to the modulator, the optical electric field is: .... David J. Williams, "Nonlinear Optical Properties of Guest-Host Polymer Structures," Nonlinear Optical Properties.
Lightwave transmission of multiple television signals using an organic polymer electro-optic phase modulator Barton A. Smith, M. Jurich, W. E. Moemer, W. Volksen, Margaret E. Best, W. Fleming, J. D. Swalen, and G. C. Bjorklund IBM Research Division Almaden Research Center, Dept. K95 650 Harry Road, San Jose, CA 95120
ABSTRACT Phase modulation and optical heterodyne detection were used to transmit and receive six cable television signals at 832 nm using a new polymeric electro-optic material. The electro-optic slab waveguide layer of the phase modulator consisted of a spin-coated, amorphous polyimide film doped with a nonlinear chromophore which had been poled in an electric field. The cladding layers, which confined the optical field within the waveguide, were composed of a photo-crosslinked acrylic polymer. Our television transmission system serves as a stringent test of the utility of the electro-optic polymer for lightwave data transmission.
1. INTRODUCTION Poled amorphous polymers with second-order optical nonlinearity show great promise for use in electro-optic devices. The materials themselves are inexpensive, and optical waveguides can be fabricated by well established spin casting and photolithographic processes. Organic polymers have much lower RF dielectric constants than nonlinear inorganic crystals, allowing higher bandwidth and lower drive power in traveling-wave devices." 2 Thus a good electro-optic polymer will have many potential applications, such as in phase modulators for optical phase-shift-keyed lightwave data transmission systems.
Current development efforts seek to improve upon available materials by: 1) Increasing the linear electro-optic coefficient, 2) Decreasing optical transmission loss, and 3) Increasing the lifetime at operating temperature of the poling induced molecular orientation necessary for the electro-optic effect.3 In addition to measuring the fundamental optical and electrical properties of the materials, it is necessary to test them in a prototype application so that any functional deficiencies in the properties of the material can be discovered early in the development cycle. Because equipment which can generate and receive television signals is readily available and inexpensive, we have chosen lightwave transmission of cable television signals with optical heterodyne detection as a functional test of our materials under development. The transmission of subcarrier multiplexed (SCM) amplitude modulation vestigial sideband (AMVSB) broadband TV signals is a demanding application requiring high analog signal-to-noise ratio and low intermodulation distortion.4 Observation of the transmitted picture and sound provides an easy and sensitive means, although somewhat subjective, of judging the quality of the transmitted signals. Heterodyne detection allows the potential utility of our electrooptic material to be demonstrated in a simple phase modulator, rather than the amplitude modulator necessary for intensity detection. The ease with which this working device was fabricated is in itself an argument for the use of polymer films in electro-optic devices.
2. TV TRANSMISSION Phase modulation and optical heterodyne detection were used to transmit and receive the analog RF cable TV signals. Figure 1 is a block diagram of the apparatus. Five cable TV signals were available on an in-house broadband cable sys-
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tern. A sixth was produced by a NTSC video camera and standard cable TV RF modulator. The RF signal comprising a block of 6 TV channels (7—12) in the frequency range 174 to 210 MHz was amplified to a total power of 1 W and applied to the electrodes of the modulator. The 832 nm light was provided by a Coherent model 899 Ti:Sapphire laser with intra-cavity etalon with a specified linewidth of less than 10 MHz. The linearly polarized light was attenuated, then split into two beams, one of which passed through the modulator and the other through an acousto-optic (AO) frequency shifter driven at 120 MHz. The up-shifted Local Oscillator (LO) beam was recombined with the phase-modulated beam at the silicon photodiode. The LO beam had a power of 4.4 mW, and the modulated beam 1.2 mW. The frequency difference components of the detected photocurrent resulting from the optical heterodyne detection appeared between 54 and 90 MHz, channels 1—6. A low pass filter was used to remove the 120 MHz beat signal. A standard TV tuner and monitor were used to observe the output signal quality, and an RF spectrum analyzer provided quantitative and objective data. Figure 2 is a schematic representation of the relationships between the frequencies present in the electrical and optical signals. Another measure of modulator performance can be obtained by substituting a single frequency RF source for the cable TV signals.5 For single-tone electro-optic modulation at frequency m produced by application of a sinusoid with peak voltage V to the modulator, the optical electric field is: (1)
E=E0exp[i ((k)C t + m Sffl(Omt )] Here E0 is the field amplitude, w is the optical frequency, and the modulation index m is given by
tL3
m=——n r V
Ad
(2)
with A the optical wavelength, L the modulator length, d the electrode spacing, n the refractive index, and r33 the relevant electro-optic coefficient. Mixing of this signal with the AO-shifted beam at
signals in the detected photocurrent, at frequencies A
yields (for m
