Single-mode Fiber Directional Couplers Research Articles

Analysis of Kerr type nonlinear single-mode triangular index fiber directional coupler by a simple method

We present the coupling characteristics of directional coupler comprising two identical single-mode triangular index fibers both in the presence and the absence of Kerr nonlinearity. Triangular index fibers of some typical V numbers are chosen for our investigation. Our analysis is based on the simple series expression for the fundamental modal field for triangular index fiber developed by Chebyshev technique. Analytical expression of the coupling length in the absence as well as in the presence of Kerr nonlinearity is prescribed. Using the said expression of coupling length, we apply a method of iteration to evaluate the said coupling parameter in the presence of Kerr nonlinearity. We also show that our results match excellently with the exact results obtainable by rigorous finite-element technique. The execution of our formalism requires little computation. Thus our simple but accurate formalism to study the triangular index directional coupler will prove a suitable alternative to the existing rigorous methods in the field of nonlinear optics. This user-friendly method will prove beneficial to the scientists and technologists working in the field of nonlinear optics.

High precision measurement technology for beat length of birefringence optical fiber

We propose a precision and flexible measurement method for the beat length of birefringence optical fiber (BOF) with a hybrid Sagnac interferometer formed with a single mode fiber directional coupler and BOFs. The measurement error equation was obtained based on error estimation theory and it was found that there is an optimum length for certain BOF under test. The measurement error can be less than 0.001 mm with 1 pm wavelength precision and 0.1 mm fiber length precision. With a high performance wavelength interrogator technology and a special designed fiber length measurement method, a beat length measurement setup was built and the measurement standard deviation of 3.08 × 10−4 mm was achieved with 250 measurement samples for solid core polarization maintaining photonic crystal fibers (PM-PCFs). The beat length stability of two PM-PCFs with different structure sizes has been measured and compared. The result demonstrated that the birefringence temperature characteristic of the PM-PCFs may be dependent on structure sizes.

Resonant Fiber Optic Gyroscope Using an Air-Core Fiber

Air-core photonic bandgap fibers (PBFs) have great potential for the resonant fiber-optic gyroscope (RFOG), because they reduce both the Kerr-induced drift and the thermal polarization instability. We experimentally studied an open-loop RFOG in which a 20-m resonant loop of 7-cell air-core fiber is closed by connecting the air-core fiber to a conventional single-mode fiber directional coupler. We measured a random walk of 0.055 °/s1/2 and a long-term drift with a standard deviation of 0.5° /s and a peak-to-peak variation of 2.5 °/s over 1 hour. We discuss the sources of error in this RFOG and identify areas for future improvement. We project that with straightforward improvements, tactical-grade performance should be possible.

Fused bitapered single-mode fiber directional coupler for core and cladding mode coupling

We demonstrate a new functional fiber component providing coupling between the LP/sub 1m/ cladding (LP/sub 1m//sup cl/) mode in one single-mode fiber (SMF) and the LP/sub 01/ core mode in another SMF. The phase-matching condition in the coupling region is achieved by prepulling the fiber sections. The experimentally demonstrated coupling efficiency between the LP/sub 01/ core and the LP/sub 13//sup cl/ modes is more than 60% over 70-nm wavelength range with less than 3% variation over 90/spl deg/C temperature range.

All-fiber single-mode fiber frequency-modulated continuous-wave Sagnac gyroscope

An all-fiber single-mode fiber unbalanced Sagnac gyroscope is described. The gyroscope is based on optical frequency-modulated continuous-wave interference and consists primarily of four Y-type single-mode fiber directional couplers and a single-mode fiber coil. The rotational velocity of the fiber coil is determined simultaneously by detection of the phase shift of the beat signal. The advantages and limitations of the gyroscope are discussed.

Flame-fused Optical Fiber Directional Couplers: Fabrication and Automated Process Control

A microprocessor controlled system for fabrication of 2 x 2 flame-fused biconical single-mode fiber directional couplers has been designed and realised. The system involves multiple motorized movements of mechanical stages, and an electronic hardware. The associated software developed for executing CPU operation enables presetting key fabrication control parameters like time of flame fusion, fiber pulling speed etc for minimising the excess loss and achieving various target specifications with regard to the coupling ratio. It has almost all the features of industrial fabrication. This automatic fabrication system has been successfully tested in our laboratory by drawing a number of couplers.

Optical fiber recirculating delay lines as controllable radio frequency notch filters

The authors analyze and demonstrate the use of optical fiber recirculating delay lines (RDLs) as electric filters at radio frequency (RF) frequencies. The electric filter is constructed from a single-mode fiber and a single-mode fiber directional coupler and is excited by an RF modulated coherent optical source. By taking advantage of a unique interaction between RF and optical effects within the RDL cavity, it is possible to control the RF filter response by varying the optical parameters of the RDL. A matrix-based formulation is used to analyze the optical and RF characteristics pc birefringent RDLs and their interactions. Several interesting features are noted. For example, there is one resonance in each free spectral range of the RDL if the optical field is not amplitude-modulated and the RDL is not birefringent. In the presence of amplitude modulation and fiber birefringence, however, the above-mentioned resonance splits into four subresonances in each free spectral range. The resonance conditions for these subresonances are controllable by adjusting the phase delay and/or the birefringence delay. Experimental confirmation of the results for RDLs with birefringent single-mode fiber excited by amplitude-modulated coherent fields is reported. The interplay of the optical and RF responses of the birefringent RDL has also been observed. The controllability of the coherent RDL RF filter is verified. It is demonstrated that the RF response of the system does indeed depend greatly upon the optical response of the RDL filter. Altering the optical path length and/or the birefringence delay alters the RF response of the filter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fiber optic beam combiner for multiple-telescope interferometry

We have constructed a five-telescope beam combiner composed of standard single-mode fiber optics and directional couplers. The device has good thermal and polarization stability, high throughput, and allows simultaneous measurement of all 10 possible interference pairs. It is connected to a small five-telescope interferometer, which is used to make synthetic aperture images.

Single-mode fiber-optic directional couplers

Abstract A new design of Y-type end-fused single-mode fiber directional couplers has been suggested and its optimum parameters to obtain losses of 0,5–1 dB have been determined. A technique has been developed for fabricating such couplers by electric arc fusion of single-mode fibers with thin claddings. Excess loss in these couplers splitting into two channels is 1—3 dB and the value is in good agreement with calculations. Cross-talk attenuation between reverse channels exceeds 60 dB, which is an indication of high directivity of the couplers. It has been found by experiments that the developed couplers maintain the state of polarization of the propagating radiation.

Mode excitation theory and experiment of single-mode fiber directional coupler with strong coupling

The mode excitation theory is presented to analyze single-mode optical fiber directional couplers with transverse strong coupling, which either the weakly coupling theory or the ray theory fails to expound. By using the equivalence between the waveguides with square and circular cross sections, regarding the coupling zone of the coupler as a single waveguide, the excitation coefficients of the propagation modes in the coupling zone excited by the modes in the input waveguide have been evaluated. The theoretical coupling amount and insertion loss have been studied. Using a special monitoring technique to control the coupling length, several couplers with various coupling amounts have been made by the polishing method. The spectral characteristic of the coupling amount has been measured and compared with the theoretically inferred result. The two are found to agree with each other. The offset effect on the coupling amount has also been measured and it is different from that of the weak coupling device-the Stanford coupler.

Wavelength-selective directional coupler made of nonidentical single-mode fibers

Low-loss asymmetrical single-mode fiber directional couplers applicable to WDM between transmission channels at 1300 and 1500 nm have been constructed. The crossover wavelength of these couplers can be tuned by a suitable intermediate layer. Theoretical calculations show that fiber multiplexers with channel spacings less than 20 nm Can be fabricated.

Analysis of tunable single-mode fiber directional couplers using simple and accurate relations

We present simple and accurate relations for the study of parallel and curved tunable single-mode fiber directional couplers employing step-index or graded-index fibers. Analytical results for the various coupler parameters such as coupling coefficient, effective interaction length, tunability, and channel wavelength separation in directional coupler wavelength filters are presented. These results must prove very useful for an easy, quick, and accurate prediction or design of tunable single-mode fiber directional couplers.

Accurate analysis of single-mode graded-index fiber directional couplers

We present an accurate coupled-mode analysis to evaluate the coupling characteristics of identical single-mode graded-index fiber directional couplers. We also give an empirical relation for calculating the coupling length of a fiber coupler formed by fibers having profile parameter q , normalized frequency V , and normalized separation d/a ; d is the distance between the fiber centers and a , the radius of the fiber core.

An all single-mode fiber Michelson interferometer sensor

A Michelson interferometer made from a single-mode fiber directional coupler with silver mirrors deposited on fiber ends is presented. This interferometer is simple to fabricate and has a theoretical visibility of unity, irrespective of the splitting ratio of the coupler. A theoretical analysis of its performance is described. Locking to quadrature was achieved by tuning the frequency of the laser source at 1.52μm. Results are presented for the interferometer operating over one kilometer of single-mode fiber, in a passive remote sensing configuration.

Single-mode fiber directional couplers fabricated by twist-etching techniques (stabilization)

Rugged single-mode fiber couplers have been fabricated by two different techniques: 1) twist-etch RTV potting, and 2) twist-etch gel glass potting. The performances, in terms of optical stability, mechanical ruggedness, and lifetime, are compared between the couplers made by these two techniques, and also with those of the original twist-etched (bottle) coupler reported earlier.