AMOS-Tralala

The purpose of these instructions is to give you a quick overview on how to work with the devices. You should still be familiar with the manual and receive a proper introduction from the lab leader or the principal users.

Lab name: Tralala (Transient Absorption Laser Laboratory) aka Fastlab

Lab location: Building 341 (ITG), room 110

Phone number: 29163

Lab leader: Ian Howard

Principal users: Michael Adams, Vu Hong Le, Marius Jakoby

Access: The building belongs to the Institute of Toxicology and Genetics (ITG), the door is secured with a card reader. To get access to the building with your KIT card you have to bring it to the ITG secretary (Selma Huber, B316 / R130). Additionally, you need a written statement (E-Mail) from the lab leader.

Optical table

The optical tables rest on vibration isolation feet (Newport PL-2000; even at Newport there seems to be no manual for the PL-2000, their service recommends to use the I-2000 manual). The isolators are passive, that means they have no direct connection to a pressurized air line. Instead, they have to be re-pressurized manually every once in a while. There is a foot pump available for this purpose. The typical air pressure according to the manual is 10 - 85psi (0.7 - 6.0 kg/cm²).

Cooling

Diagram of the cooling circuit

There are two chillers in the lab. One is located inside the rack (PolyScience Recirculator) and cools exclusively the Empower Pump Laser. The temperature setpoint is at 18°C.

The second chiller (ThermoFischer Merlin M25) is outside the rack. The cooling water (deionized water) goes through the Millenia, Tsunami and finally the Spitfire, where it cools the High-Voltage unit and the removes heat from the Peltier-cooled crystal. Most heat is generated at the crystal when the Empower output is turned on. The chiller's setpoint is at 17°C, the pressure should be below 40psi. Insufficient cooling will lead to the Millenia/Tsunami output becoming unstable.

Spitfire Laser System

Rack-mounted controls for the Spitfire Laser system

The SpectraPhysics Spitfire system is a regenerative amplifier that amplifies optical femtosecond pulses emitted by a mode-locked Ti:sapphire laser (SpectraPhysics Tsunami). A high power diode-pumped laser (SpectraPhysics Empower) is used as a pump for the Spitfire amplifier.

The Spitfire outputs femtosecond laser pulses with a center wavelength of 800nm and a repetition rate of 1kHz (can be divided). The output power is around 3.6W.

Startup procedure

The controls for the Spitfire system are located inside a temperature-controlled rack.

1. The Empower, Tsunami and TCU (Temperature Control Unit) power supply should already be on. If not, turn them on
2. Turn on the TDG (Time and Delay Generator) power supply
3. Use they keyswitches to activate the Empower, Tsunami and TDG
4. Press Laser Power on the small black Tsunami control box and wait for the diodes to warm up
5. While you wait, turn on the cooling fan next to the Tsunami (helps with stability) and switch the laser warning lamp outside the lab on
6. Once the diodes are ready, turn on the Tsunami by long-pressing the Laser Power button and wait for the output power to reach the set value (3.30W).
7. Open the OceanOptics SpectraSuite spectrometer software on the PC
8. Open the Spitfire software on the PC
9. Remove the beam tube at the Tsunami's output and modelock the laser:
   

   Example spectra for how the Tsunami's spectrum should look before (top) and after (bottom) modelocking.
   1. Use the OceanOptics spectrometer (HR2000) to monitor the spectrum of the laser
   2. Wiggle the prism knob on the Tsunami until the spectrum changes from a sharp line to a broad distribution
   3. Make sure there is no CW-breakthrough (sharp line in the spectrum)
   4. Make sure both Bandwidth Detector (BWD) indicators in the Spitfire software are on
10. Once the Tsunami is mode-locked put the beam tube back in
11. Clear the faults in the Spitfire software by clicking on them
12. Long-press Run in the software (this opens the shutter to the Empower pump laser)
13. Go to Channels and open Channels 3, 2 and 1

Shutdown procedure

1. Stop the amplifier in the Spitfire software
2. Press Laser Power on the Tsunami's control box to stop the laser output
3. Use the keyswitches to turn off the Empower, Tsunami and TDG
4. Turn off the cooling fan next to the Tsunami
5. Optional: The TDG power supply can be switched off

Important: Leave the power supply for the TCU, Tsunami and Empower on. These take very long to start up again once they have been shut off.

Maintenance

The water level in the chillers (one outside and one inside the rack box) has to be checked every three months.

Inside the TCU is a humidity filter that has to be changed roughly every six months. New filters should be kept in stock.

Troubleshooting

From time to time it occurs that the Spitfire Software fails to connect to the system and does not start. This is usually a problem with the LAN card that can be fixed the following way:

1. Open the Windows Device Manager
2. Locate the Intel network card that is connected to the Spitfire (Intel(R) 82574L Gigabit Network Connection)
3. Right-click and disable the device, a white overlay icon should appear.
4. Right-click and enable the device again
5. Now the blue progress bar in the Spitfire software should start to fill and the software should start.

Innolas Laser

Innolas Picolo AOT MOPA

The Innolas Picolo AOT 1 MOPA Sub-Nanosecond Laser is a compact oscillator/amplifier laser system. It can be used at 532nm and 355nm wavelength.

Values from the data sheet

• Max Rep-rate: up to 5kHz, optimized for 1kHz
• Max Pulse Energy at 532nm: 60µJ
• Max Pulse Energy at 355nm: 30µJ
• Pulse Width at 1064nm (FWHM): <800ps @1kHz
• Power Stability at 1064nm: < 5%
• Max Average Power: 800mW
• Jitter to external trigger: < +/- 0.4ns

Changing the wavelength

The output wavelength depends on the settings of two thermal controllers inside the laser head, TEC4 and TEC5. These temperatures can be set on the controller unit. The optimal values for 532nm or 355nm operation can be found in the manual. As of October 2016 these values are:

• 532nm: TEC4 = 44.0°C TEC5 = 54.2°C
• 355nm: TEC4 = 55.3°C TEC5 = 54.0°C

The controller "forgets" its settings after being disconnected from the power grid, you then need to set the values again after startup.

You need also to change the output caps at the laser head. Unused output ports have a beam block on them. So when changing the wavelength you need to open the required port and put the blocking cap on the previously open one.

Startup procedure

1. Turn the key on the controller to position 1
2. Wait for the temperature controllers to reach their target temperature
3. Set the desired values for TEC4, TEC5 and frequency
4. Turn the key on the controller to position 2
5. Press the On/Off button to turn laser output on

Shutdown procedure

1. Press the On/Off button to turn laser output off
2. Turn the key on the controller to position 0
3. Optional: Turn off power at the controller's rear side. This will cause the controller to "forget" manually entered settings.

Transient Absorption Spectroscopy

The Transient Absorption setup is maintained by Michael Adams.

Initial steps

1. Turn on the Spitfire laser system (see above)
2. Turn on the Thorlabs MC2000 Optical Chopper System
3. Turn on the Thorlabs FW102C filter wheel
4. Turn on the Thorlabs Delay stage controler (below the table, the switch is on the back)
5. Make sure that the tec5 electronics for the detector are powered
6. Make sure all devices are connected to the PC

Chopper Wheel

The chopper wheel reduces the pulse frequency of the 1kHz Spitfire output to 500Hz by blocking every second pulse. The Input signal (TTL) comes from the Spitfire, the Output signal (TTL) has to be connected to the tec5 PCI card inside the lab PC with a custom adapter. The controller has to be set to external reference. In order to halve the pulse frequency the harmonic multipliers have to be set to N=1 and D=2 (because f_ext = f_ref * N / D).

Pump light

400nm (SHG of Spitfire)

Part of of the Spitfire output passes through the chopper wheel, goes via the delay stage through a beam reducer onto a Beta barium borate (BBO) crystal. This crystal converts the 800nm light to a wavelength of 400nm (frequency doubling). Residual 800nm light is removed with a shortpass filter. Check the quality of the 400nm light. If there is a dark line through the spot change the phase on the chopper wheel controller.

355nm or 533nm (Innolas laser)

To-Do

Other wavelengths (TOPAS)

To-Do

Probe light (white light)

The 800nm output of the Spitfire amplifier is focussed into a sapphire crystal (located inside a cage system) to generate a white-light supercontinuum. Check the white light quality after the crystal. If no white light is visible, increase the power with the linear ND filter that is located earlier in the beampath. If the white light flickers unstably reduce the power or carefully move the crystal around. The spot should look smooth and without structure.

There will be an intense residual 800nm component in the white light that can be filtered out by placing a filter in the white light beampath. Alternatively there is a print-out spatial filter (essentially a black line printed on transparent foil) that can be placed directly in front of the detector inside the spectrometer box. You have to move it around manually so the blocking line is located where 800nm light hits the detector. This is best done in LiveView.

The white light should exit the cage not collimated but slightly focussed, so that the focus is located at the sample position. To achieve this, move the second lens in the crystal cage.

After the sample position the light passes the filter wheel and enters the spectrometer box that holds a mirror, a prism and the tec5 detector electronics. With the vertical axis of the mirror inside the box you can correct the height of the beam to properly hit the detector. Use the LiveView mode for this: move the beam up and down and look for the signal maximum. With the horizontal axis you can change the wavelength region to measure.

Software

The control software for the setup is written in LabVIEW, the project file is located under D:\Labview Programs\Michael Adams\TA_control\TA_control.lvproj.

Start and run TA_control.vi. Normally, you will need to run no other VI. Some helper VIs that can be run as standalone tools are located in the "utilities" directory.

The software can be run in a Simulation mode that does not need the lab hardware to be connected to the PC. This is useful only for development purposes.

The software should be closed with the Exit Program button and - if possible - not with LabVIEW's Abort Execution button.

Measurement

LiveView: Check Signal, Find Zerotime and Set Background

Once everything is set up, run LiveView in TA_control. The parameters here are:

• number of burst pairs: When a trigger signal arrives, this is the number of spectra pairs (pump on/off) that are measured in one burst train. You should keep this at 50, values above 200 tend to lead to errors from the tec5 electronics.
• number of burst trains: How many burst trains are to be measured.
• Integration time: This should be set to the time between two laser pulses. For standard operation of the Spitfire amplifier at 1kHz, this has to be set to 1ms accordingly. Otherwise the spectra measurements will be out of sync with the laser.

Press Start to begin taking measurements. You need to stop and restart the measurements if you want to change the parameters. After pressing Stop you have to wait for the current burst train to finish, this is normal.

In the Waveforms tab you can see current measured spectra, which is useful for alignment of the white light on the detector. Make sure that the detector is not saturated. You can also see the dT/T signal. If you switch the window back to the TA_control.vi you can manually move the delay stage. This helps you in identifying where zerotime is on the stage (note the stage position in mm).

You need to make a measurement of the background signal. Block the white light on the table (but leave the pump light on) and press Start. In Waveforms 2 you can see the accumulated spectrum of all the measurements since you last hit Start. There are two graphs, because the pump on/off background can differ in the amount of stray pump light. Use the Single Channel Viewer to look at a single channel. Usually the signal increases over time at the beginning of a measurement. If this is the case wait until the single channel signal reaches a constant level and restart the measurement. Leave it running until you are satisfied with the quality of your background spectrum, e.g. until the Difference between accumuated spectra reaches stable values. Save the backgrounds by pressing Set backgrounds. If you want to redo the background measurement reset the active backgrounds first.

Press Exit to return to TA_control.

Generate Stage Position File

The positions for the delay stage are loaded from a simple textfile. It has to contain a list of numbers (the stage positions in mm) separated by line breaks. You can create this list manually or use the Generate Stage Position File dialog. You enter the first position, the last position and where zerotime is on the stage (in mm). You can select at how many positions before and after zerotime you want to perform a measurement and choose between linear and logarithmic spacing. You can then save the file for later reference or manual editing. Close the window by clicking Done, the current positions are carried over to the main TA_control window.

Wavelength Calibration

Which detector channel receives which wavelengths depends on the sample thickness and the way it is mounted as well as the position of the mirror inside the spectrometer box. If any of those parameters are changed, a new wavelength calibration is necessary. Press Wavelength Calibration in TA_control. You can enter individual integration times for the five calibration measurements. These are performed after you click Let's go by iterating through the filters in the motorized filter wheel. These filters are:

1. 450nm Longpass
2. 532nm Bandpass
3. 633nm Bandpass
4. 730nm Bandpass
5. 850nm Bandpass
6. 950nm Bandpass

During normal TA measurements, the 450nm Longpass filter at position 1 is used to remove stray light of the 400nm pump. The wavelength calibration interates through the five bandpass filters and takes a spectrum for each. Usually there is less light in the short-wavelength region, adapt the integration time accordingly to get decent spectra for all filters with enough signal and without saturation.

The spectra are automatically fitted with a gaussian distribution to get the center channel. Select which wavelength-channel pairs you want to keep in the calibration.You can then save the calibration for later reference or manual editing. Only the selected wavelength-channel pairs are saved. How data is interpolated between them is not part of the calibration. A few possible interpolation methods can be displayed as a preview. Close the dialog by clicking Done, the current calibration is carried over to the main TA_control window.

Start Measurement

Now everything should be ready so you can start the measurement. Go to TA_control's Setup-tab and choose your settings:

• Integration time, number of burst trains, number of burst-pairs: These are the same settings as mentioned above for LiveView. Usually you can leave the integration time at 1ms and the number of burst-pairs at 50 and use only the number of trains to define how long you want to measure at each stage position.
• Sensor Work Mode: Should be set to ExteralTriggerSlope. Refer to the tec5 manual for more information.
• Stage positions file, Wavelength calibration: Select a position or calibration file to load.
• Save data directory: Select the directory where your measurement data should be saved
• Base filename: Enter the filename base without path or file extension. The filename will automatically generated as C:/path/to/file/FILENAME_001.bin, C:/path/to/file/FILENAME_002.bin, and so on.

You can check the backgrounds, stage positions and wavelength calibration before you start. Once you are ready, press Measure TA Spectrum.

The spectator window opens and you can watch your dataset grow as it is being measured. You can also import previously measured TA data to compare.

To be continued...

Cryostat

To-Do (Philipp?)