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• Abstract

SECTION I

INTRODUCTION

Widely tunable semiconductor lasers are attractive for various applications in, for example, today's dense wavelength division multiplexed (DWDM) systems and broadband sensors [1]. Tunable lasers are also key components in future optical packet and burst (OPS/OBS) switching systems to reduce the latency and increase the capacity of current optical transmission networks [2], [3]. Several typical tunable lasers have been demonstrated including the sampled-grating (SG) or superstructure grating (SSG) distributed Bragg reflector (DBR) lasers [4], [5], the grating assisted co-directional coupler with rear sampled grating reflector (GCSR) lasers [6], the digital supermode (DS) DBR lasers [7] with phase gratings [8], [9], the binary superimposed grating (BSG) DBR lasers[10] and the modulated grating Y-branch (MGY) lasers [11]. In addition, to provide a “flat-top” comb reflector which is ideal for tunable lasers, we have proposed the wavelength tunable digital concatenated grating (DCG) DBR laser [12].

In this paper, by combining the multiple reflection spectrum concatenation [12] and multiple phase shifts [13] technologies, we proposed a widely tunable four-section DBR laser based on the DCG with multiple phase shifts (MPS-DCG). The static characteristics of the proposed MPS-DCG DBR laser are simulated and analyzed with a time-domain traveling-wave model. The MPS-DCG can provide periodic multiple high reflection peaks in the wavelength range of over 100 nm. Compared to the previous DCG design, the MPS-DCG provides a narrower 3 dB bandwidth of reflection peaks which will help to increase mode selectivity. Furthermore, the grating period difference of the MPS-DCG is slightly bigger than that of the DCG, which will help to relieve the stringent requirements on fabrication tolerance of the grating.

SECTION II

GRATING DESIGN AND LASER STRUCTURE

To provide a “flat-top” comb reflector for DBR type tunable lasers, we previously proposed a DCG structure [12], and then fabricated the grating by using the nanoimprint lithography (NIL) technology [14]. Fig. 1(a) and (b) show the SEM photography of the DCG pattern on resist after the nanoimprint processing and the inductively coupled plasma (ICP) etched DCG, respectively. In Fig. 1(b), the periods of concatenated grating segments are approximately 224 nm, 228 nm, and 232 nm. In practice, we have to carefully control the fabrication tolerance as the period difference of concatenate Bragg gratings is approximately 4 nm. Besides, the 3 dB bandwidth of reflection peaks of a DCG is not narrow enough, which may result in reduced SMSR and mode instability. This can be observed from the measured lasing spectra of a demonstrated DCG-DBR laser in Fig. 2. Clearly, discrete wavelength tuning of about 32 nm is obtained with six different supermodes. We can see from Fig. 2, although, the SMSRs of these six supermodes are larger than 30 dB, the side modes are not well suppressed possibly due to the relatively large comb reflection peak 3 dB bandwidth of the DCG.

Fig. 1. SEM photographs of the fabricated DCG. (a) DCG pattern on resist after nanoimprint; (b) the ICP etched DCG.
Fig. 2. Superimposed spectra of six DCG-DBR laser supermodes.

For the above reasons, here we use the multiple phase shifts technology to improve DCG, i.e., MPS-DCG. The schematic structure of the MPS-DCG is shown in Fig. 3. A MPS-DCG consists of $M$ grating segments with different uniform periods, $\Lambda_{1}, \Lambda_{2}, \ldots$, and $\Lambda_{M}$ in one sampling period. $Z_{g}$ and $Z_{s}$ are length of grating segments and sampling period, respectively, and $Z_{s} = MZ_{g}$. The phase shift between the $k$th and $(k + 1)$th sampling period is $\varphi_{k} = 2\pi k/m$, where $m$ is the phase shift factor. A MPS-DCG consists of $M$ multiple-phase-shifts sampled gratings. By carefully designing MPS sampled grating segments periods, the reflection spectrum envelope of each grating segment is concatenated [15], and by increasing $m$, channel-counts within the 3 dB reflection envelope bandwidth could be densified. Therefore, by reducing the sampling period $Z_{s}$ to $Z_{s}/m$, the 3 dB bandwidth of reflection spectrum envelope can be increased by a factor of $m$ without changing the peak spacing.

Fig. 3. Schematic structure of a MPS-DCG.

In our design, in order to concatenate the sub-grating reflection spectrum envelope, the Bragg period of the $i$th sub-grating satisfies the following equations [15]: TeX Source \eqalignno{\Lambda_{i} - {\lambda_{c} \over 2n_{\rm eff}} = &\, \cases{{H \over 2n_{\rm eff}}\left(i - {M + 1 \over 2} \right)\Delta\lambda, & m: odd \cr {H \over 2n_{\rm eff}}\left(i - {M + 1 \over 2}\right)\Delta\lambda + {\Delta\lambda \over 4n_{\rm eff}}, & m: even} \qquad i = 1, 2, \ldots, M&\hbox{(1)}\cr \Delta \lambda = &\, {1 \over m}{\lambda_{c}^{2} \over 2n_{\rm eff}Z_{s}}&\hbox{(2)}} where $\lambda_{c}$ is the designed center wavelength of the reflection spectrum, $n_{\rm eff}$ is the effective refractive index, $\Delta\lambda$ is the channel spacing of MPS-DCG and $H$ is a positive coefficient. The optimal $H$ is equal to $m \times M$ to obtain reflection spectrum with wide wavelength range and flat peak reflectivity [15].

Fig. 4 shows the calculated reflection spectra of the MPS-DCG and DCG. The results in Fig. 4(a) indicate that by employing multiple phase shifts technology, the 3 dB bandwidth of reflection spectrum envelope can be increased. When $m = 2$, $M = 3$, multiple reflection peaks are observed in over 120 nm range with a high reflectivity of 60% $\sim$ 80%. The peak 3 dB bandwidths shown in Fig. 4(b) are 0.92 nm and 1.2 nm for MPS-DCG and DCG, respectively. The narrower peak bandwidth of the MPS-DCG has the advantages in good SMSR and mode stability. The corresponding structure parameters of the MPS-DCG and DCG are listed in Table 1. The grating coupling coefficient is 100 $\hbox{cm}^{-1}$. As shown in this table, the grating period differences between grating segments of the MPS-DCG and DCG are approximately 8.4 nm and 4.2 nm, respectively. As the MPS-DCG has a bigger period difference, it is easier to control in fabrication.

Fig. 4. (a) Reflection spectra of the MPS-DCG, $m = 2$, $M = 3$ and DCG, $M = 3$; (b) Peak 3 dB bandwidth at wavelength around 1550 nm.
TABLE 1 STRUCTURE PARAMETERS OF THE MPS-DCG AND DCG

Fig. 5 shows the schematic structure of the wavelength tunable MPS-DCG DBR laser. The MPS-DCG reflectors are formed in the front and rear passive sections. There is a slight difference of reflection peak spacings between the front and rear MPS-DCG reflectors to extend the tuning range based on the Vernier mechanism [4]. The two grating reflectors consist of three grating segments with different Bragg periods in one sampling period, which can be calculated from (1) and (2), and the phase shift factor $m$ is chosen to be 2. The structure parameters of the front and rear MPS-DCGs are listed in Table 2, and the grating coupling coefficient is 100 $\hbox{cm}^{-1}$. The corresponding reflectivities are shown in Fig. 6. This device has a designed tuning range of 88 nm. The length of the active and phase sections are 400 $\mu\hbox{m}$ and 100 $\mu\hbox{m}$, respectively. Both facets of the MPS-DCG DBR laser are AR-coating.

Fig. 5. Schematic structure of the MPS-DCG DBR laser.
TABLE 2 STRUCTURE PARAMETERS OF THE FRONT AND REAR MPS-DCGS
Fig. 6. Reflectivities of the front and rear MPS-DCGs.
SECTION III

NUMERICAL RESULTS AND DISCUSSIONS

The characteristics of the MPS-DCG DBR laser are simulated using the time-domain traveling-wave model [16] combined with a digital filter method [17], [18]. The material and physical parameters used in the simulation are given in Table 3.

TABLE 3 MATERIAL PARAMETERS USED IN SIMULATION

Fig. 7(a) and (b) show the wavelength and the SMSR tuning curves as a function of the currents across the front and rear grating sections, respectively. The active and phase sections are biased at 75 mA and 0 mA, respectively. As shown in Fig. 7(a), by tuning the front current from 0 mA to 30 mA, the mode hops to higher wavelength, after that a cycle jump of the lasing wavelength from 1581.8 to 1502 nm is observed. The mode hop spacing is about 8.9 nm, which is approximately equal to the peak spacing of rear grating due to the Vernier tuning mechanism [4]. In a similar way, by tuning the rear current from 0 mA to 40 mA, the mode hops to lower wavelength, and a wavelength cycle jump from 1509.8 to 1589.9 nm is observed with a mode spacing of around 9.9 nm, which is approximately equal to the peak spacing of the front grating. It is clear in Fig. 7(a) and (b) that the good SMSR (> 40 dB) can be achieved in the center area of each mode, while SMSR decreases at the mode boundaries. The 3-D wavelength tuning map is plotted in Fig. 8. It shows that the maximum tuning range of the device is approximately 90 nm, covering a wavelength range from 1500 nm to 1590 nm, which is slightly larger than the designed value.

Fig. 7. Tuning curves of: (a) front grating section, with ${\rm I}_{\rm r} = 29.5\ \hbox{mA}$; (b) rear grating section, with ${\rm I}_{\rm f} = 6\ \hbox{mA}$.
Fig. 8. Three-dimension wavelength tuning map.

As shown in Fig. 9, when the active section and phase section are biased at 75 mA and 0 mA, respectively, by tuning the current applied on the front and rear grating sections, the output power from the front MPS-DCG section facet varies from approximately 7.5 dBm to 10.8 dBm, which is predicted to be larger than that of the SG-DBR lasers [4]. This is because that a shorter grating is needed to provide sufficient high reflectivity for the MPS-DCG, and thus reducing optical loss in front section. However, as the thermal effects are not included in this model, the output power might be overestimated, especially when large current is injected into the active section. This is because that with the increasing of the active region current, the active region temperature increases as well, and the leakage and nod-radiative combination become more significant, which reduce the output power. Since the output optical field has to pass through the front MPS-DCG section, this causes a power variation of 3.3 dB when increasing the injected currents in grating sections. A SOA integrated in the front MPS-DCG section could reduce output power variation and improve the output power further.

Fig. 9. Contour of the output power.

In Fig. 10, the light output power is shown for three wavelength channels as a function of the current injected on active section. No kinks are observed over the tuning range from 40 mA to 120 mA. The results indicate that the threshold currents of the three different wavelengths are around 33 mA. We attribute this to the relatively low reflectivity of the front grating section as shown in Fig. 6. By increasing the reflectivity of the front MPS-DCG, the threshold current could be reduced as well as the output power. Therefore, a design tradeoff is found between the threshold current and the output power of the laser.

Fig. 10. P-I curves of the MPS-DCG DBR laser.
SECTION IV

CONCLUSION

We have proposed a new widely tunable DBR laser based on digital concatenated grating with multiple phase shifts. The static characteristics of the MPS-DCG DBR laser were analyzed. The tuning range of the laser was approximately 90 nm, while maintaining a high SMSR (> 40 dB) in the center of each mode. As the MPS-DCG has a high reflection efficiency, short gratings are needed to provide enough reflectivity. High output power could be easily obtained. Moreover, the MPS-DCG can be applied to other laser structures such as the GCSR laser or the DS-DBR laser as it can produce broadband periodic multiple high reflection peaks with a simple structure. These results show that the proposed device could be a promising candidate for future tunable laser sources.

Footnotes

This work was supported in part by the International S&T Cooperation Program of China under Grant 1016, by the National Natural Science Foundation of China under Grant 11174097, and by the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) under Grant 20100142110045. Corresponding author: Y. Yu (e-mail: yonglinyu@mail.hust.edu.cn).

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