GB2404493A

GB2404493A - Opto-electrical converter with multimode optical fibre

Info

Publication number: GB2404493A
Application number: GB0317662A
Authority: GB
Inventors: Piero Gambini; Marco Scofet; Mario Puleo
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2003-07-29
Filing date: 2003-07-29
Publication date: 2005-02-02
Also published as: GB0317662D0

Abstract

A device comprises a multi-mode optical fibre 22 and an opto-electric converter 24 such that optical radiation OS is directed onto the opto-electrical converter. The multi-mode fibre 22 radially confines the radiation OS but does not focus the radiation OS onto the opto-electric converter 24. The multi-mode fibre 22 is preferably a graded index fibre of between 1 mm and 5 mm length. The device receives radiation OS from either a single-mode or multi-mode fibre 12.

Description

"An opto-electrical converter device and a method for receiving optical

radiation" * * * The present invention relates to opto-electrical converter devices used for receiving optical radiation in opto-electrical communication systems.

In order to provide a truly satisfactory degree of operational flexibility, these systems should be capable of operating with both single-mode (i.e. monomode) and multi-mode fibers.

On the transmitter side, optical radiation can be effectively launched from a single-mode fiber into a multi-mode fiber without significant losses in the radiation power. A cost efficient solution is thus to use a "single-mode-only" transmitter, which is adapted also to operate with multi-mode fibers. In case a certain bandwidth is to be guaranteed, an offset launch into the multi-mode fiber can be easily accomplished.

The situation is more critical at the receiver side. This is particularly true at high data rates (e.g. 10 Gbit/s), where the capacitance of the photodetector must be reduced inasmuch as possible, which in turn leads to a very small size in the photodetector (e.g. a diameter of the optically active area in the order of 40 microns).

At least in principle, a lens system may be used for concentrating the optical radiation from either a multi-mode fiber (with a core diameter of 50 micron) or a single-mode fiber (with a core diameter of 10 micron) onto the photodetector.

This solution is complex and expensive from a manufacturing point of view. This also applies to lens systems including a short piece of multi-mode fiber of the kind described e.g. in US-A-5 457 759. There, a coupling arrangement between a monomode optical fiber and phototransducer is disclosed including, in succession from the phototransducer (usually a laser generating optical radiation to be injected into the fiber): a microlens, a piece of step index multi-mode fiber and a piece of graded index multi-mode fiber. In order to ensure proper operation of such an arrangement, the lengths of the pieces of multi-mode fiber must be accurately calibrated as a function of the "pitch" of the graded index multi-mode fiber. As is known, the term "pitch" is used to designate the period of the graded index multi-mode fiber, which fiber - in the prior art arrangement in question - is used as a graded index lens rather than as a fiber. The value of the pitch is determined mainly by the profile of the graded index.

The object of the present invention is to provide a solution that dispenses with the intrinsic

limitations of the prior art arrangements.

According to the present invention, that object is achieved by means of a device having the features set forth in the claims that follow. The invention also relates to a corresponding method.

Essentially, the arrangement described herein is based on the recognition of the possibility of achieving fully satisfactory operation in an optical receiver arrangement capable of operating with both single-mode (i.e. monomode) and multi-mode fibers by arranging "upstream" - i.e. before the photodetector a length of a multi-mode fiber having the purpose of radially confining without necessarily focusing the incoming radiation.

Stated otherwise, in the arrangement described herein, the multi-mode fiber, which may be a graded index fiber, is no way used as a lens, but simply as a waveguide. This in turn permits the requirements in terms of control of the length of the piece of multi- mode fiber to be extensively relaxed, since no specific, calibrated relationship is to be maintained S e.g. with respect to the multi-mode fiber pitch. This result may be achieved by ensuring that such "waveguide" fiber has a length of some millimeters (e.g. 5 millimeters) and generally no less than 1 millimeter.

When optical radiation is launched from a single mode fiber into the waveguide fiber, the optical radiation is confined within a certain diameter and not necessary focused onto the photodetector: the spot is in any case within the diameter of the photodetector, thus allowing the optical radiation to be properly detected and converted.

When a multi-mode fiber is coupled to the waveguide fiber, no major discontinuity arises there between, which allows the optical radiation to pass without significant loss in power.

Indeed, the spot of the optical radiation on the photodetector may be larger than the active area of the photodetector, whereby the optical radiation from the fiber may not be detected in its entirety. This might result in a penalty in the receiver sensitivity due to what is currently referred to as "mode selective loss".

However, this does not represent a significant problem since for highspeed transmission only short multi-mode fibers are used. The loss in optical power due to the transmission is not significant, and the minimum level of sensitivity of the photodetector in the receiver is in any case exceeded.

The invention will now be described, by way of example only, with reference to the annexed drawings, wherein: - figures 1 and 2 are schematic views of a flexible receiver as disclosed herein, - figures 3 to 5 show in detail three possible cases of different lengths of a multi-mode waveguide fiber in the arrangement disclosed herein, and - figures 6 and 7 show two possible practical embodiments of a flexible optical receiver.

In figure 1 an opto-electrical receiver 20 is shown. As better detailed in the following, this is of a "friendly" type adapted for receiving radiation from either a single-mode fiber or a multi-mode fiber.

Specifically, in the mounting arrangement shown in figure 1, the receiver 20 is coupled to a single-mode fiber 12 (with a core diameter of e.g. 9 microns).

In the mounting arrangement shown in figure 2, the receiver 20 is coupled to a multi-mode fiber 32 (with a core diameter of e.g. 50 microns).

The receiver 20 is essentially comprised of two basic elements, namely: a length of a multi-mode waveguide fiber 22 adapted to receive radiation from either of the fibers 12 and 32, and - a photodetector 24 onto which the waveguide 22 conveys the radiations received from the fibers 12 or 32.

The photodetector 24 converts the optical radiation from into an electrical signal to be fed to a processing chain of a known type (not shown).

Typically, the waveguide fiber 22 is a multi-mode fiber with a core diameter of e.g. 50 micron and a length of the order 5 millimeters. The expression "of the order" is intended to highlight that such a length is in no way to be calibrated to a fixed, absolute value related e.g. to the pitch of the fiber.

Preferably, the waveguide fiber 22 is a graded index s multi-mode fiber, even though use of a step index fiber cannot be excluded in principle.

The photodetector 24 is typically a photodiode with an optical active area 26 with a diameter of e.g. 40 microns.

Operation of the receiver 20 provides for an optical radiation OS being received from the single- mode fiber 12 or the multi-mode fiber 32 being injected/launched into the fiber 22 as a result of face-to-face alignment of either of the fibers 12 or 32 and the fiber 22. This is achieved in a manner known per se, thus making it unnecessary to provide a

detailed description herein.

Due to its length being e.g. 5 millimeters (and generally at least 1 millimeter) the waveguide fiber 22 does not act as a lens. The optical radiation propagating along the fiber 22 is not focused onto the center of the active area 26 of the photodetector 24.

In fact, the optical radiation propagating along the fiber 22 is rather just conveyed/projected to the photodetector 24 as a result of the lateral confinement action performed by the fiber 22, this applying both in the case of the single mode fiber 12 and in the case of the multi-mode fiber 32.

Three cases of varying lengths of the waveguide fiber 22 and the possible effects on the projection of the optical radiation OS onto the active area 26 of the photodetectors 24 are schematically shown in figures 3 to 5. The undulatory patterns shown in the fiber cores of figures 3 to 5 are generally intended to represent in a purely qualitative way and with reference to a graded-index fiber - exemplary propagation paths of the light signal within the fiber cores.

Since the radiation is simply conveyed onto the photodetector 24, the differences in the lengths of the fiber 22 shown in figures 3 to 5 do not appreciably affect operation of the photodetector 24.

This would not certainly be the case in any arrangement where the fiber 22 acts as a lens by focusing the radiation onto the photodetector 24.

Conversely, in the arrangement shown herein, the "spot" created by the optical radiation OS will in any case include the sensitive surface 26 of the photodiode 24.

The active area 26 of the photodetector 24 may in fact be smaller than the diameter of the core of the waveguide fiber 22 and the spot created by the optical radiation OS over the photodetector 24 exceed the active area 26. This might possibly result in a mode selective loss in that, under these circumstances, the radiation from the fiber will not be detected and converted into an (opto) electrical signal in its entirety.

The penalty in terms of receiver sensitivity induced by this mode selective loss will not however represent a significant problem. Such a loss will in fact arise only in the case the radiation OS is from a multimode fiber 32. For high speed transmission only short multi-mode fibers are used. The loss in optical power due to the transmission over the fiber 32 will not be significant, and the minimum level of sensitivity of the photodetector 24 in the receiver is in any case exceeded.

Figures 6 and 7 show two receiver structures based on the principles introduced in the foregoing.

Specifically, figure 6 shows an embodiment based on the typical layout of a conventional silicon optical bench receiver. Figure 7 shows an alternative embodiment based on a typical butt-coupled receiver arrangement.

In the arrangement of figure 6, the optical radiation OS is injected into the multi-mode waveguide fiber 22, which is arranged in a groove (typically a U- shaped groove, i.e. a groove having an essentially square cross-section) provided in the bench substrate.

The optical radiation OS propagating over the fiber 22 is reflected and deviated by a mirror surface 28 provided at the end of the groove (or comprised of a mirrored end surface of the fiber 22) and thus caused to impinge onto the active area 26 of a photodetector 24 arranged over the bench substrate.

This arrangement is substantially similar to the arrangement disclosed in EP-A-l 033 596 and has the advantage of limiting the distance between the end of the fiber 22 and the photodetector 24, thus reducing the mode selective loss.

The butt-coupled receiver arrangement of figure 7 provides for the photodetector 24 being arranged directly "behind" the fiber 22. Again, the gap between the end face of the waveguide 22 and the photodiode 24 can thus be made very small, thus minimizing any possible source of mode selective loss.

It is thus evident that, the basic principles of the invention remaining the same, the details and embodiments may widely vary with respect to what has been described and illustrated purely by way of example, without departing from the scope of the presented invention as defined in the annexed claims.

Also, terms such as ''optical", "light", "photosensitive", and that like are evidently used herein with the meaning currently allotted to those terms in fiber and integrated optics, being thus intended to apply to radiation including, in addition to visible light, e.g. also infrared and ultraviolet radiation.

Claims

l. A device including: - an opto-electrical converter (24) for receiving optical radiation (OS) propagating over a propagation path, the converter (24) converting said radiation into an electrical signal, and - a multi-mode optical fiber (22) associated with said opto-electrical converter (24), said multi-mode optical fiber (22) extending over said given propagation path to direct said optical radiation (OS) onto said opto-electrical converter (24), wherein said multi-mode optical fiber (22) conveys said radiation (OS) onto said opto-electrical converter (24) by radially confining said radiation (OS) in the absence of a focusing effect.
2. The device of claim l, characterized in that said multi-mode optical fiber ( 2 2) i s a graded-index fiber.
3. The device of either of claims l or 2, characterized in that said multi-mode optical fiber (22) extends over said given propagation path for a length of at least l mm.
4. The device of claim 3, characterized in that said multi-mode optical fiber (22) extends over said given propagation path for a length of the order of 5 mm.
5. The device of any of claims l to 4, characterized in that said multimode optical fiber (22) is arranged directly facing said opto-electrical converter (24) .
6. The device of any of claims l to 5, characterized in that said multi- mode optical fiber (22) is associated in a butt coupling arrangement with said opto- electrical converter (24) .
7. The device of claim 1, characterized in that it includes a mirror surface (28) for deflecting said optical radiation (OS) exiting said multi-mode optical fiber (22) onto said opto-electrical converter (24).
8. A method of receiving from one of a single-mode fiber (12) and a multimode fiber (32) optical radiation (OS) for conversion into an electrical signal, the method including the steps of: - providing an opto-electrical converter (24) for receiving said optical radiation from said one of a single-mode fiber (12) and a multi-mode fiber (34) over a given propagation path and converting said radiation into an electrical signal, - associating with said opto-electrical converter (24) a multi-mode optical fiber (22), said multi-mode optical fiber (22) associated with said converter (24) extending over said given propagation path to direct said optical radiation (OS) onto said opto-electrical converter (24), wherein said multi-mode optical fiber (22) associated with said converter (24) conveys said radiation (OS) onto said opto-electrical converter (24) by radially confining said radiation (OS) in the absence of a focusing effect, and - coupling said multi-mode optical fiber (22) associated with said converter (24) to said one of a single-mode fiber (12) and a multi-mode fiber (32) to receive optical radiation (OS) therefrom.
9. The method of claim 8, characterized in that it includes the step of selecting said multi-mode optical fiber (22) associated with said converter (24) as a graded-index fiber.
10. The method of either of claims 8 or 9, characterized in that it includes the step of extending said multi-mode optical fiber (22) associated with said converter (24) over said given propagation path for a length of no less than 1 mm.
11. The method of claim 10, characterized in that it includes the step of extending said multi-mode optical fiber (22) associated with said converter (24) over said given propagation path for a length of the order of 5 mm.
12. The method of any of claims 9 to 11, characterized in that it includes the step of arranging said multi-mode optical fiber (22) associated with said converter (24) directly facing said opto-electrical converter (24).
13. The method of any of claims 9 to 12, characterized in that in that it includes the step of butt coupling to said opto-electrical converter (24) said multi-mode optical fiber (22) associated with said converter (24).
14. The method of claim 9, characterized in that it includes the step of deflecting onto said opto electrical converter (24) said optical radiation (OS) exiting said multi-mode optical fiber (22) associated with said converter (24).