ULTRA-WIDEBAND ELECTRICAL PULSE GENERATOR USING PHOTOCONDUCTIVE SEMICONDUCTOR SWITCHES

Transcription

1 ULTRA-WIDEBAND ELECTRICAL PULSE GENERATOR USING PHOTOCONDUCTIVE SEMICONDUCTOR SWITCHES B. Vergne ξ, V. Couderc and A. Barthélémy IRCOM, 123 avenue Albert Thomas Limoges, France M. Lalande and V. Bertrand IRCOM / IUT GEII, 7 rue Jules Vallès Brive, France D. Gontier CEA Bruyères-le-Châtel, France Abstract We achieved the generation of unipolar and bipolar ultra-wideband electrical pulses by using a photoconductive device. It relies on a doped silicon substrate which is used in the linear mode with less optical energy than usually published. This running mode allows the synchronisation of several sources with a timing jitter less than 5ps. However it permits to control the low frequency components of the bipolar generator by means of the synchronisation of two photoconductors. So we generated unipolar pulses from 43ps to 300ps of duration (Full Width at Half Maximum: FWHM) with peak voltage up to 10700V and bipolar pulses between 200ps and 450ps of cycle length with 3000V of peak voltage. I. INTRODUCTION Nowadays the generation of ultra-wideband electrical pulses by means of photoconductive phenomenon is spreading out. Short pulses with very high peak voltage (several kilovolts) had already been performed. The most used photoconductive semiconductor for this photogeneration is GaAs which allows switching of high power pulses with a low amount of optical energy. It can be done by avalanche and lock-on phenomenon [1]. However this mode does not allow the synchronisation of several switches with a few picoseconds precision [2]. Nevertheless other materials or devices can be used to generate high power pulses. In this paper, we propose the generation of picoseconds unipolar and bipolar pulses by using the linear mode. The main limitation of this king of technique is the high optical energy needs to perform energetic pulse switching, i.e. 10kV electrical pulse needs several tens of millijoules of optical energy [3]. However we realize the generation of electrical pulses between 43ps to 400ps (FWHM) with maximum peak voltage of 10700V and by using 1.2mJ of optical energy, which is 10 to 100 times lower than the energy currently published. These specifications are obtained by using the innovative technology of the French Atomic Energy Commission (CEA) based on doped silicon substrate. The important benefit of linear mode is the low timing jitter which is involved in the switching process. Also bipolar pulses have been generated. It can be obtained by means of different devices: impulse forming networks, fast recovery diodes and shorted transmission lines, spark gap switched transmission lines or radiofrequency MOSFET switched capacitors. However, only low voltage switching has been demonstrated in picoseconds duration. Higher voltage switching was demonstrated but with longer pulse, i.e. 10ns to 100ns of duration. In this study we present experimental results on picosecond kilovolt bipolar pulses generated by using the well known Frozen-Wave Generator [4]. We demonstrated that the low frequency content of the output pulse can be easily controlled with the commutation delay between the switches constituting the generator. The total pulse duration can be modified to obtain pulses with spectral bandwidth up to 6GHz. The use of innovative switches operating in linear mode permitted to realise temporal and also spectral shaping of the generated pulses. The applications of ultra-wideband electrical pulses are many full, i.e.: phased antenna array, radar cross section measurement, ground penetrating radar, ultra-wideband systems characterisation, high speed electrical device, optoelectronic synchronisation and electroperturbation of biological cells [5]. This work was supported by DGA (French Procurement Agency) of the French Ministry of Defense. ξ bertrand.vergne@ircom.unilim.fr

2 II. EXPERIMENTAL SETUP The low optical energy consumption of our generators permits the use of large silica core fibres for generator powering. In example, the generator can be positioned close to the antenna in order to minimize the dispersion of electrical pulses before emission. In order to test our home-made generators, we used the experimental setup described in figure 1. III. UNIPOLAR AND BIPOLAR GENERATORS DESCRIPTION A. Unipolar generator The unipolar generator is composed of a photoconductive semiconductor coupled with a capacitor, figure 2. Figure 2. Ultra-wideband generator s heart description. Figure 1. Experimental setup used to generate picosecond pulses with high peak voltage. We used a 1064nm laser source which delivers 25ps pulses at a repetition rate up to 20Hz and with energy per pulse as high as 50mJ. The choice of this source was led because of the good optical absorption of the photoconductive semiconductor at 1064nm. The switched signal is recorded by means of a sampling oscilloscope with a bandwidth of 20GHz (Tektronix CSA8000 with 80E04 sampling head). This oscilloscope imposes a 20ns delay between the trigger signal and the recording process. Then we realized an optical delay line using large core silica fibre or air. We used a 15m long optical fibre with 1500µm of core diameter. A selective coupling in the optical fibre permitted to minimize modal dispersion effect. So, we performed the transmission of energy up to 2mJ with less than 20ps of temporal broadening of the input pulse. In these conditions the output pulse width was only 45ps. The photoconductor is biased by means of a DC power supply and does not require pulsed system as commonly used for high power pulses generation. The photoconductive semiconductor exhibits a high off state resistance and is insensitive to parasitic visible light. So we do not need to pay attention to thermal run-away. In case of the bipolar generator or the synchronisation of two unipolar generators another delay line is inserted after the trigger beam splitter. Also two adjustable air delay lines are used to synchronise the optical pulse before they reach semiconductor switches. It is realized in a radial geometry with a small window which permits to light the photoconductive active device. The output impedance of our device is 50 Ohms because of the output end connector (type N) and the radial geometry. Before illumination, no output signal is generated. In the same time the capacitor is continuously charged. The photo-excitation of the switch produces a sharp increase of the conductivity of the semiconductor. The duration and the temporal shape of the current pulse depend significantly on the optical excitation, the carrier life time, the bandwidth of the generator components and the stored electrical energy. B. Bipolar generator The bipolar pulse generation relies on the frozen wave generator principle [6], figure 3. Figure 3. Frozen wave generator principle and possible pulse shape depending on the two optical trigger delay. It consists on a charged line terminated on both extremities by a photoconductive semiconductor switch. The first one controls the output of the device while the

3 second one permits to realise a short cut which invert the polarity of the forward signal. When the switches are open the transmission line is charged through a resistor by using a continuous high voltage power supply. Then we can choose to generate to the load a square signal with duration of twice the transmission line length or a bipolar pulse. The square signal can be obtain by closing the output switch while the other one is open. The bipolar pulse is obtained by closing the two switches. If both switches are closed simultaneously a balanced monocycle can be observed at the output end of the generator. Else an unbalanced bipolar pulse is produced. By controlling the delay between the two optical pulses, we are able to adjust the low frequencies components in the output spectrum. IV. RESULTS A. Unipolar pulses By using the CEA s technology, we achieved the following high power performances: pulse length of 300ps, rise time of 130ps and amplitude of 10700V. Moreover, the input optical energy required for pulses switching was 1.2mJ. From our knowledge, these values are the lowest currently obtained in the linear mode for an output peak voltage of more than 10KV. The shape and the normalized spectrum of our pulses are shown on figure 4. Figure 5. Proof of the linear process involved in switching. The high stability of our generators in term of peak voltage produced is due to the limited electrical stored charges and the optical energy behaviour of the pulse shape. Then high reproducibility of output pulses was obtained for energy level between 1050µJ and 1350µJ (see figure 6). Figure 4. Shape and computed spectrum of our generated pulses. These measures had been obtained by polarising the under test generator with a voltage level of 16kV. So the electrical efficiency is about 67%. This value depends on the electrical stored energy, the electrical rise time of the device and electrical losses which are estimated close to 7%. The normalized spectrum shows a continuous distribution through frequencies up to 3GHz. This large bandwidth permits radar cross section experimentation and electromagnetic target characterization. After a complete study of the optical pulse duration impact on the electrical pulse shape, it seems that the generator s heart, materials and geometry, act as rise time limitation factors. Also one study permits to prove the linear process, figure 5. Figure 6. Pulse shape versus optical energy. A stable pulse shape is obtained for energy values between 1050µJ and 1350µJ. As a consequence the pulse amplitude stability is better than 5% and the pulse length is also very stable. Better output pulses in term of duration were obtained for lower output electrical power. These generators are based on the same architecture but with different capacitor geometry and a shorter photoconductive switch. Only 20µJ are needed to switch pulses of 43ps with peak voltage of 150V with a bias voltage of 1kV. In this configuration a rise time as short as 32ps was demonstrated (see figure 7).

4 was adjustable between 10V to 1kV. The two optical trigger pulses are obtained by splitting the original laser pulse on two adjustable delay lines. Each optical pulse is guided by a multimode fibre toward one photoconductive device. Our first results on such configuration are shown on figure 9. Figure 7. Ultra-short pulse: 43ps FWHM and a rise time of 32ps (spectrum is computed from smoothed data). The little oscillations observed on the pulse shape, figure 4, or at the pulse end, figure 7, come from unmatched line adaptations between capacitor, semiconductor s electrodes, semiconductor itself and the output N connector. These reflections decrease the maximum amplitude of the signal by dispersing the electrical energy away. Then the rise time is also longer. So the distance between the capacitor and the semiconductor was properly optimised to obtain fast rise time and low parasitic reflection. The low dispersion of experimental points is a proof of the low timing jitter and high reliability of this device, i.e. circular points on figure 4 (500 points) and figure 7 (4000points). A timing jitter of less than 5ps and a pulse to pulse distortion less than 5% was performed [7]. An example of two generators synchronisation is given on figure 8. The traces had been recorded by using two channels of our sampling head and by splitting the optical pulse. Figure 9. Example of generated pulses and their associated computed spectrum using microstrip line configuration. The cycle duration is 400ps. The two semiconductors needed less than 20µJ of optical energy per switch to be activated and the bias voltage was 1kV. Also we demonstrated the advantage of the optical command for the control of the bipolar pulses equilibrium with a very good reliability and reproducibility (timing jitter less than 5ps). So with such generator, we are able to deliver square signal or a bipolar signal with a defined balanced or unbalanced bipolar pulse. As a consequence we control the spectral content of low frequencies with the delay between the two optical commands on the photoconductive switches. Shorter bipolar generators can be realized. In this case, the charged line do not defined any more the bipolar pulses duration (200ps). The parasitic capacitance of the photoconductive device and high voltage entrance point must be taken into account. As an example the bipolar pulses on the figure 10, came from a micro-strip line of 3mm. Figure 8. Example of two high voltage synchronised generators. On the right side, the two experimental curve widths do not exceed 5ps, which correspond to the cumulated jitter of the trigger device, the sampling scope and one generator. So we produce unipolar electrical pulses from 43ps to 300ps (FWHM), with a peak voltage between 150V to 10700V and a bandwidth of 18GHz and 3GHz (-20dB) respectively. B. Bipolar pulses Using the bipolar device, we realized multiples switching experiments. The first one was carried out on a printed circuit board. The storage system was a 50Ohms microstrip line. The continuous bias voltage of the device Figure 10. Picoseconds bipolar pulses generator realized on PCB with a microstrip configuration. The cycle duration is 200ps. Nevertheless the peak-to-peak voltage decreases rapidly for shorter bipolar pulses see figure 11. Besides the parasitic signals behind the main bipolar pulse is 3 times shorter than those observed with the generator

5 producing bipolar pulse of 400ps. The amplitude of the parasitic signal is less than 10% of the main pulse. Figure 11. Examples of oscillations behind the main bipolar pulse. On the left is presented the 400ps monocycle and on the right the 200ps monocycle. To increase significantly the total output voltage of the generated pulses, we realized a new generator based on a 50Ohms radial line. The length of the hyper frequency line is quite similar to the 400ps generator. The optical switches have also the same constitution. Only the length and the diameter of the doped silicon are a little bit larger to be easily polarised with higher DC voltage. In this configuration, the generated bipolar pulse had peak-topeak amplitude of 3kV with cycle duration of 450ps. The bias voltage was fixed to 5kV which represented a switching efficiency of 60%. The two photoconductive semiconductors were optically triggered with a total optical energy lower than 1mJ. The profile and the spectrum content of the electrical pulses can be modified by using temporal delay between the switching of the two switches. A square pulse, an unbalanced and a balanced bipolar pulse were obtained and are shown in the figure 12. Figure 12. Picoseconds high voltage monocycles realized in a radial geometry. The cycle duration is 450ps and the computed spectrum is shown on the right. So we produced bipolar pulses between 200ps to 450ps with 250V to 3kV peak-to-peak voltage and a bandwidth of 2GHz to 6GHz at -3dB. V. CONCLUSION Generation of short high power electrical pulses had been demonstrated in the linear mode with 10 to 100 times less optical energy than usually published. We demonstrated the ability of such generators to be associated in a synchronized array with a precision lower than 5ps. On the other hand, we demonstrated the feasibility of medium and high voltage bipolar pulses generator using optoelectronic devices based on doped silicon substrate. We shown that the linear mode of the switching process combined with an optical delay between the switching processes of the two switches could modify significantly the spectral content of the pulse spectrum. We obtained balanced bipolar pulses up to 3kV with cycle as low as 200ps of duration. The only limitation in ultra-wideband high voltage pulses generation with our semiconductor component is its parasitic capacitance, its response time and the dielectric high voltage strength. The high reliability and large bandwidth of these kinds of electrical sources permit to use them in imagery systems such as UWB Synthetic Aperture Radar or high voltage device synchronization and the measurement of radar cross section [8]. Also they can be employed in electroperturbation of biological cells. VI. REFERENCES [1] F. Davanloo, H. Park, R. Dussart, M.C. Iosif and C.B. Collins, Progress in the developpement of stacked Blumlein pulsers commutated by photoconductive switches, Conference Record of the 1998 Twenty-Third International Power Modulator Symposium, pp , [2] W.C. Nunnally, High-power microwave generation using optically activated semiconductor switches, IEEE Transaction on Electron Devices, vol. 37, no. 12, pp , [3] A. Antonetti, M.M. Malley, G. Mourou and A. Orszag, High power switching with picosecond precision: applications to high speed Kerr cell and Pockels cell, Optics Communications 23, 1977, pp [4] C. S. Chang, M. C. Jeng, M. J. Rhee, C. H. Lee, A. Rosen and H. Davis, Direct DC to RF conversion by picosecond optoelectronic switching, IEEE MTT-S International Microwave Symp., 1984, pp [5] A. Kuthi, M. Behend, T. Vernier and M. Gundersen, Bipolar nanosecond pulse generation using transmission lines for cell elctro-manipulation, Power Modulator Conference, [6] J. M. Proud and S. L. Norman, High-frequency waveform generation using optoelectronic switching in silicon, IEEE Transactions on Microwave Theory and Techniques, vol. 26, no. 3, pp , [7] A. Antonetti, M.M. Malley, G. Mourou and A. Orszag, High power switching with picosecond precision: applications to high speed Kerr cell and

6 Pockels cell, Optics Communications 23, 1977, pp [8] G.M. Loubriel, F.J. Zutavern, A.G. Baca, H.P. Hjalmarson, T.A. Plut, W.D. Helgeson, M.W. O Malley, M.H. Ruebush and D.J. Brown, Photoconductive semiconductor switches, IEEE Transaction on Plasma Science, vol. 25, no. 2, 1997, pp