Broadband Terahertz Quantum Cascade Laser Dual‐Comb Sources under Off‐Resonant Microwave Injection

Liao, Xiaoyu; Li, Ziping; Zhou, Kang; Guan, Wanchun; Zhao, Yiran; Wang, Chenjie; Wan, Wen‐Ming; Yang, Sijia; Zhang, Zhenzhen; Wang, Chang; Cao, Juncheng; Zeng, Heping; Li, Hua

doi:10.1002/adpr.202100361

Cited by 3 publications

(3 citation statements)

References 38 publications

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“…To make the stability comparison simpler, in this work the repetition frequency of the comb laser (QCL1) is injection-locked by a stabilized microwave signal at 6.06365 GHz with a power of −25 dBm. A relatively low RF power is applied to maximally suppress the noise of the RF synthesizer, which normally presents large phase noise at a high RF power 57 . In Fig.…”

Section: Resultsmentioning

confidence: 99%

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Relative phase locking of a terahertz laser system configured with a frequency comb and a single-mode laser

Guan

et al. 2023

Adv. Photon. Nexus

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Stable operation is one of the most important requirements for a laser source for high-precision applications. Many efforts have been made to improve the stability of lasers by employing various techniques, e.g., electrical and/or optical injection and phase locking. However, these techniques normally involve complex experimental facilities. Therefore, an easy implementation of the stability evaluation of a laser is still challenging, especially for lasers emitting in the terahertz (THz) frequency range because the broadband photodetectors and mature locking techniques are limited. In this work, we propose a simple method, i.e., relative phase locking, to quickly evaluate the stability of THz lasers without a need of a THz local oscillator. The THz laser system consists of a THz quantum cascade laser (QCL) frequency comb and a single-mode QCL. Using the single-mode laser as a fast detector, heterodyne signals resulting from the beating between the singlemode laser and the comb laser are obtained. One of the heterodyne beating signals is selected and sent to a phase-locked loop (PLL) for implementing the relative phase locking. Two kinds of locks are performed by feeding the output error signal of the PLL, either to the comb laser or to the single-mode laser. By analyzing the current change and the corresponding frequency change of the PLL-controlled QCL in each phase-locking condition, we, in principle, are able to experimentally compare the stability of the emission frequency of the single-mode QCL (f s ) and the carrier envelope offset frequency (f CEO ) of the QCL comb. The experimental results reveal that the QCL comb with the repetition frequency injection locked demonstrates much higher stability than the single-mode laser. The work provides a simple heterodyne scheme for understanding the stability of THz lasers, which paves the way for the further locking of the lasers and their high-precision applications in the THz frequency range.

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Section: Resultsmentioning

confidence: 99%

“…A relatively low RF power is applied to maximally suppress the noise of the RF synthesizer, which normally presents large phase noise at a high RF power. 57 In Fig. 4(a), we show the "maxhold" traces of the intermode beatnote of QCL1 when the RF injection is on and off.…”

Section: Resultsmentioning

confidence: 99%

Relative phase locking of a terahertz laser system configured with a frequency comb and a single-mode laser

Guan

et al. 2023

Adv. Photon. Nexus

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“…Note that here a relatively low RF power is used to avoid high phase noise introduced by the RF synthesizer. [21,36] Equally-spaced dual-comb lines with a spacing of 14.5 MHz can be clearly observed in Figure 2b and the dual-comb spacing is equal to the difference between the two comb repetition frequencies, f rep1 and f rep2 .…”

Section: Frequency Comb and Dual-comb Performancementioning

confidence: 99%

Terahertz Semiconductor Dual‐Comb Sources with Relative Offset Frequency Cancellation

Zhou

et al. 2023

Laser & Photonics Reviews

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A dual‐comb source, employing two frequency combs with a slight difference in repetition frequencies, shows unique advantages in high precision spectroscopy, imaging, ranging, communications, and so on. In the terahertz (THz) frequency range, the electrically pumped quantum cascade laser (QCL) offers the possibility of realizing a compact dual‐comb source due to its semiconductor‐based chip‐scale configuration. Although dual‐comb operation of THz QCLs has been experimentally demonstrated, a full stabilization of THz dual‐comb sources is still challenging. Here, a self‐reference method is proposed for a pure THz QCL system. Without using any external locking components, one dual‐comb line is filtered out and it is beaten with the whole dual‐comb signal, which eliminates the common carrier offset frequency noise and reduces the dual‐comb repetition frequency noise. It is demonstrated that the self‐reference technique can significantly improve the long‐term stability of the dual‐comb signal. A record “maxhold” linewidth of 14.8 kHz (60 s) is obtained by implementing the self‐reference technique, while without the self‐reference, the dual‐comb lines show a “maxhold” linewidth of 2 MHz (15 s). The method provides the simplest way to improve the long‐term stability of THz QCL dual‐comb sources, which can be further adopted for various measurements.

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Terahertz quantum cascade laser frequency combs with engineered operation frequency around 4.0 THz

Wu,

Zhou,

et al. 2024

AIP Advances

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Freely engineering the operation frequency of frequency comb sources is crucial for various applications, e.g., high-precision spectroscopy, ranging, communications, and so on. Here, by employing band structure simulations, group velocity dispersion (GVD) analysis, and experimental verifications, we demonstrate that the operation frequency of terahertz (THz) quantum cascade laser frequency combs can be engineered from 4.2 to 4.0 THz. First of all, from the viewpoint of the band structure engineering, we shift the frequency corresponding to the optical transitions in the active region from 4.2 to 4.0 THz by slightly altering the thicknesses of quantum wells. Meanwhile, a GVD analysis is applied to evaluate the potential comb performance. Finally, experimental characterizations, e.g., emission spectra, inter-mode beatnote, dual-comb operation, are performed to validate the exceptional comb operation at 4.0 THz. The advancement in simulations and experimental results present a comprehensive method to customize the desired THz radiative frequency for comb generation, which facilitates the practical development of broadband, high-precision THz comb sources.

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Broadband Terahertz Quantum Cascade Laser Dual‐Comb Sources under Off‐Resonant Microwave Injection

Cited by 3 publications

References 38 publications

Relative phase locking of a terahertz laser system configured with a frequency comb and a single-mode laser

Relative phase locking of a terahertz laser system configured with a frequency comb and a single-mode laser

Terahertz Semiconductor Dual‐Comb Sources with Relative Offset Frequency Cancellation

Terahertz quantum cascade laser frequency combs with engineered operation frequency around 4.0 THz

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