DE102004008854A1 - Laser system, has control device which disconnects Q-switch when gain threshold of laser-active disk is reached for the first time - Google Patents

Abstract

The laser system has an excitation source for exciting a laser-active disk (2) arranged in a resonator which outputs a laser beam (5) with a single longitudinal mode. A Q-switch (8) is provided in the resonator. A control device (9) is provided for switching the Q-switch. The control device disconnects the Q-switch when a gain threshold of the laser active disk is reached for the first time. An independent claim is included for a method of Q-switching a laser system.

Description

The The invention relates to a laser system with a laser-active disk, an excitation source for generating an excited state of laser-active disk, a resonator in which the disk is arranged is and in which an optical radiation field the disk several times enforced so fed back is that from the resonator, a laser beam can be decoupled, whose Frequency spectrum contains only a single mode. The invention further relates a method of Q-switching such a laser system.

A disk as a laser-active medium is for example from the EP 0 632 551 known. The described there advantageous embodiment in the form of a thin disc as possible 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 basic mode (TEM ₀₀ ) and with only a single longitudinal mode starts.

Should now such a disk laser instead of continuous wave operation in one Q-switched Pulsed operation, it is not possible to use these advantageous Maintain properties of the disk laser. Compared For continuous wave operation, pulsed operation can only be achieved with a broad spectrum carried out be a so-called single-frequency operation is not possible. Furthermore, a pulse energy can be obtained, which leads to the destruction of Lead disc can.

After this The above problems 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) is there as a way out a regenerative amplifier construction proposed.

These However, proposed regenerative reinforcement requires a much more complicated structure of the laser system. So includes the laser system effectively has two disk lasers. The first disk laser cw is operated and determines the frequency of the pulsed as a seed laser second disk laser, which works as a regenerative amplifier.

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.

task Accordingly, the present invention is a disk laser to improve that while maintaining uncomplicated optical Set up a Q-switched Single mode operation can be realized in a simple manner. Further The object is to provide a method of Q-switching the improved Specify disc laser.

Of the Proposal for a solution this object comprises a laser system with a laser-active disk, with an excitation source for generating an excited state the laser-active disk and with a resonator in which the disk is arranged and in which an optical radiation field the disc repeatedly enforced so fed back is that from the resonator, a laser beam can be decoupled, whose Frequency spectrum contains only one longitudinal mode, and a method of Q-switching of the laser system. In addition, according to the invention Q-switch in the resonator and a controller for controlled switching of the Q-switch intended.

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 Güteschalter kom opened, which allows the coupling 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 straightforward Structure of a continuously operated disk laser on. by virtue of the proposed quality circuit can the laser system according to the invention operated in a pulsed operation with controllable large pulse energies become and the destruction the laser-active disc is safely avoided.

In Advantageously, in a laser system according to the invention is an optical Detector provided a portion of the optical radiation field detected and generates at least one output signal. The multiple output signals for example Information includes whether and when a certain 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 Present invention, the control device is provided, a process first output signal of the optical detector to the Q-switch to switch. This controller responds when that of the For the first time, the optical field detected a specific radiation field small value due to the then present in the resonator low quality small net reinforcement reached. If this threshold is reached, the controller switches the Q-switches free, i. this one will be complete open and permeable. The first output signal relates to a specific time. This embodiment the controller ensures the Q-switching according to the invention of the laser system.

Prefers has the laser system according to the invention 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 one more complicated structure with a variety 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 has a second output signal the optical detector is processed and the length of the linear resonator changing Actuator controls. 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 pulse energy. Maximum pulse energy is achieved when the resonator length is adjusted so that the wavelength of the laser exactly with the maximum of the transmission of mode-selective optical elements in the resonator and this with the maximum of spectral gain profile the laser active disk coincides. In this way the resonator condition is optimally fulfilled. Thereby can the laser amplifier system for a long time be operated stably in a single longitudinal mode.

following The present invention will become apparent from the detailed description two embodiments in conjunction with the attached Drawings closer explained. 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 concavely curved to achieve laser operation in a fundamental mode (TEM ₀₀ ) Diameter of the mode volume of the laser-active disk 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 and that although only one mode oscillates, but that the laser emission can "jump" to another mode due to a shift in the relative position of the gain profile and resonator modes, for many, in particular spectroscopic, applications is very disturbing In some cases it is expedient to limit the oscillation by wavelength or mode-selecting elements in the resonator to a single mode, so that only one mode resonates in the resonator, ie a single-mode longitudinal mode builds up, 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.

Even though in this embodiment the resonator is linear, is also another structure conceivable, for example, folded or in a closed geometric Form, which is called a ring resonator. What the continuous one-mode operation is concerned, ring lasers are circulating with only in one direction Wave known to be advantageous because the "spatial hole burning" in the gain medium can be avoided. However, this requires the use an optical diode in the resonator and a relatively complex one Construction. Disk laser with a laser-active disk, whose back acting as a mirror in the resonator behave similarly advantageous because all stationary ren resonator modes always have a wave node when reflected on a mirror, So at the back of the disk with regard to "spatial not burning differ. If it is by the advantageous use of a possible thin Slice with one as possible small laser-active volume and an adapted short resonator not directly succeed in operating in a single longitudinal Fashion can achieve this by using less mododeelektierender optical elements can be achieved 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. Depending on the pumping power of the excitation source and the gain to be achieved in the repetition time, the proportion is adjusted such that there is an increase in 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, is over the entire pulse duration favors no other mode to the extent that it can compete with the single mode present at the beginning of the pulse when a thin slice 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 about 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 laser active 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 well-known "Achilles heel" of the resonator structure If the mode jump of the laser is to be reliably prevented during a longer operating time, the relative frequency change of the etalon and the 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 Resona torlänge 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, the rising signal of the gain exceeding that of the photo-detector exceeds 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 falls below the value that is necessary for the self-excitation. Then turn on control device 9 ( 1 . 2 ) the Q-switch 8th again in the locked state.

Claims (6)

Laser system with: - a laser-active disk ( 2 ), - an excitation source for generating an excited state of the laser-active disc ( 2 ), and - a resonator in which the disc ( 2 ) is arranged and in which an optical radiation field ( 4 ) the disc ( 2 ) is repeatedly fed back so that from the resonator a laser beam ( 5 ) is decoupled with a single longitudinal mode, characterized in that a Q-switch ( 8th ) in the resonator and a control device ( 9 ) for switching the Q-switch ( 8th ) are provided, wherein the control device ( 9 ) the Q-switch ( 8th ) upon first reaching a threshold value of the gain ( 18 ) of the laser-active disk ( 2 ) unlocks. Laser system according to claim 1, 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 2, 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 the preceding claims by a linear structure of the resonator. Laser system according to one of Claims 2 to 4, characterized by a further control device ( 15 ), which outputs a second output signal ( 14 ) of the optical detector (1 1 ) and a the resonator length of the laser system changing adjusting device ( 17 ) in response to the second output signal ( 14 ). A method of Q-switching a laser system according to any one of claims 1 to 5, wherein a Q-switch ( 8th ) upon first reaching a threshold value of a gain ( 18 ) of an optical radiation field ( 4 ) from a first control device ( 9 ) is unlocked.