Self-adaptive integrated photonic receiver for turbulence compensation in free space optical links

Abstract

In Free Space Optical (FSO) communication systems, atmospheric turbulence distorts the propagating beams, causing a random fading in the received power. This perturbation can be compensated using a multi-aperture receiver that samples the distorted wavefront on different points and adds the various signals coherently. In this work, we report on an adaptive optical receiver that compensates in real time for scintillation in FSO links. The optical front-end of the receiver is entirely integrated in a silicon photonic chip hosting a 2D Optical Antenna Array and a self-adaptive analog Programmable Optical Processor made of a mesh of tunable Mach-Zehnder interferometers. The photonic chip acts as an adaptive interface to couple turbulent FSO beams to single-mode guided optics, enabling energy and cost-effective operation, scalability to systems with a larger number of apertures, modulation-format and data-protocol transparency, and pluggability with commercial fiber optics transceivers. Experimental results demonstrate the effectiveness of the proposed receiver with optical signals at a data rate of 10 Gbit/s transmitted in indoor FSO links where different turbulent conditions, even stronger than those expected in outdoor links of hundreds of meters, are reproduced.

Keywords: Adaptive integrated optics; Atmospheric turbulence; Free space optical communications; Silicon photonics.

Fig. 1

Effects of turbulence on a Gaussian beam. Beam with $w_0=1$ mm and $\lambda =1550$ nm propagated for 800 m for low (1st row), moderate (2nd row), and strong (3rd row) turbulence regime. The 1st column corresponds to the beam power density, the 2nd to the phase front, and the 3rd to the spatial coherence. The dashed circles correspond to the theoretical Fried parameter $r_0$.

Fig. 2

Single-aperture receiver. (a) Scheme of the single-aperture receiver that focuses a portion of the incoming beam on a single-mode fiber or PD. (b–c) Simulated time trace (b) and PDF (c) of the optical power coupled to the PD for a large and small aperture $d_R$ with respect to $r_0$. (d–f), Simulated mean received power (d), standard deviation (e), and three-sigma limit (f) of a single aperture receiver with a single-mode detector as a function of the aperture diameter $d_R$ for $C_n^2={10}^{-13}$ (yellow), ${10}^{-14}$ (blue), ${10}^{-15}$ (red), and without turbulence (purple). $\overline{P_{\mathit{rx}}}$ and ${\sigma }_{\mathit{rx}}$ have been calculated as the average of 100 simulations.

Fig. 3

Multi-aperture receiver structure. (a) Optical antenna array distribution for $M=2$, 5, 9, 16. (b) Schematic of the programmable optical processor connected to the OAA. (c–e) Mean received power (c), standard deviation (d), and three-sigma limit (e) as a function of the number of apertures for different separation distance $d_S=5$, 11 and 30 cm, and the three considered turbulences $C_n^2={10}^{-13}$, ${10}^{-14}$, and ${10}^{-15}$ $[{\text{m}}^{-2/3}]$. $d_R=4.5$ cm. $\overline{P_{\mathit{rx}}}$ is calculated by summing coherently the contributions of the M apertures. $\overline{P_{\mathit{rx}}}$ and ${\sigma }_{\mathit{rx}}$ have been calculated as the average of 100 simulations.

Fig. 4

Integrated silicon photonics FSO receiver. (a–e) Photograph of the fabricated PIC (a), the inner ring of the OAA (b), a single Mach-Zehnder interferometer (c), integrated Ge photodiodes for feedback control (d), and the chip assembled on a PCB with control electronic and fiber transposer (e).

Fig. 5

Static turbulence-induced scintillation compensation. (a–b) Schematic (a) and photo of the experimental setup (b) used for the emulation of turbulence-induced scintillation on FSO beams. (c) Image of the optical beam (c$_1^{}$) without and (c$_2^{}$) with the distortion introduced by the SLM phase screen. (d) Optical power of the FSO beam at the output port of the integrated receiver for different phase screens when the adaptive control is idle (orange) and active (blue).

Fig. 6

Real-time turbulence compensation with 5 Gbit/s signal. (a–c) Received optical power $P_{\mathit{rx}}$ (a), power spectral density (b), and probability density function (c) of the received optical power when the POP is in its native state (orange) and when the adaptive control is active (blue). (d–f) Eye diagrams of the received 5 Gbit/s OOK signal in the absence (d) and in the presence of turbulence when the adaptive control is off (e) and active (f).

Fig. 7

Real-time turbulence compensation with 10 Gbit/s signal. (a–c) Optical power (a), probability density function (b), and cumulative density function (c) of the received optical signal when the control is active (blue), idle (orange), and in the absence of turbulence (yellow). (d–f) Corresponding eye diagrams of the received 10 Gbit/s OOK.