A single-cavity triple-comb all-fiber laser is proposed by wavelength/polarization multiplexing. A variable optical attenuator is introduced to equalize the 1530-nm and 1550-nm gain profile of erbium-doped fiber for dual-wavelength pulses. Their repetition rate difference reach kHz level. Meanwhile, by further adjusting the intracavity polarization state, polarization-multiplexed dual-comb pulses with tens-of-Hz repetition rate difference in the 1550-nm gain region are obtained. The more than one-order-of-magnitude difference between the maximum and minimum repetition frequency difference and qualified passive mutual coherence of triple-frequency pulses is highlighted. These results indicate a highly potential triple-comb source for multiple-comb metrology such as triple-comb ranging and frequency measurement and so on.
In this work, we demonstrate a single-walled carbon nanotubes-based wavelength multiplexed fiber laser, which generates dual-comb pulse in the train of soliton rain. The fiber laser cavity is manipulated in repetition frequency of 16.58 MHz, 3 dB spectral bandwidth of 8.4 nm. Two asynchronous pulses constitute the soliton rain pulse sequences, which the intensity difference is about 5.72 dB between the dual frequencies. A piece of graded-index multi-mode fiber as a filter based on the multi-mode interference effect is introduced into cavity to improving the signal to noise ratio to ~62 dB, and locate the central wavelength of the dual-comb at 1556.7 nm and 1561.5 nm. The repetition rate difference of the dual-frequency is about 169 Hz with the resolution bandwidth of 1 Hz. The time delay of the dual-frequency pulse detected by cross-correlation method is 5.78 ms, which is well matched with the results in radio frequency spectrum. Different from the stable period of the general cross-correlation signal, our experimental results show several different sub-periods due to the existence of the drifting solitons in the soliton rain sequences. Meanwhile, the number of different sub-periods in the correlation decreases from six to three as the pump power reduced from 100 mA to 97.3 mA. Our work provides a new sight into the quasi-steady multi-soliton dynamics process in fiber lasers, and will be promising solutions for interference ranging, and synchronization and timing.
Due to the simple configuration, qualified passive coherence between pulses, and cost-effective characteristics, single-cavity dual-comb sources attract increasing research interest. Actually, such lasers have been experimentally verified in dual-comb metrology such as dual-comb frequency measurement and spectroscopy. Unlike the single-cavity dual-comb fiber laser multiplexed in other dimensions such as wavelength, direction and mode-locked mechanism, polarization-multiplexed pulses own the unique characteristics of overlapping spectra, intrinsic spectral coherence, and tunable repetition rate difference. They are beneficial for the simplification of additional optical amplification and the satisfaction of versatile requirements of dual-comb metrology. Here, we demonstrated a single-wall carbon nanotube saturable absorber mode-locked Er-doped fiber laser to emit wavelength-switchable polarization-multiplexed dual-comb pulses. The intracavity loss is carefully tuned by an additional optical variable attenuator to define the oscillation windows. In both the 1530- and 1550-nm gain regions, spectral-overlapping, polarization-multiplexed pulses are experimentally obtained with the fine configuration of the intracavity state of polarization. The polarization dynamics and tunable repetition rate difference are experimentally revealed. The repetition rate difference is at the tens-of-hertz level, which is somewhat lower than that of the reported polarization-multiplexed fiber laser with additionally introduced polarization-maintaining fiber. Since there are no additional birefringent media, the polarization mode dispersion for polarization-multiplexed pulses is attributed to the residual birefringence. Moreover, the passive mutual coherence is also highlighted. There results provide a simple yet effective way to design switchable and versatile single-cavity dual-comb pulses.
In our work, we experimentally demonstrate wavelength multiplexed dual-comb pulses based on multi-modal interference effect in a passively single-walled carbon nanotube mode-locking all fiber ring laser. The laser cavity achieves a variety of dual-wavelength mode-locked states by switching the polarization controller in the laser cavity. A piece of 25 cm long graded-index multi-mode fiber as a filter based on the multi-mode interference effect is introduced into cavity to fixing wavelength and to improving the signal to noise ratio. With optimized length of multi-mode fiber, we observed the two different filter state which located at 1559 nm and 1562 nm, 1561 nm and 1563 nm respectively in the different polarization dual-comb states. With suitable filtering state by stretching the multi-mode fiber, the two asynchronous pulse sequences coexist with diverse operation, which propagate with singlet and double pulses, respectively. The repetition rate of the laser is 16.59 MHz and the time period corresponding to the asynchronous pulse is ~60 ns. The repetition rate difference of dual-wavelength states reaches 100 Hz. In addition, we recorded the output modulation state of the laser cavity. Our research provides experimental basis for optical fiber sensing, wavelength division multiplexing communication system and high resolution spectroscopy.
With the advantages of high resolution, high sensitivity, wide spectral coverage and rapid measurement, the dual-comb spectroscopy technology has developed rapidly in the field of molecular and atomic spectroscopy. This article firstly uses various methods such as balanced optical cross-correlation method and beat frequency with ultra-stable laser to measure the phase noise of the dual optical comb system for different optical comb locking schemes. Then the absorption spectra of C2H2 were measured for the dual comb system under different locking schemes. The phase noise obtained under different locking schemes is compared, and the C2H2 absorption spectrum under each scheme is calculated at the same time, so as to obtain the influence of phase noise on the spectral resolution in the dual-comb spectrum measurement.
Optical frequency combs in dissipative Kerr soliton state can exist stably in a high Q-factor nonlinear cavity when a continuous wave pump laser is coupled to the microresonator as long as the double balance between anomalous cavity dispersion and kerr nonlinearity, and cavity loss and parametric gain is realized. However, due to the thermal instability of the microresonator, the generation and survival of the DKS requires keeping the pump at effective red-detuned regime. In this paper, in order to overcome the thermal instability when the intracavity power changes as the pump wavelength swept from blue-detuned to red-detuned with respect to a resonance of the microresonator, we adopt the thermal-assisted method, using an additional stable narrow linewidth laser as an assisted laser. We have studied the different mechanism of Optical frequency combs generation based on an opposite-directional pump configuration, According to the experimental results, two different initial comb states observed in the Si3N4 microresonator, but only one initial state can lead to stable Optical frequency comb generation. By adjusting the input power and stopping wavelength of the assisted laser and the pump laser respectively and properly, we observe the “step-like” transmission in our Si3N4 microresonator.
In frequency-modulated continuous-wave (FMCW) ladar, researchers usually linearize the output of the ladar’s tunable laser by a reference interferometer for high-accuracy measurement, but the reference interferometer need to be precisely calibrated. The H13C14N cell is a universal tool to complete the calibration. Here, we use the optical frequency comb instead of the traditional HCN gas cell to calibrate the path-length difference of the reference interferometer for higher precision. In this article, we present a detailed introduction of the experiment for path-length difference measurement and data processing method. The finally experimental results show that this method can give a micron precision, the standard deviation is 2.560e-5.
We demonstrate an enhancing near-infrared photoresponse in sulfur-supersaturated silicon photodiodes. We obtain microstructured silicon doped with supersaturated sulfur by femtosecond laser ablation in SF6 atmosphere. Next, we introduce annealing process to activate the dopants and improve the material quality. Then we fabricate n+ n photodiodes using the microstructured silicon. We find that the spectral response of the photodiodes exhibits gain from 400 nm to 1200 nm under the reverse bias, and under the condition of 10 V reverse bias, the photoresponse can be up to 3.69 A/W and 2.45 A/W at the wavelength of 965 nm and 1064 nm, respectively. While for the ordinary silicon photodiode, the spectral response has no gain under the reverse bias, and the photoresponse is less than 0.7 A/W and 0.35 A/W at 965 nm and 1064 nm, respectively. Our results suggest that the sulfur-supersaturated silicon diode has the great potential to improve the performance of silicon detectors in the infrared range.
As a thermal reference source, plane blackbody radiation source is widely used in infrared instrument calibration. In practical application, it is required that the normal spectral emittance of the plane blackbody radiation source should be greater than 0.95. However, the plane blackbody radiation source made by traditional sand blasting process cannot meet the requirement. Here we demonstrate a new manufacturing process for the plane blackbody radiation source. We selected aluminum material with a thickness of 10mm as the substrate, and then we roughed the substrate surface by femtosecond laser micromachining. Next, we sprayed on the prepared surface with heat resistant coatings and finished the manufacturing process. In comparison, we also made the plane blackbody radiation source by the sand blasting process with the same substrate and heat resistant coatings. Compared with the samples treated by sand blasting, we find that the samples treated by femtosecond laser show much smoother spectral emittance curves and higher spectral emissivity, and the average spectral emissivity is higher than 0.95 from 8μm to 18μm at the temperature of 500°C. Our results suggest that femtosecond laser micromachining is an effective way to obtain high quality plane blackbody radiation source.
In order to solve the problem that the length of gain fiber is difficult to be determined, and implement the femtosecond pulse features of high average power and narrow pulse width in optical measurement, the effect of erbium-doped fiber length on the pulse features is studied. In the simulation analysis, combined the rate equation of two-level system with the nonlinear Schrodinger equation describing the ultrafast pulse propagation, the variation trend of average power and pulse width with erbium-doped fiber length is numerically studied. According to the two-level model, a variable gain coefficient curve with the fiber length is obtained and applied to ultrafast pulse propagation model by polynomial curvefitting method to improve the simulation accuracy. In the experiment, a 980nm pump source is used to build a singlestage forward amplification system, and the fiber truncation method is employed to verify the numerical simulation. The results show that the simulation is consistent with the experimental data. When the average power of femtosecond pulse signal light is 50mW and the pump light is 1000mW, the optimum fiber length is 55~100cm. Compared with traditional simulation method of using fixed gain coefficient, the introduction of variable gain coefficient has an impact on simulation parameters p and d, which present the gain effect and dispersion effect in transmission model.
As a bridge connecting microwave frequency and optical frequency, the femtosecond optical frequency comb plays an important role in absolute 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 tendency to replace Ti:sapphire femtosecond optical frequency comb in some applications. Especially, due to the spectrum can be extended to visible light, fiber femtosecond optical frequency comb has a wider application prospect in the field of absolute optical frequency measurement. A frequency measurement system is set up based on an Er-doped femtosecond fiber comb. A hydrogen clock is used as a frequency standard, the optical frequency comb is traced to the hydrogen clock. Then an absolute frequency measurement of an acetylene-stabilized laser is realized by using this highly stable optical frequency comb. In addition, a narrow spectrum with a central wavelength of 633nm is achieved by Raman shifts and frequency doubling. The frequency and stability of the 633nm wavelength secondary standard are measured by beating
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.
An optical frequency comb based on a 250 MHz home-made Er-doped fiber femtosecond laser is presented in this paper. The Er-doped fiber laser has a ring cavity and operates mode-locked in femtosecond regime with the technique of nonlinear polarization rotation. The pulse duration is 118 fs and the spectral width is 30 nm. A part of the femtosecond laser is amplified in Er-doped fiber amplifier before propagating through a piece of highly nonlinear fiber for expanding the spectrum. The carrier-envelope offset frequency of the comb which has a signal-to-noise ratio more than 35 dB is extracted by means of f-2f beating. It demonstrates that both carrier-envelope offset frequency and repetition frequency keep phase locked to a Rubidium atomic clock simultaneously for 2 hours. The frequency stabilized fiber combs will be increasingly applied in optical metrology, attosecond pulse generation, and absolute distance measurement.
Femtosecond laser has been demonstrated to be a prominent tool to manufacture micro scale structure. In the processing, the focusing lens is usually used as the concentrated tool to assemble the original beam to the tiny spot to provide enough energy for ablation. What is more, different focal length means the diverse scale of the focused spot. In common use, various sizes of the spot are required to adjust to the multifarious profiles and substituting the focus lens is the general method. There is no doubt that changing the lens is a fussy job and frequent replacing the lens may cause the lack of stability. In this paper, we report the defocus of the lens to modify the scale of the spot and it is proved to be an effective way to vary the diameter of the focused spot without changing the focus lens.
We demonstrate a time-of-flight absolute distance measurement method based second harmonic generation using dual-comb with different repetition rates. A distance of about 8m is measured, compared with a laser absolute tracer, the maximum deviation is 19μm at 100ms acquisition time.
Two vibration measurement methods with femtosecond pulsed laser based on the optical cross-correlation technique are presented independently in this paper. The balanced optical cross-correlation technique can reflect the time jitter between the reference pluses and measurement pluses by detecting second harmonic signals using type II phase-matched nonlinear crystal and balanced amplified photo-detectors. In the first method, with the purpose of attaining the vibration displacement, the time difference of the reference pulses relative to the measurement pluses can be measured using single femtosecond pulsed laser. In the second method, there are a couple of femtosecond pulsed lasers with high pulse repetition frequency. Vibration displacement associated with cavity length can be calculated by means of precisely measuring the pulse repetition frequency. The results show that the range of measurement attains ±150μm for a 500fs pulse. These methods will be suited for vibration displacement measurement, including laboratory use, field testing and industrial application.
The background and principle of zero-crossing point locking technology are introduced in this paper. An experimental locking system is designed to realize fast locking of zero-crossing point, and the results of locking is studied by analyzing zero-crossing point locking signal. In the distance measurement of femtosecond pulsed laser, a crystal produces the balanced cross-correlation (BCC) signal, which signifies the time offset of the target pulses with respect to the reference pulses. By continuously pulling this signal to zero-crossing point, the locking system provides a closed loop control process, which ensures the stability of the zero-crossing point and the precision of measurement. This locking system is mainly made up by five sections. As a core section of system, P-I circuit can optimize the locking state by changing parameters. A frequency counter referenced to the rubidium atomic clock is used to measure the pulse repetition rate with a stability of 10-12 in the sampling rate of 10s in 24 hours, which is helpful to analyze the measurement precision. In the experiment, the result of zero-crossing point lock can reach to 15mV, in other words, the range of amplitude variation can be reduced to less than 15mV after locking. With the repetition rate data evaluated, the jitter of the pulse repetition rate is within 25Hz in the sampling time of 15s after locking the zero-crossing point. It is proved that the locking system designed has a high practical value in the distance and vibration measurement of femtosecond pulsed laser.
The absolute distance measurement was experimentally demonstrated by using the fiber femtosecond optical frequency comb in air. The technique is based on the measurement of cross correlation between reference and measurement optical pulses. This method can achieve accuracy better than the commercial laser interferometer. It is attained sub-micrometer resolution in large scale measurement by using the fiber femtosecond optical frequency comb. It will be benefit for future laser lidar and satellite formation flying mission.
Flattop annular beam has been predicted with good character over an increasing application, but the generating of flattop annular beam is rarely mentioned by academic article. In our paper, an optical refractive system, which is designed to achieve flattop annular beam, are proposed. The cone prism is commonly used to get an annular beam, however, the beam intensity distribution is non-uniform. In our design, an additional aspheric lens is placed in front of the cone prism along the optical axis. The lens parameters are theoretically analyzed and well optimized to homogenize the optical field. Furthermore, to lower the requirement of machining accuracy, a pair of aspheric lenses is also designed, which can be used independently to generate flattop annular beam. It combines the function of cone prism and aspheric lens, so as to replace them both. The performance of the implementations has been demonstrated in detail. Simulation result shows that the proposed design is effective and feasible. It is hope that our work would be helpful in related fields. Flattop annular beam, Aspheric lens, Cone prism
The new technique known as “The femtosecond frequency comb technology” has dramatic impact on the diverse fields of precision measurement and nonlinear optical physics. In order to acquire high-precision and high-stability femtosecond comb, it’s necessary to stabilize the repetition rate fRep and the offset frequency f0. This article presents the details of stabilizing and controlling the comb parameter fRep and finally phase lock the repetition rate of femtosecond laser to a radio frequency reference, derived from an atomic clock. In practice, the narrower the bandwidth of lock system (close-loop system), the higher stability we can achieve, but it becomes easier to be unlocked for external disturb. We adopt a method in servo unit to avoid this problem in this paper. The control parameters P and I can be adjusted and optimized more flexibly. The lock steps depend on the special servo system make it easier to find the right parameters and the lock becomes more convenient and quickly. With this idea, the locked time of repetition rate can be as long as the mode-locking time of the laser. The stability of laser can be evaluated by allan deviation. In this research, the contrast of stability of fRep between the locked laser and the unlocked is given. The new lock system is proved reasonable.
The effect of air dispersion on the femtosecond pulsed width is studied in this paper. We present the pulsed width variation with central wavelength, grating period and distance, respectively. Then the air dispersion compensation scheme based on the high-density transmissive grating is put forward to compress the pulsed width of femtosecond laser. The diffracted efficiencies variation with the groove depth and the grating period under the condition of TE and TM polarization state are also given through simulation. The scheme has advantages of compact volume and convenient operation.
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