WO2002016999A1

WO2002016999A1 - Polarization-insensitive scanning system

Info

Publication number: WO2002016999A1
Application number: PCT/US2001/026391
Authority: WO
Inventors: Andrew G. Kirk; Timothy K. Deis; Robert E. Aldrich; Daniel D. Stancil; Tuviah E. Schlesinger
Original assignee: Accelight Investments, Inc.
Priority date: 2000-08-23
Filing date: 2001-08-23
Publication date: 2002-02-28
Also published as: AU2001290563A1

Abstract

A polarization-insensitive scanning system (10) includes an optical beam splitter (12) to split an unpolarized optical beam (ab) into a first polarization (a) and a second polarization (b). This system includes a plurality of electro-optic scanning devices (14) to deflect an optical beam (ab) in response to an electrical control signal. Electrical leads (18) communicate the electrical control signals to the scanning devices (14) from a controller (20), which selects and applies the signals. A combiner element (22) is used to recombine the first polarization (a) and the second polarization (b) into the optical beam (ab). The first polarization optical beam (a) is directed through a first electro-optic scanning device (14) and the second polarization optical beam (b) is directed through a second electo-optic scanning device (14).

Description

POLARIZATION-INSENSITIVE SCANNING SYSTEM BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates generally to electro-optic telecommunications switching and, in particular, to a polarization-insensitive scanning system.

2. Brief Description of the Prior Art

[0002] In various fields, particularly in the fiber optic telecommunication field, electro-optic scanners are used in order to bend or deflect an optical beam in response to an electrical signal. See U.S. Patent Nos. 5,317,446 to Mir and 5,668,657 to Talbot. For example, an electro-optic scanner can be a crystal, such that when an optical beam is passed through the crystal, the path of the optical beam is bent. The deflection angle of the optical beam is proportional to an applied electric field. Optical beam deflection systems are presently used to deflect an incoming optical beam in proportion to the applied voltage. This deflection can be used to realize an optical switch. However, these prior art systems are typically polarization dependent. This is undesirable in optical telecommunication switches, since controlling the polarization adds considerable cost and complexity.

SUMMARY OF THE INVENTION

[0003] It is an object of the present invention to provide a polarization- insensitive scanning system that overcomes the deficiencies of the prior art. It is another object of the present invention to provide a polarization-insensitive scanning system that ensures minimum path-length differences in two parallel paths of two polarization states of an optical beam are achieved. It is a further object of the present invention to provide a polarization- insensitive scanning system for a particular application in the fiber optic and telecommunications switching industries.

[0004] Accordingly, we have invented a polarization-insensitive scanning system that includes an optical beam splitter, which splits an unpolarized optical beam into a first polarization optical beam and a second polarization optical beam. This system also includes a plurality of electro-optic scanning devices configured to deflect an optical beam in response to electrical signals applied to the devices. Electrical leads communicate electrical control signals to the scanning devices, and a controller selects and applies the electrical control signals through the electrical leads to the scanning devices. The present system also includes a combiner element for recombining the first polarization optical beam and the second polarization optical beam back into a combined optical beam. The first polarization optical beam is directed through one electro-optic scanning device, and the second polarization optical beam is directed through a second electro-optic scanning device. When appropriately deflected by each respective scanning device, the first and second polarization optical beams are recombined into a single optical beam.

[0005] a further embodiment, the polarization-insensitive scanning system includes a single electro-optic scanning device, which deflects an unpolarized optical beam in response to electrical control signals applied to the device. Electrical leads communicate electrical control signals to the device, and a controller selects and applies the electrical control signals through the electrical leads to the device. The system includes a combiner element to combine a first deflected optical beam and a second deflected optical beam into a single optical beam. This system also includes a reflector element. The first deflected optical beam is directed to the combiner element, the second deflected optical beam is directed to the reflector element, which directs the second deflected optical beam to the combiner element, and, finally, the combiner element combines the first deflected optical beam and the second deflected optical beam into a single optical beam. A dielectric retarding element can be added to the shorter path to equalize the optical paths for the two polarizations.

[0006] The present invention, both as to its construction and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of the specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Fig. 1 is a block diagram of a first embodiment polarization- insensitive scanning system in an undeflected state according to the present invention;

[0008] Fig. 2 is a block diagram of a polarization-insensitive scanning system in a deflected state according to the present invention;

[0009] Fig. 3 is a block diagram of a second embodiment of a polarization- insensitive scanning system according the present invention;

[0010] Fig. 4 is a block diagram of a third embodiment of a polarization- insensitive scanning system according to the present invention;

[0011] Fig. 5 is a block diagram of a fourth embodiment of a polarization- insensitive scanning system according to the present invention; and

[0012] Fig. 6 is a block diagram of a fifth embodiment of a polarization- insensitive scanning system according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] A first embodiment of a polarization-insensitive scanning system 10 of the present invention is generally shown in Figs. 1 and 2. The polarization-insensitive scanning system 10 includes an optical beam splitter 12, which receives an unpolarized optical beam ab (or optical beam of random polarization), which is then split into a first polarization optical beam a and a second polarization optical beam b. Typically, the first polarization optical beam a corresponds to the s-state polarization of the optical beam ab, and the second polarization optical beam b corresponds to the/7-state polarization of the optical beam ab. The optical beam splitter 12 may be a birefringent beam displacer, and, further, may be a crystal. For example, it is envisioned that the optical beam splitter 12 may be calcite, YNO4, rutile, etc.

[0014] In this embodiment, two electro-optic scanning devices 14 are used and are formed in lithium niobate with domain-inverted prisms. It is envisioned that the electro- optic scanning devices 14 are formed in an electro-optic crystal, such as lithium niobate or lithium tantalate. Further, the electro-optic crystal may be formed with prisms formed by patterned surfaces. While, as seen in Figs. 1 and 2, both of the electro-optic scanning devices 14 are resident on a common substrate 16, it is envisioned that multiple substrates 16 may be used, with each substrate 16 having one or more electro-optic scanning devices 14 displaced thereon. The electro-optic scanning devices 14 are arranged in parallel, and, further, the electro- optic scanning devices 14 may be arranged in a side-by-side configuration or a stacked configuration. When using a birefiingent beam displacer as the optical beam splitter 12, the birefringent beam displacer may have a length such that lateral separation of the first polarization optical beam a and the second polarization optical beam b on output is equal to the center separation of the two electro-optic scanning devices 14.

[0015] In order to cause deflection of the first polarization optical beam a and second polarization optical beam b in its respective electro-optic scanning device 14, an electrical control signal must be applied to the scanning devices 14. To communicate the electrical control signal, typically an electrical potential, to the scanning devices 14, electrical leads 18 are used. Further, a controller 20 selects and applies the electrical control signals through the electrical leads 18 to the electro-optic scanning device 14. After appropriate deflection, the first polarization optical beam a and the second polarization optical beam b are directed to a combiner element 22, which recombines the first polarization optical beam a and the second polarization optical beam b back to optical beam ab. [0016] In the present embodiment, a polarization rotator 24, which rotates the plane of incident polarization by 90°, is placed between the optical beam splitter 12 and the electro-optic scanning device 14 corresponding to the second polarization optical beam b. The polarization rotator 24 may be a single order half-wave plate or a multiple order half-wave plate, hi addition, the polarization rotator 24 may be a birefiingent thin film deposited on or adjacent to the path of the second polarization optical beam b as it exits from the optical beam splitter 12. For example, the birefringent thin film may also be deposited at or on the entrance surface of the substrate 16. The polarization rotator 24 is also placed on the exit path of the first polarization optical beam a from the electro-optic scanning device 14 prior to its direction to the combiner element 22.

[0017] h operation, the polarization rotator 24 corresponding with the second polarization optical beam b ensures that both the first polarization optical beam a and the second polarization optical beam b are in the same state, in this case, the s-state. By applying a voltage to the electro-optic scanning devices 14 through the electrical leads 18, the respective optical beam emerging from each scanning device 14 is deviated at a controllable angle. The first polarization optical beam a, after emerging from the electro-optic scanning device 14, is transformed to the polarization state of the second polarization optical beam b by the polarization rotator 24, which transforms the first polarization optical beam a into the -state. At this point, as both the first polarization optical beam a and the second polarization optical beam b are incident on a combiner element 22, the emerging beams are co-linear. As with the optical beam splitter 12, the combiner element may be a birefringent beam displacer, such as a crystal. To minimize the optical path length differences between the first polarization optical beam a and the second polarization optical beam b, which is particularly useful in the fiber optic switching area, the optical beam splitter 12 and the combiner element 22 should be of a substantially identical construction.

[0018] Overall, the polarization-insensitive scanning system 10 allows the first polarization optical beam a and the second polarization optical beam b to emerge with minimal polarization dispersion due to path-length differences. It is envisioned that the controller 20 may be in communication with a driver 26 applying the electrical potential to the electro-optic scanning devices 14 via the electrical leads 18.

[0019] Fig. 3 illustrates a second embodiment of a polarization-insensitive scanning system 10 according to the present invention, hi this embodiment, the optical beam splitter 12 is a polarized beam splitting/combiner cube 28 and a reflector element 30. As seen in Fig. 3, the unpolarized optical beam ab is split by the polarized beam splitting/combiner cube 28, with the first polarization optical beam a directed through an electro-optic scanning device 14 and a second polarization optical beam b directed to the reflector element 30. The second polarization optical beam b is reflected from the reflector element 30 to another electro-optic scanning device 14. It is envisioned that the polarized beam splitting/combiner cube 28 and the reflector element 30 may be separate elements or integrated into a unitary structure.

[0020] In this second embodiment, the polarization rotators 24 of the first embodiment are eliminated through the use of two electro-optic scanning devices 14 sized in length such that the longer electro-optic scanning device 14 compensates for the smaller rjj coefficient. The rjj coefficient defines the deflection of an optical beam polarized perpendicularly to the z-axis of the electro-optic scanning device 14, when an electrical field parallel to the z-axis is applied to the optical beam. Further, the r^ coefficient defines the deflection of an optical beam polarized parallel to the z-axis of the electro-optic scarming device 14, when an electrical field parallel to the z-axis is applied. This embodiment preferably utilizes a lithium niobate crystal as the electro-optic scanning device 14, the crystal is fabricated from a z-cut wafer with electrodes on the z faces. When using this geometric configuration, any pivot points of these electro-optic scanning devices 14 would need to be properly located to ensure proper beam position matching at the output. As with the first embodiment, after appropriate deflection by each respective electro-optic scanning device 14, the first polarization optical beam a and the second polarization optical beam b are recombined by the combiner element 22 into a single optical beam ab. It is also envisioned that, as opposed to using two electro-optic scanning devices 14 of different lengths, the active area of one of the electro-optic scanning devices 14 is shorter than the other, regardless of physical dimension. Although the geometric path lengths for the first polarization optical beam a and the second polarization optical beam b may be the same, the optical path lengths may differ owing to the birefringence of the substrate 16. This difference can be minimized by inserting a dielectric retarding element 32 with appropriate thickness in the branch with the shortest optical path. The thickness of the dielectric retarding element 32 is chosen so as to minimize the optical path difference between the separate polarizations of optical beam ab.

[0021] Fig. 4 illustrates a third embodiment of the present invention. The optical beam splitter 12 of this embodiment uses the polarized beam splitting/combiner cube 28 and the reflector element 30 as integrated in a unitary structure. This embodiment uses two electro-optic scanning devices 14 and two drivers 26. These drivers 26, as controlled by the controller 20, apply different voltages to each of the electro-optic scanning devices 14. A single driver 26 may be used, as the voltages could each be derived from a single drive voltage using a voltage divider to divide the applied fields, so as to compensate for the ratio x^^τ\ '-

■ K A ⁿo^ru

Alternatively, the respective thicknesses of the elecfro-optic scanning devices 14 may be sized such that appropriate deflection is achieved:

[0022] A fourth embodiment of the present invention is illustrated in Fig. 5. The fourth embodiment is a polarization-insensitive scanning system 10, which includes a single electro-optic scanning device 14 for deflecting the unpolarized optical beam flfrin response to electrical control signals. This embodiment is particularly useful for switching applications since discrete beam positions are desired. In this fourth embodiment, the first polarization optical beam a is directed through the elecfro-optic scanning device 14, deflected, and directed through the combiner element 22, in this embodiment, a polarized beam splitting/combiner cube 28. The second polarization optical beam b is directed through the electro-optic scanning device 14, deflected, and reflected to the combiner element 22 by the reflector element 30. As with the above embodiments, the combiner element 22 recombines the first polarization optical beam a and the second polarization optical beam b, thereby recreating the polarized optical beam ab. In a fifth embodiment, as seen in Fig. 6, multiple switch positions may be achieved by carefully staggering and interleaving the elements. As the present polarization-insensitive scanning system 10 is useful in the fiber optic and telecommunications industries, the combiner element 22 may be a fiber combiner.

[0023] In this manner, a polarization-insensitive scanning system 10 is provided, which ensures that two polarization states emerge with little polarization dispersion due to path-length differences. The polarization-insensitive scanning device 10 of the present invention also ensures that the two emerging beams are co-linear. This polarization-insensitive scanning system 10 has particular application in the fiber optic and telecommunication industries. In addition, this polarization-insensitive scanning system 10 is particularly useful in fiber optic switching applications.

[0024] This invention has been described with reference to the preferred embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.

Claims

We Claim;

1. A polarization-insensitive scanning system, comprising: an optical beam splitter configured to split an unpolarized optical beam into a first polarization optical beam and a second polarization optical beam; a plurality of electro-optic scanning devices configured to deflect an optical beam in response to electrical control signals applied thereto; electrical leads configured to communicate electrical control signals to the plurality of electro-optic scanning devices; a controller configured to select and apply electrical control signals through the electrical leads; and a combiner element configured to recombine the first polarization optical beam and the second polarization optical beam into the optical beam; wherein the first polarization optical beam is directed through a first of the plurality of electro-optic scanning devices and the second polarization optical beam is directed through a second of the plurality of electro-optic scanning devices.

2. The polarization-insensitive scanning system of claim 1, wherein the optical beam splitter is a birefringent beam displacer.

3. The polarization-insensitive scanning system of claim 2, wherein the birefringent beam displacer is a crystal.

4. The polarization-insensitive scanning system of claim 3, wherein the crystal is one of calcite, YNO4 and rutile.

5. The polarization-insensitive scanning system of claim 2, wherein the length of the birefringent beam displacer is sized such that lateral unpolarized optical beam separation on output is equal to center separation of the first elecfro-optic scanning device and the second electro-optic scanning device.

6. The polarization-insensitive scanning system of claim 1, wherein the plurality of electro-optic scanning devices are formed in an electro-optic crystal.

7. The polarization-insensitive scanning system of claim 6, wherein the electro-optic crystal is formed with one of domain-inverted prisms and prisms formed by patterned surfaces.

8. The polarization-insensitive scanning system of claim 6, wherein the electro-optic crystal is one of lithium niobate and lithium tantalate.

9. The polarization-insensitive scanning system of claim 1, further comprising a subsfrate, wherein the plurality of elecfro-optic scanning devices are disposed on the substrate.

10. The polarization-insensitive scanning system of claim 1, further comprising a plurality of substrates, wherein at least one of the plurality of electro-optic scanning devices is disposed upon at least one of the plurality of substrates.

11. The polarization-insensitive scanning system of claim 1, wherein the plurality of electro-optic scanning devices are arranged in parallel.

12. The polarization-insensitive scanning system of claim 11, wherein the plurality of electro-optic scanning devices are arranged in a side-by-side configuration.

13. The polarization-insensitive scanning system of claim 11, wherein the plurality of electro-optic scanning devices are arranged in a stacked configuration.

14. The polarization-insensitive scanning system of claim 1, wherein the optical beam splitter comprises a polarized beam splitting cube and a reflector element, wherein the unpolarized optical beam is split by the polarized beam splitting/combiner cube and at least one of the first polarization opticalbeam and the second polarization optical beam is directed to the reflector element.

15. The polarization-insensitive scanning system of claim 14, wherein the polarized beam splitting cube and the reflector element are integrated in a unitary structure.

16. The polarization-insensitive scanning system of claim 1, wherein the respective lengths of the plurality of electro-optic scanning devices are substantially equal.

17. The polarization-insensitive scanning system of claim 1, wherein the respective lengths of the plurality of electro-optic scanning devices are sized such that equivalent optical beam deflection is achieved.

18. The polarization-insensitive scanning system of claim 1, wherein the controller is in communication with at least one driver, the at least one driver applying an electrical potential to the plurality of electro-optic scanning devices.

19. The polarization-insensitive scanning system of claim 18, wherein each of the plurality of electro-optic scanning devices is associated with a respective driver.

20. The polarization-insensitive scanning system of claim 1, wherein the respective thicknesses of the plurality of elecfro-optic scanning devices are sized such that equivalent optical beam deflection is achieved.

21. The polarization-insensitive scanning system of claim 1, further comprising a polarization rotator, wherein at least one of the first polarization optical beam and the second polarization optical beam is directed through the polarization rotator prior to direction to one of the plurality of electro-optic scanning devices.

22. The polarization-insensitive scanning system of claim 21, wherein the polarization rotator is a single order half- wave plate.

23. The polarization-insensitive scanning system of claim 21, wherein the polarization rotator is a multiple order half-wave plate.

24. The polarization-insensitive scanning system of claim 21, wherein the polarization rotator is a birefringent thin film.

25. The polarization-insensitive scanning system of claim 24, wherein the birefringent thin film is deposited on one of the optical beam splitter and the substrate having at least one of the plurality of elecfro-optic scanning devices disposed thereon.

26. The polarization-insensitive scanning system of claim 1, further comprising a polarization rotator, wherein at least one of the first polarization optical beam and the second polarization optical beam is directed through the rotator after direction through one of the plurality of elecfro-optic scanning devices.

27. The polarization-insensitive scanning system of claim 26, wherein the polarization rotator is a single order half-wave plate.

28. The polarization-insensitive scanning system of claim 26, wherein the polarization rotator is a multiple order half-wave plate.

29. The polarization-insensitive scanning system of claim 26, wherein the polarization rotator is a birefringent thin film.

30. The polarization-insensitive scanning system of claim 29, wherein the birefringent thin film is deposited on one of the optical beam splitter and a substrate having at least one of the plurality of elecfro-optic scanning devices disposed thereon.

31. The polarization-insensitive scanning system of claim 1, wherein the combiner element is a fiber combiner.

32. The polarization-insensitive scanning system of claim 1, wherein the combiner element is a birefringent beam displacer.

33. The polarization-insensitive scanning system of claim 32, wherein the birefringent beam displacer is a crystal.

34. The polarization-insensitive scanning system of claim 33, wherein the crystal is one of calcite, YNO4 and rutile.

35. The polarization-insensitive scanning system of claim 1, further comprising a dielectric retarding element placed in the path of one of the first polarization optical beam and the second polarization optical beam to equalize the optical path length.

36. A polarization-insensitive scanning system, comprising: an elecfro-optic scanning device configured to deflect an optical beam in response to electrical control signals applied thereto; electrical leads configured to communicate electrical control signals to the elecfro-optic scanning device; a controller configured to select and apply electrical control signals through the electrical leads; at least one combiner element configured to combine a first deflected optical beam and a second deflected optical beam into the optical beam; and at least one reflector element; wherein the first deflected optical beam is directed to the combiner element, the second deflected optical beam is directed to the reflector element, which directs the second deflected optical beam to the combiner element, and the combiner element combines the first deflected optical beam and the second deflected optical beam into the optical beam.

37. The polarization-insensitive scanning system of claim 36, further comprising a dielectric retarding element placed in the path of one of the first polarization optical beam and the second polarization optical beam to equalize the optical path length.

38. The polarization-insensitive scanning system of claim 36, wherein the elecfro-optic scanning device is formed in an elecfro-optic crystal.

39. The polarization-insensitive scanning system of claim 38, wherein the electro-optic crystal is formed with one of domain-inverted prisms and prisms formed by patterned surfaces.

40. The polarization-insensitive scanning system of claim 38, wherein the elecfro-optic crystal is one of lithium niobate and lithium tantalate.

41. The polarization-insensitive scanning system of claim 36, further comprising a subsfrate, wherein the elecfro-optic scanning device is disposed on the substrate.

42. The polarization-insensitive scanning system of claim 36, wherein the controller is in communication with at least one driver, the at least one driver applying an electrical potential to the elecfro-optic scanning device.

43. The polarization-insensitive scanning system of claim 36, wherein the combiner element is a fiber combiner.

44. The polarization-insensitive scanning system of claim 36, wherein the combiner element is a birefringent beam displacer.

45. The polarization-insensitive scanning system of claim 44, wherein the birefringent beam displacer is a crystal.

46. The polarization-insensitive scanning system of claim 45, wherein the crystal is one of calcite, YNO4 and rutile.

47. The polarization-insensitive scanning system of claim 33, wherein the combiner element is a polarized beam splitting/combiner cube.