Download S-108.4010_Lecture_15.3.2006

Laser power measurements
(Ch. 63)
S-108.4010
16.03.2006
Tuomas Hieta
Outline
Why measure laser output power?
Laser fundamentals & properties
Detectors
Thermal detectors
Quantum detectors
Tools & configurations
Integrating sphere
Trap detector
Case: High fiber optic power measurement
2
Why measure laser power?
One of the fundamental
measurements
Needed in
telecommunications,
spectroscopy, industry,
characterization of light
sources(natural,
superficial)...
Light is widely used in
physics, so accurate
measurements are needed
to verify theories
3
Laser fundamentals
LASER = Light Amplification of Stimulated
Emission of Radiation
Consist from gain medium(1), Resonator(2) and
pump(3)
4
Gain medium
Is the volume where light interacts with matter
If an electron is exited to a higher energy level,
incoming photon with equal energy can stimulate the
excited electron and cause stimulated emission →
GAIN!
Stimulated photon has
same properties with
the original photon
Spontaneous emission
in all directions
5
Optical resonator
Active medium with resonator act as amplifier with
feedback → oscillator
Resonator provides positive feedback (constructive
interference) for certain wavelengths
Though the reflectivities of the mirrors are high, some
part of the field always leaks from the end mirror
6
Laser pumping
Idea is to create population
inversion in the medium i.e.
to excite electrons
Can be done by using light,
current, chemical agent,...
Population level in the upper
level must be higher than in
the ground level (stimulated
absorption)
7
Laser properties
Monochromatic light
Caused by resonator and discreet photon energy
< 1nm linewidths easy to achieve
Coherence
Phase difference between points at the wavefront remains zero =
spatial coherence
Temporal coherence if the phase doesn’t vary with time
Directionality
Cavity determines the direction
Diffraction diverges the beam from ideal
8
Laser properties(2)
Brightness or power
Even low power lasers have
much greater brightness
than conventional sources
due to directionality
Power obtained from laser
can be from microwatts to
terawatts(pulsed)
Short pulses
Even 5-10 fs pulses can be
achieved
New opportunities for
material processing
9
Detectors
The idea of detector is to convert radiation into a
measurable quantity
Operational principle is one way to categorize optical
detectors
The most common detector is, of course, the eye
Thermal detectors
Quantum detectors
Thermocouples & thermopiles Phototubes & photomultipliers
Bolometers & thermistors
Photoconductive
Pyroelectric
Photographic
Pneumatic & Golay
Photovoltaic
10
Thermal detectors
Measurable response of a thermal detector is a rise
in temperature
Absorbers are used to absorb the incoming radiation
and convert it to heat
Main virtue of thermal detectors is their relatively flat
responsivity over a wide wavelength region
Disagvantages are noisiness and slow responsivity
compared to quantum detectors, though can be used
to measure single-shot pulsed laser
11
Thermocouple
Thermocouple is based on voltage generation at
junction of two dissimilar metals
Usually thermocouple is deposited onto a light
absorbing disk
When the reference junction is held at known
temperature, the temperature of another junction can
be deduced from voltage difference
reference
12
Thermocouple(2)
Response time usually few seconds
Flat spectral response from 200nm → 20µm
Power range from 1mW → 5kW
Thermopile consists of thermocouples
connected in series → higher voltage
13
Bolometers & thermistors
Bulk device that respond to a rise in
temperature by significant change in
resistance
It’s sensitive element is either metal
(bolometer) or, more commonly, a
semiconductor (thermistor)
14
Bolometers & thermistors(2)
Incoming radiation heats the metal
→ Resistance is changed
→ Incident power can be deduced from ΔV
Thermal reservoir stabilizes
the reference voltage
Material must be
well-known
15
Pyroelectric
Ferroelectric materials have spontaneous
electrical polarization below certain
temperature(Curie point)
Incident radiation changes this polarization
Charges are induced to electrodes due to this
change and it can be to produce measurable
voltage
Output only when radiation changes!
16
Pyroelectric(2)
Used to measure pulsed
laser power
The process is
independent of wavelength
→ flat spectral responsivity
Window material used in
housing limits the the
wavelength region
17
Pneumatic & Golay
Golay cell detector is based on thermal
expansion of gas
Incident radiation heats absorer, which
causes pressure in airtight chamber
Optics can be used to detect the pressure
Sensitive to vibrations
18
Pneumatic & Golay(2)
Laser
Chamber
Detection unit
Membrane
19
Quantum detectors
Detected signal is proportional to the incident
photons per unit time
Planck’s constant h relates photon frequency
and its energy
E  hf 
hc

QDs are fast and compatible with external
electrical circuitry
Main disadvantage is the frequency
dependancy
20
Phototube & photomultiplier
Are so called photoemissive
detectors
Surface absorbs photons and
some electrons escape from the
surface if they can overcome the
work function
When anode is collecting
these electrons, the device is
called a phototube
21
Phototube & photomultiplier(2)
Photomultiplier accelerates electrons
Primary electron hits the first electrode and after that it is accelerated
towards the second electrode with higher voltage
→ High energy electron causes more emitted low energy electrones
→ cascade stucture leads to greatly amplified effect
Amplification can be several
orders of magnitude
Well-suited for low power
applications
22
Photoconductive
Are used for wavelengths over 1 µm
Too low energy to overcome work function
Are based on electron-hole pair creation, which
changes the material conductivity
Are very common
Cheap, easy to fabricate and small
Wavelengths from visible to far IF can be used by
choosing proper semiconducting compounds
23
Photoconductive(2)
Incident radiation changes the conductivity, which
leads to different photocurrent
24
Photographic
Obviously the film of a conventional camera is a
detector too
Advantage is light signal integration
long exposure time can compensate weak signal
Disadvantage is that the chemical reaction is
irreversible…
25
Photovoltaic
Most common photovoltaic detector is a p-n
junction, the semiconductor photodiode
Incident photon causes electron-hole pair in
the depletion region
Due to bias, electrons and holes drift to different electrodes,
which creates photocurrent
Wider depletion region makes photodiode
more sensitive and slower
26
Photovoltaic(2)
27
Photovoltaic(3)
Shunt resistance of the photodiode plays a
major role in determining the SNR
Some part of the photocurrent allways flows
through the shunt resistance
High shunt resistance
is preferred as it causes
only small noise current
It can vary from few
hundreds ohms to 10 GΩ
28
Photovoltaic(4)
Application determines the semiconductor material to
be used
Certain material has certain noise, spectral properties, price,
temperature dependency, etc.
1.4
Si
Ge
InGaAs
Ideal
Responsivity [A/W]
1.2
1
0.8
0.6
0.4
0.2
200
400
600
800
1000
1200
1400
1600
1800
Wavelength [nm]
29
Photovoltaic(5)
Pin photodiodes have
undoped i-region
between p and n regions
Lower speed, but higher
bandwidth
Avalanche photodiodes
are used at low power
levels
Primary carriers are
accelerated with bias and
when they collide with other
atoms, new carriers are
formed
Gains up to ~200
30
Integrating sphere
Integrating sphere is a versatile tool used in
many optical measurements
Its function is to angularly and spatially
integrate the incoming radiation
In pratice, it acts as a diffuser and an
attenuator
It consists of input and output ports and
reflective cavity coating
31
Integrating sphere(2)
The operational principle is simple;
highly reflective and diffusive coating causes multiple reflections
inside the sphere and eventually some part of it end up at the active
area of the photodiode
32
Integrating sphere(3)
Used in power measurements to attenuate (and
integrate) signal
→ cheaper detectors can be ussed when power is lower!
Comes in many sizes
Application determines the size
33
Trap detector
Trap consists of number
of photodiodes with
certain geometry to
reduce backreflection
Multiple reflections
cause more photon
absorption
→ greater QE
Well-suitable for
applications that are
sensitive to interreflections
34
Case: High fiber optic power measurement
One major problem of
fiber power
measurement is
geometry
Other issue is to find
accurate detector with
traceability to standard
for optical power near
1.55 µm
Generally, several types
of detectors can be used
for power measurement
a)
A1

b)
Detector surface
A2
35
Geometry
Problem with geometry is that the calibration is
usually done with laser beam rather than beam out of
fiber output
2
Gaussian distribution of the
output beam
-1
-2
-2
-1
0
X/mm
Spatial responsivity of
photodetector is
non-uniform!
1
2
Rel. angular power density [1/deg]
0
Y/mm
1
0.16
0.12
0.08
0.04
0.00
-10
0
Angle in degrees
10
36
Integrating sphere is a
solution to the geometry
problem
The sphere collect
almost all the ligth from
the fiber output and
delivers it to the
detector
Change from responsivity at 0-degree
Geometry(2)
0%
-10 %
-20 %
0.1 %
-30 %
0.0 %
-40 %
-0.1 %
-8
-50 %
-15
-10
-5
-4
0
0
4
5
8
10
15
Angle in degrees
Integration ”range” of ISP
37
Detector
High quality InGaAs photodetector was used
in this setup
Only major problem was that it can measure
only up to 8 mW
Tailored sphere with high attenuation in front
of it can solve this problem (~0.7% in this
case)
Detector was calibrated against pyroelectric
detector, which was calibrated against
cryogenic absolute radiometer (primary
standard)
38
Detector(2)
With high power
levels, nonlinearity
must allways be
studied
So called AC/DC
method was used to
find out weather the
ISP configuration is
nonlinear
Nonlinearity was
found and one
probable cause is
overillumination
39
The end!
Questions? Comments?
Death star with
high power laser
40