Wavelength Stabilizing unit and module for laser output

The invention provides for a unit 100 for stablizing a wavelength of light, and which includes a first light-receiver 18 to directly receive part of the laser beam irradiated from a semiconductor laser. A wavelength filter 12 directly receives part of the laser beam, and has a transmittance varying in accordance with a wavelength of the received laser beam. A second light-receiver 19 receives a laser beam having passed through the wavelength-filter 12. The first light-receiver 18 has a first edge, and the second light-receiver 19, has a second edge located in the vicinity of the first edge, and the first edge has a first linear portion and the second edge has a second linear portion extending in parallel with the first linear portion. A module may include the unit 100 and be integrated with a field-absorption type semiconductor optical modulator. The module may include two temperature controllers. One temperature controller acting on the unit 100 and the other controller independently acting on the semiconductor laser.