Electrooptic measurement systems for frequency analysis of very wide range signals

A spectrum analyzer for a variable amplitude and frequency electric signal includes an optical modulator responsive to the signal that derives a first optical wave having intensity and frequency components corresponding to the signal amplitude and frequency components. An optical analyzer responds to the first optical wave to derive a second optical wave having intensity variations at frequency components corresponding to the amplitude of the electric signal at the frequencies in the spectrum of the signal. The optical analyzer includes a tunable optical cavity responsive to the first optical wave to derive the second optical wave.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to the measurement of electrical signals 
of very wide frequency range. More particularly, the invention relates to 
the measurement of the frequential characteristics of electrical signals 
of very wide frequency range by a method of electrooptic measurement. 
2. Description of the Prior Art 
J. A. VALDMANIS and G. MOUROU recently developed a method for measuring the 
waveform of an electrical signal of very wide range by electrooptic 
sampling. This measurement method enables temporal resolution of 
approximately one picosecond to be achieved. In the article entitled 
"Subpicosecond Electrooptic Sampling:Principles and Applications" which 
was published in the IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-22, No. 
1, JANUARY 1986, J. A. VALDMANIS and G. MOUROU describe the general 
configuration of an electrooptic sampling measurement system. 
This electrooptic sampling measurement system utilizes the existence of 
birefringence in certain crystals with an electrooptic property. When a 
rectilinearly polarized light wave encounters an electrical wave produced 
by an electrical signal in a berefringent crystal, its polarization is 
rotated as a result of the interaction with the electrical wave. By 
placing itself in particular geometrical conditions, the interaction 
between the two waves induces a phase lag in the light wave. The light 
wave observed in cross-polarization then has an intensity modulated by the 
electrical signal and the variations in intensity of the light wave need 
only be measured by an optical detector to retrace the electrical signal. 
To achieve very good temporal resolution, a light wave formed by a pulse 
train of very small width is used to sample the electrical signal, it is 
possible to explore the temporal evolution of the electrical signal. The 
method is similar to stroboscopics for repetitive signals. The measurement 
of the intensity of the light wave is carried out at very low frequency 
and consequently with a conventional, very low noise and high-performance 
optical detector. 
The temporal resolution of a system of measurement by electrooptic sampling 
is higher than that of conventional measurement systems of the purely 
electronic type such as the sampling oscilloscope. This superiority is 
mainly due to the fact that the polarization rotation of the light wave in 
the electrooptic crystal is an instantaneous phenomenon which does not 
have a measurable time constant as a result of which the main limitation 
of the temporal resolution of such a system is the width of the sampling 
pulses of the light wave. 
However, the main drawback of electrooptic sampling measurement systems is 
in the fact that it is necessary to utilize a pulsing laser source which 
issues light pulses of very small width, i.e. subpicosecond. In fact, a 
laser source of this type usually has a length of several meters and is 
therefore very cumbersome. Moreover, it is difficult to adjust and is 
relatively expensive. Another major drawback of this measurement system is 
that it is only possible to measure signals with repetition frequencies 
that are multiple integers of the repetition frequency of the light pulses 
issued by the laser source. It can be conclued from these two drawbacks 
that electrooptic sampling measurement systems are as yet experimental 
systems that are difficult to industrialize and market. 
OBJECT OF THE INVENTION 
The object of this invention is to obviate the preceding drawbacks, 
particularly to provide electrooptic systems for frequency analysis of 
electrical systems with temporal performances at least equal to those of 
the electrooptic sampling measurement systems but which do not have the 
above-mentioned drawbacks. 
SUMMARY OF THE INVENTION 
Accordingly, an electrooptic system embodying the invention for frequency 
analysis of an electrical signal, therein comprises 
means for continuously producing and transmitting a first monochromatic 
coherent light wave of constant intensity, 
means for linearly modulating in amplitude the first light wave according 
to the electrical signal to be analyzed, thereby producing a second light 
wave modulated in amplitude, and 
spectroscopic means for characterizing the frequencies of the electrical 
signal to be analyzed from the second light wave modulated in amplitude. 
The electrooptic systems embodying the invention are mainly in the form of 
frequency meters and spectrum analysers. 
In the case of an electrooptic system embodying the invention in the form 
of a frequency meter, the spectroscopic means preferably comprises tunable 
means for frequency measurement to receive the second light wave and to 
issue in response a frequency light component of the second light wave, 
with a frequency equal to a frequency of which are tuned said tunable 
frequency measuring means, and means for detecting the intensity of said 
frequency light component. 
In the case of an electrooptic measurement system embodying the invention 
in the form of a spectrum analyser, the spectroscopic means preferably 
also comprises means for successively controlling the tuning of the 
tunable frequency measurement means to different frequencies in a 
frequency range to be explored, and means receiving a frequency sweep 
signal produced by said controlling means and representative of the 
explored frequency range and an intensity signal produced by said 
detecting means and representative of the intensities of the frequency 
light components of the second light wave corresponding respectively to 
the different frequencies of the explored frequency range, to visualize 
the frequency spectrum of the second light wave corresponding to the 
explored frequency range and to deduct from this the frequential 
characteristics of the electrical signal to be analyzed. 
In accordance with a further aspect of the invention a spectrum analyzer 
for an electric signal susceptible of having variable amplitude and 
frequency components over a predetermined spectrum comprises optical 
modulator means responsive to the signal for deriving a first optical wave 
having intensity and frequency components corresponding to the amplitude 
and frequency components of the signal. Optical analyzer means responds to 
the first optical wave to derive a second optical wave having intensity 
variations at frequency components corresponding to the amplitude of the 
electric signal at the frequencies in the spectrum. The optical analyzer 
means includes a tunable optical cavity responsive to the first optical 
wave for deriving the second optical wave. The tunable optical cavity is 
tuned over the frequencies of the spectrum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In reference to FIG. 1, the electrooptic spectrum analyzer embodying the 
invention comprises a laser source 1, an electrooptic modulator 2, and a 
spectroscopic device 3. 
The laser source 1 is of the monochromatic, continuous transmission type 
and has a very small line width. The laser source 1 is e.g. comprised of a 
Helium-Neon (He-Ne) type gas laser source, of stabilized frequency and 
transmitting a light i.e., optical wave L of wavelength .lambda..sub.O, of 
frequency .nu..sub.O and of very small line width .DELTA..nu.. 
The electrooptic modulator 2 is a POCKELS cell comprising a polarizer 20, a 
compensator 21, a crystal 22 with electrooptic property, and a 
polarizer-analyser 23. 
The polarizer 20 is e.g. comprised of a GLAN or NICOLL prism. It receives 
the light wave L supplied by the laser source 1 to a first side 200 and 
issues a rectilinearly polarized light wave L.sub.P by a second side 201. 
The light wave L.sub.p is applied through the compensator 21 to a first 
side 220 of the crystal 22. 
The compensator 21 is of the quarter-wave .lambda..sub.O /4 type and its 
purpose is to introduce a phase lag .GAMMA..sub.O =.pi./2 in the light 
wave L.sub.p so as to polarize the modulator 2 in a linear part of its 
frequency response curve as will appear more clearly further in the 
description. 
The electrooptic crystal 22 is e.g. comprised of a parallelepiped shaped 
ADP crystal (NH.sub.4)H.sub.2 PO.sub.4. The crystal 22 comprises 
perpendicular crystallographic axes x and z to which extraordinary n.sub.e 
and ordinary n.sub.o refraction coefficients correspond respectively. The 
first side 220 of the crystal 22 is parallel to the plane defined by the 
crystallographic axes x and z. Second and third parallel sides 221 and 222 
of the crystal 22 which are perpendicular to the first side 220 bear 
ribbon conductors R between which is applied an electrical signal V to be 
analyzed. 
The electrical signal V produces in the crystal 22 a transversal electric 
field E perpendicular to the propagation direction of the light wave 
L.sub.p and parallel to the crystollographic axis z of the crystal 22. The 
light wave L.sub.P incurs a phase lag .DELTA..GAMMA. during the crossing 
of the crystal 22. The phase lag .DELTA..GAMMA. is a function of 
electrical signal V whose amplitude variations introduce into the crystal 
22 modifications in its hirefringence property, i.e. variations in the 
refraction coefficients n.sub.o and n.sub.e, the difference n.sub.o 
-n.sub.e being substantially proportional to the electrical signal V. 
The phase lag .DELTA..GAMMA. introduced in the light wave L.sub.p during 
its crossing of the crystal 22 is expressed by the equality: 
EQU .DELTA..GAMMA.=.pi..(V/V.sub..pi.), 
whereby V.sub..pi. is a characteristic parameter of the crystal 22 and of 
the wavelength .lambda..sub.O of the light wave L.sub.p ; the parameter 
V.sub..pi. being of the order of one kilovolt. 
A light wave L.sub..phi. phase modulated according to the electrical signal 
V is produced bv the crvstal 22. The light wave L.sub..phi. is supplied by 
a fourth side 223 of the crystal 22, and is applied to a first side 230 of 
the polarizer-analyzer 23. 
The polarizer-analyzer 23 is of a type analogous to the polarizer 20. It is 
oriented crossways to the polarizer 20 and its direction is therefore at 
.pi./2 from the direction of the polarizer 20. Via a second side 231 
parallel to the first side 230, the polarizer 23 supplies a light wave 
L.sub.A in rectilinear cross-polarization by comparison with the light 
wave L.sub.p. The amplitude modulation of the light wave L.sub.A is a 
function of the electrical signal V. 
Preferably, to achieve maximum amplitude modulation of light wave L.sub.A, 
with a modulation index equal to 1, thereby ensuring maximum measurement 
sensitivity, the light wave L.sub.p has a polarization plane PL.sub.P 
oriented, as shown in FIG. 1, at .pi./4 to the crystallographic axes x and 
z of the crystal 22, which is achieved by suitably orienting the polarizer 
20 by comparison with the crystal 22. In these conditions, the intensity 
IL.sub.A of the light wave L.sub.A is expressed by the following equality: 
EQU IL.sub.A =IL.sub.O. sin .sup.2 (.GAMMA..sub.O +.DELTA..GAMMA.)/2)=(IL.sub.O 
/2)(l cos (.GAMMA..sub.O +.DELTA..GAMMA.)) 
whereby IL.sub.O is the maximum amplitude of the intensity IL.sub.A. 
The phase lag .GAMMA..sub.O introduced by the compensator 21 being equal to 
.pi./2, the intensity IL.sub.A of the light wave L.sub.A according to the 
electrical signal V is expressed by: 
EQU IL.sub.A =(IL.sub.O /2).(1+sin(.pi..V/V.sub.90 )). 
This last relation is illustrated by the response curve of the electrooptic 
modulator 2 shown in FIG. 2. 
The amplitude of the electrical signal V is usually very low in comparison 
with the value of the parameter V.sub.90 of the order of one kilovolt, as 
a result of which the ratio V/V.sub..pi. is very low and the modulator 2 
operates in a linearity zone ZL. In the linearity zone ZL the intensity 
IL.sub.A according to the signal V is expressed by the equality: 
EQU IL.sub.A .congruent.(IL.sub.O /2).(1+.pi..V/.sub.90). 
In reference to FIG. 3, the light wave L.sub.A modulated linearly in 
amplitude by the electrical signal V has a frequency spectrum S(.nu.) 
comprising a frequency line RL at frequency .nu..sub.O corresponding to 
the light wave transmitted by the laser source 1, and two sidebands 
BD(.nu.) and BG(.nu.) due to the amplitude modulation of the wave L.sub.A 
and corresponding to the frequency spectrum B(f) of the electrical signal 
V. 
The sidebands BD(.nu.) and BG(.nu.) have extreme upper and lower 
frequencies respectively equal to .nu..sub.O -f.sub.M and .nu..sub.O 
-f.sub.m, and .nu..sub.O +f.sub.m and .nu..sub.O +f.sub.M, whereby f.sub.m 
and f.sub.M are respectively extreme upper and lower frequencies of the 
frequency spectrum B(f) of the signal V. The sidebands BD(.nu.) and 
BG(.nu.) are associated with the frequency spectrum B(f) by the 
equalities: 
EQU BG(.nu.)=B(.nu..sub.O -f), and 
EQU BD(.nu.)=B(.nu..sub.O +f). 
The light wave L.sub.A therefore carries all the information relating to 
the electrical signal V to be analyzed and it is easy, knowing the 
frequency spectrum B(f) of the signal V, to deduct from it the frequency 
spectrum B(f) of the signal V. 
In reference to FIG. 1, the purpose of the spectroscopic device 3 is to 
analyze the frequency of the spectrum of the light wave L.sub.A in order 
to determine the spectrum B(f) of the electrical signal V. 
The spectroscopic device 3 is e.g. comprised of a FABRY-PEROT sweeping 
interferometer 30, a photodiode-equipped optical detector 31, and an 
oscilloscope 32. 
The FABRY-PEROT sweeping interferometer is a well known device to those 
skilled in the art and its functioning will not be described in detail 
here. It comprises essentially a tunable cavity 300 and a sweep generator 
301. The light wave L.sub.A is injected into the cavity 300 through a 
first semi-transparent wall 3000. A second semi-transparent wall 3001 of 
the cavity 300 is fitted with a piezoelectrical control device and its 
position is mobile in comparison with the first wall 3000. Displacement of 
the wall 3001 is controlled by a low-frequency sweep ramp signal BA. 
Controlled by the ramp signal BA, the cavity 300 is successively tuned to 
different frequencies in a frequency range to be explored and issues 
corresponding frequency light components CF through the second wall 3001. 
A photodiode 310 of the optical detector 31 receives the frequency light 
components supplied by the cavity 300 and issues in response a current 
that is representative of the intensity of said components. From the 
current issued by the photodiode 310, the detector 31 produces an 
intensity signal IF representing the intensity of the different frequency 
light components CF in the explored frequency range. 
The sweep ramp signal BA and the intensity signal IF are respectively 
applied at inputs X and Y of the oscilloscope 32 so as to visualize the 
frequency spectrum S(.nu.) on the cathode screen of the oscilloscope 32. 
Preferably, the cavity 300 is selected so as to have a free frequency 
interval included between the frequency .nu..sub.O and a maximum frequency 
.nu..sub.M (FIG. 3) which includes the sideband BD (.nu.) of the frequency 
spectrum S(.nu.). This free temporal interval is swept under the control 
of the sweep ramp signal BA and, the frequency line .nu..sub.O being taken 
as origin of frequency f=0 Hz, only the frequency spectrum B(f) is thereby 
displayed on the screen of the oscilloscope 32. 
The electrooptic spectrum analyzer embodying the invention can also be used 
to characterize electronic components and hyperfrequency microcircuits on 
a semiconducting substrate. In this way, e.g. when the microcircuit 
substrate is comprised of a crystal with an electrooptic property such as 
gallium arsenide (GaAs), it is then possible to analyze electrical signals 
in situ, without recourse to contact or connection by wires, by using the 
electrooptic property of the substrate to modulate the light wave. 
In reference to FIG. 4, an electrooptic modulator 2a for in situ analysis 
of an electrical signal V in a microcircuit carried out on an electrooptic 
crystalline substrate SB comprises a polarizer 20, a compensator 21, two 
analogous mirrors 22a and 22b, a focusing lens 22c, and a 
polarizer-analyzer 23. 
The polarizer 20, the compensator 21 and the polarizer-analyzer 23 are 
analogous to those included in the modulator 2 shown in FIG. 1 and have 
the same functions. In the modulator 2a, the electrooptic crystal 22 (FIG. 
1) is suppressed and the phase modulation of the polarized light wave 
L.sub.p is carried out directly in the electrooptic crystalline substrate 
SB. 
The rectilinearly polarized light wave L.sub.p coming from the compensator 
21 is oriented by the mirror 22a towards a point P in the vicinity of a 
ribbon conductor R situated on an upper side FS of the substrate SB. The 
ribbon conductor R carries the electrical signal V to be analyzed. The 
light wave L.sub.P is applied to the point P of the substrate SB through 
the focusing lens 22c. 
The purpose of the focusing lens 22c is to diminish the spatial resolution 
of the measurement by focusing the light wave L.sub.p on the point P which 
diminishes in area and can therefore be throught nearer the ribbon 
conductor R. 
The light wave L.sub.p is directed towards the point P with an angle of 
incidence substantially lower than .pi./2 and propagates in the substrate 
SB until it reaches a metallized lower side FI which is at a reference 
voltage. At the lower side FI, the light wave L.sub.p is reflected back 
towards the upper side FS. The light wave exiting from the substrate SB 
via the upper side FS is phase modulated and forms the light wave 
L.sub..phi.. In the substrate SB, the electrical signal V produces an 
electric field E substantially longitudinal to the propagation directions 
of the incident and reflected light waves. By electrooptic effect, the 
light waves propagating in the substrate SB are subjected to a phase lag 
according to the electrical signal V. 
The light wave L.sub..phi. exiting the substrate SB goes through the 
focusing lens 22c and is directed towards the first side 230 of the 
polarizer-analyzer 23 by the mirror 22b. Through the second side 231, the 
polarizer-analyzer 23 issues the light wave L.sub.A which is rectilinearly 
polarized and amplitude modulated by the electrical signal V. 
In the case of the substrate SB supporting the microcircuit to be 
characterized not having any electrooptic property, an electro-optic 
modulator 2b, shown in FIG. 5, comprising a measurement probe 22d in 
electrooptic crystal can be used. The probe 22d is then positioned near 
the ribbon conductor R carrying the electrical signal V so that the probe 
22d may be crossed by lines of the electric field E produced by the signal 
V. It is thus possible to analyze the electrical signal V without any 
connection between the ribbon conductor R and the eleotrooptic modulator 
2b. 
The invention can be embodied in many ways other than those described in 
reference to FIGS. 1 to 5. 
As regards the spectroscopic device 3 included in the analyzer and 
described in reference to FIG. 1, other types of sweeping interferometers 
can therefore be used. It is not always desirable to have the spectrum of 
the signal to be analyzed represented on the oscilloscope. In this case 
the spectroscopic device is e.g. comprised of a spectograph with a 
dispersing element issuing a spectrum recording on a paper support. 
Furthermore, when exact knowledge of the spectrum is not necessary and e.g. 
the fundamental frequency of the signal is required, the invention may 
take on the form of a frequency meter and comprise e.g. a MICHELSON 
interferometer which is manually adjusted until the photodiode-equipped 
optical detector 31 issues a maximum response indicating that the 
interferometer is tuned to the fundamental frequency. A reading of the 
interferometer's graduated slide contact then indicates the relevant value 
of the fundamntal frequency of the signal. 
The performances of an electrooptic spectrum analyzer embodying the 
invention are essentially limited by the performances of the 
interferometer included in the spectroscopic device and by the stability 
of the width of the line of the light wave produced by the laser source. A 
particular embodiment of the spectrum analyzer embodying the invention 
comprised of a FABRY-PEROT interferometer and a laser source currently 
marketed achieves the following performances for contactless in situ 
measurement: 
pass range of a few kilohertz to 8,000 GHz, 
sensitivity lower than 1 mV, 
spectral resolution substantially equal to 1 kHz, and 
spatial resolution of a few micrometers.