Irradiance Independent Spectrum Reconstruction from Camera Signals Using the Interpolation Method.

The spectrum of light captured by a camera can be reconstructed using the interpolation method. The reconstructed spectrum is a linear combination of the reference spectra, where the weighting coefficients are calculated from the signals of the pixel and the reference samples by interpolation. This method is known as the look-up table (LUT) method. It is irradiance-dependent due to the dependence of the reconstructed spectrum shape on the sample irradiance. Since the irradiance can vary in field applications, an irradiance-independent LUT (II-LUT) method is required to recover spectral reflectance. This paper proposes an II-LUT method to interpolate the spectrum in the normalized signal space. Munsell color chips irradiated with D65 were used as samples. Example cameras are a tricolor camera and a quadcolor camera. Results show that the proposed method can achieve the irradiance independent spectrum reconstruction and computation time saving at the expense of the recovered spectral reflectance error. Considering that the irradiance variation will introduce additional errors, the actual mean error using the II-LUT method might be smaller than that of the ID-LUT method. It is also shown that the proposed method outperformed the weighted principal component analysis method in both accuracy and computation speed.

Spectral Reflectance Recovery from the Quadcolor Camera Signals Using the Interpolation and Weighted Principal Component Analysis Methods.

The recovery of surface spectral reflectance using the quadcolor camera was numerically studied. Assume that the RGB channels of the quadcolor camera are the same as the Nikon D5100 tricolor camera. The spectral sensitivity of the fourth signal channel was tailored using a color filter. Munsell color chips were used as reflective surfaces. When the interpolation method or the weighted principal component analysis (wPCA) method is used to reconstruct spectra, using the quadcolor camera can effectively reduce the mean spectral error of the test samples compared to using the tricolor camera. Except for computation time, the interpolation method outperforms the wPCA method in spectrum reconstruction. A long-pass optical filter can be applied to the fourth channel for reducing the mean spectral error. A short-pass optical filter can be applied to the fourth channel for reducing the mean color difference, but the mean spectral error will be larger. Due to the small color difference, the quadcolor camera using an optimized short-pass filter may be suitable as an imaging colorimeter. It was found that an empirical design rule to keep the color difference small is to reduce the error in fitting the color-matching functions using the camera spectral sensitivity functions.

Auxiliary Reference Samples for Extrapolating Spectral Reflectance from Camera RGB Signals.

Surface spectral reflectance is useful for color reproduction. In this study, the reconstruction of spectral reflectance using a conventional camera was investigated. The spectrum reconstruction error could be reduced by interpolating camera RGB signals, in contrast to methods based on basis spectra, such as principal component analysis (PCA). The disadvantage of the interpolation method is that it cannot interpolate samples outside the convex hull of reference samples in the RGB signal space. An interpolation method utilizing auxiliary reference samples (ARSs) to extrapolate the outside samples is proposed in this paper. The ARSs were created using reference samples and color filters. The convex hull of the reference samples and ARSs was expanded to enclose outside samples for extrapolation. A commercially available camera was taken as an example. The results show that with the proposed method, the extrapolation error was smaller than that of the computationally time-consuming weighted PCA method. A low cost and fast detection speed for spectral reflectance recovery can be achieved using a conventional camera.

Gait Stability Measurement by Using Average Entropy.

Gait stability has been measured by using many entropy-based methods. However, the relation between the entropy values and gait stability is worth further investigation. A research reported that average entropy (AE), a measure of disorder, could measure the static standing postural stability better than multiscale entropy and entropy of entropy (EoE), two measures of complexity. This study tested the validity of AE in gait stability measurement from the viewpoint of the disorder. For comparison, another five disorders, the EoE, and two traditional metrics methods were, respectively, used to measure the degrees of disorder and complexity of 10 step interval (SPI) and 79 stride interval (SI) time series, individually. As a result, every one of the 10 participants exhibited a relatively high AE value of the SPI when walking with eyes closed and a relatively low AE value when walking with eyes open. Most of the AE values of the SI of the 53 diseased subjects were greater than those of the 26 healthy subjects. A maximal overall accuracy of AE in differentiating the healthy from the diseased was 91.1%. Similar features also exists on those 5 disorder measurements but do not exist on the EoE values. Nevertheless, the EoE versus AE plot of the SI also exhibits an inverted U relation, consistent with the hypothesis for physiologic signals.

Complexity and Disorder of 1/fα Noises.

The complexity and the disorder of a 1/fα noise time series are quantified by entropy of entropy (EoE) and average entropy (AE), respectively. The resulting EoE vs. AE plot of a series of 1/fα noises of various values of α exhibits a distinct inverted U curve. For the 1/fα noises, we have shown that α decreases monotonically as AE increases, which indicates that α is also a measure of disorder. Furthermore, a 1/fα noise and a cardiac interbeat (RR) interval series are considered equivalent as they have the same AE. Accordingly, we have found that the 1/fα noises for α around 1.5 are equivalent to the RR interval series of healthy subjects. The pink noise at α = 1 is equivalent to atrial fibrillation (AF) RR interval series while the white noise at α = 0 is more disordered than AF RR interval series. These results, based on AE, are different from the previous ones based on spectral analysis. The testing macro-average F-score is 0.93 when classifying the RR interval series of three groups using AE-based α, while it is 0.73 when using spectral-analysis-based α.

Long-reach 60-GHz MMWoF link with free-running laser diodes beating.

With the remote beating of two mutually incoherent laser carriers, the local-oscillator-free long-reach millimeter-wave over fiber (MMWoF) link at 60-GHz band is demonstrated. The unique schemes of the proposed MMWoF are the wavelength-locked colorless laser diode (CLD) modulator, the mutually incoherent optical carrier for heterodyne MMW generation, and the square-law power envelope detection at receiving end. By directly encoding the single-mode with the CLD modulator, the single-carrier modulated QAM-OFDM data is achieved to release the RF power fading after fiber transmission. The mutually incoherent laser beating enables the optical heterodyne MMW generation with two independent optical carriers, which provides the advantages of local-oscillator-free operation and rules out the requirement of dual-mode optical carrier delivery from central office. At the wireless receiving end, the received QAM-OFDM data is self-down-converted to the baseband by employing the square-law power envelope detection. This eliminates the requirement of local oscillator and rules out the influence of the MMW carrier frequency fluctuation between two mutually incoherent lasers (used at central office and remote node), which effectively provides the MMW carrier immunity against the down-conversion instability caused by clock jitter or carrier incoherence. This architecture ensures the transmission of 16.5-Gbit/s 32-QAM OFDM data over 50 km in SMF and 3 m in free-space with the FEC certificated error vector magnitude of 12%, signal-to-noise ratio (SNR) of 18.4 dB, and bit error rate of 3.8 × 10-3. For multi-channel DWDM-PON applications, the proposed local-oscillator-free MMWoF link can successfully perform 11 DWDM channels of 32-QAM OFDM data access at 16.5 Gbit/s per channel via the wavelength controlling of the CLD modulation stage and the detuning of the beating carrier at remote node.

DFT/IDFT-free receiving scheme for spread-OFDM signals employing low-sampling-rate ADCs.

This paper presents a DFT/IDFT-free receiving scheme for spread-OFDM signals. Leveraging sub-Nyquist sampling and proper sampling delay, the proposed scheme enables each user to receive the requested data without the need for DFT and IDFT; thus, the complexity at receiver can be greatly reduced. Nonetheless, DC component is altered in an AC coupling system, such that severe waveform distortion is caused when the process of DFT/IDFT is omitted. Thus, a DC-zeroing algorithm is proposed to guarantee constant DC after sub-Nyquist sampling, thereby eliminating such distortion. To experimentally verify the concept of proposed scheme, a 27.15-Gbit/s optical spread-OFDM signal was transmitted over fiber and received by the DFT/IDFT-free scheme with sub-Nyquist sampling. More users will reduce the required sampling rate at receiver; for the case of 16 users, the required sampling rate for the 27.15-Gbit/s signal is as low as 1 GSample/s. The experimental results show that error-free transmission was achievable, and the penalty due to lowering sampling rate (i.e., increasing the number of users) is insignificant.

Volume polarization holographic recording in thick photopolymer for optical memory.

Based on a vector wave theory of volume holograms, dependence of holographic reconstruction on the polarization states of the writing and reading beams is discussed. It is found that under paraxial approximation the circular polarization holograms provide a better distinction of the reading beams. Characteristics of recording polarization holograms in thick phenanthrenequinone-doped poly(methyl methacrylate) (PQ/PMMA) photopolymer are experimentally investigated. It is found that the circular polarization holographic recording possesses better dynamic range and material sensitivity, and a uniform spatial frequency response over a wide range. The performance is comparable to that of the intensity holographic recording in PQ/PMMA. Based on theoretical analyses and the material properties, a polarization multiplexing holographic memory using circularly polarization recording configuration for increasing storage capacity has been designed and experimentally demonstrated.

High spectral efficient W-band optical/wireless system employing single-sideband single-carrier modulation.

With broader available bandwidth, W-band wireless transmission has attracted a lot of interests for future Giga-bit communication. In this article, we experimentally demonstrate W-band radio-over-fiber (RoF) system employing single-sideband single-carrier (SSB-SC) modulation with lower peak-to-average-power ratio (PAPR) than orthogonal frequency division multiplex (OFDM). To overcome the inter-symbol interference (ISI) of the penalty from uneven frequency response and SSB-SC modulation, frequency domain equalizer (FDE) and decision feedback equalizer (DFE) are implemented. We discuss the maximum available bandwidth of different modulation formats between SSB-SC and OFDM signals at the BER below forward error correction (FEC) threshold (3.8 × 10(-3)). Up to 50-Gbps 32-QAM SSB-SC signals with spectral efficiency of 5 bit/s/Hz can be achieved.

100-GHz DD-OFDM-RoF system over 150-km fiber transmission employing pilot-aided phase noise suppression and bit-loading algorithm.

This study extended the transmission distance of a 100-GHz DD-OFDM-RoF system through the reduction of chromatic dispersion-induced phase noise. The implementation of a pilot-aided phase noise suppression (PPNS) scheme enabled the transmission of distance-insensitive 16.97-Gbps QPSK OFDM over 0~150-km fiber and 2-m air transmission via a DFB laser with linewidth of 1~10-MHz. We applied a bit-loading algorithm in conjunction with PPNS to maximize spectral efficiency, resulting in a 93% improvement in the data rate from 11.53 to 22.27 Gbps at a fiber transmission of 150 km.

High spectral efficient W-band OFDM-RoF system with direct-detection by two cascaded single-drive MZMs.

W-band wireless transmission has attracted a lot of interest due to its wider available bandwidth (i.e. 75-110 GHz). In this article, we propose a direct-detection orthogonal frequency division multiplexing radio over fiber (OFDM-RoF) system via two cascaded single-drive MZMs at center frequency of 103 GHz. We discuss maximum bandwidth of different modulation formats under forward error correction (FEC) threshold (3.8 x 10(-3)). Up to 40-Gbps 16-QAM OFDM signals is achieved over 25-km fiber and 2-m wireless transmission. To overcome the penalty from uneven frequency response, bit-loading algorithm is applied to discuss data rate and spectral efficiency with signal bandwidth from 5 to 10 GHz. With 10-GHz bandwidth, 46.4-Gb/s data rate and 4.64-bit/s/Hz spectral efficiency was achieved. To achieve 40-Gbps data rate, the required bandwidth of OFDM signal with bit-loading is 2 GHz less than that without bit-loading.

Direct-detection polarization division multiplexed orthogonal frequency-division multiplexing transmission systems without polarization tracking.

We experimentally demonstrate a direct-detection polarization division multiplexed (PDM) orthogonal frequency-division multiplexing (OFDM) scheme without dynamic polarization tracking. Simply using a polarization-diverse receiver, the proposed multiple-input multiple-output assisted system can achieve bit-error rate of 10(-) over all possible received states of polarization. Moreover, 50 Gbps PDM-OFDM transmission over 100 km single-mode fiber is successfully demonstrated without dispersion-induced penalty.

Optimization of probe-laser focal offsets for single-particle tracking.

In optical tweezers applications, tracking a trapped particle is essential for force measurement. One of the most popular techniques for single-particle tracking is achieved by analyzing the forward and backward light pattern, scattered by the target particle trapped by a trap laser beam, of an additional probe-laser beam with different wavelength whose focus is slightly apart from the trapping center. However, the optimized focal offset has never been discussed. In this paper, we investigate the tracking range and sensitivity as a function of the focal offset between the trapping and the probe-laser beams. As a result, the optimized focal offsets are a 3.3-fold radius ahead and a 2.0-fold radius behind the trapping laser focus in the forward tracking and the backward tracking, respectively. The experimental result agrees well with a theoretical prediction using the Mie scattering theory.

Highly efficient femtosecond pulse stretching by tailoring cavity dispersion in erbium fiber lasers with an intracavity short-pass edge filter.

We demonstrate highly efficient pulse stretching in Er(3+)-doped femtosecond mode-locked fiber lasers by tailoring cavity dispersion using an intracavity short-pass edge filter. The cavity dispersion is preset at around zero to obtain the shortest pulsewidth. When the cutoff wavelength of the short-pass edge filter is thermo-optically tuned to overlap the constituting spectral components of mode-locked pulses, large negative waveguide dispersion is introduced by the steep cutoff slope and the total cavity dispersion is moved to normal dispersion regime to broaden the pulsewidth. The time-bandwidth product of the mode-locked pulse increases with the decreasing temperature at the optical liquid surrounding the short-pass edge filter. Pulse stretch ratio of 3.53 (623.8 fs/176.8 fs) can be efficiently achieved under a temperature variation of 4 °C.

Photonic vector signal generation employing a novel optical direct-detection in-phase/quadrature-phase upconversion.

This work demonstrates the feasibility of the generation of an RF direct-detection vector signal using optical in-phase/quadrature-phase (I/Q) upconversion. The advantage of the proposed transmitter is that no electrical mixer is needed to generate the RF signal. Therefore, I/Q data of RF signals are processed at baseband at the transmitter, which is independent of the carrier frequency of the generated RF signal. A 10 Gb/s 16 quadrature amplitude modulation signal is experimentally demonstrated. Following transmission over a 50 km single-mode fiber, the power penalty is negligible. Moreover, I/Q imbalance of the proposed transmitter is studied and compensated by digital signal processing, which is both numerically and experimentally verified.

Transmission of 20-Gb/s OFDM signals occupying 7-GHz license-free band at 60 GHz using a RoF system employing frequency sextupling optical up-conversion.

This work describes a proposed 60-GHz radio-over-fiber (RoF) system employing a frequency sextupling optical up-conversion scheme. Based on the modified single sideband modulation scheme, spectrally efficient vector signals were transmitted with no performance degradation due to dispersion-induced fading. Wavelength-division- multiplexed optical up-conversion can be realized using the proposed system. Since the required transmitter bandwidth is significantly reduced, radio-frequency components with lower bandwidth and higher reliability can be utilized. Both 13.75-Gb/s QPSK-OFDM and 20.625-Gb/s 8QAM-OFDM signals were experimentally demonstrated. After transmission over 25-km of standard single mode fiber, no significant received power penalty was observed.

Joint compensation of CD and PMD in direct-detected OFDM transmission using polarization-time coding.

We propose and demonstrate a polarization-time coding (PTC) method which can effectively compensate both the CD and first order PMD in direct-detected OFDM transmission. Compared with the previous methods, the proposed PTC not only alleviates the need for the complex dynamic polarization controller but also exhibits superior transparencies to both the OFDM format and transmission data rate. For the proposed PTC method, we have analytically derived the transmission model with CD and first order PMD, and theoretically prove the PTC indeed can jointly compensate both CD and PMD. The numerical results show that, with the PTC method, both the previously proposed gapped and interleaved OFDM formats behave virtually immune to both CD and PMD with a price of 3-dB OSNR penalty in back-to-back (BtB). Aimed to mitigate this BtB 3-dB penalty, further partial PTC approach is proposed for trading the PMD tolerance with the BtB OSNR sensitivity. The interleaved OFDM system is found to gain profits in terms of lower sensitivity with the partial coding.

A Full duplex radio-over-fiber linkwith Multi-level OFDM signal via a single-electrode MZM and wavelength reuse with aRSOA.

This work demonstrates the feasibility of a full duplex Radio-over-fiber (RoF) link employing multi-level OFDM signal via a single-electrode Mach-Zehnder modulator and wavelength reuse for uplink utilizing a reflective semiconductor optical amplifier (RSOA). A High spectral efficiency 5-Gb/s 16-QAM OFDM signal with frequency multiplication for the RoF downstream link is demonstrated, and negligible penalty is achieved after 25-km standard single mode fiber transmission. Furthermore, wavelength reuse for a 1.25-Gb/s OOK signal via a RSOA for the upstream link is also demonstrated with a receiver penalty of less than 0.5 dB following 25-km SMF transmission.

Full duplex 60-GHz RoF link employing tandem single sideband modulation scheme and high spectral efficiency modulation format.

This study proposes a full duplex 60-GHz band radio-over-fiber (RoF) link using a modified tandem single sideband (TSSB) modulation scheme with frequency doubling. Based on the modified TSSB modulation scheme, no dispersion induced fading is observed; high spectral efficiency vector signal can be utilized; and wavelength reuse can also be achieved. Both single carrier 8-QAM and QPSK-OFDM signals for down-link transmissions are experimentally demonstrated. After transmission of 50-km SSMF, no significant receiver power penalties are observed. Wavelength reuses with 1.25-Gb/s OOK using a reflective semiconductor optical amplifier (RSOA) for up-link transmission are also demonstrated. After transmission of 50-km SSMF, no significant receiver power penalties are also observed.

A continuously tunable and filterless optical millimeter-wave generation via frequency octupling.

This work proposes a cost-effective, continuously tunable and filterless optical millimeter-wave (MMW) signal generation employing frequency octupling. Optical MMW signals with 30-dB undesired sideband suppression ratios can be obtained. Since no optical filtering is required, the proposed system can be readily implemented in wavelength-division-multiplexing (WDM) systems. V-band 60-GHz and W-band 80-GHz optical MMW signals are experimentally demonstrated. Because of the high undesired sideband suppression ratio, 60-GHz waveform with 50% duty cycle is observed. The single-sideband (SSB) phase noise of the generated 60-GHz signal is -73 dBc/Hz at 10 kHz. The proposed system is a viable solution for the future ultra-high frequency MMW applications up to 320 GHz using the external modulator with a limited bandwidth of 40 GHz.