Relevant material can be found in Chapter 2.1.2 of [1], Chapter 4.2.1 of [2], and [3].

[1] M. Seimetz, High-Order Modulation for Optical Fiber Transmission. Berlin, Heidelberg: Springer-Verlag, 2009.

[2] W. S. C. Chang, Ed., RF Photonic Technology in Optical Fiber Links. Cambridge: Cambridge University Press, 2002.

[3] D. Rörich, X. Wang, M. Bernhard, and J. Speidel, “Optimal Modulation Index of the Mach-Zehnder Modulator in a Coherent Optical OFDM System Employing Digital Predistortion,” in ITG Symposium on Photonic Networks, 2013, vol. 9, pp. 95–100. [PDF]