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Optics Communications 281 (2008) 2985–2988

Narrow band optical filter in fluorescein doped boric acidglass saturable absorber thin films

Ramesh C. Sharma a,*, Thomas A. Waigh a, Jagdish P. Singh b

a Biological Physics, Department of Physics and Astronomy and Photon Science Institute, The University of Manchester,

Manchester M13 9PL, United Kingdomb Institute for Clean Energy Technology, Mississippi State University, 205 Research Boulevard, Starkville, MS 39759-7704, USA

Received 6 December 2007; received in revised form 28 January 2008; accepted 29 January 2008

Abstract

We demonstrate narrow band optical filter like frequency response with full width half maximum (FWHM) of nearly (1.75 ± 0.25) Hzin fluorescein doped boric acid glass films [10�4 M], using modulated optical phase conjugation and a nearly non-degenerate four wavemixing technique. Modulated optical phase conjugation signals are described in the limit of a weak probe and relatively strong pumpbeams. Both pump beams are of nearly equal intensity at a wavelength of 514.5 nm from a continuous-wave Ar+ laser. The probe beamfrequency has been detuned with a ramp signal using a piezo electric mirror.� 2008 Elsevier B.V. All rights reserved.

Keywords: Nonlinear optics; Nearly non-degenerate four wave mixing; Laser materials; Spatial filtering; Optical devices; Modulated optical phaseconjugation; Active piezo mirror

1. Introduction

Saturable absorbers constitute an important class ofoptically nonlinear materials. A saturable absorber is char-acterized by a large ground-state absorption coefficient anda relatively long lifetime in its excited state. Saturableabsorbers have been extensively used for Q switching,mode locking, and pulse-shaping applications [1,2]. Non-linear absorption in fluorescein doped boric acid glasshas been studied using a single beam propagating in a sat-urable absorber [3,4]. Two beam coupling has beendescribed based on shifting phase grating in photorefrac-tive and saturable absorbers [4–9].

The term optical phase conjugation (OPC) is used todescribe a nonlinear process in which both the directionof propagation and the overall phase factor of a beam oflight are precisely reversed. Degenerate four wave mixing(DFWM) [10] is a simple method to achieve phase conjuga-

0030-4018/$ - see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.optcom.2008.01.062

* Corresponding author. Tel.: +44 161 306 3963.E-mail address: Ramesh.Sharma@manchester.ac.uk (R.C. Sharma).

tion by using the third-order nonlinearity (v3) of the electricsusceptibility tensor. In DFWM two strong counter-prop-agating pump waves and a weak probe beam with the samefrequency interact in the nonlinear medium, and a newwave is generated which is the OPC of the probe beam.This phase conjugated wave travels back along the originalpath of the incoming wave undergoing reverse phase evolu-tion. OPC by DFWM has been demonstrated in numerousnonlinear media [10–12]. Most of these experiments utilizedhigh peak power pulsed lasers. The photorefractive effectand the saturable absorber which enhance the effectivethird-order optical susceptibility make it possible to realizephase conjugation with continuous-wave lasers. OPC is avery promising technique, which can compensate for dis-persion in nonlinear media [11]. DFWM is demonstratedusing an artificial Kerr medium and is evidenced by directlyobserving the phase conjugation of a vortex signal beam[12]. OPC in quasi-linear optical transmission systems hasbeen studied previously [13]. The frequency up-conversionof coherent images by intra-cavity, measurement of com-plex v3 [14], and in the nearly degenerate four wave mixing

PrismPrism

BS1 BS2

M1

M2

M3

M5

M4

Ar +

LAS

ER

M8

L1Photodiode

Pre-Amplifier

Oscilloscope

Recorder

Piezo MirrorSupply

Thin F

ilm

Piezo Mirror

M6M7

BS3

BD

P1

P2

P3

P4

514.5nm

BS4

Fig. 1. Modulated optical phase conjugation signal experimental setup insaturable absorber solid thin films. BS1 to BS4 are beam splitters, M1–M8are mirrors, M3 is a piezoelectric mirror, L1 is the lens, P1, P2 are pumpbeams, P3 is a probe beam created via the piezo mirror with the appliedfrequency, and P4 is the optical phase conjugation beam, which isgenerated from the nonlinear medium.

2986 R.C. Sharma et al. / Optics Communications 281 (2008) 2985–2988

has been studied theoretically [15]. Nonlinear optical prop-erties have been described in photonic crystals and vapourphase using non-degenerate four wave mixing [16–22].

In this work, we have demonstrated the application oflaser induced moving dynamic gratings as a narrow bandoptical filter in fluorescein doped boric acid glass thin filmswith saturable absorbers. We report here for the first time,the use of coherent wave coupling in fluorescein dopedboric acid glass thin films based on the moving interferencepattern using pump and probe beams coupled with anapplied ramp signal at a piezo-mirror. This is demonstratedin a narrow band optical filter like frequency response withFWHM of band width (1.75 ± 0.25) Hz, using the modu-lated optical phase conjugation nondegenerate four wavemixing (OPC-NDFWM) technique at 514.5 nm in fluores-cein doped boric acid glass thin films.

2. Experimental

The samples were prepared by mixing the required quan-tities of fluorescein and boric acid, heating the mixture up toabout 240 �C, and pressing the resultant homogeneous meltbetween two glass slides preheated to around 120 �C, toobtain clear and transparent thin films [3,4,9]. The OPC sig-nal has been achieved in our experiment by using the degen-erate and nondegenerate four wave mixing method on thethin film of fluorescein doped boric acid glass [10�4 m].Two pump beams and the probe beam were derived fromthe same Ar+ laser (Spectra Physics) using a 80:20 beamsplitter BS1 (see the schematic of the experimental setupin Fig. 1). The collinear counter-propagating pump beamswere directly steered to the sample with the help of mirrorsM1, M2, M6 and M7. The probe beam was sent throughanother beam splitter BS2. The reflected beam from BS2was incident normally on a mirror M3 mounted on a piezo-electric transducer. The reflected beam from the piezo mir-ror M3 traversed through the same beam splitter on thereturn path and was then steered on to the sample withthe help of mirrors M4 and M5. Appropriate neutral-den-sity filters were introduced in the probe path to bring downthe probe power to the desired level. The probe beam wasincident at a small angle (3.5�) to one of the pump beams.A TEMoo laser beam of (2.80 ± 0.15) mm diameter wasused. Each pump beam had an intensity which was approx-imately equal to the others. The power of both pump beamswas equal to 70 mW, and the probe beam power was1.0 mW at the films. The frequency of probe beam P3 wasaltered by the piezo mirror making it slightly different fromthe pump beams P1 and P2. The optical phase conjugationbeam P4, is generated from the nonlinear optical medium inthe fluorescein solid thin film.

The piezo-transducer was driven by a triangular voltagewaveform obtained from a function generator for nearlynondegenerate four wave mixing. The calibration of themirror movement, as well as the uniformity of the mirrordisplacement throughout the range of the frequency shiftscarried out in experiments, was checked with a Michelson

interferometric setup. The ramp voltage was amplified, sothat the maximum displacement of the mirror is k/2, wherek is the wavelength of the Ar+ laser line used, 514.5 nm.This results in a frequency shift (D) of the probe beam by±2pf, where f/2 is the inverse of the ramp repetition period,the (±) sign corresponds to the up and down frequencyshifts of the probe beam. The fourth beam (the phase con-jugate beam) shifted by an amount (�D) is generated by themedium which counter-propagates along the probe beam.The phase conjugate wave is detected by a Si-photodiode,displayed on a LeCroy digital storage oscilloscope, andrecorded with a reference signal taken from the rampgenerator.

3. Results and discussion

The recorded absorption profile of the thin films, usingthe tungsten -lamp- spectrometer is given in Fig. 2. Thethickness of the thin film is determined to be L (=32 lm)[9]. The nonlinear absorption profile is recorded using thesingle beam experiment at 514.5 nm. The nonlinear absorp-tion data has been analyzed to measure the saturationintensity Is (1.02 W/cm2), and aoL (0.772) and boL

(0.389) are the unsaturated and saturated absorption,respectively in the saturable absorber four level model[3,4,9]. The nonlinear absorption profile is given in Fig. 3as transmittance versus input intensity to the thin films.The nonlinear parameters as a function of the grating pop-ulation decay time and the refractive index in fluorescein

350 550450Wavelength (nm)

Ab

sorp

tio

n (

a.u

.)

0.5

1.0

1.5

Fig. 2. Absorption profile of fluorescein doped boric acid glass solid thinfilm [10�4 M] showing the intensity versus the wavelength..

x

xx

xxx

xx

xx x

xx

x x

x

xx

x x

xx

xx x

Input Power (W)

Tra

nsm

itta

nce

(a.

u)

1010-110-310-50.40

0.50

0.60

0.70

Fig. 3. Nonlinear absorption profile of fluorescein doped boric acid glasssolid thin films[10�4 m] showing transmittance versus input power (W) at514.5 nm; (x) are the experimental data points and the solid line is the fit.

Piezo Mirror:OFF

(e) Piezo mirror: ON

+ -

(a)

Piezo mirror:OFF

(a) =0

(c) = Δ 2.0 Hz

(d) = Δ 7.0 Hz

Time (sec)

Sig

nal

inte

nsi

ty (

a.u

)

0

0.08

0.16

At = 0.70 Hz

(b) =0.70 Hz Δ=0Δ

Δ

Δ Δ

Fig. 4. Optical phase conjugation signal in degenerate four wave mixingwith (D = 0) piezo mirror OFF (a), and with Piezo mirror ON; (b–d)Nearly nondegenerate four wave mixing is shown with the modulatedoptical phase conjugate signal for fluorescein doped boric acid glass thinfilms at frequency detunings of (b) D = 0.7 Hz, (c) D = 2.0 Hz, and (d)D = 7.0 Hz at 514.5 nm and the incident ratios of the beam intensities areP1/P2 = 1 and P3/ P1 = 0.015. Triangular voltage applied to the piezoelement that upshift (positive slope, +D) and downshifts (negative slope,�D) the frequency of probe beam P3 is shown in curve (e) at D = 0.7 Hz.

R.C. Sharma et al. / Optics Communications 281 (2008) 2985–2988 2987

and Rhodamine-110 doped boric acid glass saturableabsorber solid thin films have been studied previously[3,4,9] using the two beam coupling technique. Previouslythe population decay time has been estimated as 185 msin the fluorescein doped boric acid glass thin films [4]. Opti-cal filters have been demonstrated in vapours and photoniccrystal based on the population decay time [15–17]. TheFWHM of the filter was 41 MHz in a sodium vapour,the population relaxation time is 16 ns in such a nonlinearmedium [15,16]. The FWHM of the narrow band filter is10 Hz in Cr:YALO3 crystal with the population relaxationtime 35 ms [17].

The OPC signal was recorded for a four wave mixingscheme with counter-propagating pumps P1 and P2 andprobe beam P3. When all the beams are coherent, beamP1 is scattered by the grating formed by beams P2 andP3, while beam P2 is scattered by the grating formed bybeams P1 and P3. Both scattered components combine toform the phase-conjugate wave, P4. The geometry of nearlynondegenerate four wave mixing with two counter-propa-gating pump beams of comparable intensity results in the

OPC signals decreasing symmetrically about the point ofzero detuning. This is illustrated in Fig. 5, which showsthe OPC signal for fluorescein doped boric acid glass thinfilms, for the case of equally strong pump beams P1 and P2.

The OPC-DFWM signal is recorded in Fig. 4a (at thepiezo mirror OFF condition). For the NDFWM (at piezomirror ON) a ramp signal is applied to the piezo mirrorM3. The symmetry of the signal, i.e., equal magnitude ofthe signal for frequency upshift (positive slope) in curve[Fig. 4e] and downshift (negative slope) can be clearly seen[Fig. 4e] as a reference signal. The generated fourth beamfrom the nonlinear media is modulated. The modulatedOPC signals are recorded [Fig. 4b–d] at different appliedramp frequencies to the probe beam. We have recordedOPC-NDFWM signals at different probe frequencies fromD = 0 to 25 Hz. The modulated OPC signal decreases withincreasing probe frequency as shown [Figs 4b–d] at 0.70,2.0 at 7.0 Hz respectively. Fig. 5 demonstrates the system-atic variation of the modulated OPC signal, using theexperimental data corresponding to the signal [Fig. 4].The OPC reflectivity defined as the ratio of the strengthsof the phase conjugated beam to that of the probe beamis shown in Eq. (1);

OPC signal ðRÞ ¼ I ðmodulated OPC signalÞ=I ðprobe beamÞ ð1Þ

where R is the reflectivity of the optical phase conjugation,I (modulated OPC signal) is the intensity of modulated signaland I(probe beam) is the intensity of the probe signal.

0

0.4

0.8

1.2

1.6

-20 -15 -10 -5 0 5 10 15 20

Nor

mal

ized

OPC

sig

nal (

a.u.

)

Detuning frequency (Hz)

Fig. 5. Normalized optical phase conjugation in (a.u.) versus thefrequency of the probe beam, Experimental results are plotted as (dottedpoints), and the Lorentzian Fit is a solid line.

2988 R.C. Sharma et al. / Optics Communications 281 (2008) 2985–2988

The OPC reflectivity has been plotted as a frequency ofthe probe beam [Fig. 5] using Eq. (1). A rapid decrease inthe modulated OPC signal with increasing (positive or neg-ative) probe frequency is observed. The OPC signal is dem-onstrated over a very narrow range in the probe frequency.The experimental modulated OPC signals as a function ofthe probe frequency have a very good fit to a Lorentzianprofile [Fig. 5]. Thus the modulated OPC signal undernearly non-degenerate four wave mixing condition hasthe frequency response of a narrow-band optical filter ofnearly (1.75 ± 0.25) Hz.

4. Conclusion

The fluorescein doped BAG film used in the experimenthas aoL (0.772), boL (0.389)and Is (1.02 W/cm2) a wave-length of 514.5 nm. The extremely narrow band filteringaction demonstrated in fluorescein doped boric acid glassfilm differs from the conventional filters in number of ways.A conventional filter only passes a certain spectral band-width that depends on the linear absorption and reflection

characteristics of the filter. The filter under present discus-sion however compares the spectral distributions of twolight beams with great precision. The filtering mechanismis directly related to the population response time of theoptical nonlinearity of the medium.

Acknowledgement

Authors are thankfull to Professor R.W. Boyd, Univer-sity of Rochester, USA for useful discussions.

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