The enhancement of the low frequency gain response of a microwave phase shifter based on slow light in a bulk reflective semiconductor optical amplifier (RSOA), by using forced coherent population oscillations (FCPO), is experimentally demonstrated. FCPO is achieved by simultaneously modulating the input optical power and bias current. The beat signal gain improvement ranges from 45 to 0 dB over a frequency range of 0.5 to 2.5 GHz, thereby improving the noise performance of the phase shifter. Tunable phase shifts of up to 40º are possible over this frequency range.
Reflective semiconductor optical amplifiers (RSOAs) have shown promise for applications in WDM optical networks and in fiber ring mode-locked lasers. Polarization insensitive SOAs can be fabricated using tensile-strained bulk material and a rectangular cross section waveguide. The introduction of tensile strain can be used to compensate for the different confinement factors experienced by the waveguide TE and TM modes. There is a need for models that can be used to predict RSOA static characteristics such as the dependency of the signal gain on bias current and input optical power, the amplified spontaneous emission spectrum and noise figure. In this paper we extend our prior work on non-reflective SOAs to develop a static model that includes facet reflections. The model uses a detailed band structure description, which is used to determine the wavelength and carrier density dependency of the material gain and additive spontaneous emission. The model and includes a full geometrical description of the amplifier waveguide, including the input taper and the position dependency of the TE/TM confinement factors. The amplified signal and spontaneous emission are described by detailed travelling-wave equations and numerically solved in conjunction with a carrier density rate equation. The model uses material and geometric parameters for a commercially available RSOA. The versatility of the model is shown by several simulations that are used to predict the SOA operational characteristics as well as internal variables such as the amplified spontaneous emission and signal and the carrier density.
The slow light effect in SOAs has many applications in microwave photonics such as phase shifting and filtering. Models are needed to predict slow light in SOAs and its dependence on the bias current, optical power and modulation index. In this paper we predict the slow light characteristics of a tensile-strained SOA by using a detailed time-domain model. The model includes full band-structure based calculations of the material gain, bimolecular recombination and spontaneous emission, a carrier density rate equation and travelling wave equations for the input signal and amplified spontaneous emission. The slow light effect is caused by coherent population oscillations, whereby beating between the spectral components of an amplitude modulated lightwave causes carrier density oscillations at the beat frequency, leading to changes in the group velocity. The resulting beat signal at the SOA output after photodetection, is phase shifted relative to the SOA input beat signal. The phase shift can be adjusted by controlling the optical power and bias current. However the beat signal gain is low at low frequencies, leading to a poor beat signal output signal-to-noise ratio. If the optical input and SOA drive current are simultaneously modulated, this leads to forced population oscillations that greatly enhance the low frequency beat signal gain. The model is used to determine the improvement in gain and phase response and its dependency on the optical power, bias current and modulation index. Model predictions show good agreement with experimental trends reported in the literature.
Full-field Optical coherence tomography is an en-face interferometric imaging technology capable of
carrying out high resolution cross-sectional imaging of the internal microstructure of an examined specimen in a
non-invasive manner. The presented system is based on competitively priced optical components available at the
main optical communications band located in the 1550 nm region. It consists of a superluminescent diode and an
anti-stokes imaging device. The single mode fibre coupled SLD was connected to a multi-mode fibre inserted into a
mode scrambler to obtain spatially incoherent illumination, suitable for OCT wide-field modality in terms of crosstalk
suppression and image enhancement. This relatively inexpensive system with moderate resolution of
approximately 24um x 12um (axial x lateral) was constructed to perform a 3D cross sectional imaging of a human
tooth. To our knowledge this is the first 1550 nm full-field OCT system reported.
Optical Coherence Tomography (OCT) is a promising non-invasive imaging technology capable of
carrying out 3D high-resolution cross-sectional images of the internal microstructure of examined material.
However, almost all of these systems are expensive, requiring the use of complex optical setups, expensive light
sources and complicated scanning of the sample under test. In addition most of these systems have not taken
advantage of the competitively priced optical components available at wavelength within the main optical
communications band located in the 1550 nm region. A comparatively simple and inexpensive full-field OCT
system (FF-OCT), based on a superluminescent diode (SLD) light source and anti-stokes imaging device was
constructed, to perform 3D cross-sectional imaging. This kind of inexpensive setup with moderate resolution
could be easily applicable in low-level biomedical and industrial diagnostics. This paper involves calibration of
the system and determines its suitability for imaging structures of biological tissues such as teeth, which has low
absorption at 1550 nm.
An optical delay interferometer (ODI) is employed to suppress the pattern effect manifested on a 10 Gb/s return-to-zero (RZ) data stream when amplified by a semiconductor optical amplifier (SOA) operated in deep gain saturation. The experimental results verify the competence of the scheme to confront the problem for this signal format by achieving a far better performance than that with the SOA alone.
Nonlinear polarization rotation (NPR) in semiconductor optical amplifiers (SOAs) is initially investigated. The changes of ellipticity angle of the probe light in a counter-propagation scheme are presented on the Poincaré sphere, and agree with the numerical simulations. All-optical wavelength conversions based on NPR in SOAs are realized at a bit rate of 2.5Gbit/s.
KEYWORDS: Digital signal processing, Mirrors, Heterodyning, Signal processing, Cameras, Interferometry, Signal detection, Phase measurement, Imaging systems, Interferometers
We describe a heterodyne interferometry system based on a complementary metal-oxide semiconductor digital signal processor (CMOS-DSP) camera that is utilized for full-field optical phase measurement using a carrier-based phase retrieval algorithm, with no need for electro-mechanical scanning. Camera characterization test results support the adoption of a single-pixel approach to perform quasi-instantaneous differential phase measurements, which are immune to mechanical vibrations and thermal drifts. We developed an optical configuration based on a Mach-Zehnder heterodyne interferometer to perform a static test on a mirror surface. The profiles of the mirror surface set at two angular positions, the relative displacements in the range of nanometers, and the corresponding tilt angle were determined.
Fluctuations in the output intensity and wavelength of an external cavity diode laser can introduce significant error to wavelength-tuned interferometric measurement. However, a robust phase-retrieval algorithm can compensate for these nonlinearities. Employing an inexpensive phosphor-coated charge-coupled device camera sensitive to C-band infrared, full-field interferometric phase retrieval utilizing wavelength tuning of a 1555 nm external cavity diode laser is reported. Phase measurement of a tilted mirror is presented with an estimated accuracy within 7 nm.
Digital stepping is desirable in optical metrology--operation is simple, absolute position is known, and random regions of interest can be skipped to, rapidly and accurately. However, in white-light interferometry, analog scanning has traditionally been employed because, in one operation, it achieves depth scanning of a sample and an electronically detectable optical carrier through a Doppler shift. This is not obligatory nor efficient in functional machine vision, especially if approximate preknowledge of the sample exists. Two methods, utilizing digital depth stepping and a superluminescent diode, are presented to decouple optical carrier generation from depth scanning in full-field white-light interferometry. One technique employs a complementary metal-oxide semiconductor camera and acousto-optic modulation to generate a frequency difference between two arms of a Mach--Zehnder interferometer. The other technique uses a Michelson interferometer with a piezoelectric transducer integrated to the digital stepper motor to facilitate 2λ analog scanning and an optical carrier of 4 periods, sampled with a standard charge-coupled device camera. In the former case, random depth access measurement of an engineering gauge block calibration sample is presented, while the latter demonstrates the application of the random depth access full-field white-light interferometry to a small punch test. A further benefit of these techniques is the possibility of interferometric phase retrieval on condition of path length matching; this is proven by the implementation of a heterodyne phase retrieval algorithm in the gauge block measurement. Both techniques represent an advance in optical metrology, offering an inexpensive and functional solution to machine vision and industrial measurement applications.
KEYWORDS: Cameras, Optical coherence tomography, Digital signal processing, Signal processing, Signal to noise ratio, Semiconductors, Optical signal processing, Optical filters, Light sources, 3D image processing
Full-field optical coherence tomography (OCT) using a complementary metal-oxide semiconductor (CMOS) camera with an integrated a digital signal processor (DSP) is demonstrated. The CMOS-DSP camera employed is typically used in machine vision applications and is based on an array of 1024×1024 direct readout pixels that are randomly addressable in space and time. These characteristics enable the camera to be used as a fast full-field detector in carrier-based optical metrology systems. The integrated DSP facilitates basic signal processing including real-time filtering and undersampling. The optical setup used to implement this OCT method is composed of a free-space Michelson interferometer and a superluminescent diode (SLD) light source, with an electromechanical shaker for depth scanning. Unlike classical OCT approaches, however, the setup does not require any electromechanical device for lateral scanning. A 64×30 pixel region of interest was imaged at 235 frames/s and sampled in depth, corresponding to a volumetric measurement of 875×410×150 µm. Measurements carried out on a simple calibration specimen indicated lateral and axial resolutions of 14 and 22 µm, respectively. The presented approach offers an inexpensive and versatile alternative to traditional OCT systems and provides the basis for a functional machine vision system suitable for industrial applications.
KEYWORDS: Cameras, Optical coherence tomography, Digital signal processing, Machine vision, Signal to noise ratio, Signal processing, Sensors, CMOS sensors, Aluminum, 3D image processing
Presented is a comprehensive characterisation of a complementary metal-oxide semiconductor (CMOS) and digital signal processor (DSP) camera, and its implementation as an imaging tool in full-field optical coherence tomography (OCT). The camera operates as a stand-alone imaging device, with the CMOS sensor, analogue-to-digital converter, DSP, digital input/output and random access memory all integrated into one device, autonomous machine vision being its intended application. The 1024x1024 pixels of the CMOS sensor function as a two-dimensional photodiode array, being randomly addressable in space and time and producing a continuous logarithmic voltage proportional to light intensity. Combined with its 120dB logarithmic response range and fast frame rates on small regions of interest, these characteristics allow the camera to be used as a fast full-field detector in carrier based optical metrology. Utilising the camera in an OCT setup, three-dimensional imaging of a typical industrial sample is demonstrated with lateral and axial resolutions of 14μm and 22μm, respectively. By electronically sampling a 64x30 pixel two-dimensional region of interest on the sensor at 235 frames per second as the sample was scanned in depth a volumetric measurement of 875μm x 410μm x 150μm was achieved without electromechanical lateral scanning. The approach presented here offers an inexpensive and versatile alternative to traditional OCT systems and provides the basis for a functional machine vision system suitable for industrial applications.
This paper reports on a Fibre Bragg Grating based sensor system, using a LabVIEW interface, for the measurement of strain, primarily in carbon composite materials. The process of embedding the sensors is outlined, system operation is explained and some sensitivity considerations are addressed. The system was firstly used to interrogate a single fibre Bragg grating that was embedded in a cantilever-type sample of a carbon/epoxy laminate. The accuracy of the system was examined by comparing results with those from a conventional resistive foil strain gauge. A composite "smart" panel was then constructed incorporating an array of four FBG sensors. The panel was subjected to various strain conditions, and sensor readings were used to obtain the resultant distribution of strain within the panel. Results were verified using special modelling software.
Conventional wavelength-division multiplexed (WDM) fiber optic communication systems employ semiconductor lasers operating at discrete wavelengths as carriers for the different data channels. Spectrum-slicing provides an attractive lowcost alternative by utilising spectral slices from a single broadband source, which are then fed to intensity modulators to encode data onto these slices. Such WDM spectrum-sliced systems have the potential for use in local area network fiber communication systems. A key drawback of spectrum sliced systems is the inherently high degree of excess intensity noise, which can impose severe limits on achievable system performance. In this paper we utilise the non-linear gain compression of a semiconductor optical amplifier (SOA) to suppress intensity noise of a spectrum sliced signal from a broadband source. The effects of SOA input power and bias are investigated and system experiments are carried out at 2.5 Gb/s.
KEYWORDS: Digital signal processing, Signal detection, Interferometry, Signal processing, Fiber optics sensors, Modulation, Demodulation, Fabry–Perot interferometers, Sensing systems, Fiber optics
An acquisition system pertaining to fiber-optic interferometric sensing is presented. The system consists of a digital signal processor, a codec, laser bias and modulation control circuits, an optical receiver and customized software. The system transmits a modulated light signal to the sensing interferometer, demodulates the returning interferometric signal using the Synthetic-Heterodyne detection technique, while simultaneously relaying control and signal data via PC based software. The system was tested using a low-finesse extrinsic Fabry-Perot cavity, consisting of a cleaved fiber end and a mirror mounted on a piezoelectric cylinder. Applying a sinusoidal voltage to the piezoelectric cylinder vibrated the mirror. Using the interrogation systems, successful retrieval of the vibration signal was obtained for vibrating frequencies from 44 Hz to 4.41 kHz.
The authors investigate the propagation of picosecond pulses through Semiconductor Optical Amplifiers using the measurement technique of Frequency Resolved Optical Gating by applying pulses of varying peak power, pulse width and shape. Frequency Resolved Optical Gating is a relatively new measurement system which provides complete characterisation of the pulses in both the temporal and spectral domains. We examine the pulses before and after amplification through the Semiconductor Optical Amplifier. The work shows pulse broadening, the formation of large pulse pedestals, and the generation of significant frequency chirp across the pulse in the temporal domain. In the spectral domain results exhibit spectral broadening and a shift to longer wavelengths. These properties of the output pulses would cause serious degradation in high-speed communications systems employing Wavelength Division Multiplexing and Optical Time Division Multiplexing. The resulting physical properties occurring to the pulses due to propagation through the Semiconductor Optical Amplifier are a result of Self Phase Modulation, which is due to gain saturation induced by carrier depletion and carrier heating.
A multiwavelength semiconductor ring laser is presented with more than 30 wavelength channels at uniform powers. A polarisation maintaining fibre Lyot filter forms part of the ring and forces multiwavelength operation. The cw characteristics of the system are presented with reference to the number of channels and the channel uniformity, various wavelength spacings are demonstrated. The system is harmonically mode-locked at 1 GHz producing pulsewidths of 100 ps and the pulse train synchronisation between oscillating wavelengths is shown.
Birefringence in an InGaAsP/InP tensile-strained bulk semiconductor optical amplifier (SOA) is demonstrated by a polarization resolved amplified spontaneous emission (ASE) spectrum. A mode sum/min approach is used to investigate the periodicities in the ripple for transverse electric (TE) and transverse magnetic (TM) polarizations. The signal-induced birefringence in the SOA, which leads to polarization rotation of a probe beam, is presented in the format of the Poincare sphere. All-optical wavelength conversion (inverted and non-inverted) based on cross polarization modulation (XPolM) is realized. The impact of cross gain modulation (XGM) in XPolM is initially studied.
KEYWORDS: Cameras, Digital signal processing, Optical coherence tomography, Signal processing, Glasses, CMOS sensors, Signal to noise ratio, CMOS cameras, Coherence (optics), Sensors
A comprehensive characterisation of a complementary metal-oxide semiconductor (CMOS) and digital signal processor (DSP) camera, used typically in machine vision applications, is presented in this paper. The camera consists of a direct read-out CMOS sensor, each pixel giving a direct analogue voltage output related to light intensity, with an analogue-to-digital converter and digital signal processor on the back-end. The camera operates as a stand-alone device using a VGA display; code being pre-programmed to the onboard random access memory of the DSP. High detection rates (kHz) on multiple pixels were achieved, and the relationship between pixel response time and light intensity was quantified. The CMOS sensor, with 1024x1024 pixels randomly addressable in space and time, demonstrated a dynamic logarithmic light intensity sensitivity range of 120dB. Integrating the CMOS camera with a low coherence Michelson interferometer, optical coherence tomography (OCT) axial depth scans have been acquired. The intended application is an imaging device for simple yet functional full-field optical coherence tomography. The advantages of the CMOS sensor are the potential for carrier-based detection, through the very fast pixel response with under-sampling, and the elimination of the electromechanical lateral scanning of conventional OCT by replacing it with electronic pixel scanning.
KEYWORDS: Digital signal processing, Signal detection, Signal processing, Interferometry, Fiber optics sensors, Demodulation, Modulation, Sensors, Receivers, Computing systems
An interrogation system for fiber optic interferometric sensors is presented. The system consists of a digital signal processor, a codec, laser bias and modulation control circuits, an optical receiver and customized software. The system transmits a modulated light signal to the sensing interferometer, demodulates the returning interferometric signal using synthetic-heterodyne detection, while simultaneously relaying control and signal data via PC based software. The system was tested using a low-finesse Fabry-Perot cavity, consisting of a cleaved fiber end and a mirror mounted on a piezoelectric cylinder. The mirror was vibrated using a sinusoidal voltage applied to the piezoelectric cylinder. Using the interrogation systems, successful retrieval of the vibration signal was obtained for vibrating frequencies from 44 Hz to 4.41 kHz.
A carbon composite strain sensing system using an embedded array of polymide-coated fiber Bragg grating sensors, which are interrogated by a tunable laser, is described. The system is controlled from a personal computer using a customized program. The system was tested using a composite panel consisting of 28 plies of carbon fiber. Consistent strain measurements were achieved using a laser scan resolution of 0.02 nm, which corresponds to a strain resolution of approximately 15 me. Simulations of the panel were carried out using the Marc/Mentat finite element program for advanced engineering analysis. The experimental results compared well to those obtained from the simulations.
A wideband steady-state model and efficient numerical algorithm for a tensile-strained InP/InGaAsP semiconductor optical amplifier is described. The model is applicable over a wide range of operating regimes. The relationship between spontaneous emission and material gain is clarified. Simulations and comparisons with experiment are given which demonstrate the versatility of the model.
This investigation describes the implementation of a Single Pixel Carrier Based Demodulation (SPCBD) approach on a digital CMOS-DSP camera for full-field heterodyne interferometry. A full-field vibration measurement system is presented as an alternative to a classical scanning Laser Doppler Vibrometer (LDV). The Heterodyne set-up, CMOS-DSP camera and the signal demodulation techniques adopted are described. Characterisation tests that describe the basic performance of the CMOS-DSP camera, in terms of acquisition rates and time response are presented. A simple experiment was performed to demonstrate the novel laser vibrometry system that consisted of determining the displacement of a point on the surface of a vibrating mirror. The measured velocity and displacement data were compared to the output from a commercial LDV. The integration of a CMOS sensor, DSP and a laser-doppler interferometer has lead to the development of a fully digital “functional” machine vision system that provides a flexible, compact and inexpensive tool for automated high-precision optical measurements.
This study concerns the application of a novel digital CMOS-DSP camera in full-field interferometry for vibration analysis and quasi real time processing in optical metrology. Characterisation tests on the digital camera have been performed. Both quasi-static and carrier-based approaches have been considered and will be presented in this paper. The CMOS-DSP camera is a primary component of a compact and low cost system for classical image processing and an innovative element for full field vibration measurement using a single pixel carrier based approach.
A two-section traveling-wave semiconductor laser amplifier can be utilized as a time-division demultiplexer or channel selector in all-optical networks. Light injected into the amplifier causes changes in the carrier density, which manifests itself as a change in the amplifier electrode voltages that can be processed to obtain the original data signal. Numerical analysis shows that by suitable choice of the electrode bias currents responsitivity and speed are optimized while system transparency is maintained. With optimal device design, demultiplexing speeds of the order of several gigabits per second are feasible.
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