半导体激光器与单模光纤耦合效率的分析

在高斯场分布近似的前提下,从模场半宽匹配的角度描述了半导体激光器（ＳＬＤ） 与单模光纤（ＳＭＦ） 之间的光耦合理论。对不同的ＳＬＤ ＳＭＦ 间光耦合的方式作了分析,并从耦合效率与成本双重角度给出了适合于实际工程应用的几种耦合方式的优选率     

A volume holographic optical element for optical coupling to single-mode fibers

We propose and demonstrate a volume holographic optical element (VHOE) for signal-mode fiber coupling and the sending of conjugate waves of reference light for the construction of holographic elements. Given the inherent Bragg conditions of the VHOE, the angular tolerances of the conjugate reading light in both the horizontal and vertical directions are studied under different conditions.     

Microlens fabrication on single-mode fibres for efficient coupling from laser diodes

An analysis of the etching process for forming a truncated conical microlens on the end surface of a single-mode fibre (SMF) so as to improve its coupling efficiency with a laser diode (LD) has been carried out. This is followed by a simple geometric-optics and electromagnetic analysis to calculate this coupling efficiency, when the lens is formed on a SMF fabricated by the MCVD technique. The theory is supported by measurements which gave a maximum coupling efficiency of 57%.     

Experimental investigation on coupling efficiency between semiconductor laser diodes and single-mode fibres by an etching technique

The coupling efficiency between semiconductor laser diodes (LDs) and single-mode fibres (SMFs) can be improved by matching their respective modes. This may be achieved by constructing a conical microlens on the end of an Al-coated vapour-axial deposition (VAD) SMF. A simple process to etch an SMF end selectively to produce such a conical microlens is described.A minimum coupling loss of 3 dB has been obtained using 1.3-m InGaAsP/InP buried heterostructure LDs. Experimental results on coupling efficiency and loss penalty due to lateral misalignment are also reported. This conical microlens is easy to manufacture and reproduce.     

Low-loss coupling between two single-mode optical fibers with different mode-field diameters using a graded-index multimode optical fiber

We present a method for ultra-low-loss coupling between two single-mode optical fibers with different mode-field diameters using multimode interference in a graded-index multimode optical fiber. We perform a detailed analysis of the interference effects and show that the graded-index fiber can also be used as a beam expander or condenser. The results are important for devices in which optical fibers with different mode-field diameters are coupled in series, such as in ultra-short-pulse fiber ring lasers, or in optical fiber communication links.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XVI. Depolarization and repolarization of polychromatic radiation in single-mode optical fibers with random inhomogeneities

The dependences of the degree of polarization of polychromatic radiation on the length of a single-mode optical fiber (SMF) with random inhomogeneities have been obtained by mathematical modeling. The case is considered where radiation having both polarization modes excited with equal weights of linear polarization is first introduced into a depolarizer of polychromatic radiation (a SMF segment with high linear birefringence) and arrives at an SMF with low linear birefringence. It is shown that the degree of polarization of radiation after transmission through the first segment becomes significantly suppressed and remains almost constant upon propagation through the second segment, after which it begins to sharply increase at some length; i.e., repolarization of radiation occurs. It is shown that repolarization of radiation depends weakly on the angle made by the axes of unperturbed linear birefringence of the first and second segments. The conditions for the length of the first segment (depolarizer) under which the degree of polarization remains minimum throughout the second segment are determined.     

High-quality lowest-loss-mode lasing in GaAs quasi-stadium laser diodes having unstable resonators

We obtained high-quality lowest-loss-mode lasing in quasi-stadium laser diodes having unstable resonators that consisted of two curved end mirrors and two straight sidewall mirrors. The laser diodes were fabricated by applying a reactive ion etching technique to a metal-organic chemical-vapor deposition-grown graded-index separate-confinement heterostructure single-quantum-well GaAs/AlGaAs structure. The electrode contact area of the laser diodes was formed along unstable periodic orbits, along which the optical beams are localized. Highly directional fan-out beams corresponding to the numerically obtained lowest loss mode were emitted from the end mirrors under CW operation at room temperature.     

Photopolymer microtips for efficient light coupling between single-mode fibers and photonic crystal fibers

A novel method for light coupling between single-mode fibers (SMFs) and small-core photonic crystal fibers (PCFs) is demonstrated. The method is based on growing photopolymer microtips directly on the end faces of SMFs. The shape and size of the tips can be controlled by adjusting the laser power and the exposure time for polymerization to match the mode field to the small-core PCFs. A 5 dB improvement in coupling efficiency between a SMF and a commercial small-core, highly nonlinear PCF is experimentally demonstrated. This compact and efficient butt-coupling method is particularly suitable for PCF gas sensor applications.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: Integrated statistical properties of polarization modes of single-mode optical fibers in the presence of random inhomogeneities

Using mathematical modeling, an explicit form is found for the Jones matrix of a segment of a single-mode optical fiber with random inhomogeneities, whose length is considerably greater than the correlation length of random inhomogeneities. It is shown that parameters of this matrix are of a statistical nature. A rational representation of the matrix is proposed. It is shown that, under certain conditions, one of the matrix parameters may be treated as constant, whereas the other parameter is assumed to be continuously distributed on the interval [0, 2π]. As the latter parameter is changed, which corresponds to a change from one random realization of inhomogeneities in a single-mode fiber to another, the ellipticity and the azimuth of the major axis of the polarization ellipse of natural polarization modes of a single-mode fiber simultaneously change.     

MEMS阵列光开关的光纤耦合设计

利用高斯光束传输理论进行了阵列光开关的光纤耦合设计计算。根据ABCD矩阵推导出了光束通过球透镜时的传播规律,计算了在光纤与光纤之间两个球透镜耦合的效率,分析了影响耦合效率的因素。通过设计计算,确定了光纤端部的位置,使得整个损耗达到了最小。从理论上来看,在波长λ=1 55μm时,4×4阵列光开关的耦合效率可以达到91%以上。     

Semiconductor laser diode to single-mode fiber coupling using diffractive optical elements

An integrated mode converter consisting of two diffractive optical elements (DOEs) and a Silicon slab is presented for low-loss coupling between a semiconductor laser diode (LD) and a single-mode fiber (SMF). The phase structures of the DOEs are designed using iterative phase retrieval algorithm. We introduce a new far-field amplitude constraint into the iteration to provide very high mode conversion quality. Compared with previously published mode converters, the scheme is more universal because it’s applicable for any semiconductor LD. In simulation, coupling losses lower than 0.02 dB are predicted for all the discussed LDs with aspect ratios of the elliptical fields from 1 to 9. The requirements on axial displacement and rotation angle have been removed. The tolerance for 1-dB loss increment for lateral misalignment is 0.9 μm. And the coupling loss is insensitive to tilt angle.     

Ring and axis mode lasing in quasi-stadium laser diodes with concentric end mirrors

We fabricated quasi-stadium laser diodes whose resonators consist of two concentric curved end mirrors and two straight sidewall mirrors. We observed two lasing modes that correspond to different beam propagations along the cavity axis and along a ring trajectory, and different far-field patterns with wide angular separation. The modes can be selected by control of an electrode pattern. We also show that the far-field patterns numerically obtained by the extended Fox-Li mode calculation method are in good agreement with the experimental results.     

Formation of intermediate coupling optical polarons and bipolarons in two-dimensional systems

The formation of the optical polaron and bipolaron in two-dimensional (2D) systems is studied in the intermediate electron–phonon coupling regime. The total energies of the 2D polaron and bipolaron are calculated by using the Buimistrov–Pekar method of canonical transformations. The obtained results are compared with other existing results obtained by using the Feynman path integral method and the modified Lee–Low–Pines unitary transformation method. It is shown that the electron–phonon correlation significantly reduces the total energy of the 2D polaron in comparison with the energy of the strong coupling (adiabatic) polaron. It is found that the polaron formation in 2D systems is possible when the electron–phonon coupling constant α is greater than the critical value ${\alpha }_c?2.94$, which is much lower than a critical value of the electron–phonon coupling constant α in three-dimensional (3D) systems. The critical values of the Fröhlich coupling constant α and the ratio $\eta ={\epsilon }_{\infty }/{\epsilon }_0$ (where ${\epsilon }_{\infty }$ and ${\epsilon }_0$ are the high frequency and static dielectric constants, respectively), which determine the bipolaron stability region in 2D systems, are calculated numerically. It is interesting for application to the layered cuprate superconductors that the (bi)polarons are formed more easily in quasi-2D regions than in the bulk. It is argued that the high-$T_c$ cuprate superconductivity can exist above the bulk superconducting transition temperature $T_c$ as the persisting superfluidity of polaronic (bosonic) Cooper pairs and large bipolarons at quasi-2D grain boundaries or in the CuO₂ layers above $T_c$.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XV. Dependence of integral statistical parameters of polarization modes in single-mode optical fibers with random inhomogeneities on the wavelength of light

Spectral dependences of integral statistical elements of the Jones matrix of an extended segment of a single-mode optical fiber (SMOF) with random inhomogeneities, whose length considerably exceeds the correlation length of random inhomogeneities, are found by the method of mathematical modeling. The boundaries of the ranges of optical wavelengths are determined, within the limits of which the parameters under consideration remain quasi-constant. This allows the derivation of analytical expressions for the degree of polarization of nonmonochromatic radiation in an SMOF and for the zero drift in fiber ring interferometers with the help of the approximate method of small perturbations. It is shown that the small-perturbation method leads to adequate results for SMOFs with strong and intermediate linear birefringence.     

Fabrication of high-mechanical-resistance Bragg gratings in single-mode optical fibers with continuous-wave ultraviolet laser side exposure

The mechanical resistance of single-mode fibers containing fiber Bragg gratings inscribed with cw UV laser irradiation is almost identical to that of pristine fiber. The median breaking strength of the gratings' Weibull distribution is more than 5 GPa, and the m value is of the order of 70. Based on a dynamic fatigue model, a Bragg grating lifetime of 50 years with a failure probability of 0.001 is predicted, assuming a constant applied stress of 0.96 GPa.     

Higher-order photon statistics of single-mode laser diodes and microchip solid-state lasers

We examine higher-order photon statisitics of laser diodes (LDs) and microchip solid-state lasers (SSLs). When the lasers are operated above lasing threshold, significantly different features of fluctuations are exhibited, which are the result of the asymmetric non-Gaussian distribution characteristics that are always present in SSLs, whereas, in contrast, Gaussian fluctuation persists in LDs. It is found that the deterministic dynamic trajectory in phase space, which is controlled by the ratio between the fluorescence and the photon lifetimes, is crucial to the corresponding behavior of higher-order statistics.     

Reflectometric measurement of birefringence rotation in single-mode optical fibers

We present a novel reflectometric technique for the measurement of orientation and modulus of the linear birefringence vector in single-mode optical fibers. The technique provides information also on circular birefringence, although this component, if present, appears as a rotation of the linear birefringence. A detailed theoretical analysis is reported and validated by experimental results.     

Operation of Brillouin dynamic grating in single-mode optical fibers

The first (to our knowledge) observation of Brillouin dynamic grating in conventional single-mode fibers is reported, and the characterization is demonstrated with respect to the external parameters for the grating generation. When a 100 m single-mode fiber is used, a reflectance of 8% with a spectral bandwidth as low as 2.4 MHz is achieved, which is less than 10% of ordinary Brillouin gain bandwidth.