BACKGROUND OF THE INVENTION

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Field of the Invention:

5 The present invention generally relates to local area networks that employ fiber optics.

State of the Art:

It is known to use both light emitting diodes (LEDs) and semiconductor laser diodes as light sources

10 in fiber optic transmission systems. LEDs are often preferred because they are rugged and reguire relatively simple driving circuits. Nevertheless, there are several advantages in using semiconductor laser diodes, instead of LEDs, as light sources in

15 fiber optic transmission systems. A primary advantage is that semiconductor laser diodes can be simply coupled to optical fibers because laser diodes produce narrow beams that do not require focusing for good optical coupling, while LEDs produce wider beams that

20 must be focused to achieve proper optical coupling. Furthermore, chromatic dispersion is negligible when using semiconductor laser diodes. Still further, laser diodes require lower drive current than LEDs.

In addition to the foregoing advantages, laser 25 diodes come packaged with a backfacet photodiode which allows the device to be monitored locally, generally not true for LEDs. Since the aging process of a laser is well-characterized, this information coupled with the local monitoring feature allows one to know well in 30 advance when a laser is getting weak. An important by-

product of monitoring the laser's output is the ability to set initially and maintain over the useful lifespan of the product a constant output. Thus, the 3 or 4 dB variation for aging effects that is generally allotted can be used to increase the optical power budget. This additional budget can allow an extra kilometer of fiber. Also, a substantial reduction in pulse width distortion may be achieved using a very narrow spectral width source such as a laser. Finally, lasers of the type used in accordance with the present invention are less expensive than LEDs due to highly automated packaging brought about by the high volume of such lasers sold into the CD player market. Thus, although lasers are generally regarded as less reliable than LEDs, by tracking performance of the laser and anticipating failure so that the laser may be replaced beforehand, overall system reliability may be enhanced.

Figure 1 shows an optical transmission system of the type known in the prior art for producing an optical output signal that can be transmitted through an optic fiber. In the system, an electrical information signal is provided to a preprocessing circuit 11. The output of the preprocessing circuit 11 is provided to a drive circuit 13 that drives a laser light source 17. The temperature of the laser light source 17 is monitored by temperature monitoring system 21, and the intensity of the laser light is monitored by optical monitoring system 15. The temperature monitoring system 21 controls a cooling mechanism 19 that prevents overheating of the laser light source 17. The optical monitoring system 15 provides a control signal to the drive circuit 13 for controlling the energy with which the laser light source 17 is driven.

SUMMARY OF THE INVENTION

Generally speaking, the present invention provides a low-cost, high-performance fiber optic local area network (LAN) that uses compact disk type semiconductor laser diodes as optical sources, where the output wavelengths of the CD-type laser diodes range from about 800 to about 900 nano-meters (nm) . Suitable semiconductor laser diodes can be fabricated by, for example, altering the junction geometry of a conventional diode slightly. The modified laser diodes can provide laser beams that are substantially more circular in cross section.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be further understood with reference to the following description in conjunction with the appended drawings, wherein like elements are provided with the same reference numerals. In the drawings:

Figure 1 is a functional block diagram of an optical transmission system according to the prior art;

Figure 2 is a sectional view of a modified CD-type semiconductor laser diode for use as a laser light source in a fiber optic LAN according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before embarking upon a detailed description of the present invention, it should be understood that typical CD-type lasers operate at wavelengths of about 780 nm. Further, it should be understood that it is not

heretofore known to use CD-type semiconductor laser diodes as light sources in fiber optic local area networks (LANs) ; instead, LANs typically are wire- connected with, for example, coaxial cable. Those LANs which are fiber based all currently use LEDs. One reason that CD-type semiconductor laser diodes have not heretofore been used as light sources in fiber optic LANs is that the IEEE specifications for fiber optical LANs, namely 802.3 for Ethernet and 802.5 for Token- Ring, require a light source having a wavelength of between 800-900 nm. Those IEEE specifications reflect the fact that one of the low-loss windows of commonly used glass fibers and the peak quantum efficiency of silicon detectors occurs at about 850 nm.

As mentioned briefly above, a common CD-type laser diode can be manufactured to operate at a wavelength of 850 nm, instead of the usual 780 nm. These modified lasers are used, according to the present invention, in low-cost, high-performance fiber optic LANs. In other words, these modified lasers provide an improved and inexpensive light source for fiber optic LANs. The modified lasers, as will now be described, can be manufactured by changing the doping of the aluminum- gallium arsenide junction of a conventional CD semiconductor diode and by. slightly modifying the junction structure.

Figure 2 shows a CD-type semiconductor laser diode having a double hetero-junction. The junction can comprise, for example, a very thin activation layer 111 of GaAs located between a cladding layer 131 of N-

AlGaAs and a cladding layer 151 of P-AlGaAs. In the illustrated embodiment, the double hetero-junction structure is formed on an n-GaAs substrate 171 with a P-GaAs cap layer 191. The emission wavelength of the

SUBSTITUTESHEET

laser diode is determined primarily by the ratio of aluminum to gallium in the active layer 111. To implement a fiber optic LAN according to the present invention, a suitable light source operating at a wavelength of 800-900 nm (typically 850 nm) can be obtained by changing the doping of the aluminum-gallium junction of the CD semiconductor diode to decrease the ratio of aluminum to gallium, thereby shifting the nominal output wavelength of the laser to within the foregoing range.

At this juncture, it should be noted that the CD- type laser diode has a tapered stripe structure 211 that provides a resonator in the active region 111. The resonator is narrowed at its ends so that an optical guide path is provided in the horizontal direction of the active layer. This structure results in a region 231 in which the current is supplied in the radiator length direction, with the result that laser emission occurs only in region 231.

In practice, the cross sectional shape of the laser beam is determined by the geometry of the emission region 231 as shown in Figure 2. When the emission region has a significantly greater horizontal extent than vertical extent, a laser beam of elliptical cross section is produced. However, in accordance with the present invention, the laser beam produced by the diode junction can be made substantially circular in cross section by modifying the junction geometry such that the width and height of the emission region 231 are substantially equal. At a minimum, the emission region of laser diode should have a ratio of width to height of less than about 4:1.

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Because optical fibers are of round cross section, a round laser beam can be more easily and efficiently coupled to the optical fiber than an elliptical laser beam. (If the width and height of the emission region were made exactly equal, however, there may be some undesirable collateral effects including the potential for excessive junction bypass currents.)

The present invention will find particular utility in fiber optic networks employing graded index fiber. In such networks, if an LED is used as the light source, chromatic dispersion caused by the LED contributes nearly half of the total dispersion experienced in the fiber. By using a narrow spectral width emitter, such as a semiconductor laser diode, chromatic dispersion may be dramatically reduced such that total dispersion may be nearly halved.

As described above, fiber optic LAN having improved cost-performance ratios are achieved by using a modified CD-type laser as a light source. When used as light sources in Ethernet, Token-Ring, and FDDI networks, the modified CD-type lasers reduce chromatic distortion in the optical fiber transmissions. As a result, LAN transmissions can have runs that are substantially longer than are possible when the LAN use LEDs as light sources. Moreover, in the case of Token- Ring LANs, there can be a greater number of concentrators on the ring before the signal must be regenerated or returned. The laser light source also provides for local monitoring enabling the light output to be held constant, which effectively increases the power budget without increasing the launch power or increasing the receiver sensitivity.

It will be apparent to those of ordinary skill in the art that the present invention may be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered to be illustrative and not restrictive. The scope of the invention as indicated by the appended claims rather than the foregoing description and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.