Iodine Frequency Reference on a Sounding Rocket

We build a self-contained optical absolute frequency reference at 1064 nm based on the rovibronic transition R(56)32-0 in molecular iodine and operate this instrument in space on a sounding rocket mission. The frequency reference uses a microintegrated extended cavity diode laser and a quasimonolithic spectroscopy module for modulation transfer spectroscopy, providing frequency instability of $2\times {10}^{-13}$ for averaging times of 100 s. To demonstrate the autonomous operation of our reference, we perform an absolute frequency measurement using a chip-scale atomic clock and an optical frequency comb during a 6-min-long space fight.

Figure 1

Photographs of the JOKARUS payload. (a) JOKARUS payload. The payload has dimensions of $340\times 255\times 355{\mathrm{mm}}^3$ ($w\times d\times h$), corresponding to a volume of 31 l, and has a total mass of 22.6 kg. (b) Spectroscopy tray including the spectroscopy module and the SHG modules. (c) Control computer tray including control computer, dc/dc converter, and RS232 interface. (d) rf tray including DDS, rf amplifier for the AOM, the EOM and the MTS signal, mixer and lowpass filter. (e) Laser system including the ECDL MOPA, an optical isolator, as well as the AOM and the EOM.

Figure 2

Schematic of JOKARUS. (a) Control electronics, data acquisition unit for spectroscopy signals and system monitoring, and control computer. (b) rf system. (c) Modulation transfer spectroscopy module with balanced detection. (d) Laser system with noise-canceling detector (NC Det.), thin film polarizer (TFP), polarizer (Pol.), data acquisition unit (DAQ), microintegrated optical bench (MIOB), thermoelectric cooler (TEC), second harmonic generation module (SHG), volume holographic Bragg grating (VHBG) of the ECDL, master oscillator (MO), power amplifier (PA).

Figure 3

Spectra of transition R(56)32-0 taken by the autonomy algorithm in space prior to locking to component $a_1$. The $x$ axis is calculated from the ramp applied to the injection current and the tuning coefficient of the laser. The MTS spectrum is low-pass filtered with a corner frequency of 40 kHz recorded with 2560 samples with 16-bit resolution in 500 ms, resulting in a spectral resolution of only 0.9 MHz; thus, some spectral components appear clipped. The Doppler profile is recorded from the logarithmic output of the noise-canceling detector [27] and used for the autonomous lock acquisition. The capture range of the lock between $a_1$ and $a_2$ is indicated in green.

Figure 4

Photograph of the FOKUS II system. The entire optics with two BDUs and two frequency combs are shown on the front left. The back part is used by the control and supply electronics. The payload has a volume of 7 l and the mass is 10 kg.

Figure 5

Schematic of the dual comb measurement with JOKARUS. The repetition rate of comb A is locked to a miniaturized atomic clock, while comb B is tunable via a rf frequency synthesizer. In both combs, the carrier envelope offset is self-referenced and locked to the same cesium reference. Two different beat signals with the JOKARUS payload are generated and counted simultaneously, allowing an absolute optical frequency determination.

Figure 6

Allan deviation of the beat note between the iodine frequency reference JOKARUS and a cavity-stabilized laser ($\circ$) and another iodine frequency reference ($◻$). The frequency instability of the LNCSAC is measured in comparison to a GPS disciplined OCXO and is shown for comparison ($\diamond$). The instability of the beat note between JOKARUS and FOKUS II for the different phases of the mission [ground ($▹$), flight ($△$), parachute ($▽$), postflight $◃$)] match the instability of the LNCSAC.

Figure 7

Time records of the flight. Top: Beat-note frequency between the JOKARUS and the frequency comb FOKUS II at 1064 nm ($f_o=30.9855$ MHz). The lengths of the time records are 107 (ground), 289 (flight), and 127 (parachute) samples, respectively, recorded with a sampling rate of 1 Hz. The frequency comb is intentionally unlocked 40 s before lift-off to avoid uncontrolled behavior of the feedback loops during vibrations caused by the launch. Middle: Altitude profile of the sounding rocket flight. Bottom: Acceleration during lift-off and reentry of the sounding rocket. The iodine reference and the frequency comb are locked on ground, during $\mu g$, and on the parachute.