A modular collinear femtosecond third-harmonic generation system for undergraduate laboratory education in nonlinear optics

Group velocity mismatch (GVM) and phase matching are core concepts in nonlinear optics and ultrafast physics curricula, yet undergraduate students are frequently introduced through mathematical derivations of the walk-off length formula, without a direct experimental analogue that builds physical intuition. Meanwhile, conventional femtosecond third-harmonic generation (THG) setups employ non-collinear geometries and complex temperature control, making them impractical for student-operated laboratory sessions. To address this pedagogical gap, we present a modular, fully collinear femtosecond THG system designed specifically for upper-division undergraduate laboratory education. The system uses two β-BaB2O4 (BBO) crystals for sequential frequency doubling and sum-frequency generation under Type-I phase matching, with a precision optical delay line as the key pedagogical control element. By scanning the delay line position and monitoring the 266 nm THG signal, students transform the abstract concept of GVM into an observable, quantifiable experimental variable. The collinear design reduces optical alignment to a single-axis operation, enabling students to complete assembly, calibration and data acquisition within a single 3–4 h laboratory session. The system achieves a THG conversion efficiency of 2.47%, which is comparable to typical dual-crystal cascaded THG systems, with a central wavelength of 266.8 nm (FWHM 4.6 nm), sufficient for subsequent transient spectroscopy experiments. All components are standard off-the-shelf optics, and the modular architecture allows flexible adaptation by other institutions. The system has been validated through Training Project for College Students, demonstrating its suitability for inquiry-based learning in advanced physics laboratory courses.