SP Designer Populate Current User in PeoplPickr NewForm only

Question

Hi,

 

I have a scenario where i am using sharepoint designer. In my form i have a people picker field. In that field i should auto populate the current user name(current login). But this should be apply only in New form. If some other user login and clicks on that item to edit. It should display the already saved user name. 

 

Please let me know how can we acheive it. 

 

I tried to create Infopath form(Using function i populated user name) but user name is getting changed while editing the item. Do we have any other option please let me know. 

 

Thanks