Objective The 1197 nm laser is located in the photoacoustic imaging window of the C--H bond, which can be used for the clinical diagnosis and treatment of biological tissues, such as tumors and fats. Additionally, its frequency-doubled yellow light has a wide range of applications in the field of fluorescence detection. However, the 1197 nm Raman laser suffers from low energy and poor reliability, making it unsuitable for engineering applications. In this paper, we conducted experiments to investigate the ways to improve the energy and efficiency of the 1197 nm Raman laser. On the one hand, the beam distribution and energy of fundamental light were improved to demonstrate their effect on Raman efficiency. On the other hand, the Raman laser was optimized theoretically and experimentally. A high-efficiency and high-energy solid-state Raman laser is studied and a prototype of this laser is developed to test its performance.
Methods In this study, we investigated an external resonator stimulated by Raman scattering technology and demonstrated a Ba(NO3)2 Raman laser pumped by a Q-switched 1064 nm laser. First, a xenon-lamp-pumped Nd∶YAG laser was constructed. Experiments were conducted using three different cavity length conditions to determine the best cavity length of the resonator and simultaneously increase the output energy. To improve the uniformity of the 1064 nm beam, the laser output coupler adopted a variable reflectivity output mirror. Then, the beam diameter of the Nd∶YAG laser was compressed to 5 mm using a 1.5× telescope to increase the peak power density of the fundamental light. Thereafter, we optimized the parameters of the external-resonator Raman laser according to the rate equation and radiation transmission theories, such as the optimum reflectivity, the threshold of first-order Stokes light, and the highest conversion efficiency. Moreover, the output coupler of the Raman laser was selectively coated to suppress the oscillation of high-order Stokes light. Then, the external-cavity Raman laser experiment was performed and the differences between the experimental results and theoretical values were compared. Finally, a prototype of a 1197 nm Raman laser was built to verify its reliability and stability.
Results and Discussions The experimental results of the 1064 nm and 1197 nm resonators are shown in this study. Figure 3(a) shows that 305 mm is the best cavity length under the three cavity length conditions. The maximum output energy, wavelength, and pulse width are 334 mJ, 1064.3 nm, and 10.5 ns, respectively. When the variable reflectivity output coupler is used, the 1064 nm beam is uniform with a flat top distribution [Fig. 3(b)] and the divergence angle is 1.1 mrad. The fundamental beam passed through a 1.5× telescope, and its peak power density was increased to 145.5 MW/cm 2. According to theoretical calculations of the external-resonator Raman laser, the best reflectivity of the output coupler is 55% and the highest conversion efficiency is 52%. Figures 4 and 5 show the output characteristics of the 1197 nm Raman laser. The best transmittance of the output coupler is 40%, which is consistent with the theory. When a 300 mJ 1064 nm laser was injected into the barium nitrate crystal, a steady laser at 1197.81 nm with an output energy of 135 mJ, a pulse width of 7.4 ns, and a divergence angle of 5 mrad was obtained. The maximum conversion efficiency was 46.6%, which was slightly different from the theory. This is because the second-order Stokes light starts to oscillate, and the thermal effect of Raman crystal becomes more severe. A prototype of the 1197 nm Raman laser was developed, as shown in Fig. 6(b). The energy stability (RMS) within 1 h is 0.48% [Fig. 6(a)], indicating that both the output characteristics and reliability meet the requirements of engineering applications.
Conclusions We study the solid-state laser technology based on external-cavity stimulated Raman scattering and fabricate a high-efficiency and large energy 1197 nm pulse laser. By varying the resonant cavity length and using a variable reflectivity output coupler, the energy and beam distribution of the fundamental light are optimized. The maximum output wavelength of the 1064 nm laser is 334 mJ, and the pulse width is 10.5 ns. The fundamental beam is compressed by a 1.5×telescope to increase the peak power density and improve the Raman conversion efficiency. In addition, the selective coating on the output coupler suppresses the oscillation of high-order Stokes light, increasing the output energy of the first-order Stokes light. When the injected energy reaches 300 mJ, we obtained a 1197.81 nm laser of 137 mJ with a pulse width of 7.4 ns. The maximum Raman conversion efficiency is 46.6%, and the energy stability (RMS) within 1 h is 0.48%. Generally, the prototype of the 1197 nm Raman laser has been successfully used in medical photoacoustic imaging. The system has the advantages of high output energy and conversion efficiency, good energy stability, and low cost. It can provide high-quality light sources for photoacoustic imaging and fluorescence detection and has broad application prospects.
Significance Mid-infrared (2--20 μm wavelength) photonics has extensive applications in spectroscopic analysis, environmental monitoring, medical diagnosis, free-space optical communication, and ranging, due to the distinguishable fundamental vibrational transitions of molecules and the atmospheric transmission windows (e.g. 2--2.5 μm, 3--5 μm, and 8--13 μm wavelengths) in the mid-infrared spectral region. Previously, mid-infrared applications have been mainly developed based on benchtop free-space optical instruments (e.g. Fourier-transform infrared spectrometers), which inevitably suffer from expensive, heavy, and bulky setups. To overcome this limitation, mid-infrared integrated optics has been proposed and quickly developed in the past few decades. By using the nanofabrication technology, on-chip mid-infrared devices not only significantly reduce footprints, weights, and costs of mid-infrared photonic systems, but also open an avenue to explore the light-matter interaction at the nanoscale level.
Nowadays, numerous optical materials have been investigated to develop mid-infrared integrated optics, namely, noble metals, low-dimensional semiconductors, chalcogenide glasses, and group-IV semiconductors. As for noble metals and low-dimensional semiconductors, high optical losses of the developed waveguides hinder the potential large-scale integration of on-chip systems. While chalcogenide-glass-waveguides have attracted a great attention in many mid-infrared applications due to their ultra-low optical losses. However, the fabrication of the chalcogenide-glass-waveguides is not fully compatible with the complementary metal-oxide-semiconductor (CMOS) technology. On the other hand, photonic devices based on group-IV semiconductors, namely, silicon, germanium, tin, have the notable advantages of low optical loss, excellent physiochemical stability, and full CMOS compatibility, which are critical for practical applications with low-cost and high-volume production requirements. Consequently, mid-infrared group-IV photonics has been a hot topic in the past few years.
As for the most commonly used group-IV semiconductors, silicon is first used to explore mid-infrared photonic integrated circuits. As early as 2006, Soref et al. published a paper to discuss the prospects of mid-infrared silicon photonics. Compared with the near-infrared band, silicon dioxide has huge optical absorption to the mid-infrared light, thus the silicon photonic devices utilized for the near-infrared band cannot be directly used in the mid-infrared band. Numerous novel silicon waveguide configurations, namely, suspended membrane waveguides, subwavelength-cladding waveguides, and silicon-on-sapphire waveguides, have been demonstrated. However, due to the strong multi-phonon absorption of silicon, the low-optical-loss spectral region of silicon photonic devices can only reach the functional group region (wavelengths below 8.0 μm). For silicon-germanium alloys, the photonic devices can be operated up to at least 8.5 μm wavelength. In contrast, for undoped crystal germanium material, optical absorption can be as low as 1 dB/cm within a spectral range from 1.9 μm to 16.7 μm at room temperature. Therefore, it is extremely promising to develop mid-infrared waveguides for long wavelengths based on a germanium platform.
Progress Germanium possesses advantages of wide transparency window (2--14 μm wavelength), high refractive index (~4.0), an excellent thermal optic coefficient (>10-4 K-1), large third-order nonlinear susceptibility (~10-18 m2·V-2), and low cost for high-quality and high-density device fabrication. Therefore, germanium devices could be an excellent candidate to develop mid-infrared applications, especially in the fingerprint region. Since the first germanium waveguide was developed in 2012, mid-infrared germanium photonics has been attracting increasing research attention. Currently, germanium waveguides are mainly demonstrated based on four types of integration platforms, namely, germanium-on-silicon wafer, germanium-on-silicon-on-insulator wafer, germanium-on-insulator wafer, and germanium-on-silicon nitride wafer. Based on the above germanium platforms, researchers have not only developed state-of-the-art passive optical components on a chip, such as low optical loss waveguides, grating couplers, high quality-factor microring resonators, and photonic crystal nanocavities, but also demonstrated mid-infrared waveguide-integrated lasers and electro-optical modulators. Moreover, to extend the spectral range of on-chip sensing applications to the fingerprint region, researchers have developed diverse chip-integrated gas and protein sensors by using the germanium waveguide devices. Besides, nonlinear optical phenomena, namely Kerr frequency combs and supercontinuum generation, have also been theoretically explored in the germanium devices to overcome the spectral bandwidth limitation of mid-infrared on-chip lasers.
Conclusion and Prospect In this paper, we briefly review the historical progress of mid-infrared group-IV photonics, and comprehensively summarize the development of recently emerging germanium photonics integrated circuits and their applications. In addition, the prospect of mid-infrared integrated optics is discussed. We hope this paper can not only serve as a reference for researchers specialized in mid-infrared photonics, silicon photonics, germanium photonics, optoelectronic materials, optical sensing, and spectroscopy, but also arouse attentions of researchers to mid-infrared integrated optoelectronics.
Objective Structured light has been exploited in several fields in the past two decades, such as optical tweezers, spiral interferometers, and phase-contrast microscopes. The generation of structured light is generally achieved by utilizing optical elements such as spatial light modulators or spiral phase plates. Because of the limit on output power when utilizing these optical elements to generate structured light, a direct generation of structured light is proposed in the laser cavity. Intracavity generation of structured light could be achieved based on the theory of transverse mode degeneracy. The degenerate range (or lock range), which means a special short range of cavity length, is an important condition for exploiting transverse mode degeneracy to generate structured light. When the cavity length is adjusted to the degenerate range, the degeneracy condition is met. When the cavity meets the degeneracy condition, resonant frequencies of specific transverse modes become equal. In this situation, these specific modes will coherently superpose each other and generate a light field that may exhibit spatial structure. Therefore, the accurate degenerate range plays an important role in utilizing this method to generate structured light. Thus, to the approach of measuring the accurate degenerate range becomes a significant issue, which is what we mainly consider in this research.
Methods The degenerate range is determined by the variation of output power or threshold of pump power in some articles. However, the widths of the degenerate range determined using these methods are somewhat approximate; their precision could be improved using other measurement methods. To more accurately measure the degenerate range, we try to exploit the frequency spectra of laser beams to characterize the degenerate range in this paper. As the cavity length is tuned slowly in the experiment, the frequency spectra of laser beams are detected in detail. Two peaks in the spectra that particularly considered in this study to show an obvious process of merging as the confocal resonator approaches the degenerate position. In this paper, the range of cavity length in which the two peaks become superposed is defined as the degenerate range. This type of measurement method is more accurate than the methods that utilize the variation of output power or threshold of pump power to determine the degenerate range because it directly monitors the variation of frequency spectra. We use the frequency spectra measured in experiment to analyze the change of dynamic behavior of transverse modes with different orders.
Results and Discussions The frequency data measured in the experiment are shown in Fig. 3. As the cavity length changes, each of these frequencies (ΔfL,2ΔfT,ΔfL-ΔfT,ΔfT, and ΔfL-2ΔfT) show a variation tendency consistent with the theoretical variation tendency. Variations of these detected frequencies indicate the change of dynamic behavior of transverse modes with different orders. By analyzing these frequency spectra, we find that even though competition between modes exists, transverse modes that belong to different degenerate families are not suppressed when degeneracy occurs. The frequency difference of modes with the same order is clearly detected in the vicinity of the degeneration point, and it is not observed away from the degeneration point. From the beam patterns (Fig. 4), we can observe that higher-order transverse modes are generated when the confocal resonator is tuned in the vicinity of the degeneration point. This phenomenon implies that as the resonant frequency of the high-order mode becomes equal to that of the low-order mode, the high-order mode becomes easier to generate in the cavity. The detailed frequency spectra are presented in Fig. 5, using which we determine the degenerate range. As the confocal resonator is tuned to the degeneration point, the two peaks (ΔfL-ΔfT and ΔfT) become closer to each other and finally superpose. The state in which these two peaks are superposed sustains for a short range of cavity lengths. Accordingly, we could obtain the width of the degenerate range, which is approximately 0.20 mm in this experiment. The degenerate range defined in this way ought to be more accurate because it is directly determined from frequency spectra.
Conclusions In this paper, the phenomenon of transverse mode degeneracy is researched based on the variation of frequency spectra, and a more accurate method for measuring the degenerate range (or lock range) is proposed. As the cavity length varies in the experiment, frequency spectra of laser beams are recorded in detail. The variation of dynamic behavior of transverse modes before and after the degeneracy is analyzed based on the frequency spectra. When the cavity satisfies the degeneracy condition, the two degenerate families of transverse modes (in 1/2 degeneracy) coexist in the cavity, neither of them is suppressed because of the competition between them. Meanwhile, the beam patterns measured in the experiment show that higher-order transverse modes are generated when degeneracy occurs. As the resonant frequency of the high-order transverse mode becomes degenerate with that of the low-order mode, the high-order mode becomes easier to generate, thus resonating with the low-order mode in the cavity. We achieve a more accurate measurement of the degenerate range with the frequency spectra measured in the experiment. The degenerate range is approximately 0.20 mm in this experiment. It exhibits a higher precision and is more accurate than the degenerate range determined using other methods. The results of this paper may provide some reference value for the application of transverse mode degeneracy.
Objective Laser is used as light energy and light tool in the manufacturing industry. Its characteristics such as including laser power, beam quality, and control of light, are not only the standards to measure performance of laser manufacturing system, but also the bases for selecting laser manufacturing system. Therefore, both laser power and beam quality must be considered seriously. Laser power is used as an index the processing capacity of the laser manufacturing system. Beam quality limits processing method, transmission distance, focal spot size, and processing quality. Under the condition of high average power pumping, thermal lens effect is generated, as the pump light is tightly concentrated at the laser gain medium. In the meantime, the spherical aberration of the thermal lens strongly affects the beam quality. Thus, how to improve beam quality, enhance the conversion efficiency, and shorten pulse duration are very important. In order to reduce the influence of thermal lens effect on beam quality of the solid-state laser, an Innoslab amplifier of spherical-aberration self-compensation based on ZEMAX physical optical simulation is proposed. In present study, we demonstrate a fiber-solid hybrid system based on an Innoslab amplifier with an all fiber laser and a double-passing amplifier as the seed source. This system combines the advantages of fiber laser, such as high beam quality, high electro-optical efficiency, and easy to obtain mode-locked narrow pulse, and those of solid laser, such as able to reduce the nonlinear effect and sustain high peak power.
Methods In this paper, we proposed a fiber-solid hybrid Innoslab picosecond amplifier with high beam quality and high repetition rate. The seed consisted of an all fiber laser and a double-passing end-pumped amplifier. During the experiment, an eight-passing Innoslab amplifier based on the spherical-aberration self-compensation theory was designed by using the sequence mode of ZEMAX software. The seed light realized eight-passing amplification in the slab crystal. Higher extraction efficiency was obtained by increasing the overlap area of seed light and pump light in the gain medium. In order to reduce thermal lens effect in the slab crystal thickness direction and its influence, two measures were taken. The first one was to connect the crystal with the heat sink by double-sides welding method, so that air holes in the welding layer were avoided, the flatness of the indium layer was improved, and the crystal surface was uniformly dissipated. The second one was to suppress the thermal distortion in the slab crystal thickness direction with spherical-aberration self-compensation, so that the beam quality degradation caused by the thermal lens effect was overcame.
Results and Discussions In order to better understand the phase variation of the light beams with positive spherical aberration, we used ZEMAX software to simulate the process according to the principles of geometrical optics (Fig. 7). If the two identical thermal lenses were symmetrically placed about the focus of the laser beam, the degradation of the beam quality caused by the first thermal lens can be compensated by the second thermal lens. Fig. 8 shows the Innoslab amplifier based on spherical aberration compensation theory. To reduce thermal lens effect in the slab crystal thickness direction and its influence, the method was to connect the crystal with the heat sink by double-sides welding method, so that air holes in the welding layer were avoided, the flatness of the indium layer was improved and the crystal surface was uniformly dissipated (Fig. 11). The output power from the all fiber laser was 2 W. The laser received further amplification in the double-passing end-pumped amplifier and the gain was dramatically enhanced by the Innoslab amplifier. Maximum output power of 28.4 W was achieved at a solid-state pump power of 117 W (Fig. 14). Meanwhile, the beam quality was well preserved with M2 factor of 1.3 by the Innoslab amplification structure which was favorable to the spherical-aberration compensation (Fig. 17). Figure 16 shows the spectral width of the Innoslab amplifier was narrowed to 0.21 nm, indicating remarkable gain narrowing effect. It can be seen that the gain narrowing effect broadened the pulse width of the double-passing amplifier to 10.6 ps (Fig. 10).
Conclusions In present study, we designed a picosecond fiber-solid hybrid Innoslab amplifier which was seeded by an all fiber laser and a double-passing end-pumped amplifier. The number of amplifiers and the gain were numerically simulated, and the Innoslab cavity was designed based on thermal lens effect compensation. The seed light realized eight-passing amplification in the slab crystal. The seed light injected into the Innoslab amplifier with average output power of 4.5 W and the repetition rate of 18 MHz. As a result, a laser pulse with average power of 28.4 W and pulse width of 10.6 ps was achieved under the pump power of 117 W, corresponding to an optical-to-optical efficiency of 20.4%. The beam quality factors M2 were 1.33 in the horizontal direction and 1.24 in the vertical direction. This system, which combined the advantages of the all fiber amplifier and the solid-state laser amplifier, enabled high repetition rate, and good beam quality with high gain picosecond pulses. It made significant contributions to many applications such as material micro-processing, laser ranging, and laser detection.
Objective High-brightness semiconductor lasers are preferable light sources for various applications, such as high-efficiency pumping sources, material processing, medicine and biology, and laser display. The critical design objective for high brightness semiconductor lasers is to maximize the output power while maintaining good beam quality. Conventional broad-area high-power laser diodes suffer from poor beam quality due to the excitation of the higher modes and beam filamentation. The tapered structure is introduced to enhance the output power while maintaining the high beam quality. However, the tapered lasers with common contact usually suffer from spatial hole burning and self-focusing at high continuous-wave (CW) output power. The use of separated electrode contacts for the tapered lasers can effectively suppress detrimental effects in the beam quality and increase the possibility for the improvement of the brightness. In present study, we fabricate a high brightness tapered lasers with separated contacts and analysis experimental data suitable for a better understanding of the impact of the current injected into ridge waveguide (RW) section and tapered amplifier section on output power and beam quality.
Methods The tapered laser with separated contacts is fabricated in present work. The epitaxial layer structure grown by metal organic chemical vapor deposition based on a GaAs substrate. After material growth, the ridge waveguide section, the tapered section, and the grooves are defined by standard i-line lithography and formed by inductively coupled plasma dry etching. The etched grooves are down through the active region, designed to restrict the unwanted mode travelling to the ridge waveguide section. The ohmic contact layer in taper section is etched to reduce the lateral current spreading. Then, a silica layer is deposited on the wafer by plasma enhanced chemical vapor deposition serve as electrical insulation, and insulation layer is removed from current injection windows. Next, metal p-electrode is deposited on the whole wafer by electron-beam evaporation, and then metal layer in the electrode separation area is removed using lift-off process. A metal n-electrode is formed by evaporation and metallization after substrate thinning. The wafer is cleaved and the front/rear facet is antireflection/high-reflection films coated, respectively (Rf≈0.1%, Rr≈96%). Finally, single emitters are mounted p-side down on AlN sub-mounts with AuSn solder. Golden wire bonding is used to contact the n-side with the sub-mounts for the measurements.
In this paper, we investigate tapered lasers with a ridge waveguide width of 5 μm, the RW length of 1500 μm, the taper section length 4000 μm, and taper angle of 5°. The output power, the beam propagation ratio, the intensity profile in the beam waist, and the lateral near-field profile are measured in dependence on the currents through ridge waveguide section and taper section. Device testing is performed under CW conditions.
Results and Discussions The CW output power of 7 W is achieved with the current through the ridge section IRW>300 mA. With the increase of Iridge, the threshold current of taper section decreases first, and then stays nearly constant (
Conclusions In present study, the impact of driving current on the characteristics of tapered lasers with electrically separated contacts has been investigated. Tapered lasers emitting at 975 nm providing 7 W of output power in CW operation is fabricated. The increasing of IRW induces the increase of optical gain and the reduction of the threshold current of taper section. In a certain IRW range, the improvement of beam quality on account of the decrease of pumping outside the ridge waveguide and reduction of the side lobes intensity in the RW section. At high current IRW, the unwanted higher-order modes are excited in the RW section and are amplified in the taper section, resulting in the deterioration of beam quality. At 6.4 W of the output power, the beam propagation ratio of M2=1.66 (1/e2) is obtained. The brightness of the tapered laser with separated contacts is 369 MW·cm-2·sr-1.
Objective Control of microstructure and residual stress, and optimization of process parameters based on the temperature field are important methods to improve mechanical properties of the cladding parts. A relatively accurate laser heat source model should be provided to reconstruct the temperature field during the formation of the cladding layer. First, an accurate mathematical model of hollow ring laser heat source, which should be verified by numerical simulation and the experiment results, needs to be constructed to obtain the fundamentals for a practical temperature field, microstructure distribution, residual stress control, and process parameters optimization. The temperature distribution inside the molten pool could be reflected precisely by an appropriate heat source model to simplify the numerical calculation and deeply explore the influence of temperature gradient and cooling rate on residual stress and actual solidification microstructure during the solidification and cooling process. Since Goldak proposed the classical double ellipsoid heat source in 1984, a wide variety of heat source equations have been generated based on the mathematical modeling evolution of solid heat sources. However, the mathematical model of the hollow ring heat source based on the inside-laser coaxial powder-feeding system was rarely studied. A simple and practical heat source equation should be established to verify the reliability of the model through the combination of experiment and theory analysis.
Methods Based on the finite element software called ABAQUS and the hollow ring laser system, the energy distribution discipline of the hollow ring laser was verified by the probe beam quality analyzer, and a formal hollow ring laser heat source equation was proposed. The energy density determined by numerical analysis was used to establish the final mathematical equation. Response surface method (RSM) was adopted after the hollow ring laser heat source equation was established. The fitting equation of molten pool size under mutative process parameters was conducted, and the three-dimensional size of the heat source model was parameterized through experiments.
Also, a laser-melting deposition model with certain thermal-physical properties was established, and a mobile heat source subroutine was written in Fortran language. The subroutine was called in the ABAQUS software, and the heat source was loaded by the life-and-death element method to simulate the temperature field and thermal history of a single wall under mutative cladding process parameters. To verify the reliability of the heat source model, the actual temperature of the melting pool during the experiment was measured by K-type thermocouple, paperless data recorder (HIOKI LR8501), and infrared camera monitoring equipment (InfraTec, VarioCam® hr head).
Results and Discussions The energy distribution of the hollow ring laser indicated a Gaussian distribution in the annular region, and the energy in the middle was higher than that on both sides of the annular region, as shown in
Conclusions The mathematical modeling of the three-dimensional hollow ring laser heat source was constructed to obtain an analytical formula using the idea of the normal distribution of revolution. The morphological parameters of the hollow ring laser heat source were determined by the shape of the molten pool and the value of the energy peak of the laser spot under mutative process parameters. The simulation results of the energy distribution of the heat source model indicated that the peak energy intensity was located at the edge instead of the center, and the vertical cross-section presented a “saddle”-shaped distribution. The measured results agreed well with the theory simulation, which confirmed the validity of the mathematical model.
The finite element simulation of the temperature field of the hollow ring heat source indicated that temperature peak distribution was close to the edge, which correspond to the mathematical model results. Due to the high energy at the edge, it could compensate the weak part of the central heat input to promote the uniform temperature distribution, which was consistent with the theoretical analysis of the energy intensity of the ring light source. The temperature peak continuously reduced, and the temperature trough was increasingly influenced by heat accumulation and conduction. The thermal history was simulated by the ABAQUS software, and its simulation curve was also consistent with the measured value, which confirmed that the hollow ring heat source model possessed good accuracy and applicability.
Objective Since Er 3+,Yb 3+∶glass is used in laser diodes (LD) pumped lasers, several studies on thermal properties of Er 3+,Yb 3+∶glass have been reported. However, there are still few related reports on the effect of pumping repetition frequency and the material thermo-optic coefficient on output energy. The pumping repetition frequency of LD and thermo-optic coefficient of Er 3+,Yb 3+∶glass affect the thermal focal length and ultimately change the output energy of the Er 3+,Yb 3+∶glass laser. Thus, in this study, we analyze the effect of pumping repetition frequency and the thermo-optic coefficient on output energy to guide material selection and output energy analysis for the end-pumped high output energy Er 3+,Yb 3+∶glass laser.
Methods In this paper, we design a passively Q-switched laser with LD end-pumped to measure output energy using a plano-concave cavity structure and Co 2+∶MgAl2O4 as the Q-switching crystal (Fig. 1). The measuring thermal focal length experiment is based on the critical cavity method and uses a flat-flat cavity to measure the thermal focal length. In the measurement of slope efficiency, the variation of the output energy with pumping power is measured without Co 2+∶MgAl2O4 in Fig. 1 under the same pumping conditions. Additionally, the thermal focal length formula, the matrix calculation method of the stable cavity, and the rate equation are used to analyze the phenomena in the above experiments.
Results and Discussions In a passively Q-switched LD end-pumped laser, we obtained that the output pulse energy of EY1, EY2, and EY3 decreases from 417, 401, and 378 μJ to 356, 337, and 329 μJ as the pumping repetition frequency increases from 10 Hz to 28 Hz (Fig. 2). First, we verified the phenomenon that the thermal focal length decreases with increasing pumping repetition frequency theoretically and experimentally to analyze the above phenomenon. Second, by numerical simulation of the resonator, it is found that in the plane-concave cavity, the beam radius of the front surface of the resonator decreases with increasing thermal focal length when the thermal focal length is greater than fTm(the thermal focal length corresponding to the minimum beam radius of the front surface of the resonator) (Fig. 5). Finally, the rate equation is solved using the Runge-Kutta algorithm to obtain the trend of the output pulse energy gradually decreasing with increasing pumping repetition frequency (Fig. 6). This is because the output pulse energy is positively correlated with the beam radius of the front surface of the resonator. After performing the free-running operation, the slope efficiencies of the three Er 3+,Yb 3+∶glass samples, EY1, EY2, and EY3 were 22.16%, 22.82%, and 23.14%, respectively (Fig. 7). Their thermo-optic coefficients are 34×10 -7, 29×10 -7, and 24×10 -7 K -1, respectively. The Q-switched output pulse energy after adding Co 2+∶MgAl2O4 to the above non-saturable cavity are 323, 363, and 391 μJ (Table 3). When the thermal focal length increases with a decrease in the thermo-optic coefficient (Fig. 3), the cross-sectional area of the laser beam in Er 3+,Yb 3+∶glass increases accordingly. The inversion population participating in stimulated radiation in Er 3+,Yb 3+∶glass also increases, which increases the slope efficiency. Moreover, since the Q-switched output energy is positively correlated with the beam radius of the front surface of the resonator, the Q-switched output energy also shows an increasing trend.
Conclusions In this study, we reveal the relationship among thermo-optic coefficient, pumping repetition frequency, slope efficiency, and Q-switched output pulse energy of an eye-safe solid-state laser with Er 3+,Yb 3+∶glass as the gain medium experimentally and theoretically, when the thermal focal length is greater than the value at the minimum point. The thermal focal length in Er 3+,Yb 3+∶glass decreases with an increase in the pumping repetition frequency, which decreases the cross-sectional area in the gain medium, and finally reduces the Q-switched output pulse energy. Thus, the Q-switched output pulse energy is negatively related to the LD pumping repetition frequency. Since the same pumping conditions, Er 3+,Yb 3+∶glass with lower thermo-optic coefficient has a larger thermal focal length, larger cross-sectional area of the stimulated radiation beam in Er 3+,Yb 3+∶glass, higher slope efficiency in a non-saturable cavity, and higher Q-switched output pulse energy. Thus, the Q-switched output pulse energy is negatively correlated with thermo-optic coefficient and pumping repetition frequency. Therefore, we can reduce the thermal lens effect by reducing the LD pumping repetition frequency and selecting low thermo-optical coefficient Er 3+,Yb 3+∶glass to achieve high output pulse energy, which will help human eye-safe solid-state lasers use this material further to develop the ranging, lidar, and medical fields.
Objective At present, the main research is focused on lower-noise and higher-gain-bandwidth product of an APD in order to adapt to the evolving optical fiber communication systems. Usually, lower noise is related to higher speed, because a high gain tail can result in higher noise and longer transmission time in the gain distribution. Therefore, a major issue is how to effectively reduce noises in the APD research. An effective way to reduce noises is using beneficial heterojunction structures to impact ionization engineering (I 2E) design. A heterojunction structure is used in this method to provide spaces with different ionization threshold energies. This structure is more localized than the impact ionization that can be achieved in a spatially uniform structure. Through impacting ionization engineering, it has been achieved in the GaAs/AlGaAs material system with ultra-low noise of k=0.1. Now we have known that the heterojunction structure can reduce the multiplication noise of an APD by controlling the spatial separation of electrons and holes. There is a specific design which is to place a thin narrow band gap layer with a lower threshold energy adjacent to a wider band gap layer with a higher threshold energy, so that more ionization events occur in the thin narrow band gap layer with a lower threshold energy. At present, the first choice is the use of InGaAs/InP APD detectors for optical fiber communication. Because InP material has the advantages of large electron-hole ionization ratio k and low excess noise. In addition, the lattice constants of InP and In0.53Ga0.47As are very small. High quality extension can be realized by molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD). Here, we design InP/InGaAsP SACM APD through impact ionization engineering. An InGaAsP multiplication layer is inserted into the multiplication layer of a traditional InP-based APD, and there are two regions with different ionization threshold energies in the multiplication layer to make ionization events occur more intensively. In addition, the ionization of electrons and holes is regulated spatially in order to reduce the noise.
Methods A kind of separation of absorption region-charge region-multiplication region (SACM) InGaAs/InP avalanche photodiodes is designed. Through the impact ionization engineering, an InP APD with double charge layers and double multiplication layers is designed. In order to control the location of impact ionization and to reduce the noise, a classification of ionization threshold energy is set in the multiplication region of this structure. The device simulator Silvaco is used to model this device. The energy band structure, electric field distribution, dark current, 1.55 μm pulse light response current, gain, etc. are simulated and calculated. At a 30 V voltage, a lower k value of 0.15 can be obtained for the new structure. And compared with the conventional SACM InP APD, the results show that the APD devices with new structures can obtain better noise characteristics.
Results and Discussions The device simulator Silvaco is used to calculate and simulate the light and dark currents and gain of the device. The photocurrent of the device is obtained under the condition of the incident wavelength of 1310 nm, the dark current of the APD detector is less than 10 nA under the punch-through voltage, the punch-through voltage of the device is about 20 V, and the breakdown voltage is about 30 V. The device gain is set as 1.0 at the punch-through point. When the bias voltage rises by 30 V, the gain of the device increases from 1.0 to 8.0 (Fig. 2). The ionization coefficients α and β of holes and electrons are simulated and calculated in the device simulator Silvaco. We extract the hole and electron ionization coefficient values of 20 points, integrate them to calculate the k value (α/β) of each point and take the average value to obtain the noise coefficient k=0.15. The excess noise and gain simulation diagram are obtained, and we can see that I 2E APD has a low excess noise (Figs. 3 and 4). The same treatment is also performed on the conventional InP APD. The ionization coefficient values of holes and electrons at 20 points are selected and integrated to obtain the excess noise coefficient k=0.35. The design of impact ionization engineering is to insert a thin layer with a relatively low threshold energy in the area with a higher ionization threshold energy, so that the most ionization events occur in the trap layer with a low threshold energy, which leads to more certainty in the location of impact ionization, and thus the noise is reduced. From the partial energy band structures of the two devices in the equilibrium state, we can see that the well layer structure is designed in the I 2E APD energy band (Fig. 5).
Conclusions The charge layer and multiplication layer of the conventional InP/InGaAs SACM APD are improved through impact ionization engineering, and an APD with double multiplication layers and double charge layers is designed. Two InGaAsP layers with lower ionization threshold energy are inserted between the InP multiplier layer and the InP charge layer, so that the most ionization events occur in the InGaAsP well layer. Through the structure designed by impact ionization engineering combined with the step distribution electric field in the multiplication region, the spatial modulation of the impact ionization events of electrons and holes leads to a lower excess noise with k=0.15, which has a lower excess noise compared with the conventional InP APD with the single multiplication layer. The design of this structure takes into account the positive influence of the dead zone on the excess noise and also takes into account the positive effects of energy band design and ionization threshold energy on the position of carrier multiplication. This will have a great guiding effect on device preparation in future.
Objective Planar waveguide amplifiers have the advantages of slab and fiber ones and become an essential branch of the high average power solid-state lasers. Yd∶YAG planar waveguide amplifiers have the potential for higher output than Nd∶YAG due to the less generated heat and higher extraction efficiency under the same absorption pump power. The absorption cross-section of Yd∶YAG is smaller than Nd∶YAG; a higher doping concentration of the core and end-pumping must fully absorb the pump power. To maximize extraction efficiency, the doping concentration is significantly reduced due to self-absorption. To ensure that the pump power can be fully absorbed, extending the length of the core is necessary. However, the size of the core is limited by the process conditions. If the doping concentration is sufficiently small, the end-pumping cannot meet the absorption of pump power requirements. In this study, we design a multipass pumped planar waveguide with the low-doped Yd∶YAG core to prolong the pump absorption length. The amplifier using a multipass pumped planar waveguide shows higher pump absorption, better absorption uniformity, and higher optical-optical efficiency than the double-clad planar waveguide amplifier. We hope that the new structure can provide methods and ideas for designing and optimizing planar waveguide lasers.
Methods Based on the theory of single-mode transmission and laser mode competition in the planar waveguide, the core and inner cladding of the planar waveguide are Yb∶YAG with a thickness of 0.2 mm and a doping concentration of 1% and Er∶YAG with a thickness of 0.5 mm and a doping concentration of 0.5%, respectively. The core and inner-claddings form the core area, which is 80.0 mm×16.0 mm×1.6 mm. YAG is bonded around the core area with a thickness of 1.2 mm. The outer-claddings of 1 mm cover the core and expansion areas, forming a double-cladding waveguide with the core area and a single-cladding waveguide with the expansion area. A multipass pumping ray path in the waveguide is formed through internal reflections on the waveguide surfaces. Symmetrical double-end pumped and single-pass power extraction configurations are adopted for laser amplification. Based on the Yb 3+ laser kinetics model, we develop a three-dimensional (3D) laser amplification model using ray tracing and finite element methods. We use the model to simulate double-clad planar waveguide and multipass pumped planar waveguide amplifiers. Besides, we compare their pump absorption and amplification extraction characteristics under 10 kW pump power and 200 W-injected seed power. The temperature distribution is simulated based on the results from the laser magnification model.
Results and Discussions Compared with a double-clad planar waveguide amplifier, a multipass pumped planar waveguide amplifier exhibits higher output power and optical-optical efficiency (Fig.9). The simulation results using the laser amplification model show that the planar waveguide core absorption coefficient is 0.24 cm -1, which is 77.3% lower than the passive absorption coefficient, strongly affected by nonlinear absorption (Fig.11). The pump absorption decreases with a decrease in the absorption coefficient. The multipass pump absorption efficiency is still above 90% due to an increase in the absorption length (Table 2). The absorption power density distribution of the core simulated by the laser amplification model is different from that of the passive absorption coefficient model, which is caused by considering the effects of the laser and pump intensities on pump absorption (Fig.12). Because of better pump absorption uniformity, although the absorbed power increases, the maximum temperature does not increase significantly. However, the temperature distribution in the width direction is asymmetric, which may cause low actual beam quality (Fig.13). The maximum thermal stress of core is only 18.5% of the safety limit, which is lower than that in the double-clad planar waveguide amplifier (Table.3). The extraction efficiency of the multi-channel pump is higher (Fig.16) since the high pump intensity is preferred to extract the absorbed power (Fig.15).
Conclusions A noble planar waveguide is designed for a high-power planar waveguide laser amplifier with the low-doped Yd∶YAG core. multipass pumping of the core is achieved through bonding YAG around the core area and high reflection coating on reflective surfaces. The geometric of the waveguide is reshaped and optimized to improve pump absorption and uniformity. The double-clad planar waveguide amplifier and multipass pumped planar waveguide amplifier are simulated using a 3D laser amplification simulation model combining the ray tracing and finite element methods. When the pump power is 10 kW, the optical-optical efficiency of the multipass pumped planar waveguide and double-clad planar waveguide amplifier becomes saturated with 200 W of injected seed. When the injected seed is 200 W, the multipass pumped planar waveguide threshold pump power becomes 500 W, which is almost the same as the double-clad planar waveguide amplifier. However, the slope and optical-optical efficiencies are higher than those of the double-clad planar waveguide amplifier. When the injected seed is 200 W, and the pump power is 10 kW, the absorption power density distribution simulated using the laser amplification model significantly differs from that of the passive absorption coefficient model. The simulation results using the laser amplification model showed that in the multipass pumped planar waveguide amplifier, the pump absorption efficiency is 93.3%, the output power is 7311 W, and the optical-optical efficiency is 71.1%. It is significantly higher than that of the double-clad planar waveguide amplifier. The multipass pumped planar waveguide amplifier shows better pump absorption uniformity and a smaller risk of thermal damage.