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All-fiber mode-locked laser based on microfiber polarizer Zhishen Zhang,1 Jiulin Gan,1,2,3 Tong Yang,1 Yuqing Wu,1 Qingyu Li,1 Shanhui Xu,1 and Zhongmin Yang1,2,* 1
State Key Laboratory Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China 2 Special Glass Fiber and Device Engineering Technology Research and Development Center of Guangdong Province, Guangzhou 510640, China 3 e-mail:
[email protected] *Corresponding author:
[email protected]
Received November 17, 2014; revised January 18, 2015; accepted January 20, 2015; posted January 21, 2015 (Doc. ID 226632); published February 23, 2015 A novel all-fiber mode-locked fiber laser based on microfiber polarizer is proposed and demonstrated. The microfiber polarizer is composed of two pieces of microfibers that are finely manipulated to be partly overlapped. Because of the asymmetric cross section, the microfiber polarizer shows a strong birefringence that ultimately induces a high polarization-selective feature. Compared with other polarizers, the microfiber polarizer owns the merits of simpler fabrication, lower cost, broader band, and more compact size. The polarization extinction ratio of the microfiber polarizer is 26 dB, and the stable pulse sequence with the duration of 2.9 ps is generated from this microfiber polarizer based all-fiber mode-locked laser. © 2015 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.0140) Lasers and laser optics; (230.3990) Micro-optical devices. http://dx.doi.org/10.1364/OL.40.000784
Passive mode-locked fiber lasers are quite suitable for ultrafast pulse generation, which can be widely applied to optical communications [1], materials processing [2], precision measurement [3], and nonlinear optics [4]. Various passive mode-locking modulation techniques, such as adopting a saturable absorber (SA) [5–7], nonlinear optical loop mirror (NOLM) [8,9], and nonlinear polarization rotation (NPR) [10–12] have been proposed and developed. SA solutions using a semiconductor [5], graphene [6], or a topological insulator [7] are highly dependent upon the materials’ characteristics, and the process of manufacture is also very complicated. The NOLM technique [8,9] requires figure-eight fiber cavity and is difficult to be regulated accurately. The NPR technique [10–12] owns the merits of large modulation depth and ultrashort response time so as to generate ultrafast pulses in a fiber ring cavity. However, the conventional NPR technique adopts a bulk optical polarizer that hinders the passive mode-locked fiber lasers to further miniaturize and integrate. Toward the problems mentioned, in-fiber polarizer devices [13,14] are proposed and developed as the promising option. Up to now, a segment of polarization fiber device [15] and the 45°-tilted fiber grating [16], which requires complex fabrication and rigorous fabrication accuracy, have been invented and adopted as the alternative polarizers for the passive mode-locked laser. Here, the microfiber polarizer is proposed and demonstrated with simple fabrication, low cost, broadband, and compact size. The polarization extinction ratio of the microfiber polarizer is 26 dB. Based on this microfiber polarizer, the all-fiber passively mode-locked laser using the NPR technique is demonstrated; a stable pulse sequence with the duration of 2.9 ps is generated at 1550 nm band. The microfiber polarizer is fabricated as follows. First, the microfiber [17] is drawn from standard single-mode fiber, and both ends are connected to standard fiber pigtails. Next, the microfiber is cut into two pieces of equal length. Finally, the two pieces of microfiber are micro controlled to be partly overlapped, as shown in Fig. 1. 0146-9592/15/050784-04$15.00/0
Because of van der Waals and electrostatic attraction force, the overlapped microfibers can stick together tightly. Because of the asymmetric cross section (Fig. 1) and the large refractive index difference between the silica microfiber and the surrounding air, the overlapped area shows a strong birefringence [18,19]. Simulation software (COMSOL Multiphysics) is used for calculating the effective refractive indexes of TE and TM modes in the overlapped area, as shown in Fig. 2. The diameter and refractive index of the microfiber are 1.82 μm and 1.4679, respectively. The calculated birefringence B (B NTM NTE ) is about 3.6 × 10−3 , which is about 10 times larger than the commercial polarization-maintaining optical fiber. The birefringence effect makes the overlapped microfibers an equivalent polarizer. When different linearly polarized lights are launched into the overlapped area, the birefringence makes them propagate through different refractive index distributed path. As shown in Fig. 1, the overlapped area with two conical transition regions works as an interferometer between the fundamental mode and the high order modes. Because of the different optical path difference, the transmission spectra of different linearly polarized lights own interference fringe with different peak positions. The birefringence also causes polarization related coupling efficiency. Thus, different linearly polarized lights will experience different coupling ratio. In a word, the transmission spectra of the overlapped area will be different for the different linear polarized light and the required polarizing effect will be
Fig. 1.
Schematic diagram of the microfiber polarizer.
© 2015 Optical Society of America
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Fig. 2. Field profiles and local polarization directions of TE and TM modes in the overlapped area. The effective refractive indexes (N eff ) are shown, respectively.
achieved by setting the parameters of this microfiber device properly. At 1550 nm waveband, the optimal diameter of the microfiber polarizer is in the range of 1.1–10 μm, where the high order modes exist and the birefringence B is larger than 10−4 . The overlapped length needs to be longer than 1 mm for keeping the stability of the microfiber polarizer structure. As illustrated in Fig. 1, the microfiber polarizer is composed of two sections of microfiber with a 1.82 μm diameter, and the length of L1 , L2 , L3 , L4 , and L5 at 3, 2, 1.6, 3.1, and 4 mm, respectively. The insertion loss of this microfiber polarizer is about 6 dB, which can be further reduced by improving the fabrication process [17]. The transmission spectra of this microfiber polarizer are measured by an optical spectrum analyzer (OSA Yokogawa AQ6370B) with a 0.02 nm resolution bandwidth. By adjusting the polarization of the injected linearly polarized lights, the maximum and minimum transmission spectra are shown in Fig. 3(a). The polarization extinction ratio is 26 dB at 1548 nm, with the 3 dB width of 1.4 nm, as shown in Fig. 3(b). The transmission spectra are affected by the specific parameters of the microfiber polarizer, such as the diameter, the overlapped length, and the refractive index of the microfiber. By slightly and finely adjusting those parameters, the polarizer can be adjusted to meet the requirements of the actual application. In our experiment, the polarization related transmission spectra of the microfiber polarizer are measured within the range of 1520–1580 nm, which is limited by the scope of our light source. Actually, there are more wavelengths with a high polarization extinction ratio outside the scope of the broadband light source. The microfiber polarizer is a broadband device, especially at some special wavelengths that could not be realized easily by other polarizers. Based on the above properties, the microfiber polarizer is applicable to the all-fiber mode-lock laser. Figure 4 shows the all-fiber passive mode-locked laser configuration using the NPR technique based on the microfiber polarizer. The loop contains a 980/1550 nm WDM, a 1 m long EDF, a fused OC with 10% output, a PI-ISO, a PC, and the microfiber based polarizer. Here, the light unidirectional operation is ensured by the PI-ISO, and the conventional bulk optical polarizer is replaced by our microfiber polarizer that facilitates all-fiber
Fig. 3. (a) Transmission spectra of the microfiber polarizer for two different linearly polarized light. (b) Polarization extinction ratio of the microfiber polarizer.
configuration. All the fibers utilized in the ring cavity are SMF-28e fiber, except the 1-m EDF, which induces the negative net intra-cavity dispersion. The total length of the cavity is 13.86 m, corresponding to a 14.82 MHz fundamental frequency. The laser performance is investigated in detail using a 500 MHz oscilloscope, an optical autocorrelator, and a 3 GHz spectrum analyzer (Agilent N9320A) coupled with a 12 GHz photodetector (New Focus 1554-B). Because of the self-phase modulation and cross-phase modulation, the different component of pulse will have a different phase shift, which means different polarization states. By adjusting the PC so that the center of the pulse will propagate through the polarizer with low loss, the edges of pulse will be attenuated effectively. After sufficient circles, an ultrashort pulse will be generated. Self-start mode locking is achieved at a pump power of 160 mW. The average output power is 2 mW. The
Fig. 4. Schematic of the all-fiber passive mode-locked laser. WDM, wavelength division multiplexed coupler; PC, polarization controller; EDF, erbium-doped fiber; OC, optical coupler; PI-ISO, polarization insensitive isolator; OSA, optical spectrum analyzer.
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be about 2.9 ps, corresponding to a time–bandwidth product (TBWP) of 1.17. Figure 5(d) shows the radio frequency power spectrum with a 60 dB signal-to-noise ratio, indicating that the laser is operating with lowamplitude noise. A broad harmonics spectrum is also shown in the inset of Fig. 5(d) with a 200 MHz span. From the results mentioned above, an all-fiber passive mode-locked laser based on a microfiber polarizer is demonstrated effectively. A stable pulse sequence with the duration of 2.9 ps is generated at 1550 nm band. The pulse energy and peak power are about 135 pJ and 46.55 W, respectively. Compared with traditional passive mode-locked fiber lasers, the pulse performance based on the microfiber polarizer is can be further improved. The insertion loss of the microfiber polarizer can be reduced by improving the fabrication process. During our experiment, the microfiber polarizer is exposed in the air, where the performance of this device is greatly deteriorated by the ambient dust particles, disturbance, and noise. However, referring to the mature optical coupler packaging process, we see that the microfiber polarizer can be solidified to a compact and stable device which is isolated effectively from most of the environmental disturbance. The principle of microfiber polarizer makes sure it can be used as a broadband device, especially at some special wavelengths that could not be realized easily by other polarizers. Therefore, the microfiber polarizer is a significant competitive in-fiber polarizer for generating ultrafast pulse in some special wavelength bands. In conclusion, we have demonstrated a passive modelocked fiber laser based on a microfiber polarizer. The polarization extinction ratio of the microfiber polarizer is 26 dB. A stable pulse sequence with the duration of 2.9 ps is achieved at 1550 nm band. The results show that this microfiber polarizer provides a very competitive solution for realizing an all-fiber mode-locked laser. This research was supported by the China State 863 Hi-tech Program (2013AA031502), NSFC (11174085, 51132004, and 51302086), Fundamental Research Funds for the Central Universities (2013ZP0003, 2013ZP0013, 2013ZM0032, and 2014ZM0033), The Fund of Guangdong Province Cooperation of Producing, Studying and Researching (2012B091100140), the Guangdong Natural Science Foundation (S2011030001349), and the National Science Fund for Distinguished Young Scholars of China (61325024). Fig. 5. Mode-locking characteristics. (a) Spectrum of the output pulses, (b) oscilloscope trace of the output pulses, (c) autocorrelation trace of the output pulses, and (d) radio-frequency spectrum of the output pulses.
characteristics of the output pulses are shown in Fig. 5. Figure 5(a) shows the optical spectrum of the mode-locking laser with the 3 dB width of 4.8 nm. The Kelly sidebands state clearly that the optical soliton is formed in the cavity. Figure 5(b) shows the oscilloscope trace of the output pulse train. The period is 67.5 ns, which agrees well with the cavity length. Figure 5(c) shows the autocorrelation trace and the pulse duration is estimated to
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