GB2423861A

GB2423861A - A Lens Arrangement

Info

Publication number: GB2423861A
Application number: GB0504139A
Authority: GB
Inventors: Elaine O'brien; David Healy
Original assignee: Avago Technologies Fiber IP Singapore Pte Ltd; Avago Technologies General IP Singapore Pte Ltd; Agilent Technologies Inc
Current assignee: Avago Technologies International Sales Pte Ltd
Priority date: 2005-03-01
Filing date: 2005-03-01
Publication date: 2006-09-06
Anticipated expiration: 2025-03-01
Also published as: GB0504139D0; GB2423861B

Abstract

An arrangement includes ```an optical source 1 for generating a radiation beam. A lens 2 has a substantial flat surface 2a exposed to the radiation beam from the source 1 and angled thereto. The radiation beam is partly reflected from and partly transmitted through the substantially flat surface 2a of the lens 1. A photosensitive element 3 is exposed to the radiation beam partly reflected from the substantially flat surface 2a of the lens 2. The arrangement may include a header member (10, Figure 5) for carrying the optical source 1 and the photosensitive element 3 as well as a lens housing 12 supporting the lens 2 and forming with the header a hermetic enclosure.

Description

"A lens arrangement for optical sources and related method"

Field of the invention

The invention relates to arrangements for use in connection with optical sources such as e.g. laser sources.

The invention was developed by paying specific attention to the possible use in power monitoring arrangements for semiconductor laser sources such as Vertical Cavity Surface-Emitting Lasers (VCSELs).

However, reference to this preferred field of use of the invention must in no way be construed in a limiting sense of the scope of the invention

Description of the related art

Power monitoring of laser sources can be achieved by measuring the power emitted from the back facet of an edge emitting laser such as a Fabry Perot or a Distributed FeedBack (DFB) laser. However, there is a possibility that the ratio of back and front power will vary as the laser ages/gets older.

Other types of laser sources such as VCSELs do not normally emit light from their bottom surface unless 3. subject to specific risky processing. Therefore, to * * monitor the output power from a VCSEL, a proportion of light from the top surface (front facet) needs to be tapped off and directed towards a photodiode.

Such an arrangement is adapted to be used also for other types of laser sources and is usually achieved through reflections from a thin glass window in the mechanical package. To achieve optimum coupling to an optical fibre a lens must also be used.

Object and summary of the invention

The foregoing discussion of the related art indicates that the need is felt for arrangements adapted for use e.g. in monitoring the output power from a laser that: - do not suffer from undesired variations of operation as the laser ages/gets older, - do not require the use of an angled window and, in any case, may permit a high coupling to be achieved with a fewer components.

Additionally, it would be desirable to provide an arrangement wherein the amount of light tapped off for power monitoring can be varied at will.

The object of the present invention is to provide an improved arrangement meeting the needs outlined in the foregoing. According to the present invention, that object is achieved by means of a lens arrangement as recited in the claims that follow. These claims constitute an integral part of the disclosure of the invention provided herein.

A presently preferred embodiment of the invention includes: - an optical source for generating a radiation beam, - a lens having a substantially flat surface exposed to said radiation beam and angled thereto, whereby said radiation beam is partly reflected from said substantially flat surface of said lens and partly transmitted through said substantially flat surface of said lens, and - a photosensitive element exposed to said radiation beam partly reflected from said substantially flat surface of said lens.

The presence of a lens in such an arrangement serves to shape the laser beam in some way i.e. the lens can "change" the divergent beam from a laser into a convergent beam that can be coupled into an optical fibre or other component. The lens can "change" a divergent beam from a laser into a collimated beam that can be coupled to a component such as a filter or an isolator. The flat surface of the lens can be used to affect the path of the laser beam. Beam splitters and mirrors cannot shape laser beams and can only affect the path that the laser beam can take.

In brief, in a preferred embodiment of the arrangement described herein, light from an optical source such as a laser (e.g. a VCSEL) is coupled to a flat polished lens that is positioned at an arbitrary angle to the optical axis. The angular/polished surface configuration will allow the reflected light to be coupled to a power monitoring diode such as a photodiode, and prevent back-reflection of light into the laser. Additionally, such a lens arrangement will not require costly antireflection coatings.

The light transmitted through the lens with the polished surface can be subsequently coupled to an optical fibre or any other optical component such as an isolator, a lens, and so on.

The arrangement described herein does not require the use of an angled window (only a lens with a flat polished surface is used) . This means that fewer piece parts are included in the arrangement, while conversely achieving a higher coupling. The amount of light tapped off for the photodiode can be varied by using lenses of different refractive indices. Additionally, the lens does not require costly anti-reflection coatings.

Brief description of the annexed drawings

The invention will now be described, by way of example only, by reference to the enclosed figures of drawing, wherein: - figure 1 is representative of an embodiment of the arrangement described herein, - figure 2 is representative of an alternative embodiment of the arrangement described herein, - figure 3 is representative of a further alternative embodiment of the arrangement described herein, and - figures 4 and 5 are representative of two practical circuit implementations of the arrangement described herein.

Detailed description of exemplary embodiments of

the invention Figures 1 to 3 are representative of the principles of operation of various embodiments of the arrangement described herein.

In figures 1 to 3 reference 1 designates an optical source, such as a Vertical Cav-it Surface- Emitting Laser (VCSEL). While this detailed description will refer to this type of laser source by way of example, it is self evident that the arrangement described herein and the scope of the invention are in no way limited to this specific type of light source.

Light emitted from the front facet of the laser 1 is caused to propagate towards a lens 2 having a flat polished surface 2a.

In the arrangement shown in figure 1, the lens 2 is a lens having an approximately hemi-spherjcal shape positioned with respect to the laser]. in such a way that the flat surface 2a exposed to the radiation beam from the source and angled thereto. Stated otherwise light from the laser 1 impinges onto the flat polished surface 2a at an angle thereto (i.e. not perpendicular to the surface). The light is then partly reflected from the surface 2a and partly propagated within the lens 2.

In the alternative arrangements of figures 2 and 3 the lens 2 is a spherical lens (i.e. a "ball" lens) a portion of which is flattened to form the polished surface 2a. In both alternative arrangements of figures 2 and 3, the lens 2 is positioned with respect to the laser source 1 in such a way the light of the laser source 1 enters (i.e. is injected into) the lens 2 and impinges at an angle onto the surface 2a after propagating through the lens 2. The light is then partly reflected back into the lens 2 and partly propagated outside the lens as a beam transmitted beyond the polished surface 2a.

Stated otherwi3e, in the arrangement of figure 1, the surface 2a represents a "front" surface of the lens 2 facing towards the laser 1. In the arrangements of figures 2 and 3, the polished surface 2a is a "back" surface of the lens facing away from the laser 1.

In either case, on reaching the flat surface 2a (be it a front or back surface of the lens 2), Fresnel's laws dictate that a small proportion of light will be reflected from the flat polished surface 2a.

The reflected beam produced at the surface 2a is collected by a photodiode 3 positioned in the path of the reflected beam. Typically the photodiode 3 is positioned side-by-side the laser 1. The intensity of the radiation impinging onto the photodiode 3 and thus the intensity of the photocurrent produced thereby can be used for measuring the power emitted from the laser 1. This occurs based on principles and criteria that are well known in the art thus making it unnecessary to

provide a more detailed description herein.

Mutual Positioning of the various elements shown in figure 1 is thus dictated by the reflection geometry of light at the polished surface 2a.

For instance, in the arrangement of figure 1, the radiation from the laser 1 impinges onto the surface 2a at an angle to the normal to the surface, i.e. not orthogonal thereto. This is generally an arbitrary angle: an exemplary value is e.g. 10 degrees, but for highly divergent lasers such as DFBs the lens may need to be placed a larger angle to avoid cross-talk between the incident and reflected beams.

The portion of the radiation from the iase source 1 that is not reflected by the polished surface 2a propagates within the lens 2 and surfaces from the lens 2 at a "polar" position of the quasi-hemispherical lens 2, i.e. a position approximately radially opposed to the location where the light from the laser 1 impinges onto the surface 2a. The transmitted beam surfacing from the lens 2 is a focussed beam adapted to be launched into an optical fibre 4 or another optical component such as an isolator, lens, and so on for further processing/use of any known type.

In the arrangements shown in figures 2 and 3 the relative positioning of the laser source 1 and the photodjode 3 take into account the fact that the beam reflected towards the photodiode 3 is generated at the surface 2a, such a polished surface being in this case a back surface of the lens 2. Preferably the laser source 1 is positioned in such a way that light from the laser source 1 impinges onto the lens 2 and propagates through the lens 2 in an substantially diametra]. direction. In that way, the laser 1, the centre of the lens 2 and the fibre or other optical element 4 are aligned along an almost rectilinear propagation path, with the surface 2a at an angle of e.g. 10 degrees to that path.

The arrangement of figure 3 is essentially similar to the arrangement of figure 2. However, the lens 2 of figure 2 essentially provides for light being propagated therethrough as a transmitter/collimated beam. Conversely, the lens 2 of figure 3 includes a focusing capability (provided by known means) whereby in propagating through the lens 2 light emitted from the laser source 1 undergoes focusing and thus surfaces frocc1 the lens 2 in the form of a focused beam. While not immediately appreciated, a certain degree of focusing of the beam transmitted from the lens 2 towards the optical component 4 may be achieved also in the case of the hemispherical lens 2 having the polished surface 2a as a front surface shown in figure 1.

The transmitted beam from figure 2 is essentially collimated. An instance like this might occur when optimum coupling is desired and hence this collimated beam can be coupled to another lens that focuses the beam into a component such as an optical fibre. Such a two-lens configuration tends to be much less sensitive to component movement that may occur during processing.

Another instance in which the configuration in figure 2 might occur is when it is desirable to couple the collimated beam into a component such as a filter (many filters are designed to work with Collimated light only and any deviations from normal incidence/collimation may severely affect filter performance.

The configuration of figure 3 may be used when optimum coupling may not be necessarily required or when coupling to an optical fibre and power monitoring needs to be achieved with a minimum number of components.

Figures 4 and 5 show how lens arrangements of the types disclosed in the foregoing (for instance of the type shown in figure 3) can be mounted in a so-called TO-Can type package, and in a Kovar (or such-like) mount, respectively. "TO-Can", i.e. "Transistor- Oriented Can" is a current designation f or the type of package that is also known as a capped header or capped stem.

In both figures 4 and 5 elements and parts that are identical, corresponding, or equivalent to elements or parts already described in connection with figures 1 to 3 were indicated with the same reference numerals.

Specifically, figure 4 shows the lens 2 with the flat polished (back) surface 2a incorporated in a TO- Can type package including a flat header 10 and a lens housing/cap 12. Specifically, the flat surface 2a is held at an angle (ten degrees being an exemplary non- limiting value for such an angle) in the lens cap 12, preferably by having the lens 2 press-fit/glued in place. The lens cap 12 (typically in the form of a cylindrical mount) can be welded to the header 10 to form a desirable hermetic enclosure wherein both the laser 1 and the photodiode 3 are mounted using the header 10 as a common substrate.

In the arrangement of figure 5, an L-shaped Kovar mount is shown. One arm or branch of the L-shaped Kovar mount supports the laser 1 and the photodiode 3, while the lens 2 is secured (e.g. by gluing) in a shallow, spherical inverted-dome cavity provided in the other arm of the Kovar mount 14. The Kovar mount has the added arrangement of acting as a heatsink. The type of optical sub-assembly (OSA) shown in figure 5 can be used e.g. in butterfly/dualjn..jjne type packages.

Of course, without prejudice to the underlying principles of the invention, the details and the embodiments may vary, also significantly, with respect - 10 - to what has been described in the foregoing, by way of example only, without departing from the scope of the invention as defined by the claims that follow.

Specifically, those of skill in the art will appreciate that terms such as "optical", "light" and the like are evidently used herein with the meaning currently allotted to those terms in fibre and integrated optics, being thus intended to apply, in addition to visible light, also e.g. to radiation in the infrared and ultraviolet ranges.

Claims

-II- CLAIMS

1. An arrangement including: - an optical source (1) for generating a radiation beam, - a lens (2) having a substantially flat surface (2a) exposed to said radiation beam and angled thereto, whereby said radiation beam is partly reflected from said substantially flat surface (2a) of said lens (2) and partly transmitted through said substantially flat surface (2a) of said lens (2), and - a photosensitive element (3) exposed to said radiation beam partly reflected from said substantially flat surface (2a) of said lens (2)

2. The arrangement of claim 1, characterised in that said optical source is a laser (1).

3. The arrangement of claim 2, characterjsed in that said laser (1) is an edge emitting laser.

4. The arrangement of claim 2, characterised in that said laser (1) is a VCSEL.

5. The arrangement of any of the previous claims, characterised in that said substantially flat surface (2) is a front surface of said lens (2), whereby said radiation beam is partly reflected from said front surface (2a) out of said lens (2) and partly transmitted through said front surface (2a) into said lens (2)

6. The arrangement of claim 1, characterised in that said substantially flat surface (2a) is a back surface of said lens (2), whereby said radiation beam propagates through said lens (2) before being reflected by said back surface (2a).

7. The arrangement of claim 6, characterised in that said radiation beam is partly reflected from said back surface (2a) into said lens (2) and partly transmitted through said back surface (2a) out of said lens (2)

8. The arrangement of any of the previous claims, characteriged in that said lens (2) is a spherical lens (2) having a flattened surface Constituting said substantially flat surface (2a).

9. The arrangement of claim 8, characterised in that said lens (2) is a substantially hemispherical lens.

10. The arrangement of any of the previous claims, Characterised in that said substantially flat surface (2a) is exposed to said radiation beam at an angle of about 10 degrees.

1].. The arrangement of claim 5 and claim 9, characterised in that said substantially flat surface (2a) is exposed as said front surface to said radiation beam from said optical source (1), whereby said radiation beam partly transmitted through said front surface (2a) and said lens emerges from said lens (2) as a focused beam.

12. The arrangement of claim 6 and claim 8, characterised in that said lens (2) is a focusing lens, whereby said radiation being partly transmitted through - 13 - said back surface (2a) of said lens (2) emerges from --.-1 1......_ -. - ..-- - -----..-___I t___ ab iCueu. Li. aii..LII.L. L. LeLL.j.ica&1l.

13. The arrangement of claim 6, characterjsed in that said lens (2) is arranged with respect to said optical source (1) so that the radiation beam generated by said optical source (1) propagates through said lens (2) in a substantially diametrical direction to said lens (2)

14. The arrangement of any of the previous claims, characterised in that said substantially flat surface (2a) is a polished surface of said lens (2).

15. The arrangement of any of the previous claims, characterised in that said substantially flat surface (2a) is exempt from anti-reflective coating.

16. The arrangement of any of the previous claims, characterised in that it further includes an optical element (4) receiving said radiation beam partly transmitted through said substantially flat surface (2a) of said lens (2)

17. The arrangement of any of the previous claims, characterjsed in that said photosensitive element (3) is a photodetector such as a photodjode (3).

18. The arrangement of any of the previous claims, characterised in that said optical source (1) and said photosensitive element (3) are located side-by-side.

19. The arrangement of any of the previous claims, in combination with an associated package including a header member (10) carrying said optical source (1) and said photosensitive element (3) as well as a lens housing 12) supporting oaid lens (2) and forming with said header (10) a hermetic enclosure for said optical source (1) and said photosensitive element (3).

20. The arrangement of any of claim 1 to 18, in combination with an associated an L-shaped mount member (14) including a first arm carrying said optical source (1) and said photosensitive element (3) and a second arm carrying said lens (2).

21. The arrangement of claim 20, characterised in that said second arm of said mount (14) includes a cavity having said lens (2) partly glued therein.

22. The arrangement of either of claims 20 or 21, characterised in that said mount (14) is a Kovar mount.

23. A method of tapping radiation out of an optical source (1) generating a radiation beam, the method including the steps of: - providing a lens (2) having a substantially flat surface (2a), - exposing said substantially flat surface (2) to said radiation beam by keeping said substantially flat surface (2a) at an angle thereto, whereby said radiation beam is partly reflected from said substantially flat surface (2a) of the lens (2) and partly transmitted through said substantially flat surface (2a) of the lens (2), whereby said radiation beam partly reflected from said substantially flat surface (2a) of the lens is tapped off the radiation generated by said optical source.