GB2436622A

GB2436622A - Launching optical radiation into optical fibre using attenuating optic fibre

Info

Publication number: GB2436622A
Application number: GB0606219A
Authority: GB
Inventors: Piero Gambini; Giampaolo Bendelli; Marco Scofet; Roberto Lano
Original assignee: Avago Technologies Fiber IP Singapore Pte Ltd
Current assignee: Avago Technologies International Sales Pte Ltd
Priority date: 2006-03-28
Filing date: 2006-03-28
Publication date: 2007-10-03
Anticipated expiration: 2026-03-28
Also published as: GB2436622B; GB0606219D0

Abstract

An arrangement, for launching optical radiation from a source (10) into an input end of an optical fibre (32), includes a length of optical fibre (12) to be traversed by the optical radiation. That length of optical fibre (12) includes a fibre core (120) and at least one fibre cladding (122, 124), the fibre core (120) and the fibre cladding (122, 124) having respective levels of attenuation, with the fibre core (120) having a level of attenuation that is lower than the level of attenuation in the fibre cladding (122, 124), whereby optical radiation that is unconfined to the fibre core (120) is subject to higher attenuation in the fibre cladding (122, 124). Ball lens 18 is shown.

Description

ARRANGEMENT AND METHOD FOR LAUNCHING OPTICAL

RADIATION INTO OPTICAL FIBRES

The invention relates to arrangements for launching optical radiation into optical fibres.

The invention was developed with specific attention paid to its possible use in the area of optical communications e.g. in so-called Optical Sub Assemblies (OSA's) included in transmitters or transceivers for optical communications.

In launching optical radiation generated by a.

source (such as a laser source) into the input end of an optical fibre the need frequently arises of providing a well-defined light beam quality at the optical interface of the transmitter. Specifically, the need arises of ensuring correct launch conditions into the fibre (e.g. in the case of application in a multi-mode fibre system) while also avoiding stray light that may lead to eye-safety limits being undesirably exceeded.

Additionally, in high data rate optical transmitters (such as 10Gb/s optical transmitters using directly modulated laser diodes) operational conditions may arise where the optical power emitted by the laser exceeds the power requirements at the output of the transceiver (i.e. at the fibre input) A way of countering these undesired situations involves attenuating the optical power at the output of the optical source thus ensuring that the power ranges specified by the relevant standards and/or the eye-safety limits are not exceeded.

In current practice, at least two different approaches are resorted to in order to achieve optical power attenuation in the context described in the foregoing.

A first approach involves employing a bulk optical attenuator in the optical system used to couple the light from the source into the fibre.

Another approach involves creating an intentional misalignment in the optical system that couples the light from the optical source into the optical fibre.

This misalignment can be achieved either in the x-y plane or in the z direction.

Another approach recently proposed involves using a polariser between the optical source and the fibre: the polariser is rotated until the desired power level is obtained.

However effective, these arrangements are not exempt from drawbacks.

For instance, employing a bulk optical attenuator in the optical system used to couple the light from the source into the fibre increases the complexity of the optical system and the related manufacturing process.

Additionally, this prior art solution does not properly provide any control of the spatial distribution of the beam power to achieve a well defined light beam quality.

Those arrangements based on an intentional misalignment in the x-y plane tend to be overly sensitive to any further non-intentional misalignment produced by mechanical or thermal causes. Conversely, those arrangements wherein misalignment is achieved in the z direction are exposed to the risk of coupling light with an undesired spatial distribution.

Finally, those arrangements employing a polariser between the optical source and the fibre may necessitate a rather lengthy alignment procedure, which is hardly compatible with current adjustment procedures of optical communication links.

The invention seeks to provide an improved arrangement that dispenses with the drawbacks intrinsic

to the prior art arrangements discussed in the

foregoing.

According to the present invention, there is provided an arrangement having the features set forth in the claims that follow. The invention also relates to a corresponding method.

A preferred embodiment of the invention provides an arrangement for launching optical radiation from a source into an input end of an optical fibre, the arrangement including a length of optical fibre to be traversed by said optical radiation, said length of optical fibre including a fibre core and at least one fibre cladding, said fibre core and said at least one fibre cladding having respective levels of attenuation, with said fibre core having a level of attenuation that is lower than the level of attenuation in said at least one fibre cladding, whereby optical radiation that is unconfined to said fibre core is subject to higher attenuation in said at least one fibre cladding.

In brief, the arrangement described herein includes an optical attenuator, which -instead of being comprised of a bulk optical attenuator in the optical system used to couple the light from the source into the fibre -is directly built in the ferule included in the output connector of an optical fibre transmitter or transceiver. Achieving correct alignment of such an arrangement is a relatively simple task. At the same time, such an arrangement solves both the problem of properly limiting the output power and the problem of achieving a correct spatial light distribution.

The invention will now be described, by way of example only, with reference to the annexed Figures, wherein: -Figure 1 is an axial cross-sectional view of an arrangement for launching optical radiation into an optical fibre as described herein, and -Figure 2 is exemplary of the features of a fibre adapted for use within the arrangement of figure 1.

Figure 1 is exemplary of an arrangement for launching optical radiation from a source, such as a laser diode 10 into the input end of an optical fibre.

The laser diode 10 is included within an Optical Sub System (OSA) 14 of an electro-optical device such as an optical transmitter or transmitter/receiver ("transceiver").

In a manner known per Se, the OSA 14 includes a support 16 for the laser 10 and ancillary elements such as focusing optics (e.g. a "ball" lens 18) Reference numeral 20 designates as a whole electrical contact leads carried by the OSA 14 for feeding bias and modulation signals to the laser source 10.

The OSA 14 carries a collar formation 22 centred around a hole 30 in the OSA casing in order to permit the optical radiation from the laser 10 and the lens 18 to pass therethrough. Those of skill in the art will appreciate that the exemplary OSA 14 illustrated in Figure 1 is of the type having a coaxial (or TO-based) structure. As indicated, this representation is purely exemplary and must not be construed in a limiting sense of the scope of the invention.

Reference numeral 26 indicates a tubular receptacle adapted to be inserted into the collar 22 and fixed therein during the assembly process of the OSA 14: this assembly process is standard in the art, thus making it unnecessary to provide a more detailed

description herein.

A length of optical fibre 12 is arranged in a ferule 28 of e.g. alumina or zirconia located within the tubular receptacle 26 with the partial interposition of a tubular sleeve 29 of e.g. zirconia.

The sleeve 29 extends a certain length beyond the "distal" end of the ferule 28 opposite the laser diode 10. The fibre 12 is thus arranged in the ferule 28 inserted in the tubular sleeve 29 by leaving insertion space available for the input end of a low-loss "transmission" fibre 32 arranged in a ferule 34.

This arrangement permits connection of the transmission fibre 32 to the OSA 14. Specifically, the input end of the fibre 32 is arranged in the relative ferule 34 which is in turn inserted into the sleeve 29 causing the "proximal" ends of the fibre 32 and the ferule 34 (whose outlines are illustrated in broken lines in Figure 1) to abut against the "distal" ends of the fibre 12 and the ferule 28.

This arrangement leads to the two fibres 12 and 32 being cascaded to each other in abutment relationship and precisely aligned with the direction of propagation (indicated by X10) of the optical radiation from the laser source 10.

The length of optical fibre 12 extending through the ferule 28 is doped in order to provide optical attenuation.

Doping is a well known technique in the art for the manufacture of attenuation fibres, including high-attenuation fibres. By selecting: -the material used as a dopant (typically a transition metal adapted to increase the absorption in the core of a single mode fibre), -the concentration of the dopant, and -the length of the doped fibre the characteristics of the fibre can be tailored to produce a controlled level of attenuation with a high degree of wavelength insensitivity.

High-attenuation fibres are used in optical fibre attenuators in optical transmission systems to attenuate the amount of optical power present at a certain point in a fibre link, e.g. to attenuate a signal level to the optimum sensitivity point of an optical detector or as a fibre terminator to minimise back reflections. Documents such as US-A-4 881 793, US-A-5 572 618, US-A-5 633 974 and US-B-6 498 888 are exemplary of the related technology.

The high-attenuation fibres known in the art are typically a few centimetres long. They are provided with connectors at each end to facilitate connection to a low-loss transmission fibres with the attenuation fibre secured in a ferrule to permit connection e.g. between a transmission fibre and a detection fibre.

Experiments carried out so far by the Inventors indicate that a doped fibre as disclosed in US-B-6 498 888 (which provides essentially the same attenuation at 1310 nm and 1550 nm, the two standard wavelengths for telecommunications systems) is particularly suited for use in the arrangement as described herein.

As schematically illustrated in figure 2 herein, the doped fibre 12 of US-B-6 498 888 is a so-called W-type fibre comprising: -a core 120 having a core diameter of about 9 microns and a core refractive index; - an inner cladding 122 having an inner cladding outer diameter of about 45 microns, which is less than ten times the core diameter and a refractive index less than the core refractive index; and -.7- -an outer cladding 124 having an outer diameter of about 125 microns and a refractive index higher than the inner cladding refractive index.

Specifically, the difference in the refractive indexes of the core and the inner cladding (An) is in the range of 0.0051 to 0.0046 (depending on the measurement parameters taken), while the difference in the refractive indexes of the inner cladding and the outer cladding (An') is about -0.0034.

A fibre of the type described performs a double action on the optical radiation propagated therethrough.

In the first place, the optical radiation is attenuated -as a whole -in propagating along the fibre. This solves those problems related to the optical power emitted by the laser source 10 possibly exceeding the power requirements at the output of the transceiver.

Additionally, a fibre of the type described lends itself to be produced in such a way that the fibre core exhibits a level of attenuation that is lower than the level of attenuation in the cladding or claddings 122, 124. In that way, the radiation which is not confined in the core 120 -and thus tends to propagate in the cladding or claddings 122, 124 -is strongly attenuated. This leads to a very precise control of the beam power distribution at the interface with the input of the "transmission" fibre 32.

Dopants exemplified by US-B-6 498 888 include transition-metal elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), a rare-earth elements (Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) A typical length of the doped fibre 12 for use in the arrangemnt described herein is 5 to 8 mm, a typical value being about 6 mm. This corresponds to an attenuation in the range of 3 to 10 dB, typically about dB.

Of course, the basic principle of the invention remaining the same, the details and embodiments may vary, even significantly, with respect to what has been previously described by way of example only, without departing from the scope of the present invention as defined in the annexed claims. Specifically, it will be appreciated that terms such as "optical", "light", "photodetector", and the like are used herein with the meaning currently allotted to those terms in fibre and integrated optics, being thus intended to apply l.a. to radiation including the infrared, visible and ultraviolet ranges.

Claims

CLAIMS

1. n arrangement for launching optical radiation from a source (10) into an input end of an optical fibre (32), the arrangement including a length of optical fibre (12) to be traversed by said optical radiation, said length of optical fibre (12) including a fibre core (120) and at least one fibre cladding (122, 124), said fibre core (120) and said at least one fibre cladding (122, 124) having respective levels of attenuation, with said fibre core (120) having a level of attenuation that is lower than the level of attenuation in said at least one fibre cladding (122, 124), whereby optical radiation that is unconfined to said fibre core (120) is subject to higher attenuation in said at least one fibre cladding (122, 124)
2. The arrangement of claim 1, wherein said length of optical fibre (12) is a portion of a fibre able to have cascaded thereto the input end of a low-loss transmission fibre (32)
3. The arrangement of claim 2, wherein said portion of a fibre (12) is able to have cascaded thereto in abutment relationship said input end of said low-loss transmission fibre (32)
4. The arrangement of claim 3, wherein it includes a tubular sleeve (29) for receiving therein in an abutment relationship at least a portion of said fibre (12) and said input end of a low-loss transmission fibre (32)
5. The arrangement of claim 4, wherein said portion of said fibre (12) is arranged in a ferule (28) -10 -inserted in said tubular sleeve (29) by leaving insertion space for said input end of a low-loss transmission fibre (32)
6. The arrangement of any of claims 1 to 5, wherein said length of optical fibre (12) is doped to provide optical attenuation and control of spatial light distribution of said optical radiation launched into the fibre (32)
7. The arrangement of any of the previous claims, wherein said length of optical fibre (12) includes an inner cladding (122) and an outer cladding (124)
8. The arrangement of claim 7, wherein said length of doped optical fibre (12) is of a W-type fibre wherein said inner cladding (122) has a refractive index less than the refractive index of the fibre core (120) and said outer cladding (124) has a refractive index higher than the refractive index of said inner cladding (122)
9. The arrangement of any one of the preceding claims, wherein said length of optical fibre (12) is 5 to 8 mm, preferably about 6 mm long.

10. The arrangement of any of the previous claims, wherein said length of optical fibre (12) has an attenuation of 3 to 10 dB, typically about 5 dB.

11. A method of launching optical radiation from a source (10) into an input end of an optical fibre (32), the method including the step of causing a length of optical fibre (12) to be traversed by said optical radiation, wherein said length of optical fibre (12) -11 -includes a fibre core (120) and at least one fibre cladding (122, 124), said fibre core (120) and said at least one fibre cladding (122, 124) having respective levels of attenuation, with said fibre core (120) having a level of attenuation that is lower than the level of attenuation in said at least one fibre cladding (122, 124), whereby optical radiation that is unconfined to said fibre core (120) is subject to higher attenuation in said at least one fibre cladding (122, 124) Amendments to the claims have been filed as follows 1. An arrangement for launching optical radiation from a source (10) into an input end of an optical transmission fibre (32), the arrangement including a length of optical attenuation fibre (12) to be traversed by said optical radiation upstream of the optical transmission fibre (32), said length of optical attenuation fibre (12) including a fibre core (120) and at least one fibre cladding (122, 124), said fibre core (120) and said at least one fibre cladding (122, 124) having respective levels of attenuation, with said fibre core (120) having a level of attenuation that is < lower than the level of attenuation in said at least one fibre cladding (122, 124), whereby optical radiation that is unconfined to said fibre core (120) is subject to higher attenuation in said at least one fibre cladding (122, 124) 2. The arrangement of claim 1, wherein said length of optical attenuation fibre (12) is a portion of a fibre able to have cascaded thereto the input end of a low-loss transmission fibre (32) 3. The arrangement of claim 2, wherein said portion of optical attenuation fibre (12) is able to have cascaded thereto in abutment relationship said input end of said low-loss transmission fibre (32) 4. The arrangement of claim 3, wherein it includes a tubular sleeve (29) for receiving therein in an abutment relationship at least a portion of said optical attenuation fibre (12) and said input end of a low-loss transmission fibre (32) 5. The arrangement of claim 4, wherein said portion of said optical attenuation fibre (12) is arranged in a ferule (28) inserted in said tubular sleeve (29) by leaving insertion space for said input end of a low-loss transmission fibre (32) 6. The arrangement of any of claims 1 to 5, wherein said length of optical attenuation fibre (12) is doped to provide optical attenuation and control of spatial light distribution of said optical radiation launched into the otical transmission fibre (32) 7. The arrangement of any of the previous claims, : wherein said length of optical attenuation fibre (12) includes an inner cladding (122) and an outer cladding (124) 8. The arrangement of claim 7, wherein said length of optical attenuation fibre (12) is of a W-type fibre wherein said inner cladding (122) has a refractive index less than the refractive index of the fibre core (120) and said outer cladding (124) has a refractive index higher than the refractive index of said inner cladding (122) 9. The arrangement of any one of the preceding claims, wherein said length of optical attenuation fibre (12) is 5 to 8 mm, preferably about 6 mm long.

10. The arrangement of any of the previous claims, wherein said length of optical attenuation fibre (12) has an attenuation of 3 to 10 dB, typically about 5 dB.

11. An Optical Sub Assembly including an arrangement as claimed in any preceding claim.

12. A method of launching optical radiation from a source (10) into an input end of an optical transmission fibre (32), the method including the step of causing a length of optical attenuation fibre (12) to be traversed by said optical radiation upstream of the optical transmission fibre (32), wherein said length of optical attenuation fibre (12) includes a fibre core (120) and at least one fibre cladding (122, 124), said fibre core (120) and said at least one fibre cladding (122, 124) having respective levels of attenuation, with said fibre core (120) having a level of attenuation that is lower than the level of attenuation in said at least one fibre cladding (122, 124), whereby optical radiation that is unconfined to said fibre core (120) is subject to higher attenuation in said at least one fibre cladding (122, 124) 13. tise of an optical attenuation fibre (12) having a fibre core (120) and at least one fibre cladding (122, 124), said fibre core (120) and said at least one fibre cladding (122, 124) having respective levels of attenuation, with said fibre core (120) having a level of attenuation that is lower than the level of attenuation in said at least one fibre cladding (122, 124), whereby optical radiation that is unconfined to said fibre core (120) is subject to higher attenuation in said at least one fibre cladding (122, 124), in the attenuation of optical power at the output of an optical source (10) 14. An arrangement for launching optical radiation from a source into an input end of an optical transmission fibre substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

15. An Optical Sub Assembly substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

16. A method of launching optical radiation from a source into an input end of an optical transmission fibre substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

17. Use of an optical attenuation fibre substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.