Section 53. Any police officer on duty at night, on vacation, furlough or on a day off, who attends as a witness in a criminal case pending in a district court, including the municipal court of the city of Boston, or in a juvenile court, including Boston juvenile court, shall be allowed a witness fee in the amount of three dollars for each day's attendance, including his first attendance as arresting officer in the case. Any police officer on duty at night, vacation, furlough or on a day off, who attends as a witness in a criminal case pending in a district court, including the municipal court of the city of Boston, or in a juvenile court, including Boston juvenile court, and who by reason of a continuance or postponement thereof, is required to again attend, shall be paid the amount of three dollars for each day of such further attendance. Any police officer on duty at night, vacation, furlough or on a day off, who attends the superior court as a witness for the commonwealth shall be paid a witness fee in the amount of three dollars for each day's attendance. A police officer who is a witness for the commonwealth, and who under the direction of the district attorney aids in securing the attendance of other witnesses, may receive, instead of his expenses, witness fees in the amount of three dollars for one day's attendance. Police officers shall serve subpoenas upon witnesses when requested by the district attorney, and their returns of service shall have the same force and effect as the return of a deputy sheriff or constable.