High-speed polarization scrambler with adjustable phase chirp

A fiber optic phase and polarization modulator is presented in which a thin transparent piezoelectric polyvinylidene fluoride film with indium tin oxide electrode metallization is placed directly in the path of the output of a single mode fiber. Experimental characterization is presented in terms of the linearity of response, phase shifting coefficient, and frequency response for two arrangements differing in the boundary clamping conditions. Many applications to fiber optic switches, intensity modulators and demultiplexers are indicated.

We review the evolution of modulation formats designed for direct detection, from on-off keying intensity modulation (IM) of a single polarization to three-dimensional formats where the intensity of both polarizations and the interpolarization phase are all independently modulated. We present the transmitter and receiver architectures capable of modulating an increasing number of dimensions, all orthogonal to each other. We detail the digital signal processing for Stokes vector receivers capable of recovering all three dimensions of a lightwave via self-beating direct detection, performing simultaneously polarization derotation and mitigation of intersymbol interference. We also analyze two different transmitter architectures modulating all three dimensions where the interpolarization phase is modulated over two levels using either a direct phase modulator (DPM) or a binary phase modulator, also called a BPSK modulator. We study the impact of increasing the bandwidth efficiency from 4 to 5 b/symbol by adding two level modulation of the interpolarization phase of a dual-polarization 4-level pulse amplitude modulation (PAM4) signal on the signal quality of the PAM4 IM on the polarization bearing the phase modulator by modeling the optical fields generated by both architectures. Assuming equal quality of the driving signals to the PAM4 and phase modulator, we show that a DPM minimizes the impact on PAM4 for short reach direct detect systems operating close to the zero dispersion wavelength where chromatic dispersion is negligible.

We propose and experimentally demonstrate the concurrent generation of an optical code label and payload data by use of off-the-shelf optical intensity and phase modulators in a tandem structure. This technique is capable of reconfigurable generation of an arbitrarily long optical code label of phase-shift keying concurrently with the generation of payload data, without the need for a special class of optical encoder. This technique will be powerful for all-optical packet switching networks, based on scalability and compactness, for optical code labels.

Simultaneous measurement of the Henry factor of 14 longitudinal modes is implemented using a novel method based on sinusoidal phase modulation, without using any filter. Excellent agreement with fiber transfer function technique is achieved.

A phase modulated (PM) microwave photonic link (MPL) with linearity improvement is presented. The link demodulates the PM using a phase discriminator based on a dynamically tunable, integrated, sixth-order optical lattice filter in planar lightwave circuit (PLC) technology. The linearity of the demodulation process is optimized by using electrical spectrum monitoring and a feedback algorithm to automatically choose the filter coefficients. For a 2 GHz modulation frequency, the link achieves a 6.7 dB improvement in the third-order output intercept point (OIP3) for intermodulation distortion (IMD) over a Mach-Zehnder interferometer (MZI).

The simultaneous control of amplitude and phase via metasurfaces affords us unprecedented degree of freedom in manipulating electromagnetic waves, but currently most designs suffer from low efficiency which raises certain concerns for real-world applications. Moreover, the complete amplitude, phase and polarization modulation is particularly challenging, which typically requires a combination of attenuators, optically thick wave plates and large dielectric lenses. Here, we propose an alternative scheme by introducing vertical mode cross-coupling for polarization control and high efficiency, while by involving spatially-varied orientations and structures for independent amplitude and phase modulation. The vertical mode cross-coupling is synthesized by stacking triple-layer twisted split ring resonators (SRRs) operated in transmissive scheme. Such tight cross-coupling and chirality-assisted coherent multiple resonances facilitates high cross-polarization conversion efficiency (~100%) and broadband transmission window with full phase cover. As a proof of concept, two dual-focal metalenses that are challenging to be actualized through conventional metasurfaces are designed, numerically and experimentally studied with a total thickness of λo/12 at microwave frequencies. Desirable dual focusing behavior with axial and lateral alignment of two foci are demonstrated. Our findings, not confined to microwave operation, opens up an alternative way in fine controlling light and can stimulate novel and high-performance versatile photonic metadevices.

We studied the effect of varying gas concentration, buffer gas, length, and type of fibers on the performance of optical fiber photothermal phase modulators based on C2H2-filled hollow-core fibers. For the same control power level, the phase modulator with Ar as the buffer gas achieves the largest phase modulation. For a fixed length of hollow-core fiber, there exists an optimal C2H2 concentration that achieves the largest phase modulation. With a 23-cm-long anti-resonant hollow-core fiber filled with 12.5% C2H2 balanced with Ar, phase modulation of π-rad at 100 kHz is achieved with a control power of 200 mW. The modulation bandwidth of the phase modulator is 150 kHz. The modulation bandwidth is extended to ∼1.1 MHz with a photonic bandgap hollow-core fiber of the same length filled with the same gas mixture. The measured rise and fall time of the photonic bandgap hollow-core fiber phase modulator are 0.57 µs and 0.55 µs, respectively.

A simple and low-cost method for simultaneously realizing phase-to-intensity modulation conversion and efficiency improvement of fiber-optic links is proposed and demonstrated by utilizing a fiber Bragg grating (FBG). Optical phase modulation (PM) is converted to intensity modulation (IM) by reflecting the phase-modulated light at the slope of the FBG reflection spectrum. As the optical carrier is suppressed by the FBG, the modulation depth can be enhanced at the same time. Experimental results show that the effective optical PM-IM conversion is realized and an additional 5.5 dB electrical signal gain can be obtained with appropriate suppression of the optical carrier.

We demonstrate a hybrid solution for simultaneous transfer of ultra-stable optical frequency, RF frequency and time signal on a 100 km fiber spool with the same wavelength based on coherent demodulation and optical phase-lock-loop (OPLL). The one pulse per second (1PPS) is encoded by phase and RF frequency is encoded with an intensity modulator. The noise caused by fiber link is compensated by the round-trip method. And the bad effect on optical frequency caused by optical phase modulation (PM) can be cleaned using OPLL. When the fiber link is stabilized, the transferred time deviation (TDEV) can be 2.4E-11@1s and 1.9E-12@104s, RF frequency stability can achieve 8.0E-14@1s and 2.5E-17@104s, the modified Allan deviation (MDEV) of cleaned optical frequency can be 1.4E-16@1s and 1.6E-19@104s.

A coherent laser phase shift range finder based on optical phase modulation and phase shift measurement can measure distance and velocity precisely at the same time. In this paper, methods to improve the sensitivity of the range finder is presented. Matched filter algorithm is used to calculate the range velocity in frequency domain. Experiment is conducted at different signal power. The relationship between RMSE of measured range, as well as measured velocity, and the signal power is shown in the result.

In this paper, we present a theory of an optical fiber acoustooptic phase modulator using piezoelectric zinc oxide (ZnO) coating. The effects of changing device parameters in the acoustooptic resonator are investigated for transducer design. These results are entirely different from those of the well-known; planar device structure since every part plays an important role in the resonator structure. Optical phase modulation in the frequency region up to 1 GHz is also demonstrated. Multiple sharp resonance peaks with a maximum phase shift of 0.7 radians are observed.

A scheme using one intensity modulator and two phase modulators driven directly by sinusoidal waveform to generate an optical-frequency comb (OFC) is experimentally demonstrated. It is relatively simple, where Bragg grating or specially tailored waveforms are not used. By setting the ratio of direct-current bias to half-wave voltage and phase shifts between sinusoidal waveform applied on the intensity and two phase modulators at appropriate values, 29 comb lines with spectral power variation less than 1.5 dB at 10 GHz are obtained. And the scheme has adjustability; a flat-top OFC with frequency spacing of 9.5-10 GHz is achieved.

We describe the performance of intensity and phase modulators that use an aqueous suspension of polytetrafluoroethylene (PTFE) microparticles. In this medium, the electro-optic effect is caused by the reorientation of anisotropic microparticles in an applied electric field. The intensity modulator was constructed in the Kerr geometry by the use of a sample path length of 20 µm. The response time of the modulator is less than 25 ms, and the depth of modulation was measured to be 28 dB for a switching voltage of 134 V(rms). The switching voltage necessary to achieve a π-phase shift with the phase modulator is less than 30 (Vrms).

Fast polarization scramblers have become important components in optically amplified transoceanic communication systems to eliminate anisotropic gain saturation (polarization hole burning) in the erbium-doped fiber amplifiers by depolarizing the launched optical carrier. This paper reviews the operation and implementation of compact electro-optic polarization scramblers on x-cut and z-cut lithium niobate. These integrated-optic devices employ low-loss single-mode waveguides and allow polarization modulation at variable frequencies ranging from a few 100 Hz to more than 10 GHz. We describe simple linear phase retarders that depolarize light in a fixed, well-maintained polarization state as well as cascaded multi-stage scramblers that are capable of depolarizing arbitrarily polarized light. These scramblers can be operated over a broad optical bandwidth of more than 40 nm with less than 5% residual degree of polarization and are further wavelength-tunable over more than 100 nm.

A novel calibration-free electrical spectrum analysis method for microwave characterization of electrooptic phase modulators is proposed and experimentally demonstrated based on frequency-shifted optical heterodyning. The method achieves the electrical domain measurement of the modulation efficiency of phase modulators without the need for correcting the responsivity fluctuation in the photodetection. Moreover, it extends double the measuring frequency range through setting a specific frequency relationship between the driving microwave signals. Modulation depth and half-wave voltage of phase modulators are experimentally extracted from the heterodyning spectrum of two phase-modulated signals with and without frequency shifting, and the measured results are compared to those obtained with the traditional optical spectrum analysis method to check the consistency. The proposed method provides calibration-free and accurate measurement for high-speed optical phase modulators with the high-resolution electrical spectrum analysis.