Parametric amplification in a microstructure fiber

We report an observation of parametric amplification with a gain greater than 30 dB in a 1.7-m microstructure fiber. It is found that there exists a peak pump power P_s. For peak pump powers less than P_s, the amplified twin (signal: longer wavelength, and idler: shorter wavelength) pulses are highly correlated, and the bandwidth of the amplified signal pulse broadens monotonically with pump power. For peak pump powers exceeding P_s, the correlation between the amplified twin pulses drops when the pump power increases, the bandwidth of the amplified signal pulse becomes narrower when the pump power increases, and the central wavelength of the amplified signal (idler) pulse shifts towards longer (shorter) wavelength, respectively, due to phase matching. PACS 42.65.Yj     

Stable wavelength tunable fiber ring laser for true-time delay system

In this paper, we propose and experimentally demonstrate a novel wavelength tunable fiber ring laser source for a photonic beamforming system to control phased-array antenna. In this fabrication, a Sagnac loop composed of a polarization-maintaining (PM) coupler and a piece of high-birefringence (Hi-Bi) PM fiber is acted as a comb filter to make the frequency spacing equal. The wavelength of the output signal is controlled by the tunable filter outside the Sagnac loop. The intensities of the output signals with different wavelengths increased or decreased at the same frequency spacing are equal. A five-channel true-time delay system consisting of this tunable fiber source and five grating delay lines for discrete beamsteering has been demonstrated. In the experiment, the output signals of the tunable fiber ring with the equal frequency spacing have the same intensity of about 5.9 dBm and the same high signal to noise ratio (SNR) of 40 dB. If the tunable filter in this laser is replaced by a micro-electro-mechanical systems (MEMS) tunable filter, the speed of switching wavelength will increase rapidly.     

Tunable single-longitudinal-mode Ytterbium all fiber laser with saturable-absorber-based auto-tracking filter

A broadband tunable, single-longitudinal-mode (SLM) Ytterbium fiber laser with unpumped Ytterbium-doped Sagnac loop is proposed and demonstrated experimentally. The unpumped Ytterbium-doped Sagnac loop is employed as a saturable absorber based auto-tracking filter to ensure single-longitudinal-mode oscillation. And a tunable band pass optical filter with large tuning range is applied to achieve broadband tuning ability. With 1-m Ytterbium-doped fiber as the gain medium, the SLM operation is achieved with over 60-nm wavelength tuning range at 160-mW pump power. The laser is very stable with output power of about 3 dBm and optical signal to noise ratio of higher than 50 dB in all the 60-nm tuning range.     

Broad spectral sliced multiwavelength source with a mode locked fiber laser

A simple multi-wavelength source is demonstrated based on spectral slicing of supercontinuum (SC) light using a Sagnac loop mirror. A mode locked fiber laser with pulse width of 800 fs is employed to generate the SC in photonic crystal fiber (PCF). The SC light extending from 1200 nm up to the wavelength region above 1750 nm is obtained at the amplified pulse pump power of 30 dBm. By slicing the SC with a loop mirror, a multiwavelength comb spectrum is obtained with different channel spacing for different operating wavelength region. Spacings of 1.5, 2.3, and 2.9 nm are shown at wavelength regions of 1220, 1450, and 1730 nm, respectively. It is also observed that the signal to noise ratio (SNR) increases from 15 to 20 dB as the operating wavelength increases from 1220 to 1730 nm region.     

Multi-wavelength fiber source with equal frequency spacing

A optical filter based on Sagnac interferometer was proposed to be acted as a comb filter with equal frequency spacing and good signal to noise ratio (SNR), which was composed of an 8.14 m stress-induced Hi-Bi (high-birefringence) PM (polarization-maintaining) fiber. Using this multi-wavelength Sagnac comb filter and a gain flattening Sagnac filter that made the output spectra flattening at different pump powers, a 25-channel multi-wavelength all-fiber source were successfully generated with channel spacing of 0.8 nm with respect to the center wavelength at 1550 nm and flattened gain about ±1 dB peak deviation. The channel spacing can be further reduced to 0.4 nm to produce a DWDM (dense wavelength division multiplexing) source, simply by increasing the Hi-Bi fiber to be 16.28 m. It can be used in many applications such as WDM (wavelength division multiplexing), optical amplifiers with a high SNR, narrow band filters and optical sensors.     

Single-stage gain-clamped L-band EDFA with C-band ASE saturating tone

We demonstrate a single-stage gain-clamped L-band Erbium-doped fiber amplifier with 1480 nm pump wavelength. The gain-clamping technique is achieved by utilizing the backward propagation of C-band amplified spontaneous emission (ASE). This unwanted noise is reflected back into the optical amplifier and its intensity is adjusted using the variable optical attenuator. The C-band ASE sets the population inversion level along the Erbium doped-fiber and limits the L-band signal amplification to a specific value. The whole optical bandwidth in L-band can be employed for signal amplification since the saturating tone is out of the band. The gain dynamic range of 11.7 dB is obtained between 21.7 and 10.0 dB with noise figure of less than 5.5 dB for signal power up to 2 dBm.     

Tunable fiber-optic parametric oscillator

We report operation of a tunable optical parametric oscillator that employs a nonlinear-fiber Sagnac interferometer as a parametric amplifier. The amplifier, which consists primarily of dispersion-shifted fiber that has zero dispersion at 1538 nm, is synchronously pumped with 7.7-ps pulses at 1539 nm. The wide bandwidth of the parametric gain permits tuning of the output signal pulses over a 40-nm range centered on the pump wavelength. The Sagnac interferometer decouples the pump wave from the oscillator cavity while a bandpass filter in the cavity transmits only the signal wave, thereby creating a singly resonant parametric oscillator that is phase insensitive. Whereas we demonstrate tuning over almost the entire bandwidth of Er-doped-fiber amplifiers, one could construct a similar device that operates near the 1310-nm zero-dispersion wavelength of standard telecommunication fiber.     

Dual-wavelength switchable erbium-doped fiber ring laser based on merged sagnac and intermodal interferences in sagnac loop mirror

A simple but novel concept of switchable dual-wavelength erbium-doped fiber ring laser is proposed and experimentally demonstrated. It is based on a Sagnac loop mirror incorporating a high birefringence few-mode fiber and polarization controller (PC). Since its transmission properties depend on the PC state, the proposed fiber ring laser can operate in the random combination of two wavelengths by adjusting only the PC settings. The potential lasing lines are determined by simultaneous occurrence of the Sagnac and intermodal interferences in the Sagnac loop mirror. All lasing emissions have the optical signal to noise extinction ratio higher than 45 dB and their power fluctuations lower than 0.5 dB.     

Pump-noise transfer in dual-pump fiber-optic parametric amplifiers: walk-off effects

Transfer of intensity noise from pumps to signal in dual-pump fiber-optic parametric amplifiers is simulated numerically for a realistic configuration in which both pumps are amplified and filtered before they enter the fiber. The walk-off effects induced by different group velocities of pumps, signal, and idler are fully taken into account. It is found that the optical signal-to-noise ratio can be as low as 15 dB when the amplifier length is close to 0.5 km, but it can be improved by 3 dB or so by use of longer fibers.     

Double-pumped multiwavelength fiber optical parametric oscillator based on a Sagnac loop filter

We propose a double-pumped ring cavity multiwavelength fiber optical parametric oscillator (MW-FOPO) using a highly nonlinear dispersion-shifted fiber (HNL-DSF) as the gain medium and a polarization maintained fiber based Sagnac loop filter as the comblike filter. 22-wavelength lasing of the double-pumped MW-FOPO with a ripple less than ±2.5 dB and a wavelength spacing of about 0.8 nm in a wavelength range from 1541 nm to 1558 nm is experimentally demonstrated. We discussed the power stability of the multiwavelength lasing of the double-pumped MW-FOPO. A comparison of the output spectra between the double-pumped MW-FOPO and single-pumped MW-FOPO is also presented.     

Double-pass ytterbium-doped fiber amplifier with high gain coefficient and low noise figure

We have proposed and demonstrated a double-pass ytterbium-doped fiber amplifier using an optical circulator and a fiber Bragg grating as reflector. When the signal has passed through the ytterbium-doped fiber once, it reflects off a 0.2-nm passive fiber Bragg grating filter. This reduces amplified spontaneous emission (ASE) noise from the first pass. The input signal light is amplified both forward and backward through ytterbium-doped fiber. With this double-pass configuration, 1053.15-nm unsaturated signal gain of 28 dB, gain coefficient of 1.1 dB/mW, and noise figure of less than 4 dB are achieved at 977-nm pump power of 68 mW. It is also found that this double-pass configure provides enhancing gain coefficient and improving noise figure by comparison with single-pass configuration.     

Discrimination of strain and temperature based on a polarization-maintaining photonic crystal fiber incorporating an erbium-doped fiber

A simple sensing method for simultaneous measurement of temperature and strain is investigated by using a Sagnac fiber loop mirror composed of a polarization-maintaining photonic crystal fiber (PM-PCF) incorporating an erbium-doped fiber (EDF). Amplified spontaneous emission created by a pumped EDF is transmitted to a Sagnac fiber loop mirror. The interference between two counter-propagating signals in a Sagnac fiber loop mirror generates a periodic transmission spectrum with respect to wavelength. When external temperature is increased, the transmission peak power reduces because the amplified spontaneous emission of the EDF is decreased by the applied temperature change (0.04 dB/°C). The peak wavelength is shifted into the shorter wavelength because of the negative temperature dependence of the birefringence of the PM-PCF (0.3 pm/°C). As the applied strain increases, the peak wavelength of the transmission spectrum of the Sagnac loop mirror incorporating the EDF shifts into a longer wavelength (1.3 pm/με) because the phase change of the proposed sensing probe is directly proportional to the applied strain. The transmission peak power, however, is not changed by the applied strain. Since the source and the sensing probe are integrated, the overall system configuration is significantly simplified without requiring any additional broadband light source. Therefore, it is possible to simultaneously measure temperature and strain by monitoring the variation of transmission peak power and peak wavelength, respectively.     

Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band

We present a fiber-based source of polarization-entangled photons that is well suited for quantum communication applications in the 1550 nm band of standard fiber-optic telecommunications. Polarization entanglement is created by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed orthogonally polarized pump pulses and subsequently removing the time distinguishability by passing the parametrically scattered signal and idler photon pairs through a piece of birefringent fiber. Coincidence detection of the signal and idler photons yields biphoton interference with visibility greater than 90%, while no interference is observed in direct detection of either signal or idler photons. All four Bell states can be prepared with our setup and we demonstrate violations of the Clauser-Horne-Shimony-Holt form of Bell's inequality by up to 10 standard deviations of measurement uncertainty.     

Dynamic-performance characterization of C-band EDFA using ASE-power peak-selective feedback gain-clamping

We propose and demonstrate a C-band Erbium-doped fiber amplifier (EDFA) using amplified spontaneous emission (ASE) power peaking-selective and feedback to achieve highly stabilized and wide dynamic range gain clamping performances. The gain of 16.20 ± 0.13 dB is obtained with the input signal power dynamic range of 30 dB and the maximum noise figure (NF) was 6.6 dB with the input signal power from −35 to −10 dBm. To investigate the gain variation for the probe signals, we applied a saturation tone signal to simulate 16-channel DWDM signals as it is added (dropped) into (from) EDFA. The gain variation of the proposed scheme at a appropriate ring-cavity loss (VOA = 5 dB) is less than 0.13 dB for the input signal power from −35 to −5 dBm and wavelength from 1530 to 1564 nm.     

基于长周期光纤光栅嵌入型Sagnac环光谱的折射率测量

将长周期光纤光栅(LPG)和光纤Sagnac环相结合,实现了折射率和温度的同时测量.首先利用二氧化碳激光器在保偏光纤上制作了长周期光纤光栅(PM-LPG),然后把该PM-LPG和普通单模光纤耦合器组成Sagnac环,作为传感单元.实验选择其某一透射峰作为测试对象,其波长随温度变化,强度随折射率变化,因此可实现两个参量的...     

Stable multiwavelength fiber ring laser with equalized power spectrum based on a semiconductor optical amplifier

We experimentally demonstrated a new structure of a multiwavelength semiconductor optical amplifier (SOA) ring laser based on a fiber Sagnac loop filter that can generate up to 25 stable output lasing wavelengths at room temperature. By varying the length of a polarization-maintaining (PM) fiber within the Sagnac loop filter, the wavelength spacing between the output lasing wavelengths can be changed to a desired value. By tuning a polarization controller (PC) within the Sagnac loop filter, stable multiwavelength 1550-nm operation with up to 17 lasing lines within 3 dB power level variation and with a wavelength spacing of ∼0.8 nm was achieved. The optical signal-to-noise ratios (OSNRs) of all the lasing wavelengths are greater than 40 dB.     

High gain L-band erbium-doped fiber amplifier with two-stage double-pass configuration

An experiment on gain enhancement in the long wavelength band erbium-doped fiber amplifier (L-band EDFA) is demonstrated using dual forward pumping scheme in double-pass system. Compared to a single-stage single-pass scheme, the small signal gain for 1580 nm signal can be improved by 13.5 dB. However, a noise figure penalty of 2.9 dB was obtained due to the backward C-band ASE from second stage and the already amplified signal from the first pass that extracting energy from the forward C-band ASE. The maximum gain improvement of 13.7 dB was obtained at a signal wavelength of 1588 nm while signal and total pump powers were fixed at -30 dBm and 92 mW, respectively.     

Amplitude squeezing of light by means of a phase-sensitive fiber parametric amplifier

We experimentally demonstrate generation of bright sub-Poissonian light by means of parametric deamplification in a phase-sensitive fiber amplifier that is based on a balanced nonlinear Sagnac interferometer. On direct detection, the photocurrent noise falls below the shot-noise limit by (0.6 +/- 0.2) dB (1.4 dB when corrected for detection losses). To observe the noise reduction we employed a scheme that used two orthogonally polarized pulses to cancel the noise that arises from the predominantly polarized guided-acoustic-wave Brillouin scattering in the fiber. We also present a simplified semiclassical theory of quantum-noise suppression by this amplifier, which is found to be in good agreement with the experimental results.     

Conditional preparation of a quantum state in the continuous variable regime: generation of a sub-Poissonian state from twin beams

We report the first experimental demonstration of conditional preparation of a nonclassical state of light in the continuous variable regime. Starting from a nondegenerate optical parametric oscillator which generates above threshold quantum intensity correlated signal and idler "twin beams," we keep the recorded values of the signal intensity only when the idler intensity falls inside a band narrower than its standard deviation. By this very simple technique, we generate a sub-Poissonian state 4.4 dB (64%) below shot noise from twin beams exhibiting 7.5 dB (82%) of noise reduction in the intensity difference.     

主动相位偏置折叠型萨尼亚克光纤传感阵列时分复用技术

针对折叠结构萨尼亚克(Sagnac)光纤传感阵列存在噪声光与信号光混叠的问题,提出了一种主动相位偏置时分复用方案.在传统梯形结构传感阵列的基础上,通过调整总线光纤的长度关系和附加延迟光纤的方法,使噪声光和信号光依次交替返回而不会发生混叠.进一步分析表明,通过调整延迟光纤环的长度,可以使输入光脉冲的重复频率达到标准时分复用系统重复频率的二分之一.整个阵列的相位偏置由一个与输入光脉冲同步的相位调制信号驱动集成光学芯片实现.实验演示了一个两基元的传感阵列,最小时间间隔为331.25 ns,输入光脉冲重复频率可达754.727 kHz,在5 kHz处相位灵敏度为7.3μrad√(Hz),探头间串扰约为-51.75 dB.