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Abstract
We report significant improvement in the performance of TEM00 alexandrite laser operation by employing high power fibre-coupled red diode pumping, novel cavity design, and active direct Shack-Hartmann wavefront sensor measurement of pump-induced lensing. We demonstrate 12.7 W of laser power in low-order ($M^2\sim 5$) mode operation from a compact double-end-pumped cavity, and with novel cavity design, a record power of 7.4 W in TEM00 operation with excellent beam quality ($M^2\leq 1.1$). With single-end pumping, laser power of 4.7 W ($M^2\sim 1.3$) was achieved with slope efficiencies as high as 54.9 %; a record efficiency for red-diode-pumped alexandrite. Using a birefringent filter, continuous laser wavelength tuning from 725-808 nm is achieved in diffraction-limited TEM00 mode, with laser power of 4.7 W at 765 nm, and >1 W across 730-805 nm, which is a higher tunable power than any other directly diode-pumped vibronic laser, to the best of our knowledge.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser sources emitting in the near-infrared have numerous applications including biophotonics, remote sensing and quantum technologies [1–3]. Tunable diode pumped solid-state lasers with good beam quality, high stability and low noise are well suited for these applications, and with tunability providing wavelength match to transmission/absorption features or atomic/molecular transitions. Diode pumping provides low cost, high efficiency and compactness, which is important for applications outside a laboratory. In this respect, tunable diode-pumped alexandrite (Cr$^{3+}$:BeAl$_2$O$_4$) lasers offer a promising alternative to current Ti:Sapphire laser technology pumped by frequency doubled 1 µm lasers which are expensive, low wall-plug efficiency, large footprint and with cumbersome service requirements (e.g. strong thermal management needs with water cooling).
The development of high power widely-tunable diode-pumped Ti:Sapphire lasers has been limited by the low power and poor efficiency of green laser diodes. Although higher power commercial blue diode laser pumping is available, significant parasitic issues in Ti:Sapphire with short wavelength pumping has limited operation to the 0.5 W power level [4,5]. This is in addition to the high quantum defect and heating from blue pump photons. Cr:colquiriites lasers (Cr:LiSAF, Cr:LiCAF, Cr:LiSGaF) pumped by single-mode red diodes have shown good efficiency, good beam quality and broad tunability [6] but at significantly below 1 W power level. Higher power tunability up to $\sim 2\textrm{W}$ has been achieved for Cr:LiSAF using broad area diode pumping but with spatially-multimode output [7]. Further power scaling in Cr:colquiriites is severely restricted by the poor thermo-mechanical properties with >10 W operation only suggested as a theoretical prospect in thin-disk geometry [8].
For high power operation, alexandrite is a much more promising diode-pumped broadly-tunable vibronic laser material. Its broad absorption in the visible region enables high efficiency operation when directly pumped by commercially available high power red diode lasers at 630-680 nm with low quantum defect, providing high efficiency and low heating fraction. The alexandrite material has excellent thermo-mechanical properties including high thermal conductivity and high fracture strength to make power scaling achievable [9]. Alexandrite has a wide lasing emission band from around 700-830 nm, though longer wavelength operation up to 858 nm has been achieved at high temperatures [10]. Over the previous decade there has been considerable development in red-diode-pumped alexandrite lasers, achieving output powers of >26 W [11] and slope efficiencies exceeding 54 % [12]. Due to its long upper state lifetime ($\tau _f = 260 \;$µs at room temperature) primary interest in alexandrite has been in high-energy Q-switched operation with diode-pumping achieving pulse energies of over 3 mJ [13], 1.7 mJ in single-longitudinal-mode [14] and master oscillator power amplifier systems [15]. Alexandrite pumping using LED-pumped fluorescent concentrators has also been demonstrated in oscillator and amplifier configurations [16].
Recent progress in continuous-wave operation of diode-pumped alexandrite in TEM00 operation (e.g. [12,17]) has in most cases been limited to the 1 W level. Higher power pumping using free space diode lasers have been used to achieve higher power operation but with low-order mode operation. TEM00 operation has been achieved in these systems but at the expense of overall efficiency [11,14,18,19]. The maximum power TEM00 result to date, to the best of our knowledge, is a 4.5 W (50 W pumping) bounce geometry diode-pumped alexandrite laser [18].
A significant problem for scaling high power pump delivery from multiple arrays of free-space laser diodes is the large asymmetry in the spatial output in terms of both mode size and divergence of the fast and slow axis directions. Although optics can be devised to reshape mode size and redistribute the divergence [14,17], the optical system required is complex and cumbersome in footprint size. The resultant pump mode for end-pumping of the alexandrite laser crystal is often still asymmetric and this leads to astigmatic pumping and pump-induced lensing that makes laser mode control for TEM00 operation difficult to maintain at different pump powers.
In this paper, we investigate the advantages of high-power fibre-delivery of red diode pumping to provide high beam symmetry, compact system design and ease of setup and robustness. In this work, we investigate the power scaling of diode-pumped alexandrite to 12.7 W power level using a single fibre-delivered red diode pump module. With controlled ‘top-hat’ image relay of the fibre-pump delivery, double-pass pumping and novel cavity design, we demonstrate high power scaling to 7.4 W of excellent TEM00 beam quality ($M^2\leq {1.1}$) with high optical efficiency. In single-pass pumping we show record laser slope efficiency for diode-pumped alexandrite of 54.9 %. We demonstrate a diode-pumped alexandrite laser with wide wavelength tunability and high power multi-Watt operation exceeding any other diode-pumped vibronic laser to date, to the best of our knowledge.
2. Compact plane-plane fibre-delivered dual end-pumped alexandrite laser
In the experimental work of this paper, a fibre-delivered red diode pump module was used. It had a nominal continuous wave output power of 34 W from the output of the fibre and wavelength 636 nm. The fibre had a core diameter of 200 µm and a numerical aperture $\textrm {NA}= {0.22}$. The red pump from the fibre output was collimated with an aspheric lens, had a measured beam quality of $M^2= {100}$, and was completely scrambled in polarisation (equal amount of horizontal and vertical polarisation).
The alexandrite crystal used in experimental investigations of this paper was a ${4}\times {4}\; \textrm{mm}$ square aperture, 6 mm long c-cut slab with 0.20 at.% Cr-doping and anti-reflection coated end faces at laser and pump wavelengths. The crystal was mounted in a water-cooled copper heat sink for temperature control.
In an initial laser system, an L-shaped laser cavity was used as shown in Fig. 1(a), with three plane mirrors: a back mirror (BM) that was highly reflective (HR) at the laser wavelength and highly transmissive (HT) at the pump wavelength at 0°; a dichroic turning mirror (DM) used at 45° that was HR at the laser wavelength and HT at the pump wavelength; and an output coupler (OC) with a reflectivity of $R_{\textrm {OC}}= {98.0}{\% }$. The overall physical cavity length was $\sim {20}\; \textrm{mm}$. The temperature of the alexandrite crystal was 60 °C.
Fig. 1. (a) Schematic of fibre-delivered double-end-pumped compact L-shaped alexandrite laser. (b) Measured Gaussian and ‘top-hat’ pump profile.
Fig. 2. Double-end-pumped Gaussian profile pumping results of alexandrite laser: (a) laser power, and (b) laser beam quality as a function of absorbed pump power.
Fig. 3. Double-end-pumped top-hat profile pumping results of alexandrite laser: (a) laser power, and (b) laser beam quality as a function of absorbed pump power.
In this initial experiment, the fibre delivered pump module was operated in double-end-pumping mode. The unpolarised fibre output of the pump module was split using a plate polariser into two orthogonal polarisation components which were individually rotated with half-wave plates to be parallel to the high absorption alexandrite crystal b-axis. The two pump beams were directed to either end of the crystal where they were focused by aspheric lenses to produce a waist radius of $w_p= {150}\; $µm at the alexandrite crystal end faces with a Rayleigh range (in air) of $z_R= {1.1}\; \textrm{mm}$. Two imaging systems were used: the first used collimating and focusing lenses in a long separation length and leading to the pump waist having a Gaussian profile; in a second case the collimating and focusing lenses were placed at the sum of their focal lengths and forming a relay imaging of the fibre end and delivering a ‘top-hat’ super-Gaussian pump waist. Figure 1(b) shows the measured intensity profile of the pump at focus for the two imaging systems.
Figure 2(a) shows the laser power as a function of the absorbed pump power using the Gaussian pump configuration. The pump threshold was about 2 W, and a maximum laser power of 10.1 W was obtained at an absorbed pump power of 26.8 W with an overall slope efficiency of 42.9 %. The laser typically operated on a broad spectrum that shifted to longer wavelengths with increasing pump power but was highly modulated at the maximum power as shown in the inset of Fig. 2(a).
The laser beam quality was measured at seven different pump powers with the results shown in Fig. 2(b). At low pump powers the laser operated on the fundamental TEM00 mode where at a laser power of 1.2 W the beam quality was measured to be $M_x^2= {1.02}$ and $M_y^2= {1.01}$. This is a similar result to previous work with compact plane-parallel cavity that also produced excellent beam quality at up to watt-level of output power [12,20]. However, with increasing pump power the laser beam quality was found to degrade: at laser power of 3.9 W the beam quality had degraded to $M_x^2= {2.05}$ and $M_y^2= {1.96}$ and to $M^2\sim {5}$ at 9.2 W. The beam quality degradation is due to increased pump-induced lensing and the impact of spherical aberration in the lensing of the Gaussian pump is manifest in the appearance of Laguerre-Gaussian modes, demonstrating the excellent cylindrical symmetry of the pump beam.
To investigate the prospect of improving the laser beam profile by eliminating spherical aberration in the pump-induced lensing the intensity profile of the pump was changed to the super-Gaussian top-hat form with relay-imaging and with a pump waist radius $w_p= {225}\; $µm and Rayleigh distance $z_R= {2.5}\; \textrm{mm}$. The cavity setup was the same as that shown in Fig. 1 except with $R_{\textrm {OC}}= {99.0}{\% }$ and with the water temperature at 20 °C. Figure 3(a) shows the laser power as a function of the absorbed pump power. An output laser power of 12.7 W was obtained at an absorbed pump power of 32.6 W with an overall slope efficiency of 43.0 %.
The laser beam quality for the super-Gaussian pumped alexandrite laser is shown in Fig. 3(b). There is reduction in the $M^2$ of the laser for a given laser output power, and with Laguerre-Gaussian modes no longer present (with elimination of spherical aberration) but with multi-mode operation still developing at higher absorbed pump power. These results show that high efficiency and high-power >10 W laser operation is achievable with a compact plane-plane cavity with super-Gaussian fibre-delivered pumping, however, the onset of strong pump-induced lensing requires better mode size control for TEM00 operation at the multi-watt power level. Cavity design to achieve this control is described in the following Section 3 and further analytical modelling informed by pump-induced lens measurement detailed in Section 4.
3. High-power fibre-delivered diode-pumped TEM$_{00}$ alexandrite laser design
3.1 Single-end-pumped alexandrite laser system and wavelength tuning
Figure 4 shows a single-end-pumped version of an alexandrite laser designed for maintaining TEM00 operation. A super-Gaussian pump profile was used from the fibre-delivered pump system as previously described. Better mode size control than the simple plane-plane laser cavity is obtained by using a laser cavity design with negative convex curvature ($R$) back mirror. Additionally, fuller use is made of adjustable cavity lengths $L_1$ and $L_2$, which describe the distance from the cavity mirrors to the front facet of the crystal.
Fig. 4. Schematic of single-end-pumped alexandrite laser for enhanced TEM00 operation. (A birefringent filter (BiFi) was used for wavelength tuning in an extended cavity version).
Two pump sizes were investigated. In the first system, the pump delivery was super-Gaussian profile with waist radius $w_p= {150}\; $µm and Rayleigh range $z_R= {1.1}\; \textrm{mm}$. To offset the pump-induced lens, a curved convex mirror ($R= {300}\; \textrm{mm}$) was used as the back mirror. The convex mirror was curved on both surfaces such that the transmitted pump beam was neither focused nor defocused. The measured waist size was verified to be the same as that without the convex mirror. The cavity was designed such that the laser mode size matched the pump waist size at the gain medium - further details of the cavity design, including the effect of the pump induced lensing and crystal temperature, are discussed in more detail in Section 4.
Figure 5 shows the laser power as a function of the absorbed pump power with cavity arm lengths $L_1= {10}\; \textrm{mm}$ and $L_2= {20}\; \textrm{mm}$. A laser power of 4.7 W was achieved at an absorbed pump power of 13.4 W with a beam quality of $M^2_x= {1.33}$, $M^2_y= {1.29}$. The overall slope efficiency was 54.9 % - a record for red-diode-pumped alexandrite. Increasing the pump power further achieved 5.56 W at 16.0 W absorbed pump power with a beam quality of $M^2= {1.70}$. Improved beam quality was obtained by changing the laser mode size which was done by varying $L_1$ and $L_2$, with some reduction of power, as is discussed in greater detail in Section 4.
Fig. 5. Laser power as a function of absorbed pump power for single-end-pumped alexandrite laser for TEM00 operation with $w_p= {150}\; $µm.
For wavelength tuning a 0.5 mm thick Quartz birefringent filter (BiFi) was placed between the crystal and OC, as shown in Fig. 4 but required the cavity length to be increased. For a stable cavity at maximum pump power, this required a weaker pump-induced lens dioptric power, which was obtained by increasing the pump waist radius to $w_p= {225}\; $µm and providing a longer overall cavity length ($L_1= {5}\; \textrm{mm}$ and $L_2= {55}\; \textrm{mm}$). Figure 6(a) shows the laser power of this longer cavity as a function of the absorbed pump power (without the BiFi). The increase in pump threshold from 4 W to 10 W is roughly consistent with the change in the pump waist area. An output power 5.22 W was achieved at absorbed pump power of 16.3 W. The beam quality was excellent with $M^2_x= {1.03}$ and $M^2_y= {1.07}$.
Fig. 6. (a) Laser power as a function of absorbed pump power for single-end-pumped alexandrite laser with $w_p= {225}\; $µm (b) Laser power as a function of wavelength.
The laser power as a function of the laser wavelength tuning with the birefringent tuning plate is shown in Fig. 6(b). Continuous tuning between 725 and 808 nm was achieved with >1 W over 730-805 nm for the very first time in a red-diode-pumped alexandrite laser, and with >4 W over the central region 755-780 nm. Diffraction limited output ($M^2< {1.1}$) was obtained over the entire tuning region.
The temporal power stability of the cavity was also investigated. Figure 7 shows the laser power measured for around 10 minutes for the previous laser cavity at an absorbed pump power of around 15 W. The output power was stable over this period of time with a standard deviation of just 0.01 W. No temporal measurements were taken at higher powers however the laser was typically found to run for up to an hour without any significant change in power. Temporal instabilities such as spiking and self-Q-switching which have previously been observed in alexandrite [20,21] were also not observed (using a photodiode) in any of the systems shown in this work.
3.2 Dual-end-pumped power-scaled alexandrite TEM00 laser system
For further power scaling the TEM00 cavity design of Fig. 4 was modified with the addition of a 45° dichroic turning mirror DM to produce an L-shaped cavity and dual-end-pumped using both polarization states from the fibre-delivered pump, as shown in Fig. 8. For better management of the higher pump-induced lens a stronger curved convex back mirror was used with curvature $R= {200}\; \textrm{mm}$. The pump waist size was unchanged, and the cavity length was also unchanged with $L_1= {5}\; \textrm{mm}$ and $L_2= {55}\; \textrm{mm}$.
Fig. 8. Schematic of double-end-pumped L-shaped alexandrite laser with $R= {200}\; \textrm{mm}$ radius of curvature convex mirror.
Figure 9 shows the laser power as a function of the absorbed pump power obtained at an alexandrite crystal temperature of 10 °C. The higher pump threshold of this system was due to the pump-induced lens bringing the cavity to stability with the stronger negative curvature back mirror. An output laser power 7.4 W was obtained at 26.3 W of absorbed pump power. The measured beam quality was $M_x^2= {1.12}$ and $M_y^2= {1.04}$. This is the highest power TEM00 red-diode-pumped alexandrite laser, to the best of our knowledge.
Fig. 9. Laser power as a function of absorbed pump power for double-end-pumped TEM00 alexandrite laser with $w_p= {225}\; $µm.
4. TEM$_{00}$ cavity design model with direct pump-induced lensing measurement
The optimum laser design for best output power, beam quality and laser efficiency, depends on the cavity geometry, crystal temperature, output coupling and pump induced lensing. A key requirement for TEM00 operation is to match the laser mode to the pump size at the crystal and this requires knowledge also of the pump-induced lens.
In this section, we determine the radius of the laser mode at the crystal, $w_l$, by using analytical formula for a laser cavity with an internal lens of focal length $f$ [22], with the lens corresponding to the pump-induced lens as shown in Fig. 10(a). Analysis of the laser mode size $w_l$ as a function of $f$ for different $L_2$ (and with $L_1$ fixed) is shown in Fig. 10(b). Increasing $L_2$ allows better matching to the pump size (in this case, a super-Gaussian profile with waist radius $w_p= {150}\; $µm) but reduces the region of stability. It is therefore essential to know the variation in pump-induced lensing $f$ with absorbed pump power.
Fig. 10. (a) Cavity model using pump-induced lens at pump-face of the crystal. (b) Beam width at the lens ($w_l$) as a function of pump-induced lens focal length ($f$) with different $L_2$. $R= {300}\; \textrm{mm}$ and $L_1= {5}\; \textrm{mm}$ are both fixed.
Fig. 11. (a) Shack-Hartmann measured lens dioptric power of pump-induced lens as a function of the absorbed pump power under both non-lasing and lasing conditions. (b) Beam width at crystal ($w_l$) as a function of absorbed pump power using cavity model based on measured lens dioptric power for three cavity lengths $L=L_1+L_2$ ($\textrm {A}= {15}\; \textrm{mm}$; $\textrm {B}= {30}\; \textrm{mm}$; $\textrm {C}= {50}\; \textrm{mm}$) with experimental inset showing experimental laser beam profile.
To precisely determine the lens dioptric power, a direct wavefront measurement technique with a probe beam and Shack-Hartmann wavefront sensor was used, as described in our previous work [23]. Figure 11(a) shows the lens dioptric power (inverse of focal length $f$) measured as a function of absorbed pump power under both lasing and non-lasing conditions. The measured lens dioptric power is found to be significantly weaker under lasing conditions in agreement with our previous work [23]. Using the measured values for $f$, Fig. 11(b) shows the laser mode size $w_l$ for three cavity configurations and the inset shows the experimental spatial mode at maximum pump power. In the first configuration (case A, where $L_1= {5}\; \textrm{mm}$ and $L_2= {10}\; \textrm{mm}$), output power was 4.4 W and the beam quality $M^2= {2.2}$ was multimode, this is attributed to the laser-mode under-filling the pump region. An improved beam quality of $M^2= {1.3}$ was obtained with the cavity adjusted (case B) to $L_1= {10}\; \textrm{mm}$ and $L_2= {20}\; \textrm{mm}$. Then better still (case C) with $L_1= {20}\; \textrm{mm}$ and $L_2= {30}\; \textrm{mm}$, with 5.0 W of laser power achieved at 16.0 W of absorbed pump power and near-diffraction limited beam quality of $M^2= {1.2}$.
The effect of alexandrite crystal temperature on the lasing performance was also investigated. Figure 12(a) shows the laser power as a function of the absorbed pump power for the single-end-pumped extended cavity (Fig. 4) with $w_p= {225}\; $µm and at five crystal temperatures. There is found to be a red-shift with increasing temperature (shown by the lasing spectrum at the maximum pump power in the inset of Fig. 12(a)). There was also found an increased lasing efficiency and output power with crystal temperature but at 40 °C output power is starting to roll over at maximum pump power. Increased efficiency and a red-shift in wavelength at higher temperatures is expected for alexandrite [12], however, the improved efficiency and wavelength shift is greater than might be expected for such a small temperature change. Optimum performance was not necessarily observed at elevated temperatures. In Figs. 5 and 9 optimum performance was obtained at a lower temperature of 10 °C. It is however worth noting that this is the temperature of the water, and so the temperature at the crystal centre is likely to be higher.
Fig. 12. Laser power as a function of absorbed pump power for single-end-pumped alexandrite laser with (a) $w_p= {225}\; $µm at five temperatures and (b) with $w_p= {300}\; $µm at two temperatures.
The output power and pump-induced lens strength were measured in a new cavity arrangement to further investigate the temperature dependent power curves. The new cavity arrangement: $w_p= {300}\; $µm, $R_{\textrm {OC}}= {99.0}{\% }$, $L_1= {5}\; \textrm{mm}$, $L_2= {70}\; \textrm{mm}$ and $R= {300}\; \textrm{mm}$; again showed a temperature dependent power curve, as shown in Fig. 12(b). The lens dioptric power measured for this new cavity arrangement at temperatures of 10 °C and 40 °C showed the lens dioptric power at 40 °C was as much as $\times {1.5}$ stronger than at 10 °C at the maximum absorbed pump power. The detailed reason for this is not known but suggests some importance to the temperature dependence of parameters to the pump-induced lens that warrants further investigation. Using the measured temperature dioptric lens power values, Fig. 13 shows the "predicted" laser mode beam width at the OC as a function of the absorbed pump power based on the cavity model (with $R= {300}\; \textrm{mm}$, $L_1= {5}\; \textrm{mm}$ and $L_2= {70}\; \textrm{mm}$), and comparison to experimental beam size measurement made at high (>13 W above threshold) pump power. The increasing lens dioptric power is seen to reduce the laser mode size at the OC with increasing pump power. The beam width at the two temperatures deviate from one another at around 14 W which also corresponds to the deviation in the power curve in Fig. 12(b). These results show that the temperature control of the crystal can provide significant optimisation of the output power and efficiency by altering the pump and laser mode overlap.
Fig. 13. Laser mode width radius as a function of absorbed pump power at 10 and 40 °C. Predicted values are based on measurement of the dioptric lens power under lasing conditions. Measured values are direct imaging of the OC.
5. Conclusion
We have demonstrated significant progress in continuous-wave diode-end-pumped alexandrite lasers using fibre-delivered diode-pumping, novel cavity design, and direct pump-induced dioptric lens measurement with a Shack-Hartmann wavefront sensor. Simple cavity design, coupled with low cost and small-footprint diode-pumping have been shown to be capable of providing high efficiency, broadly wavelength-tunable at high-power TEM00 laser operation far exceeding what has previously been obtained.
Output power 12.7 W in low-order mode is demonstrated in a double-end-pumped compact cavity with 43.0 % slope efficiency. Measurements of the laser beam quality as a function of the absorbed pump power is reported in detail with Laguerre-Gaussian mode selection from highly symmetric fibre-pumping and mitigation of spherical aberrations demonstrated with super-Gaussian pumping.
A novel convex-plane laser cavity design is developed to control TEM00 mode size at multi-watt level, demonstrating a record output power of 7.4 W TEM00 in excellent spatial format ($M^2\sim {1.1}$) in a robust, double-end-pumped compact cavity format. A record slope efficiency of 54.9 % is achieved in single-end-diode-pumped TEM00 alexandrite laser. Wavelength tuning is demonstrated across 725-808 nm with multi watt ($\sim {5}\textrm{W}$) results at 755-780 nm and >1 W over 730-805 nm being a record for any diode-pumped vibronic laser, to the best of our knowledge. Cavity design and physical understanding of fibre-coupled diode-pumped alexandrite was enhanced using a combination of analytical cavity design formulae and direct Shack-Hartmann wavefront measurements of the pump-induced lens. Theoretical mode-matching predictions are found to agree well with the experimental results. The effect of crystal temperature on the lasing performance is also reported to further enrich the understanding of the temperature dependent properties of alexandrite, but also suggesting the need for further investigation of the complex temperature dependence of the material parameters and their interplay in alexandrite.
These results demonstrate considerable improvement in TEM00 diode-pumped alexandrite laser performance. The novel cavity designs with symmetric fibre-delivered diode-pumping leads to a scalable route for high power TEM00 and wavelength tunable operation. The very compact designs and the engineering simplicity of fibre-delivery will be highly beneficial for moving low-cost diode-pumped alexandrite lasers to applications outside a laser laboratory including biophotonics, remote sensing and quantum technologies. Power scaling to multi-ten-Watt TEM00 operation should be possible for higher power pump fibre-delivery and with further laser design development. The designs employed in this paper can be applied to other materials such as Cr:colquirrites, and Ti:Sapphire where suitable diode-pumping is available.
Funding
European Space Agency (4000115840).
Disclosures
The authors declare no conflicts of interest.
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