# Purely Gain-Coupled Distributed-Feedback Bragg Semiconductor Laser Diode Emitting at 770 nm
## 1 Introduction  
Distributed feedback (DFB) semiconductor lasers, characterized by high power and efficiency [1, 2], narrow linewidth [3, 4], and tunable wavelength [5] have many applications in laser communication [6, 7], in radar [8], and as a pumping source [9]. The portion of the spectrum near 770 nm, which contains the transition lines of Mg, K, Fe, Ni, and many other atoms [10], also has many applications. It is used in helioseismology studies with resonance cells [11], the estimation of solar atmospheric parameters [12], and pumps for alkali-metal atomic clocks [13]. Atomic clocks are used in the timing systems of robots and laser gyroscopes are used in robots' gesture control [14]. Since robot study and application are now a booming market, cost effective laser sources are in a growing demand. However, research on 770 nm DFB lasers is very rare (although index-coupled DFB lasers with secondary epitaxy were considered more than 30 years ago [15]). Ordinarily, semiconductor lasers emitting at 770 nm are not based on DFB, but on quantum dots [16], distributed Bragg reflector arrays [17], or vertical-external-cavity surface-emitting laser arrays [18]; however, DFB lasers emitting at 770 nm still remain more explored.  
DFB lasers introduce a periodically changing perturbation on the basis of the resonant cavity of a Fabry-Perot (FP) cavity semiconductor laser, thereby, modulating the longitudinal mode of the laser to achieve single-longitudinal-mode output [19]. DFBsemiconductor lasers can be divided into index-coupled and gain-coupled semiconductor lasers [19]. A uniform-period index-coupled DFB semiconductor laser exhibits double longitudinal-mode degeneration [19]. The introduction of a quarter-wavelength phase-shift grating can effectively address this [20]; however, it leads to the spatial burning-hole phenomenon [21], causing the single-longitudinal-mode characteristic to deteriorate [22]. To avoid this, researchers have proposed many complex grating structures, for example, the multiple-phase-shift grating structure [23], corrugation pitch modulated structure [24], and apodizated structure [25]. Although such complex structures improve the performance of the device, they also considerably increase the manufacturing cost. Gain-coupled DFB semiconductor lasers do not have the problem of mode degeneration [19] and can achieve single-longitudinal-mode lasing [26]. However, the traditional gain-coupled DFB semiconductor laser requires secondary epitaxy and precision grating preparation technologies [27, 28], which are also complex and expensive.  
In this study, we only use i-line-lithography technology to form a gain contrast in the active region [29, 30] through surface periodic electrical injection, by modulating the imaginary part of the refractive index and obtaining a gain-coupled DFB semiconductor laser emitting at 770 nm that requires neither secondary epitaxy nor precision lithography. Our devices have stable single-longitudinal-mode lasing, high power (116.3 mW at 350 mA), a high side-mode-suppression ratio (33.25 dB), and a narrow 3 dB linewidth (1.78 pm). Comparing with commercial products [31], our chip shows a higher power of 116.3 mW and much simpler production process without nanoscale grating fabrication or secondary epitaxy. In addition, external cavity diode laser (ECDL) indeed achieves higher output power as comparing with DFB laser [32]. However, single chip solutions are always compact and highly integrated as compared with external cavities. Given these advantages, our devices have great commercial potential.