JPH01255803A - Optical fixed attenuator - Google Patents

Abstract

PURPOSE:To suppress the deterioration in transmission characteristic by forming a reflecting film to the clad part on both faces of an optical fiber. CONSTITUTION:The optical fixed attenuator, one end of which is used as an optical connector plug and the other end as an optical receptacle, is used as an object and the reflecting film 9 is formed to the clad part 8 of the optical fiber 1 at both end faces of the optical fiber 1. A spacer 3 is provided to the receptacle side of a terminal fitting 2 and the quantity of loss (quantity of attenuation) is determined by the length of this spacer 3. Since the reflecting film 9 is formed to the clad part 8 of the optical fiber 1, the light propagation of a clad mode in the juncture of the connector plug of a transmission line and the connector plug of the optical fixed attenuator can be suppressed. The interference of the clad mode propagated light at the other end of the optical fixed attenuator and the core mode propagated light is thereby suppressed and the deterioration in the transmission characteristic is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a fixed optical attenuator, and particularly to a fixed optical attenuator for a single mode fiber inserted into a single mode fiber transmission line.

[Conventional technology]

Referring to FIG. 2, in general, in a single mode fiber moonlight fixed attenuator, an optical fiber l held by a terminal fitting 2 is inserted and fixed on the central axis of a fixed attenuator housing (exterior member) 5. As shown in FIG.
and a cladding part 8. Further, a spacer 3 is disposed on the receptacle side of the terminal fitting 2, and the amount of loss (attenuation) is determined by the length of this surface 3.

Note that as shown in Figure 2, a part of the terminal fitting 2 is aligned with the alignment sleeve 4.
A knob 6 is provided on the outer periphery of the exterior member 5.

By the way, a laser diode is used on the transmitting side.

In a single-mode fiber optic communication system in which the PCM modulated optical output of a laser diode is passed through a single-mode fiber transmission line and received by an APD (Anonymous Transmission Diode), the optical output level of the transmitting laser diode is normally Due to the system design, the maximum reach of this system is set to an achievable value.

Therefore, when transmitting over a distance that is sufficiently shorter than the maximum reach, the transmission path loss is small.

The received light level will become higher and will deviate from the APD's optimum light receiving level. In such a case, a fixed optical attenuator for a single mode fiber having a loss that provides the optimum light reception level for the APD is inserted into the transmission line.

[Failure to solve the problem]

However, when inserting a fixed optical attenuator into a transmission line, since it is inserted through an adapter, the end face of the transmission line connector plug and the fixed optical attenuator must be connected before the spacer 3, which is responsible for the loss of the fixed optical attenuator. The end face of the connector plug will be located there. When an attenuator is inserted into the transmission line in this way,
Not only does the Fresnel reflection surface at the fiber end face increase, but also the loss due to the spacing and axis misalignment between the connector end faces increases.

Furthermore, this loss is incident on the cladding portion of the optical fiber from one end, propagates to the other end face in the cladding mode, and is emitted. Therefore, in the space part, the light propagated in the cladding mode and the light propagated in the core part do not interfere, and this interference applies intensity modulation to the light in the core part. As a result, there is a problem that the transmission characteristics deteriorate.

Referring to FIG. 4, the intensity distribution of light propagating in a two-clad mode has an inner diameter with a core diameter of 2 a 1.

This results in an annular intensity distribution with an outer diameter equal to the cladding diameter 2 a 2 . When light having this annular intensity distribution is emitted from the end face of the cladding portion, it spreads due to diffraction and causes interference. That is, as shown in FIGS. 4(a) and 4(b), the light intensity distribution (ψ) in the far field at a distance t from the cladding end face is given by the following equation.

al 1 a2 is the core radius, and the cladding radius E is the electric field strength Zi = 2fψai/λ (2) λ is the wavelength of light ψ is the beam divergence angle in the far field J1 is the first-order Bessel function M = - (3) Equation (1) shows the interference of only the cladding mode propagation light, but in the optical attenuator, the interference occurs between the core propagation light and the cladding propagation light.

The intensity distribution ■2(ψ) of light propagating through the core is sufficient if only the fundamental mode is considered, and is given by the 92nd-order equation.

I2(ψ)-8-(=)...(4)r:
Beam spread W: The interference between the Nupot size I, (ψ) and I, (ψ) is I1 (
This occurs at the overlap between ψ), I, and (ψ). The overlapping part still has an annular intensity distribution, but it is not a uniform intensity, but the (4th)
) is the intensity distribution shown by the formula. At this time, the intensity distribution of the interference light is expressed by the following equation.

Due to this intensity modulation, the intensity of light entering the optical fiber from the fixed optical attenuator becomes susceptible to disturbances such as fluctuations.

The uninvented fixed optical attenuator is intended for an optical fixed attenuator in which one end is used as an optical connector plug and the other end is used as an optical receptacle, and the invention is based on a method in which a reflective film is formed on the cladding portion of the optical fiber on both end faces of the optical fiber. It is a feature.

〔Example〕

Next, the present invention will be explained with reference to examples.

Referring to FIGS. 1 and 2, an optical fiber 1 is provided with a core portion 7 and a cladding portion 8, and a reflective film 9 is formed on the cladding portion 8 at both ends of the optical fiber 1. . As mentioned above, the spacer 3 is provided on the receptacle side of the terminal fitting 2, and the amount of loss (
amount of attenuation) is determined. As mentioned above, since the reflective film 9 is formed on the cladding part 8 of the optical fiber l, the optical propagation of the cladding mode is suppressed at the connection part between the transmission line connector plug and the optical fixed attenuation 'JE connector plug. be able to. Therefore, interference between the cladding mode propagating light and the core mode propagating light at the other end of the optical fixed attenuator can be suppressed.

〔Effect of the invention〕

As explained above, in the present invention, reflective films are formed on the cladding portions on both end faces of the optical fiber, so that the cladding mode propagating light and the core mode propagating light, ! : It has the effect of suppressing interference and suppressing deterioration of transmission characteristics.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an optical fiber applied to a fixed optical attenuator according to the present invention, FIG. 2 is a diagram showing a fixed optical attenuator, and FIG.
The figure shows a conventional optical fiber. FIGS. 4(a) and 4(b) are diagrams for explaining the intensity distribution of light. 1... Optical fiber, 2... Terminal fitting, 3... Spacer. 4... Alignment sleeve, 5... Exterior sleeve, 7...
- Core part, 8... Clad part, 9-... Reflective film. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. An optical fixed attenuator comprising an optical fiber and an attenuation member connected to one end of the optical fiber, characterized in that a reflective film is formed on a cladding portion of the optical fiber on both end surfaces of the optical fiber. optical fixed attenuator.