Low inductance laser diode bar mount

The laser mount arrangement can have a laser bar and a driver positioned adjacent to one another and secured against a connection face of a heat sink base. The heat sink base is connected to and forms a first electrical connection between the laser bar and the driver. A second electrical connection is also provided between the laser bar and the driver opposite the heat sink base, which can be in the form of a flexible metal sheet with a narrow upward fold. This arrangement can provide a low inductance path for the current.

BACKGROUND

Laser diode bars (hereinafter referred to as laser bars for simplicity) are well known electro-optical devices which typically consist of a bar of adjacent laser diodes. A laser bar has a P and a N electrical contacts on respective opposite faces to power the laser diodes via electrical connections.

Although former laser bar mounts were satisfactory to a certain degree, there remained room for improvement in the manner to mount the laser diode bar to a heat sink and to power it. For instance, the electrical connection circuit between the laser bar and the controlling electronics of former laser bar mounts had an inductance satisfactory when operating in continuous wave or quasi continuous wave modes, but there remained room for improvements when operating un pulse mode.

SUMMARY

Attempts to operate laser diode bars using former mount arrangements in pulse mode met with several limitations. One feature which is sought in pulse laser mode application is fast rise time and fast fall time. One limitation of former mount arrangements was due to the high inductance of the electrical connection circuit, which limited the ability to obtain satisfactory fast rise time and fall time.

This is illustrated inFIGS. 1A and 1Bwhich show longer rise-time and fall-time as compared withFIGS. 1C and 1D. The consequences of longer rise/fall-time are even more important when the pulses are short, for a given current amplitude, such as illustrated when comparingFIGS. 1B and 1DwithFIGS. 1A and 1C.

Range-gated active imaging, for instance, is an example of an application which can require a laser emitter having a short rise/fall time in order to achieve a satisfactory spatial discrimination.

The electrical connections of the mount assembly could be represented as a first order RL circuit, the current response rise time to a step function is:
I(t)=Imax(1−e(t/τ))   (1)Where τ=L/R

In this case, the 10-90% rise time constant is given by 2.2 T.

The system power efficiency requires a low series resistivity (R), which adversely increases the time constant. Therefore, as seen by the time constant equation, the inductance value must also be low to achieve short rise times and fall times.

The inductance value is affected by the loop surface area of the current going from the driver to the laser diode and back.

One way to reduce the loop surface area is to bring the driver (which is typically provided in the form of a printed circuit board) close to the laser bar. In an embodiment detailed herein, and schematically illustrated inFIG. 2, this has been achieved by positioning the printed circuit board driver directly onto the heat sink base, immediately adjacent the laser bar, in a manner using the heat sink base itself as an electrical connection between the driver and the laser bar. The other electrical connection can then be configured in a manner where the loop surface area, and therefore inductance, is kept much smaller than possible in former mounts.

A challenge also resided in finding a way to hold the driver and the laser bar on the heat sink base. One way which is detailed below is to use clamps in order to do this. In the case of the laser bar, this can advantageously avoid the requirement of soldering, which had been known to induced stress in the laser bar.

Finally, in the method of mounting the components to the heat sink base, it was desired that the second electrical connection (opposite to the heat sink base which acts as the first electrical connection), be adapted to lengthwise displacement, in order to account for variations in the exact positioning of the driver, while keeping a configuration which allows maintaining a satisfactorily low inductance. In an embodiment described herein, and schematized inFIG. 2, this is achieved by providing the second electrical connection in the form of a sheet of electrically conductive material having a narrow fold protruding normally from its otherwise planar body in the area corresponding roughly to between the driver and the laser bar. The elasticity of the metal in the fold allows to move the driver connection portion lengthwisely while the laser bar portion is secured to the laser bar, whereas the narrowness of the fold maintains a relatively small loop surface area given the benefit of added flexibility.

In accordance with one aspect, there is provided a laser mount arrangement comprising: a laser bar having a first electrical contact and a second electrical contact; a driver having a first electrical contact and a second electrical contact and having control electronics for driving the laser bar in pulsed mode; a heat sink base having a high thermal conductivity and an low electrical resistivity and having a connection face secured in electrical contact with both the laser bar first electrical contact and the driver first electrical contact, thereby forming a first electrical connection therebetween; a sheet of electrically conductive material secured in electrical contact with both the laser bar second electrical contact and the driver second electrical contact, thereby forming a second electrical connection therebetween.

In accordance with another aspect, there is provided a laser mount arrangement comprising: a laser bar and a printed circuit board positioned adjacent to one another and secured against a connection face of a heat sink base, the heat sink base being connected to and forming a first electrical connection between the laser bar and the printed circuit board, and a second electrical connection between the laser bar and the printed circuit board opposite the heat sink base.

In accordance with another aspect, there is provided a method of mounting a laser assembly, said method comprising, in sequence: securing a laser bar in electrical contact with a heat sink base, and a laser bar portion of an electrical connection to the laser bar; and subsequently securing a driver in electrical contact with the heat sink base and a driver portion of the electrical connection to the driver.

DETAILED DESCRIPTION

FIG. 2schematically shows an example of a low-inductance laser mount arrangement10. In this example, a heat sink base12, which is made of a material which has both high thermal conductivity and low electrical resistivity, receives both a laser bar14and a driver16. In this case, the driver16is in the form of a printed circuit board16ahaving control electronics19for driving the laser bar in pulsed mode and electrical contacts17a,17bof the driver on opposite sides thereof, and the laser bar14also has electrical contacts15a,15bon opposite sides thereof. Corresponding electrical contacts15a,17aof both the laser bar14and the driver16are connected with the heat sink base12which acts as a first electrical connection12atherebetween. A second electrical connection18configured in a manner to generate low inductance, is connected with the both the laser bar14and the driver16on the side opposite the heat sink base12. In this particular case, the second electrical connection18is provided in the form of a sheet of electrically conductive material20. More particularly, a metal sheet20ahaving a narrow fold21protruding normally from its otherwise planar body is used in this embodiment. The metal sheet20acan be seen to have a laser bar portion30connected to a corresponding electrical contact15bof the laser bar14, and a driver portion32connected to a corresponding electrical contact17bof the driver printed circuit board16a,the fold21being between the driver portion32and the laser bar portion30of the metal sheet20a.The use of the narrow fold21will be detailed below. During operation of the laser bar14, electric current travels along the loop indicated with the arrows and the laser diodes of the laser bar emit a light signal22.

The area of the heat sink base12which receives the electrical contacts15a,17ais a face which is referred to herein as the contact face23. In this specific example, the contact face23is planar and unitary, and a planar front face24is also formed in the heat sink base12.

FIG. 3shows an intermediate step of an example method for mounting the laser bar14and driver printed circuit board16ain a solderless manner. Referring toFIG. 3, it is preferred in this example to mount the laser bar14(i.e. secure the laser bar14to the heat sink base12and metal sheet20a) prior to mounting the driver printed circuit board16a.Henceforth, the laser bar14is first positioned on the contact face23of the heat sink base12, with its front facet14apositioned flush with the front face24of the heat sink base12. In this case, the laser bar14is positioned with its P-side electrical contact15aon the heat sink base12. Henceforth, the laser bar14is secured in place here by way of a laser bar clamp26which has a pressing surface27which presses downwardly as bolts40having an abutting head and a stem extending through the laser bar clamp26at opposite transversal ends thereof are screwed into threaded bores42(seeFIG. 2) provided in the heat sink base12. The laser bar14can be held in place by a temporary structure (not shown) as the laser bar clamp26is being applied. Here, the metal sheet20ais placed against the N-side electrical contact15bof the laser bar14and is maintained pressed thereagainst during use by the laser bar clamp26. To reduce the likelihood of short-circuiting the laser bar14, the laser bar clamp26can be made of a non-conductive material, in which case the pressing surface27can be directly applied to the metal sheet20a.Otherwise, a layer28of an electrically insulating material, such as a Teflon™ strip for instance, can be applied to the pressing surface27and used as an insulating bushing between the metal sheet20aand the laser bar clamp26as illustrated for example. Henceforth, the laser bar14is pressed and maintained into electrical and thermal contact with the heat sink base12at which point the temporary structure previously holding it in place can be removed.

In an alternate embodiment, the laser bar can be secured to the heat sink base by soldering, although this is not preferred here because soldering can have undesired effects on the laser bars such thermal stress, damage to the optical facets of the laser emitters and misalignment of bars individual emitters optical axis, known in the art as the ‘smile’ of the laser bar.

Referring back toFIG. 2, the next step in the example mounting method is to secure the driver printed circuit board16ato the heat sink base12and metal sheet20a.

It will be understood that the electrical contacts17a,17bof the driver16are positioned so close to the laser bar14that the printed circuit board16aand the laser bar14can be said to be immediately adjacent. For illustrative purposes, the distance between the printed circuit board16aand the laser bar14can be of the order of 500 μm for instance. This proximity contributes in reducing the loop surface area and therefore manages inductance. An electrical contact17aon one side of the printed circuit board16ais placed in direct electrical contact with the heat sink base12, whereas an electrical contact17bon the other side of the printed circuit board16ais placed in direct electrical contact with the driver portion32of the metal sheet20a.In this embodiment, the electrical contact is secured by way of a second clamp34, of non-conductive material, which maintains a pressing force sandwiching the metal sheet20aand printed circuit board16ain a similar manner than that described above in relation with the laser bar14.

In other embodiments, the electrical contacts can be maintained by soldering, or otherwise, for instance.

Those skilled in the art will recognize that given this sequence of assembly and the components involved, it is desirable here to allow a certain amount of play for lengthwise displacement of the second electrical connection20, to account in possible stresses during assembly and variations of the relative position or distance between the driver16electrical contacts17b,17aand the laser bar14electrical contacts15a,15band prevent lateral forces applied to the second electrical connection20to be directly transferred onto the laser bar14. Henceforth, both features of low inductance and flexibility are desired in the second electrical connection20.

It will be noted here that in this specific example, the second electrical connection20is provided in the form of a metal sheet20awhich has a fold21in it, the fold21protruding upwardly. The presence of this fold21, combined with the natural elasticity of the metal sheet20a,allows flexibility to move the printed circuit portion32of the metal sheet20alengthwisely to adapt to the exact position of the printed circuit board16a,while the laser bar portion30of the metal sheet20ais firmly secured to the laser bar14. Further, the fold can absorb lengthwise compression or tension forces and buffers them from affecting the laser bar14or the quality of the electrical contact. The presence of the fold21will have an effect on the inductance, but the degree of the effect can be limited by making the fold21narrow, in which case the surface area of the circuit loop is limited. In the case where the driver16and laser bar14are held by corresponding clamps26,34, as in the illustrated example ofFIG. 2, the narrow fold21can have a width of the same order of magnitude as the distance between the laser bar14and printed circuit board16a,and a height several times greater than its width, protruding into the area between the two clamps26,34, for instance.

In this particular embodiment, a flexible printed circuit board was selected. Such a flexible printed circuit board can have a thickness of 125-150 μm for instance, which is significantly less than most rigid circuit boards, and thus closer to that of the laser bar14. Having comparable thicknesses between the printed circuit board and the laser bar is another factor in managing inductance.

In the exemplary embodiment described above and illustrated, the laser was found to be satisfactorily operated in a pulsed mode at currents in the order of 40-90 A at pulse widths ranging between 150 ns and 80 μs and rise time below 125 ns at a frequency ranging between 2.5 and 100 KHz for instance. This performance is obtained given the relatively low electrical resistivity of the electrical connections between the printed circuit board16aand the laser bar14(˜3 mOhms), and relatively low inductance (˜2 nH). The thermal resistance of the laser bar14in contact with heat sink base12was satisfactory, of the order of 1.0° C./W, which led to satisfactory low temperature rise of the laser bar14. Further, using a heat sink base12made of gold plated copper and having a surface flatness below λ/2, a smile (misalignment of the bars individual emitters optical axis along an axis parallel to the heat sink base surface) of less than 0.5 μm was achieved. A laser bar14having19adjacent laser diodes was used in this embodiment although the exact number can vary strongly depending on the embodiments and desired features. In this specific example, the heat sink base and the metal sheet are made of gold-plated copper, a material which was selected here for its high electrical and thermal conductivity. Of course, in alternate embodiments, other materials having satisfactory properties can be used instead.

It will be noted that in the embodiment described above and illustrated, the driver16is provided in the form of a flexible printed circuit board16awhere the electrical contacts17a,17bof the driver16are provided directly on the printed circuit board16a.In an alternate embodiment, for instance, the driver can be provided in the form of a rigid printed circuit board connected to a transmission line, in which case a device such as a strip line flat cable can connect to the printed circuit board and provide the electrical contacts of the driver to the heat sink base and to the second electrical connection. Other embodiments are possible as well.

As can be seen therefore, the examples described above and illustrated are intended to be exemplary only. The scope is indicated by the appended claims.