This Small Business Innovation Research Program Phase II project proposes to develop a simple, single-shot, inexpensive, and complete laser-pulse measurement device for ~100-picosecond to ~10-nanosecond pulses. Long (>10 nanosecond) pulses are easily measured, and recently developed techniques completely measure ultrashort pulses (<10 picosecond). But intermediate-length, ~1-nanosecond, pulses remain only partially, roughly, and expensively measurable, and so generally remain complex and unstable. This is unfortunate because most laser pulses are in this intermediate range. The proposed measurement device is based on frequency-resolved optical gating (FROG), a very successful technique for measuring the complete intensity and phase vs. time of femtosecond pulses. The main challenge in extending FROG to much longer pulses is the generation of a many-nanosecond delay range on a single pulse-currently an unsolved problem in general. The proposed innovation solves it by tilting the input pulse by a remarkable ~89.99° without distorting it in time. As a result, one side of a ~1cm-wide beam precedes the other by over a meter. The proposed nanosecond FROG can completely measure even complex pulses and will cost less than one tenth as much as the high-bandwidth oscilloscopes currently used to only partially measure such pulses.
The broader impact/commercial potential of this project follows from the fact that most pulsed lasers, from solid-state lasers to fiber lasers, emit pulses about a nanosecond long. They are the least stable lasers in the world, yet they have billions of dollars of applications, from materials processing to distance measurements to remote sensing to medical, military, and scientific uses. With the proposed device, nanosecond lasers will finally have a previously unavailable device to monitor their performance and to diagnose problems before expensive materials are ruined or patients are harmed. It will also be essential for combining pulses from multiple fiber lasers, generally regarded as the next important step in the development of compact and convenient high-power pulsed lasers. Finally, using this device, laser engineers in general will be better able to improve the quality of nanosecond laser pulses, thus greatly benefitting all pulsed-laser applications. If the spectacular progress in much-shorter-pulse lasers that occurred after analogous complete pulse-measurement technology was introduced there is any indication, such an inexpensive and simple device for measuring nanosecond pulses should make a huge difference in the generation of cleaner, more stable nanosecond pulses and consequently in the many fields that use such lasers.
Intellectual Merit: This Small Business Innovation Research Phase II project has developed a simple, general device for completely measuring laser pulses from about 10 picoseconds to a few nanoseconds in duration. Previously, this was only a partially solved problem, and, worse, available devices were also extremely complex and expensive ($150,000). To do this, this project has extended the frequency-resolved-optical-gating (FROG) concept, which works very well for measuring much shorter, femtosecond pulses, to this much longer regime, yielding an automatically aligned, compact, single-shot, and inexpensive device for completely measuring all pulses from about 10 picoseconds to a few nanoseconds in length. In general, FROG involves splitting a pulse into two replicas and crossing them in a second-harmonic generation (SHG) crystal, which generates a light pulse of twice the frequency (half the wavelength) if the two pulses overlap in time. The trick in designing a nanosecond version of this device is to tilt one of these pulses by almost 90º (89.99º) and the other by almost -90º (-89.99º). This is not generally done with laser pulses, but this project has shown that it can be done easily, and it is the key to achieving the required range of delays between the two pulses for such a device to work. This is done by using an etalon—a piece of glass with highly parallel faces and sending one pulse into it at the bottom and the other at the top, each as a slight angle. See Figs. 1 and 2. Several versions of the nanosecond FROG were built and tested extensively. A theoretical description of all the devices’ characteristics was developed. And the final device design’s properties and limits were analyzed and found to work very well. It was used to measure various pulses, including a double pulse with known pulse separation, which is a standard test pulse because it is easy to generate and has characteristics that are easily known (the pulse separation can be measured using a ruler and the relative pulse energies is determined by the beam-splitter reflectivity), but which can be difficult to measure due to its complexity: simultaneous complex temporal and spectral structure. It was also used to measure various fiber-laser pulses, where it exposed previously unknown complexities in some of them, such as simultaneous lasing at two very different wavelengths. In previous implementations of the device developed in Phase I, due to the numerous optical components in the original design, misalignment was an issue when shipping. Fortunately, this problem was quickly remedied in all later designs by the use of more stable mounts and a more elegant beam geometry using fewer lenses. As a result, the final device is also more compact. In the course of this work, the use of laser pulses in the 10 – 100 picosecond range has also increased in many applications because cutting with such pulses is much cleaner than with longer pulses, so an analogous FROG device was designed for this pulse-length range, based on the same principles. Broader impact: Measuring such laser pulses is an important problem because lasers in this pulse-length range are the most numerous pulsed lasers in the world with many applications that require them to be clean, stable, and simple. But they are also in general the most complex and the least stable and so the most in need of an improved measurement device. They have many applications, all of which will benefit from this device. These applications include a wide range of materials processing and medical laser systems, which currently comprise over a billion dollars of laser system sales annually. Finally, using this device, laser engineers in general will be better able to improve the quality of laser pulses. If the spectacular progress in ultrafast lasers that occurred after complete ultrashort-pulse-measurement technology was introduced is any indication, such an inexpensive and simple device for measuring these longer pulses should make a huge difference in the generation of ever more stable pulses and their many applications.
