We report an all-fiber linearly-polarized (LP) supercontinuum (SC) source with high average power generated in a polarization-maintaining (PM) master-oscillation power-amplifier (MOPA). The experimental configuration comprises an LP picosecond pulsed laser and three PM Yd-doped fiber amplifiers (YDFA). The output has the average power of 124.8 W with the spectrum covering from 850 to 1900 nm. The measured polarization extinction ratio (PER) of the whole SC source is about 85% which verifies the SC an LP source. This work is, to our best knowledge, the highest output average power of an LP SC source that ever reported. The influence of PM fiber splicing method on the LP SC property is investigated by splicing the PM fibers with slow axis parallel or perpendicularly aligned, and also an LP SC with low output power is demonstrated.
© 2015 Optical Society of America
Supercontinuum (SC) generation in nonlinear fibers has been a hot research subject for more than a decade and the underlying physical mechanisms have been studied broadly and deeply [1,2 ]. SC sources have extensive applications in many fields such as optical communications [3–5 ], spectroscopy [6,7 ], optical frequency metrology [8–10 ] and optical coherence tomography (OCT) [11–13 ]. The generated continuum is generally unpolarized, as the fiber core is circularly symmetric, whereas many applications require linearly-polarized (LP) SC [14–18 ]. In order to achieve LP SC output, polarization-maintaining (PM) photonic crystal fibers (PCF) were generally utilized. Several correlative investigations on the polarization characteristics of SC generated in PM-PCFs [9–22 ] demonstrate that the spectral components of SC generated from highly birefringent PCFs exhibit the same linear polarization state [15,19 ]. Furthermore, when the input pulse is polarized close to the principal axis of the PM-PCF, SC source with a constant polarization state and a high polarization extinction ratio (PER) across the whole spectrum can be achieved [15,20 ].
Some kinds of applications e. g. hyperspectral imaging require the SC source to have not only linear polarization state but also high output power [16,18 ]. This is because that the linear polarization characteristic offers one more dimension of information when polarization-based detecting systems are adopted. The utilization of SC sources with high output power scales the measurement distance and also affords a higher signal to noise ratio. However, in the Ref. [14,15,19–22 ] the output powers of LP SC seed sources based on PM-PCFs were all limited to a low level due to the small core diameter of the fiber. Generally, there are two ways to obtain high power SC sources. The first one is pumping a section of PCF by a laser pump with high power and the other is expanding the laser spectrum to be an SC in a high power fiber amplifier. At present, the recorded highest power SC generated in a single-core PCF is 92.5 W using 2.6 m long PCF pumped by 120 W picosecond laser  and the highest SC power generated in a multi-core PCF is 116 W ranging from 800 to 1650 nm using a section of 20-m-long seven-core PCF pumped by 141 W picosecond laser . However, the raise of the output power generated from PCF is limited by the small core of the fiber and its splice with the output fiber of high power fiber laser. Recently, high power SC sources generated in fiber amplifiers have been reported. In a fiber amplifier two kinds of processes, power amplification of the pulsed signal light and spectral broadening induced by nonlinear effects, are combined to influence the signal pulse as it propagating in the gain fiber. With suitable signal light, pump light and gain fiber, the signal light will be amplified and simultaneously evolve to be an SC. By reported, over 70 and 177.6 W SC sources ranging from ~1.06 μm to beyond 1.7 μm were obtained in nonlinear Yb-doped fiber amplifiers (YDFA) [25,26 ]. Thus it is an applicable method to obtain high power LP SC sources in PM fiber amplifiers.
In this manuscript, we report a high power near-infrared LP SC source generated in a PM-YDFA. We firstly investigate the corresponding polarization characteristics of the output light by splicing PM fibers in different splice types. Then SC with the maximal average output power of 124.8 W and a broadband spectrum ranging from 850 to 1900 nm is generated in the last PM amplifier. The measured PER of the whole SC is about 85%, verifying the SC light to be linearly polarized. To our best knowledge, this is the LP SC source with the highest average power at present.
3. Experiments and theoretical analysis
The main experimental setup is a four-stage MOPA configuration comprising an LP picosecond pulsed laser and three PM-YDFAs. Figure 1 gives the schematic in the experiment and as it shows, after the second PM-YDFA the latter experiment configuration is separated into two parts: Part 1 is a section of PM-DCF (polarization-maintaining double clad fiber) and Part 2 is the final PM-YDFA. The pulsed laser seed is an electrical modulated picosecond laser source with the central wavelength of 1060 nm, the pulse-duration of 70 ps and the repetition rate of 5 MHz. The output power of the seed is 54.9 mW. The temporal shape and the spectrum of the pulse are shown in Fig. 2 . In the first PM-YDFA, the seed is amplified to 0.503 W pumped by a 2 W fiber pigtailed 976 nm laser diode (LD). About 1.5-m long PM double-clad Yb-doped fiber (DC YDF) is chosen as the gain fiber which has a core diameter of 12 μm and a clad diameter of 125 μm. The core and clad numerical apertures (NA) are 0.08 and 0.46 respectively. The peak cladding absorption at 976 nm is 11.1 dB/m. And the birefringence of the gain fiber is larger than 1.6 × 10−4 which demonstrates that the fiber is highly birefringent. A PM optical isolator is added to suppress the backward light.
The second PM-YDFA has the same structure as the first one. A PM tapper is spliced to the isolator to monitor the backward light at the In2-port. In this amplifier the signal light is amplified to 6.03 W pumped by a 16.1 W LD at 976 nm. Polarization state of the output laser is measured using a Glan-Taylor prism. The output light is collimated by a collimating lens and then propagates through the Glan-Taylor prism. The prism effectively works over the wavelength range from 250 to 2500 nm and the PER is 105. The prism is mounted in a rotatable holder and a power meter is utilized to measure the transmitted power. Firstly the prism is rotated to an optimal angle where the measured P1 power is the maximum. Then rotating the prism by 90° another power value P2 is measured. Thus the powers of two orthogonal polarization states are measured. The PER value of the output laser generated from the second PM-YDFA at different power levels can be calculated and the results are shown in Fig. 3(a) . The PER is defined by the formula PER = P1/(P1 + P2) × 100%.
From Fig. 3(a) it can be seen that the PER is maintained at a high level and the values are all larger than 99% in the power scaling process. The total output spectrum and spectra of the two orthogonal polarization states (marked by the corresponding powers P1 and P2) at the highest output power are shown in Fig. 3(b). More specifically, the total spectrum is firstly measured before the prism and then spectra of P1 and P2 are measured respectively. According to the pulse duration, repetition rate and the average power, the calculated peak power in the fiber is ~17 kW. This value is higher than the threshold of stimulated Raman scattering (SRS), thus a distinct Raman peak around 1118 nm can be observed in the spectra. Additionally, due to the low repetition rate, some weak amplified spontaneous emission (ASE) emerges around 1035 nm as shown in the figure. The three measured spectra exhibit similar profile which verifies that SRS effect and ASE will not induce polarization degradation.
3.1 Experiment at LOW power level
In order to investigate the influence of PM fiber splicing method on the LP SC property, as Part 1 shows in Fig. 1, a section of 20-m long passive PM-DCF acting as the SC-generation fiber is spliced with the Out1 port of the PM tapper. The connection point between the PM-DCF and the PM tapper is spliced in two forms: the slow axis of the tapper-fiber is parallel (Type I) or perpendicular (Type II) to the slow axis of the PM-DCF, which is shown in Fig. 4 . The power characteristics of the two splice types are depicted in Fig. 5(a) . From this figure it can be found that these two splice types are both highly efficient and the output powers have little discrepancy. The PER of the output light in different power levels are measured respectively and the result is shown in Fig. 5(b). The figure shows that the PERs of the two types are both high enough, but the result of type II is a bit better than that of type I.
Figure 6 shows the total output spectrum and spectra of P1 and P2 in the highest output power level of splice type I and type II. Spectra of the two splice types are both widely extended, so ASPER (all-spectral PER) instead of PER represents the polarization characteristic of the output source, which is defined by the formula ASPER = ∑P1(λ)/ [∑P1(λ) + ∑P2(λ)] × 100%.
As Fig. 6 (a) shows, spectra of the two polarization states (P1 and P2) are both similar to the total output spectrum which means that type I excellently maintains the polarization characteristic. While in Fig. 6 (b) spectra of P1 and P2 change distinctly at about 1400 nm, from where the light turns into the anomalous dispersion region and the two orthogonal polarization states begin to couple with each other. Correspondingly, Fig. 5 (b) shows that the ASPER of type II reduces gradually as the pump power increases, and at the same time the increase of pump power brings about the broadening of the spectrum. Thus we can come to the conclusion that the linearly-polarization characteristic of the output deteriorates when the light broadens to an SC. Due to the fact that the ASPER of type I remains more stable than that of type II as the pump power increases, the point of the last amplifier is spliced in type I to obtain high power LP SC source.
3.2 Experiment at HIGH power level
As Fig. 1 shows, in Part 2 another PM amplifier is used to substitute the PM-DCF. This main amplifier is not only the power amplifier but also the nonlinear material. The last PM-YDFA is composed of a (6 + 1) × 1 PM combiner and a section of 11 m long PM double clad YDF which has the core/cladding diameter of 25/400 μm with the NA of 0.065/0.46 and the absorption of fiber at 976 nm is about 3.2 dB/m. The birefringence of the PM gain fiber is 3.5 × 10−4. The gain fiber is pumped by two 150 W fiber pigtailed 976 nm LDs. Acting as the output fiber, a section of 1 m long passive PM-DCF with the core/cladding diameter of 25/400 μm is spliced to the gain fiber. The coat of a section of about 10 cm long passive fiber is stripped and covered by high-index glue to dump the residual pump light. And the fiber end-facet is angle-cleaved at 9° to prevent back propagated light.
The power scaling characteristic of the main PM-YDFA is shown in Fig. 7 . The output power is linearly increased versus the pump power and the pump-limited maximum output power is 124.8 W. The optical-to-optical conversion efficiency is about 47% which is much lower than those of fiber lasers. This is because that the signal laser is not only power amplified in the YDFA but also broadened to longer wavelength region due to nonlinear effects. Thus quantum defect dependent loss becomes higher and the optical-to-optical conversion efficiency drops. The output spectrum at 124.8 W is recorded and shown in the inset of Fig. 7. The output beam is collected by a multi-mode fiber patch cable and then recorded by an optical spectrum analyzers (OSAs). From Fig. 7 it can be seen that the spectrum has been extended to longer than 1900 nm.
3.3 Analysis of the results
The output spectra at different power levels are shown in Fig. 8 . The calculated zero-dispersion wavelength (ZDW) of the PM 25/400 μm YDF is 1270 nm and thus the signal light propagates in the normal dispersion region of the gain fiber. When the pump power of the final amplifier is zero the signal light is absorbed by the gain fiber to be 1.7 W from the initial 6.03 W and three Raman peaks around 1116, 1170 and 1230 nm can be obviously observed in the spectrum. When the pump power increases, the signal light and three Raman peaks are amplified to a high value due to the combination of Yb and Raman gain of the YDF. As the fourth order Raman peak is located around 1300 nm which is longer than the ZDW, the light extends to the anomalous dispersion region. This is because that SRS effect cannot form a new Raman peak but the light is affected by modulation instability (MI) and soliton effects. Under Raman-assisted soliton self-frequency shift effect, the spectrum is broadened to longer wavelength further. When the pump power increases to the maximum the output spectrum is extended to longer than 1900 nm.
In order to measure the ASPER of the obtained SC, the Glan-Taylor prism is used in the same way as mentioned in the first paragraph of section 3, and Fig. 9 (a) shows the calculated ASPER versus the output power. From the figure it can be seen that the ASPER decreases as the output power increases, which illustrates that even transferring in high birefringence fiber the PER of the output light also deteriorates due to the coupling of the two orthogonal polarization states. In comparison with the lower power results, the ASPER degrades more severely when the output power is higher. This is because that, when power in the gain fiber is in a high level, more signal power will be transferred from a polarization state to the orthogonal state, which narrows the gap between powers of the two states. This process occurs more and more intensely along the increase of the pump power, thus the ASPER continuously declines as Fig. 9(a) shows. The ASPER is higher than 90% when the output power is lower than 70 W and when the output power reaches the highest value it is about 85%. By comparing the spectra of the highest output power between the total output with P1 and P2 states as shown in Fig. 9(b), we can see that the three spectra are quite alike, which verifies that the light of the whole spectrum has an ASPER of 85%. Though the ASPER is not as high as those of narrow-band wavelength lasers, and also decreases gradually with the increase of the output power, however, based on the theoretical analyses reported previously , the generated SC source is well developed and could be treated as an LP source.
Conclusively, an all-fiber LP SC source with high power is obtained from a four-stage PM MOPA configuration. The maximal average power of the output SC is 124.8 W and the spectrum extends from 850 to above 1900 nm. The ASPER of the whole SC is also measured as about 85%, which verifies the output an LP SC. This is, to our best knowledge, an LP SC with the highest average output power that ever reported. By splicing the point between two sections of PM fibers in different splice forms, we also demonstrate that how the change of splice forms influence the characteristics of the output SC source. This kind of SC source has potential in many application fields such as hyperspectral imaging and remote sensing.
This work was supported by the State Key Program of National Natural Science of China (Grant No. 61235008), the National High-Tech Research and Development Program of China (863 program) (Grant No. 2015AA021101), the Natural Science Foundation of China (Grant No.11504424), the Postgraduate Innovation Foundation of Hunan Province, China (Grant No. CX2014B035) and the Postgraduate Innovation Foundation of National University of Defense Technology, China (Grant No.B140704).
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