Sakae Kawato

A multi-branch, fiber-based frequency comb with millihertz-level relative linewidths using an intra-cavity electro-optic modulator.

We demonstrate that fiber-based frequency combs with multi-branch configurations can transfer both linewidth and frequency stability to another wavelength at the millihertz level. An intra-cavity electro-optic modulator is employed to obtain a broad servo bandwidth for repetition rate control. We investigate the relative linewidths between two combs using a stable continuous-wave laser as a common reference to stabilize the repetition rate frequencies in both combs. The achieved energy concentration to the carrier of the out-of-loop beat between the two combs was 99% and 30% at a bandwidth of 1 kHz and 7.6 mHz, respectively. The frequency instability of the comb was 3.7x10(-16) for a 1 s averaging time, improving to 5-8x10(-19) for 10000 s. We show that the frequency noise in the out-of-loop beat originates mainly from phase noise in branched optical fibers.

Highly Efficient Continuous-Wave Laser Oscillation in Microchip Yb:YAG Laser at Room Temperature

Highly efficient continuous-wave laser oscillation in Yb:YAG microchip crystal has been realized at room temperature by high intensity Ti:sapphire laser pumping. The maximum output power of 810 mW was obtained at 1048 nm wavelength oscillation. The optical-to-optical efficiency was 81% for the incident pump power and 85% for the absorbed pump power. These optical-to-optical efficiencies are close to the quantum-defect-limit of 90%.

Optimum design of highly efficient end-pumped Yb:YAG rod amplifier

An end-pumped Yb:YAG rod amplifier is analyzed for efficient amplification of repetitive pulses for the average power of sub-kW. The laser rod and pumping parameters are optimized.

Efficient broadband-spectrum oscillation of ultrashort-pulse Kerr-lens mode-locked Yb:YAG laser with intracavity nonlinear media overcoming the fluorescence spectrum limit

Efficient broadband spectrum oscillation was obtained directly from a laser-diode-pumped Kerr-lens mode-locked Yb:YAG laser with intracavity nonlinear media. The spectrum was expanded to around 1040 nm to 1070 nm. The spectrum is much broader than the fluorescence spectrum at 1050 nm.

Design of End-Pumped Thin Rod Yb:YAG Laser Amplifiers

An end-pumped thin rod Yb:YAG laser amplifier architecture is proposed for high-power and efficient pulse amplification of quasi-four-level lasers. A thin and long laser rod with low ion concentration is used and optimum conditions of the laser crystal properties are discussed for efficient amplification of high-repetition-rate pulses. A high single-pass gain is expected and the thermal birefringence loss is significantly smaller than the single-pass gain. From these results, high optical conversion efficiency of more than 50% is expected for pulse amplification in the master-oscillator and power-amplifier (MOPA) system.

Continuous Wave Laser Oscillation of Stoichiometric YbAG Crystal

In this paper cw laser oscillation is reported in the YbAG as the active material, and in the stoichiometric Yb-doped materials by pumping shorter wavelength than 1.0 alpham without direct pumping to the upper laser level. The YbAG has several advantages over the KYbW. The results also indicate a possibility of cw and high power laser oscillation by laser diode pumping of the stoichiometric YbAG by laser diode pumping.

Nearly quantum-efficiency limited oscillation of Yb:YAG laser at room temperature

Highly efficient 1031-nm cw laser oscillation of Yb:YAG crystal has been realized at room temperature with the slope efficiency of 140% and the optical-to-optical efficiency of 89% for the absorbed pump power

Efficient ultrashort-pulse generation of Yb:YAG laser overcoming the fluorescence spectrum limit by using nonlinear medium

One-hundred-ten-fs and 72-fs pulse-widths were obtained directly from a mode-locked Yb:YAG laser with SESAM and without SESAM, respectively. The laser-spectrum-center and the fluorescence-center were almost same. The oscillation-spectra were much broader than the fluorescence.

Twenty-Watt Average Output Power, Picosecond Thin-Rod Yb:YAG Regenerative Chirped Pulse Amplifier with 200 μJ Pulse Energy

A laser-diode-pumped, ps-pulse thin-rod Yb:YAG laser amplifier was developed. The average output power of 20 W was achieved with a output pulse width of 2 ps at a pulse repetition rate of 100 kHz.

CW Oscillation of End-Pumped Rectangular Thin-Rod Yb3+:Y3Al5O12 Laser

Efficient nanosecond and sub-picosecond pulse lasers with high average power are needed for various applications such as micromachining, laser processing, and remote sensing. In comparison with the conventional neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, the diode-pumped ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser is particularly useful for high-power and ultrashort-pulse oscillation and amplification because of low heat generation with a quantum defect of 9% and a wide gain spectrum of 9.5 nm full width at half maximum (FWHM) with a long fluorescence lifetime of 0.95 ms. 1) It also exhibits a simple two-level electronic structure without undesired loss processes such as excited-state absorption and concentration quenching. The broad absorption spectrum of 18 nm is useful for efficient and temperature stable pumping using high-power InGaAs laser diodes. However, a high pumping intensity of more than 10 kW/ cm 2 is required to reduce the reabsorption loss originating from the lower level population of the quasi-four-level system. An efficient cooling mechanism is required to prevent temperature increase in the laser crystal by high intensity pumping. Several types of Yb pumping architectures have been developed for high-power and efficient oscillation, including end-pumped thin disk, 2) edge-pumped microchip or thin disk, 3) end-pumped zigzag slab, 4) edge-pumped zigzag slab, 5) end-pumped circular rod, 6) side-pumped circular rod, 7) and fiber 8,9) structures. Recently, a diode end-pumped thin-rod Yb:YAG scheme was proposed and analyzed in detail for high-average-power laser oscillation and amplification. 10) In this paper, CW oscillation characteristics of an endpumped rectangular thin-rod Yb:YAG laser are presented. A schematic of the thin-rod Yb:YAG laser gain module is shown in Fig. 1. Two fiber-coupled laser diodes (JOLD100-CAXF-15A, JENOPTIK, Laserdiode GmbH, Germany) were used for pumping. The laser diodes deliver CW 100 W output power through a fiber with a 0.6 mm core diameter with a numerical aperture of NA ¼ 0:22. The center wavelengths at maximum output power of the two laser diodes are 935 and 939 nm. The pump beam of the laser diodes is focused on the end surface of the rod by 1:1 imaging optics using two planoconvex lenses with a focal length of 50 mm. The focus diameter of the beams is 0.85 mm. A low ytterbium concentration of 0.5 at. % is selected to reduce the temperature increase in the rod. The cross section of the rectangular rod is 1 � 1 mm 2 . The pump beam axis is tilted