Single-longitudinal-mode vortex Nd:YVO4 laser using a circular Dammann grating for pump shaping

In this research, a 1064 nm single-longitudinal-mode (SLM) vortex beam was generated from an annular-pumped Nd:YVO4 microchip laser, in which a circular Dammann grating was used to reshape the 808 nm pump light into an annular intensity profile. As a result, the laser emitted a SLM LG01 beam with a maximum output power of 192 mW. A straightforward technique for producing SLM vortex beams is made available by this work.

V ortex beams are characterized by a doughnut-shaped intensity distribution and helical phase wavefront, and thus possess orbital angular momentum (OAM), [1][2][3][4][5] which brings additional dimensions for characterizing and manipulating light beams. Luerre-Gaussian (LG) beams are typical of vortex beams.6,7) Annular pumping techniques, as one of the methods for generating vortex beams, compare with other traditional methods (such as computed holograms, 8,9) spatial light modulators, 10,11) spiral phase plates, 12,13) etc.) and can maintain a good spatial overlap with the target LG mode in the gain medium, which is favorable for LG mode excitation of laser resonators with high efficiency and high modal purity.
][24][25][26] This technique has a straightforward structure, a good mode selection ability and the ability to reduce the consequences of spatial hole burning.Therefore, it is feasible to obtain a SLM vortex beam from an ultra-short cavity with an annular pumping technique.
We proposed to introduce a circular Dammann grating (CDG) into an ultra-short cavity to form a circular pumping field.][29][30] The method of adopting a CDG for annular pumping is easy to realize just by inserting the CDG element into the pump unit of a laser.Fabrication of a CDG is simple and cost-effective.
In this research, a 1064 nm SLM vortex beam was generated from an annular-pumped Nd:YVO 4 microchip laser.The pump light was reshaped into a ring structure using a CDG.Additionally, a thin Nd:YVO 4 crystal served as the resonant cavity and gain medium.As a result, a LG 01 -mode vortex beam was achieved in this simple laser resonator with up to 192 mW power for a single frequency.
Figure 1(a) shows the experimental setup of the laser.The pumping source employed was an 808 nm laser diode, and its pigtail fiber had a core diameter of 105 μm and a numerical aperture (NA) of 0.22.Lenses L1 and L2 were used to collimate and focus the pump light into the gain medium, with focal lengths of 25.4 mm and 16 mm, respectively.The gain medium consisted of an a-cut Nd:YVO 4 crystal with 2.0 at% Nd 3+ ions.The crystal had dimensions of 5 mm diameter and 345 μm thickness.The front surface of the crystal was coated with high transmission at 808 nm (R < 3%) and high reflection at 1064 nm (R > 99.8%).The rear surface, serving as the output coupler, was coated with high reflectance at 808 nm (R > 95%) and 94.5% reflectance at 1064 nm.Both surfaces of the Nd:YVO 4 crystal were enveloped with indium foil and securely mounted in a copper holder cooled by water at approximately 25 °C, and the center of the holder had a 3 mm aperture for light transmission.
A first-order CDG with a diameter of 25.4 mm was inserted in between L1 and L2, and the measured diffraction efficiency and angle were 76% and 0.0436 rad, respectively.Figure 1(b) shows a top view of the CDG taken by scanning electron microscopy.It was fabricated on a substrate of BK7 glass with a period of 20 μm and 10 μm groove width using a wet-etching method.Figure 1(c) illustrates the captured intensity distribution of the diffracted pump light at the focus of L2.As seen, a first-order diffraction ring was formed in the far field, with a radius of 650 μm and a width of 78 μm.
As is well known, the formula for the longitudinal mode spacing is where μ is the refractive index and L is the length of the laser cavity.To achieve SLM output it is only necessary to shorten the cavity length and ensure that the width of Δν q is greater than or close to the linewidth of the spectral line.Nd:YVO 4 crystal has a refractive index of 2.165 and a spectral linewidth of 0.8 nm.Based on the calculation, SLM outputs can be obtained when L is less than or close to 326 μm.However, a shorter cavity length implies lower output power.Therefore, in this experiment, a Nd:YVO 4 crystal with a length of 345 μm was chosen to potentially maintain SLM laser output and achieve higher power.
To measure the output characteristics of the laser, a dichroic mirror was employed to separate the laser wavelength from the pump wavelength.The laser power was measured with a laser power meter, and the laser beam was monitored by a CCD.The spectrum was measured with a spectrometer (AQ6370C, Yokogawa) with a spectral resolution of 0.02 nm.
The laser power as a function of incident pump power is shown in Fig. 2(a).From the graph it can be observed that the laser threshold is 0.11 W and the laser power increases linearly with incident pump power with a slope efficiency of 8.2%.When the pump power reaches 2.55 W, an output power of 192 mW is obtained.The output power does not saturate, indicating that increasing the pump power can further improve the laser power.At a low pump power, the slope efficiency is slightly lower.This is usually attributed to the laser gain near the threshold being influenced by amplified spontaneous emission and not being fully saturated.
Figure 2(b) shows the captured intensity distributions of the laser beam at different pump powers.The laser beam displays a TEM 00 -like mode at P pump = 0.11 W; at P pump > 0.22 W, the laser beam exhibits a well-defined doughnut-shaped intensity distribution, and the shape is maintained in the range 0.22 W < P pump < 0.72 W; at P pump > 0.73 W, the annular laser beam begins to gradually  042003-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd deform, and the cylindrical symmetry is disturbed, particularly at P pump = 2.55 W. This can be attributed to the pump power near the laser threshold being low, and the TEM 00 mode having the highest spatial overlap efficiency with the annular pumping light field, thereby dominating in mode competition.When the pump power increases, the spatial overlap efficiency of the doughnut-shaped mode and the annular pumping light field surpasses the spatial overlap efficiency of the TEM 00 mode and the annular pumping light field due to the thermal lens effect, making the doughnutshaped mode more prone to oscillation.Due to the use of the ultra-short cavity method in this research, as the pump power increased, the thermal lens effect became more pronounced, leading to a deformation in the doughnut-shaped mode.
The doughnut-shaped mode can be further analyzed.Figure 2(c) shows the intensity distribution of the laser beam measured at P pump = 0.43 W, with well-defined annular intensity profiles in both the near and far fields.

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd Therefore, the laser can be considered to be oscillating in LG 01 mode.Moreover, it is worth noting that the central intensity of the curve does not approach the theoretical zero value.This is because the purity of LG 01 mode output is disturbed by the oscillation induced by zero-order diffraction of the pump light through the CDG.Reducing the period of the CDG component is an effective approach to suppressing the zero-order diffraction of the pump light.By employing a CDG with a shortened period, the zero-order diffraction of the pump light can be decreased, and the efficiency of firstorder diffraction can be improved.
To further analyze the mode of the output laser, we measured the M 2 factor of the laser beam.As shown in Fig. 3(b), the variation of the waist of the laser beam with the propagation distance behind the focusing lens was measured at P pump = 0.43 W. Based on this curve, the calculated M 2 was 2.16 in the x-direction and 2.19 in the y-direction, close to the theoretical values (M 2 = 2) of an ideal lowest-order LG mode, once again confirming that the obtained laser was in the LG 01 mode.However, the beam quality will gradually deteriorate as pump power increases.We also measured the M 2 factor of the beam at P pump = 2.55 W; this was 6.15 in the x-direction and 5.63 in the y-direction.
Subsequently, we utilized interferometric techniques to measure the phase of the laser beam.Figure 4(a) illustrates the experimental setup of the Mach-Zehnder interferometer.The laser beam was divided into two beams by a nonpolarizing beam splitter (NPBS).In the upper arm, the beam was attenuated and acted as the signal wave.The beam in the lower arm was intercepted by a small aperture, and after passing through a lens it became a plane reference wave.The plane reference wave and the signal wave were then combined through another NPBS and observed by a CCD.Finally, the spectral characteristics of the output laser were measured.Figure 5 depicts the spectrum of the LG 01 mode laser at different powers.When P pump = 0.43 W, the laser output exhibits a well-defined doughnut-shaped mode with a central wavelength of 1064.06 nm and a full width at half

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd maximum (FWHM) of 0.04 nm.When P pump = 1.44 W, the central wavelength of the laser is 1064.18nm with a FWHM of 0.05 nm.When P pump = 2.55 W, corresponding to the maximum pump power in Fig. 3, the central wavelength of the laser is 1064.32 nm with a FWHM of 0.07 nm.There is a slight broadening of the spectral line under the influence of the homogeneous broadening caused by thermal vibrations in the crystal.The longitudinal mode spacing was calculated as approximately 201 GHz (0.76 nm) based on Eq. ( 1), considering the refractive index of the crystal and an effective cavity length of 345 μm.At three different power levels, the measured FWHM was much smaller than the longitudinal mode spacing.Moreover, the output waveform in the graph maintains a well-defined Lorentz distribution, with no other wavelength oscillations observed.This suggests that within the depicted output power range in Fig. 2(a), the SLM can maintain stable operation.With continuing increase in the pump power, as shown in Fig. 5(b), the Lorentzian line is disturbed and a slightly multimodal regime appears with a weak side peak to the left of the central mode.This indicates that SLM operation is no longer maintained.This phenomenon was attributed to the increase in incident pump power causing a rise in crystal temperature, leading to thermal expansion and changes in the refractive index.By optimizing the cooling of the laser crystal, spectral line broadening, deformation and multimodal issues under a high pump power can be alleviated, resulting in a SLM LG 01 beam with better quality and higher output power.In summary, we employed an annular-pumped Nd:YVO 4 microchip laser to generate a 1064 nm SLM vortex beam.The 808 nm pump light was reshaped into an annular intensity profile using a CDG.The power of the SLM LG 01 beam reached 192 mW with a slope efficiency of 8.2%.The use of a CDG and an ultra-short cavity reduced the cost of the SLM vortex laser, and the structure of the laser is simple and easy to adjust.The results of our research validates the feasibility of this approach in SLM vortex solid-state lasers.Moreover, the quality and power of the SLM vortex beam at high pump power can be further improved by optimizing the fabrication of the CDG and the cooling of the laser crystal.

Fig. 1 .
Fig. 1.(a) Experimental setup of the SLM vortex Nd:YVO 4 laser based on circular Dammann grating pump shaping.(b) Top view of the CDG taken by scanning electron microscopy.(c) Annular pump beam measured at the far field.

Fig. 2 .
Fig. 2. (a) Laser power as a function of incident pump power.(b) Intensity distributions of laser beams obtained at different pump powers.(c) Near-and far-field intensity distributions of the laser beam at P pump = 0.43 W.
Figure 3(a) depicts the radial intensity distribution of the laser beam obtained at P pump = 0.43 W. It can be observed that the measured results match the theoretical expectations by fitting the curves using a first-order Laguerre-Gaussian function.

Fig. 3 .
Fig. 3. (a) Theoretical and measured intensity distribution of the laser mode along the radial direction and (b) variations of the waist of the laser beam with the propagation distance at P pump = 0.43 W.

Figure 4 (
b) shows the measured interference pattern.A Y-shaped fork can be observed in the graph, indicating that the output laser has a helical phase with a topological charge of 1.

Fig. 4 .
Fig. 4. (a) Experimental setup of the Mach-Zehnder interferometer and (b) interference pattern of plane waves captured at P pump = 0.43 W.