Solid-state 488-nm laser based on external-cavity frequency doubling of a multi-longitudinal mode semiconductor laser

Transcription

1 Solid-state 488-nm laser based on external-cavity frequency doubling of a multi-longitudinal mode semiconductor laser Vincent Issier a, Boris Kharlamov *a, Thomas Kraft a, Andy Miller a, David Simons a, James Wong a, Simon Wong a, Andre Wong b, Kuochou Tai c, Nicolas Guerin b, Daniel Zou, Victor Rossin d, Marc von Gunten a, William Minford e, Andy Hulse a, Colette Paillet-Allison a, Krishnan Parameswaran f, Evgeny Churin g, Rob Waarts a JDSU Corp., 2789 Northpoint Parkway, Santa Rosa, CA, USA 9547 b JDSU Corp., 43 N. McCarthy Blvd. Milpitas, CA, USA 9535 c JDSU Corp., 1768 Automation Parkway, San Jose, CA, USA d JDSU Corp., 8 Rose Orchard Way, San Jose, CA, USA e JDSU Corp. 45 Griffin Road South, Bloomfield, CT, USA 62 f Physical Science Inc., 2 New England Business Center, Andower, MA, USA 181 g Institue Of Automation and Electrometry SO RAS, Novosibirsk, 639, Russia ABSTRACT Results for a new compact 488 nm solid-state laser for biomedical applications are presented. The architecture is based on a multi-longitudinal mode external cavity semiconductor laser with frequency doubling in a ridge waveguide fabricated in periodically poled MgO:LiNbO 3. The diode and the waveguide packaging have been leveraged from telecom packaging technologies. This design enables built-in control electronics, low power consumption ( 2.5 W) and a footprint as small as 12.5 x 7 cm. Due to its fiber-based architecture, the laser has excellent beam quality, M 2 <1.1. The laser is designed to enable two light delivery options: free-space and true fiber delivered output. Multi-longitudinal mode operation and external doubling provide several advantages like low noise, internal modulation over a broad frequency range and variable output power. Current designs provide an output power of 2 mw, but laser has potential for higher power output. Keywords: semiconductor laser, frequency doubling, waveguide, multiple longitudinal modes, optical fiber, external cavity laser 1. INTRODUCTION Compact and reliable sources of visible light are needed in many applications, including flow cytometry, DNA sequencing, confocal microscopy, reprographics, etc. Gas lasers, used for decades for these applications, have wellknown limitations: high energy consumption, bulkiness, and periodic replacement of gas-discharge tubes. Progress in semiconductor lasers and nonlinear optics has enabled the development of solid state lasers based on frequency conversion with similar or better performance compared to gas lasers with much lower power consumption, improved reliability, and smaller size. Currently several commercial semiconductor-based blue and green lasers for biotechnology applications are available. Several design approaches for visible CW solid-state lasers have been reported in the literature. In most cases these designs are based on an external-cavity single-longitudinal-mode semiconductor laser. Frequency doubling in single pass periodically poled nonlinear crystals (KTP or MgO:LiNbO 3 ) is attractive due to simple optical scheme, but with currently available pump power levels and conversion efficiency it is difficult to expect output power for such devices higher than several tens of milliwatts. Resonant-cavity frequency doubling has very good potential for high output power[1] but the optical design is more complicated and very sensitive to external perturbations. Recent progress in the fabrication of nonlinear waveguides [2] has enabled a third approach: frequency doubling in a periodically poled ridge waveguide [3-7]. The use of a waveguide for frequency doubling simplifies dramatically the optical design and provides, * boris.kharlamov@jdsu.com; phone ; fax Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications VII, edited by Peter E. Powers, Proc. of SPIE Vol. 6875, , (28) X/8/$18 doi: / Proc. of SPIE Vol

2 due to the high quality of the latest generation of waveguides, a way for higher power output without laser architecture changes. Very few approaches are based on a multiple longitudinal mode design. Though, multimode system has distinctive advantages. In general single-longitudinal-mode designs are more complex than multimode ones. They have other disadvantages such as limited power variation capability, high sensitivity to optical feedback etc. Multimode systems are more simple, stable, and less sensitive to external perturbation and optical feedback. It provides flexible output power and internal modulation capability. Finally multimode frequency conversion has higher efficiency. The goal of the present work was to develop an efficient, compact, reliable and commercially attractive 488-nm CW semiconductor based laser for biotechnology applications. Our multimode design has many important distinctions from other reported and/or commercially available lasers. 2. DESIGN AND PRINCIPLE OF OPERATION The optical layout of the laser is shown in Figure 1. The gain chip, with a high reflective mirror on the back and an AR coating on the front facet, is mounted on a TEC in the first butterfly package. Radiation is coupled into a PM fiber via a fiber lens. The fiber has a Fiber Bragg Grating (FBG) as the output coupler. The FBG has a reflection of about 1% at 976 nm. These elements create a multi-longitudinal mode cavity with a mode spacing in the 1 MHz range. The width of the lasing envelope depends on the diode current and typically corresponds to a few hundreds longitudinal modes. The large number of modes ensures a stable low noise operation, variable output power and modulation capability. Figure 2 shows the spectral profiles of typical pump module at various currents. Controller I DL, TEC Power loop Laser Diode FBG WG TEC Power loop Collimation, Power Figure 1. Optical layout of the laser. See details in the text. The IR radiation is coupled into a periodically poled MgO:LiNbO 3 ridge waveguide mounted on a TEC in the second butterfly package. The resulting blue radiation is coupled into the single mode PM fiber; residual IR radiation is filtered out by this fiber. The fiber serves as a perfect spatial filter for the blue. The single spatial mode is collimated by a lens. The field distribution is close to Gaussian, and M 2 is better than 1.1 with negligible ellipticity and astigmatism. A small part of the output radiation is sampled by a beamsplitter and used in the power control loop. The built-in electronics controls the function of all the elements, including laser diode and waveguide temperature, laser diode current. The laser with controller has a footprint of only 12.5 x 7 cm. The length of the waveguide is about 8 mm with a FWHM of the phase-matching curve that is well matched to the excitation envelope to avoid conversion efficiency losses. Figure 3 gives examples of phase matching curves in frequency and temperature domains. FWHM of phase matching curve in spectral dimension is λ 17 pm and T 2 C in temperature dimension. Dependence of SHG power on fundamental IR power is shown in figure 4. Slight signs of saturation can be recognized on the top of the curve. They may be caused by waveguide heating by radiation. Precise evaluation of conversion efficiency is difficult, since many parameters can only be estimated. Our estimate of the efficiency for typical waveguides is 4%/W. Proc. of SPIE Vol

3 Multimode lasers and multimode frequency converters specifically have additional sources of noise as compared with single mode lasers. In particular, noise is very sensitive to waveguide temperature and the minimum in noise does not always coincide with the maximum in power. It can be clearly seen from example in figure 3a, where noise dependence on waveguide temperature is plotted together with power. Therefore, our controller has a build-in RMS noise meter and a control loop to optimize the waveguide temperature for minimum noise. Since the noise optimum does not exactly coincide with the power maximum, this mode of operation slightly decreases conversion efficiency but provides a considerable improvement of noise..6 I=2 ma I=4 ma I=6 ma I=8 ma intensity [a.u.].4.2 wavelength [nm] wavelength [nm] current [ma] Figure 2. Generation spectra of the pump module at different currents. Left graph shows single spectral profiles at currents I=2, 4, 6 and 8 ma, right graph shows 3D color map of the generation spectrum dependence on the current. Proc. of SPIE Vol

4 power [a.u.] Power [a.u] noise [%] wavelength [nm] Temperature [C] Figure 3. Phase matching curves of typical waveguide. Left graph: spectral dependence. - are the experimental points, solid line is a fit with sinc-function. FWHM 17 pm. Right graph: temperature dependence. Solid curve power, FWHM 2 C. Dashed curve noise P P IR Figure 4. Dependence of SHG power on fundamental mode IR power. Dash curve is the data fit with equation P 488 =η*p 976 *tanh(k*p 976 *L 2 ), where η is coupling efficiency, K is a combination of constants, and L is the length of waveguide. Proc. of SPIE Vol

5 3. RESULTS Standard versions of laser now have output power up to 2 mw with a typical noise of.2-.35%. Multiple lasers are on lifetest with operating hours currently in the range of 6-12 hours. However, the output power is not limited by technical parameters of the laser. Samples with 3, 5 and 6 mw output power are now in test, and the design has demonstrated a maximum output power in single-spatial-mode operation with a PER > 2 db of about 1-12 mw (see, for example figure 4). Figure 5 shows example of performance test of laser at 6 mw power noise 62.4 power [mw] 6.3 noise [%] time [hours] Figure 5. Example of laser performance at 6 mw, measured continuously during 2 weeks. Laser shows a stable performance, power is easy to control, average level of noise is about.25 %. At high power, waveguide heating is observed, due to both IR and blue radiation absorption. Heating creates a nonuniform temperature profile along waveguide. Non-uniform waveguide heating may lead to decrease of conversion efficiency. The influence of the light-induced waveguide heating on conversion efficiency was modeled. It was found that unless the temperature difference along waveguide exceeds the width of phase matching curve, the degradation of conversion efficiency can be neglected. Another effect, caused by the waveguide heating with converted power, is a distortion of phase matching curves and potential to generation instability due to positive feedback between waveguide local temperature and second harmonic power. Both effects were found not critical issues at SHG power up to 1 mw. At high powers the heating effect may potentially be the limiting factor for efficient frequency conversion and stable operation. It was found that some waveguides show a more or less pronounced aging effect, which manifests itself in the increase of absorption and moderate decrease of efficiency. We have demonstrated that this effect saturates over time and did not cause any laser failures. The nature of the effect is not clear yet, but it definitely should be addressed not to bulk material but to waveguide structure or waveguide production process. Only small part of the total data set is shown. The total performance time of this laser is now more than 12 hours. Proc. of SPIE Vol

6 Power [mw] Power [mw] mw range x1 3 2x1 3 3x1 3 4x1 3 time [min] Noise %] Noise %] time [min] Figure 6. Demonstration of power tunability. Laser operated in constant power mode was tuned from P= to P=2 mw in steps of.2 mw. The solid curve is power, the dotted one is noise. The insert shows details in the low power range. One of the advantages of lasing in multiple longitudinal modes is flexible power tuning. Our laser shows very stable output from the lasing threshold at several tens of milliamps up to a maximum current of 8 ma (limited by controller) with continuous power tuning. Figure 6 shows the power tunability of the standard laser with maximum power 2 mw. The laser was operated in constant power mode, and the power was tuned from to 2 mw in steps of.2 mw. The noise spectrum has a peak at a power between 1 and 2 mw; in other regions, it is below.4%. The peak is the result of a coincidence of several factors and is not inherent in the architecture. If we exclude this region, there are no unstable areas with enhanced noise, practically unavoidable in the case of single mode semiconductor laser. The long-time power stability in constant power mode is better than ±2% across a broad range of ambient temperatures, 1-45 C. Figure 7 gives an example of external temperature scan in this range when laser operated in constant power mode. Proc. of SPIE Vol

7 power noise power [mw] noise [%] temperature [C].2 Figure 7. Sensitivity of output power and noise to external temperature. Steps in power are artifacts caused by digital noise in the output data format. Another benefit of the design is the capability of direct internal modulation. The laser shows stable operation with current modulation at variable frequencies in the range from several Hz to hundreds of KHz at variable modulation depth and modulation profile. Figure 8 shows two examples of internal modulation in low and high frequency ranges with rectangular and sinusoidal profiles. Modulation at frequencies in MHz range is also possible, but modulation frequency can not be tuned continuously in high frequency range, since it becomes comparable with the cavity round trip frequency (a) period T=1 ms, f=1 Hz modulation depth=1% (b) period T=5 µs, f=2 khz modulation depth=5 % power [a.u.] time [s] time [ms] Figure 8. Examples of modulation profiles: (a) low frequency rectangular modulation with 1 %depth; (b) high frequency sinusoidal modulation with 5% modulation depth. Proc. of SPIE Vol

8 Since the laser has a fiber as an output spatial filter, the mode characteristics, such as ellipticity, M 2, astigmatism are very good. The next graph illustrates this statement W X /W Y =1.1 M2 X =1.6; M2 Y =1.8; W [µm] z-shift [mm] Figure 9. Results of output beam profile measurements. The graph shows dependence of beam size on the distance from the focusing lens in x and y directions, calculated ellipticity and M 2 values. 4. CONCLUSION We have developed a compact commercial laser for biotechnology applications based on CW external cavity semiconductor laser with frequency doubling in a ridge MgO:LiNbO 3 waveguide, operating at 488 nm with nominal power of up to 2 mw. The potential for high power options is explored and demonstrated. The laser operates in multiple longitudinal modes with noise typically in the range % and has power stability better than ± 2 % in temperature range 1-45 C. Multimode operation enables power tunability on the whole range of operation power and internal modulation on a broad range of modulation frequencies and modulation signal profiles. Laser output beam has negligible astigmatism and ellipticity, and M 2 < 1.1. Telecom standard based packaging technology provides compactness, low power consumption, robustness. REFERENCES 1. Knippels, G.M.H., et al., "Moving solid-state cyan lasers beyond 2 mw", Proceedings of SPIEE, 5332(1), (24). 2. KAWAGUCHI, T., et al. "Optical Waveguide SHG Devices Using LiNbO3 Epitaxial Grown and Ultraprecision Machining Technique", in Topical Papers on Laser Technology for Next-Generation Optical Disk Systems. The Laser Society of Japan, 2: 3. Kachanov, A.A., et al., "Novel external CW frequency doubling of semiconductor lasers to generate 488 nm", Proceedings of SPIE, 577, (25). 4. Jechow, A., D. Skoczowsky, and R. Menzel, "1 mw high efficient single pass SHG at 488 nm of a single broad area laser diode with external cavity using a PPLN waveguide crystal", OPTICS EXPRESS, 15(11) (27). Proc. of SPIE Vol

9 5. KAWAGUCHI, T., et al., "High power blue/violet QPM-SHG laser using a new ridge-type waveguide", Technical Digest of CLEO Conference, 6-11 May 21 Baltimore, 21. CTul6: p. 141 (21). 6. Iwai, M., et al., "High power blue generation from a periodically poled MgO:LiNbO3 ridge-type waveguide by frequency doubling of a diode end-pumped Nd:Y3Al5O12 laser", Applied Physics Letters, 83, (23). 7. Sakai, K., Y. Koyata, and Y. Hirano, "Blue light generation in a ridge waveguide MgO:LiNbO3 crystal pumped by a fiber Bragg grating stabilized laser diode", OPTICS LETTERS, 32(16), (27). Proc. of SPIE Vol