Download Laser Spectroscopy B3 - Imperial College London

Laser Spectroscopy
B3
Head of Experiment: James McGinty
The following experiment guide is NOT intended to be a step-by-step manual for the experiment
but rather provides an overall introduction to the experiment and outlines the important tasks that
need to be performed in order to complete the experiment. Additional sources of documentation
may need to be researched and consulted during the experiment as well as for the completion of
the report. This additional documentation must be cited in the references of the report.
1
RISK ASSESSMENT
Ensure that all relevant risks or hazards are addressed
Remember, a HAZARD is an inherent property with potential to cause harm, A RISK is the likelihood of actually causing harm in the circumstances of its use.
Please note: This is your assessment carried out on your work procedure. It is to help you to identify, then eliminate, reduce or control dangers that can harm.
PHYSICS DEPARTMENT
RISK ASSESSMENT CHECK LIST
RESPONSIBLE PERSON: J. P. Cotter
Group: 3rd Year Teaching Lab
BEFORE WORK STARTS
Head of Group: Prof J Marangos
send completed assessment to DSO
Date of Assessment: 26/10/10
Location: Room: 406 A
PROCEDURE or WORK ACTIVITY: (Brief description)
SAFETY
CATEGORY
Laser
HAZARD
Substance, equipment or
process
PERSONS AT RISK
Home built infrared 780nm
Class 3B laser max power
15mW
All personnel
Eye damage risk from these
lasers
Visitors, New workers,
Emergency services, security,
Cleaners, contractors, etc.
RISK CONTROL
ASSESSMENT
REMARKS on
ASSESSMENT
Information, instruction, training provided?
State precautions taken to meet legal
requirement and comply with College Policy
- All personnel entering this room
are made aware of the hazard.
- Laser goggles are available.
- Beams are blocked when not in
use.
- Laser table has beam-blocks
around it at optical path level
- all jewelry watches are removed
whilst laser is operated
- All beams are contained within
the surface area above the table
2
Electrical
Electrocution risk from above
laser power supplies
All personnel
- Supplies used for diode laser
systems are no more dangerous
than standard electrical power
supplies within the laboratory.
Therefore usual precautions
must be taken for electrical
equipment as shown below
Mains powered electrical
equipment
All personnel
- No live wires are exposed in
electrical equipment, all items are
fully cased
- All equipment is raised above
ground level incase of lab
flooding.
- For high power equipment
requiring appropriate ventilation it
is ensured this is available.
Distribution boards
All personnel
- Boards are fixed securely in an
appropriate position off the
ground.
- boards should not be “daisy
chained” together
Cables – electrocution
All personnel
Cables should have no bare
insulation and exposed wires
3
Chemical
Rb
All personnel
- Store Rb cell when not used in
a well padded package to
minimize risk of breakage.
- Mount cell in an appropriate
stable mount
Mechanical
Cables – trip hazard
- Cables should not be trailed
along the ground.
- If possible cabling should be
bundled together and should be
transported via cable guttering
- Remove all excess cables not
being used
Head level structures – head
banging hazard
All personnel
- Padding should be placed
around most prominent points
and high visibility tape.
4
Laser spectroscopy: Doppler free saturated absorption of rubidium using a
diode laser
J. P. Cotter, Z. Diveki: July 2013, Version 0.1
Imperial College London, 3rd Year Physics Laboratory
Introduction
This experiment is designed to introduce some modern atomic and laser physics
techniques currently used in research labs around the world. Many of these techniques are
essential to laser cooling, Bose-Einstein condensation and precision measurements.
You will familiarise yourself with a home-built grating stabilised diode laser. This
will then be used to probe the internal structure of rubidium through Doppler free saturated
absorption spectroscopy enabling you to investigate the hyperfine structure in more detail.
The experiment consists of the following parts:




Section 1: Send the laser beam in the rubidium cell and measure the absorption spectrum
with a photodiode.
Section 2: Set up the optical beampath and detectors for recording of Doppler broadened
spectra.
Section 3: Set up the optical beampath and detectors for recording of Doppler free spectra.
Section 4: Determining the laser frequency with a Fabry-Peort interferometer.
The amount of work required to complete each section is not equal; to that end, you should try not
to become bogged down with the first sections. You are encouraged to leave around a week and a
half to complete section 3, which should form the main body of your report.
5
LASER SAFETY
Before you begin ensure that you have read and familiarised yourself in detail with the safety
information outlined in this section and that you have signed the risk assessment
documentation.
Any student in contradiction of these safety notices will be removed from the lab
immediately.
• The laser you will be working with is class 3B. The output is therefore extremely
hazardous to the eyes and can cause blindness if exposure occurs.
• You will be working at a wavelength of 780 nm. The response of the human eye at this
wavelength is poor (the visible spectrum is
 700 nm). This means that the beam is far
more powerful than it appears and additional precautions should be made to avoid
exposure. Your blink reflex won't be able to save you at this wavelength.
• Remove wristwatches and any other jewellery which may come into contact with the laser.
(This includes rings, long necklaces, earphones etc.)
• Wear the safety goggles provided when working with this apparatus.
• Ensure that the external laser lamp is illuminated and that the laser screens are in place if
the laser is on.
• In order to observe your beams always use either an IR card or camera.
• Ensure that all beams remain in the plane of the optical table.
• Never lower your face anywhere near the optical table. If you have to pick something
from the floor close your eyes to do so.
• Only students who have signed the appropriate risk assessment forms are permitted to enter
room 406A while the laser on light is illuminated.
• In an emergency switch of the laser using the current controller and contact the nearest
lab demonstrator or technician.
6
The Experiment
Remember NOT to increase the current controller set point above 70 mA (this can
damage the diode) and NEVER alter the temperature controller (this will significantly
alter the wavelength and could cost you many hours work).
Initial set up for fluorescence
View from above with CCD Camera
1.
2.
3.
4.
Set current & piezo modulation to zero
Increase piezo modulation until you see mode hops
Increase current modulation until mode hops disappear
Fine adjust function amplitude and signal splitter ratio to get four dips
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
Photodiode 1
Function Generator
Isolator
Signal
Splitter
Mirror 1
Rb Cell
Laser diode
Current Modulation
7
Mirror 2
Mirror 5
Photodiode 2
Probe
Stage 2
50/50 Splitter
Mirror 4
Substitute beam sampler for mirror
and add two mirrors and beam splitter
for pump and probe to reveal fine
detail obscured by Doppler broadening
Pump
Pump & Probe
Pump
Isolator
Rb Cell
R
Mirror 1
Laser diode
Mirror 3
Mirror 2
Beam sampler wedge
8
Mirror 5
Photodiode 2
Mirror 6
Optics layout for Laser
Diode Spectroscopy of
Rubidium
Probe
Photodiode 1
Mirror 4
50/50 Splitter
Pump
Mirror 7
Mirror 8
Use target with central hole
and mirrors 7&8 to align
beam with etalon and adjust
length for confocal cavity
Pump
Ref
Pump & Probe
F.P. Etalon
Isolator
R
Rb Cell
Mirror 1
Laser diode
Mirror 2
Mirror 3
Beam sampler wedge
9
You will encounter many devices around the experimental setup that will help you to obtain
results. The next layout shows the rather complicated connections between these devices. Do not
worry when doing the experiment it will all be clearer.
Layout of the applied devices
10
The Laser:
The frequency of the laser depends on 3 parameters: laser temperature, laser current and the
position and orientation of the grating. The laser temperature and current set the frequency range
over which the diode laser will operate (coarse tuning). Within this range, the laser frequency
can be continuously scanned using the position and orientation of the grating (fine tuning). Fine
tuning of the laser frequency is accomplished by a piezoelectric transducer (PZT) located in the
grating mount. The PZT expands in response to a voltage from the PZT controller. The voltage
can be adjusted manually. As the laser cavity length varies, because the number of waves in the
cavity will stay constant (for a while), the wavelength and therefore the laser frequency will vary
as well. But if the cavity length change goes too far, the number of half-wavelengths inside the
cavity will jump up or down (by one or two) so that the laser frequency will jump back into the
range set by the temperature and current. These jumps are called mode hops. Continuous
frequency tuning over the four hyperfine groups (¼ 8 GHz) without a mode hop can only be
achieved if the settings for the temperature and current are carefully optimized. The picture
below shows the layout of the laser itself.
Inside the Laser Diode Box
11
Section 1: FL UORES CENE
Synopsis: If the laser is tuned to precisely the correct wavelength then atoms within the cell can
absorb photons and be excited to the 52P3/2 energy level. These excited atoms will then
spontaneously decay back to the 52S1/2 ground state at a rate of , emitting a photon randomly
through a solid angle of 4 sr as they do so. We will refer to this scattering process as
fluorescence.
FIG. 1: Fluorescence experiment. If your laser is tuned to the correctly then you should see
bright lines your cell as the atoms which absorb light from the laser scatter photons in all
directions.
Method: To make sure the laser is close to resonance with an atomic transition let's begin by
looking for fluorescence from rubidium atoms within the cell using the infra-red camera
provided (remember your eyes can't see 780 nm light). Pass the beam reflected from the mirror
through the rubidium cell as depicted in the initial setup diagram. If the laser is correctly tuned
then the beams passing through the cell will appear as bright glowing lines on the camera,
similar to those shown in FIG. 1(b).
Because atoms can only absorb photons with very specific energies, and therefore wavelengths,
it is unlikely that when you begin working with your laser it will be correctly tuned in order to
see fluorescence. If you can't see anything try changing the wavelength of light by adjusting the
piezo voltage.
Piezo voltage adjustment: This device serves to control the incidence angle of the grating which
is placed after the diode laser. By doing so one controls the wavelength of the outcoming beam.
The adjustment consists of two devices: a) a box that controls the voltage U0 which determines
the amplitude of the grating rotation b) signal generating box that provides an alternating
voltage. The amplitude is given by U0 as an input. The frequency can be adjusted.
If you scan the piezo voltage with a frequency of ∼ 1 Hz you should see the cell flashing as the
laser sweeps through the atomic resonance. You may also need to change the current flowing
through diode itself, though be careful to only change it by small amounts at a time. The current
can be set with the control box of the laser.
12
You might like to begin looking for fluorescence with the following configuration:
Piezo
XXX V
Current
XXX mA
When you see fluorescence you know that the laser is at the correct wavelength for the
following experiments.
Section 2: DO PPL E R B RO ADE NE D ABSO RPT IO N
S PE CT RO SCO PY
Synopsis: The next step is to record the absorption of the laser by rubidium atoms. The cell you
have been provided with is filled with naturally occurring rubidium (this means it contains 72%
85Rb and 28% 87Rb atoms).
Calculation 1: Calculate the FWHM of a Doppler broadened absorption line in both isotopes of
Rb. Take the room temperature what you measure independently.
You should find that the Doppler width, ΔDop, at room temperature is much larger than the
excited state hyperfine splitting (typically ∼ 100 MHz ), and much smaller than the ground state
hyperfine splitting for both isotopes (3 GHz in 85Rb and 6 GHz in 87Rb). This means that if
Doppler broadening is present transitions to different excited states will be indistinguishable
from one another, however transitions from different ground states should be easy to observe.
Calculation 2: Why is it that the hyperfine structure is larger for the 52S1/2 than the 52P3/2?
Provide simple, physical explanation and support it with mathematical methods as well.
FIG. 2: You should be able to see four Doppler broadened dips in in your absorption spectrum
using this set-up.
13
Method: Using the same set up from your earlier fluorescence experiment replace the mirror
with the beam sampler as shown in stage 2 diagram. You should steer the path of the beam using
the opto-mechanical mount for the slide so as to maximise the DC photodiode signal. Be careful
not to let the photodiode saturate as this will stop you from taking meaningful data. If you find
that the photodiode is saturating try using one of the neutral density filters provided to attenuate
the laser beam.
Because your cell contains two isotopes of Rb, each of which have two ground states, you should
be able to observe four resolvable dips in the spectrum as you sweep the piezo voltage (two dips
for each isotope and two dips for each ground state). FIG. 2(b) shows a typical rubidium
spectrum. You probably won't be able to see all four dips in one scan as the laser doesn't have a
large enough mode-hop-free range but try to identify them all independently.
Using the scope provided, record and identify all four absorption dips. Once you're happy with
your spectra, they should look something like those displayed in FIG. 2(b), use the matlab mfiles provided to fit a Gaussian line shape to each peak.
Calculate 3: You can use this fit to measure the temperature, T. To do this you will need to
record the scan voltage that you are applying to the piezo simultaneously with your spectrum.
You will also need to know the calibration between the piezo voltage and change in optical
frequency, this is stuck to the side of your laser. Does each dip return the same temperature and
how do they compare to the temperature in the lab as measured by a thermometer?
14
Section 3: DO PPLER -FREE SATURATE D ABSORPTIO N
SPECT ROSCO PY
Synopsis: We're now going to try and remove the effect of Doppler broadening on our spectrum
using a counter-propagating saturated absorption technique.
FIG. 3: a) The set-up required in order to observe sub-Doppler features in your spectra. b) An
example of the kind of spectrum you should see from PD2.
Method: Begin by setting up the apparatus as shown in Optics layout for Laser Diode
Spectroscopy of Rubidium. Monitor the output of PD1 and PD2 on the scope as you go. When
the stronger beam (Pump) is aligned to be near anti-parallel with the weaker (Probe) beam small
peaks in the Doppler broadened profile should appear. You should steer mirrors M1 and M2 in
order to optimise the size of these peaks in your spectrum (they should look something like those
displayed in FIG. 4. It's best to optimise your scans to observe one ground state transition at a
time, recording each of your spectra as you go.
15
FIG. 4: The four Doppler broadened rubidium profiles exhibiting sub-Doppler features.
Try to balance the DC voltage of both PD1 and PD2 to be the same (the Doppler broadened dips
in both spectra should also be the same size). PD2 contains both Doppler broadened and
Doppler-free information about the vapour while PD1 contains only Doppler broadened
information. By looking at the difference between these signals we can isolate the Doppler free
information. To begin with try looking at (PD2-PD1) using the MATH function on your scope.
When you're happy your signals are well balanced try using the differential amplifier provided
instead of the scope to take the difference between your signals. Your subtracted, Doppler free
spectra should look something FIG. 5(b).
16
FIG. 5: An example of the F = 3 F’ transitions before and after Doppler subtraction.
Use your scope to record each of the four Doppler free spectra you can see. Then using your
voltage to frequency calibration try to determine the excited state hyperfine splitting for both
87Rb and 85Rb. It is easier to work with the 85Rb F = 3 spectrum as the signal is larger and less
complicated so it is advisable to start with this one. You should check that your spectra make
sense by comparing the splittings you measure by eye with the accepted values. Once you’re
happy that your numbers make sense try fitting to the peaks to extract a more accurate
measurement of the splittings. Be aware that some transitions are quite weak and therefore
produce very small peaks so you may not be able to observe every one of them.
Once you have measured the position of each observable peak try to identify them. Be careful
not to confuse cross-over resonances in your spectra with real atomic transitions. How do your
measured frequency splittings agree with the accepted values? Do the measured linewidths of
your peaks agree with the accepted natural linewidth. If not what mechanism could be causing
this and how could you overcome it?
17
Section 4: FREQUENCY CALIBRATIO N WITH A FABRY -PEROT ETALON
Synopsis: In principle all the previous measurements results in an absorption spectra as the
function of time (given by the signal generator). By recording the trace of the signal generator
you can do the mapping between time-voltage (of the piezo). The Fabry-Perot etalon helps to do
the mapping between voltage and frequency.
Method: Insert the Fabry Perot Cavity as shown in Optics layout for Laser Diode Spectroscopy
of Rubidium. Use Mirrors 7 and 8 and the florescent target provided to align the beam normal
and coaxial and adjust the cavity length to make a confocal cavity. The focal length of both
mirrors is 10cm. Connect the output of PD3 to the Ocilloscope. The oscilloscope will display
discrete peaks as the function of time that corresponds to the constructive interference of the
different laser wavelengths that fulfil the mλ=2L formula, where m is an integer and L is the
length of the cavity. From this one can deduce the frequency separation, Δν, (Free Spectral
Range) of the peaks on the oscilloscope: Δν=c/(2L), where c is the speed of light. By plotting the
trace of the signal generator and Δν one can find the frequency-voltage mapping coefficient (G).
To improve the determination of G, use the radio frequency generator to modulate the current of
the laser diode that results in the appearance of sidebands next to the main laser peaks from the
Fabry-Perot interferometer. The display of the radio frequency generator tells you the frequency
difference between the sidebands and the main peaks.
18