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Abstract
High power LP11 mode supercontinuum is generated from 25/250 large mode area (LMA) fiber. A mechanical long period grating (LPG) is utilized to control the transverse modes in the LMA fiber to realize the LP11 mode supercontinuum generation in a master oscillator power amplifier (MOPA) configuration. The generated LP11 mode supercontinuum covers the spectral range from ~900 nm to ~2100 nm with a −30-dB spectral width of ~1200 nm and 50% optical to optical conversion efficiency. The seed laser produces picosecond pulses with 1 MHz repetition rate at the wavelength of 1060 nm. After multi-stage amplification in ytterbium-doped fibers, the average output power is scaled to 54.51 W and 56.79 W respectively for LP11 and LP01 mode, accompanying supercontinuum generation. Obvious difference of supercontinuum generation between the LP01 and LP11 modes is experimentally observed due to the different dispersion characters.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since observed in 1970, supercontinuum source has been a hot research topic for its broad spectrum and high brightness [1,2]. At present, the most popular method to achieve high power supercontinuum is using a continuous wave (cw) or pulsed fiber laser with high average power to pump a piece of photonic crystal fiber (PCF) with high nonlinearity [3–5]. As for supercontinuum generated in PCFs, it can generate supercontinuum spectrum at low average power with a short length of PCF by pulse pumping. In most cases, the pump light is coupled into the PCF by focusing lens, the coupling efficiency is usually less than 80% at lower pump power and might be unstable because of external disturbance. As the pump power increases, the input end of the PCF is likely to be destroyed at high power level [6]. Also, efforts have been made to splice the laser pigtail fiber and the PCF effectively. The splicing loss between the laser pigtail fiber and the PCF is usually large due to the mismatch of mode field area, which can be reduced to less than 0.5 dB after careful PCF post processing [7–9]. However, this splicing point is still easy to be destroyed at high pump power level [10], which limits the output power of the generated supercontinuum to about one hundred watts [9].
In recent years, the technique of supercontinuum generated directly from ytterbium-doped fiber amplifier has pushed the output power of supercontinuum source to over two hundred watts [11–13]. The missing of the weak splicing point between the laser pigtail fiber and the PCF ensures the safe operation at high power level. The fibers used in the last stage amplifier in this technique are usually large mode area (LMA) fibers such as 30/250 fiber to increase the damage threshold [11], which is actually a few-mode fiber. The intermodal nonlinear effects have been considered in narrow band fiber amplifiers with LMA fibers, such as intermodal four-wave mixing [14, 15], cross-phase modulation [16] and Kerr self-cleaning [17,18]. However, high power supercontinuum in high order mode in fiber amplifier has never been reported. The possible higher order modes in this kind of supercontinuum sources cannot only affect the output beam quality, but also provide a novel supercontinuum generation mechanism different from the fundamental mode, due to their different dispersion properties and possible intermodal nonlinearities [19–23].
The purpose of this study concentrates on high power LP11 mode supercontinuum generation in an all-fiber system. Over 50 W average output power is obtained. A mechanical long period grating (LPG) acts as a mode converter with high efficiency. By applying an appropriate pressure on the mechanical LPG, the output mode can be switchable between the LP01 mode and the LP11 mode. The last stage amplifier uses 25/250 LMA ytterbium-doped fiber (YDF). To the best of our knowledge, this is the first experimental demonstration of generating high power LP11 mode supercontinuum.
2. Experimental setup
The schematic diagram of the experimental setup is shown in Fig. 1. The seed laser exhibits a repetition frequency of 1 MHz and a pulse width of 330 ps at 1060 nm. By using a combiner, the seed and pump laser are coupled into a 4 m long 10/130 LMA cladding-pumped YDF which is used as a pre-amplifier to provide sufficient signal power to the power amplifier. The core diameter and numerical aperture (NA) of the LMA fiber are 10 μm and 0.08, respectively. The pre-amplifier is pumped by two 8 W 976 nm laser diodes (LDs). After the YDF there is a high-power isolator to prevent the backward light from the followed power amplifier. A pump dumping device is arranged between the YDF and the input fiber of the isolator to let the residual pump light out. A mode field adapter is used to connect the output fiber of the pre-amplifier and the input pigtail fiber of the combiner in the power amplifier. The output pigtail fiber of the mode field adapter is 25/250 LMA double cladding fiber (DCF) that can be stressed by the mechanical LPG to realize mode converting [24, 25]. By applying an appropriate pressure to the mechanical LPG, the fundamental LP01 mode can be coupled into the LP11 mode effectively. The power amplifier is pumped with four temperature-stabilized 50 W 976 nm LDs. The active fiber is a kind of LMA-YDF with 25 μm core diameter and 0.06 NA. The 15 m gain fiber is loosely coiled on a water-cooled heat sink to reduce the bending loss of the LP11 mode. 1 m passive fiber is fusion spliced to the YDF to make a pump dumping section to strip off the residual pump light. The fiber output end is fusion spliced with 2 mm long coreless angle-cleaved fiber as an endcap to avoid harmful back-reflection. A wavelength insensitive thermal power meter is used to measure the output power. Two optical spectrum analyzers (600–1700 nm and 1200–2400 nm) were utilized to analyze the output spectrum of the system. A long-wavelength-pass filter with 1200 nm cutoff wavelength is used when measuring 1600–2000 nm spectra to eliminate measuring errors caused by high order diffractions.
Fig. 1 Schematic of the experiment. ISO, isolator. LD, laser diode. MFA, mode field adapter. LPG, long period grating. LMA-YDF, large-mode-area ytterbium doped fiber.
3. Results and discussion
The power and spectral properties of the pre-amplifier are shown in Fig. 2. The slop efficiency of the pre-amplifier is 65%. The average output power measured after the second ISO reaches 7.91 W at 12.56 W pump power, corresponding to a peak power of about 24 kW. The first Raman peak appears because of high peak power, leading to an asymmetrical spectrum broadening. The output spectrum can reach 1700 nm at the maximum pump power of around 16 W (not shown in the figure). Because of the narrow bandwidth of the mechanical LPG, we restricted the pump power of the pre-amplifier to a moderate level of 7.8 W in the followed experiments, corresponding to 4.6 W output power.
The power properties and output beam profile of the power amplifier are shown in Fig. 3. The modes can be switchable by loading/releasing the pressure on the mechanical LPG. As the pump power increases, high power supercontinuum can be generated in both LP01 and LP11 modes. The maximum average output power is 57.4 W and 54.41 W with slop efficiency of 52% and 50%, respectively for LP01 and LP11 mode. The output beam profiles shown in the inset pictures of Fig. 3 are measured at low power level.
Fig. 3 Power properties and corresponding output beam profiles of the power amplifier for (a) LP01 and (b) LP11 mode.
The output spectrum of each mode is shown in Fig. 4. Unlike the narrow band LP11 mode laser generation in [25], in this study, supercontinuum of LP11 mode is realized in the amplifier by increasing the pulse peak power and increasing the active fiber length to induce efficient spectrum broadening. The maximum average output power is 54.51W corresponding to a peak power of 165 kW, which is much higher than the peak power of 20 kW in [25]. Due to the lager mode area of the LP11 mode, the nonlinear effect is relatively weak at low power level compared with the LP01 mode. Both of the two modes can reach broadband supercontinuum spectrum. It is surprising that the LP11 mode exhibits even wider supercontinuum generation at last, as shown in Fig. 4(b).
As the pump power increases, it is difficult to accurately measure the purity of the LP11 mode due to the high output power and broadband spectrum. The response wave band of the laser intensity distribution analyzer in our laboratory is 190 nm-1100 nm. Most mode decomposition methods such as using photonic lanterns and Stochastic Parallel Gradient Descent (SPGD) algorithm are all suitable only for laser with narrow line-width. We observed the output beam through a thermal infrared viewer. As the pump power increases, the beam profile keeps in a profile similar to the LP11 mode at a certain range. When the pump power increases to higher level (over 113 W), the gap between the two lobes of the LP11 mode becomes unclear, suggesting a poor mode purity of the LP11 mode. By using bending loss method, we have a rough measurement of the proportion of the LP11 mode at 33.49 W output power. In this situation, we bend the final passive fiber into a ring with 0.06m diameter and get 28.84 W output power. As the main energy of the supercontinuum is concentrated in 1060-1100 nm, the distinction of the bending loss at different wavelengths will induce little calculation error. The bending loss of the LP11 mode at 1060 nm is 3.8 dB/m. The proportion of the LP11 mode, is roughly estimated to be about 91%. This method is not suitable for higher output power because the bending loss will induce extra heat to the passive fiber which might cause a burning-out of the fiber at high power level. When the pump power increases to 113 W, by using a thermal infrared viewer, we get a far-field spot of the output laser as shown in Fig. 5(a). The output beam still keeps in a profile similar to the LP11 mode, indicating that the main output mode is the LP11 mode. Previous research has demonstrated that the LP11 mode can be amplified in LMA-YDF [25], while in this study, it is shown that supercontinuum of LP11 mode can be generated in the amplification process.
Fig. 5 (a) The far field spot at maximum average output power of 54.51W. (b) Comparation of the output spectrum of the two modes at maximum pump power.
The final output spectrums of the two modes are shown in Fig. 5(b). Due to the lager mode field area of the LP11 mode, the nonlinear effect is relatively suppressed at low power level compared with the LP01 mode. However, it is interesting that the LP11 mode supercontinuum covers the spectrum range from ~900 nm to ~2100 nm, while the LP01 mode covers only from ~900 nm to ~2000 nm. By using the fiber intrinsic equation, we calculated the cutoff wavelength of the LP11 mode to be 1960 nm, which is shorter than the long wavelength edge of the LP11 mode supercontinuum generated in this study. As the pump power increases, the red-shift spectrum broadens over the cutoff wavelength of the LP11 mode. The main mode of the generated light longer than the LP11 mode cutoff wavelength (1960 nm) is the fundamental mode. When the fundamental mode is accumulated to a certain proportion, the intermodal effects appear, which is the main reason why the LP11 mode exhibits longer spectrum broadening at high power pumping.
The pump power can be further increased to generate higher output power with broader spectrum, but the mode purity of the LP11 mode supercontinuum will decrease due to the cutoff wavelength of the LP11 mode. The pump power is limited at 113 W in the experiment to generate high power LP11 mode supercontinuum with relatively high mode purity. There are two possible ways to further increase the output power without degradation of the LP11 mode purity. One is to increase the pulse repetition rate or pulse width to decrease the peak power thus the nonlinearity to control the spectrum broadening within the range of shorter than the cutoff wavelength of the LP11 mode. The other one is to use fibers with longer LP11 mode cutoff wavelength, such as the 30/250 fiber with 0.06 core NA that exhibits an LP11 mode cutoff wavelength of 2350 nm. In this condition, longer wavelength LP11 mode supercontinuum can be generated with high LP11 mode purity. However, the supported mode number for this kind of fiber is larger than the 25/250 fiber used here, so mode competition might make it difficult to generate pure LP11 mode output.
4. Conclusion
In conclusion, we have demonstrated an all-fiber MOPA with a mechanical LPG to control the input modes of the power amplifier to generate high power LP11 mode supercontinuum. We have obtained 54 W average output power of LP11 mode supercontinuum covering from ~900 nm to ~2100 nm, while the LP01 mode supercontinuum only reaches 2000 nm. The efficiency of the main fiber amplifier is almost immune to mode switching with 50% pump efficiency for the LP11 mode and 52% for the LP01 mode. The LP11 mode has relatively larger mode field area that can reduce the power density in the fiber core, which is helpful to increase the available output power of supercontinuum in cascade fiber amplifiers. Also, the cutoff wavelength of the LP11 mode helps the fundamental mode generation at wavelength longer than the cutoff wavelength at the maximum pump power, which induces rich intermodal effects.
Funding
National Natural Science Foundation of China (NSFC) (61235008); National High Technology Research and Development Program of China (2015AA021101).
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