GB2191007A

GB2191007A - Analyzing short electrical pulses

Info

Publication number: GB2191007A
Application number: GB08613142A
Authority: GB
Inventors: Peter Paulus; Thomas Pfeiffer
Original assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Current assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Priority date: 1986-05-30
Filing date: 1986-05-30
Publication date: 1987-12-02
Also published as: GB8613142D0

Abstract

Short electrical pulses can be analyzed using a purely electrical correlation measurement technique. A repetitive electrical signal to be analyzed is applied to input ends of a pair of transmission line sections (14 and 16). One section (14) having a fixed length of path, the other (16) having a variable length of path. The signals appearing at the output ends of the transmission line sections (14 and 16) are combined and applied to power sensitive detector means (20). The transmission line sections are coaxial lines. The outer conductor is formed by a metal block (24) and the inner conductor of the section of variable length (16) is U-shaped, the ends of which are received by the inner conductor of the fixed length section (14) telescopically. By varying the effective length of the section (16), a measure of the autocorrelation of the two signals is obtained. <IMAGE>

Description

SPECIFICATION An improved technique for analyzing short electrical pulses The present invention relates to improved techniques for analyzing short electrical pulses, i.e.

pulses with a duration in the order of picoseconds (ps) full width at half maximum (FWHM).

It is known to analyze ultrafast electrical signals by sampling oscilloscopes, whose temporal resolution is, however, limited by the finite aperture time of the sampling gate and by trigger jitter to about 40 to 50 ps. Improvements by more than one order of magnitude have been achieved by recently developed optoelectronic sampling techniques, see e.g. D.H. Auston "Picosecond Optoelectronic Devices", edited by Chi H. Lee (Academic Press, 1984), pp. 73-117.

Using the method of Auston et al., the signals of two photoconducting switches which are activated by two synchronized optical pulses are correlated. In this case, the finite response time of such a switch, caused by the capacitance of the gap in the microstrip-line, sets a lower limit to the attainable resolution. By this correlation technique correlation signals with a duration of 5.5 ps have been measured, see e.g. D. R. Bowmann, R. B. Hammond, R. W. Dutton, IEEE Trans. Electron. Device Lett. EDL-6, 502 (1985). Utilizing the considerably faster response of the electro-optical Pockels effect in Lira03 and employing femto-second (fs) optical pulses, Mourou et al. succeeded in resolving the risetime of electrical transients as short as 460 fs, see e.g. G. A. Mourou, K. E. Meyer, Appl. Phys. Lett. 45, 492 (1984).However, both these techniques need the synchronization of the electrical signal to be analysed with an optical pulse, which is used for the sampling of this signal.

In many cases, the shape of the ultrafast electrical signal to be analyzed is known so that only the duration and the amplitude are to be determined. The present invention provides a new purely electrical correlation measurement technique, which furnishes this information by applying the principle of second harmonic autocorrelation to the processing of fast electrical signals.

Thus, electrical pulses with a full width at half maximum (FWHM) of 26 ps and shorter can be resolved.

According to one aspect of the present invention there is provided a method of deriving information about repetitive ultrafast electrical signals, said method comprising the steps of dividing the signal into signal portions, delaying one signal portion with respect to the other, combining the mutually delayed signal portions, and measuring the combined signal portions.

According to a further aspect of the present invention there is provided apparatus for deriving information about repetitive ultrafast electrical signals, said apparatus comprising an input terminal, first and second delay lines having input ends coupled to siad input terminal and output ends, coupled together and to an output terminal, the delay of at least one of said delay lines being variable, detector means coupled to said output terminal.

According to an embodiment of the invention, a repetitive electrical signal to be analyzed is applied to the input ends of a pair of transmission line sections, the one of which having a fixed length the other a variable length. The signals appearing at the output ends of the wave guide sections are combined and the combined signal is applied to detector means.

The detector means may be a power-sensitive detector, as a high-frequency diode having a quadratic characteristic. Alternatively, a bolometer or any other device which is adapted to measure power at microwave frequencies may be used as the detector.

The transmission line sections are preferably coaxial lines. In a preferred embodiment, the outer conductor of these lines is formed by a metal block, and the inner conductor of the section of variable length has an U-shaped portion, the ends of which are telescopically received by fixed portions.

A not limiting embodiment of the invention will now be desctibed with reference to the drawings, in which Figure 1 is a schematic diagram of an embodiment of the invention; Figure 2 is a more detailed sectional view of a practical embodiment, and Figure 3 shows an autocorrelation signal obtained with the present invention.

The same reference numerals are used in Fig. 1 and 2 for similar eiements.

The measuring system shown in Fig. 1 comprises a pulse source 10, e.g. a pulse generator PG, having its output coupled to an input transmission line 12. The input transmission line is connected to the input ends of first and second transmission line sections 14, 16, of which at least one is of variable length. In the embodiment shown in Fig. 1 and 2, the transmission line section 14 has a fixed length while the transmission line section 16 has a variable length. The transmission line sections 14, 16 have their output ends connected with each other and through an output transmission line 18 to detector means which may comprise a high-frequency diode 20, which has a quadratic characteristic, and an integrating device 22 connected to the output of the diode 20.

The electrical voltage pulse to be measured is supplied to the input line 12 and split into two identical parts which are fed into the two separate transmission line sections 14, 16. The length of one of these lines is fixed, whereas the length of the other line can be varied thus introducing a variable time delay between the two signals. The signal from both transmission lines are recombined at the detector where the sum of the two signals arrives: V'(t)=V(t)+V(t-T) (1) By using a detector with a quadratic input/output characteristic the time averaged output signal S of the whole device in Fig. 1 is given by S=(1(2) > ce tV'2 > =2tV2(t) > +2 < V(t)-V(tz) > (2) where < . . . ... > denotes the time average. The signal S consists of two parts.The first term on the right side of Eq. (2) is independent of the time delay, leading to a constant background signal. The second term gives the well-known autocorrelation part of the signal to be measured.

As can easily be seen from Eq. (2) the two contributions yield a contrast ratio I(z=0)/l(z-co)=2/1 (3) if V(t) is assumed to be limited in time. It is well known from other autocorrelation measurements that the FWHM At of the signal V(t) can now be estimated from the FWHMAT of the second term in Eq. (2), where in most cases At~AT '2At holds, see e.g. E. P. Ippen, C. V.

Shank, in "Ultrashort Light Pulses", edited by S. L. Shapiro (Springer Series Topics in Applied Physics, Vol. 18, Berlin 1977), p. 83.

In principle, any power-sensitive device can be used as the detector, e.g. a microwave thermistor, or in the special case of small signals a microwave detector diode. For sufficiently small values of the input voltage V' the output current l(V') of such a detector diode can be approximated by l(V') =aV'+ bV'2 (4) where a and b are appropriate constants. After averaging the linear term in Eq. (4) can give rise to large additionai background signal due to the d.c. components of V'. In order to eliminate this term, a d.c. block has been inserted in front of the detector diode, in order to establish V' > =0. A microwave detector diode is preferred as detector means for the nonlinear detection of voltage pulses.

In order to achieve a large temporal resolution, the detector diode must have a large bandwidth. A suitable diode is the Wiltron model 70S50B diode, which is specified to have its -3 dB point at 34 GHz. Concerning the other components, the frequency dependent losses and the dispersion of the whole configuration should be minimized. Therefore in a practical embodiment shown in Fig. 2, all components with except to the detector diode 20 and, of course the pulse generator are integrated into a common metallic block 24 and coaxial, air-insulated lines with small diameter are used, where possible. The cut-off frequency of the device shown in Fig.

2 is about 38 GHz. The d.c. block is realized by splitting the center conductor 26 of the output transmission line 18 and by electrolytical oxidation of the resulting interface. One portion of the center conductor forming the interface may be made of aluminium. This yields a capacitance in the order of 20 to 30 pF, so that the lower cut-off frequency is about 50 to 80 MHz. The connection to the other components of the circuit is made by commercially available K-band SMA-connectors 28 and 30 (fc=40 GHz). The relative time delay between the two arms of this apparatus can continuously be varied over a range of 105 ps by sliding movement of a movable, U-shaped portion 1 6a of the coaxial line section 16.

The movable portion 1 6a consists of an U-shaped length of coaxial cable with bare outer conductor and protruding inner conductor. The outer conductor makes sliding contact with corresponding bores in the block 24 while the protruding ends of the inner conductor make sliding contact with an U-shaped inner conductor portion which forms the remainder of the transmission line 16 and the transmission line 14.

The adjustment of the effective length of the transmission line section 16 may be effected manually or by a stepping motor via a threaded shaft coupled to the movable part 16a.

If a resistor for terminating the output of the pulse source 10 is needed, this resistor can be realized by a body 32 of resistive material, as graphite, connecting a short length of the inner and outer conductors of the section 14 at its bent.

In order to test the performance of the device of Fig. 2, 26 ps pulses were generated by an optoelectronic switch. The switch was illuminated by light pulses of a synchronously pumped Rhodamine 6G dye laser with a repetition rate of 80 MHz, a pulse duration of 5 ps, and a pulse energy of 275 pJ. A d.c. voltage source and the shunt resistor 32 of 3 kilo-ohms, connecting the output of the optoelectronic switch to ground, produced a voltage of 25V. The output signal of the detector diode was measured and averaged by a conventional lock-in amplifier. The length of the variable section 16 was varied in small steps to produce a delay from -60 picoseconds to +40 picoseconds.

The resulting trace is represented in Fig. 3. As can be seen the shape of the signal is symmetrical with respect to T=0 and the contrast ratio amounts to 2:1, as predicted by Eqs. (2) and (3). The FWHM of the delay dependent part of the signal is T=36 ps. An independent optoelectronic autocorrelation measurement of the same pulse, employing the cross-like microstrip line configuration as described by Auston (I.c.) gives a FWHM of 34 ps. This is in quite good agreement with the results obtained by the present new technique. Assuming a Gaussian shape for the electrical pulses the FWHM t of the pulse is about 26 ps, using AT/At=V2.

In summary a novel, simple and low-cost technique for measuring short electrical pulses by direct autocorrelation is provided. This technique requires no ultrafast optical pulses and no trigger source. The extension to cross correlation measurement of two different electrical pulses is possible. Therefore this device may find widespread application in the analysis of electrical transients. Higher temporal resolution may be obtained by using improved coaxial transmission lines and millimeter wave detector diodes, which should be directly mounted in the correlator assembly, without using connectors. Both transmission lines and detector diodes are available today with cut-off frequencies in excess of 100 GHz, so that a temporal resolution of less than 6 ps seems to be feasible. A further modification is the use of electrically tunable delay lines instead of mechanically tuned ones. E.g.Schottky-contact transmission lines may be used as variable phase shifters. With these devices a fully integrated correlator is feasible involving modern MMIC technology.

CLAIMS 1. An improved method of deriving information about repetitive ultrafast electrical signals, said method comprising the steps -dividing the signal into signal portions, -delaying one signal portion with respect to the other, -combining the mutually delayed signal portions, -measuring the combined signal portions, and -repeating said delaying, combining and measuring steps with a series of different delays.

2. The method as claimed in claim 1, wherein said measuring step comprises the measurement of the power of the combined signal.

3. An apparatus for deriving information about repetitive ultrafast electrical signals, said apparatus comprising -an input terminal, first and second delay lines having input ends coupled to said input terminal and output ends, coupled together and to an output terminal the delay of at least one of said delay lines being variable, -detector means coupled to said output terminal.

4. The apparatus as claimed in claim 3 wherein said detector means comprises a powersensitive detector.

5. The apparatus as claimed in claim 4 wherein said detector means comprises a diode having a non-linear characteristic.

6. The apparatus as claimed in any of claims 4 to 6, characterized in that said detector means comprises a signal integrating device.

7. The apparatus as claimed in claim 4, wherein said variable delay line comprises a coaxial line having a movable U-shaped portion.

8. The apparatus as claimed in claim 4 or claim 7, characterized by a metal body shaped to form the outer conductor of a portion of the variable delay line, of the other delay line, an input transmission line coupling to the input ends of said delay lines, and of an output transmission line connecting the output ends of said delay lines to said output terminal.

9. The apparatus as claimed in claim 8, wherein one of said lines has a split portion with an interposed insulating layer forming a dc-blocking capacitor.

10. The method of acquisition of information about a fast electrical signal as described in the specification.

11. The apparatus for acquisition of information about a fast electrical signal as described with reference to any of the drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

of the detector diode was measured and averaged by a conventional lock-in amplifier. The length of the variable section 16 was varied in small steps to produce a delay from -60 picoseconds to +40 picoseconds.

The resulting trace is represented in Fig. 3. As can be seen the shape of the signal is symmetrical with respect to T=0 and the contrast ratio amounts to 2:1, as predicted by Eqs. (2) and (3). The FWHM of the delay dependent part of the signal is T=36 ps. An independent optoelectronic autocorrelation measurement of the same pulse, employing the cross-like microstrip line configuration as described by Auston (I.c.) gives a FWHM of 34 ps. This is in quite good agreement with the results obtained by the present new technique. Assuming a Gaussian shape for the electrical pulses the FWHM t of the pulse is about 26 ps, using AT/At=V2.

In summary a novel, simple and low-cost technique for measuring short electrical pulses by direct autocorrelation is provided. This technique requires no ultrafast optical pulses and no trigger source. The extension to cross correlation measurement of two different electrical pulses is possible. Therefore this device may find widespread application in the analysis of electrical transients. Higher temporal resolution may be obtained by using improved coaxial transmission lines and millimeter wave detector diodes, which should be directly mounted in the correlator assembly, without using connectors. Both transmission lines and detector diodes are available today with cut-off frequencies in excess of 100 GHz, so that a temporal resolution of less than 6 ps seems to be feasible. A further modification is the use of electrically tunable delay lines instead of mechanically tuned ones. E.g.Schottky-contact transmission lines may be used as variable phase shifters. With these devices a fully integrated correlator is feasible involving modern MMIC technology.

CLAIMS 1. An improved method of deriving information about repetitive ultrafast electrical signals, said method comprising the steps -dividing the signal into signal portions, -delaying one signal portion with respect to the other, -combining the mutually delayed signal portions, -measuring the combined signal portions, and -repeating said delaying, combining and measuring steps with a series of different delays.
2. The method as claimed in claim 1, wherein said measuring step comprises the measurement of the power of the combined signal.
3. An apparatus for deriving information about repetitive ultrafast electrical signals, said apparatus comprising -an input terminal, first and second delay lines having input ends coupled to said input terminal and output ends, coupled together and to an output terminal the delay of at least one of said delay lines being variable, -detector means coupled to said output terminal.
4. The apparatus as claimed in claim 3 wherein said detector means comprises a powersensitive detector.
5. The apparatus as claimed in claim 4 wherein said detector means comprises a diode having a non-linear characteristic.
6. The apparatus as claimed in any of claims 4 to 6, characterized in that said detector means comprises a signal integrating device.
7. The apparatus as claimed in claim 4, wherein said variable delay line comprises a coaxial line having a movable U-shaped portion.
8. The apparatus as claimed in claim 4 or claim 7, characterized by a metal body shaped to form the outer conductor of a portion of the variable delay line, of the other delay line, an input transmission line coupling to the input ends of said delay lines, and of an output transmission line connecting the output ends of said delay lines to said output terminal.
9. The apparatus as claimed in claim 8, wherein one of said lines has a split portion with an interposed insulating layer forming a dc-blocking capacitor.
10. The method of acquisition of information about a fast electrical signal as described in the specification.
11. The apparatus for acquisition of information about a fast electrical signal as described with reference to any of the drawings.