1. Introduction

Diode Pumped Alkali Lasers (DPALs), extensively studied during the past decade, attract growing interest of researchers because of their potential to produce high laser power in an excellent quality beam from single aperture. DPALs based on several alkali atoms (Cs, Rb, K) demonstrated efficient and reliable operation at moderate power levels up to several tens of Watts (see, for example [1]). However, experiments aimed at scaling DPAL output power to higher levels revealed some limiting parasitic effects such as output power degradation in time [2,3] causing a decrease in laser efficiency and, termination of lasing. Also, in some cases, catastrophic damage of the alkali vapor cell and gain medium contamination was observed [4–6]. These problems can be connected with thermal effects, ionization, chemical interactions between the gain medium components and alkali cell materials. Study of all these and, possibly, other limiting effects and ways to mitigate them is very important for high power DPAL development. In this paper we present results of our experiments on temperature measurements in the gain medium of an operating Cs DPAL at different pump power levels. To study mechanisms for heat deposition into the gain medium, we also performed measurements of the pumped gain medium temperature, when the lasing does not occur (no laser cavity).

2. Experimental apparatus and results

For contactless in situ temperature measurements, we used the interferometric technique and temperature measurement procedure, developed in our lab [7]. The diagram of the experimental setup is presented in Fig. 1. As a tool to measure temperature changes, we used a Mach-Zehnder interferometer, one leg of which was longitudinally coupled into the laser cavity of a Cs DPAL including the alkali vapor cell. The HeNe laser beam (633 nm) was coupled into the laser cavity using two dichroic mirrors, which reflect the 633 nm light and transmit both the 852 nm pump light and the 894 nm lasing light. Two 50/50 beam splitters were used to split and combine the HeNe laser beam to produce a high contrast interferogram recorded by a video camera.

 

Fig. 1 Experimental setup.

Download Full Size | PDF

The longitudinally pumped stable 40 cm long laser cavity was assembled using standard L-shape geometry with pump and lasing beams separated by a polarizing beam splitting cube (PBS). The flat output coupler had reflectivity 20% at 894nm and high reflective back mirror had a radius of curvature of 50 cm. The DPAL gain medium (Cs vapor and Methane at 600 Torr in a 2.5 cm long and 2.5 cm diameter cell) was kept at the optimal for our system temperature 125 C, which was determined experimentally using short pump pulses (from 30 μs to 1 ms) at low duty cycle, when a contribution of the thermal effects is negligible. The gain cell was pumped by a narrowband diode laser stack providing power up to 400W with a linewidth about 10 GHz. The pump beam was focused into the center of the gain cell by combination of spherical and cylindrical lenses providing beam shape inside the cell presented in Fig. 2. The size of the beam in the focal plane was 0.58 mm x 0.33 mm (FWHM).

 

Fig. 2 Pump beam FWHM inside the gain cell in two planes – vertical and horizontal.

Download Full Size | PDF

This system allows us to acquire the temperature profile in the gain medium of the operating Cs DPAL created by heat deposited from the pump and lasing beams. The HeNe laser beam propagating through the gain medium acquires a phase shift due to a gradient in the index of refraction caused by the induced temperature gradient; this causes a distortion of the interferogram corresponding to the path length change. The value of the registered phase shift can be calculated using the commercial wave front analysis software QuickFringe and then converted into the temperature rise using procedure described in [7]. The temperature measured using this setup represents its average value along the pump/lasing beam inside the alkali cell.

For measurement of the gain medium temperature rise in an operating Cs DPAL we used a pulsed pump with rectangular shaped pulses with duration 100 ms and a 5 sec pause between pulses. This pulse length is sufficiently long for the static cell to reach CW operation but is also sufficiently short to allow excluding possible contribution of thermal lensing in the cell windows. The 5 sec pause between pulses was chosen to allow deposited heat to dissipate and exclude accumulated contributions of the previous pulses into the recorded data.

Before conducting experiments on an operating Cs DPAL gain medium, we tested our setup using an empty alkali cell. Under these conditions, only the cell windows can induce a phase shift in the interferogram due to thermal lensing under the pump pulse. This test experiment showed that such distortion does exist, but its value is negligible compared to the phase shift observed in the experiments conducted on an operating DPAL described below.

Figure 3 shows the laser output pulses obtained using rectangular 100 ms pump pulses with different powers. As can be seen, the lasing power has a rapid rise at turn-on on a time scale of about 0.5 ms see Fig. 4(b) and then a rapid decay, after which it settles at the CW output power. A corresponding experimentally measured time dependence for the temperature of the gain medium during a 100 ms pump pulse with a power 370W is shown in Fig. 4. It is very well seen that the gain medium temperature rise has a significant effect on lasing power. Indeed, a rapid temperature increase at lasing turn-on causes rapid lasing degradation. As the temperature begins to plateau, the laser power converges to its CW value. When the pump pulse ends, the temperature decreases to its initial level (125 C). In a long enough run (e.g. CW mode) this process results in temperature and lasing power equilibrium.

 

Fig. 3 Static Cs DPAL laser pulses for various pump power levels. The pump pulse for each power has rectangular shape and the laser output degrades from the initial peak converging to the CW power level.

Download Full Size | PDF

 

Fig. 4 Gain medium temperature time dependence and the laser pulse for a pump power 370W (a) and details for laser power and temperature changes at turn-on (b). Estimated error for the measured temperatures is about 50 C.

Download Full Size | PDF

The experimentally measured gain medium temperature time dependences on a 1 second time scale for different pump power levels are presented in Fig. 5. Estimated error for the measured temperatures is about ± 25 C. As can be seen the gain medium thermal recovery after the end of 100 ms pump/lasing pulse is very much slower than the initial temperature increase.

 

Fig. 5 Gain medium temperature time dependences for different pump powers. Estimated error for the measured temperatures is about 50 C.

Download Full Size | PDF

We also performed comparative measurements of the gain medium peak temperatures using a 100 ms pump pulse with different powers for lasing and no-lasing conditions. For the second case, we installed an opaque screen between the gain cell and the output coupler to stop lasing, while the pump pulse was coupled into the gain medium. The results of these measurements are provided in Fig. 6. The cesium DPAL was operated using 100 ms pulses and 5 second relaxation time between pulses to ensure no residual thermal effects affected subsequent pulses. Using 100 ms pump pulses allowed the static Cs DPAL to achieve a quasi CW operation for relatively high pump power (50 – 370 W). The peak temperatures of the gain medium during lasing pulse ranged between 300 and 700 C for different pump power.

 

Fig. 6 Cs DPAL gain medium peak temperature for lasing and no-lasing conditions along with their associated uncertainties.

Download Full Size | PDF

A comparison of the temperature increase for lasing and no-lasing conditions is given in Table 1. The temperature increase for the lasing case is higher than for the non-lasing case. The higher temperature in the lasing case can be attributed to the heat deposited from spin-orbit relaxation, which is borne out by the good agreement with the theoretical analysis. These results are interesting because they indicate that the majority of heating is a result of quenching and not spin-orbit relaxation. We conducted an analysis of these results using the spin-orbit relaxation and the quenching cross-sections σ₂₁ = 21.36(0.01) Ų and σ_Q = 1.4(0.6) Ų respectively [8].

Table 1. Experimentally measured temperature rise for the lasing (ΔT_L) and non-lasing (ΔT_NL) cases along with the corresponding ratio of these values compared to the expected theoretical values. The uncertainty for the theoretical values was computed using the uncertainty in the quenching cross-section.

View Table | View all tables in this article

3. Analysis and discussion

The heat production in a DPAL arises from spin-orbit relaxation and quenching. A complete analysis of heat generation would require a fairly complex model, which was not available for this analysis. We applied a simplified model to evaluate the quenching and the spin-orbit relaxation heat contribution. The heating power P_H deposited into the unit of the gain medium that results in heating the gain medium because of quenching of both ²P states and spin-orbit mixing of these states is given by:

The powers for lasing and non lasing conditions were computed using Eq. (1) using the relative level populations for the lasing and non-lasing cases which are provided in Table 2.

Table 2. Level populations relative to Cs vapor number density N_Cs for lasing and non-lasing cases. The non-lasing cases were calculated at the peak temperatures shown in Fig. 5.

View Table | View all tables in this article

These level populations are determined by solving the laser rate Eqs. (4)-(6) under steady state conditions:

The number density is given by N_x, where x = (0, 1, 2, g, Cs) represents (S_1/2, P_1/2, P_3/2, methane, and cesium) respectively. Similarly $g_0$, $g_1$, and $g_2$ are the degeneracies for the respective levels, $\tau$ is the level lifetime, v is the velocity given by Eq. (3), ${\sigma }_{SE20}$ and ${\sigma }_{SE21}$ are the stimulated emission cross-sections for the respective levels, ${\sigma }_{21}$, ${\sigma }_{12}$ and ${\sigma }_Q$ are the spin-orbit relaxation cross-sections and the quenching cross-section, ${\Phi }_P$ is the pump laser photon number density and ${\Phi }_L$is the laser photon number density.

The solution for the non-lasing case can be obtained by assuming a sufficiently large pump photon density ${\Phi }_P$ such that $N_2=2N_0$ and dropping the lasing terms resulting in a temperature dependent relationship:

In this expression ΔE represents the energy defect between the P_1/2 and P_3/2 levels and arrives through the detailed balance relationship between the spin-orbit relaxation rate cross-sections. Using Eqs. (6) and (7), all relative populations can be found.

For the lasing case, the solution is not that simple. The solution to match the experimental conditions would result in a relationship for the relative populations that is dependent on the pump photon density and the spin-orbit relaxation rate. To simplify the calculation, we examined the case where the pump is sufficiently strong, such that the laser operation is limited by the spin-orbit relaxation rate. Under these conditions, where ${\Phi }_P$ is large, then $N_2=2N_0$, and $N_1=2N_0$ because the system is lasing, we obtain the following relationships:

Using the relationships for the population density for lasing and non-lasing cases, the power ratio P_HNL/P_HL for the three experimental cases can be computed and are listed in Table 1. It should be noted that P_HNL / P_HL = ΔT_NL / ΔT_L because ΔT is proportional to the heat deposited. These calculations agree reasonably well with our experimental results shown in Table 1. Table 3 shows a comparison of the power deposited into the laser gain medium by quenching and spin-orbit relaxation for the lasing and non lasing cases. The terms P_QL is the quenching term from Eq. (1) evaluated under lasing conditions, P_SOL is the spin orbit relaxation term of Eq. (1) evaluated under lasing conditions. Similarly, P_QNL and P_SONL are evaluated under non lasing conditions. A review of the table shows that for lasing and non lasing conditions the heat contribution from quenching is virtually the same. On the other hand, the spin-orbit contribution to heating for the lasing case is approximately three times larger than for the non lasing case. Finally, in both cases, the spin-orbit contribution to the total heating for lasing or non lasing cases is 30 percent or less.

Table 3. A comparison of the power deposition for quenching and spin-orbit relaxation for both lasing and non lasing cases.

View Table | View all tables in this article

The significant result from this analysis is that the dominant source of heat within a cesium DPAL is due to ²P states quenching.

4. Conclusion

With this paper we report our results on the first time dependent measurements of the temperature of the Cs DPAL gain medium during the laser turn on transient. These results demonstrate a significant temperature rise during this transient process. The maximum recorded temperature approached 700 C indicating a significant amount of heat deposited during laser operation. Additionally, it is clear that these thermal effects have a detrimental effect on laser operation. In an effort to understand the heat deposition process, we examined temperature rise for lasing and non-lasing conditions. These results have shown that the majority of heating within a Cs DPAL gain medium can be attributed to quenching and not to spin-orbit relaxation, which is usually considered as the only loss factor limiting DPAL efficiency [9].