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Coherent Optical CDMA Systems
Published in Paul R. Prucnal, Optical Code Division Multiple Access, 2018
Paul Toliver, Shahab Etemad, Ron Menendez
The desire to have an OCDMA system compatible with existing DWDM optical networking compels one to consider the possibility of a system with spectral range limited to a single optical passband, where the spectral width could be on the order of 100 GHz. Taking into account various coding requirements, the challenge is to be able to impart phase modulations on distinct frequency bins of width <10 GHz, much smaller than what is practical with a bulk grating–based device. Such high resolution filtering is possible through the use of either a virtually imaged phased array (VIPA) device or a micro ring resonator device discussed in the following section. Figure 5.10 shows the detailed operation of a VIPA structure as was first proposed and developed by Shirasaki [7]. The device consists of two parallel surfaces, one with 100% reflection coating and the other with >95% reflection coating. The beam is initially focused on the partially reflecting surface where, in this case, 5% of the light leaks out at the first bounce and starts propagating as a wave front. The remaining 95% bounces twice and on the third bounce 0.95 × 5% leaks as another wave front but displaced by d and time delayed. For a small incident angle, θ, a number of wavefronts propagate from virtual images of the same beam waste, but are separated by 2t, where t is the distance between the two parallel surfaces. The positions of the beam waists in the virtual images are self-aligned and there is no need to adjust their individual positions. The interference of these wavefronts leads to a collimated beam, where for a central wavelength λ0, light propagates in the direction set by the angle θ. For other wavelengths, however, light propagates at a different angle depending on the wavelength relative to λ0, with the red-shifted and blue-shifted wavelengths propagating on the opposite side of λ0. The VIPA has the following advantages over diffraction gratings: 10 to 20 times larger angular dispersion, polarization independent operation, simple structure and alignment, and compactness.
A rapid, spatially dispersive frequency comb spectrograph aimed at gas phase chemical reaction kinetics
Published in Molecular Physics, 2020
Frances C. Roberts, H. J. Lewandowski, Billy F. Hobson, Julia H. Lehman
The imaging detection system, which consists of two dispersive optical elements, an IR-sensitive camera, and a few lenses, is similar to Ref. [35] and begins at the output of the fibre (Figure 1). After the output of the fibre, the laser beam is collimated and expanded to a beam diameter of approximately 1 cm. The beam is then focussed using a cylindrical lens (100 mm focal length) into the entrance of a virtually imaged phased array (VIPA, Light Machinery). A VIPA is essentially a tilted etalon that converts the input laser beam to a series of vertical parallel outputs, which constructively interfere at a set angle depending on the wavelength of the light [36]. In essence, the VIPA provides a repeating ‘filter’ of 12.9 GHz (0.43 cm−1), which is the free spectral range (FSR). Since each mode order of the VIPA is spatially overlapped, the spectrometer requires a second dispersive element to spatially separate each mode. To accomplish this, we use the first order diffraction from a mid-infrared blazed diffraction grating (450 lines per mm, Laser Components). The total power per comb tooth is approximately 1 nW incident on the infrared camera.