OSA's Digital Library

Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 22 — Nov. 1, 2004
  • pp: 5549–5555
« Show journal navigation

Flattop mode shaping of a vertical cavity surface emitting laser using an external-cavity aspheric mirror

Zhaohui H. Yang and James R. Leger  »View Author Affiliations


Optics Express, Vol. 12, Issue 22, pp. 5549-5555 (2004)
http://dx.doi.org/10.1364/OPEX.12.005549


View Full Text Article

Acrobat PDF (1271 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Both square-shaped and circular-shaped flattop modes were experimentally demonstrated in extended-cavity broad-area VCSELs using aspheric feedback mirrors. These refractive aspheric mirrors were fabricated by electron-beam lithography on curved substrates. Excellent single-mode operation and improved power extraction efficiency were observed. The three-mirror structure of the VCSEL and the state-of-the-art fabrication of the aspheric mirror contribute to the superior VCSEL performance. The modal loss analysis using a rigid three-mirror-cavity simulation method is discussed.

© 2004 Optical Society of America

1. Introduction

Vertical cavity surface emitting lasers (VCSELs) offer tremendous advantages over traditional edge-emitting semiconductor lasers because of their low fabrication cost, low threshold current, circular-mode profile, and high modulation speeds. In addition, two-dimensional VCSEL arrays can be easily fabricated due to their planar structure and orthogonal emission to the wafer surface. While VCSELs have become the ideal choice in many low-power applications, their utilization is limited in high-power applications such as high-speed optical communication and optical storage, where high-power single-spatial-mode emitters are preferred. Single-spatial-mode VCSELs are often limited by aperture size. The resulting small gain volume restricts the maximum single-mode power to just a few milliwatts [1]. There have been numerous efforts aimed at designing large-aperture single-spatial-mode VCSELs during the past years. Among those efforts, the extended-cavity VCSELs consisting of epitaxially grown VCSELs and collinearly aligned external elements [2

2. J. G. McInerney, A. Mooradian, A. Lewis, A.V. Shchegrov, E. M. Strzelecka, D. Lee, J. P. Watson, M. Liebman, G. P. Garey, B. D. Cantos, W. R. IIitchens, and D. IIeald, “High-power surface emitting semiconductor laser with extended vertical compound cavity,” Electron. Lett. 39, 523–525 (2003). [CrossRef]

,3

3. B. J. Koch, J. R. Leger, A. Gopinath, and Z. Wang, “Single-mode vertical cavity surface emitting laser by graded-index lens spatial filtering,” Appl. Phys. Lett. 70, 2359–2361 (1997). [CrossRef]

] have attained some of the highest single-mode powers. In this paper, we describe flattop mode shaping in an extended-cavity broad-area VCSEL with an external aspheric feedback mirror.

It has been shown that the spatial mode of a laser cavity can be shaped into a specific intensity profile by using an aspheric mirror as a fixed phase-conjugate surface in a mode-conjugate resonator [4

4. C. Paré and P. A. Bélanger, “Custom laser resonators using graded-phase mirrors: circular geometry,” IEEE J. Quantum Electron. 30, 1141–1148 (1994). [CrossRef]

,5

5. J. R. Leger, D. Chen, and G. Mowry, “Design and performance of diffractive optics for custom laser resonators,” Appl. Opt. 34, 2498–2509 (1995). [CrossRef] [PubMed]

]. So far this aspheric resonator technique has only been successfully applied in high-gain lasers due to the fabricated aspheric mirror loss. In this paper, for the first time, we demonstrate flattop mode shaping in a low-gain VCSEL by using an external-cavity aspheric mirror in combination with the existing top mirror. The combination of the reduced-reflectivity top DBR of the VCSEL and the external aspheric mirror can be viewed as an etalon [6

6. Z. H. Yang and J. R. Leger, “Three-mirror resonator with aspheric feedback mirror for laser spatial mode selection and mode shaping,” IEEE J. Quantum Electron. 40, 1258–1269 (2004). [CrossRef]

]. The equivalent reflectivity of this etalon is significantly greater than either of the reflecting surfaces alone. This three-mirror resonator configuration contributes to the cavity loss reduction and enables lasing in the low-gain VCSEL medium at the desired spatial mode.

Both square-shaped and circular-shaped flattop VCSEL modes are experimentally discussed and implemented in this paper. While the square-shaped flattop mode dramatically demonstrates the mode-shaping capabilities of the aspheric extended-cavity technique, the circular-shaped flattop mode better utilizes the geometry of the circular VCSEL aperture and offers higher conversion efficiency and power performance.

2. External-cavity VCSEL configuration and aspheric mirror fabrication

The external aspheric mirror has a surface profile exactly matching the wavefront of a diffracted flattop mode whose beam waist is at the VCSEL aperture, as shown in Fig. 1. A 20th-order square SuperGaussian with a beam width of 80 µm (or a beam waist of 40 µm) was chosen as the desired mode, and the aspheric mirror was to be placed 7 mm away from the VCSEL aperture. (A nth-order SuperGaussian is defined as φ(x)=exp[-x2n0]) The same setting was used for the square-flattop mode shaping discussed elsewhere in this paper.

Fig. 1. An aspheric mirror surface for square-flattop mode shaping compared with a spherical surface with a radius of curvature of 9.3 mm.
Fig. 2. Fabricated aspheric mirror surface figure (for square-flattop mode shaping) after removing the underlying spherical envelope. (a) 2-D view, (b) 1-D slice along x at y=0.

The aspheric mirrors for mode shaping have been fabricated as diffractive elements using wet and dry etching, and as continuous reflective elements using diamond turning. The losses associated with these two techniques at near-visible wavelengths make them unsuitable for low-gain lasers. Fortunately, the surface figure of an aspheric mirror for flattop mode shaping has an underlying envelope quite close to a sphere (Fig. 1). We therefore chose to fabricate the aspheric mirror on a spherical substrate. The small phase departure from the spherical surface was fabricated by electron-beam lithography. The electron beam only needs to expose a thin resist layer, reducing the defocus through the resist layer and the proximity effect. When the commercially available concave substrate of high surface precision is combined with a somewhat less accurate micro-fabricated surface relief structure, the overall aspheric mirror quality can be significantly improved. We fabricated our aspheric mirror on a BK7 blank concave mirror with a radius of curvature of 9.3 mm. A 2-µm-thick layer of PMGI electron-beam resist was spun on top of the substrate for electron-beam exposure. Fig. 2 shows the surface of a fabricated aspheric mirror viewed with an interference microscope (Zygo New View 100) where the underlying spherical envelope has been removed. For a 512 µm×512 µm mirror size, the maximum surface relief depth was ~600 nm, while the maximum relief depth would have needed to be ~4 µm if the aspheric structure were fabricated on a flat substrate. The subsequently developed aspheric mirror was then coated with 20-nm-thick titanium followed by 100-nm-thick gold for optical reflection.

Fig. 3. Simulated modal loss ([1-γ 2] where γ is the round-trip eigenvalue) and modal discrimination ([γ0thmode2/γ1stmode2]) as a function of the external cavity length.

3. Results

Figure 4 shows the experimental square-flattop mode in the VCSEL by using an external aspheric mirror with a surface figure depicted in Fig. 2. Rectangularly pulsed current with a repetition rate of 1 kHz and pulse durations of 1 µs was used to drive the VCSEL. The same pulsed current was used in other pulse-driven-VCSEL measurements; the performance of CW-driven VCSELs will be discussed at the end. Figure 4(a) is the near-field intensity with a CCD camera imaging the VCSEL plane. Figure 4(b) is the far-field intensity with the CCD camera directly placed 10 cm away from the VCSEL aperture. Distinct side lobes at the predicted spatial locations (the expected Fourier transform of the near-field pattern) in the far field are clearly visible.

Fig. 4. Square flattop mode in circular aperture VCSEL. (a) 2-D near-field intensity, (b) 1-D slice of far-field intensity.

Fig. 5. Spectra of the extended-cavity VCSELs with different configurations under pulsed driving current of 800 mA.

Fig. 6 shows the measured single-mode power as a function of pump currents for the extended-cavity VCSELs with different configurations under pulsed driving currents. The blue dashed curve is for the VCSEL with a concave feedback mirror, and the red solid curve is for the VCSEL with an aspheric feedback mirror for the circular-flattop mode shaping. As mentioned previously, placing an external concave mirror at 2×f (focal length) away from the VCSEL aperture only results in a multi-spatial-mode oscillation. One way to obtain single-mode concave extended-cavity VCSEL is to tune the external mirror away from the 2f position. In our experiment, the external concave mirror was moved about 4 cm closer to the VCSEL aperture, resulting in an external cavity length of 5.3 cm. Single-spatial-mode operation was observed at all current levels for this concave extended-cavity VCSEL design. The L-I curves indicate that the aspheric extended-cavity VCSEL had a slightly higher threshold current due to the mirror fabrication imperfection and misalignment, but had 20% higher slope efficiency compared with that of the concave extended-cavity VCSEL.

The laser slope efficiency is defined as:

dPdI=ω2qηdwithηd=ηintαocαoc+αint
(1)

where ħω is the photon energy, q is the electron charge, ηd is the differential efficiency, ηint is the internal quantum efficiency, αoc is the output coupling loss, and αint is the total of other losses. Note that we used the pellicle beamsplitter as the output coupling element with a coupling loss of αoc , and any power loss on the external mirror is considered to be part of αint . Since the concave and the aspheric extended-cavity VCSELs have the same beamsplitter setting for the power output, they have same αoc . The fact that the concave extended-cavity VCSEL has a lower threshold implies smaller αint . Thus an improved internal quantum efficiency ηint for the aspheric extended-cavity VCSEL is deduced from the slope efficiency measurement.

Fig. 6. Single-mode power as a function of pulsed driving current for the extended-cavity VCSELs with different configurations.

The power performance of the extended-cavity VCSELs under CW driving currents was also investigated. The aspheric extended-cavity VCSEL had about the same slope efficiency as that of the concave extended-cavity VCSEL at low pumping currents, but experienced an earlier thermal rollover (at 6I th for the aspheric extended-cavity VCSEL versus 8I th for the concave extended-cavity VCSEL). This was because thermal lensing was not modeled in the aspheric extended-cavity VCSEL design, but could not be ignored in the case of CW operation. Power performance of the aspheric extended-cavity VCSEL can be improved by better heat removal or incorporation of the thermal lensing effect into the external aspheric mirror design.

4. Conclusion

Acknowledgments

The authors would like to thank Aram Mooradian of Novalux Inc for the assistance and material support.

References and links

1.

H. J. Unold, S. W. Z. Mahmoud, R. Jäger, M. Grabherr, R. Michalzik, and K. J. Ebeling, “Large-Area Single-Mode VCSELs and the Self-Aligned Surface Relief,” IEEE J. Sel. Top. Quantum Electron. 7, 386–392 (2001). [CrossRef]

2.

J. G. McInerney, A. Mooradian, A. Lewis, A.V. Shchegrov, E. M. Strzelecka, D. Lee, J. P. Watson, M. Liebman, G. P. Garey, B. D. Cantos, W. R. IIitchens, and D. IIeald, “High-power surface emitting semiconductor laser with extended vertical compound cavity,” Electron. Lett. 39, 523–525 (2003). [CrossRef]

3.

B. J. Koch, J. R. Leger, A. Gopinath, and Z. Wang, “Single-mode vertical cavity surface emitting laser by graded-index lens spatial filtering,” Appl. Phys. Lett. 70, 2359–2361 (1997). [CrossRef]

4.

C. Paré and P. A. Bélanger, “Custom laser resonators using graded-phase mirrors: circular geometry,” IEEE J. Quantum Electron. 30, 1141–1148 (1994). [CrossRef]

5.

J. R. Leger, D. Chen, and G. Mowry, “Design and performance of diffractive optics for custom laser resonators,” Appl. Opt. 34, 2498–2509 (1995). [CrossRef] [PubMed]

6.

Z. H. Yang and J. R. Leger, “Three-mirror resonator with aspheric feedback mirror for laser spatial mode selection and mode shaping,” IEEE J. Quantum Electron. 40, 1258–1269 (2004). [CrossRef]

OCIS Codes
(140.3300) Lasers and laser optics : Laser beam shaping
(140.3570) Lasers and laser optics : Lasers, single-mode
(250.7260) Optoelectronics : Vertical cavity surface emitting lasers

ToC Category:
Research Papers

History
Original Manuscript: August 11, 2004
Revised Manuscript: October 20, 2004
Published: November 1, 2004

Citation
Zhaohui Yang and James Leger, "Flattop mode shaping of a vertical cavity surface emitting laser using an external-cavity aspheric mirror," Opt. Express 12, 5549-5555 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-22-5549


Sort:  Journal  |  Reset  

References

  1. H. J. Unold, S. W. Z. Mahmoud, R. Jäger, M. Grabherr, R. Michalzik, and K. J. Ebeling, �??Large-Area Single-Mode VCSELs and the Self-Aligned Surface Relief,�?? IEEE J. Sel. Top. Quantum Electron. 7, 386-392 (2001). [CrossRef]
  2. J. G. McInerney, A. Mooradian, A. Lewis, A.V. Shchegrov, E. M. Strzelecka, D. Lee, J. P. Watson, M. Liebman, G. P. Garey, B. D. Cantos, W. R. IIitchens, and D. IIeald, �??High-power surface emitting semiconductor laser with extended vertical compound cavity,�?? Electron. Lett. 39, 523-525 (2003). [CrossRef]
  3. B. J. Koch, J. R. Leger, A. Gopinath, and Z. Wang, �??Single-mode vertical cavity surface emitting laser by graded-index lens spatial filtering,�?? Appl. Phys. Lett. 70, 2359-2361 (1997). [CrossRef]
  4. C. Paré and P. A. Bélanger, �??Custom laser resonators using graded-phase mirrors: circular geometry,�?? IEEE J. Quantum Electron. 30, 1141-1148 (1994). [CrossRef]
  5. J. R. Leger, D. Chen, and G. Mowry, �??Design and performance of diffractive optics for custom laser resonators,�?? Appl. Opt. 34, 2498-2509 (1995). [CrossRef] [PubMed]
  6. Z. H. Yang and J. R. Leger, �??Three-mirror resonator with aspheric feedback mirror for laser spatial mode selection and mode shaping,�?? IEEE J. Quantum Electron. 40, 1258-1269 (2004). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited