Modulated Light Source Research Articles

Influence of the light intensity on the operation of the self-oscillation photoacoustic setup

In the traditional approach to photoacoustic measurements, the light source modulation frequency is fixed, or a resonance tracking mechanism is applied, that adjusts the modulation frequency to the instantaneous value of the resonance frequency of the cell, especially in the case of high Q-factor cells. However, photoacoustic measurements can also be based on a novel method in which the light intensity is modulated by the photoacoustic signal induced in the cell, hence forming a photoacoustic oscillator. Frequency of such an oscillator and shape of the resulting photoacoustic signal depend mainly on the resonance properties of the cell, but also on the power of the light irradiating the investigated sample. This paper describes the influence of the intensity of light used for exciting the photoacoustic signal on the operation of such a photoacoustic generator.

Photoacoustic signal generation based on self-oscillation method

In conventional photoacoustic experiments a signal generated by photoacoustic phenomenon is produced by a forced stimulation of the investigated sample by electromagnetic radiation energy (light) which intensity is modulated at a user selected frequency. Resultant photoacoustic signal has the same frequency as the light modulation frequency. The paper presents a new technique of photoacoustic signal generation, based on self-oscillations resulting from a loopback in the signal path consisting of a modulated light source, a photoacoustic chamber, a microphone, and an amplifier. Signal from the amplifier output was used to drive the light source. Gain of the amplifier was chosen in order to obtain total loop gain of the signal path greater than one and positive feedback operation. On contrary to conventional photoacoustic methods, frequency of the photoacoustic signal in the presented self-oscillation method is determined mainly by acoustic properties of the photoacoustic cell, which in the experiments was corresponding to the resonance frequency of the cell. Taking into consideration that resonance frequency of a photoacoustic cell depends on the speed of sound, which is affected by properties of the fluid filling the cell, the method based on self-oscillations can be applied e.g. to quantitative analysis of the investigated substance.

Photoinduced absorption spectroscopy as a tool in the study of dye-sensitized solar cells

Photoinduced absorption (PIA) spectroscopy, where the excitation is provided by a square-wave modulated (on/off) monochromatic light source, is a versatile tool in the study of dye-sensitized solar cells. Spectra of transient species, such as the oxidized dye, can easily be obtained and their kinetics can be explored using frequency or time-resolved techniques. Experimental PIA conditions can be kept close to typical solar cell operating conditions, allowing extraction of relevant time constants. PIA is also a suitable method to study the quality of pore filling in case of solid hole conductors. Dye molecules that are not in direct contact with the hole conductor will have long lifetimes in their oxidized state and appear clearly in the PIA spectrum. The basic principles of PIA are explained using the example of electron injection and recombination in dye-sensitized TiO 2 in the absence of redox electrolyte.

Improvement in the Extinction Ratio of Output Optical PCM Signals From an Uncooled External Cavity Diode Laser by Using a Semiconductor Saturable Absorption Device

This paper describes the increase in the extinction ratio (ER) of an optical signal achieved by using a semiconductor saturable absorption device (SAD). We investigate a method for optimizing the SAD length in order to increase the ER, where we deal with the SAD characteristics phenomenologically. We then undertake a theoretical analysis of the bit error rate dependence on the ER increase. We clarify experimentally that by using a SAD with an effective length of 350 , the ER of the output optical signals from an uncooled directly modulated light source can be increased from 6.8 to 8.2 dB and, thus, conform to the synchronous optical network/synchronous digital hierarchy standard.

Effects of Intensity Modulation of Light Source on Brillouin Optical Correlation Domain Analysis

The effects of the intensity modulation (IM) of light source on fiber-distributed sensors based on Brillouin optical correlation domain analysis (BOCDA) are analyzed by numerical simulation, and the results are experimentally confirmed. We show that the shape of the Brillouin gain spectrum in the BOCDA system has a particular dependence on the optical spectrum of light source and that it can be controlled and tailored by proper modification of the optical spectrum using an additional IM. In the experiments, we applied several IM waveforms based on the simulation results for confirmation. Additionally, a distributed strain measurement along a 1-km optical fiber with a 30-cm spatial resolution is demonstrated by applying a proper IM scheme, which is the longest range reported using the BOCDA system

Recording invertebrate nerve activation with modulated light changes

Optical scattering techniques have the potential to provide noninvasive measurements of neural activity with good spatial and temporal resolution. We used the lobster nerve as a model system to investigate and record event-related optical signals with a modulated light source and heterodyne detection system. We observed changes in the transmitted birefringent light intensity, corresponding with electrophysiological measurements of the action potential. The photon delay was below the detection threshold, in part due to the small size of the nerve bundle. Our system allowed us to place an upper bound on the magnitude of the phase change of 0.01 degrees. The physiological stability of the preparation allows comprehensive characterization of biological and instrumentation noise sources for testing optical measurement systems.

Expansion of intensity correlation spectroscopy for lifetime measurements—application to intracellular oxygen dynamics measurements

We report on a simple correlation method for lifetime measurements using a random modulated excitation light source. We use an intensity correlation function of emission for lifetime analyses. In this method, no reference timing of the excitation is required. We apply the correlation method to measure phosphorescence decays and successfully demonstrate in the analysis of the phosphorescence decay from Pd(II) porphine in HeLa cells under aerobic and anaerobic conditions to understand the oxygen dynamics in individual cells. The method is applicable to faster decay time measurements down to a nanosecond range when the detection system is improved. Current fluorescence correlation setups can easily be modified for lifetime measurements, expanding the applicability in biological problems.

Design and analysis of a dynamic spectral encoding code-division multiple-access communication system

An optical dynamic spectral encoding code division multiple access (CDMA) communication system is proposed. In this system, an electrically controlled tunable optical filter (TOF) is used to encode the modulated broadband light source. The code depends on the function set to the controller. Two-dimensional code, named functional code, is also proposed based on a shifted sine function. The function defines the dynamic coding pattern of the central wavelength of the transmitted narrowband optical signal. Thus, the system will allow for an easy reconfiguration of the transmitter without the need for a sophisticated encoder. At the receiver, a synchronized TOF with the same function is used as a decoder. The system is modeled and analyzed taking into account the multiple access interference, phase-induced intensity noise (PIIN), and thermal noise. The performance of this system is shown to be better compared with a fast frequency hopping (FFH) system and a spectral amplitude coding (SAC) system that uses either a Hadamard code, a modified quadratic congruence (MQC) code, or a modified frequency hopping (MFH) code.

Design and analysis of synchronous dynamic frequency hopping code division multiple access communication system

An optical dynamic frequency hopping code division multiple access communication system is proposed. In this system, an electrically controlled tunable optical filter (TOF) is used to encode the modulated broadband light source. The code depends on the function set to the controller. Two-dimensional code, named functional code, is also proposed based of shifted sine function. The function defines the dynamic coding pattern of the central wavelength of the transmitted narrowband optical signal. Thus, the system will allow for an easy reconfiguration of the transmitter without the need for sophisticated encoder. At the receiver, a synchronized TOF with the same function is used as a decoder. The system is modeled and analyzed taking into account the multiple access interference, phase induced intensity noise, and thermal noise. The performance of this system is shown to be better compared with a fast frequency hopping system and a spectral amplitude coding system that uses either a Hadamard code, a modified quadratic congruence code (MQC), or a modified frequency hopping code (MFH).

A 128 × 128 Pixel Complementary Metal Oxide Semiconductor Image Sensor with an Improved Pixel Architecture for Detecting Modulated Light Signals

A complementary metal oxide semiconductor (CMOS) image sensor for the detection of modulated light under background illumination has been developed. When an object is illuminated by a modulated light source under background illumination the sensor enables the object alone to be captured. This paper describes improvements in pixel architecture for reducing fixed pattern noise (FPN) and improving the sensitivity of the image sensor. The improved 128 × 128 pixel CMOS image sensor with a column parallel analog-to-digital converter (ADC) circuit was fabricated using 0.35-mm CMOS technology. The resulting captured images are shown and the properties of improved pixel architecture are described. The image sensor has FPN of 1/28 that of the previous image sensor and an improved pixel architecture comprising a common in-pixel amp and a correlated double sampling (CDS) circuit. The use of a split photogate increases the sensitivity of the image sensor to 1.3 times that of the previous image sensor.

Optimization of Brillouin optical correlation domain analysis system based on intensity modulation scheme

We report a novel method to enhance the performance of Brillouin optical correlation domain analysis system based on the intensity modulation of light source. The suppression and the modification of the background noise in Brillouin gain spectrum is experimentally demonstrated with different intensity modulation schemes. In an optimum configuration, the signal to noise ratio is improved more than 40%, which extends the measurable strain limit and leads to the substantial increase of measurement range.

Fast frequency modulation optical code division multiple access communication system

An optical fast frequency modulation code division multiple access communication system is proposed. In this system, an electrically controlled tunable optical filter (TOF) is used to encode the modulated broadband light source. The code depends on the function set to the controller. Two-dimensional code named functional code is also proposed based on a shifted sine function. The function defines the dynamic coding pattern of the central wavelength of the transmitted narrowband optical signal. Thus, the system will allow for an easy reconfiguration of the transmitter without the need for sophisticated encoder. At the receiver, a synchronized TOF with the same function is used as a decoder. The performance of this system is shown to be better compared with a fast frequency hopping and a spectral amplitude coding systems.

Sci‐PM Fri ‐ 02: Determination of optical properties of tissue in situ using four different frequency domain models

The following problem was investigated: implant an intensity modulated light source and two detectors in tissue. From a measurement of the ratio of fluences and relative phase at the two detectors, estimate the absorption (μa) and reduced scattering (μs′) coefficients of the tissue. An initial guess of these quantities is iteratively improved by comparison of forward calculations with measurements. The model used for forward calculation must be fast, yet accurate. Four models were compared on the basis of errors in recovered optical properties: standard diffusion approximation (SDA), single Monte Carlo (SMC), delta‐P1, and isotropic similarity model (ISM). For μs′/μa>10, SDA recovered both coefficients within 5–10%, but for lower μs′/μa the errors were as large as 25%. Although the delta‐P1 model has been found to perform better under some conditions, we found no significant advantage in this application. The ISM was equivalent to SDA for μs′/μa>10 and gave errors in μa less than 7% and μs′ less than 17% for low albedo. The SMC model was the best; both coefficients were recovered to within 10% regardless of the albedo. In order to apply the SMC model, a restricted optical coefficient space must be used in the inverse problem solution.

Determination of phosphorescence lifetimes in the presence of high background signals using phase-shift measurements

Phosphorescence-based optical sensors are typically based on intensity or triplet-lifetime measurements. Lifetime determinations may be performed in the frequency-domain by exciting the luminophor with a sinusoidal modulated light source and measuring the phase shift between the excitation light and the phosphorescence emission. Modulated scattered light at the same frequency of the modulating signal must be removed in order to perform accurate measurements. A new and simple method based on phase-modulation measurements is presented to measure the triplet-lifetime of a long-lifetime phosphorescent luminophor, even if high levels of background short-lived fluorescence or scattered light are present. The method is based on the measurement of the phase shift between excitation and emission light at two different frequencies and it turned to provide reliable lifetime measurements if the background light lifetime is significantly shorter than that of the phosphorescent emission to be measured. Theoretical aspects of the proposed methodology, the design and construction of a fiber-optical measuring system and the selection of the optimal frequencies to perform the accurate lifetime measurements are discussed. Finally, the performance of the proposed measurement method has been assessed in the determination of the triplet-lifetime values of the metal chelate Al-ferron immobilized in an inorganic sol–gel support (an oxygen indicator which displays a strong fluorescence emission overlapping significantly with the measured phosphorescence emission).

Millimeter-wave WDM sources using two-mode injection-locked FP lasers

This paper concerns mm-wave WDM source based on two-mode injection-locked FP lasers. The optical master signals can be generated by modulation of a WDM CW light source with a reference RF signal. A wavelength demultiplexer is applied to distribute each pair of modes into each FP laser. The data transmission in the mm-wave band is demonstrated. Influences of optical crosstalk of the wavelength demultiplexer at the WDM source and the optical drop filters at base stations are investigated in order to perform error-free data transmission. We evaluate influence of the fiber nonlinear effects among the channels when the channel intervals are reduced to a half of the carrier frequency of the mm-wave signals. The possibility of remote signal modulation of the two-mode locked FP lasers is also investigated where an external intensity-modulated signal is injected.

Dynamics of Supercoiled and Linear pBluescript II SK(+) Phagemids Probed with a Long-lifetime Metal-ligand Complex

We extended the measurable time scale of DNA dynamics to microsecond using [Ru(phen)(2)(dppz)](2+)(phen = 1,10-phenanthroline, dppz=dipyrido[3,2-a:2',3'-c]phenazine)(RuPD) , which displays a mean lifetime near 500 ns. To evaluate the usefulness of this luminophore (RuPD) for probing nucleic acid dynamics, its intensity and anisotropy decays when intercalated into supercoiled and linear pBluescript (pBS) II SK(+) phagemids were examined using frequency-domain fluorometry with a blue light-emitting diode (LED) as the modulated light source. The mean lifetime for the supercoiled phagemids (<tau> = 489.7 ns) was somewhat shorter than that for the linear phagemids (<tau> = 506.4 ns), suggesting a more efficient shielding from water by the linear phagemids. The anisotropy decay data also showed somewhat shorter slow rotational correlation times for supercoiled phagemids (997.2 ns) than for the linear phagemids (1175.6 ns). The slow and fast rotational correlation times appear to be consistent with the bending and torsional motions of the phagemids, respectively. These results indicate that RuPD can have applications in studies of both bending and torsional dynamics of nucleic acids.

Detection of Nitrogen Dioxide by Cavity Attenuated Phase Shift Spectroscopy

We present initial results obtained from an optical absorption sensor for the monitoring of ambient atmospheric nitrogen dioxide concentrations (0-200 ppb). This sensor utilizes cavity attenuated phase shift spectroscopy, a technology related to cavity ringdown spectroscopy. A modulated broadband incoherent light source (a 430-nm LED) is coupled to an optically resonant cavity formed by two high-reflectivity mirrors. The presence of NO(2) in the cell causes a phase shift in the signal received by a photodetector that is proportional to the NO(2) concentration. The sensor, which employed a 0.5-m cell, was shown to have a sensitivity of 0.3 ppb in the photon (shot) noise limit. Improvements in the optical coupling of the LED to the resonant cavity would allow the sensor to reach this limit with integration times of 10 s or less (corresponding to a noise equivalent absorption coefficient of <1 x 10(-8) cm(-1) Hz(-1/2)). Over a 2-day-long period of ambient atmospheric monitoring, a comparison of the sensor with an extremely accurate and precise tunable diode laser-based absorption spectrometer showed that the CAPS-based instrument was able to reliably and quantitatively measure both large and small fluctuations in the ambient nitrogen dioxide concentration.

B15: Implementation of real time and stroboscobe sampling of photoacoustic signals based on CPLD circuits

The paper presents implementation of the main digital block of a photoacoustic measurement and control system in which signal processing is performed mainly in digital form. The device produces light source modulation signal and A/D converter control signals. Modulation frequency and number of samples per period of the recorded signal are programmable. The signal acquisition can be performed in real-time sampling or stroboscope sampling mode.

Evaluation of the Thermophysical Properties of Functionally Graded Materials at High Temperatures by the Photothermal Radiometry

Thermal barrier coatings and functionally graded materials (FGM) have been applied to a turbine blade exposed to high temperatures. Therefore, the evaluation of thermophysical properties of these coating materials is very important for designing turbine blade. We have adopted the photothermal radiometry (PTR) for the measurement of thermophysical properties of FGM at high temperatures. In this method, a layered sample is periodically heated by a modulated light source and periodic temperature variation is caused on the sample. The phase-lag between the modulated light source and the temperature variation on the sample contains information of thermal properties of the sample. We have measured the thermal diffusivity and thermal conductivity of round-robin FGM (single layer) at room temperature and compared the present results with other data obtained by the laser flash method. The average deviation of the other data from the present results was 14% both for thermal diffusivity and the thermal conductivity. We have measured the thermal diffusivity and thermal conductivity of FGM in the temperature range up to 1 273 K.

An electro-optic modulation technique for direct and accurate measurement of birefringence

We report a convenient and accurate technique for measuring the optical path difference induced by a birefringent crystal. The method is based on the transmission of a low-frequency signal by the electro-optic amplitude modulation of a monochromatic light source. The linear superposition principle of the induced birefringence and the electro-optic effect are employed in the analysis of the optical parameters. Birefringence measurements at the wavelength 0.6328 μm for some common nematic liquid crystal cells are presented. The measurement accuracy is close to 10 −3. In addition, the phase shift of a twisted-nematic liquid crystal spatial light modulator, which is now widely used in optical processing systems, is determined as a function of the gray-scale level.