1. Introduction

Widely wavelength tunable lasers are the key devices in the wavelength-division multiplexing (WDM) systems and the main light source in optical add-drop multiplexing (OADM) nodes. So far, different types of wavelength tunable light sources, including sampled grating DBR (SGDBR) lasers [1], grating-coupled sampled reflector (GCSR) laser [2], external cavity lasers (ECL) [3,4], tunable vertical-cavity surface-emitting (VCSEL) lasers [5] and distributed feedback laser (DFB) laser arrays have been developed [6,7]. All of these laser structures provide very wide wavelength tuning range and narrow spectral linewidth which is required in WDM and coherent communication systems, respectively. From this list, DFB laser arrays have found great interest due to their mode stability and reliability. The ability to integrate DFB lasers with other photonic components on the same chip makes them very attractive as a light source. Several types of DFB laser arrays with wavelength tuning of more than 40 nm and a spectral linewidth below 200 kHz have been reported [8,9]. The monolithic integration of tunable DFB laser array with Semiconductor Optical Amplifier (SOA), Mach-Zehnder (MZM) [10] and electro-absorption (EA) [11] modulators for the next generation 100 GbE optical networks has been demonstrated. Although DFB laser arrays have great performance, they still suffer from complex fabrication steps including the re-growth of additional material on top of the gratings and also the use of high resolution lithography to define the low order gratings present in such lasers. Therefore, the fabrication of these lasers leads to low yield and a relatively high cost.

Recently, we have demonstrated that it is possible to obtain stable single mode operation by etching high-order surface gratings using etched slots on top of the laser ridge waveguide, where the slots provide sufficient reflection for lasing to occur [12]. These types of lasers do not need any complex re-growth steps and can be fabricated by standard photolithography instead of high resolution lithography. Based on these slotted single mode lasers we developed a tunable 9-channel slotted single mode laser array [13]. The device exhibited a threshold current between 19 to 21 mA for all channels and with output power more than 35 mW with a biased SOA section. A quasi-continuous wavelength tuning of 27 nm was achieved with side mode suppression ratio (SMSR) more than 35 dB within the array.

In this work, we present an improved design of tunable slotted laser array. We added three more channels by making 12-channel laser array. The device has a wavelength tuning of 36 nm which covers the full C-band. We modified the design of slots by introducing three different slots periods forming non-uniformly spaced slots. An SMSR of 50 dB is obtained for all channels. A linewidth of less than 500 kHz is measured within the array.

2. Device structure

A microscope image of 12-channel slotted single-mode laser array is shown in Fig. 1(a). Each laser is integrated with semiconductor optical amplifier (SOA). The schematic diagram of a single laser is shown in Fig. 1(b). The overall structure and the epitaxial structure of 12 channel laser array are similar to 9-channel array which was reported earlier [13]. In brief, each laser in the array has a 2 µm wide surface ridge waveguide, the active layer consists of five AlGaInAs quantum wells with photoluminescence peak (PL) near 1530 nm. One side of the waveguide has multiple etched slots that act as an active DBR of the laser to provide enough reflection for lasing to occur. The reflection at the other end of the laser is provided by the cleaved facet. The lasers are electrically divided into two sections: the front section includes a group of slots (the active DBR) and the back section which consists of straight waveguide. This division into two contacted regions was done to remove the yield problem caused by the uncertainty of the cleaving position of the back facet. The front and back facets were coated with an anti-reflection (AR) and high reflection (HR) coatings, respectively.

 

Fig. 1 (a) Photograph of a 12-channel slotted single-mode laser array. (b) 3D schematic diagram of a single laser with non-uniformly spaced slots without SOA section.

Download Full Size | PDF

The slot parameters such as slot width, depth, number and period have been optimized in the design using the 2D scattering matrix method [14]. The slot width is chosen to be around 1.1 µm therefore, standard photolithography can be used for fabrication. The slot depth is 1.35 µm while the ridge height is 1.85 µm. For the trade-off between maximizing the reflectivity and minimizing the bandwidth of the reflection peaks while also ensuring that the laser cavity length is kept to a minimum, the slot number is optimized to be 24. Overall, these slot parameters are similar to our previous design except we have now adjusted the slot period. In our previous work, we used uniformly spaced slots with the slot period around 9 µm resulting in a 37th-order grating. In this work, we implemented a three-period structure by making non-uniformly spaced slots. We calculated that for a group of slots with uniformly spaced slots of around 9 µm, the free spectral range (FSR) is around 37 nm, which means that a mode that is one FSR longer will compete with those on the blue side of the gain peak. This behaviour was observed in 9-channel laser array where the SMSR of the lasers of the shorter wavelengths dropped from 50 dB to 35 dB due to these undesirable modes. Therefore, to eliminate these modes in this run we introduced three different slot periods with 8.5, 9.9 and 11.4 µm separation in this fabrication run as shown in Fig. 1(b). We have already demonstrated that group of such non-uniformly spaced slots can eliminate side reflection peaks when we integrated slotted single-mode laser with an EA modulator [15].

The designed lasers have the same fabrication steps as the 9-channel laser array [13]. Two steps of inductively coupled plasma (ICP) based dry etching process was used to form the ridge and the slots. Each fabricated laser in the array has the total cavity length of 590 µm, which includes 400 µm of the slotted laser length and 190 µm of SOA section. The slotted laser by itself consists of back section (straight waveguide) which is 185 µm long and front section (group of slots) which is around 215 µm long. Longer lasers were fabricated for studies of the laser linewidth. After the ridge was passivated and metal contacted (the metal electrode covered the slot are except isolation slots as shown in Fig. 1(a)) the laser bars were coated with HR and AR coatings. The final steps were to cleave the lasers into individual bars and they were mounted on AlN carriers.

3. Device characterization

The fabricated devices were mounted on a thermo-electric cooler (TEC) and tested under CW conditions at 20°C. The SOA section was set unbiased. First, light-current (L-I) curves were measured as shown in Fig. 2(a). For this, the front and back sections of the laser were connected together and injected with the same current. The output power was measured from the front SOA section as shown in Fig. 1(a) using a large Ge detector. The threshold current for all 12 lasers varies from 25 to 30 mA. These values are higher than in our previous results and the output power is less than the previous laser array. This was due to problems during fabrication. We have calculated the waveguide loss using the amplified spontaneous emission [16]. The estimated loss was 28 cm⁻¹ which is higher than in previous laser array where the waveguide loss was found to be 22 cm⁻¹.

 

Fig. 2 (400 µm cavity). (a) Measured light-current (L-I) characteristics of fabricated device with SOA section unbiased. (b) Measured output spectrum for all 12 channels.

Download Full Size | PDF

The output spectrum was measured by lensed fiber and recorded by an Agilent 86140B optical spectrum analyzer (OSA). The resolution of the OSA was set at 0.06 nm with sensitivity of −75 dBm. Figure 2(b) shows the recorded spectra for all 12 channels at 100 mA injection current with SOA section unbiased. The spacing between the channels is around 3 nm which means that temperature tuning of around 30°C is required to cover the spacing between channels (0.1 nm/°C tuning). The side mode suppression ratio (SMSR) is around 50 dB for all channels except channels 2 and 11. In these lasers the SMSR is around 45 dB however it can be improved up to 50 dB by varying the currents in the back and front sections of the laser as it was demonstrated in [13].

The next step was to check the tunability of the device. The device was set at constant 100 mA injection current and the temperature of the chip was changed from 10 to 45°C by using the TEC. Figure 3(a) shows the wavelength tuning as a function of temperature change. A wavelength tuning of 36 nm was obtained covering the wavelengths between 1532 to 1568 nm over the temperature range from 10 to 45°C. Figure 3(b) shows the SMSR for all channels during the temperature tuning and it remains close to 50 dB, which means that stable single mode operation over the entire tuning range has been achieved. It is clearly seen that an improvement in SMSR in comparison with the 9-channel laser array has occurred. Non-periodically spaced slots indeed suppress the undesirable modes caused in the case of the uniformly spaced slots and the SMSR for the lasers at the shorter wavelengths is increased. Figure 3(c) shows the spectrum for channel 2 with uniformly spaced and non-uniformly spaced slots, respectively. The emission peak is at 1537.6 nm and we can see clear suppression of modes which are 38 nm away from the gain peak resulting in the SMSR close to 50 dB for the three period grating case. The full tuning spectrum can be seen in Fig. 3(d). One can see the drop in power in each channel with increasing temperature. This is due to the decrease in internal efficiency as non-radiative recombination and thermal carrier leakage increase with temperature.

 

Fig. 3 (400 µm cavity) (a) Wavelength tuning characteristics of 12-channel slotted laser array. (b) SMSR during the temperature tuning as a function of wavelength. (c) The output spectrum for channel 2 for both uniformly and non-uniformly spaced slots cases. (d) The full tuning spectrum covering 36nm.

Download Full Size | PDF

The linewidth of 12-channel laser array was measured with the delayed self-heterodyne technique using the setup described in [17]. The measured linewidth of laser array is found to be the same as for slotted single mode laser [17] with a single period grating and remains around 1-2 MHz for lasers with cavity length of 400 µm. To improve the linewidth, the laser arrays with different cavity lengths such as 400, 700 and 1000 µm have been investigated as shown in Fig. 4. We kept the length of slotted section and extended only the non-slotted waveguide section. For the linewidth characterization, the laser arrays were mounted on the TEC at a constant room temperature of 20°C. The lasers were driven with low noise current source for measurement accuracy.

 

Fig. 4 Photograph of slotted single-mode lasers with different cavity length. The slotted section remains the same length for each laser.

Download Full Size | PDF

Figure 5(a) shows the spectral linewidth of 12 channels for different cavity length. It is clearly seen that there is a reduction in the linewidth as the laser becomes longer. For the 1 mm lasers the linewidth is below 500 kHz for all 12 channels. However, the longer lasers result in smaller mode spacing than in shorter lasers and hence the adjacent modes are positioned closer to the lasing peak. The mode spacing for 400, 700 and 1000 µm lasers is 1.2, 0.61 and 0.39 nm, respectively. This means that the adjacent modes will be closer to the lasing mode as the length increases, resulting in high mode competition and hence lower SMSR as shown in Fig. 5(b). So for 1 mm lasers, the SMSR for all 12 channels is around 40 dB, for 700 µm laser, the SMSR is around 45 dB and for 400 µm lasers, the SMSR is around 50 dB.

 

Fig. 5 (a) Measured spectral linewidth of 12-channel laser arrays with different cavity length. (b) SMSR as a function of wavelength for slotted laser arrays with different cavity length.

Download Full Size | PDF

Finally we consider how the SOA section effects the performance of the lasers. In our previous work [18] it was found that SOA had little adverse impact on laser performance. As in [18] we observe stable single mode performance with a wavelength red shift within 0.15 nm at 50mA due to the temperature rise when the SOA is biased. This can be seen for a 400 µm laser in Fig. 5(a) along with the SMSR which remains stable at or above 50 dB throughout. The effect on output power was analogous to [18] with a near linear increase of output power with SOA current. The output power reached 45 mW for 70 mA SOA bias.

The impact of the SOA on linewidth has also been considered. For these measurements, 1 mm lasers were chosen and the lasers were driven at a high 180 mA injection current. Figure 6(b) shows the linewidth for all 12 channels when the SOA section is left floating and when it is biased at 50 mA. It is seen from the figure, that the SOA does not have a strong effect on the linewidth and it remains around 500 kHz for all channels.

 

Fig. 6 (a) SMSR and wavelength as a function of SOA current with the laser section set to 120mA. (b) Spectral linewidth of 12-channel slotted single-mode laser array with cavity length of 1 mm with SOA section non-biased (blue square) and biased at 50 mA (red dots).

Download Full Size | PDF

4. Conclusion

In conclusion, we have developed wavelength tunable slotted single mode laser arrays for WDM and coherent communication systems. In the new design we implemented three different slot periods in the high-order surface grating in the laser structure to suppress undesirable modes. From the experiments, it was found that the SMSR was improved and remained close to 50 dB for all channels. Also, in the new design three additional channels were added to the laser array to increase the wavelength tunability. The wavelength tuning of 36 nm was obtained which means that the slotted laser array covers the full C-band region. The spectral linewidth of 12-channel laser array has been investigated. The laser array with cavity length of 400 µm shows a linewidth around 2 MHz. To reduce the linewidth the laser cavity length was extended and for 1 mm laser a linewidth below 500 kHz was obtained for all channels within the array. This makes these lasers suitable for coherent communication systems. The presented laser array has straightforward fabrication steps without using any re-growth steps. Therefore improved laser array structure has strong potential to become a useful optical source in WDM and coherent communication systems.