High Power Laser Science and Engineering, (2020), Vol. 8, e32, 6 pages.
doi:
10.1017/hpl.2020.32
LETTER
Mid-infrared optical frequency comb in the 2.7–4.0 µm
range via difference frequency generation from a compact
laser system
Lian Zhou
1
, Yang Liu
1
, Gehui Xie
1
, Chenglin Gu
1
, Zejiang Deng
1
, Zhiwei Zhu
1
, Cheng Ouyang
1
,
Zhong Zuo
1
, Daping Luo
1
, Bin Wu
3
, Kunfeng Chen
3
, and Wenxue Li
1,2
1
State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
2
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
3
Science and Technology on Electronic Test & Measurement Laboratory, The 41st Research Institute of CETC, Qingdao 266000,
China
(Received 4 June 2020; revised 11 July 2020; accepted 5 August 2020)
Abstract
We report on the generation of a mid-infrared (mid-IR) frequency comb with a maximum average output power of
250 mW and tunability in the 2.7–4.0 µm region. The approach is based on a single-stage difference frequency
generation (DFG) starting from a compact Yb-doped fiber laser system. The repetition rate of the near-infrared (NIR)
comb is locked at 75 MHz. The phase noise of the repetition rate in the offset-free mid-IR comb system is measured
and analyzed. Except for the intrinsic of NIR comb, environmental noise at low frequency and quantum noise at high
frequency from the amplifier chain and nonlinear spectral broadening are the main noise sources of broadening the
linewidth of comb teeth, which limits the precision of mid-IR dual-comb spectroscopy.
Keywords: fiber laser; mid-infrared; optical frequency comb
1. Introduction
Mid-infrared (mid-IR) laser sources are now becoming
enabling tools for cutting-edge applications, including
greenhouse gas sensing
[
1, 2]
, medical diagnosis
[3]
, and
security and defense
[
4]
. Many molecules and molecular
functional groups experience vibrational absorption in the
mid-IR region. Indeed, spectroscopic applications would
benefit from scaling the optical frequency comb to the
mid-IR region, leading to numerous absorption lines being
recorded with unprecedented accuracy and resolution
[
5, 6]
.
Moreover, ultrafast pulses with a stable carrier–envelope
phase permit an exhaustive understanding of molecular
structure and dynamics
[
7, 8]
, and they enable t he soft X-ray
generation to be scaled on a tabletop system
[
9, 10]
. Over the
last decade, steady progress has been made in the field of
ultrafast mid-IR frequency comb generation. There is a wide
array of innovative solutions to generate coherent mid-IR
Correspondence to: W. Li and Y. Liu, No. 500 Dongchuan
Road, Shanghai 200241, China. Email: wxli@phy.ecnu.edu.cn (W. Li);
yliu@lps.ecnu.edu.cn (Y. Liu)
laser sources, with novel gain media
[
11]
, quantum cascade
lasers
[
12]
and micro-resonators
[13]
, and supercontinuum
generation in waveguides and fibers
[
14, 15]
. Compared with
these approaches, detecting and controlling the offset
frequency of the mid-IR sources is a challenge, which is the
prerequisite for frequency comb spectroscopy. A more direct
approach is to employ nonlinear frequency conversion of
ultrashort pulses in the visible or near-infrared (NIR) regime
to generate coherent mid-IR sources. Among the different
nonlinear processes, difference frequency generation (DFG)
with signal and pump from the same oscillator offers several
advantages for a mid-IR frequency comb system, as it allows
us to use compact and well-developed fiber laser technology,
and also to achieve ultrashor t pulses and intrinsic car rier–
envelope phase stability, which reduce the complexity and
improve the quality of t he long-term performance
[
16–19]
.
In DFG systems, the mid-IR spectrumd coverage depends
on the NIR spectrum and the transmissivity of the crystal.
Limited by the gain bandwidth of laser materials, the general
NIR sources cannot directly emit pulses with such a broad
spectrum. To achieve a broad mid-IR spectrum, highly
© The Author(s) 2020. Published by Cambridge University Press in association with Chinese Laser Press. This is an Open Access article, distributed under
the terms of the Creative Commons Attribution licence (
http://creativecomm ons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and
reproduction in any medium, provided the original work is properly cited.
1