Solid state dye lasers based on 2-hydroxyethyl methacrylate and methyl methacrylate co-polymers

Solid state dye lasers based on 2-hydroxyethyl methacrylate and methyl methacrylate co-polymers

1 March 1999 Optics Communications 161 Ž1999. 163–170 Full length article Solid state dye lasers based on 2-hydroxyethyl methacrylate and methyl me...

223KB Sizes 0 Downloads 24 Views

1 March 1999

Optics Communications 161 Ž1999. 163–170

Full length article

Solid state dye lasers based on 2-hydroxyethyl methacrylate and methyl methacrylate co-polymers Shirin M. Giffin a , Iain T. McKinnie a,) , William J. Wadsworth a , Anthony D. Woolhouse b, Gerald J. Smith b, Tim G. Haskell b a b

Department of Physics, UniÕersity of Otago, P.O. Box 56, Dunedin, New Zealand Industrial Research Centre, Gracefield Research Centre, Lower Hutt, New Zealand Received 2 July 1998; revised 23 October 1998; accepted 23 December 1998

Abstract The laser performance of a range of solid state dye lasers based on rhodamine 590-doped co-polymers of 2-hydroxyethyl methacrylate ŽHEMA. and methyl methacrylate ŽMMA. has been investigated. The optimisation of preparation conditions, including polymerisation initiator and solvent for dye delivery is discussed in detail. Laser efficiency is compared for different polymeric hosts and dye concentrations with a range of output couplers, cavity lengths and repetition rates. Passive and dynamic loss have been determined for each host medium. Laser efficiencies of optimised polymers are among the highest reported for rhodamine 590-doped solid state dye lasers under these operating conditions. Highest slope efficiency of 35% and lowest threshold fluence of 0.06 J cmy2 were obtained with dimethyl sulphoxide ŽDMSO. additive in MPMMA at 10 Hz repetition rate. q 1999 Elsevier Science B.V. All rights reserved. Keywords: 2-Hydroxyethyl methacrylate; Methyl methacrylate; Solid state dye laser

1. Introduction Tunable visible solid state lasers are well-suited to diverse applications in areas such as atmospheric and underwater sensing, local area communications networks, medicine and isotope separation. For many of these applications solid state dye lasers w1–28x offer an efficient, low-cost alternative to optical parametric oscillators or indirect routes involving second harmonic generation of infrared solid state lasers. Dyes are attractive because of their high gain, broad tunability and high tolerance to pump parameters. Solid state dye lasers based on polymeric w1–14x and sol–gel glass w15–28x hosts have been widely reported. Polymer-based hosts provide enhanced compatibility with organic dyes as well as rapid and

)

Corresponding author. E-mail: [email protected]

inexpensive fabrication techniques, which lend themselves to waveguide and integrated optical systems. The main limitations of polymeric systems include thermal dissipation, a low host matrix laser damage threshold and photodegradation of the dye molecules. The most widely used polymeric host for solid state dye lasers to date has been polyŽmethyl methacrylate. ŽPMMA.. PMMA is characterised by a broad transparency range, however, the majority of conventional laser dyes are insoluble in PMMA and low molecular weight species must normally be added to enable dye doping. Low molecular weight additives, together with monomer purification, can also lead to increased laser damage thresholds w4x. Recently, a number of authors have sought to develop co-polymer matrices in which a heterogeneous monomer mixture is polymerised to provide enhanced cross-linking or dye compatibility in comparison with PMMA w4–6,9,10,13x. Among the most promising of these emerging host media are co-polymers of methyl methacrylate ŽMMA. and 2-hydroxyethyl

0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 9 9 . 0 0 0 0 8 - 5

164

S.M. Giffin et al.r Optics Communications 161 (1999) 163–170

methacrylate ŽHEMA.. HEMA provides enhanced solvation for the dye molecules, and improved laser operation has been reported with the PŽHEMA:MMA. polymers in certain specific cases w9,10x. Costela et al. w10x reported rhodamine 590-doped PŽHEMA:MMA. co-polymer dye lasers pumped transversely at wavelengths of 337 and 532 nm. In the case of ultraviolet pumping, increased laser efficiency and photostability were obtained with a PŽHEMA:MMA. polymer with 1:1 composition. To our knowledge, an optimisation of co-polymer fabrication techniques has not been reported and characterisation of laser performance under a range of operating conditions has not yet been carried out. Here, we report pulsed laser operation of rhodamine 590 dye in a range of PŽHEMA:MMA. co-polymer hosts, and in modified PMMA Ždesignated MPMMA. where the matrix is modified by the addition of dye solvating low molecular weight additives. The optimisation of preparation conditions is discussed for each matrix. Laser efficiency and photodegradation are measured under longitudinal pumping for different co-polymer compositions and dye concentrations. Laser performance is shown to be sensitive to the choice of polymerisation initiator and to the volume and type of solvent additive in MPMMA. Passive and dynamic loss processes are investigated for each host medium. A high dynamic loss, attributed to thermal lensing, is found to significantly affect laser performance. Our results indicate that efficient, long-lived laser operation of optimised dye-doped PŽHEMA:MMA. co-polymer matrices can be achieved at repetition rates of 10–20 Hz. This supports the results of Costela et al. w10x obtained in a different operating regime. The PŽHEMA:MMA. laser efficiencies are considerably higher than those previously reported for 10 Hz operation, and approach the highest efficiency reported for a co-polymer laser for 1 Hz operation w13x. However, we also find that further increases in laser efficiency and decreases in photodegradation rate can be achieved through optimisation of MPMMA polymeric hosts. Highest slope efficiency of 35% at 10 Hz repetition rate was obtained using DMSO solvation of rhodamine 590 in MPMMA. This is, to our knowledge, the highest efficiency obtained for a solid state rhodamine 590 laser operating at elevated repetition rates.

appropriate volume of dye solution. These MMA:dye solutions were deaerated by purging with dry argon and then given a brief vacuum treatment before being filtered at the 0.2 or 0.5 mm level into sealable glass ampoules. MMA:dye solution ratios of between 60:1 and 10:3 were used, corresponding to dye concentrations between 9.0 = 10y5 and 1.88 = 10y4 M. Polymerisation was initiated by either benzoyl peroxide ŽBz 2 O 2 . or 2,2X-azo-bis-isobutyronitrile ŽAIBN., present at 3 g ly1. Polymerisations were performed in the absence of light for approximately 5 days at 408C then for 3 days at 508C. The appearance and behaviour of the MPMMA polymer samples was found to depend on the initiator selected. Laser emission was very weak or absent in samples where polymerisation was initiated by Bz 2 O 2 . In contrast, when AIBN was used, strong laser emission was observed with otherwise identical samples. Fig. 1a shows fluorescence spectra for MPMMA samples incorporating ethanol, and prepared using each initiator. The ethanolic samples were prepared with a dye concentration of 1.7 = 10y4 M and from a MMA:dye solution composition of 60:1. Fig. 1b shows corresponding measurements with DMSO solvation. The DMSO samples were prepared with a dye concentration of 1.5 = 10y4 M from a MMA:dye solution composition of 5.6:1. Rhodamine 590-doped samples prepared using the AIBN initiator with either solvent show a characteristically strong fluorescence band with a maximum at 565 nm with both ethanol and DMSO additives for the MPMMA compositions shown in Fig. 1a and b. Samples prepared with Bz 2 O 2 exhibit broader and weaker fluorescence bands, blue-shifted relative to AIBN initiated polymers with maxima at 553 nm in DMSO and 539 nm in ethanol. Together with the degradation in laser performance, these spectra may indicate some dissociation of the dye during the polymerisation process when Bz 2 O 2 is used as an initiator. The emission spectra of rhodamine 590 in MPMMA delivered in DMSO and ethanol are identical to spectra obtained for the same dye in solutions of pure solvents. This suggests that the nearest neighbours of dye molecules in MPMMA are molecules of the delivery solvent. It is also notable that for identical dye concentrations and polymer composition ratios, the peak fluorescence wavelength of rhodamine 590 in MPMMA modified by DMSO is red-shifted by around 15 nm from the corresponding peak wavelength for ethanolic MPMMA. This phenomenon is the subject of a current investigation.

2. Material preparation 2.2. P(HEMA:MMA) 2.1. MPMMA Standard solutions of rhodamine 590 Žchloride form with purity of 99%. were prepared in ethanol or dimethyl sulphoxide ŽDMSO.. Dye concentrations varied between 10y2 and 10y3 M. MMA monomer was freed of inhibitor by exhaustive treatment with dilute sodium hydroxide solution which was dried before being mixed with the

Rhodamine 590 was dissolved in HEMA then mixed with inhibitor free MMA before being treated as described above. Two different compositions of the monomer mixture: PŽHEMA:MMA. 1:1 and 1:2 were prepared. The dye solutions were filtered directly into polypropylene moulds. Polymerisation took place in the absence of light for approximately 5 days at 408C then at 508C for 3 days. As

S.M. Giffin et al.r Optics Communications 161 (1999) 163–170

165

3. Experimental

Fig. 1. Fluorescence spectra of rhodamine 590-doped polymers with polymerisation initiated by AIBN Žsolid line. and Bz 2 O 2 Žbroken line.. The excitation wavelength was 500 nm. Ža. MPMMA with dye delivery in ethanol; Žb. MPMMA with dye delivery in DMSO; Žc. PŽHEMA:MMA. 1:1.

with the MPMMA materials, polymerisation was initiated by either Bz 2 O 2 or AIBN, present at 3 g ly1. However, in this case, the initiator had little or no effect on the fluorescence spectrum of the dye. Fig. 1c shows the fluorescence spectrum of rhodamine 590 in a PŽHEMA:MMA. 1:1 material with Bz 2 O 2 initiator. A strong fluorescence band with peak wavelength of around 561 nm was obtained. The spectrum obtained with the AIBN initiator was indistinguishable from this, and is not shown. All samples of MPMMA and PŽHEMA:MMA. were cylindrical in shape. The PŽHEMA:MMA. polymers were cut to form rods of 30 mm diameter and 10 mm length. The MPMMA rods were cut to 18 mm diameter and 15 mm length. End surfaces of the rods were prepared for laser experiments by conventional grinding and polishing. No attempt was made to obtain parallel end faces.

532 nm excitation for the solid state dye lasers was provided by the second harmonic of a Molectron MY34-20 lamp pumped Q-switched Nd:YAG oscillator–amplifier emitting 55 ns pulses at repetition rates of up to 20 Hz. The pump laser pulse energy was controlled by varying the amplifier flashlamp energy. The polymer samples were longitudinally pumped and a 500-mm focal length lens was used to obtain the required pump spotsize of 1 mm radius at the rod, which was placed beyond the pump beam waist. A linear two mirror planerplane resonator was used in all cases. The pump beam was introduced through one cavity mirror, coated for high reflectance between 555 and 640 nm, and transmission of 72% at 532 nm. Output couplers of 30, 50, 70, and 92% transmission between 410 and 700 nm were used. The polymer rod mount allowed for manual rotation and translation. In most cases, laser efficiency has not been optimised for particular cavity configurations or pump parameters. Rather we have sought to keep these factors unchanged in order to determine only the role of the gain media themselves. In most cases, a pump repetition rate of 20 Hz was used. The pump laser used in this work operates most stably at this repetition rate. A physical cavity length of 120 mm was chosen to accommodate the roto-translation mount, and to facilitate interchange of rods. We note, however, that higher output energy can be obtained with a shorter cavity or with lower repetition rates. The potential for obtaining improved performance of optimised MPMMA and PŽHEMA:MMA. rods by varying these parameters is reported in Section 4. Dichroic filters or prisms were used extra-cavity to separate the depleted pump from the dye laser output. Output energy measurements have been corrected for the transmission of these optics in each case. Spectral analysis was carried out using a 0.75-m Czerny–Turner spectrometer, gated integrator and storage oscilloscope. Pulse energies were determined using a fast risetime radiometer. Fluorescence spectra of the dye-doped polymer materials were recorded using a Hitachi F3010 spectrofluorimeter.

4. Results The role of preparation conditions in determining laser performance of MPMMA and PŽHEMA:MMA. polymer laser host materials has been investigated. Solvent type and volume, initiator species and monomer purity have been found to be determinants of laser performance. Each must be optimised and matched for a different host polymer. The purity of the monomer solution has a significant effect on laser efficiency. Fig. 2 shows laser performance of PŽHEMA:MMA. 1:1 rods prepared using 0.5 and 0.2 mm filters to purify the monomer. A lower threshold and

166

S.M. Giffin et al.r Optics Communications 161 (1999) 163–170

higher efficiency were obtained for the smaller pore size which minimises the number of foreign inclusions in the host matrix. Filtration at the 0.2 mm level was performed in all subsequent measurements with MPMMA and PŽHEMA:MMA. polymer rods. As already discussed in Section 2.1, the choice of polymerisation initiator is critical in determining fluorescence of MPMMA-doped materials. In fact efficient laser action was not obtained when the Bz 2 O 2 initiator was used in MPMMA and all MPMMA rods described in this section were prepared using AIBN. In contrast, in PŽHEMA:MMA. hosts the choice of initiator is relatively insignificant. Fig. 3 shows laser performance of PŽHEMA:MMA. 1:1 rods with polymerisation initiated by AIBN in one case and by Bz 2 O 2 in the other. Similar laser thresholds and slope efficiencies were measured for the two rods. These observations are consistent with the indistinguishable fluorescence spectra observed for rhodamine

590 in these materials ŽFig. 1c.. The MPMMA results could be explained if, in comparison with AIBN, Bz 2 O 2 preferentially generates free radicals from the low molecular weight solvent additives, and if it is these radicals that cause dye degradation during the polymerisation process. The absence of these particular radicals during polymerisation of the PŽHEMA:MMA. matrix could explain the lack of dye degradation observed in that case. The performance of MPMMA dye lasers has also been found to be highly sensitive to the type and volume of solvent additive. Our results indicate that for the ethanol and DMSO solvents in MPMMA, efficient laser operation is reproducibly obtained for MMA:dye solution composition ratios in excess of 10:1. Fig. 4 illustrates this, showing laser output energy as a function of pump energy for MPMMA laser rods prepared using compositions of 67% MMAr33% 6 = 10y4 M ethanolic dye solution and 91% MMAr9% 2.2 = 10y3 M ethanolic dye solution. A significant increase in efficiency is obtained with a smaller volume of solvent. Fig. 5 shows a comparison of laser efficiency for delivery of the dye by two different solvents, DMSO and ethanol, in MPMMA. In both rods, the polymer composition was 5.6:1 MMA:dye solution. Lower threshold and higher efficiency were obtained with the DMSO solvent. This may be a result of a higher dielectric constant of DMSO Ž47. compared with ethanol Ž24.. This would be expected to result in more effective solvation of rhodamine dyes in DMSO and hence to less dye dimerisation and aggregation than in ethanolic MPMMA. As dimers exhibit reduced singlet–singlet fluorescence and greater intersystem crossing as a result of exciton coupling between adjacent excited state dimers w29x, reduced laser efficiency and increased dye photodegradation would be anticipated for the ethanol additive. The influence of dye concentration on laser output has been investigated for MPMMA host polymers and for PŽHEMA:MMA. 1:1 and 1:2 co-polymers. No laser action

Fig. 3. Output energy as a function of Nd:YAG pump energy for PŽHEMA:MMA. 1:1 rods prepared using the polymerisation initiators AIBN and Bz 2 O 2 . Dye concentration was 1=10y4 M in each rod, and output coupler transmission was 30%.

Fig. 4. Output energy as a function of Nd:YAG pump energy for MPMMA rods prepared using compositions of 91% MMAr9% 2.2=10y3 M ethanolic dye solution and 67% MMAr33% 6= 10y4 M ethanolic dye solution. Dye concentration was 1.98= 10y4 M in each case.

Fig. 2. Output energy as a function of Nd:YAG pump energy for PŽHEMA:MMA. 1:1 rods with 1=10y4 M dye concentration using 30% output coupler transmission. Monomer solutions were purified with 0.2 and 0.5 mm pore size filters.

S.M. Giffin et al.r Optics Communications 161 (1999) 163–170

Fig. 5. Output energy as a function of Nd:YAG pump energy for MPMMA rods prepared using compositions of 5.6:1 MMA:dye solution for ethanol and DMSO additives. Dye concentration was 1.5=10y4 M and output coupler transmission was 30% in each case.

was observed for dye concentrations in excess of 10y3 M, or less than 10y5 M. For the rod lengths fabricated in this work, concentrations of less than 10y5 M do not provide sufficient gain. At concentrations above 10y3 M, the parasitic effects of dye aggregation described above become important. The increase in dimerisation is demonstrated by the growth of the band in the absorption spectrum, which is attributed to dye molecule dimer formation. The band, which occurs at around 490 nm, is enhanced relative to the dye monomer band with increasing dye concentration. Dye concentrations in the range of 0.9 = 10y5 to 2.0 = 10y4 M yielded efficient laser performance in all three host polymers, but within this range laser efficiency was relatively insensitive to dye concentration. For MPMMA Ž91% MMAr9% ethanolic dye solution. with 10 mJ pumping and 70% output coupler transmission, laser efficiency varied between 12% and 15%, with highest efficiency at a dye concentration of 1.5 = 10y4 M. A similar small variation in efficiency with dye concentration over this range was obtained for the two PŽHEMA:MMA. co-polymers. Optimum dye concentrations were around 1.5 = 10y4 M in the PŽHEMA:MMA. 1:1 host and around 1.0 = 10y4 M in the less polar PŽHEMA:MMA. 1:2 material. The laser performance of PŽHEMA:MMA. 1:1 and 1:2 and MPMMA Ž91% MMAr9% ethanolic dye solution. rods has also been measured for a range of output couplers. Dye concentration in each case was around 1.0 = 10y4 M. Output couplers of 30, 50, 70 and 92% transmission were used. In all cases, the laser operated with a bandwidth of around 10 nm, at a peak wavelength of close to 560 nm. Fig. 6 shows laser performance for each host. For MPMMA ŽFig. 6a., the highest output energy of 1.4 mJ at 10 mJ pump energy was obtained with 70% output coupler transmission. The lowest threshold fluence of 0.14 J cmy2 was measured with 30% output coupler transmission. Fig. 6b shows the results for the PŽHEMA:MMA. 1:1

167

co-polymer host. In this case a maximum output energy of 1.2 mJ was measured with the 70% output coupler transmission. Thirty percent output coupler transmission gave lowest threshold fluence of 0.28 J cmy2 . Results from the PŽHEMA:MMA. 2:1 rod are displayed in Fig. 6c. A roll-off in slope efficiency was observed with 70% output coupler transmission at pump pulse energies in excess of 7 mJ, and the highest output energy of 1.1 mJ was achieved with 50% output coupler transmission. The lowest threshold fluence of 0.14 J cmy2 was measured for the 30% output coupler transmission. A Findlay–Clay analysis w30x of the threshold measurements for the MPMMA and PŽHEMA:MMA. lasers has been carried out to determine the loss in each medium during laser operation. The loss due to scattering and

Fig. 6. Output energy as a function of Nd:YAG pump energy for dye-doped Ža. MPMMA, Žb. PŽHEMA:MMA. 1:1 and Žc. 1:2 rods with a range of values of output coupler transmission. An uncertainty of "0.1 mJ is estimated for each output energy measurement.

168

S.M. Giffin et al.r Optics Communications 161 (1999) 163–170

inhomogeneities in the host material has also been determined from single pass transmission measurements using a 633-nm helium neon probe laser. Table 1 summarises the attenuation coefficients measured by each method in MPMMA and PŽHEMA:MMA. 1:1 and 1:2 dye-doped polymers. The MPMMA rod had a passive attenuation coefficient of 0.04 cmy1, corresponding to a single pass loss of 6%. Similar attenuation coefficients were measured for the 1:2 and 1:1 co-polymer rods, leading to single pass loss of 3% in each case. A higher uncertainty in the Findlay–Clay attenuation coefficients arises from the extrapolation of the measured results to the case of 100% reflectance. However, it is clear that the increase in the Findlay–Clay attenuation over the passive case for each rod is significantly higher than one would predict on the basis of any residual singlet–singlet ground to excited state absorption at the laser wavelength in the dye. Although triplet–triplet excited state absorption would be expected to increase loss under optical excitation, singlet–triplet intersystem crossing is thought to be weak in rhodamine 590-doped media. We attribute the relatively high loss observed here under laser operation to thermal lensing in the low thermal conductivity polymeric medium. Previous authors have attributed a high measured divergence in a polymeric laser to thermal lensing w8x and a focal length as short as 1.5 cm has been estimated in one case w13x. The presence of thermal lensing could also contribute to an observed increase in laser efficiency with reduced cavity length as discussed below. However, a characterisation of thermal lensing in MPMMA and PŽHEMA:MMA. lasers under a range of pumping conditions would be necessary to confirm this. This is the subject of an ongoing investigation. Another key parameter in determining solid state dye laser performance is the photostability of the dye. We have investigated the photostability of rhodamine 590 in MPMMA and in PŽHEMA:MMA. 1:1 and 1:2. Measurements were carried out by monitoring the output energy of each laser as a function of time for a fixed pump energy. Dye laser energy was measured by a fast risetime radiometer, and recorded by computer through a data interface. Photostability results are presented in terms of a normalised photostability in units of GJ moly1, defined as the accumulated pump energy absorbed by the rod per mole of dye molecules which reduces the dye laser output to half of its original value w27x. Because this quantity takes account of the volume of the excited region and the concentration of

the dye, it has the advantage that it is independent of a specific experimental configuration. Photostability has been measured at repetition rates between 5 and 20 Hz using a pump pulse energy of 8 mJ and a spotsize of 1 mm. Dye concentration in each case was approximately 1.0 = 10y4 M. For MPMMA Ž91% MMAr9% dye solution. normalised photostabilities of 11 and 8 GJ moly1 were measured at 20 Hz with ethanol and DMSO solvation, respectively. When the repetition rate was reduced to 5 Hz, photostabilities increased to 20 GJ moly1 for ethanolic solvation and to 19 GJ moly1 for DMSO solvation. This decrease in photostability at higher repetition rates, and consequently higher powers, was observed in all cases and results from an increased thermal contribution to dye photodegradation. In the case of the PŽHEMA:MMA. 1:2 polymeric laser photostability increased from 13 to 19 GJ moly1 as the repetition rate was decreased from 20 to 5 Hz. With the higher HEMA content of PŽHEMA:MMA. 1:1, lower photostabilities of 8 GJ moly1 at 20 Hz and 15 GJ moly1 at 5 Hz were recorded. Costela et al. w10x have previously reported a high photostability for a PŽHEMA:MMA. 1:1 laser in comparison with other dye-doped hosts including aluminosilicate xerogel, ORMOSIL Žorganically modified silicate. and other co-polymers. Although an exact comparison with those measurements is inhibited by uncertainty in the pumped volume under transverse excitation, the normalised photostability measurements here appear to be similar to, or higher than those of Costela et al. at similar repetition rates. Rahn and King w27x have reported photostability values for rhodamine 590-doped sol–gel glass which are higher by a factor of 2.5 than the values reported here, but at a lower repetition rate of 1 Hz. It is anticipated that reduced repetition rates of around 1 Hz would also result in higher photostability measurements for the polymeric media discussed here. Further improvements would also be anticipated with more photostable perylene w31x or pyrromethene w5,32–34x dyes. Having determined optimum material preparation conditions, dye concentrations and output coupler transmissions in a standard 120 mm long cavity with 20 Hz pumping, we have been able to obtain significantly increased efficiencies for optimum laser rods at reduced pump repetition rates and with reduced cavity lengths. Both changes serve to reduce the parasitic effects resulting from thermal loading of the dye-doped polymer. Fig. 7a

Table 1 Attenuation coefficients for dye-doped MPMMA and PŽHEMA:MMA. 1:1 and 1:2 Host material

Attenuation coefficient ŽFindlay–Clay. Žcmy1 .

Attenuation coefficient Ž633 nm probe. Žcmy1 .

MPMMA PŽHEMA:MMA. 1:2 PŽHEMA:MMA. 1:1

Ž0.60 " 0.09. Ž0.22 " 0.03. Ž0.9 " 0.1.

Ž0.040 " 0.003. Ž0.030 " 0.002. Ž0.030 " 0.002.

S.M. Giffin et al.r Optics Communications 161 (1999) 163–170

169

with respect to pump light incident on the rod. The efficiency of this laser could be increased by replacing the cavity high reflector with a mirror providing higher transmission at the pump wavelength. With a readily achievable pump transmission of around 90%, a laser conversion efficiency of 40% would be obtained. The efficiencies reported here are almost a factor of two higher than efficiencies of co-polymer lasers reported recently by Costela et al. w10x. Where comparable or higher efficiencies have been previously reported for polymeric rhodamine 590 lasers, these have to our knowledge been obtained at repetition rates of around 1 Hz w3,4,13x. ŽWe note for comparison that Maslyukov et al. w12x have measured the effect of reduced repetition rate in polymeric dye lasers. They obtained a factor 4–5 enhancement in efficiency in reducing repetition rate from 20 to 3.33 Hz.. In the more general field of solid state dye lasers, 30% slope efficiency has been reported by Altman et al. w20x for 30 Hz operation of rhodamine 590 in ormosil. Where other authors have reported comparable w26x or lower efficiencies for inorganic or composite rhodamine 590 lasers, operation has generally been at lower repetition rates.

Fig. 7. Output energy as a function of Nd:YAG pump energy for four dye-doped polymeric rods: PŽHEMA:MMA. 1:1 and 1:2, and MPMMA with ethanol and DMSO dye-solvating additives. Results are shown for Ža. 20 Hz and Žb. 10 Hz repetition rate operation with 70% output coupler transmission. An uncertainty of "0.1 mJ is estimated for each output energy measurement.

shows the effect on performance when the cavity length was reduced to 20 mm. Output energy is shown as a function of pump energy for PŽHEMA:MMA. 1:1 and 1:2 rods, and for ethanol and DMSO solvated MPMMA prepared under the optimised conditions described above. The pump repetition rate was 20 Hz. Highest slope efficiency of 29%, and lowest threshold fluence of 0.06 J cmy2 were measured for the DMSO MPMMA rod. Highest output energy of 2.66 mJ was obtained at 10 mJ pump energy, corresponding to 27% efficiency. No clear trend emerges for the other materials with conversion efficiencies of between 22 and 24% and threshold fluences between 0.15 J cmy2 and 0.20 J cmy2 . A further increase in efficiency was obtained by reducing the pump repetition rate to 10 Hz. Fig. 7b shows results for the same four optimised polymeric materials. Highest efficiency and lowest threshold were once again obtained with the DMSO MPMMA rod. Similar performance was obtained for the remaining three rods. In this case, the slope efficiency for the DMSO rod was 35% and an efficiency of 32% was obtained at 10 mJ pump energy. Efficiencies of between 27% and 29% were measured for the other materials at the same pump energy. The maximum pulse energy obtained with the DMSO rod corresponds to a conversion efficiency of 44%

5. Conclusions The preparation conditions of dye-doped MPMMA and PŽHEMA:MMA. laser rods have been optimised. The role of factors such as polymerisation initiator, monomer purity and dye–solvent additive has been investigated through fluorescence spectra and laser performance. Photostability and loss in the passive and dynamic cases have been determined for MPMMA and PŽHEMA:MMA. 1:1 and 1:2 materials. The effect of output coupling, cavity length and pump repetition rate has been measured for MPMMA with ethanol and DMSO solvents, and for PŽHEMA:MMA. 1:1 and 1:2. Laser efficiencies of between 22 and 32% have been obtained for pump repetition rates of 10–20 Hz. A slope efficiency of 35% obtained with DMSO in MPMMA is, to our knowledge, the highest efficiency obtained for a rhodamine 590 solid state dye laser operating at 10 Hz repetition rate. It is anticipated that further improvements in performance can be made with more optimum input coupling of pump light. More significant improvements should be obtained by using a lower repetition rate and by utilising perylene and pyrromethene dyes. Acknowledgements This research was supported by the New Zealand Foundation for Research Science and Technology and Hughes Aircraft. The authors would like to thank Dale Watts of the University of Otago for polishing the samples, and Angel Costela of the Instituto de Quimica–Fisica in Madrid for helpful discussions. The comments of a reviewer of the manuscript are also gratefully acknowledged.

170

S.M. Giffin et al.r Optics Communications 161 (1999) 163–170

References w1x B.H. Soffer, B.B. McFarland, Appl. Phys. Lett. 10 Ž1967. 266. w2x R.M. O’Connell, T.T. Saito, Opt. Eng. 22 Ž1983. 292. w3x D.A. Gromov, K.M. Dyumaev, A.A. Manenkov, A.P. Maslyukov, G.A. Matyushkin, V.S. Nechitailo, A.M. Prokhorov, J. Opt. Soc. Am. B 2 Ž1985. 1028. w4x K.M. Dyumaev, A.A. Manenkov, A.P. Maslyukov, G.A. Matyushkin, V.S. Nechitailo, A.M. Prokhorov, J. Opt. Soc. Am. B 9 Ž1992. 143. w5x R.E. Hermes, T.H. Allik, S. Chandra, J.A. Hutchison, Appl. Phys. Lett. 63 Ž1993. 877. w6x F. Amat-Guerri, A. Costela, J.M. Figuera, F. Florido, R. Sastre, Chem. Phys. Lett. 209 Ž1993. 352. w7x M.L. Ferrer, A.U. Acuna, F. Amat-Guerri, A. Costela, J.M. Figuera, F. Florido, R. Sastre, Appl. Opt. 33 Ž1994. 2266. w8x F.J. Duarte, Appl. Opt. 33 Ž1994. 3857. w9x F. Amat-Guerri, A. Costela, J.M. Figuera, F. Florido, I. Garcia-Moreno, R. Sastre, Opt. Commun. 114 Ž1995. 442. w10x A. Costela, F. Florido, I. Garcia-Moreno, R. Duchowicz, F. Amat-Guerri, J.M. Figuera, R. Sastre, Appl. Phys. B 60 Ž1995. 383. w11x S. Chandra, T.H. Allik, J.A. Hutchinson, Opt. Lett. 20 Ž1995. 2387. w12x A. Maslyukov, S. Sokolov, M. Kaivola, K. Nyholm, S. Popov, Appl. Opt. 34 Ž1995. 1516. w13x F.J. Duarte, A. Costela, I. Garcia-Moreno, R. Sastre, J.J. Ehrlich, T.S. Taylor, Opt. Quantum Electron. 29 Ž1997. 461. w14x M.J. Cazeca, X. Jiang, J. Kumar, S.K. Tripathy, Appl. Opt. 36 Ž1997. 4965. w15x R. Reisfeld, D. Brusilovsky, M. Eyal, E. Miron, Z. Burstein, J. Irvi, Chem. Phys. Lett. 160 Ž1989. 43. w16x F. Salin, G. le Saux, P. Georges, A. Brun, C. Bagnall, J. Zarzycki, Opt. Lett. 14 Ž1989. 785.

w17x E.T. Knobbe, B. Dunn, P.D. Fuqua, F. Nishida, Appl. Opt. 29 Ž1990. 2729. w18x C. Whitehurst, D.J. Shaw, T.A. King, Proc. SPIE 1328 Ž1990. 183. w19x B. Dunn, J.I. Zing, J. Mater. Chem. 1 Ž1991. 903. w20x J.C. Altman, R.E. Stone, B. Dunn, F. Nishida, IEEE Photon Technol. Lett. 3 Ž1991. 189. w21x A. Charlton, I.T. McKinnie, M.A. Meneses-Nava, T.A. King, J. Mod. Opt. 39 Ž1992. 1517. w22x D. Lo, J.E. Parris, J.L. Lawless, Appl. Phys. B 55 Ž1992. 365. w23x M. Canva, P. Georges, A. Brun, D. Larrue, J. Zarzycki, J. Non-Cryst. Solids 147r148 Ž1992. 636. w24x D. Lo, J.E. Parris, J.L. Lawless, Appl. Phys. B 56 Ž1993. 385. w25x M. Canva, P. Georges, J.-F. Perelgritz, A. Brun, F. Chaput, J.-P. Boilot, in: T.Y. Fan, B.H.T. Chai ŽEds.., Advanced Solid State Lasers, Vol. 20, OSA Proceedings Series, Optical Society of America, Washington, DC, 1994, pp. 291–295. w26x D. Larrue, J. Zarzycki, M. Canva, P. Georges, F. Bentivegna, A. Brun, Opt. Commun. 110 Ž1994. 125. w27x M.D. Rahn, T.A. King, Appl. Opt. 34 Ž1995. 8260. w28x M.D. Rahn, T.A. King, A. Gorman, I. Hamblett, Appl. Opt. 36 Ž1997. 5862. w29x M. Kasha, H.R. Rawls, M.A. El-Bayoumi, J. Pure Appl. Chem. 11 Ž1965. 371. w30x D. Findlay, R.A. Clay, Phys. Lett. 20 Ž1966. 277. w31x G. Seybold, G. Wagonblast, Dyes, Pigments 11 Ž1989. 303. w32x J.H. Boyer, A.M. Haag, G. Sathyamoorthi, M.L. Soong, K. Thangaraj, T. Pavlopoulos, Heteroatom. Chem. 4 Ž1993. 39. w33x M.P. O’Neil, Opt. Lett. 18 Ž1993. 37. w34x T.H. Allik, R.E. Hermes, G. Sathyamoorthi, J.H. Boyer, SPIE 2115 Ž1994. 240.