CN101231369B - Waveguide Polarization-Independent Optical Circulator Based on Polarization Mode Separator - Google Patents

Abstract

The invention discloses a waveguide type polarization-independent optical circulator based on a polarization mode splitter, which is characterized in that it comprises a first polarization mode splitter (1) and a second polarization mode splitter (2). A waveguide-type Faraday rotator mirror (3) and a half-wave plate (4) connected to each other are arranged between them, and the first polarization mode splitter (1) and the second polarization mode splitter (2) respectively include a first optical power splitter device (5), second optical power splitter (6), first magneto-optical waveguide (7), and second magneto-optical waveguide (8). The beneficial effect of the present invention is that the structure is simple and easy to integrate, and compared with the separated phase adjuster, the insertion loss is greatly reduced; the insertion loss is low, the bandwidth is high and the polarization is insensitive, and it is easy to integrate with the optical communication system; the polarization crosstalk is effectively reduced .

Description

Waveguide Polarization-Independent Optical Circulator Based on Polarization Mode Separator

technical field

The invention relates to a waveguide-type polarization-independent optical circulator based on a polarization mode separator, in particular to an optical circulator based on a Mach-Zehnder type integrated waveguide polarization mode separator, which belongs to the field of optical communication applications and the field of integrated optics.

Background technique

Under the action of an external magnetic field, the magneto-optical material will affect the polarization state of the polarized light passing through it, that is, produce the Faraday magneto-optic effect. For a magneto-optical waveguide with a transverse configuration, under the action of an external magnetic field perpendicular to the direction of light propagation, the propagation constants of the forward and reverse propagation of the TM mode are different, so a non-reciprocal phase shift will occur. The non-reciprocal properties of optical materials can make optical circulators, optical isolators and other devices.

The TE-TM polarization mode separator is used to separate the TE (transverse electric field) and TM (transverse magnetic field) modes with vertical polarization in the waveguide, and plays an important role in optical fiber communication systems and optical fiber sensing systems. Based on polarization mode separators, it can form Optical switches, optical isolators, optical circulators, acousto-optic tunable filters, filters and other devices are widely used in various optical communication applications such as WDM (Wavelength Division Multiplexing) optical networks and coherent optical detection. important application.

At present, there are many kinds of TE-TM polarization mode separators with different structures. The gluing process of special refractive index material glass and multi-layer film can be made into a polarization beam splitting prism, which can project and reflect different polarized lights respectively, and can realize the separation of polarized beams. However, the polarizing beam splitter-type polarization mode splitter composed of polarizing beam splitting prisms is not conducive to integration with waveguides and optical communication systems. At the same time, the gluing process of prisms and multilayer films requires high requirements. The directional coupler-type polarization mode separator designed based on the coupled-mode principle is realized by utilizing the asymmetry of the two branch waveguides. The coupling region of one branch waveguide is covered with a metal layer, which effectively changes the real and imaginary parts of the waveguide propagation constant. , and the change to the TM mode is much greater than that of the TE mode. The absorption of the TM mode by the branched waveguide covered with the metal layer is used to suppress the coupling of the TM mode, thereby realizing the separation of the TE and TM modes. However, this structure requires a special process to reduce TM loss and polarization crosstalk, and requires a large distance between the coupler waveguides, which is not conducive to integration. Due to the small branch angle of the Y-branched polarization mode separator, not only the processing precision is high, but also the device size is large, which is not conducive to integration.

Optical circulators are an important part of optical communication systems and play an important role in WDM systems. Waveguide optical circulators have become a research hotspot because of their low price and easy integration. Most waveguide optical circulators only work in TM mode, while polarization-independent optical circulators have no requirement on the mode of the input light, and can work in both TM mode and TE mode.

At present, waveguide-type polarization-independent optical circulators are mostly based on Y-branches and directional couplers, and a few are realized by photonic crystals, blazed gratings and other devices. Due to the structural characteristics of the Y branch type and the directional coupler type, the size of the device is large, which is not conducive to the integration of the optical circulator structure.

Under the action of an external magnetic field, the magneto-optical material will affect the polarization state of the polarized light passing through it, that is, produce the Faraday magneto-optic effect. For the magneto-optical waveguide with transverse configuration, under the action of an external magnetic field perpendicular to the direction of light propagation, the propagation constants of the forward propagation and reverse propagation of the TM mode are different, so a non-reciprocal phase shift will occur, while for TE mode has no such effect.

Two optical power splitters and two phase shift arms sandwiched between the optical power splitters can form a Mach-Zehnder type polarization mode splitter. At present, power splitters are mostly realized by directional couplers, but it is not conducive to integrated. Multimode interference (MMI) couplers have attracted more and more attention because of their simple structure, low insertion loss, high bandwidth and polarization insensitivity, and their compact structure has a wide range of applications in integrated optics.

Contents of the invention

The present invention provides a waveguide-type polarization-independent optical circulator based on a polarization mode separator to address the above problems, based on a Mach-Zehnder type integrated waveguide polarization mode separator, combined with a Faraday rotator (FR) and a half-wave plate (HW )to fulfill.

The present invention adopts following technical scheme:

A waveguide type polarization-independent optical circulator based on a polarization mode splitter, characterized in that it includes a first polarization mode splitter and a second polarization mode splitter, and a waveguide type Faraday connected to each other is arranged between the two polarization mode splitters. A rotating mirror and a half-wave plate, the first polarization mode splitter and the second polarization mode splitter respectively include a first optical power splitter, a second optical power splitter, a first magneto-optical waveguide and a second magneto-optical waveguide, The two input ports of the first optical power splitter are respectively connected to the first input waveguide and the second input waveguide for inputting the forward transmission light or outputting the reverse transmission light, the first magneto-optical waveguide, the second magneto-optic waveguide The waveguide adopts a magneto-optical waveguide with a transverse configuration, the first magneto-optical waveguide constitutes a first phase adjustment unit, and the second magneto-optic waveguide constitutes a second phase adjustment unit, and the first phase adjustment unit and the second phase adjustment unit are a pair of TM The phase shifts generated by the TE mode are different by π, and the phase shifts generated by the TE mode are equal. The two output ports of the first optical power splitter pass through the first phase adjustment unit and the second phase adjustment unit and the two ports of the second optical power splitter respectively. The two output ports of the second optical power splitter are respectively connected to the first output waveguide and the second output waveguide for outputting the forward transmission light or inputting the reverse transmission light.

Preferably, the first optical power splitter and the second optical power splitter use multimode interferometers.

Preferably, the first magneto-optical waveguide and the second magneto-optical waveguide respectively adopt -π/2 phase shift and π/2 phase shift for the TM mode of the forward propagating light under the action of the magnetic field in the opposite direction, respectively. Transverse-configuration magneto-optical waveguides that generate π/2 phase shifts and -π/2 phase shifts respectively for the TM modes that transmit light, and produce equal phase shifts for the TE modes that transmit light in forward and reverse directions.

Preferably, the waveguide type Faraday rotating mirror adopts a non-reciprocal waveguide type Faraday rotating mirror capable of shifting the phase clockwise by π/4, and the half-wave plate adopts a half wave plate whose angle between the slow axis direction and the horizontal direction is π/8. wave plate.

The invention adopts two Mach-Zehnder type integrated waveguide polarization mode separators in combination with a waveguide type Faraday rotating mirror and a half-wave plate to realize a waveguide type polarization-independent optical circulator. Among them, the Mach-Zehnder integrated waveguide polarization mode separator is composed of two optical power splitters and two non-reciprocal transverse configuration magneto-optical waveguides. The phase of the TM mode produces a non-reciprocal phase change under the action of a magnetic field perpendicular to the direction of light propagation. Combined with the reciprocal phase change produced by the length of the waveguide itself, it interferes with the TE mode at different output ports and achieves mode separation.

For a magneto-optical waveguide with a transverse configuration, the applied magnetic field is perpendicular to the direction of light propagation, and the corresponding dielectric tensor is:

$\stackrel{<span\; class="notranslate">\; ^</span>}{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}<span\; class="notranslate">\; =</span>\left[\begin{array}{ccc}{\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; xx</span>}}& \mathrm{<span\; class="notranslate">\; 0</span>}& {\mathrm{<span\; class="notranslate">\; i\&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; xz</span>}}\\ \mathrm{<span\; class="notranslate">\; 0</span>}& {\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; yy</span>}}& \mathrm{<span\; class="notranslate">\; 0</span>}\\ <span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; i\&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; xz</span>}}& \mathrm{<span\; class="notranslate">\; 0</span>}& {\mathrm{<span\; class="notranslate">\; \&epsiv;</span>}}_{\mathrm{<span\; class="notranslate">\; zz</span>}}\end{array}\right]$

，其中L为TM模在磁光波导中的传输长度，Δβ为传播常数的差，显然，在正向传输和反向传输时，TM模会产生大小相等，符号相反的相位改变量。The off-diagonal elements of the dielectric tensor will couple the components of the electromagnetic field on the x, y, and z axes. Since εxy and εyz are both 0, only εxz exists, and couple the components of the electromagnetic field on the x and z axes, so that The phase of the TM mode along the x-axis direction changes without affecting the TE mode along the y-axis direction. The relationship between εxz and the Faraday rotation angle θ _F is: εxz=2nθ _F /k ₀ , where n is the refractive index of the film, and k ₀ is the wave number in vacuum. εxz will cause the propagation constants of the TM mode to be different during forward and reverse transmission, resulting in a non-reciprocal phase shift

, where L is the transmission length of the TM mode in the magneto-optical waveguide, and Δβ is the difference of the propagation constant. Obviously, the TM mode will produce phase changes of equal magnitude and opposite sign during forward and reverse transmission.

The beneficial effects of the present invention are:

1. The present invention adopts two Mach-Zehnder type integrated waveguide polarization mode splitters to combine waveguide type Faraday rotating mirror and half-wave plate to realize waveguide type polarization-independent optical circulator, wherein, adopt two optical power splitters, two The Mach-Zehnder type 4-port polarization mode separator composed of non-reciprocal transverse magneto-optical waveguides can realize the separation and synthesis of TE and TM modes, thereby easily forming a 4-port circulator with a simple structure. The polarization mode splitter adopts magneto-optical materials that are easy to integrate with semiconductor devices to realize phase adjustment, and is easy to integrate. Compared with the separated phase adjuster, the insertion loss is greatly reduced.

2. In the present invention, a multimode interferometer is used as the power divider of the Mach-Zehnder type integrated waveguide polarization mode separator, which has simple structure, low insertion loss, high bandwidth and polarization insensitivity, and is easy to integrate with optical communication systems.

3. The polarization mode separator adopted in the present invention avoids the disadvantages of large volume and difficult control of branch angle caused by the use of 3dB coupler, Y branch and other structures, and is small in size and easy to manufacture.

4. The polarization mode separator adopted in the present invention avoids the difficulties in the manufacturing process caused by changing the coupling state of the TM mode by using a metal covering layer structure, and reduces polarization crosstalk.

Description of drawings

The present invention will be further elaborated below in conjunction with the accompanying drawings and specific embodiments.

Figure 1: Schematic diagram of the structure of the waveguide-type polarization-independent optical circulator based on the polarization mode separator of the present invention;

Figure 2: Schematic diagram of the structure of the Mach-Zehnder integrated waveguide polarization mode separator used in the present invention.

Detailed ways

As shown in Figure 1, a waveguide-type polarization-independent optical circulator based on polarization mode separators includes a first polarization mode separator 1 and a second polarization mode separator 2, and mutual Connected waveguide type Faraday rotating mirror 3 and half-wave plate 4. The first polarization mode separator 1 and the second polarization mode separator 2 adopt Mach-Zehnder integrated waveguide polarization mode separators. The waveguide Faraday rotating mirror 3 adopts a non-reciprocal waveguide Faraday rotating mirror capable of deflecting the phase clockwise by π/4, and the half-wave plate 4 adopts a half-wave plate whose angle between the slow axis direction and the horizontal direction is π/8. The two output ports of the first polarization mode separator 1 are connected with the two input ports of the waveguide type Faraday rotating mirror 3, and the two output ports of the waveguide type Faraday rotating mirror 3 are connected with the two input ports of the half-wave plate 4, and the two input ports of the half-wave plate 4 are connected. The two output ports are connected with the two input ports of the second polarization mode separator 2 .

The working principle of the waveguide polarization-independent optical circulator based on the polarization mode separator is as follows:

For forward transmission light (defined according to the direction from left to right in Figure 1), the light is input from the input port of the first polarization mode separator 1, and after passing through the first polarization mode separator 1, it is divided into TM and TE modes, sequentially After the waveguide-type Faraday rotating mirror 3 and the half-wave plate 4 are converted to each other, the TM mode becomes the TE mode, and the TE mode becomes the TM mode, and then output after being coupled by the second polarization mode separator 2 to realize cross-state transmission. That is, if the light is input from the port 101 of the first polarization mode splitter 1, it will be output from the port 202 of the second polarization mode splitter 2, if the light is input from the port 102 of the first polarization mode splitter 1, then it will be output from the second Port 201 output of polarization mode separator 2.

For the reverse propagating light (defined by the direction from right to left in Fig. 1), the light is input from the input port of the second polarization mode splitter 2, and after passing through the second polarization mode splitter 2, it is divided into TM and TE modes, sequentially After passing through the half-wave plate 4 and the waveguide-type Faraday rotating mirror 3, the TM mode and the TE mode remain unchanged, and then output after being coupled by the first polarization mode separator 1 to realize straight-through transmission. That is, if the light is input from the port 201 of the second polarization mode splitter 2, it will be output from the port 101 of the first polarization mode splitter 1, and if the light is input from the port 202 of the second polarization mode splitter 2, then it will be output from the first Port 102 of the polarization mode separator 1 is output.

As shown in Figure 2, it is a schematic structural diagram of the Mach-Zehnder type integrated waveguide polarization mode separator adopted in the present invention, including a first optical power splitter 5, a second optical power splitter 6, a first magneto-optical waveguide 7 and a second optical power splitter. Magneto-optical waveguides8. The two input ports of the first optical power splitter 5 are connected to the first input waveguide 501 and the second input waveguide 502 respectively, and are used for inputting forward transmission light or outputting reverse transmission light. The first magneto-optical waveguide 7 and the second magneto-optical waveguide 8 adopt transverse configuration magneto-optical waveguides, the first magneto-optical waveguide 7 constitutes the first phase adjustment unit, and the second magneto-optic waveguide 8 constitutes the second phase adjustment unit. The two output ports of the first optical power splitter 5 are respectively connected to the two input ports of the second optical power splitter 6 through the first phase adjustment unit and the second phase adjustment unit, and the two output ports of the second optical power splitter 6 The output ports are connected to the first output waveguide 601 and the second output waveguide 602 respectively, and are used for outputting forward propagating light or inputting reverse propagating light. By selecting the appropriate length of the magneto-optical waveguide, the phase shifts produced by the first phase adjustment unit and the second phase adjustment unit to the TM mode are different by π, and the phase shifts to the TE mode are equal, so that the TM mode is in the polarization mode separator The straight-through state transmission is realized in the medium, and the cross-state transmission is realized in the TE mode, thereby realizing the separation of the TM mode and the TE mode.

More specifically, this embodiment adopts the following connection mode: the two output ports of the first optical power splitter 5 are respectively connected to the input ports of the first magneto-optical waveguide 7 and the second magneto-optical waveguide 8, and the first magneto-optical waveguide 7 1. The output ports of the second magneto-optical waveguide 8 are respectively connected to the two input ports of the second optical power splitter 6 .

In this embodiment, both the first optical power splitter 5 and the second optical power splitter 6 use multi-mode interferometers. By selecting the appropriate length of the magneto-optical waveguide, the first magneto-optic waveguide 7 and the second magneto-optic waveguide 8 respectively produce -π/2 phase shift and π/2 phase shift to the TM mode of the forwardly propagating light under the action of the magnetic field in the opposite direction. Transverse configuration magneto-optical waveguide that produces π/2 phase shift and -π/2 phase shift for the TM mode of the reverse propagating light, respectively, and produces equal phase shifts for the TE mode of the forward and reverse propagating light. That is: the TM mode of forward propagating light produces -π/2 phase shift and π/2 phase shift when passing through the first magneto-optical waveguide 7 and the second magneto-optical waveguide 8, while its TE mode passes through the first magneto-optical waveguide The waveguide 7 and the second magneto-optical waveguide 8 produce equal phase shifts; the TM mode of the reverse propagating light produces a phase shift of π/2 and -π/2 respectively when passing through the first magneto-optic waveguide 7 and the second magneto-optic waveguide 8 phase shift, and its TE mode produces equal phase shift when passing through the first magneto-optical waveguide 7 and the second magneto-optic waveguide 8 .

The working principle of this polarization mode separator is as follows:

For forward light (defined from left to right in Fig. 2), the first magneto-optical waveguide 7 makes the TM mode produce a -π/2 phase shift under the action of an external magnetic field, and the second magneto-optic waveguide 8 is in the opposite direction. Under the action of an applied magnetic field in the direction, the TM mode produces a phase shift of π/2, then the first phase adjustment unit formed by the first magneto-optical waveguide 7 makes the phase size of the TM mode change to be -π/2, and the second magneto-optical waveguide 8 constitutes The second phase adjustment unit makes the phase size of the TM mode change be π/2, and the phase size changed by the two phase adjustment units for the TM mode differs by π, so the TM mode realizes straight-through state transmission at the second optical power splitter 6 . The transverse configuration magneto-optical waveguide does not produce non-reciprocal phase shift to the phase of TE mode, the phases changed by the two phase adjustment units for TE mode are equal, and the cross-state transmission is realized at the second optical power splitter 6 . Therefore, if the light is input from the port 501 of the first optical power splitter 5, then the TM mode is output from the port 601 of the second optical power splitter 6, and the TE mode is output from the port 602 of the second optical power splitter 6; Input from the port 502 of the first optical power splitter 5 , the TM mode is output from the port 602 of the second optical power splitter 6 , and the TE mode is output from the port 601 .

For the reverse propagating light (defined by the direction from right to left in Fig. 2), the first magneto-optical waveguide 7 makes the TM mode produce π/2 phase shift under the effect of the applied magnetic field, and the second magneto-optic waveguide 8 in the opposite direction Under the action of an applied magnetic field, the TM mode produces a -π/2 phase shift, then the first phase adjustment unit formed by the first magneto-optical waveguide 7 and the second phase adjustment unit formed by the second magneto-optical waveguide 8 will change the TM mode The phase difference is also π, so the TM mode realizes straight-through state transmission at the first optical power splitter 5 . The transverse configuration magneto-optical waveguide does not produce non-reciprocal phase shift to the phase of TE mode, the phases changed by the two phase adjustment units for TE mode are equal, and the cross state transmission is realized at the first optical power splitter 5 . Therefore, if the light is input from the port 601 of the second optical power splitter 6, the TM mode is output from the port 501 of the first optical power splitter 5, and the TE mode is output from the port 502 of the first optical power splitter 5; Input from the port 602 of the second optical power splitter 6, the TM mode is output from the port 502 of the first optical power splitter 5, and the TE mode is output from the port 501.

Claims (3)

1. waveguide type polarization irrelevant optical circulator based on the polarization mode separation vessel, it is characterized in that comprising the first polarization mode separation vessel (1) and the second polarization mode separation vessel (2), be provided with interconnective waveguide type faraday rotation mirror (3) and half-wave plate (4) between two polarization mode separation vessels, the described first polarization mode separation vessel (1) and the second polarization mode separation vessel (2) comprise first optical power distributor (5) respectively, second optical power distributor (6), first magneto-optic waveguide (7) and second magneto-optic waveguide (8), two input ports of first optical power distributor (5) are connected with second input waveguide (502) with its first input waveguide (501) respectively, be used to import forward transmitted light or output reverse transfer light, described first magneto-optic waveguide (7), the transverse configurations magneto-optic waveguide is adopted in second magneto-optic waveguide (8), described first magneto-optic waveguide (7) constitutes first phase adjustment unit, described second magneto-optic waveguide (8) constitutes second phase adjustment unit, the phase shift phase difference of pi that first phase adjustment unit and second phase adjustment unit produce the TM mould, the phase shift that the TE mould is produced equates, two output ports of first optical power distributor (5) are connected with two input ports of second optical power distributor (6) with second phase adjustment unit by first phase adjustment unit respectively, two output ports of second optical power distributor (6) are connected with second output waveguide (602) with its first output waveguide (601) respectively, be used to export forward transmitted light or input reverse transfer light, described first optical power distributor (5), second optical power distributor (6) adopts the multiple-mode interfence instrument.

2. the waveguide type polarization irrelevant optical circulator based on the polarization mode separation vessel as claimed in claim 1, it is characterized in that described first magneto-optic waveguide (7), second magneto-optic waveguide (8) adopt respectively that the TM mould to forward transmission light produces respectively under in the opposite direction the action of a magnetic field-pi/2 phase moves, pi/2 phase moves, to the TM mould of reverse transfer light produce respectively pi/2 phase move ,-pi/2 phase moves, and align, the TE mould of reverse transfer light all produces the transverse configurations magneto-optic waveguide that equal phase is moved.

3. the waveguide type polarization irrelevant optical circulator based on the polarization mode separation vessel as claimed in claim 1, it is characterized in that described waveguide type faraday rotation mirror (3) adopts the nonreciprocal waveguide type faraday rotation mirror that can make the clockwise deflection π of phase place/4, it is the half-wave plate of π/8 that described half-wave plate adopts slow-axis direction and horizontal direction angle.

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