The Aladin Laser Transmitter Assembly (TXA) is one of the most challenging laser transmitter for LIDAR spaceborne applications even if compared to the homologous devices employed in the last 5 years NASA missions  .
The ALADIN TXA , fully redundant in the instrument, is organised in 3 units:
Reference Laser Head (RLH). The RLH is an ultrastable diode-pumped CW Nd:YAG Laser , used as injection seeder for the Power Laser Head (PLH). The output beam of the RLH is fed into PLH by means of an optical fibre.
Power Laser Head (PLH). The PLH is a diode-pumped, Q-switched Nd:YAG Laser working in the third harmonic. A picture of the closed unit is reported in Fig. 2.
The optical lay-out of this unit is reported in Fig. 3.
The PLH is composed of 4 main subunits described and shown hereafter:
– A low energy injection seeded, Q-switched, longitudinally laser diode pumped Nd:YAG Master Oscillator (MO).
– A first amplifier (PreA) in a double pass configuration (transversally pumped).
– A second power amplifier (PwA) in a single pass configuration (transversally pumped).
– A Harmonic Section (HS) employing two non-linear crystals.
The whole PLH operates in burst regime with a cycle of 12 s ON (7s useful) and 16 s OFF. An image of the open unit is reported in Fig. 7
Transmitter Laser Electronics (TLE). The TLE is organised in three main sections:
– Power Section, containing the Laser Diode Power Supplies for MO, PreA and PwA and High/Low voltage Power Supply;
– Interface & Control Section containing several boards dedicated to command and control PLH and RLH operations and to interface the Aladin Control and Data Management (ACDM).
– Cavity Length Control Loop (CLCL) section, dedicated to lock the MO to the RLH ensuring the PLH tunability and Single Longitudinal Mode operations.
The TXA/PLH EQM model is equivalent to the Flight Model, with the exception of some engineering grade components. The RLH and the TLE are engineering level units.
The ALADIN Instrument and TXA will be respectively the first European spaceborne Wind Lidar, and All-Solid-State laser Transmitter to be launched in 2008 for a three-year mission in space.
TXA EQM EXPERIMENTAL RESULTS
The main experimental results obtained in air with the TXA EQM are reported in Table 1.
Main UV Laser Output Performance
|Energy/pulse||107 mJ (mean during ON period)|
|Pulse duration||18.4 ns (FWHM)|
|Beam Angular Stability||24.4 μrad X (zig-zag plane)28.7 μrad Y (orthogonal to zig-zag plane)|
|Spectral linewidth||40 MHz (FWHM)|
|Frequency stability||3.7 MHz (rms value)|
Due to the transient thermal behaviour induced by the burst operations, the IR beam spatial profile and consequently the output energy vary during the useful ON period. Both causes make the UV conversion efficiency to increase from 18% to 31% from the beginning to the end of the burst.
A typical Near Field profile is shown in Fig. 10. Its dimensions at 1/e2 of the peak irradiance are 4 mm in one direction and 5.7 mm in the other one.
The good quality of the beam (M2 ≤ 3) results in a typical Far Field profile measured at the focal plane of a 500 mm lens, as the one shown in Fig. 11. Its dimensions at 1/e2 of the peak irradiance are around 77 µm in both directions.
The temporal profile width is of 18.4 ns (FWHM). It does not exhibit any temporal modulation when the RLH is injected inside the MO, indicating that the laser is operating in Single Longitudinal Mode (SLM).
The ultrastable RLH frequency characteristics guarantee a whole TXA frequency stability after harmonic conversion, as reported in the graph of Fig. 13.
The X,Y centroid displacements reported in Fig. 14, have been measured at the focal plane of a 500 mm lens.
The main physical characteristics and budgets of the TXA EQM single units are reported in Table 2.
Physical Data & Budgets
|Mass||PLH EQM = 27.3 kgRLH EM = 2.4 kgTLE EM < 22.2 kg|
|Volume||PLH = 450x350x215 mm3RLH < 150x120x75 mm3TLE < 450x345x230 mm3|
|Stiffness||PLH Resonance frequencyX axis 182 HzY axis 288 HzZ axis 251 HzRLH Resonance frequencyX axis 347 HzY axis 342 HzTLE Resonance frequencyX axis 220 HzY axis 430 HzZ axis 510 Hz|
|Cold Plate||24 ± 1 °C|
|Power consumption||440 W (measured average value)|
The EQM will be submitted in the very near future to an environmental test campaign encompassing vibration and thermal vacuum tests. The flight TXA will operate in vacuum.
TXA SPECIFIC PROVISIONS
Several provisions have been adopted in the TXA design, development and test phases to guarantee a correct operability during the mission and improve the reliability.
A high level of cleanliness, supported by continuous environmental monitoring during assembly activities, is essential to minimize surface contamination levels and to maintain the contamination within values that do not affect the instrument performance and reliability. In this contest, particulate contamination (PAC) is directly correlated to the environmental cleanliness levels, the personnel activities inside the controlled area and the duration of the various operations. All the TXA EQM assembly, alignment and test activities have been performed in a class 100 ambient. A rough formula indicating the particle accumulation on horizontal surfaces as a function of the cleanliness level of the integration ambient, is reported hereafter
this implies, as rough assessment:
|♦||CR. class 100.000||(M6.5) ⇒||≈ 275||[ppm./day]|
|♦||CR. class 10.000||(M5.5) ⇒||≈ 53||[ppm./day]|
|♦||CR, class 100||(M3.5) ⇒||≈2||[ppm./day]|
For what concerns the molecular contamination (MOC), due to a non linear relationship between the time and the amount of contaminants deposited on surfaces, an assessment can be done based on the acquisition of several typical values (on weekly and monthly basis), relevant for the specific AIT areas. An experimental graph showing the PLH EQM molecular particle accumulation during 300 days of integration activities, compared with the theoretical curve (green) and goal value (red) of 1.2 g/cm2, is reported hereafter.
It is worth noting that the contamination level remained below 1.2 g/cm2, that is considered a reasonable level obtainable with suitable precautions.
In-Flight monitoring and control
Temperatures of the key components (laser diodes and active materials of the various unit sections, cold plate, optical benches, etc.) are continuously monitored to verify the “health” status of the laser and/or detect anomalous behaviours.
Laser energy at the output of the different PLH subunits are also monitored by means of sampling photodiodes. Cross correlating this information with the temperatures, it is possible to detect ageing effects and compensate them where possible. For example, the LD current can be increased compatibly with the allocated power budget and HW limitations. In fact together with the Tele Measurements (TM) cited above, the TXA has been conceived in order to accept several Tele Commands (TC), such as laser diode currents or pulse durations, that will enable “in flight” management of the laser.
Being Single Longitudinal Mode (SLM) operation crucial for the whole Aladin Instrument and Mission performance, a monitoring of the single frequency emission of the laser is operated periodically during mission.
Temperatures, energy and currents monitors are also used to consent to operate safe transitions among the different laser Tx operational modalities at switch ON/OFF.
A series of protections, at unit, subunit and modules levels have also been implemented. They take action when some critical parameter goes out of a predefined “hazard threshold limit”, forcing the TXA in a safe operation mode (STAND-BY).
This should prevent catastrophic failures or damage of optical components and improve the success probability of the whole mission.
We whish to thank Team people from Quantel, CESI, Tesat and Nuvonix for the strong contribution and technical support given in this program.
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