实验演示平衡驱动双并行马赫-曾德尔调制器生成超平坦光频梳

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"通过平衡驱动的双并行马赫-曾德尔调制器生成超平坦光频梳" 在光学领域，光频梳（Optical Frequency Comb, OFC）是一种具有等间距频率分量的光谱，它在精密光学测量、频率合成、光钟以及量子信息处理等领域有广泛应用。本文提出的是一种利用平衡驱动的双并行马赫-曾德尔调制器（Balanced Driven Dual Parallel Mach-Zehnder Modulator）来生成超平坦光频梳的新方法。马赫-曾德尔调制器（Mach-Zehnder Modulator, MZM）是一种常见的电光调制器，它基于光的干涉原理，通过改变两个分支中的相位差来调制输入光信号的强度。当两个并行的MZM受到相同但相反的电压驱动时，可以实现对输入光信号的幅度和相位的精确控制，从而产生等间隔的频率成分。论文中提到，理论分析表明，通过这种方法可以生成五音和七音的光频梳，所有频率分量的强度完全相等。实验结果验证了这一理论，实现了平坦度分别为0.6 dB和1.26 dB的五音和七音光频梳，这与理论预测的结果相符，表明了该方法在产生高度平坦的光频梳方面的潜力。 OFC的特性，如等频率间隔、等功率谱以及灵活性，对于多种应用至关重要。等频率间隔确保了每个频率分量的精确可追踪性，而等功率谱则意味着每个分量对系统的影响一致，这对于频率标准和计量学尤其重要。此外，通过调整调制器的驱动电压，可以灵活地改变光频梳的特性，例如改变频率间隔或调整梳的带宽。超平坦的光频梳尤其适用于光学通信，因为它们可以减少由频率不均匀性引起的信号失真。在量子计算和光学传感中，高平坦度的光频梳能够提供更稳定的参考源，提高测量精度。同时，这种技术还有可能促进新型的光学频率合成和微波光子学的发展。这篇论文提出了一种创新的超平坦光频梳生成技术，通过平衡驱动的双并行马赫-曾德尔调制器，不仅理论上证明了其可行性，而且在实验上也取得了显著的效果。这项工作为未来光学和光电子领域的研究提供了新的思路和潜在的技术手段。

Generation of ultra-flat optical frequency comb

using a balanced driven dual parallel

Mach–Zehnder modulator

Jian Wang (王建)

1,2

, Haiwen Cai (蔡海文)

*, Dijun Chen (陈迪俊)

and Ronghui Qu (瞿荣辉)

Shanghai Key Laboratory of All Solid-State Laser and Applied Techniques, Shanghai Institute of Optics

and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China

University of Chinese Academy of Sciences, Beijing 100049, China

*Corresponding author: hwcai@siom.ac.cn

Received January 6, 2015; accepted April 23, 2015; posted online May 26, 2015

We propose and experimentally demonstrate an ultra-flat optical frequency comb (OFC) generator by a bal-

anced driven dual parallel Mach–Zehnder modulator. Five- and seven-tone OFC with exactly equal intensity can

be generated theoretically. Experimentally obtained five- and seven-tone OFC with flatness of 0.6 and 1.26 dB

are demonstrated, respectively, which agrees well with the theoretical results.

OCIS codes: 060.1660, 060.2330, 060.2380.

doi: 10.3788/COL201513.060604.

The optical frequency comb (OFC) generation with equal

frequency spacing, equal spectrum power, and flexible

tunability is highly desired in many fields. OFC generators

offer attractive and promising applications such as for

generating precisely spaced optical wavelengths for

dense wavelength-division multiplexing

[1]

, frequency

metrology

[2]

, optical clock

[3]

, and so on. Generally speaking,

mode-locked lasers (MLLs) and electro-optic modulator

(EOM)-based OFC generators are potential candidates.

MLL-based generators suffer from poor tunability and

stability since the resonator is easily influenced by

environment.

OFC generation for flat combs via electro-optic modu-

lation is a potential and economic method, since multiple

channels can be generated with few lasers and could be

scalable. In most cases, it is important to generate fre-

quency combs with good spectral flatness. However, it

is difficult to generate OFC with good flatness, because

the intensity of each mode is governed by Bessel function.

Schemes such as cascaded amplitude/phase modulators,

and an amplitude modulator cascaded with a phase modu-

lator provide an OFC with dozens of lines have been pro-

posed

[4–9]

which need multiple channel radio frequency

(RF) signals with different powers, and different frequen-

cies to drive the multiple EOMs, resulting in a costly and

complex structure with poor tenability. Also, O FC gener-

ation scheme based on single sideband modulation recir-

culating frequency shifter has also been proposed

adopting by dual parallel Mach–Zehnder modulator

(DPMZM) inserted in a recirculating loop

[10]

. In addition,

single EOM such as dual driven Mach–Zehnder modulator

(DDMZM), DPMZM-based OFC generator is recently

demonstrated. OF C generator with multi-lines can be ob-

tained by the unbalanced and nonlinear (super-amplified)

driven of the DDMZM theoretically

[11]

. Single-phase-

modulator-based flat OFC generator is also demonstrated

with flatness of 0.8 dB among nine comb lines

[12]

. The

only ultra-flat OFC generator with equal spectral inten-

sity is by unbalanced driven two parallel arms of the

DPMZM

[13]

. Five- and seven-tone OFC with equal spectral

intensity is obtained theoretically and experimentally, and

five- and seven-tone combs with 0 dB flatness is generated

theoretically. Experiments agree well with the theoretical

results for less than 0.1 dB for five-tone combs and less

than 1 dB for seven-tone combs. In the process of ultra-

flat OFC generation in Ref. [

13], frequency chirp is present

and its value is changeable. For unbalanced driven

DPMZM, frequency chirp is often generated, and positive

frequency chirp will degrade the performance of system on

account of fiber dispersion in long-haul optical fiber com-

munication system

[14]

In this Letter, we propose and experimentally demon-

strate another flexible ultra-flat OFC generator with a

balanced driven DPMZM by outputs of a broadband

90° hybrid coupler. Free chirp is obtained in the genera-

tion of ultra-flat OFC with our configuration. Accordi ng

to the DPMZM modulation model, both a five- and seven-

tone OFC with 0 dB flatness are theoretically generated.

Applying the theoretical analysis as a guideline, a five-

tone and seven-tone OFC with flatness within 0.6 and

1.26 dB is experimentally achieved, which is in agreement

with the theoretical results.

As shown in Fig.

1, the DPMZM is an integrated optical

device composed of a parent Mach–Zehnder interferom-

eter which is embedded by two child MZIs (MZI1 and

MZI2). MZI1 and MZI2 are balanced-driven by RF signals

with 90° phase difference. Assuming the angular frequency

and amplitude of the sinusoidal RF signal are ω and V ,

respectively. The electric field at the output (E

out

)of

the DPMZM can be expressed as

COL 13(6), 060604(2015) CHINESE OPTICS LETTERS June 10, 2015

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