U.S. patent application number 15/728279 was filed with the patent office on 2018-02-01 for coherent receiver, method, and system for coherent light source frequency offset estimation and compensation.
The applicant listed for this patent is Huawei Technologies Co., Ltd.. Invention is credited to Lei Gao, Yin Wang.
Application Number | 20180034553 15/728279 |
Document ID | / |
Family ID | 57071732 |
Filed Date | 2018-02-01 |
United States Patent
Application |
20180034553 |
Kind Code |
A1 |
Gao; Lei ; et al. |
February 1, 2018 |
COHERENT RECEIVER, METHOD, AND SYSTEM FOR COHERENT LIGHT SOURCE
FREQUENCY OFFSET ESTIMATION AND COMPENSATION
Abstract
Embodiments of the present disclosure disclose a coherent
receiver, including: a frequency offset estimation unit and a
frequency offset compensation unit, where the frequency offset
estimation unit is configured to receive signal light and local
oscillator light, where the signal light is received by a first
photoelectric detector, and a first intensity value is obtained,
the signal light is received by a second photoelectric detector,
and a second intensity value is obtained, the local oscillator
light is received by a third photoelectric detector, and a third
intensity value is obtained, and the local oscillator light is
received by a fourth photoelectric detector, and a fourth intensity
value is obtained; and the frequency offset compensation unit is
configured to obtain a frequency offset value between the signal
light and the local oscillator light according to a difference
between a first ratio and a second ratio.
Inventors: |
Gao; Lei; (Shenzhen, CN)
; Wang; Yin; (Shenzhen, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Huawei Technologies Co., Ltd. |
Shenzhen |
|
CN |
|
|
Family ID: |
57071732 |
Appl. No.: |
15/728279 |
Filed: |
October 9, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/CN2015/076338 |
Apr 10, 2015 |
|
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15728279 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04J 14/06 20130101;
H04B 10/61 20130101; H04B 10/6164 20130101 |
International
Class: |
H04B 10/61 20060101
H04B010/61 |
Claims
1. A coherent receiver, comprising: a frequency offset estimation
unit configured to: receive signal light, wherein the signal light
is received by a first photoelectric detector after passing through
an etalon and a first intensity value is obtained, and the signal
light is received by a second photoelectric detector and a second
current value is obtained, and receive local oscillator light,
wherein the local oscillator light is received by a third
photoelectric detector after passing through the etalon and a third
intensity value is obtained, and the local oscillator light is
received by a fourth photoelectric detector and a fourth intensity
value is obtained; and a frequency offset compensation unit is
configured to: obtain a first ratio according to a ratio of the
first intensity value to the second intensity value, obtain a
second ratio according to a ratio of the third current intensity
value to the fourth intensity value, obtain a frequency offset
value between the signal light and the local oscillator light
according to a difference between the first ratio and the second
ratio, and perform frequency offset compensation on the signal
light and the local oscillator light.
2. The coherent receiver according to claim 1, wherein the etalon
comprises an air type etalon or a waveguide type etalon.
3. The coherent receiver according to claim 1, wherein the
frequency offset estimation unit further comprises: a first
beamsplitter configured to split the signal light into two parts,
one part of the signal light is received by the first photoelectric
detector after passing through the etalon, and the other part of
the signal light is received by the second photoelectric detector;
and a second beamsplitter configured to split the local oscillator
light into two parts, one part of the local oscillator light is
received by the third photoelectric detector after passing through
the etalon, and the other part of the local oscillator light is
received by the fourth photoelectric detector.
4. The coherent receiver according to claim 1, wherein the
frequency offset estimation unit further comprises: a first optical
waveguide configured to split the signal light into two parts, one
part of the signal light is received by the first photoelectric
detector after passing through the etalon, and the other part of
the signal light is received by the second photoelectric detector;
and a second optical waveguide configured to split the local
oscillator light into two parts, one part of the local oscillator
light is received by the third photoelectric detector after passing
through the etalon, and the other part of the local oscillator
light is received by the fourth photoelectric detector.
5. The coherent receiver according to claim 1, wherein the
frequency offset compensation unit is further configured to perform
wavelength adjustment on the local oscillator light according to
the frequency offset value to compensate the frequency offset value
between the local oscillator light and the signal light.
6. A method for coherent light source frequency offset estimation
and compensation, the method comprising: receiving, by a first
photoelectric detector, signal light after the signal light passes
through an etalon and generating a first intensity value;
receiving, by a second photoelectric detector, the signal light and
generating a second intensity value; receiving, by a third
photoelectric detector, local oscillator light after the local
oscillator light passes through the etalon and generating a third
intensity value; receiving, by a fourth photoelectric detector, the
local oscillator light and generating a fourth intensity value;
obtaining a first ratio according to a ratio of the first intensity
value to the second intensity, obtaining a second ratio according
to a ratio of the third intensity value to the fourth intensity
value, and obtaining a frequency offset value between the signal
light and the local oscillator light according to a difference
between the first ratio and the second ratio; and performing
frequency offset compensation on the signal light and the local
oscillator light.
7. The method according to claim 6, wherein the etalon comprises an
air type etalon or a waveguide type etalon.
8. The method according to claim 6, wherein: before the signal
light passes through the etalon, the method further comprises:
splitting, by a first beamsplitter, the signal light into two
parts, wherein one part of the signal light is received by the
first photoelectric detector after passing through the etalon, and
the other part of the signal light is received by the second
photoelectric detector; and before the local oscillator light
passes through the etalon, the method further comprises: splitting,
by a second beamsplitter, the local oscillator light into two
parts, wherein one part of the local oscillator light is received
by the third photoelectric detector after passing through the
etalon, and the other part of the local oscillator light is
received by the fourth photoelectric detector.
9. The method according to claim 6, wherein: before the signal
light passes through the etalon, the method further comprises:
splitting, by a first optical waveguide, the signal light into two
parts, wherein one part of the signal light is received by the
first photoelectric detector after passing through the etalon, and
the other part of the signal light is received by the second
photoelectric detector; and before the local oscillator light
passes through the etalon, the method further comprises: splitting,
by a second optical waveguide, the local oscillator light into two
parts, wherein one part of the local oscillator light is received
by the third photoelectric detector after passing through the
etalon, and the other part of the local oscillator light is
received by the fourth photoelectric detector.
10. The method according to claim 6, wherein after obtaining a
frequency offset value between the signal light and the local
oscillator light according to a difference between the first ratio
and the second ratio, the method further comprises: performing
wavelength adjustment on the local oscillator light according to
the frequency offset value to compensate the frequency offset value
between the local oscillator light and the signal light.
11. A coherent optical receiving system, comprising: a coherent
transmitter; and a coherent receiver comprising a processor
configured to, wherein a frequency offset estimation unit
configured to: receive signal light, wherein the signal light is
received by a first photoelectric detector after passing through an
etalon and a first intensity value is obtained, and the signal
light is received by a second photoelectric detector and a second
intensity value is obtained, and receive local oscillator light,
wherein the local oscillator light is received by a third
photoelectric detector after passing through the etalon and a third
intensity value is obtained, and the local oscillator light is
received by a fourth photoelectric detector and a fourth intensity
value is obtained; and a processor configured to: obtain a first
ratio according to a ratio of the first intensity value to the
second intensity value, obtain a second ratio according to a ratio
of the third intensity value to the fourth intensity value, obtain
a frequency offset value between the signal light and the local
oscillator light according to a difference between the first ratio
and the second ratio, and perform frequency offset compensation on
the signal light and the local oscillator light.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International
Application No. PCT/CN2015/076338, filed on Apr. 10, 2015, the
disclosure of which is hereby incorporated by reference in its
entirety.
TECHNICAL FIELD
[0002] The present application relates to the field of optical
communications, and in particular, to a coherent receiver, a
method, and a system for coherent light source frequency offset
estimation and compensation.
BACKGROUND
[0003] A direct detection receiving manner is used in conventional
optical communications. However, as a communication capacity
increases, a problem such as chromatic dispersion, frequency offset
mode dispersion, a nonlinear effect, or phase noise occurs in a
fiber link, and the conventional direct detection receiving manner
is gradually replaced by an optical coherent detection
technology.
[0004] Because an optical coherent receiving manner may support
multiple modulation formats, by taking full advantage of
information about light, such as an amplitude, a phase, or
polarization, spectrum utilization may be improved, and a fiber
transmission capacity may be increased. FIG. 1 is a schematic
structural diagram of a coherent optical communications system. In
the coherent optical communications system, a signal laser at a
transmit end generates signal light, and the signal light enters a
receive end after being modulated by a modulator. A local
oscillator laser at the receive end generates local oscillator
light. Frequency mixing is performed on the signal light and the
local oscillator light by using an Integrated Coherent Receiver
(ICR). A frequency offset, between the signal light and the local
oscillator light is compensated in an electric domain by using a
Digital Signal Processor (DSP). Currently, a frequency offset
compensation capability of a DSP in a coherent system is +/-5 GHz.
Therefore, requirements of the coherent system for frequency
stability of both the signal light and the local oscillator light
are +/-2.5 GHz, and in addition, it is required that frequency
offsets between wavelengths of the signal light and the local
oscillator light and an International Telecommunication Union (ITU)
standard wavelength are separately controlled within +/-2.5 GHz.
The signal laser at the transmit end in the coherent system and the
local oscillator laser at the receive end each include a wavelength
locker. Feedback control from the wavelength locker and a
wavelength locker algorithm unit ensures that the frequency offsets
between the wavelengths of the signal light and the local
oscillator light and the ITU standard wavelength are separately
+/-2.5 GHz, that is, a frequency offset between the signal light
and the local oscillator light is +/-5 GHz. This frequency offset
value falls within a range of the compensation capability of the
DSP.
[0005] However, due to its relatively high costs, the coherent
optical communications mainly targets long-distance optical
transmission. Therefore, costs of the coherent system need to be
reduced, and the coherent system needs to be applied to a
metropolitan area or an access domain. Respectively omitting the
wavelength lockers in the lasers at the transmit end and the
receive end may reduce the costs of the coherent system. Currently,
there is a laser without a wavelength locker: a distributed
feedback (DFB) laser. If the laser without a wavelength locker is
used, wavelength control precision is generally +/-0.1 nm
(frequency stability is +/-12.5 GHz), and a frequency offset
between the signal light and the local oscillator light reaches
+/-25 GHz. However, currently, the frequency offset compensation
capability of the DSP in the coherent system is +/-5 GHz, and
consequently, using a laser without a wavelength locker in the
coherent system cannot implement frequency offset compensation.
SUMMARY
[0006] In view of this, embodiments of the present disclosure
provide a coherent receiver, a method, and a system for coherent
light source frequency offset estimation and compensation, so as to
resolve a problem of frequency offset estimation and compensation
in a coherent system when a low-cost laser without a wavelength
locker is used as a light source.
[0007] According to a first aspect, an embodiment of the present
disclosure provides a coherent receiver, including: a frequency
offset estimation unit and a frequency offset compensation unit,
where the frequency offset estimation unit is configured to receive
signal light, where the signal light is received by a first
photoelectric detector after passing through an etalon, and a first
current intensity value is obtained; the signal light is received
by a second photoelectric detector, and a second current intensity
value is obtained, where the frequency offset estimation unit is
further configured to receive local oscillator light, where the
local oscillator light is received by a third photoelectric
detector after passing through the etalon, and a third current
intensity value is obtained, and the local oscillator light is
received by a fourth photoelectric detector, and a fourth current
intensity value is obtained; and the frequency offset compensation
unit is configured to: obtain a first ratio according to a ratio of
the first current intensity value to the second current intensity,
obtain a second ratio according to a ratio of the third current
intensity value to the fourth current intensity value, and obtain a
frequency offset value between the signal light and the local
oscillator light according to a difference between the first ratio
and the second ratio, where the frequency offset value instructs to
perform frequency offset compensation on the signal light and the
local oscillator light.
[0008] According to a second aspect, an embodiment of the present
disclosure provides a method for coherent light source frequency
offset estimation and compensation, including: receiving, by a
first photoelectric detector, signal light after the signal light
passes through an etalon, and a first current intensity value is
obtained; receiving, by a second photoelectric detector, the signal
light, and a second current intensity value is obtained; receiving,
by a third photoelectric detector, local oscillator light after the
local oscillator light passes through the etalon, and a third
current intensity value is obtained, and receiving, by a fourth
photoelectric detector, the local oscillator light, and a fourth
current intensity value is obtained; and obtaining a first ratio
according to a ratio of the first current intensity value to the
second current intensity, obtaining a second ratio according to a
ratio of the third current intensity value to the fourth current
intensity value, and obtaining a frequency offset value between the
signal light and the local oscillator light according to a
difference between the first ratio and the second ratio, where the
frequency offset value instructs to perform frequency offset
compensation on the signal light and the local oscillator
light.
[0009] According to a third aspect, an embodiment of the present
disclosure provides a coherent optical receiving system, where the
system includes: a coherent transmitter and a coherent receiver,
and the coherent receiver includes a frequency offset estimation
unit and a frequency offset compensation unit, where the frequency
offset estimation unit is configured to receive signal light, where
the signal light is received by a first photoelectric detector
after passing through an etalon, and a first current intensity
value is obtained; the signal light is received by a second
photoelectric detector, and a second current intensity value is
obtained, where the frequency offset estimation unit is further
configured to receive local oscillator light, where the local
oscillator light is received by a third photoelectric detector
after passing through the etalon, and a third current intensity
value is obtained, and the local oscillator light is received by a
fourth photoelectric detector, and a fourth current intensity value
is obtained; and the frequency offset compensation unit is
configured to: obtain a first ratio according to a ratio of the
first current intensity value to the second current intensity,
obtain a second ratio according to a ratio of the third current
intensity value to the fourth current intensity value, and obtain a
frequency offset value between the signal light and the local
oscillator light according to a difference between the first ratio
and the second ratio, where the frequency offset value instructs to
perform frequency offset compensation on the signal light and the
local oscillator light.
[0010] According to the technical solutions provided in the
embodiments of the present disclosure, in a coherent receiver,
signal light and local oscillator light pass through a frequency
offset estimation unit, so that a feedback signal that reflects a
frequency offset value between the signal light and the local
oscillator light is obtained; a frequency offset compensation unit
obtains the frequency offset value between the signal light and the
local oscillator light according to the feedback signal, where the
frequency offset value instructs to perform frequency offset
compensation on the signal light and the local oscillator light,
thereby implementing frequency offset estimation and compensation
for signal light and local oscillator light in a coherent system
when a laser without a wavelength locker is used as a light source
of the signal light and the local oscillator light, and reducing
costs of the coherent system.
BRIEF DESCRIPTION OF DRAWINGS
[0011] To describe the technical solutions in the embodiments of
the present disclosure or in the prior art more clearly, the
following briefly describes the accompanying drawings required for
describing the background and the embodiments. Apparently, the
accompanying drawings in the following description show merely some
embodiments of the present disclosure, and a person of ordinary
skill in the art may still derive other accompanying drawings or
embodiments according to these drawings or description without
creative efforts, and the present disclosure is intended to cover
all these derived accompanying drawings or embodiments.
[0012] FIG. 1 is a schematic structural diagram of a coherent
optical communications system in the prior art;
[0013] FIG. 2 is a schematic structural diagram of a coherent
optical communications system used to implement an embodiment of
the present disclosure;
[0014] FIG. 3 is a schematic structural diagram of a frequency
offset estimation unit used to implement an embodiment of the
present disclosure;
[0015] FIG. 4 is a schematic structural diagram of another
frequency offset estimation unit used to implement an embodiment of
the present disclosure;
[0016] FIG. 5 is an etalon spectral transmission curve graph used
to implement an embodiment of the present disclosure; and
[0017] FIG. 6 is a demonstrative flowchart of a method for
frequency offset estimation and compensation used to implement an
embodiment of the present disclosure.
DESCRIPTION OF EMBODIMENTS
[0018] To make the objectives, technical solutions, and advantages
of the present disclosure clearer and more comprehensible, the
following further describes the present disclosure in detail with
reference to the accompanying drawings and embodiments. It should
be understood that the specific embodiments described herein are
merely used to explain the present disclosure but are not intended
to limit the present disclosure. Apparently, the described
embodiments are merely some but not all of the embodiments of the
present disclosure. All other embodiments obtained by a person of
ordinary skill in the art based on the embodiments of the present
disclosure without creative efforts shall fall within the
protection scope of the present disclosure.
[0019] The embodiments of the present disclosure propose a
technical solution in which a low-cost laser without a wavelength
locker is used as a light source in a coherent optical
communications system. FIG. 2 is a schematic structural diagram of
a coherent optical communications system according to an embodiment
of the present disclosure. As shown in FIG. 2, the coherent optical
communications system includes a transmit end and a receive end.
The transmit end includes a signal laser 21 and a modulator 22. The
receive end includes a coherent receiver, and specifically includes
a local oscillator laser 23, a beamsplitter 24, a beamsplitter 25,
an ICR 26, a DSP 27, a frequency offset estimation unit 28, and a
frequency offset compensation unit 29. Specifically, the
beamsplitter 24 and the beamsplitter 25 may be independent devices,
or may be integrated into the frequency offset estimation unit 28.
The frequency offset estimation unit 28 may be an independent
module, or may be integrated into the ICR 26.
[0020] In a specific implementation process, the signal laser 21 at
the transmit end and the local oscillator laser 22 at the receive
end may not include a wavelength locker, and therefore may have an
excessively large frequency offset range, for example, the
frequency offset range reaches +/-12.5 GHz. The signal laser 21 at
the transmit end generates signal light. The signal light is
incident to a feeder fiber after being modulated by the modulator
22. The local oscillator laser 23 at the receive end generates
local oscillator light. The signal light is split into two parts
after passing through the beamsplitter 24 at the receive end. One
part of the signal light enters the ICR 26, and the other part of
the signal light enters the frequency offset estimation unit 28.
The local oscillator light is split into two parts after passing
through the beamsplitter 25 at the receive end. One part of the
local oscillator light enters the ICR 26, and the other part of the
local oscillator light enters the frequency offset estimation unit
28. The ICR 26 performs frequency mixing on the received signal
light and local oscillator light. A signal obtained after frequency
mixing enters the DSP 27. The DSP 27 obtains phase information and
light intensity information that are of the signal light after
processing the signal obtained after frequency mixing, that is, the
DSP 27 obtains, by means of demodulation, related information
loaded by the modulator 22 at the receive end. After receiving the
signal light and the local oscillator light, the frequency offset
estimation unit 28 obtains, by means of detection, a feedback
signal that reflects a frequency offset value between the signal
light and the local oscillator light, and transmits the feedback
signal to the frequency offset compensation unit 29. The frequency
offset compensation unit 29 obtains the frequency offset value
between the signal light and the local oscillator light according
to the feedback signal, and generates a control signal according to
the frequency offset value, so as to control the local oscillator
laser 23 to perform wavelength adjustment, until the frequency
offset value meets a system requirement.
[0021] In a specific implementation process, there may be two
implementations for the frequency offset estimation unit 28. As
shown in FIG. 4, a first implementation is implementing the
operation of the frequency offset estimation unit in free space;
and as shown in FIG. 5, a second implementation is implementing the
operation of the frequency offset estimation unit by using a plane
waveguide.
[0022] FIG. 3 is a schematic structural diagram of a frequency
offset estimation unit according to an embodiment of the present
disclosure. A first implementation is implementing an operation of
the frequency offset estimation unit in free space. The
implementing an operation of the frequency offset estimation unit
in free space means that light travels through air. The frequency
offset estimation unit 28 includes a first beamsplitter 31, a
second beamsplitter 32, an etalon 33, a first photoelectric
detector 34, a second photoelectric detector 35, a third
photoelectric detector 36, and a fourth photoelectric detector 37.
Preferably, split ratios of the first beamsplitter 31 and the
second beamsplitter 32 may be the same. The etalon 33 may be an air
type etalon, and includes a pair of flat glass plated with a
partially reflective film and a parallel spacing component.
Incident light generates reflection on the two pieces of flat glass
when the incident light is incident to the etalon, and therefore
generates constructive (or destructive) interference. The first
photoelectric detector 34, the second photoelectric detector 35,
the third photoelectric detector 36, and the fourth photoelectric
detector 37 have an optical-to-electrical conversion function, and
may be specifically photodiodes or phototriodes. The signal light
is split into two beams of light after passing through the first
beamsplitter 31. The local oscillator light is split into two beams
of light after passing through the second beamsplitter 32. One beam
of the signal light is received by the first photoelectric detector
34 after passing through the etalon 33, and converted into an
electrical signal, so that a current intensity value I.sub.PD1 is
obtained; and the other beam of the signal light is directly
received by the second photoelectric detector 35, and converted
into an electrical signal, so that a current intensity value
I.sub.PD2 is obtained. One beam of the local oscillator light is
received by the third photoelectric detector 36 after passing
through the etalon 33, and converted into an electrical signal, so
that a current intensity value I.sub.PD3 is obtained; and the other
beam of the local oscillator light is directly received by the
fourth photoelectric detector 37, and converted into an electrical
signal, so that a current intensity value I.sub.PD4 is
obtained.
[0023] FIG. 4 is a schematic structural diagram of another
frequency offset estimation unit according to an embodiment of the
present disclosure. A second implementation is implementing an
operation of the frequency offset estimation unit in a plane
optical waveguide. The implementing an operation of the frequency
offset estimation unit in a plane optical waveguide means that
light is transmitted in optical waveguides located on a same plane.
The frequency offset estimation unit 28 includes a first optical
waveguide 41, a second optical waveguide 42, a first etalon
function unit 43, a second etalon function unit 44, a first
photoelectric detector 45, a second photoelectric detector 46, a
third photoelectric detector 47, and a fourth photoelectric
detector 48. The first etalon function unit 43 and the second
etalon function unit 44 may be waveguide type etalons, and have
functions and spectral transmission curves same as those of the
etalon 33 in the embodiment shown in FIG. 3. Both the first etalon
function unit 43 and the second etalon function unit 44 include a
waveguide tapered area, a cavity surface etching groove, and a
resonant cavity area. The waveguide tapered area performs beam
expansion on incident light; the cavity surface etching groove
forms a reflective surface, and the incident light generates
reflection between two cavity surface etching grooves; and the
resonant cavity area forms an FP (Fabry-Perot) cavity, and the
incident light generates constructive (or destructive) interference
within the resonant cavity area. The first photoelectric detector
45, the second photoelectric detector 46, the third photoelectric
detector 47, and the fourth photoelectric detector 48 have an
optical-to-electrical conversion function, and may be specifically
photodiodes or phototriodes. Signal light is incident to the first
optical waveguide 41, and local oscillator light is incident to the
second optical waveguide 42. The first optical waveguide 41 splits
the signal light into two beams. One beam of the signal light is
received by the first photoelectric detector 45 after passing
through the first etalon function unit 43, and converted into an
electrical signal, so that a current intensity value I.sub.PD1 is
obtained; and the other beam of the signal light is directly
received by the second photoelectric detector 46, and converted
into an electrical signal, so that a current intensity value
I.sub.PD2 is obtained. One beam of the local oscillator light is
received by the third photoelectric detector 47 after passing
through the second etalon function unit 44, and converted into an
electrical signal, so that a current intensity value I.sub.PD3 is
obtained; and the other beam of the local oscillator light is
directly received by the fourth photoelectric detector 48, and
converted into an electrical signal, so that a current intensity
value I.sub.PD4 is obtained.
[0024] FIG. 5 is an etalon spectral transmission curve graph
according to an embodiment of the present disclosure. Specifically,
the etalon 33 in FIG. 3, and the first etalon function unit 43 and
the second etalon function unit 44 that are in FIG. 4 may have the
spectral transmission curve graph shown in FIG. 5. The etalon may
be an air type etalon. A shape of a spectral transmission curve of
the air type etalon depends on a distance between flat glass
reflection surfaces and reflectivity. The distance between the flat
glass reflection surfaces determines a free spectral range, that
is, an interval between two adjacent peak points on the curve in
FIG. 5. The reflectivity of the flat glass determines steepness of
the curve. The etalon may be a waveguide type etalon. A shape of a
spectral transmission curve of the waveguide type etalon depends on
a length of the resonant cavity area and reflectivity of the cavity
surface etching groove. The length of the resonant cavity area
determines a free spectral range, that is, an interval between two
adjacent peak points on the curve in FIG. 5. The reflectivity of
the cavity surface etching groove determines steepness of the
curve. Transmissivity of the etalon is obtained by using the
following formula:
T = 1 1 + F * sin ( .DELTA. 2 ) 2 , where ##EQU00001## .DELTA. = 2
* .pi. .lamda. * 2 * L , F = 4 * r 1 - r 2 , and r = 1 - T ;
##EQU00001.2##
[0025] .lamda. is a wavelength of a light wave that is incident to
the etalon, L is a distance between the flat glass reflection
surfaces of the etalon or a length of the resonant cavity area, and
r is reflectivity of the flat glass reflection surface of the
etalon or reflectivity of the cavity surface etching groove.
[0026] In a specific implementation process, wavelengths of the
signal light and the local oscillator light may be respectively
located at two points A and B on the spectral curve in FIG. 5.
Transmissivity of the signal light and transmissivity of the local
oscillator light are different, and may be calculated by using a
current intensity value measured by a photoelectric detector.
[0027] The transmissivity of the signal light is obtained by using
the following formula:
T.sub.1=I.sub.PD1/I.sub.PD2.
[0028] The transmissivity of the local oscillator light is obtained
by using the following formula:
T.sub.2=I.sub.PD3/I.sub.PD4.
[0029] It may be learned from the foregoing formulas and the etalon
spectral transmission curve graph in FIG. 5 that the frequency
offset estimation unit 28 sets the wavelength values of the signal
light and the local oscillator light to a linear or logarithmic
linear middle part of the etalon spectral transmission curve by
using a control circuit. In this way, a linear response change
signal of the transmissivity T is obtained regardless of a specific
drift direction of the wavelength .lamda.. The transmissivity T
generated when the signal light or the local oscillator light
passes through the etalon and the wavelength .lamda. of the signal
light or local oscillator light are in a one-to-one
correspondence.
[0030] The frequency offset estimation unit 28 outputs, to the
frequency offset compensation unit 29, a feedback signal that
reflects a frequency offset value between the signal light and the
local oscillator light. Specifically, the feedback signal that
reflects the frequency offset value between the signal light and
the local oscillator light and that is output by the frequency
offset estimation unit 28 includes current intensity values
I.sub.PD1, I.sub.PD2, I.sub.PD3, and I.sub.PD4 that are
respectively output by a first photoelectric detector (34 or 45), a
second photoelectric detector (35 or 46), a third photoelectric
detector (36 or 47), and a fourth photoelectric detector (37 or
48). The frequency offset compensation unit 29 specifically
includes two implementations:
[0031] One implementation is as follows: The frequency offset
compensation unit 29 performs the following steps, so that the
frequency offset value between the local oscillator light and the
signal light meets a system requirement: obtaining I.sub.PD1,
I.sub.PD2, I.sub.PD3, and I.sub.PD4 from the frequency offset
estimation unit 28; separately calculating ratios of
I.sub.PD1/I.sub.PD2 and I.sub.PD1/I.sub.PD4; then calculating a
difference .DELTA. of I.sub.PD1/I.sub.PD2-I.sub.PD3/I.sub.PD4;
further obtaining the wavelength of the signal light because the
transmissivity of the signal light may be obtained according to
I.sub.PD1/I.sub.PD2; and further obtaining the wavelength of the
local oscillator light because the transmissivity of the local
oscillator light may be obtained according to I.sub.PD3/I.sub.PD4.
The difference .DELTA. indicates the frequency offset value between
the signal light and the local oscillator light. A control signal
is generated according to a magnitude and a direction of the
difference .DELTA., so as to perform wavelength adjustment on the
local oscillator laser 23. A wavelength adjustment process is
generally divided into two steps: coarse adjustment and fine
adjustment. In an initial stage, the difference .DELTA. is
relatively large, and the wavelength difference between the signal
light and the local oscillator light is relatively large. In this
case, the coarse adjustment is performed. When the difference
.DELTA. becomes smaller, and the wavelength difference between the
signal light and the local oscillator light becomes smaller, the
fine adjustment is performed. The adjustment process is not
completed until the frequency offset value between the signal light
and the local oscillator light meets the system requirement.
[0032] The other implementation is as follows: The frequency offset
compensation unit 29 performs the following steps, so as to perform
coarse adjustment on the frequency offset value between the signal
light and the local oscillator light: obtaining I.sub.PD1,
I.sub.PD2, I.sub.PD3, and I.sub.PD4 from the frequency offset
estimation unit 28; separately calculating ratios of
I.sub.PD1/I.sub.PD2 and I.sub.PD3/I.sub.PD4; then calculating a
difference .DELTA. of I.sub.PD1/I.sub.PD2-I.sub.PD3/I.sub.PD4;
further obtaining the wavelength of the signal light because the
transmissivity of the signal light may be obtained according to
I.sub.PD1/I.sub.PD2; and further obtaining the wavelength of the
local oscillator light because the transmissivity of the local
oscillator light may be obtained according to I.sub.PD3/I.sub.PD4.
The difference .DELTA. indicates the frequency offset value between
the signal light and the local oscillator light. A control signal
is generated according to a magnitude and a direction of the
difference .DELTA., so as to perform wavelength adjustment on the
local oscillator laser 23, until the frequency offset value between
the signal light and the local oscillator light is less than +/-5
GHz, that is, the wavelength difference between the signal light
and the local oscillator light is approximately 0.04 nm, and falls
within a range of a compensation capability of a DSP. Then the DSP
further calculates the frequency offset value between the signal
light and the local oscillator light, and generates a control
signal according to a magnitude and a direction of the frequency
offset value between the signal light and the local oscillator
light, so as to perform fine adjustment on the local oscillator
laser 23, so that the frequency offset value between the signal
light and the local oscillator light meets the system
requirement.
[0033] In this embodiment, in a coherent receiver, signal light and
local oscillator light pass through a frequency offset estimation
unit, so that a feedback signal that reflects a frequency offset
value between the signal light and the local oscillator light is
obtained; a frequency offset compensation unit obtains the
frequency offset value between the signal light and the local
oscillator light according to the feedback signal, and performs
wavelength adjustment on a local oscillator laser according to the
frequency offset value, so that the frequency offset value between
the signal light and the local oscillator light meets a system
requirement, thereby implementing frequency offset estimation and
compensation for signal light and local oscillator light in a
coherent system when a laser without a wavelength locker is used as
a light source of the signal light and the local oscillator light,
and reducing costs of the coherent system.
[0034] FIG. 6 is a demonstrative flowchart of a method for coherent
light source frequency offset estimation and compensation according
to an embodiment of the present disclosure. Specifically, the
method may be performed by a coherent receiver. The coherent
receiver may include a frequency offset estimation unit and a
frequency offset compensation unit. The method specifically
includes the following steps.
[0035] S601: A first photoelectric detector receives signal light
after the signal light passes through an etalon, and a first
current intensity value is obtained; a second photoelectric
detector receives the signal light, and a second current intensity
value is obtained.
[0036] S601 may be performed by the frequency offset estimation
unit. Specifically, the frequency offset estimation unit includes
the etalon. In a specific implementation process, when the etalon
may be an air type etalon, the signal light is split into two parts
by a first beamsplitter before passing through the etalon. One part
of the signal light is received by the first photoelectric detector
after passing through the etalon, and the first current intensity
value is obtained; and the other part of the signal light is
directly received by the second photoelectric detector without
passing through the etalon, and the second current intensity value
is obtained. Optionally, the first beamsplitter may be replaced by
a first optical waveguide. In this case, the etalon may be a
waveguide type etalon.
[0037] S602: A third photoelectric detector receives local
oscillator light after the local oscillator light passes through
the etalon, and a third current intensity value is obtained, and a
fourth photoelectric detector receives the local oscillator light,
and a fourth current intensity value is obtained.
[0038] S602 may be performed by the frequency offset estimation
unit. In a specific implementation process, when the etalon may be
an air type etalon, the etalon passed through by the local
oscillator light and the etalon passed through by the signal light
are the same etalon. The local oscillator light is split into two
parts by a second beamsplitter before passing through the etalon.
One part of the local oscillator light is received by the third
photoelectric detector after passing through the etalon, and the
third current intensity value is obtained; and the other part of
the signal light is directly received by the fourth photoelectric
detector without passing through the etalon, and the fourth current
intensity value is obtained. Optionally, the second beamsplitter
may be replaced by a second optical waveguide. In this case, the
etalon may be a waveguide type etalon, and has a spectral
transmission curve same as that of the etalon in S601.
[0039] S603: Obtain a first ratio according to a ratio of the first
current intensity value to the second current intensity, obtain a
second ratio according to a ratio of the third current intensity
value to the fourth current intensity value, and obtain a frequency
offset value between the signal light and the local oscillator
light according to a difference between the first ratio and the
second ratio, where the frequency offset value instructs to perform
frequency offset compensation on the signal light and the local
oscillator light.
[0040] S603 may be performed by the frequency offset compensation
unit. Specifically, the first ratio is obtained according to the
ratio of the first current intensity value to the second current
intensity, and the second ratio is obtained according to the ratio
of the third current intensity value to the fourth current
intensity. Because transmissivity of the signal light in the etalon
may be obtained according to the first ratio, so as to obtain a
wavelength of the signal light, and transmissivity of the local
oscillator light in the etalon may be obtained according to the
second ratio, so as to obtain a wavelength of the local oscillator
light, the difference between the first ratio and the second ratio
indicates the frequency offset value between the signal light and
the local oscillator light. Further, the frequency offset
compensation unit performs, according to the frequency offset value
between the signal light and the local oscillator light, wavelength
adjustment on a laser that generates the local oscillator light, so
as to compensate the frequency offset value between the local
oscillator light and the signal light, until the frequency offset
value between the signal light and the local oscillator light meets
a system requirement. Optionally, the frequency offset compensation
unit may perform coarse adjustment on the wavelength of the local
oscillator light, so that the frequency offset value between the
signal light and the local oscillator light is less than +/-5 GHz,
and the wavelength difference between the signal light and the
local oscillator light is approximately 0.04 nm, and falls within a
range of a compensation capability of a DSP. Then the DSP further
performs fine adjustment on the wavelength of the local oscillator
light, so that the frequency offset value between the signal light
and the local oscillator light meets the system requirement.
[0041] In this embodiment, in a coherent receiver, signal light and
local oscillator light pass through a frequency offset estimation
unit, so that a feedback signal that reflects a frequency offset
value between the signal light and the local oscillator light is
obtained; a frequency offset compensation unit obtains the
frequency offset value between the signal light and the local
oscillator light according to the feedback signal, and performs
wavelength adjustment on a local oscillator laser according to the
frequency offset value, so that the frequency offset value between
the signal light and the local oscillator light meets a system
requirement, thereby implementing frequency offset estimation and
compensation for signal light and local oscillator light in a
coherent system when a laser without a wavelength locker is used as
a light source of the signal light and the local oscillator light,
and reducing costs of the coherent system.
[0042] A person of ordinary skill in the art may be aware that, in
combination with the examples described in the embodiments
disclosed in this specification, units and algorithm steps may be
implemented by electronic hardware or a combination of computer
software and electronic hardware. Whether the functions are
performed by hardware or software depends on particular
applications and design constraint conditions of the technical
solutions. A person skilled in the art may use different methods to
implement the described functions for each particular application,
but it should not be considered that the implementation goes beyond
the scope of the present disclosure.
[0043] The foregoing descriptions are merely specific
implementations of the present disclosure, but are not intended to
limit the protection scope of the present disclosure. Any variation
or replacement readily figured out by a person skilled in the art
within the technical scope disclosed in the present disclosure
shall fall within the protection scope of the present disclosure.
Therefore, the protection scope of the present disclosure shall be
subject to the protection scope of the claims.
[0044] The foregoing are merely example embodiments of the present
disclosure. A person skilled in the art may make various
modifications and variations to the present disclosure without
departing from the spirit and scope of the present disclosure.
* * * * *