Phase-locked and mode-locked multicore photonic crystal fiber laser with a saturable absorber.

Multicore photonic crystal fiber (MC-PCF) can scale the output power with the number of cores by spatial beam combining if the in-phase mode is selected. We demonstrated simultaneous realization of phase-locked and mode-locked laser using Yb-doped 7-core MC-PCF by a semiconductor saturable absorber placed in the near-field inside a resonator. High energy 333 nJ pulses were obtained directly from a mode-locked fiber laser oscillator at a 42.4 MHz repetition rate with an average power of 14.1 W at 24 W excitation. We observed the direct output pulse width of 52 ps assuming a sech2 profile. However, it might be noise-like pulses because of no variation when we performed pulse compression. Single-pulse operation was achieved by increasing the bandwidth of the intracavity filter. In this case, 137 nJ, 42.4 MHz pulses were generated with a 5.8 W average power and the compressed output pulse width was 8.6 ps.

Kerr-lens mode-locked Yb:LuAG ceramic thin-disk laser.

A Kerr-lens mode-locked (KLM) thin-disk laser with Yb:LuAG ceramic was demonstrated. Yb:LuAG ceramic is an attractive material for high-power lasers due to its high thermal conductivity and large emission cross section. The highest output power of 17 W with a pulse duration of 130 fs was achieved. Moreover, the pulse duration of 88 fs was also obtained with a high-Q factor cavity. To the best of our knowledge, this is the first demonstration of a KLM thin-disk laser based on Yb:LuAG, including both ceramic and single crystal. The results show the usefulness of ceramic thin disks for high-power ultrashort pulse laser sources.

Sarcopenia as a Predictor of Survival Among Patients With Organ Metastatic Cervical Cancer.

BACKGROUND: This study was conducted to investigate the prognostic significance of sarcopenia in patients with organ metastatic cervical cancer. METHODS: Accordingly, the data of 40 patients with organ metastatic cervical cancer treated at our institute from December 2004 to December 2017 were retrospectively analyzed. The correlation between clinicopathological characteristics and survival was then evaluated using univariate and multivariate analyses. Psoas muscle index (PMI), calculated from the psoas muscle area at the L3 vertebral-body level using computed tomography images obtained for pretreatment evaluation, was adopted as an index of sarcopenia. RESULTS: The median follow-up period was 14 months (range, 1-91 months). Kaplan-Meier analysis showed a 3- and 5-year overall survival (OS) rate of 46.1% and 35.8% for all patients, respectively. Receiver operating characteristic curve maximizing the area under the curve showed that the optimal PMI for predicting 1-year survival was 3.72 cm2 /m2 . Patients with a PMI > 3.72 cm2 /m2 had significantly better OS than those with a PMI ≤ 3.72 cm2 /m2 (P = .046). Multivariate analysis revealed that only PMI was significantly associated with OS in patients with organ metastatic cervical cancer. Furthermore, patients with a PMI > 3.72 cm2 /m2 who underwent concurrent chemoradiotherapy (CCRT) had a longer OS than those receiving other therapies (P < .001). CONCLUSIONS: High PMI was determined to be a favorable prognostic factor for patients with organ metastatic cervical cancer. Moreover, patients with organ metastatic cervical cancer who have a PMI > 3.72 cm2 /m2 may benefit from CCRT as an initial treatment.

Prognostic value of neutrophil-to-lymphocyte ratio in early-stage ovarian clear-cell carcinoma.

OBJECTIVES: There is increasing evidence that systemic inflammatory response (SIR) markers are prognostic factors for various types of cancers. This is the first study to evaluate the usefulness of SIR markers for the prognosis of early-stage ovarian clear-cell carcinoma (OCCC). METHODS: We retrospectively investigated 83 patients diagnosed with stage I-II OCCC who underwent surgery between 2005 and 2017. Initially, receiver operating characteristic curve analysis for overall survival (OS) was used to determine optimal cut-off values for neutrophil-to-lymphocyte ratio (NLR) and platelet-to-lymphocyte ratio (PLR). Patients were stratified into 2 groups by the cut-off values (NLR=3.26, PLR=160). Univariate and multivariate analyses were performed to elucidate the significance of SIR markers as prognostic factors. RESULTS: In the median follow-up period of 64.1 months, 16 patients experienced recurrence, and nine patients died. The Kaplan-Meier curve showed that OS of the NLR-low group was significantly longer than the NLR-high group (p=0.021). There was no significant difference in progression-free survival between the 2 groups (p=0.668), but the post-recurrence survival of the NLR-low group was significantly longer than the NLR-high group (p=0.019). Furthermore, multivariate analysis showed that increase in NLR is a significant independent prognostic factor for poor prognosis (hazard ratio=7.437, p=0.017). There was no significant difference between PLR-low and PLR-high group. CONCLUSION: Results suggest that NLR can be a significant independent prognostic factor for early-stage OCCC.

Deep reinforcement learning for coherent beam combining applications.

Coherent beam combining is a method to scale the peak and average power levels of laser systems beyond the limit of a single emitter system. This is achieved by stabilizing the relative optical phase of multiple lasers and combining them. We investigated the use of reinforcement learning (RL) and neural networks (NN) in this domain. Starting from a randomly initialized neural network, the system converged to a phase stabilization policy, which was comparable to a software implemented proportional-integral-derivative (PID) controller. Furthermore, we demonstrate the capability of neural networks to predict relative phase noise, which is one potential advantage of this method.

Sub-100 fs pulse generation from a Kerr-lens mode-locked Yb:Lu2O3 ceramic thin-disk laser.

In this Letter, we demonstrate, to the best of our knowledge, the first Kerr-lens mode-locked (KLM) ceramic thin-disk laser. Ceramic laser gain media have many advantages for a high-power laser such as high fracture toughness, size scalability, low cost, and short time fabrication. By using the Yb:Lu2O3 ceramic thin disk, the pulse duration of 98 fs with the output power of 3.7 W was demonstrated. The shortest pulse duration was 86 fs with the decreased output power of 0.36 W. From our results, it was proven that a KLM thin-disk laser with ceramic gain media can become a new option for high-power, sub-100 fs pulse laser sources.

Delay line coherent pulse stacking.

Ultrafast fiber amplifiers are usually limited in terms of pulse energy and peak power due to fiber damage and nonlinearities. On the other hand, fiber amplifiers are very energy efficient. To take advantage of this efficiency, but still scale the pulse energy, coherent combining and pulse stacking have become the method of choice. Here we show how to directly and symmetrically stack up the pulses from an oscillator chain, using delay lines and phase preshaping of a 48 MHz, 1.5 µm master oscillator. This method avoids the need for high-power low-loss dumpers as in passive stack and dump cavities and, compared to divided pulse amplification (DPA), it eliminates the need for splitting pulses and pulse picking and, thus, optimally utilizes the available seed power. Furthermore, it avoids the asymmetrical gain of the pulse bursts, which typically limits the efficiency in DPA.

Yb3+-doped CaF2-LaF3 ceramics laser.

Highly transparent ceramic is an attractive gain medium for high-power lasers due to its high fracture toughness, homogeneity, and size scalability. Here we report the first Yb3+-doped CaF2-LaF3 ceramics laser. Codoping of La3+ ion can reduce the formation of Yb2+ ions and enhance the laser efficiency. In the laser experiment, the maximum output power of 4.36 W and the maximum slope efficiency of 69.5% were obtained with a 3% La, 2% Yb sample and a 2% La, 1% Yb sample, respectively. Due to the combined properties of Yb:CaF2 and a ceramic laser gain medium, Yb:CaF2-LaF3 ceramic is a promising gain medium for a high-power ultrashort pulse laser and amplifier.

Kerr-lens mode-locked Yb3+-doped Lu3Al5O12 ceramic laser.

Yb:LuAG is a promising gain medium for high-power lasers due to its high thermal conductivity and large emission cross section. Although narrowband gain media such as Yb:LuAG are not considered to be suitable for the generation of sub-100 fs pulses, we have achieved generation of 91 fs pulses with Yb:LuAG ceramic by the Kerr-lens mode-locking (KLM) technique. The obtained average power, pulse energy, and peak power were 1.64 W, 20.0 nJ, and 220 kW, respectively. These are, to the best of our knowledge, the first demonstration of KLM and the new records based on both Yb:LuAG ceramic and single crystal.

Numerical analysis of fast saturable absorber mode-locked Yb(3+) lasers under large modulation depth.

Numerical analysis of fast saturable absorber mode-locked Yb(3+)-doped solid state lasers is reported. The analysis includes a special case in which the spectral bandwidth of the short pulse is larger than the fluorescence bandwidth of the gain material. The relationship between the available shortest pulse duration and modulation depth for a standard bulk and thin disk laser geometries with several gain materials are shown. The characteristic phenomena observed in our previous Kerr-lens mode-locked laser experiments were reproduced in the simulation.

Large-mode-area hybrid photonic crystal fiber amplifier at 1178 nm.

Amplification of 1178 nm light is demonstrated in a large-mode-area single-mode ytterbium-doped hybrid photonic crystal fiber, relying on distributed spectral filtering of spontaneous emission at shorter wavelengths. An output power of 53 W is achieved with 29 dB suppression of parasitic lasing. Further power scaling is limited by parasitic lasing.

Self-focusing in multicore fibers.

Self-focusing is the ultimate power limit of single mode fiber amplifiers. As fiber technology is approaching this limit, ways to mitigate self-focusing are becoming more and more important. Here we show a theoretical analysis of this limitation in coupled multicore fibers. Significant scaling of the self-focusing limit is possible even for coupled multicore fibers if the out-of-phase mode is chosen. On the other hand the in-phase mode can - depending on the coupling strength - be prone to instabilities.

87 W, narrow-linewidth, linearly-polarized 1178 nm photonic bandgap fiber amplifier.

High-power narrow-linewidth photonic bandgap fiber amplifier was demonstrated. In order to suppress stimulated Brillouin scattering, the seed linewidth was broadened by applying a random phase noise with an electro-optical modulator. A factor of 15 in terms of Brillouin gain suppression can be theoretically expected. An 87 W linearly-polarized (11 dB PER) and narrow-linewidth (780 MHz FWHM) output was obtained.

Demonstration of a Yb3+-doped Lu3Al5O12 ceramic thin-disk laser.

Yb:LuAG ceramic is very promising for thin-disk laser and amplifier architectures since it exhibits a higher thermal conductivity at high doping concentrations and a larger emission cross section than Yb:YAG. In this Letter, we present what we believe to be the first demonstration of a thin-disk laser based on Yb:LuAG ceramic. A maximum output power of 101 W with an optical efficiency of 56% and a slope efficiency of 64% was obtained with a multimode laser resonator. Fundamental-mode laser operation with near diffraction limited beam quality (M2≈1.22) was also achieved. The fundamental-mode laser resonator showed the output power of 49 W, an optical efficiency of 31%, and a slope efficiency of 44%. A linearly polarized output beam was demonstrated in multimode operation using an intracavity Brewster window. The depolarization loss was measured to be as low as 0.15% per round trip.

Study of the thermo-optical constants of Yb doped Y2O3, Lu2O3 and Sc2O3 ceramic materials.

Thermally induced depolarization and thermal lens of three Konoshima Chemical Co. laser-ceramics samples Yb(3+):Lu(2)O(3)(C(Yb) ≈ 1.8 at.%), Yb(3+):Y(2)O(3)(C(Yb) ≈ 1.8 at.%), and Yb(3+):Sc(2)O(3) (C(Yb) ≈ 2.5 at.%) were measured in experiment at different pump power. The results allowed us to estimate the thermal conductivity of the investigated ceramic samples and compare their thermo-optical properties. The thermo-optical constants P and Q and its sign measured for these materials at the first time.

Single-frequency ytterbium doped photonic bandgap fiber amplifier at 1178 nm.

1178 nm single-frequency amplification by Yb doped photonic bandgap fiber has been demonstrated. 24.6 W output power and 12 dB gain were obtained without parasitic lasing and also stimulated Brillouin scattering. 1.8 dB suppression of Brillouin gain by an acoustic antiguiding effect has been found in the Yb doped photonic bandgap fiber.

CW and mode-locked operation of Yb(3+)-doped Lu3Al5O12 ceramic laser.

CW laser operation and first mode-locked laser operation of Yb:LuAG ceramic are reported. Efficient CW laser operation was obtained with maximum output power of 2.14 W and a 72% slope efficiency. Femtosecond mode-locked laser operation was achieved with pulse duration of 699 fs and a 200 mW average output power.

High power Yb-doped photonic bandgap fiber oscillator at 1178 nm.

An ytterbium-doped solid-core photonic bandgap fiber oscillator in an all-fiber format is investigated for high power at an extreme long wavelength. The photonic bandgap fiber is spliced with two fiber Bragg gratings to compose the cavity. The sharp-cut bandpass distributed filtering effect of the photonic bandgap fibers efficiently suppresses amplified spontaneous emission in the conventional high-gain region. Fine adjustment of the short cut-off wavelength by coiling with tighter diameter is performed to suppress parasitic lasing. A record output power of 53.6 W with a slope efficiency of 53% at 1178 nm was demonstrated.

Continuous wave and mode-locked Yb3+:Y2O3 ceramic thin disk laser.

CW and mode-locked laser operation based on an Yb3+:Y2O3 ceramic thin disk is reported. In CW laser operation, an output power of 70 W with an optical-to-optical efficiency of 57.4% was achieved. A higher slope efficiency of 70% was also obtained with a 50-W pump laser diode with more suitable emission characteristics. In mode-locked laser operation, pulses as short as 547 fs with an average power of 7.4 W were obtained. To our knowledge, this is the first demonstration of a high power CW and mode-locked laser operation based on Yb3+:Y2O3 ceramic thin disks.

Diode-pumped ultrashort-pulse generation based on Yb(3+):Sc(2)O(3) and Yb(3+):Y(2)O(3) ceramic multi-gain-media oscillator.

Diode-pumped mode-locked laser operation based on Yb(3+):Sc(2)O(3) and Yb(3+):Y(2)O(3) multi-gain-media oscillator has been demonstrated. 66-fs pulse duration with an average power of 1.5 W and 53-fs pulse duration with an average power of 1 W under 8-W laser diode pumping were achieved. The optical-to-optical efficiency was 18.8%. Additionally, 68-fs pulse duration with an average power of 540 mW from Yb(3+):Y(2)O(3) ceramic was also obtained.