Self-sweeping Laser Research Articles

Microwave-free BOTDA based on a continuous-wave self-sweeping laser.

This Letter presents the first demonstration, to our knowledge, of a Brillouin optical time domain analysis (BOTDA) system based on a self-sweeping fiber laser. The unique feature of such laser sources is the generation of a narrowband tunable radiation with a small (6 MHz) tuning step and a reasonably large tuning range (3 GHz) making self-sweeping lasers extremely attractive for use in BOTDA systems. Since the wavelength tuning occurs solely due to internal processes occurring in the laser cavity, there is no need to use complex current/temperature control and feedback systems. This makes it possible to completely eliminate microwave devices, such as electro-optic modulators, from the BOTDA design. In this work, distributed temperature measurements with sensing line length of 25 km, spatial resolution of 10 m, and sensitivity of 2°C is demonstrated in a BOTDA system based on an Er-doped self-sweeping laser. The described approaches can reduce the complexity and overall cost of the BOTDA systems.

Amplitude modulation ranging achieved by self-sweeping fiber laser in a short distance

A self-sweeping fiber laser ranging system was proposed and experimentally studied, which proved the capability of distance measurement based on the continuous wave self-sweeping pulse. Thanks to the beat-frequency signal exists in the process of central wavelength switchover, self-sweeping laser can generate amplitude modulation light without any extra active modulators. Therefore, the phase difference in laser flight times between the reference light and the reflected light can be extracted from the time domain. In experiment, we measured the phase difference and calculated the distance from different spatial lengths less than 25 cm, and the results were analyzed based on the baseline result acquired from an interferometry method. Endowing laser ranging with self-sweeping laser creates a platform that combines the individual advantages of the traditional ways of ranging, which show a promising accuracy and the ability to cross scenes.

Ultra-Narrow-Bandwidth Single-Frequency Self-Sweeping Ytterbium-Doped Fiber Laser

The sterling merits of single-frequency self-sweeping fiber laser in linewidth, compact design, wavelength and pulse dynamics make it a promising tool for laser measurement. In this letter, we proposed an ultra-narrow-bandwidth ytterbium-doped self-sweeping fiber laser by using a reflector including collimator and mirror. The self-sweeping state was achieved by a 2 m long fiber saturable absorber, and was confirmed by the unique pulse intensity. The central wavelength of laser located at 1065.382 nm along with a self-sweeping range within 2 pm. The radio frequency spectrum observed the relaxation oscillation and beat frequency signals. Especially, the nonlinear saturable absorption feature was confirmed by fitting the intervals of beat frequency signals. The behaviors of relaxation oscillation peaks were summarized and fitted by the empirical formula. This work provides a new self-sweeping laser tool with a narrow bandwidth and can be applied in laser ranging and tunable microwave signal generation.

Time-resolved mode analysis in Er-doped self-sweeping ring fiber laser

We present here a detailed study of the mode composition of an Er-doped ring self-sweeping fiber laser. Time-resolved heterodyne measurements prove that the radiation consists of slightly overlapping rectangular mode packets, each of which corresponds to generation of individual longitudinal modes. Optical frequency of each following packet sequentially increases by two free spectral ranges of the laser cavity. The presence of parametric processes during sequential growth of the neighboring mode packets is experimentally demonstrated for the first time in a self-sweeping laser. Instantaneous linewidth of the laser radiation does not exceed 4 kHz during every step of single longitudinal mode generation, which is also accompanied by a small change of the optical frequency (chirp) of ∼40 kHz associated with an integral change of the laser cavity refractive index. The results are useful for calculations and modeling of laser systems based on dynamic gratings.

Regular mode-hopping dynamics in Erbium-doped ring fiber laser with saturable absorber

We report on transition from single longitudinal mode operation to regular mode-hopping dynamics in Erbium-doped ring fiber laser with a saturable absorber. The laser can be switched from wavelength stabilization (with stability time of up to tens of seconds) to a wavelength self-sweeping regime (in the range of about 20 pm near 1560 nm) with an increase of the pump power and with elongation of the absorbing fiber length. The observed effect is associated with an interplay of amplitude and phase dynamic gratings. Additionally, a continuous wave single-frequency self-sweeping operation in an Erbium-doped fiber laser is demonstrated for the first time. The results are useful both for single-frequency and self-sweeping laser development.

Laser vibrometer-rangefinder based on self-sweeping fiber laser.

A vibrometer-rangefinder based on principles of coherent optical frequency-domain reflectometry (C-OFDR) is experimentally demonstrated. A self-sweeping ytterbium-doped fiber laser, which does not require any spectrally selective elements and drivers for wavelength tuning, with a sweeping range of 1056-1074 nm is used as a tunable source of probe radiation for the C-OFDR measurements. We demonstrate the possibility of measuring target vibrations in the frequency range from 2 Hz to 5 kHz with an amplitude of down to ∼5 nm at a distance of up to ∼13 m. The maximum measurable vibration frequency is limited by the instability of the self-sweeping laser parameters in the time domain and is estimated as ∼7.5 kHz.

Electrically driven frequency blue-chirped emission in Fabry–Perot cavity quantum cascade laser at room temperature

We present a method to produce a fast frequency swept laser emission from a monolithic mid-infrared laser. A commercially available Fabry–Pérot cavity quantum cascade laser (QCL) operating at a wavelength of 8.15 μm was electrically driven by a current pulse with a 10 μs duration and a slow front rising time of ∼2 μs. Due to the switching of the lasing emission from the vertical to the diagonal transition in the QCL and a strong quantum-confined Stark effect energy shift of the diagonal transition, the frequency of the emitted light was blue-shifting as the injection current continues to raise above the threshold. The temporal evolution of the laser spectrum was measured by a high-resolution step-scan Fourier transform infrared spectrometer. The blue-chirped emission was strongly influenced by the heatsink temperature due to the high thermal sensitivity of the threshold current and slope efficiency. By optimizing carefully the QCL operating temperature and the amplitude of the current pulse, we demonstrate a high-speed self-sweeping laser emission under room temperature operation conditions, reaching the spectral tuning range of ∼25 cm−1 within 1.8 μs.

Spectral characterization technique of self-organized distributed feedback in a self-sweeping fiber laser.

We report on spectral characterization technique of self-organized dynamical distributed feedback (DDFB) in a self-sweeping Yb-doped fiber laser. The DDFB is originated from gain and refraction index gratings formed (recorded by standing waves) in the laser's active medium and dynamically changes during frequency self-sweeping operation. Dynamic nature of the feedback requires fast characterization (reading) of corresponding reflection spectrum. The reading process can be separated from the recording one in time due to sufficiently long characteristic time of the dynamical gratings. The DDFB spectra are measured during off-state of the self-sweeping laser with a tunable probe radiation near the dynamical grating reflection maximum. The spectra reconstructed in a range of ~1 GHz from a sequence of shorter probe wave scans have narrow sharp peak with width of ~50 MHz and reflectivity of about 0.1%. A good agreement between theory and experimental results is demonstrated.

Fibre loop mirror with FBGs for the scanning range stabilisation in a fibre self-sweeping laser

A new scheme is suggested for stabilising scanning range boundaries of a self-sweeping fibre laser with fibre Bragg gratings (FBGs) and a fibre loop mirror. The mechanism for stabilising stop and/or start scanning boundaries is based on the formation of additional selective losses and/or reflection depending on the FBG position relative to the fibre mirror. It is shown that the simultaneous implementation of two selectors reduces fluctuations of the self-sweeping boundaries to several picometres, which is less by one-two orders in magnitude as compared with a case when the FBGs are absent. The approach suggested allows one to enhance predictability of emission wavelength tuning in fibre self-sweeping lasers, which is very important in practical applications.

Michelson mode selector for spectral range stabilization in a self-sweeping fiber laser.

We report on spectral range stabilization in a self-sweeping laser by adding a narrowband fiber Bragg grating (FBG) to the output mirror in the Michelson configuration. The effects of FBG reflectivity and optical path difference in the Michelson interferometer on the laser spectral dynamics are investigated. Optimization of the interferometer allows us to demonstrate broadband (over 16nm) self-sweeping operation and reduction of the start and stop wavelength fluctuations by two orders and one order of magnitude (∼100 and 15 times) for start and stop bounds, respectively (down to several picometers). The proposed approaches significantly improve quality of the spectral dynamics and facilitate application of the self-sweeping lasers.

All-fiber Brillouin optical spectrum analyzer based on self-sweeping fiber laser.

We proposed and demonstrated an all-fiber scheme for optical spectrum measurement based on stimulated Brillouin scattering and frequency self-sweeping laser without external driver and frequency tunable elements. The resolution and measuring range of proposed analyzer is measured to be 23 MHz and 5 THz respectively. The ways for improvement of the device characteristics are discussed.

Optimization and control of the sweeping range in an Yb-doped self-sweeping fiber laser

Influence of the laser cavity parameters (an active fiber length and output coupling losses) and the temperature of elements (active fiber and pump laser diode) on the sweeping range in an Yb-doped self-sweeping laser is investigated. The obtained results show that the sweeping spectral region is shifted to shorter wavelengths for shorter active fibers and with increasing absorbed power. This allows one to obtain self-sweeping operation in a broad range within a ytterbium gain bandwidth from 1028 to 1080 nm. At the same time, there are optimal cavity parameters at which the sweeping span is the broadest (>20 nm). Good agreement between the experimental sweeping range and the calculated maximum gain wavelength is demonstrated.

Application of a Self-Sweeping Yb-Doped Fiber Laser for High-Resolution Characterization of Phase-Shifted FBGs

Application of a self-sweeping self-pulsing laser for high-resolution spectral analysis of π-shifted FBGs has been demonstrated for the first time. It is shown that the self-sweeping laser generates nearly single-frequency radiation during a single pulse. This fact allows us to characterize the shift-induced dip of 14 MHz width in the FBG reflection spectrum. Nonlinear deformation of the measured ultranarrow spectral structure with increasing incident power is observed. Sampling interval during the scan is shown to be limited by pulse to pulse frequency shift, which is equal to the cavity mode spacing of 6 MHz. The main feature of the applied technique is its simplicity due to the absence of special elements for frequency scanning.

Tm-Ho co-doped all-fiber brand-range self-sweeping laser around 19 μm

We present a brand-range self-sweeping Tm-Ho co-doped fiber laser experimentally. The laser's center wavelength sweeps periodically around 1.9 μm with self-sweeping range of 4 nm ~17 nm, and the sweep rate can be changed in the range of 0.4 nm/s ~1.5 nm/s. The sweep range increases and the sweep rate declines when the pump power rises.