High Power Laser Science and Engineering, (2019), Vol. 7, e52, 7 pages.
© The Author(s) 2019. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
doi:10.1017/hpl.2019.38
Passive optimization of pump noise transfer function by
narrow band-pass filtering in femtosecond fiber lasers
Peng Qin
1
, Sijia Wang
1
, Minglie Hu
2
, and Youjian Song
2
1
Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100094, China
2
Ultrafast Laser Laboratory, Key Laboratory of Opto-electronic Information Technical Science of Ministry of Education,
School of Precision Instruments and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China
(Received 22 April 2019; revised 10 June 2019; accepted 27 June 2019)
Abstract
Fluctuation of pump power is one of the major sources of temporal and intensity noise in femtosecond fiber lasers.
In this work, the transfer functions between the relative intensity noise (RIN) of the pump laser diode (LD) and the
output RIN, between the RIN of the pump LD and timing jitter of femtosecond fiber lasers are systematically studied.
It is demonstrated, for the first time to our knowledge, that the amplitude of the pump RIN transfer function can be
effectively decreased by an intra-cavity narrow band-pass filter. In particular, for normal-dispersion lasers, the 3-dB
bandwidth of the transfer function can also be narrowed by two-thirds, with a steeper falling edge. Furthermore, with the
narrow band-pass filtering, the transfer function is almost independent of the net intra-cavity dispersion due to amplifier
similariton formation. The proposed scheme can effectively isolate the pump-induced noise without the need of complex
active pump LD control and intra-cavity dispersion management, thus providing an easy way for practical high-power,
high-stability femtosecond fiber laser design and related high-precision applications outside the laboratory.
Keywords: band-pass filter; femtosecond fiber lasers; transfer function
1. Introduction
Mode-locked fiber lasers generate trains of femtosecond
pulses with equal pulse width, intensity and timing interval,
making them ideal sources in pump–probe experiments
[1]
,
laser ablation
[2]
, timing synchronization
[3]
and precise
metrology
[4]
. To further extend their applications, various
mode-locking mechanisms have been developed to support
shorter pulse width and higher pulse energy
[5–7]
. In addition,
a number of theoretical and experimental studies have been
done to uncover the noise coupling mechanism as well
as the noise suppression methods
[8–11]
. Besides amplified
spontaneous emission (ASE) noise, which is known to
be the main source of timing jitter
[8, 12]
, pump power
fluctuations also have significant effects on both amplitude
and phase noise in femtosecond fiber lasers
[13, 14]
. On one
hand, pump power fluctuations would induce noise of the
inversion level
[15]
, and thus increase the intensity noise of
the pulse train
[8, 16]
. On the other hand, it would affect
the repetition rate and the carrier–envelope offset frequency
through self-phase modulation, third-order dispersion and
Correspondence to: S. Wang, Qian Xuesen Laboratory of Space
Technology, China Academy of Space Technology, Beijing 100094, China.
Email: sj0607@163.com
self-steepening
[13, 17]
. In order to obtain high-energy pulses
with ultra-high stability and uniformity, the noise induced by
pump power fluctuations should be sufficiently suppressed.
Due to the limited lifetime of the gain medium, the
mode-locked laser can function as a low-pass filter. The
laser response to the pump laser diode (LD) intensity fluc-
tuations has a bandwidth determined by the interplay of
gain dynamics and laser dynamics
[17]
. For mode-locked
fiber lasers, the lifetime of the gain medium is usually at
the millisecond level
[18]
. The much faster laser dynamics
can lead to a larger laser response bandwidth than that of
the gain fiber. Previously, the influence of pump noise
was mainly suppressed by feedback control. To push the
locking bandwidth beyond the response limitation, phase-
lead compensation is always used
[19, 20]
. Considering the
dominant role of laser dynamics in the pump noise response
of femtosecond fiber lasers, the transfer function between the
pump and the output can be expected to be further optimized
by improving the cavity design.
In this work, the transfer functions between the relative
intensity noise (RIN) of the pump laser diode and the output
RIN, between the RIN of the pump LD and timing jitter
of femtosecond fiber lasers are systematically studied by
inducing white noise modulation
[21]
to the pump current
1