External resonant laser module

An external resonant laser module comprising:

• a quantum cascade laser element;
• a diffraction grating unit including a movable diffraction grating constituting an external resonator for the quantum cascade laser element;
• a first lens holder disposed on a side opposite to a side where the movable diffraction grating is disposed with respect to the quantum cascade laser element and configured to hold a first lens for passing a first outgoing light from the quantum cascade laser element;
• a second lens holder disposed between the quantum cascade laser element and the movable diffraction grating and configured to hold a second lens for passing a second outgoing light from the quantum cascade laser element and a light returned from the movable diffraction grating to the quantum cascade laser element;
• and
• a mount member on which the quantum cascade laser element, the first lens holder, and the second lens holder are mounted,
• wherein the mount member includes a first mounting portion, a second mounting portion, and a third mounting portion arranged in order from a side where the first lens holder is disposed toward a side where the second lens holder is disposed along a facing direction in which the first lens holder and the second lens holder face each other,
• the first mounting portion includes a first mounting surface on which the first lens holder is mounted via a first adhesive layer made of photo-curable resin,
• the third mounting portion includes a third mounting surface on which the second lens holder is mounted via a second adhesive layer made of photo-curable resin,
• the second mounting portion includes:
• a second mounting surface which is located at a higher position than the first mounting surface and the third mounting surface and on which the quantum cascade laser element is mounted;
• a first side surface intersecting the facing direction so as to connect the second mounting surface and the first mounting surface;
• and
• a second side surface intersecting the facing direction so as to connect the second mounting surface and the third mounting surface, and
• at least one of the first side surface and the second side surface is integrally formed with a notch extending from the second mounting surface to the first mounting surface or the third mounting surface along a height direction orthogonal to the second mounting surface.