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Abstract
Stable polarization has been regarded as a key element for improving the beam quality of vertical cavity surface emitting lasers (VCSELs). However, gain isotropy in the active layer of VCSELs ineluctably leads to the uncertainty of the polarization direction. In this paper, a liquid crystal (LC)-VCSEL laser with polarization control characteristics is designed and fabricated based on the optical rotation effect and electrically controlled birefringence characteristics of orthogonal LC. The experimental results show that, electrically controlled LC could flexibly realize the scheme of polarization stable output and the switching ratio of polarization output could reach 164:1. Based on the electrically controlled birefringence characteristics of LC, the linear polarization angle of VCSEL is further regulated by applying a deflection voltage, and the polarization angle tuning range is between 0° and 90°.
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1. Introduction
With the rapid development of science and technology in the information age, high-performance VCSELs have important applications in quantum measurement fields such as optically pumped atomic clocks, magnetometers, fine spectral analysis, and gas detection due to their low cost, low power consumption, easy two-dimensional integration, and high modulation rate [1–6]. However, unlike the traditional edge emitting laser with asymmetric anisotropic gain, the gain anisotropy of the circularly symmetric VCSEL resonant cavity structure is weak, and it is easy to produce unstable orthogonal polarization states. Therefore, polarization controllable VCSEL laser with high beam quality has become an important issue. At present, methods such as optical feedback, electro-optic birefringence and grating structure are designed to achieve stable polarization output of VCSELs [7–12]. However, due to the complex preparation process and high system difficulty, the polarization switch has low comparison and limited integration. Therefore, surface LC-VCSEL laser, with a simple process and higher integration, has attracted more and more attention. Castany et al. of the Ecole Supérieure de Telecommunications Brittany in France prepared a 1550 nm LC wavelength tunable device, which realized polarized light output with a wavelength tuning range greater than 30 nm. The C. I. Wilkinson team of Sheffield University in the United Kingdom for the first time by putting the optical pump VCSEL into the LC box composition, with a polarizer and mirror to form a complete optical system, by applying voltage to the LC to achieve polarization regulation and external cavity light feedback, Yi Xie et al. of Ghent University in Belgium prepared the first electrically pumped external cavity LC-VCSEL laser in 2012 to achieve polarization control of VCSEL lasers [13–16].
In our previous work, effectively introducing LC on the external cavity could expand the stability range and optical power difference of the orthogonal linear polarization state in the VCSELs [15]. Herein, we examined the optical rotation effect in the electrically controlled orthogonal switching LC layer and proposed a configuration for VCSELs based on this layer. The experiment proves that introducing orthogonal rotation LC can restrain the phenomenon of polarization switching and achieve stable polarization output of VCSELs. The contrast ratio of the polarization switch reaches 164:1. Moreover, the polarization angles of TE and TM mode can be adjusted by applying voltage on the LC layer, and the polarization angle can reach 90°. This study offers an optimal method for controlling the polarization of VCSELs, and it has enormous potential in achieving stable polarization output.
2. Structural design and preparation
The surface LC-VCSEL laser we designed is a three-electrode structure, and the second electrode is the VCSEL upper electrode shared by the VCSEL and the LC layer. The structure of the designed surface LC-VCSEL laser is shown in Fig. 1(a). We used metal-organic chemical vapor deposition (MOCVD) method to epitaxial grow 34 pairs of λ/4 thickness Al0.12Ga0.88As/Al0.90Ga0.10As as bottom DBR on n-doped GaAs substrates. Then there are three pairs of active region structures composed of 7.2 nm thick GaAs quantum wells and 8 nm thick Al0.3Ga0.6As barriers, in which the peak wavelength of the quantum well photoinduced spectrum is 835 nm. The top part is 20 pairs of λ/4 thickness Al0.12Ga0.88As/ Al0.90Ga0.10As as the top DBR. Additionally, there is an oxidation limiting layer (Al0.98Ga0.02As) between the active region and the top DBR, which can be used for current and light field restriction after the wet nitrogen oxidation process. And an oxide aperture with a radius of 20um has been formed by controlling the wet oxidation time. After the fabrication of VCSEL mesa is completed by lithography, the alignment liquid is spin-coated on the surface of the device and the friction orientation method is used to perform friction orientation along the Y direction, so that the LC is filled and evenly arranged along the Y direction The LC layer groove is prepared by mixing a 3µm spacer and the upper surface of LC layer is an indium tin oxide (ITO) film coated with an orientation solution and oriented along the X direction (orthogonal to the Y direction). The LC is accurately dropped into the preset filling hole using a small syringe. According to the principle of capillary phenomenon, LC gradually penetrates into the LC layer until the entire LC layer is filled. The LC is arranged in a certain arrangement under the guidance of friction.So that the LC molecules injected into the LC layer rotate orthogonally. And the integrated surface LC-VCSEL laser is surface-mounted on a printed circuit board (PCB) version using conductive silver adhesive.
Fig. 1. Structure and test schematic diagram of surface LC-VCSEL laser. (a) Structure of surface LC-VCSEL. (b) LC layer with voltage. (c) Standing wave field of the surface LC-VCSEL laser (gray line: LC layer refractive index is 1.7; red line: refractive index of LC layer is 1.65; blue line: LC layer refractive index is 1.6).
LC is optically similar to uniaxial crystal and is an anisotropic material. When the incident light is linearly polarized, based on the optical rotation effect of LC, the orthogonally aligned nematic LC will cause the polarized light to rotate along the twisted direction of the molecule, and the rotation angle depends on the orientation angle between the two substrates. Therefore, when the included angle between the pre-orientation of the LC's upper and lower base plates is 90°, the polarization direction of linearly polarized light will also rotate 90° after passing through the LC [17–19]. VCSEL emit two beams of linearly polarized light named TE ([011]) polarized light and TM ([0$\bar{1}1$]) polarized light, with polarization directions perpendicular to each other. As shown in Fig. 1(a), When the driving voltage is not applied to the LC, the polarization direction of the two polarized light beams rotates 90° with the LC molecules after passing through the 90° rotating LC layer. While there is a voltage applied to the LC, due to its electrically controlled birefringence effect, the long axis of the LC molecule begins to rotate with the increase of the applied voltage. When the threshold voltage is exceeded, under the action of the electric field, most of the LC molecules tend to arrange in the direction of the electric field, the original twisted structure is destroyed, and the optical rotation effect disappears. The polarization direction of polarized light passing through the LC is no longer rotated, and the polarization direction of VCSELs when the LC is not integrated is maintained, as shown in Fig. 1(b). So based on the birefringence effect, the function of the surface LC layer is to change the surface state of the upper DBR, thereby regulating the optical feedback of the upper DBR and acting on the internal resonant cavity of the VCSEL laser. We mainly use the LC layer with DBR to form an optical mode to achieve polarization control. The influence of the birefringence effect of the LC layer on the resonator is shown in Fig. 1(c). The results show that the refractive index change of the surface LC layer has a certain influence on the standing wave field of the VCSEL.
3. Experimental results and analysis
3.1 Orthotropic LC affects the polarization output characteristics of VCSELs through the optical rotation effect
Fig. 2 shows the modulation effect of orthogonal nematic LC on the power and orthogonal polarization suppression ratio of VCSELs. In the VCSLE without LC integrated, the orthogonal linear polarization of TE and TM of VCSEL is obviously switched at the current value of 6.5 mA and 8.7 mA. According to the orthogonal polarization suppression ratio formula [20,21]:
Fig. 2. Variation of light power of VCSELs two-line polarized light TE and TM with the current. (a) Unintegrated orthogonal LC. (b) Integrated orthogonal LC. (c) Orthogonal polarization suppression ratio of TE and TM in polarization direction before and after unintegrated and integrated LC.
At this point, the orthogonal polarization suppression ratio is zero, and the polarization direction of TE and TM is switched. After the integrated LC, the TE and TM quadrature linear polarization has no polarization switching point in the entire operating current range, and the orthogonal polarization suppression ratio is always maintained as positive. It shows that there is no polarization-switching phenomenon in the whole operating current range. When the two polarization directions are switched, the orthogonal rotation LC makes the TE and TM polarization directions rotate orthogonally at the same time. So, the low-power polarized light in the two polarization directions is still switched to the low-power polarized light direction, and the high-power polarized light is still lashed to the high-power polarized light direction. In this process, one of the polarization directions is always suppressed, and random switching will not occur. Therefore, the nematic LC integrated with orthogonal rotation can eliminate the phenomenon of polarization switching and maintain stable polarized light output.
It is found that the polarization direction of TE/TM orthogonal linearly polarized light output by VCSELs rotates before and after the integration of LC. This is because the optical rotation effect of the orthogonal nematic LC causes the polarization direction of the incident linearly polarized light to spin along the twisted direction of molecules. This phenomenon can be explained by Fig. 3. Due to the optical rotation effect of the LC, when the upper and lower orientations of the LC rotate 90°, the polarized light will also rotate 90° when passing through the LC. When the surface LC is not integrated, when the polarization of the two orthogonally polarized light is switched, the TE and TM polarized light will rotate 90°, and the TE polarized light will follow the polarization direction of the original TM polarized light. Similarly, TM polarized light will also follow the direction of the original TE polarized light. At this time, the two polarized light with polarization switching rotates 90° again after passing through the LC. The TE polarized light will rotate again from the original TM polarized direction to the original TE polarized direction, and the TM polarized light will also return to the original TM polarized direction. As shown in Fig. 3 (a), when the injection current is small, the TE polarized light power is dominant. After passing through the LC, the two polarized light rotates 90°, and the TM polarized light dominates, as shown in Fig. 3(b). When the injection current is I1, the polarization of the polarized light is switched, and the TE polarized light is switched from low power polarized light to high power polarized light. After passing through the LC, the TE and TM polarized light will rotate 90° at the same time, and the TE polarized light will rotate from high power to low power polarized light again. Similarly, when the injection current is I2, the TE polarized light still remains low power without switching. In this process, one of the polarization directions is always in a suppressed state and no random switching occurs. Therefore, the two orthogonally polarized light, whether it is a single switching or frequent polarization switching, will maintain a single polarization state without polarization switching after passing through a 90° nematic LC. If the two polarized light itself does not undergo polarization switching, after passing through the LC, the two polarized light will rotate 90° and maintain a stable single polarization [22,23].
Fig. 3. (a) Polarization switching without LC. (b) Polarization switching is eliminated after integrating 90° oriented nematic LC.
However, the LC molecules are arranged along the electric field direction when a voltage is applied. The polarization direction of the incident linearly polarized light no longer rotates, and the optical rotation effect disappears. As shown in Fig. 4(a)-(b), the LC molecules will stay rotated orthogonally without voltage. The polarization direction of the incident light wave will rotate orthogonally under the action of the orthorhombic LC molecules. At this time, the TE polarization direction dominates, and the TM polarization direction is suppressed. When adding a threshold voltage of 7 V on the LC, the LC molecules will be completely deflected and arranged in parallel along the electric field direction. After the incident light passes through the LC, the incident light's polarization direction will not rotate. The TM polarization direction, which was originally at a disadvantage, gradually becomes dominant, and the TE polarization direction is suppressed, thus effectively realizing the conversion of the polarization direction of the light beam. To adjust the polarizer's polarization direction parallel to the TM polarization direction, make the necessary adjustments. When no voltage is applied to the LC, the dominant TE-polarized light passes through the polarizer after 90° rotation under the optical rotation effect of the orthogonal nematic LC. At this time, the polarization switch is on. TM polarised light's power increases linearly with the current increase, and the maximum optical power can reach about 1 mW. When the threshold voltage is applied to the LC, the LC molecules are completely deflected and aligned along the electric field direction, and the polarization switch is turned off. It can be observed that the optical power is basically unchanged with the current, and the maximum optical power is only 8µW. As shown in Fig. 4(c), the integrated orthogonal nematic surface LC-VCSEL laser has a high switching contrast ratio of 164:1. The polarization switch repeatability experiment of the integrated surface LC-VCSEL laser is shown in Fig. 4(d). The threshold voltage is intermittently applied to the LC with a cycle of 5s. In the repeatability experiment, it can be shown that the polarized light produced by the surface LC-VCSEL alternates between on and off continuously. It has been demonstrated that controlling the polarization switch with the optical rotation effect of LC has a high switching ratio and good reliability.
Fig. 4. The power-current curves of surface LC-VCSEL laser in two polarization directions. (a) without voltage applied to the LC layer. (b) with voltage applied to the LC layer. (c) The optical power of VCSEL before and after deflection voltage applied to the LC layer. (d) Polarization switch and reliability test.
3.2 Electronically controlled birefringence characteristics of orthotropic LC are used to control the polarization output characteristics of VCSEL
Figure 5 shows the influence of LC’s electrically controlled birefringence on the output characteristics of TE and TM polarization. With the increase in LC deflection voltage, the lasing threshold current of the optical power in the TE polarization direction rises, but the optical power decreases gradually. The laser threshold current of the TM polarization direction remains stable with the voltage increase while the optical power increases gradually. When the deflection voltage of the LC molecules is not reached, the LC molecules are arranged in parallel in the layer, and the deflection angle is 0°. When the voltage applied to the LC reaches the deflection voltage, the LC molecules begin to deflect with the increase of deflection voltage, and the deflection angle gradually changes from 0° to 90°. Thus, the refractive index nTE of TE polarization direction gradually decreases. The refractive index of TM polarization direction will not be affected by the applied electric field of the LC, and its refractive index nTM will always remain unchanged. When the deflection voltage reaches the threshold value, the deflection angle of the LC molecules is 90°, and the maximum torque has been reached and no longer changes with the increase of the deflection voltage. The deflection angle range of the LC molecules is 0° to 90°, as shown in Fig. 6(a)-(b). The reflectivity of TE/TM in the two polarization directions is simulated. It is evident that as the refractive index changes toward TE polarization, the reflectivity Rt (TE) gradually decreases. Since the refractive index in the TM polarization direction remains unchanged, its reflectivity Rt (TM) also remains unchanged, as shown in Fig. 6(c).
Fig. 5. Changes of threshold current in two polarization directions of surface LC-VCSEL laser with deflection voltage applied to LC. (a) TE polarization direction. (b) TM polarization direction. (c) Changes of threshold currents of TE and TM polarization directions with deflection voltage applied to LC.
Fig. 6. (a) The LC molecular deflection angle variation with different bias voltage. (b) The LC birefringence variation with different bias voltage. (c) The variation of reflectivity Rt(TE) and Rt(TM) with the refractive index of LC in TE and TM polarization directions. Inset: a simulation model of surface LC-VCSEL laser reflectivity.
According to the threshold gain formula of different polarization directions of TE and TM: [21]
From the formula (2), (3) and Fig. 7, it can be seen that, as the refractive index of LC to linearly polarized light in the TE direction decreases, the threshold gain of the TE polarization direction also increases. This causes the lasing threshold current of the TE polarization direction optical power to increase, which then causes the optical power to gradually decrease. For TM polarization direction, its refractive index does not change with the voltage increase, and its threshold gain remains unchanged. The TM polarization direction, which has been suppressed in the mode competition, gradually dominates, and the optical power in the stable TM polarization direction is stronger than in the TE polarization direction. It is proved that the optical rotation effect of orthogonal nematic LC can not only eliminate the polarization switching of VCSELs but also achieve stable polarization output and make the surface LC-VCSEL laser have a good switching ratio. In addition, it is discovered that the electrically controlled birefringence of LC has a good capacity for VCSEL spatial polarization direction selection and control.
Figure 8 show the study of the far field, TE/TM optical power in both polarization directions, orthogonal polarization suppression ratio and the range of polarization angle regulation capability of the VCSELs laser using the electronically controlled birefringence property of the LC. The TE/TM polarization direction does not change when the surface LC-VCSEL laser voltage is between 0 and 5 V because the LC molecules’ deflection voltage has not yet been reached. The TE and TM optical powers are 530µW and 21µW, respectively. The optical power fluctuates very little in this range, the far-field spot morphology does not change significantly, and the orthogonal polarization suppression ratio is relatively stable and always positive. The TE and TM polarization angles are 80° and 170°, respectively, and the angle between the two polarization directions is positive 90° without obvious changes. The TE polarization direction is dominant and stable at this moment, but the TM polarization direction is suppressed. When the LC voltage increases to 5-6 V, the LC molecules start to deflect with the increase of applied voltage. The refractive index nTE of TE polarization direction gradually decreases with the deflection of LC molecules. The optical power of the TE direction, which has the advantage, gradually decreases, which makes the polarization angle of two orthogonal polarization directions of VCSELs gradually change.
Fig. 8. (a) Changes in far-field divergence angle, optical power and polarization reduction ratio with the voltage applied to the LC. (b) Changes in TE/TM polarization angle with the voltage applied to the LC. (c) Changes in angle between TE/TM polarization directions with the voltage applied to the LC.
The orthogonal polarization suppression ratio and the angle between the two polarization directions of the surface LC-VCSEL laser gradually decrease with the increase of the applied voltage, indicating that the polarization state changes with the rise in the applied voltage. At this time, the shape of the far-field spot begins to change, and the orthogonal polarization suppression ratio and the angle between the two polarization directions are positive. When the voltage value reaches 6 V, the two polarization directions switch. The polarization angle of TE/TM is 124°. The orthogonal polarization suppression ratio and the angle between the two polarization directions are zero, and the surface LC-VCSEL laser output is stable single-polarized light. After that, when the LC voltage is further increased to 6-7 V, the TM polarization direction, which was originally at a disadvantage, gradually dominates with the deflection of LC molecules. The shape of the divergence angle of the far field spot begins to restore the initial state, the polarization suppression ratio and the angle between the two polarization directions are negative and show a decreasing trend, which indicates that the two polarization states are switched from the original TE polarization direction to the TM polarization direction. The optical power in the TE/TM direction stabilizes at LC voltages of 7-12 V. The optical power in the TM direction is 527µW, which is greater than the optical power in the TE direction, and the optical power in the TE direction is 8µW. The TE and TM direction polarization angles are 169° and 81°, respectively. The morphology of the far-field spot is restored to the original state and remains unchanged. The orthogonal polarization suppression ratio and the angle between the two polarization directions are negative and show a decreasing trend. The orthogonal polarization suppression ratio and the angle between the two polarization directions are kept stable, and the polarization angle between the two polarization directions is negative 90°. It is demonstrated that the polarization angle of the VCSELs in both polarization directions can be regulated by controlling and the regulation range is between 0° and 90°.
4. Conclusion
This paper integrates an orthogonal rotating LC with a VCSEL laser to prepare an 850 nm integrated surface LC-VCSEL laser. The optical rotation effect of orthogonal rotary LC and the regulatory capacities of electric-controlled bi-retraction characteristics on the polarization of VCSELs are studied in the paper. The results show that because one of the polarization directions of the integrated surface LC-VCSEL laser is always inhibited over the entire range of working current, no random switching of two polarization directions occurs, eliminating the phenomenon of polarization switching, and always maintaining a stable polarized light output. The integrated surface LC-VCSEL laser has good polarization switch contrast and reliability. The switch ratio can be up to 164:1, and the switching process can be repeated. By controlling the LC voltage, the polarization angle in the two polarization directions is well adjusted, and the polarization angle tuning range is between 0° and 90°. In conclusion, the orthorhombic rotary LC materials in VCSELs eliminate the polarization switching phenomenon and realize stable polarization output. By adjusting and controlling the LC voltage, it achieves the role of adjusting the polarization angle of VCSELs and forms an effective polarization control mechanism. Moreover, the integrated surface LC-VCSEL laser also has good coordination and reliability of the polarization switch, realizing good regulation and control.
Funding
National Natural Science Foundation of China (60908012, 61775007, 61575008); Beijing Municipal Natural Science Foundation, (4172011); Beijing Municipal Education Commission (040000546319525).
Acknowledgments
This work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Beijing City, China, and the Beijing Municipal Commission of Education of China.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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