The invention is related to a radio-frequency (RF)-excited gas discharge lasers, and in particular but not exclusively, to a self-oscillating dual tap RF-excited gas discharge laser.
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 a pair of electrodes. One example of a gas laser is a carbon dioxide laser. RF-excited gas lasers have found many applications because of their compact size, reliability, and relative ease of manufacture.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings, in which:
Various embodiments of the present invention will be described in detail with reference to the drawings, where like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the invention, which is limited only by the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the claimed invention.
Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” includes plural reference, and the meaning of “in” includes “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. As used herein, the term “or” is an inclusive “or” operator, and is equivalent to the term “and/or,” unless the context clearly dictates otherwise. The term “based, in part, on”, “based, at least in part, on”, or “based on” is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. The term “coupled” means at least either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, charge, temperature, data, or other signal. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the words “gate”, “drain”, and “source” includes “base”, “collector”, and “emitter”, respectively, and vice versa. Further, where an RF power grid tube may be used in place of a transistor, the scope of the words “grid”, “plate”, and “cathode” includes “gate”, “drain”, and “source” respectively, and vice versa.
Briefly stated, the invention is related to a Colpitts oscillator that includes an RF-excited gas discharge laser tube as the feedback pi-network of the Colpitts oscillator.
Laser tube 110 is a radio frequency (RF)-excited gas discharge laser tube. Virtually any RF-excitable gas discharge laser tube may be used for laser tube 110, although some laser tubes may need to be modified to ensure that that is a first tap connected to a first electrode and a second tap connected to the second electrode. Laser tube 110 has a ground input GND, a first tap Tap1 connected to node N1, and a second tap Tap2 connected to node N2. Electrode E1 is connected to node N1, and electrode E2 is connected to node N2. Also, there is a discharge region 120 between electrode E1 and electrode E2. A gas load, such as carbon dioxide or other type of lasing gas, fills discharge region 120 during operation of the laser. When excited by an RF signal provided by oscillator 100, an electric field develops between electrode E1 and electrode E2, causing plasma breakdown and therefore a discharge in the gas load in discharge region 120.
Capacitance C0 represents the lumped equivalent capacitance at node N1, and capacitance C1 represents the lumped equivalent capacitance at node N2. Inductor circuit L1 may include one coil, or by two or more coils arranged in series and/or in parallel to provide an equivalent inductance L1. Capacitances may also be includes among the coils. In one embodiment, inductor circuit L1 includes two or more inductive coils that are each in parallel with discharge region 120.
Oscillator 100 is arranged as a classic Colpitts oscillator, except that laser tube 110 is the feedback pi-network of the Colpitts oscillator. Laser tube 110 is accordingly arranged for self-oscillation for RF excitation where laser tube 110 is part of the oscillator.
RF choke 130 is provides DC voltage at its output at the operating frequency. RF choke 130 is arranged to allow DC current to flow to the drain of transistor M0 without letting any of the RF current to flow backward into the power supply. Capacitor C2 is a DC blocking capacitor. Capacitor C3 is also a DC blocking capacitor, and capacitor C3 also acts as a feedback circuit. Capacitor C3 provides a feedback signal to the gate of transistor M0 based on output the signal at Tap2, but prevents full power from going to the gate of transistor M0.
Laser tube 110 has a capacitance present between node N1 and the housing of the laser tube 110. This capacitance may be, at the very least, parasitic due to insulating structural supports for the electrodes and the free space between the electrodes and the housing. In some embodiments, this capacitance may be deliberately increased to increase the Q-factor of the laser tube. The parallel combination of this capacitance and L1 determines the resonant frequency of laser tube 110.
Although transistor M0 is used as the active device in one embodiment, in other embodiments, a different type of active device may be employed, as shown in
A simple Colpitts oscillator has a phase shift of 180 degrees. To achieve class E operation, a phase of 196 degrees is employed. Accordingly, to achieve class E operation, an embodiment of Colpitts oscillator 100 in which the phase shift is 196 rather than 180 degrees may be employed. However, the designer must keep in mind the differences between a gas load and a simple load. Design of a class E oscillator with a gas load (which tends to act as a power-dependent, frequency-dependent load) is more complex than design of a class E oscillator with a simple resistive load.
One embodiment of oscillator 200 is illustrated in
Although
Oscillator 300 is arranged as a dual Colpitts oscillator with laser tube 310 as the feedback pi-network of the dual Colpitts oscillator. The dual transistor version provides roughly double the power to the laser tube as the single transistor version. Further, the embodiment of oscillator 300 illustrated in
As previously discussed, the active device such as power transistors M0 and M1 may be replaced with other types of active devices, power grid tubes, or the like.
Bias voltages Vbias1 and Vbias2 are applied to the gate of transistors M0 and M1 respectively at a voltage close to the threshold voltage of the transistor to ensure that oscillation begins.
Reactive component 470 is mounted external to laser tube 410. In some embodiments, the reactance of reactive component 470 may be pre-selected so as to compensate for the net reactance of the oscillator circuitry outside of laser tube 410, at the frequency of oscillation of laser tube 410. In some embodiments, the frequency of operation of laser tube 410 is equal to the resonant frequency of laser tube 410. In other embodiments, the frequency of operation of laser tube 410 is relatively close to but slightly different than the frequency of oscillation of laser tube 410.
In some embodiments, the oscillation circuitry outside of laser tube 410 is net inductive. In these embodiments, reactive component 470 may be an adjustable capacitor. In other embodiments, the oscillation-circuitry outside of laser tube 410 is net capacative. In these embodiments, reactive component 470 may be an inductor.
In one embodiment, reactive component 470 is inductor L6. Inductor L6 is mounted external to laser tube 410. Inductor L6 is adjustable while the laser is operating. In one embodiment, L6 is an air-coiled inductor with an inductance that may be adjusted by physically compressing or stretching the coil, thus allowing the inductance to be adjusted by about 5% to 10% from the nominal inductance of the coil. In other embodiments, the inductance is adjustable in other ways.
The inductance value of inductor circuit L1 may vary from part-to-part. However, inductor L1 is inside the laser tube box 410 and is therefore inaccessible after laser tube 410 has been assembled. However, external inductor L6 is accessible outside of the laser tube, and therefore may be used to fine tune the total equivalent inductance between nodes N1 and N2, in order to fine-tune the frequency and the longitudinal RF voltage distribution along the gas discharge length of the laser for optimal laser performance. Taps Tap1 and tap2 may be placed on the laser tube 410 in such a way that, when the inductance between nodes N1 and N2 is properly fine-tuned by adjusting inductor L6, a uniform voltage standing wave occurs in laser tube 410. This results in improved laser performance since the electric field is therefore substantially the same everywhere in laser tube 410.
As previously discussed, a simple Colpitts oscillator has a phase shift of 180 degrees. To achieve class E operation, a phase of 196 degrees is employed. In oscillator 500, phase-shifting network 540 includes reactive components for creating an overall phase shift of approximately 196 degrees as measured between the drain and gate of one of the active devices the active device (e.g. transistor M0) of oscillator 500. Phase-shifting network 541 includes reactive components for creating an overall phase shift of approximately 196 degrees as measured between the drain and gate of the other active device (e.g. transistor M1) of oscillator 500.
Although
Internal inductive coils L1 are distributed along the length of laser tube 610. In one embodiment, internal inductive coils L1 are distributed uniformly along the length of laser tube 610. Each of the reactive components 670 is coupled between a separate corresponding pairs of taps 690. Each pair of taps 690 includes a first tap that is coupled to the first electrode E1 (not shown in
In some embodiments, some of the oscillator circuits 660 may be replaced with RF power amplifiers.
In one embodiment, the reactance of each of the adjustable reactive devices 770 may be pre-determined according to the following calculations.
In this example, Ltap, is the inductance of each of coil L1 that is placed between the taps of a dual power oscillator on a laser tube to produce a uniform voltage distribution along the length of the tube for RF operating frequency, fo, while compensating for the dual power oscillator circuit capacitances Cosc that are in shunt with each tap. The total capacitance measured at either tap of the laser tube is Ctap. (Each of the lumped capacitances C0 and C1 is Ctap/2; the parallel capacitance of C0 and C1 is measured as Ctap). The resonant frequency of the tube with only the N internal coils and none of the M dual power oscillators installed is ftap. The inductance value of each of the N internal coils is Lcoil. Adding M coils of inductance value, Lcoil, across the taps would raise the resonant frequency to fo if the capacitances, Cosc, were not present. Therefore an additional inductance, Losc, must be added across each of the taps as well to eliminate the power oscillator circuit capacitance. Capacitances Coss and Ciss represent the output and input capacitance, respectively, of each of the power devices. In one embodiment of laser device 700, an oscillator as shown in
As previously discussed, Lcoil is the inductance value of each of the N individual coils. Lcoil is pre-determined by the designer as the having an inductance corresponding to the reactance (at frequency ftap) conjugate to the total equivalent capacitance inside the tube at frequency ftap. The parallel combination of the N coils is resonant with the series combination of C0 and C1 at frequency ftap. The capacitance at C0 and C1 is Ctap/2 each, and the series combination of C0 and C1 is Ctap/4.
Cosc is the total equivalent capacitance of each single oscillator. The total equivalent capacitance of the dual oscillator is Cosc/2. If only a single Colpitts oscillator were used, the total equivalent capacitance of the oscillator would be simply Cosc. The inductance Losc is pre-determined as the inductance corresponding to the reactance conjugate of total equivalent capacitance of the dual oscillator at the operation frequency fo.
In one embodiment of the invention, Losc as given in the above equation is the inductance that is used for inductor L6.
In other embodiments, external inductors L6 may also be used as substitute positions for locations of some of the inductors L1 internal to the laser tube. For example, in the embodiment described above, there are four internal coils and three external inductors. The external inductor values may be selected in such a way that they function in a similar manner to the internal inductors, and also provide compensation for the oscillator circuit. In this way, even though there are only four internal coils, it is as if there are seven internal coils. The four internal coils are evenly spaced within the laser tube. Each pair of taps, with the corresponding external inductor L6, is placed evenly between two adjacent pairs of internal coils, which amounts to seven uniformly distributed coils, each having an inductance of Lcoil.
In this embodiment, the inductance Ltap for each inductor L6 is pre-determined as the parallel combination of Lcoil and Losc.
In this embodiment, the Ltap value calculated above is used as the nominal inductance for each of the inductors L6. During operation of the laser, the inductors L6 are further adjusted to achieve maximum laser output.
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.