Efficient laser-diode-pumped neodymium-dopedcalcium-niobium-gallium-garnet laser

Kenta Naito, Atsushi Yokotani, Takatomo Sasaki, Takashi Okuyama,Masanobu Yamanaka, Masahiro Nakatsuka, Sadao Nakai, Tsuguo Fukuda,and M. 1. Timoshechkin

Efficient lasing of Nd:Ca (Nb, Ga)2x Ga3-garnet (CNGG) disordered crystal pumped by a laser diode wasdemonstrated. In the end-pumped cw lasing experiment with a single-stripe diode, a slope efficiency of24.2% and a maximum optical conversion efficiency of 19.3% were obtained.. It was shown that thedependence of cw output power on the pump wavelength is insensitive compared with Nd:YAG because ofthe broad pump absorption bandwidth of Nd:CNGG. From the diode-bar-pumped experiment, it wasconfirmed that Nd:CNGG is a promising material for diode-bar pumping to obtain high average powerwith good beam quality, because of its advantages of a broader pump absorption band compared with thatof Nd:YAG and the higher thermal conductivity compared with Nd-doped glasses.

IntroductionIn general, Nd-doped disordered crystals have a rela-tively broader pump absorption band than Nd:YAG,which is a typical ordered crsytal, and they have ahigher thermal conductivity than Nd-doped glasses.These properties may be advantageous for diode-pumped high average-power operation using pluraldiodes or diode bars. Recently, a few kinds of diode-pumped disordered crystalline lasers have been re-ported.1-3

In this paper, we report on diode-pumped lasingperformances of Nd:Ca(Nb, Ga)2 x Ga3-garnet (CNGG)disordered crystal.3 4

Features of CNGG Disordered CrystalThe disordered structure of CNGG crystal is due tothe random distribution of the Nb and Ga ions and

K. Naito, T. Okuyama, M. Yamanaka, M. Nakatsuka and S.Nakai are with the Institute of Laser Engineering, Osaka Univer-sity, 2-6 Yamada-oka, Suita, Osaka 565, Japan. A. Yokotani iswith KimuraMetamelt Project ERATO JRDC, Satellite-2, TsukubaResearch Consortium, 5-9-9 Tokodai, Tsukuba 300-26, Japan. T.Sasaki is with the Department of Electrical Engineering, OsakaUniversity, 2-1 Yamada-oka, Suita, Osaka 565, Japan. T. Fukudais with the Institute of Material Research, Tohoku University,2-1-1 Katahira, Aobaku, Sendai 980, Japan. M. I. Timoshechkinis with the General Physics Institute, Academy of Sciences, VavilovStreet 38, 117942 Moscow, Russia.

Received 11 November 1992.0003-6935/93/367387-04$06.00/0.© 1993 Optical Society of America.

vacancies at octahedral and tetrahedral lattice sites.CNGG is an isotropic material similar to YAG.CNGG has a thermal conductivity of 0.047 W/cm C,which is three times higher than for phosphate glass.Because CNGG has an extremely low melting point of1460 C compared with that of other disordered orordered crystalline laser hosts, it can be grown by aplatinum crucible. In addition, the segregation coef-ficient of CNGG for a rare-earth ion dopant is nearunity.

The Nd:CNGG crystal used in this study wasgrown from the melt by the Czochralski method.The Nd-doping density was 2 wt. %. In Fig. 1 theabsorption spectrum around 0.8 m (419 /2-4 F5/2,

4H9 /2 )and the fluorescence spectrum around 1.06 m (4F3/ 2-4I11/2) are compared with those of Nd:YAG (0.85 at. %)and Nd:phosphate glass (2 wt. %, Hoya LHG8).From Fig. 1 it is clear that the width of the absorptionand fluorescence spectra of Nd:CNGG was inhomoge-neously broadened, which was intermediate betweenthat for Nd:YAG and Nd:phosphate glass. The ab-sorption and emission properties are summarized inTable 1.

End-Pumped Lasing Experiments Usinga Single-Stripe Diode

A coated sample (Nd-doping density 2 wt. %, 3 mm inlength, and 6 mm in diameter) was fabricated for theend-pumped lasing experiments. One end of thesample was dichroic coated (high reflection at 1.06[Lm and antireflection at 0.8 lm), and the other endwas antireflection coated at 1.06 [lm. The sample

20 December 1993 / Vol. 32, No. 36 / APPLIED OPTICS 7387

10

5

0-780

1.0

0.5

o.o L-1000

800 820

Wavelength (nm)(a)

1050 1100

Wavelength (nm)(b)

Fig. 1. Comparison of the (a) absorption spectra and (b) fluores-cence spectra of Nd:CNGG, Nd:YAG, and Nd:phosphate glass(Hoya LHG8).

was end pumped by a fiber-coupled single-stripe laserdiode (Sony Model SLD304XR). The diode outputwas collimated by a collimating lens ( f = 6.5 mm) andthen focused by an aspheric lens (f = 8.5 mm). Theseparation between the collimating lens and thefocusing lens was 150 mm. Pumping power, inci-dent onto the sample, could be varied up to 500 mW.The cavity consisted of the dichroic-coated face of thesample and a concave output mirror (curvature ra-dius of 60 mm). The cavity length was 40 mm,restricted by the sample holder and the output mirror

Table 1. Comparison of Spectroscopic Properties

HoyaNd:CNGG Nd:YAG LHG8

Laser Material (2 wt. %) (0.85 at. %) (2 wt. %)

Absorption properties(NI9/2- F5/2,

4H9 /2)Peak absorption 10.9 cm- 1 6.16 cm 1 5.47 cm 1

coefficientAbsorption band- 8.5 nm 2.1 nm 12.7 nm

width (FWHM)Peak wavelength 807 nm 809 nm 802 nm

Fluorescence properties(4F3/2-4I11 /2)

Fluorescence band- 14.7 nm - 0.5 nm 21.3 nmwidth (FWHM)

Peak wavelength 1062 nm 1064 nm 1054 nmLifetime 0.161 ms 0.230 ms 0.310 ms

holder used in this study. During the measurementof cw input-output characteristics, the temperatureof the diode was kept at the value at which the outputpower was optimized for 500-mW pump power.

Input-output cw characteristics were measured forT = 1.2%, T = 4.1%, and T = 9.6%, where T is theoutput mirror transmission. At T = 9.6%, lasingwas not observed up to the pump power of 500 mW.The best performance was obtained at T = 1.2%, asshown in Fig. 2. Lasing wavelength was measuredby a spectrum analyzer to be 1061.8 nm. The maxi-mum output power was measured to be 96 mW for aninput power of 500 mW. The extrapolated thresholdpump power, the slope efficiency, and the maximumoptical conversion efficiency were calculated to be 113mW, 24.2%, and 19.3%, respectively. The observedslope efficiency and optical conversion efficiency arethe highest reported to date on a Nd-doped disorderedcrystalline laser pumped by a laser diode. In Table 2the best results for Nd:CNGG are compared with thebest results for Nd:YAG (Nd-doping density 1.1 at. %,dichroic-coated sample, 3 mm in diameter and 7.5mm in length) by using the same experimental setup.The relatively large cavity loss for Nd:YAG is mostlydue to the poor coating quality of the sample used.

The effective emission cross section for the Nd:CNGG crystal was estimated from the measuredpump power threshold and the measured slope effi-ciency. An effective emission cross section can becalculated by Eq. (1), which is deduced from usualtheoretical expressions for the pump power thresholdand the slope efficiency of an end-pumped laser.5

_hv X T f2 r XO Pthqs Seff, (1)

where o is the effective emission cross section, hvp isthe pump photon energy, is the fluorescence life-time, Xp is the pump wavelength, X0 is the lasingwavelength, T is the output mirror transmittance, Pthis the pump power threshold, ,s is the slope efficiency,

100

A-0)

0a.

0)(0(U)-J

80

60

40

20

00 100 200 300 400 500 600

Pump Power (mW)Fig. 2. Continuous wavo input-output characteristics of theend-pumped Nd:CNGG laser using a single-stripe diode.

7388 APPLIED OPTICS / Vol. 32, No. 36 / 20 December 1993

._

00)00C0

0Cl,.0

._A

Ca)

_ IR

'

a RQ .0a

0

Table 2. Comparison of Diode-Pumped Lasing Characteristics

Nd:CNGG Nd:YAGLaser Material (2 wt. %) (1. Iat. %)

Effective emission cross sec- 13 x 10-2° cm 2 30 x 10-20 cm 2

tionFluorescence lifetime 0.161 ms 0.230 msOutput mirror transmittance 1.2% 4.1%Cavity loss -1% -3%Slope efficiency 24.2% 24.0%Pump power threshold 113 mW 119 mW

and Seff is the effective mode area taking into accountpump and lasing overlap, respectively. Here Seff wascalculated to be 5.14 x 10-4 cm2 by the analysis of thediode-pumped experiments of Nd:YAG by using thesame experimental setup. Using the measured pumppower threshold of 113 mW, the measured slopeefficiency of 24.2%, the T of 1.2% and the of 161 is,we calculated o- for the Nd:CNGG disordered crystalat 1.06 jlm to be 13 x 10-20 cm2. This value is twotimes larger than the value reported previously.4

Figure 3 shows the cw output power dependence onthe pump wavelength for the diode-pumped Nd:CNGG laser (2 wt. %, T = 1.2%) and the diode-pumped Nd:YAG laser (1.1 at. %, T = 4.1%). Thepump power was kept at 500 mW for both materials.The emission wavelength of the diode was varied bycontrolling the temperature of the diode. From Fig.3 it is seen that the output power from Nd:CNGG isnearly constant for a 4-nm variation in the pumpwavelength, and it is insensitive compared with thatof Nd:YAG, as predicted from the broader pumpabsorption band of Nd:CNGG. It can be concludedthat a diode-pumped Nd:CNGG laser relaxes thewavelength selection and tuning problem of diodes.

Diode-Bar-Pumped Lasing Experiments

When considering high-power applications, one shoulduse diode bars as a pump source. A typical diode barhas an emission bandwidth of several nanometers.

1600 mE 80

0~~~~~~~~~

~400 Nd:CNGG

0L. 20 Nd:YAG

(0

800 802 804 806 808 810 812Pump Wavelength (nm)

Fig. 3. Continuous wave output power dependencies on the pumpwavelength. Pump power was 500 mW.

From Fig. 3 it can be expected that the reduction ofthe pumping density caused by the broad emissionband of a diode bar will be smaller in Nd:CNGG thanin Nd:YAG. Higher pumping density with diode-barpumping is advantageous to obtain a higher outputpower with good beam quality.

To confirm this preferable property of Nd:CNGG,we carried out diode-bar-pumped lasing experimentsby using a conventional side-pumping configuration.A Nd:CNGG sample (Nd-doping density 2 wt. %) andNd:YAG sample (Nd-doping density 1.1 at. %), whichhad the same dimensions of 4 mm x 4 mm in crosssection and 10 mm in length, were used to comparethe lasing performances. One end face (4 mm x 4mm) was high-reflection coated at the lasing wave-length and the other end face (4 mm x 4 mm) wasantireflection coated at the lasing wavelength foreach sample. Each sample was side pumped throughan uncoated face (4 mm x 10 mm) by a quasi-cwone-dimensional diode bar (Spectra Diode Labs ModelSDL3230T) without any coupling optics. The pulsewidth of the pump light from the diode bar and therepetition rate were kept at 600 s and 10 Hz,respectively. The heat-sink temperature of the di-ode was kept for each sample at the value at which theoutput energy was optimized for the maximum pumpenergy. The time-integrated emission bandwidth ofthe diode bar during the pulsed operation was mea-sured by a spectrum analyzer to be 5 nm (FWHM).The cavity consisted of the high-reflection-coated faceof the sample and an output mirror that had atransmission of 5% and a curvature radius of 1000mm. The cavity length was 80 mm. An iris (1 mmin diameter) was inserted into the cavity to suppressthe higher transverse-mode oscillations.

The input-output characteristics obtained underfree-running operation are compared in Fig. 4.Although the output energy was monitored by anuncalibrated joulemeter, relative performances canbe discussed. From Fig. 4 it is seen that Nd:CNGGhad a 1.56 times higher slope efficiency than Nd:YAG.

Assuming that the cavity loss and the couplingefficiency between the pump light and the cavitymode are the same for the two samples, the ratio of

(0

C

L.(D

CL

0.

0.6

0.4

0.2

0 10 20

Pump Energy (mJ)Fig. 4. Quasi-cw lasing performances with side pumping using adiode bar. Pumping pulse width was 600 pLs.

20 December 1993 / Vol. 32, No. 36 / APPLIED OPTICS 7389

Table 3. Comparison of the Experimental and Theoretical SlopeEfficiencies (Normalized to Nd:YAG)

Slope Efficiency

Calculated

Effect of Pump Effect of Pump

Bandwidth Bandwidth

Laser Material Measured Not Considered Considered

Nd:YAG (1.1 at. %) 1.00 1.00 1.00

Nd:CNGG (2 wt. %) 1.56 1.21 1.63

the slope efficiency should be equal to the ratio of theabsorption efficiency. The two samples used in thisstudy had peak absorption coefficients different fromeach other. However, the calculated ratio of theabsorption efficiencies by using the peak absorptioncoefficients measured with a monochromator cannotexplain the experimental results shown in Table 3.This discrepancy can be attributed to the fact that thepump bandwidth iAp affects the absorption efficiency.Figure 5 shows the calculated dependence of theeffective absorption coefficient aeff on A Xp. We calcu-late axeff from the absorption spectrum measured witha monochromator (0.6-nm resolution), assuming thatthe pump light has a rectangular spectral shape andthe absorption length is 1 mm. For Nd:YAG, a-eff forAXp = 5 nm is reduced by 48% from the value forAXp = 0.6 nm. In contrast, for Nd:CNGG, oeff forA Xp = 5 nm is reduced by only 24% from the value forAXp = 0.6 nm, as a result of its broad pump absorp-

o

O 0c

< 0 -

0 0Or_ 0

12

10

8

6

4

2

1~~~~~~~~~~ I I I~~~~~~~

0 2 4 6 8 10

Pump Bandwidth (nm)Fig. 5. Pump bandwidth dependencies of the effective absorptioncoefficient. The effective absorption coefficient was calculatedfrom the absorption spectrum measured by a monochromator,assuming that the pump light has a rectangular spectral shape andthe absorption length is 1 mm.

tion band. The calculated ratio of the absorptionefficiencies by using eff for AX = 5 nm is almostconsistent with the experimental results, as shown inTable 3. It was concluded that the broader pumpabsorption bandwidth of Nd:CNGG permits a higherpumping density than Nd:YAG.

ConclusionEfficient laser-diode-pumped lasing of Nd:CNGG wasdemonstrated. The results are summarized as fol-lows.

First, in the end-pumped cw lasing experimentwith a single-stripe diode, the slope efficiency of24.2% and the maximum optical conversion efficiencyof 19.3% were obtained. To our knowledge, theseefficiencies are the highest reported to date on aNd-doped disordered crystalline laser with diodepumping. In addition, it was shown that the cwoutput power is insensitive to the variation of thepump wavelength compared with Nd:YAG because ofthe broad pump absorption bandwidth of Nd:CNGG.

Second, from the side-pumped experiment thatused a diode bar, it was confirmed that the broaderpump absorption bandwidth of Nd:CNGG permits ahigher pumping density than that for Nd:YAG.Higher pumping density with diode-bar pumping isadvantageous to obtain a higher output power withgood beam quality.

Nd:CNGG has a thermal conductivity that is threetimes higher than that for Nd:phosphate glass, whichis advantageous for high average-power operation.It can be concluded that Nd:CNGG is a promisingmaterial for diode-bar pumping to obtain a highaverage power with good beam quality.

References1. H. R. Verdun and L. M. Thomas, "Nd:CaYAIO4 -a new crystal

for solid-state lasers emitting at 1.08 ,um," Appl. Phys. Lett. 56,608-610 (1990).

2. F. Hanson, D. Dick, H. R. Verdun, and M. Kokta, "Opticalproperties and lasing of Nd:SrGdGa 3O7," J. Opt. Soc. Am. B 8,1668-1673 (1991).

3. A. A. Kaminskii, H. R. Verdun, V. B. Mill, A. A. Paviyuk, andA. V. Butashin, "Two new continuous lasers with diode laserpumping on the base of disordered crystals Ca3(Nb, Ga)2 Ga3aO2and KLa(MnO 4)2 with Nd3+," Russ. Acad. Sci. Neorg. Mater. 26,438-440 (1992).

4. Yu. K. Voron'ko, S. B. Bessen, N. A. Es'kov, V. V. Osiko, A. A.Sobol', M. I. Timoshechkin, S. N. Ushakov, and L. I. Tsymbal,"Spectroscopic and lasing properties of calcium niobium gal-lium garnet activated with Cr3+ and Nd 3+," Sov. J. QuantumElectron. 18, 198-201 (1988).

5. T. Y. Fan and R. L. Byer, 'Diode laser-pumped solid-statelasers," IEEE J. Quantum Electron. 28, 895-912 (1988).

7390 APPLIED OPTICS / Vol. 32, No. 36 / 20 December 1993

. .

T- I I I

Nd:CNGG

1�,

I I I I -