We report on the experimental demonstration of a tunable monolithic optical frequency comb generator. The device is based on four-wave mixing in a crystalline calcium fluoride whispering gallery mode resonator. The frequency spacing of the comb is given by an integer number of the free spectral range of the resonator. We select the desired number by tuning the frequency of the pumping laser with respect to the corresponding resonator mode. We also observe a rich variety of optical combs and high-frequency hyperparametric oscillation, depending on the experimental conditions. A potential application of the comb for generating tunable narrow band frequency microwave signals is demonstrated.
The holographic recording characteristics of phenanthrenequinone-(PQ-) doped poly(methyl methacrylate) are investigated. The exposure sensitivity is characterized for single-hologram recording, and the M͞# is measured for samples as thick as 3 mm. Optically induced birefringence is observed in this material. © 1998 Optical Society of America OCIS codes: 090.2900, 090.4220, 210.2860.The characterization of phenanthrenequinone-(PQ-) doped poly(methyl methacrylate) 1,2 (PMMA) as a recording material for holographic memories is described in this Letter. This material consists of the polymer host matrix with added PQ molecules as photosensitive dopant. High-optical-quality samples of this material were made with variable thicknesses of up to 5 mm and in a variety of shapes. This material does not shrink after exposure and is lightweight, inexpensive, and durable, making it an attractive candidate for disk-based holographic memory systems.Sample preparation consists of dissolving PQ molecules in liquid methyl methacrylate together with a polymerization initiator. This solution is then poured into molds and allowed to polymerize in a pressure chamber at an elevated temperature. The molding process allows samples to be fabricated in a variety of geometries. Disks ranging from 2.5 to 10 cm in diameter with 1 -5-mm thickness were made. For a 1-mm-thick sample doped with a concentration of 0.7% of PQ molecules before exposure, the absorption reaches a maximum of 98.8% at 445 nm and is 58% for the 488-nm line of an argon laser, which was used in all experiments described in this Letter.A hologram was recorded by a pair of 488-nm beams, each incident upon the material at an outside angle of 21.5 ± . We monitored the growth of the hologram during recording by probing the sample with a Bragg-matched He-Ne laser beam. Figure 1 shows the diffraction eff iciency (diffracted power divided by the incident power) during recording in 1-mm-thick material. The diffraction eff iciency reached a maximum of 4.3% for an exposure energy of 2.5 J͞cm 2 . If exposure was allowed to continue, the diffraction eff iciency began to drop. After 20 J͞cm 2 of exposure with a single beam the hologram decayed to approximately 0.1%. At this point the material was completely exposed, and no more holograms could be recorded.We recorded permanent holograms that do not decay with subsequent illumination by stopping the exposure before saturation was reached and then baking the sample. Figure 2 shows the strength of a hologram as a function of baking time at a temperature of 55 ± C. The diffraction efficiency reached a maximum after 12 days and remained steady with continued baking. Figure 3 shows the selectivity curves for a weak and a strong hologram (2% and 35% diffraction eff iciency, respectively). The 2% hologram has a sinc-squared selectivity curve as expected for a 1-mm-thick hologram. The stronger hologram, on the other hand, has a selectivity curve that is distorted and shifted. For a holographic memory the diffraction eff iciency is relatively sma...
A comparative analysis of phenanthrenequinone-doped poly(methyl methacrylate) materials fabricated at California Institute of Technology and National Chiao Tung University is performed in order to understand the dierences exhibited in their recording and baking dynamics. Ó 2001 Published by Elsevier Science B.V.Keywords: Holography; Photopolymer material; PQ-PMMA; Grating dynamics; Temperature eect Phenanthrenequinone-(PQ-) doped poly(methyl methacrylate) (PMMA) [1,2] has been used as a recording material in optical memories and other holographic systems [3±6]. This material consists of a polymeric basis doped with chromophores, the PQ molecules. This material is lightweight and durable, and does not suer from shrinkage. High optical quality samples of dierent shapes and thicknesses can be obtained. These properties make it an excellent candidate for holographic memory modules. In this paper, we compare the PQ±PMMA samples that we use at California Institute of Technology (Caltech) with those fabricated at National Chiao Tung University (NCTU) and try to understand the dierences in behavior they exhibit.Sample preparation consists of dissolving PQ molecules ( 6 0.7%) in liquid methyl methacrylate (MMA) together with azo-bis-isobutyrolnitrile, a polymerization thermal initiator. This solution is poured into molds and allowed to polymerize in a pressure chamber. The preparation process followed at Caltech diers from the one followed at NCTU in the temperature at which the pressure chamber is set during polymerization. For the Caltech material, the temperature of the chamber is set to 80°C. On the other hand, at NCTU the polymerization process is split into two steps [4,7]. First, the solution is let to rest at room temperature for approximately 120 h until the solution turns homogeneously viscid. At this point, the
A quality factor exceeding 5x10(9) is obtained in whispering-gallery mode (WGM) resonators fabricated of crystalline quartz. We observe significant electrical tunability of WGMs in x-cut resonators and demonstrate an electro-optic modulator with a submegahertz passband at 12 GHz. We discuss other photonics applications of the crystal quartz WGM resonators in narrowband agile tunable filters, compact narrow linewidth lasers, and microwave and millimeter wave oscillators.
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