We present wideband of 1180-2100 nm, flatly broadened supercontinuum (SC) generation using highly nonlinear hybrid fibers and femtosecond fiber laser. Stable and smooth spectra without fine structure are demonstrated. The hybrid fibers are constructed by fusion splicing fibers with different properties. The SC spectra can be properly controlled by the optimal design of the hybrid fiber based on the numerical analysis. The generated SC pulse shows the low relative intensity noise (RIN).
©2004 Optical Society of America
The broadband supercontinuum (SC) generation currently attracts a lot of attention because of the high potential for applications in the fields of the optical communications , the generation of the ultrashort pulse , the optical coherence tomography (OCT) , the optical frequency metrology , and others. Recently, the SC generation in the microstructured photonic crystal fibers (PCFs), the tapered fibers, and the highly nonlinear fibers (HNLFs) using the picosecond or femotsecond laser pulse with high peak power has been reported [5–8]. The wideband SC generation can be realized by injecting the pump pulse into the anomalous group velocity dispersion (GVD) region near the zero-dispersion wavelength (ZDW) of the fiber and the spectrum broadened over an octave has been obtained even if the injected pulse energy is less than a few-nanojoule.
However, the instability of the SC spectrum has been reported [9,10] and the appearance of the unstable spectral fine structure limits the applications of the SC pulse. The spectral fine structure is extremely sensitive to the initial pulse energy and its instability is due to the fluctuation of the pump pulse. Additionally, the noise amplification during the SC generation has been also reported [11,12].
So far, we have analyzed the SC generation in the highly nonlinear dispersion shifted fiber (HNL-DSF) with the femtosecond fiber laser both experimentally and numerically and proved the cause of the spectral fine structure . When the femtosecond pulse propagates in the extreme vicinity of the ZDW of the HNL-DSF, the spectral broadening is firstly caused by both the self-phase modulation (SPM) and the generation of the non-solitonic dispersive wave due to the fission of the higher-order soliton pulse . By increasing the propagation distance, the further spectral broadening occurs due to the soliton self-frequency shift  and the trapping effect by the red-shifted soliton pulse through the cross-phase modulation (XPM) [16,17]. Additionally, we found that the spectral fine structure gradually appeared as increasing the propagation distance. Figure 1 shows the spectrogram, spectrum, and temporal waveform in the numerical analysis when the 100 fs pulse with 2.5 kW peak power is launched into the 10-m-long HNL-DSF. We can see that each spectral component is distributed parabollically along the delay curve of the fiber. The similar spectrogram for the SC pulse generated in the PCF has been reported . The soliton pulses trap the anti-Stokes components lying in the normal GVD region across the ZDW and the trapped components are blue-shifted toward the wavelength satisfying the condition of the group velocity matching. We consider that the origin of the spectral fine structure is the results of this XPM effect and the interference between the components which are spectrally overlapped but are temporally separated. In particular, the interference in the frequency domain leads to produce the oscillating structure on the spectrum and it is one of the cause of spectral instability.
In this paper, we present the flatly and smoothly broadened wideband SC generations using the optimally designed hybrid fibers which are constructed from plural HNLFs. Each fiber in the hybrid has different role in achieving the spectral broadening. We can design the desired spectrum by changing the fiber length and the combination of the fibers according to the application of the SC pulse. The appearance of the spectral fine structure can be suppressed. The experimental results are in good agreement with the numerical ones. The relative intensity noise (RIN) of SC pulses is also discussed in terms of the configuration of fibers.
2. Flatly broadened SC generation by use of three-stages hybrid fiber
The experimental setup is shown in Fig. 2. As the pump pulse, we used a passively mode-locked erbium-doped fiber laser. It generates ~100 fs sech2-like pulses with the slight pedestal at a repetition frequency of 48 MHz, the center wavelength being 1560 nm. The root mean square timing jitter and intensity fluctuation of this light source are ~30 fs and 0.02 %, respectively.
The laser pulse is launched into the hybrid fiber which is constructed from three fibers with different properties. Figure 3 shows the dispersion curves of the fibers used in the hybrid fiber. The hybrid fiber is constructed using the fusion splicer. The splice loss is less than 0.7 dB/splice. We determined the optimal length of each fiber from the results of the numerical calculation and the actual experiment. In the numerical calculation, we solved the generalized nonlinear Schrödinger equation by using the split-step Fourier method [13,19]. This equation includes the fiber loss, dispersion, and nonlinear effects. We considered the higher-order nonlinear effects, such as the self-steepening and stimulated Raman scattering and used the nonlinear response function of the fiber that both the electronic and vibrational Raman contributions were included. For the dispersion effects, we considered the terms up to the 6th order dispersion parameters of the fibers. As the initial condition of the pump pulse, we used the reconstructed result of the fiber laser from the second harmonic generation frequency resolved optical gating method (SHG-FROG).
Figure 4 shows the experimental and numerically calculated spectra at each point in the hybrid fiber. The peak power of the pulse injected into the fiber is 3.7 kW. The flatly broadened SC pulse is generated in three stages. The pulse launched into the hybrid fiber is firstly compressed by utilizing the effect of the higher-order soliton compression in the polarization maintaining fiber (PMF). This fiber is the diameter reduced type, in which the mode-field diameter and nonlinear coefficient are 5.84 µm and 4.8 W-1km-1 at the wavelength of 1.55 µm, respectively. As increasing the propagation length, the temporal width is compressed gradually until the optimal length. When the propagation length exceeds the optimum, the pulse break-up occurs and the Raman soliton pulse is generated . We could obtain the ~30 fs pulse at the length of 17 cm. The estimated peak power of the pulse in this point is ~15 kW.
The primary spectral broadening is caused in the next HNL-DSF. The ZDW of this fiber is 1545 nm and it is almost matched with the pump wavelength. The nonlinearity of HNLFs is enhanced by doping the germanium into the silica core and by the small effective core area . The mode field diameter of the HNL-DSF is 3.7 µm and the nonlinear coefficient is as large as 21 W-1km-1. The higher peak power and the narrower temporal duration produce the widely broadened SC spectrum. However, there is an optimal propagation length that the flat spectrum is generated. When the fiber length is too long, the spectrum has the several discrete peaks and the flatness becomes to be degraded. Additionally, when the pulse break-up occurs and the spectrally broadened components become to be parabollically distributed along the delay curve of the fiber, the fine structure appears on the spectrum. We obtained the most flat and wideband spectrum at the fiber length of 4.5 cm. We also found that the spectrum without the fine structure could be generated theoretically (Fig. 4 (c)). The calculated spectrograms at each point in the hybrid fiber are shown in Fig. 5. At the output of the HNL-DSF (Fig. 5(b)), we can see that the spectral components are almost overlapped temporally.
To avoid the appearance of the spectral fine structure, we used a normal dispersion HNLF (N-HNLF) as the third fiber. The N-HNLF shows the normal GVD to the generated SC spectrum and has a large nonlinear coefficient, 23 W-1km-1. The mode filed diameter is 3.5 µm at the wavelength of 1.55 µm. In the final stage, the SC spectrum is flatted together with further broadening. We can see that the longer-wavelength component which swelled up at the output of the HNL-DSF is flatted. Seeing the Fig. 5(c), it is found that the SC pulse has the almost linear frequency chirp and the spectrally broadened components are in the condition that they do not interact with each other. The numerical calculation could accurately simulate the pulse propagation in the fiber and its results were in good agreement with the actual behavior. By use of the three-stages hybrid fiber, we realized the wideband of 1180–2100 nm SC generation without the spectral fine structure. Additionally, the flat spectrum with ±1 dB uniformity was obtained at the wavelength region of 1200-1440 nm and 1610-1920 nm. The average power of the SC pulse is 25 mW. The existence of the high-intensity component around the pump wavelength of 1560 nm is due to the temporal pedestal. A part of it is the pump pulse originally has and the other is that generated through the higher-order soliton compression in the first stage . In the HNL-DSF, since their intensity is low, they do not contribute to spectral broadening and remain in the pulse wing. The two peaks at the pump wavelength appeared in Fig. 5(c) are these remained components. In order to eliminate these components, the ideal pump pulse and the design of the hybrid fiber without the process of the higher-order soliton compression are necessary.
Next, we evaluated the noise characteristics for the SC pulse generated in the hybrid fiber. We detected the SC pulse using an InGaAs PIN photodiode which had sensitivity in whole spectral range of the generated SC pulse and measured its RF signal using an electrical spectrum analyzer. We calculated the magnitude of the RIN by dividing the product of the square of the DC power and the electrical bandwidth of the spectrum analyzer into the square of the RF noise power. Figure 6 shows the measured RIN for the obtained SC pulse at the low frequency region. We compared the result with that of fiber laser and that when the laser pulse with 2.5 kW peak power was launched into a single 10-m-long HNL-DSF. We found that the noise level increased in the whole frequency range when we used a single HNL-DSF with a long length. The increase of the noise of the SC pulse limits potential applications of this light source. For example, in the OCT imaging, the RF noise of the light source degrades the sensitivity of the signal in the heterodyne detection. Therefore, it is necessary to suppress the noise amplification during the SC generation. In the case of the hybrid fiber, we can see that the generated SC pulse keeps the equivalent low noise level to the pump pulse. By use of the hybrid fiber, we could realize the low noise SC generation.
3. Smoothly broadened SC generation with single spectral peak by use of high-power soliton pulse and two-stages hybrid fiber
Next, we present the smoothly broadened SC generation with a single spectral peak. In this scheme, since the higher-order soliton compression is not used, we can generate the extremely smooth SC spectrum. The experimental setup is shown in Fig. 7. As the pump pulse, we used the high-power soliton pulse. The power of the oscillator pulse is boosted by using the erbium-doped fiber amplifier. In the single mode fiber (SMF), the Raman soliton pulse is generated. We used a conventional SMF (SMF-28) which had a small nonlinear coefficient in order to generate the fundamental soliton pulse with high peak power. A low pass filter is used to remove the oscillator pulse at 1560 nm coming together with the soliton pulse. Additionally, a quarter-wave plate, a half-wave plate, and a polarization beam splitter (PBS) are used to pick out the high-power pulse efficiently. The generated pulse has a FWHM width of 110 fs at the wavelength of 1680 nm and is almost ideal sech2 shape (Fig. 7). The average power of the soliton pulse is 60 mW and the pulse energy is 1.2 nJ.
To generate the smooth broadband spectrum, we launched the soliton pulse into the two-stages hybrid fiber which was constructed with 5-cm-long HNL-DSF and 5-m-long N-HNLF. The injected peak power of the soliton pulse is 5.8 kW. The numerically calculated spectrograms at each point in the two-stages hybrid fiber are shown in Fig. 8. In the first stage, the pulse is spectrally broadened by the SPM effect. We can see that the pedestal-free temporal waveform is generated at the output of the HNL-DSF (Fig. 8(b)). In the next N-HNLF, the spectrum is further broadened and smoothed. And the pulse suffers the strong positive chirp in this fiber. Figure 9 shows the optical spectra in the experiment and numerical analysis for the finally obtained SC pulse. The output average power is 35 mW. We could obtain the smooth spectrum with a single peak. The spectral fine structure did not appear as in the case of the three-stages hybrid fiber. This light source is expected to apply to the OCT imaging in the infrared wavelength region. The wideband and single peak spectrum is suitable to realize the high longitudinal imaging resolution and the side-lobe free coherence function. Figure 10 shows the measured RIN for the obtained SC pulse. The noise level was almost equivalent to that of the pump soliton pulse. We could confirm that the SC generation in the hybrid fibers did not cause the noise amplification.
In this paper, we have demonstrated the generation of the wideband of 1180–2100 nm, flat, and low noise SC pulse using the highly nonlinear hybrid fibers and the femtosecond fiber laser. The hybrid fibers were optimally designed so as to realize the effective spectral broadening. The numerical analysis was used to simulate the evolution of the SC pulse in the optical fibers and it was very useful for the design of the hybrid fibers. The simulated results were in good agreement with the actually observed spectra. We presented two SC generations with the useful feature for the applications. The stable and flat or smooth spectra without fine structure were demonstrated. We found that the obtained SC pulse had the low RIN and the designed hybrid fiber did not cause the noise amplification during the SC generation.
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