What should I prepare for technical interview conducted by L1 department for algorithm developer ?

In the case that the hiring manager has not given explicit information about the interview and its content, then I believe it is best to prepare for the usual situations.
Check the position description and requirements, as this will give you already a good overview on what are the key needed requirements. If there is something that is not familiar to you, research and learn what the topic means at a high level. You can also familiarize yourself with the L1/algorithm environment, but since you are a researcher in the area you probably know the state-of-the-art algorithm topics.
There certainly might be some technical tests, but if the hiring manager has not shared specifics, you do not need to prepare with anything. If you want to be safe, you can always ask the hiring manager or recruiter for details. Good luck with the interview!