560 Mb/s optical PSK synchronous heterodyne experiment

A phase-locked optical heterodyne receiver constructed using a 1320-nm diode-pumped miniature Nd:YAG ring laser is discussed. Using this receiver and a transmitter based on another Nd:YAG laser, a 560-Mb/s phase-shift keying (PSK) synchronous heterodyne transmission was demonstrated over 78 km of single-mode fiber. With an optical phase-locked loop (PLL) natural frequency of 32 kHz and a damping factor of 1.46, the receiver sensitivity, measured at the output of the transmission link, was -48.7 dBm, or 159 photons/b. The corresponding detected sensitivity, measured on the surface of the p-i-n diode, was -51.8 dBm or 78 photons/b. This result suggests that the receiver sensitivity would have been about 82 photons/b if a balanced receiver with 0.2-dB excess coupler loss had been used. The impact of the finite intermediate frequency (IF) on heterodyne system performance was investigated; it was found that an IF of at least twice the bit rate is needed for a negligibly small penalty     

An example of a hybrid system: coherent optical system with Costasloop in receiver-system for transmission in baseband

The possibility of hybrid system construction is considered in this paper. The coherent optical system is the constituent part of this system. The first part of the system is constructed in passband (optical cable is transmission medium), and the second part of the system is constructed in baseband. The heterodyne optical receiver is used as optical receiver. Estimation is not done just after demodulation, but it is done at the end of the section in baseband. Taking into account shot noise of photodetectors, laser phase noise (transmitter laser and local laser) and noise which is generated during transmission by means of the section in baseband, we determine the performances of the proposed system     

Frequency stabilization of semiconductor lasers for applications in coherent communication systems

The stabilization of the mean frequency of a single-mode semiconductor laser used under modulation in a FSK heterodyne transmission system is achieved by a novel method. The frequency stabilization scheme locks the laser to one of the resonance frequencies of a Fabry-Perot interferometer (FPI). The laser is weakly modulated at a frequency higher than any signal frequencies. Light reflected from the FPI is detected and synchronously demodulated for feedback and current control of the laser. A laser modulated by a pseudorandom sequence with a rate of 140 Mbit/s and an optical frequency deviation of 280 MHz is stabilized with this method against variations in operating temperature and bias current. The mean optical frequency is maintained within ± 10 MHz for hours. Analysis and experiments confirm that the laser frequency remains stable even when the laser linewidth changes under weak optical feedback.     

Optical coherent broad-band transmission for long-haul anddistribution systems using subcarrier multiplexing

Signal multiplexing techniques for coherent optical transmission are compared, and appropriate application for a coherent subcarrier multiplexing (SCM) system is discussed. Optical frequency modulation (FM) using direct modulation of a distributed-feedback laser diode (DFB-LD) and a heterodyne detection is shown to be feasible. A transmission system using a local laser in the transmitter is unaffected by polarization and is cost effective. Phase noise can be suppressed by a phase-noise-canceling circuit (PNC) in a heterodyne receiver. This circuit can also effectively compensate for the frequency of instability of light sources. A theoretical simulation of a coherent SCM system showed that 100 channels of 30-MHz FM signal or 15 channels of 155-Mb/s signal can be distributed to 10000 subscribers using single-stage or double-stage optical amplifiers     

Optical heterodyne receiver providing random channel selection

The authors describe a digitally tuned optical heterodyne receiver which can randomly access the channels of an FDM optical communication system. The receiver can identify missing channels, and is insensitive to variation of the LO laser frequency relative to the tuning current.<>     

Alternate-mark-inversion optical continuous phase FSK heterodynetransmission using delay-line demodulation

Alternate-mark-inversion (AMI) optical continuous phase (CP) frequency-shift keying (FSK) heterodyne transmission using delay-line demodulation which has a higher receiver sensitivity compared to the transmission using single-filter demodulation has been achieved. Since the proposed delay-line demodulation method directly converts the received AMI-CPFSK signal into the nonreturn to zero (NRZ) baseband signal without the use of an AMI decoder, a heterodyne receiver with the same configuration as the conventional CPFSK transmission system is realized, even though AMI line coding is applied to the system. Transmission experiments at 622 Mb/s have been demonstrated for both pseudorandom 2⁷-1 and 2²³-1-b patterns at the same receiver sensitivity of -41.1 dBm (the PD input power)     

窄线宽激光器线宽测量方法

回顾了窄线宽激光器线宽测量的各种方法和发展过程,介绍了利用光外差法测量窄线宽激光器线宽的基本原理,描述了双光束外差法和延时自外差法的不同测试机理。针对延时零拍自外差法容易引起的系统误差,说明了光源调制和光路调制移频非零拍自外差法不同改进方案的优缺点。综述了窄线宽激光器测量线宽的新方法。对窄线宽激光器线宽测量方法进行了较全面的梳理,从发展过程看双光束外差法和延时自外差法有着各自的测量优势。     

Phase noise cancellation in a carrier recovered optical heterodynereceiver

The performance and application of an optical heterodyne receiver which uses a carrier recovery demodulator are described. Phase sensitive demodulators used in coherent optical transmission are compared, and the suppression of both phase noise and frequency instability of light sources by a carrier recovery (CR-) demodulator is described. A carrier recovered PSK (CR-PSK) demodulator and a phase noise canceling circuit (PNC) for a coherent SCM receiver are introduced as examples of CR-demodulators. The relationship between laser diode spectral linewidths and the delay time difference between the two paths in the CR-PSK demodulator necessary to keep the system performance within a certain power penalty is then derived. In a preliminary experiment using 560-Mb/s CR-PSK transmission, a receiver sensitivity of -51.6 dBm was obtained, and a laser phase noise suppression of about ¹/₂ that of DPSK was confirmed. The results suggest the possibility of constructing a heterodyne receiver which has no AFC-loop. Applications of a CR-demodulator to an optical frequency division multiplexing (OFDM) system and to a multivalue modulation scheme are discussed     

Modulation and demodulation techniques in optical heterodyne PSKtransmission systems

Modulation and demodulation techniques are described for an optical PSK heterodyne transmission system operating at 560 Mb/s and 1.2 Gb/s. Performance limitations affecting the receiver sensitivity in a 1.2-Gb/s DPSK system, such as laser phase noise, phase modulation depth, IF center frequency deviation, and local laser power, are studied. High receiver sensitivities for PSK systems were achieved. The applicability of the Mach-Zehnder modulator as a phase modulator for 1.2-Gb/s DPSK is also demonstrated. A 1.2-Gb/s DPSK transmission of over 100 km, using polarization diversity with novel polarization-insensitive automatic frequency control in an attempt to overcome signal fading caused by polarization fluctuation in the transmitting fiber, is also described. A receiver sensitivity of less than -42.8 dBm and varying within 1.4 dB for all states of polarization was achieved. A multichannel high-definition TV (HDTV) transmission experiment using a DPSK polarization-diversity tunable receiver is described     

Effect of external cavity semiconductor laser phase noise on a DPSKheterodyne optical receiver

The effect of an external cavity semiconductor laser on the performance of a DPSK heterodyne optical system is evaluated. The actual shape of the frequency fluctuation spectral density of the optical source is taken into account, and a comparison with a PSK coherent heterodyne system is carried out     

激光相干雷达中光单次下变频技术

在相干激光雷达远程测距中,为提高雷达的探测距离和距离分辨力,通常使用大时宽带宽积的信号如调频信号对光载波进行调制,并在接收端进行脉冲压缩处理,为降低接收端数字信号处理的数据量以提高运算的实时性,需要将接收信号下变频至合适的频段。传统的外差式雷达接收机需对光信号和电信号分别进行一次下变频,导致系统结构较复杂,且受器件非理想化的限制,下变频过程中会引入额外的噪声,此外还存在镜像频率噪声干扰的问题,导致解调信号信噪比降低。提出了一种将光信号直接下变频至脉冲压缩所需频段的方案,该方案使用正交解调的方式进行,能够简化系统结构同时抑制镜频噪声。首先将本振光进行频移并分为两束,通过控制相位使两本振光相互正交。将信号光分为两束并分别与两路本振光在光电探测器表面进行混频,接着对电信号进行采集,通过相关算法对幅相不平衡进行矫正。经仿真和实验,该方法能够在有效简化激光相干雷达接收机系统结构的同时避免镜频噪声干扰,在10 GSps采样率下,相较于外差式接收机,解调信号信噪比提高了约3 dB。     

Control of unidirectional oscillation in semiconductor orbiterlasers

Unidirectional oscillation in a ring-resonator-type semiconductor laser was demonstrated in a square-shaped orbiter laser using an optical feedback mechanism. The bistable switching phenomenon between the unidirectional and bidirectional oscillation was observed synchronized with the switching of the longitudinal lasing mode. A longitudinal-mode bistability results from the compound cavity system formed by the orbiter laser and output waveguide. The feedback light from the output waveguide end interferes with the orbiter lasing light. Switching between the unidirectional and bidirectional oscillation was controlled by injecting a current into the output waveguide, which varies the phase condition of the feedback light     

Heterodyne detection with an injection laser. II. Signal-to-noiseratio

For pt.I see ibid., vol.26, p.85-93 (1990). In part I a theory of the conversion efficiency of the self-heterodyne laser detector that was first described by R.F. Kazarinov and R.A. Suris (1974) was presented. In this device a light signal is passed into the resonant cavity of an actively oscillating injection laser, causing an electrical signal at the difference frequency between laser and signal to flow through the wire supplying the DC bias to the laser. An expression is derived for the signal-to-noise ratio of the self-heterodyne laser detector. The result shows that in the limit of ideal operation (complete population inversion and no internal losses) the signal-to-noise ratio of the self-heterodyne laser detector reaches one-half of the quantum noise limit     

Secure Chaotic Transmission on a Free-Space Optics Data Link

In this paper, we numerically demonstrate secure data transmission, using synchronized “twin” semiconductor lasers working in the chaotic regime, which represent the transmitter and receiver of a cryptographic scheme, compatible with free-space optics technology for line-of-sight communication links. Chaotic dynamics and synchronization are obtained by current injection into the laser pair of a common, chaotic driving-signal. Results of simulations are reported for the configuration in which the chaotic driving-current is obtained by photodetection of the emission of a third laser (driver), chaotic by delayed optical feedback in a short cavity scheme, selected with different parameters with respect to the laser pair. The emissions of the synchronized, matched lasers are highly correlated, whereas their correlation with the driver is low. The digital message modulates the pumping current of the transmitter. Message recovery is performed by subtracting the chaos, locally generated by the synchronized receiver laser, from the signal obtained by photodetection (at the receiver side) of the chaos-masked message transmitted in free space. Simulations have been performed with the Lang-Kobayashi model, keeping into account both attenuation of the optical signal in a line-of-sight configuration, and noise. Security has been investigated and demonstrated by considering the effect, on synchronization and message recovery, of the parameter mismatch between transmitter and receiver.      

Polarization independent coherent optical receiver

This paper describes an optical heterodyne receiver for DPSK signals which can receive an optical signal having an arbitrary polarization state. This is achieved by splitting the received signal between two orthogonal polarization axes and processing the resulting two signals as in a conventional DPSK heterodyne receiver. The sum of the two demodulated signals provides a baseband signal independent of the polarization state of the received optical signal. When the receiver noise is dominated by the shot noise of the photodetectors, the receiver provides a BER of 10^-9for an average number of 22 photon/bit. In comparison, a conventional optical heterodyne receiver requires under the same noise condition 20 photon/bit to achieve the same BER for a received optical signal polarized along the polarization axis of the local optical signal.     

Feasibility study on QPSK optical-heterodyne detection system

The feasibility of a QPSK (quadrature-phase-shift-keying) optical heterodyne detection system for a high-speed and long-span coherent transmission is investigated. The key factors, which are the modulation method and the spectral linewidth requirement for optical light sources, are discussed. Through theoretical analysis and the simulation experiment, it is confirmed that the required beat spectral linewidth for QPSK coherent detection is less than 7.5×10^-5×bit rate. It is also shown that the required spectral linewidth for QPSK differentially coherent detection is less than 2.5×10^-5×bit rate. These results show that the gigabits-per-second range QPSK optical heterodyne detection can be realized with conventional external cavity laser diodes     

Performance of Random Metal-Dielectric Film on Optical Signal Transmissions

The transmission performances of a random metal-dielectric film composed of silver metal and spherical fused silica balls were experimentally investigated for intensity- and frequency-modulated optical signals. The film was formed on the facet of a laser diode, and intensity-modulated optical signals emitted from the laser diode were successfully transmitted through it. Frequency-modulation optical signals were successfully detected by using an optical heterodyne technique. These results indicate that this random metal-dielectric film is well suited for use as a waveguide for optical signal transmission.     

无线光外差检测系统研究与仿真

本文提出了一种新的基于副载波调制的无线光外差检测方案,用Optisystem对其进行仿真,计算了系统的外差增益,分析了影响无线光外差检测系统外差增益的几个主要因素,包括信号光的偏振态、激光器线宽、本振光功率及调制速率。从仿真结果可以得出：若信号光和本振光的偏振态不一致,会导致外差增益的下降;接收机灵敏度随光源激光器线宽的增大而降低,误码率要达到10-9,线宽最大不超过25KHz;增大本振光功率可以提高灵敏度,但这个提高不是无限地,存在一个最佳本振光功率,如果大于这个最佳值,信噪比反而会降低。     

Analogue video transmission over cabled monomode fibre using optical frequency modulation with heterodyne detection

The design and receiver sensitivity of a heterodyne detection system using analogue optical frequency modulation is examined for the transmission of a single baseband colour video signal. After transmission over 30 km of cabled monomode fibre, receiver sensitivity was measured to be ?70.3 dBm for a 53 dB unified weighted video SNR.     

Full-Duplex 1.0 Gbit/s Data Transmission Over 60 GHz Radio-on-Fiber Access System Based on the Loop-Back Optical Heterodyne Technique

Full-duplex transmission over a 60 GHz radio-on-fiber access system based on the optical heterodyne technique is verified in this paper. With this technique, an optical carrier generator on the transmitting side consolidates the functions of wavelength and polarization control for optical heterodyne detection in uplinks and optical millimeter-wave generation in downlinks, on behalf of each optical transmitter/receiver. Therefore, the whole system configuration can be significantly simplified. A full-duplex 1.0 Gbit/s transmission experiment over 10 km of single-mode fiber, which represents an access network transmission, is demonstrated using the simultaneous modulation/photodetection operation of an electroabsorption modulator (EAM). Bit error rates of less than 10$^{-9}$ were obtained in both the uplink and downlink without noticeable mutual interference, even though the laser source and the EAM is shared by both links, and no significant dispersion-induced degradation was observed. Moreover, the experimental results showed that an optical link budget of 9.2 dB can be achieved with the experimental configuration.