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Abstract
We demonstrated a narrow linewidth semiconductor laser based on a deep-etched sidewall grating active distributed Bragg reflector (SG-ADBR). The coupling coefficients and reflectance were numerically simulated for deep-etched fifth-order SG-ADBR, and a reflectance of 0.86 with a bandwidth of 1.04 nm was obtained by the finite element method for a 500-period SG-ADBR. Then the fifth-order SG-ADBR lasers were fabricated using projection i-line lithography processes. Single-mode lasing at 1537.9 nm was obtained with a high side-mode suppression ratio (SMSR) of 65 dB, and a continuous tuning range of 10.3 nm was verified with SMSRs greater than 53 dB. Furthermore, the frequency noise power spectral density was characterized, from which a Lorentzian linewidth of 288 kHz was obtained.
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1. Introduction
Narrow linewidth lasers are of great interest for a wide range of applications in large-capacity optical communications [1,2], light detection and ranging [3], and atomic clocks [4]. With the rapid growth of network capacity demand, narrow linewidth lasers are of particular interest in optical communication due to their low phase noise. Advanced modulation formats with stringent requirements for phase stability have emerged in coherent optical communications to meet the demand of capacity, which requires laser linewidths in the order of 100 kHz or even lower [5,6]. Additionally, in wavelength division multiplexing based systems, tunable lasers are greatly demanded to precisely align to a certain channel [7,8]. Distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers were widely investigated for realizing single-mode operation with high-speed direct modulation [9,10], high-power [11,12], and wide-range tuning [13–17]. Conventional DBR lasers increase optical loss due to free-carrier absorption and intervalence-band absorption during DBR current injection [18], which is not beneficial to reducing laser linewidth. An active-DBR (ADBR) structure was proposed to overcome the excess loss [19,20], and active-DBR without epitaxial regrowth has advantages in simplifying the process and cutting down the fabrication cost.
Sidewall Bragg gratings with the advantage of regrowth-free were applied for fabricating single mode [21–23] and narrow linewidth [24–26] semiconductor lasers. Long cavity first-order sidewall Bragg grating DFB lasers integrated with a curved tapered semiconductor optical amplifier with a linewidth of 64 kHz were fabricated [24], and DFB lasers with a chirped-coupling sidewall Bragg grating in a 3 mm cavity were realized with a linewidth of 100 kHz [25]. The above two lasers were fabricated on 3 quantum wells AlGaInAs/InP epitaxial wafers. Epitaxial wafers with a smaller number of quantum wells lead to small optical confinement factors and small spontaneous emission factors, favoring narrow linewidths. However, first-order gratings require an electron-beam lithography process, which is costly and not conducive to mass production. Recently, a ridge waveguide semiconductor laser with 11th-order sidewall gratings was fabricated using a contact-type i-line lithography technique, and a narrow linewidth of 63 kHz was obtained for the laser with coatings on both end facets [26]. The high-order grating has a small coupling coefficient, and the aforementioned shallow-etched sidewall gratings often require long cavities for efficient coupling. Deep-etched sidewall gratings have higher coupling coefficients due to the larger refractive index difference between the wide- and narrow-slices [27], which can compensate for the low coupling coefficients of higher-order gratings. Here, deep-etched sidewall gratings with high coupling coefficients were investigated to fabricate narrow linewidth lasers using the projection i-line lithography processes technique.
In this paper, we proposed and fabricated a 1550-nm-wavelength deep-etched sidewall grating ADBR (SG-ADBR) laser on a 3 quantum wells AlGaInAs/InP epitaxial wafer. To ensure that it could be fabricated using a standard lithography process and that the grating order is small enough to have a higher coupling coefficient, we chose a DBR with a fifth-order grating. The coupling coefficients of the deep- and shallow-etched sidewall gratings were compared numerically. Based on the simulation results, deep-etched SG-ADBR lasers were fabricated by technique processes with projection i-line lithography. The SG-ADBR laser is consisted of a sidewall grating section and a gain section with separately injected currents to control the lasing wavelength. The period number of sidewall grating is selected as 500, corresponding to a length of 600 µm and the total length of the laser cavity is 1000 µm. A high side-mode suppression ratio (SMSR) of 65 dB was obtained under the sidewall grating section and gain section injection currents of ISG = 120 mA and Ig = 50 mA. By simultaneously varying ISG and Ig, 53 wavelength channels consecutively lined up with 25-GHz spacings were measured with a tuning range of 10.3 nm and SMSRs greater than 53 dB during the continuous tuning operations. The single-sided frequency noise power spectral density was characterized, from which the minimum Lorentzian linewidth of 288 kHz was measured.
2. Design and simulation
The SG-ADBR laser consisting of a gain section and a sidewall grating is presented in Fig. 1(a), where L indicates the total laser length, the sidewall grating period is Λ, the wide-slice length of the sidewall grating is τ, the narrow-slice width of the sidewall grating is W, and the extension width of the grating is D. The filling factor γ is defined by τ/Λ. Sidewall gratings satisfy the Bragg condition Λ = mλB/2neff, where m is the grating order, neff is the effective refractive index and λB is the Bragg wavelength. The end facets corresponding to the gain section and the sidewall grating are defined as the front and rear ports, respectively.
Fig. 1. (a) The schematic structure of the SG-ADBR laser. The simulation of the normalized electrical field for the transverse fundamental quasi-TE mode distribution in (b) a deeply etched waveguide (h = 4.5 µm) and (c) a shallow etched waveguide (h = 1.5 µm). (d) Corresponding effective indices of deep etched and shallow etched waveguides with varying width (W).
The laser wafer structure is shown in Table 1, with the MQWs active layer consisting of three compressively strained wells and four lattice-matched barriers, and U the meaning of undoped. The transverse-electric (TE) mode is considered, as the TE polarization dominates the modal gain in the compressively strained multi-quantum wells (MQWs) wafers. Using the two-dimensional (2D) finite element method (FEM) in a COMSOL software, the normalized electrical field distributions of the fundamental quasi-TE transverse modes are shown in Fig. 1(b) and 1(c), for a deep-etched waveguide (h = 4.5 µm) and a shallow-etched waveguide (h = 1.5 µm), respectively, where h represents the etching depth of the waveguide. And their transverse confinement factors are as low as 0.34% and 2.75%. Lasers with 3QWs have lower optical confinement factors compared to conventional lasers with 5∼6 QWs, and the laser linewidth can be effectively compressed by reducing the optical confinement factor. The effective refractive index neff was calculated for the laser waveguide confined by a bisbenzo cyclobutene (BCB) layer with a refractive index of 1.54. The effective refractive indices of the shallow etched waveguides and deep etched waveguides as a function of W are plotted in Fig. 1(d). The effective refractive index of the deeply etched laser waveguide can be fitted using a fifth-order polynomial. The narrow-slice width of W = 2 µm was selected for the following calculations.
Table 1. The epitaxial wafer structure
Using the effective refractive index, the coupling coefficient κ can be calculated based on coupled wave theory [27]
Fig. 2. (a) The coupling coefficient κ dependence on the extension width of the grating D when W, γ, and λB are 2 µm, 0.5, and 1550 nm. (b) The relationship between the coupling coefficient κ and the filling factor γ when W, D, and λB are 2 µm, 1 µm, and 1550 nm.
The deep-etched fifth-order sidewall grating with period Λ = 1.2 µm was simulated by the 2D (y-z plane) FEM [28]. The parameters W, D, and γ are selected as 2 µm, 1 µm, and 0.5 under the condition that the grating has a high coupling coefficient based on the above calculations. The model of sidewall grating is illustrated in Fig. 3(a). The refractive indices for the wide- and narrow-slice sections are calculated to be 3.2049 and 3.188. The scattering boundary condition layer is placed on the periphery of the BCB to eliminate the back reflection. The numeric port was added at the input and output of the waveguide, such that the lightwave is excited at the input port and recorded at both ports. With the defined parameters and the established model, the reflectance and transmittance spectra of the sidewall grating are successfully calculated and shown in Fig. 3(b) and 3(c). The sidewall gratings with period numbers N of 100, 300, and 500 provide reflectance of 0.148, 0.652, and 0.856 with the bandwidth of the half-maximum of 2.74, 1.27, and 1.04 nm, respectively, and the transmittances of 0.803, 0.272, and 0.062 at Bragg wavelength λB = 1524.2 nm. In our design, the end facet of the gain region is acted as the main output port, so the sidewall grating is required to have high reflectance and low transmittance to ensure that the loss caused by the transmission of the sidewall grating at the rear port is low. Therefore, the period number N = 500 is selected to prepare the SG-ADBR laser.
Fig. 3. (a) 2D design of a sidewall grating in COMSOL. (b) Reflectance and (c) transmittance spectra of sidewall gratings with the period numbers of 100, 300, and 500.
However, limited by the lithography process, the etched sidewall grating will be more like a sinusoidal grating as shown in Fig. 4(a). Using the 2D finite element method, we simulated the reflectance of the sinusoidal sidewall grating with the above parameters and the results are compared in Fig. 4(b). The peak reflectance for the sinusoidal sidewall grating is 0.682, which is slightly smaller than that of 0.856 for the rectangular sidewall grating, and can still play a good role in mode selection.
Fig. 4. (a) Schematic of a 2D sinusoidal sidewall grating. (b) Reflectance spectra of sinusoidal and rectangular sidewall gratings with the period numbers of 500.
3. Experimental and discussion
Based on the above designs, deep-etched SG-ADBR lasers with the parameters of W = 2 µm, D = 1 µm, Λ = 1.2 µm, γ = 0.5, N = 500 (corresponding to 600 µm) and L = 1000 µm are fabricated using the AlGaInAs/InP epitaxial wafer. Projection i-line lithography processes are employed to fabricate the devices. Figures 5(a) and 5(b) show the scanning electron microscopic (SEM) images of the surface view and the cross-section view of the sidewall grating after inductively coupled plasma (ICP) etching. The actual prepared sidewall grating has a narrow-slice width of 2 µm, an extension width of 0.9 µm, a period of 2 µm, and an etching depth of 4.4 µm, which is consistent with the design. To guarantee mutual electrical isolation, the ohmic contact layer between the sidewall grating section and the gain section was etched to form an isolation trench with a length of 5 µm. Figure 5(c) shows the microscopic image of an SG-ADBR laser with cleaved facets as output ports. The device is mounted on an AlN submount with temperature controlled to 287 K by a thermoelectric cooler. Continuous-wave injection currents are applied to the sidewall grating and the gain section separately. The output light is collected by a tapered single-mode fiber (SMF).
Fig. 5. Scanning electron microscopic images of the surface view (a) and the cross-section view (b) of the sidewall grating after ICP etching. (c) Microscopic image of an SG-ADBR laser.
The Keithley 2400 Source Meter was used to apply bias current on the laser and measure the voltage. The fiber coupled output power was measured by using the UC 8820 Optical Power Meter, and the lasing spectra of the laser front port were measured by using Yokogawa AQ6370D Optical Spectral Analyzer. With fixed ADBR injection current ISG = 120 mA, the variation of SMF coupling output power with the gain section injection current Ig was measured from the front and rear ports respectively. The light-current-voltage (LIV) characteristics of the SG-ADBR laser are plotted in Fig. 6(a), and the lasing spectrum at Ig = 50 mA is shown in Fig. 6(b) with a lasing wavelength of 1537.9 nm and a SMSR of 65.2 dB. The maximum SMF coupling output power is 5.6 mW at the front port when Ig = 98 mA and 2.6 mW at the rear port when Ig = 87 mA. The output power exhibits strong variation due to mode hopping caused by the heating effect of injected current as in [29,30]. The drop of output power with the increase of injection current can be explained by the wavelength mismatch between lasing mode and reflectance peak, due to the heating effect. The decrease of grating reflectance for the dominant mode will reduce the intracavity mode photon number and corresponding output power as the injected current Ig increases before mode hopping. At the same time, the reflectance corresponding to the neighboring short-wavelength FP mode (the next dominant mode) is increased. With the further increase of Ig, the short-wavelength FP mode becomes the new dominant mode as shown in Fig. 7(a). Then a stable single-mode lasing is formed again, and the output power returns to a high level. In addition, the output power of the rear port is not exactly in the same trend as that of the front port because the transmittance of the rear port is greatly affected by lasing mode wavelength as shown in Fig. 3(c). The output power at the rear port is a large loss for the laser, but we can coat the front port facet with a high reflective film to realize only one-directional high efficiency output from the rear port.
Fig. 6. (a) Applied voltage and SMF coupled powers versus Ig at fixed ISG of 120 mA. (b) Lasing spectrum for the SG-ADBR laser at ISG = 120 mA and Ig = 50 mA.
Fig. 7. (a) Lasing spectra versus Ig at ISG of 120 mA and (b) lasing wavelength and SMSR versus Ig for the SG-ADBR laser. (c) The detailed spectra of the mode hopping behavior (Solid line: Lasing spectra of different injection currents in the gain section; Dashed line: Schematic diagram of the sidewall grating reflectance spectrum). Superimposed lasing spectra (d) for 53 consecutive channels with 25-GHz spacing and (e) their corresponding lasing wavelength and SMSRs.
For the lasing spectra map versus Ig at fixed ISG = 120 mA in Fig. 7(a), single-mode lasing with the lasing mode hops to a short wavelength at 69 mA and 90 mA, respectively. Figure 7(b) shows their corresponding lasing wavelength and SMSR versus Ig. High SMSRs of greater than 60 dB are achieved for most of the current range, and SMSRs are still larger than 45 dB near the current of mode hopping with the lasing mode jumping between adjacent longitudinal modes. As shown in Fig. 4(b), the half-maximum bandwidth of the simulated reflectance spectrum is 0.82 nm, and the longitudinal mode spacing of the FP modes is about 0.6 nm. So single mode operation can be realized within a reflectance peak, i.e, single mode operation due to the mode selection of the DBR reflectance. The detailed spectra of the mode hopping behavior with ISG = 120 mA are plotted in Fig. 7(c), corresponding to the white dashed line in Fig. 7(a). As the injection current Ig increases, the reflectance spectrum of the sidewall grating and the lasing mode are both red-shifted due to heating effect of the injection current, yet the reflectance spectrum is less red-shifted than the lasing mode because of the fixed ADBR current ISG. When the injection current Ig is 52 mA, mode B is located at the center of the reflectance peak, and the SMSR is 65.2 dB. As Ig increases to 68 mA, mode B moves to the right side of the reflectance peak, while mode A moves toward the center. The lasing spectrum shows an increase in the power of mode A and a decrease in mode B. When Ig is further increased to 69 mA, the dominant mode changes from B to A, however, mode A remains at the left side of the reflectance peak. Until Ig increases to 84 mA, mode A moves to the center, and the SMSR returns to a high level of 64 dB. Therefore, continuous tunability with high SMSRs can be achieved by varying ISG to shift the reflectance peak position and adjusting Ig to fine-tune the lasing mode. The lasing characteristics of 53 wavelength channels consecutively lined up with 25-GHz spacings were measured with a tuning range of 10.3 nm as shown in Fig. 7(d). ISG and Ig were controlled to tune lasing wavelength and maximize the SMSRs. Figure 7(e) shows that the lasing wavelengths were accurately tuned from 1534.64 nm for the first channel to 1544.92 nm for the 53rd channel and their corresponding SMSRs are all over 53 dB.
The lasing spectra versus the grating section injection current ISG are given in Fig. 8(a) at the fixed Ig = 100 mA. At ISG of 109 mA and 146 mA, the lasing mode hops from short-wavelength to the adjacent long-wavelength FP modes because the FP mode redshift rate is less than that of the grating reflection peak, instead of the mode hopping to short wavelength side in Fig. 7(a). Figure 8(b) shows the lasing spectra map versus the total injection current Itotal measured as two electrodes are connected in parallel as one electrode. The lasing wavelength shifted from 1534.5 nm to 1544.7 nm as Itotal increased from 55 mA to 325 mA with only one mode hopping.
Fig. 8. (a) Lasing spectra versus ISG at Ig = 100 mA. (b) Lasing spectra of two electrodes connected in parallel as one electrode.
Finally, the single-sided frequency noise power spectral density of the SG-ADBR laser at ISG = 120 mA and Ig = 50 mA was characterized and plotted in Fig. 9(a), using self- homodyne optical coherent receiver method [31]. The frequency noise is dominated by random walk frequency modulation (FM) noise and flicker FM noise at the lower frequency range, and the frequency noise in the flat section is dominated by the white FM noise [32], from which the Lorentzian linewidth of the laser can be determined. Figure 9(b) shows the single-sided frequency noise power spectral density in a linear coordinate system, from which the white noise level is more clearly identified. And the white noise level of 91.7 kHz2/Hz is obtained corresponding to 288 kHz Lorentzian linewidth. Figure 9(c) shows the linewidths and front port power of the SG-ADBR laser as a function of Ig at a fixed ISG of 120 mA. At a constant ADBR injection current, the linewidths of the laser show oscillating variations with the injection current Ig. The variation of the linewidth is mainly affected by the front port power. The minimum linewidth measured in our experiments is 288 kHz at Ig = 100 mA. In addition, linewidths less than 400 kHz were also obtained around Ig = 50 and 75 mA. This shows that Ig and ISG can be adjusted at the same time to make the lasing mode in the center of the reflectance peak and achieve narrow linewidth.
Fig. 9. (a) Single-sided frequency noise power spectral density of the laser at ISG = 120 mA and Ig = 100 mA, showing a white noise level of 91.7 kHz2/Hz corresponding to 288 kHz Lorentzian linewidth. (b) Single-sided frequency noise power spectral density in linear coordinates. (c) Laser linewidths and front port power as a function of Ig at fixed ISG of 120 mA.
4. Conclusion
In conclusion, we have demonstrated a SG-ADBR laser with a high SMSR of 65 dB, a tuning range of 10.3 nm, and a linewidth of 288 kHz. The coupling efficiency of the fifth-order sidewall grating was studied numerically for designing the device structure. The deep-etched sidewall grating with a length of 600 µm is chosen with simulated reflectance of about 0.86. The ADBR lasers with deep-etched fifth-order sidewall gratings were fabricated, using an AlGaInAs/InP epitaxial wafer with a three-QWs active layer, by a projection i-line lithography process. The maximum SMF coupling output power is 5.6 mW at the front port and 2.6 mW at the rear port. The laser maintained stable single-mode operation at a wide range of operating currents on both the gain section and sidewall grating section. By varying ISG and Ig simultaneously, 53 wavelength channels consecutively lined up with 25-GHz spacings were measured with a tuning range of 10.3 nm and SMSRs greater than 53 dB during the continuous tuning operations. The minimum linewidth extracted from single-sided frequency noise spectrum is 288 kHz. The SG-ADBR lasers exhibit excellent performance with high SMSRs, continuous tunability, and narrow linewidths, and the gain section can be shallowly etched to avoid the troubles caused by deep etching in the active region. Furthermore, the cost-effective and efficient fabrication process provides a greater advantage for deployment in coherent optical communication.
Funding
Strategic Priority Research Program, Chinese Academy of Sciences (XDB43000000); National Natural Science Foundation of China (61935018, 62122073).
Disclosures
The authors declare no conflicts of interest.
Data availability
The 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.
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