JP2783491B2 - Laminated light polarization control device for near infrared to visible light region - Google Patents

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 device for a near infrared to visible light region. Optical polarization control elements are 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 indispensable element in configuring many optical devices and optical sensors. In recent years, there has been a demand for miniaturization, high performance, and high productivity of an optical polarization control element. To meet this demand, a multilayer laminate structure obtained by alternately laminating a dielectric and a metal or a semiconductor has been demanded. (For example, S. Kawakami, Appl. Opt., Vo)
l. 22, 2426 (1983)).

[0003] As shown in FIG. 1, this laminated optical polarization controlling 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.
Formed from O ₂ , the metal thin film is formed from 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 light polarization control element has, for example, an optical path length of three.
It shows an optical polarization control characteristic 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).

A wavelength of 0.85 μm (short wavelength region) is used for an optical fiber gyro and an optical sensor (voltage sensor).
However, if the light polarization control element having the above-described structure is used for this, the light insertion loss value becomes larger than 1 dB, so that its practical use is difficult.

In order to overcome the above-mentioned difficulties, a multilayer-structured light polarization controlling element used in a short wavelength region has been proposed by 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 IEICE,
91-47, pp11 (1991)). For example, SiO ₂
When the thickness of the layer is 1 μm and the thickness of the Ge layer is 60 Å, the obtained light polarization controlling element has an optical path length of 1 μm.
Per meter, extinction ratio 2.9 dB and insertion loss 0.018 dB
Therefore, it is suggested that light polarization control is possible in a short wavelength region.

[0006]

The short-wavelength-region laminated light polarization controlling element having a conventional multilayer laminated structure as described above has a small interfacial bonding force between its constituent films. There is a limit to the film thickness, that is, the upper limit of the total film thickness is generally about 60 μm, and it is difficult to manufacture a film having a thickness larger than this. Further, the conventional light polarization control element having the above-mentioned multilayer structure, when the polishing process,
There is a problem that the multilayer laminated structure is easily broken and the product yield is extremely low.

In general, an optical polarization controlling element having a multilayer laminated structure is bonded to an optical fiber core in most applications. When the joining step is mechanized, it is desirable that the thickness of the multilayer film be large in order to facilitate the operation. Further, in order to achieve high productivity, a high production yield of the optical polarization control element is required. further,
The optical polarization control element is the most basic optical passive element, and is one of the essential components of many optical devices and optical sensors. Therefore, it is desirable that the optical polarization control element be supplied at low cost.

In a conventional stacked optical polarization control device using a Ge film as a semiconductor film, the absorptance of light polarization that can be absorbed in the device greatly depends on the wavelength of light used. As the wavelength of the light becomes shorter, the insertion loss of the element increases, and a higher-order light propagation mode is excited in the optical waveguide of the element, so that the extinction ratio of the element decreases.

In recent years, the demand for an optical polarization control element in an optical sensor typified by an optical fiber gyro and a voltage sensor has been increasing. As such a light source, a light source having a wide spectrum is used. ing. In addition, recent remarkable advances in technology relating to light sources have promoted the shortening of the emission spectrum. Under the circumstances described above, there is a strong demand for the development of an optical polarization control element that exhibits a high extinction ratio and low insertion loss over the region of 600 to 850 nm.

The present invention has a stable multilayered structure, a large thickness, a high product yield, and a thickness of 600 to 850 nm.
It is an object of the present invention to provide a laminated light polarization control element capable of exhibiting a high extinction ratio and low insertion loss in the near infrared to visible light region.

[0011]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the problems of the prior art, and as a result, in an optical polarization control element having a multilayer laminated structure,
By arranging a specific side layer capable of stably bonding to the conductor film, the above problem was successfully solved.

The laminated light polarization controlling element for near-infrared to visible light 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. Wherein each of the semiconductor thin films is (A) germanium;
Or (B) a semiconductor thin-film core layer made of a germanium alloy, and (B) one of the elements constituting the dielectric film laminated on both sides of the semiconductor thin-film core layer, and bonded to the dielectric film. And a pair of semiconductor side layers.

Further, in the stacked optical polarization controlling 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 is only in a fundamental mode.

[0014]

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

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

In order to stabilize the multilayer structure, it is important to increase the bonding strength (adhesion) between different kinds of films. In the present invention, the dielectric film and the semiconductor thin film core layer are firmly joined via the semiconductor side layer. Kikuchi, Baba, Kanehara, “Vacuum” Vol. 29,
No. 5, p337 (1986).

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

From FIG. 3, the semiconductor side layer made of Si is
This shows that the function as a binder between the SiO ₂ dielectric film and the Ge semiconductor thin film core layer, and that the thickness of the Si semiconductor side layer functioning as the binder is about 10 Å or more is sufficient. ing.

As described above, according to the present invention, the adhesive force between the SiO ₂ dielectric film and the Ge semiconductor thin film core layer can be significantly increased, and as a result, a practical laminated film having a total film thickness of 300 μm or more can be obtained. It has become possible to manufacture a type light polarization control element.

In the laminated optical polarization controlling 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 core layer of the semiconductor thin film in the semiconductor thin film Is preferably 45 to 60 angstroms, and the thickness of the semiconductor side layer is 10 to 60 Å.
Preferably it is ２０20 Å.

In the light polarization controlling element of the present invention shown in FIG. 2, the light polarization oscillating in the Y
The light polarization oscillating in the direction is defined as a TM wave. Also, the light is d
Film thickness represented by _1, and the semiconductor thin film 3 having a complex dielectric constant represented by epsilon _S1, composed of the dielectric film 1 and having a film thickness expressed by d _2, the complex dielectric constant represented by epsilon _S2 When passing through the multilayer optical polarization control element, the light propagation constant of the semiconductor thin film 3 is P ₁ and the light propagation constant of the dielectric film 1 is P ₂ . In this case, the characteristic equations of the element with respect to the polarization components (TE wave and TM wave) that can be absorbed in the light polarization control element are expressed by the following equations (1), (2) and (3). Direction light propagation coefficient γ (= α
+ Jβ) is known to be expressed by the following equation (4) (S. Kawakami, Appl. Opt., Vo)
l. 22, 2426 (1983))

[0022]

(Equation 1)

(Equation 2)

(Equation 3)

(Equation 4)

In the laminated optical polarization controlling element, there is a possibility that a higher-order waveguide mode having a smaller absorption coefficient than the absorption coefficient α _O for the lowest-order mode of light is excited. Such higher-order waveguide modes are also described in the above (1) to
According to the equation (4), the absorption coefficient α _n (n =
0, 1, 2,...) Can be predicted by a numerical method from the thicknesses (d ₁ , d ₂ ) of the component films constituting the element and the optical constants (ε _S1 , ε _S2 ). It is.

When a higher-order guided mode is excited with respect to the TE wave in the element, the absorption of the TE wave decreases,
For this reason, the extinction ratio of the laminated light polarization control element is significantly reduced. Therefore, in order to obtain a high-performance laminated optical polarization control element, the thicknesses of the semiconductor film and the dielectric film are designed so that a high-order guided mode is not excited with respect to the TE wave in the element. It is essential.

When the thickness d ₁ of the semiconductor thin film is constant (that is, ε _S1 is also determined) and the thickness d ₂ of the dielectric film is changed, one unit of the laminated film (one 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 range of 600 to 850 nm used for the optical sensor, the higher mode excited in the device of the present invention is only the first mode for the TE wave, and the higher mode for the TM wave. Is not excited.

Therefore, in the present invention, the thickness of the dielectric film of the light polarization controlling element is adjusted so that light propagation in the element becomes only the fundamental mode and the primary mode of the TE wave is not excited.
It is calculated by a numerical solution and incorporated into the element design. Thereby, the laminated light polarization control element of the present invention is:
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 region of 600 to 850 nm used for the optical sensor, and As the wavelength becomes shorter, the concentration of the electromagnetic field on the Ge thin film becomes larger.
The light absorption by the Ge thin film increases, and the light insertion loss in the device increases.

In the present invention, the effective dielectric constant of the entire semiconductor thin film is reduced by stacking semiconductor side layers having a low complex dielectric constant on both sides of the semiconductor central thin film made of Ge, thereby reducing the complex dielectric constant. The wavelength dispersion and the localization of the waveguide mode in the semiconductor thin film are reduced. By using a semiconductor thin film having the above-mentioned laminated structure, 600
In the wavelength range of ８850 nm, the extinction ratio and the optical insertion loss of the laminated optical polarization controlling element of the present invention show almost no wavelength dependence.

[0029]

The present invention is further described by the following examples. Example 1 and Comparative Example 1 In Example 1, a Si thin film core layer made of Ge and having a thickness of 60 Å and a semiconductor side layer made of Si and having a thickness of 10 Å disposed on both sides thereof. / 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 produce a laminated optical polarization controlling element. The complex dielectric constants 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 element (600 to 850 nm),
FIG. 4 shows the relationship with the extinction ratio per optical path length of 30 μm.
Further, the wavelength (600 to 850 nm) of this element and the optical path length 3
FIG. 5 shows the relationship with the optical insertion loss per 0 μm.

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

As is apparent from FIG. 4, the extinction ratio of the conventional laminated optical polarization controlling element of Comparative Example 1 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.

As is clear from FIG. 5, Comparative Example 1
In the conventional laminated optical polarization control device of the above, the light insertion loss increases as the wavelength becomes shorter, but in the device of Example 1, the light insertion loss depends on the wavelength in the wavelength region of 600 to 850 nm. Without showing a low constant value of about 0.34 dB or less. Further, light propagation in the device of Example 1 was only in the fundamental mode.

Example 2 A laminated optical polarization controlling element was manufactured in the same manner as in Example 1 except for the following. (1) The semiconductor side layer in the semiconductor thin film is made of Si ₁ -xGe
It was formed by x alloy. (2) Complex permittivity Ge core layer: 23.112-j4.36
7 Si ₁ -xGex side layer: 12.254-j2.18
9 (3) Film thickness Ge core layer: 50 Å Si ₁ -xGex Side layer: 10 Å Semiconductor thin film: 70 Å Dielectric (SiO ₂ ) film: 0.8 μm The obtained device has a wavelength region of 600 to 850 nm. Showed a substantially constant extinction ratio of 60 dB and an optical insertion loss of 0.4 dB per 30 μm of optical path length. Light propagation in the device of Example 2 was only in the fundamental mode.

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

[0036]

[Table 1]

[0037]

According to the stacked optical polarization controlling element of the present invention, a semiconductor side layer containing a dielectric film forming element and having a low complex dielectric constant is provided between the dielectric film and the semiconductor central thin film layer. , Significantly improve the adhesion at the interface between the membranes,
As a result, the stability of the multilayer structure of the element is significantly improved, the phenomenon of destruction of the multilayer structure during element processing is prevented, and the total thickness of 300 μm or more (this is equivalent to 5 times or more the conventional thickness). Element can be obtained. In addition, the effective dielectric constant of the light absorbing layer is reduced by arranging the semiconductor side layer, which has the effect of reducing the wavelength dispersion for light absorption and the localization of the waveguide mode in the semiconductor thin film. I do. Further, by defining the thickness of the semiconductor layer and the dielectric film so as to propagate only the fundamental mode, in the wavelength region of 600 to 850 nm, for example, the extinction ratio is 54 dB or more per 30 μm of the optical path length, and the insertion loss is reduced. It can be a constant value around 0.3 dB.

[Brief description of the drawings]

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

FIG. 2 is an explanatory view showing a laminated structure of an example of a laminated light polarization controlling element of the present invention.

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

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

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

[Explanation of symbols]

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

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-87101 (JP, A) JP-A-3-75705 (JP, A) JP-B-7-66084 (JP, B2) APPLIED OPTICS, 22- 16! (1983), p. 2426-2428 Proceedings of the 1991 IEICE Spring Conference (C-273), p. 4-290 (58) Field surveyed (Int. Cl. ⁶ , DB name) G02B 5/30 G02B 5/18

Claims (2)

(57) [Claims] 1. An element comprising a multilayer film in which a plurality of semiconductor thin films and a plurality of dielectric films are alternately stacked, wherein each of the semiconductor thin films is made of (A) germanium or a germanium alloy. And (B) a pair of semiconductor side layers stacked on both sides of the semiconductor thin film core and containing one of the elements constituting the dielectric film and joined to the derivative film. A laminated light polarization controlling element for the near infrared to visible light region, comprising: 2. The near-infrared to visible light region laminated type 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 element is only a fundamental mode. Light polarization control element.