External resonator-type wavelength tunable laser device

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 (R1/R2), in which R1 denotes reflectance R1(%) of the planar reflection structure at reflection spectral peak wavelength λ1 (nm) thereof and R2 denotes reflectance R2(%) of the reflection structure at a wavelength λ2 (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 λ0 (nm) observed during carrier 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).