FIELD OF THE INVENTION

The present invention relates to a method and device of measuring the velocity of a target by laser self-mixing, wherein an output beam of a single laser source is split into two sensing beams which are directed to a surface of a moving target, and the velocity of the target is determined by measuring an intensity variation of the output beam and by determining from this intensity variation Doppler frequencies of the laser radiation of the two sensing beams back scattered from the target.

BACKGROUND OF THE INVENTION The technique of laser self-mixing can be used for sensing applications in order to determine position and/or velocity of an object. Laser self-mixing is based on the effect that laser light back scattered from an object and reentering the laser resonator influences the laser radiation emitted by said laser resulting in a modulation of the laser intensity. This modulation is measured by an appropriate photodetector. The operation principle of laser self-mixing is explained, for example, in G. Giuliani et al, "Laser Diode Self-mixing Technique for Sensing Applications", Journal of Pure and Applied Optics 6 (2002), pages 283-294.

Conventional laser Doppler velocimetry often requires expensive devices which consist of complicated optics and detectors. US 2006/0072102 Al describes a data input device and method for measuring movement of a tracking surface by detecting laser Doppler self-mixing effects of a frequency modulated laser beam. In this document a single laser is used to measure the movement of a tracking target surface. To this end, the output beam of the laser is actively frequency modulated and split into two sensing beams which are directed on different paths to the surface of the moving target. The two beam paths are selected to have different path lengths in order to be able to later on separate the influences of the back scattered light of the two sensing beams on the output beam. From the measured intensity variation of the output beam the Doppler frequencies of the two sensing beams are determined in order to obtain from these Doppler frequencies the relative movement of the tracking surface of the target. With the two sensing beams a longitudinal movement of the target is measured, i.e. a movement substantially perpendicular to the mean axis of the two sensing beams, denoted as velocity components V _x and V _y . In order to also measure the velocity component V _z in the direction of this mean axis, the so called vertical velocity, the document proposes to use a third sensing beam. An advantage of such a device is that a vertical cavity surface emitting laser (VCSEL) with integrated photodetector can be used as laser source. This provides a good measurement accuracy at very low cost level. Compared with multiple laser self-mixing Doppler velocimetry, only a single laser is required. This reduces the system complexity and costs further. Errors inherent to multiple laser source sensors owing to variation of laser characteristics are eliminated, leading to higher measurement accuracy.

Such a laser device however still requires an active modulation of the output laser beam as well as three sensing beams for measuring longitudinal and vertical velocities of a target.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and device for measuring the longitudinal and vertical velocity of a target by laser self-mixing, said method and device requiring a simpler design and simpler processing of the measurement signals. The object is achieved with the method and device according to claims 1 and 6. Advantageous embodiments of the method and device are subject matter of the dependent claims or are described in the subsequent portions of the specification.

In the proposed method of measuring the velocity of a target by laser self- mixing, an output beam of a single laser source is split into two sensing beams. Said sensing beams are directed on different beam paths to a surface of a moving target, wherein said different beam paths are selected to be symmetrical to one another. Symmetry means that both beam paths are of equal length and symmetrical with respect to a common symmetry axis which preferably is the optical axis of the laser source. Part of the laser light of the two sensing beams is backscattered from the target into the resonator of the laser source, experiencing a Doppler shift due to the movement of the target. The back scattered radiation effects a modulation of the laser radiation in the laser source which is measured as an intensity variation by an appropriate photodetector. The Doppler frequencies of the back scattered radiation of the two sensing beams appear as peaks in the frequency spectrum of the measured intensity variation. Based on the symmetric beam path geometry, at least three Doppler peaks - the two Doppler frequencies and the differential Doppler frequency - are present in the power spectrum of the undulated laser power. The vertical velocity of the moving target is then determined from a difference in the Doppler frequencies between the two sensing beams and the longitudinal velocity is determined from a sum of the Doppler frequencies of the two sensing beams which is equivalent to the peak of the differential Doppler frequency.

The proposed device for measuring the velocity of a target by laser self- mixing comprises a single mode laser source with integrated photodetector, wherein said photodetector is arranged to measure the intensity variation of laser radiation emitted by the laser. The device further comprises a beam splitting unit arranged and designed to split an output laser beam into two sensing beams and to direct said sensing beams on different beam paths to a target region, said different beam paths being symmetrical to one another. An evaluation unit is designed to determine from the measured intensity variation Doppler frequencies of laser radiation of the two sensing beams backscattered from the target region, to determine a vertical velocity of a target moving in said target region from a difference in the Doppler frequencies between the two sensing beams and to determine a longitudinal velocity of the target from a sum of the Doppler frequencies of the two sensing beams.

The proposed method and device allow the measurement of longitudinal and vertical velocities of a target with only two sensing beams with a single laser source. The method and device do not require any active modulation of the laser power for the measurement. Due to the simple evaluation of the sum and difference of the two Doppler frequencies, only a simple processing of the measurement signal is required. This simple design and processing is possible due to the selection of the two sensing beams symmetrical to one another. Due to this symmetry the sum and difference of the Doppler frequencies provide the necessary information to determine the longitudinal and vertical velocity, wherein the differential Doppler frequency is independent of the vertical movement or vibrations and the difference of the Doppler frequencies is independent of longitudinal movement. The proposed method and device can be used for many sensing applications in which the movement of a target is to be measured, in particular for measuring the longitudinal velocity and vertical vibrations of an object passing the device. The method and device can be applied to virtually any reflective moving surface. It can be used as a Doppler anemometer to measure gas (at presence of scattering seeding particles) or liquid flow rate.

In a preferred embodiment, a single longitudinal mode VCSEL laser is used as the laser source. Preferably, this laser source provides already an integrated photodiode for measuring the intensity variations. Such types of VCSEL lasers with integrated photodiode are already known in the art. With such an embodiment, a very compact and low cost device is provided.

The two sensing beams are preferably focused to the surface of the moving target. Furthermore, the symmetry axis of the two sensing beams is preferably perpendicular to the longitudinal movement or the target and the focused spots of the two split sensing beams are preferably overlapped on the target surface. This geometry can be readily achieved by checking whether the Doppler peaks of the two sensing beams overlap in the frequency domain in case of no vertical movement of the target. A tilt of the optical axis within a few degrees does not introduce significant error. Due to the measurement principle cross talk between the two sensing beams in case of overlapping spots does not produce unwanted sidebands on the measurement signals due to the symmetric optical path. On the other hand, the overlap of the focus spots on the target surface allow a quick optical alignment procedure of the device.

The beam splitting unit of the device may comprise a central beam stop, separating the output beam of the laser into two sensing beams. Furthermore, also a prism, a grating or a free form lens may be used for splitting the output beam into two sensing beams. Also other splitting components like appropriate reflectors are possible. Since the differential Doppler frequency fb is close to the second harmonic of the Doppler frequencies fϊ or f ₂ of the two sensing beams, an optical attenuator or filter can be included in the beam path to avoid excessive backscattering from the target.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described herein after.

BRIEF DESCRIPTION OF THE DRAWINGS

The following exemplary embodiments show examples of the proposed device and method with reference to the accompanying figures without limiting the scope of protection as defined by the claims. The figures show

Fig. 1 a schematic view of a first embodiment of a device according to the present invention; Fig. 2 a schematic view of a second embodiment of a device according to the present invention;

Fig. 3 a schematic view showing the symmetric beam paths of the two sensing beams when impinging on the target surface; and

Fig. 4 frequency spectra of the measurement signal of the proposed method and device in two cases of target movement.

DETAILED DESCRIPTION OF EMBODIMENTS Fig. 1 and 2 schematically show two embodiments of the proposed device for self-mixing differential Doppler velocimetry. The output beam 2 of a single mode VCSEL laser 1 is first collimated with a lens 3 and then split into two sensing beams 4, 5 by an appropriate beam splitting unit 6. The beam splitting unit 6 is designed to split up the laser output beam 2 into the two sensing beams 4, 5 such that these sensing beams 4, 5 propagate through symmetrical beam paths to the target region. The target region in figs. 1 and 2 is situated on bottom of the figures and not explicitly indicated. In fig. 1, the splitting unit 6 is formed by a lens 7 with a central beam stop 8. Fig. 2 shows a splitting unit 6 formed by a isosceles prism 9, wherein the output beam 2 enters through the base of this prism 9. The beam paths of the two sensing beams 4, 5 are symmetrically located relative to the central optical axis 11 of the VCSEL laser 1.

Fig. 3 shows the corresponding situation at the surface of the moving target 10. The incident angle of sensing beam 4 and sensing beam 5 relative to the central optical axis 11 is denoted as θ. V _x and V _z correspond to the longitudinal and vertical velocity of the moving target 10, respectively.

Both sensing beams 4, 5 are focused to the surface of the moving target 10. The focuses of the sensing beams are preferably overlapped on the surface of the 5 target 10. This allows a quick and accurate alignment of the corresponding optics. Based on the beam path symmetry, crossed talk between the two sensing beams does not lead to significant sidebands in the frequency domain.

As is obvious from fig. 3, according to the symmetric sensing beam arrangement, the Doppler frequency fϊ of sensing beam 4 is equal to the Doppler 10 frequency ^ of sensing beam 5, if no vertical movement of the target occurs.

2F sinθ f i = f ₂ = λ

In contrast to conventional single beam self-mixing laser velocimetry, the interference between the two sensing beams 4, 5 leads to an additional differential Doppler frequency component f _D .

_{I5 fD = fl + f2 = -} l^Ξi λ

Assuming a wavelength of the VCSEL of λ = 860 nm, a velocity of V = 10m/s of the target and a prism of flint glas (n = 1.6) with an apex angle of 10° for splitting the laser beam (θ ~ 6°), the differential Doppler frequency fo will be approximately 4.8 MHz. 0 In real applications, moving surfaces may also involve a vertical displacement, for example vibrations. Assuming that V _z < V _x , the corresponding Doppler frequency of the moving target at presence of vertical velocity V _z is described by

_ 2V _x sin θ 2F _z cosθ 5 Fig. 4 shows the measured signal, i.e. the measured intensity variation of the output beam of the laser, in the frequency domain. The upper diagram represents the frequency spectrum achieved in the absence of vibrations of the moving target. The Doppler frequencies fϊ, f ₂ of the two sensing beams 4, 5 are identical and the differential Doppler frequency f _D appears as a separate peak. 4F sinθ f _D = f i + f ₂ = λ

In case of vibrations of the moving target, the Doppler frequencies fϊ, ^ are shifted with equal mount but in different directions, whereas the sum of fϊ and f ₂ , i.e. the differential Doppler frequency fb, remains the same value. The Doppler frequencies fϊ and f2 are thus influenced by longitudinal and vertical velocities of the moving target. The vertical speed or vibrations is related to fi - f ₂ . Interference between sensing beam 4 and sensing beam 5 contributes to the differential Doppler frequency fb = f i + f ₂ .

Therefore, as illustrated in fig. 4, the vertical velocity of the moving target is determined by the Doppler frequency split fi - f ₂ . The longitudinal speed is determined by fo, which is independent of vertical velocity or vibrations of the target surface.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Furthermore, the description of a VCSEL laser as part of the device is only one advantageous example. In addition to a VCSEL, the laser source may be another type of single mode laser, in particular a single mode laser diode. Also, the number of split beams can be three or more depending on the specific applications. The reference signs in the claims should not be construed as limiting the scope of these claims.

LIST OF REFERENCE SIGNS:

1 VCSEL laser

2 Output beam

3 Lens

4 Sensing beam

5 Sensing beam

6 Beam splitting unit

7 Lens

8 Beam stop

9 Prism

10 Moving target

11 Central optical axis