Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2002-03-12
2004-04-13
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C398S009000, C398S025000, C398S026000, C398S065000
Reexamination Certificate
active
06720548
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-074684, filed Mar. 15, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical sampling waveform measuring apparatus and more particularly to an optical sampling waveform measuring apparatus aiming at a wider band when measuring an optical pulse waveform of an optical signal used for an optical communication and the like with a sum frequency generation light (SFG light).
2. Description of the Related Art
Generally, when constructing a new optical communication system, manufacturing a new optical transmission apparatus or inspecting such optical communication system and optical transmission apparatus periodically, it is important to measure a pulse waveform of a digital optical signal to be transmitted received in order to grasp the quality of the optical communication.
In recent years, transmission velocity of information in optical communication has been increased and currently, high-speed optical transmission of 10 Gbit/s or more has been planned.
Jpn. Pat. Appln. KOKOKU Publication No. 6-63869 has disclosed an optical sampling waveform measuring apparatus for measuring an optical pulse waveform of high-speed optical signals of more than 10 Gbit/s with the sum frequency generation light.
FIGS. 7A
,
7
B and
7
C and
FIGS. 8A and 8B
explain the measuring principle of the optical sampling waveform measuring apparatus disclosed in this Jpn. Pat. Appln. KOKOKU Publication No. 6-63869.
For example, if a measuring object light “a” having repetition frequency “f” of the pulse waveform of measurement object and a sampling light “b” having a pulse width by far narrower than the pulse width of the measuring object light “a” and having a repetition frequency (f−&Dgr;f) slightly lower than the repetition frequency “f” of the measuring object light “a” are entered into a nonlinear optical material
1
allowing type 2 phase matching to the measuring object light “a” and the sampling light “b” simultaneously, only when the two lights “a” and “b” overlap each other at the same time, a sum frequency light “c” proportional to a product of the intensities of these two lights “a” and “b” is outputted from the nonlinear optical material
1
.
Because the repetition frequency of this sum frequency light “c” is the repetition frequency (f−&Dgr;f) of the sampling light “b”, response velocity of a photo-electric converter which converts this sum frequency light “c” to an electric signal only has to be higher than the repetition frequency (f−&Dgr;f).
Further, because the time resolution of this photo-electric converter is determined depending on the pulse width of the sampling light “b”, if the envelope waveform of this electric signal is obtained after the sum frequency light “c” is converted to an electric signal by means of this photo-electric converter, the shape of the envelope of this electric signal is an optical pulse waveform “e” of the measuring object light “a” enlarged on time axis.
Next, the sum frequency light and phase matching will be described below.
If the measuring object light “a” having an angular frequency &ohgr;
D
and the sampling light “b” having an angular frequency &ohgr;
S
are entered into one face of the nonlinear optical material
1
such that the polarization directions thereof are perpendicular to each other as shown in
FIG. 8A
, in a condition that the nonlinear optical material
1
allows the type 2 phase matching to two lights “a” and “b”, the sum frequency light “c” having a sum angular frequency (&ohgr;
S
+&ohgr;
D
) is outputted from the other face of the nonlinear optical material
1
.
The phase matching refers to it that the velocity (phase velocity) of each incident light entered into the nonlinear optical material
1
and the velocity of harmonic light to the incident light like the sum frequency light excited by the incident light coincide with each other in the crystal of the nonlinear optical material
1
.
Then, the type 2 phase matching refers to phase matching which is executed when the polarization directions of two incident lights are perpendicular to each other.
Meanwhile, the type 1 phase matching refers to phase matching which is executed when the polarization directions of two incident lights agree with each other.
The velocity of light advancing within the nonlinear optical material
1
differs depending upon the wavelength (frequency) and the advance direction to a crystalline axis.
Thus, for the velocity (phase velocity) of each incident light described above within the crystal and the velocity (phase velocity) of the sum frequency light within the crystal to coincide with each other, when the direction connecting an intersection between a refractivity ellipsoid of the incident light and a refractivity ellipsoid of the sum frequency light within the three-dimensional coordinates of the crystal is regarded as phase matching direction, the optical axis of each incident light described above is matched with the phase matching direction.
Further, the polarization direction of each incident light only has to be parallel to or perpendicular to the reference axis of a crystal existing within a plane at right angle to the phase matching direction.
More specifically, the nonlinear optical material
1
is cut out in the form of a rectangular pipe or cylinder having a plane perpendicular to that phase matching direction.
Currently, as such a nonlinear optical material
1
, KTP (KH
2
, PO
4
), LN (LiNbO
3
), LT (LiTaO
3
), KN (KNbO
3
) and the like are available.
FIG. 9
is a block diagram showing the schematic structure of a conventional optical sampling waveform measuring apparatus including the nonlinear optical material
1
allowing the type 2 phase matching.
The measuring object light “a” having the repetition frequency “f” of a pulse waveform under the angular frequency &ohgr;
D
of light entered from outside is controlled in terms of its polarization direction to 90° with respect to the reference direction (0° direction) by a polarization direction controller
2
and after that, entered into a multiplexer
3
.
On the other hand, a sampling light source
4
outputs the sampling light “b” having the repetition frequency (f−&Dgr;f) of a pulse waveform under the angular frequency &ohgr;
S
different from the angular frequency &ohgr;
D
of the aforementioned measuring object light “a”.
As shown in
FIG. 7B
, the pulse width of this sampling light “b” is set by far narrower than the pulse width of the measuring object light “a”.
After the polarization direction is controlled to for example, the reference direction (0° direction) by means of a polarization direction controller
5
, the sampling light “b” outputted from the sampling light source
4
is entered into the multiplexer
3
.
The multiplexer
3
comprised of for example, a beam splitter (BS) allows the incident light to advance straight through a half mirror
3
a
and reflects it at right angle.
Therefore, the sampling light “b” having a polarization direction, which is the reference direction (0° direction) and the measuring object light “a” having a polarization direction which is at 90° with respect to the reference direction (0° direction) are entered into one face of the nonlinear optical material
1
which allows type 2 phase matching, disposed behind this multiplexer
3
and located on the optical axis of the sampling light “b” at the same time.
Consequently, a sum frequency light “c” having an angular frequency (&ohgr;
S
+&ohgr;
D
) is outputted from the other face of the nonlinear optical material
1
of type 2.
The sum frequency light “c” outputted from the nonlinear optical material
1
is entered into a light receiver
7
through an optical filter
6
.
Light outputted from the nonlinear optical material
1
contains light (sum frequency light “c”) having the sum angular frequency (
Kawanishi Satoki
Otani Akihito
Otsubo Toshinobu
Shake Ippei
Takara Hidehiko
Anritsu Corporation
Frishauf Holtz Goodman & Chick P.C.
Meyer David C.
Porta David
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