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The basic physical principle of a laser (light amplification by stimulated emission of radiation) is the induced emission of photons. With induced emission the emitted photons have identical properties and thus produce coherent light of the same wavelength. A laser consists of an optical cavity which contains the lasing material with a mirror placed at each end. The light, which is repeatedly reflected between the two mirrors, is amplified. As one of the mirrors is only partially reflecting, a small laser beam emanates from the cavity. To keep the process going, energy is supplied in order to excite the atoms in the lasing material. 2 coherence length The wave along the laser cavity is a standing wave and the cavity of length L only resonates when there is an integral number n of half wavelengths l between the mirrors: n = 2L / l The frequency W is given by W = nc/ 2L where c is the velocity of the light. The separation of the multiple wavelengths (longitudinal modes) is DW = c / 2L. However, not all possible modes in a laser cavity will be excited. Only those that are within the gain profile of the lasing medium will generate an emission. The maximum distance over which the phase relationship will exist is called coherence length. The relation between the coherence length and the bandwidth is c/Dl The laser used for laser Doppler vibrometers is a helium neon (He-Ne) laser. This laser produces a visible red beam (l = 0.6328 µm). This gas laser is an extremely low-noise light source and therefore ideally suited for this application. Such a laser can be stabilized so that only a single mode is excited. The line width is then a couple of MHz which yields coherence lengths of about 200 to 300m. Laser vibrometers are usually operated with multimode lasers where the lasers oscillate at 2-3 modes at the same time. The coherence length is only 10-20 cm because of the interference of the beat frequencies of these different modes. However, this beat frequency generates a cos2 dependence of the visibility as the distance to the object is varied. The signal has a maxima at distances of 2mL and minima at (2m-1)L where m is an integer. It is therefore possible to make measurements at very long distances with such a laser. The measuring distance should be adjusted to a visibility peak for maximum signal strength. 3 A laser Doppler vibrometer is based on the principle of the detection of the Doppler shift of coherent laser light that is scattered from a small area of a test object. The object scatters or reflects light from the laser beam, and the Doppler frequency shift is used to measure the component of velocity which lies along the axis of the laser beam. As the laser light has a very high frequency W (approx. 4.74 x1014 Hz), a direct demodulation of the light is not possible. An optical interferometer is therefore used to mix the scattered light coherently with a reference beam. The photo-detector measures the intensity of the mixed light whose beat frequency is equal to the difference frequency between the reference and the measurement beam. Heterodyne and Homodyne detection In homodyne detection, for a given relative phase shift the output is a constant (DC) signal level. This level is indirectly related to the phase shift. In heterodyne detection one modulates, usually by a frequency shift, one of two beams prior to detection. Optical heterodyne detection detects the interference at the beat frequency. The AC signal now oscillates between the minimum and maximum levels every cycle of the beat frequency. Since the modulation is known, the relative phase of the measured beat frequency can be measured very precisely even if the intensity levels of the beams are (slowly) drifting. This phase is identical in value to the phase one measures in the homodyne case. There are many additional benefits of Optical heterodyne detection including improved signal to noise when one of the beams is weak. 4 The test beam is directed to the target, and scattered light from the target is collected and interfered with the reference beam on a photodetector, typically a photodiode. Most commercial vibrometers work in a heterodyne regime by adding a known frequency shift (typically 30-40 MHz) to one of the beams. This frequency shift is usually generated by a Bragg Cell, or Acousto-Optic Modulator. 5 A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. One difference from Bragg diffraction is that the light is scattering from moving planes. A consequence of this is the frequency of the diffracted beam f in order m will be Dopplershifted by an amount equal to the frequency of the sound wave F. Photovoltaic mode a solar cell is just an array of large area photodiodes. Photoconductive mode In this mode the diode is often reverse biased. This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times. 6 In radio and signal processing, heterodyning is the generation of new frequencies by mixing, or multiplying, two oscillating waveforms. It is useful for modulation and demodulation of signals, or placing information of interest into a useful frequency range. This operation may be accomplished by a vacuum tube, transistor, or other signal processing device. Mixing two frequencies creates two new frequencies, according to the properties of the sine function: one at the sum of the two frequencies mixed, and the other at their difference. Typically only one of these frequencies is desired—the higher one after modulation and the lower one after demodulation. The other signal is either not passed by the tuned circuitry that follows, or may be filtered out. Optical heterodyne detection Since optical frequencies are far beyond any feasible electronic circuit bandwidth, all photon detectors are inherently energy detectors not oscillating electric field detectors. However since energy detection is inherently "square-law" detection, it intrinsically mixes any optical frequencies present on the detector. Thus sensitive detection of specific optical frequencies is possible by Optical heterodyne detection when two different (closeby) wavelengths of light illuminate the detector so that the oscillating electrical output corresponds to their difference frequency. This allows extremely narrow band detection (much narrower band than any possible color filter can achieve) as well as precision measurements of phase and frequency of a signal light relative to a reference light source. The heterodyne detection of the vibrometer signal The scattered or reflected light has a frequency equal to fo + fb + fd. This scattered light is combined with the reference beam at the photo-detector. The initial frequency fo of the laser is very high (> 1014 Hz), which is higher than the response of the detector. The resulted beat frequency between the two beams, which is at fb + fd (typically in the tens of MHz range). The output of the photodetector is a standard frequency modulated (FM) signal, with the Bragg Cell frequency as the carrier frequency, and the Doppler shift as the modulation frequency. This signal can be demodulated by demodulator in the instrument controller to derive the velocity vs. time of the vibrating target. 7 •An excitation (shaker, loudspeaker, hammer etc.) cause the object under investigation to vibrate. •The heterodyne detection signal is demodulated by the decoder in the controller. An output of which is proportional to the velocity of the vibration parallel to the measurement beam is achieved at the vibrometer channel. The excitation type •The voltage data on the vibrometer channel is sampled by DAQ system in the industrial PC. Final data is presented by PSV software system. •Periodic i.e. with a repeating signal (sinsusiodal, periodic chirp, periodic random, etc) •Transient i.e. with a pulse (e.g. rectangular pulse, hammer blow, etc.) •Stochastic (random) i.e. with noise (noise generator, self excited) 8 Specular surfaces, i.e. highly reflecting surfaces, obey the law: angle of incidence = angle of reflection. When making measurements from such surfaces, the optics of the LDV need to be aligned so that the reflected light returns within the aperture of the collecting optics. Diffuse surfaces scatter the incident light over a large angular area. The intensity of the scattered light power per unit solid angle follows Lambert's cosine law. It can vary greatly between shiny surfaces and dull black surfaces that absorb most of the light. Speckle patterns are always produced when a coherent light source is focused onto a rough surface. It is caused by interference effects between the beams originating at the different scattering centers on the surface. If the focused spot is very small, the number of scattering centers is small and the angular dependence of the path length differences in a given direction is also small. This leads to a large angle over which the interference condition is reasonably constant and thus a large solid angle for the speckle. 9 •Aerospace - LDVs are being used as tools in non-destructive inspection of aircraft components. •Acoustic - LDVs are standard tools for speaker design, and have also been used to diagnose the performance of musical instruments. •Automotive - LDVs have been used extensively in many automotive applications, such as structural dynamics, brake diagnostics, and quantification of Noise, Vibration, and_Harshness (NVH). •Biological - LDVs have been used for diverse applications such as eardrum diagnostics and insect communication. •Calibration - Since LDVs measure motion that can be calibrated directed to the wavelength of the light, there are frequently used to calibrate other types of transducers. •Hard Disk Drive Diagnostics - LDVs have been used extensively in the analysis of hard disk drives, specifically in the area of head positioning. 10 Polytec offers a comprehensive line of scanning laser Doppler vibrometers (SLDV). Scanning offers all the advantages of a laser vibrometer together with speed, ease of use, laser positioning accuracy and comprehensive data processing in a single automated, turnkey package. Users get a very quick, easily understood and accurate visualization of a structure's vibrational characteristics without the inconvenience of attaching and interpreting data from an array of transducers. 11 Working distance > 0.4 m (shorter distances accessible by using close-up unit) Laser wavelength 633 nm, visible beam Laser protection class Class II He-Ne laser, < 1 mW, eye-safe Sample size Several mm² up to m² range Scan grid Multiple grid densities and coordinate systems (polar, cartesian and hexagonal) each with up to 512 x 512 points 12 13 14 1. 2. 3. Position the PSV The PSV system measures velocity component parallel to the laser beam. Position scanning head and object such that: The laser beam can cover the scanning area The longitudinal axis of the scanning head is positioned perpendicular to the area to be scanned Optimize the signal level ◦ Observe the signal level indicator at the back side of the scanning head. The higher the value, the better signal-to-noise ratio will be. ◦ The surface of the area can be treated. Highly reflective and transparent surfaced usually do not scatter enought laser light back to the scanning head. Evenly matt surfaces are ideal. Water-slouble white wall paint can be used for the surface treatment. 15 Enable laser and beam shutter Adjust the camera view (Autofocus) Laser autofocus (Signal Intensity) Alignment (Scanning mirror-object surface) Defined scanning area Select the scanning parameter (excitation, decoder, frequency, etc.) Perform the scan Post processing the data 16 Alignment 17 Define scanning patterns 18 Scanning 19 Post processing 20 http://en.wikipedia.org/wiki/Vibrometer Polytec Manual 21 Thanks, 22