Patent Application
20080117944
The present invention relates generally to diode laser power supplies and, more particularly, to a method and apparatus for limiting damage to a diode laser during power cycling.
High power diode lasers, typically utilizing bar and stacked array configurations, are capable of producing in excess of hundreds of watts. In order to achieve these power levels, high drive currents are required. Unfortunately, as a result of these drive currents, a typical diode laser bar generates large amounts of heat which can lead to catastrophic failure of individual emitters or an entire laser bar. Catastrophic failure of a laser bar can, in turn, damage or destroy other system components that are in proximity to the defective laser such as optical elements, electrical components and cooling systems.
A standard laser bar power supply goes from zero output power to full power within approximately 20 milliseconds. As a result, the laser bar undergoes a rapid increase in temperature as it switches between the off and on states. Depending upon the thermal and mechanical properties of the solder used to bond the laser bar to the cooling block and the thermal properties of the cooling block, or depending upon the thermal and mechanical properties of the solder used to bond the laser bar to the submount, the thermal properties of the submount, the thermal and mechanical properties of the solder used to bond the submount to the cooling block, and the thermal properties of the cooling block, the rapid increase in temperature experienced by the laser bar may be sufficient to damage the laser bar or a portion of the laser bar (i.e., a portion of the bar's emitters). Typical damage includes both the development of cracks within the laser bar as well as laser bar delamination.
In order to overcome the problems associated with the rapid thermal cycling of laser bars, considerable effort has been expended on improving the thermal and mechanical coupling of the laser bar to the heat sink. In conventional laser bar systems, the laser bar is attached directly to the heat sink, the heat sink often being made of copper. In such systems a soft solder such as indium solder is typically used to attach the laser bar to the heat sink, a soft solder being preferred as it is capable of absorbing some of the severe CTE mismatch between the copper heat sink and the laser bar. Even with a soft solder, however, the stresses resulting from thermal cycling will often lead to cracks within the laser bar as well as delamination of the laser bar from the solder and/or heat sink. To further reduce the stresses imparted during thermal cycling, a heat sink can be selected with a CTE that closely matches that of the laser bar. Exemplary heat sink materials include copper tungsten, aluminum nitride and diamond containing a metal matrix composite. Unfortunately the thermal conductivity of such materials is typically inferior to that of copper or the materials exhibit a surface roughness that is non-conducive to forming diode bar bonds. To address the issue of limited thermal conductivity, in one approach a relatively thin substrate is fabricated from these materials, the substrate being used as a laser bar submount. The submount is then soldered to the heat sink. This architecture has the advantage of coupling the laser bar to a substrate with a similar CTE which is, in turn, coupled to a thermally efficient heat sink. This approach still, however, stresses the laser bar due, in part, to the relatively inelastic properties of the solder used to couple the laser bar to the substrate. Furthermore the inclusion of the submount and a second layer of solder results in an inefficient thermal path.
Although the prior art discloses a variety of techniques to help overcome the catastrophic effects of laser bar thermal cycling, to date these techniques have proved inadequate. Accordingly a new approach is required to alleviate these effects. The present invention provides such a system and method.
The present invention provides a method and apparatus for limiting the ramp rate of the current applied to a laser bar to a predetermined value. Preferably the predetermined value is 150 milliamps per millisecond or less. In addition to the diode laser bar and its power supply, the system includes a power supply control circuit that performs the function of limiting the power supply ramp rate. The power supply control circuit can be separate from, or integrated within, the power supply.
In an alternate embodiment, the present invention provides a method and apparatus for limiting the ramp rate of the current applied to a single emitter laser diode to a predetermined value. Preferably the predetermined value is 150 milliamps per millisecond or less. In addition to the single emitter laser diode and its power supply, the system includes a power supply control circuit that performs the function of limiting the power supply ramp rate. The power supply control circuit can be separate from, or integrated within, the power supply.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.
A control circuit 105 controls the rate that power supply 101 goes from zero output to full output power. It will be understood that full output power refers to the predetermined maximum output power (e.g., current) that is to be applied to laser diode 103, not to the maximum available output power of supply 101. Control circuit 105 can either be separate from power supply 101 as shown in system 100, or integrated within power supply 101 as shown in system 200 of
In accordance with the invention, each time that power supply 101 delivers power to laser diode 103, control circuit 105 insures that the ramp rate of supply 101 is sufficiently low to minimize damage to laser diode 103. The inventors have found that a rate of 150 milliamps per millisecond or less is preferred. This ramp rate, shown as line 301 in
The inventors have found that by significantly slowing down the ramp rate, the laser diode undergoes a much smaller temperature per time gradient than in a conventionally powered laser bar, thus minimizing strain within the laser diode since the generated heat can be more efficiently removed. As a result of not encountering large, near instantaneous temperature changes, laser diode degradation and failure due to power, and thus thermal, cycling is greatly reduced.
As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.
