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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 16 — Aug. 11, 2014
  • pp: 18949–18957
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Widely tunable six-section semiconductor laser based on etched slots

Marta Nawrocka, Qiaoyin Lu, Wei-Hua Guo, Azat Abdullaev, Frank Bello, James O’Callaghan, Thomas Cathcart, and John F. Donegan  »View Author Affiliations


Optics Express, Vol. 22, Issue 16, pp. 18949-18957 (2014)
http://dx.doi.org/10.1364/OE.22.018949


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Abstract

A six section widely tunable laser based on slots etched into the waveguide is presented. This laser is re-growth free which makes it suitable for photonics integration. To improve the laser performance, the front and the back facets are anti- reflection (AR) coated and the laser is integrated with a semiconductor optical amplifier. A tuning range of 55nm covering 12 supermodes with side mode suppression ratio (SMSR) >30dB is reported for the fabricated device using the Vernier tuning effect. This laser platform requires very simple fabrication compared with more complex superstructure gratings.

© 2014 Optical Society of America

1. Introduction

Wavelength- tunable semiconductor lasers play an important part as key components for dense- wavelength division multiplexed (DWDM) optical telecommunication systems. Tunable lasers are also needed in other important markets such as trace gas detection for environmental emission monitoring [1

1. R. Phelan, M. Lynch, J. F. Donegan, and V. Weldon, “Simultaneous multispecies gas sensing by use of a sampled grating distributed Bragg reflector and modulated grating Y laser diode,” Appl. Opt. 44(27), 5824–5831 (2005). [CrossRef] [PubMed]

]. So far, a number of monolithic wavelength-tunable lasers have been developed such as the sampled-grating distributed Bragg reflector (SG-DBR) lasers [2

2. L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: a tutorial,” J. Lightwave Technol. 22(1), 193–202 (2004). [CrossRef]

], the super-structure grating distributed Bragg reflector (SSG-DBR) lasers [3

3. Y. Tohmori, Y. Yoshikuni, H. Ishii, F. Kano, T. Tamamura, Y. Kondo, and M. Yamamoto, “Broad-range wavelength-tunable superstructure grating (SSG) DBR lasers,” IEEE J. Quantum Electron. 29(6), 1817–1823 (1993). [CrossRef]

], the digital supermode distributed Bragg reflector (DS-DBR) lasers [4

4. A. J. Ward, D. J. Robbins, G. Busico, E. Barton, L. Ponnampalam, J. P. Duck, N. D. Whitbread, P. J. Williams, D. C. J. Reid, A. C. Carter, and M. J. Wale, “Widely tunable DS-DBR laser with monolithically integrated SOA: Design and performance,” IEEE J. Sel. Top. Quantum Electron. 11(1), 149–156 (2005). [CrossRef]

] and tunable DFB [5

5. H. Ishii, K. Kasaya, and H. Oohashi, “Narrow spectral linewidth operation (<160kHz) in widely tunable distributed feedback laser array,” Electron. Lett. 46(10), 714–715 (2010). [CrossRef]

] and slotted laser arrays (TLAs) [6

6. W.-H. Guo, Q. Lu, M. Nawrocka, A. Abdullaev, M. Lynch, V. Weldon, and J. F. Donegan, “Integrable slotted single mode lasers,” IEEE Photon. Technol. Lett. 24(8), 634–636 (2012). [CrossRef]

]. However, such structures apart from that in [6

6. W.-H. Guo, Q. Lu, M. Nawrocka, A. Abdullaev, M. Lynch, V. Weldon, and J. F. Donegan, “Integrable slotted single mode lasers,” IEEE Photon. Technol. Lett. 24(8), 634–636 (2012). [CrossRef]

] are conventionally fabricated with an embedded grating, which requires at least one re-growth step complicating the device fabrication and therefore increasing the device cost. To reduce the cost, complexity and to increase the wafer yield, our group has recently demonstrated single mode lasers and wavelength tunable lasers based on slots [6,7

6. W.-H. Guo, Q. Lu, M. Nawrocka, A. Abdullaev, M. Lynch, V. Weldon, and J. F. Donegan, “Integrable slotted single mode lasers,” IEEE Photon. Technol. Lett. 24(8), 634–636 (2012). [CrossRef]

]. The merit of this laser platform is that it is re-growth free, can be fabricated by standard photolithography and is inherently suitable for photonic integration due to the high reflectivity of the mirror sections.Such a laser platform has several advantages: a significant reduction of cost, it enables the use of AlGaInAs materials for high temperature operation and also enables the integration with other optical components i.e. semiconductor optical amplifiers (SOA) to control the output power and with electro-absorption and Mach-Zehnder modulators [8

8. Q. Lu, W.-H. Guo, M. Nawrocka, A. Abdullaev, C. Daunt, J. O’Callaghan, M. Lynch, V. Weldon, F. Peters, and J. F. Donegan, “Single mode lasers based on slots suitable for photonic integration,” Opt. Express 19(26), B140–B145 (2011). [CrossRef] [PubMed]

]. In this paper, we present a six- section widely tunable laser based on this laser platform. This new design includes a phase section for fine tuning. A short section semiconductor optical amplifer (SOA) is integrated with the laser. The fabricated laser has a very wide tuning range ~55 nm with 12 supermodes with side mode suppression ratio over 30dB. Tuning maps show excellent performance with full wavelength coverage over the 55 nm range.

2. Device structure and fabrication

The schematic structure of the tunable slotted laser is shown in Fig. 1
Fig. 1 3D schematic structure of the six-section tunable laser based on slots.
. The laser has a 2.5 μm-wide surface ridge waveguide structure. The active layer of the laser consists of five AlGaInAs quantum wells which have an emission peak around 1545 nm. The laser structure consists of a SOA (semiconductor optical amplifier) section, a central gain section, a phase section, a photodiode (PD) section and two mirror sections (including a group of slots). These six sections are electrically isolated from each other naturally by the slots between them. The electrical isolation between different section is 0.4kΩ.

The whole laser cavity has a length of about 3mm of which the gain section was set to 400 µm long to provide enough gain for the laser operation. The phase section is 100 µm long. The 2D scattering matrix method has been used to optimize the laser structure [10

10. Q. Y. Lu, W.-H. Guo, R. Phelan, D. Byrne, J. F. Donegan, P. Lambkin, and B. Corbett, “Analysis of slot characteristics in slotted single- mode semiconductor lasers using the 2-D scattering matrix method,” IEEE Photon. Technol. Lett. 18(24), 2605–2607 (2006). [CrossRef]

]. To obtain a narrow reflection bandwidth, 9 slots are introduced into each mirror section to provide sufficient feedback for lasing without any cleaved facets. It is also a compromise to maximize the reflectivity and to minimize the width of the reflection peaks and keep a minimum laser length. As can be seen in Fig. 2
Fig. 2 (a) SMSR versus peak wavelength for the laser with slot period of 70 μm for the front mirror and 76 μm for the back mirror. The currents injected to the front and the back mirror were scanned together between 300 and 50 mA, (b) Output power vs. peak wavelength for the lasers with slot period of 70 μm for the front mirror and 76 μm for the back mirror. The currents injected to the front and the back mirror were scanned between 300 and 50 mA.
in [9

9. D. Byrne, J.-P. Engelstaedter, W.-H. Guo, Q. Lu, B. Corbett, B. Roycroft, J. O’Callaghan, F. H. Peters, and J. F. Donegan, “Discretely tunable semiconductor lasers suitable for photonics integration,” IEEE J. Sel. Top. Quantum Electron. 15(3), 482–487 (2009). [CrossRef]

] having a greater number of slots in each mirror section would result in providing a narrower bandwidth of a reflection peak and a higher reflection from that section, but also it would make mirror sections longer.

Tuning of this laser is observed by changing the injected current and therefore changing the refractive index in each section and using the Vernier tuning effect to determine which modes lase. To employ the Vernier effect for the improvement of the tuning range, the slot periods in the front and back mirror sections are designed slightly differently. Because each section has nine slots, the gain introduced between the slots is similar for each mirror section and therefore the reflection spectra are comparable to each other. The slots periods in the design are chosen to be 70 and 76 µm for the front and the back mirror respectively with FSR of around 4.9 and 4.5 nm, respectively. Designing a laser with short slot periods is advantageous for achieving a wide tuning range. The tuning range is determined as [11

11. H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Quasicontinuous wavelength tuning in super- structure- grating (SSG) DBR lasers,” IEEE J. Quantum Electron. 32(3), 433–441 (1996). [CrossRef]

]:
Δλ'=λfλb+1λfλb
(4)
where λf, λb are free spectral ranges FSRs for the front and back mirrors respectively. For ease of fabrication, the slot width is set to 1 µm, which makes it suitable for the standard photolithography although in this case e-beam lithography was used to set the slot positions. The slot depth is 1.80 µm. The slots are etched simultaneously with the ridge to a depth just above the active layer. To eliminate the reflection from the facets, both the SOA and PD sections are curved to generate a 7° angle towards the cleaved facet and both facets are antireflection (AR) coated. The PD section is included in this design and to keep the overall length of the lasers on the chip constant to ease fabrication. This section is not used in our present studies.

3. Characteristics

The fabricated tunable laser was mounted on an AlN carrier and placed on a copper heat sink which was mounted on a thermoelectric cooler (TEC) to control the chip temperature. The temperature was set constant to 20°C. Five independent current sources were used to inject the current into the gain, phase, SOA, and both mirrors. The currents injected into the gain and SOA sections were set at 100 mA and 20mA with the PD left unbiased. For the first set of measurements, the phase section was connected to the gain section. The currents injected into the front and back mirror sections were both scanned between 50 and 300 mA with a step of 2 mA. The mirror sections are over twice as long as the mirror sections in SG-DBR lasers. To provide enough gain and to compensate the loss coming from the slots, high currents were applied to the mirror sections to drive the laser, which will decrease the tuning efficiency. To decrease the mirror length, a lower number of slots could be used in the future design. The output spectra were recorded using an optical spectrum analyzer with a resolution bandwidth of 0.1 nm and a sensitivity of −75dB.

Figure 2 shows diagrams of the measured supermodes and their corresponding SMSRs for the device with a longer back and shorter front slot period, respectively. In order to get a higher fraction of the total emitted power from the front, it is advantageous to use a design with a shorter slot period at the front than at the back [12

12. J. Buus, M.-C. Amman, and D. J. Blumenthal, Tunable Laser Diodes and Related Optical Sources (Wiley-Interscience, 2005).

], which in our case is 70 µm for the front mirror and 76 µm for the back mirror. A discrete tuning behavior can be clearly seen over a tuning range of around 55nm. As seen in Fig. 2(a), 12 modes are accessible with SMSR greater than 30 dB for the wavelength from 1525 nm to 1580 nm. Modes on the blue end of the spectrum show lower SMSR. Generally tunable lasers suffer from a lower SMSR on blue side and red side of gain spectrum due to their large gain differences [12

12. J. Buus, M.-C. Amman, and D. J. Blumenthal, Tunable Laser Diodes and Related Optical Sources (Wiley-Interscience, 2005).

]. Therefore the super modes on red side experience a higher gain comparing with those in blue side, which results in lower SMSRs on the blue side compare with the red side.

The measured output power was plotted in Fig. 2(b) as a function of peak wavelengths. Where the SMSR is large the power is also large showing that there is good singe mode lasing at these wavelengths with a particular current injection combination. A good output power is observed for all 12 modes and the variation between the modes is within 2dBm.

Figure 3(a)
Fig. 3 (a). Wavelength tuning map versus the current injected to the front and back mirrors for the lasers with different slot period of 70 μm for the front mirror and 76 μm for the back mirror, (b) SMSR map versus injected currents into the front and back sections for the lasers with slot period of 70 μm for the front mirror and 76 μm for the back mirror.
shows the wavelength tuning map for the laser with the slot period of 70 µm for the front mirror and 76 µm for the back mirror. It can be seen that mode hopping occurs at the boundaries of each section within this map as is typical for other Vernier-tuned lasers including the SGDBR [2

2. L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: a tutorial,” J. Lightwave Technol. 22(1), 193–202 (2004). [CrossRef]

]. The wavelength tuning range is about 55 nm from 1525 nm to 1580nm. As seen from the wavelength tuning map the modes are stable during the tuning process and no mode- hopping is observed. Unexpected mode hopping might occur for high injection currents as then there is quite a lot of heating involved resulting in a mix of current and temperature tuning.The measured SMSR map versus the injected currents into the front and back mirrors is shown in Fig. 3(b). There are large regions of red colour indicating a very high SMSR but there are also some regions on the maps indicated by a blue color where the SMSR has a low value. This effect occurs due to mode competition and it can be fixed by changing the current injected into the phase section and re-tuning the lasing peak.

The continuous tuning of individual supermodes due to changing front and back current together was investigated (Fig. 4
Fig. 4 SMSR versus tuning wavelength for the lasers with slot period of 70 μm for the front mirror and 76 μm for the back mirror.
). The mode shows a continuous tuning of 0.6 nm. The rate of changing the wavelength with current for this mode is 0.012 nm/mA. This allows us to accurately set the laser to precise optical frequencies such as required within the C-band. The SGDBR laser has a continuous tuning range of < 0.4 nm for all discrete modes. The quasi- continuous tuning range is much bigger [13,14

13. B. Mason, G. A. Fish, J. Barton, L. A. Coldren, and S. P. DenBaars, “Characteristics of sampled grating DBR lasers with integrated semiconductor optical amplifiers and electroabsorption amplifiers,” In Proc. Conf. Optical Fiber Commun. (2010).

]. The continuous tuning range in SG-DBR lasers is limited by the available index change induced by pure current injections. In the laser presented here, both current variations and the temperature change contributed to the continuous tuning, making it wider than in the SG-DBR laser.

4. Phase tuning

As was mentioned before on the SMSR map, due to the mode competition, some regions are marked in blue corresponding to a low SMSR value. To improve the laser performance and get only single mode operation, we repeated the scan for those regions changing the phase current from 0mA to 50 mA with a step of 2 mA and injecting 80 mA to the gain section. The scanned regions are: (i) front current from 170 mA to 270 mA and back current from 50mA to 150 mA, (ii) front current from 230mA to 300mA and back current 150mA to 230mA, and (iii) for front current from 280 mA to 300 mA and back current from 230mA to 300 mA. The currents injected into the gain and SOA section were kept constant with values 80mA and 20 mA respectively. An example result for the phase tuning of the slotted laser for a single current set can be seen in Fig. 5
Fig. 5 (a) Phase current vs. peak wavelength of 1535nm; (b) Phase current vs. SMSR ; (c) SMSR vs wavelength.
. In this case we chose the region with high front and back currents of 230 mA and 220 mA respectively. This current setting corresponds with the wavelength of 1535 nm. It is clear the mode competition can be eliminated by re-tuning the lasing peak and therefore get SMSR values over 30 dB. For the phase current from 35 mA to 50 mA the SMSR reaches a good value of 35 dB. The lasing peak is tuned by about 0.3 nm in the process.

The original SMSR map and new data taken from the phase scan are shown in Fig. 6
Fig. 6 SMSR map with phase scan region for the lasers with slot period of 70 μm for the front mirror and 76 μm for the back mirror.
. We chose the maximum SMSR values obtained from scanning the phase current and plotted them against front and back current corresponding to a particular current value and getting a new tuning map as shown in Fig. 6. It can be seen that the phase tuning works well and it is possible to re-tune the lasing peak to get high SMSR for almost all the scanned regions. As is shown the phase tuning works well for almost all scanned regions. Very small regions with low SMSR still occurred due to two-mode competition where the two modes are separated from each other by a super-period set from the Vernier effect. These edge effects are also seen for the SGDBR laser. In the future design, adjusting the slot period would shift those regions to the edge of the tuning map.

5. Linewidth measurements

The linewidth of the laser was measured using the delayed self- heterodyne (DS-H) technique [15

15. T. Okoshi, K. Kikuchi, and A. Nakayama, “Novel method for high resolution measurement of laser output spectrum,” Electron. Lett. 16(16), 630–631 (1980). [CrossRef]

]. The experimental set-up to measure the linewidth of the slotted laser is shown in Fig. 7
Fig. 7 Self-heterodyne experimental set-up for linewidth measurements.
. A free space optical isolator giving 60dB isolation between forward and backward propagating light is included in the measurement system. This prevents backward reflections of light into the laser cavity. 1% of the emitted light is coupled to the optical spectrum analyser OSA to determine the wavelength of the super-mode. The 99% portion of the light is split again in the interferometer and one half is sent through 12 km of optical fiber to produce a delay of 60µs, the delay must be more than the coherence time of the laser. This means that the two beams will combine as if they originated from two different sources giving incoherent mixing. The other path goes through a polarization controller to ensure the polarization is closely matched to maximize the interference between the two beams. This process is equivalent to mixing two lasers withthe same linewidth and central frequency. The linewidth spectrum was recorded using an electrical spectrum analyzer (ESA) with the resolution bandwidth set to 100 kHz.

5. Conclusion

In summary, the performance of a six-section tunable laser based on etched slots has been demonstrated. To remove the facets influence, both facets of the device are applied with anti-reflection coating films. Results show a clearly discrete tuning behaviour covering 12 supermodes using the Vernier tuning effect for the fabricated device at room temperature. The extended discrete tuning range is about 55nm with SMSR>30dB within 12 supermodes. The mode competition causing low SMSR values can be eliminated by changing the current injected into the phase section. The output power is more than 2dBm with a variation < 2dBm for different supermodes during the tuning process. The measured linewidth has been also found with values from 8.6 to 31.6 MHz.

This laser structure can be fabricated by standard photolithography without any regrowth. It is also inherently monolithically integrable with other devices including amplifiers and modulators. Future studies will examine the effects of changing the periods in the front and back sections on the tuning behavior. This laser platform has strong potential to be used as a robust platform for communication applications.

Acknowledgments

Device fabrication was carried out by Compound Semiconductor Technologies (CST) LTD, Glasgow, UK. This work is supported by Science Foundation Ireland (SFI) under grant number 10/CE/I1853.

References and links

1.

R. Phelan, M. Lynch, J. F. Donegan, and V. Weldon, “Simultaneous multispecies gas sensing by use of a sampled grating distributed Bragg reflector and modulated grating Y laser diode,” Appl. Opt. 44(27), 5824–5831 (2005). [CrossRef] [PubMed]

2.

L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: a tutorial,” J. Lightwave Technol. 22(1), 193–202 (2004). [CrossRef]

3.

Y. Tohmori, Y. Yoshikuni, H. Ishii, F. Kano, T. Tamamura, Y. Kondo, and M. Yamamoto, “Broad-range wavelength-tunable superstructure grating (SSG) DBR lasers,” IEEE J. Quantum Electron. 29(6), 1817–1823 (1993). [CrossRef]

4.

A. J. Ward, D. J. Robbins, G. Busico, E. Barton, L. Ponnampalam, J. P. Duck, N. D. Whitbread, P. J. Williams, D. C. J. Reid, A. C. Carter, and M. J. Wale, “Widely tunable DS-DBR laser with monolithically integrated SOA: Design and performance,” IEEE J. Sel. Top. Quantum Electron. 11(1), 149–156 (2005). [CrossRef]

5.

H. Ishii, K. Kasaya, and H. Oohashi, “Narrow spectral linewidth operation (<160kHz) in widely tunable distributed feedback laser array,” Electron. Lett. 46(10), 714–715 (2010). [CrossRef]

6.

W.-H. Guo, Q. Lu, M. Nawrocka, A. Abdullaev, M. Lynch, V. Weldon, and J. F. Donegan, “Integrable slotted single mode lasers,” IEEE Photon. Technol. Lett. 24(8), 634–636 (2012). [CrossRef]

7.

D. Byrne, Q. Lu, W-H. Guo, J. Donegan, B. Corbett, B. Roycroft, P. Lambkin, J-P Engelstaedter, and F. Peters, “A facetless laser suitable for monolithic integration,” in Conference of Lasers and Electro-Optics, (Optical Society of America, 2008), paper JThA28.

8.

Q. Lu, W.-H. Guo, M. Nawrocka, A. Abdullaev, C. Daunt, J. O’Callaghan, M. Lynch, V. Weldon, F. Peters, and J. F. Donegan, “Single mode lasers based on slots suitable for photonic integration,” Opt. Express 19(26), B140–B145 (2011). [CrossRef] [PubMed]

9.

D. Byrne, J.-P. Engelstaedter, W.-H. Guo, Q. Lu, B. Corbett, B. Roycroft, J. O’Callaghan, F. H. Peters, and J. F. Donegan, “Discretely tunable semiconductor lasers suitable for photonics integration,” IEEE J. Sel. Top. Quantum Electron. 15(3), 482–487 (2009). [CrossRef]

10.

Q. Y. Lu, W.-H. Guo, R. Phelan, D. Byrne, J. F. Donegan, P. Lambkin, and B. Corbett, “Analysis of slot characteristics in slotted single- mode semiconductor lasers using the 2-D scattering matrix method,” IEEE Photon. Technol. Lett. 18(24), 2605–2607 (2006). [CrossRef]

11.

H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Quasicontinuous wavelength tuning in super- structure- grating (SSG) DBR lasers,” IEEE J. Quantum Electron. 32(3), 433–441 (1996). [CrossRef]

12.

J. Buus, M.-C. Amman, and D. J. Blumenthal, Tunable Laser Diodes and Related Optical Sources (Wiley-Interscience, 2005).

13.

B. Mason, G. A. Fish, J. Barton, L. A. Coldren, and S. P. DenBaars, “Characteristics of sampled grating DBR lasers with integrated semiconductor optical amplifiers and electroabsorption amplifiers,” In Proc. Conf. Optical Fiber Commun. (2010).

14.

S. Oku, S. Kondo, Y. Noguchi, T. Hirono, M. Nakao, and T. Tamamura, “Surface- grating Bragg reflector lasers using deeply etched groove formed by reactive beam etching,” in Proc. 1998 Int. Conf. Indium Phosphide Relat. Mater. 299–302.

15.

T. Okoshi, K. Kikuchi, and A. Nakayama, “Novel method for high resolution measurement of laser output spectrum,” Electron. Lett. 16(16), 630–631 (1980). [CrossRef]

16.

S. Spießberger, M. Schiemangk, A. Wicht, H. Wenzel, O. Brox, and G. Erbert, “Narrow Linewidth DFB Lasers Emitting Near a Wavelength of 1064nm,” J. Lightwave Technol. 28(17), 2611–2616 (2010). [CrossRef]

17.

D. Byrne, W. H. Guo, R. Phelan, Q. Y. Lu, J. F. Donegan, and B. Corbett, “Measurement of linewidth enhancement factorfor InGaAlAs laser diode by Fourier series expansion method,” Electron. Lett. 43(21), 1145–1146 (2007). [CrossRef]

OCIS Codes
(250.0250) Optoelectronics : Optoelectronics
(250.3140) Optoelectronics : Integrated optoelectronic circuits
(250.5960) Optoelectronics : Semiconductor lasers
(250.5590) Optoelectronics : Quantum-well, -wire and -dot devices

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: May 19, 2014
Revised Manuscript: July 6, 2014
Manuscript Accepted: July 15, 2014
Published: July 29, 2014

Citation
Marta Nawrocka, Qiaoyin Lu, Wei-Hua Guo, Azat Abdullaev, Frank Bello, James O’Callaghan, Thomas Cathcart, and John F. Donegan, "Widely tunable six-section semiconductor laser based on etched slots," Opt. Express 22, 18949-18957 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-16-18949


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References

  1. R. Phelan, M. Lynch, J. F. Donegan, and V. Weldon, “Simultaneous multispecies gas sensing by use of a sampled grating distributed Bragg reflector and modulated grating Y laser diode,” Appl. Opt. 44(27), 5824–5831 (2005). [CrossRef] [PubMed]
  2. L. A. Coldren, G. A. Fish, Y. Akulova, J. S. Barton, L. Johansson, and C. W. Coldren, “Tunable semiconductor lasers: a tutorial,” J. Lightwave Technol. 22(1), 193–202 (2004). [CrossRef]
  3. Y. Tohmori, Y. Yoshikuni, H. Ishii, F. Kano, T. Tamamura, Y. Kondo, and M. Yamamoto, “Broad-range wavelength-tunable superstructure grating (SSG) DBR lasers,” IEEE J. Quantum Electron. 29(6), 1817–1823 (1993). [CrossRef]
  4. A. J. Ward, D. J. Robbins, G. Busico, E. Barton, L. Ponnampalam, J. P. Duck, N. D. Whitbread, P. J. Williams, D. C. J. Reid, A. C. Carter, and M. J. Wale, “Widely tunable DS-DBR laser with monolithically integrated SOA: Design and performance,” IEEE J. Sel. Top. Quantum Electron. 11(1), 149–156 (2005). [CrossRef]
  5. H. Ishii, K. Kasaya, and H. Oohashi, “Narrow spectral linewidth operation (<160kHz) in widely tunable distributed feedback laser array,” Electron. Lett. 46(10), 714–715 (2010). [CrossRef]
  6. W.-H. Guo, Q. Lu, M. Nawrocka, A. Abdullaev, M. Lynch, V. Weldon, and J. F. Donegan, “Integrable slotted single mode lasers,” IEEE Photon. Technol. Lett. 24(8), 634–636 (2012). [CrossRef]
  7. D. Byrne, Q. Lu, W-H. Guo, J. Donegan, B. Corbett, B. Roycroft, P. Lambkin, J-P Engelstaedter, and F. Peters, “A facetless laser suitable for monolithic integration,” in Conference of Lasers and Electro-Optics, (Optical Society of America, 2008), paper JThA28.
  8. Q. Lu, W.-H. Guo, M. Nawrocka, A. Abdullaev, C. Daunt, J. O’Callaghan, M. Lynch, V. Weldon, F. Peters, and J. F. Donegan, “Single mode lasers based on slots suitable for photonic integration,” Opt. Express 19(26), B140–B145 (2011). [CrossRef] [PubMed]
  9. D. Byrne, J.-P. Engelstaedter, W.-H. Guo, Q. Lu, B. Corbett, B. Roycroft, J. O’Callaghan, F. H. Peters, and J. F. Donegan, “Discretely tunable semiconductor lasers suitable for photonics integration,” IEEE J. Sel. Top. Quantum Electron. 15(3), 482–487 (2009). [CrossRef]
  10. Q. Y. Lu, W.-H. Guo, R. Phelan, D. Byrne, J. F. Donegan, P. Lambkin, and B. Corbett, “Analysis of slot characteristics in slotted single- mode semiconductor lasers using the 2-D scattering matrix method,” IEEE Photon. Technol. Lett. 18(24), 2605–2607 (2006). [CrossRef]
  11. H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Quasicontinuous wavelength tuning in super- structure- grating (SSG) DBR lasers,” IEEE J. Quantum Electron. 32(3), 433–441 (1996). [CrossRef]
  12. J. Buus, M.-C. Amman, and D. J. Blumenthal, Tunable Laser Diodes and Related Optical Sources (Wiley-Interscience, 2005).
  13. B. Mason, G. A. Fish, J. Barton, L. A. Coldren, and S. P. DenBaars, “Characteristics of sampled grating DBR lasers with integrated semiconductor optical amplifiers and electroabsorption amplifiers,” In Proc. Conf. Optical Fiber Commun. (2010).
  14. S. Oku, S. Kondo, Y. Noguchi, T. Hirono, M. Nakao, and T. Tamamura, “Surface- grating Bragg reflector lasers using deeply etched groove formed by reactive beam etching,” in Proc. 1998 Int. Conf. Indium Phosphide Relat. Mater. 299–302.
  15. T. Okoshi, K. Kikuchi, and A. Nakayama, “Novel method for high resolution measurement of laser output spectrum,” Electron. Lett. 16(16), 630–631 (1980). [CrossRef]
  16. S. Spießberger, M. Schiemangk, A. Wicht, H. Wenzel, O. Brox, and G. Erbert, “Narrow Linewidth DFB Lasers Emitting Near a Wavelength of 1064nm,” J. Lightwave Technol. 28(17), 2611–2616 (2010). [CrossRef]
  17. D. Byrne, W. H. Guo, R. Phelan, Q. Y. Lu, J. F. Donegan, and B. Corbett, “Measurement of linewidth enhancement factorfor InGaAlAs laser diode by Fourier series expansion method,” Electron. Lett. 43(21), 1145–1146 (2007). [CrossRef]

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