## 3 Experiment, Results and Discussion  
The proposed switchable MDM optical power divider was fabricated by IMEC(r). The experimental setup to evaluate the proposed switchable MDM optical power divider is shown in Figure 4. We use the OFDM signal with 500 data lengths, 512 fast-Fourier transform (FFT) size, 170 subcarriers and 32 cyclic prefix (CP) length to evaluate the system. The OFDM signal is generated from an arbitrary waveform generator (AWG, Tektronix(r) AWG 70001). The OFDM modulation includes the serial-to-parallel (S/P) binary data conversion; symbol mapping, inverse fast-Fourier transform (IFFT), parallel-to-serial (P/S), CP insertion. The electrical OFDM signal is inputted to a 40-GHz bandwidth Mach-Zehnder modulator (MZM) via an electrical amplifier (Amp.) to modulate the optical signal, which is generated from different wavelength distributed feedback laser diodes (DFB-LDs). Different dense wavelength division multiplexed (DWDM) OFDM optical signals are combined via a DWDM Mux, and they are amplified by an erbium-doped optical fiber amplifier (EDFA) to compensate the transmission losses. The optical signals are coupled  
Figure 3: (**a**) Schematic of the SOI-based MDM optical power divider. FDTD simulation result of the MDM optical power divisions at (**b**) TE\({}_{0}\), (**c**) TE\({}_{1}\), (**d**) TE\({}_{2}\). (**e**) Photograph of the MDM optical power divider, showing the magnified section of the ADCs for mode conversions. into the proposed SiPh-based MDM optical power divider via an on-chip grating coupler (GC). Electrical signals are applied to the 3 \(\times\) 3 mode switch via a radio frequency (RF) probe for the different mode switching. Then, the optical power divided signals are coupled out of the chip, also via on-chip GC. DWDM Demux is used to separate different DWDM channels. Optical spectrum analyzer (OSA) is used to measure the operation wavelength window of the device. A variable optical attenuator (VOA) is used to control the received power, before launching into the optical pre-amplified receiver (Rx). An optical band-pass filter (BPF) is used to remove the out-of-band amplified spontaneous emission (ASE) noise from the pre-amplified EDFA. Finally, a 40-GHz bandwidth photodiode (PD) is used to receive the optical signal, and an 80 GS/s real-time oscilloscope (RTO, LeCroy(r) 816 ZI-B) is used to capture the waveform for the OFDM demodulation. The demodulation includes re-sampling, data synchronization, removal of CP, S/P, FFT, scaling of power, de-mapping of symbol, and BER calculation. We first experimentally evaluate the ADC for mode Mux, and Demux the TE\({}_{0}\), TE\({}_{1}\) and TE\({}_{2}\) modes. Figure 5a-c show the experimentally measured normalized mode crosstalk of the MDM Mux and Demux when the broadband light source is launched at Channel 1 (CH1) to Channel 3 (CH3) of MDM, respectively. We normalized the optical spectra measured at the three output ports with the corresponding output port spectrum. Taking CH1 for an example, the optical spectra at the CH2 and CH3 output ports are normalized with the optical spectrum at the CH1 output port when the light is launched into the CH1 input port. As a result, the optical spectrum at the corresponding output port becomes united (i.e., 0 dB), and the optical spectra at other output ports reveal the mode crosstalk. It can be observed that the typical mode crosstalk is low, which is \(<\)\(-\)30 dB in most of the wavelength windows within the C-band. Figure 4: Experimental setup to evaluate the proposed switchable MDM optical power divider. AWG: arbitrary waveform generator; MZM: Mach–Zehnder modulator; DFB-LD: distributed feedback laser diode; EDFA: erbium-doped optical fiber amplifier; Amp. : electrical amplifier; OSA: optical spectrum analyzer; VOA: variable optical attenuator; BPF: optical band-pass filter; PD: photodiode; RTO: real-time oscilloscope. Figure 5: Measured normalized mode crosstalk of (**a**) MDM CH1, (**b**) MDM CH2 and (**c**) MDM CH3. We first experimentally characterize a single 2 \(\times\) 2 switch.