3.1

Instrument architecture

The instrument parametric analysis, performed to optimise the instrument concept, has led to the following results:

The implementation of the different subsystems was performed to demonstrate the feasibility and assess the typical overall interfaces for an instrument of this class. The baseline carrier for the instrument accommodation is the International Space Station. The instrument is designed to fit a double adapter of the Express pallet, the external attachment interface structure located on the main truss.

Fig. 1:

General layout of the instrument

3.2

Instrument performance

4.1

Transmitter laser subsystem

The objectives of the transmitter laser study were the selection of the best crystal and the optimisation of the laser architecture, in order to achieve high optical energy as well as a good beam quality for the output beam. Simulations of the overall laser performance showed that Tm:Ho:YAG was the best crystal, which could lead to the optimisation of the system, both in terms of overall efficiency and thermal loads.

Concerning the laser transmitter architecture, the only feasible solution, compliant with the energy and pulse repetition frequency requirements, is a laser based on the MOPA scheme presented in figure 2.

Fig. 2:

Transmitter laser architecture (MOPA) for asingle wavelength

4.2

Frequency offsetting and stabilisation subsystem

Frequency control requirements are related to both missions of the instrument. The DIAL mission mainly constrains the transmitter laser frequency of the “on” line, due to matching the laser frequency with the atmospheric absorption line. From the receiver point of view, frequency compensation is mainly dedicated to bandwidth reduction.

The requirements for the “off” line, coming from the DIAL mission are less stringent since the transmitted frequency must be centred to the “off” frequency with an accuracy far less stringent than the “on” line.

For DWL mission, wind measurement is performed on the “off” line return. The tolerable residua! frequency error after all compensations must be of the order of magnitude of 200 KHz in order to minimise the systematical errors affecting wind velocity measurements.

The two candidates for accurate frequency stabilisation of the lasers were studied:

The Pound-Drever option was thus strongly recommended due to better performances. Likewise, two options were studied for absolute frequency reference:

Several advantages and drawbacks were identified for both options. The final choice between Perot-Fabry etalon and molecular line depends on the availability of the suitable lines and the frequency architecture selected.

An example of the frequency architecture proposed for “on” line is presented in figure 3.

Fig. 3:

Frequency control architecture for “on” line

4.3

Receiver subsystem

An optimisation of the detection chain was required to maximise the output Signal to Noise Ratio (SNR). which is the driver for the achievement of accurate and reliable measurements. Three receiver configurations have been studied, considering quantisation at different stages of the detection chain : after the pre-amplifier, after the In-phase and Quadrature (I&Q) channels, after the bandwidth reduction filters. The trade-off study has shown that the optimum electronic architecture (figure 4) is the one performing the quantisation after the I& Q demodulation stage. The overall degradation of SNR due to the detection chain is thus typically - 2 dB.

Fig. 4:

Selected receiver architecture

5.1

Laser breadboard design

The breadboard activities have been focused on the realisation of the oscillator, since the seeder has been already developed and tested in a previous contract (main characteristics of the seeder: 30 mW output energy in single longitudinal mode. < 1.6 diffraction limit. 0.5 MHz linewidth).

The oscillator under development presents an optimised laser head pumping configuration (figure 5). with minimum thermal lensing effects on the beam. The expected performance are around 70 mJ power. 10 Hz pulse repetition frequency and 200 ns pulse duration.

Fig. 5:

Laser head pumping configuration

5.2

Receiver breadboard design

The goal of the receiver breadboarding activity is to characterise the selected architecture of the coherent receiver (see § 4.3). The elements to be tested under representative signai conditions are the photodiode the transimpedance amplifier, and the matched filter (figure 6).

Fig. 6:

Receiver breadboard configuration