Difference frequency generation makes tunable mid-IR lasers possible

Mathieu Giguère

Nonlinear frequency mixing of two laser sources results in a widely tunable laser perfectly suited for molecular spectroscopy.

24 October 2013, SPIE Newsroom. DOI: 10.1117/2.1201310.005163

http://spie.org/x104001.xml?highlight=x2404&WT.mc_id=KNRLASERCE

Mid-IR lasers—which operate at wavelengths of 3–20μm—are used in a wide range of applications such as remote sensing,1 pollutant monitoring,2laser-based countermeasures, and narcotics or explosives detection.3 However, the availability of such lasers is limited. Devices operating at wavelengths longer than 1.8μm are not widely tunable nor do they exhibit the narrow linewidths needed for advanced applications such as spectroscopy. Adjustable sources like thulium-doped fiber lasers and optical parametric oscillators (OPOs)—which convert a single input laser to two output beams at lower frequencies—have not yet emerged.

So far, OPOs are the leading candidate for mid-IR laser sources but frequency adjustments are limited and require temperature or angular tuning, making their design sensitive to mechanical vibrations. More recently, quantum cascade lasers (QCLs) have surfaced, which are based on microstructured semiconductor lasers. Extensive development effort has led to relatively high power with room-temperature operation. However, the tunability of a single unit is still constrained. Moreover, the design of a widely tunable source requires the combination of many single devices, resulting in a fairly complex optical integration. The ideal mid-IR source for spectroscopic applications needs to be mechanically robust, stable during temperature fluctuations, and tunable over a broad spectral range.

Our solution is based on difference frequency generation (DFG), a nonlinear process that involves combining two photons of different energies to produce a third photon whose energy equals the difference between those of the incident photons: see Figure 1. By applying the DFG process to the pulses of a commercially available picosecond synchronized laser (SL), we can obtain a tunable mid-IR source with fairly good spectral resolution (<1cm−1).

Figure 1. Difference frequency generation (DFG) creates a photon from the frequency difference between two incident photons: the pump and signal. The DFG photon is generated only when the pump and signal laser light are present with sufficient intensity. ω: Frequency.

Our SL system (see Figure 2) consists of two fiber-based laser sources.4The first source is a picosecond programmable laser (PL) whose emitted wavelength can be tuned continuously and rapidly at up to 80,000 different wavelengths per second. The main features of the PL come from the combination of a dispersive element inside the laser cavity and an electro-optic modulator (EOM) acting as an active mode-locker. Since the laser cavity length is different for each wavelength, fast tuning is achieved by changing the driving frequency of the EOM. The second part of the SL is composed of a picosecond master oscillator power amplifier (MOPA) electronically synchronized to the PL, regardless of the wavelength at which it is being operated. Also, since the SL is electronically controlled, many intrinsic features can be exploited, such as phase dithering, rapid arbitrary wavelength tuning, and match filtering in order to increase the signal-to-noise ratio for novel detection techniques.

 

Figure 2. Schematic of our mid-IR laser source. By combining two high-peak-power lasers (red, blue) into a nonlinear DFG crystal, the output laser wavelength (blue) shifts to a different spectral region such as visible, near-IR, or mid-IR (3–4 and 6–12μm). Wavelength, synchronization, and dither are all electronically controlled. MOPA: Master oscillator power amplifier. WDM: Wavelength division multiplexer.

We have demonstrated a widely tunable mid-IR source using a PL based on a tunable thulium-doped silica fiber (1905–1990nm) along with a MOPA based on erbium-doped silica fiber (1530–1585nm). The resulting source is tunable from 6.62μm up to 9.43μm. We tested many DFG crystals—silver gallium sulfide (AGS), silver gallium selenite (AGSE), and orientation-patterned gallium arsenide (OP-GaAs)—for their conversion efficiency as well as their phase-matching bandwidth stability without any temperature or angular tuning. Each crystal has its own advantages and trade-offs (see Figure 3). For instance, AGS provides the widest bandwidth for a single crystal unit but very low conversion efficiency. We achieved up to 1000nm of bandwidth around 7.5μm with hundreds of microwatts of mid-IR radiation for watt-level SL power. On the other hand, AGSE provided fairly good efficiency (>10mW) but with only ∼100nm of bandwidth under the same experimental conditions. In the end, we used OP-GaAs: a quasi-phase-matching crystal. We found it had the best conversion efficiency (>40mW) though it exhibited the narrowest tuning range (<50nm). Even though OP-GaAs crystals do not have a broad tuning range, the quasi-phase-matching medium provides an opportunity to engineer the crystal properties in order to significantly enhance the tuning range and hopefully reach the actual DFG spectrum achievable through use of the SL.

 

Figure 3. Relative intensity and tuning range obtained with different nonlinear DFG crystals. AGS: Silver gallium sulfide. AGSE: Silver gallium selenite. OP-GaAS: Orientation-patterned gallium arsenide. a.u.: Arbitrary units.

Additionally, we demonstrated that by combining an ytterbium-doped fiber laser (1030–1090nm) with one that is erbium-doped (1530–1585nm) within a periodically poled lithium niobate crystal, it is possible to obtain a tunable source (2.9–3.8μm) with exactly the same technical advantages.5

In order to target applications such as explosives detection, we have developed a mid-IR laser source that applies DFG techniques to a synchronized laser. One of the main advantages is the fast tunability of the programmable laser. This feature allows the user to rapidly tune the laser frequency over a specified range in either a sequential or arbitrary manner. Another advantage is that the resolution and speed are fully programmable and can be independently set by the user. This is possible because the laser is driven by advanced electronics that are controlled via a graphical user interface running on a PC. With these promising results, we can next combine our mid-IR source with the right components (e.g., detectors, optical components, and software) to engineer a complete system solution for molecular detection applications.

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Mathieu Giguère

Genia Photonics

Laval, Canada

Mathieu Giguère has been a mid-IR system specialist with Genia Photonics since 2012 where he is technical lead of multiple projects in the Defense and Security sector. He has developed a strong expertise in nonlinear optics and optical system design.

References:

1. J. D. Suter, B. E. Bernacki, M. C. Phillips, Angle-resolved scattering spectroscopy of explosives using an external cavity quantum cascade laser, Proc. SPIE 8268, p. 82681O, 2012. doi:10.1117/12.908653

2. D. J. Phillips, E. A. Tanner, H. O. Everitt, I. R. Medvedev, C. F. Neese, J. Holt, F. C. De Lucia, Infrared/terahertz double resonance for chemical remote sensing: signatures and performance predictions, Proc. SPIE 7671, p. 76710F, 2010. doi:10.1117/12.853309

3. J. R. Castro-Suarez, Y. S. Pollock, S. P. Hernandez-Rivera, Explosives detection using quantum cascade laser spectroscopy, Proc. SPIE 8710, p. 871010, 2013. doi:10.1117/12.2016037

4. J. Salhany, B. Burgoyne, Fiber lasers: programmability comes to fiber lasers, Laser Focus World 48(3), 2010.

5. F. Théberge, J.-F. Daigle, A. Villeneuve, J. Salhany, B. Burgoyne, Y. Soudagar, M. Châteauneuf, J. Dubois, Tunable mid-infrared generation using synchronized programmable fiber lasers, Proc. SPIE 8381, p. 83810E, 2012. doi:10.1117/12.921500

The dangers of photonics

http://www.laserfocusworld.com/blogs/photon-focus/2013/01/the-dangers-of-photonics.html

The dangers of photonics

By Gail Overton

As I get ready to write the March "Photonics Applied: Defense & Security" feature article for Laser Focus World on whether terahertz scanners used at airports and in cargo facilities are really safe, I'm reminded that photonics has a darker, dangerous side. After all, laser weapons do exist, laser dazzlers can disable a human from a distance, fluorescentnanoparticle toxicity is still under investigation, pointing green lasers in aircraft cockpits is a punishable offense, and unmanned aerial vehicles (UAVs) roam the skies posing as birds and insects.

The dangers--whether real or media hype--are definitely in the public spotlight. Check out this alarmist video on YouTube about how the government plans to install "scanners" at airports and sporting events that can learn everything about you, from what you had for breakfast to identifying traces of explosives and narcotics. (the video link ishttp://youtu.be/K4s6HwzJwbc)

Real or hype? Unfortunately or not (depending on your conspiracy viewpoint), the claims are becoming quite real. Standoff molecular detection is real, and companies like Genia Photonics (Laval, QC, Canada) are developing picosecond programmable lasers and master oscillator power amplifier (MOPA) technologies used in coherent anti-Stokes Raman scattering (CARS) spectroscopy that are designed precisely for learning everything about the molecular makeup of a distant object.

If people think Facebook was designed for "data mining" its customers, CARS spectroscopy could certainly gather more data about your food likes and dislikes for a localized population than could any software program. And CARS doesn't lie; how many people on Facebook put in fake information specifically because they don't want to be "targeted". But is standoff detection really wrong? Personally, I think it could be beneficial because data mining could help decide, for example, which fast food restaurants or which Starbucks items should be prominent at the airport. But if I were to get a text telling me where I could grab my favorite Whopper sandwich when I land next at JFK, that might be a problem for me.

Privacy seems to be a thing of the past, and technology--much of it related to photonics and optics advances--has made population as well as individual monitoring possible. And are all of these wireless signals and lightwaves traversing our personal space safe and secure? Maybe there really is a link to cell phone use and cancer? Are the long-term effects of standoff detection well understood? I'm hoping to shed some light on the terahertz scanner safety issues in my next article, and will continue reporting on the possible dangers associated with the industry's seemingly innocuous photonics advances.

I used to think that out in the desert in the middle of nowhere, I was immune from "Big Brother is Watching" worries (not that I have anything to hide). But that dragonfly and tarantula hawk I saw the other day were just a little too mechanical looking and hung around just a little too long....

Genia Photonics tunable lasers are revolutionizing spectroscopy

I recently mentioned Genia Photonics, (GP),and the recent agreement it has entered into with In-Q-Tel in my end of the year post.  I noted  that aside from the collective "buzz", that is currently going on regarding  CMOS, solid state THz, the news about GP's, new very powerful and very highly sensitive new THz product, was the most interesting THz related story this year in my opinion.

I also mentioned that I had tried to make contact with GP, to learn more to share with you here, but that I had not had any luck. Writing about GP earlier this week peaked my interest anew, and I revisited their web-page. http://www.geniaphotonics.com/ Let me share with you what I found.

Founded in 2009 by the merger of Optav Solutions and FG2 Tech, Genia Photonics Inc. webpage states it is an innovative company specializing in high-speed picosecond fiber-based lasers and spectroscopic measurement systems. Centered around it's patented fiber laser technology, Genia’s compact, easy to use and controlled via software systems is claimed to change the methodology for various applications in biomedical, industrial as well as defense and security. Dr. Gonthier and Dr. Villeneuve boast together over 35 years of experience in transferring innovative R & D technology into profitable commercial products.

What I found to be really informative and helpful in substantiating GP's claims were articles found under the news tab, on the homepage. (Note that you can't access these pages without first signing your name & email address, but if you do access is immediate).

I found two articles to be of much interest to me, (the first of which I will provide you with  a lay-persons, non-technical review). The first is a reprint of an article In-Q-Tel, published in April of 2012, about GP, which is titled "Standoff Explosive Detection". 

In-Q-Tel, in this paper substantiates GP's claims, as IQT, notes that GP has a "technical advantage". 
'Genia Photoics offers fiber-based laser solutions that leverage the benefits of fiber, such as durability, robustness, small size, lower total cost and increased reliability, /The lasers are all software controlled which through the embedded electronics permits all the laser''s characteristic parameters [wavelength, pulse width, output power, and tuning speed] to be easily controlled. The wide wavelength tuning range and the fast tuning speed of the laser are key enablers for standoff explosive detection, allowing greater coverage for a given time interval or a reduction of the scanning time interval for a specific area." 

The article is 3 pages in length and notes a number of other technical advantages the GP fiber-based laser. The system allows for nonlinear spectroscopy as well as mid-IR spectroscopy. The detailed article concludes by noting that:
"The versatile synchronized laser system supports various standoff detection schemes.  It offers many desired features and capabilities such as wavelength tuning, pulse width variation, output power control, and repetition rate variation.  The ability of wavelengths to be swept at a very fast rate enables a specific area to be scanned in a much shorter time interval.  And, since the wavelength and repetition rate are associated, the need for a spectrometer at the receiving end is eliminated.  Multiple detectors can then be economically deployed to capture more reflected light for a better analysis.  Moreover, since all parameters are controlled electronically via software, it is possible to combine different detection techniques [bulk and trace simultaneously] in order to obtain a full detection of the targeted sample."

The second article I would recommend readers read is entitled "How Tunable Lasers are Revolutionizing Spectroscopy", which was authored internally at GP. It also more fully describes the revolutionary laser GP has developed for use in THz.
 I once again encourage readers to review these articles in their entirety on the GP webpage, which is linked above.
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US Homeland Security reportedly set to deploy Genia Photonics, ultra-sensitive spectrometer

My Note: I almost hate to post this story, since, I have a hard time believing it's true, (based upon  the TSA's tortoise like, pace in adopting new technologies). The In-Q-Tel, agreement with Genia Photonics to develop this product was not even signed until last November, Advanced Photonix, anomaly detection device which is also under review by the TSA, was (and is) the subject of a similar In-Q-Tel development contract signed in November of 2010, and the prototypes (beta version) shipped to TSA in March of 2012. I'm posting the article below, because the story is  making such a huge buzz on the net.  You can make your own decisions. I'm trying to make contact with Genia to find out the real truth of this story.  Stay posted. 

http://phys.org/news/2012-07-homeland-reportedly-deploy-ultra-sensitive-spectrometer.html
by Bob Yirka
(Phys.org) -- Online tech magazine, Gizmodo has stirred up a hornet’s nest of paranoid editorials across the globe by printing an article written by an unknown PhD student who claims that the US Homeland Security department is planning to deploy a new kind of scanning device that is so sensitive it will make all other security measures at airports moot; and worse will be able to do so at a distance allowing the process to occur without the knowledge of the person being scanned.

In the piece, the author, designated simply as NAC, says that the device has been developed by a private company called Genia Photonics, which is apparently chock full of physicists and engineers. It’s described as being able to pick up on the presence of mere molecules of suspicious substances (using apparently harmless, terahertz radiation) such as chemical weapons, gunpowder residue or even heightened levels of adrenaline in the bloodstream, all from a distance of up to 50 meters. What’s more it’s really fast, doing its work in picoseconds, and portable, meaning that DHS could set up the scanner at airports, train stations, border crossings or wherever else they believe a possible threat exists. What appears to worry some though, is the possibility of being mistakenly labeled as a suspect, criminal, terrorist, etc. People encounter many innocuous substances every day that could be construed as dangerous or even illegal. Stepping on a leftover marijuana stub without knowing it, could for example cause such a scanner to go off, as could applying fertilizer to the home garden prior to heading for the airport. Something else that seems to cause alarm is the fact that the technology behind the device appears to be sound, and in fact has apparently been done before. The difference this time is the speed at which it works; because of that, a single device could conceivably be used to scan every single person passing through an airport’s terminals, which means, that if deployed the days of singling out individuals for extra security measures would be over. If a person goes to an airport, they will be scanned, and most won’t even know it’s happened. The author of the article says an undersecretary at DHS has stated that the scanner will be ready for deployment within one or two years. © 2012 Phys.org

Genia Photonics Enters into Strategic Partnership with IQT

My note: This is old news but I just heard about it today. I  hadn't heard of Genia Photonics either, until today, but they have some interesting terahertz applications.  (Thanks to poster hemecity on the IV board, for alerting me to this story.)
http://www.iqt.org/news-and-press/press-releases/2011/Genia-Photonics.html
November 1, 2011
Genia Photonics Inc., a high technology company specializing in fiber-laser-based systems, announced a strategic partnership and technology development agreement with In-Q-Tel (IQT), the non-profit, strategic investment firm that delivers innovative technology solutions to support the missions of the U.S. Intelligence Community.
"Our partnership with Genia Photonics will build on the company's success in the commercial market," says Simon Davidson, partner on IQT's Investments team. "Genia Photonics' fiber-based technology will lead to unique possibilities for our customers in the U.S. Intelligence Community." The technology is being developed for the Department of Homeland Security Science and Technology Directorate (DHS S&T), an IQT customer agency.
This strategic partnership between Genia Photonics and IQT will promote Genia's fiber-based laser technology development to a higher level and provide new opportunities for Genia's product applications. "Genia Photonics' mission is to provide solutions that benefit society and improve quality of life," says François Gonthier, CEO of Genia Photonics. "Our partnership with IQT will create new opportunities for leveraging Genia's versatile and multifunctional fiber-based technologies into related applications."
In addition to the value for the defense and security communities, Genia Photonics' Synchronized Programmable Laser platform has applications such as non-linear spectroscopy for the biomedical and industrial communities. An important benefit of Genia Photonics' implementation as compared to existing solutions is that the entire synchronized laser system is comprised in a single, robust, and alignment-free unit that may be easily transported for use in many environments.
About Genia Photonics
Genia Photonics Inc. is an innovative technology company specializing in multi-functional measurement systems based on its patented fiber-based lasers. Genia’s easy-to-use, portable and computer controlled systems will change the methodology of various applications in the medical, industrial and defense and security communities. For more information, visit www.geniaphotonics.com.