1.1

Performance requirements

The main performance requirements for the laser part of the LASE:

1.2

Architectural Design

Several alternatives exist for providing an on-board wavelength reference. One of the most common solutions is to use a molecular absorption cell (commonly referred to as a Gas Cell). A small tube containing a specific isotope of acetylene has absorption bands at certain precise wavelengths. The laser output power and wavelength are functions of temperature and input current. By utilizing a feedback loop that compares the laser wavelength with an absorption peak of the gas cell it is possible to correct for any drift of the laser. Gas cells are wavelength stable with respect to temperature and with respect to the mechanical environment.

The ¹³C₂H₂ isotope of Acetylene has an absorption transition (P16) at λ = 1542.3837nm. This absorption line is used extensively within Wavelength Spectroscopy and so primary and secondary standard lasers are available against which the LASE may be calibrated. The P(16) line has good absorption and so a short single pass gas cell is possible reducing risk of optical misalignment, or internal etalon effects, which could upset the wavelength stability of the LASE.

Figure 1 shows the principle architecture of the LASE, which is based on the Pound Drever Hall (PDH) technique for wavelength locking [1]. The PDH circuit establishes a feedback loop that locks the DFB Laser output wavelength to the above-mentioned P16 line of Acetylene. The PDH circuit generates an error signal that is fed back to the current driver of the Laser.

Figure 1

Principle architecture of the LASE

The Laser Diode current is controlled by an FPGA using two identical external DACs that each has 12 bits resolution, one for coarse setting and one for fine setting.

The coarse DAC is used initially to turn on the laser by ramping the current up to the operating point defined in the Configuration Table stored in the LASE ROM. Then a temperature sweep is started, adjusting the laser temperature in 1 mK steps over a 1024 mK range. The absorption in the gas cell, the PDH error signal and the temperature are stored in the SRAM of the LASE at each step. In this way, a 13 GHz survey of the PDH loop is established. The temperature corresponding to midway between the minimum and the maximum PDH error is selected as a temporary temperature set point.

When the laser temperature has stabilized at the temporary set point, the fine DAC is used to perform a high-resolution scan across the absorption line, over a 600-800 MHz range, which establishes a basis for determining the laser drive current set point.

This scheme for obtaining the laser set-point is considered essential in order to make sure the system locks to the same absorption peak of the gas-cell each time it is started, despite of component ageing effects.

When the PDH loop is locked, neither laser drive current nor temperature are fixed; the drive current is adjusted to compensate for rapid, but small, wavelength deviations, while the temperature is adjusted to compensate for slow, but large, deviations.

Additionally, the LASE has a five channel programmable gain transimpedance amplifier with AVIM inputs. The amplifiers will be connect to the spectrometer outputs and converts the optical signals to electrical. Key performance parameters are low noise, high bandwidth, stable propagation time and low power consumption. A photodiode has been custom made and qualified for this amplifier.

Certain parameters of the LASE are also configurable from ground.

Figure 2

The Laser Source Electronics (LASE) unit (EQM).

The LASE development has followed the traditional approach for space.

1.3

Components qualification

For the LASE, the EEE components were either selected from the ESCC QPL or procured from suppliers being able to deliver space quality components. In case of the optical and electro optical components, it was necessary to run dedicated programs for component qualification. Figure 3 illustrates the procurement flow applied to:

Figure 3

Component procurement flow.

The evaluation phase, which is for risk reduction, focused on manufacturer evaluation, construction analyses and environmental testing. For each component category, several alternative components could be subjected to the evaluation test phase.

The qualification test campaigns were derived from [2].

3.1

Wavelength stability

Short-term wavelength measurements at stable environmental conditions were performed repeatedly throughout the LASE qualification campaign. Figure 7 shows the results obtained at the various test events, which corresponds to a relative wavelength deviation of less than ~5.3 10^-9 (i.e. less than 8.1 fm). These tests were performed with a back reflection level of -43 dB using the set-up shown in Figure 4.

Figure 7

Deviation of short term wavelength measurements relative to the average of all wavelength measurements.

The LASE wavelength was also monitored during thermal cycling between 0°C and +40°C. Figure 8 shows the measured frequency deviation and the TRP temperature versus time. The frequency anomalies occurring at TRP temperatures of approximately 33°C were due to effects occurring when the laser modulation frequency becomes exactly 10x the FPGA oscillator frequency. Even though the frequency stability requirement is met throughout the thermal cycling, measures have been implemented in order to reduce this frequency anomaly for the flight models.

Figure 8

Wavelength stability measured during temperature cycling in thermal vacuum chamber.

Figure 9 shows the wavelength RNE measured during thermal cycling. In both frequency bands, the RNE is below the requirements indicated by the red horizontal line in each plot.

Figure 9

Wavelength Random Noise Error (RNE) measurements performed during thermal cycling.

3.2

Polarization

Predicting the Polarization Extinction Ratio (PER) of a system comprising several fiber optical sections and components is difficult and often an underestimated task.

Figure 10 shows the basic optical configuration of the LASE, including the external 1m fibre at which end the Polarization Extinction Ratio (PER) shall be measured. The red, broken, line represents the wall of the LASE enclosure. AVIM and Mini-AVIM are fiber optic connectors manufactured by Diamond SA.

Figure 10

Basic Optical Configuration

The parameters influencing the LASE PER are:

Table 4 lists the characteristics of the components affecting the system PER. Additionally, the PM fibre sections contributes significantly to the system PER through the temperature and stress dependence of the fibre birefringence that shifts the angle of polarisation of the light. In this way, the temperature can compensate for the misalignment of fibre polarisation axis at fibre junctions and result in extremely high PER values for certain temperature intervals. At other temperatures, the system will reach minimum PER values.

Table 4

Component characteristics affecting the LASE PER.

*

Angle calculated from the PER value.

Figure 11 shows the measured PER, with the LASE in thermal vacuum chamber, during three temperature sweeps:

As can be seen, the three temperature responses differ. In case of the slow temperature change (+1.25 °C/hour), PER values below the 15 dB limit are found at TRP temperatures in the range 1.05 °C to 2.07 °C and 7.86 °C to 8.37 °C. For the faster temperature change (+2.67 °C/hour), violation of the 15 dB limit occurs in the range 3.99 °C to 4.74 °C.

Figure 11

Polarisation Extinction Ratio (PER) measured with LASE in thermal vacuum chamber and temperature sweeped. The lower chart shows the corresponding TRP temperature.

In case of the LASE, tested here, there will be states where the LASE PER falls below the 15 dB limit. The occurrence of these events depends upon the temperature and the temperature distribution of the fiber optic components inside the LASE. Most likely, the minimum PER will not be significantly less than the minimum value measured during temperature cycling; 13.86 dB during the +1.25 °C/hour test from 0 °C to +25 °C.

Calculating the PER using only the extreme values given in Table 4, the result is 9.75 dB. This situation, with all parameters at their worst value, is however not likely. The angular misalignments of the connectors will have an unknown distribution. A model for calculating the PER of the LASE has been established. Here, component characteristics are selected from Table 4 and the phase shifts of the fiber sections are adjusted to a worst-case situation. Initially, it is assumed that the component characteristics of Table 4 corresponds to 3σ normal distributions. Then, it is assumed that the component characteristics are uniformly distributed. Figure 12 shows the simulated probability of obtaining a system PER of less than X dB using this model. Contributions from the laser and the splitter are fixed at their worst-case values. The blue curve of Figure 12 represents uniform distributions, while the red curve represents normal distribution with boundaries equal to 3·σ.

Figure 12

Simulation results showing probability of PER ≤ X dB. The blue curve represents uniform distribution of component deviations, while the red curve represents normal distribution of components deviations.

With uniform distribution of angular misalignments, the probability of obtaining a system with PER < 15 dB is 93.8 %. In the case of normal distributions, the probability is 25.3 %.

In order to improve the resulting PER, KDA foresee a process of matching parts prior to integration in the LASE. The matching process will comprise:

If the PER requirement is met by the test system in the whole temperature range, the system parts will be allocated to one future system. If not, the test has to be repeated with a different combination of parts.

Fusion splicing of fibers is expected to give better performance with respect to PER than the connectorized junctions. The lack of space qualified processes made this alternative not feasible.