What technologies are needed to create and maintain the information superhighway? Certainly fiber optics will be one of them. Fiber-optic communications systems have the ability to transmit vast streams of data including voice (telephone conversations), video (TV channels), and data (computer networks). In fact, multimedia applications, in which voice, video, and data will be transmitted and displayed at the same time, will be highly desirable in the near future. As these services become more commonplace, the required network capacity will expand and current fiber-optic systems operating at data rates of hundreds of megabits per second will be superseded by those with rates near ten gigabits per second. Nationwide networks will give way to one that is much more international in scope. Currently, a large number of fiber-optic networks, operating at 155 megabits per second and 622 megabits per second, are in operation in the U.S.A., Europe, and Japan and new installations of 2.4-gigabit-per-second networks are near completion.

To support this high-speed, long-distance traffic, new technologies are rapidly evolving. Optical amplifiers based on rare-earth-doped optical fibers-for example, erbium-doped fiber amplifiers, or EDFAs-make it possible to overcome fiber loss and construct transparent undersea optical links thousands of kilometers long without electronic regenerators. These amplifiers can also be used as excellent noise sources to test optical components such as photodetectors (page 6). In terrestrial systems, several signals at different wavelengths can be sent over the same fiber to increase capacity (wavelength division multiplexing), and their intensity can be boosted in a single EDFA. Accurately testing the amplifier performance is critical in these systems. The HP 81600 EDFA test system (page 13) makes calibrated gain and noise figure measurements as a function of wavelength and optical power. This test system uses several high-performance lightwave products such as the HP 8168C tunable laser source and the HP 71450A optical spectrum analyzer in addition to a sophisticated software algorithm to measure the characteristics of optical amplifiers. These algorithms were designed and tested by a team consisting of engineers from California and Germany. To achieve a wide tuning range, the HP 8168C uses a new semiconductor laser chip that has been developed at HP Laboratories (Page 20) using a quantum-well indium gallium arsenide (InGaAs) structure. This device also makes it possible to obtain sufficiently high optical input power to test amplifier saturation.

Long fiber spans using optical amplifiers with no pulse reshaping compound subtle signal distortion in fiber. Particularly at high data rates, fiber polarization effects can increase dispersion problems. Therefore, it is essential to be able to measure very accurately the state of polarization of optical signals and the polarization-mode dispersion of optical components. It is also highly desirable to be able to do these measurements in real time. The HP 8509B lightwave polarization analyzer (page 27) is used in these cases. In addition, in high-performance systems, requirements on optical back-reflections are more severe. Fundamental fiber-optic test equipment like the HP 8156A optical attenuator (page 34) must be designed to these more exacting tolerances. To measure low back-reflection levels, the HP 8504B precision reflectometer (page 39) can be used. A high-performance light-emitting diode (page 43) makes it possible for the reflectometer to measure -80-dB back-reflections-as little as 1 picowatt of reflected power-with a spatial resolution of 50 micrometers. At high data rates, system timing accuracy becomes increasingly important. The HP 71501B jitter and eye diagram analyzer (page 49) is capable of making jitter measurements at rates up to 10 gigabits per second.

Monitoring network integrity becomes more complex in higher-capacity systems. The HP 81700 remote fiber test system (page 57) helps guard against breaks in the fiber causing outages. The system can monitor many fibers at once, even at different sites, while they are carrying live traffic. If a fiber break occurs, the system generates signals and alarms indicating the location of the fault.

The development of lightwave test and measurement products requires a solid photonics technology base. In addition to the quantum-well lasers and LEDs mentioned above, HP Laboratories is working on other devices and subsystems for future products. A highly stable, miniature laser using YVO (yttrium orthovanadate) crystals operating at a center frequency of 282 terahertz (page 63) is an example of excellent low-noise optical sources. The recent development of surface emitting lasers (page 67) is very promising for a variety of applications such as optical interconnect systems for data communications and generating visible light using complex laser structures (page 72). The technologies and products described in this issue show HP's strength and core competency in the area of photonics and lightwaves. A key element of the company's strength in lightwave test and measurement is the strong and healthy coupling between the research teams at HP Laboratories and the manufacturing divisions: the Lightwave Operation at Santa Rosa, California and the Boblingen Instruments Division in Germany.

Waguih Ishak, Manager Photonics Technology Department, HP Laboratories Roger Jungerman, Engineer/Scientist, Lightwave Operation

Also in this Issue

* While thorough delay testing is necessary for high-performance designs, not all circuit paths can be tested for delay. The article on page 105 proposes an algorithm for synthesizing 100% delay testable circuits. The algorithm uses a method called cube partitioning.

** The article on page 110 compares two fault diagnosis methods to determine which does a better job in CMOS circuits. They conclude that a bridging fault model with a simple diagnosis algorithm is better than a simple stuck-at-0 or stuck-at-1 fault model with a complex algorithm.