Laser Diode Array Research Articles

Non-Hermiticity and exceptional points in coherently coupled vertical cavity laser diode arrays

Coherently coupled laser arrays can be described by the temporal coupled mode theory in which the array modal behavior can be classified according to the coupling matrix, M¯¯. Accounting for a nonuniform gain/loss distribution in a laser array makes M¯¯ a non-Hermitian matrix, and experimentally we find phase-front tuning (beam steering) of the coherent supermode as a result of the non-Hermiticity. We report the experimental characterization of the supermodes in coherently coupled vertical cavity surface emitting laser diode arrays and demonstrate control of non-Hermiticity by spatially varying injection currents. Exceptional points are identified in these electrically injected microcavity diode arrays.

Efficiency and Threshold Characteristics of Spectrally Beam Combined High-Power Diode Lasers

The efficiency, threshold characteristics, and the stability of spectral beam combining (SBC) of diode laser array had been studied based on different output couplers (OC). It was found that the lower OC reflectivity, lower internal loss, and shorter cavity-length of emitter were helpful for the realization of high efficiency SBC in an external cavity. However, too low OC reflectivity would also result in an unstable combining with the characteristic of side lobes appeared in far-field and the degraded efficiency. The influence of OC reflectivity on threshold current, lasing spectra, and the beam quality of SBC was investigated and analyzed.

An advanced laser headlight module employing highly reliable glass phosphor

An advanced laser headlight module (LHM) employing highly reliable glass phosphor is demonstrated. The novel glass-based YAG phosphor-converter layers fabricated by low-temperature of 750°C exhibited better thermal stability. The LHM consisted of a 5 × 1 blue laser diode array, an aspherical lens, a glass phosphor-converter layer with an aluminum thermal dissipation substrate, and a dichroic filter to allow pass blue light and reflect yellow phosphor light. The 5 × 1 blue laser array was packaged with five blue lasers having optical power of 1.2 W per laser. The LHM exhibited total output optical power of 6 W, luminous flux of 1860 lm, relative color temperature of 4100 K, and efficiency of more than 310 lm/W. The high-beam patterns of the LHMs were measured to be 45,000 luminous intensity (cd) at 0°, 31,000 cd at ± 2.5°, and 12,500 cd at ± 5°, which were well satisfied the ECE R112 class B regulation. The proposed high-performance LHM with highly reliable glass-based phosphor-converter layer fabricated by low temperature is favorable as one of the promising LHM candidates for use in the next-generation automobile headlight applications.

Temperature distribution and thermal resistance analysis of high-power laser diode arrays

The accurate temperature measurement of high-power laser diode arrays is a considerable challenge due to their large temperature gradient and package structure. In this study, experiments based on the forward voltage method were set up to measure the transient temperature behavior of chips after the removal of heating load. A hybrid experimental/numerical method was developed to calculate multiple unknown interface thermal resistances and improve the prediction of the temperature distribution of high-power laser diode arrays by analyzing the transient temperature behavior of chips. The largest deviation between numerical and experimental results is 1.3 K. Based on the numerical results, thermal resistances for different layers and temperature distribution inside the chip were obtained and evaluated. Some suggestions for improving the thermal design of high-power laser diode arrays were derived.

紧凑型准连续泵浦调Q Nd∶YAG激光器

A compact end-pumped Electro-optical Q-switched Nd:YAG laser was designed in this paper. In order to achieve compact construction, laser diode arrays(LDAs) with collimated fast axes were used as pumping source. A spherical lens with a focal length of 40 mm and a cylinder lens with focal length of 25 mm were used as a group for the coupling lens. The LDAs' beam profiles on the incident and exit faces were calculated using Tracepro software. The simulation results indicate that when taking a Φ 6 mm×30 mm Nd:YAG with a doping concentration of 1.0at.% as the gain medium, the spots on the incident and exit faces were 5 mm(slow-axis)×4.5 mm(fast-axis) and 3 mm(slow-axis)×6 mm(fast-axis), respectively, and the absorption pumping power was about 83%. Dynamic temperature field distributions of Nd:YAG larer in 360 s at 22℃ and 60℃ were simulated by Ansys software. Output pulsed energy of 30 mJ and 25 mJ were achieved while the repetition frequencies were 30 Hz and 50 Hz, corresponding to a slope efficiency of 11.6% and 14.71%. The pulse durations were 18 ns and 16 ns, respectively. The experimental results show that the designed compact laser in this paper can achieve stable pulse laser output.

4 × 40 GHz mode-locked laser diode array monolithically integrated with an MMI combiner

We report a distributed-Bragg-reflectors-based 4 × 40 GHz mode-locked laser diode (MLLD) array monolithically integrated with a multimode interference (MMI) combiner. The laser produces 2.98 ps pulses with a time-bandwidth product of 0.39. The peak wavelength of the MLLD array can be tuned by 8.4 nm while maintaining a good mode-locked state. The four mode-locked channels could work simultaneously with the peak wavelength interval around 3 nm.

Research on 200 mJ semiconductor end-pumped MOPA Laser

Using the high-power laser diode array (LDA) end-pumped Nd:YAG laser bar method, combined with the design of the convex-concave unstable cavity, the average energy of the small beam divergence laser output was achieved about 83 mJ under 20 Hz operating conditions. Then taking the laser as the oscillation source, the same method of the LDA end-pumped Nd:YAG laser rod was used for energy amplification to form an LDA end-pumped master oscillator-power amplification (MOPA) laser, which finally obtained the pulsed laser output with a repetitive frequency of 20 Hz, an average energy of >200 mJ, a divergence angle < 2.1 mrad and an energy fluctuation < ±2.5%. The laser has a optical-to-optical conversion efficiency of about 14.6%, a volume of 175×91×49 mm3, a mass of < 1 kg, and a divergence angle of < 0.3 mrad after transmitted through an 8× transmission antenna.

A Device for Recording TV Images with High Intensity Resolution and Stroboscopic Recording Mode

A device for providing registration of grayscaled images that are formed by an analog television camera with transformation of the intensities of discrete spatial samples of an image into a 10-bit digital code is described. A method for visualizing the images with a high intensity resolution on the screen of a typical monitor, which provides rendering of more than 1000 gray tones, is proposed and its practicality is discussed. The method of implementing the stroboscopic mode of recording physical processes, which are initiated by the action of electrical pulses of the signal source, is described and the results of its application to the analysis of thermal processes in semiconductor laser diode arrays are presented.

Design of Pulse Power Supply for High-Power Semiconductor Laser Diode Arrays

This paper mainly introduces a design circuit and control method of pulse power supply for high-power semiconductor laser diode arrays. According to the high voltage and large current operating characteristics of semiconductor laser, the control strategy of multi-module cascade topology combined with multiple MOSFET connected in parallel is adopted. The topology of multi-module cascade is presented to meet the requirement of wide output voltage range. The multiple MOSFET connected in parallel in the grounding module is proposed to increase output current of the pulse power supply, and each MOSFET has exclusive closed-loop regulator. The pulse current is controlled by the gate driving voltage of the MOSFET which operates in the linear zone, which can ensure the pulse current to be smooth and stable without overshoot. Digital control and analog control are combined to make the control of the pulse power supply more flexible. An experimental platform with a pulse power of 132.3 kW is built. The parameters of pulse power supply are exhibited: pulse current 210 A, pulse voltage 630 V, pulse width 200μs, and repetition frequency 100 Hz.

Compact laser diode array based on epitaxially integrated AlGaAs/GaAs heterostructures

The main results of the development of compact laser diode mini-arrays operating under 875-nm pulsed pumping are presented and the instrumental characteristics of these arrays are studied. Specific features of these sources, in addition to a high output power (∼1.5 kW), are a narrow directional pattern (angular divergence 21° × 8°) and a small emitting area (less than 1 mm2). The use of serially integrated AlGaAs/GaAs MOCVD heterostructures with three emitting regions to develop laser diode arrays allowed us to improve their working parameters.

Effect of the spectral composition of laser diode pumping of Nd3+:YAG lasers on the reduction of pump efficiency for a temperature shift of the spectrum by 25–30 nm

The change in the absorption efficiency of laser diode emission in the gain medium of Nd3+:YAG lasers is considered for a temperature shift of the pump spectrum by 25–30 nm. It is shown that, for a typical width of the emission spectrum of modern linear laser diode arrays, the minimum efficiency can be significantly increased if the pump spectrum consists of not one but two spectral peaks of different intensities. The increase in the number of peaks to more than two does not lead to any significant improvement in efficiency.

Effect of submount thickness on near-field bowing of laser diode arrays.

Near-field bowing a laser diode bar (i.e., the "SMILE" effect) degrades the laser beam brightness, adversely affecting optical coupling and beam shaping. Due to thermally induced stress during the bonding process, the emitters in a laser diode array (LDA) are vertically displaced, which causes the SMILE effect. The mismatch between the coefficients of thermal expansion of an LDA (GaAs with 6.4 ppm/K) and a heat sink (Cu with 16.4 ppm/K) is a large obstacle in the LDA bonding process, because it provokes thermal stress and a large SMILE value, resulting in a larger divergence angle and a wider line after focusing and collimation. In this paper, the changes in stress and strain (SMILE value) and their effects on the laser bar as a function of the copper-tungsten (CuW) submount thickness were theoretically and experimentally studied. The finite element modeling simulations and experimental results show that the compression stress on the laser bar decreases with increasing CuW submount thickness because the CuW submount works as a buffer layer and can absorb stress. However, the laser bar out-of-plane strain (SMILE value) is approximately zero when the LDA is directly bonded onto the heat sink without a submount; the SMILE value is maximized when the CuW submount thickness is increased to approximately one half or 44% of the heat sink. Beyond that, the SMILE value decreases with increasing CuW submount thickness.

Projected UV-resin curing for self-supported 3D printing

This research introduces a new additive manufacturing process for support-free printing. This process uses an array of ultra-violet laser diodes to cure the jetted photopolymer at the laser intersection for rapid solidification. A prototype system using 405 nm laser is designed and built, and a process model is modified from conventional vat photopolymerization to find a proper range of printing speed. Using a speed of 1 mm/s for a flow rate about 0.5 mm3/s, a 60-degree overhanging structure can be repeatedly printed with commercial photopolymers. The study also discusses the challenges associated with fluid behaviors in such a dynamic condition.

Photonic Crystal Diode Laser Array Integrated With a Phase Shifter for Narrow Beam Divergence

A narrow far-field edge-emitting photonic crystal diode laser array integrated with a phase shifter emitting in the 980-nm range has been designed and fabricated. A phase shifter structure is integrated on the chip to convert the out-of-phase mode to the in-phase mode. The real effective-refractive-index difference produced by the phase shifter is measured according to the comparison of the experiment and the simulation results. The effects of phase shifters with different lengths on the near and far fields are investigated. With the phase and the amplitude modulation of the phase shifter, the out-of-phase mode is successfully converted to the in-phase mode. Thus, a single-lobe far-field pattern is achieved in the lateral direction. The far-field divergence angles of the lateral and vertical directions are 1.5° and 12°, respectively. Such a device is promising for narrow far-field emission with a high output power.

Amplified luminescence and nonaxial radiation modes in the active elements of high-power diode side-pumped solid-state lasers

A model system of two differential rate equations is proposed to describe the temporal characteristics of the radiation of a solid-state laser with high-energy pumping. Using a laser head with a YAG:Nd active element and a side-pumped laser diode array unit as an example, a method is discussed of determining the excitation levels of the active elements at which the amplified luminescence and the nonaxial (stray) lasing modes begin to manifest themselves in the output radiation dynamics all the way to the limitation of the maximum pulse energy generated by a solid-state laser in the Q-switched regime. The proposed approach to analyzing the role of the processes associated with the formation and evolution of amplified luminescence fluxes and nonaxial (stray) lasing modes in the active media can also be applied to other types of laser radiators.

Almost perfect in-phase and anti-phase chaotic and periodic phase synchronization in large arrays of diode lasers

Abstract In this paper we study phase synchronization in large arrays of weakly coupled single mode semiconductor laser diodes. We show that if the coupling topology is chosen appropriately, the laser array exhibits robust phase synchrony (including chaotic and non-chaotic phase synchrony). Furthermore, one can define coupling topologies that lead to chaotic anti-phase synchronization. To the best of our knowledge, chaotic anti-phase synchronization has not been observed in large arrays of coupled nonlinear oscillators. When diodes are coupled via a decayed non-local coupling scheme, the leading spatial mode can be stable. This leads to an almost-perfect phase synchronous state where the phases are synchronized, but the system is not set exactly on the synchronization manifold. This almost-perfect phase synchronous state is robust to noise and frequency disorder and can be realized under periodic (fixed-intensity limit cycle) continuous-wave and chaotic behavior. The presented result is an example of the broader phenomenon of linear transverse mode selection taking place in a coupled oscillator system with nonlinear dynamics.

Phase-locking of arrays of weakly coupled semiconductor lasers.

In this paper we study the conditions for achieving almost perfect phase locking in large arrays of semiconductor diodes. We show that decayed non-local coupling of diode lasers can provide the necessary conditions for robust phase synchronization of an entire diode laser array. Perfect global coupling is known to allow for robust synchronization, however it is often physically impossible or impractical to achieve. We show that when diodes are coupled via the decayed non-local coupling layout, the dominant transverse mode of the laser array has a uniform phase across the lasers and can be stable. This state is robust to noise and frequency disorder and can be realized under periodic (fixed-intensity limit cycle) continuous-wave and chaotic behavior of lasers.

Thermally induced chirp studies on spectral broadening of semiconductor laser diode arrays.

Spectral-width broadening has many factors. Diode lasers are not always monochromatic due to several broadening mechanisms, widening the energy distribution of emitted photons. In this paper, we report the two main factors affecting time average spectral-width broadening of a laser diode array (LDA)-a transient rise of the active region temperature of an LDA due to injection current, and the temperature and stress nonuniform distribution of different emitters within an LDA. We find that temperature and stress nonuniformity broadens the spectral width by almost 0.1-1.0nm as a function of different operating conditions, while the thermally induced chirp that is attributed to injection current plays a more signification role in spectral-width broadening.

60 mm-aperture high average output power Nd:YAG composite ceramic disk laser

A laser with 60 mm-aperture composite ceramic disk pumped by laser diode arrays was developed. By optimizing waveguide and imaging lens, a pump power peak power of 75 kW, density of 318 W/cm2 is achieved with a pump uniformity of 96% by using a hexagonal waveguide. The temperature distribution and axial deformation of the cooler were optimized, and the axisymmetric micro-channel cooler was designed. Up to 2.3 kW average output power with an optical-optical efficiency of 37% is obtained in the stable resonator when pumping with a long-pulse.

Benefits of Unidirectional Design Based on Decoupled Transmitters and Receivers in Tackling Traffic Asymmetry for Elastic Optical Networks

We consider a unidirectional design approach to tackle the traffic asymmetry in an elastic optical network (EON). We decouple a bidirectional transponder into a pair of separate unidirectional transmitters and unidirectional receivers, and consider different multi-flow transmitters that use either an array of laser diodes or a broadband laser source with filters to generate multiple sub-carriers. To evaluate the benefit of this unidirectional design, we have considered the routing and spectrum allocation optimization problem for the EON. An integer linear programming model and an efficient heuristic algorithm based on spectrum window planes have been developed. Simulation results show that the proposed unidirectional design approach can significantly improve capacity utilization and minimize network cost compared to one based on bidirectional symmetric traffic flows. It is also efficient for network scenarios with spatial traffic asymmetry and where multi-period network planning is required.