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Abstract
High-power vortex light generated directly from lasers will help drive their applications in material processing, optical manipulation, levitation, particle acceleration, and communications, but limited power has been achieved to date. In this work, we demonstrate record vortex average power of 31.3 W directly from a laser, to the best of our knowledge, using an interferometric mode transforming output coupler to convert a fundamental mode Nd:YVO4 laser into a LG01 vortex output. The vortex laser was Q-switched with up to 600 kHz pulse rate with a high slope efficiency of 62.5% and an excellent LG01 modal purity of 95.2%. We further demonstrate > 30W laser power in a high quality HG10 mode by simple adjustment of the output coupler. Experimental investigations of varying output coupling transmission are compared with theory. This successful implementation of the interferometric output coupler in a high power system demonstrates the suitability of the mode transforming method for robust turn-key vortex lasers with high efficiency and high modal purity, with scalable power and pulse rate.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
There is considerable interest with vortex modes that carry orbital angular momentum (OAM) [1] and having a stably propagating annular intensity profile. The OAM stems from a helical phase in the electric field of the beam that can act either clockwise or anti-clockwise, defining the handedness of the vortex light. The annular intensity profile results from a zero-intensity phase singularity at the centre of the vortex mode. Laguerre-Gaussian (LGpl) modes are an example of propagation-stable vortex beams when $l \ne 0$. Applications including material processing [2,3], optical manipulation and levitation [4–6], and increased information capacity optical communication [7], require vortex generation with high power, efficiency and quality. For example, material processing would benefit from industrial class high-power vortex light with short pulses and high peak powers [2,3]. The laser source should also be robust, efficient, and turn-key with a high mode purity; however, few existing vortex generation techniques have the capability to meet these requirements.
Several extra-cavity vortex generation techniques exist, for example astigmatic mode converters of high-order Hermite-Gaussian (HG) modes using a cylindrical lens pair [8] or astigmatic curved mirror pair [9], geometric phase q- and j-plates [10,11] or metasurfaces [12], Fresnel cone [13], spiral phase plate (SPP) [14–17], computer generated hologram [18], spatial light modulator (SLM) [19,20] and digital micromechanical mirror device (DMD) [21]. These variety of techniques have good attributes but also limitations: SLMs are expensive and both SLMs and DMDs suffer from low conversion efficiency and a low damage threshold [22]; SPPs require bespoke manufacture for a fixed wavelength and typically liquid-crystal q-plates also have low damage threshold; astigmatic mode convertors require pure higher-order HG modes to generate vortex beams and are limited by the maximum power of the HG beam available.
An alternative approach that could achieve better efficiency, mode purity and move towards turn-key operation is to adapt lasers that directly output the vortex mode from the laser cavity. This approach can remove the need for precise alignment of extra-cavity elements by exploiting the loss-avoiding nature of laser cavities and natural noise filtering [23]. Different intracavity laser approaches have been taken to achieve a vortex laser source. A widely researched technique is to force the internal mode of the laser to lase preferentially on the desired vortex mode instead of the usually favoured fundamental Gaussian. Example of this include: using an axial loss region to suppress the fundamental mode that is either fixed [24] or digitally controlled with an SLM [25]; gain shaping through either a tailored pump distribution with an annular pump beam [26] or off-axis pumping [27], or using a secondary coupled cavity to deplete the centre of the gain region [28]; spherical aberration in the gain medium to suppress fundamental mode and favour vortex mode operation [29,30]. Although these methods have proven successful, a common issue is being able to reliably control the vortex mode handedness. There can also be output power limitations of a few Watts due to pump-induced thermal distortions in the gain medium that can corrupt sensitive higher-order vortex modes, particularly in asymmetric pumping schemes. A second approach is to insert mode transforming elements inside the cavity such as q-plates [31,32] or metasurfaces [33]. These conversion methods have also typically been severely power limited due to passive insertion losses and mode transformation inefficiency that both compromise laser efficiency, and power damage limitations of the conversion elements due to high intracavity flux [22].
This paper follows a different high-power vortex laser scheme that overcomes most of these difficulties. The approach taken is to use an interferometric mode transforming output coupler that produces a vortex output mode while the internal laser mode remains a fundamental Gaussian beam [34]. We have previously validated this approach by replacing the output coupler mirror of the laser by a modified Sagnac interferometer [35,36]. The Sagnac is configured to operate as a vortex output coupler (VOC), which transforms an internal Gaussian mode into a vortex LG0±1 out-coupled mode, whilst providing feedback to the cavity to maintain the intra-cavity fundamental Gaussian beam [35]. We have shown that this method operates with almost the same high slope efficiency as the fundamental Gaussian cavity and can be Q-switched without detrimentally effecting the mode quality [36]. In this prior work, high-quality vortex laser generation was obtained at output powers of 3.2 W in continuous-wave operation [35] and 5.4 W in Q-switched mode [36]. Whilst these demonstrations showed the principle of operation the power achieved was limited, not by the VOC, but by the fundamental laser design.
In this paper, we have designed a diode-pumped Nd:YVO4 laser system with considerably higher power and employed the VOC method for vortex generation, demonstrating a Q-switched vortex laser with, to the best of our knowledge, a record direct from laser vortex power of 31.3 W. The LG01 vortex mode purity was high at 95.2%, measured using a phase retrieval algorithm. The laser had an exceptionally high slope efficiency of 62.5% and was Q-switched up to a pulse rate of 600 kHz. Using a simple adaptation of the output coupler configuration we also demonstrate output of a HG01 mode with > 30 W power. We perform an investigation of systematically varying the VOC transmission in both vortex LG01 mode and HG01 mode laser configurations and find a good match to results from a theoretical model of the interferometric output coupler. This high-power Q-switched vortex laser system with high pulse rate offers a promising candidate for high scan speed material processing applications [2]. Its output power is only limited by the available pump power, showing the potential for further power scaling of the interferometric transformation technique. Since the vortex method uses low-cost, high-power handling components, capable of operation across a wide wavelength range, implementation can be envisaged in a wide class of laser without need for custom-manufactured wavelength specific components. This offers robust, turn-key laser solutions for future vortex light applications.
2. Experimental vortex laser system
The vortex laser cavity schematic employed in this study is shown in Fig. 1. It was based on a diode-end-pumped Nd:YVO4 laser operating with linear polarisation on the crystal c-axis at 1064 nm. The Nd:YVO4 crystal had 0.3 at. % Nd doping, square cross-section 3 × 3 mm, length 20 mm, with additional 2.5 mm length undoped YVO4 fused endcaps. The crystal was housed in a water-cooled copper mount and diode-end-pumped through a dichroic turning mirror (TM). The diode pump was a 65 W fibre-delivered module operating at 878.6 nm. By pumping at the 880 nm transition, rather than at 808 nm as in previous work [23,24], the quantum defect heating is reduced, which when coupled with lower crystal doping and the use of endcaps reduces heat loading energy density and end bulging effects. This pump-crystal combination significantly lowers the thermally induced lensing to maintain high spatial quality and reduced crystal stress [37–39] that can lead to crystal fracture. As a result, the crystal was able to withstand the full power of the high-power diode pump module without damage and was ideally suited to investigate power-scaling of the vortex laser. While pump induced thermal lensing did occur, this was compensated for in the cavity design to maintain cavity stability and an appropriate mode overlap.
Fig. 1. Vortex laser cavity composed of: Nd:YVO4 crystal with undoped end-caps; high-reflectance back mirror (BM); intracavity lens (L) with f=−500 mm (l), 45° dichroic turning mirror (TM); VOC (highlighted in the grey-dashed box); and acousto-optic modulator (AOM) for Q-switching. The VOC is comprised of a 50% beamsplitter (BS), three HR mirrors (M1-M3) and a plane-parallel AR coated fused silica plate (P). Photograph of VOC system shown as inset.
The laser cavity was formed between a plane back mirror (BM) and the modified Sagnac interferometer acting as a vortex output coupler (VOC), highlighted by dashed grey box in Fig. 1. An intermediate 45° dichroic plane turning mirror (TM) was used with high reflectivity at the laser wavelength (1064 nm) and high transmission at the pump wavelength (878.6nm) to allow ease of access for pumping the crystal and resulted in an L-shaped cavity. The fibre-delivered pump beam was imaged to a near top-hat distribution in the crystal with a radius of 420 µm,.The large pump size was chosen to decrease the thermal lensing and TEM00 operation was achieved by using a total cavity length of 260 mm and a plano-concave lens (L) with focal length f =−500 mm in front of the back mirror for cavity mode size control. An acousto-optic modulator (AOM) placed between gain medium and back mirror provided Q-switching. The VOC was composed of a 50% beamsplitter (BS), 3 high reflectivity mirrors (M1-M3) and an AR-coated plane parallel fused silica plate for vertical beam displacement inside the Sagnac ring.
The internal components of the VOC had their alignments adjusted to transform the fundamental Gaussian intracavity mode to produce a vortex LG01 output mode and later re-adjusted to produce a HG10 laser output mode. The transmission of the VOC was determined experimentally by comparing the power outputs from the VOC and the back mirror, which had a known transmission of 0.73% for cavity monitoring purposes.
3. Vortex output coupler theory
The VOC is a modified Sagnac interferometer which, given a Gaussian input beam, can be configured to transmit a vortex mode from the beamsplitter port while reflecting an attenuated Gaussian [40]. The VOC used in this laser, highlighted by the grey dashed square in Fig. 1, consisted of a 50% beamsplitter (BS), three plane HR mirrors (M1-M3) and a 3 mm thick plane parallel AR-coated fused silica plate (P). To achieve vortex transformation, opposite horizontal angular misalignments (${\pm} {\theta _x}$) are introduced in the opposing circular paths inside the Sagnac ring using a piezo-controlled rotation of central mirror M2 by angle $\theta = {\theta _x}/2$. Orthogonal vertical opposing spatial displacements (${\pm} {d_y}$) are imparted in the two paths by vertical rotation of the plate (P) by angle $\psi $, which for small angles ${d_y} = t\psi ({1 - 1/n} )$, where t is the plate thickness and n its refractive index. The vertical displacements (${\pm} {d_y}$) can also be introduced with opposing vertical rotations of mirrors M1 and M3, but the plate P provides a simple, more precise control and continuous adjustment of parameter ${d_y}$ with a single control. The use of a piezo-controller (POLARIS-K05P2 from Thorlabs) for rotation of mirror M2 similarly provides precise (µrad precision) adjustment of parameter ${\theta _x}$.
The analysis in Ref. [41] considers the general input mode case, here we adapt the analysis for the case of a normalised fundamental Gaussian beam, $E = \textrm{H}{\textrm{G}_{00}} = \textrm{L}{\textrm{G}_{00}}$, from the laser cavity incident on the beamsplitter of the modified Sagnac interferometer. The transmitted field ${E_t}({x,y} )$ from the beamsplitter output port and the reflected field that gives feedback into the laser cavity ${E_r}({x,y} )$ are given by:
The canonical case is defined as when the normalised misalignment parameters are equal, $\varepsilon = {\varepsilon _y} = {\varepsilon _x}$, and the transmitted output field becomes
where the input Gaussian has been transformed into a first-order Laguerre-Gaussian (LG01) vortex beam with topological charge $l = 1$. The amplitude transmission $t = i\left( {\sqrt 2 \varepsilon } \right)$ and the intensity transmission $T = 2{\varepsilon ^2}$ are both determined by the VOC misalignment parameter $\epsilon $.The reflected beam in the canonical case that provides feedback into the laser cavity is
which is an attenuated version of the incident Gaussian beam. The $\textrm{H}{\textrm{G}_{00}}$ component has an amplitude reflectance $r = ({1 - {\epsilon^2}} )$ and intensity reflectance $R = ({1 - 2{\epsilon^2}} )$ (to second order in parameter $\varepsilon $). There is an additional reflected weak higher-order Laguerre-Gaussian LG02 vortex beam with intensity reflectance ${R_2} = 2{\varepsilon ^4}$, to fourth order in small parameter $\varepsilon $. The LG02 beam is not expected to be a mode of the laser cavity due to an incorrect Gouy phase and its larger size will be a poor spatial match to the gain medium for a well-designed fundamental mode laser, it will therefore tend not to grow and will be diffractively “shed” from the cavity.Equation (1a) shows that when either of the misalignments, ${d_y}$ or ${\theta _x}$, are implemented in isolation the interferometric output coupler will transform a Gaussian beam into a pure HG01 or HG10 mode, respectively. When set at the canonical condition for vortex generation, the handedness of the vortex can be controllably reversed by inverting either one of the misalignments. In Ref. [41] more general cases are considered and it is also shown that if the laser cavity operates on a higher order LG0,l mode the VOC can add or subtract vorticity and transform these to even higher-order vortex mode superpositions or transfer from azimuthal l index to radial index p. When adding vorticity, higher order LG0,l modes see larger cavity transmission loss $T = 2({2l + 1} ){\varepsilon ^2}$ than the fundamental $l = 0$ mode. This gives a self-mode filtering ability and enhances the quality of intracavity fundamental Gaussian mode operation of the vortex laser [36].
For the Gaussian intracavity laser mode, when the misalignment parameters are not small the theoretical intensity transmission of the VOC can be more exactly calculated [35] as
The transmission of the VOC is an important consideration when operating as an output coupler in a laser cavity as it can be used to optimise laser output power.
4. Vortex laser results
The laser cavity in Fig. 1 was first operated to output a LG01 mode from the VOC. The laser was operated in both CW and pulsed modes. In this section we show the power and pulse characteristics, along with analysis of the vortex mode purity and the ability to tune the VOC transmittance.
4.1 CW and pulsed power results
The average output power of the vortex laser versus absorbed pump power is shown in Fig. 2 for both Q-switched (pulsed) vortex operation at 150 kHz pulse rate (black data) and under continuous-wave (CW) operation with AOM not operated (red data). In pulsed operation, at the maximum available 62.5 W absorbed pump power 31.3 W of average vortex output power was achieved. To the best of our knowledge, this is the highest vortex power produced directly from a laser in a scalar OAM mode. The vortex laser had an impressively high slope efficiency of 62.5% and a threshold power of 13 W with the VOC at 28% transmittance. The output power under CW operation had a similar behaviour to the pulsed case. The spatial intensity profile of the pulsed vortex output is inset in Fig. 2 for Q-switched operation at 150 kHz. As a result of thermal lens and cavity mode size changes with pump power the VOC misalignments were adjusted to maintain the canonical condition and desired transmission.
Fig. 2. Average output vortex laser power versus absorbed pump power in pulsed operation at 150 kHz Q-switching rate (black) and in continuous wave (CW) operation (red). The intensity profile of the vortex output from the laser is inset and was the same in CW and pulsed regimes.
The variation of vortex output power with Q-switching pulse rate is shown in Fig. 3 at the maximum absorbed pump power of 62.5 W. Stable pulsing was achieved up to 600 kHz. At 100 kHz the pulse energy was 303 µJ and the pulse duration was 20 ns, corresponding to a peak power of 15.0 kW. Over the range of pulse rates presented the average output power stayed relatively constant and over 30 W.
Fig. 3. Average vortex power (black squares, left axis) and pulse energy (red triangles, right axis) plotted against the repetition rate of the Q-switch.
4.2 Mode purity analysis
The M2 beam propagation parameter can give a good indication of the modal content of a laser. The output vortex mode of the laser had the appearance of a high quality axially uniform annular LG01 mode, see inset Fig. 2, and it had a propagation beam parameter M2 = 2.25, which is close to the theoretical LG01 value of M2 = 2.0. This suggests that the VOC is working as intended at high power and is further supported as the intracavity mode had M2 = 1.20, close to the diffraction limit of M2 = 1 for a Gaussian beam as required in the VOC theory.
The M2 parameter together with the high-quality annular appearance of the beam is a good but not conclusive proof of a vortex mode; however, importantly, it does not quantify the spiral phase required for a pure LG01 mode. To do so the phase must be measured, which when coupled with the intensity profile allows the mode content of the beam to be determined. This analysis was performed by interfering the vortex mode with a plane wave reference in a Mach-Zehnder interferometer. The resulting interferogram was then analysed with a phase retrieval algorithm based on that of Takeda et. al. [42] to recover both the phase and intensity profile of the beam and perform the mode decomposition.
The phase retrieval process is shown in the Fig. 4 insets. The first image shows the starting interferogram, which reveals the phase singularity from the central forked fringe. This interferogram is processed to retrieve the intensity and phase profiles of the laser output, see second and third insets. In the phase profile we see the helical structure with a phase singularity at the centre. From the phase and intensity profiles the mode decomposition is performed. To verify the accuracy of the mode decomposition, the input beam is reconstructed using the calculated mode weights and phases. The reconstructed intensity is an excellent match to the recovered intensity, concluding that the decomposition is accurate.
Fig. 4. Modal decomposition of the Q-switched vortex mode using a phase retrieval algorithm on a fork interference pattern, where modal power values greater than 0.1%, have been noted. Inset: The fork interferogram, the recovered intensity and phase structure of the vortex from the interferogram, and the signal beam reconstructed from the decomposition algorithm.
The mode decomposition results are shown in Fig. 4, which shows the mode power content for $\textrm{L}{\textrm{G}_{\textrm{pl}}}$ modes up to a total mode order of 2$p + |l |= 7$. The beam demonstrates an excellent mode purity with 95.2% of the power in the LG0,−1 mode. The next most significant mode was the small 0.8% contribution from the LG1,1 mode. This could indicate a minor LG0,2 impurity in the intracavity beam created by the VOC to the reflected beam, see Eq. (3), which when returned to the VOC is converted to LG11 from higher order conversion processes [41]. On repeated measurement the LG0,−1 and LG11 components remained at their relative strengths, with other higher order mode contents randomly varying due to noise in the interferogram diagnostic signal. The output vortex beam was the same in CW and pulsed regimes.
4.3 VOC transmission control
A useful feature of the VOC is the ability to adjust the output coupling transmission simply by changing the amount of vertical shear ${d_x}$ and angular misalignment ${\theta _x}$ in the Sagnac ring. In this section we explore this behaviour and test the experimental performance compared to the theoretical transmission predicted by Eq. (4).
The displacement and angular misalignment parameters (${d_y},{\theta _x}$) were quantifiable in our laser system with the rotation of plate (P) and electronically piezo-controlled rotation of mirror M2. A 4-f image relay of the intracavity laser mode on mirror M2 allowed measurement of mode waist radius $(w$) so the divergence (${\theta _0}$) of the intracavity beam could be calculated and therefore ${\varepsilon _y} = {d_y}/w$ and ${\varepsilon _x} = {\theta _x}/{\theta _0}$. The VOC theoretical transmission is then given by Eq. (4). For LG01 output the canonical condition ${\varepsilon _y} = \; {\varepsilon _x}$ is expected to be satisfied.
The angle ${\theta _x}$ was adjusted with the piezo-controlled mirror M2, which rotates the mirror by ${\theta _x}/2$, whilst maintaining the canonical condition by adjusting ${d_x}$ with the AR plate (P). The VOC transmission was measured by comparing the vortex output power to the leakage power from the back mirror (BM) that had a known transmission of 0.73%. The resulting transmissions of the VOC are shown in Fig. 5(a) with red squares. The solid black line is a fit of the theoretical transmission function, Eq. (4), to this dataset with $\; w$ (beam radius) as a free parameter. While the fit showed excellent correlation with the data, the fitting parameter w was larger than the actual w. This discrepancy is likely due to imperfect intracavity beam quality M2=1.2 compared to the perfect fundamental mode used in the theory. As a further check, the observed versus predicted transmission based on the measured waist radius at each point is shown in Fig. 5(b). A linear fit shows that the predicted and observed transmissions are linearly correlated. The linear fit has a slope of 1.2, which further suggests that the imperfect intracavity mode quality is causing an underestimation of the transmission predicted by theory.
Fig. 5. (a) Experimentally observed transmission of the VOC in a vortex configuration (red squares), with Eq. (4) fitted to the data (solid black line) with waist radius as a free parameter. (b) The observed transmission against predicted transmission using the measured waist radius (red squares), a linear fit is also shown (solid black line).
5. Discussion
These results show the VOC’s potential for high power, high repetition rate vortex pulses for industrial-scale applications such as material processing. Being only limited by the underlying cavity, the VOC could be used to adapt existing high energy, high repetition rate material processing lasers. We believe this provides a quick route to commercial material processing vortex applications, by circumventing further research into complex, novel cavity designs built around lossy or low power handling components.
The high mode quality demonstrated separates the VOC method from other high-power capable vortex generation techniques. This makes the VOC a unique option in the structured light toolkit. Even greater mode quality is expected with a more ideal fundamental Gaussian intracavity mode.
The variable transmission of the VOC also offers another degree of freedom with which to optimise the underlying cavity power performance. The controllable misalignment that allows variable transmission also enables the VOC to adapt to any mode size within the cavity. This means that the VOC can be added to any cavity that currently operates with a plane output coupler without requiring any cavity redesign. The small footprint of our implementation also demonstrates this potential.
The combination of high power and high mode purity makes the VOC a unique tool for vortex generation. With the extra features of handedness control, variable transmission and the possibility of being built with wavelength independent optics, we believe the VOC will help drive the structured light research and industrial implementation.
5.1 Hermite-Gaussian mode laser
The VOC laser in Fig. 1 is also capable of transmitting an HG type mode, as shown in the Vortex Output Coupler Theory section. To generate an HG10 or HG01 mode either the angle or displacement adjustments are implemented $,\; {\epsilon _x}$ or ${\epsilon _y}$, instead of simultaneously for LG01 generation, see Eq. (1). This is an interesting use of the Sagnac VOC design to generate high power HG modes. One can convert HG modes into LG modes using astigmatic mode conversion; however, prior work generating HG modes with this approach have been limited to 100s mW [8,9].
The VOC was operated in the laser system outlined in Fig. 1 with only angular misalignment ${\theta _x}$ applied (${d_y} = 0$). The laser output power is plotted against absorbed pump power in Fig. 6(a), which shows over 30 W power in the HG10 mode. The laser slope efficiency was 60% and the mode intensity profile is inset. These results are almost identical to the vortex output configuration (see Fig. 2) and demonstrate that the VOC can be switched between a vortex or HG10 output mode without loss in power or laser efficiency.
Fig. 6. (a) Graph showing output power against the absorbed output power (intensity profile inset). (b) Intensity cross-section of laser output mode at maximum power (black) overlaid with the theoretical plot of $\textrm{H}{\textrm{G}_{10}}$ mode (red), the radius has been normalised to the fundamental Gaussian mode radius.
Figure 6(b) shows the cross section of the experimental output mode overlaid with a theoretical plot of a HG10 mode. The HG10 mode had beam propagation parameters of $\textrm{M}_{x,y}^2 = 3.24/1.27$, which closely match the theoretical values of $\textrm{M}_{x,y}^2 = 3.0/1.0$ expected of the HG10 mode. This measurement along with the close fit to theory demonstrates the excellent mode quality produced by the mode transforming laser cavity.
To further characterise the HG10 laser output modal decomposition was also performed, similarly to the vortex case with an interference pattern and phase retrieval algorithm. This showed that 91.4% of the power was in the desired HG10 mode. Some of the imperfection in this output mode was due to imperfect diagnostic optics and an imperfect intracavity fundamental mode quality.
5.2 HG transmission control
The transmission of the VOC in the HG mode configuration was also investigated. This was conducted in the same manner as previously with the vortex; however, the vertical shear was kept at zero (${d_y} = 0$). The piezo control of VOC mirror M2 was used to systematically vary ${\theta _x}$. The output powers from both the VOC and the BM were used to determine the transmission at each setting and the spatial profile of the beam at M2 was recorded.
The observed transmission values versus ${\theta _x}$, the relative angle imparted on beam by M2, are shown in Fig. 7(a) (red squares). The solid black line shows the fit of Eq. (4) to these points with ${\varepsilon _y} = 0$ and w as a free parameter. This fit matches the observed values very well and the fitting parameter w was close to the observed values of the beam waist. Figure 7(b) shows the observed transmission to the predicted value based on measured misalignment and the actual measured beam waist radius w at M2 via Eq. (4). A linear fit with slope of 1.0, shows an excellent correlation between predicted and observed transmission values.
Fig. 7. (a) The experimentally observed transmission of the VOC in an HG10 configuration (red squares), with Eq. (4) fitted to the data (solid black line). (b) The observed HG10 transmission against the predicted transmission (red squares) using the measured waist radius, a linear fit is also shown (solid black line).
With such excellent first-order HG mode quality and the potential to power scale far beyond the 30 W output power demonstrated here, we believe that this is a powerful use case for the VOC. The high-power, and high quality, HG mode demonstrated here could be used in vortex generation using astigmatic mode conversion [8,9].
6. Conclusion
We have designed a high-power diode-pumped Nd:YVO4 laser system operating on a fundamental Gaussian mode and employed a mode transforming vortex output coupler (VOC) based on a modified Sagnac interferometer for vortex generation. We have demonstrated a Q-switched vortex laser with, to the best of our knowledge, a record vortex power of 31.3 W produced directly from a laser. The LG01 vortex mode was measured to have a high purity of 95.2% using a phase retrieval algorithm. The laser operated with an exceptionally high slope efficiency of 62.5% and had Q-switched operation up to a high pulse rate of 600 kHz. Using a simple adaptation of the output coupler configuration, we have also demonstrated the transformation of the internal cavity mode to a HG10 mode with > 30 W power, which matched well to a theoretical HG10 mode profile and had a 91.4% purity. We performed an investigation of systematically varying the VOC transmission in both vortex LG01 and HG10 mode laser configurations and compared the results to the theoretical VOC model.
The high-power Q-switched vortex laser system with high pulse rate offers a promising candidate for high scan speed material processing applications [2]. The output power of the current laser is only limited by pump power showing the power scaling potential of the interferometric transformation technique. The technique could be automated for fast, accurate switching between modes, vortex handedness and output coupling transmission. This provides a route to industrial class speeds, for example in material processing, with control of output coupling transmission and spatial mode tunability. This may also open a new avenue into optical trapping and levitation, for example in particle physics applications, where macroscopic targets require a high-power trapping beam [5,6]. Since the vortex method uses low-cost, high-power handling components capable of operation across a wide wavelength range its implementation can be envisaged in a wide class of lasers without need for custom-manufactured wavelength specific components. This offers the attractive prospect of robust, turn-key laser solutions for future vortex light applications. The common-path VOC is also ideally suited for use in a mode-locked laser for ultra-short pulse vortex generation, which would enable access to much higher peak powers than a Q-switched design. The compatibility of the Sagnac interferometer with femtosecond inputs has been demonstrated by Naik et al. [40]. Further promise for route to higher order structured light is potentially by using cascaded VOCs [41], alternatively this could be achieved by changing the intracavity input mode to some higher order LG or HG mode. Further structured light beams can be accessed by slight modifications to the VOC demonstrated in this work, for example vortex dipole modes,, as demonstrated by Naik et al. [43]. The VOC methodology offers a versatile platform for the generation of structured light and is a unique addition to the structured light toolkit.
Funding
Engineering and Physical Sciences Research Council (EP/P510257/1, EP/R511547/1); Imperial College London (Imperial College Research Fellowship).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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