Quantum Cascade Laser Array Provides Flexibility
for IR Spectroscopy
Lynn M. Savage
Quantum cascade lasers — devices that can operate at infrared and terahertz wavelengths — are the only semiconductor lasers that can generate at room temperature wavelengths from 3 to 15 μm, the “molecular absorption fingerprint” region of the spectrum that is very important for chemical sensing.
Single-mode emission is necessary for most such sensing applications. To achieve it, quantum cascade lasers can be processed as distributed feedback lasers, which have a built-in grating for mode selection, or they can be incorporated within an external cavity with a diffraction grating that provides tunable, single-mode operation. External cavity versions are broadly tunable but complex to build, requiring careful alignment and high-quality antireflection coatings. Distributed feedback versions are more compact — a few millimeters in length — and easily can be fabricated in large quantities, but they have more limited tunability.
Shown here is a quantum cascade laser array built onto a 4 × 5-mm chip. The 32 individual lasers provide closely spaced wavelengths, enabling continuously tunable output for spectroscopic applications. Images courtesy of Applied Physics Letters.
Now a group of investigators has created a broadly tunable single-mode quantum cascade laser source that combines the advantages of external-cavity and distributed feedback devices.
The researchers — representing Harvard University in Cambridge and MIT’s Lincoln Laboratory in Lexington, both in Massachusetts, Agilent Technologies Inc. in Palo Alto, Calif., and ETH Zurich in Switzerland — based their device on an array of distributed feedback quantum cascade lasers.
They began by cladding layers of InP and InGaAs around a 2.4-μm-thick active region based on an InGaAs/AlInAs heterostructure that was lattice-matched to InP. They then fabricated an array of buried distributed feedback gratings in the material by removing the top InP cladding layer, by etching first-order Bragg gratings in the InGaAs layer next to the active region and by regrowing the InP cladding above the buried gratings. After further processing steps, they obtained an array of 32 distributed feedback lasers on a 4 × 5-mm chip. The grating periods were 1.365 to 1.484 μm, which resulted in lasing wavelengths in the array of 8.73 to 9.43 μm with 22-nm spacing.
The quantum cascade laser array’s creators used the device to attain absorption spectra of various analytes, including isopropanol (squares), methanol (triangles) and acetone (circles). The solid lines represent spectra acquired with a commercial Fourier transform infrared spectrometer.
The lasers in the array, which operated in pulse mode, were bonded individually to a circuit board that was connected to a controller. The researchers used a thermoelectric cooling and heating device to alter the temperature of the individual lasers, thus tuning the output frequencies so that they gained continuous spectral coverage.
They tested the applicability of the single-mode quantum cascade laser source for spectroscopy by using it to measure the absorption spectra of several liquid analytes, such as methanol and acetone. They fired the lasers sequentially into each analyte, measuring the intensity of the reflected spectra using a liquid-nitrogen-cooled HgCdTe detector. For comparison, they also acquired the spectra with a Fourier transform infrared spectrometer made by Bruker Optics Inc. of Billerica, Mass.
The investigators found that the spectra obtained with their device compared favorably with those obtained with the commercial system. However, the highest spectral resolution obtainable with the quantum cascade laser array — about 0.01 cm-1 — was significantly better than that of the commercial system (0.125 cm-1). In continuous mode, quantum cascade lasers can perform spectroscopy with a resolution of <0.001 cm-1.
Ongoing improvement of the laser array includes developing a method to combine the beams from the separate lasers, increasing the spectral coverage of the quantum cascade laser source and integrating the source into a number of spectroscopic devices.
Applied Physics Letters, Dec. 3, 2007, Vol. 91, 231101.
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