JPH04369888A - Semiconductor laser module - Google Patents

Abstract

PURPOSE:To provide a semiconductor laser module which is stable in transmission characteristics even if light is reflected at the joint surface of a fiber connector. CONSTITUTION:Directly modulated light 12 emitted from a semiconductor laser 1 is concentrated by an optical lens 2 and possessed of single polarization characteristics retaining its plane of polarization. Light 13 passing through an optical isolator 3 is rotated in plane of polarization by arm angle of 45 deg. but has single polarization characteristics. Then, light, 14 passing through a depolarization plate 4 is random-polarized. Even if random-polarized transmission light is multiply reflected, it hardly interferes with each other, so that a semiconductor laser module of this design excellent in transmission characteristics can be realized.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high performance semiconductor laser module necessary for optical fiber communication.

0002

2. Description of the Related Art Semiconductor laser modules can be roughly divided into a receptacle type as shown in FIG. 2 and a pigtail type as shown in FIG. The configuration and functions of each will be briefly explained below. In FIG. 2, 21 is a semiconductor laser chip, 22 is a SELFOC lens, 23 is a focused beam, 24 is a laser mount, 25 is a monitor photodiode, 26 is a package, and 27 is an optical fiber connector for extracting the laser beam. Plug, 28
is a ferrule, and 29 is a transmission optical fiber. The receptacle type module configured in this way has an FC as an optical fiber connector plug 27 for extracting laser light.
Types are commonly used.

Next, in FIG. 3, 31 is a semiconductor laser chip, 32 is a lens, 33 is a focused beam, 34 is a monitor photodiode, 35 is a thermistor, 36 is a Peltier cooler, 37 is a package, 38 is a ferrule, and 39 is an output. 40 is an optical fiber connector plug. Compared to the receptacle type module shown in FIG. 2, this type of module has a built-in temperature control function, and is also increasingly equipped with an optical isolator and the like. Furthermore, various connectors are used at the optical output end of the output optical fiber. Among them, the connector called Super PC has extremely low connection loss and return loss of -
One with a level of about 40 dB has been put into practical use.

0004

However, since the receptacle type connector uses a connector in which the tip of the ferrule 28 of the optical fiber connector plug 27 is FC polished, light returns to the laser from the end surface of the ferrule 28. Although it can be used satisfactorily in systems where the noise level does not significantly affect transmission characteristics, such as digital transmission,
It cannot be applied to systems such as analog transmission where the noise level determines transmission performance. In order to avoid the influence of this returning light, returning light from the end face of the ferrule 28 can be prevented by processing the tip of the ferrule 28 obliquely. In such an oblique system, if the coupling optical axis is made in a straight line, the coupling efficiency will deteriorate.To avoid this, if the coupling optical axis is made oblique, the incident angle of the light will change slightly each time it is attached and detached, making tracking difficult. The error becomes larger.

On the other hand, in the pigtail type, the coupling part between the focused beam 33 and the ferrule 38 is fixed during assembly, so there is no tracking error, and the package 37
Semiconductor laser chip 31 from the end face of ferrule 38 inside
The return light can also be suppressed by obliquely polishing the tip of the ferrule 38. However, although high-performance optical fiber connector plugs 40 at the fiber tips have been developed in recent years, reflections at the connector joint surface have not been completely eliminated. In particular, experience with analog transmission systems has confirmed that delicate adjustments of the optical fiber connector plug 40 often cause problems in transmission characteristics. In other words, in analog transmission, the absolute value of the light intensity itself is the information, so even if the reflectance is very small,
Appears as transmission distortion. For example, in a simple case,
There are two reflection points on the transmission path, and the distance between the reflection points is L,
If the refractive index of the transmission line is n, the transmittance of this transmission line is
It changes periodically as shown in FIG. 4 depending on the wavelength of the propagating light due to the interference effect. Here, this period T is T=c/2n
It is expressed by the formula L. Here c is the speed of light. As an example, if L=1 m, the transmittance changes at a period of about 100 MHz. On the other hand, since the laser is modulated with a current according to the transmitted information, the intensity changes at the same time as several hundred MHz/
Since the wavelength also changes at a rate of mA (in the case of a wavelength of 1.3 microns), the transmission signal strength is not transmitted accurately. Particularly in analog transmission systems, the effects of these multiple reflections directly appear on the transmission distortion characteristics, which poses a major practical obstacle.

In view of these points, it is an object of the present invention to provide a semiconductor laser module that can obtain stable transmission characteristics even if there are reflection points in the fiber connector or transmission path.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the present invention disposes a depolarizing plate on the optical path between the laser light output fiber and the semiconductor laser. Also,
An optical isolator is placed on the optical path between the depolarizing plate and the semiconductor laser.

[0008]

[Operation] With the above configuration, immediately after being emitted from the semiconductor laser, light having a single polarization characteristic becomes randomly polarized light by passing through the depolarizing plate, and as a result, there are reflection points in the transmission system that cause multiple reflections. However, the probability that multiple reflected lights will interfere with each other is drastically reduced, and as a result, the wavelength dependence of transmittance in the transmission system is extremely suppressed, making it possible to obtain stable, low-distortion transmission characteristics. .

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a configuration diagram of a semiconductor laser module according to an embodiment of the present invention. In FIG. 1, 1 is a semiconductor laser, 2 is an optical lens, 3 is an optical isolator, 4 is a depolarizing plate, 5 is a monitor photodetector, 6 is a thermistor, 7 is a Peltier cooler, 8 is a package, 9 is a ferrule, 10 is a fiber, and 11 is an optical connector plug.

The operation of the laser module constructed as described above will be explained below. The light emitted from the directly modulated semiconductor laser 1 is focused by the optical lens 2, but the light 12 at that time has a single polarization characteristic and the plane of polarization is also preserved. Although the plane of polarization of the light 13 passing through the optical isolator 3 is rotated by 45 degrees, it also has single polarization characteristics. Next, the light 14 that passes through the depolarization plate 4 becomes randomly polarized light. Here, to briefly explain the function of the depolarizing plate 4, the material forming the depolarizing plate 4 has a function of rotating the plane of polarization, and the rotation angle changes depending on the optical path length through which the light passes. Here, since the thickness of this depolarization plate 4 varies spatially, the transmitted light becomes randomly polarized light. The light 14 that has passed through the depolarizing plate in this manner becomes randomly polarized light and is coupled to the ferrule 9.

As stated in the section on the problem to be solved by the invention, when the light propagating in the fiber is single polarized light, that is, linearly polarized light, multiple reflections occur if there are multiple reflection points in the transmission path. However, in the case of randomly polarized light as in this example, even if multiple reflections occur, almost no interference occurs, so there is little distortion. Good transmission characteristics can be achieved.

[0012] In this embodiment, the case where there is only one optical lens 2 has been described, but even in a two-lens system, the location of the depolarizing plate 4 is particularly limited as long as it passes through the optical isolator 3. It's not something you do.

As described above, according to this embodiment, by arranging the depolarization plate 4 in the semiconductor laser module coupling optical system, the transmission distortion characteristics can be greatly improved.

[0014]

As described above, the present invention provides a structure in which a depolarizing plate is disposed on the optical path between a laser light output fiber and a semiconductor laser, and a structure in which a depolarizing plate is disposed on the optical path between the depolarizing plate and the semiconductor laser. Since it consists of a configuration in which isolators are placed,
Even when there are multiple reflection points in the transmission path, fluctuations in transmitted light due to multiple reflections are significantly suppressed, making it possible to provide a semiconductor laser module with improved distortion characteristics in an analog transmission system.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a semiconductor laser module in an embodiment of the present invention.

[Figure 2] Configuration diagram of a conventional semiconductor laser module

[Figure 3
] Configuration diagram of a conventional semiconductor laser module

[Figure 4] Figure 2
, a diagram showing the relationship between the transmittance of the transmission line and the change in the wavelength of light in Figure 3.

[Explanation of symbols]

1 Semiconductor laser 2 Optical lens 3 Optical isolator 4 Depolarizing plate 5 Photodetector for monitor 6 Thermistor 7. Pertier coola 8 Package 9 Ferrule 10 Fiber 11 Optical connector plug

Claims (2)

[Claims] 1. A semiconductor laser module having at least a semiconductor laser, an optical lens, and a laser light output fiber, characterized in that a depolarizing plate is disposed on an optical path between the laser light output fiber and the semiconductor laser. semiconductor laser module. 2. The semiconductor laser module according to claim 1, further comprising an optical isolator disposed on the optical path between the depolarizing plate and the semiconductor laser.