Zining Yang

Modeling of an optically side-pumped alkali vapor amplifier with consideration of amplified spontaneous emission.

Diode pumped alkali vapor amplifier (DPAA) is a potential candidate in high power laser field. In this paper, we set up a model for the diode double-side-pumped alkali vapor amplifier. For the three-dimensional volumetric gain medium, both the longitudinal and transverse amplified spontaneous emission (ASE) effects are considered and coupled into the rate equations. An iterative numerical approach is proposed to solve the model. Some important influencing factors are simulated and discussed. The results show that in the case of saturated amplification, the ASE effect can be well suppressed rather than a limitation in power scaling of a DPAA.

Theoretical model and novel numerical approach of a broadband optically pumped three-level alkali vapour laser

A model for an end-pumped double-pass alkali vapour laser is set up which has considered all the main physical features, including the fine-structure mixing rate that represents the three-level nature of this kind of laser, the spectral dependence of pump light absorption for broadband pumping, the longitudinal population variation and the distributed intra-cavity losses. To solve this model, we have proposed a searching algorithm for a single-pass configuration and developed a novel iterative algorithm for a double-pass case which has taken the single-pass solution as an initial value. The calculation process demonstrates a fast rate of convergence and high degree of accuracy. By this algorithm, some special cases, for example, the non-uniform longitudinal distribution of alkali atom concentration, can also be solved well. The model and the numerical approach described in this paper demonstrate a new method to simulate the CW end-pumped quasi-three- or three-level lasers. In this way, no significant assumptions or simplifications are needed in the model, and both the detailed intra-cavity information and pump power distribution can be obtained. To ensure the correctness of our model, a comparison between our model and Beach et als model is made. The result shows that when the non-uniformity of the longitudinal population distribution is less than 20% or the pump intensity is far beyond the threshold, the assumption of longitudinal population average is valid and the two models are equivalent. When the pump intensity is near the threshold, this assumption may be invalid due to the large longitudinal population variation. If this case happens or more detailed laser information is required, we could use this model to obtain a more accurate solution.

Experimental research of a chain of diode pumped rubidium amplifiers.

In this paper, we have set up a diode pumped rubidium MOPA system with a chain of two amplifiers. The experimental results show an amplified laser power of 26W with amplification factor of 16.3 and power extraction efficiency of 53% for a single amplifier, and an amplified laser power of 11W with amplification factor of 7.9 and power extraction efficiency of 26% for a chain of two amplifiers. The reason for lower performance of cascade amplification is mainly due to the limited total pump power, which will be not sufficient for efficient pumping when assigned from a single amplifier into two amplifiers. The situation could be well improved by increasing the seed laser power as well as the pump power for each amplifier to realize high efficient saturated amplification. Such MOPA configuration has the potential for scaling high beam quality alkali laser into high powers.

Study on photoionization in a rubidium diode-pumped alkali laser gain medium with the optogalvanic method

We use the optogalvanic method to calculate the concentration of rubidium ions produced by photoionization in a Rb diode-pumped alkali laser gain medium. With bias voltage added across the electrodes of a rubidium hollow cathode lamp, the measured optogalvanic current is 2.3×10(-7) A. Further study shows that the rubidium ion concentration is proportional to the pump intensity, and the drift velocity of rubidium ions is proportional to the bias voltage. When the photoionization process reaches dynamic equilibrium, the rubidium ion concentration will not increase with growing rubidium atom density. The calculated rubidium ion concentration is 1.5×10(5)-10(6) according to the experiment, and the ionization degree is less than 2.4×10(-7).

Real-time measurement of temperature rise in a pulsed diode pumped rubidium vapor laser by potassium tracing atom based absorption spectroscopy

In this paper, we first propose and demonstrate a novel tracing atom based absorption spectroscopy method for the real-time measurement of the temperature rise inside the pump region of a pulsed diode pumped alkali laser (DPAL). By artificially adding potassium atoms into the gain medium of an operational rubidium laser, the information of the temperature rise can be obtained from the variation of the potassium absorption signal. Some important influencing factors are studied. Typical results show that, as the pump power (2 ms duration) increases from 22 W to 92 W, the temperature rise increases from 103 K to 227 K. As the pulse duration increases from 1ms to 5 ms, the temperature rise increases from 128 K to 314 K, and the heat relaxation time increases from 3.8 ms to 8.1 ms. The method is favored for its ability for real-time detection and high sensitivity, which provides a useful way for DPAL diagnostics.

Methane-based in situ temperature rise measurement in a diode-pumped rubidium laser

We measured active zone temperature rise of an operational diode-pumped rubidium laser non-perturbatively with methane-based near-infrared tunable diode laser spectroscopy (TDLAS). For a Rb+ methane diode-pumped alkali laser (DPAL), the temperature rise was obtained. Especially, the temperature differences (∼10  K) between lasing and un-lasing cases were well identified, which demonstrated a high sensitivity of the method. To our knowledge, this is the first demonstration of extending the methane-based TDLAS method to DPAL study.

Experimental measurement of ionization degree in diode-pumped rubidium laser gain medium

We use optogalvanic method to measure the ionization degree in the hydrocarbon-free rubidium DPAL gain medium. The results show that the ionization degree increases as pump intensity and helium pressure increase, and presents rollover as rubidium concentration increases. A maximal ionization degree of ∼6.45×10(-6) has been obtained with pump intensity of 0.82  kW/cm2, temperature of 150°C and helium pressure of 500 Torr. Theoretical estimation shows that energy pooling is the main process rather than photoexcitation for the subsequent ionization processes.

Theoretical and experimental investigation of Rb-Ar excimer pumped alkali laser characteristics

Excimer Pumped Alkali Laser (XPAL) is a hopeful choice to solve Diode Pumped Alkali Laser (DPAL)’s disadvantages. Theoretical and experimental investigations of Rb-Ar XPAL were carried out in this paper. Time dependent rate equation model illustrated that extreme pump strength was needed to exceed threshold and to assure efficient cw running. 780nm lasing of four level Rb-Ar excimer was realized with a surrogate 15 ns pulsed optical parametric oscillator (OPO). Possible resonator configuration may increase pumping strength was proposed.

Frequency-narrowed external-cavity broad-area-diode for rubidium laser pumping

Using an external cavity with holographic grating, we demonstrate the spectral narrowing of a high power broad-area-diode with a single emitter. The spectral bandwidth of less than 15 GHz is obtained with output power exceeding 10 W and external cavity efficiency exceeding 60%. Absorption of 98% of the laser radiation by a 25-mm rubidium vapor cell filled with 600-torr ethane at a temperature of 368 K is acquired, which demonstrates the availability of this pump source for efficient rubidium laser pumping.

Simulation of diffraction effects of anti-reflection microstructures used as intra-cavity optical elements

The anti-reflection microstructures (ARMs) have been widely used due to their many advantages as compared with traditional films, such as high transmittance at required wavelength, high damage threshold, and resistance to corrosive environments etc. Some recent applications use ARMs as intra-cavity optical elements in laser systems. Due to the presence of microstructures, ARMs may also add diffraction effects on the features of the transmitted laser beams or the resonators eigenmodes. In this paper, we simulate the diffraction effects of ARMs that used as intra-cavity optical elements, and propose some further considerations.