Open Access
Add to BibSonomy
Abstract
To facilitate a fiber-based supercontinuum generation system, single-mode fibers with different cutoff wavelengths are introduced to serve as shortpass filters to replace conventional reflective or transmissive filters. Meanwhile, an ytterbium-doped fiber amplifier is adopted to amplify the filtrated pulses, scaling their average power to the watt level up to 4.33 W. Through this approach, ultrashort high-power laser pulses of 1.56 µm and 1.06 µm wavelengths, which are commonly used in optical communications and industrial applications, can be generated by this single system. Furthermore, it is found that the noise-like pulses still maintain their temporal features, even after they undergo multiple optical processes including amplification, supercontinuum generation, and filtration. After that, the generated pulses at 1.06 µm were launched into a photonic crystal fiber to generate a supercontinuum of 1.85 W covering a spectral range from 560 nm in the visible region to 3.5 µm in the mid-infrared region. This is one of the widest records of spectrum in broadband supercontinuum generation.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Supercontinuum (SC) generation has been widely studied for decades due to its potential for a wide variety of optical applications, such as optical communications, microscopy, and optical coherence tomography [1–6]. Because various nonlinear optical effects, including self-phase modulation, cross-phase modulation, stimulated Raman scattering, Raman self-frequency shift, and four waves mixing, are involved in the SC generation process, the power of the pump laser is one of the most important factors that lead to a broad supercontinuum spectrum [7–9]. Laser amplifiers, such as ytterbium-doped and erbium-doped fiber amplifiers, are usually utilized to scale up the output power of the pump laser before pumping a nonlinear material for SC generation [10,11]. Meanwhile, this method also enhances the power density of the SC spectrum, providing a high-power broadband spectrum covering a wavelength range of hundreds or thousands of nanometers, for various applications [5,6].
In addition to the average optical power, the duration of the excitation laser pulses is another key factor that significantly influences nonlinear generation phenomena [7]. Mode locking has been widely investigated in order to obtain ultrashort optical pulse lasers, including well-defined pulses and similaritons [12–16], which have pulses of durations in the picosecond or femtosecond ranges with high peak powers to induce the nonlinear phenomena. The optical nonlinearity and the dispersion property of the nonlinear optical material also play important roles in SC generation. Highly nonlinear fibers (HNLFs) and photonic crystal fibers (PCFs) [7,9,11,17,18] are commonly used for SC generation. Various approaches have been taken to modify the composition and the structure of a fiber in order to obtain a highly nonlinear and dispersion-flattened fiber [19,20], which is helpful for generating a broadband and smooth SC spectrum. Researchers modify the dispersion profile of an optical fiber by doping germanium into a silica fiber with a relatively high concentration, and by manipulating the distribution or the diameters of air-holes in a PCF, for example. A high-power and ultra-broadband SC spectrum covering the range from 700 nm to 3.2 µm wavelengths was generated by pumping 10 W of nanosecond pulses at a center wavelength of 1.56 µm into a highly GeO2-doped fiber [10]. To extend the short-wavelength edge of the SC spectrum, it is necessary to use a nonlinear fiber that has a short zero-dispersion wavelength (ZDW) and to pump it with high-power pulses at a center wavelength near the ZDW. Therefore, scaling up the average power of the pulses at 1.06 µm by multiple stages of YDFA before launching these pulses into a PCF is a widely used method to extend the short-wavelength edge of SC spectra. An SC spectrum ranging from 480 nm to beyond 2 µm was generated by launching pulses with extremely high average power, 556 W, into a PCF with a ZDW at 1.04 µm [11]. Another method is to use a custom-designed PCF that has a ZDW shorter than those of the commercial PCFs. An SC spectrum covering the spectral range from 350 nm to 2.4 µm was also achieved by pumping 120-ps pulses at a repetition rate of 27 MHz with a peak power of 8.8 kW from a seed laser at a center wavelength of 1.016 µm into a self-designed seven-core PCF that has a ZDW at 991 nm [20].
Noise-like pulses (NLPs) are a special type of mode-locked pulses. Because of their characteristic spectral and temporal features, they are ideal laser sources for SC generation. Such pulses, also called double-scaled pulses, consist of picosecond wavepackets with inner structures of varying femtosecond pulses, making their autocorrelation trace double-scaled with a femtosecond peak riding upon a picosecond pedestal [21–23]. NLPs have spectra that are broader and smoother than other mode-locking pulses of the same wavepacket width, with tens to hundreds of nanometers wavelength coverage, illustrating low temporal coherence of the pulses [22–25]. This property makes it suitable for various applications in different areas, such as optical metrology and optical communications [26,27]. In addition, because dispersion only slightly affects the pulse duration of the NLPs, these pulses can propagate stably without distortion or being stretched [21,28]. As for the femtosecond inner structures of these pulses, it has been found that the nonlinear phenomena that are induced by NLPs are highly related to the duration of their inner structures [8,18]. Because of these temporal and spectral advantages, high-power and ultrashort NLPs are very favorable for generating broadband and smooth SC spectra [18,23,29,30]. In this field, high-power NLPs with a 920-fs wavepacket width and a 62-fs average temporal width of the inner structures at a center wavelength of 1.06 µm and an average power of 5 W were reported [31]. NLPs of a similar average output power of 5 W, with a wavepacket having a temporal width of 3.7 ps and inner structures with an average temporal duration of 14.5 fs, were also demonstrated [32]. Both of these ultrashort and high-power NLPs were generated by using grating to compress the amplified NLPs. Furthermore, in the all-fiber system regime, NLPs of 7.5 W average power with a 63-ps wavepacket width and a 92-fs average temporal width of the inner structures were also recorded [33].
In our previous study, a high-power and octave-spanning SC at a center wavelength of 1.56 µm generated by pumping two-stage-amplified NLPs into a 1-m long HNLF was demonstrated [18]. In this study, we report a newly-designed fiber-based system that demonstrates the capability of generating SC of a high average power up to 1.85 W by pumping with NLPs at a center wavelength near 1.06 µm at an average power of 4.33 W. These pump NLPs have an average temporal width of 17.7 ps for their wavepackets and an average temporal width of 77.8 fs for their inner structures. To the best of our knowledge, these are the smallest temporal widths of both the wavepacket and the inner structures of watt-level NLPs generated by a fiber-based system, without grating pairs to compress the pulse duration, at 1.06 µm center wavelength. These newly generated 1.06 µm NLPs are launched into a 7 m-long PCF to generate a high-power and ultra-broadband SC covering a spectral range from 560 nm wavelength in the visible to 3.5 µm in the mid-infrared. Through this approach, high-power and ultrashort NLPs with the two desirable center wavelengths of 1.56 µm and 1.06 µm are generated in one single fiber-based system. By using the NLPs at these two center wavelengths as the pump sources, two different octave-spanning SC spectra at watt level can be achieved in the same system: one with a spectrum covering the range from 940 nm to 2.3 µm and the other one from 560 nm to 3.5 µm. To date, the latter is the widest broadband SC spectrum generated by a seed laser with a center wavelength in near infrared region. Even though the spectrum may not be as smooth as expected, due to the high pumping power the SC spectral power intensity is higher than ‒20 dBm/nm throughout the entire wavelength range.
2. Experimental setup
Figure 1 shows a schematic illustration of the fiber-based system in this study. It consists of four parts including the first high-power and octave-spanning SC generation, 1.08 µm NLPs filtration, high-power and ultrashort 1.06 µm NLPs generation, and the second SC spectrum generation. The first part has been previously demonstrated [18]. In this part, an erbium-doped fiber amplifier (EDFA) and an erbium/ytterbium co-doped fiber amplifier (EYDFA) are used to boost the seed laser and generate high-power NLPs at 1.56 µm wavelength. Then the amplified NLPs are pumped into a 1-m long HNLF (OFS HNLF Zero-Slope) that has an effective area of 12.5 µm2 and a nonlinear coefficient of 10.7 W−1 km−1 at 1.55 µm to generate ultra-broadband SC with a high spectral power density. Subsequently, in the second part, two single-mode fibers with cutoff wavelengths at 780 and 630 nm, respectively, are used to filtrate pulses at wavelengths shorter than 1.1 µm from the high-power SC. After that, a YDFA (Amonics YDFA-40B-R) is introduced in the third part to amplify the filtrated pulses and to generate high-power and ultrashort NLPs at 1.06 µm. Finally, in the fourth part the newly generated NLPs at 1.06 µm are launched into a 7-m PCF (NKT SC-5.0-1040) to obtain the second high-power ultra-broadband SC spectrum.
Fig. 1. Configuration of the experiment setup. HNLF, highly nonlinear fiber; SMF, single-mode fiber; YDFA, ytterbium-doped fiber amplifier; PCF, photonics crystal fiber; NDF, neutral density filter.
An optical spectrum analyzer (Anritsu MS9740A) capable of detecting wavelength spectrum in the range from 600 nm to 1.75 µm is used to analyze the spectral features of the output optical pulses from the seed laser, the pre-amplifier, the booster, and the YDFA. To thoroughly characterize the ultra-broadbands of the two SC spectra, a monochromator (Digikröm CM110) together with a photodiode detector module (SP AD131) are used to analyze the spectrum ranging from 300 nm to 4 µm. A background-free intensity autocorrelator (Femtochrone FR-103XL) is used to record the auotocorrelation traces of the optical pulses, which are displayed on an oscilloscope. The pulse train is detected with a 1.5-GHz photodiode (Electro-Optics Technology ET-3010) and displayed on a 100-MHz oscilloscope (Agilent 54622A).
3. Results and analyses
3.1 Generation of supercontinuum with a high spectral power density
By using the same system configuration as we have previously demonstrated, NLPs at 1.56 µm wavelength with a repetition rate of 15.5 MHz are used as the pump laser source for the first-stage SC generation [18]. The autocorrelation trace shows one of the most important features of NLPs with a 376-fs peak appearing on a 48.7-ps pedestal with a pedestal-to-peak intensity ratio of 0.5. After two-stage amplification, the average power of the pulses reaches 4.5 W when using a pump power of 40 W. However, a pump power of 24 W is high enough for this study to generate NLPs at 1.56 µm with an average power of 2.49 W. Figure 2 shows the parameters of the amplified NLPs. A pulse train with a repetition rate of 15.5 MHz is shown in Fig. 2(a); this figure also illustrates the fluctuation of the wavepackets. The optical spectrum of these pulses is presented in Fig. 2(b); the smoothly broadened spectrum suggests that some nonlinear phenomena already occur during the two-stage amplification. Figure 2(c) and the inset present the double-scaled autocorrelation trace of the amplified pulses; the trace consists of a 88.3-fs peak upon a 89.2-ps pedestal with a pedestal-to-peak intensity ratio of 0.5. It is noted that in this study, the width of the peak and that of the pedestal of an autocorrelation trace for the optical pulses are defined as the full widths at half-maximum of their fitting curves, assuming sech2 as pulse shapes, which are shown as red curves in the plots [34]. The picosecond pedestal indicates that the temporal duration of the wavepackets is 57.8 ps, and the femtosecond peak suggests that the inner structures have an average temporal duration of 57.2 fs. Subsequently the pulses are launched into a 1-m-long HNLF, generating an SC spectrum that has an average power of 1.85 W and a spectral power density of above ‒20 dBm/nm covering a wavelength span from 1.031 to 2.066 µm, as shown by the black curve in Fig. 2(d). To make comparison with NLPs, well-defined pulses are also launched into the same system to generate SC of high spectral power density. Well-defined pulses at 1.56 µm at a repetition rate of 19 MHz, with a temporal duration of 29.6 ps, are also amplified up to 2.6 W by two stages of amplifier. Then, the amplified well-defined pulses are launched into the same 1-m-long HNLF to generate an SC spectrum, which has a spectral power density of above ‒20 dBm/nm covering the wavelength range from 999.7 nm to 2.296 µm, as shown by the blue curve in Fig. 2(d).
Fig. 2. (a) Pulse train, (b) spectrum, (c) autocorrelation trace of NLPs from the booster that is pumped at 24 W and (d) optical spectra of the supercontinua generated by pumping the amplified NLPs (black curve) and WDPs (blue curve) into a highly nonlinear fiber. The inset in (c) shows the magnification of the center peak in the autocorrelation trace. Red curves in (c) and its inset are the fitting curves of the pedestal and the peak for sech2 pulse shapes. NLP, noise-like pulses; WDP, well-defined pulses.
3.2 Filtering by using single-mode fibers with different cutoff wavelengths
The high-power SC generated in the first stage, as described above, is coupled into a 0.5-m-long single-mode fiber (SMF) that has a cutoff wavelength of 780 nm, which is defined as the shortest wavelength that can only be transmitted in the fundamental transverse mode, but not in any high-order mode, of the fiber. Because the core diameter and the index profile of an optical fiber depend on its cutoff wavelength, the coupling efficiency into the fiber decreases as the optical wavelength increases above the cutoff wavelength. The black curve in Fig. 3(a) shows the spectrum of the SC that is generated by NLPs and then coupled into and transmitted through the SMF. It shows that the power at wavelengths longer than 1.25 µm declines dramatically, indicating that the SMF of 780-nm cutoff can be used as a 1.25-µm short-pass filter. Subsequently, another 0.5-m-long SMF that has a cutoff wavelength of 630 nm is fused after the SMF of 780-nm cutoff. The red curve in Fig. 3(a) presents the filtrated spectrum through these two fused fibers. This spectrum has a center wavelength at 1.08 µm and a spectral width Δλ of 99.5 nm, which indicates that the SMF of 630-nm cutoff can serve as a 1.15-µm short-pass filter. The blue curve in Fig. 3(a) presents the spectrum of the SC that is generated by the well-defined pulses and then filtrated by the two SMFs. Both the NLPs and the well-defined pulses of these filtrated spectra have an average power of about 10 mW, which is sufficiently high for carrying out the subsequent steps. The green curve in Fig. 3(a) shows the spectral profile of the emission cross section of the ytterbium-doped fiber [35], which indicates that further filtration would take place during the subsequent amplification of the pulses by the ytterbium-doped fiber amplifier (YDFA). Note that the filtering experiment can also be carried out by fusing the SMF of 780-nm cutoff with the HNLF, which generates SC in the previous process. However, pulses escaping from the cladding of the SMF of 780-nm would be absorbed by the coating, causing the temperature of the fiber to rise. To construct an all-fiber system, this heating problem can be solved by cooling down the fiber, such as tapping a cooling fin on the fiber. In order to conveniently avoid the heating problem, however, lenses and mirrors are used in this experiment.
Fig. 3. (a) Optical spectrum of the NLPs filtrated by a single-mode fiber of 780 nm cutoff wavelength (black curve), and then by a single-mode fiber of 630 nm cutoff wavelength (red curve). Optical spectrum of the WDPs filtrated by the single-mode fibers of 780 nm and 630 nm cutoff wavelengths (blue curve). (b) Autocorrelation trace of the NLPs filtered from the high-power supercontinuum without chirping. (c) Dependence of the average temporal widths of wavepackets (black solid square) and the inner structures (red open square) of NLPs and temporal width of the WDPs (black solid triangle) on the length of SMF. The Green curve in (a) shows the profile of the emission cross section of ytterbium-doped fiber. The inset in (b) shows the magnification of the autocorrelation trace. Red curves in (b) and its inset are the fitting curves of the pedestal and the peak of the autocorrelation trace for sech2 pulse shapes. NLP, noise-like pulses; WDP, well-defined pulses; SMF, single-mode fiber.
Figure 3(b) shows the autocorrelation feature of the filtered NLPs. This autocorrelation trace has a 41.2-fs peak riding on a 54-ps pedestal, corresponding to average widths of 26.7 fs and 35 ps for the inner structures and the wavepackets, respectively. In addition to the autocorrelation trace, the significance of dispersion on the temporal profile of the filtered pulses are measured by adding SMF-28s of different lengths. Figure 3(c) shows the temporal widths of the wavepacket and inner structures of NLPs that are pre-chirped by SMFs of 0, 10, 20 and 50 m lengths, respectively. Generally, chromatic dispersion of the regular pulses that have a center wavelength of 1.08 µm and a 3-dB spectral width Δλ of 99.5 nm are significant when they propagate through the typical SMF that has a group-velocity dispersion of about ‒25 ps/nm/km. After being chirped by a 50-m-long SMF, the average temporal width of the wavepackets of the filtered NLPs is only stretched from 35 ps to 76.3 ps, and the average width of the inner structures is only stretched from 26.7 fs to 56.7 fs. By contrast, the temporal width of the well-defined pulses under the similar condition, having a center wavelength of 1.08 µm and a spectral width Δλ of 82 nm, is stretched from 29 ps to 133 ps. These experimental data indicate that group-velocity dispersion only has a limited effect on the pulse duration and the inner structures of the NLPs. Even after undergoing the multiple processes of SC generation, filtering, and dispersion, the NLPs still maintain their ultrashort temporal features. This resilient temporal characteristic of the NLPs as compared to well-defined pulses is favorable for efficient SC generation.
3.3 Generation of high-power and ultrashort 1.06 µm noise-like pulses
The filtrated pulses are launched into a YDFA (Amonics YDFA-40B-R). The average power of the amplified pulses increases with the drive current of the amplifier. When the drive current is set at 8 A, the average power of the filtrated pulses is scaled up to the watt level, generating new high-power NLPs of a center wavelength at 1.06 µm, which is different from the center wavelength of the seed laser at 1.56 µm. As shown in Fig. 4(a), for both NLPs and well-defined pulses, the amplification efficiency of the pulses that are pre-chirped with a 50-m SMF is higher than that of un-pre-chirped pulses. The average output power from the YDFA is scaled up to 5.4 W for the pre-chirped NLPs and 4.33 W for the un-pre-chirped NLPs. Similarly, the average output power from the YDFA is scaled up to 5.35 W for the pre-chirped well-defined pulses and 4.6 W for the un-pre-chirped well-defined pulses. Figure 4(b) presents the output spectra of using and not using a 50-m SMF to pre-chirp both kinds of pulse. For the un-pre-chirped NLPs and un-pre-chirped well-defined pulses, the long-wavelength end is remarkably extended to about 1.7 µm. For both pre-chirped NLPs and pre-chirped well-defined pulses, the long-wavelength end still exceeds 1.45 µm. These observations of spectral broadening of the amplified pulses indicate that significant nonlinear processes already take place in the YDFA while the ultrashort pulses are amplified.
Fig. 4. (a) Average power of the amplified output from YDFA versus the drive current of the amplifier for the 1.06 µm NLPs that are not chirped (black) and pre-chirped (red) and the WDPs that are not chirped (blue) and pre-chirped (green). (b) Optical spectra of the amplified NLPs that are not chirped (black) and pre-chirped (red) and the WDPs that are not chirped (blue) and pre-chirped (green). (c) Autocorrelation trace of the amplified high-power NLPs without pre-chirping, when the drive current is set at 8 A. The inset in (c) shows the magnification of the peak of the autocorrelation trace. Red curves in (c) and its inset are the fitting curves of the pedestal and the peak for sech2 pulse shapes. NLP, noise-like pulses; WDP, well-defined pulses.
The amplified un-pre-chirped NLPs at 1.06 µm demonstrate excellent properties including high power and ultrashort pulse duration. The autocorrelation trace presented in Fig. 4(c) and its inset show a 120-fs peak riding upon a 27.3-ps pedestal, corresponding to an average time duration of 77.8 fs for the inner structures and 17.7 ps for the wavepackets of these amplified NLPs. For the amplified un-pre-chirped well-defined pulses, an ultrashort pulsewidth of 20 ps is also obtained after the pulses pass through the same system. Nonlinear processes can easily take place during the amplification process because of the ultrashort pulsewidth, thus broadening the output spectrum. Taking the spectral width into consideration, the spectral width Δλ of the NLPs reduces from 99.7 nm before amplification, as mentioned in Section 3.2, to 20.7 nm after amplification because the gain bandwidth of YDFA is narrower than the spectral width of the filtrated pulses. By comparison, the spectral width of the well-defined pulses reduces from 82 nm to 29.5 nm. Generally, the pulse duration tends to be stretched after being amplified, but it is compressed here. Taking the un-pre-chirped NLPs for example, the temporal duration is compressed from 35 ps to 17.7 ps through amplification. Even for the pre-chirped well-defined pulses using a 50-m-long SMF for pre-chirping, the pulsewidth is also compressed through amplification from 133 ps to 33 ps. The positive relation between the spectral width and the temporal width of the wavepackets indicates that these picosecond pulses were highly chirped before being launched into the YDFA. Indeed, the broadening of the SC spectrum on the short-wavelength side shorter than the ZDW of the nonlinear fiber is dominated by the process of dispersion-wave generation. Therefore, the pulses that are filtrated from the short-wavelength part of SC spectrum are highly chirped with a large temporal width and a large spectral width, shown by the blue curve in Fig. 3(a). Because of the narrow gain-bandwidth of the YDFA, shown by the green curve in Fig. 3(a), only a small spectral width of the filtered SC spectrum is amplified, thus substantially narrowing the temporal width of the wavepackets by significantly reducing the chirp in the amplified pulses. By contrast, only the inner structures of the largest temporal widths at the short-wavelength edge are amplified by the YDFA, resulting in an increase of the average temporal width after amplification. Nonetheless, the narrower spectral width, compared with those described in Section 3.2, indicates that the pulse energy in the spectral region near 1.06 µm is still sufficiently high to bi-directionally broaden the spectrum to both shorter and longer wavelengths during the process of SC generation, as is seen below. In addition, a pedestal-to-peak intensity ratio of 0.5 seen in the autocorrelation trace shown in Fig. 4(c) indicates that the NLPs maintain their temporal features even after going through multiple stages of amplification, SC generation, filtering, and amplification again. Through this approach, the fiber-based system can generate high-power and ultrashort NLPs at two widely-used center wavelengths of 1.56 µm and 1.06 µm, making it versatile for further applications in different fields such as biomedical detection and gas sensing, etc.
3.4 Supercontinuum generation by using newly generated noise-like pulses
After being amplified by the YDFA, the newly generated NLPs at 1.06 µm peak wavelength are coupled into a PCF that has a ZDW at 1.04 µm for SC generation at the final stage. The center wavelength of these pump pulses at 1.06 µm, which is slightly longer than the ZDW of the PCF, matches the condition for efficiently broadening the SC spectrum because of the strong soliton dynamics in the process. Figure 5(a) shows the SC spectra that are generated by the NLPs from PCFs of different lengths of 7, 4, 2, 1 m for different output SC powers of 1.85, 2.28, 3.02, 2.98 W, respectively. The average input power of the un-pre-chirped NLPs for pumping the PCF is 4.33 W from the output of the YDFA at its maximal drive current of 8A. As seen in Fig. 5(a), thanks to the ultrashort temporal duration and the high power of these pulses, an octave-spanning spectrum ranging from 560 nm in the visible to 3.5 µm in the mid-infrared can be generated by using a 7-m-long PCF (black curve in the plot), which is one of the widest broadband SC spectra that are generated with seed lasers at center wavelengths of 1.56 µm and 1.06 µm. It can be seen from Fig. 5(a) that the SC spectrum narrows as the length of the PCF is reduced from 7 m to 2 or 1 m. In particular, the spectral power density in the visible region is significantly reduced. As shown by the blue and green curves in Fig. 5(a), the short wavelength edges of the SC spectra that are generated from the 2-m and 1-m PCFs reach 660 nm approximately. By comparison, the short wavelength edges of the SC spectra that are generated from the 7-m and 4-m PCFs reach 560 nm. Furthermore, in the spectral range from 687 nm to 900 nm, the spectral power densities of the SC spectra generated by the 2-m and 1-m PCFs are largely lower than those generated by the 7-m and 4-m PCFs. Therefore, a PCF of 4 or 7 m in length is necessary to extend the SC spectrum to the visible region. However, as the length of the PCF is increased, the power density of the SC spectrum declines and fluctuates in the long wavelength region due to the strong soliton fission of the pulses and the strong absorption in the PCF at wavelengths longer than 2.2 µm. For the wavelength near 2.4 µm, the spectral power density of the SC spectrum is decreased from 2 dBm/nm to ‒8.8 dBm/nm as the length of the PCF is increased from 1 m to 7 m. For the wavelength near 3 µm, the spectral power density of the SC spectrum is greatly and unevenly reduced from 0 dBm/nm to a low level of ‒16.4 dBm/nm as the length of the PCF is increased from 1 m to 7 m.
Fig. 5. (a) Optical spectra of the supercontinuua generated by NLPs, without pre-chirping, from different lengths of the photonic crystal fiber, 7 m (black), 4 m (red), 2 m (blue) and 1 m (green). (b) Optical spectra generated from a photonics crystal fiber of 4 meters by the pre-chirped NLPs (red) and WDPs (green); black and blue curves present the optical spectra generated by NLPs and WDPs without pre-chirping, respectively. NLP, noise-like pulses; WDP, well-defined pulses.
Therefore, to flatten the SC spectrum, a shortened PCF pumped by chirped pulses is necessary. Figure 5(b) presents the SC spectra that are generated by pumping a 4-m-long PCF with amplified NLPs, shown in red curve, and well-defined pulses, shown in green curve, that are pre-chirped with a 50-m SMF, and the corresponding average powers of the SC spectra are 2.95 W and 3.35 W. To make comparison, the SC spectra generated by pumping with un-pre-chirped amplified NLPs and un-pre-chirped well-defined pulses, shown in black and blue curves, respectively, are also presented, and the corresponding average powers of the SC spectra are 2.28 W and 2.71 W. The weakened soliton-related dynamics not only make the spectra broadened more evenly, but also stretch the distance needed for broadening the spectrum [7]. When the spectrum is broadened in the PCF, the remaining PCF length tends to only cause absorption, especially in the long-wavelength spectral region. In other words, if the required PCF length for broadening the SC spectrum is longer, the absorption caused by the remaining PCF could be reduced. Therefore, the SC spectrum can be flattened by pre-chirping the pulses to weaken the soliton-related dynamics while reducing the absorption caused by the PCF. As a result, for NLPs, the output power from the 4-m PCF is increased from 2.28 W to 2.95 W by pre-chirping. For WDPs, the output power from the 4-m PCF is increased from 2.71 W to 3.35 W. As can be seen from the red and green curves in the Fig. 5(b), the SC spectra that are generated by the amplified pre-chirped NLPs and well-defined pulses are relatively flattened, compared with the SC spectra that are generated by the amplified un-pre-chirped NLPs and well-defined pulses, shown in black and blue curves. For pumping with the amplified pre-chirped NLPs, the SC spectrum has an average power of 2.95 W with a spectral power density above ‒10 dBm/nm in the wavelength range from 672 nm in the visible to 3.081 µm in the mid-IR, and a spectral power density above ‒20 dBm/nm in the range from 580 nm to 3.304 µm. For pumping with the amplified pre-chirped well-defined pulses, the SC spectrum has an average power of 3.35 W with a spectral power density above ‒10 dBm/nm in the wavelength range from 673 nm to 3.152 µm, and a spectral power density above ‒20 dBm/nm in the range from 629 nm to 3.331 µm.
4. Conclusion
In this study, high-power and ultrashort NLPs at a center wavelength at 1.06 µm are generated, which are different from the seed laser pulses at a center wavelength at 1.56 µm, for significant improvement in high-power and octave-spanning SC generation. An SC spectrum with an output power of 1.85 W and a spectral power density above ‒20 dBm/nm in the wavelength range covering from 1.031 to 2.066 µm is achieved by pumping NLPs at 1.56 µm of 2.49 W average power into a HNLF. To facilitate a fiber-based system, two single-mode fibers with cutoff wavelengths at 780 and 630 nm, respectively, are used in tandem as short-pass filters to replace the conventional reflective or transmissive filters in free space. Subsequently, the filtered pulses of a center wavelength at around 1.08 µm, are amplified by a YDFA to generate high-power and ultrashort NLPs. The newly generated NLPs, without pre-chirping, have an average power of 4.33 W and a center wavelength of 1.06 µm with wavepackets and inner temporal structures of 17.7 ps and 77.8 fs average durations, respectively. To the best of our knowledge, these are the shortest records of wavepackets and inner structures for watt-level NLPs around 1.06 µm generated in fiber-based systems without grating pairs, which is widely used for compressing pulse duration, to date. These pulses are capable of generating a high peak power of 23.4 kW. Meanwhile, it is found that these NLPs maintain their temporal features even after they undergo multiple nonlinear processes through amplifications, SC generation, and filtering. The filtered NLPs pulses are resilient to chromatic dispersion. Both the average durations of their wavepackets and inner structures are only doubled after these pulses propagate through a 50-m SMF, even though the pulses have a broad spectral width of 99.7 nm. The newly-generated high-power and ultrashort NLPs of 4.33 W average power are launched into a 7-m long PCF to generate a second SC of 1.85 W average power with a wavelength span from 560 nm to 3.5 µm. This is one of the widest broadband spectrum ever generated by a seed laser that has a center wavelength in near infrared region. The SC spectrum can be flattened by reducing the length of the PCF to reduce the absorption of the PCF in the long-wavelength region, and by pre-chirping the pulses before amplification to lower the effect of soliton-related dynamics. By using these methods, a smooth SC spectrum can be generated to have a high spectral power density above ‒10 dBm/nm covering a broad wavelength range from 672 nm to 3.081 µm. Generally, it is difficult to extend the short-wavelength edge of an SC spectrum to the visible spectral region by beginning with pump pulses in the infrared, and the strong absorption of silica limits the long-wavelength edge of the SC spectrum generated through a PCF at about 2.4 µm. The ultra-broadband SC spectrum generated by this method covers spectral range from the mid-visible region to the mid-infrared region, which is useful for many spectroscopy applications.
Funding
Ministry of Science and Technology, Taiwan (MOST 109-2221-E-009-146).
Acknowledgements
The authors would like to thank the Ministry of Science and Technology (MOST), Taiwan, for the financial support and Higher Education Sprout Project of the National Yang Ming Chiao Tung University and Ministry of Education (MOE), Taiwan.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. H. Takara, T. Ohara, K. Mori, K. Sato, E. Yamada, Y. Inoue, T. Shibata, M. Abe, T. Morioka, and K.-I. Sato, “More than 1000 channel optical frequency chain generation from single supercontinuum source with 12.5 GHz channel spacing,” Electron. Lett. 36(25), 2089–2090 (2000). [CrossRef]
2. C. Dunsby, P. Lanigan, J. McGinty, D. Elson, J. Requejo-Isidro, I. Munro, N. Galletly, F. McCann, B. Treanor, and B. Önfelt, “An electronically tunable ultrafast laser source applied to fluorescence imaging and fluorescence lifetime imaging microscopy,” J. Phys. D: Appl. Phys. 37(23), 3296–3303 (2004). [CrossRef]
3. C. Kaminski, R. Watt, A. Elder, J. Frank, and J. Hult, “Supercontinuum radiation for applications in chemical sensing and microscopy,” Appl. Phys. B 92(3), 367–378 (2008). [CrossRef]
4. M. Booth, R. Juškaitis, and T. Wilson, “Spectral confocal reflection microscopy using a white light source,” JEOS:RP 3, 08026 (2008). [CrossRef]
5. S. Ishida and N. Nishizawa, “Quantitative comparison of contrast and imaging depth of ultrahigh-resolution optical coherence tomography images in 800–1700nm wavelength region,” Biomed. Opt. Express 3(2), 282–294 (2012). [CrossRef]
6. Y.-J. You, C. Wang, Y.-L. Lin, A. Zaytsev, P. Xue, and C.-L. Pan, “Ultrahigh-resolution optical coherence tomography at 1.3 µm central wavelength by using a supercontinuum source pumped by noise-like pulses,” Laser Phys. Lett. 13(2), 025101 (2016). [CrossRef]
7. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]
8. S.-S. Lin, S.-K. Hwang, and J.-M. Liu, “Supercontinuum generation in highly nonlinear fibers using amplified noise-like optical pulses,” Opt. Express 22(4), 4152–4160 (2014). [CrossRef]
9. S.-S. Lin, S.-K. Hwang, and J.-M. Liu, “High-power noise-like pulse generation using a 1.56-µm all-fiber laser system,” Opt. Express 23(14), 18256–18268 (2015). [CrossRef]
10. D. Jain, R. Sidharthan, P. M. Moselund, S. Yoo, D. Ho, and O. Bang, “Record power, ultra-broadband supercontinuum source based on highly GeO2 doped silica fiber,” Opt. Express 24(23), 26667–26677 (2016). [CrossRef]
11. L. Zhao, Y. Li, C. Guo, H. Zhang, Y. Liu, X. Yang, J. Wang, F. Jing, and G. Feng, “Generation of 215 W supercontinuum containing visible spectra from 480 nm,” Opt. Commun. 425, 118–120 (2018). [CrossRef]
12. J. W. Nicholson, A. Yablon, P. Westbrook, K. Feder, and M. Yan, “High power, single mode, all-fiber source of femtosecond pulses at 1550 nm and its use in supercontinuum generation,” Opt. Express 12(13), 3025–3034 (2004). [CrossRef]
13. J. Takayanagi, N. Nishizawa, H. Nagai, M. Yoshida, and T. Goto, “Generation of high-power femtosecond pulse and octave-spanning ultrabroad supercontinuum using all-fiber system,” IEEE Photonics Technol. Lett. 17(1), 37–39 (2005). [CrossRef]
14. B. Kibler, C. Billet, P.-A. Lacourt, R. Ferriere, and J. Dudley, “All-fiber source of 20-fs pulses at 1550 nm using two-stage linear-nonlinear compression of parabolic similaritons,” IEEE Photonics Technol. Lett. 18(17), 1831–1833 (2006). [CrossRef]
15. J. Peng, L. Zhan, T. Chen, Z. Gu, K. Qian, S. Luo, and Q. Shen, “All-fiber ultrashort similariton generation, amplification, and compression at telecommunication band,” J. Opt. Soc. Am. B 29(9), 2270–2274 (2012). [CrossRef]
16. Y. Nozaki, Y. Nomura, M. Aramaki, and N. Nishizawa, “Octave spanning coherent supercontinuum generation using 51 fs high-power ultrashort pulse from Er-doped similariton amplifier,” Jpn. J. Appl. Phys. 53(2), 020301 (2014). [CrossRef]
17. H. Xia, H. Li, G. Deng, J. Li, S. Zhang, and Y. Liu, “Compact noise-like pulse fiber laser and its application for supercontinuum generation in highly nonlinear fiber,” Appl. Opt. 54(32), 9379–9384 (2015). [CrossRef]
18. K.-Y. Chang, W.-C. Chen, S.-S. Lin, S.-K. Hwang, and J.-M. Liu, “High-power, octave-spanning supercontinuum generation in highly nonlinear fibers using noise-like and well-defined pump optical pulses,” OSA Continuum 1(3), 851–863 (2018). [CrossRef]
19. T. Saini, A. Baili, V. Dahiya, A. Kumar, R. Cherif, M. Zghal, and R. Sinha, “Design of equiangular spiral photonic crystal fiber for supercontinuum generation at 1550 nm,” Proc. SPIE 9200, 920012 (2014). [CrossRef]
20. X. Qi, S. Chen, Z. Li, T. Liu, Y. Ou, N. Wang, and J. Hou, ““High-power visible-enhanced all-fiber supercontinuum generation in a seven-core photonic crystal fiber pumped at 1016 nm,” Opt. Lett. 43(5), 1019–1022 (2018). [CrossRef]
21. M. Horowitz, Y. Barad, and Y. Silberberg, “Noiselike pulses with a broadband spectrum generated from an erbium-doped fiber laser,” Opt. Lett. 22(11), 799–801 (1997). [CrossRef]
22. A. Zaytsev, C. Lin, Y. You, F. Tsai, C. Wang, and C. Pan, “A controllable noise-like operation regime in a Yb-doped dispersion-mapped fiber ring laser,” Laser Phys. Lett. 10(4), 045104 (2013). [CrossRef]
23. Y. Jeong, L. A. Vazquez-Zuniga, S. Lee, and Y. Kwon, “On the formation of noise-like pulses in fiber ring cavity configurations,” Opt. Fiber Technol. 20(6), 575–592 (2014). [CrossRef]
24. D. Tang, L. Zhao, and B. Zhao, “Soliton collapse and bunched noise-like pulse generation in a passively mode-locked fiber ring laser,” Opt. Express 13(7), 2289–2294 (2005). [CrossRef]
25. J. Lauterio-Cruz, J. Hernandez-Garcia, O. Pottiez, J. Estudillo-Ayala, E. Kuzin, R. Rojas-Laguna, H. Santiago-Hernandez, and D. Jauregui-Vazquez, “High energy noise-like pulsing in a double-clad Er/Yb figure-of-eight fiber laser,” Opt. Express 24(13), 13778–13787 (2016). [CrossRef]
26. S. Keren and M. Horowitz, “Interrogation of fiber gratings by use of low-coherence spectral interferometry of noiselike pulses,” Opt. Lett. 26(6), 328–330 (2001). [CrossRef]
27. S. Keren, E. Brand, Y. Levi, B. Levit, and M. Horowitz, “Data storage in optical fibers and reconstruction by use of low-coherence spectral interferometry,” Opt. Lett. 27(2), 125–127 (2002). [CrossRef]
28. S. Smirnov, S. Kobtsev, S. Kukarin, and A. Ivanenko, “Three key regimes of single pulse generation per round trip of all-normal-dispersion fiber lasers mode-locked with nonlinear polarization rotation,” Opt. Express 20(24), 27447–27453 (2012). [CrossRef]
29. S. Kobtsev, S. Kukarin, S. Smirnov, and I. Ankudinov, “Cascaded SRS of single-and double-scale fiber laser pulses in long extra-cavity fiber,” Opt. Express 22(17), 20770–20775 (2014). [CrossRef]
30. H. Chen, X. Zhou, S.-P. Chen, Z.-F. Jiang, and J. Hou, “Ultra-compact Watt-level flat supercontinuum source pumped by noise-like pulse from an all-fiber oscillator,” Opt. Express 23(26), 32909–32916 (2015). [CrossRef]
31. C. Xu, J.-R. Tian, R. Xu, Y. Wu, L. Fan, J. Guo, and Y.-R. Song, “Generation of noise-like pulses with a 920 fs pedestal in a nonlinear Yb-doped fiber amplifier,” Opt. Express 27(2), 1208–1216 (2019). [CrossRef]
32. R.-Q. Xu, J.-R. Tian, and Y.-R. Song, “Noise-like pulses with a 14.5 fs spike generated in an Yb-doped fiber nonlinear amplifier,” Opt. Lett. 43(8), 1910–1913 (2018). [CrossRef]
33. E. Aghayari and K. J. Ghaleh, “High-power supercontinuum generation by noise-like pulse amplification in Yb-doped fiber amplifier operating in a nonlinear regime,” Appl. Opt. 58(15), 4020–4024 (2019). [CrossRef]
34. J.-C. M. Diels, J. J. Fontaine, I. C. McMichael, and F. Simoni, “Control and measurement of ultrashort pulse shapes (in amplitude and phase) with femtosecond accuracy,” Appl. Opt. 24(9), 1270–1282 (1985). [CrossRef]
35. R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbium-doped fiber amplifiers,” IEEE J. Quantum Electron. 33(7), 1049–1056 (1997). [CrossRef]
