Coupling efficiency of single-mode fiber components using GRIN-rod lenses

This article analyzes the characteristics of single-mode fiber (SMF) components using graded-index-rod (GRIN-rod) lenses. The coupling efficiency of the SMF to GRIN-rod lens to SMF coupling system is derived in the general case. The problem is simplified by using the reciprocal field method. All solutions are in closed form. It is found that SMF position tolerance is about 3 μm; the angular tolerance is about 1°. The GRIN-rod lens position tolerance is much less critical while the angular tolerance is extremely small. These tolerances could be much less critical if the fiber is kept near the GRIN-rod lens axis.     

Coupling efficiency of single-mode fiber components using GRIN-rod lenses

This article analyzes the characteristics of single-mode fiber (SMF) components using graded-index-rod (GRIN-rod) lenses. The coupling efficiency of the SMF to GRIN-rod lens to SMF coupling system is derived in the general case. The problem is simplified by using the reciprocal field method. All solutions are in closed form. It is found that SMF position tolerance is about 3 μm; the angular tolerance is about 1°. The GRIN-rod lens position tolerance is much less critical while the angular tolerance is extremely small. These tolerances could be much less critical if the fiber is kept near the GRIN-rod lens axis.     

Slow and fast light based on SBS with the spectrum tailoring

In this paper, slow and fast light of a signal pulse of nanosecond duration are achieved in the same experimental device by using band-pass filters to tailor the spectra of spontaneous Brillouin scattering (SPBS) coming from 50 km single-mode optical fibers (SMF) as the pump light. The phase matching condition of slow and fast light is satisfied by changing wavelengths of band-pass filters .In our experiment, we obtain the tunable delay time for various signal pulses range from 1 ns to 20 ns. By increasing the power of the pump field to 16 mW, a 1 ns signal pulse is delayed 1.5 ns and is advanced 1.0 ns respectively, while the signal pulse almost has no distortion. The experiment has demonstrated that larger relative pulse delay of slow and fast light can be obtained in the same experimental setup when pump power is lower.     

A liquid lens-based broadband variable fiber optical attenuator

To the best of our knowledge, proposed is the first variable fiber optical attenuator (VFOA) using an electronically controlled variable focus liquid lens. The approach uses the changes in the radius of curvature of the liquid lens edge to enable an electronically controlled optical wedge that produces a varying beam tilt angle. In effect, changing beam tilt within a single mode fiber (SMF) lens free space coupling assembly leads to a polarization independent broadband VFOA design. The demonstrated VFOA experiment shows broadband operation over the 1530-1560 nm C-Band with a 40 dB dynamic range, <0.5 dB resolution, 0.3 dB polarization dependant loss, 4.3 dB fiber-to-fiber optical loss, 3 dB optical bandwidth from 1510 nm to 1700 nm, and switching time of <100 ms. Applications for this VFOA include use in hand held test and measurement equipment.     

High-power multi-mode laser to single-mode pump conversion

A novel pump-sharing module for multi-channel amplifiers based on integration of high-power multimode semiconductor lasers and planar waveguide circuits is proposed. The feedback-free modules are designed for optimum coupling, throughput and power uniformity on silicon using BPM method and fabricated via the sol-gel process of photo patternable precursors. Modal properties of the multimode lasers are considered by studying spectrum and spatial distribution of laser output power at different temperatures. Employing an 1500 mW 980 nm multimode laser (TE polarized), via lens and Butt coupling up to 1250 mW of laser output is coupled into the multimode channel waveguides, which are tapered and branched to four single mode waveguides. The measured pump power from splitter outputs, randomly polarized, exceeds 600 mW. At constant temperatures ranging from 22 °C to 30 °C, the pump-sharing modules exhibit good stability with a power uniformity of <0.5 dB over hours.     

A multi-channel full-duplex 60-GHz-band radio-over-fiber system

We propose and simulate a novel full-duplex radio-over-fiber system using a single light source at central station (CS). The scheme is employed to generate 60-GHz optical millimeter wave at CS for down-link transmission while the same optical carrier is reused at base station for up-link connection. There is no additional laser source for the upstream data generation in the base station. The bidirectional full-duplex 2.5 Gb/s data are successfully transmitted over 40 km standard single-mode fiber (SMF). The power penalty for the down-link data after transmission over 40 km SMF is less than 0.6 dB, while for the up-link data, the power penalty after transmission over 40 km SMF is neglected. This system shows good performance over long-distance delivery and has important applicable value in high radio frequency (RF) sector and multi-channel full-duplex system.     

Microlens fabrication on single-mode fibres for efficient coupling from laser diodes

An analysis of the etching process for forming a truncated conical microlens on the end surface of a single-mode fibre (SMF) so as to improve its coupling efficiency with a laser diode (LD) has been carried out. This is followed by a simple geometric-optics and electromagnetic analysis to calculate this coupling efficiency, when the lens is formed on a SMF fabricated by the MCVD technique. The theory is supported by measurements which gave a maximum coupling efficiency of 57%.     

Fusion splicing small-core photonic crystal fibers and single-mode fibers by controlled air hole collapse

A method based on controlled air hole collapse for low-loss fusion splicing small-core photonic crystal fibers (PCFs) and single-mode fibers (SMFs) was demonstrated. A taper rig was used to control air hole collapse accurately to enlarge the MFDs of PCFs which was then spliced with SMFs using a fusion splicer. An optimum mode field match at the interface of PCF-SMF was achieved and a low-loss with 0.64 dB was obtained from 3.57 dB for a PCF with 4 μm MFD and a SMF with 10.4 μm MFD experimentally.     

Self-excited brillouin–erbium fiber laser for DWDM applications

The operation of the self-excited Brillouin/erbium fiber laser (BEFL) is experimentally demonstrated. It uses a forward pumped erbium-doped fiber (EDF) as a gain medium to generate a Brillouin-pump (BP) and amplify the Stokes frequency signals generated in a single-mode fiber (SMF). The operating wavelength varies with changes of coupling ratio of the looping arm. The optimum BEFL operation is obtained with a center wavelength at 1567.5 nm by looping back 20% of BEFL output into the other end of the SMF. The BEFL output power and number of lines increases gradually as 980-nm pump power increases. At the maximum 980-nm pump power of 118 mW, a stable and strong BEFL comb of up to 16 lines with 11 GHz spacing is demonstrated. The self-excited BEFL has the potential to be used in the future dense wavelength division multiplexing (DWDM) communication system.     

New nanosecond Q-switched Nd:VO4 laser fifth harmonic for fast hydrogen-free fiber Bragg gratings fabrication

We demonstrate the fabrication of fiber Bragg gratings using a novel high-repetition rate nanosecond Q-switched Nd:VO₄ laser fifth harmonic (213 nm) source for the first time in boron and hydrogen-free, Ge doped fiber. Strong gratings are rapidly obtained with the phase mask technique in hydrogen-free B/Ge doped photosensitive fiber with relatively low average power (100 mW), as well as in standard Corning SMF28 fiber. The evolution of the refractive index change during UV-exposure is presented. Photosensitivity of fibers to the 213 nm light is compared to the fourth harmonic (266 nm) light, as well as picosecond 213 nm radiation and is shown to be significantly higher than both. We believe that the photosensitivity of SMF28 fiber is due to a single-photon rather than two-photon absorption process.     

A high temperature sensor based on a peanut-shape structure Michelson interferometer

A fiber temperature sensor with high sensitivity based on a Michelson interferometer is realized by fusion-splicing a peanut-shape structure in single-mode fiber (SMF). The theory and experimental results show that the peanut-shape structure can couple the light energy of the core mode into the cladding and re-couple the light in the cladding into the core. A high-quality interference spectrum with a fringe visibility of about 18 dB is observed. Experimental demonstration shows that the device can be heated up to 900 °C with a sensitivity of about ∼0.096 nm/°C. The device has the advantages of low-cost, high sensitivity and easy fabrication, which makes it attractive for sensing applications.     

Efficient coupling and relaxed alignment tolerances in pigtailing of a laser diode using dual ball lenses

In this paper, we report an efficient coupling scheme with relaxed misalignment tolerances. The proposed coupling scheme consists of two ball lenses of same diameter (1 mm) and different refractive indices. The second ball lens which is facing the fiber tip has a higher refractive index (1.833), whereas the first one which faces the laser diode has a refractive index of 1.5. Employing Gaussian and ABCD ray tracing optics, the theoretically obtained coupling efficiency can reach a unity with relaxed working distance (separation of the coupling system from the fiber tip) in the range between 1 and 4 mm at some optimum positions of the coupling lenses with regard to each other and to the facet of the laser diode. It has been found that if the distance between the first ball lens and the laser diode (d₁) is fixed at 1.1 mm, which is twice its focal length, the coupling efficiency and the working distance as well as the misalignment tolerances are greatly affected by variation of the separation between the two ball lenses (s), and for this proposed coupling scheme the highest coupling efficiency and largest working distance are obtained when s is in the range of 0.3-0.35 mm. Above and below this range there is a significant reduction in the values of the above-mentioned parameters. Experimentally, the Nd:YAG laser welding system has been used for the alignment and welding of the coupling components in a butterfly configuration; the experimentally obtained coupling efficiency of the proposed coupling system was around 75% with relaxed working distance. From the effect of lateral and angular offsets on coupling efficiency, it is clearly noticed that the mode field of laser diode is transformed from elliptical into circular and hence effectively matched with that of the single-mode fiber.     

Application of fiber interferometer in coherent Doppler lidar

A novel coherent Doppler lidar (CDL) system based on single-mode fiber (SMF) components and instruments is presented to measure the speed of target. A fiber interferometer used in CDL system is reported.This fiber mixer is employed as a coherent receiver to resolve the shifts of signal-to-noise ratio (SNR) and mixing efficiency induced by backscattered field's wavefront error. For a certain wavelength, the maximum coupling efficiency between signal and SMF is determined by the ratio of pupil diameter to focal length of the coupling lens. The legible interference patterns and spectrum signals show that fiber interferometer is suitable to compensate for amplitude and phase vibrations. This robust coherent receiver can achieve improved CDL system performance with less transmitter power.     

Theoretical analysis of the all-fiberized, dispersion-managed regenerator for simultaneous processing of WDM channels

The concept of the all-fiberized multi-wavelength regenerator is analyzed, and the design methodology for operation at 40 Gb/s is presented. The specific methodology has been applied in the past for the experimental proof-of-principle of the technique, but it has never been reported in detail. The regenerator is based on a strong dispersion map that is implemented using alternating dispersion compensating fibers (DCF) and single-mode fibers (SMF), and minimizes the nonlinear interaction between the wavelength-division multiplexing (WDM) channels. The optimized regenerator design with + 0.86 ps/nm/km average dispersion of the nonlinear fiber section is further investigated. The specific design is capable of simultaneously processing five WDM channels with 800 GHz channel spacing and providing Q-factor improvement higher than 1 dB for each channel. The cascadeability of the regenerator is also indicated using a 6-node metropolitan network simulation model.     

Power transfer characteristics among N parallel single-mode optical fibers

Based on the coupled-mode theory, the power transfer among “- - - -” arranged parallel single-mode optical fibers has been investigated. The analysis shows that the distances between each two of the N fiber centers have effects on the coupling coefficient and power transfer. The solution of the coupled-equations for three parallel single-mode optical fibers is given, and is studied for different initial conditions comparatively. Numerical simulations show that power transfer will be periodical during coupling among parallel single-mode optical fibers. These results can be extended to multi-parallel single-mode optical fibers.     

ABCD matrix model of quadric interface-lensed fiber and its application in coupling efficiency calculation

In order to simplify the theoretical coupling efficiency calculation of a laser diode (LD) to a single-mode fiber (SMF) with a quadric lens formed on the fiber tip, based on the Gaussian beam approximation, using the ABCD matrix method the relationship between the coupling efficiency and the structure parameters of quadric surface-lensed fiber is studied and analyzed in detail, and the coupling efficiency can reach as high as 99.87% with optimizing parameters of lensed fiber in theoretical simulation. Meanwhile, under paraxial approximation, the ABCD matrix for refraction of different quadric surface micro-lens can be formulated as a unified expression. This result can give some guidance suggestions for fabricating the quadric interface-lensed fiber.     

Raman continuum generation at 1.0–1.3 μm in passively mode-locked fiber laser based on nonlinear polarization rotation

We experimentally demonstrate cascaded Raman scattering continuum generation at 1.0–1.3 μm with pulse energy of 47 nJ by utilizing a commercially available all-normal-dispersion single-mode fiber (SMF) with a low threshold pump power. We achieve passively mode-locked all-fiber lasing by nonlinear polarization rotation technique. Continuous Raman gain is obtained by the same SMF in every roundtrip because of the ring-cavity configuration, which makes it possible to achieve high-order Raman Stokes waves by a short fiber length. Limited by the pump power, the maximum output average power and peak power are 87.4 and 339 mW, but it has great potential to be improved. These make it an ideal continuum source for many applications, such as optical coherence tomography.     

Using Fabry-Perot laser diode and reflective semiconductor optical amplifier for long reach WDM-PON system

In this investigation, we propose and investigate the simple self-injection locked Fabry-Perot laser diodes (FP-LDs) in optical line terminal (OLT); and wavelength-tunable optical network unit (ONU) using reflective optical semiconductor amplifier (RSOA) and FP-LD laser for downstream and upstream traffic in long reach (LR) wavelength division multiplexed-passive optical network (WDM-PON) respectively. The output performance of the proposed two laser sources in terms of power and side-mode suppression ratio (SMSR) has been discussed. Here, for the downstream traffic, the proposed optical transmitter can be directly modulated at 2.5 Gb/s on-off keying (OOK) format with nearly 0.4 dB power penalty at bit error rate (BER) of 10⁻⁹ through 75 km single-mode fiber (SMF) transmission. Moreover, the proposed upstream transmitter can be directly modulated at 1.25 and 2.5 Gb/s with nearly 0.5 and 1.1 dB power penalty, respectively, at the BER of 10⁻⁹.     

The thermo-optical variable optical attenuator based on SOI

A thermo-optical variable optical attenuator was studied based on silicon on insulator (SOI) substrate waveguide. It is composed by the single-mode waveguide, taper waveguide, multi-mode waveguide, and inclined electrode. By adjusting the applied voltage on the inclined electrode it can achieve continuously variable attenuation of the output light. The results we studied show that when the applied voltage is about 4.7 V (the corresponding power is 233 mW), the variation of the waveguide's core temperature is about 33 °C, the refractive index changes more than 5.0 × 10⁻³ and the attenuation will reach 35 dB, and the response time is only 35 μs.     

HDTV/Gigabit Ethernet over bidirectional WDM-PON based on injection-locked Fabry-Perot laser diodes

Bidirectional high definition television (HDTV)/Gigabit Ethernet over wavelength-division-multiplexing (WDM)-passive optical network (PON) based on injection-locked Fabry-Perot laser diodes (FP LDs) as transmitters were proposed and demonstrated. Injection-locked FP LDs, a particularly inexpensive option, are used to replace conventional distributed feedback (DFB) LDs. Services with 77 HDTV channels and 1 Gigabit Ethernet channel connection were successfully demonstrated over 40 km single-mode fiber (SMF) transport. Good performance of bit error rate (BER) and clear eye diagram were achieved in our proposed bidirectional WDM-PON.