1. Fiber Optic Sensors for relative humidity monitoring in High Energy Physics Applications

2. CONSTANT THERMAL AND HYGROMETRIC CONTROL OF THE AIR IS MANDATORY!
ATLAS
CMS
LHCb
ALICE
4 main experiments running in the LHC:
COMPACT MUON SOLENOID
The heart is the tracker
Silicon sensors measure position of particles
Radiations cause deterioration and
heat generation in silicon
Cooling at T< 0 ˚C is needed
The tracker has to be kept dry to avoid
condensation
CONSTANT THERMAL AND HYGROMETRIC
CONTROL OF THE AIR IS MANDATORY!

3. OUR SOLUTION: FIBER OPTIC SENSORS
Humidity Sensors Issues in HEP Trackers
Low mass and Small dimensions
Insensitivity to magnetic field
Operation down to -30 ˚C and in the range [0, 100] %RH
Accuracy: better than ±3 %RH
Reduced number of wires
High long term stability
Radiation resistance to dose up to 1 MGy (100 Mrad)
Currently in use at CERN:
1) Different types of miniaturized sensors (standard capacitive)
2) Measurements with laboratory instrument on air samples transferred over long distance (remote air sniffer)
PRECON
HS2000
HONEYWELL
HIH4030
SENSIRION
SHT75
- No distributed sensing
- Only time-averaged measurements
Sniffer rack in remote laboratory
NO-ONE satisfies the whole set of requirements from CERN!!!
Vaisala DRYCAP Dewpoint Transmitter DMT242
OUR SOLUTION: FIBER OPTIC SENSORS

4. WHY FIBER OPTIC SENSORS @ CERN?
Miniaturized dimension
Minimal mass
Multi parametric sensing
Possibility to work in harsh environments
Immunity to electromagnetic interference
Water and corrosion resistance
Radiation tolerant
VERY APPEALING
TO EXPLORE
HIGH ENERGY PHYSICS
ENVIRONMENTS

5. Fiber Bragg Grating fundamentals
FBG is a permanent periodic modulation of the refraction index in the core of the fiber
neff is the fiber effective refractive index (neff ~ in silica)
Λ is the grating pitch (typically ~ 500 nm)
λB is the reflected Bragg’s wavelength

6. FBG- based Strain and Temperature Sensorsε
Strain changes:
Sε≈ 1 pm/με
- Λ via direct deformation
- neff via elasto-optic effect T
ST≈10 pm/°C
Temperature Changes:
Λ via thermal expansion
- neff via thermo-optic effect
For strain measurements, discrimination between temperature and strain needed:
Strain free reference FBG sensors coupled to another FBG
(T-Compensation technique)

7. FBG AS MULTIFUNCTIONAL SENSORS Multi-parametric sensing
Functionalization: integration with appropriate materials and suitable packaging to measure physical, chemical and biological parameters
FBG + COATING:
Multi-parametric sensing
Magnetic field
Humidity
Cryo temperatures
Acoustic waves
Weight
Chemical and biological analytes ….

8. FBG-based relative humidity sensors
Bare FBGs are insensitive to humidity.
Use of a sensitive material as coating of the FBG to induce a mechanical effect.
Hygroscopic polymers swell upon adsorption of water molecules.
Development of humidity sensor by coating the FBG with a polyimide film
Water molecules absorbed by the hygroscopic coating
Coating expansion (“Swelling”)
Strain induced on the FBG
Bragg wavelength shift (ΔλB)
FBG-RH
sensor
Water molecules
Polyimide coating
Optical fiber

9. Temperature compensation
Polyimide-coated FBGs are intrinsically sensitive to temperature:
A temperature compensation scheme is required to extract RH measurements from the sensor readings
Final configuration of FBG-based thermo-hygrometers developed for CMS:
2 FBGS coupled side by side
(1 poly-coated and 1 bare)

10. PH-DT experimental set-up

11. Investigations @ CERN about RH FBG sensors
Typical FBG RH response at 20°C
λB blue shift induced by PI coating water molecules absorption
Complete agreement between FOS readings and the reference
Similar results obtained at 0 and -15°C
Temperature and Relative humidity Sensitivities
Characteristic curves
SRH=2.09 pm/%RH±20%
ST=10.08 pm/˚C±12%
Slight dependence of SRH on T and ST on RH !!
Linear response of FBG coated towards RH

12. INVESTIGATIONS ABOUT FBG SENSORS RADIATION RESISTANCE
VERY COMPATIBLE RESULTS OVER THE 4 SAMPLES!!!
Ceramic Commercial Packaged FBG Temperature sensors
4 samples belonging to 2 different batches
ϒ- irradiation campaigns up to 210 kGy
Negligible variations of ST
Polyimide Coated FOS RH sensors
SRH unaffected by ϒ-radiations!
Coating thickness:10 µm (±2 µm)
4 sensors belonging to the same batch
ϒ- irradiation campaigns up to 210 kGy
RH response found to be linear
Sensing performances unchanged after each campaign

13. FBG-based thermo-hygrometers in operation @ CMS
CMS Tracker Bulkhead:
72 FBG thermo-hygrometers installed
Commercial sensors procured (Welltech for RH/ Micron Optics for T)
In operation since December 2013 (h24)
Reliable system
During LS1 coupled to commercial RH/T sensors (which will die when LHC goes ON)
1 fiber optic array with
8 FBG-based hygrometers (MULTIPLEXING)
FBG-based thermo-hygrometer
Reference T and RH sensors mounted in a special protective casing
* Pictures from the installation of the FBG sensors in CMS

14. 12 months of FBG-RH sensors’ operation in CMS
Ex.1: Sensor F5-H-2 (Z+)
1 year of measurements
(from Dec 2013 to Dec 2014)
Some interruptions in the data acquisition
Ex.2: Sensor F5-H-14 (Z+)
Ex.2: Sensor F5-H-8 (Z+)
Different RH measurements due to different locations (and different services around)