Patent Application
20080025361
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.
Referring now to the drawings, wherein like components are designated by like reference numerals,
One step in the method is to form a diode-laser bar carrier having a set of electrical contacts which are electrically isolated from each other. In a preferred embodiment of the inventive method illustrated by
The grooving operation forms the required carrier, designated by the general numeral 16, with pads 12A-H providing the electrical contacts, electrically isolated from each other by the grooves extending into the dielectric layer. In this example, the dielectric layer is assumed to be rigid. Such a layer may be fabricated, for example, from a thermally conductive ceramic material such as aluminum nitride or beryllia (beryllium oxide). The electrical contacts may comprise, for example, a layer of highly electrically conductive material such as copper and a bonding layer of solder material such as a gold-tin (AuSn) solder. The surface of the bonding layer should be relatively flat and smooth to facilitate the subsequent bonding of a diode-laser bar.
Those skilled in the art will recognize that a carrier may be formed from a highly thermally conductive material such as copper having a relatively thin (not rigid or self supporting) dielectric layer 16 of a material such as diamond thereon, with the set of electrical contacts 12A-H formed on the electrically insulating layer. The isolated contacts can be formed by, for example, separately plating individual contacts on a carrier or by forming a large, single contact and etching patterns to form isolated contacts, or by forming a large, single contact and removing the conductive material from the region between adjacent contacts by means of mechanical sawing or laser ablation to form adjacent isolated contacts. An example of such alternative carrier is described further herein below. The present description proceeds, however, using the example of carrier 16 depicted in
The term “substantially-collinear”, referring to the alignment of the emitting regions, acknowledges that exact collinear alignment of the emitting-apertures in a diode laser bar is rarely ever achieved. Even with the most careful manufacturing techniques, the emitting apertures are usually gradually misaligned along the length of the bar with a height difference in the fast-axis (here the X-axis) of a few microns between end ones of the apertures and a central one of the apertures. This misalignment is due to stresses developed in the epitaxial-layer growing process and is whimsically termed “smile” by practitioners of the art. Smile makes fast-axis collimation of beams from all emitters with a single cylindrical lens element (a collimation method preferred by practitioners of the art) difficult.
A next step in the inventive diode-laser-array forming method is to bond diode-laser bar 22, with the epitaxial layers (epitaxial-layer side) down, to the grooved metallized surface of substrate or carrier 16 (see
Referring now to
Cuts 32 through the diode-laser bar may be made by sawing or by localized laser ablation of the diode bar. The separation may also be performed by masking and etching using, for example, reactive ion etching. Whatever method is selected the separation or division of the diode-laser bar to form the groups of emitters should not significantly change the relative alignment of the emitting themselves.
Continuing with reference to
It should be noted here that the number of wires per diode-laser bar section need not correspond to the number of emitters per diode-laser bar section. Those skilled in the art will recognize that the number of wires can be selected according to the total current drawn by the array and the current carrying capacity of individual wires, among other factors.
Other means of forming electrical connections to the diode groups are anticipated, including soldering of flexible circuit elements to the contact pads and the diode-laser top electrodes or the further bonding of a second insulating carrier assembly to the substrate side of the diode-laser bar. This second insulating carrier could have a matching set of isolated electrical contacts to make individual electrical connection to the electrically isolated groups of diode-laser emitters. Electrical connections could then be made to these electrical contacts by, for example, soldering of flexible circuit elements.
An alternative process for forming electrically isolated groups of emitters includes the step of etching grooves in the epitaxial layers 26 of the diode-laser bars, at least through the epitaxial layers and possibly partially into the substrate portion 24. This etching is preferably done at the wafer stage before the wafer is cleaved into individual diode-laser bars. The grooves would be generally in the region where the diode groups will be separated after the diode-laser bar is attached to the metallized grooved layer (carrier 16). These etched grooves are preferably made somewhat wider than the saw, laser-beam, or other cut is later used to separate the diode-laser bar into sections. In this way the cut-edge of the substrate portion 24 of the diode-laser, which is prone to mechanical damage and chipping from the saw or laser cut, is removed from the edge of the epitaxial layers 26 of the diode-laser. This step will tend to reduce any tendency for mechanical defects to propagate through the epitaxial layers to the region of the optical emitters, thus tending to improve the lifetime or reliability of the assembled diode-laser array. Those skilled in the art may devise other methods of separating the diode-laser bar into electrically isolated sections without departing from the spirit and scope of the present invention.
Regarding the number of emitters in the diode-laser bar sections or emitter groups, clearly the lowest operating current for the inventive array, for any total number of emitters, will be achieved when there is only one emitter per diode-laser bar section. For eighteen emitters having characteristics exemplified above this would be a current of about 2.5 A from a supply voltage of about 32.5 V. Three emitters per bar would require a current of about 7.5 A at a supply voltage of about 11 V, and so on.
Providing one emitter per diode-laser bar section, however, would require the greatest number of grooves and cuts and may involve a lower manufacturing yield than might be experienced with a greater number of emitters per diode-laser bar section. The choice of the number of emitters per bar will ultimately depend on factors such as the cost and availability of current controlled power supplies and the cost and yield of cutting and grooving operations. It is not necessary that the number of emitters per diode-laser bar section be the same. By way of example, a diode-laser bar having a total of nineteen emitters may be divided into five diode-laser bar sections each having three emitters, and two diode-laser bar sections each having two emitters.
A particular advantage of having the individual emitters or groups of emitters arranged with a series electrical connection is an enhanced ability to modulate the electrical drive to the diode elements. With a parallel electrical connection to the diodes, about 50 amps of current would need to be varied (modulated) to change the light output of the diode bar. While this is possible over a longer time scale, for example several milliseconds (ms), by controlling the power supply output, it is difficult or expensive to pulse this current over a short time scale, such as about one microsecond (its). With a series connection, it is relatively straightforward to switch the lower currents, say either 2.5 amps or 7.5 amps, in such a short time. Potential applications such as modulating pump-light to diode-pumped lasers would be enhanced by the ability to rapidly modulate the pump-light to these lasers.
In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.
