Visible Broadband Raman Lasers of H2∶CH4∶CO2 Gas Mixture Pumped by 532-nm Laser
Zheng Tiancheng, Cai Xianglong, Li Zhonghui, Shen Chencheng, Liu Dong, Liu Jingbo, and Guo Jingwei
Objective Mixtures of H2, CH4, and CO2 gases pumped using a 532-nm laser were studied as visible broadband Raman lasers to illuminate multispectral lasers. To conduct mixed-gas experiments, we must understand the Raman line-widths and gain coefficients of the three gases H2, CH4, and CO2, which are critical for adjusting the ratio of these three gases. The optimization process for mixing H2, CH4, and CO2 gases depends on two parameters: the input pump energy and the partial pressures of these gases. Notably, the most important step is to fully present the pressure- and pump energy-dependence of the Raman components of each order in H2, CH4, and CO2 gases. We hope to achieve simultaneous outputs of some spectral lines with nearly equal conversion efficiencies by controlling the pressure ratio between the three gases during the experiment of mixing the three Raman-active gases.Methods A schematic of the experiments used to study the multiwavelength Raman lasers generation using CH4, CO2, and H2 gases and their mixture in a single Raman cell is shown in Fig. 1. The radiation source was Nd∶YAG second harmonic at 532 nm, obtained from Beamtech Optronics Co., Ltd., with a spot diameter of ~8 mm, a divergence angle of less 1 mrad, and a pulse duration of 6 ns. All the experimental results were achieved at a fixed repetition frequency of 1 Hz. First, the pump beam was passed through a Pellin-Broca prism to avoid backward-shifted radiation. Then, it was focused using a lens L1 (f=1000 mm) at the center of the 1.80-m long Raman cell. Thereafter, the pump and Stokes beams were recollimated at the exit of the cell window using the same lens L2 (f=1000 mm) and dispersed via the same Pellin-Broca prism. Finally, the dichroic mirror was used to measure the backward scattering.Results and Discussions Fig. 3 presents the energy conversion efficiencies of the pump energy to various Raman components against the gas pressure fixed on the pump energy of 103 mJ for the H2, CH4, and CO2 gases, respectively. There are clear differences in energy distribution among the multiple orders of Stokes and anti-Stokes scattering in each of the three gases. We explain the favored BS1(the first Stokes in the backward direction) in CH4 for the large ratio between the forward and backward gain coefficients. Further, the strongest high-order Stokes in H2 can be observed owing to the biggest Raman gain. Four-wave mixing can play a considerable role in high pressure owing to the smallwave vector mismatch in CO2。Notably, various Stokes components among the three gases at the low pressure of 0.5 MPa have similar features (Fig.4): the conversion efficiencies for various Stokes components, especially for S1, will be stable at high pump energy. Fig.5(a) shows that the conversion efficiencies of different Raman components vary with the pump energy for a mixture of 0.45 MPa CH4, 0.4 MPa CO2, and 0.3 MPa H2. We can obtain 13 spectral lines (Table 2), in which the 574-, 630-, 683-, and 771-nm lasers own nearly equal conversion efficiencies (6.5%--8%) at the high energy region above 180 mJ, and the 532-, 853-, and 954-nm lasers reach 14%, 2.1%, and 1.9% efficiencies, respectively. Compared with the individual gas, we found that CH4 S1 (the first Stokes in the forward direction) yields the minimum threshold, followed by H2 and CO2. Furthermore, the S1 conversion efficiency of CH4 in the mixture is almost as good as that of the pure CH4 gas, while the S2 conversion efficiency (the second Stokes in the forward direction) is approximately two times than that of S1 owing to the additional H2 harming the BS1 in CH4. Compared with the unique H2 gas, the decrease of S1 (683 nm) in the mixture is due to a new pump laser that generates 853-nm laser by pumping CH4, which will compete with the formation of S2 (954 nm) in H2, leading to the decrease of the 954-nm laser. More attention should be given to investigate specific competitive mechanisms in the mixture.Conclusions In this paper, we have investigated the multispectral Raman system using H2, CH4, and CO2 gases as Raman media. The high-order Stokes light in H2 is the strongest among the three gases, especially S2. CH4 has the strongest BS1, causing the fierce competition with forward Raman light. The effect of S2 produced via four-wave mixing in CO2 gas is more obvious under high pressure. At low pressure, the conversion efficiency of each Stokes component (especially S1) of the three gases can reach saturation at high pump energy. We achieve the simultaneous outputs of 13 spectral lines, covering the wide visible spectral region by controlling the pressure ratio between different gases in the experiment of mixing the three Raman-active gases. A strong competition and interaction exist between the Raman-active gases in the mixture, especially between H2 and CH4gases. Thus, we conclude that combining multiple gases using a single laser and a single Raman cell can produce multispectral Raman laser outputs. Similarly, using 355-nm or other short-wavelength lasers, rich spectra can be generated, which will have paramount applications in the fields of laser color display, biomedicine, underwater communications, and atmospheric detection.
  • Oct. 18, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 21 2101006 (2021)
  • DOI:10.3788/CJL202148.2101006
Research on Frequency Stabilization Technology of Modulation Transfer Spectroscopy Based on Rb 87
Hong Yi, Hou Xia, Chen Dijun, Zhou Cuiyun, Huang Minjie, Song Tieqiang, Wang Guibing, Zhao Jian, and Chen Weibiao
Objective Compared with the traditional 780 nm saturated absorption spectroscopy, modulation transfer spectroscopy has higher sensitivity and resolution. It is very suitable for laser frequency stabilization. Nowadays, most of the researchers focus their attention on the hyperfine D2 line 5 2S1/2 (Fg = 2)→5 2P3/2 (Fe= 3) of Rb 87 modulation transfer spectrum frequency stabilization technology. However, there are few reports on the hyperfine D2 line 5 2S1/2 (Fg=1)→5 2P3/2 (Fe=0,1,2). The main reason is that the saturation absorption peak and error signal amplitude of the latter are much smaller than the former. But the frequency of hyperfine D2 line 5 2S1/2 (Fg=1)→5 2P3/2 (Fe=0,1) cross resonance peak of Rb 87, which is suitable for the re-pumping laser needed by cold atomic clock system to cool rubidium atoms, has great research significance. Therefore, it is necessary for us to study it. Methods By main oscillator power amplifier and efficient frequency doubling of 1560 nm distributed feedback laser with a periodically poled lithium niobate (PPLN) crystal, we have obtained a 780 nm laser about 200 mW. We mix the modulated probe laser with the local signal and get the error signal of modulation transfer spectrum. In order to get better signal-noise ratio (SNR), the signal is amplified and passed by a low-pass-filter. We also change the modulation frequency and modulation depth of electro-optic modulator (EOM) to increase the amplitude of the error signal. The laser frequency is locked to the D2hyperfine transition 5 2S1/2(Fg=1)→5 2P3/2(Fe=0,1) of Rb 87 via proportional-integral-derivative (PID) controller and we get a 20 h laser frequency stability data finally. We change the polarization of probe laser and pump laser by adjusting the optical structure, so as to study the influence of different polarization states on the error signal of modulation transfer spectrum. By changing the absorption length of rubidium cell (25 mm and 50 mm), we study its effect on laser frequency stability. Results and Discussions We find that the error signal on the hyperfine D2 line 5 2S1/2 (Fg=1)→ 5 2P3/2 (Fe=0,1,2) of Rb 87 can be influenced by different polarization of probe laser and pump laser. Only when the probe laser and pump laser are linearly polarized in the same direction can there be a clear and large single error signal on the D2 hyperfine transition 5 2S1/2(Fg =1)→5 2P3/2(Fe=0,1) of Rb 87(Fig. 3). There will be no locking error on this transition compared with probe laser and pump laser are perpendicular polarized or circular polarization. We finally lock the laser frequency on this transition for 20 h and measure laser frequency is 384234489.5 MHz (the theoretical calculation is 384234490.2 MHz, with a difference of 0.7 MHz). The peak to peak amplitude of frequency fluctuation is 105 kHz in 20 h. The Allan deviation of the frequency stability is 3.7×10 -11when the integration time is 1 s and 4.6×10 -12 when the integration time is 10000 s (Fig. 7). The influence of rubidium cell absorption length on frequency stability is studied. We find the amplitude of error signal in 50 mm is higher than that in 25 mm (Fig. 5). The 1 h frequency fluctuation of rubidium cell in 50 mm is 80 kHz, which is less than that in 25 mm (152 kHz). The 1 s frequency stability of rubidium cell in 50 mm is 3.7 × 10 -11, which is also less than that in 25 mm (5.4 × 10 -11), but 1000 s frequency stability about the same (Fig. 6).The results show that increasing the absorption length of a single rubidium cell will produce a higher SNR, which will be beneficial to the short-term laser frequency stability. However, its effect on the long-term laser frequency stability is not significant. Perhaps the long-term laser frequency stability depends on other factors, such as the temperature control of rubidium cell and EOM. Conclusions The reason for the difference of modulation transfer spectroscopy (MTS) error signal, which is caused by probe laser and pump laser in different polarization directions, on the hyperfine D2 line 5 2S1/2 (Fg = 1) → 5 2P3/2 (Fe= 0,1,2) of Rb 87 is analyzed theoretically. The influence of the absorption length of rubidium cell on the frequency stability is studied. The laser frequency is locked on the D2hyperfine transition 5 2S1/2(Fg =1)→5 2P3/2(Fe=0,1) of Rb 87 via modulation transfer spectroscopy, The results show that the peak to peak frequency fluctuation is 105 kHz in 20 h. The Allan deviation of the frequency stability is 3.7×10 -11when the integration time is 1 s and 4.6×10 -12 when the integration time is 10000 s, which meets the requirement of cold atom platform for cooling atoms.
  • Oct. 18, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 21 2101003 (2021)
  • DOI:10.3788/CJL202148.2101003
All Polarization Maintaining Fiber-Based Optical Comb System with Nonlinear Loop Mirror Mode-Locking
Lu Shiyu, Hao Qing, Liu Tingting, Chen Fei, Ren Xinyi, Shen Xuling, Yan Ming, and Zeng Heping
Objective Compared with the traditional laser, fiber laser has the advantages of simple structure, high stability, and small size, becoming the research focus of practical optical comb. The structure of fiber-based optical comb with nonlinear polarization rotation (NPR) mode-locking is simple, but the refractive index of non polarization-maintaining fiber is easily affected by vibration and temperature, and the long-term stability of mode-locked state and phase-locked state of the oscillator is poor. The optical frequency comb based on semiconductor saturable absorption mirror (SESAM) can improve the influence of environmental noise due to the fully polarization maintaining fiber structure and appropriate damping measures. However, SESAM has the risk of light-induced damage during long-term use. It has a relaxation time of several hundred femtoseconds to picoseconds, bringing additional phase noise and instability to the optical comb. Most of the reports about the long-term stable operation of the comb were obtained in the constant temperature laboratory. In this study, we report a compact optical frequency comb with all polarization maintaining fiber structure. The system had kept stable operation for a long time in the external environment of 10 ℃ temperature fluctuation, and the mixed gas detection experiment was completed. This paper provides a feasible method for realizing practical optical frequency comb.Methods In the study, a mode-locked fiber laser with nonlinear amplifying loop mirror (NALM) was used as the seed source of the optical comb, which was placed in a sealed cavity to achieve isolation from the external environment. Then, the optical power amplification, pulse width compression, and spectrum broadening were studied by using cascaded erbium-doped fiber amplifier, single-mode polarization-maintaining fiber (PMF), and highly nonlinear fiber (HNLF). The broadened spectrum was injected into the f-2f self-referenced detector to detect the carrier envelope offset frequency (f0) signal. Then, by precision controlling the local temperature of the mode-locked fiber oscillator, the length of a piezoelectric transducer, and the current of the pump diode, long-term repetition rate (fr) and f0 locking of the optical frequency comb was achieved. Finally, a high non-linear dispersion shifted fiber (HN-DSF) was connected to the comb application end to generate flat spectrum at 1350--1550 nm. Besides, a gas sample cell filled with 12CO and 12C2H2was used to exam the mixed gas detection experiment. Results and Discussions The repetition rate of the mode-locked laser was 75.27 MHz, and the average power was 4.1 mW. The pulsed light was amplified to 196 mW by a cascaded erbium-doped fiber amplifier, and then the pulse width was compressed to less than 100 fs by a section of PMF fiber. The supercontinuum of the compressed pulse was realized by a segment of HNLF fiber from which the spectrum covered 1000--2200 nm [Fig. 2(a)]. The f-2f self-referenced detector detected f0 signal with signal to noise ratio of 40 dB and line width of 5 kHz [Fig.2(b)--2(c)]. In long-term operation, the drifts of fr and f0 (24 h) in the open-loop were reduced from 6.2 kHz and 310 MHz to 0.51 kHz and 26.9 MHz, respectively [Fig. 4(a) and (b)]. The drifts were further reduced to 10 Hz and 700 kHz in the partially feed-back loop, which satisfied the servo capability of the phase-locked circuit of the system [Fig. 5(a) and (b)]. Finally, the standard deviations of fr and f0 after locking were 358 μHz and 248 mHz in 100 hours operation, respectively [Fig. 6(a)]. In the experiment of gas mixture detection based on the optical frequency comb, a flat spectrum from 1250--1650 nm was generated from a segment of HN-DSF fiber, covering the absorption spectra of various gas molecules including 12C2H2, 12CO, and H2O [Fig. 2(d)]. The obtained 12C2H2 gas detection data was basically consistent with the simulation results of HITRAN database, and the standard deviation of their deviations was only 2.4% [Fig. 7(c)]. Conclusions An all polarization-maintaining fiber optical frequency comb for outdoor application is proposed. The size of the optical frequency comb is 330 mm×340 mm×65 mm. The frequency stability of the optical frequency comb system is about 4×10 -12 in 1 s, and the system can keep fr and f0 stable for a long time under the temperature fluctuation of (22±5) ℃. In addition, in the experiment of wide spectrum detection of mixed gas based on the optical comb, the gas detection data obtained is consistent with the simulation results of HITRAN database, indicating that the optical comb system can be rudimentarily used for outdoor applications such as spectral analysis.
  • Oct. 18, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 21 2101002 (2021)
  • DOI:10.3788/CJL202148.2101002
Wavelength-Switchable Single-Longitudinal-Mode Thulium-Doped Fibre Laser with Multi-Channel FP-FBG
Wang Weili, Yan Fengping, and Zhang Luna
Objective The fibre laser working in the eye-safe 2.0-μm band is an important pump source for producing the ~3--5 μm mid-infrared laser, and the single-longitudinal-mode fibre laser has the characteristics of large gain, good coherence, and stable spectrum. This laser can be applied to laser optical radio, coherent optical communication, optical fibre sensing, optical fibre remote sensing, and other fields having large light source requirements. The wavelength-switchable fibre laser has a flexible output laser wavelength, which has great application value in the wavelength division multiplexing and multi-parameter sensing systems. The single-longitudinal-mode thulium-doped fibre laser operating in the 2.0-μm band has a wide range of applications, and it is necessary to further study and optimize its performance. Single-longitudinal-mode fibre lasers have excellent and better power stability than the traditional semiconductor lasers, and their modulation amplitude does not change with the modulation frequency. However, there are very few reports on the 2.0-μm-band spatial optical network. The 2.0-μm-band wavelength division multiplexing system requires the single-longitudinal-mode and multi-wavelength thulium-doped fibre laser. In our study, we reported a three-wavelength-switchable single-longitudinal-mode thulium-doped fibre laser based on a Fabry-Perot fibre Bragg grating filter. Under the normal room temperature condition, this laser can obtain the single-longitudinal-mode, high stability, ultra-high optical signal-to-noise ratio, and high power output. Therefore, this study has an important application value in space optical communications.Methods The experimental structure of the proposed laser is shown in Fig. 6. A 793-nm laser diode was used as the pump source, pumping a 2-m thulium-doped fibre. The circulator ensured the one-way operation of the optical path in the cavity; a three-channel narrow-band Fabry-Perot fibre Bragg grating filter and a fibre Bragg grating were fabricated using the phase-mask method. The fibre Bragg grating was placed on a translation stage and its reflection wavelength was changed using stress to make it overlap with each channel of the Fabry-Perot fibre Bragg grating. The optical signals passing through the filters can be selected and most of the longitudinal modes can be suppressed to vary the laser wavelength. The polarization controller balanced the gain and loss of the intracavity signal to stabilise the output with the highest optical signal-to-noise ratios. Two 50∶50 couplers formed an 8-shaped sub-cavity, which effectively increased the longitudinal mode interval in the composite cavity, so that each channel of the Fabry-Perot fibre Bragg grating filter obtained single-longitudinal-mode lasing. The 10% port of the 90∶10 coupler was used to output the laser, and the 90% port was connected to the main cavity. By reasonably adjusting the length of each resonant cavity, the single-longitudinal-mode output of the proposed laser was realized.Results and Discussions The transmission spectrum of the self-made Fabry-Perot fibre Bragg grating filter had three narrow-band filtering channels with the centre wavelengths of 1941.48, 1941.57, and 1941.65 nm; the corresponding 3-dB bandwidths were 0.060, 0.054, and 0.066 nm, respectively. The 3-dB bandwidth of fibre Bragg grating was 0.11 nm and the reflectivity was ~97%. The centre wavelength of the fibre Bragg grating was matched with the centre wavelength of the three transmission channels of Fabry-Perot fibre Bragg grating through a translation stage (Fig.5). The designed 8-shaped sub-cavity expanded the longitudinal mode interval in the cavity to 0.3 nm, which was greater than the 3-dB bandwidth of each channel of the Fabry-Perot fibre Bragg grating filter, ensuring the single-longitudinal-mode lasing in each channel (Fig. 7). Switching between different laser wavelengths was achieved by changing the reflection wavelength of the fibre Bragg grating under the normal room temperature condition and the output lasers with three different wavelengths (Fig. 8). The wavelength fluctuations were less than the minimum resolution of optical spectrum analyser, which was 0.05 nm. Each lasing’s power fluctuations were less than 0.39 dB, 0.61 dB, and 0.55 dB (Figs. 9 and 10). The experimental results indicated that the laser could work with a good stability in 50 min. Using a spectrum analyser to observe the frequency spectra of the output laser, it was seen that the laser worked at the single-longitudinal-mode operation stably (Fig. 11).Conclusions In the present study, first, the Fabry-Perot fibre Bragg grating filter is analyzed theoretically. Based on the filter, a three-wavelength-switchable single-longitudinal-mode thulium-doped fibre laser is verified. Then, the narrow-band Fabry-Perot fibre Bragg grating and fibre Bragg grating are fabricated. The laser wavelength can be switched among three wavelengths by stretching the fibre Bragg grating in the cavity through regulating the stress adjustment frame. The laser works stably at the single-longitudinal-mode operation by adjusting the polarisation controller. An 8-shaped passive sub-cavity is employed to expand the longitudinal mode spacing. At 24 ℃, the laser wavelengths were 1941.48, 1941.57, and 1941.65 nm and the corresponding optical signal-to-noise ratios were 61 dB, 61 dB, and 60 dB, respectively. The stability of lasing was measured in 50 min. The output power fluctuation of each lasing was less than 0.39, 0.61, and 0.55 dB, respectively. The wavelength fluctuations were less than 0.01 nm, which was less than the optical spectrum analyzer’s minimum resolution of 0.05 nm. Therefore, the three-wavelength switchable thulium-doped fibre laser has stable single-longitudinal-mode output characteristics and can be applied to the fields of optical communication and optical fibre sensing in the 2.0-μm band.
  • Oct. 18, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 21 2101001 (2021)
  • DOI:10.3788/CJL202148.2101001
Design and Simulation of a Colloidal Quantum Dot Vertical-Cavity Surface-Emitting Laser
Xiang Guohong, Jia Siqi, Li Depeng, Ma Jingrui, Liu Pai, Wang Kai, Hoi-Sing Kwok, Yu Mingbin, and Sun Xiaowei
Objective Featuring wide bandgap tunability, high quantum efficiency, and cost-efficient solution processible fabrication methods, colloidal quantum dots (QDs) have been studied and applied in various optoelectronic devices including photo detectors, light-emitting diodes (LEDs), and solar cells. In addition to applications based on the absorption and spontaneous emission of colloidal QDs, their stimulated emission potential has attracted extensive research interests, aiming toward a landmark target: the realization of the colloidal QD laser diodes. In the study of colloidal QD lasers, different laser architectures have been demonstrated, including Fabry-Perot cavity, distributed feedback laser cavity, whispering gallery mode cavity, and photonic crystal microcavity. The optical gain has been successfully realized in colloidal QDs under direct current pumping, demonstrating a major progress toward electrically pumped colloidal QD lasers. Furthermore, a dual function device based on specially engineered QDs that can function as an optically pumped laser and an LED is fabricated and characterized, revealing a promising pathway for realizing colloidal QD laser diodes. Different from edge-emitting lasers, vertical-cavity surface-emitting lasers exhibiting surface-emitting properties, wafer-level fabrication & characterization capability, and array integration ability have been widely used in optical fiber communication, laser printers, computer mouse, and three-dimensional facial recognition fields, etc. Here, we propose and design a colloidal quantum dots vertical cavity surface emitting laser, combining with a quantum dots light-emitting diode like current injection structure to realize the electroluminescence ability.Methods As shown in Fig. 1, the QLED-like structure containing the QD gain medium is sandwiched by two high-reflective distributed feedback reflectors to form a vertical-cavity surface-emitting laser(VCSEL)-like device. The device is designed to work under optical or electrical pumping. The DBR parameters and cavity lengths, are determined by numerical simulations with optimal performance. A DBR mirror is formed by periodically arranging two materials with different refractive indices. The reflectance spectrum is determined by both the DBR materials and DBR periods. Herein, we designed and calculated two types of DBRs with different periods (Fig. 3): (a) SiNx/SiO2 DBR and (b) TiO2/SiO2 DBR. It is found that 10 periods of the designed dielectric DBR can realize a peak reflectance of greater than 99%. The cavity length is a crucial parameter of the VCSEL device. After determining the DBR parameters, the permitted longitude modes inside the cavity can be tuned using the cavity length. Here, we use the FDTD method to build the designed QD-VCSEL device model and sweep the cavity length parameter. The current injection structure along the vertical direction includes the QD gain materials, electron and hole transmission layers, and electrodes. To tune the effective cavity length while retaining the optimized current injection capability, transparent ITO electrodes are selected and designed according to a suitable thickness. By theory, the smallest cavity length of a VCSEL device is λ/2. Thus, based on this length, the current injection structure and the thickness of the gain medium are fixed, while the thickness of the transparent ITO electrode is used to change the cavity length and then tune the resonant mode (Fig. 5). In addition to the λ/2 cavity length device, a 3λ/2 cavity length device is designed and simulated to theoretically optimize the optical parameters.Results and Discussions Under optical excitation, the designed λ/2 cavity length QD-VCSEL device can support single-mode lasing at 629.5 nm with a cavity length of 172 nm. The calculated quality factor is 259632. Alternatively, the 3λ/2 cavity length device can be optimized with a 520-nm cavity length. The lasing mode is realized at 632 nm, and the quality factor is 148291. Compared to the cavity with the smallest cavity length, a longer cavity suffers further optical loss while facilitating a thicker gain region. However, a considerably longer cavity length is not favored because of the difficulty in the formation of a very thick QD layer with a high concentration. The simulated far-field pattern reveals that the designed devices achieve a low output beam divergence, comparable to conventional VCSEL devices, which is an intrinsic advantage of this type of semiconductor laser. This work proposes a new scheme for realizing QD laser diodes, providing a theoretical basis and a parameter reference for future experimental verification.Conclusions In this work, a CdSe QD vertical-cavity surface-emitting laser is designed. The QD-VCSEL device is simulated with a QLED-like structure sandwiched by two dielectric DBR mirrors. The DBR parameters and cavity lengths are determined by numerical simulations with optimal performance. Single-longitude mode lasing can be supported by two designed cavities with different lengths with a maximum quality factor Q over 250000. The new solution toward electrically pumped colloidal QD lasers is revealed with our design, along with the theoretical model and key factors, which can be helpful in subsequent experimental work.
  • Sep. 29, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 19 1901005 (2021)
  • DOI:10.3788/CJL202148.1901005
Complex Multi-Pulse Interactions in Harmonic Mode-Locked Fiber Laser Based on Acousto-Optic Effect
Zhang Xintong, Wang Xiaocong, Huang Qi, Huang Zhiyuan, Luo Zhuozhao, Zhou Gengji, Jiang Xin, Leng Yuxin, and Pang Meng
Objective Because of their high application potential in long-distance optical communications and ultrafast laser physics, optical solitons, which are localized structures in nonlinear systems, have piqued the interest of researchers. During long-distance propagation, optical solitons can interact with each other, resulting in a variety of bound-soliton states known as “soliton molecules”. Therefore, mode-locked lasers that can support the long-distance propagation of multiple solitons within their cavities are widely regarded as ideal platforms for studying soliton interactions and dynamics. However, the studies of the complex interactions of several optical solitons are difficult in traditional passive mode-locked lasers because fast drifts and the frequent collisions of solitons caused due to intense soliton interactions can degrade the stability of the laser mode-locking operation. In this paper, we use a high-repetition-rate optomechanically mode-locked fiber laser to successfully study the complex interactions of many optical solitons. The strong optomechanical effect in a short length of solid-core photonic crystal fiber (PCF) allows forming a robust optomechanical lattice in the laser cavity, and multiple solitons can be stably trapped within each cycle of the optomechanical lattice. Experimental results reveal that complex soliton interactions can be observed and partially controlled in this optomechanically mode-locked fiber laser, highlighting the significant potential of this unique optomechanical fiber laser system for studying complex soliton dynamics.Methods To investigate the complex phenomena of multi-pulse interactions, we created an optomechanically mode-locked fiber laser with a short solid-core PCF length as the harmonically mode-locking element. Because of the strong coupling between optical and acoustic waves in the PCF, a robust optomechanical lattice formed in the laser cavity, dividing the laser cavity into two halves. Multiple solitonic pulses can be trapped within each of these time-slots, working as an optical-soliton “reactor.” The stability of the optomechanical lattice is largely enhanced by the strong optomechanical interactions in the PCF core. In contrast, the multiple solitons trapped in each lattice cycle were observed to interact intensely with each other.In the experiments (Fig. 1), an erbium-doped fiber amplifier was used with two 980 nm laser diodes as the pump sources. Two polarization controllers working together with an optical polarizer acted as an artificial saturable absorber through nonlinear polarization rotation (NPR). A time-stretch dispersion Fourier transform (TS-DFT) setup was built using a 5-km-long single-mode fiber (SMF) as the stretching element to demonstrate the detailed information of the multi-soliton interaction processes. Two 45-GHz bandwidth detectors and a 33-GHz bandwidth oscilloscope were used to record the laser output pulses' time-domain trace and DFT signal. Furthermore, the laser output spectrum was recorded using an optical spectrum analyzer with a resolution of 0.02 nm. Since most of the laser cavity was made from SMF, the cavity dispersion was strongly anomalous with a calculated average value of -23.8 ps2/km, leading to soliton operation of the laser with hyperbolic pulse shape.Results and Discussions By carefully adjusting the intra-cavity polarizer controllers, stable harmonic mode-locking at 1.89 GHz resonance frequency of the acoustic core resonance in the PCF could be realized when both of the two pump diodes have pump powers of approximately 380 mW at 980 nm. The laser output spectrum, as well as the time-domain pulse sequence, were captured. When only one soliton is trapped in each cycle of the optomechanical lattice, the stable acousto-optic mode-locking state could be obtained with a 3 dB spectral bandwidth of 2.46 nm [Fig. 2(a) and Fig. 2(d)]. At this state, a strong Kelly-sideband observed on the pulse spectrum indicates that the laser was operating in the soliton regime. Through the experiments, we found that by adjusting the intra-cavity polarizer controllers, the laser output spectrum could be varied gradually from 2.46 nm to less than 0.6 nm [Figs. 2(b)--(f)], while the optomechanical lattice remained to be stable and the average output power of the laser was kept almost constantly at approximately 70 mW, giving rise to a series of quasi-stable acousto-optic mode-locking states. In those states, multiple soliton pulses were trapped within each cycle of the optomechanical lattice. The DFT signal unveiled that intense and complex interactions between the trapped solitons occurred within each cycle of the lattice (Figs. 3--5). As an entire, this optomechanically mode-locked fiber laser system permits several solitons to coexist in its cavity, and complex interactions between these solitons in one cycle could be studied in the future using this unique platform.Conclusions In the experiments, we obtained a large number of quasi-stable states in an optomechanically mode-locked fiber laser. We discovered that each isolated cycle of the optomechanical lattice could function as robust optical-soliton “reactors,” allowing us to study complex and intense soliton interactions. We could partially adjust the number of pulses trapped in each cycle of the optomechanical lattice and, thus, the total number of solitons generated in the laser cavity by adjusting the working point of the NPR effect in the laser cavity. In this way, we could control to some extent the multi-soliton interaction processes. Compared with a traditional passive mode-locked laser, this system's stability, flexibility, and high-repetition-rate features make it an ideal experimental platform for studying complex multi-soliton interactions, providing some useful insights on soliton dynamics.
  • Sep. 29, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 19 1901006 (2021)
  • DOI:10.3788/CJL202148.1901006
Research Progress on Fiber Laser-Pumped Femtosecond Optical Parametric Oscillators
Hu Minglie, Wang Jue, and Fan Jintao
Significance Since the first demonstration of the Kerr-lens-mode-locked Ti:sapphire laser, femtosecond laser technology has attracted tremendous research interest and evolved very rapidly. Thanks to the properties of short pulse duration, broadband spectrum, and high peak power, femtosecond laser pulses can probe the high-resolution dynamics in both time and spatial dimensions, and explore new regimes of light-matter interaction. Contributing to these advantages, femtosecond laser systems could serve as powerful and reliable platforms for many cutting-edge applications, such as material processing, frequency comb generation, metrology, microscopy, spectroscopy, and nanooptics. Apart from many application fields, femtosecond laser technology has led to many breakthroughs in fundamental research fields, including attoscience, femtochemistry, and nonlinear optics. Developments in pump diodes, gain media, and saturable absorber mechanisms advance the frontiers of pulse duration and output power. Up to now, extremely short duration of pulses down to a few-optical-cycles can be achieved both directly from the oscillator and nonlinear processes outside the cavity. On the other hand, the output power level of the femtosecond laser system can reach several hundred watts. In recognition of the role of the femtosecond laser technique, Mourou and Strickland won the Nobel Prize in 2018 for chirped-pulse amplification. Apart from advancement to shorter pulse duration and higher output power, more and more research focuses are placed on ongoing efforts to expand the frequency coverage to promote femtosecond laser systems into more widespread practical applications. However, the mode-locked spectral width of femtosecond laser output is limited by the effective laser gain bandwidth due to the relatively fixed energy levels of the gain medium, which hinders its large-scale application.Nonlinear frequency conversion techniques can provide the possibility to achieve effectively tunable laser sources in a wide spectral region. Up to date, the optical parametric oscillator (OPO) has emerged as a compelling alternative to generate broadband tunable radiation, which can expand the spectral region from the UV to infrared. Among them, OPOs pumped by femtosecond fiber lasers have been recognized as ideal platforms providing tunable ultrafast pulses with formidable performance, such as high repetition rate, high output power, and broad wavelength coverage. To this end, femtosecond OPOs are appealing for numerous applications, including quantum information, laser processing, optical frequency comb generation, and biophotonics. Recent power scaling of the Yb-fiber lasers and the development of new nonlinear crystals advance the frontiers of femtosecond OPOs.Progress To fulfill more widespread applications, there remains a strong motivation to expand the spectral tuning possibilities of OPOs. The development of birefringent crystals such as BIBO, BBO, and LBO, combined with a powerful femtosecond fiber laser source, enables the generation of tunable UV radiation on an ultrafast time scale (Fig. 1). Alternatively, thanks to the unique material properties of mid-infrared materials, i.e., CSP, OP-GAP, and ZGP, the operation of femtosecond OPOs in the far-infrared at 8 μm can be realized (Fig. 3).Kerr-lens-mode-locked Ti: sapphire lasers are the most commonly used pump sources for OPOs; however, these systems suffer a limitation in terms of power scaling mainly owing to unavoidable heat load in the laser crystal. In recent years, the rapid development of a high-power Yb-laser system allows a new power scaling potential for OPOs, and W level signal output can be achieved. However, it is rather difficult for OPOs to achieve a few-cycle pulse duration directly from a femtosecond fiber laser owing to the gain bandwidth limitation and complex nonlinear control. To access even shorter pulses from OPOs pumped by a fiber laser system, chirped-pulse optical parametric oscillators and self-compressed MIR OPOs have been demonstrated by researchers in Huazhong University of Science & Technology and Tianjin University, respectively (Fig. 7 and Fig. 9).For high-speed electrooptic sampling or future optical communication applications, moving operation regime of OPOs into the gigahertz pulse repetition rate regime has advantages. OPOs operating at GHz repetition rates have been reported using both synchronous and harmonic pumping schemes.Light emission with space-variant polarization and phase distribution has become a popular topic for the research community. The development of methods to create wavelength-tunable, space-variant polarization light beams will be a very interesting topic. Hu’s research group in Tianjin University has demonstrated novel femtosecond OPOs that deliver high-order Poincaré sphere beams, cylindrical vector beams, and vortex beams (Figs. 12--14).Conclusions and Prospect In this paper, we start with the progress in Yb-doped fiber laser-pumped femtosecond OPOs in recent years. Then, we present a variety of advanced designs of fiber laser-pumped OPOs, which are categorized into widely tunable OPOs, GHz repetition rate OPOs, few-cycle optical pulse OPOs, and structured beam OPOs. Finally, the applications of femtosecond OPOs in the fields of nanophotonics and Raman spectroscopy are introduced. With further development in nonlinear materials, combined with advances in pump laser technology, as well as new design concepts, femtosecond OPOs with wider spectral coverage, higher power, higher repetition rate, and shorter pulse duration are achievable in the near future. With the growth of novel femtosecond OPOs, completely new areas in application fields will arise.
  • Sep. 26, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 19 1901001 (2021)
  • DOI:10.3788/CJL202148.1901001
Passively Q-switched Nd∶YVO4-YVO4-Cr 4+∶YAG Self-Mode-Locked Raman Laser Based on Composite Cavity
Bai Yuntao, Ding Xin, Jiang Guoxin, Lei Peng, Sun Bing, Li Tengteng, Zhang Xingrui, Li Xiangxiang, Wu Liang, Zhang Guizhong, and Yao Jianquan
Objective High-performance Q-switched mode-locked lasers can achieve high pulse-repetition-frequency (PRF) ultrashort pulse sequences with nanosecond-pulse envelopes, which are of great importance in applications such as laser remote sensing, adaptive optics, and inertial confinement fusion. Because of the clean-up effect, stimulated Raman scattering (SRS) has been regarded as a potential technical approach to achieve high-performance laser output. In particular, with the discovery of the SRS self-mode-locking phenomenon in recent years, Q-switched self-mode-locked Raman lasers with compact structure, high peak power, and high beam quality have gradually been favored by researchers. However, the mechanism of the SRS self-mode-locking phenomenon is relatively complicated, and there are few theoretical studies at present. Although some experiments have reported the SRS self-mode-locking phenomenon with corresponding explanations, most of them have yet to be verified and improved. In addition, the conversion efficiency of self-mode-locked Raman lasers needs to be further improved. Therefore, to solve the above problems, an elaborate folding coupled cavity design was employed. Taking advantage of the folding coupled cavity, the fundamental and Raman cavities can be adjusted independently. Hence, the SRS self-mode-locking effect can be verified clearly according to the experimental results, and the mode matching between the fundamental and Raman waves can also be optimized by adjusting the length of the Raman cavity to improve the conversion efficiency of SRS.Methods A schematic diagram of the Nd∶YVO4-YVO4-Cr 4+∶YAG passively Q-switched intracavity self-mode-locked Raman laser based on a folding coupled cavity is shown in Fig. 2. The fundamental resonator consisted of M1, M2, a Nd∶YVO4 crystal, and a Cr 4+∶YAG crystal. A common L-shaped Raman cavity (designated by mirror path M2-M3-M4) was adopted for the mode matching between the fundamental and Stokes waves. The radius of curvature of M1 was 150 mm, and the output coupler (OC) M2 was a flat mirror. The pump source was a fiber-coupled LD emitting at 808.2 nm with a maximum output power of 50 W. A 1∶1 multilens coupler was used to focus the pump light into an a-cut 0.3% Nd∶YVO4 crystal with a radius of ?200 μm near the incident facet of the laser gain medium, and the dimensions of the Nd∶YVO4 crystal were 3 mm×3 mm×20 mm. A 4 mm×4 mm×3 mm Cr 4+∶YAG crystal with 80% initial transmittance at 1064 nm was employed and placed as closely as possible to the OC. An a-cut 4 mm×4 mm×30 mm YVO4 crystal was used as the Raman crystal, which was 1° wedged on both facets. All the components were coated according to our requirements. The length of the fundamental cavity composed of M1 and M2 was fixed at 110 mm. By adjusting the ROC of M4 and the length of the L-shaped Raman cavity, optimization of mode matching between the fundamental and Stokes waves can be achieved effectively. Results and Discussions The linear cavity Nd∶YVO4-YVO4-Cr 4+∶YAG passively Q-switched intracavity self-mode-locked Raman laser was first studied. When the transmittance of the OC was 5%, a maximum output power of 0.81 W was obtained at 1176 nm under a pump power of 17.15 W, with an optical-optical efficiency of 4.72% (Fig. 2). The corresponding PRF and pulse width were 885.4 MHz and ?219.16 ps, respectively (Fig. 3). After that, a 45° dichroic mirror M3 was inserted into the cavity to construct an L-shaped folded Raman cavity with M4 and M2 (OC). When the radius of curvature (ROC) of M4 was 100 mm and the length of the Raman cavity was 120 mm, a maximum power of 1.23 W with 1176 nm Q-switched mode-locked output was obtained under the pump power of 17.15 W, which was an improvement of over 50% compared with the linear cavity [Fig. 4(a)]. The PRF and pulse width of the mode-locked output were 942.9 MHz and ?125.8 ps, respectively (Fig. 5). The linewidth was 0.2 nm, and the beam quality factors Mx2 and My2 were 1.39 and 1.42, respectively (Figs. 7 and 8). Replacing the ROC of M4 with 150 mm and increasing the length of the Raman cavity to 180 mm, a maximum power of 1.19 W at 1176 nm Q-switched mode-locked output was obtained at the pump power of 17.15 W, with a conversion efficiency of 6.94% [Fig. 4(b)], and the PRF was reduced to 675.6 MHz (Fig. 7). Conclusions A passively Q-switched Nd∶YVO4-YVO4-Cr 4+∶YAG self-mode-locked Raman laser based on a composite cavity was demonstrated. Taking advantage of the folded-coupled cavity, the length and mirrors of the fundamental and Raman cavities can both be adjusted independently. Hence, the SRS self-mode-locking effect has been clearly obtained in a simple manner according to the experimental results, and the mode matching between the fundamental and Raman waves can be optimized by adjusting the length of the Raman cavity. In this way, the output power and conversion efficiency of the Q-switched mode-locked Raman output can be greatly improved. In addition, the folding coupled cavity structure was proved to control the PRF of the 1176 nm mode-locked output actively by adjusting the length of the Raman cavity together with the ROC of the mirror, without a reduction of output power and efficiency.
  • Sep. 24, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 19 1901003 (2021)
  • DOI:10.3788/CJL202148.1901003
Pulse State Switchable Ytterbium-Doped Fiber Laser Based on Lyot Filter
Lin Yanlü, Huang Zinan, Huang Qianqian, Dai Lilong, Xing Zhikun, Yan Zhijun, and Mou Chengbo
Objective Various pulse shaping processes, including convention soliton, stretched pulse, similarity, and dissipative soliton, are formed in present passively mode-locked fiber lasers based on the diverse distribution positions of dispersion in the cavity. The development of soliton pulses has raised the single pulse energy to a new level, making fiber lasers cater to the needs of fields, such as optical metrology, biomedicine, and laser micromachining. Dissipative solitons are usually generated from lasers with a large net normal dispersion owing to the effects of dispersion, nonlinearity, gain, and loss. The spectral amplitude modulation introduced by the spectral filter plays a key role in forming the dissipative soliton. Therefore, various filters are used in the lasers. The birefringent filter has been widely used owing to its flexible filtering bandwidth and good fiber compatibility. In addition, noise-like pulses have also been extensively studied in normal dispersion lasers.Both dissipative solitons and noise-like pulses can be generated in Ytterbium (Yb)-doped fiber lasers by reasonably adjusting the cavity parameters such as the pump power. Although pulse state switching has been verified in many experiments, few reports on the multiple switching of dissipative soliton and noise-like pulses in Yb-doped fiber lasers are available. In this study, we design a Lyot filter with a stable and powerful comb filtering using a pair of polarization-maintaining 45° tilted fiber gratings as polarizers and section of polarization-maintaining fiber as the birefringent medium. Therefore, an all-normal-dispersion Yb-doped fiber laser can achieve stable dissipative soliton mode-locking. By increasing the pump power unidirectionally in the dissipative soliton mode-locking state, the laser realizes multiple switching of dissipative soliton and noise-like pulse.Methods Two polarization-maintaining 45° tilted fiber gratings are separated by a length of polarization-maintaining fiber with a particular splicing angle in the Lyot filter used in the experiment. It can be used as a comb filter in the laser cavity and a fiber-type polarizer because of its unique structure. To generate linear polarization light, the first grating couples the TE polarization component out of the fiber core and causes the TM polarization component to propagate in the fiber core. Linear polarization light accumulates linear phase shift in the polarization-maintaining fiber owing to the particular splicing angle between the grating and the polarization-maintaining fiber. The linear phase shift is transferred to the amplitude modulation in the second grating, resulting in comb filtering. The specific splicing angle is designed to be 45° for the filter to have the maximum-filtering modulation depth.Results and Discussions The laser realizes stable dissipative soliton mode-locking with a pump power of 177 mW by finely adjusting the polarization controller, and its spectrum is shown in Fig. 4(a). The switching of the mode-locking pulse state can be observed while keeping the polarization controller and only increasing the pump power. When the pump power is increased to 323 mW, the spectrum gradually broadens under the influence of enhanced self-phase modulation. At this time, the pulse generated by the laser is still a dissipative soliton. Then, the pump power is continuously increased up to 455 mW in Fig. 4(e).The sharp edges of the spectrum gradually disappear and become smooth. The autocorrelation trace in Fig. 4(f) has a wide base with a narrow peak, typical of noise-like pulses, indicating that the laser is working in a noise-like pulse regime. When the increasing pump power reaches 691 mW, the laser generates a dissipative soliton pulse again. The pulsed state switching of the laser can be attributed to the switching of the feedback mechanism of nonlinear polarization rotation. Fig. 5 shows the relationship between the instantaneous power and nonlinear polarization rotation transmittance. The sinusoidal transmission spectrum indicates that the nonlinear polarization rotation can occur in positive and negative states. The critical power of positive and negative feedback is named critical saturation power. After achieving stable mode-locking, the intracavity instantaneous power increases owing to the continuously increasing pump power. When the instantaneous power in the cavity exceeds the critical saturation power, the feedback mechanism of nonlinear polarization rotation switches from positive to negative feedback state, which causes the pulse state of the laser to switch from dissipative soliton to noise-like pulse. When the instantaneous power reaches the critical saturation power again, the feedback mechanism switches from the negative feedback state to the positive feedback state, so that the mode-locked pulse state of the laser also switches back to the dissipative soliton.Conclusions We integrated a compact Lyot filter with a pair of polarization-maintaining 45° inclined fiber gratings in an all-normal-dispersion Yb-doped fiber laser. The laser realizes stable dissipative soliton mode-locking at a pump power of 177 mW. Furthermore, the pulse state of the laser recognizes switching from dissipative soliton to noise-like pulse and then to dissipative soliton by only continuously increasing the pump power from 177 mW to 691 mW. As adjusting the state of the polarization controller during switching is not necessary, it has higher controllability and accuracy, and the laser can be designed as a compact multifunctional light source.
  • Sep. 23, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 19 1901004 (2021)
  • DOI:10.3788/CJL202148.1901004
A 1310 nm Band Narrow Linewidth Hybrid Integrated External Cavity Semiconductor Laser Based on Fiber Bragg Gratings
Liang Hong, Wei Fang, Sun Yanguang, Sun Guangwei, and Cai Haiwen
Objective With the development of modern communication network technology, people’s demand for communication capacity has increased dramatically, making the ultra-large capacity and long-distance optical fiber network transmission system a research hotspot in the field of optical fiber communication. As a light source with a comb-shaped spectrum in the frequency domain, the optical frequency comb can significantly increase the transmission capacity of a single optical fiber, make full use of the limited available bandwidth of optical fiber transmission, and become an ideal light source for large-capacity transmission in optical fiber network transmission systems. To improve the stability of large-capacity optical fiber communication system, it is necessary to examine the changes of the optical fiber link in the optical fiber communication system. However, the frequency coverage of the optical frequency comb is relatively wide, about dozens or even hundreds of nm, and the wavelengths needed for monitoring should avoid the band that carries information. Therefore, for a 1550 nm band optical frequency comb conventionally deployed in the current optical fiber communication system, a 1550 nm band laser cannot realize the monitoring of the optical fiber link in the optical fiber communication system. As another essential window of optical fiber communication, a 1310 nm band can realize the transmission with low attenuation and dispersion and has become an essential channel for optical fiber link detection. Therefore, it is necessary to study 1310 nm band narrow linewidth laser to improve the stability of the optical fiber communication system.Methods In this study, we have developed a 1310 nm band hybrid integrated external cavity diode laser based on the single angle facet semiconductor gain chip and fiber Bragg grating. First, the design and fabrication theory of the external cavity diode laser is presented. The fiber Bragg grating and ceramic substrate with V-groove are packaged by full-glue packaging process, which improves the fiber Bragg grating’s thermal sensitivity and mechanical stability. Then, the single angle facet semiconductor gain chip and fiber Bragg grating are coupled with a fiber-tapered lens polished at the front of the fiber Bragg grating. The fiber at the back of the fiber Bragg grating can be used directly as the output fiber. The narrow linewidth laser output is realized by the negative feedback of the sloping edge of the fiber Bragg grating reflection spectrum. The performance of the obtained external cavity diode laser is tested through experiment. Finally, our fabricated external cavity diode laser is applied in an optical fiber sensing system.Results and Discussions Our fabricated external cavity diode laser in the 1310 nm band uses the single angle facet semiconductor gain chip to provide gain and fiber Bragg grating as the frequency-selective element. It has the advantages of flexible wavelength selection, simple structure, and low cost. The performance of the fabricated external cavity diode laser is tested. At the operating temperature of 25 ℃ and operating current of 280 mA, the external cavity diode laser center wavelength is 1309.8 nm (Fig. 2), and 3 dB Lorentz linewidth is 18 kHz (Fig. 4). Moreover, under this operating temperature and current, the power and frequency fluctuation of the external cavity diode laser in 3 h is 0.6 mW and 315 MHz (Fig. 5). Additionally, the tuning characteristics of the external cavity diode laser are measured. When the external cavity diode laser operates at 25 ℃, the laser mode-hopping free current tuning range is 7 GHz, and the tuning coefficient is 47 MHz·mA -1 (Fig. 3). Conclusions A narrow linewidth hybrid integrated external cavity diode laser at the 1310 nm band is developed using a single angle facet semiconductor gain chip and fiber Bragg grating. The external cavity diode laser is integrated in a compact butterfly package. When the external cavity diode laser operating temperature and current are set to 25 ℃ and 280 mA, we obtain a wavelength, 3 dB Lorenz linewidth, power fluctuation, and frequency fluctuation of 1309.8 nm, 18 kHz, 0.6 mW, and 315 MHz, respectively, for 3 hours. Additionally, the laser mode-hopping free current tuning range is 7 GHz, and the tuning coefficient is 47 MHz·mA -1 with an operating temperature of 25 ℃. The external cavity diode laser can be used in fiber sensing and communication.
  • Sep. 22, 2021
  • Chinese Journal of Lasers
  • Vol.48 Issue, 20 2001002 (2021)
  • DOI:10.3788/CJL202148.2001002