FabricationofkW-levelchirpedandtiltedfiberBragggratingsandfilteringofstimulatedRamanscatteringinhigh-powerCWoscillators资源-CSDN文库

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High Power Laser Science and Engineering, (2019), Vol. 7, e31, 7 pages.

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.2019.18

Fabrication of kW-level chirped and tilted ﬁber Bragg

gratings and ﬁltering of stimulated Raman scattering in

high-power CW oscillators

Kerong Jiao

, Jian Shu

, Hua Shen

1,2

, Zhiwen Guan

, Feiyan Yang

, and Rihong Zhu

1,2

MIIT Key Laboratory of Advanced Solid Laser, Nanjing University of Science and Technology, Nanjing 210094, China

School of Electronic Engineering and Optoelectronic Technology, Nanjing University of Science and Technology, Nanjing 210094,

China

(Received 18 February 2019; revised 27 March 2019; accepted 4 April 2019)

Abstract

Suppression of stimulated Raman scattering (SRS) by means of chirped and tilted ﬁber Bragg gratings (CTFBGs) has

become a key topic. However, research on high-power systems is still lacking due to two problems. Firstly, after the

inscription, there are a large number of hydroxyl compounds and hydrogen molecules in CTFBGs that cause signiﬁcant

heating due to their strong infrared absorption. Secondly, CTFBGs can couple Stokes light from the core to the cladding

and the coating, which causes serious heating in the coating of the CTFBG. Aimed at overcoming these bottlenecks, a

process that combines constant-low-temperature and variable-high-temperature annealing is used to reduce the thermal

slope of the CTFBG. Also, a segmented-corrosion cladding power stripping technology is used on the CTFBG to remove

the Stokes light which is coupled to the cladding, which solves the problem of overheating in the coating of the CTFBG.

Thereby, a CTFBG with both a kilowatt-level power-carrying load and the ability to suppress SRS in a ﬁber laser has

been developed. Further, we establish a kW-level CW oscillator to test the CTFBG. Experimental results demonstrate

that the power-carrying load of the CTFBG is close to 1 kW, the thermal slope is lower than 0.015

◦

C/W, and the SRS

suppression ratio is nearly 23 dB.

Keywords: chirped and tilted ﬁber Bragg gratings; ﬁber optics components; high-power ﬁber laser; stimulated Raman scattering

1. Introduction

Fiber lasers have been widely used in many ﬁelds, in-

cluding industrial processing

[1, 2]

, biomedicine

[3, 4]

and

communication

[5]

, due to their compact structure, high

efﬁciency and high spatial beam quality

[6]

. With increasing

demand for high power, stimulated Raman scattering (SRS)

has become one of the main factors limiting the development

of ﬁber lasers

[7]

. To solve this problem in recent years,

an increasing number of researchers have focused their

attention on using specially structured ﬁber gratings to

suppress the SRS in ﬁber lasers.

One such method is to use long-period ﬁber gratings

(LPGs). LPGs, usually used in the sensing ﬁeld

[8, 9]

, can

couple the forward-propagating core modes to the forward-

propagating cladding modes. Therefore, a speciﬁc wave-

length of light can be coupled from the core to the cladding

by controlling the period of the LPG. In 2010, Nodop

Correspondence to: H. Shen, MIIT Key Laboratory of Advanced Solid

Laser, Nanjing University of Science and Technology, Nanjing 210094,

China. Email: edward bayun@163.com

et al.

[10, 11]

used this characteristic of LPGs to suppress SRS

in pulsed ﬁber lasers with an average power of 70 µW and

a suppression ratio exceeding 20 dB (99%). However, LPGs

are sensitive to temperature, strain and bending, which leads

to a drift in the cladding-mode resonances and a reduction of

the suppression ratio. Another approach is to use CTFBGs.

Unlike LPGs, CTFBGs can couple the forward-propagating

core modes to the backward-propagating cladding modes.

Furthermore, compared with LPGs, the period of a CTFBG

is much smaller, which means the cladding-mode resonances

of CTFBGs change only slightly under the inﬂuence of

temperature, strain and bending. This makes CTFBGs the

preferred method of suppressing SRS by ﬁber gratings. In

2014, Liu et al.

[12]

introduced chirp into a TFBG and real-

ized the design and inscription of a CTFBG with broadband

ﬁltering for the ﬁrst time. Moreover, potential applications

of CTFBGs in the ﬁeld of sensing and communication were

also proposed. Wang et al.

[13]

ﬁrst applied a CTFBG to

a 22 W low-power CW ﬁber laser to suppress the SRS in

2017, and reported a 25 dB (99.68%) SRS suppression ratio.

Further, they achieved inscription of a CTFBG in PS-GDF-

2 K. Jiao et al.

20/400 photosensitive ﬁber in 2018

[14]

, and applied it in

the seed of a master oscillator power ampliﬁer (MOPA)

structure ﬁber laser with a 4.2 kW output power in 2019 to

suppress the SRS in a 100 W-level seed

[15]

. However, they

did not consider the power-carrying load of the CTFBG in

the case of high power. The reason is that ﬁber gratings

made of photosensitive ﬁbers are usually used in the ﬁeld of

low-power lasers, such as sensing and communication

[16, 17]

Compared with the large-mode-area passive ﬁber gratings

used in high-power ﬁber lasers, their power-carrying load is

worse. In short, CTFBGs which can suppress SRS are still at

low power and research on CTFBGs with kW-level power-

carrying loads is still lacking.

High-power ﬁber gratings usually use LMA-GDF-20/400

double-cladding passive ﬁber and can carry thousands of

watts of signal light. Therefore, CTFBGs made of LMA-

GDF-20/400 ﬁber are one of the most promising passive

devices for SRS suppression in kW-level CW high-power

ﬁber lasers. However, no research has been carried out in

this area – mainly for two reasons. Firstly, LMA-GDF-

20/400 passive ﬁber is only weakly photosensitive, so to

realize the inscription of a high-reﬂectivity grating, the ﬁber

should be previously processed by hydrogenation. However,

a large number of hydroxyl compounds are generated in the

CTFBGs after the inscription, while there are still a large

number of unreacted hydrogen molecules in the CTFBG.

These materials cause signiﬁcant heating in the high-power

case due to strong infrared absorption, which greatly limits

the power-carrying load of CTFBGs. Secondly, CTFBGs

can couple Stokes light from the core to the cladding and

coating, which will cause serious heating in the coating of the

CTFBG in the incident direction of the laser at high power.

Aimed at overcoming these technical bottlenecks, a pro-

cess that combines constant-low-temperature and variable-

high-temperature annealing is used to reduce the thermal

slope of the CTFBG. In addition, a segmented-corrosion

cladding power stripping technology is used on a CTFBG

for the ﬁrst time to remove the Stokes light coupled to

the cladding by the CTFBG, which solves the problem of

overheating in the coating of the CTFBG in the incident

direction of the laser. Thereby, a CTFBG with both a

kilowatt-level power-carrying load and the ability to suppress

SRS in a ﬁber laser has been developed. Further, a kW-

level CW oscillator system is established to test the power-

carrying load of the CTFBG and its SRS suppression ratio.

To the best of our knowledge, this is the ﬁrst time that a

CTFBG has been applied to a kW-level ﬁber laser system

to suppress the SRS.

2. Design and simulation of high-power CTFBG

As shown in Figure 1(a), the difference between a CTFBG

and a chirped ﬁber Bragg grating (CFBG) is that there is an

angle between the ﬁber axis and the grating plane, so that the

Figure 1. (a) Schematic diagram of the structure of a CTFBG and

(b) simulated transmission spectra of a TFBG and a CTFBG with a tilt angle

of 4

◦

; the chirp rate of the CTFBG is 0.3 nm/cm.

forward core modes, which were originally transmitted only

in the core, are coupled to the backward core modes and the

cladding modes.

The spectrum of a TFBG will not only have one core-mode

resonance, but also many cladding-mode resonances

[8]

, as

shown by the solid blue line in Figure 1(b). The direction

along the ﬁber is designated as the x-axis and the direction

perpendicular to the grid plane is the x

-axis. Due to the tilted

grating plane, the period of the grating along the x

-direction

is Λ

and the period of the grating along the x-direction is

Λ. The relationship between the periods can be expressed as

Λ =

cos θ

, (1)

where θ is the tilt angle between the x-axis and the x

-axis.

Similar to a ﬁber Bragg grating (FBG), the Bragg resonance

wavelength for the TFBG can be expressed as

core

cos θ

, (2)

where n

core

is the effective refractive index of the core mode.

The cladding-mode resonance wavelength can be expressed

clad,i

+ n

core

)Λ

cos θ

, (3)

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