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Frequency Modulation Spectroscopy

Compared to conventional cw laser absorption detection using a difference amplification scheme (Amin = 1.5 × 10-3), frequency modulation (FM) spectroscopy with a minimum detectable absorption of Amin < 1 × 10-4 provides a significant higher sensitivity for radical detection behind shock waves. The sensitivity of FM spectroscopy stems from the following reasons.

  • At high modulation frequencies, the 1/f noise of the detection laser is negligible.
  • State-of-the-art lock-in techniques enable shot-noise limited detection sensitivity.
  • As a single beam technique, FM spectroscopy is less sensitive to shock tube induced noise components (e.g., shock induced birefringence of shock tube windows, beem steering effects).


fm_principle

FM Principle

FM spectroscopy is an optical heterodyne technique capable of sensitive detection of absorption and dispersion of not too much broadened spectral features. It also provides an intrinsically fast time response. The light of a continuous narrow-bandwidth laser is phase (or frequency) modulated by an electrooptic modulator. Whereas the corresponding frequency spectrum of the single mode laser light before the modulation is given by the sole frequency of the single mode detection laser (carrier frequency), the modulated light exhibit additional side bands (see Figure). The individual bands have a spacing equal to the applied modulation frequency. For a purely phase (or frequency) modulated laser beam, the (intensity) beating of the side bands against the carrier frequency perfectly cancels out such that the detection of the laser beam intensity (using a fast photodiode) generates a time independent dc signal (no amplitude modulation!). However, when the modulated light passes through an absorbing sample such that the different light components are absorbed to a different extent, the exiting laser light next to its phase modulation also shows a little bit of amplitude modulation. Absorption (or dispersion) transforms phase modulation into amplitude modulation. Now, the generated photodiode signal, next to a dc offset, also shows a superimposed ac current. Demodulation of this ac component using standard demodulation techniques results in the FM signal, which is (among other things) proportional to the concentration of the detected species.


fm_setup

FM Setup



The left Figure depicts a schematic diagram of the FM spectrometer used for radical detection behind shock waves. Main features of the FM system are:
  • High modulation frequency of 1.0 GHz.
  • High modulation index up to M = 2.0 (resonant EOM).
  • High total optical power up to 15 mW laser power.
  • Accurate phase adjustment by combination of two-polarizer setup with phase shifter.



Quantitative Detection


iod_line

Quantitative concentration measurements require the knowledge of the FM factor Δf and the electronic gain G of the FM spectrometer.


IFM = 0.5 × I0 × Δf × σ c l × G


Here, IFM is the intensity of the demodulated FM signal, I0 is the total laser intensity, σ the absorption cross section, and l the absorption path length. Whereas the FM factor can be calculated based on the spectral lineshape of the detected absorption feature, the gain factor G has to be determined experimentally from

  • a FM signal of a species with known absorption cross section.
  • a simultaneously measured FM and absorption signal.
  • a scanning etalon setup.
  • the specifications of all electronic devices.

The Figure demonstrates the determination of the gain factor G by a direct comparison of absorption and FM data. The data were taken using a low pressure iodine cell.

 


Contributing researchers:  N. Faßheber, S. Hesse, G. Friedrichs and M. Colberg, J. Dammeier (former)


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Last Updated on Wednesday, 04 January 2017 10:47
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