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Abstract
High-power, octave-spanning supercontinuum generation through pumping a highly nonlinear fiber with noise-like and well-defined optical pulses is investigated. A two-stage fiber amplifier, consisting of a pre-amplifier and a booster, is used to greatly enhance the average power of noise-like pump pulses from 14 mW to 13.1 W. Owing to the limited coupling efficiency, only a maximum optical power of 4.63 W is launched into the nonlinear fiber. As a result, a supercontinuum spectrum spanning from 940 to 2300 nm with an average power of 3.82 W is achieved with noise-like pump pulses. This is, to the best of our knowledge, the highest average power obtained over such an octave-spanning supercontinuum spectrum using noise-like pump pulses. Supercontinuum generation with a broader spectral range and a higher average power is feasible if the coupling efficiency of the pump pulse power launched into the nonlinear fiber is increased. For the purpose of comparison, well-defined pump pulses with a similar average power at a similar repetition rate are also investigated for similar octave-spanning supercontinuum generation. A supercontinuum spectrum spanning from 950 to 2500 nm with an average power of 3.62 W is obtained with well-defined pump pulses. Even though the spectrum of the resulting supercontinuum using the well-defined pump pulses is broader, a large portion of its optical power is concentrated around the center wavelength, leading to a lower spectral power density at other wavelengths; furthermore, a higher extent of spectral structure appears in its optical spectrum, resulting in a higher power variation among different wavelengths.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
There has been continuous research interest in supercontinuum generation through pumping nonlinear fibers with ultrashort optical pulses owing to its profound underlying mechanisms and revolutionary technological applications [1–6]. Depending on the features of the pump pulses, various nonlinear phenomena are involved in the process of supercontinuum generation, including stimulated Raman scattering, four-wave mixing, self-phase modulation, dispersive wave generation, Raman self-frequency shift, and soliton fission [7]. The unique characteristics of the resulting supercontinua, which typically consist of spectra over hundreds or even thousands of nanometers, find their promising applications in various areas, such as optical communications, optical coherence tomography, and optical frequency metrology [8–10].
To generate an octave-spanning spectrum with a high spectral power density through supercontinuum generation in a short nonlinear fiber, pump optical pulses that have high pulse energies ranging from tens to hundreds of nanojoules per pulse are usually required. The typical pulse energy of pump optical pulses from a laser oscillator commonly used for supercontinuum generation is, however, only sub-nanojoules or a few nanojoules. For high-power, broad-spectrum supercontinuum generation, strong amplification of the pump optical pulses before they are launched into a nonlinear fiber is necessary. Well-defined optical pulses are commonly adopted as pumps for supercontinuum generation. Before amplification, these pulses are temporally stretched using additional dispersive media so that their peak power is reduced in order to minimize unnecessary nonlinear effects when they propagate through an optical amplifier; this process is particularly necessary for well-defined optical pulses that have temporal durations of a few to hundreds of femtoseconds [1–6]. After amplification, they are then temporally compressed using extra dispersive media to enhance their peak power for supercontinuum generation [1–6]. The structure and operation of such a system for the generation of a supercontinuum to achieve an octave-spanning spectrum and a high spectral power density are highly complicated.
Noise-like optical pulses have lately been proposed as pumps for supercontinuum generation in nonlinear fibers [11–16] in order to take advantage of their smooth and broad spectra of tens to hundreds of nanometers [17–22]. These optical pulses consist of picosecond wavepackets with a fine inner structure of femtosecond pulses that have stochastically varying temporal durations and peak powers. Such a temporal feature results in a double-scaled autocorrelation trace with a femtosecond peak riding upon a wide and smooth picosecond pedestal. This suggests a feature of low temporal coherence, which has been utilized for applications in, for example, optical coherence tomography, optical sensing, optical communications, and optical metrology [23–28]. It has been demonstrated both numerically and experimentally that the temporal duration of noise-like optical pulses can hardly be stretched or compressed by the dispersive effect [17,19,20]. For the purpose of supercontinuum generation with a broad spectrum and a high spectral power density, this suggests that the processes of temporal stretching before amplification and temporal compression after amplification commonly required for well-defined pump pulses are not necessary for noise-like pump pulses. By taking advantage of this unique feature, we previously demonstrated that supercontinuum generation with a spectrum spanning from 1208 to 2111 nm and an average power of about 180 mW was achieved using a 1-m nonlinear fiber pumped by noise-like optical pulses at around 1560 nm [15]. The power (pulse energy) of the noise-like pump pulses was enhanced from 14 mW (0.8 nJ) to 202 mW (13.5 nJ) by a single-stage amplifier using erbium-doped gain fibers without considering any temporal pre-stretching or post-compression.
Yet, a supercontinuum of a broader optical spectrum and a higher spectral power density is preferred for many applications, such as broadband optical sources for various biomedical applications requiring different frequency bands and ranges. This strong application demand has continuously motivated intensive research works to develop various supercontinuum generation systems based on, for example, different nonlinear fibers, different amplification schemes, and/or different pumping approaches, for a broader optical spectrum and a higher spectral power density [29–32]. For this purpose, we have developed a second-stage amplifier using erbium/ytterbium-codoped gain fibers for optical pulses at around 1560 nm [22]. The power conversion efficiency of the second-stage amplifier was optimized through proper control of its pump wavelength. As a result, the power (pulse energy) of the noise-like optical pulses after a two-stage amplification process was strongly enhanced by about 30 dB to 13.1 W (850 nJ). By taking advantage of this significant advance in the power and pulse energy of the amplified optical pulses, in this study, the power of noise-like pump pulses launched into a highly nonlinear fiber is greatly enhanced from 14 mW to 4.63 W to generate a high-power broad-band supercontinuum that has an average power of 3.82 W and an octave-spanning spectrum covering wavelengths from 940 to 2300 nm. To the best of our knowledge, this is the highest average power obtained over such an octave-spanning spectrum through supercontinuum generation using noise-like pump pulses. The spectral power density is therefore highly enhanced. In addition, for the purpose of comparison under a similar pulse energy, well-defined pump pulses with a similar average power at a similar repetition rate are also used in this study for similar octave-spanning supercontinuum generation. A supercontinuum spectrum that has an average power of 3.62 W and a spectrum spanning from 950 to 2500 nm is obtained with well-defined pulses by launching 4.53 W into the same nonlinear fiber.
2. Experimental setup
Fig. 1 Schematics of (a) supercontinuum generation system, (b) noise-like pulse laser, (c) well-defined pulse laser, (c) pre-amplifier, and (d) booster. HNLF, highly nonlinear fiber; PLD, pump laser diode; WDM, wavelength-division multiplexer; DCF, dispersion compensation fiber; PC, polarization controller; PDI, polarization-dependent isolator; PII, polarization-independent isolator; EDF, erbium-doped fiber; EYDF, erbium/ytterbium-codoped double-clad fiber; SMF, single-mode fiber; MFA, mode-field adaptor; PS, power stripper.
The seed laser is either a noise-like or a well-defined pulse laser, as presented in Figs. 1(b) and 1(c), respectively. The noise-like pulse laser shown in Fig. 1(b) is a fiber ring laser that has a cavity length of about 7.5 m. A 0.6-m erbium-doped fiber (Liekki ER80-8/125) with a peak absorption coefficient of 80 dB/m at 1530 nm and a group-velocity dispersion coefficient of 15.7 ps/km-nm is used as the gain medium. A laser diode (JDSU S30-7602-660) operating at 976 nm with an output power of 480 mW is used to pump the gain medium through a 980-nm/1550-nm wavelength-division multiplexing coupler. To ensure unidirectional propagation and polarization selection of optical pulses, a polarization-dependent isolator between two in-line polarization controllers is used. Together with the two polarization controllers, a nonlinear switching mechanism is achieved through nonlinear polarization rotation for the generation of optical pulses. A 3-m dispersion compensation fiber with a group-velocity dispersion coefficient of −97.7 ps/km-nm at 1550 nm is used for other research purposes, which are not our consideration in this study. Noise-like optical pulses are generated by properly adjusting the polarization controllers with or without the dispersion compensation fiber [17]. An average power of 14 mW, about 10% of the intracavity optical power, is coupled out of the ring cavity and sent to the two-stage fiber amplifier. A polarization-independent optical isolator at the output of the ring cavity is used to reject optical back-reflection from the two-stage fiber amplifier.
For the well-defined pulse laser presented in Fig. 1(c), a similar fiber ring laser that has a cavity length of about 5.5 m is constructed using devices and components of the same models described above. The repetition rate of this laser increases approximately linearly from 19 MHz to 133 MHz as its pump power is enhanced from 114 mw to 240 mW. In this study, for a fair comparison between the supercontinua generated using noise-like and well-defined pulses, respectively, a similar pulse energy is obtained for each type of the pulses before they are launched into the highly nonlinear fiber so that the differences observed in this study result not from the differences in their pulse energy, at least to the best we could, but from the differences in their pulse nature. To achieve a similar pulse energy, both the noise-like and the well-defined pulse lasers are operated under a similar repetition rate and a similar average power after each stage of amplification. To keep the repetition rate close to that of the noise-like pulses, i.e. 15.5 MHz, in this study, the well-defined pulse laser consisting of a 0.52-m erbium-doped fiber as its gain medium is pumped at 114 mW accordingly, which is much lower than the power used to pump the noise-like pulse laser described above, in order to achieve a repetition rate of 19 MHz. This results in a laser output power of 720 µW. For stable generation of well-defined optical pulses, a 1.49-m dispersion compensation fiber is used. It is a common practice to temporally stretch well-defined optical pulses using a dispersive medium before amplification in order to lower their peak power and therefore to avoid inducing excessive nonlinear effects in the amplifier. Therefore, at the ring-cavity output of the well-defined pulse laser in this study, a 50-m single-mode fiber that has a group-velocity dispersion coefficient of 18 ps/km-nm is used as the dispersive medium for the purpose of temporal stretching. As noted in Fig. 1(b), such temporal stretching is not conducted for noise-like optical pulses in this study because it is not effective and thus not necessary.
The two-stage fiber amplifier consists of a first-stage pre-amplifier and a second-stage booster. For the pre-amplifier presented in Fig. 1(d), an erbium-doped fiber (Liekki ER80-8/125) of 1.2 m length is used as the gain medium, and is bidirectionally pumped by two 976-nm laser diodes (JDSU S30-7602-660 and S30-7602-600) through two 980-nm/1550-nm wavelength-division multiplexing couplers. To eliminate optical reflection back to the seed laser from the pre-amplifier or back to the pre-amplifier from the booster, two polarization-independent isolators are separately placed at the input and output ports of the pre-amplifier. For the booster shown in Fig. 1(e), a 1.5-m erbium/ytterbium-codoped double-clad fiber (Nufern LMA-EYDF-25P/300-HE) is used as the gain medium. Note that a convenient way to scale up the power level of a rare-earth-doped fiber amplifier is to use a double-clad gain fiber, in which the signal and the pump respectively propagate in the core and the inner cladding, to alter the power capacity. This requires the use of a large-mode-area fiber of a 25-µm core diameter in the booster rather than a standard single-mode fiber of a 8-µm core diameter as used in the pre-amplifier. To connect the pre-amplifier and the booster that use fibers of different core diameters, a mode-field adaptor is therefore employed. A power stripper is placed between the double-clad gain fiber and the mode-field adaptor in order to prevent the pre-amplifier from being damaged by optical signals coming from the cladding layer of the booster. Two high-power fiber-coupled diode lasers (Oclaro BMU25A-975-01-R03) are used as the pumping sources with heat sinks for properly controlling temperature to stabilize and adjust the emission wavelengths of these diode lasers for maximum power conversion efficiency. A dual-backward pumping scheme [22] is adopted in this study to achieve the highest possible power conversion efficiency. A signal–pump power combiner, which includes two fiber pigtails for coupling the two pumps into the inner cladding layer of the gain fiber, is therefore connected to the back end of the double-clad gain fiber. A standard single-mode fiber with a core diameter of 8 µm is used at the end of the booster to connect the booster with the highly nonlinear fiber. To match the different core diameters between the standard single-mode fiber used here and the large-mode-area fibers used elsewhere in the booster, a mode-field adaptor following the power combiner is thus employed. Owing to the limited coupling efficiency of about 35% that can be achieved between the two fibers of very different core diameters, significant optical power loss happens at the mode-field adaptor. As a result, while a maximum optical power of 13.1 W is achieved after the two-stage amplification, only a maximum optical power of about 4.63 W is coupled from the output of the booster into the highly nonlinear fiber.
An optical spectrum analyzer (Anritsu MS9740A) covering a spectral range from 600 to 1750 nm is used to analyze the spectral features of optical pulses at the outputs of the seed laser, pre-amplifier, booster, and highly nonlinear fiber. To fully characterize the ultrabroad band of a supercontinuum obtained at the output of the highly nonlinear fiber, another optical spectrum analyzer (APE WaveScan) covering a spectral range from 1000 to 2600 nm is also used. The pulse train is recorded with a 1.5-GHz photodiode (Electro-Optics Technology ET- 3010) and displayed on a 100-MHz oscilloscope (Agilent 54622A). A background-free intensity autocorrelator (Femtochrone FR-103XL) is used to study the auotocorrelation traces of optical pulses.
3. Results and analyses
3.1 Pump pulses before amplification
At the output of the noise-like pulse laser, a pulse train at a repetition rate of 15.5 MHz with a randomly varying peak power among pulses is generated, as presented in Fig. 2(a)
Fig. 2 (a) Pulse train, (b) optical spectrum, and (c) autocorrelation trace of noise-like optical pulses from the output of the laser oscillator. The inset in (c) shows the magnification of the autocorrelation trace. Red curves in (c) and its inset are sech2 fitting of the pedestal and the peak, respectively.
At the output of the well-defined pulse laser, a pulse train with a relatively constant peak power at the 19 MHz fundamental repetition rate of the laser is generated, as shown in Fig. 3(a)
Fig. 3 (a) Pulse train, (b) optical spectrum, and (c) autocorrelation trace of well-defined optical pulses from the output of the laser oscillator. The red curve in (c) is sech2 fitting of the trace.
3.2 Supercontinuum generation using pump pulses after one-stage amplification
The noise-like optical pulses with an average power of 14 mW from the laser oscillator are sent directly into the pre-amplifier without any temporal pre-stretching. Their average power increases linearly with the pump power of the pre-amplifier up to 202 mW at the maximum pump power of 1088 mW, an optical gain of about 12 dB. For the purpose of comparison between the supercontinua generated by noise-like and well-defined optical pulses, some features of the noise-like optical pulses are kept similar to those of the well-defined optical pulses right before pumping the highly nonlinear fiber, such as the repetition rate and the optical power. Therefore, in this study, the pump power of the pre-amplifier for the noise-like optical pulses is set at 908 mW, leading to an output power of 190 mW for the noise-like optical pulses at a repetition rate of 15.5 MHz. The optical spectrum of the noise-like optical pulses after such amplification broadens significantly, as shown in Fig. 4(a)
Fig. 4 (a) Optical spectrum and (b) autocorrelation trace of noise-like optical pulses from the output of the pre-amplifier. The inset in (b) shows the magnification of the autocorrelation trace. Red curves in (b) and its inset are sech2 fitting of the pedestal and the peak, respectively.
The well-defined optical pulses, on the other hand, are sent into the pre-amplifier after temporal pre-stretching by the 50-m single-mode fiber addressed above. Their average power also increases linearly with the pump power of the pre-amplifier. An output power of about190 mW can be achieved at the maximum pump power of 1088 mW; this 190 mW output power is the same as that of the noise-like optical pulses pumped at 908 mW. As shown in Fig. 5(a)
Fig. 5 (a) Optical spectrum and (b) autocorrelation trace of well-defined optical pulses from the output of the pre-amplifier. The red curve in (b) is sech2 fitting of the trace.
To compare with the supercontinua generated by using the noise-like and well-defined pump pulses after the second-stage amplification addressed in the following section, we first examine the supercontinua generated using pulses after the first-stage amplification, where the power of either type of pump pulses is kept at 190 mW. As presented in Fig. 6
Fig. 6 Optical spectra of supercontinua generated by using noise-like (black curve) and well-defined (red curve) pump pulses, respectively, after the first-stage amplification.
3.3 Supercontinuum generation using pump pulses after two-stage amplification
After the pre-amplifier, the noise-like optical pulses at a repetition rate of 15.5 MHz with an average power of 190 mW shown in Fig. 4 are sent into the booster under dual-backward pumping. Their average power continues to increase with the pump power of the booster at a linear rate. An amplified output power of about 13.1 W for the noise-like optical pulses, corresponding to a pulse energy of 850 nJ, is achieved from the booster at the maximum booster pump power of 40 W, an optical gain of about 18 dB. As pointed out in the introduction, however, only an optical power of about 4.63 W of the 13.1 W from the output of the booster is coupled into the single-mode fiber at the end of the booster to connect the booster with the highly nonlinear fiber owing to the limited coupling efficiency, about 35%, of the mode-field adaptor used in this study. The optical spectrum of the noise-like opticalpulses at the output of the booster is presented in Fig. 7(a)
Fig. 7 (a) Optical spectrum and (b) autocorrelation trace of noise-like optical pulses from the output of the booster pumped at 40 W. (c) Width of the peak (black symbols) and the pedestal (blue symbols) in the autocorrelation trace in terms of the pump power of the booster. The inset in (b) shows the magnification of the autocorrelation trace. Red curves in (b) and its inset are sech2 fitting of the pedestal and the peak, respectively.
The well-defined optical pulses at a repetition rate of 19 MHz with an average power of 190 mW shown in Fig. 5 are also sent to the booster under dual-backward pumping. Their average power continues to increase linearly with the pump power of the booster. Similar to the situation discussed above for the amplified noise-like pulses, only a maximum optical power of about 4.53 W from the output of the booster is coupled into the single-mode fiber that connects the booster to the highly nonlinear fiber because of the limited coupling efficiency of the mode-field adaptor used in this study. The optical spectrum of the well-defined optical pulses at the output of the booster is presented in Fig. 8(a)
Fig. 8 (a) Optical spectrum and (b) autocorrelation trace of well-defined optical pulses from the output of the booster pumped at 40 W. (c) Width of the peak (black symbols) and the pedestal (blue symbols) in the autocorrelation trace in terms of the pump power of the booster. The inset in (b) shows the magnification of the autocorrelation trace. Red curves in (b) and its inset are sech2 fitting of the pedestal and the peak, respectively.
After the second-stage amplification, the noise-like and well-defined pulses are, respectively, launched into the highly nonlinear fiber for supercontinuum generation without any temporal post-compression. While the temporal post-compression is not effective and thus not necessary for the noise-like pump pulses, it is also not conducted for the well-defined pump pulses in this study as their duration broadens considerably soon after they are launched into the nonlinear fiber. Figure 9
Fig. 9 Optical spectra of supercontinua generated by using noise-like (black curves) and well- defined (red curves) pump pulses, respectively, at four different power levels, as indicated in the upper-right corner of all plots, through fixing the pump power of the booster at 12, 24, 32, and 40 W, respectively. NL, noise-like pump pulses; WD, well-defined pump pulses.
4. Conclusion
Octave-spanning supercontinuum generation with a high average power at the watt level through pumping a highly nonlinear fiber with noise-like optical pulses is investigated. To generate an octave-spanning supercontinuum spectrum, a two-stage fiber amplifier, consisting of a pre-amplifier with a maximum gain of 12 dB and a booster with a maximum gain of 18 dB, is used to greatly enhance the power of the noise-like pump pulses launched into the highly nonlinear fiber. Temporal stretching before amplification and temporal compression after amplification using additional dispersive media is not considered because this process is not effective and thus not necessary for the noise-like pump pulses. By taking advantage of such two-stage amplification, the power of the noise-like pump pulses is greatly enhanced from 14 mW to 13.1 W. Owing to the limited coupling efficiency from the double-clad gain fiber of the second-stage booster amplifier to the single-mode fiber that connects the highly nonlinear fiber, only a maximum optical power of 4.63 W is launched into the nonlinear fiber. As a result, a supercontinuum that has an average power of 3.82 W and a broad spectrum spanning from 940 to 2300 nm is achieved. This is, to the best of our knowledge, the highest average power obtained over such an octave-spanning supercontinuum spectrum using noise-like pump pulses. For the purpose of comparison, well-defined pump pulses with a similar average power at a similar repetition rate are also investigated for similar octave-spanning supercontinuum generation. A supercontinuum spectrum that has an average power of 3.62 W and a spectrum spanning from 950 to 2500 nm is obtained. Even though the supercontinuum generated with the well-defined pump pulses has a broader spectrum than that generated with the noise-like pump pulses, a large portion of its optical power is concentrated around the center wavelength, leading to a lower spectral power density at other wavelength regions. The supercontinuum generated with the well-defined pump pulses also has larger variations in its spectral structure, resulting in higher power variations among different wavelengths, than that generated with the noise-like pump pulses.
Funding
Ministry of Science and Technology of Taiwan (MOST106-2112-M-006-004-MY3).
Acknowledgments
This work was financially supported by the Research Team of Photonic Technologies and Intelligent System at NCTU within the framework of Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.
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