Synchronous nanosecond and femtosecond pulses delivered from a low-repetition-rate Er-doped fiber laser mode-locked by nonlinear polarization evolution is experimentally proposed. Here, the repetition rate is set as ~4.5 MHz by introducing sufficiently long fiber in a ring cavity. By fully exploiting long fiber and anti-saturation absorption characteristics, it is experimentally observed that dissipative-soliton-resonance pulse with the nanosecond-level pulsewidth and femtosecond soliton pulse synchronously propagate in the same cavity. Besides, the pulsewidth of dissipative-soliton-resonance pulse and laser output power could be tailored by finely configuring the bidirectional pump powers. These results provide deep understanding of low-repetition-rate pulse laser and an intriguing way to obtain tunable dual-scale synchronous pulses, indicating the high potential for multiple-pulse laser processing and so on.
We proposed an absolute distance measurement method with a large non-ambiguity range based on a polarization-multiplexed dual-comb fiber laser. By fully exploiting the intracavity linear loss based gain profile tilting and residual birefringence, polarization-multiplexed dual-comb pulses with tunable repetition rate difference and overlapping spectra in the 1530-nm gain region are obtained. The repetition frequency difference could be continuously tuned from ~89 to ~194 Hz. The alternative sampling under different repetition rate difference is experimentally verified to be effective approach to extend the non-ambiguity range in the single-cavity dual-comb ranging. The non-ambiguity range could reach thousands of kilometer while the precision could reach at least on the order of hundreds of micrometers. These results indicate a simple and intriguing route with a free-running laser source to obtain ranging with large non-ambiguity range, showing high potential in the applications such as satellite formation flying, large-scale 3D surface morphology measurement and so on.
We report on an all–fiber polarization maintained (PM) erbium-based figure-9 structured nonlinear amplifying loop mirror mode-locked laser where a stable carrier-envelope offset frequency (fCEO) signal was detected via a fiber-packaged f-2f interferometer. At the pump power of 145 mW at 976 nm, the oscillator delivers an average output power of 6.13 mW with a repetition of 88.6 MHz and a signal-to-noise ratio of 79 dB. The spectrum of 3 dB bandwidth was measured to be 54.3 nm. Furthermore, an average power of 270 mW was obtained by a two-stage cascaded amplifier, corresponding to a single pulse energy of 3.05 nJ. Then, the lasing was injected into a 40-cm-long PM highly nonlinear fiber after optimizing the length of dispersion compensated PM single-mode fiber. At last, an fCEO beat note signal exceeding 34 dB was obtained, which implies the system can be potentially used in all–fiber optical frequency comb.
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.
The Photon Doppler Velocimetry system is widely used in transient high-speed experiments, due to its advantages of non-contact measurement, low cost, high frequency response and good traceability. When the measured target accelerates from 0 to tens of km/s within a few hundred nanoseconds, its signal frequency can reach tens of GHz, so its dynamic range and time resolution capabilities are greatly challenged. The time-stretched Photon Doppler Velocimetry uses the time-stretched characteristics of pulsed lasers to achieve high-frequency signal scaling, improves the time resolution capability of the Photon Doppler Velocimetry, and breaks the Nyquist limit of analog-to-digital conversion technology. The article completes the experimental verification and system error analysis of the time-stretched Photon Doppler Velocimetry for the simulation study of the time-stretched Photon Doppler scale factor.
In the past decade, the research on optical frequency comb and its applications has achieved rapid development, bringing revolutionary progress to the metrology field. Frequency stabilization and absolute frequency measurement of the CW laser through optical frequency combs is very important for establishing new length standards. A tunable near-infrared CW laser is phase locked to a commercial optical frequency comb referring to a Rubidium atomic clock for frequency stabilization. A home-made comb is phase locked to a hydrogen maser for measuring the absolute frequency of the frequency stabilized CW laser obtained. The procedure of frequency stabilization and absolute frequency measurement based on optical frequency combs is demonstrated and the uncertainty and stability of the frequency stabilized CW laser are estimated.
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.
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
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, 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.
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.
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.
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 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.
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