Nonlinear absorption is mainly governed by mechanisms involving excitation processes of electrons. Typically, two phenomena are considered when discussing nonlinear absorption; the multiphoton absorption where multiple photons interact directly with a single electron, and tunnel ionization, where the high electric field results in a shifting of the bandgap allowing an electron to tunnel into the conduction band. Electrons in the conduction band can be accelerated through the absorption of further photons until they obtain enough energy to excite further electrons to the conduction band, leading to runaway absorption and finally damage of the sample. By laser calorimetric measurement of the nonlinear absorption, it is expected that the laser damage threshold can be predicted without damaging the optic. Before accurate predictions can be made, the process must be thoroughly characterized and understood. The nonlinear behavior of the absorption was demonstrated with potential increases in absorption of an order of magnitude. Initial results show a noticeable impact of contaminants, though a nonlinear response is still observed.
Quantizing nanolaminates are an interesting alternative to classical coating materials with greater independence of refractive index and the optical bandgap energy. This leads to more flexibility and considerable potential to increase the laser-induced damage threshold in the ultra-short pulse regime. The following study presents and compares the design choices, characterization, and LIDT testing of different quantizing nanolaminates for the ultraviolet spectral range to classical coating materials.
This paper presents the active alignment of miniaturized, substrate-free optical thin-film filters (TFFs) according to the filters’ spectral transfer properties for integration into fiber optical networks. Optical TFFs are often designed for a specific narrow angle of incidence (AOI) range. Hence, a sufficient manufacturing precision of the angled photonic components connected to the optical filter is needed. These components then can no longer be used for different scenarios where i.e. the incident angle is changed. Conversely, the individual miniaturized optical filter chips can also vary in specification due to slight inhomogeneities during the production on a largescale wafer. Therefore, not all filter chips on the wafer meet the demanded specifications at the designed AOI, resulting in a reduced yield. Moreover, it requires a time-consuming separation into different quality classes by measuring single filter chips on the wafer. To maximize the amount of usable chips, a procedure was developed to actively align the chips inside a precision optics assembly system by measuring the transmitted power at different wavelengths while tilting them towards the optical axis. When the optimal angle is found, the chip is glued into the optical network platform. Next to maximizing the yield, the production steps can be reduced because the prior separation into quality classes becomes redundant. Manufacturing tolerances during the thinfilm deposition are equalized due to the active spectral alignment on a universal optical platform. With this technique, a more versatile process for TFF integration compared to passively aligned assemblies on fixed angle components is demonstrated.
All-silica mirrors manufactured using GLancing Angle Deposition (GLAD) are a promising approach for optics with improved LIDT. However, water content may increase over time due to the porosity of the low index silica layers, potentially changing the LIDT. Additionally, consecutive irradiation during LIDT testing may remove stored water and influence the LIDT.
Laser calorimetry, spectrophotometry and LIDT measurements, applying S-on-1 and R-on-1 methods, were used in order to determine the impact of laser-induced removal of stored water on the absorption, spectral behavior and laser damage resistance of all-silica mirrors. Influence of water reabsorption was investigated under different environmental atmospheric conditions.
We demonstrate a novel concept for an all-optical switch based on the optical Kerr-effect in thin film interference coatings. The switching between transmittance and reflectance relies on highly Kerr-active coating materials in combination with large internal intensity enhancement in thin film interference coatings. The paper investigates the switching performance as well as its relation to the laser induced damage threshold of these novel components. A modulation depth of 30 % was achieved without damage to the component, which very promising for later applications as power limiters or mode locking components.
We present a novel concept for optical switches, which is based on the optical Kerr-effect. In contrast to previous approaches, the switching is achieved by a combination of strongly Kerr-active materials with specially designed and produced optical interference filters. Intense laser irradiation causes refractive index changes in the sensitive Kerr-active layers of the components and the interference filter changes its spectral characteristics, i.e. becomes reflective, depending on design factors. The concept offers several advantages when compared to currently applied switching methods, such as the easier integration into photonic systems because of their compact nature and wide spectral application range.
Nowadays, continuous wave (cw) lasers have conquered a broad spectrum of applications in industrial laser processing and can be considered as the dominant tools in many manufacturing floors. This is reflected by the enormous average annual growth rates of 25-30 % and the continuous research efforts dedicated to this laser type leading to ever increasing output power and beam quality. This development imposes ever increasing demands on the quality of the optical laser components, that have to withstand the usually harsh industrial environment and high power levels. In fact, the corresponding of the laser components is a key factor for the efficiency and economic success of an employed laser material process. This in turn requires a thorough assessment of the quality parameters ruling the stability of such components. Among many other quality parameters, the Laser Induced Damage Threshold (LIDT) is one of the leading parameters that has to be investigated in detail. The corresponding measurement facilities and protocols as well as the evaluation of the data have to be performed with high reproducibility and comparability among different testing laboratories. As a consequence, such qualifications can only be achieved on the basis of well-defined international standards defining the complete procedure for the determination of LIDT values. We investigated the laser induced damage threshold of different types of optics using a cw laser with a wavelength of 1030 nm and power up to 6 kW, applying beam diameters of approximately 200-300 µm on the surface. The samples were irradiated for at least 30s or until damage occurred. First, it was necessary to review the existing DIN EN ISO 21254 regarding cw-irradiation of mirrors with a 25 mm diameter. An important aspect is the number of possible irradiation spots on each optic with respect to the damage size as well as the emitted debris. Both effects limit the statistical accuracy, the ISO procedure needs to be adapted to the measurement conditions. Additionally, we investigated the influence of substrate materials and coating processes on the LIDT of high reflective coatings and their damage behavior, especially regarding their thermal conductivity. The results were then compared with simulations concerning the maximum temperature within the optical component.
Atomic layer deposition (ALD) has been proven as an excellent method for coating high quality optical films due to its outstanding film quality and precise process control. Unfortunately, batch ALD requires time-consuming purge steps, which lead to low deposition rates and highly time-intensive processes for complex multilayer coatings. Recently, rotary ALD came in focus for optical applications. In this novel process concept, each process step takes place in a separate part of the reactor divided by pressure and nitrogen curtains. The substrates to-be-coated are rotated through these zones. During each rotation, an ALD cycle is completed, thus the deposition rate is mainly dependent on the rotation speed. In this study, the performance of a novel rotary ALD coating tool for optical applications is investigated and characterized with SiO2 and Ta2O5 layers. Low absorption levels of 3.1 ppm for 200 nm thick single layer of Ta2O5 and 6.0 ppm for 1032 nm thick single layer of SiO2 are demonstrated at 1064 nm, respectively, with growth rates up to 0.18 nm/s on fused silica substrates. Furthermore, excellent uniformity is also demonstrated with non-uniformity values reaching as low as 1.55 % and 2.71% for Ta2O5 and SiO2, respectively, over 120 mm on silicon wafers. Seven substrates up to a diameter of 200 mm can be coated in each run. Further investigations on uniformity improvements and multilayer coatings are currently ongoing.
We report on our recent results for output scaling by a Thulium-doped fiber Mamyshev oscillator in the spectral 2 µm region. In order to further scale the output, a highly Thulium-doped double-clad fiber was implemented in the Mamyshev oscillator. A stable pulse train at a repetition rate of 19.7 MHz with an average power of 220 mW was observed. This corresponds to a pulse energy of 11 nJ at a pulse length of 2.4 ps. Further scaling of the output parameters was limited by soliton fission, which will be mitigated by introducing normal dispersive fiber sections in future experiments.
An advantage of using additive manufacturing (AM) processes as opposed to conventional fabrication methods is that the additional degrees of freedom in design allow compact and at the same time lightweight components to be manufactured. In addition, AM reduces the material consumption, resulting in a more cost efficient production. Among others, the field of laser development benefits from the progressive implementation of AM-related opportunities. However, this integration is mostly limited to single components. In contrast, we present a compact, lightweight solid-state laser oscillator system for low-power applications based on additively manufactured optomechanical components via Fused Filament Fabrication (FFF). The laser system is based on a Nd:YVO4-crystal pumped externally with a fiber-coupled laser diode at a wavelength of 808nm and a maximum output power of 3 W. The commercial optical components, such as lenses and the crystal, are firmly embedded via FFF in quasi-monolithic optomechanics. Thereby, they are fixed at their position and thus secured against misalignment. Furthermore, sensor technology for temperature monitoring is implemented into the structure. The possibility of the FFF process to work with different materials in parallel is used here. This multi-material printing approach enables the use of the appropriate polymer for the individual mechanical and thermal requirements for any structural part. The thermal stability of the printed structures is evaluated to ensure damage-free operation of the 3D-printed polymer optomechanics. Furthermore, output power, optical-to-optical efficiency, beam pointing, and spatial beam profile of the laser system are measured for several on- and off-switching cycles as well as for long-term operation.
The principle of a Mamyshev oscillator depends on alternating spectral filtering between sections of spectral broadening by self-phase modulation. In the 2 µm wavelength range, this concept faces the difficulty that standard fibers are anomalous dispersive which limits the possible pulse energy to the pJ-regime without proper dispersion management. We applied ultra-high numerical aperture fibers with normal dispersion in order to achieve up-chirped pulses in an anomalous dispersive Thulium-doped gain fiber. With that design, we achieved mode-locked pulses with energies of 6.4 nJ and a compressed autocorrelation duration of 195 fs at a repetition rate of 16 MHz.
Recently, Mamyshev oscillators (MO) have attained a lot of attention, due to their generation of mode-locked pulses with outstanding output parameters in terms of output energy, spectral bandwidth and pulse duration. We present a MO with output pulse energies in the range of 0.5µJ, an optical spectrum ranging from 1010nm to 1060nm and an externally compressed autocorrelation duration of less than 100fs. This MO completely consists of commercially available standard step-index fibers. In order to handle the high pulse energies, we apply a few-mode gain fiber with a core-diameter of 20µm in the second arm of the oscillator.
There is an increasing demand for highly integrated optical and optoelectronical devices that provide active laser emission, adaptability and low optical losses. A well-established production technology for customized structures with high functionality and geometrical flexibility is additive manufacturing (AM). It enables new constructional degrees of freedom to overcome the limitations of substractive material processing such as milling and drilling. Commercial AM systems for metals and polymers are ubiquitous; whereas glass AM systems almost exclusively exist in scientific environments. Laser glass deposition welding allows the AM of waveguides by fusing coreless
fused silica fibers with a diameter of 400 µm and a 50 µm thick polymer coating onto a fused silica substrate. The deposition process is performed with defocused CO2-laser radiation (10.6 µm). Based on laser deposition welding, the fiber is fed laterally into the processing zone and is melted or fused by the incoming laser beam.
In order to achieve a sufficient coupling of laser radiation into and out of the fibers, a proper cleaving process for the end faces has been established. The cleaving is performed with a CO2-laser based process for optimized and reproducible results. In this contribution, the focus is on the manufacturing of bended waveguides and the feasible bending radii, which can be accomplished during the deposition process. The influence of the bending radius on the guiding efficiency is investigated. Therefore, the light transmission and beam profile of the deposited fibers is measured and compared with an untreated one. Furthermore, the appearance of the cleaved end faces and the internal stress in the glass substrate are characterized. Functional, nearly stress-free curved and straight waveguides for light transmission with high position stability are achieved, which opens a wide range of applications for optical system integration.
Solid-state white light sources gain increasing interest due to their advanced characteristics compared to conventional lighting solutions. New design challenges are introduced in the remote phosphor set-up by the substitution of the efficiency-droop-limited LEDs with laser diodes (LDs) that exhibit peak efficiencies at much higher operating currents. Although laser-excited remote phosphor (LRP) systems have already been employed in some commercial applications, the bottleneck in their performance is identified in the down-conversion process within the phosphor material. The high intensity exciting laser beam in combination with the temperature-dependent properties of phosphors can lead to thermally induced instabilities in the system. For this reason, an opto-thermal simulation framework is developed to investigate the optical and thermal interdependencies and derive the LRPS optimization parameters. The optical analysis is performed with commercial ray-tracing software, where the optical heat losses are computed and subsequently used as the volume heat source in thermal analysis implemented by the finite element method (F.E.M.). The question now arises as to how to properly model the phosphor material in such a simulation scheme. The LED experience has produced a variety of phosphors for lighting applications, most commonly powders in some appropriate resin matrix, which are treated simulation wise as bulk diffusers. As the low thermal conductivity of resins is deemed critical for their use in LRPS, recent research focuses on resin free materials such as glass phosphors, single crystals, polycrystalline dense ceramics, etc. The different modeling approaches of such solutions are investigated here as the scattering properties and surface topology of the samples can vary.
The use of additive manufacturing methods in research and industry has led to the possibility of designing more compact, light and low-cost assemblies. In the field of laser development, new opportunities resulting from additive manufacturing have rarely been considered so far. We present a compact, lightweight solid-state amplifier system for low-power applications where the optomechanical components are manufactured completely additive via Fused Filament Fabrication (FFF). The amplifier system is based on a Nd:YVO4-crystal pumped with an external, fiber-coupled diode at a wavelength of 808nm and a maximum output power of 3 W. The seed source is a Nd:YVO4-crystal based solid-state laser with an emission wavelength of 1064 nm. The commercial optical components, such as lenses and crystal, are firmly imprinted via FFF in the optomechanics and thus secured against misalignment. Additionally, sensor technology for temperature measurement is implemented into the devices. The use of FFF, in which the components are printed from polymers, results in a lightweight yet stable construction. We have shown, that optical components can be imprinted without adding mechanical stress. To increase the mechanical and thermal robustness of the system different types of polymers as well as post process treatments are tested and the use of Laser Metal Deposition for this application is investigated. The thermal stability of the printed structures is evaluated to determine the maximum power level of the system without damaging the polymer-optomechanics. Furthermore, output power, optical-to-optical efficiency, beam pointing, and beam shape are measured for several on- and off-switching processes as well as long-term operation.
We present an ultrafast fiber laser system at a central wavelength of 1750 nm for imaging applications, in particular 3-photon microscopy. It generates an output pulse train with an adjustable repetition rate ranging from 1 MHz to 21 MHz. After temporal compression the pulse duration is 220 fs and the maximum achieved pulse energy is 20 nJ.
The laser system consists of a polarization maintaining (PM) Erbium-doped fiber oscillator which emits a stable output pulse train at a fixed repetition rate of 42 MHz. The oscillator generates soliton pulses centered at a wavelength of 1560 nm and a spectral width of 7 nm. Mode-locking is initiated and stabilized by a semiconductor saturable absorber mirror. The output pulses are picked in a PM fiber coupled acousto-optic modulator to an adjustable repetition rate of 1 – 21 MHz. A consecutive Erbium-doped PM fiber amplifier (EDFA) boosts the energy of the soliton pulses from pJ to nJ level. The directly emitted pulses have a duration of 2 ps which can be compressed to a pulse duration of 115 fs by using a passive standard fiber. The uncompressed pulses are soliton-self-frequency shifted by Raman scattering to wavelengths longer than 1700 nm in 7 m of passive PM1550 fiber at a pulse energy of 1.1 nJ. The central wavelength can be adjusted by the pump power of the EDFA. To boost the pulse energy of the wavelength shifted pulses, the Raman stage is followed by a single-clad Thulium-doped fiber (TDF) amplifier. It consists of a 1560/1750 nm wavelength division multiplexer (WDM) and 0.9 m of TDF. To diminish nonlinear effects during amplification, the pulses are stretched with 25 m of normal dispersion fiber (NDF) inserted between the WDM and the TDF. Although on the very short wavelength amplification band, the pulses are amplified up to more than 40 nJ of pulse energy at an injected pump power of 4.1 W. After the fiber amplifier, the pulses are coupled out and propagate through a spectral filter, a triplet of l/4, l/2, and l/4 waveplates, an isolator, and a grating compressor. As the WDM, NDF, and TDF are not PM, the polarization state has to be readjusted to linear with the waveplates before entering the isolator. The added group delay dispersion of 2.17 ps2 by the NDF is compensated in a free space standard grating compressor built of two 600 lines/mm gratings. The transmission of the grating compressor is 60 %. To achieve optimum compression to a pulse duration of 220 fs at a pulse energy of 20 nJ, the compressor in combination with spectral filtering around 1750 nm has to be carefully adjusted. The maximum output pulse energy of 20 nJ is constant ranging from 1 MHz to 7 MHz, but is reduced at higher repetition rates down to 8.7 nJ. The output pulse duration is nearly constant at 220 fs for all repetition rates. Further amplification of the pulses is currently under investigation. This system will be used in future for the application of 3-photon microscopy.
Thulium-based fiber lasers potentially provide for the demand of high average-power ultrafast laser systems operating at an emission wavelength around 2 μm. In this work we use a Tm-doped photonic-crystal fiber (PCF) with a mode field diameter of 36 μm enabling high peak powers without the onset of detrimental nonlinear effects. For the first time a Tmdoped PCF amplifier allows for a pump-power limited average output power of 241 W with a slope efficiency above 50%, good beam quality and linear polarization. A record compressed average power of 152 W and a pulse peak power of more than 4 MW at sub-700 fs pulse duration are enabled by dielectric gratings with diffraction efficiencies higher than 98% leading to a total compression efficiency of more than 70%. A further increase of pulse peak power towards the GW-level is planned by employing Tm-doped large-pitch fibers with mode field diameters well above 50 μm. The coherent combination of ultrafast pulses might eventually lead to kW-level average power and multi-GW peak power.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.