Optical Device With a Laser-to-Fiber Coupler

1. Field of the Invention

This invention relates to an optical device and, in particular to a laser-to-fiber coupler which guides an optical beam from a semi-conductor laser to an optical fiber used for transmitting the optical beam.

2. Description of the Prior Art

Rapid progress in the development of optical fiber and the semiconductor laser is accelerating a practical use of an optical communication systems.

However, there remain still many problem to be solved.

One of these problems a connection concerns between a semi-conductor laser and an optical fiber used as transmission line. A reflected optical beam generated in an optical fiber due to a fiber-to-fiber or to other device connections, a crack, bending of a fiber, and so on, may be introduced from the input surface of the optical fiber into the active region of the semiconductor laser such as a GaAs laser, and make the operation of the laser unstable. That is, when the reflected optical beam is introduced into the active region of the semiconductor, laser, the drive current to optical output characteristic, spectrum and modulated optical output wave of the laser are effected by the reflected optical beam. The effect is proportional to the quantity of the reflected beam.

This problem is a serious one for an optical communication system. Although many types of optical connectors have been proposed, there is no optical connector that has solved the above problem

Therefore, an object of the present invention is to provide a laser-to-fiber coupler or an optical device which couples the output optical beam emanating from semiconductor laser with the optical fiber effectively, and at the same time, reduces the amount of the reflected beam impinging into the active region of the semiconductor laser.

To attain the above object, the optical device in accordance with this invention is constructed by a waveguide arranged between the input surface of a transmitting optical fiber and a semiconductor laser. The waveguide has the same construction as the optical fiber, a so-called step type optical fiber. That is, the waveguide is made by a core having a uniform reflective index, and a cladding layer having a refractive index smaller than that of the core.

Further, at least one of the core radius and numerical aperture of the waveguide is smaller than that of the transmitting optical fiber to be connected.

In the above construction, most of the reflected optical beam from the optical fiber is not coupled with the waveguide.

Accordingly, the quantity of the reflected beam injected into the active region of the semiconductor laser is extremely reduced and the operation of the laser becomes stable.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic cross sectional view of one embodiment of an optical device in accordance with this invention.

FIG. 2 is a cross sectional view of one embodiment of an optical device constructed in accordance with this invention.

FIGS. 3, 4, 5, 6, 7, 8, 9 and 10 are schematic cross sectional views of other embodiments of optical device in accordance with this invention.

FIG. 1 shows a schematic cross-sectional view of an optical device in accordance with this invention for explaning the principle of this invention. In FIG. 1, optical beam 5 is emanated from the active region 4 of a semiconductor laser 1 and most of the beam is coupled with a waveguide 2 and an optical fiber 3, where the waveguide 2 and the optical fiber have the same type of construction consisting of a core having uniform refractive index n.sub.1 and a cladding layer having refractive index n.sub.2 lower than that of the core, namely, a step type optical fiber.

The length of the waveguide is over 2.about. tens centimeter depending on the optical device, namely the laser-to-fiber coupler. The distance between the output surface of the laser 1 and the input surface of the waveguide 2 is about several micrometers.

Further, an essential feature of this invention is that at least one of the core radius D.sub.2 and numerical aperture (N.A.).sub.2 of the wave guide 2 is smaller than that D.sub.3, (N.A.).sub.3 of an optical fiber 3 to be connected. Generally, some of the optical beam in an optical fiber is reflected at points between two optical fibers or an optical fiber and another optical device and by a bending and the existence of cracks and inpurities.

Therefore, some of the reflected beam is transmitted reversely and impinges into the active region of the laser 1.

However, in the optical device in accordance with this invention described above, the quantity of the reflected beam impinging on the active region is remarkably decreased without a reduction of optical coupling from the semiconductor laser to the optical fiber.

Assuming that core radii and numerical apertures of the waveguide 2 and an optical fiber 3 are D.sub.2, D.sub.3, NA.sub.2, NA.sub.3 respectively and the reflected beam in the optical fiber is distributed uniformly over the input surface of core 3-2, the ratio R between quantity of the reflected beam emanating from the input surface of the optical fiber 3 and the quantity of the reflected beam been coupled with the waveguide 2 is represented by the following formula. ##EQU1##

The numerical aperture N.A., which is a measure of the light gathering ability of the optical fiber, is defined as follows:

As described above, D.sub.2 <D.sub.3 and or NA.sub.2 <NA.sub.3, the value of the ratio R is smaller than one.

For example, for assuming D.sub.2 =1/2D.sub.3 and/NA.sub.2 =NA.sub.3, R is equale to 0.25. By adding the condition NA.sub.2 <NA.sub.3 to the above condition D.sub.2 =1/2D.sub.3, the value R is decreased further.

In FIG. 2, which shows a cross-sectional view of an embodiment of an optical device in accordance with this invention, a (measstripe-geometry) GaAs semiconductor laser 1 with an active layer 4 (thickness of about 0.1.mu. meter) is mounted on a copper mount 11 which is attached on one inner surface of housing 10 by indium solder. The laser 1 is energized by a electrical leads 12 which extend through housing 10. Layer 21 is an insulating material. Waveguide 2 is attached through aperture of the housing 10. The surface of the waveguide 2 is fixed to the inner surface of the aperture with bonding material such as Ni, Au and In. The waveguide 2 is axially aligned with an optical fiber 3 to be connected and the active layer 4 of laser 1. The distance between the output surface of laser 1 and the input surface of the waveguide is about 5 .mu.m. The end of an optical fiber to be connected with the laser 1 is disposed in termination ferrule 13 which is provided with an aperture 14 that is tapered to facilitate the insertion of fiber 3 therein. The end of the fiber 3 and ferrule is flat and substantially perpendicular to the longitudinal axis of fiber 3.

Ferrule 13 is inserted into an aperture 22 provided in the housing 10. Sleeve 15 is threaded onto housing 10 and tighted, thereby causing end face 16 to bear upon the bottom of the aperture 22. The ferrule 13, sleeve 15 and aperture 14 form a fiber mechanically connecting means.

In the above embodiment, the core radius D.sub.2 of the waveguide 2 is 50 .mu.m and the core radius D.sub.3 of the optical fiber 3 is 80 .mu.m, and the numerical aperture NA.sub.2 of the waveguide 2 is equal to that of the optical fiber 3.

It is needless to say that this invention is not limitted to above embodiment.

To improve the coupling efficiency and the reduction of coupling of the reflecting beam, the following arrangements may be used.

In FIGS. 3, 4 and 5, the surface of at least one end of the waveguides 2-4, 2-5 and 2-6 is coarse or roughened. These embodiments are useful for effectively diffusing light and eliminating undesirable variations in the intensity of the reflected beam due to the inequality of light distribution in the optical fiber 3.

In the embodiment shown in FIG. 6, a thin transparent film 17 is interposed between the waveguide 2 and the optical fiber 3. Both surfaces of the film 17 are rough in order to obtain the same effect as those of roughened surfaces of the waveguides.

In the embodiments shown in FIGS. 7 and 8, in order to improve the optical coupling efficiency, optical lenses 18 and 19 are located between the semiconductor laser 1 and the waveguide 2.

In the embodiment shown in FIG. 9, an optical coupling part is constructed by the combination of optical lens 20 and a waveguide 2-6 of which both surfaces are roughened.

In the embodiment shown in FIG. 10, an optical coupling part is constructed by the combination of an optical lens 20, a waveguide 2 and thin film 17 having rough surfaces aligned in order.

These embodiments of optical devices have advantages that the optical coupling efficiency is improved and, at the same time, it is possible to stabilize the operation of the semiconductor laser freely from the reflected beam.