臺大光電所所訊

Coherent Tri-mode FPLD with SCM of 80 Gbit/s OFDM for 75-km Long-Reach MMWoF at 28 GHz

Professor Gong-Ru Lin’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所林恭如教授

A novel FPLD with orthogonal polarized tri-mode under single-carrier modulation (SCM) for chromatic dispersion suppression is employed to demonstrate 28-GHz remote optical heterodyne in millimeter-wave-carrier-over-fiber (MMWoF) for 75-km long-reach wireline and wireless transmission at 80 and 30 Gbit/s under bit-loaded M-QAM OFDM with spectral usage efficiency up to 10 bit/s/Hz. The 28-GHz and 75-km long-reach MMWoF constructed by an orthogonal tri-mode LD with SCM is exhibited in Fig. 1. A single-polarization light source generated from a DFBLD was adjusted a 45^o polarization between the light source and the Mach-Zehnder modulator (MZM). A 28-GHz LO sinusoidal wave was modulated onto the single-polarization light source by a MZM with null-point operation for the orthogonal tri-mode master source generation (point (a)). The orthogonal tri-mode master was divided into two paths by an optical coupler. One path of this master source injection-locked a Fabry-Perot LD (FPLD) only supporting the TE polarization. The side modes of the tri-mode source with the TM polarization would be suppressed in the FPLD cavity, which implements the SCM (point (b)). For another path, the central carrier of the tri-mode source was suppressed by a polarizer (node (c)). The injection-locked carrier and central-carrier suppressed side-modes source were orthogonal combined by another optical coupler to form an orthogonal tri-mode optical carrier with SCM (point (d)). The narrow-band 64- to 1024-QAM OFDM with the bandwidth from 166 to 100 MHz was generated by an 18-GSa/s AWG. The bit-loaded QAM-OFDM data was modulated on the injection-locked carrier with side-mode suppression by the FPLD as an optical amplifier and modulator.

Fig. 1. The microscopic image, package photo, and network scheme of a tri-mode FPLD with SCM of M-QAM OFDM for 75-km long-reach MMWoF.

To maximize the transmission capacity and meet the 5G standard, the multi-channel bit-loaded M-QAM OFDM with 100-MHz/channel bandwidth is employed. The maximal channel number in 8-GHz bandwidth is obtained as 80, and the first channel is set from 70 MHz. After demodulation, the SNR spectra of each narrow-band bit-loaded M-QAM OFDM data and the wide-band 64-QAM OFDM data delivered by orthogonal tri-mode optical carrier after 75-km transmission are exhibited in Fig. 2. Figure 3 also shows the related constellation plots for the narrow-band bit-loaded M-QAM OFDM data. The maximal bandwidth and raw data rate of the delivered wide-band 64-QAM OFDM, which can pass the FEC limit, are obtained as 8 GHz and 48 Gbit/s, respectively. For the narrow-band case, the total raw data rate of the delivered multi-channel bit-loaded M-QAM OFDM data is evaluated as 10 (1024 QAM) x6 GHz+9 (512 QAM) x2.2 GHz=80 Gbit/s.

Fig. 2. The SNR spectra of narrow-band M-QAM OFDM data and wide-band 64-QAM OFDM data delivered by orthogonal tri-mode optical carrier after 75-km transmission.

Fig. 3. The frequency spectra of the delivered multi-channel M-QAM OFDM by beat 28-GHz MMW carrier after 10-m free space transmission.

The multi-channel bit-loaded M-QAM OFDM data with the same channel bandwidth is also employed at wireless transmission. After the same demodulation, the SNR spectra of each narrow-band M-QAM OFDM data with the related constellation plots are exhibited in Fig. 3. The SNR of the 1st channel is measured as 27.2 dB to pass the 256-QAM FEC limitation with a BER of 3.1x10^-3 and an EVM of 4.4%. When increasing the channel to 25, the SNR is degraded to 22.5 dB, which can also pass the 64-QAM FEC limitation with a BER of 1.1x10^-3 and an EVM of 7.5%. Therefore, the total raw data rate of the delivered multi-channel bit-loaded M-QAM OFDM data is evaluated as 8 (256 QAM) x3 GHz+6 (64 QAM) x1 GHz=30 Gbit/s.

Minority Carrier Decay Length Extraction from Scanning Photocurrent Profiles in Two-Dimensional Carrier Transport Structures

Professor Ming-Hua Mao’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所毛明華教授

Carrier transport was studied both numerically and experimentally using scanning photocurrent microscopy (SPCM) in two-dimensional (2D) transport structures, where the structure size in the third dimension is much smaller than the diffusion length and electrodes cover the whole terminal on both sides. The scanning photocurrent simulation results surprisingly showed almost identical profiles from structures with different widths. The simulation results indicate that the integrated carrier distribution in the 2D transport structures with finite width can be well described by a simple-exponential-decay function with the carrier decay length as the fitting parameter, just like in the 1D transport structures. Furthermore, our simulation results demonstrate that the scanning photocurrent profiles in the ohmic- or Schottky-contact three-dimensional (3D) transport structures with electrodes covering the whole terminal on both sides will reduce to those described by the corresponding 1D fitting formulae. Finally, experimental SPCM on a p-type InGaAs air-bridge two-terminal thin-film device was carried out. This study allows us to extract the minority carrier decay length and to obtain the mobility-lifetime product which can be used to evaluate the performance of 2D carrier transport devices. This work has been published in Scientific Reports:

Yu‑Chien Wei, Cheng‑Hao Chu, and Ming‑Hua Mao*, “Minority Carrier Decay Length Extraction from Scanning Photocurrent Profiles in Two-Dimensional Carrier Transport Structures,” Scientific Reports, 11, 21863, (2021).

Fig. 1. (a) Schematic of the SPCM setup in an InGaAs air-bridge two-terminal thin-film device. (b) SEM images for one side of the measured InGaAs air-bridge two-terminal thin-film device. (c) Scanning photocurrent profile of the 0.2 μm wide case compared with that of the 1D transport structure. (d) Scanning photocurrent profiles with different widths.