Nonlinear Fiber Sagnac Interferometer Research Articles

Optical-Fiber Source of Polarization-Entangled Photons in the 1550 nm Telecom Band

We present a fiber-based source of polarization-entangled photons that is well suited for quantum communication applications in the 1550 nm band of standard fiber-optic telecommunications. Polarization entanglement is created by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed orthogonally polarized pump pulses and subsequently removing the time distinguishability by passing the parametrically scattered signal and idler photon pairs through a piece of birefringent fiber. Coincidence detection of the signal and idler photons yields biphoton interference with visibility greater than 90%, while no interference is observed in direct detection of either signal or idler photons. All four Bell states can be prepared with our setup and we demonstrate violations of the Clauser-Horne-Shimony-Holt form of Bell's inequality by up to 10 standard deviations of measurement uncertainty.

Broad-band mid-span spectral inversion without wavelength shift of 1.7-ps optical pulses using a highly nonlinear fiber Sagnac interferometer

We demonstrate broad-band mid-span spectral inversion without wavelength shift of short optical pulses using a highly nonlinear fiber Sagnac interferometer. The 1.55-/spl mu/m optical pulses with a 1.7-ps pulsewidth are transmitted over 40 km of standard fiber, showing the suitability of this technique for broad-bandwidth second-order dispersion compensation in high-bit-rate optical fiber transmission systems.

Polarization-independent, wavelength-shift-free optical phase conjugator using a nonlinear fiber Sagnac interferometer

We propose and experiment with a novel configuration of a wavelength-shift-free optical phase conjugator using a nonlinear fiber Sagnac interferometer. In the proposed configuration, two orthogonally polarized pump waves are fed into different ports of the Sagnac interferometer, and we can achieve polarization-independent as well as wavelength-shift-free operation.

In-line optical phase-sensitive amplifier with pump light source controlled by optical phase-lock loop

This paper describes an in-line optical phase-sensitive amplifier (PSA) with a pump laser whose optical phase is locked to that of a randomly modulated signal by using an optical phase-lock loop (OPLL). The OPLL is designed with the capability of optical phase locking at an arbitrary relative phase. Experimental evaluation is presented of the OPLL employing a newly developed external cavity semiconductor ring laser with a spectrum linewidth of less than 20 kHz. Employing this pump laser with the OPLL in conjunction with a 4.4-km long nonlinear fiber Sagnac interferometer (NFSI) yields optical phase-sensitive gain of up to 11 dB. A randomly modulated signal is successfully amplified and confirmed offering a clear eye-opening.

Tunable fiber-optic parametric oscillator

We report operation of a tunable optical parametric oscillator that employs a nonlinear-fiber Sagnac interferometer as a parametric amplifier. The amplifier, which consists primarily of dispersion-shifted fiber that has zero dispersion at 1538 nm, is synchronously pumped with 7.7-ps pulses at 1539 nm. The wide bandwidth of the parametric gain permits tuning of the output signal pulses over a 40-nm range centered on the pump wavelength. The Sagnac interferometer decouples the pump wave from the oscillator cavity while a bandpass filter in the cavity transmits only the signal wave, thereby creating a singly resonant parametric oscillator that is phase insensitive. Whereas we demonstrate tuning over almost the entire bandwidth of Er-doped-fiber amplifiers, one could construct a similar device that operates near the 1310-nm zero-dispersion wavelength of standard telecommunication fiber.

Pulsed degenerate optical parametric oscillator based on a nonlinear-fiber Sagnac interferometer

We report operation of an all-fiber degenerate optical parametric oscillator that employs a nonlinear-fiber Sagnac interferometer as a parametric amplifier. Synchronous pumping with 3.9-ps pulses at 1544 nm yields 0.83-ps output pulses. The wide bandwidth of the fiber parametric amplifier causes the oscillator to act as a pulse compressor. The output signal pulses exhibit improved spectral symmetry and a reduced time-bandwidth product compared with the pump pulses. Currently, the net group-velocity dispersion in the passive section of the fiber cavity limits the signal-pulse bandwidth and hence the minimum-obtainable pulse width. This experiment suggests the possibility of frequency conversion by operation of a similar pulsed parametric oscillator away from degeneracy.

All optical signal regularizing/regeneration using a nonlinear fiber Sagnac interferometer switch with signal-clock walk-off

All-optical signal regularizing/regeneration using a nonlinear fiber Sagnac interferometer switch (NSIS) that employs signal-clock walk-off is investigated. The NSIS realizes all-optical signal regeneration, including timing and amplitude regularizing, by switching clock pulses with amplified input signals using a walk-off-induced, wide, square switching window and intensity-dependent transmittance of the device. First, characteristics (in both the temporal and spectral domains) of the all-optical signal regeneration achieved with the NSIS are investigated theoretically and experimentally. They certify that if clock pulses are within the square switching window obtained with signal-clock walk-off, the clock pulses can be modulated according to the data that the input signals carry and retain their temporal and spectral profiles. This means that if clock pulses can be prepared that meet the system requirements, the NSIS can convert input signals that may not satisfy system requirements into high-quality output signals. Limitations on the switching contrast due to the cross-phase modulation of counterpropagating reference pulses is also discussed. Second, two possible applications of NSIS-based all-optical signal regularizing/regeneration, 1) an all-optical multiplexer with an optical clock and 2) an all-optical regenerative repeater, are discussed. Preliminary experiments with /spl sim/10-ps pulses at bit rates of /spl sim/5 Gb/s that use locally prepared optical clock pulses, show that the NSIS provides an error-free regeneration function with a certain tolerance for pulse-period irregularity if a proper optical clock is obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of crosstalk and timing jitter on all-optical time-division demultiplexing using a nonlinear fiber Sagnac interferometer switch

All-optical time-division demultiplexing using a nonlinear fiber Sagnac interferometer switch (NSIS) is studied with respect to the two main causes that degrade the bit-error-rate (BER) performance: crosstalk and timing jitter. It is shown that unwanted cross-phase-modulation in the reference signal which counter-propagates to the control pulse, as well as the poor extinction of the switch itself, seriously degrades the extinction ratio of the switch, thus increasing the crosstalk from other channels. Numerical calculations clarify the effect of the switching window width, window shape, and the multiplexed channel number on the power penalty in terms of BER performance. Timing jitter between the signal and control pulses is investigated as another degradation factor that causes an error floor in BER performance. It is found that the minimum BER is obtained when the window width is set to the time slot width and the rms value of the jitter must be less than 1/14.1 times the time slot width to ensure that BER >

Effect of timing jitter on an optically controlled picosecond optical switch

Timing jitter between optical control and signal pulses seriously degrades the bit-error-rate (BER) performance of optically controlled optical switches in the picosecond region. BER's measured in a picosecond all-optical switch based on nonlinear fiber Sagnac interferometer switching reveal that the rms signal-to-control timing jitter must be less than 1/12 of the switching window to achieve BER &lt; 10−9.

Pump-induced frequency shifting of switched pulses in an interferometric all-optical switch

The induced frequency chirping of switched pulses in an interferometric all-optical switch that uses pump-induced nonlinear phase shifts is investigated. Using a nonlinear fiber Sagnac interferometer switch, it is shown that switched pulses suffer from frequency shift and broadening and can be compressed by using an anomalous group-velocity-dispersion fiber. It is also demonstrated that picosecond probe pulses can be switched while retaining their temporal and spectral profiles if pump-probe walk-off is employed.

Polarization-independent all-optical switching

A polarization-independent all-optical switch has been demonstrated based on a nonlinear fiber Sagnac interferometer. The control signal is split and recombined with a time delay, producing two linearly polarized pulses that lie within the temporal switching window of the circuit. Less than 10% variation of the switched output signal was observed as the input polarization was varied.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

All-optical arbitrary demultiplexing at 25 Gbits/s with tolerance to timing jitter

All-optical demultiplexing has been shown with a full-duty-cycle 2.5-Gbit/s signal in a nonlinear fiber Sagnac interferometer. Complete switching of arbitrary pulse patterns in the data stream has been achieved by using two orthogonal polarization states for the switching and switched pulse trains. The polarization dispersion between the two fiber axes defines a window that allows for switching with timing errors as large as 350 ps.