In-doped n-PbTe/p-PbSnTe/Tl-doped p-PbTe LPE layers were successively grown for the fabrication of mid-infrared (lambda similar to 7.5 mu m) laser diodes (LDs) by using the constant temperature liquid phase epitaxy on (1 0 0) oriented p-type PbTe substrates. Substrate materials were prepared by the stoichiometry-controlled Bridgman method. Whereas, the present double hetero LDs have a simple broad contact structure, the threshold current density for lasing is as low as similar to 200 A cm(2). The emitted wavelength is tunable by changing the operation temperature. PbSnTe DH-LD was applied for the mid-infrared spectrum measurements of glycine and the spatial distribution imaging was obtained by using the specific absorption line (fingerprints), which vibration mode is originated from the skeleton oscillation of molecules. Another demonstration for the application of PbSnTe DH-LD spectrometer is the imaging of carrier distribution on Si wafers based on the change of free carrier absorption coefficients. (C) 2007 Elsevier B.V. All rights reserved.