Phillip A. Davies

Optical generation of millimeter-wave signals for fiber-radio systems using a dual-mode DFB semiconductor laser

This paper presents a new approach to the optical generation of millimeter-wave signals using a dual-mode multisection distributed feedback semiconductor laser. This simple device is capable of generating high power signals between 40 and 60 GHz with extremely high spectral purity and stability. The two optical modes produced by this laser are heterodyned on an ultrafast photodiode to give a beat signal at the mode difference frequency. The phase noise of the beat signal is greatly reduced by phase-locking the modes using an electrical drive signal applied to the laser at a subharmonic of the beat frequency. Millimeter-wave signals are obtained with a linewidth of less than 10 Hz, a phase noise of less than -85 dBc/Hz at 100 kHz offset, and a locking range of about 500 MHz. Millimeter-wave fiber-radio systems are seen as a major application area for these new compact optical sources. >

Analysis and Demonstration of a Fast Tunable Fiber-Ring-Based Optical Frequency Comb Generator

Fiber-ring-based optical frequency comb generators are analyzed to understand their behavior and limitations. A numerical frequency-domain model is described for studying dispersion and other phase mismatch causing effects in the fiber ring cavity, as well as for predicting the spectral and temporal evolutions of the comb in time. The results from this analysis are verified with experimental measurements. A flat optical comb, with a terahertz span within a 6-dB power envelope and containing 100 comb lines, with a suppressed central comb line, is demonstrated. The comb shows an excellent coherence dependent on the phase noise from the radio frequency synthesizer that drives the comb generator. Improvement in the error correction loop also enables the comb spacing to be set at precise 12.5-MHz intervals without having to adjust the system. Fast frequency switching of the comb line spacing is demonstrated for the first time. The comb line spacing can be switched to any operation frequency with a resolution of 12.5 MHz between 6 and 12.5 GHz, as limited only by the microwave circuit used. The switching time is less than 1 s, and the spectral profile of the comb is maintained.

Optical demultiplexing for subcarrier multiplexed systems

Subcarrier multiplexing (SCM) is an attractively simple technique for establishing multiple independent channels over a single fiber. At the receiver, electrical mixing converts a selected channel to baseband. The photodiode, oscillator, mixer, and preamplifier must, however, operate up to the highest subcarrier frequency. The use of an optical filter for channel selection allows any subcarrier to be accessed with only baseband electronics. To obtain the best performance with this approach, sub-subcarrier modulation is needed. In this paper we propose a method suitable for externally modulated CATV systems and show that the sensitivity is comparable to the best conventional receivers. >

Optical generation and transmission of 60GHz signals over 100km of optical fibre using a dual-mode semiconductor laser

An electrically-injected dual-mode DFB semiconductor laser has been used to generate 60GHz signals with relatively high power and extremely high purity (phase noise < ¿73dBc/Hz at 1kHz offset and linewidth < 10Hz). These signals have been transmitted over 100km of standard optical fibre with no observable degradation in purity. A small power penalty has been observed (5dB at 50km), which is attributed to the subharmonic nature of the modulating signal applied to the laser in conjunction with chromatic dispersion in the optical fibre. The dual-mode laser is shown to be a relatively simple and compact optical source of high purity 60GHz signals for long-reach fibre-fed millimetre-wave radio systems.

Fibre optic frequency shifters

The technology of surface acoustic wave optical frequency shifters is discussed. The construction and operation of three types of device each using interdigital transducers on a PZT4 ceramic substrate is described.

Fibre Optic Frequency Shifters

The technology of surface acoustic wave (SAW) optical frequency shifters is discussed. The construction and operation of such a device with a sideband suppression of 35 dB and a carrier suppression of 29 dB is described. A two pass version of this device gave a conversion efficiency of 10% for 2.5W input electrical power.

Integrated 1.55 μm photomixer local oscillators for heterodyne receivers at mm wavelengths

Photomixing is a flexible and efficient method of providing both local oscillator signals for heterodyne receivers and high frequency phase reference signals. Ultrafast, 70 GHz bandwidth, = 1.55 m, photodiodes from u2t Photonics AG have been incorporated into mm-wave waveguide mounts. The photomixers utilise a thin gold probe to couple power into the waveguide and a gold-on-quartz choke filter to deliver photodiode bias. A method of rapidly characterizing the frequency response of these photomixers using spontaneous-spontaneous beating of light from an EDFA is described. Recent work has been directed at increasing the degree of integration of the photodiode, waveguide probe and choke filter to reduce the frequency dependence of the output power. A simplified photomixer block manufacturing process has also been introduced. A combined probe and filter structure, impedance matched to both the coplanar output line on the photodiode chip and to 0.4 height milled waveguide, is presented. This matching is achieved over the W-band with a fixed waveguide backshort. We present modelled and experimental results showing the increased efficiency and smoother tuning. Subsequent integration steps could use the InP photodiode substrate to support the waveguide probe and rf filter. The design and frequency response of such a probe is presented.

Photonic generation and delivery of millimeter-wave reference signals

Methods for the photonic generation of stable millimetre-wave reference signals are examined and compared. In particular, the generation of optical comb lines with microwave frequency separation in an amplified fibre ring and by sideband generation with an optical phase modulator are reported. Two comb lines at a millimetre-wave difference frequency can be selected using optical filters and heterodyned. The fibre ring can produce comb lines over a broad range of up to about 1 THz, whereas the sideband generation scheme is limited to frequencies of about 160 GHz. Both methods produce stable, low-phase-noise millimetre-wave signals useful as phase/frequency references. The transmission of such reference signals through optical fibre links of up to 9 km is also investigated. Differential dispersion effects can cause a power penalty in the received millimetre-wave signal, through the interaction of chromatic dispersion and SPM/XPM effects in the fibre, and through differential polarisation changes in the fibre causing non-alignment of the two optical fields at the photodiode. For the transmission of phase reference signals, the effects of differential dispersion, both chromatic and PMD, will cause phase variations in the received millimetre-wave signal, with the PMD effect being more serious due to its stochastic behaviour.

<10Hz linewidth optical millimetre-wave source for fibre-radio systems using a dual mode dfb semiconductor laser

We demonstrate the generation of millimetre-wave signals with <10Hz linewidth and with 500MHz locking range using a dual mode semiconductor laser. A subharmonic drive signal phase locks the beat signal produced by heterodyning the two modes. Recently frequencies in the millimetre-wave band have been proposed to meet the future demand for broadband wireless access networks. Frequencies at 62-63GHz and 65-66GHz have been allocated in Europe for the Mobile Broadband System (MBS) and the 40GHz region has been identified for longer range (up to 1km) applications such as video distribution. To enable this technology with low cost, fibre distribution of radio signals at 40GHz and 60-70GHz will be required. Very few optical sources cap of modulation at these frequencies have been demonstrated [1,2].

Optical prefiltering in subcarrier systems

Subcarrier multiplexing (SCM) is a convenient method of implementing multi-access in a lightwave system. Conventionally SCM receivers detect all channels using high speed optoelectronic components, each channel then being selected using heterodyne techniques in the electrical domain. Optical prefiltering is a novel technique where channel selection is accomplished optically. This method has the primary advantage that only a low bandwidth optical detector and receiver is needed to recover the baseband information.