British Telecom trims laser costs

Aug. 1, 1995

British Telecom trims laser costs


British Telecom trims laser costsBritish Telecom plc researchers working at Optoelectronics Group Laboratories in Martlesham Heath, Suffolk, England, have developed a simple, low-cost technique for coupling more than half the light output from a semiconductor laser into an optical fiber. They have overcome alignment problems between semiconductor lasers and optical fibers by combining three basic inventions.

First, they have enlarged the spot size of the laser to be the same size as the fiber. This spot size improves the efficiency of coupling light into the fiber and simultaneously reduces the precision normally required in positioning the laser to the fiber.

The second breakthrough is the development of a precision cleaving technique that allows the position of the laser active region relative to the edge of the chip to be determined within 0.25 micron.

Third, the researchers developed the micro-machined silicon mount on which the laser sits. This mount incorporates a silica stop to which the laser is aligned by simply pushing it until contact is made against the silica wall. The fiber is positioned by laying it in a precision-etched V-groove and gluing it into place.

The combination of these three techniques has enabled British Telecom Laboratories researchers to obtain coupling efficiencies of more than 50% by purely passive alignment, removing the need for the expensive active alignment process that is currently used. In addition, within five years these techniques could reduce the cost of telecommunications lasers from $150 to $15.

The impetus for developing these technologies resulted from the low cost of semiconductor lasers and compact disk lasers, compared to the higher cost of fiber-optic telecommunications lasers. These telecommunications lasers operate in the infrared spectrum, which is not visible to the human eye. But, like compact disk lasers, if fiber-optic telecommunications lasers are to be brought into every home, then costs have to be reduced.

The high cost of fiber-optic telecommunications lasers ($150) results from the need to couple the light from the laser into an optical fiber. To achieve this coupling, the laser and the optical fiber must be aligned to within less than 1 micron because an offset of 1.2 microns halves the amount of light that can be coupled into the fiber. Achieving this accuracy has required the use of active alignment, with each laser having to be turned on while the fiber is moved around to maximize the amount of coupled light. The fiber has to be fixed in place, usually by welding with a high-power laser. This time-consuming process requires a skilled operator and expensive computer-controlled equipment.

The recently developed technology overcomes the current problem of alignment between lasers and fibers. By reducing the cost of lasers, it could enable every home to have a laser-driven link to the information superhighway.

Pick-and-place assembly

The breakthrough combines advanced laser design with silicon microbench packaging to achieve the necessary alignment passively. This approach allows inexpensive, automated, pick-and-place assembly methods to replace today`s time-consuming and costly high-precision method of active alignment. The new passive process allows the same proportion of light (greater than 50%) to be coupled into the fiber as does the more expensive active process.

Although other laboratories have reported similar passive alignment techniques, the British Telecom design is claimed to be more than seven times more efficient than the others. In the past, conventional semiconductor lasers have required expensive packaging techniques because of the mode mismatch between the semiconductor laser and singlemode optical fiber. Semiconductor lasers have an optimum active cross-sectional area that is typically 1 to 1.5 microns wide by 0.15 micron thick. The light guided by such a device has a spot size of approximately 0.75 to 1 micron.

The spot size is defined as the point at which the light intensity falls to half its value relative to the center of the spot. The optimum active dimension is determined by both the refractive indexes of the semiconductor materials used and the need to obtain high electrical current densities within the active region while maintaining a low operational current. A larger mode size--for instance, a 9-micron diameter of a singlemode optical fiber--is determined by its lower refractive index.

This modal mismatch allows only approximately 10% of the light from the laser to be coupled into a cleaved optical fiber. Because the laser is attenuated by the fiber, this poor coupling efficiency limits the distance over which the optical signal can travel before it needs to be amplified or regenerated.

Traditionally, this problem has been overcome by placing a lens between the laser and the fiber; in many cases, the lens is formed on the end of the fiber. The use of a lens typically boosts the coupling efficiency to about 50% (allowing the signal-transmission distance to be increased by as much as 25 kilometers). The use of a lens, however, greatly reduces the tolerance to movement between the fiber and the laser; a lateral movement of 1.2 microns reduces by 50% the amount of coupled power. q

Dave Wilson writes from London.

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