Open Access
Add to BibSonomy
Abstract
In this paper, we put up a robust design of a stable single-mode-operated GaSb-based laser diode emitting around 1950nm. This novel design structure with socketed ridge-waveguide enables a simple fabrication and batch production of mid-infrared laser diodes on account of the mere usage of standard photolithography. By introducing micron-level index perturbations distributed along the ridge waveguide, the threshold gains of different FP modes are modulated. Four geometrical parameters of the perturbations are systematically optimized by analyzing the reflection spectrum to get a robust single-mode characteristic. Based on the optimized geometrical parameters, 1-mm long uncoated lasers are carried out and exhibit a stable single longitudinal mode from 10 °C to 40 °C with a maximum output power of more than 10 mW. Thus, we prove the feasibility of the standard photolithography to manufacture the monolithic single-mode infrared laser source without regrowth process or nanoscale lithography.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Single-mode infrared semiconductor laser sources ranging from 1.9-4 µm are particularly in high demand for spectroscopic sensing owing to the strong molecular rotational-vibrational absorption bands of many technological and greenhouse gases [1], which has increasing application value in industrial monitoring, medical diagnosis, scientific research and so on [2,3]. The laser source also attracts a lot of attention for its potential to provide high data capacity in free-space optical communications due to the high transparency atmospheric transmission window in this spectral range [4]. Furthermore, with the growing market demand, the cost of the laser source needed to be reduced while maintaining high output performance [5].
These have led to many studies on single-mode laser diodes. GaSb-based lasers, thanks to the matched lattice constant of AlGaIn(AsSb) system with energy-band engineering, have greater output performance—high efficiency, high working temperature, and considerable continuous-wave output powers above room temperature compared to InP-based lasers in this spectral region [6,7]. The conventional F-P cavity lasers generally operate in multi-longitudinal modes. To achieve a single longitudinal operation, gratings are typically needed to filter wavelength [8]. Based on the operating mechanism of the gratings, various single-mode lasers have been developed, such as distributed feedback (DFB) lasers [9], distributed Bragg reflector (DBR) lasers [10], vertical-cavity surface-emitting lasers (VCSELs) [11], and external cavity lasers [12]. In view of convenience and compactness, the on-chip gratings are indispensable for photonic integration [13]. The low-order gratings fabricated with or without epitaxial regrowth steps, are time-consuming, expensive, and complicated due to the usage of nanoscale resolution lithography and the easy oxidation characteristics of aluminum in epitaxy. Not to mention the extremely low design and fabricating tolerances introduced by electron beam lithography or nano imprint technology [14]. Thus, these kinds of single-mode lasers are not cost-effective and not well prepared for mass production.
A simple and effective alternative approach to achieve single-mode laser is introducing micron-level index perturbations into the ridge waveguide which can be easily prepared by standard contacted photolithography [15]. Significant progress has been achieved in InP-based lasers in the near infrared region [16], proving that this approach not only has a simple and low-cost fabrication process but also portrays lots of superiorities over the DFB and DBR lasers with stable operation over a larger temperature range, narrow linewidth and low sensitivity to optical feedback [17,18]. Moreover, the convenience of monolithic integration with other electro-optical modulators [19–21], enables this kind of laser an ideal candidate for photoelectric integrated devices. However, few works have been reported in the mid-infrared region and the compatibility with GaSb-based remains not confirmed in experiments.
In this paper, by the introduction and optimized design of socketed ridge-waveguide (SR), the index perturbation is formed with the index difference between the socket region and non-socketed region of the guide mode, in which the interface makes reflection and transmission, as is shown in Fig. 1. Periodically arranging these interfaces in a certain number can modify the threshold gain values of the individual FP cavity modes by manipulating the cavity loss spectrum. By properly designing the etching depth and period length, the single-mode laser is guaranteed with the selected enhancement of a particular FP mode. What is more, we carried out a thorough simulation of the modulated longitudinal mode emission by analyzing different geometrical parameters of the socket region to get a robust design. The robust characteristic not only depends on the high lithographic tolerance of micron-level index perturbations but also benefits from the optimized geometrical parameters of sockets. Based on the robust design, we successfully fabricated the GaSb-based SR laser by standard photolithography for single-mode operation around 1950nm. The SR lasers operated in single longitudinal mode with a side mode suppression ratio larger than 30 dB from 10 °C to 40 °C with emission wavelength centered on 1950nm at 20 °C, exceeding 10 mW.
2. Simulation and design
In the simulations, the 3D ridge waveguide of multi-quantum-well GaSb-based lasers is simplified into a 2D structure with three layers as displayed in Fig. 2. The core layer consists of two 10 nm In0.22GaSb quantum wells and two 270 nm Al0.25GaAs0.02Sb separate confinement layers. And its effective reflective index is calculated by the weighted average index of the layers $n = \sqrt {\sum {d_i}n_i^2/\sum {d_i}} $, where ${n_i}$ and ${d_i}$ are the refractive index and thickness of each layer. The N-cladding layer is a 2 µm thick Al0.5GaAs0.04Sb layer, which has a lower refractive index than the core layer to confine the optical field. The ridge layer is also a 2 µm Al0.5GaAs0.04Sb cladding layer. The socket width and socket spacing respectively are ${d_s} = ({2p + 1} ){\lambda _B}/4{n_s}$ and ${d_w} = ({2q + 1} ){\lambda _B}/4{n_w}$, where p and q are integers, and $m = p + q + 1$ is the grating order, ${n_s}$ and ${n_w}$ are the effective modal index of the socket (etched) area and waveguide (unetched) area, respectively. ${\lambda _B}$ is the Bragg wavelength in a vacuum, and in this paper the Bragg wavelength is 1950nm. As for the simulation method, the rigorous coupled-wave analysis [22] is first applied to calculate the scattering matrix of a single socket and then the recursive S-matrix algorithm [23] is used to model the multiple sockets.
Fig. 2. Simplified 2D waveguide structure with sockets (Left). The unperturbed and perturbed transverse mode profile with detailed refractive index (Right). The P-mode refers to the perturbed mode in the socket region and the Unp-mode refers to the unperturbed mode in the spacing region.
In this calculation, four grating parameters: socket depth, socket width, socket spacing, and socket number, were calculated to uncover the relation with grating characteristics. Note that the calculated condition of each results figure was placed in the final caption with black brackets. The first step in the calculation is to optimize the socket depth, which determines the strength of perturbing the guided mode in the mechanism and is also used to get the effective modal index of the socket (etched) area in calculations. As Fig. 3(a) shows, the reflection spectrum is highly affected by socket depth. And the power reflectivity at 1950nm and the full width at half maximum (FWHM) were extracted with the socket depth, displayed in Fig. 3(b). We can see that the power reflectivity first rises and then falls with the maximum value at the socket depth of 1.8 µm. And the FWHM obviously increases when the socket depth is larger than 1.8 µm. We think that the socket depth of 1.8 µm is a critical condition for judging whether the socket gratings are perturbing or not. When the socket depth is larger than 1.8 µm, the sockets separate the whole F-P laser into many coupled intracavity, which is called the coupled cavity laser by other studies [24,25]. In this research, the socket depth of 1.8 µm is picked to acquire the maximum power reflectivity to guarantee lasing at 1950nm, which a is stable value during the depth from 1.7 µm to 1.9 µm. After the socket depth is fixed, the relation between the power reflectivity and different order number of sockets and spacing is analyzed, which is shown in Fig. 4. The maximum reflectivity can be got at the socket order number of 2, 4, and 7, respectively. The socket order number of 4, corresponding to the socket width of around 1.2 µm, is chosen to pattern the socket feature which can be directly fabricated by standard photolithography. The socket width of 1.2 µm is also fabrication-tolerant, in which the high reflectivity is maintained around this socket width. And the reflection spectrum versus the order number of grating by varying the order number of spacing and fixing the socket order number to 4, is present in Fig. 5(a). The power reflectivity at 1950nm, the free spectral range (FSR), and the length of one period versus the order number are also displayed in Fig. 5(b). The 29th-order grating, specifically expressed as the socket width of 1.2 µm and the socket spacing of 6.7 µm, achieves the maximum power reflectivity with the FWHM of 6.5 nm and a large FSR of about 80 nm with a length period of 7.9 µm. The large FSR of 80 nm can effectively suppress the mode hop of this kind of laser. Furthermore, the effect of socket numbers on the reflectivity spectrum is also calculated. As Fig. 6 displays, the power reflectivity and FWHM are almost unchanged when the socket number is larger than 15. In this study, the socket number of 15 is chosen to keep high reflectivity and a low FWHM of about 6.5 nm. After the above system analysis, we can see that the performance of socketed gratings is robust based on the selected gratings parameters, in which the power reflectivity and the associated FWHM are stable regardless of a process error. In a word, the optimization of the grating parameters: the depth socket of 1.8 µm and the width socket and spacing of 1.2 µm and 6.7 µm with the socket number of 15, are used to fabricate the GaSb-based SR laser emission at 1.95 µm. The power reflectivity and loss curves calculated based on the optimized sockets parameters are shown in Fig. 7. There is only one minimum loss at 1950nm in the 1.9 to 2 µm region, which indicates the SR lasers will have stable single-mode operation performance.
Fig. 3. (a) The reflection spectrum of gratings with the socket depth from 1.3 µm to 2 µm. (b) The power reflectivity at 1950nm and the full width at half maximum versus the socket depth. (Calculated at the socket and spacing order number of 4 and 22, respectively, and the socket number of 15)
Fig. 4. Contour plot of calculated reflectivity versus the order of socket and spacing. (Calculated at the etched depth of 1.8 µm, the socket number of 15, and wavelength of 1950nm)
Fig. 5. (a). The reflection spectrum of gratings with the order number from 10 to 80. 5(b). The maximum power reflectivity, the free spectral range, and the corresponding length of each grating period versus the order number from 10 to 80. (Calculated at the etched depth of 1.8 µm, the socket order number of 4, and the socket number of 15)
Fig. 6. (a) The reflection spectrum of gratings with the socket number from 1 to 20. (b) The maximum power reflectivity and the full width at half maximum versus the number of sockets. (Calculated at the etched depth of 1.8 µm and the socket and spacing order number of 4 and 24, respectively)
Fig. 7. The power reflectivity and loss curves calculated based on the optimized sockets parameters.
3. Growth and fabrication
The laser wafers were grown on a 2-inch (100) n-type GaSb substrate by Veeco solid source Gen-II molecular-beam epitaxy with valved crackers for arsenic and antimony. The heterostructures consisted of a 570-nm thickness core region and 2-µm thickness N/P cladding layers. The core region with the gain spectrum covering the 1.9 to 2.0 µm region, contained two compressively strained 10-nm In0.22GaSb quantum wells with 20-nm Al0.25GaAs0.02Sb barrier and two undoped 270 nm Al0.25GaAs0.02Sb waveguide layers. The N-type and P-type electrode contact layers were respectively n-type substrate and heavily doped 250-nm p-GaSb cap layer. In this fabrication, there are four times standard photolithography. The first time is transferring socket patterns into the upper cladding layer. In order to ensure the quality of pattern transfer, 250-nm-thick SiO2 was first grown by the plasma enhanced chemical vapor deposition (PECVD) as a hard mask layer. The inductively couple plasma (ICP) was performed twice. The first time is etching SiO2 and the second time is etching GaSb cap layer and upper-Al0.5GaAs0.04Sb. The etched depth of the design socket is set to be 1.8 µm. The second time of photolithography is to realize a 5-µm-width with a 2-µm-depth waveguide ridge to ensure the single transverse mode operation of the laser diode [26], which procedure is the same as the first time of fabricating the socket. Then a 250-nm-thick SiO2 was deposited as the electrical insulation layer. The third time of photolithography is to open the current injection window with ICP etching. The p-electrode was realized by magnetron sputtered 50 nm/50 nm/300 nm Ti/Pt/Au, followed by the fourth time of photolithography to form clean grooves to easily get the laser chip. Finally, after the wafer was thinned to 120 µm, the n-electrode was sputtered 50 nm/300 nm AuGeNi/Au and annealed under 350 °C.
Be careful that those sockets should be wider than ridges in the lateral direction for a good alignment between sockets and ridges, as presented in Fig. 8. The detailed parameters of the fabricated socket were measured, which show a little difference from the designed parameters due to process error. However, these deviations did not deteriorate the single-mode performance of SR lasers, as shown in the Results and Discussion section. Additionally, twice ICP etched were performed in the patterns of sockets out of ridge waveguides in this fabrication process, resulting in deeper notches alongside ridge waveguides. All sockets were located in the middle of the ridge waveguides in the longitudinal direction. And all of the cleaved facets were uncoated.
Fig. 8. SEM image of the 5-µm-width ridge waveguide with 45-µm-width sockets. The detailed parameters of the fabricated socket were measured.
4. Results and discussion
The 1-mm-long laser chip was tested under continuous-wave (CW) conditions at different heatsink temperatures. The output power was measured using a Thorlabs S148C integrating Sphere Sensor. Emission spectra were obtained with a Yokogawa AQ6376 optical spectrum analyzer using multimode index fiber. Figure 9(a) and (b) respectively display the typical CW current-voltage-power characteristics of a 5-µm-wide GaSb-based laser operating at 10 °C and the power-current (P-I) characteristics of a SR laser at different temperatures. The traditional narrow ridge laser and the SR laser operating at 10 °C exhibited slope efficiencies of 0.252 W/A and 0.036 W/A and threshold current densities of 300 A/cm2 and 1278 A/cm2, respectively. The inset in Fig. 9(b) shows the emission spectrum at 410 mA and 10 °C.
Fig. 9. (a) Typical CW I-V-P characteristics of a 1-mm-long and 5-µm-wide GaSb-based laser operating at 10 °C. (b) CW I-P characteristics of a 1-mm-long GaSb-based SR laser operating at different temperatures.
The wavelength and associated SMSR with current and temperature are shown in Fig. 10. There are 87% of the working conditions, in which SMSRs are greater than 30 dB. The SMSR varies with current and temperature as a result of the mismatch between the Bragg wavelength of the surface high-order gratings and the wavelength-dependent gain of the active region. The red-shift rate of gain peak against current and temperature for antimonide diode lasers are about 11 nm/A and 1.2 nm/K, respectively [27]. And the drifting rate of Bragg wavelength against current and temperature are respectively 0.0066 nm/mA and 0.182 nm/K. The maximum SMSR appears at the condition where the gain peak wavelength and the Bragg wavelength are matched. Thus, whether increasing the injection current or heatsink temperature, the SMSR generally increases at first and then decreases, which can be seen in Fig. 10. With different currents and temperatures, our SR laser exhibits a single longitudinal mode operation ranging from 1947nm to 1955nm, as shown in Fig. 11.
Fig. 10. Wavelength tuning against current and temperature (down). The associated SMSR with current and temperature (up).
The line width of the GaSb-based SR laser is measured by the Thorlabs SA200-18C Scanning Fabry-Perot interferometer with a free spectral range (FSR) of 1.5 GHz and fineness of 200 (equal to 7.5 MHz in spectral resolution), as demonstrated in Fig. 12. The mode line width was estimated to be approximately 56 MHz (equal to 0.71 pm) using the full width at half maximum (FWHM) of the interferometer resonances.
Fig. 12. Line width obtained by estimating the FWHM of the resonances of the interferometer. The inset shows the FSR of the interferometer.
5. Conclusions
We demonstrated a kind of GaSb-based single-mode laser diode fabricated by standard photolithography with socketed ridge-waveguide modulation. A robust design was obtained by systematically simulating and analyzing the relation between the socketed grating characteristic and the geometrical parameters of the socketed grating. A GaSb-based socketed ridge-waveguide laser emitting at around 1950nm was successfully fabricated based on the robust-design 29th socketed grating. The 1-mm-long uncoated lasers exhibited a single-mode emission from 10 °C to 40 °C with a maximum output power exceeding 10 mW. This laser design and manufacturing method of achieving monolithic mid-infrared GaSb-based laser sources have considerable advantages over DFB structures, as their fabrication process does not involve electron-beam lithography and requires only standard contacted photolithography. Owing to the simplicity of device fabrication, these laser diodes are cost-effective and high yield in mass production.
Funding
Innovation Program for Quantum Science and Technology (2021ZD0300801); Announce the list and take charge of the major special plan of science and technology in Shanxi Province (202201030201009); Key R&D Program of Shanxi Province (202102030201004); National Natural Science Foundation of China (62204238); Chinese Academy of Sciences and Changchun City Science and Technology Innovation Cooperation Project (21SH06); Jincheng Key Research and Development Project (20210209).
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.
References
1. L. S. Rothman, I. E. Gordon, Y. Babikov, et al., “The HITRAN2012 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transfer 130, 4–50 (2013). [CrossRef]
2. J. Wagner, C. Mann, M. Rattunde, et al., “Infrared semiconductor lasers for sensing and diagnostics,” Appl. Phys. A: Mater. Sci. Process. 78(4), 505–512 (2004). [CrossRef]
3. A. Vizbaras, I. Simonyte, S. Droz, et al., “GaSb swept-wavelength lasers for biomedical sensing applications,” IEEE J. Sel. Top. Quantum Electron. 25(6), 1–12 (2019). [CrossRef]
4. K. Zou, K. Pang, H. Song, et al., “High-capacity free-space optical communications using wavelength- and mode-division-multiplexing in the mid-infrared region,” Nat. Commun. 13(1), 7662 (2022). [CrossRef]
5. Transparency Market Research, “Mid-infrared Laser Market,” (2023), retrieved https://www.transparencymarketresearch.com/mid-infrared-lasers-market.html.
6. S. Stephan, D. Frederic, and A. Markus-Christian, “Novel InP- and GaSb-based light sources for the near to far infrared,” Semicond. Sci. Technol. 31(11), 113005 (2016). [CrossRef]
7. C. Grasse, P. Wiecha, T. Gruendl, et al., “InP-based 2.8–3.5 µm resonant-cavity light emitting diodes based on type-II transitions in GaInAs/GaAsSb heterostructures,” Appl. Phys. Lett. 101(22), 221107 (2012). [CrossRef]
8. H. Venghaus, “Wavelength filters,” in Fibre Optic Communication: Key Devices, H. Venghaus and N. Grote, eds. (Springer International Publishing, Cham, 2017), pp. 417–482.
9. Q. Gaimard, L. Cerutti, R. Teissier, et al., “Distributed feedback GaSb based laser diodes with buried grating,” Appl. Phys. Lett. 104(16), 161111 (2014). [CrossRef]
10. J. Jiang, L. Shterengas, G. Kipshidze, et al., “High-power narrow spectrum GaSb-based DBR lasers emitting near 2.1 µm,” Opt. Lett. 46(8), 1967 (2021). [CrossRef]
11. A. Andrejew, S. Sprengel, and M.-C. Amann, “GaSb-based vertical-cavity surface-emitting lasers with an emission wavelength at 3 µm,” Opt. Lett. 41(12), 2799–2802 (2016). [CrossRef]
12. T. Wang, C. Yang, Y. Zhang, et al., “High spectral purity GaSb-based blazed grating external cavity laser with tunable single-mode operation around 1940nm,” Opt. Express 29(21), 33864–33873 (2021). [CrossRef]
13. T. Zhu, Y. Hu, P. Gatkine, et al., “Arbitrary on-chip optical filter using complex waveguide Bragg gratings,” Appl. Phys. Lett. 108(10), 101104 (2016). [CrossRef]
14. R. Millett, K. Hinzer, T. Hall, et al., “Fabrication-tolerant higher order laterally coupled distributed feedback lasers,” Proc. SPIE 7099, 70991P (2008). [CrossRef]
15. J. Patchell, D. Jones, B. Kelly, et al., “Specifying the wavelength and temperature tuning range of a Fabry-Perot laser containing refractive index perturbations,” Proc. SPIE 5825, 1 (2005). [CrossRef]
16. R. Phelan, D. Byrne, J. O’Carroll, et al., “Mid-infrared inp-based discrete mode laser diodes,” in Optical Fiber Applications, G. H.-C. A. R. Imani, ed. (IntechOpen, 2019), Chap. 2.
17. C. Guignard, G. Tipaka, O. Vaudel, et al., “Low sensitivity to optical feedback and optical injection of discrete mode lasers,” in (IEEE, 2006), pp. 663–664.
18. J. O’Carroll, R. Phelan, B. Kelly, et al., “Narrow-linewidth discrete-mode laser diodes for coherent communication applications,” J. Opt. Commun. Netw. 4(9), A90–A96 (2012). [CrossRef]
19. W.-H. Guo, Q. Lu, M. Nawrocka, et al., “Integrable slotted single-mode lasers,” IEEE Photonics Technol. Lett. 24(8), 634–636 (2012). [CrossRef]
20. J. Li, X. Wang, Y. Xu, et al., “High-power semiconductor laser fabricated by standard photolithography for direct modulation,” IEEE Photonics Technol. Lett. 35(6), 289–292 (2023). [CrossRef]
21. D. C. Byrne, J. P. Engelstaedter, W. H. Guo, et al., “Discretely tunable semiconductor lasers suitable for photonic integration,” IEEE J. Sel. Top. Quantum Electron. 15(3), 482–487 (2009). [CrossRef]
22. E. Silberstein, P. Lalanne, J.-P. Hugonin, et al., “Use of grating theories in integrated optics,” J. Opt. Soc. Am. A 18(11), 2865 (2001). [CrossRef]
23. L. Li, “Formulation and comparison of two recursive matrix algorithms for modeling layered diffraction gratings,” J. Opt. Soc. Am. A 13(5), 1024–1035 (1996). [CrossRef]
24. M. Muller, H. Scherer, T. Lehnhardt, et al., “Widely tunable coupled cavity lasers at 1.9 µm on GaSb,” IEEE Photonics Technol. Lett. 19(8), 592–594 (2007). [CrossRef]
25. Y. Wang, Y. Yang, S. Zhang, et al., “Narrow linewidth single-mode slotted fabry–pérot laser using deep etched trenches,” IEEE Photonics Technol. Lett. 24(14), 1233–1235 (2012). [CrossRef]
26. T. Wang, C. Yang, Y. Chen, et al., “Promotion of specific single-transverse-mode beam characteristics for GaSb-based narrow ridge waveguide lasers via customized parameter design,” Nanoscale Res. Lett. 17(1), 116 (2022). [CrossRef]
27. S.-W. Xie, Y. Zhang, C.-A. Yang, et al., “High performance GaSb based digital-grown InGaSb/AlGaAsSb mid-infrared lasers and bars,” Chin. Phys. B 28(1), 014208 (2019). [CrossRef]
