U.S. patent application number 13/086804 was filed with the patent office on 2011-11-17 for partial dpsk (pdpsk) transmission systems.
This patent application is currently assigned to MINTERA CORPORATION. Invention is credited to Fenghai LIU, Pavel MAMYSHEV, Benny MIKKELSEN, Christian RASMUSSEN.
Application Number | 20110280588 13/086804 |
Document ID | / |
Family ID | 38556696 |
Filed Date | 2011-11-17 |
United States Patent
Application |
20110280588 |
Kind Code |
A1 |
MIKKELSEN; Benny ; et
al. |
November 17, 2011 |
PARTIAL DPSK (PDPSK) TRANSMISSION SYSTEMS
Abstract
An optical receiver includes a demodulator having a delay
interferometer comprising an optical input that receives a phase
modulated optical signal from a bandwidth limited transmission
system. The delay interferometer has a free spectral range that is
larger than a symbol rate of the phase modulated optical signal by
an amount that improves receiver performance. The receiver also
includes a differential detector having a first and a second
photodetector. The first photodetector is optically coupled to the
constructive optical output of the delay interferometer. The second
photodetector is optically coupled to the destructive optical
output of the delay interferometer. The differential detector
combines a first electrical detection signal generated by the first
photodetector and a second electrical detection signal generated by
the second photodetector to generate an electrical reception
signal.
Inventors: |
MIKKELSEN; Benny; (Newton,
MA) ; MAMYSHEV; Pavel; (Morganville, NJ) ;
RASMUSSEN; Christian; (Shrewsbury, MA) ; LIU;
Fenghai; (Nashua, NH) |
Assignee: |
MINTERA CORPORATION
|
Family ID: |
38556696 |
Appl. No.: |
13/086804 |
Filed: |
April 14, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11740212 |
Apr 25, 2007 |
7949261 |
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13086804 |
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60795121 |
Apr 26, 2006 |
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Current U.S.
Class: |
398/202 |
Current CPC
Class: |
H04B 10/5055 20130101;
H04B 10/66 20130101; H04B 10/505 20130101; H04B 10/5561 20130101;
H04B 10/5162 20130101; H04B 10/5051 20130101 |
Class at
Publication: |
398/202 |
International
Class: |
H04B 10/06 20060101
H04B010/06 |
Claims
1. An optical receiver comprising: a. a demodulator having a delay
interferometer comprising an optical input that receives a phase
modulated optical signal from a bandwidth limited transmission
system, a constructive optical output, and a destructive optical
output, the delay interferometer having a free spectral range that
is larger than a symbol rate of the phase modulated optical signal
by an amount that improves receiver performance; and b. a
differential detector comprising a first and a second
photodetector, the first photodetector being optically coupled to
the constructive optical output of the delay interferometer, the
second photodetector being optically coupled to the destructive
optical output of the delay interferometer, the differential
detector combining a first electrical detection signal generated by
the first photodetector and a second electrical detection signal
generated by the second photodetector to generate an electrical
reception signal.
Description
RELATED APPLICATION SECTION
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 60/795,121, filed Apr. 26, 2006, entitled
"Quasi Differentially Demodulated DPSK and Quasi Differentially
Demodulated DQPSK Modulation Formats (QD-PSK/QD-QPSK)", the entire
application of which is incorporated herein by reference.
[0002] The section headings used herein are for organizational
purposes only and should not to be construed as limiting the
subject matter described in the present application.
BACKGROUND SECTION
[0003] DWDM optical fiber transmission systems operating at channel
rates of 40 Gb/s and higher are highly desirable because they
potentially have greater fiber capacity and also have lower cost
per transmitted bit compared to lower channel rate systems.
Currently, many DWDM optical fiber transmission systems operate at
a channel rate of 10 Gb/s. It is desirable for these 40 Gb/s
transmission systems to be compatible with the currently existing
10 Gb/s transport architectures.
[0004] The modulation format of 40 Gb/s DWDM transmission systems
must be chosen to have high Optical Signal-to-Noise Ratio (OSNR)
sensitivity. High OSNR sensitivity means that a low OSNR is
sufficient to maintain a desired bit error rate (BER) of the
transmission or, equivalently, that the system is able to operate
at a desired BER even in the presence of a high level of optical
noise. In addition, modulation formats of 40 Gb/s DWDM transmission
systems must be chosen to be tolerant to optical filtering because
existing systems sometimes include optical multiplexers and
demultiplexers for 50 GHz channels spacing that limit the
bandwidth. Also, existing systems sometimes include many cascaded
optical add-drop multiplexers.
[0005] The Phase-Shaped-Binary-Transmission (PSBT) format has been
considered for 40 Gb/s DWDM transmission systems because of its
narrow spectrum. However, PSBT has relatively poor OSNR receiver
sensitivity, meaning that it requires a relatively high OSNR to
obtain a low BER. Also, the OSNR receiver sensitivity is dependent
on the level of applied optical filtering.
[0006] Also, Differential Phased Shift Keying (DPSK), which is
sometimes referred to as Differential Binary Phased Shift Keying
(DBPSK) has been considered for 40 Gb/s DWDM transmission systems.
DPSK transmission systems have excellent OSNR sensitivity. DPSK
transmission systems using balanced direct detection receivers,
which are sometimes referred to as differential receivers, have
been shown to have an approximately 3 dB improvement of OSNR
sensitivity compared to on-off keying systems, such as NRZ and PSBT
systems. However, DPSK transmission systems do not have good filter
tolerance.
[0007] In addition, Differential Quadrature Phased Shift Keying
(DQPSK) has been considered for 40 Gb/s DWDM transmission systems.
DQPSK uses a symbol rate that is one half of the data rate. For
example, a 43 Gb/s data rate in a DQPSK system corresponds to 21.5
Giga symbols per second. Consequently, DQPSK transmission systems
have a narrower spectral bandwidth, greater chromatic dispersion
tolerance and greater tolerance with respect to polarization mode
dispersion (PMD) compared to traditional formats and compared to
DPSK. However, DQPSK transmission systems have approximately 1.5-2
dB worse receiver sensitivity than DPSK transmission systems.
Furthermore, both the transmitter and the receiver are
significantly more complex than DPSK transmitter/receiver.
[0008] DPSK and DQPSK receivers use one or more optical
demodulators that convert the phase modulation of the transmitted
optical signal into amplitude modulated signals that can be
detected with direct detection receivers. Typically, optical
demodulators are implemented as delay interferometers that split
the optical signal into two parts, delay one part relative to the
other by a differential delay .DELTA.t, and finally recombine the
two parts to achieve constructive or destructive interference
depending on the phase which is modulated onto the optical signal
at the transmitter.
[0009] It is conventional wisdom that DPSK and DQPSK signal are
optimally received by delay interferometers that have a
differential delay .DELTA.t=n T, where n=1, 2, 3 . . . , T=1/B is
the symbol time slot, and B is the symbol rate. See, for example,
the theoretical investigation in "On the bit error rate of
lightwave systems with optical amplifiers" by P. A. Humblet et al,
J. Lightwave Technol., pp. 1576-1582, 1991. See also the
experimental investigation in "2.5 Tb/s (64.times.42.7 Gb/s)
transmission over 40.times.100 km NXDSF using RZ-DPSK format and
all-Raman amplified spans", by A. H. Gnauck et al., in proceeding
of OFC, post deadline paper FC2, February 2002.
[0010] It is also conventional wisdom that using delay
interferometers with a delay that is shorter or longer than the
symbol time slot will result in some receiver performance penalties
when receiving DPSK and DQPSK signals. See, for example, the
investigation of single-channel DPSK systems in "Degradations in
Balanced DPSK Receivers", by Peter J. Winzer and Hoon Kim, IEEE
Photonics Technology Letters, Vol. 15, 1282, No. 9, September 2003.
According to the Winzer and Kim reference, the performance penalty
increases in a nearly parabolic relationship when the differential
delay .DELTA.t of the delay interferometer deviates from the symbol
time slot or, equivalently, the free spectral range
(FSR)=1/.DELTA.t of the delay interferometer deviates from the
signal symbol rate. See also the investigation of single-channel
DPSK systems in "Athermal Demodulator for 42.7-Gb/s DPSK Signals,"
by Y. C. Hsieh et al, in proceeding of ECOC, paper Th1.5.6,
September 2005. In this reference the authors teach that a
degradation in OSNR sensitivity is typically incurred by a FSR
different from 1/.DELTA.t. However, Y. C Hsieh et al. propose to
use a delay interferometer with a FSR of 50 GHz despite the
performance penalty to be able to operate a single-channel at any
ITU frequency without the need for active control of the delay
interferometer. Neither Y. C. Hsieh et al nor Peter J. Winze et al
considered any influence of narrow optical bandpass filtering or
the use of optical filters that are typically needed for
multi-channel applications, i.e., Dense Wavelength Division
Multiplexing (DWDM) applications. Hence, the common believe today
is that DPSK and DQPSK signals are optimally received with a delay
interferometer with a differential delay that equals the symbol
time slot.
[0011] Both DPSK and DQPSK modulation formats are used in a
non-return-to-zero (NRZ) variant where the light intensity can be
constant between two neighboring symbols and a return-to-zero (RZ)
variant where the light intensity always drop or return to zero
between each symbol. The intensity returns to zero even if the data
signal includes numerous consecutive zeros or ones. Transmitters
using RZ-type modulation formats can achieve better OSNR receiver
sensitivity and tolerance to fiber nonlinearities than transmitters
using NRZ-type modulation formats. Return-to-zero modulation pulses
are typically created using pulse carving techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The aspects of this invention may be better understood by
referring to the following description in conjunction with the
accompanying drawings. Identical or similar elements in these
figures may be designated by the same reference numerals. Detailed
description about these similar elements may not be repeated. The
drawings are not necessarily to scale. The skilled artisan will
understand that the drawings, described below, are for illustration
purposes only. The drawings are not intended to limit the scope of
the present teachings in any way.
[0013] FIG. 1 shows a schematic diagram of one embodiment of a
PDPSK transmission system according to the present invention that
includes a transmitter and a PDPSK receiver that receives PDPSK
signals according to the present invention.
[0014] FIG. 2 shows a schematic diagram of a PDPSK transmission
system that illustrates individual bandwidths of the various
transmission system components.
[0015] FIGS. 3A-3C illustrate schematic diagrams of adaptive PDPSK
receivers according to the present invention.
[0016] FIG. 4A presents calculated data for electrical eye diagrams
of NRZ PDPSK signals for three different levels of optical
filtering and for three different values of the delay
interferometer FSR in a transmission system according to the
present invention.
[0017] FIG. 4B presents calculated data for electrical eye diagrams
of RZ PDPSK signals for three different levels of optical filtering
and for three different values of the delay interferometer FSR in a
transmission system according to the present invention.
[0018] FIG. 5 illustrates a schematic diagram of an experimental
transmission system used to measure eye diagrams and OSNR
sensitivity data for transmission systems according to the present
invention.
[0019] FIG. 6 presents experimental data for electrical eye
diagrams of NRZ and RZ signals for four different values of the
delay interferometer FSR in a transmission system according to the
present invention.
[0020] FIG. 7A presents experimental OSNR sensitivity data for a
NRZ DPSK signal received with a conventional DPSK receiver and with
a PDPSK receiver according to the present invention.
[0021] FIG. 7B presents experimental OSNR sensitivity data for a RZ
DPSK signal received with a conventional DPSK receiver and with a
PDPSK receiver according to the present invention.
[0022] FIG. 8A presents a comparison of calculated eye diagram data
for DPSK signals received with a conventional DPSK receiver and
received with a PDPSK receiver according to the present invention
for various levels of dispersions in the transmission line
system.
[0023] FIG. 8B presents experimental OSNR penalty data in dB for a
DPSK signal received with a conventional DPSK receiver and received
with a PDPSK receiver according to the present invention for
various levels of dispersions in the transmission line system.
DETAILED DESCRIPTION
[0024] While the present teachings are described in conjunction
with various embodiments and examples, it is not intended that the
present teachings be limited to such embodiments. On the contrary,
the present teachings encompass various alternatives, modifications
and equivalents, as will be appreciated by those of skill in the
art.
[0025] For example, it should be understood that there are numerous
variations of the PDPSK receivers according to the present
invention. In particular, it should be understood that the methods
and apparatus of the present invention are not limited to any
particular type of demodulator. In addition, it should be
understood that the methods and apparatus of the present invention
can be used with any type of multilevel phase modulation including
RZ and NRZ types of modulation.
[0026] It should be understood that the individual steps of the
methods of the present invention may be performed in any order
and/or simultaneously as long as the invention remains operable.
Furthermore, it should be understood that the apparatus and methods
of the present invention can include any number or all of the
described embodiments as long as the invention remains
operable.
[0027] The present invention is in part the recognition that the
common believe today that DPSK and DQPSK signals are optimally
received with a delay interferometer with a differential delay that
is equal to the symbol time slot does not apply to systems where
the DPSK or DQPSK signals have been narrowly filtered. On the
contrary, it has been discovered that for such narrowly filtered
systems, DPSK and DQPSK signals are optimally received with a delay
interferometer having a differential delay that is significantly
less than the symbol time slot.
[0028] The present invention features methods and apparatus for
transmitting and receiving modified DPSK (or DQPSK) modulation
formats that have improved OSNR receiver sensitivity performance.
The modified DPSK (or DQPSK) modulation format of the present
invention is referred to herein as Partial Differential Phased
Shift Keying (PDPSK). The term "PDPSK" is used herein to refer to
both the conventional DPSK and also to the DQPSK modulation
formats. The term PDPSK was referred to in U.S. Provisional Patent
Application Ser. No. 60/795,121 as Quasi Differentially Demodulated
DPSK and Quasi Differentially Demodulated DQPSK Modulation Formats.
These terms are equivalent.
[0029] In particular, the PDPSK demodulation technique of the
present invention improves performance of spectrally efficient
transmission systems that include systems with reduced transmitter,
optical link and/or receiver bandwidths, such as transmitters,
optical links and/or receivers with any kind of narrow optical
filtering as described herein. In addition, the PDPSK demodulation
technique of the present invention improves performance in systems
with significant chromatic dispersion. In one embodiment, the
modified system of the present invention is achieved by performing
DPSK/DQPSK-type delay interferometer demodulation at the receiver
using a differential delay that is less than one symbol time slot.
In one specific embodiment, the modified system of the present
invention is achieved by performing DPSK/DQPSK-type delay
interferometer demodulation in the receiver using a differential
delay that is less than 0.85 of one symbol time slot. This is in
contrast to known DPSK/DQPSK demodulation techniques which use a
differential delay that is greater than or equal to one symbol time
slot.
[0030] FIG. 1 shows a schematic diagram of one embodiment of a
PDPSK transmission system 100 according to the present invention
that includes a transmitter 102 and a PDPSK receiver 104 that
receives PDPSK signals according to the present invention. The
PDPSK transmitter 102 includes a FEC/Framer 106 having an output
that is electrically connected to an input of a precoder 108. The
output of the precoder 108 is electrically connected to an input of
a multiplexer 110. An output of the multiplexer 110 is electrically
connected to an electronic driver circuit 112.
[0031] An output of the driver circuit 112 is electrically
connected to a modulation input of a NRZ Mach-Zehnder
Interferometer (MZI) modulator 116. It should be understood that
the invention is not limited to MZI based modulator shown in FIG.
1.
[0032] An output of a laser 118 is optically connected to an
optical input of the NRZ modulator 116. The output of the NRZ
modulator 116 is optically coupled to an optical input of a pulse
carving RZ modulator 120. Alternatively, any type of pulse carving
device that converts NRZ data to RZ can be used. An output of a
sinusoidal source 122 is electrically connected to a modulation
input of pulse carving RZ modulator 120. Embodiments that generate
only NRZ-type modulated signals do not include the RZ modulator 120
and sinusoidal source 122.
[0033] Some transmitters use an optical fiber amplifier 124, such
as an erbium doped optical fiber amplifier (EDFA) to amplify the
signals generated by the RZ modulator 120. Such EDFAs are well
known in the art. In such transmitters, an optical output of the RZ
modulator 120 is optically coupled to an input of a fiber amplifier
124.
[0034] In operation, the FEC/Framer 106 in the PDPSK transmitter
102 provides a frame and the forward error correction to the data
being transmitted. The precoder 108 performs differential phased
shift keying encoding of the data. In some embodiments, the
precoder 108 is a separate component and in other embodiments the
precoder 108 is an integrated part of other components. In some
embodiments, the differential encoding function is performed at the
receiver in which case it is termed postcoding.
[0035] The multiplexer 110 multiplexes the data. The driver circuit
112 amplifies the framed multiplexed data signals with the forward
error correction to levels that are suitable for modulating with
the NRZ modulator 116. The NRZ modulator 116 modulates the encoded
data with a NRZ format on the optical signal generated by the laser
118. The RZ modulator 120 that is driven by the sinusoidal source
122 performs the pulse carving necessary to transform the modulated
NRZ signal into a modulated RZ signal. In embodiments that generate
only NRZ modulation formats, no pulse carving is performed. In some
transmission systems, the fiber amplifier 124 is used to amplify
the modulated RZ signal to the desired signal level for
transmission across the channel or transmission line (not shown).
The resulting RZ modulated signal is a DPSK/DQPSK modulated data
signal.
[0036] The PDPSK receiver 104 includes an input that receives the
transmitted DPSK/DQPSK modulated data signal across the channel.
Some receives include an optical fiber amplifier 126, such as an
EDFA, at the input of the receiver 104. The input of the receiver
104 is optically coupled to an optical input of the fiber amplifier
126. Some receives also include an adaptive dispersion compensator
(ADC) 128. In such receivers, an optical output of the fiber
amplifier 126 is optically coupled to an optical input of the ADC
128.
[0037] An optical output of the ADC 128 is optically coupled to an
input of an optical demodulator 130. In many receivers according to
the present invention, the demodulator 130 is a delay
interferometer 132 that is realized with at least one Michelson
Interferometer or at least one MZI as shown in FIG. 1. The delay
interferometer 132 can provide a fixed optical delay or can include
a variable optical delay 134. The fixed optical delay or the
variable differential delay provides a delay that is less than one
bit-period as described herein. The delay interferometer 134
includes a constructive output 136 and a destructive output
138.
[0038] In some embodiments, the variable optical delay 134 is a
continuously variable optical delay. In other embodiments, the
variable optical delay 134 is switchable between a predetermined
number of discrete optical delays. These variable optical delays
can be constructed in numerous ways. For example, a continuously
variable optical delay can be constructed with a translatable
mirror, or with a translatable collimator. A continuously variable
optical delay can also be constructed with a transparent material
having variable optical thickness in one of the arms of the delay
interferometer 132. A switchable variable optical delay can be made
by physically introducing transparent materials with different
optical thickness. A switchable variable optical delay can also be
made by positioning a rotating mirror inside the delay
interferometer 132 to switch between different paths having
different time delays. In addition, a switchable variable optical
delay can be made using various types of MEMS technology.
[0039] A first 140 and second input 142 of a balanced or
differential receiver 144 is optically coupled to a respective one
of the constructive 136 and the destructive output 138 of the delay
interferometer 134. In many PDPSK receivers, the differential
receiver 144 is realized with a first 146 and second photo detector
148. An output of the differential receiver 144 is electrically
coupled to an input of a demultiplexer 150. In some PDPSK
receivers, an electronic amplification stage (not shown) is used
between the differential receiver 144 and the demultiplexer 150. An
output of the demultiplexer 150 is electrically connected to a
FEC/Framer 152. The demultiplexer 150 typically performs the data
and clock recovery function.
[0040] In operation, optically modulated DPSK/DQPSK signals are
received at an input of the PDPSK receiver 104 and are amplified by
the fiber amplifier 126. In some PDPSK receiver systems, the ADC
128 performs dispersion compensation. The delay interferometer 132
in the optical demodulator 130 converts the PDPSK phase-modulated
signal into an amplitude-modulated optical signal at the
constructive output 136. The delay interferometer 132 also
generates an inverted amplitude-modulated optical signal at the
destructive output 138. The polarity of the data on the
constructive output 136 and the destructive output 138 can be
inverted by changing the relative phase between the two
interferometer arms of the delay interferometer 138 by
approximately .pi..
[0041] The differential delay between the two delay interferometer
outputs and the two inputs of the differential detectors is
typically less than 30% of a symbol time slot. In some embodiments
the differential delay is less than 10%. The relative optical power
propagating from the constructive port 136 and the destructive port
138 of the delay interferometer 132 is a function of the FSR of the
delay interferometer 132 and of the degree of the optical filtering
of the signal in the transmission line. For example, when the FSR
of the delay interferometer 132 increases, the optical power in the
constructive port 136 increases relative to the optical power in
the destructive port 138.
[0042] The output signals from the constructive output 136 and the
destructive output 138 of the delay interferometer 132 in the
demodulator 130 are detected with the differential receiver 144
that includes the first 146 and second photo detector 148. The
first photodetector 146 generates an electrical detection signal
that is proportional to the optical signal propagating from the
constructive output 136. The second photodetector 148 generates an
electrical detection signal that is proportional to the optical
signal propagating from the destructive output 138.
[0043] The differential receiver 144 electrically subtracts the
electrical detection signal generated by the first and second
photodetectors 146, 148 from each other to create a differential
detection signal. A DPSK and DQPSK receiver according to the
present invention can also receive phase modulation signals where
the encoded phase difference between constellation points differ
from .pi. and .pi./2, respectively. It should be understood that
the methods and apparatus of the present invention can be used in
combination with any optical or electrical equalizer.
[0044] One skilled in the art understands that the best OSNR
receiver sensitivity performance for known DPSK/DQPSK transmission
systems is obtained when the time delay .DELTA.t between the two
arms of the delay interferometer is exactly equal to an integer
number of the symbol time slots of the optical DPSK/DQPSK data
signal. Furthermore, one skilled in the art understands that the
penalty in OSNR and receiver sensitivity in these systems increases
rapidly (quadratically in most systems) when At deviates from its
optimal value. See, for example, Peter J. Winzer and Hoon Kim, IEEE
Photonics Technology Letters, vol. 15, no. 9, pages 1282-1284,
September 2003. Thus, one skilled in the art understands that the
optimum free spectral range (FSR=1/.DELTA.t) of the delay
interferometer 132 is equal to 1/nT, and in the special case of
n=1, the optimum free spectral range is equal to the symbol rate of
the signal The term "free spectral range" of an interferometer is
well known in the art as the distance (in frequency space) between
adjacent transmission peaks.
[0045] The present invention is in part the recognition that
decreasing the differential delay generated by the delay
interferometer 132 to less than one bit-period significantly
increases the transmission system's tolerance to narrow optical
filtering and to chromatic dispersion in high data rate
transmission systems. It has been discovered that the well known
equation .DELTA.t=n T, where n=1, 2, 3 . . . , T=1/B is the symbol
time slot, and B is the symbol rate, represents the optimum delay
only under certain conditions. In particular, it has been
discovered that the equation .DELTA.t=n T, where n=1, 2, 3 . . . ,
represents the optimum delay only under conditions where there is
no significant optical filtering (i.e. weak filtering) and where
the data signals being processed by the transmitter 102 and the
receiver 104 have near ideal rise/fall times. The present invention
can be understood by viewing the total system bandwidth as a
combination of individual component bandwidths as described in
connection with FIG. 2.
[0046] FIG. 2 shows a schematic diagram of a PDPSK transmission
system 200 that illustrates individual bandwidths of the various
transmission system components. Referring to both FIG. 1 and FIG.
2, the schematic diagram of the transmission system 200 illustrates
a transmitter bandwidth (B.sub.TX) 202 that corresponds to the
bandwidth of the transmitter 102. The transmitter bandwidth
(B.sub.TX) 202 includes the bandwidth of any transmitter
components, such as any NRZ modulator 116 and RZ modulators 120 and
any data formatting and driving circuits 106, 108, 110, and
112).
[0047] Also, the schematic diagram of the PDPSK transmission system
200 illustrates the optical transmission line bandwidth (B.sub.TL)
204. The optical transmission line bandwidth (B.sub.TL) 204
includes the bandwidth of the various components along the
transmission line system, such as any optical filters, any Optical
Add-Drop Multiplexers (OADMs) or Reconfigurable Optical Add-Drop
Multiplexers (ROADMs), WDM multiplexers and WDM demultiplexers.
[0048] An optical transmission system is considered bandwidth
limited when the frequency components furthest away from the center
frequency of the transmitted spectrum generated by the transmitter
are removed as the signal is transmitted from the transmitter to
the receiver. The optical transmission line bandwidth (B.sub.TL) is
considered "wide" or "narrow" depending on the relationship between
the transmission line bandwidth and the bandwidth of the signal
from the transmitter. The bandwidth of a baseband data signal is
approximately equal to one divided by its symbol time slot. The
bandwidth of a signal modulated onto a carrier wavelength is two
times its baseband bandwidth, meaning that the bandwidth of a data
signal modulated onto an optical carrier wavelength is
approximately two times its symbol rate. Therefore, the
transmission line bandwidth is considered "narrow" when it is
smaller than approximately two times the symbol rate of the
transmitter.
[0049] Also, the schematic diagram of the PDPSK transmission system
200 illustrates the demodulator bandwidth which, for the embodiment
shown in FIG. 1, is the bandwidth, e.g the FSR, associated with the
delay interferometer (B.sub.DI) 206. In addition, the schematic
diagram 200 of the PDPSK transmission system 200 illustrates the
receiver bandwidth (B.sub.RX) 208, which, for the embodiment shown
in FIG. 1, includes the bandwidth of the differential receiver 144
and the input stage of the Demux 150.
[0050] The present invention is in part the realization that
transmission system performance metrics, such as pre-FEC bit error
statistics, OSNR receiver sensitivity, dispersion tolerance can be
optimized by changing the delay interferometer bandwidth (B.sub.DI)
204 in response to changes in at least one other system component
bandwidth. That is, transmission system performance metrics can be
optimized by changing the delay interferometer bandwidth (B.sub.DI)
204 in response to changes in at least one of the transmitter
bandwidth (B.sub.TX) 202, the transmission line bandwidth
(B.sub.TL) 204, and the receiver bandwidth (B.sub.RX) 208. In other
words, the individual component bandwidths, B.sub.TX 202, B.sub.TL
204, B.sub.DI 206, and B.sub.RX 208 are partial bandwidths of a
total effective transmission system bandwidth. Thus, in order to
achieve optimum transmission system performance metrics, a change
in the bandwidth of one of the transmission system components must
be compensated for by a change in the bandwidth of at least one
other component.
[0051] For example, in order to achieve optimum transmission system
performance metrics, such as pre-FEC bit error statistics, OSNR
receiver sensitivity, and tolerance to dispersion, when at least
one of the transmission system component bandwidths, B.sub.TX 202,
B.sub.TL 204, B.sub.DI 206, and B.sub.RX 208 is decreased, at least
one other transmission system component bandwidth must be
increased. In practical high data rate transmission systems, at
least one of the transmitter bandwidth (B.sub.TX) 202, the
transmission line bandwidth (B.sub.TL) 204, and the receiver
bandwidth (B.sub.RX) 208 is likely to be reduced. Therefore, in
order to achieve at least one optimum transmission system
performance metric, the delay interferometer bandwidth (B.sub.DI)
206 must be increased. The delay interferometer bandwidth, e.g. the
FSR, (B.sub.DI) 20.6 can be increased by choosing the delay
(.DELTA.t) of the delay interferometer to be less than T, where
T=1/B is the symbol time slot, and B is the symbol rate. It has
been determined both by simulations and by experiments that
choosing a delay of the delay interferometer to be less than T can
improve pre-FEC bit error statistics, OSNR receiver sensitivity,
and tolerance to dispersion in the transmission system.
[0052] Achieving optimum transmitter performance metrics, such as
pre-FEC bit error statistics, OSNR receiver sensitivity, and
dispersion tolerance according to the present invention can also be
explained in terms of the free spectral range (FSR) of the delay
interferometer 132. The optimal FSR of the delay interferometer 132
depends on the degree of optical filtering performed in the entire
transmission system and on the degree of chromatic dispersion. In
spectrally efficient transmission systems, such as transmission
systems with tight spectral filtering of the transmitted signals,
the optimal FSR of the delay interferometer 132 for optimizing many
transmission system performance metrics is larger than the symbol
rate of the signal. That is, in transmission systems with bandwidth
limiting devices, such as reconfigurable optical add-drop
multiplexers (ROADMs), optical mux/demux interleavers, and
bandwidth limiting devices in the transmitter electronics, optical
modulators, receiver electronics, and detectors, the optimal FSR of
the delay interferometer 132 for optimizing many transmission
system performance metrics is larger than the symbol rate of the
signal.
[0053] It should be understood that the delay interferometer 132 of
the present invention can be embodied as a delay interferometer 132
with a fixed optical delay that is chosen for a particular optical
receiver performance. Alternatively, it should be understood that
delay interferometer 132 of the present invention can be embodied
as a delay interferometer 132 with the variable delay 134 that
provides a means to adjust the optical delay to change the
performance of the optical receiver or to provide a means for the
system to adapt to changing channel conditions or changing
transmission and reception conditions.
[0054] Furthermore, the performance of optical receivers according
to the present invention can be optimized by changing the ratio of
the optical power of the optical signal propagating from the
constructive port 136 relative to the optical power of the optical
signal propagating from the destructive port 138 of the delay
interferometer 132. The ratio of the optical power of the optical
signal propagating in the constructive port 136 relative to the
optical power of the optical signal propagating in the destructive
port 138 can be changed by using at least one of a fixed or a
variable attenuator and/or a variable amplifier as described in
FIGS. 3A-3C. Thus, in one aspect of the present invention, the
PDPSK receiver of the present invention is an adaptive receiver
that changes at least one of the optical delay in the delay
interferometer 132 and the gain and/or attenuation in at least one
arm of the differential receiver 144 in response to changing
transmission system condition. The adaptation scheme described
herein can be performed at installation and subsequently fixed,
continuously during operation of the system, or can be pre-set from
factory.
[0055] FIGS. 3A-3C illustrate schematic diagrams of adaptive PDPSK
receivers 300, 340, 380 according to the present invention. The
term "adaptive receiver" is defined herein to mean a receiver that
adapts or changes in response to changes in the channel or changes
in the transmitter and/or the receiver. In this aspect of the
present invention, the PDPSK receivers 300, 340, 380 all include a
delay interferometer 302 with a variable optical delay 304 that can
be adjusted to change or tune the FSR of the delay interferometer
302. The FSR of the delay interferometer 302 is changed or tuned in
response to various changes in the channel or transmission line
and/or changes in the transmitter and the receiver, such as changes
in filtering anywhere along the transmission path. The variable
optical delay 304 includes a control input 306 for controlling the
level of optical delay. The control input 306 is electrically
connected to an output of a control circuit or processor 308 that
generates a control signal related to the current transmission
system conditions, such as the transmitter bandwidth, (B.sub.TX)
202, the transmission line bandwidth (B.sub.TL) 204, and the
receiver bandwidth (B.sub.RX) 208 (see FIG. 2).
[0056] The adaptive PDPSK receivers 300, 340, 380 shown in FIGS.
3A-3C also include a means to adjust signal levels in each arm of
the differential receivers. FIG. 3A 300 shows a differential
receiver 310 that includes a balanced detector having a first 312
and second photodiode 314 in a respective one of a first and second
arm of the differential receiver. A first 316 and second attenuator
318 is electrically coupled to an output of a respective one of the
first 312 ad second photodiode 314. The first 316 and second
attenuator 318 are adjustable so as to change the signal
contributions from the first 312 and second photodiode 314 by
adding electrical attenuation. In some embodiments, the processor
308 generates a control signal related to the current transmission
system conditions to control the level of attenuation provided by
the first 316 and second attenuator 318.
[0057] FIG. 3B shows an adaptive PDPSK receiver 340 that includes a
differential detector 342. A first 344 and second optical
attenuator 346 is optically coupled between the delay
interferometer 302 and a respective one of a first 348 and second
photodiode 350 in the differential detector 342. The first 344 and
second optical attenuator 346 are adjustable so as to change the
signal contributions from the first 348 and second photodiode 350
by adding optical attenuation in one arm of the differential
receiver 340.
[0058] FIG. 3C shows an adaptive PDPSK receiver 380 that includes a
differential receiver 382 having a first 384 and second photodiode
386 in a respective one of a first and second arm of the
differential receiver 380. A first 388 and second electronic
amplifier 390 is electrically coupled to an output of a respective
one of the first 384 and second photodiode 386. The first and
second electronic amplifiers 388, 390 are adjustable so as to
change the signal contributions from the first 384 and second
photodiode 386 by adding electrical gain.
[0059] In operation, a control signal is generated by the processor
308 from measurements of transmission system parameters and
metrics. These transmission system parameters and metrics can be
related to the bandwidth of the various transmission system
components or to the level of dispersion in the transmission
system. The control signal is applied to the control input 306 of
the variable optical delay 302. The adaptive PDPSK receivers 300,
340, 380 then adjust the FSR of the variable optical delay 306 of
the delay interferometer 302 in response to the control signal
applied to the control input 306. In some embodiments, a control
signal is generated that changes the FSR of the variable optical
delay 306 automatically to optimize a certain performance metric,
such as a pre-FEC bit error statistic, OSNR receiver sensitivity,
and/or dispersion tolerance. In some embodiments, the control
signal changes the FSR of the variable optical delay 306 in a
continuously tunable manner. In other embodiments, the control
signal changes the FSR of the variable optical delay 306 between
predetermined values of FSR.
[0060] It should be understood that the methods and apparatus of
the present invention can be applied to any type of phase
modulation system, such as DPSK/DQPSK transmission systems,
including DXPSK transmission systems where X=2, 4, 8, 16 . . .
Furthermore, the methods and apparatus of the present invention can
use either an NRZ type or an RZ type modulation format. Also, it
should be understood that the methods and apparatus of the present
invention can be applied to any type of transmission system.
Furthermore,
[0061] The PDPSK transmission systems according to the present
invention have been shown to have improved OSNR receiver
sensitivity over known DPSK/DQPSK transmission systems.
Improvements have been demonstrated with both RZ-type and NRZ-type
transmission formats. The simulations and experimental results
presented herein are for a symbol rate of 43 Gb/s. However, it is
understood that the methods and apparatus of present invention can
be practiced at any symbol rate. However, the methods and apparatus
of present invention can significantly enhance receiver performance
metrics compared with known systems at data rates that are at 43
Gb/s data and higher.
[0062] FIG. 4A presents calculated data 400 for electrical eye
diagrams of NRZ PDPSK signals for three different levels of optical
filtering and for three different values of the delay
interferometer FSR in a transmission system according to the
present invention. FIG. 4B presents calculated data 450 for
electrical eye diagrams of RZ PDPSK signals for three different
levels of optical filtering and for three different values of the
delay interferometer FSR in a transmission system according to the
present invention. A second order super-Gaussian optical filter
transfer function was used in the simulations.
[0063] The eye diagram data 400, 450 presented in FIGS. 4A and 4B
indicate that when the FSR of the delay interferometer 132 (FIG. 1)
of the receiver is equal to the 43 GHz symbol rate, a wide-open eye
diagram is obtained when the optical filtering in the transmission
system is weak (not strong filtering). A wide-open eye diagram
indicates a low bit error rate and a high OSNR receiver
sensitivity, which is desirable for such transmission systems. The
term "strong filtering," which is also known in the art as "tight
filtering," is defined herein as narrow passband filtering. The 28
GHz bandwidth filtering shown in FIG. 4 is stronger filtering than
the 40 GHz bandwidth filtering, which is stronger than the 80 GHz
filtering. In contrast, the data 400, 450 presented in FIGS. 4A and
4B indicate that when the FSR of the delay interferometer 132 is
equal to the 43 GHz symbol rate with strong optical filtering, a
more closed eye diagram is obtained. The more closed eye diagram
indicates a higher bit error rate for a fixed signal-to-noise ratio
compared with an open eye diagram.
[0064] The data 400, 450 presented in FIGS. 4A and 4B also indicate
that when the FSR of the delay interferometer 132 (FIG. 1) of the
receiver is larger than the symbol rate (i.e. FSR.sub.DI equal to
50 GHz and 66.7 GHz), more distortion in the eye diagrams is
present when the filtering is weak (i.e. filter BW=80 GHz) compared
with receivers where the FSR of the delay interferometer 132 is
equal to the 43 GHz symbol rate. The increased distortion indicates
that the bit error rate has increased for a fixed signal-to-noise
ratio.
[0065] Furthermore, the data 400, 450 presented in FIGS. 4A and 4B
indicates that when the FSR of the delay interferometer 132 (FIG.
1) of the receiver is larger than the symbol rate (i.e. FSR.sub.DI
equal to 50 GHz and equal to 66.7 GHz) and the optical filtering is
strong, the eye diagrams are more open and show less distortion
compared with receivers where the FSR of the delay interferometer
132 is equal to the 43 GHz symbol rate. The lower distortion
indicates a lower bit error rate at a fixed signal-to-noise ratio.
These data visually illustrate the performance benefit that can be
achieved by using the method and apparatus of the present invention
in transmission systems with strong filtering. The bit error rate
can be improved in such systems by using a delay interferometer 132
with a differential delay that is less than one bit-period or
equivalently, a FSR that is larger than the symbol rate.
[0066] FIG. 5 illustrates a schematic diagram of an experimental
transmission system 500 used to measure eye diagrams and OSNR
sensitivity data for transmission systems according to the present
invention. The transmission system 500 includes a DPSK transmitter
502 comprising an electro-optic modulator 504. A plurality of
modulators can be used to transmit data at different wavelengths. A
driver 506 is coupled to a modulation input of the modulator 504.
The driver 506 receives a pseudo-random bit sequence (PRBS) and
then adjusts the level of the signal to a suitable level for
modulation. The driver 506 then applies the PRBS to the modulators
504.
[0067] A multiplexer 508 is optically coupled to the output of the
modulators 504. The multiplexer 508 can multiplex a plurality of
optically modulated signals onto a single output optical signal. An
output of the multiplexer 508 is optically coupled to first
interleaving devices 510. The first interleaving devices 510 are
narrow-band filter. For the experiments describe herein, the
interleaving devices are filters having a super-Gaussian shape. The
FWHM bandwidth of these super-Gaussian shaped filters was .about.42
GHz. Cascading two of these super-Gaussian shaped filters resulted
in a .about.35 GHz BW, and cascading four of them resulted in a
.about.28 GHz BW.
[0068] An adjustable noise load 514 is coupled to the output of the
first interleaving devices 510. The output of the adjustable noise
load 514 is coupled to second interleaving devices 516. The second
interleaving devices 516 are also narrow-band filters. The output
of the transmission line 512 is optically coupled to an input of a
demultiplexer 518. The demultiplexer 518 demultiplexes the optical
signals into a plurality of optical signals each with a different
wavelength.
[0069] The transmission system 500 also includes a DPSK receiver
520 comprising a demodulator 522. The demodulator 522 includes the
delay interferometer 523 that was described in connection with FIG.
1. The differential output of the demodulator 522 is optically
coupled to differential inputs of a differential detector 524. The
differential detector 524 generates a received signal at an output.
Measurement equipment 526 is electrically connected to the output
of the differential detector 524. The measurement equipment is used
to measure the experimental results, such as the eye diagrams and
OSNR data that are presented in the following figures.
[0070] FIG. 6 presents experimental data for electrical eye
diagrams 600 of NRZ and RZ signals for four different values of the
delay interferometer FSR in a transmission system where strong
optical filtering is applied to the signals by cascading four
optical filters to provide a combined FWHM of 28 GHz. Electrical
eye diagrams are shown for NRZ and RZ signals received by a
conventional DPSK receiver having a delay interferometer with a FSR
that is equal to the symbol rate and also for a PDPSK receiver
according to the present invention having a delay interferometer
with a FSR that is larger than the symbol rate as describe in the
present invention.
[0071] The electrical eye diagrams 600 visually indicate that the
conventional DPSK receiver has more inter-symbol interference for
both NRZ and RZ signals compared with the PDPSK receivers according
to the present invention. The electrical eye diagrams for the PDPSK
receivers according to the present invention show significantly
more open eye diagrams. The electrical eye diagrams also visually
indicate that there must be an optimum FSR for the delay
interferometer 523 in the demodulator 522 of the transmission
system 500 (FIG. 5). The PDPSK receivers with the delay
interferometer FSRs equal to 57 GHz appear to have less distortion
for both NRZ and RZ signals than the PDPSK receivers with delay
interferometer FSRs equal to 67 GHz.
[0072] Thus, the electrical eye diagrams 600 indicate that for a
transmission system with a particular number of filters, or
equivalently, for a transmission system with a specific effective
filtering, there can be an optimum value of delay interferometer
FSR in the PDPSK receiver that corresponds to an optimum receiver
performance. One skilled in the art will understand that both
simulations and experiments can be performed to determine the
optimum delay interferometer FSR for a demodulator according to the
present invention, which corresponds to the optimum receiver
performance in any particular range of effective transmission
system filtering.
[0073] FIG. 7A presents experimental OSNR sensitivity data 700 for
a NRZ DPSK signal received with a conventional DPSK receiver and
with a PDPSK receiver according to the present invention. FIG. 7A
shows the required receiver OSNR to achieve a certain fixed bit
error rate as a function of the number of 50 GHz interleavers. The
interleavers are narrow-band filters as described in connection
with FIG. 5. The graph 702 presents OSNR data in dB for a
2.times.10.sup.-3 bit error rate as a function of the number of 50
GHz interleavers. A BER of 2.times.10.sup.-3 is typically converted
to a BER of less than 1.times.10.sup.-15 after forward error
correction. The graph 702 shows that a PDPSK receiver does not
require as high an OSNR as a conventional DPSK receiver to achieve
a certain BER.
[0074] The graph 704 presents experimental OSNR data in dB for a
10.sup.-5 bit error rate as a function of the number of 50 GHz
interleavers. An eye diagram 706 is presented for a signal received
with a conventional DPSK receiver with four 50 GHz interleavers. In
comparison, an eye diagram 708 is presented for a signal received
with a PDPSK receiver according to the present invention with a FSR
equal to 67 GHz with four 50 GHz interleavers. The eye diagram 708
for the signal received with the PDPSK receiver according to the
present invention appears more open than the eye diagram 706 for a
signal received with a conventional DPSK receiver, which indicates
less distortion in the signal received signal by the PDPSK receiver
according to the present invention. The graph 704 shows that a
PDPSK receiver does not require as high an OSNR as a conventional
DPSK receiver in order to achieve a certain BER.
[0075] FIG. 7B presents experimental OSNR sensitivity data for a RZ
DPSK signal received with a conventional DPSK receiver and with a
PDPSK receiver according to the present invention. The graph 750
presents the experimental OSNR in dB for the FEC threshold error
rate as a function of the number of 50 GHz interleavers. The FEC
threshold is the maximum bit error rate level at which the forward
error correction circuit can remove errors (i.e., corrected BER is
less than 10.sup.-15). The graph 750 shows that a PDPSK receiver
does not require as high an OSNR as a conventional DPSK receiver to
achieve a certain BER.
[0076] Thus, the simulation and experimental data presented in
FIGS. 4A, 4B, 6, and 7A and 7B indicate that the method and
apparatus of the present invention can achieve improved receiver
performance in transmission systems with narrow optical filtering
by using the PDPSK receivers according to the present invention
with delay interferometers having FSRs that are greater than the
symbol rate.
[0077] FIG. 8A presents a comparison of calculated eye diagram data
for DPSK signals received with a conventional DPSK receiver and
received with a PDPSK receiver according to the present invention
for various levels of dispersions in the transmission line system.
The calculated eye diagrams 802 are eye diagrams for DPSK signals
received with a conventional DPSK receiver with an effective
transmitter bandwidth (Be) equal to 0.4 times the symbol rate and
with two optical filters inserted in the transmission path, each
optical filter having a bandwidth of 60 GHz. The eye diagrams 802
are presented for no dispersion, 75 ps/nm dispersion, and for 100
ps/nm dispersion. The calculated eye diagrams 800 indicate that
significantly more distortion is present in the received signal
when the dispersion is 75 ps/nm and 100 ps/nm.
[0078] The calculated eye diagrams 804 are eye diagrams for DPSK
signals received with a PDPSK receiver according to the present
invention with an effective transmitter bandwidth (Be) equal to 0.4
times the symbol rate and with two optical filters inserted in the
transmission path, each optical filter having a bandwidth of 60
GHz. The calculated eye diagrams 804 are also presented for no
dispersion, 75 ps/nm dispersion, and for 100 ps/nm dispersion. The
calculated eye diagrams 804 indicate that the dispersion tolerance
of receivers according to the present invention is increased under
some conditions when the FSR of the delay interferometer is larger
than the symbol rate of 43 Gsymbols/s
[0079] FIG. 8B presents measured OSNR penalty data 850 in dB for a
DPSK signal received with a conventional DPSK receiver and received
with a PDPSK receiver according to the present invention for
various levels of dispersions in the transmission line system. The
OSNR penalty data 850 is presented in dB. The OSNR penalty data 850
indicates that the dispersion tolerance of receivers according to
the present invention is increased under some conditions when the
FSR of the delay interferometer is larger than the symbol rate.
[0080] Thus, the calculated eye diagrams 804 and the measured eye
diagrams 850 indicate that a dispersion tolerance of .+-.100 ps/nm
is possible by properly choosing the effective transmitter
bandwidth B.sub.e, the optical filter bandwidth B.sub.o, and the
delay interferometer FSR. Also, the calculated eye diagrams 804 and
the measured OSNR penalty data indicate that the dispersion
tolerance is generally higher for a PDPSK receiver where the delay
interferometer FSR is greater than the symbol rate. In addition,
the calculated eye diagrams 804 and the measured OSNR penalty data
indicate that an optimal delay interferometer FSR exists for a
given dispersion level, effective transmitter bandwidth (B.sub.e),
optical filter bandwidth (B.sub.o), which is equivalent to
B.sub.TL, and receiver bandwidth (R.sub.RX). Similar results were
obtained for both NRZ-type and RZ-type modulated data. Also,
similar results were obtained for positive and negative
dispersion.
EQUIVALENTS
[0081] While the present teachings are described in conjunction
with various embodiments and examples, it is not intended that the
present teachings be limited to such embodiments. On the contrary,
the present teachings encompass various alternatives, modifications
and equivalents, as will be appreciated by those of skill in the
art, may be made therein without departing from the spirit and
scope of the invention.
* * * * *