高重频!宽温域!超高稳定性!量子点锁模光频梳

光学频率梳是一种特殊的激光器光源,其可在频域上产生一系列离散且等频率间隔的光谱分量。它具有各个光谱分量相位相干关系稳定的优点,从而自其被发现以来便引起了科研人员的浓厚兴趣。如今,它已被应用于许多前沿领域,包括光通信系统、光学频率精密测量、光纤信号产生和光学采样等。

在众多实现光学频率梳的技术中,半导体锁模激光器凭借其尺寸小、阈值低、调制速率高、操作便利、结构简单以及价格合理等诸多优点,被誉为最有前景的梳状谱发射器之一。

目前,随着光通信向高速率发展,市场上对低功耗、高重复频率的梳状谱发射器的需求与日俱增。在实际的系统应用中,一个梳状源往往需要在较宽的温度范围(例如-20-85℃)内保持稳定的工作。为了解决这个问题,量子点作为一个零维量子系统(三个空间维度都受限制)引起了业内的关注,它所特有的分立能级使之拥有高温度稳定性、超宽增益带宽和超快载流子动力学等先天优势。这些优异性能激发了学术界和工业界对高性能量子点锁模激光器的探索,这也促使量子点锁模激光器产生的光学频率梳在Tb/s 通信中的应用成为了研究热点。

一般而言,腔长较长的器件更易实现良好的高温特性,但较长腔长对应着较低的重频率,从而其无法被应用于密集波分复用系统之类的通信系统中。此外,虽然高重频率可以通过缩短器件长度来实现,但这对器件增益提出了更高的要求,同时也让其更难在高温条件下维持稳定的工作。简而言之,量子点锁模激光器在光通信系统应用中的主要瓶颈在于要同时满足非常宽的锁模温度范围和在整个温度范围内维持超高且稳定的重频率这两个条件。

近日,英国伦敦大学学院陈思铭博士带领的研究团队在Photonics Research 2020年第12期上(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)展示了他们最新研制的基于量子点锁模激光器开发的高性能光学频率梳。

该研究团队设计的量子点被动锁模激光器件结构示意图

此项工作的亮点在于:

1. 通过增加有源区量子点密度来提高材料增益,实现短腔长器件在25.5 GHz较高重频率状态下工作;

2. 通过优化量子点的生长条件,拉大量子点中基态与激发态能级之间的距离,让载流子更好地被限制在基态,降低了器件温度升高所引起的载流子跃迁的可能性,进而实现了迄今为止在整个量子点锁模激光器领域中最大温度范围内(20-120℃)稳定的纯基态激射;

3. 通过巧妙地设计被动锁模激光器的器件结构,使其具有在整个工作温度跨度内实现稳定锁模的操作极其简易的特点,从而便于将其应用到实际系统中。

研究人员认为,这项工作的实验结果证明了超稳定、易操作、无制冷的量子点锁模激光器作为梳状光源,在高带宽、大规模、低成本的光学通信系统中将有巨大的商业潜力和广泛的应用前景。

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.