We realize the flattening and extending of a CW-pumped supercontinuum with a high spectral intensity peak at the pump region. It is achieved by cascading a long zero-dispersion wavelength high-nonlinearity fiber with the output photonic crystal fiber, in order to improve the conversion efficiency of residual pump energy to long-wavelength continuum based on the effect of cascaded stimulated Raman scattering. Compared with the non-flattened continuum of 10.3 W with 3 dB bandwidth of 62 nm and 10 dB bandwidth of 360 nm, a flat continuum of 8 W with 3 dB spectral range of 340 nm and 10 dB spectral range of 420 nm is obtained. The spectral peak at the pump region decreases more than 5 dB, below the level of long-wavelength spectral intensity. Also, the long-wavelength edge has been extended by 60 nm.
© 2010 OSA
Supercontinua (SC) generated in photonic crystal fibers (PCFs) at continuous-wave (CW) pump have several advantages over pulse-pumped supercontinua, including higher spectral power densities (SPDs) and smoother spectra. In combination with a mature high power single mode Yb fiber laser, the low-cost high-power supercontinuum sources could be achieved, which would have a wide variety of applications such as high-resolution optical coherence tomography , biomedical imaging and chemical sensing.
The main spectral parameters of SC are spectral range and spectral flatness, both of which are crucial for fitting the technical specifications of the applications. A wide range of researches on CW-pumped SC generation have been done on the extention of SC spectral range. The SC with the shortest wavelength of 0.65 μm was demonstrated for the first time by using a PCF with a carefully engineered zero dispersion wavelength (ZDW) to accommodate the relevant phase and group velocity matching conditions . Also, the SC spectrum with the same shortest wavelength was achieved by a PCF with its ZDW decreasing along the fiber length [3–5]. On the other hand, the longest wavelength of 1.67 μm was achieved in a two-ZDW PCF used for extending the long-wavelength edge . Moreover, the broad SC spectrum covering the region from 0.47 μm to more than 1.75 μm was obtained by using a ZDW-decreasing GeO2-doped-core PCF . Although so many methods were applied for SC spectrum extention, little work has been done on the flatness of SC spectrum. Generally, there exists abundant residual pump energy from the output SC, in the spectrum of which the spectral intensity peak at pump wavelength ranges from several dB to over ten dB. Only if the pump power is very high and the pump wavelength is very close to the ZDW in the anomalous regime, the spectral peak could decrease enough [8–11]. This has considerable impact on the flatness and the effective output power of SC. The usage of a two-ZDW PCF favors to generate a flatter SC spectrum between the two ZDWs [6,8,9,12]. Also, the flatness could be further improved when increasing the fiber length, but the conversion from the pump energy to the SC is still insufficient, resulting in the highest spectral peak still located at the pump wavelength .
In this paper, we present a fiber-cascading method to deal with the insufficient energy conversion from pump region to long-wavelength region of the SC from a PCF, so as to achieve a broad flat CW-pumped supercontinuum. The cascaded fibers have been utilized in the CW multiwavelength-pumping case to control the SC spectral shape . In our experiment, a long-ZDW high nonlinearity fiber (HNLF) was cascaded with the PCF to improve the conversion of the residual pump energy to long wavelengths based on the effect of cascaded stimulated Raman scattering (SRS) dominating in the normal dispersion region of the HNLF. With this method the spectral peak intensity at the pump decreases more than 5 dB, below the level of the long-wavelength spectral intensity. And the SC up to 8 W with 3 dB spectral width of 340 nm and 0.7 dB width of 210 nm was realized compared with the non-flattened spectrum span of 62 nm at 3 dB level, also, the long-wavelength edge extends by 60 nm.
The paper is organized as follows. In section 2 we present the fibers and their properties. In section 3 we outline the experiment setup. In section 4 we give the results of supercontinuum generation and supercontinuum flattening, and discuss the mechanisms for continuum flattening as well as the characteristics of the flattened SC.
The two fibers used in this experiment, a photonic crystal fiber (PCF) and a high nonlinearity fiber (HNLF), were fabricated by Yangtze Optical Fibre And Cable Company Ltd. As described in the scanning electron micrograph (SEM) image of one end facet in Fig. 1 , the core diameter is 4.7 μm, the hole-to-hole spacing Λ 3.3 μm, and the hole diameter d is 1.9 μm. The fiber is fabricated with pure silica by the standard “stack and draw” method, with 5 complete periods between the core and the external silica jacket. Because of the significant absorption of the water band centered at 1380 nm, special measures were taken to reduce water contamination during the fiber draw, which decreases the peak attenuation at 1380 nm to about 80 dB/km. As shown in Fig. 1, the dispersion coefficient D and nonlinear coefficient γ were calculated with a finite elements method (FEM) from the SEM image, also the dispersion was measured by a chromatic dispersion system (CD 400, PE. fiberoptics). It can be seen that the ZDW is located at 1030 nm, just below the pump wavelength of 1071.5 nm, and the nonlinear coefficient and the mode field diameter (MFD) at the pump wavelength are 11 (W·km)−1 and 3.9 μm.
A nonlinear coefficient and a Raman gain coefficient of the used high nonlinearity fiber at 1550 nm are 10 (W·km)−1 and 4.8 (W·km)−1. The MFD and numerical aperture (NA) at 1550 nm, measured with a MFD/Aeff Measurement System (MA 400, PE. fiberoptics), are 3.6 μm and 0.35. The measured dispersion at 1550 nm is −5 pm/(km·nm) with a long ZDW of ~1840 nm and a cut-off wavelength of ~1520 nm. Also, the peak attenuation at 1380 nm is measured to be 5 dB/km.
3. Experiment setup
A 20 W CW Yb fiber laser (IPG Photonics) with single mode output, centered at 1071.5 nm with a line width of 0.5 nm, was directly spliced to a 200 m-long PCF, and the minimum splice loss of 0.55 dB was achieved. The 200 m-long PCF consisted of two 100 m-long section has a 0.45 dB splice loss in the midst of it. The output of the PCF was angle-cleaved to reduce the reflection from the end facet which would significantly enhance any Raman Stokes lines in the continuum or disturb the pump to lower the power. The SC output power and spectrum are measured with a power meter and an optical spectrum analyzer (EPP 2000) . In the spectrum flattening regime, a 100 m-long HNLF is cascaded with the PCF with a low splice loss of 0.3 dB. The fiber output end was also angle-cleaved.
4. Results and discussion
In this section we present the comparison of the non-flattened supercontinuum and the flattened supercontinuum, and discuss the mechanisms for continuum flattening along with the characteristics of the flattened continuum.
4.1 Supercontinuum generation
Figure 2 shows the evolution of the non-flattened supercontinua from the PCF for different pump powers. When the maximum incident pump power is 17.6 W, a 10.3 W continuum is produced with the spectral range at 10 dB level from 1055 to 1415 nm (360 nm) and 3 dB bandwidth of 62 nm (1067-1129 nm).
The continuum is originated from modulation instability producing a train of ultrashort pulses. Some of these pulses whose energy is over soliton formation threshold develop into the fundamental solitons, which then go though the soliton self-frequency shift (SSFS) forming a long-wavelength Raman-soliton continuum [3,14,15]. The red-shift wavelength edge of the continuum continues to move to longer wavelengths with the increase of pump power as shown in Fig. 3 . It also could be observed that the rate of the shift of the longest wavelength is curtailed once the water loss at 1.38 μm (16 dB in the 200 m length cf 80 dB in 1 km) is reached, which is similar to the results in .
Moreover, the first Raman Stokes line was observed at the maximum pump power from the spectrum. The wavelengths short of the pump were not observed, and this can be explained as follows. As the pump of 1071.5 nm lies far from the ZDW (1030 nm) in the anomalous dispersion region, the spectra of initial solitons generated from MI or of the anti-Stokes MI sideband could not extend into the normal dispersion region, therefore resulting in the very low efficiency of soliton trapping and four-wave mixing (FWM) [2,16] and leaving SSFS as the main mechanism for SC generation. From the Fig. 2 we can see that the ratio of the residual pump power to the continuum power even at the maximum pump power is still large with a spectral peak of 5 dB located at the pump region in the spectrum. This implies that the energy conversion efficiency from pump to SC is not high accounting for the low flatness of SC spectrum.
4.2 Supercontinuum flattening
In order to improve the pump energy conversion and attain a flattened supercontinuum, we splice a 100 m-long HNLF described above with the PCF. As can be seen from the evolution of the flattened-continuum in Fig. 4 that the proportion of residual pump power to SC power decreases with the increase of pump power, and the spectral peak transfers to longer wavelengths with the height of the peak intensity decreasing and the peak width broadening. There are three spectral peaks obviously in the flattened SC spectrum: λP1 ~1078 nm, λP2 ~1130 nm and λP3 ~1190 nm, respectively. The highest peak moves from the pump wavelength of 1078 nm to 1130 nm, and then to 1190 nm with the rising of pump power; moreover, the total SC spectrum extends to longer wavelengths. When the output power goes up to the maximum of 8 W, the SC spans from 1055 nm to 1475 nm (10 dB level) and has a 3 dB bandwidth of 340 nm (1072-1412 nm), corresponding to an average spectral power density of 23 mW/nm. What is more, a flatness of 0.7 dB over 210 nm in the range 1175-1391 nm is also attained. Consequently, the spectral flatness of the SC has been improved significantly by this SC spectrum flattening method, with 10dB spectral range extended by 60 nm and 3 dB range by 280 nm in contrast with the non-flattened case at the maximum pump power of 17.6 W. Furthermore, the peak intensity at pump region decreases more than 5 dB.
The frequency shifts between λP1 and λP2, λP2 and λP3 are calculated to be X1 ~427 cm-1, and X2 ~446 cm-1, close to the Raman shift of 440 cm-1, which proves the mechanism involved for SC flattening to be cascaded stimulated Raman scattering (SRS). The SC coupled to the HNLF lies in the normal dispersion region of the fiber, and consequently, the effect of SRS is dominant. The energy of MI-induced pulses at the pump region transfers to the first Stokes wavelength, and then to higher-order Stokes at longer wavelengths via the cascaded SRS with the high order Stokes line broader spectrally than the preceding one [17,18]. Thus, the SC spectrum extends to longer wavelengths and grows flatter and flatter. And the spectral broadening eventually limits to about 1.4 µm owing to the water loss of the two fibers and a relative low pump power.
We also investigated the output modes of the non-flattened and flattened supercontinua. The dispersed output spectra of the supercontinua are recorded using a CCD sensitive to the near infrared as shown in Fig. 5(a) , and the far-field profiles at the pump wavelength are recorded with a digital camera in Fig. 5(b) by using a low pass filter with cut-off wavelength of 1100 nm. Both of these figures show that the SC output modes in the non-flattened and flattened cases are fundamental-like in the whole spectrum.
Compared with the 6 W flattened continuum with 5 dB spectral range of 300 nm achieved by increasing the length of a two-ZDW PCF pumped by a 20 W fiber laser , the 8 W continuum with 3 dB bandwidth of 340 nm achieved in our work is flatter, broader, and of higher power with the pump region suffering sufficient energy conversion to SC. Pumped at a higher power of 50 W, a powerful flat supercontinuum with the same 3 dB bandwidth with our results and a higher spectral power density greater than 50 mW/nm has also been obtained in a carefully-engineered-ZDW PCF and a short length of two-ZDW PCF [2,6]. Even so, achieving the broad and flat supercontinuum in such a low pump power in our work has confirmed that the method of cascading a long-ZDW HNLF to a PCF is effectively for flattening and extending the supercontinuum. Also, it is presumed that the method would also play the same role in flattening the SC spectrum at pulse pump. Furthermore, the important issue in CW-pumped supercontinua generation of OH- loss at 1380 nm may be expanded beyond if cascading a HNLF with low water absorption loss which is easier to be manufactured than of PCFs. The flat SC sources obtained from this method would have a more wide range of applications.
It should be noted that there should be an optimal length of the HNLF for the flattest spectrum of the flattened-SC at different pump powers.
In conclusion, we have demonstrated a method for flattening and extending a SC spectrum at CW pump by improving the transfer efficiency of the abundant residual energy from pump region to long-wavelength continuum. A flat supercontinuum of 8 W with 3 dB spectral range of 340 nm and 0.7 dB spectral range of 210 nm has been achieved in comparison with the non-flattened continuum with 3 dB bandwidth of 62 nm and 10 dB bandwidth of 360 nm, also the long-wavelength edge has been extended by 60 nm. Furthermore, we think that the spectrum flattening method may be used for flattening the spectrum at pulse pump.
The authors acknowledge Dr Yongqin Yu and Yuan Guo (Shenzhen university) for helpful discussions, as well as Chen Guo (Shenzhen university) for the cooperation of PCF splicing.
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