We report on a monolithic 1645 nm Er:YAG nonplanar ring oscillator (NPRO) resonantly pumped by a fiber-coupled laser diode. In the experiment, an up to 550 mW single frequency laser output at 1645.2 nm was obtained, corresponding to a slope efficiency of 19.1% and an absolute efficiency of 6.0%. The beam quality M2 was measured to be 2.1 at the highest output power.
© 2013 OSA
Laser sources that operate in the eyesafe wavelength regime near 1.5–1.6 μm have applications in a number of areas,including lidar, spectroscopy, free space communications, and wavelength conversion [1,2]. In many applications, 1.6 μm Er:YAG lasers with a stable single-longitudinal-mode (SLM) operation are required. A mass of investigations have been demonstrated on the 1.6 μm SLM lasers. A linear cavity with an intracavity etalon and polarizer were used for the stable single frequency operation pumped by 1470 nm laser diode . Zhu et al. reported on a 1645 nm SLM Er:YAG laser that was resonantly pumped by a 1532 nm fiber laser, using intracavity etalons to generate SLM operation . Kim et al. reported a 1645 nm SLM Er:YAG laser by using a ring resonator composed of four mirrors and an acousto-optic modulator for enforcing unidirectional operation . Compared with the methods above, the monolithic nonplanar ring oscillator (NPRO) is very useful for eliminating spatial hole burning and obtaining high power single frequency laser output with narrow linewidth. For the development of NPRO technology case, the first monolithic unidirectional argon-ion-pumped Nd:YAG NPRO was reported in 1985 . A diode-pumped Nd:YAG NPRO was reported in 1987, obtaining cw single-axial and transverse-mode power of 25 mW at 1064 nm at a slope efficiency of 19% . As the development of solid lasers operating at 1645 nm, narrowband operation of a monolithic Er:YAG nonplanar ring oscillator resonantly pumped at 1532nm was reported in 2011 . Besides, Gao et al. demonstrated a monolithic 1645 nm Er:YAG NPRO resonantly pumped by a 1532 nm fiber laser in 2012 . However, this pumping approach adds complexity, weight and volume to the laser system. Additionally, the overall optical to optical efficiency with EDFL pumping is typically reduced due to the number of stages. Compared to the fiber lasers, laser diode is being intensely developed because of its superiorities such as simple structure and comparatively higher overall efficiency. 1532 nm LD with narrow band (less than 1 nm) is commercially available at present. Recent improvements on high power diode lasers operating at 1532 nm and studies on continuous wave and Q-switched Er:YAG laser pumped by 1532 nm narrow-band laser diode  show that there is the potential for direct diode pumping of Er:YAG NPRO. To my knowledge, the Er:YAG NPRO pumped by laser diode has not been reported so far.
In this paper, we demonstrated the Er:YAG NPRO pumped by laser diode. An up to 550 mW stable 1645 nm SLM laser output was obtained from the monolithic Er:YAG NPRO, with a slope efficiency of 19.1% and an absolute efficiency of 6.0%. The beam quality M2 was measured to be 2.1 at the highest output power. To the best of our knowledge, it is the highest 1645 nm single frequency laser output from Er:YAG NPRO pumped by 1532 nm laser diode.
2. Experimental setup
According to the principle of the NPRO to eliminate the spatial hole burning by using the unidirectional ring resonator, the Er:YAG NPRO crystal was designed as shown in Fig. 1. The facets containing B, C and D are optically polished flat surfaces where total internal reflection occurs, while the surface containing A, is designated the input/output coupler (IC/OC). The IC/OC of the NPRO crystal was coated for the 1532nm high transmission of both p-polarized and s-polarized pump, while a transmission of 5% for the s-polarized and a transmission of more than 30% for p-polarized at 1645nm laser output. With a magnetic field H, the YAG crystal itself acts as the Faraday rotator. The three total internal reflections create an effect that is analogous to a rotation by a half-wave plate, and the output coupler acts as polarizer. Thus the monolithic nonplanar ring laser is a unidirectional ring laser with no discrete intracavity elements. The parameters of the Er:YAG NPRO were designed by analyzing Jones matrices of one round trip for the s and p polarization states inside the Er:YAG NPRO. From the eigenvectors and eigenvalues of the Jones matrices, the losses and loss differences of the clockwise and counterclockwise polarized modes were calculated . According to the calculated results, a schematic of the crystal with the dimensions 8mm(width) × 4mm(height) × 13mm(length) was needed as shown in Fig. 1(a)). The round-trip path length of the Er:YAG NPRO was about 25 mm and the optical path inside the NPRO was 45 mm. The linear absorption length of Er:YAG was 25 mm since it was a nonbonding crystal. As shown in Fig. 1(b), the nonplanar angle β of our Er:YAG NPRO was chosen as 45 deg to ensure single frequency oscillation. For the defined 2.4 level of 1532/1645 Er:YAG laser , the energy-transfer upconversion (ETU) effect was influenced by the doping concentration of the Er:YAG crystal. In details, the ETU parameters scale linearly with the doping concentration [13,14].
Concerning the dependence of Er:YAG pump threshold on ETU effect and non-dependence of the slope efficiency, the doping concentration of Er3+ in our system is designed to be 0.5 at: % for reducing the ETU effect .
The In foil wrapped Er:YAG NPRO crystal was clamped in a copper block, which was mounted onto a thermoelectric cooler (TEC) to allow for pump-generated heat removal and precise temperature control. The crystal temperature was held at 17°C. In order to achieve the single directional oscillation, a permanent magnetic field, H, of 0.2 T, was applied to the NPRO in the direction shown in Fig. 2. The pumping source was a fiber coupled 25-W diode laser with a fiber diameter of 200 μm and a numerical aperture of 0.22, which was tuned to 1532 nm for the maximal absorption by Er:YAG crystal by controlling the operating temperature. The pump beam was firstly collimated by a spherical lens f1 (f=20 mm) and then focused into the Er:YAG NPRO by a spherical lens f2 (f=46 mm). To make sure the mode matching between pump light and oscillation laser, the incident angle and divergence were carefully tuned and the distance from lens f2 to the surface of crystal was 35 mm.
Before the output beam was measured, a filter (AR at1532 nm and HR at 1645 nm at 45 deg incidence) was used to block the non-absorbed pumping source. We measured the single-longitudinal-mode spectrum using a scanning confocal Fabry Perot interferometer (FPI) (finesse of~310) at the highest laser output power level. Figure 3 is a typical output signal from the scanning F-P interferometer. The FSR of the FPI was approximately 3.75 GHz, as indicated. The upper trace is the F-P ramp voltage and the lower trace is the voltage of the InGaAs detector measuring the Er:YAG laser transmission through the F-P interferometer. The absence of any peaks between the main resonances of the interferometer clearly indicates the operation on one single longitudinal and transverse frequency. Stable single frequency operation at 1645 nm was obtained at room temperature from our Er:YAG NPRO. Although more pump power was available with the 1532 nm laser diode, hops of longitudinal modes occurred with more than 550 mW of output power, indicating that the Er:YAG NPRO was not on SLM operation. Small hops appeared beside the main peaks, meaning that extra longitudinal mode came into being. The amplitude of the transmission through the F-P interferometer was also unstable as the time went on. Additionally, the observed output power fluctuation increased to ±2%.
The single frequency output power versus the incident pump power is shown in Fig. 4. We investigated the influence of the pump spot sizes on the single frequency output powers. Optimum performance was achieved when the pump diameter was set to 400 μm, yielding a slope efficiency of 19.1% and absolute efficiency of 6.0% at 550mW of output power. For the pump beam with a radius of 600 μm, the highest single frequency output power was 425 mW. The corresponding slope efficiency and absolute efficiency were 15.2% and 4.6%, respectively. Coupling optics with scales of 1:1 and 1:4, representing the pump diameter of 200 μm and 800 μm were likewise used to investigate the influence of the pump spot sizes on output power. However, we didn’t obtain laser output in these two tests. For the diameter of 200 μm case, the Rayleigh length of LD was too short to make up the loss in the crystal. Bigger pump spot size abated the mode matching, leading to the pump threshold increase and the efficiency decrease.
As shown in Fig. 5, we measured the output beam propagation by measuring the beam radius with a knife-edge technique at several positions. The data were fitted by least-squares analysis to standard mix-mode Gaussian beam propagation equations to determine the beam quality, or M2, parameter. By fitting the measured data the M2 factor was calculated to be 2.1. The two-dimensional beam profile of the single frequency laser at 550 mW output power, measured by using a Spiricon PyrocamI pyroelectric camera is shown as an inset of the Fig. 5.
The wavelength and the tuning properties of the Er:YAG NPRO were investigated by using the Burleigh WA-650 spectrum analyzer combined with a WA-1500 wavemeter (0.7 pm resolution). The emission line is centered on 1645.22 nm at the maximum output power of 550 mW, and no other emission oscillates with it simultaneously (Fig. 6).
Figure 7 shows the lasing frequency of the Er:YAG NPRO as a function of the temperature. When the temperature of the Er:YAG crystal was 17 °C, the wavelength of the output beam was 1645.220 nm. Fine tuning of the wavelength of the Er:YAG NPRO was achieved by changing the crystal temperature. The frequency tuning range of the laser was from 182.3464 THz to 182.3414 THz, which was 5 GHz without mode hopping between 17 °C and 20 °C, corresponding to the wavelength tuned from 1645.220 nm to 1644.265 nm. The obtained tuning rate of −1.67 GHz/°C is in reasonable agreement with the calculated value of −1.59 GHz/°C according to the formula 17]. The p-polarized character was demonstrated by measuring the output power with Gram Prism.
4. Discussion and conclusions
The linewidth of laser diode was not yet possibly to be made as narrow as that of fiber laser to match the absorption band of Er3+ at 1532 nm. Besides, the inherent higher brightness and beam quality available with the fiber laser pump source allowed for a longer gain medium to be used than that pumped by LD. The bad beam quality of laser diode used in this experiment reached up to 45. Consequently, the laser properties obtained in our experiment, such as highest output power, efficiency and the beam quality were not as excellent as that of Er:YAG NPRO pumped by fiber laser at 1532 nm. Nevertheless, the SLM output power and stability were enough to satisfy with the need of injection locking system. The Er:YAG lasers pumped by LD have great potential in engineering technology with the advantage of low cost, small size in volume, simplification in system integration and so on. More attention in diode pumped Er:YAG lasers will be paid due mainly to the production of semiconductor laser diodes in large quantities with well- defined characteristics, particularly with regard to wavelength stability, efficiency and life time. Higher-performance LDs at 1532 nm should be manufactured to improve the Er:YAG laser in further investigations.
In conclusion, we have obtained 550 mW single frequency output power at 1645 nm in an Er:YAG NPRO pumped by laser diode. The slope efficiency and optical efficiency were 19.1% and 6.0%, respectively. The beam quality M2 was measured to be 2.1 at the highest output power. The wavelength tuning rate of the Er:YAG NPRO was measured to be 1.67 GHz/°C. The compact Er:YAG NPRO system has potential to be used as a master laser for injection locking of a Q-switched 1645 nm Er:YAG laser.
This work was supported by Program for New Century Excellent Talents in University (NCET-10-0067).
References and links
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