The performance of fused fiber couplers is wavelength dependent. Wavelength spectral compensation is a technique to decrease the effect of the wavelength dependence, which is an essential task for many applications in fiber optic communication systems. Fiber devices such as wavelength-flattened couplers (WFCs) can be fabricated using wavelength spectral compensation methods. In this dissertation, wavelength spectral compensation techniques in fused biconical taper (FBT) couplers including both multimode and single-mode fiber couplers are studied in detail. In multimode fiber coupler operation, a novel theoretical model based on frustrated total internal reflection (FTIR) has been developed to effectively describe the power coupling and loss mechanism. Experimental results support the theoretical predictions. In single-mode fiber couplers, the conventional technique of fabricating WFCs is discussed. An alternative analytical model has been developed based upon coupled mode theory, which provides a relatively simple and mathematically sound explanation to the wavelength spectral compensation. Aiming to simplify WFC fabrication, a new way of constructing WFCs is proposed and demonstrated by connecting regular single-mode fiber couplers, some of which serve as wavelength spectral compensators. WFCs of various structures including 2x2, 1x3, 1x2ᴺ, and 4x4 have been developed, and the experimental data agree with theoretical predictions of performance. Potential applications and future research directions in wavelength spectral compensation are also presented. / Ph. D.