- Email: [email protected]
Acousto-optic filters with arbitrary spectral transmission
Optics Communications 355 (2015) 177–180
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Acousto-optic filters with arbitrary spectral transmission Konstantin B. Yushkov n, Vladimir Ya. Molchanov National University of Science and Technology “MISIS”, Acousto-Optical Research Center; 4 Leninsky prospekt, 119049 Moscow, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 28 April 2015 Received in revised form 17 June 2015 Accepted 18 June 2015 Available online 24 June 2015
We demonstrate a new method of use of acousto-optic filters consisting in adaptive modifying of the broadband arbitrary spectral transmission function of the device. The essence of the method consists in tailoring a complex-valued spectral transmission function of the acousto-optic filter and further calculating RF waveform that provides the desired spectrum. The performance of the method is experimentally demonstrated both for coherent and incoherent light. & 2015 Elsevier B.V. All rights reserved.
Keywords: Acousto-optics Filter Spectrum Transmission function
1. Introduction Acousto-optical filters (AOFs) offer a number of advantages for spectroscopic analysis in various fields of applied science, technology, astrophysics, biomedicine: atomic spectrometry, molecular spectrophotometry, fluorescence spectroscopy, Raman spectroscopy, spectropolarimetry, CARS spectroscopy, hyperspectral imaging, matched spectral filtering, etc. [1–8]. AOFs are reliable smallfootprint optical devices without moving parts that makes them usable even in space [3]. Most common are acousto-optical tunable filters (AOTFs) which operate as electronically tunable monochromators driven by single-frequency RF signals [1]. AOTFs are used as the key elements of spectrometers both for hyperspectral imaging and for collimated beams [1,3–5,8]. In a typical noncollinear AOF with uniform piezotransducer, homogeneous ultrasonic field results in a sinc2 shape of the optical transmission function. The same takes place in collinear AOTFs if the ultrasonic wave were not modulated. Generally speaking, the spectral transmission function of an AOTFs at a low efficiently limit is a Fourier transform of the ultrasound field distribution along the light path and its width is inversely proportional to the length of acousto-optic interaction [1]. Since phase gratings in AOTFs are induced by radio-frequency (RF) ultrasound, the gratings with variable period and nonuniform amplitude can be obtained. Before now, most efforts aimed at improving performance of AOTFs using complicated RF controlling were concentrated in two directions: obtaining multiple n
Corresponding author. E-mail address: [email protected] (K.B. Yushkov).
http://dx.doi.org/10.1016/j.optcom.2015.06.047 0030-4018/& 2015 Elsevier B.V. All rights reserved.
transmission windows and modifying the bandshape of AOTFs. For example, phase modulation of ultrasound can be used for modifying bandshape of collinear AOTFs [9]. Multiple transmission windows of AOTFs can be obtained using multi-frequency RF control signals, however the performance of that method is restricted because of intermodulation products [10]. Typically, multifrequency operation mode of AOTFs implies a superposition of several single-frequency ultrasonic waves in the crystal, and the number of independently controlled transmission bands is of the order of 10. Collinear and quasicollinear acousto-optic diffraction is also used for controlling spectral phase of femtosecond laser emission [11–15]. Acousto-optic devices of this type are referred to either as acousto-optic programmable dispersive filters (AOPDFs) or as acousto-optic light dispersive delay lines (LDDLs). In this paper we describe a method for synthesis of arbitrary spectral transmission functions of AOFs. That gives rise to new ways of using AOFs as equalizers and shapers of continuous spectra for both coherent and incoherent light. The wavelength interval on which the arbitrary transmission function can be defined is as wide as the whole tuning range of the AOF.
2. Method description In the following analysis, we assume collinear type of acoustooptic interaction. Let us consider the problem of arbitrary transmission function synthesis as follows: any real-valued nonnegative transmission function H (λ ) is determined on a wavelength interval [λ min , λmax ], i.e. the spectral intensity at the output of the AOF equals to Iout(λ ) = Iin(λ )H (λ ), where it is assumed that
178
a
K.B. Yushkov, V.Ya. Molchanov / Optics Communications 355 (2015) 177–180
b
c
Fig. 1. Output spectra of femtosecond laser emission after the AOF: (a) rectangular transmission window, H (λ ) = const ; (b) equalization of emission spectrum, H (λ ) ∝ Iin(λ )−1; and (c) binary modulation of equalized emission, H (λ ) ∝ Iin(λ )−1M (λ ) .
H (λ ) < 1. Our method described below is a unified algorithm for calculating RF waveforms S(t) which provide the desired transmission function of the AOF without any additional assumptions about H (λ ). The procedure of calculating RF waveforms can be implemented step-by-step as follows. First, Bragg phase matching condition provides the tuning relationship of the AOF, λ(F ). From that relationship, the central frequency of the RF signal F0 and the full frequency span ΔF corresponding to the interval [λ min , λmax ] are determined. Then, the duration of the RF waveform T0 is fixed. This duration should not exceed the acoustic wave travel time Tac through the region of acousto-optic interaction. Proper phase relations between the spectral components of RF waveforms are obtained by adding spectral phase with the second-order dispersion [13,16]. Thus, the complex-valued spectrum of the RF driving waveform is expressed as
∼ S (F ) = H1/2(λ(F ))exp[iπ (F − F0)2T0/ΔF ],
(1)
The spectrum is sampled and the complex-valued RF waveform S(t) ∼ is calculated as a discrete Fourier transform of S (F ). The magnitude of S(t) stands for amplitude modulation of the RF waveform, while the argument of S(t) stands for phase modulation. The distinction ∼ between the Fourier transforms of the complex-valued function S (F ) 1/2 and the real-valued function H (λ(F )) consists in temporal arranging of the spectral components in the resulting waveform. Since in ∼ Eq. (1) the RF spectrum S (F ) is defined with the quadratic complex phase that results in quasi-linear frequency modulation of the RF waveform. In other words, the RF waveform becomes chirped due to the exponential term in Eq. (1) as it would be after passing a dispersive element. Such type of acousto-optic spectral processing treats both input and output spectra of light as continuous functions, and RF signals with continuous spectra are applied to the piezotransducer of the AOF [14]. Since the phase grating in the AOF is dynamic and travels with the speed of sound, instantaneous spectral transmission varies with time. The full spectral window [λ min , λmax ] is transmitted by the AOF only when the whole RF waveform is located in the interaction region. The usable time window for detection of the signal equals to Tac − T0 . For a variety of amplified femtosecond laser systems, the pulse repetition period is greater than typical Tac values and each pulse can be processed independently. In this case we can imply T0 = Tac that provides the best spectral resolution and maximum diffraction efficiency. On the contrary, for both incoherent light sources and continuous-wave modelocked lasers, only stroboscopic processing of emission is possible. The waveform duration T0 can be intentionally made shorter to increase time window but it results in a decrease of spectral resolution [17]. Most of the broadband light sources are characterized by spectrally dependent intensity but it is often convenient to operate equalized flat-top spectra. AOFs with synthesized broadband
transmission can be efficiently used for that purpose. Spectral transmission function of the AOF can be determined as
H (λ ) ∝ Iin(λ )−1M (λ )
(2)
where M (λ ) is any spectral modulation function. Thus, the output spectrum is proportional to the product of rectangular window function and M (λ ). In the case of M (λ ) = const , the output spectrum is equalized on the whole interval [λ min , λmax ]. In the following section, we demonstrate consecutive realization of equalization and modulation of continuous spectra.
3. Experimental results For the commissioning of the method we used a custom-made quasi-collinear AOF based on paratellurite single crystals [18]. Spectral resolution of the AOF was equal to 3 cm 1. The AOF was driven with a 625 MSa/s arbitrary waveform generator (Agilent, N8241A) together with a broadband RF power amplifier (Amplifier Research, 10W1000C). The temporal window was equal to T0 ¼51.2 μs. Fast Fourier transform was used to calculate the sampled waveforms that were uploaded to the generator. For the experiments with coherent broadband light source we used a Ti:sapphire femtosecond laser (Femtolasers, Femtosource Synergy) with a 60 nm FWHM spectrum centered at 800 nm. The following parameter values were used for calculation of RF waveforms according to characteristics of laser emission: λ min = 745 nm , λ max = 865 nm , F0 = 75 MHz , and ΔF = 13.4 MHz. We consequently applied different transmission functions H (λ ). In Fig. 1, a series of output spectra measured with a high performance optical spectrum analyzer (Agilent, 86142B) are shown. First, the transmission was defined as a rectangular window, and Iout(λ ) ∝ Iin(λ ) was obtained. This spectrum shown in Fig. 1(a) was used as a reference for equalization of output emission according to Eq. (2). Rapid decrease of intensity takes place at the borders of the selected wavelength interval. Second, the output spectrum was equalized so that Iout(λ ) = const on the interval [λ min , λmax ]. That case is plotted in Fig. 1(b). Finally, the equalized spectrum was coded with a periodic binary pattern M (λ ), as shown in Fig. 1(c). The number of sub-bands at the output spectrum in Fig. 1(c) was equal to 25. However, this number of sub-bands was not limited by the design of the electronic driving system and could be increased several times at the cost of modulation contrast [17]. That makes principal difference with multi-frequency methods of controlling AOFs which are based on choosing a fixed number of channels and adjusting their amplitude and phase. In the experiments with incoherent light, we used a xenon arc lamp. The emission was spatially filtered with a pair of fiber collimators connected with a single-mode optical patch cord. Collimation of light is necessary to maintain appropriate spectral resolution because the bandwidth of quasi-collinear AOFs broadens
K.B. Yushkov, V.Ya. Molchanov / Optics Communications 355 (2015) 177–180
a
179
b
Fig. 2. Equalization of broadband light spectrum from the source (a) and after passing the cell with methylene blue dye (b): I0(λ ) — spectrum of the Xe arc lamp after the AOF with rectangular transmission window; I1(λ ) — equalized light spectrum after the AOF; I2(λ ) — light spectrum after the dye cell; and I3(λ ) — equalized spectrum after the dye cell.
with the divergence of the input light beam [18]. The output spectrum was captured with a grating CCD spectrometer (Ocean Optics, USB2000þ ). The emission bandwidth of 150 nm was selected. The following parameter values were used for calculation of RF waveforms: λ min = 550 nm , λ max = 700 nm , F0 ¼ 117.6 MHz, ΔF = 36.9 MHz . Spectral equalization of emission is presented in Fig. 2(a). The plain spectrum of the lamp I0(λ ) was recorded using a rectangular transmission window of the AOF. That spectrum was used as a reference for equalization. At the next step, the transmission function of the AOF was given by
H1(λ ) = H0max(I0(λ ))/I0(λ ).
(3)
The magnitude of the rectangular transmission window was set as H0 ¼ 0.5. Thus the equalized output emission had higher intensity I1(λ ) than the plain spectrum transmitted by the AOF, and the condition H1(λ ) < 1 was satisfied. In the second part of this experiment, an additional cell with water solution of methylene blue dye was used for commissioning of the spectrum equalization method. The cell was placed into the optical path before the AOF. Absorption of light by the dye resulted in a modified light spectrum I2(λ ) shown in Fig. 2(b). To compensate for the deformation of the spectrum caused by the dye, the spectral transmission function of the AOF was set as
H3(λ ) = H1(λ )max(I2(λ ))/I2(λ ).
(4)
The resulted spectrum I3(λ ) has a flat top in the whole spectral region from 550 to 700 nm. Both inhomogeneity of source emission and additional spectral absorption of the dye are well equalized. This experiment demonstrated that iterative equalization of optical spectra using AOFs with arbitrary transmission can be used for step-by-step elimination of different independent factors which determine the emission spectrum.
4. Summary The proposed method for running AOFs with arbitrary transmission function by means of dispersive RF controlling is an advanced high performance technique for spectral light processing, usable both for coherent and incoherent broadband emission. That method provides sufficient advance in flexibility and adaptiveness compared to multi-frequency controlling of AOFs. The experiments with both coherent and incoherent broadband light were performed. Different application modes of AOFs were demonstrated in the experiment, such as adaptive equalization of continuous spectra and binary modulation. AOFs with sophisticated spectral
transmission functions offer new opportunities in many dedicated applications such as matched spectral filtering, spectral encoding, spectral equalization of light, and ultrashort laser pulse shaping.
Acknowledgments The work was carried out with financial support in part from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST MISiS (Project nr. K1-2014-008), from the Russian Foundation for Basic Research (Project nr. 15-07-03714).
References [1] M. Gottlieb, Acousto-optic tunable filters, in: A. Goutzoulis, D. Pape (Eds.), Design and Fabrication of Acousto-Optic Devices, Marcel Dekker, New York, 1994, pp. 197–284. [2] L. Bei, G.I. Dennis, H.M. Miller, T.W. Spaine, J.W. Carnahan, Acousto-optic tunable filters: fundamentals and applications as applied to chemical analysis techniques, Prog. Quantum Electron. 28 (2004) 67–87, http://dx.doi.org/ 10.1016/S0079-6727(03)00083-1. [3] O. Korablev, J.-L. Bertaux, A. Grigoriev, E. Dimarellis, Yu. Kalinnikov, A. Rodin, C. Muller, D. Fonteyn, An AOTF-based spectrometer for the studies of Mars atmosphere for Mars Express ESA mission, Adv. Space Res. 29 (2002) 143–150, http://dx.doi.org/10.1016/S0273-1177(01)00581-6. [4] N. Gupta, V.B. Voloshinov, Hyperspectral imaging performance of a TeO2 acousto-optic tunable filter in the ultraviolet region, Opt. Lett. 30 (2005) 985–987, http://dx.doi.org/10.1364/OL.30.000985. [5] V.E. Pozhar, V.I. Pustovoit, Adaptive acousto-optical spectrometric systems, in: IEEE Ultrasonics Symposium 2006, 2006, p. 236 http://dx.doi.org/10.1109/ ULTSYM.2006.72. [6] S.J. Leavesley, N. Annamdevula, J. Boni, S. Stocker, K. Grant, B. Troyanovsky, T. C. Rich, D.P. Alvarez, Hyperspectral imaging microscopy for identification and quantitative analysis of fluorescently-labeled cells in highly autofluorescent tissue, J. Biophotonics 5 (2012) 67–84, http://dx.doi.org/10.1002/ jbio.201100066. [7] S.I. Chizhikov, S.G. Garanin, L.V. Goryachev, V.Ya. Molchanov, V.V. Romanov, N. N. Rukavishnikov, S.V. Sokolovskii, I.N. Voronich, K.B. Yushkov, Acousto-optical adaptive correction of chirped laser pulse spectral profile in nd-phosphate glass regenerative amplifier, Laser Phys. Lett. 10 (2013) 015301, http://dx.doi. org/10.1088/1612-2011/10/1/015301. [8] V.Ya. Molchanov, S.P. Anikin, S.I. Chizhikov, K.B. Yushkov, O.Yu. Makarov, A.M. Tatarnikov, S.A. Potanin, V.F. Esipov, Acousto-optical imaging spectropolarimetric devices: new opportunities and developments, Proc. SPIE 9147 (2014) 91472T. http://dx.doi.org/10.1117/12.2055150. [9] V.I. Pustovoit, V.E. Pozhar, Controlling characteristics of the collinear acoustooptical filter by ultrasound modulation, J. Commun. Technol. Electron. 43 (1998) 115–120. [10] D.L. Hecht, Multifrequency acoustooptic diffraction, IEEE Trans. Sonics Ultrason. 24 (1977) 7–18, http://dx.doi.org/10.1109/T-SU.1977.30905. [11] V.E. Pozhar, V.I. Pustovoit, Compression of ultrashort light pulses, Sov. J. Quantum Electron. 17 (1987) 509–511. [12] P. Tournois, Acousto-optic programmable dispersive filter for adaptive compensation of group delay time dispersion in laser systems, Opt. Commun. 140
180
K.B. Yushkov, V.Ya. Molchanov / Optics Communications 355 (2015) 177–180
(2009) 245–249, http://dx.doi.org/10.1016/S0030-4018(97)00153-3. [13] V.Ya. Molchanov, S.I. Chizhikov, K.B. Yushkov, Two-cascade acoustooptical dispersive delay line for ultrafast laser pulses, Quantum Electron. 41 (2011) 675–676, http://dx.doi.org/10.1070/QE2011v041n08ABEH014676. [14] V.Ya. Molchanov, K.B. Yushkov, Acousto-optics for femtosecond laser systems, AIP Conf. Proc. 1433 (2012) 72–75. http://dx.doi.org/10.1063/1.3703142. [15] V.Ya. Molchanov, K.B. Yushkov, Dispersive controlling of femtosecond laser radiation: new opportunities and developments, Acta Phys. Pol. A 127 (2015) 20–24, http://dx.doi.org/10.12693/APhysPolA.127.20. [16] V.Ya. Molchanov, K.B. Yushkov, Advanced spectral processing of broadband light using acousto-optic devices with arbitrary transmission functions, Opt.
Express 22 (2014) 15668–15678, http://dx.doi.org/10.1364/OE.22.015668. [17] K.B. Yushkov, V.Ya. Molchanov, MTF formalism for measurement of spectral resolution of acousto-optical devices with synthesized transmission function, Opt. Lett. 38 (2013) 3578–3580, http://dx.doi.org/10.1364/OL.38.003578. [18] V.Ya. Molchanov, V.B. Voloshinov, O.Yu. Makarov, Quasi-collinear tunable acousto-optic filters for systems of wavelength division multiplexing and selection of optical channels, Quantum Electron. 39 (2009) 353–360, http://dx. doi.org/10.1070/QE2009v039n04ABEH013970.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Acousto-optic filters with arbitrary spectral transmission Konstantin B. Yushkov n, Vladimir Ya. Molchanov National University of Science and Technology “MISIS”, Acousto-Optical Research Center; 4 Leninsky prospekt, 119049 Moscow, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 28 April 2015 Received in revised form 17 June 2015 Accepted 18 June 2015 Available online 24 June 2015
We demonstrate a new method of use of acousto-optic filters consisting in adaptive modifying of the broadband arbitrary spectral transmission function of the device. The essence of the method consists in tailoring a complex-valued spectral transmission function of the acousto-optic filter and further calculating RF waveform that provides the desired spectrum. The performance of the method is experimentally demonstrated both for coherent and incoherent light. & 2015 Elsevier B.V. All rights reserved.
Keywords: Acousto-optics Filter Spectrum Transmission function
1. Introduction Acousto-optical filters (AOFs) offer a number of advantages for spectroscopic analysis in various fields of applied science, technology, astrophysics, biomedicine: atomic spectrometry, molecular spectrophotometry, fluorescence spectroscopy, Raman spectroscopy, spectropolarimetry, CARS spectroscopy, hyperspectral imaging, matched spectral filtering, etc. [1–8]. AOFs are reliable smallfootprint optical devices without moving parts that makes them usable even in space [3]. Most common are acousto-optical tunable filters (AOTFs) which operate as electronically tunable monochromators driven by single-frequency RF signals [1]. AOTFs are used as the key elements of spectrometers both for hyperspectral imaging and for collimated beams [1,3–5,8]. In a typical noncollinear AOF with uniform piezotransducer, homogeneous ultrasonic field results in a sinc2 shape of the optical transmission function. The same takes place in collinear AOTFs if the ultrasonic wave were not modulated. Generally speaking, the spectral transmission function of an AOTFs at a low efficiently limit is a Fourier transform of the ultrasound field distribution along the light path and its width is inversely proportional to the length of acousto-optic interaction [1]. Since phase gratings in AOTFs are induced by radio-frequency (RF) ultrasound, the gratings with variable period and nonuniform amplitude can be obtained. Before now, most efforts aimed at improving performance of AOTFs using complicated RF controlling were concentrated in two directions: obtaining multiple n
Corresponding author. E-mail address: [email protected] (K.B. Yushkov).
http://dx.doi.org/10.1016/j.optcom.2015.06.047 0030-4018/& 2015 Elsevier B.V. All rights reserved.
transmission windows and modifying the bandshape of AOTFs. For example, phase modulation of ultrasound can be used for modifying bandshape of collinear AOTFs [9]. Multiple transmission windows of AOTFs can be obtained using multi-frequency RF control signals, however the performance of that method is restricted because of intermodulation products [10]. Typically, multifrequency operation mode of AOTFs implies a superposition of several single-frequency ultrasonic waves in the crystal, and the number of independently controlled transmission bands is of the order of 10. Collinear and quasicollinear acousto-optic diffraction is also used for controlling spectral phase of femtosecond laser emission [11–15]. Acousto-optic devices of this type are referred to either as acousto-optic programmable dispersive filters (AOPDFs) or as acousto-optic light dispersive delay lines (LDDLs). In this paper we describe a method for synthesis of arbitrary spectral transmission functions of AOFs. That gives rise to new ways of using AOFs as equalizers and shapers of continuous spectra for both coherent and incoherent light. The wavelength interval on which the arbitrary transmission function can be defined is as wide as the whole tuning range of the AOF.
2. Method description In the following analysis, we assume collinear type of acoustooptic interaction. Let us consider the problem of arbitrary transmission function synthesis as follows: any real-valued nonnegative transmission function H (λ ) is determined on a wavelength interval [λ min , λmax ], i.e. the spectral intensity at the output of the AOF equals to Iout(λ ) = Iin(λ )H (λ ), where it is assumed that
178
a
K.B. Yushkov, V.Ya. Molchanov / Optics Communications 355 (2015) 177–180
b
c
Fig. 1. Output spectra of femtosecond laser emission after the AOF: (a) rectangular transmission window, H (λ ) = const ; (b) equalization of emission spectrum, H (λ ) ∝ Iin(λ )−1; and (c) binary modulation of equalized emission, H (λ ) ∝ Iin(λ )−1M (λ ) .
H (λ ) < 1. Our method described below is a unified algorithm for calculating RF waveforms S(t) which provide the desired transmission function of the AOF without any additional assumptions about H (λ ). The procedure of calculating RF waveforms can be implemented step-by-step as follows. First, Bragg phase matching condition provides the tuning relationship of the AOF, λ(F ). From that relationship, the central frequency of the RF signal F0 and the full frequency span ΔF corresponding to the interval [λ min , λmax ] are determined. Then, the duration of the RF waveform T0 is fixed. This duration should not exceed the acoustic wave travel time Tac through the region of acousto-optic interaction. Proper phase relations between the spectral components of RF waveforms are obtained by adding spectral phase with the second-order dispersion [13,16]. Thus, the complex-valued spectrum of the RF driving waveform is expressed as
∼ S (F ) = H1/2(λ(F ))exp[iπ (F − F0)2T0/ΔF ],
(1)
The spectrum is sampled and the complex-valued RF waveform S(t) ∼ is calculated as a discrete Fourier transform of S (F ). The magnitude of S(t) stands for amplitude modulation of the RF waveform, while the argument of S(t) stands for phase modulation. The distinction ∼ between the Fourier transforms of the complex-valued function S (F ) 1/2 and the real-valued function H (λ(F )) consists in temporal arranging of the spectral components in the resulting waveform. Since in ∼ Eq. (1) the RF spectrum S (F ) is defined with the quadratic complex phase that results in quasi-linear frequency modulation of the RF waveform. In other words, the RF waveform becomes chirped due to the exponential term in Eq. (1) as it would be after passing a dispersive element. Such type of acousto-optic spectral processing treats both input and output spectra of light as continuous functions, and RF signals with continuous spectra are applied to the piezotransducer of the AOF [14]. Since the phase grating in the AOF is dynamic and travels with the speed of sound, instantaneous spectral transmission varies with time. The full spectral window [λ min , λmax ] is transmitted by the AOF only when the whole RF waveform is located in the interaction region. The usable time window for detection of the signal equals to Tac − T0 . For a variety of amplified femtosecond laser systems, the pulse repetition period is greater than typical Tac values and each pulse can be processed independently. In this case we can imply T0 = Tac that provides the best spectral resolution and maximum diffraction efficiency. On the contrary, for both incoherent light sources and continuous-wave modelocked lasers, only stroboscopic processing of emission is possible. The waveform duration T0 can be intentionally made shorter to increase time window but it results in a decrease of spectral resolution [17]. Most of the broadband light sources are characterized by spectrally dependent intensity but it is often convenient to operate equalized flat-top spectra. AOFs with synthesized broadband
transmission can be efficiently used for that purpose. Spectral transmission function of the AOF can be determined as
H (λ ) ∝ Iin(λ )−1M (λ )
(2)
where M (λ ) is any spectral modulation function. Thus, the output spectrum is proportional to the product of rectangular window function and M (λ ). In the case of M (λ ) = const , the output spectrum is equalized on the whole interval [λ min , λmax ]. In the following section, we demonstrate consecutive realization of equalization and modulation of continuous spectra.
3. Experimental results For the commissioning of the method we used a custom-made quasi-collinear AOF based on paratellurite single crystals [18]. Spectral resolution of the AOF was equal to 3 cm 1. The AOF was driven with a 625 MSa/s arbitrary waveform generator (Agilent, N8241A) together with a broadband RF power amplifier (Amplifier Research, 10W1000C). The temporal window was equal to T0 ¼51.2 μs. Fast Fourier transform was used to calculate the sampled waveforms that were uploaded to the generator. For the experiments with coherent broadband light source we used a Ti:sapphire femtosecond laser (Femtolasers, Femtosource Synergy) with a 60 nm FWHM spectrum centered at 800 nm. The following parameter values were used for calculation of RF waveforms according to characteristics of laser emission: λ min = 745 nm , λ max = 865 nm , F0 = 75 MHz , and ΔF = 13.4 MHz. We consequently applied different transmission functions H (λ ). In Fig. 1, a series of output spectra measured with a high performance optical spectrum analyzer (Agilent, 86142B) are shown. First, the transmission was defined as a rectangular window, and Iout(λ ) ∝ Iin(λ ) was obtained. This spectrum shown in Fig. 1(a) was used as a reference for equalization of output emission according to Eq. (2). Rapid decrease of intensity takes place at the borders of the selected wavelength interval. Second, the output spectrum was equalized so that Iout(λ ) = const on the interval [λ min , λmax ]. That case is plotted in Fig. 1(b). Finally, the equalized spectrum was coded with a periodic binary pattern M (λ ), as shown in Fig. 1(c). The number of sub-bands at the output spectrum in Fig. 1(c) was equal to 25. However, this number of sub-bands was not limited by the design of the electronic driving system and could be increased several times at the cost of modulation contrast [17]. That makes principal difference with multi-frequency methods of controlling AOFs which are based on choosing a fixed number of channels and adjusting their amplitude and phase. In the experiments with incoherent light, we used a xenon arc lamp. The emission was spatially filtered with a pair of fiber collimators connected with a single-mode optical patch cord. Collimation of light is necessary to maintain appropriate spectral resolution because the bandwidth of quasi-collinear AOFs broadens
K.B. Yushkov, V.Ya. Molchanov / Optics Communications 355 (2015) 177–180
a
179
b
Fig. 2. Equalization of broadband light spectrum from the source (a) and after passing the cell with methylene blue dye (b): I0(λ ) — spectrum of the Xe arc lamp after the AOF with rectangular transmission window; I1(λ ) — equalized light spectrum after the AOF; I2(λ ) — light spectrum after the dye cell; and I3(λ ) — equalized spectrum after the dye cell.
with the divergence of the input light beam [18]. The output spectrum was captured with a grating CCD spectrometer (Ocean Optics, USB2000þ ). The emission bandwidth of 150 nm was selected. The following parameter values were used for calculation of RF waveforms: λ min = 550 nm , λ max = 700 nm , F0 ¼ 117.6 MHz, ΔF = 36.9 MHz . Spectral equalization of emission is presented in Fig. 2(a). The plain spectrum of the lamp I0(λ ) was recorded using a rectangular transmission window of the AOF. That spectrum was used as a reference for equalization. At the next step, the transmission function of the AOF was given by
H1(λ ) = H0max(I0(λ ))/I0(λ ).
(3)
The magnitude of the rectangular transmission window was set as H0 ¼ 0.5. Thus the equalized output emission had higher intensity I1(λ ) than the plain spectrum transmitted by the AOF, and the condition H1(λ ) < 1 was satisfied. In the second part of this experiment, an additional cell with water solution of methylene blue dye was used for commissioning of the spectrum equalization method. The cell was placed into the optical path before the AOF. Absorption of light by the dye resulted in a modified light spectrum I2(λ ) shown in Fig. 2(b). To compensate for the deformation of the spectrum caused by the dye, the spectral transmission function of the AOF was set as
H3(λ ) = H1(λ )max(I2(λ ))/I2(λ ).
(4)
The resulted spectrum I3(λ ) has a flat top in the whole spectral region from 550 to 700 nm. Both inhomogeneity of source emission and additional spectral absorption of the dye are well equalized. This experiment demonstrated that iterative equalization of optical spectra using AOFs with arbitrary transmission can be used for step-by-step elimination of different independent factors which determine the emission spectrum.
4. Summary The proposed method for running AOFs with arbitrary transmission function by means of dispersive RF controlling is an advanced high performance technique for spectral light processing, usable both for coherent and incoherent broadband emission. That method provides sufficient advance in flexibility and adaptiveness compared to multi-frequency controlling of AOFs. The experiments with both coherent and incoherent broadband light were performed. Different application modes of AOFs were demonstrated in the experiment, such as adaptive equalization of continuous spectra and binary modulation. AOFs with sophisticated spectral
transmission functions offer new opportunities in many dedicated applications such as matched spectral filtering, spectral encoding, spectral equalization of light, and ultrashort laser pulse shaping.
Acknowledgments The work was carried out with financial support in part from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST MISiS (Project nr. K1-2014-008), from the Russian Foundation for Basic Research (Project nr. 15-07-03714).
References [1] M. Gottlieb, Acousto-optic tunable filters, in: A. Goutzoulis, D. Pape (Eds.), Design and Fabrication of Acousto-Optic Devices, Marcel Dekker, New York, 1994, pp. 197–284. [2] L. Bei, G.I. Dennis, H.M. Miller, T.W. Spaine, J.W. Carnahan, Acousto-optic tunable filters: fundamentals and applications as applied to chemical analysis techniques, Prog. Quantum Electron. 28 (2004) 67–87, http://dx.doi.org/ 10.1016/S0079-6727(03)00083-1. [3] O. Korablev, J.-L. Bertaux, A. Grigoriev, E. Dimarellis, Yu. Kalinnikov, A. Rodin, C. Muller, D. Fonteyn, An AOTF-based spectrometer for the studies of Mars atmosphere for Mars Express ESA mission, Adv. Space Res. 29 (2002) 143–150, http://dx.doi.org/10.1016/S0273-1177(01)00581-6. [4] N. Gupta, V.B. Voloshinov, Hyperspectral imaging performance of a TeO2 acousto-optic tunable filter in the ultraviolet region, Opt. Lett. 30 (2005) 985–987, http://dx.doi.org/10.1364/OL.30.000985. [5] V.E. Pozhar, V.I. Pustovoit, Adaptive acousto-optical spectrometric systems, in: IEEE Ultrasonics Symposium 2006, 2006, p. 236 http://dx.doi.org/10.1109/ ULTSYM.2006.72. [6] S.J. Leavesley, N. Annamdevula, J. Boni, S. Stocker, K. Grant, B. Troyanovsky, T. C. Rich, D.P. Alvarez, Hyperspectral imaging microscopy for identification and quantitative analysis of fluorescently-labeled cells in highly autofluorescent tissue, J. Biophotonics 5 (2012) 67–84, http://dx.doi.org/10.1002/ jbio.201100066. [7] S.I. Chizhikov, S.G. Garanin, L.V. Goryachev, V.Ya. Molchanov, V.V. Romanov, N. N. Rukavishnikov, S.V. Sokolovskii, I.N. Voronich, K.B. Yushkov, Acousto-optical adaptive correction of chirped laser pulse spectral profile in nd-phosphate glass regenerative amplifier, Laser Phys. Lett. 10 (2013) 015301, http://dx.doi. org/10.1088/1612-2011/10/1/015301. [8] V.Ya. Molchanov, S.P. Anikin, S.I. Chizhikov, K.B. Yushkov, O.Yu. Makarov, A.M. Tatarnikov, S.A. Potanin, V.F. Esipov, Acousto-optical imaging spectropolarimetric devices: new opportunities and developments, Proc. SPIE 9147 (2014) 91472T. http://dx.doi.org/10.1117/12.2055150. [9] V.I. Pustovoit, V.E. Pozhar, Controlling characteristics of the collinear acoustooptical filter by ultrasound modulation, J. Commun. Technol. Electron. 43 (1998) 115–120. [10] D.L. Hecht, Multifrequency acoustooptic diffraction, IEEE Trans. Sonics Ultrason. 24 (1977) 7–18, http://dx.doi.org/10.1109/T-SU.1977.30905. [11] V.E. Pozhar, V.I. Pustovoit, Compression of ultrashort light pulses, Sov. J. Quantum Electron. 17 (1987) 509–511. [12] P. Tournois, Acousto-optic programmable dispersive filter for adaptive compensation of group delay time dispersion in laser systems, Opt. Commun. 140
180
K.B. Yushkov, V.Ya. Molchanov / Optics Communications 355 (2015) 177–180
(2009) 245–249, http://dx.doi.org/10.1016/S0030-4018(97)00153-3. [13] V.Ya. Molchanov, S.I. Chizhikov, K.B. Yushkov, Two-cascade acoustooptical dispersive delay line for ultrafast laser pulses, Quantum Electron. 41 (2011) 675–676, http://dx.doi.org/10.1070/QE2011v041n08ABEH014676. [14] V.Ya. Molchanov, K.B. Yushkov, Acousto-optics for femtosecond laser systems, AIP Conf. Proc. 1433 (2012) 72–75. http://dx.doi.org/10.1063/1.3703142. [15] V.Ya. Molchanov, K.B. Yushkov, Dispersive controlling of femtosecond laser radiation: new opportunities and developments, Acta Phys. Pol. A 127 (2015) 20–24, http://dx.doi.org/10.12693/APhysPolA.127.20. [16] V.Ya. Molchanov, K.B. Yushkov, Advanced spectral processing of broadband light using acousto-optic devices with arbitrary transmission functions, Opt.
Express 22 (2014) 15668–15678, http://dx.doi.org/10.1364/OE.22.015668. [17] K.B. Yushkov, V.Ya. Molchanov, MTF formalism for measurement of spectral resolution of acousto-optical devices with synthesized transmission function, Opt. Lett. 38 (2013) 3578–3580, http://dx.doi.org/10.1364/OL.38.003578. [18] V.Ya. Molchanov, V.B. Voloshinov, O.Yu. Makarov, Quasi-collinear tunable acousto-optic filters for systems of wavelength division multiplexing and selection of optical channels, Quantum Electron. 39 (2009) 353–360, http://dx. doi.org/10.1070/QE2009v039n04ABEH013970.