Photonic Signal Processing Research Articles

Microstructure fibre array for RF photonic signal processing applications

A novel microstructure fibre array (MFA), comprising multiple singlemode fibre waveguides reduced to an overall diameter of approximately 30 µm is reported. When used in conjunction with a single photodetector, the array facilitates the efficient and stable electrical summation of a number of RF-modulated optical signals with minimal optical interference effects. The reported MFA has optical insertion loss less than 0.1 dB. An RF filter based on the array exhibits less than 0.4 dB optical insertion loss and excellent temporal stability.

Optical source coherence controller for fibre optic delay line RF/microwave signal processors

A technique for optical source coherence control that can suppress the important coherent interference effects in photonic delay line RF/microwave signal processors is presented. The concept is based on phase modulation of the optical carrier to transform the laser coherence to meet the requirements of the photonic signal processor that suppresses coherent interference. It enables conventional narrow-linewidth, high quality DFB lasers to be used for high frequency photonic signal processors, and tailors their coherence length to meet the processor design requirements. Experimental results show that the optical source coherence controller can realise photonic RF/microwave filters with robust response using narrow-linewidth sources, demonstrating the effectiveness of this technique for coherent interference suppression in photonic signal processors.

Novel synthesized photonic signal processor with hardware compression

A new grating-synthesized structure that realizes both a flat-top filter and a high rejection ratio response simultaneously is presented. The concept is based on an integrated grating synthesis approach in which the tap weights and delays are simultaneously realized in one unit. This enables multiple taps to be readily obtained with short sampling times, together with bipolar taps using a dual-output electrooptic modulator. Results demonstrate the realization of a flat-top filter response with high free-spectral range, high rejection ratio, and low passband ripple, and with compressed hardware requirements.

A microphotonic transversal RF signal processor with no laser-coherence noise

In this paper, the impact of laser coherence on transversal photonic RF signal processors is investigated and a new MicroPhotonic adaptive RF filter structure, which eliminates the phased-induced intensity noise, is proposed and demonstrated. It is shown that the phase-induced intensity noise caused by the laser-source coherence significantly degrades the RF frequency response of a transversal photonic RF filter, hence, limiting its performance. The results show that the MicroPhotonic RF filter is capable of generating an arbitrary transfer function with no phase-induced intensity noise. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 45: 540–545, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20873

A flexible and incoherent microwave photonic filter with a flat-top passband

A microwave filter realized by combing a recirculating filter based on a fiber Bragg grating (FBG) resonator and a matched two-arm delay line is presented. The proposed configuration can generate a flexible notch filter with a wide flat-top passband, which overcomes the limitations of the filter-transfer-function range of a positive-only taps system. An incoherent system is employed which avoids the coherent-interference problem of photonic signal processors. The theoretical and experimental results reveal that more than 50% free spectral range can be flattened by this method, and the ripples in the passband are less than 0.5 dB. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 132–134, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20922

All-Optical Photonic Signal Processors With Negative Coefficients

A new technique to generate all-optical negative coefficients in photonic signal processors is presented. It is based on using a dual-output Mach-Zehnder modulator, excited with two wavelengths, to obtain radio-frequency (RF) signal modulated optical carriers in opposite phase, together with a wavelength selection scheme to separate each wavelength and to perform the function of producing negative taps. This eliminates the need to convert to the electrical domain to obtain subtraction and also eliminates the speed, noise, and distortion limitations of previous optical cross-gain modulation approaches. Experimental results on a high-skirt-selectivity bandpass filter demonstrate the all-optical negative coefficient generation mechanism and show the synthesis of high-speed negative taps in the optical domain.

Erbium-Doped Fiber Amplifier Gain Characteristics Inside Active Delay Line Structures

A systematic study of how the erbium-doped fiber amplifier (EDFA) gain behaves inside an active recirculating delay line structure is presented. It is verified using a representation of an active-fiber Bragg-grating-pair filter. It shows that due to the presence of the recirculating signal, the EDFA gain inside the cavity exhibits hard saturation, and with a proper design, fluctuations in pump power have only slight effects on EDFA gain and this enables the active delay line filter to realize a stable high-Q frequency response. Results demonstrate a high-Q photonic signal processing filter that exhibits low sensitivity to pump power perturbations and which shows robust active filter operation.

Photonic Notch Filter Without Optical Coherence Limitations

A new topology for a photonic notch filter that can solve the coherent interference problem in photonic signal processors is presented in this paper. It is based on a double-pass modulation technique. The key advantage of this new structure is that it completely removes the dominant phase noise limitation, which is a fundamental problem with existing incoherent processor approaches. It also enables arbitrary narrow-linewidth telecommunication-type lasers to be used with stable and robust filter operation. Extensions to obtain tunable notch frequency and wide-passband notch filter operation are presented. Results demonstrate robust notch filter operation with a narrow-linewidth source, no phase-induced intensity noise, and a vastly superior signal-to-noise ratio performance (i.e., a nearly 60-dB increase) compared with the conventional delay line notch filter. Tunable coherence-free operation of the notch filter is also shown. Finally, a new structure that not only operates without coherence limitations but also realizes a notch filter with a wide passband of more than 50% of the free spectral range is demonstrated.

Hybrid Analog–Digital Variable Fiber-Optic Delay Line

A variable fiber-optic delay line (VFODL) is introduced that, to the best of the authors' knowledge, is the first time that a hybrid analog-digital VFODL is proposed to solve the dilemma of efficiently enabling many settable and long-duration time delays together with continuous and short time delays. In essence, this VFODL can provide near-continuous high-resolution time generation across an entire long-time-delay band. The VFODL is based on the concept of cascaded wavelength-sensitive and wavelength-insensitive time delays. A proof-of-concept VFODL built demonstrates near-continuous 0.5-ps-resolution time-delay control across an entire 25.6-ns time-delay band generating a total of 51 200 measurable time-delay bins. The experimental VFODL also gives a 4.95-dB total optical loss and a 1-ms time-delay control-setting speed. The proposed VFODL can be used in applications such as radio frequency photonic signal processing and radar testing.

High-resolution photonics-based interference suppression filter with wide passband

A new topology for a photonic signal processor, which overcomes the basic recursive frequency response problem that limits the passband range, is presented. The structure is based on a new multiple-wavelength offset-cavity structure that is cascaded with a series of unbalanced delay line structures. This not only can synthesize a very narrow notch response with good shape factor but also permits a multifold extension of the free spectral range (FSR) and passband width. Results on the interference mitigation filter demonstrate a stopband of 1% of center frequency and a fourfold increase in the FSR and passband width, while also having a very small shape factor, in excellent agreement with predictions.

RF photonics signal processing in subcarrier multiplexed optical-label switching communication systems

This paper provides theoretical and experimental studies of radio-frequency (RF) photonics processing techniques applicable in subcarrier-multiplexed optical-label switching (OLS) communications systems. The paper provides an overview of various label-coding technologies and introduces subcarrier multiplexing (SCM) as an attractive technology for OLS networks. All-optical-label extraction using optical filters, such as fiber Bragg gratings (FBGs), provides an effective means to demodulate the SCM labels without inducing RF fading effects caused by fiber dispersion. Furthermore, the role of fiber nonlinearities in the RF fading effects are theoretically and experimentally verified. The all-optical label extraction and rewriting processes constitute optical-label swapping, wherein 2R data regeneration can take place. Scalable and cascadable OLS systems are feasible by applying viable RF photonics technologies in all-optical-label processing.

Third harmonic autocorrelation and wave mixing in a thin film of poly(p-phenylenevinylene)

We show here that a thin film of a nonlinear conjugated polymer: poly(p-phenylenevinylene) (PPV) is capable of efficiently generating the third harmonic of femtosecond pulses of light at the near infrared (including telecommunication bands) wavelengths, giving coherent, low divergence beams in the visible range. By using more than one fundamental beam overlapping in space and in time on the PPV film, autocorrelation signals are observed as well as higher order mixing signals due to X(5) and higherorder nonlinear susceptibilities. These signals can be used for applications such as short laser pulse diagnostics and photonic signal processing and they provide information about the nonlinear properties of the material itself.

Intrinsic eigenstate spectrum of planar multilayer stacks of two-dimensional photonic crystals

A specific class of planar photonic crystals is investigated that provides a condensed matter combining the properties of planar multilayer stacks and two-dimensional photonic crystals in order to achieve large partial bandgaps in the eigenstate spectrum. These gaps are larger than the directionally dependent and polarization-dependent partial gaps of photonic crystal slabs. Full in-plane gaps are demonstrated numerically. Strong dispersion, waveguide confinement, high Q-cavities, and alternative photonic signal processing are feasible with these structures.

Photonic bandpass filters with high skirt selectivity and stopband attenuation

A new photonic signal processor topology that simultaneously achieves both a high-Q and a high skirt selectivity and stopband attenuation filter response is presented. It is based on a novel dual-cavity bandpass optical structure in which two pairs of active fiber Bragg grating cavities are used with an optical gain offset to control the poles and stopband attenuation characteristics of the filter. This concept enables a large improvement in the filter stopband attenuation, rejection bandwidth, and skirt selectivity to be realized. Measured results demonstrate both a narrow bandpass bandwidth of 0.4% of center frequency and a skirt selectivity factor of 16.6 for 40 dB rejection, which corresponds to a 6.5-fold improvement in comparison to conventional single cavity high-Q structures. To our knowledge, this is the best skirt selectivity reported for a photonic bandpass filter to date. The new photonic filter structure has been experimentally verified and excellent agreement between measured and predicted responses is shown.

Photonic analog-to-digital conversion by distributed phase modulation

A photonic analog-to-digital converter with a binary optical output was constructed. The architecture scheme is a pipeline analog-to-digital converter, employing phase modulators and polarization-based optics. The result is a photonic analog signal processor that maps an input analog waveform into a binary output. The operational speed of the conversion is primarily limited by the maximum slew rate of the phase modulators.

Waveguide fabrication using organic nonlinear optical materials

Among many types of nonlinear optical (NLO) polymers developed, this paper has focused on azo-dye-functionalized polymers and discussed their channel waveguide fabrication for photonic applications. After examining the critical issues of the polymer waveguide fabrication, serially-grafted quasi-phase-matching waveguide for frequency conversion, 3rd-order NLO polymer waveguide for all optical switch and trench-filled waveguide grating for wavelength filter were discussed. The serially-grafted waveguide structure described in this paper will be a key area for future photonic signal processing systems.

Photonic Signal Processing of High-Speed Signals Using Fiber Gratings

Methods for wideband photonic signal processing using fiber delay lines and Bragg grating sampling elements are described. These processors provide new capabilities for the realization of high time-bandwidth operation and high resolution performance. Multiple wavelength techniques with discrete and chirped Bragg gratings are described, which have high capacity signal processing functions using dense parallel signal processing techniques. Both fixed and tunable or reconfigurable processors using wavelength control to change the sampling time are discussed. A range of grating-based signal processors, including high Q microwave filtering, frequency discriminators, widely tunable filters, fast and adaptive signal correlators, and true-time delay beamforming in phased array antennas are described.

A novel high-Q optical microwave processor using hybrid delay-line filters

A new hybrid active-passive photonic signal processor, which achieves high-Q microwave bandpass filtering, is presented. It overcomes the problem of achieving very high-Q values, while still operating the active stage with a large gain margin. This enables a significant increase in Q to be obtained, higher filter frequencies, and robust operation. The general synthesis procedure for the hybrid filter is described. The filter response demonstrates very high-resolution microwave signal filtering with a measured Q of 801.

High-Q optical microwave filter

A new photonic signal processor topology is presented, which achieves high-Q value microwave bandpass filtering. It significantly increases the Q by means of a small free spectral range passive filter, while still operating the active stage with a large gain margin. The filter response demonstrates very high resolution microwave signal filtering with a measured Q value of 983.

Acousto-optically switched optical delay lines

High speed microsecond domain switching optical delay lines are introduced for time-delay based photonic signal processing applications such as phased array control and transversal filtering. These delay lines use acousto-optic deflectors for both optical delay path selection and optical beam realignment, producing inherently self-aligning control architectures. Both transmissive and reflective designs are introduced that can operate with either fiber, free-space, and solid-optic delay paths. High accuracy analog tuning of both beam position and beam optical power are possible with these modules.