Integrated mode-evolution-based polarization splitter

A mode-evolution-based polarization splitter suitable for high-index-contrast systems and directly integratable with a recently reported on-polarization rotator is described and its performance verified through both finite-difference time-domain and eigenmode expansion simulations. For a device length of 200 microm, greater than 22 dB of extinction is obtained across a 1.45-1.75-microm bandwidth.

Integrated mode-evolution-based polarization rotators

For the first time to our knowledge, designs for mode-evolution-based integrated polarization rotators requiring only a pair of waveguide core layers are presented. Finite-difference time-domain and eigenmode expansion simulations demonstrate the near-ideal performance of the approach. In contrast with approaches based on mode coupling, no significant wavelength sensitivity is observed.

Observation of quantum-limited timing jitter in an active, harmonically mode-locked fiber laser

We report on the observation of quantum-limited timing jitter in a harmonically mode-locked soliton fiber laser with an ultralow-noise local oscillator. The effects of amplitude and phase modulation on the spectrum are described and compared with theory.

Electromagnetic cavity with arbitrary Q and small modal volume without a complete photonic bandgap

We show how an electromagnetic cavity with arbitrarily high Q and small (bounded) modal volume can be formed in two or three dimensions with a proper choice of dielectric constants. Unlike in previous work, neither a complete photonic bandgap nor a trade-off in mode localization for Q is required. Rather, scattering and radiation are prohibited by a perfect mode match of the TE-polarized modes in each subsection of a Bragg resonator. Q values in excess of 10(5) are demonstrated through finite-difference time-domain simulations of two- and three-dimensional structures with modal areas or volumes of the order of the wavelength squared or cubed.

Self-stabilized passive, harmonically mode-locked stretched-pulse erbium fiber ring laser

We have studied a passive, harmonically mode-locked stretched-pulse erbium fiber ring laser with net positive dispersion that is self-stabilized by gain depletion and electrostriction. Periodic pulses with supermode suppression of >75 dB and picosecond jitter are achieved. The pulses are compressible to 125 fs by external chirp compensation. The repetition rate is 220 MHz, and the average power is as high as 80 mW.

Group velocity of solitons

It is shown that, in addition to the well-known phase accumulation of a traveling soliton, which may be interpreted as a change of phase velocity as a result of the Kerr nonlinearity, there is a change in the speed of travel of the envelope, the group velocity. This analysis is extended to dispersion-managed solitons, for which it is shown that the discrepancy between phase- and group-velocity changes is generally smaller.

Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime

Nonlinear optical effects due to the phase between carrier and envelope are observed with 5 fs pulses from a Kerr-lens mode-locked Ti:sapphire laser. These sub-two-cycle pulses with octave spanning spectra are the shortest pulses ever generated directly from a laser oscillator. Detection of the carrier-envelope phase slip is made possible by simply focusing the short pulses directly from the oscillator into a BBO crystal. As a further example of nonlinear optics with such short pulses, the interference between second- and third-harmonic components is also demonstrated.

Soliton squeezing at the gigahertz rate in a Sagnac loop

We demonstrate what is to our knowledge the first all-fiber squeezing experiment. A balanced Sagnac loop is used, and a record 6.1+/-0.2dB of noise reduction below shot noise has been obtained without stabilization. A gigahertz Er-doped fiber laser and a high-power double-clad Er-Yb amplifier have been developed to suppress guided acoustic-wave Brillouin scattering and to make possible the all-fiber configuration.

All-solid-state Cr:forsterite laser generating 14-fs pulses at 1.3 mum

We report the generation of 14-fs pulses at 1.3mum with 80-mW average power at 100-MHz repetition rate by an all-solid-state Kerr-lens mode-locked Cr:forsterite laser. The laser spectrum covers wavelengths of 1230-1580 nm, with a FWHM of 250 nm. Since 1.3-mum wavelengths are close to the zero dispersion wavelength of Cr:forsterite, higher-order dispersion is the main factor limiting pulse durations. We use specially designed and fabricated double-chirped mirrors in combination with high-index PBH71 prisms to compensate for the intracavity dispersion over almost 300 nm.

Planar integrated wavelength-drop device based on pedestal antiresonant reflecting waveguides and high-Q silica microspheres

Whispering-gallery modes in silica microspheres can be accessed very efficiently with the recently introduced stripline pedestal antiresonant reflecting optical waveguide (SPARROW) structure. This integrated-optics coupling technique creates novel application opportunities for the high-Q spherical cavities. We report the demonstration of a narrow-band wavelength-drop configuration utilizing SPARROW waveguides and a silica microsphere.

Gigahertz-repetition-rate mode-locked fiber laser for continuum generation

We report direct generation of <500-fs pulses at a 1-GHz rate from a self-starting passively mode-locked fiber laser by regeneratively synchronizing the pulses with a phase modulator. The pulses are amplified and passed through a dispersion-decreasing fiber and a normal-dispersion supercontinuum fiber. The resulting continuum is wider than 350 nm.

Manakov solitons and polarization mode dispersion

The effect of polarization mode dispersion (PMD) on Manakov solitons and dispersion managed solitons is treated analytically and by numerical simulation. In the analytic approach the internal motion of the Manakov soliton is represented as a damped harmonic oscillator. The PMD functions as a white noise source driving the oscillations. It is shown that the solitons can withstand PMD up to a certain instability threshold for which an analytic expression is obtained. This threshold is also evaluated for dispersion managed solitons. (c) 2000 American Institute of Physics.

Stabilization of an active harmonically mode-locked fiber laser using two-photon absorption

Two-photon absorption provided by a semiconductor mirror structure is shown to reduce amplitude fluctuations significantly in a harmonically mo e-locked fiber ring laser. Pulse dropouts are eliminated in a laser that produces picosecond pulses at a repetition rate of 2 GHz.

Reconfigurable time-domain spectral shaping of an optical pulse stretched by a fiber Bragg grating

We demonstrate a method for spectrally shaping optical pulses that is readily reconfigurable and can produce variable filter functions. This practical technique relies on a compact and robust 3.86-m-long linearly chirped fiber Bragg grating that chromatically disperses the pulse to ~30 ns. We then shape the pulse envelope, and thus the pulse spectrum, with a programmable arbitrary waveform generator and an amplitude modulator to obtain several filter functions.

Solitons and polarization mode dispersion

Analytical expressions are presented for Manakov solitons perturbed by polarization mode dispersion (PMD). Comparison is made with computer simulations. Dispersion-managed solitons are also studied. It is concluded that at high bit rates solitons are superior to linear return-to-zero propagation with regard to PMD.

Pedestal antiresonant reflecting waveguides for robust coupling to microsphere resonators and for microphotonic circuits

Strip-line pedestal antiresonant reflecting waveguides are high-confinement, silica integrated optical waveguides in which the optical modes are completely isolated from the substrate by thin high-index layers. These waveguides are particularly well suited for whispering-gallery mode excitation in high-Q microspheres. They can also be used in microphotonic circuits, such as for microring resonators. The theory and design of these structures are highlighted. Experiments that show high coupling efficiency to microspheres are also demonstrated.

Timing restoration dynamics in an actively mode-locked fiber ring laser

Using electronic phase detection, we study the dynamics that govern pulse retiming in an actively mode-locked fiber laser. We compare the dynamics for amplitude and for phase modulation and identify the characteristic time constants for each case. The retiming dynamics for amplitude modulation are revealed as a first-order exponential decay, whereas for phase modulation the dynamics are those of a damped harmonic oscillator. We show that the measured time constants agree with predictions given by the soliton perturbation theory.

Sub-two-cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser

Pulses shorter than two optical cycles with bandwidths in excess of 400 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser with a repetition rate of 90 MHz and an average power of 200 mW. Low-dispersion prisms and double-chirped mirrors provide broadband controlled dispersion and high reflectivity. These pulse durations are to our knowledge the shortest ever generated directly from a laser oscillator.

Collisions in dispersion-managed soliton propagation

We study collisions in dispersion-managed soliton propagation for wavelength-division multiplexing application at zero net dispersion when the Gordon-Haus timing jitter is removed. We show that the collisions can lead to group-velocity changes and spectral collapse. Large channel separation ameliorates the effects. Filters prevent spectral collapse but do not affect the velocity changes.

Elimination of cross talk in waveguide intersections

We present general criteria for crossing perpendicular waveguides with nearly 100% throughput and 0% cross talk. Our design applies even when the waveguide width is of the order of the wavelength. The theoretical basis for this phenomenon is explained in terms of symmetry considerations and resonant tunneling and is then illustrated with numerical simulations for both a two-dimensional photonic crystal and a conventional high-index-contrast waveguide crossing. Cross-talk reduction by up to 8 orders of magnitude is achieved relative to unmodified crossings.

Photon statistics of amplified spontaneous emission noise in a 10-Gbit/s optically preamplified direct-detection receiver

We verify experimentally, over a dynamic range of 55 dB in the probability distribution, that the amplified spontaneous emission noise of the 0's from an optically preamplified receiver is degenerate Bose-Einstein distributed. Using the noise parameters extracted from the experiment, we are able to predict the sensitivity of a 10-Gbit/s direct-detection receiver.

Second-order filtering and sensing with partially coupled traveling waves in a single resonator

The counterpropagating waves in a single traveling-wave cavity can be partially coupled by means of a small perturbation such as a notch. When it is side coupled to a waveguide, this single cavity yields a general second-order (Chebyshev) reflection response in the waveguide, which is useful for narrow-bandwidth reflecting applications. In a different application, the cavity amplifies small reflections induced by external perturbations, thus finding use in ultrafine sensing. Amplification factors as great as 10(12) are predicted for the highest-Q microsphere resonators. The analytic theory of these devices is presented.

Resonant sideband generation in stretched-pulse fiber lasers

Resonant sideband production in stretched-pulse fiber lasers operating with net normal dispersion is investigated. A theoretical formulation is compared with numerical modeling and experimental results. The inherent difference from sideband generation in soliton lasers is discussed. We also consider the implications of sideband formation in related dispersion-managed soliton transmission systems.

Dispersion-managed solitons with net positive dispersion

Propagation of dispersion-managed solitons in the net positive-dispersion regime is studied. A new form of sideband generation that is due to perturbations is identified. Some collision anomalies are pointed out.

Track changing by use of the phase response of microspheres and resonators

A microsphere or resonator that is side coupled to an incident optical beam induces a phase response in the beam. In the so-called overcoupled regime, the amplitude of the incident beam remains unmodulated, whereas the phase goes through a shift of p at resonance. This shift is insensitive to the details of the coupling geometry or the resonant mode. In conjunction with an interferometer, the phase response can be used to switch the beam between two well-defined outputs, thus offering a robust means of deploying microspheres and other microresonators in practical photonic devices.

Asynchronous phase-modulated optical fiber-ring buffer

We investigate the dynamics of a 5-Gbit/s packet of ones and zeros loaded into a 0.5-kbit, phase-modulated optical fiber-ring buffer. A new mode of operation is explored in which the cavity length of the buffer is slightly detuned from the frequency driving the phase modulator. We examine the conditions for successful packet loading and the packet storage behavior. In addition, relaxation oscillations, which affected the storage time of previous high-speed optical ring buffers, are nearly eliminated with a cw holding beam, and significantly enhanced storage times are obtained.

Direct measurement of self-phase shift due to fiber nonlinearity

We directly measure the time-independent nonlinear self-phase shift of a pulse after fiber propagation by spectral interferometry. Both the soliton and the nonsoliton case were studied. We verify numerically that the measured phase shift approximates this time-independent phase well.

Self-switching of optical pulses in dispersion-imbalanced nonlinear loop mirrors

We report a novel nonlinear filter that transmits and shortens incident pulses while rejecting cw background as well as resonant continuum. A relative extinction of 22dB is reported for the cw background.

Design and fabrication of double-chirped mirrors

We present an analytic design method for the reproducible fabrication of double-chirped mirrors to achieve simultaneously a high reflectivity and dispersion compensation over an extended bandwidth compared with those of standard quarter-wave Bragg mirrors. The mirrors are fabricated by ion beam sputtering. Use of these mirrors in a Ti:sapphire laser leads to 6.5-fs pulses directly out of the laser. The method can also be applied to the design of chirped-fiber gratings and general optical filters.

Subpicosecond solitons in an actively mode-locked fiber laser

Experimental results are presented for a study of the stability regime of an actively mode-locked polarizationmaintaining fiber ring laser used as a memory. Observations indicate that the pulse widths in the memory can be reduced (by soliton effects) by a factor of approximately 4.4 below the pulse widths predicted by standard active mode-locking theory. Stability regions for the solitons are mapped and compared with theoretical predictions.

Frequency-dependent mode size in broadband Kerr-lens mode locking

Examination of the output beam size from a 10-fs Kerr-lens mode-locked Ti:sapphire laser reveals a strong spectral dependence. These results provide insight into the details of the mode-locking mechanisms and suggest reasons for the current limitations on the shortest achievable pulses.

chi((3)) squeezed vacuum generation without a Sagnac interferometer

We show that an intense linearly polarized pulse propagating in a nonbirefringent fiber squeezes the perpendicularly polarized vacuum f luctuations through cross-phase modulation. Unlike the generation of squeezed vacuum through self-phase modulation, the mechanism discussed here does not require the use of a nonlinear Sagnac interferometer to separate the pump from the squeezed vacuum.

All-solid-state femtosecond source at 1.55 microm

We report on a stretched-pulse additive-pulse mode-locked erbium-doped fiber laser pumped by a masteroscillator/power-amplifier diode at 980 nm. This laser operates at 1.55 microm and puts out as much as 90 mW of average power at 40 MHz with pulse widths that are prism compensated to less than 90 fs. Amplitude jitter is less than 0.1%.

Measurement of the Raman gain spectrum of optical fibers

The stimulated Raman gain spectrum of optical fibers has been measured down to 6 cm(-1) by means of short pulses. Results for parallel and perpendicular polarizations are reported. With this technique, we observe spectral oscillations arising from a Brillouin mediated coupling between cw and pulsed light.

Asynchronous soliton mode locking

We report a harmonically mode-locked erbium-doped fiber ring laser with short solitons. A phase modulator running asynchronously with the pulses provides pulse start-up, continuous background cleanup, and a new form of pulse timing stabilization. Steady-state operation of picosecond solitons at a repetition rate of 1 GHz was obtained with no need for modulator drive frequency stabilization.

Orthogonal polarization fiber gyroscope with increased stability and resolution

The orthogonal polarization fiber gyroscope (OPFG) is an interferometric fiber gyroscope design that requires no phase bias in the fiber ring and is insensitive to light-source intensity noise. However, in the original OPFG [Hitachi Rev. 33, 215 (1984)], environmental changes caused first-order false rotation signals. We propose and experimentally verify modifications that eliminate the first-order sensitivity to environmental changes and that improve the gyroscope's resolution as well. We believe that this modified OPFG is the first interferometric fiber gyroscope capable of stable, high-sensitivity measurement that contains only reciprocal optical elements.

Additive-pulse mode-locking/limiting storage ring

A pulse-storage ring is an optical device capable of memorizing a pulse sequence of 0's and l's. The soliton ring of Nakazawa et al. [Electron. Lett. 29, 729 (1993)] with nearly unlimited transmission distance is such a device. We propose and experimentally demonstrate a different type of pulse-storage ring that uses intensity-dependent interferometric action to create a stronger intracavity transmission differential between the 0's and the l's.

Squeezing in a fiber interferometer with a gigahertz pump

We report 5.1 dB of squeezing from a fiber interferometer pumped with a 1-GHz pulse source that successfully eliminates guided-acoustic-wave Brillouin scattering in significant frequency regimes. The pulse source is a diode-pumped Nd:YLF laser actively mode locked at 1.314 microm. The squeezing results are consistent with the limits imposed by the Gaussian pulse shape and the detection quantum efficiency.

Additive-pulse limiting

When lasers with a long gain-relaxation time are actively mode locked at a harmonic of the round-trip frequency, the energy fluctuates from pulse to pulse unless stabilized. We report a new principle for pulse energy stabilization in harmonically mode-locked lasers.

Technique for obtaining high-energy ultrashort pulses from an additive-pulse mode-locked erbium-doped fiber ring laser

We report a self-starting stretched-pulse polarization additive-pulse mode-locked erbium-doped fiber ring laser with high output power. By using the light normally absorbed by the intracavity polarizer as the output, we obtained pulses with energies greater than 0.5 nJ at a repetition rate of 48 MHz. External chirp compensation was used to shorten the highly chirped output pulses to durations of less than 100 fs. The power levels suggest that erbium-doped fiber lasers may replace bulk solid-state lasers, such as the color-center laser, for some applications.