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IMPROVED PERFORMANCE OF BISTABLE CdSe FILTERS
O. Sahlén, U. Öhlander, U. Olin
To cite this version:
O. Sahlén, U. Öhlander, U. Olin. IMPROVED PERFORMANCE OF BISTABLE CdSe FILTERS.
Journal de Physique Colloques, 1988, 49 (C2), pp.C2-127-C2-130. �10.1051/jphyscol:1988229�. �jpa-
00227646�
JOURNAL DE PHYSIQUE
C o l l o q u e C 2 , S u p p l 6 m e n t au n 0 6 , T o m e 49, j u i n 1988
IMPROVED PERFORMANCE OF BISTABLE CdSe FILTERS
0 . S A H L E N , U . ~ H L A N D E R and U . O L I N
I n s t i t u t e o f O p t i c a l R e s e a r c h and D e p a r t m e n t o f P h y s i c s I I , R o y a l I n s t i t u t e o f T e c h n o l o g y , S - 1 0 0 44 S t o c k h o l m , S w e d e n
Rdsumd
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Dcs filtres intcrfdmmdtriques bistablcs, faits de 40 lames quartd'onde en CdSe, ont dtd fabriqub.Ces composants ont montrd de meilleures performances en comparaison avec des filtres posddant des couches plus minces. Avec un faixeau de 60 microns de diametres, ils soot rest& a peu pres stable sur une dchelle d'une heure.
Abstract
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Bistable interference filters with 40 quartcr-wavelength thick CdSe spacers have been fabricatcd.These devices show superior performance compared to earlier devices with thinner spacers. For a 60 pn spot diameter, the devices can be made almost stable on a one hour time scale.
1. Introduction
Optically bistable interference filters have been extensively studied since the first report on bistability in such devices by K q u s h k o and Sinitsyn Ill. The optical nonlinearity used in these experiments is thermal, refs I Z and f31. The polycristalline ZnS or ZnSe spacers have a long bandtail and exhibit absorption for much longer wavelengths than the corresponding bulk crystalline materials. The optimization of these devices, with respect to material parameters and resonator design, has been discussed in ref 141. Such filters have recently been used to demonstrate various types of all-optical data processing, refs I51 and 161.
Recently we demonstrated optical bistability in interference filters with CdSe spacers, ref fll. The reason for using this matcrial is that crystalline CdSc has a bandgap wavelcngth of about 710 nm. Thus the evaporated thin film deviccs have some absorption in the wavelength region 800
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850 nm, due to the bandtail, and are thcrcfore suitable for optical bistability experiments using commercial, single-mode, high-power AlGaAs laser diodes.In this paper wc present results obtained from improved deviccs with thicker spacer laycrs than those studied in fll.
2. Flltcr charactcrlstics
The filters were fabricated by thermal evaporation in vacuum, TOR. Thcy had the design:
glass L ( A L ) ~
( W M
( L H ) ~whcre L. H and H' stands for a quarter-wavelength thick layer of MgF, ZnS and CdSe respectively.
The thermeoptic coefficient has been determined to be
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dn = 1 x lo4 K-I, by measuring thc transmission dTspectrum as a function of temperature. An example is shown in Figure 1, for a (M=6, N=3) filter.
Earlier filters were fabricated with N=3 and M values of 2, 6, 10 and 18. It was found that the switch power dccreascd by using thicker spacer layers. These findings are consistent with the scheme for optimization presented in 141: An optimized device should satisfy
whcre a is the absorption coefficient, L the spaccr thickness and R,, and R, are the front mirror and back mirror rcflcctanccs, rcspcctivcly. With a mcasurcd absorption cocflicicnt of a = 500 cm-' and mirror rcflectanccs givcn by the dcsign N = 3, Eqn (1) gives an optimum for M = 20, close thc valuc 18 uscd.
It was also found that the drift of the hysteresis c u ~ c as a function of timc, when thc device was continously switched on and off at high duty cycles, decreased dramatically when we used thicker spacers. This could be duc lo cithcr a rcduccd irradiance in the cavity whcn switched on, or duc to a dccrcased tcmpcraturc rise whcn switched on. It could also bc a combination of both effects.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988229
JOURNAL DE PHYSIQUE
3. Experiments
We have now made filters with M=40 and N=2. We have also attempted to make even thicker samples, but they cracked when exposed to intense light. The reason for this is probably excessive stress in too thick layers. A way around this problem could be to increase the temperature during evaporation. All filters were now deposited at room temperature.
The experimental set-up consisted of a dye laser pumped by an argon ion laser, which generated light tunable from 790 to 930 nm. The light was intensity modulated by an acousto-optic modulator to give triangular pulses, with variable duty-cycle and amplitude. Part of the beam was deflected to a detector, the rest of the beam focussed on the filter and measured by a second detector. Normal incidence was used.
When focussing to a measured intensity radius of 4-5 pm, we routinely achieved switch-on powers below 5 mW. Several filters exhibited switch-on powers of 2
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3 mW, with a low-power record of 1.4 mW for unambigous bistable switching. A typical example is shown in Figure 2, displaying bistability with 3.5 mW input power at X =840 nm for a relatively large detuning. Bistability could be seen in these filters close to Fabry-Perot resonances in the range from 800 to 870 urn.
A major problem with bistable interference filters is that the hysteresis loop drifts as a function of time. It was found that the stability improved much when going from M=18 to M=40 filters. With 50 % duty cycle and 4-5 pm spot size the hysteresis curve drifted only by roughly 10 % during 5 minutes. With a larger spot size, 30 pn, the filters could be nearly stable one hour or more.
It was found that the drift was largest when an unused pixel was used. The drift was initially large, and then grailually became smaller. This is shown in Figure 3a and 3b. In Figure 3a an unused pixel shows a considerable drift during one hour (30 pm spot radius). The drift is due to an induced change of the refractive index in the material, moving the Fabry-Perot resonance to a shorter wavelength. When the experiment was repeated immeadi- atcly afterwards, using the same pixel but having tuned the dye laser to about 1 nm shorter wavelength. the device became almost stable, showing only small drift over one hour (Figure 3b).
The fact that the stability increases with larger spot size indicates that the imdiance rather than the tempera- ture rise is mainly responsible for the drift. This is consistent with results published by Chow et al, 181. The tem- perature rise is proportional to the phase shift upon switching, and it is independent of spot size. The irradiance, on the other hand, increases with decreasing spot size. We measured the switch-on powers for spot radii of 5 p n and 30 pm, and found that the switching irradiance is a factor = 10 larger upon switching in the former case. The. fact that the switching irradiance increases with decreasing spot size has also been demonstrated in ZnSe and ZnS filters, refs Bf and 11W.
4. Conclusion
CdSe filters with 40 quarter-wavelength thick spacers show improved switch-on powers and improved stabii- ity when compared to previous CdSe filters. The fact that they can be switched with miniwatt power and wavelengths around 830 nm makes these devices of interest in optical computing.
As another example of how to use these filters, we Enally mention a student laboratory set-up we have designed. It is shown in Figure 4, and consists of a commecial laser diode (Hitachi 8314). two polarising beam splitters, two silicon detectors, a glass plate beam splitter and a bistable CdSe filter. One of the polarising beam splitters is rotated by a motor. This, in combination with the second polarising beam splitter cube, modulates the light intensity from the laser diode. The set-up is currently used to introduce third-year physics students to non- linear optics and optical bistability.
5. Acknowledgements
Sven-Erik Htirberg is thanked for his help with evaporating the filters. This work was partially supported by the Swedish Natural Science Research Council (NFR).
6. References
111 F. V. Karpushko and G. V. Sinitsyn. J. Appl. Spectrosc. (USSR), 24 (1978) 1323.
R / S. D. Smith, J. G. H. Mathew, M. R. Taghizadeh, A. C. Walker, B. S. Wherrett, A. Hendry, Opt. Com- mun., 51 (1984) 357.
nl G. R. Olbright, N. Peyghambarian, H. M. Gibbs, H. A. Macleod, F. van Milligen. Appl. Phys. Lett.. 45 (1984) 1031.
141 B. S. Wherrett, D. Hutchings, D. Russel, J. Opt Soc. Am. B., 3 (1986) 351.
151 S. D. Smith, A. C. Walker, B. S. Wherrett, F. A. P. Tooley, N. Craft, J. G. H. Mathew, M. R. Taghiza- deh, I. Redmond, R. J. Campbell, Opt. Eng., 26 (1987) 045.
161 M. T. Tsao. L. Waog, R. Jin, R. W. Sprague, G. Gigioli, H.-M. Kulcke. Y. D. Li. H. M. Chou, H. M.
Gibbs, N. Peyghambarian, Opt. Eng., 26 (1987) 041.
f f 1 0 . Sahlen, J. Opt. Soc. Am. B., 5 (1988) 82.
/8/ Y. T. Chow, B. S. Wherret, E. van Strylaad, B. T. McGuckin, D. Hutchings, J. G. H. Mathew, A. Miller.
K. Lewis, J. Opt. Soc. Am. B.. 3 (1986) 1535.
191 I . Janossy, M. R. Taghizadeh, J. Gordon, M. Mathew, S. D. Smith, IEEE J. Quant. Electron., QE-21 (1985) 1447.
1101 0 . Sahlen, Opt. Commun., 59 (1986) 238.
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WAVELENGTH l NMI
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Transmission versus wavelength in a (?vl=6, N=3) Elter, for two different temperatures. From these meas- urements the them-optic coefficient dn can be determined.-
Optical bistability in a (M40, N=2) filter. In a) the input power (bottom) and -1ted power (top) are shown as functions of time, in b) the transmitted power (ordinate) is displayed versus input power (abscisse). ?he switch-on power is 3.5 mW and the wavelength is 840 nm.C2-130 JOURNAL DE PHYSIQUE
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Drift of the hysteresis in a ( M 4 0 , N=2) filter. In a) the output power (top) and input power (bottom) are shown as functions of time, when switching an unused pixel. The time scale is 10 ms/division. Two exposures were made, with one hour in between. The duty cycle during this hour was 50 %. The spot intensity radius was 30 p.b) The same pixel as in a) shows a decreased drift when the experiment is repeated immeadiately after the experi- ment in a). The wavelength is adjusted to a 1 om smaller value. Two exposures were made, with one hour between. The duty cycle was still 50 % during this hour. The top traces, displaying the transmitted power, almost coincide.
