Method for Operating a Pulsed Laser System

Abstract

A method for operating a pulsed laser system includes the steps of pumping a laser resonator of the pulsed laser system by means of a pump source in order to generate operating laser pulses at an operating energy level; and coupling the operating laser pulses with a focusing element into an optical fiber. A step of cleaning the optical fiber by means of cleaning laser pulses is performed prior to generating the operating laser pulses. The laser resonator of the pulsed laser system is pumped by means of the pump source in order to generate the cleaning laser pulses at one or more cleaning energy levels between a laser threshold and the operating energy level.

Claims

1. A method for operating a pulsed laser system, the method comprising the steps of: pumping a laser resonator of the pulsed laser system by means of a pump source in order to generate operating laser pulses at an operating energy level; and coupling the operating laser pulses with a focusing element into the optical fiber, wherein a step of cleaning the optical fiber by means of cleaning laser pulses is performed prior to generating the operating laser pulses, wherein the laser resonator of the pulsed laser system is pumped by means of the pump source in order to generate the cleaning laser pulses at one or more cleaning energy levels between a laser threshold and the operating energy level.

2. The method according to claim 1, wherein if the operating energy level is less than a maximum cleaning energy level, then the one or more cleaning energy levels is in a range from the laser threshold to the operating energy level and if the operating energy level is equal to or above the maximum cleaning energy level, then the one or more cleaning energy levels is in a range from the laser threshold to the maximum cleaning energy level, wherein the maximum cleaning energy level is in a range from 400 to 1000 mJ.

3. The method according to claim 2, wherein if the operating energy level is below the maximum cleaning energy level, the method comprises a second step of cleaning the optical fiber after generating the operating laser pulses by means of second cleaning laser pulses, wherein the laser resonator of the pulsed laser system is pumped by means of the pump source in order to generate the second cleaning laser pulses at one or more second cleaning energy levels between the operating energy level and a second operating energy level, wherein the second operating energy level is higher than the operating energy level.

4. The method according to claim 1, wherein the cleaning laser pulses are generated as a sequence of laser pulse groups, wherein each one of the laser pulse groups comprises at least one of the cleaning laser pulses, and wherein the one or more cleaning energy levels are increased from one laser pulse group to a following laser pulse group.

5. The method according to claim 4, wherein the one or more cleaning energy level are increased by a linear factor and/or an exponential factor from the one laser pulse group to the following laser pulse group.

6. The method according to claim 4, wherein each one of the laser pulse groups comprises at least two of the cleaning laser pulses at the same cleaning energy level.

7. The method according to claim 1, wherein the optical fiber is a reusable optical fiber.

8. The method according to claim 7, wherein a data storage device associated with the reusable optical fiber is readout in order to detect a prior usage of the reusable optical fiber, and wherein the step of cleaning the optical fiber is only carried out, if the prior usage of the reusable optical fiber is detected.

9. The method according to claim 1, wherein the method further comprises, prior to cleaning the optical fiber, a step of heating up a laser rod of the pulsed laser system by means of pre-pumping-pulses of the pump source, wherein the pump source is operated below the laser threshold.

10. The method according to claim 1, wherein the pump source is controlled by a control unit selectively at an operating voltage and/or operating current or at one or more cleaning voltages and/or one or more cleaning currents such that pump energy is provided to the laser resonator selectively at a high level for generating the operating laser pulses at the operating energy level or at one or more low levels for generating the cleaning laser pulses at the one or more cleaning energy levels.

11. A pulsed laser system, comprising: a laser resonator with a laser rod configured to generate operating laser pulses; a pump source configured to provide pump energy (P) to the laser resonator; an optical fiber; a focusing element for coupling the operating laser pulses into the optical fiber; and a control unit configured to control the pump source at an operating voltage and/or operating current such that the pump energy is provided to the laser resonator at a high level for generating the operating laser pulses at an operating energy level, wherein the control unit is configured to control the pump source at one or more cleaning voltages and/or one or more cleaning currents such that the pump energy is provided to the laser resonator at one or more low levels for generating cleaning laser pulses at one or more cleaning energy levels between a laser threshold and the operating energy level for cleaning the optical fiber.

12. The system according to claim 11, wherein if the operating energy level is less than a maximum cleaning energy level, then the one or more cleaning energy levels is in a range from the laser threshold to the operating energy level and if the operating energy level is equal to or above the maximum cleaning energy level, then the one or more cleaning energy levels is in a range from the laser threshold to the maximum cleaning energy level, wherein the maximum cleaning energy level is in a range from 400 to 1000 mJ.

13. The system according to claim 12, wherein the laser resonator of the pulsed laser system is pumped by means of the pump source in order to generate second cleaning laser pulses at one or more second cleaning energy levels between the operating energy level and a second operating energy level, wherein the second operating energy level is higher than the operating energy level.

14. The system according to claim 11, wherein the cleaning laser pulses are generated as a sequence of laser pulse groups, wherein each one of the laser pulse groups comprises at least one of the cleaning laser pulses, and wherein the one or more cleaning energy levels are increased from one laser pulse group to a following laser pulse group.

15. The system according to claim 14, wherein the one or more cleaning energy levels are increased by a linear factor and/or an exponential factor from the one laser pulse group to the following laser pulse group.

16. The system according to claim 14, wherein each one of the laser pulse groups comprises at least two of the cleaning laser pulses at the same cleaning energy level.

17. The system according to claim 11, wherein the optical fiber is a reusable optical fiber.

18. The system according to claim 17, wherein a data storage device associated with the reusable optical fiber is configured to be readout in order to detect a prior usage of the reusable optical fiber.

19. The system according to claim 11, wherein the pump source is configured to provide pre-pumping-pulses for heating up a laser rod of the pulsed laser system, wherein the pump source is operated below the laser threshold.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0033] Further features and advantages of the invention will be explained in greater detail in the following and based on the embodiment shown in the figures. The figures show:

[0034] FIG. 1 is a schematic overview of an embodiment of a pulsed laser system;

[0035] FIG. 2 is a flow chart of an embodiment of a method for operating the pulsed laser system in FIG. 1; and

[0036] FIG. 3 is a diagram showing the energy of the cleaning laser pulses and the operating laser pulses.

DETAILED DESCRIPTION

[0037] FIG. 1 shows a schematic overview of an embodiment of a pulsed laser system 1. It shows the laser resonator 10 comprising the laser rod 11, the first mirror 12 having a fully reflecting surface and the second mirror 13 having a semi-reflecting surface. The laser rod 11 is for example a holmium crystal (Ho:YAG) which emits infrared light at a wavelength in the range of 2050-2150 nm when resonating. The pump energy P is supplied to the laser rod 11 in a pulsed manner by means of the pump source 20 which may be a flashbulb or a semi conductor laser arrangement. The pump source 20 is controlled by the control unit 50 selectively at an operating voltage or at several cleaning voltages such that pump energy P is provided to the laser rod 11 either at a high level for generating operating laser pulses at an operating energy level or at several low levels for generating cleaning laser pulses at several respective cleaning energy levels. Both the operating voltage and the cleaning voltages are controlled by the control unit 50 to form a pulsed signal at a repetition rate of, for example 20 Hz. Thus, the operating laser pulses and the cleaning laser pulses have said repetition rate. Moreover, the laser resonator 10 may further comprise an amplification stage and a Q-switch.

[0038] The operating laser pulses and the cleaning laser pulses are emitted by the laser resonator 10 as the laser beam B which is focused by the focusing element 30 to form the focal point F. In this case the focusing element 30 is a coupling lens. However, it is conceivable that the focusing element 30 may be a coupling mirror or a combination of several optical elements.

[0039] Moreover, it can be further seen in FIG. 1 that the proximal end face 40a of the optical fiber 40 is located at the focal point F in order to couple the laser beam B with the operating laser pulses and the cleaning laser pulses into the optical fiber 40. The proximal end face 40a is surrounded by a cladding and embedded in the connector element 41 which is releasably attached to a mounting bracket (not shown). In this case the optical fiber 40 is a reusable fiber which can be used by an operator several times. In order to detect the number of usages, the connector element 41 comprises a data storage device 41a which can be written to and read out by the control unit 50.

[0040] The distal end face 40b of the optical fiber 40 is embedded into an operating head 42 which may be used by the operator to irradiate the laser light L on to a target area T.

[0041] Since the beam waste needs to be very small at the focal point F in order to provide a high coupling efficiency, the focal distance of the focusing element 30 is as small as possible. Consequently, the proximal end face 40a of the optical fiber 40 is located close to the focusing element 30. Thus, there is a possible risk of contaminations at the proximal end face 40a being heated up by the operating laser pulses and being accelerated towards the focusing element 30. Consequently, the focusing element 30 may become sputtered, if the optical fiber 40 has not been cleaned appropriately.

[0042] In order to prevent sputtering of the focusing element 30 the proximal end face 40a of the optical fiber 40 is cleaned by means of cleaning pulses formed according to the method 100 described subsequently.

[0043] FIG. 2 shows a flow chart of an embodiment of the method 100 for operating the pulsed laser system 1 in FIG. 1.

[0044] As a first step 101 the laser rod 11 of the pulsed laser system 1 is heated up with a series of pre-pumping-pulses for stable operation. Therefore, the control unit 50 controls the pump source 20 with a series of pulses at a pre-pulse-voltage below a threshold voltage U.sub.thr which corresponds to the laser threshold. Therefore, the laser rod 11 is heated up without actually generating laser pulses.

[0045] As the next step 102 the data storage device 41a of the optical fiber 40 is read out by the control unit 50. If by the decision 103 a prior usage of the optical fiber 40 is not detected, it is new and has been sterilized appropriately after production. Therefore, cleaning is not necessary and the method further proceeds with step 110 as described below. However, if a prior usage of the optical fiber 40 is detected, the cleaning procedure 104 is performed by means of cleaning laser pulses.

[0046] In order to minimize the risk of contaminations being sputtered onto the focusing element 30, a sequence of laser pulse groups is generated according to the procedure 105 which is described in conjunction with FIG. 3, which shows as diagram 2 the pulse number on the X-axis and the pulse energy in joules on the Y-axis for the cleaning laser pulses C and the subsequent operating laser pulses P.

[0047] As an initial step 106, a cleaning energy range is set. If the operating energy level LO is less than 600 mJ, then the cleaning energy range is set from just below the laser threshold LT to the operating energy level LO. However, if the operating energy level LO is equal to or above 600 mJ, then the cleaning energy range is set from just below the laser threshold LT to 600 mJ. Therefore, the maximum cleaning energy level MC possible is 600 mJ. Moreover, the cleaning laser energy level for the first cleaning laser pulses is set to the minimum of the cleaning energy range. As an example only, the maximum number of laser pulse groups is set to 25, wherein each pulse group has a number of two pulses.

[0048] Furthermore, the steps 107-109 are repeated until the maximum number of laser pulse groups is reached: In step 107 a first group of two cleaning laser pulses is generated at a cleaning energy level just below the laser threshold LT. As a next step 108 the cleaning energy level is subsequently increased for the first ten laser pulse groups by a first linear factor and an exponential factor and for the second ten laser pulse groups by a second linear factor, only. The cleaning energy level is increased up until the maximum of the cleaning energy range has been reached, in this case after the second ten laser pulse groups. For the last five laser pulse groups the cleaning energy level is held constant at the maximum of the cleaning energy range. In order to increase the cleaning energy levels, the cleaning voltages U.sub.FL applied to the pump source 20 are generated according to formulas (1), (2), wherein n is the index of the laser pulse group and U.sub.thr is the threshold voltage corresponding to the laser threshold:

n[1, 10]: U.sub.FL(n)=(U.sub.thr5V)+2.5V*n+e.sup.n/3.3(1)

n[11, 2]: U.sub.FL(n)=U.sub.FL(n1)+4V(2)

[0049] In other words the cleaning voltage U.sub.FL is increased for the first ten laser pulse groups by the first linear factor 2.5V and the exponential factor e.sup.n/3.3 and for the second ten laser pulse groups by the second linear factor 4V.

[0050] It is understood, that the cleaning voltages U.sub.FL represents a peak voltage of a pulsed signal applied to the pump source 20. Within each laser pulse group, the peak voltage of the pulsed signal is constant and, therefore, the cleaning energy level within each laser pulse group is constant for each one of the cleaning laser pulses. For example, the pulsed signal may have a repetition rate of 20 Hz and the pump pulse duration is set to 700 ps.

[0051] If the maximum number of laser pulse groups has been reached in step 109, the cleaning procedure is terminated and the operator may begin to generate the operating laser pulses at an operating energy level in step 110. As described above, the operating laser pulses are coupled with the focusing element 30 into the optical fiber 40 in step 110.

[0052] As a result, it can be seen in FIG. 3 that during the cleaning procedure 104-109 the laser resonator 10 of the pulsed laser system 1 is pumped by means of the pump source 20 in order to generate the cleaning laser pulses C with increasing cleaning energy levels between the laser threshold LT and the maximum cleaning energy level MC of 600 mJ, wherein the cleaning laser pulses C are generated as a sequence of laser pulse groups. Furthermore, the operating laser pulses O are generated at the operating energy level LO.

[0053] Consequently, the optical fiber 40 is cleaned prior to generating the operating laser pulses O by means of cleaning laser pulses C, wherein the laser resonator 10 of the pulsed laser system 1 is pumped by means of the pump source 20 in order to generate the cleaning laser pulses C at cleaning energy levels between the laser threshold LT and the maximum cleaning energy level MC of 600 mJ. Since the cleaning laser pulses C have less energy than the operating laser pulses O, less energy is dissipated as heat in contaminations and the cladding at the proximal end face 40a of the optical fiber 40. Consequently, particles still get removed there from, however, with less kinetic energy. Thus, the proximal end face 40a of the optical fiber 40 is cleaned by the cleaning laser pulses C without sputtering the focusing element 30.

[0054] In a further example not shown in FIGS. 1-3, the operating energy level is at 300 mJ below the maximum cleaning energy level of 600 mJ and a second operating energy level at 1000 mJ. Thus, if the optical fiber is only cleaned in steps 106-109 until the operating energy level of 300 mJ, it may become damaged when using it with the second operating energy level of 1000 mJ. Therefore, the method 100 may comprise a second step of cleaning the optical fiber 40 after generating the operating laser pulses 110 by means of second cleaning laser pulses, wherein the laser resonator 10 of the pulsed laser system 1 is pumped by means of the pump source 20 in order to generate the second cleaning laser pulses at second cleaning energy levels between the operating energy level of 300 mJ and a second operating energy level of 1000 mJ. Specifically, as the maximum cleaning energy level MC is still 600 mJ, the second cleaning laser pulses are generated between the operating energy level of 300 mJ and 600 mJ.

[0055] Afterwards, the laser resonator 10 is pumped by means of the pump source 20 after the second step of cleaning the optical fiber 40 in order to generate second operating laser pulses at the second operating energy level of 1000 mJ.

[0056] It is understood that features mentioned in the previously described embodiments are not limited to these combinations but are also possible individually or in any other combinations.