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Quantum dot mode-locked frequency comb with ultra-stable 25.5 GHz spacing between 20 ℃ and 120 ℃

Optical frequency combs (OFCs) consisting of equally spaced discrete optical frequency components have emerged as promising tools for a wide range of applications including metrology, optical communications, optical clock distribution/recovery, radio-over-fibre signal generation and optical sampling. Among all approaches for OFCs generations, semiconductor mode-locked lasers (MLLs), motivated by its ability to generate stable and cost-effective high-repetition-rate optical pulses with extremely simple structures, are being heavily investigated as light sources in optical-communications systems. Moreover, an MLL typically provides 5-10 nm bandwidth, promising comb-based transmitters.

However, the practical systems normally require a comb source to work stably over a wide temperature range (e.g. from -20 °C to 85 °C). Quantum dots (QDs) with the delta-function-like density of states have been proved to be a desirable material with inherent properties, such as temperature resilience, ultrabroad gain bandwidth and ultrafast carrier dynamics. These superior features have inspired numerous researchers in the development of high-performance QD MLLs and their applications for multi Tbit/s communications.

The QD MLLs with short-cavity could easily provide us with low power consumption, high repetition rate (thus large tone spacing) and high optical signal-to-noise ratio (OSNR) frequency combs, which are favoured by short and medium reach dense wavelength-division multiplexing (DWDM) communications systems. However, the stable mode-locking operation at high temperature, exclusively through ground state (GS) transition is essentially difficult to achieve for short devices due to the carrier escape from the GS with increasing temperature. Although one may expect a broad mode-locking temperature range from one QD mode-locked device and observe a large mode spacing from another, a key challenge is to achieve simultaneously ultra-stable mode spacing over an extremely broad temperature range from a single frequency comb light source with large mode spacing.

Recently, the research group led by Dr. Siming Chen from University College London developed an ultra-stable frequency comb source based on a passively mode-locked InAs QD with high repetition rate over the widest temperature range yet reported for any type of MLLs, which is published in Photonics Research, Volume 8, No. 12, 2020 (Shujie Pan, Jianou Huang, Zichuan Zhou, Zhixin Liu, Lalitha Ponnampalam, Zizhuo Liu, Mingchu Tang, Mu-Chieh Lo, Zizheng Cao, Kenichi Nishi, Keizo Takemasa, Mitsuru Sugawara, Richard Penty, Ian White, Alwyn Seeds, Huiyun Liu, Siming Chen. Quantum dot mode-locked frequency comb with ultra-stable 25.5 GHz spacing between 20°C and 120°C [J]. Photonics Research, 2020, 8(12): 12001937).

Schematic of the designed passively two-section QD MLL.

In this work, by developing a QD active region with high dot density and large energy separation between the GS and higher energy states, a stable 25.5 GHz QDs MLL operating over a record temperature range between 20 °C and 120 °C exclusively from the GS transition was demonstrated. With temperature increased from 20 °C to 120 °C, a marginal change in the repetition rate of 0.07 GHz. Moreover, the device emits a relatively broad comb even at an operating temperature of 100 °C with 31 total channels within the 6-dB comb bandwidth. The corresponding average relative intensity noise (RIN) for the whole lasing spectrum was measured to be −146 dBc/Hz in the frequency range from 0.5 GHz to 10 GHz.

The researchers believe that the findings pave the way for utilizing ultra-stable, easy-operating, uncooled QD MLLs as efficient frequency comb sources for high-bandwidth, large-scale, low-cost WDM in optical communications.