Differential Detection Scheme Research Articles

Decision feedback differential detection of 16-DAPSK signals

An improved differential detection scheme called DF-DD, based on decision feedback, is proposed for 16-level differentially encoded amplitude/phase shift keying (16-DAPSK). Computer simulation of BER performance in additive white Gaussian noise (AWGN) channels is presented and compared with that of 16DPSK.

Angle measurement based on the internal-reflection effect: a new method

We describe a new method of angle measurement that is based on the internal-reflection effect at an air-glass boundary. The method uses a differential detection scheme to largely reduce the inherent nonlinearity of the reflectance versus the angle of incidence in internal reflection. With nonlinearity reduced, the displacement of the angle of incidence can be determined accurately by measuring the reflectance. The resolution and measurement range are determined by the initial angle of incidence, the polarization state of the light, and the number of reflections. Compared with interferometers and autocollimators, this method has the advantage of a simple sensor design for applications ranging from a wide measurement range to extremely high resolution. Other advantages are compact size, simple structure, and low cost. A theoretical analysis of the method and some experimental results of a prototype sensor are presented. The possible applications of the method are also discussed.

Attractive-mode atomic force microscopy with optical detection in an orthogonal cantilever/sample configuration

An attractive-mode atomic force microscope is described in which the cantilever is held orthogonally with respect to the sample. The technique utilizes a linear differential optical detection scheme for the cantilever vibrations. In this design, the cantilever end is not bent to form a tip. This geometry substantially reduces the possibility of spontaneous jump-to-contact of the tip onto the sample, allowing the tip/sample separation to be set essentially to any desired value. Relatively large- and small-scale results are presented on imaging a smear of red blood cells, demonstrating the resolution and sensitivity.

Femtosecond time division interferometry technique for measuring the tensor components of χ(3)

We describe a new femtosecond time division interferometry technique for characterizing nonlinear index changes from different components of the χ(3) tensor. Pump probe interferometric measurements of nonlinear phase are performed using a time division multiplexed reference pulse. The n2∥ and n2⊥ components of the nonlinear index are measured by permuting the polarizations of the pulses and using a novel differential detection and modulation scheme. Measurements are demonstrated in an optical fiber.

On the implementation and performance of single and double differential detection schemes

A variety of schemes for performing differential detection in environments characterized by frequency offset are discussed. All of the schemes involve encoding the input phase information as a second-order difference and performing an analogous second-order differential detection at the receiver. Because of the back-to-back differential detection operations at the receiver, The performance of most of the schemes is considerably degraded relative to that of first-order differential detection schemes. However, the latter are quite sensitive to frequency offset and, in many instances, cannot be used at all. It is demonstrated that via a simple enhancement of using a 2 T s (instead of T s) delay in the second stage of the encoder and first stage of the decoder, the performance degradation can be significantly reduced. This result is significant in view of the fact that it comes without any penalty in implementation complexity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Efficient differential detection of digital FM signals in bandwidth restricted channels

The paper presents a differential detection scheme for noncoherent detection of digital FM signals received over bandwidth restricted channels. The new differential detection scheme observes the phase of the FM signal at centres of signalling intervals to derive the differential phase. Differential encoding of transmitted data is required for preventing error propagation. Numerical results show that the new differential detector yields a superior performance in narrowband channels where conventional differential detection becomes unusable. Error probabilities and optimum BT values are presented assuming that bandlimiting is caused solely by IF filtering.

Optical and electron spectroscopy of magneto-optical storage films

Magneto-optical (MO) storage films have been deposited in ultrahigh vacuum by ion beam sputtering from high purity 99.99% target material. Gradients in film composition occur due to different angular distributions of the sputter yields for different materials. The actual compositions are examined in situ by quantitative X-ray photoemission analysis with a relative accuracy of ±1%. Film growth is continuously monitored by in situ MO growth control using a differential detection scheme with a resolution in Kerr-rotation of better than 0.001° which corresponds to monolayer sensitivity for alloys like TbFeCo or GdFeCo. An example is given for the growth of multilayered FeCo/Ag films. These artificial superlattices show an enhancement of the Kerr-rotation by up to a factor of 2 without substantial reduction of the reflectivity and are therefore interesting candidates for MO storage applications.

A differential detection scheme for Faraday rotation spectroscopy with a color center laser

A differential detection scheme to reduce source noise and to improve sensitivity based upon the Faraday effect has been developed. This method is particularly useful in connection with low-power, high-noise laser sources and multipass arrangements where a degradation of polarization quality is substantial. The theoretical background of the differential method is developed. It is tested with magnetic rotation signals from transitions of stable species with low modulation sensitivity and signals from free radicals.

Signal-to-noise ratio for magneto-optic readout from quadrilayer structures

We present an experimental investigation of our earlier theoretical predictions regarding the signal-to-noise ratio for magneto-optic readout from quadrilayer structures in a differential detection scheme. Using quadrilayer structures employing amorphous Tb-Fe alloys as the magnetic material, quartz as the dielectric material, and aluminum as the reflecting material, we show that the signal enhancement predicted for quadrilayers occurs even when the numerical aperture of the readout system is large, a crucial result for their effectiveness in magneto-optic storage applications. We also show that the signal-to-noise ratios measured are in quantitative agreement with theory.

Analog read-only memory using gadolinium molybdate

In ferroelectric gadolinium molybdate a stable planar domain wall can be swept through the crystal by applying a voltage along the polar axis. The resulting polarization current can be used to read out analog information stored as a waveform pattern in the electrodes. Using a differential detection scheme, we have demonstrated the reproduction of 4 sec of typical speech stored with a density of 50 sec of speech per cm2.

A Sensitive System for Measuring Atmospheric Depolarization of Light

An improved instrument for measuring depolarization of linearity polarized light has been assembled and operated. It is a hundred times more sensitive than previous devices used for similar measurements, and it can detect depolarized light amounting to less than 10(-8) of the total transmitted light. This sensitivity is achieved by means of a high quality Wollaston prism and a differential detection scheme. Preliminary measurements over a 600-m path indicate that atmospheric depolarization is less than the detection limit of 10(-8).