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Claims  |
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What is claimed is:
1. A system for optical heterodyne detection comprising:
an intensity noise reducer for reducing the intensity noise of a first
signal;
an optical combining means for combining said noise reduced first signal
and a second signal into a combined optical signal and for outputting
light beams each including a portion of said combined optical signal, one
of said first and second signals being a swept local oscillator signal;
and
photodetectors each optically arranged to receive a different one of said
light beams, said photodetectors generating electrical signals in response
to respective ones of said light beams.
2. The system of claim 1 further including an intensity noise reducer for
reducing the intensity noise of said second signal.
3. The system of claim 1 further including an amplifier located in an
optical path before said intensity noise reducer of said first signal.
4. The system of claim 1 further including an optical pre-selector
optically connected to receive said light beams from said optical
combining means, said optical pre-selector having a passband that tracks
the wavelength of said swept local oscillator signal, said optical
pre-selector outputting filtered portions of said light beams to said
photodetectors.
5. The system of claim 1 further including a processor, arranged to receive
said electrical signals, for generating an output signal that is
substantially independent of the polarization of said first and second
signals.
6. The system of claim 1, in which said optical combining means includes an
optical combining unit arranged to receive said first signal and said
second signal, said optical combining unit operating to generate said
combined optical signal in two optical paths and to output said combined
optical signal in each of said optical paths as one of said light beams.
7. The system of claim 1, in which said optical combining means includes:
an optical combining unit arranged to receive said first signal and said
second signal, said optical combining unit operating to generate said
combined optical signal; and
a polarizing beam splitter arranged to receive said combined optical signal
and operating to split said combined optical signal into two polarized
portions and to output each of said two polarized portions as one of said
light beams.
8. The system of claim 7 further including a half-wave plate for shifting
the polarization state of one of said two polarized portions, said
half-wave plate being located between said polarizing beam splitter and
said photodetectors.
9. The system of claim 1, in which said optical combining means includes:
an optical combining unit arranged to receive said first signal and said
second signal, said optical combining unit operating to generate said
combined optical signal in two optical paths; and
a polarizing beam splitter arranged to receive said combined optical signal
from each of said two optical paths and operating to split said combined
optical signal in each of said two optical paths into two polarized
portions and to output each of said polarized portions as one of said
light beams.
10. The system of claim 9 further including two half-wave plates for
shifting the polarization states of two of said polarized portions, said
two half-wave plates being located between said polarizing beam splitter
and said photodetectors.
11. A method for monitoring an optical signal utilizing optical heterodyne
detection comprising steps of:
reducing the intensity noise of a first signal;
combining said noise reduced first signal with a second signal to generate
a combined optical signal, one of said first and second signals being a
swept local oscillator signal and the other signal being an input signal;
outputting light beams each including a portion of said combined optical
signal;
generating electrical signals in response to said light beams; and
processing said electrical signals to determine an optical characteristic
represented by said input signal.
12. The method of claim 11 further including a step of reducing the
intensity noise of said second signal before said second signal is
combined with said noise reduced first signal.
13. The method of claim 11 further including a step of amplifying said
first signal before said step of reducing the intensity noise of said
first signal.
14. The method of claim 11 further including a step of, before said
electrical signals are generated, filtering said light beams to pass a
wavelength band that tracks the wavelength of said swept local oscillator
signal.
15. The method of claim 11, in which:
in combining said input signal with said swept local oscillator signal,
said input signal is combined with said swept local oscillator signal to
generate two instances of said combined optical signal; and
in outputting said light beams, each of said two instances of said combined
optical signal is output as one of said light beams.
16. The method of claim 11, in which:
the method additionally comprises splitting said combined optical signal
into polarized portions; and
in outputting said light beams, each of said polarized portions is output
as one of said light beams.
17. The method of claim 16 further including a step of, before said
electrical signals are generated, filtering said polarized portions to
pass a wavelength band that tracks the wavelength of said swept local
oscillator signal.
18. The method of claim 17 further including a step of rotating the
polarization state of one of said polarized portions to match the other
polarized portions.
19. The method of claim 16, in which said step of processing includes steps
of:
squaring a first one of said electrical signals, thereby creating a first
squared signal;
squaring a second on of said electrical signals, thereby creating a second
squared signal; and
adding said first squared signal to said second squared signal to achieve
polarization independence.
20. The method of claim 11, in which:
in combining said input signal with said swept local oscillator signal,
said input signal is combined with said swept local oscillator signal to
generate two instances of said combined optical signal;
the method additionally comprises splitting each of said two instances of
said combined optical signal into two polarized portions to generate four
polarized portions of said combined optical signal; and
in outputting said light beams, each of said four polarized portions is
output as one of said light beams.
21. The method of claim 20 wherein said step of processing said electrical
signals includes steps of:
subtracting a first electrical signal from a third electrical signal to
electronically suppress intensity noise, thereby creating a first
subtracted signal;
subtracting a second electrical signal from a fourth electrical signal to
electronically suppress intensity noise, thereby creating a second
subtracted signal;
squaring said first subtracted signal, thereby creating a first squared
signal;
squaring said second subtracted signal, thereby creating a second squared
signal; and
adding said first squared signal to said second squared signal in order to
achieve polarization independence.
22. The method of claim 21 further including the steps of:
low pass filtering said first squared signal before said step of adding;
and
low pass filtering said second squared signal before said step of adding. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The invention relates generally to the field to optical measurements and
measuring systems, and more particularly to a system and method for
optical heterodyne detection of an optical signal.
BACKGROUND OF THE INVENTION
Dense wavelength division multiplexing (DWDM) requires optical spectrum
analyzers (OSAs) that have higher spectral resolution than is typically
available with current OSAs. For example, grating-based OSAs and
autocorrelation-based OSAs encounter mechanical constraints, such as
constraints on beam size and the scanning of optical path lengths, which
limit the degree of resolution that can be obtained.
As an alternative to grating-based and autocorrelation-based OSAs, optical
heterodyne detection systems can be utilized to monitor DWDM systems. FIG.
1 is a depiction of a prior art optical heterodyne detection system. The
optical heterodyne detection system includes an input signal 102, an input
waveguide 104, a local oscillator signal 106, a local oscillator waveguide
108, an optical coupler 110, an output waveguide 118, a photodetector 112,
and a signal processor 116. The principles of operation of optical
heterodyne detection systems are well known in the field of optical
heterodyne detection and involve monitoring the heterodyne term that is
generated when an input signal is combined with a local oscillator signal.
The heterodyne term coexists with other direct detection signals, such as
intensity noise from the input signal and intensity noise from the local
oscillator signal.
Optical heterodyne detection systems are not limited by the mechanical
constraints that limit the grating based and autocorrelation based OSAs.
The spectral resolution of an optical heterodyne system is limited by the
linewidth of the local oscillator signal, which can be several orders of
magnitude narrower than the resolution of other OSAs.
In order to improve the performance of optical heterodyne detection systems
with regard to parameters such as sensitivity and dynamic range, it is
best for the heterodyne signal to have a high signal to noise ratio.
However, the signal to noise ratio of the heterodyne signal is often
degraded by noise that is contributed by the direct detection signals,
especially in the case where the input signal includes multiple carrier
wavelengths. One technique for improving the signal to noise ratio of the
heterodyne signal, as described in U.S. Pat. No. 4,856,899, involves
amplifying the input signal before the input signal is combined with the
local oscillator signal in order to increase the amplitude of the
heterodyne signal. Although amplifying the input signal increases the
amplitude of the heterodyne signal, the amplification also increases the
intensity noise of the input signal and may not improve the signal to
noise ratio of the heterodyne signal.
It is also important in optical heterodyne detection that the polarization
of the input signal and the local oscillator signal are matched. In order
to match the polarization of the local oscillator signal to the
polarization of the input signal, the polarization state of the local
oscillator signal may be controlled by a polarization controller 120 as
indicated by the two loops in the heterodyne detection system of FIG. 1. A
disadvantage of the optical heterodyne detection system of FIG. 1 is that
detection of the input signal is highly dependent on the polarization of
the input signal.
A polarization diversity receiver can be incorporated into an optical
heterodyne detection system to provide polarization independent signal
detection. Although a polarization diversity receiver provides
polarization independent signal detection, the polarization diversity
receiver does not provide a way to separate the intensity noise from the
heterodyne signal. In order to improve the performance of heterodyne
detection systems, it is necessary to be able to clearly distinguish the
heterodyne signal from the intensity noise.
In view of the prior art limitations in optical heterodyne detection
systems, what is needed is an optical heterodyne detection system that
generates a heterodyne signal with an improved signal to noise ratio.
SUMMARY OF THE INVENTION
A system for monitoring an optical signal includes an optical heterodyne
detection system in which the intensity noise of the input signal and/or
the intensity noise of the swept local oscillator signal is reduced before
the input signal and the swept local oscillator signal are combined. An
embodiment of the optical heterodyne detection system includes an
intensity noise reducer for the input signal and/or an intensity noise
reducer for the swept local oscillator signal, an optical coupling unit,
at least two photodetectors, and may include a processor. Utilizing
intensity noise reducers for the input signal and/or the swept local
oscillator signal reduces the intensity noise that is detected by the
photodetectors and improves the signal to noise ratio and the dynamic
range of the heterodyne detection system. Optical intensity noise
reduction can be accomplished utilizing various techniques and the
particular technique utilized is not critical.
An embodiment of an optical heterodyne detection system includes an optical
amplifier for amplifying the swept local oscillator signal. The optical
amplifier increases the power of the local oscillator signal, thereby
improving the signal to noise ratio and dynamic range of the heterodyne
detection system. The intensity noise reducer for the local oscillator
signal is preferably utilized when the optical amplifier is utilized in
order to control the additional intensity noise that is contributed by the
optical amplifier.
Because intensity noise reducers for both the input signal and the swept
local oscillator signal may not be required on a single system, an
embodiment of the system may include an intensity noise reducer for the
input signal and no intensity noise reducer for the swept local oscillator
signal. Conversely, an embodiment may include an intensity noise reducer
for the swept local oscillator signal and no intensity noise reducer for
the input signal.
In an embodiment, the optical heterodyne detection system includes an
optical pre-selector connected to the output of the optical combining
unit. The optical pre-selector has a passband that tracks the wavelength
of the swept local oscillator signal. In an embodiment, the optical
combining unit includes an optical coupler for combining the input signal
and the swept local oscillator signal and for outputting light beams to
corresponding photodetectors. In another embodiment, the optical combining
unit includes an optical coupler and a polarizing beam splitter for
splitting the combined optical signal into polarized portions that are
output to corresponding photodetectors.
A method for monitoring an optical signal utilizing an optical heterodyne
detection system involves reducing the intensity noise of the input signal
and/or the swept local oscillator signal, combing the input signal and the
swept local oscillator signal to generate a combined optical signal, and
outputting light beams each including a portion of the combined optical
signal, generating electrical signals in response to the light beams, and
processing the electrical signals to determine an optical characteristic
represented by the input signal.
The method for monitoring an optical signal utilizing optical heterodyne
detection may involve additional steps. In one embodiment, the intensity
noise of both the input signal and the swept local oscillator signal is
reduced before the signals are combined. In an embodiment, the swept local
oscillator signal is amplified before it is combined with the input
signal. In another embodiment, the light beams are optically filtered
before the electrical signals are generated. The optical filtering passes
a wavelength band that corresponds to the wavelength of the swept local
oscillator signal. The passband of the filtering is adjusted in real-time
to track the changing wavelength of the swept local oscillator signal.
In another embodiment, the combined optical signal is split into polarized
portions and each of the polarized portions is output as one of the light
beams. In another embodiment, the input signal and the swept local
oscillator are combined to generate two instances of the combined optical
signal. The two instances of the combined optical signal are split into
two polarized portions and then the four polarized portions are output as
four output beams. First, second, third, and fourth electrical signals are
generated in response to the four polarized portions. The first, second,
third, and fourth electrical signals are processed to determine an optical
characteristic represented by the input signal.
In an embodiment, processing of the four electrical signals involves
subtracting the first electrical signal from the third electrical signal
in order to suppress intensity noise, thereby creating a first subtracted
signal, subtracting the second electrical signal from the fourth
electrical signal in order to suppress intensity noise, thereby creating a
second subtracted signal, squaring the first subtracted signal, thereby
creating a first squared signal, squaring the second subtracted signal,
thereby creating a second squared signal, low pass filtering the first and
second squared signals, thereby creating first and second filtered
signals, and adding the first filtered signal to the second filtered
signal in order to achieve polarization independence.
The optical heterodyne detection system and method provide an optical
measurement system that is accurate over a wide range of wavelengths. The
optical heterodyne detection system and method can be utilized as an
optical spectrum analyzer to characterize an unknown input signal. The
optical heterodyne detection system and method may also be utilized as an
optical network analyzer in which a known signal is input into an optical
network and the output signal is measured by the detection system.
Other aspects and advantages of the present invention will become apparent
from the following detailed description, taken in conjunction with the
accompanying drawings, illustrating by way of example the principles of
the invention.
BREIF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a depiction of an optical heterodyne detection system that
includes a single photodetector in accordance with the prior art.
FIG. 2 is a depiction of an optical heterodyne detection system that
includes an optical combining unit, an optical pre-selector, and two
photodetectors in accordance with the invention.
FIG. 3 is an expanded view of the optical combining unit of FIG. 2 that
includes two beams output from a coupler in accordance with the invention.
FIG. 4 is an expanded view of the optical combining unit of FIG. 2 that
includes one beam output from a coupler and split by a polarizing beam
splitter in accordance with the invention.
FIG. 5 is a graph of an unfiltered input signal and a swept local
oscillator signal in relation to the passband of an optical pre-selector
that tracks the swept local oscillator signal.
FIG. 6 is a graph of a filtered input signal and a swept local oscillator
signal in relation to the passband of an optical pre-selector that tracks
the swept local oscillator.
FIG. 7 is a depiction of an optical heterodyne detection system that
includes an optical combining unit, an optical pre-selector, and four
photodetectors in accordance with the invention.
FIG. 8 is an expanded view of the optical combining unit of FIG. 7 that
includes two beams output from a coupler and split by a polarizing beam
splitter in accordance with the invention.
FIG. 9 is a depiction of an example optical pre-selector in relation to the
optical combining unit and four photodetectors that is related to the
systems depicted in FIGS. 7 and 8 in accordance with the invention.
FIG. 10 is a graphical depiction of the signal processing that is performed
on the electrical signals that are generated from the system of FIGS. 7
and 8 in accordance with the invention.
FIG. 11A is a process flow diagram of a method for monitoring an input
signal utilizing optical heterodyne detection that includes optical
pre-selection.
FIG. 11B is a process flow diagram of process steps related to the process
flow diagram of FIG. 11A.
FIG. 11C is a process flow diagram of process steps related to the process
flow diagram of FIG. 11A.
FIG. 11D is a process flow diagram of process steps related to the process
flow diagram of FIG. 11A.
FIG. 12 is a process flow diagram of processing steps related to the
process flow diagram of FIG. 11A.
FIG. 13 is a depiction of an optical heterodyne detection system that
includes two intensity noise reducers, a local oscillator amplifier, and
an optical combining unit in accordance with the invention.
FIG. 14 is a depiction of an optical heterodyne detection system that
includes two intensity noise reducers, a local oscillator amplifier, an
optical combining unit, and an optical pre-selector in accordance with the
invention.
FIG. 15A is a process flow diagram of a method for monitoring an input
signal utilizing optical heterodyne detection that includes intensity
noise reduction.
FIG. 15B is a process flow diagram of a process step related to the process
flow diagram of FIG. 12A.
FIG. 15C is a process flow diagram of a process step related to the process
flow diagram of FIG. 12A.
FIG. 15D is a process flow diagram of a process step related to the process
flow diagram of FIG. 12A.
FIG. 16 is a depiction of an example intensity noise reducer that may be
utilized with the systems of FIGS. 13 and 14.
FIG. 17 is a depiction of an example intensity noise reducer that may be
utilized with the systems of FIGS. 13 and 14.
FIG. 18 is a graph of the power spectral density of an input signal vs.
frequency after the intensity noise has been reduced by the intensity
noise reducer of FIG. 17.
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the invention involves an optical heterodyne detection
system in which an input signal and a swept local oscillator signal are
combined and output as at least two beams. The at least two beams are
filtered by a filter that passes a wavelength band that tracks the
wavelength of the swept local oscillator signal. As the local oscillator
signal sweeps across a wavelength range, filtering of the at least two
beams is adjusted to track the wavelength of the local oscillator signal.
Filtering the beams to pass a wavelength band corresponding to the
wavelength of the swept local oscillator signal reduces the intensity
noise contributed from light sources having wavelengths that are not near
the wavelength of the local oscillator signal.
FIG. 2 depicts an optical heterodyne detection system that includes an
input signal 202, a signal fiber 204, a local oscillator signal 206, a
local oscillator fiber 208, an optical combining unit 210, an optical
pre-selector 214, two photodetectors 212, and a processor 216. It should
be noted that throughout the description similar element numbers are
utilized to identify similar elements.
The input signal 202 and the local oscillator signal 206 include optical
signals that are generated from conventional devices as is known in the
field of optical communications systems. For example, the input signal and
the local oscillator signal may be generated by lasers. The input signal
may consist of a single wavelength or the input signal may include
multiple wavelengths as is known in the field of wavelength division
multiplexing (WDM). The input signal may be an optical signal having
unknown optical characteristics, in which case the optical heterodyne
detection system can be utilized for optical spectrum analysis. The input
signal may alternatively be a delayed portion of the local oscillator
signal that is utilized for optical network analysis. When the monitoring
system is being utilized for optical network analysis, the characteristics
of a network or a single network component can be determined by inputting
a known input signal into the network or the single network component and
then measuring the response to the known signal.
Preferably, the local oscillator signal 206 is a widely tunable optical
signal generated from a wideband tunable laser. For example, the local
oscillator signal may be tunable over a range of one nanometer or greater.
During optical spectrum analysis, the local oscillator signal is typically
swept across a wavelength range in order to detect the input signal over a
range of wavelengths.
The signal fiber 204 carries the input signal 202 that is to be detected by
the system. In an embodiment, the signal fiber is a single mode optical
fiber as is known in the art, although other optical waveguides may be
utilized. In addition, although waveguides are described, optical signals
may be input into the system, or transmitted within the system, in free
space.
The local oscillator fiber 208 is an optical fiber, such as a single mode
optical fiber, that carries the local oscillator signal 206. The local
oscillator fiber may include a polarization controller (not shown) that
controls the polarization state of the local oscillator signal. Other
optical waveguides may be utilized in place of single mode optical fiber,
such as polarization preserving fiber. Alternatively, the local oscillator
signal may be transmitted through free space without the use of a
waveguide.
The optical combining unit 210 optically combines the input signal 202 and
the local oscillator signal 206 into a combined optical signal and outputs
at least two beams of the combined optical signal. The combined optical
signal includes the heterodyne signal and intensity noise from the input
signal and from the local oscillator signal. In an embodiment, the input
signal and the local oscillator signal are combined in a manner that
ensures the spatial overlap of the input signal and the local oscillator
signal, thereby allowing full interference between the input signal and
the local oscillator signal.
In an embodiment, the optical combining unit includes a coupler that
outputs the combined optical signal into two optical paths. In another
embodiment, the optical combining unit includes a coupler and a polarizing
beam splitter that outputs two polarized portions of the combined optical
signal. Although the optical combining unit is described below as
outputting two beams of the combined optical signal, it should be
understood that embodiments of the optical combining unit that output more
than two beams of the combined optical signal are possible.
FIG. 3 is a depiction of an embodiment of the optical combining unit 210 in
which the optical combining unit includes an optical coupler 320 that has
two outputs represented by two output fibers 318 and 328. As shown in FIG.
3, two portions of the combined optical signal 340 and 342 are output from
the two fibers. The two output portions of the combined optical signal are
transmitted from the coupler to the optical pre-selector and then to the
two corresponding photodetectors. In the embodiment of FIG. 3, the
resulting output signal is not independent of the polarization state of
the input signal. In the embodiment of FIG. 3, the power distribution of
the input signal and local oscillator signal between the two output fibers
is influenced by the optical coupler and can be controlled to accomplish
the desired power distribution. In an embodiment, the power of the
combined optical signal is distributed approximately evenly between the
two output fibers.
The optical coupler 320 in FIG. 3 may be an optically directional 3 dB
fiber coupler, although other optical couplers may be utilized. In an
embodiment, the optical coupler is substantially independent of the
polarization of optical signals. In an embodiment, the optical coupler
does not polarize the combined optical signal.
FIG. 4 is a depiction of an embodiment of the optical combining unit 210 in
which the optical combining unit includes an optical coupler 420, a
polarizing beam splitter 424, and a half-wave plate 426. The optical
coupler combines the input signal 402 and the local oscillator signal 406
onto at least one waveguide. As shown in FIG. 4, the optical coupler
combines the input signal and the local oscillator signal and outputs the
combined optical signal into output fiber 428. It should be noted that
although output fiber 418 is shown, there may not be two outputs from the
coupler 420. In an embodiment, the length of the output fiber 428 is
chosen to provide sufficient attenuation of any cladding modes before the
light of the input signal or the local oscillator signal reaches the end
of the output fiber. For example, the length of the output fiber should be
chosen such that cladding mode light accounts for less than one percent of
the total light emitted from the output fiber.
The polarizing beam splitter 424 separates an incoming optical beam into
two polarized beams so that an output signal that is independent of the
polarization state of the input signal can be generated. The polarizing
beam splitter may include, for example, a birefringent crystal that
provides polarization walk-off, such as a rutile walk-off crystal. As will
be described below, the polarizing beam splitter separates the combined
optical signal beam into two beams having different polarization states.
Preferably, the polarizing beam splitter separates the incoming beam into
two linearly polarized components that have orthogonal directions of
polarization. In an embodiment, the polarization of the local oscillator
signal is controlled such that the power contributed from the local
oscillator signal is split approximately evenly by the polarizing beam
splitter. Although the optical coupler 420 and the polarizing beam
splitter are shown as physically separate devices connected by optical
fiber 428, the optical coupler and polarizing beam splitter may be
integrated into a planar waveguide circuit that does not require optical
fiber connections. It should be understood that other polarizing beam
splitters may be utilized.
FIG. 4 shows how the beam of the combined optical signal carried on output
fiber 428 is split into two differently polarized beams, as identified by
the dashed lines 440 and 442. As shown in FIG. 4, the straight line beam
440 follows an "ordinary" path and is referred to as the ordinary beam.
The diagonal line beam 442 walks off in an "extraordinary" path and is
referred to as the extraordinary beam.
In a preferred embodiment, the two polarized beams 440 and 442 have the
same polarization state before the beams enter the optical pre-selector.
In order to make the two beams have the same polarization state, the
polarization state of the extraordinary beam 442 is rotated to match the
polarization state of the ordinary beam 440. When the two beams have
orthogonal polarization, the extraordinary beam is rotated by 90 degrees.
In an embodiment, a half-wave plate 426 is utilized to accomplish the
90-degree rotation. Although FIG. 4 represents one arrangement for
generating two beams with the same polarization state, other arrangements
may alternatively be utilized within the optical combining unit 210 as
would be apparent to one of ordinary skill in the field of optical
communications systems.
Whether the optical combining unit involves the embodiment of FIG. 3 or the
embodiment of FIG. 4, the end result is that two beams are output from the
optical combining unit to the optical pre-selector. Referring back to FIG.
2, the optical pre-selector 214 is a tunable bandpass filter that is tuned
to track the swept local oscillator signal 206. That is, the optical
pre-selector is tuned so that the optical pre-selector has the highest
optical transmission over a wavelength band that corresponds to the
wavelength of the swept local oscillator signal. In an embodiment, the
optical pre-selector is located between the optical combining unit 210 and
the two photodetectors 212.
Operation of the optical pre-selector 214 in a WDM system is depicted in
the signal power vs. wavelength graphs of FIGS. 5 and 6. FIG. 5 depicts an
input signal 502 as three optical carriers 506, 508, and 510 in a WDM
system in relation to a swept local oscillator signal 514 before the
combined input signal and swept local oscillator signal have entered the
optical pre-selector. For example purposes, the dashed line 518 represents
the passband of the optical pre-selector that is tuned to track the sweep
of the local oscillator signals. Optical signals within the passband
continue to be transmitted and optical signals outside of the passband are
filtered by the optical pre-selector.
FIG. 6 depicts the one optical carrier 608 that exits the optical
pre-selector after the optical signals have been filtered. As shown by
FIG. 6, the optical pre-selector filters out optical carriers that are not
near the wavelength of the swept local oscillator signal 614 (i.e.,
outside the passband of the optical pre-selector). The optical carriers
that are not near the wavelength of the swept local oscillator signal are
not necessary for optical heterodyne detection and only contribute to
noise in the detection system if not filtered. Optical bandpass filtering
that tracks the wavelength of the swept local oscillator signal is
especially useful when measuring broadband optical noise, such as
amplified spontaneous emissions from an optical amplifier.
Tunable optical pre-selectors, such as those described with reference to
FIGS. 2, 5, and 6 are well known in the field of optical communications
and can be implemented utilizing components such as diffraction gratings,
dielectric interference filters, Fabry-Perot interferometers, and other
known interferometers. The embodiment of FIG. 2 has the optical
pre-selector 214 located between the optical combining unit 210 and the
photodetectors 212 because the polarization states of the beams can be
directly controlled to provide beams with known polarization states.
Reliable bandpass filtering techniques can be utilized to filter beams
having known polarization states. Although the arrangement of FIG. 2 is
preferred, the optical pre-selector can be placed in other locations, such
a location along the optical path of the swept local oscillator signal
that is before the optical combining unit, a location along the optical
path of the input signal that is before the optical combining unit, or
both locations.
Referring to FIG. 2, the two photodetectors 212 are aligned to separately
detect the two filtered beams that are output from the optical
pre-selector 214. The two photodetectors generate electrical signals in
response to a corresponding one of the two filtered beams. The electrical
signals generated by each of the two photodetectors are individually
provided to the processor 216. The two connections between the
photodetectors and the processor are depicted in FIG. 2 by two lines 252.
Although the two photodetectors are depicted as two independent
photodetectors, the two photodetectors may be combined into a single unit
Although not shown, the photodetectors may include additional circuitry
such as signal amplifiers and filters, as is known in the field.
The processor 216 receives the electrical signals from the two
photodetectors 212 and processes the electrical signals into useful data.
The processor may include analog signal processing circuitry and/or
digital signal processing circuitry as is known in the field of electrical
signal processing. In an embodiment, analog signals from the
photodetectors are converted into digital signals and the digital signals
are subsequently processed. It should be understood that digital signal
processing involves converting the electrical signals from the
photodetectors into digital signals that are representative of the
original electrical signals.
Operation of the optical heterodyne detection system described with
reference to FIGS. 2 and 3 involves combining an input signal and a swept
local oscillator signal into a combined optical signal and outputting two
beams of the combined optical signal to the optical pre-selector 214. The
two beams are then filtered by the optical pre-selector 214. The optical
pre-selector passes a wavelength band that tracks the swept local
oscillator signal. The two filtered beams are then detected by the two
photodetectors 212. The two photodetectors generate electrical signals in
proportion to the intensity of the optical beams that are detected. The
electrical signals generated by the two photodetectors are then received
by the processor 216 and processed in a manner that maximizes the signal
to noise ratio of the heterodyne term of the combined optical signal.
Processing of the electrical signals may involve providing intensity noise
suppression. Preferably, during operation, the center wavelength of the
optical pre-selector passband tracks, in real-time, the wavelength of the
swept local oscillator signal. As is described below, the system may
require an initial calibration operation in order to provide accurate
results.
Operation of the optical heterodyne detection system described with
reference to FIGS. 2 and 4 involves combining an input signal and a swept
local oscillator signal into a combined optical signal, outputting one
beam from the optical coupler, and then splitting the beam that is output
from the optical coupler into two polarized beams. As described above, the
combined optical signal that is output from the optical coupler is split
into two polarized beams having different polarization states. The
polarization state of the extraordinary portion of the split beam is then
rotated so that the two polarized beams have the same polarization state.
The two beams are then filtered by the optical pre-selector 214. The
optical pre-selector passes a wavelength band that tracks the swept local
oscillator signal. The two polarized and filtered beams are then detected
by the two photodetectors 212. The two photodetectors generate electrical
signals in proportion to the intensity of the optical beams that are
detected. The electrical signals generated by the two photodetectors are
then received by the processor 216 and processed in a manner that
maximizes the signal to noise ratio of the heterodyne term of the combined
optical signal. Processing of the electrical signals may involve squaring
the electrical signals from the two photodetectors and then adding the two
squared terms to generate an output signal that is independent of the
polarization state of the input signal. Preferably, during operation, the
center wavelength of the optical pre-selector passband tracks, in
real-time, the wavelength of the swept local oscillator signal.
Whether or not the optical combining unit includes a polarizing beam
splitter, the combination of the optical combining unit, the optical
pre-selector, and the photodetectors creates an optical heterodyne
detection system that filters the optical signals to reduce noise and
improve the dynamic range of the system.
As described above, the optical heterodyne detection system of FIG. 2 may
include more than two beams being output from the optical combining unit
210. FIG. 7 is a depiction of an optical heterodyne detection system that
includes four beams being output from the optical combining unit.
FIG. 8 is an expanded view of an embodiment of an optical combining unit
that outputs four polarized beams. The optical combining unit of FIG. 8
includes an optical coupler 820, two output fibers 818 and 828, a
polarizing beam splitter 824, and two half-wave plates 826. As shown in
FIG. 8, the optical coupler combines the input signal and the local
oscillator signal and distributes the combined optical signal into the two
output fibers 818 and 828. The two output fibers carry the combined
optical signal to the polarizing beam splitter 824. Preferably, the power
of the combined optical signal is distributed approximately evenly between
the two output fibers. The optical coupler 820 may be an optically
directional 3 dB fiber coupler, although other optical couplers may be
utilized. In an embodiment, the optical coupler is substantially
independent of the polarization of optical signals. In an embodiment, the
optical coupler does not polarize the combined optical signal.
The polarizing beam splitter 824 splits each of the combined optical signal
beams from the output fibers 818 and 828 into two beams having different
polarization states. Preferably, the polarizing beam splitter splits each
of the incoming beams into two linearly polarized components that have
orthogonal directions of polarization. In an embodiment, the polarization
of the local oscillator signal is controlled such that the power
contributed from the local oscillator signal is split approximately evenly
by the polarizing beam splitter. Although the polarizing beam splitter is
described as a single device, the polarizing beam splitter may include
multiple beam splitters in configurations that accomplish the task of
splitting the incoming beams based on beam polarization.
FIG. 8 shows how the two beams of the combined optical signal are | | |
