The tunable single-frequency (SF) narrow-linewidth fiber laser with all-fiber complex cavity structure is designed, which is composed of an optical fiber tunable filter, a high-precision ring filter, and a fiber loop mirror. A 980-nm semiconductor laser is used as the pumping source, and the ytterbium-doped fiber is employed as the gain medium and saturable absorber, then a wide-spectrum tunable single-frequency narrow-linewidth laser output from 1030 nm to 1090 nm is successfully realized. When the pump power is up to 300 mW, the output power is 18.5 mW and the slope efficiency is 7.95% at the wavelength of 1070 nm. There is no mode hopping phenomenon within 1 h, and the standard deviation of power stability is less than 1 %. When the pump power is 200 mW, the linewidth is measured by the delay self-heterodyne method, and the average line width in the wavelength tuning range is 8.7 kHz, and the relaxation oscillation frequency is 64 kHz.

1. Tunable Broadband Single-frequency Narrow-Linewidth
Fiber Laser
Ma Xuanxuan
1
'
2
'
3
, Lu Baole
1
-
2
*
3
*, Wang Kaile
1
*
2
*
3
, Hou Yao
b2t3
,
Zhang Kailong
1,2,3
, Chen Haowei
1,2,3
, Bai Jintao
1,2,3
**
1
National Key Laboratory of Shaanxi Province for Photoelectric Technology and Functional Materials,
Xi'an, Shaanxi 710069, China ;
2
National Optical Technology and Functional Nanomaterials International Science and Technology Joint
Research Center, Xi'an t Shaanxi 710069, China;
3
Institute of Photonics and Photon-Technology, Shaanxi Provincial Key Laboratory of Photo-Electronic Technology,
Northwest University, Xif
an, Shaanxi 710069, China
Abstract The tunable single-frequency (SF) narrow-linewidth fiber laser with
all-fiber complex cavity structure is designed, which is composed of an optical fiber
tunable filter, a high-precision ring filter, and a fiber loop mirror. A 980-nm
semiconductor laser is used as the pumping source, and the ytterbium-doped fiber is
employed as the gain medium and saturable absorber, then a wide-spectrum tunable
single-frequency narrow-linewidth laser output from 1030 nm to 1090 nm is successfully
realized. When the pump power is up to 300 mW, the output power is 18.5 mW and the
slope efficiency is 7.95% at the wavelength of 1070 nm. There is no mode hopping
phenomenon within 1 h, and the standard deviation of power stability is less than
1 %. When the pump power is 200 mW, the linewidth is measured by the delay
self-heterodyne method, and the average line width in the wavelength tuning range
is 8.7 kHz, and the relaxation oscillation frequency is 64 kHz.
Key words lasers; fiber lasers; single frequency; filter; tunable lasers
Introduction
Single-frequency fiber lasers have excellent optical characteristics such as small line
width, low noise and long coherence length. They have wide application prospects in the fields
of gravity wave detection, microwave photon and fiber sensing, and wavelength tunable
single-frequency fiber lasers have The characteristics of wide-spectrum single-frequency,
in the fields of metrology, biomedicine, spectroscopy, etc., have attracted the research
interest of many researchers. At present, ultra-short linear cavity, linear cavity and
annular cavity can realize single-frequency narrow linewidth laser output. (8) Ultra-short
linear cavity is difficult to achieve wide-range tuning of wavelength due to its ultra-short
cavity length and wavelength limitation of fiber-optic Bragg grating (FBG). The linear cavity
is caused by the spatial hole burning phenomenon in the gain fiber. In the multi-longitudinal

2. mode state; the annular cavity belongs to the traveling wave cavity, which avoids the hole
burning effect caused by the linear cavity, and can insert the appropriate mode selection
component and the wavelength tuning device to realize the output of the tunable
single-frequency fiber laser "" (2) . There are many techniques for wavelength tunable
generation, such as changing the center wavelength of FBG by stress or temperature to achieve
single-frequency laser wavelength tunable, unpumped doped fiber saturable absorbers and
cascaded filter devices, etc., but these The tunable single-frequency laser realized by the
method has the disadvantages of narrow tuning range, large device loss, high cost, etc. In
2008, Zhang et al. reported the Sagnac ring using 3.5 m unpumped bait fiber as self. Inductive
FBG filters combined with tunable Fabry-Perot (FP) filters for 45 nm tunable single-frequency
fiber laser output; in 2013, Feng et al. used a fiber-optic ring (1-ring) filter to limit
The number of longitudinal modes in the laser cavity, another double coupler fiber ring (2
rings) is used to ensure that there is only one longitudinal mode oscillation in the cavity,
and the single-frequency output of the fiber laser is realized by step-by-step filtering.
Tunable FBG for wavelength selection, achieving 30 nm tunable single-frequency laser output;
in 2015, Lu et al. used 1.5 m unpumped fiber as saturable absorber to form Sagnac ring junction
The dynamic grating, the center wavelength by changing the temperature of the FBG is used
to achieve a frequency tunable single fiber laser; 2016, Feng and other mouth. The 10 nm
tunable single-frequency laser output is achieved by stretching to change the center
wavelength of the FBG while embedding a dual coupler fiber loop filter in another fiber loop
filter. In 2017, Yeh et al. used a 10 cm fiber-doped fiber (YDF) and an optical fiberscope
as interferometer filters to obtain a single longitudinal mode output of the laser and a
30 nm tunable single-frequency laser output through a tunable bandpass filter.
Based on the annular cavity, a tunable single-frequency narrow-linewidth fiber laser
with tunable bandpass filter, high-precision ring filter and fiber loop mirror (LMF) is
designed. The 980 nm semiconductor laser is used as the pumping source. The mirrored fiber
is used as the gain medium and the unpumped saturable absorber in the cavity respectively.
Through the precise adjustment and optimization of the cavity type, the stable width of
1030-1090 nm is successfully realized. The spectrum can be tuned to single-frequency narrow
linewidth laser output. There is no mode hopping within 1 h of continuous operation, and
the power instability is less than 1%. When the pumping power is 200 mW, the linewidth
measurement is performed by the delay self-heterodyne method, and the average line width
is 8.7 kHz in the wavelength tuning range, and the relaxation oscillation frequency is 64
kHz.
2 Experimental Devices and Principles
可The experimental setup for tuninga single-frequency fiber laser is shown in Figure 1. A 980 nm semiconductor
laser with a pumping power of 600 mW is used as the pumping source, andthe pumping light is output from the pigtail via
a 980/1060 nm wavelength division multiplexer (WDM). Coupling into the cavity to pump a 80 cm-length fiber with a
length of 80 cm [Yb501, Corative, Canada, the concentrationof the mirror (atomic fraction, the same below) is 0.021, the
numerical aperture is 0.13], andthe pumping light passes through 3 dB respectively. Tunable bandpass filter with 1 nm
bandwidth, high precision filter (HFRF, consistingof 80% coupling: 2% 2X2 coupler C2 and 1.5 m unpumped gain fiber)

3. and fiber loop mirror Coupling ratio is 50%: 50% of 1X2 coupler C3, three-terminal circulator (CIR) and 2 m length of
unpumped gain fiber], tunable bandpass filter for multiple longitudinal modes in the cavity Mode suppression and
wavelength tuning are performed, and then the number of modes in the cavity is suppressed by a high-precision filter.
Finally, the single longitudinal mode in the cavity is selected by a fiber loop mirror. The three-port of the circulator is
connectedwith one end of the wavelength division multiplexer to form a tunable single-frequency fiber laser composite
annular cavity structure. In order to ensure that the light remains unidirectionally transmittedduring t he transmission, the
circulator is 3 to 2 The end has a 45 dB isolation. The resulting tunable single-frequency laser is output by a 30% port of
coupler C1 with a coupling ratio of 30%: 70% between the tunable filter (TF) and the high precision filter. Sin ce the gain
fiber has broadband absorption, and the fiber loop mirror can dynamically induce the grating filter to dynamically trackthe
wavelength output of the tunable filter, the filter composed of them can be used for the wavelength tunable fiber laser, and
can obtain a wider Tuning range.
The laser cavity tunable filter, high precision filter and fiber loop mirror can reduce
the number of longitudinal modes of vibration in the cavity. Through the filtering of these
three devices, a stable wavelength tuning single frequency output can be obtained. The
tunable filter is not only used to achieve a wide range of wavelength tuning, but its 1 nm
3 dB bandwidth effectively suppresses many of the modes produced by the stimulated radiation

4. of the mirrored fiber, reducing the number of longitudinal modes that oscillate within the
cavity. The high precision filter consists of a fiber optic ring and an unpumped fiber YDF
2 . After passing through the tunable filter, the light enters a coupler with a coupling
ratio of 50%: 50%, in which one light is coupled into the fiber ring and its transmittance
is
Splitting ratio formula for coupler; g is the gain of the optical fiber loop; 3 is the angular
frequency of the light field; r = 2k / a fsr delay time, a fsr = c / "is the free spectral
range, c = 3X108. m/s is the speed of light, L|=2.3m, which is the length of the high-precision
filter. The fiber loop mirror is composed of the Sagnac ring and the unpumped fiber YDF 3.
The light passes through the circulator to enter a coupling ratio of 50%: 50%. The coupler
is divided into two paths, and the two channels of the same amplitude and polarization are
oppositely interfered in the YDF3, so that the fiber loop mirror forms a dynamic grating.
The half-height full-width bandwidth of the dynamic grating is
Where / is the coupling coefficient of the dynamic grating; input light wavelength = 1060
nm; fiber refractive index % = 1.45; dynamic grating length L = 2 m; YDF 3 refractive index
change △ "V2X10T. Therefore, △ / '<14 MHz, this value is less than 11.8 m cavity length
determined by the longitudinal mode interval of 18 MHz, indicating that the laser is in single
longitudinal mode operation.
3 Analysis and Discussion of Experimental Results
Figure 2 shows the single-frequency signal characteristics measured by the scanning
F-P etalon. The characteristics of the single-frequency laser were experimentally observed
using a scanning F-P etalon (SA210, Thorlabs, USA) and an oscilloscope (DSO9104A, Agilent
Technologies, USA). The scanning F-P standard has a free spectral range of 1.5 GHz and an
accuracy of 200, which shows that the standard has a resolution of 7.5 MHz. The black sawtooth
wave represents a voltage cycle and the red curve represents the number of single vertical
modes over a ramp voltage cycle. It can be seen from Figure 2 that there are two single
longitudinal modes in a sawtooth ramp voltage cycle, which form a smooth envelope. Even if
one mode is expanded, no other modes will appear, and there is no mode jump and mode
competition. The phenomenon proves that the laser fully realizes single-frequency laser
operation.

5. The operating wavelength of the fiber optic tunable filter is controlled by a computer
program, and a continuous single-frequency laser of different wavelengths is output. The
constant pumping power is 300 mW and interval is measured by Yokogawa spectrometer (AQ6370C,
Yokogawa, Japan, resolution: 0.02 nm). The output spectrum of a tunable single-frequency
fiber laser for 5 nm is shown in Figure 3. It can be seen that this experiment successfully
achieves continuous tunability of the output wavelength of 1030-1090 nm, its signal-to-noise
ratio is greater than 50 dB, and no mode hopping and mode competition is observed when the
control wavelength is tuned. Stable 1030-1090 nm continuous tunable single-frequency laser
output.

6. In the experiment, the continuous single-frequency laser output power with different
pumping powers at different operating wavelengths was tested (Fig. 4). It can be seen from
Figure 4 that when the operating wavelengths of the lasers are different, the corresponding
slope efficiency and output power are also different. The output characteristics of the
tunable single-frequency laser at 1070 nm when the pump power is 300 mW are studied. At this
time, the single-frequency laser output power reaches the maximum value of 18.5 mW, and the
slope efficiency reaches 7.95%. The gain of the radiation spectrum of this laser medium at
1030 nm is stronger than other operating wavelengths, and the single-frequency output laser
should have the maximum gain factor and laser output slope efficiency in this band. However,
the experimental results show that when the slope efficiency is large, the operating
wavelength of the continuous single-frequency laser is also large, mainly because the
spontaneous emission of the shorter wavelength of the laser emission is absorbed again by
the doped fiber, resulting in a single-frequency output at 1030 nm. The output power and
slope efficiency of the laser did not reach a maximum.

7. In the experiment, the MAESTRO power meter produced by Gentec of Canada was used to
study the stability of single-frequency fiber laser output power at 1070 nm. Power sampling
was performed by setting the sampling time interval to 1 s and the time length to 2 h. Figure
5 shows the single-frequency fiber. The output power stability curve of the laser
continuously working for 2 h, the power instability is less than 1%, indicating that the
single-frequency fiber laser is in stable operation.

8. Wavelength Stability and Signal-to-Noise Ratio Stability of Single-Frequency Fiber
Lasers As shown in Figure 6, wavelength stability within 1 h was measured at 5 min intervals
in a room temperature environment. As can be seen from Fig. 6(c), when the wavelength of
the optically optically tunable filter is set to 1060 nm, the spectral center wavelength
does not change significantly. It can be seen from the calculation that the instability of
the wavelength resolution is less than 0.02 nm; the optical signal-to-noise ratio fluctuation
is less than 0.22 dB. The fluctuation of the wavelength offset and the optical signal-to-noise
ratio is caused by the change of pumping power and ambient temperature. At the same time,
the wavelength stability of the wavelengths of 1040 nm [Fig. 6(a)], 1050 nm [Fig. 6(c)],
and 1070 nm [Fig. 6(d)] are measured, and their operating wavelengths and optics can be
obtained. The stability of the signal to noise ratio is high.

9. The tunable single-frequency fiber laser output linewidth is measured using a 30 km
single-mode delay fiber and a delayed self-heterodyne method. Figure 7(a) shows a continuous
single-frequency fiber laser output operating at 1060 nm at a pumping power of 200 mW. Line
width. Since the linewidth of the true single-frequency laser output is half the line width
of the heterodyne signal curve, the linewidth of the single-frequency laser can be calculated
to be about 9 kHz. In order to obtain the linewidth results of other wavelengths of
single-frequency laser output, the measurement of 13 wavelength line widths is performed
[Fig. 7(b)], and the average line width is 8.7 kHz by averaging, which indicates that the
tunable Single-frequency lasers can achieve very narrow spectral linewidths over a very large
range.

10. The relative noise intensity of a single-frequency fiber laser was measured using a
photodetector (1611, Newport, USA) with a bandwidth of 3 dB and a maximum cutoff frequency
of 1 GHz and a radio frequency spectrum analyzer (MS2724C, ANRITSU, Japan). When the pump
source has no input power, the relative noise intensity (RIN) curve (received noise) of the
spectrum analyzer is shown in the black curve in Figure 8. When the pumping power of the
pumping source is 200 mW, the peak value of the relaxation oscillation frequency is about
64 kHz (the red curve in Fig. 8). The relaxation oscillation is due to the dynamic energy
exchange process between the pumping field and the laser signal field. . By comparing the
relative noise at a frequency of 0 to 1 MHz with the received noise, it is found that when
the frequency is greater than 200 kHz, no other noise components are observed in the relative
intensity noise spectrum of the single-frequency fiber laser.

11. 4 Conclusion
A tunable single-frequency narrow-linewidth fiber laser with an all-fiber composite
ring cavity structure is designed by using optical components composed of a tunable bandpass
filter, a high-precision ring filter and a fiber loop mirror as filter elements. The 980
nm semiconductor laser is used as the pumping source. The Dode fiber is used as the gain
medium and the unpumped saturable absorber in the cavity respectively. Combined with the
optical isolation of the three-port circulator, the laser is unidirectionally transmitted
in the cavity and then passed. The computer program controls the operating wavelength of
the fiber optic tunable filter, and then outputs continuous single-frequency lasers of
different wavelengths, successfully achieving a stable wide-spectrum tunable
single-frequency narrow-linewidth laser output of 1030-1090 nm, when the pumping power is
300. At mW, the output power at the wavelength of 1070 nm is the largest, 18.5 mW, and the
slope efficiency is 7.95%. There is no mode hopping within 1 h of continuous operation, and
the power instability is less than 1%. When the pumping power of the pumping light is 200
mW, the line width is measured by the delay self-heterodyne method, and the average line

12. width is 8.7 kHz in the wavelength tuning range, and the relaxation oscillation frequency
is 64 kHz.