U.S. patent application number 11/692432 was filed with the patent office on 2008-10-02 for inter-symbol interference-suppressed colorless dpsk demodulation.
This patent application is currently assigned to NEC LABORATORIES AMERICA, INC.. Invention is credited to Philip Nan Ji, Shuji Murakami, Tsutomu Tajima, Ting Wang, Lei Xu, Yutaka Yano.
Application Number | 20080240736 11/692432 |
Document ID | / |
Family ID | 39794577 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080240736 |
Kind Code |
A1 |
Ji; Philip Nan ; et
al. |
October 2, 2008 |
Inter-Symbol Interference-Suppressed Colorless DPSK
Demodulation
Abstract
An optical device includes an interferometer for a received
optical differential phase shift keying DPSK signal, and an
equalizer integrated with the interferometer in a manner for
reducing from optical filtering effects an interference by signal
bits of the DPSK signal with signal bits of a contiguous DPSK
signal. The interferometer is a Michelson delay interferometer
type, but can also be a Mach-Zehnder delay interferometer type on
fiber, waveguide or other optical structure. The equalizer is a
Fabry-Perot type equalizer, but can be a ring resonator type or a
fiber based equalizer.
Inventors: |
Ji; Philip Nan; (Plainsboro,
NJ) ; Xu; Lei; (Princeton, NJ) ; Wang;
Ting; (Princeton, NJ) ; Murakami; Shuji;
(Herndon, VA) ; Tajima; Tsutomu; (Tokyo, JP)
; Yano; Yutaka; (Tokyo, JP) |
Correspondence
Address: |
NEC LABORATORIES AMERICA, INC.
4 INDEPENDENCE WAY, Suite 200
PRINCETON
NJ
08540
US
|
Assignee: |
NEC LABORATORIES AMERICA,
INC.
Princeton
NJ
|
Family ID: |
39794577 |
Appl. No.: |
11/692432 |
Filed: |
March 28, 2007 |
Current U.S.
Class: |
398/202 |
Current CPC
Class: |
H04B 10/677
20130101 |
Class at
Publication: |
398/202 |
International
Class: |
H04B 10/06 20060101
H04B010/06 |
Claims
1. An optical device comprising: an interferometer for a received
optical differential phase shift keying DPSK signal, and an
equalizer integrated with said interferometer in a manner for
reducing from optical filtering effects an interference by signal
bits of the DPSK signal with signal bits of a contiguous DPSK
signal.
2. The optical device of claim 1, wherein said equalizer is
integrated with said interferometer by one of: optically placing
said equalizer 807 at an input of said interferometer; optically
placing said equalizer 808 at a constructive path of said
interferometer; and optically placing said equalizer 814 having
half of a filter ripple depth at an input and a constructive path
of said interferometer.
3. The optical device of claim 1, wherein said equalizer is
integrated with said interferometer by optically placing a first of
said equalizer 811 at an input and constructive path of said
interferometer and a second of said equalizer 813 at a destructive
path of said interferometer, said first and second of said
equalizer having half of a filter ripple depth.
4. The optical device of claim 1, wherein said equalizer is
integrated by one of: optically placing a first of said equalizer
907 inside said interferometer between a beam splitter/combiner 801
and a first reflecting mirror 803 and optically placing a second of
said equalizer 909 inside said interferometer between said beam
splitter/combiner 801 and a second reflecting mirror 805; and
optically integrating a first of said equalizer 907 inside said
interferometer with a first reflecting mirror 911 and optically
integrating a second of said equalizer inside said interferometer
with a second reflecting mirror 913.
5. The optical device of claim 1, wherein said Interferometer has a
prism or 2 mirrors for its constructive and destructive reflectors
917, 919 and said equalizer is integrated by one of: optically
placing said equalizer 915 at an input of said interferometer; and
optically placing a first of said equalizer 921 at a constructive
path of said interferometer and a second of said equalizer 923 at a
destructive path of said interferometer.
6. The optical device of claim 1, wherein said Interferometer
includes a prism or 2 mirrors for its constructive and destructive
path reflectors 917, 919 and said equalizer is integrated by one
of: optically placing said equalizer 927 in both a constructive
path and a destructive path of said interferometer with a
reflective mirror or prism 925 in an optical path of said
constructive path before said equalizer 927; and optically placing
said equalizer 931 in both constructive and destructive paths of
said interferometer with a lens 929 in an optical path of both said
constructive and destructive paths before said equalizer 931 and a
reflective mirror or prism 930 in an optical path of said
constructive path before said lens 929.
7. The optical device of claim 1, wherein said interferometer is
one of a Michelson type delay interferometer, a Mach-Zehnder type
delay interferometer based on optical fiber or optical waveguide or
planar lightwave circuit.
8. The optical device of claim 1, wherein said equalizer is one of
a Fabry-Perot PT type equalizer, a ring resonator type and a fiber
based structure.
9. The optical device of claim 1, further comprising a layer of
glass in at least one optical path of said integrated
interferometer or equalizer for varying temperature of said glass
to change the index of refraction of said glass thereby varying an
optical path through said glass.
10. The optical device of claim 1, wherein said interferometer and
equalizer are integrated for reducing inter-symbol interference in
a 40 Gb/s DPSK signal in a 50 GHz spaced DWDM communication
system.
11. A method comprising the steps of: providing constructive and
destructive optical paths for a received optical differential phase
shift keying DPSK signal, and integrating an equalizer with said
providing for reducing from optical filtering effects an
interference by signal bits of the DPSK signal with signal bits of
a contiguous DPSK signal.
12. The method of claim 1, wherein said integrating includes one
of: optically placing said equalizer 807 at an input of said
providing; optically placing said equalizer 808 at said
constructive path; and optically placing said equalizer 814 having
half of a filter ripple depth at an input of said providing and
said constructive path.
13. The method of claim 11, where said integrating includes
optically placing a first of said equalizer 811 at an input and
said constructive path of said providing and optically placing a
second of said equalizer 813 at said destructive path, said first
and second of said equalizer having half of a filter ripple
depth.
14. The method of claim 11, wherein said integrating includes one
of: optically placing a first of said equalizer 907 inside an
interferometer from said providing between a beam splitter/combiner
801 and a first reflecting mirror 803 and optically placing a
second of said equalizer 909 inside said interferometer between
said beam splitter/combiner 801 and a second reflecting mirror 805;
and optically placing a first of said equalizer 907 inside an
interferometer from said providing with a first reflecting mirror
911 and optically integrating a second of said equalizer inside
said interferometer with a second reflecting mirror 913.
15. The method of claim 11, wherein said providing comprises
providing an Interferometer with a prism or 2 mirrors for its
constructive and destructive reflectors 917, 919 and said
integrating includes one of: optically placing said equalizer 915
at an input of said interferometer; optically placing a first of
said equalizer 921 at a constructive path of said interferometer
and a second of said equalizer 923 at a destructive path of said
interferometer.
16. The method of claim 11, wherein said providing comprises
providing an Interferometer with a prism or 2 mirrors for its
constructive and destructive path reflectors 917, 919 and said
integrating includes one of: optically placing said equalizer 927
in both said constructive and destructive paths of said
interferometer with a reflective mirror or prism 925 in an optical
path of said constructive path before said equalizer 927; and
optically placing said equalizer 931 in both said constructive and
destructive paths of said interferometer with a lens 929 in an
optical path of both said constructive and destructive paths before
said equalizer 931 and a reflective mirror or prism 930 in an
optical path of said constructive path before said lens 929.
17. The method of claim 11, wherein said constructive and
destructive paths are provided by one of a Michelson type delay
interferometer and a Mach-Zehnder type delay interferometer based
on optical fiber or optical waveguide or planar lightwave circuit,
and the equalizer is one of a Fabry-Perot PT type equalizer, a ring
resonator and a fiber based equalizing.
18. The method of claim 11, wherein equalizer is one of a
Fabry-Perot PT type equalizer, a ring resonator and a fiber based
equalizing.
19. The optical device of claim 11, further comprising varying
temperature of a layer of glass in an optical path of said
separating with integrated equalizing for changing the index of
refraction of said glass thereby varying said optical path through
said glass.
20. The method of claim 1, wherein separating with integrated
equalizing reduces inter-symbol interference in a 40 Gb/s DPSK
signal in a 50 GHz spaced DWDM communication system.
Description
[0001] This application is related to U.S. application Ser. No.
11/279,767, entitled "COLORLESS DIFFERENTIAL PHASE SHIFT KEYED AND
LOW CROSSTALK DEMODULATORS", filed on Apr. 14, 2006, and U.S.
application Ser. No. 11/619,499, entitled "OPTICAL EQUALIZATION
FILTERING OF DWDM CHANNELS", filed on Jan. 3, 2007, both of which
are incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to optical
communications, and, more particularly, to a colorless differential
phase shift keying demodulator for suppressing inter-symbol
interference in dense wave division multiplexing communication
systems.
[0003] Internet-based traffic has been growing exponentially due to
the rapid increase of the microelectronic processing power,
expansion of communication networks towards ubiquity, and emerging
of modern bandwidth-thirsty business and personal applications such
as video-on-demand (VoD) and storage area network (SAN). As the
backbone to provide the transportation pipelines for such traffic
volumes, the optical network has received demands for larger
bandwidth capacity.
[0004] As a result, the 10 Gb/s bandwidth per channel in dense
wavelength division multiplexing (DWDM) optical system is becoming
inadequate. DWDM system with higher transmission rate of 40 Gb/s
has begun to be deployed in long haul and metro optical
networks.
[0005] In these new DWDM systems, the conventional on-off-keying
(OOK)-based intensity modulation scheme, also called
non-return-to-zero (NRZ), has very limited performance in long-haul
optical transmission. New modulation formats such as differential
phase-shift-keying (DPSK), differential quadrature
phase-shift-keying (DQPSK) and duobinary have been developed to
mitigate the fiber detrimental effects, achieve higher ONSR
tolerance and/or deliver better spectral efficiency.
[0006] Among these new modulation formats, optical DPSK has become
a popular candidate for 40 Gb/s DWDM transmission due to its
tolerance to fiber nonlinearities and higher receiver sensitivity.
It offers 3 dB OSNR improvement with a balanced receiver and a
decrease of self-phase modulation (SPM) and cross-phase modulation
(XPM) due to the constant envelope modulation. Although DPSK has
superior transmission performance, its relatively broad spectrum
limits the spectral efficiency of DPSK-based DWDM systems. Under 50
GHz ITU grids, 40 G DPSK signal suffers from inter-symbol
interference (ISI), where optical pulse width broadening due to
narrow filtering leads to the interference between neighboring
bits. The optical filtering effect in 50 GHz spaced systems leads
to interference between neighboring signal bits. This phenomenon is
known as the inter-symbol interference (ISI). The ISI effect can
cause a dramatic increase in signal bit error rate.
[0007] FIG. 1 shows the schematic of a typical DWDM transmission
system with 40 Gb/s DPSK signal on each DWDM channel. At the input
side, each DWDM channel has a respective DPSK transmitter 101.sub.1
to 101.sub.4, which uses a differential encoder 1011 to apply
one-bit-delay exclusive OR operation on the 40 Gb/s data, and
modulates the phase of the continuous wave (CW) laser light 1012 at
the phase modulator 1013. Each laser source has its respective DWDM
wavelength on ITU-T grid. The output is an NRZ-DPSK signal. This
signal is further intensity modulated 1015 with a driving clock
1014 signal to carve the pulse and reduce the phase chirp. So the
final output of the transmitter is an RZ-DPSK or CSRZ
(carrier-suppressed RZ)-DPSK signal. These 100 GHz spaced DPSK
channels are combined using a 100 GHz AWG-based multiplexer
103.sub.1. Another set of multiplexer combined 103.sub.2, 100 GHz
spaced DPSK channels (with 50 GHz center frequency offset to the
first set) is combined with the first set using an optical 100 GHz
to 50 GHz interleaver 107. After traveling through the optical link
115.sub.1, 115.sub.2 with intermediate repeaters 105.sub.1 to
105.sub.3, the DWDM signals are separated at the receiving node by
50 GHz to 100 GHz de-interleaver 109 and 100 GHz demultiplexers
111.sub.1 111.sub.2. The demultiplexed individual channels are then
sent to DPSK receiver 113.sub.1-113.sub.4, which contains a delay
interferometer (DI) 1131 and a pair of balanced detectors
1132.sub.1, 1132.sub.2. The DI uses the interference between the
preceding bit and current bit to convert the phase modulated signal
into an intensity modulated signal. The balanced detector can use
the two output ports from the DI (the constructive port and
destructive port) and improve the sensitivity of the receiver.
[0008] This schematic of FIG. 1 shows that each DWDM signal travels
through several passive optical filter elements between the
transmitter and the receiver. These elements include a multiplexer
and demultiplexer, interleaver and de-interleaver. These optical
elements cause a strong optical filtering effect to the 40 Gb/s
DPSK signals, which broadens the 40 Gb/s optical signals and
results in the extension of signal energy into the time slots of
neighboring bits. The narrower the passband profile of these
optical filter elements, the stronger the filtering effects on the
40 Gb/s signal. Other factors such as passband shape (flat-top or
Gaussian), passband asymmetry, insertion loss ripple and center
frequency offset will also affect the level of the filtering
effect. There might be other filtering elements in the transmission
link, such as wavelength blocker in optical add/drop multiplexer
nodes. The eye diagrams of FIGS. 2A and 2B show the 33% RZ-DPSK
signal at receiver before and after the 100 GHz AWG multiplexer and
100 GHz to 50 GHz interleaver. The ISI effect caused by these
filtering elements is clearly demonstrated.
[0009] Proposed methods to mitigate ISI effect or to reduce the ISI
problem caused by the strong filtering effect include use of
spectral efficient modulation schemes; coding; side band
pre-filtering methods; electronic equalization and optical
equalization.
[0010] Use of spectral efficient modulation schemes such as optical
duobinary and DQPSK modulation schemes can achieve 33 GHz at 90%
spectral width. Therefore the signals are more tolerable to the
filtering effect caused by the optical elements. However the
duobinary signal has poor tolerance to nonlinear effect and
therefore cannot has limited transmission span. The DQPSK
modulation requires more complex and expensive transmitters and
receivers.
[0011] An advanced coding scheme can be used to introduce
correlation of the signal and control the power spectral density,
and even lead to a reduction of signal spectral width. The downside
is that the implementations are still technically challenging or
very expensive for applications at a high speed such as 40
Gb/s.
[0012] Side band pre-filtering methods such as single-side-band
(SSB) filtering and vestigial-side-band (VSB) filtering reduce the
optical signal spectral width (to as much as half) to better fit
into the passband width of the optical channel. The disadvantage is
the increased complexity and compromised signal performance.
[0013] Electronic equalization such as electronic post-detection
processing is used to improve system performance. The operation is
typically based on feed-forward equalizers (FFE), decision feedback
equalizers (DFE), maximum likelihood sequence estimation (MLSE),
etc. It is shown that electronic equalization can partially cancel
ISI and lead to an opening of the receiving signal eye. However,
the performance of EDC is limited because the phase information of
the incoming optical signals is lost due to OE conversion. Optical
equalizers can be applied together with EDC.
[0014] An optical equalizer technology developed by applicants
previously, an intra-channel optical equalizer, is a special
optical filter. It is known that for a signal pulse not to have ISI
it must satisfy the Nyquist criteria, and some popular Nyquist
pulses have raised-cosine profile for their Fourier transforms.
Therefore, it is desirable to set the transfer function of the band
limited channel to a raised-cosine shape. Based on the given
profiles of the passive optical filtering elements in the optical
link, a corresponding optical equalizer is designed to complement
them and produce an overall raised-cosine profile as shown in FIG.
3A for a multiplexer element, FIG. 3B for an interleaver element,
FIG. 3C for combined filtering effect from multiplexer element and
interleaver element, FIG. 3D for an optical equalizer element and
FIG. 3E for an overall filtering effect with optical
equalization.
[0015] The filter has a periodic profile with free spectral range
(FSR) equal to the channel spacing of the DWDM signal and center
frequency locked to the ITU-T channel grid, therefore it works on
all the DWDM channels within the band.
[0016] Simulation results show about a 6 dB Q factor improvement
for the back-to-back signals and 3 dB improvement after
transmission over about 500 km fiber.
[0017] An optical equalizer with such a scheme can be designed
based on Fabry-Perot (FP) interferometer theory and fabricated
using dielectric thin-film technology. Comparison of ISI
suppression without optical equalization, see eye diagrams 4A, 4B,
and with optical equalization, see eye diagrams 4C, 4D, for a 40
Gb/s DPSK signal shows an improvement in the receiving signal
particularly for the constructive port FIG. 4C.
[0018] A disadvantage of an optical equalizer is the requirement of
an additional optical element in the transmission link. Also, as an
athermal device without a temperature control mechanism, it might
have temperature drift and have center frequency offset to the DPSK
demodulator.
[0019] Accordingly, there is a need for an optical solution that
integrates the functions of a DPSK demodulator and optical
equalizer to reduce the inter-symbol interference ISI from the
filtering effect on the optical path.
SUMMARY OF THE INVENTION
[0020] In accordance with the invention, an optical device includes
an interferometer for a received optical differential phase shift
keying DPSK signal, and an equalizer integrated with the
interferometer in a manner for reducing from optical filtering
effects interference by signal bits of the DPSK signal with signal
bits of a contiguous DPSK signal. In an exemplary embodiment, the
equalizer is integrated with the interferometer by optically
placing a first of the equalizer 811 at an input and constructive
path of the interferometer and a second of the equalizer 813 at a
destructive path of the interferometer, the first and second of the
equalizer having half of a filter ripple depth. The interferometer
is a Michelson delay interferometer type, but can also be a
Mach-Zehnder delay interferometer type on fiber, waveguide or other
optical structure. The equalizer is a Fabry-Perot type equalizer,
but can be a ring resonator type or a fiber based equalizer.
Alternatively, a layer of glass in at least one optical path of the
integrated interferometer or equalizer can be used for varying
temperature of the glass to change the index of refraction of the
glass, thereby varying an optical path through the glass.
[0021] In another aspect of the invention, a method includes the
steps of providing constructive and destructive optical paths for a
received optical differential phase shift keying DPSK signal, and
integrating an equalizer with the providing for reducing from
optical filtering effects an interference by signal bits of the
DPSK signal with signal bits of a contiguous DPSK signal. In an
exemplary embodiment, the integrating includes optically placing a
first of the equalizer 811 at an input and the constructive path of
the providing and optically placing a second of the equalizer 813
at the destructive path, the first and second of the equalizer
having half of a filter ripple depth. The interferometer can be a
Michelson Interferometer type and the equalizer can be a
Fabry-Perot type equalizer. Alternatively, a layer of glass in at
least one optical path of the integrated interferometer or
equalizer can be used for varying temperature of the glass to
change the index of refraction of the glass, thereby varying an
optical path through the glass.
BRIEF DESCRIPTION OF DRAWINGS
[0022] These and other advantages of the invention will be apparent
to those of ordinary skill in the art by reference to the following
detailed description and the accompanying drawings.
[0023] FIG. 1 is schematic of DWDM transmission link with 40 Gb/s
DPSK-modulated signals.
[0024] FIGS. 2A and 2B are eye diagrams of ISI-induced signal
degradation in 40 Gb/s 33% RZ-DPSK signal due to filtering
effect.
[0025] FIG. 3 shows diagrams demonstrating an optical equalizing
technique to compensate for the filtering effect from AWG and
interleaver elements and generate a raised cosine profile.
[0026] FIGS. 4A and 4B are eye diagrams illustrating ISI
suppression without an optical equalizer for a 40 Gb/s DPSK
signal.
[0027] FIGS. 4c and 4D are eye diagrams illustrating ISI
suppression with an optical equalizer for a 40 Gb/s DPSK
signal.
[0028] FIG. 5A is a schematic of a Michelson interferometer based
DPSK demodulator.
[0029] FIGS. 5B and 5C show the power spectrum and group delay
(phase) spectrum of the DPSK demodulator of FIG. 5A.
[0030] FIG. 6A is a schematic of a Fabry-Perot interferometer-based
optical equalizer.
[0031] FIGS. 6B and 6C show the power spectrum and group delay
(phase) spectrum of the DPSK demodulator of FIG. 5A.
[0032] FIG. 7 is a schematic of an exemplary simulation network for
verifying performance of the inventive ISI-suppressed DPSK
demodulator.
[0033] FIGS. 8A-8D are schematics of basic Michelson interferometer
configurations for the inventive ISI-suppressed demodulator with
integrated optical equalizing elements.
[0034] FIGS. 9A-9F are schematics of modified Michelson
interferometer configurations for the inventive ISI-suppressed
demodulator with integrated optical equalizing elements.
[0035] FIG. 10 is a schematic of a non-Michelson interferometer
configuration for the inventive ISI-suppressed demodulator with
integrated optical equalizing elements.
[0036] FIG. 11A is an eye diagram of a simulated Q factor of a
received 40 Gb/s DPSK signal for an ISI suppressed DPSK demodulator
without any FP equalizing filter in the optical path.
[0037] FIG. 11B is an eye diagram of a simulated Q factor of a
received 40 Gb/s DPSK signal for an ISI suppressed DPSK demodulator
implemented in accordance with FIG. 8A.
[0038] FIG. 11c is an eye diagram of a simulated Q factor of a
received 40 Gb/s DPSK signal for an ISI suppressed DPSK demodulator
implemented in accordance with FIG. 8B.
[0039] FIG. 11D is an eye diagram of a simulated Q factor of a
received 40 Gb/s DPSK signal for an ISI suppressed DPSK demodulator
implemented in accordance with FIG. 8C.
[0040] FIG. 11E is an eye diagram of a simulated Q factor of a
received 40 Gb/s DPSK signal for an ISI suppressed DPSK demodulator
implemented in accordance with FIG. 8D.
[0041] FIG. 12 depicts insertion loss spectrum of both outputs of
ISI-suppressed DPSK demodulator compared to both outputs of a
regular DPSK demodulator.
[0042] FIGS. 13A-C show polarization dependent loss, chromatic
dispersion and differential group delay spectra of both outputs of
ISI-suppresses DPSK demodulator.
[0043] FIG. 14 is a schematic of a 40 Gb/s transmission
experimental setup with different demodulation and equalization
schemes for verifying the inventive integrated ISI-suppressed DPSK
demodulator.
[0044] FIG. 15A is an eye diagram of a DPSK demodulated signal
without any filtering effect, from the simulation setup according
to FIG. 14.
[0045] FIGS. 15B and 15C are eye diagrams for constructive and
destructive ports of a DPSK demodulator without optical
equalization, from the simulation setup according to FIG. 14.
[0046] FIGS. 15D and 15E are eye diagrams for constructive and
destructive ports of a DPSK demodulator with an external optical
equalizer for ISI suppression, from the simulation setup according
to FIG. 14.
[0047] FIGS. 15F and 15G are eye diagrams for constructive and
destructive ports of an integrated DPSK demodulator with an optical
equalizer for ISI suppression, from the simulation setup according
to FIG. 14.
[0048] FIGS. 16A-16C are eye diagrams demonstrating the effect of
optical equalization in a 100 GHz-spaced DWDM system with DPSK
modulated signal, 16A is without optical equalization, 16B is with
2.5 dB dip optical equalizer at input and 16C is with 1.25 dB dip
optical equalizers at both input and output.
DETAILED DESCRIPTION
[0049] The inventive inter-symbol interference ISI suppression
integrates the functions of a DPSK demodulator and optical
equalizer. It contains a delay interferometer to separate the
constructive and destructive ports or paths of the received DPSK
signal, and an intra-channel equalizer to mitigate the filtering
effect from the optical path and thus suppress the ISI. The
ISI-suppressed DPSK demodulator is a combination of a Michelson
interferometer (MI)-based DPSK demodulator, see FIG. 5A, and a
Fabry-Perot FP interferometer-based optical equalizer. Both of
these optical elements have periodic spectral profiles that achieve
colorless features.
[0050] For the DPSK demodulator part, a free-space Michelson
Interferometer MI structure is used. The input is through a beam
splitter/combiner 501. The mirrors 503, 505 at both arms within the
MI have reflectivity of 100% or close to 100%, and the optical path
length difference between the two interfering arms is equal to the
delay, which is 20 ps to achieve colorless operation at 50 GHz FSR
system FIG. 5A. The power and phase (group delay) spectra of the
DPSK demodulator can be expressed as:
.phi. L = .pi. v v c ##EQU00001## Power = cos 2 ( .phi. L 2 )
##EQU00001.2## GD = 1 2 .pi. .phi. L v ##EQU00001.3##
where .nu..sub.c is the 50 GHz FSR. FIG. 5B shows the power
spectrum, in log scale, having a periodic profile. FIG. 5C shows
the group delay spectrum as a constant.
[0051] The FP interferometer (or FP etalon) consists of two
partially reflective mirror surfaces 601 and a FP cavity 602, as
shown in FIG. 6A. With dielectric thin-film technology, the mirror
surfaces are constructed by stacking multiple layers of dielectric
materials with alternating higher and lower refractive indices with
carefully calculated thickness equaling to 1/4 of the longest
wavelength to be handled. The cavity between two mirror stacks is
also constructed using different transparent material. The
intensity function of a FP interferometer can be expressed as:
I T = I 0 .times. 1 1 + 4 F 2 .pi. 2 sin 2 ( 2 .pi. L .lamda. cos
.theta. ) = I 0 .times. 1 1 + 4 F 2 .pi. 2 sin 2 ( 2 .pi. nfL c cos
.theta. ) ##EQU00002## where F = Finesse = FSR FWHM = .pi. R 1 - R
##EQU00002.2## and I 0 = T 2 ( 1 - R ) 2 ##EQU00002.3##
Here R is the reflectivity of the mirrors. For the optical
equalizer application, the reflectivity value is between 10% and
20%. FIG. 6B shows the power spectra of both transmitted and
reflected signal, and FIG. 6C shows the group delay spectrum. The
transmission spectrum shows a ripple profile with a dip centered at
each 50 GHz ITU-T grid. The depth of the dip is determined by the
reflectivity of the mirror surfaces.
[0052] FIG. 7 is a schematic 701 of a simulation network used to
verify the optical performance of the inventive DPSK demodulator
703 with optical equalization 705. Six 50 GHz-spaced DWDM channels
carrying 40 Gb/s DPSK signals are multiplexed through AWG
multiplexer and interleaver elements as described in the network of
FIG. 1. The received signal of the center channel is studied
because it takes into account the crosstalk from neighboring
channels at both sides.
[0053] The data of the optical devices used in the simulation
(including the AWG multiplexer and demultiplexer, optical
interleaver) are taken from measurement data of actual devices used
in the field, which makes the simulation result closer to an actual
value. Standard Mach-Zehnder interferometer (MZI)-based DPSK
demodulator from a simulation software library is used. Its delay
time is set to be 20 ps to reflect the colorless feature and it
exhibits the same performance as an MI-based DPSK demodulator in
simulations. For the FP equalizing filter, theoretical device data
are used based on the model described for FIGS. 6A-6C. The depth of
the dip at ITU-T grid center is set to be around 2.5 dB. This is an
optimized figure based on the filtering elements in the optical
path and the results of a previous study on optical equalizing
technique.
[0054] In the simulation, the FP equalizing filter and the
colorless DPSK demodulator are represented separately so that their
relative positions in the integrated device can be varied, and the
performance of different configurations can be compared. Also, the
equalizer element can be singled out to compare the transmission
performance with and without the inventive ISI suppression
mechanism.
[0055] There are several possible configurations to integrate the
colorless DPSK demodulator and the FP optical equalizer into the
inventive ISI-suppressed DPSK demodulator. Each of them is
described for their fabrication, feasibility and performance are
compared. The configurations are grouped into three main types. The
first type is the basic MI type, which uses standard MI-based DPSK
demodulator with integrated optical equalizing elements outside its
two interfering arms. The second type is the modified MI type,
which modifies the MI structure to achieve certain features or
benefits. The third type is non-MI type, which does not use MI
structure for DPSK demodulation.
[0056] Configurations for the basic MI type, which uses a standard
MI-based DPSK demodulator with integrated optical equalizing
elements outside its two interfering arms, are shown in FIGS.
8A-8D. FIG. 8A is the most straight forward configuration. It adds
the equalizing filter 807 before the DPSK demodulator 801, 803,
805. Even though the design is simple, it is difficult to
implement. In an MI structure, the Output A and Input ports are
very close to each other and share a dual fiber collimator. It is
therefore difficult to apply the equalizer to just the Input port
without affecting the Output A port.
[0057] In the second configuration shown in FIG. 8B, equalizing
filters 807, 809 are placed at the output ports. It has a similar
issue as the first configuration, FIG. 8A, because it requires
placement of an equalizer 807 at one of the dual fiber output
ports. Requiring two equalizing filters instead of one is a cost
disadvantage.
[0058] The third configuration, shown in FIG. 8C, solves the dual
collimator issue by placing the equalizer 811 over the paths of
both Input and Output A. However the amount of equalizing and hence
the ISI suppression is not equal between the Output A path and
Output B path, because Output A path passes through the equalizer
twice while Output B path only passes through it once. To
compensate for this difference, another equalizer 813 is placed
before the Output B. This configuration requires two equalizers
also. Since the signals of both constructive and destructive ports
(Outputs A and B) go through two stages of an equalizing filter,
the depth of the equalizer ripple dips is halved to 1.25 dB
each.
[0059] The fourth configuration of the invention, FIG. 8D is
similar to the third configuration of FIG. 8C, but removes the
equalizer at Output B port, namely, the destructive port. Even
though this will cause an imbalance between the constructive and
destructive paths as described above, there might not be much of a
detriment to the signal quality because the effective portions of
the equalizer (the dips) fall in the valleys (and not the peaks) of
the destructive port.
[0060] In the Modified MI type structures, shown in FIGS. 9A-9F,
changes are applied to the standard structure of an MI to solve
some issues of the basic type device configurations discussed with
FIGS. 8A-8D, such as the dual collimator issue. Since these
modified type changes require special device designs, they cannot
be directly constructed on existing DPSK demodulator hardware.
[0061] The first modified configuration, shown in FIG. 9A, places
the equalizer 907 inside the MI structure to reduce the size of the
overall device. This modification is difficult to design because
with the equalizer in one path, the phase of the signal of the
delay interferometer will no longer be linear, and the group delay
will not be constant, which is a requirement of DPSK demodulator.
It requires a complex design of an equalizer 909 in the other path
to balance the nonlinear phase ripple. The additional optical
component inside the MI structure also requires a modification of
DPSK demodulator design
[0062] The second modified configuration, FIG. 9B, is similar to
the first modified configuration FIG. 9A, but instead of adding
equalizing filters inside the MI-based DPSK demodulator, the
equalizing filters are integrated with the reflective mirrors 803,
805 in the demodulator. This reduces the number of optical
components inside the device. In this case, the mirrors are no
longer 100% reflective, but have partial reflectivity and thus
become Gires-Tournois cavities 911, 913. Despite the structural
change, the design complexity is similar to the first modified
configuration.
[0063] The third and fourth modified, configurations, see FIGS. 9C
and 9D, use prisms or two reflective mirrors 917, 919 at each
reflective end of the MI, enabling the reflected beam to have
spatial offset from the incoming beam. Thus, the interference point
will be moved away from the position where the input light is
split. In this case, there will be a separation between the Input
port and Output A port, thus a dual fiber collimator cannot be
used. Despite the requirement of additional fiber collimator, the
optical equalizer 915 can be placed at Input port without affecting
Output A port, or the optical equalizer 921 can be placed at the
Output port A 921 in tandem with an optical equalizer 923 placed in
the path of Output port B.
[0064] The fifth modified configuration, shown in FIG. 9E, has a
structure similar to the third and fourth modified configurations
of see FIGS. 9C and 9D, but it adds a reflective mirror or prism
925 at Output A port to deflect Output A beam to the same direction
as Output B beam. Thus, the same optical equalizing element 927 can
be shared between two output ports, provided that it is wide
enough.
[0065] The configuration of FIG. 9E can be modified to produce a
sixth modified configuration, shown in FIG. 9F, to insert an
additional lens 929 at the output paths, so that the two output
beams can share a dual fiber collimator. This is also very useful
to integrate the balanced optical detector to the DPSK demodulator,
because the balanced detector will usually have a close separation
of about 250 .mu.m between two photo detectors. This will also
reduce the required width of the optical equalizing filter 931
inside the device.
[0066] An exemplary non-Michelson Interferometer MI structure for
practicing the inventive DPSK modulation is shown in FIG. 10. This
inventive ISI suppression DPSK demodulator is constructed with an
exemplary alternative to an MI structure, such as Mach-Zehnder
delay interferometer (MZDI) 1013. There are corresponding
technologies to integrate optical equalizing element inside these
DPSK demodulators to achieve ISI suppressions. The embodiment of
FIG. 10 uses a planar lightwave circuit (PLC) platform and
integrates a ring resonator 1011 in front of the MZDI-based DPSK
demodulator 1013. An equivalent optical fiber based structure can
also be constructed similarly.
[0067] Diagrams showing the ISI suppression performance of
embodiments of the invention are shown in FIGS. 11A-11E. In
addition to considerations for the structural difference and
feasibility of a particular embodiment's hardware fabrication, the
ISI suppression performance is a key factor to select the best
device design. Although ISI suppression performance for the basic
inventive DPSK with optical equalization structures are diagramed,
the other inventive DPSK structures will have similar performance
behavior because the difference in the various embodiments of the
invention is in the physical implementation and not the inventive
device design of an integrated MI interferometer and optical
equalization.
[0068] In a DPSK demodulator without any FP equalizing filter in
the optical path, the simulated Q factor of the received 40 Gb/s
DPSK signal is 19.6 dB, as shown by the simulated eye diagram of
FIG. 11A. The simulated eye diagrams of FIGS. 11B-11E correspond to
performance behavior for DPSK demodulator and equalization
configurations of FIGS. 8A-8D, respectively. The Q factor values
calculated from the eye diagrams are very similar, all within 25 to
26 dB. This represents around 5.5 dB improvement, which
demonstrates the ISI suppression performance of the exemplary
embodiments of the integrated interferometer and optical
equalization.
[0069] However, the eye diagram of FIG. 11E of the received signal
from the device configuration of 8D exhibits the least favored
balance between the peak and valley, as compared to eye diagrams of
FIGS. 11B-11C. To maintain good receiving signal quality a stronger
threshold optimization adjustment can be made.
[0070] An advantage of the device configuration according to FIG.
8B is from not having a dual collimator issue or receiving signal
balance issue. It can be implemented easily based on the existing
MI-based DPSK demodulator fabrication technology and thin-film FP
interferometer fabrication technology, which are both very
mature.
[0071] Turning now to the performance considerations of an ISI
suppressed DPSK demodulator device. Based on the configuration of
FIG. 8C, samples of a DPSK demodulator with optical equalizing
filter for ISI suppression were constructed with free-space MI
technology and dielectric thin-film technology. A combined
equalizer dip of 2.5 dB at 50 GHz ITU-T grid is required. The
device had 50 GHz FSR for colorless operation. It is athermal and
does not require a temperature control, therefore, it is a
completely passive device.
[0072] The optical performance of the ISI-suppressed DSPK
demodulator sample was measured with an analyzer, consisting of a
fast sweep laser source, an optical test head, three precision
power meters, and a PC controller. The measurement was performed
from 1520 nm to 1570 nm to over the entire C-band and beyond.
Measurement step size was 5 pm. Averaging was taken to reduce the
measurement noise, particularly the phase measurement which is very
sensitive to environmental variation such as temperature and
vibration.
[0073] The plots of FIG. 12 show a selected portion of the
insertion loss spectra of the two outputs of the DPSK demodulator
with optical equalizing filter device based on the configuration of
FIG. 8C, in comparison with a regular 50 GHz colorless DPSK
demodulator. The red curve 1201 and green curve 1203 are the
constructive port and destructive port outputs of the device
configured according to FIG. 8C, respectively, while the blue and
pink curves 1205, 1207 are constructive port and destructive port
outputs for the regular DEPSK demodulator device without ISI
suppression.
[0074] For the integrated ISI suppression DPSK demodulator, because
the slopes of the DPSK demodulator element are sharper than the
slopes of the equalizing filter element, the resultant passband of
the constructive port does not show dips at the ITU-T grid center,
but still has peaks. However, the effect of the equalizer dip is
still obvious. Because unlike the regular demodulator whose two
output ports have similar insertion loss peaks (about 0.45 dB), the
constructive port and destructive port of the integrated device
have different insertion loss peaks at around 3.75 dB and 1.45 dB
respectively. The 2.3 dB difference is the result of additional
equalizer dips. This figure is slightly smaller than the 2.5 dB
from the design. The equalizer dip occurs at the valleys of the
destructive port.
[0075] Compared to a regular colorless DPSK demodulator without ISI
suppression, there is an extra 1 dB loss experienced by the
inventive device embodiment of FIG. 8C at the destructive port
where no equalizer dip is present. This is the insertion loss from
the adding of the equalizer filter elements.
[0076] Referring now to FIGS. 13A to 13C, the curves shown are
measurements of polarization dependent loss (PDL) FIG. 13A,
chromatic dispersion (CD) FIG. 13B and differential group delay
(DGD) 13C of both ports of the inventive device embodiment of FIG.
8C. The curves 1301, 1305 and 1309 are for the constructive port
and curves 1303, 1307 and 1311 are for the destructive port of the
device. The clear channel bandwidth defined for the analysis is
.+-.12.5 GHz around the 25 GHz ITU-T grid center, giving a passband
of 25 GHz.
[0077] The measured results show similar PDL behaviors between the
two output ports, see FIG. 13A. The PDL values within the passband
are about 0.14 dB maximum for both. This is within the 0.25 dB
specified maximum value.
[0078] The trends of CD curves in the passband for both ports are
different, see FIG. 13B. The constructive port has a negative CD
slope within the passband with respect to frequency, while the
destructive port has a positive CD slope around the center of each
peak. The previous measurements on regular colorless DPSK
demodulator showed opposite results, but with smaller slope values.
The change of CD slope is caused by the CD of the thin film FP
equalizing elements, which have negative CD slopes on ITU-T 50 GHz
grids (which are the peak centers of the constructive port) and
positive CD slopes on the 25 GHz offset positions (which are the
peak centers of the destructive port). Within the 25 GHz clear
channel passband, the CD value for the constructive port is about
.+-.30 to 60 ps/nm, and for the destructive port is about .+-.100
to 130 ps/nm. The resolution bandwidth values selected for
smoothing the group delay and CD are both 50 pm (10 measurement
steps).
[0079] The behaviors of the differential group delay DGD profiles
between the two output ports are also quite different, see FIG.
13C, the constructive port has a larger DGD value. The maximum DGD
value within the 25 GHz clear channel passband is about 0.8 ps. The
average of DGD within the 25 GHz passband is about 0.5 ps. This is
sometimes defined as the polarization mode dispersion (PMD) value.
For the destructive port, the DGD values smaller, with maximum of
about 0.3 ps within the 25 GHz passband and average of less than
0.2 ps (PMD).
[0080] Turning now to the schematic of FIG. 14, a simple
experimental transmission link for performing preliminary
verification of the performance of the inventive integrated
ISI-suppressed DPSK demodulator is shown. Data at a rate of 40 Gb/s
is phase modulated 1405 onto a laser source 1401, 1403 centered on
a ITU-T 50 GHz channel. The signal is amplified 1407 and
transmitted through a 100 GHz to 50 GHz optical interleaver 1409.
The interleaver imposes filtering effect onto the 40 Gb/s signal
and causes ISI. The signal is then demodulated 1413.sub.1,
1413.sub.2 or 1415 at the output, and converted into electrical
signal by a pair of balance detectors 1417. The received signal is
displayed at a high-speed analyzer 1419. No fiber transmission is
used in this experiment.
[0081] Three demodulation configurations 1413.sub.1, 1413.sub.2 or
1415 are tested and compared. The first demodulation configuration
1413.sub.1 is with a regular 50 GHz colorless DPSK demodulator
only, the second demodulation configuration 1413.sub.2 is with
separate optical equalizer and colorless DPSK demodulator devices,
and the third configuration 1415 is with the inventive ISI-suppress
colorless DPSK demodulator containing integrated equalizing.
[0082] The receiving eye diagrams of these three demodulation
configurations 1413.sub.1, 1413.sub.2 or 1415, both constructive
and destructive ports, are shown in FIGS. 15B to 15G. The eye
diagram of FIG. 15A is of a DPSK signal without any filtering. The
eye diagrams of constructive port output 15B and destructive port
output 15C are of a signal from a DPSK demodulator without optical
equalization. The eye diagrams of constructive port output 15D and
destructive port output 15E are of a signal from a DPSK demodulator
with an external optical equalizer for ISI suppression. The eye
diagrams of constructive port output 15F and destructive port
output 15G are of a signal from an integrated DPSK demodulator with
optical equalizer for ISI suppression. It is noted that the eye
diagrams of FIGS. 15B with 15C and FIGS. 15D with 15E correspond to
the eye diagrams FIG. 4A with 4B and 4C with 4D, respectively. The
eye diagrams are compared to the DPSK eye diagram without any
filtering effect from the interleaver, see FIG. 15A.
[0083] These eye diagrams clearly show that the corresponding two
latter configurations 1413.sub.2 or 1415, with optical
equalization, produce better eye opening at the receiver compared
to the DPSK demodulation without equalization. The improvement can
be observed on both the constructive and destructive ports. The eye
opening improvement between the external optical equalizer and
integrated equalizer is small and cannot be judged by eye. However
the destructive port output power level of the integrated device
seems to be a bit lower. Similar results are obtained when the
laser wavelength is tuned to other ITU-T channels. These results
confirm that an ISI-suppressed colorless DPSK demodulator with an
integrated optical equalizing element can reduce the ISI effect and
improve the receiving signal quality after transmission.
[0084] The inventive DPSK modulator with optical equalizing
provides features and advantages unavailable with current optical
transmission techniques. The inventive ISI-suppressed DPSK
demodulator device can mitigate the ISI caused by optical filtering
effects during the transmission for 40 Gb/s DPSK signal. It has
colorless operation which allows the same device to operate on any
ITU-T 50 GHz DWDM channel, thus reducing the inventory requirement.
It does the basic function of demodulating the DPSK signal.
[0085] Compared to prior separate optical equalizer technology,
which has similar ISI suppression performance, the integrated DPSK
demodulator has the following advantages: compact, eliminates
additional fiber, smaller insertion loss, a greater tolerance to
temperature variation and lower manufacturing cost.
[0086] Compact: The size of the integrated ISI-suppressed DPSK
demodulator is only slightly larger than the regular DPSK
demodulator. This is smaller than having two separate devices, and
will reduce the footprint on the transponder line cards.
[0087] Eliminating additional fiber. With the integration, the
equalizing and demodulation elements are placed together. Therefore
the fiber between two devices is eliminated, so is the fusion
splice or connectors and adapter in between. This will make the
line cards more tidy and reduce fiber management task.
[0088] Smaller insertion loss: Even though it seems that the
integrated device has larger insertion loss than regular DPSK
demodulator (about 1 dB, based on the actual measurement), the
overall insertion loss is actually smaller than the combined loss
of separate optical equalizer and DPSK demodulator. Without
considering the 2.5 dB dip, the spec insertion loss value of the
separate optical equalizer and DPSK demodulator is 3.4 dB (1.2 for
equalizer and 2.2 for demodulator), while for integrated device the
spec value is 2.2 dB. This is because the integrated design
eliminated a pair of fiber collimators that couple the light
between free space and fiber inside the device, and the fiber
collimators are the main contributor to insertion loss.
[0089] More tolerant to temperature variation: When the two
elements, interferometer and equalizer, are integrated inside a
small hermetically sealed package, they experience similar impact
of environmental temperature change. Therefore, if there is a
wavelength drift caused by the temperature change, these two
components will more likely to have the same amount of drift
(albeit small) and maintain good relative position. Separate
equalizer and demodulator devices are likely to experience larger
relative temperature-induced wavelength drift.
[0090] Lower manufacturing cost: Because of the elimination of
components such as some fiber collimators and reducing the number
of packaging from two to one, the manufacturing cost is
reduced.
[0091] The inventive ISI-suppressed DPSK demodulator can be used in
a 100 GHz spaced system. The inventive ISI-suppressed DPSK
demodulator is designed for a 50 GHz-spaced system, and its
equalizing element imposes dips on every 50 GHz ITU-T grid
frequency. This is also optimized for a 100 GHz-spaced system where
the 40 Gb/s signal experiences very different filtering effect
(much less).
[0092] The eye diagrams of FIGS. 16A-16C are for the simulation
results for a 100 GHz spaced system with three scenarios: without
equalization FIG. 16A, with an external equalizer FIG. 16B, and
with an integrated equalizer 16C. The results show around 13 dB
worsening of the receiving signal Q factor with equalizer optimized
for 50 GHz system (from 34 dB to 21 dB), regardless of external or
internal equalizing. However even with the presence of a 50 GHz
equalizer, the received signal quality is still good and has small
inter-symbol interference ISI. Therefore, the negative effect of
the 50 GHz equalizing element is small and the signal can be
expected to satisfy the BER requirement within the system OSNR
margin.
[0093] Even though the inventive device is athermal and can operate
without any temperature control, it can experience slight frequency
shift due to temperature. A tuning mechanism may be added to not
only compensate for the frequency shift caused by temperature
variation, but can also compensate for laser drift. Tuning can be
added to the device by inserting a layer of special glass in the
optical paths. This glass material would have a larger thermal
coefficient. By varying its temperature, the refractive index of
the glass material will change and lead to the variation of optical
path length, allowing the spectrum of the DPSK demodulator to be
tuned.
[0094] The inventive optical device, the ISI-suppressed DPSK
demodulator, integrates an optical equalizer and colorless DPSK
demodulator to mitigate the filtering effect-induced ISI for a 40
Gb/s DPSK signal in 50 GHz-spaced optical DWDM system. One
exemplary embodiment of the invention includes two colorless
FP-based optical equalizing filters with half of the filter ripple
dip depth, one at the dual fiber port with Input and Output A, the
other at the Output B port. The experimentally measured optical
characteristics and ISI suppression performance demonstrate that
the invention improves the transmission performance of a 40 Gb/s
DPSK signal by reducing the ISI. Comparing to prior designs
incorporating separate optical equalizer and demodulator devices,
the inventive integrated device is more compact, has better optical
performance, e.g., smaller insertion loss and better tolerance to
temperature variation, and has lower cost. Therefore, the inventive
integrated device is useful for improving transmission in DWDM
networks.
[0095] The present invention has been shown and described in what
are considered to be the most practical and preferred embodiments.
It is anticipated, however, that departures may be made therefrom
and that obvious modifications will be implemented by those skilled
in the art. It will be appreciated that those skilled in the art
will be able to devise numerous arrangements and variations which,
although not explicitly shown or described herein, embody the
principles of the invention and are within their spirit and
scope.
* * * * *