Multi-Gigahertz Optical Modulator of Very Low RF Driving Power

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 The project objectives were as follows:
  •  To assess through testing commercial optical intensity modulators for microwave photonic applications, as well as the critical design issues, for use in space applications and operation in space environment.
  •  To develop a modulator with improved performances, particularly in term of RF half-wave voltage, able to withstand environmental tests.
  • An accurate map of the market, providing some insights on the strengths and weaknesses of each modulator product was drawn, based on an exhaustive review of the component technologies and manufacturers and a full functional and environmental test campaign carried out on a set of selected modulator products from different manufacturers and relying on different technologies
  • Different applications were reviewed making use of optical modulators as a key components and requirements were derived.
  • Several designs of lithium niobate modulators relying on different technological options were investigated, and each prototype manufactured was submitted to functional tests to assess its performances and thermal tests to evaluate its stability.
  • An exhaustive test campaign, including functional and environmental tests was carried out on a set of samples showing that the new modulator exhibits both improved performances and environmental conditions withstanding capabilities
  • Optical modulators exhibiting a RF half-wave voltage close to 6 Volts with optical insertion losses below 3 dB are now procurable from Photline. The modulators developed in this activity have proven their ability to pass all the environmental tests.
Photonic technologies are considered as a critical technology enabling the practical implementation of advanced payload concepts. In this context, optical modulators are a key building block for future photonic payloads. Their performances strongly impact the design of such systems.
The main expected benefits of this project are :
  • An improvement of the modulator performances
  •  A relaxation of the requirements on other sections of the payloads 
And more generally :
  • The availability of components meeting space and application requirements
  • A step torward space-qualifiable components 
Beyond the scope of space applications, the developments carried out in the frame of this activity strongly impacted Photline products.

Optical modulator is a key building block for future photonic payload. This activity provided the opportunity to evaluate optical modulator products and technologies for space applications, select the best technology/manufacturer to develop a new modulator with improved performances and to test the samples manufactured.

The project was divided in two phases :

Phase 1 aimed at assessing through testing commercial optical intensity modulators for microwave photonic applications, as well as the critical design issues, for use in space applications and operation in space environment.

Phase 2 aimed at developing a modulator with improved performances, particularly in term of RF half-wave voltage, able to withstand environmental tests. The design, development and manufacturing of samples of this new modulator have been carried out by the manufacturer selected on the basis of results obtained in Phase 1. 

Phase 1
As a first step towards the definition of the suitability of a component, the main photonic payload sub-system applications were identified and the overall system-level requirements were derived.
Then, an exhaustive survey of the market was carried out at the beginning of the study. A large number of optical modulator manufacturers and suppliers were identified including well-established and global suppliers of optical and optoelectronic components but also a number of SME’s manufacturing and commercializing optical modulators.
Based on the targeted use of these components in future photonic payloads and to set-up an objective metric to comparatively measure the performances of the different components, a specific figure of merit (FoM) was introduced as one of the criteria used to evaluate the different modulator technologies and products.
On the basis of several criteria, including this FoM, an initial comparative assessment of optical modulator technologies in terms of performances and merits was performed and a selection of optical modulators was carried out. 14 components relying on different technologies and from several suppliers were finally procured. 
An exhaustive evaluation followed, including functional and environmental (thermal, mechanical, thermal vacuum, radiations, fiber pulling) tests to assess both performances and ability to withstand severe environmental conditions of the procured components.


Based on the results of the test campaign, the recommended way to go for achieving improved performance, space-qualifiable modulators was to proceed with the development of new LN electro-optic modulators.
PHOTLINE was finally selected to develop an optical intensity modulator with improved performances in the frame of Phase 2.
Phase 2 
Several configurations of lithium niobate modulators have been studiedincluding modifications progressively introduced until reaching the aimed factor of merit (FoM) as well as a high thermal stability. The different options investigated were mainly based on Z-cut and X-cut architectures, Proton Exchange and Titanium indiffusion processes as well as specific designs of electrodes and optical waveguides.
Each option was manufactured and prototypes were all submitted to a test campaign including functional performance assessment as well as a stability assessment under thermal variations in order to select the best design.
All modulator developments performed contributed to a better understanding and knowledge of the different technologies available, relying on Lithium Niobate, as well as their behaviour, especially under thermal variations. The completion of the exploration of the concepts and technology available by Photline provided the opportunity to choose the best candidate for further assessment in a full test campaign.
Ten samples of the selected modulator option were finally manufactured by Photline and submitted to a full test campaign including functional and environmental tests.

Finally, the modulators developed by PHOTLINE in the frame of the project exhibit significant improvements of the performances: in particular the VπRF has been lessened by 30% to 40% (6 - 6.5 Volts) while keeping the optical insertion losses in the range of 3 dB. Also these modulators proved their ability to withstand severe environmental conditions (temperature, 
mechanical shocks and vibrations, thermal vacuum.)."

Current status

 The study is completed