Heterodynes Laser-Doppler-Vibrometer mit gekoppelten Diodenlasern zur Schwingungsanalyse von SAW-Filtern / Heterodyne laser-Doppler vibrometer with frequency-offset-locked diode lasers for vibration analysis of SAW filters

Abstract

ZusammenfassungHeterodyne Interferometer messen Schwingungsamplituden bei Frequenzen > 100MHz mit definierter Messunsicherheit im Gegensatz zu homodynen Verfahren. Die geringe Effizienz von Bragg-Zellen als Frequenzschieber bei Frequenzen > 409MHz limitieren den Einsatz von heterodynen Interferometern im Gigahertz- Bereich. In diesem Beitrag stellen wir ein heterodynes Laser-Doppler-Vibrometer-Mikroskop vor, in dem eine optoelektronische Phasenregelschleife zwei Diodenlaser auf eine Frequenzdifferenz regelt und die Trägerfrequenz erzeugt. Unser Messsystem bietet eine Auflösung von Schwingungsamplituden im Pikometer-Bereich bei Schwingungsfrequenzen von 10MHz bis 700MHz. Die Messfähigkeit der Betragsamplitude und Phase der Schwingung wird am Messbeispiel eines Oberflächenwellen-Filters demonstriert.

Heterodyne interferometry at ultra-high frequencies with frequency-offset-locked semiconductor lasers

“…Commercial balanced photodetectors with two photosensitive elements are restricted to even smaller bandwidths. In previous experiments, we showed the capability of unambiguous vibration reconstruction in amplitude and phase exemplary with vibration measurements on a SAW filter excited at 34.01 MHz [34]. Intensity and phase noise of the semiconductor lasers limit the vibration amplitude resolution (figure 4(a)).…”

“…Our LDV with offset-locked visible semiconductor lasers is able to generate a heterodyne carrier up to frequencies ≤ 1.4 GHz and measure vibration frequencies < 700 MHz [26,34]. Both is mainly limited by the bandwidth of commercial photodetectors with a single photosensitive element.…”

Signal Diversity for Laser-Doppler Vibrometers with Raw-Signal Combination

“…According to Dräbenstedt's publication, the signal reliability can be improved by combining multiple demodulated velocity signals. Using Equation (1), the combined signal S is obtained using any number n of individual velocity signals X j S = n ∑ j=1 w j X j (1) with the weighting factors w j calculated in Equation ( 2) [21] by the CNR (Carrier to Noise Ratio)…”

“…With the advancement of laser-Doppler vibrometers (LDVs), various additional applications are continuously made available in which contactless vibration measurement is possible [1][2][3]. These applications are difficult to implement with conventional methods of vibration measurement, impossible in the case of rotating or hot parts [4], or unwanted in medical applications [5].…”