Tatsuo Harada

Mechanically ruled aberration-corrected concave gratings

Aberration-corrected concave gratings with curved and variable spacing grooves are ruled with a numerically controlled ruling engine. In the design of aberration-corrected concave gratings, mechanical methods allow more freedom to choose the amount of space variation than do holographic methods. A highly efficient visible-UV monochromator and a coma-type aberration-reduced Seya-Namioka monochromator have been designed and fabricated using mechanically ruled aberration-corrected concave gratings. The gratings can be used with VUV monochromators and spectrographs with improved image focusing properties.

Development of a flat-field grazing-incidence XUV spectrometer and its application in picosecond XUV spectroscopy

A new type of grazing-incidence spectrometer with a flat focal field is developed, and XUV spectroscopy in the extreme ultraviolet region ranging from 15 to 200 A is carried out. Soft x-ray line spectra emitted from picosecond laser plasmas of aluminum and iron targets are measured and good resolutions are obtained in the XUV region. The spectral regions of detection are extended to shorter wavelengths (15 A) using a finer spaced grating. Computational studies on x-ray spectra are also performed taking into account the transient characteristics of picosecond laser-produced plasmas; the importance of the transient treatment is clearly shown. This type of soft x-ray spectrometer should be useful for time-resolved picosecond soft x-ray spectroscopy.

Corrugation-pitch modulated MQW-DFB lasers with narrow spectral linewidth

Narrow spectral linewidth lasers are developed for optical coherent transmission systems. To obtain a narrow spectral linewidth, a CPM (corrugation-pitch modulated) structure is introduced into 1200- mu m-long MQW-DFB (multi-quantum well distributed feedback) lasers. The CPM structure effectively suppresses the axial spatial hole burning effect and stabilized single-mode spectrum at high output powers. A spectral linewidth of 170 kHz was achieved at an output power of 25 mW. Details of corrugation fabrication method that makes the CPM structure possible are described. >

Novel method to fabricate corrugation for a λ/4‐shifted distributed feedback laser using a grating photomask

A novel method was developed to fabricate corrugation for a λ/4‐shifted distributed feedback (DFB) laser. Mechanically ruled grating patterns were transferred photolithographically to the surface of an InP substrate. The resultant λ/4‐shifted DFB laser operated in a single longitudinal mode at the Bragg wavelength.

Soft x-ray monochromator with a varied-space plane grating for synchrotron radiation: design and evaluation

A soft x-ray monochromator was developed for synchrotron radiation and installed at the Photon Factory in the National Laboratory for High Energy Physics. This monochromator consists of only two optical elements: a plane mirror and a varied-space plane grating with focusing properties. The 0.7-10-nm wavelength range can be covered with resolutions of 640 at 1.4 nm and 1200 at 5 nm. The output photon flux at wavelengths above 1 nm is ~10(10)-10(11) photons/s for a 100-mA beam current in the 1% spectral bandwidth.

Corrugation-pitch-modulated MQW-DFB laser with narrow spectral linewidth (170 kHz)

A spectral linewidth of 170 kHz was achieved at a 1.5- mu m wavelength in a 1200- mu m long corrugation-pitch-modulated (CPM) distributed feedback (DFB) laser having multiple quantum wells (MQW). This is the narrowest linewidth ever reported for InGaAsP DFB lasers. The CPM structure is effective in maintaining the single longitudinal mode at a high output power by suppressing the spatial hole burning effect. The linewidth-power product is 400 kHz-mW, and the extrapolated residual spectral linewidth is 150 kHz.<<ETX>>

Corrugation-pitch-modulated phase-shifted DFB laser

A corrugation-pitch-modulated phase-shifted distributed feedback (DFB) laser is proposed to reduce the spatial-hole-burning effect along the laser cavity. The structure of the proposed corrugation-pitch-modulated phase-shifted DFB laser is illustrated. A reduction in this effect is confirmed from its spectral behavior as a function of driving current. Consequently, stable single-longitudinal-mode operation up to high-output powers is obtained.<<ETX>>

Mechanically Ruled Stigmatic Concave Gratings

Concave gratings that form stigmatic images in normal incidence spectrographs are analyzed and a new method of ruling stigmatic concave gratings has been developed. The ruling engine is numerically controlled for varying the groove spacings and is capable of ruling both straight and curved grooves. Three types of concave gratings, one conventional and two stigmatic have been ruled and the imaging properties of them are examined. From the results of the examination, the possibility of producing mechanically ruled stigmatic concave gratings with the freedom of choosing stigmatic wavelengths is proved.

Ruling engine using a piezoelectric device for large and high-groove density gratings.

A ruling engine for large and/or high-groove density gratings is developed. A hybrid driving method that uses a piezoelectric device and a screw-nut mechanism is adopted to achieve stop-and-go blank motion with a closed-loop control system. Several test gratings, including a ruled area of 300(width) x 200(groove length) mm(2), groove densities of 6000 and 10,000 grooves/mm, and a uniform and varied-line space, are ruled to confirm the basic functions of the ruling engine. A set of four spherical, varied-line-space gratings with central groove densities of 6000, 4550, 3450, and 2616 grooves/mm and a ruled area of 200 x 200 mm(2) is ruled for the orbital retrievable far and extreme ultraviolet spectrometer.

A Grazing Incidence Monochromator With A Varied-Space Plane Grating For Synchrotron Radiation

A grazing incidence monochromator to scan the 5 - 100 Å wavelength range of synchrotron radiation is designed by taking advantage of the focusing property of a varied-space plane grating ruled with a numerically controlled ruling engine. The plane grating monochromator uses only two plane surfaces for reflection optics so that high throughput in the soft x-ray region is expected.