Patent Description:
High efficiency, high power levels, and high spectral and directional brightness are attractive characteristics of pigtailed diode laser modules used in in many areas, such as material processing, offset printing, medical treatment, pumping of solid state lasers. Improving all of these characteristics is important practically for all applications. It is particularly critical for laser diode pumped fiber lasers. Although the fiber laser powers continuously increase, high power fiber lasers still underperform, at least partially, due to the coupling losses of pump light. The disclosed coupling arrangement decreases the losses to about <NUM> - <NUM>%. Such a decrease is significant considering that even the loss of a fraction of percent is considered a major success.

A typical prior-art high-power multi-emitter multimode-fiber-coupled diode laser module <NUM> is illustrated in <FIG> and disclosed, for example, in <CIT> to Ovtchinnikov et al. ("the '<NUM> patent"). At the most basic level, diode laser emitters or chips <NUM> are supported by respective mounts <NUM> and output astigmatic beams <NUM> along a light path. In <FIG>, each individual chip <NUM> in the array is stacked one upon another. Various optics <NUM>, <NUM>, <NUM> and <NUM> collimate and shape the beam <NUM> of each emitter such that all light beams <NUM> are combined into a single astigmatic combined beam <NUM>. The combined beam <NUM> is guided towards a fiber <NUM>, which has a receiving core end located in the focal plane F - F of objective lens <NUM>, and focused on the core end.

Each broad-area MM chip <NUM> emits a non-circular beam <NUM> in the first direction. Due to a thin-slab geometry of diode lasers, their radiation, propagating along Z-axis, has a highly asymmetric lateral distribution of optical power density and divergence along X- and Y-axes. Each beam <NUM> is broad in its slow-axis and narrow in its fast-axis. Accordingly, the shown schematic has, as a rule, fast-axis collimator (FAC) <NUM> and slow-axis collimator (SAC) <NUM> making beam <NUM> parallel in both fast and slow axes. Multiple beams <NUM> are further combined by a set of mirrors <NUM> in combined beam <NUM> in which multiple beams <NUM> propagate in a second direction parallel to one another in the vertical plane.

As a result, combined beam <NUM> collimated in both axes is incident on and filling a region of objective lens <NUM> such that beam spot <NUM> is coupled into core end <NUM> of fiber <NUM> located in focal plane F - F of objective lens (OL) <NUM>. The '<NUM> patent teaches using as large a beam spot <NUM> as possible. As a result, the divergence of the beam in the near field is minimally possible, and the brightness of the beam, illuminating output fiber <NUM>, is relatively good.

However, the above holds true only to a point-like light source. The chip <NUM> has multiple points emitting respective rays. Thus, in contrast to the point-like source, the chip is rather elongated and further referred to as an extended light source or chip. The beam <NUM> from the extended light source is not ideally collimated at least in the slow axis. As a consequence, when such a nonparallel beam is focused in the slow axis in focal plane F - F by objective lens, OL, <NUM>, its beam spot may be excessively large for lossless or near lossless coupling into the fiber's core end, as explained below.

<FIG> illustrates a ray diagram for individual extended light source <NUM>. The chip <NUM> is located in a focal plane FP18 of SAC <NUM> having a relatively short focal length, such as less than <NUM>. If light was emitted from a point light source, it would be collimated in the slow axis (SA) by SAC <NUM> and propagate as an ideal parallel beam <NUM>, shown in dashed lines, over a distance to OL <NUM>. As a result, the point light source would have both a sharp image in focal plane F - F of OL <NUM> and a minimal beam spot or waist <NUM> formed in focal plane F - F. The focal plane is determined as <MAT>, wherein f2 and f1 are respective focal lengths of lenses <NUM> and <NUM> and △ is the size of the extended source. The diode laser <NUM> however has an array of multiple light emitting points causing the single beam <NUM> to diverge at an angle <MAT>, wherein Θ = Δ/<NUM>f<NUM>, beginning approximately from a rear focus of OL <NUM>. As a distance between lenses <NUM> and <NUM> increases, the beam progressively expands in the slow axis and finally impinges upon objective lens <NUM>, as shown in solid lines. As a consequence, waist <NUM>' of the beam in focal plane FP22 is considerably larger than the smallest beam spot <NUM> of the ideally collimated beam. The same logic should be applied to combined beam <NUM> which includes multiple beams <NUM> diverging in the slow axis and emitted by respective chips <NUM> of module <NUM> of <FIG>. Of course, for beam <NUM> to appreciably diverge, the distance between SAC <NUM> and OL <NUM> should be significant.

<FIG> discussed in conjunction with <FIG> illustrate slow axis (SA) OL <NUM> displaced off SAC <NUM> at respective first and second distances, with the first distance (<FIG>) being shorter than that the second one (<FIG>. ) In <FIG> like in <FIG>, rays R1 - R3 (or spatial modes) from respective three light emitting spots of extended diode laser <NUM> are substantially parallel to one another upon impinging SAOL <NUM> since a distance between SAOL <NUM> and the SAC is small. The OL <NUM> focuses the incident beams in its focal plane F - F in which respective waists (cross-sections) are the smallest. As a consequence, the waist of the combined focused beam is such that the focused beam is coupled into core end without substantial losses, if at all. Thus, the image of the extended light source is the sharpest in focal plane F - F with the waist of the combined beam being the smallest in the same plane.

In contrast, <FIG> illustrates a configuration in which the distance between the SAC and SAOL <NUM> is long enough for the same three - red, blue and green - rays or spatial modes to significantly diverge and impinge a large area of SAOL <NUM>. While the image of the extended source is still the sharpest in the focal plane F - F, the spatial modes however continue to converge beyond focal plane F - F. As a consequence, the smallest cross-section of the combined beam is formed beyond focal plane F - F at a distance D. Mounting an output fiber with its core end in the focal plane F - F results in the loss of light since the core diameter is smaller than the cross-section of the focused beam of <FIG> in the focal plane. The light power loss results in poor throughput and overheated module components and damaged output fiber.

A need therefore exists for an improved configuration of pigtailed MEMM diode laser module.

A further need exists for a method of manufacturing the disclosed MEMM diode laser module.

<CIT> describes an arrangement of laser diode emitters which emit broad-area light beams in a beam direction. In cross-section, each beam is broad in its slow axis and narrow in its fast axis. Groups of downstream optical components collimate, shape, stack and direct the beams along a light path towards a beam spot (which may be fiber-coupled). After collimating, stacking and directing, the beams are Fourier transformed in the fast-axis through a lens feature having a fast-axis focal length less than about <NUM> millimeters. In some embodiments, the fast-axis focal length is between about <NUM> and <NUM> millimeters. Astigmatism may be introduced between the fast axis and the slow axis in the beams upstream of the lens feature and in accordance with the fast axis focal length of the lens feature.

<CIT> discloses a semiconductor laser module including a semiconductor laser element emitting a laser light; an optical fiber, into which the laser light emitted from the semiconductor laser element is incident, guiding the laser light; and an optical-fiber-holding unit having a fixing agent and holding the optical fiber, the fixing agent being for fixing the optical fiber. The fixing agent is provided at an area in which a power of a leakage light of the laser light having been incident into the optical fiber and then emitted to outside the optical fiber is low.

The disclosed MEMM pigtailed diode laser module and method of its manufacturing differ from the known prior art by mounting a slow axis objective lens (SAOL) such that the receiving end of the output fiber is spaced from the lens at a distance exceeding the focal length of SAOL, i.e., beyond the lens's focal plane. This seemingly a counterintuitive configuration would be perfectly logical considering that the disclosure is not concerned with the image quality, which is the highest in the focal plane, but with the collection of light, i.e., brightness. In the disclosed configuration, multiple extended light sources, such as diode lasers, are located in the focal plane of respective SACs which are spaced at a distance from the SAOL sufficient for a combined beam to significantly diverge. To prevent clipping of the focused beam by the fiber's core, which is smaller than the cross-section of the focused beam in the focal plane of the SAOL, the fiber is located beyond the focal plane. The distance between the SAOL and fiber core is increased such that a cross section of the focused beam is small enough to provide substantially lossless coupling of light into the core.

In accordance to a first aspect, the invention provides a pigtailed diode laser module in accordance with claim <NUM>. According to a second aspect, the invention provides a method of manufacturing a pigtailed diode laser module in accordance with claim <NUM>. Further aspects of the invention are set forth in the dependent claims, the drawings and the following description of embodiments. The pigtailed laser module is configured with a case housing at least one row of MM diode lasers which emit respective parallel beams in a first direction. Each beam is collimated in fast and slow axes by a pair of respective FACs and SACs with the SACs being spaced downstream from respective FACs in the first direction. The disclosed module further includes multiple beam reflectors or mirrors guiding respective collimated beams, which constitute a combined beam, in a second direction, wherein the first and second directions are transverse to one another. At least one SAOL is located downstream from the last downstream reflector and operative to focus the combined beam at least in the slow axis in its focal plane. The module further has a fiber the upstream end of which is aligned with the SAOL in the second direction. The upstream end of the fiber is mounted in a plane in which the combined beam has the smallest cross-section. This plane is located beyond the focal plane.

Due to different distances of respective light sources from the SAOL, smallest cross-sections of respective beam components in the combined beam are located at different distances beyond the focal plane. The diode laser nearest to the SAOL outputs a beam component having a minimal cross-section at the shortest distance beyond the focal plane. The minimal cross-section of the beam component emitted by the diode laser farthest from the SAOL in the upstream direction is spaced downstream from the focal plane at a distance greater than that of the nearest diode laser.

Accordingly, the disclosed method further comprises a step of determining minimal cross-sections of respective beams of the nearest and farthest diode lasers downstream from the SAOL, and then determining a distance between them. Finally, the disclosed method comprises the step of displacing the SAOL upstream from its original location at the determined distance to provide substantially lossless coupling of the combined beam into the core end.

The above and other aspects and features will become more readily apparent from the following drawings, in which:.

Referring to <FIG>, a laser module <NUM> includes a plurality of spaced diode lasers or chips <NUM> (<NUM><NUM>. <NUM>n) outputting respective parallel beams <NUM> in the first direction. The chips <NUM> are each associated with a beam-shaping optic including FAC <NUM>, SAC <NUM> and mirror <NUM>.

Each chip <NUM> is aligned with designated FAC <NUM>, SAC <NUM> and mirror <NUM> in the first direction and together these components constitute a group <NUM> (<FIG>). The beams <NUM> each are collimated first in the fast axis by FAC <NUM> and then in the slow axis by SAC <NUM>. In the fast axis, beam <NUM> is SM while in the slow axis this beam includes multiple spatial modes (MM).

The beams <NUM> are further redirected by respective mirrors <NUM> in a second direction, which is transverse to the first direction, and form a combined beam <NUM>. The groups <NUM> are enclosed in case <NUM> having a bottom <NUM> which is made of heat-dissipating material and have respective chips <NUM> each coupled to mount <NUM> also made from heat dissipating material. The groups <NUM> (<FIG>) may be mounted on a common mount <NUM> or on respective individual mounts <NUM> (<FIG>) which are in contact with bottom <NUM>. The architectures of module <NUM> illustrated in respective <FIG> are well known from USPs <NUM>,<NUM>,<NUM> and <NUM>,<NUM>,<NUM>.

Specifically, <FIG> illustrates chips <NUM><NUM> - 12n mounted in a row. Typically, module <NUM> is configured with two rows of chips <NUM> of <FIG> which are mounted on respective opposite sides of combined beam <NUM> such that groups <NUM> of one row are not aligned with respective groups <NUM> of the other row in the first direction. The output fiber <NUM> is mounted in a ferrule (not shown) and aligned with fast axis objective lens <NUM> (FAOL) and SAOL <NUM> in the second direction.

It should be noted that combined beam <NUM> is astigmatic in which smallest cross-sections or waists in respective slow and fast axes are spaced from one another. Astigmatism may be corrected by installing FAOL <NUM> upstream from SAOL <NUM>, as shown in <FIG> such that respective focal planes of these lenses located in the same plane. Alternatively, it is possible to use, among others, a single spherical, aspherical, cylindrical lens <NUM>, as shown in <FIG>. Each of the schematics of <FIG> may be configured with multiple objective lenses or a single one as explained in somewhat greater detail below. However, beam <NUM> may remain astigmatic since the waist along the fast axis is very deep (Raleigh parameter is ~<NUM>. ) Thus as long as mirrors <NUM> focus combined beam <NUM> on the fiber's core, the astigmatism may not be critical.

The distance between any of SACs <NUM> and SAOL <NUM> in both <FIG> increases with the increased number of chips addressing the demands for higher output powers. The experiments show that generally when SAC <NUM> is configured with a focal length exceeding, for example, about <NUM>, the beam may significantly diverge in the slow axis.

In accordance with one of the aspects of the disclosure, SAOL <NUM> is displaced upstream from its original position, in which the SAOL, focal length f2 and original focal plane Fo - Fo all each are shown in dash lines, to its new optimal position, in which SAOL <NUM> along with focal length f2 and new focal plane Fn - Fn are shown in solid lines. A distance D between the original and optimal positions ranges between about <NUM> and <NUM> and may be determined in accordance with the disclosed method discussed below in reference to <FIG>. The output fiber <NUM> remains intact with the receiving end thereof lying in the original focal plane Fo - Fo. The focal plane of FAOL <NUM> coincides with the original focal plane Fo - Fo of SAOL <NUM> before the latter is shifted upstream. The desired distance at which SAOL <NUM> is displaced upstream from its original position is determined so that the smallest cross-section of the combined beam in the slow axis also lies in the original focal plane Fo - Fo. In other words, SAOL <NUM> and the receiving core end are spaced at a distance equal to the focal length of the SAOL and a newly determined distance D, as explained hereinbelow. The schematic of <FIG> can also be seen in <FIG>.

Referring specifically to <FIG>, diode module <NUM> has an additional row of chips <NUM>. As mentioned above, only one lens <NUM> functioning simultaneously as FAOL and SAOL is utilized in the shown configuration. According to the above-discussed salient feature of the disclosure, lens <NUM> is shifted upstream from its original position shown in dash lines and including the receiving end of fiber <NUM>, to an optimal position at the determined distance D for the reasons explained above. The optical schematic of <FIG> is also shown in <FIG>.

Referring to <FIG>, beams <NUM><NUM>. <NUM>n are output by respective chips <NUM><NUM>. <NUM>n and propagate over different optical paths before impinging upon SAOL <NUM>. Due to different optical paths, the region of SAOL <NUM> impinged by multiple beams <NUM> varies. The region with the smallest area is impinged by beam <NUM><NUM>, which propagates over the shortest optical path since chip <NUM><NUM> is the closest to SAOL <NUM> or <NUM>, whereas the largest area is covered by beam 14n which is emitted from chip <NUM>n most distant from the SAOL <NUM>. As a consequence, beams <NUM><NUM> - 14n are "focused" in slow axis at respective different distances downstream from focal plane F - F corresponding to the original position of SAOL <NUM> and including the receiving end of fiber <NUM>. The distance between the small beam cross-sections of respective beams <NUM><NUM> and <NUM>n emitted by first and last chips of module <NUM> determines the distance D at which SAOL <NUM> is shifted upstream from its original position. Alternatively, distance D may be determined as the mean of all distances of respective smallest cross-sections of beams <NUM><NUM>.

<FIG> considered in light of <FIG> helps explain the location adjustment of the SAOL in the context of the present disclosure. As one of ordinary skilled in the semiconductor arts readily understands, in mass production once a sample, such as a MEMM diode laser module, is tuned up, subsequent modules are each easily adjusted in accordance with data obtained during the tuning of the sample. Thus, the determined distance D, at which the SAOL is shifted upstream from its original position is once determined, is subsequently used in all other modules.

Accordingly, selectively turning either each of chips <NUM> in the tested module or just two chips - the closest to and most distant from the SAOL - it is possible to determine minimal cross-sections of respective beams incident on fiber <NUM>. As can be seen in <FIG>, curves <NUM> through <NUM> correspond to respective beams <NUM><NUM>. <NUM>n of <FIG>. The smallest cross-section of each beam corresponds to a bottom region of the associated curve. Thus, curve <NUM> corresponding to beam <NUM><NUM> from chip <NUM><NUM>, which is located at the shortest distance upstream from the SAOL, has its smallest cross-section downstream from focal plane F - F at the shortest a distance. The beam 14n emitted from distant chip 12n corresponds to curve <NUM> and has its smallest cross-section at a second distance greater than that one of beam <NUM><NUM>. The distance D between the smallest cross-sections of respective beams <NUM><NUM> and <NUM>n relative to the SAOL is the desired uniform distance for all subsequently tunable modules at which the SAOL is shifted upstream from its original position. The curve <NUM> illustrates the behavior of all beams after combined beam is focused in FAOL <NUM>. As can be seen, SM beams <NUM><NUM>. <NUM>n have respective beam spots in fast axis lying in the same plane as the receiving core end of output fiber <NUM>. In other words, in the fast axis beams <NUM> each are focused in focal plane F - F of SAOL <NUM> before the latter is shifted at distance D to its optimal position.

Referring to the configuration with single lens <NUM> of <FIG>, care has to be taken not only of the lens adjustment in the slow axis, but also in the fast axis. The displacement of lens <NUM> from its original position to the optimal position in the slow axis at distance D detrimentally affects the beam spot of the combined beam in the fast axis because when lens <NUM> is in its original position, the smallest beam spot in the fast axis is located in original focal plane F - F. However, the angular adjustment of mirror or mirrors <NUM> can effectively compensate for the shift of lens <NUM>. The mirrors <NUM> can be angularly adjusted such that beams <NUM><NUM>. <NUM>n, incident on the lens <NUM>, open up at a greater angle and thus could be focused in focal plane F - F of lens <NUM> when it is located in its original position. The angular position of the mirrors, like distance D, can be used for adjusting subsequent diode laser modules in mass production.

Claim 1:
A pigtailed diode laser module (<NUM>), comprising:
a case (<NUM>) housing spaced multimode, MM, chips (<NUM>) outputting respective parallel output beams (<NUM>) along a path;
an optical system configured to collimate the parallel output beams (<NUM>) into collimated beams in respective slow axes, wherein the collimated beams define a combined beam (<NUM>) which diverges along the path
wherein the optical system includes:
a plurality of slow-axis collimators, SAC, (<NUM>) each located between and optically coupled to the MM chip (<NUM>) and one focusing lens configured to collimate the output beam (<NUM>) in the slow axis; and
a plurality of fast-axis collimators, FAC, (<NUM>) coupled between respective MM chips (<NUM>) and SACs (<NUM>), the MM chips (<NUM>) being arranged in at least one row and emitting the respective output beams (<NUM>) in a first direction;
at least one focusing lens (<NUM>) for focusing the combined beam (<NUM>) in a focal plane thereof; and
an output fiber (<NUM>) coupled to the case (<NUM>) and having a core end downstream from the focal plane,
wherein the combined beam (<NUM>), coupled into the core end, has a cross-section smaller than that of the combined beam (<NUM>) in the focal plane, characterized in that
the core end is spaced downstream from the focal plane of the one focusing lens (<NUM>) at a distance corresponding to a difference between distances of respective smallest cross-sections of output beams (<NUM>), which are emitted by respective first and last MM chips (<NUM>), said respective smallest cross-sections being located downstream from the one focusing lens (<NUM>), with the first MM chip (<NUM>) being closest to the focusing lens (<NUM>), and the last MM chip (<NUM>) being farthest from the focusing lens (<NUM>); or in that
the core end is spaced downstream from the focal plane of the one focusing lens (<NUM>) at a distance corresponding to a mean value of distances between the one focusing lens (<NUM>) and respective smallest cross-sections of output beams (<NUM>) which are located downstream from the one focusing lens, with the MM chips (<NUM>) being spaced from the one focusing lens (<NUM>) at respective distances which are different from one another.