Photonic technology for lightwave communications test applications - Technical

Hewlett-Packard Journal, Feb, 1993 by Waguih S. Ishak, Kent W. Carey, Steven A. Newton, William R. Jr. Trutna

State-of-the-art fiber-optic, integrated-optic, bulk-optic, and optoelectronic devices and subsystems provide a technology base for high-speed, high-performance lightwave communications test instrumentation.

The fiber-optic systems that emerged during the decade of the 1980s have revolutionized high-speed communications by competing very well with more traditional systems as cost-effective means for information exchange. These systems can operate at speeds up to several gigabits per second, and experimental systems in Japan and the U.S.A. are aimed at 40-Gbit/s and 100-Gbit/s transmission rates. The development of high-performance optical components such as fiber amplifiers has resulted in communications networks with spans of hundreds of kilometers without electronic repeaters. These developments are continuing at research laboratories around the world and it looks very feasible to see installed > 10-Gbit/s fiber-optic communications.systems in the near future. As the technology advances, the trends toward higher speeds, lower effective cost per bit and mile, and higher performance will continue. Designers of components, subsystems, and systems for fiber-optic communications need to maximize the performance of each block in the systems and to minimize the adverse interactions among systems blocks. For this reason, the designers need new techniques and measurement tools to help them carry out their work.

At Hewlett-Packard, a major program to develop lightwave communications measurement solutions was launched in the mid-1980s. This program has resulted in an impressive set of high-performance instruments including fault locators (optical time-domain reflectometers, or OTDRs), optical sources (fixed-wavelength and tunable sources), optical signal characterization instruments (power meters, signal analyzers, polarization analyzers, and spectrum analyzers), and optical component analyzers (precision reflectometers and high-speed analyzers).

The development of these instruments required an intensive R&D program at Hewlett-Packard Laboratories and at divisional R&D laboratories to identify and develop key enabling photonics technologies for these instruments.

These technologies include integrated optic and optoelectronic devices as well as bulk-optic and fiber-optic components and subsystems. Some of these devices and subsystems have been used in some of the lightwave instruments described in this and previous issues of the HP Journal. Other devices have been used as internal characterization tools. It is the purpose of this paper to give an overview of some of these key technologies. In the first section, we will review some of the basic technologies for optical signal generation. In the second and third sections, we will discuss technologies used for analysis of optical signals and characterization of optical components, respectively. Finally, we will briefly touch on two important characterization tools that made possible the development of high-performance photonic components.

Optical Signal Generation

Generating an Optical Signal--Semiconductor Lasers. Semiconductor laser diodes play a very important role in test instruments for lightwave communications. While it is possible to purchase certain kinds of laser diodes (such as pigtailed distributed feedback lasers), it is not always possible to obtain bare laser chips with specific parameters suited for use in lightwave subsystems. The use of quantum wells in AlGaAs/GaAs lasers has resulted in impressive reductions of threshold current densities and improved temperature performance. Since then, many groups have been working to extend the quantum well technology to other material systems such as InGaAsP/InP for long-wavelength (1.3 and/.55 mm) applications with impressive results. At HP Laboratories, as part of our epitaxial material technology development, we grew and fabricated graded-index separate confinement heterostructure (GRIN-SCH) quantum well ridgel and buried heterostructure (BH) lasers. Fig. l(a) shows a cross section of a ridge laser with four quantum wells and Fig. l(b) shows the output-light-versus-threshold-current characteristic of this laser.

Generating an Extremely Stable Optical Signal--YAG Lasers. Monolithic diode-pumped unidirectional ring YAG lasers have extremely narrow linewidths and single-mode output spectra. This characteristic is useful for such applications as coherent communications and heterodyne component testing. A ring YAG laser developed earlier at HP Laboratories was very useful for the heterodyne characterization of highspeed photodetectors. For these and other applications, rapid tuning' of the laser is desirable. Since the earlier ring laser was tuned by thermal expansion or by thermally stressing the crystal, the tuning speeds were relatively low. A two-piece piezoelectrically tuned ring laser, its design derived from the earlier one-piece ring laser, was developed.2 This laser can be continuously tuned in milliseconds over more than 13 GHz. It consists of a YAG section and a magnetic glass section. The glass piece is mounted on a piezoelectric transducer. By driving the transducer at its fundamental resonance, the length of the gap between the YAG and the glass sections is changed, resulting in a change in the optical path length of the laser. This path length change produces a change in the output frequency of the laser. The laser produces more than l mW of single-mode output power at 1338 nm when pumped with a 30-mW AIGaAs semiconductor laser. Using a PZT (lead zirconate titanate) transducer, the laser was tuned over a 13.5-GHz range with a tuning rate limited only by the 4.6-kHz frequency response of the PZT.

Generating Tunable Optical Sources--External-Cavity Lasers.