Patent ID: 8189631

Claim:
An external resonator-type wavelength tunable laser device including at least a planar reflection structure enabling a reflection spectral peak wavelength to be varied, and a semiconductor gain medium, wherein the external resonator-type wavelength tunable laser device contains the semiconductor gain medium comprising a multiple quantum well in which, when reflectance difference ΔR (dB) of the planar reflection structure is defined by ΔR=10·log (R 1 /R 2 ), in which R 1 denotes reflectance R 1 (%) of the planar reflection structure at reflection spectral peak wavelength λ 1 (nm) thereof and R 2 denotes reflectance R 2 (%) of the reflection structure at a wavelength λ 2 (nm) at which a gain of the semiconductor gain medium is maximized during laser oscillation, and a wavelength at a center of an operating wavelength range of the wavelength tunable laser device is defined as λc (nm), product Γ·L of optical confinement constant Γ and semiconductor gain medium region length L (μm) of a gain layer in the semiconductor gain medium is at least 25 μm and at most 40 μm, and gain peak wavelength λ 0 (nm) observed during a time when a carrier is injected with a maximum modal gain equal to an internal loss of the semiconductor gain medium is larger than −3·ΔR/2+(λc+35) and smaller than (−(Γ·L)/7+8)·ΔR+(−(Γ·L)+λc+45), wherein the internal loss of the semiconductor gain medium is a loss of optical power absorbed at an inside of an active layer of the semiconductor gain medium, the gain peak wavelength λ 0 (nm) is a gain peak wavelength upon a carrier being injected into the active layer such that a net gain, equal to a peak gain minus the internal loss, is zero within the semiconductor gain medium.