Performance Characterization of a GaAs Based 1550 nm Ga 0.591 In 0.409 N 0.028 As 0.89 Sb 0.08 MQW VCSEL Md. Asifur Rahman, Md. Rabiul Karim, Jobaida Akhtar, Mohammad Istiaque Reja * Department of Electrical and Electronic Engineering, Chittagong University of Engineering and Technology, Chittagong-4349, Bangladesh istiaque@cuet.ac.bd (Received 03 rd February, 2018; Revised 21 st March 2018; Accepted 25 th March, 2018; Published: 02 nd April, 2018) Abstract- In high speed optical data communication networks vertical cavity surface emitting laser (VCSEL) is becoming a vital device. In this paper, the characteristics and performance of a recently designed GaAs-based 1550 nm Ga 0.591 In 0.409 N 0.028 As 0.89 Sb 0.08 multiple quantum well (MQW) VCSEL have been investigated and analyzed. The steady state carrier density is found to be 2.811 10 18 cm -3 and the steady state photon density is obtained as 1.129 10 15 cm -3. The steady state output power obtained for this VCSEL is 84.5 mw. Maximum modulation bandwidth of 20.5 GHz is obtained at 16 ma injection current. Key Words: VCSEL, carrier density, photon density, modulation bandwidth, material gain. 1. INTRODUCTION High speed optical data communication requires ideal optical sources. Due to the limitations like interference and pulse spreading of light pulses of LED, they were gradually replaced by LASERs. In the development process of LASERs, the most recent advancement is vertical cavity surface emitting laser (VCSEL). VCSELs offer highly coherent, monochromatic and focused light beam with high intensity. VCSEL is considered as a highly efficient optical source because of having these properties. VCSELs of different wavelengths have significant usefulness in different particular applications. Vertical cavity surface emitting lasers (VCSELs) have wide variety of applications in optical data communication systems recently. The importance of VCSELs to the optical data communication industry is growing fast. VCSELs based on compound semiconductors have been mostly designed for wavelength of 650-980 nm and VCSELs having wavelengths above this range is used for long distance data communication [1]. Recently there has been progress in VCSEL development process that operate at the range from 1310 to 1580 nm wavelength which is required for long-distance fiber optic communications [2]. Design and performance analysis of error free optical links at a high data rate of 0.6 Tb/s and 1 Tb/s using 683 nm VCSEL and 1550 nm VCSEL respectively are reported in recent times [3, 4]. High temperature operation of VCSEL is a big challenge which is addressed in many studies where temperature performances up to 44ºC at 1550nm at 1Tb/s bit rate [4], 80ºC at 1300nm and 1550nm [5], 85ºC at 980nm [6], 115ºC for red VCSEL [7], 128ºC at 1550nm at 15Gb/s rate [8] are demonstrated. VCSEL require very low threshold current as it has high mirror reflectivity and active region of very small volume. Very high-speed data communication at a rate around 10 Gbit/s is possible due to this little threshold current [9]. VCSELs of long wavelength possess unique characteristics like narrow beam divergence, very low power consumption, and ease of fabrication. Coupling VCSEL devices to optical fiber becomes easier because of their less divergent output beam [2]. Long wavelength (LW) VCSELs are grown mostly on GaAs and InP substrates. GaAs based LW VCSELs are widely used because they can be grown using epitaxial growth technique. InP based LW VCSELs have been designed for long distance communication but they show poor performance at wide temperature range [10]. GaAs based VCSELs have superior performance over InP based LW VCSELs [11]. InP based materials exhibit poor temperature properties. They show band gap discontinuity at high temperature, need costly cooling mechanism. On the other hand, GaAs has high refractive index contrast, low lattice mismatch and high thermal conductance [10]. InGaAsP/InP system has several critical limitations to be used for VCSELs in the long wavelength range, which include poor combination of alloys that have much lattice mismatch and cannot provide a large difference in refractive index for the distributed bragg reflectors (DBRs) required for VCSELs with reasonable thermal conductivity [9]. There are a few previous works which demonstrated GaInNAsSb/GaAs LW VCSELs operating at wavelengths greater than 1500 nm. Korpijärvi et al. designed 1550 nm GaInNAsSb/GaAs external cavity VCSEL [12]. They designed 1550 nm ridge waveguide laser using this materials for photonic integrated circuits [13]. They showed progress on 1550 nm dilute nitride lasers [14]. M. A. Wistey et al. designed GaInNAsSb/GaAs VCSEL operating at 1534 nm [15]. Recently we have designed a GaInNAsSb/GaAs MQW VCSEL operating at 1550 nm and improved material gain coefficient, photoluminescence spectra and peak output power [16]. There have been a few works on InP based LW VCSELs which operate at 1550 nm. A. Islam et al. designed InP based GaInAsN VCSEL with AlGaInAs barriers. They obtained maximum resonance frequency as 22 GHz for a maximum injection current of 45 ma [11]. T. Roy et al. designed 1550 nm Al 0.09 Ga 0.38 In 0.53 As/InP VCSEL by varying injection current. They obtained maximum resonance frequency as 12.3 GHz for 16.1 ma injection current [17]. R. R. Hasan and R. Basak showed characteristics of a designed 1550 nm Page 14
AlGaInAs/InP VCSEL and obtained maximum modulation bandwidth as 19.5 GHz and maximum resonance frequency as 12.4 GHz for 16.5 ma injection current [18]. A. V. Babichev et al. presented characteristics of an InP based single mode wafer fused 1550 nm VCSEL and they obtained maximum modulation bandwidth as 9 GHz for 10 ma injection current [19]. In this paper, the performance characteristics of our recently designed 1550 nm Ga 0.591 In 0.409 N 0.028 As 0.89 Sb 0.08 /GaAs MQW VCSEL have been analyzed. Analyzed characteristics have been presented and compared with the performance of previous works. 2. THE VCSEL STRUCTURE A. ACTIVE REGION The active region has 4 QWs of GaInNAsSb. Each QW is separated by GaNAs potential barrier layer. The thickness of each Ga 0.591 In 0.409 N 0.028 As 0.89 Sb 0.08 QW is 8 nm and the thickness of each GaN 0.047 As barrier layers is 24 nm. The active materials are sandwiched by cladding layers that are used for compensating the compressive strain [10]. In this structure, GaAs is used as cladding layer at top and bottom of active region. For the QW material refractive index is 3.6893 and for the potential barrier layer it is 3.5436. The schematic structure of this designed VCSEL is shown in Fig. 1 [16]. B. DBR MIRROR GaAs/AlAs DBR mirror is used in this structure. For n- DBR, 32 layers of GaAs/AlAs are used and for p-dbr 22 layers of GaAs/AlAs are used. Thickness of GaAs and AlAs layers are 113 nm and 132 nm respectively. At 1550 nm, the refractive index of GaAs and AlAs is 2.92 and 3.43 respectively [16]. Doping concentration of both the p-dbr and n-dbr material is 3 10 24 cm -3. Bandgap of GaAs and AlAs is 1.42 ev and 2.12 ev respectively. C. CAVITY LENGTH AND VOLUME In VCSEL, the cavity length has to be 1.5 λ. So, in this structure for 1550 nm VCSEL the desired cavity length is 2325 nm. The effective cavity length L eff is obtained adding the thicknesses of the layers of active region and the effective contribution value obtained from the top and bottom DBR mirrors. Then L eff is obtained as 2328 nm which is close to the desired cavity length. The optical length of 4 QWs is the active cavity length L active which is 47.85 nm. The active region radius is used as 8.4 x 10-4 cm. The active volume is then obtained as V a = 3.77x10-12 cm 3. The value of computed transparency carrier density N tr is obtained as 2.85 10 18 cm -3. 3. CHARACTERISTICS AND PERFORMANCE OF THE VCSEL A. CONFINEMENT FACTOR AND THRESHOLD CARRIER DENSITY The optical confinement factor is determined from (1) [20]. Here the enhancement factor is 2, 90% confinement is taken in the transverse direction. The value of optical confinement factor is 0.037. Lactive 2 0.9 L eff Where, L active is the length of the active region. The value of threshold carrier density is obtained from (2) [20]. i m N th N tr exp (2) g0 Where, i = internal loss, m= mirror loss and g0 = gain coefficient. The value of threshold carrier density N th is obtained as 1.829 10 18 cm -3. Fig. 1: Schematic structure of the VCSEL (1) B. PHOTON LIFETIME AND THRESHOLD CURRENT The photon lifetime p is calculated using (3) and the value obtained is 2.77 10-12 s [20]. 1 p v ( ) (3) g i m The threshold current of this VCSEL is computed using Eqn 4 [20]. I th qvanth (4) i c For a current injection efficiency i of 0.8, a carrier lifetime c = 2.71 10-9 s and V a = 3.77x10-12 cm 3, the threshold current I th is obtained as 0.7 ma. C. STEADY STATE CARRIER DENSITY AND PHOTON DENSITY The rate of change of carrier density is calculated using (5) [20, 21]. Page 15
volume of the active region. N and c is the carrier density and carrier lifetime respectively. vg is group velocity and a is the differential gain, Ntr and S is the transparency carrier density and photon density respectively. is the gain saturation parameter of a laser. Fig. 2: Carrier density vs. time for the VCSEL Fig. 5: Relative response vs. time for the VCSEL. Fig. 3: Photon density vs. time for the VCSEL. Fig. 6: Material gain vs. current density for the VCSEL dn dt ii qv a Fig. 4: Output power vs. time for the VCSEL. N vga NN 1S c tr Where, i and I is the injection efficiency and injection current respectively. q is the electron charge and V a is the S (5) The carrier density is plotted against time and presented in Fig. 2. From this figure, steady state carrier density of the VCSEL is obtained as 2.811 10 18 cm -3 and it is obtained after 1.2 ns from the start of operation. The rate of change of photon density is calculated using (6) [20, 22]. ds vganntrs iith S sp (6) dt 1S q Where,, sp p Ith and p is the confinement factor, spontaneous emission coefficient, threshold current and photon lifetime of the laser respectively. Photon density is plotted against time and depicted in Fig. 3. From this figure, the steady state photon density of this VCSEL is obtained as 1.129 10 15 cm -3 and it is obtained after 1.349 ns from the start of operation. Page 16
Table 1: Comparison of performance characteristics with previous works Literature Wavelength (nm) QW Material Steady State Carrier Density (10 18 cm -3 ) Steady State Photon Density (10 15 cm -3 ) Corresponding Injection Current (ma) Max. Resonance Frequency (GHz) Max. Modulation Bandwidth (GHz) Corresponding Injection Current (ma) [11] 1550 GaInAsN 3.5 0.6 15 22-45 [17] 1550 AlGaInAs 2.831 1.09941 16.1 12.3-16.1 [18] 1550 AlGaInAs 2.837 1.59941 8.5 13.4 19.5 16.5 [19] 1550 InGaAs - - - - 9 10 Our Work 1550 GaInNAsSb 2.811 1.129 16 13.5 20.5 16 D. OUTPUT POWER AND MAXIMUM MODULATION BANDWITCH The output power of the VCSEL is obtained using (7) [20, 21]. P v hsv (7) out g m p Where, Vp is the volume of cavity, is the lasing frequency. Output power is plotted against time and presented in Fig. 4. From this figure, the steady state output power of the VCSEL is obtained as 84.5 mw and it is obtained after 1.36 ns from the start of operation. The relative response transfer function of the VCSEL is related to the frequency f R and damping parameter and it is expressed as (8) [20, 21, 13]. 2 fr H( f ) (8) 2 2 f fr f j 2 The relative response of this VCSEL is calculated by using (8) varying frequency for different values of injection current. The obtained results are plotted and shown in Figure 5. From this figure, it is observed that the modulation bandwidth (-3 db cutoff frequency) increases with the increase of injection current. For 6 ma injection current modulation bandwidth is obtained as 13.5 GHz. For 9 ma the modulation bandwidth obtained is 15.5 GHz. For 11 ma the modulation bandwidth is obtained as 17.5 GHz. Maximum value of modulation bandwidth 20.5 GHz is obtained for 16 ma injection current. It is also observed from Fig. 5 that as the injection current increases, the resonance frequency increases. For injection current of 6 ma the resonance frequency obtained is 8 GHz. For 9 ma the resonance frequency is obtained as 10.5 GHz. For 11 ma the resonance frequency is obtained as 12 GHz. Maximum value of resonance frequency 13.5 GHz is obtained for 16 ma injection current. Material gain vs. current density characteristics for this VCSEL is simulated using photonic integrated circuit simulator [24] and shown in Fig. 6. It is observed that a current density of 180 A.cm -2 is required for attaining material gain coefficient of 4400cm -1 that was obtained for this VCSEL [16]. Table 1 shows the comparisons of performance characteristics of this work with that of previous works. Improvement is observed regarding maximum resonance frequency and modulation bandwidth considering injection current. 4. CONCLUSION In this work, performance characteristics of a 1550 nm Ga 0.591 In 0.409 N 0.028 As 0.89 Sb 0.08 /GaAs VCSEL are presented. The value of optical confinement factor is 0.037. The value of photon lifetime obtained is 2.77 10-12 s. The value of threshold current obtained is 0.7 ma. The steady state carrier density of the VCSEL is obtained as 2.811 10 18 cm -3. The steady state photon density of the VCSEL is achieved as 1.129 10 15 cm -3. The steady state output power of the VCSEL is obtained as 84.5mW. A maximum modulation bandwidth of 20.5 GHz and maximum resonance frequency of 13.5 GHz is obtained at 16 ma injection current. The higher modulation bandwidth obtained for this VCSEL ensures superior performance at hi-speed data transmission through optical fiber. REFERENCES [1] K. Iga, Surface-Emitting Laser Its Birth and Generation of New Optoelectronics Field, IEEE J. of Sel. Topics in Quant. Elect., vol. 6, no. 6, pp. 1201-1215, 2000. [2] D. F. Welch, A Brief History of High-Power Semiconductor Lasers, IEEE J. of Sel. Topics in Quant. Elect., vol. 6, no. 6, pp. 1470-1477, 2000. [3] M. T. Hannan, M. I. Reja, and J. Akhtar, Design and performance analysis of 0.6Tb/s 863nm VCSEL based optical link, Proc. IEEE 3rd International Conference on Electrical Information and Communication Technology (EICT), pp. 1-5, Dec. 2017, [4] M. T. Hannan, M. Asaduzzaman, M. I. Reja, and J. Akhtar, Performance analysis of 1 Tb/s RZ modulated Page 17
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