Nanosecond-scale discrete wavelength switching in feedback-controlled single-gain-section multi-wavelength lasers

Abstract

We investigate discrete wavelength switching in single-gain-section multi-wavelength lasers monolithically integrated on InP with phase-controlled optical-feedback. By modulating the feedback phase, nanosecond-scale wavelength switching is experimentally demonstrated with transition times below 2.5 ns. Measurements consistently show that the switching time decreases with stronger optical feedback and larger phase-modulation amplitudes. Transitions from lower to higher modal gain are faster. We support the experimental observations with a multi-mode extension of the Lang-Kobayashi rate-equation model. We analyze the influence of laser, feedback-cavity, and modulation parameters on the switching dynamics, and highlight the role of mode coupling. These results highlight the potential of integrated multi-wavelength lasers for compact and high-speed all-optical networking systems.

Figures (6)

• Figure 1: Wavelength switching in feedback-controlled on-chip multi-wavelength lasers. (a) Schematic of the MWL integrated circuit: the dual-cavity laser (DCL) is delimited by a multimode interference reflector (MIR) on one side and by two distributed Bragg reflectors (DBRs) on the other side and includes a semiconductor optical amplifier (SOA 1) constituting the single gain medium. The optical feedback cavity, connected with the DCL via a 1 $\times$ 2 multimode interference coupler (MMI), contains SOA 2 and an electro-optic phase modulator (EOPM) for feedback strength and round-trip phase control, respectively. (b) By applying $V_\text{EOPM}=0$ V or $V_\text{EOPM}=-5$ V, we observe the single emission at $\lambda_1=1547.9$ nm or $\lambda_2=1548.7$ nm, respectively. (c) Output power of the MWL at $\lambda_1$ and $\lambda_2$; we observe a sharp switch between the two wavelengths at $V_\text{EOPM} \approx -3.4 V$. (d) Experimental setup for switching time measurements. Two signals are recorded via the scope: the driving modulation at the DC probe level (signal A) and the photocurrent generated by the output light from the MWL at $\lambda_1$ impinging into the photodiode (PD) (signal B). (e) Schematic of the electrical connections at the chip-level for modulation of the EOPM. (f) Representative temporal responses of the phase modulation at the probe level (signal A) and the laser response (signal B) with zoomed views on the overshoot and undershoot features around wavelength transitions.
• Figure 2: (a) Representative temporal response of the laser under trapezoidal modulation of the feedback phase at a frequency of 2 MHz. (b) Detailed view of the wavelength switching transitions. The MWL optical output is oscillating between two steady-state levels: the upper state level $s_\text{up}$, corresponding to the emission at $\lambda_1 = 1547.9$, and the lower state level $s_\text{low}$, corresponding to the suppression of $\lambda_1$ being replaced by the emission at $\lambda_2 = 1548.7$ nm. For each transition edge, the switching time is measured as the elapsed time between the two reference levels $r_{20\%}$ and $r_{90\%}$. (c) Corresponding distribution of the measured rise (blue) and fall (red) switching times extracted from the complete waveform presented in panel (a).
• Figure 3: Rise (purple) and fall (green, dashed line) switching time averages as a function of the modulation amplitudes (a,b) and ramp times (c). The effect of $t_\text{ramp}$ was evaluated over two ranges with different orders of magnitude. Colored areas correspond to the standard deviation of the switching times. Details on the operational parameters of configuration A, B and C are provided in Table \ref{['tab:1-Configurations']}.
• Figure 4: Rise (purple) and fall (green, dashed line) switching time averages as a function of the DBR 1 (a) and SOA 2 (b). Colored areas correspond to the associated standard deviation of the switching times. Details on the operational parameters of configuration D and E are provided in Table \ref{['tab:1-Configurations']}.
• Figure 5: Rise (solid lines) and fall (dashed lines) switching time averages derived numerically from the time traces of the first mode for three values of cross-saturation parameter $\beta$, as a function of the: (a) phase modulation amplitude $A_\text{mod}$, (b) phase modulation ramp time $t_\text{ramp}$, (c) modal gain difference $\Delta g$ and (d) feedback strength $\kappa$. Unless modified explicitly, simulation parameters are: $\Delta g = 0$, $\kappa=0.001$, $\tau_\text{OF}=200$, $A_\text{mod}=\pi$, $\phi_\text{mod}=\pi/2$, $t_\text{ramp}=10$.