Mei Sang

Precise Simultaneous Multiwavelength Tuning by Electrical RF Signals

A method of simultaneous precise multiwavelength tuning using electrical instead of mechanical control is proposed and experimentally demonstrated. This method is achieved by using a single-side-band modulator driven by electrical radio frequency (RF) signals. It is demonstrated that the multiwavelength signals can be simultaneously tuned in frequency by an amount equal to the frequency of the RF signal. A total of 32 wavelengths with a spacing of 100 GHz (i.e., 0.8 nm at the wavelength of 1550 nm) are simultaneously tuned from 4 to 12 GHz experimentally. In principle, the tuning resolution is determined by the resolution of the RF wave generator and the line width of the light source in the system. An acceptable homogenous 20.59-dB side-band-mode suppression ratio is achieved for each wavelength output.

The simulation and experimental research on the sensing characteristics of few-mode-fiber based LPFG

Compared with FBG, long period fiber gratings (LPFGs) have higher sensing sensitivity in many aspects. With the development of few-mode fiber (FMF), few mode long period fiber gratings (FM-LPFGs) has grasped a lot of research focus in recent years. The co-propagating coupling between a core mode and a cladding mode, or two different core modes, will bring different effects on the sensing characteristics of temperature, strain and refractive index. However, the coupling characteristics between core modes and cladding modes have been rarely reported in theory before. In this paper, based on the coupled-mode theory, we simulate the transmission spectrum of the FM-LPFGs by use of MATLAB and COMSOL software. Assuming that all the power is launched in a certain mode, we calculate the coupling coefficients and the transmission spectrums between the core modes (LP01, LP11, LP21, and LP02) and the cladding modes LPlm (l=1) at 1550nm. In addition, we change some fiber parameters like grating period, refractive index modulation depth and cladding diameter to analyze their effect on the transmission spectrum. Based on the simulation, we fabricate some FM-LPFGs by the CO2 laser, and use them in temperature sensing experiments. With the certain strain, we measured the temperature sensing sensitivity of the FM-LPFGs. The measured sensitivities of temperature is -43.98pm/°C.

An FBG sensor interrogation technique based on a precise optical recirculating frequency shifter driven by RF signals

Fiber Bragg grating (FBG) sensors have numerous advantages to sense multi-physical quantities such as the temperature and strain simultaneously by monitoring the shift of the returned “Bragg” wavelength resulting from changes in these quantities. Several FBG interrogation systems have been set up using photo detectors instead of an optical spectrum analyzer (OSA) to convert wavelength to time measurements. However, in those systems, it is necessary to use mechanical tuning components to generate fast-speed wavelength-swept light sources for high-precision FBG interrogation. In this paper, a low-cost and delicate wavelength-shift detection system, without any mechanical scanning parts, is proposed and demonstrated. The wavelength scanning system is a recirculating frequency shifter (RFS) which consists of an optical amplifier, an under test FBG sensor and an optical single-sideband (SSB) modulator driven by RF signals at 10 GHz. The measurement accuracy of this system is 0.08nm.

Research on spectral resource optimization and self-healing technology of hybrid optical fiber sensing network

We propose an optical-fiber-sensing-network (OFSN) to allow hybrid fiber sensors working in the same network and it achieves self-healing function. The discrete and distributed optical fiber sensors can be connected in sub-layers of the network. WDM-OTDM technique is introduced to convert multi-wavelengths of light source into a specific arranged wavelength in each sub-layer. Thus every sub-layer can share the system spectrum resources, and sensing signals of each sub-layer are transmitted together in the backbone network. To achieve self-healing function, double-ring structure is adopted in the backbone network. Node microprocessor program is designed to make switching to the protect fiber when working fiber is broken. The experimental backbone setup of the network demonstrates the practical reliability and intelligence of the optical sensing network.

Flat pulse-amplitude rational-harmonic-mode-locking fiber lasers with GHz pulse repetition rates

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.

Precisely tunable L-band multi-wavelength fiber laser

In this paper, we propose a scheme on precisely tunable L-band multi-wavelength fiber laser. This fiber laser has two main characteristics namely broad wavelength band, uniform power spectrum and precise electronic tunability. About 65 wavelengths output within ± 1.5dB power variation with 50GHz channel spacing in broad spectrum range can be obtained at room temperature. The measured optical signal noise ratio (OSNR) and line width of each wavelength are about 20dB and 345.5MHz respectively. Theses 65 wavelengths are able to be tuned simultaneously up or down in frequency domain with a tuning step ranging from 10 MHz to 14 GHz. The tuning resolution can potentially be as low as 1 Hz in our experiment.

Precise manipulation of light properties in optical domain by RF technology

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.

Graphene thickness-dependent Er-doped Q-switched optical fiber laser

A stable Q-switched laser is useful in the area of remote sensing, range finding, optical imaging, material processing, and fiber communications. With its excellent linear and nonlinear optical characteristics, graphene has been proven to be an attractive material to generate nanosecond, picosecond and femtosecond laser pulses. It has a lot of advantages, such as lower saturation intensity, larger saturable-absorption modulation depth, higher damage threshold, sub-picosecond recovery time and an ultrabroad wavelength-independent saturable-absorption range. In this paper, we demonstrate a graphene based Q-switched fiber laser. Graphene was deposited on the fiber interface by the optically driven deposition method. The thickness of the graphene can be controlled by changing depositing time. The compact Q-switched erbium-doped fiber laser based on graphene operated stably, and got Q-switched pulse sequences output with the repetition rate of 19KHz and the average power of 1.4mW when pump power is 40mW. Higher peak power, shorter pulse duration, and higher repetition rate could be achieved by adjusting the thickness of the graphene layer appropriately. Besides, the pulse duration and output power is proved to be a function of the pump power. The repetition rate of this fiber laser had a characteristic of monotonically increasing, near-linear with the changing of pump power. The stable Q-switching pulse output can be observed on the oscilloscope with differently specific repetition rate and pump power. Theory analysis of this fiber laser and further improvement methods is also studied combined with the experimental results.

Passively mode-locked fiber laser based on single-walled carbon nanotube and graphene as co-saturable absorbers

Ultrashort pulse sources are important in the field of industry processing. With its excellent linear and nonlinear optical characteristics, graphene and single-walled carbon nanotubes(SWCNTs) have been proven to be two attractive materials to generate nanosecond, picosecond and femtosecond laser pulses. They have a lot of advantages, such as lower saturation intensity, larger saturable-absorption modulation depth, higher damage threshold, sub-picosecond recovery time. Graphene and SWCNTs were deposited on the fiber end facets by the optically driven deposition method. By utilizing two different staurable absorbers, we study the performance of three different lasers. Two Er-doped Q-switched optical fiber lasers were constructed by constructing graphene and SWCNTs separately into the ring cavity as saturable absorber. Different performances of the two fiber lasers were investigated by the physical characteristics of the two different materials. Stable pulses generated by a passively mode-locked fiber laser was obtained when two saturable absorbers were inserted into the resonator cavity of a fiber laser at the same time, the repetition rate of 8 MHz which agree with the length of the cavity proved the mode-locked state of the laser. This first time ever trial shows excellent output properties for its long time stable operation.

Energy conversion efficiency calculation model for direct-bonding planar-waveguide THz emitters based on optical rectification effects in GaAs

The generation of terahertz (THz) pulses based on optical rectification effects in GaAs has become more and more attractive and practical due to advances in high power ultrashort pulse fiber lasers. Normally coherence length is a parameter introduced for judging how the phases match by comparing the group velocity of optical pulses with the phase velocity of one of frequency components, like, for example, a component at 2 THz, of THz pulses. It is shown in this paper that the coherence length can not characterize the THz pulse generating process well because it can not count the contribution of all components in the spectrum band of the THz pulses. An energy conversion efficiency calculation model is proposed in this paper by integrating the energy of all THz components generated in the optical rectification process in a planar waveguide device. Based on the calculation model, the evolution of a THz pulse along the longitudinal direction of the waveguide is simulated and the results are used for design of the optimal waveguide structure for which the highest energy conversion efficiency could be reached to 1.5 × 10-3.