Optical Frequency Comb Generation in Monolithic Microresonators ................... Kippenberg

As a bridge connecting microwave frequency and optical frequency, femtosecond laser has important significance in optical frequency measurement. Compared with the traditional Ti-sapphire femtosecond optical frequency comb, with the advantages of compact structure, strong anti-interference ability and low cost, the fiber femtosecond optical frequency comb has a wider application prospect. An experiment of spectrum broadening in a highly nonlinear photonic crystal fiber pumped by an Er-fiber mode-locked femtosecond laser is studied in this paper. Based on optical amplification and frequency doubling, the central wavelength of the output spectrum is 780nm and the average power is 232mW. With the femtosecond pulses coupled into two different photonic crystal fibers, the coverage of visible spectrum is up to 500nm-960nm. The spectral shape and width can be optimized by changing the polarization state for satisfying the requirments of different optical frequencies measurement.

<sec>In this paper, we demonstrate an optical frequency comb (OFC) based on an erbium-doped-fiber femtosecond laser, for the measurement of cavity ring-down spectroscopy (CRDS) with wavelengths of 1064, 1083, 1240, 1380, 1500, 1600, 1750 and 2100 nm. We adopt a multi-branch structure to produce high power at the specific wavelengths to meet the requirement for application in the spectral measurement. The OFC is developed by using a mode-locked fiber ring laser based on the nonlinear amplifying loop mirror mechanism. The laser is self-starting by introducing a nonreciprocal phase bias in the cavity and insensitive to the environmental perturbation. Using the chirped pulse amplification and highly nonlinear fibers, the broad spectra at the specific wavelengths are obtained. By optimizing the parameters of the pulses, the power of per mode at each target wavelength is greater than 300 nW.</sec><sec>The<i> </i><i>f</i><sub>rep</sub> is obtained by detecting the output of the femtosecond laser directly, while the<i> </i><i>f</i><sub>ceo</sub> is detected by <i>f</i>-2<i>f</i> interference. The signal-to-noise ratio of the<i> </i><i>f</i><sub>ceo</sub> is about 35 dB with a 300-kHz resolution bandwidth. By controlling the intra-cavity electro-optic modulator and piezoactuator , the <i>f</i><sub>rep</sub> is stabilized with high bandwidth and large range (about megahertz bandwidth and 3 kHz range). The <i>f</i><sub>ceo</sub> is stabilized by using feedback to the pump current of the femtosecond laser dynamically. The in-loop frequency instability degree of the <i>f</i><sub>ceo</sub>, evaluated by the Allan deviation, is approximately 4.95 × 10<sup>–18</sup>/<i>τ</i><sup>1/2</sup> at 1 s and integrates down to 10<sup>–20</sup> level after 2000 s, while that of the <i>f</i><sub>rep</sub> is well below 5.85 × 10<sup>–13</sup>/<i>τ</i>. The all polarization-maintaining erbium fiber-based femtosecond optical frequency comb with multi-application branches we demonstrate in this paper is efficient and reliable for many other applications including optical frequency metrology and optical atomic clocks.</sec>

Practical optical frequency measurement system around 1.5 μm based on an acetylene-stabilized laser-locked optical frequency comb

Length is one of the fundamental physical quantities and its precise measurement is very important for science and technology. Laser-based distance metrology technique is a powerful tool, which is widely applied in geodetic monitoring, environmental monitoring and precision measurement engineering including in space applications. Since the invention of the optical frequency comb (OFC) at the beginning of this century, it has been used as a versatile tool for many applications, such as frequency metrology, spectroscopy, etc. OFC has played an important role in distance metrology enabling the developed techniques to achieve high accuracy in long distance measurement. In the time domain, OFC is a pulse train, which can measure distance with time of flight method or correlation detection method. In the frequency domain, spectrum of OFC consists of a series of discrete lines with equal frequency difference. Methods of dispersive interferometry, multi-heterodyne interferometry, and multi-wavelength interferometry are the techniques proposed and demonstrated in spectral domain distance metrology. Though distance metrology techniques based on OFC have been developed for tens of years, there are still challenges that need to be overcome. In most applications the measurements are completed in the air, where the results are influenced by refractive index of air. As the OFC is a multi-wavelength laser source, refractive index of air is difficult to determine. The other challenge is complexity and high cost of measurement system. Until now, to my knowledge, there is no commercial rangefinder which is based on OFC. The large size, complex configuration and high price are the bottlenecks restricting OFC’s entrance into industrial applications.

The precise measurement of difference frequencies between two lasers in a terahertz (THz) region is a basic technique for optical frequency standards and for many other applications, such as dense WDM communication systems. Frequency difference measurements based on the beat signal between two lasers become quite difficult when the frequency difference exceeds the detector bandwidth. An optical frequency comb (OFC) generator with an electro-optic (EO) phase modulator inside a Fabry-Perot cavity can overcome this problem. The OFC generator creates a large number of sidebands, which can be applied for frequency markers. In conventional OFC generation, modulation sidebands appears on both sides of the carrier, which is a disadvantage for the efficient use of frequency bandwidth. The limit of the frequency difference measurement with this scheme depends on the power degradation in the sidebands. We propose an alternative technique for generating OFC which employs a dual-electrode Mach-Zehnder Modulator (D-MZM) inside a fiber recirculating loop to generate optical single sideband (SSB). This enables the efficient use of the frequency bandwidth. An optical amplifier is used in the loop to compensate for the losses.

We study optical frequency comb (OFC) generation using cascaded injection of semiconductor lasers in this work. The OFC generation system is operated in two cascaded optical injection stages. When a master laser optically injects into the first stage with proper injection power and frequency, period-one (P1) dynamics are invoked in an optically injected semiconductor laser of the first stage. Another semiconductor laser in the second stage is then optically injected by the P1 dynamics. With proper injection power adjusted in the second stage, the P1 dynamics are regenerated, and the semiconductor laser relaxation oscillations (ROs) become undamped so that subharmonic oscillations appear. Because a subharmonic oscillation frequency is half of an oscillation frequency of the P1 dynamics, extra optical frequency components appear in the middle of the adjacent optical frequency components of the P1 dynamics, thus signaling OFC generation. The OFC signals exhibit at least 15 comb lines, resulting in a bandwidth greater than 140 GHz. Microwave comb signals are obtained after photodetection, although the microwave linewidth is on the order of a few megahertz because of the semiconductor laser noise. Thus, we propose a cascaded injection-locking scheme to stabilize the P1 dynamics and OFC signals. We have demonstrated pure microwave generations with a linewidth of less than 3 Hz and low phase noise.

We demonstrated the application of optical frequency combs in the realization of programmable microwave photonic filters. A reconfigurable high-order PF filter based on a wide-bandwidth optical frequency comb was designed, and the increasing of the tap (comb tooth) numbers results in the increasing of the order of the microwave photonic filter. In this article, the repetition frequency of the optical frequency comb is 12.5 GHz, and the optical spectral coverage reaches 25.6 nm. The order of the RF filter obtained in the experiment reaches 256, the free spectrum range (FSR) reaches 11 GHz, the 3dB-passband width is from 300 MHz to 2.6 GHz, and the sidelobe suppression is as high as 26 dB. A progressive optical pulse shaper was used to program the tap weights, which allow us to shape the bandpass of the filter. In addition, by adjusting the spectral distribution of the optical frequency comb, two types of RF filters with different passband shapes can be realized, namely, Gaussian and Sinc filters.

In the fields of ultra-wideband satellite communication, integrated radio frequency, radar and other national defense and military at present, it is necessary to realize the interconversion between baseband signals and radio frequency signals in Ka or even U band and L band to meet the processing requirements of RF front-end. Traditional electronic technology usually uses multistage local vibration mixing to realize frequency conversion, which is complicated and accompanied by serious nonlinearity and noise accumulation. As a kind of multi-wavelength light source, optical frequency beam can provide stable multiple local oscillations in the optical domain, and move the baseband/RF signals to the optical domain to achieve flexible mixing processing. In this paper, an UWB microwave photonic mixing technology based on optical frequency comb is innovatively proposed. UWB octave-spanning up-conversion from 10MHz to 60.01 GHz and down-conversion can be achieved by using only 20 GHz microwave driver, which effectively improves the RF preprocessing capability of UWB transmitter in the field of national defense and military.

Direct-comb spectroscopy techniques uses optical frequency combs (OFCs) as spectroscopic light source. They deliver high sensitivity, high frequency resolution and precision in a broad spectral range. Due to these features, the field has burgeoned in recent years. In this work we constructed an OFC-based cavity-enhanced Fourier-transform spectrometer in the near-infrared region and used it for a line-shape study of rovibrational transitions of CO perturbed by Ar. The highly sensitive measurements spanned the wavenumber range from 6270 cm−1 to 6410 cm−1, which covered both P and R branch of the second overtone band of CO. The spectrometer delivers high-resolution surpassing the Fourier-transform resolution limit determined by interferogram length, successfully removing ringing and broadening effects caused by instrumental line shape function. The instrumental-line-shape-free method and high signal-to-noise ratio in the measurement allowed us to observe collisional effects beyond those described by the Voigt profile. We retrieved collisional line-shape parameters by fitting the speed-dependent Voigt profile and found good agreement with the values given by precise cavity ring-down spectroscopy measurements that used a continuous-wave laser referenced to a stabilized OFC. The results demonstrate that OFC-based cavity-enhanced Fourier-transform spectroscopy is a strong tool for accurate line-shape studies that will be crucial for future spectral databases.

Optical frequency combs based on erbium-doped fiber lasers are attractive tools in precision metrology due to their inherent compactness and stability. Here we study a femtosecond Er:fiber comb that passively eliminates the carrier–envelope phase slip by difference frequency generation. Quantum statistics inside the all-fiber soliton oscillator governs its free-running performance. Active stabilization of the repetition rate supports a subhertz optical linewidth and does not necessitate additional intracavity elements. Direct locking to an optical atomic frequency standard enables generation of a 100 MHz microwave signal with a stability of 3.4 mHz maintained over 15 min.

Microcombs, optical frequency combs generated with integrated high Q-factor optical resonators, have emerged as a transformative tool in optics, offering compact chip-scale frequency comb sources that enable advances in metrology, spectroscopy and communications. Limiting factors include efficiency, robustness against environmental noise and long-term stability. A key requirement for many applications is the locking of two major degrees of freedom of the comb, the carrier offset and repetition rate frequencies, on which the positions of the comb teeth depend.

The basic operation, implementation and control of femtosecond laser optical frequency combs will be presented. The application of frequency combs in various scientific and technological fields will be described. 150-word Biography: Scott Diddams received the B.A. in Physics from Bethel College, St. Paul, MN in 1989 and the Ph.D. degree in Optical Science from the University of New Mexico, Albuquerque in 1996. From 1996 through 2000, he did postdoctroral work at JILA, University of Colorado. In 1998 Dr. Diddams was awarded a National Research Council fellowship to work with Dr. John Hall (also at JILA) on the development of optical combs for frequency metrology. The techniques developed at that time have had a significant impact on the fields of ultrafast optics and optical frequency metrology. Since 2000, Dr. Diddams has been a staff member and project leader at the National Institute of Standards and Technology (NIST) where he and his coworkers have continued the development of optical frequency combs for optical clocks, waveform generation, low-noise microwave synthesis, spectroscopy and other applications. Dr. Diddams was a co-recipient of a Department of Commerce gold medal for revolutionizing the way frequency is measured and was additionally the recipient of the Presidential Early Career Award in Science and Engineering (PECASE) for his work on optical frequency combs. He is a Fellow of the OSA and APS and a member of IEEE. Article not available.

The analogy between optical frequencies and RFs leads to a novel technique for RF photonic signal processing using a femtosecond laser comb modulated by a traveling-wave tunable filter, such as an acoustooptic tunable filter (AOTF) or a novel electrooptic tunable filter (EOTF). In this new scheme, the recent history of an applied RF waveform to-be-processed slides into the tunable filter and a femtosecond pulse train diffracts off the moving acoustooptically or electrooptically induced dielectric grating, producing a shaped optical pulse train with each shaped pulse as a compressed replica of the RF waveform contained within the device aperture that is sped up by the ratio between the center optical and RF frequency. For a CW RF tone input, only a narrowband group of the frequency comb lines in the incoming laser comb is spectrally filtered and modulated due to the phase-matching condition in the filter and is simultaneously Doppler shifted by that RF due to the traveling-wave grating obeying the conservation of energy. This allows us to use the traveling-wave tunable filter as a spectrally mapped Doppler-shifted modulator that encodes different RF components onto the corresponding optical frequency comb lines. RF signal processing can then be performed by using optical techniques to manipulate the spectrally modulated laser comb. To reconstruct the processed RF signal, the Doppler-shifted and optically processed pulse train is heterodyne detected by beating with a reference femtosecond pulse train from the same laser source. A high repetition rate femtosecond laser comb is modulated by an AOTF to experimentally demonstrate this novel RF photonic signal processing technique. We demonstrate an RF tunable bandpass/notch filter, an RF down-converter, and an RF jammer nuller as novel applications of ultrafast lasers.

Examination of various issues related to the generation and application of optical frequency combs (OFCs) is an important branch of modern optoelectronics. Some of the proposed OFC generation methods apply acousto-optic (AO) devices. The AO devices are used either as the element devoted to the OFC phase stabilization or they play the role of an optical radiation frequency shifting element in the frequency-shifting loop (FSL) scheme. The results of two experiments related to the application of AO cells in the FSL scheme are represented in this paper. The first experiment confirms the previously proposed effect of AO mismatch influence on all the OFC characteristics. The second experiment shows the possibility of tunable AO dual-comb downconversion with a single AO device.

Frequency comb lasers provide a phase coherent link between radio frequency sources and optical frequencies, now making spectroscopy possible at optical and near-infrared frequencies at an extremely high accuracy of 10−17 [1]. Using the comb laser for direct excitation obviates the need for a continuous wave laser at the transition wavelength, as has been demonstrated in beam experiments, vapour cells and magneto-optical traps [2–4].