Coherent light generators – Particular beam control device – Having particular beam control circuit component
Reexamination Certificate
2002-06-07
2004-09-28
Wong, Don (Department: 2828)
Coherent light generators
Particular beam control device
Having particular beam control circuit component
C372S038100
Reexamination Certificate
active
06798797
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to driver circuits for laser diodes, and more particularly to high-speed, power conditioning driver circuits for laser diodes and laser diode arrays.
2. Background
Laser diodes are continually finding new applications in the commercial, military, medical and other sectors. Laser diodes span the optical spectrum from the near infra-red (IR) through the visible wavelengths, which allows them to be used in a variety of applications, including, inter alia, optical communications, laser pointing and tracking, machining and welding, and pumping of a variety of optically-pumped lasers. Current technology trends all point toward expanded use of laser diodes, especially as efficiency and reliability are improved, and size and operating costs of laser diodes are reduced.
FIG. 1
is a schematic view of a conventional high power laser diode assembly
100
including an array of laser diodes
110
. Array
110
includes laser diodes
102
arranged in parallel (rack) and series (stack). A “rack and stack” approach enables the formation of arrays capable of generating high optical power densities (e.g., greater than 1 kW/cm
2
). Such arrays may require relatively high voltages (typically up to a few kilovolts) and high drive currents (typically up to a few kiloamperes) to operate. Array
110
is mounted on a micro-channel cooling plate
120
to dissipate heat generated by array
110
. A one-dimensional or two-dimensional array of laser diodes is referred to herein as a laser diode array (LDA).
While laser diodes have been finding new applications, the breadth of these new applications has been limited by the cost of manufacture, test, and replacement of laser diodes and laser diode arrays.
Common sources of laser diode failure arise from excessive drive currents being provided to a laser diode in an attempt to achieve high laser efficiency (where efficiency is defined as optical power output as a ratio of electrical power input). Exemplary modes of laser diode failure resulting directly or indirectly from excessive drive current include (1) dislocation of and precipitation of host atoms from the laser diode semiconductor crystal, (2) oxidation of the laser diode mirror facets, and (3) metal diffusion of the laser diode electrode and wire bonds.
Controlling the drive current of laser diodes and laser diode arrays (LDAs) to avoid excessive current is complicated by the fact that laser diode junctions are highly nonlinear, dynamic electrical loads, and output optical power can change dramatically with only a small change in input current. One example mechanism of laser diode failure resulting in the modes of failure described above is voltage breakdown of a laser diode's pn junction (also referred to herein as junction breakdown). Junction breakdown C. occurs when the drive current reaches a critical threshold, which causes strong optical absorption at a crystal defect. This in turn results in localized heating of the crystal, which causes its effective bandgap separation to shrink (and the voltage across a laser diode to decrease), giving rise to further optical absorption and increased drive current. This positive feedback process results in rapid thermal runaway, and breakdown of the pn junction.
Such voltage breakdown is illustrated graphically in
FIG. 2
, which shows a graphical representation of current versus time beginning with normal diode operation
210
, followed by the onset of junction heating
220
, during which time current increases and positive feedback begins. Ultimately catastrophic failure
230
occurs if current is not curtailed. Operation in a catastrophic failure regime can result in acute failure of a laser diode. A laser in which current has increased beyond that of normal diode operation is said to be in a “fault state.”
FIG. 3
is a schematic of a conventional power driver circuit
300
having an electrical power source
320
and a semiconductor switch
360
in series with an LDA
310
. The pulsing of semiconductor switch
360
is controlled by a switch trigger circuit
365
. Semiconductor switches used in conventional driver circuits have included power-field effect transistors (FETs) and integrated gate bipolar transistors (IGBTs).
One drawback of conventional power driver circuits, such as circuit
300
is that the laser diodes (or LDA) powered by the circuits may be exposed to excessive current or current densities in the laser diodes. For example, while switch
360
may limit the duration of excessive current to LDA
310
to prevent catastrophic failure, LDA
310
may still be exposed to excessive current in the form of short peaks in current (i.e., transients), which occur over a period of time that is relatively short compared to the duration of pulses from switch
360
or the total current through the diode might constrict within the diode medium and produce local regions of excess current density.
Excessive current or current density may be generated by power source
320
, or may be the result of changes in the operating conditions of an LDA such as constriction of the current in the laser diode medium, exposure to electromagnetic fields from other rill electric devices, electrical breakdown due to ionizing radiation from solar flares, cosmic rays or other sources of electric or magnetic interferences. Additionally, the current-voltage characteristics of an LDA itself may change over the operational lifetime of the LDA.
FIG. 4
is a graphical illustration of an exemplary current waveform
400
of a LDA driven by a conventional drive circuit. In
FIG. 4
, a semiconductor switch (e.g., switch
360
in
FIG. 3
) of the LDA driver circuit is turned on at time
410
, and turned off 20 microseconds later at time
420
. In exemplary waveform
400
, during the 10 microsecond period
430
that follows time
420
, the LDA is exposed to current transients
435
. Even if junction breakdown does not occur, cumulative effect of exposure to such current transients may limit the lifetime of an LDA and cause premature failure.
To reduce the effect of transients and thereby increase the lifetime of LDAs, conventional driver circuits have been operated at reduced average currents and powers; however, reducing the current has resulted in a reduction of the optical output power available from a given LDA assembly, and has limited the applications for which a given LDA may be used.
SUMMARY OF THE INVENTION
Accordingly, there is a need for laser diodes and laser diode arrays that operate efficiently and provide adequate optical outputs over long lifetimes to reduce the costs per unit of lifetime. To that end, aspects of the present invention are directed to a driver circuit capable of providing improved transient protection to a laser diode source. Such driver circuits are capable of terminating excessive current or current density quickly in order to reduce premature laser diode failure. An additional advantage of such driver circuits is that they allow an associated laser diode source to be driven at a higher average driver current.
A first aspect of the invention is a laser diode driver circuit to generate a drive current, comprising a laser diode source to receive an amount of the drive current, an indicator device configured to receive an input signal corresponding to the amount of the drive current, and to generate an indicator signal indicative of the amount of the drive current received by the laser diode source, and a transient snubber device coupled to the indicator device to receive the indicator signal, that in response to the indicator signal is controlled to have a first impedance state during which a first amount of the drive current is provided to drive the laser diode source, and to have a second impedance state during which a second amount of the drive current is provided to drive the laser diode source, the second amount being less than the first amount.
In some embodiments of the first aspect, the second amount is substantially zero. The transient snubber device may be in parallel w
Mangano Joseph
Petr Rodney A.
Nguyen Phillip
Science Research Laboratory, Inc.
Wolf Greenfield & Sacks P.C.
Wong Don
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