The incident occurred around 3.10am on Thursday October 21 in the St George’s Quay area. After being disturbed the suspect ran off towards the flood defences on the quay side and then onto Carlisle Bridge.
Following further enquiries detectives want to speak to Ahmed, 33, as part of their investigation.
He is described as 5ft 6in tall and is of Bangladeshi nationality.
Det Sgt Chris Hammond of Lancaster CID said: "This was a shocking and very worrying incident which has left the victim very shaken and upset.
"We realise this will have caused a lot of concern in the community and following further enquiries we now want to speak to Ahmed in connection with the offence.
"If you know where Ahmed is, or have any information about his whereabouts, please come forward.
"I would also encourage Ahmed to come forward and contact police immediately."
Anyone with information is asked to email [email protected] or call 101, quoting log 0157 of October 21, 2021.