We report on the experimental observation of a bidirectional fiber soliton laser passively mode-locked by single-wall carbon nanotubes. Two stable pulse trains in opposite directions are delivered simultaneously from the ring cavity. The counterpropagating pulses have different central wavelengths, pulse durations, and repetition rates. By adjusting the fiber birefringence and cavity length, the central wavelengths of two solitons can be the same or different. Experimental observations and analyses demonstrate that the different operating wavelengths result in the unequal repetition rates of two pulses. These unique features may be attributed to the cavity asymmetry and fiber birefringence.
©2013 Optical Society of America
Optical solitons have attracted a great deal of research interest for their extensive applications in fiber communications [1, 2] and ultrafast phenomena [3–6]. Due to the compact cavity design, environmentally stable performance, and capability of generating ultra-short pulses, passively mode-locked fiber lasers with different configurations have been proposed [7–10]. Various devices have been exploited to realize passive mode locking, such as semiconductor saturable absorber mirrors (SESAMs) , nonlinear polarization rotation techniques [12–16], nonlinear optical loop mirrors , carbon nanotubes [18, 19], graphene [20, 21], and graphite nano-particles [22, 23]. The dominant mode-locking technology in fiber lasers is based on SESAMs. However, the fabrication of SESAMs is suffered from the cost-ineffective and time-consuming process in complicated epitaxial systems . Currently, single-wall carbon nanotubes (SWNTs) have been widely investigated for mode locking due to their advantages of the ultrafast recovery time and polarization insensitivity [25, 26]. As SWNTs are direct-bandgap materials, with a gap that depends on the diameter and chirality of nanotubes, a broadband and spectral-tuning mode-locker could be achieved by mixing SWNTs with different diameters. Amongst varieties of SWNT-based mode-lockers, transmission-type SWNT-polymer composite films are generally employed in fiber lasers for their compactness, ease of fabrication, and low cost [27, 28].
In previous reports of fiber ring lasers, optical isolators were usually utilized to reduce the spurious cavity reflections and decrease the mode-locking threshold [29–31]. The isolator in a ring cavity results in less compact design and only unidirectional pulse trains. For instance, two different types of pulses/solitons were delivered from a unidirectional fiber laser [32, 33]. Recently, Kieu and Mansuripur reported an all-fiber bidirectional passively mode-locked ring laser at 1.55 μm with a fiber taper embedded in carbon nanotubes-polymer composite . In their experiments, the counterpropagating pulses exhibited the same central wavelength. Additionally, Ouyang et al. presented a bidirectional passively mode-locked fiber laser using two SESAMs . As the cavity lengths were unequal for the countercirculating light beams, two output pulse trains had different fundamental repetition rates.
In this paper, we experimentally demonstrate a bidirectional passively mode-locked erbium-doped fiber (EDF) laser with a SWNT-polyvinyl alcohol (PVA) mode-locker. Two stable solitons with the same or different central wavelengths can be achieved simultaneously in clockwise (CW) and counterclockwise (CCW) directions. Experimental observations and analyses indicate that the cavity asymmetry and fiber birefringence mainly contribute to the unique features.
2. Experimental setup
The configuration of the proposed bidirectional fiber laser is schematically shown in Fig. 1(a). A 980 nm laser diode (LD) provides pump with a 980/1550 nm wavelength-division multiplexer (WDM) coupler. A 5-m EDF with the dispersion parameter D of –9 ps/(nm·km) and an absorption of 6 dB/m at 980 nm acts as the gain media. The other fibers in the cavity, including the fiber pigtails of components, are the standard single-mode fiber (SMF) with the total length of 13.6 m and D of 17 ps/(nm·km) at 1550 nm. A polarization controller (PC) is employed to optimize the mode-locking conditions. A 2 × 2 10/90 fused optical coupler (OC) extracts the pulses in the CW and CCW directions. Different from the other fiber ring lasers, no isolator is utilized in our cavity. The net cavity dispersion and fundamental repetition rate of the laser are estimated as −0.24 ps2 and 11.05 MHz, respectively. As illustrated in Fig. 1(b), the SWNT-PVA mode-locker is assembled by sandwiching a ~2 mm2 free-standing SWNT-PVA composite film between two FC/PC fiber ferrules with a fiber connector. An optical spectrum analyzer, a commercial autocorrelator (AC), a radio-frequency (RF) analyzer, and a 6-GHz digital oscilloscope with a photodiode detector are used to monitor the laser outputs simultaneously.
3. Experimental results and analyses
With increasing the pump strength to 20 mW, self-started CW and CCW mode-locking operations can be established simultaneously. However, multiple pulses are formed in the cavity, including the harmonic mode-locking operation . By decreasing the pump power to 14 mW, stable single-pulse mode-locking operations in both directions are achieved. Figures 2(a) and 2(c) show the output spectra of the CW and CCW pulses. Stable soliton mode locking is characterized by the clear Kelly sidebands on the spectra, which originate from the constructive interference between the soliton and dispersive waves [3, 14, 37]. One can observe that the central wavelengths are 1557.7 nm for the CW pulses and 1559.9 nm for the CCW pulses. The 3-dB bandwidths of two spectra are 4.9 and 5.4 nm, respectively. Figures 2(b) and 2(d) illustrate the autocorrelation traces of the CW and CCW pulses. By using a sech2 fit, the durations of the CW and CCW pulses are estimated as 0.9 and 1.4 ps, respectively. It is worth noting that the CW and CCW pulses operate at different repetition rates (i.e., 11.055082 MHz for CW and 11.055032 MHz for CCW) with a separation of ~50 Hz, revealed in the insets of Figs. 2(b) and 2(d). The signal/noise ratios are as large as ~70 dB for both pulses. Due to the limited resolution of the oscilloscope, the round-trip times observed on the screen are equal for two pulse trains. The separation between adjacent pulses is ~90.5 ns, corresponding to the cavity length of 18.6 m. Experimental observations confirm that the proposed EDF laser operates at the stable bidirectional mode-locking state.
When two pulses with the wavelength difference Δλ propagate in a ring cavity with the length L, the round-trip time difference Δt between two pulses is given by Fig. 2(e), the observed round-trip time t is about 90.5 ns. L and Δλ are 18.6 m and 2.2 nm, respectively. Then, Δf is calculated as 49.9 Hz, which coincides well with the experimental observation of ~50 Hz. Based on aforementioned results, we conclude that the repetition rates of two pulse trains also depend on their central wavelengths.9], the pulse durations before passing through SMF are ~0.7 ps for the CW pulses and ~1.2 ps for the CCW pulses. The spectral separation between first-order sidebands for the CW pulses is calculated to be about 17 nm, which is in reasonable agreement with the experimental result.
By changing the pump power and adjusting the PC state, we can observe the bidirectional mode-locking operations with the different central wavelengths. When the pump power is ~20 mW with an appropriate PC state, the minimum difference of the central wavelengths of the CW and CCW pulses is ~1.3 nm, as illustrated in Fig. 3. Their central wavelengths are 1558.2 and 1559.5 nm, respectively. The 3-dB bandwidths of the optical spectra are 5.1 and 5.2 nm, respectively, and the corresponding pulse durations are about 0.9 and 1.4 ps by using a sech2 fit. The fundamental repetition rates of the CW and CCW operations are 11.055078 and 11.055049 MHz, respectively, as shown in the insets of Fig. 3. According to Eqs. (1)–(3), the theoretical difference between two fundamental repetition rates is ~29.5 Hz, which is in good agreement with the measured RF separation of ~29 Hz. Apart from the clear Kelly sidebands, several low-intensity sidebands are in fact recorded in Fig. 3, which originate from the nonlinear four-wave-mixing effect of Kelly sidebands with different orders, as analyzed by Lin et al. in Ref .
Our experimental observations show that the central wavelengths of the counterpropagating pulses of the bidirectional operation are different in the most cases. However, when the fiber length of cavity is designed appropriately, the central wavelengths of the counterpropagating pulses can be identical. For example, when the SMF is about 10.5 m and the other parameters are the same as Fig. 3, the bidirectional emission spectra are shown in Fig. 4. In this case, the central wavelength of the CW operation is the same as that of the CCW operation, which is 1559.7 nm. The corresponding repetition rate is 13.264637 MHz, as shown in the insets of Fig. 4. The 3-dB bandwidths are 6.0 nm for the CW pulses and 5.5 nm for the CCW pulses. The corresponding pulse durations are 1.1 and 1.5 ps.
It is found that once started, the laser can maintain stable at least for several hours, and the stable process is repeatable after the breakdown of stability. The damage threshold peak intensity of the SWNT-PVA composite film is about 570 MW/cm2. In our experiments, the counterpropagating pulses generally possess different central wavelengths, pulse durations, and repetition rates. From Figs. 2 and 3, we find that the fiber birefringence induced by the PC affects the operating wavelengths of CW and CCW pulses. Besides fiber birefringence, the asymmetry of the laser cavity may be another dominant factor for these unique features. Due to the asymmetry of cavity elements, pulses propagating in the CW and CCW directions pass through each element in reverse order and experience different cavity dispersion [34, 35]. The experimental observations of Fig. 4 are just the results from partial elimination of the asymmetric cavity dispersion.
In this paper, we have experimentally demonstrated a bidirectional soliton laser based on a SWNT-PVA mode-locker. Two counterpropagating pulse trains centered at 1557.7 and 1559.9 nm can be achieved simultaneously. It is worth noting that the CW and CCW pulses have different repetition rates with a separation of ~50 Hz. By appropriately adjusting the PC and designing the cavity length, the pulses in opposite directions have the same central wavelength at 1559.7 nm. Our results illustrate that the nonidentical central wavelengths contribute to different repetition rates of two pulses. The unique features of the bidirectional pulses mainly result from the cavity asymmetry and fiber birefringence.
The authors would like to thank Dong Mao, Dongdong Han, and Yudong Cui for assistance and fruitful discussions. This work was supported by the National Natural Science Foundation of China under Grants 10874239, 10604066, and 11204368.
Corresponding author (X. Liu). Tel.: + 862988881560; fax: + 862988887603; electronic mail: firstname.lastname@example.org and email@example.com.
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