LIGO-P R. High-Power Fundamental Mode Single-Frequency Laser

Transcription

1 LIGO-P R High-Power Fundamental Mode Single-Frequency Laser Maik Frede, Ralf Wilhelm, Dietmar Kracht, Carsten Fallnich Laser Zentrum Hannover, Hollerithallee 8, Hannover, Germany Phone: , Fax:-100, Frank Seifert, Benno Willke Albert-Einstein-Institut, Callinstr.38, Hannover, Germany Abstract: The first results on an injection-locked high-power Nd:YAG ring laser with 195W single frequency output power in a stable linearly polarized fundamental mode operation for the next generation of gravitational wave detectors will be presented Optical Society of America OCIS codes: ( ) Lasers, injection-locked; ( ) Lasers, single-mode Introduction For the fundamental research of gravitational wave detection a specific laser source is needed to achieve a detector sensitivity of Therefore, a stable and low-noise fundamental mode, single frequency laser with an output power of nearly 200W has to be realized [1,2]. Beside high amplitude- and frequency-stability the long-term stability and an easy maintenance has to be taken into account. We present the first injection-locked high-power ring laser with a single frequency output power of 195W in a stable linearly polarized fundamental mode operation. Pound-Drever-Hall locking-electronic Fig.1: Setup of the injection-locked high-power laser. HP-FI: High power isolator with thermal lens and birefringence compensation.

2 Laser concept To achieve a linearly polarized fundamental mode output power in the range of 200W a high power laser with four end-pumped Nd:YAG laser rods was developed. To compensate the thermally induced birefringence each two of the laser rods are combined with a birefringence compensation [3]. To achieve high long-term stability and an easy maintenance each laser rod is pumped with a fiber bundle of 10 fiber-coupled diodes each delivering up to 30W of pump power. The diodes were operated by nearly 75% of their nominal power to increase the diode lifetime. In case of long-term stability and maintenance, the derated operation of the diodes enables the compensation of a diode failure or diode degradation. A pump-light homogenizer was connected to the fiber ends to achieve a homogenous pump-light distribution [4]. To reduce the maximum temperature and surface stresses at the 40mm long 0.1 at.% doped Nd:YAG rod, two 7mm long undoped end caps were attached [5,6]. Furthermore, a double pass of the pump light was realized for smoothing the longitudinal temperature distribution. In order to perform injection-locking for single frequency operation the laser system was used in a ring resonator configuration. The setup of the whole system is sketched in Figure 1. To achieve a stable single frequency operation with a locking-range of a few MHz the frequency stability of a 0.8W single frequency master laser (NPRO) [7] is transferred via an intermediate 12W ring oscillator [8] to the high-power slave laser. The locking of the high-power laser via the intermediate stage increases the locking range by a factor of 4 because the locking range is proportional to the square root of the master to the slave laser power. An active length control system based on the Pound-Drever- Hall scheme [9] was used to keep the difference between master and slave laser cavity frequencies to a value within the injection-locking range. A first electronic system stabilized the 12W laser cavity to the frequency of the NPRO. If these lasers are in a non-locked condition a sawtooth voltage is applied to a piezo mirror of the 12W laser system. Therefore, the laser cavity length is changed and the cavity is scanned through her eigenfrequencies, until the cavity length fits to the frequency of the NPRO. If the frequencies coincide, the injection-locking starts and the laser operates in a unidirectional instead of a bidirectional mode. In this case a photo-diode at the output port detects twice the output power and the Pound Drever Hall stabilization locks the cavity length. To lock the high-power laser a similar procedure starts immediately as soon as the first two stages are stably locked. With these locking electronics a stable single frequency operation and a fast re-lock time was achieved. Experimental results With a pump power of nearly 800W the free running high-power laser operates with a linearly polarized output power of 180W (90W for each direction). The laser runs in a stable fundamental mode operation which is monitored by a photo diode and an electrical spectrum analyzer. In free running operation and only one circulating transversal mode, this mode beats with the circulating longitudinal modes and produces beat signals with a distance of the free spectral range (FSR) of the laser cavity. In our measurements no higher transversal modes could be detected between the two beat signals of the fundamental mode. This measurement can also be used to verify the single frequency operation, because if there is only one circulating longitudinal mode, no beat signals can be detected. Starting the locking of the system as described above, the laser starts circulating in only one direction instead of the bidirectional operation. Therefore, the laser output power increased by nearly a factor of two, see Figure 2. After a few minutes the laser achieved the thermal equilibrium and operates in a stable single frequency operation. Only external disturbances can disrupt the operation, as seen at the

3 relock peak. In this operation a linearly polarized fundamental mode output power of 195W was achieved. This is to the best of our knowledge the highest injection-locked single-frequency output power with an optical to optical efficiency > 24% Output Power [W] injection-locked slave re-lock 100 unlocked high power laser + injected 12 W laser 80 01:12 01:13 01:15 01:16 01:17 01:19 01:20 01:22 Time [mm:ss] Fig.2: Locking process of the high power ring laser. To analyse the locking range the Pound-Drever-Hall error signals have been evaluated, see Figure 3. The measured locking ranges of 4.4 MHz and 2.9 MHz are below the calculated results of 4.8 and 6.3 MHz for the intermediate and the high power slave laser, respectively. 3,0 intermediate 12 W 1 2,5 2,0 0 PD DC [V] 1,5 1,0 0,5 0,0 high power slave ,5-3 -1,0-1,5-0,0005 0,0000 0,0005 Fig.3: Pound-Drever-Hall error signals measured from the intermediate and the high power slave laser by changing the cavity length near the resonance frequency. t [s] -4

4 Especially the locking range of the high-power laser has to be improved to guaranty a long and stable locking. Independent from the locking range it was demonstrated for the first time that a slave laser with an output power above 100W can be stably coupled to single frequency operation. The first long term test showed stable locking over 8 hours with only a few re-locks caused by external disturbances. For these external disturbances like temperature or pressure changes which changed the cavity length above the piezo range or for some strong mechanical shock to the optical table an autolock feature was implemented. The dropping output power is detected by the electronic and starts immediately the re-lock of the system as described above. The re-locking process of the complete system takes less than 400 ms as shown in Figure 4. 0,0-0,4 high power laser -0,8 PD DC [V] -1,2-1,6-2,0 intermediate 12 W -2,4-0,4-0,2 0,0 0,2 0,4 t [s] Fig.4: Relock-time of the complete laser system. Measured is the output of the intermediate and the high-power laser. Conclusion We presented a high-power ring resonator with four end-pumped laser heads injection-locked via an intermediate 12W laser to a single frequency NPRO. With this setup 195W linearly polarized, singlefrequency, fundamental mode output power was demonstrated. This is the first time that injectionlocking for a high power laser was applied and from our best knowledge the highest injection-locked single-frequency output power.

5 Outlook After optimization of the locking range, in order to achieve less sensitivity to external disturbances, the system has to be further characterized in respect of beam quality as well as amplitude- and frequencynoise. Therefore, a mode-analyzer-cavity (MAC) will be used. By scanning the MAC through its free spectral range, a complete mode spectrum of the laser can be measured and all higher-order modes can be detected. By locking the MAC to the laser also the frequency stability of the laser can be measured, if thermal and acoustic coupling to the MAC is avoided. To achieve the amplitude- and frequencystabilization needed for gravitational wave detection active control loops will be implemented. References: [1] LIGO II Conceptual Project Book, LIGO M A-M (1999) [2] Advanced LIGO homepage: [3] M.Frede at al., High power fundamental mode Nd:YAG laser with efficient birefringence compensation, Optics Express 12, (2004) [4] M. Frede et al., "High-power single-frequency Nd:YAG laser for gravitational wave detection", Class.Quantum Grav. 21, (2004) [5] Jan Sulc et al., Comparison of different composite Nd:YAG rods thermal properties under diode pumping SPIE-Int. Soc. Opt. Eng, 4630, (2002) [6] M. Tsunekane et al., Analytical and Experimental Studies on the Characteristics of Composite Solid-State Laser Rods in Diode-End-Pumped Geometry IEEE, J. of Selected Topics in Quantum Electronics, 3, 9-18 (1997) [7] T.J Kane et al., Monolithic, unidirectional single-mode Nd:YAG ring laser, Opt. Lett. 10, (1985) [8] I. Zawischa et al., The GEO 600 laser system, Class. Quantum Grav. 19, (2002) [9] Eric D. Black, An introduction to Pound-Drever-Hall laser frequency stabilization, Am. J. Phys. 69, (2000)