Performance improvement and wavelength reuse in millimeter-wave radio-over-fiber links incorporating all-fiber optical interleaver

Simultaneous downlink performance improvement and uplink wavelength reuse in a full-duplex millimeter-wave (MMW) radio-over-fiber (RoF) system by using a simple and cost-effective all-fiber optical interleaver are proposed and demonstrated. The MMW RoF downlink performance improvement is based on suppressing optical carrier-to-sideband ratio (OCSR), with which the mechanism is confirmed by theoretic analysis and derived experimental results. Measured results show that, by suppressing OCSR using a fabricated all-fiber optical interleaver, the downlink optical receiver sensitivity is improved about 2.1 dB. The downlink data rate is 1.25 Gbit/s and the carrier frequency is 58.1 GHz; the link consists of 6 km optical single-mode fiber and 1 m wireless connection. On the other hand, with the interleaver suppressing downlink OCSR, simultaneously an optical carrier is recovered from the RoF downlink and is reused for RoF uplink transmission. The uplink is operated at 62.9 GHz and the data rate is the same 1.25 Gbit/s. With the recovered optical carrier, a laser-free remote access point is achieved. The principle, structure, and fabrication of an all-fiber optical interleaver are also presented in this paper.     

64 GHz optical millimeter-wave generation by octupling 8 GHz local oscillator via a nested LiNbO3 modulator

A novel scheme to generate a 64 GHz optical millimeter (mm)-wave via a nested LiNbO₃ Mach-Zehnder modulator with an 8 GHz local oscillator is proposed and simulated. Since the frequency response of the modulator and the local oscillator frequency are greatly reduced, the bandwidth requirements of the optical and electrical components in the transmitter are significantly decreased. The simulation results show that the generated optical mm-wave signal maintains good performance even after being transmitted over 20 km standard single-mode fiber.     

All-optical clock frequency divider using Fabry-Perot laser diode based on the dynamical period-one oscillation

We propose and experimentally demonstrate a frequency divider implemented by an optically injected Fabry-Perot laser diode (FP-LD) based on the nonlinear dynamical period-one oscillation. Injected by optical pulses, the FP-LD will oscillate in unstable dynamical period-one (P1) oscillation. Through changing the injected strength, emitting wavelength and bias current of the FP-LD, the oscillating frequencies of the P1 state can be varied. Once one of the harmonic frequencies is adjusted to match the repetition frequency of injected optical clock pulse, the P1 oscillation will be locked, and then a divided clock at the fundamental frequency of the P1 oscillation can be generated. By utilizing this divider, we can achieve the optical clock frequency division of divide-by-two, -three and -four in a wide input frequency range, for instance, of 9.0 to 20.0 GHz for divide-by-two. The influence of injected optical power on the timing jitter of the divided clock is also investigated. It is expected that this frequency divider can be applied to high frequency division exceeding 100 GHz due to its fast P1 oscillation.     

Enhancement of chaotic carrier bandwidth in laser diode transmitter utilizing external light injection

Semiconductor laser with optical feedback emitting chaotic optical signal can be treated as chaotic carrier transmitter. Based on laser rate equations, we numerically study the effect of external injection light on the bandwidth of chaotic carrier transmitter. Our numerical simulation shows that the bandwidth of the chaotic carrier transmitter can be enhanced significantly by external photons injection. Compared with the 2 GHz relaxation oscillation frequency of a solitary laser diode without optical injection, the bandwidth of a chaotic carrier transmitter is expanded to 14.5 GHz with injection parameter at k_inj = 0.39. Simulation results also demonstrate that the enhanced bandwidth depends obviously on the frequency detuning between the external injection laser diode and the chaotic carrier transmitter. The maximum bandwidth of the chaotic transmitter can be obtained when the frequency of the injected light is higher than the central frequency of the carrier transmitter between 2 GHz and 4 GHz.     

Experimental investigation of the impact of optical injection on vital parameters of a gain-switched pulse source

An analysis of optical injection on a gain-switched distributed feedback (DFB) laser and its impact on pulse parameters that influence the performance of the pulse source in high-speed optical communication systems is presented in this paper. A range of 10 GHz in detuning and 5 dB in injected power has been experimentally identified to attain pulses, from an optically injected gain-switched DFB laser, with durations below 10 ps and pedestal suppression higher than 35 dB. These pulse features are associated with a side mode suppression ratio of about 30 dB and a timing jitter of less than 1 ps. This demonstrates the feasibility of using optical injection in conjunction with appropriate pulse compression schemes for developing an optimized and cost-efficient pulse source, based on a gain-switched DFB laser, for high-speed photonic systems.     

Simulation of DWDM signals using optimum span scheme with cascaded optimized semiconductor optical amplifiers

We numerically simulated the ten channels at 10 Gb/s dense wavelength division multiplexing (DWDM) transmission faithfully over 17,227 km using 70 km span of single mode fiber (SMF) and dispersion compensating fiber (DCF) using optimum span scheme at channel spacing 20 GHz. For this purpose, inline optimized semiconductor optical amplifiers (SOAs) and DPSK format are used. We optimized the SOA parameters for inline amplifier with minimum crosstalk and amplified spontaneous emission noise with sufficient gain at bias current 400 mA. For this bias current, constant gain 36.5 dB is obtained up to saturation power 21.35 mW. We have also optimized the optical phase modulator bandwidth for 400 mA current which is around 5.5 GHz with crosstalk −14.2 dB between two channels at spacing 20 GHz.We show the 10×10 Gb/s transmission over 70 km distance with inline amplifier has good signal power received as compared to without amplifier, even at equal quality factor. We further investigated the optimum span scheme for 5670 km transmission distance for 10×10 Gb/s with channel spacing 20 at 5.5 GHz optical phase modulator bandwidth. As we increase the transmission distance up to 17,227 km, there is increase in power penalty with reasonable quality.The impact of optical power received and Q factor at 5670 and 17,227 km transmission distance for different span schemes for all channels has been illustrated. For launched optical power less than saturation, all channels are obtained at bit error rate floor of 10⁻¹⁰.     

High-resolution spectroscopy based on optical phase modulator and optical frequency comb

A phase modulation of an optical frequency comb has been applied to measure a fine spectrum in the 1.5 μm wavelength range by the optical heterodyne-detection method. The measurement frequency range covered 25 GHz, which satisfies the frequency interval of the optical frequency comb, with a spectral resolution of 1 MHz.     

All optical arbitrary waveform generation by optical frequency comb based on cascading intensity modulation

We propose and experimentally demonstrate an all optical arbitrary waveform generation by optical frequency comb (OFC) based on cascading intensity modulation. By selecting spectral lines of interest from OFC through optical filters, 10 GHz, 20 GHz, and 60 GHz sinusoidal signals with low phase noise and more complex waveforms, including ultra-short pulse, half-wave cosine, and single frequency modulated MMW signals, are generated easily.     

Loss-coupled distributed-feedback lasers with amplified optical feedback for optical microwave generation

Multisection semiconductor lasers for optical microwave generation have been fabricated that consist of a loss-coupled distributed feedback (LC-DFB), a phase control, and an amplifier section. High-frequency self-pulsations are generated according to the concept of a single-mode laser with short optical feedback. The effect of the optical feedback via the phase control and the amplifier section on the self-pulsation is apparently shown as a result of the superior single-mode characteristic of the LC-DFB section. Continuous frequency tuning is achieved in the range of 17-35 GHz.     

外光注入半导体激光器实现时钟分频

从理论和实验上研究了利用光注入半导体激光器对高重复速率光脉冲产生的周期振荡和时钟分频现象.结果表明，光注入半导体激光器引起的二倍周期振荡是使注入脉冲重复频率分频的直接原因.通过耦合速率方程，数值模拟了半导体激光器在外光注入时输出光的时间序列和功率谱，并且分析了激光腔内各种周期振荡的特征.研究表明，当注入光使半导体激光器出现稳定的二倍周期振荡，且注入光的重复频率为此振荡频率的二倍时，时钟分频即可产生实验中，采用重复频率为6.32GHz的光脉冲注入Fabry-Perot激光器，实现了3.16GHz时钟分频信号 关键词： 周期振荡 时钟分频 光谱侧带 光注入     

Periodically modulated emission and the possibility of generating “giant” pulses in a three-level laser using a nonuniformly excited active medium

Summary The above analysis has shown that nonuniformity of the excitation of the active medium of a laser leads, even in the single-mode approximation, to an increase in the possible motions of the system and, in particular, to the possibility of generating giant pulses and unattenuated periodic self-modulation of emission. Clearly, allowance for a large number of oscillation modes of the optical resonator and a more rigorous calculation of the active medium distribution will considerably widen the class of possible motions and will enable us to explain the complex emission-selfmodulation structure observed experimentally in solid-state lasers.Izvestiya VUZ. Radiofizika, Vol. 8, No. 5, pp. 909–919, 1965     

Mode structure of a semiconductor laser subject to filtered optical feedback

We present an analytical study of the mode structure of a semiconductor laser subject to filtered optical feedback (FOF). First, we derive expressions for the solution sets that define the structure of the modes. We then employ continuation techniques to investigate how this structure changes as filter width, filter detuning and feedback phase are varied. This leads to a comprehensive picture of the FOF laser’s mode structure. In particular, we find an equation for the boundary of the parameter region in which one finds two separate components of modes.     

High-resolution spectroscopy using interleaved 100 GHz optical frequency comb scanned by phase modulator

A high-resolution spectroscopy technique is proposed with an optical phase modulator combined with an interleaved optical frequency comb. The optical phase modulator and a frequency-locked laser light guarantee a spectral resolution less than 1 MHz on an absolute frequency axis. A wide measurement frequency range was realized using a 25 GHz optical frequency comb lying over a 4 THz frequency region. An extraction of single tooth intensity from the comb was realized by a heterodyne technique with a frequency-tunable laser used as a local oscillator. Also, the 25 GHz optical frequency comb was interleaved to generate four 100-GHz combs for removing the crosstalk from the 25 GHz neighboring sidebands in the teeth. This proposed spectroscopy technique was experimentally demonstrated with a resonator of less than 1 MHz linewidth and a H¹³C¹⁴N gas cell. Thus, a measurement frequency range higher than 4 THz (1530 nm-1560 nm) was confirmed with an effective spectral resolution 100 kHz order. In addition, the characteristics of the proposed system were compared with those of the previous system with a single-sideband (SSB) optical modulator.     

A novel actively and passively mode-locked semiconductor optical amplifier fiber ring laser

A novel actively and passively mode-locked semiconductor optical amplifier fiber ring laser was presented, where semiconductor optical amplifier provided cavity gain and introduced nonlinear polarization rotation, whereas, intensity modulator not only acted as modulator but also polarizer. The pulses with duration below 3 ps (FWHM) and peak power about 16 mW at a repetition rate of 10 GHz can be obtained in our system and the system stability may be enhanced. To investigate system parameters effects on mode-locked pulses, a theoretical model was developed.     

Chaos generation in a semiconductor ring laser with an optical injection

Two chaos generation schemes are demonstrated numerically based on a semiconductor ring laser (SRL) with either a constant or a chaotic optical injection from an external distributed feedback (DFB) laser. Under a certain bias current, the chaotic output can be generated by the SRL with appropriate injection parameters. Parameter map is achieved numerically to show types of oscillation in the SRL. Bifurcation diagrams and Lyapunov exponent spectra roughly describe the routes to chaos for the SRL with disturbance. A bandwidth-enhanced chaos generation can be describes based on the chaotic optical injection scheme. The bandwidth of the bandwidth-enhanced chaos generated is about 12 GHz, which achieves approximately a two-fold increase compared with the constant injection scheme.     

Atomic and molecular spectroscopy with a continuous-wave, doubly resonant, monolithic optical parametric oscillator

Doppler-broadened atomic and molecular spectra were observed with a one octave tunable, continuous-wave, doubly resonant, monolithic optical parametric oscillator (OPO) using 5% MgO-doped LiNbO₃ as a non-linear crystal with a birefringent phase-matching configuration. By tuning the frequency of a pump laser, longitudinal mode selection over 20 successive modes, corresponding to a 60 GHz span, was possible, owing to the simple structure of the monolithic OPO. Continuous frequency tuning was achieved using an external waveguide-type electrooptic phase modulator (EOM). By changing the modulation frequency of the EOM, frequency tuning of the optical sidebands over 12 GHz was possible, which is larger than the one free spectral range of the monolithic cavity of 3 GHz. We could observe the Cs-D₁ (894 nm), Cs-D₂ (852 nm), Rb-D₁ (795 nm), acetylene R9 (1520 nm) and P9 (1530 nm) transitions with the single monolithic OPO.     

An optical mm-wave generation scheme by frequency octupling using a nested MMI

A novel method of a filterless optical millimeter-wave (MMW) signal generation with frequency octupling via a nested multimode interference (MMI) coupler is proposed for Radio-over-fiber systems. By setting the DC bias voltage applied to the central arms of MMI-b and MMI-c accurately, the optical carrier can be completely suppressed. The OSSR can be as high as about 58 dB without optical filter and the radio frequency spurious suppression ratio (RFSSR) exceeds 32 dB, which is the best result as we know. Simulation results suggest that when the generated optical mm-wave signal is transmitted along the standard single-mode fiber, the eye diagram is still opened after being transmitted over a 50 km fiber.     

Visible-to-near infrared standard source generation using acetylene (C2H2)-stabilized comb-injection-locked technique

An ultra-stable visible laser source (VLS) was generated by the second harmonic of a selectively injection-locked distributed feedback (DFB) laser from an optical frequency comb with acetylene-stabilized laser seeding. For the second-harmonic generation (SHG) of the injection-locked DFB laser, we used periodically poled lithium niobate (PPLN) crystal and generated the VLS at a region of 771 nm, which was discretely locked to a spacing of 25 GHz within the PPLN bandwidth (180 GHz). The frequency stability of this source was estimated to be 1.1 × 10^− 12 with an average time of 1 s.     

10 and 20 Gb/s all-optical RZ to NRZ modulation format and wavelength converter based on nonlinear optical loop mirror

We present experimental and theoretical results on all-optical 10 and 20 Gb/s RZ to NRZ modulation format and wavelength converter based on a nonlinear optical loop mirror (NOLM). A vector model of converter was developed and the shape of converted pulses was found analytically for particular choice of polarization states. In the experiment, non-zero dispersion shifted fiber with a length 1200 m was used as a nonlinear medium. Pulses from a 10 GHz mode-locked semiconductor laser diode were modulated to form pseudorandom RZ signal and eventually time division multiplexed to 20 Gb/s. RZ pulses were subsequently converted to NRZ signal. The performance of the converter was evaluated experimentally using the data communication analyzer and bit error ratio tester.     

Simultaneous generation and wavelength conversion of a pulse train from multi-wave mixing in an optical fibre

The insertion of three continuous-wave optical frequencies in a low-dispersion optical fibre resulted in the generation of a high-repetition-rate train of ultra-short pulses and its simultaneous wavelength conversion. Two of the frequencies were spaced by ∼0.17 THz and multi-wave mixing generated a frequency comb to which is associated a train of 1.6 ps pulses. Wave-mixing between the generated comb and the third input optical frequency then converts the pulse train into different wavelengths. The Split-Step Fourier method was applied to numerically simulate the generation/wavelength conversion and results are in good agreement with experiments.