A directional coupled emission external ring cavity (ERC) based on an interband cascaded laser (ICL) was designed and manufactured. The laser emitted from the front cavity facet of the Fabry-Perot ICL is vertically incident on the rear cavity facet through the blazed grating and three gold-plated reflection mirrors, thereby forming a closed-loop system. The laser emitted from the front and rear cavity facets form clockwise and counterclockwise traveling waves in ERC, respectively. Since the anti-reflection film coating only on the front cavity facet introduces different losses in two directions, resulting in directional coupling in the clockwise direction. And the 0th-order reflected light generated by the directional coupling light passing through the blazed grating is used as the output light. The emitted laser from the two cavity facets enter the ICL again through ERC, enhancing the gain of the ICL. Therefore, a higher output power and wider tuning range are achieved compared to the traditional external cavity with 0th-order diffracted light as output light. The output power of ERC-ICL is 75% of the output power of the corresponding Fabry-Perot ICL, which is much higher than the output power of Littrow external cavity ICL. And single-mode operation is observed within the tuning range of 163 cm-1 (220nm), which is twice the tuning range of Littrow external cavity.
Integration of ridge array and Talbot cavity is an effective method for semiconductor laser optical power amplification. However, it is difficult for such designs to work stably in the fundamental supermode, resulting in the inability to achieve phase locking among the ridge arrays. Here, we report a phase-locked scheme that significantly increases the waveguide loss of high-order supermodes by adjusting the absorption boundary width of the ridge array, making the Talbot devices work stably in the fundamental supermode. Compared with the first-generation devices, the output power of the designed device is increased from 286 mW to 359 mW, and the central brightness is increased by twice. The demonstrated phase-locked high-brightness terahertz (THz) laser sources will have great application potential in THz spectroscopy and imaging.
We have made improvements for QCL in the thermal management to produce high output power. Unlike the previous literature, we use epilayer-down mounting and buried heterostructures to achieve high output power by improving the heat dissipation and reducing the thermal resistance. At 20 K, the continuous wave threshold current density is 110 A·cm-2 and the maximum current density is 210 A·cm−2. The maximum output power is about 250 mW at single facet. The central frequency is approximately ∼4 THz, which matches the energy band design. The thermal simulation shows that, compared with the traditional device, the heat removal performance of the optimized device is significantly improved, and the core temperature is reduced by about 20 K. It improves the heat extraction through epilayer-down mounting and buried heterostructures and leads also to significant lateral heat fluxes. The ways can facilitate the heat extraction in all in-plane directions. In conclusion, this method is beneficial to the development of high continuous wave power, especially for thick active region design. The demonstration of buried heterostructure terahertz quantum cascade lasers for epilayer-down mounting can promote the development of high-power terahertz source in continuous wave.
Spectral beam combining (SBC) is a regular approach of utilizing semiconductor laser arrays, as it can greatly improve output power and maintain the beam quality. External cavity spectral beam combining based on the six elements quantum cascade laser (QCL) array, with a 10μm ridge width and a pitch of 60μm, was realized. The divergence of the light from output coupler is 0.6mrad and 3.8mrad, for fast axis (perpendicular to the array) and slow axis (parallel to the array), respectively. Under a condition of 10kHz repetition frequency and 1μ s pulse width, the array’s peak power is 2.9W without SBC system. The peak power of 0.9W and 1.34W is achieved when utilizing a output coupler based on Ge without anti-reflection (AR) coating or with AR coating, respectively. Corresponding the beam combining efficiency is 31.0% and 46.2%, respectively. We recorded the cross-locking phenomenon when tuning array by rotating the external cavity output coupler angle, as a relatively close element spacing results in mutual locking of adjacent elements. The system could tuning over 99cm-1 (1904cm-1--2003cm-1).
Room-temperature continuous wave (CW) operation of a tunable external-cavity quantum cascade laser (EC-QCL) at center wavelength around 7.2 μm is presented. The EC-QCL was implemented in a Littrow configuration. The gain chip is based on a diagonal bound-to-continuum design with a high-reflection coating on the back facet. A two-layer antireflection (AR) coating consisting of Al2O3 and ZnSe was designed and deposited on the front facet of the chip to suppress the Fabry–Pérot modes. With this AR coating, single-mode tuning range of 128 cm−1 was achieved, from 1346.7 to 1475.3 cm−1 (6.78 to 7.43 μm). High side-mode suppression ratio over 30 dB was achieved near the center gain region. A very low-threshold current density of 0.89 kA/cm2 and a high output power of 50 mW were obtained when the EC-QCL was operated in CW mode at 20°C.
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