Note: Descriptions are shown in the official language in which they were submitted.
WO 01/65221 cA 02401977 2002-08-30 PCT/CAOI/00248
1
POLARIZATION INDEPENDENT PHOTODETECTOR DEVICE AND
METHOD OF MAKING SAME
DESCRIPTION
TECHNICAL FIELD:
The invention relates to photodetector devices, especially semiconductor
photodetector devices, and to a method of compensating for polarization
dependent
response of a photodetector of such a photodetector device. The invention is
applicable
to so-called "pigtailed" photodetector devices which have an integral optical
waveguide,
for example an optical fiber, for directing light onto a detection surface of
the detector,
and to so-called "connectorized" photodetector devices which have means for
attachment
of a separate optical waveguide.
BACKGROUND ART:
Photodetector devices preferred for use in, for example, telecommunications
systems, use semiconductor diodes, usually made of germanium or indium gallium
arsenide (InGaAs). Where relatively long wavelengths are involved, such as in
Dense
Wavelength Division Multiplex (DWDM) systems, InGaAs detectors are preferred
over
germanium detectors because the former exhibit better temperature stability
and more
uniform sensitivity, especially in the longer wavelength spectral ranges of
typical
DWDM telecommunications systems. They also have a lower dark current. Both of
these kinds of photodetector devices exhibit a response that depends upon the
state of
polarization (SOP) of the incident light. Unfortunately, however, InGaAs
photodetector
devices have a relatively high polarization dependent response (PDR) as
compared with
germanium photodetectors. For InGaAs photodetector devices, the PDR, i.e., the
difference between maximum and minimum responses, typically is around 0.02 dB
whereas, for germanium photodetectors, the typical PDR is about 0.005 dB.
Consequently, when an InGaAs photodetector device is used to measure, for
example,
polarization dependent loss, the measurement accuracy is limited because of
the relatively
high PDR of the photodetector device.
An object of the present invention is to at least ameliorate the afore-
mentioned
problem of polarization dependency of photodetector devices which have a
maximum
WO 01/65221 cA 02401977 2002-08-30 PCT/CA01/00248
2
response to incident light corresponding to a particular substantially linear
SOP of that
light.
DISCLOSURE OF INVENTION:
According to a first aspect of the present invention, there is provided a
photodetector device comprising a photosensitive detector having a
polarization
dependent response having maximum (R,~,p,X) and minimum (RM~,) values
corresponding
to substantially linear states of polarization of light incident thereupon
that are orthogonal
to each other, and at least one interface through which a light beam for
detection passes
before being incident upon a detection surface of the detector, the
arrangement being
such that the light beam traverses said at least one interface with a
propagation direction
that deviates from the normal to the interface so as to introduce polarization
dependent
transmission to compensate at least partially for the polarization dependent
response of
the photosensitive detector.
In one embodiment of the first aspect of the invention, the device has a
window
whereby, in use, the light is directed onto the detection surface, the
detection surface and
opposite surfaces of the window providing three said interfaces, cumulatively
providing
the compensation for polarization dependent response.
The photodetector device may have a connector part for releasably attaching to
the device a waveguide for directing the light to said window along an optical
axis (OA)
of the waveguide, each of said three interfaces being at an acute angle to
said optical
axis.
The window may be fixed generally parallel to the detector surface and both
tilted
to the required degree, in which case three interfaces will provide the
required
compensation. Alternatively, the detection surface may be substantially normal
to the
propagation direction of the light when incident thereupon and opposite
surfaces of the
window provide two said interfaces that cumulatively provide the compensation
for
polarization dependent response.
The photodetector device may be mounted to a mount with its maximum response
axis having a predetermined orientation relative to a reference surface of the
mount. The
mount may then be installed on or in the connector part so as to allow tilting
of the
mount about the maximum response axis or an axis parallel thereto. The
connector part
may be part of, or fitted to, equipment in which the photodetector device is
to be used.
CA 02401977 2002-08-30
WO 01/65221 PCT/CA01/00248
3
This arrangement allows PDR compensation to be provided after the
photodetector device
has already been assembled.
Alternatively, the interface may comprise an angled end surface of a
waveguide,
typically an optical fiber, through which the light is directed onto the
detector, either
directly or by way of another interface provided by a said window or an
intervening
lens. In the latter case, the required polarization dependent transmission
effect would
be provided cumulatively by the interfaces.
In a further embodiment of the first aspect of the invention, the
photodetector
device comprises a photodetector, a lens and an input optical fiber for
directing a light
beam, in use, through the lens and onto the detection surface, the optical
fiber having
its end from which the light beam emerges secured relative to the lens with a
lateral
displacement relative to the lens optical axis so that the optical axis of the
light beam
when leaving the lens will be inclined relative to the lens optical axis by
such an angle
that polarization dependent transmission introduced thereby substantially
corrects for
polarization dependent response of the photodetector per .se.
According to a second aspect of the present invention, there is provided a
photodetector device comprising a photosensitive detector and a fiber
waveguide for
directing a light beam for detection onto a detection surface of the detector,
the
waveguide being fixed relative to the detector and having an end face facing
the detection
surface, the end face being inclined by such an angle that polarization
dependent
transmission effects introduced by the end face at least partially compensate
for
polarization dependent response of the detector.
According to a third aspect of the invention, there is provided a method of
correcting for polarization dependent response of a photodetector device
comprising a
photosensitive detector having a maximum response (R,~,,~x) corresponding to
linear state
of polarization of light incident thereupon via at least one interface between
media having
different refractive indices, the interface intersecting a propagation
direction in which,
in use, a light beam for detection will be incident upon a detection surface
of the
detector, the method comprising the steps of:
determining orientation of either or both of a maximum response axis and
minimum response axis that correspond to states of polarization of
incident light for which the response of the photodetector is a maximum
or minimum, respectively; and
CA 02401977 2002-08-30
WO 01/65221 PCT/CA01/00248
4
adjusting an angle between the propagation direction and the interface in
a plane of the maximum response axis such that polarization dependent
transmission introduced by the interface at least partially compensates for
polarization dependent response of the detector.
The reference to a direction in which a light beam will, in use, be incident
upon
the detector is intended to take account of the fact that the light beam may
be collimated
or divergent. In each case, the light beam will be substantially symmetrical
about its
beam/cone axis which, preferably, extends substantially parallel to the
optical axis and
intersects the detector at or near its geometrical centre; otherwise the
detector must be
considerably wider than the beam spot.
The determination of the maximum response axis of the photosensitive detector
may comprise the steps of directing a substantially symmetrical, polarized
light beam
onto the detector and monitoring a corresponding output signal of the
detector; varying
the state of polarization between a substantial number of possible states;
identifying
either or both of maximum and minimum values of the detector output signal;
and,
while maintaining that state of polarization which provided the maximum or
minimum
value, analyzing the light to determine the orientation of the maximum
response axis of
the detector.
Preferably, the light beam is collimated, though it could be converging or
diverging.
The accuracy of the determination will depend upon the number of polarization
states selected. While it would be possible to use the subset comprising
linear SOPS, the
polarization controller permits the selection of substantially all of the
points on a
Poincare sphere. Such variable polarization controllers are well known.
While it is preferable to use a highly-polarized light beam, it is possible to
use
any light beam having a significant non-zero degree of polarization.
Accordingly, an alternative embodiment of the third aspect of the invention is
a
method of determining the maximum-response-axis response of the photosensitive
detector comprising the steps of directing linearly polarized light onto the
detector along
the optical axis, repeatedly rotating the SOP of the linearly polarized light
beam about
the optical axis, tilting the at least one interface relative to the optical
axis and about the
maximum response axis, and determining said angle for which the difference
between
maximum and minimum responses of the photodetector is substantially minimal.
CA 02401977 2002-08-30
WO 01/65221 PCT/CA01/00248
The subsequent determination of the required tilt angle may comprise the steps
of directing linearly polarized light onto the detector along the optical
axis, repeatedly
rotating the SOP of the linearly polarized light beam about the optical axis,
tilting the
interface relative to the optical axis and about the maximum response axis,
and
5 determining the tilt angle for which the difference between maximum and
minimum
responses of the photodetector is substantially minimal.
Alternatively, the required tilt angle may be determined by the steps of:
directing
a polarized light beam onto the detector and monitoring a corresponding output
signal
of the detector; varying the state of polarization between a substantial
number of possible
states; and varying the angle of the interface to select an angle for which
the difference
between the maximum and minimum values of the output signal is substantially
minimal,
the selected angle being said tilt angle.
The photodetector device may be mounted rotatably in a mount while its
maximum response axis is being determined and, once the maximum response axis
is
known, secured to the mount with the maximum response axis having a known
orientation relative to a reference surface of the mount. The mount then may
be used
to install the photodetector into a holder or other part of, or for assembly
to, the
equipment with which the photodetector is to be used. Once the mount is
installed, the
tilt angle can be determined by directing a linearly polarized light beam onto
the detector
along said optical axis; repeatedly rotating the SOP of the linearly polarized
light beam
about the optical axis, tilting the mount relative to the holder and about the
maximum
response axis and measuring the output of the photodetector so as to determine
the tilt
angle at which the difference between maximum and minimum responses of the
photodetector is minimal, whereupon the mount can be secured to the holder so
as to
maintain that tilt angle.
The interface may be at the surface of the photosensitive detector or at a
surface
of a window through which the light to be detected is incident upon the
photosensitive
detector. Such a window may provide two such interfaces.
In one embodiment of the third aspect of the invention, the photosensitive
detector
is housed in a housing having such a window; the step of tilting the interface
involves
tilting the window relative to the detector surface, and the securing step
then involves
fixing the window relative to the housing. This method is suitable for use
during
manufacture of the photodetector device.
WO 01/65221 cA 02401977 2002-08-30 PCT/CA01/00248
6
In an alternative embodiment of the third aspect of the invention, the
detector and
window are fixed relative to each other in a housing of the photodetector
device and the
step of tilting the interface involves tilting of the housing, so that the
window and the
detector surface are tilted together. This method is suitable for use after
the
photodetector device has been manufactured.
According to a fourth aspect of the invention, a method of assembling a
photodetector device according to such further embodiment comprises the steps
of
directing a polarized light beam onto the detector and varying the state of
polarization
of the light beam between a large number of possible states; adjusting the
displacement
of the optical fiber transversely relative to the optical axis of the lens
while monitoring
the difference between maximum and minimum values of the detector output
signal; and
fixing the optical fiber relative to the lens at a transverse displacement
corresponding to
the difference being a minimum.
Hence, in this case, it is not necessary to determine the maximum and minimum
polarization dependent response axes of the detector beforehand.
Preferably, the light beam emerging from the lens is substantially collimated,
though it could be converging or diverging.
The accuracy of the adjustment will depend upon the number of polarization
states
selected. While it would be possible to use the subset comprising linear SOPS,
the
polarization controller permits the selection of substantially all of the
points on a
Poincare sphere. The variable polarization controller may comprise a
polarization
scrambler for randomly varying the SOP, or a controller which varies the SOP
systematically. Such polarization controllers are well known.
While it is preferable to use a highly-polarized light beam, it is possible to
use
any light beam having a significant non-zero degree of polarization.
In one embodiment of the fourth aspect of the invention, the detector is
mounted
in a housing of the photodetector device and the lens is attached to the
housing so as to
direct light onto the detector through an opening in the housing. The lens
receives the
light from a fiber waveguide which then is offset laterally with respect to
the optical axis
of the lens to vary the angle at which the light impinges upon the detector
surface.
According to a further aspect of the invention, there is provided a method of
correcting for polarization dependent response of a photodetector device
comprising a
photosensitive detector (26;96) having a maximum response (R"~,,,X)
corresponding to
WO 01/65221 CA 02401977 2002-08-30
PCT/CA01 /00248
7
linear state of polarization of light incident thereupon via at least one
interface
(20',20",28,98) between media having different refractive indices, the
interface
intersecting a propagation direction in which, in use, a light beam for
detection will be
incident upon a detection surface (28;98) of the detector (26; 96), the
photosensitive
detector having a photodetector axis (PDA) perpendicular to its detection
surface, the
method comprising the steps of:
with the photodetector device mounted rotatably in a connector part (16) with
said
photodetector axis (PDA) extending at an arbitrary acute angle (O) to an
optical axis
(OA) of said connector part, directing polarized light onto the detection
surface (28;98);
while varying the state of polarization of the light successively between a
substantial number of points on the Poincare sphere, monitoring an electrical
output
signal from the photosensitive detector (26;96) and registering the difference
between
maximum and minimum values thereof;
rotating the photodetector device (12) step by step using the photodetector
axis
(PDA) as a rotation axis, at each step, registering the difference between the
maximum
and minimum response and determining the rotation angle for which the
difference is a
minimum;
while maintaining said rotation angle constant, varying the state of
polarization
again through a substantial number of points on the Poincare sphere,
monitoring the
output of the photosensitive detector (26;96), adjusting the acute angle (8)
with respect
to the optical axis (OA), in a step by step manner, and determining the acute
angle at
which the difference between the maximum and minimum values is a minimum; and
securing the photodetector device to the connector part ( 16) to maintain said
rotation angle and said acute angle at which the difference between the
maximum and
minimum values is a minimum.
Typically, the acute angle is between 5 degrees and 8 degrees.
Embodiments of the various aspects of the present invention will now be
described by way of example only and with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 is an exploded perspective view of a photodetector assembly assembled
by a method according to a first embodiment of the invention;
Figure 2 is an exploded plan view of the assembly of Figure 1;
WO 01/65221 cA 02401977 2002-08-30 PCT/CA01/00248
8
Figure 3 is a cross-sectional plan view of the photodetector assembly with the
parts assembled;
Figure 4 illustrates tilting of the surface of a photosensitive detector and a
window through which light is incident upon the surface;
Figure 5 illustrates, schematically, equipment for determining maximum
response
axis;
Figure 6 is a simplified cross-sectional plan view of a photodetector device
according to a second embodiment of the invention;
Figures 7 and 8 illustrate further modifications of the device;
Figure 9 is a schematic side elevation of an alternative photodetector
assembly
assembled by a method according to another aspect of the invention;
Figures 10(a) and 10(b) depict, from the side and one end, the orientations of
the
various optical axes of the components of the device of Figure 9;
Figure 11 illustrates equipment used in adjusting the device of Figure 9
during
manufacture;
Figure 12 illustrates a first modification of the device of Figure 11;
Figures 13 and 14 illustrates further modifications of the device of Figure
11; and
Figure 15 illustrates a photodetector device assembled by yet another method.
BEST MODES) FOR CARRYING OUT THE INVENTION:
Referring to Figures 1, 2 and 3, a photodetector assembly 10 comprises a
photodetector device 12, a mount in the form of a bracket 14 and a holder 16,
the latter
for securing the photodetector device 12 to a panel (not shown) of, for
example, a piece
of equipment such as an optical power meter. The photodetector device 12 has a
generally cylindrical housing 18 having at one end a glass window 20 and,
protruding
from its other end, a pair of conductors 22 and 24. As shown in Figure 3,
within the
housing 18, a photosensitive detector 26, specifically an InGaAs photodiode,
is
positioned with its light-detecting surface 28 parallel to the glass window
20. The
bracket 14 is L-shaped with a pair of perpendicular arms 30 and 32. A
cylindrical hole
34 extends through the arm 30 and is large enough to receive the photodetector
housing
18. When the photodetector device 12 is inserted fully, a lip 36 protruding
radially
around the rearmost edge of the housing 18 abuts the bracket arm 30. The other
arm
32 has a pair of holes 38 whereby it can be attached to a sidewall 40 of a
recess 42
WO 01/65221 CA 02401977 2002-08-30
PCT/CA O 1 /00248
9
(Figure 3) in one side of a flat portion 44 of the holder 16 by screws (not
shown). A
cylindrical boss 46 protrudes from the opposite side of the plate 44 and has
an external
screwthread. In use, the holder 16 will be attached to the outside of the
instrument's
panel and secured by screws (not shown) extending through holes 48 in the flat
portion
44. A fiber connector 50, of known construction, is used to connect an optical
fiber 52
to the holder 16 by way of an adapter 54. The fiber connector 50 has at one
end a
ferrule 56 to which the optical fiber 52 is connected, and a surrounding
sleeve 58 which
is screwthreaded internally. The adapter 54 has a key slot 55 to engage a
corresponding
key (not shown) of the connector 50. This allows use with industry standard
connectors.
The adapter 54 is used because the diameter of the sleeve 58 is less than the
diameter of the screwthreaded boss 46 of the holder 16. The adapter 54
comprises a
first cylindrical portion 60 which has an internal screwthread matching that
of the boss
46 and a second, smaller cylindrical portion 62 which has an external
screwthread
matching that of the sleeve 58. When the adapter 54 is screwed onto the boss
46 and
the fiber connector 50 attached to the adapter 54 by screwing the sleeve 58
onto the
adapter 54, the cylindrical axis of the optical fiber 52 is aligned with the
cylindrical axis
of the holder 16.
As can be seen more clearly in Figure 3, the bracket 14 is mounted onto the
sidewall 40 of recess 42 in the holder 16 so that the surface 28 of the
photodiode 26 and
the surfaces 20'; 20" of the glass window 20 are not perpendicular to the
optical axis OA
extending through the holder 16 and the optical fiber 52. A light beam leaving
the
optical fiber 52 will be substantially symmetrical about the optical axis OA,
and so will
be incident upon the glass window 20 and the surface 28 of the photodiode 26
in a
propagation direction which is not normal to them.
The window 20 and surface 28 both are tilted about an axis which is coincident
with, or parallel to, a maximum response axis of the photosensitive detector
26. The
angle B through which the glass window 20 and the surface 28 are tilted from
the normal
to optical axis OA will depend upon the inherent polarization-dependent
response of the
photosensitive detector 26.
It is known that tilting an interface between two light-transmissive media
relative
to the direction of incidence of a light beam will introduce polarization
dependency. As
illustrated in Figure 4, light incident upon the photodiode 26 will traverse
three such
WO 01/65221 cA 02401977 2002-08-30 PCT/CA01/00248
interfaces, namely the air/glass and glass/air interfaces 20' and 20",
respectively, at
opposite sides of the window 20 and the air/surface 28 interface of the
photodiode 26
itself. Tilting of the bracket 14 causes all three interfaces to tilt relative
to the optical
axis of fiber 52, and hence to the direction in which the light will be
incident upon the
5 photodiode 26. Because the refractive index of the semiconductor medium is
higher than
most glasses, and in particular the glass plate 20, the effect of the
polarization
dependency induced by the interface at surface 28 is considerably less than
the effect of
the polarization dependency introduced by tilting of the air-glass/glass-air
interfaces 20',
20".
10 In order to determine the axis about which the interfaces 20' 20", 28 are
to be
tilted, the PDR of the photodetector 12 is measured before assembly of the
parts shown
in Figures 1 to 3. Three procedures for doing so will be described, the first
using
equipment as illustrated schematically in Figure 5. The equipment comprises a
light
source 64, for example a laser, for supplying highly polarized light to a
polarization
controller 66 which varies, in time, the state of polarization of the light
and supplies the
polarized light via an optical fiber 68 to a collimating lens 70, for example
a GRIN lens,
which directs the collimated light onto the photodetector 12. A polarizes 72
can be
inserted into the collimated beam and rotated, as will be described later. The
conductors
22 and 24 of the photodetector 12 are coupled to a meter 74, for example an
ammeter,
which measures and displays the output signal of the photodetector 12. Where
the
photodetector 12 is part of an optical power meter, the meter 74 could
comprise the
electronics and display of the optical power meter. The photodetector 12 is
mounted
rotatably in the bracket 14, which is not shown in Figure 5 for clarity.
To determine the maximum response axis, the polarization controller 66 is
adjusted, without the polarizes 72 in the collimated beam, to vary the
polarization state
of the input light beam so as to select randomly substantially all possible
states of
polarization i.e., points on the Poincare sphere. While this is happening, the
meter 74
is monitored and the maximum and minimum levels of the output signal are
noted. It
has been found that, generally, the maximum and minimum values will correspond
to
two orthogonal states of polarization. Consequently, once one has been found,
the other
can be inferred. The output of the polarization controller 66 then is held
steady at the
state of polarization for which the maximum (or minimum) was detected and the
polarizes 72 is inserted into the collimated beam and used as an analyzer to
determine
WO 01/65221 CA 02401977 2002-08-30 PCT/CA01/00248
11
the orientation of linear SOP of the light from the controller 66. This, the
polarizes 72
is rotated until it is aligned with the SOP of the light from the controller
66
corresponding to a maximum reading, or perpendicular to the SOP, corresponding
to a
minimum, substantially zero, reading. The orientation of the state of
polarization of the
polarizes 72 then is transferred to the photodetector 12 as the maximum
response axis
R,~,p,X, or the minimum response axis RM~, as the case may be. With the
minimum
response axis R,,,,~, parallel to the outermost surface of arm 32 of the
bracket 14 (see
Figure 3), and the maximum response axis R,~,,,x orthogonal thereto, the
photodetector
12 is glued to the bracket 14. The surface of the arm 32 then can be used as
the
reference when mounting the photodetector 12 to the holder 16 and tilting the
bracket
14.
An advantage of this procedure is that it can be used to confirm that the
detector
does indeed have a maximum response axis corresponding to a substantially
linear SOP.
Thus, if rotating the polarizes through 90 degrees from the maximum response
axis does
not cause substantial extinction, the PDR of the detector is not characterized
by
substantially linear SOP response axes.
While the above-described method is preferred because the polarization
controller
66 allows substantially all polarization states to be selected, it is possible
to determine
the maximum response axis in other ways, for example, only linear SOPS could
be used,
as described below:
(i) The photodetector 12 is inserted into the hole 34 in the L-shaped bracket
14 until its lip 36 abuts the bracket and the bracket 14 then is supported
in a jig (not shown). At this stage, the photodetector 12 can be rotated
in the hole 34.
(ii) A linearly-polarized light beam is directed onto the surface 28 of the
photodiode 26 along an axis that is substantially normal to the surface 28.
(iii) Relative rotation of the state of polarization of the light beam
relative to
the photodiode 26 is effected by rotating either the SOP of the light beam
or the detector about the normal axis and the photodetector response is
measured at different angles of rotation.
(iv) The orientations of the maximum response axis R,,~,~ and minimum
response axis RMIN are determined. (Because they are orthogonal,
determination of one also gives the other.)
WO 01/65221 cA 02401977 2002-08-30 PCT/CA01/00248
12
(v) The photodetector 12 is secured to the bracket 14, conveniently using
adhesive, with the minimum response axis Rr,,~, parallel to a reference
surface of the bracket 14, specifically the outermost surface of arm 32.
As shown in Figure 3, the axis of minimum response RM~,, now extends in or
parallel to the plane of surface 28 and substantially parallel to the
outermost surface of
arm 32 of bracket 14. The axis of maximum response R,~,p,X (not shown in
Figure 3) also
lies in the plane of surface 28 but orthogonal to minimum response axis
R",,~,. In Figure
3, therefore, the maximum response axis R,~X extends out of the paper. Once
the
maximum response axis has been determined, It is necessary to determine the
angle at
which the photodetector 12 (and hence the interfaces) must be tilted to
compensate for
the inherent polarization dependency of the photodiode 26. For this, the
assembly of
parts shown in Figures 1 and 2 is used, but with the fiber 52 connected to the
polarization controller 66 and light source 64 of Figure 5.
Using a suitable jig (not shown), the bracket 14 is positioned upon the
sidewall
40 of recess 42 in the holder 16 so that the surface 28 of the photodiode 26
is generally
aligned with the optical axis extending through the centre of the holder 16.
At this stage,
the bracket 14 is not fixed.
The fiber connector 50 is attached to the holder 16 by means of the adapter 54
and polarized light is directed from the fiber 50 onto the surface 28. The
polarized light
may be supplied by the source 64 and polarization controller 66 used to
determine the
maximum response axis, as described with reference to Figure 5. The
polarization
controller 66 is adjusted to vary the SOP of the polarized light to generate,
in time,
substantially all of the possible states of polarization of the light beam,
i.e., for a
representative number of points on the Poincare sphere. Preferably, this adj
ustment of
the polarization controller 66 is under automatic control.
While the SOP is varying, the bracket 14 is tilted step-by-step and the output
of
meter 74 is monitored. It will be observed that the difference between the
maximum and
minimum values of the output signal will vary according to the angle of the
detector 12,
specifically of the interfaces at the surfaces of the glass window and the
semiconductor
device, respectively. For one tilt angle, the difference will be a minimum.
The bracket
14 will be secured to the holder 16 at this tilt angle.
WO 01/65221 CA 02401977 2002-08-30
PCT/CA01 /00248
13
Just as there were other ways of determining the orientation of the maximum
response axis, there are other ways of determining the required tilt angle.
One using
only linear SOPs will now be described.
As before, with the photodetector 12 glued in place, the bracket 14 is
positioned
on the holder 16 and linearly-polarized light from the polarization controller
66 directed
onto the photosensitive detector 26.
The SOP of the linearly-polarized light beam is rotated about the optical axis
and
the bracket 14 tilted relative to the holder 16 about the maximum response
axis R~,,,t,x (or
an axis parallel thereto), and the response of the photodetector 12 measured
so as to
determine a tilt angle B at which the difference between maximum and minimum
responses of the photodetector is a minimum. The bracket 14 then is fixed,
preferably
adhered, to the sidewall 40 of the holder 14 so as to maintain that tilt angle
B.
As illustrated in Figure 4, both the glass window 20 and the surface 28 of the
detector diode 26 then are inclined relative to the direction along which
light from the
optical fiber 52 will be incident upon the diode 26.
It should be noted that, in Figures 2 and 3, the size of tilt angle B is
exaggerated
for purposes of illustration. A typical tilt angle 8 might be about 5 - 7
degrees. The
direction of tilt will be such that the effective response of the
photodetector 12 along the
respective axes R,,,,Ax and R,,,,W will be equalized, i.e., the overall PDR of
the
photodetector device 12 will be substantially uniform.
An alternative method for aligning the photodetector device, slightly
different
from the above-described method, does not involve insertion of a linear
polarized
analyzer to determine the maximum (or minimum) response axis, but may use all
of the
other components of Figures 1, 2 and 3, with the exception of the "L bracket"
14.
Referring to Figure 15, the photodetector 12 is inserted loosely into a hole
120 in the
connector part 16 and supported by a suitable jig 121 which can be used to
rotate it
around the photodetector axis PDA and tell it to vary angle B. Initially, the
angle B is
set to an arbitrary angle B with respect to the optical beam propagation axis
(OA) in
Figures 3 and 4). A typical value of this arbitrary tilt angle would be S to 6
degrees.
The conductors 22 and 24 of the photodetector device 12 are coupled to the
electronics
of an optical power meter 74 or to an ammeter. Using a polarization
controller, the SOP
of the light from a light source 64 is varied in such a way as to cover, in
time,
substantially all of the points on the Poincare sphere. While the SOP is
varying, the
WO 01/65221 CA 02401977 2002-08-30 PCT/CA01/00248
14
signal from the photodetector device 12 is monitored with the meter 74 and the
difference between the maximum and minimum values during successive intervals
of
about 30 seconds is registered. Using the jig, the photodetector device is
rotated, step
by step, with the photodetector device axis (PDA) as the axis of rotation,
i.e.
perpendicular to the photodetector surface 28. At each step, the difference
between the
maximum and minimum response is registered, and the azimuthal orientation for
which
the difference is a minimum is the orientation for which the maximum response
axis,
R",,p,X, is perpendicular to the plane containing the axes OA and PDA. The
photodetector
device 12 is held at this rotation angle and, using the jig, the photodetector
device 12 is
tilted with respect to the optical axis, OA, in a step by step manner, to vary
the angle
B, while monitoring the output of the photodetector 12 again. When a minimum
is
reached in the registered difference between the maximum and minimum values,
the
PDR of the photodetector device should then be minimum. The photodetector
device 12
is then glued to the holder, i.e., connector part, 16 by injecting adhesive
122 between
the sides of the housing 18 and the hole 120. In effect, the first "roll"
measurement
determines the orientation of the maximum-response axis and the subsequent
"pitch"
adjustment is used to minimize the PDR of the photodetector device 12.
The invention is predicated upon the fact that the maximum and minimum
responses correspond to orthogonal, substantially linear SOPs, and that the
geometric
orientation of these axes with respect to the material of the detector is
constant with
respect to time. The invention is further predicated upon the fact that the
transmission
of light through an optical interface between two media having different
refractive
indices (e.g. air-glass) is polarization dependent if the light is not
incident upon the
interface along the normal to the interface. This arises because the
reflection of the light
beam at the interface is polarization dependent, so, by conservation of
energy, the
transmitted portion of the light beam also is polarization dependent. An
incident light
beam having a linear SOP lying in the plane of reflection ("p" polarization)
will be
preferentially transmitted past the interface as compared with a light beam
having a linear
SOP perpendicular to the plane of reflection ("s" polarization). For the case
of a light
beam passing through a glass plate, this polarization dependency is
effectively doubled
because of the presence of two air-glass interfaces. Likewise, the fraction of
the light
absorbed (i.e., transmitted past the interface at the surface of the
semiconductor material)
by a tilted semiconductor detector (e.g. InGaAs) is polarization dependent,
with the
WO 01/65221 cA 02401977 2002-08-30 PCT/CA01/00248
portion having "p" polarization being absorbed preferentially as compared with
the
portion having "s" polarization. However, on account of the higher refractive
index of
the semiconductor medium compared with most glasses, this polarization
dependence is
less marked than with transmission through an air-glass interface at the same
tilt angle.
5 It is possible, therefore, to select a tilt angle B such that the
polarization
dependency introduced by the interfaces) compensates for the inherent
polarization
dependency of the photosensitive detector. Consequently, the above-described
embodiment of the invention may be applied after the detector and the window
have been
assembled into the usual housing 18 and the window 20 has been fixed
substantially
10 parallel to the surface 28 of the photodiode 26.
Figure 6 illustrates a photodetector 12' which has been corrected for
polarization
dependent response during manufacture. The maximum response axis R,,,,,~x and
the tilt
angle B could be determined using the procedures described above, but during
manufacture, of course, the response of the detector may be measured with or
without
15 the window in place. In the photodetector 12' shown in Figure 6, the
photodetector
surface 28 is normal to the direction in which light will be incident upon it
and the
window 20 is tilted by the required tilt angle B relative to the detector
surface 28 as well
as the optical axis. The photodetector 12' will be polarization independent
and so may
be used without further tilting of the window 20 or housing 18. Hence since
there would
be no need to tilt the photodetector 12' when installing it into the usual
holder 14, the
bracket 14 could be omitted and the photodetector device 12' inserted directly
into a
suitable hole in the holder 16.
It should also be appreciated that, during manufacture of the photodetector
device
12', both the window 20 and the photodiode 26, or only the photodiode 26,
could be
installed with the required inclination to the maximum response axis of the
photosensitive
detector 26. Whether or not both the window 20 and the detector surface 28
were tilted
would depend upon the level of the inherent polarization dependent response.
It should
be appreciated, however, that tilting the window would give a greater amount
of
correction than tilting the detector surface because the window provides two
interfaces,
namely air/glass and glass/air. It should also be noted that, preferably, the
opposite
faces of the window are not parallel but rather converge very slightly, i.e.
to form a
wedge-shaped window, so as to avoid interference effects.
WO 01/65221 cA 02401977 2002-08-30 PCT/CA01/00248
16
It should be appreciated that the interface need not be within the housing of
the
photodetector device, but could be provided by a separate part, e.g. a glass
plate or the
like, disposed upstream in the optical path.
Thus, the invention embraces a photodetector having either or both of its
detector
surface and a window tilted relative to an optical axis of the device so that
it is
substantially independent of polarization state; a photodetector secured to a
bracket or
other mount with some indication as to the orientation of the maximum response
axis of
the detector, enabling the mount to be installed into equipment and the
photodetector
tilted to effect the required compensation; and an assemblage of a
photodetector and a
holder for attaching it to equipment for use.
It should also be appreciated that some combinations of fiber connector 50 and
panel holder 16 may screw together directly, in which case an adapter 54 will
not be
needed.
It is further envisaged that, as illustrated in Figure 7, where the
photodetector
device has a pigtail fiber 116 permanently attached, typically by means of a
capillary
112, the required PDT for compensating for the PDR of the detector 26 could be
introduced by angling the end facet of the fiber 116 relative to the optical
axis. The
angle of the fiber end surface would be predetermined according to the PDR
value of the
detector 26, conveniently measured using the method described herein. The
correct
compensation then would be achieved by rotating one or other of the fiber 112
and the
detector 26 relative to the other around the optical axis OA to align the
orientation of the
angle relative to the maximum response direction R,,,,~,x or minimum response
direction
RMp,,. The window 20 also would provide some correction.
As shown in Figure 8, however, the window 20 could be omitted and the
capillary 112 secured within the opening in place of the window 20. The
required
correction then would be provided solely by the single interface at the angled
end of the
fiber 112.
It should also be appreciated that, where the photodetector device has a
pigtail
fiber permanently attached during manufacture, the required polarization
dependent
transmission could be introduced by a suitable lateral displacement of the
waveguide
relative to the optical axis of a lens through which the light beam will pass
to the
detector. Embodiments of such a photodetector device, and its method of
manufacture/adjustment, will now be described with reference to Figures 9 to
14.
WO 01/65221 cA 02401977 2002-08-30 PCT/CA01/00248
17
Referring to Figures 9, 10(a) and 10(b), a photodetector device 80 comprises a
base 82 and a cup-shaped housing 84 attached by its rim to one surface 86 of
the base
82 so as to form an enclosure 88. An opening 90 is provided in the middle of a
wall 92
of the housing 84 that extends generally parallel to the base 82. The opening
90 is
covered by a glass window 94 that is adhered to the interior surface of the
wall 92. A
photodetector 96, for example an InGaAs photodiode, is mounted in the middle
of the
enclosure 88 with its detection surface 98 extending generally parallel to,
and directed
towards, the glass window 94. The photodetector 96 is supported from the base
82 by
a mount 100 and is connected to a pair of conductors 102 and 104 which
protrude from
the furthermost surface of the base 82 for connection to other equipment (not
shown).
A graded-index (GRIN) lens 106 has a first end surface adhered by suitable
optical adhesive 107, for example as marketed under the name UV NOA61 by
Norland
Inc., to the middle of the exterior surface 108' of the window 94 so that the
optical axis
LOA of the GRIN lens 106 is substantially aligned with the centre of the
detector surface
98. The opposite end surface 110 of the GRIN lens 106 extends obliquely to the
optical
axis LOA at an angle of, say, 6 degrees. A short length of fiber capillary 112
has an
end surface 114 inclined at a similar oblique angle and adhered by optical
adhesive 113
to the oblique surface 110 of the GRIN lens 106. An optical (pigtail) fiber
116 extends
coaxially through the fiber capillary 112 and its end abuts the oblique
surface 110 of the
GRIN lens 116. The end of the fiber 116 also is inclined at the same angle as
the
surface 114 of the capillary 112. This fiber 116 may either be single-mode or
multimode
at the operating wavelength of interest. As shown in Figure 10(a), the optical
axis FOA
of the fiber 116 extends generally parallel to the optical axis LOA of the
GRIN lens 106.
The window 94 is wedge-shaped, in order to significantly reduce interference
effects.
Light entering the photodetector device 80 via the fiber 116 will pass through
the
GRIN lens 106 and the window 94 and impinge upon the detector surface 98. As
shown
in Figure 9, the capillary 112 and the GRIN lens 106 are offset laterally
relative to each
other, in the direction of the minimum response axis R~,,p,,, with the result
that the beam
optical axis BOA of the light beam LB will be inclined relative to the lens
optical axis
LOA by an angle O. The size of the offset "d" will determine the size of angle
8.
Consequently, when the light beam LB is incident upon the detector surface 98
and the
window surface 108", it is not perpendicular to either. As will be described
hereinafter,
CA 02401977 2002-08-30
WO 01/65221 PCT/CA01/00248
18
during manufacture, the offset "d" is adjusted until polarization dependent
transmission
effects caused by the light beam LB leaving the lens 106 with its optical axis
BOA
inclined relative to the lens optical axis LOA substantially counteract the
inherent PDR
of the detector 96 so as to minimize the overall or effective PDR of the
photodetector
device 80.
The inclined window surface 108" serves primarily to minimize interference
effects and its angle of inclination, typically 2 degrees, is chosen to be
significantly
different from the afore-.mentioned angle of incidence O, so as to ensure low
backreflection.
As a practical example, for an InGaAs detector 96 with a PDR of 0.02 dB and
a window 94 having a refractive index of 1.5, it was found that the angle 8
should be
about 7.5 degrees, which was obtained using a GRIN lens 106 having a focal
length of
1.9 mm displaced relative to the fiber 116 by about 0.25 mm.
Adjustment of the photodetector 80 during production will now be described.
Referring to Figure 11, with the GRIN lens 106 glued to the glass window 96,
the
housing 84 is supported by a first holder 120 and the conductors 102 and 104
connected
to an ammeter 122. The fiber capillary 112, with the fiber 116 installed
inside it, is
supported in a second holder 124 and the pigtail fiber 116 coupled by a
connector or
splice 126 to a polarization scrambler 128 which varies, in time, the SOP of
light
received from a source 130 of highly polarized light.
The polarization controller 128 is adjusted to vary the SOP of the light beam
over
a wide range of possible SOPS. Ideally, the SOP is varied through
substantially all of
the possible states of polarization of the light beam, but in practice, the
number of
different SOPS will be a representative number of points on the Poincare
sphere.
While the SOP is varying, either one of the GRIN lens 106 and the capillary
112
is adjusted laterally in the plane x-y shown in Figure 10(b) and the output of
the meter
122 is monitored. The lateral movement is made in various directions to
determine that
which reduces the difference between the maximum and minimum values measured
by
the meter 122. The actual difference will vary according to the lateral
displacement "d"
between the fiber optical axis FOA and the lens optical axis LOA, since this
will control
the angle at which the light beam LB is incident upon the surface 108" and the
detector
surface 98. For a particular displacement "d", in a particular direction which
defines
the minimum response axis RMp,,, the difference between the maximum and
minimum
WO 01/65221 CA 02401977 2002-08-30
PCT/CA01 /00248
19
values will itself be a minimum. The fiber 116 and capillary 112 then are
glued to the
GRIN lens 106, using optical adhesive, to fix them at this displacement
position. As
shown in Figure 10(b), the minimum response axis RMr,,, then will intersect
the fiber
optical axis FOA and the lens optical axis LOA, the latter spaced apart by the
distance
"d". The maximum response axis R,~,p,x and minimum response axis RM~, are
orthogonal
so the maximum response axis R,r,Ax will extend perpendicular to both the
minimum
response axis RM~, and the lens optical axis LOA.
Using the above-described procedures and configurations, therefore, it is
possible
to substantially minimize the overall PDR of the photodetector device 80
because the
polarization dependency introduced by the light beam traversing the surface
108" at an
angle that is not perpendicular counteracts the inherent polarization
dependency of the
detector 96.
Very high return loss achieved by such a photodetector assembly 80 is
attributable
to the fact that little, or substantially no, residual reflection is coupled
back to the fiber
IS 116 from the surfaces I 10 and 114, because they are inclined relative to
the optical axes
LOA and FOA. The same applies at the interior surface 108" of the window 94,
and
the adjacent end surface of the GRIN lens 106, because the light beam is not
normal to
the surfaces when it traverses them. It should be appreciated, however, that
these
surfaces need not be inclined if only PDR reduction is required.
It has also been found that undesirable interference effects are negligible,
which
is attributed to the use of index matching adhesive between the fiber I 16 and
the GRIN
lens 106 and between the distal end of the GRIN lens 106 and the exterior
surface 108'
of the window 94, and to the inclination of the surfaces 110, 114 and 108" of
the fiber
116, lens 106 and window 94, respectively.
It has been found that this arrangement makes it possible to produce a
virtually
ideal detector 80 with minimum PDR, backscatter and Fabry-Perot-type
interference
effects.
Although it is preferred to use a GRIN lens 106 for reasons of compactness,
ruggedness and ease of fabrication, it will be appreciated that the concept
taught by the
present invention, i.e. using polarization dependent transmission (PDT) of an
interface
to compensate for PDR of the detector chip 96, can be implemented with other
configurations. For example, as illustrated in Figure 12, the GRIN lens 106
may be
replaced by a bulk-optic lens (e.g. "Singlet" convex lens) 106A, the capillary
112 and
WO 01/65221 CA 02401977 2002-08-30
PCT/CA01 /00248
fiber waveguide 116 then fixed with the end of the fiber 116 in the focal
plane of the
convex lens 106A so that the light beam LB leaves the lens 106A collimated.
The
required PDT then is provided by both the lens 106A and the window 94. If
desired,
the end of the fiber waveguide 116 need not be located at the focal plane, in
which case
5 the light beam LB would diverge or converge.
Of course, the GRIN lens 106 or the convex lens 106A could be fixed at a
suitable spacing from the window 94, providing the required degree of
displacement "d"
is provided and any interference effects can be tolerated.
It is also envisaged that, as illustrated in Figures 13 and 14, which show
10 modifications of the devices of Figures 9 and 12, respectively, the window
94 could be
omitted and the lens 106 or 106A installed into the opening 90 to serve also
as a
window. Depending upon the precise type of lens, either or both of the
surfaces of the
lens 106/106A would introduce the required PDT to compensate for the PDR of
the
detector chip 96.
15 It is also envisaged that the glass window or the lens could be attached or
formed
directly upon the surface of the detector (26; 96).
It should be appreciated, therefore, that the invention is predicated upon the
fact
that the transmission (T) of a light beam through an interface between two
dielectric
media having different refractive indexes becomes polarization dependent if
the light
20 beam does not traverse the interface in a direction that is normal to the
interface. The
SOPS related to maximum transmission T~,,,,,x and minimum transmission TMU,,
are
orthogonal and linear, with P-polarization having a higher transmission than S-
polarization.
INDUSTRIAL APPLICABILITY
An advantage of embodiments of the invention is that the compensation for
polarization dependent response can be effected during or after manufacture
simply by
appropriate tilting of at least one interface, i.e., at the surface of the
photosensitive
detector and/or an adjacent window and/or a fiber waveguide end facet,
relative to an
axis along which light for detection will be incident upon the detection
surface of the
photodetector or, in the case of pigtail photodetectors embodying the present
invention,
offsetting the pigtail fiber relative to the input lens.
