JPH04242984A - Solid-state laser oscillator - Google Patents

Abstract

PURPOSE:To obtain a strong laser light with higher efficiency for energy conversion from electric power by a method wherein a mechanism to turn on a Q-switch is additionally provided within a laser resonator for the laser oscillator where a titanium sapphire is used as a laser medium and an intermittent light- emission source is used as a excitation light-source. CONSTITUTION:A Q-switch element 6 is applied with a sufficient Q-switch within a titanium supphire laser oscillation. These elements make it possible that the Q-value within the oscillation is set to the state of the both oscillators that show the value with oscillation and the value without oscillation by means of a signal such as an electric signal distinctly, and the respective state is instantaneously changed faster than the width of the oscillation pulse. The switching rate of less than 500nsec is desirable. Desirable is that the excitation light-source emits light intermittently, and the pulse time width is longer than the pulse time width of the oscillation laser, with more than 500nsec in full half-width.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser oscillator using sapphire doped with titanium (doped as a different element), so-called titanium sapphire, as a laser medium. More specifically, in a laser oscillator using titanium sapphire as the laser medium, a single pulse
This invention relates to an improved laser oscillator that is capable of oscillating a pulsed laser with a variable wavelength and a large peak power and high stability.

Lasers with a short pulse width and high peak power are used for precision measurement, evaluation of high-speed operation, and the like. Furthermore, lasers using titanium sapphire as a laser medium have variable wavelengths and are used for spectroscopic measurements.

0003

2. Description of the Related Art Titanium sapphire has been used as a medium for wavelength-tunable lasers. A dye laser that uses a dye dissolved in a liquid solvent as a laser medium is also wavelength tunable. However, the wavelength tunability of dye lasers is about 50 nm, and that of titanium sapphire (500 nm).
nm), and high-output oscillation is difficult. In addition, dye lasers often use toxic liquids, are not easy to handle, and have high running costs.

Conventionally, a short pulse width laser using titanium sapphire as a laser medium uses a continuous wave argon ion laser as an excitation source, and uses an electro-optical (EO) laser as an excitation source.
) element, acousto-optic (AO) element, etc. are installed in the resonator,
It has been mainly used to perform forced mode synchronization. Mode-locking is a technique for oscillating (ie, locking) only one laser mode within a resonator. In other words, since laser light travels back and forth within the resonator many times, the optical loss of a certain optical element is small only when one laser light with a short time width (i.e., laser mode) passes through that element, and is large at other times. This is a method of selectively generating short pulse lasers. This optical element is, for example, an EO or AO element. Therefore, in order to obtain a short pulse laser using this method, a high frequency signal must be applied to this optical element at precisely the timing when the laser passes. This frequency is the reciprocal of the time it takes for the light to travel half a round trip through the resonator. Unless this frequency is the correct frequency, stable pulsed laser light cannot be obtained. Furthermore, even when obtained, the pulse shape often does not have a clear temporal waveform, such as a waveform with shoulders.

[0005] Furthermore, it is considered desirable that the excitation light source used for forced mode locking be a continuous light source. When using a discontinuous excitation light source, one use is pulsed light that is repeated at the same frequency as described above. At this time, although the input excitation light source and the signal applied to the optical element have the same frequency, a time delay occurs between them, making it more difficult to obtain the timing of the applied signal. It is also quite difficult to obtain such a high frequency light source. Another method is to lengthen the time width of one pulse of the excitation light source to produce pseudo-continuous light emission, and obtain a plurality of short pulse laser beams within the pulse application time. In this case, it is not necessary to match the timing of the excitation light sources, but the disadvantage is that the light intensity of each laser pulse is different.

In the case of a laser using titanium sapphire as a laser medium, for example, as a continuous emission excitation light source,
There are argon ion lasers and continuous wave Nd:YAG lasers. However, the energy conversion efficiency of these lasers is about 1% at most, and the energy conversion efficiency from the excitation laser to the titanium sapphire laser is 1% or less. Therefore, this method has the disadvantage that the conversion efficiency of energy from electric power is as low as 1×10 −4 or less, and that a laser beam of high intensity cannot be obtained. Furthermore, since this method requires the resonator to be assembled in two stages, the optical system of the device becomes complicated and the device inevitably becomes larger.

[0007]Also, in a titanium sapphire laser oscillator that uses a discontinuous light source such as a flash lamp as an excitation light source, pulsed light can be easily obtained by normal oscillation, but this is because there are multiple peaks on one pulse. This resulted in spike-like oscillation, and only a laser beam with a relatively small peak energy was obtained.

[0008]

[Means for Solving the Problems] As a result of various studies on laser oscillators that do not have the above-mentioned problems, the present inventors have developed a laser oscillator that uses titanium sapphire as a laser medium and uses a discontinuous light source as an excitation light source. By arranging a Q-switch element such as an EO element and applying a so-called Q-switch that instantly switches the optical loss inside the resonator from a high state to a low state, the wavelength of single pulse oscillation with high peak energy can be changed. They discovered that a tunable laser could be obtained and completed the present invention.

That is, the present invention provides a solid-state laser characterized in that a laser oscillator using titanium sapphire as a laser medium and a discontinuous light emitting light source as an excitation light source is provided with a mechanism for applying a Q switch within the laser resonator. It is related to oscillators. Next, the configuration of the present invention will be explained in detail.

The laser medium used in the present invention is titanium-doped sapphire, so-called titanium sapphire. The amount of titanium added to this medium is the same as that of commonly used titanium sapphire, for example, 0.01 to 0.5 wt%.
It is. If the amount of titanium added is less than 0.01 wt%, the amount of amplification becomes small and laser oscillation becomes difficult. Moreover, if the amount is more than 0.5 wt%, the value indicating the performance of this medium called FOM deteriorates, and laser oscillation becomes difficult.

The Q-switching element such as the EO element used in the present invention must be capable of sufficiently Q-switching within the titanium sapphire laser resonator. That is, by using these elements, it is possible to differentiate between the states of both the resonators, which take a value called the Q value in this resonator, which takes a non-oscillating value and a value that oscillates, respectively, by signals such as electrical signals, Moreover, each state can be instantaneously switched faster than the oscillation pulse width. Q-switching here refers to exciting titanium sapphire, suppressing oscillation with an EO element until sufficient energy is accumulated in titanium sapphire, and then instantaneously switching to a state where it oscillates. It refers to a method of obtaining laser light with high energy. Therefore, the switching speed is preferably 500 nsec or less, and since the applied laser is 50 nsec or less, the switching speed is also preferably 50 nsec or less. The faster the switching speed, the better the Q-switching. If the switching timing is too early, a single pulse cannot be obtained, whereas if the switching timing is too slow, a laser beam with sufficient energy cannot be obtained.

In the case of mode-locking, this switching must be repeated at a high frequency, but in the case of Q-switching, at least 1
Times are fine. Further, even if the timing of switching is shifted by several microseconds, there is no significant change in the time waveform of the oscillated laser. In the case of mode synchronization, the worst is 10 pse
It is necessary to control c.

The excitation light source used in the present invention emits light discontinuously. The pulse time width of the excitation light source is preferably longer than the pulse time width of the oscillation laser, and is preferably 500 nsec or more in full width at half maximum. If it is shorter than this, even if the oscillation is not suppressed, the laser oscillation will not occur until the state where the most excitation energy is accumulated, so there is no need to perform Q-switching. Furthermore, it is difficult to obtain a pulsed light source with a high light intensity of 500 nsec or less, or it becomes necessary to use a large-sized light source device.

No matter how long the pulse time width of the excitation light source is, Q-switching can be performed even with a continuous light source in the ultimate case. However, in the case of a continuous emission excitation light source, the longer the energy accumulation time t, the lower the efficiency of the accumulated energy amount with respect to the absorption of excitation light. This efficiency η is given by the following equation, where T is the excitation lifetime of titanium sapphire.

η=T/t {1-exp(-t/T)} From this, for example, if energy storage of 10% or more is to be performed, a pulsed light source of 30 μsec or less is desired. In addition, even with continuous emission excitation light sources, there is a method of suppressing oscillation again immediately after laser generation and performing Q-switching by repeating switching once every 30 μsec or less, but this not only takes time and effort to repeat the operation, but also The light source used to oscillate one laser pulse of energy is relatively larger than the pulsed light source. This is because light sources generate heat and are cooled to reduce their durability, but with pulsed light sources, no heat is generated during the time when light is not emitted, so there is no need for a large-scale cooling structure. This is to live in Japan. That is, a pulsed light source of 50 nsec to 30 μsec, which is a relatively simple light source, is optimal as an excitation light source, resulting in a simple laser oscillation device as a whole.

The configuration of the apparatus of the present invention will be explained with reference to one embodiment shown in the drawings.

FIG. 1 is a schematic diagram of an embodiment of a device in which a Pockels cell, which is an EO element, and a filter for wavelength selection are arranged in an oscillator. 6 in the figure indicates that both end faces are non-reflective (A
R) Coated Pockels cell, 4 is a titanium sapphire rod, 8 is a flash lamp that excites the titanium sapphire, 1 and 2 are mirrors forming a resonator, and 5 is a birefringence filter that is a wavelength selection element.

The configuration of the present invention is not limited to the above-mentioned structure; for example, the Q-switch element used in the present invention may be an AO element using a piezoelectric material. When using a Pockels cell as a Q-switch element, for example, it is set in a state where it does not oscillate when a voltage is applied, and a state where it oscillates when no voltage is applied. The switching speed varies depending on the resonator configuration, but for example, in the embodiment of FIG. 1, it may be 500 nsec or less. In this case, the faster the switching speed, the shorter the oscillation pulse width. In the present invention, these are arranged at a position within the resonator through which the oscillated laser light passes at least once, for example, between the laser medium and the mirror. It is particularly desirable for the titanium sapphire laser medium that the excitation light source has an emission wavelength of 400 to 600 nm.

In the present invention, a discontinuous light source is used as the excitation light source, and examples of the discontinuous light source include a flash lamp, a light emitting diode, and a semiconductor laser. These excitation light sources do not require the irradiation direction to be aligned with the optical axis, making it even simpler. However, irradiation from the optical axis direction is somewhat more efficient. The input energy of the excitation light source may be at least the threshold value for laser oscillation when Q-switching is not applied, but it is preferably 10 times or more the threshold value for efficient Q-switching. Further, the other configuration may be the same as that of a normal oscillator.

[0020]

Effects of the Invention The present invention has a high conversion efficiency of energy from electric power, a high-intensity laser beam, and a stable short single pulse with a high peak power of the resulting laser beam. It is a wavelength-tunable laser oscillator that can oscillate. In addition, the oscillator configuration is simple, smaller than conventional ones, and adjustment is easier. It's even safer.

[0021]

EXAMPLES Next, the present invention will be further explained with examples.

[0022]

Example 1 A laser oscillator having the configuration shown in FIG. 1 was manufactured. The amount of titanium added to titanium sapphire is 0.1wt%.
, the rod length is 50 mm, the transmittance of the output side mirror is 2 to 20%, and the energy input to the flash lamp is 2
00J, and its pulse width was 8 μsec. Oscillation stopped at a Pockels cell voltage of 3 to 4 kV. As a result of setting the switching time to 50 nsec or less and the switching timing to 1 to 5 μsec after the rise of the flash lamp current, a single pulse light with peak power approximately 5 to 20 times higher than that of Comparative Example 1 was obtained at the optimal timing. Ta. The pulse time width at this time was 20 to 100 nsec, and the pulsed laser light output intensity was 10 to 150 mJ. Energy conversion efficiency from electricity is 1.5 x 10-4 to 6.0
×10-4. Also, the variable wavelength range at this time is 730~
The wavelength range was limited to 920 nm, an effective wavelength range of the mirror used in the laser oscillator.

[0023]

[Example 2] The same procedure as in Example 1 was conducted except that the laser oscillator having the configuration shown in FIG. 1 was used and the pulse width was set to 4 μsec. Oscillation stopped at a Pockels cell voltage of 3 to 4 kV. As a result of setting the switching time to 50 nsec or less and the switching timing to 1 to 5 μsec after the rise of the flash lamp current, a single pulse light with a peak power of about 10 to 30 times that of Comparative Example 1 was obtained at the optimal timing. Ta. The pulse time width at this time is 20 to 1
00nsec, laser light output intensity is 10-150mJ
Met. Also, the variable wavelength range at this time is 730 to 920n.
was limited to the effective wavelength range of the mirror used in the laser oscillator.

[0024]

Example 3 A laser oscillator having the configuration shown in FIG. 2 was manufactured. The amount of titanium added to titanium sapphire is 0.1wt%.
, its rod length is 50 mm, the transmittance of the mirror on the output side is 2-20%, the total input power to the light emitting diode (LED) array is 3 J per pulse, and its pulse width is 5 μsec.
, the emission wavelength was 570 nm. Pockels cell voltage 3
Oscillation stopped at ~4kV. Switching time 10ns
ec or less, as a result of setting the switching timing to 1 to 10 μsec after the rise of the flash lamp current,
With optimal timing, a single pulse of light with a peak power of about 5 to 10 times was obtained. The pulse time width at this time is 30~
It was 200 nsec. Also, the variable wavelength range at this time is
The wavelength was 750 to 860 nm.

[0025]

[Comparative Example 1] Using a laser oscillator having the configuration shown in FIG. 1, laser oscillation was performed under the same conditions as in Examples 1 and 2 without applying a Q switch. As a result, the peak power is
Under the conditions of Example 1, 500 W to 2 kW were obtained, and under the conditions of Example 2, 1 kW to 4 kW were obtained. Both pulse time waveforms had a spike shape, and had a shape of 3 to 10 single pulses of 100 to 400 nsec overlapping each other. Other output laser characteristics were comparable to those of Examples 1 and 2.

[0026]

[Comparative Example 2] A laser oscillator having the configuration shown in FIG. 3 was manufactured. This is a configuration in which mode locking is performed using a titanium sapphire laser excited by a continuous wave laser. In the figure, 11 is a continuous wave argon ion laser, 4 is a titanium sapphire rod (0.1% titanium added), and 10
is an AO mode locker, 1 to 3 are mirrors forming an oscillator, and 5 is a birefringence filter. In this laser oscillator, a 10W argon ion laser is oscillated,
Tests were conducted by exciting titanium sapphire and driving the AO mode rocker. The driving frequency of the AO mode rocker is 5
0 to 200 MHz, but the frequency at which mode-locking operates can be changed by fine tuning the resonator, and can also be adjusted by 1%.
Frequency control had to be performed with the following accuracy. As a result of the test, wavelength 730-920nm, average output 100
A laser beam of ~200 mW, a single pulse output of about 1 nJ, and a peak power of about 5 kW was obtained. The amount of power input to the 10W argon ion laser was 2kW. The energy conversion efficiency from electric power was 5 x 10-5. The pulse width was shorter than in the example, and was 1 to 2 psec.

[0027]

[Comparative Example 3] A laser oscillator having the configuration shown in FIG. 4 was manufactured. This is a configuration in which mode-locking is performed for the titanium sapphire laser using the flash lamp used in Example 1 as the excitation light source. The supersaturated absorber used was IR-125 dye (6×10 −6 mol/l) dissolved in dimethyl sulfoxide (DMSO). While cooling this dye solution to 20°C, the dye solution was heated to a thickness of 0.5 mm as shown in 12 in the figure.
It was circulated through a mm cell. The dye solution used was toxic, so safety precautions had to be taken when handling it, such as wearing a mask to prevent inhalation. Furthermore, since the durability of this dye solution is relatively short, it was necessary to frequently replace it. Furthermore, changing the concentration of the dye did not result in a supersaturated absorber in this laser oscillator. In this laser oscillator, a power of 250 J is applied to the flash lamp.
As a result of the pulse input test, the wavelength was 770 nm and the output was 2.
A laser beam of 0 mJ and a pulse width of 100 psec was obtained. It was not possible to make the peak power greater than without mode locking. To tune the wavelength, it was necessary to change the concentration of the dye or to use a different dye.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a titanium sapphire laser oscillator according to one embodiment of the present invention.

FIG. 2 is a schematic diagram of a titanium sapphire laser oscillator according to another embodiment of the present invention.

FIG. 3 is a schematic diagram of a titanium sapphire laser oscillator using a continuous wave argon ion laser as an excitation source.

FIG. 4 is a schematic diagram of a titanium sapphire laser oscillator equipped with a supersaturated absorber.

[Explanation of symbols]

1: Mirror forming the oscillator 2: Mirror forming the oscillator 3: Mirror forming the oscillator 4: Titanium sapphire rod 5: Birefringence filter Mirror forming the oscillator 6: Pockels cell 7: Polarizer 8: Titanium sapphire Exciting flash lamp 9:
Light emitting diode array 10 that excites titanium sapphire
:AO mode locker 11:Laser light source 12 for exciting titanium sapphire:
supersaturated absorber cell

Claims (2)

[Claims] 1. A wavelength-tunable solid-state laser that uses titanium-doped sapphire as a laser medium, characterized in that a discontinuous emission light source is used as an excitation light source and a Q-switching mechanism is placed in the resonator. oscillator. 2. The solid-state laser oscillator according to claim 1, which is a Q-switch with a switching speed of 500 nsec or less.