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Abstract
The electrically-pumped terahertz quantum cascade laser (QCL) is characterized by high power emission, compact, broad frequency coverage, and so on, which shows abilities for frequency comb operations. Although free-running QCLs can work as frequency combs by designing the laser structure with small group velocity dispersions and/or inserting mirrors to compensate laser intrinsic dispersions, the ideal comb operation can only be obtained by firmly locking the repetition frequency and carrier frequency of a laser. In this work, we have reported a repetition frequency locking of a terahertz QCL emitting around 4.2 THz. When the 6-mm-long laser is operated in continuous wave mode without any locking techniques, the repetition frequency is measured to be ~6.15 GHz with a linewidth of hundred kilohertz. Once a phase lock loop (PLL) is applied to dynamically control the drive current of the QCL, we have demonstrated a successful repetition frequency locking of the laser with a signal to noise ratio of 80 dB. This technique can be employed for the frequency comb and dual-comb operations of terahertz QCLs for high-resolution applications.
Highlights
The terahertz wave [1,2,3], roughly defined in the frequency range between 0.1 and 10 THz, has attracted most interests over past decades due to its unique features such as covering multiple “fingerprints” of different molecules, penetration through body tissue without ionization, ultra-fast transmission speed for communications and so on
It is worth mentioning that the signal noise ratio (SNR) of quantum cascade laser (QCL) can be improved over 80 dB under the effect of phase lock loop (PLL), and it is unprecedented in other way of phase lock
Both the linewidth and SNR comparison indicate that the PLL works successfully with the terahertz QCL comb emitting at 4.2 THz
Summary
The terahertz wave [1,2,3], roughly defined in the frequency range between 0.1 and 10 THz, has attracted most interests over past decades due to its unique features such as covering multiple “fingerprints” (characteristic absorption lines) of different molecules, penetration through body tissue without ionization, ultra-fast transmission speed for communications and so on. Laser states in the multiple-quantum-well active region is one of the most efficient terahertz radiation sources in the frequency range between 1 and 5 THz [10,11]. It has been proved that QCLs are suitable for generating stable terahertz frequency combs, but additional phase lock technique, in principle, is required for ultrahigh resolution applications. The terahertz QCL can be fully stabilized by locking both repetition frequency and carrier frequency, and here focus are mainly on the former, i.e. inter-mode beat note (IBN) frequency. We experimentally demonstrate a phase lock loop (PLL) to lock the repetition frequency of a 4.2 THz QCL. The experimental setup for phase-locking the repetition frequency of the terahertz QCL is demonstrated in detail. We present the experimental results after the PLL is applied to the laser comb
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