Pedestal-free 25-GHz subpicosecond optical pulse source for 16 × 25-Gb/s OTDM based on phase modulation and dual-stage nonlinear compression.

An optical pulse generation scheme based on an ultra-short chirped seed pulse generator followed by a dual-stage fiber-based nonlinear pulse processing stage is proposed and demonstrated experimentally. After phase modulation and linear-chirp compensation, optical seed pulse with a duty cycle of 9.8% and an obvious pedestal is obtained. By soliton compression and Mamyshev reshaping, a pedestal-free optical pulse with a duty cycle of 2% and an extinction ratio of 27 dB is achieved. The optical pulse source is further applied in a 16×25-Gb/s on-off keying optical time-division multiplexing transmitter.

Study on 100-Gb/s reconfigurable all-optical logic gates using a single semiconductor optical amplifier.

We experimentally demonstrated all-optical NOR, OR and AND logic gates at 100 Gb/s with a single semiconductor optical amplifier (SOA) assisted by optical filtering. The logics can be conveniently reconfigured by deploying or not continuous wave (CW) light at the input of the SOA and adjusting the tunable optical band pass filter (OBPF) at SOA output. Correct logic functions and high quality of the output signals can be achieved as proved by clear eye opening and bit error rate (BER) measurement. Influences of optical filter parameters and the SOA device length on the logical performance are experimentally investigated.

Widely tunable monolithic dual-mode laser for W-band photonic millimeter-wave generation and all-optical clock recovery.

We demonstrate a monolithic dual-mode amplified feedback laser for photonic millimeter-wave generation and all-optical clock recovery. Dual-mode lasing with beating frequency around 100 GHz was realized by using a single-mode distributed feedback (DFB) laser with a short feedback cavity that was integrated by simple quantum-well intermixing technology. By tuning the bias currents of the laser sections, the beating-frequency can be continuously tuned from 75 to 109 GHz, almost covering the entire W-band (75-110 GHz). Furthermore, by using this device, an all-optical clock recovery for 100 Gbit/s return-to-zero on-off-keying signal was achieved with a timing jitter of 301 fs.

100-Gb/s 2R regeneration using cross gain compression in semiconductor optical amplifiers.

All-optical 2R regeneration of 100-Gb/s on-off-keying (OOK) signal is experimentally demonstrated based on cross gain compression (XGC) effect in semiconductor optical amplifiers (SOAs). It is shown that a high-quality logic-inverted signal and SOAs with faster gain recovery times are the two key enabling factors for obtaining regeneration results at such speeds. BER improvement of 1.2~2 dB is experimentally obtained at 1551 nm and regenerative results are demonstrated on a wide wavelength range from 1535 nm to 1555 nm. The tolerance of the input signal to optical signal-to-noise ratio (OSNR) deterioration is also experimentally studied for the 2R regeneration scheme at two different wavelengths.

Optical Nyquist pulse generation using a time lens with spectral slicing.

Optical Nyquist pulse generation based on a time lens with subsequent optical filtering is proposed. A nearly chirp-free 10-GHz 8.1-ps Nyquist pulse generator is experimentally demonstrated. By inserting group velocity dispersion (GVD) between cascaded phase and amplitude modulators, 11 tones ultraflat optical frequency comb (OFC) of 10-GHz frequency spacing within 0.9 dB power variation is obtained. The quasi-rectangular shape spectrum is then filtered out with a tunable rectangular-shaped optical band-pass filter (OBPF) and the quasi-linear chirp is compensated by a segment of standard single mode fiber (SSMF). By changing the wavelength of the continuous wave (CW) light, nearly chirp-free Nyquist pulses over C band are obtained. Furthermore, simultaneous dual-wavelength pulse generation is also demonstrated.

Multiwavelength 25-GHz picosecond pulse generation with phase modulation and double-side Mamyshev reshaping.

We demonstrate a simple, robust, and cost-effective method of generating high-speed multiwavelength picosecond optical pulses. This method is based on chirp compression of phase-modulated light, followed by nonlinear pulse compression and reshaping with a double-side Mamyshev reshaper. We show that this method becomes power efficient when the repetition rate is increased to 25 GHz. Wavelength-tunable optical pulses with a repetition rate of 25 GHz and pulse width of ∼2 ps, which can be temporally multiplexed to 100 GHz, are experimentally obtained over a large spectral range with moderate electrical and optical power. Simultaneous pulse generation on four wavelengths is demonstrated.

Gaussian-like dual-wavelength prescaled clock recovery with simultaneous frequency-doubled clock recovery using an optoelectronic oscillator.

A novel multifunctional frequency-doubling optoelectronic oscillator (FD-OEO) mainly based on a Mach-Zehnder modulator (MZM) cascaded with a phase modulator (PM) and a subsequent optical bandpass filter (OBPF) is proposed. We experimentally demonstrate simultaneous operations of frequency-doubled optical clock (FD-OC) recovery, low-duty-cycle dual-wavelength prescaled OC recovery and error-free fourfold time division demultiplexing with the proposed OEO injected with a 4 × 25-Gb/s optical time-division-multiplexing (OTDM) signal. We show that the proposed FD-OEO operates well for both the differential phase shift keying (DPSK) and on-off keying (OOK) modulation formats. The extracted dual-wavelength prescaled OC is proved to be nearly transform-limited with Gaussian-like shape. Furthermore, all four 25-Gb/s tributaries can be selectively demultiplexed by adjusting the phase shifters in the OEO loop. The power penalty at a bit error rate (BER) of 10(-9) is measured to be 2.4 dB, 1.2 dB for the best channel for DPSK signal and to be 2.5 dB, 1.1 dB for the best channel for OOK signal. In addition, as an extra benefit of the OEO, low-phase-noise prescaled electrical clock (EC) is also extracted.

Effective optical clock recovery and simultaneous fourfold demultiplexing of OTDM signal using an optoelectonic oscillator.

We propose and demonstrate a novel scheme for clock recovery and simultaneous fourfold optical time-division demultiplexing using a dual-parallel Mach-Zehnder modulator based optoelectronic oscillator. 25-GHz prescaled optical clock with a 23% duty cycle and a 22-dB extinction ratio is successfully extracted from both 100-Gb/s on-off keying (OOK) and differential phase-shift keying (DPSK) optical time-division-multiplexing (OTDM) signal. The timing jitters (100 Hz to 10 MHz) are measured to be 195.9 fs and 125.6 fs for the optical clock extracted from the 100-Gb/s OOK and DPSK signal, respectively. Error-free optical time-division demultiplexing is also achieved simultaneously with clock recovery. By adjusting the phase shifter in the OEO loop, all four channels can be selectively demultiplexed. The power penalties at a bit error rate (BER) of 10⁻9 for the four demultiplexed channels are measured to be between 0.8 dB and 1.2 dB for the OOK signal and between 0.9 dB and 1.5 dB for the DPSK signal.

Frequency chirp linearization for ultraflat optical frequency comb generation based on group velocity dispersion.

We propose to insert group velocity dispersion between cascaded phase and amplitude modulation for ultraflat optical frequency comb (OFC) generation. With the dispersion, the sinusoidally varied chirp of the continuous wave light induced by phase modulation becomes linear within a relatively wide time interval. This is useful to improve the flatness of the generated OFC by directly cascaded phase and amplitude modulation. Simulation shows a flat comb of 37 tones within 0.88 dB power variation when the modulation index of the phase modulation reaches 20. An ultraflat comb generator with 10 GHz frequency spacing is also demonstrated. The flatness of the 15 tones around the center wavelength has been improved to 0.98 dB.

Detailed analysis of a 40 GHz all-optical synchronization based on an amplified-feedback distributed feedback laser.

We demonstrate an all-optical synchronization using an amplified-feedback distributed feedback laser with coherent injection experimentally, which can synchronize 40 GHz degraded signals even with optical signal-to-noise ratio as low as 5 dB, or chromatic dispersion as large as 408 ps/nm. Besides, this optical synchronization has a large range of power operation, i.e. high sensitivity of synchronization, from -21.40 dBm to -8.23 dBm, as well as a large frequency-locking range of 190 MHz.

1.4 ps pedestal-free low timing jitter 10 GHz pulse source using commercial cascaded LiNbO3 modulators and fiber-based compressor.

A stable 10 GHz pedestal-free short pulse generation scheme consisting of cascaded commercial LiNbO(3) modulators has been proposed and successfully demonstrated experimentally. Fiber-based pulse compression and reshaping stages have been utilized to obtain a 1.38 ps optical pulse train with very little pedestal and 132 fs timing jitter. Excellent performance of multiplexing from 10 Gbits/s to 160 Gbits/s using this method indicates good potential for application in ultrahigh-speed optical time-division-multiplexing systems.

Optical regenerative NRZ to RZ format conversion based on cascaded lithium niobate modulators.

Optical regenerative nonreturn-to-zero (NRZ) to return-to-zero (RZ) format conversion using a lithium niobate phase modulator and a lithium niobate intensity modulator is proposed and demonstrated. The key advantage of the proposed format converter is that the converted RZ signal has a very small pulse width, which can be multiplexed to a higher bit rate using optical time division multiplexing technology. The operation can greatly reduce the timing jitter of the degraded NRZ signal due to the regenerative property of the proposed scheme. Besides, the format converter can also support multi-channel operation. An experiment is performed with the feasibility of the scheme demonstrated.

Investigation of all-optical nonreturn-to-zero-to-return-to-zero format converter based on a semiconductor optical amplifier and a reconfigurable delayed interferometer.

A simple and robust all-optical nonreturn-to-zero to return-to-zero (RZ) format converter is demonstrated and investigated using a semiconductor optical amplifier (SOA) and a reconfigurable delayed interferometer. The scheme can be used to convert input signals at different bit rates and obtain converted signals with tunable duty cycles. Format conversion at 10 Gb/s is achieved with a negative power penalty of -4.9 dB at a bit-error rate of 10(-9). Transmission of the converted RZ signal over a dispersion-managed fiber link of 80 km results in only a small power penalty of 0.9 dB. In addition, the format converter is found to be insensitive to the power fluctuation of the optical clock, as the SOA works in the saturation regime.

40 Gbits/s all-optical clock recovery for degraded signals using an amplified feedback laser.

All-optical clock recovery for the return-to-zero modulation format is demonstrated experimentally at 40 Gbits/s by using an amplified feedback laser. A 40 GHz optical clock with a root-mean-square (rms) timing jitter of 130 fs and a carrier-to-noise ratio of 42 dB is obtained. Also, a 40 GHz optical clock with timing jitter of 137 fs is directly recovered from pseudo-non-return-to-zero signals degraded by polarization-mode dispersion (PMD). No preprocessing stage to enhance the clock tone is used. The rms timing jitter of the recovered clock is investigated for different values of input power and for varying amounts of waveform distortion due to PMD.

High-performance all-optical OR/NOR logic gate in a single semiconductor optical amplifier with delay interference filtering.

The authors have proposed and experimentally demonstrated all-optical logic gates using a single SOA and delay interference filtering that enable simultaneous logic functions of OR and NOR at 40 Gbits/s. The proposed scheme, which utilizes the combinative filtering profile of a delay interferometer and an optical bandpass filter, has great merits for use in generating logic outputs with high quality in terms of pulse shape, extinction ratio, and eye diagram. Reconfiguration between the two gates is achieved by adjusting the tunable filter and the delay interferometer.

Multiwavelength erbium-doped fiber laser based on inhomogeneous loss mechanism by use of a highly nonlinear fiber and a Fabry-Perot filter.

We demonstrated a simple technique to obtain stable room temperature multiwavelength lasing in an erbium-doped fiber laser by the inhomogeneous loss mechanism. Successful reduction of the cross-gain saturation in erbium-doped fiber was achieved by incorporating a section of highly nonlinear fiber (HNLF) and a Fabry-Perot filter (FPF) in the laser cavity. More than 70 wavelengths simultaneous lasing were observed with the same frequency space of 25GHz. The laser had a total output power of ~3.2dBm, a bandwidth of 0.012nm (~1.5GHz) and a signal-to-spontaneous-noise ratio of ~44dB. Furthermore, the total output power can be increased to more than 190mW by moving the output port right after the EDFA.

Switchable single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser incorporating a semiconductor optical amplifier.

We propose and demonstrate a novel single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber ring laser incorporating a semiconductor optical amplifier. The SOA biased in its low-gain regime greatly reduces the gain competition of the two wavelengths. The stable SLM operation is guaranteed by a passive triple-ring cavity and a fiber Fabry-Perot filter. The dual-wavelength output with a 40 GHz wavelength spacing is switchable in the range of 1533-1565.4 nm.

Simple pulse compression scheme based on filtering self-phase modulation-broadened spectrum and its application in an optical time-division multiplexing system.

We propose a simple and stable pulse compression scheme based on a filtering self-phase modulation broadened spectrum in a highly nonlinear fiber. It is found experimentally that under offset filtering the compressed pulse has a better quality compared with center filtering, such as a no pulse pedestal. Furthermore, numerical simulations reveal that the scheme has the potential to improve the pulse extinction ratio during compression. Finally, based on the obtained pulse train the 4 x 10 Gbits/s optical time-division multiplexing system is studied for verification of the high pulse quality. The compression scheme has the advantages of simple configuration, being pedestal-free, and having a high pulse extinction ratio.

Combination of highly nonlinear fiber, an optical bandpass filter, and a Fabry-Perot filter to improve the signal-to-noise ratio of a supercontinuum continuous-wave optical source.

We present a theoretical study of a supercontinuum (SC) continuous-wave (cw) optical source generation in highly nonlinear fiber and its noise properties through numerical simulations based on the nonlinear Schrödinger equation. Fluctuations of pump pulses generate substructures between the longitudinal modes that result in the generation of white noise and then in degradation of coherence and in a decrease of the modulation depths and the signal-to-noise ratio (SNR). A scheme for improvement of the SNR of a multiwavelength cw optical source based on a SC by use of the combination of a highly nonlinear fiber (HNLF), an optical bandpass filter, and a Fabry-Perot (FP) filter is presented. Numerical simulations show that the improvement in modulation depth is relative to the HNLF's length, the 3-dB bandwidth of the optical bandpass filter, and the reflection ratio of the FP filter and that the average improvement in modulation depth is 13.7 dB under specified conditions.