Diode-pumped alkali-atom laser (DPAL) has attracted intense attention due to its inherently high quantum efficiency, a good beam quality, and a high potential in the power scaling. However, most of DPAL research has been confined to the continuous wave and only a few pulsed operations have been attempted with limited performances. Here, we proposed and experimentally demonstrated a new scheme using a fast mode-hopping in the pump laser diode (LD), which enabled the quasi-continuous-wave (QCW) pulse modulation in a cesium (Cs) DPAL to control both the pulse width and the repetition rate. The pump wavelength was efficiently modulated in a fast cycle within discrete spectral ranges provided by the mode-hopping in the pump LD. The spectral range was successfully adjusted to include the resonant D2 absorption line of Cs atom to result in an effective gain modulation. Using this proposed scheme, we successfully achieved Cs-DPAL QCW modulation, whose pulse width was varied from tens of microseconds to a few milliseconds and the repetition rate was also variable in a wide frequency range from 10 Hz to 7.0 kHz. Detailed pump modulation method and the corresponding laser characteristics are discussed. The proposed method can be readily applied to pulse modulation of other types of alkali vapor lasers overcoming the previous limitations of DPAL to further expand applications in various light-matter interactions.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Diode-pumped alkali laser (DPAL) has attracted intense attention as an efficient high-power laser solution with very promising advantages over its alternatives [1–3]. The most important and salient features of DPAL are: the high quantum efficiency exceeding 95%, an excellent beam quality with reduced aberrations and scatterings, and a very low thermal load  in comparison to conventional crystal, glass, and dye lasers. Furthermore, the stimulated emission cross-sections of alkali atoms which can be collision-broadened by a buffer gas are several orders larger than conventional solid-state lasers [5–7] enabling a high power output. Because of these outstanding features, DPAL is being regarded as a promising candidate for high power scalable laser systems [8–12].
Even though DPALs have demonstrated superb efficiencies due to their intrinsically high quantum efficiency, they have suffered from damages on the gas cell windows especially in continuous (CW) pumping, which has hindered stable and continuous operation . Similar to solid state gain media, the alkali vapor also has raised thermal issues such as thermal phase shift, thermal lensing, and thermally induced birefringence resulting in the output power degradation in time and causing lasing efficiency decrease or even stop lasing [13–16]. The generation of heat in the gain medium of DPALs, much like other solid state gain media, comes from state mixing and quenching of upper levels to the ground state. This energy transferred to buffer gas increasing the local temperature in the alkali gain medium.
Recently a rapid circulation of alkali vapor flow is being attempted in high power CW pumping [11,12]. The circulation of alkali vapor has required sophisticated flow control and massive bulky pumping systems, which can reduce the inherent advantage of DPAL. The QCW laser is being pursued as the most feasible method to reduce the thermal problems in DPAL , and recently QCW DPAL has been experimentally demonstrated using pulsed pump laser in time domain [14,17]. The authors experimentally demonstrated that DPAL can be operated in the QCW mode by modulating the pumping source in the spectral domain, which turned out to be as efficient as time-domain modulation method along with a wide modulation frequency range. In order to optimize the QCW operation of DPAL, the modulation frequency should be tunable in a wide range and our proposed scheme could provide the modulation frequency range from 10 Hz to 7 kHz, which was not possible in prior methods.
In order to fully take advantages of DPAL and study atomic physics, a narrow-line width single frequency pump laser is essential due to extremely narrow absorption bandwidth of alkali atoms . The external cavity diode laser (ECDL) is a one of promising candidate for optical pumping source in DPAL because of its narrow-line width and tunable single frequency [19,20]. Using ECDL, the pump wavelength could be efficiently modulated by using piezoelectric actuator with tuning range of ~20GHz in above the few kHz repetition rate . Because of extremely narrow-line width of alkali atom absorption, the gain modulation in the spectral range is feasible using a tunable single frequency ECDL. The pump wavelength was efficiently modulated in a fast cycle within discrete spectral ranges provided by the mode-hopping in ECDL. The spectral range was successfully adjusted to include the resonant D2 absorption line of Cs atom to result in an effective gain modulation.
Electro-optic or acousto-optic modulation is an obvious method to modulate the CW power, but it requires separate electrical control units and additional mechanical supports to maintain the optical alignments. Furthermore, the optical damage of the optical crystals has been the fundamental bottleneck to obtain high power output power which is essential for the power scaling. Our method utilizes the built-in integrated external cavity laser diodes without additional electrical or mechanical components. And the mod-hopping in laser diode can be utilized more easily in the high power operation and therefore more beneficial to power scaling than electro-optic modulation.
In this study, we proposed and experimentally demonstrated a new method to efficiently achieve Cs-DPAL pulse trains in the QCW mode by modulating the output of an external cavity diode laser (ECDL) pump in the spectral domain, for the first time to the best knowledge of the authors. In our scheme, the grating angle in external cavity of pump LD was variable by applying an AC voltage to a piezoelectric actuator mounting a Littrow-type grating to tune the output wavelength of ECDL whose bandwidth is tens of kHz (TOPTICAL, TA pro) [22–24]. In an appropriate condition, this piezoelectric actuator in our pump ECDL module can provoke the mode-hopping, which results in a fast shift of the LD wavelength within two discontinuous spectral bands . See the left figure on the lower column of Fig. 1, we utilized this mode-hopping, which is generally ‘unwanted’ in most LD operations, to modulate the pump laser in the spectral domain. We found optimal operating conditions in the voltage and frequency of the actuator where the overlap between the mode-hopping spectral range of the pump LD and the D2 absorption line of Cs atom  was maximized as schematically shown in the central figure of the lower column in Fig. 1. This spectral modulation of pump LD wavelength near the D2 line of Cs atom provides periodic absorption and subsequently the optical gain in the Cs vapor cell is effectively modulated as shown in the right figure of the lower column in Fig. 1. We were able to effectively achieve fast gain switching in the Cs vapor cell and successfully demonstrated control of both the pulse duration and the pulse repetition rate, which was not possible in prior method. Note that the pump LD was operated in CW in our scheme and only its wavelength was modulated maintaining the narrow linewidth even at high repetition rate, which was not possible in direct pulsed LDs in prior reports.
Taking advantage of the abrupt spectral change in the mode-hopping of the pump LD was the key to achieve a wide frequency QCW operation. Conventional frequency swept laser methods [27–29] would require additional sophisticated filtering devices and the frequency would be still lower than ours and the slope efficiency would be lower than our method.
2. Experiments and results
2.1 Continuous wave DPAL operation
The proposed laser cavity structure is schematically shown in the upper row of Fig. 1. The L-shaped 20 cm long cavity consisted of a concave high reflecting mirror with a curvature radius of 25 cm and its reflectivity was higher than 99.5% at both λ = 852 nm and 894 nm. A flat output coupler (OC) with the reflectivity of 10% at λ = 894 nm closed the cavity. A polarization beam splitter (PBS) provided separate optical paths to the pump and the laser beams. A cylindrical cell filled with Cs vapor and 500 Torr ethane buffer gas was used as an optical gain medium [20,30]. The silica cell was 2 cm long and had circular windows of 2.5 cm diameter, which were anti-reflection coated on both sides for securing a high transmission at the laser output λ = 894 nm. The temperature of Cs vapor cell was kept at 130 °C similar to prior reports [31,32] and the Cs vapor density at this temperature was estimated to be 8.41 X 1013 cm−3.
The fundamental laser cavity mode at the lasing wavelength of λ = 894 nm was calculated to have a diameter of 160 μm. The pump beam with a nearly Gaussian intensity profile at λ = 852 nm was focused at the center of the Cs vapor cell using a single lens whose focal length was 30 cm. The pump beam diameter at the center of the Cs vapor cell was WP = 162 μm. The mode overlap between pump beam and lasing beam was calculated to be WL/WP = 0.987, which was maintained during the pump wavelength modulation. Using the laser set-up shown in Fig. 1, we first built a CW laser to test the lasing stability. Figure 2(a) shows a plot of the CW Cs laser output power as a function of the pump power.
Here the Cs vapor cell temperature was maintained at 130°C. The CW output power of 1.4 W was obtained using the pump power of 2.9 W, providing an optical-to-optical efficiency of 49.2%, and the slope efficiency was 61.5%, which is comparable to prior reports [1–3]. We also measured the beam quality of the CW laser at the output power of 1.4 W and the results are summarized in Fig. 2(b). M2 values along the horizontal and vertical axes were nearly the same and were near ~1.3. M2 values did not depend much on the laser output power.
2.2 Development of QCW Cs-DPAL
As a next step to experimentally demonstrate our QCW laser, we tried to find optimal driving conditions for the piezoelectric actuator in the pump ECDL. Key parameters were the driving voltage shape, the peak-to-peak voltage, and the frequency to consistently generate the mode-hopping near the D2 line as schematically shown in Fig. 1. In order to find the optimal conditions in situ, we simultaneously measured the absorption of the pump and the QCW laser output by employing two equivalent Cs vapor cells in parallel, one for the pump absorption monitoring and the other for DPAL optical gain medium. The experimental set-up is schematically shown in Fig. 3. The pump laser was split into two paths by a beam splitter and 0.5% of it passed through the Cs absorption cell for the pump monitoring and 99.5% of it was used the Cs optical gain cell. Note that using these two Cs cells we simultaneously monitored the pump laser modulation in the spectral domain near the D2 line, and the laser output in the time domain in situ.
We experimentally found the optimal conditions for driving the actuator in the pump ECDL to serve our purpose; a triangular shape pulse with frequency of 10, 100, 1500, and 6700 Hz along with the peak-to-peak voltage of 10-50 V. These conditions were specific to our pump LD structure and would depend on the carrier dynamics in the active layer [23,33].
For optimization, it may be required to tune the external resonator to the LD resonator by means of the LD bias voltage (Vbias), the peak-to-peak voltage of the piezoelectric actuator (VPP), and the LD temperature [22–25,33]. Our notions are as follow; 1) the LD temperature and Vbias were tuned to locate the mode-hopping of LD pump near the D2 absorption line of Cs atom, and 2) VPP and the frequency of VPP were applied to provide a periodic absorption and subsequently the optical gain switching in the Cs vapor cell. By optimization of Vbias, LD temperature, VPP, and frequency of VPP, we successfully obtained QCW Cs DPAL at each repetition rate. Firstly, we applied the triangular shape voltage pulses to the piezoelectric actuator in the pump ECDL at the frequency of 100 Hz. The peak-to-peak voltage VPP was from 0 to 50 V. Utilizing this driving pulses, we could shift the pump ECDL output frequency at a rate of 0.39 GHz/V, which is similar to prior ECDLs . By adjusting the LD temperature and the bias voltage, the mode-hopping in the pump LD was successfully located within the pump LD frequency shift range, which enabled an efficient modulation of the pump LD in the spectral domain as illustrated in Fig. 1. The pump LD wavelength was modulated at the optimal frequency across the D2 line of Cs atom and when it passed through the Cs vapor cells in Fig. 3, we could simultaneously observe the periodic modulation of the pump absorption and the laser output.
The corresponding experimental observations are summarized in Fig. 4(a). Here the pump laser power through the Cs absorption monitoring cell and the corresponding QCW Cs-DPAL are shown in the black and the red curves, respectively. As we modulated the pump laser wavelength across the D2 line, the transmission through the Cs absorption cell showed the periodic behavior with the same frequency. This modulated pump subsequently resulted in the gain switching in the Cs gain cell of our Cs-DPAL. In both the pump laser transmission and the laser output, the pulse width was measured to be ~3.5 ms at the frequency of 100 Hz and we could not measure any time delay between them. In Fig. 4(b), the RF spectrum of the QCW Cs-DPAL output is shown along with the pulse train measured by an oscilloscope, which confirmed stable QCW pulsed operation as we proposed. The average laser output power was measured as a function of the average pump power and the results are summarized in Fig. 4(c). We obtained the optical-to-optical efficiency of 16.3% and the slope efficiency of 20%, which were well matched to the value of CW efficiency multiplied by the duty cycle. It is noted that the slope efficiency showed different values depending on the pump power with a lower efficiency at a higher pump power range. There might be a local distribution of the temperature by the pump absorption which subsequently could change both the alkali vapor and buffer gas densities within the cell in a higher pump power [13,15,16]. It is also known that Cs near the transmission window could affect the transmission of the windows decreasing the efficiency of laser operation. At present the available pump power is limited and behavior of laser efficiency in a higher pump power is being investigated by the authors. The optical spectrum of the laser output is shown in the inset with the central wavelength near 894.5 nm consistent to general characteristics of DPAL.
In order to endow a degree of freedom to control the repetition rate in a wide frequency rage, we further optimized the voltage of the piezoelectric actuator with the frequency of 10 Hz, 1.5 kHz, and 6.7 kHz. We successfully obtained QCW Cs-DPAL at each repetition rate as shown in Fig. 5, and confirmed that proposed method can provide tunable control of the repetition rate. Figures 5(a)-5(c) shows the time trace of the oscilloscope for QCW Cs-DPAL in various pulse repetition rates. At the repetition rate of 6.7 kHz we measured the optical-to-optical efficiency of 27.4%, the slope efficiency of 34.2% with an average output power of 797 mW, and the peak power was calculated to be ~1.66 W. We also confirmed that the pulse width could be also adjustable from few milliseconds to tens of microsecond. In Table 1, we compared the performance of our own QCW Cs-DPAL. The average power and the slope efficiency are in general affected by the duty cycle and theoretically, the slope efficiency in our QCW DPAL is expected to be equal to the CW efficiency multiplied by the duty factor. However, in our experiments, we found that the average power and the slope efficiency were greater than the CW output power multiplied duty cycle at the modulation frequency of 6.7 kHz. Detailed gain dynamics needs to be further investigated in our QCW Cs-DPAL, which is being pursued by the authors. Table 1 clearly confirmed the unique tunability of pulse characteristics in our proposed method that has not been achieved in prior techniques.
We further modified the pump laser conditions in our QCW Cs-DPAL to obtain the pulse width of 26 μs at the repetition rate of 7 kHz and the results are summarized in Fig. 6(a). Note that the pulse width of 26 μs is the shortest and the repetition rate of 7 kHz is the fastest record in QCW-DPAL to the best knowledge of the authors. The average laser power was 302 mW and its slope efficiency of 13% as shown in Fig. 6(b), which were lower than the case of 6.7 kHz repetition rate in Table 1. Detailed gain dynamics needs to be further investigated in our proposed QCW Cs-DPLA to predict the pulse characteristics and the corresponding laser performances, which is being pursued by the authors.
We proposed and experimentally demonstrated a new method to achieve quasi-continuous wave (QCW) Cesium diode-pumped alkali laser (DPAL) by spectrally modulating the pump laser. By adjusting the mode-hopping of external cavity diode laser (ECDL) pump to be located across the D2 line of Cs atom, we successfully obtained the gain switching of Cs-DPAL. We developed an in situ measurement set-up to simultaneously monitor the pump laser modulation in the spectral domain and laser output in the time domain. Using this scheme, QCW Cs-DPAL was successfully achieved obtaining unique tunability in both the pulse width and the repetition rate using a single ECDL pump. We achieved the shortest pulse width of 26 microseconds with the fastest repetition rate of 7 kHz, ever recorded in QCW Cs-DPALs. We also achieved a wide frequency range to achieve the stable QCW from 10 Hz to 7 kHz with 3.5 ms to 23.6 μs of pulse width. The proposed method could be further applied to pulse modulation of other types of alkali vapor lasers to further expand DPAL applications into the various light-matter interactions.
Research Fund of High Efficiency Laser Laboratory of Agency for Defense Development of Korea (No.UD160069BD).
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