**Author details**

Tsung‐Tse Lin

Address all correspondence to: ttlin@riken.jp

RIKEN Center for Advanced Photonics, Terahertz Quantum Device Research Team, Sendai, Japan

## **References**


compact portable size cryogenic cooling systems. We also describe the temperature perform‐ ance parameters of THz QCLs, introduce the results of an indirect injection design scheme in the THz region and modulation height active structure design with different barriers and wells for further design direction. The recent fabricated THz QCLs are combined with the liquid nitrogen cooling Dewar condenser to demonstrate the relatively compact potable THz source unit by QCLs. The different injection schemes in THz and barrier‐well height design in the active region introduce one of the directions for the further high temperature and large output

RIKEN Center for Advanced Photonics, Terahertz Quantum Device Research Team, Sendai,

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power operation of THz QCLs for real applications.

Address all correspondence to: ttlin@riken.jp

**Author details**

84 Quantum Cascade Lasers

Tsung‐Tse Lin

**References**

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Japan


## **Power Amplification and Coherent Combination Techniques for Terahertz Quantum Cascade Lasers Power Amplification and Coherent Combination Techniques for Terahertz Quantum Cascade Lasers**

Yan Xie, Yanfang Li, Jian Wang, Ning Yang, Weidong Chu and Suqing Duan Yan Xie, Yanfang Li, Jian Wang, Ning Yang, Weidong Chu and Suqing Duan

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65350

## **Abstract**

Power amplification and coherent combination are important ways to improve the output power and beam quality of single‐mode terahertz quantum cascade lasers (THz QCLs). Up to date, the tapered waveguide is the most convenient way to amplify the power of THz QCLs. The self‐focusing effect in tapered THz QCLs induces non‐ monotonic behaviours of the peak power and far‐field beam divergence, which lead to the existence of optimal structural parameters. The surface and lateral grating techniques have also been employed in tapered THz QCLs to further improve the spectral purity. For coherent combinations, the progress of facet‐emitting phase‐locked arrays of THz QCLs is still limited due to both the lack of the understanding of dynamics of coupled QCLs and the difficulties in designing high‐performance coupled wave‐ guides. We will briefly review the developments of coherent arrays of THz QCLs and present a design of monolithic QCL arrays with common coupled cavity to achieve the optical mutual injection, which may provide a new way for coherent combination of THz QCLs.

**Keywords:** terahertz, quantum cascade lasers, tapered waveguide, phase‐locked array

## **1. Introduction**

The performance achieved by state‐of‐the‐art terahertz quantum cascade lasers (THz QCLs) [1– 3] has demonstrated impressively that these compact semiconductor laser sources have become the ideal coherent radiation source for a large variety of applications, such as security check, free space optical communication, terahertz spectroscopy and imaging, particularly when

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

considering the tremendous degree of customization feasible due to the comprehensive design freedom for both the gain material and the device geometry [4]. To date the peak output power ofTHzQCLscanexceed1W.However,thehigh‐powerdevicesareusuallymulti‐modeoperated that strongly limits their applications where the purity of spectrum is desired. The power amplification and coherent combination techniques may provide ways to improve the output power of THz QCLs and meanwhile maintain the single‐mode operation. Besides, the beam divergence of a THz QCL is quite larger and the beam quality is also worse than that of mid‐ infrared QCLs or traditional semiconductor lasers since its aperture is comparable with the wavelength. Some of the power amplification techniques and the coherent combination techniques may also help to improve the brightness and the beam quality of THz QCLs.

As a method of power amplification, the tapered gain region can effectively improve the output power of a laser maintaining the single‐mode operation, which has been widely employed in diode lasers and mid‐infrared QCLs. For THz QCLs, the tapered cavity also increases the aperture, which will decrease the divergence of the laser beam in the horizontal direction.

Besides the power amplification, the coherent power combination of lasers is also a useful way to improve the output power maintaining the purity of the lasing spectrum. The phase‐locked arrays of surface‐emitting THz QCLs have demonstrated the single‐mode operation as well as the great reduction of the divergence of the laser beam. However, due to the difficulty of design and fabricate the high efficiency and low‐loss coupling waveguide, the progress of facet‐ emitting phase‐locked arrays is still limited.

This chapter will describe the tapered THz QCLs as a typical power amplification technique and the development of phase‐locked THz QCL arrays as coherent power combination techniques. For the tapered THz QCLs, we will present the basic characteristics of tapered THz QCL devices at first and introduce the development of THz tapered distributed feedback (DFB) lasers with surface grating and lateral gratings, which further ensure the single‐mode (longi‐ tude mode) operation. For the phase‐locked THz QCL arrays, we will briefly review the developments of non‐coherent arrays and phase‐locked arrays of mid‐infrared QCLs, and then study the basic dynamics of coupled THz QCLs to discuss the difficulties of developing coherent arrays for THz QCLs. Finally, we will introduce the recent developments of phase‐ locked THz QCL arrays.
