Combined Fiber Laser Research Articles

Direct observation of Kramers–Kronig self-phasing in coherently combined fiber lasers

A highly stable coherent beam-combining system has been designed to measure self-phasing in fiber lasers due to nonlinear effects. Whereas self-phasing in previous coherent combination experiments has been principally attributed to wavelength shifting, these wavelength effects have been efficiently suppressed in our experiment by using a dual-core fiber with closely balanced optical path lengths. The self-phasing from nonlinear effects could then be measured independently and directly by common-path interferometry with a probe laser. The Kramers-Kronig effect in the fiber gain media was observed to induce a phase shift that effectively canceled the applied path length errors, resulting in efficient lasing under all phase conditions. This process was demonstrated to result in robust lasing over a large range of pump conditions.

Different supercontinuum generation processes in photonic crystal fibers pumped with a 1064-nm picosecond pulse

Picosecond pulse pumped supercontinuum generation in photonic crystal fiber is investigated by performing a series of comparative experiments. The main purpose is to investigate the supercontinuum generation processes excited by a given pump source through the experimental study of some specific fibers. A 20-W all-fiber picosecond master oscillator-power amplifier (MOPA) laser is used to pump three different kinds of photonic crystal fibers for supercontinuum generation. Three diverse supercontinuum formation processes are observed to correspond to photonic crystal fibers with distinct dispersion properties. The experimental results are consistent with the relevant theoretical results. Based on the above analyses, a watt-level broadband white light supercontinuum source spanning from 500 nm to beyond 1700 nm is demonstrated by using a picosecond fiber laser in combination with the matched photonic crystal fiber. The limitation of the group velocity matching curve of the photonic crystal fiber is also discussed in the paper.

Imposing narrow spectral bandwidth in a system of passively combined fiber lasers

We investigate intra-cavity coherent beam combining in a system of two fiber lasers when a spectrally selective element is inserted in one of the lasers. We found that narrow spectral content can be imposed on the other laser, resulting in narrowband operation of the system with high combining efficiency. Furthermore, the combining efficiency decreases as the imposed wavelength is farther away from the natural lasing wavelength of the system and depends on the gain imbalance between the lasers. Our results demonstrate, for the first time, the rich dynamics associated with the spectral mode competition in the intra-cavity coherently combined system.

Multichannel polarization stabilization for coherently combined fiber laser arrays

We demonstrate a simplified approach toward active polarization control in coherently combined laser architectures. By leveraging optical phase dithers applied by a phase controller, polarization error signals are generated for an entire laser array from a single beam sample of the combined output, enabling closed-loop polarization locking of non-polarization-maintaining fibers. The concept is shown to be compatible with both hill-climbing and synchronous multidither phase control methods. Simultaneous phase locking and polarization locking was demonstrated for a five-fiber array with >99% phasing efficiency and >20 dB polarization extinction ratio.

Automated co-alignment of coherent fiber laser arrays via active phase-locking

We demonstrate a novel closed-loop approach for high-precision co-alignment of laser beams in an actively phase-locked, coherently combined fiber laser array. The approach ensures interferometric precision by optically transducing beam-to-beam pointing errors into phase errors on a single detector, which are subsequently nulled by duplication of closed-loop phasing controls. Using this approach, beams from five coherent fiber tips were simultaneously phase-locked and position-locked with sub-micron accuracy. Spatial filtering of the sensed light is shown to extend the control range over multiple beam diameters by recovering spatial coherence despite the lack of far-field beam overlap.

Breath ammonia detection based on tunable fiber laser photoacoustic spectroscopy

A new detection method for ammonia in high concentration of CO2 and H2O is reported, which uses a wavelength modulated photoacoustic spectrometer based on a near-infrared tunable erbium-doped fiber laser in combination with an optical fiber amplifier. The multi-wavelength (1522.44 nm, 1522.94 nm and 1545.05 nm) photoacoustic signal measurement is established to detect multi-spectrum signal in samples. The problem of ammonia detection in high concentration of CO2 and H2O is resolved at atmospheric pressure. The minimum detection limit of 16 ppb (signal-to-noise ratio = 1) in simulated breath samples (5.3% CO2 and 6.2% H2O (100% relative humidity at 37°C)) is achieved.

Coherently combined fiber laser system delivering 120 μJ femtosecond pulses

We report on the coherent combination of two chirped pulsed fiber lasers. The beams coming from two 100 μm core diameter ytterbium-doped rod-type fibers were coupled in a Mach-Zehnder-type interferometer by means of a polarization beam splitter. Active stabilization of the interferometer was achieved by using a piezo-mounted mirror driven by a Hänsch-Couillaud polarization detection system. Pulses with 120 μJ energy and a compressed duration of 800 fs were obtained. This, compared with the 66 μJ coming from each single amplifier, results in a combining efficiency as high as 91%.

Experimental investigation of a seven-element hexagonal fiber coherent array

The phase noises of a 10-m polarization-maintaining (PM) fiber and a 10-W PM fiber amplifier are experimentally measured. The results indicate that the 10-m PM fiber with similar phase noise could be used to investigate the architecture of master oscillator power amplifier coherent combination. A seven-element hexagonal fiber coherent array is developed to investigate the far-field distribution and phase controlling technique of a coherently combining fiber laser array. A hexagonal prism is designed as the combining and splitting component to achieve a fill factor of 0.66. The hill climbing method is employed to detect and lock the element phase.

Operating regimes of an optical coupler combined fiber laser system

Rate equations have been used to study an optical coupler combined Er 3+-doped fiber laser system, in which a common mirror attached at one of the coupler output ports is shared by two constituent lasers. Three regimes may be identified for the system depending on the relative magnitude of the wavelength difference δλ between the two constituent laser fields to the reflection band of the fiber gratings. Taking into consideration the transmission features of the coupler-mirror combination, appropriate boundary conditions have been established according to the operation regimes of the compound system. After neglecting the backgrounds losses of the fiber, rate equations have been simplified to a set of algebra equations. Based on the algebra equations, we discussed in detail the case where the laser field generated by the first laser with a wavelength slightly different from that of the field oscillating inside the second laser, is injected into the other laser by the common mirror, leading to a decreased gain for the oscillating field.

Laser-jamming analysis of combined fiber lasers to imaging CCD

To complete a successful laser jamming to imaging charge coupled device (CCD) based on combined fiber lasers, the interactions between CCD and combined fiber lasers were analyzed in detail. The saturation and crosstalk thresholds of CCD were achieved, which are lower than 10 mW/cm 2. Through theoretical analysis and numerical simulations, the thermal processes under single pulse, multi-pulses and continuous laser irradiations were developed. The simulation results have proved the possibility of hard damage caused by multi-pulses and continuous laser irradiations. The combined fiber lasers is suitable to deploy optical saturation jamming at present. The further applications of combined fiber lasers need a more powerful laser source and a more accurate tracking and pointing system.

Accurate description of the beam from a coherently combined fiber laser array

By expanding the fundamental mode of a step-index fiber in terms of Laguerre–Gaussian modes, an accurate expression for the beam emitted by a fiber laser in free space has been obtained. Based on this, the coherently combined beam from a fiber laser array has been theoretically investigated. The results are compared with those obtained by the Gaussian approximation for different distances between adjacent elements, different numbers of elements, and different values of the normalized frequency V. The study shows that the errors introduced by the Gaussian approximation decrease with increasing distance between adjacent elements and increase with the number of elements. For V, the error first decreases and then increases with increasing V. These results indicate that an expansion in terms of Laguerre–Gaussian modes can be used for an accurate description of a coherently combined beam from a fiber laser array.

Laser-jamming effectiveness analysis of combined-fiber lasers for airborne defense systems

The laser-jamming effectiveness of combined fiber lasers for airborne defense systems is analyzed in detail. Our preliminary experimental results are proof of the concept of getting a high-power laser through a beam combination technique. Based on combined fiber lasers, the jamming effectiveness of four-quadrant guidance and imaging guidance systems are evaluated. The simulation results have proved that for a four-quadrant guidance system, the tracking system takes only two seconds to complete tracking, and the new tracking target is the jamming laser; for the imaging guidance system, increasing the power of the jamming laser or the distance between the target and the jamming laser are both efficient ways to achieve a successful laser jamming.

Toward visible cw-pumped supercontinua

We report a 1 mum cw-pumped supercontinuum that extends short of the pump wavelength to 0.65 mum. This is achieved by using a 50 W Yb fiber laser in combination with a photonic crystal fiber with a carefully engineered zero-dispersion wavelength. We show that the short-wavelength generation is due to a combination of four-wave mixing and dispersive wave trapping by solitons. The evolution and limiting factors of the continuum are discussed.

Laser jamming technique research based on combined fiber laser

A compact and light laser jamming source is needed to increase the flexibility of laser jamming technique. A novel laser jamming source based on combined fiber lasers is proposed. Preliminary experimental results show that power levels in excess of 10 kW could be achieved. An example of laser jamming used for an air-to-air missile is given. It shows that the tracking system could complete tracking in only 4 s and came into a steady state with its new tracking target being the laser jamming source.

Beam quality factor for coherently combined fiber laser beams

The beam quality factor for a coherently combined beam is studied in detail. A theoretical analysis and a numerical calculation are given to illustrate that the M 2 factor is not a proper means for evaluating the beam quality of a coherently combined beam. The beam propagation factor is introduced to evaluate the beam quality. The effect of phase error on the beam quality is also studied. Results indicate that the beam quality of a coherently combined beam depends mainly on the fill factor of the laser array and not on the number of lasers. It is also revealed that the allowed phase error does not change with the number of lasers in order to ensure the energy encircled in a certain bucket.

Determine locking range of coherently combined fiber lasers joined by fiber couplers

When the wavelength difference of two fiber lasers, sharing a common reflection mirror with the aid of an optical coupler, is tuned to fall into a certain range, the array lasers can be locked to generate coherent radiation with a wavelength somewhere between the two constituent field wavelengths. Based on the fact that the laser should oscillate at the wavelength with the lowest losses and the band widths of the reflection curves of the laser tuning elements are finite, the locking range of the laser array has been quantified. For the tuning element (i.e., fiber gratings) selected in this work, the calculations show that the locking range is mainly dependent on the coupling coefficients of the gratings which ever is smaller.

Numerical analysis of the effects of aberrations on coherently combined fiber laser beams

Numerical analysis of the effects of aberrations on coherently combined fiber laser beams is presented. We prove that traditional beam quality criteria, such as the M2 factor and the Strehl ratio, do not consider necessary information to evaluate the quality of a coherently combined laser beam. The beam propagation factor (BPF) is introduced and studied as a proper beam quality factor for the coherently combined beam. Two main categories of aberrations, geometry and nongeometry factors, are numerically studied to investigate their effect on beam quality by using the BPF criterion. For a ring-distributed fiber laser array with certain vacancy factor and a RMS value of tilt error, we obtain a semianalytical equation to evaluate their effect on beam quality. We present a brief discussion of those aberrations at the end of this paper. Our generalized methodology offers a good reference for investigating coherent combining of fiber laser beams in a comprehensive way.

Rate equations studies on coherently combined fiber lasers connected by an optical coupler

Attempt has been made to apply the steady-state rate equations to study two coherently combined Er 3+-doped fiber lasers joined by an optical fiber coupler. In order to establish the boundary conditions, which are necessary for solving the equations, we have taken into consideration the fact that a constructive interference at the coupler assures that the laser radiations from both constituent lasers are directed to the coupler output port to which a reflection mirror defining both lasers is attached. Consequently, effective reflectivity, provided by the coupler–mirror combination, can be specified in terms of the laser powers of both laser signals, and the rate equations describing the compound system can be solved. Using the derived solutions, quantitative investigations can be made on the influences of different laser parameters. For example, the influence of the length difference between doped fibers, the splitting ratio of the coupler and power difference between the two pump radiations have been given in this report.

Threshold analysis of fiber-coupler-combined fiber lasers

Threshold conditions of coherently combined fiber lasers, joint by fiber couplers (FCs), have been studied. After considering the phase change of a light wave crossing the fiber inside the FC, it has been shown that, at the two input ports of the coupler connecting with the amplifying fibers, the phase difference between the two incoming fields should be equal to − π/2 if the threshold gain of the compound system is to be minimized. Starting from these input ports, following the circulations of waves, threshold conditions can be established. Expressions for the output and leakage powers, and optimum coupler splitting ratio nullifying the leakage when constituent fiber lasers giving different free-running powers have been derived. And, the threshold gain reductions of individual lasers after being coherently combined have been predicted.