Patent Application
20080240197
1. Field
The present invention relates to an improvement in excimer lasers. In particular, the present invention relates to an improvement for increasing the operational lifetime, reliability, efficiency, and/or performance of such lasers.
2. Description of the Related Art
Excimer lasers typically comprise a mixture of noble (or “rare” or “inert”) gases and halogens. When a voltage is applied to the gas mixture, the gas molecules become excited. When a noble gas is excited, it may temporarily bond with another noble gas, forming an excited dimer (or “excimer”), or much more commonly with a halogen, forming an excited complex (or “exciplex”). The spontaneous breakdown of the excimers and exciplexes, commonly referred to together as excimers, releases energy in the form of light at a specific wavelength. The excimer molecule dissociation takes on the order of nanoseconds, at which point light is no longer produced. Excimer lasers further comprise an optical cavity such that light produced by the gases resonates within the cavity.
Excimer lasers are utilized in many applications that demand approximately constant pulse energy throughout their life cycle. For example, a medical XeCl excimer laser, in which xenon is the noble gas and chlorine or HCl is the halogen, is used for phototherapy to provide substantial relief of the symptoms of several skin disorders including psoriasis. Such a laser may deliver, for example, about 10 mJ pulses between about 100 and 500 pulses per second to the diseased skin over a typical area of about 4 cm2. The number of light pulses to be delivered is determined by the skin type, location on the body, and severity of the disease. The pulses preferably have a constant energy for many applications to provide consistency in controlling applied therapeutic dosage.
As an excimer laser ages, contamination builds up within the laser gas and/or on the laser components, which reduces the pulse energy output. Eventually, the increased contamination will cause sufficient degradation in the output that the laser gas will need to be completely replaced, and eventually the chamber will need to be opened, cleaned, and refurbished. What is needed are methods for increasing the operating lifetime of the laser between such gas replacements and/or overhauls of the laser.
Various embodiments of the invention comprise an excimer laser comprising: a chamber configured to contain laser gases; first and second electrodes within the chamber, the first and second electrodes configured to energize laser gases in a region between the first and second electrodes to produce light emission from the laser gases; a plurality of reflective elements forming an optical resonant cavity configured to produce a laser beam from the light emission; a detector configured to measure an energy of the laser beam; a gas flow apparatus in fluid communication with the chamber; and a controller in communication with the gas flow apparatus and the detector, wherein the controller is configured to adjust a flow of the laser gases to alter a pressure in the chamber.
Some embodiments of the invention comprise a method of extending the lifetime of an excimer laser comprising a chamber containing laser gases, first and second electrodes within the chamber, and a plurality of reflective elements defining an optical resonant cavity, said method comprising: setting the laser gases to a first pressure; after setting the laser gases to the first pressure, applying a voltage to the electrodes, thereby propagating a laser beam in the optical resonant cavity; operating the laser for an amount of time; after the amount of time, measuring energy of the laser beam; and changing the pressure of the laser gases to a second pressure different from said first pressure.
Certain embodiments of the invention comprise a method of extending the lifetime of an excimer laser comprising a chamber containing laser gases, first and second electrodes within the chamber, and a plurality of reflective elements defining an optical resonant cavity, said method comprising: operating the laser at a first pressure of the laser gases; measuring energy of a laser beam; adjusting voltage applied to the first and second electrodes until the energy of the laser beam is substantially equal to a target energy at a first voltage; determining if the laser is operating in an optimized state; changing from the first pressure to a second pressure if the laser is not operating in an optimized state; and after changing to the second pressure, adjusting the voltage applied to the electrodes until the energy of the laser beam is substantially equal to the target energy at a second voltage.
Other embodiments are possible.
These and other features, aspects, and advantages of the invention disclosed herein are described below with reference to the drawings of preferred embodiments, which are intended to illustrate and not to limit the invention. The drawings comprise four figures in which:
Although certain preferred embodiments and examples are disclosed below, it will be understood by those in the art that the invention extends beyond the specifically disclosed embodiments and/or uses of the invention and obvious modifications and equivalents thereof. Thus, it is intended that the scope of the invention herein disclosed should not be limited by the particular disclosed embodiments described below.
As described above, as an excimer laser ages, contamination builds up within the laser gas and/or on the laser components, which reduces the pulse energy. In certain embodiments, specific materials can be used to fabricate the pressure vessel, the electrodes, the heat exchanger, and the fan of the laser to extend the lifetime of the laser. The criteria for selecting the appropriate materials for the laser can be found in U.S. Pat. No. 4,891,818, entitled “Rare Gas-Halogen Excimer Laser,” incorporated in its entirety herein by reference. Electrical energy stored in capacitors supplies energy to the laser gas via an electrical glow discharge. To maintain a constant pulse energy, the energy stored in the capacitors can be increased by increasing the charge voltage and hence the input energy, thereby compensating for degradation of the laser gas and laser components.
The efficiency of the laser is characterized by the ratio of laser pulse energy (U) to the energy stored in the capacitors (UC), as shown in Equation 1.
The energy stored in the capacitors is directly proportional to the capacitance (C) of the capacitors and the square of the charge voltage (V), as shown in Equation 2.
UC∝C·V2 (Eqn. 2)
Thus, the efficiency of the laser is inversely proportional to the square of the charge voltage, as shown in Equation 3.
Therefore, when the charge voltage is increased to keep the laser pulse energy, U, constant to compensate for gas and component degradation, the efficiency of the laser decreases with the square of that charge voltage. Although charge voltage can be continually increased to maintain output levels, there is a point at which the efficiency of the laser is so low that continued operation becomes impractical. In particular, operation at high voltage levels cause failure of laser components such as the electrodes.
Breakdown of components in the chamber, such as the electrodes, necessitates major maintenance and/or disassembly of the laser to be frequently undertaken. Such repairs and the associated downtime of the laser both introduce costs to, and reduce the productivity of, the laser. In addition, because the toxic and corrosive gases used in excimer lasers must be carefully handled during disassembly and subsequent reassembly of the laser, such procedures are complicated and potentially hazardous. This safety hazard is particularly troublesome when the excimer laser is utilized for medical procedures and is serviced proximate to locations where such medical treatment is provided. Accordingly, frequent disassembly is undesirable.
One way to reduce the frequency of such maintenance is to operate at a lower voltage or to decrease the rate at which the charge voltage is increased when adjusting the laser to maintain constant output energy. As described more fully below, by adjusting the pressure of the gases in the laser, the voltage need not be increased as quickly, thereby subjecting the entire laser system to less stress.
Excimer lasers are generally operated at an internal gas pressure that optimizes the efficiency for the initial charge voltage and electrode gap spacing. This internal pressure is traditionally not thereafter adjusted. Moreover, the pressure is traditionally not adjusted to maintain a constant optical output or to reduce the amount of increase in the charge voltage.
As described above, operation of excimer lasers at a point of increased or maximum efficiency is desirable to increase or maximize component reliability (e.g., by reducing the stress on the electrodes, etc.) and to increase or maximize the longevity of the laser gas. Referring again to Equation 3, when the charge voltage is at a lower value, the efficiency is typically higher. As charge voltage increases, the efficiency usually decreases. Accordingly, increases in the voltage to provide for constant optical output yield reduced efficiency and lifetime.
In various embodiments described herein, the optical output at different charge voltages and the pressure level of the laser gas are monitored to determine a suitable pressure and voltage. When charge voltage is to be increased to maintain pulse energy, the pressure of the laser gas is adjusted upward or downward to reduce or eliminate the change in voltage needed and thereby maintain efficiency. In certain embodiments, for example, the gas pressure is adjusted by at least about 2 psi, up to about 5 psi, or more. For example, a laser with an initial gas pressure of 35 psia and an initial charge voltage of 6,500 volts may be increased to a gas pressure of 45 psia and a charge voltage of 8,000 volts after a certain amount of time or number of pulses. This mode of operation provides a much higher overall operating efficiency and a longer service-free operating period than existing excimer lasers that modify only charge voltage and not pressure. For example, the laser lifetime may be increased from between about 5 and 12 million pulses to greater than about 60 million pulses or more. In certain embodiments, adjusting the pressure of the laser gas upward or downward can result in a laser lifetime of about 100 million pulses or more. Additionally, the use of lower charge voltages advantageously increases the lifetime of the laser gas and laser components such as the electrodes, laser windows, etc.
To understand how various embodiments described herein can be used to reduce the amount of voltage applied to the laser over time to maintain constant output, the dependency of output pulse energy on voltage and pressure is shown in
A horizontal line 210 represents a first target optical pulse energy UTarget,A selected by the user or otherwise established. This first target optical pulse energy UTarget,A desirably remains constant in certain embodiments. The curves P1, P2 intersect the line UTarget,A at a plurality of voltages 230. Thus, there is a different charge voltage that will result in the target pulse energy UTarget,A for each of the different pressures curves P1, P2. Various embodiments described in more detail below utilize this property to enable reduced voltages to be applied to the laser to maintain substantially constant optical output.
For most lasers, although there is usually a pressure that will produce the target pulse energy UTarget,A at a given voltage, an upper or maximum recommended pressure may exist for a given system (e.g., due to the strength of the seals used). A lower or minimum recommended voltage, (e.g., V1,min for the curve P1 and V2,min for the curve P2) may also exist for each pressure curve 220 in a given system. An upper or maximum recommended voltage, Vmax, (not shown) may also exist for each pressure curve 220 in a given system. In various preferred embodiments, the combination of pressure and voltage intersects the target pulse energy UTarget,A within the linear region 222.
For illustrative purposes,
The laser 10 further comprises an optical resonator 14 defining an optical path 13 at least partially included in the chamber 12 such that light propagating within the resonator 14 passes through the gas in the chamber 12. Electrodes 34, 38 are included within the chamber 12 on opposite sides of the optical path 13. A voltage applied to the electrodes 34, 38 excites the gases within the chamber 12, and particularly within the optical path 13 therebetween. Laser energy is thereby generated in the optical path 13 in the resonant cavity 14.
In the embodiment shown in
The laser energy can be coupled from the chamber 12 and delivered to another location, for example a treatment site on a dermatological patient, by using a flexible or rigid optical line (not shown) such as a fiber optic cable or liquid light guide. An example liquid light guide is provided in U.S. Pat. No. 4,927,231, entitled “Liquid Filled Flexible Distal Tip Light Guide,” which is incorporated in its entirety herein by reference. The laser energy can also be delivered by using a delivery system (not shown) including one or more mirrors. In certain such systems, the light may be guided or may propagate in free space such as through the air. Other designs are also possible.
As described above, the pulse energy delivered from the laser 10 will degrade over time. In certain preferred embodiments such as shown in
In some embodiments, the controller 20 comprises a microprocessor or computer that receives input from the detector 18 and drives the voltage supply 46 and valve control electronics 33, 35, which may comprise digital or analog electronics. Suitable A/D and D/A electronics may be used where appropriate. Other configurations are also possible.
A gas source 30 represented by a gas canister is also shown; however, this gas source 30 is not so limited. One or more gas sources 30 may be included and may provide more than one gas, either separately or in a mixture (e.g., the same mixture as in the chamber). In certain embodiments, multiple gas sources, each with separately controlled valves, supply different gases. These separate valves can be used to separately control the amount of gas introduced into chamber 12. In certain embodiments, the gas source 30 is at a higher pressure than the chamber 12 such that, upon opening of the gas inlet valve 32, laser gases flow from the gas source 30 into the chamber 12. In some embodiments, pumps may be used to flow laser gas from the gas source 30 into the chamber 12.
In order to monitor the output from the chamber 12, the detector 18 may be disposed in an optical path 13 forward of the second mirror 24 (
In certain embodiments, the gas pressure in the chamber 12 is increased by opening the inlet valve 32 and adding gas to the chamber 12 from the gas source 30 (e.g., because the gas source 30 is at a higher pressure than the chamber 12) while substantially preventing laser gases from flowing out of the chamber 12. In certain embodiments, the gas pressure in the chamber 12 is decreased by opening the outlet valve 36 (e.g., because the chamber 12 is at a higher pressure than the system downstream of the chamber 12) while substantially preventing laser gases from flowing into the chamber 12. The gas released from the chamber 12 may be vented to the atmosphere (e.g., after passing through a scrubber). An example laser configured to add and remove gas is provided in U.S. Patent Pub. No. 2007/0030876, entitled “Apparatus and Method for Purging and Recharging Excimer Laser Gases,” which is incorporated in its entirety herein by reference.
Referring again to
By contrast, as depicted in
In some embodiments, the chamber is designed to operate within a certain pressure range. As such, although a certain change in pressure without a change in charge voltage may produce the desired optical output intensity, a different change in pressure along with a change in charge voltage that also produces the desired optical output may be utilized, for example, to keep the pressure in the chamber closer to a desired operating range.
As represented by block 104, the charge voltage V is adjusted at PA such that the initial pulse energy U is substantially the same as the target pulse energy UTarget. As depicted in block 106, the laser 10 is then operated for an amount of time. The pulse energy U that is output by the laser 10 is then measured (e.g., by a detector 18 in communication with the controller 20), as shown in block 108. As represented by decision diamond 110, the controller 20 determines whether the pulse energy U is substantially the same as the target pulse energy UTarget. If the pulse energy U is substantially the same as the target pulse energy UTarget, the sequence in the blocks 106, 108, 110 is repeated. The repeated sequence includes: running the laser 10; measuring the pulse energy U; and comparing the pulse energy U to the target pulse energy UTarget. If the pulse energy U is not substantially the same as the target pulse energy UTarget, the voltage V is adjusted at PA until the pulse energy U is substantially the same as the target pulse energy UTarget (i.e., by increasing or decreasing the voltage V), as depicted in block 112.
As represented by decision diamond 114, the controller 20 determines whether the conditions of the laser 10 are optimized. If the voltage V is such that the conditions of the laser 10 are optimized (e.g., if the laser 10 is running at least at a predetermined minimum efficiency and/or if the voltage V is not greater than a predetermined voltage Vmax), the sequence in the blocks 106, 108, 110, 112, 114 is repeated. The repeated sequence includes: running the laser 10; measuring the pulse energy U; comparing the pulse energy U to the target pulse energy UTarget; continuing to run the laser 10 if substantially the same or adjusting the charge voltage V at PA until substantially the same; and determining if the conditions of the laser 10 are optimized. If the voltage V is such that the conditions of the laser 10 are not optimized (e.g., if the laser 10 is not running at a predetermined minimum efficiency or if the voltage V is greater than a predetermined voltage Vmax), the pressure is adjusted to PB, as shown in block 116.
As described above, the pressure PB may be higher than or lower than the pressure PA, depending on the target pulse energy UTarget. In some embodiments, PB is the maximum operating pressure of the laser 10. In certain alternative embodiments, PB is less than the maximum operating pressure of the laser 10. PB can be selected according to the particular design, operating characteristics and performance, applications, etc. of the laser 10 and gases in the chamber 12. In certain embodiments, it may be desirable to select PB so as to reduce (e.g., minimize) the value of the charge voltage V.
As represented by block 118, the charge voltage V is adjusted at PB such that the initial pulse energy U is substantially the same as the target pulse energy UTarget. As depicted in block 120, the laser 10 is then operated for an amount of time. The pulse energy U that is output by the laser 10 is then measured (e.g., by a detector 18 in communication with the controller 20), as shown in block 122. As represented by decision diamond 124, the controller 20 determines whether the pulse energy U is substantially the same as the target pulse energy UTarget. If the pulse energy U is substantially the same as the target pulse energy UTarget, the sequence in the blocks 120, 122, 124 is repeated. The repeated sequence includes: running the laser 10; measuring the pulse energy U; and comparing the pulse energy U to the target pulse energy UTarget. If the pulse energy U is not substantially the same as the target pulse energy UTarget, the voltage V is adjusted at PB until the pulse energy U is substantially the same as the target pulse energy UTarget (i.e., by increasing or decreasing the voltage V), as depicted in block 126.
As represented by decision diamond 128, the controller 20 determines whether the conditions of the laser 10 are optimized. If the voltage V is such that the conditions of the laser 10 are optimized (e.g., if the laser 10 is running at least at a predetermined minimum efficiency and/or if the voltage V is not greater than a predetermined voltage Vmax), the sequence in the blocks 120, 122, 124, 126, 128 is repeated. The repeated sequence includes: running the laser 10; measuring the pulse energy U; comparing the pulse energy U to the target pulse energy UTarget; continuing to run the laser 10 if substantially the same or adjusting the charge voltage V at PB until substantially the same; and determining if the conditions of the laser 10 are optimized. If the voltage V is such that the conditions of the laser 10 are not optimized (e.g., if the laser 10 is not running at a predetermined minimum efficiency or if the voltage V is greater than a predetermined voltage Vmax), then maintenance on the laser 10 is performed, as represented by block 130. This maintenance may include recharging the gases, cleaning the chamber, and/or replacing or repairing components. The laser 10 may indicate to the user that maintenance is required, for example by an alarm or indicator. In some embodiments, the controller 20 shuts down the laser 10 if charge voltage is above Vmax or if pressure exceeds Pmax.
In certain alternative embodiments, the laser 10 is configured to adjust between a plurality of pressures. For example, if the voltage V is such that the conditions of the laser 10 are not optimized (e.g., if the laser 10 is not running at a predetermined minimum efficiency and/or if the voltage V is greater than a predetermined voltage Vmax), the pressure may be adjusted to PC, as shown in block 132. The process may then continue through a set of steps similar to those represented in blocks 104, 106, 108, 110, 112, 114 and 118, 120, 122, 124, 126, 128. It will be appreciated that the number of possible pressures that the laser 10 may be set at between a minimum pressure and a maximum pressure could be large (e.g., infinite).
The pressure, P, may be increased or decreased to obtain the target optical intensity, UTarget. The voltage, V, may be increased or decreased to obtain the target optical intensity, UTarget, although in certain preferred embodiments the voltage, V, is increased as components of the laser 10 age. Such an increase in voltage, V, however, may be smaller per unit time than in systems in which the pressure, P, remains constant.
Other methods are also possible. For example, other methods of determining the suitable voltage and pressure may be used. Other parameters in addition or in alternative to Vmax and efficiency can be used to determine when maintenance is appropriate. More generally, other steps may be added, steps may be removed, or all or a portion of the steps may be reordered.
Referring again to
The different components within the feedback/control system 6 may be electrically connected using electrically conductive paths such as, but not limited to, wires and traces. However, communication may be otherwise as well. Communication and electrical connection, for example, may be wireless, via, e.g., microwave, RF, etc. Optical signals may also be used. Likewise, the components may be included in a single unit or separated by a distance. For example, the controller 20 may be remote or separate from the laser 10.
The methods and processes included herein, e.g., in the block diagram of
Additionally, some or all the processing can be performed all on the same device, on one or more other devices that communicates with the device, or various other combinations. The processor may also be incorporated in a network, and portions of the process may be performed by separate devices in the network. Display of information, e.g., user interface images, can be included on the device, can communicate with the device, and/or can communicate with a separate device.
As described above, although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while several variations of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. It should be understood that various features and aspects of the disclosed embodiments can be combined with, or substituted for, one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined by a fair reading of the claims that follow.
This application is related to, and claims the benefit of U.S. Provisional 60/920,272, filed Mar. 27, 2007, the entirety of which is hereby incorporated by reference herein and made a part of the present specification.
| Number | Date | Country | |
|---|---|---|---|
| 60920272 | Mar 2007 | US |
