U.S. patent application number 10/200183 was filed with the patent office on 2002-11-28 for method and arrangement in connection with optical bragg-reflectors.
This patent application is currently assigned to Proximion Fiber Optics AB.. Invention is credited to Asseh, Adel, Bergman, Mikael, Sahlgren, Bengt, Sandgren, Simon, Stubbe, Raoul.
Application Number | 20020176125 10/200183 |
Document ID | / |
Family ID | 24426457 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020176125 |
Kind Code |
A1 |
Asseh, Adel ; et
al. |
November 28, 2002 |
Method and arrangement in connection with optical
bragg-reflectors
Abstract
A method of establishing transmission of light through a chirped
Bragg-reflector, a method of analyzing the power spectrum of a
light signal using a chirped Bragg-reflector, and an arrangement
for analyzing the power spectrum of a light signal. The
Bragg-reflector reflects, in an unperturbed state, essentially all
incident light within a predefined wavelength range. The methods
include the steps of directing the light to be analyzed into an
input end of a light-guiding structure, such as an optical fiber,
which light-guiding structure is provided with a Bragg-reflector,
and sending an acoustic pulse along the light-guiding structure,
thereby effectively lowering the reflectance of the Bragg-reflector
for a certain wavelength at a certain time. By monitoring the light
thus transmitted through the Bragg-reflector, a power spectrum
analysis of the incident light is obtained.
Inventors: |
Asseh, Adel; (Stockholm,
SE) ; Bergman, Mikael; (Jarfalla, SE) ;
Sahlgren, Bengt; (Saltsjo-Boo, SE) ; Sandgren,
Simon; (Stockholm, SE) ; Stubbe, Raoul;
(Stocksund, SE) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Proximion Fiber Optics AB.
|
Family ID: |
24426457 |
Appl. No.: |
10/200183 |
Filed: |
July 23, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10200183 |
Jul 23, 2002 |
|
|
|
09606084 |
Jun 29, 2000 |
|
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Current U.S.
Class: |
359/10 ;
385/1 |
Current CPC
Class: |
G01J 3/1256 20130101;
G02F 2201/307 20130101; G02F 1/125 20130101 |
Class at
Publication: |
359/10 ;
385/1 |
International
Class: |
G02F 001/01 |
Claims
What is claimed is:
1. A method of establishing transmission of light, comprising the
steps of: providing a light-guiding structure incorporating a
chirped Bragg-reflector, directing light into an input end of said
light-guiding structure, and sending a longitudinal acoustic pulse
into said light-guiding structure for propagation along the same,
the acoustic pulse being such that, for each location of said
acoustic pulse in said Bragg-reflector, the reflectivity for a
wavelength associated with said location in said Bragg-reflector is
altered.
2. The method of claim 1, wherein the propagation of the acoustic
pulse along said light-guiding structure causes an alteration of a
local grating period of said chirped Bragg-reflector to provide,
for each location of said acoustic pulse in said Bragg-reflector,
transmission of a wavelength associated with said location, thereby
providing a wavelength scan of transmitted wavelengths as the
acoustic pulse propagates along said Bragg-reflector.
3. The method of claim 1, wherein the acoustic pulse has a first
part and a second part, said first part causing a temporary
increase of the local grating period of the chirped
Bragg-reflector, and said second part causing a temporary decrease
of the local grating period of said Bragg-reflector.
4. The method of claim 3, wherein the portion of said chirped
Bragg-reflector containing the first part of said acoustic pulse
has, in its unperturbed state, a longer grating period than the
portion of said Bragg-reflector containing the second part of said
acoustic pulse.
5. The method of claim 1, wherein the step of sending a
longitudinal acoustic pulse through the light-guiding structure is
performed repeatedly, whereby a plurality of acoustic pulses is
sent through said light-guiding structure, of which plurality of
acoustic pulses only one acoustic pulse is present in the chirped
Bragg-reflector at any one instant.
6. The method of claim 1, further comprising the step of detecting,
at an output end of the light-guiding structure, the intensity of
the light transmitted, the light detected at a certain instant in
time corresponding to a certain wavelength, thereby allowing
analysis of the power spectrum of the light directed into said
light-guiding structure.
7. The method of claim 6, further including the step of
prefiltering the light to be directed into the light-guiding
structure by means of a broadband filter, said filter passing
essentially the same wavelength range as the chirped
Bragg-reflector reflects.
8. The method of claim 3, wherein either one of the first and the
second part of the acoustic pulse is predominant, the acoustic
pulse thereby causing mainly an increase or mainly a decrease of
the local grating period of the Bragg-reflector.
9. The method of claim 1, wherein said light-guiding structure is
the core of an optical fiber.
10. An arrangement for analyzing the power spectrum of a light
signal, comprising: a light-guiding structure, having an input end
and an output end; a chirped Bragg-reflector in said light-guiding
structure, which chirped Bragg-reflector is provided between the
input end and the output end; an acoustic actuator connected to
said light-guiding structure; and a detector provided at the output
end of said light-guiding structure, said detector delivering a
detector output signal, wherein the input end of said light-guiding
structure is arranged to receive the light signal to be analyzed,
said acoustic actuator is operative to emit a longitudinal acoustic
pulse for propagation along the light-guiding structure, and said
detector is operative to detect the light thus transmitted through
said Bragg-reflector and to provide a detector output signal
associated with the power spectrum of said light signal.
11. The arrangement of claim 10, further including a broadband
filter provided in the light path prior to the input end of the
light-guiding structure, said filter being arranged to pass only
light of a desired wavelength range to said light-guiding
structure.
12. The arrangement of claim 11, wherein the broadband filter is
arranged to pass essentially the same wavelength range as the
chirped Bragg-reflector reflects.
13. The arrangement of claim 10, further comprising a logic unit
connected to said acoustic actuator, for supplying actuating pulses
to said acoustic actuator.
14. The arrangement of claim 13, further comprising a processing
unit, which is arranged to control the operation of the logic
unit.
15. The arrangement of claim 13, wherein the logic unit is arranged
to perform basic signal analysis of the detector output signal.
16. The arrangement of claim 15, wherein the basic signal analysis
performed by the logic unit is utilized for the control of the
acoustic actuator.
17. The arrangement of claim 10, wherein the light-guiding
structure is clamped at a first point and a second point thereof,
thereby defining a first and a second reflection point, at which
reflection points the acoustic pulse is reflected, and effectively
providing multiple passages of the acoustic pulse through the
Bragg-reflector.
18. The arrangement of claim 17, wherein the acoustic actuator is
connected to the light-guiding structure at either one of the first
and the second reflection points.
19. The arrangement of claim 18, wherein the acoustic actuator is
arranged to emit a longitudinal acoustic pulse into the
light-guiding structure at the time when a previous acoustic pulse,
after its reflection, arrives at the point where said actuator is
connected to the light-guiding structure.
20. A method of analyzing the power spectrum of a light signal,
comprising the steps of: directing the light signal to be analyzed
into a light-guiding structure incorporating a chirped
Bragg-reflector; sending a longitudinal acoustic pulse into said
light-guiding structure for propagation along the same, the
acoustic pulse being such that, for each location of said acoustic
pulse in said Bragg-reflector, the reflectivity for a wavelength
associated with said location in said Bragg-reflector is altered;
monitoring the light thus transmitted through said light-guiding
structure; and analyzing the monitored light, in order to obtain a
power spectrum analysis of the light signal directed into the
light-guiding structure.
21. The method of claim 20, wherein the propagation of the acoustic
pulse along said light-guiding structure causes an alteration of
the local grating period of said chirped Bragg-reflector to
provide, for each location of said acoustic pulse in said
Bragg-reflector, transmission of a wavelength associated with said
location, thereby providing a wavelength scan of transmitted
wavelengths as the acoustic pulse propagates along said
Bragg-reflector.
22. The method of claim 20, wherein the acoustic pulse has a first
part and a second part, said first part causing a temporary
increase of the local grating period of the Bragg-reflector, and
said second part causing a temporary decrease of the local grating
period of said Bragg-reflector.
23. The method of claim 22, wherein the portion of said chirped
Bragg-reflector containing the first part of said acoustic pulse
has, in its unperturbed state, a longer grating period than the
portion of said Bragg-reflector containing the second part of said
acoustic pulse.
24. The method of claim 20, wherein the step of monitoring the
light transmitted through the light-guiding structure includes
detecting said transmitted light by means of a detector, the
detector providing a detector output signal associated with the
power of the transmitted light signal.
25. The method of claim 20, wherein said light-guiding structure is
the core of an optical fiber.
26. The method of claim 24, wherein the step of sending a
longitudinal acoustic pulse along the light-guiding structure is
performed repeatedly, whereby a plurality of acoustic pulses is
sent along said light-guiding structure, of which plurality of
acoustic pulses only one acoustic pulse is present in the chirped
Bragg-reflector at any one instant.
27. The method of claim 26, wherein each acoustic pulse is
reflected back and forth between two reflection points in the
light-guiding structure, each successive pulse of said plurality of
acoustic pulses being sent into the light-guiding structure at the
time when the previous acoustic pulse, after its reflection,
reaches the acoustic actuator.
28. The method of claim 27, wherein the wavelength scan obtained by
a passage of an acoustic pulse through the chirped Bragg-reflector
is utilized together with a subsequent wavelength scan for spectrum
analysis of the light signal directed into the light-guiding
structure.
29. The method of claim 28, wherein correlation between different
wavelength scans in the detector output signal is performed in
order to interpret said detector output signals, to ascribe the
detector output at a certain instant to a specific wavelength.
30. The method of claim 20, further including the step of
pre-filtering the light to be directed into the light-guiding
structure by means of a broadband filter, said filter passing
essentially the same wavelength range as the chirped
Bragg-reflector reflects.
31. The method of claim 24, wherein the detector output signal is
fed to a logic unit, said logic unit processing said detector
output signal to provide a logic unit output signal indicative of
the power spectrum of the light signal directed into the
light-guiding structure.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a continuation of application Ser. No. 09/606,084,
filed Jun. 29, 2000, the disclosure of which is hereby incorporated
by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to optical Bragg-reflectors,
and more particularly to the alteration of the reflective
properties of optical Bragg-reflectors.
BACKGROUND OF THE INVENTION
[0003] The need for capacity in communications networks is
increasing exponentially, and presently capacity demands are
doubling every eighteen months or even less. Efforts are being made
in the telecom and computer business to meet this incredible
expansion of bandwidth requirements.
[0004] One obvious action is to simply draw new optical cables
between nodes of the network. However, this approach is costly and
hardly a successful way of meeting the upcoming demands. Instead,
measures are focused, when possible, on augmenting the capacity of
existing fiber networks.
[0005] One widespread method of achieving increased transfer
capacity in existing fiber networks is wavelength division
multiplexing (WDM). In WDM, a single optical fiber is used for the
transmission of several channels of information, each channel being
associated with a specific wavelength. For a background of this
technology, the reader is directed to, "A review of WDM technology
and applications", Opt. Fiber Technol., 5, pp. 3-39 (1999).
[0006] While the use of WDM in optical networks yields a
substantial increase in transfer capacity, the complexity of the
communications systems increases correspondingly. Furthermore, the
number of physical components in each link of the network
increases, the requirements on each component increase, and
detecting and localizing errors or imperfections in the network is
rendered more difficult.
[0007] The optical networks of today mainly use point-to-point
communication, wherein electronically encoded information in one
node of the network is transformed into optically encoded
information, and then transferred, through a sequence of optical
fiber amplifiers and optical transport fibers, to another node,
where the optically encoded information is transformed back into
electrically encoded information.
[0008] However, this prior art is associated with major drawbacks
and limitations.
[0009] If, for instance, an error occurs in one of the fiber
amplifiers, it is very hard to determine which amplifier, in the
sequence of amplifiers, is the faulty one.
[0010] Furthermore, technology is being pushed towards
communications systems in which the optical signal is passed via
all-optical switching nodes, where the signal is routed without any
need for optical-to-electrical conversion or vice versa. Thus, if
an error is detected in a receiving node of the communications
network, it is extremely hard to localize the origin of error.
[0011] If the light signal propagating in an optical transport
fiber could be analyzed in an accurate and simple fashion, without
interfering with the signal, any flaws in the optical
communications network could be detected at an early stage. In the
prior art, such analysis has been too complicated, too expensive
and too inaccurate to gain widespread acceptance.
[0012] Obviously, it is of utmost importance to be able to monitor
the power spectrum of the light propagating in the optical
transport fiber.
[0013] It is known in the prior art to use a phase grating in an
optical fiber in order to filter out one desired wavelength from a
broadband light signal. A phase grating will reflect one predefined
wavelength and leave other wavelengths essentially undisturbed.
[0014] An optical phase grating is a structure of essentially
periodically varying refractive index in an optically transparent
medium. An overview of the technology is given in M. C. Hutley,
"Diffraction gratings", Academic Press, London (1982). When light
is incident on an optical phase grating, a small part of the
incident light is reflected off each grating element (period). When
a multiplicity of grating elements are arranged after each other
(i.e., arranged as a phase grating), the total reflected light
would be the sum of all these single reflections. The fraction of
the incident light reflected off each grating element is determined
by the depth (amplitude) of the refractive index modulation in the
phase grating. The deeper the modulation, the larger fraction of
the incident light is reflected off each grating element. If the
incident light is essentially normal to the grating (i.e., to the
grating elements), the grating is said to act in the Bragg domain
and is named "Bragg grating". Light reflected off each grating
element will thus overlap the light reflected off the other
elements, causing interference. For a certain wavelength, all these
reflections are in phase, whereby constructive interference is
effected. Although each reflection is small, a substantial
reflection is obtained due to constructive interference. The
wavelength for which constructive interference is effected is
called the "Bragg wavelength," .lambda..sub.Bragg, and is given by
(at normal angle of incidence)
.lambda..sub.bragg=2n.LAMBDA.
[0015] where n is the average of the refractive index, and .LAMBDA.
is the grating period.
[0016] If the grating period, .LAMBDA., varies along the grating,
the grating is said to be a "chirped" Bragg grating. In a chirped
Bragg grating, different wavelengths are reflected in different
portions of the grating, in fact, making the Bragg grating a
broadband reflector, a chirped Bragg-reflector.
[0017] U.S. Pat. No. 6,052,179 discloses a system for determining
the average wavelength of light transmitted through an optical
fiber. In this system, a chirped Bragg grating is provided in an
optical fiber, the grating having a modulation amplitude that
varies from a first end to a second end of the fiber grating. Thus,
the reflectivity at one wavelength is different from the
reflectivity at another wavelength, since different wavelengths are
reflected at different portions of the grating. Based on the output
from two photo detectors, one detecting the light transmitted
through the grating and the other acting as a reference, the
average wavelength of light transmitted through the grating is
determined. However, this approach has several disadvantages.
Firstly, the light to be analyzed has to be divided into two
separate fibers, which is a serious complication in itself.
Secondly, only the average wavelength can be determined. There is
no possibility for proper spectrum analysis by the system disclosed
in the above reference. Furthermore, the system is intended for
sensing applications, where external influence changes the average
wavelength coming into the system.
[0018] Previous attempts to alter the reflective properties of an
optical Bragg grating include creation of an acousto-optic
superlattice imposed on a chirped Bragg grating. Such an
arrangement is described by Chen et al. in "Superchirped moir
grating based on an acousto-optic superlattice with a chirped fiber
Bragg grating", OPTICS LETTERS, Vol. 24, No. 22, pp. 1558-1560.
According to this reference, multiple transmission peaks are
obtained by superimposing an acoustic wave on a chirped Bragg
grating. The spacing of the transmission peaks is varied by the
acoustic frequency. However, this arrangement does not permit
transmission of one single wavelength only, and the variation of
the spacing of the transmission peaks by acoustic frequency is very
limited.
SUMMARY OF THE INVENTION
[0019] The present invention provides new methods and arrangements
for establishing transmission of light through a reflecting Bragg
grating, and for utilizing such transmission for analysis of the
characteristics of a light signal. The drawbacks and limitations
associated with the prior art are effectively eliminated by a
method and an arrangement of the general kind set forth in the
accompanying claims.
[0020] The present invention has further advantages, which will be
apparent from the detailed description set forth below.
[0021] It is a general object of the present invention to provide a
method of establishing transmission of light through a broadband,
chirped Bragg-reflector. The method can be used for analysis of the
power spectrum of a light signal, as well as for other applications
where it is desirable to transmit a certain wavelength component of
light through a structure at a certain instant in time.
[0022] Furthermore, it is an object of the present invention to
provide a method and an arrangement for spectrum analysis of a
light signal, which method and arrangement essentially eliminate
the aforementioned drawbacks and limitations of the prior art.
Briefly stated, this is obtained by establishing transmission of
light through a chirped Bragg-reflector by means of a longitudinal
acoustic pulse being present in the grating structure. The presence
of the acoustic pulse alters the reflective properties of the
Bragg-reflector, thus giving rise to transmission of a specific
wavelength of light for each position of the acoustic pulse in the
Bragg-reflector.
[0023] According to a first aspect of the present invention, a
method of establishing transmission of light through a chirped
Bragg-reflector is provided, by which method a certain wavelength
component is transmitted through the Bragg-reflector at a certain
(corresponding) instant in time. In an unperturbed state, the
Bragg-reflector is reflecting essentially all incident light within
a predefined wavelength range. According to the invention, light is
incident into an optical waveguide incorporating a chirped
Bragg-reflector. The reflective properties of said Bragg-reflector
are altered by sending a longitudinal acoustic pulse into said
waveguide for propagation along the same. For each location of said
acoustic pulse in the chirped Bragg-reflector, the reflectivity for
a wavelength associated with said location in said Bragg-reflector
is altered, thereby establishing transmission of the wavelength at
issue.
[0024] According to a second aspect of the present invention, a new
method of analyzing the power spectrum of a light signal is
provided. The method of analyzing the power spectrum is based on
the aforementioned method of establishing transmission of light
through a chirped Bragg-reflector. Briefly stated, analysis of the
power spectrum is obtained by monitoring the light transmitted
through the Bragg-reflector, and by subsequent analysis of the
monitored signal.
[0025] One advantage of this method is that it is sufficient to tap
off only a small part (typically about 1% or less) of the light
propagating in an optical transport fiber into a secondary fiber.
The light in the secondary fiber is then analyzed, and the
interfering effect on the transport fiber is negligible. The light
is tapped off from the transport fiber by non-wavelength
discriminating coupling means, as known in the art. By analyzing
the light in the secondary fiber by the method according to the
present invention, the power spectrum of the light signal
propagating in said transport fiber is determined with very high
accuracy. Furthermore, the light signal in the transport fiber is
essentially undisturbed, apart from the 1% tapped off.
[0026] Another advantage of the present invention is that it also
allows for real time supervision of a fiber-based communications
system. The information obtained from such supervision can
advantageously be utilized for controlling other equipment
connected to said system, such as amplifiers, filters, etc. In-line
amplifiers and filters can be controlled in a feedback
configuration with a spectrum analyzer according to the present
invention.
[0027] Yet another advantage of the present invention is that a
wavelength scan of transmitted light is obtained, which is very
convenient for spectrum analysis. The present invention can provide
for repeated wavelength scans, thereby facilitating interpretation
and analysis of the monitored signal.
[0028] According to a third aspect of the present invention, an
arrangement is provided for measuring and characterizing the power
spectrum of a light signal propagating in an optical fiber. Such an
arrangement includes a chirped Bragg-reflector operating in
accordance with the methods above.
[0029] The present invention is based on the general insight that
the reflective properties of a chirped Bragg grating (or chirped
Bragg-reflector) can be altered by a longitudinal acoustic pulse
propagating along the grating. More particularly, the present
invention is further based on the deeper insight that a
longitudinal acoustic pulse of carefully chosen shape propagating
along the grating can give highly accurate, and wavelength
separated in time, transmission through the chirped Bragg
grating.
[0030] According to one embodiment of the present invention, a
method of establishing transmission of light through a chirped
Bragg-reflector includes the steps of directing light into a
light-guiding structure provided with said Bragg-reflector, and
sending a longitudinal acoustic pulse along said light-guiding
structure, thereby locally and temporarily altering the reflective
properties of the Bragg-reflector for a certain wavelength
corresponding to the momentary position of the travelling acoustic
pulse.
[0031] According to another embodiment of the present invention, an
optical fiber is provided with a chirped Bragg grating. The fiber
has an input end and an output end, for the input of a light signal
into said fiber and for the output of a transmitted part of said
light signal, respectively. Connected to the optical fiber is an
acoustic actuator for the emission of a longitudinal acoustic pulse
into said fiber. The acoustic pulse is given a shape that has the
effect of lowering the reflectance of the chirped Bragg-reflector
for a certain wavelength at a certain instant in time. Preferably,
the acoustic pulse is given an anti-symmetric shape, whereby an
etalon effect is achieved in the proximity of the travelling
acoustic pulse, thereby locally lowering the reflectivity of the
chirped Bragg-reflector to essentially zero.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The following detailed description of preferred embodiments
is better understood when read in conjunction with the accompanying
schematic drawings, in which:
[0033] FIG. 1 is a schematic outline of the basic principle behind
the present invention,
[0034] FIG. 2 shows a typical transmittance profile of a chirped
Bragg-reflector in an unperturbed state,
[0035] FIG. 3 illustrates a transmittance peak caused by the
presence of an acoustic pulse in the chirped Bragg-reflector that
sweeps through the reflectance band of the Bragg-reflector,
[0036] FIG. 4 is an enlarged view of the transmittance peak caused
by the presence of an acoustic pulse in the Bragg-reflector,
[0037] FIG. 5 shows an overview of an arrangement for spectrum
analysis according to the present invention,
[0038] FIG. 6 shows the acoustic actuator and the fiber arrangement
according to the present invention,
[0039] FIGS. 7-9 shows the attachment of the fiber to the acoustic
actuator in greater detail,
[0040] FIG. 10 shows the shape of an acoustic pulse in accordance
with the present invention, and
[0041] FIG. 11 shows a detector output signal and illustrates a
correlation interpretation method according to the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0042] Typically, and in accordance with one embodiment of the
present invention, a chirped Bragg-reflector is provided in an
optical fiber. As described above, the chirped Bragg-reflector is a
distributed structure that reflects light within a predefined
wavelength range. The wavelength reflected in each portion of the
distributed structure depends on the local period of the grating
constituting the Bragg-reflector. The grating period is defined as
the distance between two adjacent grating elements. The shorter the
grating period, the shorter the wavelength that is reflected.
[0043] In a chirped grating, the grating period varies along the
grating, usually in a steady fashion (e.g., from a short period to
a longer period, or from a long period to a shorter period). The
period can vary continuously or stepwise. In a portion of the
grating (i.e., the chirped Bragg-reflector) where the grating
period is short, a short wavelength is reflected, and in a portion
of the grating where the grating period is long, a long wavelength
is reflected. In effect, different wavelengths are reflected in
different portions of the chirped Bragg-reflector.
[0044] Compression and/or elongation of the fiber can alter the
reflective properties of the chirped Bragg-reflector. According to
the present invention, this is accomplished by means of a
longitudinal acoustic pulse which causes temporary compression
and/or elongation of the fiber as it propagates along the fiber. In
fact, such compression or elongation introduces both refractive
index changes and actual path changes, the latter, however, being
predominant. The aggregate effect on light is a combination of the
change in refractive index and the change of the actual period of
the grating in the fiber. From a phenomenological point of view,
the crucial quantity is the optical path traversed by the light in
the fiber. The optical path is defined as the actual, physical path
multiplied by the refractive index of the medium in which the light
is propagating. For example, an increase of the effective grating
period in a Bragg grating in an optical fiber could be obtained
either by increasing the refractive index of the fiber, or by
increasing the physical length of the period (i.e., the fiber).
[0045] When stretching or compressing an optical fiber provided
with a Bragg-reflector, the change in refractive index is
negligible compared to the actual change in physical path.
Therefore, if any portion of a chirped Bragg-reflector is
stretched, this portion will reflect a longer wavelength than it
would do in its unperturbed state. This actually applies for any
Bragg-reflector, even with a constant grating period.
[0046] The basic principle behind the present invention will now be
described in greater detail with reference to FIG. 1.
[0047] An optical fiber 10 incorporates, in the core 102 thereof, a
chirped Bragg grating 104. The Bragg grating is incorporated into
the optical fiber by means of methods known in the art. The fiber
10 has an input end 11 and an output end 12, for receiving input of
a light signal and for delivering an output signal, respectively.
The chirped Bragg grating exhibits a modulation that is strong
enough to reflect essentially all incident light within a
predefined wavelength range when the grating is in an unperturbed
state. Due to this fact, the chirped Bragg grating is referred to
as a chirped "Bragg-reflector."In this case, it is assumed that the
Bragg-reflector reflects all light wavelengths between
.lambda..sub.0 and .lambda..sub.k. Any wavelength of light within
this range will be reflected if sent into the optical fiber
incorporating said Bragg-reflector.
[0048] In order to establish transmission of light within this
wavelength range (.lambda..sub.0 to .lambda..sub.k) through the
chirped Bragg-reflector, a light signal is first directed into the
input end 11 of the optical fiber 10, as is indicated by the arrow
111. If the optical fiber, and thus the chirped Bragg-reflector, is
in an unperturbed state, the light will be reflected back towards
the input end 11, and no light will be transmitted to the output
end 12. A longitudinal acoustic pulse 120 is then sent along the
optical fiber. In this case, and as shown in the figure, the
longitudinal acoustic pulse 120 has an anti-symmetric shape,
thereby causing in one part a localized and temporary elongation of
the optical fiber 10 and, in another part, a localized and
temporary compression of the optical fiber 10. The acoustic pulse
120 alters the reflective properties of the chirped Bragg-reflector
104, and effectively establishes transmission of light through the
optical fiber 10.
[0049] In FIG. 1, the chirped Bragg-reflector is shown to have a
grating period that decreases from left to right. This means, as
described above, that a longer wavelength is reflected in the left
portion of the Bragg-reflector than in the right portion of the
Bragg-reflector. When the longitudinal acoustic pulse travels along
the grating from left to right, and hence locally and temporarily
alters the reflective properties of the reflector, longer
wavelengths will be transmitted first, followed by shorter
wavelengths.
[0050] The mechanism behind the alteration of the reflective
properties of the chirped Bragg-reflector will now be described in
more detail.
[0051] The acoustic pulse that is sent along the optical fiber has
one elongating part causing a temporary elongation of the optical
fiber, and one compressing part causing a temporary compression of
the optical fiber, as mentioned above. In the present example, the
acoustic pulse is essentially anti-symmetric, each of said parts
being equally important. When the acoustic pulse is sent along the
chirped Bragg-reflector (the optical fiber) in a direction in which
the grating exhibits a decreasing grating period, it is essential
that the acoustic pulse has a leading portion that causes a
decrease of the grating period (i.e., a portion that causes a
compression of the optical fiber) and a trailing portion that
causes an increase of the grating period (i.e., a portion that
causes an elongation of the optical fiber). The leading portion of
the acoustic pulse will then temporarily alter the local grating
period such that the altered local grating period actually gives a
reflection for a somewhat shorter wavelength than in its
unperturbed state. The trailing portion of the acoustic pulse will,
at the same time, temporarily alter the local grating period such
that the altered local grating period actually gives a reflection
for a somewhat longer wavelength than in its unperturbed state.
Effectively, a window is created where there is reduced or no
reflection, since the grating period of the chirped Bragg-reflector
is altered in such a way that there is no grating period giving
reflection for a certain wavelength. As the longitudinal acoustic
pulse propagates along the chirped Bragg-reflector, the above
situation will successively apply for shorter wavelengths, thereby
creating a wavelength scan of transmitted light.
[0052] If the acoustic pulse propagates along the chirped
Bragg-reflector in a direction with increasing grating period, the
acoustic pulse should, of course, have the opposite phase in order
to cause a similar effect as before.
[0053] When the acoustic pulse is given an anti-symmetric shape
with a sufficiently short rise time (or fall time) between the
compressive and the elongating portion, an etalon effect is
obtained, where the reflectivity for a certain wavelength drops
essentially to zero.
[0054] In the lower part of FIG. 1, the acoustic pulse is shown in
a coordinate system at two different instants. The
compression/elongation is illustrated as a change in longitudinal
dimension .DELTA.z at a first point in time t.sub.1 and a second
point in time t.sub.2. At time t.sub.1, the acoustic pulse is in a
first portion of the Bragg-reflector where the unperturbed grating
period is large, hence causing transmission of a long wavelength
.lambda..sub.1, through the optical fiber. At a later time t.sub.2,
the acoustic pulse has propagated to a second portion of the
Bragg-reflector where the unperturbed grating period is shorter
than in the first portion, hence causing transmission of a shorter
wavelength .lambda..sub.2 through the optical fiber. Consequently,
the output power at the output end 12 of the optical fiber at a
certain point in time is associated with the power of a
corresponding wavelength. In this example, the output power at time
t.sub.1, which is denoted P(t.sub.1), is associated with the power
of the incident light at wavelength .lambda..sub.1, which is
denoted P(.lambda..sub.1). Thus, P(t.sub.1)=P(.lambda..sub.1),
P(t.sub.2)=P(.lambda..sub.2), etc., any losses neglected.
[0055] In some cases, small leakage through the unperturbed
Bragg-reflector can be allowed, since compensating for such leakage
would be straightforward.
[0056] FIG. 2 shows schematically the transmittance of the
Bragg-reflector as a function of wavelength. Illustrated in the
figure is a reflection band, where there is essentially no
transmission. The full width at half maximum (FWHM) of this
reflection band is, in the exemplary embodiment, 40 nm. Typically,
some 30 nm of the reflection band exhibits no transmission at all
in the unperturbed state of the Bragg-reflector, i.e., practically
100 percent reflection. However, almost any width of the reflection
band could conceivably be obtained by choosing the grating period
and the chirp accordingly.
[0057] FIG. 3 illustrates a narrow transmittance peak that sweeps
through the reflection band as the acoustic pulse propagates along
the Bragg-reflector. In the figure, the transmittance peak is shown
to sweep from longer wavelengths to shorter (from left to right in
the figure, as indicated by the arrow), which is the case when the
acoustic pulse propagates through the chirped Bragg-reflector in a
direction from a longer grating period to a shorter grating period.
Of course, if the acoustic pulse was propagating in the opposite
direction through the chirped Bragg-reflector, the transmittance
peak would sweep in the opposite direction, i.e., from a shorter
wavelength to a longer wavelength. When the transmittance peak
sweeps through the reflection band of the chirped Bragg-reflector,
a wavelength scan of transmitted wavelengths is effectively
achieved.
[0058] FIG. 4 is an enlarged illustration of the transmittance
peak. The width of the transmittance peak can be chosen by proper
selection of chirp of the Bragg-reflector (i.e., the rate of change
of the grating period) and the modulation depth of the grating. The
transmittance peak could be as narrow as 40 pm. However, when the
present invention is utilized in a spectrum analyzer in a WDM
system, a transmittance peak of about 0.1 nm (FWHM) is sufficient
in order to separate the different channels. As indicated in FIG.
4, the transmittance peak can reach 100 percent by choosing the
proper system parameters.
[0059] FIG. 5 shows a schematic overview of a preferred embodiment
of an arrangement for analyzing the power spectrum of a light
signal according to the present invention.
[0060] The arrangement is utilized for analysis of the power
spectrum of a light signal propagating in an optical transport
fiber. A small fraction (typically about 1 percent) of the light
propagating in the optical transport fiber is tapped of by a fiber
coupler. The light tapped off is subsequently directed into a fiber
provided with a chirped Bragg-reflector. Transmission of light
through the chirped Bragg-reflector is achieved in accordance with
the inventive method described above by means of sending an
acoustic pulse along the fiber. The light thus transmitted is
monitored and analyzed in order to provide spectrum analysis of the
light signal directed into the fiber.
[0061] The preferred arrangement according to the present invention
will now be described in greater detail with reference to FIG.
5.
[0062] The arrangement is operatively connected to an optical
transport fiber 500 through a fiber coupler 510. The coupler 510 is
a tap coupler tapping off 1 percent of the power of the light
propagating in the optical transport fiber 500. The coupler 510 is
non-discriminating in terms of wavelength, which means that each
wavelength component propagating in the fiber 500 is tapped off
with the same coupling factor. Effectively, the power spectrum of
the 1 percent tapped off has the same profile as the power spectrum
of the original light signal propagating in the transport fiber
500, but at a lower peak power.
[0063] The light tapped off the transport fiber 500 is directed
into a secondary fiber 502 and, incident upon a broadband
Bragg-reflector 520, incorporated into the fiber 502, which
Bragg-reflector operates as a broadband filter for incident light.
Only light within the reflectance band of the filtering reflector
520 is reflected back into the secondary fiber 502, the broadband
Bragg-reflector 520 thus pre-filtering the light signal to be
analyzed. Typically, this pre-filtering of the light by means of
the filtering reflector 520 selects (i.e., filters out by
reflection) the signals in the C- or L-band of the
telecommunications bands. The reflected light is subsequently
directed into a portion of the secondary fiber incorporating a
second Bragg-reflector 550. The second Bragg-reflector 550 is a
chirped Bragg-reflector reflecting essentially the same wavelength
band as the filtering reflector 520.
[0064] The second Bragg-reflector 550 is utilized for establishing
transmission of light towards a photodetector 556 provided at the
output end of the secondary fiber 502. The transmission of light
towards the photodetector 556 is obtained by sending a longitudinal
acoustic pulse along the portion of the secondary fiber 502
incorporating the chirped Bragg-reflector 550 according to the
present invention. By sending a longitudinal acoustic pulse along
the chirped Bragg-grating 550, transmission through the secondary
optical fiber 502 is achieved in a very precise fashion. The
mechanism behind the transmission is described in detail above.
[0065] The arrangement for analyzing the power spectrum, according
to the present invention, includes an acoustic actuator (not shown
in FIG. 5) for sending longitudinal acoustic pulses along the
chirped Bragg-reflector 550 in the secondary optical fiber 502; a
photodetector 556 for detecting the power of the light transmitted
through the Bragg-reflector 550; programmable logic 600, in the
form of an FPGA (Field Programmable Gate Array), for storing the
waveform of the actuating pulses, and for controlling other
hardware and performing simple signal analysis; a digital to analog
converter 610 (DAC) for converting digitally encoded output from
the FPGA 600 into analog form; an analog to digital converter 630
(ADC) for converting monitored analog signals into digital form for
further storing and processing; a memory 640 (Mem) for storing
measurement data; a delay line 650 (Del) for sub-sampling data in
order to improve measurement resolution; a clock circuit 620 (Clk)
for supplying clock pulses to the logic 600; a digital synthesizer
circuit 622 (DDS) for tuning the clock frequency and thereby tuning
the repetition rate of actuating pulses to the acoustic actuator;
and a processing unit 660 (CPU) for processing data and for
communicating with the logic 600 and other peripherals.
[0066] The longitudinal acoustic pulse to be sent along the chirped
Bragg-reflector 550 is excited by the acoustic actuator (not shown
in FIG. 5, but shown in great detail in FIGS. 6-9) attached to the
secondary fiber 502. The supply of actuating pulses to the acoustic
actuator is controlled by the programmable logic 600 (FPGA). In the
FPGA 600, the waveform of the actuating pulses is stored in digital
form. A first output 601 of the FPGA 600 is connected to a first
input of a digital to analog converter 610 (DAC). The DAC is
operative to convert the digitally encoded waveform of the
actuating pulses to an analog waveform and to send the analog
output signal to an amplifier 612. The amplified output of the
amplifier 612 is fed to the acoustic actuator as actuating pulses.
A second output 602 of the FPGA 600 is connected to the amplifier
612 in order to provide control signals from the FPGA 600 to the
amplifier 612.
[0067] The FPGA 600 is triggered by clock pulses from a clock
circuit 620 (Clk). The output of the clock circuit 620 is
operatively connected to the FPGA 600 via the digital synthesizer
circuit 622 (DDS). The digital synthesizer circuit 622 is operative
to tune the frequency of the clock circuit 620 in order to obtain
tuning capability of the actuating pulses to the acoustic actuator.
In order to control the DDS 622, a third output 603 of the FPGA 600
is connected to the DDS. The clock circuit 620 is also operatively
connected, via said DDS, to the DAC 610 and to the delay line 650
and the ADC 630.
[0068] The photodetector 556 is operative to detect the power of
the output signal from the secondary fiber 502, after transmission
through the chirped Bragg-reflector 550. As described above, the
output power at each instant corresponds to the power of a certain
wavelength component in the light signal directed into the
secondary fiber 502. The light directed into the secondary fiber,
in turn, corresponds on a one-to-one basis to the light propagating
in the optical transport fiber 500. The photodetector 556 is
arranged to provide a detector output signal, associated with the
power of the detected signal, to a preamplifier 632. The output
signal from the preamplifier 632 is passed to the ADC 630. The ADC
630 is operative to convert the amplified detector output signal
into digitally encoded form and to provide the digitally encoded
measurement data to the memory 640 for storing. The delay line 650
and the amplifier 632 (and thus the ADC 630) are controlled by the
FPGA 600 through fourth 604 and fifth 605 outputs of the FPGA,
respectively.
[0069] The contents of the memory 640 is readable to the
programmable logic 600. The programmable logic 600 is arranged to
read the stored measurement data from the memory 640 and to perform
simple signal analysis of the stored data. The analysis performed
by the programmable logic 600 is the basis for the control that the
logic 600 exerts on the DDS 622, the delay line 650, as well as for
the actuating pulses fed to the DAC 610. Furthermore, the
programmable logic 600 can communicate with a processing unit 660
(CPU) in order to transfer measurement data for further processing
and supervision of system performance.
[0070] The processing unit 660 has a memory 662 for storing both
processed and non-processed data. Also, the processing unit has
connections 664 for communicating with other peripherals (not
shown), such as a display device, a keyboard, an I/O-device for
communicating with other apparatus, etc.
[0071] Furthermore, the output of the filtering reflector 520 can
advantageously be connected to a second arrangement according to
the present invention. For example, if the C-band of the
communications bands is filtered out by a first filtering reflector
and analyzed in a first arrangement according to the present
invention, then a second arrangement can be coupled to the first
for analyzing the power spectrum of light signals in the L-band of
the communications bands. A person having ordinary skill in the art
will find it straightforward to combine two arrangements according
to the present invention, in order to analyze two different
communications bands simultaneously.
[0072] The acoustic actuator and the supply of longitudinal
acoustic pulses to the portion of the secondary optical fiber 502
incorporating the chirped Bragg-reflector 550 will now be described
in more detail with reference to FIGS. 6-9.
[0073] FIG. 6 shows schematically an acoustic actuator module 700
and the secondary fiber 502 attached thereto.
[0074] The acoustic actuator comprises a piezo device 705, made
from a piezo-electric material, arranged in a rod 706, 708 capable
of guiding acoustic pulses along its longitudinal axis. The rod is
made from aluminium, but could also be made from, for example,
copper or other suitable material. The piezo device 705 has two
connectors attached to its surface for the supply of actuating
pulses thereto. The piezo device is responsive to actuating pulses
by changing its longitudinal size. Hence, when a voltage is applied
to the piezo device 705, the size of the device changes
accordingly. If properly actuated, the acoustic actuator emits an
acoustic pulse into the rod 706, 708 (as indicated in the figure by
the four solid arrows), the rod guiding said pulse towards its
ends. One end 706 of the rod is provided with an acoustic
attenuator 707, effectively damping the acoustic pulse to
essentially zero amplitude. The other end 708 of the rod is firmly
attached to the fiber 502, thereby ensuring that the acoustic
pulse, emitted by the acoustic actuator, is transferred to the
fiber 502. When the acoustic pulse reaches the fiber end 709 of the
rod 708, some of the acoustic power is reflected back towards the
piezo device 705, and some of the acoustic power is transferred to
the fiber 502. The part of the acoustic power that is reflected
back into the rod passes through the piezo device 705 and is
effectively damped by the acoustic attenuator 707. This ensures
that only one pulse is sent into the fiber 502 by each actuation of
the piezo device 705.
[0075] The acoustic pulse propagates along the fiber 502, and thus
along the chirped Bragg-reflector 550, thereby establishing
transmission of light through the Bragg-reflector 550 and
effectively causing a wavelength scan of wavelengths transmitted
towards the detector 556.
[0076] At the output end of the secondary fiber 502, the fiber is
clamped by a clamping block 702. The clamping block 702 forms a
first reflection point 710 and is arranged to give a reflection of
the acoustic pulse back towards the rod 708 and the acoustic
actuator 705. Furthermore, when the reflected acoustic pulse
reaches the attachment of the fiber to the actuator, the pulse is
again reflected, this time back towards the output end of the
fiber, the attachment of the fiber to the actuator thereby forming
a second reflection point 720 for the acoustic pulse.
[0077] It is preferred that acoustic pulses are sent into the fiber
repeatedly, in order to provide repeated scans of transmitted
wavelengths. In this embodiment, an acoustic pulse is sent into the
fiber 502 at the time when the reflection in the fiber of a
previous pulse reaches the attachment point between the rod and the
fiber. The acoustic pulse propagating back and forth between the
reflection points 710, 720 thereby achieves some additional
amplitude each time the pulse reaches the reflection point at the
actuator 720. Preferably, the reflection of the acoustic pulse at
each reflection point 710, 720 exhibits some predefined loss. This
ensures that the acoustic pulse eventually dies away in a known
manner. Another advantage achieved by such a loss is that the
timing requirements on the acoustic pulses from the actuator are
alleviated to some extent. If an acoustic pulse from the actuator
is sent into the fiber 502 slightly off schedule, this unwanted or
untimely acoustic pulse will eventually die away, thereby
preventing the accumulation of systematic error.
[0078] As described above, it is essential that the phase of the
acoustic pulse corresponds to the chirp of the Bragg-reflector in
order to give the desired result. Conveniently, the reflection of
the acoustic pulse at each reflection point 710, 720 introduces a
phase change of 180 degrees into the acoustic pulse. The phase of
the acoustic pulse will consequently be suitable for establishing
the desired transmission through the Bragg-reflector when the
acoustic pulse is propagating back towards the actuator, as well.
This dual-pass feature of the present invention can conveniently be
utilized for correlation evaluation in order to increase resolution
and/or detect and correct any possible flaws.
[0079] The dual-pass feature is also utilized for the actual
interpretation of the detected light signals. In order to ascribe a
certain wavelength to a certain instant, i.e., to an output signal
from the detector at a certain time, a correlation procedure is
utilized. A first pass of the acoustic pulse through the chirped
Bragg-reflector causes transmission of a first spectrum profile. A
second pass of the acoustic pulse through the chirped
Bragg-reflector, now in the opposite direction, causes transmission
of a second spectrum profile, which is essentially the same
spectrum profile as the first spectrum profile, but in reversed
timely order, since the acoustic pulse now propagates in the
opposite direction through the Bragg-reflector. The acoustic pulse
is then reflected back towards the Bragg-reflector a third time,
causing transmission of a third spectrum profile. The third
spectrum profile is essentially identical to the first spectrum
profile if the first and the third pass of the acoustic pulse
through the chirped Bragg-reflector are sufficiently close in time
(since the light signal to be analyzed has not had time to change
significantly between the first and the third pass).
[0080] Now, a correlation between the first spectrum profile and
the third spectrum profile provides a first time interval,
corresponding to the passage time of the acoustic pulse to complete
one round trip in the fiber between the two reflection points. This
first time interval should be constant, unless the length of the
fiber is changed by, for example, a temperature change (i.e.,
thermal expansion or contraction of the fiber).
[0081] In order to correlate the second spectrum profile and the
first spectrum profile, the second profile must first be mirrored.
Mirroring of the second spectrum profile about a reference
wavelength, and subsequent correlation between the mirrored
spectrum and the first spectrum, provides a second time interval,
corresponding to the time elapsed between one transmission of said
reference wavelength, and the following transmission of the same
reference wavelength during the following passage of the acoustic
pulse through the chirped Bragg-reflector.
[0082] The relationship between the first time interval and the
second time interval is constant, even if the length of the fiber
changes due to, for example, temperature changes. This means that
there is a unique number (equal to the relationship between the
first and the second time intervals) associated with each reference
wavelength. By measuring the time intervals, calculating the
relationship, and comparing the relationship with a stored value in
a look-up table, the corresponding real value for each reference
wavelength is obtained.
[0083] The analysis of the spectrum profiles is preferably done
after sampling the detector output signal into a digitally encoded
form, each time interval thus corresponding to a certain number of
samples.
[0084] Furthermore, by monitoring said first time interval, a
change in fiber length due to, for example, temperature changes,
can be detected. Consequently, the repetition rate of the acoustic
pulses can be controlled to comply with the length of the fiber.
Hence, resonant feeding of acoustic pulses from the actuator is
possible even if the length of the fiber is changed.
[0085] By basic symmetry considerations, it is obvious that the
length of the piezo device 705 must be carefully selected to suit
the desired pulse length of the acoustic pulse to be sent into the
fiber 502. A person having ordinary skill in the art will find the
suitable length of the piezo device by simple tests and/or computer
simulations.
[0086] Having described the general features of the acoustic
actuator, the attachment of the fiber 502 thereto will now be
described in more detail with reference to FIGS. 7-9.
[0087] FIG. 7 shows schematically the end 708 of the actuator rod
to which the fiber 502 is attached. As illustrated in the figure,
the end of the rod is provided with a shoulder 718, which the fiber
502 is to bear against.
[0088] In FIG. 8, the end of the rod is shown in greater detail.
The fiber 502 bears against the shoulder 718 and is fixed thereto
by means of a contact block 728. The contact block 728 is tightened
to the fiber 502 by means of a wire 730, in this case a tungsten
wire, surrounding the entire rod 708. The purpose of the contact
block 728 is to distribute the force from the wire 730, and to give
a well-defined interaction length L between the fiber 502 and the
actuator rod.
[0089] The placement and the length of the shoulder 718 requires
some consideration. The placement of the shoulder 718 needs to be
close to the end of the actuator rod which, in practice, means that
the center of the shoulder should be within a fraction of the
shortest wavelength (the highest frequency) building up the
acoustic pulse. Such an arrangement of the shoulder minimizes
distortion of the acoustic pulse when transferred from the rod to
the fiber. The placement of the shoulder 718 can also be utilized
to obtain a desired distortion, e.g., a low pass filtering, of the
acoustic pulse when it enters the optical fiber 502.
[0090] FIG. 9 is a cross-sectional view of the attachment of the
fiber 502 to the actuator rod 708. The fiber 502 is clamped to the
rod 708 by means of the contact block 728, and fixed into place by
the wire 730. The wire 730 is wound one full turn around the rod
708, and is tightened by a spring 732, as shown. This arrangement
ensures firm attachment of the fiber 502 to the actuator rod 708 in
a well-defined way.
[0091] Thus, the proximity of the attachment point (i.e., the
shoulder 718) to the end of the actuator rod 708 allows for
reliable transfer of acoustic pulses into the fiber 502. More
particularly, the closeness of the shoulder 718 to the end of the
rod eliminates any influence from possible interference between a
portion of the acoustic pulse propagating in one direction and a
portion of the acoustic pulse propagating, after reflection, in the
opposite direction.
[0092] FIG. 10 shows an example of a longitudinal acoustic pulse,
illustrating its shape as it propagates in the actuator rod 708 and
in the optical fiber 502 according to the present invention. In the
shown example, the pulse is anti-symmetric and plotted in terms of
change in length .DELTA.z. The shape shown is preferred, since such
a shape gives a transmittance peak of close to 100 percent
transmission of a certain wavelength.
[0093] Although the present invention has been described with
reference to the drawings and by way of preferred embodiments, it
is to be understood that the embodiments described can undergo
several alterations and modifications without departing from the
scope of the invention as claimed in the accompanying claims.
* * * * *