Showing papers on "Chirp published in 2014"

A 56.4-to-63.4 GHz Multi-Rate All-Digital Fractional-N PLL for FMCW Radar Applications in 65 nm CMOS

Abstract: A mm-wave digital transmitter based on a 60 GHz all-digital phase-locked loop (ADPLL) with wideband frequency modulation (FM) for FMCW radar applications is proposed. The fractional-N ADPLL employs a high-resolution 60 GHz digitally-controlled oscillator (DCO) and is capable of multi-rate two-point FM. It achieves a measured rms jitter of 590.2 fs, while the loop settles within 3 μs. The measured reference spur is only -74 dBc, the fractional spurs are below -62 dBc, with no other significant spurs. A closed-loop DCO gain linearization scheme realizes a GHz-level triangular chirp across multiple DCO tuning banks with a measured frequency error (i.e., nonlinearity) in the FMCW ramp of only 117 kHz rms for a 62 GHz carrier with 1.22 GHz bandwidth. The synthesizer is transformer-coupled to a 3-stage neutralized power amplifier (PA) that delivers +5 dBm to a 50 Ω load. Implemented in 65 nm CMOS, the transmitter prototype (including PA) consumes 89 mW from a 1.2 V supply.

On the Processing of Very High Resolution Spaceborne SAR Data

Abstract: This paper addresses several important aspects that need to be considered for the processing of spaceborne synthetic aperture radar (SAR) data with resolutions in the decimeter range. In particular, it will be shown how the motion of the satellite during the transmission/reception of the chirp signal and the effect of the troposphere deteriorate the impulse response function if not properly considered. Further aspects that have been investigated include the curved orbit, the array pattern for electronically steered antennas, and several considerations within the processing itself. For each aspect, a solution is proposed, and the complete focusing methodology is expounded and validated using simulated point targets and staring spotlight data acquired by TerraSAR-X with 16-cm azimuth resolution and 300-MHz range bandwidth.

ISAR Imaging of Targets With Complex Motion Based on the Chirp Rate–Quadratic Chirp Rate Distribution

Abstract: In inverse synthetic aperture radar (ISAR) imaging of targets with complex motion such as fluctuating ships with oceanic waves and high maneuvering airplanes, the azimuth echo signals can be modeled as cubic phase signals (CPSs) after the migration compensation. The chirp rate (CR) and the quadratic chirp rate (QCR) are two important physical quantities of the CPS, which deteriorate the azimuth focusing quality due to the Doppler frequency shift. With these two quantities, other parameters can be estimated by using the fast Fourier transform (FFT). Therefore, the CPS can be uniquely determined by both CR and QCR. In this paper, based on the proposed generalized keystone transform and the parametric instantaneous autocorrelation function, a novel distribution of the CPS, known as the CR–QCR distribution (CRQCRD), is presented and applied in a newly proposed ISAR imaging algorithm for targets with complex motion. The CRQCRD is simple and only requires the FFT and the nonuniform FFT (NUFFT). Owing to the application of the NUFFT, the computational cost is saved, and the searching procedure is unnecessary for the nonuniformly spaced signal. Compared to other four representative methods for CPSs, the CRQCRD, which can acquire higher antinoise performance and no error propagation, is searching-free and more suitable for the situation of multitargets. Several simulation examples, analyses of the antinoise performance, and ISAR images validate the effectiveness of the CRQCRD and the corresponding ISAR imaging algorithm.

New chirp sequence radar waveform

Abstract: The general requirement in the automotive radar application is to measure the target range R and radial velocity v r simultaneously and unambiguously with high accuracy and resolution even in multitarget situations, which is a matter of the appropriate waveform design. Based on a single continuous wave chirp transmit signal, target range R and radial velocity v r cannot be measured in an unambiguous way. Therefore a so-called multiple frequency shift keying (MFSK) transmit signal was developed, which is applied to measure target range and radial velocity separately and simultaneously. In this case the radar measurement is based on a frequency and additionally on a phase measurement, which suffers from a lower estimation accuracy compared with a pure frequency measurement. This MFSK waveform can therefore be improved and outperformed by a chirp sequences waveform. Each chirp signal has in this case very short time duration T chirp . Therefore the measured beat frequency f B is dominated by target range R and is less influenced by the radial velocity v r . The range and radial velocity estimation is based on two separate frequency measurements with high accuracy in both cases. Classical chirp sequence waveforms suffer from possible ambiguities in the velocity measurement. It is the objective of this paper to modify the classical chirp sequence to get an unambiguous velocity measurement even in multitarget situations.

ISAR Imaging of a Ship Target Based on Parameter Estimation of Multicomponent Quadratic Frequency-Modulated Signals

Abstract: High-resolution inverse synthetic aperture radar (ISAR) imaging of a ship target is a challenging task because of fluctuation with the ocean waves. The images obtained with a standard range-Doppler algorithm are usually blurred. Consequently, the range-instantaneous-Doppler (RID) technique should be used to improve the image quality. In this paper, the received signal in a range cell is modeled as a multicomponent quadratic frequency-modulated (QFM) signal after range compression and motion compensation, and then a new RID ISAR imaging algorithm is proposed that introduces a new method for estimating parameters of the QFM signal. By defining a new function and using the scaled Fourier transform (SCFT) with respect to the time axis, the coherent integration of auto-terms can be realized via the subsequent Fourier transformation with respect to the lag-time axis, and a peak can be obtained in the 2-D frequency plane, which is appropriate for parameter estimation of the QFM signal to reconstruct RID images. The proposed algorithm is accurate and fast since the defined function has moderate order nonlinearity and the SCFT can be performed via chirp z-transform. Experiments demonstrate the performance of the new algorithm. Comparisons with existing algorithms are also given, which show that the proposed algorithm can efficiently produce a focused image with less fake scatterers.

Fading Channel Prediction Based on Combination of Complex-Valued Neural Networks and Chirp Z-Transform

Abstract: Channel prediction is an important process for channel compensation in a fading environment. If a future channel characteristic is predicted, adaptive techniques, such as pre-equalization and transmission power control, are applicable before transmission in order to avoid degradation of communications quality. Previously, we proposed channel prediction methods employing the chirp z-transform (CZT) with a linear extrapolation as well as a Lagrange extrapolation of frequency-domain parameters. This paper presents a highly accurate method for predicting time-varying channels by combining a multilayer complex-valued neural network (CVNN) with the CZT. We demonstrate that the channel prediction accuracy of the proposed CVNN-based prediction is better than those of the conventional prediction methods in a series of simulations and experiments.

Sparse Apertures ISAR Imaging and Scaling for Maneuvering Targets

Abstract: —In advanced multifunctional radar, inverse syntheticaperture radar (ISAR) imaging of sparse apertures for maneuver-ingtargets isachallengeproblem. Ingeneral,the Dopplermodula-tion of rotation motion can be modeled as linear frequency foruniformly accelerated rotation targets, which is spatial-variant intwo-dimension (2-D). The signal diversity inherently reflects themaneuverability and provides a rationale of rotation motion esti-mation. In this paper, we focus on the problem of sparse aperturesISAR imaging and scaling for maneuvering targets. The maneu-vering signal model is formulated as chirp code and representedusingachirp-Fourierbasis.Thensparserepresentationisappliedtorealize range-Doppler (RD) imaging from the sparse apertures,where the superposition of chirp parameters is acquired using themodified discrete chirp Fourier transform (MDCFT). After pre-processing,suchassampleselection,rotationcenterdetermination,and noise reduction, the chirp parameters are used to estimate theparameters of rotation motion using the weighted least square(WLS) method. Finally, a high-resolution scaled-ISAR image isachieved by rescaling the acquired RD image using the estimatedrotation velocity. Experiments are performed to confirm the effec-tiveness of the proposal.Index Terms—Inverse synthetic aperture radar (ISAR),maneuvering targets, scaling, sparse apertures, sparserepresentation.

Experimental demonstration of energy-chirp compensation by a tunable dielectric-based structure.

Abstract: A tunable energy-chirp compensator was used to remove a correlated energy chirp from the 60-MeV beam at the Brookhaven National Laboratory Accelerator Test Facility. The compensator operates through the interaction of the wakefield of the electron bunch with itself and consists of a planar structure comprised of two alumina bars with copper-plated backs separated by an adjustable beam aperture. By changing the gap size, the correlated energy chirp of the electron bunch was completely removed. Calculations show that this device, properly scaled to account for the electron bunch charge and length, can be used to remove residual correlated energy spread at the end of the linacs used for free-electron lasers. The experimental results are shown to be in good agreement with numerical simulations. Application of this technique can significantly simplify linac design and improve free-electron lasers performance.

Parametric sparse representation method for ISAR imaging of rotating targets

Abstract: Based on the linear relationship between the chirp rate of cross-range inverse synthetic aperture radar (ISAR) signal and the slant range, a parametric sparse representation method is proposed for ISAR imaging of rotating targets. The ISAR echo is formulated as a parametric joint-sparse signal and the chirp rates at all range bins are estimated by maximizing the contrast of sparse ISAR image. Comparing with homologous algorithms, the computational complexity of the proposed method is significantly reduced.

Characterization and control of peak intensity distribution at the focus of a spatiotemporally focused femtosecond laser beam.

Abstract: We report on experimental examination of two-photon fluorescence excitation (TPFE) at the focus of a spatially chirped femtosecond laser beam, which reveals an unexpected tilted peak intensity distribution in the focal spot. Our theoretical calculation shows that the tilting of the peak intensity distribution originates from the fact that along the optical axis of objective lens, the spatiotemporally focused pulse reaches its shortest duration exactly at the focal plane. However, when moving away from the optical axis along the direction of spatial chirp of the incident pulse, the pulse reaches its shortest duration either before or after the focal plane, depending on whether the pulse duration is measured above or below the optical axis as well as the sign of the spatial chirp. The tilting of the peak intensity distribution in the focal spot of the spatiotemporally focused femtosecond laser beam can play important roles in applications such as femtosecond laser micromachining and bio-imaging.

Tunable nanophotonic delay lines using linearly chirped contradirectional couplers with uniform Bragg gratings.

Abstract: We demonstrate an integrated tunable optical delay line in grating-assisted contradirectional couplers using a CMOS-compatible photonic technology. The input signal is delayed through dispersive Bragg gratings and distributedly coupled to the drop port of the coupler without backreflections. This add–drop design enables monolithic integration of grating-based delay lines without using optical circulators. The gratings are formed by slab perturbations in rib waveguides, with the index chirping realized by linearly tapering the rib widths. Both the pitch and size of the gratings are constant through the entire coupler, for a higher tolerance to fabrication errors. Continuous tuning of the optical group delay of up to 96 ps has been obtained, with a low insertion loss of less than 2 dB and a negative chromatic dispersion of −11 ps/nm that allows for bit rates of up to almost 100 Gb/s at the maximal delay. The device has a small footprint of 0.015 mm2, and can be used for on-chip optical buffering, dispersion compensation, and pulse compression.

Investigations on OFDM Signal for Range Ambiguity Suppression in SAR Configuration

Abstract: This paper presents an opportunity to cancel range ambiguities in synthetic aperture radar (SAR) configuration. One of the limitations of SAR systems is the range ambiguity phenomenon that appears with long delayed echoes. The reflected signal corresponding to one pulse is detected when the radar has already transmitted the next pulse. Thus, this signal is considered as an echo from the next pulse. This paper investigates the opportunity of coding the transmitted pulses using an orthogonal frequency-division multiplexing pulse. The results show that coded-OFDM signals outperform conventional chirp signal and make it possible to relax constraints placed upon the pulse repetition frequency.

A Parameter-Adjusting Polar Format Algorithm for Extremely High Squint SAR Imaging

Abstract: The polar format algorithm (PFA) is a wavenumber domain imaging method for spotlight synthetic aperture radar (SAR). The classic fixed-parameter PFA employs interpolation technique for data correction. However, such an operation will induce heavy computational load and cause degradation in computation precision. To optimize image formation processing performance, this study presents a novel parameter-adjusting PFA, which can implement SAR image formation at an extremely highly squint angle with obviously improved computation efficiency and imaging precision. In the parameter-adjusting PFA, radar parameters, such as center frequency, chirp rate, pulse duration, sampling rate, and pulse repeat frequency (PRF), vary for each azimuth sampling position. Due to the parameter adjusting strategy, the echoed signal can be acquired directly in keystone format with uniformly distributed azimuth intervals. In this case, range interpolation, which is necessary in the fixed-parameter PFA to convert data from polar format to keystone format, can be eliminated. Chirp z-transform (CZT) can be employed to focus SAR data along the azimuth direction. Compared with truncated sinc-interpolation, CZT was found to perform better in inducing less phase and amplitude errors in data processing. When residual video phase (RVP) compensation was accomplished for dechirped signal, the processing steps of the parameter-adjusting PFA were simplified as azimuth CZTs and range inverse fast Fourier transforms (IFFT). Lastly, computer simulation of multiple point targets validated the presented approach.

Terahertz generation with tilted-front laser pulses in a contact-grating scheme

Abstract: We present a detailed analysis of terahertz generation with tilted-front optical pulses in a scheme, where the pulse-front tilt is introduced by transmitting the laser pulse through a diffraction grating placed on the boundary of an electro-optic crystal. The analysis accounts for a finite transverse size of the pump laser beam, initial chirp of the laser pulse, and distortion of the tilted-front pulse in the crystal due to angular and material dispersion. Two situations of practical interest are explored—LiNbO3 pumped at 0.8 μm wavelength and GaAs pumped at 1.8 μm. Recommendations on increasing the efficiency of the optical-to-terahertz conversion, optimizing the generated terahertz spectrum, and improving the quality of the terahertz beam are given.

Phase-locked, chip-based, cascaded stimulated Brillouin scattering

Abstract: Compact optical frequency comb sources with gigahertz repetition rates are desirable for a number of important applications including arbitrary optical waveform generation and direct comb spectroscopy. We report the generation of phase-locked, gigahertz repetition rate optical frequency combs in a chalcogenide photonic chip. The combs are formed via the interplay of stimulated Brillouin scattering and Kerr-nonlinear four-wave mixing in an on-chip Fabry–Perot waveguide resonator incorporating a Bragg grating. Phase-locking of the comb is confirmed with real-time measurements, and a chirp of the comb repetition rate within the pump pulse was observed. These results represent a significant step towards the realization of integrated optical frequency comb sources with gigahertz repetition rates.

Amplitude-phase coupling and chirp in quantum-dot lasers: influence of charge carrier scattering dynamics

Abstract: We investigate the dependence of the amplitude-phase coupling in quantum-dot (QD) lasers on the charge-carrier scattering timescales. The carrier scattering processes influence the relaxation oscillation parameters, as well as the frequency chirp, which are both important parameters when determining the modulation performance of the laser device and its reaction to optical perturbations. We find that the FM/AM response exhibits a strong dependence on the modulation frequency, which leads to a modified optical response of QD lasers when compared to conventional laser devices. Furthermore, the frequency response curve changes with the scattering time scales, which can allow for an optimization of the laser stability towards optical perturbations.

MIMO SAR Chirp Modulation Diversity Waveform Design

Abstract: Waveform diversity design is a vital issue in multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) systems because the waveforms should have good orthogonality, high coherence, and large time-bandwidth product. However, most of the existing waveforms are not suitable for MIMO SAR and difficult to be implemented in a real-life scene. This letter investigates a scheme for designing chirp modulation diversity waveforms with large time-bandwidth product, constant modulus, implementation simplicity, good Doppler tolerance, and orthogonality. The four waveforms are orthogonal on both transmit and receive. The waveform performances are investigated through the correlation and ambiguity functions. Numerical results validate the superiorities of the designed four waveforms in MIMO SAR high-resolution imaging.

Experimental Demonstration of Electron Longitudinal-Phase-Space Linearization by Shaping the Photoinjector Laser Pulse

Abstract: Control of the electron-beam longitudinal-phase-space distribution is of crucial importance in a number of accelerator applications, such as linac-driven free-electron lasers, colliders and energy recovery linacs. Some longitudinal-phase-space features produced by nonlinear electron beam self- fields, such as a quadratic energy chirp introduced by geometric longitudinal wakefields in radio-frequency (rf) accelerator structures, cannot be compensated by ordinary tuning of the linac rf phases nor corrected by a single high harmonic accelerating cavity. In this Letter we report an experimental demonstration of the removal of the quadratic energy chirp by properly shaping the electron beam current at the photoinjector. Specifically, a longitudinal ramp in the current distribution at the cathode linearizes the longitudinal wakefields in the downstream linac, resulting in a flat electron current and energy distribution. We present longitudinal-phase-space measurements in this novel configuration compared to those typically obtained without longitudinal current shaping at the FERMI linac.

Radar system and method for determining range, relative velocity and bearing of an object using continuous-wave and chirp signals

Abstract: Determining distance and relative speed includes a transmitter transmitting electromagnetic signals defined by a plurality of sections, a first section comprising a continuous-wave (CW) signal, a second section comprising a chirp electromagnetic signal, the chirp electromagnetic signal comprising a plurality of subsections. A detector detects reflected signals being the transmitted electromagnetic signals reflected from an object, and comprising a reflected CW signal and a reflected chirp signal. The detector detects the reflected signals by: (i) generating a plurality of samples of the reflected CW signal, and (ii) generating a plurality of samples of the reflected chirp signal. A processor determines a first set of phase differences among the plurality of samples of the reflected CW signal and a second set of phase differences among the plurality of samples of the reflected chirp signal, and processes the first and second sets of phase differences to determine the distance, relative speed and bearing.

On fast chirp modulations and compressed sensing for automotive radar applications

Abstract: In the last decade radar based driver assistance systems have increased the safety in all vehicle classes and get the access to mass market applications. Currently slow FMCW waveforms are the state of the art principle for automotive radar sensors at 24 GHz and 77 GHz. This contribution presents an innovative 77 GHz multi channel radar sensor, with the capability for fast FMCW PLL locked chirp modulations down to 10 μs. It gives a brief review on short FMCW chirp modulations and its processing. Furthermore the application of compressed sensing algorithms for r/v estimation is discussed and system simulations and measurements are presented.

Coherent control of high-harmonic generation using waveform-synthesized chirped laser fields

Abstract: We show that waveform-synthesized chirped laser fields are efficient tools for coherent harmonic control A single harmonic order, or two harmonic orders, can be selectively enhanced by using a two-color field allowing a moderate linear chirp for each color Different harmonic orders within a wide spectral range can be selectively enhanced by adjusting the laser parameters Our theory bridges two current harmonic control techniques, namely, single-color phase shaping and multicolor amplitude synthesis, and opens the door to new harmonic control possibilities

Enhancement of sound by soft reflections in exponentially chirped crystals

Abstract: The enhancement of sound inside a two dimensional exponentially chirped crystal during the soft reflections of waves is experimentally and theoretically explored in this work. The control of this enhancement is achieved by a gradual variation of the dispersion in the system by means of a chirp of the lattice constant. The sound enhancement is produced at some planes of the crystal in which the wave is softly reflected due to a progressive slowing down of the sound wave. We find that the character of the sound enhancement depends on the function of the variation of dispersion, i.e., on the function of the chirp. A simple coupled mode theory is proposed to find the analytical solutions of the sound wave enhancement in the exponentially chirped crystal. Harmonic and time domain numerical simulations are performed to interpret the concept of the soft reflections, and to check the analytically calculated field distributions both in good agreement with experiments. Specially we obtain stronger sound enhancement than in linearly chirped crystals. This sound enhancement could motivate applications in energy harvesting, e.g., to increase the efficiency of detectors and absorbers.

Asynchronous Transmitter Position and Velocity Estimation Using A Dual Linear Chirp

Abstract: We present a closed-form least squares algorithm for estimating the position and velocity of a source, asynchronously transmitting known dual linear chirp signals, given times of arrival measurements obtained by spatially distributed sensors The estimates involve less computational load compared to the optimal maximum likelihood estimates Simulations show that the proposed estimates have similar mean square errors as the optimal estimates, and are close to the lower bound for small and moderate measurement noise

Chirp-based dispersion pre-compensation for high resolution Lamb wave inspection

Abstract: The dispersion of ultrasonic guided waves causes the energy of a signal to spread out in space and time as it propagates, which decreases the performance for damage detection significantly. A lot of signal processing methods have been proposed on how to obtain each mode under serious dispersion for this reason. Based on the chirp technique, a scheme is established to reduce the effect of dispersion by performing dispersion pre-compensation on the original narrowband excitation signals, and thus the time duration of received wave packet can be compressed during the extracting process. Furthermore, benefits from the broadband chirp excitation, information of multiple distinct frequency ranges can be acquired simultaneously, and thus responses to a few narrowband excitations could be readily extracted. This makes the optimal design of the excitation waveform much more efficiently. Numerical simulation and experiment are carried on aluminum specimens to investigate the behavior of the proposed method and the strategy for parameters selection. By using the proposed method, the closely distributed structural features can be recognized with ease in time domain.

Free-electron laser design for four-wave mixing experiments with soft-x-ray pulses.

Abstract: We present the design of a single-pass free-electron laser amplifier suitable for enabling four-wave mixing x-ray spectroscopic investigations. The production of longitudinally coherent, single-spike pulses of light from a single electron beam in this scenario relies on a process of selective amplification where a strong undulator taper compensates for a large energy chirp only for a short region of the electron beam. This proposed scheme offers improved flexibility of operation and allows for independent control of the color, timing, and angle of incidence of the individual pulses of light at an end user station. Detailed numerical simulations are used to illustrate the more impressive characteristics of this scheme.

Design and performance evaluation of a dispersion compensation unit using several chirping functions in a tanh apodized FBG and comparison with dispersion compensation fiber

Abstract: In this work, various dispersion compensation methods are designed and evaluated to search for a cost-effective technique with remarkable dispersion compensation and a good pulse shape. The techniques consist of different chirp functions applied to a tanh fiber Bragg grating (FBG), a dispersion compensation fiber (DCF), and a DCF merged with an optimized linearly chirped tanh FBG (joint technique). The techniques are evaluated using a standard 10 Gb/s optical link over a 100 km long haul. The linear chirp function is the most appropriate choice of chirping function, with a pulse width reduction percentage (PWRP) of 75.15%, lower price, and poor pulse shape. The DCF yields an enhanced PWRP of 93.34% with a better pulse quality; however, it is the most costly of the evaluated techniques. Finally, the joint technique achieved the optimum PWRP (96.36%) among all the evaluated techniques and exhibited a remarkable pulse shape; it is less costly than the DCF, but more expensive than the chirped tanh FBG.

Sub-20-Attosecond Timing Jitter Mode-Locked Fiber Lasers

Abstract: We demonstrate 14.3-attosecond timing jitter [integrated from 10 kHz to 94 MHz offset frequency] optical pulse trains from 188-MHz repetition-rate mode-locked Yb-fiber lasers. In order to minimize the timing jitter, we shorten the non-gain fiber length to shorten the pulsewidth and reduce excessive higher-order nonlinearity and nonlinear chirp in the fiber laser. The measured jitter spectrum is limited by the amplified spontaneous emission limited quantum noise in the 100 kHz-1 MHz offset frequency range, while it was limited by the relative intensity noise-converted jitter in the lower offset frequency range. This intrinsically low timing jitter enables sub-100-attosecond synchronization between the two mode-locked Yb-fiber lasers over the full Nyquist frequency with a modest 10-kHz locking bandwidth. The demonstrated performance is the lowest timing jitter measured from any free-running mode-locked fiber lasers, comparable to the performance of the lowest-jitter Ti:sapphire solid-state lasers.

Unambiguous evaluation of a chirp measurement standard

Abstract: This article describes an automated evaluation method for the chirp standard. Chirp calibration standards provide a way to describe the transfer behavior of different spatial frequencies as they contain sinusoidal functions of varying wavelengths (Kruger-Sehm et al 2007 chirp calibration standards for surface measuring instruments Tech. Mess. tm 74 572–76; Pehnelt et al 2011 Comparative analysis of optical surface measuring systems with a chip calibration standard Tech. Mess. tm 78 457–62). By introducing a new, automated evaluation method, an improvement for the application of the chirp standard can be achieved. The data-preprocessing for topography and profile measurement data and the fit of the geometric elements are described. Automated evaluation can reduce the labor required to evaluate measured data and make it easier to compare different evaluations in the course of standardization. The algorithm can be used to characterize the so-called 'small scale fidelity' of an optical instrument. The term 'small scale fidelity' is currently discussed in the optical group of working group no. 16 of the ISO technical committee 213.

VCSEL Based Coherent PONs

Abstract: We present a review of research performed in the area of coherent access technologies employing vertical cavity surface emitting lasers (VCSELs). Experimental demonstrations of optical transmission over a passive fiber link with coherent detection using VCSEL local oscillators and directly modulated VCSEL transmitters at bit rates up to 10 Gbps in the C-band as well as in the O-band are presented. The broad linewidth and frequency chirp associated with directly modulated VCSELs are utilized in an envelope detection receiver scheme which is demonstrated digitally (off-line) as well as analog (real-time). Additionally, it is shown that in the optical front-end of a coherent receiver for access networks, the 90 ° hybrid can be replaced by a 3-dB coupler. The achieved results show that VCSELs are attractive light source candidates for transmitter as well as local oscillator for coherent detection PONs.