- Email: [email protected]
A tunable dual frequency Tm:YAG laser
1 April 2001
Optics Communications 190 (2001) 303±307
www.elsevier.com/locate/optcom
A tunable dual frequency Tm:YAG laser G. Quehl *, J. Gr unert, V. Elman, A. Hemmerich Institut f ur Laserphysik, Universitat Hamburg, Jungiusstraûe 9, 20355 Hamburg, Germany Received 25 October 2000; received in revised form 18 January 2001; accepted 22 January 2001
Abstract A compact design for a single mode Tm:YAG laser is presented. With two thin infrasil etalons as the only intracavity tuning elements a tuning range from 1940 to 2030 nm with a gap between 2000 and 2020 nm can be realized. The laser emits in two longitudinal modes separated by 1 GHz. Pumped with 500 mW power from a Ti:sapphire laser at 786 nm the output is above 50 mW over the entire tuning range. Frequencies can be rapidly varied over a range of 4 GHz. Ó 2001 Published by Elsevier Science B.V. PACS: 42.55; 42.62.F
Tunable single mode lasers are powerful tools in many dierent areas of atomic physics, as e.g. precision spectroscopy or laser cooling and trapping. While the visible and near-infrared spectral range is basically accessible with commercially available laser systems (dye-laser, semiconductor lasers, Ti:sapphire laser) this is not the case if wavelengths above 1:9 lm are required where laser diodes are not commercially available or unreliable in performance. Several groups [2,3] have shown that Tm:YAG is a promising material for lasers operating in the range of 1.9±2.15 lm, but a tunable narrow band laser source, appropriate for spectroscopic applications, has not yet been reported. As a particularly interesting feature Tm:YAG can be pumped at 786 nm, where high power semiconductor lasers are available.
*
Corresponding author. Fax: +49-40-42838-6571. E-mail address: [email protected] (G. Quehl).
To build a tunable single frequency laser usually requires a ring resonator design with an optical diode to obtain unidirectional operation and further intracavity elements like birefringent ®lters or etalons to achieve single frequency emission. However, for low gain laser materials like Tm:YAG the number of lossy intracavity elements should be kept at a minimum in order to obtain satisfactory performance, i.e. a suciently low threshold and high slope eciency. Fortunately, a strict single mode operation is not always necessary. A second mode separated from the ®rst by a known frequency (for example 1 GHz) does not disturb spectroscopic experiments if the distances between spectral lines are much greater than between the two laser modes, while the line widths are much smaller. This applies to our experiment on laser cooling of 40 Ca atoms [1] and it should be the case for most atoms with spectral lines in the 2l region. Our laser setup might not be appropriate for spectroscopy on systems with a great manifold of levels or band structures. Dual
0030-4018/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 0 7 7 - X
304
G. Quehl et al. / Optics Communications 190 (2001) 303±307
frequency emission may be obtained in a standing wave resonator design, at the advantage of working with fewer intracavity elements. Furthermore, standing wave cavities allow a particular compact and robust setup with the merit of high passive stability. In view of these considerations we have chosen a standing wave resonator design in order to build a tunable narrow band Tm:YAG laser appropriate for precision laser spectroscopy in the 2 lm region. We employ a z-shaped standing wave cavity that has been originally developed for miniature Ti:sapphire lasers [4]. The resonator geometry, which has been redesigned to account for the peculiarities of the Tm:YAG material, is shown in Fig. 1. We use two plane and two curved mirrors with a radius of R 20 mm. The geometrical distance between the curved mirrors is 23.4 mm, each of the plane mirrors is 60 mm away from its adjacent curved mirror. The angle of incidence for each curved mirror is 13°. The curved mirrors (BK7 substrate: 1/2 in. diameter, 6 mm thickness) are highly re¯ecting for 1980 nm (99.8%) and highly transparent at 786 nm (98%). Their planeback facet received an anti-re¯ection coating for 786 nm to obtain a high transmission of the pump wavelength. One of the plane mirrors, which is highly re¯ecting at 1980 nm, is mounted on a piezoelectric element for the tuning of the cavity length. Fast tuning is facilitated by using a small mirror substrate (1/4 in. diameter, 2 mm thickness, BK7). Finally, the outcoupling mirror is a 10 mm
Fig. 1. Design for a compact tunable Tm:YAG laser. The mirrors and etalons are ®xed to aluminum ¯ex mounts. The crystal is glued with silver conduction paint to a copper mount and water cooled.
thick etalon consisting of a plane 2% outcoupling mirror (1/2 in. diameter, 3 mm thickness) and a second substrate (1/4 in. diameter, 3 mm thickness) with a single-sided AR coating (<0.2% at 2 lm) separated by a 5 mm long piezoelectric ceramic tube. These components are aligned with the help of a 2 lm laser beam to form an etalon and subsequently ®xed together with a low thermal expansion epoxy cement. The substrates are fabricated from infrasil for good transparency at 2 lm. Both substrates are not wedged although this would be preferable. This etalon acts as a 98.7% outcoupling mirror with tunable frequency response and a contrast of 1.7%. Our resonator provides a tight focus in the center (between the curved mirrors) and two collimated branches between the curved and the plane mirrors respectively. A 3 mm thick disc of 6%atomic Tm:YAG crystal is placed at Brewsters angle in the center focus of the cavity. Each crystal facet is 10.2 mm away from the adjacent curved mirror. The crystal is thermally contacted with silver conducting paint to a copper mount which can be water cooled. With a pumping power of 1 W this shows, however, no eect on the performance of the laser. The confocal parameter in the sagittal plane is adjusted to match the optical path of 3.43 mm through the crystal [5]. The laser mode before entering the crystal is almost circular (confocal parameters: tangential 1.8 mm, sagittal 2.1 mm) in order to permit easy mode matching of the circular pumping beam. The outcoupled beam is also almost circular with a waist of 197 lm for the tangential direction and 191 lm for the sagittal direction. As the only intracavity elements, we use two thin etalons (150 and 163 lm) made from infrasil and polished to k=20. They are uncoated, introducing frequency dependent losses to the resonator by re¯ecting some of the intracavity circulating power out of the resonator. The complete setup is mounted on a 9:5 18:5 1:5 cm3 aluminum base plate. For adjustment all optical elements are ®xed to aluminum ¯ex mounts. In absence of the intracavity etalons the laser emits only at the frequency which provides maximum gain (i.e. at 2014 nm, see Fig. 2). To tune the laser away from this frequency one has to insert frequency dependent losses to the resonator in
G. Quehl et al. / Optics Communications 190 (2001) 303±307
Fig. 2. Performance of the laser (a) without tuning etalons (b) tuned to 1978 nm. The insert shows the ¯uorescence spectrum of Tm:YAG [6]. The white region marks the tunability of our laser.
order to shift the maximum total gain to the desired frequency. A cascade of etalons is used for this purpose. The thickest etalon is the resonator itself. With the help of the piezo-mounted plane mirror the length of the resonator can be changed tuning the laser about one free spectral range of 1 GHz. A second frequency selective element is given by the outcoupling etalon which has a free spectral range of 25 GHz. To further increase the tuning range, an additional 150 lm thin infrasil etalon is inserted into the resonator with a free spectral range of about 600 GHz 8 nm. Even thinner etalons, which could extend the tuning range into the THz range, are not technically feasible. Instead we take advantage of the coincidences of transmission maxima of two successive etalons with slightly dierent thicknesses. These occur with a periodicity corresponding to the difference of the thicknesses of the etalons. With a dierence of 13 lm a tuning range of 100 nm is obtained. Tilting one of the thin etalons tunes the laser by a few GHz until a jump to an adjacent mode 600 GHz away occurs. To tune the laser to a wavelength of an intermediate value both etalons have to be tilted simultaneously. Fine tuning is provided by adjusting the outcoupling etalon. Synchronous adjustment of the length of the outcoupling etalon and the resonator lets us perform continuous 4 GHz scans. This value is only limited by the maximum shift of the piezo ceramic tube of 4 lm controlling the resonator length which is 4 lm.
305
With our choice of thin etalons (150 and 163 lm) it is possible to tune the laser from 1940 to 2000 nm and from 2020 to 2030 nm. Inside the gap between 2000 and 2020 nm the emission frequency is pinned to 2014 nm due to the sharp maximum in the gain pro®le at this wavelength. In this region the losses introduced by the thin etalons are not sucient to pull the laser to other desired frequencies (see Fig. 2). To narrow this region the surfaces of the etalons could be coated to have a greater re¯ectivity of up to 10%. Higher re¯ectivities are not recommended because of the increasing walk o problem. Alternatively, using a set of etalons with greater dierence in thickness would also narrow this region. To check the tuning behavior we manually set the laser to several wavelengths n 10 in the above range and recorded the threshold and slope eciency. Both parameters did not show any decrease towards the tuning range boundaries, where the laser jumped back to the sharp maximum at 2014 nm. It turned out that throughout the tuning range the achievable output power was greater than 50 for 500 mW of pump power. Since the thin etalons are tuned manually we cannot present a continuous measurement of output power versus wavelength. At an increased pump power of 800 mW a maximum of 80 mW dual frequency output power was observed at 1978 nm. Our four mirror symmetric z-shaped cavity design is particularly appropriate because it combines a number of desirable properties. Firstly, the outer collimated branches of the laser mode are ideal for insertion of the etalons without disturbing the confocal parameter or adding astigmatism. Also the walk o of the beam inside the tilted etalon is not disturbing the etalon operation much since it is collimated. Secondly, due to the ¯at outcoupling mirror, the emitted laser beam is inherently non-astigmatic. Finally, placing the crystal in the center of the resonator yields a stable two mode operation which can be seen as follows: If there was only one mode then hole burning inside the crystal would occur, because inversion persists in the nodes of a standing wave. The remaining inversion has to be removed in a controlled manner to avoid random mode competition. Longitudinal modes with an
306
G. Quehl et al. / Optics Communications 190 (2001) 303±307
Fig. 3. Fabry±Perot spectrum for the Tm:YAG. One can clearly recognize the dual frequency operation with frequencies 1 GHz apart. The spectrum shows the 1.5 GHz periodicity of the Fabry±Perot etalon.
even (odd) number of half waves show a node (anti-node) in the center of the resonator, where we placed the crystal. Using this position an odd and an even mode deplete the entire inversion. The two modes receive the same amount of gain from dierent regions of the crystal which is not the case if the crystal was placed at a dierent location. For three or more modes the additional degrees of freedom would result in an unpredictable energy distribution among those modes. A Fabry±Perot spectrum (see Fig. 3) clearly demonstrates the dual frequency operation of the laser. As intended, we did not observe any evidence for ¯uctuations due to mode competition. We obtain a lasing threshold of less than 180 mW pump power and a slope eciency of more than 11% over the entire tuning range. In evaluating the slope eciency only the fraction of the total power emitted through the outcoupling mirror is taken into account. About 50% of the power is lost due to imperfections of the HR coatings of the other mirrors (see Fig. 1). We use this additional light power to monitor the emission properties. The short time stability of each of the laser modes is better than 1 MHz in 0.5 s. This is seen by tuning the laser frequency to the side of a Fabry± Perot fringe. The long term stability of about 20 MHz in 10 s is limited by the thermal drifts of the aluminum base plate. We are planning a number of improvements in our experiment. The quality of the HR coatings of the mirrors will be improved to reduce the power
leakage in our present resonator. We will also optimize the mode matching by accounting for residual ellipticity in the pumping focus. We will reduce the emission line width below 50 kHz by means of an active stabilization to a high ®nesse cavity. Finally the pump laser which has been a Ti:sapphire in our present experiments will be replaced by a high power semiconductor laser. This last step will require a modi®cation in the geometric design of the resonator. Due to the inferior beam quality of a high power laser diode compared to our solid state laser it is necessary to get as close to the Tm:YAG crystal as possible in order to obtain an adequate match of the pumping and the laser mode. Therefore in most diode pumped lasers the crystal has a high re¯ective coating on one side to act as one of the resonator mirrors. However, with such a design several of our requirements are hard to reach, especially a narrow bandwidth. In the modi®ed design a thin plane mirror close to the crystal surface will fold the resonator in such a way that the diode can be placed directly behind the plane mirror. This way the diode will be close to the crystal which will nevertheless remain in the geometrical center of the resonator. In summary, we have presented a compact design for a Tm:YAG laser tunable from 1940 to 2030 nm. The wavelength is adjusted with two intracavity etalons. The output is a nonastigmatic circular single transverse mode Gaussian beam comprised of two longitudinal modes separated by one free spectral range of 1 GHz. A maximum of 80 mW of infrared dual frequency power at 1978 nm has been achieved with an 800 mW Ti:sapphire pump laser at 786 nm. The laser can be tuned rapidly by 4 GHz. With an emission line width of less than 1 MHz it is a precision tool for spectroscopic experiments.
Acknowledgements This work has been supported by DFGHe2334/2.1 and DAAD 415 probral/bu. We are grateful to G. Huber and collaborators for providing a Tm:YAG crystal and valuable expertise.
G. Quehl et al. / Optics Communications 190 (2001) 303±307
References [1] J. Gr unert, G. Quehl, V. Elman, A. Hemmerich, Ultracold metastable calcium ensembles, a medium for matter wave ampli®cation? J. Mod. Opt. 47 (14/15) (2000) 2733. [2] R.C Stoneman, L. Esterowitz, Ecient, broadly tunable, laser-pumped Tm:YAG and Tm:YSGG cw lasers, Opt. Lett. 16 (1990) 486. [3] C. Svelto, I. Freitag, Room-temperature Tm:YAG ring laser with 150 mW single-frequency output power at 2020 lm, Electron. Lett. 35 (1999) 152.
307
[4] C. Zimmermann, V. Vuletic, A. Hemmerich, L. Ricci, T.W. Hansch, Design for a compact tunable Ti:sapphire laser, Opt. Lett. 20 (1995) 297. [5] H. Kogelnik, T. Li, Laser beams and resonators, Appl. Opt. 5 (1966) 1550. [6] Data of the ¯uorescence spectrum downloaded from http:// aesd.larc.nasa.gov/GL/laser/spectra/spectra.htm.
Optics Communications 190 (2001) 303±307
www.elsevier.com/locate/optcom
A tunable dual frequency Tm:YAG laser G. Quehl *, J. Gr unert, V. Elman, A. Hemmerich Institut f ur Laserphysik, Universitat Hamburg, Jungiusstraûe 9, 20355 Hamburg, Germany Received 25 October 2000; received in revised form 18 January 2001; accepted 22 January 2001
Abstract A compact design for a single mode Tm:YAG laser is presented. With two thin infrasil etalons as the only intracavity tuning elements a tuning range from 1940 to 2030 nm with a gap between 2000 and 2020 nm can be realized. The laser emits in two longitudinal modes separated by 1 GHz. Pumped with 500 mW power from a Ti:sapphire laser at 786 nm the output is above 50 mW over the entire tuning range. Frequencies can be rapidly varied over a range of 4 GHz. Ó 2001 Published by Elsevier Science B.V. PACS: 42.55; 42.62.F
Tunable single mode lasers are powerful tools in many dierent areas of atomic physics, as e.g. precision spectroscopy or laser cooling and trapping. While the visible and near-infrared spectral range is basically accessible with commercially available laser systems (dye-laser, semiconductor lasers, Ti:sapphire laser) this is not the case if wavelengths above 1:9 lm are required where laser diodes are not commercially available or unreliable in performance. Several groups [2,3] have shown that Tm:YAG is a promising material for lasers operating in the range of 1.9±2.15 lm, but a tunable narrow band laser source, appropriate for spectroscopic applications, has not yet been reported. As a particularly interesting feature Tm:YAG can be pumped at 786 nm, where high power semiconductor lasers are available.
*
Corresponding author. Fax: +49-40-42838-6571. E-mail address: [email protected] (G. Quehl).
To build a tunable single frequency laser usually requires a ring resonator design with an optical diode to obtain unidirectional operation and further intracavity elements like birefringent ®lters or etalons to achieve single frequency emission. However, for low gain laser materials like Tm:YAG the number of lossy intracavity elements should be kept at a minimum in order to obtain satisfactory performance, i.e. a suciently low threshold and high slope eciency. Fortunately, a strict single mode operation is not always necessary. A second mode separated from the ®rst by a known frequency (for example 1 GHz) does not disturb spectroscopic experiments if the distances between spectral lines are much greater than between the two laser modes, while the line widths are much smaller. This applies to our experiment on laser cooling of 40 Ca atoms [1] and it should be the case for most atoms with spectral lines in the 2l region. Our laser setup might not be appropriate for spectroscopy on systems with a great manifold of levels or band structures. Dual
0030-4018/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 0 7 7 - X
304
G. Quehl et al. / Optics Communications 190 (2001) 303±307
frequency emission may be obtained in a standing wave resonator design, at the advantage of working with fewer intracavity elements. Furthermore, standing wave cavities allow a particular compact and robust setup with the merit of high passive stability. In view of these considerations we have chosen a standing wave resonator design in order to build a tunable narrow band Tm:YAG laser appropriate for precision laser spectroscopy in the 2 lm region. We employ a z-shaped standing wave cavity that has been originally developed for miniature Ti:sapphire lasers [4]. The resonator geometry, which has been redesigned to account for the peculiarities of the Tm:YAG material, is shown in Fig. 1. We use two plane and two curved mirrors with a radius of R 20 mm. The geometrical distance between the curved mirrors is 23.4 mm, each of the plane mirrors is 60 mm away from its adjacent curved mirror. The angle of incidence for each curved mirror is 13°. The curved mirrors (BK7 substrate: 1/2 in. diameter, 6 mm thickness) are highly re¯ecting for 1980 nm (99.8%) and highly transparent at 786 nm (98%). Their planeback facet received an anti-re¯ection coating for 786 nm to obtain a high transmission of the pump wavelength. One of the plane mirrors, which is highly re¯ecting at 1980 nm, is mounted on a piezoelectric element for the tuning of the cavity length. Fast tuning is facilitated by using a small mirror substrate (1/4 in. diameter, 2 mm thickness, BK7). Finally, the outcoupling mirror is a 10 mm
Fig. 1. Design for a compact tunable Tm:YAG laser. The mirrors and etalons are ®xed to aluminum ¯ex mounts. The crystal is glued with silver conduction paint to a copper mount and water cooled.
thick etalon consisting of a plane 2% outcoupling mirror (1/2 in. diameter, 3 mm thickness) and a second substrate (1/4 in. diameter, 3 mm thickness) with a single-sided AR coating (<0.2% at 2 lm) separated by a 5 mm long piezoelectric ceramic tube. These components are aligned with the help of a 2 lm laser beam to form an etalon and subsequently ®xed together with a low thermal expansion epoxy cement. The substrates are fabricated from infrasil for good transparency at 2 lm. Both substrates are not wedged although this would be preferable. This etalon acts as a 98.7% outcoupling mirror with tunable frequency response and a contrast of 1.7%. Our resonator provides a tight focus in the center (between the curved mirrors) and two collimated branches between the curved and the plane mirrors respectively. A 3 mm thick disc of 6%atomic Tm:YAG crystal is placed at Brewsters angle in the center focus of the cavity. Each crystal facet is 10.2 mm away from the adjacent curved mirror. The crystal is thermally contacted with silver conducting paint to a copper mount which can be water cooled. With a pumping power of 1 W this shows, however, no eect on the performance of the laser. The confocal parameter in the sagittal plane is adjusted to match the optical path of 3.43 mm through the crystal [5]. The laser mode before entering the crystal is almost circular (confocal parameters: tangential 1.8 mm, sagittal 2.1 mm) in order to permit easy mode matching of the circular pumping beam. The outcoupled beam is also almost circular with a waist of 197 lm for the tangential direction and 191 lm for the sagittal direction. As the only intracavity elements, we use two thin etalons (150 and 163 lm) made from infrasil and polished to k=20. They are uncoated, introducing frequency dependent losses to the resonator by re¯ecting some of the intracavity circulating power out of the resonator. The complete setup is mounted on a 9:5 18:5 1:5 cm3 aluminum base plate. For adjustment all optical elements are ®xed to aluminum ¯ex mounts. In absence of the intracavity etalons the laser emits only at the frequency which provides maximum gain (i.e. at 2014 nm, see Fig. 2). To tune the laser away from this frequency one has to insert frequency dependent losses to the resonator in
G. Quehl et al. / Optics Communications 190 (2001) 303±307
Fig. 2. Performance of the laser (a) without tuning etalons (b) tuned to 1978 nm. The insert shows the ¯uorescence spectrum of Tm:YAG [6]. The white region marks the tunability of our laser.
order to shift the maximum total gain to the desired frequency. A cascade of etalons is used for this purpose. The thickest etalon is the resonator itself. With the help of the piezo-mounted plane mirror the length of the resonator can be changed tuning the laser about one free spectral range of 1 GHz. A second frequency selective element is given by the outcoupling etalon which has a free spectral range of 25 GHz. To further increase the tuning range, an additional 150 lm thin infrasil etalon is inserted into the resonator with a free spectral range of about 600 GHz 8 nm. Even thinner etalons, which could extend the tuning range into the THz range, are not technically feasible. Instead we take advantage of the coincidences of transmission maxima of two successive etalons with slightly dierent thicknesses. These occur with a periodicity corresponding to the difference of the thicknesses of the etalons. With a dierence of 13 lm a tuning range of 100 nm is obtained. Tilting one of the thin etalons tunes the laser by a few GHz until a jump to an adjacent mode 600 GHz away occurs. To tune the laser to a wavelength of an intermediate value both etalons have to be tilted simultaneously. Fine tuning is provided by adjusting the outcoupling etalon. Synchronous adjustment of the length of the outcoupling etalon and the resonator lets us perform continuous 4 GHz scans. This value is only limited by the maximum shift of the piezo ceramic tube of 4 lm controlling the resonator length which is 4 lm.
305
With our choice of thin etalons (150 and 163 lm) it is possible to tune the laser from 1940 to 2000 nm and from 2020 to 2030 nm. Inside the gap between 2000 and 2020 nm the emission frequency is pinned to 2014 nm due to the sharp maximum in the gain pro®le at this wavelength. In this region the losses introduced by the thin etalons are not sucient to pull the laser to other desired frequencies (see Fig. 2). To narrow this region the surfaces of the etalons could be coated to have a greater re¯ectivity of up to 10%. Higher re¯ectivities are not recommended because of the increasing walk o problem. Alternatively, using a set of etalons with greater dierence in thickness would also narrow this region. To check the tuning behavior we manually set the laser to several wavelengths n 10 in the above range and recorded the threshold and slope eciency. Both parameters did not show any decrease towards the tuning range boundaries, where the laser jumped back to the sharp maximum at 2014 nm. It turned out that throughout the tuning range the achievable output power was greater than 50 for 500 mW of pump power. Since the thin etalons are tuned manually we cannot present a continuous measurement of output power versus wavelength. At an increased pump power of 800 mW a maximum of 80 mW dual frequency output power was observed at 1978 nm. Our four mirror symmetric z-shaped cavity design is particularly appropriate because it combines a number of desirable properties. Firstly, the outer collimated branches of the laser mode are ideal for insertion of the etalons without disturbing the confocal parameter or adding astigmatism. Also the walk o of the beam inside the tilted etalon is not disturbing the etalon operation much since it is collimated. Secondly, due to the ¯at outcoupling mirror, the emitted laser beam is inherently non-astigmatic. Finally, placing the crystal in the center of the resonator yields a stable two mode operation which can be seen as follows: If there was only one mode then hole burning inside the crystal would occur, because inversion persists in the nodes of a standing wave. The remaining inversion has to be removed in a controlled manner to avoid random mode competition. Longitudinal modes with an
306
G. Quehl et al. / Optics Communications 190 (2001) 303±307
Fig. 3. Fabry±Perot spectrum for the Tm:YAG. One can clearly recognize the dual frequency operation with frequencies 1 GHz apart. The spectrum shows the 1.5 GHz periodicity of the Fabry±Perot etalon.
even (odd) number of half waves show a node (anti-node) in the center of the resonator, where we placed the crystal. Using this position an odd and an even mode deplete the entire inversion. The two modes receive the same amount of gain from dierent regions of the crystal which is not the case if the crystal was placed at a dierent location. For three or more modes the additional degrees of freedom would result in an unpredictable energy distribution among those modes. A Fabry±Perot spectrum (see Fig. 3) clearly demonstrates the dual frequency operation of the laser. As intended, we did not observe any evidence for ¯uctuations due to mode competition. We obtain a lasing threshold of less than 180 mW pump power and a slope eciency of more than 11% over the entire tuning range. In evaluating the slope eciency only the fraction of the total power emitted through the outcoupling mirror is taken into account. About 50% of the power is lost due to imperfections of the HR coatings of the other mirrors (see Fig. 1). We use this additional light power to monitor the emission properties. The short time stability of each of the laser modes is better than 1 MHz in 0.5 s. This is seen by tuning the laser frequency to the side of a Fabry± Perot fringe. The long term stability of about 20 MHz in 10 s is limited by the thermal drifts of the aluminum base plate. We are planning a number of improvements in our experiment. The quality of the HR coatings of the mirrors will be improved to reduce the power
leakage in our present resonator. We will also optimize the mode matching by accounting for residual ellipticity in the pumping focus. We will reduce the emission line width below 50 kHz by means of an active stabilization to a high ®nesse cavity. Finally the pump laser which has been a Ti:sapphire in our present experiments will be replaced by a high power semiconductor laser. This last step will require a modi®cation in the geometric design of the resonator. Due to the inferior beam quality of a high power laser diode compared to our solid state laser it is necessary to get as close to the Tm:YAG crystal as possible in order to obtain an adequate match of the pumping and the laser mode. Therefore in most diode pumped lasers the crystal has a high re¯ective coating on one side to act as one of the resonator mirrors. However, with such a design several of our requirements are hard to reach, especially a narrow bandwidth. In the modi®ed design a thin plane mirror close to the crystal surface will fold the resonator in such a way that the diode can be placed directly behind the plane mirror. This way the diode will be close to the crystal which will nevertheless remain in the geometrical center of the resonator. In summary, we have presented a compact design for a Tm:YAG laser tunable from 1940 to 2030 nm. The wavelength is adjusted with two intracavity etalons. The output is a nonastigmatic circular single transverse mode Gaussian beam comprised of two longitudinal modes separated by one free spectral range of 1 GHz. A maximum of 80 mW of infrared dual frequency power at 1978 nm has been achieved with an 800 mW Ti:sapphire pump laser at 786 nm. The laser can be tuned rapidly by 4 GHz. With an emission line width of less than 1 MHz it is a precision tool for spectroscopic experiments.
Acknowledgements This work has been supported by DFGHe2334/2.1 and DAAD 415 probral/bu. We are grateful to G. Huber and collaborators for providing a Tm:YAG crystal and valuable expertise.
G. Quehl et al. / Optics Communications 190 (2001) 303±307
References [1] J. Gr unert, G. Quehl, V. Elman, A. Hemmerich, Ultracold metastable calcium ensembles, a medium for matter wave ampli®cation? J. Mod. Opt. 47 (14/15) (2000) 2733. [2] R.C Stoneman, L. Esterowitz, Ecient, broadly tunable, laser-pumped Tm:YAG and Tm:YSGG cw lasers, Opt. Lett. 16 (1990) 486. [3] C. Svelto, I. Freitag, Room-temperature Tm:YAG ring laser with 150 mW single-frequency output power at 2020 lm, Electron. Lett. 35 (1999) 152.
307
[4] C. Zimmermann, V. Vuletic, A. Hemmerich, L. Ricci, T.W. Hansch, Design for a compact tunable Ti:sapphire laser, Opt. Lett. 20 (1995) 297. [5] H. Kogelnik, T. Li, Laser beams and resonators, Appl. Opt. 5 (1966) 1550. [6] Data of the ¯uorescence spectrum downloaded from http:// aesd.larc.nasa.gov/GL/laser/spectra/spectra.htm.