Patent Application
20060198402
The present relates to combining beams from a plurality of lasers. The invention relates in particular to combining pulses from a plurality of frequency-converted, diode-pumped lasers to provide a combined pulse, and controlling the energy of the combined pulse.
Pulsed, diode-pumped, solid-state (DPSS) lasers with high power output are increasingly being favored for industrial applications such as material processing, laser machining and the like. For some industrial applications, for example UV optical lithography, more power or more energy per pulse is required than a single DPSS laser can provide. A higher power or energy per pulse for such applications can be provided by combining pulses (beams) from two or more lasers.
One problem with combining pulses from a plurality of lasers is that inevitable pulse-to-pulse energy variation in the lasers makes control of the energy in a combined pulse by controlling the energy of pulses from individual lasers very difficult to implement. Problems of pulse-to-pulse repeatability can be particularly problematical in a pulsed DPSS laser when the fundamental laser output is converted in frequency, for example, frequency-doubled or frequency-tripled, using one or more optically nonlinear crystals. This is because any pulse-to-pulse instability of the fundamental output of the laser translates to a much higher instability of the second and higher harmonics. As frequency-converted lasers deliver lower power than fundamental counterparts thereof, it is frequency converted lasers that are most likely required in combination to provide power for a desired application.
There is a need for a method of controlling pulse-energy in a pulse formed by combining pulses from a plurality of frequency-converted DPSS lasers. Preferably the method should also be applicable to controlling pulse-energy in combined pulses from lasers that deliver only fundamental radiation.
In one aspect, the present invention is directed to a method for terminating generation of frequency-converted output in a frequency-converted laser wherein at least one optically nonlinear crystal is arranged to generate the frequency-converted output from radiation plane-polarized in a predetermined polarization plane. The method comprises rotating the polarization plane of the plane-polarized radiation entering the optically nonlinear crystal.
In one preferred implementation of the method, the frequency-converted output radiation is third-harmonic (3H) radiation generated by generating second harmonic (2H) radiation from the fundamental in one optically nonlinear crystal and generating the 3H-radiation in another optically nonlinear crystal by mixing the 2H radiation with fundamental radiation. To effect the termination of 3H radiation generation, the polarization plane of the fundamental radiation is rotated by a Pockels cell before the fundamental radiation can interact with the 2H-radiation generating crystal.
In another aspect the present invention is directed to a method of delivering an amount of laser radiation having a predetermined energy to a laser beam combiner. The method comprises delivering a plurality N-1 of laser pulses from a corresponding plurality of N lasers to the beam combiner. The cumulative energy delivered by the N-1 laser pulses is determined. If the cumulative energy is determined to be less than the predetermined energy, an Nth laser delivers a portion of an Nth pulse, the portion having an energy sufficient such that the total energy delivered is about equal to the predetermined energy. Other aspects and embodiments of the present invention will be evident from the detailed description provided hereinbelow.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.
Referring now to the drawings, wherein like components are designated by like reference numerals,
Each laser includes a Q-switch 24 for providing pulsed operation of the laser. The Q-switches are controlled by an integration controller 26 via a master clock 28 and four individual delay units 30, one delay unit for each Q-switch. Master clock 28 determines the pulse repetition frequency (PRF) of the lasers (which is preferably precisely matched) and the delay units are adjusted to synchronize pulses from each laser such that there is a desired temporal overlap between the pulses, allowing the pulses to be combined and summed after being frequency converted.
Fundamental output (pulses) F from each laser are converted to second-harmonic radiation (pulses) by an optically nonlinear crystal 32. The second-harmonic (2H) radiation and the direction of propagation thereof are designated in
Laser 18 includes a Pockels cell 35 controlled by a controller 37. Pockels cell 35 is an electro-optic device that can rotate the polarization plane of radiation passing therethrough to an extent dependent on an electrical potential applied thereto by controller 37. In a normal state, the polarization of radiation transmitted by the Pockels cell is that which is required for optically nonlinear crystal 32 to effect second harmonic generation, such that crystal 34 can effect the required 3H frequency conversion.
The 3H-pulses from each optically nonlinear crystal 34, together with any residual fundamental and 2H radiation are incident on a beamsplitters or sampling mirrors 36. Each beamsplitter reflects a small sample fraction, for example, about 0.1% or less of the radiation incident thereon. The reflected fractions follow a common path 40 and are representative of the energy in the transmitted portions of the pulses. The transmitted portions of the pulses are delivered to a beam combiner for forming a single pulse. Residual fundamental and 2H content in the transmitted pulses can be removed by a dichroic filter or filters, preferably before the pulses are combined. Several beam combining methods are well-known in the art, and a detailed description of any one of these methods is not necessary for understanding principles of the present invention. Accordingly, such a detailed description is not presented herein.
In one example of operation of apparatus 10, delays 30 are adjusted such that 3H pulses delivered by the 4 lasers 12, 14, and 16, are delivered essentially simultaneously and a pulse from laser 18 is delivered after a delay time tD such that, when spatially overlapped in a beam combiner, the pulses form a continuous pulse having a peak power about three times that of any of the individual pulses, but having a longer duration than the individual pulses, dependent primarily on the delay time tD.
The samples along path 40 are intercepted by a dichroic beamsplitter 42. Beamsplitter 42 reflects the 2H and fundamental content of the samples to a beam dump 44. The 3H content of the samples is incident on a UV sensitive photodiode 46. Such a photodiode has a response time of about 100 picoseconds which is very much less than the duration of pulses delivered by the lasers in this example.
The output of photodiode 46 is integrated by an integrator 48 effectively summing the energy in all of the samples. The output of the integrator is constantly compared with a value supplied by a digital-to-analog converter (DAC) 47 and representative of a desired total energy in a summed pulse. At some time during the delivery of the pulse from laser 18, the integrator output matches the total energy signal, the comparator output triggers Pockels cell controller 37, and the controller delivers an electrical potential to the Pockels cell 35 that rotates the plane of fundamental radiation transmitted thereby to an orientation in which crystal 34 can not generate 2H-radiation, preferably through 90°. With the polarization thus rotated, laser 18 can also no longer deliver 3H-radiation. Accordingly delivery of 3H-energy in the pulse being delivered is terminated, and laser 18 no longer contributes to the combined 3H-pulse formed by the combiner. Provided that the Pockels cell is triggered after all or most of the energy in the third pulse in an overlapping sequence thereof is delivered, terminating the fourth pulse as thus described controls the energy in the combined pulse at about the desired value.
It should be noted here that the Pockels cell could be placed between optically nonlinear crystal 32 and 34 and the pulse terminating method would still function. In any sequence of optically nonlinear crystals for converting frequency to a third or higher harmonic, each crystal must receive one, two or more radiation frequencies in a particular polarization orientation to the crystal and (if there is more than one frequency) to each other. If the polarization plane of one or more of the radiation frequencies is rotated before the first crystal, or between crystals, the desired frequency conversion will cease.
In order for this method of pulse control to be effective, certain conditions should be fulfilled. Energy per pulse in all lasers must be sufficiently repeatable and energy per pulse sufficient such that all N pulses together have at least the desired energy and preferably greater that the desired energy. Preferably N-1 pulses should not, under any foreseeable circumstance, supply greater than about the desired pulse energy. Delay time tD for delivering the Nth or control pulse (here, the fourth) is preferably selected such that that pulse is not delivered until the other N-1 pulses have deposited most of the energy therein into the cumulative pulse. A discussion of the effect of delay time tD on the pulses on the effectiveness of control is presented below beginning with reference to
The pulses in the computation of
In certain applications, for example, in UV optical lithography, it is desirable to increase available pulse energy without significantly increasing pulse power. This is because the increased energy is useful for shortening exposure times and increasing throughput, while a significant increase in power could reduce the lifetime of imaging optics due to optical damage by the UV radiation.
In either of the above-discussed examples of pulse combination it is possible that delay tD can be selected such that that energy in pulses P1, P2 and P3 is all delivered before delivery of pulse P4 is initiated as discussed above with reference to
An advantage of combining pulses from lasers that operate independently is that pulse-to-pulse variation in energy in an uncontrolled sum of the N pulses would be about 1/√N of the pulse-to-pulse variation in individual pulses. This somewhat relaxes the stability requirements for the individual lasers and maximizes consistency in satisfying the above discussed energy conditions for N and N-1 pulses, which are important in effective operation on the inventive combined pulse energy control method. Clearly, however, as the number of lasers is increased, the less will be the percentage in the controlling (Nth) pulse of the total available energy and the less effective controlling energy in that pulse may be in controlling the total energy.
In considering the effectiveness of the inventive method it is important to consider the effect of an inevitable delay from the time of triggering the Pockels cell controller to the time when delivery of a 3H-pulse by laser 18 is terminated. This delay is estimated at between about 6 and 7 nanoseconds. Accordingly, pulse PSUM will always have slightly more than the desired energy value, absent any measure to compensate for the delay.
In certain instances, for example when pulses are of a sufficiently long duration, it may be possible to simply ignore the effect of the delay since only a relatively small proportion of the total pulse energy will be delivered during that delay time. Further, the percentage of excess energy that is delivered by an Nth pulse as a result of the delay in terminating the pulse will be between about one Nth and one (N-1)th of the excess energy percentage of the desired energy; By way of example, in a pulse having a width of about 150 ns, as is the case in examples of lasers exemplified above, a 6 ns overshoot will result in, at most, about 5% more pulse energy being delivered by the control pulse than is desired. In apparatus delivering 4 pulses to be combined, this would translate to an error of between about 1.25% and 1.7%in the desired energy. Such an error, in most applications would be more than acceptable. If the cutoff time occurred in the latter half of the control pulse delivery time, as would be the case in most operations, the error would be less, as energy delivery is rapidly decreasing with time in this period.
It is possible to anticipate the overshoot effect of the delay by reducing the trigger value from DAC 47 below the actual desired energy level by some amount determined from, say, an average energy per nanosecond delivered by pulse P4 times the anticipated delay in nanoseconds. Even though some of the time the actual pulse energy delivered during the delay will be more or less than this average value, this simple compensation will provide a significant reduction in any error that would otherwise occur as a result of the delay. Here again, whatever residual error there is will be reduced as a percentage of the desired energy by about the fractional contribution of the final pulse to the total available energy in all pulses. Accordingly, in an apparatus combining four 125 ns-pulses it is possible to reduce the total-energy-control error due to the reaction delay of the Pockels cell to less than 1%.
It is of course possible to compensate for the overshoot effect of the delay by including an optical delay line of appropriate length in the laser delivering the Nth or controlling pulse. This delay line must be included between the point at which the cumulative energy is sampled and the Pockels cell.
Radiation transmitted by Pockels cell 35 is normally reflected by a polarization selective reflector 72 toward the beam combiner. Pockels controller cell 35 is activated, after the above-discussed reaction delay, when measured cumulative energy reaches a value representative of a desired total energy. Pockels cell 35 is arranged to rotate the polarization plane of the 3H-radiation on activation such that the 3H-radiation is now transmitted by polarization selective reflector 72, is not directed to the beam combiner, and accordingly, is not included in the combined pulse. The transmitted 3H radiation in this case is directed to a beam dump 45.
The time required for this system to divert the unwanted portion of the laser pulses is in a range between about 6 and 7 ns as discussed above. Accordingly, turning mirrors 64, 66, and 68, and Pockels cell 35 are positioned to provide a delay of radiation in path 70 of about this time. This corresponds to an increase in the 3H-radiation path length of about 2 meters. Preferably, the length of path 70 in delay line 62 is made adjustable for “fine tuning” apparatus 60. Alternatively, an adjustable delay circuit could be built into Pockels cell controller 37 so that an additional calibration element is provided for fine tuning the delay.
It should be noted here that this arrangement of the Pockels cell and polarization selective reflector is but one possible arrangement, selected, here, for convenience of illustration. Those skilled in the art will recognize that other arrangements providing switching of 3H-radiation out of a path toward the beam combiner are possible without departing from the spirit and scope of the present invention. By way of example, the Pockels cell and polarization selective reflector can be arranged such that the 3H-radiation is normally transmitted by the polarization selective reflector, with unwanted 3H-radiation being reflected out of a path to the beam combiner when the Pockels cell is activated.
It should also be noted that if the 3H-radiation switched by the arrangement of Pockels cell and polarization select is ultraviolet (UV) radiation having a relatively high power, for example, several Watts, problems may be experienced with optical degradation of the polarization selective reflector. These problems could result from UV degradation of substrate material and optical bonding material in the bi-prism of the illustrated example, or from UV degradation or spectral shift of thin film coatings in a bi-prism or a front-surface polarization-selective reflector. Regardless of wavelength, the extinction ratio of bi-prism type polarization selective reflectors can be significantly limited by stress birefringence, either residual in the bi-prism material, or induced by stresses resulting from manufacturing and bonding operations. This could result in some “un-switched” amount of the unwanted radiation being included in a combined pulse. A particular advantage of the pulse control arrangement of
In embodiments of the inventive apparatus described above with reference to
In the method of
This “anticipation” method has an advantage that the contribution of any portion of pulse P3 that occurs during delivery of control pulse P4 is taken into account in the calculation. This allows a closer temporal spacing of the control pulse to a previous pulse than is possible in the analog method described above, thereby allowing more flexibility in determining power distribution in a combined pulse. The method also has an advantage that delays due to computation time and electronic reaction time of switching devices can be considered in calculating TCUTOFF. The effectiveness of the method will of course be limited be any pulse-to-pulse variations from the assumed nominal in the temporal power distribution of control pulse P4 in particular.
It should be noted here that while the present invention has been described with reference to four frequency-converted lasers including a master oscillator and a power amplifier, certain aspects of the invention are applicable to any frequency-converted laser with or without an amplifier. Other aspects of the invention are applicable to controlling the energy in a combination of pulses from a plurality of lasers whether or not the lasers include frequency conversion. Further, while the invention is described with reference to delivery of pulses from Q-switched lasers, the application is not limited to combining Q-switched pulses but may be applied to lasers delivering free running pulses such as would be obtained from pulsed lasers wherein pulsed operation is effected by pulsing pump energy delivered to the laser gain-medium. In general, while the present invention is described above in terms of a perferred and other embodiments, the invention is not limited to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.
