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Supercontinuum Generation
Published in Mário F. S. Ferreira, Optical Signal Processing in Highly Nonlinear Fibers, 2020
The structure of the cavity modes present in the original laser output is preserved when a normal dispersion fiber is used to generate the SC [83]. This enables the generation of an ultra-broadband frequency comb, in which the separation between peaks corresponds to the microwave mode-locking frequency of the source laser. Each peak can be considered then as a potential transmission channel. This property was used by Takara et al. [84] to generate more than 1000 optical frequency channels with a channel spacing of 12.5 GHz between 1500 and 1600 nm. Following the same principle, 124 nm seamless transmission of 3.13 Tb/s (10-channel DWDM × 313 Gb/s) over 160 km, with a channel spacing of 50 GHz, was reported [85]. Raman amplification in hybrid tellurite/silica fiber was used in this experiment to improve gain flatness. More recently, a field demonstration of 1046 channel ultra-DWDM transmission over 126 km was realized using a SC multicarrier source spanning 1.54–1.6 nm, which was mainly generated through SPM-induced spectral broadening. The channel spacing was 6.25 GHz and the signal data rate per channel 2.67 Gb/s [13]. The conservation of coherence properties was also successfully employed by Sotobayashi et al. [85] to create a 3.24 Tb/s (84-channel 40 Gbit/s) WDM source of carrier-suppressed return-to-zero pulses.
Spectrometry
Published in Chandrasekhar Roychoudhuri, Causal Physics, 2018
Sometimes, the incident pulse under analysis may contain a distribution of optical frequencies S(ν). For example, a pulse from a mode-locked laser (see Chapter 7) can contain a set of cavity mode frequencies (frequency comb). When the grating output is recorded using a slow response detector under long time integration, then each frequency ν will create its own broad fringe. The composite spectrum can be mathematically represented as the convolution [5.13a] of the pulse response function D(ν, τ) with the frequency distribution function S(ν). () Dcomp.(ν,τ)=D(ν,τ)S(ν)
Selected Applications of Absorption Spectroscopy
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
The notion that (mode-locked) ultrashort picosecond- and femtosecond-pulse laser sources can be used for ultrahigh resolution and precision spectroscopy seems somewhat odd at first sight, recalling that the spectral bandwidth of such laser light pulses is rather broad. This is a consequence of the paradigm that mode locking actually is a frequency domain concept, while mode-locked lasers themselves and their applications are typically discussed in the time domain. The central concept of understanding the underlying phenomena is that the pulse train generated by a mode-locked laser exhibits a frequency spectrum that consists of a discrete, regularly spaced series of sharp lines across the broad gain spectrum; this frequency spectrum is now normally addressed as an optical frequency comb. This idea that a regularly spaced train of pulses in the time domain corresponds to a comb of narrow frequency spikes in the frequency domain, and might thus be used to excite narrow atomic or molecular transition resonances, was first realized and exploited in the late 1970s (see, e.g., Eckstein et al. 1978). With the advent of femtosecond-laser optical frequency-comb synthesizers in the late 1990s (see, e.g., Udem et al. 2002), measurements in the field of frequency-comb metrology and precision spectroscopy were enormously simplified, which led to an avalanche of uses in fundamental science and practical applications.
Flat frequency comb generation employing cascaded single-drive Mach–Zehnder modulators with a simple analogue driving signal
Published in Journal of Modern Optics, 2021
Mahmoud Muhanad Fadhel, Haroon Rashid, Abdulwahhab Essa Hamzah, Mohd Saiful Dzulkefly Zan, Norazreen Abd Aziz, Norhana Arsad
Optical frequency comb (OFC) generators can produce a series of equally spaced frequency lines [1], which is an attractive approach to generate multiple optical channels using a single light source. OFC has been utilized in many applications, including molecular spectroscopy, ranging and coherent LIDAR, optical clocks, and astronomical spectrograph calibration [2]. There are several methods to generate OFC, such as mode-locking, micro-resonator systems, and electro-optic modulators (EOMs) [3]. The EOM technique has recently shown increasing interest among researchers, owing to its simplicity in producing tunable frequency combs with high repetition rates and high power per comb lines [4, 5]. Furthermore, generating frequency sidebands, based on EOMs, has found remarkable applications in distributed optical fibre sensors (DOFS) [6–9]. Moreover, several modulators can be fabricated with low driving voltages and high modulation bandwidths [10].
Laser spectroscopy of 176Lu+
Published in Journal of Modern Optics, 2018
R. Kaewuam, A. Roy, T. R. Tan, K. J. Arnold, M. D. Barrett
Optical frequency measurements carried out in this work are obtained by referencing the lasers to an optical frequency comb. The frequency comb is stabilized to a GPS-disciplined, rf-oscillator, which has a specified accuracy over a timescale of . Frequency measurements of the clock laser on different days are consistent with the specification. We therefore use this value in determining the uncertainties in optical frequency measurements. We also note that all microwave and rf-oscillators used in this work were synchronized to the same oscillator used to stabilize the comb.
Dual-comb Spectroscopy for Laminar Premixed Flames with a Free-running Fiber Laser
Published in Combustion Science and Technology, 2021
Ke Xu, Liuhao Ma, Jie Chen, Xin Zhao, Qiang Wang, Ruifeng Kan, Zheng Zheng, Wei Ren
Broadband light sources like optical frequency combs (OFCs) are nowadays of extraordinary interest to researchers in many fields (Picqué and Hänsch 2019). In general, the OFC source is a special mode-locked laser that emits thousands of equidistant (separated by the pulse repetition frequency) narrow-linewidth (kHz) comb teeth (optical frequencies) over a broadband spectrum. Frequency comb spectroscopy is becoming an attractive candidate for optical sensing and spectroscopy as many absorption lines of different molecules could be measured simultaneously by OFCs with a high resolution over a broad spectral range (Cundiff and Ye 2003; Diddams, Hollberg, Mbele 2007; Udem, Holzwarth, Hänsch 2002). In particular, dual-comb spectroscopy (DCS) has become a powerful tool for high-resolution molecular spectroscopy by the use of two coherent OFCs (Abbas et al. 2019; Coddington, Newbury, Swann 2016; Fllinger et al., 2019; Nakajima, Hata, Minoshima 2019). As schematically shown in Figure 1, a pair of frequency combs is normally generated from two ultrafast mode-locked lasers, which are phase-locked but with a slight difference in their repetition frequencies (i.e., fr and fr + Δfr, respectively); here Δfr is the frequency difference between the two combs. One of the OFCs (signal comb) passes through the gas medium and beats with the second comb (local comb) on a fast photodetector to generate a series of interferograms. Such an optical arrangement produces a radio frequency (RF) comb spectrum by the multi-heterodyne of adjacent comb teeth from these two OFCs. This method could be treated as an all-static spectroscopic method that performs Fourier-transform interferometry without any moving parts (Coddington, Newbury, Swann 2016).