Development of a 756 nm, 3 W injection-locked cw Ti:sapphire laser

We have developed a 756 nm, 3 W single-frequency cw Ti:sapphire laser by using the technique of injection locking. A cw Ti:sapphire laser in a ring-type configuration was forced to lase unidirectionally by use of an optical diode to prevent a high-power backward laser from disturbing the injection laser. A master laser was amplified by a broad-area laser diode and coupled into a single-mode fiber to generate a 50 mW injection laser with a Gaussian beam profile, which was enough to lock the Ti:sapphire laser at full power of 3 W. Such a high-power single-frequency Ti:sapphire laser enables a watt-level blue or near-ultraviolet single-frequency laser to be generated by frequency doubling.     

Far field intensity of a partially locked optical phased array

The effects of imperfect frequency locking on the performance of an optical phased array were investigated. An analytic expression was obtained for the far field intensity pattern in terms of the degree of mutual coherence between lasers. The results of the analytical study were applied to determine the Strehl ratio of an injection coupled optical phased array for different degrees of slave resonator length control, ratios of injected to slave laser intensities, and pulse durations.     

Effect of spatial hole burning on injection-locked vertical-cavity surface-emitting laser arrays

Injection locking of vertical-cavity surface-emitting laser arrays is analyzed at steady state, including the effect of spatial hole burning. A free-running laser array (i.e., without injection), that operates well-above threshold, can exhibit multimode oscillations. Consequently, each of the free-running modes (at different frequencies) needs a different locking injection power. For low pump levels, just above threshold, the array is single mode and, as expected, the results bear a close resemblance to those of the average gain analysis, which ignores spatial hole burning.     

Radio frequency controlled synthetic wavelength sweep for absolute distance measurement by optical interferometry

We present a new technique applied to the variable optical synthetic wavelength generation in optical interferometry. It consists of a chain of optical injection locking among three lasers: first a distributed-feedback laser is used as a master to injection lock an intensity-modulated laser that is directly modulated around 15 GHz by a radio frequency generator on a sideband. A second distributed-feedback laser is injection locked on another sideband of the intensity-modulated laser. The variable synthetic wavelength for absolute distance measurement is simply generated by sweeping the radio frequency over a range of several hundred megahertz, which corresponds to the locking range of the two slave lasers. In this condition, the uncertainty of the variable synthetic wavelength is equivalent to the radio frequency uncertainty. This latter has a relative accuracy of 10(-7) or better, resulting in a resolution of +/-25 microm for distances exceeding tens of meters. The radio frequency generator produces a linear frequency sweep of 1 ms duration (i.e., exactly equal to one absolute distance measurement acquisition time), with frequency steps of about 1 MHz. Finally, results of absolute distance measurements for ranges up to 10 m are presented.     

Influence of detuned injection locking on the relaxation oscillation frequency of a multimode semiconductor laser

Abstract

The injection locking characteristics of a multi-mode semiconductor laser are considered. A formalism is developed to investigate the stability properties of an arbitrary laser mode subject to optical injection. The formalism is used to show that the relaxation oscillation frequency (ROF) in a semiconductor laser subject to optical injection is increased relative to that of the free running laser diode. Methods of utilizing positive detuning to determine the best approach of increasing the ROF of a semiconductor laser via injection locking are considered.     

Dynamics, bifurcations and chaos in coupled lasers

Experiments and numerical modelling on two different class B lasers that are subjected to external optical light injection are presented. This presentation includes ways of measuring the changes in the laser output, how to numerically describe the systems and how to construct diagrams of the dynamical states in the plane frequency detuning between lasers and injection strength. The scenarios for the semiconductor laser include an area of frequency locking and islands of chaotic behaviour embedded in and mixed with periodic doubling regimes. Using a rate equation model, the largest Lyapunov exponent is calculated as a measure of the stability of equilibriums and the amount of chaos in chaotic regimes. In the solid-state laser case, different dynamical regions were clearly observed. The found boundaries were identified experimentally, using an identification method, and numerically, from bifurcation analysis, as Hopf, saddle-node, period-doubling and torus bifurcations.     

Coherent coupling of vertical-cavity surface-emitting laser arrays and efficient beam combining by diffractive optical elements: concept and experimental verification

A phase-locked diode-laser system based on master-slave coupling of two-dimensional vertical-cavity surface-emitting laser (VCSEL) arrays by injection locking is presented. Frequencies and phases are adjusted by laser-trimmed microresistors. Additional beam-transformation optics consisting of two diffractive optical elements (DOEs) and a Fourier lens concentrates most of the far-field power in a nearly diffraction-limited beam. Both the VCSEL array and the microlens array are monolithically integrated and mounted in a compact module. With an array of 21 slave lasers a system coherence of 95% (for several hours) and of nearly 90% (for several months) has been demonstrated without any active phase control. The scalability of the output power has been verified by locking of an array of 77 slave lasers with a system coherence of 78%. The optical system efficiency is 20-23%; with beam-transformation optics this efficiency could be improved to 44%.     

Tunable graphene Q-switched erbium-doped fiber laser with suppressed self-mode locking effect

Self-mode locking effect in a wideband tunable graphene-based passively Q-switched erbium-doped fiber laser has been observed experimentally. Q-switching is achieved by using graphene as a saturable absorber, while a tunable bandpass filter with a narrow bandwidth is used to obtain wideband tunability. We propose to suppress the modulation on each pulse from self-mode locking by introducing three subring resonators constructed with three 3 dB couplers into the laser ring cavity. Moreover, the laser output characteristics with respect to pump power are studied in detail. A stable Q-switched erbium-doped fiber laser with a tunable range from 1522 nm to 1568 nm is demonstrated experimentally.     

Three-frequency Chaos in an External Cavity Injection Laser

Abstract

We report fixed-frequency interactions in a modulated delay system. In particular, we have observed frequency locking, quasiperiodicity and the progression to a chaotic state by the collapse of a three-frequency quasiperiodic attractor, characteristic of the Ruelle-Takens-Newhouse scenario, in a modulated two-sided external cavity injection laser. While the temporal and spectral characteristics of the laser emission suggest that the laser has progressed to a chaotic state, dimensionality testing of the laser output is inconclusive.     

High-power 457-nm light source by frequency doubling of an amplified diode laser

We demonstrate the generation of 150-mW blue coherent single-mode radiation at 457 nm in a compact and inexpensive setup. The light is generated by frequency doubling the radiation of a master oscillator power amplifier (MOPA) system in an enhancement cavity with a potassium niobate (KNbO3) crystal. The MOPA consists of a 914-nm single-mode diode laser and a broad-area diode laser (BAL) as the amplifier. The BAL is a multimode laser with a specified wavelength of 938 nm. Sufficient gain at 914 nm is obtained by antireflection coating the BAL front surface and by cooling it to -10 degrees C.     

Efficient Continuous-Wave and Q-Switched Operation of a 946-nm Nd:YAG Laser Pumped by an Injection-Locked Broad-Area Diode Laser

The efficient, low-threshold operation of a 946-nm Nd:YAG laser pumped by an injection-locked broad-area diode laser is reported. The implications of pump-beam quality for efficient, low-threshold operation, particularly with intrinsically inefficient transitions, are discussed in the context of previously published models. Results are presented showing that the M(2) = 1.3 pump beam of the injection-locked diode laser enabled a cw slope efficiency of 48% and a threshold of 52 mW to be attained. When Q-switched, 335 mW of pump power gave 27-ns, 5.2-muJ pulses. These were frequency doubled to obtain 19-ns, 1-muJ pulses at 473 nm. These results represent significant improvements over similar systems pumped by free-running broad-area diode lasers or arrays.     

Improvement of the lateral-mode discrimination of broad-area diode lasers with a profiled reflectivity output facet

The emission of a broad-area laser always contains several lateral modes (along the junction plane) even at low drive levels. To increase the discrimination against high-order lateral modes, we developed simple techniques for depositing a profiled thin layer of SiO on the output facet of a broad-area laser that has a 75-μm-wide injection current stripe. The profiled coating provided a nearly Gaussian reflectivity in the lateral direction (parallel to the junction plane). The resolved near-field spectra of the uncoated and coated lasers have been compared. The maximum output power in the single-lateral-mode regime was pushed to 25 mW with the profiled coating, compared with a corresponding power of less than 1 mW before deposition. Experimental results have confirmed the behavior predicted by our numerical simulations. This method is scalable to higher-power lasers.     

Characteristics of a high-power broad-area laser operating in a passively stabilized external cavity

We narrowed the spectral bandwidth of our 2 W broad-area laser to 8+/-1 GHz with a coarse tunability of 12 nm centered near 790 nm in an external cavity. Our passively stabilized external cavity was designed for use with high-power semiconductor lasers. The cavity length and mount are modular with a fixed pivot point so that the optical elements can be changed to access a broad frequency spectrum and enable multiple applications. The bandwidth was observed to be dependent on the multimode transverse structure of the laser.     

激光器自动锁频方法研究综述

介绍了激光自动锁频的意义与目的,阐述了实现激光自动锁频的算法和常用的光学频率参考,分析了不同算法和频率参考的区别和优劣,探讨了在自动领域下实现激光锁频需要解决的问题和方法,指出了有待加强的领域,提出了部分仍待解决的困难,得出了当前激光自动锁频领域的发展方向和改良趋势,为激光器自动锁频的后续研究提供了参考。     

Tunable and frequency-stabilized diode laser with a Doppler-free two-photon zeeman lock

We describe frequency locking of a diode laser to a two-photon transition of rubidium using the Zeeman modulation technique. We locked and tuned the laser frequency by modulating and shifting the two-photon transition frequency with ac and dc magnetic fields. We achieved a linewidth of 500 kHz and continuous tunability over 280 MHz with no laser frequency modulation.     

Frequency locking of tunable diode lasers to a rubidium-stabilized ring-cavity resonator

We demonstrate a technique for locking the frequency of a tunable diode laser to a ring-cavity resonator. The resonator is stabilized to a diode laser that is in turn locked to an atomic transition in rubidium, thus giving it absolute frequency calibration. The principal advantage of the ring-cavity design is that there is no feedback destabilization of the laser. The cavity has a free-spectral range of 1.3 GHz and Q of approximately 35, which provides robust locking of the laser. The locked laser is able to track large scans of the cavity.     

Single-mode operation of a broad-area semiconductor laser with an anamorphic external cavity: experimental and numerical results

The emission of high-power broad-area semiconductor lasers inherently contains many lateral modes that increase the beam divergence and reduce the spatial coherence. Elimination of higher-order lateral modes from the output beams of commercially available broad-area lasers will be beneficial in many applications of these lasers. Experimental results obtained with a broad-area laser coupled to an anamorphic external cavity are presented and are compared with the predictions from our numerical model. We have predicted and observed with the anamorphic external cavity a greatly improved discrimination against high-order lateral modes. The measurement of the spectrally resolved near-field intensity patterns provides much more comprehensive information on their longitudinal- and lateral-mode content than do observations of near-field and far-field beam intensity profiles. With a broad-area laser of 100-mW nominal power, it has been possible to extract 40% of the maximal power in a stable single-lateral and single-longitudinal mode regime.     

Investigation of the influence of spatial coherence of a broad-area laser diode on the interference fringe system of a Mach-Zehnder interferometer for highly spatially resolved velocity measurements

Laser Doppler anemometry is a method for absolute velocity measurements that is based on a Mach-Zehnder interferometer arrangement and usually employs transverse fundamental-mode lasers. We employed inexpensive and powerful broad-area laser diodes and investigated ways in which an interference fringe system is influenced by the spatial coherence properties of a multimode beam. It was demonstrated that, owing to poor spatial coherence of the beam, interference is suppressed in the marginal regions of the intersection volume. Based on these results, a sensor for highly spatially resolved velocity measurements can be built. The inherent astigmatism of the broad-area diode is corrected by an arrangement of two crossed cylindrical lenses. An interference fringe system of length 200 microm and a relative variation in fringe-spacing of only 0.22% were demonstrated with light emitted from a broad-area laser diode with a 100 microm x 1 microm emitter size. Based on this principle a powerful, simple, and robust laser Doppler sensor has been achieved. Highly spatially resolved measurements of a boundary layer flow are presented.     

Simple method for frequency locking of an extended-cavity diode laser

We have developed an extended-cavity tunable diode laser system that has a small linewidth and a large output power (more than 90% of the free-running power) whose operating frequency can be conveniently locked to a transition line of Rb atoms. Based on flat-mirror feedback and frequency self-locking and with weak feedback, we have achieved a continuous frequency detuning range greater than 900 MHz and a short-time linewidth stability of better than 0.4%. By using a two-step locking procedure we not only can lock the laser frequency but also can detune the frequency to any desired value. The locking is quite sturdy and rugged.     

Apodizing holographic gratings for dual-wavelength operation of broad-area semiconductor lasers

Apodizing holographic gratings are designed to have a Gaussian reflectivity profile in the -1 order and a complementary reflectivity profile in the specular (0) order. They are obtained by the interference of two Gaussian beams on a glass plate covered with a photoresist. These gratings are intended to be used as the coupler of the external cavity of a broad-area semiconductor laser. When the grating is oriented to get the -1 order counterpropagating with respect to the incident beam, single-longitudinal- and single-lateral-mode operation is obtained. We report on the fabrication technique of an apodizing holographic grating designed to allow lasing on two wavelengths simultaneously, with a preselected wavelength separation. The results obtained with a commercial broad-area laser are presented.