WO2005078492A1

WO2005078492A1 - Optical coupling system and a method for manufacturing it

Info

Publication number: WO2005078492A1
Application number: PCT/FI2005/000100
Authority: WO
Inventors: Arto Salokatve; Matt Rekow
Original assignee: Corelase Oy
Priority date: 2004-02-16
Filing date: 2005-02-15
Publication date: 2005-08-25
Also published as: EP1719007A1; FI20045041A0

Abstract

The invention relates to a novel two dimensional optical structures for combining the outputs of a plurality of light sources. These structures combine the output of multiple light sources through any one or combination of spatial, polarization, and wavelength multiplexing in a monolithic optical element. The construction of these elements is such that they can be mass produced utilizing traditional glass fiber fabrication techniques resulting in small per unit cost. Furthermore, these optical structures can be employed in the construction very efficient and inexpensive systems for collecting and concentrating the light output of laser diode bars and other light sources.

Description

Optical coupling system and a method for anufactoring it

Field of the Invention

The invention relates to an optical coupling structure comprising in the axis and direction of the light propagation:

(i) a laser producing a light beam with a higher vertical divergence and a lower horizontal divergence; (ii) a waveguide having a higher refractive index core with an input surface and an output surface for the light beam, and a lower refractive index cladding on at least two opposite sides of the core (iii) an axial optical fiber having a higher refractive index core with an input surface for the light beam and a surrounding lower refractive index cladding; and (iv) coupling means for coupling the light coming from the output surface of the waveguide core to the input surface of the optical fiber.

The invention also relates to an optical coupling system, comprising, in the axis and direction of the light propagation:

(i) a transverse horizontal laser bar, having a multiplicity of lasers producing a multiplicity of light beams with a higher vertical divergence and a lower horizontal divergence; (ii) a transverse horizontal waveguide assembly, comprising at least one waveguide having a higher refractive index core with an input surface and an output surface for the light beam, and a lower refractive index cladding on at least two opposite sides of the core; (iii) a transverse horizontal fiber array comprising a multiplicity of fibers having a higher refractive index core with an input surface for the light beam and a surrounding lower refractive index cladding; and (iv) coupling means for coupling the light coming from the output surface of the waveguide core to the input surface of the optical fiber. Above, the terms "horizontal" and "vertical" have merely been used to denote two essentially perpendicular directions with respect to the direction of light propagation.

Finally, the invention relates to a method for preparing a waveguide assembly, suitable for said optical coupling system.

Background of the Invention

Over the past 15 years there has been an explosion of activity in the practical application of lasers, in particular, high power laser diodes and diode arrays. While most of the activity has centered around lasers for communications, a significant portion of this activity is for the diode pumping of solid state lasers and material processing. In the early days of this field many patents were granted concerning techniques and systems for the concentrating of light from laser diode and laser diode bars. One of the earliest patents that teaches a system for coupling light to or from an array of optical components is US Patent number 4,079,404 by Comerford. In this patent an array of optical devices is coupled to an array of optical fibers with a cylindrical lens serving as a relay between the optical devices and optical fibers.

Since then many patents have appeared with many variations and improvements on the ideas expressed in Comerford's patent. An example is US 4,763,975 by Scifres. This patent teaches a system in which the fiber geometry is altered from the typical round shape in order to better preserve the optical "brightness" available in each of the diode emitters. While this idea represents an improvement in conservation of available brightness compared to usage of round fibers it is not particularly effective at conserving the full brightness that is available in the individual laser diodes. In order to match the brightness of the diode source in the horizontal and vertical axes exactly such a system requires that the waveguide tip be compressed to an impractically small dimension to match more closely the physical dimension of the laser diode vertical axis which is typically about 1 μm. In addition to requiring extremely tight tolerances, such a structure requires multiple process steps on the fiber and fiber arrays which add cost to the system. US 5,127,068 teaches a similar technique as Scifres with the key difference being the addition of collimating optic between the laser diode array and the array of fibers. In principle, this system overcomes some of the limitations of prior art, however, there are practical limitations to how small the collimating lens can be and remain effective at collimating an array of laser diodes over a 1 cm span. As the lens size decreases more and more elaborate steps must be taken to insure that the lens remains perfectly straight. Furthermore, in this arrangement any misalignment of the lens to the diode vertical axis results in changes in beam pointing that can cause the beam to partially miss the target aperture which is some distance away.

US 5,268,978 teaches a technique similar to the above patent. However, here the vertical axis lens has the purpose of imaging the laser diode facet rather than collimating it. Po states that ideally the vertical axis of the laser diode is imaged, not collimated, on the fiber input so that the vertical axis output NA is matched to the input NA of the fiber (0.2). This, however, suffers from the same issues in that any misalignment of the lens with respect to each emitter results in deviation of the light beam (or in this case the image also) from the target aperture of the optical fiber. In order to assure that a significant fraction of the light is not lost, the fiber input apertures must be significantly larger than the beam size or the tolerances must be maintained extremely tight. In the first case the sacrifice is lost brightness and in the second case in complicity of the device.

To realize the "brightest" embodiments of the optical coupling system thus far discussed, it is also necessary to find an uncomplicated process for producing high quality rectangular optical fiber. US 5,268,978 discloses one means to make such fiber using BAK 5 and BAK 2 glass. While such glass fibers can be fabricated, they tend to be much weaker, more fragile, and much less transparent than fibers fabricated from fused silica. This introduces manufacturing complications and limits the length that such fibers can have without incurring excessive loss.

A number of approaches to the challenge of conserving all of the brightness available in a laser diode bar have appeared in the literature and in the market place as products. As a rule these systems employ free space beam shaping and macro-optics to rearrange and redistribute the output of a laser diode bar such that it has a symmetric spot and divergence angle. Such systems have achieved quite good results with a resulting brightness which can be as close as a factor of 2 from the brightest possible spot. An example of such systems that have been commercially successful can be found in US 5,513,201. This system has the drawback of requiring multiple optical components which tend to form a large fraction of the overall cost of the system. The ideal system would not only optimize the brightness conservation of the sources but would also be as simple as possible.

While the technology of laser diodes and coupling of laser diodes to fibers has advanced tremendously in recent years, unfortunately such systems are still too complicated to enable many applications. A more simple and thus economical approach is needed.

US 6,341 ,189 discloses in Figure 7B a waveguide having a curved input surface. The surface is curved more in the vertical direction to collimate more the higher NA vertical beam dimension and curved less in the horizontal direction to collimate less the lower NA horizontal beam dimension. In Figure 3, tapered waveguides are disclosed which are coupled in series with one non-tapered waveguide and in parallel with another non-tapered waveguide leading to the optical fiber. The tapered waveguide of Figure 8 is segmented into a series of cut pyramids which are parallel coupled with a non-tapered waveguide leading to the optical fiber.

EP 1 391 764 A2 describes a system including a diode laser source producing a beam with high divergence (high NA output) in the vertical plane and low divergence (low NA input) in the horizontal plane. The vertically diverging light beam is collimated by a horizontally cylindπcal lens. See Figure 4. Then, the light propagates into a waveguide through a vertically cylindrical input surface. Thereafter, the light propagates through the waveguide, which is both horizontally (Figure 4) and vertically (Figure 3) tapered in the propagation direction. Finally, the light reaches the waveguide's output surface which is butt coupled to the input surface of an optical fiber.

US 6,574,262 discloses in Figure 2A the "slow" (horizontal) plane where essentially no divergence of the laser beam takes place, and in Figure 2B the "fast" (vertical) plane where a divergence of the laser beam takes place. According to column 6, the laser beam arrives at a fiber which has a cylindrical lens receiving the light which then propagates into a tapered core of the fiber and from there to a single-mode fiber-optical cable. The cylindrical lens has its axis in the "slow" plane and thereby focuses and/or collimates the diverging beam in the "fast" plane.

US 2002/0076157 discloses an optical fiber for coupling diverging light to a waveguide. The optical fiber has a core and a cladding. The input surface is concave and curved spherically inward in order to diverge, i.e. to widen the radiation angle of the light. The output surface is butt coupled to the waveguide.

When launching power from a laser diode into an optical fiber it is desirable to arrange the system in such a way as to preserve the brightness of the diode source as much as possible. Since an optical fiber typically has an equal NA in both the horizontal and vertical axes, the diode beam must also be modified so that its NA input is approximately equal in both axes and is less than or equal to that of the fiber. Typically the horizontal axis of a laser diode intrinsically has a divergence that is compatible with common optical fiber. Thus the vertical axis divergence must be modified by a suitable optical element. In the case of collimation, this requirement leads to very short focal length lenses since the beam NA input in the vertical axis must be reduced less than or equal to the fiber NA. In the case of imaging, the lens can have relatively long focal length but must have high NA and an aberration corrected surface figure. The drawback of the first solution is very short working distance and an impractically small lens in the order of 20 μm in diameter or less. The drawback of the second solution is uncertainty in the location of the image due to system misalignments and lost coupling efficiency since the receiving fiber becomes relatively far away from the diode.

The second requirement for conserving the brightness available in the beam from a laser diode upon launching it into an optical fiber is that the geometry of the beam spot be approximately equal to that of the receiving optical fiber. Typically for a broad area laser diode stripe this implies a substantially rectangular geometry from the fiber. Previous work has focused on making such fibers from low melting point glasses but they suffer from low strength and low transparency. Summary of the Invention

It is the aim of the current invention to provide an efficient and simple light coupling element as well as a technique for combining a fiber into an efficient and inexpensive optical system which will enable many applications that have to date been unfeasible due to the complexity and inefficiency of the existing systems. The invention is based on the idea that the input surface of a waveguide is designed so that it converges a light beam of a laser into the core of the waveguide.

As was said above, the invention relates to an optical coupling structure comprising in the axis and direction of the light propagation: (i) a laser producing a light beam with a higher vertical divergence and a lower horizontal divergence; (ii) a waveguide having a higher refractive index core with an input surface and an output surface for the light beam, and a lower refractive index cladding on at least two opposite sides of the core; (iii) an axial optical fiber having a higher refractive index core with an input surface for the light beam and a surrounding lower refractive index cladding; and (iv) coupling means for coupling the light coming from the output surface of the waveguide core to the input surface of the optical fiber.

The above mentioned problems relating to such structures have now been eliminated with a new optical structure, which is mainly characterized in that the input surface of the waveguide core is cylindrical in the horizontal direction and/or that said waveguide core is tapered only in the vertical plane.

Said input surface faces the incoming light beam. Thus, it is typically cylindrical in the transverse and horizontal direction with respect to the light propagation axis. As the waveguide core is open with respect to the propagating light, tapered typically means that the vertical cross-section of the core varies when going along the axis of the propagating light. According to one embodiment of the invention, the input surface of the waveguide core is cylindrical in the horizontal direction and the waveguide core is uniform in thickness. In another embodiment, the input surface of the waveguide core is cylindrical in the horizontal direction and the waveguide core is tapered down only in the vertical plane. In a third embodiment, the input surface of the waveguide core is flat, i.e. non-cylindrical, and the waveguide core is tapered up only in the vertical plane.

In conventional waveguides, the core is usually completely surrounded by a cladding. Now, it has been realized that no cladding is needed in the transversal low NA direction of the laser beam. The divergence in this direction is low, and no guiding reflection is needed from the wall of the waveguide. Thus, the horizontal and axial dimensions of the waveguide core can be dimensioned so that the waveguide structure allows direct access of essentially the whole of the beam's low horizontal divergence to the input surface of the optical fiber core.

In one embodiment of the invention, the waveguide core has a rectangular cross-section with a longer horizontal and shorter vertical dimension. Preferably, it is sandwiched between two horizontal layers of the waveguide cladding. When making an optical coupling, the input surface of the optical fiber core is usually equal to or slightly larger than the output surface of the waveguide core. Preferably, the input surface of the optical fiber core and the output surface of the waveguide core are both rectangular.

It can be difficult to align the waveguide output with the fiber input. This puts high demands on the means for coupling them together. The coupling means for coupling the light of the output surface of the waveguide core to the input surface of the optical fiber core conveniently comprises a substrate for the waveguide. The waveguide may be fused to this substrate. It is advantageous if the substrate comprises a guiding surface or the like for guiding the fiber to an alignment with the waveguide, thus giving an exact coupling. In one embodiment, this guiding surface is sloped upwards towards the waveguide in order to catch and align the fiber with the waveguide. As was initially stated, the claimed optical coupling system comprises, in the axis and direction of the light propagation:

(i) a transverse horizontal laser bar, having a multiplicity of lasers producing a light beam with a higher vertical divergence and a lower horizontal divergence; (ii) a transverse horizontal waveguide assembly, comprising at least one waveguide having a higher refractive index core with an input surface and an output surface for the light beam, and a lower refractive index cladding on at least two opposite sides of the core; (iii) a transverse horizontal fiber array comprising a multiplicity of fibers having a higher refractive index core with an input surface for the light beam and a surrounding lower refractive index cladding; and (iv) coupling means (4) for coupling the light coming from the output surface (1.4) of the waveguide core (1.1) to the input surface of the optical fiber (3).

According to the present inventive idea, the input surface of the waveguide core is cylindrical in the transverse horizontal direction and/or the waveguide core is tapered only (when moving) in the light direction in the vertical plane. This advantageously means a waveguide profile extending in the transverse horizontal direction (the low NA or "slow" direction) in front of several or, preferably, all of the laser bar lasers.

The waveguide assembly of the claimed optical coupling system may comprise a structural junction of the at least one waveguide and the coupling means. Thereby, the coupling means preferably comprises a substrate for the waveguide. Most preferably it is a transverse horizontal substrate rod with an upper surface on which the waveguide is fastened, readily by fusing.

According to one embodiment of the invention, the transverse and horizontal waveguide profile has essentially the same transverse horizontal breadth as the substrate rod. Most advantageously, the waveguide profile and the substrate rod both extend along essentially all of the length of the laser bar, thus forming a waveguide assembly having one broad waveguide for all lasers of the laser bar. This is a very simple and practical solution and differs essentially from prior known arrangements. The substrate rod preferably comprises an upper guiding surface for guiding the fibers into alignment with the waveguide, making said coupling exact. When going opposite to the light propagation direction, the guiding surface is typically sloped upwards towards the waveguide (1) in order to catch the fibers of the fiber array when coupling and align them with the waveguide.

The other end of the fibers of the fiber array may be grouped together to form a fiber bundle and the fiber bundle may be joined to a further fiber.

Finally the invention relates to a method for preparing a waveguide assembly, suitable for an optical coupling system comprising, in the direction of the light propagation:

(i) a transverse laser bar, having a multiplicity of lasers producing a horizontal light beam with a higher vertical divergence and a lower horizontal divergence; (ii) said waveguide assembly; (iii) a transverse fiber array comprising a multiplicity of fibers having a higher refractive index core with an input surface for the light beam and a surrounding lower refractive index cladding; and (iv) coupling means for coupling the light coming from the output surface of the waveguide core to the input surface of the optical fiber core,

by forming the waveguide assembly from at least one waveguide having a higher refractive index core with an input surface and an output surface for the light beam, and a lower refractive index cladding on at least two opposite sides of the core.

The waveguide assembly preparation of the invention is mainly characterized in that the waveguide is shaped as a waveguide profile, the core of which has as its input surface a cylindrical surface parallel to the profile's axis and/or a tapering only (when moving) in a plane perpendicular to the profile's axis, by making a scaled-up waveguide profile preform, and drawing the profile preform in the direction of its axis to reduce its dimensions while maintaining its shape and proportions. This is a radically simplified waveguide preparation method. A very long, multilaser waveguide profile may easily and accurately be drawn, optionally followed by cutting into waveguides having the suitable dimensions for the desired number of lasers to be guided.

Preferably, the waveguide assembly is formed by joining structurally the at least one waveguide and the coupling means. Most the waveguide assembly is prepared by forming said scaled up waveguide profile perform, see above, forming a rod substrate preform having an upper surface, which rod substrate is said coupling means, e.g. guiding the fibers towards the waveguide, aligning the waveguide profile preform with the rod substrate preform and fusing it to said upper surface to form a waveguide assembly preform, and drawing the waveguide assembly preform in the direction of the rod substrate axis to reduce its dimensions while maintaining its shape and proportions.

The method provides significant advantages with respect to prior art. The optical structure of the present invention provides more efficient coupling of light from a laser to a fiber. The assembling of the fiber to the optical structure is also made easier by the present invention and therefore also allows more efficient coupling of light from the optical structure to the fiber. The optical structure according to the present invention is as simple, if not more so to manufacture than optical structures of prior art. The tolerances of the dimensions of the optical structure according to the present invention are not as critical as in prior art structures. Other advantages become evident on the basis of the following detailed description and claims. The substrate according to the present invention forms a rigid support structure that maintains the straightness of the waveguide.

Description of the Drawings

Fig. 1 depicts a light collection element according to an embodiment of the present invention,

Fig. 2 depicts a system comprising a light source, and an optical structure according to an embodiment of the present invention, Fig. 3 depicts an optical structure according to an example embodiment of the present invention compπsing a light collection element and a substrate,

Fig. 4 demonstrates the minimum required dimensions of an optical structure according to the present invention for achieving diffraction limited performance for one embodiment,

Fig. 5 depicts another system comprising a light source, an optical structure, the guidance surface of the substrate according to an embodiment of the present invention and a fiber,

Fig. 6a depicts general outlines of an optical structure according to another example embodiment of the present invention comprising tapered claddings,

Fig. 6b depicts general outlines of an optical structure according to yet another example embodiment of the present invention comprising tapered claddings and substantially flat input surface,

Fig. 7a depicts minimum calculated taper lengths for adiabatic tapering of waveguides for a given output aperture size,

Fig. 7b depicts one method to confine the light in the slow axis so that the taper can be lengthened,

Fig. 8 depicts an example embodiment of a system according to the present invention.

Fig. 9 depicts a system comprising means to divide the output of a laser diode into two beams,

Fig. 10 depicts a system comprising means to combine two beams or two arrays of beams into a single co-propagating array of beams, and

Fig. 11 depicts another embodiment of the optical structure of Fig. 10. Detailed Description of the Invention

In Fig. 1 there is depicted a light collection element according to an embodiment of the present invention. The key features of this element are a one-dimensional waveguide 1 consisting of an high index glass core 1.1 bounded on two sides by a cladding material 1.2 of lower index and surface of cylindrical surface figure acting as a lens. One edge 1.3 of the waveguide forms the input surface and the other edge 1.4 of the waveguide forms the output surface. The input surface1.3 can in principle have any desired surface figure but circular is preferred for simplicity of manufacture. The output surface 1.4 can also in principle have any surface figure, however, for simplicity of manufacture it is preferable flat, angled, or circular.

As mentioned, in order to effectively conserve the brightness available in the laser diode 2 (Fig. 2) it is necessary to collimate or focus (or image) the vertical axis of the laser diode beam 2.1 to a size such that the diffraction limited divergence of the beam 2.1 in the vertical axis is about equal to that of the slow axis. In both the collimated and focused cases there are resulting pointing and positional errors in the beam at the location of the receiving fibers 3. These errors require that the receiving aperture be larger than the ideal aperture in proportion to the error level that the system is designed to tolerate. In order to eliminate this limitation the system according to the current invention is constructed so that the waveguide 1 is placed as close as is possible to the focusing element by combining the two as shown in Figure 1. Immediately upon passing the focusing surface, the beam is already confined within the waveguide 1. The receiving fibers 3 can then be simply butt coupled to the waveguide 1 at a convenient distance. With this arrangement the positional tolerance of the fiber is greatly relaxed and the receiving fiber 3 need only be a few microns larger than the waveguide 1 to tolerate the manufacturing tolerances on the waveguide 1 and the fiber 3.

Another issue that arises when the effective focal length of the light collecting element is reduced down such that the vertical axis divergence is comparable to the horizontal axis divergence is that the element becomes so thin that it is difficult if not impossible to handle. For example, a rod lens made from a fiber would have to have a diameter between 20 and 30 μm to give the proper beam divergence. To use such a lens on a laser diode bar, it must be fixtured and maintained straight with submicron accuracy over a distance of 1 cm to successfully collimate or image the output of a laser diode bar. The current invention aims to overcome this limitation by creating the waveguide 1 and focusing surface 1.3 on a larger supporting substrate 4. This substrate 4, waveguide 1 and focusing surface 1.3 are manufactured in such a way that they intrinsically meet the required straightness even when the focal length of the focusing surface is as small as in the previous example of the rod lens. Furthermore during manufacturing, the substrate 4, waveguide 1 , and focusing surface 1.3 are fused together and effectively form one solid piece of glass. This final glass assembly 5 has dimensions sufficiently large so that it becomes a self supporting rigid member and is intrinsically straight to the sub micron level. It is also stiff enough to resist forces that would tend to deform it. One configuration of this assembly 5 is shown in Figure 3. One method of manufacture of the described structure is to create a preform for the waveguide of much larger scale than the final desired size and then to use standard fiber optic drawing techniques to reduce its dimension while maintaining the shape of the structure. The waveguide structure can then be fused to said mounting structure. Such manufacturing techniques can result in a per-component cost of less than 1 euro.

In prior art systems, positional errors for the fast axis collimating or focusing lens translate into position errors at the fiber input aperture as well as increases in the overall NA (numerical aperture) of the fast axis beams being launched into the fibers. In the present invention these positioning errors are eliminated since the light is confined within the waveguide immediately upon passing the lens. Hence lost coupling efficiency due to fast axis lens positioning error or laser diode bar smile is effectively eliminated. While the NA of the fast axis beams still suffers from pointing error, the fiber NA is typically much larger than the fast axis launch NA of the beams after passing the collimating or focusing optic, hence positional errors translate into an increase in the launch NA of the fast axis rather than coupling efficiency loss. In this way alignment tolerance is greatly improved.

Another problem with such a small lens is that the working distance goes nearly to zero and it becomes in practice impossible to position the lens in the proper position. For example, in the practical manufacturing of laser diode bars 6 there may be protrusions of the solder material that interfere with the positioning of the lens or the bar 6 may be recessed a few microns from the edge of the mount preventing the lens from being brought into position. The current invention overcomes this limitation by fabricating only one cylindrical focusing surface 1.3 and orienting it toward the laser diode source 2 as shown in Figure 2. This configuration yields the maximum working distance possible for a lens of a given focal length used in the collimating configuration. In this configuration , sufficiently high index material is used in the waveguide so that the NA of the light collecting surface 1.3 is high enough to collect the light 2.1 in the diode vertical axis. Generally an index of refraction for the waveguide material of 1.8 is sufficient for most commercial laser diodes.

One will immediately recognize that the configuration of the lens of Figure 2 is the worst configuration that one can have for aberrations when focusing or collimating a diode beam 2.1 in the vertical axis and, not coincidently, there is much prior art centred upon making small lenses with well corrected surface figures. However, one is also aware that a lens surface is regarded as diffraction limited as long as a wave front experiences no more than effectively one wave of aberration from the ideal surface figure. In the current invention this aberration is controlled by keeping the dimension of the collimating waveguide small enough so that a simple cylindrical profile on the surface of the waveguide delivers nearly diffraction limited focusing performance. Figure 4 demonstrates the minimum required dimension for achieving diffraction limited performance. The vertical axis is the deviation in wavelengths at 915nm of the sag of a cylindrical surface (15 μm radius, glass index 1.86) compared to a diffraction limited surface of equal focal length. The horizontal axis represents the distance from the optical axis. In this way the fabrication of the waveguide 1 and focusing surface 1.3 is greatly simplified in that one can begin with inexpensive commercially available glass rods without any need to shape the surface figure.

In order to simplify the alignment of the fibers 3 to the waveguide structure one can design into the waveguide support structure 5 features that guide the receiving fibers 3 into the proper position. Figure 5 illustrates one such variation of the support structure 5. In that embodiment the substrate 4 comprises a guiding surface 4.1 which is designed so that when installing the fiber 3 or a fiber array 8 on the substrate the guiding surface 4.1 affects the fiber 3 to move in a direction substantially parallel to the output surface 1.4 of the waveguide 1. Thus the fiber 3 can be properly aligned with respect to the output surface 1.4 of the waveguide 1 by moving the fiber 3 towards the waveguide 1 or farther away from the waveguide 1. The only tolerances that impact alignment of the receiving fibers 3 then are the tolerances of fabrication of the waveguide structure 5 and the receiving fibers 3 themselves. In practice these tolerances are expressed as a percentage of total dimension of the parts and 5 % is typical. For example, if the nominal vertical dimension of the receiving fiber 3 is 20 μm then we could expect that the minimum manufactured size would be 19 μm. One then need only to design the receiving fiber aperture to be a few microns larger than nominal to allow for the manufacturing and alignment tolerances as opposed to prior art systems in which tens of microns of allowance must be made for manufacturing and alignment tolerances. In this way the brightness and light coupling efficiency of the final system is better maintained by the current invention.

While the current invention as described gives adequate working distance in most cases, it could be useful to design a system that gives more working distance. To achieve this it is possible to arrange the claddings 1.2 of the waveguide 1 such that they form a tapered waveguide as in Figure 6a. This allows then that the radius of the input waveguide surface could be made much larger and a smaller output beam size can be achieved by tapering. Of course, to realize the optimal system performance the taper must be carefully designed so that it is adiabatic, that is, that it does not increase the divergence of the beam beyond the diffraction limit at any point along the taper.

Fig. 6b depicts another embodiment of the tapered waveguide. In this embodiment the tapering is formed so that the core 1.1 grows in the direction of laser beam propagation inside the waveguide i.e. at the input

1.3 of the waveguide the height of the core 1.1 is smaller than at the output

1.4 of the waveguide. This "upward" taper effectively collimates the diode beams in the fast axis. The input surface can be substantially flat in this embodiment because the tapered structure effectively collimates the beams eliminating the need for an optical focusing surface at the input 1 ,3.

Fig. 7a depicts examples of minimum calculated taper lengths for adiabatic tapering of waveguides for a given output aperture size. The curve 7.A demonstrates an example in which the output aperture size of the waveguide 1 is about 5 μm. The curve 7.B demonstrates an example in which the output aperture size of the waveguide 1 is about 10 μm. The curve 7.C demonstrates an example in which the output aperture size of the waveguide 1 is about 20 μm. Since the beam(s) are diverging in the slow axis, there are practical limits to how long the taper can be without resulting in excess loss. One method to confine the light in the slow axis so that the taper can be lengthened is to create a structure such as that shown in figure 7b. In the structure of Fig. 7b the tapered waveguides have been cut substantially perpendicular to the substrate so that total internal reflection confines that slow axis light to the required dimension over the length of the taper.

Example Embodiment

In an example embodiment of the current invention the laser diode 2 consists of a submount 10 (Fig. 8), either actively or passively cooled, with a diode laser bar 6 mounted at the front edge. The number of emitters in the diode laser array 6 and the fill factor is arbitrary. The fiber array 8 consists of a number of fused silica rectangular fibers 3 that corresponds to the number of emitters in the laser diode bar 6. The long dimension of the fiber 3 is chosen to be approximately equal to the horizontal dimension of each laser diode 2 in the laser bar and the vertical dimension of the fiber 3 is chosen such that the diffraction limited divergence for that aperture size is less than the NA of the fiber 3, in this case between 5 and 20 μm. The fast axis collimating structure 1 consists of two guiding glass layers and an integral mounting structure. In this example embodiment the glass waveguide layers 1.2 in the fast axis collimating structure 1 are not tapered. Furthermore the fibers are bundled together at the distal end of the array to create a single output bundle for the purposes of launching the laser light into another optical fiber or delivering it directly to a work surface.

The fibers 3 are passively aligned to the top surface of the fast axis collimating structure support structure 5 and that assembly is then aligned to the laser diode bar 6 to maximize the power through the fibers 3 while the laser diode 2 is operating. The fibers 3 are at the other end grouped together to form a fiber bundle 9. The fiber bundle 9 can then be joined e.g. by welding to another fiber (not shown) having a core the dimension of which is preferably greater or equal to the total area of the cross section of the fiber bundle 9. The another fiber can then be used for directing the light beams collected from the laser bar 6 by the fiber array 8 to a target, for example, for pumping yet another fiber, for grooving a figure to a surface of the target, for welding, etc. The structure 5 according to the present invention can improve the efficiency of the system and it can collect more power from the laser bar 6 than structures of prior art.

In a practical laser system there is often the need to sample a fraction of the laser beam for monitoring the beam power or other properties. In practice there are many ways of monitoring the power of a beam however in a fiber coupled laser diode system it can be problematic to directly monitor the diode power output in a way that is stable over time and with environmental conditions. One way of achieving a stable beam monitor is to sample a portion of the beam(s) just after they leave the laser diode. This can be accomplished by the structure of Figure 9. In Figure 9 the composite structure of Figure 2 is modified such that the waveguide consists of two separate glasses of index N1 and N2. The glass sections are then arranged so that their interface forms a partial mirror surface at an angle of about 45 degrees so that a known fraction of the light passing the interface is redirected perpendicular the primary axis of propagation. The indices N1 and N2 can be chosen to achieve a particular fractional sampling of the beam(s).

In another embodiment of the idea in Figure 9, the angled surface can be reversed and a second beam can be injected to propagate collinearly with the primary diode beam. In this configuration of course the primary beam is attenuated by the same proportion that the second beam is injected.

A configuration that makes this concept more useful would be to separate the glass components N1 and N2 of Figure 9 and apply dielectric coatings to surfaces S1 and S2 as shown in Figure 10. In this way the system can be made more versatile. For example a coating could be applied to each of the 45 degree surfaces so that the primary beam is passed with very little loss and the injected beam can be injected into the primary beam with near 100% efficiency. In a preferred embodiment of the system of Figure 10 the surfaces S1 and S2 are coated with dichroic coatings such that the primary beam is passed and the injected light beam of another wavelength is redirected down the primary optical axis. In this configuration two laser diode bars of different wavelength can be combined into the same fiber. Alternatively the injected light beam can be the output of any light emitting device and the injected beam can be utilized as an aiming beam to more easily direct the output of the primary beam to a target surface. In yet another embodiment of the system in Figure 10 the dielectric coatings on surfaces S1 and S2 can be configured such that for a given wavelength they are high reflectors of the "S" polarization and anti-reflective for the "P" polarization. In this way at a fixed wavelength two high power laser diode arrays one "S" polarized and one "P" polarized can be combined to propagate down the same optical axis. Thus the effective power output of such a system at a given wavelength can be increased by a factor of two. The input surface(s) of the entrance aperture S3 for the another light beam(s) may also be formed as a light converging surface.

The system of Figure 10 can be realized in a preferred embodiment where the first component N1 is manufactured in a similar manner to the embodiment in Figure 2 and the second component N2 is formed by approximately cutting a 45 degree angle on the end of the receiving fibers 3 as shown in Figure 11. Surfaces S1 and S2 are then coated to realize a dichroic beam combiner or a polarization beam combiner for the purposes of injecting light from a second light source into the optical fibers. One skilled in the art will recognize that said dichroic and said polarization beam combiners can be cascaded to create a system to combine several beams from several laser diode arrays into a single copropagating beam or array of beams for eventual fiber coupling. In addition AR coatings can be applied to other surfaces to reduce injection loss. Figure 11 depicts this alternate embodiment. In this embodiment the second laser diode bar would also require the integral waveguide and collimating structure to bring the light from the second laser diode bar to the entrance aperture, S3, of the combiner.

It is also possible to use the substrate 4 for an optical structure which comprises a fiber(s) and an optical element, focusing or otherwise. The guiding surface 4.1 of the substrate can then be used to guide the fiber to a proper alignment with respect to said optical element.

It is obvious that the present invention is not solely limited to the above described embodiments but it can be modified within the scope of the appended claims.

Claims

Claims:

1. An optical coupling structure (5) comprising in the axis and direction of the light propagation:

(i) a laser (2) producing a light beam with a higher vertical divergence and a lower horizontal divergence, (ii) a waveguide (1) having a higher refractive index core (1.1) with an input surface (1.3) and an output surface (1.4) for the light beam, and a lower refractive index cladding (1.2) on at least two opposite sides of the core (1.1), (iii) an axial optical fiber (3) having a higher refractive index core (3.1) with an input surface (3.3) for the light beam and a surrounding lower refractive index cladding (3.2), and (iv) coupling means (4) for coupling the light coming from the output surface (1.4) of the waveguide core (1.1) to the input surface of the optical fiber (3), characterized in that the input surface (1.3) of the waveguide core (1.1) is cylindrical in the horizontal direction and/or that the waveguide core (1.1) is tapered only in the vertical plane.

2. The optical coupling structure (5) according to claim , characterized in that the input surface (1.3) of the waveguide core (1.1) is cylindrical in the horizontal direction and the waveguide core (1.1) is uniform in thickness.

3. The optical coupling structure (5) according to claim 1 , characterized in that the input surface (1.3) of the waveguide core (1.1) is cylindrical in the horizontal direction and the waveguide core (1.1 ) is tapered down only in the vertical plane.

4. The optical coupling structure (5) according to claim 1 , characterized in that the input surface (1.3) of the waveguide core (1.1) is flat and the waveguide core (1.1 ) is tapered up only in the vertical plane.

5. The optical coupling structure (5) according to any one of the preceding claims, characterized in that the horizontal and axial dimensions of the waveguide core (1.1) are such as to allow direct access of essentially the whole of the beam's low horizontal divergence to the input surface (3.3) of the optical fiber core (3.1).

6. The optical coupling structure (5) according to any one of the preceding claims, characterized in that the waveguide core (1.1) has a rectangular cross-section with a longer horizontal and shorter vertical dimension and preferably is sandwiched between two horizontal layers of the waveguide cladding (1.2).

7. The optical coupling structure (5) according to any one of the preceding claims, characterized in that the input surface (3.3) of the optical fiber core (3.1 ) is essentially equal to or slightly larger than the output surface (1.4) of the waveguide core (1.1).

8. The optical coupling structure (5) according to any one of the preceding claims, characterized in that the input surface (3.3) of the optical fiber core (3.1) and the output surface (1.4) of the waveguide core (1.1 ) are both flat and rectangular.

9. The optical coupling structure (5) according to any one of the preceding claims, characterized in that the coupling means (4) for coupling the light of the output surface (1.4) of the waveguide core (1.1 ) to the input surface (3.3) of the optical fiber core (3.1) comprises a substrate (4) for the waveguide (1).

10. The optical coupling structure (5) according to claim 9, characterized in that the waveguide (1) is fused to the substrate (4).

11. The optical coupling structure according to claim 9 or 10, characterized in that the substrate (4) comprises a guiding surface (4.1 ) for guiding the fiber (3) to an alignment with the waveguide (1 ) in which said coupling is essentially exact.

12. The optical coupling structure according to claim 1 , characterized in that the guiding surface (4.1) is sloped upwards towards the waveguide (1) in order to catch and align the fiber (3) with the waveguide (1).

13. An optical coupling system, comprising, in the axis and direction of the light propagation:

(i) a transverse horizontal laser bar (6), having a multiplicity of lasers (2) producing a light beam with a higher vertical divergence and a lower horizontal divergence; (ii) a transverse horizontal waveguide assembly (7), comprising at least one waveguide (1) having a higher refractive index core (1.1) with an input surface (1.3) and an output surface (1.4) for the light beam, and a lower refractive index cladding (1.2) on at least two opposite sides of the core (1 .1); (iii) a transverse horizontal fiber array (8) comprising a multiplicity of fibers (3) having a higher refractive index core (3.1) with an input surface (3.3) for the light beam and a surrounding lower refractive index cladding (3.2); and (iv) coupling means (4) for coupling the light coming from the output surface (1.4) of the waveguide core (1.1 ) to the input surface of the optical fiber (3), characterized in that the input surface (1.3) of the waveguide core (1.1 ) is cylindrical in the transverse horizontal direction and/or that the waveguide core (1.1) is tapered only in the vertical plane.

14. An optical coupling system according to claim 13, characterized in that said at least one waveguide (1 ) is a waveguide profile extending in the transverse horizontal direction in front of several or, preferably, all of the laser bar (6) lasers (2).

15. An optical coupling system according to claim 13 or 14, characterized in that the waveguide assembly (7) comprises a structural junction of the at least one waveguide (1) and the coupling means (4).

16. An optical coupling system according to claim 15, characterized in that the coupling means (4) comprises a substrate (4) for the at least one waveguide (1) which preferably is a transverse horizontal substrate rod (4) with an upper surface on which the at least one waveguide (1) is fastened.

17. An optical coupling system according to claim 16, characterized in that the at least one waveguide (1) is fastened on the substrate rod (4) by fusing.

18. An optical coupling system according to any one of claims 14 to 17, characterized in that the at least one waveguide (1) is said one transverse horizontal waveguide profile (1 ) having essentially the same longitudinal dimension as the substrate rod (4).

19. An optical coupling system according to claim 16, 17 or 18, characterized in that the waveguide profile (1) and the substrate rod (4) both extend along essentially all of the length of the laser bar (6), thus forming a waveguide assembly (7) having one broad waveguide (1 ) for all lasers (2) of the laser bar (6).

20. The optical coupling system according to any one of claims 16 to 19, characterized in that the substrate rod (4) comprises an upper guiding surface (4.1 ) for guiding the fibers (3) into alignment with the waveguide (1 ), making in said coupling essentially exact.

21. The optical coupling system according to claim 20 characterized in that the guiding surface (4.1) is sloped upwards towards the waveguide (1) in order to catch and align the fibers (3) of the fiber array with the waveguide (1 ).

22. The optical coupling system according to any one of claims 13 to 20, characterized in that the other end of the fibers (3) of the fiber array (8) are grouped together to form a fiber bundle (9).

23. The optical coupling system according to claim 22, characterized in that the fiber bundle (9) is joined to a further fiber.

24. A method for preparing a waveguide assembly (7), suitable for an optical coupling system comprising, in the order of light propagation :

(i) a transverse horizontal laser bar (6), having a multiplicity of lasers (2) producing a horizontal light beam with a higher vertical divergence and a lower horizontal divergence; (ii) said waveguide assembly (7); (iii) a transverse horizontal fiber array (8) comprising a multiplicity of fibers (3) having a higher refractive index core (3.1) with an input surface (3.3) for the light beam and a surrounding lower refractive index cladding (3.2); and (iv) coupling means (4) for coupling the light coming from the output surface (1.4) of the waveguide core (1.1 ) to the input surface (3.3) of the optical fiber core (3.1 ), by forming the waveguide assembly (7) from at least one waveguide (1 ) having a higher refractive index core (1.1) with an input surface (1.3) and an output surface (1.4) for the light beam, and a lower refractive index cladding (1.2) on at least two opposite sides of the core (1.1), characterized in that the waveguide is shaped as a waveguide profile (1 ), the core (1.1 ) of which has as the input surface (1.3) a cylindrical surface parallel to the profile's axis and/or a tapering only in a plane perpendicular to the profile's axis, by making a scaled up waveguide profile preform, and drawing the profile preform in the direction of its axis to reduce its dimensions while maintaining its shape and proportions.

25. A method according to claim 24, characterized in that the waveguide assembly (7) is formed by joining structurally the at least one waveguide (1) and the coupling means (4).

26. A method according to claim 25, characterized in that the waveguide assembly (7) is made by forming said scaled up waveguide profile perform, forming a rod substrate preform having an upper surface, which rod substrate is said coupling means (4), aligning the waveguide profile preform with the rod substrate preform and fusing it to the upper surface to form a waveguide assembly preform, and drawing the waveguide assembly preform in the direction of the rod substrate axis to reduce its dimensions while maintaining its shape and proportions.