**2.1 1x7 all-optical switch**

The most advanced feature of an optically gated optical switch without any electrical parts is an easy selection of both the gating light and the signal light by selecting a suitable dye for a transparent wavelength region as the signal light and a large absorption wavelength region as the gating light. This switch is composed of four important optical components. The 1st component involves two-types of 7-bundled optical-fibres, both for incidence and collecting. The incidence optical fibre is composed of a central optical fibre for the signal light and six outer optical fibres for the gating

Fig. 1. Block diagram of the present system. The solid line is the optical fibre for data communication (wavelength of 1310-1550 nm), and the dashed line is the optical fibre for the control of a 1x7 all-optical switch (wavelength of 980 nm). The 1x7 optically gated optical

switch (1x7 all-optical switch) is connected directly to the optical interface (Opt-I/F).

The current system concerning the least stand-by power by using a 1x7 all-optical switch is composed of a 1x7 optically-gated optical switch, 7 optical interfaces for each user terminal and a reflection-type star coupler (Fig. 1). The key equipment of the least stand-by power system is both a control system and an optically-gated optical switch, which is controlled directly by each user terminal ordering to occupy the circuit by selecting the 1x7 opticallygated optical switch. To avoid collisions among the user terminals, a control system is

The most advanced feature of an optically gated optical switch without any electrical parts is an easy selection of both the gating light and the signal light by selecting a suitable dye for a transparent wavelength region as the signal light and a large absorption wavelength region as the gating light. This switch is composed of four important optical components. The 1st component involves two-types of 7-bundled optical-fibres, both for incidence and collecting. The incidence optical fibre is composed of a central optical fibre for the signal light and six outer optical fibres for the gating

**2. Current system** 

**2.1 1x7 all-optical switch** 

employed.

light. All of the collecting optical fibre is for the signal light (details of these fibres are explained in section 2.1.3). The 2nd is a dye cell made of quartz filled with a high-boilingpoint solvent and a dissolved dye for the absorbing gating light. The medium for operating an optical switch is perfectly dehumidified and deoxygenated, which brings long-life operation, even under light-irradiation (details of preparation are explained in section 2.1.2). The 3rd is a prism of a hexagonal truncated pyramid, located between a collecting lens for the dye-cell and a collimating lens for the collecting fibre. This prism brings a higher coupling efficiency to the collecting fibre. The 4th are lenses focusing both the signal light and the gating light from an incidence optical fibre to a dye-cell. Another two pairs of lenses collect signal light from the dye-cell, and focuses it to the collecting optical fibre.

Fig. 2. An external view of 1x7 optically-gated optical switch.

The typical performance of the present 1x7 optically gated optical switch is summerized in Table 1. Both the insertion loss and the crosstalk are the mean value of the 7 exit ports.


Table 1. Typical performance of a 1x7 optically gated optical switch using different axis configurations under a gating light power of 20~35 mW.

The Least Stand-By Power System Using a 1x7 All-Optical Switch 151

Fig. 4. Absorption spectrum of 0.2 wt%-YKR3080/solvent "S" in a 0.5 mm quartz cell for a

Fig. 5. Solvent "S" is composed of 4 kinds of structural isomers with the same molecular weight: 1-Phenyl-1-(2,5-xylyl)ethane, 1-Phenyl-1-(2,4-xylyl)ethane, 1-Phenyl-1-(3,4-

Another important component of the dye solution is the solvent. Here, solvent "S" is employed for a medium to form a thermal lens in the dye cell. When the gating light is

xylyl)ethane and 1-Phenyl-1-(4-ethylphenyl)ethane.

980 nm gating. The wavelength of the signal light is 1310-1550 nm.

#### **2.1.1 Principle of an optically gated optical switch**

We have developed two types of optically gated optical switches: coaxial configuration of the signal and gating light (Tanaka et al., 2007, Ueno et al., 2007), and a different axis configuration of the signal and gating light (Ueno et al., 2010). The former having a coaxial configuration can operate under a lower gating power, because the signal light is refracted at a region just heated by the gating light. This configuration possesses, however, a lower coupling efficiency to a collecting optical fibre because of ring-shaped refracted light. The later configuration of the different axis type is the currently adopted where the 1x7 optically gated optical switch (shown in Fig. 2) possesses large advantages. The basic operation of a different axis type switch is explained using a 1x2 optical switch (Fig. 3).

Fig. 3. Basic operation of a different axis configuration. Gating light (1) is mixed with the signal light (2) using a dichromatic mirror (3), and is introduced into a dye cell (thermal-lens forming element)(4) via a focusing non-spherical lens (5). The gating light and the signal light have different axis configurations. The switched light is collimated by a lens (6). In the case with gating light, the switched light is projected to a shifted position.

The reason why both the signal light and the gating light simultaneously pass through a dye-cell is due to the dichromatic property of the dye solution. An organic dye, generally, exhibits a relatively sharp absorption spectrum. An organic dye of "YKR3080", for example, exhibits both large absorption around a wavelength of 1000 nm and small absorption longer than 1200 nm, as shown in Fig. 4. This dichromatic property of an organic dye allows easy selection of both the signal and gating wavelength of light for present in the optically gated optical switch.

We have developed two types of optically gated optical switches: coaxial configuration of the signal and gating light (Tanaka et al., 2007, Ueno et al., 2007), and a different axis configuration of the signal and gating light (Ueno et al., 2010). The former having a coaxial configuration can operate under a lower gating power, because the signal light is refracted at a region just heated by the gating light. This configuration possesses, however, a lower coupling efficiency to a collecting optical fibre because of ring-shaped refracted light. The later configuration of the different axis type is the currently adopted where the 1x7 optically gated optical switch (shown in Fig. 2) possesses large advantages. The basic operation of a different axis type switch is explained using a 1x2 optical switch

Fig. 3. Basic operation of a different axis configuration. Gating light (1) is mixed with the signal light (2) using a dichromatic mirror (3), and is introduced into a dye cell (thermal-lens forming element)(4) via a focusing non-spherical lens (5). The gating light and the signal light have different axis configurations. The switched light is collimated by a lens (6). In the

The reason why both the signal light and the gating light simultaneously pass through a dye-cell is due to the dichromatic property of the dye solution. An organic dye, generally, exhibits a relatively sharp absorption spectrum. An organic dye of "YKR3080", for example, exhibits both large absorption around a wavelength of 1000 nm and small absorption longer than 1200 nm, as shown in Fig. 4. This dichromatic property of an organic dye allows easy selection of both the signal and gating wavelength of light for present in the optically gated

case with gating light, the switched light is projected to a shifted position.

**2.1.1 Principle of an optically gated optical switch** 

(Fig. 3).

optical switch.

Fig. 4. Absorption spectrum of 0.2 wt%-YKR3080/solvent "S" in a 0.5 mm quartz cell for a 980 nm gating. The wavelength of the signal light is 1310-1550 nm.

Fig. 5. Solvent "S" is composed of 4 kinds of structural isomers with the same molecular weight: 1-Phenyl-1-(2,5-xylyl)ethane, 1-Phenyl-1-(2,4-xylyl)ethane, 1-Phenyl-1-(3,4 xylyl)ethane and 1-Phenyl-1-(4-ethylphenyl)ethane.

Another important component of the dye solution is the solvent. Here, solvent "S" is employed for a medium to form a thermal lens in the dye cell. When the gating light is

The Least Stand-By Power System Using a 1x7 All-Optical Switch 153

Fig. 7. Schematic drawing of the optics of the 1x7 optically gated optical switch: 1, incidence 7-bundled optical fibre; 2, focusing lens; 3, dye solution; 4, prism of hexagonal truncated

The most important residual component for degradation of the dye solution in a quartz cell is both oxygen and water. Therefore, a dye-cell made of a quartz capillary is prepared in a vacuum glove box. First, a solvent "S" is dried using a molecular sieve for several hours. Second, a freeze-pump-thaw treatment of a dye-dissolved solution is employed several tens of times in a vacuum glove box. Next, a quartz capillary of outer diameter of 1 mm, inner diameter of 0.5 mm and 25 mm length is sealed at one end after being dried in a vacuum groove box. A dye of YKR3080 dissolved in a solvent "S" with a high concentration near saturation is charged into the capillary by a micro-syringe. Another end of the capillary is sealed temporally by glue in the vacuum glove box. At the outside of the glove box, another end of the capillary is sealed by melting quartz using a micro

Fig. 8. Micro cell made of a quartz capillary: outer diameter of 1 mm, inner diameter of 0.5 mm and 25 mm length. Both the purification and sealing processes are performed in an

Two types of a 7-bundled optical fibre are developed: one is for incidence and the other is for collecting. The incidence optical fibre is composed of a centre fibre for the signal light and 6-outer fibres for the irradiating gating-light to a dye cell. The diameter of the clad of a single mode optical fibre for optical communication is reduced from 125 μm to 40 μm, so as to reduce the centre-to-centre distance of the core to 40 μm in a 7-bundled optical fibre as

pyramid; 5, focusing lens; 6, collecting 7-bundled optical fibre.

**2.1.2 A dye-cell as one of the most important parts** 

burner (Fig. 8).

ultra-pure vacuum glove box.

**2.1.3 A 7-bundled optical fiber** 

focused into a dye solution, molecules of YKR3080 absorb energy of the irradiated light, which causes an increase in the temperature of the dye by thermal relaxation of the excited state. The transferred energy from the dye-molecule to solvent "S" (Fig. 5) around the focal point of the gating light forms a high-temperature region around the focal point of the gating light. As a relation between the refractive index (n) of solvent "S" and the temperature (T; deg.) is expressed as Equation (1); the refractive index of solvent "S" at the heated region become lower than the surrounding region (Hiraga et al., 2008).

$$\mathbf{n} = \mathbf{1.5742} \mathbf{-0.00048259 T} \tag{1}$$

This is the reason the signal light is refracted at around the focal point of the gating light. It is difficult to determine the exact shape of the region at a lower refractive index, because the dye solution has large absorbance at around 3 and the "Beer-Lambert law". We suppose the shape to be a triangular pyramid as shown in Fig. 6. We are now trying to measure the local temperature using the Raman-scattering method (Hiraga et al., 2010). In the case of different-axis configuration, the axis of the gating light (1) to be at about 40 μm parallel translation from the signal light (2) as shown in Fig. 6.

Fig. 6. Refraction of the signal light by an optical gating under different axis configurations of the gating light with the signal light: 1, gating light; 2, signal light; 3, thermal lens; 4, nonrefracted light without gating light; 5, refracted light with gating light; 6, imaginary wedgeshaped thermal lens.

Fig. 7 shows a schematic drawing of the optics of the 1x7 optically gated optical switch. It is composed of 6 kinds of optical parts. The incidence 7-bundled optical fibre (1) is employed for both the signal light and the gating light, where the centre-to-centre distance of the core is 40 μm. The collecting 7-bundled optical fibre (6) is employed for only signal light, where centre-to-centre distance of the core is 250 μm. A prism of a hexagonal truncated pyramid (4) set up between the focusing lens (5) and the collecting lens (6) is employed for enhancing the coupling efficiency to a collecting 7-bundled optical fibre (6).

focused into a dye solution, molecules of YKR3080 absorb energy of the irradiated light, which causes an increase in the temperature of the dye by thermal relaxation of the excited state. The transferred energy from the dye-molecule to solvent "S" (Fig. 5) around the focal point of the gating light forms a high-temperature region around the focal point of the gating light. As a relation between the refractive index (n) of solvent "S" and the temperature (T; deg.) is expressed as Equation (1); the refractive index of solvent "S" at the

This is the reason the signal light is refracted at around the focal point of the gating light. It is difficult to determine the exact shape of the region at a lower refractive index, because the dye solution has large absorbance at around 3 and the "Beer-Lambert law". We suppose the shape to be a triangular pyramid as shown in Fig. 6. We are now trying to measure the local temperature using the Raman-scattering method (Hiraga et al., 2010). In the case of different-axis configuration, the axis of the gating light (1) to be at about 40 μm parallel

Fig. 6. Refraction of the signal light by an optical gating under different axis configurations of the gating light with the signal light: 1, gating light; 2, signal light; 3, thermal lens; 4, nonrefracted light without gating light; 5, refracted light with gating light; 6, imaginary wedge-

Fig. 7 shows a schematic drawing of the optics of the 1x7 optically gated optical switch. It is composed of 6 kinds of optical parts. The incidence 7-bundled optical fibre (1) is employed for both the signal light and the gating light, where the centre-to-centre distance of the core is 40 μm. The collecting 7-bundled optical fibre (6) is employed for only signal light, where centre-to-centre distance of the core is 250 μm. A prism of a hexagonal truncated pyramid (4) set up between the focusing lens (5) and the collecting lens (6) is employed for enhancing

the coupling efficiency to a collecting 7-bundled optical fibre (6).

n = 1.5742-0.00048259 T (1)

heated region become lower than the surrounding region (Hiraga et al., 2008).

translation from the signal light (2) as shown in Fig. 6.

shaped thermal lens.

Fig. 7. Schematic drawing of the optics of the 1x7 optically gated optical switch: 1, incidence 7-bundled optical fibre; 2, focusing lens; 3, dye solution; 4, prism of hexagonal truncated pyramid; 5, focusing lens; 6, collecting 7-bundled optical fibre.
