Analysis Of OPS-VECSEL With Azo Materials Optical Control Layer

The reversible isomerization of trans-cis-trans under the linear polarization light, the molecules of Azo Materials have the same tropism which is vertical to the polarization of light. This presents Azo Materials anisotropy in macro scope, which means that it has photo-induced birefringence. The value of birefringence relates to optical power and the polarization angle of the light. It means we can change the birefringence of Azo Materials through modifying the status of light. Based on the characteristic, we design a new type of OPS-VECSEL which controls the polarization and frequency of the ejection laser by using Azo Materials as the optical control layer. The structure model is DBR/ Azo Materials layer /active layer/DBR. (Fig. 1)Design of DBR layerOrdinary OPS-VECSEL has a structure of single DBR, which means that one end use ejection mirror with reflection ability, the other end use the DBR. We use asymmetry double DBR in our Azo OPS-VECSEL. The growing sequence is H…L…H, and central wavelength is 600nm.Design of Optical control layerDifferent needs make us select different Azo material. There is absorption between 200-520nm and strong absorption at 488nm. We just consider the strong absorption at 488nm for simplification. Therefore we use light at the wavelength of 488nm as control light. The birefringence value of optical control layer will change when changing the intensity and polarization. Design of the active layerAccording to research of Wang lijun group in CIOMP, PVK: ALq: DCM1mixture as the active medium, Ar⁺ laser at the wavelength of 514.5nm as the pump laser, with 45°incidence angle.Analysis of controlling polarization and frequency of Azo Materials OPS-VECSELWe introduce the Azo materials into the resonator of OPS-VECSEL torealize optical control function. Through the reversible isomerization of trans-cis-trans under the linear polarization light, the molecules of Azo materials have the same tropism which is vertical to the polarization of light. This presents anisotropy of Azo materials in macroscope, which means that it has photo-induced birefringence. Based on the photo-induced birefringence effect of Azo materials, we could control the polarization and frequency of the ejection laser by using Azo materials as the optical control layer.Perfect VECSEL can be treated as isotropy medium; the fundamental mode in resonant cavity is two orthogonal modes. We call them P-polarization component and S-polarization component. The electromagnetic waves are shown like We should consider establishing the characteristic matrix of optical control layer. Through the reversible isomerization of trans-cis-trans under the linear polarization light, the molecules of Azo materials have the same tropism which is vertical to the polarization of light. This presents Azo materials anisotropy in macroscope, which means that it has birefringence. There are an effective index n_? which is parallel to the polarization of pumping light andan effective index n_? which is vertical to the polarization of pumping light andthe vector. Under this situation, they are no longer equal. The difference between them is the birefringence value.△n = |n(?)-n(?)|, (2)Under the linear polarization light, birefringence was produced. The isotropy medium now has two different optical axises n_? and n_?. Due to theimpact of birefringence, the optical admittances of S-polarization component and P-polarization component are no longer different.The plural indexes of S-polarization component and P-polarization component are as shown,N_p=n_p+i(-g_p/2k₀-K_p),N_s=(n_s+△n) + i(-g_s/2k₀-K_s), (5)According to the transfer matrix method, combining (3), (4) and (5) formula, we have the transmission spectrum of OPS-VECSEL considering the impact of birefringence. To simplify the analysis we ignore the impact of radiation angle. Then the impact of photo-induced birefringence effect of Azo materials on VECSEL with asymmetry DBR structure will be analyzed through analyzing the transmission spectrum of the resonator.Polarization Control (Radiation angle-0)We simulate the transmission spectrum of the whole resonator,according to given parameters as shown in Fig.2. As shown in Fig.2, the birefringence produced by Azo-materials make the degenerated P and S polarization component not longer in degeneration. Azo materials make the degenerated modes changed. The birefringence do not make the transmission spectrum of P-polarization component change, but make the transmission spectrum of S-polarization component far away from 600nm and the transmission rate bigger than the one of P-polarization component. If we just consider ejection laser at the 600nm wavelength, bigger birefringence valve (△n = 0.1 )lead to the ejection laser only has P-polarization component. If we consider the ejection laser at the 620nm wavelength, we could get single polarization ejection laser, which only has S-polarization component. So we conclude that the introduction of photo-induced birefringence effect of Azo materials helps us to control the polarization of the ejection laser. The bigger the value of birefringence is, the more obvious the effect is. Therefore we could see that the optical control layer of Azo materials could control the polarization of the OPS-VECSEL ejection laser.Control of wavelength (Radiation angle=0) The one central wavelength of the ejection laser changes into two central wavelengths (shown in Fig.3), when we use Azo materials as the optical control layer. As the value of birefringence is zero, only 600nm laser exist. The new wavelength's position move to long wave along with the increase of the birefringence value, which means through changing the birefringence value, we could get new laser besides 600nm laser. Therefore, the optical control layer of Azo materials acts as the selection of laser wavelength, when not considering the polarization component. When the radiation angleθand the birefringence value△n are both considered, we analysis the impact of photo-induced birefringence effect of Azo materials on VECSEL with asymmetry DBR structure will be analyzed through analyzing the transmission spectrum of the resonator. Polarization Control (Considering the radiation angle)The transmission rate of P-polarization component and S-polarization component light become bigger along with the increase of the value of birefringence, comparing the analysis when not considering the radiation angle. Whether the radiation angle is introduced or not, it do not influence the polarization control by the photo-induced birefringence of Azo materials. The only difference is that the S-polarization light will totally come out without any effective resonance when the birefringence value goes to△n = 0.08.Control of wavelength (considering the radiation angle)The radiation angle (θ= 13°)did not influenced the wavelength selectiona little. The 600nm wavelength laser never changes when the value of birefringence increases,but the wavelength of the other new laser moves to long wavelength. Besides, the transmission rate becomes bigger. Whether considering the radiation angle, the optical control layer of Azo materials could control the wavelength of the ejection laser.Theoretically when the value of birefringence of Azo material meets 0.05, this type of OPS-VECSEL could have single polarization laser. When it comes to 0.07, the tunable range reaches to 13nm.ConclusionThe value of birefringence value changes when Azo material is illuminated under 488nm control light. We can tune the birefringence value by changing the polarization and intensity of control light. Due to the changes of birefringence value, the wavelength and polarization of the ejection laser also change. Therefore we conclude that, theoretically, OPS-VECSEL with Azo material optical control layer is a new type of polarization control and wavelength tunable laser.