Chung Ghiu Lee

Experimental demonstration of 10-gb/s data format conversions between NRZ and RZ using SOA-loop-mirror

This paper describes the demonstration of a simple all-optical data format conversion scheme between return-to-zero (RZ) and nonreturn-to-zero (NRZ) that employs a semiconductor optical amplifier (SOA) in a nonlinear optical loop mirror. The format conversion has been performed between the most widely used data formats-NRZ and RZ formats. The format conversion scheme is based on gain variation by an intensity-dependent phase change in an SOA-loop mirror. The input data stream acts as a control signal that induces the phase differences between clockwise- and counterclockwise-propagating data inside an SOA-loop mirror. It is possible to change the data format of the output data stream by controlling the phase differences of the clockwise and counterclockwise pulse in an SOA-loop mirror appropriately. For the converted NRZ data from RZ data, 10-Gb/s error-free transmission up to 78 km over standard single-mode fiber has been obtained. By comparing the conventional NRZ transmission with the Mach-Zehnder modulation scheme, the proposed RZ-to-NRZ conversion shows an improved transmission performance. The NRZ-to-RZ conversion has clear eye openings up to 78 km. On the contrary, the conventional RZ binary data from a mode-locked laser has a nearly closed eye even at 52 km. The converted RZ data has a 2-dB conversion power margin to the injected NRZ data, which indicates an increase in the receiver sensitivity due to the signal format conversion. The improved transmission distance of the converted RZ signal is due to the duobinary coding effect of the SOA-loop mirror. The SOA has the possibility of high-speed operation over 40 Gb/s, and the SOA-loop mirror has the capabilities of format and wavelength conversions. Therefore, the SOA-loop mirror can be a universal building block in future all-optical networks. In addition, the proposed format conversion scheme can serve as an important format converter between the ultrafast optical-time-division-multiplexed networks and the lower line-rate wavelength-division-multiplexed networks.

Elliptical defected core photonic crystal fiber with high birefringence and negative flattened dispersion

We propose a novel design of photonic crystal fiber (PCF) using an elliptical air hole in the core as a defected core in order to enhance the performance of modal birefringence and to control the properties of chromatic dispersion at the same time. From the simulation results, it is shown that the proposed fiber has high birefringence up to the order of 10(-2), negative flattened chromatic dispersion in a broad range of wavelengths, and low confinement loss less than that of the single mode fiber. The outstanding advantage of the proposed PCF is that high birefringence, negative flattened dispersion, and low confinement loss can be achieved just by adding a small sized elliptical air hole in the core to the elliptical air hole PCF, especially at the same time.

A photonic up-converter for a WDM radio-over-fiber system using cross-absorption modulation in an EAM

A photonic frequency up-converter with a wide wavelength bandwidth is demonstrated. The up-converter is based on wavelength conversion by cross-absorption modulation in an electroabsorption modulator. A two-tone measurement is performed to investigate the linearity of the proposed up-converter. Moreover, its dependence on the wavelength and bias voltage are found to be suitable for wavelength-division-multiplexing-based radio-over-fiber application. The proposed up-converter has a spurious free dynamic range larger than 73.8 dB/spl middot/Hz /sup 2/3/ throughout the 30-nm range of the local oscillator wavelength. Also, the up-converted signal at 26 GHz has a phase noise of -76.8 dBc/Hz at a 10-kHz offset.

Photonic Frequency Upconversion by SBS-Based Frequency Tripling

We describe a photonic frequency upconversion by the stimulated Brillouin scattering (SBS)-based frequency tripling method. The frequency tripling and the photonic frequency upconversion are simultaneously obtained by incorporating a dual-electrode electrooptic modulator (EOM) and a single optical source. Each electrode of the dual-electrode EOM is driven by both an intermediate frequency (IF) and a microwave radio frequency (RF) signal, respectively, along with the optical frequency tripling scheme. The dual-electrode EOM generates appropriate optical sidebands, while the IF signal is conveyed by the pump signal. After the successive SBS shifting process, one of the third optical sidebands is amplified by the narrow gain spectrum of SBS. The carrier signal at 32.493 GHz with narrow linewidth, which is amplified by 20 dB while the other signals are suppressed more than 20 dB, is obtained after photodetection. From the simultaneous frequency upconversion and tripling, an IF signal at 1 GHz is upconverted around the 32.493-GHz signal, which is tripled from the RF signal (10.831 GHz). To verify the ability of conveying broadband data that is limited in the previous method based on SBS, the upconversion of a pilot tone at 1 GHz is demonstrated, which means that the data rate exceeds 1 Gb/s. The proposed photonic frequency upconversion shows the spurious free dynamic range of 75.1 dBldrHz2/3 , which is suitable for a wireless personal communication system adopting the analog fiber-optic link.

Modified rectangular lattice photonic crystal fibers with high birefringence and negative dispersion.

We propose a modified rectangular lattice PCF and numerically investigate its birefringence and dispersion. Based on the plane wave expansion method, it is shown that the proposed structure provides high birefringence and negative dispersion. Numerical results show that the birefringence of the modified PCF reaches 10(-2) and the leakage loss is about 1000 times smaller than that of an original rectangular PCF because the modification gives rise to the strong confinement of guided modes. Dispersion and its slope are also negative over the C band.

Coherent Heterodyne Multilevel Polarization Shift Keying With Spatial Diversity in a Free-Space Optical Turbulence Channel

This paper proposes a coherent multilevel polarization shift keying (MPOLSK) modulation scheme using the spatial diversity detection in a free-space optical (FSO) turbulence channel. The symbol error probability (SEP) analysis for the proposed MPOLSK system under various turbulence regimes has been carried out. The power penalty caused by the phase tracking error has been investigated. For instance, the error floors of 1.3×10-5 and 3.4×10-3 are observed for 8POLSK with the phase tracking variances of σΔ = 0.3 and 0.5, respectively. The maximum ratio combining technique has been applied in the MPOLSK receiver in order to improve the SEP performance. To achieve a SEP of 10-9, the spatial diversity gains required for a 16POLSK-FSO employing four detectors are ~5.8, ~15.8, and ~13.7 dB in weak, moderate, and strong turbulence regimes, respectively.

Pulse-amplitude equalization in a rational harmonic mode-locked semiconductor ring laser using optical feedback

A novel method of pulse-amplitude equalization in a rational harmonic mode-locked semiconductor ring laser is proposed and experimentally demonstrated. This pulse-amplitude equalization method is based on two key points. One is that an optical pulse train introduced to a semiconductor optical amplifier (SOA) saturates the SOA gain quickly, and this can be used for the gain modulation of SOA. The gain modulation of SOA as a mode-locking process in a semiconductor ring laser can generate stable mode-locked pulse trains. The second point is that the SOA gain is equalized from pulse to pulse because the feedback optical pulse circulates inside the ring cavity. By combining the rational harmonic modelocking using a LiNbO3 electro-optic modulator with the gain modulation of the SOA, amplitude-equalized optical pulse trains with the multiplied repetition rates of � 10.00 GHz (the third rational harmonic) and � 16.72 GHz (the fifth rational harmonic) were successfully demonstrated using optical feedback with pulse intensity matching. This scheme can be used for upgrading existing rational harmonic mode-locked semiconductor ring lasers to a semiconductor ring laser with equalized amplitudes at high pulse repetition rates. 2002 Elsevier Science B.V. All rights reserved. PACS: 42.55.Px; 42.60.Fc

Pulse-amplitude equalization in a rational harmonic mode-locked semiconductor fiber ring laser using a dual-drive Mach-Zehnder modulator

We present and demonstrate a simple method of pulse-amplitude equalization in a rational harmonic mode-locked semiconductor ring laser, using a dual-drive Mach-Zehnder (MZ) modulator. Pulse-amplitude equalization was achieved by adjusting the voltages applied to both arms of the modulator, such that each mode-locked pulse experiences the same transmission coefficient in the transmission curve of the modulator. With this method, amplitude-equalized pulse trains with repetition rates of ~7.41GHz (third rational harmonic) and ~12.34GHz (fifth rational harmonic) were successfully obtained without any additional function to the ring laser itself.

Photonic Frequency Upconversion Based on Stimulated Brillouin Scattering

We demonstrate a method of photonic frequency upconversion useful in radio-over-fiber systems, which is based on Brillouin selective amplification. One of the optical sidebands generated from the applied radio-frequency carrier at 10.831GHz is selected and amplified by Brillouin gain. By carrying the 1-GHz intermediate-frequency (IF) signal on the optical carrier and beating them with the amplified sideband, the IF signal of 1 GHz is upconverted to 11.831 GHz. This scheme can also be applied to the IF signal with bandwidth larger than the Brillouin gain bandwidth. The upconverted 11.831-GHz signal has the phase noise of -83.3dBc/Hz at 10-kHz offset. Also, the proposed upconversion scheme shows the spurious-free dynamic range of 89.5 dBmiddotHz2/3

PWM-based PPM format for dimming control in visible light communication system

We report simulation results of pulse width modulation (PWM)-based pulse position modulation (PPM) signal format for visible light communication systems. For simultaneous operation of data transmission and dimming control, PPM data format is added to PWM dimming control signal. To show dimming control, the PWM duty ratio is changed from 40 % to 80 % during PPM data transmission. The PWM frame rate is set to 1 kHz to avoid flickering in human eyes. The PPM data rate is set to 20 kb/s. The waveforms, eye diagrams and BER of this system are calculated. The simulation shows the BER of 1×10-5 is achievable.