Single frequency operation of a diode-pumped tunable injection-seeded Nd:GSAG Q-switched laser around 942nm was demonstrated. With a three-mirror ring cavity, the single frequency laser pulse with output energy of 13.2mJ was obtained at a repetition rate of 10Hz. The linewidth of the single frequency laser was less than 100MHz. The wavelength of the single frequency Nd:GSAG laser can be tuned from 942.38nm to 943.10nm.
©2010 Optical Society of America
Water vapor plays a central role in earth climate processes and is of importance for weather forecast and environment protection. For measuring the water vapor Differential Absorption LIDARs (DIAL) are often used [1,2]. For measuring the water vapor concentration from the ground level to the stratosphere, appropriate water vapor absorption lines can be found in the spectral region around 940nm with wavelengths of 935/936nm, 942/943nm, or 944nm. Traditionally optical parametric oscillators (OPOs) , Raman lasers , and Ti:Sapphire lasers  were often used to generate these wavelengths. However, for airborne or spaceborne DIAL applications, the OPOs, Raman lasers and Ti:Sapphire lasers do not accomplish the efficiency requirements . During the past few years, the laser diode (LD) pumped neodymium laser materials using the 4F3/2→4I9/2 transition have attracted wide interests, such as Nd:CLNGG, Nd:CNGG  and Nd:YGG [6,8]at 935nm, and Nd:GSAG [8–10]at 943nm. Compared with the OPO, Raman and Ti:Sapphire lasers, the diode pumped Nd-doped laser has a reduced weight and is more compact. Due to diode pumping, the Nd-doped lasers are also more reliable and have longer lifetimes.
Due to the quasi-three energy levels of the neodymium lasers around 940nm, it is more difficult to achieve stable single frequency operation than that of 1064nm in the four energy levels system [11,12]. Some works of single frequency operation of Nd laser around 940nm had been reported, such as injection-seeding Nd:YGG laser at 935nm  and injection locking Nd:YAG laser at 946nm . As a new crystal, the Nd:GSAG had been studied with the continuous wave (CW) operation, Q-switched and mode-locking experiments [8–10,14].In ref , F. Kallmeyer et al. also studied the injection-seeding of the Nd:GSAG laser. A diode pumped linear cavity Nd:GSAG laser was injection-seeded by a single frequency laser diode. However, without active controlling system the stable single frequency operation of the Nd:GSAG laser was not achieved. In this paper, the stable single frequency injection-seeding Nd:GSAG laser using a three-mirror ring cavity is reported. An active Ramp-Hold-Fire system was designed to control the ring cavity during the seeding procedure. Stable tunable single frequency laser operation around 942nm was achieved in the quasi-three level Nd:GSAG laser. The output energy was 13.2mJ, with a slope efficiency and an optical efficiency of 13.1% and 4.7%, respectively. The wavelength of the single frequency laser was tunable from 942.38nm to 943.10nm. To our knowledge, this is the first time to realize a stable single frequency Q-switched output from the Nd:GSAG laser by using a ring cavity and an active controlling system.
2 Laser design
Figure 1 shows the experimental setup of the single frequency injection-seeded Nd:GSAG laser system. The master laser is a distributed feedback laser diode (DFB) with the spectral bandwidth of less than 1MHz. By altering the temperature of the laser diode a coarse tuning of the output wavelength is achieved, with a tuning coefficient of 0.08nm/K. In our experiment the wavelength of the DFB laser was shifted to around 943nm by heating the laser diode to about 80°C. The fine tuning is achieved by changing the drive current of the laser diode. By using the above tuning modes, the wavelength of the DFB laser can be tuned from 937.5nm to 944.0nm. In order to match the fundamental mode of the slave Nd:GSAG resonator, the output beam of the DFB laser was adapted to the slave resonator by two plano-cylindrical lenses (f = 50mm and f = 100mm), two plano-convex lenses (f = 20mm and f = 1000mm), and an aperture with 1.2mm diameter. To avoid the damage of the DFB laser from the feedback of the slave laser, two optical isolators with a total attenuation of 60dB were used. Behind these optical components, the maximum available power of the master laser was 30mW.
A fiber-coupled LD end-pumped Q-switched Nd:GSAG laser was used as the slave laser. The fiber-coupled LD was operated in the quasi-continuous-wave (QCW) mode, with a pulse duration of 300μs, a repetition rate of 10Hz, and the wavelength of 804nm. Two spherical lenses with the focal lengths of 100mm and 175mm were used to collimate and focus the pump beam. In the middle of the laser rod the pump beam diameter was 1.75mm. A three-mirror ring cavity with a trip length of 2.5m was used to achieve a good mode overlap between the pump beam and the oscillating beam. Two 45° mirrors (Fig. 1) were with high reflectance at 943nm, and high transmission at 804nm (T>95%) and at 1061nm (T>90%) to transmit the pump light and the stronger 4F3/2 – 4I11/2 transition. The output coupling mirror (OC) had a transmission of ~12% at 943nm. A thin film polarizer was inserted into the cavity to achieve the polarized oscillation. Active Q-switch pulse was obtained by using an electro-optical modulator (EOM), which is a potassium dideuterium phosphate KD*P Pockels-cell (EM508M from Leysop Ltd.) and can be triggered either by the pump module or by the active controlling system. The Nd:GSAG crystal was 5.5mm in diameter and 10mm in length, doped with 0.6 at% Nd3+. Both surfaces of the crystal had a high transmission coating at 804nm, 1061nm and 943nm. The laser crystal was attached to a water-cooled copper heat-sink. The temperature of the heat-sink was stabilized to 20°C.
In order to obtain stable frequency operation by using the injection-seeding technique an active system was designed to make the master laser and the slave laser oscillate in same frequencies. A Complex Programmable Logic Device (CPLD) controlling system was used to realize the Ramp-Hold-Fire (RHF) technique . A digitized voltage ramp was amplified to drive the piezo. The interference signal was observed by a photodiode from the polarizer as in the Fig. 1. When the slave cavity resonating with the seed laser frequency, a signal peak was distinguished by the control electronics and the ramp voltage was kept and the length of the laser cavity was maintained. Then a latch signal was given to the laser diode controller and a latch signal with 300μs delay was given to switch on the EOM. During the injection-seeding procedure, the seed laser oscillated in the slave laser cavity and single frequency laser pulse was started from the seed laser.
3. Results and discussions
Figure 2 shows the Q-switched output energy as a function of the pump energy. Without the injection-seeding (free running mode), the ring Nd:GSAG laser has the same output energy in the clockwise and counterclockwise directions simultaneously. The blue circles show the output energy in one direction and the black squares show the total output energy. Without injection-seeding, the laser had maximum output energy of 7.6mJ in each direction and 15.2mJ total together. With the injection-seeding, the slave oscillator has much bigger gain in the clockwise direction than that of the counterclockwise direction. Figure 2 (red line) shows the output energy in clockwise direction as function of the input energy when the single frequency Nd:GSAG laser was injection-seeded. The maximum output energy in the clockwise direction was 13.2mJ, with a slope efficiency of 13.1% and an optical efficiency of 4.7%, respectively. The output energy in the counterclockwise direction was less than 0.05mJ with the injection-seeding. The beam profile at the maximum output energy was illustrated in Fig. 2 (right), which shows that the single frequency Q-switched pulse has a fundamental Gaussian distribution.
The pulse build-up time of the single frequency Nd:GSAG laser was shortened by 770ns when the laser was in the injection-seeding mode. Figure 3 shows the experimental result. The pulse build-up time is shorter when the injection-seeding is working. With the injection-seeding the pulse width is 310ns at the maximum output energy (13.2mJ). The reason for the build-up time reducing is the seed laser is coupled into the slave laser cavity in the injection-seeding mode. Thus a well defined intensity is already present in the cavity and the slave laser pulse starts from a higher intensity compared to the free running mode.
The spectral characteristics of the laser pulse were investigated by using a Fabry-Perot Interferometer (FPI) with plane mirrors. As shown in Fig. 4 (left), multi-mode operation was observed when the laser was in free running mode. The laser was operating in a single longitudinal mode when the laser was injection-seeded successfully, as shown in Fig. 4 (right). With the active controlling system, the pulse laser was single frequency in the experiment. The free spectral range (FSR) of the FPI was 4.17GHz formed by mirror spacing of 36mm. For a single frequency beam each ring corresponds to one interference order and the distance of two neighboring rings is the FSR of the FPI. The width of rings corresponds to the spectral linewidth of the input beam. For a ring radius r and laser frequency v, the relation between a difference in radius δr and the corresponding frequency difference Δv is given by :Fig. 5 .
The laser wavelength was measured by a laser spectrum analyzer with a resolution of 0.014nm. When laser was in free running mode, the spectrum of laser had several peaks. One example was shown in Fig. 6 (black line). These peaks were due to the etalon effect in the laser cavity. The output coupling mirror was 7mm in thickness and without high transmission coating for laser wavelength. So it had effect on the output spectrum as an etalon, which was shown as blue line in Fig. 6. When the seed-laser was injected into the cavity, the spectrum of the Q-switch laser became a single peak. The typical spectrum in injection-seeding mode was shown in Fig. 6 (red line). The wavelength of the Q-switch laser was close to the wavelength of the seed laser (green line in Fig. 6). When the wavelength of the seed laser was tuned by changing the diode current, the wavelength of the Q-switched laser followed the seed laser. The wavelength of the Q-switched laser can be tuned from the shortest wavelength of 942.38nm to the longest wavelength of 943.10nm. However, the wavelengths of the pulse laser cannot be tuned continuously because of the etalon effect and water vapor absorption in the cavity.
The laser had the highest output energy at the wavelength around 942.78nm which is near the center emission section of the laser medium. The laser can also operate at several wavelengths, even at the middle water vapor absorption line of 942.44nm . However, the maximum output energy was less than 5mJ when laser operated at 942.44nm. The absorption coefficient of water vapor absorption lines at 942.44nm and 943.08nm are 1.01 × 10−22 cm/molecule and 5.86 × 10−22cm/molecule, respectively . So when laser operating at 943.08nm, the absorption loss in the laser cavity was more than 5 times than that of laser operation at 942.44nm. The results showed that the 942.44nm absorption line was somewhat weaker and the laser can operate at this wavelength without any other step, which was also present in the earlier work . For making laser operating on strong on-line wavelength, a dry resonator full of nitrogen gas should be used.
We have demonstrated a single frequency injection-seeded Q-switched Nd:GSAG laser around 942nm region. The laser was operating with a three-mirror ring cavity and injection-seeded by a DFB laser diode. The maximum output pulse energy of the single frequency laser was 13.2mJ at 942.78nm, with a repetition frequency of 10Hz. The wavelength of the single frequency Nd:GSAG laser can be tuned from 942.38nm to 943.10nm when it was injection-seeded.
We thank Prof. G. Tränkle and Dr. A. Klehr from Ferdinand Braun Institut für Höchstfrequenztechnik (FBH) for supplying high power DFB diode lasers and Dr. L. Ackermann from the Forschungsinsitut für Mineralische und Metallische Werkstoffe Edelsteine/Edelmetalle GmbH (FEE) for supplying high quality Nd:GSAG crystals.
References and links
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