Shigeo Kubota

Very efficient speckle contrast reduction realized by moving diffuser device.

Speckle noise reduction is best tested on a precise speckle contrast measurement bench, which should be able to measure 100% contrast in fully developed speckle as well as the smallest contrast (for example, less than 10%) after its reduction. On such a test bench, we have measured very efficient speckle contrast reduction by temporal averaging using a moving diffuser on a tuning fork, which vibrates at 100 Hz over 60 microm in amplitude, a distance that is three times the surface roughness correlation length of the diffuser.

7.6 W of continuous-wave radiation in a TEM 00 mode from a laser-diode end-pumped Nd:YAG laser

We have demonstrated 7.6 W of continuous-wave radiation at λ = 1.06 μm in a TEM00 mode from a diode end-pumped Nd:YAG laser pumped by 38 laser diodes that are coupled to a bundle of 19 fibers. TEM00-mode operation was enforced by using an intracavity aperture. The absorbed power in the Nd:YAG rod was 16.4 W when the incident power upon the laser rod was 21.1 W. The optical-to-optical conversion efficiency was 46%. In multimode operation, 11 W of output power and a slope efficiency of 69% were obtained. The thermal analysis was in good agreement with interferometric measurements.

Laser-diode-pumped phase-locked Nd:YAG laser arrays

Scaling of an efficient and compact diode-end-pumped Nd:YAG laser was demonstrated by two-dimensional phase-locked arrays. The arrays of stable resonators were formed by multiple thermal lenses induced by the simultaneous end-pumping of laser-diode-coupled fibers. Because of partial overlap of each transverse mode, the output was phase locked. Various configurations of pumping fibers were tested to study phase locking. It was found that minimization of the coherently coupled intensity between adjacent array elements have good prediction for phase difference between modes. When the plane parallel cavity as long as 30 mm was pumped by 2*2 laser diode coupled fibers, phase-locked 1.8-W continuous wave 1.06- mu m output was obtained with 55% slope efficiency. 1.2-W single longitudinal mode output with single-lobe beam profile was obtained by using a spatial phase filter and the twisted mode method. >

Efficient 2 nd and 4 th harmonic generation of a single-frequency, continuous-wave fiber amplifier

We demonstrate efficient cavity-enhanced second and fourth harmonic generation of an air-cooled, continuous-wave (cw), single-frequency 1064 nm fiber-amplifier system. The second harmonic generator achieves up to 88% total external conversion efficiency, generating more than 20-W power at 532 nm wavelength in a diffraction-limited beam (M(2) < 1.05). The nonlinear medium is a critically phase-matched, 20-mm long, anti-reflection (AR) coated LBO crystal operated at 25 degrees C. The fourth harmonic generator is based on an AR-coated, Czochralski-grown beta-BaB(2)O(4) (BBO) crystal optimized for low loss and high damage threshold. Up to 12.2 W of 266-nm deep-UV (DUV) output is obtained using a 6-mm long critically phase-matched BBO operated at 40 degrees C. This power level is more than two times higher than previously reported for cw 266-nm generation. The total external conversion efficiency from the fundamental at 1064 nm to the fourth harmonic at 266 nm is >50%.

All solid-state continuous-wave frequency-quadrupled Nd:YAG laser

We report all solid-state, continuous-wave, high-power, deep ultraviolet generation. An intracavity frequency doubled Nd:YAG laser pumped by fiber-coupled diodes is used to generate the high-power green output. The significance of nonlinear coupling and spatial hole burning of the two-mode oscillations in the laser is discussed. When the incident 532 mn power on an external resonant doubler was 2.9 W, we generated 1.5 W of continuous-wave 266-nm radiation using a Czochralski-grown /spl beta/-BaB/sub 2/O/sub 4/. To achieve stable external resonance, a novel voice coil motor actuator is employed with servo bandwidth as large as 50 kHz. >

Theoretical treatment, simulation, and experiments of doubly resonant sum-frequency mixing in an external resonator

We describe the theoretical treatment of the doubly resonant sum-frequency mixing in an external resonator. The nonlinear conversion efficiency is increased by the resonantly enhanced circulating powers at both wavelengths. The model predicts the circulating powers in the resonator at two input wavelengths: the powers that are reflected off the input coupler and the output power at the sum frequency. Numerical simulation gives the design optimization of the system. The result of the simulation is compared with the result of a continuous-wave 355-nm generation experiment, where 0.66-W of output power was obtained.

Frequency-stabilized, 10-W continuous-wave, laser-diode end-pumped, injection-locked Nd:YAG laser

We describe an efficient and highly stable injection-locked, laser-diode end-pumped cw Nd:YAG laser. With 22.3-W pump input from two fiber-coupled laser diodes, combined with 700 mW of power injected by a singlefrequency master laser, the injection-locked slave laser emits 10 W of linearly polarized, TEM(00)-mode output in a single longitudinal mode. In addition to high efficiency, the injection-locked laser also possesses excellent output stability. At 300 Hz, the measured relative intensity noise and the frequency-noise spectral density are 2 x 10(-5)/ radicalHz and 50 Hz/ radicalHz, respectively. To improve frequency stability further, we lock the frequency of the injection-locked laser to an external high-finesse reference cavity. When the frequency was stabilized, a minimum frequency-noise spectral density of 40 mHz/ radicalHz was measured at the frequency-stabilization loop error point at 1 kHz.

Longitudinally diode-pumped continuous-wave 3.5-W green laser.

We have obtained 3.5 W of TEM00 output at 532 nm from a longitudinally diode-pumped intracavity-doubled Nd:YAG laser with a diode-to-green conversion efficiency of 14%. Using this laser, we obtained 0.8 W of UV radiation at 266 nm through resonant doubling.

Demonstration of long-term reliability of a 266-nm, continuous-wave, frequency-quadrupled solid-state laser using ?-BaB 2 O 4

We report what we believe to be the first operation of more than 1000 h of a 266-nm (cw) frequency-quadrupled solid-state laser with a 100-mW output. We used beta-BaB(2)O(4)(BBO) crystal grown by the Czochralski method to double the green-light (532-nm) wavelength, using an external resonant cavity. The green light was generated with an intracavity frequency-doubled Nd:YVO(4)laser pumped by a 4-W laser diode. When the incident 532-nm power on the external resonant doubler was 500 mW, we generated 100 mW of cw 266-nm radiation with the BBO crystal. The degradation rate seems to be proportional to the strength of the UV optical electric field. We also obtained a relative intensity noise of -130dB/Hz at frequencies of 2 to 10 MHz for 266-nm laser light.

Superresolution of Optical Disks Using a Small Aperture

We realized magnetically induced super resolution (MSR) using magneto-optical (MO) disks with a thermally formed aperture. The aperture size and position affect the spatial frequency response of the readout signal. Using the principle of superresolution in laser scanning microscopy, we analyze the optical transfer function (OTF) for various apertures. Applying the analysis to MSR readout, OTF of MSR is calculated using scalar diffraction theory. The calculated results agree well with experimental values.