JPH0526163B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a polarized light separating film made of a dielectric multilayer film, and particularly to a polarized light separating film that has a wide band and has a high degree of separation of polarized light components.

A polarization separation film is an optical device that separates two polarization components (vertical polarization and horizontal polarization) of light.
For example, the vertically polarized component (P liquid) is reflected and the horizontally polarized component (S wave) is transmitted, thereby separating the two polarized components. Such polarization separation films are used in optical components such as optical switches and optical isolators.

[Conventional technology and problems]

Conventionally, polarized light separation films have been used to perform polarized light separation, an example of which is shown in FIG.

Figure 1 shows an example of 23 layers, in which dielectric films TiO ₂ and SiO ₂ are alternately laminated on substrate 1.
A substrate 2 is provided outside the 23rd layer.

The first layer is λ/4 (λ is the wavelength of the light used).
0.55 times the thickness, and the second to tenth layers, the 12th to 19th layers, and the 21st layer each have a thickness equal to λ/4. Also, the 11th layer is 1.3 of λ/4
times, the 20th layer is 0.98 times, the 22nd layer is 0.8 times, the 23rd layer is
It is 0.42 times thicker. These numerical values are called the optical film thicknesses of each dielectric film, and the numbers below the second decimal point in the numerical values shown in FIG. 1 include measurement errors.

Furthermore, the refractive index of substrates 1 and 2 is 1.51.

The polarization characteristics of the polarization separation film with the above configuration are described in the seventh section.
This will be explained using figures.

In FIG. 7, the horizontal axis shows wavelength (μm), and the vertical axis shows polarization separation degree (dB). The curve C1 shows the amount of P-wave reflection at the laser beam entrance surface, and the curve C2 shows the amount of S-wave leakage at the exit surface. As can be seen from this characteristic, the bandwidth when it is desired to obtain a degree of separation of 28 dB or more is approximately 78 nm.

However, due to the nature of devices that use polarized light, the above bandwidth could not fully satisfy the required bandwidth, including wavelength fluctuations of the light source laser diode (LD), incidence angle accuracy, etc.

[Object and structure of the invention]

The present invention has been made in view of these points, and an object of the present invention is to provide a polarization separation film with a wide bandwidth.

This purpose is to create a polarization separation film in which 23 TiO ₂ layers and ₂₃ SiO ₂ layers are laminated alternately on a substrate. The thickness of the 12th layer in the center and the SiO ₂ layer is approximately 1.3 times 1/4 of the wavelength of the light used, and the thickness of the second SiO ₂ layer from the outermost layer is approximately 0.5 times 1/4 of the wavelength. This is achieved by using a polarization separation film that has a thickness approximately 0.9 times the wavelength of the light used, and the other layers have a thickness that is 1/4 of the wavelength of the light used.

[Embodiments of the invention]

The present invention will be explained below with reference to Examples.

In order to perform polarization separation over a wide band, it is necessary to make the optical wavelength region (band) where P waves and S waves are separated larger than the wavelength of light incident on the polarization separation film. In order to enlarge the optical wavelength range in which P waves and S waves are separated, light may be incident on the polarization separation film at an angle close to the Brewster angle, according to Snell's law.

However, simply changing the incident angle to the substrate changes the incident/outgoing angle of the polarization separation film, making device assembly difficult.

Therefore, the angle of incidence on the substrate is fixed at 45 degrees.
Instead, the refractive index of the substrate is made larger than the refractive index of the dielectric film formed on the substrate. In this case, the angle of incidence on the dielectric film becomes larger according to Snell's law, which corresponds to substantially increasing the angle.

Figure 2 is a graph showing the relationship between the substrate refractive index and the bandwidth at which the P-wave loss is 0.05 dB or less, and shows that the higher the refractive index, the wider the bandwidth.

However, there are restrictions on the optimization of the film thickness and the refractive index of the adhesive used when constructing the prism, so here, the refractive index on the horizontal axis was set to 1.54 to 1.60.

In this way, when the substrate has a high refractive index, the wavelength range in which the polarized light components are separated becomes larger, but the loss characteristic of the P wave with respect to the wavelength of the incident light causes ripples as shown in FIG. The fact that the P wave loss is large means that the P wave is leaking to the S wave side, indicating that the degree of polarization separation is poor. That is, if the ripple level is high, the polarized light components cannot be completely separated, resulting in leakage.

Therefore, unless this leakage of polarized light components is prevented, even if the substrate refractive index is increased and the incident angle to the polarized light separation film is increased, the bandwidth at which the desired degree of polarization separation (28 dB) can be obtained will remain narrow. be.

In order to prevent leakage of this polarized light component, in a dielectric multilayer film formed of multiple dielectric films with a thickness of λ/4 of the incident light, certain layers are made thicker or certain layers are made thinner. By performing a filter-like correction of the loss with respect to the optical wavelength, the ripple level is corrected. When performing such a filter-like correction, in order to identify the layer whose film thickness is to be changed, it is necessary to fix the substrate refractive index and change the film thickness of each layer in the multilayer film one by one. This can be determined by examining the loss characteristics.

Therefore, when examining the loss of P waves when the substrate refractive index is fixed at 1.55 and the film thickness of each layer is varied for the multilayer film having the structure shown in FIG. 1, the results are as shown in FIG. 3. In this case, the film thickness of the first layer and the 23rd layer is
The values are the same as those shown in the figure, and the thicknesses of other layers other than those whose thicknesses are changed are set to be approximately equal to 1/4 of the used light wavelength λ.

In FIG. 3, the horizontal axis represents the layer number, and the vertical axis represents the magnitude (dB) of the first ripple of the P-wave loss characteristic shown in FIG. 4. Curve C3 shows that when the film thickness is increased by +20% from the value shown in Figure 1,
If curve C4 is thickened by +30%, curve C5 is -20
% (thinner), curve C6 is -30%
This is the case. The wavelength of the light used is λ ₀ =1400 nm, and the angle of incidence on the substrate is 45 degrees.

FIG. 3 shows a certain layer, for example the second layer shown in FIG.
When the film thickness of a layer (thickness equal to λ/4) is changed between -30% and +30% with respect to λ/4, and the other layers are not changed (that is, the thickness of the other layers is The film thickness is equal to λ/4). In particular, as shown by curve C4 for the 11th layer, the case where the film thickness is increased by +30% corresponds to the case where the refractive index of the substrate is set to 1.55 in the conventional example shown in FIG.

As is clear from Figure 3, the P wave loss changes when the film thickness of each layer is changed, but when the thickness of the 12th layer is changed, the P wave loss is also small, especially when the film thickness is increased. Summer.

Therefore, we decided to increase the thickness of the 12th layer by 30% in the 23-layer multilayer structure shown in Figure 1.

Next, increase the thickness of the 12th layer by +30%, that is, λ/4.
We investigated changes in the size of the first ripple and the bandwidth when the thickness of each remaining layer was sequentially changed by increasing the thickness by 1.3 times. Also in this case, the film thickness of each layer except the first layer, 23rd layer, 12th layer, and the layer whose film thickness is changed is set equal to λ/4. The results are shown in FIG.

In FIG. 5, the wavelength of the light used is λ=1400 nm, the angle of incidence on the substrate is 45 degrees, and the refractive index of the substrate is 1.55.
Also, the horizontal axis is the layer number, and the vertical axis on the left is the P wave loss.
The bandwidth W is 0.05 dB, and the vertical axis on the right is the magnitude of the first ripple. In addition, curve C7 shows the film thickness +
The bandwidth when the film thickness is increased by 10%, the curve C8 is the bandwidth when the film thickness is increased by -10%, the curve C9 is the first ripple when the film thickness is increased by +10%, and the curve C10 is the first ripple when the film thickness is increased by -10%. It shows the size of.

As a polarization separation film, it is preferable that the bandwidth is wide and the level of the first ripple is small. From this point of view, the first ripple is the smallest when the film thicknesses of the second, fourth, 20th, and 22nd layers are -10%, but when looking at the bandwidth in this case, 4th layer, 20th layer
Bandwidth is becoming narrower for layers. Therefore,
Only the film thickness of the 2nd layer and the 22nd layer (the 2nd layer from the outermost layer) is -10%, that is, about 0.9 times λ/4, and the 4th layer and the 20th layer are the same as in Figure 1. When the film thickness is set to , the above is achieved.

In the table below in FIG. 7, the upper row shows the layer number and the lower row shows the film thickness.
The thickness of each layer is shown. As can be seen from this table, in this example, the film thickness of the 12th layer is made 30% thicker than the film thickness of 1/4 of the wavelength λ used.
The thickness of the second and 22nd layers has been reduced by 10%. The other layers have approximately the same thickness as the multilayer film shown in FIG. Note that, like FIG. 1, the film thickness values in the lower table of FIG. 7 include measurement errors below the second decimal point.

FIG. 6 is a diagram showing the relationship between light wavelength and P wave for a polarization separation film having such a configuration. As can be seen from this figure, the level of the first ripple of P-wave loss is 0.05 dB or less, which is negligible.

The graph in FIG. 7 is a graph showing the degree of polarization separation and its band of the polarization separation film having the configuration shown in the table below in FIG.

In FIG. 7, a curve C11 shows the amount of P-wave reflection on the incident surface, and a curve C12 shows the amount of S-wave leakage on the exit surface. As is clear from this figure,
In the polarization separation film according to this example, the bandwidth at which a degree of polarization separation of 28 dB or more can be obtained is 111 nm, and the 33 nm band is wider.

With a multilayer film with the configuration shown in Figure 1, the refractive index of the substrate is 1.55.
In that case, according to experiments, the bandwidth is about 85nm.
Compared to this, in this embodiment, the bandwidth has been expanded by about 26 dB.

[Brief explanation of drawings]

Figure 1 shows the structure of a conventional polarization separation film, Figure 2 shows the relationship between changes in the refractive index of the substrate and the bandwidth, and Figure 3 shows the relationship between changes in the thickness of each layer. Figures 4 and 6 are diagrams showing the change in the size of one ripple, Figures 4 and 6 are diagrams showing the relationship between wavelength and P-wave loss, and Figure 5 is a diagram showing the relationship between the wavelength and P wave loss. FIG. 7 is a diagram showing the change in the magnitude of the first ripple and the change in the bandwidth when sequentially changed, and FIG. 7 is a diagram showing the degree of polarization separation.

Claims (1)

[Claims] 1. In a polarization separation film in which 23 TiO ₂ layers and 23 SiO ₂ layers are alternately laminated on a substrate, the substrate has a refractive index of 1.54 to 1.60, and the 12th layer in the center is SiO ₂ . The thickness of the layer is approximately 1.3 times 1/4 of the wavelength of the light used, and the thickness of _the two outermost SiO layers is approximately 0.5 times 1/4 of the wavelength of the light used. eyes and 22nd layer
A polarization separation film characterized in that the thickness of _{the two} SiO layers is approximately 0.9 times 1/4 of the wavelength of the light used, and the thickness of the other layer is 1/4 of the wavelength of the light used.