Abstract: Systems for measuring a concentration of a target species include a first and second tunable diode laser generating laser light at a respective first and second wavelength each corresponding to respective absorption lines of the target species. A first optical fiber is optically coupled to the first tunable diode laser, and does not support a fundamental mode at the second wavelength. A second optical fiber is coupled to the second tunable diode laser and does not support a fundamental mode at the first wavelength. A fiber bundle includes respective distal ends of the first and second optical fibers, which are stripped of their respective coatings and arranged with their claddings adjacent to each other. A pitch head is configured to project respective optical beams from the fiber bundle through a measurement zone. A catch head located across the measurement zone receives the projected beams and directs them to a sensor.