A real time tunable electrooptic rf filter is demonstrated using a multichannel total internal reflection modu-lator to spatially filter the output of a Bragg cell. The demonstrated filter has a bandwidth to resolutionratio of 28. This ratio can potentially be increased to over 5000 with a like number of individually resolvedfrequencies.
An rf spectrum analyzer has been proposed anddemonstrated earlier.1 -3 These devices rely on elec-trical signal processing of the photodetector output todetermine the frequencies. The signal processing canbe as simple as a time delay between signals such as ina CCD array. However, this processing takes time andlimits the response of the system. The use of an elec-trooptic rf filter can be used to speed up this processingto real times. The rf filter is especially useful in the caseof a strong jamming noise masking a weak signal. Wedemonstrate a real time tunable rf filter. This filtersystem uses an acoustooptic Bragg cell to convert anelectrical signal to an optical signal and a multichanneltotal internal reflection (TIR) modulator to filter theoptical signal. An acoustic pulse imaging configurationallows the system to recover temporal information withhigh resolution.
The experimental setup is shown in Fig. 1. A polar-ized He-Ne laser is incident upon an acoustooptic (AO)Bragg cell. Appropriate optics are used to create acircularly polarized beam with approximate beam di-ameters at the Bragg cell of 1 mm sagitally by 4 mmtangentially. The Bragg cell is used to convert anelectrical signal into an optical signal. Because thediffracted angle of the optical beam is linearly propor-tional to the driving frequency of the Bragg cell, anelectrical signal to the Bragg cell of many frequenciesis converted to a like number of angularly separatedoptical beams. Each beam is an optical analog of theelectrical signal at a particular frequency. The Fourierlens transforms the angularly diffracted first-orderbeams exiting the AO cell into spatially well-separated
The authors are with Xerox Corporation, 1500 South ShamrockAvenue, Monrovia, California 91016.
beams at the multichannel TIR. A quarterwave plateis used to rotate the polarization to the required lineartangential polarization required by the TIR modulator.The TIR modulator is aligned so that each beam is in-cident to a unique channel. Application of a voltage toa TIR channel diffracts the light of only that channel.The first-order diffracted light is focused onto a pho-todiode. Because the photodiode is at the image planeof the Bragg cell, the photodiode output is a temporalanalog of the electrical signal fed to the cell. The res-olution of the imaged pulses is enhanced by use of a slitbefore the photodiode.
The Bragg cell is a TeO2 shear wave device and re-quires a circularly polarized optical beam for optimumAO bandwidth. The Bragg device is driven by an am-plified signal from two oscillators generating rffrequencies, f and f2, which could be independentlymodulated at different rates via two pulse generatorsand balanced mixers. The frequencies, f and f2, arechosen so that their respective first-order beams cor-respond to the channels of the TIR modulator. Themodulation rates are also adjusted so that the +1 and-1 optical sideband beams also fall in the center of theTIR channels. When using lower data rates of up to 100kHz, the sidebands share the same TIR channel as thecentral frequency component.
The TIR modulator is a multichannel version of thedevice demonstrated by Nishiwaki et al.
4 and Scibor-Rylski.5 The multichannel TIR modulator was fabri-cated from x-cut LiTaO3. The device had eight chan-nels with a center-to-center spacing of 1.25 mm. Eachchannel consisted of nine finger pairs of interdigitatedelectrodes at a pitch of 20 gzm. Each channel could beindependently modulated. The application of a voltageto a channel creates a phase grating which scatters thelight around the stop. If the channel-to-channelspacing corresponds to a frequency difference of Af, thebandwidth of the TIR filter is Nf, where N is thenumber of channels. The resolution of the filter isAf W/S, where W is the channel width and S is thechannel separation. The frequency difference Af is
Fig. 2. Output of the rf filter for two carrier frequencies of compa-rable amplitude modulated at different rates: (a) the combined
signal; (b) top trace f 1; bottom trace f2-
J - ORDER
c'L. LENS
ER;FORM LENS TIR
-1 +1
MIRROR
ER / STOPSFORM LENS
for the tunable rf filter.
determined by the Bragg carrier frequency and theoptical arrangement. A figure of merit for an acous-tooptic Bragg cell is the time-bandwidth product or thenumber of resolvable spots. Because an acoustoopticdevice has no inoperative areas, the bandwidth-to-res-olution ratio is equal to the number of resolvable spots.This TIR filter has inoperative areas due to the sepa-ration distance between channels so that the band-width-to-resolution ratio (NS/W) is greater than thenumber of resolvable spots (N). The TIR filter usedin this paper has eight resolvable spots and a band-width-to-resolution ratio of 28. A TIR filter is capableof larger bandwidths and/or finer resolution. A morecomplex multichannel TIR modulator6 7 with 5376 in-dividually addressable electrodes has a bandwidth-to-resolution ratio of 5376. A typical bandwidth-to-resolution ratio for a good spectrum analyzer is of theorder of 1000. The TIR filter can match the resolutionof a good spectrum analyzer.
The rise time of the TIR modulator is usually limitedby the rise time of the driver. The device in this paperhas a channel capacitance of 100 pf corresponding to a5-nsec rise time. The driver must be capable of deliv-ering up to 40 V. The rise time of the system is limitedby the Bragg cell rather than the TIR modulator forcarrier frequencies of less than the order of severalhundred megahertz. The use of an acoustic pulseimaging system has eliminated the limitations due tothe dimensions of the Bragg diffracted optical beam.With a sufficiently fast Bragg cell, this rf filter systemis capable of rise times of the order of 10 nsec.
Three sets of experiments were done to demonstratethe capabilities of the rf filter. In all cases two signalswere fed into the Bragg cell. The first case used twosignals of comparable strengths but modulated at dif-ferent frequencies /i = 25 kHz and f2 = 100 kHz. Thesecond case used a strong fi and a weak f2 signal mod-ulated at 25 and 100 kHz, respectively. The first twocases use a dc bias on the TIR modulator. The thirdcase demonstrates the tunability of the filter. The two
signals are modulated at frequencies that are not inte-gral multiples of each other while the TIR modulatoris programmed to alternately pass /i and 2 at a thirdfrequency.
Shown in Fig. 2 are oscillograms taken for the firstcase. Figure 2(a) shows the combined signal, /, + 2,when a dc bias is applied to the TIR channels corre-sponding to/ilandf/2. Figure 2(b) shows the individualsignals when a dc bias is applied only to the TIR channelcorresponding to that signal. Either signal can betransmitted or filtered.
(a)
A better idea of the dynamic- range capabilities isshown in Fig. 3. Figure 3(a) shows the combined signalfle + 2. Figure 3(b) shows only the strong signal , and
Fig. 3(c) shows only the weak signal 2. The dynamicrange, defined as 10 X lo(Ima/Imtn), is 20 dB and islimited only by the sensitivity of the detector that wasused (a silicon photodiode). Use of a more sensitivedetector on other TIR modulators has shown cross talkbetween channels to be less than -30 dB. It should benoted that the nonsinusoidal form of the modulated 2in Fig. 3(c) is real. Frequencies and 2 could not bemodulated in a truly independent fashion. Because theBragg cell was operated near the peak efficiency of/il,the amount of light deflected by 2 was strongly affectedby . When /l was turned on, most of the light was (b)deflected by . When/i was off, more light was avail-able to bedeflected byf/2. The strong signal/i robbedpower from the weak signal 2 when /i was turned on.This effect could have been reduced if the Bragg cell wasoperated at a less efficient level so that the deflectedbeam would be much less intense than the zeroth-orderbeam.
Tunability of an rf filter allows one to follow a chirpedsignal. Shown in Fig. 4 is the output of the rf filterdemonstrating tunability. Figure 4(a) shows thecombined signal / + 2, Fig. 4(b) shows /i, Fig. 4(c) 1, Ml'Xshowsf/2, and Fig. 4(d) shows alternately/ilandf/2. The 'YA' (i rate at which the filter alternates between /i and 2 isshown in the top trace of Fig. 4(d). As in Fig. 3, the (C)signals/il and 2 are not completely independent of eachother resulting in the nonuniform pulse amplitudes. Fig. 3. Output of the rf filter for two carrier frequencies of different
amplitudes: (a) the combined signal; (b) If1; () 2.
We have demonstrated the ability of a TIR modulatorto act as a tunable rf filter. This filter is fast enough(rise times of the order of 10 nsec) to be a real time de-vice with a dynamic range >20 dB. The bandwidth-to-resolution ratio can be as large as 5000. This system The authors wish to acknowledge the help of Cirilocan be used to demultiplex a complex signal or pick out Castro, Henrietta Guerrero, Ed Harrigan, Malcolma weak signal from a neighboring strong jamming Rector, and Russell Rauch in fabricating the Bragg cellsignal. and the TIR modulator.
Fig. 4. Output of the rf filter when tuned: (a) the combined signal, f1 + f2; (b) f1; (c) f2; (d) f1 and f2 alternately at the rate defined in thetop trace of (d).
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2. T. R. Ranganath, T. R. Joseph, and J. Y. Lee, in TechnicalDigest,Third International Conference on Integrated Optics and OpticalFiber Communication (Optical Society of America, Washington,D.C., 1981), paper WH3.
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