Lawrence J. Lembo

Characterization of a coherent optical RF channelizer based on a diffraction grating

A coherent optical RF channelizer has been constructed and characterized. The optical channelizer is based on a free-space optical diffraction grating, and utilizes coherent optical heterodyne detection to translate all of the frequency channels to a common intermediate frequency (IF). The designed optical channelizer has a 1-GHz channel spacing, and a nominal 5-GHz IF and can offer an instantaneous bandwidth greater than 100 GHz. The channelizing receiver has been characterized for its frequency response, crosstalk, and spur-free dynamic range, and the results are in a good agreement with the theoretical values.

CW injection locking of a mode-locked semiconductor laser as a local oscillator comb for channelizing broad-band RF signals

CW injection locking of mode-locked semiconductor lasers has been experimentally demonstrated. The phases of the mode-locked frequency comb are shown to be coherent with that of the master CW laser. The pulsewidth of the mode-locked laser remains almost unchanged (<2 ps) for a broad range of injection power (-28 to -12 dBm). Pulling of the entire mode-locked frequency comb by 400 MHz has been demonstrated. The coherent multifrequency source can be used as a local oscillator comb for coherent optical channelizers for ultrawide-band RF signals.

Low-loss fiber optic time-delay element for phased-array antennas

We present a novel concept, the fiber optic Bragg grating true- time-delay (TTD) element, for implementing true time delay in the distribution network of an optically fed phased array antenna. the device utilizes narrowband optical Bragg reflection gratings written holographically into the core of a single-mode fiber at various positions along its length. An optical carrier is modulated by the RF signal of interest and launched into this delay-line fiber. The desired RF time delay may be realized by wavelength-selectable nature of the TTD device offers the possibility for simplified beamsteering control and channel multiplexing.

Fiber optic Bragg grating true-time-delay generator for broadband rf applications

A fiber-optic approach for low-loss true time delay of wideband RF signals for phased-array-antenna beamsteering is presented. An optical carrier modulated by the RF signal of interest is launched into a delay-line fiber composed of optical Bragg reflection gratings written holographically into the core of a single-mode fiber. The desired beam steering is realized by tuning the optical carrier wavelength for reflection from the appropriate grating. Radiation testing of various fibers with Bragg gratings has been performed indicating preferable fiber types. True time delay offers much reduced beam squint and sharper antenna superior nulling compared to phase shift scanned antenna. Examples of applications of this concept showing its advantages are presented.

Optical electroabsorption modulators for wideband, linear, low-insertion-loss photonic links

We describe the characterization and development of semiconductor quantum well electroabsorption modulators (EAMs) for insertion into high-performance photonic links intended for analog applications. Limitations of existing approaches are described, motivating the potential of EAMs for exploiting the flexibility of semiconductor bandgap engineering. Relationships are established between basic modulator device characteristics and the RF system performance measures of link gain (insertion loss), bandwidth, noise figure, and dynamic range; results are then presented that have established the viability of EAMs for wideband, low-loss, linear analog photonic links.

RF performance of optical injection locking

Coherent photonic systems promise novel functionality and/or improved performance compared to direct detection photonic system, but have the disadvantage of being sensitive to optical phase noise. The most common approach to this problem is to force one laser to track the phase of the other with a phase locked loop (PLL), so that the phase noise of the lasers cancels out of the RF heterodyne beat note. Although the PLL approach has been implemented for semiconductor lasers, the large linewidth of these lasers and the resulting large PLL loop bandwidth severely constrain the design and limit performance. This disadvantage of the PLL approach is particularly relevant for many applications, since semiconductor lasers are preferred for system insertion.

Simultaneous optical amplification and splitting for lower-noise and higher-gain microwave signal distribution

Semiconductor optical amplifiers are investigated for use in large optical signal distributions systems requiring high dynamic range. The impact of amplifier length on the gain and noise figure of the microwave signal is illustrated experimentally. The performance of a device which simultaneously splits and amplifies the optical signal using the principle of multimode interference will be discussed, and it will be shown that this device has potentially higher performance that the previous generation Y-branch/amplifier combination.

Semiconductor optical modulators for high-performance analog rf photonic links and signal processing

Semiconductor based optical modulators offer flexibility in providing engineerable optical transfer characteristics that can target specific applications. Use of quantum well active regions provides the capability of efficient and linearized transfer characteristics that can benefit analog RF systems in terms of link gain, noise figure and spur free dynamic range. We present experimental results demonstrating the potential for improvements in modulator linearity and efficiency using quantum well based Mach-Zehnder modulators.

Wideband signal processing applications for coherent optical channelizers

As the frequencies and bandwidths in military RF systems escalate, we enter a realm where photonic technologies can play an important role in signal transmission and processing. Since it is very difficult to process large data bandwidths, either tunable or instantaneous, using conventional electronics, conventional receiver electronics becomes a bandwidth bottleneck, thus development is timely for optical processing of wideband signals. A review of critical mission requirements and photonics capabilities reveals a variety of military missions that can benefit immediately or in the near future from the advantages offered by photonics. In this paper we will discuss the implementation of an Integrated Sensor System (ISS) that is greatly simplified through the use of a coherent optical channelizer. In this system the received RF signal, which has been modulated onto an optical carrier, is routed via an all-optical switch to the coherent optical channelizer. A signal channelizer has the capability to process signals at frequencies of up to 100 Ghz and translate all the frequency channels to a convenient I.F. That is compatible with advanced digital receivers currently under development. The channelizer is also used on the transmit side of the system where a signal waveform is electronically synthesized at a convenient I.F. then photonically translated to a desired frequency band and transmitted to the aperture over fiber.

High power, high-frequency waveguide photodetectors

Optical RF analog applications are generating great interest for a number of reasons including the flexibility of fiber optic systems and the enormous RF signal processing capability offered by photonics. A limitation to the insertion of RF photonics has been the availability of high performance photonic components that can deliver on the advantages offered by photonic systems. In this paper we will discuss test results of InGaAsP-InP based waveguide photodetectors targeting optical RF analog applications.