Acousto-Optic Tunable Filters

Acousto-optic tunable filter and its application serial (I): Acousto-optic tunable filter principle
Acousto-optic Tunable Filters (AOTF) use the anisotropic acousto-optic effect to achieve fast wavelength selection, and have been widely used in spectral analysis, spectral imaging, optical communications and other fields. AOTF is an important acousto-optic device product of CASTECH. In this issue, CASTECH will launch the Acousto-Optical Tunable Filter serial column, which will systematically introduce the basic principle, application field, product series and other related knowledge and development direction of Acousto-Optical Tunable Filter for you in three consecutive days.

Figure 1.1 Principle of operation of non-collinear AOTFs

In serial (I), the working principle of acousto-optic tunable filters is first introduced. Acousto-optic tunable filters allow fast tuning of a single wavelength or multiple wavelengths by quickly selecting specific wavelengths from a broadband light source or a multi-line laser light source. They operate by forming an ultrasonic grating inside the acousto-optic medium, which interacts with the complex coloured light and diffracts only when a narrow band of frequencies satisfies the phase matching condition. No mechanical movement is required, and the diffraction wavelength can be quickly adjusted by changing the frequency. Compared with other traditional spectroscopic devices, AOTF has the advantages of fast tuning speed, small size, large angular aperture, high resolution, etc., and has been widely used in many applications, especially in wavelength selection and optical imaging with great potential ^[6-8].
I. Selection of acousto-optic crystals
In AOTF design, several important properties determine the suitability of the material. Firstly, the acousto-optic material must be optically transparent in the wavelength range under study. If the refractive indices are very different, it is convenient to fabricate devices for non-collinear interactions AOTF. However, if the chosen material has a very small Δn but is non-centrosymmetric, it can be used for the design of co-linear devices. In addition, the material should have good acousto-optic interaction properties.
The acousto-optic figure of merit M₂ (also known as the acousto-optic quality factor) is mainly used to measure the acousto-optic interaction performance of the device. The acousto-optic diffraction efficiency is directly proportional to M₂^[9], therefore, crystals with high acousto-optic eigenvalues are crucial for the design of AOTFs, which is defined by Eq:

where n is the refractive index of the medium, P_s is the effective acousto-optic coefficient, ρ is the material density, and v_s is the speed of sound. It can be seen that the acousto-optic optima are related to the dielectric material properties and the acousto-optic operating mode.
II. Theoretical basis of acousto-optic tunable filters
The acousto-optic tunable optical filter is made based on the principle of anomalous Bragg diffraction of anisotropic acousto-optic media and is an all-solid-state novel dispersive device ^[2,5]. It is designed to convert electrical oscillations into homodyne ultrasonic oscillations by means of a piezoelectric transducer, which are transmitted to the acousto-optic crystal through a metal bonding layer between the piezoelectric transducer and the acousto-optic crystal to form ultrasonic waves ^[2]. In AOTF, optical radiation interacts with phonons to produce polarisation or direction changes of the photons. These phenomena occur only for a selected set of photons that follow certain energy and momentum criteria ^[3]. The interaction of this ultrasound with the incident light wave is utilised to enable the AOTF to selectively diffract a single or multiple wavelengths, which can be tuned by varying the applied frequency to achieve filtering ^[2,5].
The acousto-optic tunable filters produced by CASTECH use high quality tellurium dioxide ( TeO₂ ) crystals with excellent process and crystal quality for multiple wavelength conditions.
Expanding on this, the phenomenon of AOTF can be explained by a vector equation based on particle interactions ^[3]. Incident light is imagined as a beam of photons, while sound waves moving in the crystal are imagined to be phonons. Taking TeO₂ as an example, the acousto-optic interaction geometry is shown in Fig. 1.2.

Fig. 1.2 Geometry of AOTF for non-collinear interactions ^[10]

The momentum relation is expressed as:

where h is Planck's constant, K is the incident light wave vector, K_a is the acoustic wave vector, and K_d is the diffracted light wave vector ^[2,3]. Simplifying the formula gives

In anisotropic media, polarisation rotation accompanied by refractive index change during AOTF interaction ^[3]. Since the incident light is o-light and the diffracted light is e-light, we have n_i=n_o, n_d=n_e. substitution gives:

where λ is the wavelength of light, f is the frequency of sound, and v_a is the speed of sound ^[1,3,9]. In some AOTF structures, the velocity and direction of the diffracted photons vary with the refractive index difference (Δn).
By collapsing the above equation and integrating the momentum equation for the angle of incidence θi, the tuning relation for non-collinear interacting AOTFs can be obtained:

When θ_i = θ_a = 90°, it reduces to the relational equation for the covariant interaction AOTF, i.e:

In non-centrosymmetric crystalline materials, photon and acoustic wave propagation can also be aligned (quasi-collinear) by AOTF design to maximise photon-phonon interaction ^[9].
III. Performance metrics
(1) Tuning relationship
According to the tuning relation equation of non-isotropic interaction AOTF, corresponding to the determined AOTF (off-axis angle has been fixed), the required ultrasonic frequency and optical wavelength are related to the incident angle θ_i. When the angle of incidence is fixed, the ultrasonic frequency is inversely related to the wavelength, as shown in Fig. 1.3(a). When the diffracted light wavelength is fixed, different angles of incidence need to be matched with the corresponding ultrasonic frequency, the trend is shown in Figure 1.3(b), the curve at the minimum value for the best use of the angle.

Fig. 1.3 (a) Wavelength-frequency tuning relationship; (b) Incidence angle-frequency tuning relationship
(2) Spectral width and incident angle aperture

In practice, diffraction will have a certain Bragg bandwidth, which can cause phase mismatch. In this case, the diffracted intensity of the diffracted light will drop to half the centre wavelength. The bandpass is defined here as the half peak intensity width (Δλ, or FWHM) of the diffracted light. The bandwidth relation for a non-collinear interactions AOTF is given by:

where b is the dispersion constant and L is the acousto-optic action length ^[1,9].
In some applications, the AOTF requires a sufficiently large light-collecting capacity or angular aperture in order to increase the sensitivity. When the angle θ_i ± δθ_i varies within θ_i , i.e., the diffracted intensity of the diffracted light will decrease to half of θ_i , δθ_i is called the angular aperture, and the relational equation for the angular aperture of the non-collinear interactions AOTF is:

When θi = θa = 90°, the equation reduces to the covariant interacting AOTF angular aperture relations^_[1,9,10].

Figure 1.4 (a) Angular aperture-wavelength curve;
     (b) spectral bandwidth-wavelength curve

From Fig. 1.4, it can be seen that both the angular aperture and spectral bandwidth of AOTF are positively proportional to the wavelength. For AOTF, the larger the angular aperture, the better, and the smaller the spectral bandwidth, the better, so it is necessary to make a trade-off between angular aperture and spectral bandwidth according to the actual application environment.
(3) Diffraction efficiency
Under the consideration of momentum mismatch, the diffraction efficiency of AOTF is:

P_d is the power density of ultrasound, l is the length of the transducer M₂ is the acousto-optic optima of the acousto-optic crystal ^[9].

References

[1]    Chang I C. Analysis of the noncollinear acousto-optic filter[J]. Electronics Letters, 2007,11(25):617-618.
[2] Tran C D. Principles and analytical applications of acousto-optic tunable filters, an overview[J]. Talanta, 1997,45(2):237-248.
[3]    Bei L, Dennis G I, Miller H M, et al. Acousto-optic tunable filters: fundamentals and applications as applied to chemical analysis techniques[J]. Progress in Quantum Electronics, 2004,28(2):67-87.
[4]    Dakin J P. Design and fabrication of acousto-optic devices: Editors: A. P. Goutzoulis and D. R. Pape (plus S. V. Kulakov, editor of Russian Contribution) (Marcell Dekker, Inc., New York, 512 pp., ISBN 0-8247-8930-X, $165 (subject to possible change), 1994)[J]. Acta Ophthalmologica, 2013,91(Supplement s252):0-0.
[5]    Tran C D. Acousto-optic devices. Optical elements for spectroscopy[J]. Analytical Chemistry, 1992,64(20):971A.
[6] Gupta N, Dahmani R, Bennett K, et al. Progress in AOTF hyperspectral imagers: Automated Geo-spatial Image & Data Exploitation, 2000[C].
[7]    Carnahan J W, Gillespie S R. Ultraviolet Quartz Acousto-optic Tunable Filter Wavelength Selection for Inductively Coupled Plasma Atomic Emission Spectrometry[J]. Applied Spectroscopy, 2001,55(6):730-738.
[8]    HE Zhiwei, LIAN Wenliu, CHEN Ya, et al. Influence of spectral acquisition mode on the analysis of major chemical constituents of tobacco by AOTF-near infrared spectroscopy[J]. Analytical Laboratory, 2005,24(06):28-32.
[9] YU Kuan-Xin, DING Xiao-Hong, PANG Zhaoguang. Acousto-optic principles and acousto-optic devices[M]. Science Press, 2011.
[10] Yano T, Watanabe A. Acoustooptic TeO(2) tunable filter using far-off-axis anisotropic Bragg diffraction[J]. Appl Opt,1976, 15(9):2250-2258.

Acousto-Optical Tunable Filters and Their Applications Serial (II): Advantages and Applications of Acousto-Optical Tunable Filters

In the last issue of the Cas'Tech class, acousto-optic tunable filters and their applications serial (I): acousto-optic tunable filter principle we introduced in detail the acousto-optic tunable filters related to the basic theory and the main performance indicators, in this issue of the serial, we talk about the advantages and applications of acousto-optic tunable filters.
I. Advantages of acousto-optic tunable filters
AOTF has the following significant advantages over traditional spectral devices (prisms, gratings, liquid crystals):
(1) It is small in size, no mechanical movement, strong anti-interference ability;
(2) Large through-aperture, incident angle aperture and output aperture, very suitable for application to imaging; and in the tuning range of diffracted light spectral resolution and diffraction efficiency are relatively high;
(3) Stable, reliable and wide range of wavelength tuning;
(4) Simple structure, flexible and diverse mode of work, you can use the computer to flexibly select the optical wavelength of the linear scanning output, random output or multi-wavelength mixed output and so on.
(5) wavelength switching speed, usually only a few microseconds.
Second, the application of acousto-optic tunable filter
Due to the above advantages of AOTF, AOTF has been widely used in the fields of fast spectral analysis, spectral imaging processing, environmental computing, optical computing, colour information processing, coherent light source detection and wavelength-division multiplexing technology, etc . ^[1-2]. The spectral analysis instruments of AOTF have been widely used in several spectral regions in visible to infrared spectral bands, and other spectral regions such as ultraviolet are gradually developed; meanwhile, the At the same time, AOTF technology is easy to realise the miniaturization and intelligence of the instrument, and the requirements for the working environment are relatively low, so the market prospect is broad.
2.1 The field of spectroscopic analysis
In the field of atomic molecular analysis, Fulton et al, used selenite-based AOTFs for inductively coupled plasma atomic emission spectrometry ^[3]. Gillespie ^[4] et al, extended the use of this technique in the UV spectral region using co-interacting AOTFs. C.D. Tran et al, had coupled AOTFs in combination with laser diodes into an erbium-doped fibre-optic amplifier for NIR spectrophotometric analysis ^[5]. He also used a high performance liquid chromatography analytical detector fitted with a quartz AOTF to separate chlorobenzene. Using the fast wavelength conversion property of the AOTF, spectra in the 100 nm range were also obtained when the compounds were eluted.
In the field of fluorescence spectroscopy and Raman spectroscopy, AOTF can be used instead of excitation monochromator in fluorescence spectrometers for the detection of carbocyanine dyes ^[6]. Bucher et al. ^[7] reported the use of a xenon arc lamp, excitation filters and an AOTF for the selection of emission wavelengths, and detected coumarins, rhodamine dyes, paraxylene and porphyrins using the method. e.n. lewis et al. used a krypton laser and AOTF to build a small medium resolution Raman spectrometer ^[8]. With respect to the use of lasers in Raman spectroscopy and other spectroscopic measurements, AOTFs can also be used for electron tuning and amplitude modulation of dye and gas lasers. By adjusting the acoustic power of the used AOTF, fluctuations in the incident light intensity can be compensated ^[9].
In 2011, a tellurium dioxide-based non-collinear AOTF near-infrared point spectrometer (point spectrometer) was developed at New Mexico State University and NASA/Goddard Space Flight Center to study biological samples and organic biotraces from the planets of the solar system. The spectral range is 1.7-3.4 μm, the spectral resolution is 4-12 nm, and the effective aperture is 10 mm×10 mm. Figure 2.1 shows the three-dimensional effect of this instrument ^[10-11].

Fig. 2.1 Stereoscopic diagram of a point spectrum analyser ^[10-11]

2.2 Spectral Imaging
AOTF is used in spectral imaging to obtain spatial, spectral and polarisation information about the target. Because of its wavelength tuning range from the ultraviolet band to the long-wave infrared region and its fast wavelength switching speed, AOTF, as a new force in spectroscopic components, has been developed rapidly in recent decades, and it has become an indispensable core device in the spectral imaging applications such as imaging sensing systems, space and earth observation systems and biological cell observation systems. It has become an indispensable core device for imaging sensors, space and earth observation systems, and biological cell observation systems. However, AOTFs may suffer from image degradation due to incomplete collimation of the incident light and image variation with wavelength. These effects can be compensated for by masking and mathematical data processing ^[12]. H.T. Skinner et al. developed an imaging system using an AOTF and a spatially coherent ultrafine fibre optic probe, which is capable of acquiring Raman images without clear line of sight ^[13]. The instrument was tested by acquiring a mixture of images of highly scattering samples with a resolution of about 4 mm.

Table 2.1 Main technical specifications of Chang'e-3 Visible-Infrared Spectral Imager ^[14-15]
 

Figure 2.2 Chang'e-3 mid-infrared spectral imager system programme ^[14-15]

2.3 Ultrafast laser field
In ultrafast laser systems, the group delay time dispersion from a series of amplification structures in the system widens the pulse width of the output signal due to gain narrowing and uncompensated phase errors. It therefore needs to be compensated for to optimise the performance of the system. Dispersion compensation is usually carried out by incorporating prism pairs ^[16], interferometers or dispersion mirrors in the laser cavity, but these methods can only compensate for part of the dispersion and are not programmable. In order to further reduce the optical signal pulse width, programmable devices that can compensate for a large amount of dispersion over a wide bandwidth range are needed, and at the same time can provide amplitude shaping for high contrast. 1993 Fermann proposed an acousto-optic tunable filter to shape the pulse ^[17]. 1997 Tournois proposed a programmable acousto-optic dispersion filter (AOPDF) ^[18], with a wide range of dispersion compensation . with a large range of dispersion compensation, and the first experimental demonstration of Ti:S femtosecond pulse compression in the 100 fs range was achieved with a TeO₂ acousto-optic filter, the acousto-optic filter used was based on a quasi-coherent design. Subsequently, scientists have carried out a series of studies on programmable acousto-optic dispersion filters ^[19-21], as shown in Fig. 2.3 for an optical amplification system based on programmable acousto-optic dispersion filters.

Figure 2.3 Optical amplification system based on programmable acousto-optic dispersion filters

Reference

[1] He ZH,Lian WL and Wu MJ. Determination of Tobacco Constituents with Acousto-optic Tunable Filter [J].Journal of Near Infrared Spectroscopy.2006,14(1):45-50.
[2] Takabayashi K,Takada K and Hashimoto N. Widely(132nm) Wavelength Tunable Laser using a Semiconductor Optical Amplifier an an Acousto-optic Tunable filter[J].Electronics Letter. 2004, 
[3] Horlick G, Fulton G. AOTFs as Atomic Spectrometers: Basic Characteristics[J]. Applied Spectroscopy, 1996,50(7):885-892.
[4] Carnahan J W, Gillespie S R. Ultraviolet Quartz Acousto-optic Tunable Filter Wavelength Selection for Inductively Coupled Plasma Atomic Emission Spectrometry[J]. Applied Spectroscopy, 2001,55(6):730-738.
[5] Tran C D, Gao G H. Characterization of an erbium-doped fiber amplifier as a light source and development of a near-infrared spectrophotometer based on the EDFA and an acoustooptic tunable filter.[J]. Analytical Chemistry, 1996,68(13):2264-2269.
[6] Tarazi L, George A, Patonay G, et al. Spectral characterization of a novel near-infrared cyanine dye: a study of its complexation with metal ions[J]. Talanta, 1998,46(6):1413-1424.
[7] Bucher E G, Carnahan J W. Characterization of an Acousto-optic Tunable Filter and Use in Visible Spectrophotometry[J]. Applied Spectroscopy, 1999,53(5):603-611.
[8] Lewis E N, Treado P J, Levin I W. A Miniaturized, No-Moving-Parts Raman Spectrometer[J]. Applied Spectroscopy, 1993,47(5):539-543.
[9] Tran C D, Furlan R J. Electronic Tuning, Amplitude Modulation of Lasers by a Computer-Controlled Acousto-optic Tunable Filter [J]. Applied Spectroscopy, 1992,46(7):1092-1095.
[10] Chanover N J, Glenar D A, Mcadam A, et al. An AOTF-LDTOF spectrometersuite for In Situ organic detection and characterization[C]. IEEE Aerospace Conference, 2011, 41: 1-13.
[11] Voelz D G, Xiao X, Tawalbeh R, et al. Infrared acousto-optic tunable filter point spectrometer for detection of organics on mineral surfaces[J]. 
[12] Gupta N, Dahmani R, Bennett K, et al. Progress in AOTF hyperspectral imagers: Automated Geo-spatial Image & Data Exploitation, 2000[C].
[13] Skinner H T, Sharma S K, Angel S M, et al. Remote Raman Microimaging Using an AOTF and a Spatially Coherent Microfiber Optical Probe[J]. Applied Spectroscopy, 1996,volume 50(8):1007-1014.
[14] Dai Shuwu, Wu Ji, Sun Huixian, et al. Payload of Chang'e-3 rover[J]. Journal of Space Science, 2014, 34(3): 332-340.
[15] He Z P, Wang B Y, Lv G, et al. Visible and near-infrared imaging spectrometer and its preliminary results from the Chang’E 3 project[J].Review of Scientific Instruments, 2014, 85(8): 083104.
[16] Backus S , Durfee C G , Murnane M M ,et al.High power ultrafast lasers[J].Review of Scientific Instruments, 1998, 69(3):1207.
[17] Fermann M , Silva V D , Smith D A ,et al.Shaping of ultrashort optical pulses by using an integrated acousto-optic tunable filter[J].Ol/18/18/ol Pdf, 1993.
[18] Tournois P .Acousto-optic programmable dispersive filter for adaptive compensation of group delay time dispersion in laser systems[J].Optics Communications, 1997, 140(4-6):245-249.
[19] Verluise F.Huignard JP.Tournois P.Migus A.Laude V.Arbitrary dispersion control of ultrashort optical pulses with acoustic waves[J].Journal of The Optical Society of America B-optical Physics, 2000, 17:138-145.
[20] Frédéric Verluise, Laude V , Cheng Z ,et al.Amplitude and phase control of ultrashort pulses by use of an acousto-optic programmable dispersive filter: pulse compression and shaping.[J].Optics Letters, 2000, 25(8):575-7.
[21] Pittman M ,S. Ferré, Rousseau J P ,et al.Design and characterization of a near-diffraction-limited femtosecond 100-TW 10-Hz high-intensity laser system[J].Applied Physics B, 2002, 74(6):529-535

Acousto-Optical Tunable Filters and Their Applications Serial (III): CASTECH and Acousto-Optical Tunable Filters
In Acousto-Optical Tunable Filters and Their Applications Series (II): Advantages and Applications of Acousto-Optical Tunable Filters, we learnt about the applications of acousto-optic tunable filters in the fields of spectral analysis, spectral imaging, and ultrafast lasers. Facing different applications and demands, different types and performance parameters of acousto-optic tunable filters are needed to fulfil their functions.
CASTECH's acousto-optic tunable filters are made of independently grown TeO₂ crystals LiNbO₃ crystals, which are characterised by low absorption, low loss, good uniformity, large size and high damage threshold. Precise product design, high-quality pressure welding and high-damage coating processes, and demanding processing specifications can guarantee excellent product consistency.

Figure 3.1  TeO₂  crystals grown by CASTECH

Fig. 3.2  LiNbO₃ crystals grown by CASTECH

There are two types of acousto-optic tunable filters (AOTFs) from CASTECH: non-collinear and quasi-collinear. Both the non-collinear and quasi-collinear schemes have excellent wavelength selection, with the non-collinear AOTF featuring a large incident angle aperture, which can be applied to spectral imaging analysis, and the quasi-collinear AOTF controlling the spectral phase to achieve femtosecond pulse compression. As shown in Table 3.1, CASTECH's AOTF product indicators.
 

Table 3.1 CASTECH's AOTF Product Indicators

Figure 3.3: CASTECH's (a) quasi collinear AOTF and (b) non collinear AOTF

Through extensive research and technological accumulation, CASTECH is able to provide acousto-optic tunable filters with a wide bandwidth range, and has the ability to customize special acousto-optic devices, forming a research system including device design, performance simulation, device packaging, and performance testing. According to different application requirements, product performance can be optimized and upgraded to improve customer satisfaction.
In the future, CASTECH will continue to strive to introduce better performing acousto-optic devices to the market, share more research and development achievements of acousto-optic technology with customers, and jointly promote the iteration of laser application technology, making lasers realize more possibilities.

Housing dimensions (mm)：

                          E18                                                                       C60