D. Dopheide and V. StrunckPhysikalisch-Technische Bundesanstalt, Laboratory for fluid flow measurement techniques, Bundesallee t00, D-3300 Braunschweig, E R. Germany
H. J. PfeiferGerman-French Research Institute (ISL), 12, rue de l'Industrie, F-68 301 Saint-Louis, France
Abstract. The unique advantages of high-frequency pulsed diode laser for fringe type laser Doppler anemometry is described. Unlike the known LDA-technique using cw-lasers, pulse repetition rates much higher than the Doppler frequencies and pulse durations in the nanosecond range will be applied. This technique offers a number of benefits: Firstly, in pulse mode the time-averaged number of photons may be considerably higher than in cw-mode and secondly, the output power of single pulses emitted from diodes offers very high output power and can be synchronized with the sampling process of the data acquisition system. Thirdly, multi-component velocity measurements can be made using only one receiver and one signal processing chain at identical wavelengths of all components without any crosstalk. The corresponding techniques of"pulse integration" and "coherent sampling" will be described in detail and the advantages and limitations in comparison to conventional cw-mode LDAs will be pointed out. The experimental verifications of the new techniques are described as far as they have been performed until
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H. J. PfeiferGerman-French Research Institute (ISL), 12, rue de l'Industrie, F-68 301 Saint-Louis, France
Abstract. The unique advantages of high-frequency pulsed diode laser for fringe type laser Doppler anemometry is described. Unlike the known LDA-technique using cw-lasers, pulse repetition rates much higher than the Doppler frequencies and pulse durations in the nanosecond range will be applied. This technique offers a number of benefits: Firstly, in pulse mode the time-averaged number of photons may be considerably higher than in cw-mode and secondly, the output power of single pulses emitted from diodes offers very high output power and can be synchronized with the sampling process of the data acquisition system. Thirdly, multi-component velocity measurements can be made using only one receiver and one signal processing chain at identical wavelengths of all components without any crosstalk. The corresponding techniques of"pulse integration" and "coherent sampling" will be described in detail and the advantages and limitations in comparison to conventional cw-mode LDAs will be pointed out. The experimental verifications of the new techniques are described as far as they have been performed until
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1 Introduction
The introduction of diode lasers for LDA will give the most impressive progress in the development of miniaturized optical flow sensors which offer new possibilities in flow-diagnostics. The present paper describes the benefits of a new idea for a multicomponent miniature diode laser LDA.
For a great number of flow investigations, it is of importance to know several velocity components, in particular when, e.g., the shear stresses are to be measured in highly turbulent flows. This requires the simultaneous measurement of all three velocity components.
The LDA technique allows several components to be measured at the same time; in this case, it is common practice that either all components of an individual particle are determined or that within a certain interval of time three components of different particles are measured.
The design of a miniaturized diode laser LDA for two or three velocity components will be described which needs only one wavelength for all components, one photodetector and one signal processor for all channels using the "coherent sampling" technique.
In the first part the state-of-the-art of diode laser technique will be described. The second section explains the idea of high-frequency pulsing, and at last the design of the new multicomponent technique will be outlined and discussed.
2 State-of-the-art of diode lasers for LDA
Fluid flow research can be very easily performed using diode lasers and photodiodes as has been outlined by Dopheide et al. (1986). In this publication detailed experimental studies are described proving that GaA1As-laser diodes can be used with Si-photodiodes in a very advantageous way. The authors examined in detail the most important characteristics of these devices and studied the differences between gain guided and index guided laser diodes. In addition, they carried out comparison measurements with respect to attainable signal-to-noise ratios.
Advantages of diode lasers over gas lasers are not only their small size, low energy consumption and high eliability but also, from the physical point of view, the fact that the spectral sensitivity of Si-photodiodes shows a maximum at a wavelength of about 830 nm which is the same wavelength at which GaA1As-laser diodes emit maximum power. In this region, the quantum efficiency of Si-photodiodes can attain 90% and maintain a very low self-noise. Therefore, much higher signal-to-noise ratios can be achieved as compared with the combination of gas lasers and photomultipliers as it was shown in detail by Dopheide et al. (1987 a, 1988 a).
The problems of wavelength stabilization and wavelength measurement are described in these papers. A portable wavelength stabilized backscatter LDA for velocity measurements up to 200 m/s was described by Dopheide et al.(1988b).
In 1988 several other miniature LDA systems have been presented with very interesting features. For example, Damp (1988) presented an extremely compact laser diode LDA. Brown et al. (1988) developed a battery driven LDA with infrared remote control. Bopp et al. (1988) demonstrated different applications of a simple diode LDA.
Applications of a wavelength stabilized, high performance two-component diode laser LDA for phase Doppler measurements have been described by Bauckhage et al (1989). In this paper an advanced Phase Doppler LDA for particle sizing and velocity measurements in a backscatter arrangement using diode lasers and avalanche diodes will be presented.
Rapid development of technology led to single stripe emitters with output powers up to 100 mW which are now commercially available (by SDL and SONY). Using these diodes and high performance avalanche diodes it is possible to obtain signal-to-noise ratios which are similar to signalto- noise ratios obtained with a combination of a 300 mW gas laser and a photomultiplier. Generally, the signal-to noise ratios are three or four times better than those of conventional LDAs. This is true mainly because the quantum efficiency of the photodetectors is superior by the same factor.
However, for a number of applications in fluid mechanics, e.g. in high speed measurement, much higher optical power should be very useful. Manufacturers of semiconductor components have developed phased diode arrays with output powers of some hundred milliwatts up to several watts.
Dopheide et al. have demonstrated, how to use these phased arrays for LDA (1987a) and for the laser array velocimeter (1988 c). Such a "Laser Array Velocimeter (LAV)" is very simple, extremely small and needs no wavelength stabilization. It may be very useful in aerospace and high speed applications as has been pointed out by Strunck et al. (1989).
A different way to increase the output power of "single stripe" diode emitters will now be considered for LDA use. A very interesting possibility to increase the optical output power of single element laser diodes is to operate them in pulse-mode.
It is a well-known fact that the time-averaged output power of some types of lasers may be increased considerably by pulsed-mode. Sommer and Pfeifer (1986) described a pulsed LDA system for long range applications using a frequency-doubled Nd: YAG laser. In this case, however, the pulse duration was much longer than the inverse of the Doppler frequency, i.e. longer than 50 us.
The optical output power of the laser diodes mentioned above can not be increased considerably with pulse duration in the microsecond region and above, because at such long pulses the diodes attain the thermal equilibrium, i.e. they work in quasi-cw operation. However, if the pulse durations are in the nanosecond region there are two ways to make use of the short optical pulses.
Firstly, the time averaged output power may be increased substantially if the pulse repetition rate is high enough. Some types of diode lasers can be used for LDA in very advanta-geous ways if new signal acquisition procedures are applied ("pulse integration"). Secondly, the output power of a single pulse of short duration can be increased dramatically, and if the high frequency light pulses are synchronized with the time base of a very fast analog-to-digital converter or a similar device (transient recorders), then the particles are illuminated with high laser power only, when the data acquisition is ready to record the pulses. In the time intervals in between the laser is turned off. This signal acquisition method has been called "coherent sampling" and has been described first by Dopheide et al. (1987b).
3 Pulsed diode laser LDA
The background of the pulse-mode semiconductor LDA using the data acquisition "pulse integration" and the experimental verification have already been published by Dopheide et al. (1989) and will be summarized here. Figure 1 shows in the upper part the time-history of conventional LDA signals as they are obtained in cw-operation. In the lower part the time-history of signals is sketched in the pulse mode. In reality the ratio in amplitude is much higher in favour of the pulsed mode as it is indicated in Fig. 1. The idea and the background of the pulsed mode semiconductor LDA is the fact that the laser pulse duration is significantly shorter than the transit time of the particles through the measuring control volume, and that it is even significantly shorter than the inverse of the Doppler frequency. At the same time, the pulse frequency must be higher than the Doppler frequency by at least a factor of two in order to fulfill the requirements of the Nyquist theorem. As a result two different situations have to be considered as depicted in Fig. 2. Firstly, the time-averaged optical output power/-Pu~s is higher than in cw-mode. In this case the overall time-averaged number of photons illuminating the particles is higher than in cw-operation of the diode.
The short-time pulsing of the diode laser can result in the optical output power of the laser, Ipuls, becoming much greater than the continuous wave power Icw. The repetition rate of the pulses must be matched to the actual Doppler frequency. It is recommended to select at least four or five pulses per Doppler period in practice as shown in the figure. The result is a light intensity distribution Ifringes in the measurement volume as shown in the lower part of Fig. 2. The interferential fringe field thus exists only for durations in the nanosecond range so that passing particles scatter corresponding light pulses on the photo-detector. With the pulsed system, after "signal integration" to the analog LDA burst, the signal evaluation can take place conventionally with counter-processors.
Secondly, the output peak power of a single pulse, Ipuls, can be increased very much, so that the interferential fringe system exists for the short pulse duration thus yielding much higher light intensity, and "coherent sampling" can be applied advantageously. In this mode of operation, in which the high-frequency light pulses are synchronized with the
time base of a transient recorder, the maxima of the scattered light amplitudes are measured so that the complete light power in the individual pulse contributes to the SNR.
The measurement of the amplitude of the light pulses allows the envelope of the LDA burst to be reconstructed and the Doppler frequency to be derived by suitable electronic signal evaluation methods. Then, multicomponent measurements are very easy to be performed.
4 Pulsed LDA with signal integration
The principle of data acquisition using "signal integration" is shown in Fig. 3. This method can be applied successfully, if the time averaged output power Ipuls is equal or higher than the cw-output power Icw.
A laser diode is stabilized in wavelength by a temperature control as described by Dopheide et al. (1986) and driven by a free running pulse generator using an electronic network. The cw-operating point of the diode, the pulse amplitude and the duty cycle, i.e. the pulse duration, can be adjusted in a wide range. They have to be selected for each individual type of diode in such a way that the output power of the diode increases without any loss of the output beam quality. These problems are subject of an ongoing research program at the PTB. All optical parts of the system are similar to a conventional cw semiconductor system. The differences to multicomponent measurements will be pointed out later. On the receiver side a broadband avalanche diode is used. For high frequency applications a special designed diode module with bandwidths of 400 MHz and above was designed. The pulses at the output of the avalanche diode module are then fed into an integrator which forms an analogue signal out of the digital pulses. In the most simple case, a low-pass filter is used as a signal integrator. Such a low-pass filter is always used in LDA if the signal is acquired by counter processors or by similar data acquisition systems. The authors have designed a special computer controlled integrator. The integrator offers the possibility to choose between 16 different integration times. It consists of 16 low-pass filters with a logarithmic scale of frequency limits ranging from 2 kHz to 32 MHz. The resulting LDA-bursts show exactly the same appearance as the bursts obtained in conventional cw LDAsystems, and they can be processed in the well-known counter processors or similar data acquisition systems.
Verification experiments for a set-up according to Fig. 3 have been described by Dopheide et al. (1989) using high frequency pulses with repetition rates of 50 MHz and pulse durations of 6 ns. The results clearly indicate that HF-pulsing is a powerful tool.
5 Pulsed LDA with "coherent sampling"
As pointed out in Sect. 3 "coherent sampling" can be used in a very elegant way, if the peak power of
a single diode laser pulse Ipu~s is higher than the average cw-power Icw (Fig. 2).
Figure 4 shows the principle of a pulsed LDA with "coherent sampling". As for the system with signal integration the laser diode is wavelength-stabilized by temperature and current control and is driven by a pulse generator. In this case, however, the pulse generator is not running free, but it is controlled by an HF-timebase generator. This generator serves at the time as the external clock of a transient recorder. The aperture time is synchronized with the laser pulses in this way. A variable time delay between the two instruments allows to compensate for different transit times of the pulses.
The light pulses with a duration of a few nanoseconds produce for these short time intervals the interference fringe system in the probe volume, and the particles present in the
volume scatter light of these light pulses according to the local light intensity. The pulses from the receiver are fed into the input of the transient recorder without any high- or low-pass filtering. They are sampled exactly at their highest amplitude. As indicated in Fig. 4, the signal stored in the memory of the transient recorder includes the socalled pedestal as it is observed in conventional LDA-arrangements at the output of the photodetector.
Verification experiments using "coherent sampling" according to Fig. 4 have been performed, and a digitized LDAsignal is shown in Fig. 5. As it is clearly demonstrated, the LDA-burst including the pedestal will always be measured using this technique
The following important characteristics of the new mode of LDA-operation may be summarized:
(1)The increase in signal-to-noise ratio in contrast to conventional LDA systems is exclusively determined by the ratio of the light power during the pulses and the light power in cw-mode. The optimum duty cycle, and the pulse amplitude as well as the pulse duration have to be matched to the diode under consideration by preliminary experiments.
(2) The mode of operation of the diode has to be adjusted in such a way that the emitted pulses keep their coherence and that the diode operates only in its fundamental TEMoo mode. This frequently prevents the output power from attaining its possible maximum which is about a factor of three or five above the cw-mode.
(3)The transient recorder needs a high analog bandwidth at its input due to the short pulses and the high sample rate. This demands for recorders of high quality. The authors apply A/D-converters with bandwidths of 350 MHz and 500 MHz sample frequency.
(4) The signals are neither high-pass filtered nor low-pass filtered. This means the lower frequency limit is zero Hertz and the upper limit is given by the upper limit of either the receiving diode or the subsequent amplifier circuits. This,
however, must not be considered as a disadvantage because the samples stored in the transient recorder are Fourier transformed afterwards in a PC or in a minicomputer where the filtering can be performed very effectively in a digital way.
As it is known, the wavelength of a diode laser will show a drift due to temperature change at the PN-junction when a current pulse is applied. Fortunately, however, the wavelength-drift due to pulse applications seems to be negligible for most applications and will be part of a research program started at PTB. A diffraction grating with high resolution will be used to measure the shift of the wavelength during frequency pulses of nanosecond durations.
6 Multicomponent measurement using "coherent sampling"
Conventional three-component LDAs are always based on a two colour LDA with an argon ion laser to measure two orthogonal components. A third colour or frequency shift is used for the third component; see, e.g., Pfeifer (1985). For all existing methods each channel requires a signal processing chain of its own which generally consists of counter-processors, transient recorders or burst spectrum analysers.
The following section describes a new method of multicomponent measurement which is not only basically different but also offers the advantage that only a single processing chain is necessary for all three components if "coherent sampling" is applied. This technique has the advantage, that two or three velocity components can be measured (a) with better SNR due to increased light power in a single pulse compared to cw-operation; (b) using one photodetector only; (c) using one fast transient recorder for coincident sampling of all components simultaneously.
For technical applications it is very advantageous that a miniature and very compact design of such a flow sensor is possible.
As explained in Sect. 5, "coherent sampling" can be applied very successfully, if the peak power of a single diode pulse, lpuls, is higher than the average output power, Icw. As explained, this mode of operation can be achieved very easily, if the pulse duration is in the nanosecond range. The extension of a single channel LDA to a two (three) dimensional LDA is shown together with the electronic setup in Fig.6. Two orthogonal LDAs are used in a conventional arrangement. The third optical component is not drawn in Fig. 6.
The idea underlying the new three-component LDA is to sequentially pulse three diode lasers LD1, LD2 and LD3. The photodetector then sees a sequence of pulses which alternately belong to the different velocity components vl, v2 and v 3 .
Two wavelength-stabilized diode laser LDAs are set up in a well-known way so that two orthogonal interferential fringe systems are produced. As remains to be shown, the wavelength is of no significance for the separation of the velocity components. The optical set-up for the third component has not been entered. It can, e.g., consist of another LDA which produces a third interferential fringe system at a large angle against the optical axis of the two-component LDA as it is well-known from 3D-LDV. The three diode lasers are operated by three pulse generators which are synchronized via an external time base. The HF generator serves at the same time as external time base for the transient recorder. As the time base operates with three times the frequency of the pulse generator, by phase-synchronous pulsing of the diodes, a pulse sequence is emitted by the transmitting optics which alternately comes from the diode lasers LD1, LD2 and LD3. As the aperture time of the transient recorder is synchronized with the pulses, exactly the maxima of the receiving pulses are measured so that in the memory of the transient recorder the sampled LDA signals of all three velocity components are stored.
Thus a crosstalk of the individual channels is basically not possible, so that diode lasers of the same wavelength can be used. Moreover, only one transient recorder is required for all three channels. As it is obvious, the same wavelength can be used for all components, crosstalk of the components should not occur, and the measurements will be performed coincidentally. Verification experiments of such a multicomponent LDA have not yet been performed, but they are now subject of a current research program at PTB.
However, it is possible to process the signals of the pulsed LDA in the conventional way with counter processors by prior forming the pulses into conventional analogue bursts even in multicomponent set-ups, as it is described in the following section.
7 Multicomponent measurements using "pulse integration"
Use of the advantages of the high-frequency pulsed LDA can also be made when the receiving signals are to be processed with conventional counter-processors. It will only be necessary to integrate the high-frequency receiving pulses by means of integrators to conventional LDA bursts of a continuous analogue pattern, i.e. to filter them.
A suitable test set-up for a two-component LDA is shown in Fig. 7. To facilitate understanding, the third component has not been entered. Again two frequency-stabilized diode lasers were operated by means of pulse generators and synchronized through a common time base. The output pulses of the photodiode are fed into two gated amplifiers which also operate via the common time base in the phase-synchronism with the transmitting diodes so that the pulses of diode LD1 arrive at amplifier 1 and the pulses of diode LD2 at amplifier 2 and, thus, two separate channels are available.
As described in Sect. 4, by means of a signal integrator, which in the simplest case is a low pass filter with adjustable cut-off frequency, the pulses can then be integrated to form the analog LDA burst so that they can be processed with conventional counterprocessors. For this purpose two processors are required. As an additional advantage, this synchronous operation of the amplifiers allows background light to be largely suppressed. Because the amplifier in front of the integrator is gated synchronously with the laser pulses, the background light can be reduced by an amount given by the duty cycle (time between pulses to pulse duration). This might be of importance in combustion studies
8 Photon correlation using pulsed diodes
As it has already been pointed out by Dopheide et al. (1989), signal processing using pulsed laser diodes can also be carried out with photon correlators as a data acquisition proce dure. Besides the higher averaged output power of the diode in the pulsed mode, the reduction of background light by synchronously gating the amplifier in front of the correlator with the light pulses is useful. This might be of particular interest for long range anemometry or wind tunnel applications in backscatter arrangements.
9 Characterization of pulsed LDA for multicomponent systems
The following important characteristics of multicomponent LDA-operation as described in Sects. 6-8 may be summarized and discussed:
(1) As opposed to conventional LDAs and single channel pulse LDA (Sect. 4) the increase in signal-to-noise ratio (SNR) is exclusively determined by the increase in the optical output power in pulse-operation as opposed to cw-operation. For each diode laser the optimum duty cycle is to be determined. Even in the case of strong light pulses, the diode lasers must be operated in such a way that the coherence and the transverse fundamental mode are maintained. This is a strict requirement which does not permit an optionally strong pulsing of the diode.
(2) The advantage of the multicomponent LDA with transient recorder in the mode of operation "coherent sampling" is to be seen in the fact that only a single device is necessary for all three components. This would lead to a drastic cut in the expense of a 2D or 3D-LDV. The technical requirements are very high, as a large analog bandwidth is necessary and the sampling frequency must be adjustable by means of an external time-base. In addition, when pulse synchronization as shown in Fig. 6 is selected, the scanning rate increases twofold per channel. At common velocities, sampling rates of several hundred MHz are easily reached.
(3) The receiving pulses need no longer be filtered so that high-pass and low-pass filters are no longer necessary. As generally high pulse frequencies are required and the pulses have short rise times, amplifiers with a very large bandwidth and high upper frequencies must be used. This is why the authors have developed receiving modules with bandwidths of approximately 400 MHz. The digitized signals are Fourier-transformed in a computer and digitally filtered with software so that no disadvantage results with respect to increased noise due to high bandwidths.
(4) The individual velocity components can be measured completely separately without crosstalk of the channels also at the same wavelength. The transmitting and the receiving head can be designed very small and compact (miniaturization).
(5) For the multicomponent LDA with "pulse integrator", too, the advantages of the miniaturized construction and the complete separation of the channels can be used and conventional processors employed for signal evaluation. The use of gated amplifiers in front of the integrators may reduce background light in case of synchronized operation with light pulses.
(6) A well designed wavelength stabilization of the diodes is required and an appropriate dc-bias current has to be applied to the diodes as it has been pointed out by Dopheide and Faber (1990). The application of short current pulses will yield a drift of wavelength, however, this seems not so important for most applications as preliminary experiments have shown.
10 Concluding remarksIt has been shown that the high-frequency pulsation of diode lasers offers considerable advantages as compared with cw operation. If the output power averaged over the time increases in pulse operation, the "pulse integration" can be used for evaluation. When transient recorders are used for electronic signal evaluation - "coherent sampling" -, the advantage of the improvement of the SNRs by the increased light power in the individual pulse can be fully used. As a consequence of the sequential pulsing of the diode lasers in multicomponent LDAs, the individual channels can be measured with only one photodetector without crosstalk and only one transient recorder is required.
At present the optical output power of single-stripe diodes is limited to approx. 100 mW so that for backscatter arrangements the application for cw operation is limited as well. Pulse operation can increase the optical output power,and in connection with the high quantum efficiency, such an arrangement can yield more or less the same SNRs as a 500 mW gas laser LDA with photomultiplier.
The chance of miniaturization of multicomponent LDA diode laser sensors and the simultaneously improved SNR due to pulsing and due to high quantum efficiency of the photodiodes will offer new applications, e.g., in wind tunnel research. Such a sensor can be mounted, e.g., at an arm of a computer controlled roboter machine and positioned without disturbing the flow.
It is certainly easy to forecast that laser diode development will continue to make significant progress in the future. For this reason it seemed to be of importance to us to examine possible improvements of semiconductor LDAs. But already today it is worthwhile to think about diode pumped solid-state lasers. These lasers certainly would be used best in frequency-doubled mode. Since frequency doubling is a non-linear process, the benefits described in the present paper will probably be soon surpassed by these lasers yielding a high power visible laser beam. Furthermore, the idea of pulsing a phase-coupled diode laser array whichin cw operation furnishes already 1000 mW opens aspects which are of interest for the future optical flow measurement. HF-pulsing of phased diode arrays which are used as light source in the "laser array velocimeter" as described by Strunck et al. (1989) will improve the performance of such a sensor too.
Current research at PTB regards the realization of a 2D-LDA in the "coherent sampling" mode and wave length stabilization/drift applying HF-pulsation.
Wilmer J.Sánchez
V-19358601
Seccion 1
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