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Abstract
We demonstrated a high-performance partially corrugated waveguide distributed feedback (PCW-DFB) laser with high output power, low relative intensity noise (RIN) and narrow linewidth. By introducing offset quantum-well structure that provides enough threshold gain difference for single transverse mode operation, the laser can achieve single mode behavior with an 8-µm-wide ridge waveguide. The laser has been designed by the simulation model based on the coupled wave equations, and the fabricated PCW-DFB laser with the cavity length of 1.3 mm exhibited an output power higher than 190 mW. Stable single mode characteristics have been achieved with a side-mode suppression-ratio (SMSR) over 55 dB. The RIN was less than -160.5 dB/Hz at an injection current of 470 mA, and the linewidth reached 45 kHz.
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1. Introduction
Semiconductor lasers are widely used in many fields due to their excellent optical properties, and in some cases performances in term with high power, low relative intensity noise (RIN) and narrow linewidth are strongly needed [1]. As one of the low-loss windows for atmospheric transmission, 1550 nm semiconductor lasers are widely used in free space optical communications. In recent years, silicon photonic (SiPh) has played an important role in applications such as data centers and semiconductor lasers have become the key light source for SiPh. High-power semiconductor lasers are needed to compensate for the optical loss [2,3]. At the same time, 1550 nm is a safe band for the human eye, making it suitable for light detection and ranging (LiDAR) applications such as autonomous driving and environmental monitoring [4,5]. In addition, narrow linewidth semiconductor lasers have important applications in coherent optical communications and LiDAR for speed measurement [6]. Microwave photonic links with significant dynamics require high power and low RIN semiconductor lasers [7].
Semiconductor lasers have the same noise sources of linewidth and RIN, and increasing the output power of the laser is conducive to reducing the linewidth and RIN [8]. Two methods are generally used to increase the output power of the laser, one is optimizing the epitaxial structure and the other is increasing the size of the waveguide. Through the optimization of the epitaxial structure, the injection efficiency can be improved, while the internal loss can be reduced. Optimizing the width and strain of the quantum wells and barriers [9], and using a gradient confinement layer can improve the injection efficiency [10,11]. The internal loss of the laser comes from the absorption of free carriers in the active region and the doped layer, which can be reduced by decreasing the number of quantum wells and the optical confinement in the P-type cladding layers [12]. Wang et al. reported a diluted waveguide structure to reduce the internal loss, and a distributed feedback (DFB) laser with a waveguide width of 2.3 µm achieved the output power of about 160 mW [13]. Increasing the waveguide size can reduce the intracavity carrier density. It also increases the heat dissipation area of the laser, reduces the influence of thermal effects and increases the thermal saturation output power [14]. A 1550 nm InGaAsP high-power laser fabricated by Freedom Photonics has a maximum output power of more than 300 mW and a RIN of less than -160 dB/Hz [15]. Mao et al. reported a 1550 nm laser of 4 mm length and 5 µm width with a record maximum continuous output power of 850 mW and a side-mode suppression-ratio (SMSR) of more than 50 dB [16]. Faugeron, et al. demonstrated a DFB laser with an output power of 300 mW, RIN<-162 dB/Hz and a linewidth of 130 kHz [17]. Recently, we reported a wide-waveguide high-power DFB laser. The output power reaches 170 mW with the SMSR higher than 55 dB and the RIN lower than -157 dB/Hz. Meanwhile, the linewidth of the laser is about 250 kHz [18]. Due to spatial hole burning(SHB) phenomenon, the increase of the cavity length of DFB lasers with traditional structure needs to be considered more carefully. Huang et al. proposed partially corrugated distributed feedback (PC-DFB) laser, pointing out that PC-DFB lasers have higher external feedback tolerance than DFB lasers with full gratings [19]. In recent years, PC-DFB lasers have been used to fabricate electro-absorption modulated laser (EML) devices because of their good laser stability [20]. Cheng et al reported a 100 mW high-power PC-DFB laser with a cavity length of 1 mm and a waveguide width of 2 µm [21]. PC-DFB lasers can be fabricated with longer cavity lengths and effectively avoid the SHB phenomenon to achieve higher output power. At the same time, increasing the laser cavity length is beneficial to obtain narrow linewidth.
In this work, we demonstrate the high performance partially corrugated waveguide distributed feedback (PCW-DFB) laser with high output power, low RIN and narrow linewidth. Using the offset quantum-well epitaxial structure helps to achieve low threshold current and high slope efficiency with single-mode operation at an 8-µm-wide ridge waveguide, since the confinement factor can be reduced by moving the active region away from the waveguide, and which also can reduce the optical loss [22,23]. The cavity length, grating coupling coefficient and length of the PCW-DFB laser have been designed using a simulation model based on the coupled wave equations to achieve single-mode, high-power operation. The fabricated laser achieves output power over 190 mW. It also shows a stable single-mode operation around 1550 nm with the SMSR over 55 dB within the injection current from 300 to 700 mA and the RIN below -160.5 dB/Hz. The Lorentzian linewidth is about 45 kHz at 82 mW output power for the fabricated laser.
2. Device structure and design
As schematically shown in Fig. 1. The presented laser includes the front section with first order grating and the back section without grating. Both sections use the same epitaxial structure and both have current injection. The facet at the end of the back section is high-reflection (HR) coated with the power reflectivity of around 0.95. The laser outputs from the facet of the front section which is anti-reflection (AR) coated.
The active layer consists of InGaAlAs separate confinement heterostructure (SCH) layers and the sandwiched multiple-quantum wells (MQWs). The MQWs have five compressively strained (1.2%), 6 nm-thick InGaAlAs quantum wells, and six tensile strain (-0.3%), 10 nm-thick InGaAlAs barriers. InAlGaAs materials have better confinement of electrons and better high-temperature characteristics, which can improve performance for high-power operations. The MQWs have the photoluminescence emission peak near 1.53 µm. The grating layer is 30 nm thick InGaAsP material. The p-InP layer is used to fabricate shallow ridge waveguide with a depth of about 2 µm. The lightly doped n-type InGaAsP passive waveguide layer is introduced below the active layer. A 15-nm-thick InP gap layer is introduced between the n-SCH layer and the passive waveguide layer. By adjusting the thickness of the passive waveguide layer, the optical confinement factor of the MQWs is designed to be around 3.8%. The offset quantum-well epitaxial structure also introduces optical confinement factor difference between fundamental transverse mode and higher order transverse modes, leading to enough threshold gain difference for single transverse mode operation. In our previous work, we demonstrated a 800 µm-long high-power laser by increasing single-mode operation waveguide width to 8-µm [18]. To further increase output power and realize low noise operation for application, in this work, the PCW-DFB laser structure is introduced to reduce SHB effect and enlarge reflection tolerance for low noise operation. Also wide waveguides decrease the series resistance of the laser and the photon density in the cross-section. In addition, the heat dissipation characteristics of the laser will be improved. All of these are beneficial to increase the saturated output power of the laser.
To obtain high-performance the PCW-DFB laser needs to be carefully designed in terms of its grating coupling coefficient, grating length (Lg) and laser cavity length (Lt). The time-domain and travelling-wave (TDTW) [24] modes is generally used for the analysis of the laser, where the laser cavity is divided into many segments and the output power at a specific current can be obtaining by solving the longitudinal photon density distribution of the laser. However, it is impossible to see whether the laser structure can achieve stable single longitudinal mode operation. To solve this problem, the simulation model based on the coupled wave equation has been used for the design. The coupled wave equation is a common method for analyzing gratings, which describes the coupling relationship between forward- and backward-propagating electromagnetic waves due to periodic dielectric perturbations in gratings [25,26]. The main idea of the simulation model is shown in Fig. 2. Assuming that the gain (G) in the grating section is uniform, the power reflection (R) of the grating can be calculated based on the coupled wave equation. This allows the laser to be equivalent to a Fabry-Perot laser with the power reflection of the two facets is 0.95 and R, respectively. According to the threshold condition of the laser [27], the threshold gain (G’) corresponding to the laser as a whole can be obtained to determine whether it is consistent with G. In order to make G consistent with G’, an iterative calculation of the threshold gain of the laser and the reflection coefficient r of the grating is formed. When the result of the calculation is self-consistent, the gain possessed by the grating part can be determined. Finally, the reflection spectrum and phase (${\rm \varphi }$) information determined by the grating can be obtained, the slope efficiency and threshold current can be calculated, and the single-mode operation of the laser can be judged. When the full width at half maximum (FWHM) of the grating reflection spectrum covers approximately two to three longitudinal mode spacings ($\Delta \lambda$), the adjacent longitudinal modes on both sides of the excited longitudinal mode compete simultaneously to achieve suppression of the adjacent longitudinal modes and thus to obtain the single longitudinal mode operation of the laser.
Fig. 2. Simulation model based on coupled wave equation. (λ is Bragg wavelength; ng is group refractive index; Lg, eff is the effective length of the grating; η is injection efficiency; αin is the internal loss; αm is the mirror loss; αm,2 is the mirror loss of front; h is Planck constant; ν is laser frequency; e is the electron charge, W is the width of waveguide; d is the thickness of MQWs; Nth is the density of threshold carrier; A is the line recombination coefficient; B is the spontaneous recombination coefficient, and C is the Auger carrier recombination coefficient.)
The simulation model based on the coupled wave equations is compared with the TDTW model to verify the accuracy of the model. Three sets of PCW-DFB laser structural parameters are selected as shown in Table. 1. It is seen that the proposed model agrees well with TDTW model. The deviation of the threshold current resulted from the two different simulation models is within 5%, and the consistency of the slope efficiency is better, with some of the results showing deviations of only about 3%. In addition, the threshold current and slope efficiency can be obtained more quickly for a given laser structure using the simulation model based on the coupled wave equations. It is common to judge whether a laser can operate in a single longitudinal mode based on the threshold gain difference between different longitudinal modes. However, the proposed model transforms it into a simpler way based on the longitudinal mode spacing and the FWHM of the reflection spectrum.
Table 1. Comparison of results between our proposed simulation model and the TDTW model
Based on this simulation model, the grating coupling coefficient, grating lengths and laser cavity length of the proposed high-performance PCW-DFB laser are designed to achieve low-threshold current, high slope efficiency and single longitudinal mode operation. As shown in Fig. 3(a)-(c), the number of longitudinal mode spacing covered by FWHM, slope efficiency, and threshold current versus the coupling coefficient and grating length were calculated. The laser length is around 1 mm. The number of longitudinal mode spacing covered by FWHM is at a low level near a coupling factor of 10${\rm \; c}{\rm m}^{{\rm -1}}$. With the increase of the coupling coefficient, the slope efficiency and threshold current reduce. The slope efficiency decreases approximately linearly, and the threshold current decreases in the vicinity of the coupling coefficient of 10 cm-1 significantly slows down, and then tends to stabilize. Combined with the above analysis, the coupling coefficient of the grating is set as 10${\rm \; c}{\rm m}^{{\rm -1}}$. Figure 3 (d)-(f). show the effects of the laser length and grating length on the number of longitudinal mode spacers covered by FWHM, slope efficiency, and threshold current. We expect to design a longer laser cavity length to increase the waveguide size, and increase the heat dissipation area, while decrease the series resistance. But simultaneously, the laser is expected to obtain a low threshold current, high slope efficiency, and good single-mode characteristics. It is seen that, as the grating length increases, the slope efficiency decreases faster than the threshold current at different cavity lengths. So, slope efficiency should be given priority when determining the laser structure. The grating length of the reflection spectrum FWHM covering three longitudinal mode intervals is set as the critical value, corresponding to the maximum slope efficiency at different cavity lengths. It is observed that the difference in threshold current is small (≤4%), but the difference in slope efficiency is nearly 30% at different cavity lengths and respective corresponding critical grating lengths. Therefore, considering the slope efficiency of the laser and the influence of the device size on the heat dissipation area and series resistance, the cavity length and the grating length of the designed PCW-DFB laser are set to 1.3 mm and 400 µm, respectively. The longitudinal mode spacing of the PCW-DFB laser is about 0.3 nm, and the number of longitudinal mode spacing covered by FWHM is about 3. The calculated threshold current of the laser is 83.7 mA, and the slope efficiency is about 0.33 mW/mA.
Fig. 3. The Number of longitudinal mode spacers covered by FWHM, Slope efficiency and Threshold current (a)-(c)versus the coupling coefficient and grating length. The laser length is around 1 mm; (d)-(f) versus the laser length and grating length. The coupling coefficient is around 10 cm-1.
3. Device fabrication
The designed PCW-DFB lasers are fabricated. Firstly, the gratings were patterned by electron-beam lithography (EBL) and the InGaAsP was slowly etched by dry etching using CH4/H2 /Ar. Figure 4 (a) shows the SEM image of the grating, and it can be seen that the grating is etched in good uniformity. Then the cladding and contact layers are re-grown by metal-organic chemical vapor deposition (MOCVD). The shallow ridge waveguide is etched by ICP-RIE followed by wet etching. As shown in Fig. 4(b), the width of the fabricated ridge is about 8 µm. The waveguide has a vertical and smooth profile as shown in Fig. 4 (c) of the SEM image for the etched ridge waveguide, which will help to reduce the scattering loss of the device and increase the output power. The 200 nm thick SiNx was deposited using PECVD, followed by etching to expose the contact metal, and then the P electrode was evaporated using Electron Beam Evaporation (EBE). The epitaxial wafer is rapidly annealed at 410 °C to reduce the resistance. After the epitaxial wafer is thinned, the N-electrode is deposited. The wafer was then cleaved into bar strips which were then coated with AR and HR coating at both facets for testing.
Fig. 4. (a) SEM image of the grating; (b) SEM image of the waveguide; (c) SEM image of the etched ridge waveguide; (d) Microscope image of the fabricated PCW-DFB lasers.
4. Characteristics of the fabricated device
4.1 Validation of the simulation model
The fabricated laser chip was placed on a copper heat sink mounted on a thermoelectric cooler (TEC) to control the chip temperature. A large-area photodetector is used to collect the laser emitted by the front facet of the PCW-DFB laser. The photocurrent of the photodetector is monitored using the Keithley source meter. The power received by the photodetector is calculated by the conversion efficiency. The measured light-current (LI) curve of the PCW-DFB laser at 25 °C is shown in Fig. 5(a). The threshold current of the laser is 85 mA, and the slope efficiency near the threshold current is 0.32 mW/mA, which are in good consistent with simulated results, and the deviations are both within 3%. As shown in Fig. 5(b), the longitudinal mode spacing is about 0.29 nm, which agrees with the simulation very well. Therefore, the simulation model based on the coupled wave equations provides an effective reference for the design and optimization of PCW-DFB lasers.
4.2 Performance of the PCW-DFB laser
Figure 6 shows the measured light-current-voltage (LIV) curves of the fabricated PCW-DFB laser. To realizes a maximum output power, the temperature of the laser is set at -5°C because the fabricated laser suffered from the heat dissipation problem especially at high current injection. The threshold current is about 50 m A and the slop efficiency reaches 0.29 mW/mA. The output power is over 190 mW at the injection current near 1A. The L-I curve shows good linearity above the threshold, and distortion occurs near the top of the curve as the injected current increases due to thermal effects. Laser performance can be further improved if p-side of the laser is mounted down on the AlN carrier. From the V-I curve, the turn-on voltage of the laser is 0.74 V, and the series resistance is about 1.8Ω.
We measured the optical spectrum at temperatures ranging from 5 °C to 15 °C, and the temperature shift permits a tunability of about 1.78 nm with a variation coefficient of 0.178 nm/K. Figure 7 (a) shows the optical spectra of the PCW-DFB laser at different currents at 10°C. The laser exhibits good single-mode characteristics within the injection current range of 300-700 mA. The SMSRs and center wavelengths versus the injection currents are shown in Fig. 7 (b). The center wavelength of the laser varies linearly with injection current increase, while the PCW-DFB laser behaves in a stable single-mode operation with SMSR higher than 55 dB. The LIV curves of the laser at 10°C are shown in Fig. 7(c). The threshold current is about 65 mA, and the slope efficiency near the threshold is of about 0.31 mW/mA. As the injection current increases, the temperature shift at different currents is calculated according to the temperature variation coefficient. Figure 7 (d) plots the temperature shift versus the thermal power corresponding to different currents, from which the thermal resistance of the fabricated PCW-DFB laser is calculated of about 30 K/W.
Fig. 7. (a)Optical spectra at different bias currents ;(b) Corresponding SMSRs and lasing wavelengths;(c) The L-I-V characteristics under the CW bias current; (d) Temperature shift for different thermal powers at 10°C.
We have also measured the RIN of the fabricated PCW-DFB laser. A single-mode fiber is used to couple the laser output. An optical isolator is implemented to prevent reflections from interfering with the operation of the laser. The high-speed photodetector receives the laser intensity through the variable optical attenuator (VOA), and the obtained RF signal is amplified by the electrical amplifier and read by the spectrum analyzer after passing through the DC blocking capacitor. Figure 8 shows the RIN curve measured at an injection current of 470 mA at 10°C. It is seen that the laser exhibits the RIN lower than -160.5 dB/Hz. We consider that high output power reduces the proportion of spontaneous emission, while the laser maintains stable single-mode behavior without the SHB phenomenon. Low RIN is achieved due to the effectively suppressed disturbances of photons and carriers.
We measured the linewidth of the PCW-DFB laser based on the time-delay self-homodyne method [28]. The measured linewidth of the fabricated PCW-DFB laser from 140 mA to 520 mA injection current at 10°C is shown in Fig. 9(a). It is observed that, when the injection current increases to 180 mA, the linewidth decreases to less than 100 kHz. When the injection current reaches 300 mA, the linewidth tends to about 50 kHz. As shown in Fig. 9 (b), the linewidth of the laser is inversely proportional to the output power P, according to the modified Shorel-Townes expression [29]. When the output power of the laser is about 82 mW, the linewidth reaches 45 kHz. PCW-DFB laser obtains narrow linewidth because of the low mirror loss and high output power.
Fig. 9. (a) Measured optical linewidth with different injection currents;(b) and different output powers for the fabricated PCW-DFB laser at 10°C.
5. Conclusion
We demonstrate a high-performance PCW-DFB laser with high output power, low RIN and narrow linewidth. The laser with offset quantum-well epitaxial structure reduces free carrier absorption loss to improve the saturation power of the laser. Meanwhile, the laser achieves single-mode behavior in an 8-µm-wide ridge waveguide because the offset quantum-well provides sufficient threshold gain difference between the fundamental transverse mode and the higher-order transverse mode. The simulation model based on the coupled wave equations has been used for the design of the PCW-DFB laser with a cavity length of 1.3 mm, a grating length of 400 µm and a coupling coefficient of 10 cm-1, which results that the laser can operate in a single longitudinal mode and achieve high power. The simulation model we proposed is faster than the TDTW model, and the simulation results are agreed very well with the experimental results, which can provide a reliable reference for the design of PCW-DFB lasers. The fabricated PCW-DFB laser exhibits the threshold current of about 50 mA and output power over 190 mW with slop efficiency of about 0.29 mW/mA. The RIN is below -160.5 dB/Hz and the Lorentzian linewidth reaches 45 kHz. In addition, the proposed PCW-DFB laser has the potential to avoid the SHB phenomenon, making it a good candidate of high power light sources.
Funding
National Key Research and Development Program of China (2021YFB2800504); National Natural Science Foundation of China (62274073, 62105112); Key Research and Development Program of Hubei Province (2021BAA001); China Postdoctoral Science Foundation (2021M700048).
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.
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