High Frequency Laser Based Acoustic Microscopy Using a Cw Generation Source

A laser ultrasonic system has been developed which uses a modulated CW laser source for ultrasonic wave generation. The majority of high frequency laser ultrasonic systems use picosecond pulsed laser sources to generate ultrasound in the hundreds of MHz to the low GHz range. High frequency SAW generation and high-resolution acoustic microscopy require that the generation laser spot be tightly focused on the sample surface. This places a severe restriction on the amount of energy that can be deposited in the sample without causing surface ablation and ultimately limits the sensitivity of the system. In this work, we explore the use of an amplified, electro-absorption modulated diode laser source for high frequency ultrasound generation. The laser can be amplitude modulated up to 2 GHz and the peak power is approximately 2 Watts. While LBU systems using modulated CW generation can have extremely high SNR when lock-in detection systems are employed, they have disadvantages when compared to pulsed systems in terms of signal interpretation when multiple acoustic modes are present or when signals are reflected from the boundaries of the target material leading to complex acoustic interference patterns. In order to avoid these difficulties the source frequency is scanned over the bandwidth of interest and the transient response of the specimen reconstructed from the frequency domain data. Experimental results are presented for data obtained on thin films and plates in the 0.1KHz200MHz range. The acoustic waves are detected using a stabilized Michelson interferometer fed into an RF lock-in amplifier. The reduced bandwidth of the system afforded by lock-in detection allows for a substantial improvement in signal-to-noise ratio over systems using pulsed laser generation for the same surface temperature rise. The experimental results are compared to theory and a detailed analysis of the SNR of the system given. Introduction: Conventional laser ultrasonic techniques typically use pulsed laser sources for the generation of acoustic waves [1-3]. This is due in part to the fact that modulated laser sources with enough power to generate acoustic waves with sufficient amplitude for detection with an optical probe have not been readily available. Recent developments in laser technology, geared primarily for the telecom sector, have resulted in electro-absorption modulated DFB diode laser sources that can be amplitude modulated at frequencies approaching 40GHz. In addition, erbium doped fiber amplifiers are available to amplify the output of these lasers. It is noted that these components are relatively inexpensive, especially when compared to femtosecond and picosecond solid-state laser systems. High power laser sources that can be modulated at GHz frequencies provide an attractive alternative to pulsed laser sources for laser based acoustic microscopy. 0 40 80 120 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 pulsed response