Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL SPECTRUM ANALYZER
DESCRIPTION
TECHNICAL FIELD:
This invention relates to optical spectrum analyzers and monochromators of the
kind which use a diffraction grating. The invention is especially applicable
to optical
spectrum analyzers in which the light beam to be analyzed is applied to the
diffraction
grating more than once so as to obtain improved resolution, and to
monochromators for
use therein.
BACKGROUND ART:
The invention is concerned especially with an optical spectrum analyzer of the
kind disclosed in United States patent No. 5,886,785 issued March 1999 naming
H.
Lefevre et al. as inventors. Lefevre et al. disclosed an optical spectrum
analyzer
comprising a diffraction grating and a dihedral reflector. The input light
beam for
analysis is received via an input/output port, collimated, and passed through
a
polarization beam splitter which splits the light beam into two linearly-
polarized
components having their respective directions of linear polarization mutually
perpendicular. The transmitted beam is passed through a waveplate which
rotates its
direction of polarization through 90 degrees so that the two beam components
leaving
the beam splitter are directed onto the diffraction grating with their
directions of
polarization parallel to each other and perpendicular to the groove direction
of the
diffraction grating.
Following diffraction, the light beam components are directed to the dihedral
reflector which reflects them back to the diffraction grating. Following
diffraction a
second time, the light beam components are returned to the polarization beam
splitter
which recombines them and passes the recombined light beam through the
collimator in
the opposite direction to focus it and direct it to the input/output port. In
traversing the
diffraction grating and dihedral reflector the light beam components both
follow exactly
the same path, but in opposite directions.
A disadvantage of their design arises from the fact that the light beam
components
are recombined and leave via the same exit port. Specifications for optical
spectrum
analyzers require the back-reflection caused by the analyzer to be below
certain levels
so as not to affect the equipment which is being tested. Consequently, an
optical
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circulator can be used in the approach of Lefevre et al. to separate the input
light beam
from the output light beam and avoid back reflection. The insertion loss and
isolation
of the circulator vary with wavelength and imperfections in the circulator
introduce
cross-talk, i.e coupling of energy from the input light beam directly to the
output light
beam within the circulator.
Instead of a circulator, Lefevre et al. could use a coupler to separate the
input
and output light beams and an isolator to reduce optical back reflection
significantly.
Both the insertion loss and the isolation capability of such an isolator
usually are
wavelength dependent. Moreover, the coupler would introduce insertion loss of
at least
6dB, e.g. 3dB in each direction for an ideal 3dB (50/50) coupler. Also,
directivity of
the coupler introduces cross-talk between unfiltered input and output ends.
In general, the use of components such as couplers, circulators and isolators
is
accompanied by an inevitable inherent wavelength-dependent polarization-
dependent loss,
that cannot readily be compensated for, or taken into account.
With the increasing use of Dense Wavelength Division Multiplexing (DWDM),
optical spectrum analyzers may be used to scan as many as 128 wavelengths. In
view
of this level of direct cross-talk, the optical "noise floor" existing at the
detector/receiver
will increase proportionally with the number of channels, whereas the signal
strength of
each individual channel is fixed. This degrades optical signal-to-noise ratio
(OSNR) of
the instrument.
A further disadvantage arises from the fact that the separation of the input
beam
into constituent, orthogonal polarization states occurs within the
monochromator section
of their design, where the light beams are propagating in free space. This
introduces a
complication in the optical design and the choice of components, since the
beam size may
be limited by the clear aperture of the polarization beam splitter, which for
cost and
availability reasons should be kept as small as possible. On the other hand,
the
maximum spectral resolution is obtained when the largest possible number of
grating
grooves are illuminated. The addition of a beam expander (e.g. with anamorphic
prisms)
to avoid this problem would be unsatisfactory because it would be expensive
and
unwieldy.
It is noted that Lefevre et al. apparently recognised that the need for an
optical
circulator or 3-dB coupler could be avoided by placing a separate output fiber
immediately adjacent to the input fiber. However, this modification would not
entirely
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solve the problem of significant back-reflectiori into the input fiber and
would not reduce
OSNR degradation caused by back-scattering, whereby light scattered by
components
within the monochromator is received by the output fiber.
It is desirable to avoid, or at least reduce, any undue wavelength-dependent
polarization-dependent loss in this measurement i-nstrument.
According to Lefevre et al., their optical spectrum analyzer is polarization
insensitive. In practice, however, there is a wavelength-dependent loss
resulting from
polarization dependence of components used in their design, particularly the
waveplate.
This waveplate exhibits a 7J2 retardance, result.iztg in a 90-degree rotation
of the linear
polarizatxon, for a particular wavelength. As the wavelength of the incident
light beam is
tuned away from that wavelength, the mgte of rotation provided by the
waveplate will
vary. Consequently, that beam component not having its linear state
ofpolarization (SOP)
perpendicular to the grooves of the grating will suffer increased attenuation
as compared
with the eorxaponent which has its linear SOP perpendicular to the grooves.
Although this wavelength-dependent loss can be compensated in the frmware of
the analyzer, it results in a limitation in the ultimately attainable
instrumental sensitivity.
DISCLOSURE OF INVENTION:
The present invention seeks to avoid or at least mitigate the afore-mentioned
disadvantages.
According to one aspect of the present invention, an optical spectrum analyzer
comprising a diffraction grating (DG), input mean.s ('PDM,i1,I2) comprising
means (PDM)
for decomposing an input light beam to provide first and second light beams
(LR,LT) each
having a linear state of polarization corresponding to a respective one of two
mutually-perpendicular linear states of polarization of the input light beam,
and means
(I1,I2) for directing the first and second light beams onto the diffraction
grating (DG), and
output means for directing the first and second lights beams after diffraction
to two output
ports (01,02) such that each port receives substantially exclusively
arespective one ofthe
light beams at or about a selected wavelength after diffraction by the
difl'raction grating,
the arrangement being such that, at any particular wavelengtii within an
opera.ting band of
the analyzer, the state of polarization of each of the first and second light
beams remains
substantialIy unchanged with respect to time.
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Each of the first and second light beams may be incident upon the grating with
its linear of polarization having any prescribed angle with respect to a
corresponding
plane of diffraction.
Preferred embodiments of the invention further comprise means for effecting
wavelength-independent rotation of one or both of the linear states of
polarization of the
first and second light beams so that the two linear states of polarization are
aligned
parallel to each other, and the means for directing the first and second light
beams onto
the diffraction grating does so with the linear states of polarization of the
first and second
light beams, respectively, perpendicular to grooves of the diffraction
grating.
The polarization decomposing means may comprise a polarization beam splitter
coupled to the monochromator section by a pair of polarization maintaining
fibers. One
or both of the polarization-maintaining fibers may be twisted to provide a
required
rotation of the state of polarization of the light beam passing therethrough.
According to a second aspect of the invention, there is provided a
monochromator
comprising a diffraction grating, means for directing first and second light
beams of
originally mutually-perpendicular linear states of polarization onto the
diffraction grating,
and two output ports each for receiving, substantially exclusively, a
respective one of the
linearly-polarized light beams at or about a selected wavelength after
diffraction by the
diffraction grating, the arrangement being such that, at a particular
wavelength, the state
of polarization of each of the first and second light beams is preserved.
Preferably, each of the two linearly-polarized light beams is incident upon
the
diffraction grating with its state of linear polarization parallel to a plane
of diffraction
of the diffraction grating (i.e. perpendicular to groove direction).
The monochromator may further comprise reflector means for reflecting the
light
beams leaving the diffraction grating after diffraction a first time so as to
return them to
the diffraction grating again with the same state of polarization and at a
position
displaced laterally from the position at which the light beams were first
incident
thereupon, the two output ports receiving respective ones of the linearly-
polarized light
beams after diffraction a second time.
In this specification, the term "groove" embraces both the physical grooves in
a
ruled diffraction grating and their functional equivalent in, for example, a
holographic
grating.
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Various features, advantages and objects of the invention will become apparent
from the following desctiption of a preferred embodirrrent which is described
by way of
example only with reference to the accompanying drawings.
5 BRIEF DESCRIPTTON OF THE DRAWINGS:
Figure 1 is a simplified schernatic perspective diagram of an optical spectrum
artalyzer (OSA) embodying the invention which comprises a polarization
alignment unit,
a monochro:mator section, and a pair of output ports;
Figure 2 is a simplified side view of the monochrornator section of the OSA;
Figure 3 is a schematic view showing the polarization alignment unit of the
OSA.
in more detail;
Figure 4 is a detail view illustrating a first modification to the
monochromator
section; and
Figure 5 is a detail view illustrating a second modification to the
monochromator
section.
BEST MODE(S) FOR CARRYING OUT THE INVENTION:
Referring first to Figures 1 and 2, an optical speetruan analyzer comprises a
wavelength-independent polarization demultiplexer unit PDM (not shown in
p'igure 2) and
a monoclzromator section MR. As shown in Figure 1, the wavelength-independent
polarization demultiplexer PDM has an iiiput port to which the input light
beam for
analysis is supplied via an optical fiber p' and two output ports OP I and
OI'2 for first and
second light beams LR and LT, respectively, having mutually orthogonal linear
states of
polarization. The output ports OP1 and OP2 are coupled to the zrAonochromator
section
MR by polarization-maintaining (PM) fibers PMF1 and PMF2, respectively, in
ordez that
the first and second light beams I.R and LT to the monochromator section MR
with their
respective linear states of polarization (SOP) aligned along a corresponding
one of the
birefringence axes of each oftbe said PM fibers. The two PM fibers PMFI and
P1VIF2 zrray
be single mode or multi-mode at the wavelengths of operation. The
birefringence axis of
the second polarization-maintaining fiber PMF2 is twisted through 90 degrees
relative to
that ofthe first polarization-maintaining fiberPMF1 so that, on arrival of the
two linearly-
polarized light beams LT and LR at the input ports 11 and I2 of the
monochromator section
MR, their SOPs are parallel to each other. The input pozts 11 and 12 ofthe
monochromator
section MR comprise respective ends of the first and second polarization-
maintaining
fibers PMF1 and PMF2 terminated in first and second fiber array tenninations
FPI/1 and
FPI/2, respectively, whicb, constitute a so-called fiber arrayFP1. Thus,
proximal ends of
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the polarization maintaining fibers PMF 1 and PMF2 are connected to output
ports OP 1, and
OP2, respectively, of the wavelength independent polarization demultiplexer
PDM and
their distal ends are connected to first and second fiber array terminations
FP 1/1 and FP 1/2,
respectively. The first and second fiber array terminations FP1/1 and FP1/2
may be of
known kind, for exxarnple, having two V-grooves or capillaries into which the
ends of the
fibers PMFI and PMF2 are bonded so that the spaeing between their cores is
accurately
determined and they are orientated so as to direct the light beams LR and LT
towards a
eollimatizig lens Li in the monochromator section MR.
In addition to the fiber array terminators FPl/1 and k'P1/2, and the input
collimating lens Ll, themonochrozztator scetion MR comprises an output
focusing lens L2,
a plane "folding" mirror M (to render the design more eompact), a right-angled
dihedral
reflector RAM, such as a roof mirror or Porro prism, a reflecting diffraction
grating DG
and an output fiber array FP2 formed by fiber array terminatoxs FP2/1 and
FP2/2, similar
to array terminators FP1/l and FP1/2 and output fibers OFI and OF2. The plane
mirror
M is optional. It should be appreciated that the ends of each of the input
fibers PMF1 and
PMF2 and the output fibers OF 1 and OF2 arc the equivalents of the input slits
and output
slits, respectively, of a classic moztochromator design.
Wavelength selection is effected by rotating either the dihedral reflector RAM
or
the diffraction grating DG, or both of them together. In this preferred
embodiment, the
dihedral reflector RAM is mounted upon tuaning means in the form of a
turntable device
TT, allowing it to be rotated relative to the diffraction grating DG for
scanning tbrough the
required range of wavelengths. It should be noted that the light beams from
fibers FP1/1
and FP1/2 are focused onto fibers FP2/1 and FP2/2, respectively. The
monochromator
section MR outputs the scanned light beams via the second pair of fibers Fp2/
1 and FP2/2,
respectively, respective ends of which comprise a pair of output ports 01 and
02 of the
monochromator section MR and convey the two output Iight beams LR and LT to an
output stage which comprises a pair of detectors D1 a.nd. D2, respectively,
whicb may be
photodiodes, for example. The detectors D I and D2 may be coupled to a
microprocessor
(not shown) which processes the corresponding electrical signals from the
detectors. It
should also be noted that the output fibers FP2/1 and FP2/2 could be replaced
by slits in
front of the detectors, or by an appropriate lens/detector arrangement.
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In each of the fiber pairs or "arrays" FP1 and FP2, the inter-fiber separation
is
greater than the "spot size" in the non-dispersive dimension (i.e., the
vertical direction
of Figure 1) of a signal in the focal plane, by such an amount that cross-talk
is
substantially avoided. On the other hand, the fibers are sufficiently close
together that
the two beams follow nearly parallel paths in order substantially to avoid
aberrations.
In practice, the separation between centres is about 0.25mm.
As shown in Figure 3, the wavelength-independent polarization demultiplexer
PDM comprises three fiber collimators FCl, FC2 and FC3 and a polarization beam
splitter PBS. The fiber collimator FC3 receives the input fiber-guided light
beam and
converts it into a collimated, free-space beam which it directs to the
polarization beam
splitter PBS. The latter separates the input light beam into two light beams
LT and LR,
respectively, having mutually-perpendicular linear states of polarization
(SOPs)
corresponding to original mutually-perpendicular states of polarization of the
input light
beam. The polarization beam splitter PBS directs linearly-polarized light beam
LR to
fiber -collimator FCl and directs the complementary, orthogonal linearly-
polarized light
beam LT to fiber collimator FC2. The fiber collimators FC1 and FC2 focus the
two
light beams LR and LT, respectively, into proximal ends of the polarization
maintaining
fibers PMF1 and PMF2, in each case with the linear state of polarization (SOP)
of the
launched light aligned with one of the birefringent axes ("slow" or "fast") of
the
associated one of the PM fibers PMF1 and PMF2. In this particular embodiment,
for
example, the fiber PMF1 conveys that portion of the initial beam energy
corresponding
to vertical linear polarization, while fiber PMF2 conveys that corresponding
to horizontal
linear polarization, as indicated in Figure 3.
Referring again to Figures 1 and 2, the distal ends of polarization
maintaining
fibers PMF1 and PMF2 are terminated at, and fixed in, the input fiber array
FP1 of the
monochromator section MR. Before fixing, one or both of the polarization-
maintaining
fibers PMF1 and PMF2 in order is/are manipulated to ensure that the linear
state-of-
polarization (SOP) of the light beams LT and LR exiting from the ends of these
two
fibers have the same, predetermined spatial orientation - in this case
parallel to each
other. An example of such a manipulation could be twisting of one of the two
fibers
with respect to the other.
The fibers FP 1/ 1 and FP 1/2 at the input of the monochromator section MR
direct
the two polarized light beams LR and LT, respectively, onto collimating input
lens Ll
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of the monochromator section MR and are oriented so that the light beams' SOPs
will
be parallel to the plane of diffraction, i. e. , perpendicular to the grooves,
when incident
upon the diffraction grating DG. As they traverse the other components of the
monochromator section MR, the two polarized light beams LR and LT follow
similar,
but not strictly parallel, paths. For clarity of depiction, however, the path
of only one
of the polarized light beams, LR, is shown in Figures 1 and 2. Thus, on
leaving the lens
L1, the collimated light beam LR is reflected by plane mirror M onto the
diffraction
grating DG so that the state of polarization of the light beam LR is
perpendicular to the
grooves of the grating DG. Following reflection and diffraction by the
diffraction
grating DG, the light beam LR is directed to the right-angled dihedral
reflector RAM.
The arrangement is such that the light beam LR iinpinges upon one of the
facets of the
dihedral reflector RAM at a first angle of the order of 45 degrees, and is
reflected to the
other facet, which reflects it again at the 90-degree complement of the first
angle so that
it leaves the dihedral reflector RAM in the opposite direction to that of its
arrival and
is incident upon the diffraction grating DG again, but at a position displaced
perpendicularly with respect to the plane of diffraction in which it was first
incident.
The diffraction grating DG reflects and diffracts the light beam LR again and
directs it
via plane mirror M onto output lens L2, which refocusses it into the end of
fiber FP2/1
of the second fiber pair FP2. Light beam LT follows a similar path after being
directed
onto collimating lens Ll by fiber FP1/2 but is refocussed by lens L2 into the
end of fiber
FP2/2 of the fiber pair FP2.
Upon leaving the other (output) ends of the fibers FP2/1 and FP2/2,
respectively,
the light beams LR and LT impinge upon detectors D 1 and D2, respectively.
Optical
fibers OF 1 and OF2 couple the array-fibers FP2/1 and FP2/2 to detectors D 1
and D2,
respectively, so that the diffracted and refocussed light beams LR and LT are
applied to
the detectors D 1 and D2, respectively. The detectors Dl and D2 supply their
corresponding electrical signals to a microprocessor MP for processing in the
usual way,
which might entail combining them electrically. Of course, the detectors could
be
omitted and optical fibers OF 1 and OF2 could convey the light beams LR and LT
elsewhere for subsequent detection, processing or analysis. Alternatively, the
fibers OF1
and OF2 could be omitted and the array fibers FP2/1 and FP2/2 could supply
light
beams LR and LT directly to the detectors.
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It should be noted that the polarized light beams LR and LT pass through the
respective ones of the fibers FP1/l and FP1/2 and the lens L1 to the
diffraction grating
DG and, on leaving the grating DG, via the lens L2 to the corresponding one of
the
fibers FP2/ 1 and FP2/2, respectively.
Because the decomposition of the input light beams SOP occurs outside of the
free-space optics of the monochromator section MR, one is not constrained by
such
practical issues as the clear aperture of the polarization beam splitter PBS
when
determining the working diameters of the lenses Li and L2. Hence, a relatively
large
beam diameter can be used, facilitating the illumination of a large number of
grating
grooves without having to maintain the grating at an extremely oblique and
difficult-to-
adjust grazing-incidence angle with respect to the incident beams.
Subject to practical limitations on the physical size of the equipment, the
lenses
and diffraction grating can be relatively large, so as to obtain better
resolution.
It is envisaged that either or both of the lenses Ll and L2 could be replaced
by
a concave mirror and the above-mentioned advantages still realised. Thus,
Figure 4
illustrates an off-axis paraboloid mirror PMl for receiving and collimating
the light
beams from fiber array FP1, before supplying them to the diffraction grating
DG/plane
mirror M (not shown in Figure 4). Figure 5 illustrates an off-axis paraboloid
mirror
PM2 for receiving collimated light beams from the diffraction grating DG/plane
mirror
M (not shown in Figure 5) and focusing them onto the output fiber array FP2.
When
one or more such off-axis paraboloid mirrors PM1,PM2 are used, it might also
be
possible to omit the plane mirror M and still achieve a compact design.
It should be noted that, because the above-described preferred embodiment of
the
present invention avoids the use of a waveplate, whose polarization-
transforming
properties are inherently dependent upon wavelength, the linear states of
polarization of
the light beams LR and LT exiting the two fibers FP1/1 and FP1/2,
respectively, can be
oriented so as to lie in the plane of diffraction in order to minimize the
losses in the
monochromator section MR and maximize the overall performance of the optical
spectrum analyzer across a very wide wavelength range. Likewise, the absence
of other
inherently wavelength dependent elements, such as isolators, circulators or 3-
dB
couplers, allows embodiments of the present invention to maintain their
performance
over a wide spectral range.
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It should also be noted that embodiments of the present invention which use
two
separate detectors D 1 and D2, which can be independently calibrated via a
microprocessor, allow for increased flexibility in the optical design and
alignment. For
instance, although the overall performance of embodiments of this invention,
in
5 particular their optical sensitivity and the- independence of this optical
sensitivity to the
state of polarization of the signal to be measured, is optimized when the
states of
polarization of the two light beams emanating from FP1/1 and FP1/2,
respectively, are
parallel to each other and parallel to the plane of diffraction of the
grating, embodiments
of this invention can also function with degraded sensitivity specifications
if these two
10 light beams have different, arbitrary and even wavelength-dependent states
of
polarization, provided that, when the light beams arrive at the output array
FP2, their
states of polarization do not change with time at any given wavelength. This
is a
consequence of the fact that, for a given state of polarization and wavelength
of a beam
passing through the monochromator section MR, the polarization and wavelength
dependencies of the detection can be calibrated in the microprocessor.
Hence, although it is preferable to use a twisted polarization-maintaining
fiber to
rotate the state of polarization of one or each of the light beams, it would
be possible to
use a wavelength-dependent rotation device, such as a waveplate, instead, and
calibrate
the optical spectrum analyzer (specifically the microprocessor) over the
normal range of
wavelengths so as to ensure consistent measurements at any particular
wavelength. This
is possible because the first and second light beams are not recombined after
leaving the
diffraction grating and before detection. As mentioned hereinbefore, the use
of a
waveplate could limit the ultimately-attainable sensitivity.
The beam splitter PBS may be a conventional polarization beam splitter which
can
handle a range from 400 nanometers to 2000 nanometers approximately. Such beam
splitters are readily available.
It should also be noted that the diffraction grating DG could be, for example,
a
holographic grating, or any other suitable kind of grating. It is also
envisaged that the
tuning means could rotate the diffraction grating DG instead of, or in
addition to, the
dihedral prism RAM.
It should be appreciated that the invention comprehends a monochromator formed
by omitting the detectors D I and D2, the microprocessor MP, and possibly the
rotation
device TT, and adding means for recombining the diffracted light beams LR and
LT.
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Then the two light beams LR and LT could be combined optically using, for
example,
a polarization multiplexer or the above-described polarization beam splitter
PBS in
reverse.
INDUSTRIAL APPLICABILITY
Advantageously, in embodiments of the present invention, the states of
polarization of the first and second light beams do not change substantially
with time
regardless of typical environmental changes, such as normal fluctuation in
temperature
and vibration.
Moreover, an advantage of embodiments of the invention, in which the input
ports and output ports are completely separate, is that the difficulties of
back reflection
along the optical input path, and direct cross-talk between input and output,
are avoided
substantially completely, which is very important for high density wavelength
division
multiplexing (HDWDM) applications.
