For an industrial laser application, high process throughput and low average cost of ownership are critical to commercial success. Benefiting from high peak power, nonlinear absorption and small-achievable spot size, ultrafast lasers offer advantages of minimal heat affected zone, great taper and sidewall quality, and small via capability that exceeds the limits of their predecessors in via drilling for electronic packaging. In the past decade, ultrafast lasers have both grown in power and reduced in cost. For example, recently, disk and fiber technology have both shown stable operation in the 50W to 200W range, mostly at high repetition rate (beyond 500 kHz) that helps avoid detrimental nonlinear effects.
However, to effectively and efficiently scale the throughput with the fast-growing power capability of the ultrafast lasers while keeping the beneficial laser-material interactions is very challenging, mainly because of the bottleneck imposed by the inertia-related acceleration limit and servo gain bandwidth when only stages and galvanometers are being used. On the other side, inertia-free scanning solutions like acoustic optics and electronic optical deflectors have small scan field, and therefore not suitable for large-panel processing.
Our recent system developments combine stages, galvanometers, and AODs into a coordinated tertiary architecture for high bandwidth and meanwhile large field beam positioning. Synchronized three-level movements allow extremely fast local speed and continuous motion over the whole stage travel range. We present the via drilling results from such ultrafast system with up to 3MHz pulse to pulse random access, enabling high quality low cost ultrafast machining with emerging high average power laser sources.
We introduce a novel method for characterizing the spatio-temporal evolution of ultrashort optical field by recording
the spectral hologram of frequency resolved optical gating (FROG) trace. We show that FROG holography enables
the measurement of phase (up to an overall constant) and group delay of the pulse which cannot be measured by
conventional FROG method. To illustrate our method, we perform numerical simulation to generate holographic
collinear FROG (cFROG) trace of a chirped optical pulse and retrieve its complex profile at multiple locations as it
propagates through a hypothetical dispersive medium. Further, we experimentally demonstrate our method by
retrieving a 67 fs pulse at three axial locations in the vicinity of focus of an objective lens and compute its group
Recently, we have demonstrated a hybrid diffractive optical element that combines the dispersion function of a grating
and the focusing function of a Fresnel lens (G-Fresnel) into a single device. The G-Fresnel promises a low f-number
enabling miniaturization of a spectrometer system while maintaining high spectral resolution. A proof-of-concept G-Fresnel
based spectrometer is demonstrated, yielding sub-nanometer resolution. Due to its compactness and low-cost
fabrication technique, the G-Fresnel based spectrometer has the potential for use in mobile platforms such as lab-on-a-chip
microfluidic devices and other mobile spectrometer applications.