Laser power measurements(Ch. 63)

S-108.4010

16.03.2006Tuomas Hieta

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Outline

Why measure laser output power?Laser fundamentals & propertiesDetectors

Thermal detectorsQuantum detectors

Tools & configurationsIntegrating sphereTrap detector

Case: High fiber optic power measurement

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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

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Laser fundamentals

LASER = Light Amplification of Stimulated Emission of RadiationConsist from gain medium(1), Resonator(2) and pump(3)

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Gain medium

Is the volume where light interacts with matterIf 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 hassame properties with the original photonSpontaneous emissionin all directions

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Optical resonator

Active medium with resonator act as amplifier with feedback → oscillatorResonator provides positive feedback (constructive interference) for certain wavelengthsThough the reflectivities of the mirrors are high, some part of the field always leaks from the end mirror

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Laser pumping

Idea is to create population inversion in the medium i.e. to excite electronsCan be done by using light, current, chemical agent,...Population level in the upper level must be higher than in the ground level (stimulated absorption)

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Laser properties

Monochromatic lightCaused by resonator and discreet photon energy< 1nm linewidths easy to achieve

CoherencePhase difference between points at the wavefront remains zero = spatial coherenceTemporal coherence if the phase doesn’t vary with time

DirectionalityCavity determines the directionDiffraction diverges the beam from ideal

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Laser properties(2)

Brightness or powerEven low power lasers have much greater brightness than conventional sources due to directionalityPower obtained from laser can be from microwatts to terawatts(pulsed)

Short pulsesEven 5-10 fs pulses can be achievedNew opportunities for material processing

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Detectors

The idea of detector is to convert radiation into a measurable quantityOperational principle is one way to categorize optical detectorsThe most common detector is, of course, the eye

Thermal detectors Quantum detectors

Thermocouples & thermopiles Phototubes & photomultipliers

Bolometers & thermistors Photoconductive

Pyroelectric Photographic

Pneumatic & Golay Photovoltaic

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Thermal detectors

Measurable response of a thermal detector is a rise in temperatureAbsorbers 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 regionDisagvantages are noisiness and slow responsivity compared to quantum detectors, though can be used to measure single-shot pulsed laser

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Thermocouple

Thermocouple is based on voltage generation at junction of two dissimilar metals Usually thermocouple is deposited onto a light absorbing diskWhen the reference junction is held at known temperature, the temperature of another junction can be deduced from voltage difference

reference

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Thermocouple(2)

Response time usually few secondsFlat spectral response from 200nm → 20µmPower range from 1mW → 5kWThermopile consists of thermocouples connected in series → higher voltage

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Bolometers & thermistors

Bulk device that respond to a rise in temperature by significant change in resistanceIt’s sensitive element is either metal (bolometer) or, more commonly, a semiconductor (thermistor)

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Bolometers & thermistors(2)

Incoming radiation heats the metal → Resistance is changed

→ Incident power can be deduced from ΔVThermal reservoir stabilizes the reference voltageMaterial must be well-known

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Pyroelectric

Ferroelectric materials have spontaneous electrical polarization below certain temperature(Curie point)Incident radiation changes this polarizationCharges are induced to electrodes due to this change and it can be to produce measurable voltageOutput only when radiation changes!

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Pyroelectric(2)

Used to measure pulsed laser powerThe process is independent of wavelength → flat spectral responsivityWindow material used in housing limits the the wavelength region

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Pneumatic & Golay

Golay cell detector is based on thermal expansion of gasIncident radiation heats absorer, which causes pressure in airtight chamberOptics can be used to detect the pressureSensitive to vibrations

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Pneumatic & Golay(2)

Chamber

Membrane

Laser

Detection unit

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Quantum detectors

Detected signal is proportional to the incident photons per unit timePlanck’s constant h relates photon frequency and its energy

QDs are fast and compatible with external electrical circuitryMain disadvantage is the frequency dependancy

hc

hfE

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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

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Phototube & photomultiplier(2)

Photomultiplier accelerates electronsPrimary 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 magnitudeWell-suited for low powerapplications

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Photoconductive

Are used for wavelengths over 1 µmToo low energy to overcome work function

Are based on electron-hole pair creation, which changes the material conductivityAre very common

Cheap, easy to fabricate and small

Wavelengths from visible to far IF can be used by choosing proper semiconducting compounds

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Photoconductive(2)

Incident radiation changes the conductivity, which leads to different photocurrent

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Photographic

Obviously the film of a conventional camera is a detector tooAdvantage is light signal integration

long exposure time can compensate weak signal

Disadvantage is that the chemical reaction is irreversible…

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Photovoltaic

Most common photovoltaic detector is a p-n junction, the semiconductor photodiodeIncident 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

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Photovoltaic(2)

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Photovoltaic(3)

Shunt resistance of the photodiode plays a major role in determining the SNRSome part of the photocurrent allways flows through the shunt resistance

High shunt resistanceis preferred as it causes only small noise currentIt can vary from few hundreds ohms to 10 GΩ

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Photovoltaic(4)

Application determines the semiconductor material to be used

Certain material has certain noise, spectral properties, price, temperature dependency, etc.

200 400 600 800 1000 1200 1400 1600 1800

0.2

0.4

0.6

0.8

1

1.2

1.4

Re

spo

nsi

vity

[A

/W]

Wavelength [nm]

Si

Ge

InGaAs

Ideal

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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 formedGains up to ~200

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Integrating sphere

Integrating sphere is a versatile tool used in many optical measurementsIts function is to angularly and spatially integrate the incoming radiationIn pratice, it acts as a diffuser and an attenuatorIt consists of input and output ports and reflective cavity coating

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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

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Integrating sphere(3)

Used in power measurements to attenuate (and integrate) signal→ cheaper detectors can be ussed when power is lower!

Comes in many sizesApplication determines the size

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Trap detector

Trap consists of number of photodiodes with certain geometry to reduce backreflectionMultiple reflections cause more photon absorption→ greater QEWell-suitable for applications that are sensitive to inter-reflections

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Case: High fiber optic power measurement

One major problem of fiber power measurement is geometryOther issue is to find accurate detector with traceability to standard for optical power near 1.55 µmGenerally, several types of detectors can be used for power measurement

A 1

A 2

D etector surface

a)

b)

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Geometry

Problem with geometry is that the calibration is usually done with laser beam rather than beam out of fiber output

-2 -1 0 1 2

2

1

0

-1

-2

X/mm

Y/m

m

-10 0 100.00

0.04

0.08

0.12

0.16

Rel

. ang

ular

pow

er d

ensi

ty [1

/deg

]

Angle in degrees

Spatial responsivity of

photodetector is

non-uniform!

Gaussian distribution of the

output beam

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Geometry(2)

Integrating sphere is a solution to the geometry problemThe sphere collect almost all the ligth from the fiber output and delivers it to the detector

-15 -10 -5 0 5 10 15-50 %

-40 %

-30 %

-20 %

-10 %

0 %

-8 -4 0 4 8

-0.1 %

0.0 %

0.1 %

Cha

nge

from

resp

onsi

vity

at 0

-deg

ree

Angle in degrees

Integration ”range” of ISP

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Detector

High quality InGaAs photodetector was used in this setupOnly major problem was that it can measure only up to 8 mWTailored 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)

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Detector(2)

With high power levels, nonlinearity must allways be studiedSo called AC/DC method was used to find out weather the ISP configuration is nonlinearNonlinearity was found and one probable cause is overillumination

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The end!

Questions? Comments?

Death star with

high power laser