Jul 29, 2010 –

San Diego, CA (July 2010)

Much work has been performed in the development and productization of mid-infrared laser sources in the race to surpass the performance of the glowbar, fixed-wavelength CO and CO₂ lasers, and tunable lead-salt lasers.  The evolving technology leader for accessing 4-12 µm radiation is the Quantum Cascade Laser (QCL).  These semiconductor-based lasers are relatively easy to fabricate via Molecular Beam Epitaxy (MBE) and Metal Organic Chemical Vapor Deposition (MOCVD) techniques.  They are robust and provide a morphological instead of chemical route to wavelength selection.  The current state of the art in fabrication and high barriers in the fundamental physics have not allowed access to the spectroscopically prolific wavelength region under about 3.6 µm.  Applications in the region from about 3-4 µm have been poorly served by laser sources which have largely consisted of expensive and cumbersome difference frequency generation systems, but there are some encouraging results.  Among the most promising routes to coherent, narrow-band coverage at these wavelengths is the Quantum Cascade’s cousin the Interband Cascade Laser (ICL).   A fundamentally different energy band structure allows the lasing transitions to be shifted to the blue sufficiently to reach these wavelengths.  Tentative access to this wavelength region has renewed interest in laser detection of many hydrocarbon molecules as the fundamental C-H stretching frequencies of most hydrocarbons lie between 2.9-3.5 µm.

QCL and ICL have broad gain profiles (tens to hundreds of wavenumbers) and the raw semiconductor chip in a Fabry Perot configuration has no method of line narrowing or wavelength control.  Without such control, these gain media are not appropriate for the high-resolution spectroscopic techniques necessary to detect light hydrocarbons with sufficient specificity for chemical identification.  Two routes exist to narrow the linewidth and control the wavelength: Distributed Feedback (DFB) and External Cavities (ECxcL™).  Each has its advantages and disadvantages, but the most versatile route to narrow linewidths and tunability is the external cavity.

ICLs have been used to perform trace gas sensing previously, but in the past cryogenic cooling has been necessary^[1].  Recent advances in performance have led to room temperature operation (using a TEC for temperature stabilization)^[2].  Room-temperature-operating tunable external cavity intraband cascade lasers (ECicL™) have been built that produce narrowband tunable light in the 3.13-3.24 µm range (3,090-3,200 cm^-1) and 3.55-3.72 µm (2,690-2,820 cm^-1)^[3].  The 3.2 µm laser was subsequently used to observe high resolution spectra of CH₄ by direct absorption techniques.

The spectral region accessed by these lasers is rich with absorption features for molecules with light atoms.  Figure 1 is a simulated spectrum^[4] of a number of common hydrocarbons overlaid by the tuning curves of one of our ECicL™.  In addition to accessing the C-H stretching fundamental, there is a plethora of other molecules that could be probed between 3 and 4 µm.  Table 1 is a further list of molecules that can be detected in this spectral region.  Furthermore many glasses^[5] absorb in this region.  There are interstellar features^[6]  that present at these wavelengths and a debate exists as to the identity of some 3.8-3.9 µm features observed in planetary spectra^[7],[8].  Even nickel carbonyl has detectable absorptions in the region⁴.  Work with Silicon micro-resonators is awaiting commercially viable sources from 3.5 to 4 µm.  Commercial access to narrow-band tunable lasers from 3-4 µm will greatly facilitate astronomical spectroscopy, fundamental physics, and sensing in the environmental, health and safety, and petrochemical industries.

Figure 1. Simulated spectra of common hydrocarbons overlaid by ECicL™ tuning curve

Table 1. Common molecules which are detectable in the 3-4 µm range

The “engine” of the narrow-band tunable laser is the ICL chip itself.  However, the ICL chip is not directly suitable for spectroscopic applications due to the broad, multiple-longitudinal mode performance it exhibits when operated in a Fabry Perot configuration.  An external cavity (see Figure 2) consisting of collimating optics and a rotatable grating provides narrow bandwidth tunable feedback.  Thermal management is necessary due to the large amount of heat that is dissipated in the active region of the chip.  A PID loop controls a TEC as it removes heat from the system.  Patented^[9] mezzo-optics collimate the highly divergent light from the ICL emitter.  This configuration is adopted from the Daylight Solutions ECqcL™ resonator design.  An elliptical, but near-Gaussian beam operating in single spatial and longitudinal modes is produced.  Output power in excess of 5 mW was obtained at the gain peak..

Figure 2. Optical configuration of the ECicL™

A room temperature spectrum of CH₄ was obtained (Figure 3) using one of the ECicL™ operating in cw mode.  The sample cell was evacuated and then backfilled with 10% CH₄in N₂ to a pressure of 19 psig (1290 mbar).  For over 40 cm-1 at the center of the gain curve, the spectrum makes an excellent match with the PNNL reference spectrum⁴, but at the edges of the gain curve, there is substantial deviation of the observed spectrum from the reference spectrum.  This is a result of the external cavity not being optimally aligned.  As the laser is tuned away from the gain center the gain at the desired wavelength is over powered by the gain center and multimode output occurred, thus obscuring the spectrum.  Optimization of the alignment and of cavity optics has all but eliminated this issue in further cavity iterations.

Figure 3. ECicL™ spectrum of CH₄.

Narrow linewidth tunable lasers in the 3-4 µm spectral region will have a large impact on chemical sensing, hydrocarbon leak detection, solid-state physics, and planetary astronomy.  Daylight Solutions is working to bring this technology to market.  Key developments in chip fabrication and structure, optics, and coatings are under way at Daylight Solutions to provide a robust source at this wavelength region as has been accomplished at shorter and longer wavelengths.

^[1] G. Wysocki, et al., Applied Optics, Vol. 46, No. 33, pgs. 8202-8210 (2007)

^[2] I. Vurgaftman, et al., New Journal of Physics, Vol. 11, pg. 125015 (2009)

^[3] D. Caffey et al., Optics Express Vol. 18, No. 15, pgs. 15691-15696 (2010)

^[4] Pacific Northwest National Labs Spectroscopic Data Base.

^[5] J.M. Florence, et al., Journal of Research of the National Bureau of Standards, Vol. 45, No. 2, pg. 121 (1950).

^[6] K. Sellgren, et al., The Astrophysical Journal Letters, Vol. 449, No. 1. pg. L69 (1995)

^[7] E. Palomba, et al., Icarus, Vol. 203, Issue 1, Pages 58-65 (2009)

^[8] A.S. Rivkin, et al., Icarus, Vol. 185, pgs. 563-567 (2006)

^[9] US Patent #7,535,656