116
`
`OPTICS LETTERS / Vol. 18, No. 2 / January 15, 1993
`
`Highly efficient 60-W TEMoo cw diode-end-pumped
`Nd:YAG laser
`
`S. C. Tidwell, J. F. Seamans, and M. S. Bowers
`
`STI Optronics, Inc., 2755 Northup Way, Bellevue, Washington 98004-1495
`
`Received August 7, 1992
`We have demonstrated a diode-end-pumped Nd:YAG laser that produces an output power of 60 W in a near-
`diffraction-limited beam (i.e., M2 < 1.3).
`In multimode operation, the laser produces an output power of
`92 W. The optical-to-optical efficiency (i.e., the ratio of laser power to diode power) is 26% for TEMoO operation
`and 44% for multimode operation.
`
`We have previously reported on a scalable end-
`pumped laser that used four 10-W cw laser diode
`bars to pump a laser that produced a multimode
`output power of 15 W.' This laser architecture
`has significant economic advantages resulting from
`highly efficient operation and the use of diode bars
`[which are the least-expensive source of diode power
`in terms of cost per watt (Ref. 2)]. In the present
`study we use this power-scaling approach to extend
`the multimode output power of cw diode-pumped
`lasers
`to 92 W. In addition, thermal distortion
`and stress-induced birefringence are corrected. The
`result
`is a near-diffraction-limited
`output of 60 W
`with an optical-to-optical efficiency of 26%. This
`is to our knowledge the first demonstration
`that
`the effects of higher-order thermal nonuniformities
`inherent
`to end-pumped lasers can be overcome
`without sacrificing efficiency.
`The output power of end-pumped lasers can be
`scaled by increasing the pump power delivered to a
`single end3' 4 and by combining multiple ends within
`a single cavity.5
`In cw lasers, it is important
`to
`minimize losses to provide high extraction efficiency.
`Thus, in a multiple-end laser each end should be
`pumped with the highest possible power in order
`to minimize the number of surfaces contributing to
`internal loss. The ultimate power-scaling limit for
`a single end is determined by the thermal fracture
`strength of the laser material.6 Thermal distortion
`and stress-induced birefringence can significantly de-
`grade performance at powers well below the thermal
`fracture limit.7' 8 Nd:YLF exhibits negligible thermal
`distortion and stress-induced birefringence, but many
`ends would be needed to produce high output powers
`owing to the low fracture strength.9
`In contrast, al-
`most 25 W can be extracted from a single Nd:YAG rod
`end.6 Nd:YAG is, therefore, the preferred material
`for an efficient, high-power cw end-pumped laser.
`Because Nd:YAG exhibits strong thermal distortion
`and birefringence it is essential to correct these ef-
`fects in order to achieve good beam quality, high
`polarization purity, high power, and high efficiency
`simultaneously.
`We use an angularly multiplexed pump geometry
`to deliver the diode power to the rod end. Four
`
`15-W laser diode bars (Spectra Diode Laboratories
`SDL-3450-S) are arrayed around both ends of each
`rod as shown in Fig. 1. The two sets of diodes on
`each rod are clocked 450 with respect to one another to
`produce a circular gain distribution. The divergence
`of each diode bar is reduced from ,40° to -10°
`in
`the plane perpendicular to the array by using a 2-
`mm-diameter quartz rod lens. The diode light is
`then focused into the rod end by using a 14.2-mm
`focal-length spherical lens. Unused sections of the
`spherical lenses are removed to eliminate mechani-
`cal interference and additional intracavity apertures.
`The pump light is incident upon the rod at an angle of
`30°. The edge-cooled Nd:YAG rods have a diameter
`of 6.35 mm, a length of 7.5 mm, and a doping level
`of 1.0 at.%.
`The pumping geometry allows for efficient use
`of the pump light. Both lenses have antireflection
`coatings for the pump light, and the rod ends are
`antireflection coated for both the pump and lasing
`wavelengths. A passive tuning scheme is used to set
`the center wavelengths of the diodes to within ± 1 nm
`of the optimum for absorption. The efficiency with
`which pump light is transferred from the diode and
`absorbed in the rod is over 80%. Thus approximately
`50 W of pump power is absorbed per rod end.
`The pump power is concentrated in the central
`portion of the rod, as shown in Fig. 2. The result
`is high gain and a distribution that can be extracted
`
`Fig. 1. Angularly multiplexed pump geometry used to
`focus the power of eight 15-W laser diode bars into each
`laser rod.
`
`0146-9592/93/020116-03$5.00/0
`
`© 1993 Optical Society of America
`
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`January 15, 1993 / Vol. 18, No. 2 / OPTICS LETTERS
`
`117
`
`located at the symmetry plane compensates for first-
`order thermal focusing. Neglecting aberrations, each
`rod has a thermal lens focal length of 25.7 cm and a
`TEMOO mode diameter of 2.4 mm (1/e2 ). The rods
`form the limiting apertures.
`The birefringence is corrected by placing a quartz
`polarization rotator between the two rods.'"
`If the
`thermally induced stresses and ray paths are identi-
`cal in the two rods then the depolarization and bifo-
`cusing can be cancelled by rotating the polarization of
`all rays by 90° between rods. We have demonstrated
`this technique by using two rods pumped with 20 W
`each. In this proof-of-principle test, the depolariza-
`tion in a collimated He-Ne laser passing through the
`two rods was reduced from 6% to less than 0.2% with
`the use of a quartz rotator.
`The thermally induced spherical aberration is cor-
`rected by an aspheric surface on the lens at the
`symmetry plane. The shape of the asphere is de-
`rived from calculations that use the measured ther-
`mal distortion of the rod to predict the properties
`of the aberrated mode. The thermal distortion is
`measured in a Mach-Zehnder interferometer while
`the rod is being extracted. Efficient extraction de-
`creases the distortion by approximately 25% com-
`pared with when the power is lost to fluorescence.
`This difference is a result of upconversion leading to
`a lower net quantum efficiency for the fluorescence.
`The thermal distortion is added to the phase of the
`desired Gaussian mode at the rod, and the aberrated
`wave front is propagated to the symmetry plane
`numerically. The proper asphere shape is simply
`that which reverses the phase at the symmetry plane,
`effectively making the asphere a phase conjugator.
`Radial profiles for the thermal distortion (mea-
`sured at the rods) and the correction (imposed by
`the asphere) are given in Fig. 4. The thermal dis-
`tortion is the sum of contributions from both rods.
`The transverse dimensions of the two profiles are
`different because the mode focuses down from the
`rod to the asphere. The net effect of the asphere and
`two thermally distorted rods on the mode phase front
`is equivalent to pure focus. The CaF2 asphere is
`diamond machined and postpolished to reduce scatter
`losses to less than 0.5%.
`A stable output power of 60 W with near-diffraction-
`limited beam quality is obtained with the flat-flat
`resonator and a total diode power of 235 W. The
`beam quality is calculated from measurements of the
`
`ASPHERE
`,SHR
`
`FLAT OUTPUT
`COUPLER
`
`Fig. 2. Fluorescence profile of a 6.35-mm-diameter end-
`pumped Nd:YAG rod. A total pump power of approxi-
`mately 100 W is absorbed in the rod, 70 W of which is
`encircled within the 2.4-mm mode diameter.
`
`efficiently by the fundamental mode.'0 The small-
`signal gain in the central area of each rod is approx-
`imately 0.5.
`Multimode extraction tests using a single rod were
`performed with a 14.3-cm-long resonator formed be-
`tween two 1.1-m concave mirrors with 3% output
`coupling. Output powers as high as 24 W are ex-
`tracted from a rod pumped on a single end, whereas
`a rod pumped from both ends can produce 49.5 W.
`Multimode tests using two rods are performed with
`a 44-cm cavity formed between a 1.1-m-high reflector
`and a 90% reflective 1-m output coupler. A multi-
`mode output power of 92 W is obtained with a total
`pump power of 235 W. The threshold
`for the two-
`rod oscillator is approximately 24 W, and the slope
`efficiency is 44% (based on the diode output power).
`No effort was made to correct thermal distortion
`or birefringence in the multimode tests. The beam
`quality of the multimode output ranged from 20 to
`30 times diffraction limited.
`A resonator that has a symmetry plane between the
`two rods, as illustrated in Fig. 3, was designed for
`high-beam-quality extraction tests. The symmetry
`ensures that the mode is the same size and that
`rays pass through similar areas in both rods. The
`resonator is formed by flat-end mirrors separated
`from the rods by 65 cm. The rods are separated from
`one another by 20 cm. A -16-cm
`focal-length
`lens
`
`Nd:YAG
`MOUNT.
`
`60-W
`PUMP-\
`MODULE
`
`FLAT
`-MIRROR
`
`|-
`
`65 cm
`
`Fig. 3. Symmetrical resonator used in TEMoo extraction experiments. Symmetry between the two laser rods allows
`straightforward correction of the thermal distortion and stress-induced birefringence.
`
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`118
`
`OPTICS LETTERS / Vol. 18, No. 2 / January 15, 1993
`
`the resonator length is adjusted so that the Fresnel
`number approaches unity under full pump power.
`When properly adjusted, the power is stable, and the
`mode discrimination is sufficient to assure good beam
`quality without sacrificing efficiency.
`In summary, we have demonstrated a diode-end-
`pumped cw Nd:YAG laser that produces an output
`power of 92 W in a multimode beam and 60 W in a
`near-diffraction-limited beam. The laser uses a total
`of sixteen 15-W laser diode bars pumping two short
`rods in an angularly multiplexed pump geometry. A
`diamond-machined asphere and a quartz polarization
`rotator are used in a symmetric resonator to correct
`thermal distortion and the stress-induced birefrin-
`gence for TEMoo extraction. The optical-to-optical
`efficiencies are 44% and 26% for multimode and
`TEMOO operation, respectively. The high efficiency of
`the end-pumped oscillator and the use of laser diode
`bars as pump sources provide significant economic
`advantage for this design.
`
`References
`1. S. C. Tidwell, J. F. Seamans, C. E. Hamilton, C. H.
`Muller, and D. D. Lowenthal, Opt. Lett. 16,584 (1991).
`2. For a cost comparison, the price of a 15-W laser diode
`bar is less than twice that of a 0.5-W fiber-coupled
`laser diode (Spectra Diode Laboratories, Inc., San
`Jose, Calif., 1992 Product Catalog).
`3. Y. Kaneda, M. Oka, H. Masuda, and S. Kubota, Opt.
`Lett. 17, 1003 (1992).
`4. T. Y. Fan, A. Sanchez, and W. E. DeFeo, Opt. Lett.
`14, 1057 (1989).
`5. M. S. Keirstead and T. M. Baer, in Digest of Confer-
`ence on Lasers and Electro-Optics (Optical Society of
`America, Washington, D.C., 1991), p. 490.
`6. S. C. Tidwell, J. F. Seamans, M. S. Bowers, and A. K.
`Cousins, IEEE J. Quantum Electron. 28, 997 (1992).
`7. S. C. Tidwell, M. S. Bowers, A. K. Cousins, and
`D. D. Lowenthal, in Digest of Conference on Lasers
`and Electro-Optics (Optical Society of America,
`Washington, D.C., 1991), p. 448.
`8. W. K. Koechner, Solid-State Laser Engineering
`(Springer-Verlag, New York, 1988), pp. 350-380.
`9. W. K. Koechner, Solid-State Laser Engineering
`(Springer-Verlag, New York, 1988), pp. 60-62.
`10. D. L. Sipes, Appl. Phys. Lett. 47, 74 (1985).
`11. W. C. Scott and M. de Wit, Appl. Phys. Lett. 18, 3
`(1971).
`12. V. Magni, Appl. Opt. 25, 107 (1986).
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`RADIUS (mm)
`Fig. 4. Radial profiles of the thermal distortion resulting
`from two pumped rods and the correction imposed by
`the aspheric lens. The profiles have different transverse
`scales because the mode size decreases by approximately
`1.4 times between the rod and asphere. OPD, optical
`path difference.
`
`beam size taken at the output coupler and then in
`the far field. The flat output coupler defines a waist
`for the mode, which nominally has a radius of 174
`Am. Far-field measurements were taken 2 m from
`the output coupler (the beam Rayleigh range is 9
`cm). The beam quality, or M2 factor, is simply the
`ratio of the far-field beam size to that calculated for a
`diffraction-limited beam with the same waist. The
`beam quality of this laser is 1.3 times diffraction
`limited.
`The output power and beam quality of the laser
`are sensitive to adjustments in pump power because
`the resonator Fresnel number changes rapidly as
`a function of the rod focal length. The sensitivity
`can be greatly reduced by using convex end mir-
`rors rather than flats.'2 For example, the mode size
`would be nearly constant for pump power variations
`of ±5% if the flats were replaced with -30-cm mir-
`rors at the same locations. Flat-end mirrors are
`used here for experimental convenience. In our case,
`
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