Single-point streak tube This device is designed for recording very wide passband optical signals. The system consists of a single-point streak converter tube followed by a CCD sensor for picking up the image. This tube is derived from a streak tube that has been specially optimized for this application. The optical signal is input in a photocathode, the emitted electron beam is focused on a luminescent screen. The beam is deflected by an electrostatic effect and the trace for-med on the screen is representative of the temporal development of the input signal.
Tomography is the process of obtaining two-dimensional spatial distributions (images) from one-dimensional externally derived profiles or projections. This chapter is not intended as a general review of this broad, far reaching field, but will focus on tomographic solutions to imaging problems utilizing recent developments in electro-optics and fiber optics. Some of the imaging experiments considered here are complicated by hostile environments which require transferring the data over distances of 10 meters or more to protect the recording equipment. It is also often a requirement that complete information be obtained in a single, non-repetitive experiment. These complications on the application of tomographic techniques necessitate the use of high speed photonic solutions involving fiber optic arrays combined with high speed detectors.
Although the fundamental part of any fiber optic system is the fibers, other equally important parts exist, for without appropriate components, signals might not be coupled into or out of fibers, or divided among several branches for various data handling purposes. In the following sections, a brief discussion of the operational principles of some of the more common commercially available components used in fiber systems will be presented.
The purpose of this chapter is to provide information on multimode and single mode optical fibers (SMF) which will assist in their use for high-speed analog data collection schemes. The length and type of fiber required will vary depending on the application. Generally lengths will be 1 km or less; however, some experimenters with applications such as nuclear testing or others may want to run links several kilometers to a distant recording station. This is possible if the fiber has sufficiently low attenuation and large enough bandwidth to meet the requirements. Examples of applications for fast analog data systems which have used or might conceivably utilize high bandwidth fibers are: nuclear and non-nuclear weapons testing, fusion diagnostics, laser weapons testing, electric or magnetic field measurements of pulsed rf sources, and diagnostics in experiments using pulsed particle accelerators. The properties of fibers relating to transmission of analog data only are treated in this chapter. The uses of fibers as analog sensors or as amplifiers are treated in other chapters.
Progress in the field of fiber optics depends heavily on innovation, invention, and the careful use of pretested, commercially available components. A component or device is considered to be one unit of a complete experimental diagnostic system that performs a single function and that can be used in other systems.
Photonics systems can be applied in a wide range of measurement applications. Some of these applications may require that either the photonics sensor or the optical transmission system between sensor and recorder be exposed to ionizing radiation. This exposure may be deliberate, if the sensor is designed as a radiation transducer, or may be unavoidable, if high levels of radiation exist near the measurement point. In either case, accurate data require correction for and/or understanding of potential radiation effects on optical components.
A major impetus for the development of high performance solid state optical sources is the advance in optical fiber technology. Present day off-the-shelf optical fibers offer a loss of below 1 db/km, and bandwidths from (depending on the fiber type) 1 GHz-km to 10 GHz-km. Two main categories of fibers exist today: those that operate at 0.8 um, and at 1.3 um - 1.5 um optical wavelength. In each of the above categories, there are two fiber types - multimode and single mode. By far the most common optical fibers produced today are of two types: multimode fiber at 0.8 um, and single mode fiber at 1.3 um. Single mode fibers in general offer much higher bandwidth than multimode fiber. However, single mode fibers at 0.8 um are not produced in any substantial quantity and, in this author's opinion, with unsatisfactory performances in terms of backscattering loss. Hence, for the majority of applications in photonic measurements where streak cameras are used for data recording, 0.8 um multimode fiber systems appear to be the nominal transmission medium in the near future.
Earlier papers in this book described photonic data recorders and systems, as well as some of the components used in such systems. This paper will provide an introduction to the general field of sensors, giving a background for many of the following papers.
In this chapter, we discuss photonic, single-transient radiation sensors that use optical fibers to transmit signals to a remote location. Our discussion deals with sensors for gamma rays, x rays, and neutrons. Gamma rays and x rays are both electromagnetic waves, but they have different energies and thus different penetration depths. This difference in energy necessitates the use of a different type of sensor for each type of wave. Likewise, neutrons are particles that have varying energies, so the absorption of energy as a function of depth in a material is important in designing sensor systems.
In pulsed power research, controlled fusion experiments, high-explosives-related research, and other similar undertakings, it is often necessary to measure large pulsed electric currents. The pulses can be large enough to distort conventional, electrically-conducting sensors, such as Rogowski loops. Furthermore, the measurements must frequently be made in the presence of electromagnetic interference (EMI) noise. Optical sensing of the magnetic field surrounding the current is a way to reduce or eliminate both of these problems', and in addition it can provide the other benefits of photonics recording, such as improved bandwidth and reduced cost per channel. The sensing methods developed and used for large current pulses are also finding their way into the smaller-pulse regime.
The electrooptic Kerr effect is the interaction of an applied electric field with a birefringent fluid medium resulting in a change in the refractive index of the fluid. It has found a variety of applications in the measurement of the applied voltage and electric field by analyzing the polarization rotation of light shined through the fluid. Potentials of kilovolts to megavolts, and risetimes as fast as nanoseconds have been measured.
The microballoon timing sensor is a disposable, commercially-available impact sensor. This optical-fiber sensor converts a high-pressure, mechanical impulse into an optical signal that is transmitted to a photo-detector via an optical fiber as shown schematically in Fig. 1. By analogy with electrical impact sensors known as "electrical pins," the microballoon timing sensor is often referred to as an "optical pin."
The experimental opto-electronic arrangement utilizes optical timing sensors associated with a high speed electronic streak camera. It has been developed for an operational use in hydrotests on firing sites. It replaces the classical time-arrival recording techniques : electrical contacts associated with digital time-interval meters or raster oscilloscopes.
Studies related to detonics, shock-loaded materials and explosive pulse power require timing sensors. These sensors produce timing information that enables researchers to determine the time-resolved profile of a moving wave or surface due to a dynamic deformation. The sensors may also be used as trigger signals for other diagnostic instrumentation.
Many of the applications of photonics to broadband diagnostics experiments require modulating light in response to an electrical input signal. The two general methods which are employed for this purpose are direct modulation of a source and modulation of a source by an external device. Direct modulation of a source (light emitting diode, laser diode, etc.) involves the relatively straightforward application of well known electrical tech niques and, depending on the required bandwidth, linearity, and dynamic range, may be implemented with a few components at low cost. These techniques are treated in detail in other chapters.
Integrated optic has shown a continuous growth since the sriginal papers describing the potential possibilities of this new field. Many different waveguide configurations and materials were used to realize passive as well as active devices.
We discuss the design, fabrication, and evaluation of integrated optical devices for application to high speed diagnostic systems. In particular, we focus on directional coupler modulators designed for analog modulation of large signals at 810 nm.
The Velocity Interferometer System for Any Reflector (VISAR) has become a widely used tool for measuring surface velocities of rapidly moving parts. The VISAR measures the velocity of a moving surface by measuring the velocity dependent phase change of laser light reflected from the surface. Experiments performed to date have measured velocities of several kilometers per second (millimeters per microsecond) to an accuracy of better than one percent.
The use of optical fiber for the transmission of information over relatively long distances is being recognized as the only viable solution to many data transmission problems, particularly those requiring high information density and faithful temporal content. This necessary reliance upon the optical carrier has meant that the image-tube-based optical streak camera is often the instrument of choice for recording single-shot multi-parameter events with high temporal resolution. At first, this seems to be the ideal instrumentation combination with no need for improvement.
Transmission of analog diagnostic information over fiber lightguides is a frequent practice where the signal source is a single transient event and the data link is in the kilometer length range. Certain adverse test environments, for example, require use of the data links to remotely acquire transient microwave-frequency diagnostic data. In such cases, the optical signal intensity at the receiver is often too low for the large bandwidth, wide dynamic range detection that is needed. Thus, it is desirable to amplify the signal while maintaining high fidelity. A convenient method of achieving direct in-fiber linear amplification has been developed and demonstrated that uses backward stimulated Raman scattering. In this chapter we will explain briefly the physical process, present some experimental observations, and discuss the problems related to its practical application.
During the last several years, we have witnessed rapid progress in generating ultra-short optical pulses. In addition to the effort to produce such short pulses within a laser cavity by various mode-locking techniques, a pulse compression technique using single-mode (SM) optical fibers have been demonstrated as a powerful technique to further reduce the pulse width as well as significantly increase the peak power.