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Optics Letters

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  • Editor: Alan E. Willner
  • Vol. 36, Iss. 3 — Feb. 1, 2011
  • pp: 361–363
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465 nm laser sources by intracavity frequency doubling using a 49-edge-emitters laser bar

K. Li, H. Wang, N. J. Copner, C. B. E. Gawith, I. G. Knight, H.-U. Pfeiffer, B. Musk, and G. Moss  »View Author Affiliations


Optics Letters, Vol. 36, Issue 3, pp. 361-363 (2011)
http://dx.doi.org/10.1364/OL.36.000361


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Abstract

A compact blue laser was generated by intracavity frequency doubling based on quasi-phase-matched second- harmonic generation (SHG) in a MgO-doped periodically poled lithium niobate bulk crystal. A 49 single-transverse- mode edge-emitters laser bar with antireflective coating was used as a pump source. An optical output power of 1.2 W SHG of blue lights at 465 nm is generated at 45 A injection current, equivalent to an overall wall-plug efficiency of 1.33%.

© 2011 Optical Society of America

To date, several platforms have been developed for generation of compact, high-brightness, high-efficiency blue lasers. For the first technology, frequency doubling of diode-pumped solid-state (DPSS) lasers with Nd:YVO4 lasing at 914nm and Nd:YAG lasing at 946nm are perhaps the most widely used commercial solid-state blue lasers [1

1. Z. Quan, Y. Yi, L. Bin, Q. Dapeng, and Z. Ling, J. Opt. Soc. Am. B 26, 1238 (2009). [CrossRef]

, 2

2. Y. L., J. Xia, J. Wang, A. Zhang, X. Zhang, L. Bao, H. Quan, and X. Yin, Chin. Opt. Lett. 8, 187 (2010). [CrossRef]

]. Mitsubishi engineers [3

3. Y. Hirano, S. Yamamoto, Y. Akino, A. Nakamura, T. Yagi, H. Sugiura, and T. Yanagisawa, in Advanced Solid-State Photonics,OSA Technical Digest Series (Optical Society of America, 2009), paper WE1.

] improved this platform, using an unusual planar-waveguide configuration for replacing both the fundamental and second-harmonic bulk crystal for green-light generation.

To improve the efficiency and compactness of the blue laser, it is necessary to replace the solid-state gain medium in the DPSS system by directly using a GaAs/GaAlAs/InGaAs-based diode laser and doubling it to blue. Novalux, Inc., engineers demonstrated a 460nm [4

4. J. P. Watson, A. V. Shchegrov, A. Umbrasas, D. Lee, C. A. Amsden, W. Ha, G. P. Carey, V. V. Doan, A. Lewis, and A. Mooradian, Proc. SPIE-Int. Soc. Opt. Eng. 5364, 116 (2004).

] laser based on the intracavity frequency doubling (ICFD) of a diode surface-emitting laser. High-power, narrow- bandwidth, edge-emitting diode lasers directly used for second-harmonic generation (SHG) attracted many scientists’ attention recently [5

5. A. Jechow, R. Menzel, K. Paschke, and G. Erbert, Laser Photon. Rev. 4, 633 (2010). [CrossRef]

]. External-cavity diode lasers, laser diodes based on master oscillator power amplifier systems, and distributed-Bragg-reflector tapered lasers have been reported for single-pass SHG blue [6

6. M. Chi, O. B. Jensen, J. Holm, C. Pedersen, P. E. Andersen, G. Erbert, B. Sumpf, and P. M. Petersen, Opt. Express 13, 10589 (2005). [CrossRef] [PubMed]

, 7

7. M. Maiwald, S. Schwertfeger, R. Güther, B. Sumpf, K. Paschke, C. Dzionk, G. Erbert, and G. Tränkle, Opt. Lett. 31, 802 (2006). [CrossRef] [PubMed]

, 8

8. C. Fiebig, A. Sahm, M. Uebernickel, G. Blume, B. Eppich, K. Paschke, and G. Erbert, Opt. Express 17, 22785 (2009). [CrossRef]

] and green [9

9. O. B. Jensen, P. E. Andersen, B. Sumpf, K.-H. Hasler, G. Erbert, and P. M. Petersen, Opt. Express 17, 6532 (2009). [CrossRef] [PubMed]

] light.

In this Letter, we show a compact blue laser by ICFD based on a bulk-type quasi-phase-matched (QPM) SHG in a MgO-doped periodically poled lithium niobate (MgO:PPLN) crystal. A multiemitters high-performance laser bar is used as the pump laser. 1.2W blue light at 465nm is generated at 45A injection current. No diode laser facet degradation or damage is observed during a two-hour stability test of blue-laser output power.

The experiment setup is shown in Fig. 1. The laser bar on a 25.4  mm×25.4  mm passive Cu block, manufactured at Oclaro AG in Zurich, Switzerland, comprises 49 single-transverse-mode laser emitters laid out in a regular array with a pitch of 200μm. The cavity length is 3.6mm. The front facet carries a specially adapted ultralow reflectivity coating with a target reflectivity of less than 0.1% at a wavelength of 930nm to facilitate external locking of the emission wavelength as required for efficient frequency doubling. The back facet has a standard high-reflectivity coating suited to serve as an end mirror in an external-cavity arrangement. Maximum operating light output power is around 35W with an operating current of 45A and voltage of 2.03V. The typical laser bar emission wavelength (at operating conditions) is at 930±3nm with lateral far field divergence (FWHM) of 4.6 deg in slow axis and 21 deg in fast axis. The threshold current is at 2.8A and slope efficiency is of 0.99 (W/A).

A cylindrical lens, L1 (f=121.5μm), and a microlens array, L2 (f=776μm), are used for laser bar beam shaping. L2 consists of 49 lenses in a one-dimensional (1D) array with a pitch of 200μm. Each 1/e2 beam waist spot of the 49 emitters at focus is determined to have radii of 24.7μm and 24.0μm for the slow and fast axes, respectively, with a relative defocus distance of 7.29μm. The bulk MgO:PPLN crystal provided by Covesion, Ltd. (UK), has a length of 10mm, a width of 12.7mm, and a height of 0.5mm. It was designed to have 50 gratings with widths of 150μm and each with the grating period of 4.47μm. The facets of the crystal have an antireflective (AR) coating centered both at 930nm and 465nm. The MgO:PPLN crystal is temperature stabilized in order to achieve phase matching at the laser wavelength. Also, AR coatings of R<0.5% over 20nm centered at 930nm are applied to both the front and back optical surfaces of L1 and L2 for all polarizations. L3 consists of 49 lenses in a 1D array with a pitch of 200μm and an effective focal length of 3.5mm, which transforms the output from each individual emitter and creates 49 parallel output beams with a symmetrical 1/e2 beam waist spot of around 43.5μm (radius) at the output focus plane of L3 positioned at the output coupling mirror, P3. Retroreflection of the fundamental light is achieved at P3, providing excellent stability and allowing the complete laser bar to lase. P3 is coated for high reflectivity in the near-IR range and transparency for blue light. A thin-film narrow-bandwidth IR filter, P1, is inserted in the cavity before the MgO:PPLN to restrict the spectral laser bandwidth so that optimal frequency conversion can be obtained. P1 is designed as a telecom substrate with an eternity scale of 8.5μm/°C. It is designed to provide minimal transmission loss (<0.1dB) over a given wavelength range <0.1nm and to provide higher loss outside this wavelength range, with losses as high as 1dB at ±0.5nm and 20dB at ±3nm from the center wavelength at 930nm at all polarizations and operating environments when used with light of angle of incidence of 1.3±0.5 deg. A half-wave plate, P2, having high transmission at 930nm and high reflection at 465nm, is inserted in the beam path. The alignment tolerances of L1, L2, L3, and P3 are rigorous (at micrometer level). A blue-laser prototype was built up finally with a compact volume of 44.5mm×25.4mm×14.2mm. All the optics components were assembled, fixed, and mounted onto a copper heat sink.

Figure 2 shows the measured SHG power as a function of the operating injection current. A maximum of 1.2W blue-laser output is obtained at a current of 45A with the overall wall-plug efficiency of 1.33%. The filled squares in Fig. 2 are the measured values. The solid curve shows the theoretical values obtained from the equation for SHG conversion of 49 emitters for our crystal [10

10. K. Li, A. Yao, N. J. Copner, C. B. E. Gawith, I. G. Knight, H.-U. Pfeiffer, and B. Musk, Opt. Lett. 34, 3472 (2009). [CrossRef] [PubMed]

] and shows good agreement with the measured values when a cavity loss of 72% was selected. The high cavity loss is mainly caused by the coupling and reflection loss of the laser light into the MgO:PPLN chip.

The output spectrum of the laser bar shown in Fig. 3 was measured without a feedback mirror at an injection current of 45A by an optical spectrum analyzer (Anritsu MS9710B, Anritsu Corp., Japan). The broad spectrum results from the superposition of the 49 individual emitters with AR coating, each of which is also not necessarily operating in single longitudinal mode. The spectrum of the blue-laser emission shown in Fig. 3 at an operating injection current of 45A is measured using an Ocean Optics USB2000 (Ocean Optics, Inc., USA) miniature fiber optic spectrometer. A narrow peak at an emission wavelength of 465.95nm was observed. The bandwidth of less than 1.0nm is dominated by the resolution limit of the spectrometer.

The near-field images of 49 element arrays of fundamental and SHG light are presented in Figs. 4a, 4b, respectively. They are measured at a current of 20A after beam shaping, which is imaged using a 50mm focal length lens. We found that each element of the array approximates a circular Gaussian beam, and they are incoherent to one another both for fundamental and SHG light. We can see in Figs. 4a, 4b that the beam size and intensity of each element of the array are not uniform, which is the result of the spherical aberration and coma of the lens we used. In Fig. 4b, the fringes positioned above and below the main 49 element arrays of blue light are the result of the scattering, reflecting, and degradation of the alignment of various optical elements in the laser cavity. Values of M2<1.3 were observed of each element of the blue light for both axes. Figure 4c shows the beam profile of one element in the middle of the SHG light array at a current of 20A recorded by a Thorlabs BP109 Beam Profiler (Thorlabs, Inc., USA). The M2 values of the total output green beam at an injection current of 20A were measured to be around 10.8 and 2.6 along the slow and fast axes, respectively. The optimal phase-matching temperature at a blue output power of 1.2W is around 50.4°C with an acceptance temperature bandwidth FWHM of less than 2°C. Stability testing of the blue laser was carried out by monitoring the blue output power with a powermeter. At the blue output power of 1.2W at a current of 45A, the output noise was 4.06% (rms) for 2h. The stability of blue-laser output power was obtained to be around ±4%. The center wavelength of the output spectrum of the blue- laser emission is at 466±0.4nm with a MgO:PPLN phase-matching temperature of 50.4°C. The blue-light beam quality and system stability were enhanced in comparison with the green laser [10

10. K. Li, A. Yao, N. J. Copner, C. B. E. Gawith, I. G. Knight, H.-U. Pfeiffer, and B. Musk, Opt. Lett. 34, 3472 (2009). [CrossRef] [PubMed]

] by the development of the alignment and assembly techniques and also the laser bar smile improvement.

A compact blue laser was demonstrated by ICFD of a 49-edge-emitters laser bar using a MgO:PPLN bulk crystal. A blue output optical power of 1.2W was achieved at an injection current of 45A with the optimum QPM temperature of 50.4°C, representing an overall wall-plug efficiency of 1.33%. This design for blue-laser generation can be further improved through further optimization of the design and alignment of the microlens and the use of a MgO:PPLN planar waveguide configuration.

This project was supported by the Technology Strategy Board (TSB) with DBERR Project TP/6/EPH/6/S/K2515A.

Fig. 1 Schematic of the ICFD of 49-edge- emitters laser bar.
Fig. 2 Blue-light current characteristics of the ICFD of the 49-edge-emitters laser bar.
Fig. 3 Output spectrum of the 49-edge-emitters laser bar and blue-laser emission.
Fig. 4 Array near-field images of funda mental (a) SHG, (b) light, and (c) beam profile of one SHG element.
1.

Z. Quan, Y. Yi, L. Bin, Q. Dapeng, and Z. Ling, J. Opt. Soc. Am. B 26, 1238 (2009). [CrossRef]

2.

Y. L., J. Xia, J. Wang, A. Zhang, X. Zhang, L. Bao, H. Quan, and X. Yin, Chin. Opt. Lett. 8, 187 (2010). [CrossRef]

3.

Y. Hirano, S. Yamamoto, Y. Akino, A. Nakamura, T. Yagi, H. Sugiura, and T. Yanagisawa, in Advanced Solid-State Photonics,OSA Technical Digest Series (Optical Society of America, 2009), paper WE1.

4.

J. P. Watson, A. V. Shchegrov, A. Umbrasas, D. Lee, C. A. Amsden, W. Ha, G. P. Carey, V. V. Doan, A. Lewis, and A. Mooradian, Proc. SPIE-Int. Soc. Opt. Eng. 5364, 116 (2004).

5.

A. Jechow, R. Menzel, K. Paschke, and G. Erbert, Laser Photon. Rev. 4, 633 (2010). [CrossRef]

6.

M. Chi, O. B. Jensen, J. Holm, C. Pedersen, P. E. Andersen, G. Erbert, B. Sumpf, and P. M. Petersen, Opt. Express 13, 10589 (2005). [CrossRef] [PubMed]

7.

M. Maiwald, S. Schwertfeger, R. Güther, B. Sumpf, K. Paschke, C. Dzionk, G. Erbert, and G. Tränkle, Opt. Lett. 31, 802 (2006). [CrossRef] [PubMed]

8.

C. Fiebig, A. Sahm, M. Uebernickel, G. Blume, B. Eppich, K. Paschke, and G. Erbert, Opt. Express 17, 22785 (2009). [CrossRef]

9.

O. B. Jensen, P. E. Andersen, B. Sumpf, K.-H. Hasler, G. Erbert, and P. M. Petersen, Opt. Express 17, 6532 (2009). [CrossRef] [PubMed]

10.

K. Li, A. Yao, N. J. Copner, C. B. E. Gawith, I. G. Knight, H.-U. Pfeiffer, and B. Musk, Opt. Lett. 34, 3472 (2009). [CrossRef] [PubMed]

OCIS Codes
(140.2020) Lasers and laser optics : Diode lasers
(140.3410) Lasers and laser optics : Laser resonators
(190.2620) Nonlinear optics : Harmonic generation and mixing
(190.4360) Nonlinear optics : Nonlinear optics, devices

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: October 27, 2010
Revised Manuscript: December 17, 2010
Manuscript Accepted: December 18, 2010
Published: January 26, 2011

Virtual Issues
April 21, 2011 Spotlight on Optics

Citation
K. Li, H. Wang, N. J. Copner, C. B. E. Gawith, I. G. Knight, H.-U. Pfeiffer, B. Musk, and G. Moss, "465 nm laser sources by intracavity frequency doubling using a 49-edge-emitters laser bar," Opt. Lett. 36, 361-363 (2011)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-36-3-361


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