We have developed an optical model capable of designing and evaluating lenses for high power lasers. The model is used to design two types of optical lenses suited for high power laser applications. A three element objective lens with a focal length of 150mm and NA of 0.2 is capable of achieving diffraction limited performance. However, the performance is greatly compromised when conventional fused silica is used. Both the spot size and waist location are too sensitive to lens temperature variation. To overcome the issue, an athermal lens with both fused silica and CaF2 elements can perform well over a wide range of temperature variation from 20 °C to 300 °C. Similarly, a f-theta lens with a focal length of 200mm, scanning angle of ±18° is designed. Both thermal lensing caused by material as well as environment temperature are analyzed. Overall, the f-theta lens is not as sensitive as the objective due to its small NA. Nevertheless, thermal lensing can still affect the process and needs to be addressed. With a similar approach, an athermal f-theta lens with a combination of fused silica and CaF2 elements can perform well over a wide range of temperature variation from 20 °C to 300 °C.
Germania doping is commonly used in the core of optical fiber due to its advantages compared to other materials
such as superior transparency in near-infrared telecommunication wavelength region. During fiber preform
manufacturing using the outside vapor deposition (OVD) process, Ge is doped into a silica soot preform by chemical
vapor deposition. Since the Ge doping concentration profile is directly correlated with the fiber refractive index profile,
its characterization is critical for the fiber industry. Electron probe micro-analyzer (EPMA) is a conventional analysis
method for characterizing the Ge concentration profile. However, it requires extensive sample preparation and lengthy
In this paper, a multiphoton microscopy technique is utilized to measure the Ge doping profile based on the
multiphoton fluorescence intensity of the soot layers. Two samples, one with ramped and another with stepped Ge
doping profiles were prepared for measurements. Measured results show that the technique is capable of distinguishing
ramped and stepped Ge doping profiles with good accuracy. In the ramped soot sample, a sharp increment of doping
level was observed in about 2 mm range from soot edge followed by a relative slow gradient doping accretion. As for the
stepped doping sample, step sizes ranging from around 1 mm (at soot edge) to 3 mm (at soot center) were observed. All
the measured profiles are in close agreement with that of the EPMA measurements. In addition, both multiphoton
fluorescence (around 420 nm) and sharp second harmonic generations (at 532 nm) were observed, which indicates the
co-existence of crystal and amorphous GeO2.
Laser processing has demonstrated great capabilities for processing the flat panel display glass, strengthened glass, and flexible glass for consumer electronics. In this paper, a variety of laser processing techniques and their applications are discussed. The techniques include glass cutting, drilling, and surface modification. To assess each technique, a matrix of criteria, such as speed, surface quality, strength, and process stability is proposed. Based on the matrix, future needs for laser processing of glass are outlined.
Rare-earth doped fibers for high power fiber amplifiers normally have a small refractive index difference between the
core and inner cladding, but have a larger core diameter than conventional telecom fibers. We take advantage of this
feature as well as the difference between the optical and acoustic waveguides and proposed several fiber designs to
mitigate Stimulated Brillouin Scattering (SBS) to produce higher output powers. The numerical modeling showed that
an increase in SBS threshold by 3-5 dB can be achieved using Ge-codopant while maintaining diffraction limited beam
quality. The study also highlights the need to take into account the effects of minor refractive index profile variation in
evaluating SBS performance.
Suppressing nonlinear effects such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) in high
power fiber amplifiers and lasers is crucial for scaling up output power well beyond kW levels. The paper uses a
sophisticated model to analyze many different fiber amplifier designs and compare their performance. The systematic
modeling reveals many interesting results and shows that a co-pumped amplifier can be optimized by carefully choosing
fiber lengths and applying additional heating to the fiber. It also explains why the amplifier configuration can make
great impacts on SBS characteristics. In addition, a single-polarized fiber having an effective area of 206 μm2 and cutoff
wavelength of 1100 nm is designed to suppress SRS and provide better polarization properties. The systematic
modeling concludes that in general a counter-pumped fiber amplifier has the lowest nonlinear effects and is less sensitive
to the fiber length comparing with the co-pumped amplifiers. However, the co-pumped amplifier is easy to integrate
with an all-fiber-based pump combiner without risking LD damage and it can be heated to increase SBS threshold by a
factor of 1.7.
High power fiber lasers have been recently demonstrated at the kilowatt level. The
spectral linewidths of these lasers oscillators can exceed 20 nm. Whereas, such broad
spectra are fine for many applications, such as materials processing where raw power is
the primary requirement, other applications, including coherent beam combination,
harmonic generation, or gravitational wave detection, require high powers beams with
much narrower linewidths. Amplification of narrow linewidth signals in optical fibers is
limited by stimulated Brillouin scattering (SBS). We discuss novel fiber designs that limit
SBS allowing the amplification of narrow linewidth signals to kilowatt power levels.
This paper reviews different fiber design approaches for high power lasers. First, we discuss the conventional step index
profile design and methods for achieving single mode operation in high power lasers such as bending, helical core fibers
and Yb dopant profile designs. Then we present new design approaches for reducing the SBS through profile and glass
composition designs. Finally, we describe fiber designs to achieve single polarization and at the same time to mitigate
the SRS effect.