Laser induced fluorescence using frequency modulated light

Area of Science:

• Plasma Physics
• Atomic and Molecular Physics
• Laser Spectroscopy

Background:

• Conventional laser induced fluorescence (LIF) suffers from low signal-to-noise ratio (SNR).
• Amplitude modulation with lock-in amplification is a standard method to improve SNR in LIF.
• Measuring neutral helium velocity distribution functions in rf plasmas is crucial for understanding plasma dynamics.

Purpose of the Study:

• To introduce a novel frequency modulated (FM) laser injection technique for LIF measurements.
• To enhance the SNR and precision of LIF measurements in rf plasmas.
• To compare the performance of FM-LIF against conventional amplitude modulated LIF.

Main Methods:

• Utilized a synthetic pulse train with random pulse lengths for 100% amplitude modulation.
• Employed frequency modulation (FM) of the laser beam.
• Drove an acousto-optic modulator with the FM signal in the LIF setup.
• Digitized photomultiplier tube signals and cross-correlated them with the modulation signal.

Main Results:

• The FM-based LIF signal demonstrated superior SNR compared to conventional lock-in amplification.
• The precision of the FM-LIF measurement was significantly improved.
• Successfully measured the neutral helium velocity distribution function in an rf plasma.

Conclusions:

• Frequency modulated laser induced fluorescence is an effective method for improving SNR and precision in plasma measurements.
• FM-LIF offers a significant advancement over traditional amplitude modulated LIF techniques.
• This method provides a more accurate way to determine neutral helium velocity distribution functions in rf plasmas.