Optical and Spectroscopic Techniques

Cavity-based absorption spectroscopy techniques

“…There are also 'indirectly measured' forms of absorption spectroscopy in which the excitation arising from optical absorption is detected after conversion to some other form of signal (Sigrist, 1994a(Sigrist, , 2011Demtr ö der, 2003;Svanberg, 2004;Lackner, 2008;Ashworth, 2009;Svanberg and Demtr ö der, 2012). For instance, there are many sensitive indirect techniques that are applicable in the context of absorption by gas-phase molecules.…”

“…The quality of this interaction in the optical cavity can be expressed in terms of its fi nesse F , which is defi ned as the ratio of the cavity's free spectral range (FSR, Δν FS ν ν R -the frequency interval between its periodic longitudinal modes) to the FWHM (full-width-at-half-maximum) transmission bandwidth Δν 1 2 ν ν of the cavity; its Q -factor (or resolution), which is the ratio (ν ν ν Δ 1 2 ν ν ), measures the 'sharpness' of a transmission peak at an optical frequency ν . For a linear optical cavity with mean refl ectivity R = ( R 1 R 2 ) 1/2 , in terms of the refl ectivities R 1 and R 2 of its two mirrors, the fi nesse F is π R 1/2 /(1 − R ); this reduces simply to π (1 − R ) − 1 for the very highly refl ective mirrors with R ≈ 1.000 that are commonly used in cavity-based spectroscopy (Busch et al , 1999a(Busch et al , , 1999bDemtr ö der, 2003;Svanberg, 2004;Svanberg and Demtr ö der, 2012). For such a cavity with inter-mirror distance d cavity and intracavity speed of light c , the round-trip time is (2 d cavity / c ).…”

“…Over the last 50 years, there have been steady, substantial developments in highly sensitive forms of laser absorption spectroscopy (Demtr ö der, 2003;Svanberg, 2004;Sigrist, 2011;Svanberg and Demtr ö der, 2012), many without the need for cavity-enhancement (other than via the quality of light that a laser cavity produces). Such aspects comprise a highly relevant background to our primary theme of cavity-based absorption spectroscopy.…”

“…Likewise, luminescence from electronically excited molecules results in laser-induced fl uorescence (LIF) spectroscopy (Naik et al , 2008), with particular applications to combustion diagnostics (Lucht, 1987;Wolfrum et al ., 2000;Wolfrum, 2001;Kaminski, 2005;Schulz, 2005;Hult, 2008). Other examples of indirectly measured optical absorption spectroscopy for gas-phase molecules (Demtr ö der, 2003;Svanberg, 2004;Svanberg and Demtr ö der, 2012) include: optothermal spectroscopy, detected by lensing or deposition of energy on a bolometer; resonance-enhanced multiphoton ionisation (REMPI); laser-induced photofragmentation or photoionisation, detected by mass spectrometry or optogalvanically or in molecular beams. Mention should also be made of various forms of non-linear-optical spectroscopy, such as two-photon absorption, resonant four-wave mixing and coherent Raman spectroscopies and an assortment of laser-excited double-resonance techniques.…”

Analysis of Real World Sensor Behavior for Rising Fidelity of Physically Based Lidar Sensor Models

“…The transmitted infrared light gets reflected on surfaces and the portion that reverts to the sensor's photo diode is measured. Using the time of flight principle, the distance is calculated with r = ct 2 , where c is the speed of light and t is the measured time of flight [16]. More precisely, t refers to the time interval between emitting of the light pulse and receiving a signal with a signal strength upon a predefined threshold.…”