Recent progress of integrated optics planar lightwave circuits

Planar lightwave circuits (PLCs) are waveguide devices that integrate fiber-matched optical waveguides on silicon or glass substrate to provide an efficient means of interaction for the guided-wave optical signals. PLCs provide various important and functional devices for optical wavelength division multiplexing (WDM), time division multiplexing (TDM) systems and subscriber networks. This paper reviews the recent progress and future prospects of PLC technologies including arrayed-waveguide grating multiplexers, optical add/drop multiplexers, programmable dispersion equalizers and hybrid optoelectronics integration technologies.     

RF arbitrary waveform generation using tunable planar lightwave circuits

We demonstrate photonically-assisted generation of RF arbitrary waveforms using planar lightwave circuits (PLCs) fabricated on silica-on-silicon. We exploit thermo-optic effects in silica in order to tune the response of the PLC and hence reconfigure the generated waveform. We demonstrate the generation of pulse trains at 40 GHz and 80 GHz with flat-top, Gaussian, and apodized profiles. These results demonstrate the potential for RF arbitrary waveform generation using chip-scale photonic solutions.     

Experimental demonstration of single-mode fiber coupling using adaptive fiber couplerExperimental demonstration of single-mode fiber coupling using adaptive fiber couplerExperimental demonstration of single-mode fiber coupling using adaptive fiber couplerExperimental demonstration of single-mode fiber coupling using adaptive fiber coupler

Coupling plane wave into a single-mode fiber （SMF） with high and steady coupling efficiency is crucial for fiber- based free-space laser systems, where random angular jitters are the main influencing factors of fiber coupling. In this paper, we verified a new adaptive-optic device named adaptive fiber coupler （AFC） which could compensate angular jitters and improve the SMF coupling efficiency in some degree. Experiments of SMF coupling under the angular jitter situation using AFC have been achieved. Stochastic parallel gradient descent （SPGD） algorithm is employed as the control strategy, of which the iteration rate is 625 Hz. In closed loop, the coupling efficiency keeps above 65% when angular errors are below 80/3tad. The compensation bandwidth is 35 Hz at sine-jitter of 15 ~rad amplitude with average coupling efficiency of above 60%. Also, experiments with simulated turbulence have been studied. The average coupling efficiency increases from 31.97% in open loop to 61.33% in closed loop, and mean square error （MSE） of coupling efficiency drops from 7.43% to 1.75%.     

Signal beating elimination using single-mode fiber to multimode fiber coupling

We experimentally demonstrate an all-passive fiber-based approach to prevent undesired beating during signal merging and detection. Beating occurs when optical signals of very close or the same wavelength are combined at a coupler and detected using a photodetector. Our approach is based on signal coupling from several single-mode fibers to a single piece of multimode fiber without interference, such that different signals propagate in different modes with different spatial positions inside the multimode fiber. We have investigated signal beating when the signals are coherent, partially coherent, or incoherent with each other. The measured results for single-mode to multimode coupling show signal beating is substantially reduced, resulting in widely opened eye diagrams and error-free bit error rate performance.     

Advancements in passive planar lightwave circuits and hybrid integration devices

Planar silica device technology provides the ability to filter, route, switch, and attenuate optical signals for Wavelength Division Multiplexing (WDM) based photonic networks. As network capacity expands there is a requirement to increase the complexity, scale and density of the functions whilst reducing cost. It is also important that both the optical performance and long term stability of the devices are not compromised. The following paper reviews recent progress on these types of devices for large-scale manufacture, concentrating on key performance parameters for filter type structures and hybrid integrated devices such as insertion loss and polarisation dependent loss (PDL). To cite this article: J.R. Bonar et al., C. R. Physique 4 (2003).     

Effective index method for planar lightwave circuits containing directional couplers

A simple and intuitive effective index method for planar lightwave circuits containing directional couplers is proposed. The optimal cladding refractive index of the effective two-dimensional planar waveguide is found according to the coupling length of the directional coupler involved. Numerical examples indicate that this proposed effective index method gives accurate results while saving significant computation time in simulation and design of planar lightwave circuits containing directional couplers.     

Fabrication of silica-on-silicon planar lightwave circuits by P ECVD and ECR

Plasma enhanced chemical vapor deposition (PECVD) and electron cyclotron resonance (ECR) etching were used in the development of silica layers for planar waveguide applications. The addition of GeH4 to silica was used to control the refractive index of core layers with core-to-clad index differences in the range of 0.2％-1.3％. Refractive index uniformity variance of ±0.0003 was achieved after annealing for 4-inch Si 〈100〉 wafers. The core layers with thickness up to 6 μm were etched by ECR with optimized recipe and mask material. Low-loss silica-on-silicon waveguides whose propagation loss is approximately 0.07 dB/cm at 1550 nm are fabricated.     

A graded-index fiber taper design for laser diode to single-mode fiber coupling

A tapered graded-index fiber (GIF) structure is, for the first time, proposed for the efficient fiber-laser coupling. A simple analytical approach based on the ABCD law is utilized to optimally design the parameters of the hemispherically-ended GIF taper. A wide-angle beam propagation method is then employed to compute the fiber mode fields transmitted through the fiber taper in order to evaluate the coupling efficiency. By taking one commercially available GIF as an example, the effectiveness of the new design reported here is verified with a demonstration of high coupling efficiency of nearly 90%.     

Athermal silica-based interferometer-type planar light-wave circuits realized by a multicore fabrication method

Athermal silica-based interferometer-type planar light-wave circuits were realized by a newly developed multicore fabrication method. In this method, inductively coupled chemical-vapor deposition and polishing technologies are adopted on a silica substrate with a trench-type waveguide pattern prepared by reactive ion etching. Two kinds of deposited core material, 10GeO2-90SiO2 (mol. %) and 8GeO2-5B2O3-87SiO2 (mol. %), which show wavelength temperature dependence of 9.7 and 8.1 pm/degree C, respectively, were used to prepare the waveguide sections in a device. By adjustment of the lengths of waveguide sections with these two different core materials, athermal characteristics of less than 0.5 pm/degree C were achieved for Mach-Zehnder interferometer filter devices at the 1.55-microm wavelength range while the temperature varied from -20 to 80 degrees C. The new method is also applicable for the preparation of many other kinds of functional devices.     

Experimental demonstration of single-mode fiber coupling using adaptive fiber coupler

Coupling plane wave into a single-mode fiber(SMF) with high and steady coupling efficiency is crucial for fiberbased free-space laser systems, where random angular jitters are the main influencing factors of fiber coupling. In this paper, we verified a new adaptive-optic device named adaptive fiber coupler(AFC) which could compensate angular jitters and improve the SMF coupling efficiency in some degree. Experiments of SMF coupling under the angular jitter situation using AFC have been achieved. Stochastic parallel gradient descent(SPGD) algorithm is employed as the control strategy, of which the iteration rate is 625 Hz. In closed loop, the coupling efficiency keeps above 65% when angular errors are below 80 μrad. The compensation bandwidth is 35 Hz at sine-jitter of 15 μrad amplitude with average coupling efficiency of above 60%. Also, experiments with simulated turbulence have been studied. The average coupling efficiency increases from 31.97% in open loop to 61.33% in closed loop, and mean square error(MSE) of coupling efficiency drops from 7.43% to 1.75%.     

Characterization of coupling power for single-mode fiber fusion

Fabrication of single-mode fiber coupler by heating fibers on a flame is very common and popular. However, controlling the input and the output power quantities of the fiber coupling process is neither easy nor similar. Power losses occur at the coupling length as the effect of the geometry and structure of fibers during and after fusion. In application, the power losses will affect sensor devices, e.g. optical switch. This paper proposes power splitting into the second fiber junction as a new model by deriving and integrating the coupling power with two conditions of refractive index changes. First is the change in the vertical part and second is the change in the vertical and the horizontal parts. Both conditions are studied by perpendicular and parallel directions of coupling power. The model is examined with a linear change of refractive index where power absorption and reflection are accumulated by power losses. The result shows that power increases as the effect of losses by simulation. These power losses are lost by radiation out of cladding and fiber heating.     

Orthonormal polynomials in analysis of single-mode fiber coupling

Zernike polynomials have been widely used for wave-front analysis because of their orthogonality over a uniform circular pupil. However, the pupil is not uniform but weighted by the backpropagated fiber mode in analyzing fiber coupling efficiency. Zernike polynomials are not appropriate for a weighted pupil due to their lack of orthogonality over such pupil. We emphasize the advantages of using orthonormal polynomials in fiber coupling systems. The orthonormal polynomials over weighted pupil are derived by matrix approach. The effects of primary aberrations are investigated based on the orthonormal polynomials. The accuracy of the Strehl ratio approximation for primary aberrations is evaluated.     

Semiconductor laser diode to single-mode fiber coupling using diffractive optical elements

An integrated mode converter consisting of two diffractive optical elements (DOEs) and a Silicon slab is presented for low-loss coupling between a semiconductor laser diode (LD) and a single-mode fiber (SMF). The phase structures of the DOEs are designed using iterative phase retrieval algorithm. We introduce a new far-field amplitude constraint into the iteration to provide very high mode conversion quality. Compared with previously published mode converters, the scheme is more universal because it’s applicable for any semiconductor LD. In simulation, coupling losses lower than 0.02 dB are predicted for all the discussed LDs with aspect ratios of the elliptical fields from 1 to 9. The requirements on axial displacement and rotation angle have been removed. The tolerance for 1-dB loss increment for lateral misalignment is 0.9 μm. And the coupling loss is insensitive to tilt angle.     

Experimental study on high efficiency of Ti:sapphire laser to single-mode fiber coupling

We experimentally investigate an optimum scheme of coupling a collimated light from a Ti:sapphire laser source into a standard single-mode fiber(SMF).By adjusting the effective numerical aperture(NA) of coupling lens and eliminating the chromatic aberration,a coupling efficiency of around 70% is finally obtained.This result is close to the maximum value predicted by theoretical simulation.It is well demonstrated that high coupling efficiency between Ti:sapphire laser and SMF can also be obtained by optimizing certain parameters of a coupling lens,without employing any special optical components,or the specific fiber with complex structure.     

太阳光光纤耦合光学系统设计

介绍一种长焦距、大口径太阳光光纤耦合系统的光学设计。为达到减小大口径单透镜汇聚太阳光所产生的球差和色差的目的,采用聚光透镜后增加消像差光学系统的方法,并用Zemax软件设计优化出适用于光纤耦合的光学系统,经过优化设计得到光斑半径为45.043 m,能量集中度达95 %以上,像方孔径角为9.090 2,系统总长525.584 mm,满足数值孔径为0.22,芯径直径为1.5 mm的石英光纤的耦合要求。     

Gamma-ray-radiation-induced damage to silicon single-mode fiber

The gamma-ray-radiation -induced damage to silicon single -mode fiber is reported. The immediate radiation-induced attenuation of the 10 m fiber under total dose 10⁴ rad is 8.98 dB/km, which was tolerable. Some disastrous attenuation was found a half-hour later, which rapidly increased to 642.53 dB/km. No obvious spectral variation after radiation was found, except for the decreases of the spectrum power. The optical recovery phenomenon was noticed 2 days after radiation and the output spectrum power has increased from 145.5 to 185.6 μW during 9 days. The experimental results have shown that the composition and its absorptivity of optical fibers play a more important role than total dose in determining the radiation-induced attenuation.     

Phase perturbation investigation of laser diode for the laser diode to single-mode fiber coupling system

A novel phase-perturbation method as well as an experimental system were established to study the deviation of the laser diode (LD) far-field intensity distribution from the ideal Gaussian. An analytical expression of the LD far-field intensity distribution and a numerical method based on the Monte Carlo calculation were developed to fit the measured results. The loss of the LD to the single-mode fiber (SMF) coupling system caused by this kind of deviation of the LD far-field intensity distribution was quantitatively analyzed.     

Simple ABCD matrix method for evaluating optical coupling system of laser diode to single-mode fiber with a lensed-tip

In this paper, we present a simple technique to determine the coupling efficiency between a laser diode and a lensed-tip based on the ABCD transformation matrix method. We have compared our analysis technique to that of previous work and have found that the presented method is reliable in predicting the coupling efficiency of lensed-tip and has the advantage of simplicity of coupling efficiency calculation even by a pocket calculator. The results can be useful for designing coupling optics.     

Multimode fiber devices with single-mode performance

A taper transition can couple light between a multimode fiber and several single-mode fibers. If the number of single-mode fibers matches the number of spatial modes in the multimode fiber, the transition can have low loss in both directions. This enables the high performance of single-mode fiber devices to be attained in multimode fibers. We report an experimental proof of concept by using photonic crystal fiber techniques to make the transitions, demonstrating a multimode fiber filter with the transmission spectrum of a single-mode fiber grating.