Er-doped Waveguide Research Articles

Fabrication of an erbium–ytterbium-doped waveguide amplifier at communication wavelengths for integrated optics applications

Erbium–ytterbium-doped waveguide amplifiers provide a considerable gain at telecom wavelengths, low noise, nonlinearity, and compatibility with optical networks, making it an outstanding amplification module for telecommunication systems. This study reports on the fabrication of an optical waveguide amplifier for integrated optics. The signal can be amplified by using rare-earth dopings such as erbium (Er), which works at telecommunication wavelengths, i.e., 1.55 μm. Er-doped phosphate glass waveguides can be deposited using the sol–gel method, which is convenient for preparing active films on several substrates. The Er concentration was 1–2 × 1020/cm3. The confinement and the gain of the waveguide can be increased by reducing the width of the waveguide to 0.5 μm. In such a case, more than 1dB net gain can be achieved without additional pump power. The other material used as a dopant in optical amplifiers is ytterbium (Yb). For Er energy levels, a more significant pump intensity is necessary for inversion due to the limited absorption cross-section. This issue is solved by including a substance with a large absorption cross-section that transfers energy to Er. The Ag–Na ion exchange process is then used to fabricate the buried waveguide. In such a process, ions trade between the core material and the molten salt. Then, the waveguide is immersed in the molten salt. The fabricated waveguide has low loss, and a net gain of around 2 dB at a wavelength of approximately 1.55 μm in Er:Yb:Al: phospho silicate glass is achieved. The focus of the research is on the fabrication procedure (materials and methods) of the waveguide.

On-Chip Waveguide Amplifiers for Multi-Band Optical Communications: A Review and Challenge

Erbium (Er)-doped fiber amplifier (EDFA) has achieved great success in modern optical communications for decades due to its outstanding gain performance in C-band. Nowadays, people are making efforts to make it monolithic integration due to the imperious demands from the advancement of high-performance and multi-function optoelectronic devices and chips. Except for the Er-doped waveguide amplifier, the other ion-doped on-chip waveguide amplifiers have also attracted much attention for multi-band optical communications, since they have own advantages in gain, bandwidth, selectivity of host materials, and integrability. Herein, we review the transmission characteristics of ion-doped on-chip waveguide amplifiers including gain and bandwidth, combining with the emission mechanisms and fabrication processes in applications of long-haul and short-reach optical communications. To facilitate the practical progress of the integrated waveguide amplifiers, from the perspective of gain medium, we also discuss the challenges and potential solutions for future technology roadmap. We expect that the bismuth (Bi)-doped waveguide amplifier would present impressive advantages in multi-band optical communications.

Effect of Substrate Temperature on Morphological, Structural, and Optical Properties of Doped Layer on SiO2-on-Silicon and Si3N4-on-Silicon Substrate.

A high concentration of Er3+ without clustering issues is essential in an Er-doped waveguide amplifier as it is needed to produce a high gain and low noise signal. Ultrafast laser plasma doping is a technique that facilitates the blending of femtosecond laser-produced plasma from an Er-doped TeO2 glass with a substrate to form a high Er3+ concentration layer. The influence of substrate temperature on the morphological, structural, and optical properties was studied and reported in this paper. Analysis of the doped substrates using scanning electron microscopy (SEM) confirmed that temperatures up to approximately 400 °C are insufficient for the incoming plasma plume to modify the strong covalent bonds of silica (SiO2), and the doping process could not take place. The higher temperature used caused the materials from Er-doped tellurite glass to diffuse deeper (except Te with smaller concentration) into silica, which created a thicker film. SEM images showed that Er-doped tellurite glass was successfully diffused in the Si3N4. However, the doping was not as homogeneous as in silica.

Design of an erbium-doped Al[formula omitted] optical waveguide amplifier with on-chip integrated laser pumping source

Erbium-doped optical waveguide amplifier (EDWA) is an important building block for conventional and integrated optical communication and light signal processing systems. In recent years remarkable advancement has been achieved for EDWA to meet the demand for silicon photonic chips and integrated photonic circuits. However, in the currently demonstrated EDWAs all the laser pumping sources are external or off-chip, which not only limits the application field of EDWA, complicates the amplification scheme, but also does not fulfill the expectation of monolithic integration. In this work we propose a design of EDWA using high-gain Er-doped Al2O3 waveguide, low-loss silicon-on-insulator (SOI) waveguide, and on-chip integrated semiconductor laser pumping source. The 1550 nm signal light and 1480 nm pump light are transmitted and combined in SOI waveguide and coupled into the EDWA waveguide embedded in the adjacent layer. The 1480 nm pump light is coupled into the SOI waveguide from a bonded hybrid III–V semiconductor laser. The pump and signal waveguide building scheme, the coupling method and structure, and the integration process of laser pumping source are shown and simulated. The system performance was evaluated by using the device’s structure parameters and Er3+ rate equation model. More than 1 dB/cm net gain and up to 35 dB maximum net gain can be achieved at about 100 mW pump power. It indicates that such an on-chip integration design is feasible and fully on-chip integrated optical waveguide amplifier could be achieved.

Efficient 1.54μm laser property in near- stoichiometric Er:LiNbO3 crystal

Abstract Near-stoichiometric LiNbO3 crystal heavily doped with Er3+ ions (Er:NSLN) was grown by Czochralski technique. An enhancement of 1.54 μm emission and a lengthening lifetime of 4I13/2→4I15/2 transition observed in Er:NSLN crystal would meet the requirement of the greatly shortening optical diffusion route for Er-doped waveguide amplifiers (EDWAs). Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the concentration of Er3+ ion in the crystal. Based on Judd–Ofelt theory, the spectroscopic properties of Er:NSLN crystal were discussed, and the obtained intensity parameters Ωt implied that the increased [Li]/[Nb] ratio had a large influence on the value of Ω2. The emission cross-sections of 1.54 μm emission were calculated by Fuchtbauer–Ladenburg and reciprocity methods. Studies on the gain cross-section, estimated as a function of the population inversion ratio, suggested that Er:NSLN crystal could be considered as a potential material in the application of the tunable lasers around 1.54 μm wavelength.

Graphene Q-Switched Mode-Locked and Q-Switched Ion-Exchanged Waveguide Lasers

In this letter, we present the use of monolayer graphene saturable absorbers to produce Q-switched and Q-switched mode-locked operation of Yb and Yb:Er-doped phosphate glass waveguide lasers, respectively. For the 1535-nm-wavelength Yb:Er laser, the Q-switched pulses have repetition rates up to 526 kHz and contain mode-locked pulses at a repetition frequency of 6.8 GHz. The measured 0.44-nm bandwidth should allow pulses as short as $\sim 6$ ps to be generated. Maximum average output powers of 27 mW are obtained at a slope efficiency of 5% in this mode of operation. For the 1057-nm-wavelength Yb laser, Q-switched pulses are obtained with a repetition rate of up to 833 kHz and a maximum average output power of 21 mW. The pulse duration is found to decrease from 292 to 140 ns and the pulse energy increase from 17 to 27 nJ as the incident pump power increases from 220 to 652 mW.

Population inversion and low cooperative upconversion in Er-doped silicon-rich silicon nitride waveguide

Single-mode, strip-loaded silicon-rich silicon nitride (SRSN) waveguide with 11 at.% excess Si and 1.7×10(20) cm(-3) Er was fabricated and characterized. By using a 350 nm thick SRSN:Er core layer and a 850 nm wide SiO2 strip, a high core-mode overlap of 0.85 and low transmission loss of 2.9 dB/cm is achieved. Population inversion of 0.73-0.75, close to the theoretical maximum, is estimated to have been achieved via 1480 nm resonant pumping, indicating that nearly all doped Er in SRSN are optically active. Analysis of the pump power dependence of Er3+ luminescence intensity and lifetime indicate that the Er cooperative upconversion coefficient in SRSN:Er is as low as 2.1×10(-18) cm3/sec.

320 fs pulse generation from an ultrafast laser inscribed waveguide laser mode-locked by a nanotube saturable absorber

Ultrafast laser inscription is used to fabricate the gain element for a mode-locked Er-doped bismuthate glass waveguide laser. Mode-locking is initiated and stabilized by the use of a single wall carbon nanotube saturable absorber. The waveguide laser produces 320 fs pulses at 1.56 μm with a pulse repetition rate of 40 MHz and average output power of 1.25 mW.

Ultrafast laser inscription of a high-gain Er-doped bismuthate glass waveguide amplifier

An Er-doped bismuthate glass waveguide amplifier has been fabricated using ultrafast laser inscription. Under zero pump conditions, the 87.0 mm long waveguide exhibited a fiber-to-fiber insertion loss of 4.0 dB at 1618 nm, outside the Er(3+) ion absorption band. We attribute approximately 1.8 dB of the insertion loss to coupling losses, 0.2 dB to Fresnel reflections and approximately 2.0 dB to propagation losses. When pumped using 1050 mW of 980 nm light, the amplifier exhibited a peak internal gain per unit length of 2.3 dB.cm(-1) at 1533 nm and a peak fiber-to-fiber net gain of 16.0 dB at 1533 nm. In this paper we also report the results of output power saturation and noise figure measurements.

Overlap factor between intensity profiles of signal and pump light and gain characteristics of Er-doped waveguide amplifier

The model used for analyzing the gain characteristics of Erdoped waveguide amplifier （EDWA） is based on the ratepropagation equations with intensity profiles of pump and signal light. The intensity profiles of pump and signal light for different channel opening width （DCOW） of Erdoped waveguide is discussed. And the numerical method （NM） considering the difference of intensity profiles between signal and pump light in channel waveguide with DCOW is used to solve the gain characteristics of signal light in EDWA. An overlap factor Γsp between light intensity profiles of signal and pump is introduced to modify the conventional simplified method （CSM） in EDWA, which is a revised version of the conventional simplified method （RCSM）. Results show that difference of 297 dB in gain for -10 dBm signal light at 1534 nm due to DCOW of waveguides is obtained for a 4 cm long waveguide amplifier pumped by 80 mw at 980 nm. Comparing with CSM, the gain characteristics of EDWA from RCSM is simpler and better approaches the result from NM. Therefore, the RCSM is valuable in the research of EDWA.

Theoretical and experimental research on Er-doped and Yb-Er co-doped Al2O3 waveguide amplifiers

The rate equations of Er-doped and Yb-Er co-doped systems pumped at 0.98 mm are presented, with consideration for the upconversion mechanisms such as cooperative upconversion, cross relaxation, and excited state absorption. A multi-theoretical model is founded to analyze the gain characteristics of Er-doped and Yb-Er co-doped Al2O3 waveguide amplifiers by using rate equations, a two-dimension waveguide finite element model and propagation equations with forward and backward amplified spontaneous emission. The dependence of the gain on amplifier length, pump power and doping concentration is obtained. The optimum design curve is given for designing waveguide amplifiers. The new theory is used to analyze the gain performance of a practical Yb-Er co-doped Al2O3 waveguide amplifier, and the analyzed results are in accordance with the experimental data.

Erbium-Doped Silicon-Rich Oxide Waveguides Fabricated by Using an Electro-Chemical Method

In this article, we describe and electro-chemical method for fabricating erbium-doped silicon-rich oxide (SRO) waveguides. The porosity, the Er-ion concentration and the refractive index can be controlled by varying the current density (continuous or pulsed currents) during the production process. The refractive index di erence between the core and the cladding layers and the pro le of the SRO waveguides were tested by using an m-line spectrometer and Field-emission scanning electron microscope (FE-SEM). The photo-luminescent (PL) emission of Er-ions at 1550 nm in the SRO waveguides could be obtained even when the excitation wavelength was away from the resonance absorption band of the Er-ions. This result revealed that Er-ions are possibly excited from the ground state I15=2 to the rst excited state I13=2 by energy transfer from excited Si nanocrystallites. The Er-doped SRO waveguides can be applied to optoelectronic devices compatible with Si-based integrated circuit technology.

Study on the protection of Er-doped phosphate glass waveguide surface in ion-exchange processing

A novel method, sputtering K9 glass film, is proposed to solve the surface corrosion of Er-doped phosphate glass during ion-exchange processing for optical waveguide fabrication. The corrosion causes are analyzed to be the intrinsically weak stabilization of phosphate glass structure, hydrophile and weakly acidic property of phosphate radical. Experimental results show that the K9 glass film could not only protect the Er-doped phosphate glass surface from being corroded but also give no influence on the waveguide fabrication. The effect of thickness of K9 glass film on the optical property of waveguide is also investigated and the optimal thickness is found to be 60–80 nm. It provides a good base for further fabrication of active phosphate glass optical waveguide devices.

Optical modelling of a Si-based DBR laser source using a nanocrystal Si-sensitized Er-doped silica rib waveguide in the C-band

The availability of reliable silicon-based laser sources is at the basis of the integration of photonic and microelectronic devices on a single chip with consequent development of wavelength division multiplexing telecommunication systems. A high efficiency Si-based laser source with good stability at room temperature would encourage and push the large scale of integration of electronic and photonic devices within a single chip. Several techniques have been proposed for generating light with an internal quantum efficiency some order of magnitude greater than that typical of silicon (10−6) by using either electrical or optical pumping. Among them we mention the improvement of some fabrication process steps, reduction of the channels of non-radiative recombination, quantum confinement, the use of silicon nanocrystals (Si-ncs) incorporated in a silica matrix. This last technique is used in combination with Er3+ doping to generate light emission around 1500 nm in silicon, since Er-doped Si-ncs behave as electron-hole pairs trap, and the presence of Er shifts the emission peak to around 1500 nm. In this paper we have pointed out the optical model of a Si-based DBR laser including a Si-ncs Er-doped SiO2 rib waveguide, working at a wavelength in C-band. In particular, after a brief description of the structural and optical properties of the silicon crystals, we report on the model and design of the Er:Si-nc/SiO2 rib waveguide, of the optical cavity and of the Bragg mirrors.

Fabrication of ultra-compact Er-doped waveguide amplifier based on bismuthate glass

Due to the possibility of high Er3+ doping and their broadband emission, the bithmuth-oxide-based glasses can be a promising candidate of host materials for a compact waveguide amplifier with high performance. By a novel fabrication process, we have developed a Bi2O3-based waveguide device with propagation loss as low as 0.13 dB/cm, which is comparable to the silica-based waveguides. We have achieved an ultra-compact Er-doped waveguide amplifier with 1 cm2 size by designing a spiral light-wave circuit with low bending loss. This EDWA showed gain values over 15 dB, noise figures lower than 8 dB and gain flatness less than 2 dB over the whole C-band region. The output power of this EDWA reached higher than 12 dBm.

Er$^{3+}$–Yb$^{3+}$ Codoped Silica Waveguide Amplifiers Longitudinally Pumped by Broad-Area Lasers

We present a longitudinal multimode pumping scheme which allows Yb-sensitized Er-doped silica waveguide amplifiers to be effectively pumped by high-power and low-cost broad-area lasers. The proposed configuration is based on evanescent pump light coupling from a multimode low-loss waveguide to a Er-Yb codoped active core. Theoretical predictions, based on propagation and population-rate equations for the coupled Er-Yb system, show that longitudinal multimode pumping by high-power broad-area lasers at around 980 nm can provide up to 4-dB gain per centimeter, suggesting possible integration of low-cost amplifiers with important applications in wavelength-division-multiplexing metro and access optical networks.

Correct determination of net gain in Er-doped optical waveguide amplifier from pump-on/off measurement

Starting with evolution equation of signal optical power in an Er-doped channel waveguide, rigorous theoretical expressions used for correct determination of net gain from signal enhancement measured by using pump-on/off method are derived. These expressions allow to clarify the argument on relationships between net gain, internal gain and signal enhancement in some earlier literatures. Physical implications of net gain, internal gain and signal enhancement expressions are analyzed. Standardized definitions for the three physical quantities are proposed. The definition for net gain is proposed on the basis of practical application of an EDWA in optical fiber communication and the definitions for the latter two are suggested based on the physical implication analysis of their expressions.

Fabrication and characterization of SiO2 based waveguide co-doped withSi-nanocrystal and Er3+

[ <TEX>$SiO_2$</TEX> ]thin films containing Si-nanocrystals and <TEX>$Er^{3+}$</TEX> were fabricated by the RF-sputtering method. Intense emission of <TEX>$Er^{3+}$</TEX> was observed at 1530 nm region after the annealing of the film at <TEX>$1050^{circ}C$</TEX> for 5 min. Channel waveguides were fabricated using such films for the core. The films containing Si higher than 2.4 at% exhibited the change in stress from compression to tension after annealing, which induced the fatal loss-increase in waveguide. The optical gain might be attained by the Er-doped waveguide with Si lower than 2.4 at% by a visible-light-excitation.

Optical amplification characteristics of Ti-diffused waveguides on Erbium-doped LiNbO3 crystal

We have first fabricated and characterized Ti-diffused channel waveguides on Er-doped LiNbO3 single crystal. We achieved an optical gain of 2.1dB/cm at 1531.4nm signal light (TM mode) by optical pumping at 1480nm with pump power of 200mW/port for a 40mm long Ti-diffused Z-cut 0.5mol% Er-doped LiNbO3 waveguide. The total gain of a pig tailed device was 3.8dB.

Annealing behaviour of structure and morphology and its effects on the optical gain of Er 3+ /Yb 3+ co-doped Al 2 O 3 planar waveguide amplifier

Using transmission electron microscopy (TEM) and x-ray diffraction analysis, we have studied the structural and morphological evolution of highly Er/Yb co-doped Al2O3 films in the temperature range from 600°C–900°C. By comparison with TEM observation, the annealing behaviours of photoluminescence (PL) emission and optical loss were found to have relation to the structure and morphology. The increase of PL intensity and optical loss above 800°C might result from the crystallization of amorphous Al2O3 films. Based on the study on the structure and morphology, a rate equation propagation model of a multilevel system was used to calculate the optical gains of Er-doped Al2O3 planar waveguide amplifiers involving the variation of PL efficiency and optical loss with annealing temperature. It was found that the amplifiers had an optimized optical gain at the temperature corresponding to the minimum of optical loss, rather than at the temperature corresponding to the maximum of PL efficiency, suggesting that the optical loss is a key factor for determining the optical gain of an Er-doped Al2O3 planar waveguide amplifier.