Robert F. Kalman

Dynamic range of coherent analog fiber-optic links

We investigate the performance of coherent analog optical links employing amplitude modulation (AM), phase modulation (PM), and frequency modulation (FM). The performance of these coherent links is compared to that of AM direct-detection (DD) links. The signal-to-noise ratios, nonlinearities, and-spurious-free dynamic ranges (SFDRs) of the foregoing links are evaluated. We calculate the SFDR for links using DFB and Nd:YAG lasers with typical linewidths of 10 MHz and 5 kHz, respectively. The performance of PM and FM links is dominated by phase noise above a critical value of received optical power. For a linewidth of 10 MHz, and SFDRs of PM and FM links are 30 and 31 dB, respectively, for a received optical power above -27 dBm in a 1 GHz bandwidth. For a linewidth of 5 kHz, the corresponding SFDRs above a received power level of 0 dBm are 51 and 53 dB. The performance of DD and AM links is dominated by RIN above a critical value of received optical power. For a RIN level of -155 dB/Hz, the SFDRs of DD and AM links are 49 and 47 dB, respectively, for a received optical power of 10 dBm in a 1 GHz bandwidth. The SFDRs of the DD and coherent links used for transmission of subcarrier-multiplexed (SCM) signals are also derived. We evaluate target laser parameters needed by a number of different applications. For AM video and antenna remoting applications, linewidths of >

Space division switches based on semiconductor optical amplifiers

Multiple space-division switches based on semiconductor optical amplifiers (SOAs) can be cascaded to obtain larger switching fabrics. The authors present a general analysis of optical switching fabrics using SOAs, considering noise and saturation effects associated with amplified spontaneous emission. They find that the SOA saturation output power limits the number of switches which can be cascaded. For example, a saturation output power of 100 mW limits the size of switching fabrics to 100 64*64 switches or 200 8*8 switches if distributed gain matrix-vector multiplier (MVM) switches or Benes switches are employed. The corresponding limit for lumped gain MVM fabrics is 10 64*64 or 100 8*8. The Benes switch may be more suitable for large switch size (N>16) because it requires fewer SOAs.<<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. >

Linewidth-insensitive coherent optical analog links

Coherent optical analog links offer several important advantages over conventional direct detection links including the ability to separate wavelength-division multiplexed signals, frequency translation, and utilization of angle modulation formats. However, the wide linewidth of semiconductor lasers can cause substantial performance degradation of these links. This paper analyzes the signal-to-noise ratio and the dynamic range of amplitude modulated (AM) and frequency modulated (FM) coherent links, and compares them to direct detection links. It is concluded that a properly designed AM system is insensitive to laser linewidth. For optical powers less than 1mW, the performance of coherent AM links is better than that of direct detection links; for optical powers greater than 1mW, the performance of the two links is nearly the same. Coherent FM links have the potential to increase the SNR and the dynamic range by more than 10 dB as compared to direct detection and coherent AM links. However, FM links are potentially sensitive to laser linewidth, and require elaborate phase noise cancellation techniques when semiconductor lasers are used.

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,. >

A novel analog optical link with high dynamic range

A heterodyne interferometric phase modulated link, a new analog optical link, is described. This link, based on phase modulation using an external modulator, has high linearity and suppresses the impact of laser relative intensity noise. The link increases the spurious-free dynamic range by 21 dB as compared to a conventional externally modulated AM link for 10 mW of received optical power, relative intensity noise of -130 dB/Hz, and a 15-GHz intermediate frequency.<<ETX>>

Behavior Of Broadband Fiber Sources In A Fiber Gyroscope

Two fiber laser sources, a resonant fiber laser (RFL) and a superfluorescent fiber laser (SFL), have been given initial tests as gyro sources using a medium quality gyro test bed. The RFL reacted strongly to optical feedback from the gyro circuit resulting in very large unstable errors in the gyro output. These were suppressed substantially by an optical isolator which reduced feedback from the gyro, or by a phase modulator within the laser cavity. The SFL was found to be free of errors of this type when operated at output power levels below a threshold level (about 0.5 mw) for self oscillation due to external optical circuit reflections.

Demonstration of an analog heterodyne interferometric phase-modulated (HIPM) link

We present results from a proof-of-concept demonstration of a heterodyne interferometric phase-modulated (HIPM) link we have previously proposed and analyzed. The HIPM link uses a novel frequency shifter/phase modulator and a phase discriminator receiver. The experimental HIPM link demonstrated suppression of third-order intermodulation products (IMPs) by as much as 23 dB as compared to a conventional externally modulated AM link, corresponding to an increase in spurious-free dynamic range of 7.7 dB. We compare experimental performance and theoretical predictions, and consider practical implementations of the link.<<ETX>>

1.55-um broadband fiber sources pumped near 980 nm

Stable, broadband, long-wavelength sources are required for accurate fiber sensors such as the fiberoptic gyroscope. The Er-doped superfluorescent fiber source and wavelength-swept fiber laser, which emit near 1 .55 m and can be pumped near 980 nm, are excellent candidates for this application. We discuss the design of such sources, their efficiency, pump source requirements, and the spectra they produce. The spectrum sensitivity to environmental factors such as temperature is also briefly discussed.

Design and analysis of a novel fiber-based PCS optical network

We propose a passive antenna remote unit (PARU) architecture which can be used in future optical fiber-based PCS networks. This system is robust, easy to maintain, and upgradeable to multi-GHz frequencies. By using externally modulated Nd:YAG lasers and coherent detection on the uplink, the system is far more efficient than for a direct detection (DD) uplink in terms of subcarrier multiplexed (SCM) voice and video channels per laser, meeting an spurious-free dynamic range (SFDR) requirement of 55 dB. For voice channels, a coherent FM uplink, a coherent AM uplink, and a DD uplink support, per laser with 200 mW optical power, over 1600 channels, 352 channels, and 96 channels, respectively. For moderate quality video, a coherent FM uplink supports over 1000 channels but a coherent AM uplink and a DD uplink cannot achieve the required SFDR due to the Brillouin threshold.