Coherent light generators – Particular resonant cavity
Reexamination Certificate
1999-10-12
2002-07-02
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
C372S093000
Reexamination Certificate
active
06414980
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to continuously optically pumped, solid-state, repetitively-pulsed lasers. It relates in particular to stabilization of thermal-lensing in a solid-state gain-medium in a frequency-multiplied, repetitively-pulsed laser.
DISCUSSION OF BACKGROUND ART
Optically-pumped frequency-multiplied, repetitively-pulsed lasers are finding increasing use in laser material processing operations such as precision micromachining, marking, stereo lithography, and hard-disk texturing. One preferred laser type for this purposes is an extracavity frequency-tripled, pulsed, solid-state laser including a solid state gain-medium, such as Nd:YAG or Nd:YVO
4
, which provides fundamental radiation at a wavelength of about 1.064 micrometers (&mgr;m). A detailed description of one example of such a laser can be found in U.S. Pat. No. 5,912,912 (Caprara et al.) assigned to the assignee of the present invention.
Such a laser is often designed to operate in a continuous repetitive pulsing mode at a selected pulse-repetition rate. In laser processing operations, however, the laser may be caused to deliver bursts or trains of pulses at this pulse-repetition rate for processing, with intervals between bursts when no processing occurs. The duration of such bursts depend on the processing operation. Intervals between bursts may vary, for example, according to time required to move a processing beam from one location to another on a workpiece being processed. It has been found that for short bursts of pulses, if the duration of the burst is not much greater, if at all, than the time required for the solid-state gain-medium to reach a thermal-lensing equilibrium a substantial change of thermal-lensing occurs during all, or some major portion, of a burst. Such a thermal-lensing change can lead to a variation in peak power of laser pulses and laser mode properties over the duration of the burst, which, in turn, can lead to imprecise processing operations.
Thermal-lensing is due to a spatial variation in refractive index of the solid-state gain-medium resulting from a thermal gradient in the gain-medium. This thermal gradient results, among other factors, from heating of the gain-medium by a portion of pump-light light power absorbed therein which is not extracted as laser radiation. Accordingly thermal-lensing is a function of, among other factors, pump-light power delivered to the solid-state gain-medium, and energy extracted from the gain-medium as laser radiation. In a repetitively pulsed laser, this extracted power is, in turn, dependent on the pulse-repetition rate. As described in the Caprara et al. patent, a laser-resonator can be variably configured to be adjustable for accommodating a range of equilibrium thermal-lensing effects resulting from operating the resonator at different powers and pulse-repetition rates. In operating such a laser to deliver pulse-trains or bursts (burst mode operation), a thermal-lensing change can occur as a result of a transition from a condition where no pulse is being delivered to a condition where a burst of pulses is delivered. This can occur either before a laser processing operation begins or from one pulse-burst to the next.
When no pulse-burst is being delivered, the gain-medium remains continuously pumped, but laser action is inhibited by operating an optical switch (Q-switch) which introduces a variable of controlled loss in the resonator of the laser. In one example, a Q-switch is arranged to be driven (rapidly opened and closed) during delivery of a pulse-burst by a modulated voltage applied to an acousto-optic or electro-optic crystal located in the resonator. The modulation frequency of which establishes the pulse-repetition rate.
When no pulses are being delivered, a maximum proportion of the pump-light power contributes to thermal-lensing. When a pulse-burst is being delivered some proportion of the pump-light power is extracted from the gain-medium as laser radiation. This reduces the temperature in the gain-medium and, correspondingly the thermal-lensing. This temperature-reduction occurs progressively over a time-period depending on the thermal inertia of cooling the gain-medium.
Because the resonator is adjusted to compensate for thermal-lensing when the rod reaches an equilibrium temperature (equilibrium thermal-lensing), the output power will progressively increase, once pulsing is initiated, reaching a maximum at equilibrium. Because of this, laser beam parameters such as mode-size and divergence and peak pulse-power will vary for at least an initial portion of the burst duration. This problem will be exacerbated when pulse-bursts are of different duration or have different durations therebetween, as may be required in a sequence of laser-processing operations. If pulse bursts are of shorter duration than time required to reach equilibrium the peak pulse power therein may fall short of the maximum possible for the resonator.
There is a need for a method of overcoming this peak power variation problem during a burst or repeated bursts of pulses.
SUMMARY OF THE INVENTION
The method of the present invention is directed to a method of operating a laser for performing a laser processing operation. The laser has a laser-resonator including an optically-pumped gain-medium. One or more optically-nonlinear crystals are located outside the resonator for converting laser radiation delivered by the resonator into frequency-converted radiation. The peak power of the frequency-converted radiation is dependent on delivery parameters of the laser radiation from the laser-resonator. The gain-medium exhibits a thermal-lensing effect on being optically pumped. The laser-resonator is configured to compensate for a predetermined range of the thermal-lensing. In one aspect, the method of the present invention comprises a first step of operating the laser to deliver laser radiation with delivery parameters thereof being such that frequency-converted radiation generated therefrom has insufficient peak power to perform the laser processing operation. In a second step, following this first step, the laser is operated to deliver laser radiation in the form of a train of pulses with delivery parameters thereof being such that pulses in a train of pulses of frequency-converted radiation generated therefrom have sufficient peak power to perform the laser processing operation. These may be defined as processing pulses. During the first and second steps, the laser is operated such that optical pumping power and average power of the laser radiation provide that the thermal-lensing effect is within the predetermined range for which the laser resonator is compensated.
Providing that the thermal-lensing effect is within the predetermined range during the first and second step ensures that there is no significant thermal-lensing change between intervals when processing pulses are being delivered and intervals when processing pulses are not being delivered. By this arrangement, at least the second and all other processing pulses in a train thereof have about the same peak power. By controlling loss in the resonator with a delay time before the initiation of a processing pulse train, i.e., between the first and second steps, the first pulse in the pulse-train can be controlled such that all processing pulses in the pulse-train have about the same power. Accordingly, the method of the present invention avoids the gradual increase in peak power that is experienced in pulse trains or bursts delivered by prior art lasers.
The above described first and second steps can be repeated such that frequency-converted radiation is generated as a sequence of trains of pulses having sufficient power to perform the processing operation having intervals therebetween in which insufficient frequency converted power is generated for the processing operation. Accordingly, the method of the present invention provides that the peak power of pulses in the trains of processing pulses can be maintained essentially constant independent of the duration
Hicks Acle V.
Rea, Jr. Edward C.
Wang Charles Xiaoyi
Coherent Inc.
Ip Paul
Stallman & Pollock LLP
Zahn Jeffrey
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