U.S. patent application number 12/004709 was filed with the patent office on 2008-05-08 for data transmitter and method of generating none return to zero optical signal with clock component amplification.
Invention is credited to Hyunwoo Cho, Ki Ho Han, Je Soo Ko, Wangjoo Lee.
Application Number | 20080107427 12/004709 |
Document ID | / |
Family ID | 34545771 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080107427 |
Kind Code |
A1 |
Lee; Wangjoo ; et
al. |
May 8, 2008 |
Data transmitter and method of generating none return to zero
optical signal with clock component amplification
Abstract
The present invention relates to none return to zero (NRZ)
modulation method. The NRZ optical modulation is performed by
combining a clock signal and NRZ data at a sending end and signal
distortion capable of being generated when the clock signal and the
NRZ data are combined is optimized by controlling the magnitude and
phase of the clock signal. At the receiving end, the clock signal
is extracted by performing narrow band band-pass filtering of the
detected optical signal transmitted from a transmitter and data is
recovered using the clock signal. Therefore, a receiver structure
for clock extraction is simpler, an error rate of data recovery is
lower by clearly extracting the clock signal, and transmission
distance of the optical signal is longer.
Inventors: |
Lee; Wangjoo; (Daejeon-city,
KR) ; Cho; Hyunwoo; (Seoul, KR) ; Han; Ki
Ho; (Busan-city, KR) ; Ko; Je Soo;
(Daejeon-city, KR) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
1279 OAKMEAD PARKWAY
SUNNYVALE
CA
94085-4040
US
|
Family ID: |
34545771 |
Appl. No.: |
12/004709 |
Filed: |
December 21, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10860903 |
Jun 3, 2004 |
7330664 |
|
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12004709 |
Dec 21, 2007 |
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Current U.S.
Class: |
398/183 |
Current CPC
Class: |
H04B 10/541 20130101;
H04B 10/505 20130101 |
Class at
Publication: |
398/183 |
International
Class: |
H04B 10/04 20060101
H04B010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 5, 2003 |
KR |
2003-78116 |
Claims
1. A data transmitter generating an NRZ optical signal with clock
component amplification, comprising: an attenuator which attenuates
the magnitude of an electrical clock signal; a phase shifter which
controls the phase of the magnitude-attenuated electrical clock
signal; a driver which amplifies the magnitude of electrical NRZ
data; and an optical modulator with dual RF input port driven by
the electrical NRZ data from the driver and the phase-controlled
electrical clock signal from the phase shifter.
2. The data transmitter of claim 1, wherein the NRZ
optical-modulated signal is used for clock recovery by extracting
the clock signal by a narrow band band-pass filter included in a
receiver.
3. A data transmitting method generating an NRZ optical signal with
clock component amplification, comprising: attenuating the
magnitude of an electrical clock signal; controlling the phase of
the magnitude-attenuated electrical clock signal; amplifying the
magnitude of electrical NRZ data; and performing an NRZ optical
modulation of the amplified electrical NRZ data and the
phase-controlled electrical clock signal.
4. The method of claim 3, wherein the NRZ optical-modulated signal
is used for clock recovery by extracting the clock signal by a
narrow band band-pass filter included in a receiver.
Description
BACKGROUND OF THE INVENTION
[0001] This application claims priority from Korean Patent
Application No. 2003-78116, filed on Nov. 5, 2003, in the Korean
Intellectual Property Office, the disclosure of which is
incorporated herein in its entirety by reference.
1. FIELD OF THE INVENTION
[0002] The present invention relates to a digital optical
communication system, and more particularly, to non-return to zero
(NRZ) modulation method.
2. DESCRIPTION OF THE RELATED ART
[0003] In a digital optical communication system, a sending end
outputs optical signal data synchronized by its own clock and a
receiving end recovers the received data. In a digital
communication system, data recovery is to read input data at every
instant pointed by the clock signal and to judge whether the input
data value is "0" or "1". Even if a clock frequency of the
receiving end is a little bit different from a clock frequency of
the sending end, data cannot be normally recovered. Therefore,
almost all receivers use a clock for data recovery by extracting
the clock from the input data not to have own clock.
[0004] Nowadays, in 2.5 Gbps and 10 Gbps electrical time division
multiplexing (ETDM) optical transmission system, optical
transmission using an NRZ modulation method is usually used. The
NRZ modulation method has a simple configuration and price
competition in comparison to the other modulation methods. However,
since NRZ data does not have any frequency component corresponding
to the clock signal, there's no straightforward method of
extracting clock signal from NRZ data.
[0005] A clock extracting method, which is generally used in a
digital communication system using the NRZ modulation method, uses
a phase locked loop (PLL) component. However, considering systems,
whose transmission rate is more than 10 Gbps such as 40 Gbps, it is
very difficult to manufacture an electrical component, such as the
PLL and also the price of the component becomes very expensive even
though the component can be manufactured.
SUMMARY OF THE INVENTION
[0006] The present invention provides a data transmitter and method
of modulating an optical signal in a sending end so as to easily
extract a clock in a receiving end of an optical communication
system with an NRZ modulated data using a low price band-pass
filter easy to manufacture instead of a high price PLL difficult to
manufacture.
[0007] According to an aspect of the present invention, there is
provided a data transmitter including: an attenuator which
attenuates the magnitude of an electrical clock signal; a phase
shifter which controls the phase of the magnitude-attenuated
electrical clock signal; a combiner which combines the
phase-controlled electrical clock signal and electrical NRZ data; a
driver which amplifies the magnitude of the combined signal; and an
optical modulator which performs NRZ optical modulation of the
combined signal amplified by the driver.
[0008] According to another aspect of the present invention, there
is provided a data transmitter including: an attenuator which
attenuates the magnitude of an electrical clock signal; a phase
shifter which controls the phase of the magnitude-attenuated
electrical clock signal; a driver which amplifies the magnitude of
electrical NRZ data; and an optical modulator with dual RF input
ports which performs NRZ optical modulation of the electrical NRZ
data and the electrical clock signal by receiving the electrical
NRZ data from the driver and the phase-controlled electrical clock
signal from the phase shifter.
[0009] According to another aspect of the present invention, there
is provided a data transmitting method including: attenuating the
magnitude of an electrical clock signal; controlling the phase of
the magnitude-attenuated electrical clock signal; combining the
phase-controlled electrical clock signal and electrical NRZ data;
amplifying the magnitude of the combined signal; and performing an
NRZ optical modulation of the combined signal amplified by the
driver.
[0010] According to another aspect of the present invention, there
is provided a data transmitting method including: attenuating the
magnitude of an electrical clock signal; controlling the phase of
the magnitude-attenuated electrical clock signal; amplifying the
magnitude of electrical NRZ data; and performing an NRZ optical
modulation of the amplified electrical NRZ data and the
phase-controlled electrical clock signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The above and other features and advantages of the present
invention will become more apparent by describing in detail
exemplary embodiments thereof with reference to the attached
drawings in which:
[0012] FIG. 1 is a block diagram of a transmitter according to a
preferred embodiment of the present invention;
[0013] FIG. 2 is a flowchart of an optical modulation method
performed in the transmitter of FIG. 1;
[0014] FIG. 3 is a block diagram of a transmitter according to
another embodiment of the present invention;
[0015] FIG. 4 is a flowchart of an optical modulation method
performed in the transmitter of FIG. 3;
[0016] FIG. 5 is a RF spectrum result where a 40 Gbps optical
signal modulated by a conventional NRZ optical modulation method is
detected at a receiver;
[0017] FIG. 6 is a RF spectrum result where a 40 Gbps optical
signal modulated by the transmitters of FIGS. 1 and 3 is detected
at a receiver;
[0018] FIG. 7 is an output waveform of an NRZ optical signal
generated by a conventional transmitter;
[0019] FIG. 8 is an output waveform of an NRZ optical signal
generated by the transmitters of FIGS. 1 and 3;
[0020] FIG. 9 is a waveform of a modulation signal combined without
regard to the magnitude of two signals of electrical NRZ data and a
clock signal in a transmitter;
[0021] FIG. 10 is a waveform of a modulation signal multiplexed
regarding to the magnitude and phase differences of the NRZ data
and the clock signal;
[0022] FIG. 11 is a waveform of a clock extracted by a clock
extracting circuit from a 40 Gbps optical signal modulated by the
transmitters of FIGS. 1 and 3; and
[0023] FIG. 12 shows a receiving error rate where the clock signal
for data recovery is extracted by the present invention and a
receiving error rate where the clock signal for data recovery is
extracted by a convention method of using a non-linear device
(exclusive-or) for clock component generation and band-pass
filtering it.
DETAILED DESCRIPTION OF THE INVENTION
[0024] FIG. 1 is a block diagram of a transmitter 200 according to
a preferred embodiment of the present invention. FIG. 2 is a
flowchart of an optical modulation method performed in the
transmitter 200 of FIG. 1.
[0025] With reference to FIG. 1, the transmitter 200 includes an
NRZ data generator 210, an attenuator 220, a phase shifter 230, a
combiner 240, a driver 250, an optical modulator 260, and a light
source 270.
[0026] The NRZ data generator 210 generates electrical NRZ data
ENRZ synchronized with an electrical clock signal CLK. The
attenuator 220 receives the clock signal CLK and appropriately
attenuates the magnitude of the clock signal CLK before the clock
signal CLK and the NRZ data ENRZ are combined. The phase shifter
230 receives the clock signal magnitude-attenuated by the
attenuator 220, appropriately controls a phase of the attenuated
clock signal.
[0027] The combiner 240 combines the electrical clock signal from
the phase shifter 230 and the electrical NRZ data ENRZ generated
from the NRZ data generator 210. The driver 250 amplifies the
signal input from the combiner 240 and outputs the amplified signal
to the optical modulator 260. The optical modulator 260 generates
an NRZ modulated optical signal of the light emitted from the light
source 270 in response to the output signal of the driver 250.
[0028] If distortion of an optical signal waveform occurred
exceedingly due to data and clock combined modulation of optical
signal, it is difficult to reliably recover data with an extracted
clock signal though the clock signal extraction is easy in the
receiver. For the control range of the magnitude and phase of the
clock signal CLK by the attenuator 220 and the phase shifter 230,
values determined by experiments are used.
[0029] With reference to FIG. 2, an optical modulation procedure
performed in the transmitter 200 of FIG. 1 is described.
[0030] First, the magnitude of the clock signal CLK is
appropriately attenuated in step 2200 through the attenuator 220
(for example, about one tenth the magnitude of the NRZ data ENRZ).
The phase of the clock signal CLK is appropriately controlled in
step 2300 through the phase shifter 230 (for example, 90.degree.
phase shift). The controlled clock signal CLK and the NRZ data ENRZ
are combined in step 2400 through the combiner 240. The combined
signal is amplified in step 2500 through the driver 250. Finally,
the amplified signal is applied to the optical modulator 260 in
step 2600 for the optical signal modulation.
[0031] The result is that since the transmitter 200 modulates
optical signal using the combined electrical signal of clock and
NRZ data where the clock signal is magnitude and phase controlled
through the attenuator 220 and the phase shifter 230 before the
electrical clock signal CLK and the electrical NRZ data ENRZ are
combined, the optical signal output of the transmitter 200 contains
bigger clock component with respect to typical NRZ modulated
optical signal does. Thereby, the receiver receiving the optical
signal can easily extract the clock signal from the received
optical signal. Moreover, data recovery errors are minimized by
minimizing signal waveform distortion capable of being generated by
clock component amplification of the optical signal.
[0032] FIG. 3 is a block diagram of a transmitter 300 according to
another embodiment of the present invention. FIG. 4 is a flowchart
of an optical modulation method performed in the transmitter 300 of
FIG. 3.
[0033] With reference to FIG. 3, the transmitter 300 includes an
NRZ data generator 310, an attenuator 320, a phase shifter 330, a
driver 350, an optical modulator 360 having two high frequency
signal input ports, and a light source 370.
[0034] The NRZ data generator 310 generates electrical NRZ data
ENRZ synchronized to a clock signal CLK. The driver 350 receives
the NRZ data ENRZ from the NRZ data generator 310, amplifies the
data to a predetermined level, and outputs the amplified data to
one input port of the optical modulator 360.
[0035] The attenuator 320 receives an electrical clock signal CLK
and appropriately attenuates the magnitude of the clock signal CLK.
The phase shifter 330 receives the magnitude-attenuated clock
signal from the attenuator 320, appropriately controls the phase of
the magnitude-attenuated clock signal, and minimizes signal
distortion capable of being generated when the optical modulation
is performed. The clock signal CLK, whose magnitude and phase is
controlled by the attenuator 320 and the phase shifter 330, is
input to the other input port of the optical modulator 360.
[0036] The optical modulator 360, which is driven by the electrical
NRZ data ENRZ and the electrical clock signal CLK through the two
input ports, generates clock component amplified NRZ optical signal
of the light emitted from the light source 370 in response to the
two input modulating signals.
[0037] With reference to FIG. 4, an optical modulation procedure
performed in the transmitter 300 of FIG. 3 is described.
[0038] First, the magnitude of the clock signal CLK is
appropriately attenuated in step 3200 through the attenuator 320
(for example, about one tenth the magnitude of the NRZ data ENRZ).
The phase of the clock signal CLK is appropriately controlled in
step 3300 through the phase shifter 330 (for example, 90.degree.
phase shift). The NRZ data ENRZ is amplified in step 3500 through
the driver 350 while the magnitude and phase of the clock signal
CLK is being controlled. Finally, the magnitude and phase
controlled clock signal and the magnitude amplified NRZ data are
input to the optical modulator 360 and converted to the NRZ
modulated optical signal in step 3600.
[0039] With reference to FIGS. 1 through 4, according to the number
of high frequency input ports of the optical modulators 260, 360
included in the transmitters 200, 300, structures and optical
modulation methods of the transmitters 200, 300 are a little bit
different. However, types of the modulated optical signals are all
the same and a clock component included in the optical signal
generated from the transmitters 200, 300 is more amplified than a
clock component by a conventional NRZ modulation.
[0040] FIG. 5 is a RF spectrum result where a 40 Gbps optical
signal modulated by a conventional NRZ optical modulation method is
detected at a receiver. FIG. 6 is also a RF spectrum result where a
40 Gbps optical signal modulated by the transmitters 200, 300 of
FIGS. 1 and 3 is detected at a receiver.
[0041] With reference to FIGS. 5 and 6, in a case where the
modulation method according to the present invention is adapted,
much larger peak component than that of the optical signal
modulated by the conventional modulation method is generated at 40
GHz, a clock frequency. Since one scale of the vertical axis of
FIG. 5 is 5 dB and one scale of the vertical axis of FIG. 6 is 10
dB, the difference can be seen more exactly. That is, since the NRZ
optical signal generated by the optical modulation method according
to the present invention has larger clock frequency component than
the optical signal generated by the conventional NRZ modulation
method has, it is much easier that the receiver extracts the clock
signal from the input optical signal.
[0042] FIG. 7 is an output waveform of an NRZ optical signal
generated by a conventional transmitter. FIG. 8 is an output
waveform of an NRZ optical signal generated by the transmitters
200, 300 of FIGS. 1 and 3.
[0043] With reference to FIGS. 7 and 8, a waveform of an NRZ
optical signal with an amplified clock signal component according
to the modulation method of the present invention (refer to FIG. 8)
rarely has distortion but has clock frequency component of nearly
100 times larger signal to noise ratio than a conventionally NRZ
modulated optical signal (refer to FIGS. 5 and 6)
[0044] However, if the clock signal CLK is simply combined with the
electrical NRZ data without control of the magnitude and phase, the
modulated optical signal can be harmfully distorted resulting in
high data error rate in the receiving side. In the present
invention, to prevent the harmful distortion from being generated,
the modulation is performed after the magnitude and phase of the
combined clock signal CLK is appropriately controlled through the
attenuators 220, 320 and the phase shifters 230, 330. Therefore,
the distortion is rarely generated on an optical waveform of the
modulated optical signal and also it is easier to extract the clock
signal at the receiving end.
[0045] FIG. 9 is a waveform of a modulation signal without regard
to the magnitude of two signals when electrical NRZ data and a
clock signal are combined in a transmitter. FIG. 10 is a waveform
of a modulation signal combined regarding to the magnitude and
phase differences of the NRZ data and the clock signal.
[0046] With reference to FIG. 9, (a) shows electrical NRZ data
waveform corresponding to "01011000110", (b) shows a clock signal
waveform having the same magnitude as the NRZ data waveform of (a),
(c) shows a waveform of a combined modulation signal of the
waveforms of (a) and (b), and (d) shows a combined waveform of the
NRZ data of (a) and the 90.degree. phase shifted clock signal of
(b).
[0047] In the waveforms of (c) and (d), the magnitudes of the
contained clock component is large. However, if the combined
signal, such as the waveforms of (c) and (d), is used for optical
modulation, an error rate of data recovery at the receiving end is
much higher due to the severe waveform distortion.
[0048] Since the transmitters 200, 300 according to the present
invention appropriately control the magnitude and phase of the
clock signal combined with the NRZ data, waveform distortion is
optimized. The detail description is as follows.
[0049] With reference to FIG. 10, (a) shows electrical NRZ data
waveform corresponding to "01011000110", (b) shows a clock signal
waveform having one tenth the magnitude of the NRZ data waveform of
(a), (c) shows a combined waveform of the waveforms of (a) and (b),
and (d) shows a combined waveform of the NRZ data of (a) and a
signal that 90.degree. phase transition is performed for the clock
signal of (b).
[0050] Such as in the waveforms of (c) and (d), if the clock signal
is combined with the NRZ data after the magnitude and phase of the
clock signal is controlled, waveform distortion is rarely
generated. Referring to (c) and (d) of FIG. 10 the effect of
relative phase relation of NRZ data and clock signal is not clearly
seen. Nevertheless, since surprisingly tight standards of error
rate such as one error in ten-billion number of data bits is
usually required in the optical communication, even minute
difference between the waveforms of (c) and (d) largely affects
whether the standard error rate is satisfied or not. Therefore, to
fulfill more accurate error rate, it is preferable that both of
magnitude and phase of the clock signal should be controlled.
[0051] FIG. 11 is a waveform of a 40 GHz clock signal extracted
from a 40 Gbps optical signal modulated by the transmitters 200,
300 of FIGS. 1 and 3 in a receiver. The clock signal was extracted
by transmitting a 40 Gbps NRZ optical signal, whose clock frequency
component is amplified by the optical modulation method of the
present invention, to 240 km away, detecting the optical signal
with a optical detector of the receiver, and passing the detected
signal now in electrical form through a narrow band band-pass
filter of 40 GHz center frequency. That is, if a simple circuit,
such as the narrow band band-pass filter, is added to the receiver,
the clock signal included in the received optical signal is easily
extracted with a clear waveform.
[0052] FIG. 12 shows an error rate of a case where data is
recovered by extracting a clock signal from optical data
transmitted from the transmitter according to the present invention
and an error rate (.box-solid.) of a case where data is recovered
by extracting a clock signal by a conventional method of performing
exclusive-or and band-pass filtering where the optical signal is
also typical NRZ modulated signal.
[0053] In general, the transmission error rate is the ratio of the
number of error bits to the total number of bits received at the
receiving end. Generally the larger the optical power input in the
receiving end is, the lower the error rate is. Therefore, to
discuss merits and demerits of the two clock extraction methods,
error rates at the same input optical powers are compared or the
magnitudes of two input optical powers having the same error rate
are compared. Thereby, in FIG. 12, error rates measured by the two
clock extraction methods are displayed, respectively, by changing
the received power when 240 km transmitted optical signals are
input to the receiving end.
[0054] In FIG. 12, the horizontal axis shows the received optical
power displayed in dBm unit. 0 dBm means 1 mW, -10 dBm means 0.1
mW, and -20 dBm means 0.01 mW. The vertical axis shows a common log
value of the measured error rate. For example, -10 of the vertical
axis means that the error rate is 10.sup.-10.
[0055] The error rate of data recovery by the present invention
shows 100 times lower error rate at the same optical input power
and 3 dB lower input power at the same error rate than that of the
conventional method (.box-solid.) using the exclusive-or logic.
That is, data recovery result where the clock signal is extracted
from the optical data transmitted from the transmitter according to
the present invention is very superior in comparison to the
conventional method.
[0056] As described above, in an NRZ optical signal generation
apparatus and method according to the present invention, NRZ
optical modulation is performed by combining electrical clock
signal and the electrical NRZ data at the sending end and the
signal distortion was optimized by controlling the magnitude and
phase of the clock signal. Thereby, at the receiving end, the clock
signal can be extracted using a low price band-pass filter
component easy to manufacture instead of a high price PLL component
difficult to manufacture and data can be clearly recovered.
Therefore, a receiver structure for clock extraction is simpler, an
error rate of data recovery is lower by clearly extracting the
clock signal, and transmission distance of an optical signal is
longer.
[0057] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those of ordinary skill in the art that various
changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by
the following claims.
* * * * *