At&t Bell Laboratories

Carl J McGrath

Charles B Roxlo

Edward J Flynn

Thomas Edward Darcie

George E Bodeep

Paul M Nitzsche

Jan Lipson

Mark Steven Schaefer

L C Upadhyayula

An advantageous method of fabricating lasers adapted for use in a multichannel analog optical fiber communication system, e.g., a CATV system, is disclosed. A laser generally can be used in such a communication system only if it meets, inter alia, very stringent intermodulation specifications. To identify such lasers typically requires extensive testing. It has now been discovered that certain readily determinable parameters can be used to predict the intermodulation behavior of a given device. This discovery makes possible a simpler, and therefore less costly, process of identifying suitable lasers, resulting in a more economical method of making lasers for the stated application. The method comprises measuring the light versus current (L versus I) characteristic of a given laser, determining therefrom the first, second, and possibly higher, order derivatives of L with respect to I, and determining thereform a parameter that is a predictor of the distortion behavior of the laser. Exemplarily, the quantity is the normalized second order distortion (2HD/C), defined as 20 log (mL'/4L'.sup.2), where m is the modulation index, and L' and L' are the first and second derivatives, respectively, of L. 2HD/C has a pronounced minimum at I=I.sub.op , the current at which L'=0. Advantageously, lasers are selected that have 2HD/C less than a predetermined value (e.g., -70 dbc, with m=0.04) over a relatively wide current range (e.g., at least 4 mA).

Method of producing a semiconductor laser adapted for use in an analog

An amplitude-modulated vestigial-sideband, television channel sub-carrier multiplexed optical fiber transmission system includes a laser diode operating linearly throughout the desired range of multiplex frequencies. Such linear laser diode operation is accomplished by limiting leakage current within the laser diode to a very low level. The composite second-order distortion in the optical output signal from the laser diode is down typically 50 db from the video carrier for each channel. For some laser diodes that exhibit slow turn-on in the L-I characteristic, there is a point of inflection in the L-I curve. Operation with the laser biased at or near this point results in extremely linear performance, even though the laser may have insufficient blocking of leakage current to otherwise produce acceptable linearity.

Optical sub-carrier multiplex television transmission system using a

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Atkinson, NH, United States of America

Carl J McGrath