The present invention is directed to RF-excited gas lasers and methods of manufacturing and using the lasers. The present invention is also directed to RF-excited gas lasers with electrodes disposed in a metal housing and methods of manufacturing and using the gas lasers.
A radio frequency (RF)-excited gas laser produces laser energy when a gas medium within the laser is excited by the application of RF energy between one or more pairs of electrodes. One example of a gas laser is a carbon dioxide laser. Metal sealed gas lasers have found many applications because of their compact size, reliability, and relative ease of manufacture.
RF-excited gas lasers that include RF electrodes within a metal gas envelope produce regions of high electric field strength where gas discharge can take place. Typically, the desired region is situated between the electrodes where laser gain is produced. There can be, however, other regions within the gas laser with high electric field strength where gas discharges may occur. These ancillary gas discharges are generally undesirable because they can rob RF power from the desired region. In addition, the ancillary discharges may be intermittent in nature which may lead to random fluctuations in the output power of the laser.
Undesirable regions of high RF electric field strength can include those portions of the laser where an electrode is near the metal gas envelope (i.e., metal housing.) Some conventional gas lasers have sought to suppress these ancillary discharges by making the separation between the electrodes and the housing large or by placing a dielectric material in the region to displace the gas. This can increase the size of the metal gas envelope and increase the material cost of the laser, both of which may be undesirable consequences.
One embodiment is a RF-excited gas laser including a metal housing containing a laser gas medium; and a first electrode and a second electrode disposed in the metal housing. The first and second electrodes are configured and arranged for coupling to a RF source for exciting the laser gas medium between the electrodes. The metal housing and first and second electrodes are configured and arranged for application of a DC bias between i) the metal housing and ii) at least one of the first and second electrodes.
Another embodiment is a laser system including a RF source; a metal housing containing a laser gas medium; a first electrode and a second electrode disposed in the metal housing and coupled to the RF source for exciting the laser gas medium between the electrodes; and a DC bias circuit coupled to the first and second electrodes and configured and arranged for application of a DC bias between i) the metal housing and ii) at least one of the first and second electrodes.
Yet another embodiment is a method of operating a RF-excited gas laser by providing RF energy to at least one pair of electrodes disposed within a metal housing containing a laser gas medium to excite the laser gas medium between the at least one pair of electrodes. A DC bias is applied between the metal housing and at least one of the electrodes.
Optionally, for any of these embodiments, a DC bias can be applied between i) the metal housing and ii) both the first and second electrodes. Any suitable circuitry or method can be used for applying the DC bias including, but not limited to, applying the DC bias using a DC supply or by rectifying an RF signal.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified.
For a better understanding of the present invention, reference will be made to the following Detailed Description, which is to be read in association with the accompanying drawings, wherein:
The present invention is directed to RF-excited gas lasers and methods of manufacturing and using the lasers. The present invention is also directed to RF-excited gas lasers with electrodes disposed in a metal housing and methods of manufacturing and using the gas lasers.
The RF-excited gas laser typically includes one or more pairs of electrodes disposed within a metal gas envelope containing a laser gas medium, such as carbon dioxide, helium-xenon, nitrogen, hydrogen fluoride, deuterium fluoride, copper vapor, gold vapor, and the like. Examples of RF-excited gas lasers that can be modified to include a DC bias, according to the present invention, include those described in U.S. Pat. Nos. 4,805,182; 5,602,865; and 5,953,360, incorporated herein by reference. Other suitable RF-excited gas lasers include, but are not limited to, those commercially available from Synrad, Inc. (Mukilteo, Wash.). It will be recognized that the application of a DC bias can be used with many other RF-excited gas laser configurations.
Generally, desirable gas discharge occurs within the laser gain region between the RF electrodes when a RF field is applied. Unwanted ancillary gas discharge can occur in other regions with a strong RF electric field, such as in regions between the electrodes and the metal gas envelope. This discharge does not substantially contribute to the laser energy and may divert energy away from the laser gain region. A DC bias electric field can be applied between the RF electrodes and the metal gas envelope (e.g., housing) of the RF-excited gas laser to reduce, or even substantially eliminate, the ancillary gas discharge. While not wishing to be bound by any particular theory, it is believed that the DC bias electric field sweeps electrons out of those regions where the electrodes and the metal gas envelope are close to each other. This effect can result in reduction or elimination of ancillary gas discharge.
The application of a DC bias electric field between the RF electrodes and the metal gas envelope may also shape or enhance the RF electric field between the electrodes and the corresponding desired gas discharges. It is thought that the DC bias electric field may partially, or even fully, suppress the RF electric field between the RF electrodes and the metal gas envelope except in the desired region between the RF electrodes. This suppression may even occur in those regions where there is little or no ancillary gas discharge.
Any method for applying a DC bias electric field can be used. For example, the DC bias can be generated using a DC supply. As another example, the DC bias can be generated by rectifying a RF signal. Optionally, the RF signal can be from the same RF source that is used to generate the RF electric field.
The laser 100 also includes at least one pair of electrodes 126, 128. The electrodes 126, 128 typically extend along the length of the laser gain region 10625 (
The metal housing 122, and electrodes 126, 128 can be formed of any conductive material including metals, alloys, and the like. In one embodiment, the metal housing and electrodes are formed of aluminum. The electrodes 126, 128 can be bare metal, have a metallic coating, or be anodized to form a thin coating over at least a portion of the surface of the electrode. Anodization may raise the DC breakdown voltage threshold and allow higher values of DC bias voltage to be used.
A RF source 136 is typically coupled to the electrodes 126, 128. The metal housing 122 is typically grounded. The RF source provides RF excitation energy. In at least one embodiment, the RF energy is in the 20 to 200 MHz range and preferably at about 40 or 81 MHz. The RF signal can be continuous wave (CW) or pulsed.
The electrodes 126, 128 are typically separated from the metal housing 122 using non-conductive spacers 138 leaving gaps 142. The gaps can permit exchange or flow of the laser gas medium and may provide some convection cooling of the electrode. Preferably, the gaps 142 are sufficiently narrow that substantial gas discharge does not occur. However, there are generally regions 140 where the distance between the electrodes 126, 128 and the metal housing 122 are sufficiently spaced so that gas discharge may occur if a sufficient RF electric field is present.
To reduce gas discharge in regions other than RF region 134, a DC bias 144 is applied between the metal housing 122 and the electrodes 126, 128. The sign of the bias (positive or negative) will typically depend on the laser configuration and can be easily determined.
In at least some embodiments, the magnitude of the applied DC bias is at least 50 Volts, at least 100 Volts, at least 150 Volts, or more. It was found that, in at least some instances, the reduction of ancillary gas discharge (i.e., gas discharge outside the RF region) decreased with increasing DC bias voltage until a threshold level was met when, it is believed, substantially all of the ancillary gas discharge is halted. The application of a DC bias can reduce the ancillary gas discharge by at least 25%, at least 50%, at least 75%, at least 90%, at least 99%, or more. In some embodiments, the magnitude of the threshold level is at least 50 Volts, at least 75 Volts, at least 100 Volts, at least 150 Volts, or more. Preferably, the applied DC bias exceeds the threshold level; although lower levels of DC bias will provide some reduction in ancillary gas discharge.
One method for observing or measuring the ancillary discharges includes producing a special laser tube with one or more windows installed to allow observation of the regions where ancillary discharges occur. When the laser is powered up lines of glowing gas can be observed along the regions 140 between the RF electrodes and the housing when ancillary discharges are present. As the DC bias voltage is raised the ancillary discharges will become less luminous and dark segments of unlit discharge start to appear. A further increase in DC bias voltage shrinks the ancillary discharges down to a few locations, which may flicker on and off and sometimes move about along the regions 140. When the DC bias voltage reaches a threshold level the remaining ancillary discharge extinguish completely and all regions 140 are dark.
There is generally an upper limit to the amount of DC bias that can be applied before DC breakdown occurs and current flows between the electrodes and the metal housing. The DC breakdown voltage will depend on, for example, the geometry of the metal housing and electrodes. In some embodiments, the DC breakdown voltage is in the range of 225 to 450 volts.
Application of a DC bias may also facilitate redistribution of the RF power into the desired center RF region 134. Such a redistribution is particularly advantageous because the most desirable optical mode passes through a circular cross-sectional area centered in this RF region 134. The presence of a DC field between the electrodes 126, 128 and arms 130, 132 will be strongest where the electrodes and arms are closest and may even serve to redistribute the RF power deposited within the RF region 134 toward the center of the RF region 134. In an alternative arrangement, where arms 130, 132 are not present there is generally no significant DC electric field between the RF electrodes 126, 128.
A DC bias is provided by a DC supply 156. Any DC supply that can produce the desired DC bias voltage can be used. Preferably, the DC supply is a low power supply (for example, providing up to a few hundred milliwatts). The voltage supplied by the DC supply may be fixed or variable.
Preferably, the DC bias is provided through a resistor with a relatively high resistance value (e.g., at least 100K or 1M) to limit the current and to isolate the DC supply should there be an arc between the electrodes. The high resistance can also prevent or reduce the DC supply from loading the RF driver by making the RF impedance of the DC bias circuit at the tap much larger than the RF impedance of the laser tube itself.
A series inductor 160 may optionally be added to resist the flow of RF current into the DC supply 156. In one embodiment, the inductor has a value in the range of 300 to 600 nH. The inductor 160 may also raise the RF impedance presented by the DC supply 156.
A DC blocking capacitor 162 may also be used to resist application of the DC bias to the RF driver 150. In one embodiment, the capacitor has a value in the range of 500 to 1000 pF.
Optionally a small capacitor (not shown), on the order of 10 to 500 pF, can be placed in shunt with the DC supply 156 to provide a low RF impedance path to ground for any residual RF current that manages to flow through resistor 158. This further assures that RF current will not get into the DC supply and cause interference with the normal operation of the supply.
Another optional capacitor (not shown) can be placed in shunt with the node between the resistor 158 and the inductor 160. This capacitor can also provide a low impedance path to ground for residual RF current. In one embodiment, the capacitor value is on the order of 10 to 1000 pF. This second capacitor may also perform an additional function. As mentioned above, the potential exists for an arc to occur between the electrode 126 or 128 and the laser housing 122. Such an arc may be caused by a small metal particle that is inadvertently trapped inside the laser tube during manufacturing. The trapped metal particle will have no effect on laser operation unless the particle happens to become lodged in one of the narrow gaps 142 between an electrode and the housing. Once trapped in the gap 142 the particle may produce a fault, causing a malfunction of the RF driver. However, if the second capacitor is chosen to have a value on the order of, for example, 1000 pF to 1 μF (or more) there may be enough energy stored in the capacitor to vaporize the metal particle and clear the fault. It is also thought that the capacitance of the laser tube and RF driver alone, will provide some amount of stored energy for fault clearing action, when DC bias is applied. However, adding a capacitor and an inductor, in series combination, can increase the energy storage capacitance. The RF impedance of the series inductor-capacitor combination is preferably large and the DC resistance of the inductor is preferably small.
The rectification of the RF signal is performed using a rectifier circuit 166 such as that illustrated in
The DC bias is provided in this embodiment by a RF source 164 and rectifier circuit 166. It will be recognized that other embodiments could include the DC supply of
As described above, the application of a DC bias using the DC supply 156 of
The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.