2.1

The functional design of the FRU

The functional and performance requirements of the FRU are:

In the next paragraphs, we will explain the different functional blocks of the FRU. An overview over the functional blocks is shown in Figure 1. Apart from the electronics, four main functional blocks are presented: The Maser Oscillator seeder (OSC-seed), the optical parametric oscillator seeder (OPO-seed), the wavemeter (FRUF) and the absolute frequency reference (FRUA).

Figure 1.

Functional Block Diagram of the FRU

2.2

Master Oscillator seed

The master oscillator seed (OSC-seed) provides a 10 mW laser light at 1064 nm from a temperature- and current-stabilized laser diode. By temperature tuning, the wavelength can be selected to match the gain maximum of the master oscillator laser. For reliability reasons, two laser diodes are routed to the output port through a combining tap coupler and provide a cold redundancy.

The laser diodes are available from Eagleyard in a space-qualified design flown on GAIA. Due to the larger temperature range on MERLIN, a redesign has been made which increases the stiffness of the modules against thermo-mechanical stress. In addition, optical isolators have been integrated in order to avoid mode-hopping of the lasers upon external back-reflections. To verify these changes and also the different type of chip material that is used for the required wavelength, a screening and a lot acceptance test campaign of the actual FM laser diodes will be performed along the guidelines of ESCC 23201. Environmental testing to MERLIN specifications will be performed. The component will be delivered fully qualified by eagleyard Photonics.

2.3

Optical Parametric Oscillator seed

The optical parametric oscillator seed (OPO-seed) provides 5 mW laser light at different wavelengths around 1645.55 nm from temperature- and current-stabilized laser diodes. One out of four laser diodes can be selected and routed to the output through three optical switches. In normal operation, three out of the four laser diodes are active and provide the following functions:

All laser diodes can provide any of these three functionalities. The fourth laser diode is not active and serves as a cold redundant reserve in case one of the other laser diodes fails to operate properly.

The qualification approach of the laser diodes is the same as for the OSC-seed laser diodes.

2.4

The absolute frequency reference (FRUA)

The FRUA provides the absolute frequency calibration of the FRU with respect to the methane gas absorption feature. It ensures that the λ_on wavelength maintains its defined value within 10 MHz accuracy and provides the absolute reference to the measured values of λ_on and λ_off. It consists of a beam collimator, a methane gas cell, three beam splitters and four photodiodes with associated electronics. The assembly is shown in Figure 2. A set of six nearby lines in the methane spectrum invokes a maximum in transmission in their centre, as shown in, and stems from the same absorption feature that is used by the main laser and instrument to detect the methane concentration in the atmosphere of the Earth.

Figure 2.

The FRUA assembly: The light (indicated by the red line) launches into the assembly on the upper right side from a fiber connector facing an aspheric lens to collimate the beam. The first beam splitter samples about half of the light power to the reference branch. This again incorporates a beam splitter to steer the light onto two photo didoes for redundant operation. The other part passes through the methane gas cell (shown in green) to be again split onto two photo diodes.

This feature is scanned with the reference laser diode and used to stabilize the frequency of the reference laser diode by comparing the transmission through the cell on the lower side edge with the upper side edge (red dots in the figure) and levelling them by adjustment of the reference value λ_ref, which is indicated by the dashed red line.

The λ_ref is measured on the wavemeter where the frequency measurements of all other seeders and the transmit pulses are done and can be evaluated relative to it.

Figure 3.

Transmission spectrum through the methane gas cell at 1645.55 nm. The sign of the Ref points (red dots) is defined by the delta currents w.r.t. the lambda_ref current (center position) that are applied to the laser diode in order to shift its frequency. As higher currents yield higher wavelength but lower frequency, the sign is opposite. The center frequency (dashed red line) is controlled in such a way, that both transmission values have the same level. The green dot indicates roughly the frequency where the OPO is seeded for generating the online pulse.

For the FRUA assembly, a set of InGaAs photo detectors has been procured, which were subjected to a full space qualification. The optical elements will be qualified to the vibrational and shock loads on component or unit level. The coatings are radiation hard by using the SiO₂/Ta₂O₅ material combination. A coating process that ensures dense coatings avoids problems with adsorbed water, which could release under vacuum conditions.

2.5

The wavemeter

For frequency measurements of the online and offline seeders, as well as the corresponding OPO transmit pulses, a wavemeter is used. It consists of a monolithic beam collimator, a Fizeau wedge and an InGaAs line detector, as shown in Figure 4.

Figure 4.

The FRUF wavemeter assembly

To generate a well-collimated input beam with a diameter of 12.5 mm, an ultra-high numerical aperture single mode fiber is directly spliced to a single surface aspheric lens. This allows for a very robust collimator design and a short assembly length in order to meet the demanding envelope requirements of the FRU. The Fizeau wedge has a length of 35 mm resulting in free-spectral range of 4.28 GHz. The finesse is about 24. The design does not include a cylindrical lens to focus the fringe pattern on the detector array in order to keep the design simple and to save space. Instead, a slit aperture is used to reduce stray light on the detector. The InGaAs line has 640 pixels with a width of 20 microns.

Figure 5 (bottom) shows an example of the fringe pattern generated by the Fizeau wedge and recorded with the InGaAs line detector. For frequency determination, the position of the centroid of the fringe on the line detector shall be identified. A FPGA-compatible algorithm filters the recorded pattern with a step like filter function as shown in Figure 5. The result represents the low-pass filtered derivative of the fringe. Using a linear regression on this result provides the zero-crossing point with a resolution of 1/16 of a pixel and therefore a measure for the fringe position. The filter function selects only the steep flanks of the upper 75 % of the fringe signal. The algorithm is very robust against amplitude fluctuations as well as pixel-to-pixel non-uniformities and takes about 142 μs to provide the result. Both, the continuous-wave light from the seeders as well as the OPO pulses with a duration of about 20 ns can be measured with the wavemeter. For rather symmetric input spectra, the algorithm coincides with the centroid. In case the OPO spectrum is too asymmetric, offset compensation parameters can be set. The algorithm itself as well as the InGaAs line detector design provide a certain robustness against pixel failures. Two read-out ICs in an odd-even configuration are used for data acquisition and are followed by dedicated 14-bit analogue-digital-converters. The performance of the wavemeter is sufficient with data only from odd or even pixels available; see Figure 6 a). In addition, a bad pixel map is implemented which can be filled by ground-loop actions to mask dark, bright or blinking pixels. Due to the filtering effect of the algorithm, a failure of up to 12 pixels in a row can be tolerated before the performance of the wavemeter drops below the required resolution, compare Figure 6 b).

Figure 5.

Fringe pattern and signal processing for frequency measurements

Figure 6.

a) Measurement of the wavemeter response and resolution of data recorded by both ROICs in comparison with only data from the odd and only the even pixels. b) Wavemeter linearity and resolution as a function of bad pixels. The region of interest has been zoomed in. (Note that the sign of the frequency axis is reversed, which is only defined by convention.) The first bad pixel is located at the position of Δpx = 50 and the following bad pixels are introduced at lower Δpx positions (50,49,48,…).

For the FRUF assembly a commercial InGaAs line has been procured which is undergoing the qualification process at SpaceTech. For what concerns the optical elements, the qualification approach and effort is the same as it has been described in paragraph 2.4 for the FRUA assembly.

The Fizeau’s optical length and therefore the fringe position is quite sensitive to pressure and temperature changes, which needs to be accounted for in the ground testing campaign and the operational concept in space.

2.6

The FRU harness and opto-electronics

The optical harness includes tap couplers and optical switches. The tap couplers are all fiber devices, manufactured with a fused biconical taper process. They are bidirectional devices that maintain the polarization of the input. Within the FRU two types of couplers are used: couplers in 2x1 configuration and 2x2 configuration for combination of signals within the FRU and from the ICC. Please refer to Figure 1.

High reliable switches are used for routing the signals of the different laser diodes to the submodules. The switching process relies on change of polarization of an electro-optic material by a short pulse of roughly 0.3 ms. Switching is achieved within the first ten microseconds. After switching, no electric power is necessary to maintain the current status. The device is fiber coupled and polarization maintaining. Electro-optic components within the FRU are connected by optical fiber. Therefore, tens of meters of optical fiber are integrated and contribute to the optical harness. All fibers are coated with UV acrylate and, for some components, an additional buffer of thermoplastic elastomer is needed for manufacturing. Except for a short piece of single mode (SM) fiber in front of the FIA, solely polarization maintaining fibers are used. Thus, the alignment and polarization of the optical path in the FRU is almost unaffected by temperature changes and offers great flexibility in fiber routing. Different components are connected by fiber optic splicing, external by Mini-AVIM fiber connectors. A splice qualification process ensures that the optical connections meet the requirements imposed by the space environment. This includes test of depressurization, temperature, optical performance, routing and fixation of the fibers. Long term reliability of the splices are verified by the process sustaining tensions in the 20 N regime and a rigorous screening of the systems splices including a 10 N proof-test.

The optical switches haven been procured as commercial high reliability parts and undergo a screening and qualification programme at SpaceTech.

2.7

The FRU electronics

The FRU provides all typical electrical functions and interfaces required for an optical instrument as MERLIN. The electronic design is accommodated on four modules: the power-, OSC-Seed-, OPO-Seed- and calibration module. A flex-rigid backplane connects all modules together. The power module generates the required supply voltages within the FRU from the primary power bus. The OPO-Seed module electronics can be considered as the master board of the FRU comprising the FPGA as the central element. On this module are also the 1645nm laser diodes and their control and drive electronics. The OSC-Seed module electronics include the 1064 nm laser diodes with their control and drive electronics. The calibration module provides the electrical design and interfaces to control the wavemeter and the measurement of the CH4 absorption curve by the photo diodes.

Laser Diode Driver

With regard to the optical performance, the development of the laser diode driver has to be emphasized. The frequency stability and noise requirements are challenging. The laser diode driver comprises the following features:

• • a controllable laser diode (LD) current source

• • a temperature error amplifier, proportional and integral (PI) gain controller and a linear TEC current source

• • conditioning amplifiers to monitor the laser diode chip temperature and the Laser Diode current

• • Internal Reference Voltage Generation

• • 2 Digital-to-Analogue Converters to control the TEC current and the LD modulation current

The maximum TEC current of 650 mA allows a linear power output stage which was preferred to a PWM-controlled power amplifier to avoid signal interference. A temperature conditioning circuitry measures the laser diode temperature in a range from -30°C to +80°C with a NTC thermistor installed in the laser diode module. The FPGA delivers the set value of the laser diode temperature. A PI-regulator followed by bipolar-supplied power stage represents the temperature control loop. The integrational portion of the regulator ensures that there is no remaining control deviation. The set value is limited to the optimum operating point of 30°C +/- 10 K. This offers the advantages against the full measurement range that the resolution is improved and it avoids that a failure in commanding the temperatures damages the laser diode.

Table 1.

Performance Summary of Laser Diode Driver

Parameter	OSC-Seed LD	OPO-Seed LD
Offset Current	-	130 mA
Variable Current	0 to 160 mA	0 to 6 mA
Resolution (12 Bit)	29.3uA / bit	1.5uA / bit
Modulation Range	-	0 to +/- 3mA
Max. TEC Current	650 mA
Temp. Set Range	20°C to 40°C

The laser diode operates with a current from a constant current sink. At the beginning of the project, space qualified high resolution digital-to-analogue converters (DAC) were not available. The choice fell to a 12Bit DAC from Texas Instruments. A further advantage of this device is the SPI interface.

Two in parallel operating current sinks apply the constant current to the laser diode: a fixed-offset current of 130 mA and a variable current in the range of +/-3 mA commanded by the FPGA with a resolution of 1.5 μA per step of the DAC.

Commercial laser diode driver designs can use a variety of highly sophisticated devices which have been developed for best performance such as offset, stability and noise properties. For space application such as MERLIN, initially only a small number of available components could be selected which meet the space-specific environmental requirements. Therefore, it is remarkable that the developed laser diode driver shows a very good noise characteristic comparable with off the shelf designs and as a reference from literature³ what is needed to have sufficient performance, see Figure 7.

Figure 7.

Comparison of noise performance in terms of relative intensity noise of the laser diode driver developed for the MERLIN FRU with a commercial laboratory laser diode driver. The lasers used for characterization are DFB laser diodes with a line width of 1 MHz. The shown reference is from [3].

2.8

The FPGA design

A re-programmable FPGA (RT3PE3000L) is used as the central control element of the FRU. It is connected to 10 DACs (12-bit), 4 ADCs (12-bit), 2 MUXes (8-bit), a SpaceWire RMAP port, 18 digital output and 3 input lines and implements four control loops for λref, λon, λoff, and the OPO cavity.

The functionality of the FPGA has been broken down into the following main modules (refer to Table 2.):

Table 2.

Functional modules of the FPGA

ICU Interface

Three memory-mapped TM/TC registers and four dedicated memory areas are set aside for communication with the ICU via SpaceWire RMAP:

• • Error detection and correction (EDAC) and cyclic redundancy check (CRC) safeguarded memory that serves as storage for the FRU’s operational parameters.

• • General HK1 housekeeping data, mostly temperatures, which are independent of the control loops.

• • Every PRI pulse triggers the computation of housekeeping data representing internal states of the control loops. Upon arrival of the next PRI pulse, the newly computed data is copied into the HK2 memory area that can be read by the ICU during the next PRI cycle.

• • HK3 housekeeping data allows the ICU to read the samples of each of the wavemeters’ five fringes (λ_ref, λ_on(seed), λ_off(seed), λ_on(Tx), λ_off(Tx)) that are produced during a PRI cycle in operational mode.

Figure 8.

The layout of the FPGA functional modules and the outer interfaces to the peripheral devices and the ICU.

The FRU Operating Modes

In CONFIG mode, a set of 190 operational parameters (16-bit) must be transferred via SpaceWire into the EDAC and CRC safeguarded memory inside the FPGA, because the FRU itself does not have any non-volatile storage.

In STANDBY mode, the thermo-electric-coolers of four laser diodes (λ_ref, λ_on, λ_off, and OSC) are switched on, regulated by an analogue control loop.

In DIAGNOSTIC mode, each of the four seeder diodes separately and the optical switches are stimulated in such a way that the analysis of the HK data will allow to pinpoint hardware faults.

In OPERATIONAL mode, the currents for the laser diodes are switched on and the control loops start to operate. When the control loops have synchronized, OPO cavity control data will be transmitted to the OPO via SpaceWire.

OSC CALIBRATION mode is a sub-mode of OPERATIONAL. While the control loops continue to operate generating HK data, the 1064 nm OSC laser diode will run through a configurable sequence of TEC temperatures and laser diode currents. Later analysis of the HK data allows to select the optimal operating point for the OSC laser diode.

ATMOSPHERIC CALIBRATION mode is a sub-mode of OPERATIONAL. λ_on will be swept across the six methane absorption lines, which are used as absolute frequency reference. As a consequence, OPO_on will be swept accordingly, such that the absorption lines in the backscatter signal can be recorded.

Figure 9.

FRU Mode Diagram

2.9

The mechanical housing and structural design

The mechanical housing consists of the underlying baseplate and four modules perpendicularly oriented to the baseplate. The mechanical design is driven by the functional requirements of the FRU. There are four basic modules: the Calibration module, the OPO-seed module, the OSC-seed module and the Power module. The modules are interconnected to each other by six M5 bolts. This pre-assembly is then mounted onto the baseplate. Due to the dynamic behaviour of the FRU, the components close to the baseplate are the least-stressed components. The modules are thin-walled aluminium housings with stiffening features. This ensures local stiffness. The PCB boards are bolted to the modules, the bolts are placed in a regular pattern on the stiffening ribs. The mechanical environment during launch is very demanding. The sine test loads are specified up to 150 Hz with up to 27 g. The random and shock loads are also very high for a large component (mass > 5 kg) like the FRU. The random loads are specified with 16.9 grms and 12 grms for out-of plane and in-plane respectively. The shock loads are specified with up to 1000 g at 1000 Hz. The structural design aims for minimizing the vibration loads for the optical components. The metallic structures are sufficiently strong; the design is driven by stiffness and minimum milling thicknesses. Still, the final stiffness has to be traded off against the high shock loads. A more rigid structure leads to lower shock dephasing and routes more shock to the critical large optical components.

The analysis of the structure is conducted with Nastran and all environmental tests are supported by these analyses and conducted in-house.

Figure 10.

The FRU Configuration. Overall configuration (left) and cut-away view (right)

2.10

The thermal design

The thermal design of the FRU is a classical passive system. The FRU dissipates approximately 23 W in nominal science operation mode. It utilizes the main thermal contact to the baseplate. The box housing parts are black anodized for increased heat distribution inside the box. The black surfaces outside radiate directly to the spacecraft radiators adjacent to the FRU –Z direction. The thermal design is driven by the laser diodes. The laser diode temperatures are kept constant at temperatures between 27°C and 33°C (depending on the type and function of the diode) with thermo-electric coolers (TEC). These TEC units dissipate waste heat depending on the temperature difference between interface and diode they have to bridge. In order to avoid a thermal runaway, the box temperature upper limit is the critical driver for the thermal design. The design of the box is therefore equipped with high cross-section aluminium bars between the diode interfaces and the bottom plate. The bottom plate is connected with a spacecraft-supplied heat pipe to the spacecraft-supplied radiator. The radiator-facing side of the FRU is the Power module. Most of the dissipated waste heat is at this power module side of the box. The additional heat path via radiation to the radiator is used. The analysis is conducted with ESATAN TMS, the test predictions of the analysis will be verified by TV testing in-house.

The predicted temperatures of the CDR analysis loop confirms that all component temperature limits are respected.

2.11

Main performance data

With the engineering model (EM) that is shown in Figure 11, the following main performance numbers could be achieved:

Figure 11.

The engineering model of the FRU. Only one out of the four modules has been black anodized like it will be the case for all parts in the later flight design. Three optical patch cords are attached for interfacing the test equipment and finally the laser and ICC of the engineering model of the MERLIN instrument.

The EM wavemeter incorporates the monolithic fibre collimator design that will be used in the flight model in contrast to the commercial triplet collimator that was used for the bread board model. During the tests of the EM wavemeter it turned out that the used aspheric lens introduces a non-linearity that is three times larger than what had been found for the bread board wavemeter. Further analysis revealed that the manufacturing process of the aspheric lens leads to periodic distortions in the radial direction, which act like a phase grating and modulates the intensity profile of the collimated beam in the far field. To reduce this major source of non-linearity to an uncritical level, a different production process will be used for the next models.