Broad tunability of erbium-doped fiber ring laser based on few-mode polarization maintaining fiber Sagnac interferometer

A highly stable erbium-doped fiber (EDF) ring laser with broad tunability is proposed and experimentally demonstrated. It is accomplished by using a few-mode polarization maintaining fiber (FM-PMF) Sagnac interferometer, in which simultaneous Sagnac and modal interferences occur. Continuously tunable output is obtained by exerting the longitudinal strain on the FM-PMF in combination with adjusting a polarization controller (PC) next to the FM-PMF in the Sagnac interferometer. In addition, the adverse impact of uneven EDF gain distribution on the laser output power uniformity may be offset by the PC-dependent transmission loss of the FM-PMF Sagnac interferometer and reflected backward C-band amplifier spontaneous emission from the pumped EDF. During the whole tuning process, the tunable span covers more than 40 nm with the output power flatness better than 1 dB, 3 dB spectral linewidth narrower than 0.1 nm and optical signal-to-noise extinction ratio higher than 45 dB.     

Fiber Sagnac Loop Filter with a Symmetrical Piecewise Uniform FBG

We propose a novel comb filter by using a symmetrical piecewise uniform fiber Bragg grating (S-PUFBG) in a Sagnac interferometer. The filter has wide bandwidth, equal passband spacing, and low insertion loss. The total transmission bandwidth is determined by the number and the parameters of the grating segments, while the transmission spectrum can be cosine modulated to realize comb-like filtering, and the modulating frequency can be controlled by adjusting the fiber length difference in the Sagnac loop.     

Tunable fiber-optic parametric oscillator

We report operation of a tunable optical parametric oscillator that employs a nonlinear-fiber Sagnac interferometer as a parametric amplifier. The amplifier, which consists primarily of dispersion-shifted fiber that has zero dispersion at 1538 nm, is synchronously pumped with 7.7-ps pulses at 1539 nm. The wide bandwidth of the parametric gain permits tuning of the output signal pulses over a 40-nm range centered on the pump wavelength. The Sagnac interferometer decouples the pump wave from the oscillator cavity while a bandpass filter in the cavity transmits only the signal wave, thereby creating a singly resonant parametric oscillator that is phase insensitive. Whereas we demonstrate tuning over almost the entire bandwidth of Er-doped-fiber amplifiers, one could construct a similar device that operates near the 1310-nm zero-dispersion wavelength of standard telecommunication fiber.     

Sagnac interferometer with adaptive nonlinear detection

We present a Sagnac interferometer that uses a nonlinear adaptive medium for detection. The nonlinearity of the medium being characterized by a finite response time, the detection has a finite frequency bandwidth determining two operational regimes: at low variation rate of the rotation speed the interferometer operates in the slow-light regime, providing a direct measurement of the angular acceleration. For high variation rate of the rotation speed the amplitude of the alternating component of the Sagnac phase shift is instead detected, with the continuous and low-frequency components filtered out by the adaptive nonlinear process in the medium.     

Pulsed degenerate optical parametric oscillator based on a nonlinear-fiber Sagnac interferometer

We report operation of an all-fiber degenerate optical parametric oscillator that employs a nonlinear-fiber Sagnac interferometer as a parametric amplifier. Synchronous pumping with 3.9-ps pulses at 1544 nm yields 0.83-ps output pulses. The wide bandwidth of the fiber parametric amplifier causes the oscillator to act as a pulse compressor. The output signal pulses exhibit improved spectral symmetry and a reduced time-bandwidth product compared with the pump pulses. Currently, the net group-velocity dispersion in the passive section of the fiber cavity limits the signal-pulse bandwidth and hence the minimum-obtainable pulse width. This experiment suggests the possibility of frequency conversion by operation of a similar pulsed parametric oscillator away from degeneracy.     

Amplitude squeezing of light by means of a phase-sensitive fiber parametric amplifier

We experimentally demonstrate generation of bright sub-Poissonian light by means of parametric deamplification in a phase-sensitive fiber amplifier that is based on a balanced nonlinear Sagnac interferometer. On direct detection, the photocurrent noise falls below the shot-noise limit by (0.6 +/- 0.2) dB (1.4 dB when corrected for detection losses). To observe the noise reduction we employed a scheme that used two orthogonally polarized pulses to cancel the noise that arises from the predominantly polarized guided-acoustic-wave Brillouin scattering in the fiber. We also present a simplified semiclassical theory of quantum-noise suppression by this amplifier, which is found to be in good agreement with the experimental results.     

基于多波长类噪声脉冲的掺铒光纤激光器

报道了一种基于多波长类噪声脉冲的被动锁模掺铒光纤激光器。采用980 nm半导体激光器作为泵浦源，2.5 m长的掺铒光纤作为增益介质。锁模机制为非线性放大环形镜(NALM)。通过自相关迹证明输出脉冲为类噪声脉冲。该类噪声脉冲的光谱3 dB带宽可达17.2 nm，边模抑制比为47.7 dB，重复频率为5.434 MHz，单脉冲能量为7.9 nJ。为了实现平坦的多波长输出，在NALM结构中加入Sagnac环干涉仪，获得了最大波长数为5的平坦多波长类噪声脉冲，平坦度为1.995。     

Analysis of a Sagnac interferometer with low-birefringence twisted fiber

The fiber Sagnac interferometer of low birefringence and twist is analyzed numerically in the linear region. A novel method for measurement of the birefringence of the fiber and the angle of rotation of the axes inside the fiber loop of the interferometer is also presented.     

Non-contact detection of laser-generated surface acoustic waves using fiber optic Sagnac interferometer

A high powered Q-switched Nd:YAG laser was used to excite the surface waves, and an optical fiber sensor was used to detect the out-of-plane displacements due to the propagating waves. This sensor is based on the fiber optic Sagnac interferometer, which has the path-matched configuration and does not require active stabilization. Quadrature phase bias between two interfering laser beams in the Sagnac loop is applied by controlling the birefringence in an optical path using a fiber polarization controller. A stable quadrature phase bias can be confirmed by observing the interferometer output according to the change of phase bias. Additional signal processing is not needed for the detection of ultrasonic waves using the Sagnac interferometer. The performance of the fiber optic Sagnac interferometer was investigated, and laser-generated surface wave signals were detected using this fiber optic sensor. The developed fiber optic sensor configured in this study is very simple and is effective for non-contact detection of ultrasonic waves.     

Temperature-insensitive strain measurement using a birefringent interferometer based on a polarization-maintaining photonic crystal fiber

We propose a novel and simple scheme for a temperature-insensitive strain measurement by using a birefringent interferometer configured by a polarization-maintaining photonic crystal fiber (PM-PCF). The wavelength-dependent periodic transmission in a birefringent interferometer can be achieved by using a PM-PCF between two linear polarizers. Since the PM-PCF is composed of a single material, such as silica, the peak wavelength shift with temperature variation can be negligible because of the small amount of the birefringence change of the PM-PCF with temperature change. The measured temperature sensitivity is −0.3 pm/°C. However, the peak wavelength can be changed by strain because the peak wavelength shift is directly proportion to strain change. The strain sensitivity is measured to be 1.3 pm/με in a strain range from 0 to 1600 με. The measurement resolution of the strain is estimated to be 2.1 με. The proposed scheme has advantages of simple structure and low loss without a Sagnac loop, temperature insensitivity, ease installation, and short length of a sensing probe compared with a conventional PMF-based Sagnac loop interferometer.     

一种新型侧漏型光子晶体光纤的研制及其传输特性研究

通过引入椭圆掺锗芯和侧向泄露通道, 提出并研制出一种侧漏型光子晶体光纤(photonic crystal fiber, PCF). 应用结构重构全矢量有限元数值分析法分析了设计结构和实际研制的侧漏型PCF的传输特性. 研制的侧漏型PCF, 在波长1550 nm处基模的平均模场直径为9.275 μm, 与G652标准单模光纤具有很好的适配性, 模式双折射为0.837× 10^-4, 群双折射约为1.508× 10^-4. 基于研制的侧漏型PCF光纤构建了Sagnac干涉仪, 对其群双折射进行了测量. 测量结果表明:当侧漏型PCF光纤达到一定长度时, 在1450–1750 nm波长范围内, 二阶模在光纤中不能成为有效传输模式, 光纤可以实现单模传输; 另外, 研制的侧漏型PCF群双折射实验的测量平均值, 与数值分析结果相符合. 侧向泄露通道的引入, 增强了侧漏型PCF光纤对外界参量变化的敏感性, 提高了其在扭转、弯曲、压力等参量的光纤传感和高性能光纤激光器构建等方面的应用潜能. 关键词： 光子晶体光纤 侧向泄露通道 双折射 Sagnac干涉仪     

Increase in the correlation length of nonmonochromatic radiation during propagation in a single-mode optical fiber containing random inhomogeneities and the influence of this increase on the operation of a fiber ring interferometer

An analysis is made of the conditions under which the coupling of orthogonal polarization modes at random inhomogeneities in single-mode optical fibers leads to an increase in the correlation length of a source of nonmonochromatic radiation. It is shown that when long-base fiber ring interferometers with a single-mode fiber ring system possessing weak linear birefringence are used, the correlation length of the nonmonochromatic radiation at the interferometer exit is increased, which means that the interference pattern can have satisfactory visibility even when there is an appreciable difference between the interferometer arms as a result of the Sagnac effect due to the Earth’s rotation. The calculations were made by mathematical modeling of random inhomogeneities in the fiber. Zh. Tekh. Fiz. 69, 140–143 (July 1999)     

Performance of a wavelength-tunable erbium-doped fiber laser using a Sagnac interferometer

We experimentally demonstrate a wavelength-tunable erbium-doped fiber laser that is composed of a ring cavity and a single-mode fiber Sagnac interferometer in a new and simple arrangement. We find that the fiber laser output wavelength is tunable by adjusting the filter effect of the Sagnac fiber loop through a fiber polarization controller set there. The quasi-single-wavelength continuously tunable laser outputs could be achieved within some wavelength range. The multi-wavelength laser outputs could also be observed under some appropriate settings of the polarization controller. A theoretical demonstration of the wavelength tunability about the transmission-type Sagnac loop filter has also been achieved using the Jones calculus theory.     

High-resolution, large dynamic range fiber length measurement based on a frequency-shifted asymmetric Sagnac interferometer

We propose and experimentally demonstrate a single-mode fiber length and dispersion measurement system based on what we believe to be a novel frequency-shifted asymmetric Sagnac interferometer incorporating an acousto-optic modulator (AOM). By sweeping the driving frequency of the AOM, which is asymmetrically placed in the Sagnac loop, the optical length of the fiber can be determined by measuring the corresponding variation in the phase delay between the two counterpropagating light beams. Combined with a high-resolution data processing algorithm, this system yields a dynamic range from a few centimeters to 60 km (limited by our availability of long fibers) with a resolution of approximately 1 part per million for long fibers.     

Stable multiwavelength fiber ring laser with equalized power spectrum based on a semiconductor optical amplifier

We experimentally demonstrated a new structure of a multiwavelength semiconductor optical amplifier (SOA) ring laser based on a fiber Sagnac loop filter that can generate up to 25 stable output lasing wavelengths at room temperature. By varying the length of a polarization-maintaining (PM) fiber within the Sagnac loop filter, the wavelength spacing between the output lasing wavelengths can be changed to a desired value. By tuning a polarization controller (PC) within the Sagnac loop filter, stable multiwavelength 1550-nm operation with up to 17 lasing lines within 3 dB power level variation and with a wavelength spacing of ∼0.8 nm was achieved. The optical signal-to-noise ratios (OSNRs) of all the lasing wavelengths are greater than 40 dB.     

带有对称啁啾光纤光栅的Sagnac环滤波器

通过在光纤Sagnac环中引入对称啁啾光纤光栅,对带有光纤光栅的Sagnac环梳状滤波器进行了改进,克服了原有滤波器工作带宽小、通带间隔不等的缺点.通过对光栅参量进行适当设计,在中等折射率调制度下,可使光栅同时具有高反射率和大反射带宽,结合Sagnac环可得到大带宽、等间隔梳状滤波器.     

Measurement of nonlinear phase of a SOA using an asymmetric loop mirror

We use a setup based on an asymmetric Sagnac interferometer to demonstrate an experimental methodology to characterize the nonlinear phase of a semiconductor optical amplifier as a function of input intensity.     

Soliton squeezing in a highly transmissive nonlinear optical loop mirror

A perturbation approach is used to study the quantum noise of optical solitons in an asymmetric fiber Sagnac interferometer (a highly transmissive nonlinear optical loop mirror). Analytical expressions for the three second-order quadrature correlators are derived and used to predict the amount of detectable amplitude squeezing along with the optimum power-splitting ratio of the Sagnac interferometer. We find that it is the number-phase correlation owing to the Kerr nonlinearity that is primarily responsible for the observable noise reduction. The group-velocity dispersion affecting the field in the nonsoliton arm of the fiber interferometer is shown to limit the minimum achievable Fano factor.     

2.1 μm可调谐多波长掺钬光纤激光器

为了实现2.1 μm波段光纤激光器输出多波长激光，设计了一种基于光纤Sagnac干涉仪的可调谐多波长掺钬光纤激光器。采用1.9 μm波段掺铥光纤激光器泵浦一段长3 m的掺钬石英光纤，获得2.1 μm波段的光放大；环形腔中，由保偏光纤和偏振控制器构成的光纤Sagnac干涉仪，实现2.1 μm波段周期滤波，获得了2.1 μm波段多波长激光，输出功率1 mW~15 mW可调谐，最多可观测到6个波长的激光输出。通过调节环形腔内偏振控制器，能够实现2.1 μm波段1~6个波长的调谐。     

Dynamic optical distortion in a KrF discharge amplifier

The dynamic optical distortion in a KrF amplifier was measured using a Sagnac interferometer. Maximum wavefront aberration at 249 nm was /3 using helium buffer gas, indicating the potential for transmitted beams very close to the diffraction limit.