Objective  Solid-state lasers pumped by diode lasers have the advantages of small size, long lifetime, high efficiency, high energy and good beam quality because of the high emission cross section and the matching of the emission and the absorption spectral line. In particular, diode-pumped dual-wavelength solid-state lasers are attractive for environmental monitoring, laser radar for velocity measurement, medical instruments, holography, and terahertz generation. In addition to dual wavelengths, the following laser characteristics are required for these applications. The wavelength spacing between the two lasers must be small enough to generate terahertz waves; The two beams must have nearly equal power; The relative timing of the output pulses must be controllable, so that the output pulses can be in synchrony or in succession ; The two beams must be coincident and the wavelengths must be tunable. For this purpose, a dual wavelength excimer based on spectral beam combining (SBC) is designed in this paper.

  Methods  SBC comes from the principle of wavelength division multiplexing semiconductor laser transmitters used in communications (Fig.1). Beam combining was achieved using a common external cavity containing a grating, which simultaneously forces each Nd:YAG laser to operate at a different controlled wavelength and forces the beams from the two lasers to coincide. The lasers are arranged in a line at the focal plane of a transform lens. The collimated beams overlap at the grating and the output coupler. The common external cavity forces the beams to copropagate, and each laser has a different wavelength.

  Results and Discussions   The spectrum of the combined beam shows the wavelength spread is 3.1 nm (Fig.3). At a maximum current of 200 A, the pulse energy of Laser 1 is 94 mJ and Laser 2 is 92 mJ before beam combining. The pulse energy of the combined beam is as high as 173 mJ, which corresponds to a combining efficiency of 93% (Fig.4). In the horizontal direction (beam combining direction), the M² is 2.7 and 2.1 for Laser 1 and Laser 2 before beam combining. The M² of the combined beam in the horizontal direction is 2.8. In the vertical direction (no beam combining direction), the M² is 2.2 and 1.9 for Laser 1 and Laser 2 before beam combining respectively. The M² of the combined beam in the vertical direction is 2.2 (Fig.5).

  Conclusions  In summary, we have reported a dual-wavelength laser source by SBC of two solid-state Nd:YAG lasers. The resultant output beam quality is similar to that from a single laser. For the combined beam, a pulse energy of 173 mJ, and a combining efficiency of 93% are obtained. We believe that SBC could also be applied to an actively Q-switched Nd:YAG laser to obtain ns pulse durations and higher peak powers. Furthermore, because of the existence of additional transitions of Nd³⁺: YAG, it is expected that four or more lasers with different wavelengths (e.g., 1 052 nm, 1 061 nm, 1 064 nm, and 1 072 nm) can be combined in the future.