- Email: [email protected]
Directional achromatic heterodyne fiber laser Doppler anemometer
15 February 1999
Optics Communications 160 Ž1999. 268–272
Directional achromatic heterodyne fiber laser Doppler anemometer J. Czarske ) , H. Zellmer 1, H. Welling Laser Zentrum HannoÕer, Hollerithallee 8, D-30419 HannoÕer, Germany Received 28 August 1998; revised 24 November 1998; accepted 22 December 1998
Abstract The application of diode-pumped double-clad Nd:glass fiber lasers with high fundamental mode power for laser Doppler anemometry ŽLDA. will be presented. An acousto-optic modulator ŽAOM. is used as a diffractive beam splitter, so that the LDA calibration constant is wavelength independent, i.e., achromatic. Hence, broadband emitting powerful fiber lasers can also be employed. The directional discrimination of the LDA velocity measurement was achieved using the heterodyne technique with the frequency shift of the AOM element. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Laser Doppler anemometry; Acousto-optic modulator; Heterodyne technique
1. Introduction Laser Doppler anemometry ŽLDA. is one of the standard methods for spatial and time-resolved velocity measurements in fluid flows. However, typical LDA measurements require high cw laser powers, because of the small scattering coefficient. Such powerful LDA systems conventionally employ argon-ion lasers, having a low efficiency of about 0.1%, and a bulky design. Consequentially, these LDA systems are hardly portable, so that fluid flow measurements outside the laboratory, e.g., optical in-flight measurements from air crafts w1,2x, are problematic. One approach to improve the portability of the LDA system is the use of laser diodes w3x. However, the singlemode power of conventional laser diodes is to date limited to about 0.1 W. Higher single-mode powers of about 1 W can be achieved with Master Oscillator Power Amplifier ŽMOPA. laser diodes w4x, but further power scaling is very
)
Corresponding author. E-mail: [email protected] Presently at Friedrich-Schiller-Universitat ¨ Jena, Institut fur ¨ Angewandte Physik, Max-Wien-Platz 1, D-07743 Jena, Germany. 1
difficult. Another well-known power scaling technique is diode pumping of solid-state lasers. To date, several tens of Watt of fundamental mode power are achieved by diode-pumped Nd:YAG lasers w5x, but high technical effort is necessary. Furthermore, the input of high laser powers into the single-mode fibers of fiber-coupled LDA measuring heads could be problematic. In order to overcome these drawbacks, the use of diode-pumped fiber lasers for LDA has already been proposed w6x. Fiber lasers emit to date fundamental mode powers up to the 30 W range w7,8x, have a simple, compact and reliable design, and in contrast to solid-state lasers they are not sensitive to thermal effects, due to the waveguide structure of the active fiber and the high ratio between surface and active volume. Furthermore, light generation in the single-mode active fiber avoids the abovementioned fiber launching difficulties. However, currently existing powerful fiber lasers w9x have large emission linewidths in the range of 10 nm and accurate velocity measurements require a light wavelength independent LDA calibration constant. These achromatic properties can be achieved by the use of a diffraction grating as a laser beam splitter w6,10x. In this Communication, we present an achromatic LDV system, which additionally enables directional discrimination of the scattering particle movement.
0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 6 8 4 - 1
J. Czarske et al.r Optics Communications 160 (1999) 268–272
269
The applied principle is based on the use of an acousto-optic modulator ŽAOM. w11x, which generates a moving diffraction grating. A frequency shift between the split laser beams occurs, so that directional discrimination can be realized. The combined use of the double-clad fiber laser and the AOM leads to a powerful directional LDA system, which could enhance the LDA application range significantly.
2. Principle and experimental results 2.1. Diode-pumped fiber laser The laser used is a diode-pumped double-clad Nd-doped silica fiber laser w7–9x. The light from a laser diode array, model Jenoptik CPXF-12-1L with a maximum optical power of 12 W at 40 A diode current, is directly coupled with an SMA connector into the cladding of the fiber, which has a cladding diameter of 400 mm and 0.38 numerical aperture. The fiber was manufactured by the Institut fur ¨ Physikalische Hochtechnologie, Jena, Germany. It is doped with 1300 ppm Nd, and co-doped with Al, P and Ge. Inside the pump cladding, the active Nddoped fiber core of about 11 mm in diameter and 0.16 numerical aperture is embedded Žsee Fig. 1.. By cross-talking of the pump light from the pump cladding into the active fiber core, a quasi-transversal laser pumping is achieved. However, efficient pump light absorption requires the conversion of helical pump rays to meridional pump rays, which only cross the Nd-doped fiber core. This mode conversion was enforced by a D-shaped pump fiber core, resulting in a permanent rearrangement of the pump fiber modes, so that almost each pump mode is absorbed over the fiber length of about 40 m. Conventionally, the resonator of the fiber laser has a butt-coupled dicrotic
Fig. 2. Optical spectrum of the fiber laser.
mirror with HR 1064 nm and HT 810 nm at the fiber beginning, and simply the Fresnel reflex from the fiber end. This Fabry–Perot resonator results in an emission with several longitudinal modes, having a mode spacing of D f s crŽ2 ln. s 2.6 MHz, where c is the light velocity, l s 40 m is the fiber length and n s 1.46 is the fiber phase refraction index. In consequence, a spectral comb with about 2.6 MHz mode spacing will be laying in the photodetector signal spectrum. In general, an application of the fiber laser for LDA requires suppression of the mode comb. One possibility is the enforcement of fiber laser operation with amplified spontaneous emission ŽASE.. By a reduction of the fiber end reflex with an 88 fiber cutting angle, an ASE fiber laser operation was arranged, so that the mode comb was successfully suppress. The measured maximum ASE power is about 2 W. However, using a dicrotic mirror with HR 810 nm and HT 1064 nm at the fiber end, the back-reflected pump light can be twisted absorbed. Then, an enhancement of the ASE power of more than 40% should be achievable. In Fig. 2, the optical emission spectrum of the ASE fiber laser operation is shown. The large linewidth of approximately 8 nm requires, as mentioned above, achromatic properties of the LDA system. This will be illustrated in Section 2.2. 2.2. Directional achromatic LDA system
Fig. 1. Geometry of the double-clad fiber laser. The D-shaped fiber provides a pump mode conversion, so that the absorption of the doped fiber core is significantly enhanced.
In Fig. 3, the optical arrangement of the LDA system is shown. The fiber laser output is collimated to a beam waist diameter of about 200 mm by a molded aspheric glass lens of 1.45 mm focus length. The collimated laser beam illuminates a Bragg diffraction grating inside the AOM element. The grating splits the incident beam up into the zero and first diffraction order, respectively. The beams are focused by a Kepler telescope into the measuring volume of the LDA system Žsee Fig. 3.. This can be regarded as an imaging of the moving diffraction grating, generated inside the AOM medium, into the measuring volume w6x. Therefore, it is clear that a moving fringe
270
J. Czarske et al.r Optics Communications 160 (1999) 268–272
Fig. 3. Scheme of the directional achromatic LDA system. AOM: acousto-optic modulator, APD: avalanche photo diode. Fig. 5. Measured heterodyne spectrum of the photodetector signal.
system with achromatic fringe spacing is generated in the measuring volume. The scattering light from tracer particles, moving through the measuring volume is imaged into a multimode-fiber with 400 mm diameter Žsee Fig. 3.. The light is guided to an avalanche photo diode ŽAPD type EG & G C30954E: IR-enhanced Si-diode, with transimpedance amplifier, from Laser Components, Germany., where the LDA measuring signal is generated. As can be seen from Fig. 3, integration of the backscattering receiving optics requires a sufficient laser beam separation. Hence, a high diffraction angle of the AOM is necessary to realize a compact LDA system. The diffraction angle is given by sinq s lrg s l f SrÕ S , where l is the light wavelength, g is the grating period, f S is the electrical frequency of the piezo driver for the acoustic wave generation and Õ S is the acoustic velocity of the AOM medium. Regarding this formula, a large diffraction angle is achieved by high electrical frequency together with a low acoustic velocity. Therefore, a shear-mode TeO 2 AOM Žtype AA.SHT.140, from Pegasus Optik, Germany. was chosen, which has a small acoustic velocity of 675 mrs, and allows a high electrical frequency of 140 MHz. The resulting grating period of 4.8 mm corresponds to a beam
splitting angle of 12.78, assuming a fiber laser center wavelength of 1062 nm Žsee Fig. 2.. Furthermore, this AOM type enables a high Bragg diffraction efficiency of up to 83% into the first order. However, for the presented LDA system, a symmetrical 50:50 power ratio between the zero and first diffraction order, respectively, is desirable, which can be simply adjusted using the electrical driving power of the AOM. Due to the birefringence of the TeO 2 AOM medium, a defined polarization state of the laser beam is necessary. However, a linear-polarized fiber laser is difficult to realize with D-shaped fiber cladding. Hence, linear light polarization was achieved by simply introducing a polarizer Žsee Fig. 3.. Furthermore, in the future an isolator for backward scattered light has to be introduced, in order to enhance the fiber laser power stability. Since the diffracted beam of the AOM has orthogonal polarization with respect to the non-diffracted beam, a half-wave plate is used to achieve the same polarization state of the interacting laser beams in the measuring volume. Additionally, a retardation glass plate was introduced into the other laser beam in order to minimize the path length difference between the two beams. This is necessary due to the small
Fig. 4. Quadrature demodulation technique: the generation of a quadrature signal pair is accomplished by a two-stage mixing unit and a 908C phase shifter, simply realized by a coaxial cable delay line. The phase demodulation of the signal pair and the calculation of the momentary Doppler frequency f D were implemented on a PC.
Fig. 6. Burst signal pair and determined phase curve.
J. Czarske et al.r Optics Communications 160 (1999) 268–272
Fig. 7. Burst signal pair and determined phase curve for the opposite scattering object moving direction compared to Fig. 6.
271
suring signal for a non-moving scattering object is shown. As can be seen, only the carrier frequency of the AOM driver exists, and the above mentioned mode comb is suppressed by the ASE fiber laser operation. In Figs. 6 and 7, measurements of the QDT for opposite scattering object movements are shown. Fig. 6 shows that the cosine signal leads with respect to the sine signal, resulting in a phase curve with positive slope. Fig. 7 was recorded for the opposite movement direction of the scattering object, resulting in a leading sine signal compared to the cosine signal, so that a negative phase slope results. In conclusion, the sign of the phase slope determines the flow direction, and the amount of phase slope corresponds to the momentary velocity of the fluid flow.
3. Conclusions coherence length of the fiber laser of l C s l2rD l s Ž1062 nm. 2r8 nm s 140 mm. Therefore, the path length compensation has to be achieved accurately. In Fig. 4, the proposed Doppler signal processing technique, the quadrature demodulation technique ŽQDT. w12x is presented. One advantage of the QDT compared to conventional LDV signal processing techniques is the high time-resolution of the LDA velocity measurement, which is important, e.g., for turbulent flow measurements. Furthermore, a directional discrimination of the scattering particle movement can be accomplished in the base band w12x. Hence, small velocities can be measured with high resolution. The QDT is based on the evaluation of a quadrature signal pair, which is generated by the heterodyne technique. The measuring signal with the frequency f M Ž t . s fC " f DŽ t . is down-mixed with a reference signal, having the same carrier frequency fC , which is equal to the AOM driving frequency. Using a phase shift of 908 for the carrier signal in the second mixing stage, the quadrature signal pair aSŽ t . s AŽ t .sin f Ž t ., aC Ž t . s AŽ t .cos f Ž t . is generated in the base band Žsee Fig. 4. with f Ž t . s 2p f DŽ t . t. The demodulation of the phase can be accomplished by f Ž t . s arctanŽ aSŽ t .raC Ž t ... The momentary Doppler frequency f DŽ t . can be calculated by a differentiation of the phase time curve: f DŽ t . s 1rŽ2p .Žd frdt . so that in conclusion the momentary velocity Õ Ž t . s f DŽ t . d can be determined Ž d: fringe spacing.. This procedure was implemented on a PC: First a PC-card digitized the signal pair. By using the software LabVIEW, the phase, and finally the momentary velocity can be determined from the digitized signal pair. The described LDA system was verified as follows. A glass fiber was used as scattering object, which was moved by a control motor with constant velocity through the measuring volume. Although the fiber diameter of 125 mm was significantly higher than the fringe spacing, a measuring signal with a high modulation larger than 40% was achieved. In Fig. 5, the spectrum of the heterodyne mea-
The use of a high power double-clad fiber laser for directional LDA was presented. Due to the use of an AOM as diffractive beam splitter, and together as frequency shifter, the LDA system has achromatic properties and directional discrimination. In future optimization, concerning the realization of a compact LDA system with high velocity measurement accuracy, will be accomplished.
Acknowledgements This research was partially funded by the DFG ŽCz55r4-1.. The encouragement of O. Dolle ¨ for signal processing is acknowledged. We thank Dr. Jovanovic ŽPegasus Optik GmbH, Ansgarstrasse 20, D-49134 Wallenhorst, Germany. for fruitful discussions.
References w1x H. Jentink, J. Aircraft 31 Ž1993. 444. w2x G. Grosche, H. Muller, V. Strunck, D. Dopheide, 6 Fachta¨ gung Lasermethoden in der Stromungsmeßtechnik, Essen, ¨ 1998, Paper 2. w3x M. Stiegelmeier, C. Tropea, Appl. Opt. 31 Ž1992. 4096. w4x H. Muller, D. Dopheide, in: B. Ruck, et al. ŽEds.., Proc. 7th ¨ Int. Conf. on Laser Anemometry – Advances and Applications, Karlsruhe, Germany, 8–11 September 1997, p. 3. w5x D. Golla, M. Bode, S. Knoke, W. Schone, ¨ A. Tunnermann, ¨ Opt. Lett. 21 Ž1996. 210. w6x J. Czarske, H. Zellmer, A. Tunnermann, H. Welling, H. ¨ Muller, Appl. Phys. B 64 Ž1997. 119. ¨ w7x H. Zellmer, A. Tunnermann, H. Welling, Laser Optoelek¨ tronik 29 Ž1997. 53. w8x H. Zellmer, A. Tunnermann, H. Welling, V. Reichel, Trends ¨ Optics Photonics TOPs 16 Ž1997. 137. w9x H. Zellmer, U. Willamowski, A. Tunnermann, H. Welling, S. ¨ Unger, V. Reichel, H. Muller, J. Kirchhof, P. Albers, Opt. ¨ Lett. 20 Ž1995. 578.
272
J. Czarske et al.r Optics Communications 160 (1999) 268–272
w10x J. Schmidt, R. Volkel, W. Stork, J. Schwider, N. Streibl, Opt. ¨ Lett. 17 Ž1992. 1240. w11x F. Durst, A. Melling, J. Whitelaw, Theorie und Praxis der Laser-Doppler-Anemometrie, Wissenschaft und Technik, Karlsruhe, 1987.
w12x J. Czarske, O. Dolle, in: B. ¨ I. Freitag, H. Welling, H. Muller, ¨ Ruck, et al. ŽEds.., Proc. 7th Int. Conf. on Laser Anemometry – Advances and Applications, Karlsruhe, 8–11 September 1997, p. 73.
Optics Communications 160 Ž1999. 268–272
Directional achromatic heterodyne fiber laser Doppler anemometer J. Czarske ) , H. Zellmer 1, H. Welling Laser Zentrum HannoÕer, Hollerithallee 8, D-30419 HannoÕer, Germany Received 28 August 1998; revised 24 November 1998; accepted 22 December 1998
Abstract The application of diode-pumped double-clad Nd:glass fiber lasers with high fundamental mode power for laser Doppler anemometry ŽLDA. will be presented. An acousto-optic modulator ŽAOM. is used as a diffractive beam splitter, so that the LDA calibration constant is wavelength independent, i.e., achromatic. Hence, broadband emitting powerful fiber lasers can also be employed. The directional discrimination of the LDA velocity measurement was achieved using the heterodyne technique with the frequency shift of the AOM element. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Laser Doppler anemometry; Acousto-optic modulator; Heterodyne technique
1. Introduction Laser Doppler anemometry ŽLDA. is one of the standard methods for spatial and time-resolved velocity measurements in fluid flows. However, typical LDA measurements require high cw laser powers, because of the small scattering coefficient. Such powerful LDA systems conventionally employ argon-ion lasers, having a low efficiency of about 0.1%, and a bulky design. Consequentially, these LDA systems are hardly portable, so that fluid flow measurements outside the laboratory, e.g., optical in-flight measurements from air crafts w1,2x, are problematic. One approach to improve the portability of the LDA system is the use of laser diodes w3x. However, the singlemode power of conventional laser diodes is to date limited to about 0.1 W. Higher single-mode powers of about 1 W can be achieved with Master Oscillator Power Amplifier ŽMOPA. laser diodes w4x, but further power scaling is very
)
Corresponding author. E-mail: [email protected] Presently at Friedrich-Schiller-Universitat ¨ Jena, Institut fur ¨ Angewandte Physik, Max-Wien-Platz 1, D-07743 Jena, Germany. 1
difficult. Another well-known power scaling technique is diode pumping of solid-state lasers. To date, several tens of Watt of fundamental mode power are achieved by diode-pumped Nd:YAG lasers w5x, but high technical effort is necessary. Furthermore, the input of high laser powers into the single-mode fibers of fiber-coupled LDA measuring heads could be problematic. In order to overcome these drawbacks, the use of diode-pumped fiber lasers for LDA has already been proposed w6x. Fiber lasers emit to date fundamental mode powers up to the 30 W range w7,8x, have a simple, compact and reliable design, and in contrast to solid-state lasers they are not sensitive to thermal effects, due to the waveguide structure of the active fiber and the high ratio between surface and active volume. Furthermore, light generation in the single-mode active fiber avoids the abovementioned fiber launching difficulties. However, currently existing powerful fiber lasers w9x have large emission linewidths in the range of 10 nm and accurate velocity measurements require a light wavelength independent LDA calibration constant. These achromatic properties can be achieved by the use of a diffraction grating as a laser beam splitter w6,10x. In this Communication, we present an achromatic LDV system, which additionally enables directional discrimination of the scattering particle movement.
0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 8 . 0 0 6 8 4 - 1
J. Czarske et al.r Optics Communications 160 (1999) 268–272
269
The applied principle is based on the use of an acousto-optic modulator ŽAOM. w11x, which generates a moving diffraction grating. A frequency shift between the split laser beams occurs, so that directional discrimination can be realized. The combined use of the double-clad fiber laser and the AOM leads to a powerful directional LDA system, which could enhance the LDA application range significantly.
2. Principle and experimental results 2.1. Diode-pumped fiber laser The laser used is a diode-pumped double-clad Nd-doped silica fiber laser w7–9x. The light from a laser diode array, model Jenoptik CPXF-12-1L with a maximum optical power of 12 W at 40 A diode current, is directly coupled with an SMA connector into the cladding of the fiber, which has a cladding diameter of 400 mm and 0.38 numerical aperture. The fiber was manufactured by the Institut fur ¨ Physikalische Hochtechnologie, Jena, Germany. It is doped with 1300 ppm Nd, and co-doped with Al, P and Ge. Inside the pump cladding, the active Nddoped fiber core of about 11 mm in diameter and 0.16 numerical aperture is embedded Žsee Fig. 1.. By cross-talking of the pump light from the pump cladding into the active fiber core, a quasi-transversal laser pumping is achieved. However, efficient pump light absorption requires the conversion of helical pump rays to meridional pump rays, which only cross the Nd-doped fiber core. This mode conversion was enforced by a D-shaped pump fiber core, resulting in a permanent rearrangement of the pump fiber modes, so that almost each pump mode is absorbed over the fiber length of about 40 m. Conventionally, the resonator of the fiber laser has a butt-coupled dicrotic
Fig. 2. Optical spectrum of the fiber laser.
mirror with HR 1064 nm and HT 810 nm at the fiber beginning, and simply the Fresnel reflex from the fiber end. This Fabry–Perot resonator results in an emission with several longitudinal modes, having a mode spacing of D f s crŽ2 ln. s 2.6 MHz, where c is the light velocity, l s 40 m is the fiber length and n s 1.46 is the fiber phase refraction index. In consequence, a spectral comb with about 2.6 MHz mode spacing will be laying in the photodetector signal spectrum. In general, an application of the fiber laser for LDA requires suppression of the mode comb. One possibility is the enforcement of fiber laser operation with amplified spontaneous emission ŽASE.. By a reduction of the fiber end reflex with an 88 fiber cutting angle, an ASE fiber laser operation was arranged, so that the mode comb was successfully suppress. The measured maximum ASE power is about 2 W. However, using a dicrotic mirror with HR 810 nm and HT 1064 nm at the fiber end, the back-reflected pump light can be twisted absorbed. Then, an enhancement of the ASE power of more than 40% should be achievable. In Fig. 2, the optical emission spectrum of the ASE fiber laser operation is shown. The large linewidth of approximately 8 nm requires, as mentioned above, achromatic properties of the LDA system. This will be illustrated in Section 2.2. 2.2. Directional achromatic LDA system
Fig. 1. Geometry of the double-clad fiber laser. The D-shaped fiber provides a pump mode conversion, so that the absorption of the doped fiber core is significantly enhanced.
In Fig. 3, the optical arrangement of the LDA system is shown. The fiber laser output is collimated to a beam waist diameter of about 200 mm by a molded aspheric glass lens of 1.45 mm focus length. The collimated laser beam illuminates a Bragg diffraction grating inside the AOM element. The grating splits the incident beam up into the zero and first diffraction order, respectively. The beams are focused by a Kepler telescope into the measuring volume of the LDA system Žsee Fig. 3.. This can be regarded as an imaging of the moving diffraction grating, generated inside the AOM medium, into the measuring volume w6x. Therefore, it is clear that a moving fringe
270
J. Czarske et al.r Optics Communications 160 (1999) 268–272
Fig. 3. Scheme of the directional achromatic LDA system. AOM: acousto-optic modulator, APD: avalanche photo diode. Fig. 5. Measured heterodyne spectrum of the photodetector signal.
system with achromatic fringe spacing is generated in the measuring volume. The scattering light from tracer particles, moving through the measuring volume is imaged into a multimode-fiber with 400 mm diameter Žsee Fig. 3.. The light is guided to an avalanche photo diode ŽAPD type EG & G C30954E: IR-enhanced Si-diode, with transimpedance amplifier, from Laser Components, Germany., where the LDA measuring signal is generated. As can be seen from Fig. 3, integration of the backscattering receiving optics requires a sufficient laser beam separation. Hence, a high diffraction angle of the AOM is necessary to realize a compact LDA system. The diffraction angle is given by sinq s lrg s l f SrÕ S , where l is the light wavelength, g is the grating period, f S is the electrical frequency of the piezo driver for the acoustic wave generation and Õ S is the acoustic velocity of the AOM medium. Regarding this formula, a large diffraction angle is achieved by high electrical frequency together with a low acoustic velocity. Therefore, a shear-mode TeO 2 AOM Žtype AA.SHT.140, from Pegasus Optik, Germany. was chosen, which has a small acoustic velocity of 675 mrs, and allows a high electrical frequency of 140 MHz. The resulting grating period of 4.8 mm corresponds to a beam
splitting angle of 12.78, assuming a fiber laser center wavelength of 1062 nm Žsee Fig. 2.. Furthermore, this AOM type enables a high Bragg diffraction efficiency of up to 83% into the first order. However, for the presented LDA system, a symmetrical 50:50 power ratio between the zero and first diffraction order, respectively, is desirable, which can be simply adjusted using the electrical driving power of the AOM. Due to the birefringence of the TeO 2 AOM medium, a defined polarization state of the laser beam is necessary. However, a linear-polarized fiber laser is difficult to realize with D-shaped fiber cladding. Hence, linear light polarization was achieved by simply introducing a polarizer Žsee Fig. 3.. Furthermore, in the future an isolator for backward scattered light has to be introduced, in order to enhance the fiber laser power stability. Since the diffracted beam of the AOM has orthogonal polarization with respect to the non-diffracted beam, a half-wave plate is used to achieve the same polarization state of the interacting laser beams in the measuring volume. Additionally, a retardation glass plate was introduced into the other laser beam in order to minimize the path length difference between the two beams. This is necessary due to the small
Fig. 4. Quadrature demodulation technique: the generation of a quadrature signal pair is accomplished by a two-stage mixing unit and a 908C phase shifter, simply realized by a coaxial cable delay line. The phase demodulation of the signal pair and the calculation of the momentary Doppler frequency f D were implemented on a PC.
Fig. 6. Burst signal pair and determined phase curve.
J. Czarske et al.r Optics Communications 160 (1999) 268–272
Fig. 7. Burst signal pair and determined phase curve for the opposite scattering object moving direction compared to Fig. 6.
271
suring signal for a non-moving scattering object is shown. As can be seen, only the carrier frequency of the AOM driver exists, and the above mentioned mode comb is suppressed by the ASE fiber laser operation. In Figs. 6 and 7, measurements of the QDT for opposite scattering object movements are shown. Fig. 6 shows that the cosine signal leads with respect to the sine signal, resulting in a phase curve with positive slope. Fig. 7 was recorded for the opposite movement direction of the scattering object, resulting in a leading sine signal compared to the cosine signal, so that a negative phase slope results. In conclusion, the sign of the phase slope determines the flow direction, and the amount of phase slope corresponds to the momentary velocity of the fluid flow.
3. Conclusions coherence length of the fiber laser of l C s l2rD l s Ž1062 nm. 2r8 nm s 140 mm. Therefore, the path length compensation has to be achieved accurately. In Fig. 4, the proposed Doppler signal processing technique, the quadrature demodulation technique ŽQDT. w12x is presented. One advantage of the QDT compared to conventional LDV signal processing techniques is the high time-resolution of the LDA velocity measurement, which is important, e.g., for turbulent flow measurements. Furthermore, a directional discrimination of the scattering particle movement can be accomplished in the base band w12x. Hence, small velocities can be measured with high resolution. The QDT is based on the evaluation of a quadrature signal pair, which is generated by the heterodyne technique. The measuring signal with the frequency f M Ž t . s fC " f DŽ t . is down-mixed with a reference signal, having the same carrier frequency fC , which is equal to the AOM driving frequency. Using a phase shift of 908 for the carrier signal in the second mixing stage, the quadrature signal pair aSŽ t . s AŽ t .sin f Ž t ., aC Ž t . s AŽ t .cos f Ž t . is generated in the base band Žsee Fig. 4. with f Ž t . s 2p f DŽ t . t. The demodulation of the phase can be accomplished by f Ž t . s arctanŽ aSŽ t .raC Ž t ... The momentary Doppler frequency f DŽ t . can be calculated by a differentiation of the phase time curve: f DŽ t . s 1rŽ2p .Žd frdt . so that in conclusion the momentary velocity Õ Ž t . s f DŽ t . d can be determined Ž d: fringe spacing.. This procedure was implemented on a PC: First a PC-card digitized the signal pair. By using the software LabVIEW, the phase, and finally the momentary velocity can be determined from the digitized signal pair. The described LDA system was verified as follows. A glass fiber was used as scattering object, which was moved by a control motor with constant velocity through the measuring volume. Although the fiber diameter of 125 mm was significantly higher than the fringe spacing, a measuring signal with a high modulation larger than 40% was achieved. In Fig. 5, the spectrum of the heterodyne mea-
The use of a high power double-clad fiber laser for directional LDA was presented. Due to the use of an AOM as diffractive beam splitter, and together as frequency shifter, the LDA system has achromatic properties and directional discrimination. In future optimization, concerning the realization of a compact LDA system with high velocity measurement accuracy, will be accomplished.
Acknowledgements This research was partially funded by the DFG ŽCz55r4-1.. The encouragement of O. Dolle ¨ for signal processing is acknowledged. We thank Dr. Jovanovic ŽPegasus Optik GmbH, Ansgarstrasse 20, D-49134 Wallenhorst, Germany. for fruitful discussions.
References w1x H. Jentink, J. Aircraft 31 Ž1993. 444. w2x G. Grosche, H. Muller, V. Strunck, D. Dopheide, 6 Fachta¨ gung Lasermethoden in der Stromungsmeßtechnik, Essen, ¨ 1998, Paper 2. w3x M. Stiegelmeier, C. Tropea, Appl. Opt. 31 Ž1992. 4096. w4x H. Muller, D. Dopheide, in: B. Ruck, et al. ŽEds.., Proc. 7th ¨ Int. Conf. on Laser Anemometry – Advances and Applications, Karlsruhe, Germany, 8–11 September 1997, p. 3. w5x D. Golla, M. Bode, S. Knoke, W. Schone, ¨ A. Tunnermann, ¨ Opt. Lett. 21 Ž1996. 210. w6x J. Czarske, H. Zellmer, A. Tunnermann, H. Welling, H. ¨ Muller, Appl. Phys. B 64 Ž1997. 119. ¨ w7x H. Zellmer, A. Tunnermann, H. Welling, Laser Optoelek¨ tronik 29 Ž1997. 53. w8x H. Zellmer, A. Tunnermann, H. Welling, V. Reichel, Trends ¨ Optics Photonics TOPs 16 Ž1997. 137. w9x H. Zellmer, U. Willamowski, A. Tunnermann, H. Welling, S. ¨ Unger, V. Reichel, H. Muller, J. Kirchhof, P. Albers, Opt. ¨ Lett. 20 Ž1995. 578.
272
J. Czarske et al.r Optics Communications 160 (1999) 268–272
w10x J. Schmidt, R. Volkel, W. Stork, J. Schwider, N. Streibl, Opt. ¨ Lett. 17 Ž1992. 1240. w11x F. Durst, A. Melling, J. Whitelaw, Theorie und Praxis der Laser-Doppler-Anemometrie, Wissenschaft und Technik, Karlsruhe, 1987.
w12x J. Czarske, O. Dolle, in: B. ¨ I. Freitag, H. Welling, H. Muller, ¨ Ruck, et al. ŽEds.., Proc. 7th Int. Conf. on Laser Anemometry – Advances and Applications, Karlsruhe, 8–11 September 1997, p. 73.