OPTOELECTRONICS LETTERS Vol.5 No.1, 1 January 2009

Design and fabrication of InP micro-ring resonant detectors 䕯⍋ᯢ **, HUANG Yong-qing 咘∌⏙ , CHEN Hai-bo 䰜⍋⊶ , HUANG Hui 咘䕝 , REN XiaoXIN Hai-ming 䕯⍋ᯢ 咘∌⏙ 䰜⍋⊶ 咘䕝 ਼᯳‫ܝ‬ min ӏᰧᬣ ਼᯳‫ ܝ‬ ӏᰧᬣ , and ZHOU Xing-guang Key Laboratory of Optical Communication and Light wave Technologies, Ministry of Education, Beijing University of Posts and Telecommunications, Beijing 100876, China (Received 26 November 2008) The quantum efficiency and the transient response of the InP semiconductor micro-ring resonant detector are analyzed to get the optimum design parameters. Then the side coupling micro-ring resonant is fabricated using the InP semiconductor material based on the parameters. The micro-ring resonant cavity has the raius of 80 Pm, waveguide width of 3 Pm and the coupler gap of 1 Pm. The test results show that the FSR is 0.75 nm, and the FWHM is 0.5 nm, which are consistent with the theoretical calculation results. 1673-1905(2009)01-0006-5 Document code: A Article ID: DOI 10.1007/s11801-009-8118-7

Micro-ring resonance is compact wavelength-selective element that finds usage in many applications including adddrop filters, demultiplexers, dispersion compensators, modulators. Their small size and functionality make them a key building block in densely-integrated planar light wave circuits[1,2]. Presently it is easy to fabricate the Si micro-ring resonant, but difficult to fabricate the InP micro-ring resonant because of the complexity of after-processing. In 2006, Gholamreza[3] proposed a micro-ring resonant [4] detector, but he did not realize it. In 2007, Cho et al. implemented the micro-ring resonant detector, however, the vertically coupled micro-ring resonator is fabricated with the polymer rather than semiconductor. In this letter, first, the design of the InP/InGaAsP semiconductor micro-ring resonator detector is made, then the side coupling micro-ring resonant detector is fabricated and tested. The scheme of the micro-ring resonant detector proposed by Gholamreza et al. is shown in Fig.1(a) [3]. For simplicity, we consider a PD consisting of a conventional PIN photodiode and a ring waveguide resonator which is coupled to a straight waveguide. Fig.1(b) illustrates the top view of the micro-ring resonant photo-detector (MRPD) and the optical model used in the analysis of quantum efficiency. When a single unidirectional mode of the resonator is excited and the *

The work has been supported by National “973 Program” of China (2003CB314901), the Program for New Century Excellent Talents in University of China (NCET-05-0111 ) , the National “111 Project” (B07005), and the National Program for Changjiang Scholars and Innovative Research Team in University of China (No.IRT0609) ** E-mail: [email protected]

coupling is lossless, the quantum efficiency of the micro-ring detector could be obtained as[3]

Fig.1 (a) Scheme of the micro-ring resonant detector (b) Top view and optical model of the detector

1  t (1  e 2

K

2D Lr 2

t )(1  e D Lr )

1  t 2 e 2D Lr  2te D Lr cos E Lr

,

(1)

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Optoelectron. Lett. Vol.5 No.1 gg

where t is the transmission coefficient, and 1-t2=k2, k is the couple coefficient, D is the material absorption coefficient, O is the wavelength, nr is the refractive index, R is the effective ring radius, E= 2Snr / O is the phase shift coefficient, and Lr ˙ 2SR is the resonator length. When the resonance condition of DLr=2kS and entirely coupling condition of t=exp (DLr) are satisfied, the micro-ring detector has maximum quantum efficiency. From eq.(1), if k and D are fixed, the quantum efficiency can be obtained. The coupling coefficient of micro-ring structure could be written as[5,6] S /2

k

sin[ R

³K S

//

(T ) cos 2 T d T ] ,

(2)

 /2

K // (T )

2J 12x J 22x u E k 02 ( n12  n22 )( 2  J 2 xD )

T

exp[J 2x (d  2Rsin2 )] , 2

(3)

where k is the coupling coefficient, K11 is the coupling coefficient when two waveguides couple parallelly, n1 and n2 are the refractive index of waveguide core and cladding respectively, D is the waveguide width, b is the waveguide height, d is the waveguide spacing, R is the micro-ring radius, Eis the phase shift coefficient, E, J1x and J2x have the relation2 2 2 2 2 2 2 2 2 2 ships of E k0 n1  J 1x  J 1 y and J 2 x k0 n1  k0 n2  J 1x . The simulated results of the coupling coefficient are shown in Fig.2. From the Fig.2, we could conclude that the coupling coefficient between the straight waveguide and micro-ring is decided by waveguide width, height, spacing and micro-ring radius. Fig.2 The dependence of the coupling coefficient on the relative plarameters. (a) waveguide width, (b) waveguide height, (c) waveguide spacing, and (d) micro-ring radius.

Absorption coefficient is analyzed with the Rsoft software and BPM method. For a multi-layer waveguide as shown in Fig.3, an InGaAs absorption layer is between two InGaAsP layers. 1.55 ìm light wave has very low loss when it transmits in InGaAsP, yet suffers a high loss in InGaAsP. The Fig.4 is the waveguide thickness dependence of absorption coefficient in the multilayer waveguide. We could conclude that the absorption coefficient will increase when the height of absorption layer increases, even linearly increases when

gg Optoelectron. Lett. Vol.5 No.1

the height is very small.

In order to analyze the high frequency response of micro-ring detector and improve the design of micro-ring detector, the analysis of transient response characteristic is indispensable. In this letter, the time-domain analysis is used[7]. According to Ref.[4], the forward light field at the coupling point is

Pf

Ef

1 t2

2

D L 2

1  te

Pi .

(4)

So the light field of arbitrary point in the absorption layer Fig.3 The profile of multi-layer dielectric waveguide which has an absorption layer

is

1  t e 1  te 2

P ( x)

Pf e

D x

D x

D L 2

Pi .

(5)

Assuming that f(x)=e-áx, f(x) represents the light field distribution in absorption layer. The light field distribution is related to the position in the micro-ring. It determines the photo-induced carrier distribution and high frequency response of the device. Using the method in Ref.[7], after the impulse signal transmit for time t, the average electron density and hole density in the absorption layer are

Fig.4 The waveguide thickness dependence of the absorption coefficient in the multilayer waveguide

When the coupling coefficient and absorption coefficient are fixed, the quantum efficiency could be obtained. When the resonance condition and entirely coupling condition are satisfied, the quantum efficiency of micro-ring is shown in Fig.5. Here we choose R ˙ 80 Pmˈa ˙ 1.15 Pmˈb ˙ 0.30 Pmˈd=0.40 Pm, according to Fig.2 and Fig.4. From Fig.5, the maximum value of quantum efficiency is 0.73, FSR is about 1.4 nm, FWHM is about 0.40 nm.

n (t )

1ª L N 0 f ( x ) d x º u (W n  t ) , »¼ L «¬ ³0

(6)

p (t )

1ª L N 0 f ( x ) d x º u (W p  t ) , »¼ L «¬ ³0

(7)

where u(t) is the unit speed function, Wn ˙ b / vnˈWp ˙ b / v pˈv n and v p are drift velocity of electron and hole, respectively,Wn and Wp are transit-time of electron and hole in the absorption layer, respectively, N0 is a constant coefficient, and L is the micro-ring perimeter. We need the unit speed function here because when all of the carriers drift to the boundary of absorption layer, there is no more current. When there is junction capacitance in the micro-ring resonant, the impulse response after considering RC is J imp RC ( t )

ep(t )v p u(t ))e

1 RC 

f

³0

( t W ) RC

( en (t ) v n u ( t ) 

u (t  W ) d W ,

(8)

where e is the quantity of electric charge of electron, R is the series resistance, C is the junction capacitance of the device, Fig.5 The quantum efficiency of the micro-ring resonant detector Vs wavelength

C

2SH 0 n1 aL , b

(9)

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Optoelectron. Lett. Vol.5 No.1 gg

where n1 is the refractive index of absorption layer, b is the height of absorption layer. The step response of micro-ring detector could be obtained through convolution of input signal and impulse response, as follows:

Jstep (t) Jimp RC (t)u(t)

f

³

0

JimpRC (W )u(t W ) dW .

(10)

We assume that the material of absorption layer of micro-ring detector is In0.53Ga0.47As, vn and vp are 6.5h104 m/ s and 4.8 h 104 m/s, respectively, the series resistance is 50 ¡. The wavelength of the device is 1553 nm, the width a = 1.15 Pm, the micro-ring radius R=80 Pm and micro-ring perimeter L=2SR. The transient response was simulated and shown in Fig.6 and Fig.7.

Fig.6 The impulse response of micro-ring resonant detector considering junction capacity

eters mentioned before, the high-frequency response bandwidth could be figured out to be about 90 GHz. Based on the analyses above, we designed and fabricated a side coupling micro-ring resonant with small bending losses which is composed of a straight waveguide and a micro-ring, as shown in Fig.8. The brief fabricating process is as follows: First the epitaxial layer structure was grown as shown in Fig.3 using MOCVD. The epitaxial layer structure consists of Pdoped InGaAsP(1 Pm), InGaAs absorption layer (0.15 Pm), and N-doped InGaAsP (0.35 Pm).Then the substrate was photoetched and the waveguide structure was genetated as shown in Fig.8. The waveguide width is 3 Pm, the gap between the straight waveguide and micro-ring is 1 Pm, the radius of micro-ring is 80 Pm. The waveguide didn’t strictly support single-mode operation, but worked in multimode status.

Fig.8 The scanning electron micrograph of micro-ring resonant fabricated

The normalized optical spectrum response of the microring resonant is shown in Fig.9.

Fig.7 The step response of the micro-ring resonant detector considering junction capacity

From the results, we could conclude that the transient response of micro-ring detector is closely related to the series resistance R and junction capacity C. If figuring out the Fourier transform of impulse response, we could obtain the high-frequency response bandwidth. Using the design param-

Fig.9 The optical spectrum response of the micro-ring resonant

In this Figure, FSR is about 0.75 nm, FWHM is about 0.5 nm. The FWHM could be broadened by cascading mi-

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cro-ring resonant so as to reduce the finesse and be practically applied. In summary, this letter analyses the quantum efficiency and transient response of side coupling micro-ring resonant detector. The simulation indicates that for the micro-ring detector whose radius is 80 ìm, FSR is about 1.4 nm, FWHM is about 0.4 nm, the maximum value of quantum efficiency is about 0.73. We fabricated the InGaAs/InGaAsP micro-ring detector and tested it. The test result indicates that for the micro-ring detector whose radius is 80 Pm, FSR is about mm 0.75 nm, FWHM is about 0.5 nm. There are some differences between simulation and experiment, this is because fabricated waveguide is a little wider and light is multimode propagating in the waveguide, this makes the equivalent refractive index increase and leads to the difference. This is the limitation of practically craft and could be fixed by im-

proving craft level. References [1] BIAN Zhi-xi. IEEE Journal of Quantum Electronics, 39 (2003), 859. [2] YANGJian-yi,WANG Fan,JIANG Xiao-qing, Journal of Optoelectronics·Laser, 16 (2005),1157. (in Chinese) [3] Gholamreza Abaeiani, Vahid Ahmadi, and Kamyar Saghafi. IEEE Photonics Technology Letters, 18 (2006), 1597. [4] Cho. Sang-Yeon, J. M. Nan, Appl.Phys.Lett, 90 (2007), 101105. [5] Qin Chen, Yue-De Yang. ICTON, Tu., 5 (2006), 108. [6] Chen Haibo , Huang Yongqing , Huang Hui, Journal of OptoelectronicsgLaser, 18 (2007), 543(in Chinese) [7] Huang Yongqing , Wang Qi, Huang Hui. Semiconductor Optoelectronics, 27 (2006), 148(in Chinese)