Abstract:
A laser for generating pulsed laser radiation. An element is arranged in the resonator to generate laser radiation having the first wavelength by frequency conversion of the primary radiation. The resonator is switchable into a first state in which it is open to the primary radiation, and a second state in which it is closed to the primary radiation, and is open to laser radiation of the first wavelength in both states. A control unit switches the resonator from the first to the second state so that the primary radiation begins to oscillate and the pulse generation by frequency conversion begins, switches the resonator from the second to the first state, whereby primary radiation is coupled out from the resonator. It is possible to set the duration between both steps and/or the coupling-out behavior of the resonator to adjust the pulse duration via the control unit.

Description:
RELATED APPLICATION 
   The current application claims the benefit of priority to German Patent Application No. 10 2006 006 582.4 filed on Feb. 13, 2006. Said application is incorporated by reference herein. 
   FIELD OF THE INVENTION 
   The present invention relates to a laser for generating pulsed laser radiation of a first wavelength, wherein the laser radiation is generated by frequency conversion within the laser resonator. 
   BACKGROUND OF THE INVENTION 
   It is known to convert the infrared radiation of a laser to the visible spectral range with the help of optical, non-linear crystals preferably arranged inside the resonator. Such lasers can emit the desired pulses in a Q-switched mode. 
   However, Q-switched solid-state lasers have an upper limit of the pulse repetition frequency, which is determined, for instance, by the lifetime of the fluorescence of the upper laser level, the stimulated effective emission cross-section of the laser ion, the length of the resonator, the degree of coupling-out and the pumping power density. Above this limit frequency, strong fluctuations in pulse energy occur initially between two subsequent pulses (“ping pong effect”), with every other pulse respectively having the same pulse energy, but, in an alternating manner, each pulse of a higher pulse energy is followed by a pulse of a low pulse energy. In the case of still higher pulse repetition frequencies, every other pulse drops out, or there may even be several bifurcations with respect to the pulse energy. Thus, on the whole, operation above this limit frequency no longer makes sense from a technical point of view. U.S. Pat. No. 6,654,391 describes a method for a Q-switched laser with frequency doubling inside the resonator, wherein pulse stabilization is achieved in that the pulse tail of the frequency-doubled laser radiation is respectively cut off on the descending slope. What is essential here is that part of the stored energy remains in the laser, thus achieving an improvement of the pulse-by-pulse stability at high pulse repetition frequencies and an increase in frequency doubling. However, substantial pulse shortening is not possible and, therefore, this method is suitable only for Nd:YAG or Nd:YVO4 or comparable systems having a short-lived upper laser level and large effective amplification cross-sections, which lead to shorter pulses in a Q-switched laser. In order for this method to achieve high power averages for high beam quality, Nd-doped lasers are unsuitable because the high quantum defect causes considerable heating of the laser crystal and thus opto-thermal interferences to occur, limiting the power output in the case of high beam quality. 
   These limitations do not exist in the case of Yb:YAG lasers in the disk laser arrangement as described, for example, in EP 0 632 551. However, Yb:YAG is characterized by a very long life of the upper laser level of approx. 1 ms and by a small effective amplification cross-section. In Q-switched operation, the pulses become unstable at pulse frequencies of more than 25 kHz and the pulse lengths may be up to several μs. 
   U.S. Pat. No. 4,841,528 discloses a laser assembly wherein the laser is operated in the cavity dumping niode, with the coupled-out laser radiation being frequency-doubled by means of a non-linear crystal which is arranged outside the resonator. The assembly is provided such that the part of the coupled-out laser radiation which is not frequency-doubled is coupled into the resonator again. An arrangement wherein the frequency-doubled crystal is arranged within the resonator is described as disadvantageous in this reference. 
   SUMMARY OF THE INVENTION 
   In view thereof, it is an object of the invention to provide a laser for generating pulsed laser radiation of a first wavelength, in particular using a Yb:YAG laser resonator, wherein the pulse length of the frequency-converted laser radiation can be varied and the laser can be simultaneously operated at high pulse repetition frequencies, in particular higher than in the case of Q-switched lasers. 
   According to the invention, the object is achieved by a laser for generating pulsed laser radiation of a first wavelength, comprising a resonator, a pumped active medium arranged inside the resonator, said medium emitting primary radiation of a second wavelength which differs from the first wavelength, an element arranged in the resonator and serving to generate laser radiation having the first wavelength by frequency conversion of the primary radiation, the resonator being switchable into a first state, in which it is open to the primary radiation, and into a second state in which it is closed to the primary radiation, and being open to laser radiation of the first wavelength in both states, and the laser comprising a control unit which, in order to generate a pulse of the laser radiation, switches the resonator from the first to the second state in a first step, so that at least one resonator mode for the primary radiation begins to oscillate and the pulse generation by frequency conversion using the element begins, and which control unit, in a second step following the first step, switches the resonator from the second to the first state, whereby primary radiation is coupled out from the resonator, the intensity of the primary radiation in the resonator drops and the pulse generation thus ends, it being possible to set the duration between both steps and/or the coupling-out behavior of the resonator so as to adjust the pulse duration via the control unit. 
   With this laser, the approach of the cavity dumping operation is utilized to reduce the intensity of the primary radiation very quickly (abruptly, as it were) such that frequency conversion breaks down or the intensity of the frequency-converted laser radiation drops below a desired minimum value, respectively, thereby defining the pulse duration. Since the time between both steps and/or the coupling-out behavior (e.g. the degree of coupling-out in the first state of the resonator, the switching time from the second to the first state in the second step) can be set via the control unit, the pulse duration of the pulses can be easily varied within a wide range. Using a Yb-doped medium as the active medium, pulse lengths of, for example, greater than 100 ns are possible at pulse repetition frequencies of from 20 to 200 kHz. 
   The coupling-out behavior can be modified by setting the switching time from the second to the first state (in the second step) by means of the control unit. Since the reduction in intensity of the primary radiation in the resonator (i.e. the coupling-out of the primary radiation) slows down as the switching time increases, the pulse width is increased. Thus, the pulse width can also be set and adjusted via the switching time. 
   In particular, the control unit can perform the second step already during the ascending slope of the pulse. It has been shown that this enables extremely exact setting of the pulse width with very good repeatability. 
   The control unit may perform the second step only upon reaching a predetermined value of a predetermined physical parameter (e. g. intensity, (instantaneous) power, energy) of the primary radiation or of the laser radiation. This leads to the further advantage that the pulse energy is limited and that damage to optical components can thereby be reliably prevented. For this purpose, the laser preferably comprises a measuring module, which measures the parameter of the primary radiation or of the laser radiation directly or indirectly and transmits a corresponding signal value to the control unit. The signal value then serves to determine the present value of the parameter. 
   The predetermined value of the parameter (e. g. intensity, power, energy) can be set at the control unit. 
   In the laser, the control unit can repeatedly perform the first and second steps; it is possible to set the time between a second step and the subsequent first step for adjustment of the pulse repetition frequency at the control unit. This makes it possible to set the pulse repetition frequency and the pulse width independently of each other. In particular, individual pulses can be generated as well. By controlling the pulse width via the intensity, power or energy of the primary radiation or laser radiation, excessive pulse energies of the first pulse can be avoided, for example, during burst mode operation (pulse trains). By stabilizing the pulse energy, pulse-by-pulse stabilities of less than 5% (minimum value to maximum value) are achieved. 
   The resonator of the laser may comprise a coupling-out module, which couples out more primary radiation from the resonator in the first state than in the second state. In particular, the coupling-out module may be provided such that it couples out rio primary radiation in the second state. 
   The coupling-out module may contain at least one acousto-optical or electro-optical modulator. 
   The element for frequency conversion comprises, in particular, a suitable non-linear optical material and is preferably provided as an element for frequency multiplication. Thus, for example, it may cause frequency doubling. For this purpose, a lithium triborate crystal may be used, for example. 
   Further, a method is provided for generating pulsed laser radiation of a first wavelength, wherein, in order to generate a pulse of the laser radiation, primary radiation of a second wavelength differing from the first wavelength is generated in a resonator in a first step such that at least one resonator mode begins to oscillate and laser radiation having the first wavelength is generated from the primary radiation in the resonator by frequency conversion and is coupled out from the resonator, and in a second step following the first step, primary radiation is coupled out from the resonator such that the intensity of the primary radiation for frequency multiplication decreases and the pulse generation thus ends, wherein the time between both steps and/or the coupling-out behavior (e. g. degree of coupling out in the second step, switching time from the first to the second step) can be set so as to adjust the pulse duration. 
   With this method, particularly when using a Yb-doped laser medium, for example, the pulse widths can be set over a very high range. Further, pulse repetition frequencies of greater than 20 kHz are possible, and the pulse width can be set almost independently of the pulse repetition frequency. 
   In particular, the second step can be effected even during the ascending slope of the pulse. This allows the pulse duration to be set in an extremely exact and very reproducible manner. 
   Further, the second step can be effected upon reaching a predetermined value of a predetermined physical parameter (e. g. intensity, power, energy) of the primary radiation or of the laser radiation. This procedure enables very exact setting of the pulse duration. In particular, excessively high pulse energies that might cause damage to optical elements can be avoided. 
   In the method, the first and second step can be carried out repeatedly, it being possible to set the time between a second step and the subsequent first step so as to adjust the pulse repetition frequency. This makes it possible to select the pulse repetition frequency independently of the set pulse duration. 
   The method allows the pulse width to be set and adjusted via the switching time from the first to the second step because the switching time influences the coupling-out of the primary radiation. As the switching time increases, the coupling-out is slowed down, which leads to greater pulse widths. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained hereinafter, by way of example and with reference to the Figures wherein: 
       FIG. 1  is a schematic representation of the laser according to the invention in a first embodiment; 
       FIG. 2  is a detailed representation of the laser resonator  1  of  FIG. 1 ; 
       FIG. 3  depicts the pulse shape of the infrared beam generated in the resonator, of the coupled-out infrared beam and of the frequency-doubled green beam; 
       FIG. 4  depicts the time course of three generated green, frequency-multiplied laser pulses together with the trigger signal for the Pockel&#39;s cell  14  of  FIG. 2 , and 
       FIG. 5  depicts the time course of three generated green, frequency-multiplied laser pulses together with the on/off state of the Pockel&#39;s cell  14  of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   In the embodiment schematically shown in  FIG. 1 , the laser according to the invention for generating pulsed laser radiation of a first wavelength comprises a laser resonator  1 , which is formed by two resonator mirrors  2 ,  3  and in which there are arranged a Yb:YAG medium  4  as the active medium, an optical coupling-out module  5 , a coupling-out mirror  6  as well as a non-linear optical element  7  for frequency multiplication. The laser further comprises a pumping light source  8 , a control unit  9  for controlling the coupling-out module  5 , as well as a photodiode  10 . 
   The active medium  4  is pumped with light from the pumping light source  8  (continuously, in this case) (arrow P 1 ) and emits primary radiation of a second wavelength (in the infrared range, in this case), which differs from the first wavelength (in the visible green range, in this case). The coupling-out module  5  can be switched to first and second states by means of the control unit  9 , with the generated primary radiation being coupled out from the resonator  1  in the first state (arrow P 2 ). In this case, the resonator  1  is open to the primary radiation. In the second state of the coupling-out module  5 , no primary radiation is coupled out from the resonator  1 , so that the resonator  1  is closed to the primary radiation. The resonator  1  is designed here as a laser resonator for the primary radiation. 
   The coupling-out mirror  6  couples out a small portion of the primary radiation (arrow P 3 ) and directs it to the photodiode  10  by which the intensity of the primary radiation in the laser resonator  1  can be measured. 
   In this case, the non-linear optical element  7  serves to double the frequency of the primary radiation so that the frequency-doubled green laser radiation (laser radiation of the first wavelength) is generated as the square of the intensity of the infrared primary radiation. The resonator mirror  3  is provided as a dichroic mirror, which reflects the primary radiation and transmits the frequency-doubled green laser radiation, as indicated by the arrow P 4  shown in broken lines. 
     FIG. 2  shows a specific embodiment of the laser resonator  1 , not showing the coupling-out mirror  6 , the pumping light source  8 , the control unit  9  and the photodiode  10 . 
   In addition to resonator mirrors  2  and  3 , the laser resonator  1  comprises further mirrors  11 ,  12  and  13 , and the active medium  4  is provided in the so-called disk laser assembly. 
   The coupling-out module  5  comprises a BBO Pockel&#39;s cell (BBO=beta barium borate crystal)  14  as well as a thin-film polarizer  15 . The non-linear element  7  is an LBO crystal  16  (LBO=lithium triborate). 
   Operation of the laser of  FIGS. 1 and 2  will be explained in combination with  FIG. 3 , in which the temporal pulse shape of the infrared beam inside the resonator (curve K 1  shown in broken lines), of the coupled-out infrared beam (dotted curve K 2 ) as well as of the frequency-doubled green beam (curve K 3 ) are shown, respectively standardized to one. Further, the trigger signal is indicated with a rectangular profile (curve K 4 ) for the Pockel&#39;s cell  14 . If the trigger signal has a value of 1, the Pockel&#39;s cell  14  is deactivated, so that no generated primary radiation is coupled out from the laser resonator  1 . If the trigger signal has a value of 0, the Pockel&#39;s cell  14  is activated, so that primary radiation is coupled out (arrow P 2  in  FIGS. 1 and 2 ). 
   Now, if the trigger signal is switched from 0 to 1 at the time t 1  ( FIG. 3 ), the laser resonator is switched from its open state (first state, in which primary radiation is coupled out) to the second state, in which no primary radiation is coupled out. Since the active medium  4  is being pumped continuously, at least one resonator mode begins to oscillate after a certain amount of time, so that the intensity (curve K 1 ) of the infrared signal inside the resonator (primary radiation) increases. The non-linear optical element  7  (of the LBO crystal  16 ) uses the primary radiation to generate frequency-doubled green laser radiation (curve K 3 ), whose intensity increases as the intensity of the infrared radiation inside the resonator increases. This increasing intensity forms the ascending slope of the desired green laser pulse. 
   At a time t 2 , the control unit  9  activates the Pockel&#39;s cell  14  (the trigger signal (curve K 4 ) being switched from 1 to 0), so that the generated primary radiation is then coupled out. Due to transit times of the electrical signals, the Pockel&#39;s cell  14  responds with a delay of 50 ns in the example described here. This delay is indicated in the graphic representation of  FIG. 3 . The primary radiation generated in the resonator  1  (curve K 1 ) decreases, and as a consequence the frequency-doubled laser radiation (curve K 3 ) generated by means of the non-linear element  7  also decreases. The intensity of the green laser radiation drops to zero so that the descending slope is formed and the pulse is thus terminated. In contrast thereto, the coupled-out infrared radiation (K 2 ) increases. 
   The pulse duration of the frequency-doubled green pulse (curve K 3 ) can thus be set by the activation period of the Pockel&#39;s cell. The activation period corresponds to the period Δt 1 , during which the trigger signal is 1. The control unit  9  can modify the period Δt 1  and can thus set the pulse duration or pulse width, respectively, of the generated green laser pulse (curve K 3 ) over said period. Since the period Δt 1  is approximately 10 times greater here than the pulse duration, said period is not shown to scale in  FIG. 3 . 
   The described mode of operation of the laser is similar to the so-called cavity dumping mode of operation. In the cavity dumping mode of operation, the energy in the photon field is stored in the laser resonator, and in order to generate a pulse, the desired pulse is suitably coupled out by means of an electro-optic or acousto-optic coupling-out element. In the embodiment described here, the coupling-out of the primary beam is used to terminate generation of the frequency-doubled laser beam or to allow the intensity of the frequency-doubled laser beam to drop below a desired minimum value, whereby the pulse duration of the generated frequency-doubled laser pulse can be advantageously set within wide ranges. 
   It has been shown that the pulse width of ca. 100 to 500 ns (for an activation period Δt 1  of the Pockel&#39;s cell of 2.00 to 3.50 μs) at a pulse repetition frequency in excess of 20 kHz, in particular at pulse repetition frequencies of 50 to 200 kHz, was achieved with a diffraction index M 2  of less than 5 (in particular 1). A pulse duration of 300 ns and a pulse repetition frequency of 50 and 100 kHz as well as a pumping power of 450 Watts allow to achieve an average power of the green laser pulse of approximately 100 Watts. This corresponds to an efficiency in excess of 20%. As the pumping power decreased down to 150 Watts, the average power of the green pulse decreased nearly linearly to approximately 10 Watts. The pulse width could be from less than 100 ns up to even more than 1,000 ns. 
   The setting of the pulse duration by means of the control unit  9  is carried out in the embodiment example described here by continuously detecting the intensity of the primary radiation via the dichroic coupling-out mirror  6  and the photodiode  10 . If the measured intensity exceeds a predetermined threshold value, the Pockel&#39;s cell  14  is activated. 
   However, it is also possible to arrange the coupling-out mirror  6  inside or outside the resonator  1  such that the intensity of the generated green laser radiation can be measured. In this case, the control can be effected as a function of the intensity of the green laser radiation in the same manner as with respect to the intensity of the primary radiation. 
   Further, it is possible to set the period Δt 1  to predetermined constant values and to thereby determine the pulse duration of the green laser radiation. 
   The pulse repetition frequency can be set by means of the control unit  9  by appropriately selecting the activation period of the Pockel&#39;s cell  14 .  FIGS. 4 and 5  respectively show only the generated green laser radiation for three subsequent pulses, with  FIG. 4  showing the value of the trigger signal and  FIG. 5  respectively showing the activated or the deactivated state of the Pockel&#39;s cell  14 . As is evident from  FIGS. 4 and 5 , the pulse repetition frequency can be defined by the activation period Δt 2  of the Pockel&#39;s cell  14  (i. e., the period in which the primary radiation is being coupled out). If the activation period Δt 2  is increased, the pulse repetition frequency decreases. If the activation period is shortened, the pulse repetition frequency increases. Therefore, it is possible with the laser according to the invention not only to set the pulse duration via the control unit  9 , but the pulse repetition frequency can also be set or adjusted, respectively; the pulse duration and the pulse repetition frequency can be set nearly independently of each other. 
   The Pockel&#39;s cell  14  and the polarizer  15  are designed such, in this case, that when the Pockel&#39;s cell  14  is activated, the polarizer  15  has a reflectivity of approximately 50%. When the Pockel&#39;s cell is deactivated, the polarizer has a reflectivity of (nearly) 100% (respectively related to the infrared primary radiation coming from the mirror  2  and impinging on the polarizer  15 ). However, the voltage to be applied to the Pockel&#39;s cell  14  in order to activate the Pockel&#39;s cell  14  allows to vary the polarization condition of the primary radiation and, thus, in connection with the polarizer, the degree of reflection at the polarizer  15  or its reflectivity, respectively, when the Pockel&#39;s cell  14  is activated. The degree of reflection when the Pockel&#39;s cell  14  is activated determines how quickly the resonator  1  is depleted. Increasing the degree of reflection when the Pockel&#39;s cell  14  is activated causes less primary radiation to be coupled out per time unit, so that the pulse width of the green pulse increases for the same activation period of the Pockel&#39;s cell  14 . When the reflectivity decreases by the correspondingly applied voltage when the Pockel&#39;s cell  14  is activated, the pulse duration decreases. 
   Thus, while increasing the voltage applied to the Pockel&#39;s cell  14  from 2.0 kV to 3.5 kV, the pulse duration could be decreased from approximately 400 ns to approximately 200 ns. 
   In the described embodiments, the activation time of the Pockel&#39;s cell  14  is always selected such that it is on the still ascending slope of the frequency-doubled green laser radiation. 
   It is also possible to trigger the Pockel&#39;s cell  14  by the fluorescent light of the active medium  4 , because said light increases as the inversion increases.