Open Access
Add to BibSonomy
Abstract
Surface emitting lasers are attractive light sources for silicon integrated photonic circuits. High speed direct operation is of great importance for these lasers in high capacity and low cost on-chip communication system. Here, we demonstrate a 1.3 µm surface emitting ridge-waveguide distributed feedback (DFB) laser with second order grating and λ/4 phase shift grating, which can achieve a 24 Gb/s operation over a wide temperature. The fabricated lasers can achieve low threshold current as 6.8 mA, and 12.5 mA at 20, and 70°C, respectively. Stable single mode operation has been observed with high side mode suppression ratio (SMSR) > 40 dB at all temperatures (20-70 °C). Meanwhile, the surface emitting optical power can reach 1.7 mW at high temperature as 70 °C. 3 dB bandwidth of small signal response is 21 GHz and 12 GHz at 20 °C and 70 °C respectively. The far-field divergence angle of surface emitting beam is 13.4°×20.2° of 10 µm length second order grating coupler. The proposed laser may have great advantages of single mode, high speed modulation and good temperature tolerance. In addition, compared with conventional DFB lasers, the surface emitting DFB laser has no additional manufacturing process, which is simple to fabricate and easy to integrate with silicon platform.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Semiconductor laser has high efficiency, high coherence and perfect stability and is widely used in optical communication system, pump source system and so on [1–3]. Owing to the advantages of easy formation of two-dimension arrays and hybrid integration with silicon platform, surface emitting semiconductor laser has attracted great attention [4–6]. Vertical cavity surface-emitting laser (VCSEL), a typical solution of surface emission, can extract light efficiently and has low power consumption [7,8]. But, the realization of VCSEL at long wavelength has been hindered due to the poor optical and thermal properties of DBR materials. Another type is horizontal cavity surface-emitting laser, which benefits from the good performance of conventional horizontal cavity laser like (DFB or DBR lasers) and retains the economical advantages (e.g., on-wafer testing and processing) like VCSEL devices [9–11]. The above performance makes it attractive candidate for source lasers in silicon integrated photonic circuits.
Grating coupled surface emitting distributed feedback (SEDFB) laser, one important type of horizontal cavity surface emitting laser, has obtain widespread application due to its characteristics of single mode, long wavelength, stable polarization, high speed modulation and easy fabrication [12–16]. S.H. Macomber et al. demonstrated an electrically pumped 840 nm SEDFB laser for the first time utilizing a second order grating with the slope efficiency as 0.05 mW/mA [17]. To improve the light output characteristics of the SEDFB laser, D. Botez et al. proposed a second order grating with phase shift grating. Its external differential quantum efficiency excesses 70% with relatively low threshold gains 25-40 cm-1 [11]. For the requirements of direct modulation applications, a previously designed 1.3 µm single frequency SEDFB laser can achieve 2.5 Gb/s modulation rates with slope efficiency as 0.1 mW/mA, but high threshold current as 22 mA [18].
Furthermore, to cope with the complex environment of large capacity and high temperature, semiconductor lasers need to operate at high-speed rates without an extra power consumption, high speed uncooled surface emitting laser becomes a more efficient and low-cost choice to realize compact and low-power-consumptive optical transmitters. The lens integrated surface-emitting laser (LISEL) based on temperature insensitive AlInGaAs material was reported to have 25 Gb/s direction modulation at 85 °C [10]. However, its fabrication technique is complexed and not suitable for mass manufacturing. For easy fabrication, C. Liu et al. proposed an 850 nm short-cavity grating assisted SEDFB laser with a large-area oxidized aperture, which achieves low threshold current as 1.8 mA like VCSELs and high modulation frequency bandwidth as 17 GHz at 20 °C. Limited by the intrinsic temperature sensitive characteristics of InGaAs quantum wells, the dynamic performance at high temperature was not introduced [19].
To combine the characteristics of high-speed modulation, wide temperature operation and simple fabrication process, we here demonstrate a 1.3 µm single-mode SEDFB laser with second order grating and phase shift grating based on AlGaInAs materials, with the threshold current as 6.8 mA and high output power > 4 mW. Meanwhile, large side-mode suppression ratio > 40 dB over 20-70 °C and stable wavelength controlling ability of temperature-dependent wavelength shift $\varDelta \mathrm{\lambda /\Delta T}$ as 0.077 nm/°C have also been realized. Meanwhile, the 3 dB modulation bandwidth f3dB of small signal response can reach 21 GHz. Clear eye pattern opening at 24 Gb/s Non-Return-to-Zero (NRZ) signal and low bit-error rate (BER) were successfully demonstrated at different temperatures (20 and 50 °C). Furthermore, the surface emitting output beam has good quality, whose far-field pattern divergence angles are about 13.4°×20.2° and 9.2°×20.6° of 10 µm and 20 µm length second order grating couplers, respectively.
The organization of this paper is as follows, section 2 presents the structure of device design and fabrication, and section 3 shows the results and discussion. Brief conclusion is given in the final section.
2. Device design and fabrication
Figure 1 shows the schematic diagram of the proposed SEDFB laser with first-order, second-order uniform grating and λ/4 phase shift grating. The second order grating has two diffraction orders, including feedback order and surface emitting order. In detail, the working principle of diffraction grating is as follows: the constructive interference between the adjacent diffracted waves can be written as [20]:
where M is diffraction order, λ is radiated wavelength, Λ is grating period. θi and θd are incidence angle and emergence angle, respectively. The Huygens’ constructive interference of adjacent diffracted waves of two media needs the sum or difference of optical path as the integer multiple of 2π:Due to the feedback along the parallel direction of the waveguide (θi ≈ θd ≈ π/2$ to $ sinθd = 1), the grating order P = 2Λ$\textrm{/}$λ = 2Λ/(λb/neff) equals to M. λb and neff represent the free space Bragg wavelength and effective refractive index, respectively. For first order grating (P =1), the diffraction order M = 0 and 1 indicate forward and backward propagations. While, the second order grating (P = 2) includes two horizontal diffractions (M = 0 and 2) and vertical diffraction (M = 1), which can contribute to the surface emission performance.
Figures 2(a) and 2(b) show the schematics of our proposed SEDFB lasers, which consists of a 150 µm long horizontal cavity and grating to provide optical feedback and surface beam outcoupling. The AlGaInAs/InP laser structure is grown on n+-InP substrate by metal organic chemical vapor deposition (MOCVD). The active layer consists of two thin AlGaInAs separate confinement heterostructure (SCH) layers and AlGaInAs quantum wells and barriers. The compressively strained wells are developed to improve differential gain and modulation bandwidth. 40 nm thickness InGaAsP grating layer consists of 1st order grating, 2nd order grating and λ/4 phase shift grating is formed by electron-beam lithography (EBL) system and etched by ICP etcher. A p-InP cladding layer and a p+-InGaAs contact layer are grown on top of the grating layer. The P-pad is directly evaporated on the InGaAs contact layer and SiO2 insulator. Finally, the chips are cleaved into separate bars with high-reflection (HR) side (94% reflectivity coating) and anti-reflection (AR) side (0.5% reflectivity coating). Figure 2(c) indicates the microscope image of the fabricated SEDFB laser. Although the part of second-order grating has no contact electrodes for direct current injection, the current can be pumped to this area based on the carrier diffusion effect of MQW layer. The details of epitaxial layer stack are presented in Table 1, including material, thickness and doping concentration.
Fig. 2. Schematic structure of the SEDFB laser (a) cross-sectional view; (b) longitudinal view; (c) microscope image of the fabricated laser.
Table 1. Details of the epitaxial layer stack
The SEM images of the grating layer with first order (period 202 nm, duty circle 0.5) and second order grating (period 404 nm, duty circle 0.5) are shown in Figs. 3(a) and 3(b) respectively. The λ/4 phase shift grating is used to ensure the symmetric oscillation mode operation in the laser cavity for a single-lobe far-field beam pattern [11]. In our design, λ/4 phase shift grating is optimized to be placed around one-third of the cavity section close to the HR-facet for high output power as analyzed in [21], then few asymmetric modes will be excited, causing a relatively large beam divergence. In addition, based on the phase delay introduced by λ/4 grating, the single longitudinal oscillation mode (at Bragg wavelength) can be realized and stably amplified in the cavity, recognized as the final lasing mode. Based on the upward diffraction of second-order grating, effective surface emission can be excited, achieving high SMSR and stable production yield under different conditions of facet coating reflectivity than uniform grating. The ridge width is chosen to allow fundamental lateral mode operation. Figure 3(c) indicates the image of cross section with ridge waveguide (width = 1.79 µm, thickness = 1.87 µm). The width of grating is 30 µm, bigger than the ridge width and beam size, ensuring consistent and stable optical feedback.
Fig. 3. (a) SEM image of the grating layer; (b) images of first order grating and second order grating; (c) cross-sectional SEM image of the laser structure.
3. Measurement and discussion
3.1 Static Performance
To demonstrate the optical performance of the SEDFB lasers, we here first measure those lasers with HR coating (94%) and AR coating (0.5%). The device was placed on a copper heat-sink with temperature controlled by thermoelectric cooler (TEC). The light-current-voltage (L-I-V) curve in Fig. 4(a) indicates the threshold current as 6.8 mA and high surface emitting output optical power up to 4 mW at 60 mA at 20 °C. The slope efficiency is calculated as 0.075 mW/mA. Figure 4(d) shows the temperature dependence of L-I characteristics. The threshold currents are 7.2, 8, 8.5, 11 and 12.5 mA at 30, 40, 50, 60, 70 °C, respectively. An optical output-power of about 1.7 mW with drive current of 50 mA at 70 °C was obtained. Meanwhile, the optical spectra under different inject currents is shown in Fig. 4(b), which exhibits good performance of single longitudinal mode with side-mode suppression ratio (SMSR) > 40 dB. The variation of wavelength with injection current $\varDelta {\Lambda /\Delta I}$ is about 0.12 nm/mA shown in Fig. 4(c). Figure 4 (e) shows the lasing wavelength in the temperature range from 20 to 70 °C at 50 mA, which also indicates single-mode operation with high SMSR > 40 dB. The temperature-dependent wavelength shift $\varDelta {\Lambda /\Delta T}$ shown in Fig. 4(f) is calculated as 0.077 nm/°C. In summary, static optical performance of the SEDFB lasers keeps perfect and stable over the wide temperature range.
Fig. 4. (a) L-I-V curve at 20 °C; (b) optical spectra at different injection currents; (c) the lasing wavelengths vs currents; (d) L-I curves of the SEDFB laser over 20-70 °C; (e) optical spectra under the drive current of 50 mA over 20-70 °C and (f) the lasing wavelengths vs temperatures.
In order to explore the surface emitting performance between different designs of lasers, we study the variation of surface emitting efficiency (edge emission power divided by surface emission power at 60 mA) as length, position of grating couplers and cavity length, respectively. In this work, 10 µm and 20 µm length second order grating couplers are placed in HR side, Middle and AR side of the cavity named as 10-HR, 20-HR, 10-M, 20-AR, 10-HR and 20-HR, as illustrated in Figs. 5(a)–5(c). The blue line in Fig. 5(d) indicates high surface emitting efficiency >15% of 150 µm cavity, with peak value as 22.5% of 10-HR laser. For 200 µm cavity, similar results can be drawn from the red curve shown in Fig. 5(d), with peak efficiency up to 23.8% of 20-HR laser. The good coupling efficiency of HR type lasers is attributed to the highly concentrated optical power intensity near λ/4 phase shift. Furthermore, the 30% reflective coating for both facets is also introduced to verify the surface emitting efficiency of 10-HR and 20-HR lasers shown in Fig. 5(e), which can also achieve the value > 15%, showing stable output performance.
Fig. 5. (a)-(c) The schematic diagram of the 10/20-M, 10/20-AR and 10/20-HR lasers, respectively; (d) surface emitting efficiency of 10-H, 10-M, 10-A, 20-H, 20-M, 20-A lasers with HR coating + AR coating of 150 µm and 200 µm cavity length, respectively; (e) surface emitting efficiency of 10-HR, 20-HR lasers with 30% coating for both facets of 150 µm and 200 µm length cavity, respectively; (f) wall-plug efficiency illustration for the 150 µm long cavity SEDFB lasers over 20-70 °C.
In addition, Wall-plug efficiency (WPE), the energy conversion efficiency of the electrical power to optical power, is also investigated. The wall-plug efficiency of the laser as a function of the injection current at different operating temperatures is shown in Fig. S3. It is clear that nearly 5% wall-plug efficiency can be achieved at room temperature (20 °C). The WPE decreases gradually as the temperature gets higher, whose value is low compared with conventional edge emitting DFB laser. Limit by the fabrication process of the same wafer, the thickness of the grating layer and the distance between the grating layer and the MQW layer cannot be changed. These two parameters can be optimized in our further research to effectively improve the WPE and surface emitting coupling efficiency. And, optimized epitaxial wafer with bottom DBR reflector can further enhance the upward optical output power.
3.2 Dynamic Performance
The SEDFB lasers are boned on AlN sub-mounts for radio frequency (RF) measurement. The measurement system consists of a vector network analyzer, a bias tee, a probe, and a high-speed photodiode. Thus, the small signal response of device can be expressed as [22]:
Fig. 6. (a) Small signal modulation responses under different bias currents at 20 °C for lasers with cavity radius of 150 µm; (b) under different temperatures at 50mA; (c) damping factor γ versus fr2 at 20 °C; (d) fr versus (I-Ith)1/2 at 20, 50, 70 °C.
The experimental setup for measuring large-signal dynamic characteristics is shown in Fig. 7(a). An electrical signal was generated by pulse-pattern generator. The single-ended signal was input into an RF amplifier to modulate SEDFB laser. The optical signal with pseudo-random binary sequence (PRBS) 231-1 pattern was first received by a photodetector, then into a BER tester or sampling oscilloscope. Figures 7(b) and 7(c) indicate the BER curves as function of power for 24 Gb/s NRZ signal at 20 and 50 °C, respectively. A low bit-error operation can be obtained for back-to-back (BTB) transmission. The insets in Figs. 7(b) and 7 (c) exhibit the eye patterns at 20 and 50 °C at fixed injection current 50 mA respectively, showing perfect temperature tolerance. These results demonstrate good performance of the fabricated high speed SEDFB lasers.
Fig. 7. (a) Experimental setup for 24 Gb/s signal transmission experiment; (b) BER characteristics of the SEDFB lasers for BTB transmission at (c) 20 °C and (d) 50 °C, respectively.
3.3 Beam Performance
For effective hybrid integration with Si photonic integrated circuit, the beam quality of surface emitting needs to be evaluated. A comparison of the near and far fields of SEDFB lasers with 10-HR and 20-HR out-coupler along the cavity-length direction are shown in Figs. 8(a1) and 8(a2), 8(b1) and 8(b2), respectively. The grating coupler are all located in the HR side of the cavity. In the longitudinal direction, the buried second-order grating design generates a single-lobe far-field distribution. The full-width half-maximum (FWHM) far-field beam divergences of 10-HR and 20-HR lasers are 13.4°×20.2° and 9.2°×20.6° respectively, which are smaller than that of normal edge-emitting semiconductor lasers. Meanwhile, the insets in Fig. 8 indicate the simulated near- and far- field patterns by three-dimensional (3D) finite-difference time-domain (FDTD) method, which are consistent with the intensity profiles of experimental results, with far-field beam divergences as 6.9°×13.2° and 3.2°×13.3°of 10 and 20 µm grating couplers, respectively. Due to the existence of two hybrid oscillation modes including symmetric and antisymmetric modes, the measured far-field beam divergence is larger than the pure simulation results. The nonuniform beam profile with fuzzy stripes is attributed to the background noise of the CCD camera.
Fig. 8. Experimental characterization of (a1) near field and (a2) far field beam patterns of 10 µm out-coupler at 50 mA; (b1) near field and (b2) far field beam pattern of 20 µm out-coupler at 50 mA. Simulated results (insets)
4. Conclusion
In summary, a 1.3 µm stable single-longitudinal-mode AlGaInAs SEDFB laser was developed by introducing second order grating and λ/4 phase shift grating. The fabricated SEDFB laser exhibits high optical power about 4 mW and singe mode operation with SMSR$\mathrm{\ > \;\ }$ 40 dB over wide temperature range (20-70 °C). The threshold current and slope efficiency are 6.8 mA and 0.075 mW/mA with 150 µm laser cavity. The emitting wavelength variation with current and temperature are 0.12 nm/mA and 0.077 nm/°C, respectively. The SEDFB laser has also achieved a relaxation oscillation frequency of about 12 GHz and a D factor as 1.58 GHz/(mA)1/2 at 70 °C. Clear eye diagram and low bit-error operation at 24 Gb/s NRZ signal for BTB transmission has also been demonstrated. Meanwhile, the far-field divergence angle of surface emitting beam is 13.4°×20.2° of 10 µm length second order grating coupler. High speed direct modulation, wide temperature operation and simple preparation process are the comprehensive advantage, which distinguish our grating assisted surface emitting DFB lasers from previously reported devices. Furthermore, optimized grating-coupler designs with high coupling efficiency and epitaxial wafer with higher modulated characteristics based on mature fabrication technology of DFB lasers will broaden application prospects in large scale integrated optical circuits.
Funding
National Key Research and Development Program of China (2018YFB2201500).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data used to support the findings of this study are available from the corresponding author upon request.
References
1. J. Huang, C. Li, R. Lu, L. Li, and Z. Cao, “Beyond the 100 Gbaud directly modulated laser for short reach applications,” J. Semicond. 42(4), 041306 (2021). [CrossRef]
2. L. A. Coldren, S. C. Nicholes, L. Johansson, S. Ristic, R. S. Guzzon, E. J. Norberg, and U. Krishnamachari, “High performance InP-based photonic ICs—A tutorial,” J. Lightwave Technol. 29(4), 554–570 (2011). [CrossRef]
3. E. Heumann, S. Bär, K. Rademaker, G. Huber, S. Butterworth, A. Diening, and W. Seelert, “Semiconductor-laser-pumped high-power upconversion laser,” Appl. Phys. Lett. 88(6), 061108 (2006). [CrossRef]
4. S. Kumari, E. Haglund, J. Gustavsson, A. Larsson, G. Roelkens, and R. G. Baets, “Vertical-cavity silicon-integrated laser with in-plane waveguide emission at 850 nm,” Laser & Photonics Reviews 12(2), 1700206 (2018). [CrossRef]
5. B. Song, C. Stagarescu, S. Ristic, A. Behfar, and J. Klamkin, “3D integrated hybrid silicon laser,” Opt. Express 24(10), 10435–10444 (2016). [CrossRef]
6. H. Lu, J. S. Lee, Y. Zhao, C. Scarcella, P. Cardile, A. Daly, M. Ortsiefer, L. Carroll, and P. O’Brien, “Flip-chip integration of tilted VCSELs onto a silicon photonic integrated circuit,” Opt. Express 24(15), 16258–16266 (2016). [CrossRef]
7. Z. Khan, J. Shih, R. Chao, T. Tsai, H. Wang, G. Fan, Y. Lin, and J. Shi, “High-brightness and high-speed vertical-cavity surface-emitting laser arrays,” Optica 7(4), 267–275 (2020). [CrossRef]
8. S. A. Blokhin, N. A. Maleev, M. A. Bobrov, A. G. Kuzmenkov, A. V. Sakharov, and V. M. Ustinov, “High-speed semiconductor vertical cavity surface-emitting lasers for optical data-transmission systems,” Tech. Phys. Lett. 44(1), 1–16 (2018). [CrossRef]
9. K. Adachi, T. Suzuki, T. Ohtoshi, K. Nakahara, M. Sagawa, A. Nakanishi, and S. Tanaka, “Facet-free surface-emitting 1.3-µm DFB laser,” in Proceedings of IEEE Conference on the European Conference on Optical Communication (ECOC), (2016).
10. K. Adachi, K. Shinoda, T. Kitatani, T. Fukamachi, Y. Matsuoka, T. Sugawara, and S. Tsuji, “25-Gb/s multichannel 1.3-µm surface-emitting lens-integrated DFB laser arrays,” J. Lightwave Technol. 29(19), 2899–2905 (2011). [CrossRef]
11. S. Li, G. Witjaksono, S. Macomber, and D. Botez, “Analysis of surface-emitting second-order distributed feedback lasers with central grating phase shift,” IEEE J. Select. Topics Quantum Electron. 9(5), 1153–1165 (2003). [CrossRef]
12. Y. Ao, J. Wang, D. Liu, and M. Zhang, “Surface-emitting distributed feedback laser based on high-order gratings,” Appl. Opt. 58(20), 5443–5449 (2019). [CrossRef]
13. Y. H. Liu, J. C. Zhang, F. L. Yan, Z. W. Jia, F. Q. Liu, P. Liang, N. Zhuo, S. Q. Zhai, L. J. Wang, and J. Q. Liu, “High efficiency, single-lobe surface-emitting DFB/DBR quantum cascade lasers,” Opt. Express 24(17), 19545–19551 (2016). [CrossRef]
14. C. Boyle, C. Sigler, J. D. Kirch, D. F. Lindberg, T. Earles, D. Botez, and L. J. Maws, “High-power, surface-emitting quantum cascade laser operating in a symmetric grating mode,” Appl. Phys. Lett. 108(12), 121107 (2016). [CrossRef]
15. S. Biasco, A. Ciavatti, L. Li, A. G. Davies, E. H. Linfield, H. Beere, and M. S. Vitiello, “Highly efficient surface-emitting semiconductor lasers exploiting quasi-crystalline distributed feedback photonic patterns,” Light: Sci. Appl. 9(1), 54 (2020). [CrossRef]
16. J. Luan, Y. Han, S. Yang, R. Zhang, Q. Tian, P. He, D. Liu, and M. Zhang, “Demonstration of 25 Gb/s NRZ modulated 1.3 µm surface-emitting DFB lasers,” in 2021 Asia Communications and Photonics Conference (ACP) (2021), pp.1–3.
17. S. H. Macomber, J. S. Mott, R. J. Noll, G. M. Gailatin, E. J. Gratrix, and S. L. O’Dwyer, “Surface emitting distributed feedback semiconductor lasers,” Appl. Phys. Lett. 51(7), 472–474 (1987). [CrossRef]
18. T. Masood, S. Patterson, N. V. Amarasinghe, S. McWilliams, D. Phan, D. Lee, Z. A. Hilali, X. Zhang, G. A. Evans, and J. K. Butler, “Single-frequency 1310-nm AlInGaAs-InP grating- outcoupled surface-emitting lasers,” IEEE Photonics Technol. Lett. 16(3), 726–728 (2004). [CrossRef]
19. C. Liu, P. Zhang, M. Xiang, X. Ma, C. Jiang, B. Tang, Q. Lu, and W. Guo, “Single-mode surface-emitting DFB lasers with a large-area oxidized aperture based on the surface grating,” Opt. Lett. 45(13), 3573–3576 (2020). [CrossRef]
20. G. Witjaksono, “Single-mode operation in a single-lobed beam pattern from surface-emitting second-order distributed feedback laser with central pi-phase shift grating,” Ph.D. dissertation, (2002).
21. M. Usami, S. Akiba, and K. Utaka, “Asymmetric λ/4-shifted InGaAsP/InP DFB lasers,” IEEE J. Quantum Electron. 23(6), 815–821 (1987). [CrossRef]
22. S. Matsuo and T. Kakitsuka, “Low-operating-energy directly modulated lasers for short-distance optical interconnects,” Adv. Opt. Photon. 10(3), 567–643 (2018). [CrossRef]
