The ultrafast time-stretch microscopy has been proposed to enhance the temporal resolution of a microscopy system. The optical source is a key component for ultrafast time-stretch microscopy system. Herein, we reported on the gain-guided soliton fiber laser with high-quality rectangle spectrum for ultrafast time-stretch microscopy. By virtue of the excellent characteristics of the gain-guided soliton, the output power and the 3-dB bandwidth of the stable mode-locked soliton could be up to 3 mW and 33.7 nm with a high-quality rectangle shape, respectively. With the proposed robust optical source, the ultrafast time-stretch microscopy with the 49.6 μm resolution and a scan rate of 11 MHz was achieved without the external optical amplification. The obtained results demonstrated that the gain-guided soliton fiber laser could be used as an alternative high-quality optical source for ultrafast time-stretch microscopy and will introduce some applications in fields such as biology, chemical, and optical sensing.
© 2016 Optical Society of America
Optical microscopy, as one of the most widely used diagnostic methods, has received much attention due to their numerous applications in fields from fundamental research to industrial purposes [1–4]. So far, the most common microscopy systems are charge-coupled device (CCD) and complementary metal-oxide-semiconductor (CMOS) cameras. However, there are some fundamental limitations in the above-mentioned microscopy systems, such as the long shutter speed (or exposure time) and the trade-off between the sensitivity and scan rate [5,6]. Due to these limitations, the image speeds of the high-end CCD and CMOS cameras are generally limited to be about 1 MHz. Therefore, the conventional microscopy systems could not be used to capture the high-speed dynamical phenomena. To improve the temporal resolution of the conventional image systems, recently the serial time-encoded amplified microscopy (STEAM) based on dispersive Fourier transformation (DFT) technique [7–10], also known as ultrafast time-stretch microscopy, has been proposed as a new optical imaging technology to increase the scan rate, which can readily achieve higher frame rate at the order of 10 MHz and even at GHz level [11–21]. Taking advantage of high frame-rate, the ultrafast time-stretch microscopy can be regarded as a real-time one, which was favorable for capturing fast dynamical processes such as microfluidics or single-microparticle flow analyzer [22,23], biomedical diagnostics [24,25], and so on.
For ultrafast time-stretch microscopy, the optical source is a key component in systems since the image quality is related to the optical source. For example, the scan rate is determined by the repetition rate of the ultrafast laser source. In addition, the coherent optical source with broadband and flat/rectangular spectral envelope is more favorable for its capacity of producing high quality images with uniform contrast and brightness in the time-stretch microscopy. To date, various coherent optical sources have been proposed for the ultrafast time-stretch microscopy [7–21]. As one type of the broadband light sources, the supercontinuum (SC) source was used to implement ultrafast time-stretch microscopy. However, the pulse-train of SC source suffers pulse-to-pulse amplitude fluctuations and the temporal coherence will deteriorate at the wavelength outside the pump area [26,27]. Therefore, the average operation needs to be employed to weaken the influence of the amplitude fluctuations on the image quality, which thus limits the scan rate of the microscopy system . As an alternative solution, the mode-locked fiber laser could be also used as coherent optical source for ultrafast time-stretch microscopy. Indeed, both the mode-locked fiber lasers operating at 1.55 μm and 1.06 μm wavebands have been employed to achieve ultrafast imaging process [8,29]. Regarding the ultrafast fiber laser at 1.55 μm, generally the mode-locked spectrum possesses the Gaussian-like shape with evident Kelly sidebands . The non-flat spectral shape and the strong Kelly sidebands would cause noisy effects on images. As for the ultrafast fiber laser operating at 1.06 μm, due to the all-normal dispersion regime, the Yb-doped fiber laser could deliver the mode-locked pulse with a flatter optical spectrum compared with that of conventional mode-locked pulse at 1.55 μm [31,32]. However, for the purpose of stable operation of Yb-doped fiber laser at 1.06 μm, a narrow spectral filter needs to be employed to cuts off the temporal wings of a strongly chirped pulse in the Yb-doped laser cavity, which in turn narrows the mode-locked spectrum . Therefore, the 3-dB spectral bandwidth of the stable mode-locked Yb-doped fiber laser is generally limited to be about 10 nm [34,35]. In addition, the mode-locked spectrum could also present sharp peaks around its spectral edges . Thus, it is meaningful to find an optical source possessing broadband and high-quality rectangular spectrum for ultrafast time-stretch microscopy.
In fact, by managing the intracavity dispersion, it was also found that the stable mode-locked pulse could be generated in a net/all-normal dispersion Er-doped fiber laser operating at the 1.55 μm waveband [37–40]. Since the stable mode-locking pulse was formed under the condition of the spectral filtering (gain bandwidth) of the gain fiber, this type of mode-locked pulse in Er-doped fiber lasers was referred to as “gain-guided soliton” [37–39]. The unique properties of the gain-guided soliton are that the mode-locked spectrum shows a very flat and rectangular envelope without any spectral spikes and the spectral bandwidth can extend to several tens of nanometers. Moreover, the intracavity power of mode-locked pulse can also reach a high level of about several tens of milliwatts without pulse breaking effect. We recalled that the coherent optical source with broadband and rectangular spectral envelope is good for the ultrafast time-stretch microscopy. Therefore, the high-quality of the gain-guided soliton both in the spectral and output power performances makes it suitable for the potential application as the optical source in ultrafast time-stretch microscopy systems.
In this work, we constructed a gain-guided soliton Er-doped fiber laser (EDFL) with high-quality rectangle spectrum for ultrafast time-stretch microscopy. By properly rotating the polarization controllers (PCs) and pump power level, the fiber laser delivers a stable gain-guided soliton train with a 3-dB spectral bandwidth of 33.7 nm. The output power of the mode-locked soliton could be up to 3 mW without the pulse breaking phenomenon. Therefore, no external optical amplification was needed for the ultrafast time-stretch microscopy system. Taking advantage of excellent rectangle mode-locked spectrum, the high-performance ultrafast time-stretch microscopy with a scan rate of 11 MHz was achieved. Moreover, the spatial resolution is estimated to be 49.6 μm, which is approaching the theoretically ultimate resolution of our microscopy system. The achieved results clearly indicate that the gain-guided soliton fiber laser would be an alternative high-quality optical source for ultrafast time-stretch microscopy systems.
2. Performance of the gain-guided soliton fiber laser
The experimental setup of the proposed gain-guided soliton fiber laser is shown in Fig. 1. To enable the fiber laser to operate in net-normal dispersion regime, a segment of 10-m EDF with normal dispersion of was used as the gain medium and compensates the cavity dispersion. The pumping source is a 980 nm laser diode with maximum output power of 400 mW. The other fibers are all 8.9-m standard single mode fibers (SMF). Therefore, the net cavity dispersion is positive, which is about 0.047 ps2. The polarization controllers (PCs) were employed to adjust the polarization state of the propagation light. The unidirectional operation and the polarization selectivity were provided by the polarization-dependent isolator (PD-ISO). The laser output was analyzed by an optical spectrum analyzer (OSA, Anritsu MS9710C) and a real-time oscilloscope (Tektronix DSA-70804, 8 GHz) with a photodetector (Newport 818-BB-35F, 12.5 GHz). In addition, the pulse duration was measured by a commercial autocorrelator (FR-103XL).
By properly rotating the PCs, the self-starting mode-locked operation could be easily achieved at the pump power of 300 mW. Note that the mode-locking threshold is a litter higher than those of other gain-guided soliton fiber lasers. It is probably caused by the imperfect cavity parameter settings of our fiber laser. For better performance of the fiber laser, we then increased the pump power to 380 mW. The mode-locked spectrum at the pump power of 380 mW was shown in Fig. 2(a). The mode-locked spectrum shows the typical characteristics of gain-guided soliton , where the flat top and steep edges can be seen. Here, the 3-dB spectral bandwidth of mode-locked spectrum is 33.7 nm, which is larger and flatter than that of a typical dissipative soliton Yb-doped fiber laser. The mode-locked pulse-train is presented in Fig. 2(b). The repetition rate is 11.35 MHz, which is determined by the cavity length. The pulse duration is 3.2 ps if the fit of the hyperbolic secant pulse shape is assumed, as shown in Fig. 2(c). In order to show the stability of the fiber laser, the radio-frequency (RF) spectrum was measured. As can be seen in Fig. 2(d), the fundamental peak locates at 11.35 MHz, which corresponds to the fundamental cavity repetition rate. Here, the signal-to-noise ratio (SNR) is ~51 dB, suggesting the stable mode-locking operation of the fiber laser. Due to the limited pump power level, the multiple gain-guided solitons were not observed in this experiment.
As we know, the coherent broadband optical source is suitable for the ultrafast time-stretch microscopy systems . In order to show the coherence of the gain-guided soliton fiber laser, we employed a Mach-Zehnder (M-Z) pulse interferometer to check the spectral interferometry between consecutive pulses from the laser output . The experimental setup is shown in Fig. 3. Here, the length difference between the two arms of the M-Z interferometer was carefully adjusted to match the consecutive pulse intervals, which was achieved by a free-space variable delay line with 20 cm adjustable range. Using such a setup, we could expect that the two nearby pulses at the output of the M-Z interferometer would show the evident interference patterns on the spectrum if the gain-guided solitons emitted from the fiber laser are coherent. The measured results by the conventional OSA are shown in Fig. 4, in which we can see the stable distinct fringes on the pulse spectrum. The results indicated that the gain-guided soliton from the proposed fiber laser shows the high degree of coherence, which indeed could act as a good quality optical source for ultrafast time-stretch microscopy systems.
3. Ultrafast time-stretch microscopy system
In the following, we introduced the proposed gain-guided soliton fiber laser into the ultrafast time-stretch microscopy system. The schematic of the ultrafast time-stretch microscopy system is shown in Fig. 5. The collimated gain-guided solitons emitted from the fiber laser are then injected into a transmission diffraction grating (1100 lines/mm). In this case, the spectrum of the gain-guided soliton could be dispersed into a line shape, namely, one-dimensional (1D) spectral shower. The spectral shower was focused onto the sample (USAF1951 resolution target) by a lens with a focal length of 30 mm. Note that the USAF1951 resolution test target was placed on a moving stage, which enables the consecutive 1D scans to construct a 2D image of the target. The length of spectral shower line that illumined on target was ~3.3 mm, which scanned on the sample with a step size of 5 μm. Then the spectrally encoded pulse reflected by the sample was stretched by a long segment of 20-km standard SMF with dispersion parameter of . Finally, the serial data stream is collected by a real-time oscilloscope (Tektronix DSA 70804, 8 GHz) with a 12.5 GHz high-speed photodetector.
As mentioned above, the ultrafast time-stretch microscopy system is based on DFT technique [7–10]. DFT is an optical real-time diagnostic method that enables the output temporal waveform mimic its spectrum by simply propagating in the dispersive medium. To check the pulse profile of the gain guided soliton after the DFT process, we recorded the pulse at the output of the SMF without injecting into the microscopy system by the real-time oscilloscope. As can be seen in Fig. 6, the pulse profile is very similar to mode-locked spectrum presented in Fig. 2(a), showing that the stretched pulse is a rectangular one.
4. Performance of ultrafast time-stretch microscopy system
For testing the performance of the constructed ultrafast time-stretch microscopy system, we firstly measured the optical spectrum and the temporal waveform of stretch-broadened pulses reflected by the sample. The image of scanning sample, a USAF-1951 resolution target, was presented in Fig. 7. Figure 8 is the result obtained when the profiles of the spectral shower crossing the lines at group 0, element 3 and 4, which corresponds to the red line in Fig. 7. The image-encoded spectrum measured by the commercial OSA is shown in Fig. 8 with blue curve, which was obtained from the output of circulator and carried the original information of image. To identify whether the information was still carried after the DFT process or not, the temporal waveform with DFT technique was also recorded for the purpose of comparison. The result was shown in Fig. 8 with red curve. As can be seen here, it is evident that the two profiles of curves are similar to each other. The compared results proved that the information of image was still well saved by the DFT technique, indicating the high performance of wavelength-to-time conversion. Here, the output power of the microscopy system is about 70 μW. Considering that the average output power of the gain-guided soliton is 3 mW, the total insertion loss of the microscopy system is 16 dB. Moreover, note that there are spikes in the spectral envelope recorded by the conventional OSA, while the spectrum measured by DFT is smooth. These spectral spikes could be caused by the lower resolution of the DFT system than that of OSA (0.1 nm in this work), because the spectral resolution of the DFT system is calculated to be ~0.3 nm . In this case, the spectral spikes would be sharper due to the more detected points of the OSA. It should be also noted that the lower spectral resolution of the DFT system would lose some spectral information. However, it did not influence the quality of our current stage of the microscopy system.
In the following, we scanned the USAF1951 resolution target for illuminating the spatial resolution of our system. The program for reconstructing the image is MATLAB and the type of image is set to be gray scale. Figure 9 shows the 2000 lines reconstructed image by our ultrafast microscopy system. Here, the elements which can be identified are group 0, group 2, group 3 and group 1 from left to right, respectively. Both elements of group 0 and group 1 are obvious for identification. However, in this case the elements of group 2 and group 3 with higher resolutions could not be clearly observed. For the purpose of checking the ultimate spatial resolution of the proposed ultrafast microscopy system, then we focused on the scan of group 2 and group 3 by slightly adjusting the spectral shower position. The scanning result of group 2 and group 3 was shown in Fig. 10. It can be seen that the first three elements of group 4 can be still recognized. Nevertheless, other elements with higher resolution included in Fig. 10 are too blurry to be identified. Thus, the spatial resolution of the ultrafast microscopy system would be around 49.6 μm according to the definition of USAF resolution. Note that we can still obtain the clear image without being swallowed by noises when no optical amplifiers were needed in our experiments. Therefore, we can conclude that the proposed gain-guided soliton Er-doped fiber laser could be employed to overcome the influence of noises on the image quality, which would be an excellent optical source for time-stretch imaging system.
From the experimental results above, we have obtained an image with the spatial resolution around 49.6 μm by the ultrafast microscopy system based on the gain-guided soliton fiber laser. Comparing to other results reported previously, the spatial resolution seems to be a little low. According to , the spatial resolution of the time-stretch microscopy is limited by the highest points of following three issues: the spectral resolution of the spatial disperser, the spectral resolution imposed by DFT through stationary-phase-approximation (SPA), and the temporal resolution of the digitizer. In our experiment, the parameters are 1560 nm, 1.5 mm and 30 mm for the central wavelength, the beam waist and the focal length, respectively. In addition, other parameters are 1100 lines/mm and 8 GHz for the grating period and the bandwidth of digitizer, respectively. Figure 11 presents the calculated GVD-resolution curves in the spatial domain. The vertical line in magenta corresponds to the GVD of our 20 km-long SMF, which is 0.34 ns/nm in total. In Fig. 11, the highest intersection is with the spatial dispersion (blue curve). Therefore, the factor that mostly limits the resolution of our microscopy system is the spatial dispersion. From Fig. 11, it can be identified that the theoretical spatial resolution is 31.2 μm for our microscopy system. Thus, the experimentally obtained resolution of 49.6 μm is approaching the theoretical resolution of 31.2 μm. As a method of improving the spatial resolution, we can upgrade the element of microscopy set up such as using a lens with shorter focal length. In this case, the spatial resolution can be greatly improved. It should be also noted that the direction of the spectral shower is not normal incidence. Therefore, the quality of the time-stretching image still could be also optimized by further carefully adjusting the optical paths. Finally, the spectral bandwidth of the gain-guided soliton can be enlarged if the cavity parameters such as dispersion and pump power are further optimized, which might be used as optical source in the imaging system of optical coherence tomography [43,44].
In summary, a gain-guided soliton fiber laser with high-quality rectangle spectrum for ultrafast time-stretch microscopy system was demonstrated. The output power and the 3-dB spectral bandwidth are about 3 mW and 33.7 nm, respectively. With the proposed robust optical source, the ultrafast time-stretch microscopy with a scan rate of 11 MHz was achieved without external optical amplification. By virtue of the high-quality rectangular-shape mode-locked spectrum, the DFT temporal waveform of beam reflected by a specific element has a uniform SNR along its rectangular-shape waveform. In addition, the processed image has reached a resolution of 49.6 μm, which is near the theoretically ultimate resolution of 31.2 μm. The achieved results provided the strong evidence that the gain-guided soliton fiber laser can act as an excellent optical pulse source for high-performance ultrafast microscopy system, and the constructed microscopy system will find various applications in the desirable fields.
This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 61378036, 61307058, 11304101, 11474108), the Key Program of Natural Science Foundation of Guangdong Province, China (Grant No. 2014A030311037), and the Graduate Research and Innovation Foundation of South China Normal University, China (Grant No. 2015lkxm15). Z.-C. Luo acknowledges the financial support from the Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No. 2014A030306019), Program for the Outstanding Innovative Young Talents of Guangdong Province (Grant No. 2014TQ01X220), and the Pearl River S&T Nova Program of Guangzhou (Grant No. 2014J2200008).
References and links
3. R. Zhang, Y. Zhang, Z. C. Dong, S. Jiang, C. Zhang, L. G. Chen, L. Zhang, Y. Liao, J. Aizpurua, Y. Luo, J. L. Yang, and J. G. Hou, “Chemical mapping of a single molecule by plasmon-enhanced Raman scattering,” Nature 498(7452), 82–86 (2013). [CrossRef] [PubMed]
4. D. G. de Oteyza, P. Gorman, Y. C. Chen, S. Wickenburg, A. Riss, D. J. Mowbray, G. Etkin, Z. Pedramrazi, H. Z. Tsai, A. Rubio, M. F. Crommie, and F. R. Fischer, “Direct imaging of covalent bond structure in single-molecule chemical reactions,” Science 340(6139), 1434–1437 (2013). [CrossRef] [PubMed]
5. T. G. Etoh, C. V. Le, Y. Hashishin, N. Otsuka, K. Takehara, H. Ohtake, T. Hayashida, and H. Maruyama, “Evolution of ultrahigh-speed CCD imagers,” Plasma Fusion Res. 2(2), S1021 (2007). [CrossRef]
6. “Hyper Vision HPV-2,” http://www.shimadzu.com/an/test/hpv/hpv2_1.html.
7. K. Goda, K. K. Tsia, and B. Jalali, “Amplified dispersive Fourier transform imaging for ultrafast displacement sensing and barcode reading,” Appl. Phys. Lett. 93(13), 131109 (2008). [CrossRef]
9. K. Goda, D. R. Solli, K. K. Tsia, and B. Jalali, “Theory of amplified dispersive Fourier transformation,” Phys. Rev. A 80(4), 043821 (2009). [CrossRef]
10. K. Goda and B. Jalali, “Dispersive Fourier transformation for fast continuous single-shot measurements,” Nat. Photonics 7(2), 102–112 (2013). [CrossRef]
11. A. Mahjoubfar, K. Goda, A. Ayazi, A. Fard, S. H. Kim, and B. Jalali, “High-speed nanometer-resolved imaging vibrometer and velocimeter,” Appl. Phys. Lett. 98(10), 101107 (2011). [CrossRef]
12. A. M. Fard, A. Mahjoubfar, K. Goda, D. R. Gossett, D. Di Carlo, and B. Jalali, “Nomarski serial time-encoded amplified microscopy for high-speed contrast-enhanced imaging of transparent media,” Biomed. Opt. Express 2(12), 3387–3392 (2011). [CrossRef] [PubMed]
13. C. Zhang, Y. Qiu, R. Zhu, K. K. Y. Wong, and K. K. Tsia, “Serial time-encoded amplified microscopy (STEAM) based on a stabilized picosecond supercontinuum source,” Opt. Express 19(17), 15810–15816 (2011). [CrossRef] [PubMed]
14. K. Goda, A. Mahjoubfar, C. Wang, A. Fard, J. Adam, D. R. Gossett, A. Ayazi, E. Sollier, O. Malik, E. Chen, Y. Liu, R. Brown, N. Sarkhosh, D. Di Carlo, and B. Jalali, “Hybrid dispersion laser Scanner,” Sci. Rep. 2(445), 445 (2012). [PubMed]
15. A. C. S. Chan, A. K. S. Lau, K. K. Y. Wong, E. Y. Lam, and K. K. Tsia, “Arbitrary two-dimensional spectrally encoded pattern generation—a new strategy for high-speed patterned illumination imaging,” Optica 2(12), 1037–1044 (2015). [CrossRef]
18. F. Xing, H. Chen, C. Lei, Z. Weng, M. Chen, S. Yang, and S. Xie, “Serial wavelength division 1 GHz line-scan microscopic imaging,” Photon. Res. 2(4), B31–B34 (2014). [CrossRef]
19. X. Wei, A. K. S. Lau, Y. Xu, K. K. Tsia, and K. K. Y. Wong, “28 MHz swept source at 1.0 μm for ultrafast quantitative phase imaging,” Biomed. Opt. Express 6(10), 3855–3864 (2015). [CrossRef] [PubMed]
21. X. Wei, A. K. Lau, Y. Xu, C. Zhang, A. Mussot, A. Kudlinski, K. K. Tsia, and K. K. Wong, “Broadband fiber-optical parametric amplification for ultrafast time-stretch imaging at 1.0 μm,” Opt. Lett. 39(20), 5989–5992 (2014). [CrossRef] [PubMed]
22. K. Goda, A. Ayazi, D. R. Gossett, J. Sadasivam, C. K. Lonappan, E. Sollier, A. M. Fard, S. C. Hur, J. Adam, C. Murray, C. Wang, N. Brackbill, D. Di Carlo, and B. Jalali, “High-throughput single-microparticle imaging flow analyzer,” Proc. Natl. Acad. Sci. U.S.A. 109(29), 11630–11635 (2012). [CrossRef] [PubMed]
23. L. Golan, D. Yeheskely-Hayon, L. Minai, E. J. Dann, and D. Yelin, “Noninvasive imaging of flowing blood cells using label-free spectrally encoded flow cytometry,” Biomed. Opt. Express 3(6), 1455–1464 (2012). [CrossRef] [PubMed]
26. A. Kudlinski, B. Barviau, A. Leray, C. Spriet, L. Héliot, and A. Mussot, “Control of pulse-to-pulse fluctuations in visible supercontinuum,” Opt. Express 18(26), 27445–27454 (2010). [CrossRef] [PubMed]
27. K. L. Corwin, N. R. Newbury, J. M. Dudley, S. Coen, S. A. Diddams, K. Weber, and R. S. Windeler, “Fundamental noise limitations to supercontinuum generation in microstructure fiber,” Phys. Rev. Lett. 90(11), 113904 (2003). [CrossRef] [PubMed]
28. C. Lei, H. Chen, F. Xing, M. Chen, S. Yang, and S. Xie, “Time-stretch high-speed microscopic imaging system based on temporally and spectrally shaped amplified spontaneous emission,” Opt. Lett. 40(6), 946–949 (2015). [CrossRef] [PubMed]
29. X. M. Wei, A. K. Lau, T. T. Wong, C. Zhang, K. K. Tsia, and K. K. Wong, “Coherent laser source for high frame-rate optical time-stretch microscopy at 1.0 μm,” IEEE J. Sel. Top. Quantum Electron. 20(5), 384–389 (2014). [CrossRef]
30. S. M. J. Kelly, “Characteristic sideband instability of periodically amplified average soliton,” Electron. Lett. 28(8), 806–807 (1992). [CrossRef]
31. F. W. Wise, A. Chong, and W. H. Renninger, “High-energy femtosecond fiber lasers based on pulse propagation at normal dispersion,” Laser Photonics Rev. 2(1–2), 58–73 (2008). [CrossRef]
33. B. G. Bale, J. N. Kutz, A. Chong, W. H. Renninger, and F. W. Wise, “Spectral filtering for high-energy mode locking in normal dispersion fiber lasers,” J. Opt. Soc. Am. B 25(10), 1763–1770 (2008). [CrossRef]
34. B. Ortaç, O. Schmidt, T. Schreiber, J. Limpert, A. Tünnermann, and A. Hideur, “High-energy femtosecond Yb-doped dispersion compensation free fiber laser,” Opt. Express 15(17), 10725–10732 (2007). [CrossRef] [PubMed]
35. L. Liu, J. H. Liao, Q. Y. Ning, W. Yu, A. P. Luo, S. H. Xu, Z. C. Luo, Z. M. Yang, and W. C. Xu, “Wave-breaking-free pulse in an all-fiber normal-dispersion Yb-doped fiber laser under dissipative soliton resonance condition,” Opt. Express 21(22), 27087–27092 (2013). [CrossRef] [PubMed]
36. W. H. Renninger, A. Chong, and F. W. Wise, “Dissipative solitons in normal-dispersion fiber lasers,” Phys. Rev. A 77(2), 023814 (2008). [CrossRef]
38. L. M. Zhao, D. Y. Tang, H. Zhang, T. H. Cheng, H. Y. Tam, and C. Lu, “Dynamics of gain-guided solitons in an all-normal-dispersion fiber laser,” Opt. Lett. 32(13), 1806–1808 (2007). [CrossRef] [PubMed]
40. D. Mao, S. Zhang, Y. Wang, X. Gan, W. Zhang, T. Mei, Y. Wang, Y. Wang, H. Zeng, and J. Zhao, “WS2 saturable absorber for dissipative soliton mode locking at 1.06 and 1.55 μm,” Opt. Express 23(22), 28698–28706 (2015). [PubMed]
41. A. F. J. Runge, C. Aguergaray, N. G. R. Broderick, and M. Erkintalo, “Coherence and shot-to-shot spectral fluctuations in noise-like ultrafast fiber lasers,” Opt. Lett. 38(21), 4327–4330 (2013). [CrossRef] [PubMed]
43. X. Wei, J. Xu, Y. Xu, L. Yu, J. Xu, B. Li, A. K. S. Lau, X. Wang, C. Zhang, K. K. Tsia, and K. K. Y. Wong, “Breathing laser as an inertia-free swept source for high-quality ultrafast optical bioimaging,” Opt. Lett. 39(23), 6593–6596 (2014). [CrossRef] [PubMed]
44. J. Xu, X. Wei, L. Yu, C. Zhang, J. Xu, K. K. Y. Wong, and K. K. Tsia, “High-performance multi-megahertz optical coherence tomography based on amplified optical time-stretch,” Biomed. Opt. Express 6(4), 1340–1350 (2015). [CrossRef] [PubMed]