Laser Doppler velocimetry

The laser Doppler anemometry (LDA ) is a non-contact optical measurement method for selective determination of velocity components in fluid flows (liquids or gases). Here, a laser beam is split by a beam splitter into two beams. At the measuring point, these beams cross again and it arises at the intersection of an interference fringe pattern. A particle that moves together with the fluid by the stripe pattern is generated in a photo- detector, a scattered light signal, the frequency of which is proportional to the velocity component perpendicular to the interference fringes. It is beating between the different Doppler-shifted scattered light of both laser beams. By combining three laser Doppler systems with different laser wavelengths all three flow velocity components can be recorded so selectively.

Operation

The laser -doppler technique is based on the determination of the Doppler shift of the scattered light of a moving object, which is illuminated with laser light. Since the frequency of light can not be measured directly, it is brought by superimposing a reference beam in the range of a few megahertz.

For the explanation of the signal generation, different conceptual models have been established. The interference fringes model for a two-beam arrangement is very graphic, but, strictly speaking, valid only for very small particles ( d << λ ). The slightly more complex Doppler model, from which the name laser - Doppler technique is derived, on the other hand describes the signal generation of comprehensive, includes the interference fringe model and explains the signal emergence of so-called single-beam or reference beam laser Doppler systems. Another model for Doppler equivalent approach is the description of stationary light scattering, such as Mie scattering.

Fringe model

Many laser Doppler systems in the field of flow measurement technology work with each other at an angle 2φ two intersecting coherent laser beams (see Structure of laser Doppler systems). In the crossover region, the two waves interfere with each other and there is an interference fringe system with ideally equidistant interference flat surfaces. Said interference surfaces are perpendicular to the plane defined by the laser plane and parallel to the bisector of the laser beams and have the distance

Moves a very small particles by this periodic interference fringe system, it scatters the local grid-like intensity distribution. The frequency of the detected scattered light signal is proportional to the velocity component perpendicular to the interference areas vx.

Doppler model

The direction-dependent Doppler shift of the scattered light of a particle in a laser beam can be represented on the vectorial description of the Doppler effect. In the case of laser Doppler system, the particle acts as a first moving receiver that detects the Doppler shifted frequency of a stationary transmitter, the frequency f L of the laser. The light scattered by the moving particles will be captured by a shaft fixed detector. Thus, the Doppler effect has to be applied for a second time and the result is the frequency of the scattered light

With the unit vector in the direction of the laser beam axis and the unit vector from the particles to the stationary receiver. For particle velocities V much smaller than the light velocity c, the following approximation can be indicated:

The observed Doppler shift is thus dependent on both the velocity vector, the orientation of the laser beam axis and the direction of observation. The actual frequency shift due to the Doppler effect can not be measured directly ( for direct measurement of the Doppler shift Doppler velocimetry see Global ) due to the inertia of optical detectors. Therefore, the scattered light of the frequency f is superposed with a reference wave frequency fR and similar optically mixed at the detector. The result of this mixture produces a detection signal at the difference frequency of the two shafts.

The laser Doppler systems now differ according to the direction of observation and reference frequency for the optical mixture:

• If the scattered light is detected in backscattering ( scattering angle 180 ° ) () and superimposed on the original incident wave (), the result for the measurable signal frequency

• For the determination of flow rates in biological and medical applications, the receiving optical system is often disposed at a defined angle to the vertical laser beam irradiated (). The reference wave to the mixture becomes generated by the scattering of the surrounding stationary tissue. Bearing in mind that for these applications, the velocity vector having only a component vx, e.g., blood flows in parallel to the skin surface, it can be determined by means of the laser Doppler technique. Problems occur with additional velocity components.
• Similarly, the scattered light can be detected by a moving particle at a fixed angle, and are mixed with the original laser frequency in the receiving optics. One speaks in this case of a single-beam laser Doppler system. The first realized laser Doppler systems were constructed in this manner.

• In the flow measurement technique, however, has caught the two-beam laser Doppler system. In this case, two laser beams are focused at an angle 2φ to a point on the moving structure, or in a fluid. The scattered waves of the two laser beams are Doppler-shifted differently, the position vector from the center of scattering to a stationary detector, however, is for both the scattered light signals are equal

A typical volume measurement has a length in the laser Doppler anemometry of one millimeter and a diameter of some tenths of a millimeter.

Since the flow direction of the particles is not obvious that a stationary fringe pattern, the individual laser beams are frequency-shifted by means of an opto-acoustic modulator, and thus induces a movement in the interference fringe pattern, so that the determination of the speed of direction is ambiguous. In simplified terms, there are those referred to as Bragg cell acousto -optic modulators of a crystal through which the laser beam is performed. By means of piezoelectric excitation of the crystal in the ultrasonic range of 25 MHz to 120 MHz are density differences, ie refractive index fluctuations induced on which bends the incident in the Bragg cell laser beam at the Bragg angle. The frequency of the laser, and the acoustic waves are added to an overall frequency of the outgoing laser beam, i.e., when leaving the Bragg cell, the frequency of the laser beam is shifted by the amount of the Bragg cell frequency. If only one of the laser beams has undergone a frequency shift, the fringe pattern moves in the measurement volume having the frequency of the Bragg cell used. The bursts of single particles flowing through the measuring volume to be received by a photodetector. For evaluating the signal will band-pass filtered, so that it is present symmetrically relative to a zero line, in this case the DC component of the original burst signal LDA is eliminated. An A / D converter, a processor receives the measurement signal. By evaluating the burst signals ( Counter ) and the signal time finally the frequency f Alternatively, the measuring signal with appropriate equipment (eg a transient recorder ) is replaced by the counter method can be a Fourier transform directly subjected.

The photodetector can be built in forward scattering arrangement or in the backward scattering arrangement. When the photodetector is installed in the forward scattering direction, the light scattered from the particle signal of a receiving optics in the propagation direction of the light is absorbed. In backscatter arrangement of the detector in the opposite direction of propagation of the two laser beams is arranged. In the backward scattering arrangement, the transmission optics can be designed so that it takes the same time as the receiving optical system, so that a complex adjustment between the transmitting and receiving unit is eliminated. However, the intensity of the scattered signal is an order of magnitude smaller than in the forward scattering arrangement ( Mie scattering ) in this arrangement, so that the backward scattering was only made possible by the development of powerful lasers and photodetectors.

For scientific and particularly commercial use, that is, for frequently changing measuring insert to LDA systems have prevailed in so-called back-scattering arrangement, as they are more flexible and easier to adapt. Nowadays almost exclusively LDA systems with probes containing a backscatter optics application.