Slow Wave Electrooptic Light Modulators

该项目的目标是展示使用新的调制器设计原理的极高效率和高度线性的微波-光转换。这一提议的理论基础是最近开发的，而计划中的实验将建立在德克萨斯A&M对用于导波光学的铁电材料的十多年研究的基础上。提出的电光调制器采用行波马赫-曾德尔干涉仪结构。每个干涉仪臂中的集成光栅反射器形成一个标准具，它降低了向前方向的平均光传播速度。“慢”波导结构提供了两个特征，使得调制器性能优于传统的“快”TW设计：(1)在具有高电光系数和介电常数的衬底上的光/微波速度匹配；(2)由于光在调制区的“驻留时间”而增强了电光相互作用强度。对于用传统的LiNbate(LN)衬底材料制造的器件，这两个因素导致了与传统的速度匹配设计相比，在电功率消耗方面潜在地提高了一个数量级。由于使用钨青铜衬底，例如具有比LN高得多的电光系数的铌酸锶(SBN)，预计驱动功率将进一步降低数量级。在LN和SBN中使用慢波结构也可以改善干涉型调制器的响应线性度。具有N个等间距反射器(N_3)的标准具被设计用于慢波调制器中，在宽光谱范围内表现出高透射率。由于这种结构的透射率在光频率上是周期性的，所以在密集波分复用(WDM)通信系统中，可以在任何信道上使用具有合适反射层间距的单个调制器，并且将在LN和SBN衬底上制作工作在1.5 Gm附近的调制器。传统的光刻、蚀刻和扩散技术将被用来制造波导和电极图案。以Ar激光为光源的全息位相掩模通过反应离子刻蚀或离子研磨在衬底表面形成波纹光栅，以确定0.35 Gm周期的图案。测量的电功率耗散、pi-电压(Va)和响应线性度将与LNto带宽为10 GHz的“快波”器件的报告结果进行比较。这些调制器有望在数字和模拟光纤通信系统中得到应用，在这些系统中，电力需求的数量级减少将对光传输设备的尺寸和成本产生重大影响。此外，由于消除了对极厚(~15-30~tm)电极的需要，集成光学芯片的成本可以显著降低。在gifl模式下运行的模拟光纤链路也将受益于增强的动态范围，而该动态范围目前受限于可从微波放大器获得的最大驱动功率和集成光学调制器的响应的线性。

The goal of this project is to demonstrate extremely efficient and highly linear microwave-to-optical conversion using new modulator design principles. The theoretical basis for this proposal was developed recently, while the planned experiments will build upon over a decade of research at Texas A&M in ferroelectric materials for guided wave optics. The proposed electrooptic modulators utilize a traveling wave (TW) Mach-Zehnder interferometer configuration. Integrated grating reflectors in each interferometer arm form an etalon which reduces the average optical propagation speed in the forward direction. The 'slow" waveguide structures provide two features which lead to improved modulator performance over conventional "fast" TW designs: (1) optical/microwave velocity matching in substrates with high electrooptic coefficients and dielectric constants, and (2) enhancement of electrooptic interaction strength due to the "dwell time" of the light in the modulation region. For devices fabricated in the conventional lithium niobate (LN) substrate material, these two factors lead to a potential improvement of an order of magnitude in electrical power dissipation over conventional velocity-matched designs. Additional orders-of-magnitude reduction in driving power is anticipated from the use of tungsten bronze substrates such as strontium barium niobate (SBN), which have much higher electrooptic coefficients than LN. Better response linearity in interferometric modulators is also possible using slow wave structures in both LN and SBN.Etalons with N equally spaced reflectors (N _ 3) which exhibit high transmittance over a wide spectral range have been designed for use in the slow-wave modulators. Since the transmittance of such a structure is periodic in optical frequency, a single modulator with appropriate reflector spacing could be used on any channel in a dense wavelength-division- multiplexed (WDM) communication system.Modulators designed to operate at a wavelength near 1.5 gm will be fabricated in LN and SBN substrates. Conventional lithography, etching, and diffusion techniques will be used to produce waveguide and electrode patterns. Corrugated gratings will be produced on the surface of the substrate by reactive ion etching or ion milling using a holographic phase mask with an argon laser as the light source to define the 0.35 gm-period patterns. Measured electrical power dissipation, pi-voltage (Va), and linearity of response will be compared with results reported for "fast-wave" devices in LNto bandwidths 10 GHz. These modulators are expected to find application in digital and analog fiber optic communication systems, where order-of-magnitude reductions in electrical power requirements would have a major impact on the size and cost of optical transmission equipment. Furthermore, since the need for very thick (~ 15-30 ~tm) electrodes is eliminated, the cost of the integrated optic chip can be reduced considerably. Analog fiber optic links operating in the GIfl regime would also benefit from enhanced dynamic range, which presently is limited by the maximum drive power available from microwave amplifiers and the linearity of response of integrated optic modulators.