We report on an ultraviolet-enhanced supercontinuum generation in a uniform photonic crystal fiber pumped by a giant-chirped mode-locked Yb-doped fiber laser. We find theoretically and experimentally that the initial pluses with giant chirp leads more initial energy transferred to the dispersive waves in visible and ultraviolet wavelength. An extremely wide optical spectrum spanning from 370 nm to beyond 2400 nm with a broad 3 dB spectral bandwidth of 367 nm (from 431 nm to 798 nm) is obtained. Over 36% (350 mW) of the total output power locates in the visible and ultraviolet regime between 370 nm and 850 nm with a maximum spectral power density of 1.6 mW/nm at 550 nm.
© 2014 Optical Society of America
The phenomena of supercontinuum (SC) generation originated from a combination of nonlinear effects such as modulation instability (MI), self-phase modulation (SPM), soliton fission, stimulated Raman scattering (SRS), four wave mixing (FWM), etc. has been well studied since the 1970’s . Up to now, the most favorable nonlinear medium for SC generation is a piece of photonic crystal fibers (PCF), thanks to its high nonlinearity and its flexibility in tailoring the dispersion curve [2,3]. The broadband SC spectrum from visible to mid-infrared offers a range of interesting applications. In particular, the visible part (350-800nm) of the spectrum plays as an attractive optical source in biological applications, e.g. optical coherence tomography (OCT) , confocal microscopy , and forster resonance energy transfer (FRET) . Several avenues have been explored to achieve efficient generation in the blue and ultraviolet (UV) wavelength range. In , Travers et al. proposed a method using multiple PCFs with sequentially decreasing zero dispersion wavelengths (ZDW). This enabled generation of a 1.2 W SC extending from 0.44 μm to 1.89 μm with a picosecond ytterbium pump laser operating at 1064 nm, with high spectral power in blue. In , a meter long tapered PCF fabricated in a fiber tower was pumped with a picosecond pulse laser (30 kW peak power), generating an SC spectra from 375 nm to beyond 1750 nm. Efficient generation of visible and UV light using a 1064 nm pump source was also demonstrated using a PCF with specifically designed dispersion and group index . All these work focused on tailoring the PCF to enhance the UV and visible light generation.
A linearly chirped pump source is believed to improve the quality of the SC generation [10–18] and thus may enhance the visible and UV continuum generation without using tapered or special designed PCFs. When a linearly chirped pulse propagates in the anomalous dispersion regime of PCF, the pulse adjusts its shape and width and evolves into a soliton. In this process, part of the pulse energy is dispersed to the short-wavelength, known as the dispersive waves . More than two decades ago, C. Desem et al.  pointed out that when the pulses emitted from laser sources are chirped, the effect of initial frequency chirp on soliton formation and SC generation needs to be taken into account. It was shown that with the increase of the initial chirp, more of the initial energy is channeled into the dispersive waves, bringing more energy into short-wavelength regime. Following their work, a range of numerical simulations and a few experimental studies have been carried out investigating on the influence of an input chirped pulse to the SC generation. The simulations work in  and  showed that a highly positively chirped input pulse can improve the bandwidth of the SC pumped in the anomalous regime of the PCF. In 2009, A. Fuerbach et al.  generated a high power broadband UV-enhanced SC spectrum ranging from 350 nm to 1600 nm by coupling an uncompressed chirped pulse from a Ti: Sapphire oscillator (800 nm) into a PCF.
In this paper, we report an UV-enhanced flat SC generation from a PCF with no post-processing pumped by a long cavity giant-chirped mode-locked 1060 nm fiber laser. An extremely wide optical spectrum spanning from 370 nm to beyond 2400 nm is obtained with a high 3 dB spectral flatness from 431 nm to 798 nm. Over 36% of the total SC power lies in the visible spectral ranges from 370 nm to 850 nm, corresponding to an output power of 350 mW and a maximum spectral power density of ~1.6 mW/nm at 550 nm. This is, to the best of our knowledge, the widest SC generation with the highest efficiency at the visible spectral ranges generated in a uniform PCF using a 1060 nm fiber laser. The spectral broadening dynamics of the SC generation at the anomalous dispersive region of the PCF were carefully studied in the giant-chirped pulses pump regime.
2. Numerical simulation
In order to understand the mechanism of the chirped pulse influence on the UV-enhanced SC generation, we use a generalized scalar nonlinear Schrodinger equation to model the pulse propagation inside the PCF :Eq. (1) is a convolution which is solved by split-step Fourier method.
We consider a PCF design which is typical for broadband SC generation near the 1060 nm. The diameter of the solid silica core is 4.6 μm and the ZDW is around 1015 nm. At a wavelength of 1060 nm, the nonlinear coefficient is estimated to be 15.14 W−1km−1, and the dispersion coefficient is 4.8 ps/nm/km. A 0.1 m length of PCF is considered and the fiber loss is neglected for short propagation distances.
In our simulation, the initial injected pulse is assumed to have a hyperbolic secant field profile:
The effect of the initial linear chirp on the UV-enhanced SC generation in a PCF is shown in Fig. 1. The input pulses have the same peak power P0 = 30 kW and duration T0 = 0.3 ps. We can note a significant difference in the figure. Input pulses with a larger value of C = 30 have broader bandwidth, with the short wavelength of SC spectrum extending to 400 nm. This can be understood as a positive initial linear chirp enhancing the initial pulse compression in the anomalous dispersion regime and getting higher peak power, thus enhancing the SC generation. Following the process of soliton formation and dispersive wave generation, more of the initial energy is dispersed to the short-wavelength as the UV dispersive waves. Finally, a broader and UV-enhanced SC spectrum is created by chirped pulses.
The simulation above shows the importance of the linear chirp of the pump source in the SC generation. In this simulation, for simplicity, the effect of MI has not been taken into account of. In a picosecond pump regime, MI dominates the initial SC generation, breaking up the pump pulses into shorter duration sub-pulses. This process brings more dispersive wave into the visible regime and thus is believed to further enhance the visible light generation. Simulation work in the picoseconds or nanosecond regimes including the effect of MI can be found in ref . In the following experiment, we will configure a picoseconds fiber laser with giant chirp and use it to generate an UV-enhanced supercontinuum.
3.1 Giant-chirped pulses generation and amplification
A schematic of the SC generation system used in this study is shown in Fig. 2, which consists of a giant-chirped fiber oscillator, two stages fiber amplifiers, and a piece of PCF. The giant-chirped oscillator is in a ring configuration with the cavity length of about 1010 m, as shown in Fig. 2(a). The ring cavity includes a 2 m single-cladding Yb-doped fiber (YDF) (Nufern SM-YSF-HI-6/125) with a group velocity dispersion (GVD) of −35 ps/nm/km and ~1000 m single mode fiber with GVD of −30.8 ps/nm/km. The total cavity dispersion was estimated to be 18.4 ps2. The YDF with 250 dB absorption was core-pump by a diode laser with the central wavelength of 976 nm and a maximum output power of 480 mW. A polarization-dependent isolator is used not only to ensure the unidirectional propagation of optical signals but also to define their polarization direction. By use of two polarization controllers, a nonlinear switching mechanism can be achieved through nonlinear polarization rotation (NPR), which leads to the generation of giant-chirped pulses with the 1000 m single mode optical fiber. A 70% fiber coupler is used to output the signal, which is sent into the fiber amplifiers. Three optical spectrum analyzers (Yokogawa, AQ6373, AQ6370C, and AQ6375), and a 25 GHz oscilloscope with a 25 GHz photo-detector were employed to monitor the laser output simultaneously.
As shown in Fig. 2(b), a same YDF of 2 m length is used as the gain medium for the preamplifier, which is pumped by a single mode diode operating at 976 nm with an output power of 480 mW. A polarization-independent isolator is used to prevent reflection back to the giant-chirped oscillator. A filter is used to filter out amplified spontaneous emission (ASE) and SRS generated during the amplification. The power amplifier is a 1.2 m double-cladding YDF forward pumped by a multi-mode laser diode with central wavelength of 976 nm and maximum output power of 9 Watt via a (2 + 1) × 1 fiber combiner. The gain fiber in this stage has a 7/128 μm core/cladding diameter with 0.19/0.45 NA and cladding absorption of about 5.4 dB/m at 975 nm. A short piece of single-cladding fiber is used to strip off pump light.
When the diode pump power is increased to 140 mW, a stable mode-locked pulse is achieved. Figure 3 shows a typical pulse train at repetition rate of 198 kHz. The repetition rate corresponds to the total cavity length of ~1010m. The maximum output power is 0.32 mW for 140 mW pump power, corresponding to single pulse energy of 1.6 nJ. Further increase of the pump power results in pulse splitting due to fiber nonlinear effect bring by high peak power. As shown in Fig. 4(a) and 4(b), the FWHM pulse width is 232 ps and the central wavelength is 1060.5 nm with 2.4 nm spectral bandwidth. The time bandwidth product (TBP) is calculated to be 149, which is about 473 times the transform limit (assuming a sech2 profile), indicating that the pulses are highly chirped .
The preamplifier boosts up the output power to 28 mW under 480 mW preamplifier pump power. Figure 5(a) shows the optical spectrum (FWHM = 2.4 nm) of the fiber power preamplifier at maximum average output power. Through the second-stage power amplifier, the average power of giant-chirped pulse is amplified to 1.4 W at pump power of 4.6 W, corresponding to single pulse energy of 7 μJ. Figure 5(b) shows optical spectrum (FWHM = 2.9 nm) of the fiber power amplifier at 1.4 W with clearly ASE and SRS generated. The pulse width has no obvious variation after two stages amplifiers.
3.2 Supercontinuum generation using amplified giant-chirped pluses
The PCF of 1 m length used in the experiment is provided by Yangtze Optical Fiber and Cable Company Ltd. According to their datasheet, the PCF has a core diameter of about 4.6 μm and a hole size to pitch ratio of about 0.6, with the ZDW around 1030 ± 20 nm (Fig. 6). The dispersion coefficient at 1064 nm is 8.36 ps/nm/km and the nonlinear coefficient at 1064nm is 11.16 W−1km−1.
Figure 7 shows the evolution of the SC spectra corresponding to different power levels measured by three different optical spectrum analyzers (Yokogawa, AQ6373, AQ6370C and AQ6375) with spectral resolution of 0.5 nm. The spectra are measured by directly coupled into the spectrometer slit. At low pump power, the pulse is slightly compressed due to the positive initial strong chirp in the anomalous dispersion region of the PCF. As the pump power is increased, the pulse breaks up due to MI caused in the anomalous dispersion. MI leads to temporal break-up of high power pulse into a distribution of soliton-like pulses and dispersive waves . Each of these solitons may furthermore resonantly transfer energy to the normal GVD regime if they have sufficient spectral overlap across the ZDW . The frequency shift of the first order MI gain peaks from the pump is given by :Fig. 7(a) purple curve), corresponding to the 48 mW pump power (about 1 kW peak power). The six sidebands located around 953 nm, 986 nm, 1022 nm, 1101 nm, 1146 nm and 1195 nm agree very well with the 10.64 THz peak frequency of MI gain calculated for = 1 kW and fiber parameters = −5 ps2km−1 and = 11.16 W−1km−1. When the SC average power goes up to 100 mW (Fig. 7(a) pink curve), the spectra has essentially become a SC and expanded well into the normal dispersion region below 800 nm. In this process, MI provides the extremely smooth spectral features of continuum as each soliton and dispersive wave contributes to a slightly different region of the continuum spectrum.
Once the MI dynamics breaks up the pump pulses into shorter duration sub-pulses, further spectral broadening follows the same mechanism as the case of femtosecond short pulses, whereby the long wavelength generation is due to the Raman induce soliton self frequency shift (SSFS) and the short wavelength side is determined by the FWM and dispersive wave generation in the normal dispersion region. While the soliton is being redshifted due to the self-frequency shift, it keeps a packet of dispersive waves trapped in a group-velocity matched bound state, also known as soliton trapping of dispersive waves, so that the dispersive waves are blueshifted to maintain group-velocity matching . The spectrum exhibits a continuous broadening towards both sides of the pump wavelength in this process. In addition, the peak at 530 nm can be assigned to the second harmonic generation in the PCF .
At the highest pump power of 1.4W (Fig. 7(a) black curve), a wider and flatter SC spectrum with total power of 973 mW (Fig. 8) is obtained which spans over more than 2030 nm, ranging from 370 nm to 2400 nm. The spectrum generated at the highest output power is remarkably flat, with the 3 dB spectral bandwidth from 431 nm to 798 nm. The long wavelength side of the spectrum is measured by the optical spectrum analyzer Yokogawa AQ6375 (1200 nm-2400 nm), which has no order-sorting filters. Thus, a 1200 nm cut-on long pass filter is used to prevent multi-order interference in the measurement. In addition, the intensity of long-wavelength is significantly weaker than the intensity of short-wavelength. This is because the initial pulses with giant chirp leads less energy for soliton formation in the anomalous dispersion regime and more energy transferred to the UV dispersive waves. In order to confirm this result, we compare the SC spectral power density in the visible and near infrared regime using a 669 nm band-pass filter with a bandwidth of 9.8 nm and a 1350 nm band-pass filter with a bandwidth of 12 nm. The measured spectral power density is 1.2 mW/nm at 669 nm and 0.68 mW/nm at 1350 nm. This indicates the maximum spectral power density is 1.6 mW/nm at 550 nm (Fig. 7(c)). We also measured the output power of the SC in the visible wavelength region using a short-pass filter at 850 nm. Over 36% of the total SC output power was located between 370 nm and 850 nm, corresponding to 350 mW output power. To the best of our knowledge, this is the widest SC with the highest efficiency at the visible spectral ranges generated in a uniform PCF in the picosecond pump regime using a 1060 nm fiber laser. Future work will focus on higher giant-chirped pump power for further power scaling of UV-enhanced SC.
In conclusion, we have demonstrated experimentally the UV-enhanced SC generation in a uniform PCF pumped by a giant-chirped pulse fiber laser at 1060 nm wavelength. The process can be understood as the more initial energy is dispersed from the giant-chirped pulses as the dispersive waves to the short-wavelength. At the pump average power of 1.4 W, a total power of 973 mW ultra-flat UV-enhanced SC with the spectrum range from 370 nm to 2400 nm was obtained, with the 3 dB spectral bandwidth from 431 nm to 798 nm and over 36% (350 mW) of the total SC power located between 370 nm and 850 nm.
The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC, Nos. 61235010, 61177048 and 61377098), the Beijing Municipal Education Commission (No. KZ2011100050011) and the Beijing University of Technology, China.
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