Modeling of Low Power Multilayer Vertical Cavity Surface Emitting Laser

Transcription

1 International Journal of Optics and Applications 15, 5(5): DOI: 1.593/j.optics Modeling of Low Power Multilayer Vertical Cavity Surface Eitting Laser Shashad Akther Khan *, M. A. Hasnayeen Coputer Science and Engineering, International Islaic University Chittagong, Chittagong, Bangladesh Abstract An Aluinu Indiu Galliu Indiu Phosphate based vertical cavity surface eitting laser structures with precisely adjusted resonated-cavity wavelengths. DBR odeling has been also ipleented in this paper. The theory of optical ultilayer is reviewed and the Characteristics Matrix ethod has been used to calculate the ultilayer. All the calculation was perfored using Matlab. Keywords VCSEL, Quantu well, DBR, Cavity, Refractive index, Multi-Quantu well, Reflectivity, Transisitivity, Resonator, Bragg wavelength 1. Introduction The developent of AlGaInP quantu well lasers has been driven by the requireent for short wavelength sources in optical disc inforation storage systes, the general utility of copact, battery powered visible sources and use as pup sources in solid state laser systes. Such devices are also of use in short haul optical data links using polyer fibers and waveguides and as light sources in photodynaic therapy. The applications have led to the requireent for enhanced functionality in these structures. Several aterial related liitations ake the design of the short wavelength lasers difficult and further progress requires fabricated structures and integrated with other syste coponents such as filters and detectors. At the present tie the short wavelength of ost coercial services is 635n, and is liited to low power. However, shorter wavelength lasers are required, particularly for data storage, and in the treatent of cancer using 63n absorption and PDT drugs. The objective is to reduce the use of nuber of DBR layer and increase the reflectivity. The whole task was carried out by 635n wavelength. Since we have carried out the whole work by dividing it into three parts Appendix A, Appendix B and Appendix C. We shall ention our objectives based on the respective appendix. In appendix A, Choosing the appropriate aterial cobination for first layer, second layer, substrate layer and to provide reflectivity not less than 89% with less nuber of DBR layer are the ain objective. Where as in appendix * Corresponding author: shashad.cse9@gail.co (Shashad Akther Khan) Published online at Copyright 15 Scientific & Acadeic Publishing. All Rights Reserved B, Choosing the appropriate aterial cobination for well and barrier and to provide reflectivity not less than 89% with appropriate nuber of QW are the ain objective. In appendix C, finding out the exact position to place the resonator are the ain objective.. VCSEL Fundaantals The vertical-cavity surface eitting laser (VCSEL) is a type of seiconductor laser device eits laser perpendicular to the chip surface contrary to its counterpart edge eitting seiconductor lasers (EELs). The vertical-cavity surface eitting laser (VCSEL) is a relatively new seiconductor device aong the other optoelectronics devices..1. VCSEL Structure Typical VCSEL doinant structural coponents can be divided into two coponents: the upper and botto DBRs, and the centre active region. These DBRs will create a high-finesse Fabry-Perot cavity within, which are placed integer ultiples of half wavelength-thick (λ/n) quantu-well active regions [8, 4]. On the other hand DBRs consist of repeating pairs of quarter-wavelength-thick high and low-refractive index seiconductor layers. Therefore by cobining the ultiple quarter-wavelengths thick of high-to-low refractive index layers will result in a axiu reflectance greater than 99% [8]... VCSEL Versus EEL Vertical Cavity Surface Eitting Laser (VCSEL) has any fascinating advantages over Edge Eitting Laser (EEL) that are: (1) The eitted bea characteristics of VCSEL can be controlled by the nuber of paraeters like nuber of DBRs, thickness of DBRs, type of aterial used to fabricate

2 156 Shashad Akther Khan et al.: Modeling of Low Power Multilayer Vertical Cavity Surface Eitting Laser the DBRs and other process paraeters. This kind of fine tuning is difficult to find in EELs. () The typical size of eitting circular region of VCSEL is about 5 to 5μ in diaeter that too short in contrast to eitting region of EEL. (3) A typical EELs requires 3A threshold for teleco applications while VCSELs requires 1 to A threshold current leads to low operating voltages. These all features push forward to use VCSELs for developent of high speed optical buses and interconnect [7]. 3. DBR for VCSEL Distributed Bragg reflectors consist of epitaxially grown, alternating periodic quarter-wavelength stacks of low and high refractive index seiconductors, which are used to provide high irror reflectivity in nuerous optoelectronic and photonic devices such as VCSEL. Finally, we need to look into the reflectance and transittance at an interface. Next, calculate the ultilayer using atrix Reflection and Transission of DBR A DBR is a structure fored fro ultiple layers of alternating aterials with varying refractive index, or by periodic variation in the effective refractive index in the guide. The DBR s reflectivity, R for intensity is approxiately given by N N n( n) n ( 1) [ s n R = ] (1) N N n( n) + ns ( n1) Where n, n1, n and n s are the respective refractive indices of the originating ediu, the two alternating aterials, and the terinating ediu and N is the nuber of repeated pairs of low or high refractive index aterial [6]. 3.. Methods for Calculating the Reflection Coefficient There are several ethods to calculate the reflection coefficient of a ultilayer stack, here transission ethod will be used. The phase change of electroagnetic radiation passing through the th layer. πndcos θ δ = () λ Where λ is the wavelength of the radiation, d is the thickness of the th layer that is expressed by. λ d B = (3) 4n Where λ B is the Bragg wavelength and n is refractive index of th layer [1] Reflectance and Transittance at the Interface The reflectance R and transittance T are defined as the ratio of the reflected and transitted waves respectively to the incident wave. Because the aplitudes of the reflected and transitted wave are coplex quantities so their arguents gives us phase response of reflected and transitted wave. The reflected or transitted light can be deterined by applying the boundary conditions to the Maxwell s equation [7]. We can obtain the ratios of the transitted and reflected aplitudes to incident aplitudes for each coponent of polarization that give us the expressions for the Fresnel coefficients of reflection and transission [7]. n n R 1 s = (4) n + n1 4nn T 1 s = (5) ( n + n1) Where R s is the reflection coefficient and T s is the transittance coefficient. R T = 1 (6) s + s The above expression is valid when n and n 1 are real. Figure 1. Scheatic diagra of VCSELs and EELs [9]

3 International Journal of Optics and Applications 15, 5(5): Figure. Reflection Phenoena at an interface [7] Figure 3. A Typical Resonator Structure 3.4. Transission Matrix Forulation for Multilayer The atrix ethod is easiest and versatile ethod for calculating the spectral coefficients of the layered edia. For the odeling of DBRs, if MT represents the product of characteristic atrices of the irrors and M i=1, represent the individual layers in a stack then [7]. p M T = ( M1* M* M1* M) (7) Where p is the nuber of periods, Mi is the characteristic atrix and is given by. M cos( kd r r + jαrdr) jsin( kd r r + jαrdr) = jnrsin( kd r r + jαrdr) cos( kd r r + jαrdr) (8) The above equation gives an expression to calculate coefficient B and C. Finally the transittance, the reflectance, the phase changes respectively can be calculated. T = R = 4* n n * B+ C s n n * B C n * B+ C n * B C ψ = arg n * B+ C (9) (1) (11)

4 158 Shashad Akther Khan et al.: Modeling of Low Power Multilayer Vertical Cavity Surface Eitting Laser 3.5. VCSEL Resonator Design All Deterination of the percentage of transissivity and reflectivity, copletes just one part of the work. VCSEL not only consists of DBR irrors, they also contain centre active region. So the results ust be deterined based on the resonator incorporated with ultilayer. Finally, the odified Characteristic Matrix involves calculation for upper layers, cavity region and botto layers. MTotal = ( Mu )*( Mc )*( Mb ) (1) Where M u is the characteristic atrix of upper layers, M c is the characteristic atrix of cavity region and M b is the characteristic atrix of botto layers. The centre dip represents the Bragg wavelength, 635n. We can achive the reflectivity of 3% at 635n. In Transiitivity versus Wavelength graph we obtain approxiately 7% value at 635n. The results satisfies the requireent as exhibited in equation (9) and (1) Electric Field Intensity Various studies regarding high power laser irrors have deonstrated that daage of ultilayer is caused by high intensity due to current density absorption. So it is iportant to find this region and overcoe such kind of daage by the reducing the absorption in the ultilayer. To do so we need to find the intensity inside the ultilayer [3]. Figure 5 illustrates the concept of calculating the intensity inside the ultilayer. The positive and negative going EM wave at soe point z, where z is greater than z i but less that z i+1 in the i th layer are related to aplitude reflection and transission coefficient at the interface of that layer. Using this positive and negative EM the intensity can be calculated [3]. ( ) 1 + I z = Ei ( z) + Ei ( z) (13) 4 Where the intensity is found high, the daage occurs in that interface, this region is where the ost of the current density is absorbed. In order to control the peak of the electric field intensity, a resonator is included in the ultilayer. This resonator helps to equalize the peak electric field of the interface and increase the reflectance. Figure 4. Reflectivity versus Wavelength Figure 5. Field Intensity Calculation [3]

5 International Journal of Optics and Applications 15, 5(5): Figure 6. Electric Field Intensity Distribution with Resonator Figure 6 shows the electric field intensity inside a AlGaInP VCSELs. It can be observed that strong electric field intensity occurred in the resonator and the electric field intensity starts to diinish after it has passed the resonator region. Fro this inforation, we can correctly deterine where to place our active region in the AlGaInP VCSELs. 4. Conclusions This paper has shown that by using Matlab, optial results can be obtained. It has been shown that by siply adjusting the nuber of irrors, it will significantly affect the overall reflectivity as well as transissivity. Moreover, it can be noted that by changing the substrate ediu, different reflectivity and transissivity results can be obtained. Lastly, the electric field inside the ultilayer will be reduced and finally disappear at layer 9-3. One ight look forward for following future work. Deterining the electric field intensity at non-noral incidence, Deterining the electric field intensity for different wavelength, Methods of reducing the peak electric field intensity, Deterining an optiu resonator for VCSEL, Develop the rate equation, Deterine the output of the LASER via Rate Equation. ACKNOWLEDGEMENTS Introducing the topic of y report and what I have learned, I d like to Acknowledge Thanks to all of those who have been involved in creating the present status of the report, besides those who have helped e with physically and verbally. Firstly, uch thanks to teachers, and all those who have run a routine check on the progress of y report. It s partly the, who granted e the idea for building, experience enhanceent, deeper thinking, widening y horizons and building e as a Woen of Toorrow. Secondly, a good aount of thanks, I give to y supervisor, for co-operation and assessent in the thesis process. It s ore to hi, for thesis ideas was proceeding to right direction. Thanks to you for holding y high estees towards the end of the thesis. A woan can never go by doing everything herself, so would I without your help. Thanks, and thanks a lot. Thirdly, bunch of thanks to y parents for helping e financially and spiritually. Without your encourageents, I would have accoplished nothing. Thanks to you for being y always-helping hand. Lastly, I a here with a ission fro God, and it is to be on His path and to acquire His Glory. Knowledge is His path, so is the path I have selected. If not, by thy Glory I would but be helpless creature. Glories to thee and thanks to thee for helping e, and by thy ercy accept y huble work. REFERENCES [1] Babar BASHIR., Designing of high reflectance distributed bragg reflectors (DBRs), irrors using AlGaInN aterial syste in the UV wavelength range, June 9. [] B.E. Newna, Daage resistance of dielectric reflectors for picosecond pulses, Laser Induced Daage in Optical Materials, Nat. Bur. Standards, Washington, DC, USA, xx+35, pp , [3] H.M. Liddell, Coputer-aided techniques for the design of ultilayer filter, Ada Hilger Ltd, Bristol, 198.

6 16 Shashad Akther Khan et al.: Modeling of Low Power Multilayer Vertical Cavity Surface Eitting Laser [4] J.L. Jewell, Vertical-cavity surface-eitting lasers: design, growth, fabrication, characterization, IEEE Journal of Quantu Electronics, Vol.7, pp , June [5] O.S. Heavens, Optical Properties of Thin Solid Fils, Butterworths Scientific Publications, London, [6] Sheppard, C.J.R., "Approxiate calculation of the reflection coefficient fro a stratified ediu". Pure and Applied Optics: Journal of the European Optical Society Part A 4 (5): 665. Bibcode 1995PApOp4..665S. doi: 1.188/ / 4/ 5/ 18, [7] Wee Khiang Ace Tey., Design of low threshold vertical-cavity surface-eitting lasers, Master thesis, University of Queensland, October. [8] W.W. Chow, Design, fabrication and perforance of infrared and visible vertical-cavity surface-eitting Lasers, IEEE Journal of Quantu Electronics, Vol. 33, No. 1,, pp , October [9] ples/saples.htl.