At&t Bell Laboratories

Lucent Technologies Inc.

Edward J Flynn

Charles B Roxlo

Carl J McGrath

Thomas Edward Darcie

John William Stayt, Jr

Frank Stephen Walters

George E Bodeep

Jerome Levkoff

Leonard J Ketelsen

Mark Steven Schaefer

Jan Lipson

Paul M Nitzsche

L C Upadhyayula

A precision grating period measurement system uses a pair of properly positioned photodetectors to provide sub-Angstrom resolution. That is, the absolute position of a first detector with respect to a zero point in the measurement system is assured by including a second photodetector that measures a retroreflected signal. The system is then &ldquo;zeroed&rdquo; on the retroreflected signal such that the subsequent measurements recorded by the first photodetector are a precise measurement of the grating period.

Precision grating period measurement arrangement

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

Lasers for use in multichannel analog optical fiber communication systems (e.g., of the type contemplated for CATV) have to meet very stringent requirements, including high linearity. DFB lasers are advantageously used in such communication systems. Typically only a relatively small percentage of the nominally identical DFB lasers on a wafer meet the specifications. It has now been discovered that the likelihood that a given DFB laser will meet the requirements is substantially increased if the laser comprises means that are adapted for producing a non-uniform photon density in the laser cavity, with the density of photons being larger in the rear portion of the cavity than in the front portion, such that during operation of the laser the gain in the back portion is substantially independent of the laser current, whereas the gain in the front portion is a function of the laser current. Exemplarily, lasers according to the invention have power asymmetry less than about 5. Preferred embodiments are AR/HR lasers with KL in the range 1.6-2.5. Optionally lasers according to the invention have a phase shift located in the back portion of the grating, a split contact, and/or a non-constant mesa width.

Analog optical fiber communication system, and laser adapted for use in

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Summit, NJ, United States of America

Edward J Flynn