Beam quality improvement of laser diode array by using off-axis external cavity

doi:10.1364/OE.15.011776

« Show journal navigation

Beam quality improvement of laser diode array by using off-axis external cavity

Su Zhouping, Lou Qihong, Dong Jingxing, Zhou Jun, and Wei Runrong »View Author Affiliations

Optics Express, Vol. 15, Issue 19, pp. 11776-11780 (2007)
http://dx.doi.org/10.1364/OE.15.011776

View Full Text Article

Acrobat PDF (233 KB)

Journal Search
Article Lookup

Article Tools

Citations

Alert me when this article is cited
Export Citation/Save

Article Navigation

▸ Article Outline
– Abstract
1. Introduction
2. Experiment
3. Results and discussion
4. Conclusion
– References and links

Article Tools
Full Text
Article Info
References
Cited By
Figures
Metrics
Download PDF

Viewing Equations in the
HTML Article

Optimal Viewing Recommendations

Full Text
Article Info
References (12)
Cited By
Figures (4)
Metrics

Abstract

A novel off-axis external cavity is designed for laser diode array to improve the beam quality. In this external cavity, a circle aperture with variable size is used as a spatial filter. The diameter of aperture is optimized to 1.2mm and the off-axis angle of external cavity is optimized at 2.6 deg. In the optimal case, the beam parameter product (BPP) of laser diode array is reduced to 121 mm.mrad from 1050 mm.mrad with external cavity optical efficiency of 81%.

1. Introduction

Due to their compactness, low cost, relative ease of operation and excellent efficiency, high-power laser diode arrays (LDA) are attractive as optical sources, with many applications in industry, medicine, and science [1

E. Samsøe and P. E. Andersen, “Improvement of spatial and temporal coherence of a broad area laser diode using an external-cavity design with double grating feedback,” Opt. Express 12, 609 (2004). [CrossRef] [PubMed]

–5

Y. Zheng and H. Kan, “Effective bandwidth reduction for a high-power laser-diode array by an externalcavity technique,” Opt. Lett. 30, 2424–2426 (2005). [CrossRef] [PubMed]

]. Unfortunately, the poor beam quality is a still limiting factor for many applications such as efficient coupling to optical fibers or nonlinear generation of new frequencies. So it is important to improve the beam quality. This poor beam quality is a consequence of the highly elongated emitting region, ~1µm×10mm, consisting of a linear array of individual emitters. The net result is a combined output beam that is nearly diffraction limited in the plane orthogonal to the plane of the array but that has a divergence ≥1000× diffraction limited in the array plane. This large mismatch in the beam-quality factors for orthogonal planes renders the output extremely difficult to focus into the small circular spot required by many applications (e.g., end pumping)[6

W. A. Clarkson and D. C. Hanna, “Two-mirror beam-shaping technique for high-power diode bars,” Opt. Lett. 21, 375–377 (1996). [CrossRef] [PubMed]

]. Various investigations have been developed to improve spatial beam quality. A common technique in improving beam quality of laser diode bars in the slow axis is beam shaping such as the shaping of two mirrors [6

W. A. Clarkson and D. C. Hanna, “Two-mirror beam-shaping technique for high-power diode bars,” Opt. Lett. 21, 375–377 (1996). [CrossRef] [PubMed]

], beam-shaping prism groups [7

P. Y. Wang, “Beam-shaping optics deliver high-power beam,” Laser Focus World 37, 115–118 (2001)

], and so on. In the fast axis, which is perpendicular to the array direction of a laser diode bar, the beam displays good quality and can be collimated to several milliradians by use of a microcylindrical lens. The external-cavity technique is also an effective method for laser diode bar to improve the beam quality. Xin Gao et al. reported an external cavity with a stripe mirror. The strip mirror is coated with a 500-µm pitch period, half of which (250 µm) is high reflection for feedback and the other half of which is antireflection for the output window for each laser diode element [4

X. Gao, Y. Zheng, H. Kan, and K. Shinoda, “Effective suppression of beam divergence for a high-power laser diode bar by an external-cavity technique,” Opt. Lett. 29, 361–363 (2004). [CrossRef] [PubMed]

]. However, the stripe mirror need be specially made. In this paper, we demonstrate an external cavity that consists of normal optical elements. In order to enhance further the mode selectivity of the setup, we place a circle aperture with variable size as a spatial filter (SF) in this external cavity.

2. Experiment

The experimental setup is shown in Fig. 1.

Fig. 1. Experimental setup

In our experiment, we used a commercial 17-W, 1cm-long, 49 -emitter LDA at 808nm. The dimension of each emitter is 1 µm by 80 µm with a separation between emitters of 200 µm. Light emitted perpendicular to the array (fast axis) has a divergence of 40 deg, and light emitted parallel to array (slow axis) has a divergence of 10 deg with multi-spatial mode. The asymmetric divergence of the individual emitters requires use of extra collimation optics. An aspheric lens with short focal length is used as fast axis collimator (FAC) which collimated the rapidly diverging light along y direction. The array parallel to x-axis (slow axis).

The setup of the external cavity is designed off axis. A high-reflective mirror is used as feedback element. The front facet of diode laser array is antireflection coated, so there is a laser resonator formed for each array element between the high-reflector coating on the back facet and high-reflective mirror. Thus, 49 off-axis external-cavity laser diodes are formed by feedback with a high-reflective mirror. Each external cavity laser diode is separated into a feedback branch and an output branch. The feedback branch is made up of lens1, lens2 and high-reflective mirror. And the output branch is made up of lens1 and lens3. The f/number of lens1 is f/1.875 with a focal length of 150mm. The f/number of both lens2 and lens3 is f/2 with a focal length of 40mm. The LDA is placed in the front focal plane of lens1. The lens1 acts as a transform lens that transform the near field of LDA into far field and the lens2 can be used as a collimating lens. It is well known around tens of modes oscillate in single-element laser diode on the freely running condition. Each mode has a double-lobe intensity profile in the far field [8

J.-M. Verdell and R. Freyieee, “A broad-area mode-coupling model for multiple stripe semiconductor lasers,” IEEE J. Quantum Electron. 26, 270–279(1990). [CrossRef]

–12

E. Samsøe, P. Malm, P. E. Andersen, P. M. Petersen, and S. Andersson-Engels, “Improvement of brightness and output power of high-power laser diodes in the visible spectral region,” Opt. Commun. 219, 369–375 (2003). [CrossRef]

]. For a single-element laser diode, the beam quality improvement by using off-axis external cavity technique can be understood in the following way:

Due to the different radiation angles of various spatial modes, a single lobe of one of the spatial modes may be chosen by adjustment of high-reflective mirror tilt. The selected lobe of this mode is feedback into the laser diode, the other lobe of this mode is amplified and coupled out of the external cavity as output beam of laser system and all the other modes are suppressed effectively[10

M. Chi, N.-S. Bøgh, B. Thestrup, and P. M. Petersen, “Improvement of the beam quality of a broad-area diode laser using double feedback from two external mirrors,” Appl. Phys. Lett. 85, 1107–1109 (2004). [CrossRef]

–12

]. By this, each diode will oscillate in a narrow mode range. Due to the common feedback element shared by all emitters, the output beam of all emitters will propagate in the same direction. As a result, the beam quality is improved drastically.

In order to enhance further the mode selectivity of the setup, we place a circle aperture as a spatial filter(SF) in back focal plane of lens1. The size of aperture is variable and the aperture can be moved along the back focal plane of lens1. By varying the size of aperture, moving the aperture and adjusting high-reflection mirror tilt, we can obtain the optimal laser output.

3. Results and discussion

In this paper, the beam parameter product (BPP, i.e., beam width ×beam full divergence) is used to evaluate beam quality of LDA. Figure 2 shows a typical BPP(in the slow axis) versus current for free-running LDA and external cavity feedback laser diode array(ECLDA). At the drive current of 17A, the BPP measured 1050 mm mrad and 152 mm mrad without and with external cavity respectively. When a SF with the optimal diameter (1.2mm) is inserted at the optimal position in the feedback branch, the BPP is reduced to 121mm mrad. In the following, we will discuss how to choose the optimal diameter of SF and determine the optimal position of SF. With the drive current increasing, the BPP of ECLDA and of free-running LDA is increased. The BPP of ECLDA with a SF is almost unchanged. For the ECLDA, this shows that the extra gain obtained from the higher drive current can overcome the feedback effect introduced by the external cavity [3

J. Chen, X. Wu, J. Ge, A. Hermerschmidt, and H. J. Eichler, “Broad-area laser diode with 0.02 nm bandwidth and diffraction limited output due to double external cavity feedback,” Appl. Phys. Lett. 85, 525–527 (2004). [CrossRef]

]. As a result, more extra modes will oscillate and the beam quality will be degraded. A SF placed in the feedback branch will eliminate some extra modes. Thus, the beam quality of laser output with a SF is almost unchanged.

Fig. 2. BPP variations of free-running LDA and of ECLDA versus the driver current

The output power of ECLDA measured 2.63W at the drive current of 17A or 85% of the radiated power of the free-running state; while the output power of diode array with the external cavity and a SF is 2.52W that is 81% of the radiated power of the free-running state.

Here, we define the external cavity optical efficiency as the ratio of output power of the ECLDA to the output power of the free-running LDA. The output power of 2.63W seems rather low for a 17-W LDA. This can be explained as following: The LDA used in experiment has a standard antireflection coating on the output facet with a reflectivity about 5%. For the ECLDA, due to reflection of about 5 percent in the output facet of LDA, at high current, the external feedback effect is overcome by extra gain obtained from higher driver current. As a result, the beam quality will be degraded. In order to obtain higher power output with good beam quality, output facet of LDA need be specially antireflection coated.

Figure 3 shows the BPP variation of ECLDA versus various aperture positions (the diameter of aperture is fixed to 1.2 mm).

Fig. 3. BPP variation of ECLDA versus various aperture positions

The aperture position is specified by the distance between optical axis and center of aperture. As shown in Fig. 3, the distance between optical axis and center of aperture has been changed from 3.5mm to 9nm. We can see clearly that when the aperture is moved to about 6.8mm from optical axis and the laser output with best beam quality is obtained. The off-axis angle of external cavity is defined as following:

θ = \frac{D}{f_{1}}

(1)

here D is the distance between optical axis and center of aperture and f ₁ is the focal length of lens1. According to the formula (1), the optimal off-axis angle of external cavity is calculated to θ=2.6 deg.

Figure 4 shows BPP variation of ECLDA versus various aperture sizes (the distance between optical axis and center of aperture is fixed to 6.8mm). We can see clearly from figure 4 that the optimal diameter of aperture is about 1.2 mm. When the diameter of aperture varied from 1mm to 1.5 mm, the beam displays good quality.

Fig. 4. BPP variation of ECLDA versus various aperture sizes

When the diameter of aperture is less than 1mm, with the size of aperture reducing, the beam quality drastically degraded. A major factor leading to the problem is geometric aberrations of optical elements in external cavity. Because of geometric aberrations of lens1, beams from different emitters traveling in the same direction can not converge at a common point at aperture position but a blur. Therefore, with the size of aperture reducing, some beams can not pass through the aperture. Also, because of geometric aberrations of lens2, some feedback beams can not pass through the aperture. These will lead to a great loss of feedback. As a result, the external cavity effect is reduced and the beam quality is degraded. When the diameter of aperture is more than 1.5mm and less than 4 mm, with the size of aperture increasing, the beam quality gradually degraded. This is because the function of mode-selection of SF gradually weakened, with the size of SF increasing. When the diameter of aperture is more than 4 mm, the function of mode-selection of SF is almost out of action. Therefore, with the size of aperture increasing, the beam quality is almost unchanged.

4. Conclusion

In conclusion, we have described a novel off-axis external cavity for laser diode array to improve the beam quality. A normal high-reflective mirror is used as feedback element. In order to enhance further the mode selectivity of the setup, a circle aperture with variable size is used as a SF in the external cavity. The diameter of SF is optimized to 1.2mm and the off-axis angle of external cavity is optimized at 2.6 deg. In the optimal case, the BPP of the laser diode array is reduced from 1050mm.mrad to 121mm.mrad with external cavity optical efficiency of 81%. It is expected that the output power may be increased significantly with good beam quality if the output facet of LDA can be specially antireflection coated.

References and links

1.	E. Samsøe and P. E. Andersen, “Improvement of spatial and temporal coherence of a broad area laser diode using an external-cavity design with double grating feedback,” Opt. Express 12, 609 (2004). [CrossRef] [PubMed]
2.	S. B. Bayrama and T. E. Chupp, “Operation of a single mode external-cavity laser diode array near 780nm,” Rev. Sci. Instrum 73, 4169–4171 (2002). [CrossRef]
3.	J. Chen, X. Wu, J. Ge, A. Hermerschmidt, and H. J. Eichler, “Broad-area laser diode with 0.02 nm bandwidth and diffraction limited output due to double external cavity feedback,” Appl. Phys. Lett. 85, 525–527 (2004). [CrossRef]
4.	X. Gao, Y. Zheng, H. Kan, and K. Shinoda, “Effective suppression of beam divergence for a high-power laser diode bar by an external-cavity technique,” Opt. Lett. 29, 361–363 (2004). [CrossRef] [PubMed]
5.	Y. Zheng and H. Kan, “Effective bandwidth reduction for a high-power laser-diode array by an externalcavity technique,” Opt. Lett. 30, 2424–2426 (2005). [CrossRef] [PubMed]
6.	W. A. Clarkson and D. C. Hanna, “Two-mirror beam-shaping technique for high-power diode bars,” Opt. Lett. 21, 375–377 (1996). [CrossRef] [PubMed]
7.	P. Y. Wang, “Beam-shaping optics deliver high-power beam,” Laser Focus World 37, 115–118 (2001)
8.	J.-M. Verdell and R. Freyieee, “A broad-area mode-coupling model for multiple stripe semiconductor lasers,” IEEE J. Quantum Electron. 26, 270–279(1990). [CrossRef]
9.	R. M. R. Pillai and E. M. Garmire, “Paraxial-misalignment insensitive external-cavity semiconductor-laser array emitting near-diffraction limited single-lobed beam,” IEEE J. Quantum Electron. 32, 996–1008 (1996). [CrossRef]
10.	M. Chi, N.-S. Bøgh, B. Thestrup, and P. M. Petersen, “Improvement of the beam quality of a broad-area diode laser using double feedback from two external mirrors,” Appl. Phys. Lett. 85, 1107–1109 (2004). [CrossRef]
11.	M. Chi, B. Thestrup, and P. M. Petersen, “Improvement of the beam quality of a 1000 µm wide broad-area diode laser with self-injection phase locking in an external cavity,” Proc. SPIE 5336, 33–37 (2004) [CrossRef]
12.	E. Samsøe, P. Malm, P. E. Andersen, P. M. Petersen, and S. Andersson-Engels, “Improvement of brightness and output power of high-power laser diodes in the visible spectral region,” Opt. Commun. 219, 369–375 (2003). [CrossRef]

OCIS Codes
(140.2010) Lasers and laser optics : Diode laser arrays
(140.3460) Lasers and laser optics : Lasers
(140.3520) Lasers and laser optics : Lasers, injection-locked

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: June 22, 2007
Revised Manuscript: July 28, 2007
Manuscript Accepted: August 13, 2007
Published: August 31, 2007

Citation
Su Zhouping, Lou Qihong, Dong Jingxing, Zhou Jun, and Wei Runrong, "Beam quality improvement of laser diode array by using off-axis external cavity," Opt. Express 15, 11776-11780 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-19-11776

Sort: Year | Journal | Reset

References

E. Samsøe and P. E. Andersen, "Improvement of spatial and temporal coherence of a broad area laser diode using an external-cavity design with double grating feedback," Opt. Express 12,609 (2004). [CrossRef] [PubMed]
S. B. Bayrama and T. E. Chupp, "Operation of a single mode external-cavity laser diode array near 780nm," Rev. Sci. Instrum 73, 4169-4171 (2002). [CrossRef]
J. Chen, X. Wu, J. Ge, A. Hermerschmidt, and H. J. Eichler, "Broad-area laser diode with 0.02 nm bandwidth and diffraction limited output due to double external cavity feedback," Appl. Phys. Lett. 85, 525-527 (2004). [CrossRef]
X. Gao, Y. Zheng, H. Kan, and K. Shinoda, "Effective suppression of beam divergence for a high-power laser diode bar by an external-cavity technique," Opt. Lett. 29, 361-363 (2004). [CrossRef] [PubMed]
Y. Zheng and H. Kan, "Effective bandwidth reduction for a high-power laser-diode array by an external-cavity technique," Opt. Lett. 30,2424-2426 (2005). [CrossRef] [PubMed]
W. A. Clarkson and D. C. Hanna, "Two-mirror beam-shaping technique for high-power diode bars," Opt. Lett. 21, 375-377 (1996). [CrossRef] [PubMed]
P. Y. Wang, "Beam-shaping optics deliver high-power beam," Laser Focus World 37, 115-118 (2001)
J.-M. Verdell and R. Freyieee, "A broad-area mode-coupling model for multiple stripe semiconductor lasers," IEEE J. Quantum Electron. 26, 270-279(1990). [CrossRef]
R. M. R. Pillai and E. M. Garmire, "Paraxial-misalignment insensitive external-cavity semiconductor-laser array emitting near-diffraction limited single-lobed beam," IEEE J. Quantum Electron. 32, 996-1008 (1996). [CrossRef]
M. Chi, N.-S. Bøgh, B. Thestrup and P. M. Petersen, "Improvement of the beam quality of a broad-area diode laser using double feedback from two external mirrors," Appl. Phys. Lett. 85, 1107-1109 (2004). [CrossRef]
M. Chi, B. Thestrup, and P. M. Petersen, "Improvement of the beam quality of a 1000 μm wide broad-area diode laser with self-injection phase locking in an external cavity," Proc. SPIE 5336, 33-37 (2004) [CrossRef]
E. Samsøe, P. Malm, P. E. Andersen, P. M. Petersen and S. Andersson-Engels, "Improvement of brightness and output power of high-power laser diodes in the visible spectral region," Opt. Commun. 219, 369-375 (2003). [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.

Click here to see a list of articles that cite this paper