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Abstract
We demonstrate a mode-locked all-fiber Tm laser using a single mode-step index multimode-graded-index multimode-single mode fiber structure as a saturable absorber based on the nonlinear multimodal interference. Stable fundamentally mode-locking operation was obtained at a pump threshold of 180mW. The output soliton pulses had a center wavelength, spectral width, pulse duration, and repetition rate of 1888 nm, 3.6 nm, 1.4 ps, and19.82 MHz, respectively. This is a simple, low-cost, stable, and convenient laser oscillator with many potential applications in eye-safe ultrafast photonics.
© 2017 Optical Society of America
1. Introduction
Ultrafast mode-locked fiber lasers, operating in the ~2μm range, have attracted significant attention, because Tm-doped fibers exhibit excellent power scalability and high efficiency in a wide spectrum of laser gains, ranging from 1.8μm to 2.1μm [1]. Light sources at ~2μm have wide applications in the fields of bio-medical treatment, light detection and ranging (LIDAR), processing of non-metallic materials, and infrared optical frequency combing, to name a few [2–7]. In general, ultra-short pulses are generated in fiber lasers using passive mode-locking techniques. The key device that determins the mode-locking performance is the saturable absorbers (SAs). In view of different mode-locking mechanisms, two types of SAs have been developed. SAs in the first category utilize the nonlinear absorption characteristics of materials, including semiconductor saturable absorber mirrors (SESAMs) [8,9], single-walled carbon nanotubes [10,11], graphene [12–15], MoS2, and WS2 [16]. Currently, SESAMs are the most widely used, providing saturable absorption with various characteristics. However, SESAMs are very expensive and suffer from band limitation. Many one-dimensional (1D) and two-dimensional (2D) layered materials (MoS2,WS2), represented by graphene, have been demonstrated to be saturable absorbers. Yet, these saturable absorbers require complicated fabrication processes, and their applicability is limited by their relatively low damage threshold. SAs in the second category utilize nonlinear and/or birefringent effects, such as the nonlinear polarization rotation technique [17,18], and nonlinear optical loop mirrors [19,20]. These devices have a relatively high damage threshold and broad operating wavelength range, while they are less stable and tend to be affected by environmental perturbations.
Recently, nonlinear phenomena in multimode optical fibers (MMFs), particularly nonlinear multimode interference (NL-MMI), have attracted considerable attention for mode-locked fiber laser applications [21–28]. Among MMFs, graded-index multimode fibers (GIMFs) have unique properties that make them an ideal platform to explore the nonlinear interaction of multiple modes [29]. Unlike conventional MMFs, all guided modes in a GIMF can propagate with nearly identical group velocities at special wavelengths [30]. Mafi numerically presented comprehensive analysis of NL-MMI for the first time [22] and proposed to use the single mode- graded-index multimode -single mode (SMF-GIMF-SMF) fiber device as a saturable absorber to achieve a mode locked fiber laser. He noted that the SMF-GIMF-SMF geometry can be used even with conventional commercially available fibers to mode-lock fiber lasers at the presently achievable power levels. The main advantage of this geometry is that it is a real SA which can be fully integrated compared with the existing SAs that exploit nonlinear polarization rotation, semiconductors, or carbon nanotubes. Although abundant and complex co-existing nonlinear dynamics in GIMFs have been observed, no SA based on GIMF has been demonstrated yet. Perhaps, this can be partially attributed to the complexity of nonlinear light propagation in MMFs [29].
In this paper, a SA that capitalizes NL-MMI effects of multimode fibers is proposed, to realize mode-locking in a Tm fiber laser. This all-fiber SA was fabricated using a splice of a single mode-step index multimode-graded-index multimode-single mode (SMF-SIMF-GIMF-SMF) fiber. The introduction of the step index multimode fiber can eliminate the restriction on graded-index multimode fiber length. We have experimentally shown that stable mode locking at the wavelength of 1888nm can be obtained with pulse width of 1.4ps. Tunable wavelength mode locking was also achieved. To the best of our knowledge, this is the first mode-locking operation in the 2 μm spectral region with a SMF-SIMF -GIMF-SMF fiber device as a SA. Our results demonstrate such an all-fiber SA for practical fiber lasers in the mid-infrared range.
2. Nonlinear multimodal interference
Multimodal interference (MMI) is the interference of the excited modes in a multimode optical fiber when single-mode light is coupled into a multimode optical fiber. Self imaging takes place at periodic intervals along the propagation direction of the GIMF fiber. The beat length, Lπ is defined as the distance over which the energy is maximally coupled from the single mode fiber to the GIMF fiber. Obviously, in the linear case, the transmission through the SMF-GIMF-SMF geometry is a periodic function of the GIMF length ~L and the periodic equals to a beat length. MMI in the linear case has successfully been used for various device applications such as beam shapers, sensors, filters and low-loss couplers [31–34]. In 2013, Nazemosadat and Mafi extended the linear analysis in [35] to the nonlinear regime and numerically studied nonlinear MMI in a short graded-index MMF.
In the nonlinear regime where the optical power is high, self-phase and cross-phase modulation (SPM and XPM) effects alter the refractive indices of each of the excited modes and consequently the introduced phase shifts will change the self-imaging beat length in the GIMF [36], varying the power coupling efficiency between the GIMF and SMF. In other words, the relative power transmission from the GIMF to the SMF will change as the input light power increases. Mafi noted that if the length of a GIMF is chosen exactly as the half-beat length Lπor L = nLπ, where n is an odd integer, the relative power transmission is at its minimum value for the linear case. And as the injected power increases, the relative power transmission increases as well until it reaches its maximum value. Hence, lower power signals are attenuated while higher power ones are transmitted through, resulting in a power-dependent transmission, which means the proposed SMF-GIMF-SMF configuration has the potential to operate as a saturable absorber for mode-locking fiber lasers. The basic principle of SMS is shown in Fig. 1. For low power signals, the light beam expands and experiences a large loss owing to the core diameter mismatch between the GIMF and SMF. On the other hand, for high power signals, self-focusing occurs and the light power coupled to the core of the SMF increases.
Theoretically, transmission in the SMF-GIMF-SMF structure is determined by four related parameters [22]: the length of the GIMF L, the total incident optical power P, the ratio of the mode-field diameter of the LG00 mode in the GIMF to that of the fundamental mode in the SMF η, and the total number of the excited modes p. The dependence of the transmission through the GIMF on the optical power P serves as the desired saturable absorption mechanism. The length of the GIMF L must be controlled at a specific value (L = nLπ, where n is an odd integer) to achieve optimal saturable absorption. However, as the self-imaging beat length is on the order of micrometers, the length of GIMF must be finely tuned and is difficult to be accurately controlled in practice. We can seek for other pragmatic approaches instead of fine-tuning the length.
In this work, we added an SIMF fiber segment between the GIMF and SMF fibers in the above mentioned SMF-GIMF-SMF structure, as shown in Fig. 2(a). All of the fiber segments were jointed using a simple fusion splicing machine (Fujikura 80s), demonstrating that such an SA can be easily fabricated. A microscopy image of this fiber structure is shown in Fig. 2(b),where the position of splicing can be clearly observed owing to the difference between the refractive indices of the two fibers. A ~328-μm-long section of an SIMF (105/125 Corning) was fusion-spliced with a 7.8-cm-long GIMF (62.5/125, Corning). Using an SIMF can increase the number of higher order modes and by bending SIMF the mode field of the cross-section can be changed. The bending radius dramatically affects the mode field distribution; consequently, the ratio of the LG0M mode coupling to the GIMF is adjustable. Thus, virtually any amount of coupling (in other words, the value of η) can be generated by adjusting the bending radius. In other words, the introduction of the SIMF changed the period of self-imaging. Hence, there is essentially no limit on the length of the GIMF. This means that a SMF- SIMF -GIMF-SMF structure can provide a more flexible method of fabricating NL-MMI-based SAs.
Fig. 2 (a) Schematic diagram of the proposed SMF-SIMF -GIMF-SMF structure; (b) microscopy image of SMF-SIMF -GIMF-SMF SA
3. Experiments and results
The configuration of the SMF-SIMF -GIMF-SMF mode-locked fiber laser is shown in Fig. 3. A 1570-nm-wavelength fiber laser was used as the pump source. The pump light was launched into the laser cavity using a 1550/2000 nm wavelength–division multiplexing (WDM) coupler. A 2-m-long Tm-doped single-mode fiber (SCF-TM-8/125, Coractive) was used as the gain medium. The laser was constructed inside a unidirectional ring cavity by adding a polarization insensitive isolator (ISO). One polarization controller (PC) was used to optimize the mode-locking operation as well as the intra-cavity birefringence. The SA, SMF-SIMF -GIMF-SMF device was placed after the PC. A coupler with a 10% output was employed to output the laser. The output was connected to an optical spectrum analyzer(AQ6375, Yokogawa) and a 1GHz oscilloscope, together with a 12.5GHz photodetector, to allow simultaneous measurements of the spectra and the pulse train using a 50/50 coupler.
To achieve mode-locking, the SMF-SIMF -GIMF-SMF device must be fixed at a certain curved shape. The pump threshold of the mode-locked operation was ~180mW. The measured characteristics of the output laser are summarized in Fig. 4. Figure 4(a) shows that, the laser output pulse train has a period of ~50.45ns, which matches well with the cavity round-trip time and confirms the oscillator is operating in the fundamental mode-locking state. Figure 4(b) shows that the optical spectrum is centered at the wavelength of 1888nm, which corresponds to a 3 dB bandwidth of 3.68 nm. The output spectrum exhibits Kelly sidebands, typical for solitons in anomalous dispersion fiber lasers. In order to confirm the single pulse operation, its radio frequency (RF) spectrum is measured. The first RF peak is centered at 19.82MHz, as shown in Fig. 4(c) where the signal-to-noise ratio obtained is 60dB, which confirms the high operation stability of our fiber laser. A measured autocorrelation trace is shown in Fig. 4(d). The trace is best fitted to a Gaussian shape and the pulse duration was estimated to be ∼1.4 ps. The time-bandwidth product (TBP) of the pulses is ~0.433, indicating that the output pulses are slightly chirped.
Fig. 4 Mode-locked operation at the fundamental repetition rate. (a) Pulse-train. (b) Spectrum. (c) RF spectrum at the fundamental frequency of 19.82MHz. (d) Autocorrelation trace.
In Fig. 5(a), the average output power of the mode-locked soliton laser is shown as a function of the input pump power. As the pump power increases to 180 mW, stable and self-starting mode-locking is observed and maintained up to the pump power of 980 mW without changing the polarization controller's state. In the experiment, we found that the center wavelengths of solitons did not drift with increasing the pump power, as shown in Fig. 5(b). However, under strong pumping, multiple soliton pulses were initiated. To evaluate the mode-locking stability, we examined the long-term operation stability of the fiber laser, and the results are shown in Fig. 6. The spectra were recorded every 40 minutes for 8 hours under experimental conditions in which the fibers, especially the SA structure, were carefully mounted and environmental perturbations were minimized. Apparently, the spectra are the same, suggesting good long-term stability. We note that the central spectral peak location, spectral bandwidth and spectral strength remained reasonably stable over the time period of recordings.
Fig. 5 Output characteristics as a function of the pump power. (a) Average output power versus pump power. (b) Optical spectra for different pump powers.
To validate that the introduction of an SIMF fiber can eliminate the restriction on the length of a GIMF fiber, we fabricated four SMF-SIMF -GIMF-SMF structures, with different parameters, and tested their mode-locking performance. According to the results summarized in Table 1, it can be concluded that a change in the length of the gradient fiber has nearly no effect on mode-locking. This observation can be a strong support for our expectation that adding an SIMF fiber can eliminate the restrictions on the length of a GIMF fiber. Furthermore, to obtain a low threshold and high power output, the length of the added SIMF fiber should be within 500μm.
Table 1. Properties of mode-locking output for the SMF-SIMF -GIMF-SMF structures with different parameters.
Theoretically, based on the SMF-SIMF -GIMF-SMF structure, mode-locking can be obtained at any wavelength in the bandwidth of the laser gain. The transmission characteristic of this structure is also wavelength-dependent and exhibits a periodic change [34]. Thus, tunable wavelength mode-locking can be achieved by twisting the SMF-SIMF -GIMF-SMF structure. Figure 7 shows changes in the optical spectrum of solitons obtained by varying the curvature of the SMF-SIMF -GIMF-SMF structure. The wavelength of the solitons could be continuously varied in the 1835-1886 nm range. To determine whether the mode locking operation is purely contributed by the saturable absorption of the SMF-SIMF -GIMF-SMF structure, the all-fiber SA was purposely removed from the laser cavity. In this case, no mode-locking was observed, despite increasing the pump power from zero to the maximal available power and despite rotating the polarization controller over a full range. Finally, we tested the polarization dependence of this SA and found that the device is polarization independent.
4. Summary
In conclusion, we have developed a stable passively mode-locking Tm fiber laser using the SMF-SIMF -GIMF-SMF device as a SA. The all-fiber SA based on the NL-MML of multimode fiber is easy to fabricate. The compact all-fiber laser emitted stable mode-locked soliton pulses centered at 1888 nm with spectral width of 3.68 nm and pulse duration of 1.4 ps. Wavelength tunable mode-locking was also observed and the tuning range extended from 1835 nm to 1886 nm. Such an SIMF-GIMF SA structure is promising for simple all-fiber mode-locking ultrafast laser.
Funding
Natural Science Foundation of Zhejiang Province, China (LY15F050007).
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