JPH06265834A - Lamination type optical polarization control element for near infrared and visible light region - Google Patents

Abstract

PURPOSE:To provide the optical polarization control element which has the high adhesive power between constituting component films and an excellent product yield and exhibits an extinction ratio and insertion loss having no dependency on wavelengths in near IR and visible light regions. CONSTITUTION:This laminated optical polarization control element for near IR and visible light are alternately laminated with plural dielectric films 1 and semiconductor thin films 3. These semiconductor thin films 3 consist of semiconductor thin film core layers 4 and semiconductor flank layers 5 which include dielectric constituting elements and are arranged on both sides of the core layers 4. More preferably, the dielectric films 1 have a thickness d2 regulated in such a manner that the light propagation in the element indicates only the basic mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light polarization control element for near infrared to visible light. The optical polarization control element is useful for optical devices such as optical isolators and optical switches, and various optical sensors.

[0002]

2. Description of the Related Art An optical polarization control element is one of the most basic optical passive elements, and is an essential element for constructing many optical devices and optical sensors. In recent years, there has been a demand for miniaturization, high performance, and high productivity of optical polarization control elements. To meet these requirements, a multilayer laminated structure obtained by alternately laminating dielectrics and metals or semiconductors. Has been proposed (for example, S. Kawakami, Appl. Opt., Vo.
l. 22, 2426 (1983)).

As shown in FIG. 1, this laminated type optical polarization control element is a multilayer laminated body in which a plurality of dielectric films 1 and a plurality of semiconductor or metal thin films 2 are alternately laminated. When used for optical communication wavelengths of 1.3 μm and 1.55 μm (long wavelength region), the dielectric film 1 is usually made of Si.
The thin metal film is formed of O _2, and the thin metal film is formed of Al.
In this case, the dielectric film has a thickness of 1 to 1.2 μm, and the metal thin film has a thickness of 60 to 70 Å. Such a conventional optical polarization control element has, for example, an optical path length of 3
It shows optical polarization control characteristics with an extinction ratio of 60 dB and an insertion loss of 0.3 dB per 0 μm (Sumitomo Cement Co., Ltd. optical polarization control element catalog).

The optical fiber gyro and the optical sensor (voltage sensor) have a wavelength of 0.85 μm (short wavelength range).
However, if the light polarization control element having the above-mentioned structure is used for this, the insertion loss value of the light will be larger than 1 dB, so that its practical use is difficult.

In order to overcome the above-mentioned difficulties, as a multilayer structure optical polarization control element used in the short wavelength region, S
A combination of an io ₂ film (dielectric film) and a germanium (Ge) thin film (semiconductor thin film) has been proposed (Hama,
Kawakami IEICE Spring National Convention C-273 (1
991), and Ma, Uchida, Kawakami Institute of Electronics, Information and Communication Engineers,
91-47, pp11 (1991)). For example SiO ₂
When the layer thickness is 1 μm and the Ge layer thickness is 60 Å, the optical polarization control element obtained has an optical path length of 1 μm.
Extinction ratio 2.9 dB and insertion loss 0.018 dB per m
Therefore, it is suggested that light polarization control is possible in the short wavelength region.

[0006]

The laminated optical polarization control element for the short wavelength region having the above-mentioned conventional multilayer laminated structure has a small interfacial bonding force between its constituent films, and therefore the total of the multilayered structures is reduced. There is a limit to the film thickness, that is, the total film thickness is generally about 60 μm at the upper limit, and it is difficult to manufacture a thicker film. Further, the conventional light polarization control element having the above-mentioned multi-layer structure, during the polishing process,
The multilayer laminated structure is easily broken, and the product yield is extremely low.

Optical polarization control elements, which generally have a multilayer stack structure, are bonded to the optical fiber core in most of its applications. When the bonding process is mechanized, it is desirable that the multilayer film has a large thickness in order to facilitate the operation. Moreover, in order to achieve high productivity, it is required that the manufacturing yield of the optical polarization control element is high. further,
The optical polarization control element is the most basic element as an optical passive element, and is one of the indispensable constituent elements of many optical devices and optical sensors, so it is desirable to supply it at low cost.

Further, in the conventional laminated type optical polarization control device using a Ge film as a semiconductor film, the absorptance of optical polarization that can be absorbed in the device is largely dependent on the wavelength of the light used. The shorter the wavelength of the incident light is, the larger the insertion loss of the element is, and the higher-order light propagation mode is excited in the optical waveguide of the element, so that the extinction ratio of the element is lowered.

In recent years, there is an increasing demand for an optical polarization control element in an optical sensor represented by an optical fiber gyro and a voltage sensor. As a light source for such an application, a light source having a wide spectrum is used. ing. In addition, the recent remarkable progress in the technology relating to the light source has promoted the shortening of the emission spectrum wavelength. Under the circumstances as described above, it is strongly desired to develop an optical polarization control element having a high extinction ratio and a low insertion loss over a region of 600 to 850 mm.

The present invention has a stable multi-layer laminated structure, its thickness is large, the product yield is high, and it is 600-850 mm.
Another object of the present invention is to provide a laminated optical polarization control element capable of exhibiting a high extinction ratio and a low insertion loss in the near infrared to visible light region.

[0011]

DISCLOSURE OF THE INVENTION As a result of intensive studies for solving the problems of the prior art, the present inventors have found that in a light polarization control element having a multilayer laminated structure, both sides of a semiconductor thin film are
By arranging a specific side surface layer that can be stably joined to the conductor film, the above problem was successfully solved.

The near-infrared or visible light laminated light polarization control element of the present invention is an element comprising a multilayer film in which a plurality of semiconductor thin films and a plurality of dielectric films are alternately laminated. And each of the semiconductor thin films comprises (A) germanium,
Or a semiconductor thin film core layer made of a germanium alloy, and (B) laminated on both sides of the semiconductor thin film core layer, containing one of the elements constituting the dielectric film, and being bonded to the dielectric film. It is characterized by comprising a pair of semiconductor lateral layers.

Further, in the laminated optical polarization control element of the present invention, it is preferable that each of the semiconductor thin film and the dielectric film has a thickness such that light propagation in the element becomes only a fundamental mode.

[0014]

FIG. 2 shows the structure of an example of the laminated light polarization control element of the present invention. In FIG. 2, in this device, a plurality of dielectric films 1 and a plurality of semiconductor thin films 3 are alternately laminated to form a multilayer structure. Each of the semiconductor thin films 3 is a semiconductor thin film core layer 4 made of a semiconductor material such as germanium (Ge) or germanium alloy.
And a pair of semiconductor side layers 5 that are laminated on both sides of the semiconductor thin film core layer 4 and stably join the dielectric film 1 and the semiconductor thin film core layer 4.

The dielectric film 1 is generally composed of a dielectric material such as SiO ₂ , and the semiconductor side surface layer 5 contains one of the elements constituting the dielectric film, for example, one containing Si, that is, Si alone, Or Si alloys or Si compounds, such as Si ₁
-XGex. Therefore, the semiconductor thin film core layer 4 and the pair of semiconductor side surface layers 5 exhibit semiconductor characteristics as a whole and function as a semiconductor thin film.

In order to stabilize the multilayer film structure, it is important to strengthen the bonding force (adhesive force) between different kinds of films. In the present invention, the dielectric film and the semiconductor thin film core layer are firmly bonded via the semiconductor side surface layer. Adhesive force between these different types of films is Kikuchi, Baba, Kanehara, "Vacuum," Vol. 29,
It can be measured and evaluated by the method described in No. 5, p337 (1986).

FIG. 3 shows a dielectric film made of SiO ₂ .
The adhesive force is shown when a semiconductor side surface layer (made of Si) having a thickness of 0 to 30 angstrom is arranged between the semiconductor thin film core layer made of Ge and the semiconductor thin film core layer. In FIG. 3, when there is no semiconductor side surface layer (thickness = 0), SiO ₂
The adhesion force at the / Ge interface is 5 gf. However, when a side surface layer of Si semiconductor having a thickness of 5 angstrom is provided, the adhesive force sharply increases to about 40 gf (about 8 times 5 gf), and the thickness is 1
Above 0 Å, the adhesion saturates at about 50 gf (about 10 times 5 gf).

From FIG. 3, the semiconductor side surface layer made of Si is
It is shown that the thickness of the Si semiconductor side surface layer that functions as a binder between the SiO ₂ dielectric film and the Ge semiconductor thin film core layer and that functions as a binder should be approximately 10 Å or more. ing.

As described above, according to the present invention, it becomes possible to remarkably increase the adhesion between the SiO ₂ dielectric film and the Ge semiconductor thin film core layer, and as a result, a practical laminated structure having a total film thickness of 300 μm or more is obtained. It has become possible to manufacture a type optical polarization control element.

In the laminated optical polarization control element of the present invention,
The thickness (d ₂ ) of the dielectric film is preferably 0.8 to ₁ μm, the thickness (d ₁ ) of the semiconductor thin film is preferably 55 to 100 Å, and the semiconductor thin film core layer in the semiconductor thin film is Is preferably 45 to 60 angstroms, and the thickness of the semiconductor side layer is 10 to 10.
It is preferably about 20 angstroms.

In the optical polarization control element of the present invention shown in FIG. 2, the optical polarization vibrating in the Y direction is a TE wave, and X is
An optical polarization that oscillates in the direction is called a TM wave. Also, the light is d
A semiconductor thin film 3 having a film thickness represented by ₁ and a complex dielectric constant represented by ε _S1 , a film thickness represented by d ₂ and a dielectric film 1 having a complex dielectric constant represented by ε _S2. Let the light propagation constant of the semiconductor thin film 3 be P ₁ and the light propagation constant of the dielectric film 1 be P ₂ when passing through the multilayered optical polarization control element. In this case, the characteristic equations of the element for the polarization components (TE wave and TM wave) that can be absorbed in this optical polarization control element are represented by the following equations (1), (2) and (3), and Z Light propagation coefficient γ (= α
+ Jβ) is known to be represented by the following formula (4) (S. Kawakami, Appl. Opt., Vo).
l. 22, 2426 (1983))

[0022]

[Equation 1]

[Equation 2]

[Equation 3]

[Equation 4]

[0023] In the stacked light polarization control element, in comparison with the absorption coefficient alpha _O for light lowest order mode of the high-order waveguide mode is likely to be excited with smaller absorption coefficient. Such high-order guided modes also have the above (1) to
Since the equation (4) is followed, the absorption coefficient α _n (n =
0, 1, 2, ...) can be predicted by a numerical method from the film thicknesses (d ₁ , d ₂ ) of the component films constituting the device and the optical constants (ε _S1 , ε _S2 ). Is.

When a higher-order guided mode is excited with respect to the TE wave in the element, the TE wave is absorbed less,
Therefore, the extinction ratio of the laminated optical polarization control element is significantly reduced. Therefore, in order to obtain a laminated optical polarization control element having high performance, the film thicknesses of the semiconductor film and the dielectric film are designed so that a higher-order waveguide mode with respect to the TE wave is not excited in the element. Is essential.

When the thickness d ₁ of the semiconductor thin film is fixed (that is, ε _S1 is also determined) and the thickness d ₂ of the dielectric film is changed, one unit of laminated film (1 semiconductor thin film (d ₁ ) +1
The electromagnetic field distribution of the dielectric film (d ₂ ) was analyzed, and as a result,
In the wavelength region of 600 to 850 nm used for the optical sensor, the higher-order mode excited in the device of the present invention is only the first-order mode for the TE wave and the higher-order mode for the TM wave. It became clear that he was not excited.

Therefore, in the present invention, the thickness of the dielectric film of the optical polarization control element is set so that the light propagation in the element is limited to the fundamental mode and the first-order mode of the TE wave is not excited.
It is calculated by the numerical method and incorporated into the device design. Thereby, the laminated light polarization control element of the present invention,
In the case of light having a wavelength of 850 nm, the extinction ratio per optical path length of 30 μm can be set to 54 dB.

When a conventionally proposed Ge single film is used as the semiconductor thin film, the wavelength dispersion of the complex dielectric constant of the Ge thin film is large in the wavelength range of 600 to 850 nm used for the optical sensor, and The shorter the wavelength, the greater the concentration of electromagnetic field on the Ge thin film.
The light absorption rate of the Ge thin film increases, and the optical insertion loss in the device increases.

In the present invention, by laminating the semiconductor lateral layers having a low complex dielectric constant on both sides of the semiconductor central thin film made of Ge, the effective dielectric constant of the entire semiconductor thin film is lowered, and thereby the complex dielectric constant is reduced. It reduces wavelength dispersion and localization of guided modes in semiconductor thin films. By using the semiconductor thin film having the above laminated structure, 600
In the wavelength region of ˜850 nm, the extinction ratio and optical insertion loss of the laminated optical polarization control element of the present invention show almost no wavelength dependence.

[0029]

The present invention will be further described with reference to the following examples. Example 1 and Comparative Example 1 In Example 1, Si is composed of a semiconductor thin film core layer of Ge having a thickness of 60 angstroms, and semiconductor side layers which are arranged on both sides of the core layer and which are made of Si and have a thickness of 10 angstroms. / Ge / Si semiconductor thin film and Si
Dielectric films made of O ₂ and having a thickness of 0.9 μm were alternately laminated to prepare a laminated optical polarization control element. The complex permittivities of the Ge film and the Si film are as follows: Ge core layer: 25.662-j6.460 Si side layer: 9.681-j1.811

The wavelength of this device (600 to 850 nm)
FIG. 4 shows the relationship with the extinction ratio per optical path length of 30 μm.
The wavelength of this device (600-850nm) and the optical path length 3
The relationship with the optical insertion loss per 0 μm is shown in FIG.

In Comparative Example 1, a laminated optical polarization control element was prepared in the same manner as in Example 1. However, the semiconductor thin film was composed of a Ge single layer (thickness: 60 angstrom), and the dielectric (SiO ₂ ) film had a thickness of 1 μm. The relationship between the wavelength, the extinction ratio, and the optical insertion loss of the obtained device is shown in FIGS. 4 and 5, respectively.

As is apparent from FIG. 4, in the conventional laminated optical polarization control element of Comparative Example 1, the extinction ratio decreases as the wavelength becomes shorter. However, in the device of Example 1, the extinction ratio shows a constant value of about 54 dB in the wavelength region of 600 to 850 nm without depending on the wavelength.

Further, as is clear from FIG. 5, Comparative Example 1
In the conventional laminated type optical polarization control element of No. 1, the optical insertion loss increases as the wavelength becomes shorter, but in the element of Example 1, the optical insertion loss depends on the wavelength in the wavelength range of 600 to 850 nm. However, it shows a low constant value of about 0.34 dB or less. The light propagation in the device of Example 1 was only the fundamental mode.

Example 2 A laminated optical polarization control element was produced in the same manner as in Example 1 except for the following matters. (1) The semiconductor side surface layer in the semiconductor thin film is formed of Si ₁ -xGe
x alloy. (2) Complex permittivity Ge core layer: 23.112-j4.36
7 Si ₁ -xGex side layer: 12.254-j2.18
9 (3) Thickness Ge core layer: 50 angstrom Si ₁ -xGex side layer: 10 angstrom semiconductor thin film: 70 angstrom dielectric (SiO ₂ ) film: 0.8 μm The obtained device has a wavelength range of 600 to 850 nm. In the above, a nearly constant extinction ratio of 60 dB and an optical insertion loss of 0.4 dB per 30 μm optical path length were exhibited. The light propagation in the device of Example 2 was only the fundamental mode.

Table 1 shows the interface (dielectric film / semiconductor thin film) adhesion in the optical polarization control elements of Examples 1 and 2, and the extinction ratio and insertion loss per 30 μm of the optical path length for light of wavelength 850 nm.

[0036]

[Table 1]

[0037]

According to the laminated optical polarization control element of the present invention, by providing a semiconductor side surface layer having a low complex dielectric constant and containing a dielectric film forming element between the dielectric film and the semiconductor central thin film layer. , Significantly improves the adhesive force at the interface between films,
As a result, the stability of the multi-layer laminated structure of the device is remarkably improved, the destruction of the laminated structure during the processing of the device is prevented, and the total thickness of 300 μm or more (which corresponds to 5 times or more of the conventional thickness). It became possible to obtain the element of. In addition, by arranging the semiconductor side surface layer, the effective dielectric constant of the light absorption layer is lowered, and therefore, the effect of reducing wavelength dispersion with respect to light absorption and localization of the guided mode to the semiconductor thin film is exhibited. To do. Furthermore, by defining the thicknesses of the semiconductor layer and the dielectric film so that only the fundamental mode propagates, for example, in the wavelength region of 600 to 850 nm, the extinction ratio is 54 dB or more per 30 μm of the optical path length, and the insertion loss is It can be set to a constant value around 0.3 dB.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a laminated structure of an example of a conventional laminated light polarization control element.

FIG. 2 is an explanatory diagram showing a laminated structure of an example of the laminated optical polarization control element of the present invention.

FIG. 3 is a graph showing an example of the relationship between the thickness of the semiconductor side surface layer of the laminated optical polarization control element of the present invention and the interfacial adhesion force.

FIG. 4 is a graph showing the relationship between wavelength and extinction ratio of the laminated optical polarization control elements of Example 1 and Comparative Example 1.

FIG. 5 is a graph showing the relationship between wavelength and insertion loss of the laminated optical polarization control elements of Example 1 and Comparative Example 1.

[Explanation of symbols]

1 ... The thickness of the dielectric film 2 ... semiconductor thin film (prior art) 3 ... semiconductor thin film (the present invention) 4 ... semiconductor thin core layer 5 ... semiconductor side layer d ₁ ... of the semiconductor thin film thickness d ₂ ... dielectric film L … Optical path length

Claims (2)

[Claims] 1. A device comprising a multilayer film in which a plurality of semiconductor thin films and a plurality of dielectric films are alternately laminated, each semiconductor thin film comprising (A) germanium or a germanium alloy. A semiconductor thin-film core layer and (B) a pair of semiconductor side-layers laminated on both sides of the semiconductor thin-film core layer, containing one of the elements forming the dielectric film, and bonded to the dielectric film. A laminated light polarization control element for near-infrared to visible light region, wherein: 2. The laminated type for near-infrared to visible light region according to claim 1, wherein each of the semiconductor thin film layer and the dielectric film has a thickness such that light propagation in the device is limited to a fundamental mode. Optical polarization control element