System and method for generating a constant energy laser pulse train

Abstract

A system for generating a constant energy laser pulse train, comprising: a solid-state laser resonator (23), operating in Q- switching mode, comprising a variable attenuator (32); said laser resonator (23) is pumped by means of at least one lamp (22); said lamp (22) is powered by the discharge voltage of at least one condenser (20); a control unit (15); where said control unit (15) is configured to control said variable attenuator (32) in pre-set times based on the trend of the discharge profile of said at least one condenser (20), so as to provide as an output a constant energy laser pulse train.

Description

[0001] “SYSTEM AND METHOD FOR GENERATING A CONSTANT ENERGY LASER PULSE TRAIN”

[0002] DESCRIPTION

[0003] The present invention relates to a system and method for generating a constant energy laser pulse train.

[0004] In particular, it relates to a system for generating a constant energy laser pulse train with a solid-state laser system pumped with a lamp.

[0005] The system for generating a constant energy laser pulse train can be applied, for example, in equipment for beauty and / or dermatological treatments.

[0006] Typically, the power supply of the lamp is based on the discharge of a condenser bank.

[0007] The energy of each pulse, forming part of a pulse train, decreases following the discharge trend of the condensers. This generates an imbalanced pulse train, with the first pulses being very energetic and subsequently with decreasing energy.

[0008] It is clear that such a pulse train generates different effects on the material (inorganic or organic) with which it interacts. In particular, the result will be that the first pulses, which are highly energetic, will interact more efficiently or more aggressively than the last ones, which are decidedly less energetic.

[0009] In particular, in the case of beauty and dermatological applications, where the laser radiation generated by a lamp-pumped laser system interacts with a biological tissue, such a pulse train produces an aggressive treatment with the first pulses, which degenerates into a treatment that is no longer efficient and, in some cases, even painful for the patient when the last pulses, which do not have energy and the desired clinical effect, interact on the biological tissue. It is known that these last pulses, in addition to not being completely efficient, can also be painful for the patient undergoing the treatment, since they can produce burns and necrosis of the treated tissues.

[0010] In addition, in a pulse train of this type, the more energetic pulses could exceed the threshold of damage of the optical components on the beam path.

[0011] A possible solution would be to generate a controlled-current lamp pulse with a constant profile during the pulse. Such a solution, although technologically feasible, is complex and costly, due to both the currents and the voltages to be applied to the pump lamp (typical voltages of 700V, currents higher than 1kA). Furthermore, solutions of this type are unlikely to satisfy the electromagnetic emission requirements for medical devices.

[0012] The object of the present invention is to provide a system for generating a constant energy laser pulse train.

[0013] Another object is to provide a system that is easy to manufacture.

[0014] In accordance with the present invention, these objects and yet others are achieved by a system for generating a constant energy laser pulse train in accordance with claim 1. These objects are further achieved by a method for generating a constant energy laser pulse train in accordance with claim 7.

[0015] Further characteristics of the invention are described in the dependent claims.

[0016] This solution has various advantages with respect to the solutions of the prior art.

[0017] Thanks to the present solution, a system is obtained for generating a laser pulse train for which the energy of each pulse is controllable and, in particular, is constant for all the pulses of the train.

[0018] High-power and constant energy pulses beneath the breakage limit of the remote-control fibre-optic are generated.

[0019] The system therefore obtains, in the case of beauty and / or dermatological treatments, an energetically efficient pulse train during interaction between laser radiation and biological tissue which increases the comfort zone for the patient undergoing treatment.

[0020] The system for generating the laser pulse train is the solid-state type pumped by one or more lamps.

[0021] A variable-frequency Q-Switch was used to generate the constant energy laser pulse train, thanks to which the energy of the pulses can be regulated, even with a power supply of the lamp powered by the discharge of the condenser bank. The energy of the pulses depends on the pump energy absorbed, on the active energy, between one pulse and the next one; it is therefore possible to regulate the effective pump energy of each pulse of the train in real time by varying the repetition frequency of the Q-switch. Knowing the trend of the pump lamp discharge profile and waiting the time necessary for absorption of the pump energy necessary for the active element makes it possible to obtain laser pulses with the desired energy.

[0022] Generation of the laser beam obtained by means of a resonator operating in Q-switching mode allows a laser to produce a pulsed output beam with an extremely high peak power, much higher than the one that said laser would produce if it were operating in continuous wave (CW) mode. Q-switching results in lower repetition speeds of the pulses, with higher energies of the pulses and much longer durations of the pulses.

[0023] Further characteristics and advantages of the present invention will become more apparent from the following detailed description of a practical embodiment thereof, illustrated by way of a non-limiting example in the appended drawings, in which:

[0024] Figure 1 schematically shows a system for generating a constant energy laser pulse train, in accordance with the present invention;

[0025] Figure 2 schematically shows a laser generator of a system for generating a constant energy laser pulse train, in accordance with the present invention;

[0026] Figure 3 schematically shows a graph that shows the energy of the pulses in relation to the time and to the discharge of the condensers, in accordance with the present invention. In reference to the appended drawings, a system 10 for generating a constant energy laser pulse train, in accordance with the present invention, comprises a laser generator 11 which emits a laser beam that, by means of an opportune optical element 12, is sent to an optical fibre 13 that terminates with a handle 14 that will be used by an operator.

[0027] The system 10 further comprises a control unit 15 that manages the system 10 assisted by a user interface 16. Preferably, it also comprises a cooling system 17 with a terminal alongside the handle 14 for cooling the user's skin.

[0028] The laser generator 11 comprises a condenser bank 20 followed by a circuit breaker 21 and by at least one lamp 22.

[0029] The laser generator 11 further comprises a laser resonator 23 operating in Q-switching mode.

[0030] The laser resonator 23 comprises in succession a totally reflecting mirror 30, an active element 31, a variable attenuator 32, a Brewster angle dielectric polariser 33 in quartz, a quarter-wave plate 34 and a semi-reflecting mirror 35.

[0031] The active element 31 is an Nd:YAG laser, i.e. a solid-state laser that uses as an active medium an yttrium aluminium garnet (YAG) doped with neodymium (Nd:Y3AlsOi2).

[0032] In particular, the active element 31 has a wavelength typically in the range between 300 and 500 mm, with a cylindrical or parallelepiped shape in length in the order of 100 - 200 nm, with a doping comprised between 1.0% and 1.5%. The variable attenuator 32 is controlled externally, and is preferably obtained with an acoustic-optical modulator, i.e. a Pockels cell in RTP (rubidium titanium phosphate), which guarantees the possibility of functioning up to repetition frequencies of 100kHz.

[0033] The beam at the output of the laser resonator 23, before sending it to the handle 14, if required, may be sent to a converter (not shown), which comprises a movable mirror, for example a highly reflective dielectric mirror at 45°, followed by a non-linear crystal for conversion into a second harmonic, for example in type 2 KTP (potassium titanyl phosphate).

[0034] The condenser bank 20 is loaded in a known way and the circuit breaker 21, for example an insulated-gate bipolar transistor (IGBT), opens and closes the circuit between the condenser bank 20 and the pump lamp 22 of the laser resonator 23.

[0035] The circuit breaker 21 is controlled by the control unit.

[0036] The lamp 22 is, for example, a xenon flash lamp, air-cooled and surrounded by an enamelled ceramic diffuser. Lamps of this type are typically powered with pulses, provided by the discharge of the condenser bank 20, with a voltage between 250V and 1200V and duration between 0.2ms and 20ms.

[0037] The control unit 15 generates the synchronised control pulses to control the pulse of the lamp and activation of the Q-switch, so that activation of the Q-switch occurs several times during a single pulse of the lamp. The control unit 15 regulates the frequency of the Q-switch so as to compensate for the decrease in intensity of the lamp, with an opportune increase in time between one micro-pulse and the next one (decrease of Q-switch frequency). This provides for a pulse train sufficiently spaced apart to obtain constant energy pulses, despite the decrease in the supply voltage of the lamp 22 and consequently a drop in the performance thereof.

[0038] If the discharge curve profile of the condenser bank 20 is known, it is possible to calculate and store in the control unit 15 the time necessary to achieve a particular energy value for each discharge voltage value of the condenser bank 20 and therefore the power supply of the lamp 22. The dynamics of the pulse of the lamp 22 are considered so that the control unit 15 is capable of equalising the laser pulses in the time during the decrease of the lamp pulse. Based on the information archived on the table, the number of pulses and the duration and frequency thereof are established beforehand.

[0039] Figure 3 shows the curve c of the discharge profile, as a function of the time, of the condenser bank 20, and therefore the supply voltage of the lamp 22 and schematically the energy of the pulses as a function of the time t.

[0040] At the time instant t1, the circuit breaker 21 closes and the lamp pulse, which progresses in a known way, is generated.

[0041] Applying the data saved on the table, the logic unit generates laser pulse time instants t2, t3 and t4 which, in a first embodiment, have equal energy to each other. Where time t4-t3 is higher than t3-t2, so that the pulses have the same energy.

[0042] The process described above can be iterated until a continuous and uniform sequence of pulses all having the same energy is obtained.

[0043] In particular, the number of these pulses, the duration thereof and the repetition frequency are established and known beforehand, through the information defined in the table, in a solution in which such pulses all have constant energy. In addition, in a further embodiment, the number of pulses, the duration thereof and the repetition frequency can be controlled dynamically through a feedback system 36 that opportunely modulates the aforesaid parameters, adapting them to the purpose, by measuring the previous pulse. An opportune detector 37, which could be a photodiode or a pyroelectric detector, placed in output from the resonator 23 or from the handle 14, or, more generally, placed in a point of the optical layout where it is possible to measure the number of pulses, the duration thereof and the repetition frequency, would allow measurement in real time of the pulse generated. The information read by the detector 37 is thus sent to the control unit 15 for regulation of the aforesaid parameters. In this manner, it is possible to obtain, by means of the feedback system, an optimal generation of pulses for the necessary treatment.

[0044] In particular, by regulating the temporal distance between the pulses (the frequency thereof) and their number, it is possible to regulate the duration and energy of the pulses and the total one of the train.

[0045] In the first implementation, a table provides a list of intervals between the pulses. In this implementation, the interval is regulated through a feedback.

[0046] In a second embodiment, the control unit 15 provides a laser pulse with energy higher than the previous ones at the time instant t5 followed by pulses in the times t6-t10. The new pulses at time t5-t10 are more spaced apart from each other than the previous ones, in order to take account of the higher energy required and, at the same time, the higher discharge of the condensers.

[0047] In other words, it is possible to generate a first train of pulses at a first constant energy value to each other and a second train of pulses at a second constant energy value to each other.

[0048] The first energy value can be lower or higher than the second energy value.

[0049] The various pulses, of both the first pulse train and the second pulse train, are not equally spaced apart from each other, but are more spaced apart as the discharge voltage value of the condenser bank 20 decreases.

[0050] In conclusion, and based on the conditions of application, it is possible to have several pulse trains of equal or different energy to each other but with constant energy.

[0051] For preparation of the table, the discharge curve c of the condenser bank 20 is determined, based on the supply voltage and the load of the condenser bank 20, so the curve c is calculated theoretically or, alternatively, measurements are made during calibration using the relative measuring instruments. Normally, only the first 70-80 % of the discharge curve c of the condenser bank 20 is considered, since the residual supply voltage of the lamp 22, given by the tail of the curve c, will be insufficient to provide pulses suited to the purpose.

[0052] Knowing the characteristics of the resonator 23 and simulating the behaviour thereof, it is possible to establish, upon variation of the required energy, the times needed to obtain a desired energy pulse at the various points of the curve c, and these are stored on the table. It is also possible to store the control times of the variable attenuator 32, i.e. the repetition times of the pulses, in order to provide a train of n pulses (with variable n) at the required energy.

[0053] The variable attenuator 32 will consequently be controlled by means of the control unit 15.

[0054] Based on the information on the table, the control unit 15 will activate the Q-Switch only when the lamp will have sufficiently pumped the active element 31 to obtain the pulse at the required energy, independently of the power supply value of the lamp given by the discharge curve c of the condenser bank 20.

[0055] If high-energy pulses are required, the repetition times of the pulses will be higher than in the case in which pulses with lower energy are required.

[0056] With a device in accordance with the present invention, micro pulses are obtained with a duration comprised between 10nsec and 100nsec and energy comprised between 10m J and 500m J inside a lamp pulse with a typical duration of 10msec, in any case comprised between 0.1ms and 20ms, which give rise to an envelope with a typical duration of 10ms and generally comprised between 0.1ms and 20ms, with a repetition frequency comprised between 0.5Hz and 80Hz, durations between 10 and 50ns and a train energy comprised between 0.5J and 100J, with a typical value of 60J.

Claims

CLAIMS1. A system for generating a constant energy laser pulse train, comprising: a solid-state laser resonator (23), operating in Q-switching mode, comprising a variable attenuator (32); said laser resonator (23) is pumped by means of at least one lamp (22); said lamp (22) is powered by the discharge voltage of at least one condenser (20); a control unit (15); where said control unit (15) is configured to control said variable attenuator (32) in pre-set times based on the trend of the discharge profile of said at least one condenser (20), so as to provide as an output of said laser resonator (23), a constant energy laser pulse train.

2. The system in accordance with claim 1, characterised in that it comprises a values table stored in said control unit (15), on which the times necessary to reach a particular energy value based on the discharge voltage of said at least one condenser (20) are recorded.

3. The system in accordance with claim 1, characterised in that said laser resonator (23) comprises as an active element an Nd:YAG laser.

4. The system in accordance with claim 1, characterised in that said laser resonator device (23) comprises in succession a totally reflecting mirror (30), an active element (31), a variable attenuator (32), a Brewster angle dielectric polariser (33) in quartz, a quarter-wave plate (34), and a semi-reflecting mirror (35).

5. The system in accordance with claim 1, characterised inthat the number of pulses, the duration thereof and the repetition frequency is established and known beforehand.

6. The system in accordance with claim 1, characterised in that said pre-set times are spaced apart from each other, inside a single discharge profile trend of said at least one condenser (20), so as to supply as an output a constant energy laser pulse train.

7. A method for generating a constant energy laser pulse train, comprising: a solid-state laser resonator (23), operating in Q-switching mode, comprising a variable attenuator (32); a control unit (15); where said laser resonator (23) is pumped by means of at least one lamp (22) powered by at least one condenser (20); where said control unit (15) controls said variable attenuator (32) in pre-set times based on the discharge profile trend of the supply voltage of said at least one lamp (22).

8. The method in accordance with claim 7, characterised in that said laser pulse train is formed of: a first pulse train having a first energy value; a second pulse train having a second energy value.

9. The method in accordance with claim 8, characterised in that said first energy value is different to said second energy value.

10. The method in accordance with claim 7, characterised in said storing in said control unit (15) of a table on which the times necessary to reach a particular energy value based on the discharge voltage of said at least one condenser (20) are recorded.

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