Laser power measurements (Ch. 63) S Tuomas Hieta

2 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

3 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

4 Laser fundamentals LASER = Light Amplification of Stimulated Emission of Radiation Consist from gain medium(1), Resonator(2) and pump(3)

5 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

6 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

7 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)

8 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

9 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

10 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 detectorsQuantum detectors Thermocouples & thermopilesPhototubes & photomultipliers Bolometers & thermistorsPhotoconductive PyroelectricPhotographic Pneumatic & GolayPhotovoltaic

11 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

12 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

13 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

14 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)

15 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

16 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!

17 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

18 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

19 Pneumatic & Golay(2) Chamber Membrane Laser Detection unit

20 Quantum detectors Detected signal is proportional to the incident photons per unit time Planck’s constant h relates photon frequency and its energy QDs are fast and compatible with external electrical circuitry Main disadvantage is the frequency dependancy

21 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

22 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

23 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

24 Photoconductive(2) Incident radiation changes the conductivity, which leads to different photocurrent

25 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…

26 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

27 Photovoltaic(2)

28 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

29 Photovoltaic(4) Application determines the semiconductor material to be used Certain material has certain noise, spectral properties, price, temperature dependency, etc. Si Ge InGaAs Ideal

30 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

31 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

32 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

33 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

34 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 inter- reflections

35 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

36 Geometry Problem with geometry is that the calibration is usually done with laser beam rather than beam out of fiber output Spatial responsivity of photodetector is non-uniform! Gaussian distribution of the output beam

37 Geometry(2) 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 Integration ”range” of ISP

38 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)

39 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

40 The end! Questions? Comments? Death star with high power laser