Fault tolerant laser diode package

A laser diode package (10) according to the present invention is tolerant of short-circuit and open-circuit failures. The laser diode package (10) includes a laser diode bar (12), a forward-biased diode (14), a heat sink (18), and a lid (16) which may have fusible links (86). The laser diode bar (12) and the forward-biased diode (14) are electrically connected in parallel between the heat sink (18) and the lid (16). The emitting region of the laser diode bar (12) is aligned to emit radiation away from the forward-biased diode (14). Several packages can be stacked together to form a laser diode array (42). The forward-biased diode (14) allows current to pass through it when an open-circuit failure has occurred in the corresponding laser diode bar (12), thus preventing an open-circuit failure from completely disabling the array (42). The fusible links (86), if used on the lid (16), prevent damaged active regions (90) in a laser diode bar (12) from short-circuiting and drawing more electrical current than the other active regions (90).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to lasers diodes and, in particular, to a package that is resistant to sudden short-circuit failures and resistant to sudden open-circuit failures.

2. Discussion of the Related Art

Semiconductor laser diodes have numerous advantages. They are small in that the width of their active regions is typically submicron to a few microns and their height is usually no more than a fraction of a millimeter. The length of their active regions is typically less than about a millimeter. The internal reflective surfaces, which produce emission in one direction, are formed by cleaving the substrate from which the laser diodes are produced and, thus, have high mechanical stability. The laser diode typically has several emitters, each of which is aligned with a corresponding active region.

High efficiencies are possible with semiconductor laser diodes with some pulsed junction laser diodes having external quantum efficiencies near 50%. Semiconductor lasers produce radiation at wavelengths from about 20 to about 0.7 microns depending on the semiconductor alloy that is used. For example, laser diodes made of gallium arsenide with aluminum doping (AlGaAs) emit radiation at approximately 0.8 microns (˜800 nm) which is near the absorption spectrum of common solid-state laser rods and slabs made from Neodymium doped, Yttrium-Aluminum Garnet (Nd:YAG), and other crystals and glasses. Thus, semiconductor laser diodes can be used as the optical pumping source for larger, solid-state laser systems.

Universal utilization of semiconductor laser diodes has been restricted by thermal related problems that can cause catastrophic failures. These problems are associated with the large heat dissipation per unit area of the laser diodes which results in elevated temperatures within the active regions and stresses induced by thermal cycling. Laser diode efficiency and the service life of the laser diode is decreased as the operating temperature in the active region increases.

In particular, laser diode bars containing more than one emitter are vulnerable to a sudden short-circuit failure. This short-circuit failure begins when one of the output facets of an emitter begins to absorb optical energy or when heat is not sufficiently dissipated from the emitter. As the temperature rises, the emitter becomes more inefficient and absorbs even more heat, causing a further rise in temperature and, ultimately, a thermal runaway situation. The temperature may reach levels that cause the material of the laser diode bar to melt in the area of the associated active region. Once the melting has occurred, the current and voltage characteristics of the P-N junction at the active region are locally destroyed and, thus, the active region begins to act as a simple resistor. With the P-N junction locally destroyed, current that would normally be distributed equally among all the active regions rushes through the damaged area, depriving the rest of the active regions of some or all of the available current. If the damaged region or regions are large, then it is possible for all of the available current to flow through the damaged region, and the rest of the undamaged active regions and their associated emitters on the laser diode bar become nonfunctional. Thus, an array containing the damaged laser diode bar may continue to draw current, but will have an inadequately low amount of emission or no emission whatsoever.

Additionally, laser diode bars are also vulnerable to sudden open-circuit failure. This failure mode may begin as a short-circuit failure. The emitter that has inadequate heat extraction causes the entire laser diode bar to become heated. The heat causes the soldered electrical connections between the laser diode bar and the adjacent heat sink to melt and the bar delaminates from the adjacent heat sink. Once the separation has occurred, an electrical connection no longer exists, thereby forming an open circuit. This mode of failure is more common in a diode package in which the diode bar is in intimate contact with a solid foil or a ribboned foil as the heat sink. The destruction of the solder bond between the foil and the laser diode bar forces the package into an open-circuit condition.

A need exists for a laser diode package that is not susceptible to short-circuit and open-circuit failures.

SUMMARY OF THE INVENTION

The present invention remedies the short-circuit and open-circuit failures by providing a laser diode package that includes a laser diode bar, a forward-biased diode, a heat sink, and a lid having a plurality of fusible links. The heat sink is electrically connected to the laser diode bar and the forward-biased diode with the emitters of the laser diode bar being aligned to emit radiation away from the forward-biased diode. Opposite the heat sink, the fusible links of the lid are in electrical contact with the laser diode bar, and the main body of the lid is in electrical contact with the forward-biased diode. Accordingly, the laser diode bar and the forward-biased diode are electrically in parallel between the heat sink and the lid.

The individual laser diode packages may be combined into laser diode arrays. Accordingly, the heat sink of a first package is placed in electrical contact with the lid of a second adjacent package. Numerous individual packages can be assembled in such a fashion, resulting in a multi-bar laser diode array.

The lid having the fusible links prevents short-circuit failures because each of the links, which is associated with a corresponding active region and passes the current for that active region, is destroyed like a typical electrical fuse when current levels become too high. Thus, the electrical path to the damaged active region is destroyed, causing the current to flow through the undamaged active regions.

The forward-biased diode prevents open-circuit failures when the packages are formed into laser diode arrays. If a laser diode bar has been electrically disconnected from the rest of the laser diode package due to a reflow of the solder, the forward-biased diode is activated which provides an alternate path for the electrical current. This allows a laser diode array to remain functional even though one of its laser diode bars has been electrically disconnected from the adjacent heat sink.

While the laser diode package may have each of these two mechanisms for decreasing the likelihood of open-circuit or short-circuit failures, the laser diode package can benefit from having just one of these two failure prevention mechanisms. Thus, a laser diode package according to the present invention may contain only the lid having the fusible links or only the forward-biased diode.

The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention. This is the purpose of the Figures and the detailed description which follow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

InFIG. 1, a side view is shown of a laser diode package10containing a laser diode bar12, a forward-biased diode14, a lid16, and a heat sink18. The laser diode bar12is attached to the heat sink18through a first solder layer22a. The forward-biased diode14is attached to the heat sink18through a second solder layer22b. The lid16is attached to both the laser diode bar12and the forward-biased diode14with third and fourth solder layers24aand24b, respectfully.

The heat sink18of the laser diode package10is typically made of a material that is both electrically and thermally conductive, such as copper. Electrical conductivity is required to conduct the electrical current through the laser diode bar12and produce optical energy. Thermal conductivity is needed to conduct the intense heat away from the laser diode bar12and maintain the laser diode bar12at a reasonable operating temperature. The heat is conducted from a bottom end30of the heat sink18that is coupled directly or indirectly to the ultimate thermal reservoir (e.g. a heat exchanger passing a working fluid).

The lid16may be a simple foil providing a backside of the package10. Alternatively, the lid16may be useful as a heat path and conduct heat to the ultimate thermal reservoir. In this case, the lid16essentially acts, and may be configured, like the heat sink18.

The heat sink18and the lid16of the laser diode package10may be manufactured in such a way so as to have the material for the solder layers on their exterior surfaces (i.e., “pretinned”). Such structures are described in U.S. patent application Ser. No. 09/280,783 entitled “Laser Diode Packaging” which was filed on Mar. 29, 1999, and is incorporated herein by reference in its entirety. Consequently, the first and second solder layers22aand22band the third and fourth solder layers24aand24bcan be produced by a single solder layer present on the lid16and the heat sink18, respectively. This eliminates the need to accurately locate individual solder layers for interfacing with the laser diode bar12and the forward-biased diode14, although such a methodology will also perform the desired function. Further, by pretinning the entire lid16and the heat sink18, the lid16and the heat sink18can have pre-applied solder layers26and27, respectively, allowing the package10to be soldered to an adjacent package.

The laser diode bar12has an emitting surface28and a reflective surface32that opposes the emitting surface28. Active regions29of the laser diode bar12, which are the regions in the laser diode bar12where the photons are generated from the input electrical energy, are typically closer to the heat sink18. The photons propagate through the active regions29, reflect off the reflective surface32, and are emitted from the emitting surface28. It is preferred that the emitting surface28be positioned substantially flush (i.e., substantially coplanar) with the end surface31of the heat sink18. In the preferred embodiment, the emitting surface28is positioned within about 1 mil (i.e., ±0.001 inch) of the top end31of the heat sink18.

FIG. 2shows a diagrammatic illustration of an equivalent circuit of the package10ofFIG. 1with the laser diode bar12being electrically connected in parallel with the forward-biased diode14between the heat sink18and the lid16. By providing the forward-biased diode14in parallel with the laser diode bar12, an open-circuit failure can be avoided. In particular, the forward-biased diode14has a turn-on voltage which is higher than the turn-on voltage of the laser diode bar12, usually by a small fraction of a volt. In one embodiment, the forward-biased diode14has a turn-on voltage of about 2.8 volts, while the laser diode bar12has a turn-on voltage of about 1.6 volts. In normal operation, the equivalent circuit passes electric current entirely through the laser diode bar12. During an open-circuit failure, an electrical gap results after the third solder layer24abetween the lid16and the laser diode bar12or the first solder layer22abetween the heat sink18and the laser diode bar12has been removed. The voltage across the electrical gap adjacent to the laser diode bar12begins to build. Because the forward-biased diode14is electrically connected in parallel with the laser diode bar12, the voltage across the forward-biased diode14also begins to rise. Eventually, the voltage across the forward-biased diode14reaches the tun-on voltage for the forward-biased diode14and it becomes activated. When the forward-biased diode14is activated, the electrical current can again pass from the heat sink18to the lid16, although the laser diode bar12emits no radiation.

FIGS. 3A and 3Billustrate a cross-sectional schematic view and resulting circuit, respectively, of one possible configuration for the forward-biased diode14. In this configuration, the forward-biased diode14is made of a gallium arsenide substrate32with epitaxial layers34grown on its outer surface. The substrate32is preferably an n+-type substrate. The epitaxial layers34provide a stacked diode doping profile on the substrate32that creates a two junction stacked diode with a turn-on voltage that is greater than the turn-on voltage of the laser diode bar12. An equivalent circuit40of the epitaxial layers34is shown inFIG. 3Bas two diodes in series. The forward-biased diode14constructed in this manner will provide open-circuit protection for laser diode bars that emit radiation with a wavelength of 700 nm or higher.

While not shown inFIGS. 3A and 3B, the forward-biased diode14contains metalization on its surfaces that allow it to be electrically connected with the solder layers22band24bof the heat sink18and the lid16. In addition to the electrical function of the forward-biased diode14, it also serves as a precise spacer between the heat sink18and the lid16. Thus, once the epitaxial layers34are grown on the substrate32and metalization is applied, the wafer is precisely scribed and cleaved to the proper dimensions. While the embodiment ofFIGS. 3A-3Bhas been described with the substrate32being made of gallium arsenide, silicon could also be used.

FIG. 4shows a side view of a laser diode array42composed of three stacked laser diode packages44,46and48with forward-biased diode protection. A heat sink52of the upper laser diode package44is soldered to a lid54of the middle laser diode package46. Similarly, the heat sink56of the middle laser diode package46is soldered to the lid58of the lower laser diode package48.

Under normal conditions, an equal current flows through each laser diode bar in the laser diode packages44,46and48because the laser diode packages44,46and48are electrically connected in series.FIG. 4, however, shows that the middle laser diode bar60has experienced an open-circuit failure62in which its solder layer has been melted away such that the laser diode bar58is delaminated from the lid54. According to the prior art systems, once the open-circuit failure occurs, the flow of current through the laser diode array42would be terminated and the entire laser diode array42would be nonfunctional. In the present invention, after the open-circuit failure in the damaged laser diode bar60occurs, the voltage drop across the forward-biased diode64in the laser diode package46begins to rise. When the turn-on voltage is reached, the forward-biased diode64allows current to pass through it such that the current also passes through the undamaged laser diode bars of the upper laser diode package44and the lower laser diode package48. Consequently, although an open-circuit condition existed, the laser diode array42still emits radiation from all bars except the damaged laser diode bar60.

FIGS. 5 and 6illustrate top and side views of a laser diode package80featuring a laser diode bar82that is coupled between a heat sink83and a lid84having fusible links86. The lid84is made of a metallic foil with the fusible links86being developed through etching or mechanical stamping. The laser diode bar82has active regions90emitting energy along the length of the laser diode bar82. Each active region90has a corresponding emitter92located on the emitting surface of the laser diode bar82. Electrical power is guided to the active regions90by providing more electrically conductive material within the active regions90than in areas between the active regions90.

A spacer88may be placed between the lower portions of the heat sink83and the lid84. The spacer88may simply be for maintaining the appropriate space between the heat sink83and the lid84, or can be the forward-biased diode that is used for the purpose described above with respect toFIGS. 1-4.

The utility of the fusible links86will be described with reference toFIGS. 7A-7C, all of which are end views looking at the end of the package80from which the light would be emitted from the laser diode bar82. As mentioned previously, a short-circuit failure is caused by one or more of the active regions90malfunctioning and experiencing localized melting of the material that comprises the laser diode bar82. The resulting lower resistance causes the damaged active region to pass more current than is being passed in the other active regions90of the laser diode bar82.

FIG. 7Aillustrates the normal operating condition where the current flows through each of the eleven active regions90in a substantially equal proportion. While only eleven active regions90are shown, the invention is applicable to laser diode bars82having more or fewer active regions90. Because the fusible links86are soldered to the laser diode bar82, a substantially equal portion of the current flows through each of the fusible links86. As shown, each active region90has one corresponding fusible link86. In an alternative embodiment, however, each fusible link86may create a current path for a group of active regions90.

FIG. 7Billustrates a transient condition whereby current begins to flow at an abnormally high rate through two active regions90aand90bthat have been damaged. If the damaged active regions90aand90bare large enough, all of the available current will flow through those damaged active regions90aand90band the rest of the emitters92on the laser diode bar82will become nonfunctional. Thus, the fusible links86aand86bassociated with these two active regions90aand90bbegin to achieve current levels that they cannot accommodate. The fusible links86are designed such that if a certain excessive amount of current is passed therethrough, the fusible link86destroys itself. Thus, the fusible links86are frangible structures that are destroyed at a predetermined current level.

FIG. 7Cillustrates the condition afterFIG. 7Bin which the fusible links86aand86bare destroyed, thereby inhibiting the passage of current through damaged active regions90aand90b. As such, current continues to flow through the other functional fusible links86and, thus, the rest of the active regions90of the laser diode bar82remain functional and their corresponding emitters92emit the appropriate energy.

The fusible links86prevent a short-circuit failure, which can lead to an open-circuit failure due to the intense heat concentration that may cause the reflow of the solder layers. Therefore, preventing short-circuit failure is also a preventative measure against an open-circuit failure. Thus, to some extent, the fusible links86also guard against open-circuit failures.

The lid84and, thus, the links86are preferably made of a fusible alloy, such as Indalloy™ #117, Indalloy™ #158 or Indalloy™ #281. For a known cross-sectional area, these fusible alloys have a certain resistivity per unit length such that a known current will produce a known amount of heat. When the current exceeds a predetermined value, the resultant heat causes the temperature of the link86to rise above the melting temperature of the link86so that a portion of the link86melts and the link86is no longer electrically connected to the laser diode82. For example, if the current through one link86exceeds three times the normal operating current, that amount of current produces enough heat to cause melting of the link86.