WO2013091851A1

WO2013091851A1 - Laser having a monitored optical fiber section

Info

Publication number: WO2013091851A1
Application number: PCT/EP2012/005263
Authority: WO
Inventors: Peter Heist; Matthias Hoffmann; Uwe Kutschki; Gunter SCHEIDING
Original assignee: Jenoptik Laser Gmbh
Priority date: 2011-12-21
Filing date: 2012-12-19
Publication date: 2013-06-27
Also published as: DE102011089482A1

Abstract

The invention relates to a laser having a laser source (2), which can be operated with power and generates first laser radiation as a function of the power supply, and an optical fiber section (5) having an inlet and outlet end (13, 12), into which section the first laser radiation is coupled via the inlet end (13) and which outputs second laser radiation via the outlet end (12) as a function of the first laser radiation coupled in, a measuring unit (14, 15), which measures laser radiation that emerges at a point located between the inlet and outlet end (13, 12) of the optical fiber section (5) and, on the basis thereof, generates a measured signal, and a control module (6), which continuously determines a first limiting value (S_s1(t)) and/or a second limiting value (S_s2(t)) on the basis of the power supplied to the laser source (2), continuously determines an actual signal (S₁(t)) on the basis of the measured signal from the measuring unit (14, 15), compares the actual signal (S₁(t)) continuously with the first limiting value (S_s1(t)) and switches off the laser source (2) if the actual signal (S₁(t)) is smaller than the first limiting value (S_s1(t)) and/or compares the actual signal (S₁(t)) continuously with the second limiting value (S_s2(t)) and switches off the laser source (2) if the actual signal (S₁(t)) is greater than the second limiting value.

Description

Laser with monitored fiber optic path

The present invention relates to a laser having a laser source which is operable with current and generates a first laser radiation in response to the supplied current, and an inlet and an outlet end having optical fiber link into which the first laser radiation is coupled via the input end and in Dependence of the coupled-in first laser radiation emits a second laser radiation via the outlet end. Since the leadership of the laser radiation takes place in the Lichtleitfaserstrecke, a closed light guide is in contrast to a possible open light guide, in which the laser radiation is guided by mirrors to the exit end of the laser.

When guiding the laser radiation in an optical fiber path, there is a difficulty in that until the laser radiation exits the outlet end there is no freely accessible point at which a part of the laser radiation for regulating the laser can be coupled out, for example by means of a coupling-out mirror. Since an optical fiber path as a light-guiding medium for the laser radiation usually contains glass fibers or glass fiber components, there is the risk of destruction of the optical fiber link, for example by fiber breakage or thermally induced melting of a fiber connection point. In the case of such a destruction, the laser radiation would emerge freely at least in the interior of the laser and, with a correspondingly high power of the laser (of, for example, 1 kW or more), cause considerable damage to the laser as well as endangering the operating personnel.

Particularly critical in lasers with an optical fiber link are the splices in which fiber end faces are fused together, and fiber couplers, in particular for running in the opposite direction of propagation laser radiation significantly lower damage threshold than in the intended direction of propagation. Such reverse propagation of laser radiation can occur when the laser radiation emitted by the laser is reflected back from the workpiece to be machined in the Lichtleitfaserstrecke.

In the case of a laser with an optical fiber link, there is therefore the difficulty that destruction of the laser beam which is generated primarily in the forward direction and also by laser light reflected backwards, for example by the workpiece to be machined,

CONFIRMATION OPIE can be caused. Since there are no freely accessible points for obtaining measurement signals in the optical fiber path in comparison to open beam lasers, it is already difficult to obtain corresponding measurement signals for monitoring the optical fiber path of the laser.

It is known in light-conducting cables to detect the destruction (eg in the form of a fiber break) by embedding a detection wire running next to the light-guiding fiber. The detection wire may be a wire or a conductor track deposited on the primary coating of the fiber (US 4,883,054), which changes the conductivity in case of fiber breakage. Furthermore, a second optical waveguide fiber, into which a second, very low-power light source is coupled and detected at the outlet end, can be provided, as described in DE 40 107 89 A1. Such an embedding of an electrical or "optical" wire is not or not consistently possible in a laser with an optical fiber link (especially if it is designed for high power), since such an electrical or optical wire can not be integrated in important fiber components such as a high power fiber coupler is.

Furthermore, it is known to monitor an optical waveguide by specially multilayer fiber layers, which can pass loss radiation through conversion processes from a first layer to a second layer, and which is then passed to a detector (DE 43 140 31 A1). Again, however, there is the difficulty that specially prepared glass fibers are needed, which often can not be used in a laser with an optical fiber link (especially for high power). Further, it is known to provide photodetectors for detecting stray light at the input and output ends of the fiber, respectively, in conjunction with suitable evaluation electronics which compares the two signals (US 4,812,641) or compares to a predetermined setpoint (US 7,146,073). Such a configuration thus relates to a focus in a light guide and the exit from the light guide, so the case of an interrupted beam, which is not present in a laser with a Lichtleitfaserstrecke just. In addition, the signals for the photodetectors are obtained directly at the input and the output end, for which purpose a specific preparation of the fiber ends is described in US Pat. No. 7,146,073. Such a preparation precludes use of commercially available high performance fiber optic cable provided with standard plug dimensions. Furthermore, it is known to carry out the measurement at the fiber input and at the fiber output in such a way that the signal is obtained as a function of direction at the fiber output in order to allow conclusions about the material processing process with regard to backreflection and defocusing (US Pat. No. 5,319,195).

Again, there is the case of an interrupted beam guidance in order to win the corresponding measurement signals can. The measurement signal at the output end is further influenced by reflected light from the workpiece, which is taken into account here, but the evaluation still difficult.

Proceeding from this, it is an object of the invention to provide a laser with a laser source and an inlet and an outlet end having optical fiber link, in which destruction of the optical fiber during operation of the laser as possible prevented or at least minimized.

According to the invention, the object is achieved by a laser having a laser source which can be operated or pumped with current and generates a first laser radiation as a function of the supplied current, an inlet and an outlet end having optical fiber link, into which the first laser radiation coupled via the inlet end and which emits a second laser radiation via the outlet end in dependence on the coupled first laser radiation, a measuring unit which measures the laser radiation which emerges at a point lying between the inlet and the outlet end of the optical fiber path, and generates thereon a measurement signal and a control module, based on the current supplied to the laser source continuously determines a first and a second limit, which continuously determines an actual signal based on the measurement signal of the measuring unit and constantly compares the actual signal with the first limit and the laser source switches off, who n the actual signal is smaller than the first limit value and / or the actual signal continuously compares with the second limit value and the laser source switches off when the actual signal is greater than the second limit value.

A decrease in the actual signal below the first limit indicates a loss of laser radiation in the optical fiber link, which z. B. occurs when the guided in the optical fiber laser radiation part of the optical fiber path thermally damaged or if, for example, a mechanical damage is present. With an appropriate choice of the size of the first limit value can thus be safely prevented further damage to the optical fiber link. If the actual signal exceeds the second limit, this shows z. For example, that reflected back laser radiation is coupled into the optical fiber link and it passes in the opposite direction, the optical fiber link. This can occur when machining workpieces, if z. B. during material processing a highly reflective melt is formed. With a corresponding choice of the second limit, damage that would cause the reflected back laser radiation can be prevented. Of course, can be prevented by the comparison of the actual signal with the second limit and damage by laser radiation that passes through the optical fiber for other reasons in the opposite direction. The second threshold is preferably greater than the first threshold.

Under an ongoing monitoring is here understood in particular that the system is continuously monitored, for example, with an analog electronic circuit, or that the system is monitored at successive times, the distance between successive monitoring times can be chosen so short that one in the meantime either defect would not lead to the destruction of other components of the system or would normally not at least endanger human and environmental health. The second-mentioned training of a continuous monitoring can be formed, for example, with a digital electronics, in which, for example, a controller repeatedly compares the actual values with the limit values in an infinitely loop realized in the program. For example, the time between two monitoring times may be a few ps. For completeness, it should be mentioned that the time period between two monitoring times z. B. either always be the same or different.

In the case of the laser according to the invention, the control module can switch off the laser source by switching off the power supply to the laser source.

Switching off the laser is understood here in particular to mean that the laser power is reduced to such an extent that no damage of components can occur or that no danger can emanate from the laser. The shutdown can, but need not, be made such that the laser current is brought to zero. This can be done for example by switching off the laser current or alternatively by shorting the laser current. The short-circuiting of the laser current can advantageously take place in the immediate vicinity of the laser, for example with a MOSFET or a bipolar transistor. However, it is also sufficient to switch off the laser if the current is not switched off completely but is reduced to such an extent that the laser threshold is not reached. The optical fiber path is preferably connected directly to the laser source, so that between Laser source and optical fiber link no free beam access is present. In particular, there is no free-beam access from the laser source to the exit end of the optical fiber path, so that the laser has a completely closed beam guidance. Under free-beam access is here z. B. understood that the laser radiation is conducted over a certain length in air, so that in this area z. B. a decoupling signal for decoupling a small proportion of the laser radiation can be positioned. However, the free path, for example between a diode laser and the FAC lens or within a compact fiber optic coupling optics for a diode laser is not to be understood as free beam access in this sense, because there is no space for a device for decoupling provided. In contrast, free beam access is present in an optical telescope for coupling the radiation from one fiber into another fiber, since a device for coupling out a measuring beam can be provided in the collimated beam section. However, such free beam access is not present if it is a system in which the fibers are spliced together or connected via face coupling. In such a system then no free beam access is available.

Furthermore, in the case of the laser according to the invention, the optical fiber path can have at least one optical fiber with core and cladding and the measuring unit can measure the laser radiation emerging from the cladding. This radiation is often referred to as a sheath light. The optical fiber can be formed so that it emits such a jacket light at a predetermined location. These can z. B. microscopic interference in the transition from the core to the jacket can be provided. It can also be roughened a small portion of the fiber cladding, whereby a scattering of the jacket light takes place. Additionally or alternatively, the optical fiber can be intentionally bent so that due to the bending jacket light emerges. It is also possible to apply to the lateral surface of a medium having a higher refractive index than the cladding, so that no total reflection takes place at the interface between the cladding and the applied medium and thus jacket light is decoupled.

The laser radiation measured by the measuring unit preferably exits transversely to the propagation direction of the laser radiation in the optical fiber path.

The decoupled laser radiation preferably has only a fraction of the average power of the laser radiation guided in the optical fiber path. In particular, the fraction may be in the per thousand range. In the case of the laser according to the invention, the optical fiber path can have at least two optical fibers spliced together and the measuring unit can measure the laser radiation exiting at the splice point. Furthermore, the optical fiber path may comprise a plurality of optical fibers, which are coupled by means of a fiber coupler with a single optical fiber, wherein the measuring unit measures the emerging from the fiber coupler laser radiation.

In both cases (measurement at the splice point as well as measurement at the fiber coupler), the invention makes use of the fact that the laser radiation emerging there is measured and used to monitor the safety of the optical fiber link according to the invention.

In the case of the laser according to the invention, the optical fiber link can be designed as a passive optical fiber link. In this case, only the first laser radiation of the laser source is guided over the optical fiber path, so that the second laser radiation decoupled via the outlet end of the optical fiber path corresponds to the first laser radiation.

Of course, it is also possible that the optical fiber link has an active fiber which is pumped with the laser radiation of the laser source. In this case, a fiber laser is present, so that the decoupled second laser radiation is the laser radiation of the fiber laser and the coupled-in first laser radiation is the pump radiation of the fiber laser. In this case, the pumping radiation and / or the second laser radiation generated by means of the fiber laser can be measured by means of the measuring unit as emerging laser radiation. The laser source may have at least one laser diode. It may preferably have a plurality of laser diodes which are supplied with current, for example, in series or parallel circuits. However, other laser sources are also possible which can be operated with electricity or are electrically pumpable. In particular, electrically pumped semiconductor lasers can generally be used. It is also possible to pump a solid-state laser by means of one or more electrically pumped semiconductor lasers, thereby realizing the laser source.

The control module may increase the first and / or second threshold as the pumping current increases. This takes into account the fact that with increasing pumping current, the mean laser power and thus also the risk of a defect in the optical fiber link increases. In particular, the control module performs the increase of the first limit value with increasing pumping current such that the increase increases with increasing pumping current. Thus, the larger the pumping current becomes, the greater the increase becomes, thereby providing better protection. Furthermore, the control module can carry out the increase of the second limit value such that the increase of the second limit value decreases with increasing pumping current. As the pumping current increases, the increase for the second threshold value decreases as the increase continues, which in turn leads to better protection of the laser. So z. B. the width of the window defined by both limits decrease with increasing pumping current. Only when the actual value is in the window, the laser is not switched off.

In the case of the laser according to the invention, the current supplied to the laser source can vary over time, in order, for. B. pulsed laser radiation or to vary the beam power in continuous wave mode. In this case, the actual signal as well as the first and / or second limit value preferably vary in time in the same way. This ensures that the first and / or second limit value are ideally adapted to the respective laser conditions and thus the first and / or second limit value can be selected very close to the actual value, so that the intended operation can be performed safely and at the same time the desired Security (ie the fast shutdown, if necessary) is guaranteed. The laser is designed especially for high power. This is understood in particular to mean that the average power in continuous-wave operation or the pulse peak power in pulsed operation of the laser radiation emitted via the outlet end is at least 1 kW.

The optical fiber path may also be designed such that laser radiation emerges at several points between the inlet and the outlet end. In this case, the measuring unit is preferably designed so that it respectively measures the emerging laser radiation at these multiple locations and generates corresponding measuring signals. The control module then continuously determines a corresponding actual signal for each exit point on the basis of the measurement signals and continuously compares all actual signals with the first limit value and / or the second limit value. If at least one of the actual signals is smaller than the first limit value, the control module carries out the shutdown of the laser source. Furthermore, the control module also shuts off the laser source when at least one of the actual signals is greater than the second threshold.

The optical fiber path can also be designed such that it has a plurality of light entry ends. In particular, the exit end can also be used as entry end. This is the case, for example, with a double ended end-pumped fiber laser. There, the exit end of the laser radiation of the active fiber can simultaneously be the inlet end for the pump radiation. The pump radiation and emerging laser radiation can Have different wavelengths, so that a separation of the beam paths, for example, with a wavelength-selective mirror element is possible.

The laser can emit pulsed laser radiation or continuous wave radiation.

There is further provided a method of operating a laser having a laser source which is operable with current and generates a first laser radiation in response to the supplied current, and having an inlet and an outlet end having optical fiber in which the first laser radiation over the Input end coupled and depending on the coupled-in first laser radiation, a second laser radiation is output via the outlet end, wherein the laser source power is supplied so that it generates the first laser radiation, laser radiation emerging at a lying between the inlet and the outlet end of the optical fiber link , Measured and based on a measurement signal is generated based on the current supplied to the laser source continuously a first and / or a second limit is determined based on the measurement signal continuously an actual signal is determined and the actual signal continuously with the first limit value compared and the laser source is turned off when the actual signal is smaller than the first threshold, and / or the actual signal continuously compared with the second threshold and the laser source is turned off when the actual signal is greater than the second threshold. In this case, the actual signal may be equal to the measurement signal, or the actual signal may additionally include, for example, a dependent of the time change of the measurement signal portion, which is referred to as a differential part, or for example an integral part, which depends on the accumulated over a certain period control deviation. Such a differential part leads to a faster shutdown in case of sudden deviations of the actual value from the target base value, d. H. with sudden deviations.

With the method according to the invention for operating a laser, the destruction of the optical fiber path can be reliably prevented or at least an occurring destruction can be minimized.

The inventive method for operating a laser can be further developed so that the laser according to the invention can be operated including its developments. In particular, the method according to the invention may contain the method steps described in connection with the laser according to the invention.

It is understood that the features mentioned above and those yet to be explained not only in the specified combinations, but also in others Combinations or alone can be used without departing from the scope of the present invention.

The invention will be explained in more detail for example with reference to the accompanying drawings, which also disclose characteristics essential to the invention. Show it:

Fig. 1 is a schematic view of a first embodiment of the invention

laser;

Fig. 2 is an enlarged schematic sectional view of a fiber optic element of

Optical fiber path of the laser according to the invention;

3a-3c are diagrams for explaining the monitoring of the invention

Optical fiber path of the laser according to Fig. 1;

4a-4c are diagrams for explaining the monitoring of the invention

Optical fiber path of the laser according to Fig. 1;

Fig. 5a-5c are diagrams for explaining the monitoring of the invention

Optical fiber path of the laser according to Fig. 1;

6a-6c are diagrams for explaining the monitoring according to the invention

Optical fiber path of the laser according to Fig. 1;

Fig. 7 is a schematic representation of the signal acquisition and processing by the

Control module of the laser according to the invention, and

Fig. 8 is a schematic representation for explaining the technical control

Realization of the monitoring of the optical fiber link.

The optical fiber link may include optical fibers, optical fibers and optical cables or other fiber optic components.

In the embodiment shown in FIG. 1, the laser 1 according to the invention has a laser source 2 with three laser diodes 3i, 3 ₂ and 3 ₃ , a current source 4 for operating the laser diodes 3 _r 3 ₃ , an optical fiber link 5 and a control module 6. Of course, the laser source 2 may also contain more than three or fewer than three laser diodes 3 3 ₃ . The number of three laser diodes 3 3 ₃ is only an example.

The fiber span 5, for each laser diode 3 33 a first optical fiber and thus here three first optical fibers 7, 7 ₂ and 7 _3, wherein each first optical fiber 7 7 ₃ is exactly assigned to a laser diode 3 3 ₃ so that a first end Each first optical fiber 7 7 ₃ with the associated laser diode 3, -33 is connected. The other end of the first optical fibers 7 7 ₃ is welded to second optical fibers 8 8 _{3 of} a fiber coupler 9. The fiber coupler 9 has on the output side, a third optical fiber 10, with their from the fiber coupler. 9 groundbreaking end is welded to a fourth optical fiber 11, the third optical fiber 10 facing away from an exit end 12 of the optical fiber 5 forms. The entrance end 13 of the optical fiber 5 is formed by the connected to the laser diode 3 3 ₃ ends of the first optical fiber 7 ₁ -7 ₃ .

The laser 1 further comprises a first and a second photodetector 14, 15, with which laterally from the optical fiber 5 between the inlet and outlet ends 13, 12 emerging laser radiation (indicated by arrows 16, 17) can be measured. This is used for a safety shutdown of the laser 1, as will be described in more detail below. The measurement signals of the photodetectors 14, 15 are supplied to the control module 6.

For operation of the laser 1, the laser diodes 3 3 _{3 are} supplied with current of the current source 4. The series connection of the laser diodes 3 _r 3 ₃ shown in FIG. 1 is only to be understood as an example. There are also other types of wiring possible. In particular, a parallel connection is possible.

By applying current, the laser diodes generate 3 3 ₃ laser radiation, which is coupled into the respective first optical fiber 7 7 ₃ and is guided over the fiber coupler 9 to the outlet end 12 of the optical fiber 5. In the embodiment described here, the optical fiber link 5 is formed purely passive. Thus, only the laser radiation of the laser diodes 3 3 _{3 is} guided to the exit end 12. However, it is also possible to arrange an optical fiber in the optical fiber 5, which is pumped with the laser radiation of the laser diode 3 3 ₃ . In this case, a fiber laser is present whose laser radiation is emitted as desired laser radiation via the exit end 12. The emerging at the outlet end 12 laser radiation 18 may, for. B. for cutting, welding or hardening of various workpieces W are used and here has an average power of at least 1 kW. The output current for the laser diodes 3 3 ₃ can be determined by the user of the laser 1 from the outside via a terminal 19 of the power source 4.

In the example shown in Fig. 1 Laser a closed beam guiding is present, since the laser beam of the laser diode 3 3 ₃ is directly coupled into the first optical fibers 7 ₁ -7 _3, and the optical fibers 7 7 ₃ 8 8 _3, 10 and 11 by means of splices V1 and V2, in which the fiber end faces are welded together, are connected to each other and thus the laser radiation of the laser diode 3 3 _{3 is} coupled via the outlet end 12 of the optical fiber 5. The laser according to FIG. 1 can therefore also be referred to as an all-fiber laser. The optical fibers 7 7 ₃ , 8 8 ₃ , 10 and 1 1 of the optical fiber 5 are fiber optic components that may differ depending on the specific implementation in detail, but are always similar in their basic structure. In Fig. 2 is shown schematically in an enlarged sectional view of the basic structure of such an optical fiber having a step-shaped refractive index profile, wherein the optical fiber has a light-guiding core 20, which is usually formed of quartz glass. The refractive index of the quartz glass of the core 20 is greater than the refractive index of the immediately adjacent shell 21, which leads to a total internal reflection at the boundary core-cladding and thus to the light-guiding property of the fiber. The jacket 21 may be formed, for example, from fluorine-doped quartz glass.

In order to make the fiber pliable and mechanically robust, a plastic layer 22 is applied to the casing 21, which is often referred to as a coating or buffer. The plastic layer 22 may consist of a primary, soft coating 23 and a secondary, harder coating 24. In the training as fiber optic cables still more sheaths are often provided to increase the robustness. On the other hand, for. B. on fiber couplers may be omitted on the coating 22. The optical fiber described in connection with FIG. 2 has a step-shaped refractive index profile between core 20 and cladding 21, so that such fibers are also frequently called step-index fibers. Instead of being formed as a step-index fiber, the optical fibers can also be designed as so-called gradient index fibers in which the refractive index continuously decreases towards the outside from the fiber center.

Since the laser 1 has the described closed beam guidance, the optical fiber path 5 is subject to the risk of destruction, for example in the form of fiber breakage or thermally induced melting of a splice point V1, V2. In the case of such destruction, the laser radiation could emerge freely at least in the interior of the laser 1 and at the possible high power of, for example, 1 kW and more cause significant damage to the device and a threat to the operator.

Furthermore, the laser radiation 18 emitted via the exit end 12 can undesirably be reflected back into the optical fiber path 5 by the workpiece W to be machined, so that there is an undesired reverse propagation. Especially the fiber coupler 9 can have a significantly smaller destruction threshold for laser radiation in the opposite propagation direction than in the intended propagation direction. In order to prevent a destruction of the optical fiber link 5 and a risk to persons or at least to minimize the extent of destruction of the optical fiber link 5, the fact is exploited that a clear functional relationship between the generated laser power and the current supplied to operate the laser diode 3 3 ₃ is present. This is used in such a way that, based on the current for operating the laser diodes 3 3 _3, a first limit value S _s1 (t) is derived and a (first) actual signal S t) to be compared is derived from the read signal of the first photodetector 14 , The first limit value S _s (t), which may also be referred to as the first threshold value or the first setpoint value, and the actual signal Si (t), which may also be referred to as the actual value, are continuously compared with one another in the control module 6, and if the actual signal S ^ t) is smaller than the first threshold value S _s i (t), causes the control module 6 via the power source 4, a shutdown of the laser 1. Such a decrease in the actual signal Si (t) shows that the laser power in the optical fiber 5 "lost This occurs, for example, when melting begins at a splice connection V1, V2.

This type of regulation or monitoring of the laser 1 will be described in detail below in conjunction with FIGS. 3a-3c. It is assumed that the operating current I (t) for operating the laser diodes 3 3 _{3 is} constant in time, as shown in Fig. 3a.

Based on this temporally constant operating current I (t), the control module 6 determines the first limit value S _s1 (t), as shown schematically in FIG. 3 b.

In FIG. 3c, the actual signal Si (t) determined by the control module 6 based on the measurement signal of the first photodetector 14 is shown in addition to the first limit value S _s1 (t). Since this actual signal S ^ t) is always above the first limit value S _s1 (t), the laser 1 is not switched off. In Fig. 3c is still another actual signal S '^ t) is shown, which is assumed that it at time t1 drops to the first threshold value S _s . With such an actual signal S'i (t), the control module 6 switches off the current source 4 at the time t1, so that the laser diodes 3 3 _{3 are} no longer supplied with current. This leads to the shutdown of the laser. 1

FIGS. 4a-4c show the case in which the operating current I is modulated as intended in terms of time. This then likewise leads to a time-modulated first limit value S _s1 (t), as shown in FIG. 4 b. During normal operation of the laser 1, the actual signal Si (t) should then have the same temporal modulation as long as the laser 1 has no error, as indicated in FIG. 4c. A temporally constant first limit value would not make sense for this operation, since the intended temporal modulation of the actual signal Si (t) to a Fall below the temporally constant first limit would result, so that in an undesirable manner, the laser 1 would be turned off. In order to prevent this, the distance between the temporally constant first limit value and the expected actual signal would have to be so large that it would no longer be possible to reliably prevent or at least quickly prevent destruction of the optical fiber link 5.

The laser radiation detected by the first photodetector 14 is preferably decoupled in such a way that it is only a small fraction of the light from the laser radiation exiting via the exit end 12. Typically, the fraction is in the range of a few per thousand. This can be achieved in particular by decoupling light from the cladding of the corresponding optical fiber. For such decoupling, microscopic perturbations in the transition from the core to the cladding can be provided. Furthermore, known methods for generating cladding light can be used. For example, a small area of the fiber cladding can be roughened, whereby the cladding light is scattered out. It is also possible to apply to the lateral surface a medium which has a higher refractive index than the cladding, so that no total reflection takes place at the interface between the cladding and the applied medium. Furthermore, the scattered light produced at the splices V1, V2 can also be detected. This is indicated in Fig. 1 for the splice point V1, wherein the corresponding scattered light 17 is measured by means of the second detector 15. Based on the measurement signal of the second detector 15, the control module 6 determines a second time-dependent actual signal which is continuously compared with the first limit value in the same way as the first actual signal. When the second actual signal drops to the first limit value, the control module 6 switches off the laser 1 for safety reasons.

In order to minimize the risk of laser radiation coupled into the optical fiber link 5 in the wrong direction, a second limit value S _s2 (t), which can also be referred to as the second threshold value or second setpoint value, can be determined, which is an upper limit for the actual value represents. If this upper limit is exceeded by the actual value, the control module 6 again performs a shutdown of the laser 1. In order to measure the laser radiation injected in the wrong direction, it is again possible to measure jacket light which arises when laser radiation reflected back from the workpiece W is coupled back into the optical fiber link 5 via the outlet end 12. Since, in this case, there are generally no optimum coupling-in conditions with regard to the numerical aperture of the fiber and / or with regard to spot size and angle of incidence in the fiber core, overblasting will take place, which can be detected. This type of safety shutdown of the laser 1 will be described in more detail below. In FIG. 5 a, the time profile of the current for the laser diodes 3 3 _{3 is} shown in the same way as in FIG. _{3 a} , with a current that is constant over time being assumed here.

Based on this current, the control module 6 determines, in addition to the first limit value S _s1 (t), a second limit value S _s2 (t) as an upper limit, which is shown in FIG. 5b in the same way as in FIG. 3b.

As long as the actual value S ^ t) lies between the two limit values S _s1 and S _s2 , the operation of the laser 1 as intended and proper is assumed. If, however, the actual value S "i rises above the second limit value S _s2 , as indicated in FIGURE 5c, the control module 6 shuts off the laser 1 by inhibiting the current supply.This is the case in the illustration according to FIG.

In the same way, a shutdown takes place if the actual value 8Ί reaches the first limit value S _s1 . This is the case according to the representation of FIG. 5c at the time t1.

Thus, a window for the actual value Si is predetermined by the two limit values S _s1 and S _s2 . As long as the actual value lies in this window, the operation of the laser 1 as intended is assumed. If the actual value ST leaves this window, the laser 1 is switched off.

FIGS. 6a-6c show the case with the lower and upper limit values S _s1 and S _s2 in the case of a time-varying operating current I. As long as the corresponding time-varying actual value ST is within the window prescribed by the two limit values S _s1 and S _s2 , it is assumed that the operation is as intended and the laser is not switched off. Should the actual value Si reach the upper or lower limit (ie the first or second limit value S _s1) S _s2 ), the automatic switch-off takes place.

Of course, it is possible to design the laser 1 such that only a comparison of the actual value ST with the second limit value S _{s2 is} performed.

FIG. 7 schematically shows the signal acquisition and processing by the control module 6 for the case with the lower and upper limit values S _s1 (t) and S _s2 (t). The control module 6 comprises a current detector 25 and a first to third electronic circuit 26, 27 and 28. The current detector 25 determines based on the laser diode 3 3 ₃ supplied operating current l (t) a nominal base signal S ₀ (t), for example, an electrical voltage signal is. This nominal base signal S ₀ (t) is supplied to the first electronic circuit 26, which determines therefrom the lower and upper limit values S _s1 (t) and S _s2 (t) according to the following formulas

Ssi (t) = S ₀ (t) - AS _IO w

S _s2 (t) = S ₀ (t) + AS _high

The values for AS | _0W and AS _hig h can be the same or different. The values are suitably determined to realize the desired protective function. In particular, the values AS | _0W and AS _high are selected smaller with increasing pumping current of the laser diodes 3 3 ₃ . Thus, the higher risk of destruction with increasing laser power can be considered. It should also be noted that the relative inaccuracies just above the laser threshold are particularly high. In this operating range, therefore, there is a particular risk that it will lead to faulty laser shutdown, without a fiber defect is present. This can be counteracted by allowing a larger percentage deviation AS _tow / S ₀ and / or AShigh S ₀ in the lower power range. It may, but need not, even have a greater absolute deviation for AS | _0W and AS _high in the range just above the laser threshold are allowed compared to the operation at rated power. The second electronic circuit 27 generates based on the measurement signal supplied to the first photodetector 14, the first actual signal S ^ t), which in turn may be a voltage signal.

The first actual signal Si (t) and the two limit values S _s1 (t) and S _s2 (t) are supplied to the third electronic circuit 28, which performs the described comparison of the signals (eg according to FIGS. 5c and 6c) Output signal S _out .

The third electronic circuit 28 may be formed, for example, as a window discminator, which compares the first actual signal Si (t) with the two limit values S _s1 (t) and S _s2 (t). Depending on the comparison result, the level of the output signal S _ou t is generated. If the actual value is within the limits, the level of the output signal S _ou t z. B. be low. If the actual signal is not within the limits, the level of the output signal S _{out may be} high, which then causes the current source to be turned off. Of course, the window discminator can 28 also be designed so that it only outputs the signal S _out, when the actual-value signal Si (t) is not within the limits. In this case, the current source is designed so that a shutdown is effected upon application of the signal S _out . The reaction time from the detection of a threshold value exceeding or threshold value undershooting to the switching off of the laser 1 essentially depends on the speed of the photodetectors 14, 15 and the electronics and software of the control module 6 processing the signals. Since laser-induced destruction is based on thermal effects, reaction times in the range of a few microseconds should be aimed for, in particular in the event of a preventive shutdown before the onset of fiber destruction. The need for microsecond response times can be estimated as greatly simplified as follows. If z. For example, if the fiber coupler 9 is considered critical, a small area V1 of approximately 0.01 mm ³ volume could be melted. For this purpose, a heat energy Q = p ^■ V ^■ c ^■ ΔΤ must be introduced, with a density of the quartz glass p = 2.2 g / cm ³ , a heat capacity of c = 1 J / (gK) and a melting point of the quartz glass of about 1660 ° C. This results in a heat energy Q of approximately 0.03 J = 0.03 Ws. With complete absorption of the laser radiation, a laser power of P = 1 kW must act for approximately 30 με to deposit this heat energy. Therefore, the reaction time should be in the range of a few microseconds.

If, in the case of the laser 1 according to the invention, consequential damage following the destruction of a fiber-optic component of the optical fiber link 5 is to be prevented, the reaction time can be selected as long as the housing or other shields reliably prevent the leakage of laser radiation into the environment. These reaction times are then longer than the mentioned few microseconds in the case of the preventive shutdown of the laser. 1

In Fig. 8, the control engineering view of another embodiment is shown schematically. The control deviation Ri (t) is formed as the difference from the continuously monitored measured value of the power P (t) and the instantaneous desired base value S ₀ (t) calculated from the laser current. From this, a proportional value is formed by multiplication by a coefficient P (coefficient for the proportional component) (indicated by block 30). Furthermore, an integral value (indicated by block 31) can be formed by the summation over previous control deviation values multiplied by an integral coefficient m. Further, by taking the difference between, for example, two consecutive control deviation values multiplied by a coefficient m _d for the differential component, a differential value can be formed (by block 32 indicated). These three values can be added up to the actual value S ^ t). This currently formed actual value S ^ t) is continuously monitored with a window discriminator 28 to determine whether it has a lower value of the control deviation AS | _0W or exceeds an upper permissible value of the control deviation AS _h i _9h , these two limit values being continuously recalculated from the desired basic value S ₀ (t) (indicated by blocks 33 and 34).

In this embodiment, the comparison of the actual signal S ^ t) with the limit values takes place in such a way that first the nominal base value S ₀ (t) is subtracted from the measured value P (t), to which difference in the above-mentioned manner proportional, differential and integral parts are summed and this sum (actual value) S ^ t) are then compared with the permissible control deviations. These control deviations are in each case the differences AS _h i _gh or AS | _0W from the upper limit value S _s (t) and the target base value S ₀ (t) and the target base value S ₀ (t) and the lower Gernzwert S _s2 (t). Of course, it is not always necessary to add to the control deviation Ri (t) the proportional, differential and integral parts. It can also only one of the shares or it can be added to the control deviation two of the shares. This method of performing the comparison is not only in this specific embodiment, but also generally equivalent to a direct comparison of an actual value, which is calculated on the basis of the measured value without deduction of the target base value, with the upper and lower limits S _h i _9h or S | _0W .

Claims

claims

1. Laser with

a laser source (2) which can be operated with current and generates a first laser radiation as a function of the supplied current,

an optical fiber path (5) having an inlet and an outlet end (13, 12) into which the first laser radiation is coupled via the inlet end (13) and which emits a second laser radiation via the outlet end (12) in dependence on the coupled-in first laser radiation,

a measuring unit (14, 15), the laser radiation which exits at a lying between the inlet and the outlet end (13, 12) of the optical fiber path (5), measures and based on a measurement signal, and

a control module (6),

the current supplied to the laser source (2) continuously determines a first limit value (S _s1 (t)) and / or a second limit value (S _s2 (t)),

based on the measurement signal of the measuring unit (14, 15) continuously an actual signal (S ^ t)) determines _t and

the actual signal (S ^ t)) continuously with the first limit value (S _s1 (t), compares and the laser source (2) turns off when the actual signal (S ^ t)) is smaller than the first limit value (S _s1 (t) )

and / or the actual signal (S ^ t)) continuously with the second threshold value (S _s2 (t)) compares and the laser source (2) turns off when the actual signal (Si (t)) is greater than the second threshold value (S _s2 (t)).

The laser of claim 1, wherein the control module (6) increases the first threshold (S _s i (t)) as the pumping current increases.

The laser of claim 2, wherein the control module (6) increases the first threshold (S _s1 (t)) with increasing pumping current such that the increase increases with increasing pumping current.

4. A laser according to any one of the preceding claims, wherein the control module (6) increases the second threshold (S _s2 (t)) as the pumping current increases.

5. The laser of claim 4, wherein the control module increases the second threshold (S _s2 (t)) so that the increase decreases as the pumping current increases.

6. Laser according to one of the above claims, wherein the current supplied to the laser source varies in time and the actual signal (S ^ t)) and the first and / or second limit value (S _s i (t), S _s2 (t)) in same time vary.

7. Laser according to one of the above claims, wherein the control module (6) switches off the laser source (2) by switching off the power supply to the laser source (2).

8. Laser according to one of the above claims, wherein the optical fiber path (5) comprises at least one optical fiber (7 ^ 7 ₂ , 7 ₃ ; 8 _! , 8 ₂ , 8 ₃ ; 10; 11) with core (20) and cladding (21 ) and the measuring unit (14, 15) measures the laser radiation emerging from the jacket.

9. Laser according to one of the above claims, wherein the optical fiber path (5) has at least two optical fibers spliced together (7 ^ 7 ₂ , 7 ₃ , 81, 8 ₂ , 8 ₃ , 10, 11) and the measuring unit (14, 15 ) measures the laser radiation emerging at the splice point (V1, V2).

10. Laser according to one of the above claims, wherein the optical fiber path (5) comprises a plurality of optical fibers (7i, 7 ₂ , 7 ₃ ) by means of a fiber coupler (9) with a single

Optical fiber (10) are coupled, and the measuring unit (14, 15) measures the laser radiation emerging from the fiber coupler (9).

11. Laser according to one of the above claims, wherein the Lichtleitfaserstrecke (5) is designed as a passive Lichtleitfaserstrecke (5).

12. A laser according to any one of claims 1 to 10, wherein the optical fiber path (5) comprises an active fiber which is pumped with the laser radiation of the laser source (2).

13. Laser according to one of the above claims, wherein the laser source (2) has at least one laser diode (3-i, 3 ₂ , 3 ₃ ).

14. Laser according to one of the above claims, which is designed so that the average power in the continuous wave mode or the pulse peak power in pulsed operation of the emitted via the outlet end (12) laser radiation (18) is at least 1 kW.

15. Laser according to one of the above claims, which is formed by the laser source (2) or the inlet end of the optical fiber path to the outlet end (12) of the optical fiber link (5) without free beam access.

16. Method for Operating a Laser,

a laser source which can be operated with current and generates a first laser radiation as a function of the supplied current,

and an optical fiber path having an inlet and an outlet end, into which the first laser radiation is coupled via the inlet end and a second laser radiation is emitted via the outlet end as a function of the coupled-in first laser radiation,

power is supplied to the laser source so as to generate the first load radiation, laser radiation exiting at a point located between the entrance and exit ends of the optical fiber path is measured, and based thereon a measurement signal is generated,

based on the current supplied to the laser source current, a first and / or a second threshold is determined

based on the measurement signal continuously an actual signal is determined and

the actual signal is continuously compared with the first limit value and the laser source is switched off when the actual signal becomes smaller than the first limit value and / or the actual signal is continuously compared with the second limit value and the laser source is switched off when the actual signal becomes greater than the second limit value ,