I am writing an IEEE conference paper (IEEE Photonics Society Summer Topicals) and I sometimes see that shortened version of citations are used, without the title of paper. For example, something like
 J. Sun, J. Zeng, and N. M. Litchinitser, Opt. Express 21, 2013, 14975- 14981.
 M. I. Shalaev, Z. A. Kudyshev, and N. M. Litchinitser, Opt. Lett. 38, 2013, 4288-4291
What is this style of citation called? I would like to use this style of citation in my paper to save space.
The best practice is to follow the IEEE citation style.
Having said that, in a conference paper there is going to be a lot less strict check of the actual format of the references you use and I doubt a paper might get rejected because of that. Worst case, you will be asked to correct them either during the review process or when the proceedings are being compiled by the conference organisers .
For journal articles, these will be formatted by IEEE so you do not have a choice.