Optical phase-locked PSK heterodyne experiment at 4 Gb/s

A 4 Gb/s phase-locked optical PSK (phase shift keying) heterodyne communication system is demonstrated. The receiver was implemented with a single 100-Ω loaded p-i-n photodiode and a 1320-nm diode-pumped miniature Nd:YAG laser as a local oscillator. For a 2⁷-1 PRBS (pseudorandom bit sequence), the receiver sensitivity was -34.2 dBm or 631 photons/bit. The corresponding power on the surface of the detector was -37.3 dBm or 309 photons/bit. With a 2¹⁵-1 PRBS, a 2.6 dB additional sensitivity degradation was observed due to the nonideal frequency response of the phase modulator and the receiver amplifiers     

An optical PSK homodyne transmission experiment using 1320 nmdiode-pumped Nd:YAG lasers

A second-order optical phase-locked loop was constructed using 1320-nm diode-pumped miniature Nd:YAG ring lasers. Using the loop, a 140-Mb/s PSK homodyne transmission experiment was demonstrated over 28.6 km of single-mode fiber. With a loop natural frequency of 13 kHz and a damping factor of 0.6, the receiver sensitivity was -62.8 dBm, or 25 photons/bit. The authors believe this is the highest sensitivity obtained to date with any optical communication system     

Coherent optical QPSK intradyne system: concept and digitalreceiver realization

A receiver concept based on optical quadrature phase-shift keying (QPSK) and a digital realization of synchronous demodulation including phase synchronization is presented. To keep the signal processing bandwidth low a phase diversity receiver, called an intradyne receiver, with an orthogonal electrical demodulation is proposed. Basic principles of the synchronous orthogonal and digital demodulation are described. After the evaluation of the shot noise limit some aspects of the digital phase-locked loop (PLL) are presented. In a 100-Mb/s transmission system a receiver sensitivity of -51.6 dBm has been measured. The loss in relation to the shot noise limit of -66.3 dBm (18 photons/b) is mostly due to the low local laser power and the influence of the receiver input noise     

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     

560 Mbit/s optical PSK heterodyne detection using carrier recovery

An optical PSK heterodyne synchronous detection experiment was performed at a bit rate of 560 Mbit/s using carrier recovery in the IF stage. A receiver sensitivity of -51.6 dBm was achieved, and the power penalty due to the phase noise of the laser diodes was suppressed to less than that of DPSK.<>     

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.     

Bipolar optical FSK transmission experiments at 150 Mbit/s and 1Gbit/s

Bipolar direct modulation has been used to overcome the nonuniform low-frequency modulation response of distributed-feedback laser transmitter in optical FSK transmission experiments at 150 Mb/s and 1 Gb/s. The heterodyne receiver sensitivity was -48 dBm for 150 Mb/s and -39 dBm for Gb/s, independently of the pseudorandom pattern length. There was no degradation in receiver sensitivity with transmission through more than 100 km of fiber     

4-Gb/s PSK homodyne transmission system using phase-lockedsemiconductor lasers

Homodyne detection of 4-Gb/s pilot-carrier binary-phase-shift-keyed (BPSK) optical signals using external-cavity semiconductor lasers synchronized by a linear phase-locked loop is discussed. A 2¹⁵-1 pseudorandom binary sequence (PRBS) has been transmitted through a short fiber with a receiver sensitivity of -44.2 dBm or 72 photons/bit. After transmission through 167 km of standard single-mode fiber, the sensitivity is -43.6 dBm or 83 photons/bit. A balanced PIN/HEMT transimpedance receiver which has a 3-dB bandwidth from 100 kHz to 10.1 GHz and an average equivalent input noise current of 10.8 pA/√Hz is used     

1 Gbit/s bipolar optical FSK transmission experiment over 121 km offibre

Bipolar direct modulation has been used to overcome the nonuniform low-frequency modulation response of a DFB laser transmitter in a 1 Gbit/s optical FSK transmission system. The heterodyne receiver sensitivity (nP¯) was -39 dBm for a 2²³-1 bit pseudorandom pattern, with no degradation in receiver sensitivity after transmission through 121 km of fibre     

1 Gbit/s PSK homodyne transmission system using phase-lockedsemiconductor lasers

Homodyne detection of 1 Gb/s pilot-carrier (BPSK) optical signals using phase-locked 1.5 μm external-cavity semiconductor lasers is discussed. After 209 km fiber transmission of a 2¹⁵-1 pseudorandom binary sequence (PRBS), the measured receiver sensitivity is 52.2 dBm or 46 photons/bit. Experimental evidence of the data-to-phase-lock crosstalk that potentially limits the usable ratio of linewidth to bit rate in pilot-carrier PSK homodyne systems is presented     

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     

Optimum system design for CPFSK heterodyne delay demodulationsystem with DFB LDs

An optimum system configuration for an optical continuous-phase frequency-shift-keying (CPFSK) heterodyne delay demodulation system with distributed feedback laser diodes (DFB LDs) is discussed. The optimum modulation index was determined by evaluating the LD phase noise effect and the IF noise effect. The IF noise effect was investigated in detail, considering the noise conversion effect through delay demodulation. In the case of 10-MHz IF beat spectral width, the modulation index m =1.5 is optimum for a 1.2-Gb/s system. With this optimum modulation index, a 204-km long-span transmission experiment, with -41.5 dBm receiver sensitivity, has been successfully performed. The feasibility of using stand-alone DFB LDs for a high-sensitivity CPFSK delay demodulation system has been confirmed through this experiment     

An optical FSK heterodyne dual filter detection system for takingadvantage of DFB LD applications

An optimum system configuration for an optical frequency-shift keying (FSK) heterodyne dual-filter detection system with distributed feedback laser diodes (DFB LDs) is investigated, taking into consideration LD phase noise influence. Experimental and theoretical examination show that an IF filter bandwidth greater than 10 times the beat spectral linewidth is necessary to avoid LD phase noise influence. A 301-km long-span transmission experiment has been successfully carried out with an optimum configuration for 34 Mb/s. High receiver sensitivity, -61.8 dBm with more than 10 dB improvement over the direct detection system, has also been achieved. Experimental results at higher bit rates of 140, 200, and 280 Mb/s indicate that a modulation index greater than two is desirable to avoid cross talk between mark and space signals. With sufficient frequency deviations, high receiver sensitivities of -54.7 dBm (140 Mb/s) and -52.5 dBm (200 Mb/s) have been achieved. These represent 9.6- and 9-dB sensitivity improvement, respectively, over direct detection systems. A guide for FSK dual-filter detection system design is derived from the experimental and theoretical results. Potential application regions for a dual-filter detection system with DFB LDs are determined     

565 Mbit/s optical DPSK and PSK heterodyne transmission experiment using Nd:YAG lasers at 1.3 mu m

A 565 Mbit/s optical DPSK and PSK transmission experiment is described. Stable monolithic Nd:YAG ring lasers were used as light sources. A PSK demodulation scheme achieved a receiver sensitivity of -53.4 dBm and was found to be 1.3 dB better than DPSK.<>     

A 1320 nm experimental optical phase-locked loop

An experimental balanced optical second-order phase-locked loop constructed using 1320 nm diode laser pumped miniature Nd:YAG lasers is discussed. The loop is stable and has a phase error of less than 1.8° when the received signal power is -65 dBm or more. The phase error appears to be dominated by the lasers' frequency noise as long as the signal power is more than -60 dBm     

采用NPRO光源的零差相干光通信实验

在空间相干光通信应用中,针对传统激光器线宽较宽、相位噪声大、易导致锁相环路失锁的问题,研制了单频Nd:YAG非平面环形（NPRO）激光器,其线宽小于1 kHz,相对强度噪声（RIN）低于-150 dB/Hz,具有窄线宽、低噪声的特点。搭建了光锁相环路,在信号光功率-67 dBm的情况下实现了两台NPRO激光器的相位锁定。在此基础上开展了信号频率为10 MHz和1.25 GHz的模拟通信实验,在信号光功率分别为-60 dBm和-53 dBm时可观测到较理想的眼图。在2.5 Gbps数字通信实验中,接收灵敏度达到-50 dBm,此时误码率为3.210-6。系统灵敏度可接近量子极限,明显优于传统的IM/DD方式,是一种适合长距离、大容量传输的空间通信方式。     

A high-performance optical receiver for 622 Mb/s direct-detectionsystems

A report is presented on the measurement of receiver sensitivity and noise characteristics of a high-performance optical receiver using a low-noise InGaAs avalanche photodiode (APD) and a low-noise high-electron mobility transistor (HEMT). At a bit rate of 622.08 Mb/s and a wavelength of 1.297 μm, the measured receiver sensitivity is -48.3 dBm. This is equivalent to a sensitivity of 155 photons/b and is about 12 dB away from the quantum limit of 10 photons/b     

Long-span optical FSK heterodyne single-filter detection transmission experiment using a phase-tunable DFB laser diode

High receiver sensitivity (?51.9 dBm) and long span (243 km) transmission expriments have been achieved with a 140 Mbit/s optical FSK heterodyne single-filter detection system, using a phase-tunable DFB laser diode as a transmitter. This has enabled direct FSK modulation without waveform distortion. Also, a 280 Mbit/s 204 km transmission experiment has been carried out successfully.     

High-performance dense FDM coherent optical network

The results obtained with a densely spaced frequency division multiplexing (FDM) optical star network providing random channel selection by a digitally tuned high-sensitivity heterodyne receiver are presented. The experimental system consists of six 200-Mb/s frequency-shift keyed (FSK) modulated channels, spaced by 2.2 GHz and multiplexed by a star coupler. Channel selection is performed by a computer-controlled random-access heterodyne receiver having a sensitivity of 74 photons/b at a BER of 10^-9, which is 1.7 dB from the quantum limit. The digital-tuning random-access capability (which depends on the frequency-tuning current relationship of the local oscillator (LO) laser) is protected against frequency drifts of the LO laser. The receiver can also detect the absence of a selected channel. In this case, the receiver locks to the next available channel and displays which channel is received instead. The results obtained indicate that this system has potential throughput of 2000 Gb/s     

Penalty free biphase linecoding for pattern independent FSKcoherent transmission systems

Biphase coding is used to eliminate the unwanted influence of nonuniform laser characteristics in frequency-shift-keying (FSK) transmission systems. Compared to other (ternary) linecoding schemes, biphase has the advantage that it does not give a 3-dB sensitivity penalty when properly decoded in the receiver. This has been demonstrated in a 140-Mb/s 280-Mbaud transmission experiment. The receiver features a new type of reliable unambiguous clock extraction. Due to the compact IF spectrum, as compared with ternary linecoding schemes, biphase equipped FSK systems are especially suited for efficiently spaced coherent multichannel (CMC) systems. FSK heterodyne transmission experiments show a sensitivity ηP of -56.7 dBm for 2¹⁰-1 pseudorandom patterns, only 4.8 dB from the shot-noise limit