Novel output states in the erbium-doped fiber laser near zero dispersion with semiconductor saturable absorber mirror

Accepted Manuscript Title: Novel output states in the erbium-doped fiber laser near zero dispersion with semiconductor saturable absorber mirror Author: Chuanwei Tao Kun shan Lei zhang Xizeng Hui PII: DOI: Reference:

S0030-4026(16)31577-7 http://dx.doi.org/doi:10.1016/j.ijleo.2016.12.027 IJLEO 58657

To appear in: Received date: Revised date: Accepted date:

24-11-2016 9-12-2016 9-12-2016

Please cite this article as: Chuanwei Tao, Kun shan, Lei zhang, Xizeng Hui, Novel output states in the erbium-doped fiber laser near zero dispersion with semiconductor saturable absorber mirror, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.12.027 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.

Novel output states in the erbium-doped fiber laser near zero dispersion with semiconductor saturable absorber mirror Chuanwei Tao1, Kun shan2, Lei zhang2*, Xizeng Hui3 1

Shandong Business Institute, Yantai, 264670, China

2

School of physics and optoelectronic engineering, Ludong University, YanTai 264025, China 3

Rizhao Technician College, Rizhao, 276826, China

*Corresponding author Email Address: [email protected]

Abstract: we report the observation of operation states near zero dispersion in the erbium-doped fiber laser with semiconductor saturable absorber mirror（SESAM）. The Q-switching, Q-switching mode-locking（QML）, continuous wave mode-locking (CML), harmonic mode-locking(HML), double-pulse and multiple-pulse

operation states can be

manipulated by adjusting the polarization controller with semiconductor saturable absorber mirror. The output 3dB spectral bandwidth and pulse width of operation states are also measured and analyzed. It is shown that continuous wave mode-locked state is stable and without spectral sidebands. Therefore, the proposed scheme will be useful and feasible in the optical telecommunication system. Key words: erbium-doped fiber laser (EDFL), mode locking, semiconductor saturable absorber mirror (SESAM). 1. Introduction The ultra-short optical pulse has attracted much attention because of their potential applications in communications and signal processing[1-3]. However, the self-started, passively mode-locked fiber laser is a simple and economic ultra-short pulse source. Since 1990s, passively mode-locked fiber laser at 1550nm wavelength has made great progress due to the breakthrough of Erbium-doped fiber. Several techniques including nonlinear polarization rotation[4], stretched pulse[5], additive pulse mode-locking[6], figure-8 fiber laser which utilizes the nonlinear amplifying loop mirror technique[7-8] and the dispersion-imbalanced nonlinear amplifying loop mirror technique[9] have been used to generate the ultra-short optical pulse. However, the SESAM technology has many advantages compared to the above methods, for examples, mode-locking by exploiting nonlinear polarization evolution. It offers a self-started pulse operation and good environmental stability. Therefore, SESAM technology has been widely used to generate mode-locking pulse in different solid-state and fiber lasers [10-14]. Generally, mode-locked Erbium-doped fiber lasers operate in the net negative cavity dispersion regime, where the fiber nonlinear optical Kerr effect balances the dispersion of the

cavity[15-16]. However, the solitons formed in the lasers have low pulse energy and peak power. In order to increase the pulse energy, Tamura proposed a design of dispersion-managed cavity, where the fiber laser is constructed with a segment of large positive dispersion fiber and the negative dispersion fiber. The fiber laser generates the pulse with low peak power, which reduces the effective nonlinear phase shift of the pulse within one round-trip and increases the pulse energy. And the conventional solitary wave with spectral sidebands can be formed in the fiber lasers. Compared with the conventional solitons generated in the laser, the spectrum of dispersion-managed solitons have the approximate Gaussian profile and without spectral sidebands. Moreover, the passively mode-locked fiber laser with spectral sidebands used in the high-speed optical telecommunication system will generate harmful effects. It has been shown that timing of adjacent soliton pulse can be affected by interaction with the spectral sidebands. Over long distance propagation such an interaction can be expected to generate excess bit errors. While an ideal source would generate the soliton pulse without spectral sidebands. All-fiber soliton lasers may be practical for telecommunication if the sidebands can be sufficiently suppressed.

Therefore, we suppress the spectral sidebands by control net intracavity

dispersion in the laser system. The instability of mode-locked pulse occurs when the GVD is large and the mode-locked strength becomes weak. Therefore, mode locking is most easily achieved at zero GVD. In this paper, we bring forward a new experimental scheme on the different output states of fiber laser which can be controlled by adjusting the polarization controller in the same pump power. For certain application, the fiber laser with semiconductor saturable absorber mirror generates soliton pulse without spectral sidebands and the central wavelength of the soliton pulse is stable. Switching between different short-pulse states while keeping the same averaged output power will be useful. The continuous wave (CW) and Q-switched pulse, continuous mode-locking（CML）, harmonic mode locking (HML), double-pulse and multiple-pulse state had been observed in the experiment. Moreover, we compare the 3dB spectral width and pulse width of different output states. 2. Experimental setup

The schematic experimental setup of the fiber laser is shown in Fig.1, which is a fiber laser with semiconductor saturable absorber mirror. The fiber laser has 1.7m erbium-doped fiber with the GVD parameter of 37 ps nm km , the single mode fiber have a GVD parameter of 17 ps nm km , the net GVD of cavity was controlled by changing the length

of SMF. The total cavity length is about 3.84m which includes the pigtails of all elements, corresponding to an approximate repetition period of 20.6MHz.The SESAM has an unsaturated absorption of

A0  30 0

0

and modulation depth of R  18 0 , with saturation 0

fluence of 90uJ cm2 and the reflective spectral width of fiber mirror is 30nm with central wavelength at 1550nm. Therefore, the central wavelength is stable due to the fiber mirror

acting on the generated soliton. The polarization splitter is used to output the pulse and regarded as polarizer. The wavelength-division-multiplexer (WDM) is used to couple the pump light into the cavity. The fiber laser is pumped by 980nm pump source and the output of the fiber laser is analyzed with optical spectral analyzer and oscilloscope. Therefore, the properties and evolution of mode-locked state in the laser cavity can be monitored by the optical spectral analyzer and oscilloscope. In this experimental setup, the total length of the SMF in the cavity was chosen to make the net cavity dispersion near zero. Mode-locked pulses with such spectral characteristics only appear when the net cavity GVD is around zero. We believe that its formation could be closely related to the near equal broadening and compression of the mode-locked pulse in one cavity roundtrip. Mode locking is self-started in the laser by increasing the pump power beyond a certain threshold and adjusting the orientations of the polarization controller. In this paper, we mainly control the different output states by rotating the polarization controller. 3. Theoretical analysis The pulse evolution can be treated with a modified Ginzburg-Landau equation with several nonlinear non-conservative terms. Time-dependent saturable loss  s of passive modulator can be written as a rate equation [10-11]

 s   R   s  s t  rec Esat . A 2

(1)

R is the saturable loss of an unexcited absorber ， is the optical intensity， Esat . A is the absorber saturation energy， energy

Ec

 rec is the recovery time of SESAM. The intracavity pulse

required for the stable operation of CML against Q-switched mode-locked

should obey the criterion

Ec  Esat .G Esat . AR Where

Esat .G

(2)

is the gain saturation energy of the EDFL. Therefore, if we obtain the stable

CML, the single pulse energy must satisfy the fixed requirement. The laser pulse generation process can be demonstrated by the theoretical simulation as shown in Fig.2. In the laser, the intensity-dependent loss modulator results from the combining effects of the polarization controller, the erbium-doped fiber, the single-mode fiber and the polarizer. The intensity-dependent loss will be encountered when passing the optical field through such an effective modulator, providing different transmission losses for the low and high intensity components. The transmission coefficient can be expressed as T

2

2 sin 1

s i2 n 2 

c2o1s

1 co2  s 2

2



si1n  2

s2 i n

2   c o s (3)

Where 1 is the angle between the fast axis of the fiber and the polarization controller,  2 is the angle between the axis of the polarizer and the fast axis of the fiber.    L   NL ,  NL

is the phase difference between the wave components in the orthogonal birefringent axes u and v, which is introduced by polarization controller as shown in Fig.3.  L is the linear phase shift difference. So the transmission can be tuned through adjusting the polarization controller. The transmission increases with increasing signal power and PC-SMF-Polarizer combination functions as a saturable absorber as shown in Fig.4. The SESAM used in the experiment is optimized for CML state operation, and we also obtain stable CW, Q-switching, HML state and multiple-pulse state with the help of the loss modulation mechanism induced by intracavity polarization controller. Therefore, by properly adjusting the polarization controller, one kind of the operation states can be obtained. 4. Experimental results and discussion In our experiment, the input pump power is 149mW. When the polarization of the light is approximately set, we observe the Q-switched pulse, the continuous mode-locked pulse, the multi-pulse and harmonic mode-locked operation state and so on. The CW lasing as shown in fig.4 (a) central wavelength is 1560nm. By adjusting the polarization controller, the output state can be operated in the Q-switched state as shown in fig.4 (b), and the pulse repetition rate is 33.4KHz, which can be increased by increasing the input pump power. The Q-switched mode-locked pulse envelope is shown as fig.4. The Q-switched mode-locked pulse envelope repetition rate can be increased by increasing the pump power and the Q-switched mode-locked pulse envelope is not stable. By adjusting the polarization controller, the continuous mode-locked state is obtained in the following. In the experiment, we obtain a stable train of periodic optical pulses with repetition frequency of 20.6 MHz from the fiber laser. Fig. 5(a) illustrates the generated pulse train which is

measured by using an oscilloscope. Fig. 5(b) illustrates the generated pulse spectrum from the spectral analyzer, that the 3dB spectral bandwidth is 6.6nm with the central wavelength of 1560nm. Fig.5(c) shows the 200us/div continuous mode-locked pulse train. Then an autocorrelator is employed at the output of fiber laser to measure the autocorrelation trace of the obtained optical pulse which has the FWHM of 600fs, as shown in Fig. 5(d) and the product of pulse width and spectral bandwidth is near Gaussian transform limit 0.441.Therefore, the chirped coefficient of single mode-locked pulse is very small. In our experiments, higher pulse splitting can also obtained by increasing the intracavity laser powers or accurately tuning the polarization controller. Nevertheless, all operation state, from the continuous wave to multiple-pulse state can be obtained by tuning the polarization controller. Therefore, we adjust the polarization controller and obtain double-pulse state as shown in fig.6(a). The corresponding spectrum of double-pulse state is shown in fig.6(b) and the 3dB spectral width of double-pulse state is 3.74nm. The multiple-pulse state had been observed in the fiber laser. It includes the following modes of operation: the soliton bunching mode, where several solitons tightly bunch and move together in the laser cavity.

Under special conditions, the solitons can also automatically

rearrange themselves and form the so-called harmonic mode-locked state as shown in fig.7 (a). The corresponding spectrum of harmonic mode-locked pulse is shown in fig.7 (b) and the 3dB spectral width of harmonic mode-locked pulse is 3.38nm. fig.7 (c) shows the corresponding autocorrelation measurement with pulse width of 1.5ps. It shows that the output pulse width is becoming wider with the output narrower 3dB spectral bandwidth. Based on the experimental results, apart from the gain recovery and acoustic effect, the unstable CW lasing in the cavity can be another mechanism for the formation of the passive harmonic mode locking in the fiber lasers. Because the weak CW would not affect the property of the pulse. When the intensity of CW in the laser cavity increase, all soliton pulse become unstable in the laser due to the modulation instability. Only when all the solitons are equally distributed over the whole cavity, a steady soliton distribution state is obtained and the harmonic mode-locking is obtained. In our experiments, higher pulse splitting can also obtained by future adjusting the polarization controller. Therefore, we adjust the polarization controller and obtain triple-pulse and four-pulse mode-locked pulse train as shown in fig.8(a) and fig.8(d), respectively. The corresponding spectrum of triple-pulse and four-pulse mode-locked pulse is shown in fig.8(b) and fig.8(e). The 3dB spectral width of triple-pulse and four-pulse are 3.3nm and 2.9nm, respectively. By measuring the 3dB spectral width of different operation states, it shows that the higher pulse split, the narrower the 3dB spectral width is. We measure the pulse width of triple-pulse mode-locked pulse with 2ps. In order to obtain the stable and narrower pulse for future optical communications, we should properly adjust the polarization controller to

generate single continuous mode-locked pulse near zero dispersion in the erbium-doped fiber laser with semiconductor saturable absorber mirror. Future study showed that the more operation sates can be obtained when we increase the pump power and properly adjust the polarization controller and the narrower pulse can be also generated in the same pump power. It turns out that the multiple-soliton formation in the laser is caused by the peak-power-limiting effect of the cavity. Therefore, the SESAM used in our experiment is optimized for CML operation. Moreover, we can still obtained stable CW, Q-switching, or HML operation with the help of loss modulation mechanism induced by intracavity polarization control. Different from conventional ring-cavity fiber lasers using two polarization controllers in the nonlinear polarization rotating technique, one polarization controller is used in our EDFL laser and the operation state of laser is quite stable. The fiber mirror is used in the experimental setup so that the system is compact and steady. For certain application, switching between different ultra-short pulse states while keeping the same averaged output power will be useful. 5. Conclusion We have reported on the experimental observation of many operation states in the zero dispersion erbium-doped fiber laser with SESAM by using nonlinear polarization controller. The generated solitons is characterized by its approximate Gaussian spectrum without spectral sidebands, which is distinctive different from the conventional soliton spectrum. The Q-switching, Q-switching mode-locking, continuous wave mode-locking, harmonic mode-locking and multiple-pulse can be manipulated by adjusting the polarization controller with the same pump power. The output state can be operated at stable central wavelength, depending on fiber mirror. However, the 3dB optical spectral width and pulse width is different for different output state. The pulse width of single mode-locking pulse is stable and narrower than other operation states. Therefore, the single mode-locking pulse output is the best pulse state for future optical telecommunication. References [1]. M. Jiang, K. H. Ahn, X. D. Cao, P. Dasika, Y. Liang, M. N. Islam et al, synchronization of passively mode-locked erbium-doped fiber lasers and its application to optical communication networks, J. Lightwave Technology 15 (1997) 2020-2028. [2]. M. Horowitz, C. R. Menyuk, T. F. Carruthers, and I. N. Duling, Theoretical and experimental study of harmonically modelocked fiber lasers for optical communication systems, J. Lightwave Technology, 18 (2000) 1565-1574. [3]. X. P. Xie, J. G. Zhang, Y. Wen, and W. Zhao, Experimental investigation into femtosecond fiber ring laser with passive mode-locking, Microwave and Optical Technology Letters, 51 (2009) 63-66. [4]. G. Yandong, S. Ping, T. Dingyuan, 298fs passively mode locked ring fiber soliton laser, microwave Opt. Technol. Lett, 32(2002) 320-323. [5]. I. N. Duling III, Subpicosecond all fiber erbium lasers, Electron Lett, 27(1991) 544-545. [6]. K. Tamura, C. R. Doerr, L. E. Nelson, H. A. Haus, E. P. Ippen, Technique for obtaining high-energy ultra-short pulses from an additive-pulse mode-locked erbium fiber ring laser, Opt. Lett, 19 (1994) 46-49. [7]. N. H. Seong, D. Y. Kim, A new figure-eight fiber laser based on a dispersion-imbalanced nonlinear optical loop mirror with pumped dispersive elements, IEEE Photonic Technol

Lett, 14 (2002) 459-461. [8]. A. B. Grudinin, Richardson, et al, Energy quantization in figure 8 fiber laser, Electron Lett, 28 (1992) 67-68. [9]. W. S. Wong, S. Namiki, M. Margalit, H. A. Haus, E. P. Ippen, self-switching of optical pulses in dispersion-imbalanced nonlinear loop mirrors, Opt. Lett, 22(1997) 1150-1152. [10].K. H. Lin，J. J. Kang, H. H. Wu, C. K. Lee, and G. R. Lin， Manipulation of operation states by polarization control in an erbium-doped fiber laser with a hybrid saturable absorber, , Opt. Express, 17(2009) 4806-4814. [11].M. Guina, N. Xiang, O. G. Okhotnikov, Stretched-pulse fiber lasers based on semiconductor saturable absorbers, Applied Physics B. lasers and optics, 74(2002) 192-200. [12].C. J. Chen, P. K. A. Wai and C. R. Menyuk, Self-starting of passively mode-locked lasers with fast saturable absorbers, Opt. letters, 20(1995)350-352. [13].O. Okhotnikov, A. Grudinin and M. Pessa, Ultra-fast fiber laser systems based on SESAM technology: new horizons and applications, New Journal of Physics, 6(2004) 177. [14].O. Shtyrina, M. Fedoruk, S. Turitsyn and R. Herda et al, Evolution and stablity of pulse regimes in SESAM-mode-locked femtosecond fiber lasers, J.Opt. Soc.Am.B, 26(2009) 346-352. [15].L. M. Zhao, D. Y. Tang, T. H. Cheng, H. Y. Tam, C. Lu, S. C. Wen, Period-doubling of dispersion-managed solitons in an Erbium-doped fiber laser at around zero dispersion, Opt. communications, 278(2007) 428-433. [16].H. Zhang, D. Y. Tang, L. M. Zhao, X. Wu and H. Y. Tam, Dissipative vector solitons in a dispersion-managed cavity fiber laser with net positive cavity dispersion, Opt. Express, 17(2009) 455-460.

980PUMP EDF FM

PC

PS

SESAM

980/1550WDM OUTPUT

Fig.1. scheme of the fiber laser setup. FM: Fiber mirror; EDF: erbium-doped fiber; WDM: wavelength division multiplexer; PC: polarization controller; PS: polarization

Fig.2. laser pulse generation process

u 1

u

Optical fiber v

Polarization controller

2

v

Polarizer

Fig. 3. An effective intensity-dependent loss modulator

Fig.4. Intensity transmission coefficient of laser cavity

(a)

(b)

Fig.4(a) presents the oscilloscope trace of a typical experimentally measured CW and (b) Q-switched pulse operation state.

(a)

(b)

Fig.5.Q-switched mode-locked pulse envelope train and the inset shows single pulse envelope.

(a)

(b)

(c) Fig.6(a)50ns /div continuous mode-locked pulse train,(b) the corresponding spectrum,(c) the corresponding pulse width measurement.

(a) (b) Fig.7 (a)double-pulse mode-locked pulse train,(b) the corresponding spectrum.

(a)

(b)

(c) Fig.8 (a) harmonic mode-locked pulse train,(b) the corresponding spectrum. (c) the corresponding pulse width measurement.

(a) (b) Fig.9 (a)triple-pulse mode-locked pulse train, (b)four-pulse mode-locked pulse train.