GB2413858A

GB2413858A - Optical beam-splitter with plano convex lens

Info

Publication number: GB2413858A
Application number: GB0410084A
Authority: GB
Inventors: Simon G P Meadowcroft
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2004-05-06
Filing date: 2004-05-06
Publication date: 2005-11-09
Also published as: GB0410084D0

Abstract

An optical beam-splitter 1 that splits/combines optical radiation from/to a common path into/from two paths at different focal lengths comprises an optical element (2) having a planar surface (4), and a plano-convex lens (6). The planar surface (8) of the lens (6) is in optical contact with the planar surface (4) of the optical element (2) at a planar interface (12). The arrangement is such that there are two optical paths (14',14'', 16',16'') through the beam-splitter arrangement, both of which have a mutually common section (14',16') which passes through and is focussed by a first area (18) of the convex surface (10) of the lens (6). The paths (14'',16'') diverge at the planar interface (12) where one of the paths (14'') passes through the planar interface (12) into the optical element (2) and the other of the paths (16'') is reflected by the planar interface (12,112,212) and passes through and is focussed by a second area (20) of the convex surface (10) of the lens (6).Interface 12 may include a dichroic coating or polarising layer. The beam splitter may be part of a transceiver.

Description

241 3858 - 1 Optical Beam-splitter Arrangement The present invention

relates to an optical beam-splitter, and particularly to an integrated beam-splitter arrangement that splits optical radiation from a common path into two paths or which combines optical radiation from two paths into a common path, each of which paths is focussed by the arrangement at different focal lengths.

In optical communications systems, it is often necessary to split optical radiation received along a single path, or to combine from two sources and send along a common path such optical radiation, or both to send and receive optical radiation along a common path. Depending on the application, the optical radiation may be received from a variety of devices, such as optical fibres or optoelectronic sources such as laser diodes or LEDs.

Optical radiation may be sent to any passive or active optical device, such as optical fibres, lenses, optical filters, photo-detectors, optical amplifiers or gratings.

Some devices, such as an optical fibre transceiver unit, require within the unit both a source of optical radiation such as a laser diode and a photodetector. These components are then arranged in a particular way relative to a beam-splitter arrangement within the transceiver unit such that optical radiation received or collected from the end of an optical fibre is directed along one path towards the photodetector. The arrangement is also such that the beam-splitter arrangement collects or receives optical radiation emitted by the laser diode and directs this along a second path towards the same optical fibre for onward transmission from the unit. The first and second paths overlap between the beam splitter arrangement and the optical fibre, and diverge between the beam-splitter arrangement on the one hand and the photodetector and the laser diode on the other hand.

A variety of optical components can serve as a beam- splitter, such as a half-silvered mirror, a prism or a split cube having an optical coating along an internal diagonal interface within the cube. Many arrangements have been proposed, including arrangements that use polarizing elements or dichroic coatings to separate or combine beams that have different polarizations or optical wavelengths.

In some applications there is the need to miniaturize elements of the beam-splitter arrangement. This may be simply to reduce the volume and/or weight of the optical arrangement, or to make the components more physically robust. When a beam-splitter arrangement is miniaturized, a problem arises when it is necessary to combine in close proximity with the arrangement other components, such as a photodetector or a laser diode, which may have widely different sizes, or have different positional tolerance requirements with respect to the beam-splitter arrangement. In general, different components will require different focal lengths or, equivalently, different numerical apertures from the beam-splitter arrangement. It may then be necessary either to alter the dimensions of a conventional component such as a laser diode or a photodetector or to use additional optical elements or lenses in order to align and compactly package these and the beam-splitter arrangement. - 3 -

A further problem arises from the fact that the manufacture and assembly of miniature optical components may become more expensive the smaller the components are made. In general, optical alignment tolerances will not be reduced for smaller optical components. It may also become more difficult to apply optical adhesives or otherwise bond smaller components. It therefore becomes more difficult to position, align and bond discrete miniaturized components as compared with larger equivalent components.

It is an object of the present invention to provide a more convenient and versatile compact optical beam-splitter arrangement.

According to the invention, there is provided an optical beam-splitter arrangement, comprising an optical element having a planar surface, and a piano-convex lens, the piano-convex lens having a planar surface and a convex surface, the planar surface of the lens being in optical contact with the planar surface of the optical element at a planar interface, the arrangement being such that there are two optical paths through the beamsplitter arrangement, both of which have a mutually common section which passes through and is focussed by a first area of the convex surface of the lens, said paths diverging at the planar interface where one of the paths passes through the planar interface into the optical element and the other of the paths being reflected by the planar interface and passing through and being focussed by a second area of the convex surface of the lens.

The arrangement is therefore such that an optical beam incident on the convex surface of the lens is either focussed once or twice by the lens, depending on which path the optical radiation takes through the beamsplitter arrangement.

The arrangement therefore naturally provides two optical paths with different focal lengths and hence numerical apertures. This is particularly useful when optical radiation is directed towards and/or originates from different optical components having different physical dimensions or optical requirements such as focal length or numerical aperture.

The optical element and the lens are preferably permanently bonded to each other at the planar interface.

The optical element may be any convenient element having a planar surface, but in a preferred embodiment of the invention is a prism. For example, the prism may be a right-angled prism in which case, the planar surface of the optical element is a hypotenuse of this prism.

The convex surface may be any type of convex surface, but will most conveniently be a spherical surface.

In a preferred embodiment of the invention, the lens is a hemispherical lens.

The beam-splitter surface may split the beams using any of a variety of techniques. For example, the interface may - 5 include one or more layers forming a dichroic coating for splitting optical beams having different wavelengths.

Alternatively, the interface may include a semi-silvered coating for splitting optical beams having the same or a different wavelength.

In a further alternative embodiment, the interface may include a polarizing layer or layers for splitting optical beams having different polarizations.

Also according to the invention, there is provided an optical transceiver unit, having a source of optical radiation, a photodetector, and an optical port for receiving and for transmitting optical radiation to/from the unit, and an optical beam-splitter arrangement for directing said received and transmitted optical radiation respectively to/from the photodetector and the source of optical radiation, wherein the optical beam splitter arrangement is according to the invention.

In the optical transceiver unit the optical path which is focussed twice by the convex surface of the piano-convex lens leads from the source of optical radiation to the optical port, and the optical path focussed once by the piano-convex lens leads from the optical port to the photodetector.

The optical beams along the common path between the plano- convex lens and the optical port may be focussed by the lens into an end of an optical fibre, for example a fibre stub as part of an optical connector. - 6

The entrance face of the fibre stub may include a polarizing element, for example a polarising isolator for suppressing back-reflection of the optical radiation into a laser diode. In this case, the beam-splitter may conveniently be a polarizing beam-splitter.

Also according to the invention, there is provided a method of splitting or combining beams of optical radiation using an optical beam-splitter arrangement, the lO arrangement comprising an optical element having a planar surface, and a piano-convex lens, the piano-convex lens having a planar surface and a convex surface, the planar surface of the lens being in optical contact with the planar surface of the optical element, wherein the method comprises the steps of: i) directing along a first path a first beam of optical radiation towards and through a first area of the convex surface of the lens so that the optical radiation is focussed by the lens, the direction of the first beam being such that this beam passes through the planar interface and the optical element on its way to or from the first area; ii) directing along a second path a second beam of optical radiation towards and through a second area of the convex surface of the lens so that the optical radiation is focussed by the lens, the direction of the second beam being such that this beam is reflected by the planar interface on its way to or from the second area; and iii) directing along the second path the second beam of optical radiation towards or from said reflection at the planar interface, the second path having a common section with the first path leading to the first area of the convex surface of the lens through which the second beam of optical radiation is focussed by the lens.

The first beam of optical radiation may have first optical properties and the second beam of optical radiation may have second optical properties, for example, first and second wavelengths and/or first and second polarizations.

The invention will now be described in further detail, and by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an optical beam splitter arrangement according to a first embodiment of the invention, having an optical element with a planar surface, and a piano-convex lens in optical contact with the planar surface of the optical element at a planar interface, the arrangement being such that there are two optical paths through the beamsplitter arrangement one of which leads from a laser diode and the other of which leads to a photodiode; Figures 2 is a schematic view of an optical beam splitter arrangement according to a second embodiment of the invention similar to the first but having a third optical path through the optical element leading from the laser diode to a second photodiode; and 8 Figure 3 is a schematic view of an optical beam splitter arrangement according to a third embodiment of the invention similar to the first but having coatings on surfaces of the optical element to absorb or pass optical radiation having a particular wavelength.

Figure 1 shows schematically an optical beam-splitter arrangement 1, comprising an optical element 2, here a prism, having a planar surface 4, and a piano-convex lens 6. The lens 6 has a planar surface 8 and a hemispherical convex surface 10, which optionally is coated with a broadband antireflection coating. The planar surface 8 of the lens 6 is in optical contact with the planar surface 4 of the optical element 2 at a planar interface 12, the lens 6 and optical element 2 being bonded permanently together at the interface 12 by means of a transparent index-matched optical adhesive 13 to form an integrated beam-splitter arrangement.

Shown in Figure 1 are two optical paths through the beam- splitter arrangement 1. One of the paths has a central line or axis indicated by a short dashed line 14',14", and the other path has a central line or axis indicated by a long dashed line 16',16" both of which have a mutually common section 14',16' which passes through and is focussed by a first area 18 of the convex surface 10 of the lens 6. The paths 14',14",16',16" diverge at the planar interface 12 where one of the paths 14" passes through the planar interface 12 into the prism 2. As will be explained in further detail below, the other of the 9 - paths 16" involves a reflection by the interface 12 and focussing by a second area 20 of the convex surface of the lens.

In the illustrated embodiments, the optical beam splitter arrangement is part of an optical transceiver unit, having a source of optical radiation 22, here an infra-red laser diode, and a photodetector 24, here a photodiode. The arrangement also includes an optical fibre 26, which may be a single mode fibre stub by which coherent optical radiation is coupled into and out of an optical port (not illustrated) of the transceiver unit.

The optical fibre 26 directs optical radiation 15 having a first wavelength Al along the first path 14',14" towards the first area 18 of the convex surface 10 of the lens 6 so that this optical radiation 15 is focussed by the lens 6, towards the interface 12. The planar interface 12 includes a dichroic coating formed from a multilayer interference coating which allows the first wavelength \1 to pass through the interface 12 into the prism 2 along the path 14". This transmitted optical radiation 15 then exits a first planar rear surface 28 of the prism 2 and is incident on the photodiode 24. A second planar rear surface 30 of the prism 2 extends at right angles to the first rear surface 23.

The dichroic coating may be provided either as part of the prism planar surface 4 or as part of the lens planar surface '3.

The laser diode 22 emits and directs optical radiation 25 having a second wavelength A2 along the second path 16" towards the second area 20 of the convex surface 10 of the lens 6 so that this optical radiation 25 is focussed by the lens 6, towards the dichroic coating interface 12. The dichroic coating 12 blocks transmission of the second wavelength A2 and reflects this wavelength back into the lens 6 and towards the first area 18 of the convex surface lo along the path 16'. This reflected optical radiation 25 then exits and is focussed a second time by the lens 6 towards the optical fibre 26.

This arrangement is particularly useful because the collimation and focussing of the laser diode optical radiation 25 requires a higher numerical aperture than does the focussing of the optical radiation 15 towards the photodiode 24. The lens 6 appears to the laser diode optical radiation 25 as a ball lens, and to the photodiode as a piano-convex lens. The laser diode 22 is also a smaller component than the photodiode, and may need to be positioned about 100 Am from the lens surface 10. The spherical surface 10 of the piano-convex lens 6 may have a radius of about 0. 5 mm, and the dimensions of the back surfaces 28, 30 of the prism 2 may be about 1. 5 mm square.

The arrangement provides a sufficient spacing between the laser diode 22 and photodiode 24 so that these components can be correctly positioned with respect to the lens 6 and prism 2.

The wavelengths Al and A2 may be any convenient wavelength that may be generated by a laser diode 22 and detected by 1 1 a photodiode 24. In optical communications systems, these wavelengths may be at or near 1310 nm and 1550 nm.

Reference is now made to Figure 2, which shows a second embodiment of an optical beam-splitter arrangement 101 in which features common with the first embodiment 1 are indicated by the same reference numerals. The second embodiment differs from the first embodiment in that the planar interface 112 includes a dichroic coating that is only partially reflective to the second wavelength 25, so that a substantially minor proportion 25', for example 1% to 10%, is transmitted through the interface 112 into the prism and is directed along a third path 16''' towards and through the second rear face 30 of the prism to a collimating lens 32 (if needed). A substantially major proportion of the optical radiation 25", for example between 99% and 90% is reflected at the interface 112 towards the lens first area 18 and focussed towards the optical fibre 26 as described above.

The partially reflective dichroic coating may be provided either as part of the prism planar surface 4 or as part of the lens planar surface 8.

The collimating lens 32 can focus the transmitted optical radiation 25' so that this is incident on a second photodetector 34, here a photodiode. The second photodiode 34 may be used as a monitor photodiode to monitor the output power of the laser diode 22.

As with the first embodiment 1, the second embodiment of the invention permits a close and compact arrangement of a number of optical components, namely a laser diode 22, two photodiodes 24, 34 and an optical fibre 26. The flow of optical radiation 15, 25, 25', 25" between these components is directed and controlled by just three optical elements, namely a prism 2, a piano-convex lens 6 and a collimating lens 32.

Figure 3 shows a third embodiment of an optical beam- splitter arrangement 201 in which features common with the first and second embodiments 1, 101 are indicated by the same reference numerals. In the third embodiment 201, both beams of optical radiation 115, 125 have the same wavelength Al and the planar interface 212 includes a non wavelength selective beam-splitting coating, for example a 40% reflective and 40% transmissive aluminium coating. The aluminium coating may be provided either as part of the prism planar surface 4 or as part of the lens planar surface 8. The third embodiment 201 includes in place of a laser diode a light emitting diode (LED) 122. Although LED- based optical communications systems do not have as high a data rate as laser diode based systems, LED based systems are potentially very inexpensive.

Optical radiation 115 from a multimode optical fibre 126 is partially transmitted 115' by the interface 212 and passes into the prism 2 and is incident on the photodiode 24. Some of the optical radiation 115 from the optical fibre 126 is, however, reflected by the interface 212 and is directed towards the LED 122. In an LED-based communications system, however, this may not be significant disadvantage, because LEDs are less susceptible to optical feedback induced instability than are laser diodes.

The optical radiation 125 from the LED is partially reflected and partially transmitted at the interface 212, with a transmitted component 125' passing into the prism 2 along the third path 16''' towards the second rear face 30 of the prism, which is coated with a layer 38 which is absorbing to optical radiation at the wavelength Al of the LED 122. This absorking layer 38 helps to suppress back reflections, which could otherwise be directed by a second reflection at the interface 212 towards the photodiode 24.

The reflected component 125" is passed along the common section of the path 16' and is focussed by the first area 18 of the lens surface 10 towards the optical fibre 126.

Optionally, the first rear surface 28 of the prism 2 may be coated with a narrow band-pass optical filter 36 which allows the wavelength Al to pass towards the photodiode 24, but which blocks other wavelengths. This may be useful in a multi- wavelength optical communication system in which more than one wavelength of optical radiation may be used to transmit information along the same optical fibre 26. For the same reason, such a wavelength-selective coating 36 may also be useful in the first and second embodiments 1, 101 described above.

As can be seen from Figures 1, 2 and 3, the optical radiation from the optical source 22, 122 is focussed directly, i.e. without the need for any other focussing elements, from the source 22, 122 into the optical beam splitter components 2, 6, 12, 112, 212. Similarly, in the - 14 illustrated arrangements, there is no need for any other focussing elements between either the optical fibre 26, 126 or the photodetector 24 and the optical beam-splitter components.

In all cases, once the desired optical orientation has been achieved between the beam-splitter components 2, 6, 12, 112, 212 and other optical components 22, 122, 26, 126, 24, 32, 34, the orientation between the beam-splitter components and the other optical components may be fixed, for example by means of an adhesive bonding the arrangement and other components to a support structure (not shown).

The invention provides a number of benefits. First, the beam-splitter arrangement 1, 101, 201 facilitates the compact arrangement of optical components of differing sizes or having different requirements for alignment tolerances or for numerical aperture, owing to the fact that one optical path 16', 16" is focused twice by the piano-convex lens 6, and the other optical path 14', 14" is focussed only once by the lens 6.

Since the lens 6 and planar surface 4 of the optical element 2 are in optical contact, and preferably bonded 13 permanently together, the alignment between the lens 6 and prism 2 is assured during the manufacturing process.

Furthermore, variations of the beam paths 14', 14", 16', 16" resulting from mechanical changes in the assembly, for example those caused by thermal or ageing effects, or the component fixing process, are less pronounced and make the optical performance of the assembly more stable. 15

It is also possible to use a single standard arrangement of components in a number of different ways, which may reduce manufacturing costs.

The invention is well-suited to optical beam-splitter arrangements where it is necessary to separate or combine, either partially or fully, along two paths two beams having different or the same wavelengths. The beam splitter arrangement according to the invention is particularly useful in optical communications links operating at near-infrared wavelengths around 1.3 Am and 1.5 m.

Optionally, the arrangements described above may be combined with linear or circular polarizing elements and a polarising interface in order to provide the two optical paths through the arrangement.

The invention therefore provides a convenient and versatile integrated optical beam-splitter arrangement. - 16

Claims

Claims 1. An optical beam-splitter arrangement (1,101,201), comprising an

optical element (2) having a planar surface (4), and a piano-convex lens (6), the piano-convex lens having a planar surface (8) and a convex surface (10), the planar surface (8) of the lens (6) being in optical contact with the planar surface (4) of the optical element at a planar interface (12,112,212), the arrangement being such that there are two optical paths (14',14", 16',16") through the beam- splitter arrangement, both of which have a mutually common section (14', 16') which passes through and is focussed by a first area (18) of the convex surface (10) of the lens (6), said paths (14",16") diverging at the planar interface (12,112,212) where one of the paths (14") passes through the planar interface (12,112,212) into The optical element (2) and the other of the paths (16") being reflected by the planar surface interface (12,112,212) and passing through and being focussed by a second area (20) of the convex surface (10) of the lens (6).
2. An optical beam-splitter arrangement (1,101,201) as claimed in Claim 1, in which the optical element (2) and the lens (6) are permanently bonded to each other at the planar interface ( 12,112,212) .
3. An optical beam-splitter arrangement (1,101,201) as claimed in Claim 1 or Claim 2, in which the optical element is a prism (2) .
4. An optical beam-splitter arrangement (1,101, 201) as - 17 claimed in Claim 3, in which the prism (2) is a right- angled prism and the planar surface (4) of the optical element is a hypotenuse of the prism (2).
5. An optical beam-splitter arrangement (1,101,201) as claimed in any preceding claim, in which said convex surface (10) is a spherical surface.
6. An optical beam-splitter arrangement (1,101) as claimed in any preceding claim, in which the beam-splitter interface (12,112) includes either a semi-silvered or a dichroic coating for splitting or combining optical beams (15,115,25,125) having, respectively, the same or different wavelengths.
7. An optical beam-splitter arrangement as claimed in any of Claims 1 to 5, in which the planar interface includes a polarising coating for splitting or combining optical beams having different polarizations.
8. An optical transceiver unit, having a source (22,122) of optical radiation (25,125), a photodetector (24), and an optical port for receiving and for transmitting optical radiation (15,115,25,125) to/from the unit, and an optical beam-splitter arrangement (1,101,201) for directing said received and transmitted optical radiation respectively to/from the photodetector (24) and the source of optical radiation (22, 122), wherein the optical beam splitter arrangement is as claimed in any preceding claim.
9. An optical transceiver unit as claimed in Claim 8, in which the optical path (16',16") focussed twice by the - 18 convex surface (10) of the piano-convex lens (6) leads from the source (22,122) of optical radiation (25,125) to the optical port, and the optical path (14',14") focussed once by the piano-convex lens (6) leads from the optical port to the photodetector (24).
10. An optical transceiver unit as claimed in Claim 8 or Claim 9, in which an optical beam along the common path (14',16') between the piano-convex lens (6) and the optical port are focussed by the lens (6) into an end of an optical fibre (26,126).
11. A method of splitting or combining beams of optical radiation using an optical beam-splitter arrangement (1,101,201), the arrangement comprising an optical element (2) having a planar surface (4), and a piano-convex lens (6), the piano-convex lens having a planar surface (8) and a convex surface (10), the planar surface (8) of the lens (6) being in optical contact with the planar surface (4) of the optical element (2), wherein the method comprises the steps of: i) directing along a first path (14',14") a first beam of optical radiation (15,115) towards and through a first area (18) of the convex surface (10) of the lens (6) so that the optical radiation (15,115) is focussed by the lens (6), the direction of the first beam (15,115) being such that this beam passes through the planar interface (12,112,212) and the optical element (2) on its way to or from the first area (18); ii) directing along a second path (16',16") a second beam of optical radiation (25,125) towards and through a second area (20) of the convex surface (10) of the lens (6) so that the optical radiation (25,125) is focussed by the lens (6), the direction of the second beam (25,125) being such that this beam is reflected by the planar interface (12,112,212) on its way to or from the second area (20); and iii) directing along the second path (16',16") the second beam of optical radiation (25,125) towards or from said reflection at the planar interface (12,112,212), the second path having a common section (14',16') with the first path leading to the first area (18) of the convex surface (10) of the lens (6) through which the second beam of optical radiation (25,125) is focussed by the lens (6).