Transcript Thermal detectors for Far-Infrared

Thermal detectors for Far-Infrared
SP-ANR meeting, 15 March 2013
Main technologies of IR detection
Thermocouples
Thermal detectors
Resistive bolometers
Pyroelectric detectors
Dielectric detectors
Dielectric bolometers
Semi-conducting
Photonic detectors
Photodiodes
Photocapacitance
Monocrystal
Photoconductor
Heterostructure
Multiple quantum wells
Principle of thermal detectors
Incident flux
variation
Probe
temperature
change
Physical
parameter
variation
Absorption
• Volume change: Thermometers, Golay cells
Sensitivity
Noise
Response time
• Electric resistance: Bolometers
• Potential difference: Thermocouple, thermopile
• Electric charge: Pyroelectric detectors
• Electric permittivity: Dielectric detectors
Tension or
current
change
Pyroelectric detectors
• Ferroelectric material: a spontaneous polarisation exists
under a certain temperature (Curie point)
• Dependence on the temperature: pyroelectricity
Thermal
Pyroelectricity
Types
of energy
in a crystal
Electric
Kinetic
Piezoelectricity
Pyroelectric detectors
Incident flux
variation
Probe
temperature
change
Physical
parameter
variation
Absorption
Crystal dilatation
 Dipole moment variation
 Charge on the surface
 Current when coming back to equilibrium
state
Tension or
current
change
Pyroelectric detectors
• Properties:
– Works at ambiant temperature, providing TC>Tamb
LiTaO3 : TC = 618°C
– Sensitive to temperature variations only
– Sensitivity depends on the used crystal
Surface
Pyroelectric coefficient P in C/K : dq=PdT
– Current source
Thermopiles
• Based on thermocouples: 2 different conductors in
contact, voltage production when there is a temperature
difference
Thermopiles
• Sensitivity = Tension variation depending on incident flux
variation
S : thermal sensitivity (V/K) depending on Seebeck effect of the thermocouple
materials
Z: thermal impedance (K/W)
Independent of the spectral domain
Thermopiles
• Properties:
– Sensitive to temperature difference between the two
junctions
– Works at ambient temperature
– Sensitivity depends on the materials
– Voltage source