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High-speed Continuous-wave Stimulated Brillouin Scattering Spectrometer for Material Analysis
Chapters
Summary September 22nd, 2017
We describe the construction of a rapid continuous-wave-stimulated-Brillouin-scattering (CW-SBS) spectrometer. The spectrometer employs single-frequency diode-lasers and an atomic vapor notch-filter to acquire transmission spectra of turbid/non-turbid samples with high spectral-resolution at speeds up to 100-fold faster than those of existing CW-SBS spectrometers. This improvement enables high-speed Brillouin material analysis.
Transcript
The goal of this experiment is to construct and operate a continuous-wave stimulated Brillouin scattering spectrometer to acquire transmission stimulated Brillouin spectra of turbid and non-turbid samples with high spectro resolution and speed. This method can advance the use of Brillouin spectroscopy and imaging for investigating the mechanics of biomaterials such as cells and tissue. The main advantage of this technique is that it can provide rapid acquisition of Brillouin spectra of turbid and non-turbid matter.
To begin the experiment, verify that the components of the CW-SBS spectrometer are securely mounted on an optical board. Check that the custom data acquisition software is receiving data from the microwave frequency counter, the lock-in amplifier, the pump and probe laser controllers, and the function generator. Then fill with distilled water a sample chamber made of two 25 millimeter diameter 0.17 millimeter thick glass cover slips and 500 micrometer thick polytetrafluoroethylene tape.
Mount a chamber holder on a three-axis motorized translation stage. Secure the water sample in the chamber holder. Translate the sample to the common focus point of the probe and pump focusing lenses.
Then adjust the current knob of the tapered amplifier pump laser controller until the pump laser power is above 250 milliwatts. Set the lock-in amplifier time constant to one second, the low-pass filter to 24 decibels per octave, and the sensitivity to one millivolt RMS. Set the phase shift between the amplifier reference and signal inputs to zero.
While monitoring the stray pump reflections on channel one, slowly adjust the pump laser current until the observed reflections are at a minimum. Then set the rubidium vapor cell heating assembly power supply to 17 volts DC.Once the rubidium cell stabilizes at 90 degrees Celsius, place the power meter before the probe laser focusing lens. Slowly adjust the probe laser current until the measured power is over 10 milliwatts.
Next, move the power meter behind the rubidium cell. Adjust the probe laser temperature until the measured power is at a minimum. Then adjust the probe laser current until the measured power stabilizes above 10 milliwatts.
Alter the probe laser current to set the frequency detuning between the pump and probe lasers to approximately the Brillouin shift of the sample. Set the lock-in amplifier sensitivity to 100 microvolts RMS and set the phase shift back to zero. Then carefully turn the pitch and yaw screws of the pump beam folding mirror kinematic mount and transfer the pump focusing lens along the optical axis of the system to optimize the crossing efficiency of the pump and probe lasers.
Block the probe beam and note the levels of the stray pump laser reflections while monitoring channel one of the lock-in amplifier. It is critical to properly reject pump stray reflections from the chamber and sample. If needed, slightly close the iris placed in front of the rubidium cell and slightly increase the cross angle between the pump and probe laser to improve the rejection levels.
Unblock the probe beam and continue adjusting the pump laser mirror and focusing lens until the stimulated Brillouin gain signal is at a maximum above two microvolts RMS and the stray pump reflections are at a constant minimum. Once the system has been optimized for the sample, create a calibration curve from a plot of probed modulation current versus pump-probe frequency detuning. Set the lock-in amplifier time constant to greater than or equal to 100 microseconds.
Set the function generator connected to the laser controller current modulation input to channel one. Select a triangle waveform, set the amplitude to a peak to peak voltage of 150 millivolts, and set the frequency to 50 Hertz. Set the data acquisition unit sampling rate to less than or equal to 100, 000 samples per second per channel.
Using the custom data acquisition software, record the SBG and probe laser modulation current signals for at least 10 milliseconds. Import the acquired data into computational software. Use the calibration curve to convert the measured probe laser modulation current values to the pump-probe frequency detuning values.
Subtract the average noise floor. Plot the SBG measurements against the pump-probe frequency detuning values to generate the SBG spectrum. Fit the spectrum with a Lorentzian curve using the amplitude, frequency position, and full width at half of the highest point of the spectrum as initial values.
Calculate the Brillouin shift and the line width from the frequency positions of the maximum and the full width at half maximum of the Lorentzian fit respectively. Use values from repeated Lorentzian fitted spectra to generate a corresponding histogram. SBG spectra of distilled water and lipid emulsion tissue phantom samples were generated using this method with a spatial resolution of approximately eight micrometers.
The water samples were acquired in 10 seconds and 10 milliseconds and the lipid samples were acquired in 10 seconds and 100 milliseconds. The mean Brillouin shifts determined from the rapidly acquired spectra were 5.08 gigahertz for water and 5.11 gigahertz for the tissue phantom. These values were consistent with those calculated from the spectra recorded over 10 seconds and with literature values.
200 successive measurements were performed on each sample and the standard deviations of the distributions of estimated Brillouin shifts were evaluated. The standard deviations were 8.5 megahertz and 33 megahertz for the water and tissue samples respectively. After watching this video, you should have a good understanding of how to assemble and operate a stimulated Brillouin scattering spectrometer to rapidly measure stimulated Brillouin gained spectra of water and tissue-like samples in transmission mode.
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