An all-fiber linearly polarized supercontinuum (SC) laser source with 93 W average output power and spectrum ranging from 520 nm to 2300 nm is experimentally demonstrated. The linearly-polarized SC is generated in a piece of 2.6 m long polarization-maintaining photonic crystal fiber (PM-PCF), pumped by a polarization-maintaining picosecond Yb-doped master oscillator power amplifier (PM-MOPA). The source exhibits a flat spectrum from 600 nm to 1880 nm at −10 dB level except for the residual pump peak. A new method is proposed to measure the polarization extinction ratio (PER) of each single wavelength of the broadband supercontinuum at a high-power level, resulting in larger than 16 dB PER from 900 nm to 1600 nm and larger than 15 dB PER from 540 nm to 650 nm. To our knowledge, this is the first demonstration of hundred-watt level linearly polarized visible SC and the first demonstration of PER measurement of each single wavelength within such a wide spectrum range.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Visible supercontinuum (SC) sources have attracted considerable attention due to potential applications in optical frequency metrology spectroscopy, optical coherence tomography and hyperspectral lidar [1–4]. In particular, high-power SC source is one of the hot research topics in this field. Photonic crystal fibers (PCFs) based on silica are particularly attractive candidates for high-power SC generation which require low-loss materials in the wavelength range of interest, as well as appropriate nonlinear coefficient and controllable dispersion [5–7]. In 2016, C. Sun et al used a picosecond ytterbium-doped fiber laser to pump a piece of 5 m long PCF to obtain 67.9 W SC with spectrum coverage of 500-1700 nm . In 2018, X. Qi. et al demonstrated an 80 W SC with spectra covering 350-2400 nm from multicore PCFs . In the same year, L. Zhao et al realized a 215 W SC from 480 nm to over 2000 nm from PCF .
However, all these hundred-watt level visible SC sources from PCFs are randomly polarized. In some applications, linearly polarized SC source is needed . Polarization-maintaining (PM) PCFs are suitable medium for linearly polarized SC generation. Many studies have been conducted on this subject, particularly in literatures [12–16]. However, owing to the mode field mismatch and large coupling loss between the conventional PM fiber and the PM-PCF, the output power of linearly polarized SC source from PM-PCF is no more than ten Watts up to now. An exception is the linearly polarized SC directly generated from fiber amplifier , however, with a very limited spectral broadening that can hardly reach the visible range, due to difficult dispersion management in conventional fibers.
In this paper, a high-power all-fiber linearly polarized SC source with 93 W power is experimentally demonstrated. The spectrum of the linearly polarized SC source covers from visible to near-infrared band. A new method based on 45° angled fusion splicing between two PM fiber sections and rotating the polarization beam splitter (PBS) is proposed for the first time to measure the PER of the high-power linearly polarized SC.
The schematic diagram of the linearly polarized SC laser system is shown in Fig. 1. A picosecond pulsed seed source is amplified by an ytterbium-doped fiber amplifier (YDFA) chain. The 10/125 μm PM circulator (PM-CIR) following the YDFA chain protects the seed from backward propagation light. A piece of 4 m long 10/125 µm double-cladding PM-YDF with a nominal absorption coefficient of 5 dB/m at 976 nm is adopted as the gain medium in the main stage amplifier, which is cladding pumped by two 90 W fiber-pigtailed multimode laser diodes via a (2 + 1) × 1 polarization-maintaining fiber combiner.
The amplified pulsed laser is then launched into a piece of 2.6 m long PM-PCF, as shown in the inset of Fig. 2(a). The PM-PCF has a core diameter of 4.8 μm, an air-hole pitch of 3.32 μm and an air-hole diameter of 1.36 μm, as shown in Fig. 2(b). The birefringence is at 1064 nm and the zero-dispersion wavelength (ZDW) is calculated to be 1070 nm for the fiber. To reduce the coupling loss induced by the mismatch of mode fields between the main amplifier fiber and the PM-PCF, a 10/125 μm switching to 6/125 μm PM mode field adaptor (PM-MFA) is added. The splicing point between the PM-MFA and the PM-PCF is important and needs to be handled elaborately. By selecting suitable splicing parameters, a polarization-maintaining point with transmission rate of ~91% is achieved. The splicing points, the main amplifier and the PM-PCF are carefully dealt with heat management. At the output port, an angle cleaved end-cap is made to eliminate back reflection and protect the PM-PCF from dust, humidity and other contaminants.
3. Results and discussion
3.1 Performance of the high-power linearly polarized SC source
Figure 3(a) shows the spectral evolution of the main amplifier, which operates at 1064 nm with 132.6 W highest average output power. The Raman effect are stimulated at 1120 nm when the output power exceeded 100 W with a peak 40 dB lower than the main peak. The main amplifier delivers 166 ps pulses at a high repetition rate of 200 MHz, as shown in Fig. 3(b). The average output power of the main amplifier as a function of the incident pump power is plotted in Fig. 4, showing a slope efficiency of ∼73%.
Figures 5(a) and 5(b) show the spectral and power properties of the linearly polarized SC. As pump power increases, multiple nonlinear effects lead to a broadening of the spectrum . The Four Wave Mixing (FWM) and Stimulated Raman scattering (SRS) dominant the initial spectrum broadening. Modulation instability (MI) gives birth to solitons after the boundary of the spectrum reaches and rises above the ZDW of the PM-PCF. The solitons undergo soliton self-frequency shift (SSFS) mechanism and thus induce a large continuum up to 2300 nm . At the same time, dispersive wave generation broadens the spectrum to short wavelengths of 520 nm. With further pump power transferred to longer wavelengths, it will be attenuated by the silica fiber losses. Finally, the output power of the linearly polarized SC source increases monotonously to 93 W, limited mainly by the available pump power at present. The spectrum covers from 520 nm to 2300 nm with an average spectral power density of 52 mW/nm in the entire wavelength range and exhibits a flat spectrum from 600 nm to 1880 nm at −10 dB level except for the residual pump peak. Figure 5(b) plots the curve of linearly polarized SC average power versus incident pump power, showing a slope efficiency of about 68.2%. Inset in Fig. 5(b) shows that the pulse width of the linearly polarized SC source is broadened to 181 ps.
3.2 The PER of the high-power linearly polarized SC
Since the SC has a wide spectral range, the PER of each wavelength within the bandwidth should be guaranteed to ensure the polarization property of the whole SC source. Z. Zhu et al utilized a half-wave plate, a quarter-wave plate and an analyzer while B. Zhang adopted a Glan-Taylor prism to measure it [20,17]. Since the wave plates are wavelength dependent, the measured spectral range is limited. The Glan-Taylor prism is applied in wide spectrum range but the results measured by plates and prism both are a synthesis of power intensity in a range of wavelengths, it cannot reflect the polarization characteristics of each single wavelength in the linearly polarized SC spectrum range. Moreover, the low damage threshold of the plates indicates that it is not suitable for measuring linearly polarized SC polarization properties at high power level. Based on Lyot filter principle and related measurement approaches [21,22], we propose a new method to measure the PER of high-power linearly polarized SC on spectral-domain. The measurement configuration is shown in Fig. 6. The PM-PCF is spliced to a piece of PM germanium-doped fiber (GDF) with 6 μm core and birefringence of Δn, the spliced angle is set to be 0° to ensure the linearly polarized SC light propagate along slow axis. Then the PM-GDF is 45° angled spliced to a length of L1 PM-GDF, which play as a wave plate. The beam of light collimated by the lens 1 enters the PBS which can be rotated 360° on X-Y plane, and is then focused by the lens 2 onto a single-mode patch cord to an OSA.
Through the 45° splicing point, the incident linearly polarized SC light polarized along the slow axis would be equally decomposed into both fast and slow axes of the PM-GDF. After the transmission through the PM-GDF with length of L1, the phase difference Δ ϕ generated by light at wavelength λ propagating in the fast and slow axes can be expressed as Eq. (1).Eq. (2).Eq. (3), where Eλ1(s), Eλ2(f) represents the intensity of light propagating parallel to slow axis and fast axis at wavelength of λ1 and λ2.
The PER of the pulsed seed laser transmitting after the PM-PCF is 16.4 dB, measured by a polarization extinction ratio meter at a ~mW power level. Limited by the bandwidth of the PBS, two representative spectra are selected for PER measurement. By adopting the measurement method that we proposed above and measuring at the highest power level, the spectral scanning results with 0.02 nm resolution show that the PER of 900-1600 nm are above 16 dB, as can be seen from Fig. 7(a). The degradation of PER near the central wavelength of 1064 nm is considered as that the pump light is converted into light of other wavelengths by nonlinear effects. We also measure the PER of short wavelength spanning from 520 nm to 650 nm, which are great than 15 dB averagely in the range of 540-650 nm. As the signal intensity around 520 nm is too weak for the OSA to be directly detected, the measured PER is only 10 dB. The measurement results show that there is no PER degradation when the power increases and the linearly polarized SC has a good degree of single-wavelength polarization PER in the whole spectrum. The results also verify the feasibility of this PER measurement method for wide spectrum light sources. However, since it is not a straight measurement on the real PER of each single wavelength, a precondition for the measurement to be correct is that the PER of the broadband light source do not change or change slightly with wavelengths.
In conclusion, a high-power all-fiber linearly polarized SC source is experimentally generated from a piece of 2.6 m long PM-PCF pumped by a picosecond Yb-doped PM-MOPA, which operates around 1064 nm with pulse duration and repetition rate of 166 ps and 200 MHz respectively. The output spectrum ranges from 520 nm to 2300 nm with a maximum 93 W average output power under 132.6 W pump power. By adopting a new method for measuring the PER of each single wavelength within a broad spectrum at a high-power level, over 16 dB and 15 dB PER are experimentally verified in the spectra of 900-1600 nm and 540-650 nm.
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