A new scientific article published in The Journal of Lightwave Technology, with the contribution from the Institute of Telecommunications, KTH Royal Institute (Applied Physics Department), RISE Research Institutes of Sweden, Zhejiang University (College of Information Science and Electronic Engineering) and mirSense. The article shows the latest potential solution for improving 5G transmission systems to support orders of magnitude faster data transfer with much lower latency than the deployed solutions nowadays.
Why is it relevant?
A roadmap for future wireless communications is expected to exploit all transmission-suitable spectrum bands, from the microwave to the optical frequencies, to support orders of magnitude faster data transfer with much lower latency than the deployed solutions nowadays. The currently under-exploited mid-infrared (mid-IR) spectrum is an essential building block for such an envisioned all-spectra wireless communication paradigm. Free-space optical (FSO) communications in the mid-IR region have recently attracted great interest due to their intrinsic merits of low propagation loss and high tolerance of atmospheric perturbations. Future development of viable mid-IR FSO transceivers requires a semiconductor source to fulfill the high bandwidth, low energy consumption, and small footprint requirements. In this context, quantum cascade laser (QCL) appears as a promising technological choice. In this work, we present an experimental demonstration of a mid-IR FSO link enabled by a 4.65-µm directly modulated (DM) QCL operating at room temperature. We achieve a transmission data rate of up to 6 Gbps over a 0.5-m link distance. This achievement is enabled by system-level characterization and optimization of transmitter and receiver power level and frequency response and assisted with advanced modulation and digital signal processing (DSP) techniques. This work pushes the QCL-based FSO technology one step closer to practical terrestrial applications, such as the fixed wireless access and the wireless mobile backhaul. Such a QCL-based solution offers a promising way towards the futuristic all-spectra wireless communication paradigm by potentially supporting the whole spectrum from the MIR to the
terahertz (THz).
What has been demonstrated and what results have been achieved?
In this work, we have demonstrated a directly modulated QCL-based MIR FSO link with three amplitude modulation formats, i.e., NRZ-OOK, PAM4, and PAM8, operating at room temperature. Without any post-equalization, successful transmissions of NRZ-OOK signals of 1.3 Gbps and 1.6 Gbps are achieved with BER performances below the KP4-FEC and the 6.7%-OH HD-FEC limits, respectively. Assisted with effective post equalization techniques, up to 4.5 Gbps data rate to meet the KP4-FEC threshold and up to 6 Gbps data rate to reach the HD-FEC threshold are successfully demonstrated over the 0.5-m MIR FSO link. Enabled by proper beam collimation, thus the enhanced receiver SNR, this demonstration shows a 50% higher achievable data rate with ten times longer reach than the previous record of 4 Gbps data rate over 5 cm link distance. Since the atmospheric attenuation at this wavelength window is below 1 dB/km and the power margin of the current MIR FSO setup is considerably large, we can confidently expect a much longer transmission distance at the same data rates for all three tested modulation formats with ideal alignment and beam collimation. Therefore, in the near term, we plan to take extra laser safety measures and make further research efforts towards higher data rates and longer-distance transmissions in the MIR region. In the mid to long term, research towards the all-spectra communication paradigm by opening the spectrum window from the MIR to the THz can be well-envisioned with such QCL-based technologies. The optimal choice of modulation formats in this spectral region will also need to be explored by jointly considering the tradeoff between the bandwidth and the SNR, as well as the propagation characteristics of the dynamic atmospheric channel.
The work is open access, click on this DOI link to read the full paper.
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