Showing papers on "Single-mode optical fiber published in 1994"
01 Jan 1994
TL;DR: In this article, a method to determine the individual strain-optic coefficients in single-mode fibers is described, based on two photoelastic experiments, namely, the polarimetric measurement of optical activity induced by mechanical twist and the interferometric measurement of optic-path-length change induced by static longitudinal strain.
Abstract: A method to determine the individual strain-optic coefficients in single-mode fibers is described. It is based on two photoelastic experiments, namely, the polarimetric measurement of optical activity induced by mechanical twist and the interferometric measurement of optical-path-length change induced by static longitudinal strain. For fibers with pure silica core and B/sub 2/O/sub 3/ doped cladding, the optical activity per unit twist rate and the phase change per unit fiber elongation have been measured to be 0.1472 and 1.150*10/sup 7/ rad/m, respectively. The strain-optic coefficients have been measured to be p/sub 11/=0.113 and p/sub 12/=0.252, 7% lower than those of bulk silica. >
244 citations
TL;DR: In this article, the extrema density and mean-level crossing densities of a single-mode fiber were derived for the measurement of the expected value of polarization-mode dispersion (PMD) in the transmission spectrum.
Abstract: Routine characterization of polarization-mode dispersion (PMD) in single-mode fiber, both at the manufacturing stage and in installed systems, requires an easy-to-implement measurement technique. One method that is particularly simple to implement involves counting the number of extrema (i.e., maxima and minima) per unit wavelength interval in the transmission spectrum measured through a polarizer (analyzer) placed at the output of a test fiber. In this paper, we establish accurate equations relating both the extrema density and mean-level crossing densities in such spectra to the expected value of PMD. These equations are used to measure several fiber samples, and are compared to measured values obtained with a commercially available test set. It is shown that measuring both mean-level crossings and extrema densities provides a simple means for establishing whether a fiber is scaling as /spl radic/L (long-length regime) or L (short-length regime). Using Monte Carlo simulations, the accuracy of the fixed-analyzer measurements is examined as a function of the width of the wavelength interval over which measurements are made. In addition, the simulations indicate that fixed-analyzer measurements are quite robust with respect to the presence of polarization-dependent loss (PDL) in the span, an important consideration for measurements in amplified systems. >
233 citations
TL;DR: In this paper, a spatial mode-converter is used to selectively excite a higher order mode in specially designed multimode fiber, which can be used to compensate the positive dispersion in conventional single-mode fiber spans.
Abstract: Higher order spatial modes in optical fibers exhibit large, negative chromatic dispersion when operated near their cutoff wavelength. By using a spatial mode-converter to selectively excite a higher order mode in specially designed multimode fiber, this dispersion can be used to compensate the positive dispersion in conventional single-mode fiber spans. In this paper, issues related to compensating fiber and mode-converter design are explored. Experimental measurements in specially designed two-mode fibers operated in LP/sub 11/ mode show negative dispersion as large as -70 ps/nm/spl middot/km at 1555 nm. Pulse propagation and system experiments employing spatial mode-converters to excite LP/sub 11/ mode in a two-mode fiber demonstrate the feasibility of this technique for dispersion compensation in lightwave systems. >
233 citations
TL;DR: In this article, an analytical model for two-, three-, and four-level system rare-earth-doped fiber amplifiers and lasers is presented, where fiber-amplifier gain is expressed in terms of attenuation coefficients, intrinsic saturation powers, and cross-saturation powers at the pump and signal wavelengths.
Abstract: An analytical model for two-, three-, and four-level system rare-earth-doped fiber amplifiers and lasers is presented. The theory is applicable to dopants such as erbium, neodymium, thulium; praseodymium, and ytterbium. Fiber-amplifier gain is expressed in terms of attenuation coefficients, intrinsic saturation powers, and cross-saturation powers at the pump and signal wavelengths. These parameters can be directly determined from one- and two-beam fiber-transmission measurements. System-independent formulas are given for the slopes and thresholds of ring and linear fiber lasers. Good agreement between theory and experiment has been shown for erbium-doped fiber amplifiers and lasers and thulium-doped fiber lasers. Because of the finite-pump-level lifetime, three- and four-level models predict a flattening of the fiber laser slope at higher pumping powers when the fiber is shorter than the optimum length. Approximate system-independent solutions are also given for fiber amplifiers with excited-state absorption at either the pump or signal wavelengths. A novel technique, requiring only one tunable light source, is proposed for finding the best pump wavelength when pump ESA is present. The two-level analytical model recently developed for erbium-doped fibers is a special case of this theory. >
206 citations
TL;DR: In this article, the effect of rib etch depth, width, and interface roughness on loss and mode characteristics have been studied at wavelengths of 1.15 and 1.523 microns.
Abstract: Optical rib waveguides with rib heights of 3.17 and 7.67 microns with various widths have been formed in separation by implantation of oxygen (SIMOX) based silicon-on-insulator (SOI) structures. The effect of waveguide rib etch depth, width, and interface roughness on loss and mode characteristics have been studied at wavelengths of 1.15 and 1.523 microns. The experimental results support the hypothesis that certain rib dimensions can lead to single mode SOI waveguides even though planar SOI waveguides of similar multimicron dimension are not single mode. Mode loss was found to be strongly dependent on interface roughness and mode confinement. >
199 citations
TL;DR: In this article, the authors proposed a nonlinearly tapered waveguide with a non-Gaussian shape and an expanded mode field with a very small cross-section and a nonlinear shape.
Abstract: The novel waveguide structures described in this paper have nonlinearly tapered shapes that result in low radiation losses despite their relatively short lengths. The core at the waveguide endface connected with the fiber has a very small cross section and an expanded mode field with a non-Gaussian shape. The taper structures are analyzed by using an improved step-transition method. This method is a based on the theory of enclosing a waveguide within electrical walls and that can therefore treat the radiation modes in a tapered waveguide as discrete mode spectra. Analyzing the relationships between the lengths and shapes of the tapers and the radiation loss due to the tapers show that appropriately tapered semiconductor waveguides operating at an optical wavelength of 1.55 /spl mu/m and having a taper length of less than 0.7 mm can have a radiation loss of only 0.1 dB and a coupling loss with a conventional single-mode fiber of less than 0.5 dB. >
154 citations
TL;DR: In this article, the authors studied optical micro-resonators with dimensions between 0.1 and 10 microns and showed that one can isolate a single mode of the optical field in a cube a half-wavelength on a side with perfectly reflecting walls.
Abstract: Studies of optical microresonators with dimensions between 0.1 and 10 microns are now under way in a wide variety of condensed matter systems. Ideally, one can isolate a single mode of the optical field in a cube a half-wavelength on a side with perfectly reflecting walls. Liquid droplets, polymer spheres and semiconductor Fabry-Perot microcavities with dielectric mirrors are examples of microresonators with which one can approach this ideal limit and nearly isolate a few modes of the electromagnetic field from the continuum of surrounding free-space modes.
152 citations
TL;DR: In this paper, the authors analyzed the operation of a reset-free polarization controller comprising three endlessly rotatable wave plates: a first quarter wave plate followed by a half-wave plate and a second quarter waveplate rotated synchronously with the first wave plate.
Abstract: We analyze the operation of a reset-free polarization controller comprising three endlessly rotatable wave plates: a first quarter-wave plate followed by a half-wave plate and a second quarter-wave plate, which is rotated synchronously with the first quarter-wave plate We show that for any arbitrary angular offset between the two quarter-wave plates; the controller allows continuous, reset-free transformations from any varying general input state of polarization into any general output state An integrated-optic realization of this scheme on z-propagation LiNbO/sub 3/ offers control speeds that well exceed the speed of natural polarization fluctuations in standard single-mode fibers, thus allowing fast automatic polarization stabilization in fiber-optic transmission systems >
148 citations
TL;DR: In this article, the authors proposed a method of dispersion-supported transmission of 10 Gb/s signals at 1.53 /spl mu/m wavelength on standard single-mode fiber with zero dispersion.
Abstract: Using the new method of dispersion-supported transmission, 10 Gb/s signals at 1.53 /spl mu/m wavelength are transmitted on standard single-mode fiber with zero dispersion at 1.3 /spl mu/m via the record length of 253 km without in-line regeneration. Detailed experiments with a directly modulated laser demonstrate the system performance for different fiber lengths ranging from 0 up to 253 km. >
127 citations
TL;DR: In this article, the characteristics of square, circular and diagonal quadruple-ridged waveguides, including cutoff frequencies, attenuation, impedance and modal field distributions, are for the first time systematically analyzed and reported.
Abstract: In a previous paper, a unified approach has been proposed for the analysis and design of single- and double-ridged waveguides by a magnetic field integral equation (MFIE) formulation (see ibid., vol. 41, no. 11, p. 1965-71, Nov. 1993). This paper presents a continuing work with emphasis on the design of quadruple-ridged waveguides. The characteristics of square, circular and diagonal quadruple-ridged waveguides, including cutoff frequencies, attenuation, impedance and modal field distributions, are for the first time systematically analysed and reported. Distinct to being in a single- or double-ridged waveguide, the fundamental-mode in a quadruple-ridged waveguide has a cutoff frequency very close to that of the second-lowest mode, thus the natural single mode bandwidth is very small. However, when the second-lowest mode is effectively suppressed or not excited, a very wide bandwidth (6:1) can be achieved. This unique property, plus the capabilities of dual-polarization, high power, and low impedance, makes the quadruple-ridged waveguides well-suited to many antenna and microwave applications. >
114 citations
TL;DR: In this paper, a linearly chirped in-fibre Bragg grating was used to compensate for the pulse broadening arising from negative group-delay dispersion in single mode optical fibre.
Abstract: A linearly chirped in-fibre Bragg grating has been used to compensate for the pulse broadening arising from negative group-delay dispersion in single mode optical fibre. Numerical simulations of the fibre and the Bragg grating produce good agreement with the experiment.<
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TL;DR: In this article, a compensation technique based on propagating the signals through a specially designed fiber with large negative dispersion for the LP/sub 01/mode, thereby ending up with zero net pulse spreading is presented.
Abstract: The practical implementation of erbium-doped fiber amplifiers with gain at 1.55 /spl mu/m allows long unrepeatered transmission distances. However, in order to realize high data rates over these distances with already installed standard single mode fiber, techniques must be found to overcome the pulse spreading due to the positive chromatic dispersion of the transmission fiber in this window, we review a compensation technique based on propagating the signals through a specially designed fiber with large negative dispersion for the LP/sub 01/ mode, thereby ending up with zero net pulse spreading. The basis of the concept are discussed and a key figure of merit for dispersion compensating devices is defined. The design and optimization of dispersion compensating (DC) fiber is described with special attention to practical concerns including packaging and manufacturability. We describe experimental fabrication results of DC fiber, results of using the fiber to make compact dispersion compensating modules, and the outcome of recent systems experiments incorporating the fiber. >
TL;DR: In this article, a simple and inexpensive calibration scheme for a continuous-wave difference frequency spectrometer is presented, based on the stabilization of an open transfer cavity by locking onto the output of a polarization stabilized HeNe laser.
Abstract: A novel, simple, and inexpensive calibration scheme for a continuous‐wave difference frequency spectrometer is presented, based on the stabilization of an open transfer cavity by locking onto the output of a polarization stabilized HeNe laser High frequency, acoustic fluctuations of the transfer cavity length are compensated with a piezoelectric transducer mounted mirror, while long term drift in cavity length is controlled by thermal feedback A single mode Ar+ laser, used with a single mode ring dye laser in the difference frequency generation of 2–4 μm light, is then locked onto a suitable fringe of this stable cavity, achieving a very small long term drift and furthermore reducing the free running Ar+ linewidth to about 1 MHz The dye laser scan provides tunability in the difference frequency mixing process, and is calibrated by marker fringes with the same stable cavity Due to the absolute stability of the marker cavity, precise frequency determination of near infrared molecular transitions is achi
TL;DR: The field of fiber optics still reserves some surprises as mentioned in this paper and the past few years have seen stunning developments in reaction to advances in rare earth-doped fiber amplifiers and nonlinear (or soliton) pulse propagation in fibers.
Abstract: After nearly two decades of research in fiber‐optics technology, what could possibly remain to be discovered? Clearly, the design of optical fibers cannot be improved forever. With current technology, fibers can transmit infrared (1.5‐micron wavelength) light pulses in a single electromagnetic mode with a minimal power loss of 5% per kilometer and without dispersion; such performance is forever fixed by Maxwell's equations and the physics of silica glass waveguides. Yet the field of fiber optics still reserves some surprises. Indeed, the past few years have seen stunning developments in reaction to advances in rare‐earth‐doped fiber amplifiers and nonlinear (or soliton) pulse propagation in fibers.
TL;DR: A single-mode Nd(3+)-doped tellurite glass fiber laser operating at 1.061 microm is described, believed to be the first demonstration of a single- mode fiber laser in tellurites glass.
Abstract: A single-mode Nd(3+)-doped tellurite glass fiber laser operating at 1.061 microm is described. We believe this is the first demonstration of a single-mode fiber laser in tellurite glass. A lasing threshold of 27 mW of 818-nm absorbed pump power and a slope efficiency output power versus pump power of 23% emitted from one end were observed in the fiber cavity with 11.9% Fresnel reflection at both ends.
TL;DR: In this paper, a chirped Bragg reflector was used to stabilize the single mode output of a hybrid laser with a side-mode suppression ratio of over 55 dB at 27.5 mW and a relative intensity noise (RIN) below 160 dB/Hz.
Abstract: We describe hybrid lasers combining a semiconductor gain section and fiber cavity with integrated chirped Bragg reflector. These devices have produced output powers of 27.5 mW in a narrow linewidth (400 KHz) stable single longitudinal mode. The use of a chirped reflector to stabilize the single mode output, and correct grating orientation are described. The laser output has a side‐mode suppression ratio of over 55 dB at 27.5 mW output, and relative intensity noise (RIN) below 160 dB/Hz.
TL;DR: In this paper, the authors measured the physical quantity by measuring the variation in polarization mode beat frequency, which is linearly proportional to the external physical quantity externally applied, such as pressure, temperature, strain etc.
Abstract: Apparatus for measuring a physical quantity such as pressure, temperature, strain etc., by using a beat frequency of a longitudinal mode, a transverse mode or two polarization modes of the optical fiber laser. In the rare-earth optical fiber laser as a resonator, there are two different polarization modes normal to each other. Since the optical path lengths of the optical fiber laser for the two polarization modes are different, longitudinal mode frequencies of each polarization mode are different depending on the birefringence of resonator which is varied as an external physical quantity such as pressure, temperature, strain etc. Due to the variation in the birefringence of the resonator, the longitudinal mode frequency difference between two polarization modes becomes varied. This variation is linearly proportional to the physical quantity externally applied. The difference in longitudinal mode frequency between polarization modes can be measured from a beat signal of the polarization modes. Accordingly, it is possible to measure the physical quantity by measuring the variation in polarization mode beat frequency.
TL;DR: The role of third-order dispersion on the propagation, stability, and interactions of solitons in optical fiber transmission links and fiber lasers is analyzed.
Abstract: We analyze the role of third-order dispersion on the propagation, stability, and interactions of solitons in optical fiber transmission links and fiber lasers.
TL;DR: In this paper, the authors developed optical subsystem interconnections using long-wavelength laser diode arrays and single-mode fiber arrays, and demonstrated and channel 200-Mb/s/ch 100m transmission using compact (0.18 cc/ch/module), low power (total 280 mW/ch), fully integrated transmitter and receiver modules with an ECL (emitter coupled logic) interface.
Abstract: Synchronous parallel optical-fiber transmission is an attractive method for providing increased interconnection throughput and higher density in advanced information systems. Skew suppression (reducing transmission delay time variation), error-free fully DC-coupled data transmission capability, compactness, and low power consumption of modules are important requirements. In order to meet these requirements, we developed optical subsystem interconnections using long-wavelength laser diode arrays and single-mode fiber arrays. The major design criteria are discussed, especially as they are related to skew due to laser diode turn-on delay and receiver input optical power variation. The use of low-threshold-current laser diode arrays is an important design requirement. Based on the design criteria discussed in this work, we demonstrated and channel 200-Mb/s/ch 100-m transmission using compact (0.18 cc/ch/module), low power (total 280 mW/ch), fully integrated transmitter and receiver modules with an ECL (emitter coupled logic) interface. These modules include our new laser diode arrays with low threshold current of 3.2 mA. Performance results showed that these modules are very effective as interconnections between synchronously operating subsystems. >
TL;DR: In this article, an intracavity horizontally tapered active stripe was used to reduce the output beam divergence of a 1.48μm InGaAsP/InP strained quantum well laser.
Abstract: We propose and demonstrate the application of an intracavity horizontally tapered (in width) active stripe to adiabatically reduce the output beam divergence of a 1.48‐μm InGaAsP/InP strained quantum well laser. We achieve far‐field full width at half‐maximum divergences as low as 13° together with 100 mW emitted power into stable single transverse mode and 400 mA driving current. Maximum coupling efficiency to lensed single mode fiber of 73% is obtained.
TL;DR: In this paper, the authors proposed a novel approach to combating the pulse-broadening effect of group-velocity dispersion (GVD) in a fiber-optic communication link, where linear loss in the fiber is balanced by a chain of periodically spaced, phase sensitive optical amplifiers.
Abstract: Recently, we proposed a novel approach to combating the pulse-broadening effect of group-velocity dispersion (GVD) in a fiber-optic communication link (IEEE Photon. Technol. Lett., vol. 5, pp. 669-672, 1993). In our scheme, linear loss in the fiber is balanced by a chain of periodically spaced, phase-sensitive optical amplifiers. In this paper, we present a detailed analysis of pulse propagation in such a fiber line showing that, due to attenuation in the quadrature orthogonal to the amplified quadrature, it is possible for a pulse to propagate without significant broadening over lengths many times longer than the usual dispersion length of the fiber. An averaged equation for pulse evolution over distances longer than the amplifier spacing is derived. The effect of optical-phase fluctuations between the propagating pulse and the amplifiers is also considered. Since our proposed scheme does not rely on the formation of solitons in the fiber, it can be implemented in both the positive and negative GVD regions, and it does not require a minimum peak power for the pulses. >
TL;DR: In this paper, computer simulations are presented which yield lower dispersion penalties for mixed FM and AM compared to pure AM, and a laser modulated at a bit rate of 10 Gbit/s, with a peak to peak frequency chirp of 3 GHz and an extinction ratio of 4:1 does operate in the desired regime.
Abstract: The dispersion penalty increases with the spectral width of the modulated optical source. Pure AM leads to a two sided optical spectrum. Mixed FM and AM, produced by a directly modulated low-chirp laser diode, can lead to the partial suppression of one of those sidebands. Results of computer simulations are presented which yield lower dispersion penalties for mixed FM and AM compared to pure AM. A laser modulated at a bit rate of 10 Gbit/s, with a peak to peak frequency chirp of 3 GHz and an extinction ratio of 4:1 does operate in the desired regime. 180 km of fiber with a chromatic dispersion of 17 ps/km/spl middot/nm lead to 2-dB dispersion penalty at those operating conditions. >
TL;DR: In this article, all optical guided modes able to propagate in ideal metallic-dielectric structures having the symmetry of a step index fiber are classified and discussed, and the geometric and frequency dispersion of the effective index of the modes is discussed as a function of the optogeometrical parameters of the fibers.
Abstract: We classify and discuss all optical guided modes able to propagate in ideal metallic-dielectric structures having the symmetry of a step index fiber. The cases of a metallic core with a dielectric cladding and a dielectric core with a metallic cladding are considered. In each case, the intensity profile of the allowed guided modes is shown. The geometric and frequency dispersion of the effective index of the modes is discussed as a function of the optogeometrical parameters of the fibers. >
TL;DR: In this paper, a new type of fiber acousto-optic device based on tapered single-mode fiber was described, where the overlap between the acoustic and optical waves is complete, so very little acoustic power is required.
Abstract: A new type of fiber acousto-optic device, based on tapered single-mode fiber, is described. Unlike earlier designs, the overlap between the acoustic and optical waves is complete, so very little acoustic power is required. Maximum coupling in an experimental device was attained for an electrical drive power of just 0.5 mW. >
TL;DR: The generation of optical masks that permit the spatial shaping of a laser beam so that one can selectively excite single modes of a multimode parabolic graded-index optical fiber are reported on.
Abstract: We report on the generation of optical masks that permit the spatial shaping of a laser beam so that one can selectively excite single modes of a multimode parabolic graded-index optical fiber. The amplitude masks are determined by the analytical radial function of the fiber mode in the weakly guiding approximation. Phase reversal is encoded on the transparency through a shifted-grating technique. Effective injection and propagation of a single mode in a largely multimode fiber are shown as examples.
01 Jun 1994
TL;DR: In this article, a theoretical and experimental comparison of the coupling efficiency between a laser diode and a single-mode fiber on the end face of which a matching conical or hemispherical lens is formed is made.
Abstract: A theoretical and experimental comparison at 1.5 µm wavelength is made of the coupling efficiency between a laser diode and a single-mode fibre, on the end face of which a matching conical or hemispherical lens is formed. The same analysis applied to a hyperbolic microlens also gives good agreement with a recently published experimental result for this alternative matching lens.
TL;DR: In this paper, a new approach to study the transmission impairment of polarization mode dispersion in a digital optical fiber system, based on numerical simulation and statistical description of the problem, is discussed in detail.
Abstract: A new approach to studying the transmission impairment of polarization mode dispersion in a digital optical fiber system, based on numerical simulation and statistical description of the problem, is discussed in detail. A new figure of merit, the penalty probability, has been calculated and used to set up an overall criterion for system design considerations involving polarization mode dispersion in a single mode fiber. Simulation examples have shown that the design bound obtained holds well against experimental results. With practical system parameters and operation conditions known, the method can be used for effectively assessing potential system impairment and producing guidelines for achieving design compromises. >
TL;DR: In this paper, a near field measurement of the intensity profile in the transverse direction parallel to the waveguide surface was performed across one of the optical channel waveguides and similar transverse measurements of light propagating through a directional coupler were performed at many locations along the coupler.
Abstract: Near field microscopy is used to investigate the guided mode intensity distribution of optical channel waveguides and directional couplers with subwavelength spatial resolution. The directional coupler consisted of two single mode optical ridge channel waveguides formed with silicon nitride deposited on a lower cladding layer of SiO2 on a silicon substrate. A near field measurement of the guided mode intensity profile in the transverse direction parallel to the waveguide surface was performed across one of the optical channel waveguides. These variations are compared with model calculations. Similar transverse measurements of light propagating through a directional coupler were performed at many locations along the coupler, providing a view of the evolution of optical power transfer.
Patent•
01 Dec 1994
TL;DR: An intrusion-alarmed optical fiber communication system, where light from two or more sources are launched with a planar channel waveguide launcher into two different modes of a multimode graded-index transmission fiber, is disclosed in this article.
Abstract: An intrusion-alarmed optical fiber communication system, where light from two or more sources are launched with a planar channel waveguide launcher into two or more modes of a multimode graded-index transmission fiber, is disclosed. Input fibers containing the source light waves and the output fibers are in direct contact with the waveguide channels. Waveguide channels redistribute the light from input channels to output channels by rerouting, crossing, merging, or splitting channels. Waveguide launcher precisely launches data light into the fundamental mode and intrusion monitor light into high order modes of a multimode graded-index fiber. Fiber intrusion attenuates light in high order modes, but much less of the data in the fundamental mode, thereby forming a basis for the intrusion-alarmed system. Waveguide launcher permits light from a plurality of sources to be launched selectively into several types of fibers: singlemode fibers, multimode fibers, multicore fibers, multimode fibers having high refractive-index ring profile within the fiber core boundary. Waveguide channel configuration permit other versatile functions to be performed; these include optical time domain reflectometry, channel feedback stabilization of the diodes, launching light from more than one light source into the fundamental mode of a multimode graded-index fiber, and other functions. Means for launching and propagating the fundamental mode in multimode graded-index fiber in order to increase bandwidth capacity of said fiber are disclosed. Optical-electronic intrusion-alarmed systems with synchronous phase sensitive detection of intrusion are described, including reference recovery means. In order to decrease false alarm rates, information on transmitter light source variations are transmitted to the receiver via digital bit stream. A precise fiber optic sensor system based on the waveguide launcher is disclosed.
TL;DR: In this paper, a tapered fiber optical probe with an aperture of around 100 nm, served as the means for delivering pump radiation while simultaneously collecting the Stokes radiation from the diamond specimen.
Abstract: Raman spectroscopy with subwavelength spatial resolution of a diamond sample was recorded using a tapered fiber optical probe in conjunction with a conventional Raman spectrometer. The experiment demonstrates the potential of suboptical wavelength resolution analytical spectroscopy. The tapered fiber optical probe with an aperture of around 100 nm, served as the means for delivering pump radiation while simultaneously collecting the Stokes radiation from the diamond specimen. Comparing the magnitude of the Raman scattering measured with the submicron single mode fiber probe to similar signals obtained with a nontapered probe made of the same type of fiber, illustrates the potential increase in effective optical aperture resulting from the close approach of the fiber to the surface.