Coherent Optical Detection Research Articles

Wavelength-Swept VCSELs

The ability to continuously tune the wavelength of a laser is of critical importance and is a fundamental building block for many optical systems, including wavelength division multiplexed optical fiber communication systems. Vertical cavity surface emitting laser (VCSEL) structure offers a unique advantage to engineer the lasing wavelength because of its ultrashort cavity length, inherently supporting a single Fabry–Perot mode. Hence, a continuous change in VCSEL cavity thickness leads to a continuous sweep of VCSEL wavelength. Wavelength-tunable VCSELs have been a subject of intense interest for the last two decades. Incorporating part or entire top mirror of a VCSEL in an optical microelectromechanical structure (MEMS), continuous wavelength sweeps have been reported. The monolithic integration brings together the best of both technologies and leads to an unprecedented performance in the continuously swept wavelength range. In addition, with the advances of ultrathin high contrast gratings and metastructures (HCG/HCM), the wavelength tuning speed of MEMS-VCSELs has been increased to 1–10 MHz range. Such lasers, now referred as wavelength-swept lasers, are enabling new applications in optical coherent tomography and light detection and ranging systems. In this paper, we review various structures and recent progress of MEMS-VCSELs. We summarize some of the early breakthroughs in designs and properties of micromechanical tunable VCSELs, including advances in HCG/HCM-based MEMS-VCSELs emitting at 850, 1060, and 1550 nm wavelength regimes. In addition, we report a brand new design leading to a record high Δλ/λo = 6.9% tuning ratio with 600 kHz speed at center wavelength of 1060 nm.

Flexible-rate optical packet generation/detection and label swapping for optical label switching networks

Abstract In recent years, optical label switching (OLS) gains lots of attentions due to its intrinsic advantages to implement protocol, bit-rate, granularity and data format transparency packet switching. In this paper, we propose a novel scheme to realize flexible-rate optical packet switching for OLS networks. At the transmitter node, flexible-rate packet is generated by parallel modulating different combinations of optical carriers generated from the optical multi-carrier generator (OMCG), among which the low-speed optical label occupies one carrier. At the switching node, label is extracted and re-generated in label processing unit (LPU). The payloads are switched based on routing information and new label is added after switching. At the receiver node, another OMCG serves as local oscillators (LOs) for optical payloads coherent detection. The proposed scheme offers good flexibility for dynamic optical packet switching by adjusting the payload bandwidth and could also effectively reduce the number of lasers, modulators and receivers for packet generation/detection. We present proof-of-concept demonstrations of flexible-rate packet generation/detection and label swapping in 12.5 GHz grid. The influence of crosstalk for cascaded label swapping is also investigated.

基于光学相干和微弱信号检测的干涉探测技术

基于光学相干和微弱信号检测的干涉探测技术

Affordable dispersion mitigation with an analog electrical filter.

A new method for dispersion mitigation is presented for low-cost and simple networks. The method does not require dispersion-compensating fibers, special optical filters, coherent detection, or external modulation. It can work in a direct modulation scheme and with a standard optical detector. The disadvantage of this method is the requirement to operate in the weak modulation regime of the signal. In this regime, the dispersive channel can be regarded as linear in the power domain, and not only in the field domain, so that the effects of dispersion can be reduced with a proper electronic filter. Since electronic filters are usually considerably cheaper than coherent optical solutions, this solution can be implemented in low-cost networks, where dispersion is a more severe problem than noise. We show that by adding the filter to a low-noise on-off keying (OOK) system it is possible to transmit data at bit rates of 50 Gb/s to distances at least sixfold larger than its OOK limit (6 km in this case), i.e., 40 km and beyond.

Analytical Investigations on Carrier Phase Recovery in Dispersion-Unmanaged n-PSK Coherent Optical Communication Systems

Using coherent optical detection and digital signal processing, laser phase noise and equalization enhanced phase noise can be effectively mitigated using the feed-forward and feed-back carrier phase recovery approaches. In this paper, theoretical analyses of feed-back and feed-forward carrier phase recovery methods have been carried out in the long-haul high-speed n-level phase shift keying (n-PSK) optical fiber communication systems, involving a one-tap normalized least-mean-square (LMS) algorithm, a block-wise average algorithm, and a Viterbi-Viterbi algorithm. The analytical expressions for evaluating the estimated carrier phase and for predicting the bit-error-rate (BER) performance (such as the BER floors) have been presented and discussed in the n-PSK coherent optical transmission systems by considering both the laser phase noise and the equalization enhanced phase noise. The results indicate that the Viterbi-Viterbi carrier phase recovery algorithm outperforms the one-tap normalized LMS and the block-wise average algorithms for small phase noise variance (or effective phase noise variance), while the one-tap normalized LMS algorithm shows a better performance than the other two algorithms for large phase noise variance (or effective phase noise variance). In addition, the one-tap normalized LMS algorithm is more sensitive to the level of modulation formats.

Photonic frequency up‐conversion of baseband pulses for IR‐UWB using optical beating and balanced coherent detection

ABSTRACTThis article describes a photonic scheme that performs frequency up‐conversion of baseband pulses for potential applications on Impulse Radio‐Ultra Wideband systems. Mixing a laser modulated by a pulse with a wavelength‐tunable laser (local oscillator) and a coherent photodetection scheme we obtain high order pulses comparable with nth derivatives of a Gaussian pulse. With our approach is possible to perform the frequency translation of the baseband pulses so that UWB pulses within standardized UWB mask are obtained. Baseband components produced in the optical mixing are removed by using a balanced photodetection stage. Our scheme exploits two different microwave photonics technics (microwave generation and coherent detection) for accomplish a simple generator of high order pulses avoiding the use of complex schemes. Additionally, the scheme can be implemented with commercially available components and it is reconfigurable in a very simple way. The performance of the proposed scheme is analyzed and evaluated in this work.© 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2749–2755, 2016

Optical digital coherent detection technology enabled flexible and ultra-fast quantitative phase imaging.

Quantitative phase imaging has been an important labeling-free microscopy modality for many biomedical and material science applications. In which, ultra-fast quantitative phase imaging is indispensable for dynamic or transient characteristics analysis. Conventional wide field optical interferometry is a common scheme for quantitative phase imaging, while its data acquisition rate is usually hindered by the frame rate of arrayed detector. By utilizing novel balanced-photo-detector based digital optics coherent detection techniques, we report on a method of constructing ultra-fast quantitative phase microscopy at the line-scan rate of 100 MHz with ~2 μm spatial resolution.

Tracking performance of optical phase locking loop with frequency discrimination and control subloop

Optical phase locking is a key technique in homodyne coherent optical communication, coherent optical detection, and active coherent laser beam combination. In these applications, environmental temperature variation and mechanical vibration would affect the accuracy of phase locking, or even cause losing lock. These disturbances are generally equivalent to introducing phase jitter, phase step, frequency ramp, and frequency step in the loop. A frequency discrimination and control subloop is introduced to improve the frequency acquisition, and the tracking performance is studied experimentally. The loop can track phase step in 0.2 ms, and precisely track ±π/2 sine phase jitter for jittering frequency lower than 1 kHz. For frequency ramp, the residual phase error is unaffected for ramping rates slower than 40 MHz/s. The frequency discrimination and control subloop makes the loop lock quickly under a frequency step larger than the pull-in frequency. The mean tracking time is 31 ms for a 1 MHz frequency step. The maximum trackable frequency step is around 160 MHz. Continuous or step variation of phase and frequency could be tracked by the loop with the frequency discrimination and control subloop.

Single-Fiber Lightwave Centralized WDM-OFDMA-PON With Colorless Optical Network Units

We propose and experimentally demonstrate a carrier-reuse,single-feeder,wavelength-division-multiplexed, orthogonalfrequency-division-multiple-accesspassiveoptical network (WDM-OFDMA-PON) with colorless direct-detection optical network units and coherent detection optical line terminals. We examine two strategies by adjusting the frequency occupancy and the modulation format of the uplink (UL) and downlink (DL) signals. We investigate the impact of the DL signal-to-carrier ratio on the performance of both the UL andDLsignals via simulations and identify impairments limiting system performance. As a proof of concept,we demonstrate on a single wavelength channel a realization of each of the two scenarios investigated using orthogonal frequency division- multiplexing (OFDM). A quadrature phase-shift keying approachwith a wide spectrum and a narrow guard band achieves 21.6 Gb/s.A32-aryquadrature amplitudemodulation approach with a narrow spectrum and a wide guard band achieves 14.5 Gb/s and a span of over 80 km.

Self‐homodyne detection technique using extracted local oscillator light by GS‐SOA in optical coherent transmission

ABSTRACTA self‐homodyne coherent detection technique using gain‐saturated SOA (GS‐SOA) is proposed. In the proposed system, local oscillator (LO) for optical coherent detection is extracted from the optical signal which is sent from the transmitter using the fact that the modulated downlink data can be erased with SOA operation in saturation region. Therefore, carrier frequency offset noise does not occur because LO light which is extracted from the received optical signal has totally same wavelength with the optical signal. In order to operate SOA operates in deep saturation region, high optical input power is needed. The proposed scheme was experimentally demonstrated, and the feasibility was discussed with the experimental results. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:561–565, 2016

Orthogonal-band-multiplexed offset-QAM optical superchannel generation and coherent detection.

Nowadays the Internet not only has fast growing data traffic, but also has a fast growing number of on-line devices. This leads to high demand of capacity and flexibility of the future networks. The conventional Orthogonal Frequency Division Multiplexing (OFDM) and Nyquist pulse shaping signals have the advantage of high spectral efficiency when consisting of superchannels in the Wavelength-Division-Multiplexing (WDM) way. However, they face a cost issue when the spectral granularity of the superchannel is decreased to support more users. This paper proposes for the first time the scheme of Orthogonal-band-multiplexed offset-Quadrature Amplitude Modulation (OBM-OQAM) superchannel. OBM-OQAM superchannel provides large capacity and high spectral efficiency. Furthermore, it has the advantage of offering subbands of variable symbol rate without changing the system configuration. We provide a proof-of-principle demonstration of OBM-OQAM superchannel transmission. In our experiment, 400 Gbps 16 Quadrature Amplitude Modulation (QAM) OBM-OQAM superchannel transmission over 400 km Standard Single Mode Fiber (SSMF) is conducted. The experimental results show that the OBM-OQAM signal has low penalty in multi-band aggregation.

Theoretical and experimental investigations on measuring underwater temperature by the coherent Brillouin scattering method.

In this paper, a new method of measuring a water-stimulated Brillouin scattering (SBS) frequency shift by optical coherent detection is presented, in order to remote-sense the underwater temperature of the ocean. A single longitudinal mode, passively Q-switched pulsed Nd:YAG laser is used as the light source, the water SBS beam is used as the signal beam, and a portion of the incident laser beam is used as the local oscillator. The heterodyne is detected by a high-speed photodetector, and the heterodyne frequency is the Brillouin frequency shift. Therefore, the underwater temperature can be determined according to the relationship between the Brillouin frequency shift and the water temperature. To test and verify its practicability, the heterodyne waveforms at different water temperatures are recorded in the laboratory with a wide-band oscilloscope, and the Brillouin frequency shifts are deduced by a Fourier transform. The experimental results are consistent with the theoretical analysis. This work provides the foundation for the development of a water temperature measurement system based on coherent Brillouin scattering.

Photonic Downconversion and Linearization of Microwave Signals from the <inline-formula> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula>- to <inline-formula> <tex-math notation="LaTeX">$K\!$ </tex-math></inline-formula>-Band

A linearized photonic radio frequency (RF)-to-intermediate frequency (IF) conversion system is proposed and experimentally demonstrated based on balanced detection and digital signal postcompensation. The optical carrier suppression and remodulation scheme is employed to significantly reduce the local oscillator frequency requirements and common mode optical phase noises. Photonic IF downconversion is enabled by using a reconfigurable multiport optical processor and balanced coherent detection of the spectrally separated lower and upper first-order sidebands of the phase modulated signals. Moreover, the digital linearization technique is used to suppress the third-order intermodulation components by more than 21 dB and an improvement of the spurious free dynamic range as large as 7 dB for RF signals ranging from 8–20 GHz is also achieved.

Laser-phase-fluctuation-insensitive offset-frequency-spaced two-tone optical coherent detection scheme and its transmission for radio-over-fiber systems

We propose a new optical coherent detection scheme with a "two-tone" local light, which is also in principle insensitive to the laser phase fluctuation and is assisted by the digital signal processing technique for radio-over-fiber (RoF) systems. The main feature of our proposal is that the frequency separation of the two-tone local light is different from that of the RoF signal, which is called "offset-frequency-spaced" in this paper. First, we explain the principle of our new proposal and experimentally demonstrate the data recovery. Then, the influence of the frequency detuning between photo-detected modulated and unmodulated signals is discussed. Moreover, the transmission performance with an error vector magnitude (EVM) is evaluated for the optical coherent detection of a 10-Gbaud quadrature-phase-shift-keying RoF signal after a 20-km-long standard single-mode fiber transmission.

Novel optical scanning cryptography using Fresnel telescope imaging.

We propose a new method called modified optical scanning cryptography using Fresnel telescope imaging technique for encryption and decryption of remote objects. An image or object can be optically encrypted on the fly by Fresnel telescope scanning system together with an encryption key. For image decryption, the encrypted signals are received and processed with an optical coherent heterodyne detection system. The proposed method has strong performance through use of secure Fresnel telescope scanning with orthogonal polarized beams and efficient all-optical information processing. The validity of the proposed method is demonstrated by numerical simulations and experimental results.

Research of 100Gbit/s DP-QPSK Based on DSP in WDM-PON System

This paper introduces the DP-QPSK modulation coherent reception technology,the overall design and other key is sues for 100 Gbit/s DP-QPSK modulation. Using the technologies based on DSP, it realizes the long-range transmissions of 100 Gbit/s optical systems to achieve optical signal dispersion compensation, polarized solution reuse and phase estimation. The effect of this scheme is verified with OptiSystem and the simulation results indicate that this with the help of DSP module for processing of the received signal, the last constellation is ideal and scheme is implemented simply and has high reliability, and it also has reference significance for the optimization of coherent optical detection. The results show that the technical plan is easy and reliable, which has reference significance on optimization of coherent optical detecting hardware.

OLT Receiver for Power Normalization of Burst OFDM Signals Enabling OFDM/TDMA-PON

An optical line terminal receiver, composed of an optical gain-control stage based on an optical coherent detection followed by an electrical front-end amplifier, is verified to be able to normalize upstream burst orthogonal frequency division multiplexing (OFDM) signals with different power levels, thus alleviating a substantial limiting factor in OFDM/time division multiplexing access-passive optical network (PON) system development. A receiver dynamic range of 25 dB and system power budget of 29 dB are achieved. Experiments confirm that the receiver meets the linearity requirement posed by OFDM signals while satisfying the dynamic range and power budget specified by the current PON standards.

Performance Optimization of Nyquist Signaling for Spectrally Efficient Optical Access Networks [Invited

In this work, Nyquist pulse-shaping technology is exploited for future implementations of optical access networks. The paper first presents technical issues regarding digital implementation of Nyquist spectral shaping for low-symbol-rate systems. Second, the potential benefits regarding mitigation of cross talk (linear and nonlinear) of Nyquist pulse shaping in symmetric and bidirectional wavelength-division multiplexing passive optical networks (WDM-PONs) are presented. Lastly, the paper proposes a WDM-PON solution employing self-coherent detection at the optical network unit and heterodyne coherent detection at the optical line terminal. In this proposed architecture, a multi-objective optimization strategy is developed to enhance the transmission performance of the Nyquist-shaped bands in full-duplex mode.

Homodyne OFDM with Optical Injection Locking for Carrier Recovery

Homodyne detection provides the simplest digital signal processing (DSP) solution to optical coherent detection and minimizes the receiver bandwidth requirements. These features make it promising for high spectrally efficient formats such as optical orthogonal frequency domain multiplexing (OFDM), which has a flat optical spectrum and which is thus inherently sensitive to high-frequency distortions, e.g., due to limited detector bandwidth. The key to homodyne detection is recovery of the carrier from the received signal all optically (as opposed to frequency offset compensation via DSP. Herein, we use optical injection locking (OIL) in conjunction with carrier tone-assisted OFDM to achieve this. In contrast to previous reports, we show that OIL carrier recovery with subsequent homodyne detection can operate without the need for any optical prefiltering. First, we evaluate the performance as a function of the carrier tone guardband bandwidth. Further, we improve the robustness of this technique using a slow-phase lock loop that compensates for drift in the laser's temperature/current control electronics. Using this improved setup, we compare our all-optical-carrier-recovered homodyne and the “traditional” DSP-assisted intradyne detection for the case of OFDM-16QAM signals. Finally, we compare the computing complexity necessary for both approaches and estimate the intradyne performance limitations due to the carrier-local oscillator frequency offset.

Timing and phase jitter suppression in coherent soliton transmission.

We have revisited soliton transmission in the new context of coherent optical detection optimizing and comparing digital backward propagation and in-line optical filtering as a means to suppress soliton timing and phase jitter. We find that in-line optical filtering allows one to improve the reach of the soliton system by up to the factor of 2. Our results show that nonlinear propagation can lead to performance beyond the nonlinear Shannon limit.