Sequential detection technique to measure the shape of short THz pulses in the presence of a large jitter in the trigger signal .

J.N. Hovenier, R.W. van Es, T.O. Klaassen, W.T. Wenckebach, F. Klappenberger, M. Kratschmer, S. Winnerl, E. Schomburg, G.M.H. Knippels and A.F.G. van der Mew-.

Abstract: The possibilities and restrictions for the use of the simple experimental method - differential electronic gating (DEG) - developed to determine the shape of repetitivc subnano- second THz pulses in the presence of a large jitter in the trigger signal are investigated. This method a modification of the recently reported differential optical gating method, is based on subpicosecond electronic gating employing a high frequency sequential oscilloscope. AS the ultimate test, the shape of l o p s FWHM THz pulses from the free electron laser FELIX has been measured.

1 Introduction

Nowadays, many techniques are available to measure the shape of repetitive optical pulses as short as a few femto- seconds. These have in common the use of non-linear optical effects such as frequency doubling, frequency sum or difference generation, 2-photon absorption, etc. to determine the intensity autocorrelation of thc pulse. With the many excellent non-linear optical materials available, the shape of even low intensity optical signals can be determined conveniently using such a sequential scanning technique. An essential prerequisite for such a method to be successful is the repetitive nature of the optical signal, both in shape and in time. As most often these very short pulses are generated by CW mode-locked lasers, these conditions are easily fulfilled.

Some work has been done using such techniques also at longer wavelengths. For instance, usin two-photon induced absorption across the 590cm-' bandgap of p-type HgxCd,-,Te for ~ ~ 0 . 2 0 9 , the width of (sub-) picosecond pulses of the Dutch free electron laser FELIX in the 2 0 4 0 p m wavelength range has been studied [ I ] . The use of this method is, however, limited to a maximum wavelength of about 40 pm, set by the minimum bandgap that can be 'engineered' reliably by varying the Hg concentration.

0 IEE, 2002 IEE f'mceeding.? online no. 20020260 Dol: IO. l049lip-opt:20020260 Paper first received 10th October 2001 and in revised form 24th Janiiaiy 2002 J.N. Hovenicr, R.W. van Es. T.O. Klaasscn and W.T. Wenckehach are with the Depanment of Applied Physics, DclA University of Technology, PO Box 5046, 2600 GA, The Netherlands F. Klappenberger. M. Kriitschmei, S. Winnerl and E. Schomburg are with the lnstitut fur Experimentelk und Angewandte Physik, Universitat Regensburg, Germany G.M.H. Knippels and A.F.G. van der Meer are with thc FOM Institute for Plasma Physics, Edironharn 14. 3439 MN Nicuwegein, The Netherlands

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Recently, it was demonstrated that a GaAs/AIAs snper- lattice detector could be used as an autocomelator for THz pulses in the 1-7 THz frequency band [2].

A problem for a more general application of non-linear techniques at far-infrared wavelengths is caused by the steep decrease of non-linear optical susceptibilities of materials with increasing wavelength. Evidently, for the study of very short pulses in the far-infrared development of other detection techniques should be useful. Two such combined 'creation and detection' techniques for THz pulses already exist and are widely used. They are based on the use of high intensity subpicosecond visible or near- infrared optical pulses to create very short electromagnetic bursts containing broadband THz radiation, and use the same optical pulses for their detection through either optical or electronic gating [3, 41. As this detection is coherent, both phase and amplitude of the electric field of this THz burst is determined. A direct consequence of such a 'combined' technique is the intrinsic perfect synchroni- sation between the broadband THz pulse and the gating pulse, resulting in a very good time resolution. These techniques yield a very good sensitivity, which is also due to the very high repetition rate of the pulses; typically a l00MHz mode-locked Ti:sapphire laser is used as the excitation source.

However, recently other sources for the creation of short pulses in the THz region have been developed such as the free electron laser and the mode locked p-Ge laser [5] . As these THz pulses do not rely on short pulse optical pumping, serious detection problems arise. With such THz carrier signals no coherent detection is possible, and the much lower repetition rate of these sources leads to a smaller signal to noise ratio for the detection. Moreover, gating synchronisdtion can be problematic. For a proper performance of a gated detection, a very well defined time relation between THz pulse and gating pulse has to be established. If the THz carrier pulse and the gating pulse are created by independent sourccs, the time jitter between them constitutes the largest problem. It is evident that the temporal resolution with which the THz pulse can he determined is set by the amplitude of this time jitter.

99

Recently we developed the ‘differential electronic gating’ method (DEG) [6], based on the earlier published ‘differential optical gating’ method (DOG) [7], which enables the study of THz pulses with a good time resolu- tion, also in the absence of adequate synchronisation hetween THz and gating pulse. This technique does not rely on optical, but rather on electronic, gating.

2 Principle of differential gating technique

In the differential electronic gating technique the optical- signal is first detected by a fast detector and the electronic signal is subsequently split equally over two coaxial cables (see Fig. I ) . Both electronic signals are now electronically gated and recorded at slightly different times I and r + A t . So similar to the DOG technique, both the intensity I and the time derivative of the intensity F(I) of the signal are measured simultaneously:

[I(I + A t ) - I ( t ) ] - [I(I + A t ) +/(I)] I =

2 F(7) =

AI Consequently, instead of data on intensity as a function of time, /(I), as in a conventional sequential gating method, now data on the time derivative of the intensity as a function of intensity, F(0, are collected. Such a data set will not be influenced by time jitter in the trigger, as the time as an explicit variable has been eliminated. However, just as for all other ‘sequential’ techniques, good results can he obtained only with a good shot to shot reproduci- bility of the THz pulse shape.

The actual pulse intensity as a function of time can now he reconstructed by integration along the closed contour in the F-l plane:

To keep the integral well defined the function F(I) must not contain any zeros. This condition will pose probleins for even the simplest pulse shapes, since the dcrivative will always be zero at the pulse maximum. Therefore, it is possible to reconstruct only the rising and falling edges of a pulse shape. The exact shape of the top of the signal can be found through inspection of the relative nuinher of maximum intensity/zero derivative data points (scc [7] for a detailed discussion of this problem).

h Fig. 1 4-channel, 34 FHz digitiiing oscilloscope is uscd

100

Experimental serup for DEG method

In the case that pulses with different shapes are present within the time window of ohscrvation, they all will contribute to the data set, and yield their differently shaped curves of F against 1. They can be distinguished by thcir difference in shape. However, if these different pulses do not overlap, the time delay between them cannot be determined unambiguously. A great advantage of this method, above autocorrelation techniques, is the property that any asymmetry of the pulse shape is preserved.

Because in this technique it is the elcctronic signal, resulting from the optical detector, that is manipulated, it is clear that this detection method can also he applied to repetitive signals of pure electronic origin.

3 Experimental setup

The measurement setup, shown in Fig. I , used for the DEG technique is extremely simple. The optical signal is focused on a fast room temperature quantum heterostruc- ture detector [2] and its video output is split by a Pico- second Pulse Labs Model 5335 high frequency power splitter. One signal is delayed with respect to the other, simply using two coaxial lines with slightly different lengths. The signals / ( 1 + A f ) and / (I) are now measured at the same time on two different channels of an HP 541208 gating oscilloscope with 54123A front-end, featur- ing a 34 GHz bandwidth and a 200 fs electronic gate width. The knowledge of the exact value of the time delay between the channels is very important, as it determines directly the timescale of the reconstructed pulse. This delay is measured very conveniently by sending the scope’s TDR pulse, which has a steep rising edge, into the splitter, and comparing the time delay of the signals on the two channels. At the same time, also the accuracy o f the power splitting, equally important for this technique, can be checked.

4 Results: electronic pulse

To further investigate the necessary experimental condi- tions for a correct application of this technique, we have used a well defined, reproducible electronic signal with an FWHM of the order of 300 ps from an externally triggered electronic pulse generator. By adding a noise generator signal to the external trigger input, an artificial time jitter in the pulse delivered by the generator is created.

One set of data {l(t) , ( + A t ) } is taken at each pulse, using a normal sequential data collection routine of the oscilloscope, with, in this case, a I ps stepwise increase of the time delay between external trigger and electronic gate and a gate width of 0.25 ps. The time delay between the two channels has been set at A I = 12.4 ps. The results, for the case that there is no jitter in the trigger signal, are shown in Fig. 2a; the data from the two ‘channels are distinguished by solid and dotted lines, respectively. In Fig. 26 these data are plotted as F against I , and the reconstructed pulse shape is given in Fig. 2c. The FWHM of this reconstructed pulse equals 2 IO ps.. which compares very well with the value of 205 ps as obtained froin the direct signal in Fig. 2a.

To show the influence of the choice of the time delay AI between the channels, in Fig. 3 the corresponding results are shown for At=230ps. As this value is of the same order as the FWHM, it is too large to yield a correct derivative. The F-I curve therefore shows a peculiar shape, and the FWHM of the reconstructed pulse equals 410ps, considerably larger than the 3 I5 ps FWHM seen in Fig. 3a.

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b

0: " 200 ' 400 a " 600 800 to00 time, ps

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Fig. 2 wiihoui jit/er A I = 1 2 . 4 ~ 5 u Data from two channels b F-l contour (bold dots are 'avcnged' points used far integration procedurc) c Reconstructed pulic

Results of detennbrolion of shape of electronic puhe

It must be mentioned that if A i is chosen to be very small compared to the FWHM , the primary signal may he rather noisy because the differences between I(/) and I ( t + A i ) can be of the order of the signal noise. Proper signal averaging of the F-l curve is found, however, to result in a correct reconstruction of the shape. A clear drawback of such a choice is the unnecessary increase in the data acquisition time.

For the experimental results shown in Fig. 4, an addi- tional noise signal was added to the signal triggering the pulse generator, to cause a time jitter between pulse and the regular trigger used for the data collection. A very 'noisy' sequential data trace is observed, with only part of the rising edge of the pulse discernable. The F-I plot, however, shows the signature of a single pulse; the reconstructed

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200 400 600 800 1000

time. ps , c

Fig. 3 Results of derern!inution of shape of elecmnic pulse wilhoirr jitter A1=230 ps a Data from two channels b F-I contour c Reconstructed pulse

pulse in Fig. 4c has an FWHM equal to 350ps, only slightly larger than the 315 ps of the actual pulse. It is evident that this procedure of pulse reconstruction works properly as long as At is chosen correctly.

5 Results: THz pulse

The first experiment using THz pulses and the quantum heterostsucture detector was performed using the output of our mode-locked p-Ge THr laser [ 5 ] . Earlier experiments using a 6GHz bandwidth single shot 7250 Tektronix oscilloscope proved that this source was able to produce 60 ps FWHM pulses. In view of the 50 ps overall rise time of the electronic system used, it seemed conceivable that

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the actual pulse width would he even less. In this experi- ment we used a 40 ps time delay between the two channels, and the optical signal itself was used as a trigger. In Fig. 5a the sequential data from two channels are shown, whereas in Fig. 5b the F against I plot of a large data set is given. The reconstructed shape, together with a pulse shape obtained from an averaged sequential scan, is shown in Fig. 5c.

Although the FWHM value of the reconstructed pulse is ahout equal to that of the averaged direct scan signal, the overall shape is quite distorted. Clearly this DEG method does not work properly in this case. The reason for that is

102

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time. ns a

-3 0 0.1 0.2 0.3 0.4 0.5

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Fig. 5 TH: pulse 12 I75 pm; A f = 4 0 ps a Sequential data with optical trigger h F-1 contour

Re.su1i.s of deterntinotion of shape of mode locked p-Ge

c Reconstructed pulse (solid line) and averaged sequential signal (dotted line)

the strong shot to shot variation of the intensity and shape of the pulse, now known to he due to strong beating of transverse modes of the laser [SI.

To avoid problems with strong shot to shot variations of the pulse shape, experiments were performed using the very short THz pulses from the Dutch free electron laser for infrared experiments (FELIX). Its stable and wave- length tunable output consists of about 4 ps long macro- pulses, with a repetition rate of 5 Hz, that contain a train of micropulses with a repetition frequency of either 1 GHz or 25 MHz. We have used micropulses of ahout I O ps FWHM at a wavelength of approximately 150 pm and a micropulse repetition frequency of 1 GHz.

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The first experiment was performed with the FELIX cavity detuned such.-as to give a double micro pulse. Because of the limited optical power available in this experiment, a Picosecond Pulse Labs model 5828 amplifier with a 28 ps rise time was used to amplify the signal from the detector. As trigger we used the macropulse trigger of FELIX, which was found to have a jitter of about 1 ns ~ much larger than the micropulse width.

The time delay between the two channels was set at Ar = 7 ps. and the gate width was 0.2 ps, with a step size of 1 ps. The sequential data are given in Fig. 6a; no pulse can he observed at all. The F-1 plot in Fig. 6b, however, shows two different contours, indicating the presence of two unequal pulses. The reconstructed pulse shape in Fig. 6c is compared with a single shot signal taken with thc 6 GHr real time bandwidth 7250 Tektronix oscilloscope. The

0.4r

> E i

delay, ns

double pulse structure of the signal is perfectly reproduced because of the overlap hetween the two pulses, no amhi- guity in the time interval between the pulses occurs. The slight difference between the two shapes is probably mainly related to the longer rise time of the 6GHz oscilloscope.

The second test ofthe DEG methbd, with a near ultimate time resolution, considering the time constants of the gating scope, was performed with the FELIX cavity tuned properly to yield a single micropulse with an

-FWHM of --lops. Because now the full optical power was used, no amplifier was necessary. The detector was connected with a short cable directly to the power splitter. The FELIX micropulse trigger, with a jitter of - I O ps, was

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25

20 15

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Fig. 6 Resulrs of dereniiinution of shupe of FELIX double pulse ;.= 150 pn; Ar-7 ps o Sequcntial data with inacropulse tripgcr b F-1 contour c Reconstructed pulse (solid line) and single shot 6 GHz bandwidth signal (dotted line)

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Re.wlls ofdeferminotion of shape ofFELIX single pulse

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used to trigger the oscilloscope. The experiment was performed with a time delay between the channels of I ps and a gate width of 0.2 ps, and the sequential steps were 0.2 ps.

In Fig. l a the sequential intensity data of one of the channels is given; a noisy pulse with an FWHM of 25 ps is observed. The reconstruction of the pulse using the F-l plot (Fig. 76) reveals a micropulse shape with an FWHM of I O f 2 ps. The fall time of the asymmetric pulse (an intrinsic property of FELIX) is 1 ps.

6 Conclusion

We have shown that the use of the differential electronic gating method (DEG) enables the detection of repetitive picosecond THz or electronic pulses, even when the jitter of the synchronisation signal is much larger than the width of the pulse. The time resolution of this method is limited by the electronic rise time of the optical detcctor and the other data acquisition electronics; in the present system the minimum rise time is found to be 5 7 ps.

Crucial conditions to employ this method successfully are: good shot to shot pulse stability and the use of a time delay between the two channels that is properly matched to the pulse width. Inspection of the shape of the F-1 curve can be used to inspect the right choice of the time delay.

7 Acknowledgments

The authors thank M.I.W. Vermeulcn from the Delft Inter- faculty Reactor Institute for the use of the 6 GHz oscillo- scope and help with the data acquisition.

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References

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S~~ppdrlurrices Micrmtmct.. 1999. 25. p. 57-60 ZHANG, X.-C.. JIN. Y., and MA. X.F.: 'Coherent measurement ofTl.1~ optical rectification from elcctro-optic crystals', Appl. Phy.7. Lett., 1'192, 61 pp. 2764-2766 VAN EXTER, M.. and CRISCHKOWSKY. D.R.: 'Characterization of an omoelectmnic tenhenr bcam wstcm' IEEF Trarrms. Microw Thcwv T&. 1990.38, p. 1684 HOVENIEK, J.N., DIEZ. M.C., KLAASSEN, T.O., WENCKEBACH, W.T.. MURAVJOV, A.V, PAVLOV. S.C.. and SHASTIN. VN.: 'The p- Oe THI layer nmnciiies under nulsed and mode locked meration'. .~ ~~~~~~~~~

IEEE Trans. Mi&i,,~Throni Tich:. 2000, 48, pp. 670-676 HOVENIEK, J.N., VAN ES. R.W., KLAASSEN, 1.0.. WENCKEBACH. W.T., KRATSCHMEK, M.. KLAPPENBERGER, F~ WINNERL. S . KN1PPFI.S OM.H 2nd VAN DER MEER.

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A:F.C.;~'Differentldi electron&~&ing: a method to measure the shave of short THr vulsees with a ooorlv define