Comb Signal Research Articles

High-Speed Coherent Transmission Using Advanced Photonics in Terahertz Bands

SUMMARY A terahertz-wave communication system directly connected to an optical fiber network is promising for application to future mobile backhaul and fronthaul links. The possible broad bandwidth in the terahertz band is useful for high-speed signal transmission as well as radio-space encapsulation to the high-frequency carrier. In both cases, the low-latency feature becomes important to enhance the throughput in mobile communication and is realized by waveform transport technology without anydigital-signal-processing-based media conversion. A highlyprecise optical frequency comb signal generated by optical modulation and the vector signal demodulation technology adopted from advanced optical fiber communication technologies help perform modulation and demodulation with impairment compensation at just the edges of the link. Terahertz wave, radio over fiber, waveform transport, coherent detection, multilevel modu

An Efficient Group Delay Measurement Method for Frequency Converting System

The existing group delay measurement methods can only get the group delay at a certain aperture or frequency perturbation in one or two measurements. A new method based on comb signal is proposed. If modulo delay ambiguity can be ignored, then the method permits measurement without frequency perturbation of the test signal in one single measurement. The whole phase character of the test band can be obtained, and the group delay characteristics can be calculated by polynomial fitting of the phase curve at diverse aperture. A group delay ambiguity resolution method based on extended Chinese remainder theorem (CRT) is detailed. If modulo delay ambiguity cannot be ignored, then the extended CRT method can be used to obtain the unambiguity delay, releasing the operator from the constraint of keeping the aperture or frequency perturbation small. Computer simulations and field experiments are presented to verify the proposed methods.

High-power mid-infrared frequency comb from a continuous-wave-pumped bulk optical parametric oscillator.

We demonstrate that it is possible to obtain a mid-infrared optical frequency comb (OFC) experimentally by using a continuous-wave-pumped optical parametric oscillator (OPO). The comb is generated without any active modulation. It is based on cascading quadratic nonlinearities that arise from intra-cavity phase mismatched second harmonic generation of the signal wave that resonates in the OPO. The generated OFC is transferred from the signal wavelength (near-infrared) to the idler wavelength (mid-infrared) by intracavity difference frequency generation between the OPO pump wave and the signal comb. We have produced a mid-infrared frequency comb which is tunable from 3.0 to 3.4 µm with an average output power of up to 3.1 W.

GN-Model Validation Over Seven Fiber Types in Uncompensated PM-16QAM Nyquist-WDM Links

We report on a set of transmission experiments over seven different uncompensated fiber links carrying a 22-channel Nyquist-WDM signal comb based on polarization-multiplexed 16-quadrature amplitude modulation at RS=15.625 GBd(RB=125 Gb/s). The channel spacing was Δf=16 GHz. The experimental transmission performance was compared with analytical GN-model predictions showing a good agreement for all seven considered fiber types. Moreover, we used the experimental results for the validation of a fiber figure of merit based on the maximum reach, showing its effectiveness for approximate fiber performance prediction. Within the experimental tests, a maximum transmission distance of 3810 km at a bit error-rate of 1.5×10-2 was achieved using a pure-silica-core fiber with 150 μm2 effective area, at a net spectral-efficiency-distance product of 24700 (b·km)/(s· Hz), in a good agreement with the GN-model forecast.

Discretely tunable comb spacing of a frequency comb by multilevel phase modulation of a periodic pulse train

We experimentally demonstrate tunable comb spacing of an original 10-GHz periodic frequency comb by spectral Talbot effect over an unprecedented range of even and odd comb spacing division factors, from 2 to 9. The implementation has been achieved by periodic electro-optic (EO) temporal phase modulation of the original comb (conventional mode-locked optical pulse train) with multilevel modulation functions, produced by an arbitrary waveform generator (AWG). These comb spacing division processes have been observed through the use of a high-resolution (20-MHz) optical spectrum analyzer. Comb spacing tuning is achieved without essentially affecting the spectral bandwidth and total energy of the original comb signal. Our results also confirm that the spectral Talbot method does not require carrier-envelope phase stabilization in the input frequency comb. Numerical studies on the impact of deviations in the applied phase modulation functions confirm the robustness of the technique, in agreement with the experimental results.

Monolithically Integrated All-Optical Regenerator for Constant Envelope WDM Signals

All-optical regeneration has attracted growing interest due to the possibility to increase the all-optical reach. However, to be really competitive with optoelectronic regenerators, all-optical regenerators should process an entire wavelength division multiplexed (WDM) signal comb at once and significantly reduce the footprint and the power consumption, as possible, in principle, by exploiting photonic-integrated circuits (PICs). Moreover, since constant envelope modulation format signals have come into play in recent years, all-optical regenerators dealing with a multiplex of these signals are of particular interest. This paper presents a monolithically integrated indium-phosphide-based PIC acting as an all-optical regenerator for constant envelope WDM signals. The regeneration scheme is based on hard-limiting amplification in saturated semiconductor optical amplifiers (SOAs), which removes the signal intensity noise. The presented PIC, designed and fabricated within the JePPIX technology platform, can handle up to four WDM signals, which are demultiplexed and enter an array of SOAs to undergo regeneration before being multiplexed again. The channel-by-channel regeneration of both polarization shift keying and frequency shift keying signals at 10 Gb/s is experimentally demonstrated in terms of Q-factor improvement.

Broadband parametric multicasting via four-mode phase-sensitive interaction

Optical frequency multicasting with significantly enhanced signal-to-noise-ratio (SNR) is demonstrated over wide wavelength range. High-fidelity multicasting relies on a four-mode phase-sensitive (PS) parametric process. Four-mode seeding was used to drive dual-pump, multistage mixer and achieve high-efficiency frequency comb generation and signal replication assisted by field interference. New PS mixer was measured to possess near 12-dB of spectrally uniform optical SNR advantage over conventional, phase-insensitive (PI) parametric multicaster. In signal attenuation regime, PS operation was measured with better than 10dB over the entire mixer band, indicating highly controllable PS interference. The role of the new PS mixer in near-noiseless signal replication is elaborated.

In-Orbit Measurement of the AOS (Acousto-Optical Spectrometer) Response Using Frequency Comb Signals

The in-orbit response characteristic of the AOS (Acousto-Optical Spectrometer) was estimated from spectral profiles taken by using a frequency comb generator, which was originally used for frequency axis calibration. Frequency comb signals taken under various environmental conditions in orbit were consolidated to deduce the response profile of the spectrometer. The fluctuation bandwidth calculated from the deduced spectral response compared well with that derived from the noise characteristic of the spectrometer.

Laser frequency comb techniques for precise astronomical spectroscopy

Precise astronomical spectroscopic analyses routinely assume that individual pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons. Intra-pixel sensitivity (IPS) variations may already cause small systematic errors in, for example, studies of extra-solar planets via stellar radial velocities and cosmological variability in fundamental constants via quasar spectroscopy, but future experiments requiring velocity precisions approaching ~1 cm/s will be more strongly affected. Laser frequency combs have been shown to provide highly precise wavelength calibration for astronomical spectrographs, but here we show that they can also be used to measure IPS variations in astronomical CCDs in situ. We successfully tested a laser frequency comb system on the Ultra-High Resolution Facility spectrograph at the Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded in a single CCD exposure, we find that the average IPS deviates by <8 per cent if it is assumed to vary symmetrically about the pixel centre. We also demonstrate that series of comb exposures with absolutely known offsets between them can yield tighter constraints on symmetric IPS variations from ~100 pixels. We discuss measurement of asymmetric IPS variations and absolute wavelength calibration of astronomical spectrographs and CCDs using frequency combs.

Polarization-independent tunable spectral slicing filter in Ti:LiNbO_3

A two-port polarization-independent tunable spectral slicing filter at the 1530 nm wavelength regime is presented. The design utilizes an asymmetric interferometer with a sparse index grating along its arms. The sparse grating makes it possible to select equally spaced frequency channels from an incident WDM signal and to place nulls between them to coincide with the signal comb frequency. The number of selected channels and nulls between them depends on the number of coupling regions used in the sparse grating. The free spectral range depends on the spacing between the coupling regions. The Z-transform method is used to synthesize the filter and determine the spectral response. The operation of a device with six coupling regions is demonstrated, and good agreement with theoretical predictions is obtained. A 3 dB bandwidth of ∼1 nm and thermal tuning over a range of ∼13 nm are measured.

Terahertz Optical Frequency Comb Generation by Spectral Filtering of Broadband Spontaneous Amplified Emissions From a Semiconductor Optical Amplifier

In this paper, we describe a fiber-optics-based system for the generation of optical frequency comb using amplified spontaneous emissions from a semiconductor optical amplifier (SOA) as signal source. The continuous-wave (CW) spectrum from the SOA ranges from 1480 nm to 1680 nm. A Solc-Sagnac birefringent interferometer filters the CW into a comb of discrete spectral lines with line spacing of 600 GHz, producing comb signals than span 182-230 THz.

Evaluating SOA Nonlinearity Effects by High-Order Susceptibility on Constant Envelope Signals in WDM Systems

Nonlinearity effects, arising within a semiconductor optical amplifier (SOA), have been deeply investigated with particular interest for exploiting them in wavelength conversion. This application shrinks the research area to a precise scenario: a strong pump signal travels through the active medium simultaneously with a modulated signal to be converted. In this case, it is possible to neglect the spectral distribution of the optical power and phase relationship between propagating signals. SOAs have been also studied as line amplifiers for wavelength-division multiplexing (WDM) phase-modulated signals. Aim of this paper is to extend the propagation model in an SOA, in order to simulate nonlinearities occurring in the amplification of a WDM comb signal evaluating-wave mixing on constant envelope modulation schemes (differential phase-shift keying). This can be carried out due to an enhanced analytical model based on the optical field and medium interaction, overcoming the wide-used rate equation approach, which is based on the overall photons density.

Broadband-Frequency-Tunable Sub-Terahertz Wave Generation Using an Optical Comb, AWGs, Optical Switches, and a Uni-Traveling Carrier Photodiode for Spectroscopic Applications

We present a monochromatic sub-terahertz signal generation technique using an optical comb signal, arrayed waveguide gratings (AWGs), and a uni-traveling carrier photodiode (UTC-PD) for spectroscopic applications. This scheme offers random or continuous frequency tuning in the range between 100 GHz and up to 1 THz. In addition, since a RF synthesizer is employed as a reference signal source of the photonic frequency multiplier, frequency locking with external instruments and reliable operation are offered. Highly coherent optical comb signal for the photonic frequency multiplication provides a narrow linewidth and very low phase noise in the generated sub-terahertz signal. For 125 GHz, the phase noise is approximately -92 dBc/Hz at the offset frequency of 10 kHz. This is larger than that of the 25-GHz RF source by about 13 dB and agrees well with the theory regarding phase noise multiplications due to frequency multiplication. For generating monochromatic signals, unwanted spurious signals are suppressed in the optical domain over a wide range with two AWGs, and the suppression ratio is expected to be better than 46 dBc. Utilizing the implemented sub-terahertz signal generator with a J-band UTC-PD module, absorption lines of N2O were measured in the frequency range between 240 and 360 GHz and compared with theoretical calculations.

Subpicometer wavelength accuracy with gain-switched laser diode in high-resolution optical spectrometry

We present a method to generate an optical reference comb signal with subpicometer wavelength accuracy. XPM nonlinear effect in a fiber is used to get an optical frequency comb signal, free of frequency chirp and wavelength instabilities, from a pulsed gain-switched laser diode. Principles of such comb generation are presented as well as the application of this comb as a ruler to measure frequency differences in high resolution optical spectrum measurements. To confirm this method, Brillouin filtering optical spectrometric technique is used to characterize a modulated optical source. Typical resolution of this technique allows 0.08 pm wavelength accuracy.

The First Radioastronomical Observation with Photonic Local Oscillator

Abstract We present the results of experimental radioastronomical observations of an interstellar molecular spectral line with an SIS (superconductor–insulator–superconductor) receiver pumped by a photonic local oscillator (LO). A millimeter-wave LO signal was generated by feeding an optical comb signal into a W-band (75–110 GHz) photomixer using a uni-traveling-carrier photodiode (UTC-PD). A CS $J = 2 \hbox{--} 1$ line at 98 GHz was observed toward a high-mass star-forming region W 51 ($\mathrm{H}_2 \mathrm{O}$). We compared the obtained spectra with those obtained with a conventional Gunn oscillator as an LO. As a result, the line parameters, such as the width, intensity, and frequency, were consistent with each other. The results presented here show that the photonic LO is a very promising source for heterodyne receivers in millimeter–and submillimeter-wave astronomy.

The comb signal and its Fourier transform

In this paper, we study the aperiodic comb signal from the point of view of the Fourier transform. The comb is very important in the theory of ideal sampling. The knowledge of its properties is crucial for the establishment of suitable interpolation schemes. Here, we present sufficient conditions so that the Fourier transform of an aperiodic comb is an aperiodic comb. We use this result to propose: (1) an alternative approach to the definition of an almost periodic signal and its anharmonic Fourier series; (2) a generalisation of the Shannon–Whittakker sampling/reconstruction for the irregular sampling case. Application of this theory to pulse duration modulation and pulse position modulation is also presented.

A Digital Noise Reducer for Encoded NTSC Signals

Until recently, most of the random noise problems that have long plagued television engineers could only be attacked at the source. The recent advent of practical digital television technology, however, now makes it possible to process a television signal on an element-by-element basis. The noise reduction system described in this paper uses a digital frame store operating as an adaptive recursive filter, under the control of a motion detector employing comb filtering and signal modification. Using these techniques, an improvement of 15 dB can readily be achieved without introducing objectionable artifacts into the picture. Among the many areas of application of the digital noise reducer are: improvement of ENG material (both live and taped, especially at low light levels); processing of multigeneration 1-in or 2-in videotape; microwave reception; CATV satellite reception; telecine film-grain reduction (monochrome and color); and off-air reception.