(1) Complete Characterization of Ultrafast X-Ray Pulses and, (2)
Chemical Remote Sensor for Proliferation

Both of these grants are Phase 1 SBIRs

TECHNOLOGY

Pulse Measurement: Frequency Resolved Optical Gating (FROG)

Ultrafast laser systems, which generate laser pulses with durations of approximately 10 picoseconds or less, have a large number of applications in biochemistry, chemistry, physics, and electrical engineering. Such systems may be used to explore kinetics in proteins or examine carrier relaxation in semiconductors. They are also used as an ultrafast probe in electronic circuits. By using ultrafast diagnostic systems, highly advanced semiconductors, electronic circuitry, and even biomedical products can be developed and tested for commercial applications. Unfortunately, ultrafast laser systems can be difficult to develop and maintain because few diagnostics are available to characterize the ultrashort laser pulses.

Real ultrafast laser pulses are not perfect, smooth pulses. Even though they are short, they exhibit temporal structure in their amplitude and/or phase. The most common structure is called chirp. Chirped pulses can be viewed as changing color. In the case of positive chirp, the wavelengths change from red at the beginning of a pulse to yellow at the end. This chirp is the result of a change in phase of the light during the pulse that can be induced by dispersion or nonlinear optical processes. Pulse chirp is not just detrimental; chirp in ultrafast laser pulses can be specifically designed to excite molecules more efficiently and to generate sculpted quantum states. Chirp also increases the length (duration) of a pulse. Ultrafast laser systems require a diagnostic tool that can measure both the intensity (or amplitude) and the phase of the optical pulses.

Frequency-resolved optical gating, or FROG, measures both the intensity and phase (chirp) of ultrafast laser pulses. Consequently, it is the only true pulse measurement technique available. Other methods, such as MIIPS and SPIDER, are only frequency domain measurement technologies—they only measure the spectral phase of the ultrafast laser pulses. By combining the spectral phase with the pulse spectrum, the pulse shape can be determined. Unfortunately, measuring the pulse spectrum is not as easy as you might think. Amplified spontaneous emission (ASE) from amplified ultrafast laser broadens the measured ultrafast laser pulse spectrum. This broadened spectrum, together with the spectral phase can make the pulses seem shorter than they are. Worse, some ultrafast laser manufacturers use this strategy to make their specifications appear better than they really are. FROG, on the other hand, provides a true pulse measurement independent of the spectral measurement.

FROG characterizes ultrashort laser pulses by interacting one or more pulses in a nonlinear medium. One pulse forms a “gate” that lets a time slice of the other pulse pass to a spectrometer. This signal pulse is spectrally resolved and recorded as a function of the delay between the input pulse and the gate. This record is called a spectrogram or FROG trace. The spectrogram is a plot of signal intensity vs. frequency and time which contains all of the information about the laser pulse. In the chirp example given above, the spectrogram would show that the pulse was red at early times, changing to orange in the middle and yellow at the end of the pulse. The target information, the temporal and spectral profile of the input pulse (intensity and phase), can be obtained from the FROG trace using two-dimensional phase retrieval methods. ]]> http://www.mesaphotonics.com/2011/04/frog/feed/ 0 http://www.mesaphotonics.com/2011/04/technology2/ http://www.mesaphotonics.com/2011/04/technology2/#comments Mon, 25 Apr 2011 19:26:10 +0000 MesaBlogAdmin http://www.mesaphotonics.com/?p=580 READ MORE →]]> Mesa Photonics is working in four areas of optical absorption spectroscopy. All are aimed at detection of trace compounds, particularly trace gases. Optical absorption is described by the Beer-Lambert law: 

where and , respectively the intensity of light before and after passing through a sample, is the concentration of a target compound in the sample, is the target compound’s absorption cross section, and is the length of the optical path through the sample. The cross section, , is a known property of the target compound and distance is a design parameter of the spectrometer. The concentration, , of a compound can then be determined by measuring . Measuring trace concentrations – i.e., determining small values of – requires either determining small changes in optical intensity, – , or using long optical paths, or both.

Our spectroscopy technology emphasizes working with and developing spatially coherent light sources that can be propagated easily over long distances or can be injected efficiently into optical resonators. These approaches provide the long optical paths that are needed for high sensitivity.

Two of Mesa Photonics spectroscopic technologies use near-infrared diode lasers – the types of lasers developed for long haul fiber optic telecommunications – for trace gas analysis. This work will lead highly miniaturized trace gas analyzers that are useful for emissions monitoring, environmental studies, and industrial safety application. The diode lasers have the advantages of being small, operating at low power, and, for most cases, producing eye safe output. The lasers operate at room temperature and are housed in hermetically sealed packages that are only 2.5 × 1.5 × 0.7 cm. The light is emitted through an optical fiber. Mesa Photonics researchers have over 30 years combined experience in diode-laser-based spectroscopy for trace gas detection.

One of the technologies is based on optical absorption spectroscopy with diode lasers and will, we anticipate, provide part per billion detection limits for a variety of gases with pocket-sized analyzers that cost less than half the price of existing optical analyzers. This proprietary method can also be applied to detection carbon monoxide, carbon dioxide, ammonia, hydrogen sulfide, methane, hydrogen chloride, nitric oxide, and ethylene. In the case of hydrogen sulfide detection for the petrochemical industry, the new technology should offer significant advantages over existing methods for hydrogen sulfide monitoring at wellheads and in refineries. It is also likely that our approach will be useful for isotope ratio measurements in carbon dioxide, water vapor, and methane.

Mesa Photonics is also working on detecting trace gases in exhaled breath using a technique called NICE-OHMS, standing for Noise-Immune, Cavity-Enhanced, Optical Heterodyne Spectroscopy. NICE-OHMS was invented by John Hall, Jun Ye, and co-workers at NIST in Boulder, CO. Their work and subsequent research by other groups demonstrated phenomenal detection sensitivity. But, the research used specialized lasers and was limited to low pressure gas samples. We are developing real-world applications including exhaled breath analysis by implementing NICE-OHMS using low-cost, telecommunications diode lasers and atmospheric pressure samples. Sensitivity is not as good as achieved at NIST, but it doesn’t need to be. We can trade off some performance for simplicity, and end up with better than part per billion sensitivity for some of the gases listed above.

We are also investigating new implementations of Fourier transform infrared (FTIR) spectroscopy. This technology area combines recent advances in optical sources, interferometry technology, and a proprietary signal analysis algorithm to implement compact, inexpensive spectrometers. Proof-of-principle applications were successful using near-infrared light sources. The next step is shifting to mid-infrared wavelengths.

Mesa Photonics interests in ultrafast lasers and optical spectroscopy are combined in the development and application of broad-band optical sources generated by femtosecond lasers. We own two ultrashort lasers – a Ti:sapphire oscillator from KM Labs and a 1550 nm fiber laser purchased from Menlo Systems – that are used for continuum generation. Wavelengths generated extend from the near-ultraviolet to 3 µm. New fiber materials will make it possible to extend the long wavelength range to 5 µm, and beyond. Temporal synchronization of the laser pulses simplifies cavity-enhanced spectroscopy techniques, and the broad-band sources can be used for optical spectroscopy with our new FTIR sub-system. ]]> http://www.mesaphotonics.com/2011/04/technology2/feed/ 0 http://www.mesaphotonics.com/2011/04/technology1/ http://www.mesaphotonics.com/2011/04/technology1/#comments Mon, 25 Apr 2011 19:25:26 +0000 MesaBlogAdmin http://www.mesaphotonics.com/?p=578 READ MORE →]]>

TECHNOLOGY| FROG Retrieval Algorithm

Fast analysis of FROG spectrograms

Using our proprietary algorithm, our VideoFROG software system can perform the phase retrieval in real time. Single-shot FROG geometries have allowed 30 Hz measurement rates to be obtained. Now you can tweak up your laser system systems while watching the phase, intensity, and duration of the pulses displayed on a computer screen. The ultrafast oscilloscope has arrived.

FROG is experimentally simple. A single pulse is split into two equal pulses, with one pulse used as the gate and the other pulse as the probe. The nonlinear medium can be as simple as a piece of optical quality quartz. The inset shows that a signal pulse is sliced out where the gate and probe overlap.

Our FROG Scan system works by step scanning an optical delay, and reading the spectrum at each delay using a mini-spectrometer. Because of the unique zero-backlash servo, data acquisition is fast rapid—it can be less than 300 ms. The resulting spectrogram provides immediate, qualitative information about the pulse. However, because VideoFROG seamlessly couples data acquisition with the inversion algorithm, true pulse measurement is achieved in real-time.

To obtain the our robust, real-time measurements, we use an algorithm developed by Dan Kane, CEO and founder of Mesa Photonics, called the Principal Component Generalized Projections algorithm (PCGP), that is very fast and easy to implement for common FROG geometries. This algorithm coupled with data acquisition in a multishot second harmonic generation (SHG) FROG device, is the basis of our FROG Scan femtosecond oscilloscope that displays the intensity and phase of the extracted pulse at rates of several Hertz. ]]> http://www.mesaphotonics.com/2011/04/technology1/feed/ 0 http://www.mesaphotonics.com/2011/04/news3/ http://www.mesaphotonics.com/2011/04/news3/#comments Fri, 22 Apr 2011 23:10:46 +0000 MesaBlogAdmin http://www.mesaphotonics.com/?p=563 June 11 – June 13, 2013

Upcoming CLEO Conference in San Jose, CA

Please visit our Booth (#2421) at CLEO 2013.
June 11-13, 2013 at the San Jose McEnery Convention Center – San Jose, CA

www.cleoconference.org

Dr. Kane is the 2011 Recipient of the Harold E. Edgerton Award

Dr. Daniel Kane was named, by SPIE, the 2011 recipient of the Harold E.
Edgerton Award. The award is presented annually for outstanding
contributions to optical or photonic techniques in the application and
understanding of high speed physical phenomena.

Dr. Kane was recognized for his contributions to the characterization and
application of ultrashort light pulses, including the first realization of time
domain retrieval of amplitude and phase using frequency-resolved
optical gating.