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 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.

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.

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.

Ballistic imaging of biological media with collimated illumination and focal plane detection.

A simple, affordable method for imaging through biological tissue is investigated. The method consists of (1) imaging with a wavelength that has a relatively small scattering coefficient (1310 nm in this case) and (2) collimated illumination together with (3) focal plane detection to enhance the detection of the ballistic photons relative to the diffusive light. We demonstrate ballistic detection of an object immersed in a 1-cm-thick cuvette filled with 4% Intralipid, which is equivalent to ∼1 to 2 cm of skin tissue. With the same technology, a ballistic image of a 1-mm-wide object in 10-mm-thick chicken breast is also presented.

Effect of measurement on the ballistic-diffusive transition in turbid media.

The dependence of the transition between the ballistic and the diffusive regimes of turbid media on the experimental solid angle of the detection system is analyzed theoretically and experimentally. A simple model is developed which shows the significance of experimental conditions on the location of the ballistic-diffusive transition. It is demonstrated that decreasing the solid angle expands the ballistic regime; however, this benefit is bounded by the initial Gaussian beam diffraction. In addition, choosing the appropriate wavelength according to the model's principles provides another means of expanding the ballistic regime. Consequently, by optimizing the experimental conditions, it should be possible to extract the ballistic image of a tissue with a thickness of 1 cm.

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.