ2, TI (T (T2, T
The Faraday rotation angle θ for the above-mentioned minimum and maximum rotation angles always exists between the minimum and maximum rotation angles.

f5), (Even though the conditions of (61) are met, (T2 (Tt) is chosen as the relationship of T1.T2 like the force equation, (λ1.Tt), (λ2, Tz) are the maximum and minimum values. This means that the distribution of the Faraday rotation angle θ is not determined only by the temperature and wavelength at both ends.

If so, λ1 × λ × λ2 (9) Naturally, T2<Tl, and TI and T2 are selected.

In this way, the present invention connects two optical isolators in cascade.

FIG. 1 shows the configuration of an isolator according to the present invention.

The polarizers 1.2.3 are installed so that their planes of polarization are twisted by 45 degrees. With respect to the polarization plane of polarizer 1, the polarization plane of polarizer 2 is 45 degrees, and the polarization plane of polarizer 3 is 9 degrees.
Twisted by 0°.

Polarizers have been known for a long time to be manufactured by appropriately cutting crystals such as calcite, quartz, KDP, and ADP. Such a polarizer may also be used. However, if a smaller polarizer is desired, the polarizer invented by the present inventor (Japanese Unexamined Patent Publication No. 60-97304, published on May 31, 1983)
It is better to use This is a stack of multiple thin metal layers and thicker dielectric layers. It is called a metal-dielectric multilayer.

FIG. 2 shows an explanatory perspective view of the metal dielectric multilayer body.

The metal dielectric multilayer body includes a dielectric layer 10 having a thickness d and a thickness g
Metal layers 11 (d>g) are alternately laminated. Light passes in a direction parallel to the layer plane.

The propagation direction of light is 21 dielectric layers, and the normal direction of the metal layer is Y. It is assumed that these layers are parallel to the XZ plane.

A mode with an electric field component perpendicular to the layer plane is called a TM mode. Since this has an electric field in the normal direction of the metal layer, almost no current is generated in the metal layer. Therefore, it can pass through with almost no attenuation.

Light with an electric field component parallel to the layer plane is called TE mode. This causes a current to flow through the metal layer, so energy is lost.
Attenuates quickly.

After all, only the TM mode having an electric field component in the Y direction can pass through this metal-dielectric multilayer body. This means that only light whose polarization plane coincides with the Y-axis direction passes through, so it functions as a polarizer.

The metal layers are AJ, Au, Ag, etc.
For example, quartz 5i02 is used as the negative dielectric layer as long as it is a transparent dielectric material. - The thickness d of the layer is approximately 4000 to 10000 A.

The thickness T of the metal dielectric multilayer body may be approximately several μm to several hundred μm. It can be significantly smaller than conventional polarizers.

The number of layers of metal and dielectric material to be laminated depends on the beam diameter, but it may be, for example, about 1000 layers each.

Polarization, the deviation of the plane of polarization between photons 1 and 2, polarizers 2 and 3 is 4
Set to 5°.

45° is natural for optical isolators, but in the present invention, the Faraday rotation angle θ1.5 of the Faraday rotation element 4°5. Since θ2 is set to be above and below 45°, care must be taken.

The plane of polarization of polarizers 1, 2.3 is set to θ1. θ2. Assuming θ3, B2-B1-45° α0) θ3
-θ2=45° (1υ.

The first and second Faraday rotation elements 4 and 5 sandwiched between the polarizers have a Faraday rotation angle θl.

02, but these have a reference wavelength λ0 and a reference temperature TO
Assume that the angle is not 45°, but is shifted above and below 45°.

In T-TO1λ=λO, B1>45° and B2<45° α, or 01<45° and B2>45° 03).

The angular relationship of the polarizers 1.2.3 is 45°, which is the same as the known one.

However, λ0. of the Faraday rotation element 4.5. The Faraday rotation angle at TO is shifted up and down by 45°.

However, the definition of (13, 03 is still not sufficient).

The optical isolator Ll consists of polarizers 1 and 2 and a Faraday rotation element 4. The optical isolator L2 consists of a polarizer 2.3 and a Faraday rotation element 5.

When deviating from the reference wavelength and reference temperature, θ1. It is desirable that θ2 changes in a complementary manner. In other words, if one decreases, the other increases.

That is, as well as Just add this relationship.

Same as these definitions, two optical isolators Ll and B2 are connected vertically, and λ1 . λ2 is the reference wavelength λ
When the value is on both sides of 0, the optical isolator Ll
is λ1. Has optimal optical isolator characteristics in TI,
The optical isolator L2 can also be defined as having optimal optical isolator characteristics at λ2°T2.

As already mentioned, the optimal optical isolator characteristics are those with maximum reverse loss and minimum forward loss. In one optical isolator, λ, T
When fixed, the conditions are the same.

The magnetic field applying mechanism 6 is made of a permanent magnet and applies a magnetic field to the Faraday rotation element. Apply a magnetic field until the Faraday rotation element is saturated. Must be a strong permanent magnet. For example, Nd-Fe-B magnets, rare earth magnets, etc. are used.

(:8) The action reverse loss B(λ, T) is the two optical isolators Ll
, B2 reverse loss Bt (λ, T), B2 (λ, T
) is given by the sum of

B(λ,T) = Bt(λ,T)+B2(λ,T
) (16) B1 is λ1. The maximum value is taken at TI. B2
takes its maximum value at λ2°T2. These functions are continuous except for these points. Reference temperature and reference wavelength are T1
~T2, between λ1 and λ2. Therefore, in this range, the reverse direction loss B(λ, T) has a considerably large value.

In other words, excellent optical isolator characteristics can be provided even if there are fluctuations in temperature and wavelength.

(ri) Effect: Even if there are temperature fluctuations, it is possible to provide an optical isolator with high reverse loss.

It is possible to provide an optical isolator in which reverse direction loss does not decrease even if there are variations or fluctuations in the oscillation wavelength of a light source.

By using the metal-dielectric multilayer body invented by the present inventor as a polarizer, it is possible to create a small-sized, easy-to-use optical isolator.

Kasumi Example A cascade-connected optical isolator as shown in FIG. 1 was manufactured. The Faraday rotation element is a trinium iron garnet (GBIG) single crystal with bismuth substitution, and the polarizer is a metal dielectric multilayer.

Figure 5 shows the reverse loss for light with λ = 1.57 μm.
The forward loss is determined as a function of temperature.

The horizontal axis is temperature. The left vertical axis is the reverse direction loss, which is shown as a solid line. The right vertical axis is the forward loss, which is shown by the dashed line.

The reverse loss is at its maximum at 0°C. Even at 50°C, the reverse loss is at its maximum.

The reference temperature is 25°C, but even at this time it is 53 dB or more. Between -10℃ and 60℃, reverse loss is 5Q
dB or more.

Each peak corresponds to the optical isolator L1. It corresponds to either L2.

FIG. 6 shows the reverse loss and forward loss determined as a function of wavelength at T=25°C. λ=1.55
The reverse direction loss takes a maximum value at μm and λ=1.59 μm.

Even at the reference wavelength λ=1.57 μm, there is a reverse loss of 55 dB.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of a cascade-connected isolator of the present invention. FIG. 2 is a perspective view for explaining the metal dielectric multilayer body invented by the present inventor. FIG. 3 is a graph showing temperature changes in reverse loss and forward loss of a conventional optical isolator. FIG. 4 is a graph showing the wavelength change of the light source in the reverse direction loss and forward direction loss of a conventional optical isolator. FIG. 5 is a graph showing changes in reverse loss and forward loss of the optical isolator of the present invention depending on temperature. FIG. 6 is a graph showing changes in reverse loss and forward loss depending on wavelength of the optical isolator of the present invention. 1.2.3...Polarizer 4.5...Faraday rotation element 6...
Magnetic field application mechanism Figure 1 Optical isolator Optical isolator
Figure 2 Polarizer gold layer dielectric multilayer temperature (°C)

Claims (2)

[Claims] (1) A polarizer 1, a Faraday rotation element 4 installed behind it, and a second polarizer installed behind it so that the polarization direction makes an angle of 45° with that of the polarizer 1. 2, a Faraday rotation element 5 installed behind it, and a third polarizer 3 installed behind it so that the polarization direction makes an angle of 90° with that of the polarizer 1, Reference temperature T_0 Reference wavelength λ
A cascade-connected isolator characterized in that, in _0, one of the two Faraday rotation elements 4 and 5 has a Faraday rotation angle smaller than 45°, and the other Faraday rotation angle is set larger than 45°. . (2) The polarizers 1, 2, and 3 are metal-dielectric multilayer bodies formed by alternately laminating multilayer dielectric thin films and metal thin films. Cascading isolators.