The Optical Wireless Communication Technology Giulio Cossu, Ali Wajahat, Raffaele Corsini , Ernesto Ciaramella Scuola Superiore Sant’Anna Pisa, Italy

Optical free-space transmission: not a new idea! 800 BC: fire beacons (Romans) 150 BC: Smoke signals (Native-Americans) 1790: optical telegraph (Claude Chappe) 1960: Laser >1970 Laser FSO for military secure applications 1993: IrDA standard

Free Space Optics (FSO) systems Optical wireless can flexibly replace fiber links over short distances (because of practical/time/cost issues) Main issues: connection to fiber links capacity (limit: 80 Gbit/s) stability (few minutes) In-field experiments in CNR area : record capacity (1.28 Tb/s) demonstrated in a stable duoble-pass link

1.28 Terabit/s (32x40 Gb/s) FSO experiment Transparent connection fiber-FSO-fiber between S.Anna building and CNR “A” building (2x210 m) EDFA1 (Booster) 32- DFBs AWG . PCs 40 Gbs PRBS (231-1) IM 40 Gbs Clock and data recovery Pre-amplified Receiver OC1 FSOT 1 FSOT 2 OC2 OTBF EDFA2 EDFA3 PD OA NZDF 210 m

Considered for terrestrial and satellite links FSO conclusions Considered for terrestrial and satellite links Useful and potentially very high bandwidth Affected by weather conditions (especially fog/clouds)  Complex alignment and tracking, may not always solve beam-wandering effects, and cannot give enough availability 

A new player LED

A key technology advancement: LED Progress in LED luminous efficiency opens up new possibilities LED used everywhere (consumer electronics, traffic signalling, illumination etc.) because they are good ... and very cheap

Light Emitting Diodes (LEDs) Lower power consumption, lower voltage, longer lifetime, smaller size, cooler operation and faster response Phosphorescent white LED Blue chips + phosphorus layer Limited bandwidth due to the slow phosphor layer (2-3 MHz) Original frequency response restored with blue filter (10-15 MHz) RGB white LED Mix of Red + Green + Blue chips Full bandwidth without optical filter Allows Wavelength Division Multiplexing (WDM)

Differences between OW and RF technologies Property of medium Radio Optical Bandwidth regulation Yes No Power limitation Radio law Eye safety/illumination Multipath fading Passes though walls Physical security Low High Input x(t) Amplitude Power (always positive) Detection type Coherent/Incoherent Incoherent

Applications and more...

Challenges Improve data rate Provide uplink: most likely using IR Compatibility with other illumination sources Feeding VLC: may have a non-negligible impact, especially in existing building

Discrete Multitone Modulation Principle used in ADSL Allocate orthogonal subcarriers over wide frequency range Probe the channel and allocate power and capacity depending on estimated SNR frequency Freq. response Bit loading

High-speed OWC link – Introduction Goal: Highest speed operation (with low-cost components) Minimization of power losses -> Directed Line-of-sight configuration Narrow emission beam Narrow acceptance angle WDM operation Achieved results (2012) 1 Gbit/s @ 15 cm – Phosphorescent LED (single channel) [Photonics journal] (2012) 2.1 Gbit/s @ 15 cm – RGB LED (WDM channel) [ECOC] (2012) 3.4 Gbit/s @ 15 cm – RGB LED (WDM channel) [Optics Express] (2014) 5.2 Gbit/s @ 3 m – RGBY LED (WDM channel) [ECOC]

High-speed OWC link – Experimental Setup

High-mobility OWC link – Introduction Goal: High speed operation in high mobility Non Directed Line-of-sight scheme Trade-off between diffuse links and high speed of LOS links. Scenario closer to typical indoor topology: synergy illumination and data Robust to indoor ambient light Achieved results (2013) 200 Mbit/s @ 2.4 m – Phosphorescent LED (uni-directional) (2014) 250 Mbit/s @ 2 m – Phosphorescent LED/IR-LED (bi-directional) (2014) 400 Mbit/s @ 2 m – RGB LED/IR-LED (bi-directional)

High-mobility OWC link set– Experimental up

Current demo Artemide-Sant’Anna

Other applications for OWC Use in environment rich of RF (or where RF cannot be used), as alternative to RF wireless Use for localization in indoor environments Use in Internet of Things

OWC for HEP - Introduction Design a Multi Gigabit OWC system for particles detectors CMS used as a case study: data transmission between layers of CMS sensor Motivations: Reduce the material budget Reduce latency Requirements: Transmission distance: 10-15 cm Transmission bitrate: 2.5 Gbit/s Target bit error rate (BER): 10-12 Tolerance to misalignment >250 µm Low latency HEP environment Thanks to F. Palla et al.

OWC for HEP – Experimental setup Tx: Vertical Cavity Surface Emitting Laser (VCSEL) Relatively high output optical power: 0 dBm (1 mW) Medium divergence angle: 16° Emission wavelength: 1550 nm (no absorption with silica material) Rx: PIN Photodiode Active area: 60 µm diameter Ball lens to increase received power

OWC for HEP – System characterization We transmitted 2.5 Gb/s at 10 cm distance Experiment with 3, 4 and 5 mm ball lens ± 1mm tolerance to misalignment with all ball lenses at BER of 10-12.

OWC for HEP - X-rays irradiation test Optical components were tested under X-ray radiation facility in Padova We tested the electrical behavior of the devices for a long period VCSEL PIN photodiode with ball lens Bare PIN photodiode Radiation area: 2x2 cm2

OWC for HEP - X-rays irradiation test results The irradiation lasted for 22 hours: total dose > 200 Mrad 223 VCSEL PIN PD with ball lens bare PIN PD

OWC Applications for Medical Physics

OWC for medical physics Design OWC to transmit the PET/MRI data to the back-end processes Motivations: Place the elecronics far from the PET/MRI gantry Requirements: Transmission distance: 1-2 m Different scenarios... Thanks to JJ. Vaquero and G. Konstantinou

OWC For MRI Compatible PET insert In collaboration with UC3M, we studied three scenarios to explore the possibilities of OWC for MRI compatible PET insert. Main aim here is to reduce the electronics inside the MRI gantry. OWC between internal logic unit and external one (1.5 Gbps approx. per ring). (Study Completed) OWC between the output of ADC and an external logic unit (65 Gbps approx. per ring) (Not Feasible) Analogue OWC directly after SiPM (Work in progress)

OWC For MRI – Scenario III Results Necessary measurement for PET: Transmission jitter: < 100 ps Distance: 1.5 m Amplified output Important elements such as rising (falling) slope within specifications and with excellent SNR SiPM output Noise seems to be an issue, but preliminary results are very encouraging

Single channel OWC transmission to analyze FFD. 1 channel from Argus PET module on OWC link and rest three channels as direct: analysis based on Field Flow Diagram (FFD) The scintillating crystals are clearly visible in the FFD. We defined the Resolvability Index 𝑅𝐼= 𝐹𝑊𝐻𝑀 𝐷 Lower is RI, better is the pixels resolvability

OWC Signal Transmission using Delay Lines To reduce the number of OWC links per detector we multiplexed the 4 analogue channels As first trial, we used the Time Division Multiplexing technique deploying analogue delay lines The image we received is not the best but it is depicting the scintillating crystal pixels. The main reason is interference from neighboring channels.

Thanks for your attention g.cossu@sssup.it

Backup slides

Link configuration – Directed Line-of-sight Maximized capacity Maximized power efficiency Minimized ambient light noise Requires precise aligment Limited user mobility Complex design

Link configuration – Diffuse (Non Directed – Non Line of Sight) No tracking required Highest mobility High connection robustness Very low signal power Low bit rate Ambient light noise

Link configuration – Non-Directed Line-of-sight No tracking required High mobility Low signal power Ambient light noise Trade-off: robustness of the diffuse link and high capacity D-LOS link

4.25 Gb/s OWC (Preliminary Results) Because of obtaining high SNR in 2.5 Gb/s setup we tried to improve the data rate up till 4.25 Gb/s at an expense of reduced tolerance to misalignment range (±0.75mm).

Introduction Optical components and ball lens were tested under X-ray radiation facility in Padova. The aim of the experiment is to test the optical components especially the quartz ball lens under ionizing radiations with higher dose. Each component is tested with total dose as shown below in the table . Dose per hour values are w.r.t Si and SiO2 as a reference We prepared boards with line of sight CW transmission of the VCSEL and photodiode for testing each component. The following components were tested Quartz ball lens PIN Photodiode (Kyosemi 2.5 GHz) VCSEL (Raycan 4.25 GHz)   Dose per hour Total dose for 22 hours Si dose 10.8 237.6 SiO2 dose 6.17 135.74

Experimental setup Figure presents the experimental setup for X-rays irradiation test. The board with optical components were placed in a box inside X-ray chamber. The device to be tested was placed under the radiation beam while other components were properly shielded. Bias current and forward voltage of VCSEL, dark/received current of photodiode were recorded using Keithley power supply.

Quartz Lens Test (1) A quartz ball lens packaged with PIN photodiode was irradiated up till 135 Mrad of dose (SiO2). Irradiation effect on ball lens was observed by measuring the received current at the PIN photodiode. Below mentioned figure shows the irradiated quartz lens packaged with PIN photodiode. No darkening effect due to X-ray irradiations is observed on the lens. Quartz lens

Quartz Lens Test (1) Figure provides the dark current and received current of the PIN photodiode at fixed VCSEL bias current of 7mA. Dark current is observed to analyze the behavior of photodiode during irradiation. Pre irradiation value (0 dose level) of dark current is lower than post irradiation because of X-rays effect. The stable values of received current illustrates that there is no change in the Quartz lens properties (refractive index or transmittance) Rx current at VCSEL Ibias of 7mA

No darkening is observed for 2nd quartz ball lens. Quartz Lens Test (2) No darkening is observed for 2nd quartz ball lens. The received current also remained approximately same throughout the experiment. Dark current also show stagnant behavior w.r.t dose levels. Rx current at VCSEL Ibias of 7mA

Photodiode is tested independently. Photodiode test Photodiode is tested independently. Same setup of VCSEL and photodiode on board was used. No change in dark current as well as received current was observed during irradiation as shown below. Rx current at VCSEL Ibias of 7mA

VCSEL test Below mentioned figure shows the characteristics of the VCSEL under test w.r.t bias current. It is evident that no change in L-I-V characteristics is observed during the irradiation process. VCSEL

Conclusion We tested the quartz ball lens as well as PIN photodiode and VCSEL under X rays with 135Mrad of dose (SiO2 as reference). No change in quartz ball lens properties was observed under such high radiations, which means the lens can be used in optical wireless communication link designed for HEP particle detectors. As expected PIN photodiode and VCSEL also showed no degradation under X rays.