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Energy Express

  • Editor: Bernard Kippelen
  • Vol. 18, Iss. S3 — Sep. 13, 2010
  • pp: A300–A306
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L-band tunable external cavity laser based on 1.58 μm superluminescent diode integrated with spot-size converter

Su Hwan Oh, Ki-Hong Yoon, Ki Soo Kim, Jongbae Kim, O-Kyun Kwon, Dae Kon Oh, Young-Ouk Noh, and Hyung-Jong Lee  »View Author Affiliations


Optics Express, Vol. 18, Issue S3, pp. A300-A306 (2010)
http://dx.doi.org/10.1364/OE.18.00A300


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Abstract

We report a 1.58 μm superluminescent diode (SLD) with a spot-size converter (SSC) designed and fabricated as a light source for a tunable external cavity laser (T-ECL). The active section of the SLD is fabricated by using a planar buried heterostructure (PBH) for low-threshold current and high-output power operation at a low injection current. The SSC structure of the SLD is designed to possess a buried deep-ridge waveguide (BD-RWG) and show a beam of less divergence. The full-width at half maximum (FWHM) of the horizontal and vertical far-field patterns (FFPs), due to the beam of the less divergence, are 14° and 13°, respectively. We also confirm that an L-band T-ECL employing the SSC SLD operates well enough to prove the characteristics of high performance.

© 2010 OSA

1. Introduction

In this paper we propose an L-band SLD integrated with an SSC to improve the performance of the L-band T-ECL. The structure of the L-band SLD is in general composed of a double-waveguide core, a planar buried heterostructure (PBH), and an SSC with a BD-RWG. The length of the SSC region is intentionally shortened only to decrease the propagation loss of a wave in the SLD, but the structure of the SSC region is optimized to make a narrow and circular beam and to obtain an FFP of less than 15 degrees. The output power of the L-band T-ECL is higher than that of the C-band T-ECL if the optimized L-band SLD is used as a gain source of the L-band T-ECL.

2. Design and fabrication

For optical access networks, performance of the C-band SLD is discussed in [12

12. S. H. Oh, K. S. Kim, J. J. Ju, M.-S. Kim, K.-H. Yoon, D. K. Oh, Y.-O. Noh, and H.-J. Lee, “Tunable external cavity laser employing uncooled superluminescent diode,” Opt. Express 17(12), 10189–10194 (2009). [CrossRef] [PubMed]

]. In the paper, the SLD is utilized as a source of a T-ECL operating at 1.25 Gbit/s over a 20 Km transmission in the C-band. The structure of the SLD, however, should be improved to obtain high output power in the L-band T-ECL, because the gain decrease in the L-band SLD leads to a reduction of the output power when compared with the output power of the C-band SLD. The output power of the L-band SLD is increased simply by reducing the length of the SSC in the SLD by 200 μm in comparison with that in [14

14. S. H. Oh, D.-H. Lee, K. S. Kim, Y.-S. Baek, and K.-R. Oh, “High-performance 1.55-μm superluminiscent diode with butt-coupled spot-size converter,” IEEE Photon. Technol. Lett. 20(11), 894–896 (2008). [CrossRef]

] because shortening the length suppresses the propagation loss in the SSC region. The FFP characteristic for the ridge width of the SSC region is shown in Fig. 1
Fig. 1 FFP characteristic with the ridge width of SSC region.
. Though the angle of the FFP decreases along with the increase of the ridge width of SSC, it does not form a circular beam because of both the reduction of the SSC length and the characteristic of the ridge waveguide. The FFP angle is not varied if the ridge width is over 10 μm. On the other hand, to suppress the lasing characteristics of SLD the waveguide is tilted by 7° with respect to the cleaved facet. Therefore, the angle of emission beam is tilted about 24° with respect to the output facet of the SLD. It is very difficult to achieve high coupling efficiency between SLDs with 24° tilted emission angles and a polymer Bragg reflector, even though aspheric microlenses in a transistor outline (TO) packing can are used. The FFP of the SLD is the most important factor in the structure of hybrid integration, such as the T-ECL for high coupling efficiency. The circular beam of the FFP in the SLD is indispensible for increasing the output power of the T-ECL by means of improving the coupling efficiency between the SLD and the polymer waveguide. Therefore, we propose that the structure of the L-band SLD should possess an SSC with a BD-RWG, as shown in Fig. 2
Fig. 2 (a) Schematic configuration of the L-band SSC SLD having novel SSC; (b) schematic configuration of the front facet in SSC region.
. Figure 2(a) represents a schematic configuration of the SSC SLD, and Fig. 2(b) shows a schematic diagram of the BD-RWG in the SSC region. The width of the passive waveguide core is set to 2 μm to reduce the FFP angle of the vertical direction, while the width of the BD-RWG is designed at 10 μm. The present SLD is fabricated as a BD-RWG type instead of a B-RWG type [16

16. S. H. Oh, K. S. Kim, O. K. Kwon, and K. R. Oh, “InGaAsP/InP buried-ridge waveguide laser with improved lateral single-mode property,” ETRI J. 30(3), 480–482 (2008). [CrossRef]

] in order to make the SLD generate a more circular beam at the front facet.

The SLD is fabricated to possess the structure of a double-waveguide core with an active waveguide in a PBH and a passive waveguide in a BD-RWG shape. The SLD with a cavity length of 600 μm includes four sections: a straight active, a bending active, an SSC, and a passive waveguide with lengths of 400 μm, 60 μm, 110 μm, and 30μm, respectively. The SSC (λg = 1.3 μm) is butt-coupled to an active layer (λg = 1.60 μm) of multiple quantum wells (MQWs) with a strained separate confinement heterostructure (SCH). According to our numerical simulation, maximum coupling efficiency between the SLD and the polymeric waveguide with a Bragg reflector in the T-ECL is achieved if the FFP angle of the SLD is less than 15°.

For high-speed operation over 1.25 Gbit/s, the parasitic capacity of the SLD should decrease. Therefore, trenches are formed in the lateral sides of the active region by using selective wet etching. The depth and width of the BD-RWG region are defined as 6 and 10 μm, respectively. To reduce reflectivity and to obtain a ripple of less than 3 dB, the passive waveguide is tilted by 7° towards the [110] direction of the cleavage facet, and the front facet is AR-coated with two layers of SiO2/TiO2. The reflectivity of the SSC facet is estimated to be less than 10−4. The rear facet in the active region is high-reflection (HR)-coated with multilayers of SiO2/TiO2 with a reflectivity of 98%.

3. Result and Discussion

The FFPs observed from the SSC facet at the injection current of 50 mA are shown in Fig. 4
Fig. 4 FFP characteristics from the SSC facet of SLD
. In the figure, the black and red symbols represent the horizontal and vertical FFPs whose FWHMs are 14° and 13°, respectively. The result is almost consistent with the designed value. When compared with the FFPs in [12

12. S. H. Oh, K. S. Kim, J. J. Ju, M.-S. Kim, K.-H. Yoon, D. K. Oh, Y.-O. Noh, and H.-J. Lee, “Tunable external cavity laser employing uncooled superluminescent diode,” Opt. Express 17(12), 10189–10194 (2009). [CrossRef] [PubMed]

], more narrow and circular FPPs are obtained from the proposed BD-RWG of the SSC SLD with a passive waveguide core of 2 μm width and a B-RWG of 10 μm width.

The L-band T-ECL is fabricated by using the L-band SLD, an aspheric microlens, and a tunable polymer Bragg reflector. The structure of the T-ECL is described in [12

12. S. H. Oh, K. S. Kim, J. J. Ju, M.-S. Kim, K.-H. Yoon, D. K. Oh, Y.-O. Noh, and H.-J. Lee, “Tunable external cavity laser employing uncooled superluminescent diode,” Opt. Express 17(12), 10189–10194 (2009). [CrossRef] [PubMed]

] in which the thermo-electric cooler (TEC) in a TO can is not included. The fabrication procedure and the performance of the tunable polymer Bragg reflector are detailed in [15

15. Y.-O. Noh, H.-J. Lee, J. J. Ju, M.-S. Kim, S. H. Oh, and M.-C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express 16(22), 18194–18201 (2008). [CrossRef] [PubMed]

]. The L-I curves of the T-ECL are shown in Fig. 5
Fig. 5 L-I characteristics of T-ECL with no TEC.
. The temperature of the polymer Bragg grating is fixed by using a TEC, but the SLD temperature is not intentionally controlled. The black and red lines stand for the L-I characteristics of the C-band T-ECL [11

11. S. H. Oh, J.-U. Shin, K. S. Kim, D.-H. Lee, S.-H. Park, H.-K. Sung, Y.-S. Baek, and K.-R. Oh, “200 GHz-spacing 8-channel multi-wavelength lasers for WDM-PON optical line terminal sources,” Opt. Express 17(11), 9401–9407 (2009). [CrossRef] [PubMed]

] and the L-band T-ECL in the present study, respectively. The L-band T-ECL shows small kinks due to mode hopping at the current level of 47 mA and 62 mA. The kinks are caused by the temperature instability of the SLD by dint of the injection current. It is possible to reduce these kinks by using a TEC in the SLD TO can, as explained in [12

12. S. H. Oh, K. S. Kim, J. J. Ju, M.-S. Kim, K.-H. Yoon, D. K. Oh, Y.-O. Noh, and H.-J. Lee, “Tunable external cavity laser employing uncooled superluminescent diode,” Opt. Express 17(12), 10189–10194 (2009). [CrossRef] [PubMed]

]. The maximum output power is 7 mW and the slope efficiency is 0.138 mW/mA at 50 mA. Consequently, the output power is 0.6 mW higher and the slope efficiency is 0.03 mW/mA higher than those of the C-band T-ECL because of the less divergent beam.

Figure 6
Fig. 6 Wavelength tuning characteristics of the polymer Bragg grating tunable laser for 16 channels; (a) C-band T-ECL, (b) L-band T-ECL.
shows the superimposed CW spectra of 16 channels spaced by 1 nm with a resolution of 0.01 nm. The SLD current is set to be 50 mA and the temperature of the Bragg reflector in the polymer waveguide is set to be 25°C. Figure 6(a) presents the output spectra of the C-band T-ECL, while Fig. 6(b) shows the superimposed spectra of the L-band T-ECL. The lasing wavelength of both T-ECLs are blueshifted linearly over a 15 nm wavelength range due to the thermo-optic effect of the Bragg reflector in the polymer waveguide, if the electric power is applied to the polymer waveguide from 0 to about 71 mW. The tuning range of the C- and the L-band T-ECLs is from 1545.8 to 1530.8 nm and from 1587.3 nm to 1572.3 nm, respectively. During the tuning, the output power of the C-band T-ECL varied from 3.5 to 2.46 dBm, with a variation of about 1 dB. The spectral peak power of the L-band T-ECL is higher than 3.73 dBm, with a small variation of 0.6dB. This result indicates that the spectral characteristics of the L-band are better than those of the C-band, and that is because the coupling efficiency is improved by using the L-band SLD with a more narrow and circular beam. The 20 dB bandwidth in both of the T-ECLs is less than 0.1 nm, and the side-mode suppression ratio (SMSR) is larger than 40 dB.

4. Conclusion

We have developed an L-band SLD with a SSC that possesses the characteristics of high output power and a narrow and circular beam for a light source of a T-ECL. The characteristics of the SLD are improved through optimizing the SSC structure of the SLD with a passive waveguide core of 2 μm in width and a BD-RWG of 10 μm in width. The average output power is 9 mW at the injection current of 60 mA, and the ripple is less than 3 dB. The horizontal and vertical FWHMs are 14° and 13°, respectively. Remarkably, the fabricated L-band T-ECL employing the novel SLD operates well enough for high performance. The device shows a maximum output power of 7 mW, and the slope efficiency is 0.138 W/A. These results imply that the proposed design of the device is useful as a light source for a T-ECL.

Acknowledgments

This work is supported by the IT R&D Program of MK/IIT (2008-S-008-1) in Korea.

References and links

1.

S. J. Park, C.-H. Lee, K.-T. Jeong, H.-J. Park, J.-G. Ahn, and K.-H. Song, “Fiber-to-the-home services based on wavelength-division-multiplexing passive optical network,” J. Lightwave Technol. 22(11), 2582–2591 (2004). [CrossRef]

2.

J. Yu, O. Akanbi, Y. Luo, L. Zong, T. Wang, Z. Jia, and G. K. Chang, “Demonstration of a novel WDM passive optical network architecture with source-free optical network units,” IEEE Photon. Technol. Lett. 19(8), 571–573 (2007). [CrossRef]

3.

H. S. Chung, B. K. Kim, and K. Kim, “Effects ou upstream bit rate on a wavelength-remodulated WDM-PON based on Manchester of inverse-return-to-zero coding,” ETRI J. 30(2), 255–260 (2008). [CrossRef]

4.

H. D. Kim, S. G. Kang, and C. H. Lee, “A low cost WDM source with an ASE injected Fabry–Perot semiconductor laser,” IEEE Photon. Technol. Lett. 12(8), 1067–1069 (2000). [CrossRef]

5.

H.-C. Ji, I. Yamashita, and K.-I. Kitayama, “Cost-effective colorless WDM-PON delivering up/down-stream data and broadcast services on a single wavelength using mutually injected Fabry–Perot laser diodes,” Opt. Express 16(7), 4520–4528 (2008). [CrossRef] [PubMed]

6.

J. G. Kim, C.-J. Chae, and M.-H. Kang, “Mini-slot-based transmission scheme for local customer internetworking in PONs,” ETRI J. 30(2), 282–289 (2008). [CrossRef]

7.

H.-S. Kim, B.-S. Choi, K.-S. Kim, D. C. Kim, O.-K. Kwon, and D. K. Oh, “Improvement of modulation bandwidth in multisection RSOA for colorless WDM-PON,” Opt. Express 17(19), 16372–16378 (2009). [CrossRef] [PubMed]

8.

H. Takesue and T. Sugie, “Wavelength channel data rewrite using saturated SOA modulator for WDM networks with centralized light sources,” J. Lightwave Technol. 21(11), 2546–2556 (2003). [CrossRef]

9.

E. K. MacHale, G. Talli, P. D. Townsend, A. Borghesani, I. Lealman, D. G. Moodie, and D. W. Smith, “Signal-induced Rayleigh noise reduction using gain saturation in an integrated R-EAM SOA,” in Optical Fiber Communication Conference (OFC), (2009), Paper OThA6.

10.

areS. H. Oh, J.-U. Shin, Y.-J. Park, S.-B. Kim, S. Park, H.-K. Sung, Y.-S. Baek, and K.-R. Oh, “Multiwavelength lasers for WDM-PON optical line terminal source by silica planar lightwave circuit hybrid integration,” IEEE Photon. Technol. Lett. 19(20), 1622–1624 (2007). [CrossRef]

11.

S. H. Oh, J.-U. Shin, K. S. Kim, D.-H. Lee, S.-H. Park, H.-K. Sung, Y.-S. Baek, and K.-R. Oh, “200 GHz-spacing 8-channel multi-wavelength lasers for WDM-PON optical line terminal sources,” Opt. Express 17(11), 9401–9407 (2009). [CrossRef] [PubMed]

12.

S. H. Oh, K. S. Kim, J. J. Ju, M.-S. Kim, K.-H. Yoon, D. K. Oh, Y.-O. Noh, and H.-J. Lee, “Tunable external cavity laser employing uncooled superluminescent diode,” Opt. Express 17(12), 10189–10194 (2009). [CrossRef] [PubMed]

13.

K. H. Yoon, S. H. Oh, K. S. Kim, O.-K. Kwon, D. K. Oh, Y.-O. Noh, and H.-J. Lee, “2.5-Gb/s hybridly-integrated tunable external cavity laser using a superluminescent diode and a polymer Bragg reflector,” Opt. Express 18(6), 5556–5561 (2010). [CrossRef] [PubMed]

14.

S. H. Oh, D.-H. Lee, K. S. Kim, Y.-S. Baek, and K.-R. Oh, “High-performance 1.55-μm superluminiscent diode with butt-coupled spot-size converter,” IEEE Photon. Technol. Lett. 20(11), 894–896 (2008). [CrossRef]

15.

Y.-O. Noh, H.-J. Lee, J. J. Ju, M.-S. Kim, S. H. Oh, and M.-C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express 16(22), 18194–18201 (2008). [CrossRef] [PubMed]

16.

S. H. Oh, K. S. Kim, O. K. Kwon, and K. R. Oh, “InGaAsP/InP buried-ridge waveguide laser with improved lateral single-mode property,” ETRI J. 30(3), 480–482 (2008). [CrossRef]

OCIS Codes
(140.3600) Lasers and laser optics : Lasers, tunable
(230.1480) Optical devices : Bragg reflectors
(230.3120) Optical devices : Integrated optics devices
(250.5960) Optoelectronics : Semiconductor lasers
(130.5460) Integrated optics : Polymer waveguides

ToC Category:
Integrated Optics

History
Original Manuscript: April 15, 2010
Revised Manuscript: June 10, 2010
Manuscript Accepted: June 16, 2010
Published: June 30, 2010

Citation
Su Hwan Oh, Ki-Hong Yoon, Ki Soo Kim, Jongbae Kim, O-Kyun Kwon, Dae Kon Oh, Young-Ouk Noh, and Hyung-Jong Lee, "L-band tunable external cavity laser based on 1.58 μm superluminescent diode integrated with spot-size converter," Opt. Express 18, A300-A306 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-S3-A300


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References

  1. S. J. Park, C.-H. Lee, K.-T. Jeong, H.-J. Park, J.-G. Ahn, and K.-H. Song, “Fiber-to-the-home services based on wavelength-division-multiplexing passive optical network,” J. Lightwave Technol. 22(11), 2582–2591 (2004). [CrossRef]
  2. J. Yu, O. Akanbi, Y. Luo, L. Zong, T. Wang, Z. Jia, and G. K. Chang, “Demonstration of a novel WDM passive optical network architecture with source-free optical network units,” IEEE Photon. Technol. Lett. 19(8), 571–573 (2007). [CrossRef]
  3. H. S. Chung, B. K. Kim, and K. Kim, “Effects ou upstream bit rate on a wavelength-remodulated WDM-PON based on Manchester of inverse-return-to-zero coding,” ETRI J. 30(2), 255–260 (2008). [CrossRef]
  4. H. D. Kim, S. G. Kang, and C. H. Lee, “A low cost WDM source with an ASE injected Fabry–Perot semiconductor laser,” IEEE Photon. Technol. Lett. 12(8), 1067–1069 (2000). [CrossRef]
  5. H.-C. Ji, I. Yamashita, and K.-I. Kitayama, “Cost-effective colorless WDM-PON delivering up/down-stream data and broadcast services on a single wavelength using mutually injected Fabry–Perot laser diodes,” Opt. Express 16(7), 4520–4528 (2008). [CrossRef] [PubMed]
  6. J. G. Kim, C.-J. Chae, and M.-H. Kang, “Mini-slot-based transmission scheme for local customer internetworking in PONs,” ETRI J. 30(2), 282–289 (2008). [CrossRef]
  7. H.-S. Kim, B.-S. Choi, K.-S. Kim, D. C. Kim, O.-K. Kwon, and D. K. Oh, “Improvement of modulation bandwidth in multisection RSOA for colorless WDM-PON,” Opt. Express 17(19), 16372–16378 (2009). [CrossRef] [PubMed]
  8. H. Takesue and T. Sugie, “Wavelength channel data rewrite using saturated SOA modulator for WDM networks with centralized light sources,” J. Lightwave Technol. 21(11), 2546–2556 (2003). [CrossRef]
  9. E. K. MacHale, G. Talli, P. D. Townsend, A. Borghesani, I. Lealman, D. G. Moodie, and D. W. Smith, “Signal-induced Rayleigh noise reduction using gain saturation in an integrated R-EAM SOA,” in Optical Fiber Communication Conference (OFC), (2009), Paper OThA6.
  10. areS. H. Oh, J.-U. Shin, Y.-J. Park, S.-B. Kim, S. Park, H.-K. Sung, Y.-S. Baek, and K.-R. Oh, “Multiwavelength lasers for WDM-PON optical line terminal source by silica planar lightwave circuit hybrid integration,” IEEE Photon. Technol. Lett. 19(20), 1622–1624 (2007). [CrossRef]
  11. S. H. Oh, J.-U. Shin, K. S. Kim, D.-H. Lee, S.-H. Park, H.-K. Sung, Y.-S. Baek, and K.-R. Oh, “200 GHz-spacing 8-channel multi-wavelength lasers for WDM-PON optical line terminal sources,” Opt. Express 17(11), 9401–9407 (2009). [CrossRef] [PubMed]
  12. S. H. Oh, K. S. Kim, J. J. Ju, M.-S. Kim, K.-H. Yoon, D. K. Oh, Y.-O. Noh, and H.-J. Lee, “Tunable external cavity laser employing uncooled superluminescent diode,” Opt. Express 17(12), 10189–10194 (2009). [CrossRef] [PubMed]
  13. K. H. Yoon, S. H. Oh, K. S. Kim, O.-K. Kwon, D. K. Oh, Y.-O. Noh, and H.-J. Lee, “2.5-Gb/s hybridly-integrated tunable external cavity laser using a superluminescent diode and a polymer Bragg reflector,” Opt. Express 18(6), 5556–5561 (2010). [CrossRef] [PubMed]
  14. S. H. Oh, D.-H. Lee, K. S. Kim, Y.-S. Baek, and K.-R. Oh, “High-performance 1.55-μm superluminiscent diode with butt-coupled spot-size converter,” IEEE Photon. Technol. Lett. 20(11), 894–896 (2008). [CrossRef]
  15. Y.-O. Noh, H.-J. Lee, J. J. Ju, M.-S. Kim, S. H. Oh, and M.-C. Oh, “Continuously tunable compact lasers based on thermo-optic polymer waveguides with Bragg gratings,” Opt. Express 16(22), 18194–18201 (2008). [CrossRef] [PubMed]
  16. S. H. Oh, K. S. Kim, O. K. Kwon, and K. R. Oh, “InGaAsP/InP buried-ridge waveguide laser with improved lateral single-mode property,” ETRI J. 30(3), 480–482 (2008). [CrossRef]

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