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A three phase shifted (3PS) distributed feedback (DFB) semiconductor laser based on Reconstruction-Equivalent-Chirp (REC) technique is experimentally demonstrated for the first time. The simulation results show that the performances of the equivalent 3PS DFB semiconductor laser are nearly the same as that of the true 3PS laser. However, it only changes the μm-level sampling structures but the seed grating is uniform. So, its cost of fabrication is low. The measurement results exhibit its good single longitudinal mode (SLM) operation even at high bias current and surrounding temperature.
©2012 Optical Society of America
1. Introduction
Distributed feedback (DFB) semiconductor lasers are one of the most widely used light sources in the high speed optical communication system due to its excellent properties such as compactness, high efficiency and reliability, etc [1]. Single longitudinal mode (SLM) operation is one of the most important characteristics and it has been widely studied up to now. Quarter-wave-phase shifted (QWS) DFB laser is the well-known structure to improve the SLM property. But it sometimes also suffers decreased SLM due to the spatial hole burning (SHB) caused by the high injection current or larger index coupling coefficient [2]. Therefore, some other complex grating structure such as three phase shifted (3PS) structure [3], corrugation-pitch-modulated (CPM) structure [4], distributed coupling coefficient (DCC) structure [5], have been proposed to overcome the drawback of the SHB. These kinds of lasers have much better SLM property than QWS DFB lasers.
Moreover, some specially designed grating structures with unique laser performances are usually required for particular applications [6, 7]. On the other hand, if these lasers are integrated into a multi-wavelength laser array, the difficulty of the fabrication is also further increased accordingly [8]. Until now, the most commonly used fabrication method is the Electron-Beam lithography (EBL) [9]. Though it can accurately control each grating line, it is time-consuming and high-cost. Therefore, many efforts are made for seeking other low-cost techniques of fabrication, such as bent waveguide and phase mask [10, 11]. But they usually lack enough flexibility or the performance is limited for actual manufacture. Thus, a simultaneously simple, reliable and low cost method is still a challenge.
Recently, the Reconstruction-Equivalent-Chirp (REC) technique was successfully applied to fabricate the QWS DFB semiconductor laser and laser array [12, 13]. Some fine structure such as chirp, phase shift can be equivalently realized by pre-designed sampled grating which is formed by the uniform seed grating and the µm-level sampling structure. So it can be easily fabricated only by conventional holographic exposure combined with an additional corresponding µm-level conventional photolithography. The other processes are the same as that of the normal DFB lasers. Therefore, the cost is greatly reduced and the yield is also improved. In this paper, equivalent 3PS DFB semiconductor laser is experimentally demonstrated based on the REC technique for the first time. It exhibits good SLM property at high bias current and high surrounding temperature, which shows a promising way of the REC technique for low cost fabrication of the DFB semiconductor lasers and even the integrated laser arrays with complex grating profile.
2. Theory and simulation analysis
2.1 Principle
Based on the Fourier series expansion and supposing there is an abrupt shift ΔP in the sampling structure, the index modulation of the sampled Bragg grating can be expressed as [14],
where P is the sampling period, Λ0 is the seed grating period, Δns is the index modulation of the seed grating and m denotes the mth order Fourier series. If an abrupt shift of ΔP is introduced to the sampling structure, the phase shift of θm = −2mπΔP/P is achieved in each channel. Furthermore, if the −1st sub-grating is used as the working grating and ΔP = P/3 is equally distributed along the cavity as shown in Fig. 1 , three phase shifts of θ-1 = 2π/3 can be achieved. The −1st sub-grating period is given byIf selecting the suitable sampling period P, the −1st sub-grating Bragg wavelength can locate within the gain region, while the others are outside the gain region.
Fig. 1 Schematic of the grating structure with equivalent 2π/3 phase-shifts at three different positions.
2.2 Simulation analysis
The static characteristics of the equivalent 3PS DFB semiconductor laser are studied using the transfer matrix method (TMM) [15]. The sampling period P is 4.2μm. The abrupt shift ΔP in sampling structure is 1.4μm. The grating coupling coefficient κtrue of the true DFB structure is 83.3 cm−1. In order to obtain the same index modulation strength, the coupling coefficient κseed of seed grating should be 250 cm−1. So the index modulation of −1st sub-grating is κseed/3 which is equal to κtrue [14]. The cavity lengths of both of the two lasers are 300µm. The seed grating period is 232 nm. To obtain the same lasing wavelength, the grating period of the true 3PS laser is 245.56nm. Other parameters are same.
As shown in Fig. 2(a) , it can be seen that the curves of light intensity distributions along the cavity of two different structures are almost overlapped. The output powers of the two lasers are both about 4.3 mW at the same bias current of 40mA.
Fig. 2 (a) Light intensity distributions along the laser cavities with the two different structures at the same bias current of 40mA, (b) the corresponding simulated lasing spectra of the two DFB lasers.
The spectra of the two DFB lasers can be calculated based on the model in ref [16], as shown in the Fig. 2(b). The spectrum of the equivalent 3PS laser is nearly the same as that of the other one.
3. Fabrication and experimental results
The device is fabricated by a conventional two-stage lower-pressure metal-organic vapor phase epitaxy (MOVPE). An InP buffer layer, a lower optical confinement layer, a multiple quantum-well (MQW) active structure and an upper optical confinement layer are successively grown on an n-InP substrate in the first epitaxial growth. The MQW structure contains five-layer undoped 6nm-thick 1.2% compressive strain AlGaInAs wells separated by six-layer 9nm-thick −0.45% tensile-strain AlGaInAs barriers. The sampled grating is then formed on the upper separate-confinement-heterostructure (SCH) layer by a conventional holographic exposure combined with conventional photolithography. The cavity length is about 300μm. The measured seed grating period is around 232nm. Therefore, the 0th channel is located nearly 1485nm where estimated effective index is around 3.2. The sampling period is 4.6µm. Figure 3(a) shows the microscope image of the sampling photomask and (b) is the fabricated sampled grating. Figure 3(c) shows the cross-section of the fabricated DFB laser.
Fig. 3 The microscope images of (a) the sampling photomask and (b) the sampled grating, (c) the cross-section of the fabricated DFB laser.
3.1 Optical power-current characteristics
One laser was selected in the laser bar, the power-current (P-I) curves and threshold currents at different surrounding temperature are measured as shown in Fig. 4 . Good slope efficiency was obtained. The slope efficiency is 0.362W/A and the threshold current is 18.25mA when the temperature is 25 °C. With the increase of the temperature, the slope efficiency decreases but also can reach up to 0.303W/A even at temperature of 55 °C.
3.2 Optical spectra
The SLM property was measured. The wavelength resolution of the Optical Spectrum Analyzer is 0.02nm. Figure 5(a) shows the lasing spectra when the bias current is 150mA and the surrounding temperature is from 25 °C to 55 °C. Good SLM operation is obtained with average side mode suppression ratio (SMSR) of 57.10dB as shown in Fig. 5(b).
Fig. 5 (a) The measured lasing spectra of the fabricated laser when bias current is 150mA and surrounding temperature is from 25°C to 55°C, (b) the corresponding SMSRs.
3.3 Heat induced wavelength shift
The feature of the heat induced wavelength shift is an important characteristic especially for thermal fine tuning to achieve the accurate lasing wavelength [10, 17]. For the normal DFB semiconductor laser, the heat induced wavelength shift is mainly caused by refractive index change and the grating period change is too small that can be ignored. The situation is the same for the REC based DFB laser. If there is a change of the cavity length ΔL caused by the temperature change, the change of the seed grating period is given by,
Here, L is the cavity length. The −1st sub-grating period change can be further expressed as,Therefore, it can be found that the period change of the REC based DFB laser is amplified by a factor of . But the factor is also very small. For example, if P = 5µm and Λ0 = 232nm, the factor is only 1.0997. So it nearly doesn’t contribute to the wavelength shift. Figure 6 is the measured lasing wavelength versus the temperature. The wavelength shift ratios are both about 0.09nm/°C at bias currents of 70mA and 110mA, respectively. This value is equal to that of the normal DFB semiconductor laser. Thus, if the thermal tuning is applied, the same tuning scheme can be used.Fig. 6 The measured lasing wavelengths versus the temperature at the bias currents of 70mA and 110mA.
4. Discussion
We also designed the two lasers with different sampling periods of 4.4µm and 4.1µm respectively for their lasing wavelength space of 6.4nm in the same laser bar. Figure 7 shows the measurement results. The measured wavelength spacing is 6.31nm. The relative error is only about 1.4%. It shows that the grating period difference of about 1.0nm can be controlled by sampling difference of 0.3µm according to the Bragg condition (assuming neff is 3.2). The fabrication precision is largely relaxed and it denotes that the integrated multi-wavelength DFB semiconductor laser array can be easily achieved.
Fig. 7 The lasing spectra of the two fabricated equivalent 3PS DFB semiconductor laser with different sampling period in the same laser bar.
Furthermore, according to Eq. (2), we can obtain the grating deviation caused by the deviation of the sampling period Δp as follows,
When the 0th wavelength is 1485nm, P is 5530nm and −1st wavelength is 1550nm, a deviation of 0.125nm for corresponds to a Δp of 65.2nm, where the error tolerance is largely relaxed by about 520 times. Equation (5) shows it is reasonable that the fine complex grating structure can be controlled by the µm-level pre-designed sampling structure. This also well benefits the fabrication of the DFB laser array with complex grating structure which requires the accurate control of the each lasing wavelength. If REC technique is applied, the fabrication tolerance should be highly relaxed. Finally, it should be mentioned that though the larger P can lead to the higher precision, the wavelength spacing between the 0th order and −1st order sub-grating becomes smaller for larger P. So in order to simultaneously ensure the SLM operation, P should be comprehensively considered.5. Conclusion
The 3PS DFB semiconductor laser based on REC technique is analyzed and experimentally realized. The good optical performance of the laser was obtained. Thanks to the equivalently realization of the fine grating profile by the pre-designed sampling pattern, the fabrication is easy and the cost is low. In addition, because each laser with different wavelength can be individually controlled by the sampling period and shares the same uniform seed grating, the laser array with three phase shifts should be also easily realized.
Acknowledgment
This research was supported by the National Nature Science Foundation of China under Grant 61090392 and 60877043, National “863” project under Grand 2011AA010300, the Fundamental Research Funds for the Central Universities and PAPD, Jiangsu Province, China.
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