Polarization detection system

Abstract: A polarization detector system for an optical data storage system including a substrate 32 having a pair of overlapping optical couplers 38 and 40 for separately coupling first and second polarization components of a light beam into first and second waveguide channel 50 and 54. The channels are oriented with their longitudinal axes 52 and 56 approximately perpendicular to each other. First and second optical detectors 60 and 70 are located in the first and second waveguide channels, respectively, for measuring the light intensity of the first and second polarization components. Comparison of the measurements provides polarization data.

Description: This invention relates to polarization detector systems and particularly, though not exclusively, to optical data storage systems including such detector systems. Polarized light is used in a number of applications such as thin film depth measurement and magneto-optic recording. In such systems it is necessary to be able to determine the state of polarization, i.e. direction of the plane of polarization. Magneto-optic recording systems allow erasable storage of data. A laser beam is focused onto a spot on a magneto-optic disk medium and heats the material to a temperature at which the magnetic domain of the medium may be changed. A magnetic field is then applied in one of two directions and orients the magnetic domain of the spot in either an upward or downward direction. The disk is read by focusing a low-power polarized laser beam onto the spot on the disk. The Kerr effect causes the plane of polarization of the reflected beam to be rotated either clockwise or counter-clockwise depending upon the direction of the magnetic domain of of the recorded spot. The difference in the rotation is detected and represents the digital data. The disk can be re-recorded by heating the spots and switching the directions of the magnetic domains. The detection of the direction of polarization is critical to these optical recording systems. Some of the these systems use discrete optical components, such as Wollaston prisms, to separate the reflected light beam into two orthogonal polarization component beams. The intensity of these beams is then detected and compared. The exact direction of the plane of polarization of the original reflected beam can then be determined. Another way to separate the orthogonal polarization components is to use optical grating couplers. Examples include US-A-4,868,803; "Focusing Grating Couplers for Polarizntion Detection" by Shogo Ura et al., Journal of Lightwave Technology , Vol. 6, No. 6, June 1988; and " An Integrated Optic Detection Device for Magneto-optical Disk Pickup", by Sunagawa et al., International Symposium On Optical Memory 1987, September 16-18, 1987, Tokyo, Japan. These references show a combination of transverse electric (TE) and transverse magnetic (TM) grating couplers positioned side by side in the beam path of light returning from a magneto-optic disk. The TE and TM gratings each couple a different orthogonal polarization component into a waveguide. A problem with this approach is that only portions of the beam are being sampled for each polarization component. The beam is often non-uniform in its intensity, and errors in detection can occur. US-A-4,868,803 also discloses use of a waveguide device to couple the output of two different gratings to separate optical detectors. EP 0 322 714 discloses a polarization detector system which includes a three division element with a substrate and first and second cross gratings for diffracting first and second polarized light beam components to first and second optical detectors, what is reflected in the preamble of claim 1. The present invention seeks to provide an integral polarisation detection system with a focus device incorporated into a waveguide coupling the grating to the detector. Accordingly the invention provides a polarization detector system comprising a first grating coupler for receiving a polarized light beam; a first optical detector for detecting a first polarized component light beam; a second grating coupler for receiving a polarized light beam; a second optical detector for detecting a second polarized component light beam. In the polarization detector system the first and second polarized components are approximately orthogonal components. The invention is characterised in that the system further comprises a first waveguide channel located along a first axis, receiving a polarized light beam from said first coupler, and in optical communication with said first optical detector; a second waveguide channel, located along a second axis, said second axis being approximately perpendicular to said first axis, receiving a polarized light beam from said second coupler, and in optical communication with said second optical detector; a first beam splitter located in the first waveguide channel; a first focus optical detector in optical communication with the first beam splitter behind a first lens having a focal point behind said first focus optical detector ; a second beam splitter located in the second waveguide channel; and a second focus optical detector in optical communication with the second beam splitter behind a second lens having a focal point in front of said second focus optical detector. Other features of the invention are set out in the appended claims. How the invention may be carried into effect is hereinafter particularly described with reference to the accompanying drawings, in which:- Fig. 1 is a schematic diagram of nn optical data storage system of the present invention; Fig. 2 is a schematic diagram of the opto-electronic channel portion of the system of Fig.1; Fig. 3 is a diagram of a circuit for the system of Fig. 2. Fig. 4 is a schematic diagram of a portion of the system of Fig. 1 as used in a rotary actuator system; and Fig. 5 is a schematic diagram of the opto-electronic channel in a second embodiment of the present invention. A data recording system 10 (Fig. 1) according to the present invention includes a laser 12 for providing a beam of polarized light. Laser 12 is preferably a laser diode such as a semiconductor diode laser, although other types of light sources may be used. A laser power control circuit 14 is connected to laser 12. A collimating lens 16 collimates the light from laser 12 and a focusing lens 18 focuses the collimated light on to an optical recording medium 20 in the form of a disk of a magneto-optic material. The position of the lens 18 is determined by a focus motor 22 to which the lens 18 is attached and which is connected to a focus control circuit 24 that controls movement of the motor 22. An opto-electronic channel 28 is located between lens 16 and lens 18. Laser 12, lenses 16 and 18, opto-electronic channel 28 and motor 22 constitute an optical head 30. The opto-electronic channel 28 (Fig. 2) has a glass substrate 32 with a crossed grating 36 constituting of a pair of polarization grating couplers 38 and 40. Couplers 38 and 40 are aligned approximately perpendicular to each other. Coupler 38 is oriented to couple light of a first orthogonal polarization component into a waveguide 50 along an axis 52. Coupler 40 is oriented to couple light of a second orthogonal polarization component into a waveguide 54 along an axis 56. Axes 52 and 56 are both located in a plane parallel to the plane of substrate 32. Couplers 38 and 40 are both transverse electric (TE) type grating couplers, and are Bragg matched to the lowest order TE mode of their respective waveguides 50 and 54 couplers. Alternatively, couplers 38 and 40 may be transverse magnetic (TM) type grating couplers Bragg matched to the lowest order TM mode of waveguides 50 and 54, respectively. Waveguides 50 and 54 are preferably single-mode planar-type waveguides formed on substrate 32 and extending over the entire top surface of substrate 32. The boundaries of waveguides 50 and 54 are determined by the couplers 38 and 40. The axes 52 and 56 are oriented approximately perpendicular to each other. An optical detector 60 in two separate sections 62 is located within or on waveguide 50. An optical detector 70 in two separate sections 72 and 74 is located within or on waveguide 54. A beamsplitter 80 is located within or on waveguide 50 between the grating 38 and the detector 60. Beamsplitter 80 directs a portion of the light in waveguide 50 along an axis 81 orthogonal to axis 52, to a lens 82 which focuses the light at focal point 84. A beam splitter 86 is located within or on waveguide 54 between the grating 40 and the detector 70. Beamsplitter 86 directs a portion of the light in waveguide 54 along an axis 87 orthogonal to axis 56, to a lens 88 which focuses the light at a focal point 90. Lenses 82 and 88 are formed within or on waveguide, 50 and 54, respectively. Alternatively, beamsplitters 80 and 86 and lenses 82 and 88 may be formed within or on substrate 32. An optical detector 92 is located along axis 81 a distance in front of focal point 84. An optical detector 94 is located along axis 87 beyond focal point 90. When light from focusing lens 18 is in focus on medium 20 (Fig. 1), the distance between detector 92 and point 84 will be approximately equal to the distance between detector 94 and point 90. The detectors 92 and 94 are smaller than the beam diameter at that point, so that, when in focus, light spills over the edge of the detectors 92 and 94. An optical detector 96 is located on substrate 32 on axis 52 on the opposite side of grating 38 to waveguide 50. An optical detector 98 is located on substrate 32 on axis 56 on the opposite side of grating 40 to waveguide 54. A detector circuit 100 (Fig. 3) includes a data detector circuit 102, a track detector circuit 104, a focus detector circuit 106 and a power detector circuit 108. Circuit 102 has summing amplifiers 110, 112, 114 and 116 and a differential amplifier 118. Summing amplifier 110 is connected to the outputs of detectors 62 and 64. The output from amplifier 110 and the output from detector 92 are connected to summing amplifier 112. Summing amplifier 114 is connected to the outputs of detectors 72 and 74. The output from amplifier 114 and the output from detector 94 are connected to summing amplifier 116. The outputs from amplifiers 112 and 116 are connected to differential amplifier 118 whose output is the data signal. Circuit 104 has summing amplifiers 120 and 122, and a differential amplifier 124. Summing amplifier 120 is connected to the outputs of detectors 62 and 72. Summing amplifier 122 is connected to the outputs of detectors 64 and 74. The outputs of amplifiers 120 and 122 are connected to differential amplifier 124 whose output is the track error signal. Circuit 106 has a differential amplifier 130 which is connected to the outputs of detectors 92 and 94, and whose output is the focus error signal. Circuit 108 has a summing amplifier 132 which is connected to the outputs of detectors 96 and 98, and whose output is the power signal. In a rotary actuator (Fig. 4), the disk medium 20 is mounted on a disk drive spindle 160. The optical head 30 (of which only the opto-electronic channel 28 is shown) is attached to the end of a rotary actuator arm 162 mounted about a pivot point 164 and positioned underneath disk 20. A plane of polarization 168 of the incoming light from laser 12 is approximately radial or perpendicular to concentric tracks 169 on disk 20. The track detector circuit 104 is connected to a tracking control circuit 172 which controls the movement of a voice coil motor 170 attached to arm 162. The focus detector circuit 106 is connected to focus control circuit 24 (Fig. 1), and the power circuits 108 is connected to laser control circuit 14. In operation, incoming polarized light from laser 12 is collimated by lens 16 and passes through grating 36 of opto-electronic channel 28. The laser 12 is oriented so that the plane of polarization 168 of the incoming light is offset at an equal angle from both the axis 52 and 56. A portion of the incoming light is diverted by couplers 38 and 40 to the optical detectors 96 and 98, respectively. Coupler 38 diverts a first orthogonal component of the incoming light and coupler 40 diverts a second orthogonal component of the incoming light. Because the plane of polarization 168 is offset at equal angles to each coupler 38 and 40, the intensities of the first and second orthogonal components which hit detectors 96 and 98 are approximately equal. The outputs of detectors 96 and 98 are added by summing amplifier 132 and a power signal is generated from circuit 108. This power signal is then transmitted to laser control circuit 14 which adjusts the laser 12 to the proper power level. The incoming light which is not coupled to detectors 96 and 98 continues through coupler 36 and is focused by lens 18 onto disk 20. When the incoming light reflects from the recorded data spots on the disk 20 its plane of polarization is rotated either clockwise or counter clockwise depending upon whether a one or a zero is recorded. This reflected light then returns through lens 18 to couplers 38 and 40. Coupler 38 diverts a first orthogonal component of the reflected light along axis 52 and into waveguide 50. Coupler 40 diverts a second orthogonal component of the reflected light along axis 56 and into waveguide 54. Couplers 38 and 40 are able to distinguish between the incoming and reflected light because channel 28 is positioned at a slight angle (1° to 3°, for example) relative to the plane perpendicular to the direction of propagation of the light. Thus, in order to ensure that the light will be coupled in the proper directions, axes 52 and 56 should represent the projections of the orthogonal polarization components of the incident light on the channel 28. The angles at which axes 52 and 56 are offset from the plane perpendicular to the direction of light polarization are equal. The first polarization component light travels along waveguide 50 to beamsplitter 80. A portion of the light is diverted to lens 82 and then to detector 92. The remaining light continues along waveguide 50 to detectors 62 and 64. The second orthogonal polarization component of the light travels along waveguide 54 to beamsplitter 86. A portion of the light is diverted to lens 88 and detector 94. The remaining light continues along waveguide 54 to detectors 72 and 74. Because the recorded spots on the disk 20 cause the plane of polarization of the reflected light to he rotated either clockwise or counter clockwise relative to the plane of polarization 168 of the incoming light, the first polarization component of light in waveguide 50 has a greater intensity than the second polarization component in waveguide 54, or vice versa . The intensity of the first component is determined by summing the outputs of detectors 62, 64 and 92 in circuit 102. The intensity of the second component is determined by summing the outputs of detectors 72, 74 and 94 in circuit 102. The difference between the two is then determined and this represents the data signal. When the incoming light is focused directly on a track of disk 20, the reflected light will be symmetrically distributed across couplers 38 and 40. Detectors 62 and 72 will receive the same amount of light as detectors 64 and 74. If the incoming light drifts off track then the sum of the outputs of detectors 62 and 72 will be greater than the sum of the outputs of detectors 64 and 74, or vice versa . Circuit 104 determines this difference and sends a track error signal to the tracking control circuit 172 which in turn operates motor 170 to adjust the arm and keep the light on track. When the incoming light is perfectly focused by lens 18 on to disk 20, the reflected light will return to channel 28 as parallel beams of light. The light passing through lens 82 will converge at focal point 84 and the light passing through lens 88 will converge at focal point 90. Detectors 92 and 94 are positioned on opposite sides of focal points 84 and 90, respectively, and the distances between focal points 84 and 90 and detectors 92 and 94, respectively, are approximately the same. The outputs of the detectors 92 and 94 will be the same when the lens 18 is focused on disk 20. The detectors 92 and 94 are positioned relative to focal points 84 and 90, so the detectors are slightly overfilled with light when the light 20 is in focus on the disk 20. If the incoming light is not correctly focussed by lens 18, then the reflected light will not be parallel but will be slightly convergent or divergent. This causes the focal points 84 and 90 to be shifted either forward or backward relative to lenses 82 and 88. The amount of light falling on one of detectors 92 or 94 will increase while the light falling on the other will decrease. This difference is detected by circuit 106 which transmits a focus error signal to the focus control circuit 24. Circuit 24 causes motor 22 to move lens 18 up or down relative to disk 20 to return the light to the proper focus. Other embodiments of the present invention are possible. For example, instead of making the optical channel all on one side of substrate 32, parts can be made on separate sides. The coupler 38, waveguide 50 and related parts can be made on a first side of substrate 32 and coupler 40, waveguide 54 and related parts can be made on the second side. Coupler 38 is then directly above coupler 40. The top view of the assembly will look exactly like the channel 28 of Fig. 2. In a second embodiment of the present invention, an optical channel 200 (Fig. 5) may be used in place of channel 28 of system 10. Channel 200 is comprised of a pair of substrates 202 and 204. Substrate 202 has a coupler 238, a waveguide 250, an axis 252, a beamsplitter 280, a lens 282, detectors 292 and 296, and detectors 262 and 264 which are similar to coupler 38, waveguide 50, axis 52, beamsplitter 80, lens 82, detectors 92 and 96 and detectors 62 and 64, respectively, of channel 28. Substrate 204 has a coupler 240, waveguide 254, an axis 256, beamsplitter 286, a lens 288, and detectors 294 and 298 and a pair of detectors 272 and 274 which are similar to coupler 40, waveguide 54, axis 56, beam splitter 86, lens 88, and detectors 94 and 98, and detectors 72 and 74, respectively, of channel 28. Substrate 202 is positioned either above or below substrate 204 such that coupler 238 is either above or below coupler 240. In effect the couplers 238 and 240 overlap. The substrates are adjusted so that the axes 252 and 256 are approximately perpendicular to one another. The operation of channel 200 is similar to that described in connection with channel 28. There are several ways to manufacture an optical channel according to the present invention, however the preferred method is by photolithography and ion milling. The substrate may be made of glass or other transparent material. The grating coupler pattern is placed on the substrate by photolithography. The crossed grating is made by forming two TE grating patterns at approximately right angles to one another. The pattern is then etched into the surface of the substrate by ion milling. The crossed grating is preferably etched and ion milled into the substrate, but it may also be formed on top of the waveguide. The fabrication of optical grating couplers is described in detail in the book by H. Nishihara, M. Haruna, and T. Suhara, Optical Integrated Circuits, McGraw Hill, New York, 1989. After the coupler is formed, the waveguide is deposited on the substrate. A film of glass is deposited on the substrate to a depth of sufficient thickness for a single mode planar waveguide. A typical depth may be two microns. Waveguide formation is described in the Nishihara book, as well as in the articles by S. Dutta et al., "Extremely Low-Loss Glass Thin-Film Optical Waveguides", Journal of Applied Physics , Vol. 52, pp. 3873 (1981) and M. Gupta, "Laser Annealing Of Corning 7059 Glass Waveguides," Applied Optics , Vol. 29, pp. 2307 (1990). The beamsplitters and lenses are formed next on the waveguide surface by the combination of photolithography and ion beam etching. Alternatively, the beamsplitters and lenses may be formed within or on the substrate. The manufacture of these devices is also described in the Nishihara book. Finally, the optical detectors are formed. An amorphous silicon layer (such as a hydrogenated amorphous silicon (a-Si:H) is deposited to provide an active layer. The detector area is defined on the waveguide surface by a combination of photolithography and plasma etching. The detector structure is completed by formation of Cr and Au interdigitated fingers (Shottky barrier metal contacts) using photolithography, liftoff and metal thin-film deposition techniques. After the electrical signal lead routing is done for the detectors, they are optionally encapsulated by a dielectric thin-film layer. The formation of the optical detectors is described in R.G. Hunsperger, Integrated Optics: Theory and Technology , Springer-Verlag, Berlin, 1984, pp 251-252; D. L. Rogers, "Interdigitated Metal Semiconductor Metal Detectors", OSA Proceedings On Picosecond Electronics and Optoelectronics, (1989). Although the orthogonal couplers, waveguides and related components are preferably formed on the same surface of the substrate, they may also be formed with each one of the orthogonal waveguides located on a separate side of the substrate. Advantages of the present invention will now be readily appreciated. The entire opto-electronic channel may be formed on a single substrate by photolithography. This greatly simplifies the manufacturing process and ensures that the components will be properly aligned. The use of crossed or overlapping grating couplers provides another advantage. Systems of the prior art used a TE and a TM coupler located side by side in order to couple the two orthogonal components of the reflected beam. In effect one side of the beam was sampled for the first polarization component and the other side of the beam was sampled for the second polarization component. Half of the beam was transmitted through the coupler and not even detected. This problem is critical because the reflected light beam is not perfectly symmetrical but tends to shift from one side to the other. In contrast the overlapping couplers of the present invention sample each polarization component across the entire beam diameter. This gives the system a much greater sensitivity and ensures more reliable data detection. A system according to the present invention is also insensitive to the wavelength changes common to systems which use semiconductor laser diodes as sources. Prior art systems use a TE and TM coupler together to couple the separate polarization components. However, the efficiency of these two couplers is different at different light frequencies. Minor variations in the laser frequency add errors to the data signals. The present invention solves this problem by using two of the same type of coupler (TE or TM) oriented perpendicular to each other. As the frequency of the light changes, the efficiency of both couplers changes by the same amount and the data signal is reduced, but the differential detection system is not unbalanced. This provides the present invention with larger common mode noise rejection than previous art. The invention has other uses. For example, the invention could be used in any application where it is necessary to determine the orientation of a plane of polarized light. While the invention has been particularly shown and described with references to particular embodiments thereof, it will be understood by those skilled in the art that various other changes in form and details may be made therein without departing from the scope of the invention.

Claims: A polarization detector system comprising: a first grating coupler (38;238) for receiving a polarized light beam; a first optical detector (60;262,264) for detecting a first polarized component light beam; a second grating coupler (40;240) for receiving a polarized light beam; a second optical detector (70;272,274) for detecting a second polarized component light beam; and wherein the first and second polarized components are approximately orthogonal components; characterised in that the system further comprises: a first waveguide channel (50;250), located along a first axis, receiving a polarized light beam from said first coupler, and in optical communication with said first optical detector; a second waveguide channel (54;254), located along a second axis, said second axis being approximately perpendicular to said first axis, receiving a polarized light beam from said second coupler, and in optical communication with said second optical detector; a first beam splitter (80;280) located in the first waveguide channel; a first focus optical detector (92;292) in optical communication with the first beam splitter behind a first lens (82) having a focal point behind said first focus optical detector (92,292); a second beam splitter (86;286) located in the second waveguide channel; and a second focus optical detector (94;294) in optical communication with the second beam splitter behind a second lens (88) having a focal point in front of said second focus optical detector (94,294). A system according to claim 1, including a focus detection circuit (106) connected to the first and second focus optical detectors for comparing the intensity of light at the first and second optical detectors and generating a focus error signal responsive thereto. A system according to claim 1 or claim 2 including a detection circuit (102;506) connected to the first and second optical detectors (60,70) for producing a detection signal responsive to the difference in intensity of the first and second polarized components. A system according to any one of claims 1 to 3, wherein the first and second grating couplers are (38,40;238,240) transverse electric (TE) couplers. A system according to any one of claims 1 to 3, wherein the first and second grating couplers (38,40;238,240) are transverse magnetic (TM) couplers. A system according to any one of claims 1 to 3, wherein the first and second couplers (38,40) are comprised of a crossed grating coupler (36;336). A data storage system comprising a polarization detector system according to any preceding claim, a light generation means (12,16,18) for providing a polarized light beam; a data storage medium (20) for receiving said polarized light beam and providing a reflected light beam, the first grating coupler (38;238) being in optical communication with the data storage medium (20) for receiving the reflected light beam, and the second grating coupler (40;240) being in optical communication with the data storage medium (20) for receiving the reflected light beam. A system according to claim 7, including a third optical detector (96;296) in optical communication with the light generation means via the first grating coupler (38;238;338) for generating a first signal, a fourth optical detector (98;298;398) in optical communication with the light generation means via the second grating coupler (40;240) for generating a second signal, and a power control circuit (108) connected to the light generation means and the third and fourth optical detectors for generating a power signal responsive to the first and second signals. A system according to claim 7 or claim 8, wherein the first and second optical detectors each have first and second sections, (62,64) 72,74;262,264,272,274) and a track detection circuit (104) is connected to the first and second sections of the first and second optical detectors for generating a track error signal. A system according to claim 9, including a lens (18) located between the data storage medium (20) and the first and second grating couplers for focusing light onto the medium, and a lens movement means (22,24) connected to the lens (18) for moving the lens relative to the medium responsive to the focus error signal. A system according to claim 9 or 10, wherein the light generation means, first and second waveguide channels, first and second grating couplers, and first and second optical detectors comprise an optical head (30) and an optical head movement means (162,170,172) is connected to the optical head (30) for moving the optical head relative to the medium responsive to the track error signal.

Inventors: KURDI BULENT NIHAT, SINCERBOX GLENN TAVERNIA, ZAVISLAN JAMES MATTHEW, KURDI, BULENT NIHAT, SINCERBOX, GLENN TAVERNIA, ZAVISLAN, JAMES MATTHEW
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: G11B11
IPC codes: G11B7, G01J4, G11B11