Optical pulse shaping at moderate power using a twisted-fibre NOLM with single output polarisation selection

We demonstrate theoretically and experimentally that efficient signal shaping operation can be obtained at moderate power by using the transmission characteristic of a power-symmetric nonlinear optical loop mirror (NOLM) including highly twisted fibre and operating through nonlinear polarisation rotation, when the circular polarisation state orthogonal to the input polarisation is selected at the NOLM output. By adjusting the angle of the quarter-wave retarder inserted in the loop, the phase bias of the transfer characteristic can be adjusted precisely to enable proper signal shaping for moderate values of input power, remaining well below switching power. The tolerance of the procedure to deviations of the input polarisation from the ideal circular case is investigated numerically. We demonstrate experimentally the capabilities of this setup for both power equalisation and extinction ratio enhancement. Finally, we show that this setup is also useful to shape ultrashort optical pulses from the relaxation oscillations of a DFB semiconductor laser. In comparison with other NOLM-based techniques, the proposed approach allows to reduce by a factor of 8-10 the peak power required for pulse shaping, for the same fibre length and Kerr coefficient.     

Easily tuneable nonlinear optical loop mirror including low-birefringence,highly twisted fibre with invariant output polarisation

We investigate theoretically the operation of a versatile nonlinear optical loop mirror (NOLM) structure to be used in optical communication systems. The proposed device is a fibre Sagnac interferometer that includes a low-birefringence, highly twisted fibre and a quarter-wave plate retarder in the loop. We study, both analytically and numerically, the evolution of the intensity-dependent NOLM transmission for both output polarisation components, using different models for the NOLM. From this analysis, we propose an easy way to adjust the position of the NOLM maximum transmission, simply by tuning the angle of the retarder. This procedure is particularly useful for amplitude regularisation of an optical signal. We also demonstrate that, if a tuneable optical attenuator is inserted in the loop, the positions of both maximum and minimum transmission can be tuned separately, using a perfectly reproducible procedure. It is therefore possible to optimise the NOLM transmission for both pedestal and amplitude fluctuations removal in an optical pulse train. For a circular input polarisation, this procedure ensures the highest possible contrast between minimum and maximum transmission, and an output polarisation state that is linear and independent of the input power. Finally, we demonstrate that the transmission characteristic of this NOLM is robust to environmentally induced changes in the fibre birefringence. Thanks to its versatility, robustness and polarisation invariance, this device is thought to be of primary interest for applications such as passive mode locking, pulse compression and pedestal suppression, amplitude regularisation in harmonically mode-locked, rational-harmonically mode-locked or subharmonic synchronous mode-locked lasers, as well as damping of relaxation oscillations.     

Optimum splitting ratio for amplifier noise reduction by an asymmetric nonlinear optical loop mirror

We theoretically and experimentally analyze the influence of the splitting ratio and the input power on the noise reduction capability of an asymmetric nonlinear optical loop mirror (NOLM) for different input noise levels. An easy method to calculate the optimum parameters for noise reduction is also presented. The best noise reduction is found at NOLM input powers at which the nonlinear transfer function has a slope close to zero. Additionally, the splitting ratio of the NOLM has to be adapted to its input noise level to suppress amplitude fluctuations effectively. Since the noise reduction by the NOLM is due to the Kerr nonlinearity, which has a timescale below a few femtoseconds, the noise reduction is applicable to short pulses in the picosecond and femtosecond range. This makes the NOLM applicable as an optical regenerator in an optical data transmission system at high bit rates, such as 160 GBit/s.     

Experimental investigation of self-starting operation in a F8L based on a symmetrical NOLM

We experimentally analyze the self-starting operation of a figure-eight mode-locked fiber laser. The design is based on a power-balanced nonlinear optical loop mirror (NOLM) with highly twisted low-birefringence fiber and a quarter-wave (QW) retarder in the loop. The NOLM operates by nonlinear polarization rotation. Self-starting mode-locking requires a careful adjustment of the NOLM low-power transmission, which is easily realized with our setup by adjusting the angle of the QW retarder. The laser is capable of generating ∼20 ps pulses at the fundamental repetition frequency of 0.78 MHz.     

Generation of amplitude squeezed green light from a high efficiency PPKTP frequency doubler

We report on a high-efficiency 532 nm green light conversion from an external cavity-enhanced second harmonic generation of a home-made 1064 nm single-frequency Nd:YVO₄ laser with a periodically poled KTP crystal. A stable green power of 60 mW with a conversion efficiency of 75% was measured. Meantime, we investigate the quadrature amplitude noise of the green light at the same experimental setup and 0.6 dB green light squeezing was experimentally observed (taking into account the total detection efficiency of 58%, the squeezing should be 1.1 dB). The squeezing as a function of input power was also studied and we found qualitative agreement with theoretical prediction.     

All-fiber passive mode-locked laser to generate ps pulses based in a symmetrical NOLM

We experimentally investigate an all-fiber passively mode-locked laser generating ps pulses. The experimental setup is a figure eight fiber laser configuration, including a power-symmetric Nonlinear Optical Loop Mirror (NOLM) with highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by polarization asymmetry between the counter-propagating beams in the loop. We used a Quarter-Wave Retarder in the loop to break the polarization symmetry. Using a polarizer beam-splitter cube as the NOLM output we got the best quality output pulses from the laser. At this output, we are monitoring the output pulse polarization component which is parallel to the input NOLM component. We achieved stable generation of ～25 ps pulses at the repetition frequency of 0.78 MHz with milliwatts average output power. The mode-locked laser ran in stable operation for hours.     

Sub-terahertz on-off switch based on a two-dimensional photonic crystal infiltrated by liquid crystals

A sub-terahertz switch is realized by infiltration of a two-dimensional photonic crystal (PC) with the liquid crystal 5CB. On-off switching is based on a shift of the bandgap of the PC by applying an external electric field which rotates the 5CB molecules. We confirm theoretically and experimentally that rotating the optical axis of the 5CB molecules considerably affects the transmission of the electromagnetic waves of TM polarization in the stop band. The effect can be used for on-off switching of the electromagnetic waves in the sub-terahertz range. Experimentally we demonstrate an extinction ratio of 13.3 dB at 91 GHz.     

Large amplitude noise reduction in ultrashort pulse trains using a power-symmetric nonlinear optical loop mirror

We analyse numerically the capabilities of a power-symmetric nonlinear optical loop mirror (NOLM) in the ultrashort pulse regime for high-quality amplitude regeneration of an optical signal. The device, which operates through nonlinear polarisation rotation, includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. For particular adjustments of the retarder orientation, and a circularly polarised input beam, the output energy characteristic flattens near the switching energy, a property that can be used to eliminate large amplitude fluctuations in an optical signal. The group velocity mismatch between polarisation components introduced by twist is mitigated by the interplay between anomalous dispersion and the nonlinear Kerr effect, although strong twist should be avoided as it still introduces substantial pulse distortion. Contrary to other designs, where a plateau characteristic requires a large power imbalance between the counter-propagating beams, both pulses in the present scheme can be simultaneously close to fundamental solitons, which allows a substantial widening of the plateau for particular pulse parameters. Good quality, nearly transform-limited pulses are obtained in this case at the NOLM output. The device is applicable for the regeneration of ultrafast data streams in which the signal-to-noise ratio is severely deteriorated.     

Frequency noise suppression in a diode laser locked to a travelling wave resonator incorporating a Brewster prism

A polarisation locking technique was applied to stabilise an extended cavity diode laser using a travelling wave resonator incorporating a Brewster prism. Despite the fact that the employed unbalanced detection was sensitive to optical power fluctuations, the in-loop photodetector measured 90 dB of noise suppression at 10 Hz in comparison to the free running frequency noise spectrum. Excess intensity noise measured with an out-of-loop detector, indicated the presence of correction-correlated noise in the output of the stabilised diode laser.     

Design of terahertz wave polarizing beam splitter

In this paper, we design a THz wave polarizer based on a periodic symmetrical thin film structure, which operates over a broadband THz frequency region (1.0–2.0 THz) and over an effective wide range of incident THz wave (72–84°). The spectral performance of this structure is characterized by transfer matrix method calculations. Results of simulations show that the polarizer is highly transmittance for TE polarized THz wave as well as highly reflecting for TM polarized THz wave.     

All-optical NRZ-OOK-to-RZ-OOK format conversions with tunable duty cycles using nonlinear polarization rotation of a semiconductor optical amplifier

We experimentally demonstrate an all-optical 10 Gb/s format conversion from non-return-to-zero (NRZ) on-off-keying (OOK) to return-to-zero (RZ)-OOK with tunable duty cycle in the whole C-band using nonlinear polarization rotation (NPR) arising in an semiconductor optical amplifier (SOA). The experimental results show that, by tuning the polarizer at the SOA output, an RZ signal with tunable duty cycle from 33% to 66% could be obtained with an extinction ratio(ER) over 10 dB. In addition, we show that the NRZ-to-RZ conversion with duty cycle of 33-66% can be obtained with less than 1 dB power penalty at the bit error ratio (BER) of 10⁻⁹. The device can facilitate the cross-connection between optical transmission networks employing different modulation formats.     

Differential group delay monitoring using an all-optical signal spectrum-analyser

We demonstrate an all-optical device that monitors differential group delay (DGD) degradation of picosecond optical pulses. This device is based on an ultrafast all-optical signal analyser that uses nonlinear effects (cross-phase modulation) to transfer rapid temporal fluctuations into frequency domain effects that can be measured on an conventional optical spectrum analyser (incorporating a slow-detector). This monitoring scheme will enable rapid dynamic monitoring and compensation of DGD in ultrafast optical networks, at 160 Gb/s data rates and beyond, where electronic monitoring techniques cannot operate. We discuss the required signal polarisation condition.     

Improved chromatic dispersion monitoring technique

We consider and investigate an improved chromatic dispersion monitoring method using two RF tones with an inserted dispersion offset. This improved technique can be used to monitor both the positive and negative accumulated dispersion caused by optical fibers as well as other optical components in optical networks. We experimentally demonstrate that the monitoring range of the improved technique can be greater than 1150 ps/nm and the monitoring sensitivity better than 0.064 dB/ps/nm by selecting appropriate RF frequencies and dispersion offsets. Our investigations reveal that the RF modulation index should be greater than 10% but less than 20% so as to acquire a large monitoring range with a small power penalty. We also examine the CD monitoring errors caused by polarization mode dispersion (PMD) and self-phase modulation, and show that the use of a dispersion offset can effectively reduce the PMD-induced monitoring errors.     

Fabrication of low OH loss holey fibers with varying air hole sizes and their optical properties

We experimentally investigate a flexible fabrication technique for low OH and transmission losses holey fibers with a Ge-doped core and air holes in a silica cladding region. Versatile holey fibers of different size, pitch, and shape of air holes were achieved by controlling the temperature and heating time of the holey fiber preform. In addition, we suppress the OH loss of less than ∼0.323 dB/km at 1383 nm. After fabricating holey fibers, we measure their optical properties including cut-off wavelength, mode field diameter, splicing loss, dispersion, bending loss, and polarization dependent loss based on the size of air holes. The total transmission loss was measured to be ∼0.226 dB/km at 1550 nm by improving the fabrication process. After fabricating optical patch cord based on holey fibers, we measured the long-term stability of the fabricated holey fiber by using the temperature cycling technique for 24 and obtained low power fluctuation of 0.2 dB. We achieve the high quality holey fiber with a low bending loss of ∼0.04 dB/turn under a bending radius of 2.5 mm at 1550 nm. We also obtain a tunable band rejection filter with a number of bending turns.     

Analysis of a polarisation-maintaining NOLM switch for OTDM applications

We propose a polarisation-maintaining NOLM switch design to be used as optical regenerator or wavelength converter in dense optical time-division multiplexed (OTDM) systems. The Sagnac loop is made of a piece of high birefringence fibre which is cut and cross-spliced in the middle. If pump and probe polarisations are linear and aligned in the co-propagating direction, the cross-splice ensures that the counter-propagating probe beam will be orthogonal to the pump, so that the parasitic cross-phase modulation between counter-propagating beams is minimised. This architecture also allows easy control of the optical phase bias, through squeezing a short section of the fibre, without any other modification of the setup. The performances of the proposed architecture are studied analytically and numerically, and compared with those of conventional schemes. It appears that, although the proposed setup reduces the interaction between counter-propagating beams only by a factor 3, it yields an extinction ratio improvement of a factor 10 or higher in comparison with conventional schemes. If there is substantial walkoff between pump and probe, a 10-fold reduction of the relative intensity noise of the emerging signal is also obtained when the mark ratio of the incoming data is variable.     

Design and simulation of a novel 32 × 32 photonic bandgap power switch based on SOI waveguide

In this paper, we propose a novel 32 × 32 photonic bandgap (PBG) power switch based on silicon-on-insulator (SOI) substrate. The 32 × 32 PBG power switch merges the design concept of PBG structure and multimode interference (MMI) structure. With controlling the voltages of 256 blocks for changing the refractive index, we can demonstrate that the light propagation direction can be switched to the assigned output port as a 32 × 32 power switch. According to the simulations, the power switching control tables are searched and selected for successful switching. The size of our designed 32 × 32 PBG switch can be reduced to 252 × 6040 μm which is much smaller than the conventional opto-electronic switches.     

Transmission intensity disturbance in a rotating polarizer

Random disturbance was observed in transmission intensity in various rotating prism polarizers when they were used in optical systems. As a result, the transmitted intensity exhibited cyclic significant deviation from the Malus cosine-squared law with rotation of prisms. The disturbance spoils the light quality transmitted through the polarizer thus dramatically depresses the accuracies of measurements when the prim polarizers were used in light path. A rigorous model is presented based on the solid basis of multi-beams interference, and theoretical results show good agreement with measured values and also indicate effective method for reducing the disturbance.     

Wavelength dependence of the phase retardation of a quarter-wave plate

We describe an optical heterodyne polarization interferometer that can be efficiently used for the precision measurement of the change in the state of polarization of a light wave induced by polarizing optical devices. This technique is used to measure the change with wavelength of the phase retardation of a quarter-wave plate. A theoretical derivation is presented to permit computation of the wavelength dependence of the phase shift induced by a quarter-wave plate.     

基于光弹调制技术的波片相位延迟量测量方法

提出了一种基于光弹调制技术的波片相位延迟量测量方法,利用米勒矩阵对其进行了理论推导和误差分析。测量光路包括激光器、起偏器、光弹调制器、被测波片、检偏器和光电探测器,利用探测信号的归一化基频分量和二次谐波分量精确计算出被测波片的相位延迟量。该方法能测量紫外到红外光谱范围内任意相位延迟量的波片,误差分析表明其误差小于0.05°。实验验证了该测量方法的有效性,波片相位延迟量的重复测量精度为0.0048°。     

The use of NOLM for investigations of initial development of supercontinuum in fibers with anomalous dispersion

We performed a numerical study of the transmission through a nonlinear optical loop mirror (NOLM) of a sequence of solitons generated at the initial stage of the supercontinuum (SC) generation. We found that the NOLM exhibits a selective transmission that critically dependents on the amplitude of the input solitons and on the NOLM loop length. The results demonstrate that by properly selecting these parameters the NOLM behaves as an optically controlled switch that allows the transmission of solitons with similar amplitude and width. The results obtained by employing this method are reasonably good and can be used to analyze the soliton formation at the initial stage of SC generation.