Cylindrical vector beams (CVBs) with axially symmetric field-amplitude distributions and radial or azimuthal polarization states have attracted great interest in recent years for a multitude of applications including optical trapping [1], high-resolution imaging [2], and material processing [3,4]. In the field of material processing, high-power CVBs have been used to obtain enhanced cutting speeds to reduce material spatter in deep-penetration welding and to achieve better micro-hole drilling quality [4]. The direct generation of CVBs under both continuous-wave (CW) and pulsed operation at average powers exceeding 100 W has been recently demonstrated from a ${\mathrm{CO}}_2$ laser [5], Nd:YAG rod amplifier [6], thin-disk multipass amplifier [7], and a single crystal fiber amplifier [8]. We recently demonstrated the efficient generation of CVBs from Yb-doped fiber (YDF) lasers and amplifiers based on conventional few-mode YDFs operating at several tens of watts of average output power in both CW and pulsed modes by employing a nano-grating-based spatially variant half-wave plate in the laser cavity/amplifier chain, achieving comparable optical efficiencies to more conventional YDF systems [9–11]. Further power scaling is of interest and the YDF-master oscillator power amplifier (MOPA) architecture, which has previously been demonstrated to provide a compact and robust approach to amplify picosecond pulses to the hundreds of watts of average output power level with diffraction-limited beam [12], represents a promising way to generate high-performance, high-power CVBs in the picosecond regime.

In this Letter, we report a gain-switched laser-diode-seeded YDF-MOPA system capable of generating picosecond pulses with high pulse energy, high peak power, and narrow bandwidths in a radially polarized beam at average powers in excess of 100 W.

A schematic of the experimental setup, shown in Fig. 1, consists of a four-stage YDF amplifier chain. The seed is a 1030 nm Fabry–Perot laser diode (Oclaro LC96A1030-20R) which is gain-switched using a train of sinusoidal RF pulses at a repetition rate of 87.5 MHz. The seed diode is self-seeded using a uniform fiber Bragg grating with a reflectivity of 12.5% to improve the spectral and temporal quality of the gain-switched pulses and produces $\sim 150\mathrm{ps}$, $\sim 4\mathrm{pJ}$ pulses at 1034.7 nm with a 3- B spectral bandwidth of 0.03 nm. The first pre-amplifier stage consists of a $\sim 85\mathrm{cm}$ long length of polarization-maintaining (PM)-YDF (Nufern PM-YDF-5/130) with a 5 μm core diameter and 130 μm cladding diameter. It was forward core-pumped by a 975 nm single mode laser diode to amplify the pulses to $\sim 20\mathrm{mW}$ average power (signal gain of $\sim 18\mathrm{dB}$). A fiber pigtailed electro-optic modulator (EOM) (Photline NIR-MX-LN-10), which has an extinction ratio of $\sim 26\mathrm{dB}$ and an excess insertion loss of $\sim 2.7\mathrm{dB}$, was used as a pulse picker to reduce the repetition frequency to any desired subharmonic of 87.5 MHz. A second pre-amplifier, having a similar configuration to the first pre-amplifier, was employed after the EOM to ensure adequate seeding of the following cladding-pumped pre-amplifier. A fiber pigtailed acoustic-optic modulator (AOM), synchronized to the EOM, was used to remove inter-pulse amplified spontaneous emission (ASE) which builds up within the prior core-pumped pre-amplifiers. The third pre-amplifier consisted of a $\sim 2.5\mathrm{m}$ long dual-clad PM-YDF (Nufern PLMA-YDF-10/125-VIII) with a core diameter of 10 μm (0.085 NA) and a cladding diameter of 125 μm. This amplifier was forward cladding-pumped with a multimode 975 nm pigtailed laser diode.

 

Fig. 1. Schematic of the picosecond YDF-MOPA for an amplifying radially polarized beam.

Download Full Size | PDF

A free-space bandpass filter (BPF) with a 3 dB bandwidth of 4 nm was inserted after the third pre-amplifier to remove any excess out-of-band ASE. The final amplifier is composed of a $\sim 2.2\mathrm{m}$ length commercial few-mode YDF (LIEKKI Yb1200-25/250) which has a core diameter of 25 μm with an NA of 0.075 and an inner cladding diameter of 250 μm with an NA of 0.46. The V-number of the YDF at 1035 nm is 5.3, which means it is capable of supporting the propagation of the two lowest order scalar modes (${\mathrm{LP}}_{01}$ and ${\mathrm{LP}}_{11}$). The fiber was loosely coiled with a large bend diameter of $\sim 25\mathrm{cm}$ to avoid any excess propagation loss and to reduce the intermodal coupling as the effective index difference of the four vector modes in the LP11 mode group are relatively small. Both ends of the fiber were spliced to $\sim 1.2\mathrm{mm}$ long silica endcaps with a diameter of 250 μm to suppress any potential parasitic lasing. The input end facet was perpendicularly cleaved, and the output end facet was angle cleaved with an angle of $\sim 8$ degrees.

A commercially available vortex retarder (also called a q-plate, Thorlabs WPV10L-1064) placed at the input to the final amplifier was employed as the transverse spatial mode converter. The q-plate is composed of a thin liquid crystal polymer film sandwiched between two 1 mm thick N-BK7 glass plates with antireflection coatings at 1 μm. The q-plate provides a constant half-wavelength retardance at $\sim 1.064\mathrm{\mu m}$ across its clear aperture; however, the orientation ($\theta$) of the fast-axis continuously rotates with respect to the azimuthal angle ($\varphi$) over the plate according to the equation: $\theta =\varphi /2+\delta$ (where $\delta$ is the orientation of the fast axis at $\varphi =0$). Therefore, with proper orientation of the q-plate with respect to the incident linearly polarized Gaussian-shaped beam, different CVBs, including the radial and azimuthal polarization, can be formed. In our case, the q-plate was oriented to convert the input Gaussian beam into a donut-shaped radially polarized beam which was then carefully launched into the final amplifier to excite the radially polarized ${\mathrm{TM}}_{01}$ mode. The q-plate has a high transmission efficiency of $>98\%$, and the resultant converted vector beam experiences negligible degradation in terms of beam quality. A typical converted radially polarized beam measured in the far field is shown in Fig. 2(a), exhibiting a pronounced donut-shaped intensity profile. The dark dot in Fig. 2(b) illustrates the measured one-dimensional intensity profile across the beam center [red dashed line in Fig. 2(a)], which could be well fitted (red line) with the first higher order Laguerre–Gaussian mode (${\mathrm{LG}}_{01}$). The propagation parameter $M^2$ of the converted donut-shaped beam was measured to be $\sim 2.06$, which is very close to the theoretical value of 2 for a radially polarized beam. One of the major advantages of the q-plate is that the resultant donut-shaped mode has negligible intensity at the beam center, as shown in Fig. 2(b). This is critical, as any residual intensity at the beam center will be amplified considerably due to the fact that the population inversion in the center of the core is not accessed by the donut-shaped beam, allowing a very high population inversion to be built up at the center of the fiber core. The two-lobed intensity profile beam was passed through a rotated linear polarizer, as shown in the Fig. 2(c), indicating that the resultant vector beam should have a high modal purity. The mode extinction ratio (MER) was measured to be $\sim 18\mathrm{dB}$, according to the method described in Ref. [10].

 

Fig. 2. Radially polarized beam generated by the q-plate.

Download Full Size | PDF

In order to get a high peak power pulse with a narrow spectral bandwidth, the fiber length and gain of the pre-amplifier chain were optimized so that the self-phase modulation (SPM)-induced spectral broadening and stimulated Raman scattering (SRS)-induced power transfer to longer wavelength were minimized. Figure 3 shows the output optical spectra of different pre-amplifier stages measured with an optical spectrum analyzer (OSA) with a low resolution of 2 nm. The seed had an optical signal-to-noise ratio (OSNR) of $\sim 35\mathrm{dB}$. In order to extract high pulse energy from the MOPA system, the repetition frequency was reduced by a factor of 64 to 1.367 MHz with the aid of the EOM. In this case, the EOM introduced a total loss of $\sim 21\mathrm{dB}$, resulting in a relatively low seeding power of $\sim 100\mathrm{\mu W}$ into the second core-pumped pre-amplifier stage. A high gain of $\sim 17\mathrm{dB}$ was extracted from the second pre-amplifier, yielding $\sim 5\mathrm{mW}$ of seed signal into the cladding-pumped pre-amplifier stage. The EOM, in combination with the synchronized AOM effectively suppressed the ASE to ensure that the signal after the core-pumped pre-amplifier stages had an OSNR (blue dashed line) similar to that of the seed (black line). An average output power of $\sim 200\mathrm{mW}$ (corresponding to a peak power of 1.3 kW) was obtained from the cladding-pumped third pre-amplifier stage. The spectrum (green dashed–dotted line) shows that it had an OSNR of $\sim 25\mathrm{dB}$ with a component of broadband ASE around 1070 nm. In order to avoid seeding SRS in the final power amplifier stage with this broadband ASE, a BPF was employed after the pre-amplifier chain to remove the longer wavelength ASE, resulting in a clean narrow band spectra (red dot) that could be used to seed the final amplifier stage.

 

Fig. 3. Spectra ($\text{resolution}=2\mathrm{nm}$) measured after the seed, the core-pumped preamplifier, and cladding-pumped preamplifier stages.

Download Full Size | PDF

The q-plate was mounted on an adjustable flip mount so that it could easily be inserted into, or removed from, the signal beam path allowing the MOPA system to be operated either in the donut-shaped ${\mathrm{TM}}_{01}$ mode or in the fundamental ${\mathrm{LP}}_{01}$ mode as required. Figure 4(a) shows the average output power of the MOPA system as a function of the launched pump power when operating either on the ${\mathrm{LP}}_{01}$ (black triangle) or ${\mathrm{TM}}_{01}$ (red circle) mode. A maximum output power of $\sim 42\mathrm{W}$ was obtained for the radially polarized ${\mathrm{TM}}_{01}$ mode at a launched pump power of $\sim 61\mathrm{W}$, corresponding to a slope efficiency of $\sim 76\%$ with respect to the launched pump power. In comparison, the fundamental ${\mathrm{LP}}_{01}$ mode yielded a maximum output power of $\sim 29\mathrm{W}$ with a similar slope efficiency. The approximately equivalent slope efficiency for the ${\mathrm{LP}}_{01}$ and ${\mathrm{TM}}_{01}$ modes can be attributed to the similar spatial overlap of each mode with the pump beam distribution within the fiber core. Further power scaling was mainly limited by significant nonlinear distortions experienced by the amplified pulses, as well as the transfer of energy to the SRS line. We stopped further power scaling for both modes when the SRS peak reached a level of $\sim -30\mathrm{dB}$ relative to the signal. The temporal profiles of the optical pulses were directly measured by a 32 GHz bandwidth photodetector (Agilent 83440D) and a 20 GHz bandwidth digital communication analyzer (Agilent HP 86100C). The pulse duration of the seed pulses was measured to be $\sim 150\mathrm{ps}$, as shown in Fig. 4(b), and decreased to $\sim 110\mathrm{ps}$ at the maximum output power of 42 W, which is likely due to the strong negative chirp associated with the input seed pulse.

 

Fig. 4. (a) Average output power of the TM01 and LP01 modes versus the launched pump power. (b) Temporal pulse shapes of the seed (blue) and at maximum TM01 output (red).

Download Full Size | PDF

Figures 5(a) and 5(b) show the measured beam intensity profiles in the far field at the maximum output power for the ${\mathrm{LP}}_{01}$ and ${\mathrm{TM}}_{01}$ modes, respectively. The $M^2$ values of the ${\mathrm{LP}}_{01}$ and ${\mathrm{TM}}_{01}$ modes were measured to be 1.3 and 2.2, respectively. The donut-shaped intensity profile is well maintained as the output power is increased, and the radial polarization can be preserved with the aid of a pair of half-wave plate and quarter-wave plates at the output as reported in Ref. [10,11]. The bottom row of Fig. 5 shows the intensity distribution of the ${\mathrm{TM}}_{01}$ mode when passed through a rotated linear polarizer. The MER was measured to be $\sim 10\mathrm{dB}$ at 42 W.

 

Fig. 5. Typical beam profiles at maximum output power: (a) the LP01 mode at 29 W and (b) the TM01 mode at 42 W. The bottom row shows the beam intensity distributions of the TM01 mode passing through a rotated linear polarizer.

Download Full Size | PDF

A comparison of the output optical spectrum (0.5 nm resolution) for both the ${\mathrm{LP}}_{01}$ and ${\mathrm{TM}}_{01}$ modes at the maximum output power is illustrated in Fig. 6(a). It clearly indicates that the donut-shaped ${\mathrm{TM}}_{01}$ mode has a higher SRS threshold than the fundamental ${\mathrm{LP}}_{01}$ mode. For the ${\mathrm{LP}}_{01}$ mode (black line), the SRS peak at $\sim 1090\mathrm{nm}$ becomes apparent ($\sim 30\mathrm{dB}$ below the signal peak) when the average output power reaches $\sim 29\mathrm{W}$, corresponding to a pulse energy of 21.2 μJ and a peak power of $\sim 190\mathrm{kW}$. However, at the same SRS peak suppression, the average output power of the ${\mathrm{TM}}_{01}$ mode (red line) can be scaled up to 42 W, corresponding to a pulse energy of 30.7 μJ and a peak power of $\sim 280\mathrm{kW}$. Our results show that $\sim 45\%$ more pulse energy can be extracted for the donut-shaped ${\mathrm{TM}}_{01}$ mode than for the fundamental ${\mathrm{LP}}_{01}$ mode. This is mainly due to the significantly larger effective mode area of the ${\mathrm{TM}}_{01}$ mode. Our calculations show that for the few-mode YDF used in our setup, the ${\mathrm{TM}}_{01}$ mode has an effective mode area of $\sim 460{\mathrm{\mu m}}^2$, which is 40% larger than the ${\mathrm{LP}}_{01}$ mode ($\sim 330{\mathrm{\mu m}}^2$), consistent with our SRS observations. Figure 6(b) plots the spectral profiles of each transverse mode with an OSA resolution of 0.02 nm at the maximum output power. The 3 dB bandwidth of the output pulse was broadened to 0.69 nm for the ${\mathrm{LP}}_{01}$ mode (black line) and 0.73 nm for the ${\mathrm{TM}}_{01}$ mode (red line), mainly due to SPM (maximum acquired nonlinear phase shift of $6.5\pi$). It is clear, however, that the detailed nonlinear spectral evolutions of the ${\mathrm{TM}}_{01}$ mode and the ${\mathrm{LP}}_{01}$ mode are somewhat different, likely due to the presence of four-wave mixing (FWM) for the ${\mathrm{LP}}_{01}$ mode operation. This could be attributed to the presence of the ${\mathrm{LP}}_{11}$ mode, as the $M^2$ value of the ${\mathrm{LP}}_{01}$ mode was 1.3. The two modes have different dispersive properties likely resulting in enhanced phase-matching contributions to the FWM process [13]. By integrating the spectra in Fig. 6(b), we estimated that $\sim 56\%$ of the total pulse energy was contained within the 3 dB bandwidth for the ${\mathrm{LP}}_{01}$ mode. In comparison, $\sim 80\%$ of the total pulse energy was contained within the 3 dB bandwidth for the ${\mathrm{TM}}_{01}$ mode.

 

Fig. 6. (a) Spectra ($\text{resolution}=0.5\mathrm{nm}$) measured for the LP01 and TM01 modes at different output powers. (b) Spectra ($\text{resolution}=0.02\mathrm{nm}$) measured at the maximum output power of the ${\mathrm{LP}}_{01}$ mode (black line) and ${\mathrm{TM}}_{01}$ mode (red line).

Download Full Size | PDF

To further scale the average output power of the radially polarized ${\mathrm{TM}}_{01}$ mode, the repetition rate of the MOPA system was increased four times to 5.468 MHz. In this case, an average power of $\sim 500\mathrm{mW}$ of donut-shaped radially polarized beam was obtained after the pre-amplifier chain which was then coupled into the final power amplifier stage. As shown in Fig. 7(a), the average output power of the donut-shaped ${\mathrm{TM}}_{01}$ mode was linearly increased to $\sim 106\mathrm{W}$, corresponding to a gain of $\sim 23.3\mathrm{dB}$ and slope efficiency of 77% with respect to the launched pump power. This corresponds to a maximum pulse energy of $\sim 19.4\mathrm{\mu J}$ with a peak power of $\sim 176\mathrm{kW}$. Figure 7(b) shows the measured spectral profiles of the seed and output beam at the maximum output power of 106 W with an OSA resolution of 0.5 and 0.02 nm (insert), respectively. The spectra indicate that both the ASE and SRS were well suppressed. The 3 dB bandwidth was broadened to 0.46 nm, containing $\sim 80\%$ of total pulse energy. The evolution of the beam intensity distribution of the ${\mathrm{TM}}_{01}$ mode as a function of output power is shown in the Figs. 8(a)–8(d). The beam profile exhibits a pronounced donut shape with slight ellipticity and little variation at different power levels. The ellipticity is believed to be due to the aberration induced to the output beam, resulting from slight deviation of the output beam from the optic axis due to the slightly curved angle-cleaved fiber end facet. The intensity at the beam center gradually increases with an increase in output power and becomes quite appreciable at the maximum output power of 106 W, as shown in Fig. 8(d). Figure 8(e) plots the one-dimensional intensity profile across the beam center for Fig. 8(d). The measured intensity profile (black dots) was fitted using an incoherent superposition of the LP01 and TM01 modes (red line), indicating that $\sim 7\%$ of power appears to be in the LP01 mode. This could be attributed to the fast buildup of ASE, as well as amplification of the residual LP01 mode under high pump power conditions. The MER was deteriorated to $\sim 8\mathrm{dB}$, and the $M^2$ was measured to be 2.38 at the maximum output power.

 

Fig. 7. (a) Average output power of the TM01 mode at a repetition rate of 5.45 MHz versus the launched pump power and (b) spectra ($\text{resolution}=0.5\mathrm{nm}$) measured for the seed and maximum output power of 106 W. The insert shows the corresponding spectra measured with a resolution of 0.02 nm.

Download Full Size | PDF

 

Fig. 8. (a)–(d) Measured intensity profiles of the radially polarized beam at different output powers; (e) one-dimensional intensity profile across the beam center at 106 W fitted with an incoherent superposition of the LP01 and TM01 modes; (f)–(g) the beam intensity distributions after passing through a rotated linear polarizer.

Download Full Size | PDF

In summary, we demonstrated a gain-switched diode-seeded YDF-MOPA system capable of generating high average output power, high pulse energy and high peak power picosecond pulses with a radially polarized donut beam profile. The maximum pulse energy reached 30.7 μJ with a peak power of $\sim 280\mathrm{kW}$ at a repetition rate of 1.367 MHz. The extracted pulse energy in the radially polarized mode is $\sim 45\%$ higher than the fundamental ${\mathrm{LP}}_{01}$ mode. The average output power of the radially polarized beam was further scaled up to 106 W at a repetition rate of 5.467 MHz, corresponding to a pulse energy of $\sim 19.4\mathrm{\mu J}$ and a peak power of $\sim 176\mathrm{kW}$. To the best of our knowledge, this is the highest average output power and highest peak power obtained for a CVB from a conventional step-index few-mode fiber laser. Such a high average power, high peak power, and high pulse energy CVB source should be attractive for a variety of applications in high-throughput material processing and imaging.

The data from this Letter is available in Dataset 1, Ref. [14].