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Abstract—In this paper, we numerically study and
experimentally achieve a low-threshold, high-efficiency random
fiber laser, featuring a linear output as well. The lasing cavity
incorporates a section of standard single mode fiber and a
band-selective point reflector placed at the far end of the fiber.
The numerical result indicates that most energy is further pushed
towards the pump side comparing with the open cavity scheme,
producing a high efficiency output. Then we analyze the
dependence of threshold and slope efficiency on cavity length and
pumping wavelength. Most importantly, shorter cavity length
would yield higher efficiency, and for different pumping
wavelengths there will be different cavity lengths corresponding
to the lowest lasing threshold. Finally, we deliberately choose the
parameters and experimentally achieve an 1145nm random fiber
laser with 7.13W output and >90% slope efficiency (with 10W
pump), while the slope efficiency is almost constant above the 2W
lasing threshold. This work provides a comprehensive guideline
for designing such random fiber lasers with tailored performance.
Index Terms—random media; Raman scattering; Rayleigh
scattering; optical fiber lasers.
I. INTRODUCTION
he random fiber laser (RFL) has attracted lots of interests
since the first demonstration reported by Turitsyn et al [1].
The RFL is a new type of laser since the feedback of the lasing
light solely depends on the random distributed Rayleigh
scattering along the fiber, rather than reflective point-mirrors as
in conventional fiber lasers. The fiber itself provides both
optical amplification and distributed feedback. RFL shows the
properties of “mode-less” spectrum [1], long-distance signal
delivery ability [2], stable output with little thermal sensitivity
[3] and single-transverse-mode profile. Due to these advantages,
attentions have been paid to their applications in fiber-optic
sensing and communication [3-5]. Also, various aspects of RFL
have been studied: RFL has been designed to be narrow
bandwidth [6], multiwavelength [7, 8] wavelength-tunable [9],
high power [10-12] and generating high order Stokes waves
[13,14]. Besides, the concept of RFL can be further developed
by using Brillouin gain instead of Raman gain [15].
This work is supported by Natural Science Foundation of China (61205048,
61290312), Research Fund for the Doctoral Program of Higher Education of
China (20120185120003), Fundamental Research Funds for the Central
Universities (ZYGX2012J002), and PCSIRT (IRT1218), and the 111 project
(B14039).
The authors are with the Key Lab of Optical Fiber Sensing &
Communications, University of Electronic Science & Technology of China,
Chengdu, Sichuan, China 611731(email: znwang@uestc.edu.cn).
Most of the previous RFL studies focus on the long fiber
cavity with a length of tens of kilometers; however, RFL with
short cavity has a unique advantage to generate high power
mode-less Stokes wave, because of the very high thresholds of
higher order Stokes waves, but this subject is yet to be fully
explored. For short-cavity RFL the lasing threshold of the
1st-order Stoke wave is also very high, therefore the cavity
design and the fiber selection are essential in order to make it
operational. Recently, some achievements about high power
RFL were reported [10-12]. In Reference [10], near the 5.5W
lasing threshold, more than 2W of output power is generated
from only 0.5W of pump power excess over the threshold, and
finally they obtained 7.3W output with 11W pump source,
while the input-output curve is nonlinear. However, a linear
output is generally preferred in most applications. In Reference
[11], 73.7W output power with 74.7% optical efficiency is
obtained. But it should be noted that the output power is a sum
of forward power and backward power rather than the power
from the same output port. Also, in both of the two
achievements, the purpose of high power output is at the
expense of the relatively high lasing threshold. In reference [8],
linear input-output dependence with backward pumping is
demonstrated, also in the recent review [16], brief numerical
results are given, while leaving space for further study,
particularly for simultaneously realizing low-threshold and
high-efficiency in such RFLs.
In this paper, we present the detailed analysis for the key
factors of forming a low-threshold, high-efficiency random
fiber laser with linear output. The cavity incorporates a short
section of single mode fiber (SMF) and a point reflector placed
at the far end of the fiber. With the numerical calculations, we
elaborately analyze the optimal cavity length and operating
wavelength for the purpose of achieving low-threshold and
generating high-power random lasing. As the verification, we
experimentally demonstrate a high power (7.13W output),
highly efficient (more than 90% slope efficiency) RFL at
1145nm using 10W 1090nm pump and 5km standard SMF,
with a threshold of only 2W. The experimental results coincide
with the theoretical model well, and this work provides a
comprehensive guideline for the design of such RFLs.
II. THEORETICAL ANALYSIS
The schematic setup for the low-threshold, high-power
random fiber laser is shown in Fig.1. Without loss of generality,
the pump wavelength is at first set to 1090nm and the
corresponding 1st-order Stokes wavelength is 1145nm for
Low-threshold, high-efficiency random fiber
laser with linear output
Mengqiu Fan, Zinan Wang, Han Wu, Wei Sun, and Li Zhang