We proposed and demonstrated an all-fiber laser to generate cylindrical vector beam. A pair of fiber-based collimators was used to select the cylindrical vector beam operating. The radially and azimuthally polarized modes can be switchable just by applying pressure to a section of fiber in our fiber laser system. A 70 cm long Yb-doped fiber was used as gain medium and the lasing wavelength was around 1030 nm.
©2010 Optical Society of America
In the past two decades, the unique properties of vector beams especially those with cylindrical vector (CV) symmetry, radial polarization (RP) or azimuthal polarization (AP), have attracted more and more interests. Electric field distributions focusing through high numerical aperture have different characters for RP and AP beam. For radially polarized beam, the existence of a large longitudinal field component may lead to a smaller focal spot. For azimuthally polarized beam, there is doughnut spot in focal plane of the objective lens .
Theoretical and experimental researches have demonstrated the useful applications of CV beams including single molecule imaging , atoms guiding , atom manipulation , optical tweezers [5,6], material processing  and surface Plasmon exciting . Various techniques, passively [9–13] and actively [14–18], have been developed to generate such cylindrically polarized fields.
Fiber lasers have attracted greatly interests because of their high efficiency, compactness and flexibility. In previous fiber lasers [14–18] for CV beams, a volume element was always used to select the desire polarization mode inside the laser cavity, which would increase the complexity of the system and instability of the laser output, unavoidably.
In this paper, we proposed and demonstrated an all-fiber laser to generate CV beam. The laser system was composed of a fiber loop mirror, 977 nm pump laser, 980/1060 nm WDM, a couple of fiber based collimators, polarization controller, Yb-doped fiber, and a section of few mode fiber. A couple of fiber-based collimators were used to select the cylindrical vector beam operating. The radially and azimuthally polarized modes can be switchable just to apply pressure to a section of fiber in our fiber laser system. A 70 cm long Yb-doped fiber was used as gain medium and the lasing wavelength was around 1030 nm.
2. Experimental setup
The experimental setup is illuminated in Fig. 1 . The laser system was composed of a fiber loop mirror, 977 nm pump laser, 980/1060 nm WDM, a couple of fiber based collimators, polarization controller, Yb-doped fiber, and a section of few mode fiber. A couple of fiber-based collimators were used to select the cylindrical vector beam operating in our fiber laser system. A 977 nm laser diode was used as pump laser to pump the Yb-doped fiber through 980/1060 nm WDM. A 70 cm long Yb-doped fiber with absorption of 470 dB/m was used as gain medium and the lasing wavelength was around 1030 nm. The corresponding NA and core diameter of Yb-doped fiber are 0.14 and 6 µm, respectively. In order to gain effective coupling between the fundamental mode and high order mode, one fiber collimator was selected for 1060 nm wavelength, while the other for 1550 nm wavelength. Red solid line illustrated in Fig. 1 is SMF-28 fiber, which is connected to the 1550 nm collimator. The cutoff wavelength of the SMF-28 fiber is about 1260 nm, that is say, the SMF-28 fiber is few-mode fiber for 1060 nm band. Misalignment in angle and transverse dimension were adjusted to excite high order mode in SMF-28 fiber. One of the cavity mirrors was a fiber loop mirror with the reflectivity of 80%, while the other was cleaved end face with reflectivity of 4%. The spectrum of fiber laser is measured using optical spectrum analyzer (Ando 6317B).
A linear polarizer was placed with optical axis paralleling the laser beam. The polarization analyzer used in our experiment was Glan prism. By rotating the polarizer, the intensity profiles in different polarization direction were recorded by CCD.
3. Results and discussions
Figure 2 (a) shows the intensity distribution of the fiber laser output. The doughnut-shaped beam indicates high order mode output. The fibers used in our experiment are two types, single mode fiber and few modes fiber (SMF-28) for 1060 nm band, only the fiber connecting to the 1550 nm collimator is SMF-28 fiber (highlight in red color in Fig. 1). To excite the high order mode in SMF-28 fiber, we adjusted the misalignment between two fiber collimators. Mode conversion takes place when the laser signal propagates from left collimator to right one, and vice versa.
As shown in Fig. 1, the red solid line represents SMF-28 fiber connected to the 1550nm collimator, and the black solid line represents the 1060 nm band single mode fiber connected to the 1060 nm collimator. In the laser cavity, the 1060 nm band single mode fiber can only support the fundamental mode, and SMF-28 fiber (Corresponding normalized frequency V is about 3.5 at 1030 nm) can support fundamental mode and high order mode. There exist two kinds of mode couplings from 1060 nm collimator to 1550 nm collimator. One is fundamental mode to fundamental mode (F-F coupling). The other is fundamental mode to high order mode (F-H coupling). The CV beam generation is corresponding to F-H coupling. If F-F coupling, the laser operated at regular state, no CV beam output was observed. In order to introduce the F-H coupling and obtain the CV beam, we misaligned the1060 nm and 1550 nm collimators. The misalignment is to excite high order mode in the few mode fiber, when light translates from 1060 nm collimator to 1550 nm collimator, which is similar to reference .
When a polarizer was inserted after laser beam, two lobs appeared in intensity profile. Figure 2(b)-(e) are intensity profile distributions for radial symmetry in different polarized direction. The white arrows indicate different polarization directions. Radial polarization property is proved where the arrow direction was orthogonal to the dark line of the intensity profile.
The output power variation as the pump power is shown in Fig. 3 . It can be seen in Fig. 3 that the curve stands the laser operating, the pumping threshold for the laser system is about 30 mW. When the pump power exceeded the threshold, the output power increased linearly according to the pump power. The laser behavior was demonstrated through the threshold characters. In consideration of the intensity distribution in Fig. 2, the output beam is laser beam with vector polarization property. The slope efficiency for our fiber laser is about 1.5%. We believe that the low slope efficiency is mainly owing to the loss of coupling and the coupling efficiency can be improved and tuned, and potential improvement will be achieved by using more precise instrument. We also measured the laser spectrum with pump power of 50 mW.
There was no filter in the laser cavity, the most possible oscillating wavelength is around the peak emission of Yb3+ ions. For Yb-doped fiber, the emission cross-section peak of Yb3+ ions is located near 1030nm . Figure 4 shows the laser spectrum and the laser wavelength is about 1032nm under 50mW pump power.
The polarization states of the CV beam, radial symmetry and azimuthal symmetry, can be switchable when apply the pressure to the SMF-28 fiber. Pressing the few mode fiber is to introduce transverse deformation, which will change the propagating constant βx and βy in the few mode fiber, as result, the coupling between TE mode and TM mode will be induced. Thus the switching between radial symmetry and azimuthal symmetry is obtained. The intensity profile distributions of azimuthal polarized beam are shown in Fig. 5(a)-(d) with linear polarizer in different polarized directions. Azimuthal polarization property is proved where the arrow direction was parallel to the dark line of the intensity profile.
By changing the polarization state via polarization controller, the spectrum could change from one peak to multi peaks. We have recorded multi-peak laser spectrum in long term laser spectra shown in Fig. 6 . The two peaks are around 1028nm and 1035nm, respectively. It should been noted that the output spectrum is stable when the CV beam generated in our fiber laser, either the spectrum with one peak or with multi-peak. The output spectrum was unstable when the fiber laser operated in regular polarization state because of the competition in homogeneous gain.
In summary, CV beam with radial and azimuthal polarization was obtained from all-fiber structure laser with two fiber collimators as mode filter. By introducing misalignment of two fiber collimators, high order mode in few-mode fiber has been excited and oscillated in fiber cavity with sufficient gain. Fundamental mode was suppressed in this structure with proper transverse and angle misalignment.
According to our knowledge, it is the first time to report work of generating CV beam in all-fiber structure laser. Here we have to mention that the CV beam generated from laser is quasi-radial or azimuthal polarization where modal symmetry needs further improvements. The switching of the different polarization symmetry can be obtained by applying pressure to the few mode fiber and repeatable control of the switching will be optimized in the future work.
The authors acknowledge Professor Q. W. Zhan’s helpful discussion. The authors gratefully acknowledge financial support from the National Basic Research Program of China under Grant No. 2006cb302905, Chinese Universities Scientific Fund.
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