For science, by science, he emergence and circulation of conflict of interest as a protest

Introduction

Nowadays, the condemnations of the harmful effects of pesticides on human health and the environment are a highly visible political cause in France. Many actors drive this cause, including environmental activists, victims' associations, investigative journalists, law firms, and concerned scientists. Over the past decade, throughout a series of controversies involving numerous substances (neonicotinoids, chlordecone, glyphosate, or succinate dehydrogenase inhibitors), they have criticized the lack of objectivity and the industry ties of the institutions in charge of pesticide assessment at both the national level (ANSES, French Agency for Food, Environmental, and Occupational Health and Safety) and the European level (EFSA, European Food Safety Authority). To denounce these phenomena, they often use the term "conflict of interest" (COI), which has become a central element of their protest repertoire.

This way of framing opposition to pesticides is relatively new in terms of the political history of these substances. French farmers used pesticides on a massive scale starting in the second half of the 20th century. From the 1960s, pesticides were part of the effort to modernize agriculture and, as such, received strong support from public authorities. Very soon after their widespread diffusion, these substances were denounced by unions and NGOs who criticized their effects on health and the environment, and the industrialization of agriculture in general. However, activists rarely brought up the pro-industry bias present in the pesticide risk assessment process and in pesticide regulation. The absence of this theme in their mobilization efforts is surprising when we recall that at that time it played a much bigger role in other countries. In the United States, especially, where agriculture already relied even more heavily on synthetic inputs, activists often expressed their doubt that regulatory authorities could act objectively without bending to pressure from powerful corporate interests. This transatlantic time difference invites us to examine more closely how the framework used to denounce pesticides evolved over time and circulated in different social contexts. This chapter explores the social conditions that led to the issue of industry influence on pesticide regulation becoming part of anti-pesticide activists' arsenal in France. We highlight three social processes that contributed to the emergence and diffusion of this issue: the institutionalization of risk assessment, the development of investigative environmental journalism, and the professionalization of environmental health advocacy organizations. All three have contributed to the successful framing of the fight against pesticides in terms of COIs. Within this framing, anti-pesticide movements in the United States and in France criticize the way scientific data are produced and used as part of marketing authorization procedures for these products. By doing so, they are helping to change the way pesticides are regulated, but they are also indirectly reinforcing the idea that the best way to control pesticides is to always rely on more science. This chapter is based on a survey conducted in France and the United States, using interviews with various people involved in the controversies surrounding pesticides (activists, researchers, lawyers, risk assessment professionals, victims' groups), as well as archival documents, most of which have been published.

Denouncing COIs in the fight against pesticides: The emergence of the pesticide industry's influence as a prominent issue

As several scholars have shown, the concept of COI emerged decades ago and has been used to describe different issues over time [START_REF] Parascandola | A turning point for conflicts of interest: the controversy over the national academy of sciences' first conflicts of interest disclosure policy[END_REF] Hauray, Chapter 1). Among anti-pesticide activists, this term is often used in a rather broad way to refer to the influence of firms on the regulation of pesticides and their capacity, in particular, to shape the production of scientific knowledge and expertise. This use emerged in the 1970s in the United States, where multiple consumer and environmental activist movements started to condemn pesticide-producing firms' ability to sell dangerous products, and their capacity for influencing risk assessments. These issues would spread internationally only gradually, becoming central in France 30 years later.

The US: The crucible of the denunciation of the pesticide industry's influence on regulation Pesticides began to be used intensively in the United States at the beginning of the 20th century, encouraged by a coalition of actors working to promote more productive agricultural practices: the US Department of Agriculture (USDA), as well as a host of farmers' unions and researchers in the fields of agronomics and entomology [START_REF] Whorton | Before Silent Spring: pesticides and public health in pre-DDT America[END_REF]. The USDA was put in charge of approving these products by the Federal Insecticide Act of 1910, which banned the sale of pesticides whose plant protection properties differed from what was advertised on their label. In 1947, the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) replaced this law and made it compulsory for industrial firms looking to put a new pesticide on the market to secure a preliminary license from the USDA. These licenses were based on an assessment of the products' health risks for exposed populations and for wild plants and animals.

As the work of various historians and political scientists [START_REF] Dunlap | DDT: scientists, citizens, and public policy[END_REF][START_REF] Daniel | Toxic drift: pesticides and health in the post-World War II south[END_REF] has shown, the birth of the anti-pesticide movement in the United States paralleled the adoption of these early legislations. At first, this movement was made up mostly of agriculture experts and garnered little attention. It began to receive more publicity in the 1960s, at a moment sometimes described as the "toxicity crisis" [START_REF] Vogel | Is it safe? BPA and the struggle to define the safety of chemicals[END_REF][START_REF] Boudia | Powerless science? Science and politics in a toxic world[END_REF]. At the time, more and more controversies were coming to light about the health effects of the increasing presence of dangerous substances and technologies in the environment, with pesticides among these. In this respect, the publication of Rachel Carson's book, Silent Spring (1962), marked a turning point. It brought worrying information to light about the harmful environmental and health effects of the large-scale use of pesticides in food production and in the environment in general. Carson focuses on one pesticide in particular, dichlorodiphenyltrichloroethane (DDT). Although she mainly discusses its harmful effects rather than the strategies used by the companies that produced it to influence regulation, in a few passages she refers to this wider issue. Indeed, she criticizes the chemical industry's influence on university agriculture laboratories and on the USDA. To describe this influence, she does not use the term "conflict of interest" but talks about the "biases" in favor of industry displayed by many scientists assessing the harmfulness of these products.

Such themes went on to become central to the political debates that followed the book's release. A bestseller, Silent Spring received significant television coverage [START_REF] Kroll | The "silent springs" of Rachel Carson: mass media and the origins of modern environmentalism[END_REF], leading President Kennedy to assert his position on the matter. He claimed to stand with Carson, and in 1963 he created a special panel of the President's Science Advisory Committee to study the issue of pesticides. Congress also held a series of hearings, with the purpose of debating the conditions of the sale of pesticides and their risk assessment. The USDA received regular criticism, especially for its clear unwillingness to share the information in its possession about the harmful effects of pesticides with other administrative bodies [START_REF] Bosso | Pesticides and politics. The life cycle of a public issue[END_REF]. At the end of the decade, a report from the Government Accountability Office sharply criticized the way the USDA had implemented the FIFRA, as well as its inability to effectively limit the environmental and health impacts of pesticides. In 1970, a new administration was created to assess pesticide risks and to authorize their sale in the place of the USDA: the Environmental Protection Agency (EPA). This was followed in 1972 by an overhaul of the FIFRA legislation, on the initiative of President Nixon. These institutional responses aimed to make pesticide risk assessment more objective, a clear response to the publication of Silent Spring and its political aftermath.

Questioning the influence that agricultural economic interests had on the institutions charged with regulating plant protection industry became a central issue for the actors involved in the fight against pesticides. Some of these actors were agricultural scientists who had long been engaged in denouncing the negative effects of these substances in particular. Among these was Robert Van den Bosch, a professional entomologist who had testified during the first major hearings against DDT in the late 1960s and early 1970s. In The Pesticide Conspiracy, a book published in 1978, he uses the term "pesticide mafia" to refer to the chemical industry's influence on the USDA and on entomological societies. Other players in the rapidly developing environmental movement also took an interest in this issue from the late 1960s. This included the Environmental Defense Fund (EDF), an association founded in 1967 to push for a ban on DDT [START_REF] Dunlap | DDT: scientists, citizens, and public policy[END_REF]. The EDF gradually expanded its field of action, and in the 1970s moved to block sale authorizations for certain organochlorine pesticides, including Aldrin and Dieldrin, on the grounds of alleged large-scale fraud that took place in some product risk assessments [START_REF] Gillespie | Carcinogenic risk assessment in the United States and Great Britain: the case of Aldrin/Dieldrin[END_REF]. Ralph Nader's Center for Study of Responsive Law, another key actor involved in the movement for tighter regulations on chemical products in the United States, raised similar criticisms at that time. In 1972, it published a well-researched and widely read report entitled Sowing the Wind: Food Safety and the Chemical Harvest, which took a critical look at industry influence on food and pesticide regulations. The report uses the term "conflicts of interest" to indicate a lack of neutrality in the laboratories hired by industry firms to conduct testing to secure authorization for their pesticides (Wellford, 1972, p. 351). It revealed that one of the leading laboratories that provided such services to chemical firms had submitted intentionally falsified toxicity data to public health authorities to hide internal organizational issues and to ensure client loyalty. This led to a lawsuit that lasted until 1983, as well as to congressional hearings that would play a central role in the development of EPA guidelines for producing and collecting data about pesticide toxicity [START_REF] Jasanoff | The fifth branch: science advisers as policymakers[END_REF].

Throughout the 1980s, criticisms of corporate influence on the pesticide risk assessment process in the United States evolved in a context where the institutions in charge of regulating these products were growing weaker. The presidencies of Ronald Reagan and George H. W. Bush saw fewer resources invested in environmental protection [START_REF] Fredrickson | History of US presidential assaults on modern environmental health protection[END_REF]. The result was that social movements grew disenchanted with the idea that hazardous substances could be properly controlled by making risk assessment bodies stronger. While private research funding was on the rise 1 these movements shifted their focus further upstream, to the production of the scientific data used in pesticide regulation, with the help of public health scientists. Several prominent figures got involved in organizations that opposed the negative environmental and health impacts of industrial activities. For example, Dr. Samuel Epstein, a researcher whose political importance has already been highlighted [START_REF] Paehlke | Environnementalisme et syndicalisme au Canada anglais et aux États-Unis[END_REF][START_REF] Proctor | Cancer wars: how politics shapes what we know and don't know about cancer[END_REF], was directly involved in publicizing the dangers of pesticides. He served as an advisor to the Center for Study of Responsive Law for the above-mentioned Sowing the Wind report. More broadly, his own work was influential in drawing anti-pesticide activists' attention to the many dimensions of the COIs that influence pesticide regulation. Epstein used the concept of COI several times in his important and widely read work The Politics of Cancer (1978). In this book, he stressed the bias of USDA policies in favor of the interests of industrial agriculture organizations and denounced instances of fraud in laboratories involved in toxicological risk assessment. At the end of the 1970s, as part of a lawsuit, he obtained internal documents from asbestos-producing firms that provided him with a clearer understanding of firms' strategies with regard to science both within and beyond the asbestos industry [START_REF] Epstein | The politics of cancer[END_REF][START_REF] Epstein | The politics of cancer[END_REF]. In the years that followed, other documents revealed that these strategies were used with several toxic products. In the 1980s in particular, archives obtained during lawsuits against cigarette manufacturers 2 allowed journalists, activists, and researchers in public health 3 and the social sciences to understand the details of these firms' strategies for influencing scientific data. Activists drew on these revelations to broaden their criticisms against the pesticide industry's influence on regulation. They increasingly used the concept of COI to denounce the multifaceted industry strategies to leverage pesticides regulation and the production of scientific data itself.

In the 1990s, this broader framing of industry influence on the regulation of toxic substances led to debates within the environmental and public health scientific community about the declaration of interest policies. Such policies, which started to be implemented in biomedical journals in the 1980s (see Hauray, Chapter 1), were gradually adopted in public health, environment, and occupational health and safety journals, before becoming standard for all scientific disciplines [START_REF] Resnik | Conflict of interest and funding disclosure policies of environmental, occupational, and public health journals[END_REF][START_REF] Daou | Public health journals' requirements for authors to disclose funding and conflicts of interest: a cross-sectional study[END_REF]. One of the first journals in this field to implement a mandatory policy of disclosing financial COIs was the American Journal of Industrial Medicine, in 1994, at the time headed by Philip J. Landrigan, a researcher who had worked on childhood exposure to pesticides. In 2003, Toxicological Sciences, the journal of the Society of Toxicology, also decided to adopt a disclosure policy (Lehman- [START_REF] Lehman-Mckeeman | Guidelines governing conflict of interest[END_REF], followed in 2005 by the leading journal Epidemiology, which had long been hostile to COI declarations. The systematic adoption of disclosure policies helped environmental activists to intensify their criticisms of industry influence on toxic substance regulations in general, and pesticide regulation in particular. Indeed, it has provided them with information to substantiate, in books or reports, the hypothesis of a proindustry bias in pesticide risk assessment (see, for example, [START_REF] Fagin | Toxic deception. How the chemical industry manip-ulates science, bends the law and endangers your health[END_REF][START_REF] Melnick | Testing toxic pesticides in humans: health risks with no health benefits[END_REF][START_REF] Sass | Industry testing of toxic pesticides on human subjects concluded "no effect", despite the evidence[END_REF].

The late importation and circulation of COIs as a campaigning issue in France

French agriculture started to use synthetic pesticides at almost the same time as the United States, during the decades immediately after the Second World War. Along with mechanization, greater field sizes and the use of synthetic fertilizers constituted one of the pillars of French agricultural development policies [START_REF] Fourche | Contribution à l'histoire de la protection phytosanitaire dans l'agriculture française (1880-1970[END_REF]. However, the denunciation of corporate influence and COIs in pesticide regulation only gained prominence within anti-pesticide activists' repertoire at the end of the 20th century.

The first wave of public interest in the harmful effects of pesticides came after the publication of Printemps silencieux, the French translation of Silent Spring, in 1963, one year after the original edition was published in the United States. Roger Heim, a prominent biologist, well-known in the media for criticizing the environmental damage of technological progress, wrote the book's preface. In the 1968 edition, he stressed the responsibilities of pesticide producers and regulators:

We arrest gangsters, we shoot at hold-up men, we guillotine assassins, we execute despots -or alleged despots -but who will jail the public poisoners who distribute every day the products that synthetic chemistry provides for their profit and their recklessness? [START_REF] Heim | Préface[END_REF] This quote, however, does not accurately capture the controversies that surrounded the book's publication. These controversies focused essentially on whether or not the book's analysis on the dangerousness of pesticides was well-founded. On these grounds, Printemps silencieux's conclusions on the dangers of DDT were attacked by researchers from the National Institute of Agricultural Research (INRA), France's leading agronomic research institution, as well as by representatives of the French Ministry of Agriculture, who were in charge of authorizing pesticides for sale [START_REF] Fourche | Contribution à l'histoire de la protection phytosanitaire dans l'agriculture française (1880-1970[END_REF][START_REF] Jas | Public health and pesticide regulation in France before and after Silent Spring[END_REF]. The media also heavily discussed the book and received it with skepticism, "revealing the overwhelming trust in science to be found in the press at that time" [START_REF] Trespeuch-Berthelot | La réception des ouvrages d'alerte environnementale dans les médias français (1948-1973)[END_REF]. Conversely, the book received a more enthusiastic reception among several activists' organizations, although interest in pesticides in general, and especially in industry influence on pesticide regulation, remained peripheral among these organizations.

For instance, the French organizations promoting organic farming, very active in the 1960s, focused much more on the issues of land access policies or on the impact of the widespread diffusion of mineral fertilizers on soils than on the use of pesticides [START_REF] Pessis | Défendre la terre. Scientifiques critiques et mobilisations environnementales des années 1940 aux années 1970[END_REF]. In the 1970s, a large "farmerlaborer" union movement was created to denounce the consequences of the socalled modernization policies promoted by the state and the majority farmers' union. This movement criticized the social consequences of industrialized agriculture and its dependence on pesticides. However, for this movement, the impact of pesticides on health and the environment, and industry influence on their regulation, were rarely considered a full-fledged issue. Instead, they were considered a one of the many examples of industrial capitalism's nefarious grip on agriculture and of farmers' growing dependence on technological progress [START_REF] Martin | Histoire de la nouvelle gauche paysanne[END_REF]2015;[START_REF] Pessis | Défendre la terre. Scientifiques critiques et mobilisations environnementales des années 1940 aux années 1970[END_REF].

French activist organizations were not completely unaware of the US works and campaigns denouncing the influence of industry on pesticide regulation. They circulated among some of the environmental organizations that emerged in France in the 1960s and 1970s. For example, the newspaper La Gueule ouverte, which played an important role in facilitating the emergence of a left-wing libertarian environmental movement in France between 1972 and 1980 [START_REF] Vrignon | Journalistes et militants. Les périodiques écologistes dans les années 1970[END_REF], mentioned one of Ralph Nader's lectures in its first issue. Nader's work was also known to the nascent French consumer movements (Lepiller, 2012, pp. 359-360). However, it was not until the late 1990s that the agrochemical industry's influence on pesticide regulation became central in the framing of opposition to pesticides.

At that time, several environmental health organizations, sometimes focused mainly on the fight against pesticides, began to dedicate important resources to denouncing the agrochemical industries' leverage on risk assessment agencies and on the production of scientific data on pesticide hazards. Several books that have highlighted and encouraged this shift in the protest repertoires of pesticide opponents have been published since then by NGO directors, scientists, journalists, and national political figures (see Box 10.1). Some achieved a great deal of commercial success and were accompanied by documentary films or television programs.

These books and their success indicate changing attitudes toward science and technology. Indeed, the groups that were most involved in denouncing pesticides in the 1970s mostly fought against techno-scientific progress in general, and its grip on farming practices [START_REF] Bécot | Improbables mais fécondes: les rencontres entre scientifiques critiques et syndicalistes dans les "années 1968[END_REF]. In contrast, the organizations that have been working on pesticides since the 1990s often express their faith in science's ability to provide objective risk assessments, as long as it is purged of the biases created by COIs between institutions and researchers on one side, and industry firms on the other. These books also illustrate the influence of US activists on French protest movements. In fact, most of the French authors mentioned above (see Box 10.1) cite US publications, including journalistic and activist writings about industry strategies to influence public health policy, beyond the sole issue of pesticide risks. In particular, they very often cite Robert Proctor's work on the tobacco industry (2011), David Michaels' work on chemical firms (2007), and the work by Naomie Oreskes and Erik Conway on energy producers (2010). They also sometimes directly discuss their personal relationships with scientists who are speaking out against the COIs that bias industry regulation in the United States. 4 Beyond such personal connections between French and US activists, major changes in the institutional and social context of French activism have supported this re-framing of the dangers of pesticides as an issue of industry influence and COIs.

A changing institutional and social context: Explaining the success of COIs as a protest repertoire

Putting the French situation into perspective with the US one, we identified three interrelated dynamics that have contributed to the rise of COIs as a prominent issue in the mobilizations against pesticides in France. These dynamics have involved pesticide regulation institutions, the media, and the field of environmental activism.

The institutionalization of risk assessment

In the United States, anti-pesticide organizations' focus on COIs was closely related to institutional changes in pesticide regulation. The new administrative bodies and regulatory assessment procedures put into place in the 1960s and 1970s to respond to concerns about the threats these substances posed to health and the environment created new expectations among social movements. They triggered a fresh round of activism, inclined to resort to legal action and attentive to the industry's influence on the regulation of toxic products. A similar phenomenon was observed in France, but 30 years later. French pesticide policies have historically been the responsibility of the Ministry of Agriculture. For decades, it was in charge of assessing and managing all of these products' environmental and health risks and was able to fulfill this task following its own rules and procedures. Starting in the 1980s, however, the ministry's monopoly on these issues was called into question. As the creation of the European single market launched a first stage in the harmonization of pesticide risk assessments among its Member States, national-level authorities found themselves with less room for maneuver. Adopted in 1991, Council Directive 91/414/EEC formalized the common rules that all Member States must follow when considering authorization for pesticide sale requests. At the end of the 1990s, after a series of public health scandals, new public health and safety administrative bodies were created to shape risk assessment policies, further eroding the Ministry of Agriculture's monopoly and autonomy over pesticide regulation.

In 1998, the French Food Safety Agency (AFSSA) was created in France, followed four years later by the European Food Safety Authority (EFSA) at the European Union (EU) level [START_REF] Demortain | Standards of scientific advice. Risk analysis and the formation of the European food safety authority[END_REF]. Pesticides gradually came under the purview of these "new technocratic bureaucracies" [START_REF] Benamouzig | Les agences, alternatives administratives ou nouvelles bureaucraties techniques? Le cas des agences sanitaires[END_REF]. In France, this change was supported in particular by beekeepers' organizations, who claimed that neonicotinoid pesticides posed a serious threat to domestic honeybees. They focused particular attention on the Ministry of Agriculture's ComTox ("Commission on Toxics"), which had been responsible for pesticide risk assessments in France since 1943. In the 1970s, this commission had formed a "bee group", in charge of creating a "bee label" required for the authorization of pesticides to be applied during foraging periods. The commission, however, included interests that would be subject to any new pesticide regulations, with industry firms and the consultants handling their authorization requests allowed to participate. 5 This participation was regularly criticized by beekeeper unions and their political allies. The ComTox's credibility collapse led to its dissolution in 2006, with its pesticide risk assessment responsibility transferred to the AFSSA [START_REF] Jouzel | La normalisation des alertes sanitaires. Le traitement administratif des données sur l'exposition des agriculteurs aux pesticides[END_REF][START_REF] Jouzel | Pesticides, comment ignorer ce que l'on sait[END_REF].

Just as in the United States in the 1970s, such an institutional dynamic had several important consequences on anti-pesticide activism. By creating specific risk assessment agencies, lawmakers sought to protect themselves from accusations of COI [START_REF] Boudia | La production d'un instrument générique de gouvernement. Le "livre rouge" de l'analyse des risques[END_REF]. However, in many ways, the creation of these new agencies had the opposite effect. Indeed, it generated expectations among social movements and activist organizations in terms of the independence and transparency of the pesticide regulation process. The formalization of pesticide assessment rules, concerning either the nature of scientific information to be taken into account or to what extent industry firms could participate in the process, gave them levers to criticize industry influence on pesticide regulation. The work of Corporate Europe Observatory (CEO) illustrates this link between institutional transformations and activism. This NGO was founded in 1999 to scrutinize the influence of corporations on European public policy. Starting in the 2010s, it launched several campaigns and investigations that specifically looked at COIs in pesticide regulation. These investigations posit that the EU agencies do not respect the rules of transparency and independence they purport to uphold. These rules not only created expectations among the NGO activists but also helped them to access information. For instance, Stéphane Horel, a French journalist who worked with CEO, describes in detail in her works how she used transparency procedures to access European institutions' administrative files on pesticides and other toxic substances [START_REF] Horel | Intoxication: perturbateurs endocriniens, lobbyistes et eurocrates: une bataille d'influence contre la santé[END_REF]. In 2011, CEO launched an initiative to analyze COIs at the EFSA. More recently, CEO partnered with the NGO Pesticides Actions Network (PAN) Europe to uncover upstream industry influence on the tools and protocols that are used to conduct regulatory science (PAN Europe, 2018).

This denunciation of industry influence on pesticide regulation by CEO and other activist organizations received heavy press coverage in France. This increased publicity can be explained by the long-term changes in the press and the media that indirectly encouraged activists to invest time and resources in exploring the issue of COIs.

From activist journalism to investigative journalism

In the United States, environmental investigative journalism developed in the 1970s, and was readily critical of the chemical industry [START_REF] Friedman | Alar and apples: newspapers, risk and media responsibility[END_REF][START_REF] Friedman | The changing face of environmental journalism in the United States[END_REF]. Stories about plant protection industry firms committing fraud on their commercial authorization requests became a regular feature of investigative journalism, both in activist newspapers with limited circulation (such as Mother Jones, published by the Center for Investigative Reporting) and in national publications. In France, it was only over the course of the next two decades, with the environment becoming a regular topic in the media [START_REF] Comby | Quand l'environnement devient "médiatique[END_REF] and investigative journalism beginning to develop [START_REF] Marchetti | Genèse et consecration d'une spécialité journalistique[END_REF], especially in the public health field [START_REF] Marchetti | Quand la santé devient médiatique[END_REF], that a similar journalistic treatment of information about industrial pollution emerged. Several journalists progressively covered this issue through the lens of COIs and the bias they introduce into risk assessment procedures. They highlighted that these are long-standing problems, and that scandals have significant moral, legal, and political ramifications. This treatment aligned with the prevailing ethos in professional journalism, allowing the journalists using it to base their criticisms on scientific authority, as well as helping them to not appear too close to environmental activist groups. It was also a way for journalists to set themselves apart in an increasingly competitive information market.

During the first decade of the 21st century, the issue of industry influence on pesticide regulation was initially taken up by independent journalists of a more investigatory bent, who tracked down COIs among scientists and administrators in charge of assessing these products' risks [START_REF] Jouzel | Des journalistes qui font les victimes? Le traitement médiatique des maladies professionnelles liées aux pesticides[END_REF]. More recently, this issue has begun to appear regularly in some mainstream newspapers. The way that the newspaper Le Monde has handled information about pesticides is a particularly interesting illustration of this shift. As the leading daily general-interest newspaper in France, Le Monde began to cover environmental issues in the 1970s, publishing its first dedicated "Environment" section in 1972. Up through the end of the 1990s, as in other national daily papers, coverage of these issues was intermittent, 6 peripheral, and handled by journalists who were often personally aligned with political ecology and environmental activist groups [START_REF] Comby | Quand l'environnement devient "médiatique[END_REF]. In the 2000s, under the direction of editor-in-chief Éric Fottorino, this coverage intensified. The "Planet" and "Science" departments beefed up their staff and hired new generalist and scientific journalists.

While their colleagues in more prestigious and established departments (International, Politics) sometimes suspected them of being political ecology "activists", they offered "critical expertise", combining a "refusal of political journalism with the ability to offer critique based on their technical knowledge of the issues at hand" (Neveu, 1999, p. 40), leading to more regular coverage of environmental issues. Stéphane Foucart was one of the journalists recruited during this period. He had a background in science and science journalism. Upon joining Le Monde in 2000 he initially covered technology, but later joined what would become the "Planet" department in 2009. Once in this department, he published several papers on the controversies surrounding climate change, the effects of neonicotinoids, and endocrine disruptors. While he did not specifically look at issues of COI in his early career, he came to write more and more on the topic as he became aware of industry firms' strategies for manipulating science. Many of his articles and books explore this theme, based on the work of French and US researchers, NGOs, and his own investigations. Foucart has published several articles about the impact of pesticides on health and the environment, where he questions the validity of regulatory agencies' toxicity tests. In 2018, along with Stéphane Horel, he received the European Press Prize's Investigative Reporting Award for a series of articles they wrote together using internal Monsanto documents obtained as part of a glyphosate lawsuit that was brought in California after the International Agency for Research on Cancer (IARC) classified the substance as a carcinogen in 2015. His work has been an important driver for communicating and legitimizing criticisms of pesticide regulation based on COIs in the French media. It has contributed to establishing a new way of discussing certain agricultural issues, which is not purely political or sectorbased (as one might see in the "Agriculture" section of newspapers), but rather is based on a "scientific" point of view that goes beyond popularizing science and expressing scientific concepts in layman's terms.

The professionalization of environmental health organizations

In the United States, the coalitions of activists and scientists dedicated to environmental health protection that formed in the 1970s played a central role in placing industry influence on pesticide regulation in the spotlight. Similar coalitions emerged in France, but later. From the 1980s onward, French environmental organizations became more professional and invested their resources heavily in protest actions based on technical expertise [START_REF] Ollitrault | Les écologistes français, des experts en action[END_REF]. Throughout the 2000s, a new sector formed in the non-profit activism field, made up of organizations that devoted resources and technical expertise to denouncing the harmful effects of environmental pollution on human health. Despite their diverse backgrounds, these organizations had something in common: they brought together activists who had a strong scientific social capital (researchers, doctors, science teachers).

They would base their claims on this capital, casting themselves as "whistleblowers" and drawing on the authority of science to condemn industry influence on the regulation of hazardous substances [START_REF] Guilleux | L'institutionnalisation de la santé environnementale en France. D'une approche globale homme/environnement à la sectorisation d'actions de santé publique[END_REF].

Within this sector, organizations have specialized in pesticides and played a prominent role. This was notably the case for the Mouvement pour la défense et le respect des générations futures (Movement for the Defense and Respect of Future Generations), founded by teacher and Greenpeace activist François Veillerette in 1996, and renamed Générations futures in 2011 [START_REF] Jouzel | Mettre en mouvement les agriculteurs victimes des pesticides. Émergence et évolution d'une coalition improbable[END_REF]. Its repertoire of contention includes science-based arguments and criticisms of COIs in the regulation and sale of pesticides. Over the last 20 years, Veillerette's editorial activity and the organization's alliances and campaigns have made it a force to be reckoned with. In 2002, with support from NGO PAN Europe (of which he became the administrator in 2003), he published Pesticides: le piège se referme, which popularized a lengthy review of toxicological and ecotoxicological literature. He followed this five years later with Pesticides: révélations sur un scandale français, co-written with Fabrice Nicolino, an environmental activist journalist who has written several books over the course of his career on the devastating effect of industrialized agriculture on the planet. This second book, which begins with the quote from Roger Heim's preface cited above, was the first fully documented critique of industry influence on pesticide regulation in France. It describes this influence at many levels: the personal relationships between industry firms and some representatives from the public authorities; the pressure from the Ministry of Agriculture and from industry lobbyists on expert agencies; and the financial ties between some scientists and industry firms. After selling tens of thousands of copies, it firmly established pro-industry bias in pesticide risk assessment as a serious issue for French organizations fighting against these products. Today, Générations futures continues to promote this issue, drawing on the work of journalists, activist organizations, and public health researchers, such as Annie Thébaud-Mony and André Cicolella. It also advocates for better procedures that would limit industry influence on pesticide regulation, as well as these companies' ability to "manufacture doubt".

Conclusion

This chapter describes three related dynamics -the institutionalization of risk assessment, the development of investigative environmental journalism, and the professionalization of environmental health advocacy organizationsthat contributed to establishing the issue of industry influence as a central element of the protest repertoire of the US and French activists engaged in the fight against pesticides. For them, COI is a central concept that includes not only interpersonal financial relations between public agents and industries, but more broadly the corporate leverage on public expertise and decision covering different forms (manipulation of science, revolving doors, and so on). The concept is instrumental in connecting disparate phenomena, bringing to light various "incidents" or "scandals", and revealing recurring malpractices and repeat offenders. It helps these activists to generalize their criticisms of pesticide regulation and link them to other products (genetically modified organisms, endocrine disruptors, and so on), revealing the systemic nature of industry leverage on the expertise and scientific data used in formal risk assessment.

The critical discourse surrounding COIs is part of a particular historical process that began in the United States. We thus shed light, more generally, on how protest movements and the institutionalization of pesticide risk assessment are intertwined. New risk assessment procedures and expert agencies were created to manage pesticides and to respond to criticisms of COIs, just as in other sectors. The outcome of these institutional changes, however, was quite different than expected. They did not put an end to the denunciation of COIs. Instead, they encouraged activist organizations to shift their target further upstream in the risk assessment process, and to focus on the production of scientific data itself. What happened in France echoed what happened in the United States decades earlier: as risk assessment procedures became more formalized, they only created expectations of neutrality that were never met, to the disappointment of pesticide opponents. This chapter stresses that these expectations are largely based on the idea that risk assessment should be informed by "robust" and "pure" science. By condemning COIs and industry pressure on the production of scientific knowledge about pesticides'hazards, those who oppose their use are calling, more or less explicitly, for more science and expertise to guarantee that the environment and exposed populations will be protected. Over time, one may wonder whether or not this framing might disconnect the critiques of pesticides from a more radical condemnation of the hold of the ideology of technological progress over farmers' knowledge and practices.

Notes

1. The Bayh-Dole Act was passed in 1980, allowing universities to retain intellectual property rights to any innovations they develop using public research money. The law led to a boom in public-private partnerships, sparking a debate about the consequences this could ultimately have for researchers' independence. 2. Cipollone vs. Liggett (1988) was the first case to provide significant access to industry documents subpoenaed by the courts. 3. For example, Stanton Glantz, a researcher at the University of California.

Sometimes called the "Ralph Nader of tobacco", Glantz has published several articles and has worked to make internal industry documents publicly available. 4. For example, influential journalist and activist Marie-Monique Robin describes how Devra Davis's work on industry influence inspired her own investigations. See https://blog.m2rfilms.com/la-fabrique-du-doute/#_ftn1 (last accessed December 23, 2020).

5. Of the 44 members of the Commission named in 2001 to serve in its penultimate term (of 3 years), there were 8 former industry representatives who had become consultants or directors of learned societies. It should be noted that this openness to economic interests went beyond industry representatives. For instance, the ComTox "bee group" was mostly made up of researchers who were also beekeepers themselves. 6. In 1982, for example, Le Monde replaced its dedicated "Environment" journalist with an editor who was only assigned to cover these issues part-time [START_REF] Bodt | La 'cité écologique' dans l'espace public médiatique: trajectoires de controverses environnementales dans la presse généraliste française[END_REF].


Keywords: acousto-optic devices, wavelength filtering devices, birefringence, polarization-selective devices

Equal-efficiency diffraction of unpolarized wideband light with acousto-optic filters

We present the diffraction of unpolarized light over a broad wavelength range using paratellurite Acousto-Optic Tunable Filters. The diffraction of unpolarized light usually relies on a specific operating point that is sensitive to the wavelength and angle of incidence, resulting in an operating ability over a narrow bandwidth. We present a detuning strategy to eliminate the angular dependence and obtain equal diffraction efficiency for both the linear polarization states over a broad bandwidth. We explain how to find the optimum operating frequency and present its dependency as a function of the wavelength. With the quasi-phase matching condition presented, the diffraction of unpolarized light over the 400 nm to700 nm band is possible with an efficiency of over 80%.

Introduction

Acousto-Optic (AO) devices have multiple uses including frequency shifters, modulators, deflectors, or filters [START_REF] Kaczmarek | Laser Doppler vibrometry with acoustooptic frequency shift[END_REF][START_REF] Voloshinov | Acousto-optical modulation of radiation with arbitrary polarization direction[END_REF][START_REF] Trypogeorgos | Precise shaping of laser light by an acousto-optic deflector[END_REF][START_REF] Li | Quasi-4D laser diagnostics using an acousto-optic deflector scanning system[END_REF][START_REF] Kb | Acousto-optic filters with arbitrary spectral transmission[END_REF]. Acousto-Optic Tunable Filters (AOTFs) are commonly used to measure the spectral composition of a light source [START_REF] Kozlova | Tunable transportable spectrometer based on an acousto-optical tunable filter: Developpement and optical performance[END_REF]. They can also split an unpolarized beam into two orthogonal linear polarized beams [START_REF] Kastelik | Bifrequency acousto-optic beam splitter[END_REF]. Knowledge and control of the polarization state of an optical beam are of crucial importance in applications such as sensing [START_REF] Caucheteur | Polarization-Assisted Fiber Bragg Grating Sensors:Tutorial and Review[END_REF][START_REF] Levenson | Polarization selective optical heterodyne detection for dramatically improved sensitivity in laser spectroscopy[END_REF][START_REF] Yan | General review of optical polarization remote sensing[END_REF] or laser beam modulation [START_REF] Antonov | Wide-Angle Polarization-Independent Paratellurite-Based Acousto-Optic Laser Radiation Modulator[END_REF]. When combined with an imaging system, the ability to analyse the polarization can provide important information about the object observed [START_REF] Voloshinov | Spectral and polarization analysis of optical images by means of acousto-optics[END_REF][START_REF] Sergey | AOTF transmission shaping for spectral polarimetric imaging[END_REF] and it is beneficial in various applications including biomedical analysis [START_REF] Obata | Imaging of a retinal pigment epithelium aperture using polarization-sensitive optical coherence tomography[END_REF], the military sector to detect targets and mines [START_REF] Cheng | AOTF polarimetric hyperspectral imaging for mine detection[END_REF], or environmental sciences and agriculture to provide information about plants and crops [START_REF] Dekemper | ALTIUS: a spaceborne AOTF-based UV-VIS-NIR hyperspectral imager for atmospheric remote sensing[END_REF][START_REF] Rondeaux | Polarization of light reflected by crop canopies[END_REF].

The diffraction of unpolarized light using AOTFs, which can be used to analyse linear polarization, is possible over a limited wavelength range as the diffraction efficiency is wavelength-sensitive at a given incident angle [START_REF] Kastelik | Bifrequency acousto-optic beam splitter[END_REF]. Indeed, in birefringent acousto-optic crystals, an unpolarized input beam is divided into two orthogonally polarized beams. The simultaneous interaction of these two beams with the same efficiency is possible at a given wavelength for one incident angle and one operating frequency. These two parameters constitute the "Double Diffraction" (DD) operating point. When the wavelength is changed, a new incident angle-interaction frequency pairing must be determined but when a polychromatic incident beam is analysed, maintaining a fixed incident angle can be necessary for practical reasons. Consequently, as the wavelength of interest moves away from the operating point wavelength corresponding to the incident angle chosen, the more the interaction efficiency of the two polarization states differs. Hence with polychromatic light, knowledge of the polarization state of the incident beam is normally only possible over a very limited wavelength band.

In this paper, we analyse theoretically the characteristics of simultaneous diffraction over a wide optical range when a fixed incident angle is considered. In this configuration, we determined the conditions to maintain equal diffraction efficiencies without the need for realignment. To reach this goal, we optimized the choice of the acoustic operating frequency and the wavelength, which determines the initial incident angle. First, we show that for each optical wavelength there is one acoustic frequency corresponding to interaction efficiencies that are equivalent for the two polarization states. We then show that equal-efficiency Double Diffraction of unpolarized wideband light is possible by carefully choosing the initial incident angle. The properties of paratellurite (TeO2) and the slow shear wave were taken into consideration in our calculations because numerous commercially available devices are based on this highly efficient configuration.

Simultaneous diffraction of unpolarized light

The acousto-optic diffraction efficiency can be very high under the Bragg phasematching condition, the latter being dependent on the incident angle, the acoustic frequency, and the optical wavelength. The efficiency depends on the nature of the crystal (material properties and device geometry) and adherence to phase-matching conditions. In practice, for a given AOTF design, the diffraction efficiency for an optical wavelength λ depends on the optical incident angle θi and the acoustic frequency f.

In birefringent crystals, an efficient anisotropic acousto-optic interaction is possible. In such crystals, two propagation modes exist that correspond to the wellknown "ordinary" and "extraordinary" rays. Two types of anisotropic diffraction can thus occur, one from the extraordinary to the ordinary ray (e→o) and the other from the ordinary to the extraordinary ray (o→e). From a theoretical point of view, one of the two diffraction types can be selected by adjusting the characteristics of the incident light: polarization state and direction of propagation (i.e. incident angle). When the incident optical wave is unpolarized, the two diffraction types generally occur at two different frequencies. Nevertheless, simultaneous diffraction of both linear polarization states with equal efficiency is possible at a very specific operating point (see [START_REF] Kastelik | Bifrequency acousto-optic beam splitter[END_REF] for a detailed description). In the latter case, there is "Double Diffraction" [START_REF] Yan | General review of optical polarization remote sensing[END_REF] with two diffracted light beams at the crystal output, as well as potentially a transmitted light beam (that fades out under appropriate acoustic power), as illustrated in figure 1. The two simultaneously diffracted beams are orthogonally polarized. The vector diagram corresponding to this special case is represented in figure 2. The diagram illustrates that Double Diffraction (DD) is possible for a unique acoustic vector K and a particular incident angle θi. K is proportional to the acoustic frequency and so the vector diagram can be used to determine the interaction parameter values.

Figure 3 depicts the evolution of the interaction frequency as a function of the incident angle for the two anisotropic diffraction cases for a slow wave in a TeO2 crystal. As can be seen, the (e→o) and (o→e) curves intersect at a single operating point specific to the wavelength λ considered. The coordinates of one of these points define the wavelength-sensitive DD point: (fDD(λ), θDD(λ)). At this operating point, the exact phase-matching condition (synchronism) is respected simultaneously for the two diffraction types and the efficiencies of both interactions are identical and maximum for the wavelength λ considered. From 400 nm to 700 nm, the DD frequency can evolve over an operating range of 86 MHz and the incident angle over 0.15°. From figure 3, it is clear that when the incident angle is no longer θDD(λ), the curves corresponding to (e→o) and (o→e) split from one another. Hence, the synchronism conditions for each interaction are obtained with two different frequencies. Similarly, when the frequency is no longer fDD(λ), the synchronism conditions for each interaction are obtained with two different incident angles. Finally, when the optical wavelength is changed, large variations in the efficiency are observed as the DD parameters are significantly different, as illustrated for three different wavelengths in figure 1.

Consequently, the case of Double Diffraction for one wavelength can only be achieved for one specific parameter pairing: the acoustic interaction frequency (proportional to K) and the incident angle [START_REF] Kastelik | Bifrequency acousto-optic beam splitter[END_REF]. As it is considered that the third parameter, the optical wavelength, depends on the characteristics of the incident light to be analysed, these characteristics cannot be tuned experimentally. However, when considering wideband diffraction, the variation in the refractive index, n, as a function of the wavelength has to be taken into account: the optical wave vector loci are inversely proportional to n. The Sellmeier formulas are introduced to deal with wavelength dependence [START_REF] Mccarthy | Optical rotatory power in crystals of the mercurous halides and tellurium dioxide[END_REF]: Figure 4 illustrates the vector diagram when the exact Bragg conditions are not met. In Figure 4a, an offset from the Bragg angle of incidence is depicted. As can be seen, a momentum mismatch, Δk, is introduced. In Figure 4b, Δk results from a change in the optical wavelength which modifies the wave vector loci. The momentum mismatch value is deduced from the wave vector loci and geometrical considerations.

Efficiency of simultaneous diffraction of unpolarized light

The analysis of the phase-matching conditions leads to an expression for the evolution of the diffraction efficiencies when the operating point departs from the exact Bragg conditions: the momentum mismatch Δk leads to a phase mismatch ΔΦ = w* Δk that affects the interaction efficiency. The diffraction efficiency η was evaluated using the well-known formula [START_REF] Kastelik | Bifrequency acousto-optic beam splitter[END_REF]: [START_REF] Kaczmarek | Laser Doppler vibrometry with acoustooptic frequency shift[END_REF] Where P is the power of the acoustic wave, P0 is the power required to obtain maximum efficiency, and ΔΦ is the phase mismatch parameter of the interaction. This parameter results from an offset from the phase-matching conditions of the acoustic frequency, the optical wavelength, or the incident angle. To analyse the consequences of the phase mismatch alone, we can choose P/P0=1.

Using the efficiency expression [START_REF] Kaczmarek | Laser Doppler vibrometry with acoustooptic frequency shift[END_REF], it is possible to determine the evolution of the efficiencies for the interactions (e→o) and (o→e) as a function of frequency, wavelength, or incident angle.

Figure 5 shows the evolution in efficiency as a function of the wavelength (5a.) and the acoustic frequency (5b). In both figures, the wavelength λ0 = 671 nm is considered. As can be seen, the efficiencies are simultaneously at their maximum for the two types of diffraction only at the DD point and decline rapidly when the conditions depart from synchronism.

Close inspection of Figure 5b highlights that the two types of diffraction have different radio frequency bandwidths with (o→e) being narrower than (e→o). Figure 2 explains the difference in bandwidth: the same variation of K will result in two different offsets from synchronism due to the difference in wave vector loci geometries. The difference in radio frequency bandwidth means that even a slight change in the acoustic frequency causes a relative change in the diffraction efficiencies.

As figures 5a and 5b illustrate the evolution in the efficiency when the interaction parameters are set around the DD point, it is clear that the Double Diffraction phase-matching is the only operating point that ensures the same efficiency for both the polarization states. Hence, the DD point is appropriate for linear polarization analysis when a single wavelength is considered.

Simultaneous diffraction of broadband unpolarized light with a fixed incident angle

When the wavelength of the light beam is changed, it is necessary to modify the frequency and the angle of incidence accordingly to recover the wavelength-sensitive DD point. Nevertheless, in some cases, for instance with a broadband optical source, it is not desirable to continuously change the incident angle while scanning the wavelength over the entire optical band.

We now consider that the incident angle for the entire band is set to the Double Diffraction value for one single wavelength, λ0, called the central wavelength. Once this angle θDD,λ0 is set, the double diffraction frequency fDD(λ) is no longer optimal for wavelengths λ different from λ0. Consequently, if the fDD determined for λ is applied, then the efficiencies of the two diffraction types will differ and the measured intensities of the two diffracted light beams will no longer reflect the relative proportion of the linear polarization states of the light source. Figure 6 illustrates this phenomenon. The evolution of the (e→o) and (o→e) diffraction efficiencies are plotted as a function of the wavelength in the vicinity of λ = 405 nm when the incident angle is chosen for double diffraction at λ0 = 671 nm and the acoustic frequency used is fDD (λ = 405 nm). In this case, and despite the choice of fDD, the two diffraction efficiencies at λ = 405 nm are significantly different (intersection of the two curves and the red line). However, the efficiencies are identical for another wavelength, which guarantees that an optimal operating condition can be found. Table 1 presents the diffraction efficiencies for three different wavelengths with a central wavelength of λ0 = 671 nm. Two operating conditions were evaluated: the first with the selection of the Double Diffraction frequency and the second with the selection of the optimal frequency. The existence and determination of the optimal frequency are discussed in the next section.

From table 1 it is obvious that the (e→o) and (o→e) diffraction efficiencies obtained with the Double Diffraction frequency chosen remain close to each other in the vicinity of λ0 but become very different when considering wavelengths that are not within the vicinity of λ0. A ratio of more than 2 exists between the two efficiencies for wavelengths close to the UV band (at 405 nm for example). Such a huge difference confirms that the choice of fDD is not applicable for wideband equal-efficiency diffraction with a fixed incident angle, and consequently, nor for wideband linear polarization analysis. Such applications would benefit from optimized operating conditions to keep diffraction efficiencies constant even when not within the vicinity of λ0.

Next, we present a method that enables equal diffraction efficiency to be maintained over a wide band with a fixed incident angle, thanks to the choice of an optimal acoustic frequency.

Wideband simultaneous diffraction analysis

For use with a single wavelength, the analysis of linear polarization is simple: the incident angle and the acoustic frequency of the system are set according to the double diffraction point: (fDD(λ), θDD(λ)), as explained in section 2. For a small wavelength shift from DD conditions, the differences are relatively negligible if fDD is still used.

However, for a wideband polychromatic light beam, the DD conditions are too different from one wavelength to another across the entire band (for example, the entire visible band), as explained in section 3. We analyse hereinafter this issue and show that it is possible to obtain the same efficiencies for both diffraction types with a proper operating frequency strategy, which requires no realignment and with quasi-phase-matching leading to high efficiency.

First, the incident angle for each simulation was set to θDD(λ0 = 671 nm) in the context of wideband use of the system across the 400 nm to 700 nm band. We examined the evolution in efficiency as a function of the operating radio frequency and developed a strategy ensuring equal efficiency for both diffraction cases. Indeed, there is still a frequency for which the diffraction efficiency is the same for both the polarization states, even when a wavelength other than λ0 is chosen. The procedure to derive this frequency was initially developed for double pass systems [START_REF] Champagne | Study of the spectral bandwidth of a double-pass acousto-optic system[END_REF] and was adapted as follows. We set a wavelength λ out of the vicinity of λ0 and determined the phase mismatch factor ΔΦ as a function of the radio frequency for the two diffraction cases in the vicinity of fDD. We calculated the corresponding diffraction efficiencies using (1), determined the relative efficiencies (simply the ratio of (e→o) and (o→e) efficiencies), and identified the frequency necessary to obtain a unity ratio. Figure 7 depicts the evolution of the relative efficiencies as a function of the acoustic frequency for λ = 405 nm.

As shown in figure 7, the two curves intersect; there is a frequency for which the efficiencies are equal, leading to relative efficiencies equal to unity. The choice of this specific frequency for a wavelength λ corresponds to an optimal choice, fopt, (λ), as it ensures equal efficiency for the two diffraction cases without the need for realignment.

The choice of the optimal operating frequencies enables the analysis of a linearly polarized light source over a broad bandwidth. Once a practical value of λ0 was set, the procedure to determine the optimal frequencies was performed numerically for every wavelength λ across the entire band of interest: 400 nm to 700 nm. The evolution of fopt, is presented as a function of λ in figure 8.

Figure 9 compares the relative efficiencies of the interactions for the two operating frequency strategies: fDD and fopt, in the case of a fixed incident angle. The relative efficiency is 1 when the interactions present the same efficiencies, i.e. when η(e→o) = η(o→e). As expected, this is the case when the chosen operating frequency strategy corresponds to the optimal frequency fopt. From the curves, it can be seen that for higher wavelengths (λ > 550 nm), the efficiencies are very close whatever the operating strategy used, as the frequencies fDD and fopt are close to each other in the vicinity of λ0, making the efficiency error relatively small. However, for lower wavelengths the efficiencies obtained with fDD are different. Indeed, for example at λ = 405 nm, there is a ratio of less than 0.4 between the efficiencies when the double diffraction frequency is chosen. Hence, for lower wavelengths, the use of fDD is not valid to obtain equal diffraction efficiency for the two polarization states when analysing unpolarized light over a broad bandwidth. Contrariwise, with the optimal operating frequency strategy presented, this capability is maintained across the entire wavelength range.

We then investigated the influence of the choice of central wavelength λ0 on efficiency. As an example, with λ0 = 671 nm, the efficiency curves in figure 6 intersect at an equal efficiency of 0.13. However, is it possible to obtain a higher value with a different central wavelength for alignment? The answer can be found by studying the evolution of the efficiency (equal efficiency) for several central wavelengths. This evolution is plotted in figure 10. Each one of the curves depicted corresponds to the optimal frequency strategy when fopt (λ) is recalculated to satisfy equal efficiency phasematching for each λ0 under consideration. Figure 10 shows that an alignment at λ0 = 437 nm is very efficient. When the system is aligned at this central wavelength, both polarization states are diffracted with an equal efficiency higher than 0.84 over the entire visible band. Finally, it can be noted that the practical implementation of the strategy presented is compatible with modern programmable radio frequency generators based on Direct Digital Synthesis (DDS). The transposition to higher wavelength bands is also possible, which would result in a higher overall efficiency thanks to reduced birefringence.

Conclusion

The use of Acousto-Optic Tuneable Filters for the simultaneous diffraction of wideband unpolarized light has been presented. AOTFs can be used under the usual synchronism condition at the Double Diffraction point over a limited optical band. Herein, we have shown that they can also be used with equal efficiency over a wide bandwidth thanks to optimal operating radio frequency conditions. The usual procedure requires the realignment of the setup for each wavelength, whereas we have studied optimal conditions to avoid the need for realignment. We have presented the evolution of the radio frequency needed to maintain equal diffraction efficiencies for both polarization states over a wide bandwidth and the choice of an appropriate central wavelength for alignment. Under such conditions, diffraction of unpolarized light is possible over the entire visible band with quasi-phase matching leading to high efficiency for both polarization states. This can be useful for evaluating linear polarization. [001]

[110]

Figure 1 .

 1 Figure 1. Scheme of an AOTF and double diffraction of a light beam.

Figure 2 .

 2 Figure 2. Vector diagram of (e→o) and (o→e) interactions simultaneously at synchronism. Wave vectors are represented in blue (light blue for diffracted optical beams, dark blue for incident beam) and red for acoustic. Dotted lines represent the wave vectors loci.

Figure 3 .

 3 Figure 3. Interaction frequencies at synchronism as a function of incident angle for three wavelengths, solid line o->e case; dashed line e->o case. The double diffraction point are highlighted on the graphs. The acoustic cut is a = 8°.

Figure 4 .

 4 Figure 4. Vector diagram of (e→o) and (o→e) interactions when synchronism conditions are not met; the momentum mismatch, Δk, results from: (a) an offset from the Bragg angle of incidence; (b) Δk results from a change in the optical wavelength which modifies the wave vector loci.

Figure 5 .

 5 Figure 5. Evolution of the two diffraction type efficiencies with θ = θDD for λ0 = 671 nm; (a) as a function of wavelength when f = fDD. The efficiencies are maximum for λ = λ0; and (b) as a function of acoustic frequency when λ = λ0. The efficiencies are maximum when f = fDD.

Figure 6 .

 6 Figure 6. Diffraction efficiency evolution for the two interaction types as a function of λ, for θDD (λ0 = 671 nm) and fDD (λ = 405 nm).

Figure 7 .

 7 Figure 7. Relative efficiency of the interactions for λ = 405 nm as a function of the acoustic frequency, with  = DD(λ0 = 671 nm). fDD and fopt are highlighted.

Figure 8 .

 8 Figure 8. Optimal frequencies, θi = θDD (λ0 = 671 nm) and θa =8°.

Figure 9 .

 9 Figure 9. Relative efficiency evolution for the Double Diffraction frequency strategy and the optimal frequency strategy, as a function of wavelength. λ0 = 671 nm.

Figure 10 .

 10 Figure 10. Evolution of the efficiency for the optimal frequency strategy, as a function of wavelength, for different values of central wavelength λ0.

Table 1 .

 1 Diffraction efficiencies as a function of frequency for incident an angle equal to θDD (λ0 = 671 nm).

	Wavelengh Non optimised (double	Optimised for θ i =
		diffraction frequency)		θ DD (671 nm)
	Λ	f DD	η e→o η o→e E	f opt	η e→o =η o→e
	[nm]	[MHz] %	%	%	[MHz] %
	671 (λ 0 )	87,23 100 100 0	87,23 100
	532	116,27 98	98,3 0,3 116,27 98,2
	405	174,19 20,7 10	10,7 174,21 53,2