U.S. patent application number 09/764271 was filed with the patent office on 2001-07-26 for optical pulse position detecting circuit and an optical pulse generating apparatus and their methods.
This patent application is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Edagawa, Noboru, Kitayama, Tadayoshi, Komiya, Takeshi, Matsushita, Kiwami, Mizuochi, Takashi, Morita, Itsuro, Shimizu, Katsuhiro, Suzuki, Masatoshi, Taga, Hidenori, Yamamoto, Shu.
Application Number | 20010009469 09/764271 |
Document ID | / |
Family ID | 26421109 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009469 |
Kind Code |
A1 |
Shimizu, Katsuhiro ; et
al. |
July 26, 2001 |
Optical pulse position detecting circuit and an optical pulse
generating apparatus and their methods
Abstract
Detection of an optical pulse position uses an optical pulse
string with a determined repetitive ratio and an electric clock
signal with a same frequency as the repetitive ratio of the optical
pulse string. A phase of the electric clock signal oscillator is
shifted and supplied to an optical modulator. The optical modulator
modulates the optical pulse string based on the electric clock
signal and outputs a modulated optical signal. A photo detector
converts the modulated optical signal output from the optical
modulator to an electric signal. The phase shift amount of the
electric clock signal is controlled to maximize an output from the
photo detector. Additionally, a dither signal may be used in the
control of the phase shift, more than the optical modulator may be
employed, and/or more than color light source may be employed. The
use of at least one of feed forward and feedback control provided
by maximizing an output of the photo detector allows an optical
pulse having a short width to be realized.
Inventors: |
Shimizu, Katsuhiro; (Tokyo,
JP) ; Mizuochi, Takashi; (Tokyo, JP) ; Komiya,
Takeshi; (Tokyo, JP) ; Matsushita, Kiwami;
(Tokyo, JP) ; Kitayama, Tadayoshi; (Tokyo, JP)
; Suzuki, Masatoshi; (Tokyo, JP) ; Taga,
Hidenori; (Tokyo, JP) ; Yamamoto, Shu; (Tokyo,
JP) ; Edagawa, Noboru; (Tokyo, JP) ; Morita,
Itsuro; (Tokyo, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha
|
Family ID: |
26421109 |
Appl. No.: |
09/764271 |
Filed: |
January 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09764271 |
Jan 19, 2001 |
|
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|
09052072 |
Mar 31, 1998 |
|
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|
6236488 |
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Current U.S.
Class: |
398/197 |
Current CPC
Class: |
G02F 1/0327 20130101;
H04J 14/08 20130101; G02F 2203/26 20130101; G01J 11/00
20130101 |
Class at
Publication: |
359/187 ;
359/135 |
International
Class: |
H04B 010/04; H04J
014/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 1997 |
JP |
HEI 9-80069 |
Aug 8, 1997 |
JP |
HEI 9-214215 |
Claims
What is claimed is:
1. An optical pulse position detecting circuit comprising: an
optical pulse string input terminal receiving a optical pulse
string with a determined repetitive ratio; an oscillator for
outputting an electric clock signal with a same frequency as the
repetitive ratio of the optical pulse string; a phase shifter for
receiving the electric clock signal outputted from the oscillator,
shifting a phase the electric clock signal, and outputting a
shifted electric clock signal; an optical modulator for receiving
the shifted electric clock signal outputted from the phase shifter
and the optical pulse string received by the optical pulse string
input terminal, modulating the optical pulse string based on the
electric clock signal and outputting a modulated optical signal; a
photo detector for converting the modulated optical signal
outputted from the optical modulator to an electric signal and
outputting the electric signal; a phase controlling circuit for
receiving the electric signal outputted from the photo detector and
controlling a phase shift amount of the phase shifter to maximize
an output from the photo detector; and a phase shift amount output
terminal for outputting the phase shift amount of the phase
shifter.
2. The optical pulse position detecting circuit of claim 1, wherein
the phase controlling circuit comprised a dither signal generating
circuit for outputting a dither signal; a phase comparator for
detecting synchronization of the output from the photo detector and
the dither signal outputted from the dither signal generating
circuit and outputting an error signal; and a phase shifter
controlling circuit for superimposing the dither signal outputted
from the dither signal generating circuit and the error signal
outputted from the phase comparator, and outputting a control
signal for controlling the phase shift amount of the phase
shifter.
3. The optical pulse position detecting circuit of claim 1, further
comprising: an optical pulse string generating light source driven
by the electric clock signal outputted from the oscillator; a
transmission line connecting the optical pulse string generating
light source and the optical puls string input terminal, wherein
the oscillator further outputs electric clock signals at two or
more frequencies; and a transmission time detecting circuit
receiving the phase shift amount outputted from the phase shift
amount output terminal and detecting a transmission time of an
optical pulse in the transmission line based on the frequency of
the electric clock signal outputted from the oscillator.
4. The optical pulse position detecting circuit of claim 1, wherein
the optical modulator includes a semiconductor electro-absorption
type optical modulator.
5. An optical pulse generating circuit comprising: a light source
for outputting an optical signal with a determined wavelength; an
oscillator for outputting an electric clock signal with a
determined frequency; a first optical modulator connected to the
oscillator for modulating a power of the optical signal based on an
electric clock signal and outputting a first modulated optical
signal; a second optical modulator for modulating a power of the
first modulated optical signal outputted from the first optical
modulator based on the electric clock signal and outputting a
second modulated optical signal; an optical multiplexer, receiving
one of the first modulated optical signal and the second modulated
optical signal, outputting a part of the optical signal and
branching a part of the optical signal; a photo detector for
converting the optical signal multiplexed by the optical
multiplexer to an electric signal; a phase changing unit for
changing a phase of the signal; and a controlling circuit for
receiving the electric signal outputted from the photo detector and
controlling a phase change amount of the phase changing unit to
maximize the output from the photo detector.
6. The optical pulse generating apparatus of claim 5, wherein the
controlling circuit comprises: a dither signal generating circuit
for outputting a dither signal; a phase comparator for detecting
synchronization of the output from the photo detector and the
dither signal outputted from the dither signal generating circuit
and outputting an error signal; and a circuit for superimposing the
dither signal outputted from the dither signal generating circuit
and the error signal outputted from the phase comparator and
outputting a control signal for controlling the phase change amount
of the phase changing unit.
7. The optical pulse generating apparatus of claim 3, wherein the
phase changing unit comprises a phase shifter for shifting a phase
of the electric clock signal outputted from the oscillator and
outputting the electric clock signal to one of the first optical
modulator and the second optical modulator, wherein the controlling
circuit comprises a phase shifter controlling circuit for
controlling the phase shift amount of the phase shifter.
8. The optical pulse generating apparatus of claim 5, wherein the
phase changing unit comprises an optical delayer for delaying the
optical signal outputted from the first optical modulator and
outputting the optical signal to the second optical modulator,
wherein the controlling circuit comprises a delay controlling
circuit for controlling a delay amount of the optical delayer.
9. The optical pulse generating apparatus of claim 5, wherein the
phase changing unit comprises: a phase shifter for shifting a phase
of the electric clock signal outputted from the oscillator and
outputting to one of the first optical modulator and the second
optical modulator; and an optical delayer for delaying an optical
signal outputted from the first optical modulator and outputting to
the second optical modulator. wherein the controlling circuit
comprises a delay controlling circuit for controlling a delay
amount of the optical delayer, wherein the phase shifter receives a
dither signal and shifts a phase of the electric clock signal
outputted from the oscillator.
10. The optical pulse generating apparatus of claim 8, wherein the
first and second optical modulators are configured in a single
optical modulator, wherein the light source outputs an optical
signal in a first polarization state, wherein the optical pulse
generating apparatus, further comprises: a first polarization
multiplexer/demultiplexer connected between the light source and
the single optical modulator; a second polarization
multiplexer/demultiplexer connected between the single optical
modulator and the optical delayer; and a polarization state
converter for converting a polarization state of the optical signal
outputted from the optical delayer and outputting to the second
polarization multiplexer/demultiplexer, wherein the optical
multiplexer outputs a part of the optical signal outputted from the
first polarization multiplexer/demultiplexer with a converted
polarization state and branches a part of the optical signal via
the second polarization multiplexer/demultiplexer, the optical
modulator and the first polarization multiplexer/demultiplexer.
11. The optical pulse generating apparatus of claim 5, wherein: the
light source includes a first light source for outputting a first
optical signal with a first wavelength and a second light source
for outputting a second optical signal with a second wavelength;
and further comprising: first and third optical modulators
corresponding to the first and second light sources; and an optical
demultiplexer for demultiplexing optical signals outputted from the
first and third optical modulators and outputting to the second
optical modulator.
12. The optical pulse generating apparatus of claim 5 comprising: a
temperature detecting circuit for detecting a temperature in an
apparatus; a reference voltage generator; and a temperature drift
detecting circuit for receiving an output signal from the
temperature detecting circuit and an output signal from the
reference voltage generator, generating a compensation signal for
compensating the phase change amount and outputting the
compensation signal to the controlling circuit.
13. The optical pulse generating apparatus of claim 5, comprising:
a wavelength detecting circuit for detecting a wavelength of the
optical signal outputted from the light sources; a reference
voltage generator; and a wavelength drift detecting circuit for
receiving an output signal from the wavelength detecting circuit
and an output signal from the reference voltage generator,
generating a compensation signal for compensating the phase change
amount and outputting the compensation signal to the controlling
circuit.
14. The optical pulse generating apparatus of claim 5, wherein the
optical modulator is a semiconductor electro-absorption type
optical modulator.
15. The optical pulse generating apparatus of claim 5, wherein a
plurality of sets of the second optical modulator, the phase
changing unit, the photo detector and the controlling circuit is
provided in parallel, wherein an optical signal outputted from the
first optical modulator is branched and processed in parallel.
16. The optical pulse generating apparatus of claim 5, wherein a
plurality of sets of the second optical modulator, the phase
changing unit, the photo detector and the controlling circuit is
provided serially, wherein an optical signal outputted from the
first optical modulator is processed serially.
17. An optical pulse position detecting method comprising:
inputting an optical pulse string with a determined repetitive
ratio; oscillating an electric clock signal with a same frequency
with the repetitive ratio of the optical pulse string and
outputting an oscillated electric clock signal; shifting a phase of
the oscillated electric clock signal and outputting a shifted
electric clock signal; modulating the optical pulse string based on
the shifted electric clock signal; converting the optical signal
outputted from the modulating step to an electric signal and
outputting the electric signal; controlling, in response to the
electric signal, a phase shift amount in the shifting step to
maximize the electric signal outputted from the converting step;
and outputting the phase shift amount for the shifting step.
18. The optical pulse position detecting method of claim 17,
wherein the controlling step comprises: generating a dither signal;
detecting synchronization of the electric signal and the dither
signal and outputting an error signal; and superimposing the dither
signal and the error signal, and outputting a control signal for
controlling the phase shift amount in the shifting step.
19. An optical pulse generating method comprising: outputting an
optical signal with a determined wavelength; outputting an electric
clock signal with a determined frequency; first modulating a power
of the optical signal based on the electric signal and outputting a
first modulated optical signal; second modulating a power of the
first modulated optical signal based on the electric signal and
outputting a second modulated optical signal; multiplexing one of
the first modulated optical signal and the second modulated optical
signal, outputting a part of a multiplexed optical signal and
branching a part of the multiplexed optical signal; converting the
multiplexed optical signal to an electric signal; changing a phase
of the optical signal; and controlling, in accordance with the
electric signal, a phase change amount in the changing step to
maximize the electric signal outputted from the converting
step.
20. The optical pulse generating method of claim 19, wherein the
controlling step comprises: generating a dither signal; detecting
synchronization of the electric signal and the dither signal and
outputting an error signal; and superimposing the dither signal and
the error signal, and outputting a control signal for controlling a
phase change amount in the changing step.
21. An optical pulse generating apparatus comprising: a light
source for outputting an optical signal with a determined
wavelength; an oscillator for outputting an electric clock signal
with a determined frequency; a first optical modulator connected to
the oscillator for modulating a power of the optical signal with
the electric clock signal and outputting a first modulated optical
signal; a second optical modulator for modulating a power of the
first modulated optical signal outputted from the first optical
modulator with the electric clock signal and outputting a second
modulated optical signal; an optical multiplexer for receiving one
of the first modulated optical signal inputted to the second
optical modulator and the second modulated optical signal outputted
from the second optical modulator, outputting a part of a
multiplexed optical signal and branching a part of the multiplexed
optical signal; a photo detector for converting the multiplexed
optical signal from the optical multiplexer to an electric signal;
a phase changing unit for changing a phase of an optical signal;
and a controlling circuit for receiving the electric signal
outputted from the photo detector and the electric clock signal
outputted from the oscillator, and controlling a phase change
amount of the phase changing unit to match a phase of the electric
signal and a phase of the electric clock signal.
22. The optical pulse generating apparatus of claim 21, wherein the
controlling circuit comprises: a clock re-generating circuit for
re-generating a clock signal from the electric signal outputted
from the photo detector; and a phase comparator for comparing a
phase of the clock signal re-generated in the clock re-generating
circuit and a phase of the electric clock signal.
23. The optical pulse generating apparatus of claim 21, wherein the
phase changing unit comprises a phase shift for shifting the phase
of the electric clock signal outputted from the oscillator and
outputting a shifted electric clock signal to one of the first
optical modulator and the second optical modulator, wherein the
controlling circuit comprises a phase shifter controlling circuit
for controlling a phase shift amount of the phase shifter.
24. The optical pulse generating apparatus of claim 21 wherein the
phase changing unit comprises an optical delayer for delaying an
optical signal outputted from first optical modulator and
outputting a delayed optical signal to the second optical
modulator, wherein the controlling circuit comprises a delay
controlling circuit for controlling a delay amount of the optical
delayer.
25. The optical pulse generating apparatus of claim 21 further
comprising a modulation signal generating circuit for receiving the
electric clock signal, receiving a dither signal and outputting a
data signal synchronized with electric clock signal as a modulation
signal to one of the first and second optical modulators.
26. The optical pulse generating apparatus of claim 21 wherein the
second modulator is a polarization scramber.
27. An optical pulse generating method comprising: outputting an
optical signal with a determined wavelength; outputting an electric
clock signal with a determined frequency; first modulating a power
of the optical signal with the electric clock signal and outputting
a first modulated optical signal; second modulating a power of the
first modulated optical signal outputted with the electric signal
and outputting a second modulated optical signal; multiplexing one
of the first modulated optical signal and the second modulated
optical signal, outputting a part of a multiplexed optical signal
and branching a part of the multiplexed optical signal; converting
the multiplexed optical signal to an electric signal; changing a
phase of an optical signal; and controlling, in response to the
electric signal and the electric clock signal, a phase change
amount in the changing step to match a phase of the electric signal
and a phase of the electric clock signal.
28. The optical pulse generating method of claim 27, wherein the
controlling step comprises: a clock re-generating step for
re-generating a clock signal from the electric signal outputted
from the optical detecting step; and a phase comparing step for
comparing a phase of the clock signal re-generated in the clock
re-generating step and a phase of the electric clock signal and
outputting an error signal to the phase changing step.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an optical transmission system,
particularly to an optical time division multiplexing technique.
This invention can be applied a measuring system, particularly to a
temperature sensor and a pressure sensor.
[0003] 2. Description of the Related Art
[0004] In an optical transmission system according to the related
art, a transmitting apparatus performs TDM (Time Division
Multiplexing) of a plurality of low-speed signals by processing in
an electronic circuit and generates high-speed signals. A receiving
apparatus or a node performs DEMUX (Demultiplexing) of the
high-speed signals by processing in an electronic circuit and
regenerates the low-speed signals. A transmission system with a
transmission level of 10 Gb/s (bits per second) has been already
realized by adopting the TDM technique in an electronic circuit.
However, a processing speed of the electronic circuit is likely to
become a bottle neck for a future transmission system with a larger
capacity. Therefore, the time division multiplexing technique
(optical TDM technique) for performing optical processing is
currently under intense studies.
[0005] According to the optical TDM technique, an optical signal is
processed without being converted to an electric signal. Optical
pulse strings inputted from various transmission lines must be
multiplexed synchronously. However, the optical pulse strings are
transmitted in the transmission lines at different transmission
rates depending on environmental factors such as a temperature,
etc. Further, pulse positions (phases) of inputted optical pulse
strings change constantly. Therefore, it is necessary to provide a
method for detecting the pulse positions. The pulse positions are
relative time relationships of standard clock signals with
repetitive frequencies of the optical pulse strings and inputted
pulses.
[0006] A technique for detecting a pulse position is disclosed by
Ohteru, et. al.
[0007] A block chart in "Optical Time-Division-Multiplexer Based on
Modulation Signal to Optical Modulators," B-1118 in Proceedings of
the 1996 Institute of Electronics, Information and Communication
Engineers (IEICE) General Conference is revised in FIG. 43.
[0008] In FIG. 43, an optical pulse string input terminal 1, an
optical multiplexer 2, an optical modulator 3 for modulating an
optical power level, a phase shifter 4, an oscillator 5, optical
power meters 6a and 6b for detecting power levels of optical
signals and a transmittancy rate detector 7 are illustrated.
[0009] In FIG. 43, an optical pulse string is inputted from the
optical pulse string input terminal 1 and multiplexed into two
transmission lines by the optical multiplexer 2 A first output from
the optical multiplexer 2 is inputted to the first optical power
meter 6a and a second output from the optical multiplexer is
inputted to the optical modulator 3. A standard clock signal
outputted from the oscillator 5 drives the optical modulator 3 via
the phase shifter 4. An optical signal outputted from the optical
modulator 3 is inputted to the second power meter 6b. The
transmittancy rate detector 7 performs a comparative operation of
outputs from the first and second optical power meters 6a and 6b,
and detects a transmittancy rate of a pulse in the optical
modulator. Since the comparative operation of the outputs from the
first and second power meters 6a and 6b is performed, even if the
optical power level of the inputted optical pulse string
fluctuates, the transmittancy rate in the optical modulator 3 can
be measured. As discussed below, the transmittancy rate of a pulse
in the optical modulator is determined from a phase of a clock
signal for driving the optical modulator and a position (phase) of
an optical pulse string inputted to the optical modulator.
Therefore, a pulse position can be known from the transmittancy
rate. The phase shifter 4 is controlled manually to increase a
value of the transmittancy rate Accordingly, the phase of the
inputted optical pulse and the phase of the standard clock signal
can be synchronized.
[0010] An operation of FIG. 43 is discussed with reference to FIG.
44.
[0011] In FIG. 44, pulse positions (a) in an optical pulse string
inputted to the optical modulator 3 are illustrated. A relation (b)
of a time and a transmittancy rate in the optical modulator 3 is
also shown. The relation corresponds to the clock signal which
drives the optical modulator 3. A relation (c) of a time and a
transmittancy rate of a pulse is also shown.
[0012] In FIG. 44, three pulse positions (a) of pulse 1, pulse 2
and pulse 3 in the optical pulse string correspond to transmittancy
rates 1, 2 and 3 in (c). Since the pulse positions and the
transmittancy rates correspond, the pulse positions can be known by
detecting the transmittancy rates.
[0013] In technique illustrated in FIG. 43, it is assumed that a
pulse position detector is provided as an error signal detecting
circuit for controlling pulse positions. Therefore, it is not
necessary to detect an accurate pulse position. It is only
necessary to detect a sign relationship (left-or-right from
position A in (b) of FIG. 44) of the detected pulse position.
[0014] However, when it is necessary to detect the pulse positions
accurately, following problems arise from the technique illustrated
in FIG. 43. As apparent from (c) of FIG. 44, the transmittancy
rates 1, 2 and 3 correspond to pulses 1', 2' and 3' as well as
pulses 1, 2 and 3. A range for detecting positions is limited to
field T in (b) of FIG. 44. Since the transmittancy rate in the
optical modulator corresponds to two phase shift amounts, it is
difficult to optimize the phase shift amounts. As shown in (c) of
FIG. 44, the relation of the transmittancy rate and the pulse
position is not a straight-line but a sine function curve, a
complicated operation circuit is necessary to detect the accurate
pulse positions.
[0015] It is necessary to detect the accurate pulse positions to
simplify a controlling circuit of the pulse positions and to
perform more complicated optical processing. Detection of the
accurate pulse positions is also necessary for various sensors that
utilize changes of a transmission delay time in transmission
lines.
[0016] Another technique for detecting a pulse position is
disclosed in Japanese Unexamined Published Patent Application HEI
2-1828. FIG. 1 in HEI 2-1828 is revised in FIG. 45 for this
specification.
[0017] In FIG. 45, the optical pulse string input terminal 1, an
optical demultiplexer 33, a fully-optical modulator 43 for
modulating an optical pulse string with an optical clock pulse and
a photo detector 8 are illustrated. In FIG. 45, an optical delayer
controlling circuit 34, an optical clock pulse generating circuit
44, an optical delayer 24 and a phase shift amount output terminal
10 are also illustrated. An optical signal is inputted from the
optical pulse string input terminal 1 and inputted to the photo
detector 8 via the fully-optical modulator 43. Then, the photo
detector 8 outputs a signal to the optical clock pulse generating
circuit 44, and the optical clock pulse generating circuit 44
outputs an optical clock pulse. The optical delayer 24 delays the
optical clock pulse and the optical demultiplexer 33 inputs the
delayed optical clock pulse to the fully-optical modulator 43. The
fully-optical modulator 43 is designed to have a higher
transmittancy rate when an optical signal with a higher power is
inputted. Therefore, when phases of the optical signal inputted
from the optical pulse string input terminal 1 and the optical
clock pulse synchronize, the photo detector 8 detects a maximum
optical power. When the optical delayer 24 is controlled to
maximize the output from the photo detector 8, the optical clock
pulse synchronizes with the inputted optical pulse. A pulse
position of inputted optical pulse string can be detected by
monitoring an amount of delay for the optical delayer 24 from the
phase shift amount output terminal 10.
[0018] In the technique illustrated in FIG. 45, the optical clock
pulse generating circuit 44 needed to detect an accurate pulse
position is complicated. In particular, the optical clock pulse
generating circuit 44 needs to output an optical clock pulse
synchronized with an inputted optical signal. However, it is not
desirable that such a complicated optical clock pulse generating
circuit 44 is provided only to detect the pulse position. Besides,
even though some embodiments of the fully-optical modulator 43 are
known, they are not available in a market. Further, it is difficult
to produce the optical delayer 24 for controlling a longer delay
time more accurately than an electric delayer (phase shifter).
[0019] An importance of realizing a short pulse generating circuit
according to the optical TDM technique is discussed. The short
pulse is an optical pulse with a short pulse width.
[0020] When a transmission capacity increases, an optical pulse
with a shorter pulse width is required. Therefore, generation of
the short pulse is important in the optical TDM technique. For
example, in a transmission system with a transmission capacity of
20 Gb/s according to the optical TDM technique, an optical pulse
with a pulse width of 20 ps (pico second) or less is necessary. In
a transmission system with a transmission capacity of 100 Gb/s, an
optical pulse with a pulse width of 4 ps or less is necessary.
[0021] One known pulse generation method uses an optical modulator
which includes a pulse type gate. A technique of connecting optical
modulators which include pulse type gates in multi-layers and
thinning an effective gate width to generate a short pulse is
disclosed in "Super High Speed Optical Technique, 2. Chapter 2," by
Tatsuo Yajima, Maruzen Co.
[0022] In order to generate the short pulse by connecting the
optical modulators in multi-layers, it is necessary to balance a
phase of an electric signal for driving each of the optical
modulators with a phase of an optical pulse inputted to each of the
optical modulators.
[0023] Generally, optical amplifiers are provided between the
optical modulators to compensate an insertion loss each of the
optical modulators. However, since the optical signals are
transmitted at different transmission rates in the optical
amplifiers and the transmission lines depending on environmental
temperatures, it is difficult to balance the phases of the electric
signal and the optical pulse without providing a system for
absorbing a fluctuation in a delay time of the optical signal.
[0024] A technique for balancing the phases is disclosed by
Tomioka, et. al.
[0025] A block chart in "A Control Method of Phase between
Ultrashort Optical Pulses and Modulation Data," B-112J in
Proceedings of the 1996 Institute of Electronics, Information and
Communication Engineers (IEICE) General Conference is revised for
FIG. 46.
[0026] In FIG. 46, a light source 26, a first optical modulator 3a,
an optical amplifier 29, a second optical modulator 3b, RF (Radio
Frequency) amplifiers 28a and 28b, the phase shifter 4, and the
oscillator 5 are illustrated. In FIG. 46, a 2:1 multiplexing
circuit 32, a pulse pattern generator 31, the optical multiple 2, a
modulation light output terminal 30 and an optical power meter 6
are also shown.
[0027] Operations are performed as follows.
[0028] A clock signal is outputted from the oscillator 5 and
branched to the pulse pattern generator 31 and the phase shifter 4.
A phase of the clock signal is shifted by the phase shifter 4 and
amplified by the RF amplifier 28a. The amplified clock signal is
inputted to the optical modulator 3a. The optical modulator 3a
modulates an optical signal outputted from the light source 26 by
the clock signal outputted from the RF amplifier 28a, and outputs
an optical pulse. The optical pulse outputted from the optical
modulator 3a is amplified by the optical amplifier 29 and inputted
to the second optical modulator 3b. An output from the pulse
pattern generator 31 is RZ (Return to Zero) encoded by the 2:1
multiplexing circuit 32, and amplified by the RF amplifier 28b. A
RZ signal synchronized with the clock signal outputted from the
oscillator 5 is inputted from the RF amplifier 28b to the second
optical modulator 3b. The second optical modulator 3b modulates the
optical pulse outputted from the optical amplifier 29 with the RZ
signal outputted from the RF amplifier 28b. At this time, it is
necessary that the optical pulse inputted to the optical modulator
3b and the RZ signal are synchronized.
[0029] A part of an output from the optical modulator 3b is
branched by the optical multiplexer 2 and inputted to the optical
power meter 6.
[0030] An operation principle is discussed with reference to FIG.
47. A relation between the phase shift amount of the phase shifter
4 and the transmittancy rate in the optical modulator 3b is shown
in (b). In (b) of FIG. 47. when the optical pulse inputted to the
optical modulator 3b and the RZ signal are synchronized, the phase
amount is a phase shift amount 2. When the phase shift amount is
too small (phase shift amount 1) or too large (phase shift amount
3), the transmittancy rate in the optical modulator 3b decreases.
Therefore, a phase relation of the optical pulse inputted to the
optical modulator 3b and the RZ signal can be optimized by
controlling the phase shift amount of the phase shifter 4 manually
to maximize an optical power level of the output from the optical
power meter 6.
[0031] The technique illustrated in FIG. 46 is not intended to
generate a short pulse but to generate a RZ modulated optical
signal. Therefore, the optical modulator-3b is driven by the RZ
signal. However, when the RZ signal for driving the optical
modulator 3b is in an ideal short wave, the phase shift amount
cannot be measured from the transmittancy rate in the optical
modulator 3b. Further even if it is intended to generate a proper
pulse waveform, since the relation between the transmittancy rate
and the pulse position is not a straight-line but a sine function
curve, a change (.DELTA.P) of the transmittancy rate in the optical
modulator 3b against a change (.DELTA..PHI.) of the phase shift
amount becomes smaller around the optimal phase shift amount (phase
shift amount 2). Hence, a control accuracy drops. Besides, since
the transmittancy rates in optical modulators correspond to two
phase shift amounts as in technique illustrated in FIG. 43, it is
difficult to optimize the phase shift amount in the technique
illustrated in FIG. 46. Further, a comparison of the outputs from
the first optical power meter 6a and the second optical power meter
6b is not performed in the configuration illustrated in FIG. 46
differing from the configuration illustrated in FIG. 43. Therefore,
when one of the optical power outputted from the light source 26,
the transmittancy rate in the optical modulator 3a and a gain of
the optical amplifier 29 is changed, it becomes impossible to
measure the transmittancy rate in the optical modulator 3b.
[0032] In a long distance optical amplifying relay transmission
system, it is well known that an optical S/N ratio (Optical Signal
to Noise Ratio) deteriorates or fluctuates by polarization hole
burning in an optical amplifying delayer and a polarization
reliance loss in transmission lines. In order to improve the
optical S/N ratio, polarization scramble is performed. The
polarization scramble is a method for transmitting a signal by
switching two kinds of independent polarizations from time to time.
The polarization scramble is performed one or more times for a
signal of one bit. Particularly, the polarization scramble must
performed at a speed of a signal bit rate or higher to average out
a fluctuation (signal fading) of the optic S/N ratio. Generally, a
Lithium Niobate optical phase modulator is used to perform
polarization scramble.
[0033] Particularly, in the polarization scramble, phase modulation
occurs simultaneously with polarization modulation. It is published
in Japanese Unexamined Published Patent Application HEI 8-111662
that the polarization scramble is used to compensate wave-form
deterioration due to dispersion of transmission lines (differences
in optical transmiission rates according to frequencies based on
transmission line characteristics). In HEI 8-111662, a polarization
scrambler must be driven by a data clock synchronized with a data
fluctuation bit. Therefore, it is discussed that a significant
improvement in a sign error rate in a super long distance optical
amplifying relay transmission system across the ocean. Hence, when
a determined relation between a phase of a driving signal of the
polarization scrambler and a phase of a data is maintained, an
opening of an eye can be further enlarged to an advantage for
distinguishing a signal, even if a fiber dispersion (differences in
transmission rates of light according to frequencies based on fiber
characteristics) and an amplitude fluctuation at an inps terminal
by fluctuation of non-linear refractive index occur.
[0034] FIG. 43 illustrates a technique for controlling phases
between the optical modulators based on optical power level.
However, since two pulse positions are assumed from a transmittancy
rate, it is impossible to know a direction of the phase shift only
from the optical power level. Further, since the phase shifter is
controlled manually, it is difficult to control the phases between
the optical modulators automatically reflecting achange in a
transmission line length between the optical modulators which
fluctuates from time to time. Further, since the relation between
the transmittancy rate and the pulse position is not a
straight-line, a complicated operation circuit is necessary.
[0035] FIG. 45 illustrates another technique for detecting the
pulse position. However, a complicated optical clock pulse
generating circuit, a fully-optical modulator which is difficult to
obtain and an accurate optical delayer which is difficult to be
controlled are necessary.
[0036] FIG. 46 illustrates a technique for solving a phase
balancing problem caused by fluctuation in a delay time to generate
an optical pulse. However, in this technique, a control accuracy
drops around an optimal phase shift amount. Further, when an
optical signal power changes, it becomes impossible to measure a
transmittancy rate in the optical modulator. Further, since two
phase shift amounts are assumed from a transmittancy rate in an
optical modulator as in the technique illustrated in FIG. 43, it is
difficult to optimize the phase shift amount.
[0037] In HEI 8-111662, an effect of synchronizing a driving signal
of the polarization scrambler and a data is discussed. However, a
solution for disturbance in synchronization (a fluctuation of a
transmission delay time of a fiber due to a temperature
fluctuation, for example) is not disclosed. In HEI 8-111662, a
circuit configuration for detecting a phase of a data signal
inputted to the polarization scrambler and driving the polarization
scrambler in an optimal synchronized phase is not disclosed.
SUMMARY OF THE INVENTION
[0038] This invention is intended to solve the above mentioned
problems in the related art.
[0039] Primarily, this invention is intended to provide an optical
pulse position detecting circuit for detecting a pulse position
without a complicated operation circuit.
[0040] Secondly, this invention is intended to provide an accurate
optical pulse position detecting circuit without using an optical
clock pulse generating circuit, a fully-optical modulator and an
optical delayer.
[0041] Thirdly, this invention is intended to provide an optical
pulse generating apparatus for generating an optical pulse by
optimizing a phase shift amount.
[0042] Fourthly, this invention is intended to provide an optical
pulse generating apparatus for outputting a plurality of short
pulses with different wavelengths simultaneously.
[0043] Fifthly, this invention in intended to provide a pulse
generating apparatus for modulating synchronously with an optical
pulse.
[0044] According to one aspect of this invention, an optical pulse
position detecting circuit comprises an optical pulse string input
terminal receiving an optical pulse string with a determined
repetitive ratio, an oscillator for outputting an electric clock
signal with a same frequency as the repetitive ratio of the optical
pulse string, a phase shifter for receiving the electric clock
signal outputted from the oscillator, shifting a phase of the
electric clock signal, and outputting a shifted electric clock
signal, an optical modulator for receiving the shifted electric
clock signal outputted from the phase shifter and the optical pulse
string received by the optical pulse string input terminal,
modulating the optical pulse string based on the electric clock
signal and outputting a modulated optical signal photo detector for
converting the modulated optical signal outputted from the optical
modulator to an electric signal and outputting the electric signal,
a phase controlling circuit for receiving the electric signal
outputted from the photo detector and controlling a phase shift
amount of the phase shifter to maximize an output from the photo
detector, and a phase shift amount output terminal for outputting
the phase shift amount of the phase.shifter.
[0045] According to another aspect of this invention, an optical
pulse generating circuit comprises a light source for outputting an
optical signal with a determined wavelength, an oscillator for
outputting an electric clock signal with a determined frequency, a
first optical modulator connected to the oscillator for modulating
a power of the optical signal based on an electric clock signal and
outputting a first modulated optical signal, a second optical
modulator for modulating a power of the first modulated optical
signal outputted from the first optical modulator based on the
electric clock signal and outputting a second modulated optical
signal, an optical multiplexer, receiving one of the first
modulated optical signal and the second modulated optical signal,
outputting a part of the optical signal and branching a part of the
optical signal, a photo detector for converting the optical signal
multiplexed by the optical multiplexer to an electric signal, a
phase changing unit for changing a phase of the signal, and a
controlling circuit for receiving the electric signal outputted
from the photo detector and controlling a phase change amount of
the phase changing unit to maximize the output from the photo
detector.
[0046] According to another aspect of this invention, an optical
pulse position detecting method comprises inputting an optical
pulse string with a determined repetitive ratio, oscillating an
electric clock signal with a same frequency with the repetitive
ratio of the optical pulse string and outputting an oscillated
electric clock signal, shifting a phase of the oscillated electric
clock signal and outputting a shifted electric clock signal,
modulating the optical pulse string based on the shifted electric
clock signal, converting the optical signal outputted from the
modulating step to an electric signal and outputting the electric
signal, controlling, in response to the electric signal, a phase
shift amount in the shifting step to maximize the electric signal
outputted from the converting step, and outputting the phase shift
amount for the shifting step.
[0047] According to another aspect of this invention, an optical
pulse generating method comprises outputting an optical signal with
a determined wavelength, outputting an electric clock signal with a
determined frequency, first modulating a power of the optical
signal based on the electric signal and outputting a first
modulated optical signal, second modulating a power of the first
modulated optical signal based on the electric signal and
outputting a second modulated optical signal, multiplexing one of
the first modulated optical signal and the second modulated optical
signal, outputting a part of a multiplexed optical signal and
branching a part of the multiplexed optical signal, converting
the-multiplexed optical signal to an electric signal, changing a
phase of the optical signal, and controlling, in accordance with
the electric signal, a phase change amount in the changing step to
maximize the electric signal outputted from the converting
step.
[0048] According to another aspect of this invention, an optical
pulse generating apparatus comprises a light source for outputting
an optical signal with a determined wavelength, an oscillator for
outputting an electric clock signal with a determined frequency, a
first optical modulator connected to the oscillator for modulating
a power of the optical signal with the electric clock signal and
outputting a first modulated optical signal, a second optical
modulator for modulating a power of the first modulated optical
signal outputted from the first optical modulator with the electric
clock signal and outputting a second modulated optical signal, an
optical multiplexer for receiving one of the first modulated
optical signal inputted to the second optical modulator and the
second modulated optical signal outputted from the second optical
modulator, outputting a part of a multiplexed optical signal and
branching a part of the multiplexed optical signal, a photo
detector for converting the multiplexed optical signal from the
optical multiplexer to an electric signal, a phase changing unit
for changing a phase of an optical signal, and a controlling
circuit for receiving the electric signal outputted from the photo
detector and the electric clock signal outputted from the
oscillator, and controlling a phase change amount of the phase
changing unit to match a phase of the electric signal and a phase
of the electric clock signal.
[0049] According to another aspect of this invention, an optical
pulse generating method comprises outputting an optical signal with
a determined wavelength, outputting an electric clock signal with a
determined frequency, first modulating a power of the optical
signal with the electric clock signal and outputting a first
modulated optical signal, second modulating a power of the first
modulated optical signal outputted with the electric signal and
outputting a second modulated optical signal, multiplexing one of
the first modulated optical signal and the second modulated optical
signal. outputting a part of a multiplexed optical signal and
branching a part of the multiplexed optical signal, converting the
multiplexed optical signal to an electric signal, changing a phase
of an optical signal, and controlling, in response to the electric
signal and the electric clock signal, a phase change amount in the
changing step to match a phase of the electric signal and a phase
of the electric clock signal.
[0050] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are give by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
[0051] Other objects features, and advantages of the invention will
be apparent from the following description when taken in
conjunction with the accompany drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] The present invention will become more fully understood from
the detailed description give hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not limitative of the present invention, and wherein:
[0053] FIG. 1 shows a fundamental configuration block chart of an
optical pulse position detecting circuit according to embodiment 1
of this invention;
[0054] FIG. 2 illustrates a phase shifter in the optical pulse
position detecting circuit of FIG. 1;
[0055] FIG. 3 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 1;
[0056] FIG. 4 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 1;
[0057] FIG. 5 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 1;
[0058] FIG. 6 shows a fundamental configuration block chart of an
optical pulse position detecting circuit according to embodiment 2
of this invention;
[0059] FIG. 7 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 6 when a phase
shift amount is optimal;
[0060] FIG. 8 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 6 when the phase
shift amount is optimal;
[0061] FIG. 9 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 6 when the phase
shift amount is more than the optimal phase shift amount;
[0062] FIG. 10 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 6 when the phase
shift amount is more than the optimal phase shift amount;
[0063] FIG. 11 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 6 when the phase
shift amount is less than the optimal phase shift amount;
[0064] FIG. 12 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 6 when the phase
shift amount is less than the optimal phase shift amount;
[0065] FIG. 13 shows a fundamental configuration block chart of an
optical pulse position detecting circuit according to embodiment 3
of this invention;
[0066] FIG. 14 shows a fundamental configuration block chart of an
optical pulse position detecting circuit according to embodiment 4
of this invention;
[0067] FIG. 15 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 14;
[0068] FIG. 16 shows an explanatory chart of an operation of the
optical pulse position detecting circuit of FIG. 14;
[0069] FIG. 17 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 5 of
this invention;
[0070] FIG. 18 shows an explanatory chart of an operation of the
optical pulse generating apparatus of FIG. 17 when a phase-shift
amount is optimal;
[0071] FIG. 19 shows an explanatory chart of an operation of the
optical pulse generating apparatus in FIG. 17 when the phase shift
amount is more than the optimal phase shift amount;
[0072] FIG. 20 shows an explanatory chart of an operation of the
optical pulse generating apparatus in FIG. 17 when the phase shift
amount is less than the optimal phase shift amount;
[0073] FIG. 21 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 6 of
this invention;
[0074] FIG. 22 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 7 of
this invention;
[0075] FIG. 23 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 8 of
this invention;
[0076] FIG. 24 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 9 of
this invention;
[0077] FIG. 25 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 10 of
this invention;
[0078] FIG. 26 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 11 of
this invention;
[0079] FIG. 27 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 12 of
this invention;
[0080] FIG. 28 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 13 of
this invention;
[0081] FIG. 29 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 14 of
this invention;
[0082] FIG. 30 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 15 of
this invention;
[0083] FIG. 31 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 16 of
this invention;
[0084] FIG. 32 shows an explanatory chart of an operation of the
optical pulse generating apparatus of FIG. 31 when a phase shift
amount is less than the optimal value;
[0085] FIG. 33 shows an explanatory chart of an operation of the
optical pulse generating apparatus of FIG. 31 when a phase shift
amount is more than the optimal value;
[0086] FIG. 34 shows an explanatory chart of an operation of the
optical pulse generating apparatus of FIG. 31 when a phase shift
amount is optimal;
[0087] FIG. 35 shows a fundamental configuration block chart of an
optical pulse generating apparatus of FIG. 31 including an
adder;
[0088] FIG. 36 shows a fundamental configuration block chart of an
optical pulse generating apparatus of FIG. 31 with a phase shifter
relocated;
[0089] FIG. 37 shows a fundamental configuration block chart of an
optical pulse generating apparatus of FIG. 31 with a multiplexer
repositioned;
[0090] FIG. 38 shows a fundamental configuration block chart of an
optical pulse generating apparatus of FIG. 36 with a multiplexer
repositioned;
[0091] FIG. 39 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 17 of
this invention;
[0092] FIG. 40 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 18 of
this invention;
[0093] FIG. 41 shows a fundamental configuration block chart of an
optical pulse generating apparatus according to embodiment 19 of
this invention;
[0094] FIG. 42 shows a block chart showing relationship a data
signal D and a polarization scramble signal S for the apparatus of
FIG. 41;
[0095] FIG. 43 shows a block chart for controlling phases between
optical modulators;
[0096] FIG. 44 shows an explanatory chart for FIG. 43;
[0097] FIG. 45 shows a block chart for detecting pulse
position;
[0098] FIG. 46 shows a block chart for generating an optical pulse;
and
[0099] FIG. 47 shows an explanatory chart for FIG. 46.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0100] Embodiment 1.
[0101] FIG. 1 shows a configuration block chart of an optical pulse
position detecting circuit in an embodiment of this invention.
[0102] In FIG. 1, the optical pulse string input terminal 1 for
inputting an optical pulse string in which optical pulses are
generated in a determined repetitive ratio and the optical
modulator 3 for receiving the optical pulse string, modulating an
optical power of the optical pulse string based on an electric
clock signal and outputting an optical signal are illustrated. In
FIG. 1, the phase shifter 4 for shifting a phase of the electric
clock signal, the oscillator 5 for generating an electric clock
signal with a same frequency as the repetitive ratio of the optical
pulse string as a driving signal for the optical modulator, the
photo detector 8 for converting an optical power of an optical
signal to an electric signal, a phase controlling circuit 90 for
outputting a control signal for controlling the phase shift amount
of the phase shifter 4 and the phase shift amount output terminal
10 for outputting the phase shift amount of the phase shifter are
also illustrated.
[0103] As the optical modulator 3, optical modulators for
controlling an output optical power level by an electric signal,
e.g., a Lithium Niobate (LiNbO3) Mach-Zehnder type optical
modulator can be used. The phase shifter 4 is a device for
controlling the phase shift amount with an electric signal.
[0104] FIG. 2 illustrates an example in an embodiment of the phase
shifter 4.
[0105] In FIG. 2, a phase shifter input terminal 13, a circulator
14, a phase shifter output terminal 15 and a varactor diode 16 are
illustrated. Since a capacity of the varactor diode changes with a
voltage, a reflective phase of a microwave at the varactor diode is
controlled by a voltage of a control signal outputted from the
phase controlling circuit 90. Many types of phase shifters. such as
a phase shifter for switching a transmission line length by a
digital control, an analog phase shifter which uses a balanced
modulator, a phase shifter for switching a transmission line length
by a switch or a motor, are available in a market. For these phase
shifters, control voltages indicate the shift amounts of the phase
shifters. Therefore, when the control voltage outputted from the
phase controlling circuit 90 is monitored by the phase shift amount
output terminal 10, a phase shift amount, i.e., a pulse position
can be detected. As the photo detector 8, a photo diode (PD) etc.
for converting an optical signal to an electric signal can be
used.
[0106] An operation is explained with reference to FIG. 3
[0107] According to the technique of this invention, a pulse
position is a phase relationship of the inputted optical pulse
string and a standard electric clock signal outputted from the
oscillator 5. An optical pulse is inputted from the optical pulse
string input terminal and outputted to the optical modulator 3. The
optical modulator 3 is driven by an electric clock signal outputted
from the oscillator 5. A frequency of the electric clock signal
outputted from the oscillator 3 same as a repetitive ratio of the
optical pulse string inputted from the optical pulse string input
terminal 1 The phase shifter 4 is controlled to maximize an optical
power level outputted from the optical modulator 3 by the phase
controlling circuit 90. As shown in FIG. 3, when the phase shifter
is controlled to maximize the output from the optical modulator,
the phase shift amount corresponds to the pulse position.
[0108] In FIG. 3, three optical pulses at pulse 1, pulse 2 and
pulse 3 in the optical pulse string correspond to phase shift
amount 1, phase shift amount 2 and phase shift amount 3. Even if an
optical pulse string with pulse position is inputted, the phase of
the electric clock signal is automatically shifted by the phase
shifter and the optical power level of the optical signal outputted
from the optical modulator 3 is maximized.
[0109] A technique for controlling the phase shifter 4 to maximize
the optical power outputted from the optical modulator 3 is shown
in FIG. 4.
[0110] The phase controlling circuit 90 controls the phase shift
amount of the phase shifter by monitoring an output from the photo
detector 8. When the phase is increased by .DELTA..PHI. (step S2),
if the output from the photo detector increases (step S3 and YES
for step S4), the phase is further increased by .DELTA..PHI. (steps
S8 and S2). When the phase is increased by .DELTA..PHI., if the
output from the photo detector is not increased (step S3 and N0 for
step S4) the phase is once reduced by .DELTA..PHI. and returned to
the original phase (step S5). Then, the phase is further reduced by
.DELTA..PHI. (step S6), and the output from the photo detector is
detected (step S7). If the output from the photo detector is
increased by repeatedly reducing the phase by .DELTA..PHI., the
phase shifter 4 can be controlled to maximize the optical power
level outputted from the optical modulator 3.
[0111] FIG. 5 shows actual operations.
[0112] In FIG. 5, the phase of the electric clock signal shifts as
indicated by arrows 1, 2, 3, 4, 5, 6 and 7, the pulse position
almost matches with a target position A of the electric clock
signal. According to the algorithm in FIG. 4, a shift direction for
increasing the optical power is detected by either increasing or
decreasing the phase by .DELTA..PHI.. Therefore, even if an optical
power level corresponds to two phase shift amounts, the pulse
position and the target position A can coincide. Further, in the
algorithm of FIG. 4, since the optical power levels before and
after increasing or decreasing the phase by .DELTA..PHI. are
compared, it is not necessary to obtain the transmittancy rate in
the optical modulator. The phase controlling circuit 90 in FIG. 1
can be realized by providing a simple logic circuit or a simple CPU
and software. An algorithm besides the algorithm in FIG. 4 can be
also used.
[0113] The pulse position which is detected according to this
invention may be used when processing of a signal synchronized with
the phase of the inputted optical pulse string is necessary. The
processing of the signal synchronized with the phase of the
inputted pulse includes modulating of the signal, time division
multiplexing by optical processing, time division demultiplexing by
optical processing, etc.
[0114] Embodiment 2.
[0115] FIG. 6 shows a configuration block chart of the optical
pulse position detecting circuit in another embodiment of this
invention.
[0116] In FIG. 6, the optical pulse string input terminal 1, the
optical modulator 3, the phase shifter 4, the oscillator 5, the
photo detector 8 for converting an optical signal to an electric
signal, a phase shifter controlling circuit 9 for controlling a
phase shift amount of the phase shifter 4, the phase shift amount
output terminal 10 for outputting the phase shift amount of the
phase shifter, a dither signal generating circuit 11, a phase
comparator 12 for detecting synchronization and the phase
controlling circuit 90 are illustrated. The phase shifter
controlling circuit 9 includes an adder 20 and an amplifier 21. The
adder 20 and the amplifier 21 can easily be realized by operation
amplifiers. The phase comparator 12 includes a mixer 17, an
amplifier 18 and a low-pass filter 19. The phase controlling
circuit 90 includes the dither signal generating circuit 11, the
phase comparator 12 and the phase shifter controlling circuit
9.
[0117] The dither signal generating circuit 11 outputs a dither
signal D with a micro amplitude in a low frequency f of 1 kHz to 15
kHz. The dither signal D which is an alternating current element is
added to an error signal E which is a direct current element
outputted from the phase comparator 12 by the adder 20, and a
controlling voltage V is generated. The controlling voltage V is
applied to the phase shifter 4. Synchronization of an electric
signal P outputted from the photo detector 8 and the dither signal
D is detected by the phase comparator 12. An output signal from the
phase comparator 12 is inputted to the adder 20 as the error signal
E. Thus, a feedback circuit is configured.
[0118] A basic operation of FIG. 6 is similar to FIG. 1, except
that FIG. 6 includes a step of detecting synchronization to detect
a phase shift amount for maximizing an optical power level of an
optical signal outputted from the optical modulator in addition to
the operations shown in FIG. 1. An operation principle is discussed
with reference to FIGS. 7-12.
[0119] FIGS. 7 and 8 show an operation when the phase shift amount
of the phase shifter 4 is a value for maximizing an output from the
optical modulator. When the transmittancy rate in the
optical-modulator 3 is as shown in (b) of FIG. 7 and a phase of a
driving signal for the optical modulator is modulated to a micro
amplitude by the dither signal D with the frequency f which is as
shown in (a) of FIG. 7, a low frequency signal element of an
optical signal as shown in (c) of FIG. 7 is outputted from the
optical modulator. As also shown in FIG. 8, the output (c) in FIG.
7 from the optical modulator is a low frequency signal element with
a frequency 2f. Since an element with the frequency f does not
exist, an output from the phase comparator 12 after detecting
synchronization of the output (c) in FIG. 7 from the optical
modulator 3 and the dither signal (a) in FIG. 7 is zero, and the
error signal E of zero is outputted from the phase comparator
12.
[0120] FIGS. 9 and 10 show an operation when the phase shift amount
of the phase shifter is more than the value for maximizing the
output from the optical modulator.
[0121] When the transmittancy rate in the optical modulator is as
shown in (b) of FIG. 9 and a phase of a driving signal for the
optical modulator is modulated to a micro amplitude by the dither
signal D with the frequency f as shown in (a) of FIG. 9, a low
frequency signal element of an optical signal as shown in (c) of
FIG. 9 is outputted from the optical modulator. Since the phase of
the output (c) in FIG. 9 from the optical modulator and the phase
of the dither signal (a) in FIG. 9 are inverted as also shown in
FIG. 10, an output from detecting synchronization is negative.
Hence, a negative error signal E is outputted from the phase
comparator 12. The error signal E is a direct current element. A
phase shift amount (controlling voltage V) corresponding to a value
of the error signal E is generated and outputted by the phase
shifter controlling circuit 9. When the outputted error signal E is
negative, the phase shift amount is reduced.
[0122] FIGS. 11 and 12 show an operation when the phase shift
amount of the phase shifter is less than the value for maximizing
the output from the optical modulator. When the transmittancy rate
in the optical modulator is as shown in (b) of FIG. 11 and a phase
of a driving signal for the optical modulator is modulated to a
micro amplitude by the dither signal D with the frequency f as
shown in (a) of FIG. 11, a low frequency signal element (c) of an
optical signal in FIG. 11 is outputted from the optical modulator.
Since the phase of the output (c) in FIG. 11 from the optical
modulator and the phase of the dither signal (a) in FIG. 11 are the
same, the output from detecting synchronization is positive. Hence,
a positive error signal E is outputted from the phase comparator
12. When the positive error signal E is outputted, the phase shift
amount is increased.
[0123] Since synchronization is detected according to this
invention, an accurate feedback circuit is realized in a relatively
simple configuration. Since synchronization is detected, the dither
signal can be modulated to a micro amplitude and sensitivity can be
improved. Since the frequency of the dither signal for detecting
synchronization does not relate to the repetitive frequency of the
inputted optical pulse string, a low frequency of around 1 kHz for
easy processing can be selected. Since the dither signal is an
alternate current element, an error is not caused for the
controlling voltage V indicating the phase shift amount which is
the direct current element outputted from the phase shifter
controlling circuit 9.
[0124] Embodiment 3.
[0125] FIG. 13 shows a configuration block chart of the optical
pulse position detecting circuit according to another embodiment of
this invention.
[0126] In FIG. 13, in addition to the elements in FIG. 6, a light
source 22 for generating an optical pulse string, an optical fiber
23 for a transmission line, a transmission time detecting circuit
25 and a transmission time output terminal 52 are provided. The
light source 22 for generating the optical pulse string is driven
by an output from the oscillator 5 for generating an electric clock
signal. As the light source 22 for generating the optical pulse
string, a gain switching operation of a semiconductor laser, a mode
lock operation of the semiconductor laser, a mode lock operation in
an external oscillating structure, etc. can be used.
[0127] The oscillator 5 generates electric clock signals with two
or more different frequencies. The transmission time detecting
circuit 25 measures a transmission time of an optical pulse in the
optical fiber 23 for the transmission line by storing and operating
a change in an output from the phase shift amount output terminal
10 by changing the frequency of the electric clock signal from the
oscillator 5. The measured transmission time is outputted from the
transmission time output terminal 52 to various sensors.
[0128] An operation is as follows.
[0129] A time for which an optical pulse is transmitted in the
optical fiber 23 for the transmission line is T (sec) and a
frequency of an electric clock signal of the oscillator 5 is f
(Hz). (The frequency f of the electric clock signal is different
from the frequency f of the dither signal D.) A relative pulse
delay time calculated from an output from the phase shift amount
output terminal 10 is t (sec). Then, a following equation is
obtained:
T=N/f+t (1)
[0130] In equation (1), N is a natural number and it is a number of
optical pulses which exist simultaneously in the optical fiber 23
for the transmission line.
[0131] When the equation (1) is differentiated by f, a following
equation is obtained:
N=f.sup.2.multidot.(dt/df) (2)
[0132] Therefore, the time T for which the optical pulse is
transmitted in the optical fiber for the transmission line is
calculated as follows:
T=f.sup.2.multidot.(dt/df)+t (3)
[0133] The equation (3) shows that the time T (sec) for which the
optical pulse is transmitted in the optical fiber 23 for the
transmission line is obtained from the frequency f (Hz) of the
electric clock signal of the oscillator 5, the relative pulse delay
time t (sec) calculated from the output from the phase shift amount
output terminal 10 and dt/df. The dt/df is obtained by measuring an
amount of change dt of the pulse delay time t when the clock
frequency f outputted from the oscillator 5 is changed by df.
Accordingly, the time T for which the optical pulse is transmitted
in the optical fiber 23 for the transmission line is obtained from
the equation (3) which is extended from the equation (1). The
transmission time detecting circuit 25 is a circuit in which the
frequency of the electric clock-signal outputted from the
oscillator 5 is changed by df, an amount of change dt of an output
from the phase shift amount output terminal 10 is inputted and the
time T for which the optical pulse is transmitted is calculated by
the equation (3). The transmission time detecting circuit 25 is
easily realized by a computer.
[0134] Since a transmission rate of an optical pulse in the optical
fiber 23 for the transmission line is generally known, a length W
of the optical fiber 23 for the transmission line is obtained by
measuring the time T for which the optical pulse is transmitted in
the optical fiber for the transmission line. According to this
invention, the length W of the optical fiber 23 for the
transmission line is measured from the time T for which the optical
pulse is transmitted in the optical fiber for the transmission
line. Further, this invention provides a method for measuring the
length of the transmission line accurately in a very dynamic range.
This invention may be applied to a transmission line, e.g., a
waveguide, space, etc., which transmits the optical pulse besides
the optical fiber.
[0135] Since the length W of the transmission line can be measured
accurately according to this invention, this invention can be
applied to sensors. For example, since a fiber length and
refractive rate of the optical fiber change according to an
environment temperature, the environment temperature of the optical
fiber can be obtained by measuring the transmission time in the
optical fiber. Similarly, this invention may also be used to
measure a pressure applied to the optical fiber. This invention may
also be applied to an optical time domain reflectometer (OTDR:
Optical Time Domain Reflectometer) for measuring a transmission
time of an optical pulse from a fault location in the transmission
line.
[0136] Embodiment 4.
[0137] FIG. 14 shows a configuration block chart of the optical
pulse position detecting circuit according to another embodiment of
this invention.
[0138] The modulator in FIG. 6 is replaced by a semiconductor
electro-absorption type optical modulator 42 in FIG. 14. The
semiconductor electro-absorption type optical modulator 42 is a
device in which an optical absorption coefficient changes in
accordance with an applied voltage.
[0139] An operation is as follows.
[0140] A basic operation is same as in FIG. 6.
[0141] As shown in FIG. 15, when the applied voltage increases, a
transmittancy rate of the semiconductor electro-absorption type
optical modulator 42 sharply drops. Generally, a logarithm of the
transmittancy rate is proportional to the applied voltage.
Therefore, as shown in FIG. 15, the semiconductor
electro-absorption type optical modulator driven by the electric
clock signal in a sine function curve is a steep gate in a pulse
form.
[0142] FIG. 3 for explaining an operation principle is revised in
FIG. 16 to explain an operation of the semiconductor
electro-absorption type optical modulator.
[0143] Since the transmittancy rate of the optical modulator is a
steep gate, when the phase shift amount of the phase shifter
changes even slightly by .DELTA..PHI., an optical power level of a
light which is transmitted through the optical modulator is changed
even by .DELTA.P. Accordingly, the phase shift amount for
maximizing the optical power level transmitted through the optical
modulator can be detected sensitively.
[0144] Embodiment 5.
[0145] FIG. 17 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0146] In FIG. 17, the light source 26 for oscillating an optical
signal with a determined wavelength, the first optical modulator
3a, the optical amplifier 29, the second optical modulator 3b, the
optical multiplexer 2, an optical pulse output terminal 27, the
oscillator 5 and the phase shifter 4 which is a kind of a phase
changing unit according to this invention are illustrated. In FIG.
17, the photo detector 8, the phase comparator 12, the phase
shifter controlling circuit 9, the dither signal generating circuit
11 and the phase controlling circuit 90 which is a kind of a
controlling circuit according to this invention are also
illustrated. The phase shifter controlling circuit 9 includes the
adder 20 and the amplifier 21. The adder 20 and the amplifier 21
can be easily realized by operation amplifiers. The phase
comparator 12 includes the mixer 17, the amplifier 18 and the
low-pass filter 19. The phase controlling circuit 90 includes the
phase shifter controlling circuit 9, the dither signal generating
circuit 11 and the phase comparator 12.
[0147] The dither signal generating circuit 11 outputs a dither
signal D in a micro amplitude with a low frequency f of 1 kHz to 15
kHz. The dither signal D is added to an error signal E outputted
from the phase comparator 12 by the adder 20 and applied to the
phase shifter 4. Then, synchronization of an electric signal P
outputted from the photo detector 8 and the dither signal D is
detected by the phase comparator 12. An output signal from the
phase comparator 12 is inputted to the adder 20 as the error signal
E. Accordingly, a feedback circuit is configured.
[0148] As the light source 26, a semiconductor laser diode, fiber
laser, solid laser, etc. may be used. As the optical modulators 3a
and 3b, Lithium Niobate (LiNbO3:Lithium Niobate) Mach-Zehnder
(Mach-Zehnder) type optical modulator, etc. for controlling an
optical power level by an electric signal may be used. The phase
shifter 4 is a device for controlling a phase shift amount by an
electric signal. As discussed above regarding FIG. 1, many types of
devices for the phase shifter are available in the market. As the
optical amplifier 29, a fiber amplifier for using an optical fiber
doped with a rare-earth element, a semiconductor optical amplifier
for using a semiconductor, an optical amplifier for using
non-linear effect such as Raman effect, etc. may be used.
[0149] A power level of an optical signal outputted from the light
source 26 is modulated by an electric clock signal outputted from
the oscillator 5 in the optical modulator 3a and an optical pulse
signal is generated. Then, the power level of the optical pulse
signal is amplified by the optical amplifier 29. The optical pulse
signal is outputted from the optical amplifier 29 and the power
level of the optical pulse signal is further modulated by the
optical modulator 3b. Since the phase shifter 4 controls the
optical modulator 3b to synchronize with the inputted optical pulse
signal, a pulse width of the optical pulse outputted from the
optical modulator 3b is less than a pulse width of the optical
pulse inputted to the optical modulator 3b (W1>W2). The phase
shifter controlling circuit 9 receives a dither signal and an
output from the phase comparator 12, and controls the phase shifter
4 to maximize the output from the photo detector 8. This operation
is discussed with reference to FIGS. 18, 19 and 20.
[0150] In FIG. 18, the phase shift amount of the phase shifter is
optimal.
[0151] The phase shift amount is set to maximize a transmittancy
rate in the optical modulator 3b as shown in (b) of FIG. 18. When
the phase shift amount of the phase shifter is modulated to a micro
amplitude by the dither signal D with the frequency f as shown in
(a) of FIG. 7 and the dither signal is superimposed on the driving
signal of the optical modulator 3b, a low frequency signal element
of the optical signal as shown in (c) of FIG. 7 is outputted from
the optical modulator 3b. As shown in FIG. 18, the output from the
optical modulator 3b is a low frequency signal element with a
frequency 2f and an element with a frequency f does not exist.
Therefore, an output from detecting synchronization of an output
from the optical modulator 3b which is (c) in FIG. 18 and the
dither signal which is (a) in FIG. 18 is zero and the error signal
is zero.
[0152] FIG. 19 shows an operation when the phase shift amount of
the phase shifter is more than the optimal value.
[0153] The transmittancy rate in the optical modulator 3b is shown
in (b) of FIG. 19. When the driving signal of the optical modulator
3b is modulated to a micro amplitude by the dither signal with the
frequency f as shown in (a) of FIG. 19, a low frequency signal
element as shown in (c) of FIG. 19 is outputted from the optical
modulator. Since phases of the output signal from the optical
modulator 3b as shown in (c) of FIG. 19 and the dither signal as
shown in (a) of FIG. 19 are inverted, an output from detecting
synchronization is negative. Therefore, a negative error signal is
outputted.
[0154] FIG. 20 shows an operation when the phase shift amount of
the phase shifter is less than the optimal value.
[0155] The transmittancy rate in the optical modulator 3b is shown
in (b) of FIG. 20. When a driving signal of the optical modulator
3b is modulated to a micro amplitude by a dither signal with the
frequency f as shown in (a) of FIG. 20, a low frequency signal
element as shown in (c) of FIG. 20 is outputted from the optical
modulator. Since phases of the output signal from the optical
modulator 3b as shown in (c) of FIG. 20 and the dither signal as
shown in (a) of FIG. 20 are matched, an output from detecting
synchronization is positive. Hence, a positive error signal is
outputted.
[0156] Since the phase shift amount is constantly optimized by a
feedback control, even if there is a fluctuation in a transmission
delay time of the optical signal in the optical amplifier 29 and
the transmission lines, driving signals of two optical modulators
3a and 3b constantly synchronize with the optical signal. Since the
optical modulators synchronize, a short pulse is outputted from the
optical pulse output terminal 27.
[0157] Since synchronization is detected according to this
invention, an accurate feedback circuit is realized in a relatively
simple configuration. Further, since a frequency of the dither
signal for detecting synchronization does not relate to a
repetitive frequency of the inputted optical pulse string, a low
frequency of 1 kHz to 15 kHz for easy processing can be selected.
Besides, since the dither signal is an alternate current element in
a micro amplitude, an error is not caused in the phase shift amount
which is a direct current element.
[0158] A RF amplifier may be used to amplify the electric clock
signal for driving the optical modulators 3a and 3b in the
embodiment of this invention. The electric clock signal for driving
the optical modulators 3a and 3b may be doubled. Further, even if
the optical amplifier 29 is not provided, since there is a delay in
transmission of the optical signal in the transmission line between
the optical modulator 3a and the optical modulator 3b, the
embodiment of this invention is still possible. Besides the phase
controlling circuit 90 may be configured according to the algorithm
as shown in FIG. 4.
[0159] Embodiment 6.
[0160] FIG. 21 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0161] In FIG. 21, an output signal from the optical modulator 3a
is branched to two transmission lines by the optical multiplexer
2a. This operation is not in FIG. 17.
[0162] In FIG. 21, the first optical modulator 3a, a first optical
amplifier 29a, the second optical modulator 3b, a second optical
multiplexer 2b, a first optical pulse output terminal 27a, the
oscillator 5, a first phase shifter 4a, a photo detector 8a, a
phase comparator 12a, a first phase shifter controlling circuit 9a,
the dither signal generating circuit 11, a first phase controlling
circuit 90a and a second phase controlling circuit 90b are
illustrated. The phase shifter controlling circuit 9a includes an
adder 20a and an amplifier 21a. The phase comparator 12a includes a
mixer 17a, an amplifier 18a and a low-pass filter 19a. In FIG. 21,
a second optical amplifier 29b, a third optical modulator 3c, a
third optical multiplexer 2c, a second optical pulse output
terminal 27b, a second phase shifter 4b, another photo detector 8b,
a phase comparator 12b and a phase shifter controlling circuit 9b
are illustrated. The phase shifter controlling circuit 9b includes
an adder 20b and an amplifier 21b. The phase comparator 12b
includes a mixer 17b, an amplifier 18b and a low-pass filter
19b.
[0163] The dither signal generating circuit 11 outputs a dither
signal in a micro amplitude with a low frequency.
[0164] When the optical pulse is branched by the optical
multiplexer 2a, an optical pulse is inputted to the optical
amplifier 29a. Then, the optical pulse in a short pulse is
outputted from the optical pulse output terminal 27a as in FIG. 17.
Similarly, when an optical signal which is branched by the optical
multiplexer 2a inputted to the optical amplifier 29b, an optical
pulse in a short pulse is outputted from the optical pulse output
terminal 27b. Since the optical signal is branched and processed in
parallel in embodiment 6, two optical pulse output terminals 27a
and 27b can be provided. In FIG. 21, the first and second phase
controlling circuits 90a and 90b receive dither signals from a
dither signal generating circuit 11. However, when characteristics
of the first and second optical amplifiers 29a and 29b are
different or lengths of the transmission lines are different, the
dither signal generating circuits 11 may be provided respectively
to the first and second phase controlling circuits 90a and 90b.
[0165] Obviously, a number of branching from the optical
multiplexer 2a and a number of the optical pulse output terminals
may be increased, if necessary.
[0166] Embodiment 7.
[0167] FIG. 22 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0168] In FIG. 22, the light source 26, the first optical modulator
3a, the first optical amplifier 29a, the second optical modulator
3b, the first optical multiplexer 2a, the second optical amplifier
29b, the third optical modulator 3c, the second optical multiplexer
2b and the optical pulse output terminal 27 are illustrated. In
FIG. 22, the oscillator 5, the phase shifter 4a, the photo detector
8a, the phase comparator 12a, the phase shifter controlling circuit
9a, the dither signal generating circuit 11a, the first phase
controlling circuit 90a and the second phase controlling circuit
90b are also illustrated. The phase shifter controlling circuit 9a
includes the adder 20a and the amplifier 21a. The phase comparator
12a includes the mixer 17a, the amplifier 18a and the low-pass
filter 19a. In FIG. 22, the second phase shifter 4b, the photo
detector 8b, the phase comparator 12b, the phase shifter
controlling circuit 9b and a dither signal generating circuit 11b
are also illustrated. The phase shifter controlling circuit 9b
includes the adder 20b and the amplifier 21b. The phase comparator
12b includes the mixer 17b, the amplifier 18b and the low-pass
filter 19b.
[0169] In FIG. 22, the third optical modulator 3c which is not in
FIG. 17 is provided to process an optical signal serially. In FIG.
22, the photo detector 8b, the dither signal generating circuit
11b, the phase comparator 12b, the phase shifter controlling
circuit 9b and the phase shifter 4b are also provided to optimize
the phase of the electric clock signal for driving the optical
modulator 3c.
[0170] An operation principle of FIG. 22 is same as FIG. 21.
However, in FIG. 22, since three serial optical modulators are
driven synchronously, an optical pulse signal with a shorter pulse
width than the optical pulse outputted from the optical pulse
generating apparatus in FIG. 17 can be outputted. In FIG. 22, since
the phase shifters 4a and 4b must be controlled independently, the
dither signal generating circuits 11a and 11b for performing the
feedback control output dither signals with different frequencies.
For example, when the dither signal generating circuit 11a outputs
the dither signal with the frequency of 10 kHz, the dither signal
generating circuit 11b outputs the dither signal with the frequency
of 15 kHz.
[0171] In addition to the configuration of FIG. 22, fourth and
fifth optical modulators may be connected serially to perform a
similar feedback control. In that case, an optical pulse signal
with an even shorter pulse width can be outputted.
[0172] Embodiment 8.
[0173] FIG. 23 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0174] In FIG. 23, the phase shifter 4 controls the phase of the
electric clock signal for driving the optical modulator 3a. Since a
fluctuation in a transmission delay time of an optical signal
generated in the optical amplifier 29 is compensated by controlling
the phase of the optical pulse signal outputted from the optical
modulator 3a, phases of the optical pulse inputted to the optical
modulator 3b and the optical pulse outputted from the optical pulse
output terminal 27 do not change. Therefore, as in FIG. 17, an
optical pulse signal with a short pulse width can be generated, and
the phase of the optical pulse outputted from the optical pulse
output terminal 27 can be kept constant.
[0175] Embodiment 9.
[0176] FIG. 24 shows a configuration block chart of an optical
pulse generating apparatus according to an embodiment of this
invention.
[0177] In FIG. 24, the optical delayer 24 which is a kind of the
phase shifter according to this invention, the optical delayer
controlling circuit 34 and a delay controlling circuit 91 which is
a kind of a controlling circuit according to this invention are
illustrated. The optical delayer controlling circuit 34 includes
the adder 20 and the amplifier 21 which can be easily realized by
operation amplifiers. The delay controlling circuit 91 includes the
dither signal generating circuit 11, the phase comparator 12 and
the optical delayer controlling circuit 34. The delay controlling
circuit 91 may also be configured by the algorithm as shown in FIG.
4.
[0178] The optical delayer 24 controls a transmission delay time of
the optical signal by a control signal outputted from the optical
delayer controlling circuit 34. As the optical delayer 24, a device
for changing a length of an optical transmission line by a step
motor, a device for controlling the transmission delay time by
giving a stress on an optical fiber by a piezo element, a device
for changing a delay time by switching transmission lines, etc. may
be used.
[0179] A basic operation of FIG. 24 is same as in FIG. 17. An
optical delay amount of the optical delayer 24 is constantly
optimized by a feedback control. Therefore, even if there is a
fluctuation in a transmission delay time of the optical signal in
the optical amplifier 29, driving signals for two optical
modulators 3a and 3b constantly synchronize with the optical
signal. Since two optical modulators synchronize, a short pulse is
outputted from the optical pulse output terminal 27. Since the
phase of the optical pulse signal outputted from the optical
modulator 3a is controlled to compensate the fluctuation in the
transmission delay time of the optical signal in the optical
amplifier 29, phases of the optical pulse inputted to the optical
modulator 3b and the optical pulse outputted from the optical pulse
output terminal 27 do not change. Therefore, the phase of the
optical pulse outputted from the optical pulse output terminal 27
can be kept constant.
[0180] Embodiment 10.
[0181] FIG. 25 shows a configuration block chart of another optical
pulse generating apparatus according to an embodiment of this
invention.
[0182] In FIG. 25, a delay controlling circuit 91a which is a kind
of a controlling circuit according to this invention is
illustrated. The phase changing unit according to this invention
includes the phase shifter 4 and the optical delayer 24. The
optical delayer controlling circuit 34 includes the amplifier
21.
[0183] In contrast to FIG. 24, a dither signal is superimposed on
the phase shift amount of the phase shifter 4 in FIG. 25. Since the
photo detector 8 detect a same signal even if the dither signal is
superimposed in the optical delayer 24 as shown in FIG. 24 or the
dither signal is superimposed in the phase shifter 4 as shown in
FIG. 25, operations are same. However, it is easier to superimpose
the dither signal in the phase shifter than the optical delayer 24.
Since many realized optical delayers include a moving unit with a
relatively low response frequency, e.g., such as a step motor,
piezo element, etc., the optical delayers tend to respond to a
dither signal which includes an alternate current element with a
low frequency. In FIG. 25, the dither signal with a low frequency
is not inputted to the optical delayer 24. Instead, a control
signal with a direct current element generated from the error
signal E is inputted to the optical delayer 24. Hence, an optical
delayer with a relatively slow response can be used.
[0184] Embodiment 11.
[0185] FIG. 26 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0186] In FIG. 26, the first and second optical modulators are
replaced by a single optical modulator.
[0187] In FIG. 26, a first polarization multiplexer/demultiplexer
35a, a second polarization multiplexer/demultiplexer 35b and a
polarization state converter 45 are illustrated.
[0188] A polarized wave of an optical signal outputted from the
light source 26 is adjusted to a determined polarized wave
condition (P wave). A power level of the optical signal outputted
from the light source 26 is modulated by an electric clock signal
outputted from the oscillator 5 in the optical modulator 3 and an
optical pulse signal is generated. Then, a power level of the
optical pulse signal is amplified by the optical amplifier 29. The
polarization multiplexer/demultiplexers 35a and 35b are devices
which output optical signals from different ports according to the
polarized wave condition (P or S wave) of the optical signal. The
polarization multiplexer/demultiplexers are sold as polarization
beam splitters and polarization prisms. Since the polarized wave
condition of the optical signal outputted from the light source 26
is adjusted to a determined polarized wave condition (P wave), the
polarization multiplexer/demultiplexer 35a outputs the optical
signal received from the light source to the optical modulator 3.
The optical signal outputted from the optical modulator 3 is
outputted to the optical delayer 24 by the polarization
multiplexer/demultiplexer 35b.
[0189] When a determined delay is provided to the optical signal by
the optical delayer 24, a power level of the optical signal is
amplified by the optical amplifier 29 and the optical signal is
inputted to the polarization state converter 45. Then, a polarized
wave of the optical signal is converted (P wave-S wave) by the
polarization state converter 45 and inputted to the polarization
multiplexer/demultiplexer 35b. The optical signal is returned to
the optical modulator 3. The optical pulse re-modulated by the
optical modulator 3 is outputted to the optical multiplexer 2 by
the polarization multiplexer/demultiplexer 35a. Since the
polarization state (S wave) is vertical to the polarization state
(P wave) outputted from the light source 26, the optical pulse is
not inputted to the light source 26.
[0190] As stated, the optical signal is transmitted to the optical
modulator 3 twice and the power level of the optical signal is
modulated twice by the optical modulator 3. Since the optical
modulator 3 modulates synchronously with the inputted optical pulse
signal by controlling the optical delayer 24, an optical pulse with
a short pulse width is outputted from the optical pulse output
terminal 27. A control principle of the optical delayer is same as
in FIG. 24.
[0191] The polarization multiplexer/demultiplexers 35a and 35b may
be replaced by optical multiplexer/demultiplexers. As the optical
multiplexer/demultiplexers, cheap devices such as an optical
coupler may be used. However, since the optical coupler has
demultiplexing and branching loss fundamentally, the optical
coupler is not appropriate as the optical multiplexer/demultiplexer
judging from an optical S/N ratio (Optical Signal Noise Ratio).
[0192] Embodiment 12.
[0193] FIG. 27 shows a configuration block chart of an optical
pulse generating apparatus according to an embodiment of this
invention.
[0194] In FIG. 27, a first light source 26a, a second light source
26b, the first optical modulator 3a, the second optical modulator
3b, the optical demultiplexer 33, the optical amplifier 29, the
third optical modulator 3c, the first phase shifter 4a, the second
phase shifter 4b, a first phase controlling circuit 90a and a
second phase controlling circuit 90b are illustrated. The first and
second phase controlling circuits 90a, 90b include first and second
phase shifter controlling circuits 9a, 9b, first and second dither
signal generating circuits and first and second phase comparators
12a, 12b, respectively.
[0195] In FIG. 27, two light sources for outputting optical signals
with different wavelengths are provided in addition to the elements
in FIG. 23 and short pulses with different wavelengths are
outputted from the optical pulse output terminal. An optical signal
outputted from the first light source is modulated to a short pulse
by the first optical modulator 3a and the third optical modulator
3c. An optical signal outputted from the second light source 26b is
modulated to a short pulse by the second optical modulator 3b and
the third optical modulator 3c. A phase of an electric clock signal
for driving the optical modulator 3a is controlled by the first
phase shifter 4a.
[0196] A driving signal for the first phase shifter 4a is outputted
by the first phase shifter controlling circuit 9a. The first dither
signal generating circuit 11a outputs a dither signal in a micro
amplitude with a low frequency and the dither signal is applied to
the phase shifter 4a by an adder 20a. Synchronization of the
electric signal outputted from the photo detector 8 and the dither
signal is detected by the first phase comparator 12a, which
includes a mixer 17a, an amplifier 18a and a low-pass filter 19a.
An output signal from the first phase comparator 12a is inputted to
the adder 20a as an error signal. The output from the adder 20a is
amplified by an amplifier 21a and outputted as the driving signal.
Thus, a feedback circuit is configured.
[0197] Similarly, a phase of an electric clock signal for driving
the optical modulator 3b is controlled by the second phase shifter
4b. A driving signal for the second phase shifter 4b is outputted
by the second phase shifter controlling circuit 9b. The second
dither signal generating circuit 11b outputs a dither signal in a
micro amplitude with a low frequency. The dither signal is applied
to the phase shifter 4b by an adder 20b. Synchronization of the
electric signal outputted from the photo detector 8 and the dither
signal is detected by the second phase comparator 12b, which
includes a mixer 17b, an amplifier 18b and a low-pass filter 19b.
An output signal from the second phase comparator 12b is inputted
to the adder 20b as an error signal, the output of which is
amplified by an amplifier 21b, and a feedback circuit is
configured.
[0198] The first dither signal generating circuit 11a and the
second dither signal generating circuit 11b output dither signals
with different frequencies. For example, the first dither signal
generating circuit 11a outputs a dither signal with a frequency of
10 kHz and the second dither signal generating circuit 11b outputs
a dither signal with a frequency of 15 kHz.
[0199] In addition to the configuration in FIG. 27, third and
fourth light sources may be connected in parallel to perform a
similar feedback control. Then, short pulses with three or four
different wavelengths can be outputted.
[0200] Embodiment 13.
[0201] FIG. 28 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0202] In FIG. 28, a delay controlling circuit 92 is illustrated.
The phase shifter controlling circuit 9 includes the adders 20a and
20b and the amplifier 21a. In FIG. 28, a temperature detecting
circuit 41, a reference voltage generator 37, a temperature drift
detecting circuit 40, a subtractor 39 and the amplifier 21b are
also illustrated.
[0203] In FIG. 28, the temperature detecting circuit 41, the
reference voltage generator 37 and the temperature drift detecting
circuit 40 are provided in addition to the elements in FIG. 17 to
perform feed-forward controlling. The temperature detecting circuit
41 detects a temperature in an apparatus such as the optical
amplifier 29. As the temperature detecting circuit 41, a thermistor
element of which resistance value changes according to a
temperature or a semiconductor element for detecting a temperature
may be used. A change in a temperature of the optical amplifier 29
can be detected by calculating a difference between a voltage of an
output signal from the temperature detecting circuit 41 and a
reference voltage outputted from the reference voltage generator by
the subtractor 39. A fluctuation of a delay time of an optical
pulse inputted to the optical modulator 3b is primarily caused by a
fluctuation of a transmission delay time in the optical amplifier
29. The transmission delay time in the optical amplifier 29 is
caused by a change in a temperature of the optical amplifier.
Therefore, a fluctuation of a delay time of the optical pulse
inputted to the optical modulator 3b can be predicted by detecting
a change in a temperature in the optical amplifier. A feed-forward
control of the phase shift amount generated by the phase shifter 4
can be performed to compensate the predicted fluctuation of the
delay time by inputting the output signal from the temperature
drift detecting circuit 40 to the adder 20b.
[0204] Since the feed-forward control is performed, an error which
must be compensated by the feedback circuit can be reduced. Hence,
accurate processing can be achieved.
[0205] Further, even if a very big error is caused and the error is
beyond a feeding range of the feedback circuit, the error can be
reduced to the feeding range of the feedback circuit by
feed-forward controlling. Hence, controlling for a wide dynamic
range can be achieved and the optical pulse generating apparatus
can operate in a wider temperature range.
[0206] Embodiment 14.
[0207] FIG. 29 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0208] In FIG. 29, a wavelength detecting circuit 36, the reference
voltage generator 37, a wavelength drift detecting circuit 38, the
subtractor 39 and the amplifier 21b are illustrated.
[0209] In FIG. 29, the wavelength detecting circuit 36, the
reference voltage generator 37 and the wavelength drift detecting
circuit 38 are provided to perform feed-forward controlling in
addition to the elements in FIG. 17. The wavelength detecting
circuit 36 can use an output from a temperature controlling circuit
of the light source 26. a wave meter, etc. A change in a wavelength
of an optical signal outputted from the light source 26 can be
detected by calculating a difference between an output signal from
the wavelength detecting circuit 36 and a reference voltage
outputted from the reference voltage generator 37 by the subtractor
39. A fluctuation in a delay time of an optical pulse inputted to
the optical modulator 3b is primarily caused by a fluctuation in a
transmission delay time of the optical amplifier 29. The
transmission delay time of the optical amplifier 29 is caused by a
change to a wavelength of an optical signal. Therefore, a change in
a delay time of an optical pulse inputted to the optical modulator
3b can be predicted by detecting a change in a wavelength of an
optical signal inputted to the optical amplifier. When an output
signal from the wavelength drift detecting circuit 38 is inputted
to the adder 20b, a feed-forward control of a phase shift amount
generated by the phase shifter 4 can be performed to compensate a
fluctuation in the predicted delay time.
[0210] Since the feed-forward controlling is added, an error which
must be compensated by the feedback circuit can be reduced. Hence,
more accurate processing can be achieved.
[0211] Further, even if a very big error is caused and the error is
beyond a feeding range of the feedback circuit the error can be
reduced to the feeding range of the feedback circuit by
feed-forward controlling. Hence, optical pulse generating apparatus
can operate in a wider dynamic range.
[0212] When both feed-forward controlling by the temperature drift
detecting circuit as shown in FIG. 28 and feed-forward controlling
by the wavelength drift detecting circuit as shown in FIG. 29 are
performed, a control accuracy is further improved. The feed-forward
controlling by the temperature drift detecting circuit FIG. 28 and
the feed-forward controlling by the wavelength drift detecting
circuit in FIG. 29 may be used in other embodiments of this
invention.
[0213] Embodiment 15.
[0214] FIG. 30 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0215] The optical modulators 3a and 3b in FIG. 17 are replaced by
semiconductor electro-absorption type optical. modulators 42a and
42b in FIG. 30.
[0216] As discussed for FIG. 11. since the semiconductor
electro-absorption type optical modulators can realize steep gates,
an optical pulse with a shorter width can outputted. When a phase
shift amount of the phase shifter changes even slightly, an optical
power which is transmitted through the optical modulators changes
considerably. Therefore, an optimal phase shift amount can be
detected sensitively. Since the semiconductor electro-absorption
type optical modulators provide steep gates, when the phase shift
amount is much different from an optical value, an optical power
outputted from the semiconductor electro-absorption type optical
modulator 42b becomes small, and the error signal might not be
detected. In that case, feed-forward controlling by the temperature
drift detecting circuit as shown in FIG. 28 or feed-forward
controlling by the wavelength drift detecting circuit as shown in
FIG. 29 should be performed additionally.
[0217] Embodiment 16.
[0218] FIG. 31 shows a configuration block chart of an optical
pulse generating apparatus according to another embodiment of this
invention.
[0219] In FIG. 31, the light source 26 for oscillating an optical
signal with a determined wavelength, the first optical modulator
3a, the optical amplifier 29, the second optical modulator 3b, the
optical multiplexer 2, the optical pulse output terminal 27, the
oscillator 5 for outputting an electric clock signal with a
determined frequency and the phase shifter 4 which is a kind of a
phase changing unit according to this invention are illustrated. In
FIG. 31, the photo detector 8, a clock re-generating circuit 92,
the phase comparator 12, the phase shifter controlling circuit 9, a
first RF (Radio Frequency) amplifier 28a which is a kind of a first
modulation signal generating circuit according to this invention
and a second RF amplifier 28b which is a kind of a second
modulation signal generating circuit according to this invention
are illustrated.
[0220] In FIG. 31, a phase controlling circuit 99 which is a kind
of a controlling circuit according to this invention is also
illustrated. The phase controlling circuit 99 receives an electric
signal from the photo detector 8 and an electric clock signal
outputted from the oscillator 5 and controls a phase change amount
of the phase shifter 4 to match a phase of the electric signal and
a phase of the electric clock signal.
[0221] The clock re-generating circuit 92 includes limiter
amplifiers 93a and 93b and a band-pass filter 94, for example. The
phase shifter controlling circuit 9 includes the amplifier 21. The
amplifier 21 can be easily realized by an operation amplifier. The
phase comparator 12 includes the mixer 17, the amplifier 18 and the
low-pass filter 19. The phase controlling circuit 99 includes the
clock re-generating circuit 92, the phase comparator 12 and the
phase shifter controlling circuit 9.
[0222] As the light source 26, a semiconductor laser diode, fiber
laser, solid laser, etc. may be used. As the optical modulators 3a
and 3b, Lithium Niobate (LiNbO3:Lithium Niobate) Mach-Zehnder
(Mach-Zehnder) type optical modulator, semiconductor
electro-absorption type optical modulator, etc. for controlling an
optical power level by an electric signal may be used. The phase
shifter 4 is a device for controlling a phase shift amount by an
electric signal. As discussed regarding FIG. 1, many types of
devices for the phase shifter are available in the market. As the
optical amplifier 29, a fiber amplifier for using a fiber doped
with a rare-earth element, a semiconductor optical amplifier for
using a semiconductor, an optical amplifier for using non-linear
effect such as Raman effect, etc. can be used.
[0223] A power level of an optical signal outputted from the light
source 26 is modulated by an electric clock signal outputted from
the oscillator 5 in the optical modulator 3a and an optical pulse
signal is generated. The power level of the optical pulse signal is
amplified by the optical amplifier 29. The optical pulse signal is
outputted from the optical amplifier 29 and the power level of the
optical pulse signal is further modulated by the optical modulator
3b. Since the phase shifter 4 controls the optical modulator 3a to
modulate synchronously with the optical pulse signal inputted to
the optical modulator 3b via the phase controlling circuit 99, a
pulse width of the optical pulse outputted from the optical
modulator 3b is less than a pulse width of the optical pulse
inputted to the optical modulator 3b. The operations and effects
are same as in FIG. 17.
[0224] The clock re-generating circuit 92 receives the electric
signal from the photo detector 8 and outputs a re-generating clock
signal. The phase comparator 12 receives the re-generating clock
signal from the clock re-generating circuit 92 and the electric
clock signal from the oscillator 5 and compares the phase of the
re-generating clock signal and the phase of the electric clock
signal. Then, the phase comparator 12 controls the phase shifter 4
to make an error to zero. This operation is discussed with
reference to FIGS. 32, 33 and 34.
[0225] FIG. 32 shows an operation when the phase shift amount of
the phase shifter is more than an optimal value.
[0226] An output signal from the photo detector 8 is shown as (a)
in FIG. 32. The clock re-generating circuit 92 extracts a
re--generating clock signal as (b) in FIG. 32 from the signal from
the photo detector 8. The re-generating clock signal outputted from
the clock re-generating circuit and the electric clock signal
inputted to the second modulation signal generating circuit (RF
amplifier 28b in this case) as (c) in FIG. 32 are inputted to the
mixer 17. Then, the mixer 17 outputs a signal as (d) in FIG. 32.
When the output signal is inputted to the low-pass filter 19, an
error signal with a positive value is outputted and the phase shift
amount of the phase shifter is decreased. Accordingly, a phase of
the electric clock signal as (c) in FIG. 32 is shifted to a
direction of an optimal pulse position (left in FIG. 32).
[0227] FIG. 33 shows an operation when the phase shift amount of
the phase shifter is less than the optimal value.
[0228] An output signal from the photo detector 8 is shown as (a)
in FIG. 33. The clock re-generating circuit 92 extracts a
re-generating clock signal as (b) in FIG. 33 from the re-generating
clock signal. The re-generating clock signal outputted from the
clock re-generating circuit and the electric clock signal inputted
to the second modulation signal generating circuit (RF amplifier
28b in this case) as (c) in FIG. 33 are inputted to the mixer 17.
Then, the mixer 17 outputs a signal as (d) in FIG. 33. When the
output signal is inputted to the low-pass filter 19, an error
signal with a negative value is outputted and the phase shift
amount of the phase shifter is increased. Accordingly, a phase of
the electric clock signal as (c) in FIG. 33 is shifted to a
direction of an optimal pulse position (right in FIG. 33).
[0229] In FIG. 34, the phase shift amount of the phase shifter is
optimal.
[0230] An output signal from the photo detector 8 is shown as (a)
in FIG. 34. The clock re-generating circuit 92 extracts a
re-generating clock signal as (b) in FIG. 34 from the signal from
the photo detector 8. The re-generating clock signal outputted from
the clock re-generating circuit and the electric clock signal
inputted to the second modulation signal generating circuit (RF
amplifier 28b in this case) as (c) in FIG. 34. are inputted to the
mixer 17. Then, the mixer 17 outputs a signal as (d) in FIG. 34.
When the output signal is inputted-to the low-pass filter 19, an
error signal of zero is outputted. Accordingly, the phase of the
electric clock signal is not changed.
[0231] As discussed, the phase shift amount is constantly
controlled. Therefore, even if the transmission delay time of an
optical signal in the optical amplifier 29 and a transmission line
fluctuates, phases of the driving signals for two optical
modulators 3a and 3b are kept constant.
[0232] A transmission delay time from the optical multiplexer 2 to
the second optical modulator 3b, a transmission delay time from the
optical multiplexer 2 to the mixer 17, a transmission delay time
from the oscillator 5 to the mixer 17, a transmission delay time
from the oscillator 5 to the first optical modulator 3a and a
transmission delay time from the oscillator 5 to the second optical
modulator 3b are all different. Therefore, even if the mixer 17
controls to reduce the error to zero., the optical modulator 3a
does not alway synchronize. In order to synchronize two optical
modulators 3a and 3b completely, an adder 100 for offering a proper
offset voltage V to an output from the phase shifter controlling
circuit 9 may be provided as shown in FIG. 35 in the configuration
to control the phase relationship. Since the optical modulators
synchronize completely, a short pulse is outputted from the optical
pulse output terminal 27.
[0233] Since synchronization is detected according to this
invention, a highly accurate control circuit can be realized in a
relatively simple configuration. Further, it is not necessary to
superimpose a dither signal according to this invention.
[0234] As discussed regarding FIG. 17, controlling is riot
performed to maximize an output according to this invention.
Therefore, output wave-forms from the optical modulators 3a and 3b
are not relied on. Therefore, in FIG. 31, RF amplifiers are used as
the first modulation signal generating circuit and the second
modulation signal generating circuit. However, as the modulation
signal generating circuits, frequency multiplexer, wave form
former, modulators, etc. may be also used.
[0235] Even if the optical amplifier 29 does not exist, a delay in
transmission of an optical signal in a transmission line exists,
this invention can still be applied.
[0236] When the oscillator 5 generates an electric clock signal
with a sufficient power, the first modulation signal generating
circuit and the second modulation signal generating circuit are not
necessary.
[0237] When the phase shifter 4 is provided at an input side of the
second modulation signal generating circuit as shown in FIG. 36,
this invention can still be applied.
[0238] In the configuration of FIG. 36. a change in a controlling
object (phase shifter 4 in this case) is not detected to return to
the controlling object. Therefore it is a kind of feed-forward
circuit. The inventors have confirmed through experiments that
effects of this invention can be achieved even in such feed-forward
circuits.
[0239] As illustrated in FIGS. 37 and 38, the optical multiplexer 2
can be provided at an output side of the optical modulator 3b
instead of the input side of the optical modulator 3b. Further, in
the configuration of FIG. 17, the optical multiplexer 2 may be
provided at an output side of the optical modulator 3b instead of
the input side of the optical modulator 3b.
[0240] Embodiment 17.
[0241] FIG. 39 shows a configuration block chart of a pulse
generating circuit according to an embodiment of this
invention.
[0242] A distingruisher 97 may be realized with a D-type flip-flop,
for example. The D-type flip-flop delays a data inputted from a
data input D by a clock signal inputted from a clock input C and
outputs from an output Q. Hence, data inputted from the data signal
input terminal 98 is synchronized with the clock signal and
outputted to the optical modulator 3b. Accordingly, an optical
signal inputted to the optical modulator 3b and a modulation signal
for driving the optical modulator 3b can be synchronized. Other
operation principles are the same as in FIG. 31.
[0243] FIG. 39 shows a case in which a signal with RZ (Return to
Zero) format is modulated by NRZ (Non Return to Zero) signal.
[0244] A phase relationship of modulation signals for driving the
optical modulators 3a and 3b can be controlled by providing the
adder 100 for providing a proper offset voltage V to an output from
the phase shifter controlling circuit 9 as shown in FIG. 35. Even
if a phase fluctuation amount changes in the optical amplifier 29,
other transmission lines and circuits, the phase relationship of
the optical modulators can be kept constant by controlling the
offset voltage V.
[0245] According to this embodiment, since the optical pulse
generated by the optical modulator 3a is modulated by a data signal
inputted from the data signal input terminal 98, an optical pulse
generating apparatus for generating a modulated pulse can be
provided. As modulation methods for this embodiment, there are a
power level modulation method, phase modulation method,
frequency-modulation method, polarization plane modulation method,
etc.
[0246] For the configuration in FIGS. 35-38, the distinguisher 97
in FIG. 39 may be provided.
[0247] Embodiment 18.
[0248] FIG. 40 is a configuration block chart of a pulse generating
apparatus according to an embodiment of this invention.
[0249] In FIG. 40, the light source 26 for oscillating an optical
signal with a determined wavelength, the first optical modulator
3a, the delayer 24, the optical amplifier 29, the second optical
modulator 3b, the optical multiplexer 2, and the optical pulse
output terminal 27 are illustrated. In FIG. 40, the oscillator 5,
the photo detector 8, the clock re-generating circuit 92, the phase
comparator 12, the optical delayer controlling circuit 34, the
first RF amplifier 28 which is a kind of the first modulation
signal generating circuit, the distinguisher 97 which is a kind of
the second modulation signal generating circuit according to this
invention. the data signal input terminal 98, and a delay
controlling circuit 101 which is a kind of a controlling circuit
according to this invention are also illustrated.
[0250] The clock re-generating circuit 92 includes limiter
amplifiers 93a and 93b and the band-pass filter 94. The optical
delayer controlling circuit 34 includes the amplifier 21. The
amplifier 21 can be easily realized by an operation amplifier. The
phase comparator 12 includes the mixer 17, the amplifier 18, and
the low-pass filter 19. The delay controlling circuit 101 includes
the clock re-generating circuit 92, the phase comparator 12, and
the optical delayer controlling circuit 34.
[0251] Fundamental operations are same as in FIG. 39. The optical
modulator 3a and the optical modulator 3b can be synchronized by
inputting an error signal from the phase comparator 12 to the
optical delayer 24.
[0252] The optical delayer 24 may be connected to an output side of
the optical amplifier 29 instead of an input side of the optical
amplifier 29.
[0253] The optical multiplexer 2 may be connected to an output side
of the optical modulator 3b instead of an input side of the optical
modulator 3b as shown in FIGS. 37 and 38.
[0254] Embodiment 19.
[0255] FIG. 41 shows a configuration block chart of a bit
synchronization circuit which includes a polarization scrambler
according to an embodiment of this invention.
[0256] In FIG. 41, the light source 26 for oscillating an optical
signal with a determined wavelength, the optical modulator 3,
the-distinguisher 97 which is the first modulation signal
generating circuit, the data signal input terminal 98, the
oscillator 5, the optical amplifier 29 for compensating a loss of
the optical modulator 3 and the polarization scrambler 95 which is
an embodiment of the optical modulator are illustrated. In FIG. 41,
the optical multiplexer 2, the optical signal output terminal 27,
the RF amplifier 28 which is an embodiment of the second modulation
signal generating circuit, the photo detector 8, the clock
re-generating circuit 92, the phase comparator 12, the phase
shifter controlling circuit 9, and a phase controlling circuit 102
which is a kind of the controlling circuit according to this
invention are also illustrated.
[0257] The clock re-generating circuit 92 includes the limiter
amplifiers 93a and 93b, a multiplexer 96 and the band-pass filter
94. The phase shifter controlling circuit 9 includes the amplifier
21. The amplifier 21 can be easily realized by an operation
amplifier. The phase comparator 12 includes the mixer 17, the
amplifier 18, and the low-pass filter 19.
[0258] Operations are as follows.
[0259] A power level of an optical signal outputted from the light
source 26 is modulated by the optical modulator 3. Generally, a
modulation format of NRZ (Non Return to Zero) is used. A driving
signal for the optical modulator 3 is generated from a clock signal
outputted from the oscillator 5 and a data signal inputted from the
data signal input terminal 98 by the distinguisher 97. An output
from the distinguisher 97 is generally amplified by a voltage
amplifying circuit (not illustrated), if necessary.
[0260] The optical amplifier 29 compensates for a loss in the
optical signal of which poser level is modulated. As discussed in
other embodiments, the optical amplifier 29 includes optical fibers
of several to several tens of meters, a fluctuation in transmission
delay time due to environment temperatures cannot be ignored.
Polarization scramble is performed for an optical signal inputted
to the polarization scrambler 95 which is modulated in power level
by a clock signal outputted from the RF amplifier 28. However, the
phase relationship of the data signal and the polarization scramble
signal S must be kept constant.
[0261] A part of an optical signal outputted from the polarization
scrambler is branched by the optical multiplexer 2. Then, an
optical signal is converted to an electric signal by the photo
detector 8. Since a power level is not changed by the polarization
scramble, an electric signal detected by the photo detector 8 is
also an electric signal like the electric signal detected from a
power modulation signal applied by the optical modulator 3.
[0262] The electric signal is converted and a re-generating clock
signal is re-generated from the clock re-generating circuit 92. The
clock re-generating circuit 92 includes the multiplexer 96 which
includes the mixer, the band-pass filter 94, the amplifier and the
limiter. A phase-lock loop circuit may be provided as the clock
re-generating circuit 92 to achieve a same effect. Phases of the
re-generating clock signal and the electric clock signal outputted
from the oscillator 5 are compared in the mixer 17.
[0263] A voltage signal corresponding to a relationship between the
phase of the electric clock signal and the phase (data phase) of
the clock signal re-generated from the electric signal detected by
the photo detector 8 in the mixer 17 is compared with a determined
reference voltage in the amplifier 21. Then, an error signal is
outputted. The phase shifter 4 is driven by the error signal and a
phase of the data signal D in the polarization scrambler and a
phase of the polarization signal S are synchronized.
[0264] For example, as shown in FIG. 42, controlling may be
performed to make the polarization scramble signal S to zero at a
center of each bit of the data signal D. When the polarization
scramble signal S crosses a zero level, polarization is switched.
Accordingly, polarization is switched at a center of each bit. The
relationship of the phases can be changed by modifying the
reference voltage in the amplifier 21.
[0265] In this discussion, it is assumed that the optical amplifier
29 causes uncertain fluctuations of the phases. It is assumed that
connection lines for the electric signals and an optical line
between an output from the polarization scrambler to the photo
detector are short enough and the fluctuations can be ignored.
Therefore, even when an optical signal inputted to the polarization
scrambler 95 is branched by the optical multiplexer 2 and inputted
the photo detector 8, a same effect can be achieved. When such
fluctuations cannot be ignored, the adder 100 is provided to input
the offset voltage V as FIG. 35.
[0266] According to the optical pulse position detecting circuit
and method discussed, for example, in connection with FIG. 1, an
optical pulse string is inputted to the optical modulator and a
phase of an electric clock signal for driving the optical modulator
is controlled to maximize an optical signal power outputted from
the optical modulator. Since a phase shift amount for maximizing an
optical signal power is outputted from the optical modulator, an
optical pulse position can be detected accurately.
[0267] According to the optical pulse position detecting circuit
and method discussed, for example, in connection with FIG. 6, an
optical pulse string is inputted to the optical modulator and a
dither signal is superimposed on a phase of an electric clock
signal for driving the optical modulator. Then, synchronization of
a dither signal element extracted from an optical signal outputted
from the optical modulator and the dither signal superimposed on
the electric clock signal is detected and a feed back is outputted
from detecting synchronization to the phase shifter. Accordingly,
the phase shift amount of the phase shifter is controlled to
maximize an optical power outputted from the optical modulator.
Since a phase shift amount for maximizing an optical signal power
from the optical modulator is outputted, an optical pulse position
can be detected accurately.
[0268] According to the optical pulse position detecting circuit in
FIG. 13, a repetitive ratio of an optical pulse string is changed
and a relationship of the repetitive ratio of the optical pulse
string and a phase shift amount is operated. Accordingly, a
transmission time of the optical pulse string in a transmission
line can be detected for a dynamic range.
[0269] According to the optical pulse position detecting circuit in
FIG. 14, a semiconductor electro-absorption type optical modulator
is used as the optical modulator Therefore, a pulse position can be
detected more accurately.
[0270] According to the optical pulse generating apparatus and
method discussed in connection with FIG. 17, a phase of an optical
signal is changed to maximize an output from the photo detector.
Therefore, a feedback control is performed to optimize a phase
change amount constantly. Since the first optical modulator and the
second optical modulator are synchronized at this time an optical
pulse with a short width can be outputted.
[0271] According to another aspect of the optical pulse generating
apparatus and method of the present invention, a dither signal
element is added to a phase change a and synchronization of the
dither signal element extracted from an optical signal outputted
from the optical modulator and the dither signal is detected. Then,
a feedback of an output from detecting synchronization is
performed. Therefore, a maximum optical signal power is outputted
from the optical modulator.
[0272] According to another aspect of the optical pulse generating
apparatus of the present invention, the phase shifter controlling
circuit receives a dither signal and an output from the phase
comparator and controls the phase shifter to shift a phase of a
signal for driving the second optical modulator to maximize an
output from the photo detector. Therefore, a phase shift amount is
constantly optimized by feedback controlling. Since the first and
second optical modulators synchronize, an optical pulse with a
short width can be outputted.
[0273] When a fluctuation in a transmission delay time of an
optical pulse signal inputted to the second optical modulator is
compensated, a phase of an outputted optical pulse can be kept
constant.
[0274] According to another aspect of the optical pulse generating
apparatus of the present invention, the optical delayer controlling
circuit receives the dither signal and the output from the phase
comparator and controls the optical delayer for delaying an optical
signal inputted to the second optical modulator to maximize an
output from the photo detector. Therefore, a feedback control is
performed to optimize the delay time constantly. Since the first
and second optical modulators synchronize, an optical pulse with a
short pulse width can be outputted.
[0275] When a fluctuation in a transmission delay time of an
optical pulse signal inputted to the second optical modulator is
compensated, a phase of an outputted optical pulse can be kept
constant.
[0276] According to another aspect of the optical pulse generating
apparatus of the present invention. a dither signal is superimposed
on a phase of a signal for driving the first and second optical
modulators and the optical delayer for delaying an optical signal
inputted to the second optical modulator is controlled to maximize
an output from the photo detector. Since the dither signal is
superimposed on the phase shifter, a device with a low response can
be used as the optical delayer.
[0277] According to another aspect of the optical pulse generating
apparatus of the present invention, an optical signal is
transmitted to the optical modulator twice and the optical pulse
inputted to the optical modulator and a driving signal of the
optical modulator are synchronize Therefore, an optical pulse with
a short width can be outputted.
[0278] Since only a single optical modulator is necessary, a
configuration can be simplified.
[0279] According to the optical pulse generating apparatus
discussed in connection with FIG. 27, an optical pulse outputted
from a first light source and an optical pulse outputted from a
second light source can be modulated to short pulses
simultaneously. Therefore, optical pulses with two wavelengths can
be outputted.
[0280] According to the optical pulse generating apparatus
discussed in connection with FIG. 28, a phase shift amount of the
phase shifter is controlled by a feed-forward control to compensate
a fluctuation in a predicted delay time based on an output signal
from the temperature drift detecting circuit. Therefore, an optical
pulse with a short width can be outputted more accurately in a
dynamic range. Further, the optical pulse generating apparatus can
operate in a wider temperature range.
[0281] According to the optical pulse generating apparatus
discussed in connection with FIG. 29, a phase shift amount of the
phase shifter is controlled by a feed-forward control to compensate
a fluctuation in a predicted delay time based on an output signal
from the wavelength drift detecting circuit. Therefore, an optical
pulse with a short width can be outputted more accurately in a
dynamic range.
[0282] According to the optical pulse generating apparatus
discussed in connection with FIG. 30, the semiconductor
electro-absorption type optical modulator is used as the optical
modulator. Therefore, an optical pulse with a shorter width can be
outputted.
[0283] According to the optical pulse generating apparatus
discussed in connection with FIG. 21, optical pulses are processed
in parallel. Therefore, a plurality of optical pulse strings can be
outputted.
[0284] According to the optical pulse generating apparatus
discussed in connection with FIG. 22, an optical signal is
processed serially. Therefore, an optical pulse string with short
pulses can be outputted.
[0285] According to another aspect of the optical pulse generating
apparatus and method of the present invention, a phase of a
re-generating clock signal extracted from an optical signal
inputted to the first or second optical modulator and a phase of an
electric clock signal for driving the first or second optical
modulator are controlled to be matched. Accordingly, the first and
second optical modulators synchronize, and an optical pulse with a
short pulse width can be outputted.
[0286] According to another aspect of the optical pulse generating
apparatus and method of the present invention, the phases of the
re-generating clock signal and the electric clock signal are
compared and a fluctuation in a delay time is, detected. Therefore,
a highly accurate control circuit can be realized with a simple
configuration.
[0287] According to another aspect of the optical pulse generating
apparatus of the present invention, the phase shifter controlling
circuit receives an output from the phase comparator and controls
the phase shifter for shifting a phase of a driving signal for the
optical modulator. Therefore, a phase shift amount is constantly
optimized.
[0288] According to another aspect of the optical pulse generating
apparatus in the present invention, the optical delayer controlling
circuit receives an output from the phase comparator and controls
the phase delayer for delaying an optical signal inputted to the
optical modulator. Therefore, a delay time is constantly
optimized.
[0289] According to another aspect of the optical pulse generating
apparatus of the present invention, the optical modulator modulates
with a data signal and outputs a modulated optical pulse.
[0290] According to another aspect of the optical pulse generating
apparatus of the present invention, a phase of an electric clock
signal for driving the polarization scrambler and a phase data can
be synchronized in determined relationship. Therefore, an opening
of an eye can be enlarged to be an advantage for distinguishing a
signal even if an amplitude fluctuates at a receiving terminal due
to a fiber dispersion and a change in non-linear reflective
rate.
[0291] Having thus described several particular embodiments of the
invention, various alterations, modifications, and improvements
will readily occur to those skilled in the art. Such alterations,
modifications, and improvements are intended to be part of this
disclosure, and are intended to be within the spirit and scope of
the invention. Accordingly, the foregoing description is by way of
example only and is limited only as defined in the following claims
and the equivalents thereto.
* * * * *