Fabrication lithium niobate

Progress in the design and fabrication of high-quality optical microresonators is closely related to the development of novel optical materials and technologies. The key material systems used for microresonator fabrication include silica, silica on silicon, silicon, silicon on insulator, silicon nitride and oxynitride, polymers, semiconductors such as GaAs, InP, GalnAsP, GaN, etc, and crystalline materials such as lithium niobate and calcium fluoride. Table 2 smnmarises the optical characteristics of these materials (see Eldada, 2000, 2001 Hillmer, 2003 Poulsen, 2003 for more detail). 

There are two principal types of materials that can function as piezoelectrics the ceramics and polymers. The piezoelectric materials most widely used are the piezoceramics based on the lead zirconate titanate, PZT, formations, mixed sodium and potassium niobates, lithium niobate, and quartz. The advantages of these piezoceramics are that they have a high piezoelectric activity and they can be fabricated in many different shapes. 

Modulation can be accomplished in a variety of ways. For example, the output of a laser can be modulated directly or external modulation can be applied using electro-optic or electro-absorptive modulators. Currently, commercial electrooptic modulators are fabricated from lithium niobate. The performance characteristics of one family of lithium niobate modulators, produced by Lucent Technologies (Breinigsville, PA), are given in Table 1. 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

Two approaches are proposed to resolve this difficulty. The first is based on generation of a high-purity acoustic field in a spherical, unharmonic resonator, and the second is based on comparison of a set of transducers with limited but overlapping dynamic ranges. In the second approach, simultaneous measurement of a broadband signal produces an accurate characterization over the entire range. Candidates for sensor fabrication include solid dielectric capacitive sensors, piezoelectric transducers, and high-temperature lithium niobate or quartz transducers. 

For a low-frequency, Mach-Zehnder modulator, the impedance is dominated by the electrode capacitance and resistance as shown in Fig. 9.57. A Smith chart plot for a MZM is shown in Fig. 9.58. Unlike the laser diode, a modulator on lithium niobate has no junctions consequently the impedance is independent of modulator bias. As was the case with the diode laser, it is often important to match the modulator impedance to the system impedance, typically 50 or 75 real. However, unlike the diode laser, modulators fabricated in electro-optic materials that are also piezoelectric can have perturbations in their impedance due to coupling of some of the modulation signal into compressional waves. The coupling can be significant at frequencies where the modulator crystal is resonant. The dashed curve in Fig. 9.58 shows an acoustic resonance at 150 MHz. A number of techniques (Betts, Ray and Johnson, 1990) have been developed to suppress these acoustic modes to insignificant levels, as is evidenced by the solid curve in Fig. 9.56. 

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