Superheterodyne Detection of High Frequency Acoustic Waves

A laser ultrasonic system that allows for the excitation and detection of high frequency (GHz range) acoustic waves using low frequency, low cost detection electronics is presented. A continuous wave diode laser source, operating at a wavelength of 1544nm, is amplitude modulated using an integrated electro-absorption modulator and used to excite high frequency, narrow bandwidth acoustic waves. A path-stabilized Michelson interferometer, using a second electro-absorption modulated diode laser operating at 1550nm, is used for detection. Both lasers can be amplitude modulated over a bandwidth of approximately 10 GHz using low power signal generators. In the superheterodyne approach, the detection laser serves as the local oscillator and is modulated at a frequency offset from the excitation laser modulation frequency by a fixed amount. The amplitude and phase information of the acoustic signals are measured at the intermediate frequency using a low-frequency photodetector and lock-in amplifier. The modulation frequencies of the two lasers can be scanned over a broad frequency range while the signals are always measured at the fixed (low) intermediate frequency, obviating the need for high frequency detection electronics. The sensitivity and limitations of the technique are discussed, and results are presented demonstrating surface wave dispersion measurements in aluminum films on a silicon substrate over a 1GHz bandwidth.