Detection of CO_2 and CO at 1.58 μm (6322 cm^−1) using an InGaAsP diode laser and mode control is described. Mode control is a technique whereby a short cavity, external to the laser, is used to force the laser to operate in a single mode. By monitoring the voltage across the terminals of the laser and using electronic feedback it is possible to optimize continually the external cavity so that the laser operates reliably in the mode of choice for scans >3.5 cm^−1. This technique offers the possibility of high-sensitivity detection over a region of ~30 cm^−1 with continuous coverage in overlapping sections of ~3 cm^−1 with conventional (and hence inexpensive) multimode diode lasers.