Improving the Beam Quality of a High Power Yb:YAG Rod Laser

Falgun D. Patel, Dennis G. Harris, and Charles E. Turner, Jr. The Boeing Company

West Hills, CA

ABSTRACT An investigation has been made of improving beam quality of a high power diode pumped solid state rod laser. It was determined that the beam quality is limited by aberrations present in the medium, which may be due to mode-medium effects in the Yb:YAG laser. A diffractive optics propagation model was developed to predict the phase distortions present on the laser wavefront at the resonator mirror. Resonator mirrors were then fabricated with the proper correction applied to correct the distorted wavefront. The beam quality improved from M2 = 2.0 to M2 =1.4. Keywords: Yb:YAG laser, beam quality

1.0 INTRODUCTION Diode Pumped Solid State Lasers (DPSSLs) offer compact efficient high power sources of laser radiation, but have been limited to power levels of a few hundred watts with good (M2 ~ 1.5) beam quality. To date, to retain good beam quality at higher powers has required nonlinear optical techniques such as SBS phase conjugation. An approach that will provide good beam quality with an uncomplicated design is very appealing. The dual rod Yb:YAG laser being pursued by Boeing offers promising possibilities as a compact robust device of simple design.

1.1 Rod Laser Architecture An investigation has been made of scaling issues of a kilowatt class Yb:YAG rod laser which is of a robust and compact design. It is of interest to understand the issues and limitations which will allow this architecture to scale to high powers while retaining a beam quality of about 1.5. It is also desired to keep the resonator design simple, effective and in concert with the robust yet simple philosophy of the architecture, and without the reliance upon nonlinear phenomena SBS or STS phase conjugation requiring complex additions to the architecture. Yb:YAG was chosen because of its low quantum defect translating to higher quantum efficiency and less heat deposited into the host as compared to Nd:YAG. The quasi three level nature of Yb does, however, require high pump rates to achieve efficient lasing.

The laser is an end pumped Yb3+:YAG system that has been described elsewhere.1 An end-pumped dual rod architecture has been chosen as shown in Figure 1. The thermally induced birefringence compensation is achieved using a dual rod stable cavity configuration with a quartz optical rotator located between the rods2. The rotator has the effect of changing the polarization and making the resonator symmetrical. The rods are end pumped by diode arrays. The diode pumping radiation is transferred to the rods by hollow lensducts; at the entrance to the lensduct is a focusing lens which focuses most of the radiation into the laser rod. The lensduct captures that pumping radiation escaping from the optical system and redirects it into the rod. Once the pumping rays enter the 2mm diameter Yb:YAG rod, they uniformly fill the cross section by total internal reflection.

Solid State Lasers XV: Technology and Devices, edited by Hanna J. Hoffman, Ramesh K. Shori, Proc. of SPIE Vol. 6100, 610018, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.649358

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Figure 1. Schematic of the end pumped dual rod laser.

Figure 2 is a photograph of the typical layout of the dual rod laser which may be operated either as 1 kW laser with beam quality of approximately 10, or it can be operated with 500 W and Beam Quality (BQ) of approximately 1.5. It is desired to improve the power, efficiency, and BQ of such lasers without completely redesigning them, but rather by prudently improving the resonator design.

Output Coupler

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Figure 2. Typical layout of end pumped dual rod Yb:YAG laser.

Several models have been developed to describe and to predict performance of the dual rod laser. The models are well anchored with experimental data to provide fidelity and credibility to the scaling projections. The models include a ray tracing model to describe the 940 nm pumping radiation beginning with the diode arrays with microlenses, following the rays through the lensduct with focusing lens and into the laser rod as depicted in Figure 3. With the water jacket around the rod, the total internal reflection of the pumping rays is then followed down the rod. An energetic model is used to optimize power extraction of the end pumped laser, operating either in cw or Q-switched mode. The model considers the quasi-three level nature of Yb:YAG, taking into account ground state depletion, gain saturation, fill factor, clipping loss of the beam profile by the laser rods, and M2 beam quality. The resonator properties are modeled in Paraxia3, based upon an ABCD matrix approach, is used to describe the performance of the TEM00 Gaussian beam.

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Figure 3. Raytrace from diode pump array, through light duct, into laser rod.

2.0 General & Current Status The initial investigations of scaling this design were carried out with close coupling of the rods. That is, the laser rods were located as near as feasible to the compensation lens and quartz rotator with little regard to optical imaging or ray transfer. Typical lasing data versus the pumping rate, i.e. diode current, is shown in Figure 4. It was noted that laser power at good beam quality (typically M2 between 1.5 and 4.5) always reached a ceiling or plateau, then turned over. Changing the thermal compensation lens provided a modest increase in power, but would also reach a plateau. Even though power levels of greater than a 1 kW had been observed, the beam quality at such power levels was poor. It has been suggested that laser performance may be improved by an imaging resonator, one in which the laser rods are imaged onto each other with a lens optical system4. Such a configuration will reduce thermally induced birefringence, improving beam quality and laser extraction efficiency. Such a resonator was configured, but laser performance did not significantly improve. Consequently, other sources limiting performance were considered. Interferometry was performed to interrogate the rod and investigate the effect of thermal aberrations during the laser operation.

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Figure 4. Laser output reaches performance ceiling for low order Gaussian modes.

2.1 Scaling Path: aberration limited It has been noted, during laser testing that the lasing mode size in the rod is smaller than one would prefer for efficient extraction of lasing energy. Efforts to enlarge the mode, to more completely fill the laser rod, have not been successful. This limitation is believed to be attributed to thermal aberrations in the rod not permitting a larger mode to operate. That is, aberrations cause radiation to be scattered out of the resonator. Additionally, others5,6,7 have noted that spherical aberration caused by thermo-optic distortions in the gain medium can be an impediment to scaling solid state lasers to high powers. If the pump distribution is uniform, then spherical aberration can be attributed to at least two different effects: temperature dependence of the thermal conductivity and lasing induced heating (or cooling) of the gain medium. The former effect has been observed in Nd3+:YAG8. In Yb3+:YAG however, it appears that the observed lasing induced aberration is the dominant effect in our experiments. Unlike the diode pump distribution, the mode intensity distribution can be non-uniform, especially in stable resonators with near diffraction limited beam quality. Consequently, if the lasing mode can be made to fill the laser rod, and extract the gain uniformly, the thermal aberrations will be minimized. A static diffractive element may be useful in controlling the laser mode. Rather than use a transmissive element, we have chosen to alter the laser resonator mirrors, by adding an appropriate figure to them.

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3.0 Mirror Profile Design Calculation The designs for the mirror profiles were developed using a diffractive propagation computer code. The approach is to use static phase conjugating elements, reflective rather than transmissive, to define the preferred mode (one with the least losses), in a resonator or amplifier. Briefly, a super-Gaussian beam was propagated from the center of the cavity, through the medium to a resonator mirror, where the conjugate phase was then calculated. From here the mode was propagated back to the center of the cavity and using a Fox-Li analysis it was verified to be a low order eigenmode of the resonator. Inputs to the propagation model consist of optical schematic element parameters, distances, focal lengths, indices of refraction (no and n2-focusing), initial mode (intensity, phase, wavelength and polarization) and flags for controlling the number round trip iterations, parametric sensitivities, and plots. The code outputs include plots/tables of the fields at designated locations, the radial phase profile of the phase conjugating elements and iteration histories (power loss, fill factors and phase Strehl).

4.0 DOE fabrication Several approaches to fabrication of the profile were considered: Chemical etching, ion machining, photolithographic techniques, laser ablation, and magneto-rheological finishing (MRF). The first attempt at constructing a DOE utilized the photolithographic approach of building up layers from the mirror substrate. Initially, a single layer was attempted to investigate the feasibility of such an approach. The results were inconclusive. The step did not provide sufficient loss to the higher order modes and allowed too many transverse modes were to oscillate. More sophisticated modeling indicated that 8 steps would be required to produce a digital binary step optic which would have the necessary fidelity to emulate the calculated profile. Each step in the process requires a photolithographic mask to be fabricated and aligned to the previously built step. We did not pursue this approach because there was concern about the number of procedural steps required. The technique of laser ablation was also investigated as a possible method to produce the optic. The technique, while relatively new, is able to controllably remove small amounts of material, from the fused silica substrates. However, the surface roughness of the produced optic is generally too high. The resulting surface may be corrected further by a post-ablation chemical polish. While the technique seems to hold much promise, given that all of these processes (ablation & chemical etch) would have to be calibrated, it was deemed to require too much development.

The technique of magneto-rheological finishing (MRF) has held much promise and has shown excellent results previously for polishing optical elements. QED Technologies of Rochester, NY is the premier company in this area. Unfortunately, until recently the machines in use were not able to polish such small areas (approximately 2 mm diameter) as those required for this project. However, a new machine recently became available in to fabricate surfaces on small parts.

4.1 Fabrication The diffractive optics code was used to develop three designs of mirrors to provide super-Gaussian modes. The model uses symmetry to calculate the modes in the left and right half of the resonator. There are n2r and n2phi indices of refraction. The code calculated each based on the operating point determined experimentally in the laboratory. Figure 5 shows the experimentally measured profile of two substrates compared to the desired mirror profile. Additionally, a mirror design was calculated for the average of the indices of refraction. The designs for all three mirrors are shown in the next section.

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Design vs. Fabrication

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Figure 5. Fabricated vs. Designed profiles for the n2avg mirror.

5.0 Results and Discussion As described in the previous sections, the dual rod Yb:YAG architecture is a scalable and rugged design. From a power budget analysis of the laser power, we estimated that approximately 30% more power can be extracted from the rods. To date the extraction efficiency and mode quality have been hindered by the onset of higher order aberrations (predominantly 4th order spherical). The testing of the analog diffractive mirrors (ADMs) was conducted in a Yb:YAG resonator as shown in Figure 6. Test results were compared with a baseline case of -25 cm radius of curvature spherical mirrors. There were three designs for the ADMs and the results from each data set as well as permutations of each of the designs are considered in the following sections. In all cases the length of the cavity (104 cm), the negative compensating lens (f = -25 mm, biconcave), and the output coupler reflectivity (20% at 1030 nm) were kept the same as in the baseline case.

Figure 6. Schematic of the dual rod resonator design used for testing the analog diffractive mirrors (ADMs). Details of the individual elements of the resonator are described in earlier sections.

5.1 Experimental Layout The pictures of the laboratory setup are shown in Figures 2 and 7. A comprehensive set of qualitative and quantitative diagnostics were utilized to characterize the laser output. When the laser exits the output coupler, it is collimated by a

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broadband AR coated plano-convex lens. After collimation the beam is transmitted through an uncoated wedge and an AR coated negative lens to a power meter, which measures 92% of the total power. The 8% reflection from the uncoated wedge is sent to another wedge, where another 8% reflection (0.64% of the total power) is sent to more diagnostics. This reduced power beam is picked off by a third wedge and the reflected beam sent to a camera. The camera optics are positioned to image a collimated near field of the beam. After passing through the third wedge the beam travels through a 1 m plano-concave lens (far-field lens), reflects off a turning mirror, and is imaged by a camera positioned 1-f away from the far-field lens. Beam quality measurements (M2) are conducting by taking images of the beam before and after focus and analyzing the second moments with in-house software. The far-field camera can also be positioned within the dynamic range of the stage to image the mode in the rod as well as the mode on the output coupler. In addition to this set of beam diagnostics, the mode on the HR is also imaged to a camera. A representative collection of data from the diagnostics is shown in Figure 8.

Collimating lens

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Figure 7. Laboratory layout of the diagnostics used to characterize the output from the HPI.

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Far Field

Collimated Near FieldImage of mode on HR

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Figure 8. Representative collection of images obtained from beam diagnostics. The magnifications are different for each image. There are two pictures titled “Image in rod.” One shows a saturated image so that the outline or edge of the rod is well defined. The other is an attenuated image showing the laser mode inside the rod. Calibration of the size of the modes are in agreement with theoretical calculations.

5.2 Baseline Data The best performance with the spherical mirrors was obtained at an incident pump power on the rods of 2980 W. The output power was on average 517 W (which accounts for the 8% reflection loss from the first wedge) at an average M2 of 2.0. The error in the power measurement is negligible due to the instrument. The mode stability was very good at this optimal point, with power variations of approximately +/- 5 W at the worst. Typically the power variation at the optimal point was +/- 2 W, which is less than 0.5 % of the total power. The error in measurement of the M2 was approximately +/- 0.15. The M2 measurement is prone to error due to several factors: the background noise subtraction of the image, the bit depth of the camera, the camera settings, and the algorithm of calculating the second moments. We varied all the parameters in our measurements using a good quality beam as reference (632 nm HeNe beam) and determined that our M2 measurement system was accurate. When measuring the beam quality of the Yb:YAG laser, we also varied the parameters for the measurement to determine a system error bar +/- 0.1. The optimal experimental baseline data was therefore: Pout = 517 W at an M2 = 2.0 at an incident input power of 2980 W. The data at the optimal point was reproducible day to day. We removed and replaced the mirrors many times, following a standard alignment procedure, and were able to reproduce the same power and BQ consistently. The performance of the laser was also not limited to the same incident power on the rods. Similar performance to the design point was also achieved for 2980W +/- 40W. The M2 calculation indicates a slightly astigmatic beam, but the final M2 value falls within the error range of +/- 0.1. The fit of the second moments is also excellent.

5.3 Design 1: n2avg mirrors The n2avg mirrors, which were designed by anchoring our model to an average of the n2r and n2phi values of the two rods, performed the best of the three designs we tried. With this profile mirror in place (HR and 20% reflectivity OC) the best

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results obtained were an output power of 551 W at an M2 of 1.34 at the design point of incident pump power of 2980 W. We recorded similar results (M2 ~ 1.34 – 1.37) on three independent occasions (the mirrors had been taken out and replaced). The power level at each of these three optimal points varied from Pout = 551-568 W, all for the same input power of 2980 W. The M2 result and fit was best for the case where we obtained 551 W. The stability of the lasers using these mirrors was very good. The amplitude jitter in the power measurement (mode stability) was very low, indeed, the same as for spherical mirrors.

5.4 Design 2: n2r mirrors The second design did not yield an improved brightness than the baseline or design 1. At an input power of 2980 W, the output power was 609 W with an estimated M2 = 4-5.

However, these mirrors did yield an interesting result beyond the baseline design point. In the current laser resonator configuration, the spherical mirrors optimized (power and BQ) at approximately 3 kW of pump power incident on the rods. At powers above 3.1-3.2 kW the mode quality of the output beam and the output power were severely degraded. The power curve essentially rolled over and no significant improvement was possible at higher pump powers. With the n2r mirrors in place, the roll over threshold was extended beyond 3 kW to approximately 3.6 kW, yielding more output power (~ 745W) at an M2 of 5. An interesting note is that the BQ essentially remained the same with higher input pump power. This result is promising for the current state of the art in kW class solid state lasers. The beam quality for 1 kW class lasers can be M2 > 10. With a simple change to our resonator (Design 2 mirrors) we were able to achieve very high powers (745W) with a M2 of 5.

5.5 Design 3: n2phi mirrors The third design was chosen to optimize for the n2phi index of refraction in the rods. At the design point, this mirror resulted in only 364 W of output power and the BQ was poor. We were not able to obtain an improvement or to equal the results from the spherical mirror.

6.0 Conclusion The onset of aberrations has limited the beam quality of the two rod Yb:YAG laser. The performance has been improved via mode control and mode discrimination at high powers from 527 W and M2 = 2.0 to 555 W with M2 = 1.34. The improvement was based upon a diffractive optics model of propagation of the laser resonator and the ability to fabricate the require mirror figure using MRF technology. The success of the current mirrors is promising for scaling the existing resonator to higher powers with increased extraction efficiency.

Acknowledgements This work was supported by the Joint Technology Office as well as Internal Research and Development funds of the Boeing Company. We wish to acknowledge the contributions of Michael Johnson.

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