1

INTRODUCTION

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 [2] [3].

The ALADIN TXA [4], 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 [5], 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.

Fig. 1.

RLH Unit.

Fig. 2.

PLH Unit.

The optical lay-out of this unit is reported in Fig. 3.

Fig. 3.

PLH Optical Lay-Out.

The PLH is composed of 4 main subunits described and shown hereafter:

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

Fig. 4.

MO Subunit.

Fig. 5.

Pre and Power Amplifier Subunits.

Fig. 6.

HS Subunit.

Fig. 7.

PLH Unit: On-Ground integration configuration (upper), Testing/In-Flight configuration (lower) without cover.

Transmitter Laser Electronics (TLE). The TLE is organised in three main sections:

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.

2

TXA EQM EXPERIMENTAL RESULTS

The main experimental results obtained in air with the TXA EQM are reported in Table 1.

Table 1.

Main UV Laser Output Performance

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.

Fig. 8.

TLE Unit.

Fig. 9:

IR/UV Energy during 12 s. The reported values refer to last 7 s useful period.

Fig. 10:

Typical UV Near Field profile.

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.

Fig. 11:

Typical UV Far Field profile.

Fig. 12:

Typical UV pulse temporal profile.

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.

Fig. 13

Optical frequency stability.

The X,Y centroid displacements reported in Fig. 14, have been measured at the focal plane of a 500 mm lens.

Fig. 14:

UV beam angular stability during a burst.

Fig. 15.

MOC exposition PLH EQM curve.

Fig. 16.

Example of various PLH internal components temperature monitoring.

Fig. 17.

TXA functional mode diagram and mode control parameters.

The main physical characteristics and budgets of the TXA EQM single units are reported in Table 2.

Table 2.

Physical Data & Budgets

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.

3

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.

Cleanliness/Contamination

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.