JP2000187108A - Small-sized laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an inexpensive, high speed and small-sized wavelength laser by using a photonic crystal formed by a grating in place of a resonator mirror. SOLUTION: A photonic crystal formed by a grating in place of a resonator mirror is used. Synchronization with exciting light is not executed in a stage wherein dye laser beams of picosecond or femtosecond order pulses are generated, and resonance is repeated within the time of one pulse width. For this reason, a resonator length should be set shorter than the distance wherein light advances in picoseconds or femtoseconds. Thereupon, a laser medium containing material 4 is held with two diffraction gratings which face across a distance within 1 mm, or one mirror and one diffraction grating which face across a distance within 1 mm and excited with a laser 2. For example, excitation with laser 2 can easily be realized by using photonic crystals using the grating as the mirror. Consequently, even if synchronization with the exciting light is not satisfied, high speed dye laser oscillation can be obtained, and the device can be miniaturized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high-speed small laser.

[0002]

2. Description of the Related Art In recent years, as research on optical communication, optical recording, nonlinear optical materials, sensors, and the like has progressed, high-speed phenomena of material properties have been regarded as important, and femto (10 ⁻¹⁵ ) seconds and pico (1
^0-12 ) Many optical properties in the time domain on the order of seconds have been studied. For this reason, lasers with time widths on the order of femtoseconds and picoseconds have become increasingly important,
The number of types and products that are marketed and developed are increasing.

On the other hand, a dye laser is a very useful device for converting the wavelength of a certain laser beam, and the wavelength of a single dye can be changed within a certain range. Can be oscillated. The oscillation wavelength of a gas or solid laser medium is determined, and a dye laser is often used to obtain laser light of another wavelength. In recent years, Ti: sapphire lasers have become widespread as solid-state tunable lasers, but the oscillation range is limited to 700 nm to 1 μm. Although there is a method of expanding the oscillation wavelength range by using optical parametric oscillation and second and third harmonics, it is actually expensive and very difficult to handle.

In the case of a continuous light oscillation (CW) dye laser or a pump light having a wide pulse width of about (10 ⁻⁹ ) seconds or more, if the cavity mirror spacing of the dye laser is several cm to several tens cm, the inside of the resonator may be reduced. Since light reciprocates at least several times, oscillation is established. This is because the speed of light in the air is about 3
Considering that it is × 10 ⁸ m / s, it can be easily understood. However, picosecond and femtosecond laser dye lasers are difficult to reciprocate within a practical resonator length within one pulse time width, and must be synchronized with the excitation pulse light. In many cases, measurement of optical properties requires that the pulse interval be long to some extent. For example, oscillation at an interval of 12n (nano) sec, that is, 82 mega (10 ⁶ )
Hz is often used. In this case, the resonator spacing is about 1.8 m. Assuming that the resonator length is L, the frequency is f, and the high speed is c, it is easy to understand if L = c / 2 · f. In other words, if the excitation laser beam is 82 MHz, the dye laser also has a frequency of 82 MHz, and the cavity length must be about 1.8 m. Therefore, pico and femtosecond lasers are 2m
It will be close in size. Conversely, when synchronization is performed with a resonator length of 2 cm, for example, 7.5 G (giga = 10
⁹ ) Hz, the pulse interval is about 130 psec, and the repeated pulse interval is too short to be suitable for application.

[0005]

As described above, it is difficult to miniaturize a conventional wavelength conversion laser such as a dye laser having a picosecond or femtosecond pulse width.

An object of the present invention is to provide an inexpensive, high-speed, small-sized wavelength conversion laser which has solved the above-mentioned problems.

[0007]

The present invention is characterized in that a photonic crystal formed by a grating is used in place of a resonator mirror. Specifically, a picosecond or femtosecond pulse dye laser beam is used. Resonance is repeated within the one pulse width time without synchronizing with the excitation light at the generation stage.

For this purpose, the cavity length must be set shorter than the distance that light travels in picoseconds and femtoseconds.
This is an ordinary Fabry-Pe formed by two mirrors.
It is virtually impossible with a rot resonator. It is noted that this can be easily achieved by using a photonic crystal as a mirror, particularly a photonic crystal using a grating.

An optical pulse having a time width of 100 femtoseconds travels about 30 μm in the air and about 20 μm in an organic solvent at that time, though it depends on the organic solvent. In order to obtain a visible dye laser beam, a photonic crystal having a period of about the wavelength (several hundreds of nm) may be used, which corresponds to a resonator mirror, and the resonator length is set within the pulse time. An effect equivalent to reciprocating on the order of several tens of times can be obtained. As a result, high-speed dye laser oscillation can be obtained even when not synchronized with the excitation light, and the device can be miniaturized.

Next, a photonic crystal and a photonic band will be described. (See, for example, E. Yablono
pitch, J.M. Opt. Soc. Am. B
10 , 283 (1993); Toshihiko Baba, Solid State Physics 3
2 , 859 (1997)) The photonic band can be well understood when compared with an energy band formed by electrons in the crystal, particularly a band formed by an electron wave function such as a conduction band of a semiconductor or a metal band. These electrons in the crystal form a relationship between the wave vector k and the energy E by Bragg reflection at the crystal lattice point. Similarly, the photonic band is formed by periodically arranging two or more kinds of substances having different refractive indexes. Light is Bragg-reflected by these materials with different refractive indices and forms a band in the same way as an electron wave. A system that forms a photonic band is a photonic crystal.
That is, 2 having a different refractive index from the photonic crystal
At least one substance is arranged in a periodic manner as if it were a crystal, and the period is about the wavelength of light. Using photonic band, transmission spectrum, absorption spectrum, reflection spectrum, light progression, diffraction, reflection, scattering, phase velocity,
The group velocity, the refractive index and the like can be formed separately from those of the constituent materials. These change when the arrangement of the photonic crystal or the refractive index of the constituent material changes.

In a photonic crystal, the angle of diffraction varies depending on the wavelength of light traveling, and at a certain wavelength, the light travels as it is, and at a certain wavelength, it is reflected backward. That is, the effect of diffraction in the traveling direction of light in the photonic crystal is the same as that of the wavelength selection mirror. Therefore, the photonic band can be used as a resonator effect in a laser oscillator. If the wavelength at which the excited electrons originally emit light is a wavelength at which the electrons cannot travel at all in the photonic crystal, the emission is suppressed, and if the electrons can move energetically to the wavelength at which the electrons can travel, they emit light there. That is, the oscillation efficiency also increases.

When a laser medium is inserted in a photonic crystal, the oscillation wavelength of the laser medium can be
Oscillation in which direction and at which wavelength is determined by the transmittance, reflectance, diffraction direction, and diffraction efficiency of light of each wavelength in various directions of the photonic crystal.

The formation of a photonic band can reduce the group velocity of light (the traveling speed of light pulses). Assuming that each frequency of light is ω and the wave number is k, the slope d of the dispersion relation of ω−k
Although ω / dk is the group velocity, dω / dk can be changed, and when dω / dk is small, the progress of the light pulse is slow. When the group velocity is reduced, the interaction between the laser dye molecules and the excitation light is amplified, and the cavity length is substantially shortened, so that the lasing threshold can be reduced in laser oscillation.

In general, photonic crystals are generally difficult to form. When fabricating a photonic crystal that operates in the near-infrared and visible regions, a semiconductor formation process is used, but it has not been possible to fabricate it regularly over many cycles. In addition, when a photonic crystal is used as a laser resonator, a laser medium must be put in the photonic crystal, which is also difficult.

A photonic crystal can be easily formed by using a grating (disclosed in PH 10083005).
It is easy and efficient to face two gratings, but the same effect can be obtained with a combination of a grating and a mirror. When two diffraction gratings are used, the photonic band or the effect of the photonic band depends on the polarization of light. The resonator mirror effect proceeds in a direction perpendicular to the grating lines, and occurs only for light whose polarization is perpendicular to the grating lines. That is, the polarization of the oscillating light also has this polarization direction. In many cases, the excitation light does not transmit when the polarized light and the grating line are perpendicularly incident. Therefore, even when the light is laterally excited (incident perpendicular to the grating surface) or perpendicular to the grating line, the polarized light is parallel to the grating line. Is more convenient. The purpose of facing the grating or two gratings and the mirror is to provide a light confinement effect in this direction. A laser dye-containing polymer is placed in a waveguide shape on one grating, and the oscillated light is It may be confined inside. In this case, the excitation light may be excited from any direction. As the laser medium, a dye solution is used, but a dye-containing polymer may be used as described above.
Inorganic substances such as i-containing sapphire are also applicable.

Although the oscillation direction is also emitted in the direction parallel to the grating surface, it can spread by diffraction. Oscillation is oscillated in two front and rear directions, but if a total reflection mirror is provided on one side, the oscillation direction can be made one direction. However, some air such as air bubbles may be present. Further, since the grating type photonic crystal has polarization characteristics, it is not necessary to insert a polarization selection element. The grating coated with a metal is more convenient because the diffraction efficiency is increased.

In the case of a photonic crystal using this grating, if the light transmission wavelength is band-passed by using the band in which the photonic band is folded first, calculation of the number of grating lines of the grating, etc. In this case, the oscillating wavelength is in the light transmission wavelength region. In a non-bandpass region, light emission itself is limited, and energy is shifted to a transmission wavelength to increase the number of oscillating electrons.

The wavelength to be selected depends on the spacing between the gratings and the refractive index of the dye solution, and it is difficult to shift the wavelength. However, this can be calculated in advance, and when changing the wavelength, the grating may be replaced.

Although the structure is similar to a distributed feedback structure (DFB laser) in a semiconductor laser, since current is injected into a semiconductor and oscillated for the purpose of wavelength selection, the purpose and adaptation method are different. It is.

[0020]

Embodiments of the present invention will be described below based on examples. Example 1 DCM in ethanol (concentration: 5 × 10
^-4 mol / L), two 1800 gratings / mm were arranged facing each other at an interval of 15 μm. 2
The grating lines of the gratings were arranged so as to be parallel to each other. A total reflection mirror is installed on one side of the oscillation light direction,
Efficient oscillation to one side. (FIG. 1). The grating surface is coated with aluminum. The size of the surface of one diffraction grating is 12.5 × 12.5
mm and a thickness of 10 mm. Here, a spot is set to 3 using a cylindrical lens and a convex lens in parallel with the grid line.
The DCM solution between the gratings was irradiated with a second harmonic (wavelength: 532 nm) of a mode-locked YAG laser so as to have a size of 00 μm × 15 μm. The long side of this rectangular spot was parallel to the grating so as to enter between the gratings. The time pulse width (full width at half maximum) of this excitation light laser is 10 psec (10 picoseconds; 10 × 10
^{-1 2} seconds), the excitation energy is 10 pJ.

At this time, a laser beam is emitted in a direction perpendicular to the grid lines.
A shake has occurred. The oscillation wavelength was 645 nm. At this time
The full width at half maximum of the oscillation pulse time was about 8 psec. This
The oscillation pulse width is determined by the excitation pulse time.
The shorter time is due to the non-linear effect. Ma
The polarization of the oscillating light was perpendicular to the diffraction grating. (Embodiment 2) Next, the excitation light is irradiated with a full width at half maximum time of 120 fsec
(100 femtoseconds; 100 × 10 ^-15Seconds) mode
Lock Ti: 2nd harmonic of sapphire laser, wavelength 470
Irradiation was performed by changing to nm. Oscillation also occurred in this case,
The width was about 1 psec. Also, change the excitation pulse width.
The oscillation time width does not change even with the excitation of 70 fsec.
Almost never. Furthermore, the long side of the spot is 300 μm
When it is shortened to 100 μm, the time width is about 300 f
sec. This means that the oscillation time width is sufficiently short for the excitation time
If not, the spot size is determined. Greaty
Oscillations in the excitation spot of the DCM solution between rings
This is because the difference in location determines the oscillation time width.

[0022]

According to the present invention, since a photonic crystal formed by a grating is used instead of a resonator mirror, a high-speed and small-sized wavelength conversion laser can be obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view for explaining a first embodiment of the present invention.

[Explanation of symbols]

 1. 1. Diffraction grating 2. Excitation laser light Oscillation light 4. 4. Dye solution Total reflection mirror

Claims (2)

[Claims] 1. A laser medium containing a laser medium sandwiched between two diffraction gratings facing each other within a distance of 1 mm or one mirror and one diffraction grating facing each other within a distance of 1 mm. A small laser pumped by 2. A small laser in which a laser dye solution, a laser medium glass, or a laser medium polymer is brought into close contact with a diffraction grating, excited by laser light, and oscillates laser oscillation light in a direction parallel to the grating surface.