The characteristics of polarization stable microstructure vertical-cavity surface-emitting lasers (PS-MS-VCSELs) with low threshold current and single fundamental mode (SFM) operation were theoretically and experimentally investigated. Elliptical air hole photonic crystal (EPC) structure was incorporated in the top mirror of the MS-VCSELs to realize single fundamental mode operation. By controlling the mode loss difference among the two orthogonal modes, the fundamental mode and other high order modes of the MS-VCSELs, through suitable oxide aperture shape and EPC parameters, a high performance PS-MS-VCSEL is achieved with output power of 1.6 mW, low threshold current of 0.8 mA as well as more than 20 dB orthogonal polarization suppression ratio (OPSR).
© 2015 Optical Society of America
Vertical-cavity surface-emitting lasers(VCSELs) are key devices in optical fiber communication systems due to their important characteristics such as high modulation bandwidth, circular beam shape, small divergence angles, low threshold current and low power consumption, as well as high reproducibility in fabrication [1,2 ]. However, VCSELs usually suffer from the shortcomings of multi-transverse modes operation and polarization instability, which may limit their applications [1–4 ]. To improve the device performance, various types of microstructures have been proposed and incorporated on the top mirror of VCSELs [5–7 ]. Through introducing mode loss difference among transverse modes and the two orthogonal polarization modes by the microstructures, polarization stable fundamental mode operation can be realized. Nowadays, considerable progress has been made for high performance VCSELs. Polarization stable (PS) fundamental mode (SFM) VCSELs have been successful demonstrated by incorporating different microstructures, such as surface relief grating (SRG) , elliptical air hole photonic crystal (EPC)  and high-index-contrast grating (HCG) [10,11 ]. However, the MS-VCSEL is with higher threshold current and lower quantum efficiency than the normal VCSEL due to the additional loss caused by the microstructures. Therefore, it is essential to analyze the mode characteristics of the device to improve the device performance [12–15 ]. With reasonable mode loss and loss difference between the lasing fundamental mode and other high order modes, high performance VCSEL with low threshold current and high efficiency can be achieved [16, 17 ]. In this manuscript, a three dimensional Finite Difference Time Domain (FDTD) model is used to analyze the mode loss and loss difference for several types of MS-VCSELs. Based on the analysis results, a high performance EPC-VCSEL is demonstrated with SFM output power of 1.6 mW, low threshold current of 0.8 mA as well as more than 20 dB OPSR.
2. Design and fabrication
In the MS-VCSEL, microstructures were patterned on the top distributed feedback reflector (DBR) mirror to introduce mode loss difference among transverse modes. With proper mode loss difference between fundamental mode and other high order transverse mode, the high order modes can be suppressed and only the fundamental mode is lasing. Similarly, single polarization can also be realized by introducing mode loss difference between the two orthogonal polarization modes of SFM-VCSEL. As reported, the polarization can be controlled through incorporating asymmetric structure in SFM-VCSEL [7, 9, 18, 19 ]. Therefore, stable linearly polarized fundamental mode can be realized and the polarization switching is suppressed. As all know, photonic crystal (PC) is one of the most useful and reliable microstructures for polarization and mode control in VCSEL. It can introduce reasonable mode loss in the cavity by adjusting the air hole depth, shape, diameter, and arrangement of the PC, so as to easily demonstrate high performance VCSELs with polarization stable, high SFM output, high modulation speed, and small divergence angle [20,21 ]. Thus, the MS-VCSELs with EPC structure on the top mirror were fabricated and polarization control analysis were carried out in this paper.
The mirror loss αm in the MS-VCSEL can be calculated by the equation of αm = 1/L(ln(1/(Rtop × Rdown)1/2)), where L is the effective cavity length (about 1 μm); Rtop and Rdown are the reflectance of the top and bottom DBRs of the devices, respectively. Since Rdown is 1, the mode loss caused by the microstructure is only determined by Rtop. The schematic of the MS-VCSEL was shown in Fig. 1 . FDTD method was used to analyze the polarization dependent mode loss in the VCSEL. Perfectly matched layer (PML) boundary conditions were used in the simulation domain to analyze the asymmetric structure. Plane wave light source with different polarization were adopted in the model. TE polarization mode means the source polarization along the short axis of elliptical air hole and TM polarization mode is along the longer axis as shown in Fig. 1. Above the structure, frequency domain profile monitor was used to collect the transmitting spectrum. The reflective index and thickness of every layer material of the VCSELs were selected from the reference [12–16 ]. In this simulation, several types of EPC-VCSELs were analyzed. The effects of period, b/a ratio (a is the long axis of the elliptical air hole and b is the short one), air hole depth, the oxide aperture diameter and shape of the EPC-VCSEL were analyzed to optimize the device performance. For comparison, the conventional VCSEL with unpatented top DBR and circular oxide aperture was also analyzed in this paper.
Figure 2 shows the reflectance spectrum of the DBRs and the DBRs coupled with EPC structure. In the simulation, the period of the EPC structure is 4 μm, the longer axis of the air hole a is 2 μm, the b/a ratio of the EPC is 0.7, the depth of the air hole is 2 μm. The oxide aperture diameter is 10 μm in both of the simulation structures. In the conventional VCSEL, both TE and TM modes reflectivity are close to 99.8%, and there is no difference between the two modes. That means both of the two modes has the same mirror loss and the polarization of the fundamental mode is randomly oriented in the plane of the active layer due to the symmetric of the device structure. The reflectance spectrum of the EPC structure is very different from that of DBRs. In the EPC structure, the reflectance of TE and TM modes are different. The reflectance of TE mode is larger than 99.5% and also higher than that of the TM mode. Notice that the reflectance of the TM mode is less than 99.5%, which means the cavity mirror reflectivity is not high enough for TM mode lasing due to the introduction of larger TM mode mirror loss . Considering the relation between the mode loss and reflectance, the mode mirror loss is calculated and shown in Fig. 3 . We can see that both the TE and TM mode loss in the EPC structure are higher than those in the conventional DBR structure. An additional mode loss is introduced by incorporating the EPC structure on the top surface of the VCSELs, which makes the EPC-VCSEL operate with single fundamental mode. Moreover, a larger mode loss difference has been observed between the TM and TE modes in the EPC-VCSEL due to the use of elliptical air holes rather than circular air ones. The TM mode lasing can be suppressed due to the higher mode loss, while only the TE mode would be lasing. Thus a polarization stable VCSEL can be realized. From Fig. 3, we can also find that the device can work with stable polarization in a very large wavelength bandwidth. It means that the device can work with stable polarization against many external disturbances, such as injection current adjustment, spectral red shifting, and self-hearting. Based on the above analysis, a simultaneous polarization and mode controlling approach is presented by changing the air hole shapes of the PC in the PC-VCSELs.
To further improve the performance of the EPC-VCSEL, the effect of b/a ratio of the EPC on mode loss is analyzed and the results are shown in Fig. 4 . From the simulation results, we can know that the mode loss difference between TE and TM modes decreased with the b/a ratio. When the b/a ratio is 0.7, the mode loss difference is the largest among these simulation structures. When the b/a ratio reduces to 0.6, the mode loss reduces to the lowest. However, the loss difference of this b/a ratio is also reduced and several intersection points appear in the spectrum which may affect the polarization stability of the device. Based on the above analysis, the most suitable b/a ratio for the device with single fundamental mode and polarization stable operation is 0.7. Except the b/a ratio, the depth of the air hole is another important parameters of the EPC structure. Figure 5 shows the effect of the EPC air hole depth on mode loss. As shown in Fig. 5, we can find the mode loss is rapidly increased with the etching depth. The mode loss in the depth of 2.5 μm is about 20 times larger than that in the depth of 1.5 μm. Moreover, the loss difference is also enlarged in the deeper EPC structure. When the depth is 1.5 μm, the loss difference between the two orthogonal modes is about zero, only a little loss difference is introduced. When the depth is 2.5 μm, nearly 50 cm−1 loss difference is achieved. Although the large mode loss differences are essential for the polarization control, the large mode loss would lead to increased threshold current as well as reduced quantum efficiency of the device. The worst case is that device does not lase due to large loss. Therefore, a suitable etching depth of the EPC is essential to improve the device performance. Based on the above the analysis, the EPC depth is 2 μm in this paper.
We also analyze the effect of oxide aperture shape and diameter on the mode loss of the VCSELs. The simulated results are shown in Fig. 6 . When the oxide aperture reduces from 10 μm to 8 μm, the mode loss introduced by the EPC also reduces and the loss difference between the TE and TM modes is nearly constant. The results suggest that the oxide aperture can be used to change the mode loss with constant loss difference. Considering both the mode and polarization characteristics of the VCSEL, the oxide aperture of 8 μm is a good choice in the EPC-VCSEL. The mode loss is lower and current injection area is smaller for the VCSELs with aperture of 8 μm than the devices with aperture of 10 μm. After the analysis, all of the analyzed structure EPC-VCSELs have been produced. All of these structure were patterned using electron beam lithography (EBL) and transferred to the top DBR mirror using the inductively coupled plasma (ICP) etching. The depth of the air hole is 2.0 μm. The SEM images of the fabricated air hole on the top DBR mirror were shown in Fig. 7 .
3. Results and discussion
All the fabricated devices were carefully measured and characterized. The devices can simultaneously operate with single fundament mode and stable polarization when the b/a ratio is defined as 0.7.The laser optical output was collected by a photodetector located in front of the top DBR mirror. A polarizer inserted before the photodetector allows the detector to sense these two different states of polarization (TE and TM).Fig. 8 shows the measured L-I -V curve of the fabricated EPC-VCSEL. From Fig. 8, we can see that the device can work with stable polarization during the increase of the injection current. The polarization is maintaining even when the device is saturated at high injection currents. This means that the mode loss difference between TE and TM is larger enough to avoid polarization switch. In addition, more than 20 dB orthogonal polarization suppression ratio (OPSR) are obtained. The threshold current of the devices is 0.8 mA and the output power is more than 1.6 mW. To confirm the fundamental mode operation, the far field distribution and spectrum of the device is also measured. The divergence angle is less than 10 degree with only one peak which reveals good single fundamental mode operation of the devices as shown in Fig. 9(a) . The lasing spectrum is measured by an optical spectrum analyzer (OSA) and the measured results are shown in Fig. 9(b). The spectral line width of the devices is less than 0.1 nm, emission wavelength is centered at about 843 nm with side mode suppression ratios (SMSRs) more than 30 dB at the injection current of 12 mA which shows an excellence single mode performance. For comparison, the device with the air hole depth of 2.5 μm has also been fabricated, the threshold current of the device is larger than 20 mA as shown in Fig. 10 . The high threshold current is caused due to the destruction of the active region by deep air hole.
In conclusion, a high performance polarization stable low threshold current single fundamental mode VCSEL is demonstrated by incorporating elliptical air hole photonic crystal (EPC) structure in the top mirror of the device. 3D-FDTD method is used to analyze the mode loss and loss difference in the devices. We find that the mode loss and loss difference between the two orthogonal modes of EPC-VCSELs are highly dependent on size of the oxide aperture and EPC structure. By controlling the mode loss difference among the two orthogonal modes, the fundamental mode and other high order modes of the devices through suitable oxide aperture shape and EPC parameters, high performance polarization stable device can be achieved. The fabricated VCSELs shows output power of 1.6 mW, low threshold current of 0.8 mA and more than 20 dB OPSR.
The authors gratefully acknowledge financial support by fund from the National Key Basic Research Program of China (Nos. 2011CB933102 and 2011CB933203), the Nation Natural Foundation of China (Grant Nos. 61378058, 61036002, 61036009, 60978067, 61076044 and 61335010), China Postdoctoral Science Foundation (No. 2014M550796) and Science and Technology Research Funding of State Cultural Relics Bureau (No. 20110135).
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