Single-Longitudinal Mode Nd:YVO /YVO /KTP Green Solid State Laser

OPTO−ELECTRONICS REVIEW 18(1), 75–79

DOI: 10.2478/s11772−009−0028−5

Single-longitudinal mode Nd:YVO4/YVO4/KTP green solid state laser

J.Z. SOTOR*, A.J. ANTOŃCZAK, and K.M. ABRAMSKI

Institute of Telecommunications Teleinformatics and Acoustics, Wrocław University of Technology, 27 Wyb. Wyspiańskiego Str., 50−370 Wrocław, Poland

We present the concept and practical realization of a single frequency, tuneable diode pumped Nd:YVO4/YVO4/KTP micro− chip laser operating at 532 nm. Theoretical analysis of the single mode operation of such a laser configuration is presented. The single frequency operation has been obtained in a birefringent filter, where an YVO4 beam displacer acts as an ideal polarizer. Experimental results are in good agreement with theoretical analysis. We have obtained stable single frequency operation, tuneable over 0.6 nm in the spectral range around 1064 nm. The laser operated with output power up to 110 mW at 53 nm. The total optical efficiency (808 nm to 532 nm) was 14%.

Keywords: single mode operation, solid state laser, green laser, birefringent filter.

1. Introduction plate in the laser configuration with a birefringent filter. It ensures perfect spatial separation of s and p polarized laser Compact single frequency solid state lasers with intracavity radiation. Consequently, selectivity of the birefringent filter second harmonic generation are attractive coherent sources is improved. The insertion of the YVO crystal enables us for many applications including high−resolution spectros− 4 building a compact and monolithic laser resonator. copy, holography, precise interferometry, and coherent op− tical communication. These applications require stable, nar− row linewidth laser sources with output power levels of tens 2. Theoretical analysis of birefringent filter of mW or higher. The above requirements are well fulfilled Figure 1 shows the concept of a microchip laser with an in− by single mode microchip lasers. ternal nonlinear KTP crystal and a birefringent filter. The Most solid state lasers are based on the Fabry−Perot type filter consists of a polarization element (in our case the optical resonators [1] with standing−wave operation. Hence, YVO beam displacer) and the nonlinear KTP crystal. Crys− spatial hole−burning in active materials causes multi−longi− 4 tals configuration and their places related to each other in tudinal mode operation and, consequently, the amplitude the laser cavity is depicted in Fig. 2. fluctuations of output power (called the green problem, The polarization of laser radiation generated by the Ref. 2) are observed. To avoid this undesirable effect, single Nd:YVO gain crystal has s and p components. After pass− mode operation is needed. In order to obtain single fre− 4 ing the YVO beam displacer, the polarization components quency operation, several techniques have been developed, 4 are spatially separated. Hence, when the output mirror is short laser cavity [3,4], the set−up with an external etalon in− p side the laser cavity [5], the set−up with a quarter plate inside spherical, the polarization is redirected from the main axis the cavity [6,7] or special ring configuration resonator – non and lost for internal radiation. The laser beam at 1064 nm s planar ring oscillator [8]. Single mode operation can also be has only one ( ) polarization component. Polarization state obtained in a laser configuration with a birefringent filter, of the infrared laser beam is changed while passing through where the filter is formed by Brewster plate and KTP crystal the KTP crystal (Fig. 1). [9–11]. Single frequency radiation is generated in the Nd:YVO4/KTP laser configuration but the temperatures of the crystals have to be controlled to within 0.1°C [12], and in the monolithic microchip green laser with pumping current – controlled from 950 mA to 1040 mA [13]. In order to extend the temperature and pumping current range of single frequency operation we put an YVO4 beam displacer between the Nd:YVO4 and the KTP crystals. The YVO4 crystal plays a similar role to that of the Brewster

*e−mail: [email protected] Fig. 1. A concept of microchip laser with birefringent filter.

Opto−Electron. Rev., 18, no. 1, 2010 J.Z. Sotor Single−longitudinal mode Nd:YVO4/YVO4/KTP green solid state laser

The phase shift f between the ordinary and the extraor− dinary waves caused by the double−pass via the KTP crystal, is given by 4p f =×Dl . (3) l optKTP

The optical length DloptKTP of the KTP crystal is rela− tively strongly dependent on the temperature. This depend− ence is given by the following equation

æ dn dn y ö DDlTl()=-+ç z DDnTa÷ , (4) optKTP 3 è dT dT ø Fig. 2. Crystals configuration in the investigated laser cavity. –5 –1 –5 –1 where dnz/dT =1.6×10 K , dny/dT =1.3×10 K thermo− For the settled conditions only (temperature, length of optic coefficients for the axes z and y of the KTP crystal, a = the KTP crystal, polarization state of input laser radiation), 2.8×10–5 K–1 – thermal expansion coefficient along the opti− the beam passing through the KTP crystal reproduces its in− cal axis. Combination of Eqs. (3) and (4) yields the tempera− put polarization after full round trip travel. Next, the laser ture change DT = 18.5°C that is required to tune the bire− beam travels trough the beam displacer without any losses fringent filter trough one period. and the birefringent filter operates like multiplication of Temperature tuning of a single frequency laser with a half−wave plate. For other conditions, the polarization a birefringent filter causes mode hopping between longitu− does not match its input state and the walk−off introduces dinal modes (laser resonator mode in Fig. 2). In order to ob− losses. Hence, the configuration with birefringent filter tain lasing on specific longitudinal mode, the temperature of forces single frequency operation. The filter has periodical DT D the laser resonator must be controlled to q given by Eq. 5. spectral characteristic with maxima spaced by vFSR, In our laser, the temperature change DTq = 0.39°C c Dv = , (1) D FSR D × Dn vq 2 nl3 DT = × . (5) q æ ö dn z dn y v0 D ç -+Dna÷ where n = 0.0844 is the natural birefringence of KTP and è dT dT ø l3 is the geometrical length of KTP. Birefringent filter cha− racteristic and its relation with the Nd:YVO4 gain curve and resonator modes is shown in Fig. 3. 3. Experimental set-up D In order to obtain single frequency operation, vFSR should be greater than the gain width of the medium which According to the theoretical analysis presented above, D is about vG ~ 260 GHz for 3% doped Nd:YVO4. To meet a compact single mode microchip laser was designed. Figu− these criteria, the length of the KTP must fulfil re 4 shows the experimental set−up. The laser resonator con− sists of the following items, 0.5 mm−long Nd:YVO gain c 4 l < . (2) medium with a high reflectance coating at 1064 nm and 3 DD× 2 nvG a high transmittance coating at 808 nm on the entrance side Taking into account the parameters of the crystals being and an antireflection coating at 1064 nm on the other side, used and Eq. (2), length of the KTP crystal must be less than 5−mm−long YVO4 beam displacer with an antireflection 6.9 mm. We used commercially available KTP crystal with coating at 1064 nm on both surfaces, 5−mm−long KTP crys− the length l = 5.3 mm. tal with an antireflection coating at 1064 nm on both sur− 3 faces and 50−mm (radius) curvature concave mirror with a high reflectance coating at 1064 nm and a high transmit− tance coating at 532 nm. The crystals were enfolded in in− dium foil and positioned in a special aluminium block. This way of crystals mounting ensures symmetric heat flow in the laser resonator. The so constructed laser resonator was pumped by a 1−W laser diode at 808 nm. The pump beam was focused on the gain crystal by the aspheric collimator. In the experiments, the laser spectrum was measured with an optical resolution of 0.01 nm using the optical spec− trum analyzer (Ando 6317b). Laser spectrum was investi− Fig. 3. Relations between transmission characteristics of individual gated at 1064 nm. Output power was measured with a power elements of Nd:YVO4/YVO4/KTP laser. meter (Coherent LabMax Top with head PS10).

4. Results

Wavelength of longitudinal modes for the Nd:YVO4/YVO4/ KTP laser was measured as a function of temperature. It was changed from 15°Cto53°C. The temperature wavelength tuning is shown in Fig. 5. These measurements were carried out with a 600−mW pump power. In order to observe proper operation of the birefringent filter, the spectrum of the laser output without the YVO4 beam displacer was measured. The laser operated in a multimode regime, Fig. 6(a). The green output power fluc− tuations were observed. After that, the YVO4 beam displacer was inserted and the laser started operating with one longitudinal mode. Single frequency operation of the designed laser is shown in Fig. 6(b). In order to show the method of temperature wavelength Fig. 5. Wavelength of longitudinal modes as a function of laser tem− tuning, the temperature was changed from 20°Cto30°C with perature.

Fig. 6. Optical spectra of the laser: (a) multimode operation of the laser without the YVO4 beam displacer and (b) single mode operation with the YVO4 beam displacer.

Opto−Electron. Rev., 18, no. 1, 2010 J.Z. Sotor 77 Single−longitudinal mode Nd:YVO4/YVO4/KTP green solid state laser

Fig. 7. Spectrum of: (a) mode−hoping (19.1 GHz) due to temperature tuning of DTq = 0.8°C and (b) overall single frequency tuning range of 0.62 nm. astepof0.1°C. Single frequency operation was obtained in the range from 20.2°C to 29.6°C. In this range, the wave− length of the laser was tuned from 1063.76 nm to 1064.38 nm [Fig.7(b)]. Mode−hopping phenomena appeared. Free mode−hopping conditions were also investigated. The Nd:YVO4/YVO4/KTP laser operated on one specific longitudinal mode when the temperature was controlled to within 0.8°C. Temperature change exceeding 0.8°C results in a mode−hop to the next longitudinal mode [Fig. 7(a)]. The frequency and the thermal free mode−hopping range are 19.1 GHz and 0.8°C, respectively. It is approximately twice the mode spacing in the designed cavity and twice the theoretically calculated value of DTq. This effect is caused by the spatial hole burning and the energy diffusion in the Nd:YVO4 crystal [14]. The single mode operation was observed at the optical Fig. 8. The output power of laser as a function of pump power. spectrum analyzer. However, because the spectral resolution Dl l = 0.01 nm (about 3 GHz around = 1 μm) was not high 5. Conclusions enough to observe high order transverse modes, the simple beating experiment was performed. Beating signals between The concept and practical realization of a single frequency the laser Nd:YVO /YVO /KTP under investigation and the 4 4 widely tuneable Nd:YVO4/YVO4/KTP microchip laser very short (length of the laser cavity 0.3 mm) single fre− were presented. The theoretical background of the single quency Nd:YVO4 laser were observed on the RF spectrum mode operation was introduced. Obtained experimental re− analyzer (HP9648C). The spectral analysis in the range up to sults are in good agreement with theoretical analysis. Single 20 GHz of beating signals was performed by the fast frequency operation in the spectral range above 0.6 nm at photodetector (discovery semiconductors 35 GHz pin diode). 1064 nm was demonstrated. Output power up to 110 mW at It showed nice one beating heterodyne signal – evidence of 532 nm in a single mode regime was obtained. The total op− the single−frequency operation of the investigated laser. tical efficiency (808 nm to 53 nm) of the laser is 14%. The output power at 532 nm of single mode operation was measured as a function of pump power as shown in Acknowledgements Fig. 8. Experiments were carried out at the laser temperature of 28°C. Figure 8 shows that the lasing threshold is about This work was partly supported by the European Union in 150 mW pump power. The maximum power was about 110 the framework of the European Social Fund – the GRANT mW (1−W pump diode was available). The light to light project. It was also performed under the status grant ob− (808 nm to 53 nm) efficiency was 14%. tained by our research group.

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