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We report a diode-pumped Q-switched Nd:YAG-KGW Raman laser operating in two-color modulation. The output wavelength can be switched between 1159nm and 1177nm using an E-O switch, and the Raman output modulated in spectra-time domains was achieved. Raman pulse energy up to 114mJ at 1177nm and 98mJ at 1159nm were obtained respectively, corresponding to an overall Diode-Stokes conversion efficiency of 15.3% at 1177nm and 13.2% at 1159nm.
©2010 Optical Society of America
1. Introduction
The stimulated Raman scattering (SRS) in crystalline materials provides an efficient method for frequency conversion [1–4]. The cascaded nature of SRS process has been utilized to develop practical multi-wavelength lasers [5–10], especially solid-state Raman lasers based on KGd(WO4)2, in infrared [5], visible [6,7], and ultraviolet [8] regions of spectrum. In cascade SRS operation, the fundamental and all Stokes wavelengths can be produced simultaneously by an output mirror with broad-band coating [9]. In wavelength-selectable operation, one or several wavelengths can be generated by optimizing the output coupler [5,7] and tuning angle/temperature [6,8].
In previous works [6–8], by manual rotating the KGW 90°about the fundamental propagation axis, the operating wavelength can be tuned and the accessible wavelengths can be extended. However in some applications requiring quick wavelength switching, such as modulation and coding in the spectra-time domains, manual method is not a practical choice. Electrically controllable multi-wavelength operation is of interest for applications [11] in countermeasures, lidar and bio-sensing.
In this paper, we report a diode-pumped Q-switched Nd:YAG-KGW Raman laser operating in two-color modulation. A λ/2 electro-optical switch is introduced in the Raman laser so that the linear polarization direction of fundamental beam can be conveniently switched. By virtue of the polarization-dependent property of KGW crystal, we can further control a T-shape folded cavity to operate in two operation modes—the wavelength-selective mode and output-modulated mode. The Raman line selected out was achieved, and the Raman output was also modulated successfully in spectra-time domains, for which we believe to be the first time.
With the incident pump energy of 745mJ at the center wavelength of 808nm, Raman pulse energy up to 114mJ (peak power of 19MW) at 1177nm and 98mJ (peak power of 15MW) at 1159nm were obtained respectively, corresponding to an overall Diode-Stokes conversion efficiency of 15.3% at 1177nm and 13.2% at 1159nm, a slope efficiency of 23.3% at 1177nm and 20.2% at 1159nm. To our knowledge, these are the best performance reported to date for an EO Q-switched tunable folded cavity Nd:YAG-KGW Raman laser with conductive cooling and air exchanging.
2. Experiment setup
The experimental arrangement is shown in Fig. 1 . We used a T-shape folded cavity configuration, in which the fundamental resonator and the Stokes resonator were separated and operated independently. The design can guarantee high intracavity power density and good mode-matching condition. Particularly, a λ/2 electro-optical switch SW was introduced in the configuration, by which we can conveniently control the linear polarization direction of fundamental beam. By virtue of the polarization-dependent property of KGW crystal, the purpose of controlling and selecting Raman wavelengths can be finally reached.
In Fig. 1, a high-Q fundamental resonator is defined by the plane mirror M1 and the plane mirror M2. M1 has a high reflectivity of 99.8% at the fundamental wavelength of 1064nm. M2 has a high reflectivity at 1064nm (HR≈99.97%), 1177nm (HR≈99.95%) and 1159nm (HR≈99.98%), whilst high transmissivity of approximate 98.5% at the second order Stokes wavelength of 1317nm and 1272nm. The energy source of fundamental wave is provided by a Nd:YAG Module. An electro-optical (EO) Q switch consists of a λ/4 wave-plate WP, Pockels-cell PC, and polarizer PL. The transmission of polarizer PL is 95.78% for P polarized wave, and is 0.4% for S polarized wave respectively. Furthermore, the polarization direction of fundamental beam at 1064nm between the SW and M2 can be controlled by a λ/2 electro-optical switch SW in the fundamental resonator.
Whereas the Stokes cavity is defined by the plane mirror M2 and M3, where M3 is the first order Raman output coupler with 62% partial transmission at the first order Stokes wavelength of 1177nm and 67% partial transmission at the other first order Stokes wavelength of 1159nm. The Raman active medium is a 3 at. % KGd(WO4)2 crystal with the dimensions of Ф5 × 50mm which is cut for laser propagation along NP-axis and AR coated for 1064nm and the first order Stokes wavelengths. The dichroic folding mirror DM placed along 45°direction has a high transmissivity of HT>98.9% at 1177nm, HT>97.3% at 1159nm, and a high reflectivity of HR>97.2% at the fundamental wavelength. The fundamental optical field and Stokes field can be separated by the dichroic folding mirror. The length of fundamental resonator is around 391mm, while that of Stokes resonator is about 107mm.
Figure 2 depicts the experimental structure of the Nd:YAG Module. We employed a side LD arrays pumping structure by the periodical arrangement between the circular LD arrays (LA) mounted on heat sinks (HS) and holders (Hld) of heat sinks. The LD arrays with a wide spectrum of 6nm are used and the pump source efficiency is about 66% in the spectra width of 3nm at the center wavelength of 808nm. The LD bars work at the repetition rate of 11.5Hz with the pump pulse duration of about 230μs. A 1% at. Nd:YAG rod with the dimensions of Ф5 × 65mm, placed in the middle of the module, is held by the holders of heat sinks periodically. In addition, the cooling methods employed in the module are conductive cooling and forced air. The heat sinks play an important conductive cooling role in the process of heat removal, which conduct the heat produced by the LD arrays and Nd:YAG crystal to the outside TE and heat exchangers (HE). The TE coolers are responsible for temperature-control and the forced air is used to exchange heat by the two fans in the most outside. Thus the temperature of heat sinks can be well controlled. Further, the LD wavelength shift with temperature can be efficiently prevented and the thermo-optic effects can be avoided to the most extent.
Without a λ/2 voltage on the optical switch SW, the polarization direction of linearly polarized fundamental light does not change and thus the optical field in the fundamental resonator keeps linearly P polarized wave which is experimentally arranged to coincide with the Nm axis of optical indicatrix in the KGW crystal. According to the polarized spontaneous Raman spectra properties in KGW crystals, we can obtain the first order Stokes line of 1177nm. However, with a λ/2 voltage on the optical switch SW, the linearly polarized direction of fundamental light after the SW will change by 90°. The optical field between M1 and the optical switch SW keeps linearly P polarized wave yet. While the optical field between the optical switch SW and the M2 changes to be S polarized wave which is parallel to the spatial orientation of the Ng axis of optical indicatrix in the KGW crystal. So we can have the first order Stokes line of 1159nm.
Furthermore, the coated mirror M2 only supports the first order Stokes to resonate and suppresses the higher order Stokes mode from resonating. As a result, the Stokes resonator can only produce the Raman wavelength of 1177nm or 1159nm.
3. Results and discussion
We investigate the two operation modes of the Raman laser—the wavelength-selective mode and output-modulated mode.
The wavelength-selective operation mode is first investigated. Without a λ/2 voltage on the optical switch SW, the output Raman spectrum was recorded by a spectrum analyzer (Agilent 86140B) as shown in Fig. 3 . The spectrum width was less than 0.4nm. The measured Raman spectrum validated that only the first order Stokes wave at 1177nm was observed with a shift of 901.5cm−1 relative to the fundamental wave of 1064nm, which was in agreement with the Raman frequency shift on the Nm axis in KGW. In contrast, with a λ/2 voltage on the optical switch SW, the spectrum information of Raman output was shown in Fig. 4 and the spectrum width was less than 0.4nm too. The measured Raman spectrum testified that only the first order Stokes wave at 1159nm was observed with a shift of 767.5cm−1 relative to the fundamental wave, which was also in agreement with the Raman frequency shift on the Ng axis. Consequently, the Raman line selective out of two wavelengths was achieved.
The temporal characteristics of Raman pulse are also investigated. The pulse repetition ratio was measured to be about 11.5Hz. Figure 5 and Fig. 6 illustrate the temporal evolution characteristic of Raman pulse at 1177nm and 1159nm respectively. With Raman line selected out at 1177nm, the pulse width of Raman pulse was about 7.09ns and fundamental pulse was approximately 10.4ns. While, with Raman line selected out at 1159nm, the pulse width of Raman pulse was also about 7.09ns but fundamental pulse was approximately13.5ns. The fundamental pulse was measured from the position of P1 (see Fig. 1.), recorded by an oscilloscope (Agilent 54845A). The temporal compression ratio is less than 2, which can be understood by the lower threshold behavior as a result of the high circulating power density. The delay between Raman and fundamental pulse was about 8.5ns.
In the wavelength-selective operation mode, the near-field profiles measured by a Spiricon beam analyzer (M2-200s) are illustrated in Fig. 7 . The near-field profile of Raman output at the wavelength of 1177nm is shown in Fig. 7a and that at the wavelength of 1159nm in Fig. 7b. At the pump energy of 745mJ and the repetition rate of 11Hz, the laser beam had a divergence angle of 3.1mrad in the x direction and 4.1mrad in the y direction at Raman wavelength of 1177nm (corresponding M2 x of 6.9 and M2 y of 11.4), whereas a divergence angle of 3.8mrad in the x direction and 4.8mrad in the y direction at Raman wavelength of 1159nm (corresponding M2 x of 7.9 and M2 y of 12.6). The larger waist diameter of about 4mm was observed and thus supported more high-order modes which may deteriorate beam quality.
Fig. 7 The near-field profiles of Raman output: (a) near-field profile at 1177nm. (b) near-field profile at 1159nm
In the wavelength-selective operation mode, Raman pulse energy (peak power) at the wavelength of 1177nm and 1159nm were recorded respectively by a Molectron EPM-2000. Figure 8 shows Raman output pulse energy, corresponding peak-power at 1177nm, and the fundamental losses P1 and P2 at 1064nm as functions of the incident pump energy at 808nm. With the first order Stokes output at 1177nm, both pulse energy and corresponding peak-power, increased near linearly with the LD pump energy once the pump energy was above the Raman threshold. The Raman output of the pulse energy up to 114mJ and the corresponding peak power of 19MW were obtained at the incident pump pulse energy of 745mJ, corresponding to an overall Diode-Stokes conversion efficiency of 15.3% and a slope efficiency of 23.3%. The fundamental losses P1 and P2 grew very slowly with the increase of incident LD pump energy, and were always kept at a low level below 20mJ. Comparing the loss P1 with P2, we found that the loss P1 was always slightly lower than that of P2 and no surge of deviation from linearity with the increase of LD pump, which implied that the thermo-optic effects, such as thermally induced birefringence and depolarization, did not occur in the folded cavity.
Fig. 8 Raman output pulse energy, corresponding peak-power at 1177nm, and the fundamental losses P1 and P2 at 1064nm as functions of the incident pump energy at 808nm.
Comparing with the output at 1177nm, the first order Stokes output at 1159nm is shown in Fig. 9 . Both pulse energy and corresponding peak-power showed similar linear trend as the output at 1177nm and were slightly less than those at 1177nm, which was mainly the result of different stationary gain increment, loss and coupling output between the two neighboring Stokes waves. With the incident pump energy of 745mJ, we obtained the first-order Stokes output of 98mJ at 1159nm and corresponding peak power up to 15MW, corresponding to an overall Diode-Stokes conversion efficiency of 13.2% and a slope efficiency of 20.2%.
Fig. 9 Raman output pulse energy, corresponding peak-power at 1159nm as functions of the incident pump energy at 808nm.
In the wavelength-selective operation experiment, the output fluctuation was around 10% and no saturation was observed. In addition, no coating and crystal damage were observed, which was benefited from those of the larger waist diameter of about 4mm, the AR-coating with a transmissivity of higher than 99.8%, the Nd:YAG damage threshold of higher than 15J/cm2 and the KGW damage threshold of 10GW/cm2.
Next, we periodically switched the λ/2 optical-switch at a period of approximately 15 seconds at the pump repetition rate of 11.5Hz. The spectral information was monitored by a wide range optical spectrum analyzer (Agilent 86140B). The measured Raman spectrum showed an alteration between 1177nm and 1159nm, which testified that the output was modulated successfully in spectra-time domains. It is significant because the multi-wavelength laser detecting technology of output-modulated plays an important role in lidar, reconnaissance and laser range finder, especially in optoelectronic countermeasure environment [11].
Since the response time of the λ/2 optical switch SW is in the magnitude of microsecond, the spectra-time domain coding of Raman output can be potentially realized. Regrettably, the experiment of coding was not carried out due to the lack of available beam splitter between 1177nm and 1159nm.
4. Conclusion
In summary, a diode-pumped Q-switched Nd:YAG-KGW Raman laser operating in two-color modulation has been demonstrated. By virtue of the polarization selectivity of KGW crystal and a λ/2 electro-optical switch, Raman laser successfully operated in two operation modes—the wavelength-selective mode and output-modulated mode.
The design of the symmetrical periodical side pumping provides a way to get rid of the complex liquid cooling system and balance the space conflict between pump and heat-removal device due to the demands for larger lateral surface of a laser rod. It can be predicted that this design has great potential to be used in compact and miniature laser systems.
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