ICTON 2010 Mo.P.15

978-1-4244-779 - /10/$26.00 ©2010 IEEE

Single Mode Lasing in MMI Coupled Square

Semiconductor Ring Resonators

Kyung-Sook Hyun

Department of Optical Engineering, Sejong University 98 Kunja-Dong, Kwangjin-Ku, Seoul, 143-747, Republic of Korea

Tel: (82) 3408 3792, Fax: (82) 3408 4337, e-mail: kshyun@sejong.ac.kr

ABSTRACT

This work reports the spectral characteristics of coupled-square ring semiconductor resonators. A simple square cavity showed complex peak lasing patterns with a broad envelope. For a single mode operation, the square ring cavities coupled with multimode interferometers are proposed and fabricated using the epitaxial layers of 1.55 µm center wavelength InGaAsP-InP multiple quantum wells. A stable single spectral lasing mode was obtained in the proposed multimode interferometer square cavities and the lasing characteristics with power and temperature were also presented. Keywords: square ring, coupled-square ring resonator, single mode lasing, InGaAsP-InP multiple quantum well,

multimode interferometer.

1. INTRODUCTION

Circular micro cavities, such as disk or ring-type resonators, are versatile components that can perform filtering, switching, routing and modulating in integrated photonic circuits [1, 2]. Accordingly, devices using circular resonators have been widely studied and realized to date in several different technologies in spite of some fabrication difficulties[3,4]. In most semiconductor resonators, the radius should be many times of the operating wavelength to attain high Q factors. In the case of using these resonators, the multimode operation is associated and the closely spaced resonances are inevitably obtained. However, the narrow free spectral range (FSR) of the output spectra is not desirable in most photonic device applications, such as passive filters and laser cavities. Thus, achieving a moderate FSR or a single mode operation in an optical resonator is essential, to be used as an integrated optical component.

In this work, the coupled resonator is focused, which enables one to expand FSR of the output spectra considerably. The two coupled-resonator cavity using Vernier effect can be a commonly accepted method to get single mode lasing or wide FSR in devices. Usually, the coupling types can be classified into two types, such as a vertical coupling and a side coupling. Regardless of the coupling type, the interaction between cavities is accomplished by evanescent electric field coupling, which is associated with exponential decrease of the interaction strength as the gap increases.

However, the ring or disk-type resonator requires very small gap (shorter than the operating wavelength) between the cavities to get strong interaction because those have very short coupling length. The small gap distance between the cavity and the waveguide can be obtained by a vertical coupling. However, it needs multiple layers which have different refractive indices and intricate fabrication such as epitaxial regrowth process. On the other hand, the side coupling has advantages in simple epitaxial layer structures. But, it also has difficulties in achieving sub micrometer-gap by conventional contact photo lithography. The very small gap distance between the cavities in semiconductors can be obtained with complicated fabrication processes such as e-beam lithography in side coupling or a regrowth process in vertical coupling structures.

For the various applications of the resonators, the previously mentioned difficulties should be resolved. The square cavities are focused on because it substantially enhances the interaction length between the cavities. As a candidate, a multimode interferometer (MMI) is proposed to couple the square ring resonator, which is easy to establish the coupling between resonators.

2. MMI COUPLED SQUARE RING CAVITY

Before the study of coupled ring square cavity, the single square ring cavity was fabricated and characterized shown in Fig. 1. The square ring cavity has 45º-tilted flat corners. When the corners are symmetrically 45º-tilted, closed orbits, formed by total reflections with an incident angle 45º at the corner, are possible. The lasing spectrum of the single square resonator showed multi mode and many closely spaced resonance peaks of 4 nm as expected.

It is desirable to have a specific operating wavelength for the cavities, to be used as a wavelength filter or an active device such as a laser. One of the possible methods to obtain a large free spectral range is to couple the cavities. The difficulty of coupling is resolved using a MMI coupled square ring structure.

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Figure 1. Typical lasing spectrum of the single square ring cavity.

Figure 2 presents the schematic diagrams of the proposed structures. The large cavity and small cavity were named as Cavity 1 and Cavity 2, respectively. The two square cavities share its side with each other. Thus, the coupled waveguide width is twice the square waveguide width and can be treated as an MMI. When the light enters a coupled waveguide region, it suffers abrupt waveguide broadening. Finally, the light intensities are divided and coupled separately into Cavity 1 and Cavity 2. As a result, the lasing wavelength is determined by the common resonance of Cavity 1 and Cavity 2.

w

MMI

Figure 2. Schematic diagram of an MMI coupled square ring resonators.

In this work, the length of Cavity 1 is fixed as 50 µm and Cavity 2 is 20 µm. The waveguide width has been tried

4 µm. By varying the cavity lengths and the coupling types, diverse cavity structures are fabricated to investigate the resonance characteristics of the MMI coupled-square cavities. From the beam propagation calculation, the coupling efficiencies between adjacent cavities calculated to 0.24.

For the calculations of the resonant wavelengths, the optical path length L can be given as follows:

( )4L m d w= − , (1)

where m is the effective refractive index of the semiconductor materials and d and w is the length of the cavity and waveguide width. In addition, the resonance condition is given by:

( )nL m l nλ λ= = , (2)

where n is an integer and l is the round trip length. Based on above equation (1) and equation (2), the adjacent

resonant mode wavelength spacing (∆λ = λn-1 – λn) can be derived as follows:

2 2

', where ' 1 .

1

dmm m

dm d mm lml

d m

λ λ λλλ λ

λ

∆ = = = −−

(3)

These Fabry-Perot like resonant equations are valid when the cavity sizes are much larger than the operating wavelength. For an exact calculation of mode spacing, FDTD (finite-difference time domain) method is required, in particular for small cavities with the wavelength comparable dimensions. Using the above Eq.(1), Eq.(2) and

Eq.(3), the FSR, which is equal to adjacent resonant wavelength spacing, was calculated as 3.28 nm for 50 µm

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and 8.20 nm for 20 µm single square ring cavities. The parameter m' was estimated 3.55 at 1544 nm depending on the work of single square cavities, which were fabricated on the same epitaxial layer structure.

2.1 Semiconductor Epitaxial Layers and Measurements

InP/InGaAsP based semiconductor epitaxial layers were chosen for the proposed resonator fabrication. The active layers consisted of 6 layers of undoped InGaAsP/InGaAs MQW (multiple quantum well)s with

a center wavelength, λg = 1.55 µm and are sandwiched by n- and p-type InGaAsP confinement layers; p-InP on active layers and n-InP buffer on InP substrate were used as cladding layers.

The devices are fabricated by a conventional III-V compound semiconductor process. Particularly, the elaborate etching process using CH4/H2 reactive ion etching is needed to obtain the refined mesa surface of

a device. In addition, the etch depths are controlled exceeding 4.2 µm to laterally confine the travelling wave and to avoid radiation losses into the substrate.

As a measurement of the cavity functions, optical pumping was used. An Nd:YVO4 laser, operating

wavelength of 1.06 µm, is adapted to optically pump MQWs. With this wavelength, most of the pump light penetrated the clad layer and was strongly absorbed by the MQW layers. To minimize the thermal variations of the refractive indices in semiconductor, the laser was acousto-optic (AO) Q-switched (10 kHz) for reducing average pump power. The pulse width of the pump beam was measured at approximately 300 ns and was illuminated vertically by a lens. For uniform illumination upon the coupled cavity, the pump beam was loosely

focused and the beam diameter was measured at ~500 µm on semiconductor wafer. The pump beam was

identified by CCD camera made of Si, which can detect 1.06 µm. The lasing signal from the coupled-cavity was collected through a tapered fiber launched around the cavity corner and its spectrum was measured by an optical spectrum analyzer (HP 81818)

2.2 Single mode lasing characteristics

Figure 3 shows the microphotograph of the fabricated MMI-coupled square cavity and its lasing characteristics by pulsed optical pumping. Logarithmic values of the lasing intensities are also shown in the inset.

The waveguide width is 4 µm and the cavity lengths are 50 µm and 20 µm. The center wavelength was measured 1543.7, which is in the range of the PL spectrum of the MQW. The lasing spectra of the MMI coupled cavity shown in Fig. 3 represent single mode operation with full width at half maximum (FWHM) of 1.30 nm and side mode suppression ratio (SMSR) of 20.24 dB. From these results, single mode operations can be obtained in the MMI coupled square cavity.

Figure 3. Microphotograph and lasing spectrum of the inside MMI coupled square ring resonator.

2.3 Lasing Characteristics According to Input Power

The devices with various combinations of cavity sizes and waveguide width are fabricated to observe the resonant characteristics and the stability of the single mode. Single mode operations can be found on most of the devices. The center wavelength of the resonating mode is distributed from 1538.00 nm to 1563.56 nm, which corresponds to the range of photoluminescence spectra of the epitaxial InGaAsP MQW structures. From these results, the center wavelength can be tunable by the combination of Cavity 1, Cavity 2 and waveguide thickness. In addition, one of the important characteristics of resonator is the full width at half maximum (FWHM), which is directly related to the Q factor of the resonators. Hence, the value of FWHM is calculated as 1.30 nm and the

quality factors are estimated to be ~1 .2×103, which is enough to be used as a communication device.

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The improved value of the Q factor founded in this work compared to single resonator is due to the Vernier configuration effect.

It is well known that the Q factors of the microcavities are degraded as the pumping power increases. To obtain the Fig. 4, the input power level is increased several times larger than the threshold average input power level for each measurement step. The threshold of the MMI coupled cavity is measured ~2 mW. The lasing spectrum at the threshold input power showed narrow FWHM with poor SMSR in MMI coupled square cavities. By increasing the pump power up to several tens of mW, the lasing peak power is rapidly increased and the single mode lasing is maintained with an enhanced SMSR of ~20 dB represented as solid line in Fig. 4. Beyond this power level, the Q factors are also degraded in MMI coupled cavities. As a result, the quality factors were decreased as expected due to de-tuning effect.

Figure 4. The lasing spectra of the MMI coupled square ring resonator by increasing the optical input power.

3. CONCLUSIONS

Design and characteristics of the MMI coupled square resonators were demonstrated using InGaAsP MQW semiconductor layers. The single mode lasing was stably obtained and the lasing center wavelength was controlled by the combinations of Cavity 1, Cavity 2 and waveguide widths. In addition, the single mode lasing was maintained for the wide range of input optical power. From the experimental results, it was concluded that single mode lasing in MMI coupled square resonators can be obtained by relatively simple fabrication processes and the resonators can also be applied to any integrated passive or active building blocks in semiconductor devices.

ACKNOWLEDGEMENTS

This work was supported by the Korea Science and Engineering Foundation (KOSEF) Grant Funded by the Korean Government (R01-2007-000-21036-0).

REFERENCES

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[2] S. Xiao et al.: Multiple-channel silicon micro-resonator based filters for WDM applications, Opt. Express,

vol. 15. pp. 7489-7498, Jun. 2007. [3] K. Djordjev et al.: High-Q vertically coupled InP microdisk resonators, IEEE Photonics Tech. Lett., vol. 14.

pp. 331-333, Mar. 2002. [4] W. M. J. Green et al.: Hybrid InGaAsP-InP Mach-Zehnder racetrack resonator for thermooptic switching

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