Light source

[0002] 1. Field of the Invention

[0003] The present invention relates generally to light sources used in optical communication systems, and particularly to laser light sources capable of generating steady light with controlled intensity.

[0004] 2. Description of Related Art

[0005] Semiconductor lasers are used in many fields including optical communication systems and optical memory systems. Semiconductor lasers include edge-emitting lasers and surface-emitting lasers. Edge-emitting lasers require high threshold current, therefore surface-emitting lasers are frequently preferred. In particular, vertical-cavity surface-emitting lasers are being adopted in many new systems because they are relatively easy to manufacture and are cost-efficient. Such surface-emitting laser must be mounted in a package to prevent dirt and external temperature variations from impairing the performance of the laser, and to prevent components within the package from aging prematurely. The package provides a stable working environment for the laser, and the light intensity of the laser can be maintained at a steady level.

[0006] FIG. 1 shows a conventional method for packing a laser adopting TO-CAN structure, as disclosed in U.S. Pat. No. 5,835,514. A light source 1 comprises a header 4, electrical conductors 5, 6 and 7, a light sensor 8, a laser 9, a can 2 and a convex beam-splitting lens 11. The light sensor 8 is mounted on the header 4. The laser 9 is mounted on the light sensor 8, with a light receiving area of the light sensor 8 surrounding a periphery of the laser 9. A window 10 is mounted in a top portion of the can 2. The can 2 is mounted on the header 4, and the laser 9 and the light sensor 8 are mounted within the can 2. The convex beam-splitting lens 11, such as a ball lens, is mounted within the window 10. The electrical conductors 5, 6 and 7 electrically connect with and extend from the header 4 to connect with an external control circuit. Thus, the laser 9 and the light sensor 8 are connected with the external control circuit. Part of the light beam generated by the laser 9 is reflected by the convex beam-splitting lens 11 to the light sensor 8. The light sensor 8 generates an electrical signal in response to the reflected light beam, and transfers the signal to the external control circuit via the electrical conductors 5, 6 and 7. The light intensity of the laser 9 is thereupon adjusted to maintain the emission power of the laser 9 at a steady level.

[0007] The above-described light source has serious drawbacks. The convex beam-splitting lens 11 is a ball lens. Thus, the reflected light beam is scattered over a wide area including areas outside a periphery of the light sensor 8. Furthermore, the laser 9 is mounted on the light sensor 8, thereby blocking part of the light receiving area of the light sensor 8. These factors in combination result in only part of the reflected light beam being received by the light sensor 8. Furthermore, part of the reflected light beam 14 goes back to the laser 9 itself. This interferes with the light being emitted by the laser 9, and results in unsteady emission of the laser 9.

[0008] FIG. 2 is a graph showing the relationship between emission power P₁and the drive current I of the laser 9 at different external temperatures T₁, T₂. It can be seen that the P₁-I characteristic curve 12 changes into the P₁-I characteristic curve 13 in response to a change of the external temperature from T₁to T₂. That is, at an identical drive current, the laser 9 generates different emission powers in response to different external temperatures. In order to maintain the emission power of the laser 9 at a steady level, an external control circuit needs to be adopted to adjust the drive current to the laser 9 and thereby control the emission power of the laser 9.

[0009] Referring to FIG. 3, curves 23, 24 respectively show the relationships between reflected light power P₂of the laser 9 and the light receiving area X₁-X₂, X₃-X₄of the light sensor 8 at different external temperatures T₁, T₂. That is, the light energy distribution characteristic curve 23 of the light receiving area X₁-X₂, X₃-X₄of the light sensor 8 changes into the characteristic curve 24 in response to the change of the external temperature from T₁to T₂. A ratio of the light energy received by the light sensor 8 to the total reflected light energy is variable. Accordingly, the light sensor 8 inaccurately measures the output light power of the laser 9. The external control circuit cannot control the laser 9 precisely, and the emission power of the laser 9 is unstable.

[0010] FIG. 4 shows another conventional method for packing a laser, which includes an inclined top structure. A laser 19 and a light sensor 20 of a light source 111 are separately mounted on a header 17. A planar beam-splitting lens 22 is placed within a window 21 of an inclined top of a can 15. This design eliminates the problem of reflected light going back to the laser 19 itself. However, the beam-splitting lens 22 does not converge the reflected light beams. The light sensor 20 still receives only a fraction of the reflected light beams, which results in imprecise measurement of a ratio of the reflected light to the emitted light. Furthermore, the beam-splitting lens 22 is sensitive to changes in temperature. The beam-splitting lens 22 distorts with any change in temperature, resulting in a variable spread of reflected light beams reaching the light sensor 20. Thus, the external control circuit cannot precisely control the light intensity of the laser 19, and the emission power of the laser 19 is unstable.

[0011] Referring to FIG. 5, curves 25, 26 respectively show the relationships between reflected light power P₃of the laser 19 and the light receiving area X₁-X₂of the light sensor 20 at different external temperatures T₁, T₂. That is, the light energy distribution characteristic curve 25 of the light receiving area X₁-X₂of the light sensor 20 changes into the characteristic curve 26 in response to the change of the external temperature from T₁to T₂. A ratio of the light energy received by the light sensor 20 to the total reflected light energy is variable. Accordingly, the light sensor 20 inaccurately measures the output light power of the laser 19. The external control circuit cannot control the laser 19 precisely, and the emission power of the laser 19 is unstable.

[0012] In view of the above, it is an object of the present invention to provide a light source, the emission power of which can be maintained at a steady level.

[0013] It is another object of the present invention to provide a light source having a beam splitter which can converge light beams to obviate the effects of changes of the external temperature.

[0014] In order to achieve the objects set above, a light source in accordance with the present invention generates an output light beam having a controlled intensity. The light source comprises a header, electrical conductors, a light sensor, a laser, a can, a convex lens, and an external control circuit. A window is defined in an inclined top of the can, and a convex lens serving as a beam-splitter is mounted within the window. A convex surface of the inclined convex lens faces away from the light sensor. The laser and the light sensor are separately mounted on the header. The electrical conductors extend from the header to electrically connect with the external control circuit. Emitted light beams of the laser are divided by the convex lens into output light beams and reflected light beams. The output light beams are transmitted through the convex lens as an optical carrier wave for signals. The reflected light beams are reflected toward the light sensor as feedback light for measurement. The light sensor generates electrical signals representing the intensity of the light energy falling on it. The external control circuit adjusts the emission power of the laser according to the electrical signals it receives from the light sensor. The inclined convex lens has a pre-determined curvature such that emitted light beams of the laser reflected from the lens converge onto the light sensor. Accordingly, the reflected light beams received by the light sensor are accurately proportional to the emitted light power of the laser. Thus the external control circuit can precisely control the light intensity of the laser to maintain the emission power of the laser at a steady level.

[0015] Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:

[0016] FIG. 1 is a schematic cross-sectional view of a conventional light source;

[0017] FIG. 2 is a graph showing a relationship between emission power of the laser of the light source of FIG. 1 and drive current of the light source of FIG. 1, at different external temperatures;

[0018] FIG. 3 is a graph showing the relationship between reflected light power of a laser of the light source of FIG. 1 and a light receiving area of a light sensor of the light source of FIG. 1, at different external temperatures;

[0019] FIG. 4 is a schematic cross-sectional view of another conventional light source;

[0020] FIG. 5 is a graph showing a relationship between reflected light power of a laser of the light source of FIG. 4 and a light receiving area of a light sensor of the light source of FIG. 4, at different external temperatures;

[0021] FIG. 6 is a schematic cross-sectional view of a light source in accordance with a preferred embodiment of the present invention; and

[0022] FIG. 7 is a graph showing a relationship between reflected light power of a laser of the light source of FIG. 6 and a light receiving area of a light sensor of the light source of FIG. 6, at different external temperatures.

[0023] Reference will now be made to the drawings to describe the present invention in detail.

[0024] Referring particularly to FIG. 6, a light source 27 in accordance with the present invention is depicted. The light source 27 comprises a header 30, electrical conductors 31, 32 and 33, a light sensor 40, a laser 39, a can 28, a convex lens 36 and an external control circuit 34. The laser 39 is mounted on a middle of the header 30, and the light sensor 40 is mounted on a periphery of the header 30. The electrical conductors 31, 32 and 33 electrically connect with a circuit of the header 30 and the control circuit 34. The can 28 is mounted on the header 30 to enclose the laser 39 and the light sensor 40 therein.

[0025] A top of the can 28 is an inclined plane, with a window 35 defined therein. A convex lens 36 is mounted within the window 35. A convex surface of the convex lens 36 faces away from the light sensor 40. The convex lens 36 comprises a concavity 37. A curvature of the concavity 37 is pre-determined so that the emitted light beams of the laser 39 reflected from the lens 36 converge onto the light sensor 40. The emitted light beams of the laser 39 are divided by the concavity 37 into output light beams 51 and reflected light beams 52. The output light beams 51 are transmitted through the convex lens 36 as an optical carrier wave for signals. The reflected light beams 52 are reflected toward the light sensor 40 as feedback light for measurement.

[0026] The inclined top of the can 28 of the light source 27 cooperates with the convex lens 36 to converge the reflected light beams, such that all reflected light beams are received by the light sensor 40. No reflected light beams are fed back to the laser 39, thereby avoiding adverse effects on the stability of the laser 39. Referring to FIG. 7, curves 41, 42 show the relationships between the reflected light power P₄of the laser 39 and the light receiving area X₁-X₂of the light sensor 40 at different external temperatures T₁, T₂respectively. A light energy distribution characteristic curve 41 of the light receiving area X₁-X₂of the light sensor 40 changes into a characteristic curve 42 in response to a change of the external temperature from T₁to T₂. When comparing FIG. 7 with FIGS. 3 and 5, it can be seen that the convex lens 36 of the present invention is less susceptible to variation in the external temperature in reflecting the emitted light beams from the laser 39. Furthermore, the convex lens 36 converges the reflected light beams. Accordingly, the reflected light beams received by the light sensor 40 are accurately proportional to the emitted light power of the laser 39. Thus, the external control circuit can precisely control the light intensity of the laser 39 to maintain the emission power of the laser 39 at a steady level.

[0027] While the preferred embodiment in accordance with the present invention has been shown and described, equivalent modifications and changes known to persons skilled in the art according to the spirit of the present invention are considered within the scope of the present invention as defined in the appended claims.