JPH03185890A - Short-wavelength semiconductor laser module - Google Patents

Abstract

PURPOSE:To obtain good transmission characteristics by a method wherein a long- wavelength band single mode fiber having a 0-order mode and a primary mode, whose propagation delay times to a short-wavelength light source are large, is used as an optical fiber for output use and the length of a fiber, the optical path difference between the modes of which is sufficiently longer than the coherence length of a semiconductor laser light source, is used as the length of the optical fiber for output use. CONSTITUTION:When a difference between the propagation delay times of a 0-order mode and a primary mode of an optical fiber for output use (a long-wavelength band single mode fiber) 6 is assumed DELTAtau, an optical path difference DELTAI between the modes at an output end of the fiber is represented by DELTAI=(DELTAtauXL)/Vf. Here, the Vf is the mean propagation velocity of an optical signal in the fiber and the L is a fiber length. When a coherence length of a semiconductor laser light source is assumed Ic, there is a need to set on the condition of Ic<<DELTAI for reducing sufficiently a coherence between the modes at the output end of the optical fiber. Here, a length L' necessary to the fiber for suppressing the generation of modal noise at a node between the film 6 and an optical fiber line 5 becomes a length of L'>>(IcXVf)/DELTAtau and a necessary optical fiber length for output use is inversely proportional to the difference between the propagation delay times.

Description

[Detailed Description of the Invention] <Industrial Application Fields> The present invention relates to a short wavelength semiconductor laser module that is applied to an optical communication system in which a short wavelength semiconductor laser is used as a light source and a long wavelength band single mode fiber is used as an optical transmission line. It's about configuration.

(Prior Art) When transmitting a short wavelength optical signal below the cutoff wavelength through a long wavelength band single mode fiber, the single mode condition is not satisfied, resulting in multimode transmission.

In multimode transmission, interference between propagation modes occurs due to the coherence of the light source, and a speckle pattern appears at the fiber output end. If there is axis misalignment at the connector or fusion splice point, fluctuations in the speckle pattern become modal noise and deteriorate transmission characteristics.

The propagation mode in an optical communication system that combines a short-wavelength light source and a long-wavelength single-mode fiber is essentially two-mode transmission, the zero-order mode and the first-order mode. The effect is extremely large compared to modal noise in a multimode fiber optical line.

Conventionally, the following methods have been adopted to reduce modal noise.

FIG. 4 is a diagram showing an example of the configuration of a short wavelength semiconductor laser module by a conventional method, in which 1 is a short wavelength semiconductor laser module, 2 is a short wavelength semiconductor laser diode, 3 is a coupling lens, and 4 is a semiconductor laser module. Reference numeral 5 denotes an optical fiber line, which is an output single mode fiber having a cutoff wavelength shorter than the emission wavelength. In this configuration example, by using a single mode fiber with a cutoff wavelength shorter than the emission wavelength of the short wavelength semiconductor laser as the output optical fiber,
This is intended to suppress the generation of higher-order modes, avoid multimode transmission, and suppress the generation of modal noise. However, in this configuration example, it is difficult to always ensure conditions for suppressing the generation of higher-order mode components in the connection between the optical output single mode fiber 4 and the optical fiber line 5.
In order to actually realize this, it had the disadvantage of requiring highly accurate characteristics in the connector, fusion splicer, and fiber core eccentricity. Even if the connection of the line 5 is made well, if there is a connection point with a large axis misalignment in the middle of the fiber line and cable handling etc. is performed, 1
The next mode will occur there, so it is not a fundamental countermeasure.

(Problems to be Solved by the Invention) The present invention prevents the generation of modal noise at the optical line connection point by reducing the coherence between modes during optical output fiber propagation, An object of the present invention is to provide a short wavelength semiconductor laser module that can be applied to an optical communication system using a mode fiber as an optical transmission path and obtain good transmission characteristics.

(Means for Solving the Problems) The short wavelength semiconductor laser module of the present invention uses a long wavelength band single mode fiber having a large propagation delay time of the 0th mode and the 1st mode with respect to the short wavelength light source. A fiber is used as the output optical fiber, and its length is such that the difference in optical path length between modes that occurs during propagation is sufficiently longer than the coherent length of the light source.

<Example) Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a configuration diagram of an embodiment of a short wavelength semiconductor laser module of the present invention, and 6 is a long wavelength band single output for output with a large propagation delay time of the 0th mode and the 1st mode for a short wavelength light source. It is a mode fiber. The operation of this embodiment will be explained below.

If the propagation delay time difference between the 0th mode and the 1st mode of the output optical fiber is △τ, the optical path length difference at the fiber output end is Δ
■ is expressed by the following formula.

ΔI=(Δτxt,)/vr (t) where ■, is the average propagation speed of the optical signal in the fiber, and L is the fiber length. Furthermore, if the coherent length of the semiconductor laser light source is IC, then in order to sufficiently reduce the coherence between modes at the output end of the optical fiber, the following steps must be taken: (1). It is necessary to set Δ■(2). Here, from equations (1) and (2), the length L' required for the output fiber in order to suppress the generation of modal noise at the connection point between the output fiber and the optical fiber line is given by the following equation.

L'〉〉〉(■oxvf)/Δτ[3] The required output optical fiber length is inversely proportional to the propagation delay time difference. That is, the larger Δτ is, the shorter the output fiber needs to be, and the smaller the short wavelength semiconductor laser module can be realized.

Figure 2 shows an example of the shape of the refractive index distribution of a long wavelength band single mode fiber that can be used as an output optical fiber.
is a step type, and b] is an example of a shape with a hang and a tail. FIG. 3 shows an example of the propagation delay time characteristic for a short wavelength optical signal of a long wavelength band single mode fiber having the refractive index distribution shape shown in FIG. 2.

The refractive index distribution shape of A shown in Figure 3 is a step type, and the cutoff wavelength (λ, ) is 1.3 μm, and the mode field diameter (MOP) is 8.5 μm. Type: λce=1.1, MDF
It has a propagation delay time that is approximately three times that of a 10.5 μm fiber. That is, by using a fiber with a refractive index distribution shape A as an output optical fiber, it is possible to suppress the generation of modal noise with an output optical fiber that is 1/3 shorter than a fiber with a refractive index distribution shape B.

(Effects of the Invention) As explained above, the short wavelength semiconductor laser module of the present invention can suppress the generation of modal noise in an optical fiber line, has a simple configuration, is small in size, and has a long wavelength band single layer. When applied to an optical communication system using a one-mode fiber as an optical path, there is no need for special restrictions on connectors, fusion splicers, fiber core eccentricity, fiber handling conditions, etc., and extremely large effects can be expected.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of an embodiment of a short wavelength semiconductor laser module of the present invention, Fig. 2 is a diagram showing an example of the shape of the refractive index distribution of a long wavelength band single mode fiber, and Fig. 3 is a diagram showing an example of the shape of the refractive index distribution of a long wavelength band single mode fiber. A diagram showing an example of propagation delay time characteristics for a short wavelength optical signal of a one-mode fiber, and FIG. 4 is a diagram showing an example of the configuration of a conventional short wavelength semiconductor laser module. 1...Short wavelength semiconductor laser module 2...Short wavelength semiconductor laser diode 3...Coupling lens 4...Single mode fiber for output 5 having a cutoff wavelength shorter than the emission wavelength of the semiconductor laser...・Optical fiber line 6...Long wavelength band single mode fiber for output with large propagation delay times of the 0th mode and 1st mode for a short wavelength light source Patent applicant Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. A long wavelength single mode fiber with a large propagation mode delay time difference for a short wavelength optical signal is used as an output optical fiber of a short wavelength semiconductor laser module, and the length of the output optical fiber is set as the length of the output optical fiber for a short wavelength optical signal. The optical path length difference between the 0th mode and the 1st mode at the fiber optical output end is
A short wavelength semiconductor laser module characterized by a coherent length that is sufficiently longer than the coherent length of a light source.