Is a cw 3 μm holmium laser possible? A spectroscopic study

J. Quant. Specrrosc. Radiar. Transfer Vol.52, No. 5, pp. 545-554. 1994 Copyright 0 1994 Elsevier ScienceLtd

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IS A cw 3 pm HOLMIUM LASER POSSIBLE? A SPECTROSCOPIC STUDY P. J. MORRIS,~$ W. L~~THY,$ H. P. WEBERJ S. YA. RUSANOV,§A. I. ZAGUMENYI,~ I. A. SHCHERBAKOV,~and A. F. UMYSKOV~ IInstitute of AppliedPhysics, Universityof Bern, 3012 Bern, Switzerland, and $General Physics Institute, Academy of Sciences, Moscow, Russia (Received 15 February 1994)

Abstract-A thorough spectroscopic examination of the co-doped Yb: Ho : YSGG system was performed with the goal of achieving cw laser action on the 3 pm Ho j16- 51, transition. Yb has very good pump absorption properties, and a highly efficient process transfers the excitation to the Ho upper laser level, which has a lifetime of -400 psec. The lower laser level lifetime can be reduced from 9 msec to 1.5msec by the addition of 2% Eu as a deactivator without adversely affecting the upper laser level population. An undesirable upconversion process was discovered, though its effect can be minimised using high Yb and Ho concentrations, as these make the energy transfer rate so fast that it dominates the upconversion rate. The ideal composition derived from our studies is 50% Yb: 8% Ho:2% Eu, and laser operation at duty cycles in excess of 0.25 appears possible.

1. INTRODUCTION Water, the main constituent of biological tissue, has a local absorption maximum at 2.93 ,um with an absorption coefficient of > lo4 cm-’ which corresponds to an absorption depth of < 1 pm. This property is ideal for cutting and drilling in medicine as such a minute absorption depth ensures precise ablation with negligible thermal damage to surrounding regions in the tissue.’ It is therefore of no surprise that erbium lasers, currently the only solid state lasers which emit in the 3 pm region, have seen an intense research effort over the last 10 years. Lasing on the 3 pm erbium transition has been demonstrated in a variety of crystal hosts by pumping with flashlamps,* and by diode (or simulated diode) pumping at 800 nm and 970 nm.3’4 Currently, the preferred choice of crystal and pump wavelength are YLF and 970 nm, respectively. Holmium also has a 3 pm transition, between the ‘Z6 and ‘Z, electronic energy levels as shown in Fig. 1. Fluorescence and rudimentary lasing has been demonstrated on this transition in a number of hosts.5-‘0 This transition is self terminating due to a longer ‘1, level fluorescence lifetime than the ‘Z, level. In addition, holmium has no absorption lines which suitably match the emission wavelengths of diode lasers and is therefore not attractive for diode pumping. Co-doping with ytterbium and either europium or terbium, however, could overcome these problems and provide a viable cw 3 pm source. Ytterbium has strong absorption bands over the range 91&1030 nm and a near resonant energy transfer to the Ho ‘Z6 level and so can be

used as a sensitiser. Terbium and europium have electronic energy levels which lie very close to the holmium ‘Z, level and therefore near resonant energy transfer and/or a coupled Boltzmann system can be expected to rapidly de-excite the 5Z, level and drastically reduce its fluorescence lifetime. In this letter we report on a thorough spectroscopic examination to examine the possibility of cw lasing on the 3 pm transition in the co-doped Yb: Ho: Eu:Tb:YSGG system.

tTo whom all correspondence

should be addressed. 545

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2. EXPERIMENT

The crystal samples were prepared from oxides of 99.995% purity. The appropriate compositions were computed, the oxide powders weighed, thoroughly mixed and pressed at 1.5 MPa for 1 min to produce ceramic rods 2 mm in diameter and 50 mm long. Single crystal samples were obtained via laser heated pedestal growth. 20 W of 10.6 pm laser power was directed onto the side surface at one end of the rod using a circular donut focus to achieve uniform melting. The melt drop, with a diameter of approximately 3 mm was then cooled by gradually reducing the CO, power to produce transparent single crystal samples of spherical form without cracking. These were then cut to lengths of 400pm, polished and mounted between thin glass plates. The pump source for the fluorescence dynamics studies was a flash pumped Nd phosphate glass laser operating at a wavelength of 1.054 pm, as the Yb absorption at 1.054 pm is significantly stronger than at 1.06 pm. The laser was passively Q-switched to produce pulses of 100 nsec in length, with pulse energies between 10 and 20 mJ. The beam was focused to a diameter of approximately 3 mm at the crystal using a 200 mm lens and the fluorescence was detected using fast germanium or photomultiplier detectors connected to a digital oscilloscope. Absorption measurements were performed on a Perkin Elmer Lambda-9 spectrophotometer. Emission spectroscopy was possible by pumping over the range 910-990 nm using a Titanium-sapphire laser. The beam, with a power of 200mW, was focused to a diameter of approximately 100 pm at the crystal and the resulting fluorescence was collimated using a 38 mm lens. This was coupled into a monochromator (Sopra 1.15 m, 600 lines/mm) which was flooded with dry nitrogen to reduce water vapour absorption in the 2 and 3 pm regions. The signal was recorded using a PbS detector in combination with a lock-in amplifier. The system was not calibrated for detector or filter response. 3. RESULTS

3.1. Host crystal The co-doped Yb:Ho electronic level and transition scheme can be seen in Fig. 1. Absorption of a pump photon in the range 91&1030 nm excites the ytterbium ion from the 2F,,2ground state to the ‘Fsi2 level 0. The Yb 2F5,2and Ho ‘Z,,levels lie very close together and near resonant energy transfer occurs via a dipoledipole interaction 0. The excited Ho 5Z6level can decay radiatively

2ot 18188 550nm

Yb Fig.

1.Electronic energy levels and transition

..

Eu

Ho mechanisms

of the co-doped

Yb: Ho: Eu:Tb:

Tb YSGG system

Is a cw 3 pm holmium laser possible?

900

950

547

1000

1050

Wavelength (nm) Fig. 2. Absorption spectrum of a 280pm thick 100% Yb:SGG sample.

or non-radiatively to the Ho sZ, level (0 and a), or directly to the ‘Zs ground state 8. The branching ratio of these transitions is dependant on host crystal: for hosts with large phonon energies and broad phonon spectra, heavy oxides such as YAG for example, multi-phonon decay is fast, the ‘Z6lifetime is short and a large proportion of the decay is non-radiative. In hosts with low phonon energies and therefore narrower phonon spectra, YSGG or YLF for example, multi-phonon decay is slower, the 5Z6lifetime lengthens, the proportion of non-radiative decay decreases, and the branching ratio shifts in favour of radiative decay. We therefore chose YSGG as host crystal in order to obtain a large radiative branching ratio and a long ‘Z, fluorescence lifetime. 3.2. Activator As holmium has no suitable absorption bands in the region of commercially available diode lasers, around 800 nm and 930-970 nm, an activating ion is required for effective pumping. As mentioned in the introduction, ytterbium has very large absorption bands which well match the emission wavelength of InGaAs laser diodes. The room temperature absorption spectra of a 280pm thick YbSGG sample (a quasi 100% Yb doped YSGG) can be seen in Fig. 2. This is composed of a very broad peak at 930 nm, a narrower peak at 970 nm and a long wavelength shoulder. The largest peak has a maximum absorption coefficient of 92.3 cm-’ at 929 nm and a width of 28.15 nm (FWHM) whilst the peak at 969.6 nm has a maximum of 76.7 cm-’ and a FWHM of 3.8 nm. Due to this large bandwidth at 930 nm, diode pumping of Yb at this wavelength would be considerably simpler than in other solid state lasers. These suffer from narrow absorption bands which result in a decreased absorption co-efficient due to the considerable bandwidth of the diode emission and requires, in addition, rigorous temperature control of the diodes to tune the emission to the absorption maximum. With such high absorption coefficients, very short crystal lengths can be used without compromising the absorption efficiency. This means that very tightly focused and/or highly divergent diode laser pump beams can be used in the end-pumping scheme, as the radiation is absorbed before it can diverge out of the resonator mode, considerably simplifying the design of beam shaping optics. In addition, short crystal lengths can reduce thermal loading when the active mirror scheme is used to cool the back face. Heat flow is then longitudinal and the temperature difference between front and back faces reduces with crystal length. 3.3. Lasing ion Efficient pumping of Yb can be achieved between 915 and 950 nm, and at 970 nm. However, for efficient population of the holmium upper laser level, efficient energy transfer from the Yb *Fsi2to

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the 5Z,level is required. This can be achieved via a near resonant dipole-dipole interaction, though this is heavily dependant on the separation between Yb and Ho ions, and is therefore concentration dependent. The dependence of the Yb *Fsj2+ *F,,* fluorescence was measured as a function of both Ho and Yb concentration. In a 50% Yb:YSGG sample this lifetime shortens from 770 psec for 0% Ho to less than 20 psec for 1% Ho and continues to shorten at higher concentrations. This drastic shortening indicates fast energy transfer to holmium with efficiencies in excess of 90% at Ho concentrations above 0.5% . In a 1% Ho sample the lifetime shortens from 240 psec at 5% Yb to 18 psec at 50% Yb. This is again indicative of an increased Yb + Ho energy transfer at higher concentrations, where energy migration within the Yb population and multipole-multipole Yb-Ho energy transfer interactions may additionally occur. For good laser operation a long 5Z6fluorescence lifetime is desirable. In addition to radiative and non-radiative multiphonon de-excitation from the 5Z6level, the upconversion process, 0 in Fig. 1, can depopulate this level causing shorter lifetimes and a reduced population inversion. The fluorescence lifetime of the ‘I, level was measured for both the 3 pm 5Z6-5Z,and the 1.2 pm 5Z6-5Zs transitions; the data was derived from the 1.2 pm fluorescence. Figure 3 shows the lifetime as a function of Yb concentration for a 1% Ho : YSGG sample and as a function of Ho concentration for 20% and 50% Yb samples. The 5Z6level lifetime has a local minimum at 10% Yb, though is relatively unchanged at higher or lower Yb concentrations. There is a definite, though slight reduction in lifetime with increasing Ho concentration up to 4% Ho in both 20% Yb and 50% Yb, though this then lengthens again at 8% Ho. This suggests that the upconversion losses increase up to a maximum at around 4% holmium and then fall off. This can be explained by considering the upconversion rate, W, and the Yb + Ho energy transfer rate, Wt. With upconversion the Ho 5Z6level lifetime, tO, shortens according to 1 -=‘+ 7” 70

W,N,

(1)

Yb Concentration 0

10

20

30

(atoh) 40

50

550

500

I 0

350

-

20% Yb

300 0

2

4

6

Ho Concentration Fig. 3. Fluorescence sample

lifetime of the Ho ‘I6 level as a function and as a function of Yb concentration

6

10

(at %)

of Ho concentration for a 1% Ho:YSGG

for a 10% Yb:YSGG sample.

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2800

549

holmiumlaser possible?

2900

Wavelength (nm) Fig. 4. 3 pm emission

spectrum

of Yb:Ho:YSGG

when pumped

at 925 nm.

where T, is the new lifetime and N, is the population of the Yb ‘Fsj2 level. N, is itself a function of W, and W, and is reduced by both upconversion and energy transfer to Ho. At low concentrations both W, and W, are negligible and the lifetime is not shortened. As the Yb or Ho concentration is increased W, increases and r, is reduced. At even higher concentrations, however, W, and W, become so large that N, becomes negligible. With N, z 0, Eq. (1) tends to r0 and the lifetime shortening is reduced. Thus at high concentrations the Ho sZ, lifetime reverts to its original value, as energy transfer to Ho dominates over upconversion. The 3 pm laser transition is from Stark sub levels of the Ho ‘Z, level to Stark sub levels of the ‘1, level. The room temperature emission spectra due to these transitions is shown in Fig. 4. It is continuous from 2.75 to 3 pm and peaks at 2.85 pm. This lies directly between the two most used erbium laser lines, 2.94 pm (at the water absorption maximum) in YAG and 2.79 pm in YSGG. The 2.85 pm Ho line is very interesting as the absorption peak in heated water, >3OO”C as is typically encountered in medicine,” is shifted to shorter wavelengths. The absorption maximum shifts off Er : YAG and is centred on Ho: YSGG. The 3 pm fluorescence emission strength was measured as a function of Yb and Ho concentration and the results are plotted in Fig. 5. In a 1% Ho sample the lowest value is for 5% Yb, as at such low concentrations the YbHo transfer efficiency is low and the excited Yb ions decay non-radiatively. Above 10% the yield is greater and nearly constant, indicating more efficient energy transfer and only limited upconversion losses. The variation with Ho concentration shows a decreasing emission strength up to 4%, indicative of upconversion losses, though the emission increases at higher concentrations, suggesting upconversion is reduced or repopulation via non-linear Ho interactions occurs. discussed in the next section. 3.4. Deactivator The holmium ‘1, level can decay either radiatively or non-radiatively (0 and 0 in Fig. 1), though with a large energy gap to the ground state, N 5000 cm-‘, multi phonon decay is very slow and the ‘1, lifetime is long, typically an order of magnitude longer than the ‘Z6 level. The 5Z6-5Z, transition is therefore self terminating. By operating in the cascade laser scheme, however, where the 2 pm ‘Z,-‘I, transition @ follows the 3 pm transition, the ‘1, level can be depopulated sufficiently fast to allow cw action on the 3 pm transition. 8 This has the additional benefit of reducing the thermal load in the crystal as the ‘I, excitation is removed in photon instead of phonon form. The 5Z,-5Z, transition is quasi three level in nature and can only be operated efficiently with low Ho concentrations, short crystal lengths or at low temperatures. Such constraints, however, can

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seriously compromise the efficient action of the 3 pm transition. As good 2 pm sources already exist’2-‘4 and the simultaneous use of 2 pm and 3 pm radiation in medicine is not normally necessary, a depopulation mechanism for the Ho 5Z,level is required which does not adversely affect the 3 pm laser action. Two deactivator candidates exist, europium and terbium.6 These ions have very similar electronic energy level structures, as can be seen in Fig. 1, with the seven lowest levels of Eu and Tb extending to 5000 and 5500 cm-’ respectively, suitable for energy transfer from the Ho ‘1, level which begins at 5200 cm-‘. Furthermore the next energy levels lie at 17 and 20 cm-’ for Eu and Tb respectively, and so negligible interactions between these levels and the Ho or Yb ions can be expected, as the activation energy required is significantly higher than the pump or fluorescence photon energy. Energy transfer from Ho 5Z,to either the Eu or Tb ion (@ in Fig. 1) can be brought about in two ways: a dipole-dipole cross relaxation, or Boltzmann coupled Stark levels. The former simply requires resonance between the two transitions and a sufficiently small inter-ionic separation. A coupled Boltzmann population occurs if the Stark levels of the different ions lie sufficiently close together and the inter-ionic separation is sufficiently small. The combined population of the levels is then distributed amongst their Stark sub levels according to the temperature dependant Boltzmann relation and at anything but very elevated temperatures the majority is found in the lower Stark levels. Thus if a deactivator level lies directly beneath that of the Ho ‘1, level, rapid and near total energy transfer can be expected to take place. The room temperature absorption spectra of EuSGG and TbSGG over the range 1900-2200 nm were measured, with Tb exhibiting a somewhat featureless, flat and weak profile corresponding to the broadened shoulder of absorption peaks due to higher and lower lying levels. Eu on the other hand, exhibits three strong absorption peaks at 1.94, 1.97, and 2.01 pm, corresponding to Stark levels at - 5150, 5080, and 4980 cm-’ respectively. These lie very close to, and below, those of the Ho ‘1, level and so coupling of these levels can be expected, in addition to cross relaxation which

Yb Concentration (at%)

-

2

4

6

20%Yb

8

Ho Concentration (at%) Fig. 5. 3 pm fluorescence emission strength as a function of Ho concentration for a 10% Yb:YSGG sample and as a function of Yb concentration for a 1% Ho:YSGG sample when pumped at 925 nm.

Is a cw 3 pm holmium laser possible?

S 3 ._i! z z A 2 r

551

500

10000

400

6000

300

6000

200

4000

100

2000 0

-I

-.

0

0 0

1

2

3

Deactivator Concentration (at%) Fig. 6. Fluorescence lifetime of the Ho ‘I6 and ‘I, levels as a function of terbium and europium concentrations for a 10% Yb: 1% Ho:YSGG sample.

would proceed at a faster rate than Tb due to improved resonance. Eu could be expected to depopulate the ‘1, faster than Tb and therefore act as a better deactivator. Figure 6 shows the dependence of the Ho 5Z, level fluorescence lifetime on Eu and Tb concentrations for a 10% Yb: 1% Ho : YSGG sample. Both Tb and Eu are very good deactivators of the Ho 5Z,level and the lifetime is halved at deactivator concentrations of 0.2%. Above 0.8%, however, the Eu curve crosses that of the Tb, indicating that Tb is the better deactivator at high concentrations (contrary to expectations) with the ‘1, lifetime reduced to less than 1 msec for a Tb concentration of 2.5% . In addition to effective shortening of the ‘1, level, a good deactivator should not adversely affect the 3 pm ‘Z6--+‘1, fluorescence. If cross relaxation interactions such as @ in Fig. 1 occur, the ‘Z6 level is depopulated, resulting in a shortened lifetime and a reduced population inversion on the 3 pm transition. The interaction strength is dependent on the resonance between these transitions and the inter-ionic separation. An estimate of this resonance can be obtained by comparing the Ho 3 pm fluorescence with the Eu and Tb absorption spectra. The room temperature absorption spectra between 2.7 and 3.1 pm were measured for EuSGG and TbSGG samples. Tb exhibited both stronger absorption and more pronounced features than Eu over this range and so could be expected to suffer larger cross-relaxation. The results of lifetime measurements on the Ho ‘Z6level are shown in Fig. 6 for the 1.2 pm fluorescence. Lifetime shortening clearly occurs in Tb, with the ‘Z6 level lifetime shortening from 476 to 190psec as predicted. Eu on the other hand, exhibits negligible shortening, with the ‘Z6 lifetime in excess of 400 psec at high concentrations. Eu is therefore the preferred choice for deactivator as it effectively depopulates the ‘Z, level whilst leaving the 5Z6level relatively unaffected. The resulting lifetime ratio, 1580 psec: 413 psec means that the 3 pm transition is still self terminating. This is nevertheless a drastic improvement on before, 20 : 1, and means operation at a duty cycle approaching 0.25 should be possible. CW lasing may occur if the branching ratio of the ‘1, level is small as in this case the condition for cw lasing:

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satisfied, where r6 and t, are the lifetimes of the ‘I, and ‘1, levels, and fl12is the branching ratio from the upper to the lower lasing level. I5 Larger duty cycles or even true cw operation on this transition could be expected using fluoride host crystals. This is due to the substantially longer ‘I6 lifetimes in these materials,“j a result of their very small phonon energies. Furthermore, non-linear effects at high intrinsic Ho concentrations under strong flashlamp pumping” have been shown to reduce the Ho ‘1, level lifetime due to the interaction: ‘1’ + ‘Z,-+ 5Z6+ ‘I,. This effect would be stronger in resonantly end-pumped systems and has the additional benefit of populating the upper laser level, thereby increasing the Stokes efficiency and reducing pump heating. is

3.5. Upconversion Upconversion in the co-doped Yb : Ho : YSGG system occurs according to Yb ‘Fsjz-+ 2F7,2,Ho ‘Z6+ ‘Sz ; process @ in Fig. 1. The excited Ho ‘S, level can then decay radiatively to the ‘1, level, the 5Z7level or the ground state, emitting around 1 pm, 750 nm and 550 nm, respectively. The room temperature emission spectra of the green fluorescence, @ in Fig. 1, was measured and found to be continuous from 535 to 555 nm with two broad peaks centred at 537 and 547 nm respectively. To compare the green emission strength of the various samples it is important to minimise reabsorption losses, due to the quasi three level nature of the ground state terminated transition, as the differing Ho and Yb concentrations could mask the underlying trends. We therefore measured the magnitude of the green fluorescence at 547.9 nm, in the long wavelength tail of the fluorescence spectra corresponding to transitions to the higher lying Stark levels of the ground state which are subject to negligible reabsorption. The results plotted in Fig. 7 are normalised for absorbed pump power. With a constant Ho concentration the green fluorescence increases with Yb concentration, with a maximum at lo%, and then falls off. Comparison of this green fluorescence curve with the Ho 5Z6level lifetime curve, Fig. 3, at this point would be advantageous. The lifetime has a minimum at 10% Yb which also suggest maximum upconversion around this point. The Yb Concentration 0

10

20

(at%) 40

30

50

70 - . . 50% Yb -

60

5 3 c G

50

f

40

-

20% Yb

-

l%Ho

fi g

30

s 9 3 c

20

10

0

2

4

6

Ho Concentration Fig. 7. Green fluorescence sample and as a function

6

10

(at%)

emission strength as a function of Ho concentration for a 10% Yb:YSGG of Yb concentration for a 1% Ho:YSGG sample when pumped at 925 nm.

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upconversion process can therefore be interpreted as follows: at low concentrations it is low as there is poor energy transfer to the Ho ‘Z6level and consequently negligible population of this level. At higher concentrations the transfer is more efficient, both Yb and Ho excited states have high populations and the upconversion rate increases to a maximum. At even higher concentrations the energy transfer is so complete that, even though the Ho ‘Z6 population is very large, the Yb population is so small that the upconversion rate decreases. The fluorescent emission strength as a function of Ho concentration mirrors this trend. With 50 and 20% Yb, the energy transfer is already much faster than that of the 10% Yb considered above, and the upconversion rate is dominated by energy transfer. Increasing Ho concentrations further reduce the Yb population and reduce the upconversion rate. This is seen in Fig. 7, with a monotonous fall in green fluorescence with Ho concentration for both 50 and 20% Yb. The 20% Yb: 8% Ho point does not fit this interpretation; with a value of 27, the yield is more than an order of magnitude higher than expected. Further work is planned to clarify this behaviour. 4.

SUMMARY

AND

CONCLUSIONS

Yb : Ho : YSGG can be resonantly pumped across the range 915-950 nm and at around 970 nm due to very broad and strong absorption bands in Yb. These would make diode pumping considerably simpler than conventional solid state laser systems. Highly efficient energy transfer (> 90% ) from Yb to Ho is achieved with Ho concentrations in excess of 0.5% and this directly populates the ‘Z, upper laser level. This level has a lifetime in the region of 400 psec, and transitions to the ‘Z, level result in a fluorescence spectrum which is continuous from 2.75-3.0pm, peaking at 2.85 ,um. This wavelength lies directly between those of Er: YAG and Er: YLF and would be very interesting for medical applications. The 5Z,level normally has a lifetime in excess of 9 msec, though by co-doping with 2% europium this can be reduced to under 2 msec, without adversely affecting the 3 pm emission (co-doping with terbium resulted in even better shortening of the ‘1, level, though with simultaneous and undesirable shortening of the ‘Z, level). Nd has also been shown to be a good deactivator, though co-doped. Nd:Ho systems also suffer from loss mechanisms.‘* Upconversion occurs in this system whereby the excited Yb *F5,*and Ho 5Z, levels interact to produce a ground state Yb and an excited Ho ion which fluoresces in the green. This undesirable process depletes the population of the Ho ‘Z, upper laser level and simultaneously shortens the lifetime. Using high concentrations, however, ensures such a fast and effective energy transfer mechanism between Yb and Ho that there is negligible population of the Yb level. Upconversion requires large populations in both Yb and Ho excited states, so at large concentrations its effect is reduced and losses are minimised. True cw laser operation on this transition in YSGG cannot be expected as it is still self terminating, though operation with duty cycles of 0.25 should be possible, and this could be increased with increased deactivator concentrations which were not available for this study. Use of hosts with longer ‘Z6lifetimes, fluorides, for example, would still further increase the duty cycle, with cw action possible. These hosts have further benefits as the reduced Stark splitting of the levels results in narrowing of all the levels. The upconversion process, which is not very resonant, would become even less so and its rate would decrease. Yb-Ho energy transfer is much more resonant and would not be so adversely affected and so upconversion can be expected to be seriously reduced. The best combination for 3 pm laser action, as derived from our investigations, is 50% Yb: 8% Ho with 1% Eu as deactivator. Such a combination has good absorption and transfer efficiency, a long upper laser level lifetime, short lower laser level lifetime and a low upconversion rate. Acknowledgements-We thank H. J. Weder for his help with the figures. This work was supported in part by the Swiss Commision for the Encouragement of Science. REFERENCES 1. 2. 3. 4.

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