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Abstract
We demonstrate a polarization-stable and single-mode grating-coupled surface-emitting laser (GCSEL) with high side-mode suppression ratio (SMSR) of ∼40 dB and orthogonal polarization suppression ratio (OPSR) of ∼25 dB around 795 nm. The fabricated devices have low threshold current of ∼4.8 mA and low electrical resistance of 53 Ω at 25 °C. Meanwhile, a low thermal resistance of ∼1 K/mW is achieved, which is comparable with that of the record of ever reported for vertical-cavity surface-emitting lasers (VCSELs). The far-field divergence angle of surface-emitting beam is ∼14.5°x14.7° at an injection current of 12 mA indicating a relatively good beam quality. Our results open what we believe is a new way to produce polarization-stable single-mode surface-emitting lasers with simple fabrication process. While the GCSEL is specifically designed for quantum sensing applications such as atomic clocks, magnetometers, and gyroscope, its performance in terms of low-power consumption, low thermal resistance, good beam qualities, and wafer-level testing are of particular interest for a wide range of applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Polarization-stable, single-mode semiconductor lasers are compelling light sources for quantum sensing applications, including microscale atomic clocks, magnetometers, and gyroscopes, that rely on spectroscopic interrogation of alkali atoms contained in a gas vapor cell [1–3]. Among the numerous types of semiconductor lasers, vertical-cavity surface-emitting lasers (VCSELs) feature low power consumption, high beam quality and easy integration, have been widely applied to quantum sensing field over the last few years [4–6], However, conventional VCSELs typically operate in multi-transverse mode due to a relatively large transverse dimension. An oxide aperture of less than 3 µm is required to ensure single transverse mode operation of VCSELs, which leads to high electrical resistance and reduced lifetimes due to higher current densities and increased internal temperatures caused by higher thermal and electrical resistances [7]. Other alternative approaches used to realize single-mode VCSELs including photonic crystal structure [8], Zn-diffusion method [9] and surface relief structure and so on [10]. Furthermore, VCSELs are prone to unstable polarization due to their cylindrical symmetry of resonators and isotropic gain of quantum wells (QWs). The major approaches to achieve polarization-stable VCSELs including introducing asymmetric resonators and polarization-dependent gain or mirror loss [11–13]. While these methods have shown effectiveness in controlling the transverse mode and polarization state of VCSELs, they typically require additional structural complexity and stringent fabrication control.
As an alternative way to realize surface-emitting lasers, grating-coupled surface-emitting lasers (GCSELs) have the advantages combined both edge-emitting lasers and VCSELs, such as single mode, low power consumption and narrow spectral linewidth [14–16]. Moreover, single-mode GCSELs with high output power have been demonstrated from mid-infrared to terahertz range in the previous reports [17,18]. More recently, single-mode GCSELs with high modulation frequency bandwidth of 17 GHz and 21 GHz have been achieved at 850 nm and 1.3 µm respectively [19,20]. Our previous works have demonstrated single-mode polarization-controlled VCSEL [12] and single-mode distributed-feedback (DFB) laser with surface gratings [21]. In this paper, we present the design and experimental results of polarization-stable single-mode GCSELs with an emission wavelength around 795 nm corresponding to the D1 line of rubidium. Stable single-mode operation is realized at all injection currents and heat-sink temperatures, and the side-mode suppression ratio (SMSR) and orthogonal polarization suppression ratio (OPSR) were as high as ∼40 dB and ∼25 dB respectively, indicating efficient single-mode and polarization selection. Moreover, the fabricated GCSELs exhibited low thermal resistance of ∼1 K/mW which is comparable with that of the record of ever reported for VCSELs. Meanwhile, the far-field divergence angle of surface-emitting beam is ∼14.5°x14.7° at an injection current of 12 mA indicating a relatively good beam quality.
2. Device design and fabrication
Figure 1(a) shows a schematic of the device structure. The epilayers for GCSELs grown by metal organic chemical vapor deposition on a n-type GaAs substrate was primarily composed of a GaAs-tunnel junction, a p-Al0.65Ga0.35As cladding layer, an oxide layer, an active region, and a n-Al0.32Ga0.68As cladding layer. The active region sandwiched between n-type and p-type cladding layers was composed of three Al0.07Ga0.93As QWs separated by Al0.35Ga0.65As barrier layers aiming to get the photoluminescence emission peak around 779 nm. The top electrodes were positioned on both sides of the surface grating. Considering the high mobility of electrons as well as reducing optical absorption loss, a tunnel junction was adopted to make N-side up. The detailed epitaxial structure including the material composition, thickness and doping level of each layer is plotted in Fig. 1(b).
For a periodic Bragg grating shown in Fig. 2(a), the condition of constructive interference between the diffracted waves can be described as [22]:
where m and λ are diffraction order and radiated wavelength respectively, θi and θd are angle of incidence and diffraction respectively. The grating period is defined as ${\mathrm{\Lambda }_g} = \frac{{p\lambda }}{{2{n_{eff}}}}$, where p and neff are grating order and effective index respectively. In our device, the Bragg grating placed along the surface of ridge waveguide is consisted of a 2nd order grating sandwiched between the 1st order gratings. The propagating optical modes in a ridge waveguide have an incident angle of π/2. Thus, there is only the 1st order diffraction composed of forward diffractive wave (θd=π/2) and backward diffractive wave (θd = -π/2) for the 1st order grating which provides the in-plane optical feedback. As for the 2nd order grating, there are the 1st order and 2nd order diffractions. Surface emission is provided by the 1st order diffraction composed of upward diffractive wave (θd = 0) and downward diffractive wave (θd=π). Schematic of in-plane feedback and surface emission provided by 1st order grating and 2nd order grating is presented in Fig. 2(b). In order to achieve a stable lasing of Bragg wavelength, λ/4 phase shift is inserted between the 1st order and 2nd order gratings to break the degeneracy [23].
Fig. 2. (a) Diffraction from a periodic Bragg grating. θi and θd are angle of incidence and diffraction respectively. ${\mathrm{\Lambda }_g}$ is grating period. (b) In-plane feedback and surface emission provided by the 1st and 2nd order grating. (b) The cross-sectional diagram of GCSELs indicating the electrical and optical confinement.
In our design, an oxide-confined structure generally adopted in VCSELs is used to realize electrical and transverse optical confinement. As shown in Fig. 2(c), an oxide aperture with higher effective refractive index than outer region is formed after wet oxidation of the oxide layer. The oxidized Al0.98Ga0.02As turned into an insulative AlxOy with lower refractive index, thus an electrical confinement as well as transverse optical confinement is achieved. In order to maintain stable single transverse mode operation of GCSELs, the aperture width is optimized. Figure 3(a) and 3(b) display the simulated mode distribution of the fundamental mode (TE00) and the first-order mode (TE01) for an aperture width of 2.5 µm. It is seen that the single-lobe TE00 mode field is mainly confined within the center of the oxide aperture., while the double-lobe TE01 mode has more intensity distributed near the edge of oxide aperture. This leads to a higher scattering loss for the TE01 mode. Figure 3(c) and 3(d) plots the calculated effective index and confinement factors of the TE00 and the TE01 versus aperture width. The effective index difference and confinement factor difference decrease as the aperture width increases, which leads to multi-transverse mode lasing eventually. In a practical device, a relative larger threshold gain difference will be obtained between the TE00 mode and the TE01 mode owing to the non-uniform current distribution in the active region and higher oxide-aperture scattering loss in the TE01 mode, which favors the fundamental mode get sufficient gain to lase with the higher-order modes suppressed. Therefore, the aperture width has been optimized to be around 2.5 µm to ensure stable single transverse mode operation as well as a moderate electrical resistance.
Fig. 3. Simulated mode distribution of the fundamental mode TE00 (a) and the first-order mode TE01 (b) for an oxide-confined ridge waveguide. Calculated effective index (c) and optical confinement factor (d) of the fundamental mode and the first-order mode for the oxide-confined ridge waveguide with different aperture width.
The fabrication process of GCSEL device is based on our previous work [21]. The process on the epi-wafer was start with the selective wet etching of GaAs by citric acid/H2O2 so as to expose the AlGaAs grating layer. Then, ridge waveguides were etched through inductively coupled plasma reaction ion etching (ICP-RIE) followed by a wet thermal oxidation at a temperature of 400 °C in N2/H2O atmosphere to form the oxide aperture. The infrared microscope image of the oxide aperture is displayed in Fig. 4(a) with a rectangle aperture of ∼2.6 × 50 µm. Afterwards, the SiNx passivation layer covering the upper surface of the entire device was deposited through plasma enhanced chemical vapor deposition (PECVD). A current aperture and emission window revealing the n + -GaAs and grating region for the formation of ohmic contact and surface grating was defined by photolithography and etching off the SiNx layer by ICP-RIE. The surface grating consisted of 1st order grating and 2nd order grating was formed by electron-beam lithography (EBL) system and etched by ICP-RIE. Figure 4(b) presents the scanning electron microscopic (SEM) image of the fabricated surface grating with 1st order grating period of ∼120 nm and 2nd order grating period of ∼240 nm. The width of the grating is 10 µm which is easy for aligning with the aperture as well as maintaining sufficient optical feedback considering the aperture width of ∼2.6 µm. The length of 2nd order grating designed to be ∼2.52 µm has a similar dimension to the aperture width, which is good for achieving output beam with circular far-field beam profile. After the N-electrode is deposited, the n-GaAs substrate was thinned down to 100 µm and the P-electrode is deposited. The N-electrode and P-electrode were both composed of AuGeNi-Au forming ohmic contacts on n + -GaAs. Figure 4(c) presents the fabricated GCSELs under wafer-level testing. Meanwhile, the SEM image of a cleaved single chip with 200-µm cavity length is displayed in Fig. 4(d).
Fig. 4. (a) Fabricated oxide-confined ridge waveguide with a ∼50 × 2.6-µm rectangle oxide aperture. (b) SEM images of the fabricated surface gratings. (c) Picture of the fabricated GCSELs under wafer-level testing. (d) SEM image of a cleaved single chip with 200-µm cavity length.
3. Experimental results and discussions
The cleaved single chip was integrated on a copper heat-sink with temperature controlled by thermoelectric cooler. The temperature-dependent light-current-voltage (L-I-V) of the GCSEL chips used in the following characterization are illustrated in Fig. 5(a). The device has typical diode I-V curves with electrical resistance about 53 Ω and hardly varies with temperature. This value is far lower than the resistance (∼200 Ω) of a conventional GaAs-based single-mode VCSEL with small oxide aperture diameter. Meanwhile, relatively low threshold currents of 3.9 mA, 4.8 mA and 6.4 mA at the temperatures of 15 °C, 25 °C and 35°C are extracted. The threshold current density is as low as ∼3 kA/cm2 considering the active region area of 130 µm2. At higher injection currents, the output power tends toward saturation due to the internal heating. The slope efficiency of the device is estimated to be about 0.13 mW/mA, which can be improved by adding a bottom reflector to reflect the down-radiated light into the upward direction [16]. Figure 5(b) presents the optical spectra under different inject currents at a heat-sink temperature of 25 °C. Stable single-mode operation with high SMSR is obtained through all the injection currents, which is attributed to the careful designed oxide aperture width and λ/4 phase shift. The -3-dB spectral width of the fabricated device decreased from 0.12 nm to 0.076 nm as the injection current increased from 6 mA to 14 mA, which is limited by the resolution of the Optical Spectrum Analyzer (0.02 nm resolution). For a traditional VCSEL structure, it has a relatively short cavity length of ∼ 1 µm, which leads to a large spectral linewidth of ∼20-100 MHz [5,24]. In our GCSEL structure, it has a cavity length of 200 µm, thus a potential narrower linewidth is expected considering the inverse relation of cavity length to linewidth. The extracted peak lasing wavelength and SMSRs versus injection currents are plotted in Fig. 5(c) indicating a current-wavelength coefficient of ∼0.14 nm/mA and high SMSRs of ∼40 dB. The increase of the lasing wavelength is mainly caused by a temperature induced change of the refractive indices due to internal heating as the injection current increases. To relieve the thermal effect, a more effective thermal management method can be adopted, such as packaging in a heatsink with higher thermal conductivity. The single-mode lasing and the possibility of its fine tuning by modifying the injection current make our GCSEL a promising candidate for quantum sensing applications.
Fig. 5. Lasing characteristics of the GCSEL: (a) temperature-dependent L-I-V; (b) optical spectra under different injection currents; (c) SMSR and peak lasing wavelength as a function of injection current.
Laser sources for quantum sensing applications such as atomic clocks must feature strictly polarization-stable and single-mode emission. An undefined polarization generally increases the relative intensity noise of laser, which degrades the performance of quantum sensing system [5,25]. Figure 6 presents the polarization-resolved operation characteristics of the GCSEL. A polarizer was inserted between the collimating lens and the detector to obtain polarization resolved L-I curves as shown in Fig. 6(a). The device maintains polarization-stable up to thermal roll-over with a maximum magnitude of the OPSR around 17 dB, where OPSR is defined as $10\mathrm{\ast log}({{P_{orth}}/{P_{par}}} )$. Meanwhile, the polarization resolved spectra were measured at an injection current of 14 mA, and the results are shown in Fig. 6(b). The difference between the dominant and the suppressed polarization states defined as peak-to-peak OPSR is about 25 dB. This value is higher than the magnitude of the OPSR estimated from the powers in the two polarization states for the same current, which is attributed to ignored spontaneous emission in the peak-to-peak definition of the OPSR. The physical mechanism of the stable polarization is originated from modal scattering loss difference among two polarization states. In the longitudinal direction, the 1st order gratings provide strong optical feedback, while weak optical confinement is achieved by a small effective refractive index difference in the transverse direction. Therefore, the two orthogonal polarization states experience different scattering losses. The polarization anisotropy in the scattering losses contributes to the polarization-stable operation of the GCSEL.
Fig. 6. Polarization-resolved operation characteristics of the GCSEL: (a) L-I and OPSR; (b) optical spectra.
A fine tuning of emission wavelength by modifying the injection current and operation temperature is important for sensing applications. Figure 7(a) presents the red-shift of the peak emission wavelength with dissipated power and operation temperature. It is seen that the peak emission wavelengths are a linear function of the dissipated power defined as ${P_{diss}} = I \times V - {P_{output}}$. The peak emission wavelengths of different ambient temperature without internal heating are extracted at Pdiss = 0 mW with a linear fit of ∼0.069 nm/°C as shown in Fig. 7(b). Then, the thermal resistance of GCSEL can be determined by the equation [26]
Fig. 7. (a) Measured peak emission wavelength versus the dissipated electrical power for different heat-sink temperatures. (b) Extracted values of peak emission wavelength at Pdiss = 0 mW with a linear fit of 0.069 nm/°C.
The thermal resistance is estimated to be about 1 K/mW at the temperature of 25 °C, which is much lower than conventional GaAs-based single-mode VCSELs and is comparable with that of the record of ever reported for VCSELs [26–28].
One of the advantages of VCSELs over edge-emitting lasers is the circular output beam profiles with small divergence angles simplifying the design of beam-shaping optics. Figure 8 presents the measured far-field profiles of our GCSEL. The far-field divergence angle is defined as the full width at half maximum (FWHM). The transverse divergence angle (Fig. 8(a)) increased from ∼13° to ∼16° as the injection current increased from 8 mA to 14 mA, while the longitudinal divergence angle (Fig. 8(b)) is increased from ∼14.5° to ∼14.8° hardly varied as the increasing of injection current. A near-circular far-field profile with relatively small divergence angles of ∼14.5°x14.7° was achieved at an injection current of 12 mA. The increasing of divergence angles is caused by thermal lensing. The relatively small dimension of oxide aperture in the transverse direction of GCSEL leads to a faster increasing of divergence angle than the longitudinal direction.
Fig. 8. Far-field profiles of the GCSEL at different injection currents: (a) transverse direction; (b) longitudinal direction.
4. Conclusion
In conclusion, we have demonstrated a polarization-stable and single-mode 795 nm GCSEL with SMSRs and OPSRs as high as ∼40 dB and 25 dB. The device has a relatively low threshold current of ∼4.8 mA and a low electrical resistance of 53 Ω. Meanwhile, the thermal resistance is as low as ∼1 K/mW, which is comparable with that of the record of ever reported for VCSELs. A near-circular far-field profile with relatively small divergence angles of ∼14.5°x14.7° was achieved by our careful design of the GCSEL structure. We conclude that the GCSELs have potential in the field of quantum sensing applications such as atomic clocks, magnetometers, and gyroscope.
Funding
National Natural Science Foundation of China (62134008, 62204237, 62074141).
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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