U.S. patent application number 15/412872 was filed with the patent office on 2018-07-26 for low noise colorless, directionless, contentionless reconfigurable optical add/drop multiplexer.
The applicant listed for this patent is Fujitsu Limited. Invention is credited to Youichi Akasaka, Takeshi Hoshida, Tadashi Ikeuchi.
Application Number | 20180212701 15/412872 |
Document ID | / |
Family ID | 62906640 |
Filed Date | 2018-07-26 |
United States Patent
Application |
20180212701 |
Kind Code |
A1 |
Akasaka; Youichi ; et
al. |
July 26, 2018 |
LOW NOISE COLORLESS, DIRECTIONLESS, CONTENTIONLESS RECONFIGURABLE
OPTICAL ADD/DROP MULTIPLEXER
Abstract
Methods and systems for implementing a low noise CDC ROADM
include incorporating individual stages of an optical PSA before
and after WSSs included in the CDC ROADM. The WSSs may be used to
route the pump and idler signals, as well as to perform phase
tuning for optimal phase-sensitive amplification.
Inventors: |
Akasaka; Youichi; (Allen,
TX) ; Hoshida; Takeshi; (Kawasaki, JP) ;
Ikeuchi; Tadashi; (Plano, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fujitsu Limited |
Kawasaki-shi |
|
JP |
|
|
Family ID: |
62906640 |
Appl. No.: |
15/412872 |
Filed: |
January 23, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 14/0212 20130101;
H04B 10/2916 20130101; H04Q 2011/0016 20130101; H04J 14/0221
20130101; H04Q 11/0005 20130101; H04Q 2011/0049 20130101 |
International
Class: |
H04J 14/02 20060101
H04J014/02; H04Q 11/00 20060101 H04Q011/00 |
Claims
1. A method for operating reconfigurable optical add/drop
multiplexers (ROADM) in optical transport networks, the method
comprising: receiving an input wavelength division multiplexing
(WDM) optical signal at an input degree of a ROADM; transmitting
the input WDM optical signal through a phase sensitive amplifier
(PSA) stage I, wherein the PSA stage I comprises a first non-linear
optical element (NLE) through which the input WDM optical signal
and a first pump wavelength are passed to generate a PSA stage I
optical signal comprising the input WDM optical signal, the first
pump wavelength, and an idler signal; transmitting the PSA stage I
optical signal through a first wavelength selective switch (WSS);
receiving wavelengths for a PSA stage II optical signal at a second
WSS, the PSA stage II optical signal comprising wavelengths
included in an output WDM optical signal, the first pump
wavelength, and corresponding idler signals; subsequent to the
second WSS, transmitting the PSA stage II optical signal to a PSA
stage II, wherein the PSA stage II comprises a second NLE through
which the PSA stage II optical signal is amplified to generate the
output WDM optical signal; and at the PSA stage II, prior to the
second NLE, transmitting the PSA stage I signal through a third NLE
through which a second pump wavelength is counterpropagated,
wherein the third NLE and the second pump wavelength perform Raman
amplification on the PSA stage I signal.
2. The method of claim 1, further comprising: passing the PSA stage
II optical signal through an optical band pass filter to remove the
pump wavelength and the idler signals to isolate the output WDM
optical signal; transmitting the output WDM optical signal through
an output degree of the ROADM.
3. The method of claim 2, wherein the output degree of the ROADM is
a drop port.
4. The method of claim 1, wherein the input degree of the ROADM is
an add port.
5. The method of claim 1, further comprising: using the first WSS
to phase tune the PSA stage I optical signal, wherein respective
phases of the input WDM optical signal, the pump wavelength, and
the idler signal are aligned.
6. The method of claim 5, wherein the respective phases are aligned
based on a feedback signal from an optical channel monitor
monitoring the output WDM optical signal.
7. (canceled)
8. A reconfigurable optical add/drop multiplexer (ROADM),
comprising: a phase sensitive amplifier (PSA) stage I to receive an
input wavelength division multiplexing (WDM) optical signal at an
input degree of the ROADM, wherein the PSA stage I comprises a
first non-linear optical element (NLE) through which the input WDM
optical signal and a first pump wavelength are passed to generate a
PSA stage I optical signal comprising the input WDM optical signal,
the first pump wavelength, and an idler signal; a first wavelength
selective switch (WSS) cross connect comprising a first WSS enabled
to receive the PSA stage I optical signal; a second wavelength
selective switch (WSS) cross connect comprising a second WSS
enabled to receive a PSA stage II optical signal, the PSA stage II
optical signal comprising wavelengths included in an output WDM
optical signal, the first pump wavelength, and corresponding idler
signals; a PSA stage II to receive the PSA stage II optical signal,
wherein the PSA stage II comprises a second NLE through which the
PSA stage II optical signal is amplified to generate the output WDM
optical signal; and a third NLE at the PSA stage II, prior to the
second NLE, receiving the PSA stage I signal and receiving a second
pump wavelength in a counterpropagating direction to the PSA stage
I signal, wherein the third NLE and the second pump wavelength
perform Raman amplification on the PSA stage I signal.
9. The ROADM of claim 8, further comprising: an optical band pass
filter through which the PSA stage II optical signal is passed
through after the PSA stage II to remove the pump wavelength and
the idler signals to isolate the output WDM optical signal at an
output degree of the ROADM.
10. The ROADM of claim 9, wherein the output degree of the ROADM is
a drop port.
11. The ROADM of claim 8, wherein the input degree of the ROADM is
an add port.
12. The ROADM of claim 8, wherein the first WSS is used to phase
tune the PSA stage I optical signal, wherein respective phases of
the input WDM optical signal, the pump wavelength, and the idler
signal are aligned.
13. The ROADM of claim 12, wherein the respective phases are
aligned based on a feedback signal from an optical channel monitor
monitoring the output WDM optical signal.
14. (canceled)
Description
BACKGROUND
Field of the Disclosure
[0001] The present disclosure relates generally to optical
communication networks and, more particularly, to low noise
colorless, directionless, contentionless reconfigurable optical
add/drop multiplexers.
Description of the Related Art
[0002] Telecommunication, cable television and data communication
systems use optical networks to rapidly convey large amounts of
information between remote points. In an optical network,
information is conveyed in the form of optical signals through
optical fibers. Optical fibers may comprise thin strands of glass
capable of communicating the signals over long distances. Optical
networks often employ modulation schemes to convey information in
the optical signals over the optical fibers. Such modulation
schemes may include phase-shift keying (PSK), frequency-shift
keying (FSK), amplitude-shift keying (ASK), and quadrature
amplitude modulation (QAM).
[0003] Optical networks may also include various optical elements,
such as amplifiers, dispersion compensators,
multiplexer/demultiplexer filters, wavelength selective switches
(WSS), optical switches, couplers, etc. to perform various
operations within the network. In particular, optical networks may
include costly optical-electrical-optical (O-E-O) regeneration at
colorless, directionless, contentionless reconfigurable optical
add-drop multiplexers (CDC ROADMs) when the reach of an optical
signal is limited in a single optical path.
[0004] As data rates for optical networks continue to increase,
reaching up to 1 terabit/s (1 T) and beyond, the demands on optical
signal-to-noise ratios (OSNR) also increase, for example, due to
the use of advanced modulation formats, such as QAM and PSK with
dual polarization. In particular, noise accumulations resulting
from cascading of optical amplifiers in an optical network
operating at very high data rates may limit the reach of an optical
signal at a desired level of OSNR and may result in an increased
number of O-E-O regenerations, which is economically
disadvantageous.
SUMMARY
[0005] In one aspect, a disclosed method for operating ROADMs in
optical transport networks includes receiving an input wavelength
division multiplexing (WDM) optical signal at an input degree of a
ROADM, and transmitting the input WDM optical signal through a
phase sensitive amplifier (PSA) stage I. In the method, the PSA
stage I may include a first non-linear optical element (NLE)
through which the input WDM optical signal and a first pump
wavelength are passed to generate a PSA stage I optical signal
comprising the input WDM optical signal, the first pump wavelength,
and an idler signal. The method may also include transmitting the
PSA stage I optical signal through a first wavelength selective
switch (WSS), and receiving wavelengths for a PSA stage II optical
signal at a second WSS, the PSA stage II optical signal comprising
wavelengths included in an output WDM optical signal, the first
pump wavelength, and corresponding idler signals. The method may
further include, subsequent to the second WSS, transmitting the PSA
stage II optical signal to a PSA stage II. In the method, the PSA
stage II may include a second NLE through which the PSA stage II
optical signal is amplified to generate the output WDM optical
signal.
[0006] In any of the disclosed embodiments, the method may further
include passing the PSA stage II optical signal through an optical
band pass filter to remove the pump wavelength and the idler
signals to isolate the output WDM optical signal, and transmitting
the output WDM optical signal through an output degree of the
ROADM.
[0007] In any of the disclosed embodiments of the method, the
output degree of the ROADM may be a drop port.
[0008] In any of the disclosed embodiments of the method, the input
degree of the ROADM may be an add port.
[0009] In any of the disclosed embodiments, the method may further
include using the first WSS to phase tune the PSA stage I optical
signal. In the method, respective phases of the input WDM optical
signal, the pump wavelength, and the idler signal may be
aligned.
[0010] In any of the disclosed embodiments of the method, the
respective phases may be aligned based on a feedback signal from an
optical channel monitor monitoring the output WDM optical
signal.
[0011] In any of the disclosed embodiments, the method may further
include, at the PSA stage II, prior to the second NLE, transmitting
the PSA stage I signal through a third NLE through which a second
pump wavelength is counterpropagated. In the method, the third NLE
and the second pump wavelength may perform Raman amplification on
the PSA stage I signal.
[0012] In a further aspect, a disclosed ROADM includes a phase
sensitive amplifier (PSA) stage I to receive an input wavelength
division multiplexing (WDM) optical signal at an input degree of
the ROADM. In the ROADM, the PSA stage I may include a first
non-linear optical element (NLE) through which the input WDM
optical signal and a first pump wavelength are passed to generate a
PSA stage I optical signal comprising the input WDM optical signal,
the first pump wavelength, and an idler signal. The ROADM may
further include a first wavelength selective switch (WSS) cross
connect comprising a first WSS enabled to receive the PSA stage I
optical signal, and a second wavelength selective switch (WSS)
cross connect comprising a second WSS enabled to receive a PSA
stage II optical signal, the PSA stage II optical signal comprising
wavelengths included in an output WDM optical signal, the first
pump wavelength, and corresponding idler signals. The ROADM may
still further include a PSA stage II to receive the PSA stage II
optical signal. In the ROADM, the PSA stage II may include a second
NLE through which the PSA stage II optical signal is amplified to
generate the output WDM optical signal.
[0013] In any of the disclosed embodiments, the ROADM may further
include an optical band pass filter through which the PSA stage II
optical signal is passed through after the PSA stage II to remove
the pump wavelength and the idler signals to isolate the output WDM
optical signal at an output degree of the ROADM.
[0014] In any of the disclosed embodiments of the ROADM, the output
degree of the ROADM may be a drop port.
[0015] In any of the disclosed embodiments of the ROADM, the input
degree of the ROADM may be an add port.
[0016] In any of the disclosed embodiments of the ROADM, the first
WSS may be used to phase tune the PSA stage I optical signal. In
the ROADM, respective phases of the input WDM optical signal, the
pump wavelength, and the idler signal may be aligned.
[0017] In any of the disclosed embodiments of the ROADM, the
respective phases may be aligned based on a feedback signal from an
optical channel monitor monitoring the output WDM optical
signal.
[0018] In any of the disclosed embodiments, the ROADM may further
include a third NLE at the PSA stage II, prior to the second NLE,
receiving the PSA stage I signal and receiving a second pump
wavelength in a counterpropagating direction to the PSA stage I
signal. In the ROADM, the third NLE and the second pump wavelength
may perform Raman amplification on the PSA stage I signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] For a more complete understanding of the present invention
and its features and advantages, reference is now made to the
following description, taken in conjunction with the accompanying
drawings, in which:
[0020] FIG. 1 is a block diagram of selected elements of an
embodiment of an optical network;
[0021] FIG. 2 is a block diagram of selected elements of an
embodiment of a low noise CDC ROADM with an optical phase-sensitive
amplifier;
[0022] FIG. 3 is a block diagram of selected elements of an
embodiment of a phase-sensitive optical amplifier stage I;
[0023] FIG. 4 is a block diagram of selected elements of an
embodiment of a phase-sensitive optical amplifier stage II; and
[0024] FIG. 5 is a flow chart of selected elements of a method for
operating a low noise CDC ROADM.
DESCRIPTION OF PARTICULAR EMBODIMENT(S)
[0025] In the following description, details are set forth by way
of example to facilitate discussion of the disclosed subject
matter. It should be apparent to a person of ordinary skill in the
field, however, that the disclosed embodiments are exemplary and
not exhaustive of all possible embodiments.
[0026] Throughout this disclosure, a hyphenated form of a reference
numeral refers to a specific instance of an element and the
un-hyphenated form of the reference numeral refers to the element
generically or collectively. Thus, as an example (not shown in the
drawings), device "12-1" refers to an instance of a device class,
which may be referred to collectively as devices "12" and any one
of which may be referred to generically as a device "12". In the
figures and the description, like numerals are intended to
represent like elements.
[0027] Referring now to the drawings, FIG. 1 illustrates an example
embodiment of optical network 101, which may represent an optical
communication system. Optical network 101 may include one or more
optical fibers 106 to transport one or more optical signals
communicated by components of optical network 101. The network
elements of optical network 101, coupled together by fibers 106,
may comprise one or more transmitters 102, one or more multiplexers
(MUX) 104, one or more optical amplifiers 108, one or more optical
add/drop multiplexers (OADM) 110, one or more demultiplexers
(DEMUX) 105, and one or more receivers 112.
[0028] Optical network 101 may comprise a point-to-point optical
network with terminal nodes, a ring optical network, a mesh optical
network, or any other suitable optical network or combination of
optical networks. Optical network 101 may be used in a short-haul
metropolitan network, a long-haul inter-city network, or any other
suitable network or combination of networks. The capacity of
optical network 101 may include, for example, 100 Gbit/s, 400
Gbit/s, or 1 Tbit/s. Optical fibers 106 comprise thin strands of
glass capable of communicating the signals over long distances with
very low loss. Optical fibers 106 may comprise a suitable type of
fiber selected from a variety of different fibers for optical
transmission. Optical fibers 106 may include any suitable type of
fiber, such as a Single-Mode Fiber (SMF), Enhanced Large Effective
Area Fiber (E-LEAF), or TrueWave.RTM. Reduced Slope (TW-RS)
fiber.
[0029] Optical network 101 may include devices to transmit optical
signals over optical fibers 106. Information may be transmitted and
received through optical network 101 by modulation of one or more
wavelengths of light to encode the information on the wavelength.
In optical networking, a wavelength of light may also be referred
to as a channel that is included in an optical signal (also
referred to herein as a "wavelength channel"). Each channel may
carry a certain amount of information through optical network
101.
[0030] To increase the information capacity and transport
capabilities of optical network 101, multiple signals transmitted
at multiple channels may be combined into a single wideband optical
signal. The process of communicating information at multiple
channels is referred to in optics as wavelength division
multiplexing (WDM). Coarse wavelength division multiplexing (CWDM)
refers to the multiplexing of wavelengths that are widely spaced
having low number of channels, usually greater than 20 nm and less
than sixteen wavelengths, and dense wavelength division
multiplexing (DWDM) refers to the multiplexing of wavelengths that
are closely spaced having large number of channels, usually less
than 0.8 nm spacing and greater than forty wavelengths, into a
fiber. WDM or other multi-wavelength multiplexing transmission
techniques are employed in optical networks to increase the
aggregate bandwidth per optical fiber. Without WDM, the bandwidth
in optical networks may be limited to the bit-rate of solely one
wavelength. With more bandwidth, optical networks are capable of
transmitting greater amounts of information. Optical network 101
may transmit disparate channels using WDM or some other suitable
multi-channel multiplexing technique, and to amplify the
multi-channel signal.
[0031] Optical network 101 may include one or more optical
transmitters (Tx) 102 to transmit optical signals through optical
network 101 in specific wavelengths or channels. Transmitters 102
may comprise a system, apparatus or device to convert an electrical
signal into an optical signal and transmit the optical signal. For
example, transmitters 102 may each comprise a laser and a modulator
to receive electrical signals and modulate the information
contained in the electrical signals onto a beam of light produced
by the laser at a particular wavelength, and transmit the beam for
carrying the signal throughout optical network 101.
[0032] Multiplexer 104 may be coupled to transmitters 102 and may
be a system, apparatus or device to combine the signals transmitted
by transmitters 102, e.g., at respective individual wavelengths,
into a WDM signal.
[0033] Optical amplifiers 108 may amplify the multi-channeled
signals within optical network 101. Optical amplifiers 108 may be
positioned before or after certain lengths of fiber 106. Optical
amplifiers 108 may comprise a system, apparatus, or device to
amplify optical signals. For example, optical amplifiers 108 may
comprise an optical repeater that amplifies the optical signal.
This amplification may be performed with opto-electrical or
electro-optical conversion. In some embodiments, optical amplifiers
108 may comprise an optical fiber doped with a rare-earth element
to form a doped fiber amplification element. When a signal passes
through the fiber, external energy may be applied in the form of an
optical pump to excite the atoms of the doped portion of the
optical fiber, which increases the intensity of the optical signal.
As an example, optical amplifiers 108 may comprise an erbium-doped
fiber amplifier (EDFA).
[0034] OADMs 110 may be coupled to optical network 101 via fibers
106. OADMs 110 comprise an add/drop module, which may include a
system, apparatus or device to add and drop optical signals (for
example at individual wavelengths) from fibers 106. After passing
through an OADM 110, an optical signal may travel along fibers 106
directly to a destination, or the signal may be passed through one
or more additional OADMs 110 and optical amplifiers 108 before
reaching a destination.
[0035] In certain embodiments of optical network 101, OADM 110 may
represent a reconfigurable OADM (ROADM) that is capable of adding
or dropping individual or multiple wavelengths of a WDM signal. The
individual or multiple wavelengths may be added or dropped in the
optical domain, for example, using a wavelength selective switch
(WSS) (see also FIG. 2) that may be included in a ROADM. ROADMs are
considered `colorless` when the ROADM is able to add/drop any
arbitrary wavelength. ROADMs are considered `directionless` when
the ROADM is able to add/drop any wavelength regardless of the
direction of propagation. ROADMs are considered `contentionless`
when the ROADM is able to switch any contended wavelength (already
occupied wavelength) to any other wavelength that is available.
[0036] As shown in FIG. 1, optical network 101 may also include one
or more demultiplexers 105 at one or more destinations of network
101. Demultiplexer 105 may comprise a system apparatus or device
that acts as a demultiplexer by splitting a single composite WDM
signal into individual channels at respective wavelengths. For
example, optical network 101 may transmit and carry a forty (40)
channel DWDM signal. Demultiplexer 105 may divide the single, forty
channel DWDM signal into forty separate signals according to the
forty different channels.
[0037] In FIG. 1, optical network 101 may also include receivers
112 coupled to demultiplexer 105. Each receiver 112 may receive
optical signals transmitted at a particular wavelength or channel,
and may process the optical signals to obtain (e.g., demodulate)
the information (i.e., data) that the optical signals contain.
Accordingly, network 101 may include at least one receiver 112 for
every channel of the network.
[0038] Optical networks, such as optical network 101 in FIG. 1, may
employ modulation techniques to convey information in the optical
signals over the optical fibers. Such modulation schemes may
include phase-shift keying (PSK), frequency-shift keying (FSK),
amplitude-shift keying (ASK), and quadrature amplitude modulation
(QAM), among other examples of modulation techniques. In PSK, the
information carried by the optical signal may be conveyed by
modulating the phase of a reference signal, also known as a carrier
wave, or simply, a carrier. The information may be conveyed by
modulating the phase of the signal itself using two-level or binary
phase-shift keying (BPSK), four-level or quadrature phase-shift
keying (QPSK), multi-level phase-shift keying (M-PSK) and
differential phase-shift keying (DPSK). In QAM, the information
carried by the optical signal may be conveyed by modulating both
the amplitude and phase of the carrier wave. PSK may be considered
a subset of QAM, wherein the amplitude of the carrier waves is
maintained as a constant.
[0039] Additionally, polarization division multiplexing (PDM)
technology may enable achieving a greater bit rate for information
transmission. PDM transmission comprises independently modulating
information onto different polarization components of an optical
signal associated with a channel. In this manner, each polarization
component may carry a separate signal simultaneously with other
polarization components, thereby enabling the bit rate to be
increased according to the number of individual polarization
components. The polarization of an optical signal may refer to the
direction of the oscillations of the optical signal. The term
"polarization" may generally refer to the path traced out by the
tip of the electric field vector at a point in space, which is
perpendicular to the propagation direction of the optical
signal.
[0040] In an optical network, such as optical network 101 in FIG.
1, it is typical to refer to a management plane, a control plane,
and a transport plane (sometimes called the physical layer). A
central management host (not shown) may reside in the management
plane and may configure and supervise the components of the control
plane. The management plane includes ultimate control over all
transport plane and control plane entities (e.g., network
elements). As an example, the management plane may consist of a
central processing center (e.g., the central management host),
including one or more processing resources, data storage
components, etc. The management plane may be in electrical
communication with the elements of the control plane and may also
be in electrical communication with one or more network elements of
the transport plane. The management plane may perform management
functions for an overall system and provide coordination between
network elements, the control plane, and the transport plane. As
examples, the management plane may include an element management
system (EMS) which handles one or more network elements from the
perspective of the elements, a network management system (NMS)
which handles many devices from the perspective of the network, and
an operational support system (OSS) which handles network-wide
operations.
[0041] Modifications, additions or omissions may be made to optical
network 101 without departing from the scope of the disclosure. For
example, optical network 101 may include more or fewer elements
than those depicted in FIG. 1. Also, as mentioned above, although
depicted as a point-to-point network, optical network 101 may
comprise any suitable network topology for transmitting optical
signals such as a ring, a mesh, and a hierarchical network
topology.
[0042] As discussed above, the amount of information that may be
transmitted over an optical network may vary with the number of
optical channels coded with information and multiplexed into one
signal. Accordingly, an optical fiber employing a WDM signal may
carry more information than an optical fiber that carries
information over a single channel. Besides the number of channels
and number of polarization components carried, another factor that
affects how much information can be transmitted over an optical
network may be the bit rate of transmission. The higher the bit
rate, the greater the transmitted information capacity. Achieving
higher bit rates may be limited by the availability of wide
bandwidth electrical driver technology, digital signal processor
technology and increase in the required OSNR for transmission over
optical network 101.
[0043] In operation of optical network 101, as data rates approach
1 T and beyond, a correspondingly high OSNR becomes desirable to
maintain economic feasibility by avoiding excessive numbers of
O-E-O regenerators. One source of OSNR reduction is the noise
accumulation resulting from cascaded optical amplifiers 108 at
various points in the transmission path. For an optical amplifier,
OSNR may be represented as a noise figure (NF), given by Equation
1, where OSNR.sub.in is the input OSNR, OSNR.sub.out is the output
OSNR, and dB is decibels.
NF=10
log(OSNR.sub.in/OSNR.sub.out)=OSNR.sub.in[dB]-OSNR.sub.out[dB]
Equation (1)
[0044] Current designs for optical amplifiers may include optical
phase-sensitive amplifiers (PSA), which may exhibit a low noise
figure, such as less than 3 dB in many instances. Some PSA designs
may exhibit 0 dB noise figure. The lower noise figure may enable an
increased optical reach for a given optical signal, which is
desirable.
[0045] A typical phase-sensitive optical amplifier will have
different stages, including an idler stage to initially generate an
idler signal using an optical pump and an amplification stage to
amplify the input signal using the optical pump and the idler
signal. In between the idler stage and the amplification stage, an
intermediate stage may be implemented in the phase-sensitive
optical amplifier. The intermediate stage may involve complex
signal processing and pump power recovery to adjust the power level
of the input signal and the idler signal. In typical
phase-sensitive optical amplifiers, the optical paths of the input
signal, the optical pump, and the idler signal may be separated in
the intermediate stage in order to independently modulate power of
each of the signals. When the separated optical paths are
recombined, a phase adjustment may be performed to re-align the
phase of the signals. The phase adjustment may involve an optical
phase lock loop to re-align the phases of the input signal and the
idler signal with the optical pump.
[0046] A CDC-ROADM may provide extensive flexibility in the
operation of optical network 101. However, the CDC ROADM includes
many components that inherently exhibit optical signal attenuation
that is typically compensated using optical amplifier 108, which is
often an EDFA. In a CDC ROADM, two WSSs are generally present on
each optical path that passes through the CDC ROADM, while total
attenuation losses are typically greater than about 20 dB, which is
comparable to an equivalent attenuation of one entire fiber span
stretching 100 km in length.
[0047] As will be described in further detail, methods and systems
are disclosed herein for implementing a low noise CDC ROADM that
integrates a PSA among the WSSs present in the CDC ROADM. In some
embodiments, a Raman amplification stage may be also be used in the
PSA integrated within the CDC ROADM. The low noise CDC ROADM
disclosed herein integrates a PSA by including the idler stage
(also referred to as "stage I" herein) and the amplification stage
(also referred to as "stage II" herein) among the components of the
CDC ROADM. The low noise CDC ROADM disclosed herein may utilize the
capabilities already included in the WSSs to operate the PSA
integrated within the CDC ROADM, such as wavelength-selective
switching and phase matching functionality. The PSA described
herein and integrated before and after two WSSs in the low noise
CDC ROADM may also be implemented in other nodes that have at least
two WSSs, such as gain equalization nodes, for example.
[0048] Referring now to FIG. 2, selected elements of an example
embodiment of a low noise CDC ROADM 200 with an optical PSA is
shown. As shown, CDC ROADM 200 is a schematic illustration and is
not drawn to scale. It will be understood that, in different
embodiments, CDC ROADM 200 may be implemented with fewer or more
components than illustrated in FIG. 2. In particular, it will be
understood that CDC ROADM 200 may be expanded by adding additional
input and output degrees, as well as additional add ports and drop
ports, for use in optical networks of different sizes,
topographies, and complexity. For example, additional output
degrees 208 shown with WSS 214-1 may be used for expanding the
capacity of CDC ROADM 200 and it will be understood that
corresponding additional input and output degrees may be used to
extend the capacity of CDC ROADM 200. Although not shown in FIG. 2
for descriptive clarity, additional output degrees, such as output
degrees 208, may also be used with WSS 214-2.
[0049] In FIG. 2, CDC ROADM 200 operates by transmitting WDM
optical signals in an optical network, such as optical network 101
(see FIG. 1). Specifically, CDC ROADM 200 may receive WDM input
signal 210-1 in one direction of propagation, and simultaneously
receive WDM input signal 210-2 in the counterpropagating direction
from the same optical path. Additionally, CDC ROADM 200 includes
drop port 222, where wavelengths in WDM input signal 210-1, 210-2
(comprising one or more channels or wavelengths) may be dropped and
output as WDM output signal 211-3, as well as add port 224 where
wavelengths in WDM input signal 210-3 may be added to WDM output
signals 211-1, 211-2. Thus, add port 224 and drop port 222 may
respectively add/drop WDM optical signals in either one of the
propagation directions. Because of the CDC nature of CDC ROADM 200,
it will be assumed that CDC ROADM 200 can internally resolve any
wavelength conflicts that may arise during adding of WDM optical
signals, for example by shifting wavelengths using an O-E-O
converter, or other means (not shown). Then, CDC ROADM 200 may
output amplified WDM output signal 211-2 in one direction of
propagation, and simultaneous output amplified WDM output signal
211-1 in the counterpropagating direction.
[0050] Accordingly, various wavelengths in WDM input signals 210
are received by CDC ROADM 200 and may be directly transmitted to
amplified WDM optical signals 211. Some wavelengths in WDM input
signals 210 may be dropped by CDC ROADM 200 from the optical path,
and are available for output at drop port 222. New wavelengths may
be added to amplified WDM optical signals 211 and are received at
add port 224. Because CDC ROADM 200 is used to add or drop
wavelengths from WDM input signals 210, amplified WDM optical
signals 211 may have different wavelengths, or may have different
information modulated on a given wavelength, than WDM input signals
210.
[0051] Instead of conventional optical amplifiers 108 that are used
before and after a typical CDC ROADM, CDC ROADM 200 incorporates
various instances of PSA stage I 204 and PSA stage II 206 along the
different input and output optical paths. As a result of the
configuration of CDC ROADM 200, each wavelength passes through at
least two WSSs. Along each possible optical path within CDC ROADM
200, PSA stage I 204 is located prior to a first WSS, while PSA
stage II 206 is located subsequent to a second WSS.
[0052] For example, as WDM input signal 210-1 is received as an
input to CDC ROADM 200, PSA stage I 204-1 (idler stage) adds an
optical pump and an idler signal using a first non-linear element
(NLE), such as a highly non-linear fiber (HNLF) to generate a PSA
stage I optical signal 230-1. Then, PSA stage I optical signal
230-1 is routed by WSS 214-1 in WSS cross connect 212-1. WSS 214-1
may be a 1.times.K WSS, where K is an integer number of output
degrees, such as 10, 20, 40, or more output degrees, in various
embodiments. One output degree 234 of WSS 214-1 may connect with
WSS 216-2 (K.times.1 WSS) in WSS cross connect 212-2 and may
transmit at least certain portions of PSA stage I optical signal
230-1, such as the optical pump, along with the wavelengths of WDM
input signals 210-1 that are retained in CDC ROAM 200. Thus
wavelengths in WDM input signals 210-1 that are passed through CDC
ROADM 200 are routed through WSS 214-1 as the first WSS and WSS
216-2 as the second WSS. After WSS 216-2, the wavelengths that are
passed through CDC ROADM 200, along with new wavelengths in WDM
input signals 210-3 added at add port 224, are transmitted to PSA
stage II 206-2, which includes a second NLE to perform
amplification, as PSA stage II optical signal 232-2. PSA stage II
optical signal 232-2 may include the optical pump along with
respective idler signals that are added at PSA stage I 204-1 or
204-3.
[0053] In CDC ROADM 200, wavelengths in WDM input signals 210-1
that are to be dropped may be routed by WSS 214-1 to WSS 220-1,
which may be a M.times.N WSS, where M and N are integer number of
switchable degrees, in WSS add/drop 218. From WSS 220-1, the
dropped wavelengths may be output to PSA stage II 206-3 as PSA
stage II optical signals 236. PSA stage II 206-3 may output WDM
output signals 211-3, comprising the dropped wavelengths, to drop
port 222.
[0054] In CDC ROADM 200, wavelengths in WDM input signals 210-3
that are to be added may be received by PSA stage I 204-3. The
resulting WDM input signals 210-3, along with a corresponding
optical pump and idler signals, may then be routed by WSS 220-2 to
WSS 216-2, for further transmission as WDM output signals 211-2, or
to WSS 216-1 for further transmission as WDM output signals
211-1.
[0055] Additionally, in a substantially similar manner as described
above with respect to WDM input signals 210-1, WDM input signals
210-2 may be transmitted through PSA stage I 204-2 to generate a
PSA stage I optical signal 230-2. Pass through wavelengths in PSA
stage I optical signal 230-2 are then switched by WSS 214-2 and WSS
216-1 to PSA stage II 206-1. Dropped wavelengths in PSA stage I
optical signal 230-2 are switched by WSS 214-2 and WSS 220-1 to PSA
stage II 206-3. Added wavelengths in PSA stage I optical signal
210-3 are switched by WSS 220-2 and WSS 216-1 to PSA stage II
206-1. In the counterpropagating direction (right to left on the
page of FIG. 2), PSA stage II optical signal 232-1 is generated and
amplified by PSA stage II 206-1 to generate WDM output signals
211-1.
[0056] Also shown in FIG. 2 is optical channel monitor 226, which
may monitor information regarding WDM output signals 211. In FIG.
2, tap 228 is shown splitting WDM output signal 211-2 as an example
configuration, to provide an optical input to optical channel
monitor 226. Optical channel monitor 226, may monitor various
properties of WDM output signal 211, such as wavelength, power,
residual chromatic dispersion, polarization mode dispersion, and
OSNR. Optical channel monitor 226 is shown in a feedback
configuration providing electrical control signals to WSS 214-1 and
WSS 216-2, to enable phase matching of various wavelengths, such as
the WDM optical signals 211-2, the optical pump, and the idler
signals, based on monitored optical power of WDM output signal
211-2, for example. Although optical channel monitor 226 is only
depicted in the propagating direction at PSA stage II 206-2 in FIG.
2 for descriptive clarity, it will be understood that optical
channel monitor 226 may also be used in the counterpropagating
direction at PSA stage II 206-1, or with drop port 222, for
regulation and control of WDM output signals 211. In this manner,
WSSs, which are equipped with internal phase tuning capability, can
have their existing capabilities leveraged to enable integration of
a PSA stage I and II, as shown in FIG. 2, in CDC ROADM 200.
[0057] Referring now to FIG. 3, selected elements of an embodiment
of an optical PSA stage I 204 are depicted. In FIG. 3, optical PSA
stage I 204 is shown in a schematic representation and is not drawn
to scale. It is noted that, in different embodiments, optical PSA
stage I 204 may be operated with additional or fewer elements as
shown in FIG. 3.
[0058] In FIG. 3, optical PSA stage I 204 receives WDM input signal
210 and adds optical pump 308 using coupler 306. Intermediate stage
I signal 312, comprising WDM input signal 210 and optical pump 308
are then sent to NLE idler 314, which is a non-linear optical
element. In the presence of optical pump 308 and WDM input signal
210, simple four wave mixing (FWM) may occur at NLE idler 314 to
generate idler signals 318, resulting in PSA stage I optical signal
230. In PSA stage I 204, simple four wave mixing (FWM) may occur to
generate so-called "idler signals", which are conjugate wavelengths
of an optical signal, such as WDM optical signal 210, relative to a
pump wavelength. In FWM, the idler signals appear when the optical
signal and the pump wavelength are passed through a non-linear
element (NLE idler 314), which may include a highly non-linear
fiber (HNLF). In various embodiments, other NLEs may also be used
to facilitate FWM, such as optical crystals or other optical
materials. A non-linear optical element (NLE) may include a doped
optical fiber, periodically poled lithium niobate (PPLN), aluminium
gallium arsenide (AlGaAs) or another semiconductor material that
exhibits desired optical non-linearity. In NLE idler 314, photons
are converted from the pump wavelength and the optical signal to
the idler signal by non-linear processes.
[0059] Also shown in FIG. 3 are spectra of the different signals
transmitted in optical PSA stage I 204. In spectra 210-S, optical
signal 310 represents one or more wavelengths included in WDM input
signal 210. In spectra 312-S, corresponding to intermediate stage I
signal 312, optical pump 308 is added to optical signal 310. In
spectra 230-S corresponding to PSA stage I optical signal 230,
idler signal 318 has been added, representing corresponding one or
more wavelengths of optical signal 310, but spectrally spaced
symmetrically with respect to optical pump 308.
[0060] Referring now to FIG. 4, selected elements of an embodiment
of an optical PSA stage II 206 are depicted. In FIG. 4, optical PSA
stage II 206 is shown in a schematic representation and is not
drawn to scale. It is noted that, in different embodiments, optical
PSA stage II 206 may be operated with additional or fewer elements
as shown in FIG. 4.
[0061] In FIG. 4, optical PSA stage II 206 receives PSA stage II
optical signal 232 which is passed through isolator 402 to prevent
back propagation of Raman pump 424, before sending PSA stage II
optical signal 232 to NLE Raman amplification 422, which receives
Raman pump 424 using coupler 406. PSA stage II optical signal 232
may include optical signal 310, which comprises the wavelengths in
WDM output signal 211, as described above, along with corresponding
idler signals 318 and optical pump 308. Because of the attenuation
in various components of CDC ROADM 200, the signal intensity of PSA
stage II optical signal 232 may be relatively low, as shown by a
reduced intensity level in spectrum 232-S.
[0062] In optical PSA stage II 206, NLE Raman amplification 422 may
comprise a Raman amplifier that includes Raman pump 424, which may
be a laser source, that is directed through an NLE as a gain medium
in a counter propagation direction to the optical signal being
processed (PSA stage II optical signal 232). Raman pump 424 may be
selected based on the gain medium used. For example, a 13 THz
optical pump may be used with GeO.sub.2/SiO.sub.2 single mode
fibers (SMF) as the NLE, while a 40 THz optical pump may be used
with P.sub.2O.sub.5-doped SiO.sub.2 phosphate-doped fiber (PDF) as
the NLE. Furthermore, modulation or modification of the optical
power of Raman pump 424 may be used to determine or modify an
optical gain of NLE Raman amplification 422. It is noted that Raman
amplification may be optional in some embodiments of optical PSA
stage II 206, such that isolator 402, NLE Raman amplification 422,
coupler 406, and Raman pump 424 may be omitted (not shown).
[0063] The output of NLE Raman amplification 422 is shown as Raman
amplified optical signal 412, which is directed to NLE
amplification 418, which is a non-linear optical element described
above. In the presence of Raman amplified optical signal 412,
one-pump four wave mixing (FWM) may occur at NLE amplification 418
to amplify the WDM optical signal and the idler signals, at the
expense of optical pump 308. NLE amplification 418 may include
components for performing one-pump optical four-wave mixing (FWM).
The one-pump FWM may be accomplished by passing the input signal,
or filtered portions thereof, through a non-linear optical element
(NLE), such as a doped optical fiber, periodically poled lithium
niobate (PPLN), aluminium gallium arsenide (AlGaAs) or other
semiconductor material that exhibits desired optical
non-linearity.
[0064] After NLE amplification 418, optical signal 414 includes the
amplified WDM optical signals 310, and idler signals 318, which
have increased in intensity, along with optical pump 308, which has
decreased in intensity. An optical bandpass filter (OBPF) 408 may
then be applied to isolate WDM output signal 214.
[0065] Also shown in FIG. 4 are spectra of the different signals
transmitted in optical PSA stage II 206. In spectra 232-S, optical
signal 310 represents one or more wavelengths included in WDM
output signal 211, while idler signals 318 are conjugates of
optical signal 310 with respect to optical pump 308. Spectra 232-S
corresponds to PSA stage II optical signal 232, in which idler
signals 318 have been added and which has been switched through at
least two WSSs, resulting in optical power attenuation (as compared
to PSA stage I optical signal 230), which is depicted by a
decreased signal intensity (as compared to spectra 230-S). In
spectra 412 -S, corresponding to Raman amplified optical signal
412, optical signal 310, optical pump 308, and idler signals 318
are Raman amplified, and show higher intensity (as compared to
spectra 232-S). In spectra 414-S, optical signal 310 and idler
signals 318 are amplified by NLE amplification 418 at the expense
of optical pump 308 (as compared to spectra 412-S). In spectra
211-S, optical signal 310 is isolated in amplified form by optical
bandpass filter 408, which removes idler signals 318 and optical
pump 308. Accordingly, spectra 211-S shows the spectrum of WDM
output signal 211.
[0066] Referring now to FIG. 5, a block diagram of selected
elements of an embodiment of method 500 for operating a low noise
CDC ROADM, as described herein, is depicted in flowchart form.
Method 500 may be performed using CDC ROADM 200 (see FIG. 2). It is
noted that certain operations described in method 500 may be
optional or may be rearranged in different embodiments.
[0067] Method 500 may begin at step 502 by receive an input WDM
optical signal at an input degree of a ROADM. At step 504, the
input WDM optical signal is transmitted through a PSA stage I,
where the PSA stage I comprises a first NLE through which the input
WDM optical signal and a first pump wavelength are passed to
generate a PSA stage I optical signal comprising the input WDM
optical signal, the first pump wavelength, and an idler signal. At
step 506, the PSA stage I optical signal is transmitted through a
first WSS. At step 508, wavelengths for a PSA stage II optical
signal are received at a second WSS, the PSA stage II optical
signal comprising optical signal wavelengths included in an output
WDM optical signal, the first pump wavelength, and corresponding
idler signals. Optical signal wavelengths 310 included in output
WDM optical signal 211 may include added optical signal wavelengths
and may exclude dropped optical signal wavelengths, if any optical
signal wavelengths are added or dropped by the ROADM from input WDM
optical signal 210. If no optical signal wavelengths are added or
dropped by the ROADM, optical signal wavelengths 310 included in
output WDM optical signal 211 will be the same as in input WDM
optical signal 210. At step 510, subsequent to the second WSS, the
PSA stage II optical signal is transmitted to a PSA stage II, where
the PSA stage II comprises a second NLE through which the PSA stage
II optical signal is amplified to generate the output WDM optical
signal.
[0068] As disclosed herein, methods and systems for implementing a
low noise CDC ROADM include incorporating individual stages of an
optical PSA before and after WSSs included in the CDC ROADM. The
WSSs may be used to route the pump and idler signals, as well as to
perform phase tuning for optimal phase-sensitive amplification.
[0069] The above disclosed subject matter is to be considered
illustrative, and not restrictive, and the appended claims are
intended to cover all such modifications, enhancements, and other
embodiments which fall within the true spirit and scope of the
present disclosure. Thus, to the maximum extent allowed by law, the
scope of the present disclosure is to be determined by the broadest
permissible interpretation of the following claims and their
equivalents, and shall not be restricted or limited by the
foregoing detailed description.
* * * * *