RESUMO
An IF-over-fiber (IFoF)-based analog transport technology for mobile fronthaul applications has recently attracted significant attention. However, most previous studies have employed discrete optical components. For the analog transport technology to be a more cost-effective and power-efficient solution, it is necessary to utilize existing integrated optical transceivers. In this paper, we demonstrate IFoF transmission using a commercial off-the-shelf transmitter optical sub-assembly (TOSA). Although the TOSA was developed for a digital system employing non-return-to-zero (NRZ) signals, we show that it is also possible for the TOSA to support high-capacity analog transmission. As a demonstration, by using the TOSA, we could successfully transmit 64- and 256-ary quadrature-amplitude-modulated (64/256QAM) orthogonal-frequency-division-multiplexed (OFDM) signals with net bit rates of 54.74 and 36.49 Gbps per wavelength, respectively. Since the TOSA has four wavelength channels, the total capacities are 218.94 and 145.98 Gbps, respectively. To the best of our knowledge, these rates are the highest among all the demonstrations using analog transport technology.
RESUMO
Wideband signal generation using frequency/phase modulation (FM/PM) is the key fundamental function for various applications such as radar and analog communication systems. It is well known that analog FM/PM communication systems can dramatically improve signal quality by spectral expansion. In classical communication theories, the Armstrong indirect method is one of the most popular methods for bandwidth expansion of FM signals. In the Armstrong method, a narrowband signal is converted to a broadband signal with the help of a nonlinear frequency multiplier. In this paper, we propose a photonic Armstrong method enabled by direct detection. By utilizing the nonlinearity caused by direct detection, we can increase bandwidth so that it is double that of the original signal in the photonic Armstrong method. Also, it completely eliminates signal-to-signal beat interference (SSBI) at the same time. We conducted an experiment to verify the concept and confirmed these advantages experimentally.
RESUMO
Up to 18-Gbps direct encoding of blue laser diode (BLD) is demonstrated for free-space data transmission. By reshaping the orthogonal frequency multiplexed (16-QAM OFDM) stream with sidelobe filtering, the raw data rate expedites from 17.2 to 18.4 Gbps. Employing an ultrafast p-i-n photodiode with smaller active area diameter and lower noise equivalent power significantly enlarges the data rate by 1.6 Gbps or upgrades the signal-to-noise ratio (SNR) by 0.2 dB. Replacing the 80-mW BLD with the 120-mW one essentially increases the received SNR by 0.4 dB under enhanced modulation throughput. Reinforcing the beam collimation and collection by increasing the numerical aperture with a plano-convex hyper-hemispherical lens further improves the SNR by 0.6 dB. After optimization, the 16-QAM OFDM data with and without sidelobe filtering are respectively delivered at raw data rates of 16.4 and 18 Gbps with spectral-density usage efficiency as high as 4 bit/s/Hz over 16 m in free space, wherein the BLD carried QAM-OFDM data stream remains its capacity after reformation with sidelobe filtering as the superior inter-carrier-interference immunity reinforces.
RESUMO
For high-speed optical OFDM transmission applications, a comprehensive comparison of the homemade multi-/few-/single-transverse mode (MM/FM/SM) vertical cavity surface emitting laser (VCSEL) chips is performed. With microwave probe, the direct encoding of pre-leveled 16-QAM OFDM data and transmission over 100-m-long OM4 multi-mode-fiber (MMF) are demonstrated for intra-datacenter applications. The MM VCSEL chip with the largest emission aperture of 11 µm reveals the highest differential quantum efficiency which provides the highest optical power of 8.67 mW but exhibits the lowest encodable bandwidth of 21 GHz. In contrast, the SM VCSEL chip fabricated with the smallest emission aperture of only 3 µm provides the highest 3-dB encoding bandwidth up to 23 GHz at a cost of slight heat accumulation. After optimization, with the trade-off set between the receiving signal-to-noise ratio (SNR) and bandwidth, the FM VCSEL chip guarantees the highest optical OFDM transmission bit rate of 96 Gbit/s under back-to-back case with its strongest throughput. Among three VCSEL chips, the SM VCSEL chip with nearly modal-dispersion free feature is treated as the best candidate for carrying the pre-leveled 16-QAM OFDM data over 100-m OM4-MMF with same material structure but exhibits different oxide-layer confined gain cross-sections with one another at 80-Gbit/s with the smallest receiving power penalty of 1.77 dB.
