Prabhu Ramamoorthy was convicted in August after a five-day trial.

An Indian techie from Tamil Nadu was sentenced to nine years of imprisonment on Thursday on charges of a "brazen sexual assault" against a fellow female passenger inside a commercial airplane early this year.

Prabhu Ramamoorthy, 35, who came to the US on a H-1B visa in 2015, will be deported after he serves his sentence, a federal court in Detroit said as it sentenced the Indian national to nine years of imprisonment.

Judge Terrence Berge hoped it would be grave enough to deter others from committing similar crimes. Federal prosecutors had sought 11 years of imprisonment for Ramamoorthy.

"Everyone has the right to be secure and safe when they travel on airplanes. We will not tolerate the behaviour of anyone, who takes advantage of victims who are in a vulnerable position. We appreciate the victim in this case for her courage to speak out," US Attorney Matthew Schneider said after the sentencing was announced.

Ramamoorthy was convicted in August after a five-day trial that was conducted before United States District Judge Terrence Berg. The jury deliberated approximately three-and-a-half hours before returning the guilty verdict.

According to the evidence presented at the trial, on January 3, Ramamoorthy had engaged in a sexual act with a female, while she was sleeping on a flight from Las Vegas to Detroit. Ramamoorthy was seated along with his wife, when he sexually assaulted his co-passenger.

During the trial, the government admitted evidence demonstrating that Ramamoorthy had digitally penetrated the woman sleeping next to him during the overnight flight. The sexual assault woke her up, she found her pants unbuttoned and unzipped, and sought help from the flight attendants.

The sentence was proof that the safety and security of all aircraft passengers was a priority for the FBI, said Timothy R Slater, Special Agent in Charge, Detroit Division of the FBI.

In recent years, a number of Indian nationals have been arrested or charged with sexual assaults inside a plane. The FBI says airplane sexual assault is a serious offence that is on the rise. FBI statistics indicate that investigations of mid-air sexual assaults increased by 66 per cent from 2014 to 2017.

That was in part due to increasingly "cramped, confined spaces; alcohol and drugs; fewer flight attendants; and dark cabins on night flights" -- factors that "likely embolden offenders" -- the FBI said in a recent report.

The cramped style of airplane seating can exacerbate trauma for victims. According to mental health professionals, victims who are violated in confined spaces feel even more helpless, vulnerable and powerless. Victims may also feel intimidated by the person sitting next to them since the seating arrangement means that the perpetrator is effectively blocking the victim from getting up, according to the FBI.

According to court papers, Ramamoorthy is from Tamil Nadu, where he grew up. His parents are farmers. He came to the US in July, 2015 after his graduation to work as an IT professional.

During his trial, Ramamoorthy presented himself to the court as a confused man stranded in America without English language skills despite his sophisticated technological job, federal prosecutors alleged. "His lack of empathy and remorse should not go unnoticed by this court," they argued, seeking nearly 11 years of imprisonment for him.