WO2012001413A1

WO2012001413A1 - Fibre optical coupling

Info

Publication number: WO2012001413A1
Application number: PCT/GB2011/051232
Authority: WO
Inventors: Adrian Janssen; Sang-Ki Park; Edmund Wolak
Original assignee: Oclaro Technology Limited
Priority date: 2010-07-01
Filing date: 2011-06-29
Publication date: 2012-01-05
Also published as: GB201011058D0

Abstract

An optoelectronic device which comprises an optical source (1) for emitting a light beam (6) of a first optical power, and an optical fibre (2) for receiving a light beam of a second optical power different to the first optical power. The device also comprises a first cylindrical lens (4) having a first longitudinal plane, and a second cylindrical lens (5) having a second longitudinal plane transverse to the first longitudinal plane. The focal length of the first lens (4) and the focal length of the second lens (5) are selected to convert the first optical power to the second optical power acceptable to the optical fibre (2). Advantageously the manufacturing of the cylindrical lenses is simple and cost effective.

Description

FIBRE OPTICAL COUPLING

Field of the Invention

This invention relates to fibre optical coupling and is more particularly, but not exclusively, concerned with an optoelectronic device for coupling of a semiconductor laser to an optical fibre.

Background of the Invention

Semiconductor lasers have been used for fibre optical pumping or other applications for which high power can be obtained from such lasers. For such an application, it is possible to use a lens system between the laser and the optical fibre. It is desirable that the power or brightness obtained from the laser is transformed to the optical fibre without much power loss.

US 6370298 relates to a lens system for optical coupling of a solid state laser to an optical fibre. The lens system achieves minimum coupling-loss of the optical fibre to the laser, and is specifically concerned with coupling efficiency and not with fibre pumping.

US 5140608 relates to an optical system for focusing a light beam on to an image plane particularly suited for compensating manufacturing tolerances associated with laser diodes having variable divergence. The system needs several lenses to be adjusted and fixed in position which incurs considerable expenditure.

Thus there is a need for an optical device for fibre pumping applications which will deal with power transformation from the laser to the optical fibre. It is an object of the present invention to provide such an optical device which is cost effective and provides good power transformation from the laser to the optical fibre. Summary of the Invention

According to one aspect of the present invention there is provided an optoelectronic device comprising:

an optical source for emitting a light beam of a first optical power,

an optical fibre for receiving a light beam of a second optical power different to the first optical power, and

a first lens and a second lens for optical coupling of the source to the optical fibre by coupling the emitted light beam to the received light beam,

wherein the focal length of the first lens is selected to match the width and the optical divergence of the emitted light beam in a first plane to the width of the optical fibre and the optical divergence of the received light beam in the first plane, and the focal length of the second lens is selected to match the width and the optical divergence of the emitted light beam in a second plane to the width of the optical fibre and the optical divergence of the received light beam in the second plane, so as to convert the first optical power to the second optical power acceptable to the optical fibre.

This arrangement ensures that the maximum power is transferred from the source (laser) to the optical fibre (receiver) by considering both the source width and optical divergence of the light beam. This also makes sure that the optical density or the optical power at the laser does not exceed a level which can limit the reliability of the laser. The first and second planes may be transverse to each other. The first lens may be a cylindrical lens having an axis parallel to the first plane and the second lens may be a cylindrical lens having an axis parallel to the second plane. The first and second lenses may be aspheric cylindrical lenses comprising non-circular section geometric surfaces having spheric terms and/or any number of odd or even aspheric terms and/or elliptic terms. The first and second lenses may be circular-section rod lenses. The first lens may be a fast axis lens and the second lens may be a slow axis lens. In one embodiment, one of the first and second lenses has part circular or aspheric section having constant section in one direction. The rod lenses are made of glass which is simple and cost effective to manufacture, as even for aspheric sections a rod lens can be drawn in one direction to give constant section for any reasonable length. According to another aspect of the present invention there is provided a method of optically coupling an optical source to an optical fibre, the method comprising:

emitting a light beam of a first optical power from the source,

directing the emitted light beam from the source through a first lens and a second lens, and

receiving a light beam of a second optical power different from the first optical power from the first and second lenses to the optical fibre,

Brief Description of the Drawings

In order that the invention may be more fully understood, a number of embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a plan view of an optoelectronic device according to the present invention; Fig. 2 is a schematic representation of an optical coupling of a laser to a multimode optical fibre according to the present invention;

Fig. 3a to Fig. 3f show simulation results of ray distribution of a laser at different distances from a lens;

Fig. 4a shows the variation of the coupling as a function of lens focal length;

Fig. 4b shows the variation of the coupling as a function of source (laser) to lens distance;

Fig. 5a shows the optical power distribution as a function of optical divergence angle of an optical fibre;

Fig. 5b shows the optical power distribution as a function of width of an optical fibre; Fig. 6a to Fig. 6c are modelling illustrations of ray tracing into a multimode optical fibre; Fig. 7a is a plan view of an optoelectronic device using two cylindrical rod lenses, and Fig. 7b is vertical view of the arrangement shown in Fig. 7a. Detailed Description of Preferred Embodiments

Fig. 1 is a plan view of an optoelectronic device according to the present invention provided with a wide stripe laser diode 101 and a multimode optical fibre 102. In one embodiment, the width or the core dimension of the optical fibre 102 is about 105 μηη. Two rod lenses 104, 105 are located in between the laser 101 and the optical fibre 102 in order to establish an optical coupling of the laser 101 to the optical fibre 102. The first rod lens 104 is positioned transversely to the second rod lens 105. The first lens 104 receives a light beam from the laser 101 in a first plane of the light beam and the second lens 105 receives the light beam in a second plane transverse or orthogonal to the first plane of the light beam. The first lens 104 adjacent the laser diode 101 has a smaller diameter than that of the second lens 105 adjacent the optical fibre 102. The second lens 105 is a section of a rod lens having a planar front surface which is also referred to as a plano-convex lens.

Fig. 2 is a schematic representation of an optical coupling of a laser to a multimode optical fibre according to the present invention. The lens shown in this figure can be any one of the first and second lenses 104, 105 shown in Fig. 1 . In order to transfer the maximum power from the laser (source) 201 to the optical fibre (receiver) 202, an image of the width (core dimension) of the laser emitter 201 needs to be within the width (core dimension) of the coupling fibre 202. It is therefore necessary that both the width, w1 , and the optical divergence (NA), Θ1 , of the laser emitter 201 are transformed within the width, w2, and the NA limit, Θ2, of the optical fibre or receiver 202. The arrangement of lens 204 can achieve this transformation.

The fundamental limit to achieving the power transformation from the laser 201 to the optical fibre 202 is provided by conservation of brightness. This is the optical power derived from unit area of laser emitter per unit solid angle of emission (divergence). The brightness of the laser emitter 201 is defined as the product of the linear width (core dimension) and its beam divergence. Consequently, using the lens 204 of Fig. 2 with a paraxial approximation, the width and the optical divergence of the optical fibre 202 would be:

w2=f1 ^* Θ1 and Θ2 =w1 ^* f1

Here f1 is the focal length of the lens 204. This provides the conservation of brightness condition:

w2^* Θ2 = w1 ^* Θ1 It can be shown that a minimum size optical image of the width, w1 , and the forward divergence, Θ1 , of the laser emitter occurs at a focal point after a principal plane of the lens, which is dependent on the variation of the distance between the laser and the lens.

Fig. 3a to Fig. 3f show simulation results of ray distribution of a laser located at different distances from the lens. In this configuration, the focal length of the lens 4 is given by:

f1 = w1 / Θ2 or f1 = w2 / 01

The selection of focal length determines the optimum lens design in the paraxial approximation in order to transfer maximum power from the laser to the optical fibre. It will be appreciated that for simple spherical lenses there will be additional aberrations.

As shown in different arrangements of Figs. 3a to 3f, an image of the laser width has been formed at the focal point 303. The distance between the focal point 303 and the lens 304 is the focal length, f. The formation of the image of the laser width at the focal point 303 is dependent on the variation of the distance between the laser and the lens 304 as a function of the focal length, f. For example, in the arrangement of Fig. 3a, the image of the laser width is formed at the focal point 303 when the distance between the lens 304 and the laser is about zero. In the arrangement of Fig. 3b, the image is formed when the distance between the lens 304 and the laser is less than the focal length. Fig. 3c shows that the image of the laser width is formed when the distance between the lens 304 and the laser is equal to the focal length. In the arrangements of Fig. 3d to Fig. 3f, the image of the laser width is formed at the focal point 303 when the distance between the lens 304 and the laser is 1 .5 times the focal length, twice the focal length and four times the focal length respectively. The optimum image of the laser width is formed when the distance between the laser and the lens is between half and twice the focal length.

Fig. 4a and Fig. 4b show the variation of the coupling between the laser (soruce) and the optical fibre (receiver) as a function of lens focal length and laser to lens distance respectively. The optimum coupling occurs when the laser and the optical fibre have the same brightness. In one embodiment, for a laser width of 60 μηη, a beam divergence from the laser of 5 degrees, an optical fibre width of 30 μηη and a beam divergence acceptable to the optical fibre of 10 degree, the focal length condition is given by:

f 1 = w1 / Θ2 = 0.06 / 0.1763 = 0.34mm f1 = w2 / Θ1 = 0.03 / 0.0882 = 0.34mm

In Fig. 4a, the coupling 401 varies with lens focal length and the maximum coupling occurs at the focal length of 0.34 mm. In such an arrangement, the source to lens distance and the lens to receiver distance are equal to the focal length, e.g. 0.34 mm.

In Fig. 4b, the coupling 402 varies with the source to lens distance. The maximum coupling occurs when the source to lens distance is 0.34 mm, which is equal to the optimum focal length (image to lens distance).

Fig. 5a and Fig. 5b show the optical power distribution as a function of the optical divergence angle and the width of the optical fibre (receiver), respectively. For the results shown in these figures, a focal length of 0.34 mm has been used and the power distribution correspond to the acceptable optical power of the optical fibre. In Fig. 5a, the power distribution or intensity distribution 501 within +/-1 1 degree angular divergence limit of the optical fibre is shown. The small spreads beyond this limit are caused by spherical aberration. In Fig. 5b, the power distribution or intensity distribution 502 within +/- 30 μηη width limit of the receiver is shown. Fig. 6a to Fig. 6c are modelling illustrations of ray tracing into multimode optical fibre according to the present invention. The arrangement of Fig. 6a shows that the first rod lens 604 and the second rod lens (or section of a rod lens) 605 are arranged so that the ray from the source is accommodated within the acceptable width (core dimension) and optical divergence of the fibre 602. The coupling of the first and second lenses 604, 605 is limited by the lateral far-field. As shown in Fig. 2, the image width is the product of the object far-field divergence and the focal length of the lens at which the object is placed in front of the lens. This condition gives the limiting divergence within which the optical fibre 602 is able to accept all the power from the source (object) 601. For reduced source divergences the acceptable power is also optimum but for higher divergences the fractional power accepted is reduced in accordance with the fraction of total power from the source within the limiting solid angle. In such an arrangement, it is assumed that the lens does not suffer from any aberrations and that it has unlimited numerical aperture, i.e. it is able to accept any divergence from a source and re-image without aperture loss and acts as a paraxial lens. It will be appreciated that the vertical divergence is imaged well within the core diameter or aperture using a piece of single- mode fibre. Fig. 6b is a vertical view of the arrangement shown in Fig. 6a. Fig. 6c is a horizontal view of the arrangement shown in Fig. 6a. The optical fibre 602 shown in Figs. 6b and 6c comprises a vertical mode under-fill at the input 607. Since the vertical width of the source is very small, there is a considerably greater margin in defining the focal length of the first (fast axis) lens 604 and the conditions for optimum coupling can be relaxed. This means that the source 601 can be treated as if it were a point source and the lens focal length and position chosen to produce and image at the fibre with arbitrarily low divergence (NA). However the product of divergence and focal length is smaller than the fibre core width, or the ratio of the fibre divergence to that of the source is defined by the reciprocal of the system magnification i.e. the ratio of the source-lens and image-lens distances.

Fig. 7a is a plan view of an optoelectronic device using two cylindrical rod lenses according to the present invention. The first lens 704 is a fast axis cylindrical lens which has its plane of constant section positioned vertically and the second lens 705 is the slow axis lens which has its plane of constant section positioned horizontally. The slow axis lens 705 is positioned such that it receives the slowest rays having the smallest NA or divergence from the laser 701. The fast axis lens 704 is positioned to receive relatively faster rays having the largest NA or divergence. Despite the use of the slow and fast axis lenses 704, 705, all rays take about the same time to arrive at the focal point. The operation of fast and slow lenses depends on the relative divergences in the horizontal and vertical planes. The considerations detailed hereinbefore therefore relate to both fast and slow axis lenses equally. Fig. 7b is a vertical view of the arrangement shown in Fig. 7a. In this figure, the modelling ray distribution 6 tracing into the optical fibre 702 through the first and second lenses 704, 705 is represented. Although the earlier descriptions have been made based on an ideal thin lens, the invention is also applicable to thick lens arrangements such as shown in Fig. 7b. In this case the lens in composed of an entrance surface 708 and an exit surface 709 separated by the diameter 710 of the cylinder and is therefore classed as a thick lens where the refracting surfaces are separated from the principle plane of the lens by the radius. In Fig. 7b the rays are directed to the image close to the focus by the combined action of refraction from both surfaces.

The ends of the first lens 704 are bonded to the facet/carrier of the laser 701 to achieve fast axis collimation. No resin is applied in the optical path. The alignment of the slow axis lens 705 takes place along the ray propagation direction only (X-axis) and no height adjustment for the slow axis lens 705 is required.

A suppression coating (AR-HR coating), having a reflection at a wavelength about 1060nm and a transmission (AR) at the source wavelengths, can be applied on the slow axis rod Iens705. The suppression coating can be an anti-reflective (AR) to high reflective (HR) coating which suppresses back-reflected wavelengths. The use of the coating enables cost effective and compact rod lenses to be used.

The slow axis rod lens 705 can be made of a holographic grating material to lock the wavelength. The holographic material can be written on a surface either by modification of the refractive index or by modification of the surface geometry on a wavelength scale. Alternatively a volume hologram can be used to provide a filter function for optical reflection back to the laser.

The surfaces of the first lens and/or the second lens can be in the form of Fresnel lenses providing flat plates for both lenses and giving a similar performance to the rod lenses described hereinbefore.

When the brightness of the receiver is substantially greater than that of the source, there is a wide regime of acceptable configurations which will achieve efficient power transfer. However when this is not the case, the design limits defined above need to be applied.

Claims

CLAIMS: 1 . An optoelectronic device comprising:

an optical source for emitting a light beam of a first optical power,

2. An optoelectronic device according to claim 1 , wherein the focal length of the or each lens corresponds approximately to the width of the light beam from the source and the optical divergence of the light beam acceptable to the optical fibre in the or each plane of the light beam.

3. An optoelectronic device according to claim 1 , wherein the focal length of the or each lens corresponds approximately to the width of the optical fibre and the optical divergence of the light beam from the source in the or each plane of the light beam.

4. An optoelectronic device according to any preceding claim, wherein the first and second lenses are spaced from the source by a distance between half and four times the focal length.

5. An optoelectronic device according to any preceding claim, wherein the source is a laser diode, preferably a wide stripe laser diode and the optical fibre is a multimode optical fibre.

6. An optoelectronic device according to any preceding claim, wherein the first and second planes are transverse to each other.

7. An optoelectronic device according to any preceding claim, wherein the first lens is a cylindrical lens having an axis parallel to the first plane and the second lens is a cylindrical lens having an axis parallel to the second plane.

8. An optoelectronic device according to any preceding claim, wherein the first and second lenses are aspheric cylindrical lenses comprising non-circular section geometric surfaces having spheric terms and/or any number of odd or even aspheric terms and/or elliptic terms.

9. An optoelectronic device according to any preceding claim, wherein the first and second lenses are circular-section rod lenses.

10. An optoelectronic device according to any preceding claim, wherein the first lens is a fast axis lens and the second lens is a slow axis lens.

1 1 . An optoelectronic device according to any preceding claim, wherein the first lens has a diameter less than the diameter of the second lens.

12. An optoelectronic device according to any preceding claim, wherein one of the first and second lenses comprises a holographic grating material.

13. An optoelectronic device according to any preceding claim, wherein one of the first and second lenses comprises a suppression coating.

14. A method of optically coupling an optical source to an optical fibre, the method comprising:

emitting a light beam of a first optical power from the source,