JPH095522A - Non-polarizing beam splitter - Google Patents

Abstract

PURPOSE: To provide a non-polarizing beam splitter which is compact and inexpensive and where the ratio of the S polarized component and the P polarized component of emitted light is equal to that of incident light over a wide wavelength band. CONSTITUTION: A plate-like transparent base plate 10 is provided with multilayer films 20 and 30 obtained by alternately laminating a high refractive index thin film and a low refractive index thin film on both surfaces. Since the ratio of the S polarized component and the P polarized component of the emitted light reflected on and transmitted through the respective surfaces of the base plate 10 is equal to that of the incident light, and the multilayer films 20 and 30 are formed on both surfaces of the base plate 10, the ratio of the S polarized component and the P polarized component of the emitted light reflected on and transmitted through the respective surfaces is freely selected, and the effective wavelength area of non-polarized light becomes very wide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a beam splitter which divides an optical path in an optical device or the like, and more particularly to a non-deflecting beam splitter which is provided in an optical system of an optical fiber communication device and branches monitor light from signal light. It is a thing.

[0002]

2. Description of the Related Art In general, a beam splitter has a structure in which an optical film 50 having a constant reflectance is provided between the inclined surfaces of prisms 40 and 41, as shown in FIG. The light incident on the beam splitter is split into transmitted light and reflected light by the reflectance characteristic of the optical film 50,
The reflected light is normally emitted in the direction of 90 ° with respect to the incident light.

[0003]

By the way, due to the recent development of optical fiber communication technology, an optical system for handling coherent light, such as a high-quality laser diode light source or an optical fiber amplifier, is provided between an incident light and an outgoing light. There is a demand for non-polarized optical elements that do not cause changes in polarization characteristics. In particular, in a beam splitter for a signal light monitor, there is no difference in polarization characteristics between the signal light that passes through the beam splitter and the monitor light. , The quality of the signal light itself is degraded, and inaccurate monitoring is performed.

However, conventionally, as a characteristic of the optical film 50 of the beam splitter shown in FIG. 6, there is a tendency that the ratio of the S-polarized component of the reflected light is much larger than that of the P-polarized component, that is, the P-polarized component of the transmitted light. Tends to be much higher than the proportion of S-polarized component. Therefore, in order to make the ratio of the S-polarized light component and the P-polarized light component of the light emitted from the beam splitter equal to that of the incident light, it is proposed that the optical film 50 has a multilayer structure. However, there is a problem that the ratio cannot be equalized in a wide wavelength range and cannot be used in an optical system of a recent wavelength division multiplexing optical communication device.

Further, the beam splitter using two prisms has a problem that it is difficult to miniaturize the beam splitter itself, and the cost required for processing the members, forming the multilayer optical film, and assembling them is expensive. It was

The present invention has been made in view of the above problems of the conventional beam splitter, and it is small and inexpensive, and has a wide wavelength band usable for an optical system of a wavelength division multiplexing optical communication device. It is an object of the present invention to provide a non-polarization beam splitter in which the ratio of S-polarized component and P-polarized component of incident light is equal to that of incident light.

[0007]

In order to achieve the above object, the non-polarizing beam splitter of the present invention comprises a flat transparent substrate, both surfaces of which a high refractive index thin film and a low refractive index thin film are alternately laminated, It is characterized in that the ratio of the S-polarized component and the P-polarized component of the emitted light is made equal to that of the incident light.

In the non-polarizing beam splitter of the present invention, the refractive index (N _S ) of the transparent substrate, the refractive index (N _H ) of the high refractive index thin film, and the refractive index (N _L ) of the low refractive index thin film. And 1.4 <N _S <1.8 1.7 <N _H <2.5 1.3 <N _L <1.8.

The non-polarizing beam splitter of the present invention comprises an S-polarized component and a P-polarized component of a multilayer film on each surface of a flat transparent substrate.
By setting the film refractive index, the optical film thickness, and the number of film layers so that the distributions of the reflectance ratios of the polarization components cancel each other out over the wavelength range used, the light is reflected and transmitted on both sides of the substrate. The ratio of the S-polarized light component and the P-polarized light component of the incoming outgoing light is made equal to that of the incoming light. The expression "equivalent" is meant to include not only the same case but also a case where there is a difference that is practically acceptable. Specifically, for example, assuming that the loss due to the absorption of the substrate is 0% in the predetermined wavelength range of light, the reflectance of the S-polarized component is 2% and the transmittance is 98% on the incident side of the substrate. P
When the reflectance of the polarized component is 3% and the transmittance is 97%, the reflectance of the S-polarized component is 3% and the transmittance is 97% on the transmitting side, and the reflectance of the P-polarized component is 2% and the transmittance is 98%. In this case, the beam splitter reflects 5% of both the S-polarized component and the P-polarized component of light, and transmits 95% thereof.

It is used for the flat transparent substrate of the present invention and has a refractive index of 1. in the wavelength band of 1400 to 1700 nm.
As a material having a range of 4 to 1.8, for example, BLC (borosilicate glass manufactured by Nippon Electric Glass Co., Ltd., a refractive index of 1.48 to
1.49), quartz glass (refractive index 1.43 to 1.4
6), sapphire (refractive index 1.73 to 1.76), etc. can be used, and as a material of the high refractive index thin film having a refractive index in the range of 1.7 to 2.5, for example, a dielectric is used. TiO ₂
(Refractive index 2.20 to 2.30), Ta ₂ O ₅ (refractive index 1.90 to 2.10), ZrO ₂ (refractive index 1.90 to
2.00) or the like can be used, and the refractive index is 1.3 to 1.
As the material of the low refractive index thin film in the range of 8, for example, SiO ₂ (refractive index is 1.44 to 1.46) which is a dielectric, Mg
₂ F (refractive index 1.35 to 1.36), Al ₂ O ₃ (refractive index 1.50 to 1.60) and the like can be used. These dielectrics hardly absorb the light in the infrared region used in optical communication, and the strength of the film itself is high, and the adhesion strength of the film to the substrate is also high, and the physical and chemical durability is further improved. Excellent and easy to handle.

[0011]

In the non-polarizing beam splitter of the present invention, a high refractive index thin film and a low refractive index thin film are alternately laminated on both sides of a flat transparent substrate, and the ratio of the S polarization component and the P polarization component of the emitted light is adjusted. Since it becomes the same as the incident light, by adjusting the ratio of the S-polarized component and the P-polarized component of the light reflected and transmitted by each surface of the substrate with the multilayer film of each surface, The ratio of the P-polarized component can be made equal to that of the incident light, and since the multilayer film is formed on both surfaces of the substrate, the ratio of the S-polarized component and the P-polarized component of the reflected light and the transmitted light at the multilayer film can be freely selected. The effective wavelength range of the polarization characteristics can be sufficiently widened.

[0012]

1 is a conceptual diagram showing an embodiment of the present invention, FIG.
Shows an enlarged sectional view of a main part. In these figures,
1 and 3 are incident side high refractive index thin films, 2 and 4 are incident side low refractive index thin films, 5 and 7 are transmission side high refractive index thin films, 6 and 8 are transmission side low refractive index thin films, and 10 is a plate-like transparent substrate. Reference numeral 20 denotes an incident side multilayer film, and 30 denotes a transmission side multilayer film.

BLC of borosilicate glass (manufactured by Nippon Electric Glass Co., Ltd., refractive index 1.48 to 10) as the flat transparent substrate 10.
1.49) as a material and the dimensions are 4 mm × 4 mm × 0.
After precision cleaning a 5 mm glass plate with mirror surfaces on both sides,
On each surface of this substrate, using Ta ₂ O ₅ as a high refractive index thin film and SiO ₂ as a low refractive index thin film as film materials,
The theoretical design is such that the center wavelength is 1550 nm, the incident angle is 45 °, and the reflectance is 5%. As a result, a non-polarizing beam splitter was produced.

[0014]

[Table 1]

First, T is provided on the incident side of the flat transparent substrate 10.
Multilayer film 20 in which four layers of a ₂ O ₅ and SiO ₂ are alternately laminated
Is formed into a sample and a wavelength of 140
Light of 0 to 1700 nm is incident at an incident angle of 45 °,
The reflectance of each of the S-polarized component and the P-polarized component was measured. The measurement results are shown in the spectral characteristic diagram of FIG.

Separately from the sample having the above-mentioned multilayer film 20, Ta ₂ O ₅ and SiO _{2 are provided} on the transparent side of the flat plate-like transparent substrate 10.
_A sample is created by forming a multilayer film 30 in which four layers of ₂ and ₂ are alternately laminated, and a wavelength of 1400 to 1700n is applied to the sample.
The light of m was incident at an incident angle of 45 °, and the reflectances of the S-polarized component and the P-polarized component were measured. The measurement result is shown in the spectral characteristic diagram of FIG.

A multilayer film having the spectral characteristics shown in FIGS. 3 and 4 is formed by laminating the multilayer films 20 and 30 on each of the incident side and the transmitting side of the flat transparent substrate 10. 1400 to 17 for the beam splitter of the product
Light having a wavelength of 00 nm was incident at an incident angle of 45 °, and the reflectances of the S-polarized component and the P-polarized component were measured. The measurement result is shown in the spectral characteristic diagram of FIG.

As a result, the S-polarized component and P shown in FIG.
From the distribution of the reflectance of the polarized component, the reflectance of 5.0 ± 1.0
%, The effective wavelength range is 1480 nm to 1600 nm under the condition that the reflectance difference between the S-polarized component and the P-polarized component is within 0.3 dB.
It was confirmed that a wide-band unpolarized beam splitter covering 120 nm was manufactured.

[0019]

According to the non-polarizing beam splitter of the present invention, the ratio of the S-polarized component and the P-polarized component of the emitted light is made equal to that of the incident light, and the effective wavelength range of the polarization characteristic is sufficiently wide. Therefore, it can be used for monitoring the signal light of an optical system that handles coherent light such as a high-quality laser diode light source or an optical fiber amplifier and an optical system of a wavelength division multiplexing optical communication device.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing the configuration of the present invention.

FIG. 2 is an enlarged cross-sectional view of a main part of FIG.

FIG. 3 is a spectral characteristic diagram of an incident side multilayer film according to an example of the present invention.

FIG. 4 is a spectral characteristic diagram of a transmission-side multilayer film according to an example of the present invention.

FIG. 5 is a spectral characteristic diagram of a beam splitter according to an embodiment of the present invention.

FIG. 6 is a conceptual diagram showing a configuration of a conventional technique.

[Explanation of symbols]

 1, 3 Incident side high refractive index thin film 2, 4 Incident side low refractive index thin film 5, 6 Transmission side high refractive index thin film 7, 8 Transmission side low refractive index thin film 10 Plate transparent substrate 20 Incidence side multi-layer film 30 Transmission side multilayer Film 40 Incident side prism 41 Transmission side prism 50 Optical film

Claims (2)

[Claims] 1. A high-refractive-index thin film and a low-refractive-index thin film are alternately laminated on both surfaces of a flat transparent substrate so that the ratio of S-polarized light component and P-polarized light component of emitted light is equal to that of incident light. A non-polarizing beam splitter characterized by that. 2. The refractive index (N _S ) of the transparent substrate, the refractive index (N _H ) of the high refractive index thin film, and the refractive index (N _L ) of the low refractive index thin film are 1.4 <N _S < The non-polarizing beam splitter according to claim 1, wherein 1.8 1.7 <N _H <2.5 1.3 <N _L <1.8.