A noise-like pulse (NLP) with broadband emission spectrum and superior beam quality from a dispersion managed mode-locked Yb-doped fiber laser has been demonstrated based on stimulated Raman scattering. After insertion of a 150 m long single mode fiber into the laser cavity, the second order stoke wave from 1.3 MHz repetition rate of NLP can be excited. With a 320 mW pump power, the highest pulse energy of NLP was about 35.1 nJ and the emission spectrum was extended from 1000 to 1160 nm. Through a multi-mode fiber laser, the broad bandwidth NLP can produce relatively low speckle noise imaging with contrast below 0.04. The generated NLPs can be used as a superior light source for the biomedical diagnosis and laser projection in the near future.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Nowadays, passively mode-locked fiber lasers (PML-FLs) have attracted considerable attention owing to their peculiar soliton dynamics and a great variety of potential applications. A number of industrial technologies like micro-machining, 3D printing, lithography, and biomedical diagnosis and treatment can be accurately and precisely achieved by means of PML-FLs. Unlike CW lasers, PML-FLs possess unique characteristics such as high peak intensity that can induce incredible nonlinear optical effects  such as self-phase modulation, four-wave mixing, and stimulated Raman-scattering, etc., as they interact with the nonlinear medium. In addition, the induced multiple photon absorption and high harmonic generation in bio-tissue excited by the PML-FLs has been applied to obtain the morphology of structure with high spatial resolution . Furthermore, a relatively broad spectrum bandwidth of PML-FLs can be used for optical metrology, sensing, spectroscopy and optical coherent tomography (OCT) . OCT is a widely used non-invasive technology that can perform high resolution cross-sectional in vivo imaging of biological tissues in real time . For the enhancement of axial resolution in OCT, the increase in the spectrum bandwidth of PML-FL  becomes a critical factor that can be realized through the injection of pulsed light into the nonlinear fiber or photonic crystal fiber .
PML-FLs are primarily produced by insertion of a saturable absorber (SA), such as a semiconductor saturable absorber, carbon nanotubes or graphene , graphene oxide  and topological insulator  into the laser cavity. In addition, artificial saturable absorbers like nonlinear polarization rotation (NPR) [10–12], nonlinear optical loop mirror (NOLM) , and nonlinear amplifier loop mirror (NALM), have also been widely used for the production of PML-FLs. Typically, NPR is one of the most reliable techniques that has been adopted in net anomalous dispersion fiber lasers for producing high intensity pulses with ultra-short pulsewidth. By means of the NPR, the dissipative solitons (DSs) can also be demonstrated in a net normal or all-normal dispersion fiber laser with considerably large pulse energy [14, 15]. Apart from the stabilized pulses, scientists are also fascinated by a variety of intriguing nonlinear dynamics such as multiple pulses , bound soliton , and soliton rain , which exist in certain configurations of PML-FLs. Since the first report by Horowitz et al.  on Erbium doped fiber laser (EDFL), the specific operation state called noise-like pulse (NLP) has attracted considerable attention with its unique property of relative broad spectrum bandwidth and double scale IAC traces. Apart from EDFL , the behavior of NLPs has also been studied in YDFL [21, 22]. The tuning of the spectrum bandwidth and the duration of NLPs from the net normal dispersion Yb-doped fiber laser (YDFL) can be achieved by using the iris as a filter behind the grating pair . In addition, the switching between the DSs and NLPs operation states has been demonstrated by the insertion of a slit between the grating pair as a bandpass filter .
In contrast to the conventional solitons, NLPs possess higher energy pulsed light with a broad emission spectrum that is beneficial for micro-machining and OCT measurement. Based on the Raman effect, the NLP with a spectrum bandwidth of about 61.4 nm has been demonstrated through appropriate adjustment of the polarization controller . Nevertheless, the output characteristic of NLP including beam profile and speckle noise are dominated for certain applications but have seldom been investigated. In this work, we investigated the generated mechanism and output characteristic of NLP with a relatively broad spectrum bandwidth. The speckle noise suppression of pulsed light has also been realized through a multi-mode fiber (MMF). In order to generate an even broader spectrum bandwidth, we insert different lengths of the single mode fiber (SMF, HI1060) into the laser cavity to reduce the Raman threshold for the generation of an even higher order Raman scattering effect.
2. Experimental setup
The schematic setup of a PML-YDFL with ring cavity configuration is shown in Fig. 1(a). The gain medium was a 40-cm-long ytterbium-doped single mode fiber (with the absorption of 280 dB/m at 920 nm), which was pumped by a pigtail laser diode with the central wavelength of around 976 nm through a 980/1060 nm wavelength division multiplexer (WDM). The mode-locked mechanism of the YDFL relied upon the (NPR) that was comprised of two quarter-wave-plates (λ/2), one half-wave-plate (λ/4) and a polarization beam splitter (PBS) cube in free space. In addition, the dispersion delay line, comprising the grating pair (GP) with separation around 3.5 cm and 1000-lines/mm groove density, was used to induce anomalous dispersion inside the laser cavity. The estimated net group delay dispersion (GDD) inside laser cavity is around −0.0389 ps2. The optical isolator was also put in free space to ensure uni-directional propagation of pulsed light inside the laser cavity. The laser output was taken from the NPR rejection port by the PBS and coupled to the collimateor. The time traces of PML pulses were detected by a high-speed photodiode (EOT Inc.) and monitored through a 2 GHz high-speed oscilloscope (OSC, 200 GHz sampling rate, WaveRunner 620Zi, LeCroy Inc.) and the optical spectrum was recorded by an optical spectrum analyzer (OSA, AQ-6370, Ando Inc.). We used an autocorrelator (FR-103XL, Femtochrome Research inc.) to obtain the pulsewidth of the NLPs. Different additional lengths of the SMF (HI1060), 25 to 150 meters, were added inside the laser cavity to reduce the Raman threshold and to extend the spectrum bandwidth of the NLPs. With the 150 m long SMF inside cavity, the estimated net GDD of the oscillator is around 3.313 ps2.
In order to confirm the quality of the output beam, the M-square was measured by the experimental setup as shown in Fig. 1(b). The collimated light from the collimator was focused by a lens (focus length of 200 mm). The beam profiler (SP-928, Ophire Inc.) was translated along the z-axis to record the beam profile at different locations. The setup of objective speckle measurement (according to IEC 62906-1-2:201528) is shown in Fig. 1(c). After projection of the collimated beam of YDFL at different operating states onto the screen, the monochromatic speckle patterns were also recorded in the beam profiler (SP-928). In addition, we connected a 5-meter long MMF (Corning, cladding diameter 125 μm, core 62.0 μm, NA 0.271) at the laser output to eliminate speckle noise and get higher laser projection quality.
Without the insertion of extra-length SMF into the laser cavity, the NLPs can be generated through appropriately adjusting the wave-plate inside the laser cavity at certain pump powers. Figure 2(a) shows the time trace of NLP and the long-term monitoring of the pulse trains [Inset of Fig. 2(a)], respectively. The time interval between sequential pulses is about 39.4 ns, which corresponds to the pulse repetition rate of about 25.4 MHz. The plots in Fig. 2(b) reveal the spectrum evolution of the NLP as pump power (Ppump) increases from 224 mW to 325 mW. With Ppump = 325 mW, a relatively broad spectrum bandwidth (NLP-I state) with Δλ (3-dB bandwidth) around 65.2 nm was obtained. In comparison with the dissipative soliton state, the smoothing and broad spectrum bandwidth of about 61.4 nm of the NLPs from ANDi YDFL have been attributed to the stimulated Raman scattering (SRS) effect . Thus, the emission spectrum in Fig. 2(b) can be decomposed into two solitons, with peak wavelengths at 1030 nm and 1080 nm, and a revealed 13.4 THz frequency shift owing to the stimulated Raman effect. The inset of Fig. 2(b) illustrates the double scale IAC traces at various pump powers, which is the main characteristic of NLP. By the fitting of the Gaussian function, a sub-picosecond narrow coherent spike (CS) on the top of the broad pedestal with the duration of around 312 fs (τs) and 18.6 ps (τp), respectively, can be obtained with Ppump = 325 mW.
The measured output power (Pout) and estimated pulse energy (Eout) versus the pump power for the PML-YDFL in operation in the NLP-I state are shown in Fig. 3(a). Both Pout and Eout increase linearly with Ppump. Once YDFL was mode locking as Ppump at 207 mW, the NLP was generated with the minimum Pout about 31 mW that corresponded to Eout = 1.1 nJ. With the maximum Ppump at 325 mW, the maximum Pout of YDFL was about 57 mW that corresponded to the Eout = 2.1 nJ. We also fitted the IAC trace by the Gaussian function [Inset of Fig. 2(b)] to obtain the FWHM (τp) of the pedestal. The measured τp versus the Ppump are shown by blue squares in Fig. 3(b). As Ppump increases from 224 mW to 325 mW, the τp increased slightly from 15.7 ps to 18.6 ps. In considering the output power of YDFL and the width of the pedestal, the estimated peak power of NLP increased from 73.0 W to 113.6 W as shown by the red triangles in Fig. 3(b).
Through proper adjustment of the PC, the YDFL can operate in the other NLP states (NLP-II) with narrower spectrum bandwidths as shown in Fig. 4(a). The peak wavelength of NLP-II was located at 1030 nm. The corresponding IAC trace [inset of Fig. 4] still showed the characteristic of double scale shape. Comparing the NLP-I state with τp = 18.6 ps, the duration of the pedestal at NLP-II became narrower with τp of about 6.8 ps. We suppose that the shrinkage of spectrum bandwidth in the NLP-II state might be attributed to the invisible filter  from the NPR technique to which filters out long wavelength spectrum components stoke wave). In order to confirm the hypothesis, we inserted a real filter between the grating pair and end mirror [Fig. 1] with transmission wavelength around 1010 nm to 1060 nm to filter out the long wavelength component of NLP. By means of a real filter, the optical spectrum and IAC trace in the NLP-III state [Fig. 4(b)] shows a similar shape of the spectrum and the pedestal width to that of NLP-II in Fig. 4(a).
The critical power for the stoke wave generation (Raman scattering) is defined by :1], and Leff = [1−exp(−αL)] / α is the effective length of the fiber laser with α= 0.345 km−1 being the absorber coefficient of HI-1060 at 1060 nm. In considering the average power Pout=63 mW, repetition rate Rrep=24 MHz and duration τ p= 6.84 ps of NLP-II, the estimated peak power inside the laser cavity is about 772.5 W which is higher than the Raman threshold of about 688 W. Thus, the stoke wave with central wavelength at 1080 nm could be generated after properly adjusting the PCs to remove the invisible filter.
From Eq. (1), the critical power of the Raman threshold can be reduced after increasing the extra-lengths of SMF (Lext), from 25 to 200 m, inside the cavity of PML-FL to induce a higher order stoke wave at the fixed pump power. With the fixed pump power of 325 mW, the evolution of the optical spectrum in the NLP state by alternating the extra-length of SMF is shown in Fig. 5 (a). It is obvious to see that the first stoke wave (1st-SW) with peak wavelength at 1076 nm as Lext = 25 m [black solid curve in Fig. 5(a)]. While we further increased the length of SMF inside the cavity, the enhancement of the intensity of the first stoke wave can be obviously seen, e.g., Lext = 50 m and 75 m. As the extra-length of SMF was increased above 100 m, the second stoke wave [2nd-SW, green solid curve in Fig. 5(a)] could be excited. Similarly, the intensity of the 2nd-SW increased with an additional insertion length of SMF, e.g., Lext = 125 m and 150 m. Thus, the emission spectrum of YDFL can be further extended toward the long wavelength at the fixed pump power by the increase of SMF inside the cavity.
The emission spectrum of NLP revealed the most flattened and broad spectrum bandwidth with Lext = 150 m [red solid curve in Fig. 5(a)]. In this state, the double scale IAC trace can still be observed [inset of Fig. 5(d)], which indicates the operation of YDFL in the NLP state. The frequency shifts between three emission peaks, i.e., pump wave (λp= 1030 nm), 1st-SW (λp= 1076 nm), and 2nd-SW (λp= 1135 nm), which are quite close to 13 THz. Thus, the extension of the emission spectrum toward the long wavelength from YDFL is ascribed to the stimulated Raman scattering (SRS). If we further increased the length of SMF above 150 m inside the laser cavity, the output spectrum of PML-YDFL did not broaden even more. Owing to the normal dispersion induced by the SMF (HI1060), the duration of NLPs broadens or even breaks, resulted in the peak power of NLPs decreasing after further increasing the length of the SMF inside the cavity. Then, the induced nonlinearity declined and the emission spectrum of YDFL shrunk as Lext was larger than 150 m.
With Lext = 150 m, the output pulse trace of YDFL in Fig. 5(b) shows that the time interval between sequential NLPs is about 764 ns, which corresponds to the 1.3 MHz repetition rate. Long-term operation of pulse trains is shown in the inset of Fig. 5(b). Figure 5(c) reveals the spectrum evolution of YDFL as the pump power increases. Once YDFL was mode-locked with pump power at 188 mW, the 1st-SW was generated. As the pump power increased, the depth of the gap between the pump wave and the 1st-SW declined apparently. When the pump power increased above 257 mW [green solid curve in Fig. 5 (c)], the 2nd-SW started to be excited. We can see the broadest emission spectrum and the shallowest gap between the pump and the stoke wave with pump power at 325 mW. It is attributed to the SPM and XPM effect which resulted from the high intensity of the ultrashort duration of the coherent spike. The linear scale of the optical spectrum from the YDFL in operation at the NLP-IV state is also shown in the inset of Fig. 5(c) with Ppump=325 mW. The measured output power (Pout) and estimated pulse energy (Eout) versus the pump power for the PML-YDFL in operation in the NLP-IV state are shown in Fig. 5(d). As Ppump increased from 188 mW to 325 mW, the Pout of YDFL increased linearly from 10.8 mW to about 57 mW that corresponded pulse energy Eout = 8.2 nJ and 35.1 nJ, respectively.
The beam quality of YDFL in operation in the NLP state is identified by the M-square. Figure 6(a) illustrates the variation of beam radius in the x-and y-directions (Wx:blue squares and Wy: red triangle) as a function of position (z). The intensity distribution of the projected beam onto the beam profiler reveals a circular shape as shown in the inset of Fig. 6(a). The evolution of the beam radius can be fitted by the formula :Fig. 6(a) are the fitting curves by Eq. (2). This illustrates that the beam waist in the x-axis and y-axis are 41.65 μm () and 45.25 μm (), respectively. Moreover, the values of M-square in the x- and y-directions are 1.09 () and 1.06 (), respectively. Furthermore, and can be obtained by the same measurement as the laser is operated at the NLP-IV state. This demonstrates that the YDFL in operation in the NLP state still reveals good beam quality.
In this work, we also compared the generated speckle noise from YDFL in operation in the CW state and NLP state. Figures 6(b)–(d) show the speckle imaging of YDFL in operation in CW state with a central wavelength of 1030 nm as well as in the NLP-I (1st-SW) and NLP-IV (2nd-SW) states with a relatively broad spectrum bandwidth. The speckle contrast of image was estimated by : , where is the root mean square (RMS) light intensity, N is the total pixel number of the digital image Iave is the average light intensity and In is the light intensity for each speckle. The grabbed image from the CW state in Fig. 6(b) reveals an obviously granular pattern with many dazzling spots resulting from the interference of multiple beam-let from the project plane. In comparison to that in operation in the CW state, the temporal coherence of YDFL in operation at NLP-I state reduced obviously owing to the coexistence of various sub-ps coherent spikes with random amplitude and phase. Thus, the generated dazzling spots from the projected beam become unapparent from a light source with broad spectrum bandwidth as shown in Figs. 6(c) and 6(d). By the estimation, the speckle contrast will reduce from 0.5675 in the CW state to 0.1959 in the NLP-I state. Owing to the broadening spectrum bandwidth in operation at NLP-I state, the estimated contrast is slightly lower than the C=0.216 for the dissipative soliton from the all normal dispersion YDFL. Furthermore, the speckle contrast fell dramatically to 0.0894 in the NLP-IV state with the occurrence of the 2nd-SW.
In order to suppress the speckle noise of amplified spontaneous emission (ASE) light source, a MMF  has been adopted to increase the number of mutually incoherent spatial modes that can produce distinct speckle patterns. By means of the summation from the intensity of speckle patterns, the contrast can be greatly reduced according to , where Nt is the number of spatial modes. For further diminishing the spatial coherence of NLP, a five-meter long MMF has been connected to the output of the YDFL. The grabbed speckle images from YDFL in operation in the CW, NLP-I and NLP-IV states are shown in Figs. 6(e)–(g). The estimated contrast (C) of CW, NLP-I and NLP-IV experienced a dramatic reduction, to 0.1958, 0.0402 and 0.0378. Thus, the produced light source can be applied in OCT measurement with almost no speckle noise.
The NLPs with broad spectrum bandwidth from a passively mode-locked Yb-doped fiber laser based on the NPR technique has been investigated in this work. As a result of stimulated Raman scattering, the noise-like pulse (NLP) with 3-dB broad spectrum bandwidth of around 65.2 nm has been demonstrated with superior beam quality, having M-squared values of about 1.09 and 1.06 in the x and y directions, respectively. After insertion of additional length of SMF inside the laser cavity to reduce the Raman threshold, the 2nd-SW can be generated to extend the spectrum of NLP toward the long wavelength at lower pump power. When the inserted length of SMF is around 150 m, the 1.3 MHz repetition rate NLP with 35.1 nJ pulse energy can reveal the most flattened and broadest emission spectrum from 1000 nm to 1160 nm with the pump power at 325 mW. In addition, the speckle imaging of YDFL was investigated to reveal a great reduction in speckle noise in operation in the NLP state with broad spectrum emission resulting from the generation 2nd-SW. To further reduce coherence and to obtain a lower speckle contrast value, a five-meter long multi-mode fiber was connected to excite more spatial modes and produce the low speckle image with contrast of about 0.0378. All the works indicated that the high beam quality YDFL with broad spectrum bandwidth through a MMF can be a promising light source for use in OCT measurement in the near future.
Ministry of Science and Technology of Taiwan (MOST) (105-2112-M-027-001-MY3); National Taipei University of Technology (NTUT-CGMH-106-09); Chang Gung Memorial Hospital (CGMH-NTUT-106-CORPG1G0011)
The authors would like to thank National Taipei University of Technology and Chang Gung Memorial Hospital Joint Research Program (NTUT-CGMH-106-09) and Chang Gung Memorial Hospital and National Taipei University of Technology Joint Research Program (CGMH-NTUT-106- CORPG1G0011) for grants supporting this study. This project was reviewed and approved by the Human Research Protections Program (IRB Number:201701028B0C501)
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