Evaluation of Chirp Reduction in Polymer-Based Tunable External-Cavity Lasers

We evaluate the chirp reduction achievable using the detuned loading effect in the polymer-based tunable externalcavity laser (ECL) capable of operating at 10 Gb/s. In particular, we estimate the achievable linewidth enhancement factor (LEF) of this ECL considering the stable operation region increased by the nonlinear gain. The results show that, due to the nonlinear gain, we can detune the lasing mode further away from the nominal detuning limit (i.e., half of the mode spacing) by ~4 GHz for an ECL with a mode spacing of ~30 GHz under the continuous-wave (CW) operating condition. As a result of this additional detuning, the effective LEF is reduced from 1.3 to 0.8, although its material LEF is ~5. However, when we change the operating wavelength of this ECL, the minimum achievable LEF is deteriorated due to the parasitic reflection occurring at the antireflection (AR)-coated facet. For example, when the optical power reflected at the AR-coated facet is ~5% of the feedback power from the polymer Bragg grating reflector (PBR), the effective LEF is increased from 0.8 to 3.4. To verify these results experimentally, we fabricate a polymerbased tunable ECL and measure its chirp characteristics. For example, we directly modulate this ECL at 10 Gb/s and measure its effective LEF as a function of the detuned frequency. The result shows that we can reduce the effective LEF to ~1 by detuning the operating mode by 18 GHz. This value is slightly larger than that achievable under the nearly CW operating condition (i.e., 0.8) due to the chirp-induced mode hopping. We also confirm that, by optimizing the detuning condition, this ECL can be used for the transmission of a 10-Gb/s signal over 20 km of a standard single-mode fiber (SSMF) with a power penalty of <;2 dB. However, an error-free transmission cannot be achieved at some wavelengths due to the parasitic reflection. To avoid this problem, it is needed to suppress the parasitic reflection to be <;0.1% of the feedback power from the PBR.