High-brightness laser diodes (LDs) play an important role in many applications with their high reliability and low-cost advantages, such as optical pumping of solid-state lasers, material processing, and nonlinear frequency conversion. According to past research, optimizing the LD beam quality and enhancing the LD output power have been proven to be two effective means to increase the laser brightness^[1]. Reducing the spatial mode distribution of the emitter can optimize the beam quality of the LD. In 2008, Wenzel et al. achieved stable fundamental transverse mode operation by optimizing the groove width of ridge waveguide lasers^[2]. Still, the output power of narrow ridge waveguide lasers is relatively low. Tapered waveguides have become a focus to enhance the output power of the LD further. The narrow ridge stripe region filters out the higher-order modes and preserves the fundamental spatial mode, and the tapered amplification region optically amplifies the fundamental spatial mode to obtain a single-mode LD with high beam quality and high power. In 2016, Ma et al. introduced one-dimensional longitudinal photonic crystals into tapered lasers to achieve stable single-mode operation in the vertical direction^[3]. However, the tapered cavity introduces astigmatism and modal mismatch between the ridge cavities^[4,5]. In 2022, Fu et al. proposed an electrically injected double-ridge stripe parity-time-symmetric laser diode (PTLD), which obtains a single-lobe far-field pattern under high currents^[6]. Compared with a single-ridge stripe PTLD, the double-ridge stripe PTLD can output more power. A single LD has limited output power due to catastrophic optical damage (COD)^[7]. In order to break the limit of a single emitter, beam combining has been proven to be an effective way to increase the output power of LDs, and the spectral beam combining (SBC) structure is one of the most effective and convenient methods to achieve high-power and high-quality laser output^[8,9]. In 2000, the Lincoln Laboratory first demonstrated that SBC technology can improve semiconductor lasers’ power and beam quality^[10]. However, in conventional SBC technology, polarization-dependent gratings lead to a reduction in combining efficiency. In 2023, Zhang et al. improved the spectral combination efficiency by polarization multiplexing, but the degree of polarization (DOP) of the output beam was still not linearly polarized^[11], which is not conducive to the subsequent polarization beam combination. In order to improve the DOP of the SBC output, Hu et al. proposed a polarization full-feedback spectral beam combination (PFF-SBC) structure that separates the wavelength locking with beam-combining output^[12]. The transverse electric (TE) polarization state is used for the full feedback of the external cavity for wavelength locking, and the transverse magnetic (TM) polarization state is used for open-loop SBC output. The combining efficiency of the structure is increased by 11.26 percentage points, and the DOP is increased by 7.26%. PFF-SBC is a power amplification method highly correlated with DOP, and PTLD is a laser with large differences in beam quality between the two polarization states. Therefore, combining PTLD with PFF-SBC is considered an effective way to obtain high-brightness semiconductor lasers.

In this paper, in order to solve the problem that the TM polarization state destroys the beam quality in the PTLD, we introduce a new SBC structure with three PFF-PTLDs. In each PFF-PTLD structure, the TM polarization state is fed back to the PTLD to lock the wavelength, while the TE polarization state is used for output. Three output beams of PFF-PTLD are focused to a transmission grating (TG) by a transform lens (TL), which forms an open-loop SBC structure. The PFF-SBC structure can optimize the beam quality of the PTLD and improve the DOP of the SBC output. To the best of our knowledge, it is the first report of using PFF-SBC to improve the brightness of the PTLD.

Each PTLD possesses a double-ridge waveguide structure, with a width of 5 µm for each ridge waveguide; the gap of the double-ridge waveguide is 1 µm and the length is 2 mm. The schematic diagram of the PTLD chip structure is shown in Fig. 1.

In order to produce the parity-time (PT)-symmetric optical complex potential, only the left waveguide has an electrically injected window to get a variable gain for the injected current, while the right waveguide is not injected to keep the intrinsic loss constant all the time^[6]. The complex propagation constants of the supermodes of the PT-symmetric laser can be expressed asβc=βr±κ2−γ2,(1)where βr is the propagation constant of the waveguide element, γ is the variable mode gain of the gain waveguide on the left side, and κ is the coupling constant. When the current is small (γ<κ), the real part of the complex propagation constant of the two supermodes splits and the imaginary part degenerates to zero; at this time, the two supermodes are in the PT symmetry phase. When the current is large enough (γ>κ), the real part of the complex propagation constant of the two supermodes degenerates and the imaginary part splits. At this time, the two supermodes are in the PT symmetry-breaking phase; the electric field of one supermode is mainly localized in the gain waveguide, while the electric field of the other supermode is mainly localized in the loss waveguide. Because the coupling constant of the higher-order modes is greater than the coupling constant of the fundamental mode, when the supermode obtained by the fundamental mode coupling has a PT symmetry-breaking phase, while the other supermodes obtained by the higher-order mode coupling are in the PT-symmetric phase and have no gain, in the spatial domain, only the gain supermode obtained by the fundamental mode coupling has a gain and can be lased.

The emission wavelength of the laser diode is approximately 964.8 nm, and the spectral width for free-running is 1.04 nm. The angle divergence of the PTLD is 30.45° in the fast-axis direction and 14.44° in the slow-axis direction. The front facet of the PTLD has an anti-reflection coating with a reflectivity of less than 0.2%, and the rear facet has a high-reflection coating with a reflectivity larger than 99%. The test results of PTLD’s self-excited emission spectrum and free-running power curve are shown in Figs. 2(a) and 2(b).

The experiment is at 20°C; in order to reduce the influence of temperature changes, the PTLD is placed with the n-side facing up on the H-shaped heat sink for heat dissipation through the water cooling system.

The experimental setup of the PFF-SBC structure of the three laser diode emitters is shown in Fig. 3(a). The experimental setup of the conventional SBC structure is shown in Fig. 3(b). The PFF-PTLD structure is shown in Fig. 3(c).

In PFF-PTLD, due to the influence of quantum well strain and external stress, when the current is 2.3 A, the initial DOP of the PTLD rod is 96%. The laser beam from the PTLD is collimated by an aspheric lens (AL) and then passes through a half-wave plate (HWP-1). The aspheric lens has an effective focal length (EFL) of 8 mm and a numerical aperture (NA) of 0.5 to collimate both axes’ directions. The residual divergence after collimation is approximately 0.027° in the fast-axis direction and approximately 0.046° in the slow-axis direction. The HWP-1 converts the power ratio of the transmitted TE polarization state of polarization beam splitter (PBS) and the reflected TM polarization state to 85:15 at a current of 2.3 A. The polarized beam in the TM polarization state will be reflected by the PBS-1, and the polarized beam in the TE polarization state will directly pass through PBS-1. The TM polarization state is converted to the TE polarization state along the vertical direction through another HWP (HWP-2) with a specific rotation angle to match the highest diffraction efficiency of the reflection grating (RG). For the RG with 1800 lines/mm, the diffraction efficiency of the TE polarization state reaches more than 96%. The first-order diffracted beam is fed back to the emitter by the RG to form a PFF-PTLD structure. Through the RG feedback, the LD is locked to a specific wavelength. The TE polarization state propagates along the horizontal direction without any feedback. This way ensures that a single mode of PTLD is locked and is not affected by the feedback beam of the other laser diodes. In the PFF-SBC structure, the TE polarization states of the three PTLDs use a TL with a focal length of 200 mm to converge on the TG (1851 lines/mm) to form an open-loop SBC structure. For the conventional SBC structure shown in Fig. 1(b), the parameters of the optical components are the same as the PFF-SBC, and the partial reflectivity of the output coupling mirror (OC) is 15% in this structure. Compared to the conventional SBC structure, the PFF-SBC structure can ensure that the incident beams on the gratings are linearly polarized, thereby reducing the diffraction loss of the grating and improving the combining efficiency.

The output spot distributions are shown in Figs. 2(a) and 4(b). Figure 4(a) shows the spot distribution before the TG, where three spots are arranged side by side, indicating that when the three emitters simultaneously output, the beam quality will decrease by more than three times. Figure 2(b) shows the spot distribution after the TG, where the final output spot size of the three emitters is close to that of a single emitter, and the intensity is the superposition and sum of the three emitters. The intensity difference between the three spots was caused by the gain spectrum of the laser diode having a limited bandwidth. The central PTLD operates at the peak wavelength of the gain spectrum, which enables maximum gain acquisition. In contrast, the left and right PTLDs operate at wavelengths located at the weaker edges of the gain spectrum, resulting in reduced gain acquisition capabilities.

The spot profiles of PTLDs under different polarization conditions are presented in Fig. 5. All spot profiles in Fig. 3 were measured when the PTLD operated in free-running without an external cavity. The “TM + TE” represents the spot profile without polarization splitting in the PTLD, while “TM” and “TE” correspond to the spots observed at both ends of the polarizing beam splitter after polarization separation. For emitters without polarization state separation, the supermode evolution demonstrates a distinct PT symmetry transition: below the current of 1 A, the laser maintains a PT-symmetric phase with double-lobed emission, whereas, beyond 1 A, the system enters a PT symmetry-breaking phase accompanied by single-lobed emission^[6]. Notably, the TM polarization state exhibits multimode operation across all currents, while the TE polarization state shows current-dependent mode suppression. Specifically, the TE polarization state evolves from a double-lobed to a single-lobed profile with increasing current, demonstrating significantly fewer transverse modes compared to the non-separated case. These findings suggest that implementing polarization state separation and selectively outputting the TE polarization state can effectively improve the beam quality of PTLDs.

Figure 6 illustrates the relationship between beam quality and current under different polarization states along the x direction. The black curve represents the TE polarization state of the PTLD without an external cavity, while the red curve corresponds to its TM polarization state. The blue and green curves show the TE and TM polarization states of the PFF-PTLD structure, respectively. All four curves exhibit a trend of ascent followed by descent near 1 A, which is due to the breaking point existing around this current. Below the breaking point, both ridges are excited, which leads to a decrease in beam quality, thus forming an upward curve. After the breaking point, the emission area is reduced to half due to the transition to single-ridge operation, resulting in improved beam quality and the descending curve. It is worth noting that the beam quality under the TE polarization state shows superior performance compared to the TM polarization state under the same conditions, which is the same as the observed spot distribution. This is because the epitaxial material exhibits a higher gain threshold for TE polarization but a lower gain threshold for TM polarization. Under the same current condition, TM polarization preferentially excites multimode emission. The beam quality of the PFF-PTLD structure is better than that of a free-running single emitter because this structure removes the influence of TM polarization beam quality on PTLD beam quality through polarization separation. Therefore, the PFF-PTLD structure can further improve the beam quality of PTLD.

Figure 7(a) shows the variation of the output power and the power ratio of the TM polarization state of a single PTLD with the continuous-wave current. The black curve shows the relationship between the output power and current of the free-running PTLD without an external cavity. The red curve represents the ratio of TM polarization state power to the output power of the same PTLD, which is measured by separating the beam with a polarizing beam splitter. The reason for the decrease in DOP of PTLD at small currents is the existence of multiple longitudinal modes with different polarization states, and the reason for the decrease in DOP near the breaking point is the decrease in the power of the TE polarization state. When the injected current exceeds 1 A, the power of the TM polarization state approximately accounts for 3% of the total power. The variation of the grating diffraction efficiency in PFF-SBC and the conventional SBC with the continuous wave current is shown in Fig. 7(b). The diffraction efficiency of a grating is defined as the ratio of the power after grating diffraction to that before grating diffraction. The diffraction efficiency of the grating in the conventional SBC decreases with the decrease of the polarization degree of the laser, while the diffraction efficiency of the PFF structure grating is less affected by the decrease of the polarization of the laser, and the diffraction efficiency remains above 90% after the breaking point. The variation of the beam combination efficiency in PFF-SBC and the conventional SBC with the continuous wave current is shown in Fig. 7(c). The definition of the combined efficiency is the ratio of the output power of the SBC to the output power in the free-running mode. The reason why the combined efficiency is higher than 100% under a small current is that the external cavity of the conventional SBC reduces the threshold of the laser diodes, and after the PTLD breaks, the combined efficiency of PFF-SBC is 5% higher than that of the conventional SBC.

Figure 8(a) shows the output power and brightness of the PFF-SBC at different currents. The maximum output power of 2.71 W is obtained at a current of 2.3 A, and the maximum output brightness of 116.2MW·cm−2sr−1 is obtained at a current of 1.6 A. The definition formula of beam brightness isB=Pλ2MX2MY2.(2)

When determining brightness, P represents the output power, λ represents the central wavelength, and MX2 and MY2 represent the beam quality along the slow axis and fast axis, respectively. The reason for the decrease in brightness after 1.6 A is that the beam quality of the TE polarization state becomes worse. Figure 8(b) shows the combined laser spectrum of the PFF-SBC when the current is 1.6 A. Due to the high-quality feedback and dispersion effect of RG, the spectrum is clear and consists of three independent spectral peaks without any crosstalk. These three spectral peaks correspond to the three emitters in the LD source module. The total spectral width is 13.4 nm, and the wavelength separation between adjacent peaks is determined by both the emitter spacing and the wavelengths of individual lasers. Figure 8(c) shows the beam quality of PFF-SBC under different currents, which shows that the beam quality of the PFF-SBC structure is inferior to that of the polarization separation full feedback single emitter. The difference primarily stems from alignment errors introduced during the manual adjustment of beam spacing in the spectral beam combining process. The beam quality was measured by the BSQ-SP920 (Ophir Spiricon) in the justification criterion at D4σ.

Figures 9(a) and 9(b) show the beam quality of the PFF-SBC and the single PTLD at a current of 1.6 A; the beam quality of the PFF-SBC structure is improved compared to the single emitter without an optical cavity.

In conclusion, we proposed a PFF-SBC structure for three PTLDs, which improves the beam quality of the PTLDs and the combining efficiency of SBC through polarization separation and full feedback, which we call PFF-SBC. At the same time, the PBS in the PFF-SBC splits the beam into two linearly polarized beams with mutually perpendicular polarization directions, making the combined beam linearly polarized. The PFF-SBC structure can reduce the diffraction loss of the grating and be conducive to the subsequent polarization beam combination. At a pump current of 2.3 A, an output power of 2.71 W can be obtained, and the combined efficiency is 83.4%. Compared with the conventional SBC, the combining efficiency is increased by more than 4%. The maximum brightness of 116.2MW·cm−2sr−1 is obtained when the pump current is 1.6 A, which is 3.5 times that of a single PTLD. We combined the optical power from three PTLDs while achieving superior beam quality compared to a single PTLD. These effects result in the measured brightness exceeding three times that of a single PTLD. This structure can expand and combine more laser diodes, which provides a new way to improve the beam brightness of PTLDs.