JP2007019056A - Tunable laser resonator and wavelength sweeping method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tunable laser resonator, along with its wavelength sweeping method, in which wavelength width is narrower than conventional one. <P>SOLUTION: The resonator is arranged with a double diffraction gratings to enhance dispersion for a narrower oscillation wavelength width. Wavelength sweeping is performed by rotation around the support point of the diffraction grating and translation of a total reflection mirror synchronized with it. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a wavelength tunable laser resonator using two circuit gratings and a wavelength sweeping method thereof.

Conventionally, there is one in which a resonator is configured by using a single diffraction grating and folding back the diffracted light by a total reflection mirror (see, for example, Patent Document 1).
Hereinafter, a conventional resonator will be described with reference to FIG. The resonator is constituted by a total reflection mirror 3, a diffraction grating 1, and a wavelength selection total reflection mirror 6 with a laser medium 5 as a center. Conventionally, the wavelength selecting total reflection mirror 6 has been used for the purpose of selecting the wavelength of the diffracted light from the diffraction grating 1 and returning it to the diffraction grating 1 again (see, for example, Patent Document 1).

That is, in the resonator shown in FIG. 2, the light from the light source is incident on the wavelength tunable solid-state laser element that is a laser medium, and the wavelength selection of the obtained excitation light is performed by the wavelength selection total reflection mirror through the diffraction grating. However, it is described that a single longitudinal mode laser beam is oscillated. The single longitudinal mode laser light has a single stable longitudinal mode and a stable spectrum with a narrow energy width that can selectively excite a small energy width.
JP-A-4-33365

  In the conventional method, the oscillation wavelength width is wide and single longitudinal mode oscillation is not easy. The present invention is intended to solve the problems of such a conventional configuration, and aims to narrow the oscillation wavelength width and more easily realize single longitudinal mode oscillation. .

  In the present invention, as shown in FIG. 1, for the purpose of reflecting the diffracted light of the diffraction grating 1, by using the diffraction grating 2 instead of the conventional wavelength selecting total reflection mirror, the oscillation wavelength width is narrow, and Single longitudinal mode laser light is oscillated. Further, the wavelength sweep (continuously changing the wavelength of the oscillation light from the resonator) causes the total reflection mirror 3 to rotate at the wavelength in synchronization with the rotation about the fulcrum C of the diffraction grating 2 and the rotation. The total reflection mirror is moved to a position that satisfies the diffraction angle condition of the diffraction grating 1 and the diffraction grating 2 and satisfies the conditions to be satisfied by the single longitudinal mode laser beam at the same time. .

  The diffraction angle condition is a condition that the diffraction angles of the two diffraction gratings should satisfy at a certain oscillation wavelength λ, and are the conditions of the following formulas (1) and (3). By satisfying the two conditions by changing L2 (see FIG. 4) so that the condition of the following expression (4) is always satisfied by the following expressions (1) and (3) and the conditions at a certain wavelength λ. is there. Therefore, the oscillation wavelength is determined by the rotation angle of the diffraction grating 2, and when the wavelength is swept and changed by the rotation of the diffraction grating 2, when the diffraction grating 2 is at a certain rotation angle, the oscillation wavelength is a wavelength determined by the angle. In order to satisfy the mode condition required for this wavelength, the length of the resonator is changed by a moving mechanism.

The technical background of the present invention will be described as follows.
(About the narrow oscillation wavelength)
The dispersion relationship between the diffraction angle of the diffraction grating, which is a wavelength dispersion element, and the wavelength of incident light is generally expressed by the following equation.
[Equation 1]
d (sin α + sin β) = mλ (1)
d: diffraction grating period m: diffraction order λ: wavelength α: incident angle of incident light β: diffraction angle
In the present invention, as shown in FIG. 4, compared to the case where a conventional reflector is used, the use of the second diffraction grating further increases the degree of dispersion and allows the wavelength to become a standing wave in the resonator. Can be made narrower. That is, the second diffraction grating further increases the dispersion of the wavelength, and the wavelength width constituting the standing wave in the resonator is narrowed, whereby the width of the output light can be narrowed.
(About single vertical mode)
When the length of the resonator is L and the wavelength of incident light is λ, the relationship between these longitudinal modes is expressed by the following equation.
[Equation 2]
L = n · (λ / 2) (2)
n is an integer. The longitudinal mode is at an interval c / 2L as shown in FIG. In this case, if the oscillation wavelength width is wide as shown by the broken line in FIG. 3B, the obtained oscillation light has a plurality of modes and the wavelength width becomes wide as shown by the broken line in FIG. . However, by narrowing the wavelength width as shown by the solid line in FIG. 3B, the oscillation wavelength can be limited to one mode as shown by the solid line in FIG.
(About geometric relationships)
With respect to the second diffraction grating, as shown in FIG. 4, the angle of incident light and outgoing light is the same γ, so the relationship between it and the wavelength λ of incident light is expressed by the following equation.
[Equation 3]
2d · sinγ = m′λ (3)
m ′: Diffraction orders (1) and (3) are taken together, and the conditions for obtaining the diffraction angles β and γ of light having a wavelength λ incident at a certain incident angle α are the geometric conditions of the diffraction grating. . When the wavelength is changed, the rotational angle θ of the second diffraction grating required for the wavelength change also has a geometrical relationship.
(About movement of the total reflection mirror)
As shown in FIG. 4, the condition of the single longitudinal mode is that the sum of the resonator lengths L1 and L2 is the mode condition at the wavelength λ, as in the equation (2).
[Equation 4]
L1 + L2 = n · (λ / 2) (4)
Is to satisfy.

On the other hand, the distance of L2 is changed
[Equation 4 ']
L2 = n · (λ / 2)-L1 (4 ')
By changing so as to satisfy the condition, the condition of the single vertical mode can always be satisfied.

  For the purpose of reflecting the diffracted light of the diffraction grating 1 in the present invention, a tunable laser light source having a narrow wavelength width can be provided by using the diffraction grating 2 instead of the conventional total reflection mirror for wavelength selection.

Hereinafter, an embodiment of the present invention will be described with reference to FIG.
The resonator is composed of a total reflection mirror 3 around a laser medium 5 and a diffraction grating 1 and a diffraction grating 2 which are wavelength dispersion elements. Laser light obtained from the laser medium is incident on the diffraction grating 1. The first-order diffracted light from the diffraction grating 1 is incident on the diffraction grating 2, and the diffracted light is returned to the diffraction grating 1 again to oscillate a single longitudinal mode laser beam with a narrow oscillation wavelength width.

  The wavelength sweep of the resonator is performed by rotation around the fulcrum C of the diffraction grating 2. At this time, the geometrical relationship between the diffraction angle and the rotation angle of the diffraction grating 1 and the diffraction grating 2 at each wavelength is satisfied, and at the same time, a resonator from the total reflection mirror to the diffraction grating 2 through the diffraction grating 1. By moving the total reflection mirror 3 in synchronization with the rotation of the diffraction grating 2 by the total reflection mirror moving mechanism 4 so as to satisfy the condition that the length is an integral multiple of half the oscillation wavelength, a single longitudinal mode is obtained. Sweeps the wavelength of the resonator that maintains

  Using a titanium sapphire crystal as a laser medium, two 1800 diffraction gratings / mm diffraction gratings and a total reflection mirror having a central reflection wavelength of 800 nm were used to form a resonator similar to that shown in FIG. The resonator length was about 7.8 cm. A single longitudinal mode oscillation could be achieved at an oscillation wavelength of 811 nm by excitation with the second harmonic of the yag laser. At this time, an oscillation light output of about 1.1 mJ was obtained with an excitation light input of about 25 mJ.

  The present invention is used as a wavelength tunable light source for isotope separation, high resolution spectroscopy, uranium enrichment, separation and recovery of spent nuclear fuel fission products, and the like.

It is the top view which showed the structure of the resonator of this invention. It is the top view which showed the structure of the conventional resonator. It is a figure which shows the principle ((addition)) of single longitudinal mode oscillation. It is a figure which shows the rotation angle of the diffraction grating of the resonator of this invention, and the angle of diffracted light.

Explanation of symbols

1: First diffraction grating 2: Second diffraction grating 3: Total reflection mirror 4: Total reflection mirror moving mechanism 5: Laser medium 6: Total reflection mirror for wavelength selection

Claims (4)

  A resonator that uses, as a wavelength dispersion element, a first diffraction grating arranged obliquely and a second diffraction grating for diffracting the diffracted light.   The laser medium is composed of a total reflection mirror, a first diffraction grating, and a second diffraction grating. The laser light obtained from the laser medium is incident on the first diffraction grating, and the first-order diffracted light from the first diffraction grating is the first. 2. The resonator according to claim 1, wherein a laser beam having a narrow oscillation wavelength and a single longitudinal mode is oscillated by entering the second diffraction grating and returning the diffracted light to the first diffraction grating 1 again.   A wavelength sweeping method characterized in that wavelength sweeping and longitudinal mode maintenance are enabled by rotating the second diffraction grating about its fulcrum and simultaneously moving the total reflection mirror by a moving mechanism. The resonator is composed of a total reflection mirror, a first diffraction grating, and a second diffraction grating with the laser medium as the center, and laser light obtained from the laser medium is incident on the first diffraction grating, and the first time from the first diffraction grating. The folded light is incident on the second diffraction grating, and the diffracted light is returned to the first diffraction grating again. At this time, the diffraction angles of the first diffraction grating and the second diffraction grating and the rotation angles of the second diffraction grating at the respective wavelengths. So that the resonator length from the total reflection mirror through the first diffraction grating to the second diffraction grating is an integral multiple of half the oscillation wavelength. 4. The method according to claim 3, wherein the total reflection mirror is moved in synchronism with the rotation of the second diffraction grating by the moving mechanism to oscillate a laser beam having a narrow oscillation wavelength width and a single longitudinal mode.

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