# Online Measurement of Internal Parameters in a Quasi-CW Diode-Pumped Nd:YAG Laser
## 3 Experimental Arrangement  
### Online Measurement Experiment  
The experimental arrangement of online measurement of internal parameters by a measurement cell is shown in Figure 3. In the Nd:YAG laser product, a flat-flat cavity was employed. The plane mirror M1 had a high reflectivity of 99.8% at the wavelength of 1064 nm. The plane mirror M2 was an output coupler with a partial transmission at the wavelength of 1064 nm. The length of the resonator was around 300 mm. The energy source of the solid-state laser was provided by a quasi-CW diode-pumped Nd:YAG module. In the module, each LD array (LA) mounted on the upper and lower heat sink (HS) had a semicircular structure, and the orientation of the LD bars was arranged along the axis of the Nd:YAG crystal. The face-to-face configuration of the upper and lower semicircular LD arrays formed a circular periodical pumping. The HS was designed in a \"comb\" shape with four rod holders (Hld) that are responsible for supporting and cooling the rod. A 1% doped Nd:YAG rod with a dimension of \(\Phi 5\times 65\) mm in the middle of the circular structure was clamped by the upper and lower HS. The LD arrays had a length of 10 mm roughly along the axial direction of the laser rod and the length of the holders was about 8 mm. In addition, the methods for heat removal that were employed in the module are conductive to cooling and forced air. The heat produced by the LD arrays and the Nd:YAG crystal was conducted to the outside heat exchanger (HE). Furthermore, two fans were used to generate air flows that pass over the HEs to dissipate the heat into the surrounding air. LD arrays with a wide spectrum (6 nm) were used, and the pump source efficiency was about 66% in the spectrum width of 3 nm at the center wavelength of 808 nm. Each LD array had approximately 10 LD bars, and the incident peak power of each array was near 1 kW. The LD bars work at a pumping rate of 12 Hz with a pump pulse duration of 250 \(\upmu\)s.  
A measurement cell with a rotating angle accuracy of approximately 1\({}^{\circ}\) was placed between mirror M1 and the Nd:YAG Module. The measurement cell and the total reflector mirror M1 form another variable output coupler. The output \(E_{MO}\) of the measurement cell is the sum of the output loss \(E_{MO1}\) and \(E_{MO2}\) as shown in Figure 3. The outputs \(E_{MO1}\) and \(E_{MO2}\) include the polarization reflective loss and the round-trip insertion loss of the measurement cell. A three-dimensional (3D) view of the measurement cell is shown in Figure 4. The polarization analyzer PL2 with a diameter of 10 mm was assembled at a mirror bracket (Mb) with an external thread roller. The roller screws into the threaded hole of a support stand (St) on the bed plate as shown in Figure 4. As a result, PL2 rotates around the axis of the roller. The polarizer PL1, with a diameter of 10 mm, was fixed to a holder on the bed plate. PL1 and PL2 were each designed as a polarizer of a 45\({}^{\circ}\) incident angle, and the minimum spacing distance was about 8 mm. The dimensions of the measurement cell are approximately 28 mm (L) \(\times\)12 mm (W) \(\times\)18 mm (H). The thicknesses of PL1 and PL2 were about 1 mm, which caused a smaller variation of 1 mm in the laser cavity's optical length relative to the cavity optical length of 300 mm. If a laser is operated without any longitudinal mode selection elements in the resonator, then the spectral output will consist of a large number of discrete frequencies. The separation of the longitudinal modes in a laser cavity [13] is given by  
\[\Delta\lambda=\frac{\lambda_{0}^{2}}{2L}, \tag{20}\]  
where \(L\) is the optical length of the resonator and \(\lambda_{0}\) is 1064 nm.