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VC 2011 Wiley Periodicals, Inc.

A STUDY OF ERBIUM–YTTERBIUM-CODOPED POLYMER WAVEGUIDEAMPLIFIER

Dan Zhang,1,2 Daming Zhang,2 Xu Liang,3 Fei Wang,2

and Donghui Guo1

1 Department of Electronic Engineering, Xiamen University, Xiamen361005, China; Corresponding author: 6504912@qq.com2 State Key Laboratory on Integrated Optoelectronics, College ofElectronic Science and Engineering, Jilin University, Changchun130012, China3 Xiamen Microelectronic Integrated Technology R&D Center,Mobile, AL, USA

Received 6 December 2010

ABSTRACT: In this article, organic/inorganic hybrid matrix dopedwith Er3þ,Yb3þ nanoparticles was synthesized and used as the corematerial to fabricate the waveguide amplifier. The absorption and

luminescence spectra of the material were characterized. An X-rayphotoelectron spectroscopy analysis was performed to characterize the

changes on the surface elemental composition of the material in thereactive ion etching processes. An embedded channel waveguides basedon this material were demonstrated. Optical gains of 2.3 and 3.5 dB/cm

at 1534 and 1550 nm wavelengths, respectively, were obtained indifferent dimensions of waveguides. VC 2011 Wiley Periodicals, Inc.

Microwave Opt Technol Lett 53:2157–2160, 2011; View this article

online at wileyonlinelibrary.com. DOI 10.1002/mop.26193

Key words: optical gain; polymer; erbium-doped waveguide amplifier

1. INTRODUCTION

In recent years, rare earth-doped polymer waveguide amplifiers

have attracted much attention in dense wavelength division mul-

tiplexed systems [1–7]. They can be used to compensate various

losses at the optical telecommunication windows because of

their near-infrared emissions. Polymer materials [8–10] are

promising candidates as hosts of rare earth ions because of their

ease of fabrication, compatibility with other materials, and low

optical loss in the near-infrared wavelength range. However, the

bottlenecks are the insolubility of rare earth ions in polymers

and the luminescent quenching from CAH and OAH bonds in

polymer. These problems can be overcome by encapsulating

rare earth ion with organic ligands [11] and doping them into a

organic/inorganic hybrid matrix host.

In this work, Er3þ,Yb3þ-codoped nanoparticles with organic

ligands were synthesized. The nanoparticles were doped in or-

ganic/inorganic hybrid matrix as the core material to fabricate

the waveguide amplifier. The absorption and photoluminescence

spectra were characterized. Optical gains at 1534 and 1550 nm

wavelengths were demonstrated. The fabrication method of

waveguides was studied.

2. FABRICATION AND CHARACTERIZATION OF POLYMERCHANNEL WAVEGUIDES

2.1. MaterialsOleic acid, ErCl3�6H2O (99.99%), YbCl3�6H2O (99.99%), and

LaCl3�7H2O (99.99%) were used to prepare the oleic acid-modi-

fied LaF3:Er,Yb nanoparticles according to the method of the lit-

erature [12]. The molar ratio of LaF3:Er,Yb was 15:1:4. LaF3

was an ideal host of lanthanide elements because of its low pho-

non energy, thus the nonradiative loss of the excited state of the

lanthanide ions could be minimal [13]. The organic/inorganic

hybrid matrix, of which the main components were metacrylo-

propyltrimethoxysilane (Aldrich, 98%) and zirconium tetraiso-

propoxide (Aldrich, 70 wt.% in propanol) [14], was selected

as a host of nanoparticles. (Diphenylphosphoryl)(mesityl)

Figure 1 The absorption spectrum of the nanoparticles powder

Figure 2 The room-temperature fluorescence spectrum of Er3þ in

LaF3:Er,Yb nanoparticle-doped organic/inorganic hybrid matrix, excited

at 976 nm with 90 mW pump power

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 9, September 2011 2157

methanone was added as photoinitiator to perform the photopo-

lymerization. The concentration of nanoparticles in organic/inor-

ganic hybrid matrix could be up to 50 wt.%. The refractive

index of the organic/inorganic hybrid matrix doped with

LaF3:Er,Yb nanoparticles was 1.512 at 1534 nm wavelength.

2.2. Spectroscopic CharacterizationFigure 1 shows the absorption spectrum of the nanoparticles

powder. The spectrum consisted of seven absorption bands, cor-

responding to the transitions from the ground state 4I15/2 to the

excited states 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, and 4F7/2.

The exploitation of the efficient energy transfer from Yb3þ to

Er3þ ions and the strong pump absorption from Yb3þ ions were

key issues to achieve good performance. Figure 2 shows the

room-temperature fluorescence spectrum for the 4I13/2 to 4I15/2

transition of Er3þ in LaF3:Er,Yb nanoparticle-doped organic/inor-

ganic hybrid matrix, excited at 976 nm with 90 mW pump power.

2.3. XPS for the FilmTo obtain sufficient amplification in the centimeter-long device,

high concentration of nanoparticles was required. However, inor-

ganic components, which largely existed in the material, made it

difficult to directly etch waveguides with rib or rectangular core

cross-section. An X-ray photoelectron spectroscopy (XPS) anal-

ysis was used to characterize the changes on the surface elemen-

tal composition of the sample in the RIE processes. One sample

was prepared by spin coating the core material onto silicon wa-

fer to form a 1.5-lm thick film. Then the film was etched by

SF6/O2 gas. Table 1 shows the elemental composition of the

sample’s surface before and after etch for 30 min. The binding

energy peaks of lanthanum (La3d; 851.9, 835.9 eV), fluorine

(F1S; 684.8 eV), oxygen (O1S; 531.5 eV), and carbon (C1S; 284.5

eV) elements were obtained before and after etch. The fraction of

La remained almost unchanged, while the O and C content

decreased and the F content increased after etch. The O and C

elements existed as silicon oxide and CAH bonds were formed in

the core material. They could react with the chemically reactive

gases SF6 and O2, respectively. Therefore, their concentrations

decreased after etch. The small quantity of O and C elements was

mainly because of the gas surface absorption when the sample

was exposed into air. The high concentration of F element mainly

resulted from the gas surface absorption when the sample was

etched by SF6. As the contents of Zr and Si elements were low

in the material, they were not reflected in the Table 1.

According to the XPS analysis, we conclude that when the

sample is sufficiently etched by chemically reactive gases, the

main remainder of this material is LaF3. For the sample with

high content of nanoparticles, the area density of LaF3 is large.

It is very difficult to directly etch waveguides with rib or rectan-

gular core cross-section. Therefore, the embedded waveguides

were designed and fabricated instead.

2.4. Fabrication ProcedureThe organic/inorganic hybrid matrix doped with LaF3:Er,Yb

nanoparticles was used as the core material for the waveguide.

A polymethylmethacrylate-glyciclyl-methacrylate (PMMA-

GMA) with a 10-lm thickness was first spin coated onto a Si sub-

strate to form the bottom cladding layer. Grooves were fabricated

in PMMA-GMA by standard photolithography and then were filled

by the core material. A 4-lm-thin PMMA-GMA was finally spin

coated onto the core to form the top cladding. The refractive index

TABLE 1 Composition and Elemental Loss Determined byXPS for the Sample, Before and After 30 min RIE Process

Element Eb (eV)

Atomic % of Elements

Before After

C1S 284.5 44 12

O1S 531.5 24 11

F1S 684.8 24 67

La3d 851.9, 835.9 8 10

Figure 3 A schematic of the channel waveguide structure

Figure 4 A schematic of the experimental setup for the optical gain measurement. [Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com]

2158 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 9, September 2011 DOI 10.1002/mop

of cladding was 1.489 at 1534 nm wavelength. A schematic of the

channel waveguide structure was given in Figure 3.

2.5. Measurement of GainWe used a tunable laser source operating between 1510 and

1590 nm (Santec TSL-210) as the signal source and a 976 nm

laser diode as the pump source. They were coupled into the

channel waveguides using a 980/1550 nm wavelength-division

multiplexing fiber coupler. An optical spectrum analyzer (Ando

AQ-6315A) was used to receive the output signal. Figure 4

shows a schematic of the experimental setup for the optical gain

measurement. Figure 5 displayed a 4.5 dB relative gain at 1534

nm wavelength measured with a cross-section of 8 � 6 lm2

waveguide in a 2-cm-long device. The launched pump power was

128 mW, and the input signal power was 0.12 mW. Because of

the 80-nm wide FWHM of the luminescence spectrum, the gain

could also be observed at 1550 nm wavelength. Figure 6 shows

the output power at 1550 nm wavelength as a function of pump

power. The input signal power was varied from 0.05 to 0.5 mW.

For the fixed input signal power, the gain gradually increased

with the increment of pump power and then followed by gain sat-

uration. At 120 mW pump power and 0.05 mW signal power, a

maximum gain of 3.5 dB/cm was obtained in a cross-section of

12 � 8 lm2 waveguide, as shown in Figure 6.

3. CONCLUSIONS

In conclusion, organic/inorganic hybrid matrix doped with

Er3þ,Yb3þ nanoparticles was synthesized and used as the core

material to fabricate the waveguide amplifier. The absorption

and photoluminescence spectra of the material were measured.

The XPS analysis was performed to characterize the changes on

the surface elemental composition of the core in the RIE proc-

esses. The embedded waveguides were designed and demon-

strated. When the pump power was 128 mW and the input sig-

nal power was 0.12 mW, an optical gain of 4.5 dB at 1534 nm

wavelength was obtained with a cross-section of 8 � 6 lm2

waveguide. Because of the wide bandwidth of the gain spec-

trum, a maximum gain of 3.5 dB/cm was obtained in a cross-

section of 12 � 8 lm2 waveguide at 1550 nm wavelength.

ACKNOWLEDGMENTS

This work is supported by Fujian Provincial Natural Science Foun-

dation (Nos: 2009J05157 and 2009H0043), National Natural Sci-

ence Foundation of China (No: 60807029), Program for New

Century Excellent Talents of Xiamen University.

REFERENCES

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A. Barberis, and S.M. Pietralunga, Demonstration of net gain at

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Figure 6 The output power at 1550 nm wavelength as a function of

pump power (waveguide length: 20 cm, cross-section: 12 � 8 lm2).

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com]

Figure 5 A 4.5 dB optical gain at 1534 nm wavelength measured with a cross-section of 8 � 6 lm2 waveguide in a 2-cm-long device. [Color figure

can be viewed in the online issue, which is available at wileyonlinelibrary.com]

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nel waveguide amplifiers, Opt Lett 34 (2009) 473–475.

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properties of planar polymer waveguides doped with organo-lantha-

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hyay, Effect of symmetric substitution on the phenyl groups of

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ties, J Lumin 130 (2010) 1952–1957.

9. L. Guo, B. Yan, and Y. Li, Photoactive ternary rare earth complex

hybrids with sulfoxide functionalized silica and PMMA(or Phen),

Photochem Photobiol 86 (2010), 813–820.

10. S.Y. Cheng, K.S. Chiang and H.P. Chan, Polarization-insensitive

polymer waveguide bragg fratings, Microw Opt Technol Lett 48

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11. J. Yang, M.B.J. Diemeer, G. Sengo, M. Ploonau, and A. Driessen,

Nd-doped polymer waveguide amplifiers, IEEE J Quant Electron

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VC 2011 Wiley Periodicals, Inc.

FET FREQUENCY DOUBLER WITH OUT-OF-PHASE SWITCHABLE OUTPUT ANDAPPLICATION IN BALANCEDFREQUENCY DOUBLER

Ning Yang, Christophe Caloz, and Ke WuPoly-Grames Research Center, Ecole Polytechnique de Montreal,2500 Chemin Polytechnique, Montreal, Quebec, Canada H3T 1J4;Corresponding author: n.yang@polymtl.ca

Received 9 December 2010

ABSTRACT: A field effect transistor (FET) frequency doubler, with 180�

out-of-phase alternate output by switching the bias status of FET at pinch-

off (Vgg � Vt) or saturation (Vgg � 0) is proposed. With such biasconditions, the former conducts in the positive half-cycle of the input

signal, while the latter conducts in the negative half-cycle. Based on thisconcept, a new frequency doubler architecture is proposed. Without aninput balun, it is more compact than conventional balanced frequency

doubler. In addition, it provides an alternate doubler with differentialoutput. To demonstrate this concept, a single-ended 5–10 GHz doublertested at the two bias conditions is designed and demonstrated

experimentally to provide opposite-phase outputs for the two cases. Theoutput phase difference error is less than 4� and over 42-dB fundamental

frequency rejection is achieved. The conversion gain measured at 10 GHzis 2.5 dB for an input power of 5 dBm. VC 2011 Wiley Periodicals, Inc.

Microwave Opt Technol Lett 53:2160–2164, 2011; View this article online

at wileyonlinelibrary.com. DOI 10.1002/mop.26172

Key words: frequency doubler; balanced doubler; pinch-off and

saturation biasing; FET

1. INTRODUCTION

Millimeter-wave wireless communication and radar systems

require increasingly high-quality millimeter-wave sources. Fre-

quency multipliers are widely used in such high-frequency appli-

cations to extend the upper frequency limit of sources using

fixed or variable frequency oscillators. Indeed, the use of fre-

quency doublers cascaded with lower frequency oscillators to

form millimeter-wave sources provides more reliability and lower

phase noise than the direct design of millimeter-wave frequency

oscillators. Compared with the doublers using p-n junction varac-

tors, it is more attractive to use FETs as frequency multipliers as

they are easy process in microwave monolithic integrated circuit

(MMIC) technology, and because they can provide conversion

gain over a broad frequency band and inherent isolation between

the input and output ports [1].

However, the design of single-ended frequency doubler

requires a filter or a quarter-wavelength open-circuited stub to

effectively suppress the fundamental frequency energy present at

the multiplier’s output. In microwave integrated circuit design,

this requires significant chip area and also limits the bandwidth.

Therefore, a balanced structure is mostly used in actual designs,

which drives 180� out-of-phase inputs to two single-ended dou-

blers. Without the need of filters or resonators in the output, the

balanced topology can suppress the fundamental frequency and

other odd-order harmonic components automatically in a broad

bandwidth, while combining the required second harmonic in-

phase at the output port. Another advantage of the balanced

design is higher output power due to the ‘‘push–pull’’ operation

of the FET. However, due to the large area occupied by the

input balun or hybrid, balanced doublers often need large area.

This increases the cost, which represents a major application ob-

stacle of a balanced frequency doubler. A miniature broad-band

balanced frequency doubler comprised of a reduced-size 180�

rat-race hybrid and two distributed doublers to form a balanced

doubler configuration has been reported in Ref. 2. Another mini-

aturized balanced frequency doubler combining common gate

(CG) and common source (CS) FETs has been proposed in Ref.

3. However, the phase distance error at the harmonic output is

relatively high (20�), which limits the output gain. In Ref. 4, a

wideband CG/CS active balun is placed before the balanced

FET doubler as an integrated MMIC.

This article proposes a doubler with switched output with

180� out-of-phase. The switching is performed by changing the

gate bias voltage between the pinch-off (Vgg � Vt) and satura-

tion (Vgg � 0) states. Then, a balanced frequency doubler is pro-

posed with one FET biased at pinch-off and another FET at sat-

uration. The two bias conditions conduct the FET in opposite

cycles of the input signal alternately, and provide almost identi-

cal nonlinearity and transducer gain. When combined to form a

balanced frequency doubler, an input balun is not required in

contrast to traditional balanced frequency doublers. Besides, the

benefit of a balanced doubler is kept, i.e., suppression of funda-

mental frequency in a wideband without the needs of output fil-

ters. In addition, it provides an alternate balanced doubler with

differential output. This is beneficial to direct integration with

differential components, such as differential amplifier pairs, or

dipole antennas. To validate the concept, a single-ended doubler

with alternate 180� distance output phase is designed and meas-

ured, where the phase is controlled by the gate biasing circuit.

The measured output phase difference error is less than 4� and

the fundamental frequency rejection is higher than 42 dB. The

2160 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 9, September 2011 DOI 10.1002/mop