The JOKARUS payload, schematically shown in Figure 2, features a laser system based on a ECDL with an integrated optical amplifier operating at 1,064 nm, a spectroscopy module including the quasi-monolithic setup for modulation transfer spectroscopy of molecular iodine and two periodically poled lithium niobate (PPLN) waveguide modules for second harmonic generation (SHG), as well as RF and control electronics for frequency stabilizing the ECDL laser to the spectroscopy setup. The individual subsystems are presented in the following sections.
3.1 Laser system
The laser system is housed in a module as shown in Figure 3(a) and includes the laser, an electro-optic modulator (EOM) and an acousto-optic modulator (AOM) for preparation of the spectroscopy beams. The laser is a micro-integrated master-oscillator-power-amplifier-module (MOPA) developed and assembled by the Ferdinand-Braun-Institut (FBH), see Figure 3(b). The MOPA consists of a narrow-linewidth ECDL master oscillator operating at 1,064 nm and a high-power power amplifier. A previous generation of the laser module is described in [28] and details on the performance of the laser module and the technology applied for its integration will be given elsewhere [29]. The ECDL master oscillator provides an emission linewidth of less than 50 kHz (1 ms, FWHM) and allows for high bandwidth frequency control via control of the injection current. The laser module provides a fiber-coupled optical power of 500 mW at an injection current of 1,500 mA. This module was subject to a vibration qualification according to the requirements of the TEXUS program (8.8 grms), while similar laser modules of previous generations even passed 29 grms and 1,500 g pyroshock tests [28].
In the laser system, see Figure 2, the MOPA is followed by an optical isolator (Thorlabs) and a 99:1 fiber splitter, where few mW are separated from the main beam for frequency measurements with the frequency comb that is part of another payload aboard the TEXUS 54 mission. A second fiber splitter divides the main beam into pump and probe beam for modulation transfer spectroscopy (MTS). The probe beam is connected to a fiber-coupled PPLN wave guide module (NTT Electronics) for second harmonic generation. The pump beam is frequency shifted by 150 MHz using a fiber-coupled AOM (Gooch & Housego) to shift spurious interference between pump and probe beam outside the detection bandwidth of the MTS signal. The frequency-shifted beam is then phase-modulated at ≈300 kHz by a fiber-coupled EOM (Jenoptik) and will later also be frequency-doubled by a second SHG module. Taking nominal losses of the components, splice connections and the conversion efficiency of the SHG modules into account, we expect an optical power of 10 mW and 3 mW for the pump and probe beam at 532 nm, respectively, which is sufficient for saturation spectroscopy. The power of the pump beam can be stabilized by using a voltage controlled attenuator (VCA) and a feedback loop (see Figure 2). Several fiber taps are used for power monitoring at various positions in the laser system during flight. The pump and probe beam are finally guided from the laser module to the spectroscopy module by polarization maintaining fibers at 1,064 nm and mating sleeves connectors.
All components of the laser system were qualified at a random vibration level of 8.8 grms (hard-mounted) to ensure their integrity after the boost phase of the rocket launch.
3.2 Spectroscopy module
The spectroscopy setup is housed in a separate module as shown in Figure 4 together with the SHG modules (cf. Figure 2). It is based on previous iterations of an iodine reference for deployment in space missions, developed at ZARM Bremen, DLR Bremen and the Humboldt-Universität zu Berlin [9]. The optical setup is realized using a special assembly integration technique [30], where the optical components are bonded directly on a base plate made from fused silica with a footprint of 246 mm × 145 mm resulting in a quasi-monolithic, mechanically and thermally stable spectroscopy setup as shown in Figure 4(b). An iodine setup using this assembly technique was subjected to environmental tests including vibrational loads up to 29 grms and thermal cycling from −20°C to +60°C [25].
In the JOKARUS MTS setup, the pump beam is launched from a fiber collimator (Schäfter + Kirchhoff) with a beam diameter of 2 mm and is guided twice through an iodine cell with a length of 15 cm, resulting in an absorption length of 30 cm. Behind the cell, the pump beam is reflected at a thin film polarizer (TFP) and focused on a photo detector for optional power stabilization. The probe beam is launched with the same beam diameter and is split using a TFP into a probe and a reference beam for balanced detection using a noise-canceling detector adapted from [31], as shown in Figure 2. The iodine cell is provided by the Institute of Scientific Instruments of the Academy of Sciences of the Czech Republic (ISI) in Brno, filled with an unsaturated vapor pressure of ≈1 Pa [32]. From the experimental parameters of the spectroscopy module we estimate the expected operating linewidth Γ [20]
$$ \varGamma =(\varGamma _{\mathrm{nat.}}+\varGamma _{\mathrm{press.}}+\varGamma _{\mathrm{tof}}) \frac{\sqrt{1+S_{\mathrm{sat.}}}+\sqrt{1+S_{\mathrm{pr.}}}}{2}, $$
(1)
with the natural linewidth \(\varGamma _{\mathrm{nat.}}\), the contributions from pressure broadenig \(\varGamma _{\mathrm{press.}}\) and time-of-flight broadening \(\varGamma _{\mathrm{tof}}\) and the saturation parameter S of the saturating and the probe beam respectively. We take the natural linewidth \(\varGamma _{\mathrm{nat.}}\) of 220 kHz [20], the pressure broadening of 120 kHz Pa−1 [20] adds \(\varGamma _{\mathrm{press.}}\) of 120 kHz and the time-of-flight broadening \(\varGamma _{\mathrm{tof}}\) is 22 kHz for the beam diameter of 2 mm. The saturation parameter for the saturating and the probe beam will be set to ≈2 and ≈1, respectively. The expected operating linewidth Γ thus evaluates to ≈570 kHz.
3.3 Electronics
The electronic system for JOKARUS is segmented in 3 functional units. First, the RF electronics for the optic modulators shown in Figure 2. It is based on a direct digital synthesizer (DDS9m, Novatech Instruments), referenced to an oven-controlled crystal oscillator. The DDS provides a 150 MHz signal for the AOM and two signals for phase modulation of the pump beam via the EOM and analog demodulation of the MTS signal. Second, a stack of compact electronic cards by Menlo Systems based on the FOKUS flight electronics [12] are used for temperature control of the SHG modules and the diode laser, as current source for the ECDL-MOPA and for realizing the feedback control for laser frequency stabilization. The cards are controlled by an ARM based embedded system via a CAN interface, also providing an interface to higher level data acquisition. The third unit contains a 16-bit DAQ card that is used for data acquisition and the x86-based flight computer (exone IT). It runs the experiment control software that provides coarse tuning of the laser frequency, identification of the fine transition R(56)32-0 as well as invoking and controlling a PID feedback control for frequency stabilization to the selected hyperfine transition.
3.4 Payload assembly
The subsystems presented above are integrated in individual housings made from aluminum that share a common frame as a support structure shown in Figure 5. A water-cooled heat sink is integrated into the base frame for temperature control until liftoff. During flight, we expect an average temperature increase of about 3 K throughout the mechanical structure, based on nominal power consumption of 100 W. The optical fiber connection between the laser and spectroscopy units are realized via mating sleeves. The total payload has a dimension of 345 mm × 270 mm × 350 mm and a total mass of 25 kg, which allows for integration into the TEXUS sounding rocket format.
We estimated the performance of the JOKARUS system in terms of amplitude spectral density (ASD) of frequency noise in comparison to the frequency stability of an iodine reference developed, characterized and reported previously [9, 25], called elegant breadboard model (EBB). The frequency noise achieved with the EBB is shown in Figure 6 (green graph) together with the frequency noise of the free-running Nd:YAG laser (blue graph) used in this setup. The EBB fulfills the requirement on the frequency noise of planned space missions like LISA and NGGM.
For the JOKARUS instrument we expect to achieve frequency noise on a level of 10 Hz/Hz1/2 (red dashed graph) at Fourier frequencies below 10 Hz which corresponds to a fractional frequency instability of \(2.4 \times 10^{-14}/\sqrt{ \tau }\) for averaging times above 100 ms. The performance was estimated from the frequency noise of the free-running ECDL (orange graph) assuming a control bandwidth of ≈100 kHz. The white frequency noise floor was estimated from the equation
$$ \sigma (\tau)=\frac{\varGamma }{\nu _{\mathrm{opt.}}}\frac{1}{S/N}\frac{1}{\sqrt{\tau }}, $$
(2)
which gives the fractional frequency instability of our laser locked a transition with Q-factor \(\nu _{\mathrm{opt}}/\varGamma \) and resolved with a signal-to-noise ratio \(S/N\) in a 1 Hz bandwidth. The signal-to-noise ratio \(S/N\) for FM-spectroscopy is estimated from [20]
$$ S/N=\frac{1}{2}J_{0}(\beta)J_{1}(\beta)\sqrt{ \frac{\eta }{2e B}}\Delta \alpha p L \sqrt{P_{\mathrm{pr.}}} $$
(3)
with the pressure p and absorption length L given by the spectroscopy setup, a probe beam power \(P_{\mathrm{pr.}}\) of 200 μW, detection efficiency \(\eta = 0.25\) A W−1, bandwidth \(B = 1/(2\pi)\mbox{ Hz}\) and the Bessel functions \(J_{n}(\beta)\) for the phase-modulation index β of ≈1. A factor 1/2 accounts for 3 dB above shot-noise detection expected from the noise canceling photo detector [31]. The peak absorption contrast Δα is calculated from [20]
$$ \Delta \alpha = \alpha _{0} \biggl( \frac{1}{\sqrt{1+S_{\mathrm{pr.}}}}- \frac{1}{\sqrt{1+S_{\mathrm{pr.}}+S_{\mathrm{sat.}}}} \biggr) $$
(4)
with \(\alpha _{0} = 1.3\times 10^{-3}\mbox{ cm}^{-1}\mbox{ Pa}^{-1}\) for transition R(56)32-0 [20]. With Eqs. (3), (4) and (1), the white frequency noise in terms of fractional frequency instability, Eq. (2), evaluates to \(\sigma (\tau) = 2.4\times 10^{-14}/\sqrt{\tau }\), which corresponds to a frequency noise amplitude spectral density of \(S_{f}(f)=\sqrt{2}\sigma \nu _{\mathrm{1{,}064 nm}}\), which evaluates to 10 Hz/Hz1/2 at 1,064 nm. The estimated performance is in agreement with the performance of the EBB, taking into account a factor of three shorter absorption length of the JOKARUS spectroscopy module compared to the EBB, and otherwise similar experimental parameter. We therefore expect the JOKARUS instrument to fulfill the requirements on the frequency noise of space missions like LISA and NGGM, where the long term instability needs to be characterized in beat-note measurements, e.g., with the EBB setup, to determine the Flicker noise floor and random walk noise of the JOKARUS frequency reference.