Hyunhwan Choi

A Review of the Polarization-Nulling Technique for Monitoring Optical-Signal-to-Noise Ratio in Dynamic WDM Networks

The polarization-nulling technique utilizes the different properties of optical signal and amplified spontaneous emission (ASE) noise for accurate monitoring of the optical-signal-to-noise ratio (OSNR) in dynamic optical networks. However, the performance of this technique is bound to be deteriorated if the signal is depolarized by polarization-mode dispersion and/or nonlinear birefringence or the ASE noise is partially polarized due to polarization-dependent loss (PDL) in the transmission link. The authors analyze these effects on the performance of the polarization-nulling technique and introduce several techniques to overcome these problems. These improved versions of the polarization-nulling techniques could monitor the OSNR with accuracy of better than plusmn1 dB, even when the differential group delay is as large as 60 ps. These techniques could also negate the effect of the signal depolarization caused by nonlinear birefringence in a highly nonlinear transmission link. The effect of the partially polarized ASE noise due to PDL is found to be not severe in most cases, as long as the PDL/span is smaller than 0.2 dB. To verify the possibility of using the polarization-nulling technique in real systems, the OSNR of the wavelength-division-multiplexed (WDM) signals transmitted through a 120-km-long aerial fiber link is measured for one week. No significant degradation in the monitoring accuracy is observed during this long-term measurement. In addition, the performance of the polarization-nulling technique in an ultralong-haul transmission link is evaluated by using a 640-km-long recirculating loop. The results show that this technique could accurately measure the OSNR in the transmission link longer than 3200 km. From these results, the authors conclude that the polarization-nulling technique is well suited for monitoring the OSNR in dynamic WDM networks

Bidirectional WDM PON using light-emitting diodes spectrum-sliced with cyclic arrayed-waveguide grating

We proposed and demonstrated a bidirectional wavelength-division-multiplexing passive optical network by using spectrum-sliced light-emitting diodes (LEDs) with cyclic arrayed-waveguide gratings (AWGs). When the spectrum-slicing technique is used, it is often difficult to secure an adequate amount of system margin due to the large slicing loss and low output power of LED. To overcome this problem, we reduced the slicing loss by using all the peaks of the spectrum-sliced light (separated by the free-spectral range of AWG) and improved the receiver sensitivity by using the forward-error correction technique. The excessive dispersion penalty (caused by using multiple peaks) was suppressed by using a simple dispersion precompensation circuit.

Measurement of Differential Phasor Diagram of Multilevel DPSK Signals by Using an Adjustment-Free Delay Interferometer Composed of a 3

We develop an adjustment-free differential-phase demodulator based on a delay-interferometer (DI) made of a 3 times 3 optical coupler, which is used as a 120-degree optical hybrid, and demonstrate the possibility of using it as a phasor monitor for the multilevel differential phase-shift keyed (DxPSK) signal. The key features of the proposed demodulator are twofold: (a) in-phase and quadrature (I-Q) components and the phasor diagram can be obtained only by using a single DI, and (b) the phase delay of the DI can be derived from the output power of the DI without the knowledge of the signal under test. These features enable us to demodulate the I-Q components of DxPSK signals without any adjustments. We formulate a theoretical model for deriving the differential phasor from the output signals of the 3 times 3 coupler regardless of unbalances in the phase retardations and the splitting ratios of the 3 times 3 coupler. Thus, the discrepancy from the ideal 120-degree optical hybrid is not a problem, and hence the requirements of the 3 times 3 couplers are greatly relaxed. For a demonstration, we implement the proposed demodulator by using a fiber Michelson interferometer with Faraday rotator mirrors and use it as a differential phasor monitor capable of the plug-and-play (wavelength- and polarization-independent) operation. We show that this phasor monitor can be used for diagnosis of differential phase-shift keyed (DPSK) and differential quadrature phase-shift keyed (DQPSK) transmitters by identifying the sources of impairments from the measured constellation diagram and phasor trajectories. The proposed phasor monitor can also be used for monitoring the optical signal-to-noise ratio (OSNR) of DPSK signal.

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This study presents a liquid lens using electrowetting that employs an oil phase floating in between the conducting fluids. The lens shape has double-sided surfaces and operates with a bias of 0-60 V. The focal length of the lens, with an aperture size of 2 mm, is ~5.8 mm, and it is converted into an optical power of 172. The lens is sufficient to suppress the fluctuation of fluids due to the external vibration. An image seen through the lens clearly resolves the element better than 6.35 LP/mm on USAF 1951 1×.

3 Optical Coupler

A fluidic lens with double-sided polydimethylsiloxane membranes of different thicknesses that complement each other to reduce spherical aberration was fabricated. It is operated with magnetic repulsion induced by the current in the voice coil. The optical power of the proposed lens was observed to be considerably higher compared to that of a typical convex glass lens. The focal lengths of rays passing through the optical axis and the edge of the lens was measured and compared with the simulation data. The spherical aberration of the proposed lens was observed to be negligibly small compared to that of a plano-convex lens.

Fluidic lens of floating oil using round-pot chamber based on electrowetting.

We develop a new technique for simultaneously monitoring the amplified spontaneous emission (ASE) and multi-path interference (MPI) noises in distributed Raman amplified (DRA) systems. This technique utilizes the facts that the degree-of polarization (DOP) of the MPI noise is 1/9, while the ASE noise is unpolarized. The results show that the proposed technique can accurately monitor both of these noises regardless of the bit rates, modulation formats, and optical signal-to-noise ratio (OSNR) levels of the signals.

Adaptive double-sided fluidic lens of polydimethylsiloxane membranes of matching thickness

This letter presents a liquid lens that uses an electrowetting principle while also employing a floating lens between actuating fluids. The lens shape is a double-sided curved surface, and it has powerful optical properties, showing a focal length of 12.6 mm with a 2 mm aperture diameter, and operating with a bias voltage 0-60 V. The lens has shown mechanical reliability sufficient to hold a floating shape that is robust against vibration of 200 RPM. An image seen through the lens can resolve an element better than 5.65 LP/mm on the USAF 1951 1x standard.

Simultaneous monitoring technique for ASE and MPI noises in distributed Raman Amplified Systems.

This letter presents a fluidic lens whose power can be rapidly adjusted. It is fabricated using a polydimethylsiloxane (PDMS) membrane combined with a ring-shaped magnet to form a single lens. The actuating force of this lens is generated by a voice-coil that exerts magnetic force onto the ring-shaped magnet, which moves upward and downward on the optical axis. It shows improved controllability that is smooth and fast, with a higher plus optical power than the current liquid lens of PDMS membrane. The focal length is measured to be 25 D, and demonstrates clearness by showing images captured by a digital camera.

Fluidic Lens of Floating Water Using Intermediate Hydrophilic Layer Based on Electrowetting

A full fluidic lens system consisting of multi-layers of PDMS membranes, which each builds single convex/concave lens, whose lens aperture is ~4mm, was fabricated. It was operated with magnetic attraction/repulsion induced by voicecoil coiled in between magnets adhesively bonded to each PDMS membrane. It has shown a smooth and fast controllability of the focal-length control versus the applied voltage and quite good image of the target at the distance from 10 mm to 5 m. To verify zooming function of lens, images captured by digital camera was attached, and it also shows good image magnification.