Nonlinear susceptibility tensor

It takes at least 45 GW of power to generate the filaments at these mid-infrared wavelengths, he says, so this device easily meets that requirement, and the team proved that it did indeed work as expected. That now opens up the potential for detecting a very wide range of compounds in the air, from a distance.

"I think there is an agreement in the ultrafast laser community that the exploration of the mid-infrared spectral domain is going to be a new frontier in ultrafast laser science," Polynkin adds. "The extension of intense atmospheric propagation regimes into the mid-IR spectral range certainly holds a lot of promise to overcome the limitations associated with the very well-explored near-IR spectral range, namely the very unstable propagation dynamics in the near-IR. The authors tapped into a new domain of intense nonlinear optics. Without a doubt there will be follow-up work."

Show mathematically how the frequency doubled term arises in SHG

Focusing errors are the main problem you will face. After the most careful focusing possible through an eyepiece magnifier, it still helps to bracket focus a little and go by the sharpest frame/photo for that lens and aperture.

The above chart is too small to download and print. For actual lens testing, a higher resolution version is here.  This is a large file. Right click and save the file to your computer, then print it on photographic paper at about 3x3 inches in size at your printer's highest resolution photo setting.

Set your digital camera to ISO 100 at its highest capture resolution. If you are shooting film, load your camera with a high resolution film like Tech Pan (black and white) or Fuji Velvia slide film.

One way to test your own lenses is with the USAF 1951 lens testing chart. The best way to do this is to download a high resolution file of the chart from the link at the bottom of the page, and print the file out at about 3x3 inches in size on photo paper at your printers highest resolution (with many printers this is 1440 or 1880 ppl). You will need to print several copies of the test chart

Phase matching in nonlinear optics

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The results of your test will not correlate with anyone's published lines per mm results. This procedure is only for comparing results from your own lenses.

Researchers at MIT and elsewhere have found a new way of using mid-infrared lasers to turn regions of molecules in the open air into glowing filaments of electrically charged gas, or plasma. The new method could make it possible to carry out remote environmental monitoring to detect a wide range of chemicals with high sensitivity.

Take your printed (or photocopied) copies of the chart and attach them to the center, corners, and edges of a flat piece of foam core, or other flat board, about 24 inches by 36 inches in size. Add more copies of the chart between the center and edges.

But it is the mid-infrared (mid-IR) wavelengths, rather than the near-IR, that offer the greatest promise for detecting a wide variety of biochemical compounds and air pollutants. Researchers who have tried to generate mid-IR filaments in open air have had little success until now, however.

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Hong explains that such filaments, as generated by lasers in the near-infrared part of the electromagnetic spectrum, have been widely studied already because of their promise for uses such as laser-based rangefinding and remote sensing. The filament phenomenon, generated by high-power lasers, serves to counter the diffraction effects that usually take place when a laser beam passes through air. When the power level reaches a certain point and the filaments are generated, they provide a kind of self-guiding channel that keeps the laser beam tightly focused.

If you are shooting film, you can not accurately judge the results on film with a standard film loupe or by projecting the images with a slide projector. You will need a simple microscope that will give you a magnification of about 50x.

Nonlinear optomechanics

Laserpulse stretching

You will be able to see important differences in sharpness between different lenses set at the same aperture, and differences in apertures with the same lens.

If you are shooting film, when you get your processed film back, look at it with a microscope of about 50X magnification. Simple, inexpensive microscopes are sold by Edmund Scientific. You want the Scientifics catalog.

The new system makes use of a mid-infrared ultra-fast pulsed laser system to generate the filaments, whose colors can reveal the chemical fingerprints of different molecules. The finding is being reported this week in the journal Optica, in a paper by principal investigator Kyung-Han Hong of MIT's Research Laboratory of Electronics, and seven other researchers at MIT; in Binghamton, New York; and in Hamburg, Germany.

Only one previous research team has ever succeeded in generating mid-IR laser filaments in air, but it did so at a much slower rate of about 20 pulses per second. The new work—which uses 1,000 pulses per second—is the first to be carried out at the high rates needed for practical detection tools, Hong says.

More information: Houkun Liang et al. Mid-infrared laser filaments in air at a kilohertz repetition rate, Optica (2016). DOI: 10.1364/OPTICA.3.000678 Journal information: Optica

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This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

Laserchirp definition

Use two flash units at 45 degree angles to the chart as your light source. It is best not to use ambient light due to possible errors from vibration due to mirror slap. If you must use ambient light, make sure the light level is high enough or low enough that you aren't in the 1/60 to 1/2 second shutter speed range if you are testing a lens longer than 100mm.

You can see from the small version of the chart below that the lines are grouped in sets of six, three horizontal and three vertical. Each set of six lines is slightly smaller than the preceding set.  The smaller the lines that are are distinguishable on film, the greater the resolution, or in other words, the sharper the lens is at that aperture.

Femtosecondlaser filament

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Chirped pulse amplification Nobel Prize

This research "is one of the very first investigations of self-channeling of ultraintense mid-IR laser pulses in the air," says Pavel Polynkin, an associate research professor of optical sciences at the University of Arizona, who was not involved in this work. "Whether there will be new and exciting applications, time will show."

Put a removable sticky label on the chart for each lens and aperture in use. Change labels when changing lenses or apertures. This eliminates confusion later when looking at slides or negatives and wondering which frames were taken with which lens and at what aperture.

Put your camera and lens on a tripod about one inch in distance from the chart for every mm of focal length on the lens being tested. For a 50mm lens, you will be 50 inches away. For a 300mm lens, you will be 300 inches away.

The following procedure will not give you exact lines per mm resolution, but will allow you to test one lens against another, or compare different f-stops on the same lens.

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"People want to use this kind of technology to detect chemicals in the far distance, several kilometers away," Hong says, but they have had a hard time making such systems work. One key to this team's success is the use of a high-power femtosecond laser with pulses just 30 femtoseconds, or millionths of a billionth of a second, long. The longer the wavelength, the more laser peak power is needed to generate the desired filaments, due to stronger diffraction, he says. But the team's femtosecond laser, coupled with what is known as a parametric amplifier, provided the necessary power for the task. This new laser system has been developed together with Franz X. Kaertner in Hamburg and other group members for last several years. At these mid-IR wavelengths, Hong says, this device produces "one of the highest peak-power levels in the world," producing 100 gigawatts (GW, or billion watts) of peak power.

Using spectrally broadened mid-IR laser filaments, "we can detect virtually any kind of molecule you want to detect," Hong says, including various biohazards and pollutants, by detecting the exact color of the filament. In the mid-IR range, the absorption spectrum of specific chemicals can be easily analyzed.

Don't be surprised if the center chart has sharper lines than the edge charts. Most lenses are sharper in the center of the frame, especially at wide apertures. At the smallest apertures, all lenses get less sharp due to diffraction (bending of the light as it passes through the small aperture).

So far, the experiments have been confined to shorter distances inside the lab, but the team expects that there's no reason the same system wouldn't work, with further development, at much larger scales. "This is just a proof-of-principle demonstration," Hong says.

If you can't print your own chart, you can also find it on page 105 in the book SPECIAL PROBLEMS, one of the Time-Life series of photography books. You can get this book at your library, or have your library get it on inter-library loan. After obtaining a copy of the chart from the above book (or from the link at the bottom of the page), photocopy the chart. Resize or shrink the test chart "square" down to about 3 inches by 3 inches in size and print several copies