1. Introduction

While high resolution filtering is commonly commercialized in the fields of optical telecommunications or spectroscopy, wavelength tuning is usually carried out mechanically, which implies a speed limitation [1]–[3]. Fast tuning is possible with acousto-optic tunable filters (AOTF)[4]–[7] but their selectivity is not sufficient for numerous applications. The association of a diffraction grating with an acousto-optic cell gives a filter with the resolution of the former and the tuning speed of the latter. The use of a deflector followed by a diffraction grating has been proposed by Paek et al[8]. This configuration allows good resolution but the efficiency cannot be uniform on the whole tuning range of the deflector because of Bragg detuning. This is a disadvantage for instance in Dense Wavelength Division Multiplexing (DWDM) [9].

We propose a new configuration, which ensures a constant efficiency on a wide acoustic frequency range as well as the high resolution of the grating/acousto-optics association.

2. Filtering principle

 

Fig. 1. Filtering principle: schematic view of the whole device

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Figure 1 is a schematic view of the whole device. The polychromatic beam is dispersed by the diffraction grating so that the incidence angle on the AO cell depends on the wavelength. Figure 2 shows the details of the diffraction in the acousto-optic (AO) cell for three distinct incidences when it is tuned to the angle θ ₁. Figure 2(a) shows the variation of the diffraction efficiency of the AO cell versus incidence angle, when it is tuned to the angle θ ₁.The acoustic frequency is adjusted so that the chosen wavelength (λ ₁), incident at θ ₁, fulfills the Bragg condition λ ₁ = 2Λsinθ ₁ where Λ is the acoustic grating period. Depending on their incidence, the beams of different wavelengths experience full diffraction, (θ ₁) partial diffraction (θ ₂) or no diffraction (θ ₃). Figure 2(b) shows the beams wavevectors at wavelength λ ₁ and λ ₂: as the grating fringes are perpendicular to the input surface of the AO cell, only the wavelength at Bragg incidence can be diffracted symmetrically and θ′₁ = -θ ₁, (θ′₁ is the diffraction angle), whereas the beams whose incidences are close to Bragg resonance (θ ₂), are diffracted in a direction θ′ ≠ -θ ₂.

A mirror is set at the back of the cell, perpendicularly to the grating fringes, so that the forward incident and diffracted beams are reflected in directions which are symmetrical to the incident ones (see Fig. 2(b) and 2(c)). Consequently, the beam obtained by reflection of the forward diffracted beam (RD) is exactly in the same direction as the one resulting from the diffraction of the reflected incident beam (DR) and they both contribute to the global efficiency. Actually, diffraction is taking place continuously, all along the round trip of the beam in the AO cell, and this special configuration of intracavity Bragg grating[10] (with no reflectivity on the front surface) is equivalent to a double length Bragg grating, as far as diffraction efficiency is concerned. The optimum power required for the acoustic wave is, thus divided by 2.

As can be seen on Fig. 2(b) and (c), only the Bragg resonant incident beam, at θ ₁, gives rise to a reflected diffracted beam in exact contra-propagation to the incident beam. All reflected diffracted (and diffracted reflected) light (at wavelengths λ ₂ and λ ₁ on Fig. 1 and 2) can easily be retrieved using a beam splitter (see Fig. 1) or a circulator [11]. Spatial filtering by a lens and a pinhole or a fiber can then be performed to select a narrow wavelength bandwidth in the returning beam collected by the beam splitter. The reflected diffracted beam at λ ₁, in exact contrapropagation to the incident beam is centered on the pinhole, whereas the beam at λ ₂ is stopped. The light arriving far from Bragg incidence (λ ₃ on Fig. 1) is not diffracted but merely reflected by the back mirror. All the reflected light (the λ ₃ beam and the non-diffracted part of the λ ₂ beam) can be retrieved. In order to set these wavelengths parallel again, one can use a second diffraction grating, with a symmetrical orientation to the first one, with respect to the mirror normal (see Fig. 1).

 

Fig. 2. The tuning principle: (a) Diffraction efficiency of the AO cell, versus incidence angle, when it is tuned to the angle θ ₁. Depending on their incidence, the beams of different wavelengths experience full diffraction (θ ₁) partial diffraction and partial reflection (θ ₂) or no diffraction and full reflection (θ ₃). (b) Beams wavevectors (modulus 2π/λ) of the Bragg incident beam at λ ₁ and the θ ₂ incident beam at λ ₂ and the corresponding diffracted (D), reflected (R) and reflected diffracted or diffracted reflected (RD=DR) beams. K is the AO grating wavevector .(c) AO cell, when it is tuned to θ ₁ (period Λ = λ ₁/2sinθ ₁), with its back mirror

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The device efficiency is intrinsically uniform, as the selected wavelength, at Bragg resonance, whatever its value within the tuning range, always benefits from the maximum possible diffraction efficiency in the AO device. This is a major advantage and novelty, compared to the previously mentionned device[8], which, using a beam deflector, operated in the usual way (with a fixed incidence, identical for all wavelengths), is necessarily affected by Bragg detuning. The diffraction efficiency is then strongly related to the wavelength and the filtering efficiency cannot be uniform.

Neglecting the dispersion of the AO grating, the spectral bandwidth is δλ = λ/2N_R, the tuning range Δλ = λT_aΔF/2N_R and the number of resolvable wavelengths N = T_aΔF. In these expressions N_R is the number of diffraction grating periods illuminated by the beam, T_a the access time (time for the acoustic wavefront to cross the light beam) and ΔF the acoustic frequency tuning range. Since the filter spectral bandwidth essentially depends on the number of illuminated diffraction grating periods, and the number of resolvable lines on the time-bandwidth product, it is easy to optimise them separately.

3. Experimental results

The operation principle of the filter is tested using a Titanium-Sapphire laser and a laser diode tuned around 760 nm. In order to obtain a gaussian beam, the laser light is injected into a monomode fiber and collimated in a 0.5 mm waist beam. The diffraction grating has a 555 nm period and is used at grazing incidence. The acousto-optic cell used in our experiment is originally a modulator. Its active medium is a 15 mm long TeO ₂ crystal. An acoustic wave is produced by a piezo-electrical transducer excited by a radio frequency generator at a frequency ranging from 85 to 135 MHz. This longitudinal acoustic wave propagates at a speed of 4200 m.s ^-1. This gives rise to a sinusoidal index modulation with a period ranging from 31 to 49 μm. The Bragg angle associated to this acousto-optic grating is ranging from 7.5 to 12 mrad. The angular selectivity of this acousto-optic cell being equal to 8 mrad, a wide range of wavelengths is diffracted simultaneously but a good resolution can still be achieved by spatial filtering with a lens and a pinhole. Let us stress that a cell designed for this purpose would allow a much better Bragg selectivity, therefore completing the demonstration of a high performance add/drop filter. A first experiment is carried out with a 70° incidence of the beam on the diffraction grating. For five different Ti-Sa laser wavelengths, the transmission of the filter is measured as a function of the acoustic frequency. The corresponding spectra are shown in Fig. 3(a) (the efficiencies of 77% displayed in Fig. 3 and 4 do not account for the losses on the diffraction grating and the beam splitter).

Figure 3(a) demonstrates that this filter exhibits a uniformity better than ±0.2 dB on the whole tuning range, which is compatible with telecommunication applications requirements. The acoustic Full Width at Half Maximum (FWHM) is 1.6 MHz, which gives the possibility to resolve 30 different optical frequencies within the 50 MHz tuning range of the AO modulator used in our experiment. Sidelobes can be seen, due to diffraction on the AO cell aperture. This can be explained by the widening of the beam after diffraction on the grating, making its diameter comparable to the aperture. However, these sidelobes are 20 dB bellow the peaks maxima and they are quite harmless for most applications. The optical resolution of the filter was directly measured using the laser diode, its optical wavelength being varied continuously by changing the injection current. The transmission peak in Fig. 3(b) exhibits an optical FWHM of 0.075 nm, which is constant over the whole spectral range of the filter. The device tuning range shown on Fig. 3(a) is 2.2 nm.

 

Fig. 3. 70° incidence on the diffraction grating. a) diffraction efficiency as a function of acoustic frequency for 5 optical wavelengths ranging from 759.73 to 760.77 nm. b) optical spectrum for an acoustic frequency of 110 MHz.

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4. Optimization of the device

Using the same AO device it is possible to improve the number of resolvable wavelengths of the filter by increasing the incidence angle on the diffraction grating up to 85°. In this case, the beam is wider than the aperture, giving rise to lower efficiency and higher sidelobes (-12 dB). The resolution is nevertheless increased to 0.045 nm and the tuning range to 2.3 nm, corresponding to 46 resolvable frequencies (see Fig. 4(a) and 4(b)).

In order to improve the filter efficiency, it is possible to reduce the beam size without increasing the FWHM (i.e. without decreasing N_R, the number of grating periods illuminated by the beam) : a prism is inserted between the diffraction grating and the modulator (Fig. 5(a)) in order to reduce the beam width before the aperture (expressions of δλ, Δλ and N remain unchanged in this case).

 

Fig. 4. 85° incidence on the diffraction grating. a) diffraction efficiency as a function of acoustic frequency for 5 optical wavelength ranging from 759.73 to 760.77 nm. b) optical spectrum for an acoustic frequency of 110 MHz.

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The narrowing of the beam decreases the FWHM to 0.026 nm and the noise to less than -26 dB (Fig. 5(b)), the measurement accuracy being limited by the electronic digitalization. However, the tuning range is reduced to 0.37 nm and the number of resolvable frequencies to 14.

These small values are nevertheless only limited by the AO cell used in our experiment. For instance, an AO cell of TeO₂ used in longitudinal mode with a 17.5 mm aperture and a tuning range of ΔF = 150 MHz (equivalent to an octave if the center frequency is 230 MHz) can be achieved, allowing to resolve N=625 frequencies. For a DWDM application, with a 1200 gr/mm grating it is possible to obtain a bandwith of δλ=0.2 nm (25 GHz) and a tuning range Δλ of 125 nm around 1.55 μm. The peaks being three times wider at -25 dB than at 3 dB, it would be possible to use 208 channels with a maximum crosstalk of -25 dB.

Another possible improvement would be to use an AO cell with a good angular selectivity (a cell diffracting only a narrow angle range around the Bragg angle). In that case all the wavelengths that are not selected would be retrieved with negligible losses by the second diffraction grating set symmetrically to the first one (see Fig. 1) and an add/drop filter could be made.

It is interesting to note that this device could also be well adapted to make a tunable laser by placing a high gain medium on the light path and closing the cavity with a second mirror. Such a laser would be tuned quickly without any mechanical move. Due to the frequency shift of the light diffracted by the AO cell, the linewidth of such a laser would not be very small. It could nevertheless find applications in particular to DWDM testing.

 

Fig. 5. 85° incidence on the diffraction grating with a prism. a) setup. b) optical spectrum for an acoustic frequency of 110 MHz

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5. Conclusion

A filtering technique combining a diffraction grating and an AO cell has been proposed. A good resolution is obtained. The choice of the grating period, the incidence angle and the possible use of a prism make the selectivity and the tuning range easily adaptable to a specific application. What is more, the number of resolvable wavelength could be greatly increased using a cell designed for this purpose.