Large and tunable optoelectronic chromatic dispersion in PIN-type photodiodes.

It is known that PN-type photodiodes possess high optoelectronic chromatic dispersion (OED). Here we present a theoretical and experimental study of OED in PIN-type photodiodes. Applying the modulation phase-shift technique, a Ge PIN photodiode exhibits ∼0.5 deg/nm phase-shift sensitivity at 10 MHz modulation, corresponding to a dispersion of 1.4 ×109ps/(n m ×k m), many orders of magnitude larger than high-dispersion optical materials such as chalcogenide glass. A striking feature of the PIN device is the ability to tune the amount and sign of the OED through the bias voltage. Electronic tuning between -0.8 deg/nm and +0.5 deg/nm is shown. The PIN photodiode is an on-chip device possessing significant tunable dispersion for applications in optical sensing and spectroscopy.

Large optoelectronic chromatic dispersion in PN-type silicon photodiodes and photovoltaic cells.

Optoelectronic chromatic dispersion (OED) is a significant source of effective chromatic dispersion in photodiodes. We present an experimental and theoretical study of OED in PN-type Si photodiodes and photovoltaic cells and report on a very large effective chromatic dispersion in these devices. As measured with the modulation phase-shift technique at a frequency of 4 kHz for these slow devices, the OED spectral sensitivity for a commercial Si photodiode is approx. 0.02 deg/nm in the 720-850 nm wavelength band and increases to 0.25 deg/nm at λ = 1µm. For a Si photovoltaic cell, the OED is approx. 0.09 deg/nm in this spectral region. These values translate into an effective chromatic dispersion parameter of approx. 1012ps/(n m ×k m) for these sub-millimeter device lengths, which is over eight orders of magnitude larger than high-dispersion materials such as chalcogenide glass. The enormous dispersion in these sub-millimeter sized silicon-based devices can be utilized for on-chip optoelectronic sensors such as wavelength monitoring and spectroscopy. The substantial OED of photovoltaic cells can be utilized for the characterization and optimization and new applications for optical sensing with these self-powered devices.

Multitasking FBG sensors for condition monitoring with a wideband (DC-MHz) interrogation system.

We demonstrate diagnosis of several machine-condition failures using wide-frequency-band interrogation of fiber Bragg grating (FBG) sensors. In collaboration with Israel's national water company Mekorot Ltd., a scaled-down version of a semi-submerged pumping system was constructed. By monitoring broadband signals from DC to ultrasound (>M H z), at different points of the engine and the submersed pump, the system was able to diagnose incipient cavitation, faulty bearings, and submerged dynamic water-level measurements. In addition, a metal embedded FBG sensor was investigated, revealing the potential of using FBGs in applications where bonding is problematic such as bearing housing. These results prove that wideband data acquisition, together with advanced analytics, could open a variety of new applications in the fields of structural health and machine-condition monitoring.

Spectral sensor of the ethanol concentration in water based on photodiode optoelectronic chromatic dispersion.

Optoelectronic chromatic dispersion (OED) of a PN-type germanium photodiode is used for spectral sensing of ethanol concentration in water. A concentration sensitivity of 70 ppm is achieved. Spectral sensors based on OED in PN-type photodiodes can serve as low-cost on-chip devices for optical spectroscopy.

Femtometer-resolved wavelength monitor based on photodiode optoelectronic chromatic dispersion with RF phase-shift amplification.

The spectral sensitivity of photodiode-based optoelectronic chromatic dispersion is enhanced by phase-shift amplification using RF interferometry. With phase-shift amplification of G=4⋅104, a peak phase-shift sensitivity of Δθ = 27 deg/pm is achieved, corresponding to a spectral resolution of Δλres = 1 fm. This all-electronic solid-state technology can serve as an on-chip inexpensive technique for femtometer-resolved wavelength monitoring.

Optoelectronic chromatic dispersion and wavelength monitoring in a photodiode.

The optoelectronic process of light absorption and current formation in photodiodes is shown to be a significant source of optoelectronic chromatic dispersion (OED). Simple design rules are developed for fabricating a photodiode-based dispersion device that possesses large, small, zero, and either positive or negative OED. The OED parameter is proportional to a spectrally-dependent absorption term α-1dα/dλ . Silicon-based devices are predicted to display significant OED throughout the near IR, while Ge and InGaAs have high OED in the C- and L-bands and 1650 nm region, respectively. The OED of a commercial Ge PN photodiode is measured to be 3460 ps/nm at 1560 nm wavelength with 500 kHz modulation, demonstrating 8 pm spectral resolution with the phase-shift technique. Temperature-tuning of the OED in the Ge photodiode is also demonstrated. The ubiquitous photodiode is a tunable OED device, with applications in high-resolution optical spectroscopy and optical sensing.

Optoelectronic chromatic dispersion in germanium PN photodiodes: wavelength monitoring and FBG interrogation.

Optoelectronic chromatic dispersion (OED) has recently been shown to be a significant source of chromatic dispersion in photodiodes. We characterize the OED in a commercial germanium PN-type photodiode and determine the optimum conditions for maximum OED sensitivity and wavelength monitoring. A peak OED sensitivity of 1 deg/nm is measured in a spectral range of 1550-1558 nm with 4 MHz modulation. We also demonstrate an application of OED in fiber Bragg grating (FBG) interrogation. Quasi-static and vibration strains are monitored, with a spectral and strain sensitivity of 1.25pm/Hz and 1.08µÎµ/Hz, respectively. Photodiode OED can form the basis of inexpensive chip-scale grating-less spectral analysis.

Heterodyne phase-shift-amplified interferometry with improved shot-noise-limited sensitivity and immunity to RIN.

Phase-shift-amplified interferometry (PAI) is demonstrated using a heterodyne detection scheme. We demonstrate a sensitivity amplification factor of 35, giving $7.9 \cdot {10^{ - 4}}$7.9⋅10-4 rad, or 40 pm displacement, resolution. This was achieved due to the improved immunity of PAI to the total relative intensity noise (RIN) of the system. In addition, we predict a factor of $\sqrt 2 $2 fundamental improvement to shot-noise-limited phase-shift sensitivity as compared to a regular heterodyne Mach-Zehnder interferometer.

Phase-shift-amplified interferometry.

We present a new technique for improving the sensitivity of an interferometer, phase-shift-amplified interferometry (PAI), which is based on two embedded interferometers. The internal interferometer, which is biased in anti-phase, amplifies the phase shift; the external interferometer converts this into an amplified intensity shift. PAI can improve the sensitivity of standard interferometers by an order of magnitude or more. The theory of PAI, including its enhanced immunity to relative intensity noise, phase noise, and other post-detection noise and distortion components, is presented. We experimentally demonstrate a phase-shift amplification factor of 11.

Detuned Brillouin amplification of OTDR signals with an enhanced signal-to-noise ratio.

The technique known as external Brillouin amplification of Rayleigh scattering is characterized in the detuning regime. When employed as an amplifier of OTDR signals, it is shown that the signal-to-noise ratio is significantly improved for optimum pump Stokes detuning, leading to enhanced nanostrain sensitivity. This effect is due to the Brillouin bandwidth dependence on the Stokes power.

Phaseless incoherent optical frequency domain spectroscopy.

We describe a new technique for incoherent optical frequency domain spectroscopy (I-OFDS) that does not require measurements of the RF phase spectrum in order to reconstruct the optical spectrum. It is based on the addition of either an optical or electronic reference line to the I-OFDS system. Compared to the spectrum acquired by a regular I-OFDS system, high accuracy (error<1%) is predicted and achieved.

Affordable dispersion mitigation method for the next generation RF-over-fiber optical channels.

Next-generation analog radio frequency over fiber (RFoF) links will require dispersion compensation. Most dispersion compensation methods are based on additional optical elements. Therefore, these solutions may be inadequate for low-cost channels. In this work, we suggest a novel low-cost dispersion compensation solution for RFoF links. The method is based on two properties, which are common in these links: the modulation depth in analog RFoF links is lower than in digital links (and to avoid nonlinearities, it may be deliberately set to a small value), and the data are carried by a high-frequency carrier. It is shown that with these properties, the optical channel behaves approximately as a linear system. The distortion occurs in the linear domain and in the third-harmonic regime. Since the third-harmonic distortions are usually smaller than the back-to-back distortions, they are linear in the power domain. We therefore suggest using a simple low-cost electronic filter to compensate the dispersion distortions. The performance of this technique is presented both analytically and with a numerical simulation. These preliminary results are very encouraging and point to an inexpensive solution for next-generation RFoF links to the home.

Affordable dispersion mitigation with an analog electrical filter.

A new method for dispersion mitigation is presented for low-cost and simple networks. The method does not require dispersion-compensating fibers, special optical filters, coherent detection, or external modulation. It can work in a direct modulation scheme and with a standard optical detector. The disadvantage of this method is the requirement to operate in the weak modulation regime of the signal. In this regime, the dispersive channel can be regarded as linear in the power domain, and not only in the field domain, so that the effects of dispersion can be reduced with a proper electronic filter. Since electronic filters are usually considerably cheaper than coherent optical solutions, this solution can be implemented in low-cost networks, where dispersion is a more severe problem than noise. We show that by adding the filter to a low-noise on-off keying (OOK) system it is possible to transmit data at bit rates of 50 Gb/s to distances at least sixfold larger than its OOK limit (6 km in this case), i.e., 40 km and beyond.

Brillouin amplification and processing of the Rayleigh scattered signal.

Brillouin amplification of Rayleigh scattering is demonstrated using two different configurations. In the first technique, the Rayleigh scattering and amplification occurs simultaneously in the same fiber. In the second technique, the amplification takes place in a second fiber. The differences between the two techniques are delineated. Using the second technique, we demonstrate single-sideband off-resonant Brillouin amplification of the Rayleigh signal. This technique is shown to enhance the SNR of a signal that is due to vibration-induced strain on the fiber.

Photonic radio frequency phase-shift amplification by radio frequency interferometry.

We present a new technique for radio frequency (RF) phase-shift amplification based on RF interferometry and demonstrate it in an optical system. A striking feature of this amplifier is that the input phase noise is not amplified together with the input phase signal, so the phase sensitivity improves with higher phase amplification. We also predict that in the case of correlated amplitude noise, the sensitivity is not affected by the amplitude noise. With 600 MHz of modulated light and a phase amplification of 100, we demonstrate a phase resolution of 0.2 mrad, giving a distance resolution of 8 µm. We postulate that nanometric distance resolution can be achieved with sub-gigahertz modulation.

Generic propagation of beams with sharp spatial boundaries.

The propagation of spatial beams with initially sharp transverse boundaries is investigated theoretically and experimentally with the paraxial wave equation (PWE). The sharp boundaries generate a universal pattern, which is a consequence of the Schrödinger-like nature of the paraxial dynamics. As a consequence, an approximate analytical expression can be derived for the longitudinal propagation dynamics of the beam. Furthermore, it is shown that the validation of the derived analytical approximation is not limited to the zone in which the PWE is valid, but it is valid in the entire space. Therefore, this solution is a good approximation for the solution of the scalar wave equation (and to the Maxwell wave equation whenever the aperture is much wider than the wavelength of light) in the entire space. Good agreement between the analytical expression and experiment results is presented.

Transition from the ballistic to the diffusive regime in a turbid medium.

By varying the absorption coefficient and width of an intralipid-India ink solution in a quasi-one-dimensional experiment, we investigate the transition between the ballistic and the diffusive regimes. The medium's attenuation coefficient changes abruptly between two different values within a single mean free path. This problem is analyzed both experimentally and theoretically, and it is demonstrated that the transition location depends on the scattering coefficient as well as on the measuring solid angle.

Kilohertz laser frequency sensing with Brillouin mutually modulated cross-gain modulation.

Using mutually modulated cross-gain modulation, Stokes optical frequency changes are converted into modulation phase changes with high sensitivity. In the slow-light transition regime, we demonstrate kilohertz sensitivity to the Stokes optical carrier frequency. The sensitivity is inversely proportional to the modulation frequency of the pump and Stokes beams.

Optical imaging of hidden objects behind clothing.

Optical impulse-response characterization of diffusive media can be of importance in various applications, among them optical imaging in the security and medical fields. We present results of an experimental technique that we developed for acquiring the impulse response, based upon the Kramers-Kronig algorithm, and have been applied for optical imaging of objects hidden behind clothing. We demonstrate three-dimensional imaging with 5mm depth resolution between diffusive layers.

Brillouin cross-gain modulation and 10 m/s group velocity.

We introduce a method of achieving cross-gain modulation and slow light using the Brillouin nonlinearity in an optical fiber. We demonstrate approximatetely 10 m/s group velocity using this technique with milliwatts of optical power at room temperature.