LASER SYSTEM HAVING A DUAL PULSE-LENGTH REGIME

Abstract

A single loop hardware-based system for producing laser pulses in a microsecond scale operational mode includes a GUI to enable a user to select the operational mode of the system; a laser source for producing one or more laser beam pulses, the laser source being a diode laser pump source module; a DSP which enables and disables a hardware-based FPGA. The FPGA controls the diode pump source module. When a user selects one or more microsecond scale laser sub-pulses on the GUI, the DSP transmits to the FPGA the sub-pulse energy level and the sub-pulse on-time selected by the user on the GUI. A photodetector operatively connected to the hardware-based system measures the power of the laser pulse beam that was transmitted to the photodetector and, in a feedback mode, transmits a feedback signal of that power measurement to the FPGA. The FPGA compares the power of the laser beam measured by the photodetector to the power of the laser beam selected by the user on the GUI. If the power level read by the FPGA is higher than the selected power level, the FGPA decreases the power level to the pumping source module for any subsequent laser pulses; and if the power level read by the FPGA is less than the selected power level, the FGPA increases the power level to the pumping source module for subsequent laser pulses.

Claims

1.-17. (canceled)

18. A method for producing laser pulses with a single loop hardware-based system: the system consisting of a single loop hardware-based device; the method providing the single loop hardware-based system capable of producing laser pulses in a microsecond scale operational mode, the system further comprising: a laser source for producing one or more laser beam pulses, the laser source being a diode laser pump source module controlled by a hardware-based control system; wherein the method further comprises: selecting one or more microsecond scale laser sub-pulses, the control system receiving the sub-pulse energy level and the sub-pulse on-time selected; a detector operatively connected to the hardware-based system measuring the power of the laser pulse beam that was transmitted to the detector and, in a feedback mode, transmitting a feedback signal of that power measurement to the control system; the control system comparing the power of the laser beam measured by the detector to the power of the laser beam selected; and, the control system decreasing or increasing the power level for any subsequent laser pulses to the pumping source depending on, respectively, whether the power level read is lower or higher than the selected power level.

19. The method of claim 18, wherein the hardware-based control system comprises a field programmable gate array (FPGA).

20. The method of claim 19, wherein the detector is a photodetector.

21. The method of claim 20, further comprising a graphical user interface (GUI), further comprising the step of when a user sets on the graphical user interface (GUI) the desired pulse power level, the field programmable gate array (FPGA) causes the laser module to provide one or more pulses to be measured by the photodetector to determine whether the set desired pulse level is reached; and, if so, the set power level is stored in a memory of a computer system.

22. The method of claim 21, further comprising a calibration device to calibrate the power of one or more pulses in the microsecond scale of operation.

23. The method of claim 22, wherein the calibration device performs the calibrating using a two-step algorithm to stabilize the energy profile of the microsecond operational mode.

24. The method of claim 23, wherein the algorithm includes a sequence of: a first energy step of a set energy value, followed by a first delay period, then a second energy step of a set value followed by a second delay period.

25. The method of claim 24, further comprising the step wherein, after the second delay, the step of the field programmable gate array (FPGA) sampling the photodetector at a high rate of frequency in the microsecond operational mode and comparing the sampled measurement from the photodetector to the selected energy level.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIGS. 1, 2, 3, 5, and 7 are screen shots showing a GUI in the operation of the present invention.

[0020] FIGS. 4, 6 and 8 are oscilloscope screen shots showing the operation of the present invention.

[0021] FIG. 9 is a schematic illustrating the structure of components of the present invention.

[0022] FIGS. 10, 11A-11C, 12 and 13A-13B show a screenshot and graphical representation of calibration techniques of the present invention.

[0023] FIGS. 14 and 15 illustrate a phase slip schematic and phase slip tuning diagram in accordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0024] The apparatus of the present invention will be described below in connection with the schematic diagram of FIG. 9. In that figure, reference numeral 910 relates to the human interface into the system, and is marked as a GUI (for Graphical User Interface). The operation of the present invention will now be described in connection with what appears on such GUI.

[0025] FIG. 1 shows a screen shot 100 that appears on the GUI of a laser system of the present invention. The laser system is configured to deliver in a first state, laser pulses having a duration on the scale of milliseconds for providing suprathreshold treatments. The laser system is also configured to deliver in a second state, laser pulses having a duration on the scale of microseconds for providing subthreshold treatments. When button 110 is in Off position the system is configured to operate in the first state and when button 110 is in On position the system is configured to operate in the second state.

[0026] In the first state, button 120 is configured to switch the system from single pulse mode into a repeat mode. In a single pulse mode, each activation of the system (such as by a footswitch) will cause the laser system to shoot a single pulse. In a repeat mode, as long as the activating mechanism is turned on, e.g., as long as the footswitch is pressed, the system will continue to generate repeated pulses. The time delay between consecutive pulses, the pulse-off time, can be selected by a user through button 130 shown in FIG. 2. Pulse duration 140, which is the pulse on-time, and energy level per pulse 150 may also be selected by a user.

[0027] FIG. 3 shows the laser system in a first state in a repeat mode which is selected to deliver a sequence of pulses every 50 milliseconds, each pulse is configured to deliver 100 mW through a pulse on-time of 20 milliseconds. A control system which will be described below turns the laser system on when a pulse starts and turns it off at the end of a pulse.

[0028] As can be seen in FIG. 4 shown on an oscilloscope, in the above-mentioned example of a 20 millisecond pulse on-time, each pulse 400 is characterized by spikes 410 at the beginning of pulse 400 followed by a relatively flat zone 420. Raised energy spikes associated with the activation of a laser system are known and are a result of multiple optical and electrical parameters associated with the laser. As can be seen in FIG. 4, the relative percentage of spikes 410 in a relatively long pulse in the millisecond scale is almost negligible and hardly changes the energy average of the pulses 400.

[0029] FIG. 5 shows the laser system in the second state. Button 510 here has been switched on and this has switched the laser system to operate on a microseconds laser pulse scale. In this second state, the laser system is configured to deliver a train of sub-pulses each having a corresponding sub-pulse on time and sub-pulse off time as indicated in area 520. Duty cycles are calculated by the ratio of (sub-pulse on time) over the (sub-pulse on time+sub-pulse off time) and is provided in percentage representation. In this example in FIG. 5, the duty cycle is seen to be 5%. The laser system is configured to allow a user to select a sub-pulse on time and duty cycle and based on that the system calculates the corresponding sub-pulse off time.

[0030] In the example of FIG. 5, a sub-pulse on time of 0.5 millisecond (500 microsecond) was chosen with a duty cycle of 5%. The corresponding sub-pulse off time calculated by the laser system is 9.5 millisecond. Further, FIG. 5 shows that the system is configured to deliver a train of sub-pulses for a period of 20 milliseconds, duration, and that each sub-pulse is configured to deliver 100 mW. Since the laser system is also in a repeat mode as indicated by the position of button 530, the same train of pulses mentioned above will be delivered repeatedly by the laser system as long as it is activated with off times between one train of pulses to the next train of pulses, of 50 milliseconds.

[0031] The oscilloscope image in FIG. 6 shows a pulse 600 of 500 microseconds per the above-mentioned settings having an unstable area 610 and a more stable area 620. As can be seen, in the case of a microsecond pulse scale, energy fluctuations associated with the rise time of pulse 600 consumes significant time of the entire pulse and therefore influences dramatically the energy profile reaching a target tissue.

[0032] FIG. 7 shows a laser system in the second state which is configured to deliver a train of sub-pulses having sub-pulse on time of 100 microsecond. Such sub-pulses are shown on the oscilloscope image of FIG. 8 as pulses 800. As can be seen, due to the short pulse, the entire pulse profile is characterized by strong energy fluctuations.

[0033] It is one aspect of the present invention to reduce these fluctuations by providing a better control system and method as described below. Referring now to FIG. 9, this figure shows a general schematic of a laser system 900. According to this aspect of the invention, a GUI module 910 provides features such as described above in reference to FIGS. 1-8. A computer system having a processor and a memory may be operatively associated with the GUI module 910. DSP module 920 is configured to enable and disable FPGA module 930. FPGA module 930 is a single hardware control loop which controls pumping diode module 940 which is configured to pump laser 950. Laser beam 9500 is split by a suitable device such as a prism or mirror setup 931 into a treatment laser beam 951 which is configured to reach target tissue 970 and a feedback laser beam 952 which is configured to be detected by photodetector module 960. For safety reasons, photodetector module has a first main photodetector 961 and a second redundancy photodetector 962. FPGA 930 is configured to receive input signals from the photodetector unit 960. Such signals provide an indication of the intensity of feedback laser beam 952 which can be converted by the FPGA into power values of treatment laser beam 951. In the first state of the laser system which is configured to deliver millisecond pulse scale, DSP 920 is configured to transfer to FPGA 930 the pulse energy level and the pulse on-time selected by a user. If the system is in a repeat mode, DSP 920 also transfers to FPGA 930 the time delay between consecutive pulses as selected by a user.

[0034] In the second state of the laser system which is configured to deliver trains of microsecond sub-pulses, DSP 920 is configured to transfer to FPGA 930 the sub-pulse energy level and the sub-pulse on-time selected by a user. If the system is in a repeat mode, DSP 920 also transfers to FPGA 930 the time delay between consecutive trains of sub-pulses as selected by a user. During laser operation, whether in the first or second state, once DSP 920 enables FPGA 930, FPGA turns on pumping diode 940 to activate laser 950 and to irradiate laser beam 9500. A feedback signal is delivered from photodiode module 960 to FPGA 930. FPGA module 930 reads such feedback signal once every 1 microsecond to compare measured power to the power selected by a user. If the power level read by FPGA 930 is higher than the power level selected by the user, FPGA 930 decreases the current level to pumping diode 940. If the power level read by FPGA 930 is lower than the power level selected by the user, FPGA 930 increases the current level to pumping diode 940.

[0035] Referring now to FIG. 10, shown is an example of a current calibration table 1100 for a laser system 1000 as generated in the factory or during service in the field, which is configured to operate in the second, microsecond sub-pulse state. A similar current calibration table, having different numerical values of course, which is configured to operate the laser in a first millisecond state, is also generated in a similar mechanism as will be described below and for simplicity is not shown and duplicated.

[0036] The purpose of the calibration process is to search for electrical current values (DAC values) to be delivered by FPGA 930 to pumping diode module 940 which correspond to specific optical output power of laser 950. During the calibration process, laser system 1000 is configured to shoot a long series of short pulses. For example, such short pulses may be 50 microsecond pulses or 100 microsecond pulses. According to one example, in order to calibrate laser system 1000 to produce optical output power of about 50 mW, laser system 1000 will measure the electrical current DAC value which provides measured optical power of about 50 mW and store it in row no. 1 as seen in calibration table 1100. In this example, the measure DAC value is shown as 508. Then the system will step to find the DAC value corresponding to a second optical power. In this example, the second step was chosen to be 100 mW. As shown in row no. 2 of calibration table 1100, DAC value 599, for this specific laser system 1000, provides optical power output of 97 mW which is close to and about 100 mW. In a similar way, according to this specific example, laser system 1000 will search for DAC values which provide 200 mW, 500 mW, 1000 mW and 1500 mW. Those values were found to be 719, 910, 1260 and 1588 respectively.

[0037] It should be mentioned that other energy steps can be chosen to calibrate laser system 1000. Once calibrated, calibration table 1100 is stored in the computer system. According to one aspect of the invention, a two-step algorithm may be provided for operating the laser. The two-step algorithm has been found to be effective in stabilizing the energy profile of short microsecond pulses which tend to fluctuate as mentioned in the above discussion of FIGS. 6 and 8. According to this aspect of the invention, the hardware control loop of FPGA 930 is configured to send pumping diode 940 a two-step signal.

[0038] As shown in FIG. 12, once the DSP 920 enables FPGA 930 there is an initial DAC set up period of about 10 microseconds. According to another embodiment of the present invention, DAC set up period may be anywhere between 0 to 30 microseconds. During the DAC set up period, FPGA 930 is set to deliver a first energy step to pumping diode 940. According to one embodiment of the invention, such a first energy step may be 50 mW. According to another embodiment such a first energy step may be anywhere between 50 mW and 150 mW.

[0039] Referring now to FIG. 11, the DAC set up period can be seen as 1110 in FIG. 11b and first energy step 1130 is seen in FIG. 11a. A second energy step 1140 with another DAC value is triggered by FPGA 930 after a first delay period 1120. The DAC value of second energy step 1140 is calculated and extrapolated based on calibration table 1100 and lies along line 1300 of graph 1200 in FIG. 10 and is correlated to the selected energy level as was selected by the user. After a second delay period 1150, FPGA 930 starts to sample the photodetector at a high frequency, such as once every 1-10 microseconds. In a differential algorithm period 1160, following the second delay period 1150, FPGA 930 compares each sampled measurement of the photodetector to a selected energy power which was selected by a user.

[0040] As shown in FIG. 13a, if the measured energy level as read from photodetector 960 is lower than the user selected energy level, FPGA 930 raises DAC value to pumping diode 940 by one DAC number. As shown in FIG. 13b, if the measured energy level as read from the photodetector 960 is higher than the user selected energy level, FPGA 930 reduces DAC value to pumping diode 940 be one DAC number.

[0041] According to another aspect of the invention and referring now to FIG. 14, when pumped, gain in the laser material [4] creates an optical standing wave oscillation bounded by the mirrors [1,3,5] at the fundamental wavelength. The standing wave can be decomposed as the sum of two traveling waves which propagate in opposite directions. Due to the property of nonlinear polarization inside the nonlinear crystal [2], a harmonic wave may be generated, and the power of the fundamental may be transferred efficiently into the harmonic wave. The fundamental wave, as it travels from the harmonic separator [3], has its first opportunity to interact with the field of a harmonic wave as it passes through the nonlinear crystal [2] the first time. After reflection at the end mirror [1], the fundamental wave has a second opportunity to interact with the field of the harmonic wave as it passes through the nonlinear crystal the second time. When the fundamental next passes the harmonic separator, the harmonic is removed from the laser resonator and moved into the output. The system of FIG. 15 also includes a pumping laser diode [6], and cylindrical lenses [7] and [8].

[0042] The rate of flow of energy from the fundamental to the harmonic depends on the magnitude of the nonlinearity and the magnitude of the field of the fundamental. The flow of energy also depends on the matching of the phase of the fundamental to the phase of the harmonic throughout the length of the interaction. Phase mismatching inside the nonlinear crystal is described by k=k.sub.22k.sub.1, where k1 is the wavenumber of the fundamental wave and k2 is the wavenumber of the harmonic wave, considering the refractive indices of the nonlinear material. In order to achieve efficient frequency conversion, the nonlinear crystal is designed by some means to achieve a value of k close to 0, so that transfer of power can occur throughout a long enough interaction length, or, in the best case, across the entire length of the nonlinear crystal. For large k, the harmonic wave will become out of phase with the fundamental wave, until the flow of power is reversed and the fundamental is regenerated.

[0043] For a standing wave laser, the phase difference on reflection of the end mirror [1], defined as .sub.22.sub.1, where .sub.1 is the reflected phase of the fundamental and .sub.2 is the reflected phase of the harmonic, is also important for efficient harmonic generation. In the worst case if k=0, but .sub.22.sub.1=180, then the harmonic will build power during the first pass through the nonlinear crystal. But then during the second pass after reflection, the two fields will be out of phase, so that all the harmonic power will return to the fundamental and the harmonic output will be zero. In order to achieve the optimal case where the power is continuously transferred toward the harmonic throughout the length of the crystal in two passes, the phase difference on reflection should be minimized .sub.22.sub.10, and also the phase mismatch should be minimized k0.

[0044] Even in the case in which the end mirror [1] is created by accident where .sub.22.sub.1=180, the laser can still be operated by adjusting away from k=0, for example by adjusting the temperature of the nonlinear crystal. In this case the harmonic output will not be as high as the case where .sub.22.sub.1=0 and k=0, but it will be greater than zero.

[0045] FIG. 15 illustrates an experiment in which a temperature sweep on an LBO crystal is used to sweep across a wide range of values of k, in this example k=0 occurs at approximately 41 C. In this experiment, a wedged glass component is placed in between the laser crystal and the end mirror. The glass component has dispersion, and because of its wedge it can be used to adjust the effective of phase difference on reflection .sub.22.sub.1. Each line in the chart therefore belongs to a distinct value of .sub.22.sub.1. This illustrates that even though a solution exists for efficient harmonic generation for every value of .sub.22.sub.1, there is an optimal value of .sub.22.sub.1 for best efficiency. Theoretically we know that the optimal value must be .sub.22.sub.1=0.

[0046] Generally, it may be possible to increase harmonic generation efficiency by means other than control of the phase difference on reflection .sub.22.sub.1, primarily reduction of laser beam diameter, increase of the length of the nonlinear crystal, or reduction of residual losses at the fundamental, such as scattering from defects or rough surfaces. However, each of these design parameters exhibits some practical limitation. Reduction of beam diameter may be limited by laser damage limits. Eventually the reduction of residual losses becomes cost-prohibitive. The Rayleigh range of the fundamental laser beam is one limit to the length of the nonlinear crystal. The nonlinear crystal length is also inverse to the acceptance bandwidth of harmonic conversion process. If the acceptance bandwidth is reduced below the gain bandwidth of the laser material, then the laser wavelength will escape the harmonic conversion by running away at a wavelength where no conversion occurs. In this way, the gain bandwidth of the laser material defines a limit on the length of the nonlinear crystal. Longer crystal lengths can be used in this case only with the addition of some wavelength bandwidth filter inside the laser, such as an etalon or a birefringent Lyot filter.

[0047] The phase difference on reflection has been controlled in the past by using an element with adjustable chromatic dispersion, for example a glass element located in between the nonlinear crystal and the end mirror, in which the temperature of the glass is controlled and adjustable in order to achieve exactly the correct phase difference. Air also has enough dispersion that the spacing between the mirror and the nonlinear crystal can be made to be adjustable. In both cases, a random phase difference can be compensated, however these implementations suffer from additional complexity and adjustment and are often not considered worthwhile.