cw, 35nm, 1mW semiconductor laser system as potential substitute for HeCd gas lasers T. Schmitt 1, A. Able 1,, R. Häring 1, B. Sumpf, G. Erbert, G. Tränkle, F. Lison 1, W. G. Kaenders 1 1) TOPTICA Photonics AG, Munich, Germany, Tel: +49 (89) 85 837-, email: info@toptica.com ) Ferdinand-Braun-Institut für Höchstfrequenztechnik,Berlin, Germany, Tel: +49 (3) 63 9 659 ABSTRACT In the last decades, diode laser systems conquered the spectral range step-by-step from conventional gas lasers, wherever they can match or outperform in optical specifications. Although highly anticipated in the ultraviolet wavelength range, for instance in high-resolution lithography, biological and medical fluorescence applications or holography, cw single frequency operation of sufficient power has been a challenge for diode or other solid state laser systems. Currently this scope is still dominated by the HeCd gas laser, emitting at 35 nm with powers of up to 1 mw. In this paper we present a diode laser system emitting at 35 nm offering the same output power by efficient second harmonic generation (SHG) of a master oscillator power amplifier (MOPA) at 65 nm. For the master oscillator a ridge waveguide diode is anti-reflection coated and used in an external cavity diode laser (ECDL) with grating feedback in Littrow configuration. This setup features a MHz line width (coherence length of 1m), a coarse tuning range from 649 nm to 657 nm and a mode hope free tuning of GHz. In a second step, we use a tapered amplifier to boost the output from the ECDL to a level of 4 mw, for powering an efficient second harmonic generation process in an enhancement cavity. Faraday isolators on both ends of the amplifier stage prevent back reflection and stabilize the single mode operation of the system. Together with astigmatism compensation this yields to a high spatial quality (M²<1.5) of the amplified beam. The frequency doubling is achieved by using a four mirror bow-tie enhancement resonator fitted with a Beta-Barium Borate (BBO) crystal. The cavity length is actively locked to the laser frequency using the Pound-Drever-Hall method 1. With this set-up, stable and reliable laser operation is achieved. After a few minutes warm-up time, fixed frequency and tunable UV output power of more than 1 mw could be generated, opening this important wavelength range for future product development. Keywords: semiconductor lasers, UV laser, noinear optics 1. INTRODUCTION Lasers emitting high quality UV-radiation for industrial applications like high resolution lithography, biological and medical fluorescence applications, holography and semiconductor processing are of high interest. For scientific applications like spectroscopy, atom cooling and trapping, quantum optics and quantum information more complete wavelength coverage or tuneability of single frequency radiation is required. At present, several types of gas lasers are available producing fixed frequency light in the UV range, like Argon-ion for 364 nm, XeF operating at 351 nm, N for 337 nm or HeCd operating at 35 nm. In the past many applications suffered from the typical drawbacks of gas lasers, such as high power consumption, large space requirement and big heat dissipation. In the case of air-cooled lasers, cooling fans introduced vibration problems and noise, while water cooling meant bulky chillers and higher operating costs. Finally, these gas lasers are delicate because they incorporate plasma tubes that are neither durable nor portable. Although some of the demands can be fulfilled in principle by frequency doubling of diode pumped solid state lasers or direct diode lasers there is still no full alternative for some of the gas lasers mentioned above. The development of GaN diode laser material during the last 1 years has produced laser light in the UV and blue spectral range at certain wavelength in the range from 373 nm to 473 nm. Unfortunately, these devices do not produce laser radiation with High-Power Diode Laser Technology and Applications VI, edited by Mark S. Zediker, Proc. of SPIE Vol. 6876, 68761, (8) 77-786X/8/$18 doi: 1.1117/1.761864 8 SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol. 6876 68761-1
properties required for most of the applications mentioned, especially high cw output power in combination with single frequency operation, a narrow line width and a good spatial beam quality. Even though commercial UV laser systems based on frequency conversion of Nd:YVO 4 lasers are delivering higher powers at 66 nm or 355 nm with good spectral and spatial properties, these systems are equally limited in spectral coverage and can not always be tailored to the needs of specific applications i.e. in life-science. The more flexible approach of frequency doubling of an optically pumped Ti:Sapphire laser using a noinear optical crystal inside an external enhancement cavity has been developed successfully. These laser systems provide narrow line width and tuneable output in the entire range from 35-5 nm with adequate power up to 1 W. However, these systems tend to be complex, delicate, and expensive, restricting the usage to laboratory applications. Therefore frequency doubling of a robust visible direct red semiconductor-based laser system is a challenging but very attractive approach to realise compact and tuneable laser sources emitting in the UV spectral range. In this paper we report on continuous-wave second harmonic generation of a visible red semiconductor based laser system emitting >3 mw output at 65 nm using an external enhancement cavity to generate >1 mw of tuneable, single frequency laser radiation at 35 nm. The power. THEORETICAL BACKGROUND P ω of the generated frequency converted light can be written as ( ) with P ω = E Pω = KPω lk1h σ, B, ξ K ω d eff 3 =. ε n n c 1 π Here, E denotes the single-pass conversion efficiency while P w is the fundamental power. The conversion efficiency E depends on several parameters: l is the length of the crystal, k 1 the absolute value of the wave-vector, n 1 and n the indices of refraction of the fundamental and the second harmonic wave, d eff the effective noinearity, ε the dielectric constant and c the vacuum speed of light. The h-function describes the influence of the focusing parameterξ = l b, the phase mismatch σ and the walk-off parameter B on the conversion efficiency and is defined as For a given crystal, ( σ, B,ξ ) b ξ ξ iσ ( τ τ ') B²( τ τ ')² / ξ 1 e e h( σ, B, ξ ) = dτdτ ' 4ξ (1 + iτ )(1 iτ '). ξ ξ h can be maximized with respect to ξ and σ, while B is a constant, representing double 1 = πω λ, the optimum beam waist in the crystal is defined. refraction. With The value of E is usually a small number in the range from 1-5 W -1 to 1 - W -1, depending on the crystal properties and beam parameters. A detailed description of noinear frequency conversion can be found elsewhere 3. The power of the second harmonic radiation generated by single pass frequency doubling is limited by the available power of the fundamental laser source, the crystal length and the effective non-linearity of the crystal. Thus, higher second-harmonic power can be achieved with significantly higher fundamental power. A promising method to increase the available fundamental power is the resonant enhancement of cw radiation in a high finesse cavity. Proc. of SPIE Vol. 6876 68761-
The enhancement in the cavity depends critical on the transmission of the input coupler. Optimum coupling is achieved, if the input coupler equals the linear and non-linear cavity losses. The linear losses L are fixed and determined by the mirror and crystal surface reflectivity, while the non-linear losses L depends on the circulating power P circ inside the cavity. The noinear losses can be described by 4. The optimum input is determined by P ω = E Pω = L P ω T L = + L 4 otp ic + E P ω The enhancement factor for the incident radiation in a high finesse cavity can be calculated, according to Pcirc Tic = P T L E P inc ( 1 (1 1 )(1 )(1 circ )) Here, T ic denotes the transmission of the in-coupling mirror, L the linear losses of the remaining resonator without the incoupling mirror, Pcirc is the fundamental power build up in the cavity and Pinc is the fundamental power incident on the input coupler. Since the second-harmonic power increases quadratically with the intra-cavity fundamental power, it is of high importance to minimize any passive cavity losses, which would otherwise reduce the intra-cavity power. Thus, the utilized noinear crystal ideally shows low absorption and scattering losses. 3. EXPERIMENTAL SETUP Our setup is shown in Figure 1 and is similar to the system reported in 5 : Optical Isolator Master Oszillator Tapered Amplifier Folding Mirror Modematching Lens Adjustable Mirror Fast Photodiode BIBO Crystal Lens Red-Filter P4 P1 P Incoupling Mirror Piezo Stack P3 Figure 1: Experimental setup The laser system consists of a frequency stabilized external-cavity diode laser in Littrow configuration, operating at 65 nm with an output power of 4 mw. This laser light is single pass amplified in a tapered amplifier up to 4 mw after the tapered amplifier chip, whereas the spectral properties of the diode laser are maintained. An optical isolator with Proc. of SPIE Vol. 6876 68761-3
an extinction of 6 db between the external-cavity laser and the tapered amplifier is used to avoid optical feedback into the laser diode. A second optical isolator is placed behind the amplifier to prevent feedback into the amplifier. The output beam of the tapered amplifier passes a spherical lens to match the beam to the mode of the enhancement cavity. With this setup, stable and reliable laser operation can be achieved. After a few minutes warm up time, the UV output power is constant for several hours without any realignment. 4. MEASUREMENTS AND RESULTS 4.1 The tapered amplifier system Generation of the fundamental laser beam is realized by a master oscillator power amplifier system (MOPA) that is seeded by an extended cavity diode laser (ECDL) in Littrow configuration 6. Both master and amplifier diode have been processed from the same vertical semiconductor heterostructure with peak emission at 651 nm at C and a gain width of 1 nm at 6 dbc (Fig.). 5-5 rel. intensity [dbm] -1-15 - -5-3 -35-4 6 63 64 65 66 67 68 wavelength [nm] Figure : Amplified spontaneous emission of the semiconductor gain material at C. Peak emission is at 651 nm, gain width is 1 nm at 6 db below peak. The seed laser diode is designed as a ridge waveguide diode with a stripe width of 7.5 µm and a length of 1.5 mm. For higher stability and wider tuning range in the external cavity setup, the diode is antireflection coated to <1-3 on the front facet and high reflection coated on the back facet. Total tuning range is 649 nm to 657 nm at a maximum power of 4 mw and a suppression of amplified spontaneous emission (ASE) of 4 db. The line width is typically about 1 MHz and the mode hop free tuning range (MHFTR) is GHz. For the amplifier diode a tapered design has been chosen. This allows high output powers while maintaining a nearly Gaussian shaped beam profile. The total length of the tapered amplifier diodes (TA) is mm. It is obvious that broader output facets are more resistant to catastrophic optical mirror damage (COMD), but usually at the cost of inferior beam quality. To evaluate this behaviour, output facets with widths of 35 µm 5 µm and 7 µm have been compared. Figure 3 shows the optical output power vs. injection current for the smallest and largest chip geometries at low currents near threshold and at high currents in the thermal rollover regime. As expected, the diode with the smaller facet width has much higher slope efficiency. Also the peak power of 4 mw at 18 C is 35% higher. Lowering the temperature, significantly improves the gain of the amplifiers. At 1 C similar output powers of up to 5 mw are achieved with both geometries at a seed power of 17 mw. This corresponds to an amplification of about 15 db. Although peak power and efficiency are important parameters for the system, reliability and long term stability also have to be taken into account. As one can see in Figure 3b, the chip with the smaller output facet did not survive the measurements. The tests have shown, that 35µm wide diodes are prone to COMD at high output powers. Proc. of SPIE Vol. 6876 68761-4
5 (a) 35µm; 18 C 7µm; 18 C 5 (b) output power [mw] 15 1 5 output power [mw] 4 3 1 7µm; 1 C 7µm; 18 C 35µm; 1 C 35µm; 18 C,,1,,3,4,5,6 injection current [ma],8 1, 1, 1,4 1,6 injection current [ma] Figure 3: Optical output characteristics at low currents near threshold and at high currents in the thermal rollover regime. Seeding power was 17mW. Furthermore, to achieve high coupling efficiency into the enhancement resonator, the beam shapes of the amplifier diodes have been compared. Figure 4 shows the profiles of the collimated beams. Both beams are characterized by small side lobes in fast axis originating from aberrations introduced by the collimating optics. Beam shape in the crucial slow axis is very smooth with a single maximum and nearly Gaussian distribution. M², as measured with a Spiricon M²- beam propagation analyzer, are well below 1.5 for both diodes and both axes. In particular, no significantly inferior beam quality can be observed with the broader facet diodes. Parallel to junction (slow axis) Parallel to junction (slow axis) (b) perpendicular to junction (fast axis) perpendicular to junction (fast axis) Figure 5: Profile image of collimated beam for chips with 7 µm (a) and 35 µm wide output facet (b). A cylindrical lens of 35 mm focal length was used for slow axis collimation. In conclusion, no significant deterioration of the beam shape can be observed for all three geometries. We could not see any degradation of the chip over a test period of around 1 h for all geometries, but we observed some higher tendency for COMD when overpowered a low temperature with the smallest facet size. For setting up the second harmonic generation, we choose the 5 µm facet as optimum of efficiency and reliability. Proc. of SPIE Vol. 6876 68761-5
4. The second harmonic generator Our enhancement cavity is a four-mirror bow-tie resonator, with two plane and two curved mirrors with a radius of curvature of 38 mm. The geometrical distance between the curved mirrors is 47.5 mm, the total distance for one complete round trip is 67.5 mm. We use a 14 mm long BBO crystal, cut for critical phase matching and normal incidence and antireflection coated surfaces, coated for 65 nm and 35 nm. The angle of incidence on the mirrors is 1. This setup provides a stable resonator with a beam waist in the crystal center of 19.38 µm in the tangential and.44 µm in the sagittal plane, which is close to the optimum waist of 5 µm. The size of the beam waist between the two plane mirrors is 1 µm and 155 µm in the tangential and sagittal plane, respectively. The amplifier output beam is matched to the cavity mode with a lens with 5 mm focal length. In order to lock the cavity length to the laser frequency, we employ the Pound-Drever-Hall (PDH) method, which uses MHz shifted side-bands to the center wavelength, imprinted to the spectrum by current modulation of the laser diode. The PDH lock uses these side-bands to generate a phase sensitive error signal for locking top-of-fringe. As actuator, one of the plane mirrors is attached to a piezo stack to control the cavity length. To achieve maximum power enhancement, the transmission of the in-coupling mirrors is calculated to be 1% for a fundamental power of 315 mw, passive resonator losses of.5%, a spatial coupling efficiency of 7% and a conversion efficiency E of 1.31 1-4 W -1. Due to the sensitivity of the enhancement cavity to losses it was advantageous to use a slightly higher transmission of the in-coupling mirror. Thus, mirrors with reflectivity of 98.5% and 98.1%, respectively, have been tested (see Figure 5). The calculated values show good agreement with the measurements. To characterize the system we measured the fundamental power in front of the in-coupling mirror and the second harmonic power behind a filter to block the residual red light. 1 1 (a) Measured Calculated 1 1 (b) Measured Calculated SHG Power [mw] 8 6 4 SHG Power [mw] 8 6 4 5 1 15 5 3 35 Tapered Amplifier Power [mw] 5 1 15 5 3 35 Tapered Amplifier Power [mw] Figure 5: Measured SHG Power as function of the tapered amplifier output power with R ic =.985 (a) and R ic =.981 (b) Proc. of SPIE Vol. 6876 68761-6
Conversion Efficiency,35,3,5,,15 R=.985 R=.981,1 5 1 15 5 3 35 Tapered Amplifier Power [mw] Figure 6: Measured conversion efficiency with R ic =.985 and R ic =.981 Figure 6 shows the second harmonic conversion efficiency as function of the tapered amplifier power for the two incoupling mirrors. The conversion efficiency is increasing from low to high power, because of the selected transmission of the input coupler, which is optimized for high fundamental power. A maximum conversion efficiency of 35% is obtained for a fundamental power of 315 mw. 5. CONCLUSION AND OUTLOOK We present a diode-based all solid state laser system operating at 35nm. The single frequency laser light from an amplified diode laser is frequency-doubled in an external enhancement cavity. As noinear optical crystal a BBO crystal of 14 mm length is used. The crystal facets were cut for normal incidence and antireflection coated. At a fundamental power of 315 mw, more than 1 mw of UV radiation @ 35 nm were obtained. The conversion efficiency thus amounted to 35%. With the locked cavity, stable laser operation could be maintained for several hours with a laboratory set-up. These results show that semiconductor-based laser systems are now also powerful enough to be promising candidates for scientific and in the future also industrial applications, where either tunability or narrow line width in the UV is required. 6. REFERENCES 1 3 4 5 6 R. W. P. Drever, J. L. Hall, and F. V. Kowalski, Appl. Phys. B 31 (1983) 97-15 G. D. Boyd and D. A. Kleinman, Journal of Applied Physics 39 (8) (1968) 3597-3639 W. P. Risk, Compact Blue-Green Lasers, Cambridge University Press, (3) F. S. Polzik and H. J. Kimble, Opt. Lett. 16 (18) (1991) 14-14 T. Schmitt, A. Deninger, F. Lison, W. G. Kaenders, Proc. SPIE 577, (5) 16- TOPTICA AG, www.toptica.com/ Proc. of SPIE Vol. 6876 68761-7