Analog optical link with external modulation and digital output for L-band signals: experimental evaluation of the effective number of bits

When transmitting analog signals through optical link, the signal quality deteriorates due to losses, nonlinear distortions, and noise, leading to a reduction in the effective number of bits in the transmitted signal subjected to analog-to-digital conversion. The papaer presents the results of an experimental study on the operation of a ultra-wideband ultra-high-frequency analog optical channel with a digital output. The aim of the work is to assess the signal transmission quality of the studied channel in terms of digital metrics. A comparison of the digitization results of the generator signal and the same signal transmitted through the analog optical link has been performed. By analyzing the radio frequency spectra of the digitized signals, the contribution of the optical part of the system to the change in signal quality has been evaluated. Measurements have shown that the level of nonlinear distortions and noise introduced into the signal by the optical channel was 14.7 dB, which is equivalent to a reduction in the effective number of bits by 2.44, primarily due to power losses in the optical part.

1. Cox III C.H. Analog Optical Links: Theory and Practice. Cambridge University Press, 2004. doi: 10.1017/CBO9780511536632

2. Urick V.J., McKinney J. D., Williams K.J. Fundamentals of microwave photonics (Urick/Fundamentals of Microwave Photonics). Wiley & Sons, 2015. doi: 10.1002/9781119029816

3. Chen N., Okada M. Toward 6G Internet of Things and the Convergence With RoF System. IEEE Internet of Things Journal, 2021. Vol. 8. No. 11. Pр. 8719–8733. doi: 10.1109/JIOT.2020.3047613

4. Yang H., Tian W., Li J., Chen Y.. Multi-Band Analog Radio-over-Fiber Mobile Fronthaul System for Indoor Positioning, Beamforming, and Wireless Access. Sensors, 2025. Vol. 25. No. 7. Pр. 2338. doi: 10.3390/s25072338

5. Sung M., Kim S., Kim E. S., Moon S.R., Kim M. et al. Photonic THz Communications Based on Radio-Over-Fiber Technology for 6G Mobile Network: Design and Opportunity. IEEE Journal of Selected Topics in Quantum Electronics, 2023. Vol. 29. no. 5. Pр. 1–11. doi: 10.1109/JSTQE.2023.3308899

6. Hoff M.T., Ralph S.E., Stevens R.C. Optical Signal Transport on Aerospace Platforms: Analog vs. Digital Links. IEEE Aerospace and Electronic Systems Magazine, 2025. doi: 10.1109/MAES.2025.3554480

7. Fu Y., Zhang X., Hraimel B., Liu T., Shen D. Mach-Zehnder: A Review of Bias Control Techniques for Mach-Zehnder Modulators in Photonic Analog Links. IEEE Microwave Magazine, 2013. Vol. 14. no. 7. Pр. 102–107. doi: 10.1109/MMM.2013.228033

8. Walden R. Analog-to-digital converter survey and analysis. IEEE J. Sel. Areas Communic., 1999. Vol. 17. Pр. 539–550.

9. Walden R. Analog-to-digital conversion in the early twenty-first century / The Wiley Encyclopedia of Computer Science and Engineering. John Wiley & Sons Inc., New York, 2008. Pр. 126–138.