Fiber Lasers: Mid-IR laser source is widely tunable for standoff explosives detection
MATHIEU GIGUÈRE, VINH N. DANG, and JOSEPH SALHANY
The detection of explosives and explosive-related compounds has become a high priority in recent years for homeland security and counterterrorism applications. There has been a huge increase in research within this area both through the development of new, innovative detection approaches and the improvement of existing techniques. Efforts have been aimed at developing ruggedized field-deployable systems with larger stand-off distances, which can be achieved by improving selectivity and sensitivity.
The objective of standoff detection is to eliminate the need for any direct contact, allowing detection to take place farther away from objects being analyzed—for example, people or vital assets—and thus reducing the potential for any harm or damage. Many techniques have been considered for chemical detection, including mass spectroscopy and chromatography. However, standoff detection is based on techniques that can be efficient while avoiding direct contact with the samples.
Standoff-detection applications currently being being developed are based upon optical techniques such as hyperspectral imaging, fluorescence, Raman scattering, laser-induced breakdown spectroscopy (LIBS), and differential-absorption lidar (DIAL). While these techniques have been shown to be efficient, they are restricted by their limited specificity and sensitivity, as well as a lack of eye-safe capability.
Among these optical techniques, infrared (IR) spectroscopy in the fingerprint region (5 to 15 μm) is the most promising, as it provides strong and unique signatures for many chemical substances. Recent advances in explosive detection have enabled the use of Fourier-transform infrared (FTIR) instruments combined with a mid-IR fiber probe. However, poor signal-to-noise ratio (SNR) due to the use of a lamp light source and fiber loss restrict the distance of the sample from the fiber probe tip to a maximum of a few centimeters. For stand-off detection of samples, a transition from lamp-based sources to high-spatial-coherence laser sources is required.
Mid-IR laser sources: A potential solution
Mid-IR lasers have been developed for applications in diverse fields such as remote sensing, pollutant monitoring, laser-based countermeasures, and narcotics/explosives detection. Other applications in the biomedical and pharmaceutical industry—such as cell and lipid detection and identification of pharmaceutical raw materials—also have emerged using this technology.
Optical parametric oscillators (OPOs) are used as mid-IR laser sources, but their tunability is still limited and requires temperature and/or angular tuning, making their design inherently sensitive to mechanical vibrations.
Quantum-cascade lasers (QCLs) have also emerged as potential mid-IR sources. They are based upon multiple-quantum-well semiconductor lasers that rely on intersubband transitions to create light; extensive development of QCLs has led to relatively high power outputs at room temperature. The tunability of a single unit, however, is still limited. As a result, the design of a widely tunable QCL-based source requires the combination of many single devices, requiring fairly complex optical integration.
Tunable mid-IR from difference frequency generation
A widely tunable mid-IR fiber-based source represents an ideal solution for standoff explosives detection, as its wavelength range covers most of the fingerprint region with the benefits and flexibility of fiber-based technology. For quality spectroscopic measurements, and specifically for standoff-detection measurements, a mid-IR source needs to have specific properties: high laser SNR, narrow linewidth to obtain high selectivity and sensitivity, low source noise and low amplitude modulation, low temperature and current tuning rates to minimize wavelength jitter, and rapid wavelength tunability for fast response and high data-acquisition rates.
Genia Photonics has developed a mid-IR laser source based upon nonlinear difference frequency generation (DFG) of its commercially available picosecond synchronized laser (SL). DFG is a nonlinear process (see Fig. 1) in which two photons of different energies are mixed in a proper nonlinear material to produce a third photon with the energy corresponding to the difference in frequency of the two incident photons. By application of DFG to the rapid spectrally tunable picosecond pulses from the laser, it becomes possible to obtain a tunable mid-IR source with impressive spectral resolution (better than 3 cm-1).
The synchronized laser system consists of two fiber-based laser sources: the programmable laser (PL) (wide tuning) and a master oscillator power amplifier (MOPA). The PL is a dispersion-tuned actively mode-locked laser based on an electro-optic modulator coupled with high-speed electronics and is capable of generating optical pulses with durations as short as 25 ps (see Fig. 2). The emitted wavelength of the PL can be rapidly and continuously tuned up to 10,000 times per second while keeping both optical pulses synchronized with the use of a low-jitter function generator. This same function generator also gives flexibility to the system by enabling features such as phase dithering, rapid arbitrary wavelength tuning, and match filtering.
To access the mid-IR spectral region, a PL based on thulium-doped fibers was developed with a capability to spectrally tune from 1893 nm up to 2000 nm. By coupling this PL with erbium-doped lasers (1542 nm and 1597 nm wavelengths), it is possible to obtain DFG wavelengths ranging from 6800 nm to 10,200 nm by using the proper nonlinear medium.
Most of the common nonlinear crystals exhibit very strong absorption in the mid-IR spectral region. The crystal of choice for a widely tunable source based on Genia's SL needs to have a very wide phase-matching bandwidth associated with the highest nonlinear index possible. For these reasons, orientation-patterned gallium arsenide (OP-GaAs), using quasi-phase-matching, is the best candidate.
This domain-engineered crystal allows the flexibility of customizing the phase-matching properties required to generate DFG from the PL and MOPA. To do so, two crystals with different quasi-phase-matching period profiles are matched with the corresponding DFG spectral range. The desired output of each of the crystals is then combined via polarization to enable a widely tunable mid-IR source ranging from 6800 nm to 10,200 nm with power levels reaching the milliwatt range. The emitter is integrated in a ruggedized mechanical package; no temperature tuning is required to access the full spectrum, and the system has no moving parts.
Mid-IR and molecular detection
This tunable mid-IR spectroscopic source provides four important performance characteristics: sensitivity, selectivity, fast response time, and compactness. Combined with the right components-detectors, optical components, and software-it can be engineered into a complete system for molecular detection and can be operated under many configurations to address different applications.
As this mid-IR source evolves in capability, an emphasis on the precision of trace chemical detection becomes essential. Upcoming issues and challenges may not solely depend on the laser source, but on other factors that may be application-dependent, such as environmental conditions, the type of samples being analyzed, and the extent of the detection component. It will thus be crucial to address these important key factors while keeping the system simple from a deployment and operational standpoint.
Mathieu Giguère, Vinh N. Dang, and Joseph Salhany are at Genia Photonics, Laval, QC, Canada; email: [email protected]; www.geniaphotonics.com.