Plane gratings for high-resolution grazing-incidence monochromators: holographic grating versus mechanically ruled varied-line-spacing grating

Comparative studies have been made on the holographic plane grating and the ruled varied-line-spacing (VLS) plane grating designed for two kinds of objective Monk-Gillieson type high-resolution grazing incidence monochromator, I and II. The ray-traced performance of monochromator types I and II on a synchrotron radiation beam line was evaluated in terms of resolving power and spectral purity by the introduction of new concepts of effective Gaussian line and purity profiles. The resolving power defined on the basis of the effective Gaussian profile is consistent with the spectral purity of the beam emerging from the exit slit and is more realistic as compared with those defined in the conventional manner, especially when spectral images have asymmetric profiles. It is concluded that holographic plane gratings recorded with a spherical and an aspheric wave front are capable of providing high resolution with high spectral purity and are fully interchangeable with the corresponding ruled VLS plane gratings. This interchangeability provides more flexibility for users in choosing a proper grating for a high-resolution grazing incidence monochromator of the Monk-Gillieson type.     

Spectral linewidth narrowing and tunable two-color laser operation of two diode laser arrays

We propose and implement a common external cavity to narrow spectral linewidth of two broad-area laser diode arrays (LDAs) and align their center wavelengths. The locked center wavelength of two LDAs can be tuned in the range of ~10 nm by tuning the tilted angle of the diffraction grating. The output beams of two LDAs are spatially overlapped through the polarization beam splitter of the common external cavity, and the total output power equals the power of two LDAs. The center wavelength of each LDA can be independently tuned by shifting the corresponding fast-axis collimation lens. As a result, the high-power two-color LDA operation is demonstrated with the tunable wavelength difference of up to 2 nm (~1 THz).     

Grazing-incidence Monk-Gillieson monochromator based on surface normal rotation of a varied-line-spacing grating

A geometric theory of a grazing-incidence varied-line-spacing plane-grating monochromator system whose scanning is made by a simple grating rotation about the grating normal has been developed for designing Monk-Gillieson monochromators capable of covering an energy range of 0.6-2.5 keV. Analytic expressions are given for the grating equations, focal conditions, dispersion, spectral image shape, and optimization of groove parameters. On the basis of the theory, two monochromator systems have been designed: system I for moderate resolution and system II for relatively high resolution. The validity of the analytic formulas and the expected performance of the designed systems have been evaluated by means of ray tracing. The results show that the analytic formulas are sufficiently accurate for practical applications and that systems I and II would provide resolving power of approximately 1450-600 and 7500-2000, respectively, in the wavelength region of 0.5-2.0 nm.     

Optimum design of a grazing-incidence flat-field spectrograph with a spherical varied-line-space grating

A grazing-incidence spectrograph is designed by use of the flat-field image-focusing property of a spherical varied-line-space grating. Optimum grating parameters for mechanical ruling are selected by application of genetic algorithms. Two gratings, one for 2-5-nm and the other for 5-20-nm spectral regions, are designed, and their fabrication tolerances are analyzed.     

Sub-micrometre distance measurements with a broadly tunable short-external-cavity InGaAsP/InP diode laser

Measurement of distance using a custom-designed, broadly tunable InGaAsP/lnP short-external-cavity diode laser is described. A tuning range of over 100 nm was achieved with the custom-designed laser in a diffractive optical element short external cavity. This tuning range made it possible to achieve a sub-micrometre resolution in measurement of distance with a single laser source for an interferometer. A non-linear, least squares fitting method was used to extract the displacement from the raw data. This fitting method showed a potential for extraction of accurate displacement in the presence of noise.     

Ultraviolet groove efficiency of a holographic grating: implications for a dual-order spectrograph

The ultraviolet groove efficiency for a holographically ruled diffraction grating with a trapezoidal profile has been measured. The efficiencies for the +/-1 and the zero orders are in good agreement with those derived from scalar theory. The +/-1 orders have equal efficiency as a function of wavelength. The peak of the sum of fitted groove efficiency functions is 76%, a level that is competitive with the groove efficiency of a mechanically blazed grating. We suggest that a normal-incidence grating mount with detectors at both orders will offer a system with twice the efficiency and provide a built-in redundancy. We discuss design considerations for reducing astigmatism equally in both orders in such dual-order mountings.     

Kalman filtering of tunable diode laser spectrometer absorbance measurements

A recursive Kalman time-series filter was applied to absorbance measurements obtained with a tunable diode laser spectrometer. The spectrometer uses frequency modulation spectroscopy and a nearinfrared diode laser operating at 1.604 μm to monitor the CO(2)-vapor concentration in a 30-cm absorption cell. The Kalman filter enhanced the signal-to-noise ratio of the spectrometer by an order of magnitude when an absorbance of 6 × 10(-5) was monitored.     

Linear focusing by a plane grating with curved grooves

We study the field diffracted by a plane grating with curved (parabolic) grooves. We will demonstrate that when a monochromatic plane wave is incident on a grating with parabolic grooves the diffracted field has a focal line whose position depends on the curvature radius of the parabolas and the incidence angle of the light onto the grating. The effect described has potential applications in grating-based devices for focusing light without requiring any additional optics.     

Combined time- and wavelength-division-multiplexing demodulation technique of fiber grating sensor arrays using a tunable pulsed laser

Combined time- and wavelength-division-multiplexing demodulation technique of fiber Bragg grating (FBG) sensor arrays using a tunable pulsed laser is proposed and demonstrated. A tunable fiber laser based on a matched FBG is applied. The wavelengths of the sensing FBGs are obtained by detecting the maximum voltages with a photodiode that avoids the complex demodulation process. The advantages of this scheme are simple structure, high signal-to-noise ratio, and the sensing signals obtained by detecting the maximum voltages.     

Degenerate four-wave mixing with a tunable excimer laser

We use a simple, forward-geometry degenerate four-wave mixing setup to monitor weak absorptions of various gas-phase molecules with a tunable excimer laser. With this technique, state-selective detection of H(2), CO, H(2)O, and O(2) is demonstrated. The dependence of the forward-geometry degenerate four-wave mixing signals on various experimental parameters (e.g., pressure, laser power) is determined. Several mechanisms for the generation of the observed signals are discussed, including diffraction from density gratings or thermal gratings and two-photon degenerate four-wave mixing.     

Fabrication of a thermally actuated tunable grating and its application as a CO2 laser beam profile analyzer

We propose a thermally actuated tunable grating for measuring the beam profile of a CO(2) laser. The grooves of a transmissive grating are filled with a liquid whose refractive index depends on temperature. A visible laser as a probe and a CO(2) laser as a heat source are illuminated on the grating. The CO(2) laser is absorbed, and depending on its beam profile, a temperature profile is induced on the grating. The refractive index of the heated liquid is changed, resulting in a change of efficiency of the grating for the probe laser. By using the 1st orders of diffraction in a 4f imaging system, the beam profile of the CO(2) laser is imaged onto a CCD camera by the probe laser.     

Concave grating and convex mirror double dispersion spectrograph for optical network applications

The concave grating stationary anastigmatic spectrograph has been modified into a novel retroreflective scheme by introducing a convex mirror concentric to the intermediate image. This provides double dispersion, diffraction-limited imaging of a long entrance slit, and substantial compensation of the stationary chromatic astigmatism. One can realize 140 independent multi/demultiplexers with 160 channels each simultaneously in the described device for optical network applications. Ray-tracing spot diagrams are presented.     

Development of a pumping laser system for x-ray laser research

A two-beam chirped-pulse-amplification Nd:glass laser system dedicated to x-ray laser research is described. Each beam provides an output energy of 20 J with a typical pulse duration of 1.3 ps. A prepulse of variable duration is generated by use of a novel, to our knowledge, optical system. A reflection optical system, comprised of an off-axis parabolic mirror and a spherical mirror, produces a line focus with 6-mm length and 165-microm width without chromatic aberration. By use of this pumping laser system, the nickel-like silver x-ray laser at a wavelength of 13.9 nm has been demonstrated.