JPH03283686A - Laser apparatus - Google Patents

Abstract

PURPOSE:To obtain stable and high conversion efficiency by using a single crystal of lithium niobate containing a specific ratio of magnesium as a non- linear optical crystal. CONSTITUTION:A laser apparatus makes semiconductor exciting solid laser light into a non-linear optical crystal as primary laser light to generate a non- linear phenomenon in said single crystal wherein light of a higher order wavelength such as a half of the wavelength (secondary harmonic) of the primary laser light wavelength (fundamental wave) is secondary output light. A single crystal of lithium niobate containing 5at% or less of magnesium is used as this non-linear optical crystal. Thus stable and high conversion efficiency can be realized. Further by eliminating a slight shift between retention temperature of the crystal and retention temperature of semiconductor laser, phase conformity conditions are matched and harmonic waves can be generated with high efficiency.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to the improvement of harmonic generation elements used as short wavelength light sources in fields such as optical recording. The present invention relates to a harmonic generation element with improved characteristics of a lithium oxide single crystal.

[Prior art] In the field of information processing technology, there is a demand for even higher speeds and higher densities in each process such as information transmission, recording, and reproduction. Many applied technologies are beginning to be used. In the future, this speed increase/
It is expected that the trend toward higher density will further accelerate, but in order to achieve this, shorter wavelengths (e.g. 400 to 700
nm), and in practical terms it is a light source of several mW~
A fast-response light source with an output on the order of several watts is needed. .

As such a light source, there is a wavelength variable element that converts the wavelength of laser light. Regarding the conversion method, SHG (Secondary, l
larmonics, Generation (second harmonic generation)) elements are known (for example, Nikkei Electronics, July 14, 1986 (No. 399)),
(Refer to P. 89-90), some of which have recently been commercially available, and are moving toward the stage of practical use.

However, lithium niobate crystals pose a problem of optical damage. That is, in the field of optical transmission η using a light source with a long wavelength of 1.3 μm or 1.55 μm, for example, when a lithium niobate crystal is used in an optical modulator, etc., the energy intensity of the light source is about 10 mW. Unless it is above, the occurrence of optical damage does not pose a major problem in practical use.

On the other hand, when short-wavelength light is used for optical recording, etc., the occurrence of optical damage becomes a serious problem even when the energy intensity of the light source is about 0.1 mW. For this reason, various proposals have been made, one of which has been reported to be a method of reducing optical damage by adding magnesium (D,
A, Bryan, et al, [Appl, Pby
s, Lett, J, Vol, 44, P, 847.
1984).

According to this report, in lithium niobate crystals containing magnesium, the optical damage resistance increases with the amount of magnesium added, and this strength remains constant when the amount of magnesium added is 5 at% or more (hereinafter referred to as at%). It is known that Therefore, when using magnesium-doped lithium niobate crystals in optical elements where optical damage is a problem, the amount of magnesium added should be 5 at% (M
When it is added as gO, it is common knowledge in this field that the amount is 5 mol % or more.

Since wavelength conversion efficiency is proportional to the square of the incident laser power density, it is important to increase the power density as much as possible by confining it in a waveguide or fiber, or using a resonator structure to obtain high output harmonics. Become.

The configuration shown in FIG. 1 has been proposed as a device that achieves phase matching using a Cerenkov radiation method using a waveguide. However, although a conversion efficiency of approximately 1% has been obtained from semiconductor lasers, the problem is that the beam shape of the emitted harmonics is crescent-shaped, making it difficult to collimate it into an easy-to-use beam shape. be. On the other hand, highly efficient wavelength conversion can be achieved by forming an external resonator with an extremely high pitch for the fundamental wave, increasing the electric field at the resonant frequency, and placing a nonlinear optical crystal inside the resonator to generate the second harmonic. This has recently attracted attention, and the device shown in FIG. 2 has been proposed.

This is because phase matching is achieved by heating a monolithic magnesium-doped lithium niobate crystal with a length of 1.25 crri to 107°C in an oven.9 At this time, the conversion efficiency from semiconductor laser light to harmonics is 6.
It has even reached %. However, this structure has not been put into practical use because of the following problems.

■) Since the temperature phase matching method is used for phase matching (matching the refractive index of the fundamental wave and harmonics), which is essential for nonlinear optical effects, precise temperature control is required for phase matching.

2) Since the matching temperature is high, when miniaturizing the device, the temperature of parts around the crystal becomes high, which limits its structure.

[Problems to be solved by the invention] As mentioned above, lithium niobate single crystals are being put into practical use as a material for harmonic generation elements, but in practical use, it is necessary to suppress the occurrence of optical damage and increase conversion efficiency. It is required that the phase matching be made stable and easy to achieve. An object of the present invention is to provide a harmonic generation element using a lithium niobate single crystal that achieves stable and high conversion efficiency and has optical damage characteristics to the extent that there are no practical problems.

[Means for Solving the Problems] In order to achieve the above object, the present invention makes a semiconductor-excited solid-state laser beam enter a nonlinear optical crystal as a primary laser beam to generate a nonlinear phenomenon in the single crystal. , half the wavelength (second harmonic) of the wavelength (fundamental wave) of the first laser beam
A laser device that outputs light at a high-order wavelength as secondary output light, characterized in that the nonlinear optical crystal is a monocrystalline monocrystalline titanium niobate containing less than 5 at% magnesium. It is.

By the way, as mentioned above, when using a magnesium-doped lithium niobate single crystal in an optical element where optical damage is a problem, it is common knowledge that the amount of magnesium added should be 5 at % or more.

However, the present inventors focused on the fact that as the amount of added magnesium increases, grain boundaries occur in the grown crystals and light transmittance decreases near the absorption edge, which impairs their usefulness as optical applications. , conducted various experiments and studies.

As a result, in the case of lithium niobate single crystals for SHG devices, it is expected that the addition of magnesium will increase the photodamage resistance, but contrary to conventional wisdom, adding too much magnesium does not necessarily result in good results. It was discovered that it was not possible to obtain an SHG element with such characteristics. Furthermore, it has been revealed that the optimum amount of magnesium added to generate harmonics exists in a range of less than 5 at%, depending on the wavelength of the fundamental wave used in the laser device and the holding temperature of the nonlinear optical crystal.

Therefore, the amount of magnesium added was increased from a trace amount to 5aL%.
The present invention was achieved by growing lithium niobate single crystals with various amounts exceeding 10%, examining the SHG characteristics of these crystals, and determining the characteristics of a lithium niobate single crystal that is optimal for SHG use.

FIG. 3 is a diagram showing the relationship between magnesium content and phase matching temperature. The three wavelengths shown in the figure as the fundamental light are N
d: YAG, Nd: YLF were generated by a laser and an optical parametric oscillator, and these were incident on a magnesium-doped lithium niobate single crystal, and the temperatures at which harmonics were generated by non-critical phase matching were determined. If the amount of magnesium contained in the crystal changes, the phase matching temperature at which harmonics can be generated changes even if the fundamental wave used is the same, and within this concentration range, the matching temperature increases with the amount of magnesium. Furthermore, if the wavelength of the fundamental light changes, the amount of magnesium that generates harmonics and the value of the matching temperature will change. Even if it is non-critical, the allowable phase matching width is small in the case of lithium niobate single crystals, so it was found that temperature control is essential.

From the above results, depending on the wavelength of the laser beam used in the laser device and the temperature at which the nonlinear optical crystal is maintained,
It can be seen that there is an optimal magnesium content that allows efficient generation of harmonics. Alternatively, when using a lithium niobate single crystal containing a certain amount of magnesium as a nonlinear optical crystal based on the wavelength of the laser beam, it is sufficient to maintain the crystal at a humidity that efficiently generates harmonics. , have a taste.

In particular, when using light generated from a semiconductor laser-excited solid-state laser as the fundamental wave, the temperature of the semiconductor laser must be controlled to suppress variations in the oscillation wavelength of the semiconductor laser. However, the temperature at this time is near room temperature, and is within a temperature range of about ±10° C. from room temperature at most.

The large difference between this temperature and the temperature to be maintained by the nonlinear optical crystal poses a major obstacle in miniaturizing the device. It goes without saying that it is desirable that the control temperatures of the semiconductor laser and the magnesium-added lithium niobate single crystal be the same if possible. Generally, once the type and oscillation wavelength of the solid-state laser are determined, the oscillation wavelength of the semiconductor laser and the temperature to be controlled are determined, and the magnesium content is determined accordingly. From Figure 3, the optimum range for this magnesium is less than 5 at%.6 The present inventors further eliminated the slight difference between the holding temperature of the crystal and the holding temperature of the semiconductor laser, thereby achieving better phase matching conditions and achieving high efficiency. We discovered that a way to generate harmonics in the field is to adjust the refractive index of the crystal using the electro-optic effect.

[Example] Hereinafter, the present invention will be explained in more detail based on Examples.

(Example 1) FIG. 4 shows the first example. Magnesium lat%
From the crystal containing, each edge is parallel to X+V+ and 2
Cut out a square block of OXloXloXlO,
The two y-planes were mirror polished. These bulk samples were excited in the axial direction from one y-plane using a single mode semiconductor laser with a wavelength of 0°809 μm, and a 1.06 μIn Y
AG laser light was applied. At this time, the semiconductor laser was placed on a Bertier element, and its temperature was controlled at 25°C.

Since the non-critical phase matching temperature of a crystal with a magnesium content of lat% is 30°C, this temperature was controlled by placing the crystal on a Vertier element. A harmonic of 1.2 mW was obtained for the laser diode excitation light of 400 mW,
A laser device was fabricated without keeping the crystal at high temperature.

(Example 2) FIG. 5 shows a second example. Excite from the axial direction with a single mode semiconductor laser with a wavelength of 0.809 μm to 1.0
A YAG laser beam of 6 μm was incident. At this time, the semiconductor laser was placed on a Bertier element, and its temperature was controlled at 25°C. A crystal containing 0.8 at% magnesium and having a phase matching temperature of 25° C. was placed in the same temperature control element as the semiconductor laser element. Harmonics 1. OmW can be obtained, a laser device can be manufactured without holding the crystal at a high temperature, it is smaller than Example 1, and it has the advantage of having one temperature control.

(Example 3) FIG. 6 shows a third example. Excite from the axial direction with a single mode semiconductor laser with a wavelength of 0.809μ to 1.0
A YAG laser beam of 6 μITI was applied. At this time, the semiconductor laser was placed on the Bertier element, and its temperature was 25
The temperature was controlled at ℃. A crystal containing 0.8 aL% of magnesium and having a phase matching temperature of 25° C. was placed in the same temperature control device as the semiconductor laser device. Electrodes were formed on both Z planes of the crystal, and slight deviations from matching conditions were corrected by applying voltage. Laser diode excitation light 400mW
Compared to Example 1, a harmonic of 1.4 mW was obtained, the laser device was manufactured without keeping the crystal at a high temperature, and compared to Example 1, it had the advantage of being smaller and having only one temperature control. Note that in this example, the conversion efficiency is not high because the crystal is not installed in the resonator structure, but this can be improved by using the resonator structure.

[Effects of the Invention] As described above, according to the present invention, a laser device using a lithium niobate single crystal that achieves stable and high conversion efficiency and has optical damage characteristics to the extent that there are no problems in practical use can be achieved. Can be provided. The device according to the present invention can be applied in a wide range of fields such as light sources for laser printers, optical start-up light sources, and optical information processors.

[Brief explanation of the drawing]

Figure 1 is a schematic configuration diagram of a laser device using a conventional wavelength conversion element, Figure 2 is a schematic configuration diagram of a laser device using a conventional wavelength conversion element, and Figure 3 shows the relationship between magnesium content and phase matching temperature. FIGS. 4 to 6 are schematic diagrams of the laser apparatuses of the first to third embodiments of the present invention. l... Semiconductor laser, 2... Half-wave plate, 3...
Condensing lens, 4... Optical waveguide, 5... Lithium niobate single crystal, 6... Second harmonic, 7, 15... Semiconductor laser excitation Nd: YAG laser, 8, 14,
...Magnesium-doped lithium niobate single crystal, 9.
...Oven (107°C), 10...Dichroic mirror 11...Harmonic detector, 12...Fundamental wave detector, 13...Feedback control by Pockels effect, 16...
・Temperature controller, 17... Electrode, 18... Mirror 1 Fig. 16 (25°C) 16 (30”C) Fig. 16 (25° C) Figure 6 Procedural amendment (spontaneous) Heisei IT 3/6/128, 1990 Patent Application No. 4258 Relationship to the case of the person amending the name of the invention laser device Patent applicant address Marunouchi, Chiyoda-ku, Tokyo 2-chome 1-2 name (
508) Koji Matsuno, Representative of Hitachi Metals Co., Ltd.

Claims (6)

[Claims] (1) A semiconductor laser-excited solid-state laser beam enters a nonlinear optical crystal as a primary laser beam, and a light with a higher-order wavelength whose fundamental wave is the wavelength of the primary laser beam is used as a secondary output beam. A laser device characterized in that, in the harmonic generation element, a lithium niobate single crystal containing less than 5 at% magnesium is used as the nonlinear optical crystal. (2) The laser device according to claim 1, further comprising means for controlling the temperature of the nonlinear optical crystal. (3) The laser device according to claim 1, further comprising means for controlling the temperature of the semiconductor laser. (4) The laser device according to claim 1, wherein the element for controlling the temperature of the nonlinear optical crystal and the element for controlling the temperature of the semiconductor laser are integrated. (5) Semiconductor laser excitation solid-state laser light is incident on the nonlinear optical crystal as primary laser light, and light with a higher-order wavelength whose fundamental wave is the wavelength of the primary laser light is used as secondary output light. 2. The laser device according to claim 1, wherein in the harmonic generation element, phase matching is performed by non-critical phase matching. (6) A means for changing the refractive index of the crystal or a surrounding medium by an electro-optic effect is provided inside or near the nonlinear optical crystal to make the phase matching condition variable. A laser device as described in any of the sections.