Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL
RESHAPING ELEMENT
Reference To Pending Prior Patent Applications
This patent application:
(i) is a continuation-in-part of pending prior U.S. Patent Application Serial
No. 10/289,944, filed 11/06/02 by Daniel Mahgerefteh et al. for POWER
SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM
(Attorney's Docket No. TAPE-59474-00006);
(ii) is a continuation-in-part of pending prior U.S. Patent Application
Serial No. 10/308,522, filed 12/03/02 by Daniel Mahgerefteh et al. for
HIGH-SPEED TRANSMISSION SYSTEM COMPRISING A COUPLED
MULTI-CAVITY OPTICAL DISCRIMINATOR (Attorney's Docket No.
TAYE-59474-00007);
(iii) is a continuation-in-part of pending prior U.S. Patent Application
Serial No. 10/680,607, filed 10106/03 by Daniel Mahgerefteh et al. for FLAT
DISPERSION FREQUENCY DISCRIMINATOR (FDFD) (Attorney's Docket
No. TAYE-59474-00009);
(iv) claims benefit of pending prior U.S. Provisional Patent Application
Serial No. 60/548,230, filed 02/27/2004 by Yasuhiro Matsui et al. for OPTICAL
SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING
ELEMENT (Attorney Docket No. TAYE-31 PROV);
(v) claims benefit of pending prior U.S. Provisional Patent Application
Serial No. 60/554,243, filed 03/18/04 by Daniel Mahgerefteh et al. for FLAT
CHIRP INDUCED BY FILTER EDGE (Attorney Docket No. TAYE-34 PROV);
(vi) claims benefit of pending prior U.S. Provisional Patent Application
Serial No. 60/566,060, filed 04/28/04 by Daniel Mahgerefteh et al. for A
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METHOD OF TRANSMISSION USING PARTIAL FM AND AM
MODULATION (Attorney Docket No. TAYE-37 PROV);
(vii) claims benefit of pending prior U.S. Provisional Patent Application
Serial No. 60/567,737, filed 05/03/04 by Daniel Mahgerefteh et al. for
ADIABATIC FREQUENCY MODULATION (AFM) (Attorney Docket No.
TAYE-39 PROV);
(viii) claims benefit of pending prior U.S. Provisional Patent Application
Serial No. 60/569,769, filed 05/10/04 by Daniel Mahgerefteh et al. for FLAT
CHIRP INDUCED BY AN OPTICAL FILTER EDGE (Attorney Docket No.
TAPE-40 PROV);
(ix) claims benefit ofpending prior U.S. Provisional Patent Application
Serial No. 60/569,768, filed 05/10/2004 by Daniel Mahgerefteh et al. for
METHOD OF TRANSMISSION USING PARTIAL FM AND AM
MODULATION (Attorney's Docket No. TAPE-41 PROV);
a
(x) claims benefit of pending prior U.S. Provisional Patent Application
Serial No. 60/621,755, filed 10/25/04 by Kevin McCallion et al. for SPECTRAL
RESPONSE MODIFICATION VIA SPATIAL FILTERING WITH OPTICAL
FIBER (Attorney's Docket No. TAYE-47 PROV); and
(xi) claims benefit of pending prior U.S. Provisional Patent Application
Serial No. 60/629,741, filed 11/19/04 by Yasuhiro Matsui et al. for OPTICAL
SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING
ELEMENT (Attorney's Docket No. TAYE-48 PROV).
The eleven above-identified patent applications are hereby incorporated
herein by reference.
Field Of The Invention:
This invention relates to signal transmissions in general, and more
particularly to the transmission of optical signals and electrical signals.
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Background Of The Invention
The quality and performance of a digital fiber optic transmitter is
determined by the distance over which the transmitted digital signal can
propagate
without severe distortions. The bit error rate (BER) of the signal is measured
at a
receiver after propagation through dispersive fiber and the optical power
required
to obtain a certain BER, typically 10-12, called the sensitivity, is
determined. The
difference in sensitivity at the output of the transmitter with the
sensitivity after
propagation is called dispersion penalty. This is typically characterized the
distance over which a dispersion penalty reaches a level of ~ ldB. A standard
10
Gb/s optical digital transmitter, such as an externally modulated source can
transmit up to a distance of ~ 50 km in standard single mode fiber at 1550 nm
before the dispersion penalty reaches the level of ~1 dB, called the
dispersion
limit. The dispersion limit is determined by the fundamental assumption that
the
digital signal is transform limited, i.e. the signal has no time varying phase
across
its bits and has a bit period of 100 ps, or 1/ (bit rate). Another measure of
the
quality of a transmitter is the absolute sensitivity after fiber propagation.
Three types of optical transmitters are presently in use in prior art fiber
optic systems: (i) directly modulated laser (DML), (ii) Electroabsorption
Modulated Laser (EML), and (iii) Externally Modulated Mach Zhender (MZ).
For transmission in standard single mode fiber at 10 Gb/s, and 1550 mn, it has
generally been assumed that MZ modulators and EMLs can have the longest
reach, typically reaching 80 km. Using a special coding scheme, referred to as
phase shaped duobinary, MZ transmitters can reach 200 km. On the other hand,
directly modulated lasers (DML) reach < 5 km because their inherent time
dependent chirp causes severe distortion of the signal after this distance.
By way of example, various systems for long-reach lightwave data
transmission (> 80 km at 10 Gb/s) through optical fibers which increase the
reach
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of DMLs to > 80 km at 10 Gb/s in single mode fiber are disclosed in (i) U.S.
Patent Application Serial No. 10/289,944, filed 11/06/02 by Daniel Mahgerefteh
et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER
OPTIC SYSTEM (Attorney's Docket No. TAPE-59474-00006); (ii) U.S. Patent
Application Serial No. 10/680,607, filed 10/06/03 by Daniel Mahgerefteh et al.
for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD) (Attorney's
Docket No. TAYE-59474-00009); and (iii) U.S. Patent Application Serial No.
10/308,522, filed 12/03/02 by Daniel Mahgerefteh et al. for HIGH-SPEED
TRANSMISSION SYSTEM COMPRISING A COUPLED MULTI-CAVITY
OPTICAL DISCRIMINATOR (Attorney's Docket No. TAPE-59474-00007);
which patent applications are hereby incorporated herein by reference. The
transmitter associated with these novel systems is sometimes referred to as a
Chirp Managed Laser (CML)TM by Azna LLC of Wilmington, Massachusetts. In
these new systems, a Frequency Modulated (AFM) source is followed by an
Optical Spectrum Reshaper (OSR) which uses the frequency modulation to
increase the amplitude modulated signal and partially compensate for
dispersion
in the transmission fiber. In one embodiment, the frequency modulated source
may comprise a Directly Modulated Laser (DML). The Optical Spectrum
Reshaper (OSR), sometimes referred to as a frequency discriminator, can be
formed by an appropriate optical element that has a wavelength-dependent
transmission function. The OSR can be adapted to convert frequency modulation
to amplitude modulation.
In the novel system of the present invention, the chirp properties of the
frequency modulated source are separately adapted and then further reshaped by
configuring the OSR to further extend the reach of a CMLTM transmitter to over
250 km on standard single mode fiber at 10 Gb/s and 1550 nm. The novel system
of the present invention combines, among other things, selected features of
systems described in (i) U.S. Provisional Patent Application Serial No.
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60/548,230, filed 02127/2004 by Yasuhiro Matsui et al. for entitled OPTICAL
SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING
ELEMENT (Attorney Docket No. TAYE-31 PROV); (ii) U.S. Provisional Patent
Application Serial No. 60/554,243, filed 03/18/04 by Daniel Mahgerefteh et al.
for FLAT CHIRP INDUCED BY FILTER EDGE (Attorney Docket No. TAYE-
34 PROV); (iv) U.S. Provisional Patent Application Serial No. 60/566,060,
filed
04128/04 by Daniel Mahgerefteh et al. for, A METHOD OF TRANSMISSION
USING PARTIAL FM AND AM MODULATION (Attorney Docket No. TAYE-
37 PROV); (iv) U.S. Provisional Patent Application Serial No. 60/567,737,
filed
05/03/04 by Daniel Mahgerefteh et al. for ADIABATIC FREQUENCY
MODULATION (AFM) (Attorney Docket No. TAYE-39 PROV); (v) U.S.
Provisional Patent Application Serial No. 60/569,769, filed 05/10/04 by Daniel
Mahgerefteh et al. for FLAT CHIRP INDUCED BY AN OPTICAL FILTER
EDGE (Attorney Docket No. TAYE-40 PROV), which patent applications are
hereby incorporated herein by reference.
Summary Of The Invention
This invention provides an optical spectrum reshaper (OSR) which works
in tandem with a modulated optical source which, by modifying the spectral
properties of the modulated signal, results in extending the optical
transmission
length well beyond the dispersion limit. The OSR can be defined as a passive
optical element that imparts an optical frequency dependent loss and frequency
dependent phase on an input optical signal. This invention also provides a
modulated laser source and an optical spectrum reshaper system that increases
tolerance to fiber dispersion as well as converting a partially frequency
modulated
signal into a substantially amplitude modulated signal.
The optical spectrum reshaper (OSR) may be a variety of filters such as a
Coupled Multicavity (CMC) filter to enhance the fidelity of converting a
partially
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frequency modulated signal into a substantially amplitude modulated signal. '
The
OSR may also partially compensate for the dispersion of the fiber. In one
embodiment of the present invention, a modulated laser source may be provided
that is communicatably coupled to an optical filter where the filter is
adapted to
lock the wavelength of a laser source as well as converting the partially
frequency
modulated laser signal into a substantially amplitude modulated signal.
In one form of the present invention, there is provided a fiber optic
communication system comprising:
an optical signal source adapted to receive a base binary signal and
produce a first signal, said first signal being frequency modulated; and
an optical spectrum reshaper adapted to reshape the first signal into a
second signal, said second signal being amplitude modulated and frequency
modulated;
characterized in that:
the frequency characteristics of said first signal, and the optical
characteristics of said optical spectrum reshaper, being such that the
frequency
characteristics of said second signal are configured so as to increase the
tolerance
of the second signal to dispersion in a transmission fiber.
In another form of the present invention, there is provided an optical
transmitter comprising:
a frequency modulated source for generating a first frequency modulated
signal, and
an amplitude modulator for receiving the first frequency modulated signal
and for generating a second amplitude and frequency modulated signal.
In another form of the present invention, there is provided a method for
transmitting an optical signal through a transmission fiber comprising:
receiving a base binary signal;
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operating an optical signal source using the base binary signal to produce a
first signal, said first signal being frequency modulated;
passing the frequency modulated signal through an optical spectrum
reshaper so as to reshape the first signal into a second signal, said second
signal
being amplitude modulated and frequency modulated;
the frequency characteristics of said first signal, and the optical
characteristics of said optical spectrum reshaper, being such that the
frequency
characteristics of said second signal are configured so as to increase the
tolerance
of the second signal to dispersion in a transmission fiber; and
passing the second signal through a transmission fiber.
In another form of the present invention, there is provided a method for
transmitting a base signal, comprising:
using the base signal to produce a frequency modulated signal; and
providing an amplitude modulator for receiving the frequency modulated
signal and for generating an amplitude and frequency modulated signal.
In another form of the present invention, there is provided a fiber optic
communication system comprising:
an optical signal source adapted to produce a frequency modulated signal;
and
an optical spectrum reshaper adapted to convert the frequency modulated
signal into a substantially amplitude modulated signal;
characterized in that:
the operating characteristics of the optical signal source and the optical
characteristics of the optical spectrum reshaper combine to compensate for at
least
a portion of a dispersion in an optical fiber.
In another form of the present invention, there is provided a method for
transmitting an amplitude modulated signal through a fiber comprising:
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providing a laser and providing a filter having selected optical
characteristics;
inputting the amplitude modulated signal into the laser, and operating the
laser, so as to generate a corresponding frequency modulated signal;
passing the frequency modulated signal through the filter so as to generate
a resulting signal and passing the resulting signal into the fiber;
the laser being operated, and the filter being chosen, such that the resulting
signal
is configured to compensate for at least a portion of the dispersion in the
fiber.
In another form of the present invention, there is provided a fiber optic
communication system comprising:
an optical signal source adapted to produce a first signal, said first signal
being frequency modulated; and
an optical spectrum reshaper adapted to convert said first signal into a
second signal, said second signal being amplitude modulated and frequency
modulated;
characterized in that:
the frequency characteristics of said first signal, and the optical
characteristics of said optical spectrum reshaper, being such that the
frequency
characteristics of said second signal are configured so as to extend the
distance
said second signal can travel along a fiber before the amplitude
characteristics of
said second signal degrade beyond a given amount.
In another form of the present invention, there is provided a fiber optic
communication system comprising:
a module adapted to receive a first signal and convert said first signal into
a second signal, said second signal being amplitude modulated and frequency
modulated;
characterized in that:
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the frequency characteristics of said second signal are configured so as to
extend the distance said second signal can travel along a fiber before the
amplitude characteristics of said second signal degrade beyond a given amount.
In another form of the present invention, there is provided a system
adapted to convert a first signal into a second signal, said second signal
being
amplitude modulated and frequency modulated;
the improvement comprising:
tailoring the frequency characteristics of said second signal so as to extend
the distance said second signal can travel along a fiber before the amplitude
characteristics of said second signal degrade beyond a given amount.
In another form of the present invention, there is provided a fiber optic
communication system comprising:
an optical signal source adapted to receive a base signal and produce a
first signal, said first signal being frequency modulated; and
an optical spectrum reshaper adapted to convert said first signal into a
second signal, said second signal being amplitude modulated and frequency
modulated;
characterized in that:
the frequency characteristics of said first signal, and the optical
characteristics of said optical spectrum reshaper, being such that the
frequency
characteristics of said second signal are configured so as to extend the
distance
said second signal can travel along a fiber before the amplitude
characteristics of
said second signal degrade beyond a given amount.
In another form of the present invention, there is provided a fiber optic
communication system comprising:
an optical signal source adapted to produce a first signal, said first signal
being frequency modulated; and
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an optical spectrum reshaper adapted to convert said first signal into a
second signal, said second signal being amplitude modulated and frequency
modulated;
characterized in that:
the frequency dependent loss of the optical spectrum reshaper is adjusted
to increase the dispersion tolerance of the second signal.
In another form of the present invention, there is provided a fiber optic
system comprising:
an optical source adapted to produce a frequency modulated digital signal;
characterized in that:
said digital signal has a time varying frequency modulation which is
substantially constant across each 1 bit and equal to a first frequency and
substantially constant over each 0 bit and equal to a second frequency,
wherein
the difference between said first frequency and said second frequency is
between
0.2 times and 1.0 times the bit rate frequency.
In another form of the present invention, there is provided a method for
generating a dispersion tolerant digital signal, comprising:
modulating a DFB laser with a first digital base signal to generate a first
optical FM signal,
wherein said first FM signal has a ~t phase shift between 1 bits that are
separated by an odd number of 0 bits, and
modulating amplitude of said first optical FM signal with a second digital
base signal to produce a second optical signal with high contrast ratio.
Detailed Description Of The Preferred Embodiments
Many modifications, variations and combinations of the methods and
systems and apparatus of a dispersion compensated optical filter are possible
in
light of the embodiments described herein. The description above and many
other
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features and attendant advantages of the present invention will become
apparent
from a consideration of the following detailed description when considered in
conjunction with the accompanying drawings wherein like numbers refer to like
parts and further wherein:
$ Fig. 1 illustrates an optical digital signal with concomitant amplitude
modulation and frequency modulation (i.e., flat-topped chirp);
Fig. 2 illustrates the instantaneous frequency and phase of a 101 bit
sequence for flat-topped chirp values of $ GHz and 10 GHz for a 10 Gb/s
digital
signal;
Fig. 3 illustrates a 101 bit sequence with (CML output) and without
(Standard NRZ) flat-topped chirp before and after propagation;
Fig. 4 illustrates a Gaussian pulse with adiabatic chirp profile before an
OSR and the resulting pulse shape and flat-topped chirp after an OSR;
Fig. 5 illustrates the instantaneous frequency profile of the pulse and
1$ definitions of the pulse;
Fig. 6 illustrates the receiver sensitivity after 200 km as a function of the
rise times and fall times of the instantaneous frequency profile;
Fig. 7 illustrates the instantaneous frequency profile and intensity profile
after an OSR with two different slopes;
Fig. ~ illustrates the optical spectrum of an adiabatically chirped signal,
the spectrum of the OSR, and the resulting reshaped spectrum;
Fig. 9 illustrates receiver sensitivity after 200 km of 17 ps/nn~/km fiber for
various values of adiabatic chirp, and the spectral shift of signal relative
to the
OSR, which in this example is a 3 cavity etalon filter;
Fig. 10 illustrates an example of a non-Gaussian OSR and the spectral
position of the signal relative to the OSR spectrum;
Fig. 11 illustrates the definition of slope of slope on an OSR;
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Fig. 12 illustrates Bessel filters used as OSR provide the desired slope of
slope;
Fig. 13 illustrates optical and electrical eye diagrams before and after
transmission through 200 km (3400 ps/nm) of fiber;
Fig. 14 illustrates eye diagrams for back-back and after 200 km of fiber for
a chirp managed laser (CMLTM) transmitter with transient chirp at the output
of
the laser;
Fig. 15 illustrates measured slope and slope of slope for a 2 cavity etalon;
Fig. 16 illustrates transmission and slope of az~ edge filter used as an OSR;
Fig. 17 illustrates an example of an OSR with its dispersion profile;
Fig. 18 illustrates sensitivity versus fiber length of dispersion in 17
ps/nxn/km fiber with and without dispersion of the OSR taken into account;
Fig. 19 illustrates FM optical source with a DFB FM modulator and
separate amplitude modulator;
Fig. 20 illustrates FM optical source with a modulated DFB
and integrated Electro-absorption modulator;
Fig. 21 illustrates the temporal profiles of the AM and FM signals; and
Fig. 22 illustrates an optical FM/AM source with a bandwidth limiting
OSR or filter.
Detailed Description Of The Preferred Embodiments
In one embodiment of the present invention, the CMLTM generates a
digital optical signal having concomitant amplitude and frequency modulation,
such that there is a special correlation between the optical phases of the
bits. This
phase correlation provides a high tolerance of the resulting optical signal to
dispersion in the optical fiber; further extending the reach of the CMLTM.
In one preferred embodiment of the present invention, the CMLTM consists
of a directly modulated DFB laser and an optical spectrum reshaper (OSR). The
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distributed feedback (DFB) laser is modulated with an electrical digital
signal,
wherein a digital signal is represented by 1 bits and 0 bits. The DFB laser is
biased high above its threshold, for example, at 80 mA, and is modulated by a
relatively small current modulation; the resulting optical signal has
amplitude
modulation (AM), the 1 bits having larger amplitude than the 0 bits. The ratio
of
the amplitude of the 1 bits to the 0 bits is typically referred to as the
extinction
ratio (ER). Importantly, the modulated optical signal has a frequency
modulation
component, called adiabatic chirp, which is concomitant with the amplitude
modulation and nearly has the same profile in time, an example of which is
shown
in Fig. 1. The extinction ratio (ER) of the optical output can be varied over
a
range depending on the FM efficiency of the laser, defined as the ratio of the
adiabatic chirp to the modulation current (GHz/mA). A higher modulation
current increases ER, as well as the adiabatic chirp.
The chirp property of directly modulated lasers has been known for some
time. When the laser is modulated with an electrical digital signal, its
instantaneous optical frequency changes between two extremes, corresponding to
the 1 s and Os, and the difference in the frequency changes is referred to as
adiabatic chirp. In addition to adiabatic chirp, which approximately follows
the
intensity profile, there are transient frequency components at the 1 to 0 and
0 to 1
transitions of the bits, called transient chirp. The magnitude of transient
chirp can
be controlled by adjusting the bias of the laser relative to the modulation
current.
In one embodiment of the present invention, the transient chirp component is
minimized by using a high bias and small modulation. The signal is then passed
through an optical spectrum reshaper (OSR), such as the edge of an optical
band
pass filter with a sharp slope. The OSR modifies the frequency profile of the
input optical signal, generating a flat-topped and square shaped frequency
profile
such as that shown in Fig. 1. In the preferred embodiment of the present
invention, the magnitude of the resulting flat-topped chirp is chosen to be
such
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that it provides a special phase correlation between the bits, as described
below.
Given an FM efficiency value, r~FM , the desired adiabatic chirp, Ov specifies
the
modulation current, ~i = Ov~r~F,~ , which in turn determines the extinction
ratio,
ER = l O log I b Irn + ~i ,where Ib is the bias current, and Ith is the
threshold
Is -Ia, -~i
current of the laser. The magnitude of the flat-topped chirp after the OSR is
determined by the magnitude of the adiabatic chirp at the output of the laser
and
the slope of the OSR. For a 10 Gb/s NR~ signal, for example, the desired
adiabatic chirp is ~ 4.5 GHz, and the ER ~ 1 dB for a DFB laser with FM
efficiency ~ 0.2 GHz/mA. Passing this optical signal through an OSR with
average slope of approximately 2.3 dB/GHz increases this chirp magnitude to
about 5 GHz. The significance of this value is the desired phase correlation
between the bits as described below.
One important aspect of the present invention is the realization that as the
frequency of an optical signal is changing with time, due to the chirp, the
optical
phase of the bits changes as well, depending on the bit period, rise fall
times and
the amount of chirp. It should be noted that when monitoring the optical
Garner
wave, which is a sine wave, it can be observed that at some point in time,
phase is
a particular position on the carrier wave. The phase difference between the
crest
of the wave and its trough, for example, is ~. Frequency describes the spacing
between the peaks; higher frequency means the waves are getting bunched up and
more crests are passing by per unit time. Mathematically, phase is the time
integral of optical frequency. When the laser is modulated by a digital signal
with
bit period T, the optical phase difference between two bits depends on the
flat-
topped chirp, as well as on the total time difference between the bits. This
phase
difference can be used to increase the propagation of the signal in the fiber
as is
shown in the following example.
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An optical electric filed is characterized by an amplitude envelope and a
time varying phase and a carrier frequency as follows:
E(t) = A(t) exp(-iwot + i~(t)) (1 )
where A(t) is the amplitude envelope, c~ is the optical carrier frequency, and
~(t)
is the time varying phase. For example, for a chirp-free, or so-called
transform
limited, pulse, the time varying phase is zero. The instantaneous frequency is
defined by the following equation:
1 d~(t) (2)
.f (t) _ - ~~. dt
Note that the negative sign in Equation 2 is based on the complex notation
convention that takes the carrier frequency to be negative frequency. Hence
the
optical phase difference between two time points on the optical filed is given
by:
0~ _ ~(t2 ) - ~(t~ ) = 2~t f tz .f (t)dt
r,
Let's consider a 101 bit sequence at the output of a CMLTM having a
certain magnitude flat-topped chirp. Taking the frequency of the 1 bits as a
reference frequency, we obtain the plot shown below in two cases for a 10 Gb/s
digital signal (100 ps pulse duration) for flat-topped chirp values of 5 GHz
and 10
GHz. The pulses are assumed to have ideal square shape amplitudes and flat-
topped chirp with 100 ps duration. Significantly, for 5 GHz of flat-topped
chirp
there is a ~ phase shift between the two 1 bits separated by a single zero.
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0~ = 2~t x S GHz x 100 ps = ~' (4)
Following Equations 3 and 4, the phase shift is 2~ between two 1 bits
separated
by two 0 bits, and 3~ fox two 1 bits separated by three 0 bits and so on. In
general, two 1 bits separated by an odd number of 0 bits are ~ out of phase
for 5
GHz of chirp, and a 10 Gb/s signal. For 10 GHz of chirp and 10 Gb/s square
pulses the 1 bits separated by odd number of bits are in phase; i.e. phase
difference is 2~.
The significance of this phase shift is realized when the 101 bit sequence
with 5 GHz of flat-topped chirp is propagated through dispersive fiber, where
each pulse broadens due to its finite bandwidth. Fig. 3 shows that the ~ phase
shift causes the two bits to interfere destructively at the center of the 0
bit,
therefore keeping the 1 and 0 bits distinguishable by the decision circuit at
the
receiver. The decision threshold chooses a threshold voltage above which all
signals are counted as 1 and below which they are counted as 0 bits. Hence,
the
phase shift helps differentiate between the 1 and 0 bits and the pulse
broadening
does not reduce the BER for this bit sequence. Therefore, the ~t phase shift
constructed, based on the preferred embodiment of the present invention,
increases tolerance to dispersion. For intermediate chirp values, there is
partial
interference, which is enough to extend transmission distance, but not to
distances
in the case described above.
Optical Spectrum Reshaping
In one embodiment' of the present invention, the FM modulated signal
generated is passed though an optical spectrum reshaper so as to change the
instantaneous frequency profile of the signal across the 1 and 0 bits in such
a way
so as to increase the tolerance of the signal to dispersion. In the prior art,
such as
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UK Patent No. GB 2107147A by R. E. Epworth, the signal from the FM source is
filtered to produce an intensity modulation, which is higher modulation depth
after passing through the filter than that before passing through the filter.
In the
present invention, optical spectrum reshaping, rather than increase in
amplitude
modulation alone, can be achieved using an optical spectrum reshaper (OSR). In
one embodiment of the present invention, the instantaneous frequency profile
of
the output signal is modified across its bits after the OSR, so as to increase
the
distortion free propagation distance.
In a preferred embodiment of the present invention, a semiconductor laser
is directly modulated by a digital base signal to produce an FM modulated
signal
with adiabatic chirp. The output of the laser is then passed through an OSR,
which, in this example, may be a 3 cavity etalon filter used at the edge of
its
transmission. The chirp output of a frequency modulated source, such as a
directly modulated laser, is adiabatic. This means that the temporal frequency
profile of the pulse has substantially the same shape as the intensity profile
of the
pulse.
In a preferred embodiment, the OSR converts the adiabatic chirp to flat-
topped chirp, as described in U.S. Provisional Patent Application Serial No.
60/554,243, filed 03/18/04 by Daniel Mahgerefteh et al. for FLAT CHIRP-
INDUCED BY FILTER EDGE (Attorney Docket No. TAPE-34 PRO, which
patent application is hereby incorporated herein by reference.
Fig. 4 shows the optical intensity and the instantaneous frequency profile
of a Gaussian pulse before and after an OSR. The Gaussian pulse has adiabatic
chirp before the OSR, i.e., its instantaneous frequency profile has the same
Gaussian shape as its intensity profile. After the OSR, both the amplitude and
instantaneous frequency profiles are altered. The ratio of peak power-to-power
in
the background (extinction ratio) is increased, and the pulse narrows slightly
in
this example. An important aspect of the present invention is the flat-topped
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instantaneous frequency profile resulting from passage through the OSR,
indicated by the dotted horizontal green line in Fig. 4. The flat-topped chirp
is
produced when the spectral position of the optical spectrum of the signal is
aligned with the edge of the OSR transmission. The optimum position depends
on the adiabatic chirp and the slope of the OSR transmission edge.
The instantaneous frequency profile of a flat-topped chirp pulse is
characterized by a rise time, a fall time, duration and a slope of the flat-
top, and a
flat-topped chirp value as shown in Fig. 5. The slope, in turn, can be defined
by
. the two frequency values f~ and f~. In an embodiment of the present
invention the
rise time, fall time, duration, and slope of the top-hat portion of the
frequency
profile are adjusted relative to the rise time, fall time, duration of the
amplitude
profile, in order to increase the transmission distance of the signal beyond
the
dispersion limit.
The importance of reshaping the instantaneous frequency profile of the
pulses can be realized by simulation which shows the bit error rate of such a
spectrally reshaped 10 Gb/s pulse after propagation though 200 km of
dispersive
fiber having 17 ps/nm/lan dispersion. Fig. 6 shows that for a given flat-
topped
chirp value as measured in the instantaneous frequency profile of the signal
after
the OSR. In such a case, the BER sensitivity can be optimized by varying the
rise
time and fall time. Also, for a given rise time and fall time of the
instantaneous
frequency profile, the chirp value can be varied over a range from 3 GHz to 10
GHz in order to achieve a desired BER sensitivity after propagation through
fiber.
The following conclusions can be drawn from this example calculation:
(i) the optimum adiabatic chirp after the OSR is SGHz, with short rise
time and fall time for the instantaneous frequency profile; this achieves the
lowest
sensitivity after fiber propagation; .
(ii) any chirp in the range of 3-l OGHz can be used to extend
transmission relative to the case of no chirp. The rise time and fall times
have to
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be adjusted based on the adiabatic chirp value. In the above example, a rise
time
and fall time of < 30ps is always optimum; and
(iii) the rise time and fall time of the instantaneous frequency can be
reduced by increasing the slope in dB/GHz of the transmission profile of the
OSR.
Slope of top-hat portion of the frequency profile is determined by the
dispersion
of the OSR and provides further dispersion tolerance.
Fig. 7 shows another example, where the rise time and fall time of the
instantaneous frequency profile are reduced after the OSR by increasing the
slope
in dB/GHz of the OSR, here by a factor of 2. In one embodiment of the present
invention, the output of a frequency modulated signal is passed through an OSR
and the rise time and fall time of the frequency profile are reduced by
increasing
the slope (in dB/GHz) of the OSR.
Suectral Narrowing
1 S Simultaneous frequency modulation and amplitude modulation with the
same digital information reduces the optical bandwidth of the signal and
suppresses the carrier frequency. This effect is most marked for a chirp value
that
is 1/z the bit rate frequency; i.e., SGHz chirp for 10 Gb/s. This corresponds
to the
phase change of 0 to ~ between 1 bits separated by an odd number of 0 bits,
i.e.,
optimum correlation between the phases of the otherwise random bit sequence.
For an approximate range of chirp values between 20% to 80% of the bit rate
frequency (2-8 GHz for 10 Gb/s bit rate) the Garner is significantly
suppressed
and the spectrum is narrowed. For 0 value of chirp or for chirp equal to the
frequency of the bit rate frequency, the carrier is present and the spectrum
broadens again. This is because the phase of all the pulses becomes equal for
these two cases and the phase correlation is lost. As shown in Fig. 8, the
narrowing of the spectrum by application of amplitude modulation and frequency
modulation narrows the spectrum on the high frequency side. Note that in this
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example the chirp is ~ 7.5 GHz for 10 Gb/s. The spectral position of the
signal
relative to the peak transmission of the OSR is adjusted such that the
spectrum in
on the low frequency edge of the OSR. This further reduces the spectral width
on
the low frequency side. Reducing the spectral bandwidth extends the
transmission distance.
In one embodiment of the present invention the Bandwidth (BW) of the
OSR is less than the bit rate. The spectrum of a digital signal is determined
by the
product of the spectrum of the digital information and the Fourier transform
of the
pulse shape. Using the correct amount of FM modulation (5 GHz of chirp for 10
Gb/s data rate) which gives a ~t phase shift between 1 bits separated by odd
number of 0 bits as prescribed above, reduces the information BW. In order to
increase tolerance to dispersion it is still necessary to reduce the spectrum
of the
pulse shape. This is done by a bandwidth limiting OSR in the preferred
embodiment of the present invention.
Fig. 8 shows that_for a given value of adiabatic chirp, the spectral position
of the signal relative to the peak transmission of the OSR can be adjusted to
increase the transmission distance. Fig. 8 shows the sensitivity for a 10 Gb/s
signal at the transmitter (Back-back) and after propagation through 200 km of
fiber having l7ps/run/lan of dispersion as a function of the spectral shift
relative
to the OSR. Sensitivity is defined as the average optical power (in dBm)
required
to achieve a bit error rate of 10-12. The OSR in this example is a 3 cavity
etalon.
It is therefore an embodiment of the present invention to adjust the adiabatic
chirp
of the frequency modulated source as well as the spectral position of the
resulting
spectrum relative to the OSR in order to achieve a desired bit error rate
after
propagation through dispersive fiber.
Fig. 9 shows an example of an OSR, formed by a non-Gaussian shaped
band pass filter. Fig. 9 shows the transmission profile in dB scale as well as
the
derivative, or frequency dependent slope, of the OSR. Fig. 9 also shows the
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spectral position of the input FM signal to be reshaped. It is a preferred
embodiment of the present invention that the optimal spectral position of the
FM
signal on the OSR be such that the 1 s peak frequency be near the peak
logarithmic
derivative of the transmission profile of the OSR. In this example, the
derivative
is not linear on the dB scale, indicating that the OSR has a non-Gaussian
spectral
profile. A Gaussian OSR would have a linear slope as a function of frequency.
Fig. 9 also shows the position of the clock frequency components of the input
FM
signal, which are reduced substantially after the OSR. This in-turn reduces
the
clock frequency components in the RF spectrum of the resulting second signal
after the OSR. In this example, the peak slope is 2.7 dB/GHz, and the 3 dB
bandwidth of the OSR in this case is approximately S GHz.
It is an embodiment of the present invention for the OSR to also reduce
the clock frequency components, 10 GHz for a 10 Gb/s NRZ signal, in the RF
spectrum of the signal resulting after the OSR.
The optimum OSR shape is one for which the transmitter has good performance
both at its output (Back-to-back) as well as after transmission. The back-to-
back
performance is determined by having minimum distortion of the bits in the eye
diagram, while after transmission performance is determined by a low
dispersion
penalty. As described in U.S. Provisional Patent Applications Serial No.
60/554,243 (Attorney Docket No. TAPE-34 PROV) and 60/629,741 (Attorney's
Docket No. TAYE-4S PROV), which patent applications are hereby incorporated
herein by reference, a certain value of filter slope is required to convert an
adiabatically chirped input signal to one having flat-topped chirp. It was
shown
that the OSR converts the first derivative of the amplitude of the input pulse
to
blue shifted transient chirp at the edges. For an optimum value of slope the
added
transient chirp increases the chirp at the edges to produce a nearly flat top
chirp.
U.S. Provisional Patent Applications Serial No. 601554,243 (Attorney
Docket No. TAPE-34 PROV) and 60/629,741 (Attorney's Docket No. TAPE-4~
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PRO disclose that a significant parameter of the OSR is the slope of its
slope.
As defined in the present invention, slope of slope (SoS) is the ratio of the
peak
logarithmic derivative of the transmission (in dB/GHz) to the frequency offset
of
this peak form the transmission peak (in GHz), as illustrated in Fig. 11. In
one
embodiment of the present invention, the slope of slope of an OSR is adjusted
to
optimize both the back-to-back transmitter BER and to reduce the BER after
fiber
transmission. For example, for a 10 Gb/s transmitter good back-to-back eye
diagram, as well as low BER after transmission is obtained if the slope of
slope is
approximately in the range of 0.38 dB/GHz2 to 0.6 dB/GHz2. In addition the
slope of the OSR near the center of the transmission needs to be approximately
linear. Deviations from linearity introduce distortions in the resulting
output eye
diagram and thus cause increased bit error rate. A linear slope corresponds to
a
round-top shape filter. So, for example, a flat-topped filter, which has a
near zero
slope near the center is not desirable. The 3 dB band width of the band-pass
OSR
has to be in the range of 65% to 90% of the bit rate.
Two examples of such OSRs, as shown in Fig. 12, are 2nd order Bessel
filters having a 6 GHz or 5.5 GHz band widths. The 2nd order Bessel filter
shape
is well known to the skilled in the art and is described mathematically by w
T(p) = 1 (6)
3+3p+p2
where p = 2zf ~Of3aB . Here T is the field transmission, f is the optical
frequency
offset from the center of filter, and ~f3dB is the 3 dB band width of the
filter. The
measured quantity is the optical transmission of the filter, which is the
absolute
square of the field transmission in Eq. 6, IT(p)IZ and is plotted in Fig. 12.
The
Bessel filter is usually used as an electrical low pass filter because it
minimizes
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distortion in its pass band. In one embodiment of the present invention, the
Bessel filter is an optical filter, and it is chosen because it provides the
desired
slope of slope and linear slope near its peak transmission. The slope of slope
for
the 2nd order Bessel filter with a 6 GHz bandwidth is 0.46 dB/GHz2, and the
slope
of slope for the 5.5 GHz bandwidth 2nd order Bessel filter is 0.57 dB/GHz2.
These examples show that the bandwidth of the filter can be adjusted to change
SoS to be the desired value.
Another example of a filter that can be used in accordance with the present
invention is a 4th order Bessel filter with a band width of 7.5 GHz, also
shown in
Fig. 12. This OSR has a slope of slope of 0.41 dB/GHz2. The field transmission
of the 4th order Bessel filter is given as a function of the normalized
frequency by
1
T(p)-15+15p+6p2+p3 (7)
Fig. 13 shows examples of calculated eye diagrams for back-back and
after 200 km of fiber having 3400 ps/nm dispersion. In this example, the 2nd
order Bessel filter with 5.5 GHz bandwidth was used. The eye diagrams on the
left column are the back-back optical eye (so-called O-eye) of transmitter
(top)
and the eye transmitted after 200 km (3400 ps/nm). The eye diagrams on the
right
column are the eye diagrams measured after an optical to electrical converter
with
a typical ~~ GHz band width, which is called electrical eye (E-eye). The
electrical eye is that at the output of the receiver, which converts the
optical to
electrical signal and provides it to the decision circuit for distinguishing
the 1 and
0 bits.
A directly modulated laser produces transient chirp, which occurs at the 1
to 0 and 0 to 1 bit transitions, in addition to adiabatic chirp. In a
conventional
directly modulated laser, transient chirp is detrimental as it hastens pulse
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distortion and increases BER after transmission. However, in the present
invention, it has been found that when used as the FM source, where the
directly
modulated laser is followed by an OSR, some transient chirp at the output of
the
laser is desirable. Fig. 14 shows the results of simulation of a transmitter
in
accordance with the present invention. In this example, the adiabatic chirp of
the
laser is 4.5 GHz and the OSR is a 2 cavity etalon filter operated near its
transmission edge.
Fig. 14 shows the eye diagrams of a 10 Gb/s transmitter at its output
(back-back), as well as the eye after propagation through 200 km of fiber with
3400 ps/nm dispersion. The transient chirp at the output of the laser, before
the
OSR, is either nearly zero (~ 0.2 GHz) (left column) or 2 GHz (right column).
Looking at Fig. 14, it is clear that the case having 2 GHz transient chirp
produces
a less distorted eye back to back. The eye after 200 km of fiber is also more
open
and has less inter-symbol interference (ISI) in the case having 2 GHz
transient
chirp. It is, therefore, one embodiment of the present invention to adjust the
transient chirp of the frequency modulated source as well as the slope of
slope of
the optical spectrum reshaper to obtain the desired transmitter output having
minimum distortion and to increase the error free propagation length of the
transmitter beyond the dispersion limit.
In practice, an optical filter such as a rnulticavity etalon may not have the
desired transmission shape and slope of slope. Therefore, in another
embodiment
of the present invention, the angle of incidence and the beam divergence of
the
optical signal impinging upon the filter are adjusted to obtain the desired
SoS.
Fig. 15 shows an example of the measured slope as well as slope of the slope
as a
function of angle of incidence for a 2 cavity etalon. The peak slope initially
decreases for increasing angles, reaches a minimum, and then increases again.
The increase in slope at large angles is caused by spatial filtering, as
described in
U.S. Provisional Application Serial No. 60/621,755, filed 10/25/04 by 10/25/04
et
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al. for SPECTRAL RESPONSE MODIFICATION VIA SPATIAL FILTERING
WITH OPTICAL FIBER (Attorney's Docket No. TAYE-47 PROV), which
patent application is hereby incorporated herein by reference. For the same
range
of angles the slope of slope monotonically decreases from 0.75 dB/GHz2 to 0.35
dB/GHz2 because the peak position is increasing with increasing angle. In this
example, the optimum value of 0.45 dB/GHz~' is obtained by adjusting the angle
of incidence to 1.5 to 2 degrees.
In the above described examples, the optical spectrum reshaper (OSR) was
a multicavity etalon filter. In another preferred embodiment of the present
invention the OSR may be an edge filter, as shown in Fig. 16. The edge filter
has
a substantially flat transmission with frequency over a frequency range and a
sharp edge on one side of the peak transmission. The position of the first
optical
signal in this case will be substantially on the slope of transmission.
OSR Dispersion
The OSR can also provide some dispersion compensation as well as the
spectral reshaping. Fig. 17 shows the transmission characteristics of a filter
and
its corresponding dispersion profile.
The filter dispersion can compensate for a portion of the fiber dispersion.
For example, if the laser frequency spectrum substantially overlaps with the
normal dispersion peak, having a negative dispersion, the transmission for a
standard single fiber having positive dispersion is extended. If the laser
frequency
spectrum substantially overlaps with the anomalous dispersion peak, where
dispersion is positive, it reduces the transmission distance for a standard
fiber
with positive dispersion, but extends the reach over negative dispersion fiber
such
as Dispersion Compensating Fiber (DCF). Fig. 18 shows the sensitivity as a
function of fiber distance for a case of an OSR with and without dispersion.
The
laser spectrum substantially overlaps with the negative dispersion peak of the
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OSR. As shown in Fig. 18, the negative distance indicates a fiber having
negative dispersion of that length. So, for example, -100 km indicates a 100
km
dispersion compensating fiber having -17 ps/nm/km dispersion.
FM Sources
The present invention teaches a variety of methods for generation of a
dispersion tolerant FM signal with high extinction ratio (ER). In one
preferred
embodiment of the present invention the FM signal is generated in two steps.
First, a base digital signal is chosen to modulate a directly modulated DFB
laser so as to generate an FM signal with adiabatic chirp such that the phase
difference between two 1 bits separated by an odd number of 0 bits is an odd
integer multiple of ~t. As an example, for a 10 Gb/s NRZ signal with 100 ps
pulses and near square shaped instantaneous frequency profile, this is 5 GHz.
Next, the resulting optical signal is sent through a second amplitude
modulator, such as a LiNb03 modulator or an electro-absorption modulator, as
shown in Fig. 19. The amplitude modulator is modulated by a second digital
base
signal, which is a replica of the first digital base signal. The base signal
supplied
to the modulator may be inverted relative to that modulating the laser,
depending
on the transfer function of the modulator. This is the case, for example, if a
higher signal increases the loss of the modulator. Hence, a high signal
produces a
higher amplitude optical signal from the laser and a corresponding low signal
is
supplied to the modulator. The AM modulator may be a variety of optical
amplitude modulators such as a LiNb03 modulator, or an electro-absorption
modulator. The DFB and EA may be integrated on the same chip, as shown in
Fig.20.
In one preferred embodiment of the present invention, the first and second
base signals supplied to the laser and modulator may be adapted to generate FM
and AM signals, respectively. These FM and AM signals are different in
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temporal profiles, as demonstrated in Fig. 21, in that there may be a phase
difference between the two digital base signals. Also, the rise time and fall
time
of the instantaneous frequency of the first signal and the rise time and fall
time of
the resulting second signal after the AM modulator may be different. In
addition,
the durations of the FM and AM pulse profiles may be different. In a preferred
embodiment of the present invention the duration, rise time and fall time,
adiabatic chirp, amplitude modulation depth, and the phase delay between the
two
digital base signals are varied, as described by the prescriptions and
examples
above, so as to increase the dispersion tolerance of the transmitted signal to
fiber
dispersion. These parameters for the frequency and amplitude profiles are
defined
in Fig. 21.
In another embodiment of the present invention, and as shown in Fig. 22,
there may be a bandwidth limiting filter or an OSR placed after the FM/AM
source described above. The OSR or filter is chosen so as to reduce the
optical
frequency components that are at, or higher than, the bit rate frequency, 10
GHz
for a 10 Gb/s NRZ signal, for example.
Parameter Ranges
In various embodiments of the present invention, for longer distance
transmission of signal, performance after the optical spectrum reshaper needs
to
be optimized, leading to the following preferred characteristics:
(i) AM ER < 3 dB (i.e., the extinction ratio of the laser's intensity output
is preferably less than 3 dB in order to minimize transient chirp);
(ii) adiabatic chirp in the range 2.5-7.5 GHz (i.e., the adiabatic chirp of
the laser's output ~f = fl-fo~ 2.5-7.5 GHz for optimum transmission); and
(iii) Optical spectrum reshaper bandwidth is in the range of 5-10 GHz (i.e.,
the OSR has a filter bandwidth of 5-10 GHz to maximize spectral narrowing
effect).
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Modifications
It will be appreciated that still further embodiments of the present
invention will be apparent to those skilled in the art in view of the present
S disclosure. It is to be understood that the present invention is by no means
limited
to the particular constructions herein disclosed and/or shown in the drawings,
but
also comprises any modifications or equivalents within the scope of the
invention.
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