Modular systems can be used for optical analysis, including in-situ analysis, of stimulated liquids. An excitation module can include a radiation sources, e.g., a laser, LED, lamp, etc. A detection module can include one or more detectors configured to receive spectral and/or temporal information from a stimulated liquid. Such systems can be used to identify or measure optical emissions including fluorescence or scattering. The efficient excitation of liquid samples and collection of emissions from the samples provides substantial, up to four-fold increase in the emission signal over prior systems. In an example, emission measurements can be conducted in an isolated sample compartment, such as using interchangeable modules for discrete sampling, flow-through sampling, or sampling via fiber probe. The systems and methods described herein can be used to characterize natural aquatic environments, including assessments of phytoplankton pigments, biomass, structure, physiology, organic matter, and oil pollution.