A former UBS AG (UBSN.S) banker who helped U.S. authorities prosecute the Swiss bank in a tax fraud case has asked for permission to travel to France to comply with a subpoena in another investigation of the company, according a court document. Bradley Birkenfeld, who received more than $100 million for being a whistleblower but also served 30 months in prison in the U.S. case, has been subpoenaed to take part in the French case later this month, according to a motion his lawyer filed in the U.S. District Court of Southern Florida.