Fiber Sagnac Loop Research Articles

Single and triple output coherence‐free microwave photonic filter using a 3 × 3 collinear coupler in a Sagnac fiber loop

AbstractNew structures that can realize single and triple output coherence‐free microwave photonic filter are presented. They are based on a Sagnac loop interferometer containing a single drive intensity modulator and a 3 × 3 collinear coupler, and no non‐reciprocal bias unit is required. Notch responses are obtained by modulating the clockwise and counterclockwise propagating waves inside the Sagnac loop at different times. Theoretical analysis is obtained for both DFB laser source and broad‐band source. Measured results verify the theoretical expressions and demonstrate robust notch filter responses. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 744–747, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23218

Microwave photonic notch filter with an electro-absorption modulator in a Sagnac fiber loop

Coherence-free microwave photonic filter configurations are presented. The configurations are based on a Sagnac loop interferometer containing an electro-absorption modulator (EAM) without a nonreciprocal bias unit. Notch responses are obtained by modulating the clockwise and counterclockwise propagating waves inside the Sagnac loop at different times. Measured results verify the theoretical expressions and demonstrate a robust notch filter response extending to high frequency without reducing notch depth.

Wavelength-tunable picosecond pulses from a passively mode-locked figure-eight Erbium-doped fiber laser with a Sagnac fiber filter

We experimentally demonstrated a wavelength-tunable passively mode-locked Erbium-doped figure-eight fiber laser. Wide tunability is achieved using a high-birefringence (hi-bi) fiber Sagnac loop. The filter transmission pass wavelength is controlled by thermally adjusting the hi-bi fiber loop length. The output pulses are wavelength tunable over a range from 1525 nm to 1555 nm. The FWHM of the autocorrelation trace is about 3.1 ps and the pulse spectrum has a FWHM of 1.5 nm. The pulse temporal and spectral widths remain constant over the tunable range.

一种用于应力传感的基于光纤Bragg光栅及高双折射Sagnac环镜的新型光纤激光器

A novel fiber-laser-based strain sensor is proposed and experimentally demonstrated. The laser cavity is composed of a high-birefringence Sagnac fiber loop mirror (HiBi-SFLM) and a fiber Bragg grating (FBG) which also acts as a strain-sensing element. In the linear region of the HiBi-SFLM reflection spectrum, when the strain applied on the FBG makes the Bragg grating wavelength shift, the laser output power changes due to reflectivity variation of the HiBi-SFLM. Experimental results show that the laser output power varies almost linearly with the applied strain. The measurement of the output power can be performed by a conventional photo-detector.

Experimental schemes for developing fiber-based source of entangled photon pairs

Based on the theory of third-order nonlinear Kerr effect in optical fiber, we experimentally demonstrate the generation of photon pairs by exploiting spontaneous four-wave mixing. The photon pairs are produced by pumping a fiber Sagnac loop and a piece of straight fiber with a pulsed laser, respectively. By comparing the experimental results, we analyze how to optimize the experimental parameters to obtain entangled photon pairs with high purity. Our results are useful for developing the all-fiber source of photon pairs and single photon source, which are well suited for quantum communication.

Fast-interleaving of dual-wavelength fiber ring laser using switchable fiber Bragg gratings

We propose a novel lasing wavelength-switchable fiber ring laser based on the gain competition between two nearby band-pass wavelengths in a homogeneously broadened erbium-doped fiber gain. The cascaded two band-pass filters are demonstrated with fiber Sagnac loop interferometers including fiber Bragg gratings and polarization controllers. The fast electro-optic interleaving operation of dual-wavelength lasing outputs can be achieved by modulating the polarization state of one band-pass filter.

Coherence-free microwave photonic notch filter with a single driver intensity modulator in a Sagnac fiber loop

A coherence-free microwave photonic filter configuration is presented. The configuration is based on a Sagnac loop interferometer containing a single-drive intensity modulator without a nonreciprocal bias unit. A notch response is obtained by modulating the clockwise and counterclockwise propagating waves inside the Sagnac loop at different times. A general theoretical analysis for both the reflected signal and the transmitted signal is obtained. Measured results verify the theoretical expressions and demonstrate a robust notch filter response.

Multifrequency erbium-doped fiber laser operating at room temperature

A method to realize room-temperature operation of a multifrequency Er-doped fiber laser with low-frequency shift feedback placed within a linear laser cavity is theoretically proposed and experimentally demonstrated. Simultaneous multiwavelength lasing with 0.5 nm wavelength spacing is experimentally demonstrated by applying a sinusoidal signal of 10 kHz to a fiber phase modulator inserted within the linear cavity to prevent single wavelength steady-state oscillation. In the linear cavity, an all-polarization-maintaining fiber Sagnac loop is used as a periodic filter, and a single-mode fiber loop with a polarization controller is used as a partial reflector and also as an output port.

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 and injection‐switchable erbium‐doped fiber laser of line structure

AbstractA tunable and injection‐switchable erbium‐doped fiber (EDF) laser is proposed based on a line structure formed by a fiber Sagnac loop reflector and an fiber Bragg grating (FBG). Wavelength switching is achieved by controlling the power of the tunable injection laser. The self‐seeded wavelength corresponding to the Bragg wavelength of the FBG can be tuned by, for example, heating the FBG, and the injection wavelength can be tuned over a wide range of about 50 nm. The characteristics of the wavelength switching for different levels of the EDF pump power and different wavelengths of the injection laser are studied experimentally. The present fiber laser has the advantages of tunability, stability, low amplified spontaneous emission noise, and high injection efficiency when compared with a fiber ring laser. Rapid wavelength switching is expected and the transient switching response of the laser is also studied. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 765–768, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22262

A fiber sensor for measuring gas concentration based on laser’s transient regime

A novel fiber sensor for measuring gas concentration is reported, which is based on the transient regime of a Er-doped fiber laser. A Sagnac fiber loop and a FBG that also acts as the sensing element form the linear fiber laser cavity. By adjusting the central wavelength of the grating mirror, the oscillating wavelength is set to 1531.56 nm, which is a strong absorption peakof C2H2. The gas concentration can be obtained by measuring the laser build-up time, which is closely related with the cavity loss and therefore determined by the gas concentration. In the present experiment, the sensor's sensitivity and resolution can be adjusted simply by controlling the pumping level. When the pump pulse is at high level and low level of 25 mW and 5.9 mW, respectively, a sensitivity of 100 ppm/μs and resolution of 20 ppm are achieved.

Individual switching of multi-wavelength lasing outputs based on switchable FBG filters

We demonstrate an individually-switchable multi-wavelength fiber ring laser using semiconductor optical amplifier with novel switchable band-pass filters. The switchable band-pass filter is based on a fiber Sagnac loop interferometer incorporating both fiber Bragg gratings and polarization controller. The proposed multi-wavelength fiber ring laser shows a high output performance of independent switching at arbitrary lasing wavelength and high extinction ratio for wavelength-division multiplexing source.

A all-optical switching based on highly nolinear photonic crystal fiber Sagnac loop mirror

We designed a novel and low switching-power all-optical fiber switch. The highly nonlinear photonic crystal fiber and the bi-directional pumped Er-doped fiber amplifier are inserted into the Sagnac loop mirror simultaneously. The switching is realized based on cross-phase modulation because the symmetry of the loop is broken. The theoretical analysis shows that the switching-power is inverse proportional to the product of the amplifier gain and the nonlinear coefficient of the photonic crystal fiber. In our experiment, 40 mW switching-power and 15.9 dB switching-extinction are obtained; furthermore, the transmission of signal light is a cosine function of the peak power of the control pulse. All the results of experiment well agree with the theory.

Measurement of SOA Linewidth Enhancement Factor With a Sagnac Fiber Loop

We demonstrate a novel method to measure the linewidth enhancement factor (alpha-factor) of the semiconductor optical amplifier (SOA). We derived theoretically the quantitative relationships among the alpha-factor, the cross-gain modulation, the cross-phase modulation, and the contrast ratio of an SOA-based Sagnac fiber loop. We found that the alpha-factor value can be calculated directly from the maximum power contrast ratio measurement. Our experiment results show that the obtained alpha-factor fluctuates within a small range of 5.23 to 6.83 when the bias current varies from 130 to 240 mA. Compared with existing measurement methods, our method is more attractive because of its simple configuration and better stability

High-fidelity transmission of quantum polarization states through birefringent optical fibers

High fidelity transmission of quantum polarization states is essential for realizing quantum networks. The degenerate ${\ensuremath{\Psi}}^{\ensuremath{-}}$ state is an eigenstate of a transmission fiber. In this paper we propose a method for preparing the degenerate ${\ensuremath{\Psi}}^{\ensuremath{-}}$ state using spontaneous four wave mixing (SFWM) in a fiber Sagnac loop excited by two orthogonally polarized pump waves. This configuration includes a 45\ifmmode^\circ\else\textdegree\fi{} Faraday rotator, which creates a relative phase of $\ensuremath{\pi}$ between the two counterpropagating entangled photon pairs with indistinguishability both in frequencies and generation paths. We confirm that only the amplitude is degraded in the output wave form of nonlinear sum frequency generation in the Bell state analyzer after propagation through the fiber due to the polarization mode dispersion.

Measuring chromatic dispersion of optical fiber using time-of-flight and a tunable multi-wavelength semiconductor fiber laser

We have developed a tunable multi-wavelength semiconductor fiber laser (SFL) for chromatic dispersion measurements of optical fiber based on the time-of flight (TOF) or pulse-delay technique. The SFL incorporates a programmable high-birefringence fiber Sagnac loop to select the separation of the lasing wavelengths between 3.2 and 1.6 nm. The SFL emits 5 and 11 wavelengths for separations of 3.2 and 1.6 nm, respectively, all within the C-band and with an output power uniformity within 3.2 dB. Results from TOF measurements are compared with standard phase-shift techniques; the percent differences between the two methods are within ±1.34% for measurements on various lengths of standard single mode fiber.

Temperature stability improvement of a multiwavelength Sagnac loop fiber laser using a high-birefringent photonic crystal fiber as a birefringent component

We experimentally demonstrate a temperature-stable multiwavelength erbium-doped fiber laser source using a high-birefringent photonic crystal fiber (HiBi-PCF) as the birefringent component of the Sagnac loop filter within the laser cavity. Three different high-birefringence (Hi-Bi) fibers are used in the loop filter to compare the temperature stability of the fiber laser systems: polarization-maintaining erbium-doped fiber (PM-EDF), panda Hi-Bi fiber, and HiBi-PCF. Because of the high birefringence and low temperature sensitivity of the HiBi-PCF, fiber length in the loop is greatly reduced and the temperature stability of the system is dramatically enhanced.

A novel fiber strain sensor based on laser’s transient regime

A novel fiber strain sensor based on the fiber laser's transient regime is reported. A Sagnac fiber loop and a fiber Bragg grating(FBG) that also acts as the sensing element form the linear Fabry-Perot cavity, in which a long-period grating(LPG) is inserted. The original peak wavelength of the FBG, whose bandwidth is 0.23nm, is 1557.98nm and located on the left side of the LPG dip (1557.4nm). The light reflected from the Sagnac loop and the FBG is transmitted through the LPG twice in each round trip, and the transmission loss increases with the strain-induced wavelength shift of the FBG, which in turn modifies the laser build-up time. The strain value can be obtained by measuring the build-up time of the laser. In the present experiment, the sensor's sensitivity and resolution can be adjusted by simply controlling the pumping level. When the pump pulse's high level and low level are 32 and 6mW, respectively, a sensitivity of 7×10-6ε/μs and resolution of 7×10-7ε are achieved.

Multi-point interrogation of FBG sensors using cascaded flexible wavelength-division Sagnac loop filters

A simple optical signal demodulation technique is demonstrated for multi-point interrogation of fiber Bragg grating sensors. The wavelength-division responses of polarization-maintaining fiber Sagnac loop filters are flexibly controlled to interrogate the simultaneous optical intensity changes from the independent wavelength shifts of multiple fiber Bragg grating sensors. Experimental performance is also demonstrated to show the simultaneous detection of the separated FBG sensing signals by individual photo detectors.

Sagnac-loop-based equivalent negative tap photonic notch filter

A new structure that can realize an all-optical equivalent two-tap negative tap photonic notch filter response is presented. It is based on a Sagnac fiber loop interferometer containing an optical phase modulator. An equivalent two-tap negative tap notch filter response is obtained by phase modulating the clockwise and counterclockwise propagating optical waves inside the Sagnac loop at different times. Results demonstrate a robust notch filter response using a narrow-linewidth optical source with no phase-induced intensity noise and no second-order harmonic component present at the output.