John B. Georges

Dynamic range requirements for optical transmitters in fiber-fed microcellular networks

Analog fiber-optic links can be used for antenna remoting in microcellular networks. Using a statistical model of user access in a wireless network, it is shown that by accepting a modest (<0.5%) percentage of blocked calls, a modest optical link dynamic range of 91 dB (1 Hz) is required. By using multiple fiber-fed antennas per cell and proper network protocol, the required dynamic range is dramatically reduced to <80 dB for the same blocking probability.<<ETX>>

Techniques for improving in-building radio coverage using fiber-fed distributed antenna networks

The characteristics of an in-building fiber-fed distributed antenna network are addressed by simulation of an indoor non-shadowed radio environment at 900 MHz, 1.8 and 1.9 GHz. The performance of the network is discussed in terms of the number and placement of antennas, and the method of diversity combination used. Statistical analysis of the radio environment shows that the arbitrary placement of two fiber-fed antennas in the room is as good as conventional half-wave diversity. Furthermore, the performance of multiple distributed antennas (>3) is typically superior to non-distributed architectures.

Optical transmission of narrowband millimeter-wave signals

We describe experimentally and theoretically three techniques used to transmit narrowband millimeter-wave (MM-wave) analog signals over optical fiber: 1) narrowband MM-wave optical transmitters based on resonant modulation of monolithic semiconductor lasers, 2) feedforward optical modulation, and 3) a passively mode-locked laser operating in an optoelectronic phase-locked loop. The resonant modulation response at the cavity round-trip frequency is fully characterized for multiple-contact lasers under various bias conditions. Issues such as modulation efficiency, passband bandwidth, noise, and intermodulation distortion are addressed. A system implementation of resonant modulation is presented in which two simultaneous 2.5-Mb/s BPSK channels centered at a subcarrier frequency of 41 GHz is transmitted over 400 m of single-mode fiber. Simple microstrip matching circuits are fabricated at 41 GHz to couple the MM-wave signals into the laser. Resonant modulation of single-contact lasers is also reported. Next, implementation of a tunable MM-wave (30-300 GHz) optical transmitter based on feedforward optical modulation is presented, and the fundamental performance of this technique investigated in terms of noise and dynamic range. Feedforward modulation is used to transmit 300-Mb/s data at 39 GHz over 2.2 km of single-mode fiber. Finally, a passively mode-locked monolithic semiconductor laser operating in an optoelectronic phase-locked loop is implemented as a narrowband MM-wave optical transmitter at 46 GHz. The phase-locked loop bandwidth, MM-wave tracking capability, and fundamental limit to the stability of the MM-wave subcarrier is established. The relative merits of the three techniques are discussed and compared. The MM-wave subcarrier transmission results presented here represent the highest reported to date. >

Millimeter wave, multigigahertz optical modulation by feedforward phase noise compensation of a beat note generated by photomixing of two laser diodes

A technique is described for high frequency (up to 100 GHz), narrowband ( approximately GHz) optical modulation by encoding the millimeter-wave signal onto a beat note produced by photomixing of radiation from two 1.3 mu m DFB lasers. The phase noise on the beat note is compensated by a feedforward technique, using a low-frequency external optical modulator which simultaneously encodes the information to be transmitted. The modulation band can be tuned by varying the lasing frequency of one or both of the lasers. The fundamental performance limit of this technique is investigated, and the transmission of a pulsed RF signal at 40 GHz is demonstrated.<<ETX>>

Self-pulsating laser diodes as fast-tunable (<or=1 ns) FSK transmitters in subcarrier multiple-access networks

The authors demonstrate the use of self-pulsating laser diodes as fast-tunable (<or=1 ns), frequency-shift keyed (FSK) microwave subcarrier transmitters in local-area subcarrier frequency-division multiple access (SFDMA) lightwave networks. They show that by using commercially available compact-disk self-pulsating lasers, a network supporting four users each transmitting at 150 Mb/s FSK is feasible, and can be extended to 12 FSK subcarrier channels at 200 Mb/s if the self-pulsating frequency is extended to 12 GHz.<<ETX>>

Multichannel millimeter wave subcarrier transmission by resonant modulation of monolithic semiconductor lasers

We study the multichannel analog and digital performance of narrowband millimeter-wave optical transmitters based on resonant modulation of monolithic semiconductor lasers. Two-tone measurements are performed under various bias conditions at a cavity round-trip frequency of 41 GHz, and optical transmission over 400 m of single-mode fiber of two simultaneous 2.5 Mb/s BPSK channels centered at a subcarrier frequency of 41 GHz is demonstrated.<<ETX>>

Stabilization of millimeter-wave frequencies from passively mode-locked semiconductor lasers using an optoelectronic phase-locked loop

The use of an optoelectronic phase-locked loop to stabilize the pulsation frequency of a three-section, passively mode-locked quantum well laser diode at 41 GHz is discussed. It is shown that the free-running mode-locked signal, with a radio frequency (RF) linewidth of 770 kHz, can be stabilized to the linewidth of a reference RF oscillator (<1 kHz). The stabilized mode-locked signal has a phase noise of -70 dBc/Hz at 370 kHz offset. Tracking of the mode-locked signal to the external reference RF oscillator is maintained over 11 MHz. This is, in effect, an actively mode-locked laser source at 41 GHz.<<ETX>>

800 Mb/s microwave FSK using a self-pulsating compact-disk laser diode

Microwave frequency-shift keying (FSK) is demonstrated at 800 Mb/s using a low-cost, compact-disk, self-pulsating laser diode. The FM response of the self-pulsation at various drive currents is also given. The response exhibits a 3-dB bandwidth of 700 MHz and is independent of the peak current over a large range of deviations. The results confirm that an inexpensive, self-pulsating, compact-disk laser diode is a viable alternative to conventional microwave oscillators as a microwave subcarrier FSK transmitter with bit rates from 125 to 800 Mb/s, a range unachievable with commercially available microwave oscillators.<<ETX>>

On distributed microwave effects in semiconductor lasers and their practical implications

Recently, it has been shown that when a semiconductor laser is directly modulated at high frequencies, the modulation signals suffer substantial loss and phase shift propagating from the wire feed point along the length of the laser. It was suggested that these distributed microwave effects lead to a further bandwidth degradation in electrical current injection over the single‐pole roll‐off predicted by a lumped RC model. We show, however, that this degradation is significant only when the laser is driven directly by a voltage source. In contrast, when the laser is driven, commonly, through a 50 Ω transmission line the degradation is minimal, and the total injection current is still RC limited.

On the characteristics of narrow‐band resonant modulation of semiconductor lasers beyond relaxation oscillation frequency

We consider practical issues related to narrow‐band resonant modulation of laser diodes at frequencies above relaxation oscillation. We established the limits to the performance of this approach based on bandwidth, noise, and intermodulation distortion considerations, as well as the proper trade‐offs between these three quantities.