Fiber Delay Interferometer Research Articles

Carrier-Suppressed Single Sideband Signal for FMCW LiDAR Using a Si Photonic-Crystal Optical Modulators

We have studied frequency-modulated continuous-wave (FMCW) light detection and ranging which uses externally modulated light instead of a wavelength-chirped laser diode and a complicated clock sampling circuit. In this method, the FMCW beat signal is accompanied by a 3 dB average loss in principle when pseudo frequency-modulated light simply generated by an intensity modulation with frequency-swept signals is used. In this article, we first fabricated a Si in-phase/quadrature-phase modulator equipped with compact photonic-crystal phase shifters and generated carrier-suppressed single sideband modulation signals. Furthermore, we obtained almost ideal frequency modulation, with a side-mode suppression ratio of 22.3 dB, by temporally controlling the frequency and phase of the modulation signals. When the FMCW beat signal was produced with this modulated signal in a fiber delay interferometer, its intensity was increased by 5.3 dB, as compared with that using intensity modulation. This improvement corresponds to an 80% increase in the limit distance of FMCW ranging. We also generated similar signals using a single modulator with a bypass waveguide, which simplified modulation. The side-mode suppression ratio for the frequency-swept modulation was limited to 9.8 dB, while comparably large increase in the beat intensity was confirmed.

All-optical UWB doublet pulses generation by using a delay interferometer

We demonstrated a simple scheme to generate ultra wideband (UWB) doublet pulses by inputting a dark return-to-zero (RZ) signal into a fiber delay interferometer (FDI). An 0.625-Gbit/s dark-RZ pulse train where the pulse width is 120ps was inputted into a FDI where the free spectral range (FSR) is 0.16nm (∼20GHz, according time delay is ∼50ps) and the extinction ratio (ER) is 9dB, and the phase difference of the two fiber arms was changed and controlled by adjusting the operation temperature of the FDI, by do so, UWB doublet pulses were directly generated at an output port of the FDI. The system parameter effects on the output UWB pulses were discussed. Moreover, we also numerically demonstrated that the UWB quadruplet pulses can be generated in the same set by optimizing system parameters. This scheme has some distinct advantages including easy integration, convenient tuning, good stability, and so on. Presented method also accords with the general features in future applied UWB-Over-Fiber communication system, such as, single optical source input, simple configuration and passive device.

Photonic ultrawideband impulse radio generation and modulation over a fiber link using a phase modulator and a delay interferometer

We propose and experimentally demonstrate a simple and flexible photonic scheme for generation and modulation of ultrawideband (UWB) using a phase modulator and a fiber delay interferometer (DI)-based multichannel frequency discrimination. By introducing a Gaussian signal to the phase modulator, the UWB polarity-switchable doublet pulses can be achieved by combining the pair of UWB monocycle pulses with inverted polarities at the DI outputs under proper time delay. Furthermore, the pulse shape modulation, pulse position modulation, and on-off keying can be performed by coding the electrical data patterns and adjusting the time delay between the two monocycle pulses. Only a laser source introduced in the architecture guarantees the excellent dispersion tolerance over 75 km optical fiber link for UWB pulse sequence, which has potential application in future high-speed UWB impulse radio over optical fiber access networks.

Multi-channel multifunctional optical differentiator based on phase modulation and linear filtering

Multifunctional photonic differentiation is robust in optical computing and optical signal processing. We theoretically propose a multifunctional optical differentiator based on phase modulation and linear filtering. We also experimentally demonstrate two kinds of differentiators by using a phase modulator cascaded a fiber delay interferometer (DI) whose transmission spectrum is approximately linear. Difference between the two differentiators relies on whether the differentiated signals are formed by the optical intensity or the optical field of the output signal, which in turn depends on the relative shift between the signal wavelength and the closest notches of a linear filter. We also show the average errors of all kinds of differentiations and results show that the increase of the degree of linearity of the transmission spectrum of the DI will reduce the average error. Moreover, we demonstrate multi-channel multifunctional differentiation due to the comb spectrum of the DI.

All-optical binary phase-coded UWB signal generation for multi-user UWB communications

An all-optical incoherent scheme for generation of binary phase-coded ultra-wideband (UWB) signals is proposed and experimentally demonstrated. The binary phase coding is performed based on all-optical phase modulation in a semiconductor optical amplifier (SOA) and phase modulation to intensity modulation (PM-IM) conversion in a fiber delay interferometer (DI) that serves as a multichannel frequency discriminator. By locating the phase-modulated light waves at the positive and negative slopes of the DI transmission spectra, binary phase encoded UWB codes (0 and π) are generated. We also experimentally demonstrate a bipolar UWB coding system with a code length of 4, operating at 1.25 Gb/s. And the decoding is analyzed as well. Our proposed system has potential application in future high-speed UWB impulse radio over optical fiber access networks.

Simultaneous all-optical multi-channel RZ and CSRZ to NRZ format conversion

We propose and demonstrate multi-channel all-optical return-to-zero (RZ) and carrier-suppressed RZ (CSRZ) to non-return-to-zero (NRZ) format conversions using a same fiber delay-interferometer (DI). The conversions are based on simultaneous constructive interference induced by the periodical comb-like filtering characteristic of the DI. By using a DI with free spectral range of 40 GHz, 8 channel RZ and CSRZ signals at 20 Gb/s can be converted to corresponding NRZ signals at the same time. Furthermore, a duplicate output can be obtained for multi-channel CSRZ input. Bit error ratio measurements show 4 and 3.5 dB penalties for the proposed multi-channel RZ and CSRZ to NRZ conversions respectively. Further transmission for the converted NRZ signals verifies the good performance of the proposed all-passive multi-channel format converter. The potential of multi-channel RZ signals with mixed duty-cycle to NRZ conversions is also investigated.

Stable and high-performance multiwavelength erbium-doped fiber laser based on fiber delay interferometer

In this paper, we proposed a novel scheme to realize the multiwavelength erbium-doped fiber lasers. By adding a length of dispersion shifted fiber (DSF) in the ring cavity, we can suppress the cavity mode competition resulting from homogeneous line broadening (HLB) effect. In addition, a comb filter based on fiber delay interferometer (DI) is used for frequency selecting. To enhance the extinction ratio while maintaining the free space range (FSR), the proposed isolator-assisted double-pass DI is utilized into the laser cavity, and a stable 7-wavelength simultaneous lasing spaced at 21.5 GHz is accordingly achieved with an extinction ratio of higher than 40 dB. The lasers are stable with a maximum power fluctuation per channel of less than 0.6 dB during an hour test.

PPLN-based all-optical 40 Gbit/s ODB/AMI/FSK wavelength conversion and FSK logic NOT gate

We propose and demonstrate all-optical wavelength conversion for optical duobinary (ODB), alternate-mark inversion (AMI), and frequency-shift keying (FSK) signals and a logic NOT gate for a FSK signal based on cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) in a periodically poled lithium niobate (PPLN) waveguide. ODB/AMI/FSK are generated from the demodulation of differential phase-shift keying (DPSK) using one-bit-delay fiber delay interferometer (FDI). PPLN-based 40 Gbit/s ODB/AMI/FSK wavelength conversion and FSK logic NOT gate are simultaneously implemented in the experiment.

Ultrafast all-optical NOR gate based on semiconductor optical amplifier and fiber delay interferometer

An ultrafast all-optical logic NOR gate based on a semiconductor optical amplifier (SOA) and a fiber delay interferometer (FDI) is presented. For high-speed input return-to-zero (RZ) signal, nonreturn-to-zero (NRZ) switching windows which satisfy Boolean NOR operation can be formed by properly choosing the delay time and the phase shift of FDI. 40Gb/s NOR operation has been demonstrated successfully with low control optical power. The factors that degrade the NOR operation have been discussed.