JPH06152015A - Short pulse optical functional device - Google Patents

Abstract

PURPOSE:To provide a device which stably generates sort pulse light in the femto second area by high power. CONSTITUTION:A Fabry-Perot laser oscillator 11 is provided with laser medium 1, a pair of high-dispersion Brewster prisms 4 and a saturable absorbing pigment jet 5. Continuous light beams 21 are incident on the resonator 11 from an exciting laser light source 12 for excitation, pulse train 22 which is in synchronism with constant continuous mode is generated and amplifying exciting pulse light 23 are projected from an amplifying laser light source 13 for the operation of the high gain amplifying medium. Thus, high power and stable short pulse light 24 in the femto-second area are generated.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The present invention is useful in the field of quantum electronics in the ultra-short time region such as ultra-high-speed precision optical measurement, optical communication, optical device characteristic evaluation, and ultra-fast physical phenomenon spectroscopic measurement. , A picosecond (ps = 10 ⁻¹² seconds) to femtosecond (fs = 10 ⁻¹⁵ seconds) short pulse light with high output and stably.

[0002]

2. Description of the Related Art In recent years, short pulsed light in the femtosecond region can be generated by the development of techniques for generating short pulsed light such as various mode locking methods and pulse compression methods. Until now, the generation of such short-pulse light in the ultra-short time region and its application research have been limited to dye lasers with a wide gain band.

The pulse width of the shortest pulse light obtained at present is 6 fs by the dye laser system (RL
Fork et al: Opt. Lett., 12, 483 (1987)). In this system, first, a phase-compensated collision mode-locked ring dye laser (JAValdmanis et al: Opt. Lett., 10, 131 (1985)) is used.
The short pulse light of about 50 fs is generated by the reference. Next, since it is difficult to generate ultrashort pulsed light of 10 fs or less from an ordinary oscillator having a low peak intensity, a multiple optical path (6 round trips) dye amplifier (WHKnox et al: J.Quant. ., 24, 388 (1988)) or cwQ-switched Nd: YAG laser pumped dye amplifier (see Y. Ishida et al: Optics Commu., 68, 295 (1988)) to enhance the peak output (≧ 1 MW). Further, the amplified pulsed light is compressed by a pulse compressor that combines nonlinearity (optical Kerr effect) in a quartz optical fiber and a negative dispersion element (diffraction grating pair) (H. Nakatuka et al. : Phy.Rev.Lett., 47,910 (19
81), also WJ Tomlinson et al: J.Opt.Soc.Am., Bl, 1.
(See (1992)).

[0004]

By the way, in the above-mentioned dye laser system, the dye material (solution) which is the laser medium is remarkably deteriorated, and the dye material 100 f is emitted from the laser oscillator.
In order to stably maintain the short pulse light of s or less, it was necessary to frequently exchange the dye solution and to adjust the resonator very carefully each time. In addition, the gain duration is short (≤5
ns) As long as a dye amplifier is used, accurate timing coincidence (≦ 1 ns) between the amplified pulsed light and the excitation pulsed light is required, and it is not easy to stably obtain a high-output short pulsed light, and control is possible. There were problems in terms of reliability, reliability and operability.

In view of the above-mentioned conventional problems, it is an object of the present invention to provide an apparatus capable of stably generating short pulse light in the femtosecond range with high output.

[0006]

In order to achieve the above object, the present invention uses a Fabry-Perot type laser resonator including a laser medium, a pair of phase compensation prisms and a saturable absorption medium as a light source to be amplified, and a laser light source for amplification. It proposes a short pulse light functional device which causes the laser medium to act as a high gain amplification medium by condensing the short pulse light generated from the laser medium in the light source to be amplified.

[0007]

According to the present invention, the short pulse light generated from the laser light source for amplification is focused on the Fabry-Perot type laser resonator which generates the pulse train in the steady continuous mode-locked state, and this is strongly excited. As a result, an optical amplification effect due to gain switching is caused to generate ultrashort pulsed light.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the present invention, in which 1 is a continuous mode-locked solid soliton laser medium, here a titanium sapphire (Ti: Al ₂ O ₃ ) rod (Y. Ishida et.
al: Ultrafast Phenomena VII, Springer-Verlag, Berlin,
p.75 (1990), also N. Sarukura et al: Opt. Lett., 16,15.
3 (1991)), 2 is a total reflection plane mirror, 3 is a transmittance of 5
% Output plane mirror, 4 is a high dispersion Brewster prism pair made of SF ₆ glass for phase compensation, and 5 is HITC
A saturable absorbing dye jet made of I solution, 6 a wavelength selecting element made of a quartz plate, 7, 8, 9, 10 concave mirrors, which constitute a Fabry-Perot type laser resonator 11. Further, 12 is a laser light source for excitation, here a cw argon ion laser having an average output of 6 W. Further, 13 is a laser light source for amplification, here a cwQ-switched Nd: YAG laser (second one with an output energy of about 1 mJ).
With a harmonic generation crystal). Further, 14 is a condenser lens, and 15, 16 and 17 are mirrors.

In the above structure, the Fabry-Perot type laser resonator 11 is excited by the continuous light 21 from the exciting laser light source 12 and generates the pulse train 22 in the steady continuous mode-locked state. The pulse width of the pulse train 22 can be varied from 90 fs to 10 ps by adjusting the group velocity dispersion amount (prism interval) of the high dispersion Brewster prism pair 4. The repetition cycle of mode synchronization is T _c = 2
It is given by L / c (about 12 ns). Here, L is the resonator length, and c is the speed of light.

In the state where the pulse train 22 in the steady continuous mode synchronization state is generated, the amplification pumping pulse light 23 generated from the amplification laser light source 13 is collected by a condenser lens (f =
(500 mm) 14 and the concave mirror 10 to focus on the laser medium 1. The amplification pump pulse light 23 and the pulse train 22 are spatially matched in the laser medium 1.
CwQ switch N used as amplification laser light source 13
The d: YAG laser is compact and highly repeatable (1 to 1
Since it is a solid-state laser capable of 0 kHz), maintenance and operability are significantly improved as compared with the conventionally used copper vapor laser, excimer laser and the like. If high-power semiconductor lasers emerge in the future, it will become a more compact system.

The pump power density is sufficiently increased (0.
1 J / cm ² ), a net gain occurs, and the pulse train 22 that reciprocates in the resonator 11 is exponentially amplified to a level at which a saturation effect occurs each time when passing through the laser medium 1, and the output pulse is output. It becomes the light 24. After the pump pulse light for amplification (70 ns width) 23 is cut off, the pulse train 22 is attenuated by the lifetime of photons in the resonator 11 (t _c = 400 ns), and undergoes transient relaxation oscillation (10 to 20 μs), The original steady continuous mode synchronization state is restored.

FIG. 2 shows the envelope waveform of an actually amplified pulse train measured with a high-speed semiconductor optical device and an oscilloscope. In the figure (a), the sweep speed is set to 5 μs / div.
The waveform is such that the amplified transient response portion and the subsequent non-amplified portion (steady continuous mode-locked state) are all visible. Further, FIG. 6B is a waveform in which the sweep speed is increased to 200 ns / div. And only the amplified transient response portion is enlarged. When the excitation power density is 0.5 J / cm ² , 150
A double amplification is easily obtained. After amplification, about 15μ
It can be seen that a steady state is reached at s. From this, 5
High repetitive amplification of 0 kHz or more is possible. The light intensity in the resonator 11 is 10 to 15 times higher than that of the light extracted to the outside. Therefore, if a cavity damper element is inserted between the concave mirrors 9 and 10, the output can be further increased by one digit.

The pulse widths of the transient response portion and the amplified portion were measured by a second harmonic generation autocorrelator. If the original pulsed light is in the 10 ps region, it will not spread even after amplification. 100f
With pulsed light of s or less, the dispersion effect in the resonator 11 and the self-phase modulation effect are combined and spread to 300 fs, but this point can be compressed by phase compensation, as described later.

Even if the same amplifying medium is used, it is necessary to raise the pumping power density by about one digit ( ² J / cm ² ) in order to obtain the same degree of amplification in the conventional external multiple optical path (2 to 4 round trips). The high efficiency operation and effectiveness of the device of the present invention are well demonstrated.

According to this embodiment, since the laser medium acts as an amplifying medium during the time when it is strongly excited, the conventional multi-path / multi-stage amplifier (P.
Georges et al: Opt. Lett., 16, 144 (1991)) and a regenerative amplifier (see J. Squier et al: Opt. Lett., 16, 324 (1991)) do not require any complicated optical system. Therefore, a timing control system for the pulse train to be amplified and the pump pulse light is also unnecessary. Further, in this apparatus, since titanium sapphire (Ti: Al ₂ O ₃ ) rod, which is a new solid material, is used for the laser medium, there is no problem of deterioration as in the dye solution. Unlike the conventional solid medium, this laser medium has a wider gain bandwidth (700 to 1100 nm) and a higher saturation energy (1 J / cm ² ) than the dye, and thus has a wide wavelength tunability, a short pulse, and a high pulse width. Suitable for output. In addition, the stability of the system is greatly simplified,
Problems such as improved beam quality, reliability, operability, and maintainability can be solved at once. In addition, a large number of optical elements (lens,
(Mirror, polarizing element, etc.) are not required, so the pulse broadening effect and optical loss due to their group velocity dispersion can be ignored,
It is advantageous for ultrashort pulse amplification.

FIG. 3 shows a second embodiment of the present invention. In the figure, the same components as those of the first embodiment are designated by the same reference numerals. That is, 11 is a Fabry-Perot type laser resonator, 12 is a laser light source for excitation, 13 is a laser light source for amplification, 18 is an acousto-optic device, and 19 is a diffraction grating pair for phase compensation (pulse width compression).

The output pulsed light 24 of the Fabry-Perot type laser resonator 11 is shaped by the following procedure before entering the diffraction grating pair 19. The output pulsed light 24 is amplified in synchronization with the repetition frequency of the amplification pumping pulsed light 23 described in the first embodiment. The amplification duration is determined by the energy relaxation time of the laser medium 1 and the amplification pump pulse light 23.
It varies depending on the width of 1 to 2 μs under the condition of this system.
(Corresponding to 100 to 200 pulses). In order to extract only this amplified portion, an electrical gate pulse corresponding to the amplification width is generated from a high-speed photodetector (not shown), and this is applied to the acousto-optic element 18 that responds at high speed.
Alternatively, a single pulse extractor using an electro-optical element can be used to extract only one pulse train.
This makes it possible to obtain the gated pulse light 25 in which the main part of amplification is separated from the output pulse light 24 and selected.

On the other hand, the output pulsed light 24 reciprocating in the resonator 11 reaches the high peak intensity (> 10 GW / cm) because it is condensed by the concave mirrors 9 and 10 when passing through the laser medium 1. ² ). In order to obtain the same light intensity with the external amplifier, the amplification degree of 10 ⁴ times or more is required.
When the optical field in the medium becomes strong, self-phase modulation (Kerr effect), which is a third-order nonlinear optical process, is induced.

Further, even if the medium length (2 cm) is short, the effective length of the resonator 11 increases by 1 to 2 digits due to the reciprocating effect of the resonator 11, so that the chirping (frequency modulation) due to the self-phase modulation strongly appears. As a result, the pulsed light causes sufficient spectral broadening. That is, the same effect can be effectively produced in the short laser medium 1 without causing external self-phase modulation by using a long optical fiber as in the prior art. Moreover, the response time in the laser medium 1 is fast (up to 1 f
s). If a broad spectrum broadening and Δν (positive linear chirp) are positively generated on the basis of high peak intensity and phase compensation is performed, an ultrashort pulse (10 fs or less) corresponding to the reciprocal of the broadening of the spectrum is directly emitted from the laser.
It can occur.

FIG. 4 shows the spectrum of the output pulsed light 24 (10 ps) obtained from the resonator 11. In the figure,
Reference numeral 31 indicates a case where no light is incident from the amplification laser light source 13, and 32 indicates a case where light is incident. In this figure, the vertical axis is displayed on a logarithmic scale in order to see the condition of the tail.

When the energy amplification degree is 30 times, the pulsed light after amplification has a spectral spread (the original full width at half maximum is 2 to 3).
.Times..times..DELTA..lamda. = 2.3 nm) and has a positive linear chirp characteristic due to high-speed response, so that a diffraction grating pair (1200 l / mm) having a negative dispersion characteristic as shown in FIG.
The pulse width (t _p = Δν ⁻¹ ) of the pulsed light 26 after compression is compressed to the reciprocal of the spectral spread simply by passing through 19 or a prism pair. The distance between the diffraction grating pair 19 is adjusted in order to perform optimum phase compensation according to the spectrum spread. As a result, the compression ratio when compressed to the Fourier limit becomes about 1/30.

FIG. 5 shows an example of gated pulsed light cut out by the acousto-optic device 18, and the sweep speed is 5
It is 00 ns / div. From FIG. 5, it was possible to selectively extract the chirping light pulse generated together with the high output. It should be noted that compressed pulsed light can be obtained from the gated pulsed light by a conventional technique using an optical fiber or the like.

[0023]

As described above, according to the present invention, 1
One laser medium can be effectively operated as a laser oscillator and an amplifier, and also as a high-speed nonlinear optical element, and the whole system can be made very simple. Therefore, all laser light sources such as solid-state / semiconductor lasers and their laser sources and their It can be expanded to integrated devices, and in the future it will be possible to generate high-power ultrashort pulsed light over a wide wavelength range.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a waveform diagram showing an example of output pulsed light in the device of FIG.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

4 is a diagram showing spectra of pulsed light before and after amplification obtained by the apparatus of FIG.

FIG. 5 is a waveform diagram showing an example of gated pulsed light.

[Explanation of symbols]

1 ... Laser medium, 2 ... Total reflection plane mirror, 3 ... Output plane mirror, 4 ... High dispersion Brewster prism pair, 5 ...
Saturable absorption dye jet, 6 ... Wavelength selection element, 7, 8,
9, 10 ... Concave mirror, 11 ... Fabry-Perot type laser resonator, 12 ... Excitation laser light source, 13 ... Amplification laser light source, 14 ... Condensing lens, 15, 16, 17 ... Mirror, 18 ... Acousto-optic element, 19 ... Diffraction grating pair, 21 ...
Continuous light, 22 ... Pulse train to be amplified, 23 ... Excitation pulse light for amplification, 24 ... Output pulse light, 25 ... Gated pulse light, 26 ... Compressed pulse light.

Claims (2)

[Claims] 1. A Fabry-Perot type laser resonator including a laser medium, a pair of prisms for phase compensation and a saturable absorbing medium is used as a light source to be amplified, and short pulse light generated from the laser light source for amplification is used as a laser in the light source to be amplified. A short pulse light functional device characterized in that the laser medium is caused to act as a high gain amplifying medium by focusing on the medium. 2. The laser medium in the light source to be amplified is caused to act as a high-speed nonlinear optical element.
Short pulse light functional device described.