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We experimentally demonstrate tunable Brillouin comb generation in a ring cavity composed of a piece of tellurite single mode fiber. The tunable Brillouin comb lines are separated with frequency space of 7.9 GHz, which is equal to the Brillouin frequency shift in the tellurite fiber. Due to the high Brillouin gain coefficient of the tellurite fiber, a tellurite fiber as short as 200 m was inserted into the ring cavity. Cascaded stimulated Brillouin Stokes and anti-Stokes lines are generated in the ring cavity leading to multi-wavelength Brillouin comb generation bi-directionally. By tuning the narrow-linewidth tunable laser, we can tune the Brillouin comb in the wide range of 1533 nm to 1565 nm.

We perform a sub-Doppler spectroscopy of the D 2 line of 87Rb atoms in a vapor cell using a mode-locked 1 GHz femtosecond laser and a cw diode laser. The experiment explores the high resolution of the frequency comb and the possibility of the atoms interacting with more than one mode of the comb. We probe several velocity groups or a selective group of atoms using a velocity-selective and a repetition rate spectroscopy, in a co-propagating configuration of the beams. By detecting the diode transmission we can distinguish the resonances associated with the hyperfine structure of the 5 P 3 ∕ 2 level and follow the competition of the optical pumping processes, in the regime that the two beams have approximately the same intensities. The results are well described in the frequency domain picture, using rate equations for a four level system interacting with three cw lasers.

An approach is reported to acquire an optical frequency standard based on precise frequency locking between optical frequency comb and continuous wave. A stabilized erbium-fiber-based frequency comb is created by locking to the two-photon absorption resonance of rubidium atoms. An Erbium-doped distributed-feedback narrow linewidth lasers is presented with an actively stabilized all fiber Mach–Zehnder interferometer. With this structure, many lines of the optical frequency comb can make a contribution to the error signal. The signal-to-noise ratio (SNR) is enhanced compared to traditional way which utilizes only one comb line. An acousto-optic modulator (AOM) is used to compensate the noise in fiber which is induced by environment. The relative frequency offset instability is 2 × 10−13 at 1 s and 6 × 10−15 at 1000 s. This stabilized system can be wisely used in optical communication at 1550 nm. The main advantage of this structure is the ability for long distance frequency transmission. Without dispersion, the stability and the robustness of the transmission is greater than the scheme only with the frequency comb. We transferred the signals along 10km optical fiber link with it. The relative frequency stability loss of the frequency signal is 1.1 × 10−15 at 1 s and reaches 7.6 × 10−19 at 1000 s.

We successfully constructed a pulse laser whose fundamental repetition rate is 2.6-GHz in Q-switched mode-lock oscillation. To the best of our knowledge, this is the highest repetition rate among silica glass fiber lasers. The laser cavity was constructed by a 40-mm-long Nd 3 + -doped single-mode silica glass fiber (Nd-SMF) and a SESAM as a saturable absorber. The core glass of the Nd-SMF was fabricated by a zeolite method and its concentration of Nd2O3 was 1.25 wt%. The maximum laser power reached 27.2 mW, and its slope efficiency and lasing thresholds were 19.6% and 7.3 mW. The peak wavelength of the oscillation was 1062.7 nm. The RF spectrum and the time-dependent waveform from a 20-GHz-bandwidth oscilloscope show a 2.6-GHz repetition rate of an Nd-SMF laser. We also discuss the conditions for achieving a CW-mode-lock (CML) oscillation, which is critical laser output P out of 180 mW. Such a simple multi-GHz repetition rate laser will be available to make a compact frequency comb or a laser oscillator that will achieve ablation-cooled material removal in laser processing.

A simple and quasi-tunable optical frequency comb (OFC) generator is proposed and experimentally demonstrated using a C-band passively Fabry-Perot quantum dot mode-locked laser and a dual-driven LiNbO 3 Mach–Zehnder modulator. A 16-nm bandwidth OFC with 81, 58 and 30 comb lines at frequency interval of 23.3 GHz, 35 GHz and 70 GHz respectively is obtained experimentally. Measured average optical signal to noise ratio of 10-dB bandwidth OFCs is 36.3 dB, 38.5 dB and 40.8 dB at frequency interval of 23.3 GHz, 35 GHz and 70 GHz, respectively. Besides, single-sideband phase noise of the 23.3 GHz and 35 GHz frequency comb is −110 dBc/Hz and −102 dBc/Hz at an offset of 1 kHz, respectively. RF linewidth of the 23.3 GHz and 35 GHz OFC is about from 275 Hz to 289 Hz. This is considered a very simple OFC generator with a broadband and seamless spectrum.

Here we investigate a phase-coupled parity-time symmetric plasmonic system, and theoretically achieved the all optical on-chip plasmonic diode using the coupled mode theory. The proposed symmetrical system consists of one loss cavity and one gain cavity each coupled with the waveguide, and we find that the controllable amplification of the input field can be achieved by changing the power coupling fraction between the resonators and the waveguide. Moreover, this loss–gain symmetric system could work as a frequency comb filter, and the operation on the device could be controlled by tuning the coupling strength between the two plasmonic cavities by tuning the coupling distance between the cavities and the waveguide.

In this study, we attempt the separation of overlapped interference fringes arising from multiple pulse trains of a femtosecond optical frequency comb for length measurement. Based on an optical experiment, we test the performance of the separation of two overlapped interference fringes by time division for an absolute length measurement, which is about one adjacent pulse repetition interval length. We compare our results with those of a commercial He–Ne interferometer system. The two sets of results show an agreement within 0.7 µm.

In this paper, we design a nitrobenzene-core photonic crystal fiber (NC-PCF) with all-normal dispersion profile. The nonlinear coefficient of the designed NC-PCF at wavelength 1560 nm is as high as 1.43 W−1 m−1, which is 100 times larger than that of the traditional silica PCF. We investigate the influences of the pump center wavelength, pulse width, peak power, and fiber length on the supercontinuum (SC) generation. The highly coherent SC spanning from 1.0 to 2 . 3 μ m is generated when the pump pulses with the center wavelength of 1560 nm, peak power of 20 kW, and width of 120 fs are launched into the 3 cm long NC-PCF. Finally, the SC-based optical frequency comb is obtained by assuming a 50 pulse pump source at a repetition rate of 1 GHz.

The femtosecond fiber-based optical frequency combs have been proved to be powerful tools for investigating the energy levels of atoms and molecules. In this paper, an Er-doped fiber femtosecond optical frequency comb has been implemented for studying the polarization dependence of 5 S –5 D two-photon transitions in thermal gas of atomic rubidium 87 using an entirely symmetrical optical configuration. By changing the polarization states of the counter-propagating light beams, the polarization dependence of direct two photon transition spectrum is demonstrated, and a dramatic variation (up to 5.5 times) of the two-photon transitions strength has been observed. The theory for the polarization dependence of two photon transition based on the second-order perturbation was established, which is in good agreement with the experimental results. The measurement results indicate that the polarization state manipulation with the existing frequency comb is used for femtosecond optical frequency comb based two photon transition spectroscopic purposes, which will improve the precision measurement of the absolute transition frequency and related applications.

A novel approach to realize photonic compressive sensing (CS) with a multi-tap microwave photonic filter is proposed and demonstrated. The system takes both advantages of CS and photonics to capture wideband sparse signals with sub-Nyquist sampling rate. The low-pass filtering function required in the CS is realized in a photonic way by using a frequency comb and a dispersive element. The frequency comb is realized by shaping an amplified spontaneous emission (ASE) source with an on-chip micro-ring resonator, which is beneficial to the integration of photonic CS. A proof-of-concept experiment for a two-tone signal acquisition with frequencies of 350 MHz and 1.25 GHz is experimentally demonstrated with a compression factor up to 16.

We experimentally demonstrate a delay subsystem that utilizes a combination of three types of fiber-Bragg gratings (FBGs), wavelength conversion, and wavelength multicasting using a frequency comb source. While the linearly chirped FBG together with a channelized dispersion compensation one, provide the continuous delay, an array of standard FBGs utilizes the multicast copies to extend the delay in steps determined by the array geometry. Following the characterization of the group-delay dispersion performance of the FBGs, a 10 Gbaud quadrature phase-shift keying (QPSK) signal is continuously delayed by 2.2 ns and discretely delayed in 3 steps of ∼ 2 ns, to provide a total gap-less delay of 8.1 ns. The delayed signal is finally recovered to its original wavelength with an optical signal-to-noise ratio (OSNR) penalty of ∼ 2.3 dB. Furthermore, we show that the penalty can be reduced to 1.2 dB after equalization and report delaying a 20 Gbaud QPSK signal. The demonstrated high-quality delay performance is quite scalable. The continuous delay can be extended to tens of nanoseconds, limited only by the performance of the used components, while the discrete delay is determined by the number of multicast copies and propagation distance between the reflections in the array of FBGs.

Master–slave common carrier phase recovery (MS-CCPR) is a method for reducing computational complexity of the phase recovery in multi-channel systems where the channels have correlated phase noise. However, these systems will suffer performance penalties if there are phase-noise differences between the channels. Since MS-CCPR schemes rely on phases of the subchannels being same, the intra-channel phase-drift will inevitably influence the design and performance of such schemes. In addition, fiber nonlinearities may introduce further different phase perturbations on the subchannels of a superchannel. This paper presents a master–slave carrier recovery method with compensation of the phase-noise differences based on the blind phase search method for optical frequency comb-based WDM systems. Specifically, we investigated in details using simulations, for the first time, the impact of this phase-noise differences on the performance of master–slave scheme in long-haul transmission systems with laser and nonlinear phase noise effects. In the MS-CCPR strategy used, the slave subchannels are preprocessed with the master–slave scheme, then a low-complexity slave phase-tracker is used to correct the phase-noise differences remaining on them. The concept has been numerically verified in simulations for square 16-QAM format, where the BER limit for soft-decision FEC could still be achieved in a link of 3000 km.

A new approach for the generation of an optical frequency comb, based on chirping of modulators, is proposed and numerically demonstrated. The setup includes two cascaded Mach–Zehnder Modulators (MZMs), a sinusoidal wave oscillator, and an electrical time delay. The first MZM is driven directly by a sinusoidal wave, while the second MZM is driven by a delayed replica of the sinusoidal wave. A mathematical model of the proposed system is formulated and modeled using the Matlab software. It is shown that the number of the frequency lines is directly proportional to the chirp factor. In order to achieve the highest number of frequency comb lines with the best flatness, the time delay between the driving voltages of the two MZMs is optimized. Our results reveal that at least 51 frequency lines can be observed at the output spectrum. In addition, 27 of these lines have power fluctuations of less than 1 dB. The performance of the proposed system is also simulated using a split-step numerical analysis. An optical frequency comb, with tunable frequency spacing ranging from 5 to 40 GHz, is successfully generated.

We propose and simulate an on-chip mid-IR frequency comb generator based on the four-wave mixing (FWM) effect in a coupled silicon microring resonator. The resonator is formed by suspended ridge waveguides that are designed to have low group-velocity dispersion. A continuous-wave pump laser coupled into the resonator excites other longitudinal modes of the resonator through FWM-based parametric oscillations. In comparison with the device based on a single-ring resonator, we numerically demonstrate that the pump threshold can be dramatically reduced by introducing an auxiliary ring resonator to compensate the phase mismatch between the pump and the generated comb components. The proposed frequency comb generator has a low pump threshold of ~40 mW and a high efficiency of ~25% at the pump power of 100 mW.

We investigate the dynamics of a pair of short laser pulse trains propagating in a medium consisting of three-level Λ-type atoms by numerically solving the Maxwell–Schrodinger equations for atoms and fields. By performing propagation calculations with different parameters, under conditions of electromagnetically induced transparency, we compare the propagation dynamics by a single pair of probe and coupling laser pulses and by probe and coupling laser pulse trains. We discuss the influence of the coupling pulse area, number of pulses, and detunings on the probe laser propagation and realization of electromagnetically induced transparency conditions, as well on the formation of a dark state.

A flat and broadband optical frequency comb generator based on two cascaded intensity modulators is proposed and experimentally demonstrated. In this scheme, we first use even or odd harmonic sidebands to generate optical frequency comb, so the bandwidth can be improved. By adjusting the power of the microwave signals and the direct current bias applied to the modulators, a flat and broadband optical frequency comb can be achieved. The scheme is relatively simple and adjustable, where the frequencies spacing vary with microwave frequency applied on modulators. 15 and 20 comb lines with the comb flatness within 1 dB can be generated. The generated spectrums can meet the requirement of OFDM modulation, and can be used for high capacity optical transmission systems in the future.

We investigate the problem of two-level atoms driven by an ultrashort pulse train in the frequency domain. At low intensity regime, we obtain a perturbative analytical solution that allows us to discuss the role of the mode number of the frequency comb near or at resonance in the temporal evolution of the atomic coherence. At high intensities, the effect of the number of modes is analyzed on the steady-state regime through numerical calculations.

Stable, near-infrared laser sources were generated by an electro-optic modulator with selected comb-mode seeding. The single comb mode was selected from the fiber laser comb using a Fabry–Perot cavity (FPC) of 1.25 GHz spacing, and injection locking technique. An electro-optic frequency comb (EOFC) with spacing of 25 GHz was generated as the side mode of the injection-locked comb mode. Using a single mode of the EOFC, we measured the frequency of the acetylene stabilized laser used as the calibration reference frequency light source for optical communication. The absolute frequency and estimated stability of the acetylene stabilized laser were 194 369 569 384.6 kHz and 2.3×10 −12 (average time 1 s), respectively.

An optical resonator was installed as an incoherent frequency comb generator into a two-dimensional single-shot profilometry and tomography system to increase the measurement range up to 30 times compared with a conventional white-light interferometer. The dynamic range of the sample set position was experimentally confirmed to be ∼28 mm, equivalent to the coherency of the resonator. Scanless measurement can be achieved through high-order comb interference. In this study, we demonstrated tomography with a resolution of ∼11 μm and effective high-order interference fringe discrimination.

We propose a novel and simple scheme to achieve NRZ-to-RZ format conversion and simultaneous wavelength multicasting based on a single-stage dual-arm electro-optic Mach–Zehnder modulator (MZM) and a short single mode fiber (SMF). The format conversion and wavelength multicast process are achieved by chirp compensation under the condition of generation of optical flat frequency comb. 40 Gb/s NRZ-to-RZ conversion with one-to-five multiple-wavelength channel multicasting and transmission of the NRZ and the converted signals over 200 km dispersion-managed fiber-link are successfully demonstrated by numerical simulation. Research results show that 40 Gb/s 2 ps RZ signal with wavelength-preserving can be obtained after format conversion. The converted RZ signal presents good transmission performance and can easily be multiplexed to 160 Gb/s using optical time division multiplexing (OTDM) technology. All the multicast channels can be error free after 50 km transmission. Besides, the conversion operation can also greatly reduce the timing jitter of the degraded NRZ signal due to the retiming function of the proposed scheme.

A high-resolution spectroscopy technique is proposed with an optical phase modulator combined with an interleaved optical frequency comb. The optical phase modulator and a frequency-locked laser light guarantee a spectral resolution less than 1 MHz on an absolute frequency axis. A wide measurement frequency range was realized using a 25 GHz optical frequency comb lying over a 4 THz frequency region. An extraction of single tooth intensity from the comb was realized by a heterodyne technique with a frequency-tunable laser used as a local oscillator. Also, the 25 GHz optical frequency comb was interleaved to generate four 100-GHz combs for removing the crosstalk from the 25 GHz neighboring sidebands in the teeth. This proposed spectroscopy technique was experimentally demonstrated with a resonator of less than 1 MHz linewidth and a H 13 C 14 N gas cell. Thus, a measurement frequency range higher than 4 THz (1530 nm–1560 nm) was confirmed with an effective spectral resolution 100 kHz order. In addition, the characteristics of the proposed system were compared with those of the previous system with a single-sideband (SSB) optical modulator.

We investigate the generation of high-rate optical pulse trains by spectral phase-only filtering of a frequency comb derived from an electrooptically phase-modulated continuous-wave laser. The technique is initially analyzed as a two-step filtering process. First, a fundamental pulse-train with repetition-rate equal to the modulation frequency is obtained by line-by-line phase-cancellation of the electrooptic frequency comb. Second, the temporal Talbot-effect is considered so that the output pulse repetition-rate is an integer multiple of the electrooptic modulation frequency. Nonidealities found in the fundamental train lead, in general, to multiplied trains with important degradations. We numerically analyze optimum modulation conditions for generation of output pulse trains with minimum peak-to-peak variations and/or maximum extinction level. On the other hand, a genetic algorithm is considered to numerically find optimum line-by-line phase-only filters that generate output multiplied trains with minimized degradations. Numerical simulations show that, in general, this second approach allows for improvement in the quality of the resultant multiplied trains, in terms of the uniformity degree and/or noise-level, compared with the pulse trains resulting from the Talbot-effect-based approach.

Real wedge interferometers of the Fizeau-type do not allow for fringes in case of a spectral broadband source – or in short: for white light fringes. Here, the use of a suitable frequency comb source will help to overcome this limitation on the one hand and on the other will offer the capability for enhanced phase sensitivity in high precision measurements of surface deviations. Frequency combs can be produced either by using a pulse train from a fs-laser or by passive filtering of the light emitted by a broadband source as a superlum-diode or a fs-laser. The frequency comb produced by a common fs-laser is extremely fine, i.e., the frequency difference of consecutive peaks is very small or the distance of consecutive pulses of the pulse train might be of the order of 1 m. Therefore, the pulse train produced by passive filtering of a broadband source is better adapted to the needs of surface testing interferometers. White light fringes are either applied for the profiling of discontinuous surfaces and/or can serve as an indication for the correct choice of multiplication factors in superposition interferometry. During the last decennium it became more and more clear that spatially incoherent sources provide better measuring accuracy in surface measurements due to the reduced influence of dust diffraction patterns. The advantage of laser illumination can nevertheless be maintained if the laser light is made spatially incoherent through moving scatterers in the light path. Here, we will discuss the application of spatially incoherent broadband light frequency filtered through a Fabry–Perot filter. The main applications are in the following fields: (1) surface profiling applications using two-beam Fizeau interferometers, (2) selection of single cavities out of a series of interlaced cavities, and (3) sensitivity enhancement for multi-beam interferometers for planeness or sphericity measurements. Some of the discussed possibilities will be experimentally demonstrated.

A phase modulation of an optical frequency comb has been applied to measure a fine spectrum in the 1.5 μm wavelength range by the optical heterodyne-detection method. The measurement frequency range covered 25 GHz, which satisfies the frequency interval of the optical frequency comb, with a spectral resolution of 1 MHz.

We demonstrate a Fourier-transform infrared (FTIR) spectrometer without moving parts based on frequency-combs where the interference of two coherent frequency-comb beams occurs via wavefront combination, thereby eliminating the need of a dielectric combiner. This enables dispersionless operation over eventually the complete THz-to-visible spectrum in a single instrument. Furthermore we demonstrate the use of a single GaSe crystal for generating both beams, in an implementation for the 22–36 THz mid-infrared range. Spurious cross-modulation effects and a way to eliminate them are experimentally demonstrated. The compact comb-FTIR can be equally well applied for spectroscopic microscopy as for long-path standoff chemical monitoring.

The generation of standard reference frequencies close to the ITU channels is essential for the frequency calibration of DWDM systems. This paper describes the generation of a reference frequency comb based on the combination of a cascaded four-wave mixing in two semiconductor optical amplifiers and Raman amplification in a dispersion-shifted optical fiber. As a result we have achieved a stable frequency comb with 36 reference lines separated by a constant frequency spacing of 177 GHz. The seed of the comb is combination of two narrow-linewidth semiconductor lasers which are locked to two absorption lines of the acetylene (12C2H2)., Ministerio de Educación y Ciencia, Comunidad de Madrid

We report frequency measurements at the rovibronic transition P(42)1-14 (772 nm) and R(114)2-11 (735 nm) from the electronic X 1 Σ g + → B 3 Π 0 u + transition of the iodine molecule 127I2 with the help of a frequency comb as a reference. By using Doppler-free saturation spectroscopy a frequency precision in the 7 × 10−10 region is reached and two iodine cells both operated at 550–600 °C are compared. To relate our results to other measurements, the absolute transition frequency of the hyperfine structure line P(148)1-14 a1 at 780 nm with an already known transition frequency was also determined.

We have developed a practical and precise frequency measurement system at 1.5 μm telecommunication band. An electro-optic-modulator based optical frequency comb is phase-locked to a dither-free acetylene-stabilized laser to realize an optical frequency comb with frequency uncertainty of 10 kHz (5 × 10 −11 ) and the linewidth of 15 kHz. The present frequency comb can be also used as an optical frequency reference grid defined by ITU-T (International Telecommunication Union, Telecommunication Standardization Sector). Using the present frequency measurement system, we have demonstrated the first optical frequency measurement of 12 C 16 O overtone absorption lines around 1.56 μm with the uncertainty of lower than 900 kHz.

The insertion of three continuous-wave optical frequencies in a low-dispersion optical fibre resulted in the generation of a high-repetition-rate train of ultra-short pulses and its simultaneous wavelength conversion. Two of the frequencies were spaced by ∼0.17 THz and multi-wave mixing generated a frequency comb to which is associated a train of 1.6 ps pulses. Wave-mixing between the generated comb and the third input optical frequency then converts the pulse train into different wavelengths. The Split-Step Fourier method was applied to numerically simulate the generation/wavelength conversion and results are in good agreement with experiments.

The hyperfine structure splittings of the P(10)14-1, R(15)14-1, and R(99)15-1 transitions at 585 nm, P(62)17-1 at 576 nm, and P(80)21-1 at 565 nm in 127 I 2 are measured by heterodyne spectroscopy using two dye lasers. In addition, the absolute frequencies of the hyperfine components P(10)14-1 a 15 and P(80)21-1 a 10 are determined using a self-referenced frequency comb. These frequencies are used in an experiment testing relativistic time dilation by laser spectroscopy on a fast ion beam.

We present a frequency comparison and an absolute frequency measurement of two independent I2-stabilized frequency-doubled Nd:YAG lasers at 532 nm, one set up at the Institute of Laser Physics, No vosibirsk, Russia, the other at the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. The absolute frequency of the I2-stabilized lasers was determined using a CH4-stabilized He-Ne laser as a reference. This laser had been calibrated prior to the measurement by an atomic cesium fountain clock. The frequency chain linking phase-coherently the two frequencies made use of the frequency comb of a Kerr-lens mode-locked Ti:sapphire femtosecond laser where the comb mode separation was controlled by a local cesium atomic clock. A new value for the R(56)32-0:a10 component, recommended by the Comite International des Poids et Mesures (CIPM) for the realization of the metre [1], was obtained with reduced uncertainty. Absolute frequencies of the R(56)32-0 and P(54)32-0 iodine absorp tion lines together with the hyperfine line separations were measured.

We have demonstrated optical frequency link of two frequency-stabilized laser diodes operating at 1542 nm (a rotational-vibrational line of isotope acetylene molecule) and 778 nm (an Rb two photon transition) using a two-color mode-locked fiber laser. The laser produces a frequency comb (300 fs pulse) in the 1560 nm region and, through the second harmonic generation (SHG) process, another frequency comb (100 fs pulse) in the 780 nm region. Beat note signals have been observed between each of the stabilized laser radiation and a mode of the mode-locked fiber laser in the vicinity of the stabilized laser frequency. The S/N ratios were 25 dB and 30 dB with 100 kHz resolution bandwidth for beat notes at λ=1542 nm and λ=778 nm, respectively. Preliminary result has shown the potential of the system for frequency measurements with the accuracy of below 100 kHz. This limitation will be overcome by improving the frequency counting system using a phase-locked tracking oscillator and installing a PZT element in the fiber laser cavity. Supposing the accuracy of the mode spacing is similar to the case of a mode-locked Ti:sapphire laser, the accuracy of kHz level is very presumable.

An injection-seeded Nd: YLF laser incorporating frequency shifted feedback generates a frequency comb with a 140 GHz bandwodth and a tunable frequency spacing. Near bandwidth-limited pulses of 31 ps duration have been produced in the seeded laser by matching the frequency shift to a multiple of the cavity roundtrip frequency.