DE102004008854B4 - Laser system with a laser-active disk and method for operating a laser system - Google Patents

Abstract

Method for operating a laser system with a laser-active disk (2), an excitation source for continuously pumping the laser-active disk (2) into an excited state and a resonator in which the disk (2) is arranged, in which an optical radiation field (4) the disc (2) is repeatedly fed back through so that from the resonator, a laser beam (5) can be coupled out, and in which a Q-switch (8) is arranged, with which the laser beam (5) is generated in the form of pulses, characterized in that the Q-switch (8) is controlled such that an increasing gain due to the pumping of the laser-active disk (2) causes an incipient laser action with a single longitudinal mode and then completely opens the Q-switch (8) when a threshold value of the gain is exceeded so that the laser pulse is built in a single longitudinal mode.

Description

The invention relates to a method for operating a laser system with a laser-active disk, an excitation source for continuously pumping the laser-active disk in an excited state and a resonator in which the disc is arranged, in which an optical radiation field, the disc repeatedly passing through so fed back in that a laser beam can be coupled out of the resonator and in which a Q-switch is arranged with which the load beam is generated in the form of pulses.

The invention further relates to a laser system with a laser-active disk, an excitation source for continuously pumping the laser-active disk in an excited state and a resonator in which the disc is arranged, in which an optical radiation field the disk through several times is fed back so fed back from that Resonator a laser beam is coupled out, and in which a Q-switch is arranged, with which the laser beam is generated in the form of pulses.

A disk as a laser-active medium is for example from the EP 0 632 551 A1 known. The described there advantageous embodiment in the form of the thinnest possible disc allows to build such a disk laser with simple optical means so that it in a continuous wave operation, also with cw operation (continuous wave) referred to, in a fundamental mode (TEM ₀₀ ) and with only a single longitudinal mode starts.

In the article "Q-switched Yb: YAG thin-disk laser" by I. Johannsen, S. Erhard and A. Giesen (OSA TOPS, Vol. 50, advanced solid-state lasers, pp. 191-196, 2001), one can such disk laser instead of being operated in continuous wave mode in a Q-switched pulse mode. In this case, a pulse energy can arise, which can lead to the destruction of the laser-active disk. As a way out, a regenerative amplifier construction is suggested in the article. However, this requires a much more complicated structure of the laser system. Thus, the laser system effectively includes two disk lasers. The first disk laser is operated continuously (CW operation) and determines as a seed laser the frequency of the pulsed second disk laser, which works as a regenerative amplifier.

The pulsed operation of the disk laser is possible only with a broad spectrum. A so-called single-frequency operation with a single laser mode is therefore not possible.

From the US 4,197,513 is an actively Q-switched single-mode laser known. In the laser system described there, which does not require a seed laser, the Q-switching takes place in two steps in order to produce a longitudinal single-mode operation at high output power. When there is a known procedure known as the two-stage Q-switching, the Q-switch is opened in two stages. First, only the level of the lasing threshold is switched, and then, in a second step, the Q-switch is fully opened to maximize the amplification pulse. The second step takes place after the first occurring peak of the relaxation oscillation when laser action begins. However, this method does not work satisfactorily with high power solid state lasers to achieve single mode longitudinal operation.

That is why in the US 4,197,513 proposed to hold in the first step, the relatively high level of loss of the Q-switch until a mode has stabilized. Only then, in a second step, the Q-switch is fully opened. For this purpose, the optical radiation power in the resonator is monitored and a signal is generated for each peak of the relaxation oscillation. A trigger circuit switches to the lossless state as soon as the second or another subsequent peak of the relaxation oscillation is registered because no stable single-mode operation of the laser system is obtained at the first peak.

The US 5,365,532 discloses a Q-switched laser system whose method of Q-switching is termed "laser cavity dump pulse". The laser system has a circuit so that the coupled-out pulsed laser beam has a constant amplitude. For this purpose, when a threshold value of the radiation field is reached, a Q-switch is switched off and consequently a laser pulse is generated. The presented laser system is a gas laser, in particular a CO ₂ laser.

US 5,157,677 discloses a solid state laser, in the resonator next to a laser rod, a Q-switch is arranged. The light emerging from the laser rod due to the pumping with the flashlight only leads to a laser pulse when the Q-switch is switched on. Before switching on, a pre-laser phase is created (pre-lase stage) in which the Q-switch deflects the light beam onto a detector. When switching the Q-switch then creates a laser pulse, which does not have a defined frequency or a defined mode, since the laser pulse can occur in different frequencies. A problem does not arise when the laser is used in an interferometric operation, because for an interferometric evaluation only a single-frequency laser is needed, the exact frequency is not important.

From the prior art, there is thus the problem that the pulsed operation applied to increase the intensity of the laser output power, unlike the continuous wave operation, can not be operated safely with a predetermined single mode.

The present invention is therefore based on the object to improve a disk laser of the type mentioned in such a way that with a consistently uncomplicated optical design, a Q-switched one-mode operation can be realized.

To solve this object, the method of the type mentioned above is characterized in that the Q-switch is controlled so that an increasing laser effect due to the pumping of the laser-active disc an incipient laser action is effected with a single longitudinal mode and exceeding a threshold value of the gain Then fully opened, so that the laser pulse is built in a single longitudinal mode.

To solve the above object, a laser system of the type mentioned is further characterized in that a control device controls the Q-switch so that an increasing laser effect due to the pumping of the laser active disk causes a laser action with a single longitudinal mode and when exceeding a threshold of Reinforcement of the Q-switch is fully opened so that the laser pulse is built up in a single longitudinal mode.

In the off state, the Q-switch is controlled in such a way that it can adjustably cause high resonator losses. Due to continuous pumping by the excitation source, the occupation of the excited state of the laser-active disk increases. If the associated gain of the disk reaches a threshold value for the first time, which just compensates for the high resonator losses, the laser action begins. Then the Q-switch is completely opened, which allows the extraction of a giant pulse from the resonator. Both in the onset of laser action prior to opening and in the build-up of the giant impulse immediately thereafter, the special properties of a disk as a thin gain medium on a resonator mirror favor the oscillation of the laser in a single longitudinal mode. In this case, the problems described above do not occur and consequently waiting for a mode stabilization with decreasing relaxation time is not required.

The proposed laser system has a simple and uncomplicated structure of a continuously operated disk laser. Due to the proposed Q-switching, the laser system according to the invention can be operated in a pulsed operation with controllable large pulse energies and the destruction of the laser-active disc is reliably avoided.

Advantageously, in an inventive laser system, an optical detector is provided which detects a portion of the optical radiation field and generates at least one output signal. The plurality of output signals may, for example, contain information as to whether and when a particular value of the radiation field is reached or in relation to the energy of a laser pulse. These output signals are available for controlling the laser system.

In a preferred embodiment of the present invention, the controller is arranged to process a first output signal of the optical detector to switch the Q-switch. This controller responds when the radiation field detected by the optical detector first reaches a certain small value due to the small net gain then present in the low-Q resonator. When this threshold is reached, the controller releases the Q-switches, i. H. this becomes completely open and permeable. The first output signal relates to a specific time. This embodiment of the control device ensures the inventive Q-switching of the laser system.

The laser system according to the invention preferably has a linear structure of the resonator. Other structures, for example, with a folded or annular course of the radiation field are in principle also possible, but require a more complicated structure with a plurality of mirrors with additional resonator losses. The linear structure, however, ensures a short resonator structure with few optical means.

In a further advantageous embodiment of the present invention, a further control device is provided, which processes a second output signal of the optical detector and drives a length of the linear resonator changing adjusting device. The second output signal of the optical detector is proportional to the time integral of the radiation field over the duration of a laser pulse or to the pulse energy. Maximum pulse energy is achieved when the resonator length is adjusted so that the wavelength of the laser is exactly equal to the maximum of the transmission of mode-selecting optical elements in the resonator and this coincides with the maximum of the spectral gain profile of the laser active disk. In this way the resonator condition is optimally fulfilled. This allows the laser amplifier system to be operated stably for a long time in a single longitudinal mode.

Hereinafter, the present invention will be explained in detail with reference to the detailed description of two embodiments in conjunction with the accompanying drawings. These show in

1 A schematic sketch of a first embodiment of the laser system according to the invention;

2 A schematic sketch of a second embodiment of the laser system according to the invention;

3 - A graph of gain and output over time.

1 shows a schematic sketch of a first embodiment of the laser system according to the invention. Shown is the simplest structure, a linear laser resonator with a highly reflective and partially transparent mirror.

The resonator according to the invention comprises a laser-active disk as the laser-active medium 2 For example, a platy Yb: YAG crystal. The disc 2 is with the back, which also serves as a mirror with high reflection, on a cooling device 1 arranged. The cooling device 1 is a cold finger or a Peltier element, wherein the temperature of the disc 2 is maintained at a certain set point whose deviations are less than about 0.1 ° C. The laser-active disc 2 arranged opposite is a Auskoppelspiegel 3 so that an optical radiation field 4 between the two (end) mirrors 2 . 3 of the resonator is imaged in itself. The inside of the Auskoppelspiegel 3 that is, the side facing the resonator is slightly curved concavely to achieve laser operation in a fundamental mode (TEM ₀₀ ), where the diameter of the mode volume of the laser active disc is defined by the size of a pump light spot. With the numeral 5 is the decoupled radiation field, that is, the laser light generated by the laser system referred to.

The laser-active disc 2 is excited by a suitable excitation source, which is not shown for clarity. The excitation source or pumping means may be, for example, a diode laser which is inserted into the disc 2 incident pumping radiation field generated, preferably on the laser-active disc 2 is focused and its unabsorbed portion is repeatedly refocussed to the same location of the disc.

The possibility that several longitudinal modes in the gain profile of the laser-active disk 2 can lie and simultaneously oscillate, or that although only one mode oscillates, but by a shift in the relative position of gain profile and resonator modes, the laser emission can "jump" to another mode, for many, especially spectroscopic, applications very disturbing. In these cases it is expedient to limit the oscillation by wavelength-selecting elements in the resonator to a single mode. In order for only one mode to oscillate in the resonator, ie, a single-mode longitudinal mode to build up, are in the beam path of the optical radiation field 4 at least one etalon 6 and a Brewster angle double refractive filter 7 (Lyot filter) provided, which also acts as a polarizer.

Although in this embodiment, the resonator is formed linearly, also another structure is conceivable, for example, folded or in a closed geometric shape, which is referred to as a ring resonator. As far as continuous single-mode operation is concerned, ring lasers having a shaft rotating in only one direction are known to be advantageous because the "spatial hole burning" in the gain medium can be avoided. However, this requires the use of an optical diode in the resonator and a relatively complex construction. Disk lasers with a laser-active disk, the back of which acts as a mirror in the resonator behave similarly advantageous, because all stationary resonator modes always have a wave node when reflected on a mirror, so do not differ on the back of the disk with respect to "spatial hole burning". If the advantageous use of the thinnest possible slice with the smallest possible laser-active volume and a matched short resonator does not directly succeed in achieving operation in a single longitudinal mode, this can be achieved by using less mode-selective optical elements in the resonator.

For higher peak powers of the emitted laser light 5 in pulse mode, a quality modulation (Q-switch) is performed. For this purpose, a Q-switch is in the resonator 8th , For example, an acousto-optic crystal introduced, with which the losses in the resonator and thus its quality can be controlled in time.

In the case of the invention, the Q-switch is operated in a locked state so that it is responsible for a small portion of the optical radiation field 4 is permeable. Depends on the Pumping power of the excitation source and to be reached in the repetition time gain, the proportion is adjusted so that there is an increase of the optical radiation field 4 comes when the gain of the laser active disk 2 just big enough to pass the key through the Q-switch 8th caused to compensate for high resonator losses. In this process, which takes many resonator cycle times, the mode with the least losses is preferred.

The timing of the Q-switch 8th done by a control electronics 9 , As regards the nature of the coupling of the low proportion 10 from the optical radiation field 4 No details were given in connection with the previous explanations. This advantageous embodiment provides that the proportion 10 from a frontal reflex of the Lyot filter 7 which comes under Brewster angle in the beam path of the optical radiation field 4 stands. An alternative or complementary possibility for an optical signal 10 for a detector 11 is any reflection from other optical components in the resonator or a small portion of the decoupled laser beam 5 ,

The detector 11 converts the receiving optical signal 10 into an electronic trigger pulse 12 in order to the control electronics 9 is forwarded. This process the electronic trigger pulse 12 and turns on the Q-switch 8th by changing a control signal 13 free. In other words, upon reaching a certain threshold of the optical signal 10 That is, at a certain threshold of amplification in the resonator, the Q-switch is disabled as described above 8th fully open. Then the previously already slowly grown optical radiation field 4 fed back with low losses and so efficiently with each pass through the laser-active disk 2 reinforced to a giant impulse.

Although the intense radiation field circulating in the linear resonator forms a linearly polarized wave which does not undergo amplification in vibration nodes and therefore leads to spatial hole burning in the amplification medium, no other mode is fostered over the entire pulse duration to such an extent with the Beginning of the pulse existing single mode can compete if a thin disc is used. For example, a slice thickness of 250 μm has proven to be sufficiently thin. At a laser wavelength of about 1 μm, this corresponds to about 500 knots. This already requires careful selection of the optical elements which select the wavelength or modes in the cw mode, precise adjustment and a thermally stable encapsulated resonator structure, in particular with a resonator length of approximately 500 mm. With the presented Q-switching method, the spectral quality achieved in continuous wave mode can be obtained in a single longitudinal mode even in pulsed mode.

In the 2 a second embodiment of the laser amplifier system according to the invention is shown in a schematic sketch, compared to the first example of 1 is extended. The same and equivalent parts are given the same reference numbers and their explanations are omitted to avoid unnecessary repetition at this point.

The advantage of the extension is a reduction of the high demands on the precision of the adjustment and the stability of the structure with regard to the maintenance of the optimal resonance condition. The boundary condition of a standing wave in the resonator restricts the possible frequency spectrum of the radiation field 4 to discrete values. Longitudinal resonator modes are only possible if the optical path length in the resonator is between the two mirrors 2 . 3 is an integer multiple of half the wavelength (λ / 2). Optimal operation requires that such wavelength coincide with the maximum of the gain spectrum of the lasing slice, which is modulated with the wavelength-dependent transmission of the mode-selecting optical elements in the resonator. In this case, the exact match of the frequency of a resonator mode with a transmission maximum of the etalon with the smallest free spectral range is the known "Achilles heel" of the resonator structure. Namely, a typical optical length of the resonator of about 500 mm means that adjacent modes differ by only one ppm in their frequency. If a mode jump of the laser is to be reliably prevented during a longer operating time, the relative frequency change of etalon and resonator mode must be less than 10 ^-7 .

To achieve this, the photo detector captures 11 additionally the intensity of the optical signal 10 and generates an intensity-dependent electronic signal 14 , which is sent to a second control electronics. When the resonator length is optimally set to the emission frequency, the intensity of the optical signal reaches 10 the greatest value. Accordingly, the second drive electronics processed 15 the received electronic signal 14 and generates a control signal 16 to an adjusting device 17 head for. In this embodiment, the adjusting device 17 at the Auskoppelspiegel 3 arranged to shift it, so that the maximum signal and thus a stable single-mode operation is maintained. A resonator length changing adjusting device 17 is preferably realized with a piezoelectric element. But any other finely tunable optical cavity length adjustment device may alternatively be used. Without changing the operating principle, the required match, for example, by adjusting the temperature of the laser-active disk 2 and / or the temperature or tilt of the etalon.

3 is a graphical representation of the gain and output power of the laser system from the 1 and 2 over time to explain the principle of Q-switching. In this count shows the line 18 the time course of the amplification within the laser-active disk. The time course of the output power of the laser system is with the line 19 played. For both graphic lines 18 . 19 the abscissa is the time axis.

In order for a resonator mode to actually resonate in the laser system, a corresponding small optical radiation field must be used 4 when passing through the disc 2 experience a gain that compensates at least the resonator losses. As long as the Q-switch 8th (from the 1 . 2 ), constant optical pumping by the excitation source results in an increase in gain 18 in the laser-active disk 2 , In contrast to a conventional Q-switching, in the laser system according to the invention the losses of the Q-switch are set only so high that from a time t ₀ the gain exceeds the losses, first for the mode favored by the mode-selecting optical elements in the resonator. The low net gain leads to an oscillation of this mode. At time t ₁ , the rising signal exceeds the gain from the photo detector 11 ( 1 . 2 ), for the first time a trigger threshold s ₁ . The Q-switch will then be activated for a short time. The losses in the resonator sink very rapidly and then the fully effective gain leads to the build-up of a large pulse from the existing mode. This will cause inversion of the laser active disk 2 degraded and the gain drops below the value necessary for the self-excitation. Then the controller switches 9 ( 1 . 2 ) the Q-switch 8th again in the locked state.

Claims (6)

Method for operating a laser system with a laser-active disk ( 2 ), an excitation source for continuously pumping the laser-active disc ( 2 ) in an excited state and a resonator in which the disc ( 2 ) is arranged, in which an optical radiation field ( 4 ) the disc ( 2 ) is repeatedly fed back so that from the resonator a laser beam ( 5 ) and in which a Q-switch ( 8th ) is arranged, with which the laser beam ( 5 ) is generated in the form of pulses, characterized in that the Q-switch ( 8th ) is controlled so that by an increasing gain due to the pumping of the laser-active disc ( 2 ) an incident laser action is effected with a single longitudinal mode and when exceeding a threshold value of the gain of the Q-switches ( 8th ) is then fully opened so that the laser pulse is built up in a single longitudinal mode. Laser system with a laser-active disk ( 2 ), an excitation source for continuously pumping the laser-active disc ( 2 ) in an excited state and a resonator in which the disc ( 2 ) is arranged, in which an optical radiation field ( 4 ) the disc ( 2 ) is repeatedly fed back so that from the resonator a laser beam ( 5 ) and in which a Q-switch ( 8th ) is arranged, with which the laser beam ( 5 ) is generated in the form of pulses, characterized in that a control device ( 9 ) the Q-switch ( 8th ) is controlled so that by an increasing gain due to the pumping of the laser-active disc ( 2 ) causes an incipient laser action with a single longitudinal mode, and when a threshold value of the gain of the Q-switches ( 8th ) is opened completely so that the laser pulse is built up in a single longitudinal mode. Laser system according to claim 2, characterized by an optical detector ( 11 ), which has a share ( 10 ) of the optical radiation field ( 4 ) or the laser beam ( 5 ) and at least one output signal ( 12 ) generated. Laser system according to claim 3, characterized in that the control device ( 9 ) a first output signal ( 12 ) of the optical detector ( 11 ) and in response to the first output signal ( 12 ) the Q-switch ( 8th ) switches. Laser system according to one of claims 2 to 4, characterized by a linear structure of the resonator. Laser system according to one of Claims 3 to 5, characterized by a further control device ( 15 ), which outputs a second output signal ( 14 ) of the optical detector ( 11 ) and a the resonator length of the laser system changing adjusting device ( 17 ) in response to the second output signal ( 14 ).

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