You Min Chang

Multilayered graphene efficiently formed by mechanical exfoliation for nonlinear saturable absorbers in fiber mode-locked lasers

An efficiently prepared graphene from a bulk graphite using mechanical exfoliation is experimentally investigated for the first practical application to ultrafast photonics. Overcoming the limitations of the method in its size and atomic layer control, the multilayered graphene guarantees a nonlinear intensity modulation. After confirming its excellent crystal quality and few-layered nanostructure employing Raman analysis and atomic force microscopy the graphene layer is introduced into a fiber laser as an intracavity saturable absorber to realize the passive mode-locking that produces picosecond pulses at the repetition rate of 10.9 MHz. Extinction ratio of the resultant pulsed output is higher than 40 dB.

A detailed experimental study on single-pump Raman/EDFA hybrid amplifiers: static, dynamic, and system performance comparison

This paper presents an experimental study on the performance comparison of three different schemes of single-pump dispersion-compensating fiber (DCF)-based Raman/erbium-doped fiber amplifier (EDFA) hybrid amplifiers together with a DCF-based Raman-only amplifier in terms of static properties, dynamic properties, and system impact: Raman-only amplifier (Type I), Raman/EDFA hybrid amplifiers recycling residual Raman pump in a cascaded EDF located either after (Type II) or prior to (Type III) a DCF, and a Raman-assisted EDFA (Type IV), the concept of which was proposed by Kurosawa et al. With respect to the overall gain and system impact based on bit error rate (BER) measurements in a transmission system, the hybrid amplifier of Type II was found to have the best performance among the four types while the Raman-only amplifier shows the best tolerance to transient response.

Dispersion-compensating Raman/EDFA hybrid amplifier recycling residual Raman pump for efficiency enhancement

We experimentally demonstrate a novel concept of the dispersion-compensating Raman/erbium-doped fiber amplifier hybrid amplifier recycling residual Raman pump for increase of overall power conversion efficiency. The proposed dispersion-compensating hybrid amplifier system has only one pump source for Raman amplification in the dispersion-compensating fiber (DCF) and the residual pump power after the DCF is recycled for secondary signal amplification in an erbium-doped fiber cascaded to the DCF. Using the proposed scheme, we achieve the significant enhancement of both signal gain and effective gain-bandwidth by 15 dB (small signal gain) and 20 nm, respectively, compared to the performance of the Raman-only amplifier.

Raman amplifier-based long-distance remote, strain and temperature sensing system using an erbium-doped fiber and a fiber Bragg grating

We demonstrate a novel, Raman amplifier-based long-distance sensing system for simultaneous measurement of temperature and strain using a combined sensing probe of an erbium-doped fiber (EDF) and a fiber Bragg grating. By recycling residual Raman pump power for generation of amplified spontaneous emission in the EDF after distributed Raman amplification in the transmission fiber, the overall system configuration was significantly simplified without requiring any additional broadband light source. We obtain a remote sensing operation of simultaneous temperature and strain measurement at a location of 50 km. High quality of sensing signals with a ~ 11 dB signal-to-noise ratio (SNR) is readily achieved even after the 50 km transmission with distributed Raman amplification.

2~5 times tunable repetition-rate multiplication of a 10 GHz pulse source using a linearly tunable, chirped fiber Bragg grating

We experimentally demonstrate a simple scheme for the tunable pulse repetition-rate multiplication based on the fractional Talbot effect in a linearly tunable, chirped fiber Bragg grating (FBG). The key component in this scheme is our linearly tunable, chirped FBG with no center wavelength shift, which was fabricated with the S-bending method using a uniform FBG. By simply tuning the group velocity dispersion of the chirped FBG, we readily multiply an original 8.5 ps, 10 GHz soliton pulse train by a factor of 2 ~ 5 to obtain high quality pulses at repetition-rates of 20 ~ 50 GHz without significantly changing the system configuration.

Fully reconfigurable photonic microwave transversal filter based on digital micromirror device and continuous-wave, incoherent supercontinuum source

The combined use of a programmable, digital micromirror device (DMD) and an ultrabroadband, cw, incoherent supercontinuum (SC) source is experimentally demonstrated to fully explore various aspects on the reconfiguration of a microwave filter transfer function by creating a range of multiwavelength optical filter shapes. Owing to both the unique characteristic of the DMD that an arbitrary optical filter shape can be readily produced and the ultrabroad bandwidth of the cw SC source that is 3 times larger than that of Er-amplified spontaneous emission, a multiwavelength optical beam pattern can be generated with a large number of wavelength filter taps apodized by an arbitrary amplitude window. Therefore various types of high-quality microwave filter can be readily achieved through the spectrum slicing-based photonic microwave transversal filter scheme. The experimental demonstration is performed in three aspects: the tuning of a filter resonance bandwidth at a fixed resonance frequency, filter resonance frequency tuning at a fixed resonance frequency, and flexible microwave filter shape reconstruction.

Ultrawideband doublet pulse generation based on nonlinear polarization rotation of an elliptically polarized beam and its distribution over a fiber/wireless link.

Proposed herein is an alternative photonic scheme for the generation of a doublet UWB pulse, which is based on the nonlinear polarization rotation of an elliptically polarized probe beam. The proposed scheme is a modified optical-fiber Kerr shutter that uses an elliptically polarized probe beam together with a linearly polarized control beam. Through theoretical analysis, it was shown that the optical-fiber-based Kerr shutter is capable of producing an ideal transfer function for the successful conversion of input Gaussian pulses into doublet pulses under special elliptical polarization states of the probe beam. An experimental verification was subsequently carried out to verify the working principle. Finally, the system performance of the generated UWB doublet pulses was assessed by propagating them over a 25-km-long standard single-mode fiber link, followed by wireless transmission. Error-free transmission was successfully achieved.

Detailed Theoretical and Experimental Study on Single Passband, Photonic Microwave FIR Filter Using Digital Micromirror Device and Continuous-Wave Supercontinuum

We perform a detailed theoretical and experimental study on the impact of the finesse (F) of a multiple wavelength channel optical filter on output microwave filter transfer function in a spectrum slicing-based photonic microwave finite-impulse response (FIR) filter. For this paper, we use the combination of a digital micromirror device-based spatial light modulator and a continuous-wave, depolarized supercontinuum, since it allows us to perform flexible reconfiguration of output filter transfer function. It is found that the superimposed broadband, low-pass RF transfer function induced by the wide linewidth single-wavelength channel effectively suppresses higher order, periodic resonance peaks as the F is reduced. The optical filters with F=2 are found to provide the most effective suppression of higher order peaks, whatever the single channel waveform is: sinusoidal or Gaussian.

Active Q-switching in an erbium-doped fiber laser using an ultrafast silicon-based variable optical attenuator

Presented herein is the use of an ultrafast Si-based variable optical attenuator (VOA) as a Q-switch for rare earth-doped fiber lasers. The ultrafast VOA is based on a forward-biased p-i-n diode integrated with a ridge waveguide, which was originally designed and optimized for WDM channel power equalization in optical communication systems. By incorporating a Si-based VOA with a transient time of ~410 ns into an erbium-doped fiber-based Fabry-Perot cavity it has been shown that stable Q-switched pulses possessing a temporal width of less than ~86 ns can be readily obtained at a repetition rate of up to ~1 MHz. The lasers peak power of ~38 W is shown to be obtainable at 20 kHz with a slope efficiency of ~21%.

High

We experimentally demonstrate the combined use of an ultrabroadband, incoherent, continuous-wave (CW) supercontinuum (SC) and a dispersion-profiled fiber to implement a photonic microwave filter with a -factor as high as 140. In our scheme based on spectrum slicing, such a high -factor can be readily achieved due to the following two reasons. The -factor restriction due to the limited wavelength tap number caused by narrow spectral bandwidth of conventional optical broadband sources can readily be solved owing to the ultrabroad bandwidth of the CW SC source. Second, the delay medium based on dispersion-profiled fiber compensates the nonuniform relative time delay between each wavelength filter tap caused by inherent wavelength chirping of conventional frequency domain multichannel comb filters. A fiber dispersion profile is provided for the nonuniform relative time delay compensation.