A nonlinear polarization rotation based all-fiber passively mode-locked Tm3+-doped fiber laser is demonstrated by using a 45° tilted fiber grating (TFG) as an in-line polarizer. The 45° TFG centered at 2000 nm with polarization dependent loss (PDL) of >12 dB at 1850 nm~2150 nm range was UV inscribed for the first time in SM28 fiber using a 244 nm Ar+ continuous wave laser and a phase mask with 25 mm long uniform pitch and titled period pattern of 33.7° with respect to the fiber axis. Stable soliton pulses centered at 1992.7 nm with 2.02 nm FWHM bandwidth were produced at a repetition rate of 1.902 MHz with pulse duration of 2.2 ps and pulse energy of 74.6 pJ. As increased pump power, the laser also can operate at noise-like regime with 18.1 nm FWHM bandwidth and pulse energy of up to 250.1 nJ. Using the same 45° TFG, both stable soliton and noise-like mode-locking centered at ~1970 nm and ~2050 nm, were also achieved by shortening and extending the length of Tm3+-doped fiber, respectively, exhibiting advantages of broadband and low insertion loss at 2 µm band.
© 2014 Optical Society of America
Passively mode-locked lasers operating in the 2 µm eye safe region have attracted increasing interests because of their potential applications in biology, remote sensing, free-space communication, and mid-IR supercontinuum generation. Compare to 2 µm mode-locked solid-state lasers, all-fiber structure fiber lasers without coupling between fiber and bulk elements have great stability, compact design, good beam quality and low cavity loss albeit they have comparative low pulse energy. Three main kinds of mode-locking techniques have been already employed in ultra-fast pulsed Tm3+-doped fiber lasers to generate ps or fs-level ultra-fast pulses. The first is material saturable absorber based mode-locking such as semiconductor saturable absorber mirror , single-wall carbon nanotube saturable absorber [2, 3], graphene [4, 5] and very recently topological insulator . The second approach is nonlinear switching-based mode-locking such as nonlinear polarization rotation (NPR) [7–10], nonlinear amplifying loop mirror (NALM)  and nonlinear optical loop mirror (NOLM) . The third one is hybrid structure-based mode-locking combining saturable absorber and the nonlinear switching [13, 14]. Currently, the single-wall carbon nanotube (SWCNT) saturable absorber and graphene have been integrated into all-fiber structure [2–5, 13, 14], especially with improved damage threshold and insertion loss.
NPR in the second approach can overcome the mentioned limitations of material saturable absorber and also has simpler structure compared to NALM and NOLM. However, bulk polarizers employed in this technique break down the benefit of all-fiber structure and induce high insertion loss. Currently, the insertion loss of the commercial polarizers at 2 μm is typically as high as ~1.0 dB . The high loss will enhance the laser threshold, decrease the efficiency and influence the mode-locking stability. Several kinds of in-fiber polarizers have been demonstrated at 1.55 μm, such as evanescent field coupling based polarizers [16, 17], chiral grating structure based circular polarizer [18, 19], and polarizing fiber based linear polarizer [20–22]. For the evanescent field coupling based polarizers, it is easy to achieve a high PER, but the fabrication method is complex as a part of the fiber cladding will need to be removed and replaced with a layer of film. It also cannot be used for high power system because the coating layer is easily burned. For the chiral grating structure based circular polarizer, the integrity of fiber is maintained and its operating wavelength range is easily changed by changing the twisting period. However, this type of polarizer requires special fiber (non-circular core), thus inducing high insertion loss. For the polarizing fiber based linear polarizer, its fabrication method is the simplest one because the PER only depends on the length of the fiber. Recently, a specifically designed polarizing fiber as the polarizer has been successfully applied in an NPR-based hybrid mode-locked Er3+-doped fiber laser . The polarizing fiber has a high PER of >20 dB from 1460 nm to 1600 nm and short length of only 0.7 m, indicating that this method could be extended to 2 μm band. However, the polarizing fiber required for achieving a high PER usually needs small bend radius, which will introduce a several dB insertion loss. Especially the current polarizing fiber at 2 μm band has larger bend loss compared to 1.55 μm band. On the other hand, Tm3+-doped fiber has a very broad gain spectral band covering from 1850 to 2100 nm which provides more than 200 nm of available bandwidth . Mode-locked Tm3+-doped fiber lasers with the shortest center wavelength at 1886 nm and the longest wavelength at ~2040 nm have been reported [25, 26]. Currently, the bandwidth of the commercial fiber-based polarizers at 2 μm is only ~60 nm , which is much narrower than the gain spectral band of Tm3+-doped fiber. That means several such polarizers must be used to cover the whole Tm3+-doped fiber gain bandwidth.
The tilted fiber gratings (TFGs) showing very excellent polarization property was first demonstrated by Meltz et al. in 1990 . Since then, many theoretical papers have reported that the TFGs with a 45° tilt structure could be used as an in-fiber polarizer [28, 29]. In 2005, the first UV-inscribed 45°-TFG based polarizer was demonstrated with a high PER . It based on the pile of tilted index plates created inside of the fiber can tap out the s-polarization light from fiber core and propagates the p-light with low transmission loss. As an in-fiber polarizer, it has many advantages such as all-fiber structure, extra-low insertion loss, high extinction ratio, broad bandwidth, and low cost. At 1.55 μm band, a series of applications based on 45° TFG have been demonstrated such as PER equalizer , polarimeter  and Lyot filter . Recently, 45° TFG has been used in all mode-locked erbium doped and ytterbium doped fiber lasers [34–36] as the polarizer. By using the 45° TFG in the NPR structure, C. B. Mou et al have demonstrated a mode-locked Er3+-doped fiber laser with pulse duration of 600 fs and pulse energy of ~1 nJ . In a similar way, an all fiber normal-dispersion Yb3+-doped fiber laser with pulse duration of 4 ps have been also demonstrated . Afterwards, Z. Zhang et al introduced the 45° TFG as a polarizer into an NPR-based Er3+-doped ring cavity with proper dispersion management and the dissipative stretched pulses with duration of 90 fs and energy of 1.68 nJ were obtained . However, there has no report about 45° TFG-based applications at the 2 μm band.
In this paper, we report the inscription of 45° TFGs at 2 μm band for the first time and employed it into an all-fiber mode-locked Tm3+-doped fiber laser with NPR structure. The laser can operate at both solitary and noise-like regimes with center wavelength of 1992.7 nm. Using the same TFG, stable soliton mode-locking pulses centered at 1970.8 nm and 2051.3 nm were also achieved by changing the length of Tm3+-doped fiber.
2. Experiment setup and results
The 45° TFGs centered at ~2000 nm were UV inscribed for the first time into SMF-28e fiber by using the phase-mask scanning technique. The UV laser we used is a 244 nm UV source from a continuous wave frequency doubled Ar+ laser (Coherent Sabre Fred®). The phase-mask (Ibsen, Denmark) has a 25 mm long uniform pitch and 33.7° tilted angle with respect to the fiber axis. It can produce the internal titled index fringes at 45° in the fiber core with polarizing function response to a broad band light around 2000 nm. Before the inscription, the SMF-28e fiber was hydrogen loaded at 150 bar at 80°C for 48 hrs to enhance photosensitivity. After the inscription, the grating samples were subjected to an annealing treatment at 80°C for 48 hours to stabilize the grating structure. Figure 1 (a) (inset) shows the microscopic image of the TFG giving the titled index fringes at 45°. The effective length of the TFG was 24 mm.
The polarization extinction ration (PER) of the 45° TFG was measured by using the polarization scanning technique. The typical experimental setup is shown in Fig. 1(a). In this setup, the light source is a supercontinuum source (Fianium SC480, UK) which can generate light from 450 nm to 2400 nm. The polarizer (Thorlabs LPNIRA050-MP, USA) is a bulk component which has a high PER of >50 dB at a range from 1550 nm to 2450 nm. To measure the PER of the 45° TFG at a specific wavelength, the central wavelength of optical spectrum analyzer (Yokogawa AQ6375, Japan) was set at the measuring wavelength, and the span was set to zero. The maximum and minimum transmission through the 45° TFG can directly be measured by adjusting polarization controller. Figure 1(b) shows the simulated and measured PER of the 45° TFG at the range from 1600 nm to 2400 nm. The simulation method we used is volume current method, of which the details have been reported in [37, 38]. In the simulation, we set the core radius was as 4.5 μm, the period of grating as 0.990 μm and the length of grating as 24 mm. The refractive index modulation induced by the UV light was set to be 0.0017. It is observed that the simulation result agree well with the measured result. The measured bandwidth of PER is broader than 400 nm and the PER is larger than 12 dB from 1850 nm to 2150 nm which covers the whole gain band of the Tm3+-doped fiber. Although the PER of the 45° TFG at 2000 nm (~24 dB) is still low compared to the commercial in-line polarizers at 1550 nm, it is comparable with current commercial in-line polarizers at 2000 nm (~25 dB). Moreover, the PER of the 45° TFG could be further increased by extending the length of grating and optimizing the UV scanning power and speed. Using the same setup, we also measured the insertion loss the 45° TFG, which was essentially identical at ~0.6 dB during the range of 1850 nm~2150nm.
The experimental arrangement for the NPR mode-locked double-clad TDF based on 45° TFG is shown in Fig. 2. Two 793 nm diode lasers (Lumics, Germany) with maximum launched pump power up to 7.56 W were launched through a (2 + 1) × 1 pump combiner (ITF, Canada) to 7.0 m double-clad Tm3+-doped fiber (Coractive, DCF-TM-10/128) with an octagonal shaped inner cladding with 128 µm diameter and 0.45 numerical aperture (NA), and fiber core with 10 µm diameter and 0.22 NA. The measured absorption at 793 nm is ~4.0 dB/m. The selected fiber length of 7.0 m provides >95% pump absorption efficiency. A polarization independent optical isolator with an insertion loss of 0.76 dB and an extinction ratio of 35 dB at 2 µm (Advanced Photonics, USA) was used to ensure the unidirectional laser operation. A 45° TFG and a pair of polarization controllers (PC1 and PC2) positioned before and after the 45° TFG form the NPR based saturable absorber. A 95:5 fiber coupler centered at 2 µm with an insertion loss of 0.62 dB (Advalue Photonics, USA) was employed to output 5% laser power from the cavity. The measured splicing loss was ~0.3 dB due to the mismatch of the NA between the active fiber and SMF-28e fiber. Note that the coupling ratio chosen as 95:5 is to increase the cavity power for ensuring enough nonlinear phase shift difference. The SM2000 fiber (Thorlabs, USA) designed for 2 μm operation with a cut-off wavelength at 1.7 μm was chosen as the passive fiber in the NPR for the merit of its relatively low background loss and bend loss around 2 µm compared to SMF-28e and SMF-28e + fiber. The length of SM2000 fiber was optimized to 95.0 m to ensure enough nonlinear phase shift difference for stable mode-locking as well.
The total fiber laser cavity is 105.0 m including 7.0 m Tm3+-doped fiber, 95.0 m SM2000 fiber and 3.0 m SMF-28e pigtail fiber from the pump combiner, isolator and coupler. The anomalous dispersion values of the Tm3+-doped fiber and the SM2000 fiber at 1.993 µm were about −84 ps2/km and −73 ps2/km respectively, which are provided by the fiber producer. The dispersion of the SMF-28e at 1.993 µm was estimated to be −80 ps2/km according to the measured dispersion value at 1.9 µm . Thus, the net dispersion in the cavity was estimated to be ~−7.76 ps2, suggesting that the laser was operating at a large anomalous dispersion regime. For the measurements of the laser output, an InGaAs photodetector (EOT ET-5000F, USA) with a response time of approximately 28 ps connected to a 2.5 GHz digital oscilloscope was used to measure the pulse train and pulse waveforms. The pulse duration was measured by an interference autocorrelator (APE, Germany).
Since the net GVD is anomalous, the GVD and SPM counter-balance to give soliton pulses. Initially, the oscillator started to operate at continuous wave regime at a launched pump power of 1.229 W. Then, self-staring multiple soliton pulses were observed by adjusting the two PCs at a launched pump power of 1.320 W. It is well known that the nonlinear polarization rotation in this cavity is power dependant. Only at sufficient pumping and nonlinear polarization rotation, the mode-locking can be self-started. In our experiment, the pump was increased to a value that is capable of generating enough nonlinear polarization rotation for initiating the mode-locking, but the power in the cavity has beyond the limitation of single soliton pulse. Therefore, the multiple soliton pulses were generated firstly and then the single soliton pulse was achieved by decreasing the pump power, as similar as the soliton regime in  and . Gradually reducing the pump power from 1.320 W to 1.232 W, the reduction of wave-breaking in soliton from multiple pulses to 4, 3, 2-pulses and finally a stable single pulse at a slope efficiency of 6.2%, as shown in Fig. 3(a). The measured pulse waveform and sequence at this launched pump power are also shown in the inset of Fig. 3(a). The obtained repetition rate of 1.902 MHz matches well with the theoretically cavity length dependent value suggesting that only one pulse was generated per round trip. Maximum output power of 142 μW was achieved for the single pulse regime corresponding to a pulse energy of 74.6 pJ. Note that this energy includes the energy in the Kelly sidebands, and it is much larger than the theoretically calculated limited output energy of 13 pJ for the single soliton based on the formula in . Once the mode locking was achieved by the initial adjustment of the PCs, the single pulse operation can be achieved by reducing the pump power from self-starting multiple pulses and no longer needed to adjust the PCs. Figure 3(b) shows the measured interference and intensity autocorrelation trace of the mode-locked pulses at a scanning range of 10 ps. The shoulder to peak ratio is 1:8 confirming that the oscillator operated at typical solitary mode-locking regime. The FWHM is 3.4 ps corresponding to the pulse duration of 2.2 ps when sech2-pulse fit is assumed. Figure 3(c) shows the optical spectrum of the soliton pulses with a resolution of 0.05 nm. The central wavelength and FWHM were 1992.7 nm and 2.02 nm, respectively, thus giving a time-bandwidth product of 0.335 indicating the pulse is almost transform-limited. Typical Kelly sidebands originating from spectral interference of dispersive waves were observed suggesting the operation in this case was the conventional solitary mode-locking. Moreover, the absence of additional narrow spectral line also confirms that continuous wave components have been blocked. Figure 3(d) shows the RF spectrum of the output at fundamental frequency for a scanning range of 10 kHz with a resolution of 100 Hz. The signal noise ratio of ~60.5 dB and the absence of modulation in 20 MHz broad RF spectrum [see inset of Fig. 3(d)] indicated it is stable mode-locking and no Q-switching modulation. The smooth noise floor also indicated that the oscillator operated with low amplitude noise.
The laser can also be switched to stable noise-like regime by adjusting the PCs as the pump ramps up to 1.82 W, as shown in inset of Fig. 4(a). The measured repetition rate of 1.902 MHz indicated that the oscillator operated at the fundamental mode-locking regime. Figure 4(b) shows the measured interference and intensity autocorrelation trace at a scanning range of 150 ps. A narrow spike riding on a broad pedestal extended over the entire measurement window was observed, which was the typical autocorrelation trace of noise-like mode-locking [3, 9]. A typical smooth and broad optical spectrum for noise-like pulses shown in Fig. 4(c) reveals a center wavelength of 1994.2 nm and FWHM of 18.1 nm. Figure 4(d) is the RF spectrum at a scanning range 10 kHz with a resolution of 100 Hz. The lower signal noise ratio of ~47 dB compared to that of solitary pulses is still in the typical range of 40~50 dB for NPR-based noise pulses [39, 42], but much lower than that obtained in NOLM structure . No side lobes were observed in the RF spectrum which are always obvious in the noise-like regime suggesting the absence of appreciable fluctuations of pulse duration. The noise-like mode-locking state can be maintained to the maximum launched pump power of 7.56 W at a slope efficiency of 7.5% yielding maximum output power of 475.8 mW. This corresponds to 250.1 nJ per noise-like pulse bundle. Such high pulse energy and high spike peak power could lead to some potential applications involving laser radar, large electric field interactions with molecules, material processing and pump source of mid-IR supercontinuum generation.
To investigate the long-term stability of our mode-locked laser, we monitored its performance under the laboratory condition for 24 hours and found it was always maintained at stable mode locking state once the mode locking was obtained by adjusting the PCs and it was not sensitive to the environmental fluctuations. However, the single soliton operation compared to multiple soliton operation or noise-like operation was more sensitive to the direct influence on the fiber in the cavity. It is noted that such a long SM2000 fiber was used in our experiment to accumulate nonlinear phase shift as a result of the lower nonlinear parameter at 2 μm compared to that at 1 μm and at 1.55 μm. As decreasing the SM2000 fiber length from 95 m, it becomes harder to obtain even multiple soliton pulses. Until that of 85 m, only noise-like pulses can be achieved at a higher pump power level. In contrast, if the SM2000 fiber length is increased from 95 m, it is easier to obtain the multiple soliton pulses and noise-like pulse regime, however, it is harder to achieve the stable single soliton pulses. As the fiber length was increased to 120 m, the stable single soliton regime cannot be obtained. Replacing the 45° TFG by a commercial polarizer (AFR), the required SM2000 fiber for initiating the mode locking was still at 100 m level but only with an increased laser threshold due to the increased insertion loss. This indicates that the required long SM2000 fiber for mode-locking initiating was not caused by the 45° TFG.
In order to show the broad bandwidth advantage of 45° TFG, the length of Tm3+-doped fiber was shortened to 3.0 m and also extended to 10.0 m to shift the emissions center wavelength to ~1970 nm and ~2050 nm from the ring oscillator, respectively, resulting from the weakening and strengthening re-absorption. Stable mode-locked single soliton pulse for both 3.0 m and 10.0 m active fiber lengths was achieved by reducing the pump power from self-starting multiple pulses to 1.175 W and 1.342 W, respectively. Figure 5 shows the measured optical spectra of soliton mode-locking with different TDF lengths, giving the central wavelength and FWHM of 1971.2 nm and 2.15 nm for 3.0 m fiber, and 2051.3 nm and 1.89 nm for 10.0 m fiber, respectively. The corresponding pulse duration was 1.94 ps and 2.40 ps for the two lengths. This behavior indicates that shorter wavelength has narrower pulse duration as a result of the increasing net cavity dispersion with wavelength. Note that further decreasing or increasing the fiber length cannot further extend the wavelength. In a similar way to the soliton pulses, the noise-like pulses were also achieved for above fiber lengths with almost same bandwidth of 17.97 nm.
In summary, we have experimentally demonstrated the soliton and noise-like pulses delivered from an all-anomalous all-fiber Tm3+-doped fiber ring cavity oscillator based on the 45° TFG. The achieved soliton pulse centered at 1992.7 nm has a pulse duration of 2.2 ps and a FWHM bandwidth of 2.02 nm. The achieved noise-like pulse centered at 1997.2 nm has a FWHM bandwidth of 18.1 nm and a pulse energy of up to 250.1 nJ owing to the employing of double-clad Tm3+-doped fiber. Moreover, the broad band advantage of 45° TFG also provided the facility for large wavelength range mode-locking. Mode-locked pulses centered at 1971.8 nm and 2051.3 nm were also achieved based on the same 45° TFG through changing the active fiber length. Further deceasing of the cavity length and pulse duration can be expected by utilizing a hybrid structure combining a 45° TFG and an SA with a proper dispersion management. As a result, the 45° TFG pave a new way for the realization of all-fiber, compact, high-power, single-polarization mode-locking Tm3+-doped fiber pulse sources at 2 μm band, which can also easily to be extended to 2.1 μm or 2.2 μm band for mode-locked Ho3+ doped fiber lasers.
This work was supported by National Natural Science Foundation of China (Grant No. 61377042, 61107037, 61435003 and 61421002), and European Commission's Marie Curie International Incoming Fellowship (Grant No. 911333) and Marie Curie Project “CarbonNASA” (Grant No. 295208). The work was also partly supported by Open Foundation of Key Laboratory of high energy laser science and technology, CAEP (Grant No. 2013005580), Open Fund of Medical Optical Key Laboratory of Jiangsu Province (Grant No. JKLMO201403), and Open Fund of State Key Laboratory of Modern Optical Instruments of Zhejiang University.
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