demultiplexer for CWDM system

Journal Pre-proof Design of 4-channel AWG Multiplexer/demultiplexer for CWDM system Yu Zheng, Xionghui Wu, Lianqiong Jiang, Yao Wu, Ji’an Duan

PII:

S0030-4026(19)31411-1

DOI:

https://doi.org/10.1016/j.ijleo.2019.163513

Reference:

IJLEO 163513

To appear in:

Optik

Received Date:

27 June 2019

Accepted Date:

1 October 2019

Please cite this article as: Zheng Y, Wu X, Jiang L, Wu Y, Duan J, Design of 4-channel AWG Multiplexer/demultiplexer for CWDM system, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163513

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Design of 4-channel AWG Multiplexer/demultiplexer for CWDM system

Yu Zheng, Xionghui Wu*, Lianqiong Jiang, Yao Wu, Ji’an Duan State key Laboratory of High Performance Complex Manufacturing, College of Mechanical and Electrical Engineering, Central South University, Changsha, Hunan,

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410083, China

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∗ Corresponding author: [email protected], [email protected]

Abstract-Arrayed Waveguide Grating (AWG) for Coarse wavelength division

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multiplexing (CWDM) system is a key component of above 100Gb/s high-speed optical transmission module in telecommunication and interconnects for data center.

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Based on the theory of light transmission, the relationships between structure parameters and optical performance of AWG chip are analyzed. Four-channel AWG MUX/DEMUX chips for CWDM system at O-band are designed and simulated

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according to the relationships. The simulation results show that the insertion loss of the MUX is below 0.8 dB with non-uniformity of 0.07 dB. The bandwidths at 1 dB

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and 3 dB are more than 10 nm and 17 nm, respectively. For DEMUX, Multi-mode interference (MMI) couplers are introduced at the start of output waveguides to

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produce flat-top spectral response with 1 dB bandwidth of 12 nm. The lowest insertion loss and adjacent crosstalk are 2.78 dB and below -20 dB, respectively.

Keywords: CWDM；AWG；MUX/DEMUX；Structure parameters；Insertion loss； MMI

1. Introduction Wavelength Division Multiplexing (WDM) devices have many advantages such as low insertion loss, high reliability [1], low crosstalk and so on, which are widely applied in optical fiber communication and sensor networks [2]. With VR/AR, Internet of things, 5G technologies appearing, there are much higher requirements for transmission bandwidth and rate. However, traditional discrete components can no longer satisfy the demands. CWDM MUX/DEMUX chips based on AWG can realize above 100Gb/s optical transmission, which have a very broad prospect in telecommunication and data center [3]. The chips have high requirements for optical

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performance. MUX chips require low insertion loss with low non-uniformity and large bandwidth while DEMUX chips need to perform low crosstalk besides those of

MUX chips. Therefore, it is of great significance and challenge to design chips with good optical performance.

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In recent years, much work has been done on the design and optimization of the CWDM AWG chips. C. J. Leo et al. designed and fabricated a polymer AWG as

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MUX/DEMUX chip for the first time. The insertion loss is about 7 dB with non-uniformity of 1.5 dB [4]. SOI-based DEMUX chips have been designed by N.

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Juhari et al. The simulation results show that the average insertion loss is 5.04 dB and the non-uniformity of insertion loss is less than 1.3 dB [5]. Compared with these

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studies above, we achieved much lower insertion loss and loss non-uniformity. Zou, Jun, et al. succeeded in making crosstalk less than -14 dB [6], while the crosstalk of DEMUX chips are generally required less than -18 dB at least. In order to solve the

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conflict between bandwidth and insertion loss of AWG, T. Demeester et al. proposed a multi-mode output waveguides AWG [7]. S. Kamei et al. designed and fabricated

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8-channel multi-mode output waveguides AWG as DEMUX chip, which successfully achieved low insertion loss of 1.7 dB, low crosstalk of -29 dB and large 1 dB bandwidth of 14 nm [8]. The optical performances are excellent but vertical tapered waveguides are difficult to fabricate and the multi-mode output waveguides cannot be coupled with single-mode fiber [9]. K. Hassan et al. proposed a design method of cascaded AWGs with insertion loss less than 4 dB and crosstalk less than -60 dB [10]. However, the technique made the footprint size of the chip larger which was not

conducive to integration. In this paper, the beam propagation method (BPM) under TE mode polarization was used to simulate the propagation of light in AWG chips [11]. The relationships between AWG structure parameters and optical performance are analyzed. The structure parameters include the arrayed waveguides spacing (Da), the output waveguides spacing (Do), the number of arrayed waveguides (Na) and the ratio of Roland circle to grating circle radius (r/R). The optical performance includes the insertion loss (IL), the insertion loss non-uniformity (IL non-uniformity), the bandwidth at 1 dB and 3 dB, the adjacent crosstalk. According to the International

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Telecommunication Unit (ITU) wavelength grid of CWDM network, 4-channel

SiO2-based AWG chips with channel spacing of 20 nm are designed (1271nm, 1291nm, 1311nm, 1331nm). The spectral response is Gaussian type for MUX, and

MMI structures are introduced at the start of output waveguide to produce flat-top

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2. AWG basic design

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spectral response for DEMUX [12].

Fig. 1. 1×4 channel AWG basic schematic layout

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Fig. 1 shows a schematic layout of a typical AWG, which consists of input/output

waveguides, arrayed waveguides with a fixed length difference and two slab waveguides based on Roland circle structure. When AWG is used as demultiplexer, the light is launched into an input waveguide, and Gauss far-field diffraction occurs when it enters the first slab waveguide also called as free propagation region (FPR) and continues to transmit in the arrayed waveguides with the same phase. When the light is transmitted to the end of the arrayed waveguides, linearly phase change is

generated due to the path length difference. Consequently, the light will be titled, and the focal will shift along the second slab waveguide. By placing output waveguides at proper positions along the second slab waveguide, separation of the different wavelength channel can be achieved. Also, AWG can be used as a multiplexer when the light is transmitted in the opposite direction. The basic parameters of AWG can be designed according to the following formulas: 𝑚𝜆0

(1)

𝑛𝑐

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𝛥𝐿 =

Where ΔL is the adjacent length difference between two adjacent arrayed waveguides, m is the diffraction order, the central wavelength is λ 0, and the effective

𝜆0 𝑛𝑐

𝑣

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𝐹𝑆𝑅 =

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refractive index of the rectangular waveguide is nc.

(2)

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𝑚𝑛𝑔

Where FSR is a free spectral range, which represents the minimum wavelength separation between two wavelength channels that map to the same output port. ng is

na

the group refractive index.

(3)

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𝐹𝑆𝑅 ≥ 𝑁𝛥𝜆

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Where N is the number of output channels and Δλ is the channel wavelength spacing. FSR determines the maximum number of output channels of AWG devices.

𝑅=

𝐷𝑜 𝐷𝑎 𝑛𝑠 𝑛𝑐 𝛥𝜆𝑚𝑛𝑔

(4)

Where R is the radius of grating circle, Do is the spacing of output waveguides, Da is the spacing of arrayed waveguides, and ns is the effective refractive index of slab

waveguides. The core size is 4.5 μm×4.5 μm with buried type waveguide. The relative refractive index difference of core and cladding material (SiO2) is △=1.5%, which make the minimum radius of bending waveguide is 2 mm [13,14]. The AWG simulation was based on BPM algorithm which will give accurate data such as radiation and loss in the waveguide.

3. Optimization of optical performance

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3.1 Insertion Loss CH1 CH2 CH3 CH4

6

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IL /dB

4

0

6

8

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2

10

12

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Da /μm

Fig. 2. Relationship between the insertion loss and the arrayed waveguides spacing.

na

10

6

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IL /dB

8

CH1 CH2 CH3 CH4

4 2 0

0

10

20

30

40

Na

Fig. 3. Relationship between the insertion loss and the number of arrayed waveguides.

Fig.2 shows the relationship between the insertion loss and the arrayed waveguides

spacing. It can be seen that the insertion loss increases as the arrayed waveguides spacing increases, showing a near linear correlation. The arrayed waveguides spacing increases from 6 μm to 12 μm, and the corresponding insertion loss increases by about 5 dB. It can be seen that the arrayed waveguides spacing has a great influence on the insertion loss. So, when design AWG, the arrayed waveguides spacing is as small as possible, which means width of gap between two adjacent arrayed waveguides should be very small. Fig.3 shows the relationship between the insertion loss and the number of the arrayed waveguides. The insertion loss decreases with the number of the arrayed waveguides increasing from 6 to 36. When the number of the arrayed

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waveguides is more than 25, there is no obvious change with the insertion loss. While the number of the arrayed waveguides is less than 25, the insertion loss is affected significantly. It is necessary to design the number of the arrayed waveguides large

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enough to make sure of low insertion loss.

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3.2 IL non-uniformity

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1.5

1.0

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IL nonuniformity /dB

2.0

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0.5

0

10

20

30

40

Na

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Fig. 4. Relationship between the insertion loss non-uniformity and the arrayed waveguides spacing.

IL nonuniformity /dB

4

3

2

1

0.4

0.6

0.8

1.0

1.2

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r/R

Fig. 5. Relationship between the insertion loss non-uniformity and the ratio of Roland circle to grating circle radius.

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Fig. 4 shows the relationship between the insertion loss non-uniformity and the

number of arrayed waveguides. The insertion loss non-uniformity decreases rapidly

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with the increase of the number of arrayed waveguides in the range smaller than 15. When the number of arrayed waveguides increases to more than 15, the trend of the

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insertion loss non-uniformity slows down. Therefore, the optimal scheme for lower insertion loss non-uniformity is to design a large enough number of arrayed waveguides. The slab waveguides of typical AWG devices adopt Roland circle

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structure where curvature radius of grating (R) is twice large as radius of Roland circle (r). However, the insertion loss non-uniformity produced by this structure is large. Fig. 5 describes the relationship between insertion loss non-uniformity and r/R

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under the condition of R being an invariance. It can be seen from the figure that the insertion loss non-uniformity decreases significantly with the increase of r/R in the

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range from 0.4 to 0.9 and the decreasing trend tends to be stable in the range from 0.9 to 1.2. In order to facilitate the design, r/R is set to be 1, which reduces the insertion loss non-uniformity by around 1.2dB compared with conventional structure. Undoubtedly, this is a simple and effective method of reducing the insertion loss non-uniformity in comparison with other proposed techniques [15,16].

3.3 1dB bandwidth

1dB bandwidth /nm

8 CH1 CH2 CH3 CH4

7

6

5 6

8

10

12

14

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Do /μm

Fig. 6. Relationship between the 1 dB bandwidth and the output waveguides spacing.

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CH1 CH2 CH3 CH4

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12

8

4 6

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1dB bandwidth /nm

16

12

18

24

30

36

na

Na

Fig. 7. Relationship between the 1 dB bandwidth and the number of output waveguides.

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Fig. 6 shows the relationship between the 1 dB bandwidth and the output waveguide spacing. In the illustrated range, the 1 dB bandwidth decreases with the increase of the output waveguides spacing and the change trend becomes slower.

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When the output waveguides spacing increases from 7 μm to 14 μm, the 1 dB bandwidth of each channel decreases by about 2.5 nm. Fig. 7 shows the relationship between the 1 dB bandwidth and the number of arrayed waveguides. In the illustrated range, the 1 dB bandwidth decreases as the number of arrayed waveguides increases. When the arrayed waveguide number exceeds 25, the 1 dB bandwidth almost does not change. When the number of arrayed waveguides increases from 6 to 36, the 1 dB bandwidth of each channel decreases by about 9.5 nm.

3.4 3 dB bandwidth Fig. 8 shows the relationship between the 3dB bandwidth and the output waveguide spacing. In the illustrated range, the 3dB bandwidth decreases as the output waveguides spacing increases and the change trend becomes slower. When the output waveguides spacing increases from 7 μm to 14 μm, the 3dB bandwidth of each channel decreases by about 4.4 nm. Fig. 9 shows the relationship between the 3dB bandwidth and the number of arrayed waveguides. In the illustrated range, the 3dB bandwidth decreases with the increase of the arrayed waveguide number, and the

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change trend becomes slower. When the arrayed waveguide number exceeds 25, the

3dB bandwidths almost do not change. When the number of arrayed waveguides increases from 6 to 36, the 3dB bandwidth of each channel decreases by about 15.5

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nm to 16.6 nm.

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CH1 CH2 CH3 CH4

12.0

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10.5

9.0

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3dB bandwidth /nm

13.5

6

8

10

12

14

Do /μm

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Fig. 8. Relationship between the 3 dB bandwidth and the output waveguides spacing.

3dB bandwidth /nm

25 CH1 CH2 CH3 CH4

20

15

10

6

12

18

24

30

36

Na

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Fig. 9. Relationship between the 3 dB bandwidth and the number of arrayed

waveguides.

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3.5 Adjacent crosstalk

Fig.10 shows the relationship between the adjacent crosstalk and the output

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waveguides spacing. Each channel has left crosstalk and right crosstalk, which are recorded as R and L respectively. In the illustrated range, the crosstalk of adjacent

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channels decreases with the increase of the output waveguide spacing, and the change trend becomes slower. It can be seen that when the output waveguides spacing is small, the adjacent crosstalk is greatly affected by the change of the output

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waveguides spacing. With the increase of the spacing of the output waveguides, the crosstalk of the adjacent channels no longer decreases obviously. Fig.11 shows the relationship between the crosstalk of adjacent channels and the number of arrayed

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waveguides. In the illustrated range, the crosstalk of adjacent channels decreases as the number of arrayed waveguides increases. When the number of arrayed

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waveguides increases from 6 to 36, the crosstalk of adjacent channels is reduced at least by 23.8 dB and the maximum is reduced by 30 dB. When the number of arrayed waveguides exceeds 25, the crosstalk of adjacent channels changes slightly.

Adcent crosstalk /dB

-6

CH1R CH2L CH2R CH3L CH3R CH4L

-9

-12

-15

4

6

8

10

12

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-18

Do /μm

Fig. 10. Relationship between the adjacent crosstalk and the output waveguides spacing.

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0 CH1 R CH2 L CH2 R CH3 L CH3 R CH4 L

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-10 -15 -20 -25 -30 -35

10

na

5

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Adcent crosstalk /dB

-5

15

20

25

30

35

40

Na

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Fig. 11. Relationship between the adjacent crosstalk and the number of arrayed waveguides.

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4. MUX/DEMUX design 4.1 MUX design

Transmission loss /dB

0

CH4 CH3 CH2 CH1

-5

-10

-15

-20

1.26

1.28

1.30

1.32

1.34

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Wavelength /μm

Fig. 12. Spectral response of MUX under TE mode polarization

According to the requirements of the insertion loss, the insertion loss

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non-uniformity and the bandwidth, an AWG MUX chip with Gaussian spectrum was designed based on the influence of structure parameters on optical performance. The

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main parameters are shown in Table 1. For low insertion loss, the arrayed waveguides spacing is set to be 5 μm, indicating the width of gap between adjacent arrayed

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waveguides is 0.5 μm. According to formula (2), the diffraction order m is 8, meaning that the corresponding FSR can contain 8 output channels, which is a traditional method of reducing insertion loss non-uniformity. We set the number of arrayed

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waveguides and the output waveguides spacing to be 11 μm and 5 μm respectively in order to broaden bandwidth. Fig. 12 shows the output spectrum of MUX under TE mode polarization at a central wavelength of 1301 nm. The simulation results show

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that the maximum insertion loss is 0.8 dB in CH1 with non-uniformity of 0.07 dB. The 1dB bandwidths of all 4 channels are larger than 10nm and the 3dB bandwidths

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are more than 17nm, which are 50% and 85% of channel spacing respectively. The chip size is 1.2mm × 7.8 mm.

4.2 MMI-based DEMUX Design

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（a）

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（b）

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（c）

Fig.13. MMI. (a)Structure, (b) Optical field distribution, (c）Location.

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The traditional AWG spectral response is usually Gaussian type, which is sensitive

to wavelength drift. It cannot satisfy low crosstalk and large bandwidth at the same

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time. In order to solve the problem, many methods have been proposed to obtain flat-top spectrum, such as using an MMI [17] or a Mach-Zehnder interferometer (MZI) [18] in the input waveguide, multiple gratings [19] and so on. In this paper, we present the method of introducing MMI at the start of the output waveguides [12]. Based on the property of self-imaging, one or multiple images of the input field profile can be reproduced periodically along the propagation direction of the guide [20]. When structure parameters are set appropriately, a hump type field can be

generated at the output of MMI and the flat-top spectral response can be formed by convolution with the input Gaussian field. In order to reduce loss caused by the structural change, MMI is designed to consist of a parabolic and a rectangular waveguide. The MMI structure, optical field distribution and location in AWG are shown in Fig.13 (a), (b) and (c), respectively. 0

CH4 CH3 CH2 CH1

-10 -15 -20 -25 -30 -35 1.27

1.28

1.29

1.30

1.31

1.32

1.33

1.34

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-40 1.26

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Transmission loss /dB

-5

Wavelength /nm

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Fig. 14. Spectral response of DEMUX under TE mode polarization

Table 2 shows the main parameters of AWG DEMUX. The arrayed waveguides

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spacing and the diffraction order m with corresponding FSR of the DEMUX are the same as those of the MUX. We set the number of arrayed waveguides and the output waveguides spacing to be 30 and 11.6 μm respectively in order to obtain low adjacent

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crosstalk. Fig. 14 shows the output spectrum of DEMXU under TE mode polarization at a central wavelength of 1301 nm. The simulation results show that the minimum

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channel insertion loss is 2.78 dB in CH3 with non-uniformity of 0.15 dB. The 1 dB bandwidths of all 4 channels are above 12 nm which is 60% of channel spacing. The 3

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dB bandwidths are larger than 16 nm or 75% of channel spacing. The adjacent crosstalk is less than -20 dB. The overall size of the chip is 1.6 mm ×8.4 mm. Compared to the Gaussian AWG, the 1 dB bandwidth is broadened around 2 nm and the flat-top spectrum is successfully obtained. As a trade-off, the insertion loss increases about 2 dB mainly due to mode conversion loss of MMI.

5.Conclusion In this paper, we present the design and optimization for AWG MUX/DEMUX chips for CWDM system, which have advantages of good optical performance, simple design and fabrication and compact chip size. By analyzing the influence of AWG structure parameters on its optical performance, it is found that the relationships between them are complicated. The arrayed waveguides spacing is the most important factor affecting the insertion loss. The structure parameter r/R of slab waveguide is set to be 1, which greatly improves the insertion loss non-uniformity. There is a conflict between bandwidth and adjacent crosstalk in traditional AWG. Introducing MMI

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structures at the start of output waveguides effectively flatten the top of spectrum but increase additional loss around 2 dB. This paper will provide certain reference value for the design and optimization of CWDM AWG chips.

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Acknowledgments:

Supported by the National Key Research and Development Program of China (Grant

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No. 2017YFB1104800), the National Natural Science Foundation of China (Grant No. 51475479), the Key Research and Development Program of Hunan Province, China

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(Grant No. 2016GK2098), the State Key Laboratory of High Performance Complex Manufacturing, Central South University (Grant No. ZZYJKT2017-07), and the Key

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Laboratory for Precision & Non-traditional Machining of Ministry of Education,

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Dalian University of Technology (Grant No. JMTZ201804).

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[11] M. K. Smit, C.V. Dam, PHASR-based WDM-devices: Principles, design and application, IEEE J. Sel. Top. Quantum Electron. 2 (1996) 236-250. [12] H. C. Lu, W. S. Wang, Cyclic arrayed waveguide grating devices with flat-top passband and uniform spectral response, IEEE Photon. Tech. Lett. 20 (2008) 3-5. [13] Hibino, Y, Recent advances in high-density and large-scale AWG multi/demultiplexers with higher index-contrast silica-based PLCs, IEEE J. Sel.Top.

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Table 1. Main parameters of DEMUX

Parameters

Value

Central wavelength

1.301μm

Channel spacing

20nm

Diffraction order

8 7.121μm

Path length difference FPR length

228.932μm

Number of arrayed waveguides

11

Output waveguides spacing

5μm

Arrayed waveguides spacing

5μm 162.625nm

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Free Spectrum Range

Table 2. Main parameters of DEMUX

Parameters

Value

1.301μm

Channel spacing

20nm

Diffraction order

8

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Path length difference FPR length

7.121μm

549.436μm 30

Output waveguides spacing

11.6μm

Arrayed waveguides spacing

5μm

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Number of arrayed waveguides

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Free Spectrum Range

162.625μm

Width of rectangular waveguide

10.1μm

Length of rectangular waveguide

82μm

Length of parabolic waveguide

20μm

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Central wavelength