1.89 kW all-fiberized and polarizationmaintained amplifiers with narrow linewidth
and near-diffraction-limited beam quality
Pengfei Ma, Rumao Tao, Rongtao Su, Xiaolin Wang, Pu Zhou,* and Zejin Liu
College of Opticelectric Science and Engineering, National University of Defense Technology, Changsha 410073,
China
*
[email protected]
Abstract: In this manuscript, we demonstrate high power, all-fiberized and
polarization-maintained amplifiers with narrow linewidth and neardiffraction-limited beam quality by simultaneously suppressing detrimental
stimulated Brillouin scattering (SBS) and mode instability (MI) effects.
Compared with strictly single frequency amplification, the SBS threshold is
scaled up to 12 dB, 15.4 dB, and higher than 18 dB by subsequently using
three-stage cascaded phase modulation systems. Output powers of 477 W,
1040 W, and 1890 W are achieved with full widths at half maximums
(FWHMs) of within 6 GHz, ~18.5 GHz, and ~45 GHz, respectively. The
MI threshold is increased from ~738 W to 1890 W by coiling the active
fiber in the main amplifier. Both the polarization extinction ratio (PER) and
beam quality (M2 factor) are maintained well during the power scaling
process. To the best of our knowledge, this is the first demonstration of allfiberized amplifiers with narrow linewidth, near linear polarization, and
near-diffraction-limited beam quality at 2 kW power-level.
©2016 Optical Society of America
OCIS codes: (140.3480) Lasers, diode-pumped; (140.3510) Lasers, fiber; (060.2320) Fiber
optics amplifiers and oscillators.
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1. Introduction
High power fiber laser with narrow linewidth and near-diffraction-limited (NDL) beam
quality has widely applications in various regimes, such as nonlinear frequency conversion
(NFC) [1], remote communication [2], beam combination [3], and gravitational wave
detection [4]. As for power scaling of this type of monolithic fiber source, stimulated
Brillouin scattering (SBS) effect has been become one of the most primary limitations in
previous studies. Over the past decade, several techniques were employed in high power
amplifiers for SBS suppression, such as imposing thermal and/or strain distributions [5–8],
design special active fiber [9–11], using large core size and/or highly doped active fiber [2,
12], directly using multi-longitudinal-mode oscillator for amplification [13, 14], and
employing phase modulation technique [15–19]. Within these SBS suppression techniques,
phase modulation technique is a preferable approach to achieve laser sources with narrow
linewidth beyond kilowatt-level outputs [16–19]. Notably that 2.3 kW output power with
narrow linewidth and NDL beam quality has been presented by using two-stage phase
modulation systems to suppress SBS effect quite recently [19]. Despite impressive result
demonstrated, it has been shown that the mode instability (MI) effect will become another
serious limitation for further high brightness scaling [19].
A point should be noted is that the polarization states in most of the aforementioned high
power demonstrations are stochastic. In fact, except for high brightness operation, linear
polarization is also strongly required in many applications of fiber lasers with narrow
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© 2016 OSA
Received 13 Jan 2016; revised 15 Feb 2016; accepted 16 Feb 2016; published 19 Feb 2016
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.004187 | OPTICS EXPRESS 4188
linewidth. Due to that the effective Brillouin gain in polarization-maintained (PM) fiber is
typically higher than that in non-PM fiber, SBS suppression in high power narrow linewidth
and PM amplifiers is more challenging [2, 11]. More importantly, previous study shows that
the MI threshold in PM amplifier seems to be remarkably lower than that in non-PM
amplifier [20]. Consequently, power scaling of fiber lasers with narrow linewidth and near
linear polarization by directly using PM amplifiers is more difficult. Nowadays, the record
output power of this kind of fiber source has been still remained at 1 kW power-level [1, 20,
21].
In this manuscript, we present narrow linewidth, all-fiberized, and polarization-maintained
amplifiers operating at maximum output power of 1.89 kW by simultaneously suppressing the
dual impacts of SBS and MI effects. The SBS effect in such high power amplifiers is
suppressed by using three-stage phase modulation systems. By subsequently imposing the
three-stage phase modulation signals into the seed laser, the SBS threshold is scaled up to 12
dB, 15.4 dB, and higher than 18 dB compared with the single frequency amplification
process. Output powers of 477 W, 1040 W, and 1890 W are obtained with full widths at half
maximums (FWHMs) of within 6 GHz, ~18.5 GHz, and ~45 GHz, respectively. In the
experiment, the MI threshold is increased from ~738 W to 1890 W by simply coiling the
active fiber in the main amplifier. The polarization extinction ratio (PER) at maximum output
power is measured to be 15.5 dB and low degradation of the far-field intensity distribution is
observed in the whole power scaling process. As far as we know, this is the highest
demonstration of all-fiberized amplifiers with narrow linewidth, near linear polarization, and
NDL beam quality.
2. Experimental Setup
The experimental setup of high power all-fiberized and polarization-maintained amplifiers
with narrow linewidth is shown in Fig. 1, which is based on conventional master oscillator
power amplification (MOPA) structure. The master oscillator (MO) is a linear-polarized,
single-frequency (line-width <20 kHz) laser with output power of 40 mW and central
wavelength of 1064.4 nm, which is based on an ultra-short-cavity configuration [22]. Output
power of the seed laser is firstly amplified to be 150 mW by using a PM pre-amplifier (P-AI).
After P-AI, three-stage phase modulation systems (PMSs) are used to broaden the linewidth
of the seed for SBS suppression. The PMSs include three cascaded phase modulators and
three sine-signal generators (SG1, SG2, and SG3 shown in Fig. 1). The modulation
frequencies and depths generated by SG1, SG2, and SG3 are 17 GHz with 8.9 V peak-vale
(PV) voltage, 6 GHz with 11.2 V PV voltage, and 100 MHz with 36 V PV voltage,
respectively. The typical half-wave voltages of the three cascaded phase modulators are 4 V,
4V, and 2.2 V, respectively. The output power after PMSs is measured to be about 20 mW,
and the power loss is mainly attributed to the insertion loss of the three phase modulators.
Then, the linewidth-broadened seed laser is subsequently amplified to be 0.5 W and 20 W by
using two-stage PM preamplifiers (P-AII and P-AIII), respectively. At the rear end ports of PAI, P-AII, and P-AIII, three PM isolators (ISO1, ISO2, and ISO3) are incorporated into the
MOPA structure to block off the backward powers from the following amplifications. After
ISO3, the pre-amplified laser is coupled through a PM fiber coupler (PM-C) to the main
amplifier for further power scaling. The coupling ratios of the out port of the PM-C to
backward monitor port and injected signal port are 0.1% and 99.9%, respectively. The main
function of the PM-C is to split a small portion of the backward power for diagnosing the
SBS effect during the power scaling process.
The main amplifier is pumped by using five wavelength-stabilized, 500 W power-level
laser diodes (LDs) with 976 nm central wavelength via a (6 + 1) × 1 PM pump combiner. The
active fiber in this stage is large mode area (LMA) and double clad PM Yb-doped fiber with a
core diameter of 20 μm and an inner cladding diameter of 400 μm. The cladding absorption
coefficient is about 1.7 dB/m at 976 nm and 8.5 m long active fiber is employed for high
power scaling. About 1 m long PM passive fiber and a high power fiber end-cap with~1.5 m
long PM passive fiber are successively fused to the rear end of the active fiber for power
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Received 13 Jan 2016; revised 15 Feb 2016; accepted 16 Feb 2016; published 19 Feb 2016
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.004187 | OPTICS EXPRESS 4189
delivering. The core and inner cladding diameters of the two pieces of passive fibers are
remained the same as the active fiber. In the 1 m long passive fiber, ~40 cm high-index gel
section is made for stripping out the residual pump and cladding light. The delivering laser
through the fiber end-cap is collimated into free space by using a high power beam
collimator.
Fig. 1. The experimental setup of high power all-fiberized and polarization-maintained
amplifiers with narrow linewidth.
3. Experimental Results and Discussions
3.1 Power scaling of the strictly single frequency seed
For comparison of the SBS suppression effects with cascaded phase modulation systems, we
firstly investigate the power scaling ability of the high power MOPA structure with strictly
single frequency seed. In this situation, all the three sine-wave signal generators are turned
off. The actual backward power as a function of the output power is shown in Fig. 2. From
Fig. 2, it is shown that nonlinear increase of the backward power occurs when the output
power is beyond 30 W. This is attributed to the fact that the signal power dramatically
transforms into the Stokes light due to the SBS effect. The SBS threshold in this manuscript is
defined as the output power of the MOPA architecture before nonlinear increase of the
backward power. Thus, the SBS threshold with strictly single frequency amplification is
about 30 W in our experiment.
Fig. 2. The actual backward power as a function of the output power.
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Received 13 Jan 2016; revised 15 Feb 2016; accepted 16 Feb 2016; published 19 Feb 2016
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.004187 | OPTICS EXPRESS 4190
3.2 Power scaling of the seed by just imposing one stage phase modulation signal
In this section, we investigate the SBS suppression effect by just imposing the phase
modulation signal of SG3 to broaden the linewidth of the single frequency seed. From our
previous study [15], the broadened linewidth of the seed in this situation is well within 6
GHz. The output power and actual backward power as a function of the absorbed 976 nm
pump power are shown in Fig. 3. As shown in Fig. 3, output power of 477 W can be attained
with an optical to optical conversion efficiency of ~73% before nonlinear increase of the
backward power, which indicates that the SBS threshold is about 477 W in the experiment.
Compared with strictly single frequency amplification, the SBS threshold is scaled up to 12
dB by just employing the phase modulation signal of SG3.
Fig. 3. The output power and backward power as a function of the absorbed pump power.
3.3 Power scaling of the seed by imposing two cascaded phase modulation signals
For further SBS suppression, we simultaneously impose the phase modulation signals of SG2
and SG3 to broaden the linewidth of the single frequency seed. Output power scaling
characteristic along with the absorbed pump power in the main amplifier is shown in Fig.
4(a). As shown in Fig. 4(a), the output power is increased near linearly when the pump power
is below 960 W while abnormal increase trend is observed when the pump power is higher
than 960 W. Specifically, the optical to optical efficiency decreases from 76% to 72.4% when
the pump power is increased from 960 W to 997 W. Further investigation of the temporal
instability of the output beam by using an InGaAs photo-detector (PD) with 150 MHz electrooptical bandwidth, abrupt temporal instabilities are observed when the pump power is beyond
960 W. The normalized time-serial signals collected by the PD at pump powers of 960 W and
997 W are shown in Fig. 4(b) and their Fourier spectral distributions are shown in Fig. 4(c).
From the experimental results shown in Fig. 4(b), the standard deviations of the temporal
signals are respectively calculated to be 0.79% and 2.38% at the two specific pump powers,
which is increased more than 3 times. As shown in Fig. 4(c), compared with the Fourier
spectral distribution at pump power of 960 W, some noise-like protuberances exist within the
frequency range of 0-5 kHz at pump power of 997 W. According to the temporal and Fourier
spectral characteristics of MI effect [23], we confirm that the MI effect occurs when the pump
power is beyond 960 W. The MI threshold is defined as the output power at pump power of
960 W, which is 738 W in the experiment.
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Received 13 Jan 2016; revised 15 Feb 2016; accepted 16 Feb 2016; published 19 Feb 2016
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.004187 | OPTICS EXPRESS 4191
Fig. 4. (a) Output power scaling characteristic along with the absorbed pump power; (b) the
time-serial signals at pump powers of 960 W and 997 W; (c) the corresponding Fourier
spectral distributions of the time-serial signals at pump powers of 960 W and 997 W.
It is to be noted that the active fiber in the main amplifier is loose coiling with radius of
about 0.4 m in the above experiments. As shown above, the MI limited output power is just
about 738 W with this loose coiling radius. In order to further power scaling, MI suppression
technique is strongly required in our experiment. According to the theoretical and
experimental studies [24–27], coiling active fiber to increase the relative losses of higher
order modes is a simple and effective method to suppress MI effect in practice. More
impressively, as for the active fiber used in the present setup, our theoretical analysis shows
that the MI threshold can be increased to be three times by coiling the active fiber with the
radius of ~5.5cm [27]. Thus, we reconstruct the main amplifier in the PM MOPA structure by
coiling the active fiber with radius of ~5.5 cm and re-scale the output power of the narrow
linewidth PM amplifiers. The output power and actual backward power as a function of the
absorbed 976nm pump power are shown in Fig. 5(a). As shown in Fig. 5(a), the backward
power is increased dramatically when the pump power is beyond 1365 W, which indicates
that the SBS effect occurs at this pump power-level. At 1365 W pump power, 1040 W output
power is achieved with an optical to optical efficiency of 75.6%, which indicates that the SBS
threshold is scaled up to 15.4 dB compared with the single frequency amplification process.
Figure 5(b) shows the optical spectrum of the PM amplifiers at 1040 W output power, which
is measured by using an optical spectrum analyzer with resolution of 0.02 nm. As shown in
Fig. 5(b), higher than 38 dB signal-to-noise ratio (SNR) is attained with a resolution-limited
full width at half maximum (FWHM) of ~0.07 nm (18.5 GHz). The normalized time-serial
signal and the corresponding Fourier spectral distribution at 1040 W output power are shown
in Figs. 5(c) and 5(d), respectively. From Figs. 5(c) and 5(d), it is shown that the temporal
characteristic is quite stable without any noise-like protuberances in the Fourier spectral
regime, which indicates that the MI effect is suppressed effectively at 1040 W output power.
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© 2016 OSA
Received 13 Jan 2016; revised 15 Feb 2016; accepted 16 Feb 2016; published 19 Feb 2016
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.004187 | OPTICS EXPRESS 4192
Fig. 5. (a) The output power and actual backward power as a function of the absorbed 976 nm
pump power; (b) the optical spectrum at 1040 W; (c) the time-serial signal at 1040 W; (d) the
Fourier spectral distribution of the time-serial signal at 1040 W.
3.4 Power scaling of the seed by imposing three cascaded phase modulation signals
As shown in section 3.3, with effective MI suppression technique, further power scaling of
the PM amplifiers is still limited by SBS effect. In this section, we add all the three cascaded
phase modulation signals to broaden the linewidth of the seed for further SBS suppression. In
this situation, the power scaling process with the injected pump power is shown in Fig. 6(a),
and the backward power as a function of the output power is shown in Fig. 6(b). As shown in
Fig. 6(a), 1890 W output power is ultimately achieved with a linear-fitting slope efficiency of
74%. Figure 6(b) shows that the backward power is also increased near linearly with a slope
efficiency of ~2.4% in the power scaling process, which denotes that SBS effect is suppressed
effectively in such high power PM amplifiers. Compared with the single frequency
amplification process, the SBS threshold is increased to be more than 18 dB in this
experiment. At 1890 W output power, the normalized time-serial signal and the
corresponding Fourier spectral distribution are also measured in the experiment, which are
shown in Figs. 6(c) and 6(d), respectively. Experimental results in Figs. 6(c) and 6(d) denote
that the PM amplifiers are operated at MI-free state. Consequently, we conclude that the MI
threshold can be scaled to be more than 2.5 times (from 738 W to more than 1890 W) by tight
coiling the active fiber to suppress MI effect, which is compatible with our previous
theoretical analysis [27].
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© 2016 OSA
Received 13 Jan 2016; revised 15 Feb 2016; accepted 16 Feb 2016; published 19 Feb 2016
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.004187 | OPTICS EXPRESS 4193
Fig. 6. (a) The power scaling process with the increase of pump power; (b) the backward
power as a function of the output power; (c) the time-serial signal at 1890 W; (d) the Fourier
spectral distribution of the time-serial signal at 1890 W.
The emission spectra of the PM amplifiers at 102 W and 1890 W are shown in Fig. 7(a).
From Fig. 7(a), it is shown that the residual pump power, the amplified spontaneous emission
(ASE), and the stimulated Raman scattering (SRS) effect are not observed with SNR of
~44dB at 1890 W. The spectral details at 102 W and 1890 W are shown in the inset of Fig.
7(a). The resolution-limited FWHMs are measured to be 0.19 nm (51 GHz) at 102 W and
0.17 nm (45 GHz) at 1890 W, respectively. Besides, the spectral linewidths within 20 dB are
measured to be 0.84 nm at 102 W and 0.7 nm at 1890 W, respectively. This linewidth
narrowing effect is attributed to the gain competition effects in the power scaling process.
Figure 7(b) shows the captured far field intensity distributions of the PM amplifiers at output
powers of 552 W, 1048W, 1520 W and 1890 W, respectively. As shown in Fig. 7(b), low
degradation of the far- field intensity distribution is observed during the power scaling
process. At 1520 W output power, the beam quality (M2 factor) of the high power PM
amplifiers is investigated for long time (15 minutes) operation. Five groups of M2 data
(measured by using M2-200) are subsequently obtained during the observation time and the
beam quality is maintained well during the whole investigation process. Figure 7(c) gives a
typical M2 measurement result, which shows that the beam quality of the output beam is neardiffraction-limited (M2x ~1.19, M2y ~1.27). Besides, the output power of the PM amplifiers is
stable during the observation time of 15 minutes. Figure 7(d) shows the polarization
extinction ratio (PER) as a function of the output power, which is measured by using
assemble components of a half-wavelength plate and a polarization beam combiner operating
at central wavelength of 1064 nm. As shown in Fig. 7(d), the measured PER changes between
15.5 dB (97.2%) and 20.8 dB (99.2%), which just fluctuates within 2% during the power
scaling process. At maximum output power, the PER of the PM amplifiers is measured to be
15.5 dB (97.2%).
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© 2016 OSA
Received 13 Jan 2016; revised 15 Feb 2016; accepted 16 Feb 2016; published 19 Feb 2016
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.004187 | OPTICS EXPRESS 4194
Fig. 7. (a) The emission spectra of the PM amplifiers at 102 W and 1890 W; (b) the far field
intensity distributions at output powers of 552 W, 1048 W, 1520 W and 1890 W, respectively;
(c) the M2 measurement result at 1520 W; (d) the polarization extinction ratio (PER) as a
function of the output power.
4. Conclusion
High power, all fiberized and polarization-maintained amplifiers with narrow linewidth and
NDL beam quality are presented based on a conventional MOPA configuration. During the
power scaling process, the SBS effect is effectively suppressed by subsequently using threestage cascaded phase modulation systems and the MI effect is managed by simply coiling the
active fiber in the main amplifier. With increase of phase modulation signals, the SBS
threshold is scaled up to 12 dB, 15.4 dB, and higher than 18 dB compared with the strictly
single frequency amplification process. Output powers of 477 W, 1040 W, and 1890 W are
achieved with FWHMs of within 6 GHz, 18.5 GHz, and 45 GHz, respectively. The MI
threshold is increased from 738 W to 1890 W, which is scaled up to 2.5 times in the
experiment. The PER just fluctuates within 2% during the power scaling process and as high
as 15.5 dB is obtained at 1890 W output power. Low degradation of the far-field intensity
distribution is observed along with increase of output power. At 1520 W, the PM amplifiers
are operated stably without degradation of beam quality (M2 factor) for long time observation.
The M2 factor is measured to be within 1.3 (M2x ~1.19, M2y ~1.27) at 1520 W output power.
To the best of our knowledge, this is the first demonstration of all-fiberized amplifiers at 2
kW power-level with the characteristics of narrow linewidth, near linear polarization, and
NDL beam quality.
Acknowledgements
This research is sponsored by the National Natural Science Foundation of China (NO.
11274386) and the innovation projects of Hunan Province and National University of Defense
Technology for graduate students.
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© 2016 OSA
Received 13 Jan 2016; revised 15 Feb 2016; accepted 16 Feb 2016; published 19 Feb 2016
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.004187 | OPTICS EXPRESS 4195