High-brightness laser diodes (LDs) have gained a good reputation because of their high electro-optical conversion efficiency, small size, light weight, and wide wavelength range. However, due to the limitation of poor spatial brightness and catastrophic optical damage (COD), they cannot be directly used in many applications^[1]. The spatial brightness of the LDs is positively correlated with the output power and beam quality. According to past research, coherent beam combining (CBC) and spectral beam combining (SBC) have been proven to be two effective approaches for overcoming the COD limitation of the ridge waveguide LD and for improving the brightness of the LD.

The CBC can effectively improve laser output power and not lead to undesired sidelobes in the far field. Therefore, the CBC ensures the beam quality of the laser while increasing the output power, which can lead to a high-brightness output. In 2016, G. Schimmel et al. employed a Michelson extended cavity for CBC of two diode lasers, achieving a combining efficiency of 82% through phase locking^[2]. In 2017, they coherently combined five diode lasers that were passively locked through an external cavity containing diffractive optical elements, which presented a combining efficiency of 76%^[3]. Passive CBC technology is more suitable for constructing high-brightness semiconductor laser modules in industrial applications due to its simplicity. But the increase in the number of beam combining units in the structure of external-cavity locked phase-coherent optical beams significantly reduces beam combining efficiency^[4,5]. According to previous predictions based on cold cavity models, the upper limit for a passive beam combining unit number with good coherence is between 8 and 10 elements^[6]. In order to efficiently combine a larger number of lasers, researchers have shifted their focus to active phase control in the study of active CBC. In 2012, Creedon, Redmond, and others demonstrated the highest power output and the greatest number of phase-locked semiconductor amplifiers actively phase-controlled CBC systems^[7,8]. Actively phase-controlled CBC allows for the combining of a larger number of semiconductor LDs, but it also increases the complexity of the system due to the need for real-time active phase correction from an external control system. The SBC has been proven to combine many low-power beams into a high-power beam without deteriorating the beam quality^[9]. In 2022, Zhang et al. achieved wavelength locking and beam combining functions separately through dual-wavelength division multiplexing external cavities, which further improves the SBC efficiency by reducing the effective front-cavity reflectivity^[10].

Combining multiple coherent beams through spectral combining has been demonstrated to be an effective way to achieve high-brightness fiber lasers. In 2008, Fridman et al. proposed a method to enhance the brightness of fiber lasers by coherent–spectral beam combining (CSBC) to overcome the amplification limitations of each individual method and achieved a total combining efficiency of more than 80%, with a beam quality approaching that of an individual laser^[11]. In 2015, Ma et al. demonstrated CSBC by employing eight 20 W all-fiber amplifiers, which achieved an output power of 142.1 W with the system’s combining efficiency exceeding 90%^[12]. However, previously there had been no research to improve the brightness of semiconductor lasers by CSBC.

In this Letter, a CSBC structure is demonstrated for four emitters to improve the brightness of semiconductor lasers. The CSBC structure combines two groups of CBC structures with two LDs using SBC to achieve a high beam combining efficiency. The CBC structure with two LDs has the highest theoretical beam combining efficiency. This CSBC structure allows for scalability to higher output powers without concerns about the failure of CBC when the combining units increase.

The experimental setup of the CSBC structure of the four LD emitters is shown in Fig. 1(a). The standard CBC structure is shown in Fig. 1(b), and the standard SBC structure is shown in Fig. 1(c). Four ridge LDs are used in the experiment with an active zone of 5.5 µm in the slow axis and a length of 2 mm. The angle divergence of the LDs is 30.4° in the fast-axis direction, and 13.9° in the slow-axis direction. The emission wavelength of the LD is approximately 962.6 nm, with a spectral width of 0.8 nm when free-running. The external cavity is composed of the back surface of the LD coated with a highly reflective layer greater than 99% and an external output coupler (OC). The front surface of the LD has an anti-reflection coating with a reflectivity of less than 0.5%.

In the experiment, in order to reduce the influence of temperature changes, the LDs were placed with the n-side facing up on the H-Mount for heat dissipation through the water cooling system. Four aspherical lenses with an effective focal length (EFL) of 8 mm and a numerical aperture (NA) of 0.5 were selected to collimate both axes. The residual divergence after collimation is approximately 0.026° in the fast-axis direction and 0.042° in the slow-axis direction. The laser beam from LD1 and LD2 passes through the aspheric lenses for collimation, then goes through a mirror (M) and a beamsplitter (BS) to form a quasi-CBC structure, generating sub-beam 1. The BS is a polarization-insensitive beam splitter device that can separate an incoming beam power into two outgoing beam powers in a ratio of 50%±5%. Another set of LD3 and LD4’s beam passes through the aspheric lenses for collimation, then goes through a mirror and a BS to form another quasi-CBC structure, generating sub-beam 2.

Figure 1(b) shows the schematic diagram of a standard filled aperture CBC structure, which optimizes beam quality compared to spliced aperture structures while avoiding additional far-field sidelobes. After the beam is partially reflected by the OC mirror, it returns to the LD, and through the self-organizing process of laser emission, the CBC establishes the external cavity to achieve a “supermode” with a locked phase. Figures 2(a) and 2(b) show the output beam profile in the CBC direction and in the non-CBC direction, respectively. Figure 2(c) shows the beam profile of the LD when it is free-running. The profile shown in Fig. 2 was detected in the setup in Fig. 1(b). It can be seen that the output power of the filled aperture CBC path is more concentrated, while the destructive interference path has a high-order mode with a central void in the beam profile, indicating that the structure effectively filters out high-order modes. The setup in Fig. 1(a) also exhibits central void beam profiles in two non-CBC directions. The two non-CBC directions are not connected. A black metal plate is positioned to reduce the damage of energy escaping from the CSBC setup to other elements in the optical system.

Sub-beam 1 and sub-beam 2, using a transmission lens TL with a 200 mm focal length, converge onto the transmission grating G (1851 lines/mm) to form a quasi-SBC structure. Figure 1(c) shows the schematic diagram of a standard SBC structure composed of a transmission lens and a diffraction grating used to combine the emitted beams from two different wavelengths of LD into a single beam. The grating G achieves a diffraction efficiency of over 98% for the 965–980 nm wavelength range in the S-polarization state. After diffraction by grating G, a plane mirror OC with a partial reflectance of 5% is used as the external cavity feedback. The beam, after partially reflecting off the OC mirror, then returns to the LD to form the combined beam cavity.

Through the OC feedback, sub-beam 1 and sub-beam 2 were locked to two specific wavelengths.

At a test temperature of 20°C, Fig. 3 shows the relationship between the beam combining efficiency and the injection current of the laser transmitter with the CSBC structure and the comparison of power between the CSBC structure and a single laser transmitter. The experimental results of a single laser transmitter were detected after the aspherical lens without the BS, TL, or OC. Under a current of 0.55 A, the optical power emitted by the CSBC structure is 1.422 W, whereas the optical power emitted by a single LD is 0.418 W. It can be observed that the output power of the CSBC structure is approximately 3.4 times that of an individual laser emitter. The power loss at each BS is approximately 2.5% of the output optical power, and the power loss at the diffractive grating is approximately 10% of the output power. Due to the weak beam feedback provided by the external cavity, the threshold of the semiconductor laser is reduced, resulting in a higher measured power with the external beam feedback at low operating currents compared to free running power^[10]. As the operating current increases, the proportion of higher-order mode power in the laser output increases. The increase in higher-order mode power has almost no effect on the output power of the CBC. Therefore, as the operating current increases, the efficiency of beam combining gradually decreases.

The combined beam’s M2 was characterized using the beam quality analyzer (BeamSquared SP920) under a current of 0.55 A. Figure 4(a) shows the measured results for the beam quality of a single laser emitter at 0.55 A, and Fig. 4(b) shows the measured results for the beam quality of the CSBC structure at 0.55 A. When there is no external cavity, the slow-axis beam quality of a single emitter is M4σX2=1.623, and the fast-axis beam quality is M4σY2=1.687. The destructive interference path of the CBC in the CSBC structure acts as a filter for the high-order transverse modes, and the beam quality of the constructive interference path of the CBC in the CSBC structure is improved compared to that of a single laser emitter. The slow-axis beam quality is M4σX2=1.508, and the fast-axis beam quality is M4σY2=1.692 of the four lasers combined by the CSBC structure. They are significantly reduced in overall beam quality.

The improvement is based on the definition formula of beam brightness, B=Pλ2MX2MY2,(1)when determining brightness, with P representing the output power, λ representing the central wavelength, and MX2 and MY2 representing the beam quality along the slow-axis and fast-axis, respectively. Based on Eq. (1), the brightness of a single laser emitter at 0.55 A is 16.02MW·cm−2·sr−1, while the brightness of the CSBC structure is 58.65MW·cm−2·sr−1, which is 3.66 times the brightness of a single laser emitter. This is attributed to the structure that increases the output power and improves the beam quality.

Figure 5 shows the spectral distribution of the CSBC structure. The laser output spectrum is composed of two spectral peaks corresponding to the two groups of CBC structures. The two peaks are located near 963.5 and 969.5 nm, respectively. The wavelength spacing of the adjacent spectral peaks is 6 nm, and according to the grating formula^[13], the adjacent distance of the beam output from the two groups of CBC is approximately 4.5 mm, which is consistent with the actual distance of the lasers.

In summary, we have demonstrated the achievement of high output optical power and brightness in LD sources using the CSBC structure. In this structure, the CBC parts optimize the beam quality and improve the output power, and the SBC part combines the beam from two groups of CBC into one beam, which further increases the output power. A maximum output power of up to 1.422 W was obtained, and the brightness of the CSBC structure is 58.65MW·cm−2·sr−1 with a combining efficiency of 87.2%. The entire structure avoids the significant decrease in combining efficiency observed with the increase in the number of composing units in the standard passive CBC structure, while maintaining a beam quality that is superior to the standard SBC structure. Thus, the proposed structure provides a new approach to increasing the beam brightness of LDs.