Paul M. Hill

Subcarrier multiplexed lightwave systems for broad-band distribution

Microwave subcarrier multiplexing (SCM) is an important approach to the design of lightwave systems for broadband distribution. Recent progress in the design and performance of both analog and digital multichannel SCM systems is reviewed. The application of broadband SCM systems to both passive and optically amplified distribution networks is discussed. The discussion covers the general features of SCM systems; the electrooptic components that have been used in the system experiments described here, including laser intensity noise and noise due to intermodulation products; the carrier-to-noise ratio requirements; some multichannel FM systems experiments; and a 20-channel digital systems experiment; and a hybrid system carrying 60 FM SCM channels plus a 100-Mb/s baseband digital channel. Several approaches to broadband subscriber distribution networks are analyzed. >

Optical polarization division multiplexing at 4 Gb/s

An optical polarization multiplexing system using BPSK coherent heterodyne detection to transmit 4 Gb/s over a 45-km standard single-mode fiber optical link at a receiver sensitivity of -35 dBm is described. The 4-Gb/s system has a potential receiver sensitivity of -40 dBm while using 2-Gb/s baseband electronics.<<ETX>>

A 20-channel optical communication system using subcarrier multiplexing for the transmission of digital video signals

Microwave subcarriers have been used to demonstrate an optical communication system suitable for the distribution of digital video channels. The design, performance, and optical link requirements are discussed for a 2-Gb/s subcarrier-multiplexed (SCM) system with 20 microwave subcarriers modulated at 100 Mb/s and using a frequency-shift-keyed format. The system uses the 2-6-GHz bandwidth and represents the first example of an SCM system using more than one octave of bandwidth. The bit error rate was measured for all 20 channels; results for five typical channels are shown and discussed. The performance results show that a digital SCM approach is attractive in an optical link for a multichannel wide bandwidth video distribution system made from standard microwave components. >

Multigigabit subcarrier multiplexed coherent lightwave system

A wideband coherent detection optical communications system using two microwave subcarriers to transmit 8 Gb/s with a RF bandwidth efficiency of 1 b/s/Hz is described. It is demonstrated that two subcarriers each carrying 4 Gb/s QPSK, compactly spaced at just twice the data rate, can achieve a receiver sensitivity ( eta P) of -26.5 dBm with as little as 0.5-dB crosstalk between subcarriers. How the various components affect the overall performance of QPSK-SCM systems, including a CNR model for multichannel systems is discussed. >

20 channel coherent FSK system using subcarrier multiplexing

A 20-channel multioctave coherent frequency shift keying (FSK) system using subcarrier multiplexing techniques is described. The microwave subcarriers range from 2.1 to 5.9 GHz with a 200-MHz channel spacing. The receiver consists of a p-i-n photodiode, electrical isolator, low-noise amplifier, and bandpass filter to provide channel selection. The channels are modulated at 100 Mb/s, corresponding to one digitized video channel. Results for the optimal phase modulation index and degradation due to second- and third-order intermodulation distortion are discussed. For the worst-case channel, a receiver sensitivity of -27 dBm was achieved.<<ETX>>

Five-channel coherent heterodyne subcarrier multiplexed system

Results of a coherent digital-subcarrier multiplexed (SCM) lightwave system are described. Five frequency-shift-keyed channels at 100 Mb/s each were transmitted using microwave subcarriers. With a phase modulation index of beta =0.19, a bit error rate of 10/sup -9/ was achieved with a detected optical power of -33 dBm. This is a 14-dB improvement in receiver sensitivity over a comparable direct-detection SCM system.<<ETX>>

Novel carrier and clock-recovery circuit for multigigabit/second lightwave systems

The first use of a clock pilot-tone to simultaneously recover the carrier and clock signals in a suppressed carrier baseband BPSK optical transmission system was demonstrated. Three signals are obtained from the received pilot-tone, including the reference carrier used to synchronously demodulate the BPSK data, a reference to establish automatic frequency control, and a pattern-independent clock. The pilot-tone method for simultaneous signal extraction was demonstrated for the first time in a baseband multigigabit/second coherent heterodyne detection lightwave system. The recovered clock had a measured rms jitter of less than 1.5 degrees and a spectral width of +or-5 Hz.<<ETX>>

Eight-Gb/s QPSK-SCM over a coherent detection optical link

The first 8 Gb/s coherent optical detection system using two closely spaced subcarriers with 4 Gb/s QPSK on each subcarrier while maintaining an RF bandwidth efficiency of 1 bit/sec/Hz is reported. The receiver sensitivity is -26.5 dBm, with less than 0.5 dB penalty due to the other channel. It is expected that these techniques can be extended to transmit 16 Gb/s while using only 2 Gb/s baseband electronics.

Coherent Fiber Optic Subcarrier Multiplexed Systems

The results for coherent fiber optic digital subcarrier multiplexed (SCM) system experiments are described. With a frequency shift keying (FSK) format, 20 video channels at 100 Mb/s each were transmitted on one optical carrier using microwave subcarriers in a multi-octave configuration. A receiver sensitivity of -27 dBm was obtained when using narrow linewidth Nd YAG lasers, representing a 14 dB improvement over a 20 channel direct detection SCM system. It is shown that intermodulation distortion, in addition to shot noise and thermal noise, limits the obtainable receiver sensitivity. Theoretical and experimental results are compared and conditions for optimal performance as a function of channel number and phase modulation index are given. Analog system experiments using standard DFB lasers with a combined linewidth of 50 MHz were also performed. By employing a simple phase noise cancellation circuit, 12 FM video channels with a 30 MHz bandwidth and a 40 MHz channel spacing were successfully transmitted.