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We have achieved 4.1W of 3.5-micron output from a non-critically phasematched (NCPM), type II, KTiOAsO4 (KTA) optical parametric oscillator (OPO) pumped within the cavity of a Q-switched diode-pumped Nd: YALO laser operating at 10kHz. We adopted the simplest configuration with a compact diode-pumped Nd: YALO module pumping the singly resonant KTA OPO. Besides 4.1W of 3.5um, 10.9W of 1.5 micron and 11.3W of 1-micron radiation were obtained simultaneously.
©2001 Optical Society of America
1. Introduction
There are many applications for high repetition rate, high average power mid-IR (3–5µm) laser sources, such as remote sensing (eg. LIDAR) [1, 2, 3] and medical applications, however, it is not trivial to develop such a device. High power with high repetition rate (>1kHz) operation often requires pumping with continuous wave (CW) diode arrays. When Q-switched by an acoustic-optic modulator, the pulsewidth of such a CWdiode-pumped laser is typically few tens of nanoseconds with pulse energy typically up to a few millijoules. It is difficult to pump an external OPO efficiently, such as KTP or KTA OPO, with such a low peak power laser. One way to pump OPOs efficiently with such high average but low peak power lasers is to put the OPO inside the cavity of the pump laser. This takes advantage of the high fluence inside the laser cavity. Moreover, putting the OPO within the cavity of the laser permits recycling of the nondepleted 1-µm pump beam (which is resonating inside the laser cavity) by multiple passes through the intracavity OPO. In addition, the intracavity configuration allows one to obtain compact laser head design compared to externally pumped OPO.
Intracavity mid-IR OPO’s have been investigated by several researchers [4~7], however, mostly with average power of less than 2W. In Ref [4], Larry R. Marshall has demonstrated 1.4W of 3.77-µm with intracavity KTA OPO pumped by an Nd: YLF laser. However, the thermomechanical properties of Nd: YLF limits its power handling capability. In this work, we report our result of >4-watts of mid-IR intracavity OPO output with noncritical phase matching (NCPM) KTA crystal, which was pumped by the Nd: YALO laser.
We have recently reported the high average power, Q-switched operation of our diode pumped Nd: YALO laser [10]. The main advantage of this Nd: YALO laser lies in its natural optical anisotropy which provides a simple solution (with much less optical elements) to generate polarized laser light for nonlinear wavelength conversion, such as OPO. On the other hand, the KTA crystal has significantly low absorption in the 2~5µm regime[8]. The large acceptance angle for NCPM also permits efficient OPO operation even with a multi-transverse- mode pump laser, such as our Nd: YALO laser with an M2 of around 20. KTA has already demonstrated its capability of up to tens of watts of power handling [8, 9], however, such high power configurations are very complex and easily occupy a lot of space.
In this paper, we will present an intracavity NCPM KTA OPO pumped within this Nd: YALO laser, where we have managed to push the 3.5-µm output power to over 4W at a repetition rate of 10 kilohertz. Our combination of Nd: YALO pump source with intracavity NCPM KTA OPO provides a simple solution to generate multi-watt level mid-IR output, together with more than ten watts of eye-safe (1.5-um) radiation and more than ten watts of 1-µm radiation in a very compact laser head (<40cm×10cm×10cm in size).
2. Experimental Setup
The set-up for our intracavity KTA OPO is shown in Fig. 1. The Nd: YALO pumping module consists of a b-axis (Pbmn) cut, 4-mm diameter by 97-mm-long water-cooled Nd: YALO rod side-pumped by five close-coupled CW diode arrays [13]. Its end surfaces are curved (ROC=-0.3meter). The rod is aligned with its c-axis parallel to the vertical axis and the output of this 1079nm laser is linearly polarized along this c-axis. The maximum diode pumping power available is approximately 570W. The laser cavity, ~33cm long, is formed by high reflector M1 (R>99% @ 1-µm) and output coupler M2 (for which we use 1-µm reflectivities in the range from 60 to ~100%). We did not use R=100% for M2 in this experiment because we wanted to have three wavelengths output simultaneously. In addition, this also prevented optical damage due to fluctuations of high laser intensities when tuning the OPO (which in this case acted as a non-linear output coupler for the laser). The KTA OPO consists of a type II, x-cut (ϕ=0°, θ=90°), 7×7×35mm KTA crystal (from Crystal Associates, Inc), which is AR coated at 1.34-µm, placed within mirrors M3 and M1. This KTA crystal is mounted on a water-cooled heatsink with indium thermal contacts. The Y-axis of this KTA crystal is parallel to the vertical axis, which is the c-axis of the Nd: YALO rod. Mirror M3 is AR coated at 1-µm and highly reflective (R>99%) at both 1.5 and 3~4-micron. Besides acting as a high reflecting mirror at 1-µm, M1 was also the output coupler for the OPO, which is AR coated at 3~4-µm and has 1.5-µm reflectivities in the range of 60 to 100%. Hence, this KTA OPO is a singly resonating OPO (SRO). Because Mirror M1 is AR coated at 3~4-µm, the mid-IR output is fully extracted. We have experimented with different output coupling for the OPO, using R=60% to 100% @ 1.5µm for mirror M1. We achieved the maximum mid-IR output power with R=90%. The distance between the surface of the laser rod and M2 is 12cm, while the distance between the other surface of the rod and M1 is 11.5cm. All mirrors used in the experiment were flat. The stable laser cavity mode is derived from the thermal lensing produced in the strongly pumped Nd: YALO laser rod with its two concave end faces. The KTA OPO cavity is 4cm long and placed as near as possible to mirror M1, where the position of the laser beam waist is. The AO Q-switch is also positioned as near as possible to mirror M2 (the location of the other waist) in order for better hold-off at high gain of the 1-µm laser.
Fig. 1: Schematic diagram of intracavity NCPM KTA OPO, where M1 is the HR mirror for 1-µm laser and output coupler for KTA OPO, M2 is the output coupler for 1-µm laser and M3 is the HR mirror for KTA OPO.
3. Experimental Results
3.1 Performance of diode-pumped Nd: YALO laser
Without KTA crystal and OPO mirror M3, we have obtained up to 100W of CW 1079nm laser output. During Q-switched operation, the maximum average power was 90W at 30kHz, and drop to 77W at 10kHz, and this is obtained with output coupling mirror M2 having 1-um reflectivity of R=80%. The 1-um laser average output power versus the repetition rate is shown in Fig. 2. The pulsewidth (FWHM) of the 1-um output is also shown in Figure 2, with pulsewidths of 155ns at 20kHz and 97ns at 10kHz.
Fig. 2: 1-µm laser average output power and pulsewidth versus A-O Q-switch repetition rate, obtained using an R=80% output coupler.
We also measured the M2 factor for the laser beam at different repetition rate (3–30kHz). This was done by focusing the beam with an f=25cm lens, sampling the second moment beam diameter along various positions with the Spiricon Beam Analyzer (Model LBA-300PC), and then fitting these data with the standard beam propagation equation. We found that the 1079nm-laser mode was quite stable with such cavity design, the M2 remaining quite constant at different repetition rates, the M2 parallel to c-axis is within 18–20 and the M2 parallel to a–axis is within 13–15. This indicated that the thermal lensing was quite insensitive to the repetition rate. The reason the M2 parallel to the c-axis was different from the M2 parallel to the a-axis is due to the natural anisotropy of the YALO host resulting in different thermal lensing in the two axes. Within the range of the measured thermal focal length, the TEM00 mode size remained at around 300~400µm. This contributed to our observed stable operation of the intracavity KTA OPO over the operation range of 5–15kHz.
When only KTA crystal was placed inside the laser cavity, the maximum CW output power was ~93W. When both KTA crystal and mirror M3 are placed inside the laser cavity, the CW output power dropped to ~80W and this was achieved after careful alignment of mirrors M1 and M3 to the KTA crystal surfaces. We observed that when the KTA crystal was slightly out of alignment, the output power dropped dramatically. This is because the AR coatings on the surfaces of the KTA crystal is for 1.34-µm but not for 1-µm, with a measured single pass transmission of ~70% at 1079nm. With good alignment, the losses due to the coatings can be significantly reduced by the Fabry-Perot effect between the mirrors M1 and M3 and the KTA surfaces.
Fig. 3: Average output power of 1.5-µm and 3.5-µm from NCPM KTA OPO, together with 1-µm laser output power versus diode pump power.
3.2 Results of Intracavity NCPM KTA OPO
We have achieved the maximum output power of 4.1W @3.5-µm with R=90%@1.5-µm reflectivity for mirror M1 and at a repetition rate of 10kHz. Fig. 3 shows the 3.5-µm and 1.5-µm output power versus the diode pumping power using R=90% @1.5-µm reflectivity for mirror M1 and at a repetition rate of 10kHz. The maximum 4.1W of 3.5-µm output was obtained together with 10.9W of 1.5-µm, both of them coming out from mirror M1. At the same time, there was also 11.3W of 1-µm output produced from the laser output coupler mirror M2 (with R=95% @ 1-µm). All these were obtained at a diode pumping power of 570W. This correspond to 14.2% conversion efficiency to signal and 5.3% conversion efficiency to idler, with respect to the 77W of 1-um pump laser output power at 10kHz repetition rate. We observed the OPO threshold almost coincided with the laser threshold, which reflect the much lower threshold of our intracavity OPO compared to externally pumped OPOs. By varying the repetition frequency from 5 kHz to 15 kHz, the 3.5-µm OPO power peaked at 10kHz and stayed above 3Wthroughout this entire frequency range, which is shown in Fig. 4.
The wavelengths for the KTA OPO signal and idler outputs were measured to be 1563nm and ~3490nm, respectively. The pulsewidths of 1-µm and 3.5-µm were also measured with a fast ORIEL InGaAs detector and HgCdZnTe detector, respectively. At the highest 3.5-µm OPO power of 4.1W, the pulsewidths (FWHM) of the 3.5-µm and 1-µm laser pulses were 59ns and 97ns, respectively. We have also measured the M2 values for the 3.5-µm outputs at the maximum output power of 4.1W to be 17.8 (vertical) and 17.6 (horizontal).
Fig. 4: Average output power of 1.5-µm and 3.5-µm from NCPM KTA OPO, together with 1-µm laser output power versus A-O Q-switch repetition rate.
4. Conclusion
In conclusion, we have obtained 4.1W of 3.5-µm from an intracavity KTA OPO, pumped by a diode-pumped Nd: YALO laser. To the best of our knowledge, this is the highest intracavity mid-IR OPO power reported to date. Besides, we also have 10.9W of 1.5-µm and 11.3W of 1-µm outputs simultaneously, and to the best of our knowledge, this is again the highest reported 1.5-µm average output power obtained from an intracavity SRO. To achieve such high average power output with multi-wavelengths, we have adopted the NCPM KTA OPO pumped within the linearly polarized Nd: YALO laser. Our intracavity configuration also resulted in a simple and compact laser device.
5. Acknowledgments
The authors would like to thank the Directorate for Research and Development, Singapore (DRD) for funding this experiment and Mr. Philip Chan and Dr. Teo Kien Boon for their continued support. Besides, we would also like to thank Mr. Phua Poh Boon for his contributions and fruitful discussions.
References and Links
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