Introduction
Widely tunable sampled grating distributed Bragg reflector (SGDBR) lasers have attracted much attention in high-capacity photonic networks based on dense wavelength division multiplexing and wavelength routing [1]. SGDBR lasers provide wide tuning range, fast wavelength switching time, and excellent side-mode suppression ratio (SMSR). Furthermore, SGDBR lasers have an important advantage of being easily monolithically integrated with other devices such as semiconductor optical amplifier (SOA) and electro-absorption modulator (EAM) to create photonic integrated circuits with more functionality without significantly increasing the fabrication complexity [2]. The work presented here is concentrated on an SOA-integrated SGDBR laser. The integrated SOA in front of the SGDBR laser, can be used not only as a variable optical power controller, but also as a shutter during fast wavelength switching of the laser. It has been demonstrated that an integrated SOA can be used to blank the spurious modes generated by the tunable DBR lasers during wavelength channel switches [3,4]. Previous results also indicated that high facet reflection of an integrated SOA section will degrade the SMSR and linewidth of such lasers [5].
In this paper, the influence of facet reflection of the integrated SOA on blanking characteristics of the SOA-SGDBR devices is investigated by simulations and experiments.
Model description
A combined model of the transmission-line laser model (TLLM) with the digital filter approach is developed to simulate the blanking characteristic of an SOA integrated with an SGBDR laser. The scheme of the combined model is illustrated in Fig. 1. The active regions, the phase regions and the SOA regions can be described easily in the time domain by the TLLM [6]. For the two grating reflectors of the SGDBR lasers, however, it is difficult to represent them in the time domain, due to their complex structure of sampled grating. Here, the transfer matrix method (TMM, a frequency domain model) is used to characterize the front and rear sampled grating (F/RSG) regions, and then the reflection and transmission coefficients of F/RSG are coupled into the proposed TLLM via the digital filter approach. It has been verified that signals in time domain and frequency domain can be transformed into each other by using the finite impulse response (FIR) filter as a bridge [7,8].
The basic principle of the TLLM is that the laser cavity length can be longitudinally divided into a number of equal length sections , with a length . In each section, a scattering matrix is used to represents the optical process, including stimulation emission, spontaneous emission, and attenuation. The matrices of these sections are then connected by transmissionlines, which account for propagation delays of the optical wave. Each section includes a propagation delay of , which is related to the laser’s cavity length and the group velocity inside the cavity, and is given by
The connecting processes can be represented by a connecting matrix . From the iterations of scattering and connecting processes, the output optical field in the time domain can be obtained. Then, with the help of the fast Fourier transform (FFT), the laser output spectrum can be easily acquired. The independent carrier rate equation is used in each section to govern the carrier-photon interaction.
The scattering processes represent the optical process, including stimulated emission, spontaneous emission, and attenuation, in each section, and can be expressed as [9]where is the incident wave at the input of the n section at the k time step, is the reflected wave from that section, and is the wave due to the spontaneous emission. represents the scattering matrix.
The connection matrix describes the cross coupling between forward and backward traveling waves. It provides the incident wave of the scattering section from the reflected wave of the previous section, which may be described by
F/RSG sections are first modeled by TMM [10,11] to get the reflection and transmission coefficients, then the FIR filter coefficients in the time domain can be obtained easily by a reverse FFT transformation of the reflection and transmission coefficient function in the frequency domain, is given by [7]where represents the reflection or transmission coefficients of FSR/RSG sections in the frequency domain.
The transmitted output optical field or the reflected field can be expressed by the input field and reflectivity and transmission FIR filter coefficients at the specific time as [7]where are the digital filter coefficients.
Simulation and experiment results
The SOA integrated SGDBR laser for simulation includes a 450-μm-long active section, a 150-μm-long phase section, a 600-μm-long SOA section, a 10-period front sampled-grating mirror with 6-μm-wide bursts using a 58.5-μm period and a 12-period rear sampled-grating mirror with 6-μm-wide bursts and 64.5-μm period. Other simulation parameters used are given in Table 1.
Tab.1 Simulation parameters |
| parameter | value | |
|---|---|---|
| waveguide width | 2 μm | |
| waveguide thickness | 0.05 μm | |
| waveguide loss | active region | 3000 m-1 |
| pssive region | 200 m-1 | |
| waveguide confinement factor | active region | 0.35 |
| passive region | 0.5 | |
| effective refractive index | 3.23 | |
| nonradioactive linear recombination coefficient | 1 × 108 s-1 | |
| bimolecular recombination coefficient | 1 × 10-16 m3·s-1 | |
| auger recombination coefficient | active region | 2.5 × 10-41 m6·s-1 |
| passive region | 7 × 10-41 m6·s-1 |
The active, FSG, RSG and phase currents of the SGDBR laser are kept at 100, 3, 8, and 0 mA, respectively, to ensure that the laser has a stable output power to be launched into the SOA section. Simulated SOA gain versus the bias current of the SOA section is shown in Fig. 2. As can be seen from Fig. 2, the bias current of the SOA section needs to be switched from over 100 to 0 mA in order to get enough output power suppression for blanking applications.
Blanking characteristics of the SOA-SGDBR laser are simulated. The bias current of the integrated SOA section is first switched between 120 and 0 mA, under two cases of facet reflections of the SOA. Figure 3(a) is the case which the facet reflection of the SOA is 10-4, while Fig. 3(b) is the case which the facet reflection of the SOA is 10-3. We can see that there is a small oscillation on the turn-on edge when the facet reflection increases to 10-3. The oscillation can be observed more clearly when the bias current of the SOA increases. Figure 4 shows the simulated results when the bias current of the SOA is switched between 160 and 0 mA, under same two cases of facet reflections of the SOA. In comparison with Fig. 3(b), the oscillation on the turn-on edge in Fig. 4(b) is much more pronounced.
From Figs. 3 and 4, we can find that the turn-on edge of the blanking process will oscillate when the facet reflection of the SOA is increased. Meanwhile, higher bias current into the SOA section will aggravates this oscillation.
We attribute the oscillation phenomenon to unstable optical output from the SGDBR laser. With the help of the developed model, we could investigate more detail of this phenomenon. The active, FSG, RSG and phase current of SGDBR laser are set as the above currents to ensure that the SGDBR laser has a stable output field to be launched into the SOA section. The output powers at the facet of the FSG section and the SOA section are simulated respectively, as shown in Fig. 5, where the facet reflection of the SOA is 10-3. Due to the high facet reflection of the SOA, the reflected optical field from the SOA section results in an unstable output field from the SGDBR laser, which then affects the final output of the SOA section in turn, as shown in Fig. 5.
The oscillation on the turn-on edge during the SOA shuttering can be observed by experiments as well. Figure 6 shows the experimental set-up. To achieve fast control of the SOA-integrated SGDBR device, a high-speed current driving board has been developed which has a similar design with the one in Ref. [12]. In this case, an additional current driving circuit for the SOA section was included. The output of the SOA-integrated SGDBR lasers is directed to an optical spectrum analyzer (OSA) and an oscilloscope. Figure 7 reveals measured results of evolution of the output from the SOA under the blanking, where bias current of SOA section is switched between 140 and 0 mA.
Conclusions
We investigated the influence of facet reflection of the integrated SOA on blanking characteristics of the SOA-SGDBR laser by both simulations and experiments. Oscillation occurs on the turn-on edge of the SOA blanking process when the facet reflection is relatively high, and it can be aggravated by larger bias current into the SOA section. This is due to the facet reflection of the SOA, which causes the power fluctuation of the output of the SGDBR laser.