Alan Y. Arai

Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for rapid writing of low-loss optical waveguides in transparent glasses. A novel femtosecond fiber laser system (IMRA America, FCPA muJewel) providing variable repetition rate between 0.1 and 5 MHz was used to study the relationship between heat accumulation and resulting waveguide properties in fused silica and various borosilicate glasses. Increasing repetition rate was seen to increase the waveguide diameter and decrease the waveguide loss, with waveguides written with 1-MHz repetition rate yielding ~0.2-dB/cm propagation loss in Schott AF45 glass. A finite-difference thermal diffusion model accurately tracks the waveguide diameter as cumulative heating expands the modification zone above 200-kHz repetition rate.

“Quill” writing with ultrashort light pulses in transparent materials

A remarkable phenomenon in ultrafast laser processing of transparent materials, in particular, silica glass, manifested as a change in material modification by reversing the writing direction is observed. The effect resembles writing with a quill pen and is interpreted in terms of anisotropic trapping of electron plasma by a tilted front of the ultrashort laser pulse along the writing direction.

Waveguide writing in fused silica with a femtosecond fiber laser at 522 nm and 1 MHz repetition rate

We report on waveguide writing in fused silica with a novel commercial femtosecond fiber laser system (IMRA America, FCPA microJewel). The influence of a range of laser parameters were investigated in these initial experiments, including repetition rate, focal area, pulse energy, scan speed, and wavelength. Notably, it was not possible to produce low-loss waveguides when writing with the fundamental wavelength of 1045 nm. However, it was possible to fabricate telecom-compatible waveguides at the second harmonic wavelength of 522 nm. High quality waveguides with propagation losses below 1 dB/cm at 1550 nm were produced with 115 nJ/pulse at 1 MHz and 522 nm.

Self-assembled periodic sub-wavelength structures by femtosecond laser direct writing

Self-assembled, sub-wavelength periodic structures are induced in fused silica by a tightly focused, linearly polarized, femtosecond laser beam. Two different types of periodic structures, the main one with period (Lambda(E)) in the direction of the laser beam polarization and the second with period (Lambda(k)) in the direction of the light propagation, are identified from the cross-sectional images of the modified regions using scanning electron microscopy. We demonstrate the spatial coherence of these nanogratings in the plane perpendicular to the beam propagation direction. The range of effective pulse energy which could produce nanogratings narrows when the pulse repetition rate of writing laser increases. The period Lambda(E) is proportional to the wavelength of the writing laser and period Lambda(k) in the head of the modified region remains approximately the wavelength of light in fused silica.

A spectroscopic comparison of femtosecond-laser-modified fused silica using kilohertz and megahertz laser systems

Waveguides were written in fused silica using both a femtosecond fiber laser with a 1MHz pulse repetition rate and a femtosecond amplified Ti:sapphire laser with a 1kHz repetition rate. Confocal Raman and fluorescence microscopies were used to study structural changes in the waveguides written with both systems. A broad fluorescence band, centered at 650nm, associated with nonbridging oxygen hole center (NBOHC) defects was observed after waveguide fabrication with the megahertz laser. With the kilohertz laser system these defects were only observed for pulse energies above 1μJ. Far fewer NBOHC defects were formed with the megahertz laser than with kilohertz writing, possibly due to thermal annealing driven by heat accumulation effects at 1MHz. When the kilohertz laser was used with pulse energies below 1μJ, the predominant fluorescence was centered at 550nm, a band assigned to the presence of silicon clusters (Eδ′). We also observed an increase in the intensity of the 605cm−1 Raman peak relative to the to...

Changes to the network structure of Er-Yb doped phosphate glass induced by femtosecond laser pulses

Changes to the glass network structure after modification with tightly focused 1043nm, 400fs laser pulses have been studied in Er–Yb doped phosphate glass using in situ confocal Raman microscopy. For femtosecond laser writing conditions that result in heat accumulation, the 710 and 1209cm−1 Raman peaks, which are due to the (POP)sym and (PO2)sym network vibration modes, respectively, shift to both higher and lower wavenumbers. The differences in refractive index are shown to correlate spatially with the 1209cm−1 Raman signal shifts. Systematic shifts in this Raman peak to higher and lower wavenumbers indicate an overall expansion and/or contraction of the phosphate network that depends on the femtosecond laser writing conditions.

Three-dimensional micromachining inside transparent materials using femtosecond laser pulses: New applications

At high repetition rate, nonlinear absorption of tightly-focused femtosecond pulses produces a point-like heat source that can be located inside a transparent material, allowing glass welding with weld lines that are located on internal surfaces.

Ultrashort pulse micromachining with the 10-μJ FCPA fiber laser

IMRAs ultrashort pulse fiber laser products continue to evolve to expand the application scope. The latest prototype FCPA produces pulses with less than 500-fs pulse duration at a 50-kHz repetition rate. At the fundamental wavelength of 1045 nm, the pulse energy is greater than 10 mJ. The increase in pulse energy over the standard FCPA μJewel permits greater flexibility in the focusing conditions applicable for micromachining, enabling a wider variety of laser-machined structures and profiles. This paper describes the latest micromachining application areas being studied with this new laser.

Laser ablation threshold and etch rate comparison between the ultrafast Yb fiber-based FCPA laser and a Ti:sapphire laser for various materials

Ti:Sapphire lasers remain the most widely used utlrafast laser. However, precise optical alignment and environmental control are necessary for continuous, long-term stable operatoin of the laser. IMRAs FCPA laser is an air-cooled, Yb fiber-based ultrafast laser designed to operate in an industrial environment and provide a stable, high-quality laser beam. In this work, the micromachining performance of the FCPA laser is directly compared with a conventional Ti:Sapphire regenerative amplifier laser. An experimental study was conducted to determine the ablation threshold and etch rate for a variety of materials (including metals, semiconductors, and dielectrics). The materials chosen for the experiments cover a wide range of optical, mechanical and physical properties. Similar focusing conditions were used for both lasers in order to ensure that any differences in the results are primarily due to the different characteristics of each laser. For materials with a relatively low ablation threshold, the full energy of the Ti:Sapphire laser is not needed. Furthermore, it is near the ablation threshold where ultrafast laser processing provides the benefit of minimal thermal damage to the surrounding material. Although the relatively low pulse energy of the FCPA limits its ability to ablate some harder materials, its high repetition rate increases the material processing speed and its good beam quality and stability facilitates tight, efficient focusing for precise machining of small features.

Fiber chirped pulse amplification system for micromachining

Chirped Pulse Amplification (CPA) is widely used for generating high-energy femtosecond pulses. This is most commonly done with a solid-state Ti:Sapphire crystal through a free-space optical path. The high energy density in the crystal and the precise optical path required with the use of bulk optics make it difficult to design a simple system with good stability and beam quality over the environmental conditions typically encountered in the manufacturing environment. A CPA system using fiber architecture reduces the need for precise beam guiding since the light follows the fiber. The pump energy is more evenly distributed along the length of the amplifier fiber, reducing the thermal dissipation that is required (no water chiller is required) and improving the overall efficiency. The fiber architecture also produces a superior quality beam that does not require great care to maintain. IMRAs latest FCPA μJewel uses the inherent advantages of the FCPA architecture, along with extensive engineering, to produce a compact and stable femtosecond fiber laser system. Its high repetition rate and stable performance enables applications that were difficult to achieve previously. This paper will review the general design architecture of the FCPA μJewel and discuss several applications.