Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-03-02
2003-09-16
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S214100, C356S366000
Reexamination Certificate
active
06621067
ABSTRACT:
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 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 formed from a material having a polarization dependent response having maximum (R
MAX
) and minimum (R
MIN
) 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 light beam traversing said at least one interface with a propagation direction that is not normal to the interface, the arrangement being such that said at least one interface is tilted about an axis substantially parallel to the direction of the linear state of polarization corresponding to said maximum (RMAX) value and by such a tilt angle that polarization dependent transmission introduced by the at least one interface will compensate at least partially for the polarization dependent response of the material 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. 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
MAX
) 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
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 correspo
Babin François
Breton Marc
He Gang
Potvin Steeve
Schinn Gregory Walter
Adams Thomas
EXFO Electro-Optical Engineering Inc.
Pyo Kevin
Sohn Seung C.
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