In a recent decade, miniaturized optical sensor devices based on photonic integrated circuits (PICs) have been investigated vigorously due to their exceptional advantages such as high speed, high sensitivity, high-density integration, low cost, compactness, and robustness against the environmental fluctuation by eliminating the assembly and alignment of the free-space optical components. Especially, near- and mid-infrared photonics have intrigued a great interest in various sensing applications including the physical object/environment imaging and mapping, material identification, and chemical/biomolecule sensing due to the strong fundamental vibrational-rotational signatures of the distinct chemical bonds and high transmittance in an ambient atmospheric environment or in-vivo mammalian bio-cells. The basic components constituting the PICs are the micro/nano waveguides to guide the light by the total internal reflection, and the most matured PIC material platforms are typically based on silicon platforms. However, depending on the targeting application's operation scheme and wavelengths, appropriate material platforms and structures should be designed and optimized to minimize material loss and maximize performance. This Ph.D. dissertation focuses on the design, fabrication, and experimental characterization of the various passive photonic components in different material platforms and photonic integrated sensor devices for the mid-infrared (MIR) absorption trace gas sensors, near-infrared (NIR) on-chip spectrometers, and biosensing applications. The first chapter covers the MIR trace gas sensor devices based on the suspended InGaAs membrane waveguides. We designed and compared two different waveguide structures on the III-V material platform considering the monolithic integration of the sensing waveguides with the light sources and detectors in MIR wavelengths. The second chapter demonstrates the NIR spatial heterodyne Fourier transform spectrometer (SHFTS) integrated with a subwavelength grating coupler (SWGC) for the dual-polarization bandpass sampling on the Si3N4 platform to solve the intrinsic trade-off limitation between the resolution and bandwidth. After that, the micro-ring resonator (MRR) biosensor devices integrated with SHFTS for the lab-on-a-chip optical biosensor platform on the silicon-on-insulator wafer were presented which can substitute the external optical spectrum analyzers for detecting the resonance wavelength shift. Finally, a thorough study on the point-of-care SARS-CoV-2 lab-on-a-chip (LoC) optical biosensor platforms is presented, and the recommendations for future research will be discussed