KR100458367B1 - Polarization splitter/combiner by using side-polished fiber coupler with a metal-layer - Google Patents

Abstract

The present invention relates to a structure of a polarization splitter / compositor using a side polished optical fiber coupler including a metal thin film and a method of manufacturing the same. The proposed polarization splitter / synthesizer separates TE mode from TM mode by controlling the thickness of the metal and the bonding gap. Since the optical fiber for communication is used as it is, the connection loss with other systems is small, and insertion loss is also very low at 0.3 dB in the TM mode or less than 0.2 dB in the TE mode, and the polarization extinction ratio is very high at 28 dB.

Description

Polarization splitter / combiner by using side-polished fiber coupler with a metal-layer}

Fiber-optic polarization splitters are key components in coherent optical communications and optical sensors, and are widely used in a variety of applications, including optical network switches in free space, magneto-optical data storage systems, imaging systems, single-polarized optical receivers, polarization-dependent optical filters, and optical gyroscopes. It is a necessary element.

Conventional polarizers can be broadly classified into integrated optical waveguide device technology, beam splitter (beam sp-litter) technology, fusion splicer technology using a special optical fiber and technology using a liquid crystal crystal.

First, M.H.Hu et al., IEEE Photon. Technol. Lett., Vol. 9, No. 6, June 1997 show the pointed Y-branch type on the GaAs-AlGaAs substrate in the symmetrical and asymmetrical forms at the input and output terminals, respectively. It is a gender polarizer.

M.B.J. Diameer et al., United States Patents 5,056,883, Field at Jan. 25,1991 describe polarization splitters by selectively polling only one branch in an Y-branched optical waveguide fabricated using a nonlinear polymer optical waveguide to excite optical birefringence. Is a patent.

P.-K. Wei et al., IEEE Photon. Technol. Lett., Vol. 32, NO.4, pp. 245-247, 1994 describe the polarization splitter using optical birefringence and Y-branched optical waveguide, which are caused by internal diffusion of various kinds of elements onto a lithium niobate substrate. Produced.

And in F. Ghirardi et al., IEEE Photon. Lett., Vol. 5, No. 9, pp. 1047-1049, 1993, fabricated a directional coupler-type optical waveguide on a lithium phosphite (InP) substrate and fabricated a polarization separator using the coupling distance difference according to the polarization.

The conventional polymer optical waveguide polarizer is manufactured by adjusting the birefringence, the structure of the integrated optical waveguide, or the coupling length in the nonlinear polymer. It is necessary to apply the proper birefringence only to certain parts, and to design and manufacture waveguides in um carefully.

Next, Akira Sakamoto et al., Optics and Electronics Lab., Fujukura Ltd., Japan, presented a fusion coupler-type polarizer using PANDA optical fiber, which has a limited wavelength region as a pumping light source element of 980um wavelength. It is difficult to use in the area. It also requires a high degree of alignment to align the axis.

Lastly, many polarizers using beam splitters and polarizers using liquid crystal crystals have been reported. The above two methods mainly use the birefringence of crystals, which require large scale and multiple additional devices due to separation of polarized light in free space. Do.

In order to solve the shortcomings of the above-mentioned conventional polarizers, the present invention introduces a technique of side polishing a single mode optical fiber, depositing a thin metal film thereon, and then combining the same side polished optical fiber. It is an object of the present invention to provide a new type of polarizer and synthesizer that is simple, has excellent mechanical reliability, has a low loss, and has a high polarization separation ratio.

1 is a structural diagram of a side polished optical fiber coupler including a metal layer to which the present invention is applied.

2 is a structural diagram equivalent to a cross-sectional structure and a multilayer planar waveguide of the side polished optical fiber coupler of the present invention.

Figure 3 is a device structure divided into a plurality of linearized linear section bent optical fiber for device analysis.

Fig. 4 shows the transfer characteristics of the device according to the thickness (s) of the cladding remaining after polishing the equivalent planar wave, tm = 5 nm.

5a to 5c is a characteristic of the transmission process of the optical fiber mode according to the thickness of the metal layer according to the present invention, (5a) tm = 0nm, (5b) tm = 5nm, (5c) tm = 10nm.

In order to achieve the above object, the present invention forms a metal thin film in the middle of the directional coupler using two side polished optical fibers having the same structure as shown in FIG. TE polarization does not cause coupling between the two optical fibers by the metal thin film layer, but optical coupling occurs between the two optical fibers because the TM polarization component can penetrate the metal. A thin metal film layer of suitable thickness only allows dissipation coupling between the two waveguides. 1 can separate two orthogonal polarization components into both optical fibers of the output terminal. In FIG. 1, Pa denotes power of input light and optical power of two optical fibers at Pb and Pc output terminals. In the proposed coupler structure, the cross-sectional structure of FIG. 1 is replaced with an equivalent planar waveguide as shown in FIG. In FIG. 2, the thickness of the metal thin film is tm, the remaining cladding thickness after polishing is d, the refractive index of the optical fiber core is nco, and the refractive index of the cladding is represented by ncl. s is the thickness of the cladding of the equivalent planar waveguide. For the device analysis, as shown in FIG. 3, the coupling region of the optical fiber bent at a radius of curvature R of 25 cm was divided into a plurality of linearized coupling structures. The total bond length was set to 5 mm and the combined area was divided into 2000 linearized segments. Orthogonal mode theory is introduced for the analysis of equivalent planar waveguides. In FIG. 3, it is considered that the distance between the two optical fiber cores is far enough so that dissipative coupling does not occur in the region 0 and the section N. The light input to the coupler input stage excites the orthogonal modes of the multilayer waveguide when it encounters the first coupling region (section 1). The field distribution at the start of the first coupling region is represented by the linear combination of the fields of each orthogonal modes excited from the input light. In the case of TE polarization, symmetric and asymmetric modes exist, but in the case of TM polarization, when the thickness is several nm or more, symmetric and asymmetric modes as well as surface plasmon modes that proceed along the metal layer exist. Each orthogonal mode proceeds to the end of the first coupling region with its respective phase velocity, which in turn delivers energy to the orthogonal modes of the second coupling region. Through the above-described analysis in FIG. 4, the coupling characteristics of the TM polarized light input according to the thickness s of the cladding of the equivalent planar waveguide are analyzed. Here the thickness of the remaining cladding of the two polished optical fibers is considered to be the same. It is shown that there are two cladding thicknesses (s) that can be transferred almost to the opposing optical fiber of the TM mode first input. When the thickness of the cladding is 6 µm or more, little energy exchange occurs between the two optical fibers. The radius a of the core of the optical fiber 4.1 m, the refractive index nco of the core 1.4485 and the refractive index ncl of the cladding were set to 1.444. The refractive index of aluminum is 1.44 + j16 at a wavelength of 1.55 mu m.

As the thickness of the metal film increases gradually, the TE mode energy is concentrated in the core layer of the planar waveguide, whereas in the TM mode, due to the optical property that the energy is confined at the boundary between the metal and the dielectric due to the surface plasmon effect, The intensity increases further. If the thickness of the metal thin film is 4 nm or more, the TE mode hardly penetrates the metal thin film, and the intensity of the dissipation field in the uppermost layer is negligibly small. When the thickness of the metal thin film is 10 nm or more, the TM mode progresses along the metal layer and newly forms a surface plasmon mode having an optically large absorption loss. Thus, the thicker the metal thin film in the coupler structure of FIG. Increases. When the metal thin film is 0.2nm, the TE mode has the maximum loss. However, when the metal film is thicker enough, the loss is drastically reduced. This is because the strength of the field in the metal is greatly reduced as the metal thickens. In TM mode, losses increase rapidly as the thickness of the metal film increases. This is because TM mode can penetrate deep into the metal layer. From the above results, it is considered that the thickness of the metal layer placed in the intermediate layer of the coupler is suitable for several nm, and as the metal thickens, the bond between the TE modes is drastically decreased while the bond between the TM modes is slightly increased.

In an embodiment of the present invention, a side polished optical fiber coupler including a metal layer having a structure as shown in FIG. First, in order to make a side polished optical fiber, a V-groove having a radius of curvature (R) of 25 cm was formed on a quartz block having a width of 1 cm, a length of 2.5 cm, and a height of 0.5 cm by using a diamond wheel (100 μm in thickness) of a groove maker. Next, the core diameter was 8.2 μm, the cladding diameter was 125 μm, and the blocking wavelength was 1234 nm. 2.3 cm of the jacket of the single mode optical fiber for general communication was removed, and the epoxy was fixed to the V-groove. After the epoxy was hardened, the side surface of the optical fiber was polished using a polishing film and a solution of water and alumina powder diluted 1: 1. At this time, the scattering of light is observed during polishing using a visible light DFB laser of 630nm wavelength. In the polishing finishing step, the thickness of the cladding was 1.8 μm using a 0.1 μm abrasive film, and the roughness of the polishing surface of the optical fiber was minimized. The thickness of the cladding was determined using the liquid-drop method. Using the order and method listed above, make two side-film fiber optic blocks of the same structure. Next, a metal thin film was formed on the lower side side polished optical fiber using a thermal evaporator. As the material of the metal thin film, aluminum and silver were used. The thickness of the metal thin film was measured using an Ellipso-meter. The upper side side polished optical fiber was combined with the lower side side polished optical fiber in which the metal thin film was formed, and the bonding region of the upper and lower side side polished optical fibers was centered using an micrometer-zig and fixed with epoxy.

3 (a) -3 (c) are state diagrams illustrating a process of transmitting an optical fiber mode of a device according to a thickness of metal (aluminum) in the structure of FIG. 1. 3 (a) shows that the TE and TM modes are the same as the transmission state of the optical fiber mode when the thickness of the metal layer is 0 nm. 3 (b) shows that when the thickness of the metal layer is 5 nm, the TE mode proceeds to the applied optical fiber while the TM mode completely transfers energy to the opposite optical fiber. This means that the TE and TM polarization components can be completely separated. In this case, the absorption loss by the metal is 0.2dB or less in TE mode and 0.3dB or less in TM mode, and the polarization extinction ratio satisfies 28dB or more in both output stages. With this property, this coupler can be used as a polarizer as well as a polarization separator. If the b output stage is used, it can be used as a TE pass polarizer, and if the c output stage is used as a TM pass polarizer. 3 (c) shows a transfer mode when the thickness of the metal layer is 10 nm. In the TM mode, energy is coupled to the surface plasmon mode that is trapped in the metal layer and proceeds, resulting in large loss. From the above results, it can be seen that the light energy of the TM mode is transferred from the first input optical fiber to the opposite optical fiber and the light energy of the TE mode can maintain the original optical fiber path by appropriately adjusting the thickness of the metal and the bonding gap. Appropriate control of the device structure has shown that this coupler can be used as a polarization splitter / synthesizer and polarizer. In addition, it was found that aluminum (Al) or silver (Ag) may be used as a metal layer placed in the intermediate layer of the coupler because of good adhesiveness with a glass optical fiber for optical communication.

As described above, in the present invention, in fabricating a polarization separator using a side polished fiber coupler including a metal thin film, a single mode optical fiber for optical communication can be used to minimize connection loss with various devices or systems. In addition, the fabrication process is simplified, optical power loss is less than 0.3dB, and the polarization extinction ratio is more than 28dB. One device of the present invention can also be used as a TE / TM synthesizer by inversely combining the TM pass polarizer, TE / TM separator and polarized light. By securing a simple, stable and low-cost polarization separator manufacturing technology, the device of the present invention will be applied to various fields, and optical communication will be extended to the bottom of society.

Claims (4)

As shown in FIG. 1, a metal thin film is placed on the intermediate layer, and the upper and lower layers are combined with the side polished optical fiber, and then the light is incident to the side polished optical fiber input terminal of the upper layer. Or a polarization splitter configured to separate light from the TM mode to the output of the lower layer side polished optical fiber by injecting light into the side polished optical fiber input terminal of the lower layer. The method of claim 1, A polarization separator in which the side polished optical fibers of the upper layer and the lower layer have a symmetrical structure around a metal thin film with the thickness of the cladding of the side polished optical fiber of the upper layer and the lower layer being equal to 1.8 μm. The method of claim 1, A polarization separator having a structure in which aluminum and silver are used as a material of a metal thin film of an intermediate layer, and an aluminum or silver thin film having a thickness of 5 nm is used as an intermediate layer of a side polished optical fiber coupler. The method of claim 1, A polarization synthesizer having a structure in which polarized light is incident on an input end or an output end of a polarization splitter having a side polished optical fiber combiner structure including a metal thin film to synthesize polarization.