A novel temperature-independent multi-mode fiber (MMF) lateral strain sensor based on a core-offset interferometer is
presented and demonstrated experimentally. Slightly misaligning a splice between an MMF and a single-mode fiber
(SMF), high extinction ratio of the interferometer based on SMF-MMF-SMF structure can be obtained. When the lateral
strain is applied to a short section of the MMF, the extinction ratio of the interferometer will decrease accordingly while
the interference phase remains almost constant. Temperature variation only leads to shift in the transmission power
spectrum of the interferometer and does not affect the extinction ratio. Experimental results show that there is a good
quadratic relationship between the lateral strain and the extinction ratio. The proposed strain sensor has the advantages of
temperature-independency, high extinction ratio sensitivity, good repeatability, low cost, and simplicity in structure.
A novel lateral force sensor based on a core-offset multimode fiber interferometer with intensity-based interrogation
technique is reported. An offset between the cores of the single-mode fiber and multimode fiber is made to produce high
extinction ratio. When a lateral force is applied to a short section of the multimode fiber, the extinction ratio decreases
with the interference phase almost unchanged. In addition to serving as a sensing head, the multimode fiber can also act
as a filter to realize lateral force measurement by determining the power change from a power meter. Experimental
results show that the power ratio change has a linear relationship with respect to the applied lateral force, and the
resolution of the sensor configuration is about 0.01 N.
We propose a novel efficient dispersion compensating filter that is relatively simple and much less expensive to implement. It is a multicavity Gires-Tournois etalon based all-pass filter and the length of each of the cavities is slightly mismatched. We present a theoretical study of the filter for chromatic dispersion compensation. Our simulation results show that a linear group delay response can be obtained by properly choosing the mismatched cavity length and the chromatic dispersion value can be controlled by suitably adjusting the reflection coefficients of the mirrors. For applications in fiber disperison compensation, the disperson slope can be achieved by slightly modulating the reflection coefficients of the reflectors relative to wavelength.
A periodic optical filter based on a Michelson interferometer with a three-mirror Gire-Tournois resonator in one of the two arms is analyzed. We derive the general condition for the occurrence of flattop spectra. The results show that flattop spectra can be obtained by suitably choosing the reflective coefficients of individual mirrors.
Erbium-doped fiber (EDF) lasers with active coupling cavities have recently shown to enable the production of multiwavelength oscillations with various novel functions. This paper reviews the advances of such lasers. The lasers may be designed to produce multiwavelength oscillations in both C and L bands, either as a combined band or as two separate bands. In the C band, the laser can operate in a multiwavelength regime with controllable output powers and tunable wavelengths for any one individual lasing line. Wavelength-switching is also possible; switching responses of about 100 Hz are typical. In the L band, the laser exhibits multiwavelength bistable phenomena. The bistable hysterisis behaviors for a dual-wavelength laser evolve in antiphase with respect to each other. The width of the bistable region can be controlled; it could be designed to be as wide as a few hundred milliwatts. The two wavelengths can be switched from one to the other, with typical switching times of 4 to 15 ms. By suitably controlling the bistability, simultaneous multiwavelength oscillations in the L band alone, or in combination with the C band, can be obtained. A new wavelength-tuning and switching mechanism can be achieved by using such bistabilities. With this scheme, widely tunable and switchable lasers in fiber ring and linear configurations have been constructed with two commercially available tunable ordinary fiber Bragg gratings (FBGs). The lasers can be tuned over the whole L-band although the tunable range of each of the FBGs is only about 15 nm.
Optical bandpass filters with ripple-free spectral response are highly desirable for dense wavelength-division-multiplexed (DWDM) systems. We study and analyze the transmission characteristics of an optical bandpass filter, based on a three-mirror Gires-Tournois resonator (GTR) in a Michelson interferometer (MI). The three-mirror GTR (R<sub>1</sub>, R<sub>2</sub>, R<sub>3</sub>) is actually an all-pass filter with the reflectivity of the end mirror R<sub>3</sub> being unity. In this paper, we present an analytical expression for the optimum design which has a ripple-free spectral response. The results show that flattop spectra can be obtained by suitably choosing the reflectivities R<sub>1</sub>, R<sub>2</sub> of the two mirrors. Whereas a two-mirror GTR-based bandpass filter has only one unique optimum reflectivity, a three-mirror GTR-based filter has many sets of optimum values, making it easier to be designed to give much better performance. The effects of the reflective coefficients of the mirrors on the optical performance were also discussed.
A new compound Bragg grating structure with periodic nonlinearity and gain is presented. It works as a DFB laser structure that could be switched between two resonance frequencies. A mathematical model has been developed and used to investigate the structure. The laser can be switched between two resonance frequencies by a triggering optical pulse. A numerical simulation of the switching dynamics in the time domain is described. At one of the frequencies, the structure is found to have a self-pulsing behavior.
Optical bistability in an L-band dual-wavelength erbium-doped fiber laser with linear overlapping cavities is investigated. The hysterisis bistabilities of the two lasing wavelengths evolve in antiphase. It has a large bistable region which may be controlled and adjusted by the EDF length and the cavity losses. Simultaneous dual-wavelength oscillations in the L-band are also achieved.
An erbium-doped fiber ring laser, with a single- longitudinal-mode, operating at 1548.5 nm, has achieved traveling wave operation without the use of an optical isolator. The unidirectional operation is guaranteed by a combination of an output coupler and an external fiber Bragg grating. The laser can provide up to 19.6 mW output power. The linewidth of the laser was measured to be < 24 kHz.
Linear fiber lasers can have single-mode operations especially if their cavities are short. The smallest length achieved is 100 micrometers . Such lasers have low thresholds, slope efficiencies and output powers. Ring lasers have much higher slope efficiencies and output powers, and narrow linewidths (a few kHz), but suffer from instabilities such as mode-hoppings. If double- ring resonators are used, the mode-hoppings can be eliminated without associated frequency shifts by dithering the two ring cavities together with a feedback mechanism. Double-clad fibers can be effectively used to increase the coupling of pump light into the core, while the absorption of the pump light can be greatly increased by codoping an Er<SUP>3+</SUP>-doped fiber with ytterbium.
The temperature sensitivities of three common types of commercially available high- birefringent PM fibers (bow-tie, PANDA and elliptical core fiber) have been measured by a dynamic polarimetric method. The phase retardation of two eigen polarizations in these fibers have been obtained and compared, with the temperature changing from -10 °C to 100 °C. An oil bath was used to heat the fibers and dry ice to produce low temperatures. The results show that within the above temperature range, the phase retardation of these three fibers increase almost linearly with temperature. The temperature sensitivities of the bow-tie, PANDA and elliptical core fibers are 7.35, 7.57 and 1.1 rad/m °C, respectively.
This paper shows a method to minimize the temperature sensitivity by coating an elliptical core fiber with an additional thin elliptical cladding. We analyze the temperature and the strain sensitivities of the fiber and discuss the design of such temperature-insensitive fibers with high strain sensitivities, suitable for smart structures and skin sensors.