If anyone from modern day is compared or associated with the characters of the prophet is indeed a "dirty" man which includes myself.
The most common and simplest definition of secularism is exclusion of religion from state's affairs. Only a fool would imagine that the glorious prophet was a secular when he himself established a governance system based on Islamic principles which means inclusion of religion in state's matter.
Secularism in the western sense meant separation of Church and state because the churches laws themselves were discrimatory against non-Christians. The Islamic state does not have this problem so is secular by nature.
As for "inclusion of religion in state's matters" well there isn't a country on earth which doesn't have "secular laws" that can't be traced back to religious text. Islamic laws are usually secular in that they don't distinguish between Muslim and non-Muslim and justice is given to both in equal terms.