OPTICAL ISOLATOR

Abstract: An optical isolator (1) comprises an isolator element (4) having an optical axis and comprising a Faraday rotator (6) and at least one polarisor (7a, 7b) and a magnet (2). The magnet (2) is an open-core magnet shaped so that when placed on a substantially planar surface an orifice (3) is defined by the magnet (2) and the substantially planar surface. The isolator element (4) is located within said orifice so that the said optical axis passes through said orifice. Such an isolator is well adapted for use in semiconductor laser modules.

Description: OPTICAL ISOLATOR The present invention relates to an optical isolator, and to optical devices employing optical isolators, in particular semiconductor laser modules. An optical isolator permits the passage of light in one direction through the isolator, but prevents (or at least substantially attenuates) the passage of light in the opposite direction through the isolator. To achieve such non-reciprocal operation an optical isolator must comprise a component which behaves differently, dependent on the direction in which the light passes through it. This component is generally a Faraday rotator. A Faraday rotator comprises a material, typically a crystalline material, which is capable of rotating the plane of polarisation of light passing through it in response to the application of an external magnetic field. The direction of rotation of the plane of polarisation is dependent on the direction of passage of the light through the Faraday rotator, relative to the direction of the applied external magnetic field. The Faraday rotator is combined with one, or more, usually two, polarisors or birefringent walk off crystals, in order to form an isolator. The rotation of the plane of polarisation provided by the Faraday rotator in one direction allows light to pass through both polarisors, whereas in the opposite direction the plane of polarisation is rotated so that the passage of the light through the isolator is blocked by the polarisor. Optical isolators find particular application in optical communications systems, especially for use with semiconductor lasers. As the transmission rates used in optical communications systems have increased, for example to several Gbits per second, the performance required of lasers used in such systems has also increased. It is well known that light reflected back from some part of an optical communications system will adversely affect the operation of such a high performance laser, for example leading to fluctuations in the spectrum, line width, or intrinsic noise of the laser. Optical isolators are utilised to protect such high performance semiconductor lasers from these reflections, and are increasingly being used not just in systems for research purposes, but in systems which are deployed for commercial use. To prevent isolators from adding an excessive additional cost to laser transmitters, isolators must be miniaturised, easily aligned to a laser, and of low cost. A number of miniaturised optical isolators are known. WO 93/20465 discloses a laminated isolator element which is disposed within an optical fibre feedthrough tube. The fabrication and assembly technique for this isolator is complex, and thus expensive Furthermore, although the isolator element itself is small, the cylindrical magnet employed renders this isolator arrangement bulky and cumbersome. The optical isolator arrangement of US 4,966,444 attempts to reduce the number of components employed by utilising a Faraday rotator as a lens. In addition to comprom i sing the performance of the isolator, this arrangement remains bulky due to the large rad i us of curvature required by the Faraday rotator lens, and the large cylindrical magnet employed. US 5,452,122 discloses a further isolator arrangement which utilises laminated i solator elements, and a cylindrical magnet. Due to the use of a cylindrical magnet the i solator must be rotationally aligned to the output mode of the semiconductor laser. According to a first embodiment of the present invention, there is provided an optical isolator mountable on a planar surface, the optical isolator comprising an isolator element, having an optical axis and comprising a Faraday rotator and at least one polarisor or birefringent walk off crystal and a magnet, wherein the magnet is an open-core magnet shaped so that when placed on a substantially planar surface an orifice is def i ned by the magnet and the substantially planar surface, and wherein the isolator element is located within said orif i ce so that the said optical axis passes through said orifice. The present invention thus provides a surface mountable optical isolator by the use of an open-core magnet. The present applicants have discovered that an open-core (i.e. non cylindrical) magnet can provide sufficient magnetic flux to saturate a Faraday rotator. Preferably, in embodiments of the present invention, the magnet is generally U- shaped in cross-section, and preferably the isolator element is a laminate comprising a plate shaped Faraday rotator bonded between plate shaped polarisors. In embodiments of the present invention in which the edges of the orifice defined by the magnet def i ne a rectangle, the rotational alignment of the isolator element within the magnet orif i ce is def i ned by this geometry. Since the non-cylindrical magnet is surface mountable the rotational alignment between the magnet and any source of optical radiation, for example a semiconductor laser, is also pre-defined by the geometry of the magnet. Thus, no rotational alignment is required in the assembly of optical isolators according to embodiments of the present invention. According to a second aspect of the present invention there is provided an optical device comprising a source of optical radiation for generating a beam of optical radiation, an optical waveguide for receiving the beam, focusing means for focusing the beam onto the optical waveguide and an optical isolator according to the present invention, wherein the focusing means directs a converging beam of optical radiation through the isolator and onto the optical waveguide. Embodiments of the present invention thus provide a particularly compact optical isolator arrangement which utilises a converging beam of optical radiation passing through the isolator. Additionally, the use of a converging optical beam through the isolator prevents reflections from components of the optical isolator from reaching the source of optical radiation e.g. the semiconductor laser. Preferably the focusing means comprise a single, preferably spherical lens. This focusing means is particularly simple, effective, and of low cost. Alternatively, a third aspect of the present invention provides an optical device comprising a source of optical radiation for generating a beam of optical radiation, an optical waveguide for receiving the beam, focusing means for focusing the beam onto the optical waveguide and an optical isolator according to the present invention, wherein the focusing means directs a parallel beam of optical radiation through the isolator, and the optical axis of the isolator element is at a non-zero angle to the parallel beam. Thus, in isolator arrangements where it is desirable to provide a parallel beam of optical radiation through the isolator, the present invention provides a particularly simple and effective means to angle the optical isolator element to the parallel beam, for example to avoid reflections from components of the isolator element from reaching the source of optical radiation. Since, according to the present invention, the isolator element is within the orifice of the surface mountable magnet, the whole optical isolator may be easily angled relative to the parallel optical beam by rotating the magnet within the plane of its mounting surface. Such rotation will not affect the rotational alignment of the isolator element about its optical axis relative to the output of the source of optical radiation. According to a forth aspect of the present invention there is provided a semiconductor laser module comprising a package within which is mounted an optical device, which device comprises an optical isolator according to the present invention. The particularly compact optical isolators provided by the present invention, in combination with their ability to be surface mounted, allow them to be directly mounted within the package of a semiconductor laser module. This leads to a number of advantages. Packaging the optical isolator within the same package as the semiconductor laser leads to cost reductions both in component parts, and in assembly costs. Furthermore, the optical isolator benefits from the controlled environment provided within the package for the semiconductor laser. For example, many semiconductor laser modules are provided with temperature control which improves and stabilises the performance of the semiconductor laser. Since the Faraday rotation angle of Faraday rotators varies with temperature, the performance of the optical isolator can be dramatically improved at no additional cost if its temperature is controlled due to its position within a semiconductor laser module. Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: - Figure 1 is a schematic representation of an optical isolator according to an embodiment of the present invention, Figure 2 is a schematic representation of a first embodiment of a semiconductor laser module according to the present invention, Figure 3 is a schematic representation of a magnet utilised in an embodiment of the present invention, showing the results of a theoretical calculation of the magnetic field strength within the magnet, and Figure 4 is a schematic representation of a second embodiment of a semiconductor laser module according to the present invention. Figure 1 is a perspective view of an optical isolator according to an embodiment of the present invention. The isolator 1 comprises a magnet 2 having a square sided orifice 3 in which is located an isolator element 4. The magnet 2 is generally U-shaped in cross-section, and comprises two mounting areas 5, which lie within the same plane, for mounting the optical isolator 1 on a planar surface. The isolator element 4 is a laminate comprising a Faraday rotator 6 sandwiched between two polarisors 7a and 7b. The isolator element 4 comprises an optical axis 8 which passes through the orifice 3 in the magnet 2. The fabrication of this laminate will be described below. Figure 2 shows a schematic, perspective view of a semiconductor laser module according to an embodiment of the present invention, which utilises an optical isolator 1. A semiconductor laser 9 is mounted on a heatsink 10 within a laser package 11. Due to the geometry of the optically active element of the semiconductor laser 9, the output mode (TEoo) of the semiconductor laser is horizontally polarised with respect to the planar base 12 of the semiconductor laser package 11. Between the optical isolator 1 and the semiconductor laser 9 is a zirconia ball lens 13. The ball lens 13 has a diameter of 500μm, and serves to collect the output light beam from the semiconductor laser 9, and to focus this light beam through the optical isolator 1 and on to a single mode optical fibre The fabrication of the isolator element 4, and the magnet 2 will now be described in greater detail. The isolator element 4 is formed as a three-layer sandwich assembly. The central layer of the .solator element 4 is the Faraday rotator 6 which serves to rotate the plane of polaπsanon of light. Any suitable material, such as a crystalline material, may be used for th i s purpose. In the preferred case the Faraday rotator 6 of the isolator element 4 is a b i smuth iron garnet ("BiO") crystal wafer. The outside layers of the isolator element 4 are formed by polarisors 7a and 7b. The polarisors 7a and 7b are secured to the major surfaces of the water 6 by a suitable adhesive. Although any appropriate crystal may be used, in the preferred instance the crystal 6 i s der i ved from an LPE Garnet Film Optical isolator wafer [chemically, <TbBi)>Fe,0„] sold by the Non-Ferrous Metals Division of Sumitomo Corporation of America, New York, New York. Tne crystal 6 has a rotation angle of 45 degrees, +/- 1 degree, for the centre wavelength desired. To achieve high yields in manufacture the wafer from which the crystal 6 is derived must have rotation uniformity across the entire wafer and a transm i ttance of greater than approximately 90%. A preferred centre wavelength is 1550 nanometres, although the wavelength of 1300 nanometres is not uncommon. To obtain a 0.01 % or lower reflectivity from the isolator element 4 for the desired cemre wavelength the major surfaces of the crystal 6 should be covered with a three-layer h i gh quahty anti-reflection (AR) coating. A suitable coating may be provided by KML Opt i cal Industries, Hauppauge, New York. Each coating is designed to index-match the crystal 6 to the adjacent adhesive layer. Tne AR coatings are preferably applied by ion deposition. J Although other polarisors may be used, the preferred polarisors 7a and 7b are each formed from the material manufactured by Corning Incorporated, Coming, New York, and sold as Coπung Code 1550 HC Polarcor™ polarisors. One of the polarisors 7b exhibits an edge angle of 45° (i.e. the plane of polarisation passed by the polarisor is at 45" to the edge of the polarisor) while the edge angle of the other polarisor 7a is 0" . The polarisors 7a and 7b are attached to the coated major surfaces of the crystal 6 by a respective layer of index-matching adhesive. Preferably, the adhesive is UV curable epoxy adhesive manufactured by Epoxy Technologies Inc. of Billerica, Massachussetts, USA and sold as Epotek 353 ND adhesive. The isolator element 4 is fabricated by securing the wafers of polarisor material 7a and 7b (each approximately 10x10x0.2 mm) to the major surfaces of the crystal (approximately 10x10x0.35 mm) using the adhesive. The resulting assembly is cured and thereafter diced into the isolator elements 4 used in the isolator 1. The dicing is accomplished using a diamond dicing saw with a 0.07 mm kerf blade. The resulting isolator elements 4 are approximately 0.4 mm x 0.4 mm. One edge of the polarisor 7a is marked so as to distinguish polarisor 7a from polarisor 7b, and to indicate the direction of the plane of polarisation passed by polarisor 7a. The magnet 2 has a width of 2 mm, a height of 1.2 mm, and a depth of 0.7 mm. When the magnet 2 is placed on a planar surface, an orifice 3 is defined which is dimensioned so as to accept an isolator element 4 which is 0.4 mm wide and 0.4 mm high. The magnet may be formed of a number of magnetic materials, but preferably is formed of samarium cobalt, or neodymium iron boron. A solid block of magnetic material may be machined to form the magnet, or preferably due to lower costs, the magnet may be moulded from resin bonded material. In this case a typical composition would include approximately 6% resin. The magnet is magnetised subsequent to fabrication by subjecting it to a 40 KGauss magnetic field. The magnetic field within the orifice of the magnet 2 has been theoretically modeled, and Figure 3 shows a schematic outline of the magnet 2 with a super-imposed theoretically calculated field strength for the magnetic field generated in a vertical plane within the orifice 3. From this figure it can be seen that the magnetic field strength varies from 3250 Gauss at the top of the orifice 3, to 2000 Gauss at the bottom of the orifice 3. Since the saturation field strength of the Faraday rotator 6 is 1000 Gauss, the Faraday rotator 6 will be fully saturated when placed within the orifice 3 of the magnet 2. The isolator element 4 is then bonded within the orifice 3 of the magnet 2. The isolator element 4 is orientated so that the polarisor 7a, whose plane of polarisation is parallel to an edge of the isolator element 4, is mounted within the magnet so as to face the semiconductor laser 9, and so as to pass horizontally polarised light. With reference to Figure 2, the assembly of the semiconductor laser module will now be described. The heatsink 10 is first mounted on the planar base 12 of the semiconductor laser package 11. This provides a surface on which the semiconductor laser „ 9 can be mounted which is parallel to the base 12 so that the polarisation of the output mode of the semiconductor laser 9 is now accurately aligned to the base 12. Furthermore, rotational alignment of the optical isolator, about its optical axis, is simply achieved by the placing the mounting areas 5 of the optical isolator in contact with the planar base 12 of the semiconductor laser package 11. The optical isolator 1 is then mounted on the base 12. This can be achieved "passively" i.e. without activating the semiconductor laser 9, since the orifice 3 and isolator element 4 are sufficiendy large. A "dummy" optical fibre 14 is then utilised to aid the accurate, active alignment of the ball lens 13 to the semiconductor laser 9. One end of the dummy fibre 14 is temporarily mounted to the laser package 11, while the other end of the dummy fibre 14 is inserted into an optical detector. The semiconductor laser 9 is electrically powered, so that it emits optical radiation, and the ball lens 13 is aligned so as to maximise the optical power coupled from the semiconductor laser 9 to the dummy optical fibre 14. Once the ball lens 13 has been fixed in place, using light curing resin, the dummy optical fibre 14 is replaced with a permanent optical fibre 14 mounted within a fibre tube. The fibre 14 is actively aligned, and the fibre tube is laser welded to the wall of the laser package 11. In use, horizontally polarised light is emitted by the semiconductor laser 9, passes through the lens 13 and enters the isolator 1, via the polarisor 7a. The polarisor 7a passes this light with little optical loss, since its plane of polarisation is horizontally aligned. The Faraday rotator 6, being within the magnetic field of the magnet 2, causes the plane of polarisation of the light to be rotated through 45°. The light is then passed out of the isolator through the polarisor 7b, which because its plane of polarisation is aligned at 45° to the edges of the polarisor, once again passes the light with little optical loss. Any light reflected by the end 15 of the optical fibre 14, or any other component to which the optical fibre 14 is attached, is prevented from reaching the semiconductor laser 9 by the optical isolator 1. Reflected light which is not aligned with the plane of polarisation of the polarisor 7b is attenuated severely by the polarisor. Reflected light which is aligned with the plane of polarisation of polarisor 7b passes through polarisor 7b with a low optical loss, and is rotated through 45° by Faraday rotator 6 so that its plane of polarisation is vertical relative to the base 12 of the laser module 11. Since the plane of polarisation of polarisor 7a is horizontal, this light is severely attenuated by polarisor 7a. Any light from the semiconductor laser 9 which is, despite the AR coating applied to various surfaces, reflected from components of the isolator 1 back towards the semiconductor laser 9, is unlikely to reach the semiconductor laser 9 due to the converging nature of the light beam striking the isolator. The use of a converging beam through the optical isolator 1 is facilitated by the use of polarisors 7a and 7b which have a high optical extinction for optical beams received at large angles to the optical axis. The polarisors employed have an optical extinction greater than 30db for optical beams received at angles of less than 10° to the optical axis. Measurements performed on optical isolators according to the present embodiment indicate that typically isolation of 33dB is provided, with insertion losses of 0.5dB. Figure 4 shows an alternative embodiment of the present invention in which two ball lenses 13 are employed, in order to provide a parallel beam through the optical isolator 1. The optical isolator 1 is mounted at an angle of approximately 2° to the parallel optical beam, in order to prevent any reflections from the optical isolator from reaching the semiconductor laser 9. This angling of the optical isolator is easily achieved by rotating the isolator about a vertical axis, and does not affect the alignment, about the optical axis 8, of the polarisor 7a to the plane of polarisation of the semiconductor laser. Although embodiments of the present invention have been described in relation to semiconductor lasers, the invention also finds application with fibre lasers. Furthermore, it will be appreciated that optical isolators according to the present invention can advantageously be utilised with opto-electronic integrated circuits (OEICs) and can be mounted on planar surfaces comprising metallic, ceramic or silicon substrates. Such substrates may be provided with location steps or v-grooves.

Claims: Q CLAIMS 1. An optical isolator mountable on a planar surface, the optical isolator comprising: - an isolator element, having an optical axis and comprising a Faraday rotator and at least one polarisor or birefringent walk off crystal, and a magnet, wherein the magnet is an open-core magnet shaped so that when placed on a substantially planar surface an orifice is defined by the magnet and the substantially planar surface, and wherein the isolator element is located within said orifice so that the said optical axis passes through said orifice. 2. An optical isolator as claimed in Claim 1 , wherein the magnet comprises two mounting areas, which lie substantially within the same plane, for mounting the magnet on the planar surface. 3. An optical isolator as claimed in Claim 1 and 2, wherein the magnet is generally u-shaped in cross-section. 4. An optical isolator as claimed in any preceding claim, wherein the magnetic flux generated by the magnet within said orifice is substantially parallel to the optical axis of the isolator element. 5. An optical isolator as claimed in any preceding claim wherein the edges of said orifice define a rectangle. 6. An optical isolator as claimed in Claim 5, wherein the edges of said orifice substantially define a square. 7. An optical isolator as claimed in any preceding claim, wherein the isolator element is a laminate comprising a plate-shaped faraday rotator bonded between plate- shaped polarisors. 8. An optical device comprising :- a source of optical radiation for generating a beam of optical radiation, an optical waveguide for receiving the beam, focusing means for focusing the beam onto the optical waveguide, and an optical isolator according to any preceding claim, wherein the focusing means directs a converging beam of optical radiation through the isolator and onto the optical waveguide. 9. An optical device as claimed in Claim 8, wherein the focusing means comprises a single lens. 10. An optical device as claimed in Claim 8 or 9, wherein the or each polarisor of the optical isolator has a high optical extinction for optical beams received at a large angle to the optical axis. 11. An optical device as claimed in Claim 10, wherein the or each polarisor has an optical extinction greater than 30dB for optical beams received at less than 10° to the optical axis. 12. An optical device as claimed in any one of Claims 8 to 11, wherein the or each polarisor is of the grating type. 13. An optical device comprising :- a source of optical radiation for generating a beam of optical radiation, an optical waveguide for receiving the beam, focusing means for focusing the beam onto the optical waveguide, and an optical isolator according to any one of Claims 1 to 7, wherein the focusing means directs a parallel beam of optical radiation through the isolator, and the optical axis of the isolator element is at a non-zero angle to the parallel beam. π 14. An optical device as claimed in Claim 13, wherein the focusing means comprise two confocal lenses. 15. An optical device as claimed in any of Claims 8 to 14, wherein the optical waveguide is an optical fibre. 16. An optical device as claimed in any one of Claims 8 to 14, wherein the optical waveguide is a planar optical waveguide. 17. An optical device as claimed in any one of Claims 8 to 16 wherein the source of optical radiation is a semiconductor laser. 18. An optical device as claimed in Claim 8 to 16 wherein the source of optical radiation is a fibre laser. 19. A semiconductor laser module comprising a package within which is mounted an optical device according to any one of Claims 8 to 17.

Inventors: PARK CHRISTOPHER ANTHONY, PARK, CHRISTOPHER, ANTHONY
Applicants: HEWLETT PACKARD CO, PARK CHRISTOPHER ANTHONY, HEWLETT-PACKARD COMPANY, PARK, CHRISTOPHER, ANTHONY
Application year: 1997
Main classification: G02F1
IPC codes: G02B6, G02B27, G02F1