As data centers are becoming the nerve system of the modern information society, transmission of massive data to meet future demands requires continuous advancement in fiber-optic data link technology for data center networks. Advanced modulation formats with high spectral efficiency, such as DPQPSK, m-QAM, PAM etc. have been broadly used recently to improve the performance of high speed, large capacity fiber-optic links for telecom and datacom. These modulation formats will be reviewed for the needs of data center networks which require large throughputs with low cost, both in dollars and in watts. Alternative multi-dimensional polarization-based modulation formats, such as PolSK (Polarization Shift Keying) and SVM (Stokes Vector Modulation) will be explored for data center applications. Polarization-based modulation formats can provide 2-D, 3-D, or even 4-D symbol spaces using direct detection, which provide a possible way to improve data rate with low cost devices and components. Fundamental issues of polarization-based multi-dimensional modulation techniques will be evaluated and performance of Stokes vector receiver designs will be analyzed.