Electro-optic sampling oscilloscope

Details Electro-optic sampling oscilloscope Electro-optic sampling oscilloscope

1999-04-27

2001-03-13

Lee, John R. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S2140AG, C250S2140RC, C356S218000, C356S222000, C324S754120, C324S750010, C324S754120

Reexamination Certificate

active

06201235

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-optic sampling oscilloscope used in various signal measurements.
This application is based on patent appellation No. Hei 10-122514 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In recent times, electro-optic sampling oscilloscopes are favored that can measure ultra-fast bit-rate signals of the order of 2.4 Gbps on a target circuit, without disturbing the operation of the target circuit.
In such an electro-optic sampling oscilloscope, an electro-optic probe based on electro-optic effects is used to detect signals in the target circuit.
Such electro-optic sampling oscilloscopes are favored for measurements in communication technologies that are constantly evolving into ultra-fast systems, because of the following features of the device.
(1) Measurement process is facilitated because the technique does not required a ground line.
(2) Metal pin placed at the end of an electro-optic probe is insulated from the test circuit resulting an extremely high input impedance, so that measurement process hardly affects the performance of the target circuit.
(3) Because optical pulses are used, measurements are possible over a wide bandwidth of the order of GHz.
FIG. 5
is a schematic side view showing the components of an electro-optic probe used in a conventional electro-optic sampling oscilloscope. An electro-optic probe
4
is based on a principle that when an electro-optic crystal, that is being subjected to an electrical field generated by the target signal, is irradiated with a laser beam, the polarization state of the laser beam is altered.
In the electro-optic probe
4
, a metal probe
5
with a tapered tip touches a signal line in a circuit. The metal pin
5
serves the purpose of facilitating the electrical field of the target signal to affect the condition of the electro-optic (e-o) crystal
7
. A circular shaped insulator
6
is provided to contact the end of the metal pin
5
in the center of the rear surface of the insulator. In other words, the metal pin
5
is surrounded by the insulator
6
. The e-o crystal
7
is a cylindrical crystal of BSO (B
12
SiO
20
), and has a property, known as the Pockels' effect, that the primary opto-electric effect, which is its refraction index, is altered in response to an electrical field coupled through the metal pin
5
.
A reflection mirror
8
is a dielectric film laminated mirror and is made by vapor deposition of a reflecting substance on the rear surface of the e-o crystal
7
. A reference laser beam La
0
transmitted through the e-o crystal
7
is reflected by the reflection mirror
8
, which is bonded to the front surface of the insulator
6
.
A cylindrical casing
9
is comprised by a tube section
9
b
and an end piece
9
a
of a tapered-shape integrally formed at one end of the tube section
9
b
having a hole through the axial center. The end piece
9
a
houses the metal pin
5
, insulator
6
, e-o crystal
7
and the reflection mirror
8
.
An optical fiber
10
is a polarization-maintaining optical fiber, and connects a connector
11
and a laser generator (not shown). The laser generator generates a linearly polarized reference laser beam La
0
. The reference laser beam La
0
is comprised by base band component signal that does not contain signal components in the measurement band. The connector
11
is disposed so that the reference laser beam La
0
output from the ejection end
11
a
will be injected at right angles to the e-o crystal
7
and the reflection mirror
8
. A collimator lens
12
is disposed on the left of the connector
11
, and converts the reference laser beam La
0
to a parallel beam of light.
A polarized beam splitter
13
is disposed on the left of the collimator lens
12
, and transmits a polarized component of the reference laser beam La
0
parallel to the plane of the paper in a straight line, while the polarized component of the reference laser beam La
0
is bent at 90 degrees to the plane of the paper, and the bent beam is transmitted as the second signal light La
2
in a straight line. A Faraday element
14
is disposed on the left of the polarized beam splitter
13
, and rotates the polarized component of the reference beam La
0
, transmitted through the polarized beam splitter
13
, at 45 degrees to the plane of the paper.
A half-wave plate
15
is disposed on the left of the Faraday element
14
in such a way that the orientation of its crystal axis in inclined at 22.5 degrees, and re-directs the polarized beam rotated by the Faraday element
14
in a direction parallel to the plane of the paper. A polarized beam splitter
16
is disposed on the left of the half-wave plate
15
, and has the same structure as the polarized beam splitter
13
, and splits a portion of the reference laser beam La
0
reflected from the reflection mirror
8
as the first signal light La
1
. A full-wave plate
17
is disposed on the left of the polarized beam splitter
16
, and adjusts the S/N (signal to noise) ratio of the output signals ultimately obtained from the e-o probe
4
, by adjusting the intensity balance of the reference laser beam La
0
transmitted through the polarized beam splitter
16
. Adjustment of S/N ratio is performed by varying the angle between the reference laser beam La
0
and the wave plate
17
by rotating the wave plate
17
.
A first photo-diode
18
is disposed above the polarized beam splitter
16
, and converts the first signal light La
1
(a portion of the reference laser beam La
0
split by the polarized beam splitter
16
) into first electrical signals and outputs the electrical signals to a positive (+) terminal of a differential amplifier
30
. A second photo-diode
19
is disposed above the polarized beam splitter
13
, and converts the second signal light La
2
(a portion of the reference laser beam La
0
split by the polarized beam splitter
13
) into second electrical signals and outputs the electrical signals to a negative (−) terminal of the differential amplifier
30
.
In such an apparatus, when the metal pin
5
shown in
FIG. 5
is made to contact a signal line (not shown), an electrical field of a magnitude, corresponding to the level of the signal in the target circuit, propagating in the signal line, and couples with the e-o crystal
7
. Accordingly, refraction index of the e-o crystal
7
changes with the strength of the electrical field. In this condition, a reference laser beam La
0
is injected into the front surface of the e-o crystal
7
, through the output end
11
a
of the connector
11
, collimator lens
12
, polarized beam splitter
13
, Faraday element
14
, ½ wave plate
15
, polarized beam splitter
16
and the wave plate
17
.
Under this condition, the polarization state of the reference laser beam La
0
propagated through the e-o crystal
7
is changed. Polarization-affected reference laser beam La
0
is reflected from the reflection mirror
8
, and is output from the front surface of the e-o crystal
7
, and is separated in the polarized beam splitter
16
. The first signal light La
1
produced by this splitting process is converted into first electrical signals in the first photo-diode
18
, and the first electrical signal S
1
are input in the (+) terminal of the differential amplifier
30
.
In the meantime, the second signal light La
2
produced by the polarized beam splitter
16
is diverted by the polarized beam splitter
13
to the second photo-diode
19
, and is converted into second electrical signal S
2
in the second photo-diode
19
, and the second electrical signal S
2
are input in the (−) terminal of the differential amplifier
30
.
Accordingly, the first and second electrical signals S
1
, S
2
are amplified in the differential amplifier
30
, in such a way that the in-phase noise components contained in the reference laser beam La
0
generated by fluctuation and other factors are canceled.
Differentially amplified signals are input as detection signal SO

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