Background: Using an optical method based on tunable diode laser absorption spectroscopy, we previously assessed oxygen (O₂) and water vapor (H₂O) content in a tissue phantom of the preterm infant lung. Here we applied this method on newborn piglets with induced lung complications. Methods: Five mechanically ventilated piglets were subjected to stepwise increased and decreased fraction of inspired oxygen (FiO₂), to atelectasis using a balloon catheter in the right bronchus, and to pneumothorax by injecting air in the pleural cavity. Two diode lasers (764 nm for O₂ gas absorption and 820 nm for H₂O absorption) were combined in a probe delivering light either externally, on the skin, or internally, through the esophagus. The detector probe was placed dermally. Results: Calculated O₂ concentrations increased from 20% (IQR 17−23%) when ventilated with room air to 97% (88−108%) at FiO₂ 1.0. H₂O was only detectable with the internal light source. Specific light absorption and transmission patterns were identified in response to atelectasis and pneumothorax, respectively. Conclusions: The optical method detected FiO₂ variations and discriminated the two induced lung pathologies, providing a rationale for further development into a minimally invasive device for real-time monitoring gas changes in the lungs of sick newborn infants. Impact: Optical spectroscopy can detect pulmonary complications in an animal model.Oxygen concentrations can be evaluated in the lungs.Presents a novel minimally invasive method to detect lung oxygenation and complications.Potential to be developed into a lung monitoring method in newborn infants.Potential for bed-side detection of pulmonary complications in newborn infants.