Optical transmitter module

An optical transmitter module comprising:

• a housing that houses a semiconductor laser element, and an optical modulation element for modulating a laser beam by an electrical signal;
• lead pins for electrically connecting the inside and outside of the housing;
• and
• a flexible printed circuit board connected to the lead pins,
• wherein said flexible printed circuit board includes:
• a signal pattern to be connected to the optical modulation element;
• first and second ground conductor patterns provided on both sides of the signal pattern;
• a third ground conductor pattern provided on the back side of the signal pattern;
• a first insulating layer for covering the signal pattern as well as the first and second ground conductor patterns;
• a second insulating layer for covering the third ground conductor pattern;
• a first covering conductive layer for covering the first insulating layer;
• and
• a second covering conductive layer for covering the second insulting layer, and
• wherein said first covering conductive layer at least has a slit formed over the portion of the signal pattern.