JPH0850261A - Optical circulator - Google Patents

Abstract

PURPOSE:To obtain the same functions as heretofore by simple optical circuits which are shorter in their lengths and are fewer in constituting element circuits by inserting 45 deg.C nonreciprocal polarization rotators and half-wave plates inclined at a specific angle from the circuit plane in this order into optical paths without holding a double refractive optical medium. CONSTITUTION:This optical circulator consists of two pieces of 3dB directional couplers 1, an M-Z type optical interference circuit consisting of two pieces of the optical paths A, B of an equal optical path length therebetween, two pieces of the nonreciprocal 45 deg. polarization rotators 3 inserted into the optical paths A and B and the half-wave plate 2 connected thereto without holding the double refractive optical medium therebetween. The 45 deg.C nonreciprocal polarization rotator 3 and the half-wave plate 2 are inserted in this order into the optical path A in the part where the Z direction is the optical axis. The half-wave plate 2 is so installed that its optical axis has an angle of +22.5 deg.C when viewed in the Z direction. On the other hand, the half-wave plate 2 and the 45 deg. nonreciprocal polarization rotator 3 are inserted in this order 10 the optical path B by reversing the order from the order in the optical path A in the part where the Z direction is the optical axis. Further, the half-wave plate 2 is so installed that its optical axis has an angle of -22.5 deg. when viewed in the Z direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical circulator which is one of the constituent elements in constructing various waveguide type optical circuits used in the fields of optical communication and optical information processing. is there.

[0002]

2. Description of the Related Art A conventional waveguide type optical circulator has a structure as shown in FIG. That is, in the optical interference circuit of Mach-Zehnder (Mach-Zehnder type: hereinafter abbreviated as “MZ type”) including two 3 dB directional couplers 1 connected in cascade, an optical path length difference of half a wavelength is provided. At the same time, a half-wave plate 2 arranged at 90 degrees is inserted on the short optical path side to form a waveguide type polarization separation circuit. Between these polarization separation circuits, a 45 degree non-reciprocal polarization rotator 3 and a 22.5 degree arrangement half are arranged. Two 0-90 degree non-reciprocal polarization controllers composed of the wave plate 2 were installed.

In such a structure, when the light incident from the port 1 is emitted to the port 3, the light incident from the port 3 is emitted to the port 2 by the action of the 0-90 degree nonreciprocal polarization controller. The optical circuit exhibits an optical circulator operation having an input / output relationship as shown in FIG.

[0004]

In such a conventional structure, the circuit length becomes long as is apparent, and the insertion loss inevitably increases, and one optical circuit incorporating this optical circulator circuit is provided. There was a problem that the number of circuit wafers that could be taken was limited.

An object of the present invention is to provide an optical circulator having the same function as that of a conventional optical circulator circuit, but with a simple optical circuit having a shorter length and a smaller number of component circuits.

[0006]

SOLUTION TO THE PROBLEMS Two input / output optical waveguides, two 3 dB directional couplers, and a waveguide having two equal optical path lengths sandwiched between the two directional couplers. An optical circulator having a wave-type Mach-Zehnder interferometer as a basic structure, wherein one optical path is 45 degrees away from the propagation direction of light incident from the input circuit to the Mach-Zehnder interferometer. A reciprocal polarization rotator and a half-wave plate whose optical principal axis is tilted +22.5 degrees from the circuit plane in a plane perpendicular to the optical axis are inserted in this order without sandwiching a birefringent optical medium, and the other side. In the optical path of, a half-wave plate whose optical main axis is tilted by -22.5 degrees from the circuit plane in a plane perpendicular to the optical axis and a 45-degree non-reciprocal polarization rotator in the same polarization rotation direction as above are placed in this order. It is characterized in that the birefringent optical medium is inserted between the both without being sandwiched.

In the above optical circulator, the 45
The non-reciprocal polarization rotator comprises a magneto-optical waveguide.

In the above optical circulator, the half-wave plate is made of polyimide.

The contents of the optical circulator according to the present invention will be described below.

FIG. 1 shows the configuration of a waveguide type optical circulator of the present invention. The optical circulator has an MZ type optical interference circuit including two 3 dB directional couplers 1 and two optical paths A and B having the same optical path length between them, with the xz plane in the figure as a circuit surface, and optical paths A and B. It is composed of two non-reciprocal 45-degree polarization rotators 3 inserted therein and a half-wave plate 2 which is connected to them without sandwiching a birefringent optical medium. In the optical path A, a 45-degree non-reciprocal polarization rotator 3 and a half-wave plate 2 are inserted in this order from the left side of the figure at a portion having the optical axis in the z direction. The half-wave plate 2 is
The optical axis (here, for the sake of convenience, the low refractive index direction) is installed so as to form an angle of +22.5 degrees when viewed in the z direction. On the other hand, in the optical path B, the half-wave plate 2 and the 45-degree non-reciprocal polarization rotator 3 are inserted in this order from the left in the part having the optical axis in the z direction in the reverse order of the optical path A. Further, the half-wave plate 2 is set so that the optical axis forms an angle of −22.5 degrees when viewed in the z direction. The polarization rotation directions and the optical path lengths of the two 45-degree non-reciprocal polarization rotators 3 are set to be the same in both the optical paths A and B. Here, for the light propagating in the z direction, for convenience. Turn clockwise.

As is apparent from FIG. 1, the operational characteristic of the present invention is that two polarization separation circuits are not required as compared with the conventional example shown in FIG.

Next, the operation of the optical circulator of the present invention will be described with reference to FIG. A general optical waveguide has birefringence induced by stress at the interface between the waveguide and the substrate. The optical axis of this birefringence is generally in the circuit plane parallel to the substrate and in the direction perpendicular to the circuit. The propagation of light in such an optical waveguide is divided into two independent modes, a TE mode in which the electric field is in the circuit plane and a TM mode in which the electric field is in the direction perpendicular to the circuit plane. Therefore, in the following, light propagation will be divided into these two independent modes.

Now, port 1 or ports 2 to 3d
Consider the light that has passed through the B optical coupler 1 and propagated along the optical path A. At time t = 0, as shown in the case where the vibration vectors of both the TE mode and TM mode electric fields are A _f (f is forward, forward direction) in the figure, 45 degrees non-reciprocal in the state of maximum amplitude in the x direction and the y direction. It is assumed that the light is incident on the polarization rotator 3.
Then, when the refractive index of the rotator 3 is n ₁ and the length is d ₁ , at the exit of the rotator 3 at time t = n ₁ d ₁ / c, the maximum amplitude is obtained when the polarization direction is rotated clockwise by 45 degrees. Become. Here, c is the speed of light.

Next, when the light enters the half-wave plate 2, the time is further n.
The polarization state at the exit of the half-wave plate when ₂ d ₂ / c has passed is
The low-refractive-index optical axis of the half-wave plate is tilted by 22.5 degrees from the x-axis, and the component perpendicular to this is delayed by half a wavelength, so the vibration vector is line-polarized with the optical axis 2'in between. The maximum amplitude is reached when the Here, n ₂ and d ₂ are the refractive index corresponding to the low refractive index optical axis 2 ′ of the half-wave plate 2, and the half-wave plate 2
Is the thickness of.

Next, the optical path B will be considered. Ports 1 and 2
It is assumed that the light that has entered from is incident on the half-wave plate 2 at time t = 0 as shown by B _f . The states of the vibration vectors in both TE and TM modes are the same as those considered in the optical path A. The polarization state of the transmitted light at the time t = n ₂ d ₂ / c at the exit of the half-wave plate 2 is a polarization angle that is line-symmetric with respect to the optical axis 2 ′ inclined by −22.5 degrees from the x direction. Maximum amplitude.

In this state, the polarization state at the exit of the rotator when it is incident on the 45-degree non-reciprocal polarization rotator 3 and has passed time n ₁ d ₁ / c is the maximum amplitude state rotated clockwise by 45 degrees. Becomes At time t = 0, the light enters the non-reciprocal polarization controller composed of the polarization rotator 3 and the half-wave plate 2 at the same time, and the time t = n ₁ d
Comparing the vibration vectors of the light on the optical paths A and B emitted after the passage of ₁ / c + n ₂ d ₂ / c, the directions of the vectors are opposite in both TE and TM modes, and the phases are shifted by half a wavelength. become.

Next, consider the case where light is incident from the ports 3 and 4. At this time, the light travels in the -z direction, the tilt angle of the optical axis 2'of the half-wave plate 2 feels opposite to the above, and the polarization rotation direction of the 45-degree nonreciprocal polarization rotator 3 is , -Counterclockwise when viewed in the -z direction.

Considering these points and considering the polarization state after passing through the non-reciprocal polarization controller in the same manner as in the case of the optical path A, A _b (b is backward, reverse direction), B in the figure. _As can be seen from the comparison of _b , the light passing through the optical paths A and B is TE,
Both TM modes have the same phase.

In the MZ type optical interference circuit composed of two 3 dB directional optical couplers 1, when the phase relationship when passing through the optical paths A and B deviates by half a wavelength, the light is emitted to the through port and the same phase. If it is, it is emitted to the cross port. From this characteristic and the above consideration, in the optical circuit of the present invention, port 1,
Alternatively, the light incident from 2 is emitted to the through port 3 or 4 with the phase shifted by a half wavelength, while the light incident from the port 3 or 4 is emitted in the same phase to the cross port 2 or 1. The optical circulator operation shown in FIG. 6 is realized. Moreover, since both TE and TM modes operate in the same manner, polarization independent operation is achieved.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below. FIG. 3 is a schematic diagram of the optical circulator according to the present embodiment. The main components of the optical circulator of this embodiment are
These are the MZ type optical interference circuit 12 including the magneto-optical waveguide 11 having the function of the 45-degree non-reciprocal polarization rotator 3 and the 3 dB directional optical coupler 1 and the half-wave plate 13. First, the production of these three points will be described below.

1) Magneto-Optical Optical Waveguide The magneto-optical optical waveguide 11 was manufactured as follows. First, gadolinium gallium garnet (Gd ₃ Ga ₅ O
₁₂ ) The (111) plane of the single crystal is used as a substrate, and yttrium iron garnet (YFe ₅ O ₁₂ , commonly known as YI
G) A buried YIG optical waveguide array was manufactured by a liquid phase epitaxial growth technique for a single crystal film, an ion beam etching technique for a garnet film, and the like. The steps are: under clad layer growth, core layer growth, waveguide core pattern etching, and over clad layer growth. The optical axis direction is the <110> direction, the core is 4 μm square, and the waveguide interval is 250 μm. The length was 3.02 mm at which the polarization rotation angle was 45 degrees (wavelength 1.55 μm). The required magnetic field strength was 1400e in the optical axis direction. A non-reflective coating was applied to the end face so as to match the refractive index with an interference circuit described later. The shape of the manufactured magneto-optical waveguide 23 was as shown in FIG. 3 (the growth surface was downward in the figure), and the overclad layer immediately above the core had a convex surface.

2) MZ type interference circuit The magneto-optical waveguide 11 is inserted between the MZ type interference circuits. Here, as shown in FIG. 3, a magneto-optical waveguide is inserted in the interference circuit. The waveguide 11 is inserted with the growth surface facing downward, and at this time, a height such that the core 14 of the magneto-optical waveguide 11 and the core 15 of the interference circuit are aligned in the height direction (y direction) at the insertion site. A waveguide circuit provided with the pedestal 16 was manufactured by the following steps. First, at a position where the magneto-optical waveguide 11 of the (100) silicon single crystal substrate 17 is to be installed,
The table 16 is placed in an arrangement avoiding the convex surface right above the core of the waveguide 11.
Was produced. The patterning was performed by etching with potassium hydroxide using the thermal oxide film as a mask. Next, an under-cladding layer containing quartz as a main component was deposited by a flame direct deposition method to a height of the base 16 or higher, and the surface of the base 16 was polished and flattened. Next, after further stacking an underclad layer at a desired height, a core layer was deposited, and the core layer was dry-etched into an MZ type interference circuit pattern. Next, an overclad layer was deposited. The size of the waveguide core is 6 μm square,
The refractive index difference is 0.75%. The distance between optical paths A and B is 3.0 m
m, the total circuit length is 3.5 cm. At the same time, a linear monitor waveguide 18 used for positioning the magneto-optical waveguide was set outside the interference circuit at a distance of 250 μm. A heater 19 is formed on the optical paths A and B outside the installation site of the magneto-optical waveguide 11 for the phase adjustment by the thermo-optic effect with a chromium vapor deposition film. Finally, the portion into which the magneto-optical waveguide 11 was inserted was chamfered with a dicing saw so as to have a width of 3.04 mm, and the glass in between was chamfered with buffer hydrofluoric acid to fabricate the interference circuit shown in FIG.

3) Half-wave plate As the half-wave plate 13, a polyimide plate was used. When the polyimide plate is stretched, birefringence occurs and the refractive index in the stretching direction increases. The one used here is a fluorinated polyimide having birefringence Δn = 0.053 (wavelength 1.55 μm) and has a thickness of 14.6 μm. A rectangle having sides having angles of 22.5 degrees and -22.5 degrees from this axis was cut out from the half-wave plate 13 with an optical axis perpendicular to the stretching direction.

Next, the assembly will be described. First, an appropriate amount of epoxy adhesive is applied to a portion of the interference circuit where the magneto-optical waveguide is inserted, and then the magneto-optical waveguide 11 is inserted therein. The magneto-optical waveguide 11 was aligned at the optimum position and held in that state, and the adhesive was cured and fixed. At that time, a space for inserting the half-wave plate 13 was secured on one side. The latter half wave plate 13 with the two magneto-optical waveguides fixed was inserted, and the space between the interference circuit, the magneto-optic waveguide 11, and the half wave plate 13 was filled with a refractive index matching ultraviolet curing resin and cured. Finally, a magnet 20 was placed on the circuit as shown in FIG. 4 to complete the optical circulator. The insertion loss of this circulator is 2.1-
It was 2.6 dB and the isolation was 15 to 20 dB (wavelength 1.55 μm). By passing a current through the phase adjustment heater 19, the isolation was improved to 25 dB. An optical circulator of the conventional configuration shown in FIG. 1 was also manufactured at the same time, but the insertion was ˜5 dB and the total circuit length was 7 cm.

[0025]

As described above, according to the present invention, the polarization separation circuits on both sides of the non-reciprocal polarization controller are not required as compared with the conventional example, the circuit configuration is simplified, and the circuit length is reduced. It can be greatly shortened. Therefore, the yield is improved and the insertion loss can be significantly reduced. The optical circulator circuit is often used in combination with other circuits in addition to being used alone, and in that case, it is clear that the degree of freedom in optical circuit design is greatly increased from the above features.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical circulator of the present invention.

FIG. 2 is an explanatory diagram of the operation of the optical circulator of the present invention.

FIG. 3 is a first diagram of an embodiment of the optical circulator of the present invention.

FIG. 4 is a second diagram of an embodiment of the optical circulator of the present invention.

FIG. 5 is a diagram showing a configuration of a conventional waveguide type optical circulator.

FIG. 6 is an explanatory diagram of the operation of the optical circulator.

[Explanation of symbols]

 1 3 dB directional optical coupler 2 half-wave plate 2'wave plate optical axis 3 45 degree non-reciprocal polarization rotator 11 magneto-optical waveguide 12 MZ type optical interference circuit 13 half-wave plate 14 core of magneto-optical waveguide 15 M-Z type optical interference circuit core 16 units 17 (100) silicon substrate 18 monitor waveguide 19 phase adjustment heater 20 magnet

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akiyuki Tate 1-6-1, Saiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. Two input / output optical waveguides, two 3 dB
A directional coupler, an optical circulator having a waveguide-type Mach-Zehnder type optical interference circuit composed of two optical paths having the same optical path length sandwiched between the two directional couplers as a basic configuration, wherein the Mach -For the propagation direction of the light incident from the input circuit to the Zener type optical circuit, a 45-degree non-reciprocal polarization rotator is provided in one optical path, and +22 from the circuit surface in the plane where the optical main axis is perpendicular to the optical axis. A half-wave plate tilted at an angle of 5 degrees is inserted in this order without sandwiching a birefringent optical medium between them, and the other optical path has a main optical axis in the plane perpendicular to the optical axis from the circuit surface -22. 5
Half-wave plate tilted by 45 degrees and 45
An optical circulator characterized in that a non-reciprocal polarization rotator is inserted in this order without sandwiching a birefringent optical medium between them. 2. The optical circulator according to claim 1, wherein the 45-degree nonreciprocal polarization rotator comprises a magneto-optical waveguide. 3. The optical circulator according to claim 1, wherein the half-wave plate is made of polyimide.