Compact electro-optic polarization scramblers for optically amplified lightwave systems

To increase the transmission capacity of a wavelength-division multiplexed (WDM) system, numerous works concentrated on the extension of erbium-doped fiber amplifier (EDFA) bandwidths. Recent reports showed that ultra-wide bandwidths, wider than 80nm were possible by using both the C-band (1550nm band) and L-band (1580nm band). System experiments showed the possibility of their application to real systems. In spite of the large bandwidth of the single ultra-wideband EDFA, bandwidth-distance products on the demonstrations limited to less than 24K nm-km (55.6nm/spl times/400km, 40nm/spl times/600km). The key issues are how to maintain the flat gain with transmission distance increase and how to achieve low noise figure for L-band EDFAs. In the work, we demonstrate a 124K nm-km (52nm/spl times/ 2400nm) transmission of 51 channels at 17dB of the optical signal to noise ratio (OSNR), by employing a low-noise wideband amplifier in conjunction with an all-fiber acousto-optic tunable filter. We have employed a re-circulating EDFA loop which consists of acousto-optic modulators, a single mode fiber, an attenuator, an isolator, a polarization scrambler, and an wideband EDFA (WEDFA).

We demonstrate a compact electro-optic polarization scrambler that depolarizes arbitrarily polarized light with less than 2.5% residual degree of polarization and variable depolarization times in the microsecond to millisecond range. The integrated-optic depolarizer is fabricated on lithium niobate and operates with a single-mode waveguide designed for a 1.5-μm wavelength. The scrambler introduces negligible intensity modulation of less than 1.6% in the depolarized output light.

We report on the operation and performance of a novel electrooptic polarization scrambler that is capable of generating any desired combination of optical phase and polarization modulation. The scrambler comprises a dual-drive Mach-Zehnder-type intensity modulator on lithium niobate and an external microoptic polarization splitter which combines the two complementary outputs of the integrated-optic intensity modulator in orthogonal polarization states. The modulation indexes of the sinusoidal phase and polarization modulation in the output light can be varied independently of each other via the amplitudes and phases of the two drive voltages to the Mach-Zehnder modulator. The scrambler has a 3-dB modulation bandwidth of 14 GHz and operates in the 1.5-1.6-/spl mu/m wavelength range with an overall fiber-to-fiber insertion loss of less than 5 dB. At 5 GHz modulation frequency and electric drive powers between 390-2030 mW, we demonstrate phase-chirped and chirp-free depolarization with a residual degree of polarization of less than 0.02 and continuously variable phase modulation to 2.3-rad peak phase deviation.

We analyzed the dry etching of perovskite oxides using argon-based inductively coupled plasmas (ICP) for photonics applications. Various chamber conditions and their effects on etching rates have been demonstrated based on Z-cut lithium niobate (LN). The measured results are predictable and repeatable and can be applied to other perovskite oxides, such as X-cut LN and barium titanium oxide (BTO). The surface roughness is better for both etched LN and BTO compared with their as-deposited counterparts as confirmed by atomic force microscopy (AFM). Both the energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) methods have been used for surface chemical component comparisons, qualitative and quantitative, and no obvious surface state changes are observed according to the measured results. An optical waveguide fabricated with the optimized argon-based ICP etching was measured to have -3.7 dB/cm loss near 1550 nm wavelength for Z-cut LN, which validates this kind of method for perovskite oxides etching in photonics applications.

A serious problem facing wavelength-division multiplexed (WDM) networks with fiber amplifier cascades is transient cross-gain saturation or gain dynamics of fiber amplifiers. Attention has been focused primarily on circuit-switched scenarios. When the number of WDM channels transmitted through a circuit-switching network varies, channel addition/removal will tend to perturb signals at the surviving channels that share all or part of the route. Even more serious bit error rate deterioration can arise in WDM packet switched burst mode networks. In this paper, we present experimental and theoretical results demonstrating the effect of fast power transients in erbium-doped fiber amplifiers (EDFAs) on packetized traffic transmitted through a chain of five EDFAs. Traffic of a local-area network has been transmitted over three channels. The effect of EDFA cross-gain saturation due to the burstiness of the traffic has been observed in a continuous-wave monitoring channel. The stabilizing effect of gain clamping of the first EDFA in the cascade has been investigated. The experimental results are extended to eight-channel WDM system using a large signal numerical analysis.

In Er-doped fiber amplifiers (EDFA), ASE polarized orthogonal to a saturating signal experiences more gain than ASE with the same polarization as the saturating signal.1 The accumulated enhancement of orthogonal ASE degrades the performance of long systems.1,2 This effect has been observed in single EDFAs using differential measurement techniques.3,4 A computer model of polarization hole-burning (PHB) has been presented.5 It postulates that each ion has an ellipsoid-like surface representing the emission and absorption cross-sections. The ratio of the major to minor cross-sections is the anisotropy parameter:

Erbium-doped fiber amplifiers (EDFAs) are attractive for various optical fiber transmission systems and networks owing to their several advantages such as high gain and low noise. A EDFA with its high output saturation power and wide gain profile is specially suitable for wavelength-division multiplexing (WDM) applications such as multichannel CATV distribution systems [1]. In these applications, the EDFA is normally used as a post (or a booster) amplifier and operates at a saturated region. Although it has been demonstrated that the EDFA is immune to patterning effects and interchannel interference, and has low gain saturation induced crosstalk [2], [3], there is little information about effects of wavelength spacing and fiber length in a saturated EDFA for multichannel applications. In this paper, we report the experimental results on wavelength and fiber length dependence of gain for saturated EDFAs in a two-channel amplification system. The experimental results show that when an EDFA operates at the saturated region in a multichannel system, a shorter fiber length than that in single-channel should be selected to get a flat gain profile, and that the Ge/Al/Er-doped silica fiber is desirable for multichannel amplification use since a gain hole-burning is observed for the Ge/Er-doped silica fiber.

The erbium-doped fiber amplifier (EDFA) has had a profound impact on the design, operation and performance of transoceanic cable transmission, and is central to the expected proliferation of cable systems. Important strides have been made in areas of dispersion management, gain equalization, modulation formats and polarization scrambling which have made possible the demonstration of large data transmission capacity. In this paper we review the recent advances in these techniques, and report a transoceanic length transmission experiment with 160 Gb/s total capacity.

In this contribution, we investigate second harmonic generation (SHG) in periodically poled lithium niobate (LN) on insulator waveguides and examine under what conditions such waveguides suffer from undesirable loss due to lateral leakage. We investigate the lateral leakage losses in X-cut and Z-cut LN for the fundamental (1550 nm) and second harmonic (775 nm) wavelengths. Our findings show that Z-cut lithium niobate on insulator (LNOI) is more likely to suffer from lateral leakage and has a lower SHG efficiency. We further provide design guidelines for highly efficient nonlinear optical waveguides in LNOI and show how lateral leakage can be avoided.

Polarization controllers and scramblers can substantially increase the transmission capacity in high-speed fiber-optic communication systems by reducing undesired polarization effects and through polarization multiplexing. In this paper we review the operation of these polarization modulators and their applications in high-speed lightwave systems.

The authors report on the fabrication of buried waveguides in both lithium niobate and periodically poled lithium niobate. First a low insertion loss waveguide is fabricated in z-cut lithium niobate using femtosecond laser waveguide inscription. To fabricate a waveguide exhibiting both low propagation and coupling losses, we used the multiscan fabrication technique to control the size of the waveguide cross section. We measured coupling losses of 1.1 dB/facet and propagation losses as low as 0.6 dBcm -1 . Optical waveguides have been also inscribed in periodically poled lithium niobate by femtosecond laser pulses with the same multiscan technique. Second harmonic generation experiments from a fundamental wavelength of 1567 nm demonstrate that the nonlinear optical coefficient in the waveguides is preserved, yielding a conversion efficiency of 18 % W -1 .

In saturated erbium-doped fiber amplifiers (EDFAs), signal gain and power conversion efficiency are known to be greater in the backward pumping configuration. This study shows that the difference between pumping schemes is primarily an effect of amplified spontaneous emission. It is concluded that the highest conversion efficiency achievable in highly saturated EDFAs is predicted with fair accuracy by ASE noise-free models, but is approached most nearly with backward-pumped EDFAs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Bit Error Rate (BER) performance of Erbium Doped fiber amplifiers (EDFA) is analysed in this paper. An analysis of the BER from quantum statistics is compared to the BER obtained from semiclassical approaches. In earlier work [1], the authors showed how the predictions of the two treatments approached each other in the case of ideal amplifiers for large gain. In this paper, the analysis is carried out for non-ideal, i.e., saturated EDFA and it is found that convergence of the two treatments occurs at a lower gain for saturated amplifiers than for ideal amplifiers.

Summary form only given. In-line amplifier transmission using erbium-doped fiber amplifiers (EFDAs) has been realized in 5-Gbit/s transoceanic systems in 1995-1996. In Japan, a 900-km in-line amplifier system with a data rate of 2.5 Gbit/s, upgradable to 10 Gbit/s, started service in May 1995. All of these systems use nonreturn-to-zero (NRZ) signal formats. Solitons are another candidate for long-distance transmission. This paper compares the choices for the next step in large-capacity long-distance systems.

We present a rate-equation model that accounts for polarization hole burning in erbium-doped fiber amplifiers. This model yields analytical expressions for the polarization sensitivity of the amplifier for arbitrary signal polarization states. We investigate the influence of the birefringence properties of the fiber and calculate the average polarization properties of an amplifier chain.