A 2.8 W Single-Frequency Laser Output at 1064 nm from a Gradient-Doped Composite Ceramic Non-Planar Ring Oscillator

Abstract

An efficient Nd: A YAG single-frequency laser was demonstrated using a gradient-doped ceramic non-planar ring oscillator (NPRO). A thermal model of the gradient-doped ceramic NPRO was built to analyze the temperature field and thermal focal length. By employing a gradient-doped gain structure, the thermal distribution within the NPRO can be effectively smoothed to reduce thermal lensing effects. Up to 2.8 W of single-frequency output power at 1064 nm from the gradient-doped ceramic NPRO was obtained, with a slope efficiency of 38%.

1. Introduction

Single-frequency lasers are employed in many scientific and technological applications such as optical frequency standard, precision laser spectroscopy, gravitational wave detection, medical research, space high-precision laser interferometry measurement, etc. [1,2,3,4]. In such applications, besides stringent requirements for intensity and phase stability, a high-output power laser is essential to achieve the desired detection sensitivity. Diode-pumped non-planar ring oscillator (NPRO) invented by Kane and Byer is well suited for the above-mentioned applications because of its excellent frequency stability and low noise in single-frequency operations [5]. In addition, the resonator of NPRO was composed by a single crystal, and the length of the resonator was limited in several ten millimeters. When the NPRO laser worked at the situation of high pump power, the laser fundamental transverse mode related to the thermal lensing effect would be too small to match with the beam size of high-power pump diodes, thus limiting the power-scaling possibilities. In recent years, attempts were made by different groups to increase output power by reducing the thermal effect of the NPRO laser. A direct upper-level pumping method was used to decrease the quantum deflection, thus reducing the thermal effect and increasing the efficiency of the NPRO laser [6]. By using a negative curved front facet when compensating for the thermally induced lensing inside the crystal, the mode matching was improved in the situation of high pumping power, leading to a stable 1064 nm single-frequency output power up to 2 W [7]. A single-frequency output of 1.4 W was generated from a composite resonator combined with an undoped NPRO resonator and an end-pumped thin disk laser, which had been proven to have good thermal property in high-power operations [8]. However, most of them suffered from highly complicated manufacturing and precise alignments. The exponential decay of the pump absorption distribution for the homogeneous dopant laser crystal in the end-pumped structure is the main reason for the high thermal lensing effect. Therefore, by changing the non-uniform pump absorption of the gain crystal, the thermal lensing effect of the crystal can be effectively reduced. The multi-segmented composite crystal manufactured with the diffusion-bonded technique with increasing doping concentrations in the pump direction provides more homogeneous pump absorption, which can flatten the longitudinal temperature distribution under high-power operations to permit a higher pump power [9,10]. However, multi-layer composite crystals processed using diffusion-bonded techniques suffer from high manufacturing complexity and time consumption. The bonding quality of the interfaces greatly influences the polarization state of oscillating beams in the laser, which is particularly important for monolithic non-planar ring oscillators (NPROs) that rely on polarization loss differences to achieve single-longitudinal mode operations. Due to numerous significant advantages such as high doping concentration, high optical uniformity, short fabrication cycle, low cost, large size and composite structures, rare-earth doped transparent ceramic has attracted more and more attention as a substitute for single crystal [11,12,13]. A composite ceramic, easily fabricated through the vacuum sintering process without any obvious interface, will have better performance in achieving a more uniform pump absorption with a gradient-doped structure. In this work, we focus on the study of the gradient-doped Nd: YAG ceramic as a gain medium in combination with an NPRO structure in a single-frequency operation. A near-diffraction-limited laser beam with a maximum single-frequency output power of 2.8 W was achieved at 1064 nm. To the best of our knowledge, this is the first time that gradient-doped ceramics have been applied to monolithic non-planar ring oscillator (NPRO) technology to scale the output power.

2. Design of Gradient-Doped Ceramic NPRO

A schematic drawing of the gradient-doped ceramic blank used for processing the NPRO is shown in Figure 1a. Considering the ease of ceramic crystal fabrication and uniform pump light absorption, we employ a ceramic structure with three gradient-doped segments in the middle and undoped regions at both ends. The thickness of each of the three gradient-doped segments is 2.5 mm, for a total length of 7.5 mm, which matches the 10 mm geometric length of AB of the NPRO. The longer non-doped areas at both ends of the gradient-doped ceramic blank are designed for the convenience of measuring the dihedral angle between the reflective surfaces, especially the reflective surfaces containing points B and D during NPRO processing [14]. The structure of the gradient-doped ceramic monolithic NPRO, as shown in Figure 1b, is identical to the parameters previously reported [15], with dimensions of 12.5 mm × 12 mm × 4 mm. The facets containing B, C, and D are optically polished flat surfaces on the Nd: YAG composite ceramic where total internal reflections occur. The front surface is dielectrically coated for high transmission at the pump wavelength of 808 nm and a partial output coupling coating of the s-polarized beam at 1064 nm. The doping concentration gradient of the ceramic crystal increases along the direction perpendicular to the incident surface containing A. The first ceramic segment, with a width of 2 mm near surface A, is undoped to avoid spatial hole burning effects, which can prevent laser operation in a stable fundamental mode at high pump power levels [16,17]. The pump light can be bi-directionally pumped along the AB and AD within the NPRO, with AB and AD lengths of approximately 10 mm and an angle of 46 degrees.

The doping concentration distribution in various segments of gradient-doped composite ceramics is a critical parameter that determines the temperature profile of the ceramic. The Poisson equation for gradient-doped ceramic NPRO was developed to identify the optimal parameters of the doping concentration distribution, as shown below [18]:

$${\displaystyle \frac{{\mathsf{\partial }}^{2}T}{{\mathsf{\partial }}^{2}x}}+{\displaystyle \frac{{\mathsf{\partial }}^{2}T}{{\mathsf{\partial }}^{2}y}}+{\displaystyle \frac{{\mathsf{\partial }}^{2}T}{{\mathsf{\partial }}^{2}z}}=-{\displaystyle \frac{q}{K_c}}$$

(1)

where $K_c$ is the heat conductivity of Nd: YAG ceramic and $q$ is the heat power of unit column, which can be expressed by:

$$q={\displaystyle \frac{2\eta P}{\pi w(z{)}^2\left[1-\mathit{exp}(-\alpha l)\right]}}\times \mathit{exp}[{\displaystyle \frac{-2(x^2+y^2)}{w(z{)}^2}}-\alpha z]$$

(2)

where $\eta$ is the pump efficiency, $P$ is the pump power, $w\left(z\right)$ is the beam width of the pumping beam at position $z$, $\alpha$ is the absorption factor, and $l$ is the length of the doped medium. Using Equations (1) and (2), the temperature field distribution of the gradient-doped composite ceramic NPRO, as shown in Figure 2a, was obtained by the finite element analysis method. As a contrast, the temperature field distribution of diffusion-bonded NPRO shown in Figure 2b was calculated too. This diffusion-bonded NPRO consists of a 2 mm thick undoped YAG crystal at the front end, followed by a second crystal segment made of active Nd: YAG gain medium with a doping concentration of 1.0 at.% for the laser process. It could be observed that the gradient-doped composite ceramic NPRO with increasing doping concentrations in the pump direction provides a more homogeneous temperature distribution to permit a higher pump power. Balancing the overall pump absorption efficiency, we selected a gradient doping concentration parameter of 0.6 at.%–0.9 at.%–1.1 at.% for fabricating the ceramic by comparing the thermal distribution uniformity of different gradient doping parameters through simulation.

The resonator of the NPRO was produced with four planar reflecting surfaces, in which case the beam properties are determined by the focal length of the thermally induced lens. A larger thermal lens focal length will result in a larger laser oscillation mode, which is more conducive to achieving good mode matching with the pump light. The thermal focal length of the gradient-doped ceramic NPRO can be calculated using the following formula [19]:

$$f_{th}=-{\displaystyle \frac{r^2}{2\cdot \left[\mathsf{\Delta }OPD(r,z)-\mathsf{\Delta }OPD\left(\mathrm{0,0}\right)\right]}}$$

(3)

where r is the radial coordinate, $\mathsf{\Delta }OPD$ is the optical path difference induced by temperature distribution, and $\mathsf{\Delta }OPD\left(\mathrm{0,0}\right)$ is the optical path difference in the laser material center. In Figure 3, the thermal focal length under different concentration distributions is shown as a function of pump power. Simulation results indicate that, compared to the diffusion-bonded monolithic non-planar ring oscillator, the gradient-doped composite ceramic structure exhibits significantly reduced thermal effects at higher pump power. Therefore, it can achieve good mode matching at higher pump power, leading to higher output power.

3. Experiment Setup and Results

To estimate laser performance of the gradient-doped composite ceramic NPRO, experiments to compare diffusion-bonded NPROs were carried out (see Figure 4). The pump modules, using a bidirectional pumping method, were two 808 nm fiber-coupled diode lasers (BWT) with a core diameter of 105 μm and a numerical aperture of 0.22. Two planar convex lenses with 25 mm focal length were used to focus the pump beam into the gradient-doped ceramic NPRO. Two 45° dichroic mirrors (HT at 808 nm and HR at 1064 nm) were used to extract the oscillating laser light from the bidirectional pumping structure. The NPRO was designed to have a high transmission coating at 808 nm and 2% output coupling coating of the s-polarized beam at 1064 nm. To achieve unidirectional oscillation, a magnetic field of 0.3 T was applied along the NPRO. The temperature of the NPRO was controlled at 24 °C using a thermal electric cooler.

In Figure 5, the maximum single-frequency output power of 2.8 W for a gradient-doped composite ceramic NPRO laser was obtained with a total pump power of 8 W, giving a slope efficiency of 38%. As a contrast, the maximum single-frequency output power of 2.4 W for a diffusion-bonded NPRO laser was achieved with a total pump power of 4.4 W. Increasing the pump power further will lead to multimode oscillation, which is consistent with theoretical analyses.

Figure 6 shows the typical longitudinal spectrum of the single-frequency laser from the gradient-doped composite ceramic NPRO, measured at an output power of 2.8 W using a scanning confocal Fabry–Perot (F-P) interferometer with a free spectral region of 3.75 GHz and a finesse of approximately 100. The emission wavelength of 1064.4768 nm was measured with an optical spectrum analyzer (YOKOGAWA AQ6370D, Yokogawa Electric, Tokyo, Japan) with a side-band suppression of more than 50 dB (see Figure 7). To investigate the linewidth properties, a heterodyne experiment between a gradient-doped ceramic NPRO laser and a diffusion-bonded NPRO laser was carried out [5]. A spectrum analyzer (Agilent, N9020A, Santa Rosa, CA, USA) trace of the heterodyne signal from the two NPRO lasers is shown in Figure 8. The −3 dB width of the heterodyne signal determines the upper limit of the linewidth of the two NPRO lasers. Therefore, it can be concluded that the linewidth of the gradient-doped ceramic NPRO laser is less than 5 kHz over a 14.6 msec spectrum analyzer sweep time.

Figure 9 shows the experiment result of power stability of a gradient-doped NPRO laser measured during a 4 h period. At the output power of 2.8 W, the standard deviation of the power fluctuation of output power is within 0.3%. In Figure 10, the beam quality of the single-frequency laser at 2.8 W output power was measured using Spiricon PY-III, Ophir Optronics, Jerusalem, Israel. By fitting the beam sizes at different positions along the beam propagation direction, the beam propagation factors (M²) are calculated to be 1.12 and 1.13 in both x and y directions, respectively.

4. Discussion

From Figure 5, it is evident that the threshold of the gradient-doped monolithic non-planar ring oscillator (NPRO) is higher than that of the bonded crystal NPRO, and the output slope efficiency is lower compared to the bonded crystal. Due to the more uniform thermal distribution in the gradient-doped ceramic, stable single-frequency output can be achieved at higher pump power. The higher pump threshold and lower slope efficiency of the ceramic crystal monolithic non-planar ring oscillator may be attributed to greater scattering losses and grain boundary losses in the ceramic crystal. Furthermore, the relaxation oscillation frequency of a single-longitudinal mode laser can intuitively reflect the relationship between the pump rate and losses, and their relationship is expressed in the following equation [20]:

$$L={\displaystyle \frac{nl{\tau }_f{\left(2\pi f_{RO}\right)}^2}{c(r-1)}}-T$$

(4)

where $L$ is the round-trip cavity loss, $f_{RO}$ is the frequency of relaxation oscillation, $r$ is the ratio of the pump power to the threshold pump power, ${\tau }_f$ is the Nd: YAG upper-state lifetime, $l$ is the round-trip length of NPRO cavity, $n$ is the index of refraction, and $T$ is the transmission of the output coupler.

Table 1. The value of parameters in Equation (4).

Symbol	Parameter	Value
n	Index of refraction of YAG ceramic @1064 nm	1.82
l	Round-trip length of NPRO cavity	30.6 mm
${\tau }_{\mathit{f}}$	Upper-state lifetime of Nd: YAG	230 μs
T	Transmission of the output coupler	2%
$P_{th}$	Threshold pump power of gradient-doped NPRO	0.7 W

provides relevant values for the parameters of Equation (4).

The relaxation oscillation center frequencies of the ceramic NPRO laser output at different pump powers were measured using a high-speed detector (Newfocus1811AC, Irvine, CA, USA) and a spectrum analyzer (Agilent N9020A), as shown in Figure 11. By substituting these values into the formula, the intracavity loss coefficient was calculated.

The internal loss of the gradient-doped ceramic NPRO can be approximately obtained by averaging the loss values calculated at different pump powers, with the loss value being approximately 0.08. As a contrast, the internal loss of the diffusion-bonded NPRO based on Nd: YAG crystal is 0.04. Therefore, further improvement in the fabrication process of the ceramic crystals used in the monolithic non-planar ring oscillator, aimed at reducing internal losses, is expected to lower the laser oscillation threshold, increase the output slope efficiency, and achieve higher-power single-longitudinal mode laser output.

5. Conclusions

In conclusion, the thermal model of the gradient-doped ceramic NPRO was built to analyze the temperature field and thermal focal length. Compared with the uniform-doped NPRO, the gradient-doped ceramic NPRO can smooth the thermal distribution efficiently and reduce the thermal lens. We have demonstrated a good single-frequency operation in a diode-pumped and gradient-doped ceramic Nd: YAG NPRO, and a 2.8 W stable single-frequency output at 1064 nm was obtained with a slope efficiency of 38%.