JP2000058976A - Saturable absorption reflector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a wide band semiconductor saturable absorption reflector having high reflectance by additionally providing a thin film of a dielectric substance transparent to light in a specified wavelength range and exhibiting refractive index lower than that of a semiconductor multiplayer film structure for the light in the specified wavelength range between a metal face and a semiconductor multilayer film structure. SOLUTION: A transparent substance 7 having different refractive index is arranged between the metallic reflective film 3 of a wide band semiconductor saturable absorption reflector and semiconductor layers 4, 5, 6 in order to enhance reflectance. The transparent substance 7 must be transparent over the entire wavelength range of optical spectrum in a pulse laser utilizing a wide band semiconductor saturable absorption reflector. In order to attaing a significant effect, refractive index of the transparent substance must be sufficiently lower than that of the semiconductor layers 4, 5, 6 and sufficiently higher than that of the metal. A substance having such a refractive index can be selected generally among so-called dielectrics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a saturable absorption characteristic, that is, an increase in light intensity, in an industrial field in which a device for oscillating, amplifying, and controlling a short pulse laser is manufactured and used. The present invention relates to a saturable absorbing mirror having a characteristic that the rate temporarily decreases.

[0002]

2. Description of the Related Art The present invention generally relates to a broadband semiconductor saturable absorber.
r Mirror). Hereinafter, a conventionally manufactured broadband semiconductor saturable absorption mirror will be described.

[0003] Among laser devices, a device called a pulse laser is characterized in that laser light is emitted intermittently and autonomously, and is widely used for many purposes. . In a continuous wave laser, laser light is uniformly distributed in an optical resonator, whereas in a pulse laser, light reciprocates in a resonator as a group. Normally, the continuous oscillation state is more stable, so that a mechanism that promotes a group of lights must be added in order to realize a pulsed laser state. To date, several mechanisms have been proposed and realized, one of which is to install a saturable absorber inside the resonator.

[0004] A saturable absorber is a substance that functions as an absorber when low-intensity light is incident, but saturates its ability as an absorber when high-intensity light is incident, and functions as a transparent body. When the saturable absorber is present in the optical resonator, the light loss in continuous oscillation, in which low-intensity light is uniformly distributed in the resonator, increases, while a pulse in which a group of high-intensity light reciprocates. The light loss in the oscillation state is small, and the pulse oscillation state is promoted. In the pulse oscillation state, the spectrum of the laser beam is broadened, but the saturable absorber must exhibit absorption characteristics in at least half of the wavelength range of the spectrum, and preferably in the entire range.

[0005] The principle of saturable absorption is that molecules, atoms, electrons, etc. in the absorber are directly responsible for light absorption, and the state after light absorption has no absorption ability, and the state after light absorption. It takes a certain amount of time to relax to the state before absorption. Therefore, although the saturable absorption characteristics themselves are widely seen, an appropriate substance is selected according to the requirements of light intensity, absorption wavelength, absorption characteristic recovery time, and the like necessary for causing saturable absorption. One of the desirable substances in the visible to near infrared region is a semiconductor.

FIG. 4 shows the structure of a conventional broadband semiconductor saturable absorbing reflector (R. Fluck et al., OPTICS LETTERS Vol. 2).
4, 743 (1996)). This reflecting mirror was developed for a pulse laser having a center wavelength of 760 nm. This reflecting mirror comprises a metal reflecting film 3 made of silver and a semiconductor layer joined to the upper surface thereof. Silver is the substance having the highest reflectance in the wavelength range of interest, and is considered to have been employed to increase the reflectance of the reflector. The semiconductor layer has a structure in which gallium arsenide as the saturable absorption semiconductor layer 5 is sandwiched between aluminum arsenide as the transparent semiconductor layer 4 and gallium aluminum arsenide as the transparent semiconductor layer 6.

To manufacture this reflector structure, a crystal growth semiconductor substrate (not shown) made of gallium arsenide is prepared, and a transparent semiconductor layer 6, a saturable absorption semiconductor layer 5, and a transparent semiconductor layer The crystals are grown and stacked in the order of 4, and
The metal reflection film 3 is deposited by a vacuum deposition device. Thereafter, the surface of the sample is adhered to the reflector holding substrate 1 by the adhesive layer 2, and mechanical polishing and composition-selective dissolution with a solution are performed to remove all the substrates, thereby obtaining a desired reflector structure.

When the above-mentioned broadband semiconductor saturable absorbing mirror is used as a mirror at one end of a laser resonator, light in the resonator is reflected by the above-mentioned mirror, and the reflectance is the same as that of the above-described embodiment. According to the example, 93 93
About 94%, and as the light intensity increases, the reflectivity becomes
It is improved by about 3.5% and becomes about 96 to 96.5%. The reflector at the other end of the resonator is used to output light to the outside.
Assuming that there is a% reflection loss, amplification of 7-8% for low intensity light and 4.5-5% for high intensity light must occur during the passage of light through the laser medium. In a normal continuous oscillation state, the electron-hole pairs generated by light absorption in the saturable absorption semiconductor layer 5 can quickly recombine and absorb light again, so that absorption saturation does not occur. On the other hand, when the light in the resonator reciprocates in a group of pulses, the instantaneous intensity of the light exceeds 10,000 times that of continuous oscillation. The electron-hole pairs cannot recombine within the time when the light pulse passes, and absorption saturation occurs. In this case, the reflectivity of the reflecting mirror is 93 to 94% for the front end of the light pulse, but is improved to 96 to 96.5% for the center and the rear end of the light pulse. Higher reflectance can be achieved as compared with the case of continuous oscillation light.

[0009] While the continuous oscillation state is relatively suppressed by the difference in the reflectivity, the pulse oscillation state is relatively promoted, so that a pulsed laser can be realized.

[0010]

However, the pulse laser realized by the above method has a problem that the output is relatively small and the minimum energy required for oscillation is large. Specifically, even with high intensity light, there is a loss of 3.5 to 4% upon reflection, and an amplification factor enough to overcome the loss must be realized in the laser medium. The energy has to grow. Further, for the same reason, the lasing threshold of the laser, that is, the minimum energy required for lasing is increased.

In the above conventional example, a sapphire crystal in which titanium atoms are dispersed in a laser medium (commonly known as titanium sapphire) is used. However, this laser medium is a crystal capable of realizing a high amplification effect, and has the above-mentioned drawbacks. Although this may cause a decrease in the performance of the device, it does not hinder the realization of the laser device.

However, the laser medium must be selected according to the required center wavelength of light, and titanium sapphire can be used only in the center wavelength range of 700 to 900 nm. Since titanium / sapphire is a material well known for its high amplification efficiency among various laser media, if another laser medium is selected, the above problem becomes extremely serious.

In order to solve this problem, it is indispensable in a broadband semiconductor saturable absorption mirror to improve the reflectance when the light intensity is high. In the above conventional example, the reflectivity for high-intensity light is 96 to 96.5%, but it can be confirmed by calculation that 98% is the limit.
The discrepancy between the values of the calculation and the example is presumed to be due to problems such as surface flatness in the example, and it is suggested by the calculation, though it is likely to be improved by a more sophisticated manufacturing process. The problem is that the theoretical limit is less than 100%. It is known that a semiconductor reacts with a metal such as gold or silver even at a low temperature of about 400 ° C. If this fact reduces the reflectance of the above-described conventional mirror, the reflectance is reduced. It is difficult to improve the reflection mirror, and even if this is not the case, the durability and reliability of the reflector are reduced.

[0014]

In order to achieve the above object, according to the present invention, as shown in FIG. 1, between a metal reflection film 3 of a broadband semiconductor saturable absorption mirror and semiconductor layers 4, 5, 6; A transparent substance 7 having a different refractive index is arranged to improve the reflectance. The transparent material layer 7 must be transparent over the entire wavelength range of the light spectrum of the pulse laser using the broadband saturable absorbing reflector. In order to obtain a remarkable effect, the refractive index of the transparent material is limited to the semiconductor layers 4, 5,
6, must be sufficiently smaller than the refractive index of the metal. The refractive indices of the semiconductor layers 4, 5, 6 are gallium arsenide, aluminum arsenide,
Indium arsenide and their mixed crystals have a refractive index of 3 to 4 in the visible to near-infrared region. On the other hand, the refractive index of metal, which is generally used in gold and silver, is less than 2 μm in wavelength. 1 or less. Therefore, it is desirable that the transparent substance 7 has a refractive index of about 1.5 to 2 in a visible to near infrared region. Substances having such a refractive index value can be selected from those generally called dielectrics.

In determining the thickness of the transparent material layer 7,
It is most appropriate to obtain the reflectance of a composite system of a metal layer, a dielectric layer, and a semiconductor layer by calculation, and design the reflectance so as to maximize the value.

As a method for approximately determining the thickness of the transparent material layer 7, the metal layer is assumed to be a layer having a low refractive index and not absorbing light, and the above-described virtual transparent layer is considered. And, when the layer of the intermediate refractive index and the layer of the high refractive index value are stacked in that order on the layer of the low refractive index, the reflectance becomes maximum, and the thickness of the two stacked layers is obtained. 2
It can be concluded that the thickness of each layer is an odd multiple of 1/4 wavelength. Although an actual metal has a large light absorption coefficient, a sufficiently high reflectance can be achieved by using the thickness determined in this way, though not optimally.

As the above-mentioned dielectric substance, the use of silica, ie, SiO ₂ , for which application to a dielectric laminated film has been established and a stable film can be formed, is not required unless other restrictive conditions cause an obstacle. Is the right way. Also, if the dielectric substance is selected to have a strong adhesiveness to a metal thin film, it is appropriate to use alumina having a proven track record as a tough protective film on the metal surface, that is, Al ₂ O _3. Is the way.

As described above, according to the present invention, the reflectance of the broadband semiconductor saturable absorbing reflector is increased by adding the transparent dielectric layer 7. In calculating the reflectance, even if the transparent semiconductor layer 4, the saturable absorption semiconductor layer 5, and the transparent semiconductor layer 6 are replaced by one average transparent semiconductor layer, the refractive indexes of the semiconductors are close to each other. Calculation can be performed with high accuracy. As a method of increasing the reflectivity of such a multilayer structure, both the average transparent semiconductor layer and the transparent dielectric layer 7 have a thickness of 1/4 wavelength.

In order to obtain a higher reflectivity, the thickness of the average transparent semiconductor is fixed to a thickness of 1/4 wavelength, and then the reflectivity is maximized by changing the thickness of the transparent dielectric. The thickness may be determined, and the transparent dielectric layer 7 may be formed according to the thickness.

[0020]

FIG. 2 shows an element sectional structure of an embodiment of the broadband semiconductor saturable absorption mirror according to the present invention. In the element cross-sectional structure, a substrate for explaining a formation process from semiconductor crystal growth is shown, but after the element is formed, the element becomes an extremely thin film so as to avoid that robustness and flatness cannot be maintained. The reflector holding substrate 1 and the adhesive layer 2 for bonding the same are omitted.

The semiconductor layer has a structure in which a saturable absorption semiconductor layer 5 is sandwiched between a transparent semiconductor layer 4 and a transparent semiconductor 6. An anti-reflection coating can be applied to the upper part of the transparent semiconductor layer 6. In the present embodiment, 1.3 μm was selected as the center wavelength of the reflected light, so that the saturable absorbing semiconductor layer 5 was called indium gallium arsenide, and the transparent semiconductor layers 4 and 6 were selected so that the lattice constant matched it. Further, the transparent semiconductor layers 4 and 6 are made of indium aluminum arsenide. In forming the transparent semiconductor layers 4 and 6 and the saturable absorption semiconductor layer 5, indium phosphide, which is a crystal growth substrate 8 having the same lattice constant as both semiconductors, is used to grow the etch stop layer 9. And a method of growing a transparent semiconductor layer and a saturable absorption semiconductor layer.

The reason why the metal reflection film 3 is formed in a thin film state is because of the manufacturing process of the reflection mirror. That is, in order to fabricate the reflector, the transparent semiconductor layer 6, the saturable absorption semiconductor layer 5, and the transparent semiconductor layer 4 are grown and laminated in this order on the surface of the semiconductor substrate for crystal growth made of indium phosphide. Further, a metal (gold) reflection film 3 is deposited by a vacuum deposition apparatus. Thereafter, the sample surface is adhered to a reflector holding substrate (not shown), and mechanical polishing and composition-selective dissolution using a solution are performed to remove all the substrates, thereby obtaining a target reflector structure. At this time, the substrate is removed using an etching solution having a high etching rate with respect to indium phosphide and a small etching solution with respect to the etch stop layer 9, and further having a high etching rate with respect to the etch stop layer 9, and a transparent semiconductor layer. The etching solution is changed to a smaller etching solution and etching is further continued to obtain a reflector structure. In this step, the element is designed based on the judgment that the thin film forming step is most appropriate for forming the metal reflecting film 3 on a smooth metal surface for reflecting light. . The transparent semiconductor layer 4 and the transparent semiconductor layer 6 may have the same composition, or may have different compositions as in the prior art. However, the composition of the transparent semiconductor layer 6 is determined by judging whether the etch stop layer 9 can be dissolved sufficiently quickly in the solution dissolving step and a solution having a property that the dissolution rate of the transparent semiconductor layer 6 is sufficiently low can be obtained. Should be determined.

FIG. 3 shows the wavelength dependence of the reflectance of the broadband semiconductor saturable absorption mirror according to the present invention in comparison with the reflectance of the conventional broadband semiconductor saturable absorption mirror. is there. In the calculation of the reflectivity, the conventional structure is designed to have a thickness of a half wavelength over the entire semiconductor layer so that the highest reflectivity can be achieved. FIG.
The horizontal axis represents the wavelength of the incident light, and the vertical axis represents the reflectance. The dashed line in the figure indicates the reflectance of the conventional broadband semiconductor saturable absorption mirror when there is no saturable absorption semiconductor layer. The solid line indicates the reflectance of the broadband semiconductor saturable absorption mirror according to the present invention when no saturable absorption semiconductor layer is present. Thus, the improvement of the reflectance according to the present invention is remarkable. The broken line and the dotted line in the figure are the reflectivity when the saturable absorbing semiconductor layer having the absorptance of 1% and 2% exists in the broadband semiconductor saturable absorbing mirror according to the present invention, and of course, in the case of the conventional example. Comes close to the reflectance,
Even in the conventional example, when the saturable absorbing semiconductor layer is present, the reflectance is reduced, so this is not a drawback of the present invention.

[0024]

As described above, according to the present invention, a broadband semiconductor saturable absorption mirror having a high reflectance can be manufactured, and the oscillation threshold of a laser using the broadband semiconductor saturable absorption mirror can be reduced. Remarkable effects of reduction, improvement of output, and improvement of efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a cross-sectional structure of a broadband semiconductor saturable absorption reflecting mirror of the present invention.

FIG. 2 is a cross-sectional structure of a 1.3 μm band broadband semiconductor saturable absorption reflecting mirror element as one embodiment of the present invention.

FIG. 3 is a diagram showing a comparison between the reflectance of a 1.3 μm band broadband semiconductor saturable absorption reflector according to one embodiment of the present invention and the reflectance of a conventional broadband semiconductor saturable absorption mirror of a conventional structure.

FIG. 4 is an explanatory diagram showing a cross-sectional structure of a conventional broadband semiconductor saturable absorbing reflector.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 reflector holding substrate 2 adhesive layer 3 metal reflection film 4, 6 transparent semiconductor layer 5 saturable absorption semiconductor layer 7 transparent dielectric layer 8 crystal growth substrate 9 etch stop layer

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[Procedure amendment]

[Submission Date] May 20, 1999 (1999.5.2
0)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Taro Itaya 1-1-4 Umezono, Tsukuba, Ibaraki Pref. Inside the Institute of Electronics and Technology (72) Inventor Takeyoshi Sugaya 1-4-1, Umezono, Tsukuba, Ibaraki (72) Inventor Kenji Torizuka 1-1-4 Umezono, Tsukuba, Ibaraki Prefecture F-term (reference) 5F072 AB13 FF03 FF04 FF08 RR01 5F073 AA53 AA83 CB20

Claims (4)

[Claims] 1. A smooth metal surface for reflecting light,
A semiconductor multilayer structure including a semiconductor thin film having a saturable absorption characteristic for light in a predetermined wavelength range, and after the incident light in the predetermined wavelength range passes through the semiconductor multilayer structure, the semiconductor multilayer structure A saturable absorption mirror that is reflected by a metal surface adjacent to the metal surface, between the metal surface and the semiconductor multilayer structure, is transparent to light in the predetermined wavelength range and has a refractive index for light in the wavelength range. Wherein a thin film of a dielectric substance smaller than the semiconductor multilayer structure is added. 2. The method according to claim 1, wherein the thickness of the dielectric material is 1/4 wavelength with respect to light having a central value in the predetermined wavelength range, or
2. The saturable absorption mirror according to claim 1, wherein a thickness of (1 + 2n) / 4 wavelengths is selected with n being an arbitrary positive integer. As claimed in claim 3, wherein the dielectric material, silica, ie claim 1 or 2, characterized by using the SiO ₂
The saturable absorption mirror according to 1. 4. The saturable absorption mirror according to claim 1, wherein alumina, that is, Al ₂ O ₃ is used as the dielectric material.