GB2535803A

GB2535803A - Fibre collimator

Info

Publication number: GB2535803A
Application number: GB1503435.8A
Authority: GB
Inventors: Mark Norman Adrian
Original assignee: Gooch and Housego PLC
Current assignee: Gooch and Housego PLC
Priority date: 2015-02-27
Filing date: 2015-02-27
Publication date: 2016-08-31
Anticipated expiration: 2035-02-27
Also published as: GB201503435D0; GB2535803B

Abstract

An optical fibre collimator 300 and method of fabrication. The collimator has a housing 310 with a through-hole or cavity 305. The through-hole has a longitudinal axis, a first longitudinal portion 305a having a first internal diameter and a second longitudinal portion 305b having a second internal diameter. The collimator has a lens 303 moulded directly into the second portion, where the first portion is arranged to receive an optical fibre ferrule. The first internal diameter may be greater than the second internal diameter and the first and second portions may be connected by a shoulder 307. The method of fabrication involves boring a first hole forming a first portion in a housing, machining a second hole forming a second portion in a housing, inserting a moulding tool into the first portion and directly moulding a lens into the second portion.

Description

FIBRE COLLIMATOR

Field

The present disclosure relates in general to collimators. More particularly, the present disclosure relates to optical fibre collimators. More particularly still, the present disclosure relates to imaging fibre optic modes and creating collimated free space propagating modes and vice versa. The present disclosure also relates to a method for manufacturing and assembling an optical fibre collimator.

Background

Fibre collimators are needed whenever laser light needs to be coupled into the fibre or is exiting the fibre and needs to be collimated. Examples are fibre coupling from laser diode or making a fibre-laser mode accessible to experimentation. Another common configuration using two collimators is when a bulk optic device needs to be integrated into the fibre path. This can be an isolator, an electro-optic or acousto-optic modulator. A collimator may take guided light and collimate it into a laser mode propagating in free space. The direction of light can equally well be reversed.

The major challenge in constructing such collimators is precise relative alignment of the collimating lens and the fibre end. Lateral displacement of the lens with respect to the fibre axis causes the light to fall on the lens at an angle and causes not only beam pointing errors, but also beam distortions because off-axis rays are generally not imaged optimally.

Axial displacement, which creates a distance between the fibre end and the focal point of the lens, can lead to diverging or converging beams instead of well-collimated beams.

Collimators manufactured to date typically contain components, such as the lens, the lens housing, the main tube and the fibre, plus some means of manipulation to achieve and 30 maintain focus and pointing. These components are aligned, and then held together using either mechanical fixturing, laser welding methods, or adhesives.

A build-up of tolerances in piece parts causes the core of the fiber not to be co-aligned to the center of the lens, this causes poor pointing. Various designs to assist in manipulating the core with respect to the lens center can cause the assembly to be temperature sensitive, typically over complicated, large and more expensive to produce. Poor PER (polarization extinction ratio) inherently occurs when the lens is attached to the main tube, soft adhesives have a tendency to creep and move over time and temperature, hard adhesives cause stress in the lens causing poor PER. Laser welding has been used successfully however this can be expensive due to piece part tolerances to achieve the performance and process equipment to achieve a consistent weld.

The lens is typically moulded in to metal surrounds. These moulded lenses are then attached to another component using adhesive or laser welding. This joint can cause stress to the micro lens affecting polarization of the lens. In addition to this the mounted lens moves when fixed to the higher level assembly causing poor beam pointing stability. This movement will also take place if the temperature of the assembled component varies.

The lateral alignment can be aided by machining a collimator tube that holds both the ferrule containing the fibre and the moulded lens in its housing. A drawback of this method is that the lens housing needs to be attached to the collimator tube. This attachment typically induces strain in the lens, causing poor PER and associated yield losses.

The present disclosure provides a fibre collimator that can quickly and accurately be assembled with a fibre end mounted in a ceramic ferrule. The present disclosure also provides an assembled collimator which can easily be incorporated in other optical assemblies with automatic angular keying of the main mode axes in PM fibre. The present disclosure also provides a collimator which offers stable operation even under extreme temperature changes that would cause poor PER or beam pointing stability in prior art devices.

The present disclosure therefore addresses a need for a laser fibre collimator with improved collimator pointing stability over temperature, better pointing specification, and 30 improved PER (polarization extinction ratio). The present disclosure also addresses a need for a fibre collimator that is simple to align and cost effective.

Summary

Aspects of an invention are defined in the appended independent claims.

There is provided a collimator tube fabricated from stainless steel having: a flat outside surface parallel to the collimator axis; a cylindrical precision-bore that is matched to receive a ceramic ferrule having a precise outside diameter; a breathing opening on the side of the tube that allows air to escape during assembly; and a moulded glass lens opposite the opening, wherein the moulded glass lens has well aligned axis with the cylindrical precision bore.

There is provided a fibre collimator including: a (stainless steel) part having a first cylindrical opening defining the collimator axis, a second cylindrical opening opposite the first opening having same said collimator axis, a flat outside surface being parallel to said collimator axis, and a smaller third opening on the side; and a glass lens moulded into said second opening whereas the first opening is utilized in the moulding process to ensure the lens axis is centred on said collimator axis.

There is also provided a collimator further comprising a precision ceramic ferrule 20 containing concentric fibre inserted into said first opening and affixed to said collimator after axial alignment The term "directly" is used herein to indicate the absence of intermediate mounting components.

Brief description of the figures

Embodiments of the present disclosure will now be described with reference to the accompanying drawings in which: Figures I a, lb and lc show a moulded lens in a lens housing;

Figure 2 shows a prior art fibre collimator;

Figure 3 shows a fibre collimator according to an embodiment of the present disclosure; Figure 4 shows a fibre collimator according to a further embodiment of the present disclosure; and Figure 5 shows a flow chart of a process in which a fibre is aligned and mounted in a fibre collimator in accordance with embodiments.

In the figures, like reference numerals refer to like parts.

Detailed description of embodiments

Some improvements have been developed for individual components used in a collimator. A glass or plastic lens directly moulded into a metal tube can serve the purpose of auto-aligning the lens axis with the metal outside cylinder surface. Such lenses are available, but they offer no provision to align the optical axis of the lens to that of the fibre. Direct moulding allows the lens to be aligned with features of the lens housing with a much higher accuracy than other lens mounting techniques such as mechanical mounting, adhesive mounting and laser welding techniques. Mechanical mounting requires high precision machining of the housing mounting surfaces and the corresponding mounting surfaces of the lens; adhesive mounting can give rise to the position of the lens in its housing being dependent on temperature; and laser welding can be expensive.

Figures la, lb and lc each show different views of a lens directly moulded into a lens mount. Specifically, Figure 1 a shows isometric views of three different lens mounts 101 arranged to contain a moulded lens. Figures lb and lc show a cross-sectional view and a plan view, respectively, of a lens mount containing a moulded lens 103. A lens mount 101 is shown into which a collimating lens 103 is directly moulded. The lens mount 101 includes a cylindrical lens mount body 110 with a lens mount cavity 105 extending from a first end 111 to a second end 112 of the lens mount body HO. The lens mount cavity 105 is a through-hole with a cylindrical first portion 105a having a first diameter and a cylindrical second portion 105b having a second diameter. Both the first portion 105a and the second portion 105b are coaxial with the lens mount body 110. The first portion 105a extends from the first end 111 through a first length of the lens mount body 110. The second portion 105b extends from the second end 112 through a second length of the lens mount body 110. The sum of the first length and second length is equal to the total length of the lens mount body 110. The first portion 105a and the second portion 105b form a through-hole with a shoulder 107 located at the point where the first portion 105a and second portion 105b meet. The first diameter is larger than the second diameter such that the shoulder 107 has an annular surface which lies in a plane perpendicular to the axis of the lens mount body 110.

The collimating lens 103 is moulded into the lens mount 110 such that it straddles the shoulder 107 and is located partly in the first portion 105a and partly in the second portion 105b of the lens mount cavity 105. The collimating lens 103 has a first surface 103a spanning a cross-section of the first portion 105a of the lens mount cavity 105 and a second surface 103b spanning a cross-section of the second portion 105b of the lens mount cavity 105. The first and second surfaces each have a radially symmetric convex profile, the first surface 103a having a convex profile with a smaller radius of curvature than that of the second surface 103b.

Single-Mode fibres, both regular and polarization-maintaining (PM) varieties are typically mounted in precision ferrules that guarantee a precise positioning of the single-mode fibre-optic core in the centre of the cylinder defined by the outside of the ferrule. This precise positioning, with errors much less than 1 pm, allows alignment of other optical elements with respect to the cylinder axis rather than the fibre core without incurring mode-mismatch losses.

Despite advances in the design of individual components used in fibre collimators, the problem of how to ensure a reliable alignment of the individual components remains. This problem is illustrated with reference to a prior art fibre collimator shown in Figure 2. Figure 2 shows a fibre collimator 200 which includes the collimating lens 206 and lens mount 202 along with fibre 220, a ferrule 230 and a main housing 210.

The main housing 210 includes a main housing cavity 205 extending from a first end 211 to a second end 212 of the main housing 210. The main housing cavity 205 has a first section 205a with a first diameter, a second section 205b with a second diameter and a third section 205c with a third diameter. The first section 205a, second section 205b and third section 205c are cylindrical and are arranged in order end-to-end along a common axis and form a through-hole from the first end 211 to the second end 212 of the main housing 210. The first section 205a extends from the first end 211 of the main housing along a first length of the main housing 210. The third section 205c extends from the second end 212 along a third length of the main housing 210. The second section 205b extends along a second length of the main housing 210 between the first section 205a and the third section 205c. The first section 205a and the second section 205b form a first shoulder 207 where they meet. The first section diameter is larger than the second section diameter such that the first shoulder 207 has an annular surface which lies in a plane perpendicular to the cylindrical axis of the first and second sections. The second section 205b and the third section 205c form a second shoulder 208 where they meet. The second section diameter is larger than the third section diameter and the second shoulder 208 has a chamfer such that it has a shallow conical annular surface.

The ferrule 230 is cylindrical and includes a through-hole 231 for receiving a fibre 220. The through-hole 231 is coaxial with the ferrule 230. The fibre 220 is mounted in the through hole such that the fibre end 221 is aligned with a first end 233 of the ferrule.

The diameter of the lens mount 202 is generally equal to the diameter of the first section 205a of the main housing 210. The lens mount 202 is inserted into the main housing 210 and mounted such that the annular surface of the first shoulder 207 and one end of the lens mount 202 are mated. The ferrule diameter is generally equal to the diameter of the third section 205c of the main housing cavity 205. The ferrule 230 is inserted into the main housing 210 and mounted such the first end 233 of the ferrule 230 is spaced apart from the collimating lens 206. The ferrule 203 is fixed in place using an epoxy-based adhesive or other means.

Notably, in the fibre collimator of Figure 2, the lens 206 and ferrule 230 are directly mounted in different housings. Specifically, lens 206 is directly moulded in the lens mount 202 and the lens mount is then mounted into the main housing 210 (usually with an epoxy-based adhesive). The ferrule 230 is directly mounted in the main housing 210 (also usually with an epoxy-based adhesive). This leads to conventional design collimators, such as these, requiring an extensive alignment procedure. The epoxy adhesives used to fix the relative position of the lens 206 and the ferrule 230, once aligned, often degrade and cause performance problems in subsequent use. In the case of polarization maintaining (PM) fibres, the rotational orientation of the fibre 220 also needs to be controlled and remain stable even when undergoing environmental stress. The sum of alignment errors of individual parts leads the core of the fibre 220 to be mis-aligned from the centre of the lens 206, degrading pointing stability.

The alignment procedure using conventionally fabricated parts requires moving the fibre with respect to the lens centre by tens of microns which can be elaborate and labour intensive. The thick epoxy layers required to fill the space can lead to the assembly shifting with temperature changes. Poor pointing stability occurs when the lens is attached to the main tube because the soft adhesives used in the attachment have a tendency to creep and move overtime and temperature. If hard adhesives are used, it can cause stress in the lens causing poor polarization extinction ratio (PER). Laser welding has been used successfully; however this can be expensive due to piece part tolerances to achieve the 15 performance and process equipment to achieve a consistent weld.

The inventors have provided a single housing into which the lens is directly moulded and the ferrule is directly mounted such that more accurate and reliable alignment of the fibre and the collimating lens in a fibre collimator can be carried out.

An embodiment is illustrated in Figure 3 Figure 3 shows a fibre collimator 300 which includes a housing 310, a moulded lens 303, a ferrule 330 and a fibre 320. The housing 310 has a collimator cavity 305 extending from a first end 311 to a second end 312 of the housing 310. The collimator cavity 305 is a through-hole with a first longitudinal portion 305a having a first internal diameter and a first length and a second longitudinal portion 305b having a second internal diameter and a second length. The first longitudinal portion 305a and the second longitudinal portion 305b are coaxial. The first longitudinal portion 305a extends from the first end 311 along a first length of the housing 310. The second longitudinal portion 305b extends from the second end 312 along a second length of the housing 310. The sum of the first and second lengths is equal to the total length of the housing 310 such that the first longitudinal portion 305a and the second longitudinal portion 305b form a through-hole with a shoulder 307 located at the point where the first longitudinal portion 305a and second longitudinal portion 305b meet. The first longitudinal portion 305a is suitable for directly receiving the ferrule 330 and the second longitudinal portion 305b is suitable for directly receiving the moulded lens 303.

The moulded lens 303 is moulded into second longitudinal portion 305b of the collimator 5 cavity 305. The moulded lens 303 has a first surface 303a spanning a cross-section of the second longitudinal portion 305b towards the shoulder 307 and a second surface 303b spanning a cross-section of the second longitudinal portion 305b towards the second end 312. In this embodiment, the first and second surfaces each have a radially symmetric convex profile, the first surface 303a having a convex profile with a smaller radius of 10 curvature than that of the second surface 303b. The ferrule 330 has a through-hole into which the fibre 320 is mounted. The end of the fibre 320 is aligned with a first end 333 of the ferrule 330 and the optical axis of the fibre 320 is coaxial with the ferrule 330.

It may therefore be understood that there is provided an optical fibre collimator including: a housing comprising a through-hole having a longitudinal axis, wherein the through-hole comprises a first longitudinal portion having a first internal diameter and a second longitudinal portion having a second internal diameter; a lens directly moulded into the second longitudinal portion; wherein the first longitudinal portion is arranged to directly receive an optical fibre ferrule. In embodiments, the first and/or second longitudinal portions are substantially cylindrical. In embodiments, the first longitudinal portion is continuous with the second longitudinal portion.

Advantageously, a fibre collimator is provided in which the fibre and collimating lens may be aligned more easily and with greater reliability. Further advantageously, a fibre collimator is provided in which there are fewer components to align, and therefore fewer steps of alignment are needed. In particular, it may be understood that the present collimator does not comprise a separate lens mount component.

It is advantageous, though not essential, that the first internal diameter is greater than the second internal diameter. It may therefore be understood that, in embodiments, the first internal diameter is greater than the second internal diameter. Advantageously, this allows the second surface to receive an anti-reflection coating without any shadowing of the second surface by the first longitudinal portion. This necessarily creates a shoulder 307 as shown in Figure 3. In Figure 3, the first internal diameter is larger than the second internal diameter such that the shoulder 307 has an annular surface which lies in a plane perpendicular to the housing axis. It may therefore be understood that, in embodiments, the first and second longitudinal portions are connected by a shoulder which extends in a direction substantially perpendicular to the longitudinal axis. The shoulder provides its own further advantages as will become apparent later.

In embodiments, the dimensions of the first longitudinal portion are such that it is suitable for receiving a moulding tool of generally equal diameter for moulding the collimating lens. Therefore, in embodiments, the first longitudinal portion is arranged to receive a lens moulding tool, optionally, wherein the first internal diameter is approximately equal to that of a lens moulding tool. Further optionally, wherein the first internal diameter is in the range 2 to 3 mm, yet further optionally, 2.4 to 2.6 mm.

The first longitudinal portion is such that the lens moulding tool used in the moulding of 15 the collimating lens has a suitable clearance between its outer surface and its moulding surface. Furthermore, it allows the lens moulding tool to be sufficiently rigid in construction.

The shoulder as shown in Figure 3 provides a mating surface for a lens moulding tool (not 20 shown). Therefore, in embodiments, the shoulder is arranged to provide a stop for a lens moulding tool. Advantageously, the stop allows the axial position of the second surface of the collimating lens to be moulded with greater accuracy and reliability.

By using a suitable lens moulding tool, the moulded lens in Figure 3 may be readily moulded with an optical axis which is collinear with the axis of the first longitudinal portion. Therefore in embodiments, the lens has an optical axis which is substantially collinear with the longitudinal axis. Advantageously, the position of the optical axis of the lens may be located or referenced more easily.

The first diameter of the first longitudinal portion is substantially equal to the diameter of the ferrule such that the ferrule has a sliding fit or running fit when inserted into the first longitudinal portion. Therefore in embodiments, the first internal diameter is approximately 3 to 19 microns greater than the diameter of the optical fibre ferrule.

Advantageously, the optical axis of the fibre may be more easily aligned with the axis of the first longitudinal portion, and hence with the optical axis of the lens.

In embodiments, the housing has additional features which provide additional advantages. 5 Figure 4 shows the housing of Figure 3 with the additional features of a cylindrical outer surface portion 313, a flat outer surface portion 314 and a hole 315.

The outer surface of the housing 310 has a cylindrical outer surface portion 313. The cylindrical outer surface portion 313 is coaxial with the first longitudinal portion 305a and 10 second longitudinal portion 305b.

Advantageously, the cylindrical outer surface portion 313 may be used as a reference to align other components to the cylindrical axis of the first longitudinal portion 305a, which is not easily referenced when the fibre collimator is in use.

Figure 4 shows the outer surface of the housing 310 having a flat outer surface portion 314. The flat outer surface portion 314 is parallel to a plane which passes through the cylindrical axis of the first longitudinal portion 305a. Therefore, in embodiments, the housing comprises a flat outer surface portion being substantially parallel to said longitudinal axis.

Advantageously, the flat outer surface portion acts as a surface from which to reference a relative rotational alignment of the housing and the fibre. Further advantageously, the flat outer surface may be clamped easily during such an alignment or be used to reference a 25 relative rotational alignment of an external component and the fibre.

The hole 315 passes through the housing 310 from the curved wall of the first longitudinal portion 305a to an outer circumferential surface of the housing 310. The hole 315 is positioned towards the first shoulder 307 and has an axis which is perpendicular to the cylindrical axis of the first longitudinal portion 305a. It may be understood that the hole can take other forms or be located in a different place to that shown in Figure 4. For example, in other embodiments, the hole has a square shaped cross-section, is located on the shoulder and has an axis which is parallel to the cylindrical axis of the first longitudinal portion such that is passes from the shoulder through the housing to the second end of the housing. Therefore, in embodiments, the housing comprises a hole extending from the first longitudinal portion to the outer surface of the housing.

Advantageously, the hole provides a passage to allow the exchange of gases between an 5 outside of the housing and an internal space of the housing situated between the first end of the ferrule and the second surface of the collimating lens.

By choosing appropriate materials for the housing and the lens, a more robust collimator may be formed. The housing material must be suitable to withstand the lens moulding process and the lens material must be suitable for moulding and suitable to receive high energy laser light. The main body of the collimator consists of a part machined from stainless steel. Therefore, in embodiments, the lens is a glass lens and, optionally, the housing is a stainless steel housing.

The apparatus as herein described may be formed by a process which facilitates the alignment of the optical axis of the lens with the optical axis of the fibre. The end result of this process is the apparatus shown in Figures 3 or 4 with a ferrule inserted into the first longitudinal portion such that optimum collimation of the light from a fibre housed in the ferrule is achieved with optimum axial and rotational alignment In an embodiment, a process starts with machining a piece of raw material to form the housing 310 shown in Figures 3 or 4. First, to form the first longitudinal portion 305a, a precision bore is drilled into one end; with the depth of this cylindrical bore such as not to create a through-hole. The bore diameter is chosen to directly accept the ceramic ferrule 330. If the viscosity of an epoxy adhesive used to mount the ferrule is high, a small gap is impractical. On the other hand, the high viscosity improves ensuring the ferrule is centred in the first longitudinal portion during axial movements and this allows for looser tolerances without sacrificing alignment accuracy. The nominal clearance between the ferrule diameter and the bore diameter needs to allow for fabrication tolerances but is chosen as small as possible to ensure good alignment accuracy. The optimal gap is therefore dependent on the viscosity of the epoxy used to fix the ferrule 330 in the first longitudinal portion 305a. In an embodiment, the gap between 3 and 19 Rm. For reasons described below, we chose a ceramic ferrule diameter of 2.5 mm. The bore cylinder axis defines the collimator axis, and the present disclosure relies on the moulded lens 303 being accurately positioned on this same axis. The second longitudinal portion 305b is formed by a machined hole wherein the centre of the machined hole is referenced to the collimator axis defined from the bore.

The machined housing 310 is then used in a glass moulding process to receive the moulded lens 303 that is directly moulded into the second longitudinal portion 305b. The mould used typically defines an aspheric lens shape optimized to collimate the fibre mode to a free-space TEMOO mode. One or both of the surfaces of the moulded lens 303 may be shaped by the mould. In an embodiment, the tooling during this moulding process is designed to reference the collimator axis from the inside cylinder face of first longitudinal portion so that the axis of the moulded lens 303 coincides with the collimator axis. In another embodiment, the collimator axis is referenced from the outer surface of the first longitudinal portion.

There is therefore provided a method of fabricating an optical fibre collimator, the method including the steps of boring a first hole to form a first longitudinal portion having a first internal diameter and defining a longitudinal axis, wherein the first hole is not a through-hole and is arranged to directly receive an optical fibre ferrule; machining a second hole having a second diameter to form a second longitudinal portion having a second internal diameter by referencing the centre of the second hole to the longitudinal axis, wherein the second hole is a through-hole to the first hole and has a second diameter less than the first internal diameter; inserting a moulding tool in to the first longitudinal portion; and directly moulding a lens into the second longitudinal portion using the moulding tool.

Lens moulding typically references the moulding tool with respect to the outside cylinder surface of the lens mount (in this case the collimator). This is because traditional optical assemblies typically mount the lens into a machined body that holds and references this outer surface. Rather than using a second machined body to reference to both the ferrule and to the lens mount outside mount, the inventors have designed an embodiment in which the collimator to ensure proper alignment of the lens, not to the outside of the body, but to the inner surface of the first longitudinal portion.

In embodiments, the first longitudinal portion is bored to have a tight tolerance to enable the axis of the lens to be accurately aligned with the first longitudinal portion axis. Unless this surface is utilised to reference the lens moulding tooling from, there can be no guarantee that the lens is well aligned.

Therefore in embodiments, the method includes the further the step of referencing the axis of the moulding tool to an inside surface of the first hole. Advantageously, the process allows the moulded lens to be accurately and reliably positioned in an optical fibre collimator such that the optical axis of the lens is collinear with the axis of the first longitudinal portion. This technique is unconventional in the art. The skilled person will understand that other methods of referencing the moulding tool may be suitable.

The flat outer surface portion 314 on the outside of the collimator 300 is also machined, and referenced off the axis defined by the first longitudinal portion 305a. Therefore, in embodiments, the method further includes the step of machining a flat outer surface portion into an outer surface of the optical fibre collimator, wherein an edge of the flat portion is substantially parallel to said longitudinal axis.

Finally, a small opening is machined through the wall of the housing 310 at a direction roughly perpendicular to the collimator axis. This hole 315 is positioned to allow for air or other gases to be exchanged between the outside of the collimator 300 and the inside of 20 collimator cavity 305, even after the moulded lens 303 and the ferrule 330 are in place.

After the lens is moulded, it needs to be anti-reflection coated. The diameter of the precision first longitudinal portion 305a needs to be sufficiently large for two reasons. First, the moulding tool inserted into the first hole needs a certain stiffness and clearance between the optical surface and outside of the tool. Second, the first hole needs to be larger than the diameter of the moulded lens 303, defined by the diameter of the second hole, to avoid shadowing of the AR coating materials during thin film deposition. A 2.5mm bore diameter works well to satisfy those conditions.

Therefore, in embodiments, the method further includes the step of applying an anti-reflection coating to a first surface of the moulded lens through the first hole and, optionally, further comprising applying an anti-reflection coating to a second surface of the moulded lens through the second hole.

The collimator as machined makes assembly with fibres easy. The flow chart shown in Figure 5 outlines one embodiment. It may be understood that the method could be performed in a different order or by exchanging, adding or omitting steps without departing from the inventive concepts disclosed herein.

In step S501 of Figure 5, the housing is placed in a groove, such as a V-groove, with the flat surface facing upwards the housing 310 is then securely clamped into tooling. Such tooling could for example consist of fixed groove receiving the housing 310 and a pneumatically driven flat piston that clamps down onto the flat outer surface portion 314.

Such tooling aligns the collimator axis to the groove and controls the rotation, important when aligning PM fibres.

In step 5502, the fibre at the opposite end of the ferrule is connected to a light source and the ferrule is inserted into the first longitudinal portion. In embodiments, the method 15 therefore further comprises inserting an optical fibre ferrule into the first longitudinal portion.

The end of the fibre is imaged using a beam analyser. A CCD array is used to observe the collimated beam emerging from the collimator during alignment Laser light is launched into the fibre and the beam can be observed via the CCD. In step 5503, the longitudinal position of the ferrule with respect to the lens is adjusted until the desired beam width is achieved at 2 or 3 different predetermined positions on optical axis. Step S503 may be considered a longitudinal (axial) alignment step which achieves a well-collimated beam over a large range of distances.

Therefore, in embodiments, the method further comprises the step of comprising aligning the optical fibre ferrule in the first longitudinal portion, wherein the aligning comprises: launching light into an optical fibre mounted in the optical fibre ferrule; detecting light transmitted through the moulded lens using a detector; and moving the optical fibre ferrule relative to moulded lens. In further embodiments, the detector is a photodetector array, optionally, a CCD array.

The ferrule can be moved forward or back in the bore such that the observed spot size corresponds to optimal collimation. Because of the tight fit of the ferrule in the bore, the lateral position has no adjustable degrees of freedom, and the pointing direction of the collimated beam needs no adjustment. Therefore, in embodiments, the method includes observing the spot size. Optionally, the optical fibre ferrule is moved longitudinally relative to the moulded lens in response to the observed spot size.

If the fibre is of a polarization maintaining (PM) type, the method proceeds to step 5508 before proceeding to step S509. If not, then the method proceeds directly to step S509.

In step 5508, the PM-type fibre is aligned in an azimuthal adjustment. A fixed polarizer is positioned in front of a power head and this is positioned in the beam path. The ferrule is rotated around its axis until extinction is achieved. Alternative launching of both polarization modes into the fibre will allow both the axial and azimuthal adjustment to proceed simultaneously. In embodiments, step 5508 ensures that the polarization of the mode is either parallel or perpendicular to the flat surface 314.

Therefore, in embodiments, a polarizer is positioned in front of the detector. In further embodiments, the method further includes rotating the ferrule around its optical axis, optionally, rotating the ferrule until extinction is achieved. Optionally these steps may be performed after launching both polarisation modes in the optical fibre.

The ferrule can be fixed in the collimator bore by epoxy or some other means. In an embodiment, a small amount of two-component epoxy is applied to the outside of the ferrule. Because of the tight fit, most of the applied epoxy will be scraped off when the ferrule is inserted, only retaining a thin and uniform layer to fill the gap between the ferrule and the bore. In embodiments, a two-component epoxy that cures at 150°C in 1 minute to 1 hour is used. In step S509 of Figure 5, the fibre is moved a predetermined distance out of the housing and an appropriate resin is applied to the ferrule. In step 5510, the ferrule is moved back in the opposite direction by the same predetermined distance of step 5509.

Step 5503 is performed again until the beam width is within the required predetermined ranges at each position of the beam analyser. For cases in which the fibre is polarization maintaining, the step 5508 is additionally performed again until the desired polarisation extinction ratio has been achieved. The resin is cured in step S511 and, optionally, a check of the alignment is performed to check it is within accepted tolerances. If the final alignment step is failed ("N" branch of "alignment check after curing"), the product is scrapped.

In a final step 5512, the fibre is disconnected from the light source and the fibre pigtail is 5 coiled. The flat face of the tube is cleaned with propanol and the fibre collimator is lifted out of the groove and packaged for shipment.

Therefore, in embodiments, the method includes fixing the optical fibre ferrule in the first longitudinal portion. Optionally, using an adhesive, further optionally, an epoxy-based 10 adhesive.

The hole 315 allows air to exit or enter the chamber as the ferrule is inserted and adjusted to result in good collimation. This "breathing" hole is advantageous during alignment and epoxy curing. The small gap between first longitudinal portion and ferrule is filled with epoxy whilst the ferrule must still be able to move in and out in order to adjust the position of fibre end to the collimating lens. The cavity defined by the ferrule, some of the first longitudinal portion surface, the shoulder and the lens would be closed, trapping a fixed amount of air if this breathing hole were not present. This would result in the compressed air pushing against the desired motion, making it more difficult to ensure a reliable axial fibre-lens displacement after curing. The breathing hole allows for air to escape from the cavity when inserting the ferrule or moving it closer to the lens during adjustment, and for air (or a processing atmosphere such as dry nitrogen) to flow in from the environment when the ferrule is pulled away from the lens. Once the correct position is determined, the epoxy curing is initiated. Owing to the breathing hole, the airspace in the cavity does not exert forces on the ferrule during curing which may otherwise make the ferrule move.

Therefore, in embodiments, the method further includes machining a hole extending from the first longitudinal portion to an outer surface of the optical fibre collimator. Advantageously, if this opening were not present, the air trapped inside the collimator cavity between the ferrule 330 and the first surface 303a of the moulded lens 303 would be expand during cure and potentially push the ferrule back, leading to a converging beam.

This opening is also important in subsequent environmental sealing of a larger enclosure that contains one or two collimators and some active component such as an acousto-optic modulator. Hermetic sealing starts with evacuating all spaces, and then refilling the larger enclosure with a dry, inert gas. The hole 315 ensures that the collimator cavity between the ferrule 330 and the first surface 303a of the moulded lens 303 also gets filled with the inert gas and no moisture or oxygen is enclosed.

Therefore, in embodiments, the method further includes hermetically sealing the optical fibre collimator, optionally, in an outer housing.

It may be understood that embodiments are provided by way of example only and the 10 scope of the claims extends beyond the embodiments and examples described.

Claims

What is claimed is: 1. An optical fibre collimator comprising: a housing comprising a through-hole having a longitudinal axis, wherein the through-hole comprises a first longitudinal portion having a first internal diameter and a second longitudinal portion having a second internal diameter; a lens directly moulded into the second longitudinal portion; wherein the first longitudinal portion is arranged to directly receive an optical fibre ferrule.
2. The optical fibre collimator of claim 1, wherein the first and/or second longitudinal portions are substantially cylindrical.
3. The optical fibre collimator of claim 1 or 2 wherein the first longitudinal portion is continuous with the second longitudinal portion.
4. The optical fibre collimator of claim 1, 2 or 3, wherein the first internal diameter is greater than the second internal diameter.
5 The optical fibre collimator of any preceding claim, wherein the first and second longitudinal portions are connected by a shoulder which extends in a direction substantially perpendicular to the longitudinal axis.
6. The optical fibre collimator of any preceding claim, wherein the first longitudinal portion is arranged to receive a lens moulding tool.
7. The optical fibre collimator of any preceding claim, wherein the first internal diameter is approximately equal to that of a lens moulding tool.
8. The optical fibre collimator of any preceding claim, wherein the first internal diameter is in the range 2 to 3 mm, optionally, 2.4 to 2.6 mm.
9. The optical fibre collimator of any preceding claim, wherein the shoulder is arranged to provide a stop for a lens moulding tool.
10. The optical fibre collimator of any preceding claim, wherein the lens has an optical axis which is substantially collinear with the longitudinal axis.
11. The optical fibre collimator of any preceding claim, wherein the first internal diameter is approximately 3 to 19 microns greater than the diameter of the optical fibre ferrule.
12. The optical fibre collimator of any preceding claim, wherein the housing comprises a flat outer surface portion being substantially parallel to said longitudinal axis.
13. The optical fibre collimator of any preceding claim, wherein the housing comprises a hole extending from the first longitudinal portion to the outer surface of the housing.
14. The optical fibre collimator of any preceding claim, wherein the lens is a glass lens.
15. The optical fibre collimator of any preceding claim, wherein the housing is a stainless steel housing.
16. A method of fabricating an optical fibre collimator, the method comprising the steps of: boring a first hole to form a first longitudinal portion having a first internal diameter and defining a longitudinal axis, wherein the first hole is not a through-hole and is arranged to directly receive an optical fibre ferrule; machining a second hole having a second diameter to form a second longitudinal portion having a second internal diameter by referencing the centre of the second hole to the longitudinal axis, wherein the second hole is a through-hole to the first hole and has a second diameter less than the first internal diameter; inserting a moulding tool in to the first longitudinal portion; and directly moulding a lens into the second longitudinal portion using the moulding tool.
17. The method of claim 16 further comprising the step of referencing the axis of the moulding tool to an inside surface of the first hole.
18. The method of claim 16 or 17, further comprising machining a flat portion into an outer surface of the optical fibre collimator, wherein an edge of the flat portion is substantially parallel to said longitudinal axis.
19. The method of any one of claims 16 to 18 further comprising applying an anti-reflection coating to a first surface of the moulded lens through the first hole.
20. The method of any one of claims 16 to 19 further comprising applying an anti-reflection coating to a second surface of the moulded lens through the second hole.
21. The method of any one of claims 16 to 20 further comprising inserting an optical fibre ferrule into the first longitudinal portion.
22. The method of claim 21 further comprising aligning the optical fibre ferrule in the first longitudinal portion, wherein the aligning comprises: launching light into an optical fibre mounted in the optical fibre ferrule; detecting light transmitted through the moulded lens using a detector; and moving the optical fibre ferrule relative to moulded lens.
23. The method of claim 21 or 22 wherein the detector is a photodetector array, optionally, a CCD array.
24. The method of claim 22 or 23, further comprising observing the spot size.
25. The method of claim 24 wherein optical fibre ferrule is moved longitudinally relative to the moulded lens in response to the observed spot size.
26. The method of claim 22 or 23 wherein a polarizer is positioned in front of the detector.
27. The method of claim 22, 23 or 26 further comprising rotating the ferrule around its optical axis.
28. The method of claim 27 comprising the step of rotating the ferrule until extinction is achieved.
29. The method of any one of claims 22 to 26, further comprising launching both polarisation modes in the optical fibre and performing the method of claims 27 and 28.
30. The method of any one of claims 17 to 29 further comprising fixing the optical fibre ferrule in the first longitudinal portion.
31. The method of claim 30 wherein the fixing comprises using an adhesive, optionally, an epoxy-based adhesive.
32. The method of any one of claims 17 to 31 further comprising machining a hole extending from the first longitudinal portion to an outer surface of the optical fibre collimator.
33. The method of claim 32 further comprising hermetically sealing the optical fibre collimator, optionally, in an outer housing.
34. An optical fibre collimator or method of fabricating an optical fibre collimator substantially as hereinbefore described with reference to the accompanying drawings