Preetpaul S. Devgan

Long-Haul Analog Photonics

A paper on long-haul analog photonics is presented using theory and experimental results. Various analog fiber-optic modulation formats are reviewed, including intensity modulation with direct detection, phase modulation with interferometric demodulation, and suppressed-carried techniques. Modulation linearization methods are reviewed. The limitations of and requirements for photodiode detectors are described.

Phase Modulation With Interferometric Detection as an Alternative to Intensity Modulation With Direct Detection for Analog-Photonic Links

We analyze the performance of analog-photonic links employing phase modulation with interferometric demodulation and compare it to intensity-modulation direct-detection links. We derive expressions for RF gain, RF noise figure, compression dynamic range, and spurious-free dynamic range for both architectures. We demonstrate theoretically and experimentally that phase-modulated links can outperform intensity-modulated links over substantial frequency ranges.

Analysis of an Analog Fiber-Optic Link Employing a Low-Biased Mach–Zehnder Modulator Followed by an Erbium-Doped Fiber Amplifier

In this paper, a complete analysis of an analog fiber-optic link employing a low-biased Mach-Zehnder modulator followed by optical amplification with an erbium-doped fiber amplifier is presented. The expressions for RF gain, RF noise figure, second- and third-order intercept points, and spurious-free dynamic range are derived. Experimental data are employed to support the analytical results, and the optimal bias points for the metrics are demonstrated.

Improvement in the Phase Noise of a 10 GHz Optoelectronic Oscillator Using All-Photonic Gain

We have investigated the improvement in the phase noise of a 10 GHz optoelectronic oscillator using all-photonic gain as compared to using an electronic amplifier in the cavity. The optoelectronic oscillator achieves the necessary RF gain for oscillation by using the carrier-suppression technique of a low-biased Mach-Zehnder modulator (MZM) followed by optical amplification. The measured RF gain due to this all-photonic technique is as high as 15 dB and matches well with theoretical predictions. The phase noise of the generated 10 GHz signal is at least 10 dB lower than the signal from the same oscillator using an electronic amplifier. The improvement in the phase noise is due to the lower RF noise figure of the all-photonic gain process as compared to the electronic amplifier configuration.

Photodiode linearity requirements for radio-frequency photonics and demonstration of increased performance using photodiode arrays

The impact that photodiode nonlinearities have on intensity-modulation direct-detection analog photonic links is described. The second- and third-order linearity requirements for photodiodes, quantified in terms of output intercept points, are calculated. Measured data at 1 GHz for a high-current (60-mA) partially-depleted-absorber photodiode are presented. A four-photodiode 26-mA array at 5 MHz is demonstrated to achieve modulation-limited third-order performance and a second-harmonic output intercept point of 84 dBm.

Detecting Low-Power RF Signals Using a Multimode Optoelectronic Oscillator and Integrated Optical Filter

We have assembled and characterized a multimode optoelectronic oscillator with integrated optical filter for detecting low-power radio-frequency (RF) signals. The system can selectively amplify RF signals from 1 to 6 GHz. The input signals can be as low as - 83 dBm with a compression dynamic range of 72 dB. Using an integrated silicon optical filter facilitates channelization of the amplified RF signal from 1 to 3 GHz by reducing the gain for signals above 3 GHz. Future system improvements are discussed.

Even-order harmonic cancellation for off-quadrature biased Mach-Zehnder modulator with improved RF metrics using dual wavelength inputs and dual outputs

We present a technique using a dual-output Mach-Zehnder modulator (MZM) with two wavelength inputs, one operating at low-bias and the other operating at high-bias, in order to cancel unwanted even-order harmonics in analog optical links. By using a dual-output MZM, this technique allows for two suppressed optical carriers to be transmitted to the receiver. Combined with optical amplification and balanced differential detection, the RF power of the fundamental is increased by 2 dB while the even-order harmonic is reduced by 47 dB, simultaneously. The RF noise figure and third-order spurious-free dynamic range (SFDR(3)) are improved by 5.4 dB and 3.6 dB, respectively. Using a wavelength sensitive, low V(pi) MZM allows the two wavelengths to be within 5.5 nm of each other for a frequency band from 10 MHz to 100 MHz and 10 nm for 1 GHz.

Detection of Low-Power RF Signals Using a Two Laser Multimode Optoelectronic Oscillator

We present a two laser, multimode optoelectronic oscillator (OEO) for detecting unknown low-power RF signals. By switching between one or two lasers, the OEO cavity will become sensitive to an interleaved set of frequencies. The system has an input sensitivity of -78.4 dBm, which is only 14.2 dB higher than the thermal limit for a 140-MHz bandwidth. In addition, mode spacing has been increased to 100 MHz, a 20 times increase from previous demonstrations.

Cascaded Noise Penalty for Amplified Long-Haul Analog Fiber-Optic Links

The noise penalty (NP) metric is useful in long-haul analog link design, as it predicts degradation of the RF noise figure (NF), 1-dB compression dynamic range (CDR), and the spurious-free dynamic range (SFDR) for a single-amplifier link. We extend the NP theory to predict the cascaded NP of a multiamplifler fiberoptic link from the NP due to each of the individual amplifiers. A cascaded NP expression is derived and compared to experimental results from a 60-km link consisting of three amplifiers. The measured NP of 11.8 dB for the link matches well with our theoretical prediction. The link gain of -7 dB, RF NF of 31 dB, 1-dB CDR of 148 dB ldrHz, and of 108 dB ldrHz2/3 are also measured and matched to predicted values for the measured NP.

Cancellation of photodiode-induced second harmonic distortion using single side band modulation from a dual parallel Mach-Zehnder

We have theoretically and experimentally investigated using a dual parallel Mach-Zehnder modulator (DP-MZM) in an RF photonic link to cancel the second harmonic distortion due to the photodiode. Biasing the DP-MZM for single sideband modulation, the second harmonic generated by the DP-MZM can be set out of phase with the second harmonic generated at the photodiode. We measure the output intercept point of the second harmonic distortion of the link to be 55.3 dBm, which is an improvement of over 32 dB as compared to only the photodiode.