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.     

Photon-number squeezing in the normal-dispersion regime

We experimentally demonstrate the generation of amplitude-squeezed light in the normal-dispersion regime and measure by direct detection 1.7+/-0.1 dB (33%) and, with correction for linear losses, 2.5+/-0.2 dB (47%) of noise reduction below the shot-noise level. The dependence of the noise behavior on dispersion is investigated both experimentally and theoretically.     

Gain-clamping techniques in two-stage double-pass L-band EDFA

Two designs of long-wavelength band erbium-doped fiber amplifier (L-band EDFA) for gain clamping in double-pass systems are demonstrated and compared. The first design is based on ring laser technique where a backward amplified spontaneous emission (ASE) from the second stage is routed into the feedback loop to create an oscillating laser for gain clamping. The gain is clamped at 18.6 d B from -40 to -80 dBm with a gain variation of less than ±0.1 dB and a noise figure of less than 6 dB. Another scheme is based on partial reflection of ASE into the EDFA, which is demonstrated using a narrowband fiber Bragg grating. This scheme achieves a good gain clamping characteristic up to -12 dBm of input signal power with a gain variation of less than ±0.3 dB from a clamped gain of 22 dB. The noise figure of a 1580 nm signal is maintained below 5 dB in this amplifier since this scheme is not based on lasing mechanism. The latter scheme is also expected to be free from the relaxation oscillation problem.     

Experimental error filtration for quantum communication over highly noisy channels

Error filtration is a method for encoding the quantum state of a single particle into a higher dimensional Hilbert space in such a way that it becomes less sensitive to noise. We have realized a fiber optics demonstration of this method and illustrated its potentialities by carrying out the optical part of a quantum key distribution scheme over a line whose phase noise is too high for a standard implementation of BB84 to be secure. By filtering out the noise, a bit error rate of 15.3% +/- 0.1%, which is beyond the security limit, can be reduced to 10.6% +/- 0.1%, thereby guaranteeing the cryptographic security.     

Group-delay ripple correction in chirped fiber Bragg gratings

Group-delay ripple (GDR) introduced by systematic and random errors in chirped fiber Bragg grating fabrication is the most significant impediment to application of these devices in optical communication systems. We suggest and demonstrate a novel iterative procedure for GDR correction by subsequent UV exposure by use of a simple solution of the inverse problem for the coupled-wave equation. Our method is partly based but does not fully rely on the accuracy of this solution. In the experiment we achieved substantial reduction of the low-frequency group-delay ripple, from +/- 15 to +/- 2 ps, which resulted in dramatic improvement of the optical signal-to-noise-ratio system penalty, from 7 to less than 1 dB, for a chirped fiber Bragg grating used as a dispersion compensator in a 40-Gbit/s carrier-suppressed return-to-zero system.     

均衡功率的超宽带可调谐掺铒光纤激光器

报道了一种功率均衡的超宽带可调谐掺铒光纤激光器.该激光器采用窄线宽可调谐FP滤波器、掺铒光纤、波分复用器和耦合器构成光纤环形腔激光器，并通过部分反馈光功率自动控制，对输出激光功率进行均衡.实现了87 nm带宽（1525~1612 nm）范围内，输出激光功率波动小于0.2 dB的均衡结果.输出激光3 dB线宽为0.26 nm，边模抑制比大于55 dB.     

Ultrathin fiber probes with extended depth of focus for optical coherence tomography

We report on a novel scheme for extending the depth of focus (DOF) of ultrathin (125 μm diameter) fiber probes for optical coherence tomography (OCT) using a simple phase mask consisting of graded-index (GRIN) fiber. The technique is compatible with existing all-in-fiber probe fabrication techniques, and our simulations show that it can provide a DOF gain of ~2 at a modest ~5 dB reduction of peak sensitivity. In a prototype device using commercially available GRIN fiber, a DOF gain of 1.55 is obtained, validated by beam profiling and OCT imaging.     

Efficient polarization squeezing in optical fibers

We report on a novel and efficient source of polarization squeezing that uses a single pass through an optical fiber. Using the fiber's two orthogonal polarization axes produces two identical squeezed beams. Combining these in a Stokes measurement generates polarization squeezing of up to 5.1 +/- 0.3 dB. Furthermore, this scheme enables us to directly measure, for both polarizations, the noise of any given quadrature.     

基于长周期光纤光栅滤波器的掺铒光纤放大器理论和实验研究

从理论和实验上研究了带有高频二氧化碳激光写入的低成本长周期光纤光栅（LPFG）掺铒光纤放大器（EDFA）.结果表明,单波长和多波长EDFA的性能都可以通过在掺铒光纤（EDF）中插入长周期光纤光栅用作自发辐射噪声（ASE）滤波器或增益平坦器来提高性能.优化设计了带LPFG噪声滤波器的线放EDFA,与没有LPFG噪声滤波器相比,线放的噪声和小信号增益分别被减小和提高了约0.5 dB和7 dB。通过在多波长EDFA的EDF中插入一个LPFG增益平坦滤波器的方法,获得了1.5 dB的增益平坦度,与没有LPFG平坦器相比,EDFA的噪声被减小了0.1 dB,增益被提高了1 dB.     

1.50-mum-band gain-shifted thulium-doped fiber amplifier with 1.05- and 1.56-mum dual-wavelength pumping

We propose and demonstrate a 1.50-mum- band gain-shifted thulium-doped fiber amplifier (TDFA) by using a novel dual-wavelength (1.05and1.56 -mum) pumping scheme for what we believe is the first time. By providing a small amount of 1.56-mum power to a conventional upconversion-pumped TDFA, we successfully demonstrated a small-signal gain larger than 25 dB and a noise figure of 5 dB in a 1475-1510-nm band.     

Generation of continuous variable Einstein-Podolsky-Rosen entanglement via the Kerr nonlinearity in an optical fiber

We report on the generation of a continuous variable Einstein-Podolsky-Rosen (EPR) entanglement using an optical fiber interferometer. The Kerr nonlinearity in the fiber is exploited for the generation of two independent squeezed beams. These interfere at a beam splitter and EPR entanglement is obtained between the output beams. The correlation of the amplitude (phase) quadratures is measured to be 4.0+/-0.2 (4.0+/-0.4) dB below the quantum noise limit. The sum criterion for these squeezing variances 0.80+/-0.03<2 verifies the nonseparability of the state. The product of the inferred uncertainties for one beam (0.64+/-0.08) is well below the EPR limit of unity.     

Soliton squeezing at the gigahertz rate in a Sagnac loop

We demonstrate what is to our knowledge the first all-fiber squeezing experiment. A balanced Sagnac loop is used, and a record 6.1+/-0.2dB of noise reduction below shot noise has been obtained without stabilization. A gigahertz Er-doped fiber laser and a high-power double-clad Er-Yb amplifier have been developed to suppress guided acoustic-wave Brillouin scattering and to make possible the all-fiber configuration.     

Frequency noise reduction in erbium-doped fiber distributed-feedback lasers by electronic feedback

A novel technique for suppressing frequency noise in an erbium-doped fiber distributed-feedback laser incorporated into a master-oscillator-power-amplifier configuration by an electronic feedback technique is presented. The frequency noise is suppressed by locking of the laser emission to a fiber interferometer. The frequency noise spectral density of the laser is reduced by as much as 20 dB over the frequency range 1 Hz-10 kHz to 1.5 Hz/Hz(1/2) +/-25% at 1 kHz with a relative intensity noise spectral density below -120 dB/Hz over the frequency range 10 Hz-1 kHz. These lasers will have applications as sources for fiber-optic interferometry, high-resolution spectroscopy, and high-bandwidth communications.     

Raman effects in a highly nonlinear holey fiber: amplification and modulation

We experimentally demonstrate that a short length of highly nonlinear holey fiber (HF) can be used for strong L(+) -band (1610-1640-nm) Raman amplification and ultrafast signal modulation. We use a pure silica HF with an effective area of just 2.85mum(2) at 1550 nm, which yields an effective nonlinearity ~15 times higher than in conventional silica dispersion-shifted fiber. Using a 75-m length of this fiber, we obtained internal Raman gains of more than 42 dB and a noise figure of ~6 dB under a forward single-pump scheme, and the Raman gain coefficient was experimentally estimated to be 7.6 chi 10(-14)m/W . Also, an 11-dB signal extinction ratio in a Raman-induced all-optical modulation experiment was achieved with the same fiber.     

Experimental evidence for Raman-induced limits to efficient squeezing in optical fibers

We report new experiments on polarization squeezing using ultrashort photonic pulses in a single pass of a birefringent fiber. We measure what is to our knowledge a record squeezing of -6.8+/-0.3 dB in optical fibers, which when corrected for linear losses is -10.4+/-0.8 dB. The measured polarization squeezing as a function of optical pulse energy, which spans a wide range from 3.5-178.8 pJ, shows a very good agreement with the quantum simulations, and for the first time we see the proof experimentally that Raman effects limit and reduce squeezing at high pulse energy.     

Transmission performance of optical OFDM signals with low peak-to-average power ratio by a phase modulator

We have experimentally generated optical orthogonal frequency-division multiplexing (OOFDM) signals by a phase modulator (PM). The generated OOFDM signal can tolerant higher nonlinear effects in fiber than that generated by an intensity modulator because of its lower peak-to-average power ratio (PAPR). It is shown that, by using a PM, the PAPR of the OOFDM signal has a 2-3 dB reduction and the input power in fiber can be improved over 3 dB.     

Co(2+)-doped flatband optical fiber attenuator

The attenuation properties of Co(2+) -doped single-mode optical fibers were experimentally and analytically investigated for preparation of a stable flatband attenuator. By controlling the Co(2+) -doped region in a fiber core, we could prepare a flatband attenuator in which the attenuation in wavelengths 1530-1610 nm was held to within 9.9-10.2 dB (3% variation) and the fluctuation of attenuation was less than 0.1 dB with 1-W input power for 100 h.     

Compact polarization rotators for integrated polarization diversity in InP-based waveguides

We present the design, fabrication, and operation of a polarization converter based on angled waveguides in the InP/InGaAsP material system. By combining design elements from mode evolution and birefringent devices, the total device length is kept short (less than 50 microm) and the insertion efficiency high at 81%+/-19%, which corresponds to an insertion loss of 1 dB. Devices operate broadband, i.e., the polarization conversion exceeds 15 dB over a 100 nm wavelength range. A polarization rotator with these specifications is a prime candidate for use in an integrated polarization diversity scheme.     

波长可调谐超短光脉冲的简便高质的生成技术

提出了一种以处于增益开关调制状态下的法布里珀罗(Fabry-Pérot)半导体激光器作为光源,采用简单的自激注入锁定方式,以生成波长可调谐的超短光脉冲的实验系统。该实验装置中,激光器的外腔包括两个串联在一起的布拉格光纤光栅,一个掺铒光纤放大器(EDFA),一个光耦合器和两个环行器,其作用是选模,以及增强并控制反馈回激光器内腔的光强。该实验系统简单而高效,在24nm的波长调谐范围内获得了边模抑制比高于40dB的单模光脉冲输出;而在1521.8nm和1550.0nm之间28.2nm的波长调谐范围内,边模抑制比高于35dB。所得到的各个单波长激光脉冲的时域半峰全宽为140~260ps,各个脉冲的光谱半峰全宽皆为0.1nm。     

On duty cycle selection of RZ optical pulse to optimize the performance of dispersion compensated 10 Gbps single channel optical communication system using dispersion compensating fibers

We present results for duty cycle selection of optical RZ pulse to optimize the performance in 10 Gbps single channel dispersion compensated optical communication system. The system has link length of 240 km with two spans. Each of the spans consists of 120 km standard single mode fiber (SSMF) of 16 ps/nm/km, whose chromatic dispersion is compensated using pre-, post- and symmetrical-dispersion compensation schemes by 24 km dispersion compensating fiber (DCF) of −80 ps/nm/km. The performance of the three compensation schemes is compared by taking 8, 10, 12 and 14 dBm Er-doped fiber amplifier (EDFA) power levels in the link with a duty cycle range (0.1-0.9) of RZ optical pulse. The graphical results obtained show a relationship among the duty cycle, EDFA power and dispersion compensation scheme which predicts the best performing duty cycle case. To optimize performance of the system, we recommend in general, duty cycle less than 0.3 and EDFA power below 8 dB irrespective of compensation scheme. However, with post compensation duty cycle less than 0.7 and EDFA power below 12 dBm give optimum performance. The results conclude that for the high value of duty cycle, post dispersion compensation scheme should be used.