Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-12-06
2002-11-19
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06483624
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical pulse generation system and, more particularly, to a method and device for outputting optical pulses having high duty ratio, and an optical sampling pulse generation apparatus using the device.
Conventionally, as a device for generating high-duty optical pulses which have a repetition frequency ranging from several GHz to 10 GHz used in an optical communication system and a small pulse width, an optical pulse generation device using an electroabsorption optical modulator has been proposed, as shown in
FIG. 14
(Jpn. Pat. Appln. KOKAI Publication No. 5-283804).
In this optical pulse generation device, for example, light a, which is a laser beam output from a single-wavelength light source
1
such as a laser beam source or the like, and has a continuous, single wavelength, becomes incident on the light entrance surface of an electroabsorption optical modulator
2
.
The modulator electrode of this electroabsorption optical modulator
2
receives a pulse modulation signal c, which is obtained by adding a negative DC voltage Va output from a DC voltage source
4
to a sine-wave signal b output from a sine-wave generator
3
.
The electroabsorption optical modulator
2
outputs, from its light exit surface, optical pulses d obtained by modulating the light a, which becomes incident on its light entrance surface and has a continuous, single wavelength, by the pulse modulation signal c input to its modulator electrode.
The electroabsorption optical modulator
2
has light absorption characteristics A shown in
FIG. 15
with respect to a negatively applied DC voltage V.
A DC voltage V is plotted in a linear scale on the abscissa of the light absorption characteristics A, while the light intensity to be output is plotted in a logarithmic (decibel: dB) scale along the ordinate.
Hence, when the sine-wave signal b is applied to the modulator electrode of this electroabsorption optical modulator
2
at a bias point that has shifted in the negative direction by the negative DC voltage Va, as shown in
FIG. 16A
, optical pulses d shown in
FIG. 16B
are output from the light exit surface of this electroabsorption optical modulator
2
.
More specifically, as the optical pulses d, pulses
5
that define a waveform in which minus portions of the sine-wave signal b are compressed and its plus portions are amplified in a linear scale appear at periods Ta of the sine-wave signal b, as shown in FIG.
16
B.
The pulse width Ts of each pulse
5
that forms the optical pulses d output from the light exit surface of the electroabsorption optical modulator
2
, i.e., from the optical pulse generation device is expressed by the width of a portion 3 dB below the top of that pulse
5
.
The pulse width Ts of the portion 3 dB below is greatly smaller than the period Ta of the sine-wave signal b.
Hence, using the pulse modulation signal c obtained by adding the negative DC voltage Va to the sine-wave signal b, the high-duty optical pulses d having the pulse width Ts greatly smaller than the repetition period Ta can be obtained.
In recent years, in an optical communication system, the transfer rate of optical signals is increasing, and an optical pulse generation device capable of generating optical pulses d having a pulse width Ts still smaller than the repetition period Ta is required.
Such requirement is also adamant in an optical sampling pulse generation device that outputs optical sampling pulses used to sample light to be measured, which becomes incident on a light sampling unit in a light sampling waveform measurement apparatus.
Hence, since the pulse width Ts of the optical pulses generated by the optical pulse generation device shown in
FIG. 14
depends on the period Ta of the sine-wave signal b, i.e., a frequency f
A
of the sine-wave signal, a pulse width still smaller than the repetition period Ta cannot be obtained.
To combat this problem, an optical pulse generation device shown in
FIG. 17
has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 9-133901).
This optical pulse generation device shown in
FIG. 17
comprises a plurality of sine-wave generators
3
a
and
3
b
for outputting sine-wave signals b
1
and b
2
(see
FIG. 18B
; only b
1
is illustrated) having different frequencies with respect to a sine-wave generator
3
for outputting a fundamental sine-wave signal b (see
FIG. 18A
having a frequency f
A
.
The sine-Wave signals b
1
and b
2
are respectively delayed a predetermined amount by delay circuits
6
a
and
6
b
, and the delayed signals are added to the fundamental sine-wave signal b, thus obtaining a pulse modulation signal c having sharp peaks, as shown in FIG.
18
C.
By applying the pulse modulation signal c having sharp peaks to the modulator electrode of an electroabsorption optical modulator
2
, the pulse width Ts alone can be shortened without changing the repetition period Ta of optical pulses d output from the light exit surface of this electroabsorption optical modulator
2
.
Furthermore, conventionally, an optical pulse generation device using a rectangular wave signal shown in
FIG. 19
has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 5-283804).
In the optical pulse generation device shown in
FIG. 19
, light a output from a single-wavelength light source
1
is modulated into optical pulses d
1
by a first electroabsorption optical modulator
2
a
, and is then modulated into optical pulses d
2
by a second electroabsorption optical modulator
2
b.
A rectangular wave signal e
1
(
FIG. 20A
) output from a rectangular wave generator
7
is applied to the first electroabsorption optical modulator
2
a.
On the other hand, a rectangular signal e
2
(FIG.
20
B), which is obtained by delaying the rectangular wave signal e
1
output from the rectangular wave generator
7
a predetermined period of time by a delay circuit
8
, is applied to the second electroabsorption optical modulator
2
b.
As a result, optical pulses d
2
output from the second electroabsorption optical modulator
2
b
include pulses
5
having a pulse width Ts corresponding to the overlapping time between the rectangular wave signals e
1
and e
2
, as shown in FIG.
20
C.
Hence, when a short overlapping time between the rectangular wave signals e
1
and e
2
is set by adjusting the delay time of the delay circuit
8
, as shown in
FIGS. 20A
,
20
B, and
20
C, only the pulse width TS can be shortened without changing the repetition period Ta of the output optic al pulses d
2
.
However, the optical pulse generation devices shown in
FIGS. 17 and 19
still suffer the following problems.
In the optical pulse generation device shown in
FIG. 17
, in order to obtain the pulse modulation signal c having sharp peaks shown in
FIG. 18C
, the delay amounts in delay circuits
6
a
and
6
b
must be set with high precision while synchronizing sine-wave generators
3
,
3
a
, and
3
b.
In this case, since the frequency of the sine-wave signal b is as very high as several GHz to 10 GHz, complicated setting adjustment is required to set the delay amounts with high precision, and the delay amount may vary soon even after they are set with high precision.
Furthermore, the optical pulse generation device shown in
FIG. 17
requires a large number of sine-wave generators
3
,
3
a
, and
3
b
, and delay circuits
6
a
and
6
b
, resulting in a complicated circuit arrangement.
In the optical pulse generation device shown in
FIG. 19
, since the repetition frequency of the rectangular wave signal e
1
output from the rectangular wave generator
7
is as very high as several GHz to 10 GHz, jitter of around 1 ps is produced at the leading and trailing edges of the rectangular wave.
As a consequence, jitter of around 1 ps is produced in the optical pulses d
2
output from this optical pulse generation device.
The jitter of around 1 ps is not negligible for optical pulses which are required to have a pulse width Ts of 3 to 4 ps.
Consequently, in this pulse generation device, decreasing the pulse width Ts of the output optical pulses d
Otani Akihito
Otsubo Toshinobu
Anritsu Corporation
Chan Jason
Frishauf Holtz Goodman & Chick P.C.
Kim David S.
LandOfFree
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