SINGAPORE - A full-time police national serviceman abused his authority by sneaking up on couples behaving intimately at HDB staircases, recording their acts on video and then using his status to extort money from them.

The third time he did this, Muhammad Shazwan Sapuwan, 21, went a step further - he raped a 16-year-old girl he had "caught" in intimate acts with her 16-year-old boyfriend.

On Monday, Shazwan, now 23, was jailed 18 years and ordered to be caned 15 strokes after he pleaded guilty to rape, oral sex, outrage of modesty and extortion. Five other charges for extortion, cheating and sexual assault were taken into consideration.

The court heard that on Oct 24, 2014, Shazwan followed the couple and used his phone to take pictures of them getting intimate.

He then approached them, identifying himself as a police officer. At the time, he was wearing a Jurong Police Division polo T-shirt, his blue police trousers and a lanyard that displayed his police warrant card.

Shazwan asked for their particulars and threatened to report them for public nudity. He told the couple that he would "submit" their names as he did not want to lose his job.

After an hour of talks, he suggested that they repeat their intimate acts and he be allowed to record them. He told them that if he were to lose his job, he would use the video recording to threaten them for money.

The girl, who did not want her parents to find out about the acts, convinced her boyfriend to comply.

While the couple were performing sexual acts, Shazwan touched the girl's breast and forced her to perform oral sex on him.

He then told the boyfriend to have sex with the girl but the teenager refused. Shazwan then raped the girl.

After leaving the scene, Shazwan communicated with the boyfriend on WhatsApp and demanded $40 by the next morning. The boyfriend eventually transferred the money to him.

Three days later, the girl started crying in front of her friends in school and told them what had happened. Her friends managed to find out Shazwan's identity on Facebook and encouraged her to make a police report.