Question

Which poet first popularized the sonnet form?

Answer 1

Francesco Petrarch is the first poet who popularized the sonnet from :)

Answer 2

It was Francesco Petrarch.
The Petrachan or Italian sonnet is named for the 14-century Italian poet Francesco Petrarch who popularized the sonnet form. The last six lines, or sestet, can have various rhyme schemes.
Hope that this helps you, have a great day! =)