基于光子晶体光纤和拉锥式单模光纤的超连续光谱产生的实验研究

本文使用重复频率为250 MHz、脉冲宽度为135 fs、最大功率为2.2 W的锁模掺镱光纤激光作为种子源,利用光子晶体光纤和自制的拉锥式单模光纤两种高非线性光纤研究了超连续光谱的产生特性,通过对比两种光纤的结构、色散等特性,分析了拉曼孤子、色散波及其他非线性效应对产生的超连续谱形状的影响,并均得到了大于一个倍频程的超连续光谱,特别是拉锥式单模光纤产生的超连续光谱,耦合效率达到60%,这为众多研究领域,尤其是光学频率梳的建立提供了实用的超连续光源.     

Experimental investigation of continuous-wave supercontinuum ring laser composed of clad-pumped Er/Yb codoped fiber and highly-nonlinear optical fiber

We experimentally demonstrate a practical benefit of our proposed continuous-wave (CW) supercontinuum (SC) generating ring laser scheme in terms of low-cost device implementation and then investigate its output intensity noise property, as a further study to our previous paper of Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599]. First, we implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Unlike the scheme in Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599] the gain medium of an Er/Yb fiber is clad-pumped by low quality multimode pump LDs to underline the fact that our proposed scheme does not require any high power single-mode laser pump. A SC of a bandwidth larger than 470 nm is readily achieved. Next, we measure relative-intensity-noise (RIN) of the generated SC and compare it with that of a pumped Er/Yb fiber ASE. The SC laser is found to have a much higher RIN than the ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process.     

Signal-to-noise ratio improvement of a supercontinuum continuous-wave optical source using a dispersion-imbalanced nonlinear optical loop mirror

A multiwavelength continuous-wave (cw) optical source is based on supercontinuum (sc) produced in a highly nonlinear fiber. Noise properties of this optical source are studied through numerical simulations based on the nonlinear Schrödinger equation. The numerical simulations show that the amplified spontaneous emission (ASE) generation in an erbium-doped fiber amplifier and the ASE amplification during the sc generation result in a serious degradation of the signal-to-noise ratio (SNR). A novel scheme for improvement of the SNR of the multiwavelength cw optical source based on sc by using a dispersion-imbalanced nonlinear optical loop mirror is presented and the numerical simulation results are given. As a result, the average level of the SNR improvement is 11 dB and the eye-opening penalty is reduced by 0.91 dB due to the SNR improvement. In addition, the scheme can also be applied for improvement of the SNR of a pulse-type multiwavelength light source. PACS 42.50.Lc; 42.65.Re; 42.79.Sz; 02.60.Cb     

Experimental investigation of the effect of pump incoherence on nonlinear pump spectral broadening and continuous-wave supercontinuum generation

The development of high-power cw fiber lasers has triggered a great interest in the phenomena of nonlinear pump spectral broadening and cw supercontinuum generation. These effects have very convenient applications in Raman amplification, optical fiber metrology, and fiber sensing. In particular, it was recently shown that pump incoherence has a strong impact in these processes. We study experimentally the effect of pump incoherence in nonlinear pump spectral broadening and cw supercontinuum generation in optical fibers. We show that under certain experimental conditions an optimum degree of pump incoherence yields the best performance in the broadening process. We qualitatively explain these results, and we point out that these results may have important implications in cw supercontinuum optimization.     

Nonlinear-polarization-rotation based multiwavelength erbium-doped fiber lasers with highly nonlinear fiber

A multiwavelength erbium-doped fiber laser based on nonlinear polarization rotation with incorporation of a highly nonlinear fiber has been demonstrated. Stable multi-wavelength operation at room temperature is realized owing to the intensity-dependent transmission in fiber laser with nonlinear-polarization-rotation effect. And benefiting from the four-wave mixing in the highly nonlinear fiber, the uniformity and stabilization of the output multi-wavelength lasing have been greatly improved, in comparison with similar laser using single-mode fiber. Moreover, the wavelength spacing can be arbitrarily chosen according to the requirement through changing the PMF length.     

孤子光脉冲的产生及其应用研究

利用非线性偏振旋转技术实现自起振被动锁模.在掺铒光纤环形腔激光器中产生了中心波长为1563.3 nm、重复频率为12.5 MHz、脉冲宽度为352.0 fs、3 dB光谱宽度为7.8 nm的孤子光脉冲.采用该孤子光脉冲作为抽运光源，经掺铒光纤放大器放大后，输入到101 m长的高非线性光子晶体光纤中，获得了20 dB带宽约为240 nm的超连续激光光谱.实验详细观测了光脉冲随抽运功率的变化及超连续激光光谱的形成过程，分析了其形成机理.研究表明：当抽运功率较低时，光谱加宽主要由高阶孤子的分裂引起；随着抽运功率的增加，高阶孤子分裂成基本孤子的数目逐渐增大，光谱进一步加宽；当抽运功率增加到受激拉曼散射的阈值时，受激拉曼散射成为光谱展宽的主要原因；抽运功率进一步增加时，受激拉曼散射、参量四波混频等非线性的共同作用将使光谱进一步加宽且变得光滑. 关键词： 孤子光纤激光器 超连续 光子晶体光纤     

Supercontinuum generation in a standard fiber pumped by noise-like pulses from a figure-eight fiber laser

We report the experimental study of broadband spectrum generation in a piece of standard fiber (SMF-28) using as the pump a train of noise-like pulses, or sub-nanosecond packets of sub-ps pulses with randomly varying amplitudes. The pulses are generated by an erbium-doped figure-eight fiber laser, and present a wide (∼50 nm) optical spectrum, which represents a significant advantage to seed the generation of new frequencies. Another advantage of the pulses is their relatively large energy, as they are made up of a large number of ultrashort pulses. After amplification with an Erbium Doped Fiber Amplifier (EDFA), the pulses were injected in a 0.75 km length of SMF-28 fiber. We obtained experimentally at the end of the fiber an output signal spectrum extending from 1530 nm to at least 1750 nm (the upper limit of the spectrum analyzer) for pump pulses with an average power of 20.4 mW, corresponding to a few kilowatts peak power. The spectral broadening is due to Raman self-frequency shift (SFS). It is noteworthy that the spectrum of the newly created frequencies was extremely uniform over the range of measurement. Considering that the Raman shift is directly related to the pump pulse duration, spectral flatness is a direct consequence of the random distribution of amplitudes and durations of the pulses in the packet. Finally, the results show the capabilities of noise-like pulses from a fiber laser for applications in supercontinuum generation based on nonlinear phenomena such as Raman SFS.     

Coherent fiber supercontinuum for biophotonics

Biophotonics and nonlinear fiber optics have traditionally been two independent fields. Since the discovery of fiber‐based supercontinuum generation in 1999, biophotonics applications employing incoherent light have experienced a large impact from nonlinear fiber optics, primarily because of the access to a wide range of wavelengths and a uniform spatial profile afforded by fiber supercontinuum. However, biophotonics applications employing coherent light have not benefited from the most well‐known techniques of supercontinuum generation for reasons such as poor coherence (or high noise), insufficient controllability, and inadequate portability. Fortunately, a few key techniques involving nonlinear fiber optics and femtosecond laser development have emerged to overcome these critical limitations. Despite their relative independence, these techniques are the focus of this review, because they can be integrated into a low‐cost portable biophotonics source platform. This platform can be shared across many different areas of research in biophotonics, enabling new applications such as point‐of‐care coherent optical biomedical imaging.     

Generation of a 660-2100 nm laser frequency comb based on an erbium fiber laser

We present a multibranch laser frequency comb based upon a 250 MHz mode-locked erbium-doped fiber laser that spans more than 300 THz of bandwidth, from 660 nm to 2100 nm. Light from a mode-locked Er:fiber laser is amplified and then broadened in highly-nonlinear fiber to produce substantial power at ～1050 nm. This light is subsequently amplified in Yb:fiber to produce 1.2 nJ, 73 fs pulses at 1040 nm. Extension of the frequency comb into the visible is achieved by supercontinuum generation from the 1040 nm light. Comb coherence is verified with cascaded f-2f interferometry and comparison to a frequency stabilized laser.     

Extended continuous-wave supercontinuum generation in a low-water-loss holey fiber

We report on the development of a 2.5 microm core photonic crystal fiber with a substantially reduced water-peak loss around 1.38 microm, which allows extended Raman-soliton supercontinuum generation up to 1.55 microm with a cw ytterbium fiber laser pump source. The resulting broadband, high-spectral-power-density, low-coherence light source can be employed for advanced, submicrometer resolution optical coherence tomography.     

Supercontinuum generation in short dispersion-shifted fiber by a femtosecond fiber laser

The supercontinuum generation has been obtained in short conventional dispersion-shifted fiber using the femtosecond pulses from a passively mode-locked erbium-doped fiber laser. In the experiment, the supercontinuum spectrum of >300 nm has been observed by injecting 70-fs pulses into a several-meter dispersion-shifted fiber. The simulation of the evolution in the fiber shows that spectral broadening arises from soliton dynamics when pumping using femtosecond pulses in the anomalous group velocity dispersion regime of the fiber.     

中红外超连续谱在氟化物光纤中的产生

为获得高功率全光纤中红外超连续谱,采用自制的掺Er锁模光纤激光器作为种子源,激光经过两级放大至1.67W,泵浦氟化物光纤,获得光谱覆盖1000~2400nm的超连续光谱;光谱宽度随着泵浦功率的增加而展宽,当输出功率达到1.21W时,转换效率为72%,并且产生的超连续谱被强烈的调制,在多个波长点处谱功率密度调制到0mW/nm,但调制波长与泵浦功率无关。     

Multi-wavelength hybrid gain fiber ring laser based on Raman and erbium-doped fiber

A stable and uniform multi-wavelength fiber laser based on the hybrid gain of a dispersion compensating fiber as the Raman gain medium and an erbium-doped fiber (EDF) is introduced. The gain competition effects in the fiber Raman amplification (FRA) and EDF amplification are analyzed and compared experimentaUy. The FRA gain mechanism can suppress the gain competition effectively and make the present multi-wavelength laser stable at room temperature. The hybrid gain medium can also increase the lasing bandwidth compared with a pure EDF laser, and improve the power conversion efficiency compared with a pure fiber Raman laser.     

Subwatt threshold cw Raman fiber-gas laser based on H2-filled hollow-core photonic crystal fiber

We report on what is, to our knowledge, the first cw pumped Raman fiber-gas laser based on a hollow-core photonic crystal fiber filled with hydrogen. The high efficiency of the gas-laser interaction inside the fiber allows operation in a single-pass configuration. The transmitted spectrum exhibits 99.99% of the output light at the Stokes wavelength and a pump power threshold as low as 2.25 W. The study of the Stokes emission evolution with pressure shows that highly efficient Raman amplification is still possible even at atmospheric pressure. The addition of fiber Bragg gratings to the system, creating a cavity at the Stokes wavelength, reduces the Raman threshold power below 600 mW.     

高重复频率锁模光纤激光器及其超连续谱产生

设计并实验研究了一种结构简单的主动调制锁模高重复频率窄脉宽光纤激光器。采用窄线宽连续激光调制4 GHz高重频后,通过拉曼增益孤子压缩效应将脉宽由27 ps压窄至2.6 ps。该高重频锁模激光泵浦一段300 m长高非线性光纤,同时脉冲被展宽至7.4 ps。产生的超连续谱平坦度20 dB宽带可达250 nm,功率波动为±0.2 dB。     

掺铒光纤光栅的开关动力学特征研究

研究在连续光入射下掺饵光纤光栅的开关动力学特征，发展其开关过程有类似于相变的特征。数值结果表明，主动光纤光栅在连续光条件下更适合于脉冲串产生。     

非均匀微结构光纤中双折射现象的研究

报道了利用飞秒脉冲激光与非均匀微结构光纤相互作用中产生超连续光谱后在非均匀微结构光纤传输中双折射拍频现象的研究.利用35?fs的飞秒激光脉冲在高双折射微结构光纤中的传输过程中直接观察到了拍频现象.并利用有限元方法对该光纤进行了模拟计算分析，计算得出在600?nm处拍频长度为毫米量级.所得结果与实验一致. 关键词： 双折射效应 微结构光纤 超连续光谱 有限元法     

Room temperature multiwavelength erbium-doped fiber ring laser using a highly nonlinear photonic crystal fiber

A multiwavelength laser source is demonstrated with a high power erbium-doped fiber amplifier as the gain medium. A highly nonlinear photonic crystal fiber (PCF) is inserted in the ring cavity to provide nonlinear gain by four-wave mixing. A Sagnac loop is incorporated in the ring cavity serving as a comb-like multichannel filter. The comparison between fiber ring laser without PCF and with PCF shows that the highly nonlinear PCF can generate a larger number of excited wavelengths and help stabilize the output power.     

Real-time, ultrahigh-resolution, optical coherence tomography with an all-fiber, femtosecond fiber laser continuum at 1.5 microm

Real-time, ultrahigh-resolution optical coherence tomography (OCT) is demonstrated in the 1.4-1.7-microm wavelength region with a stretched-pulse, passively mode-locked, Er-doped fiber laser and highly nonlinear fiber. The fiber laser generates 100-mW, linearly chirped pulses at a 51-MHz repetition rate. The pulses are compressed and then coupled into a normally dispersive highly nonlinear fiber to generate a low-noise supercontinuum with a 180-nm FWHM bandwidth and 38 mW of output power. This light source is stable, compact, and broadband, permitting high-speed, real-time, high-resolution OCT imaging. In vivo high-speed OCT imaging of human skin with approximately 5.5-microm resolution and 99-dB sensitivity is demonstrated.     

Phase-locked, erbium-fiber-laser-based frequency comb in the near infrared

A phase-locked frequency comb in the near infrared is demonstrated with a mode-locked, erbium-doped, fiber laser whose output is amplified and spectrally broadened in dispersion-flattened, highly nonlinear optical fiber to span from 1100 to >2200 nm. The supercontinuum output comprises a frequency comb with a spacing set by the laser repetition rate and an offset by the carrier-envelope offset frequency, which is detected with the standard f-to-2f heterodyne technique. The comb spacing and offset frequency are phase locked to a stable rf signal with a fiber stretcher in the laser cavity and by control of the pump laser power, respectively. This infrared comb permits frequency metrology experiments in the near infrared in a compact, fiber-laser-based system.