Coherent laser radar for vibrometry: robust design and adaptive signal processing

Abstract

A coherent laser radar system based on semiconductor laser technology has been designed and built. The compact design and the absence of adjustments makes the system mechanically robust and easy to use. The present system has an output power of 50 mW and a line width of 280 kHz (HWHM). The laser radar system has been used in vibrometry measurements. For vibrometry of moving objects, adaptive signal processing is required in order to obtain the vibration signature. Especially for unresolved objects, interference between di erent vibrating parts will complicate the analysis. Modelbased estimation techniques are used to obtain the parameters which determine the dynamics of the re ecting object. Key words: coherent laser radar, vibrometry, modelbased signal processing 2 INTRODUCTION Coherent laser radar (CLR) systems have been investigated over several decades primarily for military applications. In order to receive a more general industrial acceptance, compact, robust, cost-e ective systems and more general signal processing tools must be developed. Semiconductor laser technology can achieve both high power and narrow line width without using external cavity designs. This allows the design of very compact systems without any m o vable parts such as pzt-elements. A critical feature is the tolerance designing, allowing for laser welding of the interferometric block. The design is also compatible with distributed ber sensor systems. The compact design and the absence of adjustments makes the system mechanically robust and easy to use. Semiconductor lasers can easily be broadband frequency modulated, and various applications can be addressed by making use of this property. For range/Doppler measurements, chirping or frequency shift keying can be applied. If necessary, a frequency shifted signal can, be obtained by using ,e.g., an acousto-optic modulator. CLR

FM demodulation in the presence of multiplicative and additive noise

“…Finally, we should emphasize that the analysis reported here is based on well-established models for speckle and additive noise although with a somewhat idealized model for the demodulator. This is in line with the approach which has been adopted by previous authors and which has been shown to be consistent with experimental data [6][7][8]. We are currently undertaking experiments to test the validity of our results in the low carrier-to-noise ratio regimes which have not been analysed thoroughly before.…”

“…The principles of laser vibration sensing have been described in detail elsewhere [6][7][8]. The optical set-up is illustrated schematically in figure 1.…”

Efficient Frequency Estimation from a Particular Almost Periodic Function with Application to Laser Vibrometry

“…We aim to compare the performance of our algorithm with that of the microdoppler developed in Renhorn et al (1995) and classically used in laser vibrometry to analyse the vibrations of an object with a translation motion. Further details concerning its implementation can be found in Le´vy-Leduc (2004).…”

Semiparametric estimation of the frequency of unknown periodic functions and its application to laser vibrometry signals

“…We will see in this section that the penalized cumulated periodogram provides an accurate estimation of at SNR much smaller than those required by the microdoppler approach, which is a classical method used in laser vibrometry for specific signals like (12) (more details on the microdoppler method can be found in [12] and [18]). This is an important point in practice, because it means that the vibration analysis can be achieved even if the vibrating object is far from the laser.…”