U.S. patent application number 11/178863 was filed with the patent office on 2005-12-08 for pulse control in laser systems.
Invention is credited to Smart, Donald V..
Application Number | 20050271095 11/178863 |
Document ID | / |
Family ID | 22258140 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050271095 |
Kind Code |
A1 |
Smart, Donald V. |
December 8, 2005 |
Pulse control in laser systems
Abstract
A pulsed laser system includes a laser pump, a laser rod, a
reflector interposed between the laser pump and the laser rod,
through which energy from the laser pump enters the laser rod, an
output reflector through which energy is emitted from the laser
rod, a switch interposed between the laser rod and the output
reflector, and a control device. The switch, when closed, causes
energy to be stored in the laser rod and, when opened, allows
energy to be emitted from the laser rod during an emission period.
The control device allows a primary laser pulse emitted from the
laser rod during the emission period to impinge on a workpiece and
blocks from the workpiece secondary laser emission occurring during
the emission period after emission of the primary pulse. The pulsed
laser system is operated over a range of repetition rates, so as to
cause laser energy to be emitted during a plurality of emission
periods at each repetition rate. At least a portion of the laser
energy emitted during the emission periods is directed toward the
target structure in order to perform passive or functional trimming
of the target structure. The switch is closed for a fixed,
predetermined period of time prior to each emission period
regardless of repetition rate of the primary laser pulse within the
range of repetition-rates in order to store energy in the laser
rod. The pump is operated continuously at constant power.
Inventors: |
Smart, Donald V.; (Boston,
MA) |
Correspondence
Address: |
William E. Hilton, Esq.
Gauthier & Connors LLP
Suite 3300
225 Franklin Street
Boston
MA
02110
US
|
Family ID: |
22258140 |
Appl. No.: |
11/178863 |
Filed: |
July 11, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11178863 |
Jul 11, 2005 |
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11005981 |
Dec 7, 2004 |
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11005981 |
Dec 7, 2004 |
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09633837 |
Aug 7, 2000 |
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6831936 |
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09633837 |
Aug 7, 2000 |
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09096600 |
Jun 12, 1998 |
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6339604 |
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Current U.S.
Class: |
372/26 ; 372/13;
372/25 |
Current CPC
Class: |
H01S 3/10046 20130101;
H01S 3/10038 20130101; H01S 3/0085 20130101; H01S 3/08068 20130101;
H01S 3/1068 20130101; H01S 3/117 20130101; H01C 17/242 20130101;
H01G 5/019 20130101; B23K 26/0622 20151001; B23K 26/064 20151001;
H01S 3/0057 20130101; B23K 26/0648 20130101 |
Class at
Publication: |
372/026 ;
372/013; 372/025 |
International
Class: |
H01S 003/117; H01S
003/10 |
Claims
What is claimed is:
1-39. (canceled)
40. A method of providing a pulsed laser output, said method
comprising the steps of: providing a laser pump, a laser rod, a
reflector interposed between the laser pump and the laser rod, and
through which energy from the laser pump enters the laser rod, an
output reflector through which energy is emitted from the laser
rod, and a switch interposed between the laser rod and the output
reflector configured to be closed to cause energy to be stored in
the laser rod for a desired period of time, and to be opened to
allow energy to be emitted from the laser rod during an emission
period; selecting a pulse shape to be produced by the laser source,
based on known properties of a target material to be processed on a
workpiece, said step of selecting a pulse shape being independent
of a time interval between at least two successive transmissions of
pulses onto the workpiece; and causing the pulsed laser system to
be pulsed, by causing the switch to be closed for a fixed,
predetermined period of time prior to each emission period
regardless of the time interval between the at least two successive
transmissions of pulses onto the workpiece, so as to cause the
laser source to process the target material on the workpiece, with
the set time interval between the at least two successive
transmissions of pulses onto the workpiece, while the pulse shape
remains as preset regardless of the time interval, without setting
of the time interval affecting the pulse shape.
41. The method as claim 40, wherein the step of cause the laser
source to process the target material on the workpiece involves
causing micromachining of a target material on the workpiece.
42. The method as claimed in claim 41, wherein the micromachining
of the target material involves micromachining a semiconductor
circuit on a silicon substrate.
43. The method as claimed in claim 41, wherein the micromachining
of the target material involves trimming a trimmable component.
44. The method as claimed in claim 41, wherein the micromachining
of the target material involves trimming a thick-film electrical
element.
45. The method as claimed in claim 41, wherein the micromachining
of the target material involves trimming a thin-film electrical
element.
46. The method as claimed in claim 41, wherein the micromachining
of the target material involves trimming a resistor.
47. The method as claimed in claim 41, wherein the micromachining
of the target material involves trimming a capacitor.
48. The method as claimed in claim 41, wherein the micromachining
of the target material involves laser marking the target
material.
49. The method as claimed in claim 41, wherein the micromachining
of the target material involves micromachining a worksurface with a
controlled laser pulse width.
50. A method of providing a pulsed laser output, said method
comprising the steps of: providing a laser pump, a laser rod, a
reflector interposed between the laser pump and the laser rod, and
through which energy from the laser pump enters the laser rod, an
output reflector through which energy is emitted from the laser
rod, and a switch interposed between the laser rod and the output
reflector configured to be closed to cause energy to be stored in
the laser rod for a desired period of time, and to be opened to
allow energy to be emitted from the laser rod during an emission
period; selecting a time interval between at least two successive
transmissions of pulses onto a workpiece, based on known properties
of a target material to be processed on a workpiece, said step of
selecting a time interval being independent of a pulse shape to be
produced by the laser source; and causing the pulsed laser system
to be pulsed, with the pulse shape set independently of the time
interval, by causing the switch to be closed for a period of time
prior to each emission period that is fixed and predetermined for
the set pulse shape regardless of the time interval between the at
least two successive transmissions of pulses onto the workpiece, so
as to cause the laser source to process the target material on the
workpiece, while the time interval remains as set regardless of the
pulse shape, without setting of the pulse shape affecting the time
interval.
51. A method of providing a pulsed laser output, said method
comprising the steps of: providing a laser pump that is operable
continuously at constant power, a laser rod, a reflector interposed
between the laser pump and the laser rod, and through which energy
from the laser pump enters the laser rod, an output reflector
through which energy is emitted from the laser rod, and a switch
interposed between the laser rod and the output reflector
configured to be closed to cause energy to be stored in the laser
rod for a desired period of time, and to be opened to allow energy
to be emitted from the laser rod during an emission period;
selecting a pulse energy characteristic to be produced by the laser
source, based on known properties of a target material to be
processed on a workpiece, said step of selecting a pulse energy
characteristic being independent of a time interval between at
least two successive transmissions of pulses onto the workpiece;
and causing the pulsed laser system to be pulsed, while the laser
source is continuously pumped at constant power, so as to cause the
laser source to process the target material on the workpiece, with
the set time interval between the at least two successive
transmissions of pulses onto the workpiece, while the selected
pulse energy characteristic remains as set regardless of the time
interval, without setting of the time interval affecting the pulse
energy characteristic.
52. A method of providing a pulsed laser output, said method
comprising the steps of: providing a laser pump, a laser rod, a
reflector interposed between the laser pump and the laser rod, and
through which energy from the laser pump enters the laser rod, an
output reflector through which energy is emitted from the laser
rod, and a switch interposed between the laser rod and the output
reflector configured to be closed to cause energy to be stored in
the laser rod for a desired period of time, and to be opened to
allow energy to be emitted from the laser rod during an emission
period; selecting a time interval between at least two successive
transmissions of pulses onto a workpiece, based on known properties
of a target material to be processed on a workpiece, said step of
selecting a time interval being independent of a pulse energy
characteristic to be produced by the laser source; and causing the
pulsed laser system to be pulsed, with the pulse energy
characteristic set independently of the time interval, by causing
the switch to be closed for a period of time prior to each emission
period that is fixed and predetermined for the set pulse energy
characteristic regardless of the time interval between the at least
two successive transmissions of pulses onto the workpiece, so as to
cause the laser source to process the target material on the
workpiece, while the time interval remains as set regardless of the
pulse energy characteristic, without setting of the pulse shape
affecting the time interval.
53. A method of providing a pulsed laser output, said method
comprising the steps of: providing a laser pump that is operable
continuously at constant power, a laser rod, a reflector interposed
between the laser pump and the laser rod, and through which energy
from the laser pump enters the laser rod, an output reflector
through which energy is emitted from the laser rod, and a switch
interposed between the laser rod and the output reflector
configured to be closed to cause energy to be stored in the laser
rod for a desired period of time, and to be opened to allow energy
to be emitted from the laser rod during an emission period;
selecting a pulse width to be produced by the laser source, based
on known properties of a target material to be processed on a
workpiece, said step of selecting a pulse width being independent
of a time interval between at least two successive transmissions of
pulses onto the workpiece; and causing the pulsed laser system to
be pulsed, while the laser source is continuously pumped at
constant power, so as to cause the laser source to process the
target material on the workpiece, with the set time interval
between the at least two successive transmissions of pulses onto
the workpiece, while the selected pulse width remains as set
regardless of the time interval, without setting of the time
interval affecting the pulse width.
54. The method of claim 53 wherein the laser source is a
continuously pumped laser source and wherein the method further
includes closing the switch to cause energy to be stored by the
laser source for a desired period of time, and opening the switch
to allow energy to be emitted from the laser source during an
emission period.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to controlling pulses in laser
systems and more particularly relates to controlling the width and
energy of pulses at differing repetition rates during
micromachining procedures such as resistor trimming or capacitor
trimming.
[0002] The pulse width of a laser typically increases with
increased repetition rate (i.e., the rate at which pulses are
emitted by the laser). This is because at high repetition rates the
time to store energy in the laser rod prior to each pulse is short
and at low repetition rates the time to store energy in the laser
rod prior to each pulse is long. Hence, on a per pulse basis, there
is great variation in the energy output and temporal pulse width as
the repetition rate is varied.
[0003] This effect is due to the fact that the energy that can be
extracted from a laser rod depends on the energy stored in the rod.
For example, at a repetition rate of 30 kilohertz there are only
about 33 microseconds available to store and open a Q-switch to
allow a laser pulse to be emitted, whereas at 1 kilohertz there are
about a thousand microseconds available to store and Q-switch. The
gain in a laser is proportional to quantity of energy stored in the
rod. Therefore, when a laser pulse is instigated at a low
repetition rate it sweeps up much more quickly than it would at a
higher frequency because there is more energy stored in the rod,
resulting in a shorter temporal pulse width.
[0004] For a given energy per pulse the peak power varies inversely
with the laser pulse width. Therefore, the peak power of a
300-nanosecond pulse is much less than the peak power of a
100-nanosecond pulse having the same total energy. The total energy
per pulse delivered to the workpiece is typically controlled by a
device that attenuates the beam; laser pulses at 1 kilohertz would
be attenuated more than laser pulses at 10 kilohertz in order for
the pulses in each instance to have the same total energy.
[0005] It is possible to widen laser pulses provided by a given
laser at low repetition rates by lowering the energy stored in the
laser rod when the laser is operated at low repetition rates. This
can be accomplished by lowering the amount of energy that enters
the rod from the laser pump. The Light Wave Electronics Model 110
laser works according to this principle.
[0006] It is also possible to ensure similar pulse widths at
differing repetition rates by pumping energy into the laser rod
prior to each laser pulse for about the same storage time period
regardless of the repetition rate. After this high energy storage
time but prior to opening of the Q-switch, the energy that is
pumped into the laser rod is reduced to a level that is just above
a threshold required to compensate for losses in the energy stored
in the laser rod. This reduced energy level can be maintained until
the Q-switch is opened to allow a pulse to be released from the
laser rod.
[0007] General Scanning's M320 pulsed laser system is an example of
a system that does not ensure similar pulse widths at differing
repetition rates. In this system, an acousto-optic modulator (AOM),
is placed between the laser and the workpiece. As the laser scans
over a workpiece, the acousto-optic modulator blocks laser pulses
from impinging on the workpiece except when a laser pulse is needed
to remove a link on the workpiece. In order to remove a link, the
acousto-optic modulator allows a single pulse, emitted immediately
after opening of the Q-switch, to impinge on the link The
acousto-optic modulator can allow only a fraction of the energy of
the pulse to impinge on the link, as desired.
[0008] Togari et al., U.S. Pat. No. 5,719,372 describes a laser
marking system in which laser pulses create holes in a workpiece
that form a marking. Each emission period, during which the
Q-switch is off (open), is sufficiently long to allow the laser to
emit a primary emission pulse and a plurality of secondary emission
pulses, all of which impinge upon the workpiece. The intensities of
these primary and secondary emission pulses are less than the
intensity of the single emission pulse that would be emitted if the
emission period were shorter and the repetition rate kept the same.
The low-power secondary emissions deliver extra energy to the
workpiece. The patent claims that the low-power secondary emissions
result in improved visibility of marking lines in a workpiece that
includes a resin film containing carbon.
SUMMARY OF THE INVENTION
[0009] One aspect of the invention features a pulsed laser system
that includes a laser pump (e.g., a continuous wave (CW) pump), a
laser rod, a reflector interposed between the laser pump and the
laser rod, through which energy from the laser pump enters the
laser rod, an output reflector through which energy is emitted from
the laser rod, and a switch (e.g., a Q-switch) interposed between
the laser rod and the output reflector. Further there is a control
device, which may be external to the laser resonator. The Q-switch,
when closed, causes energy to be stored in the laser rod and, when
opened, allows energy to be emitted from the laser rod during an
emission period. The control device allows a primary laser pulse
emitted from the laser rod during the emission period to impinge on
a workpiece and prevents at least a portion of secondary laser
emission occurring after the primary pulse during the emission
period from impinging on the workpiece.
[0010] The diode pumped laser technology according to the invention
provides flexibility in and control over the pulse width, along
with the repetition rate, in order to optimize performance. The
invention makes it possible to use a laser that has short pulse
widths at high repetition rates to process a workpiece (for
example, to perform resistor trimming) at low repetition rates
without emitting unduly short pulses. The low repetition rates may
be especially useful for certain applications such as trimming high
valued resistors.
[0011] The invention does not require any reduction in the output
of the laser pump in order to provide wide pulses at low repetition
rates. Thus, it is not necessary to redesign or otherwise
accommodate the power supply electronics and feedback circuitry
that are designed to ensure a stable output of the laser pump.
Also, the invention does not require energy to be pumped into the
laser rod at a reduced level during the portion of the emission
period following emission of the primary laser pulse. Thus, the
invention need not concern itself with errors that might be
introduced into the total energy stored in the laser rod following
emission of the primary laser pulse, which errors would be
especially significant at low attenuation.
[0012] Because a control device is provided that prevents unwanted
output emitted during the emission period after emission of the
primary pulse from impinging on the workpiece, this portion of the
laser output does not affect the temperature of the workpiece, and
therefore does not affect measurements that might take place prior
to each primary pulse, which may be temperature-sensitive, and does
not affect performance of the workpiece. For example, in trimming
of thick-film resistors, resistance measurements might take place
immediately prior to each primary pulse. In micromachining of a
semiconductor circuit on a silicon substrate, elimination of
secondary pulses and a continuous wave output can prevent undue
heating of the silicon substrate and thereby protect the silicon
substrate against damage.
[0013] Another aspect of the invention features a method in which
the pulsed laser system is operated over a range or repetition
rates, so as to cause laser energy to be emitted during a plurality
of emission periods at each repetition rate. At least a portion of
the laser energy emitted during the emission periods is directed
toward the target structure. The switch is closed for a fixed,
predetermined period of time prior to each emission period
regardless of repetition rate of the primary laser pulse within the
range of repetition rates. The pump is operated continuously at
constant power.
[0014] Numerous other features, objects, and advantages of the
invention will become apparent from the following detailed
description when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a block diagram that discloses the major
components of a laser system according to the invention.
[0016] FIG. 2 is a block diagram that discloses the major
components of an alternative laser system according to the
invention.
[0017] FIG. 3 is a set of waveforms illustrating operation of the
laser system of FIG. 1 at a high, fixed repetition rate, and also
at a low repetition rate.
[0018] FIG. 4 is a diagram illustrating the power of the output of
the laser system of FIG. 1 as a function of time in the absence of
an acousto-optic modulator.
[0019] FIG. 5 is a diagram similar to FIG. 4, having a reduced time
scale.
[0020] FIG. 6 is a diagram illustrating the power of the output of
the laser system of FIG. 1 as a function of time, where an
acousto-optic modulator is used to dump a continuous wave output
from the laser onto a heat sink. Thus, FIG. 6 is the same as FIG. 4
with the secondary, unwanted pulses removed.
DETAILED DESCRIPTION
[0021] In trimming of thick-film resistors, the optimal peak laser
pulse power, pulse width, and pulse energy depends on the type and
thickness of the resistor paste material.
[0022] For example, high-ohm pastes generally contain less metal
than low-ohm pastes and are generally thicker than low-ohm pastes.
High-ohm pastes generally require a longer laser pulse width than
low-ohm pastes because heat conduction in the semi-insulating
high-ohm pastes generally takes more time than in low-ohm
pastes.
[0023] In contrast, low-ohm pastes tend to contain a great deal of
metal. These pastes have a tendency to conduct heat laterally away
from the kerf (cut) produced by the laser pulse. Short laser pulses
tend to limit the likelihood of this lateral conduction in these
low-ohm pastes.
[0024] Thick-film resistors typically are about 5 microns thick,
and therefore it takes time for a laser pulse to heat through the
entire resistor due to thermal diffusivity. Typically, 100
nanoseconds is a good pulse width for such a resistor, but below 70
nanoseconds the pulse might not have enough time to penetrate all
the way through the resistor and hence might leave some resistor
material at the bottom of the kerf. This material can promote
leakage currents and compromise resistor performance. At a pulse
width of 300 nanoseconds, on the other hand, the resistor will be
penetrated completely, but heat might tend to dissipate laterally
through the resistor because the pulse is so long. This lateral
heat conduction can result in a melting zone and residue at the
edge of the resistor kerf, which can change the temperature
coefficient of resistance (TCR) and cause microcracking.
Microcracking can, in turn, cause long-term resistance drift.
[0025] In addition to pulse width, the energy of the laser pulses
is also important because a certain amount of energy is required to
vaporize the resistor material.
[0026] Also, the speed of trimming is important, with high trimming
rates typically being desirable. The ultimate limit to the trimming
rate is a function of the amount of energy per pulse that is to be
delivered to the resistor. This energy per pulse is approximately
200 to 300 microjoules per pulse, depending on the type of resistor
paste that is used. For a laser having an average power of 7 watts,
if the desired energy per pulse is 200 microjoules, the repetition
rate of the pulses cannot exceed 7 watts divided by 0.0002 joules,
or 35 kilohertz. If the desired energy per pulse is 300 microjoules
then the repetition rate of the pulses cannot exceed 7 watts
divided by 0.0003 joules, or about 23 kilohertz. The higher energy
per pulse of 300 microjoules would typically be used for low-ohm
materials, which are ordinarily trimmed at lower repetition rates
anyway.
[0027] The dynamic pulse width control technique described below is
implemented using a high-power, short pulse width, diode-pumped
laser. This laser system can provide 30-nanosecond pulses at low
repetition rates and 125-nanosecond pulses at 50 kilohertz, in
comparison to lamp-pumped laser systems that provide 70-nanosecond
pulses at low repetition rates and 300-nanosecond pulses at 40
kilohertz. Nevertheless, the dynamic pulse width control technique
can alternatively be implemented using a lamp-pumped laser
system.
[0028] The dynamic pulse width control described below can allow a
laser, such as the Spectra Physics DPL laser system, for example,
to provide 125-nanosecond pulse widths at any repetition rate, from
a single pulse to 50 kilohertz.
[0029] While high repetition rates are typically desirable, such a
laser might be operated at low repetition rates so as to allow
resistance measurements to be made, between the pulses, while the
resistor is being trimmed. If the resistance to be measured is very
high, then it might typically take a relatively long time to
perform each measurement accurately, and thus a lower repetition
rate might be desirable. According to the invention, such a laser
can be operated at a low repetition rate of about 1 kilohertz, for
example, at a pulse width of about 125 nanoseconds, rather than 30
nanoseconds (which would be typical without the dynamic pulse width
control). The dynamic pulse width control ensures that the pulse
width is long enough to cut through the resistor material to the
bottom of the resistor.
[0030] With reference to FIG. 1, laser 10 includes an energy
storage rod 12 and a diode pump 14, which is pumped continuously.
Energy From the diode pump enters the laser rod 12 through a lens
16 and a 100-percent reflector 18. An acousto-optical Q-switch 20,
which is essentially an optical switch, is switchable on and off to
cause energy to remain stored in laser rod 12. When Q-switch 20 is
turned on, lasing action is inhibited, thereby allowing energy from
the laser diode 14 to be delivered to the rod 12. The energy stored
in laser rod 12 will increase, because Q-switch 20 blocks emission
of the laser beam. About a microsecond or two alter Q-switch 20 is
turned off (opened) a laser pulse is emitted from laser rod 12
through reflector 24. An X-scanning mirror and a Y-scanning mirror
(not shown) move the pulsed laser beam to perform trimming of a
thick-film resistor.
[0031] The period of time during which Q-switch 20 is off is the
"emission period." The rate at which Q-switch 20 is activated is
known as the "repetition rate."
[0032] In one application of the resistor trimming system of FIG.
1, laser 10 has an intrinsically short pulse width at all
repetition rates. However, the energy, pulse width, and peak power
of the laser pulse depend upon the amount of energy storage. At low
repetition rates the energy storage is high and therefore the pulse
width is short and the energy per pulse and peak power are high. As
is explained in detail below, during operation of the resistor
trimming system at low repetition rates, Q-switch 20 remains open
after the laser pulse subsides, and a secondary emission that
includes a series of secondary pulses and continuous wave (CW)
output is emitted from laser 10. These secondary pulses result from
the fact that energy is continuously being pumped into energy
storage rod 12 by diode pump 14. Whenever the stored energy
resulting from this continuous input exceeds a threshold (the
minimum energy required to overcome losses in the energy storage
rod system), a secondary pulse is emitted.
[0033] The energy of the primary pulse is essentially equal to the
power of pump diode laser 10 multiplied by the which the Q-switch
RF power is on, causing the Q-switch to be closed. If the storage
time is about 30 microseconds and the power of the laser is about 7
watts, the total energy in the primary pulse is about 210
microjoules.
[0034] At one kilohertz, the primary pulse as well as the
continuous wave output together have a total energy far in excess
of 210 microjoules. The primary pulse output is deflected towards
the workpiece by an acousto-optic modulator (AOM) 26, which
operates in synchronization with Q-switch 20. The acousto-optic
modulator 26 is sufficiently fast in operation to allow the primary
laser pulses to deflect to the workpiece and then, by switching
off, to dump the continuous wave output and secondary pulses from
the short-pulse laser onto a heat sink 28. The acousto-optic
modulator 26 deflects the primary laser pulses with at least eighty
percent efficiency, and more preferably about ninety percent
efficiency or higher, and all of the continuous wave output and
secondary pulses of short-pulse laser 10 are dumped onto heat sink
28.
[0035] Other optical shutters, such as an electro-optic modulator,
a liquid crystal modulator, or a high-speed optical switch, may be
substituted for acousto-optic modulator 26.
[0036] According to an alternate method, the primary beam passes
through acousto-optic modulator 26, which deflects the unwanted
continuous wave output and secondary pulses with ninety percent
efficiency as shown in FIG. 2. The method to be used is determined
by the nature of the micromachining application. Thus, according to
the method of FIG. 2, an optical shutter diffracts, deflects,
redirects, or otherwise shutters unwanted laser output away from
the workpiece, whereas according to the method of FIG. 1, an
optical shutter diffracts, deflects, redirects, or otherwise
shutters desired laser output toward the workpiece.
[0037] FIG. 3 illustrates operation of the mechanism for dynamic
pulse width control using the primary beam deflection technique as
shown in FIG. 1. The top three waveforms of FIG. 3 illustrate
operation of the short-pulse laser at a high, fixed repetition rate
without the dynamic pulse-width control. In this case the Q-switch
is operated at the maximum desired repetition rate (40 Khz is shown
for this example). The rise time of a laser trigger pulse 30
triggers activation of an RF Q-switch control signal 32, which in
turn is applied to the Q-switch to control the on/off state of the
Q-switch. While RF Q-switch control signal is activated 32, the RF
signal, which is applied to the Q-switch, causes the Q-switch to be
in its "on" (closed) state, and the Q-switch blocks emission of the
laser beam. This causes energy to be stored in the laser rod. The
falling of laser trigger pulse 30 causes RF Q-switch control signal
32 to be de-activated, and because there is no RF signal applied,
the Q-switch is caused to be in its "off" (open) state. This causes
optical power to build up within the laser cavity (the laser cavity
consists of everything located between the 100 percent reflector
and the output mirror), and a short time later a laser pulse 34 is
emitted during the "emission period." After the emission period,
another laser trigger pulse 30 activates RF Q-switch control signal
32 again to cause energy to be stored in the laser rod. At this
high repetition rate, the "on" time for RF Q-switch control signal
32 is set to allow the maximum laser storage time between pulses
34. For example, at a repetition rate of 40 kilohertz, the storage
time would be about 25 microseconds minus the time required for a
pulse 34 to build up in the laser cavity and emit (which is about
two to three microseconds). In alternative embodiments, the
Q-switch may be an electro-optical Q-switch, and a high-voltage
Q-switch control signal may be used instead of an RF Q-switch
control signal.
[0038] The bottom five waveforms in FIG. 3 illustrate operation of
the short-pulse laser at a low, fixed repetition rate using the
dynamic pulse width control. RF Q-switch control signal 32 is
activated only for a period of time necessary to provide the
desired energy storage per pulse and the desired pulse width,
which, for this example, is the same energy storage per pulse and
pulse width as in the high-repetition-rate example of the top three
drawings in FIG. 3. After the time required for energy storage has
elapsed, RF Q-switch control signal 32 is deactivated, which causes
the Q-switch to be turned off so as to allow the laser to emit an
output. The Q-switch is maintained in its off state until storage
for the next emission period is desired. According to this method,
the energy storage time corresponds to the desired energy per pulse
and the desired pulse width. At low repetition rates, as shown in
the bottom five waveforms in FIG. 3, the laser will build up and
lase in a continuous wave mode output 38 during the emission
period, after the primary pulse 36 has been emitted. The output 38
after the primary pulse consists of a series of secondary pulses
emitted from the laser followed by a CW output. In the embodiment
of FIG. 1, an acousto-optic modulator signal 40 triggers the
acousto-optic modulator just before the primary pulse 36 is to be
emitted, in order to deflect the primary pulse towards the
resistor, and then switches the AOM off so that the unwanted
secondary and CW output is dumped on the heat sink as illustrated
in FIG. 1. Accordingly, unwanted heating of the resistor to be
trimmed is prevented.
[0039] FIGS. 4 and 5 illustrate the power of the laser output,
including primary pulse 36, and secondary pulses and continuous
wave output 38, as a function of time in the absence of an
acousto-optic modulator. FIG. 6 illustrates the power of the laser
output where the acousto-optic modulator has dumped the continuous
wave output from the laser onto a heat sink.
[0040] The operator can choose a desired laser pulse width by
computer control. The computer is preprogrammed using a look-up
table to provide the correct Q-switch storage time for the desired
laser pulse width. The computer also provides the correct timing
signal for the AOM deflector. Once the operator has chosen a
desired pulse width, then the laser can be operated at any
repetition rate below the maximum repetition rate that corresponds
with this storage time, without change in the total energy per
pulse or the pulse width. Thus, the energy delivered to the
resistor, the pulse width, and the peak power are fixed at constant
values over all repetition rates.
[0041] There have been described novel and improved apparatus and
techniques for controlling pulses in laser systems. It is evident
that those skilled in the art may now make numerous uses and
modifications of and departures from the specific embodiment
described herein without departing from the inventive concept. For
example, while applications of the apparatus to thick-film resistor
trimming have been disclosed, other applications of the technology
are also possible, such as thick-film capacitor trimming,
micro-machining of semiconductor circuits on silicon substrates,
thin-film trimming of resistors or capacitors, link blowing,
etc.
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