- Email: [email protected]
Multiwavelength passively mode-locked fiber laser with ultra-broad bandwidth
Accepted Manuscript Title: Multiwavelength passively mode-locked fiber laser with ultra-broad bandwidth Author: Xing Li Jianping Chen Shixun Dai PII: DOI: Reference:
S0030-4026(16)30865-8 http://dx.doi.org/doi:10.1016/j.ijleo.2016.08.002 IJLEO 58019
To appear in: Received date: Revised date: Accepted date:
31-3-2016 31-7-2016 1-8-2016
Please cite this article as: Xing Li, Jianping Chen, Shixun Dai, Multiwavelength passively mode-locked fiber laser with ultra-broad bandwidth, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.08.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Multiwavelength passively mode-locked fiber laser with ultra-broad bandwidth Xing Li1, 2 *, Jianping Chen3 and Shixun Dai1, 2
1Laboratory
of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo 315211, China 2Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province, Ningbo, 315211, China 3State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China *Email: [email protected]
Abstract: We demonstrate a stable multiwavelength passively mode-locked erbium-doped fiber laser with ultra-narrow wavelength spacing and ultra-broad bandwidth. It is realized by exploiting an intracavity birefringence-induced comb filter and inhomogeneous loss mechanism based on nonlinear polarization rotation. Stable 808-line wavelength lasing with wavelength spacing of 0.07 nm and 3-dB bandwidth of 55.75 nm is achieved at room temperature. If 5-dB bandwidth of 86.15 nm is considered, the number of the lasing lines reaches 1248. In addition, the lasing wavelength lines and spacing could be flexibly tuned by modifying polarization-dependent cavity loss via properly rotating the waveplates. Index Terms: Multiwavelength lasers, passively mode-locked, Er-doped fiber lasers, birefringence filter.
1. Introduction Multiwavelength pulsed fiber lasers have attracted much attention for potential applications in optical signal processing [1], optical instrumentation [2], optical fiber sensing [3], and wavelength division multiplexing (WDM) communication systems [4]. The actively mode-locked technique was an effective way to generate multiwavelength optical pulses due to the advantages of high repetition and narrow linewidth [5-9]. However, actively mode-locked fiber lasers are usually locking only a few wavelengths, resulting in the limitation of some applications. Furthermore, a modulator is required to be inserted in the cavity, which increases the cost and the insertion loss. The passively mode-locked fiber laser can also be used to generate multiwavelength ultrashort pulses [10-24]. The main challenge of stable multiwavelength erbium-doped fiber lasers at room temperature is the strong homogeneous line broadening or mode competition. It can be efficiently suppressed by the intensity-dependent loss induced by the nonlinear polarization rotation (NPR) technique [16, 17]. For example, stable multiwavelength passively modelocked fiber lasers using the polarization maintaining fiber (PMF) have been demonstrated [18]. Its shortage lies in the difficulty to perform the wavelength tuning operation because the wavelength spacing is determined by the length of PMFs. Some techniques have been used to realize the tunable operation of multiwavelength lasing, such as using cascaded multi-section polarization-maintaining fiber [19], incorporating a semiconductor saturable absorber mirror [20] and employing a Mach–Zehnder interferometer [21] or a Sagnac loop with fiber Bragg grating in the cavity [22]. Nonetheless, they were unfavourable for the compactness of a fibre laser and also introduce larger cavity loss. In this letter, we demonstrate a tunable multiwavelength pulsed fiber laser with ultra-narrow wavelength spacing and ultra-broad bandwidth. Up to 808-line wavelength mode-locked pulses with a wavelength spacing of 0.07 nm and 3-dB bandwidth of 55.75 nm are successfully obtained. The number of the lasing wavelengths is 1248 with 5-dB bandwidth of 86.15 nm. By simply rotating the waveplates and thus modifying the intracavity comb filter, the lasing wavelength lines and spacing can be flexibly tuned.
2. Experimental Setup As shown in Fig. 1, the basic experimental setup of the multiwavelength pulsed fiber laser is a typical one of passively mode-locked fiber laser based on NPR [25]. The cavity is organized by a set of fibers and free-space components, all of which have appropriate group velocity dispersion (GVD). The fibers in the laser cavity include a 37 cm long highly Er-doped gain fiber (Liekki ER110-4/125, with GVD of 12 fs2/mm) which counter-pumps by two 700 mW, 974 nm diodes (Oclaro LC96U) that are polarization multiplexed to output up to 1.1 W, an 30 cm long
OFS-980 fiber (with GVD of 1.8 fs2/mm) pigtail of a 980 nm/1550 nm WDM, and two leading fibers (SMF28, with GVD of -22 fs2/mm) of the collimator1 and collimator2 with lengths of 9 cm and 12 cm, respectively. The free-space components consist of a polarization beam splitter (PBS), waveplates, and an isolator. The PBS acts both as the polarizer for the NPR mode locking and as the laser output coupler. A polarization-dependent isolator (PD-ISO) with a polarization isolation of 35 dB is inserted in the cavity to allow unidirectional operation and provides the functions of the tunable comb filter by combing with the intracavity birefringence. The polarization states of the multiwavelength mode-locked fiber laser are optimized by a half waveplate and two quarter waveplates. Considering the GVD of free-space bulk components, the net GVD of the whole cavity is managed to be close to zero at 1550 nm.
3. Experimental Results and Discussions The NPE based mode-locking can be easily initialized by rotating the wave plates when the pump power is above the threshold of 480 mW. When the pump power is increased above 640 mW (for instance, 700 mW in our experiment), the fiber laser switches to multiwavelength mode-locking state under careful adjustment of the waveplates. As shown in Fig. 2(a), the fundamental repetition rate of pulse trains is 201.6 MHz recorded by an oscilloscope (Agilent DSO9254A). No free-running pulses is observed, which indicates that the passive modelocking is successfully achieved. Figure 2(a) depicts a typical spectrum of multiwavelength mode-locked lasing centered at 1562.9 nm with 3-dB bandwidth of 39.45 nm, which is measured by an optical spectrum analyzer with a resolution of 0.02 nm (Yokogawa AQ6370C). More than 564 wavelength lasing lines in the 3-dB bandwidth are generated. If 5-dB bandwidth of 66.55 nm is considered, the number of the lasing lines reaches 951. The decrease of spectrum at 1530 nm is particularly serious which may be due to the fact that the ground state absorption in the gain fiber was not fully pumped by 980 nm pump light. The zoomed-in spectrum range from 1570 nm to 1571 nm is plotted in Fig. 2(b). It shows that the wavelength spacing is 0.07 nm, regular for all output wavelengths, and the linewidth is 0.041 nm. The average power of direct multiwavelength pulse output is as high as 58.5 mW at the pump power of 700 mW. When the pump power was increased to the maximum available pump power of 900 mW (after a 980nm ISO), the fiber laser can output multiwavelength lasing spectrum with ultrabroad 3-dB bandwidth of 55.75 nm and central wavelength of 1571.35 nm by readjusting the waveplates. As shown in Fig. 2(c), up to 808 wavelength lasing lines are generated at the 3-dB flattened spectrum from 1543.48 nm to 1599.23 nm. The number of the lasing wavelengths is 1248 with 5-dB bandwidth of 86.15 nm, ranging from 1515.13 nm to 1601.28 nm. Figure 2(d) also show the zoomed-in spectrum range from 1570 nm to 1571 nm. It can be seen that the wavelength spacing and the linewidth have no obvious change compared with the previous spectrum (2(b)). To our best knowledge, this is the the widest spectrum reported to date from a multiwavelength mode-locked fiber laser. The tunability of the multiwavelength mode-locked operation in the fiber laser by rotating the waveplates in the cavity is also investigated. Figures 4 show other three typical output spectra under different polarization states when the pump power is fixed at 700 mW. The number of the lasing wavelengths, the 3-dB bandwidth and the wavelength interval of these spectrum are summarized in Table 1. For clear comparison, the narrowest one (see Fig. 3(a)) is also included. It is found that the 3-dB bandwidth almost remains the same but the number of wavelength lines and the wavelength spacing as well can be widely tuned. Such feature of large multiwavelength bandwidth and tremendous wavelength lines is physically contributed to the intensity-dependent saturable absorption or loss. In other words, the wavelength components with higher intensity have larger loss, which balances power distribution of multiwavelength lasing induced by saturated NPR effect. Furthermore, the ring configuration with two polarizers (i.e. two quarter waveplates) connected by optical fibers is in principle equivalent to a birefringence filter as schematically shown in Fig. 5(a). Its transmittivity is dependent on the lasing wavelength ( ) as follows [26-27]:
1 T cos2 1 cos2 2 sin 2 1 sin 2 2 sin 21 sin 2 2 cos L NL 2
(1)
and
L 2 L ny nx
(2)
NL 2 PL cos 21 Aeff Where
(3)
L and NL are the linear and the nonlinear phase shift in the cavity, 1 ( 2 ) is the angle between the
polarization directions of the polarizers and the principal axes of the fiber, L is the fiber length of laser cavity, and n y are the refractive indices of the fast and slow axes of the optical fiber,
nx
is the nonlinear coefficient, P is
the instantaneous power of input signal, and Aeff is the effective fiber core area. A numerical calculation of Eq. (1) is depicted in Fig. 5(b), which shows a spectral comb filtering response with the wavelength spacing being dependent on the angles of 1 and 2 . It can well explain the tunability of the spectral spacing and number of the wavelength lines (see Fig. 4) via rotating the waveplates. Additionally, we investigate the effects of the pump power on the multiwavelength generation. The polarization states of the laser cavity is kept unchanged and the pump power is tuned from 700 to 610 mW with a step of 30 mW. The experimental results are depicted in Fig. 6(a). The 3-dB bandwidth keeps almost unchanged while the 5dB bandwidth becomes slightly narrower with the decrease of the pump power. For example, the 5-dB bandwidths are 66.65 nm, 65.62 nm and 62.57 nm under pump powers of 700 mw, 670 mw and 640 mw, respectively. As shown in Fig. 6(b), the wavelength spacing has no obvious change. Hence lowering pump power leads to the decrease of the lasing lines. The physical reason can be explained as follows. The total length of fiber in our cavity is only 88 cm. As long as the pump power is above certain value, say 640 mW, the accumulated nonlinear phase shift is sufficiently strong to generate the multiwavelength pulsed lasing. When the pump power is under 610 mW, the passive mode-locking becomes unstable and no lasing starts (see Fig. 6(a)). By assuming the intracavity power decreases from 150 mW to 120 mW, we calculate the spectral comb filtering response according to Eqs. (1)- (3). The result is shown in Fig. 6(c), clearly confirming no obvious change of wavelength spacing except for a tiny power fluctuation. Finally, we repeat measurement of the optical spectra of the fiber laser under 700 mW and 900 mW pump power over 3 hours to verify the stability of the proposed multiwavelength passively mode-locked fiber laser. The results per 30 minutes are compared in Fig. 7(a) and Fig. 7(c), the zoomed-in ones from 1570 nm to 1571 nm are depicted in Fig. 7(b) and Fig. 7(d). No significant wavelength drifts and power variations for all lasing lines are observed, and the wavelength spacing and the linewidth keep stable all across the -5dB bandwidth. These results indicate that the multiwavelength operation is stable at room temperature.
4. Conclusions In conclusion, we have demonstrated a stable multiwavelength passively mode-locked fiber laser with ultra-narrow wavelength spacing by use of an intensity-dependent-loss mechanism with a short cavity length. The repetition rate of the multiwavelength pulses is higher than 200 MHz. Stable multiwavelength lasing with multiwavelength lines of up to 808 and wavelength spacing as small as 0.07 nm in 3-dB bandwidth of 55.75 nm was successfully achieved at room temperature. Based on the intracavity birefringence-induced comb filter consisting of two polarizers and optical fibers, wide tunability was realized by simply rotating the wave plates. It can flexibly tune the lasing wavelength spacing and lines. The mode-locking once initialized is self-starting and highly stable with no need to re-adjust wave plates. Under the condition of stable multiwavelength lasing, laser operation is maintained for 24 hours with negligible variation of the characteristic parameters in the laboratory environment. The effect of pump power and the stability under a fixed pump power have been also experimentally investigated.
Acknowledgements This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 61435009), Natural Science Foundation of Ningbo (Grant Nos. 2016A610062), Science Research Fund Project of Ningbo University (Grant Nos. XYL16003), Open Fund of the Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices (South China University of Technology, Grant Nos. 2016-3), and was sponsored by the K. C. Wong Magna Fund in Ningbo University.
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Collimator1
λ /2 λ /4
λ /4 PD-ISO
Collimator2
SMF28
SMF28 PBS
Er:fiber
WDM
OFS980 974 nm Pump
Fig. 1. Configuration of the laser. PBS, polarization beam splitter; ISO, polarization-dependent isolator; λ/2, half waveplate; λ/4, quarter waveplate; WDM, wavelength-division multiplexer.
-20
Power(dBm)
Signal intensity(a.u.)
0.8 0.6 0.4 0.2
-40 -60 -80
0.0 -20
-10
0
10
Delay(ns) (a)
20
-100 0
200
400
600
800
Frequency(MHz)
1000
(b)
Fig. 2. (a) Output pulse trains measured by an oscilloscope. (b) The RF spectrum from 0 to 1 GHz with a resolution bandwidth of 10 kHz
-25
-30
5dB
Output power(dBm)
Output power(dBm)
-20 3dB
-40
-50
BW=39.45nm
-60
Bandwidth=66.63nm
-70 1480
1510
1540
1570
Channel Spacing~0.070nm
-30
-35
Linewidth~0.041nm
1570.0
1600
1570.2
1570.4
1570.6
1570.8
Wavelength(nm)
Wavelength(nm)
(a)
(b)
1571.0
-30
5dB
Output power(dBm)
Output power(dBm)
-20 3dB
-40 BW=55.75nm -50 Bandwidth=86.15nm
-60
-70 1480
1530
1580
Wavelength(nm)
(c)
1630
-25
Channel Spacing~0.070nm
-30
Linewidth~0.041nm -35 1570.0
1570.2
1570.4
1570.6
1570.8
1571.0
Wavelength(nm)
(d)
Fig. 3. (a) Multiwavelength spectrum with 3-dB bandwidth of 39.45 nm and (b) The zoomed-in spectrum of (a) from 1570 nm to 1571 nm when the pump power is 700 mW. (c) Multiwavelength spectrum with ultra-broad 3-dB bandwidth of 55.75nm and (d) The zoomed-in spectrum of (c) from 1570 nm to 1571 nm when the pump power is 900 mW.
60 30 0 -30
10
Channel Spacing~0.107nm
Output power(dBm)
Output power(dBm)
3.110nm 0.788nm 0.107nm 0.070nm
0
-10 -20
Channel Spacing~0.070nm
-30
-60 1500
1520
1540
1560
Wavelength(nm) (a)
1580
1600
-40 1570.0
1570.2
1570.4
1570.6
1570.8
1571.0
Wavelength(nm) (b)
Fig. 4. (a) Tunability of the number of the wavelength lines and pacing by rotating the waveplates. (b) The zoomed-in spectra with ~0.107-nm and ~0.07nm wavelength spacing. The vertical axes (output power) in (a) and (b) are artificially decreased by 25 dB from top to the bottom in order to show the spectra without overlap.
V
ᶿ
1
V
Poarizer 1
Poarizer 2
ᶿ
H
Optical fiber
2
H
Spectral intensity(a.u.)
0.6
0.4
0.2
0.0 1570
1572
1574
1576
1578
1580
Wavelength(nm)
(a)
(b)
Fig. 5. (a) Schematic of the birefringence comb filter in the multiwavelength pulsed fiber laser. (b) Calculated transmission spectra for different angles of polarizers, which is equal to the rotation of waveplates. Solid curve: 1 and 2 are
3 , respectively. Dashed curve: 1 and 2 are 6 and 8 , respectively.
5
and
-80
-120
-40
-80
-120
-160
-160 1480
700mW 670mW 640mW 610mW
0
1500
1520
1540
1560
1580
Wavelength(nm)
(a)
1600
1570.0
1570.2
1570.4
1570.6
1570.8
Wavelength(nm)
(b)
1571.0
0.6
Spectral intensity(a.u.)
-40
Output power(dBm)
Output power(dBm)
700mW 670mW 640mW 610mW
0
150mw 120mw
0.4
0.2
0.0 1570
1572
1574
1576
1578
Wavelength(nm)
(c)
Fig. 6. (a) Multiwavelength spectra under different pump power. (b) The zoomed-in spectra from 1570 nm to 1571 nm. (c) Calculated transmission spectra of the comb filter when the intracavity power is assumed to reduce from 150 mW to 120 mW. The vertical axes (output power) in (a) and (b) are artificially decreased by 25 dB from top to the bottom.
1580
Output power(dBm)
0
3h 2.5 h
-40
2h
1.5 h
-80
1h 0.5 h
-120
0h 1570.0
1570.2
1570.4
1570.6
1570.8
1571.0
Wavelength(nm)
(b)
Output power(dBm)
0
3h 2.5 h
-40
2h 1.5 h
-80
1h 0.5 h
-120
0h 1570.0
1570.2
1570.4
1570.6
1570.8
1571.0
Wavelength(nm)
(d) Fig. 7. Repeatability test of the multiwavelength spectra with 30-minute interval (a) and the zoomed-in spectra from 1570 nm to 1571 nm (b) when the pump power is 700 mW. Repeatability test of the multiwavelength spectra with 30-minute interval (c) and the zoomed-in spectra from 1570 nm to 1571 nm (d) when the pump power is 700 mW. The vertical axes (output power) in (b) and (d) are artificially decreased by 20 dB from top to the bottom.
Table 1. Summary of the multiwavelength tunability Cases
Wavelength lines
3-dB bandwidth(nm)
Wavelength spacing (nm)
1
12
35.17
3.110
2
50
39.22
0.788
3
366
39.13
0.108
4
564
39.45
0.07
S0030-4026(16)30865-8 http://dx.doi.org/doi:10.1016/j.ijleo.2016.08.002 IJLEO 58019
To appear in: Received date: Revised date: Accepted date:
31-3-2016 31-7-2016 1-8-2016
Please cite this article as: Xing Li, Jianping Chen, Shixun Dai, Multiwavelength passively mode-locked fiber laser with ultra-broad bandwidth, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.08.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Multiwavelength passively mode-locked fiber laser with ultra-broad bandwidth Xing Li1, 2 *, Jianping Chen3 and Shixun Dai1, 2
1Laboratory
of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo 315211, China 2Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province, Ningbo, 315211, China 3State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China *Email: [email protected]
Abstract: We demonstrate a stable multiwavelength passively mode-locked erbium-doped fiber laser with ultra-narrow wavelength spacing and ultra-broad bandwidth. It is realized by exploiting an intracavity birefringence-induced comb filter and inhomogeneous loss mechanism based on nonlinear polarization rotation. Stable 808-line wavelength lasing with wavelength spacing of 0.07 nm and 3-dB bandwidth of 55.75 nm is achieved at room temperature. If 5-dB bandwidth of 86.15 nm is considered, the number of the lasing lines reaches 1248. In addition, the lasing wavelength lines and spacing could be flexibly tuned by modifying polarization-dependent cavity loss via properly rotating the waveplates. Index Terms: Multiwavelength lasers, passively mode-locked, Er-doped fiber lasers, birefringence filter.
1. Introduction Multiwavelength pulsed fiber lasers have attracted much attention for potential applications in optical signal processing [1], optical instrumentation [2], optical fiber sensing [3], and wavelength division multiplexing (WDM) communication systems [4]. The actively mode-locked technique was an effective way to generate multiwavelength optical pulses due to the advantages of high repetition and narrow linewidth [5-9]. However, actively mode-locked fiber lasers are usually locking only a few wavelengths, resulting in the limitation of some applications. Furthermore, a modulator is required to be inserted in the cavity, which increases the cost and the insertion loss. The passively mode-locked fiber laser can also be used to generate multiwavelength ultrashort pulses [10-24]. The main challenge of stable multiwavelength erbium-doped fiber lasers at room temperature is the strong homogeneous line broadening or mode competition. It can be efficiently suppressed by the intensity-dependent loss induced by the nonlinear polarization rotation (NPR) technique [16, 17]. For example, stable multiwavelength passively modelocked fiber lasers using the polarization maintaining fiber (PMF) have been demonstrated [18]. Its shortage lies in the difficulty to perform the wavelength tuning operation because the wavelength spacing is determined by the length of PMFs. Some techniques have been used to realize the tunable operation of multiwavelength lasing, such as using cascaded multi-section polarization-maintaining fiber [19], incorporating a semiconductor saturable absorber mirror [20] and employing a Mach–Zehnder interferometer [21] or a Sagnac loop with fiber Bragg grating in the cavity [22]. Nonetheless, they were unfavourable for the compactness of a fibre laser and also introduce larger cavity loss. In this letter, we demonstrate a tunable multiwavelength pulsed fiber laser with ultra-narrow wavelength spacing and ultra-broad bandwidth. Up to 808-line wavelength mode-locked pulses with a wavelength spacing of 0.07 nm and 3-dB bandwidth of 55.75 nm are successfully obtained. The number of the lasing wavelengths is 1248 with 5-dB bandwidth of 86.15 nm. By simply rotating the waveplates and thus modifying the intracavity comb filter, the lasing wavelength lines and spacing can be flexibly tuned.
2. Experimental Setup As shown in Fig. 1, the basic experimental setup of the multiwavelength pulsed fiber laser is a typical one of passively mode-locked fiber laser based on NPR [25]. The cavity is organized by a set of fibers and free-space components, all of which have appropriate group velocity dispersion (GVD). The fibers in the laser cavity include a 37 cm long highly Er-doped gain fiber (Liekki ER110-4/125, with GVD of 12 fs2/mm) which counter-pumps by two 700 mW, 974 nm diodes (Oclaro LC96U) that are polarization multiplexed to output up to 1.1 W, an 30 cm long
OFS-980 fiber (with GVD of 1.8 fs2/mm) pigtail of a 980 nm/1550 nm WDM, and two leading fibers (SMF28, with GVD of -22 fs2/mm) of the collimator1 and collimator2 with lengths of 9 cm and 12 cm, respectively. The free-space components consist of a polarization beam splitter (PBS), waveplates, and an isolator. The PBS acts both as the polarizer for the NPR mode locking and as the laser output coupler. A polarization-dependent isolator (PD-ISO) with a polarization isolation of 35 dB is inserted in the cavity to allow unidirectional operation and provides the functions of the tunable comb filter by combing with the intracavity birefringence. The polarization states of the multiwavelength mode-locked fiber laser are optimized by a half waveplate and two quarter waveplates. Considering the GVD of free-space bulk components, the net GVD of the whole cavity is managed to be close to zero at 1550 nm.
3. Experimental Results and Discussions The NPE based mode-locking can be easily initialized by rotating the wave plates when the pump power is above the threshold of 480 mW. When the pump power is increased above 640 mW (for instance, 700 mW in our experiment), the fiber laser switches to multiwavelength mode-locking state under careful adjustment of the waveplates. As shown in Fig. 2(a), the fundamental repetition rate of pulse trains is 201.6 MHz recorded by an oscilloscope (Agilent DSO9254A). No free-running pulses is observed, which indicates that the passive modelocking is successfully achieved. Figure 2(a) depicts a typical spectrum of multiwavelength mode-locked lasing centered at 1562.9 nm with 3-dB bandwidth of 39.45 nm, which is measured by an optical spectrum analyzer with a resolution of 0.02 nm (Yokogawa AQ6370C). More than 564 wavelength lasing lines in the 3-dB bandwidth are generated. If 5-dB bandwidth of 66.55 nm is considered, the number of the lasing lines reaches 951. The decrease of spectrum at 1530 nm is particularly serious which may be due to the fact that the ground state absorption in the gain fiber was not fully pumped by 980 nm pump light. The zoomed-in spectrum range from 1570 nm to 1571 nm is plotted in Fig. 2(b). It shows that the wavelength spacing is 0.07 nm, regular for all output wavelengths, and the linewidth is 0.041 nm. The average power of direct multiwavelength pulse output is as high as 58.5 mW at the pump power of 700 mW. When the pump power was increased to the maximum available pump power of 900 mW (after a 980nm ISO), the fiber laser can output multiwavelength lasing spectrum with ultrabroad 3-dB bandwidth of 55.75 nm and central wavelength of 1571.35 nm by readjusting the waveplates. As shown in Fig. 2(c), up to 808 wavelength lasing lines are generated at the 3-dB flattened spectrum from 1543.48 nm to 1599.23 nm. The number of the lasing wavelengths is 1248 with 5-dB bandwidth of 86.15 nm, ranging from 1515.13 nm to 1601.28 nm. Figure 2(d) also show the zoomed-in spectrum range from 1570 nm to 1571 nm. It can be seen that the wavelength spacing and the linewidth have no obvious change compared with the previous spectrum (2(b)). To our best knowledge, this is the the widest spectrum reported to date from a multiwavelength mode-locked fiber laser. The tunability of the multiwavelength mode-locked operation in the fiber laser by rotating the waveplates in the cavity is also investigated. Figures 4 show other three typical output spectra under different polarization states when the pump power is fixed at 700 mW. The number of the lasing wavelengths, the 3-dB bandwidth and the wavelength interval of these spectrum are summarized in Table 1. For clear comparison, the narrowest one (see Fig. 3(a)) is also included. It is found that the 3-dB bandwidth almost remains the same but the number of wavelength lines and the wavelength spacing as well can be widely tuned. Such feature of large multiwavelength bandwidth and tremendous wavelength lines is physically contributed to the intensity-dependent saturable absorption or loss. In other words, the wavelength components with higher intensity have larger loss, which balances power distribution of multiwavelength lasing induced by saturated NPR effect. Furthermore, the ring configuration with two polarizers (i.e. two quarter waveplates) connected by optical fibers is in principle equivalent to a birefringence filter as schematically shown in Fig. 5(a). Its transmittivity is dependent on the lasing wavelength ( ) as follows [26-27]:
1 T cos2 1 cos2 2 sin 2 1 sin 2 2 sin 21 sin 2 2 cos L NL 2
(1)
and
L 2 L ny nx
(2)
NL 2 PL cos 21 Aeff Where
(3)
L and NL are the linear and the nonlinear phase shift in the cavity, 1 ( 2 ) is the angle between the
polarization directions of the polarizers and the principal axes of the fiber, L is the fiber length of laser cavity, and n y are the refractive indices of the fast and slow axes of the optical fiber,
nx
is the nonlinear coefficient, P is
the instantaneous power of input signal, and Aeff is the effective fiber core area. A numerical calculation of Eq. (1) is depicted in Fig. 5(b), which shows a spectral comb filtering response with the wavelength spacing being dependent on the angles of 1 and 2 . It can well explain the tunability of the spectral spacing and number of the wavelength lines (see Fig. 4) via rotating the waveplates. Additionally, we investigate the effects of the pump power on the multiwavelength generation. The polarization states of the laser cavity is kept unchanged and the pump power is tuned from 700 to 610 mW with a step of 30 mW. The experimental results are depicted in Fig. 6(a). The 3-dB bandwidth keeps almost unchanged while the 5dB bandwidth becomes slightly narrower with the decrease of the pump power. For example, the 5-dB bandwidths are 66.65 nm, 65.62 nm and 62.57 nm under pump powers of 700 mw, 670 mw and 640 mw, respectively. As shown in Fig. 6(b), the wavelength spacing has no obvious change. Hence lowering pump power leads to the decrease of the lasing lines. The physical reason can be explained as follows. The total length of fiber in our cavity is only 88 cm. As long as the pump power is above certain value, say 640 mW, the accumulated nonlinear phase shift is sufficiently strong to generate the multiwavelength pulsed lasing. When the pump power is under 610 mW, the passive mode-locking becomes unstable and no lasing starts (see Fig. 6(a)). By assuming the intracavity power decreases from 150 mW to 120 mW, we calculate the spectral comb filtering response according to Eqs. (1)- (3). The result is shown in Fig. 6(c), clearly confirming no obvious change of wavelength spacing except for a tiny power fluctuation. Finally, we repeat measurement of the optical spectra of the fiber laser under 700 mW and 900 mW pump power over 3 hours to verify the stability of the proposed multiwavelength passively mode-locked fiber laser. The results per 30 minutes are compared in Fig. 7(a) and Fig. 7(c), the zoomed-in ones from 1570 nm to 1571 nm are depicted in Fig. 7(b) and Fig. 7(d). No significant wavelength drifts and power variations for all lasing lines are observed, and the wavelength spacing and the linewidth keep stable all across the -5dB bandwidth. These results indicate that the multiwavelength operation is stable at room temperature.
4. Conclusions In conclusion, we have demonstrated a stable multiwavelength passively mode-locked fiber laser with ultra-narrow wavelength spacing by use of an intensity-dependent-loss mechanism with a short cavity length. The repetition rate of the multiwavelength pulses is higher than 200 MHz. Stable multiwavelength lasing with multiwavelength lines of up to 808 and wavelength spacing as small as 0.07 nm in 3-dB bandwidth of 55.75 nm was successfully achieved at room temperature. Based on the intracavity birefringence-induced comb filter consisting of two polarizers and optical fibers, wide tunability was realized by simply rotating the wave plates. It can flexibly tune the lasing wavelength spacing and lines. The mode-locking once initialized is self-starting and highly stable with no need to re-adjust wave plates. Under the condition of stable multiwavelength lasing, laser operation is maintained for 24 hours with negligible variation of the characteristic parameters in the laboratory environment. The effect of pump power and the stability under a fixed pump power have been also experimentally investigated.
Acknowledgements This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 61435009), Natural Science Foundation of Ningbo (Grant Nos. 2016A610062), Science Research Fund Project of Ningbo University (Grant Nos. XYL16003), Open Fund of the Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices (South China University of Technology, Grant Nos. 2016-3), and was sponsored by the K. C. Wong Magna Fund in Ningbo University.
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Collimator1
λ /2 λ /4
λ /4 PD-ISO
Collimator2
SMF28
SMF28 PBS
Er:fiber
WDM
OFS980 974 nm Pump
Fig. 1. Configuration of the laser. PBS, polarization beam splitter; ISO, polarization-dependent isolator; λ/2, half waveplate; λ/4, quarter waveplate; WDM, wavelength-division multiplexer.
-20
Power(dBm)
Signal intensity(a.u.)
0.8 0.6 0.4 0.2
-40 -60 -80
0.0 -20
-10
0
10
Delay(ns) (a)
20
-100 0
200
400
600
800
Frequency(MHz)
1000
(b)
Fig. 2. (a) Output pulse trains measured by an oscilloscope. (b) The RF spectrum from 0 to 1 GHz with a resolution bandwidth of 10 kHz
-25
-30
5dB
Output power(dBm)
Output power(dBm)
-20 3dB
-40
-50
BW=39.45nm
-60
Bandwidth=66.63nm
-70 1480
1510
1540
1570
Channel Spacing~0.070nm
-30
-35
Linewidth~0.041nm
1570.0
1600
1570.2
1570.4
1570.6
1570.8
Wavelength(nm)
Wavelength(nm)
(a)
(b)
1571.0
-30
5dB
Output power(dBm)
Output power(dBm)
-20 3dB
-40 BW=55.75nm -50 Bandwidth=86.15nm
-60
-70 1480
1530
1580
Wavelength(nm)
(c)
1630
-25
Channel Spacing~0.070nm
-30
Linewidth~0.041nm -35 1570.0
1570.2
1570.4
1570.6
1570.8
1571.0
Wavelength(nm)
(d)
Fig. 3. (a) Multiwavelength spectrum with 3-dB bandwidth of 39.45 nm and (b) The zoomed-in spectrum of (a) from 1570 nm to 1571 nm when the pump power is 700 mW. (c) Multiwavelength spectrum with ultra-broad 3-dB bandwidth of 55.75nm and (d) The zoomed-in spectrum of (c) from 1570 nm to 1571 nm when the pump power is 900 mW.
60 30 0 -30
10
Channel Spacing~0.107nm
Output power(dBm)
Output power(dBm)
3.110nm 0.788nm 0.107nm 0.070nm
0
-10 -20
Channel Spacing~0.070nm
-30
-60 1500
1520
1540
1560
Wavelength(nm) (a)
1580
1600
-40 1570.0
1570.2
1570.4
1570.6
1570.8
1571.0
Wavelength(nm) (b)
Fig. 4. (a) Tunability of the number of the wavelength lines and pacing by rotating the waveplates. (b) The zoomed-in spectra with ~0.107-nm and ~0.07nm wavelength spacing. The vertical axes (output power) in (a) and (b) are artificially decreased by 25 dB from top to the bottom in order to show the spectra without overlap.
V
ᶿ
1
V
Poarizer 1
Poarizer 2
ᶿ
H
Optical fiber
2
H
Spectral intensity(a.u.)
0.6
0.4
0.2
0.0 1570
1572
1574
1576
1578
1580
Wavelength(nm)
(a)
(b)
Fig. 5. (a) Schematic of the birefringence comb filter in the multiwavelength pulsed fiber laser. (b) Calculated transmission spectra for different angles of polarizers, which is equal to the rotation of waveplates. Solid curve: 1 and 2 are
3 , respectively. Dashed curve: 1 and 2 are 6 and 8 , respectively.
5
and
-80
-120
-40
-80
-120
-160
-160 1480
700mW 670mW 640mW 610mW
0
1500
1520
1540
1560
1580
Wavelength(nm)
(a)
1600
1570.0
1570.2
1570.4
1570.6
1570.8
Wavelength(nm)
(b)
1571.0
0.6
Spectral intensity(a.u.)
-40
Output power(dBm)
Output power(dBm)
700mW 670mW 640mW 610mW
0
150mw 120mw
0.4
0.2
0.0 1570
1572
1574
1576
1578
Wavelength(nm)
(c)
Fig. 6. (a) Multiwavelength spectra under different pump power. (b) The zoomed-in spectra from 1570 nm to 1571 nm. (c) Calculated transmission spectra of the comb filter when the intracavity power is assumed to reduce from 150 mW to 120 mW. The vertical axes (output power) in (a) and (b) are artificially decreased by 25 dB from top to the bottom.
1580
Output power(dBm)
0
3h 2.5 h
-40
2h
1.5 h
-80
1h 0.5 h
-120
0h 1570.0
1570.2
1570.4
1570.6
1570.8
1571.0
Wavelength(nm)
(b)
Output power(dBm)
0
3h 2.5 h
-40
2h 1.5 h
-80
1h 0.5 h
-120
0h 1570.0
1570.2
1570.4
1570.6
1570.8
1571.0
Wavelength(nm)
(d) Fig. 7. Repeatability test of the multiwavelength spectra with 30-minute interval (a) and the zoomed-in spectra from 1570 nm to 1571 nm (b) when the pump power is 700 mW. Repeatability test of the multiwavelength spectra with 30-minute interval (c) and the zoomed-in spectra from 1570 nm to 1571 nm (d) when the pump power is 700 mW. The vertical axes (output power) in (b) and (d) are artificially decreased by 20 dB from top to the bottom.
Table 1. Summary of the multiwavelength tunability Cases
Wavelength lines
3-dB bandwidth(nm)
Wavelength spacing (nm)
1
12
35.17
3.110
2
50
39.22
0.788
3
366
39.13
0.108
4
564
39.45
0.07