Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Reexamination Certificate
2001-01-11
2003-04-01
Oda, Christine K. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
C359S326000, C359S199200
Reexamination Certificate
active
06541951
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical measurement of the waveform of target light and more particularly, to a method of measuring the waveform of target light and an apparatus for measuring the same, which are applicable to measurement of the waveform of ultra-high speed pulsed light used for optical communication and/or optical information processing.
2. Description of the Related Art
In recent years, the capacity of data to be transmitted in optical communications systems has been increasing rapidly and accordingly, not only the techniques for the wavelength multiplexing method that transmits the data using different wavelengths of signal light but the techniques for raising the data transmission rate in each wavelength to 100 Gb/s or higher have been being researched and developed actively. Under such circumstances, there have been the increasing need to develop the techniques for generating stable, coherent, ultra-high speed optical pulses and to measure the waveform of the ultra-high speed optical pulse train in real time with sufficiently high time resolution. In particular, the “eye pattern measurement” that measures directly an optical pulse train modulated by random bit data is essential to evaluate the characteristics of optical transmission systems.
A typical one of the known methods of measuring optical pulse trains is to use a ultra-high speed photoelectric converter and an electrically sampling oscilloscope. In this case, the “eye pattern measurement” can be performed, but in the present circumstances, the higher end of the measurable frequency range of light is, at most, approximately 40 GHz. As a result, it is difficult to measure the waveform of ultra-high speed optical pulse trains having a data transmission rate that exceeds about 40 Gb/s in each wavelength in real time with sufficiently high time resolution.
To solve the above-described difficulty, a method of measuring the waveform of target light has been developed and actually used. In this method, pulsed target light to be measured and pulsed sampling light having a sufficiently narrower pulse width than the target light is supplied to a specific nonlinear optical member, thereby generating intensity cross-correlated light between the target light and the sampling light due to nonlinear optical effects. On the basis of the cross-correlated light thus generated, the waveform of the target light is measured. In this method, the target light can be optically sampled and therefore, the above-described difficulty can be solved. Specifically, the waveform of ultra-high speed optical pulses having a data transmission rate that exceeds about 40 Gb/s in each wavelength can be measured in real time with sufficiently high time resolution.
Examples of the prior-art apparatuses of this type for measuring the waveform of target light pulses using the above-described method are disclosed in the Japanese Non-Examined Patent Publication No. 8-29814 published in 1996 and the Japanese Non-Examined Patent Publication No. 9-160082 published in 1997.
FIG. 1
shows a typical one of the prior-art apparatuses of this type, in which thick lines with arrows indicate the flow of optical signals while thin lines with arrows indicate the flow of electrical signals.
The prior-art measuring apparatus
200
comprises a driving signal oscillator
262
, a sampling light source
263
, a nonlinear optical member
264
, an optical filter
265
, an optical detector
266
, an electrical signal processing circuit
267
, and a display device
268
. The apparatus
200
itself is electrically and optically connected to an external apparatus
261
.
The external apparatus
261
includes a driving signal oscillator
271
that oscillates an electrical driving signal SD
1
with a frequency f
0
and a target light source
272
that is driven by the oscillator
271
to emit pulsed target light LT
0
. The target light LT
0
thus emitted has a repetition frequency equal to the frequency f
0
of the driving signal SD
1
. An example of the waveform of the target light LT
0
is shown by the waveform a in FIG.
14
.
The oscillator
262
, which is electrically connected to the oscillator
271
provided in the external apparatus
261
, oscillates a driving signal SD
2
having a frequency f
S
synchronized in phase with the driving signal SD
1
having the frequency f
0
. The reason why the oscillator
262
is electrically connected to the oscillator
271
is to synchronize the phase of the target light LT
0
with the phase of the sampling light LT
S
. Because of the phase synchronization between the light LT
0
and LT
S
, the fluctuation of time difference &dgr;t of each pulse of the target light LT
0
from each pulse of the sampling light LT
S
, (i.e., mutual jitter), is decreased. Thus, the time resolution can be prevented from degrading. In principle, possible time resolution is approximately equal to the pulse width of the sampling light LT
S
.
The sampling light source
263
is driven by the driving signal oscillator
262
, emitting the pulsed sampling light LT
S
. The sampling light LT
S
thus emitted has a repetition frequency f
S
, where f
S
=(f
0
/N)−&Dgr;f, f
0
is the repetition frequency of the target light LT
0
, &Dgr;f is a frequency difference, and N is a natural number (i.e., N=1, 2, 3, 4, . . .). The repetition frequency f
S
of the sampling light LT
S
is slightly different by &Dgr;f from the divided frequency of the target light LT
0
by N, i.e., (f
0
/N). For example, when N=1, the sampling light LT
S
has a waveform b shown in FIG.
14
. In this case, each pulse of the sampling light LT
S
has a time difference &dgr;t from the corresponding pulse of the target light LT
0
.
The target light LT
0
and the sampling light LT
S
thus generated enters the nonlinear optical member
264
, emitting intensity cross-correlated light LT
CC
between the light LT
0
and LT
S
thus supplied.
The nonlinear optical member
264
may be made of a ferroelectric crystal such as KTP (KTiOPO
4
) that causes a secondary nonlinear optical effect, e.g., the Sum Frequency Generation (SFG). Alternately, the member
264
maybe formed by a semiconductor optical amplifier or a quartz-system optical waveguide such as an optical fiber that causes a tertiary nonlinear optical effect, e.g., the Four Wave Mixing (FWM). The member
264
is used to emit the intensity cross-correlated light LT
CC
between the pulses of the target light LT
0
and the sampling light LT
S
. For example, the cross-correlated light LT
CC
thus emitted has a waveform shown by the waveform c in FIG.
14
. The cross-correlated light LT
CC
has a repetition frequency equal to the repetition frequency f
S
of the sampling light LT
S
.
Here, the time difference &dgr;t of the pulse of the sampling light LT
S
from the corresponding pulse of the target light LT
0
corresponds to the sampling time. Thus, it is expressed by the following equation (1).
δ
t
=
1
f
s
-
N
f
0
≅
Δ
f
f
s
2
(
1
)
For example, when the repetition frequency f
S
of the sampling light LT
S
is set as 1 GHz and the frequency difference &Dgr;f is set as 100 kHz, the time difference &dgr;t is given as 0.1 ps (picosecond) by the equation (1).
The optical filter
265
removes the target light LT
0
and the sampling light LT
S
and their secondary and higher harmonics (which serve as background light LT
B
of the intensity cross-correlated light LT
CC
), allowing only the cross-correlated light LT
CC
to pass through the filter
265
.
The optical detector
266
photoelectrically converts the cross-correlated light LT
CC
thus passed through the filter
265
to generate a pulsed electrical signal S
CC
. The signal S
CC
is supplied to the signal processing circuit
267
.
The detector
266
needs to have a frequency band equal to or higher than the repetition frequency f
S
of the sampling light LT
S
. This is due to the fact that each pulse of the cross-correlated light LT
CC
needs to be photoelectr
Kurita Chizuko
Kurita Hisakazu
Shirane Masayuki
Yamada Hirohito
Yokoyama Hiroyuki
Kurita Chizuko
NEC Corporation
Oda Christine K.
Scully Scott Murphy & Presser
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