|
Optoelectronics Interferometry and Polarimetry Metrology for Optical Fiber and Components Nanostructure Fabrication and Metrology Semiconductor Growth and Devices Spectral and Non-linear Properties of Optical Fiber and Components Return
to Electronics and Electrical Engineering Lab contents |
Division Contact: Kent Rochford Accurate characterization of optoelectronic sources and detectors is important in the development and use of technologies such as lightwave telecommunications, laser-based medical instrumentation, materials processing, photolithography, data storage, and laser safety equipment. This project focuses on selected critical parameters intrinsic to sources and detectors, especially the calibration of optical-fiber power meters and laser power or energy meters at commonly used wavelengths and powers or energies. In addition, special test measurements are available for linearity, spectral responsivity, and spatial and angular uniformity of laser power meters and detectors. Project members participate in national and international standards committees developing standards for laser safety, laser radiation measurements (such as beam profile and pointing stability), and optical-power-related measurements. They extend and improve source and detector characterizations, including development of low-noise, spectrally flat, highly uniform pyroelectric detectors; high-accuracy transfer standards for optical fiber and laser power measurements; and advanced laser systems for laser power and energy measurements. We have historically used electrically calibrated laser calorimeters to provide traceability to the SI units for laser power and energy. We recently developed a new measurement capability based on a laser optimized cryogenic radiometer (LOCR), which provides improvement in accuracy by an order of magnitude for laser power measurements. To meet the increasing demands for higher accuracy over a larger range of optical power and wavelength, it is necessary to improve the accuracy of calibration services through the development of better transfer standards, traceable to LOCR. Advances in laser technology are continuously producing lasers with new wavelengths and power levels. We are involved in an ongoing effort to expand wavelength and power-range capabilities through implementation of new tunable solid-state laser technology to keep up with customer needs for calibration services at NIST. These new laser systems will be capable of providing a new wavelength requested by a customer with a minimum of development time and cost by having a flexible suite of laser systems available in the laboratory. Contact: John Lehman High-bandwidth measurements support systems that take advantage of the potential bandwidth of optical fiber. Current systems operate at 2.5 to 10 gigabits per second using pure time-division multiplexing (TDM). Next-generation TDM systems will operate at 40 gigabits per second or higher. Methods are needed to accurately characterize the scalar and vector frequency response of high-speed sources, detectors, and instrumentation at three to five times the system modulation rate. Increasingly tight tolerances in both digital and analog systems require frequency-response measurements with low uncertainty. We have developed highly accurate heterodyne techniques at 850 nm, 1319 nm, and 1550 nm for measuring frequency response of detectors. A calibration service has been established for frequency-response transfer standards operating at 1319 nm and is capable of measuring response from 300 kHz to 110 GHz or more. We have similar capabilities at 850 nm and 1550 nm, along with a service for calibrating the frequency-response magnitude of bare photodiodes to 110 GHz. Optoelectronic phase response, when combined with the magnitude response, is called the vector response and is required for design of high-speed optoelectronic systems. At present there are no accepted standard methods for measuring optoelectronic vector frequency response. We are addressing this problem and have demonstrated time-domain techniques for measuring optoelectronic vector response with verifiable accuracy up to 30 GHz using electro-optic sampling. Optical communications analyzers or reference receivers used for measuring digital eye-patterns are important for system characterization. They can be calibrated over a very high bandwidth because they do not require band-limited microwave calibrations. This calibration on typical measurements, however, possesses some unique problems, and we are currently applying our expertise in receiver measurements to these problems in collaboration with the Statistical Engineering Division. The project is also developing optical-noise measurement systems for calibrating laser relative-intensity noise (RIN) and optical-amplifier noise-figure measurements. We have documented a calibration artifact for RIN and are currently investigating ways of applying this artifact to noise-figure measurements. Contact: Paul Hale Accurate measurement methods and standards for characterizing pulsed-laser sources and detectors are critical in a number of industrial applications. Project members work closely with industry to develop standards, new technology, and appropriate measurement techniques for pulsed-laser measurements. These efforts include standards development, advising customers on in-house measurements, and pulsed-laser calibration services for laser power and energy using excimer and pulsed Nd:YAG lasers. In all areas, our focus is to identify and anticipate future demands for pulsed-laser metrology and to meet these demands in a timely fashion. An example of this is the expansion of our existing pulsed Nd:YAG laser power and energy calibration services to include high pulse energies and to reduce the overall uncertainty associated with these measurements due to the increase in demand for more accurate laser target ranging and designation systems. The majority of our work concentrates on ultraviolet (UV) laser metrology using excimer lasers. Excimer lasers are used in corneal sculpting procedures for vision correction and in micromachining of small structures such as ink jet printer nozzles. A major application of these lasers is optical lithography, since the demand for higher-density chips has led to the introduction of deep-ultraviolet (DUV) laser-based lithographic tools for semiconductor manufacturing, using KrF (248 nm) and ArF (193 nm) excimer lasers. Accurate measurements at the DUV laser wavelengths are needed and, with SEMATECH support, we have developed primary standard calorimeters for 193 nm and 248 nm excimer laser power and energy measurements. We are also developing calibrations for F2 (157nm) excimer lasers needed for next-generation tools. In addition to existing laser measurement services, we have added the capability to make accurate measurements of laser dose (energy density), where the detector samples a fraction of the total laser beam. Accurate measurements of laser dose are important because small-area detectors are used widely to monitor laser-pulse energy density and are needed for the development of new mask resist materials. Finally, we have recently developed a system to measure the non-linear response of an excimer laser detector. This system, which complements our other excimer laser power and energy calibration services, provides a quantitative measurement of a detector's response over a large energy and/or power range. Most detectors typically are used over a power and/or energy range that is larger than the range covered by a single calibration point. Measurement uncertainty can be introduced if the detector response is not linear and the non-linearity is not quantified. Range discontinuity, e.g., due to change in detector amplification, also can introduce additional measurement uncertainty. Possible sources of detector non-linearity include, but are not limited to, the dependence of detector response on input signal, range discontinuity of the detector system or readout electronics, and background reading. Contact: Christopher Cromer Newly emerging material systems offer great promise in optoelectronics. The group III-nitride semiconductors, for example, are now regarded as the material system of choice for high-powered visible and ultraviolet light-emitting diodes and laser diodes in applications ranging from solid-state lighting and displays to optical data storage to biological agent detection. More mature materials technologies, including the III-arsenides and LiNbO3, are now enjoying substantial economic impact due to the large markets for components, including lasers and high-speed integrated optical modulators, that are required for optical telecommunications. The Optical Materials Metrology Project is applying methods of high-spatial resolution optical spectroscopy, non-linear optics, and ultrafast optics in order to characterize these materials. Highly collaborative, the project works closely with various laboratories, universities, research institutes, and other NIST groups. For example, a study of the refractive index of AlGaN is in progress. In this work, the refractive index and birefringence are correlated with Al composition, band edge absorption, and lattice constants. Earlier studies of LiNbO3 combined non-linear optical analysis methods with high-resolution X-ray diffraction imaging to correlate defect and composition uniformity with device performance. High-spatial resolution spectroscopy and non-linear optical methods are being used in conjunction with complementary structural and analytical measurement techniques to investigate the optical, electrical, and structural uniformity of intrinsic and doped GaN in the form of bulk single crystals and thin epitaxial films. Our refractive index measurements of the III-nitrides support accurate device modeling in the industry. We also develop advanced optoelectronic devices for metrology and other applications. Compact and stable solid-state lasers (pulsed and cw) are being developed for the measurement of frequency response in high-speed photoreceivers and the study of low jitter pulsed sources. We are fabricating passively mode-locked waveguide lasers for use in photonic analog-to-digital conversion applications. These sources also may play a significant role in emerging sensor and medical applications of femtosecond lasers. Ultrafast measurement capabilities over a broad spectral range support this effort. Contact: Norman A. Sanford Nanostructure Fabrication and Metrology Nanoscale periodic dielectric structures, known as photonic crystals, are contributing to the development of an entirely new family of integrated optical components, including modulators, waveguides, wavelength-division multiplexing filters, resonators, and channel add/drop elements. Quantum dots, embedded in photonic crystals, are leading to ultralow-threshold lasers and countable single-photon emitters via quantum-confined cavity effects. This project focuses on nanotechnology to serve the metrology needs of the optoelectronics industry. Project members are developing a single photon turnstile inside a photonic crystal nanocavity and photon number state generators that will enable NIST to define quantum-based radiometric standards. These devices also will enable future applications such as quantum cryptography and optical-based quantum computation and communication. This effort requires the precise anisotropic etching of photonic crystals and the comprehensive modeling of photonic nanostructures. Additional project research includes the epitaxial growth of semiconductor quantum dots and their optical, electrical, and structural characterization. We are studying the ultrafast properties of quantum dots in a waveguide to support the realization of the next generation of optical devices such as laser diodes, photodetectors, and semiconductor optical amplifiers. We are developing broadband tunable lasers based on quantum wells for use in the measurement of detector spectral responsivity. The fabrication and characterization of semiconductor saturable absorbers are being pursued for compact mode-locked solid-state lasers for signal processing applications. Contact: Richard P. Mirin Semiconductor Growth and Devices The rapid growth of the U.S. optoelectronics industry is dependent on the high yield manufacture of devices with increasingly tight specifications. Compound semiconductor materials form the basis for LEDs, lasers, photodetectors, and modulators critical to optical communication, display, data storage, and many other applications. Materials purity and uniformity issues are at the foundation of device yield and performance. Measurements of starting materials and epitaxial layers must be supported by standard procedures and reference materials. Increasingly, the needs are for accurate in-process measurements. This project supports the semiconductor optoelectronics industry with research related to the epitaxial growth of III-V materials and fundamentally new device structures. We are working with industry to improve the accuracy of alloy composition measurements and in situ measurements during epitaxial growth to simplify the purchase of epitaxial growth services and to facilitate device modeling. Interlaboratory comparisons of ex situ measurements of AlGaAs and InGaAsP epilayers are an important part of this effort. Our development of highly sensitive measurements of impurity concentrations in semiconductor source materials (e.g., water in phosphine) helps quantify problems of irreproducibility faced by device manufacturers. We are studying the materials properties of native oxides made from wet oxidation of AlGaAs for better understanding and control of their use in optoelectronic devices, particularly vertical cavity surface-emitting lasers. We also are collaborating with industry to develop new types of structures including highly ordered quantum dot arrays. Contact: Kristine A. Bertness Spectral and Non-linear Properties of Optical Fiber and Components This project concentrates on metrology of optical fiber and components used in high-bandwidth optical fiber communication systems. We develop techniques for the measurement of spectral and non-linear properties and develop wavelength calibration transfer standards. In an optical fiber communication system, wavelength division multiplexing (WDM) increases bandwidth by using many wavelength channels. Most systems employ 50 or 100 GHz channel spacing in the 1540-1560 nm region, with narrower channel spacing planned. Systems may be implemented in other wavelength regions as well, possibly covering the entire range from about 1280 to 1630 nm. We are developing spectral characterization techniques and wavelength calibration transfer standards. The project currently produces four wavelength reference Standard Reference Materials (SRMs) based on fundamental molecular absorption lines. Together, these SRMs cover the 1510 to 1630 nm region and can be used to provide wavelength calibration over the WDM C-band (1530 to 1565 nm) and L-band (1565 to 1625 nm). To reach arbitrary wavelengths, we are developing a hybrid reference incorporating superimposed fiber Bragg gratings and a molecular absorption cell. Increases in the channel count and transmission distance lead to higher optical power in the fiber, and, in turn, will increase the importance of non-linear effects. These non-linear effects, such as cross-phase modulation, self-phase modulation, and 4-wave mixing can cause pulse broadening, pulse distortion, and crosstalk, and ultimately limit system performance. Beneficial non-linear effects, such as Raman amplification, can be used to improve overall system performance through lower noise, lower amplified power, and the ability to amplify over the full WDM region. Because of the increase in non-linearities, we are developing measurement techniques to characterize these effects in optical fiber. We have also begun measurements in supercontinuum generation in microstructure and/or tapered optical fiber. This supercontinuum generation is important for the generation of broad optical frequency combs, which, in turn, can be used for accurate frequency measurements and wavelength standards. Contact: Sarah Gilbert Interferometry and Polarimetry Metrology for Optical Fiber and Components This project uses interferometric and polarimetric techniques to provide calibrating measurements, standards, and expertise in support of the optoelectronics industry, primarily telecommunications. In fiber optic communications systems, increasing the data rate reduces a system's tolerance to chromatic dispersion, relative group delay, polarization-mode dispersion, polarization-dependent loss, and polarization-dependent spectral transmission. We are developing new measurement techniques for these parameters and are working to increase the spectral and temporal resolution of existing methods. Chromatic dispersion in optical fiber causes pulse broadening that leads to intersymbol interference and bit errors. With our RF phase shift system we are able to perform certifying measurements of chromatic dispersion and zero-dispersion wavelength of customer artifacts. Similarly, wavelength-dependent relative group delay (RGD) in components degrades system performance. Often, RGD must be measured in components that have sub-nanometer optical bandpass regions; at the same time, high data rates require that RGD be measured with sub-picosecond temporal resolution. Our RGD measurement work is directed toward improving temporal resolution, reducing the necessary measurement bandwidth and establishing fundamental standards with theoretically predictable RGD profiles. This same dilemma is experienced in polarization-mode dispersion (PMD) metrology where propagation velocity depends on the polarization state of the light. Communication systems require PMD measurement with a few tens of femtoseconds of resolution in a bandwidth of a few tens of picometers. We have developed a Standard Reference Material (SRM) that simulates mode-coupled PMD in optical fiber, with certified mean differential group delay from 1480 to 1570 nm. We have also developed a non-mode-coupled PMD SRM that simulates the PMD in discrete components. We are working toward improved understanding of uncertainties in PMD measurement, improving temporal resolution, decreasing measurement bandwidth requirements, and developing new measurement techniques. Non-temporal polarization effects such as polarization-dependent loss (PDL) and polarization-dependent wavelength shift (PDW) cause intensity variations as a function of polarization and wavelength. These effects generally distort transmission signals in a way that can vary with time and system conditions—again causing bit errors. We are developing measurement techniques for accurate wavelength-dependent characterization of PDL and measurements with reduced detrimental effects due to system birefringence. We are also developing accurate characterization of the bandpass spectrum of narrow optical filters in both the frequency domain (tunable-laser based system) and in the Fourier domain (Fourier transform spectroscopy). Contact: Paul Williams We are developing the measurement technology for the characterization of advanced image display systems. A set of meaningful performance specifications is needed that can be used to assess display quality and that can be applied across the wide spectrum of display technologies that either are available or will become available shortly. Display quality issues are not simply a matter of light measurement, power efficiency, display environment, or signal quality. Rather, many of these factors act in concert to affect display quality, with an important addition-the complexities of human visual perception. Our research topics include the development of radiometric and colorimetric measurements of emissive and non-emissive displays, the automation of such measurements, investigation of the visual perception of the eye, and modeling of display characteristics using the Princeton Engine video supercomputer. Contacts: Paul Boynton
Date
created:November
8, 2001 |