New laser sources benefit terahertz and mid-infrared remote sensing

Many molecules exhibit strong absorptions at mid-infrared (mid-IR) and terahertz (THz) wavelengths, which lie roughly between 3-30μm and 30μm-1mm, respectively. Applications for the mid-IR include: environmental/safety monitoring of greenhouse gases and hydrocarbons; engine emissions reduction; prospecting for oil and gas reserves; medical diagnostics; and infrared countermeasures. Although metal and water absorb THz light, many common materials, drugs, explosives, biochemical agents, and living tissues are semi-transparent at THz wavelengths. THz light also provides sub-millimeter spatial resolution for imaging and, unlike x-rays, is non-ionizing. These unique properties are driving an array of THz applications: these include security checks of people, mail, or luggage; cancer detection; and non-destructive testing.

The utility of mid-IR and THz wavelengths has spurred the development of various methods to generate laser light in these spectral regions. For widespread implementation, however, no device thus far has appeared to have an optimal mix of properties: high brightness, for sensitivity and range; wide wavelength coverage, for use with many substances; small size, for portability; room-temperature operation, for practical convenience and low power consumption; and low-enough cost. Our Firefly-IR and Firefly-THz tunable laser sources have changed this situation.

Both our sources are based on novel optical ‘engines’ that were first developed by the University of St. Andrews, UK.^1,2 Although the Firefly-IR generates mid-IR light from >2.4 to 4.7μm, and the Firefly-THz tunes from >1.2 to 3THz, both variants use a small, pulsed, diode-pumped solid-state laser (DPSSL) operating at ∼1μm to energize or ‘pump’ an optical parametric oscillator (OPO), a widely tunable frequency converter. The Firefly platform owes its small size and high power to the placement of the OPO inside the DPSSL cavity, which results in highly efficient conversion of pump pulses to OPO wavelengths as well as near diffraction-limited beam quality (see Figure 1). Automated mechanical adjustment and/or temperature tuning of the OPO gain medium easily tunes the OPO. We have achieved mid-IR average powers >800mW at a pulse repetition rate of 350kHz, and more than 10μW at 400Hz at THz wavelengths.

Figure 1. Typical Firefly-IR beam profile.

The Firefly design provides many advantages over other methods of generating mid-IR^1,3 or THz radiation.^2,4 Lead salt lasers and quantum cascade lasers can produce mid-IR or THz wavelengths, but need low temperatures. Difference frequency generation (DFG) and Raman shifting generally suffer from low power, limited wavelength coverage, or both. Electronic THz emitters offer power, but backward wave oscillators require high magnetic fields, and both synchrotrons and free electron lasers are building-sized. Optical THz sources include far infrared molecular gas lasers (expensive) and DFG (typically limited to tens of nanowatts of power). The Auston switch,⁵ in which a femtosecond laser gates a photoconductive dipole antenna, produces a broadband THz pulse, but requires relatively expensive femtosecond lasers, time delay sweeping to record the pulse, and Fourier transform data processing (which can introduce noise artifacts). Atmospheric water absorption lines can also strongly deplete parts of the pulse, hampering pulse propagation.

Figure 2. Methane gas imaging with the Firefly-IR. Left: a frame captured from conventional video shot under normal lighting (Play video).  Right: mid-IR image recorded using a Firefly-IR tuned to a methane absorption line at 3.35μm. The emerging methane plume is clearly visible (Play video). Courtesy of the University of St. Andrews.

In contrast, the design of the Firefly-THz and Firefly-IR has a number of application-friendly attributes. The Firefly-THz and Firefly-IR have relatively narrow bandwidths (<50GHz and <2cm⁻¹ respectively), which are well suited to the rotational-vibrational linewidths of most molecules. Absorption features can be mapped out by scanning directly in the frequency domain, without needing Fourier transforms. Tuning the Firefly-THz away from water absorption lines also helps increase propagation distance. Both lasers also have high average power, high peak power (by virtue of their sub-10ns pulse widths), and low intrinsic beam divergence. These give high spectral brightness, essential for high signal-to-noise and long standoff distances.

Imaging demonstrations carried out by the University of St. Andrews demonstrate the capabilities of these instruments. For example, the Firefly-IR's high repetition rates make real-time video-rate imaging possible. Figure 2 shows frames (with accompanying video clips^6,7) captured during methane gas imaging experiments. The mid-IR video images⁶ clearly show leaking methane, while the conventional video clip,⁷ recorded synchronously, shows nothing. This highlights the power of detecting the temporal evolution of gas plumes, rather than just their presence.

Figure 3. Imaging with the Firefly-THz tuned to 1.35THz. a) Conventional color image of a salami slice taken under normal lighting. b) Grayscale THz image (85×60mm scan area). c) Overlay of conventional image with a false color plot of the THz scan. Photo courtesy of the University of St. Andrews.

The University of St. Andrews’ team also showed recently how effectively THz uncovers new information and adds contrast in imaging applications. Using a simple THz imaging system, a Firefly-THz tuned to 1.35THz was used to probe a biological tissue sample (in this case, rather resourcefully, salami). The THz images in Figure 3 clearly reveal tissue areas with higher fat content.

A slew of important applications—including environmental safety and security monitoring—could benefit from next-generation mid-IR and THz laser sources. Firefly-IR and Firefly-THz offer a new blend of capabilities that should allow users to push their application boundaries. With a goal of providing mid-IR and THz sources that aid all application niches, we are working to expand our Firefly product line. CW ultra-narrow linewidth, for instance, would allow for higher resolution spectroscopy. Other developments we would like to see include further line narrowing of our pulsed lasers and amplification of Firefly-IR to millijoule pulse energies: ideal for photoacoustic spectroscopy.