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IEEE Lasers and Electro-Optics Society Annual Meeting (LEOS)

Code-selective RF photonic mixing for use in optical CDMA demultiplexers

Adithyaram Narasimha, Eli Yablonovitch

Published Nov 13, 2002• 3 citations
Publisher

Abstract

We demonstrate a technique that coherently frequency shifts a direct sequence encoded optical channel by mixing it with an identically encoded RF local oscillator in a dual-electrode Mach Zehnder modulator (DE-MZM). The spectral separation would reduce the effects of cumulative shot noise and speckle, and full code orthogonality is possible since the bipolar nature of the optical field is recognized by the frequency shifting process.

DOI
10.1109/leos.2001.969272

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We demonstrate a technique that coherently frequency shifts a direct sequence encoded optical channel by mixing it with an identically encoded RF local oscillator in a dual-electrode Mach Zehnder modulator (DE-MZM). The spectral separation would reduce the effects of cumulative shot noise and speckle, and full code orthogonality is possible since the bipolar nature of the optical field is recognized by the frequency shifting process.
We introduce and experimentally demonstrate a simple and reliable optical technique for matching between two periodic numerical sequences based on optical single-shot measurement of their broadband cross-correlation function in the frequency domain. Each sequence is optically encoded into the shape of the different broadband femtosecond pulse using pulse-shaping techniques. The two corresponding shaped pulses are mixed in a nonlinear medium together with an additional (amplitude-shaped) narrowband pulse. The spectrum of the resulting four-wave mixing signal is measured to provide the cross-correlation function of the two encoded sequences. For identical sequences it is the auto-correlation function that is being measured, allowing also the identification of the sequence period. The high contrast achieved here between cross-correlation and auto-correlation functions allows to determine with a very high reliability whether the two encoded sequences are identical or not. The demonstrated technique might be employed in an optical implementation of CDMA communication protocol.
Single mode laser fields oscillate at frequencies well outside the realm of electronics, but their phase/frequency fluctuations fall into the radio frequency domain, where direct manipulation is possible. Electro-optic devices have sufficient bandwidth for controlling and tailoring the dynamics of a laser field down to sub-nanosecond time scales. Thus, a laser field can be arbitrarily reshaped and in particular its phase/frequency fluctuations can be in principle removed. In practice, the time evolution of a reference laser field can be cloned to replace the fluctuations of another laser field, at a close-by frequency. In fact, it is possible to exploit a partially stabilized optical comb to perform the cloning across a large frequency gap. We realize this long-haul phase transfer by using a fibered Mach-Zehnder single-sideband modulator driven by an appropriate mix of the beat notes of the master and the slave laser with the comb.
We present a quantum mechanical model of dual parallel Mach Zehnder electro-optic modulators (DPMZMs), derived using the quantum scattering formalism [1,2]. We obtain the complete, unrestricted transformation of single-photon and coherent input states through the DPMZM architecture, including independent radiofrequency drive conditions on each internal modulator, and recover the symmetric push pull operating point as an exact special case. We numerically evaluate the resulting frequency comb structure, quantify the device sensitivity to bias error, modulation-index imbalance, and RF noise, and use the model to design a DP-MZM-based frequency-bin qubit source, reporting its fidelity as a function of these impairments.
Abstract Electro-optic modulators are widely used for the generation of optical sidebands for various applications. Here, we report on a technique enabling control of the relative amplitudes of optical sidebands generated by electro-optic modulators. The technique makes use of a phase modulator and Mach–Zehnder amplitude modulator, connected in series to break the symmetry of the sideband amplitudes. The generated optical sideband spectrum can be controlled by the two radio frequency (RF) modulation amplitudes, the attenuation level of the amplitude modulator, and the relative RF phase of the two modulations. We demonstrate near-complete suppression of one first-order sideband and with simultaneously achieving equal amplitudes of the carrier and the other first-order sideband with > 94% purity. The technique can be utilised to produce a spectrum with effectively two frequency components. This enables application in atomic physics experiments such as the minimisation of off-resonant light shifts that can limit the performance of atomic clocks and interferometers. We demonstrate operation in the near-infrared, at 795 nm, using commercial-off-the-shelf components and circumventing the need for frequency doubling of lasers in the optical communications region.
Optical CDMA technology has shown promise in optical communications, particularly in local-area optical fiber networks. We present a novel O-CDMA scheme with programmable and reconfigurable bipolar code capability using liquid crystal (LC) Spatial Light Modulators (SLMs). The key to our system performance depends on constructing a decoder that implements a true bipolar correlation using only unipolar signals and intensity detection. This has been accomplished using two unipolar correlations that can be performed optically, followed by a subtraction. In our coding system, the power spectrum of a broadband light source is encoded and decoded by programming the SLMs. The high polarization selectivity of these components coupled with the polarization rotation ability of liquid crystal elements makes switching possible with high extinction ratio and low crosstalk. Experimental results including the correlation measurements are presented. Good contrast between the autocorrelation and cross correlation values shows that a binary information symbol can be recovered by an appropriate threshold operation.
We demonstrate a photonic radio-frequency transmission system via optical fiber. Optical radio-frequency signal is generated utilizing a Mach-Zehnder modulator based on double-side-band with carrier suppression modulation scheme. The phase error induced by optical fiber transmission is transferred to an intermediate frequency signal by the dual-heterodyne phase error transfer scheme, and then canceled by a phase locked loop. With precise phase compensation, a radio frequency with high-phase stability can be obtained at the remote end. We performed 20.07-GHz radio-frequency transfer over 100-km optical fiber, and achieved residual phase noise of -65 dBc/Hz at 1-Hz offset frequency, and the RMS timing jitter in the frequency range from 0.01 Hz to 1 MHz reaches 110 fs. The long-term frequency stability also achieves 8×10(-17) at 10,000 s averaging time.