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Letter

Vol. 40, No. 22 / November 15 2015 / Optics Letters

Multi-wavelength distributed feedback laser array with very high wavelength-spacing precision JUN LU,1 SHENGPING LIU,1 QI TANG,2 HAIMING XU,2 YUTAO CHEN,1

AND

XIANGFEI CHEN1,*

1

College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China Wuhan Huagong Genuine Optics Tech Co., Ltd., Wuhan, Hubei 430223, China *Corresponding author: [email protected]

2

Received 11 September 2015; revised 10 October 2015; accepted 10 October 2015; posted 14 October 2015 (Doc. ID 249817); published 2 November 2015

Multi-wavelength laser arrays (MLAs) with a high wavelengthspacing precision are designed and fabricated. The MLAs are realized by the reconstruction-equivalent chirp (REC) technique, and anti-reflection structures are used to decrease the wavelength deviation. Six multi-wavelength laser arrays are fabricated and tested. Statistical results show that 81.1% of all 95 lasers have wavelength deviations within 0.1 nm, and all of them are within 0.2 nm. The mean wavelength spacing is 1.605 nm, and the standard deviation is 0.132 nm. © 2015 Optical Society of America OCIS codes: (230.3120) Integrated optics devices; (250.5960) Semiconductor lasers. http://dx.doi.org/10.1364/OL.40.005136

Multi-wavelength arrays (MLAs) are very important for photon– integration circuits (PICs) in wavelength division multiplexing (WDM) systems. Compared with combining many separated distributed feedback (DFB) lasers to obtain a multi-wavelength laser source, MLAs have the advantages of compact size, low energy consumption, and low cost. In large-scale PIC devices, MLAs with a large channel number and accurate channel spacing are in great need. To achieve such an MLA, many techniques are employed in fabrication, such as selective area growth [1,2], nano-imprint lithography [3], and electron-beam lithography (EBL) [4–7]. However, there are still some basic difficulties preventing MLAs from being widely applied in WDM systems. One crucial difficulty is that the wavelength spacing precision still needs to be improved. EBL may be the finest fabrication technique for MLAs, but it is found that a λ∕4-phase-shift DFB laser fabricated by EBL still has a wavelength uncertainty of ∼3 nm for typical resolution [7]. In Bellcore’s statistical results in 1996, any laser in MLAs that was made by EBL had only about 35% probability within a 0.2 nm range of the wavelength demanded by constant channel spacing, and about 74% probability within a 0.5 nm range [4]. Another problem is that the cost of such an MLA is usually high for its long-time fabrication. In 2007, the reconstructionequivalent chirp (REC) technique was introduced into the grating of semiconductor DFB lasers by us [8] and then used to 0146-9592/15/225136-04$15/0$15.00 © 2015 Optical Society of America

achieve MLAs [9–11]. The technique employs the equivalent response of a sampled and phase-shifted grating to obtain a laser resonance. Theoretical study showed that the fabrication error tolerance in REC grating can be relaxed by about two orders of magnitude [12]. Besides, the REC technique only uses holographic lithography and conventional lithography in MLA fabrication, so it is fast and inexpensive. Our recent experiment study has achieved 60-wavelength arrays with 100 GHz channel spacing, and the statistical analysis shows that about 83% of the lasers have a wavelength deviation within 0.2 nm [13]. However, in future 100 GHz channel spacing WDM systems, the deviation is required to be less than 20% of the channel spacing [14], corresponding to ∼  0.16 nm around 1550 nm. Therefore, more precise wavelength-spacing control is necessary. According to the reported results, it still seems to be a great challenge to achieve such a goal. In this Letter, the design of REC-based MLAs was improved to obtain higher channel spacing precision. Statistical results show a high yield, and most lasers have a wavelength deviation within 0.1 nm, which has shown that based on REC technique, more precise wavelength spacing is possible. These REC MLAs are very suitable laser sources for large-scale PIC chips in WDM systems. The REC technique employs the high-order frequency response generated by the sampling pattern on a homogeneous grating to achieve laser resonance. The resonance wavelength λp is obtained from the following equation [12]: 2np ∕λp  1∕Λ  1∕P;

(1)

where the symbols are Λ, pitch of holographic grating; P, sampling pitch; Λ, resonance wavelength; and nP , effective refractive index of λp . When Λ is fixed, the wavelength can be adjusted by changing the sampling pitch P. An equivalent phase shift can be arranged in the middle of the sampling pattern and will lead to an equivalent phase-shift response at the resonance wavelength λp . In MLA fabrication, homogenous grating Λ is fabricated by holographic lithography. Its Bragg wavelength is several tens of nanometers longer than the lasing wavelength, so that new resonance can be generated in the required wavelength range while unwanted lasing is avoided. Sampling period P is varied to adjust the real lasing wavelength according to the requirement.

Letter Usually, the sampling pitch is several micrometers in length and can be made by conventional lithography. An important factor that affects the output wavelength of a λ∕4-phase-shift DFB is the random phase shift at the facets. The facets’ position of the laser usually cannot be exactly controlled. The uncertain position leads to a random phase shift at the end facet of the laser. When the laser is reflected back into the laser cavity by the end facet, it will carry this random phase shift and affect the intra-cavity resonance. This causes a wavelength shift of the output light. A numeric simulation is carried out to study the wavelength shift. In the simulation, the value of the coupling coefficient κ is set to a relatively low value of 20 cm−1 , since the REC grating reduces the κ value to 1∕π of the normal DFB grating. Phase shift was introduced into both facets with the same value. When the phase-shift value varies from 0 to π, the value of the wavelength deviation is shown in Fig. 1. Figure 1 shows that, when facet reflectivity has a value of R  0.5%, the phase shift causes a maximum wavelength deviation of ∼0.34 nm. When the reflectivity is reduced to R  0.01%, the deviation is less than 0.1 nm. It shows that the reduction of facet reflection can reduce the random wavelength shift, so usually an AR/AR coating on two facets is employed on a λ∕4 phase-shift DFB MLA. However, in real fabrication, a high-transmission AR coating is expensive, and sometimes the coating cannot reach an ideal transmission. High transmission AR coating increases the cost, and imperfect coating affects the yield of accurate MLA. Thus, in this Letter, anti-reflection laser structure is used to improve the wavelength spacing precision of the MLA. Tilt facet is used at the back end of the laser to prevent the returning light from being reflected back into cavity. At the front end, an absorption section is used to reduce the returning light power because we need to keep the waveguide straight to couple the output light into a passive light combiner. Such an absorption section has the same epi-growth structure as the ridge waveguide laser, except it is not covered by the metal electrode. Because the current cannot be injected into this section, photons of laser light are partially absorbed by the active layer; thus the returning light power is reduced. Figure 2 is a photo of our MLA. Each laser in this laser array is an equivalent λ∕4 phase-shift DFB laser with a length of 450 μm. The holographic grating has a Bragg wavelength of 1634 nm, and the sampling period varies from 3.55 to 5.25 μm to make the lasing wavelength around 1550 nm. Equivalent λ∕4 phase shift is arranged at the middle of the sampling pattern. A 50 μm long curved waveguide is used at the back end

Fig. 1. Wavelength deviation of a λ∕4-phase-shift DFB laser at different facet phase shift and facet reflectivity: R  0.5% (solid line), R  0.1% (dashed line), and R  0.01% (dashed-dotted line).

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Fig. 2. Photo of the fabricated MLA with anti-reflection structure.

Fig. 3. Typical P-I diagram of the MLAs.

of the laser to make a tilt facet and, at the front end, another 50 μm long absorption section is used to reduce the returning light power. The MLA is designed around the center wavelength of 1550 nm, and the channel spacing is 200 GHz, which is about 1.62 nm in a 1550 nm range. Two kinds of arrays are designed. One is composed of 20 lasers, and the other is composed of 12 lasers. Six laser arrays were made, and each of the two kinds has three samples. Figure 3 shows a typical P-I measurement result of the MLAs. Most lasers have a threshold value of around 30–35 mA. The relative high threshold is mainly due to the small coupling coefficient κ of the REC gratings because the theoretical coupling coefficient of the REC grating is only 1∕π of normal DFB grating. The measured lasing wavelengths of the MLAs are in the range of 1525–1555 nm. Typical spectral diagrams of the MLAs are shown in Fig. 4(a), which shows a good single-mode property of the lasers. The output wavelength changes in good linearity with the channel number in all MLAs, as shown in Fig. 4(b).

Fig. 4. (a) Typical spectral diagram of two kinds of MLAs. (b) Wavelengths at different channels of all six laser bars.

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Letter Table 1. Linear Fitting Results of All Six MLAs Linear-Fitting Equation: y  b  ax

MLA01 MLA02 MLA03 MLA04 MLA05 MLA06

Slope Value a, (nm/ channel)

Intercept Value b (nm)

−1.5911 −1.6063 −1.6172 −1.5959 −1.6110 −1.6054

1557.3876 1557.4836 1557.3940 1549.0886 1549.5719 1549.5287

Fig. 5. (a) Wavelength deviation of all lasers in six bars. (b) Deviation distribution of all lasers.

Figure 5(a) shows the wavelength deviation from linear fitting of all laser bars. There are a total of 95 lasers taken into account in all six bars. In them, 18 lasers have wavelength deviations larger than 0.1 nm, and the other 81.1% are within this range. All lasers’ wavelength deviations are within 0.2 nm, and the distribution is shown in the bar diagram of Fig. 5(b). To find out if the wavelength spacing accuracy is improved by anti-reflection structures, an MLA without anti-reflection structure is also fabricated on the same wafer for comparison. Its wavelength deviations are shown in Fig. 6. In this MLA, seven of all 14 lasers have a wavelength deviation larger than 0.1 nm, and three of them exceed 0.2 nm. Apparently, this laser array has a larger wavelength spacing deviation than those arrays with anti-reflection structures. Because all these MLAs are fabricated on the same wafer and under the same processing conditions, the anti-reflection structures are considered to be the main cause of the difference. According to the simulated wavelength shift caused by facet reflection, we thought the anti-reflection structure lowered the reflectivity value by ∼1 order. Absolute wavelength deviation is another important aspect for industrial mass production. The wavelength spacing should meet the requirement of ITU-T standard. To find absolute deviation, the average wavelength spacing of all MLAs is needed. This is obtained from the slope values of the linear fitting results shown in Table 1. The average slope value of all six MLAs is −1.6045 nm∕ channel. At this fixed slope value, the deviations of all 95 lasers in six MLAs are shown in Fig. 7.

Fig. 6. Deviation of the laser bar without anti-reflection structures.

Fig. 7. Absolute wavelength deviations of all 95 lasers.

When all six MLAs are considered, the average channel spacing is 1.605 nm, and the standard deviation is 0.132 nm. The absolute wavelength deviations are larger than those of a single MLA, mainly due to the nonuniformity in different areas of the epi-wafer and the imperfection in fabrication. However, the uniformity of epi-wafer and fabrication quality can be further improved to obtain higher absolute wavelength accuracy. In summary, MLAs with 20 and 12 λ∕4-phase-shift DFB lasers are realized by the REC technique. Two-hundred GHz channel spacing is achieved by changing the sampling periods of different lasers. The single-mode yield in all 96 lasers is ∼99%, except for only one laser which did not operate. By suppressing the end-facet reflection, random wavelength deviation is reduced, and the statistical results show a high yield and high wavelength spacing precision in these MLAs. 81.1% of the lasers have a wavelength deviation within 0.1 nm, and 100% have a wavelength deviation within 0.2 nm. An analysis of absolute deviations shows a mean wavelength spacing of 1.605 nm of all 95 lasers, and the standard deviation is 0.132 nm. To the best of our knowledge, this is the highest wavelength spacing precision achieved in REC MLAs and proved that end-facet reflection is a crucial cause of wavelength deviation in REC arrays. That is to say, if the end-facet reflection is small enough, the wavelength spacing can be controlled well using the REC technique, and it will meet the requirement of the ITU-T standard. This provides a compact and costefficient light source for large-scale PIC devices. Funding. National Natural Science Foundation of China (NSFC) (61435014, 61306068); Nature Science Foundation of Jiangsu Province for the Youth (BK20140414, BK20130585); Huawei Innovation Research Program (YB2014030052).

Letter REFERENCES 1. C. Zhang, S. Liang, H. Zhu, and W. Wang, in 12th International Conference on Optical Communications and Networks (ICOCN) (IEEE, 2013), pp. 1–3. 2. C. Zhang, H. Zhu, S. Liang, L. Han, and W. Wang, IEEE Photon. J. 5, 1400407 (2013). 3. J. Zhao, X. Chen, N. Zhou, X. Huang, M. Cao, L. Wang, and W. Liu, Opt. Commun. 339, 78 (2015). 4. T. P. Lee, C. E. Zah, R. Bhat, W. C. Young, B. Pathak, F. Favire, P. S. D. Lin, N. C. Andreadakis, C. Caneau, A. W. Rahjel, M. Koza, J. K. Gamelin, L. Curtis, D. D. Mahoney, and A. Lepore, J. Lightwave Technol. 14, 967 (1996). 5. Y. Kotaki and K. Morito, in 28th European Conference on Optical Communication (IEEE, 2002), pp. 1–2. 6. H. Ishii, K. Kasaya, H. Oohashi, Y. Shibata, H. Yasaka, and K. Okamoto, IEEE J. Sel. Top. Quantum Electron. 13, 1089 (2007).

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7. M. Zanola, M. J. Strain, G. Giuliani, and M. Sorel, IEEE Photon. Technol. Lett. 24, 1063 (2012). 8. Y. Dai and X. Chen, Opt. Express 15, 2348 (2007). 9. J. Li, X. Chen, N. Zhou, J. Zhang, X. Huang, L. Li, H. Wang, Y. Lu, and H. Zhu, Proc. SPIE 7631, 763104 (2009). 10. Y. Shi, X. Chen, Y. Zhou, S. Li, L. Lu, R. Liu, and Y. Feng, Opt. Lett. 37, 3315 (2012). 11. J. Li, S. Tang, J. Wang, Y. Liu, X. Chen, and J. Cheng, IEEE Photon. Technol. Lett. 26, 1593 (2014). 12. Y. Shi, S. Li, L. Li, R. Guo, T. Zhang, R. Liu, W. Li, L. Lu, S. Tang, Y. Zhou, J. Li, and X. Chen, J. Lightwave Technol. 31, 3243 (2013). 13. Y. Shi, S. Li, X. Chen, L. Li, J. Li, T. Zhang, J. Zheng, Y. Zhang, S. Tang, L. Hou, J. H. Marsh, and B. Qiu, Sci. Rep. 4, 7377 (2014). 14. “40-Gigabit-capable passive optical networks 2 (NG-PON2): Physical media dependent (PMD) layer specification,” ITU-T G.989.2 Recommendation (ITU, 2014).

Multi-wavelength distributed feedback laser array with very high wavelength-spacing precision.

Multi-wavelength laser arrays (MLAs) with a high wavelength-spacing precision are designed and fabricated. The MLAs are realized by the reconstruction...
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