Coherent light generators – Particular component circuitry
Reexamination Certificate
1998-11-24
2001-01-30
Arroyo, Teresa M. (Department: 2874)
Coherent light generators
Particular component circuitry
Reexamination Certificate
active
06181719
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Fields of the Invention
The present invention relates generally to an apparatus for and a method of applying a radio frequency power supply to ignite a gas laser and operate it, and, more particularly, to such a supply which uses multiple, independent RF generators to provide different frequencies which enable optimal performance of each operation of the gas laser.
2. Discussion of Background And Prior Art
In order to obtain good performance of a CO
2
gas laser with RF excitation, the RF power supply has to provide a high ignition voltage to the electrodes of the plasma tube to ignite the discharge in a laser gas mixture and, after ignition, it must provide high RF power to sustain the laser radiation output. These two independent conditions (maximum ignition voltage and maximum power) are difficult to achieve in actual practice because both ignition voltage and RF power depend on the output impedance of the power supply and the impedance of the tube. To further complicate the issue, the impedance of the tube is different for the two states of the tube (discharge ON and discharge OFF). Thus, once the RF power supply is tuned to provide maximum ignition voltage, the RF power would not be a maximum and vice versa. FIG.
1
.
From an electrical point of view, the conventional tube for the laser with RF excitation is a resonant circuit with a capacitance and an inductance associated with the electrodes, and the ignition of the discharge is the equivalent of adding a resistance and a capacitance to the circuit. As a result, the impedance of the tube is lower with the discharge ON than it is with the discharge OFF, and the resonance frequency of the tube when the discharge is ON is about 1-3 MHz lower than the resonance frequency of the same tube when the discharge is OFF. These issues make the problem of the design of the RF power supply a difficult one. As described in greater detail below, numerous attempts have been made in the prior art to solve these complex problems.
a. Optimal Power Transfer At The Operating Frequency
To deliver the optimum power to the gas laser discharge the impedance of the oscillator output circuit should be matched to the impedance of the plasma discharge at its operating frequency. In U.S. Pat. No. 4,343,126 Chenausky taught that when the discharge is made a tuned circuit as by coupling a coil between the electrodes, and if the real (ohmic) impedance of the discharge is matched to the output impedance of the driving oscillator while the imaginary (reactive) impedance of the tube is canceled by careful choice of the coil, then, in that case, the optimal operating frequency is a frequency about 3% lower than the resonant frequency, and continuous operation at this lower non-resonant optimal frequency is characterized by a great increase in the power transferred to the discharge.
b. Dual Impedance Matching
However, as pointed out above, there is a significant difference in conditions of the laser plasma after plasma breakdown compared to before plasma breakdown. This difference manifests itself in the large decrease in the laser plasma tube impedance after plasma ignition. Importantly, an RF excited CO
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laser will not readily ignite if the rf excitation is initially applied at the frequency and power level which is optimal for full power continuous operation. Nourrcier U.S. Pat. No. 5,150,372 (“Nourrcier”)
In an early attempt to solve the problem of requiring a much higher starting or ignition voltage than is required for normal operation or running, Sutter proposed in U.S. Pat. No. 4,455,658 using a power source having dual fixed element impedance matching circuits that have each been adjusted to provide a compromise between a first impedance match required for efficient, steady operation and a second impedance matching device coupling the electrodes to cancel the pre-ignition reactive impedance of the elongated chamber best for applying a high starting voltage to the electrodes. However, in actual practice, such compromise circuits cannot be optimized for either starting or efficient running.
As a proposed solution to this limitation, in U.S. Pat. No. 4,451,766 Angle (“Angle”) attempted to provide an RF power supply that provides a high voltage for starting and also an impedance match between the power supply and the gaseous medium of the laser for high efficiency of energy transfer during steady state operation, by providing a variable impedance matching circuit controlled by a feedback voltage applied to a varactor diode. However, while this approach gives good performance at low RF power levels, it is unreliable at high RF power levels when the RF voltage on the veractor exceeds 300-500 volts.
c. Low Energy Ignition Pulses And High Energy Emission Pulses At The Same RF Frequency
A further proposed solution to control laser output power in a system which uses pulse-width modulation of the RF power applied to the laser plasma tube is discussed in
Series
48
Lasers, Operation and Service Manual
, (Synrad, Inc.), Release v. 2.0, Oct. 18, 1995, page 10 (“Synrad”). In order to shorten the delay between a user's ON command pulse and laser emission, Synrad delivers a short 1 &mgr;s ignition pulse, having an energy level just below the laser emission threshold, at a 5 KHz pulse repetition rate which pre-ionizes the laser gas and a wider pulse thereafter which adds enough energy to the plasma to cause laser emission. This method allows the laser to respond predictably and almost instantaneously to the user's command signal. The problem with this approach is that the radio frequency of the short pulse is the same as the radio frequency of the long pulse so the ignition and operation of the laser are not optimized.
Thus, due to the inherent compromises necessary to provide two impedance levels matched to the amplifier, the high impedance prior to breakdown, and the low impedance after breakdown, an RF power supply tuned to a single frequency in a traditional implementation simply cannot satisfy optimum conditions for both ignition and pulsed or CW operation of a gas laser.
d. Single Fixed Frequency Generator With Manual Frequency Shifting
In an attempt to provide a multiple frequency power supply as a proposed solution to the problems experienced by Angle, Synrad, and others, Hesterman describes in U.S. Pat. No. 4,748,634 an RF power supply for gas lasers working on one frequency optimized for CW operation of a laser in which, in order to ignite the laser tube, the operating frequency of the single RF generator is momentarily shifted upwardly to the laser resonant frequency by changing the voltage of a variable capacitor in the bias circuit of the crystal oscillator to change the crystal frequency and thereby provide a large transient voltage for the laser ignition while using considerably less starting power.
While this approach is quite good for starting a laser and for CW operation and results in a smaller, lighter, and less expensive power supply, nevertheless, it suffers from low pulse to pulse reproducibility of the laser energy, especially in applications demanding a rapid and random change of pulse rate and pulse width of a laser (e.g., imaging, engraving and material cutting applications). The main reason for such lack of consistency in the pulse mode in this prior approach is the laser tube impedance dependency on the level and duration of the previous pulses.
e. Single Generator With Automatic Frequency Shifting Feedback Loop
Peter Laakman tried to solve Angle's problem in U.S. Pat. No. 4,837,772 by including the tuned circuit discharge in a circuit path feeding back via a quarter-wave impedance transformer 10% of the output power to the RF frequency power oscillator input which automatically self-adjusted its frequency depending upon the before or after ignition state of the discharge of the laser plasma tube. However, in this case, too, there is still a problem because it requires a very narrow range of the feedback parameters, a requirement that leads to significant difficulties in obtaining maximu
Mavroleon George M.
Ryskin Mikhail E.
Schultz David W.
Sukhman Yefim P.
Arroyo Teresa M.
Inzirillo Gioacchino
Lisa Donald J.
Universal Laser Systems, Inc.
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