We present highly efficient sum-frequency generation between two CW IR lasers using periodically poled KTP. The system is based on the 1064 and 1342 nm laser-lines of two Nd:YVO4 lasers. This is an all solid-state light source in the yellow-orange spectral range. The system is optimized in terms of efficiency as well as stability. We compare the performance of a singly and a doubly resonant system, and find that the stability of the singly resonant system is superior to that of the doubly resonant system. We find that the overall conversion efficiency of the single resonant system is higher than for the doubly resonant configuration.
©2005 Optical Society of America
Lasers in the yellow spectral region have over the past decade found an increasing number of applications. The yellow spectrum corresponds to the peak absorption of several popular fluorescent dyes and of the sodium “D” and “d” lines around 594 nm. A great need exists, in the field of biomedical optics, for light sources in the yellow-orange spectral range, due to the high absorption in hemoglobin. This makes the yellow spectral range very interesting for a number of applications both in dermatology and ophthalmology. Today copper vapor lasers (578 nm) and various dye-lasers are used in the medical clinics for dermatological treatment. These lasers are, however, generally large in size, inefficient (need water cooling), complicated to operate, and both the gasses and liquids used in these lasers are highly toxic. These lasers can be used for dermatology where pulsed radiation is needed. However, in ophthalmology there exists a need for stable continuous wave light sources in the 1–2 W power range.
The aim of this project is to make an all solid-state yellow-orange light source that can reach the required power level and stability necessary for ophthalmologic applications. The system should be cost efficient, compact in size and with a high efficiency and stability, allowing operation without water-cooling or special power installations.
In this paper, a diode laser pumped all-solid-state system, able to generate more than 750 mW of 593.5 nm continuous-wave power with a total diode pump power of 6.6 W, is presented. The system is based on sum-frequency generation between the 1064 and the 1342 nm laser lines of two Nd:YVO4 lasers using periodically poled KTP.
Different approaches have been used to pursue an all solid-state yellow laser. Recently, milliwatts of 605 nm radiation have been generated using frequency-doubled quantum-well semiconductor lasers . However, the power level of these devices is still too low and the wavelength too long for many medical applications. In the pulsed regime Raman shifted 1064 nm lasers with subsequent second harmonic generation have been shown to generate average powers in the W-level . More exotic materials such as Cr:Forsterite with large tuning range have been used to generate tens of milliwatts in the orange spectral range . The approach described in this paper is based on sum-frequency generation of two infrared Nd-based laser lines at 1064 and 1342 nm. Similar approaches have been demonstrated previously by a number of groups, both in the pulsed regime [4, 5] and continuous-wave [6, 7]. In the case of CW operation, these systems were based on extra-cavity SFG, whereas some of the pulsed systems used intra-cavity mixing. The reason for this choice was the difficulty in obtaining stable CW operation in the intra-cavity SFG system. In order to obtain sufficient SFG conversion efficiency it is necessary to use a doubly resonant external cavity [8, 9].
We have investigated a system based on SFG between the intra-cavity field of one laser and a single-pass field of another laser, as shown in Fig. 1. Different cavity configurations have been tested and compared both in terms of conversion efficiency and stability. The highest conversion efficiency as well as the best stability were obtained in the setup shown in Fig. 1. The setup was tested both in a single-pass configuration with the 1064 nm laser and as a doubly resonant system where the non-linear SFG crystal was intra-cavity to both lasers. As expected, the conversion efficiency at low power levels could be increased by using the doubly resonant configuration. However, as the power was increased, the increase in conversion efficiency was at the expense of stable CW operation; the system became highly unstable and the average power generated never exceeded 750 mW obtained in the single-pass configuration. Using the single-pass configuration it was possible to obtain high conversion efficiency, and maintain a high degree of stability in the system, even when the system was oscillating on several longitudinal modes [10, 11].
2. Setup for efficient sum-frequency generation
The setup consists of a folded cavity 1342 nm laser with an intra-cavity periodically poled KTP crystal. The 1342 nm laser comprises a 3×3×3 mm3 Nd:YVO4 crystal with a doping level of 1.0 atm%, LC2, folding mirrors M2 (planar) and M3 (r=-100 mm) and the end mirror M4 (r=-100 mm). The 1064 nm laser consists of LC1, a 3×3×3 mm3 Nd:YVO4 crystal (1.0 atm%) and output coupling mirror M1 (r=-100 mm, R=95 % @ 1064 nm). Mirrors M2 and M4 are coated for high transmission at 1064 nm, and M3 is highly reflecting at 1064 nm. Mirrors M2, M3 and M4 are coated for high reflection at 1342 nm. Mirror M4 has a transmission coefficient of approx. 75 % at 593.5 nm.
The distance from LC2 to folding mirror M3 is 277 mm and the distance between M3 and M4 is 158 mm, resulting in an intra-cavity focus between the curved mirrors of approx. 47 µm. The length of the 1064 nm laser is 50 mm, giving a beam waist of approx. 100 µm in the laser crystal. This beam is transformed into a beam waist of 36 µm by the combined effect of focusing lens L (f=150 mm) and mirror M3. The position of this beam waist corresponds to the intra-cavity beam waist of the 1342 nm laser.
The PP:KTP crystal is poled with a period of 12.65 µm corresponding to first order quasi-phase matching for 1064 and 1342 nm sum-frequency generation. Using the temperature dependent dispersion equations of ref. [12, 13] the phase match temperature was calculated to be 40 °C. The optimum phase match temperature was measured to be 51 °C, see Fig. 3. The total crystal length was 11 mm, whereas poling was made over a length of 9 mm. The PP:KTP crystal was AR coated at both ends for 1064, 1342 and 593 nm. Each laser was pumped with an RPMC-3415-808 laser diode capable of delivering up to 4 W of pump power.
3. Measurements of yellow-orange light conversion
Using the setup described above, more than 750 mW of yellow-orange light was generated using 3.2 W of diode pump power incident on each of the two laser crystals. This corresponds to an optical to optical conversion efficiency of 11 %.
The 1064 nm laser was able to generate 1.8 W of power in a single transverse mode using 3.2 W of diode pump power. This beam was focused by lens L and aligned to match the beam waist of the 1342 nm laser at the position of the PP:KTP crystal. The 1342 nm laser was able to form an intra-cavity circulating power of more then 45 W incident on the PP:KTP crystal, in the absence of the 1064 nm field (without nonlinear conversion).
Figure 2 shows measurements of generated yellow power (diamonds) as a function of diode laser pump power for the 1064 nm laser (left Fig.) and 1342 nm laser (right Fig.), respectively. Squares are the 1064 nm power transmitted through the PP:KTP; triangles show the 1064 nm power incident on the PP:KTP crystal. Circles indicate the 1342 nm power.
Clearly, the generated power as a function of the 1064 nm pump power shows the smoothest behavior. However, the variations as a function of the 1342 nm pump diode power are also seen without nonlinear conversion, and they probably originate from thermal lensing rather than from instabilities induced by the nonlinear process. Operating the system at constant pump power and with a proper temperature stabilizing circuit for the PP:KTP crystal, we obtain a very stable output power. When the 1064 nm laser is in a single-pass configuration as described above, no indications of mode beating between longitudinal modes in the two lasers are seen. Furthermore, the transverse beam profile of the generated yellow light is very close to the fundamental Gaussian distribution, however slightly elliptical due to the small off-axis incidence on the curved mirror M3. The ellipticity in the generated yellow beam corresponds to the focused beam profile of the 1064 nm beam and can easily be compensated by adjustment of that laser.
Using the measured values of the 1064 and the 1342 nm power incident on the PP:KTP crystal and using the plane wave approximation, an effective nonlinear (bulk) coefficient of the nonlinear crystal was found to be deff=11 pm/V, which is in good agreement with reference .
Figure 3 shows the measured yellow power as the temperature of the PP:KTP crystal is scanned. As expected, the generated yellow power varies as a sinc-function, and the optimum temperature is found to be 51 °C.
Now an additional mirror was placed after mirror M4, in order to return the 1064 nm light and make non-linear conversion also in the backward direction. This mirror was high reflecting at 1064 nm and high transmitting at 593.5 nm. At low pump power levels approximately the same amount of yellow light was generated it the two directions. However, as the pump power was increased, the 1064 nm laser started to work as a coupled cavity system with the new cavity formed between the laser crystal and the newly added mirror and with mirror M1 as the coupling mirror. Reaching this point at approximately 0.75 W of diode pump power, the system became highly unstable, and the average power did not increase further with increasing pump power, as seen in the left part of Fig. 4.
The corresponding measured single-pass yellow power is shown in Fig. 4 (diamonds). The right part of Fig. 4 shows the amount of generated 593.5 nm power as a function of the PP:KTP temperature. An enhancement of the field is seen as increased power in the side loops of the sinc-function, but as the non-linear coupling is increased by optimizing the phase-match temperature, unstable operation is again seen between 45 and 56 °C.
4. Conclusion and future improvements on the system
The system presented in this paper shows very stable and efficient performance as long as the 1064 nm laser is single-passed through the non-linear media. At the power levels used no saturation or beam degradation effects have been observed. It is therefore expected that this system can be scaled to even higher power levels. It is the aim to increase the power level to 2 W of CW yellow light.
The dynamics of the system, when the 1064 nm laser is operated intra-cavity, needs to be investigated further. Preliminary measurements show that it is possible to obtain stable pulsed operation of the system, if the non-linear SFG crystal is intra-cavity to both the 1064 and the 1342 nm laser. Preliminary results also show that 100 nsec pulses with a repetition rate of 50 kHz and a peak power of approx. 20 W at 593.5 nm can be obtained. Further investigation of these lasers is needed, in order to understand the dynamics of the system as it enters the pulsed regime.
This work was supported by the Danish Technical Research Council, grant 26-02-0210.
References and Links
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