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Abstract
We describe a method of line narrowing and frequency-locking a diode laser stack to an alkali atomic line for use as a pump module for Diode Pumped Alkali Lasers. The pump module consists of a 600 W antireflection coated diode laser stack configured to lase using an external cavity. The line narrowing and frequency locking is accomplished by introducing a narrowband polarization filter based on magneto-optical Faraday effect into the external cavity, which selectively transmits only the frequencies that are in resonance with the 62S1/2 → 62P3/2 transition of Cs atoms. The resulting pump module has demonstrated that a diode laser stack, which lases with a line width of 3 THz without narrowbanding, can be narrowed to 10 GHz. The line narrowed pump module produced 518 Watts that is 80% of the power generated by the original broadband diode laser stack.
1. Introduction
During the past decade, Diode Pumped Alkali Lasers (DPALs) have received significant attention [1–5] because they combine many positive features of Gas Lasers and Diode Pumped Solid State Lasers. The DPAL has the feature of being electrically excited using a diode laser similar to solid state lasers but has the heat management and inherent good beam quality of a gas phase laser. In recent years, DPALs have demonstrated high average power of 1.5 kW [6] and showed great promise for future power scaling. In addition to the experimental efforts, there has been significant effort in the theoretical analysis of DPALs [7-8]. A key element of any DPAL is the diode laser pump module. The current state-of-the-art pump module is a Volume Bragg Grating (VBG) line narrowed diode laser bar or stack [9–13]. The problem with using multiple VBGs, required for narrowbanding stacks, is the requirement to be individually tuned to the resonant frequency. Also, they have an inherently broad line width of about 0.3 nm, which is about an order of magnitude broader than the alkali absorption line broadened by one atmosphere buffer gas. An alternative method used for line narrowing of low power single emitter diodes is to use a Faraday filter which has been demonstrated by [14–25]. In this paper we demonstrate the first high power line narrowing of a 600 watt diode laser stack to the linewidth of 10GHz and locking this line to the 62S1/2 → 62P3/2 transition of Cs atom using a Faraday filter in an external cavity of a diode pump module.
2. Experimental apparatus and results
In Fig. 1 we provide a diagram of the pump module. The pump module consists of a diode laser stack with an antireflection coated output facet, which is configured to lase using an external stable cavity containing a frequency sensitive element. The diode laser stack used was a Dilas 600 W designed to produce light at 852 nm.
The frequency sensitive element is a Faraday filter, which is an alkali vapor cell, 6 inches in length, placed in a magnetic field between two crossed linear polarizers. The output coupler of the external laser cavity had a reflectivity of 20%. The outcoupler was flat therefore, to satisfy the cavity stability requirement, a lens with a focal length of 25 cm was placed inside the cavity one focal length away from the outcoupler. The alkali vapor cell evacuated to pressure of 10−7 torr and filled with a visible amount of metallic Cs was heated to a temperature of 96 C corresponding to a vapor pressure of 1x1013/cm3 which provided optimal performance. The alkali vapor cell used anti reflection coated high quality fused silica windows and did not experience any birefringence. A magnetic field of 700 Gauss was applied to the alkali vapor cell using permanent cylindrical neodymium magnets. The magnetic field was applied parallel to the laser cavity optical axis and caused a rotation of the laser light polarization that is in resonance with the cesium 62S1/2 → 62P3/2 transition. The narrow-banding occurs because only the polarization of the spectral components of the diode laser emission, which are near the alkali atomic transition 62S1/2 → 62P3/2 of the cesium atomic vapor, can be rotated 90° and thus transmitted by this magneto-optical filter in both directions and provide feedback to the emitters in the diode stack. All other spectral components will not experience this rotation of polarization, resulting in significant losses and will not lase. This means that lasing occurs only in a narrow spectral range which matches the Cs 62S1/2 → 62P3/2 transition absorption line observed in a 1 atm pressure broadened alkali gain medium (about 12 GHz).
We examined the spectral output of the unlocked diode laser stack. The spectrum of the unlocked stack was produced by lasing the diode stack using the external cavity but with the Faraday filter removed. The spectrum was recorded using an Ocean Optics USB 2000 + spectrometer with spectral resolution of 2 nm (see Fig. 2). The unlocked stack lases with a line width of approximately 8 nm or 3 THz (FWHM). Such a broad spectral line is clearly unacceptable for pumping a 20 GHz pressure broadened cesium D2 line. On the other hand, when the Faraday filter is used, then a significant line narrowing occurs as it is also shown in Fig. 2. It should be noted that the line shape of the narrowed line is the instrument function of the Ocean Optics spectrometer and does not provide an accurate representation of the quality of the line narrowing.
In order to record this spectrum with sufficient resolution, we used a Burleigh Fabry-Perot interferometer with a free spectral range of 30 GHz and resolution about 0.3 GHz. The recorded spectrum is shown in Fig. 3 together with the Cs D2 pressure broadened line.
Fig. 3 High resolution spectrum, recorded with a Fabry-Perot interferometer, of the line narrowed diode laser stack reveals a bimodal distribution. Also shown is the calculated D2 pressure broadened line shape with 200 Torr of methane and 400 Torr of helium. The x-axis is centered on the 62S1/2 → 62P3/2 fine structure transition.
The high resolution spectrum shows a bimodal distribution. The pump radiation spectrum is well matched to the Cs D2 line shape pressure broadened using 200 Torr CH4 and 400 Torr He. The pressure broadened line shape was calculated taking into account the hyperfine splitting of the S and P states using values from Steck [26] and pressure broadening and line shift reported by Pits [27]. The low intensity Faraday transmission through the filter is provided in Fig. 4 and was calculated using the methodology outlined in [28].
Fig. 4 Low intensity Faraday filter transmission spectrum along with the transmission of the cesium vapor cell without the polarizers. These were calculated using the methodology reported in [28]. The x-axis is centered on the 62S1/2 → 62P3/2 fine structure transition.
The calculated Faraday transmission spectrum indicates that the diode stack should frequency lock at approximately ± 16 GHz relative to the 62S1/2 → 62P3/2 fine structure transition. This would be true in the low intensity limit but at the high powers of this pump module there is significant optical pumping of the ground state atoms resulting in an effective reduction of the alkali number density. This reduction in number density will have the effect of moving the two transmission peaks towards the center. Also, because the diode stack has sufficient gain it will lase simultaneously on both peaks resulting in a bimodal output which quite fortuitously is well matched to a one atmosphere pressure broadened 62S1/2 → 62P3/2 transition line shape.
We next examined the total power output from the narrow-banded diode laser stack pump module. The output power versus power supply current is shown in Fig. 5.
Fig. 5 The triangles indicated the power output of the diode stack in an external cavity without the Faraday filter and the circles show the Narrow-banded pump module output power versus input current. Also included are the linear least squares fit to the data.
The output power of the pump module appears to be linear with pump current similar to a free running diode stack. The maximum line narrowed power achieved was 518 watts which is about 80% of the broad band value (650 W). Such a loss in narrow-banded output power is expected when introducing losses from optical elements inserted into the external cavity.
3. Conclusion
The pump module reported in this paper was built using a commercial 600 W diode laser stack originally emitting with broadband radiation with line width about 8 nm. Using a Faraday filter in the external cavity of this stack we succeeded in narrowing spectral line of output radiation of this pump module to the value about 10 GHz, which is well matched to a 12 GHz pressure broadened (1 atm) cesium atomic vapor transition 62S1/2 → 62P3/2. The produced narrowband radiation power was 80% of the stack initial broadband power. This pump module is well matched to the Cs alkali metal pump line and will significantly improve the overall efficiency of a DPAL system. An important feature of this method is that it not only provides significant narrow-banding, but automatically locks the frequency of the pump module to the absorption line of the alkali atom. Such a pump module can also find application in spin-polarization of nuclei such as 3He and 129Xe for use in magnetic resonance imaging of lungs and other organs of the human body [29, 30].
Funding
Directed Energy Joint Transition Office (JTO-14-UPR-0525).
Acknowledgments
We acknowledge the Directed Energy Joint Transition Office for their funding and support.
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