阅读说明：本技术 一种提高偏振消光比的方法、装置及系统 (Method, device and system for improving polarization extinction ratio ) 是由 任梅珍 周来 于 2020-12-08 设计创作，主要内容包括：本申请公开了一种提高偏振消光比的方法、装置及系统,其中,提高偏振消光比的方法,包括：确定宽波导的长度；确定宽波导的长度后,对相移波导的长度进行调节,获得相移波导的均衡后长度；确定相移波导的均衡后长度,再对宽波导的长度进行调节,获得宽波导的调节后长度；根据相移波导的均衡后长度和宽波导的调节后长度对TE模和TM模的消光比进行判断,若满足预设条件,则确定均衡后相移波导的长度和调节后宽波导的长度为调节参数,根据调节参数进行偏振消光比调节；若不满足预设条件,则重新确定宽波导的长度。本申请同时优化宽波导L1w和相移波导L2n-1的长度,调节MZI两臂之间的双折射和臂长差,使PBS的两个输出端的PER同时得到提高。(The application discloses a method, a device and a system for improving polarization extinction ratio, wherein the method for improving the polarization extinction ratio comprises the following steps: determining the length of the wide waveguide; after the length of the wide waveguide is determined, the length of the phase shift waveguide is adjusted to obtain the balanced length of the phase shift waveguide; determining the balanced length of the phase shift waveguide, and adjusting the length of the wide waveguide to obtain the adjusted length of the wide waveguide; judging the extinction ratios of the TE mode and the TM mode according to the equalized length of the phase shift waveguide and the adjusted length of the wide waveguide, if the preset conditions are met, determining the length of the equalized phase shift waveguide and the adjusted length of the wide waveguide as adjusting parameters, and adjusting the polarization extinction ratio according to the adjusting parameters; and if the preset condition is not met, re-determining the length of the wide waveguide. The method simultaneously optimizes the lengths of the wide waveguide L1w and the phase-shift waveguide L2n _1, and adjusts the birefringence and the arm length difference between the two arms of the MZI, so that the PERs of the two output ends of the PBS are simultaneously improved.)

1. A method for increasing polarization extinction ratio, comprising:

determining the length of the wide waveguide;

after the length of the wide waveguide is determined, the length of the phase shift waveguide is adjusted to obtain the balanced length of the phase shift waveguide;

determining the balanced length of the phase shift waveguide, and adjusting the length of the wide waveguide to obtain the adjusted length of the wide waveguide;

judging the extinction ratios of the TE mode and the TM mode according to the equalized length of the phase shift waveguide and the adjusted length of the wide waveguide, if the preset conditions are met, determining the equalized length of the phase shift waveguide and the adjusted length of the wide waveguide as adjusting parameters, and adjusting the polarization extinction ratios according to the adjusting parameters; and if the preset condition is not met, re-determining the length of the wide waveguide.

2. The method of claim 1, wherein the predetermined condition is that the extinction ratio of both TE mode and TM mode is greater than 30 dB.

3. The method of claim 1, wherein the equalized length of the phase-shifting waveguide is 1920_μAdjusted length of m, wide waveguide 3650_μm。

4. A polarizing beam splitter, comprising: a first coupler and a second coupler; wherein the first coupler has a first output terminal and a second output terminal; the second coupler has a third input terminal and a fourth input terminal;

the first output end is connected with the first narrow waveguide, the first tapered transition waveguide, the wide waveguide, the second tapered transition waveguide and the second narrow waveguide in sequence, and the second narrow waveguide is connected with the third input end;

the second output end is connected with the phase shift waveguide, the third tapered transition waveguide, the fourth tapered transition waveguide and the third narrow waveguide in sequence, and the third narrow waveguide is connected with the fourth input end;

wherein the adjusted length of the wide waveguide and the equalized length of the phase-shifted waveguide are obtained by adjusting the method for improving polarization extinction ratio according to any one of claims 1 to 3.

5. The polarization beam splitter of claim 4 wherein the first coupler and the second coupler each employ any one of a directional coupler, a Y-type coupler, or an MMI-type coupler.

6. A decoding chip, comprising: the polarizing beam splitter, half-wave plate, asymmetric mach-zehnder interferometer, and phase modulating electrode of claim 4 or 5; the polarization beam splitter is connected with the half-wave plate; the half-wave plate is connected with the asymmetric Mach-Zehnder interferometer; the asymmetric Mach-Zehnder interferometer is connected with the phase modulation electrode.

7. The decoding chip of claim 6, wherein the polarization beam splitter is connected to the half-wave plate through a waveguide; the half-wave plate is connected with the asymmetric Mach-Zehnder interferometer through a waveguide; the asymmetric Mach-Zehnder interferometer and the phase modulation electrode are connected through a waveguide.

8. A coding and decoding system, which is characterized in that the coding end and the decoding end are connected through an optical fiber, wherein the decoding end comprises the decoding chip of claim 6 or 7.

9. The encoding and decoding system of claim 8, wherein the encoding end comprises a picosecond pulse laser, an encoding chip, a lithium niobate phase modulator A and an attenuator which are connected in sequence; the decoding end also comprises a lithium niobate phase modulator B, and the lithium niobate phase modulator B is respectively connected with the attenuator and the decoding chip.

10. The encoding and decoding system according to claim 9, wherein the picosecond pulse laser, the encoding chip, the lithium niobate phase modulator a and the attenuator are connected in sequence by an optical fiber; the lithium niobate phase modulator B is respectively connected with the attenuator and the decoding chip through optical fibers.

Technical Field

The present application relates to the field of integrated optics technologies, and in particular, to a method, an apparatus, and a system for improving a polarization extinction ratio.

Background

A silica-on-silicon Polarizing Beam Splitter (PBS) has the advantages of low loss and easy integration. PBS was made based on silica-on-silicon platforms, with several protocols: the first is to sputter an amorphous silicon (amorphous-Si) film on the upper cladding layer to control the birefringence of the waveguide, and this scheme requires a Laser Trimming (Laser Trimming) technique to precisely control the birefringence of the waveguide, which is a complicated process. The second is to insert a thin film of a lambda/4 wave plate into a symmetric MZI (Mach-Zehnder interferometer) to change the optical path difference of TE and TM modes (where lambda represents the wavelength, and TE and TM represent the polarization modes of light), which can reduce the temperature and wavelength dependence of the device. The third is to etch trenches in both arms of the MZI and fill the low index material to change the birefringence of both arms, which can achieve high polarization extinction ratios (greater than 30 dB). And the fourth is to realize PBS by using the effect of the birefringence of the silica-on-silicon waveguide and the width of the waveguide core area, and the proposal does not need special processes, such as amorphous silicon film deposition, laser trimming technology, groove etching and the like, and has low insertion loss and polarization extinction ratio more than 20 dB. The PBS realized by the fourth scheme has the advantages of simple process, low loss, convenience in integration with other devices of the PLC platform and the like.

The Polarization Extinction Ratio (PER) of the Polarization beam splitter is an important evaluation index. The PER of a PBS based on the birefringence principle of waveguides of different widths is generally small due to the small birefringence of silica waveguides and process errors. To solve this problem, the general approach is to introduce a phase difference in both arms of the MZI to compensate for the phase error between the TE and TM modes. After compensation, the PER of the device can reach 20 dB.

Disclosure of Invention

The method, the device and the system for improving the polarization extinction ratio adopt the simultaneous optimization of the lengths of the wide waveguide L1w and the phase-shift waveguide L2n _1, and adjust the birefringence and the arm length difference between two arms of the MZI, so that the PER of two output ends of the PBS is improved simultaneously.

To achieve the above object, the present application provides a method for improving a polarization extinction ratio, comprising: determining the length of the wide waveguide; after the length of the wide waveguide is determined, the length of the phase shift waveguide is adjusted to obtain the balanced length of the phase shift waveguide; determining the balanced length of the phase shift waveguide, and adjusting the length of the wide waveguide to obtain the adjusted length of the wide waveguide; judging the extinction ratios of the TE mode and the TM mode according to the equalized length of the phase shift waveguide and the adjusted length of the wide waveguide, if the preset conditions are met, determining the equalized length of the phase shift waveguide and the adjusted length of the wide waveguide as adjusting parameters, and adjusting the polarization extinction ratios according to the adjusting parameters; and if the preset condition is not met, re-determining the length of the wide waveguide.

As above, the preset condition is that the extinction ratios of the TE mode and the TM mode are both greater than 30 dB.

As above, where the equalized length of the phase-shifting waveguide is 1920 μm, the adjusted length of the wide waveguide is 3650 μm.

The present application further provides a polarizing beam splitter comprising: a first coupler and a second coupler; wherein the first coupler has a first output terminal and a second output terminal; the second coupler has a third input terminal and a fourth input terminal; the first output end is connected with the first narrow waveguide, the first tapered transition waveguide, the wide waveguide, the second tapered transition waveguide and the second narrow waveguide in sequence, and the second narrow waveguide is connected with the third input end; the second output end is connected with the phase shift waveguide, the third tapered transition waveguide, the fourth tapered transition waveguide and the third narrow waveguide in sequence, and the third narrow waveguide is connected with the fourth input end; wherein the adjusted length of the wide waveguide and the equalized length of the phase-shift waveguide are obtained by the method for improving the polarization extinction ratio.

As above, the first coupler and the second coupler each employ any one of a directional coupler, a Y-type coupler, or an MMI-type coupler.

The present application further provides a decoding chip, including: the polarization beam splitter, the half-wave plate, the asymmetric Mach-Zehnder interferometer and the phase modulation electrode; the polarization beam splitter is connected with the half-wave plate; the half-wave plate is connected with the asymmetric Mach-Zehnder interferometer; the asymmetric Mach-Zehnder interferometer is connected with the phase modulation electrode.

The method, wherein the polarization beam splitter and the half-wave plate are connected through a waveguide; the half-wave plate is connected with the asymmetric Mach-Zehnder interferometer through a waveguide; the asymmetric Mach-Zehnder interferometer and the phase modulation electrode are connected through a waveguide.

The application also provides a coding and decoding system, which comprises a coding end and a decoding end which are connected through an optical fiber, wherein the decoding end comprises the decoding chip.

As above, wherein the encoding end comprises a picosecond pulse laser, an encoding chip, a lithium niobate phase modulator A and an attenuator which are connected in sequence; the decoding end also comprises a lithium niobate phase modulator B, and the lithium niobate phase modulator B is respectively connected with the attenuator and the decoding chip.

As above, wherein the picosecond pulse laser, the coding chip, the lithium niobate phase modulator a and the attenuator are connected in sequence through the optical fiber; the lithium niobate phase modulator B is respectively connected with the attenuator and the decoding chip through optical fibers.

The method simultaneously optimizes the lengths of the wide waveguide L1w and the phase shift waveguide L2n _1, and adjusts the birefringence and the arm length difference between two arms of the MZI, so that PERs of two output ends of the PBS are simultaneously improved.

Drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments described in the present application, and other drawings can be obtained by those skilled in the art according to the drawings.

FIG. 1 is a schematic diagram of an embodiment of a polarizing beam splitter;

FIG. 2 is a schematic structural diagram of an embodiment of a decoding chip;

FIG. 3 is a schematic structural diagram of an embodiment of a QKD dual-polarization phase encoding and decoding system;

FIG. 4 is a flow chart of one embodiment of a method of adjusting a polarization extinction ratio;

FIG. 5 is a graph of the loss variation of TE and TM modes at two output ports by adjusting the length of the phase-shifting waveguide L2n _1 when the length L1w of the wide waveguide is 4000 μm; wherein IL is the insertion loss of the device;

FIG. 6 is a graph showing the loss variation of TE and TM modes at two output ports when L2n _1 is 1920 μm and the length of the wide waveguide L1w is changed.

Detailed Description

The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

As shown in fig. 1, the present application provides a polarization beam splitter, including a first coupler 1 and a second coupler 2; wherein the first coupler 1 has a first output 11 and a second output 12; the second coupler 2 has a third input 21 and a fourth input 22. The first output end 11 is connected with the first narrow waveguide L1n _1, the first tapered transition waveguide, the wide waveguide L1w, the second tapered transition waveguide and the second narrow waveguide L1n _2 in sequence, and the second narrow waveguide L1n _2 is connected with the third input end 21. The second output end 12 is connected with the phase shift waveguide L2n _1, the third tapered transition waveguide, the fourth tapered transition waveguide and the third narrow waveguide L2n _2 in sequence, and the third narrow waveguide L2n _2 is connected with the fourth input end 22; a tapered transition waveguide is also introduced in this arm of the MZI to balance the optical path difference and transmission loss of the two arms. Wherein the adjusted length of the wide waveguide L1w and the equalized length of the phase-shifted waveguide L2n _1 are obtained by adjusting the polarization extinction ratio as described below.

Further, the first coupler 1 and the second coupler 2 each employ any one of a directional coupler, a Y-type coupler, or an MMI-type coupler.

Further, as shown in fig. 2, the present application provides a decoding chip, including: the above-mentioned polarization beam splitter, Half-Wave Plate (HWP), Asymmetric Mach-Zehnder interferometer (AMZI), and phase modulation electrode; the polarization beam splitter is connected with the half-wave plate; the half-wave plate is connected with the asymmetric Mach-Zehnder interferometer; the asymmetric Mach-Zehnder interferometer is connected with the phase modulation electrode. The decoding chip is of a single-chip integrated structure, and is smaller and higher in precision compared with an existing decoding system realized through optical fibers.

Further, the polarization beam splitter is connected with the half-wave plate through a waveguide; the half-wave plate is connected with the asymmetric Mach-Zehnder interferometer through a waveguide; the asymmetric Mach-Zehnder interferometer and the phase modulation electrode are connected through a waveguide.

Specifically, in the BB84 quantum key distribution system (QKD), the polarization beam splitter can be used to implement a QKD dual-polarization phase decoding chip, and the schematic diagram of the decoding chip is shown in fig. 2. The decoding chip combines polarization multiplexing and phase decoding, can increase the code rate of the QKD system by two times, and does not need a high-precision single-photon detector. The PBS is used to separate the two pulses of orthogonal polarization, where one pulse (arriving first at Bob) first passes through the HWP, rotating the polarization state by 90 degrees, thus keeping it the same as the other pulse; and then through a delay line (with a delay time consistent with the Alice side) so as to be consistent in time with another pulse. Another pulse (arriving back at Bob) passes through the short arm of the AMZI, and a phase modulating electrode on the short arm can be used to correct the phase. Two pulses that are temporally coincident and have the same polarization then interfere at the directional coupler at 50/50, and only one photon pulse is received by the detector, which reduces the time resolution requirements of a single photon detector.

As shown in fig. 3, the present application provides a codec system (QKD dual-polarization phase codec system) including an encoding end (Alice encoding end) and a decoding end (Bob decoding end) connected by an optical fiber, where the decoding end includes the above decoding chip.

Furthermore, the encoding end comprises a picosecond pulse Laser (Laser), an encoding chip, a lithium niobate Phase Modulator A (PMA) and an Attenuator (ATT) which are connected in sequence; the decoding end also comprises a lithium niobate Phase Modulator B (PMB), and the lithium niobate phase modulator B is respectively connected with the attenuator and the decoding chip.

Further, the picosecond pulse laser, the coding chip, the lithium niobate phase modulator A and the attenuator are sequentially connected through optical fibers; the lithium niobate phase modulator B is respectively connected with the attenuator and the decoding chip through optical fibers.

Specifically, at an Alice encoding end, a 1550nm picosecond pulse laser emits pulse laser with repetition frequency of 1GHz and pulse width of 40ps, the pulse light is input into a 3dB coupler and then is divided into two parts, and after one part is transmitted through a waveguide, the polarization state is kept unchanged; the other part passes through a Polarization Rotator (PR) and a delay line, AMZI has a delay time Δ t, PR is implemented by a HWP plate, and PR can rotate the Polarization state of the light by 90 °. At this time, the two pulses are separated by Δ t in time and orthogonal in polarization, and the two pulses are input to a lithium niobate Phase Modulator a (Phase Modulator, PM) through a 3dB coupler and then loaded with phases at randomFour options are 0 °, 90 °, 180 °, 270 °. After strong attenuation of the pulse pairs, the average intensity of the pulses input into the fiber is such that each pulse pair contains 0.1-0.2 photons. The pulse pair reaches the Bob decoding end after being transmitted by the optical fiber, and is randomly loaded with phase through a lithium niobate phase modulator BThere are two options for 0 ° or 90 °. Then the pulse pair is input into a decoding chip, the working principle of the decoding chip is as described above, the output result is related to the phase difference between the phase modulators of the Alice encoding end and the Bob decoding end, and when the phase difference is between the phase modulators of the Alice encoding end and the Bob decoding endThen the output pulse will be output from Arm "0"; when the phase difference is betweenThen the output pulse will be output from Arm "1"; when the phase difference is betweenOr 270 deg., the output pulse may appear at APD (single photon detector) "0" or at APD "1", and this portion of the data will be discarded after the public screening.

As shown in fig. 4, the present application provides a method for improving polarization extinction ratio, comprising the steps of:

s610: the length of the wide waveguide is determined.

Specifically, the length of wide waveguide L1w is determined, where the length of wide waveguide L1w has the values: 2000-6000 microns.

S620: after the length of the wide waveguide is determined, the length of the phase-shift waveguide is adjusted to obtain the equalized length of the phase-shift waveguide.

Specifically, after the length of the wide waveguide is determined, the length of the phase shift waveguide L2n _1 is adjusted, and the length of the phase shift waveguide L2n _1 that makes the extinction ratios of the TE and TM modes more balanced is determined to be the balanced length of the phase shift waveguide L2n _ 1.

As shown in FIG. 5, for the silica waveguide having a refractive index difference of 0.75%, when the narrow waveguide width is 6 μm and the wide waveguide width is 19 μm, the birefringence due to the waveguide width is maximized, and Δ B ≈ 2.5 × 10^-4And the birefringence is primarily stress birefringence. When the length L1w of the wide waveguide is 4000 μm, the length of the phase shift waveguide L2n _1 is adjusted, the loss change curves of the TE mode and the TM mode at two output ports are shown in FIG. 5, and when L2n _1 is 1890 μm, the TE mode reaches the maximum extinction ratio of 40.19 dB; when L2n _1 is 1940 μm, the maximum extinction ratio of the TM mode is 39.17 dB. If L2n _1 is selected to be 1890 μm, the extinction ratio of TE mode is 40.19dB, and the extinction ratio of TM mode is 17.82 dB; if L2n _1 is selected to be 1940 μm, the extinction ratio of TE mode is 18.51dB, and the extinction ratio of TM mode is 39.17 dB. The TE and TM modes cannot simultaneously achieve the maximum extinction ratio, which reduces the PER of the overall device.

S630: and determining the balanced length of the phase shift waveguide, and adjusting the length of the wide waveguide to obtain the adjusted length of the wide waveguide.

Specifically, the equalized length of the phase-shift waveguide is determined, the length of the wide waveguide is adjusted, and the length of the wide waveguide L1w that maximizes the extinction ratio between the TE and TM modes is determined as the adjusted length of the wide waveguide.

S640: judging the extinction ratios of the TE mode and the TM mode according to the equalized length of the phase shift waveguide and the adjusted length of the wide waveguide, if the preset conditions are met, determining the length of the equalized phase shift waveguide and the adjusted length of the wide waveguide as adjusting parameters, and adjusting the polarization extinction ratio according to the adjusting parameters; if the preset condition is not satisfied, S610 is executed again.

Specifically, the preset condition is that the extinction ratios of the TE mode and the TM mode are both greater than 30 dB. Judging the extinction ratios of the TE mode and the TM mode according to the equalized length of the phase-shift waveguide and the adjusted length of the wide waveguide, if the extinction ratios of the TE mode and the TM mode are both larger than 30dB, the equalized length of the phase-shift waveguide L2n _1 and the adjusted length of the wide waveguide L1w reach the best, and if at least one of the extinction ratios of the TE mode and the TM mode is smaller than or equal to 30dB, executing S610 again.

Further, as an example, the equalized length of the phase-shifting waveguide L2n _1 is 1920 μm, and the adjusted length of the wide waveguide L1w is 3650 μm.

Specifically, when the equalized length of the phase-shift waveguide L2n _1 is 1920 μm, the extinction ratios of the TE mode and the TM mode are more equalized, and the length of the wide waveguide L1w is changed to obtain the loss change curves of the TE mode and the TM mode at two output ports as shown in fig. 6, and when the adjusted length of the wide waveguide L1w is 3650 μm, the extinction ratio of the TE mode is 35.75dB, and the extinction ratio of the TM mode is 37.23 dB. At this time, the extinction ratios of both TE and TM are greater than 35dB, and the polarization extinction ratio of the whole polarization beam splitter is greater than 35 dB.

To obtain a better PER, in the selection of parameters for actually making a PBS, it is necessary to adjust not only the arm length difference between the arms of the MZI, but also the birefringence between the arms (i.e., the length of the wide waveguide). This is related to the working principle of the MZI-type PBS.

Specifically, the MZI PBS operates according to the following principle:

wherein, I₁Input power of port 1; i is₃Is the output power of port 3; i is₄Is the output power of port 4; k is the coupling efficiency of the two couplers (assuming the coupling efficiency of the two couplers is equal); delta phi is the phase between the two arms of the MZI interferometerAnd (4) poor.

Where Δ φ is the phase difference between the two arms, λ is the wavelength in vacuum, Δ n is the change in refractive index in the waveguide, and Δ L is the change in arm length difference.

TE and TM modes are output from ports4 and ports3, respectively, when the following conditions are satisfied:

wherein, is_TEIs the phase difference of the TE mode between the two arms; delta phi_TMIs the phase difference of TM mode between two arms; n and M are integers.

TE and TM modes are output from ports3 and ports4, respectively, when the following conditions are satisfied:

wherein, is_TEIs the phase difference of the TE mode between the two arms; delta phi_TMIs the phase difference of TM mode between two arms; n and M are integers.

Deriving the phase difference between the TE mode and the TM mode between the two arms to obtain:

where λ is the wavelength in vacuum, w_nIs the width, w, of the narrower waveguide_wIs the width of the wider waveguide, n_TEAnd n_TMMode refractive indices, L, of TE and TM, respectively₁(w_n) Is the length of waveguide L1n _1+ L1n _ 2; l is₂(w_n) Is the length of the waveguide L2n _1+ L2n _2, and the path difference between the two arms is L₂(w_n)-L₁(w_n)-L₁(w_w) And Δ L is the amount of change in arm length difference，ΔL＝L₂(w_n)－L₁(w_n)；L₁(w_w) Is the length of wide waveguide L1 w.

Then:

combining equation (6) with equation (3) yields:

then, the following results were obtained:

where Δ B is the birefringence of the waveguide, and Δ B ═ n_TM-n_TE，n_TEAnd n_TMMode refractive indices, L, of TE and TM, respectively₁(w_w) Is the length of L1 w.

As known from the working principle of the PBS, the polarization extinction ratio is related to the phase difference between TE modes or TM modes of two output ports, and when the phase difference is pi, the ideal polarization extinction ratio can be obtained. After Δ B is determined, the phase difference is compared with Δ L and L in equation (8)₁(w_w) Regarding, that is, the lengths of the wide waveguide L1w and the phase-shift waveguide L2n _1 in fig. 1, the length of the wide waveguide L1w and the length of the phase-shift waveguide L2n _1 are optimized at the same time when determining the parameters, so that the PER of the two output ends of the PBS can be improved at the same time.

The method simultaneously optimizes the lengths of the wide waveguide L1w and the phase shift waveguide L2n _1, and adjusts the birefringence and the arm length difference between two arms of the MZI, so that PERs of two output ends of the PBS are simultaneously improved.

While the preferred embodiments of the present application have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, the scope of protection of the present application is intended to be interpreted to include the preferred embodiments and all variations and modifications that fall within the scope of the present application. It will be apparent to those skilled in the art that various changes and modifications may be made in the present application without departing from the spirit and scope of the application. Thus, if such modifications and variations of the present application fall within the scope of the present application and their equivalents, the present application is intended to include such modifications and variations as well.