# Recent Advances in Tunable External Cavity Diode Lasers
## 3 Research Progress of ECDL
### External Cavity Quantum Well Laser
#### 3.1.1 Grating-Feedback Configuration
Specifically, it has been demonstrated that the polarization direction of the beam can be converted from p-polarization to s-polarization by rotating the half-wave plate placed between the lens and grating. At the same time, the first-order diffraction efficiency of grating increased from 25% to 55%. [PERSON] et al. observed that the laser polarization direction affected the grating's mode selection efficiency due to its polarization sensitivity [33]. The relative positions of the chip and the grating in the experiment are shown in Figure 3a and Figure 3b, respectively. For configuration A, the grating lines were oriented perpendicularly to the p-n junction of the gain chip. In contrast, for configuration B, the gain device aligned the grating lines parallel to the p-n junction. The measured linewidth for configurations A and B were 0.28 nm and 0.1 nm at an injection current of 1100 mA, respectively. This difference can be attributed to the feedback wavelength range of the grating, which is proportional to the emitter size. The emitter size for configuration A and B were 15 um and 1 um, respectively. The experimental results showed that configuration B exhibited superior spectral characteristics compared to configuration A.  
Mode stability is a critically important performance attribute of external cavity diode lasers, particularly in achieving wide continuous mode-hop-free tuning. Ensuring mode stability is challenging due to several influencing factors, including competition among the internal diode cavity mode, mechanical vibrations, and temperature changes. For the Littrow configuration, research indicates that mode stability is influenced by key design  
Figure 3: **(a)** Schematic diagram of configuration A: grating lines are perpendicular to the p-n junction; **(b)** Schematic diagram of configuration B: grating lines are parallel to the p-n junction [33]. ©Elsevier. Copyright 2017 Optics and Laser Technology. parameters, including cavity length, the alignment and placement of the diffraction grating, and the positioning of a temperature sensor [34]. By optimizing geometric and optical design configurations, continuous wavelength tuning without mode hopping can be achieved. For instance, rotating the grating around a specific pivot point has enabled a mode-hop-free tuning range of 35 GHz at 780 nm. Moreover, long-term frequency stability of the laser has been shown to correlate strongly with the stability of the intracavity temperature. Improved frequency stability can be achieved by positioning the temperature sensor close to both the thermoelectric cooling device (TEC) and the external cavity support device. For ECDLs, the focal length of the collimating lens significantly impacts the output light performance, such as tuning range. To investigate this effect, five different structures with varying lens focal lengths were studied [35]. The results demonstrated that both the tuning range and power initially increased and then decreased as the cavity length increased. Among the five structures, the ECDL with a collimating lens focal length of 4.51 mm exhibited superior performance. This configuration achieved a maximum output power of 42.36 mW and a broad tuning range of 152.3 nm, spanning from 1458.2 nm to 1610.5 nm. The Littman configuration is another widely used external cavity structure, as shown in Figure 2b. In this setup, the first order diffracted light from the grating is entirely reflected by an external cavity mirror and retraced its path back to the grating. Subsequently, the first-order diffracted light from the reflected beam returns to the laser chip, while the zero-order diffraction beam serves as the output light. Compared to the Littrow configuration, the Littman structure achieves a narrower spectral linewidth by incorporating a reflecting mirror. In 2000, [PERSON] et al. presented a narrow-band tunable external cavity semiconductor laser utilizing the Littman configuration [36].