M. Tabara

Improving the dynamic range of a coherent AM analog optical link using a cascaded linearized modulator

An AM coherent analog optical link is experimentally demonstrated to achieve the highest spurious-free dynamic range reported to date for a coherent system by employing a linearized cascaded electro-optic modulator. The link gives a spurious-free dynamic range of 115 dB/spl middot/Hz/sup 2/3/, with a 34-dB reduction in third-order intermodulation distortion over conventional Mach-Zehnder modulators. With the linearized modulator, our experimental results are just 3 to 6 dB away from the fundamental limit of this link; the remaining penalty is mainly due to RF components.<<ETX>>

Linewidth-insensitive coherent AM optical links: design, performance, and potential applications

The use of coherent detection in analog optical links offers several advantages over direct detection: improved receiver sensitivity, inherent frequency translation, and the ability to utilize angle modulation and separate wavelength division multiplexed (WDM) signals. In this paper, we investigate an externally modulated coherent AM optical link. We study the dynamic range of the coherent AM link, considering receiver noise, laser phase noise, laser relative intensity noise (RIN), and system nonlinearities. With proper selection of the receivers IF bandwidth, the coherent AM link can be made insensitive to the laser linewidth. For optical powers less than 5 mW, RIN of less than /spl minus/160 dB/Hz reduces the spurious-free dynamic range (SFDR) by less than 3 db with the use of a balanced receiver. The external modulator nonlinearity is the dominant nonideal effect; it reduces the SFDR by 5-19 dB from the theoretical limit for 100% modulation index. We compare the performance of the coherent AM link with that of a conventional direct detection link for two applications: point-to-point links and distribution networks. When the received optical power is less than 1 mW, the coherent link can provide higher SFDR than the direct detection link. Thus, coherent links are well-suited for long distance point-to-point links and FM video distribution systems. >

Experimental linewidth-insensitive coherent analog optical link

We constructed an experimental linewidth-insensitive coherent analog optical link. The transmitter utilizes an external electro-optic amplitude modulator and a semiconductor laser. The receiver consists of a heterodyne front-end, a wideband filter, square law detector and narrowband lowpass filter. We performed experimental measurements and theoretical analyses of the spurious-free dynamic range (SFDR), link gain and noise figure for both the coherent AM and the direct detection links; we investigated the dependencies of the foregoing parameters on the received optical signal power, laser linewidth, IF bandwidth, and the laser relative intensity noise (RIN). By selecting a wide enough bandpass filter, we made the coherent AM link insensitive to laser linewidth. The coherent AM link exhibits a higher SFDR than the corresponding direct detection link when the received optical signal power is less than 85 /spl mu/W. The noise figure for the coherent link is greater than that for the direct detection link under all conditions investigated. For received optical signal powers greater than 4 /spl mu/W, the link gain for the direct detection link is greater than that for the coherent AM link. The following are the link parameters that have been achieved for the coherent AM link investigated: SFDR=88 dB/spl middot/Hz/sup 2/3/, link gain=-25 dB and noise figure=78 dB; this performance has been obtained with a received optical signal power of 85 /spl mu/W, and a local oscillator power at the photodetector of 228 /spl mu/W. The link performance can be further improved by auxiliary subsystems such as a balanced receiver and impedance matched transmitter and receiver ends; and/or by using better optical and electrical devices like higher power lasers, linearized optical modulators, low-noise and high gain RF amplifiers, and optical amplifiers,. >

Coherent analog optical links: design, performance and potential applications

Coherent analog optical links offer several important advantages over conventional direct detection links: better receiver sensitivity, possible use of angle modulation and easy separation of WDM signals and frequency translation. We propose a novel coherent AM-WlRNA optical link, and analyze and optimize its performance. With proper system design, the coherent AM link is insensitive to the laser linewidth. We then compare the performance of the coherent link with that of a direct detection link for two applications: point-to-point links and distribution networks. For typical system parameters, the coherent AM link provides better performance than the direct detection link.<<ETX>>

Dynamic range of externally modulated analog optical links: optical amplification versus coherent detection

Two externally modulated analog optical links (a direct detection link using an optical preamplifier and a coherent AM link) are analyzed and compared with the conventional direct detection link. For received optical power less than 1 mW, the optical preamplified link and the coherent AM link have a better performance than the direct detection link. However, the coherent AM link is more susceptible to relative intensity noise (RIN) than the direct detection links (both amplified and unamplified) and have a RIN-limited spurious-free dynamic range (SFDR) of 4 dB less than that of the direct detection links. Therefore, optically preamplified direct detection links may be more attractive in improving the SFDR of analog systems.<<ETX>>

Experimental Investigation Of A Linewidth-insensitive Coherent AM Optical Link

Video delivery and antenna remoting are two important applications of analog optical links [l]. High dynamic range is needed for both applications. At low signal power, the dynamic range of coherent systems is larger than that of direct detection systems [2]. In many applications, semiconductor lasers have to be used as light sources to make the system compact. However, laser diodes have relatively large linewidth (due to phase noise) that may cause signal degradation. To reduce the impact of phase noise, some kind of phase noise cancellation is required. Theoretical investigation of one cancellation technique AM-WIRNA (WIRNA stands for the receiver structure: Wide bandpass filter Rectifier NArrow lowpass filter) heterodyne system has been reported in [2]. That theory predicts that, with WIRNA detection, distortion is quite small.

A linewidth-insensitive coherent AM optical link: a comparison of direct and external modulation formats

In this paper we investigate experimentally the effect of the modulation methods on the AM-WIRNA (wide bandpass filter-rectifier-narrow lowpass filter) system. We use two modulation methods: external modulation and direct modulation. Direct modulation is simpler, has a more linear response and requires smaller RF input power as compared to external modulation.<<ETX>>

Theoretical and experimental investigations of the dynamic range of high-frequency coherent AM optical links using semiconductor lasers

In this paper, we theoretically and experimentally investigate the spurious-free dynamic range of an externally modulated coherent AM optical link. The coherent links performance is also compared to a similar direct detection link. Semiconductor lasers are used as the optical transmitter and local oscillator lasers. However, the wide linewidth of semiconductor lasers can cause substantial performance degradation of coherent links. We show that by using a coherent receiver structure consisting of a wideband filter, rectifier and narrowband lowpass filter, and by selecting a wide enough bandpass filter, the coherent AM link can be made linewidth-insensitive. We also show that for low optical powers, the coherent AM link has a better spurious-free dynamic range than a similar direct detection link, and that laser RIN is not a significant effect.