Photonic digital-to-analog conversion using a blue frequency chirp in a semiconductor optical amplifier.

In this Letter, we present a photonic digital-to-analog conversion (DAC) technique based on blue-chirp spectral slicing using a semiconductor optical amplifier (SOA). Because the gain change in an SOA leads to a refractive-index change based on the change in intensity of the input data signal, the probe signals experience a dynamic frequency shift to a shorter-wavelength side called a blue-chirp. After passing through the SOA, the probe signals corresponding to the logic level of the input digital signal are extracted by filtering only the blue-chirp component of the probe signals using rectangular-shape filters. In this study, we experimentally demonstrate a 10-Gb/s, 2-bit photonic DAC from a 10-Gb/s digital signal with various data patterns to a four-level amplitude signal assuming an analog signal. In addition, we evaluate the resolution performance of the photonic DAC in terms of differential and integral nonlinearities and an effective number of bits.

Photonic analog-to-digital conversion using a red frequency chirp in a semiconductor optical amplifier.

A photonic analog-to-digital conversion (PADC) based on intensity-to-frequency conversion using a frequency chirp in a semiconductor optical amplifier (SOA) is proposed. The presented PADC has a simple scheme whereby optical quantization is achieved using a single SOA with multiple rectangular bandpass filters placed in parallel. In this Letter, we successfully achieve 10 GSamples/s, eight-level optical quantization using a quantum-dot SOA. The PADC also has much lower input signal pulse power requirements for optical quantization, compared with conventional PADCs.

Optically powered radio-over-fiber system based on center- and offset-launching techniques using a conventional multimode fiber.

We demonstrate radio-over-fiber (RoF) transmission with a 10-W feed power-over-fiber (PWoF) using a conventional multimode fiber (MMF). In this scheme, the modal dispersion and feed light crosstalk in the MMF are effectively mitigated by the combination of center-launching (CL) and offset-launching (OL) techniques. The CL is used for propagating the feed light into lower-order modes in the MMF, while the OL is used not only for propagating the optical data signals into higher-order modes in the MMF, but also for mitigating the modal dispersion. We successfully achieved significant improvement in the RoF transmission performance with the 10-W feed PWoF and extended the link length up to 4 km, owing to the two techniques.

60 W power-over-fiber feed using double-clad fibers for radio-over-fiber systems with optically powered remote antenna units.

We experimentally demonstrated the ability to feed 60 W of optical power into a power-over-fiber link using a double-clad fiber for radio-over-fiber systems with optically powered remote antenna units. In order to improve the available optical feed power and the power transmission efficiency of the power-over-fiber link, we have designed a specially customized tapered fiber bundle divider for the remote antenna unit. We demonstrated, for the first time, bidirectional radio-over-fiber transmission over a 300 m double-clad fiber with an improved power transmission efficiency and optical power feeding up to 60 W input.

Experimental investigation of chirp properties induced by signal amplification in quantum-dot semiconductor optical amplifiers.

We experimentally show the dynamic frequency chirp properties induced by signal amplification in a quantum-dot semiconductor optical amplifier (QD-SOA) for the first time. We also compare the red and blue chirp peak values and temporal chirp changes while changing the gain and injected signal powers of the QD-SOA with those of a common SOA.

Reflection noise reduction effect of graded-index plastic optical fiber in multimode fiber link.

We experimentally demonstrate that a graded-index plastic optical fiber (GI POF) can significantly reduce reflection noise in a multimode fiber link with a vertical-cavity surface-emitting laser (VCSEL). By directly observing beams backreflected to the VCSEL, we show that the noise reduction effect is closely related to random mode coupling because of light scattering by microscopic heterogeneities in the GI POF core material. This suggests that intrinsic mode coupling can lower the self-coupling efficiency of the light backreflected to the VCSEL cavity through beam quality degradation. Using GI POFs, low-cost radio-over-fiber systems for indoor networks can be realized without optical isolators or fiber end-face polishing.

Evaluation of modal noise in graded-index silica and plastic optical fiber links for radio over multimode fiber systems.

We have evaluated and compared modal noise induced in a graded-index silica multimode fiber (GI-MMF) link and a graded-index plastic optical fiber (GI-POF) link with the misaligned fiber connections. In radio over fiber (RoF) systems using these optical fibers, modal noise appears as unwanted amplitude modulation in the received signal, and results in degradation of the RoF transmission performance. In this work, we have evaluated the modal noise induced in GI-MMFs and GI-POFs with its same core diameter of 50 µm. Our results show that GI-POFs have an inherently higher tolerance to misaligned connection and less modal noise than GI-MMFs in terms of both the error-vector magnitude and the speckle pattern of the transmitted signals.

Multichannel transmission of intensity- and phase-modulated signals by optical phase conjugation using a quantum-dot semiconductor optical amplifier.

We report the demonstration of optical phase conjugation using a quantum-dot semiconductor optical amplifier. The transmission performance of wavelength-multiplexed 10-Gbit/s intensity- and phase-modulated signals is investigated for different channel alignments. In a 100-km transmission, we have successfully achieved high transmission performance with low power penalties for all the transmitted signals.

Error-free 320-to-40-Gbit/s optical demultiplexing based on blueshift filtering in a quantum-dot semiconductor optical amplifier.

We present an ultrahigh-speed optical demultiplexing concept based on optical blue-shift filtering in a quantum-dot semiconductor optical amplifier (QD-SOA). Using a simple scheme, a QD-SOA and an optical bandpass filter, we have successfully achieved error-free operations at 40 Gbit/s on all the extracted tributaries from an aggregated traffic at 320 Gbit/s.

Generation of multi-wavelength picosecond pulses with tunable pulsewidth and channel spacing using a Raman amplification-based adiabatic soliton compressor.

We experimentally demonstrate pulsewidth-tunable picosecond multi-wavelength pulse generation at 10 Gb/s by the use of a Raman amplification-based adiabatic soliton compressor (RA-ASC). Multi-wavelength seed pulse trains are generated by a commercially available electroabsorption modulator and then compressed by using the RA-ASC. The pulsewidths of the compressed pulses can be simultaneously controlled from 16.0 ps to 2.0 ps by adjusting Raman pump power. Operating wavelength range of our scheme are also investigated, showing the possibility for wide channel spacing operations.

320 Gbit/s wavelength conversion using four-wave mixing in quantum-dot semiconductor optical amplifiers.

In this study, we demonstrate error-free all-optical wavelength conversion of ultrahigh-speed intensity modulated signals by means of four-wave mixing in a quantum-dot semiconductor optical amplifier. Error-free performance at a bit rate of 320 Gbit/s is measured for the extracted 40 Gbit/s tributaries with a 3.4 dB average power penalty to the original signal.

Ultrahigh-speed and widely tunable wavelength conversion based on cross-gain modulation in a quantum-dot semiconductor optical amplifier.

We present ultrahigh-speed and full C-band tunable wavelength conversions using cross-gain modulation in a quantum-dot semiconductor optical amplifier (QD-SOA). In this study, we successfully demonstrated error-free 320-Gbit/s operation of an all-optical wavelength converter (AOWC) using the QD-SOA for the first time. We also demonstrated full C-band tunable operation of the AOWC in the wavelength range between 1535 nm and 1565 nm at a bit rate of 160-Gbit/s.

Multichannel wavelength conversion of 50-Gbit/s NRZ-DQPSK signals using a quantum-dot semiconductor optical amplifier.

We demonstrate simultaneous four-channel wavelength conversion of 50-Gbit/s non-return-to-zero differential quadrature-shift-keying signals with a channel spacing of 100-GHz using a quantum-dot semiconductor optical amplifier. Error-free operations with low-power penalties are successfully achieved with various channel configurations.

Polarization-insensitive and widely tunable wavelength conversion for polarization shift keying signal based on four wave mixing in highly non-linear fiber.

A fiber-based polarization-insensitive and widely tunable all-optical wavelength conversion for polarization shift keying (PolSK) signal is experimentally demonstrated by means of four-wave-mixing (FWM) in highly non-linear fiber. Polarization sensitivity of our scheme is compared with the conventional one and the conversion performance of the PolSK signal is investigated. In our scheme, preservation of state of polarization (SOP) of input PolSK signal is possible on both of the converted spectral components in FWM spectrum. In addition, over 23 nm of wavelength tunable operation with high conversion performance is achieved and detailed bit-error-rate (BER) characteristics are measured for both the up- and down-converted signal.

Wideband wavelength-flexible all-optical signal regeneration using gain-band tunable Raman amplification and self-phase-modulation-based spectral filtering.

We demonstrate operating wavelength-flexible all-optical signal regeneration with 80 nm wavelength range by simply controlling the pump wavelength of the Raman preamplifier and the passband of broadened signal spectral filtering. In the operating wavelength range, improvements in receiver sensitivity of more than 2.2 dB are successfully achieved.

Multiple-channel optical signal processing with wavelength-waveform conversions, pulsewidth tunability, and signal regeneration.

A multiple-channel multiple-function optical signal processor (MCMF-OSP) including wavelength-waveform conversions, pulsewidth tunability, and signal regeneration is realized through AND logic gate based on optical parametric processing with a pulsewidth-tunable RZ clock pump. The proposed scheme simultaneously offers four signal processing functions which are useful in wavelength-division multiplexing (WDM) transmission systems, and at network nodes with the necessity for multiple-channel data processing. After the discussions on the concept of MCMF-OSP, a proof-of concept experiment is demonstrated on four 10 Gb/s nonreturn-to-zero (NRZ) data format channels using nonlinearities in semiconductor optical amplifier (SOA) and highly nonlinear fiber (HNLF). A wavelength and waveform conversions to return-to-zero (RZ) modulation format are obtained together with pulsewidth-tunable range from 20% to 80% duty cycles for all input signals. The converted signals inherit the timing and waveform of the RZ clock pump, thus resulting in a time regeneration and large tolerance to narrow-band optical filtering (NAOF) and fiber accumulated chromatic dispersion (CD).

Transmission performance of a wavelength and NRZ-to-RZ format conversion with pulsewidth tunability by combination of SOA- and fiber-based switches.

An all-optical signal processing scheme coupling wavelength conversion and NRZ-to-RZ data format conversion with pulsewidth tunability into one by combination of SOA- and fiber-based switches, is experimentally demonstrated, and its transmission performance is investigated. An 1558 nm NRZ data signal is converted to RZ data format at 1546 nm with widely tunable pulsewidth from 20 % to 80 % duty cycle at the bit-rate of 10 Gb/s. The investigation on transmission performance of the converted RZ signals at each different pulsewidth is carried out over various standard single-mode fiber (SSMF) links up to 65 km long without dispersion compensation. The results clarify a significant improvement on transmission performance of converted signal in comparison with the conventional NRZ signal through tunable pulsewidth management and show the existence of an optimal pulsewidth for the RZ data format at each transmission distance with particular cumulative dispersion. The optimal pulsewidths of the converted RZ signal and its corresponding power penalties against the NRZ signal are also investigated in different SSMF links.

Broadband regenerative wavelength conversion and multicasting using triple-stage semiconductor-based wavelength converters.

We demonstrate broadband wavelength conversion with a 320 nm operating wavelength range and channel spacing flexible wavelength-division-multiplexing (WDM) multicasting from a 1550 nm signal using a triple-stage cascaded semiconductor-optical-amplifier-based wavelength converter.

Discrete wavelength tuning characteristics of a single-frequency fiber laser with dual-wavelength external frequency-stable light injection.

We investigate discrete wavelength tuning characteristics of a single-frequency fiber laser locked to either of two external lights. Frequency locking is achieved by the cooperatively induced spatial-hole burning (SHB) of a saturable absorber in the laser cavity. We show that lasing frequency is well locked to either of the two external lights when the lasing wavelength of the fiber laser is adjusted to the corresponding wavelength of the external light by tuning the bandpass filter in the laser cavity. The locked frequency stability of the fiber laser is as high as that of the employed external light source.

Performance improvement of optical RZ-receiver by utilizing an all-optical waveform converter.

A practical receiver scheme with all-optical waveform conversion is proposed and demonstrated. To mitigate influence of the timing jitter of the received signal, the proposed receiver employs a semiconductor optical amplifier (SOA)-based waveform converter, which can generate signal pulses with a rectangular-like profile. We have evaluated the receiver performances of the conventional and proposed schemes. The receiver sensitivity improvement of 0.7 dB and the phase-margin enlargement of 60 % were simultaneously achieved in comparison with the conventional receiver scheme.