IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 50, NO. 4, AUGUST 2001 1003

Diode-Pumped 2�m Optical Oscillatorfor High-Resolution Spectroscopy

and Frequency MetrologyG. Galzerano, Member, IEEE, C. Svelto, Member, IEEE, and E. Bava, Associate Member, IEEE

Abstract—Diode-pumped Tm:YAG and Tm-Ho:YAG lasers op-erating near room temperature with continuous-wave emission at2 m have been developed. A thorough characterization in bothmulti- and single-longitudinal mode regimes has been performedby experimentally determining the corresponding threshold andslope efficiency values. Continuous-wave output powers of morethan 70 mW and 35 mW have been obtained in single-longitu-dinal mode operation near room temperature for Tm:YAG andTm-Ho:YAG lasers, respectively.

Index Terms—Diode-pumped laser, frequency metrology,holmium, solid state laser, thulium, 2 m laser.

I. INTRODUCTION

H IGH-RESOLUTION spectroscopy and frequencymetrology in the 2 m region of the electromagnetic

spectrum are assuming an important scientific interest forrealizing high-accuracy frequency standards and frequencyconnection chains between optical standards at differentwavelengths [1]. Lasers with emission at a wavelength of 2mare also useful sources for applications that require eye-safepropagation through the atmosphere. Such applications includecoherent Doppler-LIDAR (LIght Detection And Ranging),DIAL ( DIfferentialAbsorptionLIDAR) measurements of COand H O, and Doppler velocimetry (wind shear detection,global wind speed measurement both from satellite and groundstations) [2], [3]. Moreover, these sources may be useful inmedical applications because the water molecule absorbsstrongly in this wavelength region [4]. All these applicationsrequire significant output power, wide frequency tuning range,and low amplitude- and low frequency-noise.

In this work, single-frequency diode-pumped Tm:YAG andTm-Ho:YAG lasers with continuous-wave emission around2 m have been developed at room temperature. The resultsachieved on the characterizations in multi- and single- longitu-dinal mode regimes are hereby reported.

II. FIBER-PUMPED TM:YAG AND TM-HO:YAG LASERS

The Tm:YAG and Tm-Ho:YAG crystals used in our experi-ments, supplied by Scientific Material, have a nominal dopant

Manuscript received May 4, 2000; revised April 9, 2001. This work was sup-ported in part by the Italian Space Agency (ASI) and by MURST under Con-tracts ARS-98-59 and Cofi. 98.

G. Galzerano, C. Svelto, and E. Bava are with the Dipartimento di Elettronicae Informazione, Politecnico di Milano, INFM and CSTS CNR, Milano, Italy(e-mail: Gianluca.Galzerano@PoliMI.IT).

Publisher Item Identifier S 0018-9456(01)07347-8.

Fig. 1. Energy levels of Tm and Ho ions in YAG. d represents thedegeneracy of the energy levels.

concentration of 9.3% with Tm ions and 5.7% with Tm and0.36% with Ho , respectively. A schematic diagram of the rel-evant energy levels for both the Tmand Ho ions in the YAGhost is shown in Fig. 1. The upper and lower laser levels areFand H for the Tm:YAG system whereas in the Tm-Ho:YAGmedium the laser levels are theI and I of the Ho ions. Foreach absorbed pumping photon at 785 nm, giving aH Htransition, two Tm ions are raised in theF excited state bymeans of a cross-relaxation process between the TmlevelsH F and H F . The population inversion in the

Ho system is obtained by means of a resonant energy transferprocess between the Tm (F H ) and Ho ( I I ) ions.

To build the laser cavity, we used an end-pumped hemi-spherical resonator as shown in Fig. 2. The end facets of theactive material are polished flat and covered with dichroicmultidielectric coatings. The first facet of the crystal is coatedfor high reflectivity at the laser wavelength % andfor high transmission at the pump wavelength % . Thesecond facet is antireflection coated at the laser wavelength

% and for a high reflectivity at pump wavelength% . Approximately 69% (42%) of the incident pump

power at 781 nm is absorbed during a single pass through the3 mm long Tm:YAG (Tm-Ho:YAG) laser crystal. Using thedouble-pass pumping configuration, more than 90% (65%) ofthe available pump power can be usefully absorbed within theTm:YAG (Tm-Ho:YAG) active medium. In the longitudinalpumping scheme the gain element cannot be arbitrarily long,thus achieving maximum pump absorption, because in thesequasithree-level lasers, a significant population inversionalways needs to be maintained in order to avoid reabsorptionlosses [5]. Therefore, when choosing the optimal length ofthe gain medium, there is a trade-off between the pumping

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1004 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 50, NO. 4, AUGUST 2001

Fig. 2. Schematic diagram of the Tm:YAG and Tm-Ho:YAG laser cavity.

efficiency and the optimal profile for the population inversion.The reflective coating at 781 nm on the second facet of theactive medium, in addition to providing for higher pumpabsorption, also guarantees a much more uniform inversionprofile along the active medium. A spherical output couplingmirror (transmission %), with radius of curvature ofmm, is placed at mm geometrical distance from the flatmirror (pump entrance facet of the crystal).

The laser was originally end pumped by a fiber-coupled Al-GaAs laser diode (OPC-D010-785-HB/250) emitting up to 5 Waround 781 nm. The 1 m long optical fiber carrying the pump ra-diation has a core diameter of 250m with a numerical apertureof 0.22. The pump beam was focused into the laser crystal, witha beam waist of m, by means of two spherical lenseswith focal lengths of 40 mm and 25 mm, respectively. Recently,a novel high-brightness fiber-coupled pump diode has been usedinstead of the previous device in the same end-pumping con-figuration but using an output coupler with radius of curvatureof mm. The new pump source has a 100m fiber core(diameter) with a numerical aperture of 0.22 and a maximumoutput power of 3 W at 781 nm. In this case, an optical imagingsystem with two spherical lenses of focal lengths mmand mm, respectively, has been adopted, resulting ina m beam waist in the active medium. With this moreintense pump beam, the population inversion within the activemedium reaches a much higher value than in the previous case.This results in lower threshold power and higher laser efficiency.In both schemes, the pump wavelength is tuned to the Tmab-sorption peak, near 781 nm [6], using a thermoelectric cooler(TEC) to control the temperature of the laser diode at C.The YAG crystal is held in a copper structure directly mountedon a Peltier cooler, which is used to remove the pump-generatedheat and to actively control the crystal temperature.

III. M EASUREMENTS ANDCHARACTERIZATION

The optimized laser configuration (with the higher brightnesspump diode) achieved rather low threshold values in the multi-longitudinal mode regime for both the active media:

Fig. 3. Output power of the Tm-Ho:YAG laser as a function of the incidentpump power in single transverse mode.

mW for the Tm:YAG laser and - mWfor the Tm-Ho:YAG laser. The corresponding slope efficien-cies, with respect to the incident pump power, turned out to be

% and - %, respectively. Fig. 3 showsa typical Tm-Ho:YAG laser output, in multi-axial mode regime,versus the incident pump power (for the two pumping systemconfigurations). The high threshold value achieved with the ini-tial pumping scheme (diamond data points in the diagram) is adirect consequence of the quite large pump spot size realizedin the active medium (125m) compared to the laser spot di-mension (140 m) [7]. A significant improvement in terms ofperformance is observed when a better mode matching is real-ized using the higher brightness pump diode (square data pointin the diagram). We measured the transverse dimension of boththe pump beam and laser mode by observing a 10-times magni-fied image of the laser beam waist on a Vidicon camera (

m) connected to a beam profiler system. The laser shows apure beam spot, corresponding to a single spatial mode(see Fig. 4). By taking both near-field and far-field measure-ments, the value for the laser beam was calculated, pro-viding similar values in the two propagation planes, ,

GALZERANO et al.: DIODE-PUMPED 2 m OPTICAL OSCILLATOR FOR HIGH-RESOLUTION SPECTROSCOPY 1005

and always being less than 1.2 (diffraction limited laser beam[8]) for all operating conditions.

The laser performance has been characterized in terms of thecrystal temperature and cavity length (Figs. 5 and 6). For bothlaser systems, slope efficiencies and threshold pump powershave been measured as a function of the operating crystaltemperature. Due to the intrinsic behavior of a quasithree-levellaser system, the threshold pump power is significantly reducedand the slope efficiency is increased, as the temperature isdecreased. Typical measurement results are shown in Fig. 5for the Tm-Ho:YAG laser (the Tm:YAG system behaves in asimilar way but with lower sensitivity to temperature changes).Temperature coefficients of 3.84 mW/C and % Cwere experimentally determined for the laser threshold andslope efficiency parameters, respectively. The output powerversus cavity length characterization, depicted in Fig. 6 in thecase of the Tm:YAG laser at an operating temperature of 14C,shows an optimum cavity length of 37 mm, which is near theoptical instability point (hemispherical cavity configuration)where the pump to laser mode matching is maximized.

Single-frequency operation of the Tm:YAG laser is achievedusing an uncoated high refraction index (SF10) etalon with 200

m thickness. Note that a round-trip loss discrimination of lessthan 1% is enough, in this low-gain active material, to ensuresingle-mode oscillation. A maximum single-frequency outputpower of 70 mW is achieved with an incident pump power of2.7 W for the Tm:YAG laser.

Due to the wider gain bandwidth of the Tm-Ho:YAG system,single-mode selection is more critical for this kind of laser.Fig. 7 shows the Tm-Ho:YAG laser spectrum in multimoderegime as measured with a scanning Fabry-Perot interferometerwith a Finesse of 50 and free spectral range (FSR) of 380 GHz.Seven of the eight observed peaks have different longitudinalmode frequencies separated by 28 GHz, i.e., laser cavityFSR, around 2.097m, whereas the last peak is a lasing modenear a wavelength of 2.092m. In order to select the singlelongitudinal mode, it was necessary to place within the cavitya 135 m etalon with 80% reflectivity and a Lyot filter (basedon a 5 mm thick quartz plate at Brewster angle) at the sametime. In this way, a maximum single-frequency output powerof 35 mW is achieved with an incident pump power of 2.7 W.Fig. 8 shows the single-frequency spectrum of the Tm-Ho:YAGlaser analyzed by means of a scanning confocal Fabry-Perotcavity (Finesse 100 and FSR of 150 MHz) and a photodetectorconnected to a digital scope.

No special precautions have been taken for long-term stabilitywith the current hardware setup. The laser mirrors are mountedon an aluminum baseplate; in the near future, we will replacethese mounts with super Invar baseplates.

IV. CONCLUSIONS ANDDEVELOPMENTS

Characterization of multi- and single-longitudinal modeTm:YAG and Tm-Ho:YAG lasers with continuous-waveemission around 2 m has been presented. Output powersof more than 70 mW and 35 mW have been obtained insingle-longitudinal mode operation near room temperature forTm:YAG and Tm-Ho:YAG lasers, respectively. These types

Fig. 4. Transverse profile of the Tm:YAG and Tm-Ho:YAG laser beam. Dots:measured data. Line: Gaussian fit.

Fig. 5. Slope efficiency and threshold pump power as a function of theTm-Ho:YAG crystal temperature.

Fig. 6. Output power of the Tm:YAG laser operating at 14C as a function ofthe cavity length. Square: measured data. Continuous line: theoretical fit. Dottedline: theoretical laser power at a working temperature of 77 K (right ordinateaxis).

of optical oscillators are interesting sources in high-resolutionspectroscopy, metrology, and high-sensitivity optical sensors.

For these applications, it is very important to achieve highperformance in terms of amplitude and frequency stability. Inthis sense, and in particular for the Tm-Ho:YAG laser, the nextexperimental work will address active amplitude stabilization(relative intensity noise suppression by means of an optoelec-tronic control loop) and to the frequency stabilization (againstresonant cavities or molecular absorption lines, e.g., HBr) of thedeveloped laser system.

1006 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 50, NO. 4, AUGUST 2001

Fig. 7. Spectrum of the multimode Tm-Ho:YAG laser.

Fig. 8. Spectrum of the single mode Tm-Ho:YAG laser.

ACKNOWLEDGMENT

The authors are grateful to Andrea Maffiolini and FedericoScandroglio for their valuable help during the first part of theexperimental activities.

REFERENCES

[1] J. E. Bernard, B. G. Whitford, and A. A. Madej, “A Tm:YAG laser foroptical measurements: Mixing 148 THz with COlaser radiation,”Opt.Commun., vol. 140, pp. 45–48, 1997.

[2] S. M. Henderson, C. P. H. Hale, J. R. Magee, M. J. Kavaya, and A. V.Huffaker, “Eye-safe coherent radar system at 2.1�m using Tm,Ho:YAGlasers,”Opt. Lett., vol. 16, pp. 773–775, Oct. 1991.

[3] M. E. Storm, “Coherent 2�m sources burst into wind shear detection,”Laser Focus World, pp. 117–122, Apr. 1991.

[4] J. A. Curcio and C. C. Petty,J. Opt. Soc. Amer., vol. 41, p. 302, 1951.[5] T. Y. Fan and R. L. Byer, “Modeling and CW operation of a quasithree-

level 946 nm Nd:YAG laser,”IEEE J. Quantum Electron., vol. QE-23,p. 605, May 1987.

[6] M. G. Jani, R. J. Reeves, R. C. Powell, G. J. Quarles, and L. Esterowitz,“Alexandrite-laser excitation of Tm-Ho:YAl O laser,”J. Opt. Soc.Amer. B, Opt. Phys., vol. 8, pp. 741–746, 1991.

[7] D. G. Hall, “Optimum size criterion for low gain lasers,”Appl. Opt., vol.20, pp. 1579–1583, Sept. 1981.

[8] T. F. Johnston, Jr., “M concept characterizes beam quality,”LaserFocus World, pp. 173–183, May 1990.

Gianluca Galzerano (S’94–M’00) was born inTeramo, Italy, on August 14, 1968. He receivedthe Laurea degree in electronic engineering fromthe Politecnico di Milano, Milano, Italy, in July,1994, and the Ph.D. degree in metrology from thePolitecnico di Torino, Torino, Italy, in February,1999.

His research interests include frequency stabiliza-tion of solid state lasers and high-resolution opticalmeasurements.

Cesare Svelto(S’94-M’97) was born in Bari, Italy,on August 9, 1968. He received the Laurea degreein electronic engineering (cum laude) from the Uni-versity of Pavia, Pavia, Italy, in 1993 and the Ph.D.degree from the Politecnico di Milano, Milano, Italy,in 1997.

In 1996, he joined the Electronic and InformationScience Department of the Politecnico di Milano asan Assistant Professor of electronic measurements.His current research activities are in the fields of op-tical frequency standards, phase noise measurements,

and frequency stabilization of laser and electronic oscillators.Dr. Svelto is a member of the Italian CNR Group on Electrical and Electronic

Measurements, the North Italy Chapter of the IEEE-Instrumentation and Mea-surement Society, and AEI. In 1997, he was awarded the Philip Morris Prize forhis Ph.D. thesis on the absolute frequency stabilization of solid-state lasers.

Elio Bava (A’84) was born in 1940. He received thedegree in electronic engineering from the Politecnicodi Torino, Torino, Italy, in 1964.

In 1966, he joined the IEN in Torino, where,for a few years, he was involved in satellite com-munication research and microwave metrology.Afterward, he was interested in coherent frequencysynthesis from UHF up to the submillimeterregion, in the characterization and stabilization offar-infrared lasers and in the development of an Mg

submillimetric atomic-beam frequency standard. Since 1990, he has been afull Professor at the Politecnico di Milano. His current scientific interests areconcerned with optical laser frequency stabilization and measurements.