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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.

We proposed and demonstrated a linearly frequency-swept multi-wavelength laser source for optical coherence tomography (OCT) eliminating the need of wavenumber space resampling in the postprocessing progress. The source consists of a multi-wavelength fiber laser source (MFS) and an optical sweeping loop. In this novel laser source, an equally spaced multi-wavelength laser is swept simultaneously by a certain step each time in the frequency domain in the optical sweeping loop. The sweeping step is determined by radio frequency (RF) signal which can be precisely controlled. Thus the sweeping behavior strictly maintains a linear relationship between time and frequency. We experimentally achieved linear time-frequency sweeping at a sweeping rate of 400 kHz with our laser source.

Photonic true-time delay (TTD) beamforming has been considered as a promising technique for future wide-band phased array antenna systems. A TTD beamforming system based on a linear chirped fiber grating (LCFG) requires a tunable multi-wavelength laser source, in which the wavelengths are tuned simultaneously with equally increased or decreased wavelength spacing. We present a novel approach of a TTD laser source employing a DQPSK modulator and a highly nonlinear fiber (HNLF). The wavelength tuning is achieved by tuning the frequency of the radio frequency signal generator that drives the DQPSK modulator. Up to 41 lasing wavelengths with 0.08 nm spacing, 29 lasing wavelengths with 0.06 nm spacing and 31 lasing wavelengths with 0.12 nm spacing in 3 dB bandwidth were obtained. In theory, the multi-wavelengths can be simultaneously tuned from 10 MHz to 20 GHz in our experiments based on the fact that the frequency range of the RF signal source is from 10 MHz to 20 GHz and the bandwidth of DQPSK modulator is up to 22 GHz. The tuning precision of the RF signal generator is up to 1Hz and phase noise is about -104dBc/Hz. Therefore, the wavelengths can be tuned precisely and stably.

We investigate gain equalization of a four-mode-group and six-mode-group erbium-doped fiber amplifiers with a genetic algorithm for mode division multiplexing transmission systems by optimizing rare earth dopant profile and pump modes powers, respectively. The optimizing gains are calculated to be > 28 dB between 1520 nm – 1565 nm with a maximum differential modal gain of ~ 2 dB among the mode groups only by optimizing the rare earth ring doping profile in the few-mode erbium-doped fiber.

Light detection and ranging (lidar) is used for various applications such as remote sensing, altimetry and imaging. Continuous wave frequency modulated (CWFM) lidars rely on linearly ramping the optical frequency of a laser and interfering the delayed echo signal with a reference signal to produce a beat frequency. The range resolution of lidar is determined by the swept frequency range of the linearly chirped continuous light waves in a frequency modulation system. For example, for 1mm range resolution, it needs the frequency sweep range of more than 150 GHz. In this paper a system which can generate a linearly chirped optical waveform within a super broad-wide band of 200 GHz, using cascaded phase modulators (PM) and single side band (SSB) modulator, is designed as a light source in a high resolution lidar system.

Spectral broadening and the generation of supercontinuum are inherent features of nonlinear optics, and have been studied intensively for many years. Supercontinuum generation has been found numerous applications in such various fields as spectroscopy, pulse compression, and the design of tunable ultra-short femtosecond laser sources. Generally, supercontinuum is mainly generated by hyperbolic-secant soliton pulse. It is rarely investigated about the spectral broadening when pumping with pulse of a rectangle or quasi-rectangle shape, which is an interesting direction of supercontinuum generation. In this paper, supercontinuum generation based on soliton, quasi-rectangle, and rectangle pulses are investigated numerically. Firstly, we introduced the general nonlinear Schrodinger equation (GNLSE) to deduce the temporal and spectral evolution of pulse broadening in the photonic crystal fiber. Secondly, with different types of pulses, including hyperbolic-secant, ultra-Gaussian and rectangle pulses, we investigated the generation of new frequency and spectral broadening by numerical methods. Comparisons of characteristics of spectral and temporal evolution with different pulses are made in the following parts of paper. It is found that the generation of dispersive waves and Raman effect play main roles in the spectral broadening and generation of supercontinuum, as the broadband wave is generated by the coupling of Raman soliton and dispersive waves through cross-phase modulation. Apparent side-bands can be observed in our simulation under the condition of quasi-rectangle and rectangle pulses pumping, which affect the smooth of spectrum. Thus, to our acknowledge, a flat supercontinuum can be generated by changing the parameter of pulse to make a better coupling of the side-bands.

In recent years,tunable optical delay line has attracted much research interest for its applications in optical communications,optical signal processing and optical control of phased array antennas in microwave communications. Compared with optical delay line based on slow light techniques, all-optical wavelength conversion followed by signal propagation in a dispersive medium presents an alternative approach to control the delay time because it is simpler and more controllable. In this paper, a tunable optical delay line with a large time delay up to 650ps based on four-wave mixing (FWM) wavelength conversion in 420m high nonlinearity fiber (HNLF) and dispersion in 8km standard single mode fiber (SMF) is experimentally demonstrated.Our experimental results are in good agreements with the principle.

Rational harmonic mode locking (RHML) in an active mode-locked fiber laser can increase the output pulse repetition rate a number of times the modulation frequency of an optical modulator in a cavity when driven by gigahertz (GHz) RF. The amplitudes of the output optical pulse train in a high order RHML operation are not equalized and flat due to the GHz RF drive signals. A modified RHML technique using standard instrumentation that generates 1 GHz electrical square wave signals to accomplish up to 6th order RHML in fiber lasers is presented for improving the flatness of the amplitudes of the output optical pulse train at the pulse repetition rate of up to 12 GHz.

In this paper, the manipulation of light properties in the pure optical domain and its limits are briefly reviewed, followed by the general descriptions of the advantages and the feasibility of altering the light properties by using RF signals. The principles of manipulating light, including continuous wave (CW) light and pulsed light sources, are introduced, respectively. Several research systems which have recently emerged for potential applications in optical communication and optical sensing networks are discussed in order to demonstrate the understanding of how light can be modified by RF signals.

Passively mode-locked fiber laser (MLFL) has been widely used in many applications, such as optical communication system, industrial production, information processing, laser weapons and medical equipment. And many efforts have been done for obtaining lasers with small size, simple structure and shorter pulses. In recent years, nonlinear polarization rotation (NPR) in semiconductor optical amplifier (SOA) has been studied and applied as a mode-locking mechanism. This kind of passively MLFL has faster operating speed and makes it easier to realize all-optical integration. In this paper, we had a thorough analysis of NPR effect in SOA. And we explained the principle of mode-locking by SOA and set up a numerical model for this mode-locking process. Besides we conducted a Matlab simulation of the mode-locking mechanism. We also analyzed results under different working conditions and several features of this mode-locking process are presented. Our simulation shows that: Firstly, initial pulse with the peak power exceeding certain threshold may be amplified and compressed, and stable mode-locking may be established. After about 25 round-trips, stable mode-locked pulse can be obtained which has peak power of 850mW and pulse-width of 780fs.Secondly, when the initial pulse-width is greater, narrowing process of pulse is sharper and it needs more round-trips to be stable. Lastly, the bias currents of SOA affect obviously the shape of mode-locked pulse and the mode-locked pulse with high peak power and narrow width can be obtained through adjusting reasonably the bias currents of SOA.

In the modern intelligent fiber Bragg grating (FBG) sensing network, the spectrum interrogation should not only have high speed and wavelength resolution, but also use the limited spectrum resource efficiently. In this paper, we propose a spectrum interrogation technique by using a wavelength-division multiplexing (WDM) to optical time-division multiplexing (OTDM) converter, which could map the sensing signals from the wavelength domain to the time domain quickly and efficiently. The converter consists of an electro-optic switch, time delay unit and a multi-wavelength converter. The continuous WDM sensing signals are converted into pulsed WDM signals by using a 10 GHz LiNbO 3 intensity modulator as the electro-optic switch. A segment of single mode fiber (SMF) is used as the time delay unit for delaying the pulsed WDM signals overlapped in time. By using a high speed 40 GHz electro-absorption modulator (EAM) as the multi-wavelength converter, the pulsed and delayed WDM signals are converted into OTDM signals with the same wavelength. At last, the light signals after demodulation are detected by photoelectric detector (PD) which has high response speed and low cost. The mapping of the wavelength domain to the time domain in our spectrum interrogation can allocate the spectrum resource efficiently and dynamically. It is experimentally demonstrated that two signals in wavelength domain can be converted into time domain signals carried by a wavelength with a roughly equal conversion efficiency.

High resolution wavelength-tunable lasers are essential to sensing applications. For sensing applications, high resolution is needed to improve the spatial resolution and/or measurement accuracy, and fast tuning (sweeping) is required to enhance the measurement speed for dynamic sensing. However the demand of high resolution conflicts with the requirement of fast continuous wavelength tuning. The solution to this issue is tuning the wavelength of the output in a quasi-continuous way in which the length of each step is dependent on the frequency of a RF generator which is used to drive a single-sideband (SSB) modulator in the wavelength-swept optical system. In this paper, a principle of the step-tunable wavelength-swept optical system is proposed and demonstrated. The two optical features of narrow bandwidth and fairly high optical output power make the system unique for improving the accuracy of the measurement of the center-wavelength of a fiber Bragg grating (FBG) sensor. In addition, changing the tuning-step by adjusting the frequency of a RF generator electrically is user-friendly compared to the conventional wavelength swept systems by tuning optical elements mechanically.

Erbium-doped fiber lasers have become a reliable source of sub-100 fs pulses at 1550 nm; however dispersion and nonlinearities in optical fiber make generation of multi-nJ pulses difficult. In this paper a two-stage fiber amplification system which consists of two sub-fiber-amplifiers was designed by introducing a filter between the two sub-amplifiers to manage or modify the spectrum of the pulse amplified by the first sub-fiber-amplifier before it goes into the second one. A systematic simulation, both in time domain and in frequency domain, for this fiber amplifier system was conducted. The role of the filter in the system is presented and discussed. The parameters of this two-stage fiber amplification system are determined by calculation results based on the high-concentration Er-doped fiber as gain media and high power laser diodes as pump light sources, All of the components in the system are commercially available. In addition, the optimal output of the amplification system was obtained based on simulations.

In this paper, a novel soliton compressor based on dispersion decreasing fiber (DDF) assistant by Raman gain is studied. The influence of Raman gain on compression quality is analyzed. The compressor is optimized by choosing proper gain. An optimization of the linear decreasing DDF compression method is made through distributed optical fiber Raman amplification with gain coefficient of 4.5dB/km. The pulse with width of 10ps is compressed down to 878.7fs, and the compression factor is 11.38. Our research results show that distributed Raman amplification in DDF not only increases the compression factor, but also improves the quality of compressed pulse.

We demonstrate an ultra-long cavity all-fiber Erbium-doped fiber laser that is passively mode-locked by nonlinear polarization rotation. The length of the resonant cavity amounts to 4.046 km, which is achieved by incorporating a 4 km single mode fiber. The laser generates stable mode-locked pulses with a 50.90 kHz fundamental repetition rate. The maximum average power of output pulses is 2.73 mW, which corresponds to per-pulse energy of 53.63 nJ.

Optoelectronic technology played a pivotal role in the unprecedented information revolution in the past two decades. One of the remaining grand challenges is the ability to store an optical signal in optical format. So slowing down the velocity of light have recently attracted substantial interest. In various mechanisms of slow light generation, semiconductor optical amplifier (SOA) attracts much attention because it offers the advantage of compactness, room temperature operation, electric-optical controllable and easy integration with existing optical communication systems. In this paper, slow light generation in SOA using four wave mixing (FWM) effect is analyzed. The dynamic changes of the signal light time delay with the outside controllable parameters, such as the injection current into SOA, the pump light power, the detuning frequency between pump light and signal light, are numerically solved on the basis of the theory of refractive modulation-index and the sub-sections model of SOA. This method has the advantage of accurate simulated results and providing the explicit relationships between the controllable parameters with the signal light time delay for the practical experiment.

In this paper, an adiabatic soliton compression (ASC) compressor using a loss management fiber link (LMFL) comprised of alternating erbium-doped fiber (EDF) and ordinary single-mode fiber (SMF) is proposed. The EDF provides quasi-distributed gain instead of distributed gain provided by Raman amplification along the fiber link to compensate the fiber loss. With appropriate choice of length, segments numbers of erbium fiber and pump power, ASC will be realized. The input optical pulse will get narrower through adiabatic soliton compression effect when it propagates in the LMFL. Numerical simulation shows that a fundamental soliton pulse with repetition rate, width and peak power of 40GHz, 10ps and 478mW, respectively, can be compressed to a 1.40ps-width pulse with small pedestal by using the LMFL. The main advantages of this method are as follows: Firstly, by changing the pump power of EDF, the compressed optical pulses with different compression factor can be obtained. Secondly, compared with other adiabatic soliton compression techniques based on Raman amplification, dispersion-decreasing fiber or comb-like dispersion profiled fiber, this method is easy to control and costs less.

The paper demonstrates a novel multi-wavelength ring laser for light source in dense wavelength division multiplexing (DWDM) system, which consists of semiconductor optical amplifier (SOA), high-birefringence fiber loop mirror (Hi-Bi FLM), polarization controller, etc. Among these devices, SOA is used as optical gain medium; Hi-Bi FLM serves as wavelength selecting comb filter. When SOA is driven with higher injection current, the number of output multi-wavelength, and the single channel power and total power all increase gradually due to the inhomogeneous broadening in SOA gain medium. When SOA is driven with a 280-mA dc current at room temperature, it tends to gain saturation state, and simultaneous stable oscillation of 18 lines with the equal line spacing of 0.8 nm (100 GHz) is obtained in the range of 1571.3 nm ~ 1587.1 nm. The power fluctuation among the lasing lines is less than 6 dB and each line-width is less than 0.118 nm. Furthermore, the signal to noise ratio is greater than 25 dB. By adjusting the polarization controller in Hi-Bi FLM, the frequency tuning range of the overall output multi-wavelength is up to 50 GHz.

In this paper, an ultra-broadband all fiber light source for optical coherence tomography was obtained. This light source was obtained by supercontinuum (SC) generated in matched fibers with different characteristics. It was showed that the spectrum quality was affected by dispersion and nonlinear characteristics of the fiber pumped firstly. SC spectrum flatness on the short wavelength side of pumping wavelength was affected by characteristic of the fiber pumped secondly. Dispersion shifted fiber (DSF) was connected after highly nonlinear fiber (HNLF) to optimize spectral shape. Widely broadened SC spectrum was generated in the combination of 120m HNLF+4.5km DSF+2km single mode fiber (SMF) with 55 mW average pump power. 140.6nm 3 dB bandwidth spectrum on the short wavelength was obtained. About 4.1 μm longitudinal resolution could be obtained theoretically around ~1300nm by using this SC spectrum as the light source of optical coherence tomography system. A Fourier-domain OCT (FD-OCT) system was set up by using this light source. Dielectric film plane mirror, a three-layer microscope cover glass with 160 μm thick and onion scarfskin were used as test sample and their depth signal at one point was obtained respectively.

Acousto-optic modulator (AOM) and electro-optic modulator (EOM) drived by voltage controlled oscillator (VCO) are widely used to control and modulate laser, especially semiconductor lasers. We designed a frequency generator based on direct digital synthesizer (DDS) technology which can be used to replace VCO. The frequency generator with high frequency stability can reach 160 MHz bandwidth and frequency resolution up to 1 Hz. The output frequency is programmable and controlled by input TTL edge. These advantages are not available with VCO. We obtain a output radio frequency (RF) power as high as 1W with a power amplifier in order to drive an AOM. This device has been used to scan the wavelength of external cavity laser diode (ECLD) in our laser cooling experiments.

In this paper, the application in return-zero (RZ) light source based on regeneratively mode-locked fiber laser (RMLFL) is demonstrated. By using Mach-Zehnder modulator and pseudo-random code modulation, the back-to-back eye-diagrams with good performances are obtained and RZ format light source with pulse duration of 5ps based on RMLFL is achieved, which quite meets the requirements of 10 Gbit/s long haul transmission. Furthermore, such a 10 GHz RZ source could be multiplexed into 40 GHz RZ light source simply by 4 fold optical time division multiplexing (OTDM).

In this work, 10 GHz over 58 nm 20 dB bandwidth supercontinuum (SC) is generated in 4.5 km dispersion-shifted fiber (DSF) pumped by a 10 GHz regeneratively mode-locked fiber laser (RMLFL). The optimized SC with 51.5 nm 20 dB bandwidth is obtained by properly adjusting the pump power. A 25 nm spectral region from 1535 nm to 1560 nm and a 12nm spectral region of 1567-1579 nm with ±3 dB uniformity are generated. 10GHz, over 30 channels were sliced by an array waveguide grating (AWG) with 100 GHz channel spacing. Timing jitter of each channel is less than 1.25 ps.