Photonic technology for lightwave communications test applications - Technical

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.

As the performance of optical components is improved, new and improved measuring techniques and tools are needed. Particularly important are accurate measurements of the performance of components and systems as a function of wavelength. For example, with the rapid development of broadband erbium-doped fiber amplifiers in communications links, a widely tunable optical source is needed for wavelength characterization. The external-cavity semiconductor laser is a device that can satisfy this requirement. At HP Laboratories, we developed alignment-tolerant, singlemode, and widely-tunable external-cavity lasers in the 1.3 and 1.55-mm wavelength ranges. One of the interesting configurations of a grating-tuned external-cavity laser (Fig. 2) incorporates a gradient-index rod lens and a pair of silicon prism beam expanders.3 This laser achieves the following objectives simultaneously:

* Ability to operate at any external-cavity longitudinal mode without tuning gaps

* Stable feedback coupling between the laser diode and the external cavity

* Narrow optical linewidth

* A high degree of external cavity side-mode suppression

* A very wide tuning range.

The results obtained showed complete wavelength coverage over more than 100 nanometers with single-mode operation over most of the tuning range. The linewidth was less than 100 kHz and the side-mode suppression ratio was greater than 70 dB. We also developed other external-cavity laser configurations to tune over wider tuning ranges and to enhance the single-mode properties of the laser. One of these configurations forms the heart of the HP 8167A and 8168A tunable laser sources (see articles, pages 11 and 20).

Protecting the Optical Sources--Isolators. High-performance optical isolators are playing an increasingly important role in lightwave instruments. Their purpose is to protect optical sources from reflections and backscattering that cause output instabilities or unwanted changes in the output spectra of the optical sources. The HP isolator combines rutile birefringent walk-off crystals and bismuth-doped yttrium iron garnet films (Bi-YIG) in a proprietary design that provides high isolation, low insertion loss, high return loss, and polarization independence over wide wavelength and temperature ranges.4

Modulating the Optical Sources--Modulators. HP is credited with developing the world's first integrated optic modulator for a commercial instrument application.5 The Mach-Zehnder lithium niobate (LiNb03) modulator used in the HP 8703A lightwave component analyzer uses titanium-diffused optical waveguides and exhibits a bandwidth of more than 20 GHz. The limitation on the bandwidth of this modulator is mainly the mismatch between the optical and microwave velocities. Building on the success of this 20-GHz modulator, HP Laboratories developed a new modulator configuration6 that is velocity matched to frequencies in excess of 50 GHz with excellent loss-drive ratio and contrast characteristics over the l.3-to-l.55-mm wavelength range. The modulator is a Mach-Zehnder type. It uses a thick-electrode, buffer-layer geometry that results in a device that achieves almost exact velocity matching to the optical index, maintains high impedance, and has a very low voltage-length product. The structure uses a narrow ground plane whose width is only slightly larger than the electrode width, resulting in higher impedance and increased microwave velocity. This introduces an extra degree of freedom that allows exact matching of the optical and microwave indexes at a reasonably high impedance. The optical waveguides in the active section of the device are fabricated with a high enough index difference to be multimode at 1.3 mm. The input and output waveguide sections are reduced in width so as to be singlemode over the entire range from 1.3 to 1.55 mm. The symmetric, adiabatic nature of the Y junction connecting these two regions ensures that no coupling occurs to the higherorder modes of the active section.

Several cuts of LiNbO3 wafers were used to verify the design of this modulator and excellent agreement with theory was obtained. Fig. 3a shows a cross section of the modulator design and Fig. 3b shows the exceptional bandwidth characteristics of this device. Measurements showed more than 60 GHz of bandwidth with a voltage-length product as low as 8.3 volt-cm over the 1.3-to-1.55-mm range.

Optical Signal Analysis

Measuring the Amplitude of Optical Signals--Photodetectors. One of the first challenges facing HP Laboratories designers was to develop a high-speed infrared (1200 to 1600 nm) photodetector, which would form the basis for the optical receivers needed for many lightwave instruments. This required high-quality epitaxially grown layers of indium gallium arsenide (InGaAs) on indium phosphide (InP) substrates and careful device design to minimize spurious capacitance and maximize the photodetector responsivity. For material growth, metalorganic chemical vapor deposition (MOCVD) was the method of choice because of the uniform thickness and low defect density in the grown films. A frontilluminated, circular, p-i-n device structure was chosen, and the layer thicknesses were designed to produce the 22-GHZ response needed for the first-generation lightwave receivers. The InGaAs/InP p-i-n photodetector technology has been extended to develop and produce higher-frequency detectors (up to 50 GHz). Other work in GaInAs/InP photodetectors has concentrated on special needs for extended wavelength response (600 to 1600 nm achieved), and custom configurations such as a chip with two photodiodes with a precise separation for a polarization diversity receiver. This dual photodetector is used in the HP 8504A precision reflectometer (see article, page 39).

Measuring the Polarization of Optical Signals--Polarimeters. To measure the polarization sensitivity of optical components, an efficient real-time technique for accurately measuring the state of polarization and degree of polarization of an optical signal is needed. A scheme to measure the state of polarization and degree of polarization in real time was developed at HP Laboratories. This approach forms the basis for the HP 8509A lightwave polarization analyzer. The signal under test is transmitted out of an optical fiber and diverges onto four mirrors in a specific arrangement. The reflected beams are processed using bulk optic components and then transformed into electrical signals by four photodetectors to produce the needed Stokes parameters.7 From these parameters, the final state of polarization is determined using electronic postprocessing. The results are displayed in various formats (polarization ellipse or Poincare sphere) on a personal computer capable of providing the user with menudriven software suited for several important polarization measurements. 8

Optical Component Characterization

The optical sources and optical receivers described above can be used to build optical component analyzers such as the HP 8702A and 8703A.9,10 These instruments are broadband optical subcarrier network analyzers that are capable of measuring the RF response of optical or optoelectronic components and networks out to a frequency of 20 GHz in the case of the HP 8703A. By calculating the inverse Fourier transform of these responses, a time-domain picture of the network under test can be obtained. Thus, these instruments can also serve as optical frequency-domain reflectometers (C)FDRs) that are capable (in the case of the HP 8703A) of resolving reflections spaced less than a centimeter apart.

Coherent FMCW (frequency modulated continuous wave) reflectometry techniques can provide high sensitivity coupled with high resolution. Using tunable miniature YAG lasers, we were able to implement an FMCW reflectometer capable of resolving reflections spaced 5 cm apart.

Some reflectometry applications require that a user be able to look inside of a componcnt, that is, to resolve reflections that are spaced less than 1 mm apart. The HP 8504A precision reflectometer (see article, page 39) was developed in response to these needs. This instrument is based on the well-known principle of white-light interferometry, in which coherent interference between reflections in each of two arms of an interferometer can be detected only if their distances to the splitting and recombining point are equal, to within the coherence length of the source. By locating the device under test at the end of one interferometer arm, scanning the position of a reference mirror at the end of the other, and detecting the positions where coherent interference takes place, the instrument effectively scans through the device under test, mapping out the positions and amplitudes of reflections. By using a broadband source with a short coherence length, reflections spaced less than 50 micrometers apart can be resolved. Using proprietary calibration techniques, accurate measurements of reflection amplitudes as small as -80 dB can be obtained. This resolution and reflection sensitivity are orders of magnitude better than those of other commercially available reflectometers.

The research team at HP Laboratories is a world leader in this technology. Early in 1992, they reported a reflectometry experimentIl in which they demonstrated a world-record reflection sensitivity of-148 dB as shown in Fig. 4.

Characterization Tools

Two important characterization tools made possible the development of the photonics devices mentioned in this article. The first is photoluminescence and the second is electrooptic sampling. Photoluminescence (continuous wave and time-resolved) is used to characterize III-V compound semiconductor epitaxial films and is crucial to the development of high-quality films. For example, Fig. 5 shows time-resolved photoluminescence from AIGaInP structures for five excitation energies. The persistence of the long decay times at the low energy levels indicates high material quality. This AiGaInP is used in HP's new highbrightness yellow and orange LEDs.

Electrooptic sampling is a tool for characterization of highspeed devices. It was very important to the time-domain characterization of the InGaAs photodetectors. At HP Laboratories, we continue to invest in photoluminescence measurements and electrooptical sampling to enhance their capabilities.

Conclusions

The success of the Hewlett-Packard lightwave instruments program is due in part to the strong photonics technology base at HP Laboratories and the healthy coupling and interaction between HP Laboratories and the divisional R&D programs. This interaction will continue to be an integral factor in the success of the program and the new products that are yet to come.

Acknowledgments

The results described in this paper came from the work of many people within HP Laboratories. We would like to acknowledge the entire team at the photonics technology department of the Instruments and Photonics Laboratory. We want to thank Bill Shreve for his support and for useful technical discussions. We would like to thank Ron Moon, Mike Ludowise and Bill Perez of HP Laboratories for the excellent III-V epitaxial material work, and Randy Coverstone, Greg Gibbons, Duncan Gurley, and Dick Allen for innovatire software support. Special thanks to Bob Bray and his team for the excellent interaction and joint development. This partnership has laid the foundation for a strong photonics program at HP. We would also like to thank the laboratory and section managers and their teams at the lightwave divisions for their support and help (Hugo Vifian, Roger Wong, Jack Dupre, Werner Berkel, and Steve Hinch). Finally, the efforts of Bob Allen to enhance the technical interactions between the various HP entities are very much appreciated.

References

1. M.J. Ludowise, T.R. Ranganath, and A. Fischer-Colbrie, "Continuously graded-index separate confinement heterostructure multiquantum well [Ga.sub.1-x] [In.sub.x][As.sub.1-y][P.sub.y]/InP ridge waveguide lasers grown by low-pressure metalorganic chemical vapor deposition with lattice- matched quaternary wells and brakets," Applied Physics Letters, Vol. 57, no. 15, October 8, 1990, p. 1493.

2. W.R. Trutna, Jr. and D.K. Donald, "Two piece, piezoelectrically tuned, single-mode Nd:YAG ring laser," Optics Letters, Vol. 15, 1990, pp. 369-371.

3. P. Zorabedian, "Characteristics of a grating-external-cavity semiconductor laser containing intracavity prisrm beam expanders," Journal of Lightwave Technology, Vol. 10, March 1992, pp. 330-335.

4. K.W. Chang, et al, "A High-Performance Optical Isolator for Lightwave Systems," Hewlett-Packard Journal, Vol. 42, no. 1, February 1991, pp. 45-50.

5. R.L. Jungerman and D.J. McQuate, "Development of an Optical Modulator for a High-Speed Lightwave Component Analyzer, ibid, pp. 41-45.

6. D.W. Dolfi and T.R. Ranganath, "50 GHz velocity-matched, broadband wavelength LiNb03 modulator with multimode active section," Electronics Letters, 1992.

7. P. Hem day and R. Cross, "Polarization Measurements in Lightwave Applications Using a New Real-Time Polarization Analyzer," Hewlett-Packard Lightwave Test Seminar 1992.

8. R. Cross, B. Heffner, and P. Hem day, "Polarization measurement goes automatic," Losers and Optotronics, November 1991, pp. 14.

9. R.W. Wong, et al, "High-Speed Lightwave Component Analysis," Hewlett-Packard Journal, Vol. 40, no. 3, June 1989, pp. 35-51.

10. P.R. Hem day, et al, "Design of a 20-GHz Lightwave Component Analyzer," Hewlett-Packard Journal, Vol. 42, no. 1, February 1991, pp. 13-22.

11. W.V. Sorin and D.M. Baney, "Measurement of Rayleigh backscattering at 1.55 mm with 32-mm spatial resolution," Photonics Technology Letters, Vol. 4, April 1992, pp. 374-376.

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