Until now, spectroscopic devices using gratings have not been heavily used in the communications industry (save for optical-spectrum analyzers) because of insertion loss and polarization dependence. This new spectroscope has a novel optical system that eliminates problems related to insertion loss. In addition, by configuring the echelle grating using Littrow geometry, designers can create a compact device with high-resolution capability (see figure).
The structure is very simple. Light from an optical fiber is incident on an echelle grating; the resulting 14th-order diffracted light is linearly focused. Optical fibers and sensors at the image plane pick up specific wavelengths based on position.
High insertion loss in previous grating-based spectroscopic methods was caused by low diffraction efficiency of the grating itself, aberrations inherent in the optical system of the spectrometer, and distortion due to anamorphic properties. Polarization dependence emerged when diffraction gratings designed for first- and second-order fringes were used, with the grating pitch being on the order of the wavelength.
The Littrow optical system solves these problems by eliminating aberration and incorporating telecentric imaging. Anamorphic properties of the grating are also reduced. As a result, the insertion loss is reduced to below 2.5 dB when single-mode optical fibers are used on the image-formation side. In addition, the grating pitch of the echelle grating can be enlarged to 12 μm, reducing polarization dependence to less than 0.15 dB without using polarization-reduction components.
By using higher-order diffracted light, the focal length of the optical system becomes smaller, bringing the size of the device down to 30 x 77 x 16 mm. In addition, by changing the grating, the device can be used for any spectral region from the 0- to the U-band, or 1260 to 1675 nm. Nikon anticipates using this device to extract a narrow range of wavelengths for wavelength-adjustable filters, channel monitors for wavelength-division multiplexing, and optical components for spectrometers.
Courtesy O plus E magazine, Tokyo