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

Abstract

Modern broadband telecommunications require microelectromechanical filters which mechanically vibrate at ultra-high frequencies up to several gigahertz. Heterodyne interferometers, so-called laser-Doppler vibrometers (LDVs), provide a sensitive and contactless measurement technique for vibrations in such filters, but are limited in GHz heterodyning by the efficiency drop of acousto-optic frequency shifters. Heterodyning by frequency-offset locking of two lasers in an optoelectronic phase-locked loop (OPLL) overcomes this limitation. This is demonstrated with our LDV setup with heterodyning up to 1.4 G H z via offset locking of two semiconductor lasers at visible wavelength. The experiments show a vibration-amplitude resolution of less than 1 p m per H z for frequencies higher than 50 M H z up to 700 M H z . The bandwidth is only limited by our photodetectors. This amplitude resolution already qualifies our LDV for vibration measurement of microelectromechanical filters at ultra-high frequencies. We present a comprehensive model for the vibration-amplitude resolution of a LDV with this technique including the laser linewidths, the OPLL transfer function, and interferometer delays. The experiments with our LDV validate the model predictions from numerical simulations. Finally, we discuss the collapse of the heterodyne carrier at vanishing mutual coherence due to interferometer delays, the transition to shot-noise-limited detection, and provide design recommendations.

Lateral resolution limit of laser Doppler vibrometer microscopes for the measurement of surface acoustic waves

“…For the experimental validation, we employed our heterodyne LDV, which we describe in detail in a further publication 8 . This LDV achieves high-frequency measurement capability via heterodyning with two frequency-offset-locked semiconductor lasers at and broadband photodetectors.…”

“…Further, with heterodyning, an (angular) frequency offset is introduced which is denoted as (heterodyne) carrier frequency. Other (differential) phase fluctuations of the laser sources are neglected here (find a more extensive discussion in our further publication 8 ).…”

“…One challenge for this instrumentation is to achieve heterodyning techniques capable of measuring vibrations unambiguously at several gigahertz. We, therefore, have proposed heterodyning via two semiconductor lasers which are frequency-offset locked by an optical phase-locked loop 8 . This allows to achieve sub-picometer amplitude resolution for vibrations at several GHz only limited by (half of) the bandwidth of commercially available photodetectors with a flat frequency response.…”

Compressional-Wave Effects in the Operation of a Quartz Crystal Microbalance in Liquids:Dependence on Overtone Order

“…Applying a heterodyne detection scheme, the motion of the surface at the laser spot can be reconstructed. Frequencies may range up to a few gigahertz [33]. Most instruments resolve amplitudes down to the femtometer regime (at 1 Hz bandwidth), which is much below the amplitudes under study here.…”

Bestimmung des Anregungsspektrums für kontaktlose breitbandige Messungen der Übertragungsfunktion mit Plasmaanregung und Laser-Doppler-Vibrometrie

“…breitbandige Schwingungsmessungen benötigt, um die Sensitivität bei der Schadenserkennung zu verbessern, weil Schädigungen an Bauteilen bei Resonanzfrequenzen im höheren Frequenzbereich eine stärkere Veränderung erwirken. Laser-Doppler-Vibrometer können problemlos Schwingungen mit so großen Schwingungsbandbreiten messen (Bandbreiten bis in den Gigahertz-Bereich sind möglich [9]). Allerdings muss für eine breitbandige FRF-Messung bis z.B.…”