AN immigration officer who forged official stamps on a woman overstayer's visa and taught her how to escape prosecution was jailed for 18 months yesterday. Magistrate Patrick Li also rejected an application by Chiu Sin-wai, 33, to be released on bail pending an appeal. North Kowloon Court heard Chiu first met Yu Zhang Biyun, 27, on July 30, 1994, at Lowu checkpoint when the mainlander came to Hong Kong to visit her husband. On August 10, Chiu stamped two new chops on Mrs Yu's two-way permit which purportedly showed an eight-week extension of stay. He asked the woman to be his 'girlfriend'. Chiu had denied a count of forgery, and pleaded not guilty to a charge of assisting Mrs Yu, who was later granted immunity, to escape from the attention of the Immigration Department.