U.S. patent application number 14/594313 was filed with the patent office on 2015-06-18 for spectral alignment of a wdm device to a specified frequency grid.
The applicant listed for this patent is ALCATEL-LUCENT USA INC.. Invention is credited to Pietro A.G. Bernasconi, David T. Neilson.
Application Number | 20150171986 14/594313 |
Document ID | / |
Family ID | 46577440 |
Filed Date | 2015-06-18 |
United States Patent
Application |
20150171986 |
Kind Code |
A1 |
Bernasconi; Pietro A.G. ; et
al. |
June 18, 2015 |
SPECTRAL ALIGNMENT OF A WDM DEVICE TO A SPECIFIED FREQUENCY
GRID
Abstract
A WDM device having a controller that individually controls the
operating parameters of tunable lasers and the temperatures of an
optical multiplexer and etalon. The device employs a spectral
analyzer to measure the spectral composition of the optical output
signal produced by the device. Based on the analyses of the
measured spectral composition, the controller sets the temperatures
of the tunable lasers, optical multiplexer, and optical etalon to
values that cause: (i) middle frequencies of transmission bands of
the optical multiplexer to be spectrally aligned with the
corresponding frequencies of the specified frequency grid, (ii)
each laser line to be properly positioned within the corresponding
transmission band, and (iii) transmission resonances of the optical
etalon to be properly positioned with respect to the laser
lines.
Inventors: |
Bernasconi; Pietro A.G.;
(Origlio, CH) ; Neilson; David T.; (Old Bridge,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ALCATEL-LUCENT USA INC. |
Murray Hill |
NJ |
US |
|
|
Family ID: |
46577440 |
Appl. No.: |
14/594313 |
Filed: |
January 12, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13294410 |
Nov 11, 2011 |
8983306 |
|
|
14594313 |
|
|
|
|
61437861 |
Jan 31, 2011 |
|
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Current U.S.
Class: |
398/79 |
Current CPC
Class: |
H04B 10/07 20130101;
H04B 10/506 20130101; H04B 10/572 20130101; H04J 14/02
20130101 |
International
Class: |
H04J 14/02 20060101
H04J014/02; H04B 10/07 20060101 H04B010/07 |
Claims
1. An optical method, comprising: receiving light from a plurality
of lasers, each laser lasing at a corresponding lasing wavelength
and being connected to a different optical input port of an optical
multiplexer that is periodic in its frequency response;
multiplexing, using the optical multiplexer, the light received
from the lasers to produce an optical output signal; measuring,
using a spectral analyzer, a spectral composition of the optical
output signal over one or more wavelength ranges, wherein each of
said one or more wavelength ranges excludes the lasing wavelengths;
and adjusting a spectral location of optical pass bands of the
optical multiplexer based on the spectral composition measured by
the spectral analyzer, said adjustment being performed by changing
a temperature of the optical multiplexer.
2. The method of claim 1, wherein the adjusting includes operating
a first temperature-control unit to change the temperature of the
optical multiplexer based on the spectral composition measured by
the spectral analyzer.
3. The method of claim 1, wherein the adjusting includes adjusting
a heater or cooler located along part of the optical
multiplexer.
4. The method of claim 1, wherein the adjusting includes changing
the temperature of the optical multiplexer to cause a series of
regularly spaced pass bands thereof to be spectrally aligned with a
regular grid.
5. The method of claim 1, wherein the one or more wavelength ranges
include a first wavelength range and a second wavelength range,
said first and second wavelength ranges being disjoint from one
another by a wavelength interval that includes the lasing
wavelengths.
6. The method of claim 5, wherein one or each of the first and
second wavelength ranges has a respective width of at least a free
spectral range of the optical multiplexer.
7-8. (canceled)
9. The method of claim 5, wherein wavelengths of the first
wavelength range are shorter than wavelengths in the wavelength
interval, and wavelengths of the second wavelength range are longer
than the wavelengths in the wavelength interval.
10. The method of claim 1, further comprising measuring, using the
spectral analyzer, a spectral composition of the optical output
signal over a wavelength interval that includes the lasing
wavelengths.
11. The method of claim 10, further comprising operating a
plurality of laser-control units, each laser-control unit coupled
to a corresponding one of the lasers, to change at least one of the
lasing wavelengths of the lasers based on the spectral composition
measured over said wavelength interval.
12. The method of claim 11, wherein the change of the at least one
of the lasing wavelengths is performed by changing at least one of
a temperature and an injection current of at least one of the
lasers.
13. The method of claim 2, wherein the multiplexing includes:
passing light received from an output port of the optical
multiplexer through an optical etalon to produce a filtered optical
signal, wherein said filtered optical signal is the optical output
signal; and operating a second temperature-control unit to change a
temperature of the optical etalon based on the spectral composition
measured by the spectral analyzer.
14. The method of claim 13, wherein the change of the temperature
of the optical etalon is configured to cause a transmission
spectrum of the optical etalon to shift by at least a free spectral
range of the optical etalon.
15. The method of claim 13, wherein the measuring includes:
recording the spectral composition at a first temperature of the
optical etalon; recording the spectral composition at a different
second temperature of the optical etalon; and adding or averaging
the spectral compositions recorded at the first temperature and the
second temperatures.
16-20. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Patent Application No. 61/437,861 filed on Jan. 31, 2011 and
entitled "SPECTRAL ALIGNMENT OF A WDM DEVICE TO A SPECIFIED
FREQUENCY GRID," which is incorporated herein by reference in its
entirety.
[0002] This application is a divisional of U.S. patent application
Ser. No. 13/294,410, filed on Nov. 11, 2011, and entitled "SPECTRAL
ALIGNMENT OF A WDM DEVICE TO A SPECIFIED FREQUENCY GRID," which is
incorporated herein by reference in its entirety.
BACKGROUND
[0003] 1. Field
[0004] The present invention relates to optical communication
equipment and, more specifically but not exclusively, to WDM
(wavelength division multiplexing) devices.
[0005] 2. Description of the Related Art
[0006] This section introduces aspects that may help facilitate a
better understanding of the invention(s). Accordingly, the
statements of this section are to be read in this light and are not
to be understood as admissions about what is in the prior art or
what is not in the prior art.
[0007] A typical WDM system uses a multiplexer at the transmitter
site to join individual modulated optical signal streams together
for transmission over the optical link, and a de-multiplexer at the
receiver site to split the streams apart for detection and
decoding. The system may also employ optical add-drop multiplexers,
frequency converters, signal regenerators, and a variety of optical
filtering devices, such as etalons and phase masks. To achieve
seamless operation, different components of the WDM system usually
are appropriately spectrally aligned to a specified frequency
grid.
SUMMARY
[0008] Disclosed herein are various embodiments of an optical WDM
device having a controller that can individually control the
operating parameters of tunable lasers and the temperatures of an
optical multiplexer and etalon. The optical device employs a
spectral analyzer to measure the spectral composition of the
optical output signal produced by the device. Based on the analyses
of the measured spectral composition, the controller sets the
operating parameters for the tunable lasers and the temperatures of
the optical multiplexer and etalon to values that cause: (i) middle
frequencies of transmission bands of the optical multiplexer to be
spectrally aligned with the corresponding frequencies of the
specified frequency grid, (ii) each laser line to be appropriately
positioned within the corresponding transmission band, and (iii)
transmission resonances of the optical etalon to be appropriately
positioned (e.g., spectrally aligned or controllably shifted) with
respect to the laser lines.
[0009] According to one embodiment, provided is an optical method
comprising the steps of: in an optical multiplexer that is periodic
in its frequency response, receiving light from a parallel array of
lasing lasers, each laser of the array being connected to a
different optical input of the optical multiplexer; measuring
spectral intensities of light output from the optical multiplexer
in response to the receiving light therein, the measuring being
performed over one or more wavelength ranges, each of the one or
more wavelength ranges excluding wavelengths at which the lasers
lase; and adjusting a spectral location of one or more optical pass
bands of the optical multiplexer based on the measured spectral
intensities.
[0010] According to another embodiment, provided is an apparatus
comprising a plurality of lasers, each laser configured to produce
light having a corresponding optical-carrier frequency, and a
plurality of laser-control units, each unit coupled to a
corresponding one of the lasers. The apparatus further comprises an
optical multiplexer configured to multiplex the light generated by
the lasers to produce an optical output signal; a first
temperature-control unit thermally coupled to the lasers and the
optical multiplexer; and a controller operatively connected to
control the laser-control units and the first temperature-control
unit based on a spectral composition of the optical output
signal.
[0011] According to yet another embodiment, provided is a method of
operating an optical device. The method has the step of generating
an optical output signal having a plurality of optical-carrier
frequencies using a plurality of lasers and an optical multiplexer
that wavelength multiplexes the light produced by the lasers. The
method further has the steps of measuring a spectral composition of
the optical output signal using a spectral analyzer; regulating a
plurality of laser-control units using a controller that is
operatively connected to individually control each of the
laser-control units based on the measured spectral composition,
wherein each of the laser-control units is coupled to a respective
one of the lasers; and regulating a first temperature-control unit
using the controller, which is further operatively connected to
control the first temperature-control unit based on the measured
spectral composition, wherein the first temperature-control unit is
thermally coupled to the plurality of lasers and the optical
multiplexer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Other aspects, features, and benefits of various embodiments
of the invention will become more fully apparent, by way of
example, from the following detailed description and the
accompanying drawings, in which:
[0013] FIGS. 1A-1D show a WDM transmitter and its characteristics
according to one embodiment of the invention;
[0014] FIG. 2 shows a flowchart of a method that can be used to
configure the transmitter shown in FIG. 1A for optimal operation
according to one embodiment of the invention;
[0015] FIG. 3 graphically shows a representative spectrum that
might be recorded at one of the steps of the method shown in FIG.
2;
[0016] FIG. 4 graphically illustrates the effect of a
representative etalon on the spectrum shown in FIG. 3; and
[0017] FIG. 5 shows a flowchart of a method that can be used to
implement one of the steps of the method shown in FIG. 2 according
to an alternative embodiment of the invention.
DETAILED DESCRIPTION
[0018] The most common frequency grid that is used in fiber-optic
communication systems is defined by the ITU-T G.694.1 (2002)
Recommendation, which is incorporated herein by reference in its
entirety. This frequency grid extends from about 191.7 THz to about
196.1 THz, with a 100 GHz spacing of the channels therein. While
defined in frequency units, this grid can equivalently be expressed
in wavelength units, in which case the grid covers the wavelength
range from about 1528.8 nm to about 1563.9 nm, with approximately
0.8 nm spacing between the centers of neighboring channels. For
certain applications, the frequency grid can be extended to cover
the range from about 186 THz to about 201 THz and/or further be
divided to provide a 50 GHz and/or 25 GHz spacing between the
centers of neighboring channels. Other frequency grids are also
used in fiber-optic communication systems, albeit less
frequently.
[0019] For illustration purposes, various embodiments of the
invention are described below in reference to an ITU frequency grid
having a spacing of 100 GHz. However, the invention is not limited
to this grid. From the provided description, one of ordinary skill
in the art will be able to practice various embodiments while using
other frequency grids.
[0020] FIGS. 1A-1D show a WDM transmitter 100 and certain
characteristics of this WDM transmitter according to one embodiment
of the invention. More specifically, FIG. 1A shows a block diagram
of transmitter 100. FIG. 1B shows a cross-sectional side view of
transmitter 100. FIG. 1C shows a representative emission spectrum
of one of lasers 110 in transmitter 100. FIG. 1D shows transmission
characteristics of an optical multiplexer (MUX) 130 in transmitter
100.
[0021] Referring to FIG. 1A, transmitter 100 has an array of N
tunable lasers 110.sub.1-110.sub.N, each feeding a corresponding
one of optical modulators 120.sub.1-120.sub.N. In operation, each
laser 110 generates a respective light beam 112 having a specified,
different carrier frequency (wavelength) located on the ITU grid so
that the array, as a whole, generates an emission comb with a
peak-to-peak comb-line spacing of 100 GHz. Each of optical
modulators 120 receives a respective drive signal (not explicitly
shown in FIG. 1A) that causes the modulator to modulate the
respective carrier frequency with data intended for transmission to
a remote receiver. The resulting modulated optical signals
122.sub.1-122.sub.N are applied to (N.times.1) optical multiplexer
(e.g., an arrayed-waveguide-type optical multiplexer) 130, where
they are wavelength multiplexed to produce a WDM signal 132. In
certain embodiments, WDM signal 132 is subjected to filtering in an
optional optical etalon 140. The resulting filtered WDM signal 132'
is then applied to an optical link (e.g., optical fiber) for
transmission to the intended receiver(s). In a representative
embodiment, N=10, but N may have any selected, larger or smaller
positive integer value in an alternative embodiment.
[0022] In one embodiment, each of lasers 110.sub.1-110.sub.N is a
distributed-feedback (DFB) laser. A DFB laser is a type of laser
diode, quantum-cascade laser, or fiber laser in which the active
region has a periodic structure. The periodic structure acts as a
one-dimensional Bragg grating that defines the laser cavity and
provides the optical feedback for the laser emission to occur. The
grating is typically constructed so that a single longitudinal
lasing mode is supported in the laser cavity. A change in the
temperature of the device and/or a change in the injection current
cause the pitch of the grating to change, thereby changing the
wavelength of the supported lasing mode and thus the carrier
frequency of light beam 112.
[0023] FIG. 1C graphically shows a representative emission spectrum
of laser 110. More specifically, when laser 110 is biased to
operate above the lasing threshold, the laser's output consists of
(i) a relatively strong and narrow laser line 114 that is produced
by stimulated emission and corresponds to the supported
longitudinal lasing mode and (ii) a relatively weak and broad
emission band 116 that is produced by amplified spontaneous
emission (ASE). The wavelength corresponding to the maximum of
laser line 114 is sometimes referred to as the lasing wavelength.
Note that ASE band 116 is typically present on both sides of laser
line 114 regardless of the intensity or spectral location of the
laser line. The relative intensity of laser line 114 and ASE band
116 can be controlled, for example, by controlling the injection
current applied to the laser cavity.
[0024] In an alternative embodiment, a coupled laser-modulator pair
(e.g., laser 110.sub.1 and modulator 120.sub.i) in transmitter 100
can be replaced by a directly modulated laser (DML). Similar to DFB
lasers, DML lasers are temperature-tunable and produce an ASE band
that is similar to ASE band 116, in addition to the corresponding
laser line that is similar to laser line 114. DML lasers suitable
for use in transmitter 100 are commercially available, e.g., from
JDS Uniphase Corporation headquartered in Milpitas, Calif.
[0025] In one embodiment, optical multiplexer 130 is implemented
using an arrayed waveguide grating (AWG). A representative AWG is a
planar waveguide circuit having a plurality of waveguides coupled
between two optical star couplers. Wavelength selection in an AWG
is based on light interference, which causes only certain
wavelengths applied to the input ports of the input star coupler to
interfere constructively at the output port of the output star
coupler. Since the interference depends on the index of refraction
of the waveguide material(s), the passband wavelengths in an AWG
shift when the temperature of the AWG changes.
[0026] FIG. 1D graphically shows a transmission spectrum of a
representative AWG that can be used to implement optical
multiplexer 130 in transmitter 100. Note that the shown
transmission spectrum corresponds to N=10, but, as already
indicated above, N may be different from 10 in alternative
embodiments. The transmission spectrum is periodic, with the
spectral period usually referred to as the FSR (free spectral
range). A spectral range that covers approximately three FSRs is
shown in the FIG. 1D. Each FSR order has ten transmission bands
(also sometimes referred to as pass bands), with each band
corresponding to a different respective input port of optical
multiplexer 130. As indicated in FIG. 1D, the spectral separation
between adjacent transmission bands is 100 GHz, which causes the
transmission bands to be closely bundled together so that the
transmission-band bundles corresponding to different FSR orders of
optical multiplexer 130 are separated from each other by spectral
regions in which the optical multiplexer transmits very little
light. The spectral width of the gap between transmission-band
bundles may vary depending upon the specific design of the AWG. For
example, the gap may be as wide as a relatively large number of
transmission bands or be reduced to zero, thereby causing different
transmission-band bundles to form a continuous sequence of
passbands. For all practical purposes, a temperature change causes
the spectrum shown in FIG. 1D to shift along the abscissa, with
substantially no change in the relative positions of the bands and
band bundles with respect to one another.
[0027] In one embodiment, etalon 140 is implemented using a
transparent plate (e.g., made of fused silica) having its two
parallel reflecting surfaces coated with a dielectric coating. This
type of an etalon is often referred to as a solid Fabry-Perot
etalon. Similar to the transmission spectrum of an optical
multiplexer (FIG. 1D), the transmission spectrum of a solid
Fabry-Perot etalon is temperature tunable. However, the
transmission resonances of etalon 140 are typically significantly
narrower than the transmission bands of optical multiplexer 130. In
various alternative embodiments, other etalon versions, such as
air-spaced Fabry-Perot, VIPA (virtually imaged phased array),
Gires-Tournois, and multi-cavity, can similarly be used. In various
embodiments, etalon 140 can be implemented so that the band spacing
in optical multiplexer 130 is an integer multiple of the FSR value
of the etalon.
[0028] Maximum transmitted power, which is often considered to
constitute an approximate optimum operating condition for
transmitter 100, is typically achieved under the following
conditions. The maxima of the transmission bands in a selected FSR
order of optical multiplexer 130 (e.g., the 0-FSR order in FIG. 1D)
are approximately spectrally aligned with the corresponding
frequencies (wavelengths) of an ITU frequency grid. Laser line 114
(FIG. 1C) of each laser 110 is approximately centered within the
corresponding transmission band of the 0-FSR order of optical
multiplexer 130 to reduce the insertion loss in the optical
multiplexer. Transmission resonances of etalon 140 are
approximately spectrally aligned with the laser lines 114 of lasers
110.sub.1-110.sub.N to reduce the ASE noise and increase the output
power. It is also worth noting that, in certain configurations, the
transmission resonances of etalon 140 may be intentionally shifted
with respect to the ITU frequency grid to introduce a desired
spectral filtering of the signals that may positively affect signal
transport in the optical link.
[0029] Referring now to FIGS. 1A-1B, to enable transmitter 100 to
operate in a desired regime, the transmitter may have a controller
170 that can individually control, via a corresponding one of
control signals 172.sub.1-172.sub.N+2, each of laser-control units
118.sub.1-118.sub.N, temperature-control unit 180, and
temperature-control unit 190. Temperature-control unit 180 is
thermally coupled to regulate the temperature(s) of lasers
110.sub.1-110.sub.N, modulators 120.sub.1-120.sub.N, and optical
multiplexer 130. For example, temperature-control unit 180 may be
affixed to a base plate (e.g., circuit board) 102, with each of
lasers 110, modulators 120, and optical multiplexer 130 being
attached to temperature-control unit 180 as indicated in FIG. 1B.
Each of laser-control units 118.sub.1-118.sub.N is a circuit that
can change the injection current applied to the cavity of the
corresponding one of lasers 110.sub.1-110.sub.N and/or can regulate
the temperature of the laser without changing the injection
current. Note that laser-control unit 118 is designed to provide
localized temperature control in the area it is attached to and, as
such, has little effect on the temperature of modulator 120,
optical multiplexer 130, or other lasers 110. Temperature-control
unit 190 is thermally coupled to regulate the temperature of etalon
140. The relatively poor heat conductance of base plate 102 and air
gaps between optical multiplexer 130 and etalon 140 and between
temperature-control units 180 and 190 ensure that
temperature-control unit 180 and temperature-control unit 190
efficiently and separately control the temperatures of optical
multiplexer 130 and etalon 140, respectively. In various
embodiments, each of control units 118.sub.1-118.sub.N, 180, and
190 can be implemented using a respective thermoelectric cooler
and/or a respective resistive heater. Note that certain models of
thermoelectric coolers are operationally reconfigurable and,
depending on the configuration, the same device can operate as a
cooler or as a heater.
[0030] Controller 170 generates control signals
172.sub.1-172.sub.N+2 based on a feedback signal 162 that is
generated by an optical spectral analyzer 160 based on a portion of
output signal 132' (or 132, if optional etalon 140 is omitted) that
the spectral analyzer receives via an optical tap 150. Spectral
analyzer 160 is designed to measure the spectral composition of the
received optical signal at sufficient spectral resolution within a
certain spectral window, whose position can be moved (tuned) within
a specified spectral range. Controller 170 uses a control signal
174 to control the position of the spectral window captured by
spectral analyzer 160 and also to configure the spectral analyzer
to start and stop spectrum acquisition. Spectral analyzer 160
provides the acquired spectra to controller 170 via feedback signal
162. In one embodiment, the width of the spectral window captured
by spectral analyzer 160 in one acquisition is smaller than or
about the same as one FSR of optical multiplexer 130 (see FIG. 1D).
In another embodiment, the width of the spectral window captured by
spectral analyzer 160 in one acquisition is greater than or about
the same as three FSRs of optical multiplexer 130.
[0031] FIG. 2 shows a flowchart of a method 200 that can be used to
configure transmitter 100 (FIG. 1A) for operation according to one
embodiment of the invention. Note that method 200 corresponds to an
embodiment of transmitter 100 in which etalon 140 is absent.
Modifications of method 200 that can be used when etalon 140 is
present are described further below in reference to FIGS. 4-5.
[0032] Prior to step 202, method 200 is initiated with the initial
value of an index i set to zero. Then, at step 202, index i is
incremented by 1.
[0033] At step 204, controller 170 may use control signal
172.sub.N+1 to configure temperature-control unit 180 to set the
temperature of optical multiplexer 130 to T.sub.i. In one
implementation of step 204, the following set of temperatures
T.sub.i can be used: T.sub.1=20.degree. C.; T.sub.2=70.degree. C.;
T.sub.3=30.degree. C.; T.sub.4=60.degree. C., etc. Controller 170
uses control signals 172.sub.1-172.sub.N to configure laser-control
units 118.sub.1-118.sub.N to set the temperatures of lasers
110.sub.1-110.sub.N to a selected nominal value, e.g., 30.degree.
C.
[0034] At step 206, one or more of lasers 110.sub.1-110.sub.N are
configured to operate above the lasing threshold so that the
corresponding ASE band(s) 116 (FIG. 1C) are relatively prominent
(e.g., have sufficiently high intensity to enable spectral analyzer
160 to detect and measure the corresponding light after it passes
through optical multiplexer 130 and optical tap 150). Note that, in
this embodiment, modulators 120.sub.1-120.sub.N are configured not
to perform data modulation at this time. In an alternative
embodiment, modulators 120.sub.1-120.sub.N may be configured to
perform data modulation during step 206.
[0035] At step 208, controller 170 may use control signal 174 to
configure spectral analyzer 160 to record the spectrum of the
resulting output signal 132 over at least three FSR orders of
optical multiplexer 130, for example, the -1, 0, and +1 FSR orders
shown in FIG. 1D. Depending on the width of the spectral window
captured by spectral analyzer 160 in one acquisition, more than one
spectrum acquisition may need to be performed, with the concomitant
shift of the spectral window between the acquisitions, to cover the
three FSR orders with sufficient spectral resolution. The recorded
spectrum is then transferred via feedback signal 162 to controller
170 for processing.
[0036] FIG. 3 graphically shows a representative spectrum that
might be recorded at step 208. The recorded spectrum typically has
(i) a first set 316.sub.1 of bands corresponding the +1-FSR order
of optical multiplexer 130, (ii) a second set 314 of peaks
corresponding the 0-FSR order of the optical multiplexer, and (iii)
a third set 316.sub.-1 of bands corresponding the -1-FSR order of
the optical multiplexer. The bands of set 316.sub.1 are produced by
the light of ASE bands 116 (FIG. 1C) that have been subjected to
optical filtering in the +1-FSR order of optical multiplexer 130
(FIG. 1D). The bands of set 316.sub.-1 are similarly produced by
the light of ASE bands 116 that have been subjected to optical
filtering in the -1-FSR order of optical multiplexer 130. Since ASE
bands 116 are relatively broad and substantially spectrally flat
over each transmission-band bundle, the middle frequency of each
band in each band set 316 matches with relatively high accuracy the
middle frequency of the corresponding transmission band of optical
multiplexer 130 (FIG. 1D).
[0037] Each of the peaks in set 314 has a relatively narrow line
that corresponds to laser line 114 of the corresponding laser 110.
This narrow line sits on top of a wider base that is produced by
the light of ASE band 116 (FIG. 1C) subjected to optical filtering
in optical multiplexer 130 (FIG. 1D). Note that the individual
laser lines 114 are not necessarily centered on their respective
bases at this stage of the spectral-alignment procedure because the
temperatures of lasers 110.sub.1-110.sub.N are set to nominal
values (see step 204) and the injection currents are set to produce
a relatively high intensity in ASE bands 116, which settings may
differ from the settings that center the laser lines within the
respective transmission bands of optical multiplexer 130.
Furthermore, in certain situations, some or all of the individual
laser lines 114 might even be located outside the respective
transmission bands of optical multiplexer 130.
[0038] If spectral analyzer 160 has a spectral window that is about
one FSR in width, then step 208 may be implemented with two
spectrum acquisitions, one for capturing the bands of set
316.sub.-1 and another one for capturing the bands of set
316.sub.1. The spectrum acquisition in which the spectral window
covers the spectral region containing set 314 may be omitted. In
this case, the spectrum transferred to controller 170 for
processing may have no data in the middle portion of the spectrum
shown in FIG. 3.
[0039] Referring back to FIG. 2, at step 210 of method 200,
controller 170 analyzes the spectrum recorded at step 208. For
example, each of band sets 316.sub.-1 and 316.sub.1 or a respective
portion of each of these band sets may be fitted with a multi-peak
function to determine the middle frequencies of the individual
bands in the band set. In one exemplary implementation, the fitting
function for fitting one band set 316 may be a multi-Gaussian
function given by Eq. (1):
I ( f ) = A 0 + n = 0 N - 1 a n exp - ( f - f 0 + n .DELTA. f 2
.sigma. ) 2 ( 1 ) ##EQU00001##
where f is the frequency; I(f) is the spectral intensity as a
function of frequency in the recorded spectrum; A.sub.0 is a
constant; a.sub.n is the amplitude of the n-th Gaussian peak;
f.sub.0 is the middle frequency of the Gaussian peak corresponding
to n=0; .DELTA.f is the band spacing; and .sigma. is the variance
of the Gaussian curve. During the fit, the values of A.sub.0,
a.sub.n (n=0, . . . , N-1), f.sub.0, .DELTA.f, and .sigma. are
adjusted until the fit-convergence criteria are met. In other
implementations, other suitable fitting functions may similarly be
used. In one alternative implementation, Eq. (1) can be
appropriately modified to take into consideration and fit fewer
than N bands per band set. In principle, this approach may even
work, albeit with lower accuracy, with a single transmission band
per each of band sets 316.sub.-1 and 316.sub.1, provided that there
is some prior or nominal knowledge of .DELTA.f that enables one to
obtain the middle frequencies of the other transmission bands in
the band set by extrapolation from that single fitted transmission
band.
[0040] Note that the spectral data corresponding to peak set 314
may not lend themselves to sufficiently accurate determination of
the middle frequencies of the transmission bands in the 0-FSR order
of optical multiplexer 130. This statement is true, for example,
because laser lines 114 significantly distort the shapes of the
peak bases that represent the 0-FSR transmission bands (see FIG.
1D) in peak set 314. However, the middle frequencies of the
transmission bands in the 0-FSR order of optical multiplexer 130
can be determined with sufficiently high accuracy by interpolation
from the middle frequencies of the corresponding bands in band sets
316.sub.-1 and 316.sub.1. For example, in one implementation, Eq.
(2) may be used for such interpolation:
f n ( 0 ) = 1 2 ( f 0 ( + 1 ) + n .DELTA. f ( + 1 ) + n .DELTA. f (
- 1 ) ) ( 2 ) ##EQU00002##
where f.sub.n.sup.(0) is the middle frequency of the n-th
transmission band in the 0-FSR order of optical multiplexer 130;
f.sub.0.sup.(+1) is the middle frequency of the Gaussian peak
corresponding to n=0 in band set 316.sub.1; f.sub.0.sup.(-1) is the
middle frequency of the Gaussian peak corresponding to n=0 in band
set 316.sub.-1; .DELTA.f.sup.(+1) is the band spacing in band set
316.sub.1; and .DELTA.f.sup.(-1) is the band spacing in band set
316.sub.-1. In other implementations, other suitable interpolation
methods may similarly be used.
[0041] At step 212, it may be determined whether or not another
spectrum analogous to that shown in FIG. 3 is to be recorded at a
different temperature T.sub.i. If the answer is affirmative, then
the processing of method 200 is directed back to step 202.
Otherwise, the processing of method 200 continues on to step 214.
Note that, often, spectral data corresponding to at least two
different temperatures T.sub.i are collected.
[0042] At step 214, controller 170 may process the spectral data of
steps 210 performed at least at two different temperatures T.sub.i
to determine temperature T.sub.MUX that approximately best aligns
the middle frequencies of the transmission bands in the 0-FSR order
of optical multiplexer 130 with the specified frequency grid (e.g.,
an ITU grid). For example, if spectral data have been collected at
temperatures T.sub.1 and T.sub.2, then the following exemplary
procedure may be used.
[0043] At a first step of this procedure, a function of temperature
(f.sub.n.sup.(0)(T)) given by Eq. (3) is constructed:
f n ( 0 ) ( T ) = f n ( 0 ) ( T 2 ) - f n ( 0 ) ( T 1 ) T 2 - T 1 (
T - T 1 ) + f n ( 0 ) ( T 1 ) ( 3 ) ##EQU00003##
where f.sub.n.sup.(0)(T.sub.1) and f.sub.n.sup.(0)(T.sub.2) are the
interpolation data of step 210 corresponding to temperatures
T.sub.1 and T.sub.2, respectively (also see Eq. (2)). Although Eq.
(3) uses a linear approximation, other suitable approximations may
alternatively be used. Although one function f.sub.n.sup.(0)(T),
e.g., for n=0, may be sufficient for the determination of
temperature T.sub.MUX, two or more (e.g., up to N) such functions
corresponding to different n values may similarly be
constructed.
[0044] At a second step of the exemplary procedure, temperature
T.sub.MUX is determined as the temperature value at which the
deviations of the values of f.sub.n.sup.(0)(T.sub.MUX) from the
frequency grid are acceptably small. For example, if a single
function f.sub.n.sup.(0)(T) (e.g., corresponding to one selected n)
has been constructed in the first step of the procedure, then
T.sub.MUX can be determined by directly solving Eq. (3). If
multiple functions f.sub.n.sup.(0)(T) (e.g., corresponding to two
or more different n's) have been constructed in the first step of
the procedure, then T.sub.MUX can be determined by minimizing the
sum of square deviations of values f.sub.n.sup.(0)(T.sub.MUX)
corresponding to different n's from their respective grid
frequencies. In alternative implementations, other suitable
error-optimization methods may similarly be invoked to find
temperature T.sub.MUX.
[0045] As part of step 214, after temperature T.sub.MUX has been
determined, controller 170 configures temperature-control unit 180
to set the temperature of optical multiplexer 130 to T.sub.MUX.
[0046] At step 216, each of lasers 110.sub.1-110.sub.N may be tuned
to place the respective carrier frequency (wavelength) on a
particular line of the frequency grid. The tuning of each
individual laser 110 may be performed, e.g., in two sub-steps. In
the first sub-step, the injection current applied to the laser
cavity is set to a value corresponding to normal operation of the
laser, which may be different from the injection-current value that
was set at step 206. In the second sub-step, controller 170
configures the laser-control unit 118 attached to the laser, via
the corresponding one of control signals 172.sub.1-172.sub.N, to
set the injection current and temperature of the laser to values
that cause the carrier frequency (wavelength) generated by the
laser to be on the frequency grid. More specifically, spectral
analyzer 160 is configured to monitor the frequency (wavelength) of
the corresponding peak in peak set 314 while the injection current
and temperature are being adjusted to find a proper operating
value. In various implementations of step 216, lasers
110.sub.1-110.sub.N may be tuned one at a time, more than one at a
time, or all at the same time. After each of lasers
110.sub.1-110.sub.N has been appropriately tuned, the result may be
that its respective laser line 114 (FIG. 1C) is appropriately
positioned near the middle of the corresponding transmission band
in the 0-FSR order of optical multiplexer 130 (FIG. 1D).
[0047] FIG. 4 graphically illustrates an effect of representative
etalon 140 on the spectrum shown in FIG. 3. More specifically, the
data shown in FIG. 4 correspond to an etalon 140 having an FSR that
is one half of the transmission-band spacing in optical multiplexer
130 (see FIG. 1D). The dashed lines in FIG. 4 show a spectrum
corresponding to band set 316.sub.-1 (FIG. 3) that would have been
recorded at step 208 of method 200 (FIG. 2) in the absence of
etalon 140. The solid lines with solid squares in FIG. 4 show the
spectrum that is observed when etalon 140 is present.
[0048] One skilled in the art will appreciate that the spectrum
shown in FIG. 4 by the solid lines/squares is a product of the
spectrum shown by the dashed lines and the transmission spectrum of
etalon 140. In effect, the presence of etalon 140 causes optical
analyzer 160 to sample the spectrum of signal 132 at 50-GHz
intervals, with the sampling points being the points corresponding
to the transmission resonances of the etalon. Therefore, the entire
dashed-line spectrum can be reconstructed by shifting the
transmission resonances of etalon 140 along the abscissa of FIG. 4
and measuring the spectrum of signal 132' at different positions of
the resonances. The shift of transmission resonances can be
performed in a continuous gradual manner or in several discrete
steps.
[0049] As already indicated above in reference to FIGS. 1A-1B,
controller 170 can configure temperature-control unit 190, via
control signal 172.sub.N+2, to change the temperature of etalon
140. Provided that the temperature of optical multiplexer 130
remains constant, this temperature change will shift the
transmission spectrum of etalon 140 with respect to the
transmission spectrum of the optical multiplexer, thereby sampling
the transmission spectrum of the optical multiplexer at different
frequencies.
[0050] FIG. 5 shows a flowchart of a method 500 that can be used to
implement step 208 of method 200 (FIG. 2) in the presence of etalon
140 according to an alternative embodiment of the invention. Note
that method 500 is based on the above-described concept of changing
the temperature of etalon 140 to shift its transmission resonances
in a manner that enables controller 170 to reconstruct the
transmission spectrum of optical multiplexer 130 even though the
etalon's presence prevents a direct measurement of that
transmission spectrum from one acquisition.
[0051] At step 502 of method 500, controller 170 determines a
suitable temperature interval in which a temperature change from
the lower to the higher boundary of the interval causes a shift of
the transmission spectrum of etalon 140 by an integer number of the
etalon's FSRs. For example, for an etalon corresponding to the data
of FIG. 4, controller 170 may find a temperature interval in which
the transmission spectrum of the etalon shifts by about 50 GHz, 100
GHz, or 150 GHz. Controller 170 can make this determination, e.g.,
by configuring spectral analyzer 160 to acquire, at each of several
different temperatures of etalon 140, the spectrum of output signal
132' and then analyzing the acquired spectra to determine the rate,
in GHz/K, at which the transmission resonances of the etalon shift
with temperature.
[0052] At step 504, temperature-control unit 190 is configured to
set the temperature of etalon 140 at a selected temperature within
the temperature interval determined at step 502.
[0053] At step 506, temperature-control unit 190 is configured to
ramp the temperature of etalon 140 up and down within the
temperature interval determined at step 502 an integer number of
periods. One temperature-ramp period generally consists of starting
at the temperature that has been set at step 504, then going to one
boundary of the temperature interval, then reversing the direction
and going to the other boundary of the temperature interval, and
finally coming back to the starting temperature. At the same time,
spectral analyzer 160 is configured to run in an averaging mode or
a cumulative mode, in which the recorded spectra are automatically
averaged over time or added up to create a cumulative spectrum. One
skilled in the art will appreciate that the resulting averaged or
cumulative spectrum is an approximation of the spectrum that would
have been recorded at step 208 of method 200 (FIG. 2) in the
absence of etalon 140.
[0054] In an alternative embodiment, step 506 may be implemented
using the sub-steps of: (i) recording the spectrum of output signal
132' at the temperature that has been set at step 504, with the
temperature remaining constant during the spectrum acquisition;
(ii) changing the temperature of etalon 140 to a next selected
temperature from the temperature interval determined at step 502
while the spectrum acquisition is halted; (iii) recording the
spectrum of output signal 132' at said next temperature, with the
temperature remaining constant during the spectrum acquisition;
(iv) repeating sub-steps (ii) and (iii) for a desired number of
times; and (v) adding or averaging all of the recorded spectra,
e.g., in digital form.
[0055] Further modifications of method 200 that can be used to
accommodate the presence of etalon 140 may include one or more of
the following. Prior to performing step 216 (see FIG. 2),
temperature-control unit 190 may be configured to set the
temperature of etalon 140 to a value that causes the appropriate
transmission resonances in the transmission spectrum of the etalon
to be located approximately near the middle of the corresponding
transmission bands in the transmission spectrum of optical
multiplexer 130. After performing step 216, temperature-control
unit 190 may be configured to fine-tune the temperature of etalon
140 to more-accurately position the transmission peaks in the
transmission spectrum of the etalon with respect to the
corresponding laser lines 114. This fine-tuning may be
accomplished, e.g., by changing the temperature of etalon 140 so
that the optical power of signal 132' is maximized.
[0056] While this invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Although embodiments of the
invention have been described above in reference to a WDM
transmitter, the invention is not so limited. Based on the provided
description, one of ordinary skill in the art will be able to
practice various embodiments of the invention using other WDM
devices, such as optical add-drop multiplexers, frequency
converters, signal regenerators, and various optical filtering
devices. Various modifications of the described embodiments, as
well as other embodiments of the invention, which are apparent to
persons skilled in the art to which the invention pertains are
deemed to lie within the principle and scope of the invention as
expressed in the following claims.
[0057] Unless explicitly stated otherwise, each numerical value and
range should be interpreted as being approximate as if the word
"about" or "approximately" preceded the value of the value or
range.
[0058] The use of figure numbers and/or figure reference labels in
the claims is intended to identify one or more possible embodiments
of the claimed subject matter in order to facilitate the
interpretation of the claims. Such use is not to be construed as
necessarily limiting the scope of those claims to the embodiments
shown in the corresponding figures.
[0059] Although the elements in the following method claims, if
any, are recited in a particular sequence with corresponding
labeling, unless the claim recitations otherwise imply a particular
sequence for implementing some or all of those elements, those
elements are not necessarily intended to be limited to being
implemented in that particular sequence.
[0060] Reference herein to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic
described in connection with the embodiment can be included in at
least one embodiment of the invention. The appearances of the
phrase "in one embodiment" in various places in the specification
are not necessarily all referring to the same embodiment, nor are
separate or alternative embodiments necessarily mutually exclusive
of other embodiments. The same applies to the term
"implementation."
[0061] Also for purposes of this description, the terms "couple,"
"coupling," "coupled," "connect," "connecting," or "connected"
refer to any manner known in the art or later developed in which
energy is allowed to be transferred between two or more elements,
and the interposition of one or more additional elements is
contemplated, although not required. Conversely, the terms
"directly coupled," "directly connected," etc., imply the absence
of such additional elements.
[0062] The description and drawings merely illustrate the
principles of the invention. It will thus be appreciated that those
of ordinary skill in the art will be able to devise various
arrangements that, although not explicitly described or shown
herein, embody the principles of the invention and are included
within its spirit and scope. Furthermore, all examples recited
herein are principally intended expressly to be only for
pedagogical purposes to aid the reader in understanding the
principles of the invention and the concepts contributed by the
inventor(s) to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions. Moreover, all statements herein reciting principles,
aspects, and embodiments of the invention, as well as specific
examples thereof, are intended to encompass equivalents
thereof.
[0063] The functions of the various elements shown in the figures,
including any functional blocks labeled as "processors" and
"controllers" may be provided through the use of dedicated hardware
as well as hardware capable of executing software in association
with appropriate software. Moreover, explicit use of the term
"processor" or "controller" should not be construed to refer
exclusively to hardware capable of executing software, and may
implicitly include, without limitation, digital signal processor
(DSP) hardware, network processor, application specific integrated
circuit (ASIC), field programmable gate array (FPGA), read only
memory (ROM) for storing software, random access memory (RAM), and
non volatile storage. Other hardware, conventional and/or custom,
may also be included.
* * * * *