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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 10 — May. 9, 2011
  • pp: 9255–9261
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Design of phase-shifted hybrid silicon distributed feedback lasers

Sudharsanan Srinivasan, Alexander W. Fang, Di Liang, Jon Peters, Bryan Kaye, and John E. Bowers  »View Author Affiliations


Optics Express, Vol. 19, Issue 10, pp. 9255-9261 (2011)
http://dx.doi.org/10.1364/OE.19.009255


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Abstract

We present data on the design and performance analysis of phase shifted distributed feedback (DFB) lasers on the hybrid silicon platform. The lasing wavelength for various input currents and temperatures, for devices with standard quarter-wavelength, 60 μm and 120 μm-long phase shift are compared for mode stability and output power. The pros and cons of including a large phase shift region in the grating design are analyzed.

© 2011 OSA

1. Introduction

Optical interconnects to and on silicon chips are necessary to address the issues of increasing power consumption and limited communication bandwidth faced by conventional electrical interconnects [1

1. D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]

]. Wafer-bonding compound semiconductors to SOI substrates, combines the superior gain characteristics of compound semiconductors with the superior passive waveguide characteristics of silicon. Laser diodes fabricated on this hybrid platform make good light sources, which are useful for optical interconnects on silicon [2

2. D. Liang and J. E. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics 4(8), 511–517 (2010). [CrossRef]

]. Distributed-feedback (DFB) lasers, useful because of its single longitudinal mode output and lithographically-defined cavity length, can produce milliwatts of output power with relatively low threshold currents, making them attractive candidates for on-chip light sources [3

3. G. Morthier and P. Vankwikelberge, Handbook of Distributed Feedback Laser Diodes (Artech House, Inc., 1997), Chaps. 10 and 11.

]. Earlier, Fang et al. successfully designed and fabricated a quarter-wave shifted DFB laser structure on the hybrid silicon platform [4

4. A. W. Fang, E. Lively, Y. Kuo, D. Liang, and J. Bowers, “Distributed Feedback Silicon Evanescent Laser,” in National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper PDP15.

]. Figure 1
Fig. 1 Schematic of a symmetric phase-shifted DFB silicon laser.
shows the schematic longitudinal cross section of a hybrid DFB silicon laser. A standard DFB laser, without phase-shifts, relies on perturbing reflections to destroy the degeneracy of the two modes on either side of the Bragg

wavelength. A single quarter-wave shifted DFB design eliminates the degeneracy and becomes resonant at the Bragg wavelength. However, the intense electric field concentrated at the phase shifted region of the cavity limits its performance primarily because of spatial hole burning [5

5. H. Ghafouri-Shiraz, Distributed Feedback Laser Diodes and Optical Tunable Filters (Wiley, 2003), Chaps. 3 and 5.

].

In Section 2, we discuss the design and fabrication of the DFB lasers with different phase shift lengths. The theoretical and experimental threshold current and maximum power for the relevant devices is studied in Section 3. In Section 4, we analyze the spectral data to discuss mode stability and in Section 5, devices are compared from a thermal performance perspective.

2. Device design and fabrication

The devices were fabricated using the same procedure as in Ref [4

4. A. W. Fang, E. Lively, Y. Kuo, D. Liang, and J. Bowers, “Distributed Feedback Silicon Evanescent Laser,” in National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper PDP15.

]. The rectangular surface corrugations in this work are made on silicon, which enables us to have any form of grating structure with relatively low tolerance on dimensions because of advanced high-resolution CMOS lithography tools. The grating pitch is 238 nm and the stop band is designed around 1600 nm. Figure 2
Fig. 2 Hybrid silicon chip showing 300 DFB lasers with on-chip photodetectors.
shows an image of a processed hybrid silicon chip which contains 36 DFB laser designs distributed among 300 devices with a laser yield of over 95%.

The coupling coefficient, κ, of the grating was designed for 250 cm−1; this is large compared to gratings on passive silicon rib waveguides of similar dimension since the index difference between air and silicon is quite large and the index perturbation is located nearer to the center of the optical mode [6

6. W. Alexander, Fang, “Silicon evanescent lasers,” Ph.D. dissertation (Dept. of Elect. and Comp. Eng., Univ. of California, Santa Barbara, CA, 2008), pp. 104–105.

]. The κL product, where L is the length of the grating, is varied by changing L. In this paper, we compare the performance of eight designs, primarily distinguished by the phase-shift length introduced at the center of the grating (see Fig. 3
Fig. 3 Grating structures with phase-shift lengths equal to (a) one quarter wavelength (λ0 = 1600 nm), (b) 60 μm, and (c) 120 μm.
). Long grating lengths were used in the DFB lasers of an earlier generation to minimize the negative effects of device heating, which lead to poor power extraction and low differential quantum efficiency. The goal of this paper is to find the effects of changing the phase-shift length and grating length on output power and mode stability. Two on-chip hybrid silicon photodetectors are integrated to detect the output power from both sides. We assume a responsivity of 1 A/W to conservatively estimate the device output power.

3. Threshold current and maximum output power

Devices with III-V junction side up sit on a copper stage whose temperature is actively controlled during characterization. The stage temperature (T) was kept constant at 20 °C for all the measurements. Figure 4
Fig. 4 L-I-V curve for a DFB laser with 120 μm long phase-shift and κL = 3. The total device length is 240 μm.
shows a typical L-I-V curve for one of the DFB laser designs under cw operation. The total cavity length is 240 μm with 120 μm long phase-shift region, resulting in κL = 3. The series resistance for all devices lie between 20 Ω and 35 Ω.

Figure 5
Fig. 5 Threshold current (a) and maximum output power (b) plotted against κL for three phase-shift lengths, one quarter wavelength (green line and diamonds), 60 μm (blue line and triangles) and 120 μm (red line and circles).
shows the experimental values of threshold current and maximum output power versus the grating coupling coefficient-grating length product, κL, alongside the theoretical curves. The error bars indicate the full range of measured values from various positions on the chip. We expect the threshold and output power to vary as described in Eqs. (1) and (2) respectively,

Ith=Ith0e((ZT(PDPout)+T)T0)
(1)
Pout=ηi(αmαi+αm)(hνq)e((ZT(PDPout)+T)T1)(IIth)
(2)

Using only the data from Fig. 5, large phase-shift lengths and lower κL products seem to have high power extraction. This is largely due to reduced device thermal impedance (Eq. (2)). However, we do not know the mode stability and side mode suppression ratio (SMSR) of these lasers which is important for interconnect operation given the relatively harsh conditions on the silicon chip.

4. Spectral Analysis

5. Thermal Impedance

The thermal performance of silicon evanescent lasers is currently limited by heat extraction from the active region, due to the presence of silicon dioxide lower cladding in the SOI substrate [7

7. M. N. Sysak, H. Park, A. W. Fang, J. E. Bowers, R. Jones, O. Cohen, O. Raday, and M. J. Paniccia, “Experimental and theoretical thermal analysis of a hybrid silicon evanescent Laser,” Opt. Express 15(23), 15041–15046 (2007). [CrossRef] [PubMed]

]. This limits the maximum lasing temperature and degrades the device performance. The ratio of change in lasing wavelength to the change in input electrical power (dλ/dPelec) and change in stage temperature (dλ/dT), for the eight designs under cw operation are plotted in Figs. 9(a) and (b)
Fig. 9 (a) The ratio of change in lasing wavelength to change in input electrical power for different laser designs in cw operation. The lines are a linear fit to the data points. (b) The ratio of change in lasing wavelength to change in stage temperature for the same designs.
respectively. The data points in Fig. 9(b) are concentrated around the value of 0.1nm/°C for all designs. Thus the value of dλ/dPelec is a good tracking point for the thermal impedance of the device.

Since the electric field intensity peaks in the phase shift region much of the heat is generated here in the laser. Thermal impedance of the device scales inversely with device length. Hence, we expect the thermal impedance of the lasers with phase-shift length of 60 μm and 120 μm not to increase significantly with decrease in grating length when compared to the quarter-wave shifted laser. The slopes in Fig. 9(a) corresponding to phase-shift lengths of ¼ λ, 60 μm and 120 μm are 0.372, 0.344 and 0.268 respectively, consistent with this hypothesis.

6. Conclusions

We designed and studied eight symmetric phase-shifted DFB lasers on a hybrid silicon platform. The effects of incorporating long phase-shift lengths are studied from device, electrical, optical, spectral and thermal perspectives. Phase-shift lengths much longer than one quarter wavelength can provide good power extraction, while keeping the thermal impedance of the device low. However, very long phase-shift lengths lead to mode instability and degraded SMSR. Optimal designs are determined by the application requirement.

Acknowledgments

The authors would like to thank Hui-Wen Chen and Yongbo Tang for valuable discussions and fabrication assistance, and NSF funded NNIN fabrication facility at the University of California, Santa Barbara. This research was supported by the DARPA CIPhER project.

References and links

1.

D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]

2.

D. Liang and J. E. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics 4(8), 511–517 (2010). [CrossRef]

3.

G. Morthier and P. Vankwikelberge, Handbook of Distributed Feedback Laser Diodes (Artech House, Inc., 1997), Chaps. 10 and 11.

4.

A. W. Fang, E. Lively, Y. Kuo, D. Liang, and J. Bowers, “Distributed Feedback Silicon Evanescent Laser,” in National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper PDP15.

5.

H. Ghafouri-Shiraz, Distributed Feedback Laser Diodes and Optical Tunable Filters (Wiley, 2003), Chaps. 3 and 5.

6.

W. Alexander, Fang, “Silicon evanescent lasers,” Ph.D. dissertation (Dept. of Elect. and Comp. Eng., Univ. of California, Santa Barbara, CA, 2008), pp. 104–105.

7.

M. N. Sysak, H. Park, A. W. Fang, J. E. Bowers, R. Jones, O. Cohen, O. Raday, and M. J. Paniccia, “Experimental and theoretical thermal analysis of a hybrid silicon evanescent Laser,” Opt. Express 15(23), 15041–15046 (2007). [CrossRef] [PubMed]

OCIS Codes
(050.5080) Diffraction and gratings : Phase shift
(140.3490) Lasers and laser optics : Lasers, distributed-feedback

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: January 3, 2011
Revised Manuscript: February 25, 2011
Manuscript Accepted: February 28, 2011
Published: April 27, 2011

Citation
Sudharsanan Srinivasan, Alexander W. Fang, Di Liang, Jon Peters, Bryan Kaye, and John E. Bowers, "Design of phase-shifted hybrid silicon distributed feedback lasers," Opt. Express 19, 9255-9261 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-10-9255


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References

  1. D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]
  2. D. Liang and J. E. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics 4(8), 511–517 (2010). [CrossRef]
  3. G. Morthier and P. Vankwikelberge, Handbook of Distributed Feedback Laser Diodes (Artech House, Inc., 1997), Chaps. 10 and 11.
  4. A. W. Fang, E. Lively, Y. Kuo, D. Liang, and J. Bowers, “Distributed Feedback Silicon Evanescent Laser,” in National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper PDP15.
  5. H. Ghafouri-Shiraz, Distributed Feedback Laser Diodes and Optical Tunable Filters (Wiley, 2003), Chaps. 3 and 5.
  6. W. Alexander, Fang, “Silicon evanescent lasers,” Ph.D. dissertation (Dept. of Elect. and Comp. Eng., Univ. of California, Santa Barbara, CA, 2008), pp. 104–105.
  7. M. N. Sysak, H. Park, A. W. Fang, J. E. Bowers, R. Jones, O. Cohen, O. Raday, and M. J. Paniccia, “Experimental and theoretical thermal analysis of a hybrid silicon evanescent Laser,” Opt. Express 15(23), 15041–15046 (2007). [CrossRef] [PubMed]

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