JPS5837711B2 - Mode mode laser touch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a mode-locked laser device that stably generates high-power ultrashort optical pulses.

Conventionally, pulse-excited solid-state lasers that are passively [Q-switched and mode-locked] by the nonlinear absorption of saturable dyes or the Force-Ichi effect have been used to obtain high-power ultrashort optical pulses.

However, the optical pulse obtained in this way is the result of the wave height being selected from the noise at the initial stage of oscillation by a modulator such as a saturable dye, and there is an unavoidable probability that a satellite pulse will be generated in addition to the main pulse. It has been difficult to obtain high-power ultrashort optical pulses with good reproducibility.

Efforts have been made to reduce the stochastic nature of such pulse generation by external control, and Utility Application No. 49-112393 "Mode Locking Device" uses a forced loss internal modulator in combination with a saturable dye. It is stated that the probability of such satellite pulse occurrence can be significantly reduced.

However, with this device, it is necessary to precisely match the period of the forced modulation that occurs while the laser is oscillating with the round-trip period of the optical pulse inside the resonator, which has been difficult to achieve in practice.

For example, in the case of a pulse-excited glass laser in which the Q-switching time from the start of oscillation reaches 50 to 100 μS, the time width in which the satellite pulse can exist can only be achieved by using forced modulation with a modulation depth of 100%. The pulse width is about 8 Ps, which is about the same as the obtained pulse width, and it is finally possible to obtain a stable single pulse without satellite pulses.

However, the positional accuracy of the resonator mirror required at this time is 1~
2 μm, which cannot be fully adjusted using normal mechanical parts.

Furthermore, even if the above-mentioned precision synchronization can be effectively achieved by finely adjusting the modulation frequency of the forced modulator, thermal expansion and refractive index changes of the laser medium due to mechanical vibration of the flash lamp and heat generation due to optical excitation may occur. , resulting in a change in the optical path length during oscillation and breaking the synchronization of the two periods.

If ideal modulation is not possible in this way, the time range in which satellite pulses can exist is expanded, and satellite pulses will still occur with a certain finite probability, although this is less than when forced modulation is not used.

The generation rate of satellite pulses thus generated is also premised on the fact that the excitation level is lowered to just below the oscillation threshold, and there are also troublesome problems such as fluctuations in the threshold causing instability of oscillation.

Of course, if the excitation level is expanded beyond this, the satellite pulse generation rate will increase accordingly.

On the other hand, there is also an example of extracting a single pulse from the output of a normal continuous mode-locked laser, introducing it at the initial stage of oscillation of a Q-switched laser, and amplifying the pulse to obtain a stable high-power ultrashort pulse. However, since there is no pulse width compression function, the
Only a relatively wide pulse around P8 is obtained.

In order to effectively amplify the extracted pulse, the optical paths of the two oscillators must be perfectly aligned through the pulse extractor, which is technically troublesome.

Furthermore, in order for pulse extractors to function, high voltage pulses, typically several nanoseconds long, must be created and applied to a medium that exhibits an electro-optic effect, but such high-speed switching often requires complete impedance. Matching is not possible and the shape of the applied voltage pulse is distorted.

As a result, the next weak pulse is extracted at a time delayed by the resonator round trip time of the continuous mode-locked oscillator, and this pulse becomes the seed of the satellite halus.

To prevent this, the resonator lengths of the two oscillators had to be exactly matched, which also required troublesome adjustment.

The purpose of the present invention is to easily eliminate the stochastic nature of ultrashort pulse generation seen in ordinary pulse-excited mode-locked solid-state lasers without the need for the troublesome adjustment described above, and to eliminate the influence of fluctuations in the oscillation threshold. First, it is an object of the present invention to provide a laser oscillator that generates high-output ultrashort pulses narrower than the pulse width of a continuous mode-locked state with extremely high reproducibility and without generating any satellite pulses.

According to the present invention, within the resonator of a continuous wave mode-locked solid-state laser device, a means for further exciting the active medium in a pulsed manner and a loss that exactly cancels the temporally increasing excessive gain caused by the excitation are provided. , and then a loss means that can rapidly reduce the loss at any time, and the stronger the optical electric field inside the resonator, the lower the loss. A mode-locked laser device including a saturable loss body in which the loss does not decrease can be obtained.

Next, a description will be given of how the present invention produces stable high-power ultrashort optical pulses.

The laser of the above configuration can be operated by continuous pumping of the active medium and forced or passive modulation to provide a gain that overcomes the cavity losses and the saturable absorber losses. A continuous oscillation mode locking state is achieved.

The optical pulses obtained in this way have a weaker optical intensity and a wider pulse width compared to the Q-switched and mode-locked state of a pulse-pumped solid-state laser, but they do not generate satellite pulses and have very good reproducibility. It has the following characteristics.

If there were an additional function to amplify this pulse and narrow the pulse width, it would be possible to create a picosecond pulse generator that is much more stable than a normal pulse excitation mode locking device.

This role is played by a pulsed excitation means for the active medium, a loss means capable of canceling out excess gain and then turning it off, and a saturable loss body.

In other words, if the active medium is further excited by pulsed excitation at an arbitrary time after achieving a steady continuous mode-locked state, most of the electrons that were in the ground state will be excited and a larger population inversion will be formed. The gain of the active medium gradually increases with a certain rise time determined by the relaxation time unique to the medium and the method of excitation.

At this point, the optical pulses are reciprocated within the resonator due to continuous mode locking, and should be gradually amplified by the gain that has begun to increase. When the increasing device is functioning, the intensity of the optical pulse is kept at a steady value of 1=-1 during continuous mode locking, and only the population inversion of the active medium is increased.

Then, if the loss that had been increasing when a population inversion sufficient to suppress the Q-switch is suddenly reduced, the Q-switch is clearly activated.

After this, the optical pulse becomes greatly amplified, saturates the saturable loss body in the resonator, and also receives the effect of compressing the pulse width, which leads to Q-switch mode locking, which is key to ordinary pulse-pumped solid-state lasers. This means that a high-power ultra-short pulse of the same order of magnitude can be obtained.

Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a block diagram of the first embodiment, in which a solid-state laser rod 2, a continuous optical excitation device 3, a forced loss modulator 6, and a driving power supply I are installed in a resonator formed by a resonator mirror 14. is installed to maintain the normal continuous oscillation mode synchronization state.

At this time, at the intra-cavity light intensity, the threshold for oscillation is higher than in the normal case because there is a saturated dye solution 5 in the resonator whose absorption (loss) is not saturated.

1. A modulator 11 that can temporally control loss.
Since the drive power supply systems 12 to 15 reduce the loss to zero during continuous oscillation mode locking, the mode locking is not affected.

The pulse width at this point is about 70 ps when a 4-level Nd-doped YAG crystal is used as the active medium to facilitate continuous oscillation.

Next, if the trigger pulse output from the signal generator 15 is amplified by the amplifier 10 and a pulse excitation discharge circuit separate from the continuous excitation device 3 is discharged using the trigger line 9, the trigger pulse will not decrease much during continuous oscillation mode synchronization. The distribution number of ground states decreases with strong excitation, and the population inversion gradually increases.

If only the increased gain is added to the above-mentioned continuous wave mode locking state, the pulse of about 70 Ps before this excitation will be amplified, but then, as is well known, relaxation oscillation will be reached, and the effective amplification and pulse Width compression becomes impossible.

Therefore, if the controllable modulator 11 is used to provide a loss that offsets only the increasing excess gain at the time when the excess gain reaches its maximum, and then this loss is made to disappear, the The light pulse is amplified to provide a high power picosecond pulse with no satellite pulses, combined with effective pulse width compression by the saturable dye.

Specifically, the loss that offsets this 4 gains is created as follows.

The trigger pulse from the signal generator 15 is split into two parts, one of which passes through a variable delay circuit 14 for synchronizing the discharge of the pulse exciter initiated by the other;
Enter the waveform generator 13.

There, an electrical signal is created that has the same waveform as the temporal change in excess gain and becomes zero at any subsequent time, and the amplification rate of the amplifier 12 is adjusted and modulated to exactly cancel out the excess gain. is applied to the device 11.

As the modulator, for example, an ultrasonic loss modulator, a Pockels cell or a Kersel in combination with a polarizing element can be considered.

When the modulator is used in such a way that the response to the applied voltage is non-slow, the pulse waveform generator 13 generates a waveform that takes this into consideration in advance.

FIG. 2 shows the function of each mechanism of this device in terms of time.

The horizontal axes all represent time t, and the vertical axes represent the waveform P of the excitation light in FIG. 2a) and the resulting gain G in FIG. 2b).

The maximum gain G occurs with a delay from its maximum thickness, depending on the energy relaxation time in the active medium used and the time width of the waveform P.

Figure 2 C) is the loss L inserted to cancel the gain G,
The gain G is decreased to zero from just before it reaches its maximum value.

Figure 2d) shows the resulting substantial excess gain (GL)
It is.

In reality, the optical pulse lengthened with this excessive gain suffers from population inversion, so the gain G decreases greatly along with the Q switch, and the oscillation light intensity returns to the original continuous oscillation mode locking state.

In the end, the features of the present invention are that although the output obtained from a continuous wave mode-locked laser is weak and the pulse width is wide, it has excellent reproducibility and stability, and the optical pulse without satellite pulses can be used as the initial oscillation of a pulse-excited mode-locked laser. Compared to conventional pulse-excited passive mode-locking, in which the initial condition of oscillation was a random noise distribution, the reproducibility and stability are improved, and no satellite pulses are generated.

Next, a second embodiment shown in FIG. 3 will be described.

The major difference from the first embodiment is that the active medium to be excited by the pulsed excitation means is provided separately from that for continuous mode locking, and that the active medium to be excited by the pulsed excitation means is provided separately from that for continuous mode locking. The point is that passive elements are used.

1 to 5 in FIG. 3 are exactly the same as the first embodiment in FIG. 1, have the same function, and in phase with an appropriate saturable dye 31, produce a continuous wave mode locking state.

Then, another active medium 32 is started to be excited using conventional pulse excitation means.

That is, the output of the high voltage trigger device 35 is led to the trigger wire 34 which is wound around the pulse excitation circuit 330 lamp, and the discharge of the excitation lamp is started.

Compared to the first embodiment, using separate active media in this way has drawbacks such as the resonator length becoming thicker, but it becomes easier to design a condenser for the active media.

The functions of this embodiment will be explained below with reference to FIG. 4, which shows the change in gain over time.

Excess gain G (4th
Figure C) and loss L due to passive loss element 36Vc (Figure 4b)
) to obtain an effective excess gain (G-L FIG. 4d) and obtain the same effect as in the first embodiment.

The passive loss L introduced here is proportional to the excitation light p [but
A specific example of such an absorber is a polymethine dye that has strong absorption in the visible to near infrared range, and one unit of its energy is given in FIG.

51 represents the singlet ground state, and 52 represents its excited state.

This absorption between levels (wavelength input p) has a peak from the visible region to the near infrared 1 depending on the individual dye, and the wavelength width of absorption reaches several hundred AVC.

Of the broad and strong emission spectrum of the pulse-excited 1,000-stage lamp 33, the components that are not absorbed by the active medium 32 are
Since the oscillation light strongly leaks in almost the same direction as the oscillation light, if the spectrum of the light overlaps with the absorption spectrum of the absorber, the absorption cross-section is large and the dye is effectively excited.

As is well known, the electron once excited to the excited level 52 immediately undergoes a non-radiative transition to the triplet ground level 53.

This level has a relatively long lifetime and typically provides effective absorption of another wavelength band (up to the peak wavelength) between the triplet excited state 54 and the triplet excited state 54.

However, the relaxation times involved in these levels are all sufficiently short compared to the light emission time of the lamp, so if you choose a dye whose absorption band overlaps with the laser oscillation wavelength, this dye can be Excitation light waveform P (Figure 4a)
The absorption L of the laser light (FIG. 4b) approximately coincides with .

In order for this absorption L to cancel out the excess gain G due to the excitation, at least in the initial stage of the pulsed excitation, the dye concentration may be adjusted.

The effective gain (GL) obtained in this way starts to rise earlier and more slowly than in the first embodiment, but
If the speed rises faster than the relaxation oscillation, the Q-switch will occur and the objective will be achieved.

Further, in this embodiment, two active media are used, but in this way, it is possible to use a different rod depending on the purpose.

For example, rod 2 is made of Nd:YAG, which can stably generate high-output continuous oscillation, and rod 32 is made of Nd:glass, which provides a high amplification degree and has a wide bandwidth, making it possible to generate shorter pulses. You can also use

If a four-level system is selected as the active medium, it is clear that the rod 32 during non-excitation does not cause any loss to the laser beam.

By the way, if there is a possibility that the continuous excitation light of the active medium 2 may disturb the operation of the passive absorber 36, the laser light is transmitted and
A filter 37 that absorbs light at the absorption wavelength p of the absorption dye
may be placed between the absorbent body 36 and the active medium 2.

The features of this second embodiment over the first embodiment, that is, the use of two active media and the use of a passive element as a loss body for canceling excess gain, are not mutually necessary, so each has its own advantages. It is of course possible to separately combine the first embodiment with the first embodiment.

Furthermore, it is clear that there is mutual compatibility between using a forced modulator and a self-directed modulator for continuous mode locking.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the structure of the first embodiment, in which 1 is a total reflection mirror, 2 is a laser rod containing an active medium, 3 is a continuous excitation means, 4 is an output mirror, 5 is a saturable absorber, and 6 is a laser rod containing an active medium. 7 is a loss modulator for continuous mode locking, 7 is its driving power source, 8 is a pulse excitation means,
9 is a trigger line, 10 is an amplifier, 11 is a controllable modulator, 14 is a variable delay circuit, 13 is a pulse waveform generator, 1
2 is an amplifier for its output, and 15 is a trigger pulse generator. Figure 2 is a diagram chronologically showing the function of each mechanism in Figure 1.
Figure 2 a) is the emission waveform of the pulsed excitation means, Figure 2 b)
2C) shows the waveform of the increase in gain, FIG. 2C) shows the waveform of the electrically generated loss, and FIG. 2D) shows the substantially increased gain. FIG. 3 is a block diagram of the second embodiment, in which 31 is a saturated dye;
32 is an active medium for pulse excitation, 33 is an electric circuit for excitation, 34 is a trigger wire, 35 is a high voltage power supply, 36 is a loss element, and 37 is a filter. FIG. 4, like FIG. 2, is a diagram showing the function of each mechanism of the second embodiment. FIG. 5 is an engineering level diagram of an example of the absorber 36.

Claims (1)

[Claims] 1. A continuous wave mode-locked laser device including an active medium and a forced loss modulator in a resonator, including means for further exciting the active medium in a pulsed manner, and providing a loss that cancels out excessive gain increased by the excitation. A mode-locked laser device, further comprising means for rapidly reducing the loss after an arbitrary period of time, and a saturable loss body for the optical electric field within the resonator.