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Abstract
We experimentally demonstrate an all-fiber, ultraviolet-enhanced, supercontinuum generation in a specifically designed seven-core photonic crystal fiber pumped by a picosecond Yb-doped master oscillator power amplifier (MOPA). The MOPA source is seeded by a giant-chirped Yb-doped mode-locked fiber laser operating in the dissipative-soliton-resonance (DSR) region. The DSR is achieved by using a nonlinear optical loop mirror (NOLM) with a fundamental repetition rate of 4.5 MHz and a central wavelength of 1035 nm. An extremely wide optical spectrum spanning from 350 nm to 2400 nm is obtained with a total output power of 6.86 W.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The supercontinuum sources with spectra from ultraviolet to mid-infrared region offer a wide range of attractive applications like frequency metrology, micro-spectroscopy [1,2]. In particular, the visible part (380 nm~800 nm) of the supercontinuum sources plays a very important role in biomedical and medical applications [3–6]. However, most of the traditional supercontinuum sources are limited on the short-wavelength edge, normally around 450 nm [7]. Many efforts have been devoted to extend the supercontinuum spectrum into the ultraviolet region and to enhance the intensity of the visible part of supercontinuum sources. In the picosecond pump regime, the spectral broadening is initially driven by modulation instability that breaks the pump pulse into shorter sub-pulses and brings more dispersive waves into visible part [1]. The supercontinuum spectrum is determined in a complex process in which the Raman-redshifting-solitons trap the dispersive waves and then force them to blueshift with the same group velocity of redshifting solitons [8]. With this mechanism, the short-wavelength edge of the supercontinuum can be predicted by the group velocity profile. Moreover, the pump wavelength is required to approach the zero-dispersion wavelength (ZDW) of the photonic crystal fiber (PCF). If the ZDW shifts away from the pump wavelength, the intensity of the visible part of supercontinuum source decreases correspondingly [7]. Meanwhile, when a chirped pulse propagates in the anomalous dispersion region of PCF, it is believed to bring more energy to short-wavelength region and to enhance the supercontinuum generation [9].
Generally, using tapered or special designed PCF, the blue-enhanced or ultraviolet supercontinuum can be achieved efficiently. In 2011, Blue-extended supercontinuum spectra down to 372 nm in simply designed high air-filling fraction (> 0.9) PCFs was reported [10]. In [11], a flat spectrum in cascade tapered PCF down to 352 nm was obtained in 2013. Although the supercontinuum spectrum is efficiently extended in these works, the intensity of visible part of supercontinuum is not very high. Another effective way to obtain an enhanced ultraviolet and visible supercontinuum generation is using a chirped pump source [12–14]. In 2009, a broadband supercontinuum spectrum spanning from 350 nm to 1600 nm was obtained by coupling the chirped pulse into PCF at 800 nm [15]. In 2014, Gao et al. [14] generated an ultraviolet-enhanced supercontinuum pumped by giant-chirped pump laser. The spectrum was ranging from 370 to 2400 nm with over 36% of the total output power locating in the visible and ultraviolet region. In 2017, a high-power supercontinuum pumped by a giant-chirped laser with output power of 30 W and spectrum spanning from 385 nm to 2400 nm was reported [16].
In this paper, we experimentally demonstrate an ultraviolet-enhanced supercontinuum generation from a specifically designed seven-core PCF pumped by a giant-chirped mode-locked Yb-doped fiber laser operating in dissipative soliton resonance (DSR) region. The extremely wide optical spectrum spanning from 350 nm to 2400 nm is obtained with output power of 6.86 W. Over 59% of the total supercontinuum output power locates in the visible and ultraviolet region from 350 nm to 850 nm according to the spectral integration. To the best of our knowledge, this is the first demonstration of wide band supercontinuum generation from PCF pumped by DSR laser, and is also the widest supercontinuum generation with the highest efficiency of visible part from seven-core PCF.
2. Experimental setup
A schematic of the experimental setup in this study is shown in Fig. 1, which consists of a seed laser, one stage fiber amplifier, and a piece of seven-core PCF. The seed laser is built in an all-fiber figure-eight configuration and consists of two loops, as shown in Fig. 1(a). The inner loop in our configuration is a nonlinear optical loop mirror (NOLM) made of a 35:65 coupler, a polarization controller and a piece of non-PM fiber. The outer loop of the laser contains 1.84 m double-cladding Yb-doped fiber (YDF, LMA-10/125). The YDF is pumped by an 8 W multi-mode diode laser operating at 976 nm via a 976/1030 nm combiner. A polarization-independent fiber-optic isolator ensures the unidirectional operation of the laser. A bandpass filter with 8 nm pass band at 1030 nm is utilized to choose the pulse center wavelength and forces an all-normal dispersion laser cavity regime [17]. The laser is coupled out through the 60% port of coupler2 which simultaneously provides 40% laser feedback. The total cavity length and cavity dispersion is estimated to be 39 m and 0.4 ps2 at 1030 nm, respectively. The seed pulse width is measured by a 60 GHz sampling oscilloscope with a 45 GHz high speed InGaAs photodetector and a 5 GHz photodetector (as a trigger signal).
Fig. 1 Scheme of the all-fiber ultraviolet-enhanced supercontinuum source (ISO: Isolator, LD: laser diode, YDF: Yb-doped fiber, NOLM: nonlinear optical loop mirror, BPF: band-pass filter, PC: polarization controller).
As shown in Fig. 1(b), the gain fiber used in the amplifier is a piece of double-cladding YDF (LMA-15/130) of 2.5 m length, which is pumped by a multi-mode laser diode at 976 nm with over 20 W output power via a combiner. A polarization-independent isolator is used to prevent back reflection to the oscillator.
In order to obtain the ultraviolet-extended supercontinuum generation, a novel seven-core PCF is designed [18] and manufactured using the stack and draw technique. The 6 m PCF has seven identical cores of about 5 μm with an air-filling fraction 0.85 and a ZDW around 1020 nm. The inset of Fig. 2(a) presents the scanning electron microscopy (SEM) image of the seven-core PCF. The dispersion properties and the group-velocity of the structure are numerically modeled by Finite Element Method (FEM). The calculated dispersion and group-velocity of the PCF is presented in Fig. 2. Owing to the material absorption of silica, the soliton redshift is limited at ~2.4 μm and then the short-wavelength edge can be predicated by the group velocity matching from this wavelength. The group-velocity matched short-wavelength edge is 452 nm, as shown in Fig. 2(b). Because of the serious mismatch of the mode field area between the pump pigtail and the seven-core PCF, the air hole collapse technique [19] is used to reduce the splicing loss and to obtain an all-fiber supercontinuum source. By using the air hole collapse technique, a low loss splicing with 0.68 dB is achieved between the laser pigtail fiber and the seven-core PCF. The output end of the seven-core PCF is angle cleaved at 10° to avoid optical feedback. The output optical spectrum is measured by three optical spectrum analyzers (350 nm~1200 nm, 600 nm~1700 nm, and 1200 nm~2400 nm) simultaneously.
Fig. 2 (a) Calculated dispersion curve and the inset shows the SEM picture of the PCF. (b) Calculated group velocity curve.
3. Experimental results and discussion
3.1 Chirped seed pulses amplification
The characteristics of the seed laser are depicted in Fig. 3. By rotating the PC properly, a self-started stable mode-locked seed laser is obtained at the pump power of 2.8 W. Once mode-locked, the laser produces a stable train of pulses with an interval of ∼222 ns, corresponding to a fundamental repetition rate of ∼4.5 MHz, as shown in Fig. 3(a). The graphs depicted in Fig. 3(b) shows that the pulse width increases linearly from ~151 ps to ~353 ps with the increase of the pump power from ~2.83 W to ~7.8W. Figure 3(c) gives the auto-correlation trace of the pulse at the highest pump power. Due to the limitation of the scan range, only a part of the auto-correlation trace is shown and the half-width is 179 ps. There are not any significant spikes on top of the auto-correlation trace, which rules out the existence of noise-like components and ensures the coherence of the pulses.
Fig. 3 Property of the DSR laser. (a) Pulse train at 4.5MHz repetition rate. (b) Average output power and pulse width variation with pump power. (c) Auto-correlation trace at the highest pump power. (d) Pulse peak power and 3-dB spectral width variation with pump power. (e) Optical spectra under different output power.
Although the output pulses broaden linearly with an increasing pump power, the peak power of the output pulse almost keeps constant at ~830 W, caused by an effect named as peak power clamping [20], as shown in Fig. 3(d). The maximum output power is 1.4 W for 7.8 W pump power, corresponding to a single pulse energy of 311 nJ and a slope efficiency of 21.9%. The spectral shape is approximately triangular and the central wavelength is always located at ~1035 nm with a 3-dB spectral bandwidth of ~9.2 nm, as can be seen from Fig. 3(d) and 3(e). All those characteristics are typical phenomena of DSR, indicating that our seed laser operates in DSR regime [21–25]. The slight peak at 1085 nm corresponds to the stimulated Raman scattering (SRS) which is in excellent agreement with the Raman data for the fused silica of wavelength at 1035 nm, as shown in Fig. 3(e). The SRS in the spectrum can be attributed to the absence of effective filter and high peak power. We can also notice that the relative intensity between the 1035 nm peak and the SRS peak at 1085 nm does not change significantly while the spectra of them increase simultaneously. This also indicates that the peak power remains constant when pump power increases. At the highest output power, the measured pulse width is 353 ps and the 3-dB spectral bandwidth is 9.08 nm. Therefore, the time-bandwidth product is calculated to be around 898, showing that the seed pulse has a giant chirp which is beneficial for ultraviolet-enhanced supercontinuum generation [14,16].
Figure 4(a) shows the optical spectrum of the fiber power amplifier under different average output power. After the amplification, the center wavelength of the laser shifts toward longer wavelength around 1045 nm at the highest pump power. Due to the high peak power of the laser, the spectrum broadens progressively and the SRS is increasing as the pump power increases. The peak around 976 nm is due to the unabsorbed pump light. The average output power of the amplifier as a function of the incident pump power (black line with squares) is shown in Fig. 4(b). The amplifier boosts up the output power to 15.9 W under 24.8 W LD pump power. When the pump power reaches 15 W, the slope efficiency decreases gradually, probably limited by the nonlinear effects (such like SRS) caused by high peak power.
Fig. 4 (a) Optical spectrum of the amplifier at various average output power (b) Average output power of the MOPA and the supercontinuum source variation with pump power
3.2 Supercontinuum generation using amplified chirped pluses
At the maximum pumping power, 6.86 W supercontinuum power is obtained in the experiment, as shown in Fig. 4(b). If more powerful supercontinuum is needed, the power of the amplifier should be increased first. The pulse repetition rate or pulse duration should also be increased to properly control the peak power so as to avoid the power roll over effect shown in Fig. 4(b). The splicing point between the laser pigtail fiber and the PCF, and the end cap at the output of the supercontinuum source might be damaged at high power level, which may limit the ultimate output power of the supercontinuum source. According to a high-power supercontinuum generation work in [26] which also use a seven-core PCF as the nonlinear medium, it is hopeful that the output power of the supercontinuum in this study can be further enhanced to the level of one hundred watt.
After the amplification, the center wavelength of the laser shifts toward longer wavelength around 1045 nm, which still locates in the anomalous dispersion region of the PCF and is close to the ZDW of 1020 nm. The laser pulses transmit in the anomalous dispersion region of the PCF will break up into shorter sub-pulses by modulation instability. Then long-wavelength component generates due to soliton-self-frequency-shift induced by Raman and short-wavelength component generates by four-wave mixing and dispersive wave generation in the normal dispersion region. The concept of group-velocity matching between the longest and the shortest wavelength is considered as a limiting factor in blue and ultraviolet supercontinuum generation [7]. Figure 5(a) shows the output supercontinuum spectra under different powers, which are measured by three different optical spectrum analyzers (350 nm-1200 nm, 600 nm-1700 nm and 1200 nm-2400 nm, respectively) with spectral resolution of 2 nm. The middle-wavelength part and the long-wavelength part of the spectrum is filtered out in front of the optical spectrum analyzers by two long-wavelength-pass filters with cut-off wavelength of 900 nm and 1600 nm respectively, in order to eliminate the high order diffraction caused measuring error. With an increasing pump power, the spectrum of the supercontinuum source broadens obviously, especially the visible part of the spectrum. At the highest pump power, a wide supercontinuum spectrum ranging from 350 nm to 2400 nm is obtained with the intensity fluctuation of about 20 dB (Fig. 5(b)). For the short wavelength range, the spectrum has a 7 dB spectral bandwidth of 560 nm from 390 nm to 950 nm. The inset of Fig. 5(b) shows the output light spot taken by mobile phone. As we can see, the spot is essentially white. The group-velocity matched short-wavelength of our PCF is calculated to be only 452 nm. However, the short wavelength reaches 350 nm in the experiment finally. It may be caused by the interactive nonlinearity mechanisms like cross-phase modulation and four-wave mixing which lead to the generation of shorter wavelengths than dispersion wave [27].
Fig. 5 (a) Supercontinuum output spectra at different power levels. (b) Spectral power intensity of supercontinuum at the highest output power.
The output beam profiles at various wavelengths are shown in Fig. 6. The output of the supercontinuum is roughly collimated by an uncoated lens and illuminated on a white paper to take the photo. In the visible regime, the beam is filtered by a <900 nm short wavelength pass filter. Bandpass filters at 700nm, 660 nm, 546 nm and 470 nm are used respectively after the short wavelength pass filter to take the photo of the light spot at different wavelengths. The light is not Gaussian-shaped but are emitted in a cone. High order modes might be excited especially at short wavelengths. An in-depth study of spatial mode characteristics will be carried out in future.
In order to investigate the influence of the chirp, the supercontinuum spectra with different pulse widths at the same peak power of 9.5 kW are measured and shown in Fig. 7(a). The time-bandwidth product is calculated to be 497, 603 and 898, and the pulse energy is calculated to be 160 nJ, 196 nJ and 311 nJ, respectively for pulse width of 193 ps, 235 ps and 353 ps. The three spectra are almost the same except for a slight difference in the visible part. As depicted in the inset of Fig. 7(a), the spectra from 380 nm to 650 nm is enhanced with an increasing pulse width which can also attribute to an increasing chirp. The intensity of long-wavelength is significantly weaker than the intensity of short-wavelength. This is because the pump central wavelength is close to the ZDW of the PCF. Furthermore, initial pulses with chirp enhance the initial pulses compression in the anomalous dispersion regime and getting higher peak power, thus enhancing the supercontinuum generation [14].
Fig. 7 (a) Supercontinuum output spectra at different pulse duration. (b) The intensity of the supercontinuum with the pulse width at 353 ps.
Figure 7(b) shows the spectral power density of the supercontinuum at the output power of 6.86 W with pulse width of 353 ps. The optical-to-optical conversion efficiency of the supercontinuum at the pump power of 15.1W is calculated to be ~45%. According to spectrum integration, the intensity of the supercontinuum in the short wavelength region ranging from 350 nm to 850 nm occupies about 59% of the total output power. The intensity of supercontinuum in the ultraviolet wavelength region ranging from 350 nm to 400 nm occupies 1.1% of the total output power, namely 75 mW. To the best of our knowledge, this is the widest supercontinuum with the highest efficiency at the visible spectral ranges generated in a seven-core PCF in the picosecond pump regime using a fiber laser operating in DSR region.
4. Conclusion
In conclusion, we have experimentally demonstrated an all-fiber, ultraviolet-enhanced supercontinuum generation in a piece of specifically designed seven-core PCF pumped by a picosecond Yb-doped MOPA system. The MOPA source is seeded by a giant-chirped mode-locked fiber laser operating in DSR region with a fundamental repetition rate of ∼4.5 MHz and a tunable pulse width from 115 ps to 353 ps. At the pump average power of 15.1 W, a total power of 6.86 W ultraviolet-enhanced supercontinuum with the spectrum range from 350 nm to 2400 nm is obtained.
Funding
National Natural Science Foundation of China (NSFC) (61235008); National High Technology Research and Development Program of China (2015AA021101).
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