γ-irradiation tests of volume-holographic Bragg gratings
Spaceborne quantum optical sensors for high-precision measurements rely on compact and robust narrow linewidth laser systems. Monolithic lasers, while small in volume and resilient against mechanical and acoustic interference, do not meet the related frequency stability requirements. One approach to enhance frequency stability is to provide optical feedback from a distant, frequency-selective reflector to a ridge waveguide diode laser chip (extended cavity diode laser) . Here, one cavity mirror is realized, e.g., by a volume-holographic Bragg grating (VHBG) .
The VHBG consists of a photo-thermo-refractive (PTR) glass . It features a periodic refractive index modulation which implements the volume-holographic Bragg reflector, providing frequency-selective feedback to the laser diode. This feedback controls the laser frequency and reduces the laser linewidth. A typical VHBG transmission function, corresponding fit function, and residuals are depicted in Fig. 1. The frequency selectivity is determined by the Bragg frequency fBragg, the maximum diffraction efficiency hmax, which can be calculated from experimental data.
Deployment in space requires stability of the VHBGs under ionizing radiation. It is known that γ-irradiation induces additional absorption in PTR glass mainly in the visible and UV spectral ranges . Radiation could also affect the background refractive index and/or the refractive index modulation depth of the VHBG, which has not been studied so far with high resolution. Both effects would cause a shift of the Bragg wavelength and a modification of the diffraction efficiency. To show that VHBGs do not degrade significantly through γ-irradiation under low-earth-orbit conditions, samples have be subject to a total ionizing dose of 15 krad.
The experimental setup employed is shown in Fig. 2. Measurement principle and setup are selected to minimize the influence of systematic errors which arise from uncontrolled temperature variations or angular misalignment of the VHBG. Laser light at 1064 nm is coupled into a polarization-maintaining optical fiber, sent through a fiber optical circulator, of which the second port is connected to a fixed fiber collimator delivering the laser beam for the measurement. The beam is then directed through a thin film polarizer and a wedged plate beam splitter, which splits the beam into three parts. The beam transmitted through the wedged plate beam splitter is directed onto a power meter. The reflected beams are directed towards the reference (REF) and the device-under-test (DUT) VHBGs. In case of optimal alignment, the wave vectors of the incident laser beams are parallel to the VHBG grating vectors. This is achieved by maximizing the coupling of the diffracted power back into the fixed fiber collimator. Further, the VHBGs are mounted on a common temperature-stabilized heat sink. This way, a differential measurement is implemented for the Bragg resonance frequency which suppresses the effect of temperature variations.
The frequency of the laser and optical power transmitted through the wedged plate beam splitter WPB and through the two VHBGs are recorded simultaneously. The Bragg frequency and the diffraction efficiency of the VHBGs are then determined by fitting the diffraction function to the measured data in a range of ± 15 GHz centered at the Bragg frequency. Differential analysis of REF and DUT results provides information about potential shifts due to radiation influence.
Fig. 3 shows the difference between the Bragg frequency of REF and DUT and the diffraction efficiency for three VHBG temperatures, each before and after γ-irradiation. The analysis reveals a differential shift of the Bragg frequency of less than 80 MHz, which is well within the error budget, i.e., no frequency shift is observed. The diffraction efficiency of the DUT shows a barely significant change of 1.3 %, while that of the REF remains unchanged as expected.
The results underline the radiation hardness of the VHBGs for typical LEO applications.
This work is supported by the German Space Agency (DLR) with funds provided by the Federal Ministry of Economic Affairs and Energy (BMWi) (Grant. No. 50WM1755).
 E. Luvsandamdin, C. Kürbis, M. Schiemangk, A. Sahm, A. Wicht, A. Peters, G. Erbert, and G. Tränkle, "Micro-integrated extended cavity diode lasers for precision potassium spectroscopy in space, " Opt. Express 22, 7790-7798 (2014).
 H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J. 48, 2909 (1969).
 L.B. Glebov, N.V. Nikonorov, E.I. Panysheva, G.T. Petrovskii, V.V. Savvin, I.V. Tunimanova, V.A. Tsekhomskii, “New ways to use photosensitive glasses for recording volume phase holograms,” Opt. Spectrosc. 73, 237 (1992).
 L. Glebov, L. Glebova, E. Rotari, A. Gusarov, F. Berghmans, “Radiation-induced absorption in a photo-thermo-refractive glass,” Proc. SPIE 5897, Photonics for Space Environments X, 58970J (2005).