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

2. 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

3. 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

4. 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

5. 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 

6. A new player LED

7. 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

8. 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)

9. 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

10. Applications
and more...

11. 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

12. 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

13. 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) Gbit/s @ 15 cm – RGB LED (WDM channel) [ECOC]
(2012) cm – RGB LED (WDM channel) [Optics Express]
(2014) m – RGBY LED (WDM channel) [ECOC]

14. High-speed OWC link – Experimental Setup

15. 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) Mbit/s @ 2.4 m – Phosphorescent LED (uni-directional)
(2014) Mbit/s @ 2 m – Phosphorescent LED/IR-LED (bi-directional)
(2014) Mbit/s @ 2 m – RGB LED/IR-LED (bi-directional)

16. High-mobility OWC link set– Experimental up

17. Current demo Artemide-Sant’Anna

18. 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

19. 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: 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.

20. 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

21. 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

22. 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

23. 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

24. OWC Applications for Medical Physics

25. 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

26. 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)

27. 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

28. 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

29. 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.

30. Thanks for your attention

31. Backup slides

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

33. 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

34. 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

35. 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).

36. 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

37. 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.

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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.