Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
1999-11-10
2002-05-07
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
C324S647000, C250S205000, C250S2140AG
Reexamination Certificate
active
06384590
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light receiving circuit for use in an electro-optic sampling oscilloscope which receives an optical signal polarized so as to reflect a signal to be measured and which reproduces an electric signal according to the signal to be measured on the basis of the polarization state of the optical signal.
2. Description of the Related Art
An electro-optic sampling oscilloscope (hereinafter abbreviated as an “EOS oscilloscope”) which optically samples a signal to be measured and reproduces the waveform of the signal has conventionally been used for observing the waveform of, for example, an internal signal of an integrated circuit.
In the EOS oscilloscope, an electro-optic crystal whose plane of polarization is changed by an electric field is connected to a portion where an internal signal to be measured (hereinafter abbreviated as a “target signal”) appears, and the target signal is reproduced on the basis of the deflected state of light reflected by the electro-optic crystal. The EOS oscilloscope comprises an electro-optic probe equipped with a built-in optical system for acquiring an optical signal whose polarization state corresponds to the target signal, and a light-receiving circuit which receives the optical signal and reproduces an electric signal corresponding to the polarization state of the optical signal.
In contrast to a conventional sampling oscilloscope using an electric probe, the EOS oscilloscope has the following characteristics:
1) Since the EOS oscilloscope does not need a ground line at the time of measuring a signal, the signal can be readily measured.
2) A metal pin provided at the tip of the electro-optic probe is electrically insulated from circuitry of the oscilloscope, and hence the waveform of a target signal can be measured without distorting the state of the target signal.
3) The EOS oscilloscope utilizes an optical pulse signal, and hence the EOS oscilloscope can measure a wide frequency range extending up to giga hertz. Because of these merits, the EOS oscilloscope receives attention.
An exemplary configuration of an electro-optic probe used in the EOS oscilloscope will now be described by reference to FIG.
2
. In the drawing, reference numeral
1
designates a probe head formed from an insulator, and a metal pin
1
a
to be brought into contact with an area where a target signal appears is fitted into the center of the probe head
1
. Reference numeral
2
designates an electro-optic element (i.e., electro-optic crystal) whose plane of polarization is changed by an electric field. A reflection film
2
a
is provided on the side of the electro-optic element
2
facing the metal pin
1
a
and remains in contact with the metal pin
1
a.
Reference numeral
4
designates a half-wave plate;
5
designates a quarter-wave plate;
6
and
8
designate polarization beam splitters;
7
designates a Faraday element;
9
designates a laser diode which emanates a laser beam in accordance with a pulse signal (control signal) output from the EOS oscilloscope main unit (not shown); and
10
designates a collimator lens which collimates a laser beam emitted from the laser diode
9
in a single direction. The electro-optic element
2
, the half-wave plate
4
, the quarter-wave plate
5
, the polarization beam splitters
6
and
8
, and the Faraday element
7
are disposed on the optical path of a laser beam A collimates by the collimator lens
10
.
Reference numerals
11
designates a collective lens for collecting a laser beam separated by the polarization beam splitter
6
;
13
designates a collective lens for collecting a laser beam separated by the polarization beam splitter
8
; and
101
and
104
designate photodiodes constituting a light-receiving circuit to be described later. The photodiode
101
converts the laser beam collected by the collective lens
11
into an electric signal and outputs the electric signal to the EOS oscilloscope main unit, and the photodiode
104
converts the laser beam collected by the collective leans
13
into an electric signal and outputs the electric signal to the EOS oscilloscope main unit.
Reference numeral
15
designates a probe body; and
17
designates an isolator comprising the quarter-wave plate
5
, the two polarization beam splitter
6
and
8
, and the Faraday element
7
. The isolator
17
permits transmission of the laser beam emitted from the laser diode
9
and separates orthogonal polarization component of the light reflected by the reflection film
2
a.
The exemplary configuration of the conventional light-receiving circuit used in the EOS oscilloscope will be described by reference to FIG.
3
. In the drawing, reference numeral
100
designates a bias power supply;
101
and
104
designate photodiodes;
102
and
105
designate resistors;
103
and
106
designate amplifiers;
107
designates a current monitor;
108
designates an analog-to-digital-converter;
109
designates a differential amplifier comprising resistors
109
A to
109
D and an operational amplifier
109
E;
110
designates a resistor; and
111
designates an analog-to-digital converter.
In the light-receiving circuit, the photodiode
101
is biased by the bias power supply
100
, to thereby produce an electric current. The electric current is amplified by the respective amplifiers
103
and
106
, and a difference between the signals output from the amplifiers
103
and
106
is amplified by the differential amplifier
109
, thus producing a target signal. The value of a signal output from the differential amplifier
109
is subjected to analog-to-digital conversion by the analog-to-digital converter
111
. Electric currents produced by the photodiodes
101
and
104
are monitored by the current monitor
107
, and the values of the currents are subjected to analog-to-digital conversion by the analog-to-digital converter
108
.
The operation of the conventional light-receiving circuit will now be described.
The laser diode
10
shown in
FIG. 2
is activated by a pulse signal (control signal) and emits the pulse-like laser beam A having a sampling frequency. The laser beam A is converted into collimated light by the collimator leans
9
, and the thus-collimated light travels straight ahead through the polarization beam splitter
8
, the Faraday element
7
, and the polarization beam splitter
6
and enters the electro-optic element
2
by way of the quarter-wave plate
5
and the half-wave plate
4
.
The laser beam that has entered the electro-optic element
2
is reflected by the reflection film
2
a
formed on the end face of the electro-optic element
2
facing the metal pin
1
a.
When the metal pin
1
a
is brought into contact to a point of measurement, the electric field corresponding to the voltage applied to the metal pin
1
a
is propagated to the electro-optic element
2
, thus causing a phenomenon in which the double refractive indices of the electro-optic element
2
are changed by the Pockels effect. The polarized state of the light is changed when the laser beam emitted from the laser diode
9
propagates through the electro-optic element
2
. As a result, the laser beam reflected by the end face of the electro-optic element
2
includes a polarization component corresponding to the voltage of the target signal.
The laser beam reflected by the end face of the electro-optic element
2
passes again through the half-wave plate
4
and the quarter-wave plate
5
. A portion of the laser beam (a polarization component corresponding to the voltage of the target signal
9
is separated by the polarization beam splitter
6
. The thus-separated beam is collected by the collective lens
11
, and the thus-collected beam enters the photodiode
101
constituting the light-receiving circuit. The remaining laser beam that has transmitted through the polarization beam splitter
6
is separated by the polarization beam splitter
8
, and the thus-separated laser beam is collected by the collective lens
13
. The laser beam then enters the photodiode
104
Banjo Nobukazu
Endou Yoshio
Kikuchi Jun
Nagatsuma Tadao
Shinagawa Mitsuru
Ando Electric Co. Ltd.
Finnegan Henderson Farabow Garrett & Dunner LLP
Metjahic Safet
Nguyen Vincent Q.
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