Errors in an analog to digital converter that cause generated digital codes to deviate from expected values are corrected. A spark plug generates a spark to combust the gaseous mixture in the combustion chamber. The pump chamber is delimited by a displacement member which is movable to and fro between a first position and a second position. A bottom collar has a recessed cross-member for holding at least a part of the sabiki rig, the bottom collar being attached to the second outer end of the outer tube. Since the pump light for Raman amplification propagates at a low loss through the optical fiber for Raman amplification, a great Raman amplification gain can be attained by the Raman amplifier.