This Small Business Innovation Research (SBIR) Phase I project proposes to develop a simple, single-shot, inexpensive, and complete laser-pulse measurement device for ~100ps to ~10ns pulses. While long (>10ns) pulses are easily measured, and recently developed techniques completely measure ultra-short pulses (<10ps), intermediate-length ~1ns, pulses remain only partially and roughly measurable, and, consequently, such pulses generally remain complex and unstable. This is unfortunate because most commercial pulsed lasers emit pulses in this intermediate range. The proposed measurement device extends Frequency-Resolved Optical Gating (FROG), a very successful technique for measuring the complete intensity and phase versus time of ultra-short pulses, which operates by measuring the pulse's spectrogram. The main challenge in extending FROG to much longer pulses is the generation of a many-ns delay range on a single shot - currently an unsolved problem in general. The proposed innovation solves it by tilting the input pulse by a remarkable ~89.9Â° without distorting it in time. As a result, one side of a ~1cm-wide beam precedes the other by over a meter. The proposed ns FROG can measure even very complex pulses and will cost about 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 will extend to most pulsed lasers, from Q-switched and gain-switched solid-state lasers to fiber lasers, which emit ~ns pulses and have many applications. All such applications will benefit from this device. First, ~ns-laser users will now have a device to test their laser?s performance, and it will be simple, easy to use, single-shot, and relatively inexpensive. It will be essential in attempts to coherently combine pulses from multiple fiber lasers, generally regarded as the next important step in high-power fiber-laser development. Injection-seeded Qswitched and gain-switched lasers, which endeavor to emit very clean ~ns pulses, are also in need of such a method for performance confirmation. Finally, laser engineers in general will be better able to improve the quality of ~ns-laser pulses, greatly benefitting all ~ns-pulsed-laser experiments and applications. If the spectacular progress in ultrafast lasers that occurred after complete ultra-short-pulse-measurement technology was introduced is any indication, such an inexpensive and simple device for measuring ns pulses should make a huge difference in the generation of ever more stable ~ns pulses and consequently in the many fields that use such lasers, from welding to surgery to material processing to distance measurements to remote sensing.
Shortly after the development of the first lasers, fifty years ago, researchers learned a valuable lesson: laser pulses were not very useful if their beam quality was poor. And it was. Variations in the light intensity from place to place in the beam and from pulse to pulse, made experiments noisy and applications unreliable. A good beam shape—a simple, unstructured, stable beam—was critical for essentially all experiments and applications. Fortunately, the human eye can see the beam of visible lasers, and cameras can more quantitatively measure it for all lasers. Using such measurements, researchers were able to improve laser-beam quality considerably over the decades. And today, a simple, unstructured, stable beam is considered critical. Lasers with complex, structured, unstable beams are of little use. In addition, the beam phase (that is, its wavelength or color) can vary from place to place in the beam and from pulse to pulse. This is also a problem for the same reasons, and it was also slowly solved over the years. Over the same years, researchers also learned how to generate shorter laser pulses, a few nanoseconds (billionths of a second) in length. And just as simple, unstructured, stable beams in space are important, equally important for the same reasons are analogously simple, unstructured, and stable pulses in time. Unfortunately, even the fastest light detectors and oscilloscopes could not resolve few-nanosecond pulses in time. Ultrashort-pulse-measurement techniques eventually emerged, but yielded only rough measures, not the required pulse intensity and phase vs. time. Nevertheless, researchers managed to generate even shorter laser pulses in the succeeding decades, achieving lasers that emitted pulses picoseconds long (thousandths of a billionth of a second). But pulse-measurement methods continued to lag far behind beam-measurement methods. And it was not until the 1990s, when pulses reached femtosecond lengths (millionths of a billionth of a second), that pulse-measurement methods emerged that could completely measure their intensity and phase vs. time. As a result, over the past couple of decades, femtosecond pulses could be optimized and stabilized. Femtosecond lasers—now the most stable light sources ever developed—are the basis of spectacularly precise time-measurement applications. But what about nanosecond laser pulses? Alas, pulse-measurement researchers essentially forgot about them! Worse, measuring nanosecond pulses actually turned out to be much more difficult than measuring femtosecond pulses because, among other reasons, devices generally scale in size with the length of the pulses, and nanosecond pulses are meters long, while femtosecond pulses are less than a millimeter long. In short, there remains no commercial device for completely measuring nanosecond pulses in time. Because it has not been possible to measure them, the engineering necessary to improve them has not been possible, and currently most nanosecond laser pulses are far from ideal. This is unfortunate because most of the pulsed lasers in use throughout the world emit pulses nanoseconds long and vastly outnumber femtosecond lasers. Also, applications of nanosecond pulses are far more numerous, from welding to surgery to materials processing to distance ranging to laser radar. Swamp Optics is developing and proposes to market a nanosecond pulse-measurement device, which can measure essentially everything about even a single, individual nanosecond laser pulse. It solves the long-standing problem of generating the required very long delay range for a single pulse in a very novel manner: by actually tilting the laser pulse to be measured by 89.99° without distorting it. In Phase I, a prototype of the device was demonstrated and found to work well. Could even measure fairly complex pulses from a fiber laser. This device will allow greater control over the light used in many applications, so it should improve the quality of a wide range of fields, from materials processing to the vast array of scientific and military applications of these lasers. For example, Q-switched Nd:YAG lasers currently emit very complex and unstable nanosecond pulses, but most modern applications of this laser, such as ophthalmological, oncological, dental, and other forms of surgery, which must avoid the generation of undesired thermal effects, will benefit greatly from simpler, more stable pulses. Newer potential applications of YAG lasers, such as improved automotive spark plugs for improved fuel efficiency will also require improved pulse simplicity and stability. Other important common applications, such as range-finding and LIDAR (laser radar), which use both solid-state and fiber lasers, will see vastly improved accuracy from improved pulse shapes and stability—and also from the mere ability to measure the pulse shapes from their lasers.