Round-trip Phase Shift Research Articles

Continuous-wave pumped dispersion-compensated ultrashort-pulse doubly resonant oscillator studies

We have reported theoretical analysis of broadband phase-coherent spectral generation with stable short pulse output based on optical parametric oscillator (OPOs) pumped by low-cost single-mode continuous-wave (cw) laser considering the incorporation of mode-locking element in the cw dispersion-compensated OPO in doubly-resonant oscillator (DRO) configuration. Exploiting the control of dispersion in the cw mode-locked cavity, the intracavity round-trip phase shift, phase coherence among resonating waves, carrier envelope offset phase of mode-locked output and enhancement of efficiency of system using active mode-locking element driven at high modulation frequency has been studied. The study paves the way of realization of compact, low-cost, efficient ultrashort pulse OPO under cw pumping.

Control scheme for polarization circulation speed meter using a dual-retardation waveplate

In interferometric gravitational wave detectors, quantum radiation pressure noise, which is a back action of the measurement, will limit their sensitivities at low frequencies. Speed meters are one of the solutions to reduce the back action noise and improve the sensitivities, and furthermore, they can surpass the standard quantum limit over a wide range of frequencies. The Polarization Circulation Speed Meter is the latest incarnation of the speed meter concept in the sense that it requires a slight modification in the conventional interferometer designs; however, its control scheme has not been developed. The main difficulty is the length and alignment control of the cavity formed by the polarization circulation mirror and the input test masses, whose round-trip phase shift should be kept to $\pi$. In this article, we propose a new control scheme using a dual-retardation waveplate, called Dual-Retardance Control (DRC). In addition, we compare the shot noise level of the DRC to another simpler scheme by dithering. Finally, we design the experimental setup for the demonstration of the DRC and show the expected results through the transfer function measurement.

Raman silicon nanocavity laser with efficient light emission from the edge of an adjacent waveguide.

A Raman nanocavity laser can emit light into free space and into a properly designed waveguide adjacent to the cavity by mode coupling. In common device designs, the emission from the edge of this waveguide is relatively weak. However, a Raman silicon nanocavity laser with strong emission from the waveguide edge would be advantageous for certain applications. Here we investigate the increase in the edge emission that can be achieved by adding photonic mirrors to the waveguides adjacent to the nanocavity. We experimentally compare devices with and without photonic mirrors: the edge emission for devices with mirrors is 4.3 times stronger on average. This increase is analyzed using coupled-mode theory. The results indicate that the control of the round-trip phase shift (between the nanocavity and the mirror) and an increase of the quality factors of the nanocavity are important for further enhancement.

Planar hot-electron photodetector utilizing high refractive index MoS2 in Fabry–Pérot perfect absorber

Hot electron photodetection (HEPD) excited by surface plasmon can circumvent bandgap limitations, opening pathways for additional energy harvesting. However, the costly and time-consuming lithography has long been a barrier for large-area and mass production of HEPD. In this paper, we proposed a planar and electron beam lithography-free hot electron photodetector based on the Fabry–Pérot (F–P) resonance composed of Au/MoS2/Au cavity. The hot electron photodetector has a nanoscale thickness, high spectral tunability, and multicolour photoresponse in the near-infrared region due to the increased round-trip phase shift by using high refractive index MoS2. We predict that the photoresponsivity can achieve up to 23.6 mA W−1 when double cavities are integrated with the F–P cavity. The proposed hot electron photodetector that has a nanoscale thickness and planar stacking is a perfect candidate for large-area and mass production of HEPD.

Study of optical bistability in a fully optimized laser Fabry-Perot system

The analytical study of optical bistability is concerned in a fullyoptimized laser Fabry-Perot system. The related phenomena ofswitching dynamics and optimization procedure are also included.From the steady state of optical bistability equation can plot theincident intensity versus the round trip phase shift (φ) for differentvalues of dark mistuning 12,6,3,1.50 , o    or finesse (F= 1, 5, 20,100). In order to obtain different optical bistable loops. The inputoutputcharacteristic for a nonlinear Fabry-Perot etalon of a differentvalues of finesse (F) and using different initial detuning (φ0) are usedin this research. When the cavity finesse values increase, the switch-ON intensity increases for the same value of (φ0), because thebistable loop width increases with increasing the cavity finessevalues.

A free-standing wire scattering technique to monitor calibration variations of the DIII-D density profile reflectometer.

Real-time phase calibration of the ITER profile reflectometer is essential due to the long plasma duration and expected waveguide path length changes during a discharge. Progress has been recently made in addressing this issue by employing a phase calibration technique on DIII-D that monitors calibration variations that occur during each plasma discharge. By installing a thin free-standing metallic wire (1 mm diameter) near the end of the overmoded waveguide transmission system (oriented perpendicular to the waveguide axis), the round-trip phase shift from the wire is detected simultaneously with the plasma phase shifts. Variations in the reflectometer round trip path length (∼26 m) are then calculated after each DIII-D plasma discharge, allowing the calibration phase to be accurately monitored and updated. The round-trip reflectometer path length is observed to vary by ∼3 mm (root mean square value) during a typical DIII-D discharge. Using the variations in calibration phase, the density profile measurement accuracy can be improved. Since the wire retro-reflected power is ∼0.01 of the plasma signal, minimal effect is observed on the reflected signal from the plasma. Importantly, through a suitable choice in wire diameter, the calibration signal can be made approximately independent of the V-band reflectometer launch polarization. This is particularly important on DIII-D since orthogonal X- and O-mode polarized beams are coupled into the same transmission waveguide and launch antenna.

Optical design and suspension system of the KAGRA output mode-cleaner

KAGRA is a Japanese large scale, underground, cryogenic gravitational telescope which is under construction in the Kamioka mine. For using cryogenic test masses, the sensitivity of KAGRA is limited mainly by quantum noise. In order to reduce quantum noise, KAGRA employs an output mode-cleaner (OMC) at the output port that filters out junk light but allows the gravitational wave signal to go through. The requirement of the KAGRA OMC is even more challenging than other telescopes in the world since KAGRA plans to tune the signal readout phase so that the signal-to-noise ratio for our primary target source can be maximized. A proper selection of optical parameters and anti-vibration devices is required for the robust operation of the OMC. In this proceeding, we show our final results of modal-model simulations, in which we downselected the cavity length, the round-trip Gouy phase shift, the finesse, and the seismic isolation ratio for the suspended optics.

Laser frequency stabilization using folded cavity and mirror reflectivity tuning

A new method of laser frequency stabilization using polarization property of an optical cavity is proposed. In a standard Fabry–Perot cavity, the coating layers thickness of cavity mirrors is calculated to obtain the same phase shift for s- and p-wave but a slight detuning from the nominal thickness can produce s- and p-wave phase detuning. As a result, each wave accumulates a different round-trip phase shift and resonates at a different frequency. Using this polarization property, an error signal is generated by a simple setup consisting of a quarter wave-plate rotated at 45°, a polarizing beam splitter and two photodiodes. This method exhibits similar error signal as the Pound–Drever–Hall technique but without need for any frequency modulation. Lock theory and experimental results are presented in this paper.

Reflective vertical cavity quantum-well saturable absorber as an all-optical nonlinear phase-shifting element

A scheme of an all-optical nonlinear phase-shifting element is proposed, based on a reflective vertical cavity semiconductor (quantum-well) saturable absorber (R-VCSSA). The nonlinear round-trip phase shift in a low-intensity resonant VCSSA is obtained at an input intensity due to the saturating nonlinearity in index change and optically induced thermal effects. An enhanced large nonlinear positive or negative phase shift incurred in a reflected signal is investigated analytically with the wavelength tuning of the probe signal around the low-intensity resonant wavelength of a Fabry–Perot cavity. The device may be useful in compensating the phase of an input signal, either positive or negative, in long-haul fiber communications. A representative InGaAs/InP quantum-well-based VCSSA is considered to demonstrate the phase shift in the pump–probe configuration. With a device speed of 10 GHz, maximum phase shifts of −0.24 and +0.14  rad at pump power of 19.2 mW are observed for the operations at probe wavelengths of 1555.6 and 1557.8 nm, respectively.

Non-Markovianity of a quantum emitter in front of a mirror

We consider a quantum emitter ("atom") radiating in a one-dimensional (1D) photonic waveguide in the presence of a single mirror, resulting in a delay differential equation for the atomic amplitude. We carry out a systematic analysis of the non-Markovian (NM) character of the atomic dynamics in terms of refined, recently developed notions of quantum non-Markovianity such as indivisibility and information back-flow. NM effects are quantified as a function of the round-trip time and phase shift associated with the atom-mirror optical path. We find, in particular, that unless an atom-photon bound state is formed a finite time delay is always required in order for NM effects to be exhibited. This identifies a finite threshold in the parameter space, which separates the Markovian and non-Markovian regimes.

XOR and XNOR operations at 125 Gb/s using cascaded carrier-depletion microring resonators

We report the implementation of the XOR and XNOR logical operations using an electro-optic circuit, which is fabricated by CMOS-compatible process in the silicon-on-insulator (SOI) platform. The circuit consists of two cascaded add-drop microring resonators (MRRs), which are modulated through electric-field-induced carrier depletion in reverse biased pn junctions embedded in the ring waveguides. The resonance wavelength mismatch between the two nominally identical MRRs caused by fabrication errors is compensated by thermal tuning. Simultaneous bitwise XOR and XNOR operations of the two electrical modulating signals at the speed of 12.5 Gb/s are demonstrated. And 20 Gb/s XOR operation at one output port of the circuit is achieved. We explain the phenomena that one half of the resonance regions of the device are much more sensitive to the round-trip phase shift in the ring waveguides than the other half resonance regions. Characteristic graphs with logarithmic phase coordinate are proposed to analyze the sensitivity of the demonstrated circuit, as well as several typical integrated optical structures. It is found that our circuit with arbitrary chosen parameters has similar sensitivity to MRRs under the critical coupling.

Quasi synchronous tuning for grating feedback lasers

A general analytical form of the round trip phase shift in grating feedback diode lasers is proposed. Using the new form, it is obvious that the round trip phase shift can be independent of rotation angle in first order approximation when only one restriction condition is met. We call this the quasi synchronous tuning (QST) condition. In the QST region, a considerably large mode hopping free tuning range can be obtained. An adjustment structure with only one freedom is needed to accurately find and locate the quasi synchronous pivot, which is not strictly confined on the grating surface and its extension. It means that the external cavity diode lasers design can be easier and the laser can be more stable and reliable.

利用Gires-Tournois标准具的色散特性来获得窄线宽激光

A longer laser resonator length is benefit for laser linewidth, but harmful for single-frequency operation. A novel way is suggested to narrow the bandwidth of a Fabry-Perot (FP) cavity through increasing the derivative of one round-trip phase shift with respect to the frequency. It can be implemented by replacing one of two FP mirrors with a Gires-Tournois etalon (GTE), called FPGT, as a dispersion element. FPGT resonator has additional axial modes due to the GTE reflection phase shift. Theoretical analyses show that the bandwidth of additional axial modes can be 1% of that of a conventional FP cavity. A distributed feedback (DFB) laser diode can employ FPGT resonator to achieve ultra-narrow linewidth laser. It is shown that the effect of refractivity fluctuation in the gain medium on the linewidth is little, and kilohertz linewidth is achievable for such a device.

Coherent Array of Nonlinear Regenerative Fiber Amplifiers

A coherent array of high power fiber amplifiers (lasers) with a nonlinear refractive index is described and discussed. By adding reflectivity to the inputs of the fibers, each fiber laser is transformed into a regenerative amplifier, with an internal circulating power that varies depending on the round trip phase shift of the field within the fiber. The variation of the internal circulating power results in a preferential shift in the output phase of each amplifier due to the high nonlinearity within the fiber. The resulting phase shift is predicted to substantially compensate for the inherent randomness in the output phase distribution usually associated with the long lengths of fiber amplifiers. The individual outputs of the coherent array are shown to exhibit a ldquofuzzyrdquo phase distribution that can be partially quantified based on a statistical analysis and is dependent on the parameters of the array. A simple example is given of a coherent array of fiber lasers in a Self-Fourier cavity and the degree of coherence is predicted under simplifying assumptions.

Interferometric fiber-optic sensor embedded in a spark plug for in-cylinder pressure measurement in engines.

Pressure sensing in an internal combustion engine with an intrinsic fiber Fabry-Perot interferometer (FFPI) integrated with a spark plug is demonstrated for the first time. The spark plug was used for the ignition of the cylinder in which it was mounted. The FFPI element, protected with a copper/gold coating, was embedded in a groove in the spark-plug housing. Gas pressure inthe engine induced longitudinal strain in this housing, which was also experienced by the fiber-optic sensing element. The sensor was monitored with a signal conditioning unit containing a chirped distributed-feedback laser. Pressure sensitivities as high as 0.00339 radians round-trip phase shift per pounds per square inch of pressure were observed. Measured pressure versus time traces showed good agreement with those from a piezoelectric reference sensor mounted in the same engine cylinder.

Generation of wavelength-tunable single-mode picosecond pulses from a self-seeded gain-switched Fabry–Perot laser diode with a high-birefringence fiber loop mirror

We demonstrate a simple self-seeding scheme for generating wavelength-tunable single-mode picosecond pulses at a fixed repetition rate with a Fabry–Perot (FP) laser diode. In this scheme, the FP laser diode is self-seeded from a high-birefringence (HiBi) fiber loop mirror that acts as a periodic multichannel wavelength filter. By setting the channel spacing of the filter properly, stepwise wavelength tuning over a wide range can be achieved by either changing the FP cavity roundtrip phase shift or tuning the fiber loop mirror filter. With this scheme, fast electrical tuning is also possible by incorporating a phase-control section into the FP cavity. Using a HiBi fiber loop mirror with a channel spacing of 1.56 nm and a FP laser diode with a mode spacing of 1.45 nm, we were able to generate picosecond pulses at a repetition rate of approximately 2 GHz, whose wavelength could be tuned stepwise over ten successive laser cavity modes by either changing the temperature of the laser diode or tensioning/pressing a section of the HiBi fiber in the loop mirror. A side-mode suppression ratio over 15 dB over the entire tuning range was achieved.

Wavelength tuning in self-seeded gain-switchedFabry-Perot laserdiode with Moiré grating

The authors propose a simple method for generating wavelength tunable singlemode picosecond pulses at a fixed repetition rate. This method is based on self-seeding a Fabry-Perot (FP) laser diode having a fibre external cavity formed by a moire grating with a proper channel spacing. Stepwise wavelength tuning over a large range can be achieved by changing the FP cavity roundtrip phase shift, and fast electrical tuning is possible by incorporating a phase-control section into the FP cavity. By using a two-channel moire grating and a temperature-controlled FP laser diode, tuning of picosecond pulses at a repetition rate of /spl sim/2 GHz between two wavelengths /spl sim/11 nm apart is demonstrated. A sidemode suppression ratio of >25 dB has been achieved.

Mirrorless laser action by randomly distributed feedback in amplifying disordered media with scattering centers

A novel mirrorless mechanism for coherent laser action is investigated, where effective feedback is provided by randomly distributed scattering particles of mesoscopic size, yielding a reverse coupling with the backscattered par- tial wave satisfying the Bragg condition. In contrast to other models, which can only explain an enhanced stimulated emis- sion, this novel feedback mechanism yields conditions for steady-state round-trip gain, as well as for a round-trip phase shift necessary for coherent superposition of traveling waves in real lasers. Threshold gain, linewidth narrowing and slope efficiency are calculated and compared with experimental ob- servations.

Calculation of a transverse optical mode and diffraction loss in a supercavity

In this paper, we present a simulation of the dynamic procedure of the photon storage in a practical supercavity ( R = 99.99%) pumped by a circular symmetric Gaussian beam. The Fox-Li technique has been used to numerically calculate the field distribution, the diffraction losses, and the round-trip phase shift of TEM 0 n modes. A concept of an effective Fresnel number is put forward. Details of the Fox-Li type calculation are introduced along with the discussions of the convergence in solutions of the iterative equations. The effects of the refraction index of the e-beam and the loss of Compton backscattering by the e-beam inside the cavity are discussed.

An exact phase shift model for image states on metal surfaces with alkali metal overlayers

Within the framework of multiple reflection theory, an exact solution is developed for image and surface states on a nearly-free-electron metal with a flat adlayer potential. Wave function matching conditions are used to determine the round-trip phase shift at a given energy, and a Bohr-like quantization condition is applied to determine bound state energies. Also, a model is developed for determining the effective mass of image state electrons in the presence of an adlayer. Binding energies obtained from two-photon photoemission experiments on Na/Cu(111) are in excellent agreement with the computed results.