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Abstract
We present a cylindrical rod of single-crystal Nd:YAG fabricated from a bulk crystal using femtosecond laser-induced preferential etching. The rod is pumped at 808 nm, and the laser characteristics at 1064 nm emission and the thermal stability are investigated. The slope efficiency was determined with a maximum optical-to-optical efficiency of 7.9%±0.29% and a FWHM linewidth of 299 ± 63 pm. The etched rod shows parameters consistent with existing Nd:YAG gain crystals. This fabrication technology will find use in composite micro-optical devices where microfluidics, active and passive optics, and structures can be etched out of many different materials and combined into a single device.
Published by Optica Publishing Group 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
The manufacture of optical components down to the micrometer scale and their integration into complex photonic devices has become one of the major driving issues in modern photonics due to the volume of consumer devices produced each year which rely on them [1–3]. Micro-optics and micro-photonics such as laser sources, waveguides, gratings and lenses are found across a broad range of consumer goods and industries [1–4]. Micro-optic systems in handheld devices must often be both robust and precisely positioned [1,5], and composite micro-fabricated optical devices using preferential etching present an alternative to existing fabrication methods.
Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) is an industry standard laser medium for near-IR applications, finding applications in biomedical science [6], material processing [7], remote sensing [8], and spectroscopy [9].
Using a four-level atomic transition which emits at 1064 nm, Nd:YAG lasers used Krypton or Xenon flashlamps as pump sources for much of the 1960s [10], but developments in laser diode technology lead to more efficient diode-pumped Nd:YAG lasers becoming commercially available [11]. These lasers found further applications with other emission wavelengths due to alternative atomic transitions [12], and second-, third-, and fourth frequency harmonics leading to laser light in the green, blue, and UV [13–16].
The procedures to fabricate Nd:YAG rods, blocks, and fibers are mature technologies in their own right [17]. Typically for bulk components, a large single-crystal boule of doped YAG crystal is diced into blocks or rods are extracted using a core drill [18]. These techniques can mass-manufacture identical optical components, but any bespoke shaping of components require additional processing e.g., a rod with a planar facet along its length must be ground back using chemical-mechanical polishing techniques, adding time and cost onto the process.
Chemical etching, on the other hand, can provide a route to achieving more complicated device geometries than mechanical methods, but this comes requires expensive and precise pre-processing stages [19,20].
In this paper we demonstrate that the optical properties of an Nd:YAG rod fabricated through Ultrafast Laser Inscription (ULI)-induced preferential chemical etching [21,22] are comparable to Nd:YAG crystals which have been neither etched nor laser inscribed [23,24]. In this work, no etching of the unmodified material was observed, such that the etchant did not damage the surface polish quality of the bulk sample, demonstrating an ideal etching ratio - and this etching contrast removes the need for many of the pre-processing stages in traditional etching.
This rapid and precise manufacturing process is another step towards a new generation of micro-optic components which can combine several materials in a single hybrid photonic system. Existing knowledge of the ULI-induced preferential etching properties of other readily available photonic materials (such as sapphire and fused silica [25–28]) will allow for substrates to be created which enclose many different active materials in a robust single-chip device at low cost and processing time. Composite material devices using components of arbitrary three-dimensional design will allow for miniaturised and readily modifiable photonic systems.
2. Experiment
A cylindrical volume was directly inscribed into bulk Nd:YAG using a ytterbium-doped fiber amplified femtosecond laser (Amplitude Systems Satsuma HP2), described elsewhere [29,30]. The laser pulses had a width of 250 fs and a pulse energy of 180 nJ, with a pulse repetition rate of 500 kHz. Individual linear ULI modifications were used to build up an outline of the cylindrical rod as shown in Fig. 1 (a). To ensure that the volume modified by the laser was continuous, the line density used was 1.5 lines per μm of diameter i.e., a rod of diameter 400 μm was inscribed with 600 individual modification lines around the perimeter of the volume. The sample was translated through the laser focus at a speed of 5 mm/s, and the incident laser light was circularly polarised and focused in by an aspheric lens with an NA of 0.4.
Fig. 1. (a) Three rods inscribed in a sample which has subsequently been ground back and polished to optical quality; (b) the same sample after 71 hours in Nitric Acid – the modified areas have been completely etched away but the material left impedes the removal of the rods. The facet surface remains visibly unetched; (c) a second sample with a single rod and structure to aid removal written and etched for 148 hours until the modified lines were completely removed; (d) the rod prior to removal, with the section above it removed.
The sample was then immersed in Nitric Acid with a 50% concentration at 100°C. The regions modified by the femtosecond pulses showed an etching susceptibility many times above the unmodified bulk material such that no detectable etching of the unmodified YAG was observed at all.
Table 1 shows the observed etching rates of lines which had been inscribed with increasing pulse energies. High pulse energy modifications correspond to greater etching rates but cause more stress in the immediate crystal structure. For this reason, a lower etching rate was chosen (105 µm/hour) to ensure that the crystal would not suffer any damage from the density of modifications. The sample was immersed for 148 hours to allow the complete removal of all modified areas.
Table 1. Inscription pulse energy and the observed etching rate after a five-hour etch of 50% Nitric Acid at 101 °C.
Four rods were manufactured, with diameters of 1000 µm, 800 µm, 600 µm, and 400 µm. Further lines were inscribed from the widest horizontal point of each rod to the surface of the sample, shown in Fig. 1(c) and 1(d). The volume immediately above the rods could thus be removed, freeing the rods to be lifted out without placing any strain on them.
As the unmodified material was undamaged by the etchant (as shown by Fig. 1), no surface polishing was necessary post-etch. After repeatedly mounting and unmounting the laser rods of different diameter into the laser cavity for characterization, the facet of the 600 µm rod suffered damage which was subsequently polished out and the facet restored to optical quality.
The 600 µm diameter rod was chosen for characterization as it was the rod with the smallest diameter which demonstrated lasing with the beam setup shown in Fig. 2. It was placed into the cavity and end-pumped using a high-power multimode laser diode (BWT K808CAHFN-15.00W) at 808 nm to generate the laser emission at 1064 nm. The rod had a length of 7.53 mm, and the cavity was 8.97 mm long from reflector to reflector. The resonator used a planar mirror (99.9% reflective at 1064) placed as close as possible to one facet of the rod without making contact, while an output coupler with a radius of curvature of 50 mm was positioned 1.44 mm away from the other rod facet.
Fig. 2. Diagram of the pump beamline and laser cavity; (A) 0.8 ND filter, (B) Couplet with back focal length = 100 mm, forward focal length = 50 mm, (C) Planar mirror HR 1064 nm, (D) Nd:YAG rod, (E) Curved output coupler with R = 50 mm, (F) Collimation lens, and (G) Long-pass dichroic mirror with a cut-on of 950 nm to deflect the pump light.
The pump laser was operated at 40% of its maximum power to avoid fluctuations at threshold. For alignment of the pump beam, the end facet of the rod was imaged through the output coupler onto a CCD camera. As the pump source is highly multimodal, the light was coupled into the rod to be guided through it by total internal reflection rather than optimizing spatial mode matching of the pump and laser. This “overfilling” of the rod decreases the efficiency of the laser but results in a much simpler alignment procedure.
The rod was mounted onto a finned aluminum mount, secured using SPI Supplies silver conducting paint (SPI 05002-AB) to allow for effective conduction of heat between the rod and the mount as a thermal control measure. Figure 3(a) shows the 600 µm rod mounted in the v-groove of the aluminum mount, while Fig. 3(b) shows a side view of the 800 µm rod.
Fig. 3. (a) Microscope view of an etched rod end facet (d = 600 µm) after mechanical polishing, secured in an aluminum mount; and (b) a side view of an unmounted rod (d = 800 µm) after mechanical polishing of the end facets.
The emission spectrum was measured using a Yokogawa AQ6373B Optical Spectrum Analyser, and the beam profiles taken using a Thorlabs BP209-VIS/M scanning-slit optical beam profiler. The M2 was determined using a Spiricon M2-2000 laser characterization system, while all optical power measurements were performed with a Thorlabs S121C power sensor and a PM100D power meter console.
3. Results and discussion
3.1 Power, threshold, and slope efficiency
The output power of the laser crystal was measured with output couplers of four different transmissions at 1064 nm. These output couplers (1%, 2%, 4.5%, and 15%) were used to determine the slope efficiencies and lasing thresholds of the laser. These are compiled in Table 2 and shown in Fig. 4.
Fig. 4. Absorbed Pump Power vs Output Power for 1% (downwards triangle), 2% (upwards triangle), 4.5% (square), and 15% (cross) transmission output couplers. The linear fit for the slope above threshold is plotted as a solid line.
Table 2. Slope Efficiency and Thresholds with Output Coupling
The absorbed pump power was determined by measuring the pump power coupled into the rod, the pump power transmitted longitudinally through the rod, and then the pump power scattered through the top surface of the rod was collected and measured using an integrating sphere. The former measurement minus the latter two gives the absorbed power inside the rod. The slope efficiency of the laser increases with the percentage transmission of the output coupler, up to an optical-to-optical efficiency of 7.9%. The threshold of the laser increases linearly with the output coupling transmission, which is expected for these low output coupler transmissions [31].
The rod was treated as a multimode waveguide due to its large radius and the multimode pump source, and the pump beam was guided along the volume largely by total internal reflection. As a result, a much larger volume of the rod was excited than resonated inside the laser cavity. This difference in volume determines the maximum efficiency achievable in this laser cavity design.
The pump thresholds found are well within the achievable range of commercial single-mode laser diodes at 808 nm. Single-mode operation and mode matching is achievable with the current rod geometry, and this would increase the efficiency significantly. Additionally, the design freedom of preferential etching induced by laser inscription allows for a multitude of alternate pump and rod geometries to be trialed quickly and cheaply.
3.2 Thermal stability
The laser demonstrated significant stability over time with no active thermal management. When operated with a 2.5% transmission output coupler and 970 mW of incident pump power absorbed, the laser emission had a mean output power of 5.41 mW over thirty minutes, and a standard deviation of 0.26 mW (4.7% of the mean). This long-term stability is such that no further thermal management is necessary, as the laser crystal reaches a thermal equilibrium of 30.1 °C with the aluminium mount acting as a heatsink to conduct and disperse the accumulated heat.
3.3 Emission spectrum
The laser emission spectrum, measured with the Yokogawa AQ6373B Optical Spectrum Analyser, shows an average half-maximum width of 299 ± 63 pm from measurements taken over twenty minutes – a snapshot of one of these measurements is shown in Fig. 5. The linewidth is well within the published values for the spontaneous emission gain bandwidth at 1064 (quoted as 450 pm) [32,33], while the maximum possible value for emission linewidth given by Nd:YAG crystal manufacturers is 600 pm [34,35]. The optical cavity length gives a calculated longitudinal mode spacing of 990 MHz. The spectrum shows evidence of some multimode operation in the interference fringe pattern visible along the peak of the trace caused by adjacent modes
Fig. 5. Spectrum of the laser emission in wavelength (black) with an applied Gaussian fit (dotted, blue). The fit allows for simple estimation of the FWHM.
3.4 Beam profile
Using the BP209-VIS/M scanning-slit beam profiler, a cross-section of the beam was imaged in both the horizontal (X) and vertical (Y) axes (relative to the optical bench) and a Gaussian fit was applied to a one-dimensional intensity measurement taken across the centroid of the beam. The beam profile of the collimated beam, shown in Fig. 6 as a snapshot, shows little ellipticity, determined on average to be 96% (X/Y diameters) over one hour. The beam had 4.78% variation in the Gaussian diameter for the X axis; 3.62% in the Y diameter; and the ellipticity had a standard deviation of 0.03.
Fig. 6. A beam profile of the expanded beam (radius = 1 mm), and curves representing a one-dimensional measurement of the intensity across the beam profile in each axis (yellow trace) and the Gaussian curve fitted to this intensity profile (red curve). The beam shows a minor deviation from ideal Gaussian behavior indicative of multimode operation with low numbers of modes oscillating.
The M2 was measured using a CCD M2 beam profiling device and was found to be 6.061 in the X axis and 4.777 in the Y axis, using the four-sigma definition of the diameter of the beam as equal to four standard deviations of the mean (D4σ).
4. Conclusion
It has been demonstrated that Ultrafast-Laser-Inscribed laser rods written in Nd:YAG using femtosecond pulses and preferentially etched out of the bulk crystal have optical properties which are comparable to existing bulk Nd:YAG lasers. This Nd:YAG rod showed a maximum slope efficiency of 7.9% when pumped at 808 nm and emitting at 1064 nm. The pump diode and the laser emission were multimodal in both longitudinal and transverse cases, and the laser achieved a FWHM linewidth of 299 ± 63 pm.
While the cylindrical geometry of this rod is readily achieved using existing manufacturing methods, the demonstration of laser properties comparable to published values opens the door to arbitrary three-dimensional designs written and etched directly from single-crystal Nd:YAG. This capability may allow for bespoke optical components to be fabricated quickly and cheaply compared to current cleanroom fabrication procedures, enhancing the already notable versatility of Nd:YAG as a laser source.
The greatest advantage of this technique lies in the shaping of both substrate and active medium through femtosecond laser induced etching susceptibility. This would allow for hybrid photonic devices manufactured with micrometer accuracy which could combine multiple optical properties and techniques on a small scale and could even utilize miniaturised modular or replaceable components in their designs.
Funding
Engineering and Physical Sciences Research Council (EP/L01596X/1).
Acknowledgements
This work is funded by the UK Engineering and Physical Science Research Council (EPSRC) (EP/L01596X/1) via the CDT in Applied Photonics.
Disclosures
The author declares no conflict 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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