Chunfeng Ge

Multi-wavelength lasers with suppressed spectral linewidth of 10 kHz

High coherent multi-wavelength or multi-tone light source are in high demand for optical density wavelength division multiplexed (DWDM) networks as the telecommunication capacity expands exponentially. However the linewidths of commercial multi-wavelength semiconductor lasers are typically a few MHz which is not acceptable when the frequency spacing of the multi-tones is 10 GHz. In this paper, a novel and simple method to suppress the linewidths of the multi-wavelength from ~6 MHz to ~10 kHz using an all-optical approach is proposed and demonstrated. The linewidths of the multi-wavelength are suppressed by a factor of 600 and the noise level of the multi-wavelength is decreased by nearly 20 dB. Each wavelength of the multi-wavelength operates in single longitudinal mode. Finally, more than 8 wavelengths over 10 nm are suppressed simultaneously through the approach and scheme presented in this work.

Over 10-GHz Peak-Equalized Pulse Train Source With Pulsewidth Shortening

Peak-flattened and width-shortened pulses have been successfully generated in an improved rational harmonic mode-locking (RHML) fiber laser in which an intensity modulator is driven by square waveform signals at gigahertz (GHz) repetition rates rather than typically used sinusoidal radio frequency (RF) signals. The repetition frequency of a fifth-order RHML under a driving signal at 3.125 GHz has been up to 15.7 GHz in our system. The equalized peaks of pulses generated reached to a minimum standard deviation of 0.2%, which is the best performance to our knowledge. The pulsewidth is shortened to 17.8 ps in the same RHML fiber laser driven by the square-wave signals compared with 30.3 ps driven by traditional sine-wave signals. The 3-dB spectral width of the RHML pulses was increased almost two times and the spectrum has a clear comb-like structure which can be developed into a frequency comb with GHz frequency spacing.

Study on all-optical 2R regeneration at 40 Gbit/s based on SPM

In recent years, optical fiber communication system has made a great development, but the quality of the optical signal will be seriously deteriorated by some factors, such as amplified spontaneous emission (ASE) noise, group velocity dispersion and so on. The traditional regeneration technology is accomplished within the electrical domain, and the opticalelectrical-optical conversion consumes vast amounts of energy. So it is necessary for all optical regeneration technology. This paper introduces the principle of an all-optical 2R regenerator based on self-phase modulation(SPM) and offset filtering technology. The deteriorated optical signal at 40 Gbit/s has been regenerated by the combination of SPM in highly nonlinear fiber (HNLF) and offset filtering. The factors influencing the regeneration have been analyzed by changing related parameters. It is concluded that by reasonably choosing parameters, we can get the best result of all-optical 2R regeneration.

A fiber Bragg grating dynamic stress sensing interrogation system with high interrogation rate

Dynamic stress sensing interrogation has become into a potential research for its wide applications. In order to test and monitor the dynamic stress in real-time, new types of fiber Bragg gratings (FBGs) sensing interrogation systems have been gradually developed. In this paper, a high interrogation rate dynamic stress sensing interrogation system by using a wavelength-swept laser with high sweeping rate and flexible-controlled sweeping step is proposed and demonstrated. In this system, a single sideband (SSB) modulator driven by radio-frequency (RF) signal is used to realize the abovementioned parameters of the wavelength-swept laser, which overcomes the limitation of traditional mechanical sweeping devices. The interrogation rate of the whole system is decided by the sweeping rate of the source, which is significantly improved and tunable with a broad range. Owing to the advantages of the sweeping source, our sensing interrogation system is effective to deal with different vibration sensing interrogations. Experimentally, the sweeping rate of the wavelength-swept laser can be tuned from 40 kHz to 200 kHz as sweeping step tuned from 5 GHz to 15 GHz. The highspeed vibration generated by a piezoelectric transducer (PZT) on the FBGs is sensed and interrogated in real-time. The interrogation of frequency vibration sensing from 5 Hz to 11 kHz are obtained. A 25 μs transient vibration mutation is also successfully interrogated by this system.

Nyquist pulse generator by techniques of frequency synthetization

Nyquist pulses, which are defined as responses of Nyquist filter, can be used in time-division multiplexing transmission which can simultaneously achieve ultrahigh data rate and spectral efficiency (SE). Generally, the methods for Nyquist pulse generation are based on optical Nyquist filters, nonlinear effects in fiber and phase-locked frequency comb. In this paper, we focus on the third method of phase-locked frequency comb. However, this method has a problem which the large duty cycle of generated Nyquist pulses limits their applications. To address this issue, we proposed a new setup in which one optical intensity modulator and an electrical arbitrary function generator (AFG) are employed. The various duty cycles of ideal Nyquist pulses are generated using one optical intensity modulator so that the phase-locking between the different RF signals is no need any more. And the ideal Nyquist pulses in microwave domain are generated successfully. The duty cycles ranging from 21% to 11% are obtained by programming the number of frequency comb lines in the RF signal which is generated by the AFG. The method has a potential to generate ideal Nyquist pulses in radio frequency domain if a high bandwidth AFG is used to replace the low bandwidth AFG used in this paper.

Generations of linear frequency modulation continuous

An external modulation technique for the generation of optical linearly chirped frequency continuous waves is proposed and experimentally demonstrated. The output from a distributed feedback (DFB) laser is modulated by a single sideband (SSB) modulator which is driven by a linearly chirped electrical signal generated by an arbitrary waveform generator (AWG). A high precise linear optical frequency sweep with a tuning range of 2.5GHz is successfully achieved. Due to the good modulation quality of SSB and high stability of AWG, the chirp of the electrical signal can be perfectly modulated onto the optical signal. The linearity of the optical signal is almost consistent with the linearity of the electrical signal. The best linearity of 0.9996 and the lowest jitter rate of 4.76 % are obtained from the experimental measurements which are reported in this paper.

Picosecond-pulse generation over 10 GHz repetition rate based on cascaded semiconductor optical amplifiers

A cavity-free setup to generate short pulses at high repetition rate is introduced, which is based on four-wave mixing (FWM) in cascaded semiconductor optical amplifiers (SOAs). High repetition rate picosecond-pulse is important in optical communication and all-optic information processing systems. Cavity-free setup based on SOA means without ring cavity, which is stable and easy to be large-scale integrated. In this paper, we obtain picosecond-pulse around 10GHz repetition rate and the side-mode suppression ratio is 23 dB. Moreover, the repetition rate and center wavelength of optical pulse is tunable.

External Modulation Method for Generating Accurate Linear Optical FMCW

Frequency modulation continuous wave (FMCW) lasers are key components in modern optical imaging. However, current intracavity modulation lasers do not exhibit low-frequency jitter rate and high linearity due to the inherent relaxation oscillations. Although this may be compensated in a direct modulation laser diode using an optoelectronic feedback loop, the available sweep speed is moderately small. In this letter, a special external modulation method is developed to improve the performance of FMCW. Since only the first sideband optical field is used during the entire generation process, phase noise is kept to a minimum and is also independent of the sweep speed. We demonstrate that the linearity and jitter rates do not deteriorate appreciably when the sweep speed is changed over three orders of magnitude, even up to the highest sweep speed of 2.5 GHz/

Experimental study on slow light based on all-optical wavelength conversion and dispersion medium

\mu \text{s}

Experimental studies on WDM to TDM signal conversions using gigahertz electro-absorption modulators

.