Thermal treatment method for tuning the lasing wavelength of a DFB ...

International Workshop on Photonics and Applications. Hanoi, Vietnam. April 5-8, 2004Theory BasisThe single frequency of a DFB laser is obtained by creating a phase-shift of π/2 in theDFB structure. The single wavelength of the laser is equal to the Bragg wavelength of thestructure [3].The Bragg wavelength of the grating depends on the effective index of refraction ofthe core and the periodicity of the grating. The effective index of refraction, as well as theperiodic spacing between the grating planes, will be affected by changes in temperature.Consequently, the shift ∆λ B in the Bragg center wavelength λ B is dependent on the changesin the temperature, which is given as:∆λ= λBα Λ+ αBo(Λ)FBGWhere ∆T FBG =(T H -T O ) is the heating temperature in degree oC, λ Bo is the Braggwavelength at the reference temperature T O ,⎡ 1 ∂Λ ⎤αΛ = is the thermal expansion coefficient⎢⎣Λ∂Τ ⎥⎦∆Tαn⎡ 1= ⎢⎢⎣neff∂Λ ⎤⎥ is thermo-optic coefficient∂Τ ⎥⎦The rate of refractive index change is higher than the period changes, and which is themain contributor to the wavelength shift. The wavelength shows high sensitivity at highervalues of T H . Though, under 100 o C, the change is considered linear [4]. In addition, thefiber Bragg gratings show excellent temperature stability in the temperature under 300 o C[5]. It is, therefore, permissible to think of a thermal method for tuning the lasingwavelength of a DFB fiber laser.ExperimentThe DFB fiber laser used in the experiment has the following specifications: gratinglength: 5cm, single wavelength: 1551nm, single polarization mode, linewidth: 30KHz,pumping power 9mW, maximum power peak: 40mW, SNR >50dB, SMSR > 40dB [6].The Figure 1 shows the optical spectrum of the laser.0Output Power dBm-20-40-60-801550.0 1550.5 1551.0 1551.5 1552.0WavelengthFigure 1: Optical spectrum of the laser280