A Duplexed CTFBG with Broad Bandwidth for SRS Suppression in High Power Lasers

Abstract

CTFBGs with different center wavelengths are cascaded to broad the bandwidth to 20 nm (FWHM). The homemade duplexed CTFBG assists a 20.88 kW output of single fiber laser system with 18.6 dB of SRS suppression.

Full text

A Duplexed CTFBG with Broad Bandwidth for SRS
Suppression in High Power Lasers
Shan Huang, Wenjie Wu, Fengyun Li, Yue Li, Yuwei Li, Rumao Tao, Xi Feng, Min Li, Benjian Shen ,
Jianjun Wang, and Yu Liu＊
Laser Fusion Research Center, China Academy of Engineering Physics, Sichuan 610200, China
Corresponding author: liuyu_ly@foxmail.com
Abstract: CTFBGs with different center wavelengths are cascaded to broad the bandwidth to 20
nm (FWHM). The homemade duplexed CTFBG assists a 20.88 kW output of single fiber
laser system with 18.6 dB of SRS suppression.
1.Introduction
Due to the compact structure, high efficiency, and excellent brightness, high power fiber lasers have attracted
constant research and industrial interests [1-2]. To achieve higher efficiency and better performance for applications,
a higher power fiber laser beam is required. With the rapid development of laser diode and high power fiber
manufacture craft, the average output power of the single mode fiber lasers has been scaled up to multi-kW level.
However, sustained improvement of output power is limited by effects related to nonlinear effects, optical damage,
and mode instability, among which the stimulated Raman scattering (SRS) presents a major restriction [3-5]. The
SRS effects could cause the decline of the signal power, fiber components damage and beam quality deterioration,
which will seriously affect the normal operation and performance of the whole system [6-7]. Thus, SRS suppression
is becoming essential for the high power fiber lasers beyond multi-kW output.
Over the past decades, many techniques have been proposed for SRS suppression, such as the application of
large-mode-area (LMA) fibers, spectrally selective fibers, long-period fiber gratings (LPFGs) and chirped tilted fiber
Bragg gratings (CTFBGs) [8-10]. Due to the continuous broadband spectral profile, better stability, and convenient
adjustable wavelength range, CTFBG is considered a comparatively suitable components for SRS suppression in
high power fiber laser systems. In this paper, we introduce a duplexed CTFBG (DCTFBG) to broaden the bandwidth
for SRS suppression. The spectrum of DCTFBG after annealing shows that the transmission valley has a full width
at half maximum of 20 nm, which reduces the SRS level by ~18.6 dB in a homemade high power single-fiber laser
system.
2. Fabrication of CTFBGs
The method of ultraviolet (UV) side-writing with phase masks is used to fabricate CTFBGs [11]. A cw UV laser
with a wavelength of 248 nm and an output power of 100 mW is utilized as the writing laser source. The laser is
reflected by a UV reflector to a cylindrical lens (focal length =200 mm), which is set to longitudinally compress the
laser spot for higher power density. The reflector and cylindrical lens are both installed on a high-precision step
motor to conduct horizontal scanning. A phase mask (period 392nm, chirp rate=2 nm/cm, length=37 mm) is fixed on
a six-dimensional optical bench and a piece of hydrogen-loaded fiber (LMA-GDF-20/400-M) is placed closely
behind of it. The tilt angle of the phase mask varies from 2° to 4° for shifting the wavelength of loss peak. As for the
spectrum monitoring during CTFBG fabrication, detection light from a single-mode super-luminescent diode (SLD,
1100-1160nm) is injected into the CTFBGs through a circulator and a mode field adapter (MFA), and then the
transmitted light is coupled into an optical spectrum analyzer(OSA) through another MFA.
In the fabrication process, we first inscribe a single CTFBG at a power level of 50 mW for a duration of one hour.
To achieve a stable spectrum and high power handling capability, the CTFBG is annealed by sequential high-
temperature (300℃) and low-temperature (70℃) processing. The transmission spectrum of the single CTFBG is
shown in Fig. 1, from which we can observe that the reflection peak is located at 1122.5 nm with a ~15dB loss and
the full-width at half maximum (FWHM) is about 14 nm. Then, we adjust the tilt angle of the phase mask to shift
the loss peak wavelength, and fabricate a cascaded CTFBG on the same fiber. . The two sub-CTFBGs of the
DCTFBG device have 3.5-cm inscribing length each and are separated by ~5 cm. In Fig. 1, the spectrum of
DCTFBG after annealing shows that the reflectivity of the CTFBG is ~20 dB at the reflection peak, and the FWHM
is broadened to 20 nm.
JW3B.58 CLEO 2022 © Optica Publishing Group 2022
© 2022 The Author(s)

Fig. 1. Transmission spectrum of DCFBG Fig. 2. Capability of SRS suppression in laser system
3. Capability of SRS suppression
By adjusting the inscribing parameters and phase mask tilt angle, we fabricate another DCFBG for a 1080nm high-
power single-fiber laser system which demonstrates the SRS suppression capability of the DCFBG. The high power
laser system is shown in Fig. 3. The DCFBG is utilized between the oscillator and amplifier to filter out the Raman
frequency band and improve Raman suppression. Finally, the amplifier realizes an output power of 20.88 kW. The
Optical spectrum at maximum output power is shown in Fig. 2, which indicates that an excellent SRS suppression of
~18.6 dB has been achieved and no typical Stokes peak is observed in the Raman band.
QBH
LD
LD
LD
LD
LD
LD
LD
combiner CPS
pump
pump
pump
pump
combiner
pump
osicllator amplifier
DCTFBG
HR OC
Fig. 3. Schematic diagram of single-fiber laser system
4. Conclusion
In summary, a new DCFBG device aimed at broadening the bandwidth for Raman suppression is presented. The
experimental observation indicates the DCFBG’s SRS suppression capability at a high level (~18.6 dB) and wide
bandwidth (~20 nm), which provides strong support to the 20.88 kW output of the single-fiber laser system.
Reference
[1] W. Shi, Q. Fang, X. Zhu, et al., “Fiber lasers and their applications [Invited]”. Appl. Opt., 2014, 53(28): 6554-6568.
[2] B. Shiner. “The fiber laser: Delivering power”. Nature Photon, 2010, 4:290.
[3] J. W. Dawson, et al., “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power.”. Opt. Exp., 2008,
16(17):13240-13266.
[4] T. Eidam, et al., “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers,” . Opt. Exp., 2011,
19(14), 13218-13224.
[5] D. J. Richardson, et al., “High power fiber lasers: current status and future perspectives [Invited]”. J. Opt. Soc. Amer. B., 2010, 27(11), B 63-
92.
[6] H. W. Zhang, P. Zhou, et al., “Toward high-power nonlinear fiber amplifier”. High Power Laser Sci. Eng. 6,e51(2018)
[7] R. Tao, H. Xiao, et al., “Dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers in the presence of mode
instabilities”, Opt. Express 26(19), 25098-25110(2018)
[8] J. Kim, P. Dupriez, et al., ”Suppression of stimulated Ra man scattering in a high power Ybdoped fiber amplifier using a W -type core with
fundamental mode cut-off”, Opt.Lett. 31(17),2550-2552(2006)
[9] T. Taru, J. Hou, et al.,”Raman gain suppression in all -solid photonic bandgap fiber”, IEICE Tech. Rep.107(271),29(2007)
[10] T.Osuch, et al.,”Spectral tra nsmission characteristics of weakly tilted and tilted chirped fiber grating comparative studies”, Proc. SPIE 8903,
89030W (2013)
[11] H. Song, D. Yan, et al.,”SRS suppression in multi -kW fiber lasers with a multiplexed CTFBG”, Opt Exp, 20535, 29(13), 2020
single
CTFBG
duplexed
CTFBGs
JW3B.58 CLEO 2022 © Optica Publishing Group 2022