A monolithic lens-window-prism (LWP) device, made of lithium fluoride (LiF) or magnesium fluoride (MgF₂), was proposed. When either of the devices was fixed onto one end of a gas cell filled with Xe, it becomes a “wedge-crystal”-like device and was used to convert a 1 MHz femtosecond 347 nm laser to its third harmonic radiation at 10.7 eV. This led to an improved beam profile and a more compact and less lossy configuration. A stable output power of ～11 μW was demonstrated for 2 h using LiF-LWP. In addition, MgF₂-LWP was also verified for its practicability at 10.7 eV.

Efficient high harmonics generation (HHG) was demonstrated at 10 MHz repetition rate with an external femtosecond enhancement cavity, seeded by a $｛\sim｝70~\text｛fs｝$ post-compressed 10 MHz fiber chirped pulse amplifier (FCPA) laser. Operation lasting over 30 min with 0.1 mW outcoupled power at 149 nm was demonstrated. It was found that shorter pulse was beneficial for alleviating the nonlinear plasma effect and improving the efficiency of HHG. Low finesse cavity can relax the plasma nonlinearity clamped intra-cavity power and improve the cavity-locking stability. The pulse duration is expected to be below 100 fs for both 1040 nm and 149 nm outputs, making it ideal for applications such as time-resolved photoemission spectroscopy.

The group-delay dispersion of an optical fiber was measured with the time-of-flight method, using fingerprint-like characteristic spectra from a mode-locked fiber laser source. To determine the group-delay dispersion up to the fourth order, least-squares fitting was applied to the overall time waveform mapped on the time axis for the fingerprint- spectral broadband pulses through a long optical fiber. The analysis of all 4003 data points reduced statistical uncertainty, and provided second-, third-, and fourth-order dispersion with uncertainties of 0.02%, 0.4%, and 4%, respectively.