Open Access
Add to BibSonomyAbstract
This work reports the spectral characteristics of coupled square ring semiconductor resonators. For a single mode operation, the square ring cavities coupled with multimode interferometers (MMIs) are proposed and fabricated using the epitaxial layers of 1.55 μm center wavelength InGaAsP-InP multiple quantum wells. Resonant characteristics can be tailored by varying the parameters of the cavity size or the waveguide width. By using the MMI coupled square ring cavity, a stable single spectral lasing mode was obtained in various combinations of square cavities.
©2010 Optical Society of America
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 [1–5]. Accordingly, devices using circular resonators have been widely studied and realized to date in several different technologies in spite of some fabrication difficulties [6–11]. In most semiconductor resonators, the radius should be many times the operating wavelength to attain high quality factors(Qs). In the case of using these resonators, the multimode operation is associated and the closely spaced resonances are inevitably obtained. However, the large free spectral range (FSR) of the output spectra is desirable in most photonic device applications, such as passive filters and laser cavities. Therefore, achieving an expanded FSR or a single mode operation in an optical resonator is essential, to be used as an integrated optical component.
In this study, the coupled resonator is focused because it enables one to expand FSR considerably. A coupled-resonator cavity using the Vernier effect is a commonly accepted method to get single mode lasing or large FSR in devices [10]. Usually, coupling types can be classified into two types, such as a side coupling [12-14] and a vertical coupling [6,15,16]. Regardless of the coupling type, the interaction between cavities is accomplished by evanescent electric field coupling, which is associated with an exponential decrease of the interaction strength as the gap increases. Hence, the ring or disk-type resonator requires a very small gap (shorter than the operating wavelength) between the cavities to get a strong interaction. This is because they have a very short coupling strength with simple conventional fabrication method.
Using a vertical coupling, the small gap distance between the cavity and the waveguide can be more easily obtained rather than using a side coupling. However, it needs multiple layers which have different refractive indices employing epitaxial regrowth process. On the other hand, the side coupling has advantages in simple epitaxial layer structures. Nevertheless, 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 a regrowth process in vertical coupling or e-beam lithography in side coupling structures.
In this work, the square ring cavities are focused on because it substantially enhances the interaction length between the cavities compared to the circular ring cavities and simplifies the fabrication processes. Besides, the square cavities can provide more efficient cavity-to-straight-waveguide coupling than circular resonators [17,18]. Relying on the previous results of the single square resonator [19], the square resonator has a small Q factor and many closely spaced resonance peaks which are expected. For the actual applications of the resonators, the previously mentioned difficulties, such as weak coupling between cavities and fabrication difficulties, should be resolved. The difficulty of coupling can be solved by using an MMI coupled square ring structure, which consists of two square rings with different cavity sizes. The MMI is proposed to couple directly the two square ring resonators, which could provide simple fabrication processes, improved resonator characteristics of large FSR and an increased Q factor.
2. MMI coupled square ring resonators
Figures 1(a) and 1(b) show the schematic diagrams of the proposed structures. The hollow square cavities have 45°-tilted flat corners. When the corners are symmetrically 45° -tilted, closed orbits, formed by total reflections with an incident angle 45° at the corner (so called ring type WGMs) are possible. The large cavity and small cavity were named as Cavity 1 and Cavity 2, respectively. The two square cavities are directly coupled by attaching the sides as shown in Fig. 1. 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.
Fig. 1 (a) Schematic diagram of an outside MMI coupled square ring resonators. (b) Schematic diagram of an inside MMI coupled square ring resonators.
In this work, the length of Cavity 1 is fixed as 100 μm and Cavity 2 is varied from 30 μm to 45 μm. The waveguide width has been tried in three cases: 3 μm, 4 μm and 8 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 vary from 0.02 to 0.24 depending on waveguide width and MMI lengths.
For the calculations of the resonant wavelengths, the optical path length L can be given as follows:
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:where n is an integer and l is the round trip length. Based on above Eq. (1) and Eq. (2), the adjacent resonant mode wavelength spacing (Δλ = λn-1 – λn) can be derived as follows: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 1.64 nm for 100 μm and 5.49 nm for 30 μm single square ring cavities. The parameter m' was estimated 3.55 at 1544 nm depending on the previous work of single square cavities [19], which were fabricated on the same epitaxial layer structure.3. Experimental results and discussions
InP/InGaAsP based semiconductor epitaxial layers were chosen for the proposed resonator fabrication. The detailed epitaxial layer structure used for the coupled-square ring cavities are shown in Fig. 2 . 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. According to the data provided, the photoluminescence spectral linewidth of the MQW epi layers is 54 meV with a center wavelength of 1550 nm.
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).
Figure 3 and Fig. 4 show 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. Figure 3 shows the outside coupled-square cavity and its lasing spectrum. The waveguide width is 4 μm and the cavity lengths are 100 μm and 30 μm. The center wavelength was measured 1558.35, 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 0.40 nm and side mode suppression ratio (SMSR) of 20.2 dB. The inside coupled square cavity has the same structure parameters of the outside coupled cavity except the location of Cavity 2, which is inside of Cavity 1 as shown in Fig. 4. The inside coupled cavity also exhibits single mode operations at the center wavelength of 1561.94 nm with 0.37 nm FWHM and 20.3 dB SMSR. From these results, single mode operations can be obtained in the MMI coupled square cavity regardless of whether it is an inside coupled or outside coupled type.
We also checked the mode degeneracy by fabrication defect in the MMI coupled cavity, such as clockwise and counterclockwise mode, by changing the fiber launching direction. Apparent change in the lasing wavelength was not observed in this experiment though the lasing intensities were usually different each other.
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 corresponding characteristics of resonators with different device parameters are summarized in Table 1 . 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. It is apparent that FWHM slightly increases as the waveguide thickness increases. Hence, the average values of FWHM are calculated as 0.28 nm, 0.41 nm and 0.43 nm for the resonator waveguide width of 3 μm, 4 μm to 8 μm, respectively. This is attributed to the wide reflection area for the thick waveguide device. The area of the reflection mirror surface increases directly proportional to the waveguide width. Increase of the mirror surface area could raise the probability of irregular reflection, which results in the reduction of Q factor. Depending on the data shown in Table 1, the quality factors are estimated to be ~3x103 to ~8x103, which is enough to be used as a communication device. The improved value of the Q factor founded in this work compared to single resonator [19] is due to the Vernier configuration effect.
Table 1. Lasing characteristics of various types of MMI coupled square ring resonators
It is well known that the Q factors of the microcavities are degraded as the pumping power increases. To obtain the Fig. 3 and Fig. 4, the input power level is increased about ten times larger than the threshold average input power level of the MMI coupled cavity which 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.
Beyond the power level used to obtain Fig. 3 or Fig. 4, the Q factors are also degraded in MMI coupled cavities. To observe the lasing characteristics by the high pumping level, the optical input power level in increased 2, 4, and 8 times larger than that used in Fig. 3 or Fig. 4. As a result, the quality factors were decreased as expected and even multimode lasing due to single resonator mode is detected due to de-tuning effect. The effect of out of tune between two cavities was also found from some configuration of the cavity parameters even at a lower pumping power as simultaneous lasing of multiple side modes caused by single square ring cavity resonance with a wider FWHM. Under the continuous wave excitation without AO-Q switching, we cannot obtain lasing spectrum possibly due to local heating effect on the MQW region.
Table 1 shows that the amount of change in lasing center wavelengths with the Cavity 2 length, is large for the devices with a narrow waveguide width. As the Cavity 2 length changes from 30 μm to 45 μm, the lasing wavelength is shifted about 25.56 nm from 1563.56 to 1538.00 nm for the 3 μm-waveguide outside coupled devices, which is larger than the changes of 10.89 nm for 4 μm-waveguide devices or 3.09 nm for 8 μm-waveguide devices. This suggests that the wide tuning of the lasing wavelength of the MMI coupled cavity is provided in the narrow waveguide width devices. From the above results, the center wavelength is widely adjustable by the various combinations of the cavity size. Moreover, the MMI coupled square cavity scheme is very useful in order to get the single mode lasing.
Thermal dependence of the MMI coupled cavity was also measured by changing the substrate temperature using thermo electric cooler for the outside coupled 100-45-4 device. As illustrated in Fig. 5 , the lasing peak was shifted to 1.25 nm longer wavelengths when the ambient temperature increased from 20 °C to 35 °C. Usually, a very weak temperature dependence of ~0.1 nm/°C are reported in distributed feedback(DFB) laser devices compared to 0.5~1 nm/ °C of Fabry-Perot(FP) laser diode [20,21]. This is due to the existence of a wavelength selection mechanism in DFB laser diode. Within the measured temperature range, the lasing wavelength shift in MMI coupled square resonator exhibits 0.08 nm/°C with a stable single mode operation under the pulsed measurement condition. This result also supports that wavelength filtering mechanism worked well in the proposed resonator and the weak temperature dependence of the MMI coupled resonator raises the possibility which will then be used as an element of an integrated optical circuit.
Fig. 5 Thermal dependence of the MMI coupled cavity by changing the substrate temperature and the corresponding lasing spectrum for the outside coupled 100-45-4 device
4. Conclusion
Design and characteristics of the MMI coupled square resonators were demonstrated using InGaAsP MQW semiconductor layers. With various combinations of square resonators, single mode lasing was stably obtained and the lasing center wavelength was controlled by the combinations of Cavity 1, Cavity 2 and waveguide widths. The resonator performances, such as wide tuning of lasing wavelength and reasonable Q factor for applications, were good when the waveguide width is narrow. In addition, the temperature dependent lasing wavelength showed a rate of shift about 0.08 nm/°C, which is weak enough to be applicable as an integrated element. 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 Research Foundation Grant Funded by the Korean Government (MOEHRD) (KRF-2006-531-D00018) and Korea Science and Engineering Foundation (KOSEF) Grant Funded by the Korean Government (R01-2007-000-21036-0).
References and links
1. S. Suzuki, Y. Kokubun, M. Nakazawa, T. Yamamoto, and S. T Chu,“Ultrashort optical pulse transmission characteristics of vertically coupled microring resonator add/drop filter,” J. Lightwave Technol. 19(2), 266–271 (2001). [CrossRef]
2. S. L. McCall, A. F. J. Levi, R. E. Slusher, S. J. Perton, and R. A. Logan, “Whispering gallery mode micro disk lasers,” Appl. Phys. Lett. 60(3), 289–291 (1992). [CrossRef]
3. T. Baba, “Photonics crystals and microdisk cavities based on GaInAsP-InP system,” IEEE J. Sel. Top. Quantum Electron. 3(3), 808–830 (1997). [CrossRef]
4. S. Xiao, M. H. Khan, H. Shen, and M. Qi, “Multiple-channel silicon micro-resonator based filters for WDM applications,” Opt. Express 15(12), 7489–7498 (2007). [CrossRef] [PubMed]
5. D. G. Rabus, Integrated Ring Resonators:The compendium, Springer series in optical sciences (Springer, 2007)
6. K. Djordjev, S. J. Choi, S. J. Choi, and P. D. Dapkus, “High-Q Vertically Coupled InP Microdisk Resonators,” IEEE Photon. Technol. Lett. 14(3), 331–333 (2002). [CrossRef]
7. W. M. J. Green, R. K. Lee, G. A. Derose, A. Scherer, and A. Yariv, “Hybrid InGaAsP-InP Mach-Zehnder racetrack resonator for thermooptic switching and coupoing control,” Opt. Express 13(5), 1651–1659 (2005). [CrossRef] [PubMed]
8. Y. Huang, G. T. Paloczi, J. Scheuer, and A. Yariv, “Soft lithography replication of polymeric microring optical resonators,” Opt. Express 11(20), 2452–2458 (2003). [CrossRef] [PubMed]
9. S. J. Choi, Z. Peng, Q. Yang, S. J. Choi, and P. D. Dapkus, “Tunable narrow linewidth all-buried heterostructure ring resonator filters using Vernier Effects,” IEEE Photon. Technol. Lett. 17(1), 106–108 (2005). [CrossRef]
10. B. E. Little, S. T. Chu, J. V. Hryniewicz, and P. P. Absil, “Filter synthesis for periodically coupled microring resonators,” Opt. Lett. 25(5), 344–346 (2000). [CrossRef]
11. D. G. Rabus and M. Hamacher, “MMI-Coupled Ring Resonators in GaInAsP-InP,” IEEE Photon. Technol. Lett. 13(8), 812–814 (2001). [CrossRef]
12. B. Liu, A. Shakouri, and J. E. Bowers, “Passive microring-resonator coupled lasers,” Appl. Phys. Lett. 79(22), 3561–3563 (2001). [CrossRef]
13. R. Grover, T. A. Ibrahim, T. N. Ding, Y. Leng, L.-C. Kuo, S. Kanakaraju, K. Amarnath, L. C. Calhoun, and P.-T. Ho, “Laterally Coupled InP-Based single mode microracetrack notch filter,” IEEE Photon. Technol. Lett. 15(8), 1082–1084 (2003). [CrossRef]
14. V. Van, P. P. Absil, J. V. Hryniewicz, P.-T. Ho, J. V. Hryniewicz, and P.-T Ho, “Propagation loss in single-mode GaAs-AlGaAs microring resonators: measurement and model,” J. Lightwave Technol. 19(11), 1734–1739 (2001). [CrossRef]
15. M. Cai and K. Vahala, “Highly efficient optical power transfer to whispering-gallery modes by use of a symmetrical dual-coupling configuration,” Opt. Lett. 25(4), 260–262 (2000). [CrossRef]
16. S. Suzuki, Y. Kokubun, M. Nakazawa, R. Yamamoto, and S. T. Chu, and S. T. Chu, “Ultrashort Optical pulse transmission characteristics of vertically coupled microring resonator Add/Drop Filter,” J. Lightwave Technol. 19(2), 266 (2001). [CrossRef]
17. A. W. Poon, F. Courvoisier, and R. K. Chang, “Multimode resonances in square-shaped optical microcavities,” Opt. Lett. 26(9), 632–634 (2001). [CrossRef]
18. C. Manolatou, M. J. Khan, S. Fan, P. R. Villeneuve, H. A. Haus, and J. D. Joannopoulos, “Coupling of modes analysis of resonant channel add-drop filters,” IEEE J. Quantum Electron. 35(9), 1322–1331 (1999). [CrossRef]
19. H.-J. Moon and K.-S. Hyun, “Selective lasing of guided modes from hollow square semiconductor Microcavities,” Jpn. J. Appl. Phys. 46(12), L274–L276 (2007). [CrossRef]
20. H. Lu, C. Blaauw, and T. Makino, “Single-Mode operation over a wide temperature range in 1.3 mm InGaAsP/InP distributed feedback lasers,” J. Lightwave Technol. 14(5), 851–859 (1996). [CrossRef]
21. M. Kondow, T. Kitatami, K. Nakahara, and T. Tanaka, “Temperature dependence of lasing wavelength in a GaInNAs laser diode,” IEEE Photon. Technol. Lett. 12(7), 777–779 (2000). [CrossRef]
