External Phase Modulator Research Articles

Pulse Timing Stabilization of a Mode-Locked Laser Using an External Phase Modulator

A new technique is proposed and demonstrated for stabilizing the pulse timing of a mode-locked laser by the use of an electro-optic phase modulator placed outside the laser cavity. The output pulses are phase-locked to an electrical reference oscillator by controlling the phase modulator and the laser cavity length simultaneously. The experiment is carried out using a laser diode-pumped Cr:LiSAF laser operating at 100 MHz and a timing jitter of 4.2 fs relative to the reference has been achieved for a bandwidth of 25 mHz–10 kHz.

Interferometric optical time-domain reflectometry for distributed optical-fiber sensing

The technique of optical time-domain reflectometry is analyzed to determine the effect of an optical phase modulation on light backscattered in an optical fiber. It is shown that the spatial distribution along the fiber of an external phase modulation can be measured with a spatial resolution close to that of optical time-domain reflectometry. A distributed interferometric sensor arrangement that employs this technique is investigated experimentally, and a satisfactory interrogation of more than 1000 resolution intervals is demonstrated.

Experimental verification of the phase-shaped binary transmission (PSBT) effect

Using an external phase modulator, a phase signal has been superimposed to a 10-Gb/s chirp-free nonreturn-to-zero (NRZ) signal in order to extend the dispersion limit. A transmission over 146 km of standard dispersive fiber has been achieved. The phase signal shape is based on a three-bit analysis using the concepts of phase-shaped binary transmissions (PSBT). By removing or adding the phase signal, we have experimentally shown the physical concepts involved in PSBT in agreement with our simulations.

Coherent optical frequency domain reflectometry using phase-decorrelated reflected and reference lightwaves

We report a new scheme for coherent optical frequency domain reflectometry (C-OFDR) where the coherence length of the lightwaves does not limit the measureable fiber length. In this scheme, we use the beat spectrum which results when we mix reflected and reference lightwaves whose phases are not correlated. We demonstrated this scheme using a narrow-linewidth-lightwave source and an external electro-optical phase modulator. We measured Rayleigh backscattering and Fresnel reflections from a 30-Ion optical fiber, and achieved a spatial resolution of 5 m for two neighboring Fresnel reflectors located at the far end of the fiber. We estimated the expected spatial resolution and single-way dynamic range for our new scheme and show that it is capable of measuring long optical fibers with high-spatial resolution.

Coherent optical CDMA with limited phase excursion

We examine the bit error rate (BER) of the conventional matched-filter detector for an optical code division multiple access (CDMA) system employing coherent homodyne detection. We assume a phase shift keying (PSK) modulation format where the phase is modulated by one of two methods: (1) an external phase modulator or (2) injection locking. Phase excursion is limited to /spl plusmn/0.42 /spl pi/ when using injection locking. We examine the bit error rate performance for both external modulation and modulation of the injection current and find only a moderate penalty (0.55 dB) associated with the limited phase excursion.

Absolute frequency synthesis of pulsed coherent light waves through phase-modulation active optical feedback

A novel method for the broadband absolute frequency synthesis of pulsed coherent lightwaves is demonstrated. It is based on pulse recirculation around an active optical feedback ring containing a delay-line fiber, an external phase modulator, an acousto-optic frequency shifter (AOFS), and a high-finesse Fabry-Perot étalon. The modulation frequency F(M) and the frequency shift F(AO) that are due to AOFS are designed so that their sum or difference equals the free-spectral range of the étalon and F(AO) is set at larger than the half-width at full maximum of its resonant peaks. If one of the peak frequencies is tuned to the frequency of the initial pulse, the frequency of the recirculating pulse jumps to the next peak for each round trip. In the experiment the absolute frequency is synthesized over a frequency span of 700 GHz around the initial stabilized frequency of the master laser.

Coherent optical frequency domain reflectometry for a long single-mode optical fiber using a coherent lightwave source and an external phase modulator

Coherent optical frequency domain reflectometry (C-OFDR) is demonstrated for a long single-mode optical fiber at a wavelength of 1.55 /spl mu/m by using a highly coherent lightwave source and an electrooptical (EO) external phase modulator. Rayleigh backscattering and Fresnel reflection from a 16.4-km optical fiber are observed when the modulation frequency is swept from 4 GHz to 8 GHz for 40 ms. The insertion of an optical fiber delay line in a probe lightwave path makes it possible to distinguish the Rayleigh backscattering profile due to the first-order modulation sideband in the phase-modulated lightwave from those due to the higher order sidebands. A spatial resolution of better than 100 m is achieved for two neighboring Fresnel reflectors located near the far end of the test fiber. >

Frequency locking of a Nd:YAG laser using the laser itself as the optical phase modulator

We report frequency locking of a Nd:YAG tunable laser to a 2000 finesse 87-cm-long Fabry–Pérot cavity in air using the Pound–Drever technique. The novelty is that the necessary phase modulation of the beam is obtained using the laser directly instead of an external phase modulator (Pockels cell). The spurious relative amplitude modulation using this method was ∼3×10−5 with a modulation index β∼1 and the spectral density of the frequency difference between the laser and the cavity is below 1 mHz/√Hz in the region 1–500 Hz.

Interferometers with Internal and External Phase Modulation: Experimental and Analytical Comparison

Optical intensity noise in a light source easily degrades the sensitivity of a shot-noise-limited interferometer which is directly detecting low frequency phase or displacement variations. In this paper we describe and compare two experimental methods in which we use high frequency optical phase modulation to shift low frequency phase signals in an interferometer to a shot noise limited region of the photocurrent spectrum. This phase modulation is applied either within the interferometer arms-internal modulation-or in a local oscillator beam tapped off the main interferometer and coherently recombined with the interferometer output-external modulation. he photocurrent is mixed electronically with the high frequency modulating waveform to extract the signal information free from laser intensity noise. In our experiments, we have been able to detect interferometrically low frequency signals with true shot-noise-limited sensitivity. We find, theoretically and experimentally, that the interferometric sensitivity achievable in each scheme depends critically on non-ideal factors, such as imperfect interferometric fringe contrast and electronic noise in the detectors or amplifiers. This paper examines the relative merits and operating requirements of both modulation schemes in practical interferometers.

Parametric resonances in synchrotrons with two rf systems.

When a double rf system is subjected to sinusoidal phase modulation, the Poincar\'e surfaces of the section display a rich spectrum of resonance islands. Stable and unstable fixed points of these resonance islands form a tree of bifurcation branches which can be explained as parametric resonances generated by external phase modulation. A semianalytic determination of the condition for the bifurcation of fixed points is presented for an autonomous Hamiltonian of one degree of freedom with sinusoidal time dependent perturbation.

An all optical coherent receiver for self-homodyne detection of digitally phase modulated optical signals

A novel optical phase-locked loop (OPLL) system for the self-homodyne detection of digitally phase modulated optical signals is introduced. A Mach-Zehnder type interferometer is used to self-homodyne binary phase-modulated optical signals with an external phase modulator inserted in the control arm of the interferometer. In the absence of thermal noise, this arrangement is 3 dB less sensitive than a homodyne receiver for a background noise limited operation. For a negligible background noise level and when the receiver thermal noise is ignored, a power penalty of 6 dB is incurred,. >

Frequency stabilization of a semiconductor laser using an external phase modulator

A Ti:LiNbO(3) waveguide phase modulator is used as an external frequency stabilizer to reduce the linewidth of an AlGaAs semiconductor laser. A ring interferometer is used as a frequency discriminator, and a wideband servo controller (dc to 30 MHz) is constructed in which feed-forward control of the external phase modulator is combined with feedback control of the injection current. A linewidth of 45 kHz is obtained.

Modulation, signal, and quantum noise in interferometers.

We discuss the generation of quantum noise in interferometers. A heuristic model is described that enables different characteristics of the noise to be understood and calculated. In particular, the effects of modulation on signal and noise in interferometers are discussed. It is seen that the mixing in of quantum noise from different modes of the vacuum can degrade the signal-to-noise ratio of a measurement in a variety of apparently different physical situations. Michelson interferometers with both internal and external phase modulation are specifically considered, but the results are easily extendable to any type of interferometer. We also comment upon some implications for the use of squeezing in interferometers. For examples, we argue that it is possible, even in lossy narrow-band interferometers, for squeezing to reduce significantly the effective quantum-noise power.

Highly sensitive 565 Mb/s DPSK heterodyne transmission experiment using direct current modulation of a DFB laser transmitter

A comparison between external phase modulation and phase modulation obtained through direct current modulation of a multiquantum-well distributed feedback (DFB) transmitter is reported for a sensitive 565 Mb/s differential-phase-shift-keying (DPSK) heterodyne transmission experiment. A limited sensitivity penalty for direct phase modulation is expected from the driving current pulse shape, the transmitter FM response and the receiver characteristics. The experimentally observed 0.5 dB penalty is in agreement with theory and highlights the potential of direct DPSK modulation as compared to both standard DPSK and continuous-phase frequency shift keying (CPFSK) schemes. >

1 Gbit/s injection-locked DPSK communication experiments for space applications

An experimental 1 Gbit/s, 120mW, differential phase shift keying (DPSK) communications system has been demonstrated at 830 nm with an external phase modulator and an injection-locked diode array. A comparison of the bit error rate with and without the injection-locked array showed no degradation. This is the highest power DPSK result reported to date.

Optical fiber length measurement by external phase modulation type optical heterodyne reflectometry

A method for measuring optical fiber length is proposed and demonstrated experimentally with external phase modulation optical heterodyne reflectometry using a He-Ne laser as a light source. The system offers a simple length-measuring method with satisfactory resolution. The reflected waves from fiber faults, if any, also can be detected by this method by observing two signals which occur at fault points and fiber end. >

Integrated-optic, narrow-linewidth laser

We report the demonstration of the single-frequency and narrow-linewidth (<100 kHz) operation of an extended cavity laser consisting of a Ti:LiNbO3 single-mode waveguide circuit and an InGaAsP waveguide gain medium. A parallel coupled cavity, electro-optic elements for tuning the resonant frequencies of the cavities, and an external phase modulator are integrated on the lithium niobate crystal.

Interference between broadened sidebands of a pulse-amplified phase-modulated continuous-wave dye laser

The spectrum of a narrow-band cw laser may be expanded by the generation of equally spaced sidebands through external phase modulation. The spectral profile of the sidebands, which are phased linked, depends on the modulation index. Pulsed amplification of the modulated cw laser broadens each sideband, and thus interference between the sidebands is introduced. It is theoretically and experimentally shown that the resulting spectrum is a function of that interference and depends on the modulation frequency Ω, the amplifier pulse width T, and the phase relation between the periodic refractive index in the modulator and the amplifier pulse. Various spectral line shapes could be generated by varying the modulation parameters and the amplifier pulse timing in relation to the modulator phase.

Frequency-modulation spectroscopy: a new method for measuring weak absorptions and dispersions

A new type of wavelength-modulation laser spectroscopy is accomplished by utilizing an external phase modulator driven at radio frequencies large compared to the width of the spectral feature of interest. The spectral feature is probed by a single frequency-modulated (FM) sideband, and the associated absorption and dispersion are measured by monitoring the resulting radio-frequency beat signal. Experimental results are presented for the measurement of Fabry–Perot resonances, I2 vapor absorption lines, and saturation holes in Na vapor.