Compact two-photon fluorescence microscope based on a single-mode fiber coupler

We present a two-photon fluorescence microscope based on a three-port single-mode optical fiber coupler. It is found that the coupler behaves as a low-pass filter that can deliver an ultrashort-pulsed laser beam of as much as 150 mW of power in the wavelength range from 770 to 870 nm as well as collect a two-photon fluorescence signal in the visible range. As a result of using the fiber coupler, the new two-photon imaging system exhibts a number of advantages, including a compact arrangement, freedom from vibration from lasers and electronic devices, self-alignment, reduction of multiple scattering, and an enhanced optical sectioning effect. The effectiveness of the new instrument is demonstrated with a set of three-dimensional images of biological samples. This instrument may make two-photon fluorescence endoscopy possible for in vivo medical applications.     

Two-photon excited spectroscopies of ex vivo human skin endogenous species irradiated by femtosecond laser pulses

Two-photon excited spectroscopies from ex vivo human skin are investigated by using a femtosecond laser and a confocal microscope (Zeiss LSM 510 META). In the dermis, collagen is responsible for second harmonic generation (SHG); elastin, nicotinamide adenine dinucleotide (NADH), melanin and porphyrin are the primary endogenous sources of two-photon excited autofluorescence. In the epidermis, keratin,NADH, melanin and porphyrins contribute to autofluorescence signals. The results also show that the SHG spectra have the ability to shift with the excitation wavelength and the autofluorescence spectra display a red shift of the spectral peaks when increasing the excitation wavelength. These results may have practical implications for diagnosis of skin diseases.     

Double-clad photonic crystal fiber coupler for compact nonlinear optical microscopy imaging

A 1 x 2 double-clad photonic crystal fiber coupler is fabricated by the fused tapered method, showing a low excess loss of 1.1 dB and a splitting ratio of 97/3 over the entire visible and near-infrared wavelength range. In addition to the property of splitting the laser power, the double-clad feature of the coupler facilitates the separation of a near-infrared single-mode beam from a visible multimode beam, which is ideal for nonlinear optical microscopy imaging. In conjunction with a gradient-index lens, this coupler is used to construct a miniaturized microscope based on two-photon fluorescence and second-harmonic generation. Three-dimensional nonlinear optical images demonstrate potential applications of the coupler to compact all-fiber and nonlinear optical microscopy and endoscopy.     

Arrangement of a 4Pi microscope for reducing the confocal detection volume with two-photon excitation

The main advantage of two-photon fluorescence confocal microscopy is the low absorption obtained with live tissues at the wavelengths of operation. However, the resolution of two-photon fluorescence confocal microscopes is lower than in the case of one-photon excitation. The 4Pi microscope type C working in two-photon regime, in which the excitation beams are coherently superimposed and, simultaneously, the emitted beams are also coherently added, has shown to be a good solution for increasing the resolution along the optical axis and for reducing the amplitude of the side lobes of the point spread function. However, the resolution in the transverse plane is poorer than in the case of one-photon excitation due to the larger wavelength involved in the two-photon fluorescence process. In this paper we show that a particular arrangement of the 4Pi microscope, referenced as 4Pi′ microscope, is a solution for obtaining a lateral resolution in the two-photon regime similar or even better to that obtained with 4Pi microscopes working in the one-photon excitation regime.     

Selective corneal imaging using combined second-harmonic generation and two-photon excited fluorescence

A multiphoton microscope employing second-harmonic generation (SHG) and two-photon excited fluorescence (TPF) is used for high-resolution ex vivo imaging of rabbit cornea in a backscattering geometry. Endogenous TPF and SHG signals from corneal cells and extracellular matrix, respectively, are clearly visible without exogenous dyes. Spectral characterization of these upconverted signals provides confirmation of the structural origin of both TPF and SHG, and spectral imaging facilitates the separation of keratocyte and epithelial cells from the collagen-rich corneal stroma. The polarization dependence of collagen SHG is used to highlight fiber orientation, and three-dimensional SHG tomography reveals that approximately 88% of the stromal volume is occupied by collagen lamellae.     

73-nm tuning of a double-clad Yb<Superscript>3+</Superscript>-doped fiber laser based on a hybrid array

We report on a wide wavelength tuning in a double-clad ytterbium-doped fiber laser. The laser cavity consists of an array of broadband high-reflection fiber Bragg gratings and a bulk grating as the output coupler and wavelength selection element. The proposed fiber laser configuration combines a low intracavity loss of the fiber Bragg grating mirrors with a wide wavelength tuning of the bulk gratings. We demonstrate a >70-nm wavelength tuning range, limited only by the available fiber Bragg gratings.     

Simultaneous multichannel nonlinear imaging: combined two-photon excited fluorescence and second-harmonic generation microscopy

Simultaneous two-photon excited fluorescence (TPF) and second-harmonic generation (SHG) imaging is demonstrated using a single femtosecond laser and a scanning microscope. This composite nonlinear microscopic technique was applied to imaging DNA and chromosomes, and it was shown that the two different interaction mechanisms provide complementary information on the structure and nonlinear properties of these biological materials, beyond that achievable using either TPF or SHG imaging alone. The use of separate modes of detection, in reflection and transmission respectively, and the simultaneous nature of the acquisition of the two images allows pure TPF and SHG images in precise registration to be obtained.     

单到单和单到双信道皮秒脉冲全光波长转换的实验研究

基于周期极化反转铌酸锂（PPLN）光波导级联倍频和差频（SHG+DFG）的二阶非线性效应,提出并实验研究了皮秒脉冲的可调谐波长转换以及单信道到双信道的波长转换.信号光采用重复频率为40 GHz,脉宽为1.57 ps的脉冲信号.连续抽运光由光纤环形腔激光器（FRL）提供.不同于传统的SHG+DFG型波长转换,信号光固定在PPLN光波导倍频过程的准相位匹配（QPM）波长处,通过调节抽运光的波长实现了转换空闲光的可调谐输出.当使用两个抽运光时实验观察到了单信道到双信道的波长转换.     

Imaging the structure and organization of mouse cerebellum and brain stem with second harmonic generation microscopy

To visualize the structure and organization of the brain is a fundamental requirement in the research of neuroscience. Here, combining with two-photon excitation fluorescence microscopy and transgenetic mouse GAD67,we demonstrate a custom-built second harmonic generation(SHG) microscope to discriminate brain layers and sub regions in the cerebellum and brain stem slices with cellular resolution. In particular, the cell densities of neurons in different brain layers are extracted due to the cell soma appearing as dark shadow on an SHG image.Further, the axon initial segments of the Purkinje cell are easily recognized without labeling, which would be useful for guiding micropipettes for electrophysiology.     

Wavelength dependent Stark compensation in isotope selective Doppler-free two photon ionization

A novel wavelength dependent Stark shift compensation method for isotope selective ionization in Doppler free two-photon spectroscopy is theoretically investigated. In the present paper, certain wavelengths for the ionization step termed as “magic wavelengths” are identified for compensating the Stark shift induced by the excitation laser. The numerical calculations indicate that, ionization efficiencies can be maximized by the judicious choice of the intensities of the excitation and the ionization lasers without much degradation in the optical selectivity. The effects of Stark shift induced asymmetry and its reversal by selecting the appropriate magic wavelength for the ionization step for various excitation and ionization laser intensities have been investigated. The wavelength dependent Stark compensation method can be adapted for the isotope selective Doppler free two-photon excitation followed by non-resonant ionization process for any atomic system. In the present work, the theoretical results pertaining to the specific example of ⁴¹Ca has been presented.     

Linear cavity Brillouin fiber laser with improved characteristics

A configuration for linear cavity Brillouin fiber laser (BFL) generation is demonstrated using a standard single-mode fiber, two optical circulators, a 3 dB coupler, and a 95/5 coupler to allow high efficiency. With a Brillouin pump (BP) power of 13 dBm, the laser peak power is 12.3 dB higher than a conventional linear cavity BFL at an upshifted wavelength of 0.086 nm from the BP wavelength. In addition, it is revealed that the BFL peak power can be higher than the transmitted BP peak power when the BP power exceeds the second Brillouin Stokes threshold power.     

Three-photon-induced fluorescence of diphenylhexatriene in solvents and lipid bilayers

We observed the emission of l,6-diphenyl-l,3,5-hexatriene (DPH) when excited with the fundamental output of a fs Ti:sapphire laser at 860 nm. The emission spectra of DPH were identical to that observed for one-photon excitation at 287 nm. The dependence of the DPH emission intensity on laser power was cubic, indicating three-photon excitation of DPH at 860 nm. At a shorter wavelength of 810 nm, the dependence on laser power was quadratic, indicating a two-photon process. At an intermediate wavelength of 830 nm the mode of excitation was a mixture of two- and three-photon excitation. At 830 nm the anisotropy is no longer a molecular parameter, and the mode of excitation and anisotropy of DPH depends on laser power. Frequency-domain anisotropy decays of DPH in triacetin revealed the same rotational correlation times for two- and three-photon excitation. However, the time 0 anisotropy of DPH was larger for three-photon excitation than for two-photon excitation. Steady-state anisotropy data for DPH-labeled membranes revealed the same transition temperature for one- and three-photon excitation. These anisotropy data indicate that membrane heating was not significant with three-photon excitation and that three-photon excitation may thus be of practical usefulness in fluorescence spectroscopy and microscopy of membranes.     

基于少数层石墨烯可饱和吸收的锁模光纤激光器

本文利用化学气相沉积法高温分解甲烷在铜箔上制得单层石墨烯薄膜, 测量了石墨烯的拉曼光谱. 将石墨烯薄膜逐层转移到光纤跳线的氧化锆插芯端面上做成可饱和吸收材料, 实验研究了环形腔掺铒光纤脉冲激光器的输出特性, 获得了峰值波长为1560.1 nm, 3 dB带宽为0.27 nm, 重复频率为7.69 MHz, 脉冲宽度为58.8 ps 的锁模脉冲序列. 时间带宽积为1.98, 表明脉冲出现了啁啾. 最后, 通过改变两个光纤活动接头之间的空气腔的长度, 激光波长实现了4 nm的调谐. 关键词： 光纤激光器 石墨烯 锁模     

Pressure sensing utilizing linear cavity erbium-doped fiber laser

In this paper, we report a fiber laser pressure sensor based on linear cavity erbium-doped fiber laser. The fiber laser structure comprises of an erbium-doped fiber amplifier, a circulator, an optical coupler and a fiber Bragg grating (FBG) which acts simultaneously as a lasing wavelength selecting components as well as a pressure sensor. The FBG is fitted to the shock tube where the pressure is applied. The fiber laser pressure sensor has a low threshold power of 7 mW, an output power of 2.28 mW and an optical signal to noise ratio over 55 dB. The proposed fiber laser sensor is expected to be an attractive choice for long-distance pressure monitoring.     

Integrated multimodal endomicroscopy platform for simultaneous en face optical coherence and two-photon fluorescence imaging

We report an all-fiber-optic scanning, multimodal endomicroscope capable of simultaneous optical coherence tomography (OCT) and two-photon fluorescence (TPF) imaging. Both imaging modalities share the same miniature fiber-optic scanning endomicroscope, which consists of a double-clad fiber with a core operating in single mode at both the OCT (1310 nm) and two-photon excitation (1550 nm) wavelengths, a piezoelectric two-dimensional fiber-optic beam scanner, and a miniature aspherical compound lens suitable for simultaneous acquisition of en face OCT and TPF images. A fiber-optic wavelength division multiplexer was employed in the integrated platform to combine the low coherence OCT light source and the femtosecond two-photon excitation laser into the same optical path. Preliminary imaging results of cell cultures and mouse tissue ex vivo demonstrate the feasibility of simultaneous real-time OCT and TPF imaging in a scanning endomicroscopy setting for the first time.     

Single- and two-photon excitation of a GaAs random laser

We have demonstrated an anti-Stokes random laser emission at lambda approximately 885 nm in micrometer-size powder of GaAs. The maximal pumping wavelength, lambda=1,300 nm, exceeded the lasing wavelength by nearly 50%. We conclude that both one- and two-photon pumping contribute to optical excitation of the material, with the two-photon pumping being a key process in creating the population inversion and gain.     

基于双芯光纤耦合器的梳状滤波器及其CO2激光调节

基于光纤耦合器的波长特性,分析了光纤耦合器作为梳状滤波器的光谱特性;将长度不同的双芯光纤(twin-core fiber,TCF)熔接在两根单模光纤(single mode fibors,SMF)之间,实验制得具有不同峰值波长间隔的基于双芯光纤耦合器的全光纤型梳状滤波器,其消光比可达25 dB. 首次使用CO₂激光对其进行光谱调节,调节导致光谱向长波方向漂移和消光比的变化. 这种调节是基于CO₂与光纤相互作用时的残余应力释放和熔融变形机理,使用氢载TCF可以在相同实验参数下得到更大的波长调节激光. 关键词： 双芯光纤耦合器 梳状滤波器 2激光')" href="#">CO₂激光 残余应力     

Two-photon fluorescence endoscopy with a micro-optic scanning head

A major obstacle in the race to develop two-photon fluorescence endoscopy is the use of complicated bulk optics to transmit an ultrashort-pulsed laser beam and return the emitted fluorescence signal. We describe an all-fiber two-photon fluorescence microendoscope based on a single-mode optical fiber coupler, a microprism, and a gradient-index rod lens. It is found that the new endoscope exhibits an axial resolution of 3.2 microm and is capable of imaging transverse cross sections of internal cylindrical structures as small as approximately 3.0 mm in diameter. This device demonstrates the potential for developing a real-time diagnostic tool for biomedical research without the need for surgical biopsy and may find applications in photodynamic therapy, microsurgery, and early cancer detection.     

Miniaturized two-photon microscope based on a flexible coherent fiber bundle and a gradient-index lens objective

We present a miniature, flexible two-photon microscope consisting of a fused coherent optical fiber bundle with 30,000 cores and a gradient-index lens objective. The laser focus of a standard two-photon laser-scanning microscope was scanned over the entrance surface of the fiber bundle, resulting in sequential coupling into individual cores. Fluorescent light was detected through the fiber bundle. Micrometer-sized fluorescent beads and pollen grains were readily resolved. In addition, fluorescently labeled blood vessels were imaged through the fiber bundle in rat brain in vivo.     

超冷里德堡原子的产生以及探测

利用激光冷却与俘获技术获得冷原子,由双光子激发产生超冷里德堡原子,利用场电离法得到了里德堡原子ns和nd态的离子谱图;再将激光波长固定在6p3/2-34d态的共振跃迁线上,得到了离子和里德堡原子的TOF(Time of Flight)图,并对实验结果做了分析.