A frequency stabilized single broad area laser in a V-shaped external cavity is used for Second Harmonic Generation (SHG) in a waveguide channel with dimensions of 3 μm × 5 μm × 10 mm of a PP-MgO:LN crystal. A maximum coupling efficiency of 63% could be obtained. An optical output power of 100.4 mW of visible light at 488 nm could be generated with 265 mW of coupled infrared light. This results in a single pass conversion efficiency of 37.8%. No photorefractive damage or saturation effects were observed.
© 2007 Optical Society of America
Compact and efficient blue green light sources are needed for various applications such as optical data storage, medical diagnostics or spectroscopic analysis. Since there is a lack of efficient solid state laser materials with direct intersections in the visible spectrum around 500 nm this wavelength range is still dominated by gas lasers such as Argon-ion lasers. Those gas lasers are bulky, show short lifetimes and high operation costs due to low wall plug efficiencies and short service intervals.
A common way to built lasers in that spectral region is frequency doubling of infrared laser light in the range of 900 to 1100 nm. Compact diode pumped solid state lasers (DPSSL) with Second Harmonic Generation (SHG) have attracted a lot of interest in the past few years. Most common are the Nd:YAG based systems at 532 nm and 473 nm. With those systems SHG single pass conversion efficiencies of up to 42% could be realized . However DPSSL systems are limited to certain wavelengths and show low wall plug efficiencies. Another way is frequency doubling of Yb-doped fiber lasers. Those lasers have the advantage of being tunable around 980 nm by the use of diffraction gratings. Single pass conversion efficiencies of up to 26% could be realized by the use of a periodically poled magnesium doped lithium niobate (PP-MgO:LN) waveguide . Nevertheless the wall plug efficiency of the fiber lasers is still rather low. Further optically pumped semiconductor lasers (OPSL) are suitable pump sources for SHG into the visible. With high pump powers and intracavity SHG output powers up to several watts could be achieved .
Diode lasers are the most efficient laser sources available. Extensive research efforts have been done to build direct emitters in the blue spectral range available. Those diode lasers are now available at 405 – 470 nm, but are still expensive and suffer on short lifetimes, show whether low output powers or poor beam quality. Until now no direct emitters around 490 nm are available.
Since periodically poled materials covering all wavelengths in the visible spectrum with high quality and reliability have become broadly available, single pass SHG with low or medium power densities has gained importance. That’s why diode lasers in the near infrared region have become more and more interesting for SHG as well. Several approaches for SHG with periodically poled materials and diode lasers have been done including the use of external cavities , fiber coupled diodes , ridge lasers , tapered diodes  and MOPA systems [8, 9]. Bulk crystals as well as waveguide structures have been used to generate powers up to 600 mW and single pass conversion efficiencies of 31% could be reached.
Single emitter broad area laser (BAL) diodes with high output powers have the disadvantage of showing poor beam quality and broad spectral emission. This makes them unsuitable for any nonlinear application including SHG. Recently an external cavity diode laser (ECDL) system for beam shaping and frequency stabilization of a BAL diode was presented . With that resonator setup up to 800 mW of diffraction limited light at 976 nm could be obtained and it was possible to generate 23 mW SHG at 488 nm by the use of a 1 cm bulk PP-MgO:LN crystal . In this letter this ECLD system is used for SHG with a 1 cm PP-MgO:LN waveguide channel. For the first time it was possible to achieve SHG of a BAL diode by the use of a waveguide structure.
2. Experimental setup
Figure 1 shows the scheme of the V-shaped external cavity in direction of the slow axis. The cavity consists of a BAL diode as gain medium, two cylindrical lenses in slow axis, a diffraction grating for frequency stabilization and tuning and a slit aperture.
The BAL diode has a width of 400 μm (slow axis), a height of 1 μm (fast axis) and a length of 1500 μm. To suppress the internal longitudinal and transversal modes of the diode itself the front facet is anti-reflection (AR) coated in the order of R ≥ 10-5. Without AR-coating the diode provides about 1.8 W optical output power at an injection current of I = 2.5 A with an M2 > 60 and a line width of 2 nm centered around 976 nm. With the AR-coating applied the diode shows a broad luminescence spectrum of more than 30 nm (FWHM).
Because of the small dimension of the diode in the direction of the fast axis only one transversal mode will propagate. The highly divergent and diffraction limited light is collimated by an aspherical lens (FAC) with f = 0.9 mm and a high numerical aperture of NA = 0.7 . The light in fast axis direction will remain uninfluenced by the external cavity.
In direction of the slow axis the diode is 400 times larger and many transversal modes can arise. Thus the slow axis beam quality is the major drawback of broad area diodes. To obtain gain guiding a stripe array structure in direction of the slow axis is realized. Therefore contact stripes with a pitch of d = 20 μm are applied onto the active region.
The V-shaped resonator is explicitly designed to take advantage of the slow axis substructure of the gain guided BAL and is split into a feedback branch and an outcoupling branch. Feedback is centered around the angle αFB that is determined by an out of phase oscillation of the contact stripes. It is given by αFB = λ/d. With the pitch of the contact stripes d = 20 μm and the wavelength λ = 976 nm follows αFB = 48.8 mrad = 2.8°.
The feedback branch contains a slit aperture which is placed in the focal plane of the two cylindrical lenses L1 (f = 40 mm) and L2 (f = 300 mm) and the diffraction grating in a Littrow configuration. The combination of the first cylindrical lens L1 in slow axis and the slit aperture forces the light to travel under the angle αFB respectively to the surface normal of the emitter. This leads to a drastically improvement of the beam quality of the diode by coupling the chip internal transversal modes. The second cylindrical lens L2 in the slow axis path in combination with the diffraction grating and the slit aperture acts as spectral filter.
The outcoupling branch contains a rectangular aperture for selecting the fundamental transversal mode TEM00 and to suppress amplified spontaneous emission (ASE). This aperture is optional because at low output powers only the fundamental transversal mode will propagate. At higher injection currents the coupling between the chip internal mode is weaker and higher transversal modes can occur. Nevertheless no higher transversal modes will be coupled into the waveguide channel but to protect the surface of the waveguide crystal from damage the aperture remained inserted during the experiments.
Behind the aperture an attenuator consisting of two crossed polarizers and a rotary half waveplate is placed. This has the advantage that it is possible to change the laser output power without changing the injection current. That’s why no thermal drift or detuning was observed.
Since the diode shows a strong astigmatism a beam shaping system for astigmatic correction and collimation is necessary. Therefore two crossed cylindrical lenses L3 in slow axis (f = 150 mm) and L4 in fast axis (f = 60 mm) and an achromatic lens (L5, f = 75 mm) are set up before the focusing lens. For coupling the infrared light into the waveguide an aspherical lens (L6) with a focal length of f = 3.3 mm and a numerical Aperture of NA = 0.47 is used. With the same lens more than 60% of the ECDL laser output power could be coupled into a single mode fiber of 5 μm diameter.
The waveguide crystal is temperature stabilized and placed on a 3-axis translation stage. For collimation of the blue light another aspherical lens (L7) with a focal length of 8 mm and NA = 0.5 is used. For the separation of the pump light and the SHG a dichroic mirror (HT@488 nm and HR@976 nm) is applied. To check and guarantee that no residual pump light is measured a spectral filter (Schott BG18) was introduced behind the dichroic mirror.
3. Experimental results
With the V-shaped cavity several hundred milliwatts of diffraction limited light with M2 < 1.2 and a bandwidth below 25 pm could be achieved . The slow axis brightness could be improved by a factor of 12 and the brilliance was increased by a factor of 960 compared to the free running BAL diode.
For generation of the Second Harmonic a periodically poled, magnesium doped LiNbO3 crystal made of congruent melt is used. The crystal was manufactured by HCPhotonics, has a length of 10.2 mm, a width of 3 mm and a height of 0.5 mm and an AR coating for 976 nm and 488 nm. It has 16 waveguide channels each with dimensions of 3.5 μm × 5 μm with an interaction length of 10 mm applied. The poling period of the crystal is 3.6 μm resulting in a quasi-phase matching (QPM) wavelength of 488 nm at a temperature of 120°C. For optimal heat conduct an oven with a lapped copper surface is used.
To avoid damage of the crystal surface and to maintain stable operation the ECDL was fixed at an injection current of 2.5 A. This resulted in an output power of 420 mW behind the attenuator at maximum transmission. A coupling efficiency of more than 63% could be obtained resulting in 265 mW of infrared light inside the waveguide. For measuring the coupled infrared light the laser is slightly detuned out of the QPM wavelength so that no light is converted into the visible spectrum.
Figure 2 shows the power of the second harmonic (PSHG) as a function of the power of the fundamental wave coupled into the waveguide (PIR-coupled).
To obtain the data the half waveplate of the attenuator was rotated to change the infrared output power while the injection current remained fixed at 2.5 A. With 265 mW infrared light a maximum SHG output power of 100.4 mW could be generated. This equals to a conversion efficiency of 37.8% without incoupling losses and an overall conversion efficiency of 23.9% including coupling losses.
For a lossless nonlinear waveguide crystal  the power of the second harmonic light as a function of the incident fundamental power can be described by:
A fit through the data points in Fig. 2. resulted in a normalized conversion efficiency of η0 = 200%/W∙cm. The measured data match very well to the theoretical prediction. The overall wall plug efficiency for the generated blue light at a pumping current of 2.5 A and an output power of 100 mW was 3.7%.
One example of spectrum of the generated blue light is depicted in Fig. 3. The measured band width of 50 pm is equal to the resolution limit of the optical spectrum analyzer (OSA). The side band suppression is in the range of 60 dB.
For the measurement of the beam quality a moving slit technique was used to determine the second moments of the intensity distribution. Therefore an 80 mm lens was used to focus the collimated beam. A hyperbolic fit through the data was used to obtain the M2 value. This follows completely the ISO11146 standard.
Figure 4 shows the caustics of the blue light at 81 mW output power resulting in M2 = 1.05± 0.4 for the y-axis and M2 = 1.03 ± 0.4 for the x-axis. The beam shows a slight astigmatism and an elliptical profile which is caused by the different dimensions of the waveguide in x and y direction. Values of M 2 < 1.15 were observed in all cases and for both axis.
By changing the crystal temperature it was possible to tune the emission wavelength of the blue light (Fig. 5). From room temperature to 120°C a tuning range of more than 3.5 nm could be achieved. The wavelength varies linearly as a function of the crystal temperature with a coefficient of α = 0.039 nm/ K . The laser was running at an optical output power of 100 mW for several hours. During that period no drop in output power and no change in the QPM wavelength which could be the result of photorefractive damage were observed.
Based on previous results an external cavity for a broad area laser diode at a wavelength of 976 nm was used for Second Harmonic Generation. With this cavity the slow axis brilliance of a BAL diode was increased by a factor of 960 which makes it suitable for SHG. 63% of the infrared light could be coupled with a beam shaping optic into a 3.5 × 5 μm2 waveguide with 1 cm length of a PPLN crystal. With 265 mW incoupled infrared light it was possible to generate an SHG power of 100.4 mW of blue light at a wavelength of 488 nm. This results in a conversion efficiency of 37.8% without concerning coupling losses and 23.9% including coupling losses. A normalized SHG conversion efficiency of η0 = 200%/W∙cm inside the waveguide could be obtained. To our knowledge this represents the highest normalized conversion efficiency achieved with diode lasers at that wavelength. Further it was possible to tune the blue light over 3.5 nm around 488 nm by changing the crystal temperature. The beam quality of the blue light was measured to be better than M2 = 1.15 in all cases. An overall wall plug efficiency of 3.7% could be reached. Further no photorefractive damage was observed.
We thank Karin Wu of HCPhotonics and Sven Kern of GWU Lasertechnik for helpful discussion and providing the nonlinear crystal as well as Dirk Heinrich for the construction of the crystal oven.
References and links
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