I.

INTRODUCTION

Among the different optical modulator technologies available such as polymer, III-V semiconductors, Silicon, the well-known Lithium Niobate (LN) offers the best trade-off in terms of performances, ease of use, and power handling capability [1-9]. The LN technology is still widely deployed within the current high data rate fibre optic communications networks. This technology is also the most mature and guarantees the reliability which is required for space applications [9]. In or der to fulfil the target specifications of opto-microwave payloads, an optimization of the design of a Mach-Zehnder (MZ) modulator working at the 1500nm telecom wavelength was performed in the frame of the ESA-ARTES “Multi GigaHertz Optical Modulator” (MGOM) project in order to reach ultra-low optical insertion loss and low effective driving voltage in the Ka band. The selected modulator configuration was the X-cut crystal orientation, associated to high stability Titanium in-diffusion process for the optical waveguide. Starting from an initial modulator configuration exhibiting 9 V drive voltage @ 30 GHz, a complete redesign of the coplanar microwave electrodes was carried out in order to reach a 6 V drive voltage @ 30GHz version. This redesign was associated to an optimization of the interaction between the optical waveguide and the electrodes.

Following the optimisation steps, an evaluation program was applied on a lot of 8 identical modulators. A full characterisation was carried out to compare performances, showing small variations between the initial and final functional characteristics. In parallel, two similar modulators were submitted to both gamma (10-100 krad) and proton irradiation (10.10⁹ p/cm²) with minor performance degradation.

II.

OPTICAL MODULATOR APPLICATIONS AND REQUIREMENTS

During past years, innovative payload concepts based on photonic technologies were investigated by Thales Alenia Space [10-11]. Several architectures were elaborated covering different application cases including (but not limited to) flexible analogue repeaters based on a photonic centre section and photonic receiver front-ends for advanced antennas allowing digital beam forming. Such architectures rely on photonic subsystems able to assist, complement, replace and/or extend the capabilities of conventional RF subsystems including:

In this context, optical modulators play a key role in the implementation of such functions.

For instance, the optical distribution of LO’s requires to produce a microwave LO signal, under optical form, with low phase noise and optical power high enough to be deliveredto a large number of receivers while meeting system requirements. The transfer of a high-frequency signal onto an optical carrier through direct modulation of the laser current is not applicable at high frequencies and requiresexternal electro-optical intensity modulators. In particular, optical double sideband modulation with carrier suppression (DSB-CS) is a LO generation technique making use of a high-power CW laser and a MZ electro-optical modulator (EOM) biased at minimum optical transmission, as shown in Figure 1. When the modulator is driven by a microwave signal at ω_LO/2 frequency, the optical output signal mainly contains the first two modulation side-bands. Optical heterodyning at the photo receiver generates a microwave signal at the ω_LO frequency.

Fig. 1.

Microwave photonic Local Oscillator (LO) based on DSB-CS Schematic

Photonic RF frequency mixing for both up and down conversion of microwave signals can be achieved optically by means of EOMs. The sinusoidal transfer function of MZ modulators make them attractive as mixers. In addition to a very wide bandwidth and an infinite LO to RF input port isolation, a remarkable feature of electro-optical mixing is its ability to perform simultaneously multiple frequency-conversions. In this concept, shown in Figure 2, the optical mixer is fed by several optical LO’s through wavelength-division-multiplexing (WDM). In this way, the RF signal at the ω_RF frequency driving the modulator is mixed to the different LO’s and frequency-converted to several signals at various ω_IF frequencies. The most critical requirements for the EOM in previously depicted applications are low optical loss, high dynamic extinction ratio, high optical power handling capability and low RF half-wave voltage. More exhaustively, an efficient optical modulator shall, in particular:

Fig. 2.

Optical Multi-Frequency Conversion concept

The Table 1 hereafter summarizes typical specifications for optical intensity modulators used in these applications.

Table 1:

Typical modulator specifications

(Note 1) Peak-to-peak RF voltage (or power) for 100% modulation at 30 GHz, i.e. half-wave voltage for Mach-Zehnder modulator.

(Note 2) Carrier-to-Intermodulation ratio, with 2-tone @ 30 GHz, and power per tone 25 dB below RF drive power

(Note 3) Polarisation-maintaining fibre / Polarisation-maintaining fibre or Single-mode fibre

It is practical in microwave analogue applications to use the following figure of merit (FoM) :

Where α_dB is the optical insertion loss (dB), and V_πRF is the effective half-wave voltage (V) at the operating frequency. One should notice that the RF gain of a non-amplified link is directly proportional to this FoM, and that the RF noise figure is inversely proportional to this FoM. The factor of merit described here will be a guideline in the following description. We describe in the next paragraphs the MZ modulator.

III.

NEW MODULATOR DESIGN

A previous assessment of different modulator technologies highlighted the suitability of LN modulator as a mature and promising technology for space applications [9].

Figure 3 shows a conventional broadband modulator. It is constructed from a Mach-Zehnder waveguide integrated in a LN substrate. Depending on the aimed performances, two crystals orientations are possible. In many cases, for thermal stability reasons, the crystal orientation is X-cut. The reason is that the Z-cut orientation is sensitive to pyroelectric effects requiring thus special antistatic layers in the fabrication to avoid failures due to temperature variations. Single drive Z-cut modulators are also characterized by chirp which can be a drawback for DSB-CS modulation.

Fig. 3.

Microwave electro-optic Mach-Zehnder modulator

Two technologies can be used for the waveguide fabrication. However, the technology called Titanium indiffusion waveguide (TI-WG) is preferred to proton exchange for thermal stability reasons. For an amplitude or intensity modulator, the design of a waveguide is the Mach-Zehnder configuration with one input waveguide, a Y-branch section for spatial separation of the input beam in two waves, two parallel waveguide arms, a second Y-branch section and an output waveguide playing the role of modal filter.

Coplanar waveguide microwave electrodes are patterned in order to allow the electro-optic interaction between the electric signal and the optical light field propagating into the waveguide. A silica buffer layer is introduced between the electrodes and the waveguide in order to achieve the 50 Ω characteristic impedance. The thickness of the buffer is selected in order to reach the phase matching condition, i.e. the microwave effective index is equal or very close to the optical effective index. Moreover, the thickness of the gold electrodes is adjusted in order to reduce the conductor loss. Finally, the gap between the central line and the lateral ground plane is designed to optimize the driving voltage.

In order to reach the targeted performances required in microwave photonic payloads, described in section I, it was necesary to improve the initial version of the Photline broadband analog modulator. This initial version exhibited a half-wave voltage in excess of 9 V at 30 GHz and the RF and DC electrodes were separated. Following multiple microwave and optical simulations, a new version was designed. One single RF electrode allowed driving both the RF and DC voltage via a dedicated bias tee termination. The CPW electrodes and optical waveguide designs were deeply remodeled in order to ensure an effective driving voltage reduced by 30% while keeping a minimal footprint. The housing was also improved in order to reach a smaller form factor (85 mm long packaging while previous version was 100 mm long).

To validate the new modulator design, prototypes were submitted to a functional test campaign and thermal tests were carried out to assess their stability. The measured low frequency half-wave voltage on the RF electrodes was typically 4.1 V. The S₂₁ electro-optic response at 30 GHz in the Ka band was -4 dB. The corresponding effective half-wave voltage of 6.5 V is thus expected to reach the aim target of 6-7 V.

The X-Cut and TI-WG devices showed optoelectronic results in compliance with the original target. Their high stability under temperature variations was also confirmed. The devices were hermetically sealed and submitted to 6 steps of temperature ranging from -30°C, -20°C, 0°C, 20°C, 40°C & 70°C. Each steps lasted 120 minutes. The insertion loss (IL) variations (Figure 4, blue line) and the bias voltage drift (Figure 5, blue line) were recorded and plotted for each of them, while the temperature variations are represented by a red dashed line.

Fig.4 :

Insertion loss variation (blue line) during temperature steps from -30°C up to +70°C

Fig.5 :

DC bias drift (blue line) with temperature variations, from -30°C up to +70°C

The tested modulators exhibited a very high bias voltage stability over the whole temperature range, with peak to peak drift of 3 V for the worst case (1 V in the case of Fig. 5) while at stable temperature, the drift was very low. In all cases, the insertion loss remained stable (< 0.5 dB) over the whole temperature range.

Based on this optical modulator prototype dedicated to microwave photonic payloads, an evaluation based on space applications consideration was launched. Next paragraphs describe the adopted test plan.

IV.

TEST PLAN

In order to fully assess the suitability of this new modulator design for the previously mentioned space applications, 8 identical samples were fabricated by Photline and evaluated through a functional and environmental test campaign. They were screened and then submitted to an initial characterisation. Two additional samples (#1946-12 & #1946-14) of similar technology (X-cut, Ti-WG) were used for irradiation (gamma & proton) tests.

The test plan sequence for the different samples is shown in table 2, and it can be summarised as follows:

Table 2.

Test plan sequence and samples distribution (T: test, R: reference)

The initial and final characterisations were carried out by Thales Alenia Space, while intermediate characterisations were carried out by Alter Technology in collaboration with Universidad Politécnica de Madrid.

The measured characteristics were the following:

In next paragraphs, we report the results obtained on the different samples after each tests and we will check the main drift thus obtained. We focus in particular on the variations of the insertion loss and of the effective half wave drive voltage at 30 GHz, in order to conclude on the stability of the aimed FoM at the end of this space assessment.

V.

TEST RESULTS

B.

Overall test results

Fig. 6 shows the variations of the key parameters (Insertion loss IL and drive voltage Vπ_RF) measured after each main steps (mechanical shocks, sine vibrations, random vibration, thermal vacuum, endurance) with all intermediate characterisations.

Fig. 6:

Insertion loss IL (a) and drive voltage Vπ_RF (b) drifts during tests achievements (red line: reference modulator)

During the environmental test campaign no deleterious effect was observed. In particular, thermal and mechanical tests were successfully passed by the modulators tested: no dramatic variations of the main parameters (IL & Vπ_RF) were measured. However, thermal vacuum tests highlighted different behaviours among modulators submitted to this test possibly due to slight differences in the design. All samples were fully operational after the complete mechanical series, with very good stability on the DC Vπ and RF Vπ @30 GHz. Slight variation of IL parameter after random vibration could be observed on some samples.

Following the environmental tests, a final functional test campaign was performed in order to evaluate the variations of the FoM defined in (1) between the beginning and the end of the full test campaign. The same parameters as during the initial functional test were measured. Fig. 7 summarises the FoM variations of the samples on a chart taking into account both critical parameters (IL and Vπ_RF).

Fig. 7:

Figure of merit of the samples before (blue circles) and after (red circles) the tests campaign.

VI.

CONCLUSION

The design of the modulators allowed reaching low insertion loss (2.6 dB at best) concurrently with relatively low driving voltage (6V). Thus, the reached figure of merit meets the requirements of photonic microwave payload in particular LO generation technique. The prototypes were submitted to a space assessment. The final characterisation showed no significant drift, in particular relative to insertion loss. The average insertion loss variation was found to be -0.12 dB while the average driving voltage drift was found to be -0.125 V when comparing the initial and final characterisations performed at 30 GHz.

This evaluation carried out on new multi GigaHertz optical modulators, anticipates a full space qualification process. The low drift of physical parameters after irradiation, vacuum thermal cycling in particular confirms the suitability of this technology of optical modulators to operate in space with a proven level of reliability within future microwave photonic payload.