Optical Frequency Comb

—A new scheme for generatinga flat optical frequency comb using three Laser sourcesbased on cascading of Dual Drive Mach Zehnder Modulator (DD-MZM) and Electro Absorption Modulator (EAM) through an optical nonlinear fiber loop is explored in this paper. A single RF signal of 25 GHz having different amplitudes feed into the two arms of DD-MZM and then feed the signal into EAM to modulate the pulse train. By controlling the DC bias of both modulators as well as the RF signal frequency, the scheme is successful in generating evenly spaced 15 flat comb lines among 25 lines with a spectral power variation of about 2dB.The paper compares the performance of the proposed scheme against other methods where only one modulator was used. The scheme is also analysed for producing an optimum number of spectral lines based on number of sources, channel spacing and RF signal frequency. Finally, the performance of the system was successfully evaluated in a preliminary WDM setup of five channels/modes at 10Gb/s for a channel of 1 km length. Results in the form of BER and Q factor are presented in this paper.

Optical frequency combs (OFC) constitute an array of phase-correlated equidistant spectral lines with nearly equal intensities over a broad spectral range. The adaptations of combs generated in mode-locked lasers proved to be highly efficient for the calibration of high-resolution (resolving power > 50000) astronomical spectrographs. The observation of different galaxy structures or the studies of the Milky Way are done using instruments in the low- and medium resolution range. To such instruments belong, for instance, the Multi Unit Spectroscopic Explorer (MUSE) being developed for the Very Large Telescope (VLT) of the European Southern Observatory (ESO) and the 4-metre Multi-Object Spectroscopic Telescope (4MOST) being in development for the ESO VISTA 4.1 m Telescope. The existing adaptations of OFC from mode-locked lasers are not resolvable by these instruments.

Within this work, a fibre-based approach for generation of OFC specifically in the low- and medium resolution range is studied numerically. This approach consists of three optical fibres that are fed by two equally intense continuous-wave (CW) lasers. The first fibre is a conventional single-mode fibre, the second one is a suitably pumped amplifying Erbium-doped fibre with anomalous dispersion, and the third one is a low-dispersion highly nonlinear optical fibre. The evolution of a frequency comb in this system is governed by the following processes: as the two initial CW-laser waves with different frequencies propagate through the first fibre, they generate an initial comb via a cascade of four-wave mixing processes. The frequency components of the comb are phase-correlated with the original laser lines and have a frequency spacing that is equal to the initial laser frequency separation (LFS), i.e. the difference in the laser frequencies. In the time domain, a train of pre-compressed pulses with widths of a few pico-seconds arises out of the initial bichromatic deeply-modulated cosine-wave. These pulses undergo strong compression in the subsequent amplifying Erbium-doped fibre: sub-100 fs pulses with broad OFC spectra are formed. In the following low-dispersion highly nonlinear fibre, the OFC experience a further broadening and the intensity of the comb lines are fairly equalised. This approach was mathematically modelled by means of a Generalised Nonlinear Schrödinger Equation (GNLS) that contains terms describing the nonlinear optical Kerr effect, the delayed Raman response, the pulse self-steepening, and the linear optical losses as well as the wavelength-dependent Erbium gain profile for the second fibre. The initial condition equation being a deeply-modulated cosine-wave mimics the radiation of the two initial CW lasers. The numerical studies are performed with the help of Matlab scripts that were specifically developed for the integration of the GNLS and the initial condition according to the proposed approach for the OFC generation. The scripts are based on the Fourth-Order Runge-Kutta in the Interaction Picture Method (RK4IP) in combination with the local error method.

This work includes the studies and results on the length optimisation of the first and the second fibre depending on different values of the group-velocity dispersion of the first fibre. Such length optimisation studies are necessary because the OFC have the biggest possible broadband and exhibit a low level of noise exactly at the optimum lengths. Further, the optical pulse build-up in the first and the second fibre was studied by means of the numerical technique called Soliton Radiation Beat Analysis (SRBA). It was shown that a common soliton crystal state is formed in the first fibre for low laser input powers. The soliton crystal continuously dissolves into separated optical solitons as the input power increases. The pulse formation in the second fibre is critically dependent on the features of the pulses formed in the first fibre. I showed that, for low input powers, an adiabatic soliton compression delivering low-noise OFC occurs in the second fibre.  At high input powers, the pulses in the first fibre have more complicated structures which leads to the pulse break-up in the second fibre with a subsequent degradation of the OFC noise performance. The pulse intensity noise studies that were performed within the framework of this thesis allow making statements about the noise performance of an OFC. They showed that the intensity noise of the whole system decreases with the increasing value of LFS.

In this letter, we present a scalable high-speed W-band (75-110-GHz) fiber wireless communication system. By using an optical frequency comb generator, three-channel 8.3-Gb/s/ch optical orthogonal frequency-division-multiplexing (OOFDM) baseband signals in a 15-GHz bandwidth are seamlessly translated from the optical to the wireless domain. The W-band wireless carrier is generated from heterodyne mixing the OOFDM baseband signal with a free-running laser. A W-band electronic down-converter and a digital signal processing-based receiver are used. Three-channel QPSK-OFDM W-band wireless signals are transmitted over 0.5- and 2-m air distance with and without 22.8-km single-mode fiber, respectively, with achieved performance below the forward error correction limit.

Recent experimental work on semiconductor-based harmonically mode-locked lasers geared toward low noise applications is reviewed. Active, harmonic mode-locking of semiconductor-based lasers has proven to be an excellent way to generate 10 GHz repetition rate pulse trains with pulse-to-pulse timing jitter of only a few femtoseconds without requiring active feedback stabilization. This level of timing jitter is achieved in long fiberized ring cavities and relies upon such factors as low noise rf sources as mode-lockers, high optical power, intracavity dispersion management and intracavity phase modulation. When a high finesse etalon is placed within the optical cavity, semiconductor-based harmonically mode-locked lasers can be used as optical frequency comb sources with 10 GHz mode spacing. When active mode-locking is replaced with regenerative mode-locking, a completely self-contained comb source is created, referenced to the intracavity etalon.

This paper refers to a research about 5-channel wavelength division multiplexed passive optical network (WDM-PON) based on optical frequency combs generation (OFCG), using Mach-Zehnder modulator (MZM). The simulations were applied in standard single-mode fiber (SSMF) like an alternative for next generation-PON (NG-PON). OFCG using MZM has been widely studied due to the fact that the spectral spacing is directly related with the RF frequency, and the number of sideband components depends on the RF power. These parameters affect the bit-error rate (BER). Therefore, OFCG shows that suitable channel spacing is required for WDM-PON systems with high data-rate at 10 Gbps. In addition, WDM-PON based on OFCG using MZM at 2,488 Gbps can reach lengths up to 37 Km; this system also provides high performance over the link length up to 22 Km with BER <10−12, 10 Gpbs and flat optical spectrum.

Broadband sensing and spectroscopy in millimeter-wave and subterahertz frequencies can be facilitated with CMOS coherent detectors as a low-power, low-cost solution. This paper presents a fully integrated coherent detector with a broadband frequency comb as an on-chip reference signal. The receiver incorporates a frequency comb generator, which provides a broadband comb as a reference for heterodyne detection, and an NMOS field-effect transistor as the heterodyne detector. It also includes an on-chip elliptical antenna with a periodic structure which enhances its directivity up to +10 dBi without using a lens. The receiver detects any arbitrary spectrum from 50 GHz to 280 GHz with resolution of 2 Hz by tuning the comb spacing of the reference signal from 100 MHz to 5 GHz. The chip is implemented in a standard 65-nm CMOS process and it consumes 34 mW dc power.

—This paper presents the research work for a proposed novel scheme for generating a flat and equally spaced optical frequency comb using a simulated layout using Optisystem. Three pump sources at 1 nm wavelength apart are injected along with a 25 GHz RF signal having different amplitudes into a cascade of Dual-Drive Mach-Zehnder Modulator and Electro-Absorption Modulator and multiple highly nonlinear fiber loops. The initial scheme is using Four Wave-Mixing and yields a comb of 27 lines with uneven gain and a 6 dBm power deviation. After characterizing the response by exploring critical parameters such as pump power, frequency spacing, dispersion effect, Erbium Doped Fiber Amplifier and DC bias of the modulators, the researchers were able to reduce the power deviation down to 2 dBm at the expense of some comb lines. This final optimized scheme was then evaluated for a preliminary Wavelength Division Multiplexing setup having only five channels/modes at 10 Gb/s for a channel length of 1 km. Each of the received de-multiplexed channels was evaluated through recorded data in the form of Bit Error Rate and Quality-factor and the final results are shown to be quite promising.

In this letter, we present a scalable high-speed W-band (75-110-GHz) fiber wireless communication system. By using an optical frequency comb generator, three-channel 8.3-Gb/s/ch optical orthogonal frequency-division-multiplexing (OOFDM) baseband signals in a 15-GHz bandwidth are seamlessly translated from the optical to the wireless domain. The W-band wireless carrier is generated from heterodyne mixing the OOFDM baseband signal with a free-running laser. A W-band electronic down-converter and a digital signal processing-based receiver are used. Three-channel QPSK-OFDM W-band wireless signals are transmitted over 0.5- and 2-m air distance with and without 22.8-km single-mode fiber, respectively, with achieved performance below the forward error correction limit.

We present a new generation of compact and rugged mid-infrared (MIR) difference-frequency coherent radi-ation sources referenced to fiber-based optical frequency comb synthesizers (OFCSs). By coupling the MIR radia-tion to high-finesse optical cavities, high-resolution and high-sensitivity spectroscopy is demonstrated for CH 4 and CO 2 around 3.3 and 4.5 µm respectively. Finally, the most effective detection schemes for space-craft trace-gas monitoring applications are singled out.

We designed and demonstrated a power-efficient highly-integrated photonic system, requiring a total power consumption of 1.7 W and producing a spectrally-pure coherent optical signal with a wavelength range of 23 nm in the C-band. The system consists of a compact, low-power InP-based photonic integrated coherent receiver, microresonator-based Kerr frequency comb and agile electronic circuits. The photonic coherent receiver contains a 60-nm widely-tunable Y-branch local oscillator (LO) diode laser, a coupler, and a pair of photodetectors. It consumes a record-low (approximately 184 mW) electrical power. The optical frequency comb reference has excellent spectral purity, <4 kHz optical linewidth and good frequency stability. The spectrally-pure tunable optical source was produced by offset locking the on-chip local oscillator (LO) laser of the integrated receiver to this frequency comb source. A possibility of further stabilization of the frequency comb repetition rate by locking to an external radio frequency synthesizer was demonstrated.

The atmospheric chemistry community suffers a lack of fast, reliable and space resolved measurement for a wide set of very reactive molecules (e.g. radicals such as OH, NO3, BrO, IO, etc.). Due to their high reactivity, these molecules largely control the lifetime and concentration of numerous key atmospheric species. The concentrations of radicals are extremely low (ppbv or less) and highly variable in time and space. Measuring their concentration is often extremely laborious, expensive and requires heavy equipment (chemical sampling and treatment followed by mass spectrometry and/or chromatography). We recently introduced an optical spectroscopy technique based on a femtosecond laser oscillator, "Mode-Locked Cavity-Enhanced Absorption Spectroscopy", that we propose to develop into an instrument for in situ measurement of local concentration of traces of reactive molecules [1-3]. We have already demonstrated the possibility of measuring part in 1E12 by volume concentrations of radicals of high atmospheric interest, such as IO or BrO [4], as needed for monitoring these species in the environment. We apply cavity-enhanced absorption spectroscopy in the near UV range using a frequency-doubled Ti:Sa modelocked femtosecond laser. Efficient broadband injection of a high finesse cavity is obtained by matching this optical frequency-comb source to the comb of cavity transmission resonances. A grating spectrograph and a detector array disperse and detect the spectrum transmitted by the cavity carrying the absorption features of intracavity molecules. IO traces were obtained by mixing together controlled flows of gaseous iodine and ozone inside a high finesse cavity (F~6000). A Chameleon Ultra II ML-Laser (gracefully lent during 1 month by Coherent Inc.) was frequency doubled to address an absorption band of IO at 436 nm. A locking scheme allowed the cavity transmission to be smooth and stable. The transmitted light was dispersed using a high resolution (0.07nm) grating spectrograph (HR4 Jobin-Yvon) and collected with a cooled CCD linear array (1024 pixels). As the cavity finesse was measured by the ringdown time, and the IO absorption cross section is known, we could estimate that we had produced 0.2 ppbv of IO. The absorption noise level (7E-10cm-1 after 10s averaging) corresponded to a detection limit of 3 ppt par racine de Hertz. It is to be noted that the mirrors yielding a finesse of 6000 around 436 nm were centered at 460 nm, where the cavity finesse was measured to be about 20 000. Moreover, modern solid state ML lasers are compact and could function reliably in environmental research stations. The spectral range from 500 down to 225 nm, accessible by efficient frequency doubling and tripling, includes strong absorption bands of important species; in particular, given their known optical cross sections, we estimate detection limits in the range 1-10 pptv for IO, BrO, OClO, OIO, and OBrO, sufficient for environmental measurements. ClO, HONO, HCHO, NO2, SO2, and O3 should be detectable down to 10-100 pptvs. The OH radical should be visible at 308 nm below 0.1 pptv, not far from typical ambient concentrations. [1] T. Gherman and D. Romanini, 2002, Mode-locked cavity-enhanced absorption spectroscopy, Optics Express 10, 1033-1042. [2] T. Gherman, S. Kassi, A. Campargue, and D. Romanini, 2004, Overtone spectroscopy in the blue region by Cavity-Enhanced Absorption Spectroscopy with a Mode-Locked femtosecond laser: Application to acetylene, Chem. Phys. Lett 383, 353-358. [3] T. Gherman, E. Eslami, D. Romanini, S. Kassi, J.-C. Vial, and N. Sadeghi, 2004, High sensitivity broad-band mode-locked cavity-enhanced absorption spectroscopy: Measurement of Ar*(3P2) atoms and N2+ ions densities, J. Phys. D: Appl. Phys. 37, 2408-2415. [4] G. Méjean, S. Kassi, and D. Romanini, 2008, Cavity Enhanced absorption spectroscopy with an Optical Comb: Detection of atmospheric radicals in the near UV, Optics Letters. 33, 1231-1234 (2008)