Silicon Photonics MDM Optical Power Divider with NOMA for Flexible Data Rate Allocation

Introduction

Silicon photonics (SiPh) integrated circuits have emerged as a promising technology for optical communication systems and interconnects. To meet the ever-increasing demand for higher transmission capacity, advanced multiplexing techniques like mode-division-multiplexing (MDM) have been employed. MDM utilizes multiple spatial modes in a single waveguide to transmit independent data streams, effectively multiplying the overall capacity.

In this tutorial, we explore a novel SiPh MDM optical power divider that supports single (TE0), dual (TE1) and triple (TE2) modes. By incorporating non-orthogonal multiple access (NOMA), this power divider enables flexible data rate allocation to multiple users, making it an attractive solution for system-on-chip applications.

Device Architecture

The proposed MDM optical power divider consists of two main sections, as shown in Fig. 1(a). The first section is the mode up-conversion stage, realized using asymmetric directional couplers (ADCs). The second section is a Y-branch splitter that divides the higher-order modes into equal power outputs.

Fig. 1. (a) Schematic of the SiPh MDM optical power divider. FDTD simulation results of the MDM optical power divisions at (b) TE0, (c) TE1, (d) TE2. (e) Photo of the device, illustrating the magnified section of the ADCs.

The mode up-conversion is achieved through the ADCs, where the input TE0 mode is converted to TE1, TE1 is converted to TE3, and TE2 is converted to TE5. The Y-branch then splits the higher-order modes into two equal power outputs, effectively dividing the TE0, TE1, and TE2 modes, as illustrated in Figs. 1(b)-(d).

Fig. 1(e) shows the fabricated device, where the silicon input waveguide dimensions are 0.45 μm × 0.22 μm, supporting the TE0 mode. The buried oxide layer (BOX) thickness is 2 μm, and the bus waveguide widths are optimized for TE1 and TE2 mode conversion.

Experimental Setup and NOMA Algorithm

To demonstrate flexible data rate allocation, the MDM optical power divider is integrated with NOMA-OFDM, as shown in Fig. 2(a). The NOMA-OFDM signal is generated by an arbitrary waveform generator (AWG) and modulated onto an optical carrier using a Mach-Zehnder modulator (MZM). The modulated signal is coupled into and out of the SiPh MDM power divider via on-chip grating couplers (GC).

Fig. 2. (a) Experimental setup of the proposed MDM optical power divider with NOMA for flexible data rate allocation to users. (b) Schematic diagram of the NOMA operation.

The NOMA algorithm, depicted in Fig. 2(b), utilizes superposition coding and successive interference cancellation (SIC). First, channel estimation is performed to obtain channel state information (CSI) and determine the signal-to-noise ratio (SNR) distribution. Based on the channel coefficients, the decoding order is established, with the user having the better channel decoding first.

In this experiment, data-1 uses a smaller power P1, and data-2 uses a larger power P2, as channel 1 has better channel coefficients than channel 2. The power ratio (PR) is set to P2/P1. At the receiver, maximum likelihood (ML) detection is used to decode data-2 first, followed by data-1 after canceling the interference from data-2.

Results and Discussion

To evaluate the performance and flexibility of the MDM optical power divider with NOMA, different power ratios (PR) are used. Figs. 3(a)-(f) show the SNR distribution and bit-loading for user-1 and user-2 at TE0, TE1, and TE2 modes, with PR = 2.0 and PR = 3.0.

Fig. 3. SNR and bit-loadings for different mode channels and power ratios. (a) User-1 @ TE0, (b) User-2 @ TE0, (c) User-1 @ TE1, (d) User-2 @ TE1, (e) User-1 @ TE2, (f) User-2 @ TE2.

As the power ratio increases from PR = 2.0 to PR = 3.0, the SNR distribution of user-1 decreases, while the SNR of user-2 increases, as expected due to the higher power allocation to user-2.

Figs. 4(a)-(c) show the measured bit error rate (BER) performance for user-1 and user-2 at TE0, TE1, and TE2 modes, respectively.

Fig. 4. Measured BER for different users and PRs at (a) TE0, (b) TE1, (c) TE2.

For TE0 and TE1, the data rates of user-1 and user-2 are 25.76 Gbit/s and 12.8 Gbit/s at PR = 2.0, and 15.08 Gbit/s and 26 Gbit/s at PR = 3.0, respectively. For TE2, the data rates are 22.65 Gbit/s and 12.65 Gbit/s at PR = 2.0, and 12.12 Gbit/s and 24.6 Gbit/s at PR = 3.0, respectively.

All measured channels satisfy the hard-decision forward-error-correction (HD-FEC, BER = 3.8 × 10-3) threshold, demonstrating the feasibility of the proposed MDM optical power divider with NOMA for flexible data rate allocation.

Conclusion

In this article, we have presented a silicon photonics MDM optical power divider that supports TE0, TE1, and TE2 modes. By incorporating NOMA-OFDM, the power divider enables flexible data rate allocation to multiple users, making it an attractive solution for optical interconnects and system-on-chip applications. The experimental results showcase successful data transmission at different power ratios, satisfying the HD-FEC threshold for all measured channels.

This work paves the way for further exploration of advanced multiplexing techniques in silicon photonics, enabling higher transmission capacities and more efficient resource utilization in integrated optical systems.

Reference

[1] Y.-Z. Lin, Y.-H. Jian, C.-W. Chow, and C.-H. Yeh, "Utilizing Non-Orthogonal Multiple Access (NOMA) for Flexible Data Rate Allocation in Integrated Mode Division Multiplexing (MDM) Optical Power Divider," Department of Photonics, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan; Department of Photonics, Feng Chia University, Seatwen, Taichung 40724, Taiwan, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.