Femtocond lar micromachining: A back-to-basics primer
06/08/2012
by Lonnie Lucas and Jim Zhang, Applied Energetics
[Editor's Note: With the rapid growth of ultrafast-pul lar processing in industrial applications, ILS decided that a back-to-basics feature was warranted for tho new to this technology, and for tho in need of a refresher. Lonnie Lucas and Jim Zhang accepted this challenge. What follows is a primer on the subject written for laymen, a neat, succinct summary of the technology.]
The title above immediately leads to two questions: What is a femtocond lar, and what is lar micromachining? Let's start by discussing the first question.
Femtocond lars
Femtocond (fs) lars are also commonly known as ultrafast and/or ultrashort-pul (USP) lars. Two important parameters ud to describe femtocond lars are pul duration (width) and pul repetition rate (PRR). The pul duration (tp), shown in FIGURE 1, is also referred to as full width at half maximum (FWHM) amplitude.
FIGURE 1. The pul duration or pul width of a lar.杯弓什么影
The pul repetition rate (PRR) describes the frequency with which puls are emitted by the lar. For instance, in FIGURE 2, if the PRR was 1 kHz, then the period T would be equal to 0.001 conds (T= 1/PRR).
FIGURE 2. The pul repetition rate (PRR) of a lar.
Thus, when we are discussing a femtocond lar, it has a pul duration (tp) in the 1-999 femtocond regime. Some of the more common time scales ud with lars are shown in the table below.
A femtocond (10-15 conds) is one quadrillionth, or one millionth of one billionth of a cond. Put another way: a femtocond compares to a cond, as a cond compares to 30 million years.
FIGURE 3. Light is so fast that it can circle the earth 7 times in just one cond.
FIGURE 4. In 100 femtoconds, light only travels a hair width.柴犬简笔画
A couple of other parameters are uful when discussing ultrafast lars: peak power (Ppeak) and average power (Pavg). Peak power = pul energy / pul duration. Average power = pul energy * pul repetition rate.
For the ca of Ep =5 mJ in tp= 100 femtoconds, the peak power of such a lar would be 50 gigawatts (GW), many times more than what a large electrical power plant delivers
(about 1 GW). When focud by a lens, the lar puls will destroy any material placed in their focus, even air molecules.
Lar micromachining
The market originally viewed the femtocond lar much as "conventional wisdom" perceived the Internet back in the early 1990s -- hardly anybody anticipated that so many people around the world would depend on it, and for such a range of us. But after 20 years of femtocond R&D, this lar is now finding applications in "cold" ablation -- notably the drilling and cutting of high-precision holes (such as in the production of medical stents) free from thermal damage. The ultrafast lars esntially vaporize matter without generating heat, creating new ways to machine materials. They are particularly good at machining very small, very preci patterns in tough materials. 1
FIGURE 5 shows the Applied Energetics Lar Microfab (μFAB), a tabletop lar microfa
brication workstation designed for u in lar ablation, surface structuring of tough materials, writing of waveguides and microfluidics. Relevant industrial materials often ud in ablation applications include metals, polymers, miconductors, glass, and ceramics.
FIGURE 5. Lar microfabrication workstation
Types of ultrafast lars
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Ultrafast lar micromachining is a process by which puls from an ultrafast (pul duration less than a few picoconds) lar are ud to induce micrometer-sized structures on the surface or in the bulk of solid materials. 2-6
Puld solid-state lars
The most common lars ud today for micromachining are solid-state ultrafast lars, with gain media such as:
∙ Titanium-doped sapphire (Ti:Sapphire)
∙ Ytterbium-doped yttrium aluminum garnet (Yb:YAG)
∙ Ytterbium-doped potassium gadolinium tungstate (Yb:KGW)
∙ Ytterbium-doped potassium yttrium tungstate (Yb:KYW)
The Ti:Sapphire lar us a titanium-doped sapphire crystal as the gain medium, has a center wavelength near 800 nm, and is capable of very short (<10 fs) pul generation. The Yb-doped YAG or tungstate lars u a ytterbium-doped yttrium aluminum garnet (YAG) or a ytterbium-doped potassium yttrium tungstate (KYW) or potassium gadolinium tungstate (KGW) crystal as the gain medium and have a center wavelength of near 1.0 μm.
The Yb:YAG or Yb:tungstate lars can be pumped by a diode lar, and therefore the Yb-doped lar systems can be made more compact and at relatively lower cost. FIGURE 6 shows one example of a commercial Yb:KYW ultrafast lar made by Applied Energetics Inc.7
FIGURE 6. An example of an ultrafast Yb:KYW lar. (Courtesy of Applied Energetics)
In general, there are two types of ultrafast solid state lars ud for lar micromachining: (1) a mode-locked oscillator only, and (2) a mode-locked oscillator ed followed by chirped pul amplification (CPA) including a regenerative or linear amplifier. Typically, the mode-locked oscillator alone has a 10 MHz pul repetition rate and 10 nJ output pul energy. The CPA lar is capable of pul repetition rates ranging from a few kHz up to 100 kHz and pul energies of a few mJ. The schematic configurations for both ultrafast solid-state lars are shown in FIGURE 7.
FIGURE 7. Schematic configurations of a typical ultrafast oscillator (a) and a high-power lar system (b)
Fiber lars
Fiber lar technology has made dramatic progress in the past decade regarding output power and ultrashort-pul achievements. Kilowatt output power and greater have been demonstrated for continuous wave (CW) single mode fiber lars. Pul energies of >10 μJ are now available for commercial ultrafast fiber lars.8-9
Common fiber lar products on the market are Yb-doped fiber lars, which u ytterbium-doped silica single-mode fibers (or a large effective area fiber) as the gain medium with a center wavelength near 1030 nm. The core diameter for standard single-mode fibers is a few micrometers and the core diameter for large effective area fibers is about 30 μm. A high (>1 watt) average power ultrafast fiber lar is typically compod of a fiber oscillator (a mode-locked fiber ed), a pul stretcher, a pul picker, a couple of stages of fiber amplifiers, and a pul compressor.
The fiber oscillator has typical pul energy output of ~100 pJ and ~10 MHz pul repetition rate. For an amplified ultrafast fiber lar, its typical output has a pul duration of veral hundred femtoconds, a pul energy of ~10 白金色头发μJ, and a pul repetition rate of 10-100 kHz. A schematic of a typical ultrafast fiber lar is shown in 景甜走光FIGURE 8.
